JP2008511182A - Injection-locked high power laser system - Google Patents

Abstract

  A high power laser system comprising a master laser and a primary slave laser oscillator having a resonator including a rare earth doped fiber. The primary slave laser oscillator is actively injection locked to the master laser, and the resonator provides an optical power output exceeding 1W.

Description

  The present invention relates generally to high power laser systems including active synchronization of a high power primary slave laser oscillator to a low power master laser, and more particularly to a high power rare earth doped double clad fiber hybrid primary slave laser oscillator.

While high power laser systems with a low power master laser injection locked to a primary (slave) laser oscillator are known, such laser systems utilize a solid (ie, solid laser crystal) gain medium. Laser crystals are generally as long as about 60 mm, diameters are as small as about 1.6 mm, and are cut into Brewster angles, resulting in a narrow optical aperture in the crystal. Due to the thermal laser oscillation of the laser crystal and the aperture sandwiched between the laser crystals, it is necessary to keep the laser resonator length short, generally about 50 cm. Therefore, the free spectral range f _cav of the resonator is considerably (about 10 times) larger than the modulation frequency f _mod of the electro-optic modulator required for injection locking.

  Such solid state laser systems do not provide a diffraction limited output, especially when scaled up to operate at high power. Furthermore, optical birefringence induced at high power (due to high thermal stress) results in mode instability and depolarization. In these types of laser systems, the solid (crystal) laser medium has a crystal that absorbs some of the pumping light and loses it as heat, thus reducing the efficiency of the laser system and making stable operation at high output power difficult. There is a problem of heat dissipation. The formation of crystalline thermal lenses, aberrations and flaws due to thermal stress when operated at high power is commonly known. These thermal effects cause instability (fluctuation) in both the spectral behavior and mode behavior of the laser system output.

  The injection-locked laser system described above can be used to generate light of secondary or higher harmonic frequencies that utilize a frequency converter crystal that is appropriately phase matched. For example, light having a deep ultraviolet (DUV) wavelength of 198 nm can be generated by sum frequency generation (SFG) of light having an infrared (IR) wavelength of 1064 nm and light having an ultraviolet (UV) wavelength of 244 nm. However, in the method of generating such deep ultraviolet light, the optical damage that the frequency converter crystal is subjected to must be considered. This damage is mainly caused by the simultaneous presence of both IR light and UV light. This limits the operating time length of the frequency converter crystal until optical damage begins to progress, causing a significant reduction in conversion efficiency.

  For example, one manufacturer of an industrial laser generates a frequency converter crystal (CLBO) when generating 200 W DUV light power at 198 nm from 500 W intracavity IR light power at 1064 nm and 600 mW UV light power at 244 nm. ) Reveals that the maximum operating time for any given spot above is about 70 hours. To increase the total lifetime of the laser system, the frequency converter crystal had to be moved laterally to another operating spot after 70 hours of operation. That is, before the frequency converter crystal had to be completely replaced, about 100 position indexing was required to extend the life until it exceeded 5000 hours. Similarly, when another laser manufacturer uses a frequency converter crystal (CLBO) to generate second harmonic wavelength (266 nm) light from 290 W intracavity power at a fundamental wavelength of 532 nm, It reports 3 hours of operation at a power level of 3 W at 266 nm.

  An object of the present invention is to provide means for suppressing the occurrence of instability of the spectrum and mode of the output in the injection-locked laser system and avoiding damage to the frequency conversion crystal.

  One aspect of the present invention is a high power laser system including a master laser and a primary slave laser oscillator synchronized with the master laser having a resonator including a rare earth doped fiber, and the resonator provides optical power exceeding 1 W. It is. In some embodiments, the output optical power exceeds 50 W, 100 W, or even 150 W.

  According to one embodiment of the invention, the optical path length in the rare earth doped fiber is longer than the passive path length in the primary laser oscillator.

  According to one embodiment of the invention, the resonator has a phase modulator that can stretch at least a portion of the rare-earth doped fiber to synchronize the optical signal frequency, the phase modulator as a mode filter Function.

  According to one embodiment of the invention, the rare earth doped fiber is a polarization maintaining fiber. According to another embodiment of the invention, the rare earth doped fiber is a single polarization fiber.

  According to some embodiments, the resonator has a second harmonic generator.

  The laser system according to the present invention offers several advantages of high output power, eg, several hundred watts, high output spectral purity and high operational stability, while also having the advantages of small size and high light damage resistance.

  Additional features and advantages of the invention will be set forth in the following detailed description, and in part will be apparent to those skilled in the art from the description, including the following detailed description, the claims, and the accompanying drawings. It will be appreciated by practicing the invention as described in the document.

  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 characteristics of the invention as recited in the claims. It is natural that it is done. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in 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.

Referring to FIG. 1a, a low power master laser 12 and a high power slave laser oscillator 14 (also referred to herein as a primary laser oscillator) having a length of rare earth doped fiber 16 as an active medium are provided. A schematic diagram of the optical and electrical systems of an exemplary laser system 10 is shown in this figure. “Oscillator” means that the high-power slave laser oscillator 14 can independently generate a coherent laser output by itself without any input from the master laser 12, as occurs when the master laser 12 is not injection-locked. . If active injection locking is not achieved, the spectral linewidth of the high power slave laser oscillator will be wide, for example 20 nm if Yb-doped fiber is utilized. If active injection locking is achieved, the spectral line width of the high power slave laser oscillator will be quite narrow, for example 10 pm wide. That is, active injection locking provides high output power from the slave laser oscillator 14 while maintaining the spectral characteristics of the master laser 12. In this embodiment, the high power slave laser 14 has a Yb-doped double clad fiber (DCF) 18. In this embodiment and some exemplary embodiments, the optical power output of the laser 14 exceeds 50 W, 100 W, or even 150 W. For example, active injection locking can be achieved by a feedback circuit that changes the path length in the resonator of the primary slave laser oscillator 14 appropriately. In this embodiment, the well-known Pound-Drever-Hall (PDH) synchronization technique is used to actively inject-lock the laser 14 to the single frequency master laser 12. it can. When active injection locked, the wavelength of the slave laser 14 will be the same as the wavelength of the master laser 12. The operating wavelength of 1064 nm shown for the master laser 12 and slave laser 14 integrated injection locking device is only representative, and the master laser 12 and slave laser 14 are independent over the entire Yb emission range, ie from 1020 nm to 1180 nm. Can be tuned. The low power single frequency output of the master laser 12 is sent through an electro-optic modulator (EOM) 20 driven by a driver 20a. The electro-optic modulator (EOM) 20 generates two sidebands B, each separated from the carrier frequency A corresponding to the optical frequency of the master laser 12 by a frequency difference f _mod as seen in FIG. The frequency difference f _mod between each sideband B and the carrier frequency A is equal to the (electric) drive frequency of the electro-optic modulator (EOM) 20. The semi-reflective mirror 22 guides part of the output light of the master laser 14 into the photodetector 24. The electrical output of the photodetector 24 is mixed with a reference electrical signal from the driver 20a using a double balanced mixer 26. The high frequency component of the output of the mixer 26 is filtered by the electric filter 28. The electrical signal output from the filter 28 is guided to the integrated integration circuit device 30 including the high-speed integration circuit 30a and the low-speed integration circuit 30b. In the present embodiment, the electrical output of the high-speed integration circuit 30 a is guided to the high-speed response unit (F) 32 a of the (optical) phase modulator 32. Similarly, the electrical output of the low speed integration circuit 30 b is guided to the low speed response unit (S) 32 b of the phase modulator 32. Together, the mixer 26, filter 28, integrated integrator circuit device 30 and phase modulator 32 form a feedback unit 34. Phase modulator 32 generally couples two sections of rare earth doped fiber 16 to two piezoelectric cylinders 32a by wrapping and joining one section of rare earth doped fiber 16 onto one piezoelectric cylinder (not shown). , 32b can be individually wound and joined. The diameter of the piezoelectric cylinder 32a is smaller than the diameter of the piezoelectric cylinder 32b, and thus acts as a high-speed phase modulator (the piezoelectric cylinder 32b having a large diameter serves as a low-speed phase modulator). Alternatively, the piezoelectric cylinder 32a can be replaced with two piezoelectric half-cylinder clip-on sets (not shown). The optical path length (the product of the refractive index n and the actual length d) of the optical fiber 16 is modulated (changed) by an electrical signal that drives the two piezoelectric cylinders 32a and 32b. For example, the optical path length can be extended (shrinked) by stretching (pushing) a fiber segment wound around a piezoelectric cylinder. The change in the optical path length corresponds to the modulation of the optical phase in the resonator 36 of the primary slave laser oscillator 14.

  The free oscillation wavelength of the laser 14 (that is, when the primary slave laser oscillator 14 is not synchronized with the master laser) is close to the wavelength of the master laser 12 and within the synchronization range of the feedback unit 34, and the master laser 12 and the slave laser. In order to match the 14 wavelengths, a phase change of the appropriate sign and magnitude is made by the phase modulator 32.

In the present embodiment, the laser 14 includes a rare earth doped fiber 16 (active fiber) having an optical power gain and an optical path length n ₂ d ₂ . Here, n ₂ is the effective refractive index of the optical fiber, and d ₂ is the actual length of the optical fiber. The optical fiber length of the active fiber is the passive non-guided optical path length in the resonator:

Shorter. Where n _1i is the refractive index of the optical medium along the passive optical path and d _1i is the corresponding distance or thickness of the medium along the passive optical path. Optical path length:

Is kept as small as possible, and the total optical path length L of the laser resonator 36 of the laser system 10 of this embodiment is:

Is selected such that the free spectral range f _cav (see FIG. 1b) of the laser resonator can remain at a minimum of 5 MHz corresponding to a total optical path length L of about 60 cm. Furthermore, in this embodiment, the optical path length L is selected so that the free spectral range f _cav of the resonator 36 is not equal to the modulation frequency f _mod of the electro-optic modulator 20. More specifically, in this embodiment, the laser system further comprises an EOM coupled to the EOM driver, and the resonator has a resonator length L such that f _mod > f _cav . Here, f _mod is the frequency of the EOM driver, and f _cav is the resonator interval (that is, the signal mode interval when there is no EOM) as determined by the resonator length L of the primary slave laser oscillator. The length L is preferably greater than 0.25 m, and more preferably greater than 1 m.

  The double clad structure of the Yb-doped optical fiber 18 allows high optical power from the multimode output of a pumping laser 38 (eg, 980 nm pump) to be coupled to the inner cladding of the fiber 18. This coupling is facilitated by the pump-signal combiner 40. The pump light at 980 nm is coupled to the inner cladding, for example by superimposed waveguides on the Yb-doped core of the fiber 18, allowing optical power gain for light emission (ranging from 1020 nm to 1180 nm) in the Yb-doped core. Other methods of pumping the laser fiber 18 can also be used, such as side pumping by use of a V-groove or prism and end face pumping by use of a dichroic mirror.

  In the present embodiment, the Yb-doped fiber 18 plays a dual role as an optical power gain medium and as an optical phase element that can be modulated by the piezoelectric cylinders 32a and 32b.

  Further, due to the small diameter of the high speed piezoelectric cylinder 32a, whenever the core of the Yb-doped double clad fiber 18 supports a higher order mode, the phase modulator 32 can further assume the role of an (optical) mode filter. It becomes possible. As the core diameter of the rare earth doped fiber 18 increases, the end face coupling efficiency of the fiber to incident light will increase. However, if the core diameter is large, for example 15 μm or larger, higher order modes will be supported by the fiber core. Thus, wrapping the fiber 18 onto the piezoelectric cylinders 32a and 32b for phase modulation purposes also allows higher order mode radiation from the core of the fiber 18. The amount of suppression of higher order mode propagation in the fiber 18 depends on the bending loss difference between the fundamental mode and the higher order mode. Therefore, in this embodiment, the optical phase modulator 32 also serves as a double mode as a useful mode filter.

  A mode-matched optical component 42, such as a microscope objective and / or telescope, allows for intracavity light injection into and extraction from the Yb-doped double-clad fiber 18. If necessary, the internal loss increases, but a suitable polarization control light element 44 can be added. In the input-output coupler, which is a mirror 46 in this embodiment, a high optical power output from the laser 14 (higher than 1 W, preferably higher than 10 W, more preferably higher than 50 W) is provided. In the present embodiment, the high reflection mirror 48 reflects light (counterclockwise) toward the input-output coupler 46. In this embodiment, the input-output coupler is a semi-reflective mirror. In order to match the internal loss of the laser resonator 36, the light transmittance of the input-output coupler 46 is selected based on the theoretical optical impedance matching principle. For example, if the loss in the laser resonator 36 is 4%, the transmittance of the input-output coupler (mirror) 46 should be 4%. A portion of the light that arrives from the mirror 48 and is subsequently reflected by the input-output coupler 46 is coupled to the fiber 18 using a mode-matching optical element, such as the microscope objective lens 42. The semi-reflector 22 is utilized to branch a small portion of the light exiting the input-output coupler 46, for example 1% or 2%, to the photodetector 24.

  Since it is important to reduce the internal loss as much as possible, the reflection loss at the interface of the mode matching optical system 42 can be minimized by using the antireflection film for the light wavelength in the laser resonator 36. .

Damage when focusing very high intracavity power onto a small fiber core is also minimal if the core diameter of the fiber 18 is greater than 10 μm, preferably greater than 15 μm, and the mode area is preferably greater than 150 μm ^2. Can be suppressed.

  Introduction of the rare earth doped optical fiber 16 as a gain medium into the slave laser 14 reduces the self-focusing and associated thermal problems that occur in non-fiber type solid state laser media. The introduction of the rare earth doped fiber medium 16 (eg, Yb doped fiber 18) also provides the great advantage of tunability of the injection locked slave laser 14 when the master laser 12 is tuned. Unlike fiber lasers, non-fiber type solid high power lasers have poor wavelength tunability. It is also possible to improve the spectrum fidelity by passively injection-locking a pulse slave laser to the master laser 12.

  Further, the rare earth doped fiber 18 may be a polarization maintaining type or a single polarization type fiber. If a polarization maintaining fiber is utilized, the optical polarization at the input-output coupler 46 and within the laser resonator 36 will be stable. Correspondingly, if a single polarization fiber is utilized, the optical polarization at the input-output coupler 46 and within the laser resonator 36 will be linearly polarized. Such single polarization rare earth doped fibers are, for example, Joohyun Koh, Christine Louise Tennent, Donnell Thaddeus Walton, Ji Wang. And U.S. Patent Application Publication No. 2005/0158006, filed July 21, 2005, in the name of Luis Alberto Zenteno. That is, one of the main advantages of the laser system 10 of this embodiment is that the fiber 18 of the slave laser 14 has several roles at the same time: (i) optical gain medium, (ii) polarization maintaining waveguide path, (Iii) a polarization waveguide path, (iv) an optical phase modulator, and (v) an optical birefringence modulator / when properly wound on a paddle and properly rotated to form a waveplate It can play one or more of the roles of the controller.

  The main advantages of the injection locking technique are the spectral purity (single frequency operation) and high fidelity of the low power master laser 12, otherwise it is very (e.g. when not synchronized to the master laser) To a high power slave laser 14 that would have a very wide wavelength spectrum (eg, 20 nm), along with instabilities associated with the strong multi-longitudinal mode properties of long (eg, 40 m fiber length) resonators. This injection locking approach eliminates or minimizes the use of frequency selective elements such as etalons in resonators and direction selective elements such as isolators, both of which result in high intracavity losses. Further, such devices are known to degrade or be damaged when very high (eg, several hundred watts) intracavity optical power circulates in the resonator 36. Unidirectional operation in the high power / large cavity length slave laser 14 in the same direction as the master laser light coupled to the slave laser 14 by the input-output coupler 46 is achieved without the need for an optical isolator.

  Furthermore, the introduction of the fiber 16 as a gain medium generally results in a small laser (small ground area) by wrapping a long fiber over a small size (typically less than 3 inches) piezoelectric cylinder. can get.

  For those skilled in the art, other synchronization techniques such as the Hansch-Couillaud method and (though less preferred because it is more complex) are also schematically shown in FIG. 1a. It will be appreciated that the present invention is applicable by making modifications suitable for optical / electrical systems.

  Another embodiment of the present invention is a resonator in the high power slave laser 14 that is injection-locked to the master laser 12 and coincident with the generation of visible, ultraviolet, deep ultraviolet or intermediate wavelength light. Includes internal optical frequency conversion. This simultaneous frequency conversion and thereby the generation of new wavelengths is possible by having high intracavity optical power at near IR wavelengths, eg 1064 nm, with high spectral purity and stability when injection locking is achieved. become.

  FIG. 2 shows a schematic diagram of the optical and electrical systems of an exemplary laser system 10 with a high power slave laser 14 injection-locked to the master laser 12. Simultaneous frequency conversion is performed while the fundamental light recirculated in the resonator 36 is injection-locked to the master laser 12. As in the previous embodiment, the laser 14 has a length of rare earth doped fiber 16 as the (active) optical gain medium. The laser 14 of this embodiment is similar to the laser shown in FIG. 1a, but with an optical frequency converter 50 added. The optical frequency converter 50 is a crystal such as lithium triborate (LBO), potassium phosphotitanate (KTP), periodically poled KTP (PPKTP), periodically poled lithium niobate (PPLN), magnesium oxide doped periodically poled niobium. Lithium oxide (MgO: PPLN), magnesium oxide doped periodically poled stoichiometric lithium niobate (MgO: PPSLN), periodically poled lithium tantalate (PPLT), magnesium oxide doped periodically poled lithium tantalate (MgO: PPLT) Alternatively, it can have magnesium oxide doped periodically poled stoichiometric lithium tantalate (MgO: PPSLT), or other suitable crystals that are appropriately phase matched. Waveguides can also be incorporated into the periodically polarized crystals, allowing longer interaction lengths. When radiated light of wavelength λ enters such a second harmonic generator crystal, part of the optical energy is converted to an optical signal having a frequency twice that of the original signal wavelength λ. . For example, if an optical signal with a wavelength of 1064 nm enters such a crystal, a portion of the light exiting from the frequency converter 50 will have a wavelength of 532 nm.

Alternatively, the frequency conversion process can be performed by utilizing the Raman effect. For example, a crystal such as barium tungstate (BaWO ₄ ) that generates a primary Stokes wavelength of 1180 nm from a fundamental wavelength of 1064 nm can be used as a Raman converter. Over the same approach, the same crystal can be used to generate higher order Stokes wavelengths. Another extension of the same frequency conversion approach is the generation of the first 1156 nm primary Stokes wavelength utilizing a Raman converter, such as a lithium iodate (LiO ₃ ) crystal, followed by lithium triborate (LiB ₃ O ₅ ) There is a conversion to a second harmonic wavelength of 578 nm by the crystal.

  In generating 532 nm output from a fundamental wavelength of 1064 nm, the mirror 48a of the present embodiment will transmit most of the 532 nm light, thus providing 532 nm laser output and reflecting most of the 1064 nm towards the mirror 46. . As in the previous example, the transmittance of the input-output coupler 46 is a theoretical optical impedance match to match the coupled internal loss of the laser resonator, which in this example includes the loss of the fundamental light due to the frequency conversion process. Chosen based on the principle. For example, if the loss in the laser resonator is 5%, the transmittance of the mirror 46 should be 5%.

  In this embodiment, the mode matching optics 42a and 44a are optimized to include the effects of introducing the frequency converter crystal 50. In general, due to the optical birefringence introduced by the crystal, it will be necessary to reorient the polarization component within the polarization control optical element 44a, and at the same time, it is necessary to focus the fundamental light within the crystal 50. Therefore, it will be necessary to convert the mode matching characteristics to enable highly efficient optical coupling to the fiber 18.

  FIG. 3 shows a schematic diagram of the optical and electrical systems of another exemplary laser system 10 that includes a high power primary slave laser oscillator 14 injection-locked to the master laser 12. As in the previous embodiment, the laser 14 has a length of rare earth doped fiber 16 as the active medium. The laser 14 of this embodiment is similar to the embodiment shown in FIG. 2, but in this embodiment, an optical frequency converter 50 is disposed between the mirror 48 and the mirror 46, and is near the mirror 48. The exemplary laser system 10 further includes a secondary resonator 52 associated with the secondary laser. The secondary resonator 52 shares a common path with the primary resonator 36 of the laser 14 to extend the frequency conversion to the third harmonic wavelength, for example 354.6 nm. When leaving the optical frequency converter 50, both the light of the primary wavelength (for example, 1064 nm) and the light of the second harmonic wavelength (1 / 2λ, for example, 532 nm) are both the second frequency converter 54 (in this example, 354 Propagating towards the third harmonic crystal generating .6 nm. In the present embodiment, the second frequency converter 54 is a lithium triborate (LBO) crystal.

  In this embodiment, the secondary resonator 52 or coupler of the secondary laser also has three mirrors 56a, 56b and 56c. The input-output coupler 56a is selected to have a transmissivity of approximately 1% to 10% semi-reflective, selected to match the internal loss of the secondary resonator 52 at the second harmonic wavelength, in this case 532 nm. It is a vessel. The input-output coupler 56a is a dichroic mirror (wavelength separator) having a high transmittance at 1064 nm. The mirror 56b is also a dichroic mirror that has high transmittance for light with wavelengths of 1064 nm and 354.6 nm and high reflectance for light with a wavelength of 532 nm. When entering the mirror 56c, the 532 nm light is reflected toward the mirror 56a. Accordingly, the 532 nm light recirculates in the secondary resonator 52. The mirror 56c is attached to the piezoelectric plate 56'c, and the position of the mirror 56c is modulated by an electric signal supplied to the piezoelectric plate 56'c. The change in the position of the mirror 56 changes the resonator length of the secondary resonator.

  The reflected light (of the second harmonic wavelength) at the mirror 56a and the leaked light (of the second harmonic wavelength, which exits the mirror 56a after circulating through the resonator 52) interfere with each other, and the integrated Hansch-Kellows servo device 62 Give input light to. More specifically, the integrated Hansch-Killow servo device 62 has a feedback circuit including a quarter-wave plate 58a, a polarizing beam splitter 58b, two photodetectors 58c, an electrical subtractor 58d, and an integrating circuit 60. The integration error signal from the integrated servo device 62 is sent to the piezoelectric plate 56'c.

  The 532 nm light beam resonates within the resonator 52 when the assembled Hansch-Kellows servo device keeps the secondary resonator 52 in resonance with the 532 nm input light coming from the second harmonic crystal 50. The 1064 nm light in the primary resonator 36 (ie, the resonator of the primary slave laser oscillator) and the 532 nm light resonating in the secondary resonator 52 are phased so as to generate a sum frequency of 354.5 nm from the 1064 nm light and the 532 nm light. Aligned. Mixed in crystals 54. A dichroic mirror (harmonic separator) 64 separates the 354.6 nm light from the 1064 nm light beam of the primary resonator 36 and the residual 532 nm light leaking from the mirror 56 b of the secondary resonator 52. The 1064 nm light beam that passes through the dichroic mirror 64 travels toward the mirror 46 that directs the 1064 nm light beam toward the mode matching optics 42 and the rare earth doped fiber 18.

  At least one optical component of the secondary resonator, such as mirror 56c, does not share a common path with the primary resonator. That is, at least one section of the secondary resonator is not in the primary resonator (ie, not in the resonator of the primary slave laser oscillator). In this example, the mirror 56c is appropriately joined to the actuator 56'c, for example, of a piezoelectric type, and is movable.

  The secondary resonator 52 can be, for example, a closed triangular resonator as shown in FIG. 3 or a bowtie resonator (not shown), both types providing unidirectional propagation. Supports but does not support bidirectional operation as in linear resonators or folded L or V type resonators (not shown).

  It is pointed out that sharing of the path in the common resonator with the primary resonator of the secondary resonator (that is, the resonator of the primary slave laser oscillator 14) results in a very small laser system with reduced ground area. Keep it. The same basics based on harmonic generation in injection-locked resonators for some applications where primary and secondary resonators need not be superposed or small size is not so important Light operation can be utilized, which is equivalent to the embodiment of FIG.

  FIG. 4 shows a schematic diagram of the optical and electrical systems of another exemplary laser system 10 that includes a high power slave laser 14 injection-locked to the master laser 12. As in the previous embodiment, the laser 14 has a length of rare earth doped fiber 16 as the active medium. The laser 14 of this embodiment is similar to the embodiment shown in FIG. 3, except that the third harmonic generator (crystal 54) is replaced with a fourth harmonic generator (crystal 66). Instead of using an anti-reflective coating on the frequency converter (crystal 66) and polarizer 68, the crystal 66 is cut to the Brewster angle as shown in FIG. 4 and is similar to the crystal 54 shown in FIG. Can be aligned in a manner. The integrated Hansch-Killow servo device 62 described above with reference to FIG. 3 for third harmonic generation applies equally to FIG. 4 for fourth harmonic generation. The crystal 66 (fourth harmonic generator) is phase matched to convert incident second harmonic light, eg, 532 nm green light, to 266 nm fourth harmonic. The generated 266 nm light is then separated from the intracavity 1064 nm light in the primary resonator 36 and the 532 nm light resonating in the secondary resonator 52.

  FIG. 5 shows a schematic diagram of the optical and electrical systems of another exemplary laser system 10 that includes a high power slave laser 14 injection-locked to the master laser 12. As in the previous embodiment, the laser 14 has a length of rare earth doped fiber 16 as the (active) optical power gain medium. The laser 14 of this embodiment is similar to the embodiment shown in FIGS. 3 and 4, but in this embodiment, a secondary resonator 52 is disposed near the (secondary) frequency converter 70a (third order). ) It further has an optical frequency converter 70b. Therefore, the laser system shown in FIG. 5 starts with high-power fundamental light that is injection-locked to the master laser 12 and emits light of the fifth harmonic frequency. More specifically, the laser system 10 of the present exemplary embodiment includes the first nonlinear crystal 70a (in the secondary resonator) that generates the third or fourth harmonic in the secondary resonator 52. Another nonlinear crystal 70b arranged appropriately generates a 213 nm laser beam of the fundamental harmonic of 1064 nm and the fifth harmonic. If the crystal 70 a is phase matched to generate the third harmonic of the fundamental light 1064 nm that resonates in the primary resonator 36, the crystal 70 b is the second harmonic of 532 nm that is resonating in the secondary resonator 52. And phase matching so that the fifth harmonic is generated from the sum frequency generation of the third harmonic generated by the crystal 70a. If the crystal 70a is phase matched to generate the fourth harmonic of the 1064 nm fundamental light that resonates in the primary resonator 36, the crystal 70b is the 1064 nm fundamental light that is resonating in the primary resonator 36. And phase matching so that the fifth harmonic is generated from the sum frequency generation of the fourth harmonic generated by the crystal 70a.

  A notable feature of this configuration is that the spectral width of the fifth harmonic light is changed from single frequency operation to multiple axes by changing the length L ′ of the secondary resonator 52 relative to the length L of the primary resonator 36. It can be changed to mode operation. For example, as the secondary resonator 52 becomes longer, more axial modes can be supported, and all such axial modes fall within the single-frequency light line width of the primary resonator 36.

  The laser systems shown in FIGS. 3, 4 and 5 are very compact laser systems because in each case the secondary resonator 52 shares a significant portion of that resonator with a significant portion of the primary resonator 36. It is an embodiment.

  Alternatively, for example, as shown in FIG. 5, the laser system 10 can be phase matched harmonics due to birefringence made with one crystal (54 in FIG. 3 or 66 in FIG. 4) or two optical crystals (70a and 70b in FIG. 5). Instead of using wave generation, a self-phase matched Raman frequency shift in the crystal can be used to create a light beam of the desired wavelength. The first nonlinear medium in the secondary laser resonator is: (a) a resonator resonating in the primary resonator 36; (b) a resonator resonating in the secondary resonator 52; A very novel laser system when generating Raman shift frequencies from 532 nm light or from both 1064 nm and 532 nm intracavity light beams as described in (a) and (b) above (c) Is obtained. This Raman shift frequency technique allows the use of a wide range of frequencies (and thus optical wavelengths). The Raman shifted light is then either (a) recirculated the Raman shifted wavelength from the 1064 nm fundamental light, or (b) recirculated the second harmonic 532 nm light with the Raman shifted light from both the 1064 nm wavelength and the 532 nm wavelength. When mirrors 56a, 56b and 56c with suitable coatings are selected, they can resonate in secondary resonator 52. The Raman shifted light is then separated from the 1064 nm light and the residual 532 nm light by a suitable dichroic mirror 64b.

  The second crystal 70b of the secondary resonator 52 in FIG. 5 mixes any of the Raman-shifted light generated in the first crystal 70a of the secondary resonator 52 with the fundamental light of 1064 nm or the second harmonic light of 532 nm. Phase matching can be achieved.

  Here, the same light detection circuit and electrical system for the integrated servo device as shown briefly in FIGS. 3, 4 and 5 can be used to generate Raman shifted light or to generate Raman shifted light with 1064 nm light and / or 532 nm light. It can be applied to mixing.

  FIG. 6 shows a laser system 10 that utilizes the combined operation of two independent injection-locked primary laser oscillators 15a and 15b. The two primary laser oscillators 15a and 15b have two different starting fundamental wavelengths, for example 976 nm and 1064 nm. Further, in order to generate the 244 nm light of the fourth harmonic of the primary laser oscillator 15a that resonates at 976 nm, an external resonator 74 that does not have any optical gain medium therein has two primary resonators 15a and 15b. Arranged between. This coupling system is described in further detail below.

  The primary laser resonator 15a generates a light output of 488 nm, which is the second harmonic of the 976 nm resonance fundamental light in the primary resonator 36a. Here, the crystal 72 converts 976 nm light into 488 nm light. Master laser 12a (ie, master laser) operates at a wavelength of 976 nm, and pumping laser 38a operates at a wavelength of 915 nm. A pumping light combiner 40a couples pumping light having a wavelength of 915 nm and a resonance wavelength of 976 nm. As described above, the primary resonator 15a is injection-locked to the master laser 12a using the electro-optic modulator 21, the PDH servo integration circuit 34, and the phase modulator 32.

  The 488 nm output of the primary laser oscillator 15a is incident on the external resonator 74, in which the second harmonic generator crystal 82 is placed. Crystal 82 converts the resonant 488 nm light in the resonator to 244 nm light. The 244 nm light output is then separated from the resonant 488 nm light in the resonator 74 by the curved dichroic mirror 78b. The resonator 74 is maintained in a resonance state with respect to the incident 488 nm light by using the integrated Hansch-Killow servo device 62 as described above. The feedback signal from the integrated servo device 62 is given to a piezoelectric actuator 76'b attached to the mirror 76b. If necessary, a polarizer 80 for performing Hansch-Killow polarization analysis can be added in the resonator.

  The 244 nm light output from the resonator 74 is then injected into the resonator 36b of the primary laser oscillator 15b through the dichroic mirror 48b. The resonator 36b resonates at an optical wavelength of 1064 nm incident on the crystal 86 together with the incoming 244 nm light 84. Crystal 86 mixes two wavelengths, 1064 nm and 244 nm, to generate 198 nm light. The 244 nm light does not resonate in the primary resonator 36b. The primary resonator 36b is maintained in resonance with the master laser 12 operating at 1064 nm by utilizing the PDH injection locking technique described above. Dichroic mirrors 48b and 46a are high reflectors at 1064 nm and are transparent at 244 nm and 198 nm. The dichroic mirror 22a separates 198 nm light from 1064 nm or 244 nm residual light.

  One very important advantage of embodiments of the present invention, as described above and shown schematically in FIG. 6, is that a substantial reduction in optical damage to the nonlinear optical frequency converter crystal is obtained. This advantage increases the infrared light power of eg 1064 nm in the resonator and simultaneously and correspondingly lowers the input / internal UV light power of eg 244 nm to maintain an output deep ultraviolet light power level of eg 198 nm. Is achieved. For example, if the IR light power is doubled and the UV light power is reduced to ½ at the same time, the output power obtained at the 198 nm wavelength is the same, but damage to the CLBO crystal 86 is avoided. This concept takes advantage of the linear dependence of power at deep ultraviolet wavelengths to power at ultraviolet wavelengths, along with known damage mechanisms in crystal 86, when the infrared light power far exceeds the ultraviolet light power. In our embodiment, the preferred range of infrared (IR) light power (for example at a wavelength of 1064 nm) in the resonator (resonator 36b of the primary slave laser oscillator 15b in FIG. 6) is greater than 500 W, ( The preferred range of UV light power (for example at a wavelength of about 244 nm) is less than 600 mW. More preferably, the range of intracavity IR light power (at a wavelength of 1064 nm) is greater than 1000 W, and the preferred range of UV light power is less than 300 mW. Most preferably, the intracavity IR light power is greater than 2000 W and the preferred range of UV light power is less than 150 mW.

  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. Therefore, it is intended that the present invention cover such modifications and variations as come within the scope of the appended claims and their equivalents.

1 is a schematic diagram of a laser system according to an embodiment of the present invention. The separation f _mod between the carrier frequency A and the sideband B and the modulation frequency f _cav of the primary laser oscillator resonator are _simply shown. 4 is a schematic diagram of the configuration of optical and electrical systems for a laser system according to a second embodiment of the present invention. 4 is a schematic diagram of a laser system according to a third embodiment of the present invention. 6 is a schematic diagram of a laser system according to a fourth embodiment of the present invention. 6 is a schematic diagram of a laser system according to a fifth embodiment of the present invention; 6 is a schematic diagram of a laser system according to a sixth embodiment of the present invention;

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Laser system 12 Low power master laser 14 High power slave laser oscillator 16 Rare earth doped fiber 18 Yb dope double clad fiber (DCF)
20 Electro-optic modulator (EOM)
DESCRIPTION OF SYMBOLS 22 Semi-reflective mirror 24 Photodetector 26 Double balanced mixer 28 Electric filter 30 Integration circuit assembly element 32 Optical phase modulator 34 Feedback unit 36 Resonator 38 Pumping laser 40 Pump-signal combiner 42 Mode matching optical system 44 Polarization control light Element 46 Input-output coupler 48 High reflection mirror

Claims (15)

In high power laser systems,
Master laser,
A primary slave laser oscillator having a resonator including a rare earth-doped fiber and actively injection-locked to the master laser;
With
A high power laser system wherein the resonator provides an optical power output exceeding 1 W.   2. The high power laser system according to claim 1, wherein an active optical path length in the rare earth doped fiber is longer than a passive optical path length in the primary slave laser oscillator.   The resonator of the primary slave laser oscillator comprises a phase modulator capable of modulating the optical phase within at least a portion of the rare earth doped fiber to synchronize the optical signal frequency, the phase modulator being a mode filter The high power laser system according to claim 1 or 2, wherein   4. The high power laser system according to claim 1, wherein the rare earth doped fiber is a polarization maintaining fiber or a single polarization fiber.   The high power laser system according to any one of claims 1 to 4, wherein the resonator of the primary slave laser oscillator includes a second harmonic generator.   The laser system includes a secondary resonator, the secondary resonator includes a high-order harmonic generator, and the high-order harmonic generator includes a third-order harmonic generator, a fourth-order harmonic generator, or a fifth-order harmonic generator. The high power laser system of claim 1 including a second harmonic generator.   The high power laser system according to claim 1, wherein the laser system includes a secondary resonator, and the secondary resonator includes a Raman converter crystal.   The laser system comprises a secondary resonator, the resonator of the primary slave laser oscillator and the secondary resonator share at least one common optical component, and at least one section of the secondary resonator is the primary 5. The high power laser system according to claim 1, wherein the high power laser system is outside the resonator of a slave laser oscillator.   9. The high power laser system of claim 8, wherein the common optical component is a frequency converter crystal. The laser system further comprises an electro-optic modulator connected to an electro-optic modulator driver, the resonator having a resonator length L such that f _mod > f _cav , where f _mod is the EOM ( 2. The high frequency according to claim 1, wherein f _cav is a resonator interval determined by the resonator length L, and L is longer than 0.25 m. Power laser system.   11. The high power laser system according to claim 1, wherein the resonator does not have a linear polarizer operating with the fiber.   The resonator of the primary slave laser oscillator comprises a phase modulator capable of stretching at least a portion of the rare earth doped fiber to synchronize the optical signal frequency. The high power laser system described in 1.   The high power laser system according to claim 12, wherein the phase modulator functions as a mode filter.   The high power laser system according to claim 1, further comprising a second primary slave laser oscillator, wherein the second primary slave laser oscillator is injection-locked to the second master laser.   15. The high power laser system according to claim 14, further comprising an intermediate external resonator connected in a manner operable with both the primary slave laser oscillator and the second primary slave laser oscillator.

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