Semiconductor optical amplifier

Abstract

A semiconductor optical amplifier includes: a laminated structure sequentially including a first compound semiconductor layer composed of GaN compound semiconductor and having a first conductivity type, a third compound semiconductor layer having a light amplification region composed of GaN compound semiconductor, and a second compound semiconductor layer composed of GaN compound semiconductor and having a second conductivity type; a second electrode formed on the second compound semiconductor layer; and a first electrode electrically connected to the first compound semiconductor layer. The laminated structure has a ridge stripe structure. When widths of the ridge stripe structure in a light output end face and the ridge stripe structure in a light incident end face are respectively W_out, and W_in, W_out>W_in is satisfied. A carrier non-injection region is provided in an internal region of the laminated structure from the light output end face along an axis line of the semiconductor optical amplifier.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor optical amplifier.

2. Description of the Related Art

In these days, in the advanced scientific region researches using laser light with the pulse time on the attosecond time scale or on the femtosecond time scale, the ultrashort pulse and ultrahigh power laser is actively used. Further, the high power and ultrashort pulse laser diode device that is composed of GaN compound semiconductor and that has light emitting wavelength of 405 nm band has been expected to be a light source for a volumetric optical disc system expected as a next generation optical disc system displacing the blu-ray optical disc system or has been expected to be a light source demanded in the medical field, the bio imaging field and the like.

As the ultrashort pulse and ultrahigh power laser, for example, titanium/sapphire laser is known. Such a titanium/sapphire laser is an expensive and large solid laser light source, which is a main factor to inhibit spread of the technology. If the ultrashort pulse and ultrahigh power laser is realized with the use of a laser diode or a laser diode device, significant miniaturization, price reduction, and high stability are able to be realized, which is expected to become a breakthrough for promoting its wide usage in these fields.

Meanwhile, short pulsation of the laser diode device has been actively researched since 1960s in the communication system field. As a method of generating short pulses in the laser diode device, gain switching method, loss switching method (Q switching method), and mode locking method are known. In these methods, high output is pursued by combining the laser diode device with a semiconductor amplifier, a nonlinear optical device, an optical fiber and the like. The mode locking is further categorized into active mode locking and passive mode locking. To generate light pulses based on the active mode locking, an external oscillator is configured by using a mirror or a lens, and further high frequency (RF) modulation is added to the laser diode device. Meanwhile, in the passive mode locking, light pulses are able to be generated by simple direct current drive by using a laser diode device having a multiple electrode structure.

In the laser light source, obtaining high power is a big challenge. As a means for amplifying light from the laser light source, the semiconductor optical amplifier (SOA) has been keenly examined. The optical amplifier is an amplifier that directly amplifies an optical signal in a state of light without converting the optical signal to an electric signal. The optical amplifier has a laser structure without resonator, and amplifies incident light by light gain of the amplifier.

In the past, the optical amplifier has been mainly developed for optical communication. Thus, for practical application of the semiconductor optical amplifier in 405 nm band, very few precedent cases exist. For example, based on Japanese Unexamined Patent Application Publication No. 5-067845, the semiconductor optical amplifier in 1.5 μm band that uses GaInAsP compound semiconductor and that has a tapered ridge stripe structure has been known. In the technique disclosed in the foregoing Japanese Unexamined Patent Application Publication No. 5-067845, in the semiconductor optical amplifier, a light guide width is gently extended in tapered shape from the narrow input-side-light guide satisfying single mode conditions to output-side-light guide. Thereby, mode field is expanded along the light guide width to improve maximum output of the semiconductor optical amplifier.

SUMMARY OF THE INVENTION

However, it becomes clear by studies by the inventors of the invention as follows. That is, in the semiconductor optical amplifier composed of GaN compound semiconductor, even if the light guide width on the output side is widened, the width of an outputted near-field image is not expanded and is narrower than the light guide width. The foregoing fact may lead to inhibition of increase of the maximum output of the semiconductor optical amplifier, and instability of laser light outputted from the semiconductor optical amplifier.

Accordingly, in the invention, it is firstly desirable to provide a semiconductor optical amplifier composed of GaN compound semiconductor that is able to retain higher light output. Further, it is secondly desirable to provide a semiconductor optical amplifier with which there is no possibility that laser light outputted from the semiconductor optical amplifier is unstable.

According to a first embodiment to a third embodiment of the invention to attain the foregoing first and the second objects, there is provided a semiconductor optical amplifier including: a laminated structure in which a first compound semiconductor layer that has a first conductivity type and is composed of GaN compound semiconductor, a third compound semiconductor layer that has a light amplification region (carrier non-injection region, gain region) composed of GaN compound semiconductor, and a second compound semiconductor layer that has a second conductivity type different from the first conductivity type and is composed of GaN compound semiconductor are sequentially layered; a second electrode formed on the second compound semiconductor layer; and a first electrode electrically connected to the first compound semiconductor layer, wherein the laminated structure has a ridge stripe structure. When a width of the ridge stripe structure in a light output end face is W_out, and a width of the ridge stripe structure in a light incident end face is W_in, W_out>W_in is satisfied.

In the semiconductor optical amplifier according to the first embodiment of the invention to attain the foregoing first object, a carrier non-injection region is provided in an internal region of the laminated structure from the light output end face along an axis line of the semiconductor optical amplifier.

In the semiconductor optical amplifier according to the second embodiment of the invention to attain the foregoing second object, a width of the second electrode is narrower than the width of the ridge stripe structure.

In the semiconductor optical amplifier according to the third embodiment of the invention to attain the foregoing second object, when a maximum width of the ridge stripe structure is W_max, W_max>W_out is satisfied.

In the semiconductor optical amplifiers according to the first embodiment to the third embodiment of the invention, where the width of the ridge stripe structure in the light output end face is W_out, and the width of the ridge stripe structure in the light incident end face is W_in, W_out>W_in is satisfied. That is, the light guide width is broadened from the light guide on the light output side having a narrow width satisfying single mode conditions to the light guide on the light output side having a wide width. Thus, mode field is able to be expanded according to the light guide width, high light output of the semiconductor optical amplifier is able to be attained, and laser light is able to be optically amplified while single lateral mode is maintained.

Further, in the semiconductor optical amplifier according to the first embodiment of the invention, the carrier non-injection region is provided in the internal region of the laminated structure from the light output end face along the axis line of the semiconductor optical amplifier. Thus, the width of laser light outputted from the light output end face is able to be broadened. Therefore, higher light output is able to be attained, and reliability is able to be improved. Meanwhile, in the semiconductor optical amplifier according to the second embodiment of the invention, the width of the second electrode is narrower than the width of the ridge stripe structure. In the semiconductor optical amplifier according to the third embodiment of the invention, when the maximum width of the ridge stripe structure is W_max, W_max>W_out is satisfied. Thereby, stable lateral mode amplified light is obtained, and there is no possibility that laser light outputted from the semiconductor optical amplifier becomes unstable.

Other and further objects, features and advantages of the invention will appear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual view of a light output device of a first example including a semiconductor optical amplifier.

FIG. 2 is a schematic cross sectional view of the semiconductor optical amplifier where the semiconductor optical amplifier is cut along a virtual vertical plane (YZ plane) including an axis line (Z direction) of the semiconductor optical amplifier of the first example.

FIG. 3 is a schematic cross sectional view of the semiconductor optical amplifier where the semiconductor optical amplifier is cut along a virtual vertical plane (XY plane) orthogonal to the axis line of the semiconductor optical amplifier of the first example.

FIG. 4 is a schematic perspective view of the semiconductor optical amplifier of the first example.

FIG. 5 is a schematic plan view of a ridge stripe structure in the semiconductor optical amplifier of the first example.

FIGS. 6A and 6B are respectively near-field images of laser light outputted from the semiconductor optical amplifier of the first example and a semiconductor optical amplifier of Comparative example 1.

FIG. 7 is a conceptual view of a light output device of a second example including the semiconductor optical amplifier.

FIG. 8 is a schematic cross sectional view of the semiconductor optical amplifier where the semiconductor optical amplifier is cut along a virtual vertical plane (YZ plane) including an axis line (Z direction) of the semiconductor optical amplifier of the second example.

FIG. 9 is a schematic cross sectional view of the semiconductor optical amplifier where the semiconductor optical amplifier is cut along a virtual vertical plane (XY plane) orthogonal to the axis line of the semiconductor optical amplifier of the second example.

FIG. 10 is a schematic end view taken along the direction in which a resonator of a mode locking laser diode device in the second example extends.

FIG. 11 is a schematic perspective view of the semiconductor optical amplifier of the second example.

FIG. 12 is a schematic plan view of a ridge stripe structure in the semiconductor optical amplifier of the second example.

FIG. 13 is a graph schematically illustrating change of a current flown in the semiconductor optical amplifier in the case where a given value of voltage is applied to the semiconductor optical amplifier while making laser light enter the semiconductor optical amplifier of the second example from the laser light source and XYZ stage is moved in the X direction.

FIG. 14A is a conceptual view of a modification of the light output device of the second example, and FIG. 14B is a conceptual view of a monolithic semiconductor optical amplifier.

FIGS. 15A and 15B are schematic perspective views of semiconductor optical amplifiers according to a third example and a fourth example.

FIG. 16 is a schematic plan view of a ridge stripe structure of the semiconductor optical amplifier of the third example illustrated in FIG. 15A.

FIGS. 17A and 17B are schematic perspective views of semiconductor optical amplifiers of modifications of the third example and the fourth example.

FIG. 18 is a schematic plan view of a ridge stripe structure of the semiconductor optical amplifier of the modification of the third example illustrated in FIG. 17A.

FIGS. 19A and 19B are views respectively and schematically illustrating a system of performing mode locking drive by configuring an external resonator from the mode locking laser diode device in the second example and a mode locking laser diode device in a sixth example.

FIGS. 20A and 20B are views respectively and schematically illustrating a system of performing mode locking drive by configuring an external resonator from a mode locking laser diode device in a fifth example, and FIG. 20C is a view schematically illustrating a system of performing mode locking drive by using the mode locking laser diode device in the sixth example.

FIG. 21 is a schematic view viewed from above of a ridge section in a mode locking laser diode device of a seventh example.

FIGS. 22A and 22B are views respectively and schematically illustrating a system of performing mode locking drive by using a mode locking laser diode device in an eighth example and a mode locking laser diode device in a ninth example.

FIG. 23 is a schematic end view taken along a direction in which a resonator of a modification of the mode locking laser diode device in the second example is extended.

FIG. 24 is a schematic end view taken along a direction in which a resonator of another modification of the mode locking laser diode device in the second example is extended.

FIGS. 25A and 25B are graphs illustrating reverse bias voltage dependence measurement results of relation between an injection current and light output (L-I characteristics) in the second example and a second referential example.

FIGS. 26A and 26B are views illustrating results obtained by measuring light pulse generated in the second example and the second referential example by a streak camera.

FIG. 27 is a graph illustrating result obtained by measuring an electric resistance value between a first section and a second section of a second electrode of the mode locking laser diode device obtained in the second example by four terminal method.

FIGS. 28A and 28B are graphs respectively illustrating results of measuring RF spectrum of the eighth example and an eighth referential example.

FIGS. 29A and 29B are schematic partial cross sectional views of a substrate and the like for explaining a method of manufacturing the mode locking laser diode device in the second example.

FIGS. 30A and 30B are schematic partial cross sectional views of a substrate and the like for explaining a method of manufacturing the mode locking laser diode device in the second example following FIG. 29B.

FIG. 31 is a schematic partial end view of a substrate and the like for explaining a method of manufacturing the mode locking laser diode device in the second example following FIG. 30B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the invention will be hereinafter described based on examples with reference to the drawings, the invention is not limited to the examples, and various numerical values and materials in the examples are exemplification. The description will be given in the following order:

1. Semiconductor optical amplifiers according to a first embodiment to a third embodiment of the invention and overall description

2. First example (the semiconductor optical amplifier according to the first embodiment of the invention)

3. Second example (modification of the first example)

4. Third example (the semiconductor optical amplifiers according to the second embodiment and the third embodiment of the invention)

5. Fourth example (modification of the third example)

6. Fifth example (modification of the mode locking laser diode device in the second example)

7. Sixth example (another modification of the mode locking laser diode device in the second example)

8. Seventh example (another modification of the mode locking laser diode device in the second example)

9. Eighth example (another modification of the mode locking laser diode device in the second example)

10. Ninth example (another modification of the mode locking laser diode device in the second example) and others

Semiconductor Optical Amplifiers According to a First Embodiment to a Third Embodiment of the Invention and Overall Description

In the semiconductor optical amplifier according to the first embodiment of the invention, W_out may be 5 μm or more. Though the upper limit of W_out is not limited, for example, 4×10² μm can be exemplified as the upper limit of W_out. Further, in the semiconductor optical amplifier according to the first embodiment of the invention, W_in may be from 1.4 μm to 2.0 μm both inclusive. The foregoing preferred embodiments may be applied to the semiconductor optical amplifiers according to the second embodiment and the third embodiment of the invention.

In the semiconductor optical amplifier according to the second embodiment of the invention, the value of (width of a second electrode)/(width of a ridge stripe structure) is desirably from 0.2 to 0.9 both inclusive, and is preferably from 0.6 to 0.9 both inclusive. The width of the second electrode and the width of the ridge stripe structure mean the width of the second electrode and the width of the ridge stripe structure obtained where the semiconductor optical amplifier is cut in a certain virtual plane orthogonal to the axis line of the semiconductor optical amplifier.

Further, in the semiconductor optical amplifier according to the third embodiment of the invention, 0.2≦W_out/W_max≦0.9 is desirably satisfied, and 0.5≦W_out/W_max≦0.9 is preferably satisfied.

In the semiconductor optical amplifiers according to the second embodiment and the third embodiment of the invention, as in the semiconductor optical amplifier according to the first embodiment of the invention, a carrier non-injection region may be provided in an internal region of a laminated structure from a light output end face along the axis line of the semiconductor optical amplifier. In the semiconductor optical amplifiers according to the first embodiment to the third embodiment of the invention, further, the carrier non-injection region may be also provided in an internal region of the laminated structure from a light incident end face along the axis line of the semiconductor optical amplifier.

Further, in the semiconductor optical amplifiers according to the first embodiment to the third embodiment of the invention, it is possible that the second electrode is not provided in the carrier non-injection region, or it is possible that the second electrode is composed of a first section and a second section that are separated by an isolation trench, and the second section of the second electrode is provided in the carrier non-injection region. In the latter case, a voltage equal to or less than a built-in voltage is desirably applied to the second section of the second electrode. Specifically, a voltage equal to or less than (1.2398/λ) is desirably applied to the second section of the second electrode. λ represents wavelength of incident laser light to the semiconductor optical amplifier (unit: μm), and “1.2398” represents constant number. For example, in the case where 0.4 μm wavelength laser light enters the semiconductor optical amplifier, a voltage equal to or less than 3.0995 volt is desirably applied. Though not limited, as the lower limit value of voltage applied to the second section of the second electrode, −20 volt can be exemplified. Light amplification as an inherent function of the semiconductor optical amplifier is able to be performed by applying a voltage to the first section of the second electrode, while monitoring light intensity and measurement for position adjustment and the like are able to be performed by applying a voltage to the second section of the second electrode. For such a point, a description will be given in detail later. Further, near-field image is able to be controlled.

Further, in the semiconductor optical amplifiers according to the first embodiment to the third embodiment of the invention, the axis line of the semiconductor optical amplifier may intersect with the axis line of the ridge stripe structure at a given angle. As given angle θ, 0.1 deg≦θ≦10 deg can be exemplified. The axis line of the ridge stripe structure is a straight line that connects the point obtained by equally dividing the line between both ends of the ridge stripe structure in the light output end face with the point obtained by equally dividing the line between both ends of the ridge stripe structure in the light incident end face.

Further, in the semiconductor optical amplifiers according to the first embodiment to the third embodiment of the invention, a low reflective coating layer formed from a laminated structure composed of at least two types of layers selected from the group consisting of a titanium oxide layer, a tantalum oxide layer, a zirconia oxide layer, a silicon oxide layer, and an aluminum oxide layer may be formed in the light incident end face and the light output end face.

Further, in the semiconductor optical amplifiers according to the first embodiment to the third embodiment of the invention, though not limited, the light intensity density of laser light outputted from the semiconductor optical amplifier may be 60 kilowatt or more per 1 cm² of a third compound semiconductor structuring the light output end face, and may be preferably 600 kilowatt or more.

Further, in the semiconductor optical amplifiers according to the first embodiment to the third embodiment of the invention, the value of (width of the ridge stripe structure in the light output end face)/(width of laser light outputted from the semiconductor optical amplifier) may be from 1.1 to 10 both inclusive, and may be preferably from 1.1 to 5 both inclusive.

Further, in the semiconductor optical amplifiers according to the first embodiment to the third embodiment of the invention (hereinafter, in some cases, such semiconductor optical amplifiers will be generically and simply referred to as “semiconductor optical amplifier of the embodiment of the invention,”) the semiconductor optical amplifier may be composed of a transmissive semiconductor optical amplifier. However, the semiconductor optical amplifier is not limited to the transmissive semiconductor optical amplifier. For example, the semiconductor optical amplifier may be composed of a monolithic semiconductor optical amplifier.

In the semiconductor optical amplifier of the embodiment of the invention, W_out>W_in is satisfied where the width of the ridge stripe structure in the light output end face is W_out, and the width of the ridge stripe structure in the light incident end face is W_in. In this case, each end section of the ridge stripe structure may be composed of one line segment (the semiconductor optical amplifiers according to the first embodiment and the second embodiment of the invention), or may be composed of two or more line segments (the semiconductor optical amplifiers according to the first embodiment to the third embodiment of the invention). In the former case, for example, the width of the ridge stripe structure is gently and flatly extended in tapered shape from the light incident end face to the light output end face. Meanwhile, in the latter case and in the semiconductor optical amplifiers according to the first embodiment and the second embodiment of the invention, for example, the width of the ridge stripe structure is firstly the same, and is next gently and flatly extended in tapered shape from the light incident end face to the light output end face. Further, in the latter case and in the semiconductor optical amplifier according to the second embodiment of the invention, for example, the width of the ridge stripe structure is firstly widened, and is next narrowed after the width exceeds the maximum width from the light incident end face to the light output end face.

In the semiconductor optical amplifier according to the first embodiment of the invention or the semiconductor optical amplifiers according to the second embodiment to the third embodiment of the invention, the carrier non-injection region is provided in the internal region of the laminated structure from the light output end face along the axis line of the semiconductor optical amplifier. As length of the carrier non-injection region along the axis line of the semiconductor optical amplifier (width of the carrier non-injection region) L_NC, a value from 0.1 μm to 100 μm both inclusive can be exemplified.

Further, in the semiconductor optical amplifier according to the first embodiment of the invention or the semiconductor optical amplifiers according to the second embodiment to the third embodiment of the invention, the second electrode is composed of the first section and the second section that are separated by the isolation trench, and the second section of the second electrode is provided in the carrier non-injection region. When the length of the first section is L_Amp-1 and the length of the second section is L_Amp-2, 0.001≦L_Amp-2/L_Amp-1≦0.01 is desirably satisfied, and 0.0025≦L_Amp-2/L_Amp-1≦0.01 is preferably satisfied. The electric resistance value between the first section and the second section of the second electrode in the semiconductor optical amplifier is 1×10²Ω or more, is preferably 1×10³Ω or more, and is more preferably 1×10⁴Ω or more. Further, the electric resistance value between the first section and the second section of the second electrode is 1×10 times or more the electric resistance value between the second electrode and the first electrode, is preferably 1×10² times or more the electric resistance value between the second electrode and the first electrode, and is more preferably 1×10³ times or more the electric resistance value between the second electrode and the first electrode. Further, the width of the isolation trench that separates the second electrode into the first section and the second section is desirably 1 μm or more and 50% or less the length of the semiconductor optical amplifier, and is preferably 10 μm or more and 10% or less as much as the length of the semiconductor optical amplifier. Further, as the width of the isolation trench, a value from 3 μm to 20 μm both inclusive can be exemplified. As the length of the second section of the second electrode L_Amp-2, a value from 3 μm to 100 μm both inclusive can be exemplified.

In the semiconductor optical amplifier of the embodiment of the invention, a laser light source may be composed of a mode locking laser diode device, and pulse laser light outputted from the mode locking laser diode device may enter the semiconductor optical amplifier. In this case, the laser light source may output pulse laser light based on mode locking operation. However, the laser light source is not limited thereto. A known continuous oscillation laser light source, known various types of pulse oscillation laser light sources such as a gain switching laser light source and a loss switching laser light source (Q switching laser light source), and a laser light source such as a titanium sapphire laser are able to be used. The semiconductor optical amplifier of the embodiment of the invention is an amplifier that directly amplifies an optical signal in a state of light without converting the optical signal to an electric signal. The semiconductor optical amplifier of the embodiment of the invention has a laser structure excluding resonator effect as much as possible, and amplifies incident light by light gain of the semiconductor optical amplifier. That is, the semiconductor optical amplifier of the embodiment of the invention may substantially have the same composition and the same configuration as those of the laser diode device structuring the laser light source of the embodiment of the invention, and may have a composition and a configuration different from those of the laser diode device structuring the laser light source of the embodiment of the invention.

In the semiconductor optical amplifier of the embodiment of the invention, in the case where the laser light source is composed of the mode locking laser diode device as described above, the mode locking laser diode device (hereinafter referred to as “mode locking laser diode device of the embodiment of the invention”) may include: a laminated structure in which a first compound semiconductor layer that has a first conductivity type and is composed of GaN compound semiconductor, a third compound semiconductor layer that has a light emitting region composed of GaN compound semiconductor, and a second compound semiconductor layer that has a second conductivity type different from the first conductivity type and is composed of GaN compound semiconductor are sequentially layered; the second electrode formed on the second compound semiconductor layer; and a first electrode electrically connected to the first compound semiconductor layer. The laminated structure may be formed on a compound semiconductor substrate having polarity. The third compound semiconductor layer may have a quantum well structure including a well layer and a barrier layer. In addition, though not limited, the thickness of the well layer is from 1 nm to 10 nm both inclusive, and is preferably from 1 nm to 8 nm both inclusive. The impurity doping concentration of the barrier layer is from 2×10¹⁸ cm⁻³ to 1×10²⁰ cm⁻³ both inclusive, and is preferably from 1×10¹⁹ cm⁻³ to 1×10²⁰ cm⁻³ both inclusive.

In driving the mode locking laser diode device of the embodiment of the invention, light pulse may be generated in the light emitting region by flowing a current from the second electrode to the first electrode through the laminated structure. Further, in the mode locking laser diode device of the embodiment of the invention, light pulse may be generated in the light emitting region by flowing a current from the second electrode to the first electrode through the laminated structure.

To enable mode locking operation of the laser diode device, the light emitting region and a saturable absorption region should be provided for the laser diode device. Based on arrangement state of the light emitting region and the saturable absorption region, the laser diode device is able to be generally categorized into SAL (saturable absorber layer) type or WI (weakly index guide) type in which the light emitting region and the saturable absorption region are arranged in vertical direction, and multielectrode type including bisection type in which the light emitting region and the saturable absorption region are arranged in line in the resonator direction. In the mode locking method, in a cubic (mainly GaAs) laser diode device, it has been confirmed that light pulse with time width of 0.6 psec is able to be generated (see “Appl. Phys. Lett. 39 (1981) 525,”
As described above, since the high etching selection ratio exists between the second electrode 162 and the second compound semiconductor layer 150, the second electrode 162 is able to be surely etched without etching the laminated structure (or even if the laminated structure is etched, the etching amount is slight).

Step-240

After that, the n-side electrode 161 is formed, the substrate is cleaved, and further packaging is made. Accordingly, the mode locking laser diode device 110 is able to be fabricated.

In general, resistance R (Ω) of a semiconductor layer is expressed as follows by using specific resistance value ρ (Ω·m) of a material composing a semiconductor layer, length of the semiconductor layer X₀ (m), cross section area S of the semiconductor layer (m²), carrier density n (cm⁻³), electric charge amount e (C), and mobility μ (m²/V sec).
R=(ρ·X₀)/S=X₀/(n·e·μ·S)

Since mobility of the p-type GaN semiconductor is two-digit or more smaller than that of the p-type GaAs semiconductor, the electric resistance value gets high easily. Thus, it is found that the electric resistance value of the laser diode device having a ridge stripe structure with small cross section area being 1.5 μm wide and 0.35 μm high becomes a large value based on the foregoing formula.

FIG. 27 illustrates a result obtained by measuring an electric resistance value between the first section 162A and the second section 162B of the second electrode 162 of the fabricated mode locking laser diode device 110 of the second example by four terminal method. When the width of the isolation trench 162C was 20 μm, the electric resistance value between the first section 162A and the second section 162B of the second electrode 162 was 15 kΩ.

In the fabricated mode locking laser diode device 110 of the second example, forward bias state was obtained by flowing a direct current from the first section 162A of the second electrode 162 to the first electrode 161 through the light emitting region 141, and electric field was applied to the saturable absorption region 142 by applying reverse bias voltage V_sa between the first electrode 161 and the second section 162B of the second electrode 162, and thereby mode locking drive was performed.

Further, the electric resistance value between the first section 162A and the second section 162B of the second electrode 162 is ten times or more the electric resistance value between the second electrode 162 and the first electrode 161, or 1×10²Ω or more. Thus, flow of leakage current from the first section 162A of the second electrode 162 to the second section 162B of the second electrode 162 is able to be inhibited securely. In the result, the light emitting region 141 is able to be in forward bias state, the saturable absorption region 142 is securely able to be in reverse bias state, and mode locking operation is able to be securely performed.

Further, the semiconductor optical amplifier 200 is able to be manufactured by the same manufacturing method as that of the mode locking laser diode device 110, except that the structure of the second electrode is different. Thus, detailed description thereof will be omitted.

To promote better understanding of the mode locking laser diode device of the second example, a mode locking laser diode device of a second referential example was fabricated. In the mode locking laser diode device of the second referential example, the structure of the third compound semiconductor layer 140 in the layer structure illustrated in Table 2 was as illustrated in the following Table 3.

TABLE 3
		Second referential
	Second example	example
Well layer	8 nm	10.5 nm
Barrier layer	12 nm	14 nm
Impurity doping concentration	Non-doped	Non-doped
of well layer
Impurity doping concentration	Si: 2 × 1018 cm−3	Non-doped
of barrier layer

In the second example, the thickness of the well layer is 8 nm, the barrier layer is doped with Si at a concentration of Si: 2×10¹⁸ cm⁻³, and QCSE effect in the third compound semiconductor layer is modified. Meanwhile, in the second referential example, the thickness of the well layer is 10.5 nm, and the barrier layer is not doped with impurity.

A light condensing external resonator was formed from the mode locking laser diode devices of the second example and the second referential example, and mode locking driving was performed (refer to FIG. 19A). In the light condensing external resonator illustrated in FIG. 19A, the external resonator is configured of the end face of the mode locking laser diode device in which a high reflective coating layer (HR) is formed on the saturable absorption region side and the external mirror 13, and light pulse is extracted from the external mirror 13. A low reflective coating layer (AR) is formed on the end face (light output end face) of the mode locking laser diode device on the light emitting region (gain region) side. As the optical filter 12, a bandpass filter is mainly used, which is inserted for controlling laser oscillation wavelength. Repetition frequency f of light pulse train is determined by the external resonator length Z′ as expressed by the following formula, where c represents light velocity and n represents reflective index of waveguide.
f=c/(2n·Z′)

Mode locking is determined by a direct current applied to the light emitting region 141 and the reverse bias voltage V_sa applied to the saturable absorption region 142. FIGS. 25A and 25B illustrate reverse bias voltage dependence measurement results of relation between an injection current and light output (L-I characteristics) of the second example and the second referential example. In FIGS. 25A and 25B, measurement results affixed with referential symbol “A” are results in the case of the reverse bias voltage V_sa=0 volt, measurement results affixed with referential symbol “B” are results in the case of the reverse bias voltage V_sa=−3 volt, measurement results affixed with referential symbol “C” are results in the case of the reverse bias voltage V_sa=−6 volt, and measurement results affixed with referential symbol “D” are results in the case of the reverse bias voltage V_sa=−9 volt. In FIG. 25A, the measurement result in the case of the reverse bias voltage V_sa=0 volt almost overlaps the measurement result in the case of the reverse bias voltage V_sa=−3 volt.

Based on comparison between FIGS. 25A and 25B, it is found that in the second referential example, as the reverse bias voltage V_sa is increased, the threshold current I_th at which laser oscillation is started is gradually increased, and change is shown at lower reverse bias voltage V_sa compared to in the second example. It indicates that in the third compound semiconductor layer 140 of the second example, effect of saturable absorption is electrically controlled more by the reverse bias voltage V_sa.

FIGS. 26A and 26B illustrate results obtained by measuring light pulse generated in the second example and the second referential example by a streak camera. In FIG. 26B obtained in the second referential example, subpulse component is generated before and after main pulse. Meanwhile, in FIG. 26A obtained in the second example, subpulse component is inhibited from being generated. The results may be all caused by increased effect of saturable absorption since QCSE effect is moderated by the structure of the third compound semiconductor layer 140.

Drive conditions and the like of the mode locking laser diode device of the second example illustrated in FIG. 19A are exemplified in the following Table 4. I_th represents a threshold current.

TABLE 4

Mode locking drive conditions: 0 < 1gain/Ith ≦ 5
−20 ≦ Vsa (volt) ≦ 0
High reflective coating layer (HR): 85 ≦ reflectance RHR (%) < 100
Low reflective coating layer (AR): 0 < reflectance RAR (%) ≦ 0.5
Optical filter: 85 ≦ transmittance TBPF (%) < 100
0 < half bandwidth τBPF (nm) ≦ 2.0
400 < peak wavelength λbpf (nm) < 450
External mirror: 0 < reflectance Roc (%) < 100
External resonator length Z′
0 < Z′ (mm) < 1500

More specifically, in the second example, the following conditions were adopted as an example:

• I_gain: 120 mA
• I_th: 45 mA
• Reverse bias voltage V_sa: −11 (volt)
• Reflectance R_HR: 95%
• Reflectance R_AR: 0.3%
• Transmittance T_BPF: 90%
• Half bandwidth τ_BPF: 1 nm
• Peak wavelength λ_BPF: 410 nm
• Reflectance R_OC: 20%
• External resonator length Z′: 150 mm

Meanwhile, in the second referential example, the same conditions as those of the second example were adopted except for the following conditions:

• I_gain: 95 mA
• I_th: 50 mA
• Reverse bias voltage V_sa: −12.5 (volt)
• Reflectance R_OC: 50%

As illustrated in the conceptual view of FIG. 14A, part of light output of laser light outputted from the semiconductor optical amplifier 200 is extracted by using a beam splitter 32, and extracted light enters a photodiode 34 through a lens 33. Thereby, the light output of laser light outputted from the semiconductor optical amplifier 200 may be measured. In the case where light output is changed from a desired value, alignment method of the semiconductor optical amplifier of the second example is executed again. That is, a given value of voltage V₀ is applied to the semiconductor optical amplifier 200 while laser light enters the semiconductor optical amplifier 200 from the laser light source 100, and thereby the relative position of the semiconductor optical amplifier 200 with respect to laser light entering the semiconductor optical amplifier 200 is adjusted again so that a current flown in the semiconductor optical amplifier 200 becomes the maximum. In the case where result of readjustment of the relative position of the semiconductor optical amplifier 200 with respect to laser light entering the semiconductor optical amplifier 200 is the same as the relative position of the semiconductor optical amplifier with respect to laser light entering the semiconductor optical amplifier 200 before readjustment, light path through which the laser light outputted from the semiconductor optical amplifier 200 passes is adjusted. Such adjustment may be performed by, for example, laying a reflective mirror 31 on an XYZ stage 35. The XYZ stage 35 may be moved by an operator. Otherwise, the XYZ stage 35 is able to be automatically moved by direction of the semiconductor optical amplifier control device 400 based on the voltage and measurement result of the photodiode 34. In FIG. 14A, elements of the light output device located in the upstream of the semiconductor optical amplifier 200 are the same as the elements of the light output device of the second example, and thus the elements of the light output device located in the upstream of the semiconductor optical amplifier 200 are not illustrated in the figure. By adopting such a method, in the case where change occurs in the light output monitor, it is possible to easily determine whether or not such change is caused by relative position change of the semiconductor optical amplifier 200 with respect to laser light entering the semiconductor optical amplifier 200 (that is, change of efficiency of coupling of the entrance laser light and the light guide of the semiconductor optical amplifier).

Third Example

The third example relates to the semiconductor optical amplifiers according to the second embodiment and the third embodiment of the invention. FIGS. 15A and 16 illustrate a schematic perspective view of the semiconductor optical amplifier and a schematic plan view of a ridge stripe structure according to the second embodiment of the invention of the third example. The width of the second electrode 262 is narrower than the width of the ridge stripe structure. In this case, (width of the second electrode)/(width of the ridge stripe structure) satisfies a value from 0.2 to 0.9 both inclusive. Further, FIGS. 17A and 18 illustrate a schematic perspective view of the semiconductor optical amplifier and a schematic plan view of a ridge stripe structure according to the third embodiment of the invention of the third example. Where the maximum width of the ridge stripe structure is W_max, W_max/W_out is satisfied, and 0.2≦W_out/W_max≦0.9 is satisfied. In FIG. 18, though the second electrode 262 is not illustrated, the second electrode 262 is formed from the p-type GaN contact layer corresponding to the top face of the ridge section to part of the top face of the p-type AlGaN cladding layer as in the first example.

A composition and a structure of the semiconductor optical amplifier of the third example are the same as the composition and the structure of the semiconductor optical amplifier described in the first example except for the foregoing points or except that the carrier non-injection region is not provided, and thus detailed description thereof will be omitted.

As illustrated in FIG. 6B, in the case where the width of the near-field image is narrower than W_out, there is a possibility that light field becomes unstable depending on drive conditions and light output conditions such as the light density, the carrier diffusion length, and device temperature. Thus, in the third example, by adopting the foregoing composition and the foregoing structure, mode instability is modified.

Fourth Example

The fourth example is a modification of the third example. FIG. 15B illustrates a schematic perspective view of a modified example of the semiconductor optical amplifier illustrated in FIGS. 15A and 16, and FIG. 17B illustrates a schematic perspective view of a modified example of the semiconductor optical amplifier illustrated in FIGS. 17A and 18. As illustrated in FIG. 15B and FIG. 17B, in the fourth example, differently from the third example, a carrier non-injection region is provided in the internal region of the laminated structure from the light output end face along the axis line of the semiconductor optical amplifier. A composition and a structure of the semiconductor optical amplifier of the fourth example are the same as the composition and the structure of the semiconductor optical amplifier described in the third example except for the foregoing points, and thus detailed description thereof will be omitted. In the forth example, the second electrode may be separated into the first section and the second section by the isolation trench as in the second example.

Fifth Example

The fifth example is a modification of the mode locking laser diode device in the second example. FIGS. 19B, FIG. 20A, and FIG. 20B illustrate an example in which an external resonator is structured by the mode locking laser diode device of the fifth example.

In the collimation type external resonator illustrated in FIG. 19B, the external resonator is formed from the end face of the mode locking laser diode device in which a high reflective coating layer (HR) is formed on the saturable absorption region side and the external mirror, and light pulse is extracted from the external mirror. A low reflective coating layer (AR) is formed on the end face (light output end face) of the mode locking laser diode device on the light emitting region (gain region) side. The drive conditions and the like of the mode locking laser diode device of the fifth example illustrated in FIG. 19B are similar to those of the foregoing Table 4.

Meanwhile, in the external resonator illustrated in FIGS. 20A and 20B, the external resonator is formed from the end face of the mode locking laser diode device in which a reflective coating layer (R) is formed on the saturable absorption region side (light output end face) and the external mirror, and light pulse is extracted from the saturable absorption region 142. A low reflective coating layer (AR) is formed on the end face of the mode locking laser diode device on the light emitting region (gain region) side. The example illustrated in FIG. 20A is light condensing type, and the example illustrated in FIG. 20B is collimation type. The drive conditions and the like of the mode locking laser diode device of the fifth example illustrated in FIGS. 20A and 20B are similar to those of the foregoing Table 4. However, the reflective coating layer (R) may be as illustrated in the following Table 5.

TABLE 5

Reflective coating layer (R)
0 < reflectance RR (%) < 100

Specifically, reflectance R_R was set to 20%. A composition and a structure of the mode locking laser diode device in the fifth example are the same as the composition and the structure of the mode locking laser diode device described in the second example, and thus detailed description thereof will be omitted.

Sixth Example

The sixth example is also a modification of the mode locking laser diode device of the second example. In the sixth example, as illustrated in FIG. 20C, the mode locking laser diode device is monolithic type. The drive conditions and the like of the mode locking laser diode device of the sixth example are similar to those of the foregoing Table 4. Other composition and other structure of the mode locking laser diode device of the sixth example are similar to the composition and the structure of the mode locking laser diode device described in the second example, and thus detailed description thereof will be omitted.

Seventh Example

The seventh example is also a modification of the mode locking laser diode device in the second example. The mode locking laser diode device of the seventh example is a laser diode device having a ridge stripe type separate confinement heterostructure with oblique light guide. FIG. 21 illustrates a schematic view viewed from above of a ridge section 158A in the mode locking laser diode device of the seventh example. The mode locking laser diode device of the seventh example has a structure in which two straight line-like ridge sections. A value of angle θ′ of intersection of the two ridge sections desirably satisfies, for example, 0<θ′≦10 (deg), and preferably satisfies 0<θ′≦6 (deg). By adopting the oblique ridge stripe type, reflectance of the end face provided with low reflective coating is able to be closer to 0% as the ideal value. In the result, generation of light pulse that would revolve in the laser diode device is able to be prevented, and generation of sub-light pulse associated with main light pulse is able to be inhibited. The oblique ridge stripe type mode locking laser diode device of the seventh example is applicable to the second example, the fifth example, and the sixth example. Other composition and other structure of the mode locking laser diode device in the seventh example are similar to the composition and the structure of the mode locking laser diode device described in the second example, and thus detailed description thereof will be omitted.

Eighth Example

The eighth example is also a modification of the mode locking laser diode device in the second example. In the eighth example, a current is flown from the second electrode 162 to the first electrode 161 through the light emitting region 141, and an external electric signal (RMS jitter Δ_signal) is superimposed on the first electrode 161 from the second electrode 162 through the light emitting region 141. FIG. 22A schematically illustrates a system of performing mode locking drive by using the mode locking laser diode device of the eighth example. The external electric signal is sent from a known external electric signal generator to the second electrode 162. Thereby, light pulse is able to be sync with the external electric signal. That is, RMS timing jitter Δt_MILD is able to be kept down as the following formula: Δ_signal≦Δt_MILD.

The drive conditions and the like of the mode locking laser diode device of the eighth example illustrated in FIG. 22A are similar to those of the foregoing Table 4. Voltage maximum value V_p-p (unit: volt) of the external electric signal desirably satisfies 0<V_p-p≦10, and preferably satisfies 0<V_p-p≦3. Further, frequency f_signal of the external electric signal and repetition frequency f_MILD of a light pulse train desirably satisfy 0.99≦f_signal/f_MILD≦1.01.

More specifically, in the eighth example, the following conditions were adopted as an example:

• I_gain: 120 mA
• I_th: 45 mA
• Reverse bias voltage V_sa: −11 (volt)
• Reflectance R_HR: 95%
• Reflectance R_AR: 0.3%
• Transmittance T_BPF: 90%
• Half bandwidth τ_BPF: 1 nm
• Peak wavelength λ_BPF: 410 nm
• Reflectance R_OC: 20%
• External resonator length Z′: 150 mm
• V_p-p: 2.8 volt
• f_signal: 1 GHz
• f_MILD: 1 GHz
• Δ_signal: 1 picosecond
• Δt_MILD: 1.5 picosecond

Meanwhile, in the eighth referential example, a current was flown from the second electrode 162 to the first electrode 161 through the light emitting region 141 without superimposing an external electric signal on the first electrode 161 from the second electrode 162 through the light emitting region 141. RF spectrum was measured. FIGS. 28A and 28B illustrate measurement results in the eighth example and the eighth referential example. In the eighth referential example, the same conditions as those of the eighth example were adopted except for the following conditions:

Reflectance R_OC: 50%

FIGS. 28A and 28B show that in the eighth example, the area of bottom component of RF spectrum is decreased more than in the eighth referential example. Such a fact shows that the eighth example is a drive method in which the phase noise and the timing jitter are smaller compared to those of the eighth referential example.

Other composition and other structure of the mode locking laser diode device in the eighth example are similar to the composition and the structure of the mode locking laser diode device described in the second example, the fifth example, the sixth example, and the seventh example, and thus detailed description thereof will be omitted.

Ninth Example

The ninth example is also a modification of the mode locking laser diode device in the second example. In the ninth example, an optical signal enters from one end face of the laminated structure. FIG. 22B schematically illustrates a system of performing mode locking drive by using the mode locking laser diode device of the ninth example. The optical signal (RMS jitter: Δ_opto) is outputted from an optical signal generator composed of the laser diode device, and enters one end face of the laminated structure through a lens, an external mirror, an optical filter, and a lens. Thereby, light pulse is able to be sync with the optical signal. That is, the RMS timing jitter Δt_MILD is able to be kept down as the following formula. Δ_opto≦Δt_MILD.

Other composition and other structure of the mode locking laser diode device in the ninth example are similar to the composition and the structure of the mode locking laser diode device described in the second example, the fifth example, the sixth example, and the seventh example, and thus detailed description thereof will be omitted.

Descriptions have been hereinbefore given of the invention with reference to the preferred embodiments. However, the invention is not limited to the foregoing embodiments. The compositions and the structures of the semiconductor optical amplifier, the light output device, the laser light source, and the laser diode device described in the embodiments are just exemplified, and modifications may be made as appropriate. Further, in the embodiments, though various values have been shown, such various values are just exemplified as well, and thus it is needless to say that, for example, if specifications of the semiconductor optical amplifier, the light output device, and the laser diode device to be used are changed, values are also changed. For example, the second electrode 162 may have a laminated structure including a lower metal layer composed of palladium (Pd) having a thickness of 20 nm and an upper metal layer composed of nickel (Ni) having a thickness of 200 nm. In performing wet etching with the use of aqua regia, the etching rate of nickel is about 1.25 times the etching rate of palladium.

In the embodiments, the semiconductor optical amplifier is composed of a transmissive semiconductor optical amplifier. However, the semiconductor optical amplifier is not limited thereto. As illustrated in a conceptual view of FIG. 14B, the semiconductor optical amplifier may be composed of a monolithic semiconductor optical amplifier. The monolithic semiconductor optical amplifier is an integrated body composed of a laser diode device and a semiconductor optical amplifier.

In the embodiments, the mode locking laser diode device 110 is provided on the {0001} plane, which is the C plane as the polarity plane of the n-type GaN substrate 121. Alternately, the mode locking laser diode device 110 may be provided on A plane as {11-20} plane, M plane as {1-100} plane, non-polarity plane such as {1-102} plane, {11-2n} plane including {11-24} plane and {11-22} plane, or a semi-polarity plane such as {10-11} plane and {10-12} plane. Even if piezoelectric polarization or intrinsic polarization is thereby generated in the third compound semiconductor layer of the mode locking laser diode device 110, piezoelectric polarization is not generated in the thickness direction of the third compound semiconductor layer and piezoelectric polarization is generated in the direction approximately perpendicular to the thickness direction of the third compound semiconductor layer. Thus, adverse effect resulting from piezoelectric polarization and intrinsic polarization is able to be excluded. {11-2n} plane means a non-polarity plane making 40 deg approximately with respect to the C plane. In the case where the mode locking laser diode device 110 is provided on a non-polarity plane or on a semi-polarity plane, limitation of the thickness of the well layer (from 1 nm to 10 nm both inclusive) and limitation of the impurity doping concentration of the barrier layer (from 2×10¹⁸ cm⁻³ to 1×10²⁰ cm⁻³ both inclusive) are able to be eliminated.

The number of the light emitting regions 141 and the saturable absorption regions 142 is not limited to 1. FIG. 23 illustrates a schematic end view of a mode locking laser diode device in which one first section 162A of the second electrode and two second sections 162B₁, and 162B₂ of the second electrode are provided. In the mode locking laser diode device, one end of the first section 162A is opposed to one second section 162B₁ with one isolation trench 162 C₁ in between, and the other end of the first section 162A is opposed to the other second section 162B₂ with the other isolation trench 162C₂ in between. Further, one light emitting region 141 is sandwiched between saturable absorption regions 142₁, and 142₂. Further, FIG. 24 illustrates a schematic end view of a mode locking laser diode device in which two first sections 162A₁, and 162A₂ of the second electrode and one second section 162B of the second electrode are provided. In the mode locking laser diode device, an end section of the second section 162B is opposed to one first section 162A₁ with one isolation trench 162 C₁ in between, and the other end of the second section 162B is opposed to the other first section 162A₂ with the other isolation trench 162 C₂ in between. Further, one saturable absorption region 142 is sandwiched between two light emitting regions 141₁, and 141₂.

Further, as a modification of the second embodiment, it is possible that a given value of current is applied to the semiconductor optical amplifier while laser light enters the semiconductor optical amplifier from the laser light source, and thereby the relative position of the semiconductor optical amplifier with respect to laser light entering the semiconductor optical amplifier is adjusted so that voltage applied to (added to) the semiconductor optical amplifier becomes the maximum. In this case, in the case where light output of laser light outputted from the semiconductor optical amplifier is measured and the light output is changed from a desired value, it is possible that a given value of current is applied to the semiconductor optical amplifier while laser light enters the semiconductor optical amplifier from the laser light source, and thereby the relative position of the semiconductor optical amplifier with respect to laser light entering the semiconductor optical amplifier is adjusted again so that voltage applied to (added to) the semiconductor optical amplifier becomes the maximum. Further, in the case where result of readjustment of the relative position of the semiconductor optical amplifier with respect to laser light entering the semiconductor optical amplifier is the same as the relative position of the semiconductor optical amplifier with respect to laser light entering the semiconductor optical amplifier before readjustment, light path through which the laser light outputted from the semiconductor optical amplifier passes is able to be adjusted. Specifically, where a voltage applied to (added to) the semiconductor optical amplifier in the case where a given value of current I₀ is flown to the semiconductor optical amplifier while laser light does not enter the semiconductor optical amplifier from the laser light source is V₁, and a voltage applied to (added to) the semiconductor optical amplifier in the case where a given value of current I₀ is flown to the semiconductor optical amplifier while laser light enters the semiconductor optical amplifier from the laser light source is V₂, the relative position of the semiconductor optical amplifier with respect to laser light entering the semiconductor optical amplifier may be adjusted so that value of ΔV=(V₂−V₁) becomes the maximum. As a given value of current, 0 milliampere<ΔI≦20 milliampere is able to be exemplified.

Further, as a modification of the second embodiment, it is possible that a given value of voltage is applied to the semiconductor optical amplifier while laser light enters the semiconductor optical amplifier from the laser light source, and thereby the relative position of the semiconductor optical amplifier with respect to laser light entering the semiconductor optical amplifier is adjusted so that a current flown to the semiconductor optical amplifier becomes the maximum. In this case, when light output of laser light outputted from the semiconductor optical amplifier is measured and the light output is changed from a desired value, it is possible that a given value of voltage is applied to the semiconductor optical amplifier while laser light enters the semiconductor optical amplifier from the laser light source, and thereby the relative position of the semiconductor optical amplifier with respect to laser light entering the semiconductor optical amplifier is adjusted again so that current flown in the semiconductor optical amplifier becomes the maximum. Further, in the case where result of readjustment of the relative position of the semiconductor optical amplifier with respect to laser light entering the semiconductor optical amplifier is the same as the relative position of the semiconductor optical amplifier with respect to laser light entering the semiconductor optical amplifier before readjustment, light path through which the laser light outputted from the semiconductor optical amplifier passes is able to be adjusted. Specifically, when a current flown in the semiconductor optical amplifier in the case where a given value of voltage V₀ is applied to the semiconductor optical amplifier while laser light does not enter the semiconductor optical amplifier from the laser light source is I₁, and a current flown in the semiconductor optical amplifier in the case where a given value of voltage V₀ is applied to the semiconductor optical amplifier while laser light enters the semiconductor optical amplifier from the laser light source is I₂, the relative position of the semiconductor optical amplifier with respect to laser light entering the semiconductor optical amplifier may be adjusted so that value of ΔI=(I₂−I₁) becomes the maximum. As a given value of voltage, 0 volt≦ΔV≦5 volt is able to be exemplified.

The present application contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2010-149345 filed in the Japanese Patent Office on Jun. 30, 2010, the entire contents of which is hereby incorporated by reference.

It should be understood by those skilled in the art that various modifications, combinations, sub combinations and alternations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.

Claims

1. A semiconductor optical amplifier comprising:

a laminated structure including (i) a first compound semiconductor layer that has a first conductivity type and is composed of GaN compound semiconductor, (ii) a second compound semiconductor layer that has a second conductivity type different from the first conductivity type and is composed of GaN compound semiconductor, and (iii) a third compound semiconductor layer that has a light amplification region composed of GaN compound semiconductor, the first compound semiconductor layer, the third compound semiconductor layer, and the second compound semiconductor layer being sequentially layered;

a second electrode on the second compound semiconductor layer; and

a first electrode electrically connected to the first compound semiconductor layer,

wherein,

the laminated structure has a ridge stripe structure,

when a width of the ridge stripe structure in a light output end face is W_out, and a width of the ridge stripe structure in a light incident end face is W_in, W_out>W_in is satisfied, and

a carrier non-injection region is provided in the laminated structure, the carrier non-injection region extending from the light output end face of the ridge stripe structure and towards the light incident end face of the ridge stripe structure along an axis line of the semiconductor optical amplifier, the first, second, and third compound semiconductor layers being present in the carrier non-injection region, and the second electrode not being present in the carrier non-injection region.

2. The semiconductor optical amplifier according to claim 1, wherein W_out is greater or equal to 5 μm.

3. The semiconductor optical amplifier according to claim 1, wherein W_in is from 1.4 μm to 2.0 μm, both inclusive.

4. The semiconductor optical amplifier according to claim 1,

wherein the second electrode is composed of a first section and a second section separated by an isolation trench, and

the second section of the second electrode is provided in the carrier non-injection region.

5. The semiconductor optical amplifier according to claim 4, wherein a voltage equal to or less than a built-in voltage is applied to the second section of the second electrode.

6. The semiconductor optical amplifier according to claim 1, wherein the axis line of the semiconductor optical amplifier intersects with an axis line of the ridge stripe structure at a given angle that ranges from 0.1 to 10 degrees.

7. The semiconductor optical amplifier according to claim 1, wherein a value of (width of the ridge stripe structure in the light output end face)/(width of laser light outputted from the semiconductor optical amplifier) is from 1.1 to 10, both inclusive.

8. A light output device comprising:

a laser light source; and

a semiconductor optical amplifier according claim 1 that optically amplifies laser light from the laser light source and outputs amplified light.

9. The light output device according to claim 8, further comprising a mirror and a light isolator positioned such that laser light output from the laser light source passes through the light isolator and then is reflected by the mirror.

10. The light output device according to claim 9, further comprising a half-wave plate and a lens positioned such that laser light reflected by the mirror passes through the half-wave plate and lens and then enters the semiconductor optical amplifier.

11. The light output device according to claim 10, wherein the light isolator is positioned to prevent light returned from the semiconductor optical amplifier from returning to the laser light source.

12. The light output device according to claim 11, further comprising an output lens, wherein the laser light is optically amplified in the semiconductor optical amplifier, and is output outside the system through the output lens.

13. The light output device according to claim 8, wherein the laser light source is a mode locking laser diode device, and pulse laser light output from the mode locking laser diode device enters the semiconductor optical amplifier.

14. A light output device comprising:

a laser light source;

a semiconductor optical amplifier according claim 1;

an alignment device that adjusts a relative position of the semiconductor optical amplifier with respect to laser light entering the semiconductor optical amplifier; and

a semiconductor optical amplifier control device that controls operation of the semiconductor optical amplifier.

15. The light output device according to claim 14, further comprising a mirror and a light isolator positioned such that laser light output from the laser light source passes through the light isolator and then is reflected by the mirror.

16. The light output device according to claim 15, further comprising a half-wave plate and a lens positioned such that laser light reflected by the mirror passes through the half-wave plate and lens and then enters the semiconductor optical amplifier.

17. The light output device according to claim 16, wherein the light isolator is positioned to prevent light returned from the semiconductor optical amplifier from returning to the laser light source.

18. The light output device according to claim 17, further comprising an output lens, wherein the laser light is optically amplified in the semiconductor optical amplifier, and is output outside the system through the output lens.

19. The light output device according to claim 14, wherein the laser light source is a mode locking laser diode device, and pulse laser light output from the mode locking laser diode device enters the semiconductor optical amplifier.

20. The light output device according to claim 14, wherein the reflecting mirror, the half-wave plate, and the lens are on the alignment device.

21. The light output device according to claim 20, wherein the alignment device comprises an XYZ stage and when the thickness direction of the laminated structure in the semiconductor optical amplifier is the Y direction and an axis line direction of the semiconductor optical amplifier is the Z direction, the reflecting mirror and the lens are moved in the X direction, the Y direction, and the Z direction by the alignment device.

22. A light output device comprising:

a laser;

a semiconductor optical amplifier that amplifies light output by the laser;

a light isolator between the laser and the semiconductor optical amplifier;

a mirror between the light isolator and the semiconductor optical amplifier;

a half-wave plate between the mirror and the semiconductor optical amplifier; and

a lens between the half-wave plate and the semiconductor optical amplifier,

wherein,

(1) the semiconductor optical amplifier comprises

(a) a laminated structure including (i) a first compound semiconductor layer that has a first conductivity type and is composed of GaN compound semiconductor, (ii) a second compound semiconductor layer that has a second conductivity type different from the first conductivity type and is composed of GaN compound semiconductor, and (iii) a third compound semiconductor layer that has a light amplification region composed of GaN compound semiconductor, the first compound semiconductor layer, the third compound semiconductor layer, and the second compound semiconductor layer are sequentially layered,

(b) a second electrode on the second compound semiconductor layer, and

(c) a first electrode electrically connected to the first compound semiconductor layer,

(2) the laminated structure has a ridge stripe structure, and

(3) a carrier non-injection region is provided in the laminated structure, the carrier non-injection region extending from the light output end face of the ridge stripe structure and towards the light incident end face of the ridge stripe structure along an axis line of the semiconductor optical amplifier, the first, second, and third compound semiconductor layers being present in the carrier non-injection region, and the second electrode not being present in the carrier non-injection region.

23. The light output device of claim 22, wherein, for the semiconductor optical amplifier, when a width of the ridge stripe structure in a light output end face is W_out and a width of the ridge stripe structure in a light incident end face is W_in, W_out>W_in is satisfied.