Continuously tunable and precisely wavelength-switchable fiber lasers combine fiber Bragg gratings and the transmissive filtering properties of high finesse fiber Fabry-Perot filters. This laser arrangement adapts to multiple wavelength ranges based on the selections of fiber Bragg grating and gain medium and their arrangement to create a wavelength-modulatable and simultaneously rapidly wavelength-switchable narrow linewidth all-fiber laser design. This laser arrangement further results in narrow-linewidth outputs with fast switching speeds between the selected wavelengths.
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1. A precisely and rapidly wavelength-tunable CWR continuous wave (CW) or pulsed laser, said laser comprising:
a gain medium;
a pump source, said pump source for providing light to said gain medium;
a laser cavity resonator having a transmissive filter loop and at least a first arm, said gain medium disposed in said laser cavity resonator and said pump source optically aligned with said laser cavity resonator;
a rapidly wavelength-tunable filter disposed in said laser cavity resonator transmissive filter loop, wherein said wavelength-tunable filter is tunable by application of voltage across said wavelength-tunable filter;
a multi-wavelength grid filter disposed in said laser cavity resonator first arm; and
means for removing an output signal from said laser cavity resonator.
27. A precisely wavelength-tunable laser, said laser comprising:
a gain medium;
a pump source, said pump source for providing light to said gain medium;
a laser cavity resonator, said gain medium disposed in said laser cavity resonator and said pump source optically aligned with said laser cavity resonator;
a rapidly wavelength-tunable filter disposed in said laser cavity resonator, wherein said wavelength-tunable filter is tunable by application of voltage across said wavelength-tunable filter;
a multi-wavelength grid filter disposed in said laser cavity resonator; and
means for removing an output signal from said laser cavity resonator, wherein said laser cavity resonator has a gain loop and at least a first arm and a second arm attached to said gain loop; said rapidly wavelength-tunable filter disposed in said first arm; and said multi-wavelength grid filter disposed in said second arm.
42. A precisely wavelength tunable laser, said laser comprising:
a gain medium;
a pump source, said pump source providing light to said gain medium;
a laser cavity resonator, said gain medium disposed in said laser cavity resonator and said pump source optically aligned with said laser cavity resonator;
a rapidly wavelength-tunable filter disposed in said laser cavity resonator, wherein said wavelength-tunable filter is tunable by application of voltage across said wavelength-tunable filter;
a multi-wavelength grid filter disposed in said laser cavity resonator;
a coupling means; and
means for removing an output signal from said laser cavity resonator;
wherein said laser cavity resonator has a transmissive filter loop and at least a first arm; said rapidly wavelength-tunable filter disposed in said transmissive filter loop; and said multi-wavelength grid filter disposed in said first arm, said transmissive filter loop coupled by said coupling means to said first arm.
15. A precisely wavelength-tunable laser, said laser comprising:
a gain medium;
a pump source, said pump source for providing light to said gain medium;
a laser cavity resonator having a transmissive filter loop and at least a first arm, said gain medium disposed in said laser cavity resonator and said pump source optically aligned with said laser cavity resonator;
a rapidly wavelength-tunable filter disposed in said laser cavity resonator transmissive filter loop, wherein said wavelength-tunable filter is tunable by application of voltage across said wavelength-tunable filter;
a multi-wavelength grid filter disposed in said laser cavity resonator first arm, wherein said multi-wavelength grid filter is selected from the group consisting of Reflective Waveguide Bragg Gratings (WBGs); FBGs (Fiber Bragg Gratings); SWBG (Sampled Waveguide Bragg Gratings); SFBGs (Sampled Fiber Bragg Gratings); transmissive WBGs; phase-shifted FBGs (with single or multiple passbands); Fabry-Perot (FP) micro-etalons and filters; fiber FP filters and micro-optic; and micro-machined FP filters; semiconductor FP filters; Micro-ring resonators; waveguide couplers; Interferometric-waveguide-based filters; Michelson Interferometric (MI) waveguide filters; Mach-Zehnder Interferometric (MZI) waveguide based filters; Arrayed Waveguide Gratings (AWGs); and polarization interferometric (PI) waveguide-based filters; fiber loop mirrors; and bulk-optic-based PI filters; and
means for removing an output signal from said laser cavity resonator.
0. 2. The precisely and rapidly wavelength-tunable CWR pulsed laser recited in
3. The precisely and rapidly wavelength-tunable CWR continuous wave (CW) or pulsed laser recited in claim 2 1, further comprising a second arm in said laser cavity resonator, and second arm coupled to said first arm and said transmissive filter loop.
4. The precisely and rapidly wavelength-tunable CWR continuous wave (CW) or pulsed laser recited in
5. The precisely wavelength-tunable laser recited in claim 2 1, further comprising an optical coupling feedback device for combining optical feedback from said transmissive filter loop and said at least a first arm.
6. The precisely wavelength-tunable laser recited in
7. The precisely wavelength-tunable laser recited in
8. The precisely and rapidly wavelength-tunable CWR continuous wave (CW) or pulsed laser recited in
9. The precisely and rapidly wavelength-tunable CWR continuous wave (CW) or pulsed laser recited in
10. The precisely and rapidly wavelength-tunable CWR continuous wave (CW) or pulsed laser recited in
11. The precisely and rapidly wavelength-tunable CWR continuous wave (CW) or pulsed laser recited in
12. The precisely wavelength-tunable laser recited in
13. The precisely and rapidly wavelength-tunable CWR continuous wave (CW) or pulsed laser recited in
14. The precisely and rapidly wavelength-tunable CWR continuous wave (CW) or pulsed laser recited in claim 1 13, further comprising means for tuning said rapidly wavelength-tunable filter.
0. 16. The precisely wavelength-tunable laser recited in
17. The precisely wavelength-tunable laser recited in claim 16 15, further comprising a second arm in said laser cavity resonator, and second arm coupled to said first arm and said transmissive filter loop.
18. The precisely wavelength-tunable laser recited in
19. The precisely wavelength-tunable laser recited in claim 16 15, further comprising an optical coupling feedback device for combining optical feedback from said transmissive filter loop and said at least a first arm.
20. The precisely wavelength-tunable laser recited in
21. The precisely wavelength-tunable laser recited in
22. The precisely wavelength-tunable laser recited in
23. The precisely wavelength-tunable laser recited in
24. The precisely wavelength-tunable laser recited in
25. The precisely wavelength-tunable laser recited in
26. The precisely wavelength-tunable narrow linewidth laser recited in
28. The precisely wavelength-tunable laser recited in
29. The precisely wavelength-tunable laser recited in
30. The precisely wavelength-tunable laser recited in
31. The precisely wavelength-tunable laser recited in
32. The precisely wavelength-tunable laser recited in
33. The precisely wavelength-tunable laser recited in
34. The precisely wavelength-tunable laser recited in
35. The precisely wavelength-tunable laser recited in
0. 36. A precisely wavelength-tunable laser, said laser comprising:
a gain medium;
a pump source, said pump source providing light to said gain medium;
a first laser cavity resonator, said pump source optically aligned with said first laser cavity resonator;
a second laser cavity resonator, said gain medium disposed in either said first laser cavity resonator or said second laser cavity resonator;
an optical coupling feedback device for combining optical feedback from said first and second laser cavity resonators;
a rapidly wavelength-tunable filter disposed in said first laser cavity resonator, wherein said wavelength-tunable filter is tunable by application of voltage across said wavelength-tunable filter;
a multi-wavelength grid filter disposed in said second laser cavity resonator; and
means for removing an output signal from one of said laser cavity resonators.
0. 37. The precisely wavelength-tunable laser recited in
0. 38. The precisely wavelength-tunable laser recited in
0. 39. The precisely wavelength-tunable laser recited in
0. 40. The precisely wavelength-tunable laser recited in
0. 41. The precisely wavelength-tunable laser recited in
43. The precisely wavelength tunable laser recited in
44. The precisely wavelength tunable laser recited in
45. The precisely wavelength tunable laser recited in
46. The precisely wavelength tunable laser recited in
47. The precisely wavelength-tunable laser recited in
48. The precisely wavelength-tunable laser recited in
49. The precisely wavelength-tunable laser recited in
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This application makes reference to co-pending U.S. Provisional Patent Application No. 60/110,056, entitled “Novel Wavelength-Modulatable and Continuously Tunable Narrow-Linewidth Fiber Lasers and Precisely Wavelength-Switchable Narrow-Linewidth Laser for Optical Telecommunications and Spectroscopic Applications,” filed Nov. 25, 1998, and U.S. patent application Ser. No. 09/246,125, entitled “Tunable Bragg Gratings and Devices Employing the Same,” filed Feb. 8, 1999, the entire contents and disclosures of which are hereby incorporated by reference.
This invention is made with government support under contract numbers MDA-972-94-1-0003 and MDA-972-98-1-0002, awarded by DARPA. The government may have certain rights in this invention.
1. Field of the Invention
The present invention relates generally to tunable fiber lasers.
2. Description of the Prior Art
Tunable lasers have applications in a wide variety of fields, including optical communications and spectroscopy. For trace gas monitoring, compact tunable (20-30 GHz), wavelength modulatable sources with output powers on the order of approximately 100 μW and linewidths better than 100 MHz are in great demand for wavelength modulation spectroscopy. The availability of high performance erbium-doped fiber amplifiers and pumped lasers allows for tunable fiber lasers. Modulation and switching of optical signals are basic functions in an optical communication system. Through modulation, the information to be communicated is expressed in one or more parameters of a light signal, such as the amplitude, the polarization, the phase or frequency of the field, or of the magnitude or spatial distribution of the power and/or intensity. Through switching, the light signal may be routed through a network of optical nodes and connections.
Precisely wavelength-switchable narrow linewidth laser sources are of great interest for many photonic applications, such as for tuning “on and off” narrow absorption lines in spectroscopic measurements, including the monitoring of resonantly absorbing species in DIAL (differential absorption LIDAR)-type applications. Wavelength-switchable narrow linewidth laser sources are also critically needed for several applications in WDM-based (wavelength division multiplexed) fiber optic communication systems. Key requirements for such multi-wavelength switchable sources for WDM/DWDM systems are: (1) an accurate match with the wavelength channels on the WDM/DWDM ITU grid, (2) an arbitrary set of such channels, (3) a capability for switching reliably to any channel between such a preselected arbitrary set of channels, (4) low crosstalk, and (5) microsecond (or faster) switching speeds.
Past multi-wavelength switchable sources have in general been limited to schemes that are either difficult to scale to a large number of wavelengths, or have relatively slow (millisecond) switching speeds. Laser arrangements such as those found in U.S. Pat. No. 5,504,771 also require the use of stable external “wavelength lockers” to prevent wavelength drift from the FFPs PZT tuning assembly. Multi-frequency lasers based on integrated-optic arrays of DBR and DFB lasers, or SOA (semiconductor optical amplifiers arrays integrated with AWGs (arrayed waveguide gratings) seem to satisfy most of the above requirements. However, these are relatively difficult and expensive to manufacture, particularly in small volumes or for custom applications that may require a combination of numerous arbitrarily-spaced channels on the WDM/DWDM ITU grid.
Therefore, there is still a need for a rapidly switchable multi-wavelength source that is relatively easy to manufacture for any customized set of arbitrary channels on the ITU grid.
The term “rapidly wavelength-tunable filter” for the purpose of the present invention refers to a device that provides the means for rapid (<50 μs) tuning of the optical filter passband across the spectral range of interest. The means include but are not limited to: thermal or strain tuning, electro-optic tuning, and electro-absorptive modulation. The device may have single or multiple wavelength passbands. The device may be composed of a single optical filter or a set of such optical filters. Specific examples of rapidly wavelength-tunable filters include but are not limited to: Reflective Waveguide Bragg Gratings (WBGs) such as tunable FBGs (Fiber Bragg Gratings), and tunable SFBGs (Sampled Fiber Bragg Gratings); Transmissive WBGs such as phase-shifted tunable FBGs (with single or multiple passbands); PZT-based FFP (fiber Fabry-Perot) filters; LCD-based FFP (fiber Fabry-Perot) filters; Microelectromechanical system (MEMS)-based FP (Fabry-Perot) filters; Electro-optic FP filters (i.e. LCD-based, poled glass, poled crystals); Micro-ring resonators (i.e. poled glass, electro-absorptive material); Grating-assisted tunable waveguide couplers; Electro-optic waveguide coupler; Tunable Michelson interferometer (MI) waveguide-based filter; Tunable MZI (Mach-Zehnder Interferometer) waveguide-based filter; and Tunable AWGs, etc.
The term “optical feedback coupling device” for the purpose of the present invention refers to a device for combining the optical feedback from transmissive and reflective filters in the same laser resonator cavity. Specific examples of such devices include, but are not limited to, N-port optical circulators (N=3, 4, etc.), fused fiber couplers, and waveguide-Y junctions, etc.
The term “gain medium” for the purpose of the present invention refers to a medium that provides gain to the optical signal. Examples of such mediums include, but are not limited to doped fiber optic waveguides or semiconductor optical amplifiers.
The term “pump source” for the purposes of the present invention refers to an optical or electrical tunable or switchable emission.
The term “wavelength comb” for the purposes of the present invention refers to refers to a set of wavelengths that are equally spaced or whose spacing are multiples of a common interval.
Before the preferred embodiments are described, we will first discuss the invention in a general fashion. It should be appreciated that there are two distinct embodiments of the invention. The first embodiment addresses precisely wavelength switchable narrow linewidth lasers. This first embodiment is mainly used in WDM telecommunication systems. The second embodiment addresses precisely wavelength tunable narrow linewidth lasers. This second embodiment is mainly used for tuning “on and off” narrow absorption lines in spectroscopic measurements, including the monitoring of resonantly absorbing species in DIAL (differential absorption LTDAR)-type applications.
In the precisely wavelength switchable narrow linewidth laser embodiment, the design is based on two narrow band filters: (1) a stable multi-wavelength filter where each selected wavelength is precisely matched to a standard grid of WDM channels, and (2) a rapidly (<50 μs) wavelength-tunable filter. In the 1.5 um telecommunications spectral region, both narrow linewidth filters should have bandwidths of at least 5 GHz (preferably <1 GHz). These bandwidths are determined by the precision required of each channel in the standard grid of WDM channels. This laser source can have single- or multiple-wavelength rapidly wavelength-switchable emission in the channels specified by the multi-wavelength grid filter.
In the precisely wavelength tunable narrow linewidth laser embodiment, the device is based on two filters: (1) a stable multi-wavelength filter where each selected wavelength is precisely matched to a pre-designated set of wavelength channels (such as that which coincides with significant absorption peaks of chosen molecular species), and (2) a rapidly wavelength-tunable filter. The multi-wavelength filter should have individual wavelength channels that have bandwidths that are at least as broad as the tuning range desired (i.e. bandwidth of molecular absorption feature). The rapidly wavelength-tunable filter should have a bandwidth that is at least 10 times (preferably<20 times) narrower than the bandwidth of the above-mentioned absorption feature. In the spectroscopy applications for the 1.5 um near-infrared spectral region, the significant molecular absorption features are normally 10-30 GHz wide, so the multi-wavelength filter wavelength channels should have a bandwidth that is at least as wide (10-30 GHz), while the rapidly tunable wavelength filter should have a bandwidth less than 1 GHz (preferably<100 MHz). This laser source can have single- or multiple-wavelength rapidly wavelength-tunable emission in the wavelength channels specified by the multi-wavelength grid filter.
In either embodiment, the laser resonator can be wholly or in part of ring, or linear geometry. A pump source provides energy for the gain medium and a means for coupling this energy to the gain medium is provided. The gain medium may be a doped fiber optic waveguide or a semiconductor optical amplifier. The optical field in the laser resonator has a roundtrip path that is facilitated in part or wholly by various media which include, but are not limited to: optical fiber, planar waveguides in bulk glass, planar waveguides in semiconductors, as well as free space open cavity resonators in MEMS-based designs.
In a ring geometry, transmissive filters may be directly inserted in the roundtrip path. In the linear geometry, reflective filters can be directly inserted in both ends of the cavity roundtrip path. In a cavity which is part linear and part ring, the means to couple the feedback from both filters is provided by the optical feedback coupling device. An example of such a device is a 3-port circulator which transforms a reflective FBG into a transmissive filter.
The multi-wavelength filter is stabilized through an ultrastable compensation means that is usually accomplished through a reference wavelength wavelengthref which is locked to a frequency standard. This reference wavelength(s) wavelengthref is a subset of the pre-designated multi-wavelength grid. The multi-wavelength filter may be a set of discrete filters or a single filter.
The tunable wavelength filter may be a set of discrete filters or a single tunable filter. The means for rapid tuning of wavelength-tunable filter is provided.
The source design of the invention as shown, for example, in
Optical isolators are used, where appropriate, to ensure unidirectional operation and, in the case of embodiments employing FFP transmissive filters, to prevent feedback arising from wavelengths reflected off the non-transmissive FFP bands. Optical circulators, in combination with reflective filters (usually fiber Bragg gratings), are employed in specific embodiments to allow for efficient low-loss precise wavelength feedback from the pre-designated set (single or multiple) of wavelengths. Polarization control is introduced at appropriate places in the cavity to ensure optimal output powers and stability.
Now that the basic concepts of the invention have been described, we will now describe specific structures that utilize the teachings of the two embodiments above. Turning now to
The transmissive filter of the present invention may be an FFP with coated fiber mirrors, FFP with FBG mirrors, phase-shifted FBG-based transmission filters, waveguide ring resonator filters, MEMs-based FFPs, etc. Electronically tunable filters may be PZT-tuned FBGs, EO-tunable FBGs, PZT-tuned FFP, MEMs-based tunable FFP, etc. FBGs may be single frequency, multiple frequency (sampled FBG), etc.
Preferably, a 0.08 nm linewidth fiber Bragg grating (FBG), with a reflectivity of 25%, centered at 1531.08 in a Sagnac loop geometry may be used. The fiber Fabry-Perot (FFP) provides intracavity wavelength tunability to the source. The filter has a 20 GHz free spectral range (FSR) and a 20 MHz linewidth (LW). The isolators prevent unwanted reflective feedback from the high reflectivity FFP mirror surfaces.
An instrument-limited linewidth of 0.05 nm and 15 GHz of maximum tuning centered in the vicinity of the FBG peak are demonstrated. The observed output powers (2 mW max), however, show 10% fluctuations, and spectral instability (estimated to be approximately 1 GHz).
The standing wave laser of
The switchable fiber lasers of the present invention may be designed using linear, ring, multiple cavity configurations, etc. The configurations may be a linear cavity standing wave laser, a Sagnac ring (based on fused fiber coupler) travelling wave laser, a ring cavity (based on optical circulators), etc. The configuration may also incorporate multiple cavities. A feature of each of these configurations is the use of an FFP having a switching speed of less than 1 milliseconds, or less than 100 microseconds in another preferred embodiment, and most preferably less than 10 microseconds.
Although only switching between two pre-designated wavelengths has been shown for the embodiments described above, it should be appreciated that switching between multiple precise DWDM channels is within the scope of the present invention. This switching is achieved by (1) dynamically adjusting the input voltage to the FFP; (2) dynamically adjusting the resonant frequency of the FBG; or (3) a combination of methods (1) and (2).
Turning now to
Turning now to
As shown in
One example of a design wherein the FFP FSR<Δf involves eight channels with Δf=50 GHz. A coordinate FFP has an FSR of about 44 GHz, which requires a δftolerance of about 5 GHz.
Multi-wavelength switchable operation has also been illustrated in
The speed of switching may be measured using a tunable bandpass filter and an isolator in front of a 1 GHz InGaAs photodetector. The bandpass filter may be tuned to one channel or another, to give a 30 dB rejection between the two channels. Rapid switching between the two wavelengths is accomplished with the use of a 1 KHz square wave input voltage source (˜60 ns rise time) to drive the tunable FFP.
When the FFP transmission peak is not tuned to either grating's reflective bands, i.e., FFP-unassisted emission at 1551 nm (λ1), large output power fluctuations of approximately 50% is exhibited. This effect is caused by a combination of several factors: slight differences in reflectivity between the two gratings, the formation of an effective laser cavity defined by the gratings on one end and stray reflections from the 3 dB coupler on the other, and to the gain coefficient differential between λ1, and λ2, which is estimated to be as much as approximately 1 m−1 for similar Er:Silica fibers pumped to high inversion levels.
Instrument limited linewidths of 0.05 nm are also observed. Linewidth measurements using scanning Fabry-Perot interferometry and self-homodyne techniques for similarly-configured fiber laser sources as shown in
The switching time between λ1, and λ2 was measured to be approximately 25 μs based on a capacitive loading at the FFP PZT inputs. The switching time between the two wavelengths (separated by 300 GHz) is more accurately 18 μs, and may reach sub-μs speeds with the use of Bessel-function pre-filtered drivers for the FFP.
In an alternate embodiment of the present invention, the desired comb of wavelength channels may be obtained from a single sampled FBG. In addition, the accuracy of the switched wavelengths may be assured by using tighter (i.e. <10 GHz) 0.5 dB bandwidth specifications for the FBGs. Highly stabilized fixed FFPs may also be used to generate the ITU grid directly.
Additionally, the laser cavity round trip time may be further reduced by reducing laser cavity lengths to approximately 1 m by using high doping density Er/Yb:Silica fibers or semiconductor optical amplifiers. For a 16-wavelength (50 GHz channel separation) switchable source using an FFP with an optimized FSR switching speed of 5 μs, the switching time is approximately 0.75 μs.
While the above discussion has focused on the switching between two wavelengths λ1 and λ2 or λa and λb, it should be appreciated that runability from λ1 to λn is achievable by utilizing an AC input voltage having the correct frequency and magnitude.
It should be appreciated that numerous embodiments describe a fiber laser using only one fiber Fabry-Perot filter (FFP) in combination with discrete fiber Bragg gratings (FBGs). Among the advantages of using FBGs, we can list the following:
Although the present invention has been fully described in conjunction with the preferred embodiment thereof with reference to the accompanying drawings, it is to be understood that various changes and modifications may be apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims, unless they depart therefrom.
Jain, Ravinder, Libatique, Nathaniel C.
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