Extended source transmitter for free space optical communication systems

Abstract

An apparatus for transmission of free space optical communication system signals employing a spatially-extended light source and method of using the same. A laser beam source directs an optical signal into a free end of a segment of multimode fiber. As the optical signal passes through the segment of multimode fiber, the optical signal is converted into a mode-scrambled optical signal. This mode-scrambled signal may then be used as a spatially-extended light source that is directed outward as an optical beam through the use of a collimating lens.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

where NA is the numerical aperture, θ is the half angle of the incident light beam, n₁, is the index of refraction for the optical fiber core, and n₂ is the index of refraction for the optical fiber cladding.

Light rays launched within the angle specified by the optical fiber's numerical aperture excite optical fiber modes. The greater the ratio of core index of refraction to the cladding index of refraction results in a larger numerical aperture.

Launch conditions corresponding to an under-filled and substantially filled numerical aperture are illustrated in FIGS. 12a and 12b, respectively. In FIGS. 12a and 12b, optical signals 1202A and 1202B are respectively launched from segments of optical fiber 1200A and 1200B. As the optical signals impinge upon a collimating lens 1204, the signals are (substantially) collimated into respective transmitted signals 1206A and 1206B, which are received by a FSO terminal (not shown) to complete the link. In these Figures, the dashed lines illustrate relative intensity values, wherein the heavier the line, the greater the intensity.

At the right hand of each figure is an intensity distribution diagram that depicts the relative power distribution P of the optical signal vs. angle Θ relative to a centerline of the signal. In practice, the actual intensity distribution comprises a three-dimensional profile, with the two-dimensional profiles shown in FIGS. 12a and 12b being for illustrative purposes.

FIG. 12a illustrates two intensity distributions 1208A and 1210. Intensity distribution 1210 is illustrative of a theoretical Gaussian profile. As discussed above, the conventional single-point launch produces a Gaussian-like profile at the launch point (i.e., exiting the launch fiber); as the optical signal traverses the atmosphere and/or passes through optics and windows, uneven optical effects cause distortion to the Gaussian curve, which are illustrated in intensity distribution 1208A. Generally, the peak intensity will be located near the center of the profile, although the encountered optical effects may cause it to be offset.

In contrast, the signal intensity profile produced by embodiments of the present invention, as illustrated by an intensity distribution 1208B, is in the shape of a “top hat,” which is a desirable intensity distribution for optical communication. For example, one advantage of the “top hat” intensity distribution is that, for a given safety classification of laser product, it allows for more energy to be transmitted out of the transmit aperture than the Gaussian distribution characteristic of a single mode transmission, or large peak and valley profile common to prior art mode-scrambled signals.

Another advantage of launching a mode-scrambled signal with a substantially-filled numerical aperture is that the optical signal is pre-distorted such that effects such as atmospheric scintillation and/or window wave front aberration are small compared to the scrambling generated on the transmitting end. This means that the light beam power distribution at the receiving aperture is more homogenous and the intensity fluctuations caused by atmospheric scintillation and/or window wave front aberration are practically transparent.

A top hat intensity, extended source distribution is an improvement over a Gaussian distribution for the additional following reasons:

(1) The Gaussian vacuum eigenmode can never be allowed to fill the exit aperture because intermediate field diffraction effects (Fresnel diffraction) will produce unmanageable diffraction maxima and minima; the Gaussian mode field diameter must be much less than the clear aperture of the optical system. Such beams also focus with high brightness on the retina. In contrast, a top hat beam, specifically from an extended source, has a certain amount of natural divergence and can also “fill” the exit aperture without excessive loss and without concentrating the power in the center of the aperture. The eye safety power limit is greater as a result of this combination of filling the aperture and extended source divergence. The filled aperture distributes the power more evenly, lowers the radiance, and the extended source divergence reduces the focused irradiance at the eye's retina. An extended source that has a nearly top hat shape that fills the exit aperture will greatly increase the total eye-safe power out of the aperture without resulting in noticeable Fresnel diffraction effects.

(2) The Gaussian vacuum eigenmode is not an eigenmode of the FSOCS optical system and is not an appropriate choice. Considering the entire communication link as the optical system (including air turbulence, window aberrations, etc.) requires one to recognize that the Gaussian eigenmodes will never be the appropriate choice. The practical eigenmode is one that does not significantly change as it propagates across the link. An extended source produces a beam that is significantly the same from one end of the link to the other (provided the link is not excessively long or the transmit aperture is not excessively small.) This top hat pseudo-eigenmode is essentially unaltered by atmospheric turbulence or window aberrations (unless the aberrations are so severe that one can see the aberrations or turbulence, such as mirage effects, with the naked eye.)

(3) When the light source is from a single mode fiber, the power distribution has a bell shape that is approximately Gaussian. This smooth shape is compromised with any modest number of scratches or dust on the fiber tip. Alternatively, light directly from a laser diode facet is elliptical and, from one laser to the next, this elliptically can vary by several degrees of divergence. An extended source allows one to build an optical system that does not need to compensate for the vagaries of these light sources, since variations between different light sources are lost in the mode-scrambling. It is therefore possible to make a simpler optical design and improve the manufacturability of the total FSOCS.

(4) A larger transmit divergence in FSOCS translates into reduced tracking requirements, but also geometric power loss at the receiver. While not a complete solution to this problem, increasing the transmit divergence using extended sources also allows some of the power loss to be reduced since higher powers are allowed out of the transmit aperture.

(5) Lowering tolerances on laser sources allows the use of lower cost lasers and components.

(6) Using large core optical fiber in the extended source allows the optical head to be de-coupled from the electronics in the mechanical assembly. This promotes modularity of design, which has obvious advantages.

An exemplary FSOCS transceiver 1300 that employs spatially-extended transmitter elements discussed above is shown in FIG. 13. FSOCS transceiver 1300 employs a binocular configuration including a transmit optic 1302 and a receive optic 1304. A laser beam source assembly 1306 generates laser light that is launched into a first end of an Si fiber segment 1308. After traversing the Si fiber segment, the light passes through a coupling 1310 that couples the Si fiber segment to a GI fiber segment 1312. The light then passes through GI fiber segment 1312, which excites a large number of modes, resulting in mode-scrambled light 1314 exiting an exit fiber end 1316 of GI fiber segment 1312. The exit fiber end 1316 is held by a fiber mount 1318. As mode-scrambled light 1314 impinges on transmit optic 1302, it is collimated into a mode-scrambled optical beam 1320, which is transmitted to be received at a receiver optic 1304 on another transceiver (not shown).

In the illustrated embodiment, laser source 1306 includes a laser (not shown) mounted to a heat sink 1322, which, in turn, is mounted to a circuit board 1324. Fiber mount 1318 and a fiber mount 1326 in which the receive end 1327 of a receiver fiber (not shown) are coupled to a plate 1328. Transmit and receive optics 1302 and 1304 are coupled to a plate 1330. Plates 1328 and 1330 are coupled via a rear cross-plate 1332 and mid and front cross plates (both removed for clarity), thereby forming a frame assembly 1334.

All of the illustrated components of FSOCS transceiver 1300 are mounted within a housing, which is not shown for clarity. Under a typical use, the housing is mounted to a support member, or is otherwise operatively coupled to a building member (e.g., wall or floor). Typically, respective FSOCS are mounted in offices of buildings that are within line-of-sight of one another, wherein the optical signals are transmitted through building windows. Optionally, one or both of the transceivers may be mounted on the exterior of a building.

In the foregoing detailed description, the method and apparatus of the present invention have been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present invention. The to present specification and Figures are accordingly to be regarded as illustrative rather than restrictive. Furthermore, it is not intended that the scope of the invention in any way be limited by the above description, but instead be determined entirely by reference to the claims that follow.

Claims

1. A free-space optical communication system (FSOCS) transmitter An apparatus, comprising:

a spatially-extended light source including comprising:

a laser; and

to generate a mode-scrambled optical signal, the spatially extended light source further including a segment of multimode fiber having a first end positioned to receive a laser optical signal produced by the laser and a second end from which a mode-scrambled optical signal is emitted and, wherein a portion of the segment of multimode fiber is configured in a series of alternating loops;

a power controller, operatively coupled to drive the laserand

output optics, optically coupled configured to receive the mode-scrambled optical signal from the spatially-extended light source and direct the mode-scrambled optical signal outward from the FSOCS transmitter apparatus as an optical beam having a controlled divergence;

wherein the multimode fiber comprises two or more separate pieces of multimode fiber segments that are joined together to be a continuous single strand.

2. The apparatus of claim 1, wherein the first end of the segment of multimode fiber is operatively coupled to the laser so as to produce an offset-launched optical signal.

3. The apparatus of claim 1, wherein the segment of multimode fiber consists of two or more separate pieces of multimode fiber that are joined together to be a continuous single strand.

4. The apparatus of claim 3 1, wherein there are two joined segments the two or more separate pieces of multimode fiber wherein the segments comprise a first segment piece of multimode fiber comprises segment having a 62.5 micron graded-index core, and the a second segment piece of multimode fiber comprises segment having a 200 micron step-index core.

5. The apparatus of claim 3 1, wherein the two or more segments separate pieces of multimode fibers fiber segments are operatively coupled together using one or more fusion splices.

6. The apparatus of claim 1, wherein a laser beam carrying the laser optical signal converges or diverges at an angle that substantially matches a numerical aperture of the multimode fiber.

7. The apparatus of claim 1, wherein the mode-scrambled optical signal beam has a power intensity distribution that has a shape substantially similar to a top hat.

8. The apparatus of claim 1, wherein an output of the spatially-extended light source is located coincident with in or adjacent a focal plane of a collimating lens comprising the output optics.

9. The apparatus of claim 1, further comprising a data modulator operatively coupled to the spatially-extended light source, wherein the data modulator is configured to modulate the optical beam.

10. The apparatus of claim 1, wherein the optical beam has a wavelength from in a range of about 400 to 1400 nanometers.

11. A method for generating a free space optical communication system (FSOCS) spatially-extended light signal, comprising:

producing a mode-scrambled modulated optical signal with a spatially-extended light source by:

operatively coupling a segment of multimode fiber configured in a series of alternating loops; and

directing an optical signal a laser beam produced by a laser to the a first end of the segment of a multimode fiber to generate a launched optical signal that is received by the first segment of multimode fiber;, wherein the multimode fiber is configured in a series of alternating loops,

wherein, as the launched optical signal laser beam passes through the segment of multimode fiber, it is converted into a the spatially-extended light signal that is mode-scrambled optical signal that serves as a spatially extended light source;

passing the modulated optical signal through a collimating lens to output an optical beam comprising the FSOCS signal;

wherein the multimode fiber comprises two or more separate pieces of multimode fiber segments that are joined together to be a continuous single strand.

12. The method of claim 11, wherein the segment of multimode fiber consists of two or more separate pieces of multimode fiber, with possibly differing core sizes and index profiles, that have been joined together to form a continuous single strand.

13. The method of claim 12 11, wherein there are the two joined segments or more separate pieces of multimode fiber wherein the segments include a first segment of multimode fiber comprises having a 62.5 micron graded-index core, and the a second segment of multimode fiber comprises having a 200 micron step-index core.

14. The method of claim 11, wherein the optical signal laser beam is directed towards the first end of the segment of multimode fiber such that it is received at an offset angle relative to a centerline of an end portion of the segment of multimode fiber.

15. The method of claim 11, further comprising focusing the optical signal laser beam into an the first end of the segment of multimode fiber such that the optical signal laser beam is launched into the first end at a point that is offset from a centerline of the multimode fiber.

16. The method of claim 11, further comprising focusing the optical signal laser beam such that it converges at an angle that substantially matches a numerical aperture of the segment of multimode fiber.

17. A free-space optical communication system (FSOCS) transmitter An extended light source, comprising:

means for generating a spatially-scrambled optical signal that functions as an extended light source, wherein the means for generating a spatially-scrambled optical signal comprises an optical fiber segment configured in a series of alternating loops and configured to generate a spatially-scrambled optical signal from a laser beam; and

focusing means, positioned to receive the spatially-scrambled optical signal that is generated and to direct the spatially-scrambled optical signal outward from the FSOCS transmitter as a spatially-scrambled optical beam;

wherein the optical fiber comprises two or more separate pieces of multimode fiber segments that are joined together to be a continuous single strand.

18. The FSOCS transmitter extended light source of claim 17, wherein the means for generating a spatially-scrambled optical signal optical fiber comprises:

lasing means for generating a light signal; and

means for converting the light signal laser beam into a mode-scrambled signal.