Patent Application
20220181837
The present invention relates to wavelength tunable fiber lasers and, more particularly, to polarization-maintaining fiber lasers that are tunable across a portion of the spectrum in the two micron region.
Tunable, narrow-linewidth single-frequency lasers in the two micron region are important as sources for mid-IR generation, LIDAR, DWDM and coherent communication systems, as well as instrumentation applications such as infrared spectroscopy and gas sensing. Tm-doped fiber lasers (TDFLs) are known as operable sources in this region, providing output powers of 1 mW and tunable over a region within the bandwidth of about 1650-2000 nm.
While having success at tuning the output wavelength of a TDFL over a usable range, these sources to date have not maintained the desired polarization stability required for many of the above-cited applications, particularly as required for coherent systems and many spectroscopy applications. While there have been efforts at realizing a polarization-maintaining (PM) TDFL, these have been found to be limited in operation to a single wavelength, with no tuning capability.
The needs remaining in the prior art are addressed by the present invention, which relates to polarization-maintaining fiber lasers that are tunable across a portion of the spectrum in the two micron region.
In accordance with the teachings of the present invention, a wavelength tunable fiber laser is based upon a ring laser geometry and includes sections of polarization-maintaining (PM) optical fiber for supporting propagation of the circulating laser radiation around the ring. At least one gain module is included in the ring and is formed of polarization-maintaining active optical fiber including a core region that is doped with either Thulium (Tm) or Holmium (Ho), or co-doped with both of these rare earth materials. In the presence of a pump beam operating at a suitable wavelength, the gain module(s) will generate laser radiation at a wavelength within the two micron region. In further accordance with the present invention, a tunable bandpass filter (BPF) that is also formed as a polarization-maintaining component is included within the ring and used to control/adjust the wavelength of the output beam provided by the fiber laser. The PM-based tunable BPF may be formed from a number of different components (gratings, dichroic filters, etc.), with the tuning range and linewidth of the inventive PM fiber laser being a function of the particular design of the PM-based tunable BPF.
One or more embodiments of the present invention including an amplifier booster stage coupled to the output of a tunable PM fiber ring laser for applications that require a higher output power (e.g., multi-watt power levels).
As exemplary embodiment of the present invention may take the form of a wavelength-tunable, PM fiber laser operable in the two micron region, the fiber laser based upon a PM ring resonator structure, with a separate pump source used to generate an input beam at a wavelength known to create gain in the presence of a particular dopant. In particular, PM ring resonator structure includes a gain module of PM optical fiber including a dopant selected from the group consisting of: Thulium (Tm), Holmium (Ho), and Thulium-Holmium (Tm—Ho), a PM-based, tunable BPF for selecting a particular laser output wavelength from within the two micron region, and a PM output coupler for out-coupling a defined portion of the laser radiation generated at the selected output wavelength within the gain module, a remaining portion of the laser radiation continuing to circulate within the resonator provide a continuous laser radiation output from the ring resonator structure.
Another embodiment of the present invention is configured to generate higher output power levels by adding a power boosting fiber amplifier to the output from the ring resonator.
Other and further advantages and aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.
Referring now to the drawings,
A fiber ring laser may be constructed by using a closed fiber loop to form a ring-shaped optical resonator. The fiber ring laser includes at least a portion of a doped fiber as the laser gain medium to produce an optical gain within a desired spectral range in response to an optical pump beam. As mentioned above, the optical pump beam is selected to operate at pump wavelength (or within a specified pump spectral range) that is known to interact with the particular dopant in the gain medium.
A laser oscillation in the fiber ring occurs at a laser wavelength within the gain spectral range when two operating conditions are met. First, the total optical gain at that laser wavelength exceeds the total optical loss in the fiber ring, and secondly, an accumulated optical phase delay of 360° (or a multiple thereof) associated with a round trip around the fiber ring.
In accordance with the principles of the present invention, a polarization-maintaining, tunable bandpass filter (BPF) 16 is included within tunable PM-fiber ring laser 10 and controlled (adjusted) to select the specific wavelength (defined herein as “λ0”) within the two micron region that is to be used as the output wavelength of tunable PM-fiber ring laser 10. PM-based tunable BPF 16 may comprise a single wavelength device (e.g., a combination of an optical circulator with a reflective fiber Bragg grating (FBG) structure), or a broadband device based on a dichroic filter or grating-based structure or a tunable electro-optic filter. Various other types and structures of BPFs are well-known in the art and are useful in the fiber laser of the present invention. Polarization control within the BPF itself is typically achieved by positioning a linear polarizer at the input to the BPF, and another linear polarization at the output, thus ensuring that only the desired polarization is preserved within the ring. The tuning range, as well as the linewidth of the filtered beam, are design considerations associated with the selected tunable BPF. The adjustment of the center wavelength of PM-based, tunable BPF 16 to a desired value of λ0 may be accomplished by using temperature adjustments, electrical signal controls and/or mechanical movements of an associated grating or dichroic filter structure, as well as other tuning techniques well-known in the art. In some cases, the wavelength tuning is performed during assembly of the fiber ring laser and is thereafter “fixed”. For some applications, however, it is desirable to provide a tunable fiber laser where the output wavelength may be actively tuned during operation of the laser; for these situations, PM-based, tunable BPF 16 is designed as an active device with a filter center wavelength that is tunable as a function of time (i.e., λ0(t)).
Since the laser radiation circulates around the ring in a continuous fashion, it passes through PM-based, tunable BPF 16 multiple times, thus maintaining stability of the laser output wavelength as related to the essentially continuous filtering. For the purposes of discussion, it is presumed that the generated laser radiation R is propagating at a wavelength λ0 selected from the two micron region, extending generally across the spectral region of 1700-2150 nm, as controlled by the type of dopant used within the gain fiber and the parameters of PM-based, tunable BPF 16.
In accordance with the principles of operation of a fiber ring laser, a pump beam P is used to initiate and maintain the generation of the laser output emission. In the particular example of
Pump source 30 may take the form of a discrete semiconductor laser designed to emit at the pump wavelength, or an amplified laser (e.g., MOPA) configuration, or (as shown here) a fiber laser arrangement. In this particular configuration of a fiber laser arrangement for pump source 30, an uncooled diode laser 32 operating at a wavelength of 940 nm is used as the light source, with the output from diode laser 32 coupled into a fiber laser 34 that comprises a section of Er—Yb co-doped optical fiber 36 (which does not need to be polarization-maintaining for use in the pump source) disposed between a pair of fiber Bragg gratings (FBGs) 38 designed to create an output at the desired pump wavelength λP of 1567 nm.
As shown in
A polarization-maintaining output coupler 22 is shown as an additional component of tunable PM-fiber ring laser 10, where output coupler 22 is designed to out-couple (tap) a defined portion of the circulating laser radiation R as the laser output “O” (operating at λ0) from tunable PM-fiber ring laser 10. In an exemplary embodiment, an 80/20 coupler may be used, with 80% of the radiation power forming laser output beam O, and the remaining 20% continuing to circulate and provide the continuous amplification required for lasing (it is to be understood that this power splitting ratio is exemplary only, and other values may be more appropriate for a specific application). In accordance with the principles of the present invention, the use of PM optical fiber and PM components within the ring structure ensures that the laser output beam O is polarized as well, a requirement for various applications.
Tunable PM-fiber ring laser 10 maintains its uni-directional circulation of the laser radiation in this embodiment through the use of polarization-maintaining optical isolators. In particular, tunable PM-fiber ring laser 10 includes a first PM optical isolator 14 disposed between the output of gain module 12 and the input to PM-based, tunable BPF 16. A second PM optical isolator 26 is disposed between PM output coupler 22 and polarization-maintaining WDM 28. Individual lengths of PM optical fiber 18 are utilized to interconnect the various PM optical components to form the “loop” topology of the ring structure.
In accordance with the principles of the present invention, the presence of a pump beam operating at λp of 1567 nm within a section of polarization-maintaining Tm-doped optical fiber (such as used within gain module 12) results in creating laser radiation R operating at a wavelength λ0 within the two micron region, as selected by tunable BPF 16. Tunable filters useful for this purpose may provide a linewidth of less than about 0.05 nm, providing suitable isolation between adjacent wavelengths that may be selected. The combination of polarization-maintaining components as shown in
In accordance with this embodiment of the present invention, three-port PM optical circulator 29 controls the movement of incoming pump beam P and circulating laser radiation R within the ring structure of the fiber laser, particularly with respect to their interaction within gain module 12. As shown, pump beam P from pump source 30 (operating at circulator-appropriate pump wavelength λP) is introduced into a first port A of optical circulator 29, and propagates around to exit at a second port B that is coupled to gain module 12. Thus, pump beam P is introduced into the doped PM optical fiber forming gain module 12 (either Ho-doped PM fiber or Tm—Ho co-doped PM fiber, for the reasons mentioned above). The generated output laser radiation R from gain module 12 (operating at selected output wavelength λ0 from the Ho related wavelength range of about 2000-2150 nm) is shown as propagating counter-clockwise within the ring structure so as to enter optical circulator 29 at second port B and thereafter propagate along a polarization-maintaining path within the circulator to exit at a third port C. As with the arrangement of
For the configuration of tunable PM-fiber ring laser 10C, PM output coupler 22 is shown as coupled to output port C of PM optical circulator 29-1 by a section of PM optical fiber 18. In the particular embodiment of
In particular, the retained remaining portion of the circulating amplified radiation R from output coupler 22 is coupled to a first port A2 of second PM optical circulator 29-2, propagating along a PM signal path within optical circulator 29-2 and exiting a second port B2. As shown, PM-based, tunable BPF 16 is positioned along a linear signal path at second port B2, with a reflector 17 disposed beyond the termination of tunable BPF 16. The circulating laser radiation R exiting at second port B2 propagates through tunable BPF 16 in a first direction, and is then re-directed back into tunable BPF 16 by reflector 17. The filtered radiation exiting tunable BPF 16 is thereafter re-introduced into second port B2 and continues to propagate through second PM optical circulator 29-2, exiting at a third port. PM optical fiber 18 is shown as coupled between third port C2 and the input to gain module 12, where the use of an optical circulator along this path eliminates the need for a PM optical isolator to be in place between tunable BPF 16 and gain module 12.
Again, it is to be understood that the circulator-based configuration of tunable PM-fiber ring laser 10C cannot be used with a Tm-doped gain module, since the pump beams that may be used to react with the Tm dopant are outside of the transmission bandwidth of an optical circulator configured for use with signals within the two micron wavelength region.
Instead of using a pair of three-port polarization-maintaining optical circulators, it is possible to utilize a single four-port polarization-maintaining optical circulator to form a fiber ring laser in accordance with the principles of the present invention.
In this configuration, pump source 30 is shown as coupled to first port α, allowing an input pump beam P (operating at pump wavelength λP) to propagate through optical circulator 29A to exit at second port β. The signal path between first port a and second port β may comprise a conventional single mode fiber or the like. Upon reach second port β, pump beam P exits and is injected into gain module 12 in the same manner as discussed above. The amplified laser radiation R created within PM gain module 12 is directed into second port β of circulator 29A, where it propagates along a polarization-maintaining signal path and exists at third port γ. As shown, the combination of tunable BPF 16 and reflector 17 is disposed along a linear signal path from third port γ. Thus, similar to the operation of second three-port optical circulator 29-2 discussed above, the amplified laser radiation R exiting from gain module 12 passes back and forth along BPF 16 and thereafter re-inserted into four-port optical circulator 29A at third port γ. The amplified, filtered laser radiation R then propagates along a polarization-maintaining path to output port Δ of four-port circulator 29A, where it is then directed into a section of PM optical fiber 18. As with various other embodiments described above, output coupler 22 is disposed beyond PM optical fiber 18 and used to out-couple a portion of the circulating laser radiation R as the output laser beam O (operating at a selected λ0 controlled by adjusting the center wavelength of tunable BPF 16) from tunable fiber ring laser 10D.
In accordance with this embodiment of the present invention, pump beam P from pump source 30 first passes through gain module 12, and is thereafter coupled into second gain module 12-2. Since the same pump beam is used to generate amplification in both gain modules, the same type of PM gain fiber is used to form each module (i.e., both using Tm—Ho co-doped PM fiber or both using Ho-doped PM fiber). Inasmuch as the pump wavelength used for Tm-based applications cannot be transmitted through PM optical isolator 42, this configuration is workable only for H-doped gain modules or Tm—Ho co-doped gain modules.
As with the embodiments described above, PM-based, tunable BPF 16 functions to provide an adjustment of the filter's center wavelength within a wavelength range associated with the particular dopant within the PM gain fiber forming modules 12, 12-2. When using Ho-doped gain fiber (with a tunable output laser wavelength range of about 2000-2150 nm), a pump beam operating at a wavelength λP of 1850 nm or 1940 nm may be used. The input power of pump beam P, as well as the lengths L1, L2 and absorption coefficients σ1, σ2 of the doped gain fibers within modules 12 and 12-2, respectively, may all be designed and controlled to ensure that sufficient pump power remains to generate additional gain within the laser radiation propagating through second gain module 12-2. The remainder of the PM optical components of tunable fiber ring laser 40 (i.e., PM isolators 14 and 26, polarization-maintaining tunable BPF 16, PM output coupler 22, and polarization-maintaining WDM 28) function in the same manner as discussed above, where these components are coupled together in a ring structure using sections of PM optical fiber 18 in accordance with the principles of the present invention.
In the arrangement as shown in
As with the embodiment of
Similar to the configuration of
Laser radiation R (propagating at the “tuned” output wavelength λ0) is then applied as a first input to a second polarization-maintaining WDM 28-2, where second pump sub-beam P2 (operating at λP) is shown as applied as a second input to polarization-maintaining WDM 28-2. WDM 28-2 functions to multiplex both laser radiation R (propagating at tuned wavelength λ0) and second pump sub-beam P2 (propagating at pump wavelength λP) onto a section of PM optical fiber 18 that directs both radiation R and pump sub-beam P2 into second gain module 12-2. Amplified laser radiation R exits second gain module 12-2 and passes through PM optical isolator 48-3 before entering PM output coupler 22. Here, the portion of laser radiation R that is not out-coupled continues to propagate along PM optical fiber 18 prior to being applied as an input to a tunable BPF 16 (also formed of PM optical fiber). It is to be recalled that tunable BPF 16 may comprise any suitable device that is able to achieve wavelength tuning over a defined range while maintaining polarization of the beam.
In accordance with this embodiment of the present invention, the use of two separate, spaced-apart amplifying stages allows for the laser power to be boosted immediately prior to out-coupling a portion of beam, thus providing a higher power laser output than possible with the configuration shown in
In particular, tunable fiber laser 70 can be contemplated as comprising a tunable fiber ring laser 10 or 40 (including their various configurations) as discussed above in association with
For the particular embodiment as shown in
A separate pump beam PC from pump source 78 is used in accordance with this embodiment of the present invention to increase the optical power of the signal propagating through gain module 76. As shown, pump beam PC propagates along a section of conventional single mode fiber 79, and is thereafter applied as an input to a polarization-maintaining WDM 80. Polarization-maintaining WDM 80 is positioned at the output of gain module 76 and used to couple pump beam PC (operating at a suitable pump wavelength λP) into gain module 76 to be used as a counter-propagating pump beam. The amplified output from gain module 76 is thereafter directed by WDM 80 onto an output signal path 82 that outputs the amplified signal via an optical isolator, as shown.
Similar to the configurations discussed above in association with
A second three-port polarization-maintaining optical circulator 84-2 is shown as included in the output, power amplifying stage that is disposed beyond the ring structure (used for output wavelength tuning) of tunable fiber laser 70A. In this case, a second pump source 78-2 is coupled to port A2 of second PM optical circulator 84-2, passing along the circulator to exit at port B2. As shown, the output from gain module 76 is coupled to port B2, allowing the applied pump beam to counter-propagate through gain module 76. The amplified laser output O from gain module 76 is subsequently introduced to second PM optical circulator 84-2 at port B2, where it propagates along to exit at port C2, which is coupled to polarization-maintaining output fiber 82.
While not explicitly illustrated in
First pump beam P-1 is passed through an optical power splitter 90, which directs a first portion of this beam into the ring structure (denoted as P-2H) and a second into the booster structure (denoted as P-2T). In order to create a pump beam operating at a wavelength suitable for use with the polarization-maintaining Ho-doped gain fiber within gain module 12-H (i.e., creating a pump wavelength for use within the ring laser), first pump portion P-2H is shown as applied as an input to a fiber laser 92. As shown, fiber laser 92 comprises a section of Tm-doped gain fiber 94 disposed between a pair of FBGs 96. The interaction of first pump portion P-2H with fiber laser 92 creates an output pump beam P-O operating at a wavelength λPH of 1940 nm (or perhaps 1860 nm).
Pump beam P-O is thereafter applied as an input to polarization-maintaining WDM 28, where as discussed above the circulating laser radiation R operating at a selected wavelength λ0 is supplied as a second input to polarization-maintaining WDM 28. Thereafter, in a manner similar to those described above, the combination of pump beam P-O and laser radiation R passes through the Ho-doped polarization-maintaining fiber within gain module 12-H, increasing the gain of laser radiation R prior to reaching power splitter 72.
In the particular configuration as shown in
Summarizing, a tunable polarization-maintaining Tm-doped and/or Ho-doped optically amplified fiber ring laser is proposed. Various configurations described above require a minimal number of individual components and, as a result, may be assembled in a compact OEM package suitable for integration into manufacturing or laboratory applications. The inventive fiber laser has a tuning range of 1890-2050 nm, an experimentally measured linewidth of <0.05 nm (<4 GHz), and peak fiber coupled output powers of >3.5 W CW. Applications include sources for component and system evaluation, mid-IR generation, LIDAR, DWDM, and coherent systems, and infrared spectroscopy and gas sensing applications.
While there is shown and described herein certain specific structures embodying the present invention, it will occur to those skilled in the art that various modifications and re-arrangements of the components may be made without departing from the spirit and scope of the underlying inventive concept; thus the same is not limited to the particular forms herein shown and described except insofar as indicated by the scope of the claims appended hereto.
