Patent Publication Number: US-2011064096-A1

Title: Mid-IR laser employing Tm fiber laser and optical parametric oscillator

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to optical fiber lasers (“fiber lasers”); and particularly to mid-infrared (mid-IR) fiber lasers used in combination with optical parametric oscillators (OPOs). 
     BACKGROUND ART 
     Lasers that generate light at mid-IR wavelengths have uses for a wide variety of industrial, commercial and military applications such as spectroscopic analysis, biomedical applications (e.g., “laser scalpels”), and IR countermeasures. For many if not most applications, it is preferred that the mid-IR laser be compact and lightweight and generate a high average power and spectral brightness at a relatively high repetition rate without consuming undue amounts of electrical power. It is also preferable that the output wavelength be tunable in the mid-IR, e.g., over the range from about 3000 nm to about 10,000 nm. 
     Conventional mid-IR lasers such as quantum cascade lasers (QCLs) have relatively low power levels (e.g., &lt;100 mW) and virtually fixed output wavelengths. Other types of conventional lasers, such as gas lasers (e.g., frequency doubled CO 2  lasers), semiconductor lasers and chemical lasers can provide mid-IR output but have similar shortcomings, whether it be inefficiency, complexity or limited output wavelength tunability. 
     U.S. Patent Application Publication No. 2005/0286603, which is incorporated by reference herein, describes a thulium- (Tm-) based pump laser to drive a ZGP (i.e., ZnGeP 2 ) optical parametric oscillator (OPO) to generate mid-IR laser light. The Tm-based pump laser uses a Tm-doped crystal such as a YAlO 3  laser rod. However, this crystal-based laser has a number of shortcomings, including limited average power (e.g., no more than about 10 W, depending on the particular crystal used), a relatively limited repetition rate, and thermal lensing. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention is a laser system that generates light in the mid-IR wavelength range. The laser system includes an optical fiber laser comprising a Tm-doped optical fiber gain medium and configured to generate pump light of at least one wavelength. The laser system also includes an OPO arranged to receive the pump light and configured to generate therefrom, via spontaneous parametric downconversion (SPDC), mid-IR wavelength output light having an average output power of greater than 5 W. 
     Another aspect of the invention is a laser system for generating light in the mid-IR wavelength range from about 3,000 nm to about 10,000 nm. The laser system includes a Q-switched, Tm optical fiber laser configured to generate pump light having at least one pump-light wavelength in a tunable range from about 1950 nm to about 2100 nm. The laser system also includes an OPO arranged to receive the pump light and that comprises input and output couplers with an OP-GaAs crystal disposed therebetween so as to generate, via SPDC, idler light and signal light from the received pump light. 
     A further aspect of the invention is a method of generating mid-IR light in a wavelength range from about 3,000 nm to about 10,000 nm. The method includes generating pump light of at least one pump light wavelength from an optical fiber laser having a section of Tm-doped optical fiber that serves as the gain medium. The method also includes providing the pump light to an OPO configured to generate, via SPDC, mid-IR wavelength output light having an average output power of greater than 5 W. 
     Additional features and advantages of the invention will be set forth in the detailed description that follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description that follows, the claims, as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operations of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a generalized embodiment of the mid-IR laser system according to the present invention; 
         FIG. 2  is a close-up view of the pump lasers and Tm-doped fiber section for the Tm fiber laser, illustrating an example embodiment where pump light from the pump lasers is free-space coupled to the Tm-doped fiber section; and 
         FIG. 3  is a schematic “phase-matching tuning curve” plot of the signal and idler wavelengths λ S  and λ I  (nm) as a function of the pump wavelength λ 20  (nm) for an orientation-patterned gallium arsenide (OP-GaAs) crystal having a period of 61.2 μm. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a schematic diagram of an example embodiment of a mid-IR laser system (“laser”)  10  according to the present invention. Laser system  10  has associated therewith an optical axis A 1 . Laser  10  includes a Tm fiber laser  20  that generates laser light  20 L, and an optical parametric oscillator (OPO)  120 , both of which arranged along optical axis A 1 . In various example embodiments, optical axis A 1  is folded and is curved to correspond to the optical path of laser light  20 L through Tm fiber laser  20  and OPO  120 . 
     Tm fiber laser  20  includes a Tm-doped “active” optical fiber section  26  doped with Tm +3  ions and that serves as the gain medium for the Tm fiber laser. In an example embodiment, Tm-doped fiber section  26  includes a silica fiber having a 25-μm core diameter. In one example embodiment, Tm-doped fiber section  26  is optically connected (e.g., spliced via splices  28 ) to upper and lower undoped optical fiber sections  32 U and  32 L. Tm fiber laser  20  further includes a set  38  of one or more pump lasers  40  having a pump wavelength λ 40  and that are optically connected by respective one or more pump combiners (e.g., splices, 1×2 couplers, etc.) to one of optical fiber sections  32 U or  32 L. Pump lasers  40  are shown in  FIG. 1  as pigtailed to respective fiber sections  41 . In an example embodiment, the one or more pump lasers  40  comprise 79X nm laser diodes, where “X” indicates that the pump-laser wavelength λ 40  can vary within the 790 nm to 800 nm range. In an example embodiment, optical filters  46  are disposed in fiber sections  41  to prevent laser light  20 L from reaching pump lasers  40 . 
     In an example embodiment, pump lasers  40  are optically coupled to Tm-doped fiber section  26  via a tapered fiber bundle. In yet another example embodiment illustrated in the close-up view of Tm fiber laser  20  of  FIG. 2 , pump lasers  40  are optically coupled to Tm-doped fiber section  26  through free-space using dichroic mirrors MD and focusing lenses  94  as needed. Dichroic mirrors MD are configured to reflect pump light  40 L at wavelength λ 40  and transmit laser light  20 L at wavelength λ 20 . 
     With reference again to  FIG. 1 , Tm fiber laser  20  also includes a Q-switching device  50 , such as a free-space acousto-optic modulator (AOM) unit  52  that includes, for example, an AOM  54  and a focusing lens  56 . Arranged adjacent Q-switching device  50  and opposite Tm-doped fiber section  26  is a wavelength-selecting element  70 , such as a reflective diffraction grating  74 . In an example embodiment, wavelength-selecting element  70  is operably supported by a rotation stage  78 . Wavelength-selecting element  70  is configured (and in the case of diffraction grating  74  is selectively spatially oriented) to reflect at least one select wavelength of laser light  20 L back into upper fiber section  32 U. In addition to using a conventional diffraction grating  74  for wavelength selection and tuning, other example wavelength-selecting elements  70  include, for example, fiber Bragg gratings (FBGs), volume Bragg gratings (VBGs), guided mode resonant filters (GMRFs), and combinations thereof. 
     Wavelength-selecting element  70  is used in instances where the pump phase-matching bandwidth of the non-linear crystal  140  (introduced below) in OPO  120  is small. For example, in the case of a non-linear crystal  140  in the form of an OP-GaAs crystal, the pump phase-matching bandwidth is less than 2 nm, thereby requiring a relatively narrow bandwidth Δλ 20  for pump light  20 L generated by Tm fiber laser  20  for efficient energy conversion. 
     In one example embodiment, wavelength-selecting element  70  is configured to provide feedback at a single central wavelength λ 20  with a defined width and some range over which the central wavelength can be tuned. In another example embodiment, wavelength-selecting element  70  is configured (e.g., via a monolithic or stacked arrangement of one or more of the above-described example elements) that selects two or more separate wavelengths—say, λ 20A , λ 20B , etc. In this latter embodiment, Tm fiber laser  20  generates laser light  20 L at the two or more separate wavelengths. 
     Lower optical fiber section  32 L has an output end  32 E adjacent to which is arranged a focusing lens  94  and a fold mirror MF 1  arranged along optical axis A 1 . Also arranged along optical axis A 1  on the downstream side of fold mirror MF 1  is a half-wave plate  96  and an optical isolator  98 , such as a Faraday isolator. As described below, laser light  20 L from Tm fiber laser  20  is used as pump light for OPO  120 , and so is thus referred to below as “pump light”  20 L, which is not to be confused with pump light  40 L from pump lasers  40 . 
     With continuing reference to  FIG. 1 , OPO  120  is arranged along optical axis A 1  downstream of optical isolator  98  and is optically coupled to Tm fiber laser  20 . OPO  120  includes a fold mirror MF 2  and a focusing lens  124  arranged along optical axis A 1 . In an example embodiment, focusing lens  124  has a focal length of about 150 mm. OPO  120  also includes input and output couplers  130 -I and  130 -O arranged along optical axis A 1 . 
     Disposed in between input and output couplers  130 -I and  130 -O is a non-linear crystal  140  capable of converting input (pump) light  20 L from Tm fiber laser  20  into signal light and idler light  180 L and  182 L via SPDC. In example embodiments, non-linear crystal  140  is one of orientation-patterned gallium arsenide (OP-GaAs), zinc germanium phosphide (ZGP), silver gallium selenide (AgGaSe 2 ), silver gallium sulfide (AgGaS 2 ), silver gallium indium selenide (AGIS), cadmium silicon phosphide (CdSiP 2 ), and periodically poled lithium niobate (PPLN). 
     Input coupler  130 -I is highly transmissive at the pump wavelength λ 20 , e.g., in the range from about 1950 nm to about 2100 nm. Input coupler  130 -I is also highly reflective in the mid-IR wavelength range. Output coupler  130 -O is also highly transmissive at the pump wavelength λ 20  and is as low as ˜20% reflective in the mid-IR wavelength range. 
     In an example of the operation of laser  10 , one or more pump lasers  40  generate pump light  40 L of wavelength λ 40  in the range of 790 to 800 nm. Pump light  40 L travels through pigtail fiber section(s)  41 , through non-doped lower optical fiber section  32 L and then through Tm-doped fiber section  26 , thereby optically pumping the gain medium. In the alternative embodiment shown in  FIG. 2 , pump light  40 L reflects from dichroic mirrors MD and is optically coupled into an end  26 E of Tm-doped fiber section  26  via focusing lens  94 . 
     Activation of Q-switching device  50  and the configuration of wavelength-selecting element  70  causes Tm fiber laser  20  to lase (i.e., generate laser light  20 L) at a wavelength λ 20  of about 2,000 nm (e.g., between about 1,950 nm and 2,100 nm). Thus, in the configuration shown in  FIG. 1 , laser light  20 L of wavelength λ 20  is outputted at lower optical fiber section end  32 E. This light is collimated by focusing lens  94  and continues to propagate through free-space to fold mirror MF 1 , which directs light  20 L to optical isolator  98 . Optical isolator  98  serves to protect Tm fiber laser  20  from back reflections. Light  20 L then travels through optical isolator  98  to OPO  120  and as mentioned above, serves as the pump light for the OPO. 
     In OPO  120 , light  20 L is focused by focusing lens  124  into the center of non-linear crystal  140 . Thus, the focused light  20 L passes through input coupler  130 -I and enters non-linear crystal  140 , where SPDC converts a substantial portion (e.g., on the order of up to 50% or even more) of this light into signal photons (“signal light”)  180 L of wavelength λ S  and idler photons (“idler light”)  182 L of wavelength λ I . As discussed above, input coupler  130 -I is highly transmissive at the pump light wavelength λ 20  from about 1,950 nm to about 2,100 nm and is highly reflective at mid-IR wavelengths. Output coupler  130 -O is also highly transmissive at the pump light wavelength λ 20  and is as low as 50% reflective in the mid-IR wavelength range associated with the signal and idler wavelengths λ S  and λ I . Thus, signal light  180 L, idler light  182 L and unconverted pump light  20 L are emitted from output coupler  130 -O, and constitute “output light”  200  outputted by laser  10 . The selection of the particular wavelengths λ S  and λ I  for signal light  180 L and idler light  182 L is based on the pump light wavelength λ 20 , as discussed in detail below. 
     In many instances, it is desirable to limit output light  200  to just one or two of signal light  180 L, idler light  182 L and unconverted pump light  20 L. For example, if one wished to use pump light  20 L, one can just pick off part of the pump light beam using a beamsplitter prior to this light reaching OPO  120  rather than having this light travel through the OPO, which effectively attenuates the pump light due to SPDC. Also, one may only wish to utilize a small wavelength band in the mid-IR associated with just one of signal and idler light  180 L and  182 L. 
     Thus, in an example embodiment, at least one wavelength-selecting element  70  such as one or more of a filter, a grating, etc., is arranged adjacent output coupler  130 -O so as filter at least one of signal light  180 L, idler light  182 L and unconverted pump light  20 L, thereby providing a greater degree of wavelength selection for laser  10 .  FIG. 1  shows by way of example a single wavelength-selecting element  70  arranged adjacent output coupler  130 -O and configured to filter idler light  182 L and residual pump light  20 L. Two such elements  70  configured to respectively filter idler light  182 L and residual pump light  20 L while transmitting signal light  180 L could also be used to achieve this result. 
     A preferred embodiment of laser  10  utilizes a non-linear crystal  140  in the form of an OP-GaAs crystal. Present-day manufacturing limitations restrict this non-linear crystal&#39;s clear aperture to &lt;2 mm, with most OP-GaAs crystals having a thickness &lt;500 μm. Consequently, for such non-linear crystals  140 , pump light  20 L is focused by focusing lens  124  to a beam waist of &lt;200 μm (full width at 1/e 2  of the maximum intensity) at the center of the crystal. Prior art OPO cavities generally consist of a flat-flat mirror set that partially recycles both the signal and idler photons. For a typical OP-GaAs crystal length of 15 mm to 20 mm, the OPO  120  of the present invention has a length of about 25 mm to about 30 mm, which ensures efficient energy conversion of the pump light  20 L to signal light  180 L and idler light  182 L. Conversion efficiencies of greater than 50% from pump light  20 L to signal light  180 L and idler light  182 L have been achieved in laser  10 . 
       FIG. 3  is a schematic “phase-matching tuning curve” plot of the signal and idler wavelengths λ S  and λ I  (nm) as a function of the pump wavelength λ 20  (nm), as adapted from the article by K. L. Vodopyanov et al., entitled “Optical parametric oscillation in quasi-phase-matched GaAs,” Opt. Lett. 20, p. 1912 (2004), which article is incorporated by reference herein.  FIG. 3  shows the correspondence between the pump wavelength λ 20  and the signal and idler wavelengths λ S  and λ I  for signal light  180 L and idler light  182 L generated by SPDC for an all-epitaxially-grown OP-GaAs crystal  140  that is 0.5 mm thick, 5 mm wide, and 11 mm long, with a domain reversal period of 61.2 μm. The regions of the curve associated with signal light  180 L and idler light  182 L are labeled on the plot, along with the degeneracy point DP where both the signal and idler light have the same wavelength. 
     The shape of the phase matching curve of  FIG. 3  depends upon the orientation period of non-linear crystal  140 . Therefore, the choice of the particular non-linear crystal  140  should be “matched” to Tm fiber laser  20 . It is also common with a PPLN non-linear crystal  140  to create a waveguide with several domains near one another so that changing the position of the beam in the crystal changes the output wavelength(s) of OPO  120 . 
     The plot of  FIG. 3  shows how to select signal and idler wavelengths λ S  and λ I  by selecting the pump wavelength λ 20 . By tuning the pump wavelength λ 20  from 1,950 nm to 2,100 nm, the wavelength λ S  of signal light  180 L is made to vary from 2,750 nm to 3,500 nm while the wavelength λ I  of the idler light  182 L is made to vary from 4,900 nm to 7,000 nm for the particular OP-GaAs crystal. For example, for a pump wavelength of λ 20 =2,000 nm, the signal light  180 L will have a wavelength λ S =3,000 nm while the idler light  182 L will have a wavelength λ I =6,000 nm. 
     In an example embodiment, the phase-matching curve of  FIG. 3  is also tuned by varying the temperature of the OP-GaAs crystal to shift the signal and idler wavelengths λ S  and λ I , so that laser  10  is capable of emitting light in the mid-IR wavelength range from about 3,000 nm to about 10,000 nm. Such the temperature tuning does not typically provide a strong increase in range. However, it can be used to shift the effective phase matching such that the signal and idler shift by about ±1,000 nm from standard operating conditions. 
     Different non-linear crystals  140  have similar types of phase-matching tuning curves. Likewise, variations in the configuration of the particular non-linear crystal  140 , such as the poling period of an OP-GaAs crystal, give rise to variations in the phase-matching tuning curve. 
     The selection of the output signal and idler wavelengths λ S  and λ I  depends upon the choice of the pump wavelength(s) λ 20  and the phase-matching condition—for example, the period of the orientation patterning in an OP-GaAs non-linear crystal  140 . As such, in a preferred embodiment the bandwidth of pump light  20 L is narrow (e.g., sub-nanometer), which generates correspondingly narrow linewidths for the signal light  180 L and idler light  182 L. Wavelength tuning of the signal and idler output wavelengths λ S  and λ I  occurs as a result of changing either the pump wavelength λ 20  and/or the phase-matching condition of non-linear crystal  140  so as to trace out as much of the phase-matching curve ( FIG. 3 ) as possible. 
     As described above, in an example embodiment Tm fiber laser  20  is configured (via wavelength-selecting element  70 ) to simultaneously produce more than one wavelength λ 20 . In this example, OPO  120  is configured to output multiple signal and idler output wavelengths λ S  and λ I  (e.g., λ SA , λ SB , . . . and λ IA , λ IB , . . . ). By way of example, it may be desirable to have one output wavelength from Tm fiber laser  20  in the range from 2,500 nm to 3,000 nm and simultaneously have output from 4,500 nm to 5,000 nm. However, as can be seen from  FIG. 3 , these two wavelength ranges cannot be covered simultaneously using an OPO  120  pumped by with a single wavelength λ 20 . 
     On the other hand, if Tm fiber laser  20  is configured to generate output light  20 L having two wavelengths—say λ 20A  at about 1,980 nm and λ 20B  at about 2,050 nm—the signal light  180 L of the shorter wavelength pump λ 20A  can have a signal wavelength λ SA  the range from 2,500 nm to 3,000 nm and the idler light  182 L from the longer wavelength pump λ 20B  can have an idler wavelength λ IB  in the range from 4,500 nm and 5,000 nm. Thus, a multi-wavelength Tm fiber laser  20  allows for greater tunability of the output signal and idler wavelengths λ S  and λ I . 
     The reflectivities of the input and output couplers  130 -I and  130 -O of OPO  120  play a secondary role in controlling the signal and idler output wavelengths λ S  and λ I . Generally, it is preferred that the reflectivities of input and output couplers  130 -I and  130 -O are chosen to provide as wide a mid-IR output wavelength range as possible. In an example embodiment, input and output couplers  130 -I and  130 -O are highly reflective from 3,000 nm to 5,000 nm. In other example embodiments, input and output couplers  130 -I and  130 -O are more closely optimized to only the signal wavelength λ S  or only the idler wavelength λ I . 
     The use of Tm-doped fiber  26  as the gain medium provides laser  10  with a number of advantages over the prior art laser systems, including a smaller form factor, better mode quality, and higher average power. While the prior art lasers are generally limited to average output powers of just a few Watts, Tm fiber laser  20  is cable of generating pump light  20 L of greater than about 50 W and up to about 100 W, which allows laser  10  to generating output light  200  in the form of 100 ns optical pulses with about 400 μJ energy at an average power of greater than 5 W, and even several tens of Watts, and in one example embodiment greater than about 50 W. 
     These average power values represent the power of signal light  180 L or idler light  182 L and ignore residual pump light  20 L. Generally, signal light  180 L and idler light  182 L have about the same amount of power, with the idler light having slightly less power due to the higher energy per photon. For each photon of signal light  180 L, there should, in principle, be one photon of idler light  182 L. Variations in the number of signal and idler photons can of course vary due to attenuation along the optical path both in the OPO and beyond. 
     In an example embodiment, the repetition rate of system  10  is in the MHz range for average powers in excess of 250 W. In example embodiments where lower average power, &lt;50 W, is used, the repetition rate is between about 20 KHz and about 100 kHz. A general range for the repetition rate is between 50 KHz and 1 MHz. 
     The photon bandwidths of signal and idler light  180 L and  182 L are typically similar to that of pump light  20 L. Typically, the pump bandwidth Δλ 20 &lt;2 nm for pumping an OP-GaAs-based OPO  120 , so the signal and idler bandwidths are typically &lt;10 nm. Instabilities in Tm fiber laser  20  can lead to variations in the signal and idler bandwidths. 
     The various configurations of laser  10  are amenable to modular design and compact and rugged packaging. Laser  10  is thus suitable for use in a variety of applications, such as for infrared countermeasures, spectroscopic analysis, and “laser scalpel” applications relying upon specific absorption resonances of biological tissue in the mid-IR. Laser  10  could replace a wide variety of other sources currently used in such applications, such as QCLs, mid-IR FELs, and periodically poled lithium niobate- (PPLN-) based OPOs. Direct tuning of the laser wavelength λ 20  and/or the OPO crystal temperature enables laser  10  to produce high-power mid-IR output tunable from about 3,000 nm to about 10,000 nm with the use of appropriate OPO input and output couplers  130 -I and  130 -O and non-linear crystal  140 . 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.