JP5269764B2 - Pulsed UV and visible Raman laser system - Google Patents

Description

Explanation of related applications

  This application is filed on April 28, 2006, US Provisional Patent Application No. 60 / 79,915, entitled “Pulsed UV and Visible Raman Laser System”, and US Provisional Patent Application No. 60/810520, filed June 2, 2006, with the name “Laser Systems for Producing UV and Visible Light Output” Insist on benefits.

  The present invention relates generally to solid state lasers, and more particularly to fiber lasers that use nonlinear wavelength conversion to produce output in the ultraviolet (UV) wavelength range and / or the visible wavelength range.

  Many important for coherent light sources in the visible wavelength range (400-775 nm) and in the UV to deep UV (DUV) wavelength range (150-400 nm) (such as medicine, life sciences, material processing, photolithography and metrology) There are various uses. In general, high output power is desired and different applications require different wavelengths.

  However, in contrast to the widely available light sources developed for the near IR spectral range, the choice of light sources with shorter wavelengths (eg, visible or UV) is extremely limited. Excimer lasers are often used to generate 248 nm, 193 nm, and 157 nm UV light. However, these lasers are expensive, expensive to maintain, have relatively poor beam quality, and are not wavelength tunable.

  To convert the IR (infrared) wavelength output of a diode pumped solid state (DPSS) laser to the UV and visible range, harmonic conversion in a nonlinear crystal is commonly used. Unfortunately, only a few discrete wavelengths are available from DPSS lasers, so the output wavelengths generated by this method are also fundamental or pump wavelengths (eg, secondary, tertiary, quaternary). Limited to harmonics. Such laser outputs are, for example, 532 nm, 355 nm, and 266 nm generated by harmonic conversion of the 1064 nm Nd: YAG laser output.

  Optical parametric oscillation (OPO) can be used with a DPSS laser to further provide output wavelength variability if there is a nonlinear crystal with suitable transmission wavelength range and phase matching conditions. This is not always possible. Furthermore, since the output wavelength from the OPO is determined by the phase matching condition of the nonlinear crystal, the laser system using the OPO is generally more complex and less stable than the laser system using the harmonic conversion. Have

  Another drawback of DPSS lasers is that the average power output is limited to relatively low values (10-25 W) due to thermal problems (heat dissipation in the laser crystal). In order to achieve the high peak optical power values required for highly efficient nonlinear frequency conversion, DPSS lasers generally have a Q-switch (long 30-50 ns pulse) scheme where the pulse repetition frequency is limited to a few kHz, or the output spectral width. It is operated in a mode-locked (5-10 ps) mode, which is quite broad and therefore the coherence length of the laser output is shorter than the coherence length of a continuous wave laser or CW laser. Thus, such a DPSS laser generates a pseudo CW output where the optical pulse is long enough to keep high coherence and at the same time the repetition frequency is high enough so that the output light effectively looks CW for a particular detector. Not suitable for.

  A new class of diode-pumped lasers, rare earth doped glass fiber lasers, have attracted considerable attention recently and the first industrial applications have been found. Due to the long active medium length (several meters), the heat dissipation problem is not as great a problem with fiber lasers as DPSS lasers, and therefore the average power output is considerably higher while maintaining a perfect transverse single mode beam quality. be able to. That is, fiber lasers are perfect candidates for forming high power CW sources or pseudo CW sources in the visible and UV range by harmonic conversion. Furthermore, the fiber laser output wavelength is not limited to discrete values and can be tuned over a relatively wide range, for example 1030 to 1120 nm for Yb doped devices. However, even with this additional flexibility, not all desired output wavelengths can be achieved by harmonic generation. Although OPO can provide additional wavelengths, as mentioned above, laser systems using OPO are generally complex and have the problem of being less stable than laser systems using harmonic converters.

  Therefore, there is still a need to develop a high-power, high-efficiency, stable, pseudo CW laser source in the range of 0.15 to 0.775 μm.

  An embodiment of the present invention includes: (i) a pulsed light source that generates a pulsed light having an optical spectral range centered on the original wavelength; (ii) a nonlinear Raman conversion fiber coupled to the pulsed light source, wherein the pulsed light is Passing through a nonlinear Raman conversion fiber and converted into a first pulsed light output having an optical spectrum centered on a first output wavelength longer than the original wavelength by a multistage stimulated Raman scattering process; and (iii) a nonlinear Raman conversion fiber A first pulsed light such that the harmonic generator generates a final light output of a final output wavelength within a range of 150 to 775 nm. A Raman laser system comprising a harmonic generator for receiving the output and converting it to a higher optical frequency.

  One embodiment of the present invention is a light source that generates light, having a fiber laser or fiber amplifier operating in the visible spectral range, and a frequency converter operably coupled to the light source, A frequency converter for receiving light supplied by the light source and converting it to a higher optical frequency, such that the frequency converter generates a light output of a final light wavelength that is in the range of 150-400 nm; Is a laser system. The final output wavelength is preferably in the range of 150 to 300 nm.

  Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art, or may include the following detailed description and claims, It will be readily appreciated by practice of the invention as described herein, including the accompanying drawings.

  Both the foregoing general description and the following detailed description are intended to provide an overview or framework for presenting embodiments of the invention and for understanding the nature and features of the invention as claimed. It is natural that it has been 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.

  Sub-200 nm laser light sources are very important for metrology applications in the semiconductor industry. As the minimum dimensions of integrated circuits shrink, the wavelength of light used for photolithography becomes shorter. Therefore, the same or similar DUV light wavelength is required for inspection of masks and wafers and also for the production of optical elements. Currently used solid-state laser sources, harmonic conversion and OPO based systems generally operate at very low repetition rates, are very large, cumbersome, complex, expensive, frequent and cumbersome maintenance is necessary.

  A new class of diode-pumped lasers, rare earth doped glass fiber lasers, have attracted considerable attention recently and the first industrial applications have been found. Due to the long active medium length (several meters), the heat dissipation problem is not as great a problem with fiber lasers as DPSS lasers, and therefore the average power output is considerably higher while maintaining a perfect transverse single mode beam quality. be able to. That is, fiber lasers are perfect candidates for forming high power CW sources or pseudo CW sources in the visible and UV range by harmonic conversion. Further, the fiber laser output wavelength is not limited to a discrete value and can be made variable over a relatively wide range, for example, about 570-630 nm for an Sm-doped fiber laser or amplifier. This tunability provides tuning or adjustment of the final output wavelength supplied by the laser system.

  A cavity-free or resonance-free method and apparatus for generating coherent light is taught herein. According to some embodiments of the present invention, multistage stimulated Raman scattering (SRS) of pulse pump light, preferably in an optical fiber having normal dispersion (negative dispersion), can be obtained by multiplying an initial short wavelength into a longer wavelength. Used in the method and apparatus of the present invention to produce a next wavelength shift.

  The harmonic generator then produces a final output wavelength optical output in the range of 150 to 775 nm (eg, second, third, or fourth order) harmonic generation so that the harmonic generator generates the final Stokes light. Used to convert power output to higher optical frequencies (shorter wavelengths).

  Reference will now be made in detail to presently preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like elements. One embodiment of the laser system of the present invention is shown in FIG. 1 and is designated generally by the reference numeral 10 throughout this specification.

Referring to FIG. 1, a laser system 10 includes a pulsed light source 102 that generates a pulsed light 104 having an optical spectrum centered on the original wavelength λ _p (eg, a “parent oscillator—having a first“ parent oscillator ”pulsed light source 102 ′). Power amplifier "(MOPA). A Raman wavelength converter 106 having a nonlinear Raman conversion fiber 106 ′ is operably coupled to the pulsed light source 102. Pulse light 104 passes through the nonlinear Raman conversion fiber 106 ', (longer than the original wavelength lambda _p) by multistage stimulated Raman scattering process in the first pulse light output signal 108 having a light spectrum centered on the output wavelength lambda _{1 output} The output wavelength λ ₁ corresponds to the Nth-order Stokes light wavelength (N = 1, 2, 3 or 4) of the original wavelength λ _p with respect to the glass material of the nonlinear Raman conversion fiber 106 ′, and the optical power from the pulse light source 102 Is converted so that a significant portion (> 25%) is converted to Nth-order Stokes light in a single pass through the nonlinear Raman conversion fiber 106 '. The pulse width and peak pulse power provided by the pulsed light source 102, the Raman conversion fiber length, and the dispersion parameters of this fiber are preferably the final output wavelength in a single pass of the original power nonlinear Raman conversion fiber 106 '. It should be chosen to maximize the conversion of the next Stokes light to power output.

More specifically, a Raman wavelength shift in the optical fiber is used to generate a coherent light output of any wavelength where the glass material used to make the fiber is transparent. A double-order stimulated Raman scattering (SRS) generator is used in a transparent window of glass material used to make optical fibers, where the first wavelength i-order Stokes light acts as a pump for the generation of (i + 1) -order Stokes light. Is used as a multistage wavelength converter (from short to long) to produce a new output wavelength λ _{1 output} that would not be available with other types of laser sources .

Due to the self-phase matching of the stimulated Raman scattering process, when an optical pulse with sufficiently high peak power is projected onto the single-mode optical fiber 106 ′ and propagates through the fiber 106 ′, the pump light wavelength (ie, the original wavelength) λ _p Will be continuously converted to first, second, third or higher Stokes wavelengths. The intra-spectral or optical frequency spacing between pump light and Stokes light is due to the so-called Stokes shift (ie, the wavelength (frequency) shift with the highest Raman gain in the particular glass material from which the optical fiber 106 'is made). It is determined. Since the conversion occurs continuously, the order of Stokes light existing at the output of the Raman conversion fiber is uniquely determined by the input peak power and the fiber length. Some control can further be performed by causing wavelength dependent losses (eg, by bending the fiber) so that the conversion terminates at a given Stokes order. The Raman-converting optical fiber 106 ′ is a single-mode nonlinear fiber having uniform chromatic dispersion over the fiber length such that chromatic dispersion is normal dispersion for the original wavelength, the output wavelength, and the plurality of intermediate order Stokes light wavelengths. can do. The values of chromatic dispersion at each of the original wavelength, the output wavelength, and the plurality of intermediate order Stokes light wavelengths include requirements for minimizing nonlinear light effects that are detrimental to conversion efficiency, including spectral broadening caused by four-wave mixing. It is determined at least in part by the requirement to minimize the pulse walk-off for the two consecutive conversion order lights (wavelengths). The length of the Raman conversion optical fiber 106 ′ is such that the stimulated Raman scattering threshold is superior to the Nth-order Stokes light, but is not superior to the (N + 1) th-order Stokes light. Is preferably determined from the peak pulse power of the light source and the Raman gain, attenuation and effective area of the fiber.

  Alternatively, the Raman-converting optical fiber 106 'can be made of a series connection of nonlinear Raman fiber segments, each fiber segment specifying an input wavelength, an output wavelength and an intermediate order Stokes light wavelength passing through a particular fiber segment. The requirement for minimizing non-linear light effects that are detrimental to conversion efficiency, including spectral broadening caused by four-wave mixing, with normal chromatic dispersion for each combination of wavelengths. And the requirement to minimize the pulse walk-off for any two consecutive conversion order lights (wavelengths). Each fiber segment has a segment length, and the fiber segment length is dominated by the orders of Stokes light generated in a particular segment and the orders of Stokes light passing through that particular segment, and the final fiber The segment length is long enough to dominate the Nth-order Stokes light threshold, but not enough to dominate the (N + 1) th-order Stokes light threshold, resulting in the majority of the original power being converted to Nth-order optical power. Thus, the peak pulse power of the primary Stokes light entering the segment and the Raman gain, attenuation and effective area of the fiber segment are preferably determined in advance.

In order to receive the first pulsed optical output signal 108 of wavelength λ _{1 output} and convert it to a higher optical frequency, the harmonic generator 110 is operably coupled to the nonlinear Raman conversion fiber 106 ' The wave generator 110 generates a final pulsed light output 112 having an output wavelength λ _output within a range of 150 to 775 nm. The harmonic generator 110 can be, for example, a second-order, third-order or fourth-order harmonic generator, for example, where the harmonic generator 110 outputs the first output wavelength λ _{1 output} , λ _output = λ _This means that the wavelength is converted to the final wavelength of _{1 output} / 2, λ _{1 output} / 3, or λ _{1 output} / 4.

The Raman conversion of the pulsed light occurs in a single pass through the optical fiber 106 'and does not require any resonant cavity or wavelength selective element. However, for such free-running conversion, the Raman gain spectrum of the glass is relatively wide, and the generation of a new order of Stokes light is caused by amplification of natural Raman radiation, so that each subsequent order of Stokes light It will have a slightly broader spectrum than the order Stokes light. Further spectral broadening occurs due to the influence of four-wave mixing. For these reasons, accurate selection of the output wavelength can be difficult. Accordingly, the inventors suggest that it is preferable to provide seed light at the Raman fiber input for the first pulsed light output wavelength λ _{1 output} and (if necessary) for each intermediate order Stokes light wavelength. To do. This is shown in FIG. 1, for example showing a seed light source 112 (for a first pulsed light output) and a seed light source 114 (for one of the intermediate order Stokes lights). In principle, it is sufficient to give a few microwatts of power in each seed beam, but by giving higher power (up to 10 mW), the required Raman fiber length is shortened and the conversion efficiency is slightly It will be higher and the noise will be slightly lower. Both the pulsed light source 102 (also referred to herein as the pump) and the laser 104 that seeds the intermediate-order Stokes light are highly sensitive due to the relatively weak Raman gain (less than 1 nm is generally sufficient). Must have a narrow spectral output and can have a multiple longitudinal mode structure. The seed light source 112 for the seed light source 112 for the first output wavelength λ _{1 output} preferably provides a narrow spectral line, and more preferably provides a single frequency. Since it will be broadened by stimulated Raman scattering, the spectral width of the output of the Raman wavelength converter 106 will be approximately the same as the spectral width of the seed light source 112. That is, having a narrow spectral line will ensure that the high coherent seed light source 112 (preferably with a line width narrower than 100 pm) can achieve high conversion efficiency in the harmonic conversion stage. External cavity type, DFB type, DBR type or vertical cavity type semiconductor diode lasers are most suitable, but single frequency DFB type or single frequency DBR type fiber lasers and solid state lasers are also of the first output. It can be used as the seed light source 112.

  Since the Raman process is polarization selective, all of the pump light 102 and the seed lights 112 and 114 are polarized, and the polarization states of the respective outputs are aligned at the input of the Raman conversion fiber 106 ′. is important. The Raman conversion fiber 106 ′ is also preferably a polarization maintaining fiber; otherwise, the ellipticity of the converter output is corrected and the optical axis of the nonlinear crystal used in the harmonic generator (converter) 110 is corrected. In order to align the respective polarization states, a combination of a half-wave plate and a quarter-wave plate will be necessary. As is well known in the art, the optical birefringence properties of nonlinear crystals are commonly used to achieve phase matching or to ensure that harmonic light of different orders propagates through the crystal at the same speed.

The Raman converter 106 must be configured according to the type of Raman conversion fiber 106 'used. Our numerical calculations and preliminary experiments have shown that the silica-based fiber 106 'has a sufficiently large Raman gain to achieve high conversion efficiency in a relatively short length, as described below. . Depending on the desired peak power level and average power level, the fiber may need to be configured to have a large mode area and to avoid optical damage. The Raman gain for silica-based fibers has two adjacent peaks that are approximately 13.4 THz and 14.6 THz from the optical frequency of the pump (ie, pulsed light source 102). More preferably, the pump light wavelength λ _p , the intermediate order Stokes light wavelength and the output seed light wavelength are given so that any two in the optical frequency domain are preferably between 12.5 THz and 14.7 THz. Chosen to be exactly at the maximum Raman gain for the converted fiber.

  According to this embodiment, the pulse width provided by the light source 102 is 0.1 to 100 ns, and the pulse duty cycle is 1: 2 to 1: 1000. For optimum efficiency, it is important to consider the requirements for optical pulse width and repetition frequency. In principle, there is no upper limit to the optical pulse width. However, for pulses longer than a few nanoseconds, the maximum amount of power that can be converted by stimulated Brillouin scattering (SBS) in the fiber can be limited, ensuring that the pump light spectrum is wide enough to suppress SBS. It must be. In order to increase the nonlinear conversion efficiency in the crystal, it is desirable that the duty cycle (ratio of pulse width and repetition time, which is also the ratio of average power to peak power) is lower than 1: 100. For pulses longer than 10 ns, the repetition frequency will be limited to values below 1 MHz, which is undesirable if the target is to form a pseudo CW source. On the other hand, if the light pulse is too short, the Raman conversion efficiency will be affected by the time walk-off between pump light and Stokes light caused by fiber dispersion. Therefore, unless the Raman conversion fiber is specifically designed to match the group velocities of the pump light and Stokes light, it is preferable not to make the pulse shorter than 300 ps (0.3 ns). Therefore, the preferred range for the pulse width is 0.3 to 10 ns, and the corresponding repetition frequency range is 1 to 30 MHz. It is also preferred that the pulse shape be as close to a perfect rectangle as possible, since the presence of any pulse “wing” will cause an incomplete conversion. Another important issue is the magnitude of the spectral broadening imposed on the Raman converter output by a non-linear light effect known as self-phase modulation (SPM), which can be detrimental to the efficiency of subsequent harmonic conversion. As is well known in the art, if the light intensity changes over time, the SPM only changes the optical frequency, resulting in spectral broadening. Therefore, a nearly perfect rectangular pulse with a flat top side and the shortest possible rise and fall times is preferred to minimize the effects of SPM.

  As defined herein, the term “non-linear fiber” is an optical waveguide (including a fiber), where an optical power threshold for the generation of at least one (primary) Stokes light constitutes the optical waveguide. It refers to an optical waveguide having a sufficiently large Raman gain, a sufficiently large length, a sufficiently low attenuation, and a sufficiently small effective area for a fundamental waveguide mode that does not exceed the optical damage threshold for a certain material.

A suitable glass material that is transparent (ie low loss, ie less than 1/2 of the Raman gain given only by Raman fiber) and has sufficient Raman gain at the pump wavelength and all required orders of Stokes wavelength In the presence of, an arbitrary magnitude of wavelength shift can be achieved by a multistage stimulated Raman scattering process. Thus, in principle, any desired output wavelength, including wavelengths in the visible and UV ranges, can be generated using a pulsed laser, Raman wavelength conversion process and a suitable harmonic generator. The nonlinear Raman conversion fiber 106 'can be made of optical glass that is transparent in the mid-infrared wavelength range, and the glass can be sulfide, selenide, telluride, germanate (based on GeO ₂ ), (Al ₂ It is preferably made of a material selected from the group consisting of aluminates (based on O ₃ ) and tellurites (based on TeO ₂ ).

  The output power of the laser system 10 is limited only by optical damage to the glass used to make the Raman fiber 106 'and can in principle reach several hundred watts. The choice of glass material for making the Raman fiber is defined by the output wavelength and output power requirements. Quartz glass-based materials are preferred because of their high transparency, high optical damage threshold, and sufficiently high Raman gain to achieve high efficiency conversion at relatively short fiber lengths in the wavelength range of 800-1900 nm. Other types of glass can be used if even higher Raman gains or different wavelength ranges are desired.

If the pump light pulse from the pulse light source 102 having a high peak power is used, the conversion can be completed in the Raman fiber 106 'having a relatively short length. The theoretical maximum power conversion efficiency is defined by the ratio of the Raman fiber output 108 to the photon energy of the pump 104, and can thus reach (or even exceed) 90%. For example, for conversion to secondary Stokes light 1178nm in silica fiber pump wavelength lambda _p of 1056.3Nm, the theoretical maximum efficiency is η = 1056/1178 = 0.897.

  If necessary, in order to amplify the pulsed light 104 so that the average power and peak pulse power of the pulsed light source 102 can be increased, an initial pulsed light source 102 ′ (in this embodiment, a pulsed master oscillator) and a nonlinear Raman conversion are used. A high power optical amplifier 105 is coupled between the fibers 106 '. In this way, a cost effective pump source based on well developed fiber amplifier technology can be used for the amplifier 105. The high power optical amplifier 105 forms a parent oscillator-power amplifier (MOPA) with the initial pulse light source 102 '. The method and apparatus of the present invention is particularly suitable for pump sources including Yb-doped or Er-doped fiber optical amplifiers, but can also be used with other pump sources having other types of power amplifiers 105. More particularly, Yb-doped fiber based lasers or MOPA can provide light output in the range of 1030 to 1120 nm, Er, Tm and Nd doped silica fiber based lasers or MOPA are respectively 1503-1610 nm, 1800-2000 nm and Note that an output in the range of 890-930 nm can be provided.

In accordance with some embodiments of the present invention, the light source 102 has a tunable laser for tuning the original wavelength λ _p and can fine tune the final output wavelength λ _output by tuning the original wavelength.

  Since the lifetime of the excited state is very long (several ms), rare earth (eg Er or Yb) doped fiber amplifiers essentially amplify the average power of the incoming signal, and for very small duty cycles the average power A very large peak pulse power can be generated by the amplifier 105 having a medium output. For example, a 1 ns long pulse from the parent oscillator 102 ′ (eg, by direct modulation of a distributed feedback (DFB) type laser diode) can be amplified to a peak power of 20 kW in the multistage Er or Yb doped amplifier 105, but at a repetition rate Is 100 kHz (peak power is 10000 × average power), the average output power of the power amplifier 105 is only 2 W. Such a large peak power would allow highly efficient Raman conversion to be performed in silica-based fibers that are only a few meters long.

  The direct modulation of the semiconductor laser diode by the electric pulse generator for setting the pulse width or the connection of the diode output to the individual electro-optic intensity modulator is further amplified or not amplified (ie using the optical amplifier 105). It can be used to create the initial pulsed light source 102 for generating the pulsed light 104 (with or without). As mentioned above, to maximize conversion efficiency (minimize the effects of incomplete conversion in pulse wing) and minimize spectral broadening (due to self-phase modulation (SPM)), rectangular pulses are used. It is preferable to form. Since the Er, Yb, Tm and Nd doped amplifiers 105 have a relatively wide (several tens of nanometers) spectral gain bandwidth, the pulsed light source 102 or the entire laser system 10 (directly modulated or individually Tunable wavelength can be achieved by using a tunable master oscillator pulse source 102 '(such as an external cavity semiconductor laser) coupled to a modulator.

To illustrate the sequential nature of multi-stage Raman wavelength changes, Figure 2 shows the papers by G. Vareille, O. Audouin, and E. Desurvire (Electronics Letters, 1998). 34, No. 7, p.675) shows the result of a simplified numerical model calculation based on the combination of the power equation presented in FIG. In the model, the pulsed nature of the light source 102 is ignored (for the sake of simplicity) and instead a 1000 W continuous wave (CW) pump light power with a primary wavelength λ _p of 1056.3 nm is the primary Stokes light and the secondary Stokes light. Together with a small amount (10 ⁻⁶ W) of seed power for light, it is supplied directly to the input of the nonlinear Raman conversion fiber. FIG. 2 is a graph of the optical power versus the length of the Raman conversion fiber 106 ′. As shown in FIG. 2, the output light power of the pump wavelength λ _p is completely converted to the primary Stokes light having a wavelength of 1114 nm before the conversion step to the secondary Stokes light starts. The graph requires a fiber length of approximately 12.5 m for each conversion stage, and the total power is converted to secondary Stokes light (1178 nm output wavelength λ _{1 output} ) with 90% efficiency at a length of 25 m. It also shows that. The conversion is accomplished in a single pass of the fiber and is less affected by variations in fiber dispersion, attenuation or effective area along the length of the fiber due to the non-resonant nature and self-phase matching of stimulated Raman scattering. That is, the operation of the pulsed multistage Raman wavelength converter is advantageous in that it does not depend on dispersion, loss, and effective area variation of the nonlinear conversion fiber 106 ′.

In order to understand the present invention, it is important to discuss the requirements for chromatic dispersion of the nonlinear Raman conversion fiber 106 '. The key to the operation of the pulsed multi-stage Raman wavelength converter 106 is that at any point along the length of the nonlinear Raman conversion fiber 106 ′, at least the original wavelength λ _p and the first output wavelength λ _{1 output} , and its It has ordinary (negative) dispersion for any wavelength of intermediate Stokes light passing through the point. Ordinary (negative) dispersion is a multistage Raman transformation, such as pulse splitting caused by soliton effects, four-wave mixing, noise amplification and spectral line broadening caused by spontaneous emission due to parametric or modulation instability effects, and "parasitic" spectral line generation. Required to prevent nonlinear light effects that are detrimental to efficiency.

  Known techniques of fiber waveguide design can be used to ensure that the fiber dispersion is normal (negative) dispersion. Standard doped fibers or microstructured fibers can be used, where the cladding layer includes a number of holes that run along the fiber length.

  Determining the minimum absolute value of fiber dispersion that remains sufficient to suppress the detrimental nonlinear effects discussed above is difficult because it depends on many factors, including pump pulse rise and fall times It is. In modern optical communication systems, a dispersion of about 2-3 ps / nm / km is usually considered sufficient to suppress four-wave mixing. However, the power level used for optical transmission is typically much lower than the power level discussed herein. Thus, the present invention teaches that it is preferable if the fiber dispersion is more negative than -5 ps / nm / km. The fiber dispersion is preferably in the range of −10 to −100 ps / nm / km.

  On the other hand, if the absolute value of the nonlinear Raman conversion fiber dispersion is too large, a “dispersion walk-off” (ie, due to dispersion) with respect to the time of pulses of different orders of Stokes light, which results in reduced conversion efficiency and pulse shape distortion. In order to avoid separation of light pulses of different wavelengths). The tolerance depends on the pulse width and the fiber length required to complete one conversion step (12.5 m in the example of FIG. 2), but in general it is preferred that the walk-off amount is less than 10% of the pulse width. .

The inventors conducted a preliminary analysis of the possibility of obtaining both high conversion efficiency and high average output power in the Raman converter 106 described above. An optical fiber 106 ′ having a 22 μm-diameter core was prepared, in which the pure silica clad layer and the GeO ₂ were slightly doped so that the difference in refractive index, that is, the refractive index delta between the core and the clad layer was 0.0007. The numerical model of the Raman converter 106 takes into account the time dependence of the light intensity, non-linear light effects such as self-phase modulation and cross-phase modulation and four-wave mixing, such as effective mode area, chromatic dispersion and Raman gain. The measured parameter values of the fiber are incorporated. FIGS. 3A and 3B show model calculation results for a laser system 10 comprising a pulsed light source 102 that provides a 400 ps long optical pulse with a rise time and fall time of 100 ps and a peak power of 25 kW. The inventors assumed that the seed light sources 112, 114, etc. gave 1.0 mW CW power to all of the Stokes light up to the fourth order. FIG. 3A illustrates the dependence of the secondary power of Stokes light on average position (averaged over the duty cycle) in the fiber 106 '. As is apparent from FIG. 3A, an average power conversion efficiency to the secondary Stokes light wavelength of about 80% can be achieved for the 26 m long fiber 106 ′. For conversion to third-order Stokes light, the model predicts an average power conversion efficiency of up to 69% for a fiber length of 36 m (not shown). FIG. 3B shows the secondary Stokes optical signal pulse shape at various locations in the fiber. As can be seen in FIG. 3B, the peak power conversion efficiency is less than the input pulse due to incomplete conversion in the pulse wing, and is closer to a rectangle. It can be concluded that it is even higher.

Experiments were also conducted to pump the Raman conversion fiber 106 ′ with a 1064 nm 400 ps long pulse from an Nd: YAG laser. FIG. 4 shows the output pulse signal spectrum at the end of the nonlinear multi-man conversion fiber 106 ′ for a fiber length of 38 m. As explained above, the choice of fiber length for a given input pulse peak power uniquely determines which order of Stokes light will terminate the conversion (secondary Stokes light in the case of FIG. 4). It is determined. FIG. 4 shows the relative power corresponding to the original wavelength λ _p , the primary Stokes light wavelength, and the secondary Stokes light wavelength at the output of the Raman conversion fiber. As can be seen from FIG. 4, the conversion efficiency from the 1064 nm light supplied by the Nd: YAG laser to the secondary Stokes light is approximately 60% (ie, ˜4 / 7), but the primary Stokes light and the secondary Stokes light. No seed light is provided, and the input pulse shape is double exponential, very different from the optimal rectangle, resulting in a significant amount of pump light (λ _p ) and first order Stokes Light is still present at the output. The measured value of the secondary Stokes light pulse energy is about 10 μJ, which means that the output pulse peak power was about 25 kW. For a duty cycle of 1: 100, this would correspond to an average output power of 250 W from the Raman converter 106. Based on the presented numerical calculation results and experimental results, the pulsed light signal can be converted into secondary and tertiary Stokes light having an average output power of several hundred watts and a peak output power of several tens of kW with a conversion efficiency exceeding 50%. It can be concluded that Raman wavelength conversion is completely possible. Stokes light having a higher order than the third order can also be generated, but the efficiency gradually decreases.

As already mentioned above, the Yb-doped fiber laser system can directly handle an output wavelength range of approximately 1030 to 1120 nm. This range can be effectively extended to 1030 to 1340 nm by Raman wavelength conversion (by the Raman converter 106) up to third-order Stokes light. Therefore, the range of 515 to 770 nm by second harmonic generation, the range of 343.3 to 446.7 nm by third harmonic generation, the range of 257.5 to 335 nm by fourth harmonic generation, and the like, the same output wavelength λ _Output can be generated. Er-Yb co-doped for the 1530-1570 nm range, Nd-doped (operating with 3 level transitions) for the 890-930 nm range, and Tm-doped for the 1800-2000 nm range, Other types of fiber laser systems can also be used to pump the Raman wavelength converter 106, such as systems with silica-based fibers. According to an embodiment of the present invention, the only requirement is the availability of a narrow spectral line seed source suitable for the output wavelength of the corresponding Raman converter 106 (seed light supply to the intermediate Stokes light wavelength is not critical) Will improve conversion efficiency). A further output wavelength λ _output can be generated by sum frequency mixing in a suitable nonlinear crystal of the output of two different fiber lasers, possibly followed by a Raman wavelength converter and a harmonic generator. In accordance with the teachings of the present invention, any desired output wavelength λ _{output in} the range of 150-775 nm, as described below, one or two pulse fiber lasers, Raman converters, harmonic generators, and It can be generated by a suitable combination of sum frequency mixing stages.

  The inventors present below examples of laser systems 10 that generate a number of specific output wavelengths, focusing on specific applications. In the following exemplary embodiment, the borate salt used in the harmonic generator 110 is known to have the highest optical damage threshold, and thus can generate higher power through harmonic conversion. The inventors choose to design the harmonic generator 110 limited to crystals (LBO, BBO, CLBO). Those skilled in the art will recognize that these examples are only a few of the almost infinite number of possibilities opened by the use of Raman wavelength conversion techniques, and that other nonlinear crystals can also be used. Let's go.

Laser system for generating λ = 589.16 nm sodium D2 line Sodium beacons generated by atomic sodium excitation in the upper atmosphere can be used as a “guide star” for adaptive optics, and therefore advanced laser-based defense systems It has attracted a great deal of attention in its use. For this purpose, a high power CW source or pseudo CW source tuned to a sodium D2 absorption line with a wavelength of 589.1 nm is required.

An exemplary laser system 10 for generating an output of 589.1 nm is shown in FIG. In the embodiment shown in FIG. 5, the pulsed light source 102 is a Yb-doped fiber system (parent oscillator-power amplifier (MOPA)). This MOPA 102 supplies the Raman converter 106 with pulsed light with λ _p = 1056.3 nm. The 1056.3 nm pulsed light 104 is converted by the Raman converter 106 into a first output signal 108 having an optical spectrum centered on the _{first output} wavelength λ _{1 output} = 1178.32 nm through a multistage stimulated Raman scattering process. Is done. Since λ _{1 output} = 11178.32 nm corresponds to the secondary Stokes light wavelength 1056.3 nm for quartz glass, a silica-based conversion fiber 106 ′ with low loss and high optical damage threshold can be used for the Raman converter 106. Is an advantage. The harmonic generator 110 includes a non-linear crystal LBO (lithium triborate (LiB ₃ O ₅ )) crystal 110A and, if necessary, the first light from the Raman converter 106, all indicated by reference numeral 110B. A second harmonic generator having a lens that focuses the output 108 and a lens that collimates the final light output from the LBO crystal 110A. The second harmonic generator 110 of the present embodiment changes the _{first output} wavelength λ _{1 output} = 1178.32 nm to the final output wavelength of λ _output = λ _{1 output} / 2 = 1178.32 / 2 = 589.26 nm. Convert.

In order to design the laser system 10 to operate at the desired output wavelength λ _output , the problem can be processed retrogradely. That is, it is known from the data available for the nonlinear crystal that λ _output = 589.16 nm can be generated in the LBO crystal 110A as a second harmonic of 1178.32 nm. Therefore, if λ _{1 output} = 1178.32 nm can be used, λ _output = 589.16 nm can be obtained. From the data available on the Raman gain of silica-based optical fiber, it is known that 1178.32 nm is the second-order Stokes shift wavelength from 1056.3 nm, which falls within the wavelength range that can be handled by Yb-doped fiber laser systems. It is done. Thus, a laser system with an output of 589.16 nm according to the present invention can be designed starting with a Yb-doped MOPA with λ _p = 1056.3 nm. Note that if the required final wavelength λ _output is slightly different from 589.16 nm (eg, within the range of 576-622 nm), the Yb-doped MOPA can be tuned to provide the appropriate wavelength λ _p . More specifically, the Yb-doped MOPA can be tuned to provide an output of 1030 to 1120 nm, and this wavelength range can be shifted in the silica fiber to the second-order Stoke light wavelength 1152-1244 nm, which is This corresponds to a λ _output of 576 to 622 nm.

  As described above with reference to FIG. 1, in the exemplary embodiment shown in FIG. 5, seed light for the Raman converter output wavelength, ie, 1178.32 nm, must be provided to the input of the Raman conversion fiber 106 ′. The corresponding seed light source 112 can be an external cavity diode laser or a longitudinal cavity laser, or a solid state laser, preferably with a single narrow spectral line output. If necessary, the second seed light source 114 also provides a second seed light having a wavelength of the primary Stokes light of 1114 nm in order to increase the conversion efficiency. Since 1114 nm is in the wavelength range obtained with a Yb-doped laser, the second seed light source 114 can be a Yb-doped fiber laser with a fiber Bragg grating reflector, but of course other diode lasers, fiber lasers or solids A laser can also be used.

  An exemplary configuration of the MOPA (light source 102) of FIG. 5 is shown in FIG. The initial light source 102 ′ (parent oscillator) has a laser diode 102A ′ and an electro-optic modulator (EOM) 102B ′ for generating pulsed light 104 supplied to the optical amplifier 105 (power amplifier). This modulator is used to form an optical pulse and set the pulse width. The laser diode 102A ′ of this embodiment is a fiber Bragg grating (FBG) stabilized CW diode laser that emits 100 mW at 1056.3 nm, and the EOM 102B ′ forms a 2 ns long pulse at a repetition rate of 2 MHz (duty cycle 1: 250). ) Is a lithium niobate intensity modulator. If the insertion loss by the EOM 102B ′ is 6 dB, the average optical power at the output of the EOM 102B ′ is reduced to 0.1 mW. The power amplifier portion of the MOPA has four stages of fiber amplifiers 105A to 105D, Yb-doped single mode polarization maintaining fiber is used for the two amplification stages 105A and 105B, and Yb is used for the two amplification stages 105C and 105D. Doped polarization maintaining double clad fiber is used. The Yb-doped fiber is pumped with 976 nm pump light for amplification. The first amplifier stage 105A provides an average power output of 30 mW, the second amplification stage 105B amplifies this to 400 mW, the third amplification stage 105C further amplifies to 5 W, and the final amplification stage 105D has a repetition rate. Provides an average optical power output of 140 W and a peak optical output power of 35 kW in a 2 ns pulse at 2 MHz. If the modulator 102B ′ has an electrical bandwidth greater than 5 GHz, it should be able to generate a substantially rectangular pulse with rise and fall times shorter than 100 ps. A narrow-band optical filter 105E is inserted after each of the amplification stages 105A to 105C except for the final amplification stage 105D. The two filters following the single mode fiber stage 105A, 105B are constructed using an optical circulator and an FBG, and the final filter of the amplification stage 105C is a thin film device in a free space optical isolator, thus simultaneously separating the amplification stages from each other. And to reduce the accumulation of amplified spontaneous emission with a wide spectral bandwidth. Those skilled in the art will recognize that many variations on this exemplary configuration are possible with different fibers, different numbers of amplification stages, different powers, different pulse widths and different repetition rates. Another type of CW light source with an external modulator or a directly modulated diode laser can be used as the initial light source for MOPA.

The MOPA output mixed with the Raman seed light from the seed light sources 112 and 117 is projected onto the Raman conversion fiber 106 ′ as shown in FIG. The wavelength conversion proceeds to 1178.32 nm output secondary Stokes light via 1114 nm primary Stokes light. The frequency shift between the pump light and the first Stokes light and the frequency shift between the first Stokes light and the second Stokes light are both 14.69 THz corresponding to one of the Raman gain peaks in the quartz glass. To be elected. Based on the results of the model calculations and preliminary experiments described above, it is expected that at least 50% of the MOPA output power, ie 70 W, will be converted to a single frequency output of 1178.32 nm. As described above, the first output signal 108 is then provided to the harmonic generator 110, which in this embodiment has an LBO crystal 110A (see FIG. 5). The LBO crystal 110A is a second harmonic generator. LBO crystal 110A receives a _{first output} wavelength λ _{1 output} of 1178.32 nm from Raman converter 106 and provides the desired wavelength λ _output of 589.16 nm. More specifically, for the second harmonic of 1177.82 nm, the LBO crystal temperature is adjusted to 313 K (θ = 90 °) to achieve non-critical phase matching with an effective nonlinear coefficient value of 0.84 pm / V. The so-called non-critical phase matching in this allows maximum angular and spectral acceptance for second harmonic generation and is characterized by zero or nearly zero birefringence walk-off of the pump and second harmonic light beams. This is the most preferred type of phase matching because it allows long crystals to be used to achieve high conversion efficiency at moderate peak power levels).

Table I below gives an overview of the parameters of the crystal (LBO 110A) used in the laser system 10 of Example 1.

In Table I, as in all of the following examples, the type of nonlinear crystal used is shown in the first row, and the nonlinear process performed by the crystal is shown in the second row. The output wavelength and the two input wavelengths (if the nonlinear process is second harmonic generation, the two input wavelengths are the same) are shown in the third to fifth rows. The sixth row gives the crystal temperature, and the seventh to eighth rows give the propagation direction angle with respect to the crystal optical axis necessary for phase matching (LBO is a so-called biaxial crystal). Line 9 presents the effective nonlinear coefficient (a measure of how much conversion efficiency can be for a given input power and crystal length), and line 10 is the input and output beams produced by the birefringence of the crystal. Angle walk-off (a slight angle shift between the input light and the harmonic light in the crystal).

  Noncritical phase matching (NCPM) at θ = 90 ° (φ = 0 ° for biaxial crystals) is acceptable without causing a significant decrease in conversion efficiency and angle and spectral maximum acceptance for second harmonic generation. With a birefringence walk-off of the pump light beam and second harmonic light beam being zero or nearly zero, which results in high conversion efficiency at moderate peak power levels This is the most preferred type of phase matching because it makes it possible to use long crystals to achieve

  The calculation of the phase matching angle and temperature for this and subsequent examples was performed using SNLO, a free software package by Sandia National Lab., Along with the calculation of the effective nonlinear coefficient of borate crystals. The second harmonic generation efficiency can reach at least 50%, and an output of 35 W is considered to be obtained at a wavelength of 589.16 nm. By using higher MOPA output power and / or higher Raman conversion efficiency, it is possible to construct a laser system that gives an average output power greater than 50 W in the sodium D2 line.

Laser system I for generating an output of λ = 198.7 nm
Sub-200 nm laser light sources are very important for metrology applications in the semiconductor industry. As the minimum dimensions of integrated circuits shrink, the wavelength of light used for lithography becomes shorter. Therefore, the same or similar DUV light wavelength is required for inspection of masks and wafers and also for the production of optical elements. Currently used solid-state laser sources, harmonic conversion and OPO based systems generally operate at very low repetition rates, are very large, cumbersome, complex, expensive, frequent and cumbersome maintenance is necessary.

FIG. 7 shows an example of a 198.7 nm laser system 10 according to one embodiment of the present invention. As can be seen, the 1067.6 nm Yb-doped fiber MOPA 102 pumps a Raman wavelength converter 106 that produces a narrow linewidth output of 1192.2 nm secondary Stokes light wavelength λ _{1 output} according to the design principles disclosed above. To do. In this case, if the primary Stokes light wavelength is 1126.5 nm, the Stokes frequency shift is 14.67 THz, which corresponds to one of the Raman gain peaks in quartz glass. The first output signal 108 from the Raman wavelength converter 106 then has, in this embodiment, two LBO crystals 110A, 110B and one BBO (beta-barium borate (β-BaB ₂ O ₄ )) crystal 110C. It is supplied to the harmonic generator 110. The three nonlinear crystals, LBO110A, LBO110B and BBO110C, are (i) second harmonic generation (SHG) by LBO110A generating a wavelength of 596.1 nm, and (ii) (LBO110B generates a wavelength of 298.05 nm) It is used to generate sixth harmonic light with a wavelength of 1192.2 nm by one SHG and (iii) sum frequency mixing (SFM) of 596.1 nm and 298.05 nm by BBO110C generating 198.7 nm output. More specifically, LBO crystals 110A and 110B are second harmonic generators (SHG). The LBO crystal 110A receives a _{first output} wavelength λ _{1 output} of 1192.2 nm from the Raman converter 106 and supplies an output of 596.1 nm to the second LBO crystal 110B. Any residual light with a wavelength of 1192.2 nm is filtered out by the filter (dichroic mirror) 110D and removed from the system. The LBO crystal 110B receives 596.1 nm light and converts a part (1 to 90%, preferably 50%) of it to 298.05 nm light. This 298.05 nm light exiting the LBO crystal 110B then generates light of the desired wavelength λ _output = 198.7 nm by sum frequency mixing (SFM) along with residual (99% -10%) 596.1 nm light. To the BBO crystal 110C. As predicted by the SNLO software, the optimum temperature for the first LBO crystal 110A is 304 Kelvin. At this temperature, the crystal has an effective nonlinear coefficient of d _effective = 0.84 pm / V and an essentially zero birefringence walk-off for second harmonic generation of 1192.2 nm (with respect to the optical axis of the crystal). Operating with non-critical phase matching (for light propagating at angles of θ = 90 ° and ψ = 0 °). The second LBO crystal 110B cannot be non-critical phase matched. For light propagating at θ = 90 ° and ψ = 64.6 ° with respect to the optical axis, the phase matching temperature for second harmonic generation of 596.1 nm is 453 K (180 ° C.), and the effective nonlinear coefficient is d _Effective = 0.4 pm / V and birefringence walk-off is 14 milliradians (mrad). The BBO crystal 110C is a uniaxial crystal. For light propagating at θ = 81.3 ° with respect to the optical axis, the phase matching crystal temperature for a non-linear process of 596.1 nm and 298.05 nm sum frequency mixing is 453 K, and the effective non-linear coefficient is d _effective = 0.59 pm / V and the birefringence walk-off is 29.2 mrad. Table II gives a summary of the crystal parameters used in the laser system 10 of Example 2.

  In FIG. 7, (i) one of the lenses associated with each nonlinear crystal, one for converging the beam and maximizing the conversion efficiency for the given crystal, the other being the output Two lenses L1 and L2, (ii) a dichroic mirror M that removes 1192.5 nm light that is not used, (iii) a custom wave plate WP, and (iv) harmonic separation optics, which are lenses for recollimating light An element set 110E is also shown. The optimal waist diameter for beam focusing depends on the peak power of the light being converted, the crystal length and optical damage threshold, the amount of birefringence walk-off, and other parameters, either from model calculations or experimentally. Can be determined. Waveplate WP is custom designed to rotate either (but only one) of these polarizations so that the polarizations of 596.1 nm and 298.05 nm light are the same at the input of the BBO crystal Is done. Harmonic separation optics set 110E is used to separate 198.7 nm light output from both 596.1 nm light and 298.05 nm light, and typically uses one or several prisms made of dispersion glass. Have. For simplicity, these auxiliary optical elements are not shown in the optical block diagrams given for the following examples. One skilled in the art will be able to determine when and where such an element should be used.

  The advantage of the laser system 10 of FIG. 7 is that a minimal number (only three) of nonlinear crystals are used to generate a sub-200 nm wavelength output. However, considerable birefringence walk-off can occur in the BBO crystal 110C. A large walk-off does not allow for tight focusing of the laser beam, and therefore a lower crystal or larger beam diameter (lower optical power density) must be used, resulting in lower conversion efficiency. The effect of the walk-off can be mitigated by using multiple crystals of the same type rotated 180 °, but the useful life of the device will be shortened because more surfaces will be exposed to high optical power. An extra exposed crystal surface can be eliminated by seamlessly joining the crystals rotated 180 ° using diffusion bonding or non-adhesive bonding. From numerical model evaluation, the total conversion efficiency, including Raman shift, is operating at a duty cycle of 1: 250 (eg, 2 ns pulse at 2 MHz repetition rate or 0.5 ns pulse at 8 MHz repetition rate), average It can reach 0.2% for a MOPA with a power of 100 W and a peak power of 25 kW. As a result, an average power output of 200 mW can be obtained at 198.7 nm.

Laser system II for generating an output of λ = 198.7 nm
Another embodiment of a laser system 10 for generating 198.7 nm light output is shown in FIG. In this embodiment, the pulse light source 102 is 1041 nm Yb-doped MOPA. This light source 102 generates an output wavelength λ _{1 output} of 1228.2 nm tertiary Stokes light via intermediate 1096.7 nm (first order) Stokes light and 1158.7 nm (second order) Stokes light (14.63 THz step). The wavelength converter 106 is pumped. The harmonic generator 110 has four conversion stages: three LBO crystals 110A, 110B, 110C and one CLBO (lithium cesium borate (CsLiB ₆ O ₁₀ )) crystal 110D. The two LBO crystals 110A and 110B are used to sequentially generate the second harmonic (614.1 nm) and the fourth harmonic (307.05 nm) of the Raman converter output by frequency multiplication or SHG. The third LBO crystal 110C causes the residual (10 to 90%, preferably 50%) 1228.2 nm light in the output of the first LBO crystal 110A to be output from the second LBO crystal 110B by SFM (sum frequency mixing). Produces a light output of 245.64 nm (the fifth harmonic of 1228.2 nm). A second high power amplification stage 102B is added to the MOPA for generation of a second output of 1041 nm (the reason for having two separate high power stages is that the peak power of each high power stage is reduced by nonlinear distortion to the amplifier fiber. This 1041 nm light is mixed with the 245.64 nm light in the CLBO crystal 110D, and the SFM finally produces an output wavelength λ _{output of} 198.7 nm. Table III gives a summary of the parameters of the crystals used in the laser system 10 of Example 3.

In the design of the laser system 10 according to this embodiment of the present invention, the “color” (ie) different wavelengths at the inputs of the third and fourth conversion stages (SFM), which in this embodiment correspond to the LBO 110C and CLBO 110D. Special care must be taken to ensure that the light pulses are aligned with respect to time. This can be achieved, for example, by carefully aligning the optical path lengths by introducing auxiliary mirrors or parallel plane glass blocks. In addition to 198.7 nm light output, residual 614.1 nm light, 307.05 nm light and 245.64 nm light can be provided at the laser system output if required, or wavelength selective. It can be filtered out, such as a (eg dichroic) mirror or harmonic separator. One skilled in the art will recognize that in this and other embodiments, different nonlinear crystals can be used to perform the necessary conversions, even for the exact same wavelength combination. For example, in the second conversion stage (fourth harmonic generation), an ADA (ammonium dihydrogen arsenate (NH ₄ H ₂ AsO ₄ )) crystal is used instead of LBO110B, and the third conversion stage (fifth harmonic by SFM) is used. Generation) using ADP (ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ )) crystals instead of LBO110C, both non-critical phase matching by temperature adjustment to the corresponding nonlinear process (ie, Θ = 90 ° and ψ = 0 ° with respect to the biaxial crystal. The laser system 10 of FIG. 8 is more complex than the laser system of FIG. The laser system 10 of FIG. 8 uses four crystals instead of three crystals, and the Raman converter supplies third and second order Stokes light. However, the laser system 10 of FIG. 8 has one very important advantage. The last crystal (CLBO) is even closer to the non-critical phase matching condition, so the birefringence walk-off can be almost ignored. Furthermore, high peak IR (1041 nm) optical power is supplied directly from the Yb-doped MOPA. As a result, the conversion efficiency with respect to the UV light power can be close to 80%, and therefore, to achieve the same DUV (deep UV) light power output, the UV light power entering the CLBO crystal is minimized, thus CLBO. It will be necessary to minimize photodamage to the crystal. It should be noted that the optical power values shown in FIG. 8 as well as the temperature, phase matching angle, effective nonlinear coefficient, and birefringence walk-off value are only given as guidelines. Other configurations and operating temperatures can also be used. Based on the predictions of SNLO software, two branches of Yb-doped MOPA, each delivering an average power of 100W and 32W, can generate a 2W output at 198.7 nm, which increases overall conversion efficiency. In terms of conversion, it becomes 1.5%, which is at least 7 times higher than the overall conversion efficiency of the laser system 10 of FIG.

Laser system III for generating an output of λ = 198.7 nm
The laser system 10 of FIG. 9 is the same as the laser system of FIG. The laser system 10 of FIG. 9 includes the same pulsed light source 102 and a Raman converter 106 that supplies the first crystal 110A and the last crystal 110D with a light output having a wavelength λ _{1 output} of 1228.2 nm. However, the 1228.2 nm third harmonic (409.4 nm) light output of the Raman converter is generated by the SFM that mixes the second harmonic (614.1 nm) light and the original 1228.2 nm light in the LBO crystal 110B. .5 nm fifth harmonic (245.64 nm) light is here generated by the SFM that mixes second harmonic (614.1 nm) light and third harmonic (409.4 nm) light in CLBO crystal 110C. . The crystals 110B and 110C in the second and third conversion stages have higher effective nonlinear coefficients and higher power is available at the input to the second conversion stage (ie, to the LBO crystal 110B) (each figure is a figure). 9), the overall conversion efficiency (predicted by the numerical model) for the laser system 10 of this embodiment is even higher (over 2.1%) than the overall conversion efficiency of the laser system of FIG. At the output of the fourth conversion stage (CLBO crystal 110D) of the vessel 110, at least 3W of 198.7 nm DUV light should be obtained. Table IV gives a summary of the parameters of the crystals used in the laser system 10 of Example 4.

Laser system I for generating an output of λ = 193 nm
One embodiment of a 193 nm laser system 10 is shown in FIG. The laser system 10 of the present embodiment includes a pulse light source 102 having one parent oscillator and preamplifier and two high power optical amplifiers. More specifically, the output of the 1049.5 nm Yb doped preamplifier is split and amplified individually by two high power amplifiers. The first amplifier 105 ₁ pumps a Raman converter 106 ₁ that generates a 1169 nm secondary Stokes optical signal, and the second amplifier 105 ₂ includes another Raman converter 106 ₂ that generates a 1106 nm primary Stokes optical signal. Pump. In this case, the configuration can be modified in several ways, for example, a single Raman converter can be configured to generate an optical output at the lower power of the first and second Stokes wavelengths simultaneously. it can. In the configuration shown, it is possible to use first the same narrow linewidth 1106nm light source to the seed light supplied to the final output wavelength for the relative Raman converter 106 ₁ secondary and intermediate following Stokes light. Second harmonic (584.5 nm) light of 1169 nm light is generated by frequency multiplication (SHG) in the first LBO crystal 110A, and third harmonic (389.7 nm) light is generated by SFM in the second LBO crystal 110B. Is done. The second and third harmonic lights are mixed by the SFM in the CLBO crystal 110C to generate a 233.8 nm fifth harmonic light. Finally, 233.8Nm light is mixed with another CLBO crystal 110D at the second output of the Raman converter 106 ₂ (1106nm) Light, 193 nm light output is generated.

Table V gives a summary of the parameters of the crystals used in the laser system 10 of Example 5.

Due to the fact that three of the four nonlinear crystals 110A, 110B, 110C, 110D operate in non-critical phase matching and all four crystals exhibit a relatively high effective nonlinear coefficient for the corresponding transformation. From the 200 W IR power available from the two high power Yb-doped amplifiers 105 ₁ , 105 ₂ , where this configuration approach can achieve the highest conversion efficiency (about 2.0%) of the previous four embodiments. Starting out, we expect to get an average power output of 4W at 193nm.

Laser system II for generating an output of λ = 193 nm
Another embodiment of the 193 nm laser system 10 is shown in FIG. In the laser system 10 of the present embodiment, the Yb-doped fiber MOPA 102 operating at λ _p = 1041 nm supplies pulsed light of this wavelength to the Raman converter 106. Next, the Raman converter 106 converts the light having the wavelength λ _p into the second-order Stokes light having the λ _{1 output} = 1158 nm. The harmonic generator 110 includes four nonlinear crystals—LBO 110A, LBO 110B, BBO 110C, and CLBO 110D—that generate second, third, fifth, and sixth harmonic light of wavelength 1158 nm by SHG and SFM, respectively. The wavelength of the final output sixth-order harmonic light is λ _output = 193 nm. The calculated values of the non-linear transformation parameters are shown in Table VI. As can be seen from this table, the operation of the BBO crystal 110C is accompanied by a considerable birefringence walk-off, so depending on the power level used, either a short crystal or (the major axis of the ellipse is in the walk-off direction). It may be necessary to use elliptical beam focusing (along) or any of these. Table VI gives a summary of the parameters of the crystals used in the laser system 10 of Example 6.

Laser system for generating an output of λ = 195.1 nm In this embodiment shown in FIG. 12, two pulsed fiber-based MOPA sources—Nd-doped MOPA 102 operating at 913 nm (so-called three-level transition for Nd ions) Yb-doped MOPA102 ₂ − operating at ₁ and 1081 nm is used. Nd-doped MOPA 102 ₁ pumps a Raman converter 106 that generates a first output of 952 nm (first-order Stokes light wavelength for 913 nm pump light in a silica fiber). The harmonic generator 110 has three nonlinear crystals. The first two crystals 110A and 110B are both LBOs, which generate second order (476 nm) and fourth order (238 nm) harmonic light of the 952 nm output of the Raman converter, respectively. The third crystal 110C is CLBO, which generates a final output light of 195.1 nm by performing sum frequency mixing of 238 nm light and 1081 nm light. Nonlinear transformation parameter calculations are presented in Table VII. This exemplary embodiment uses an Nd-doped fiber MOPA system operating at a three level transition, which is more difficult to achieve than the normal 1064 nm four level transition operation, but using only three nonlinear crystals, each of which is all There are significant advantages in operating at or near non-critical phase matching (NCPM) conditions.

  Note that in the above example, the embodiment of the laser system 10 does not use OPO and therefore produces a stable output at the desired output wavelength. Further, although the output from the OPO is determined by the phase matching conditions, the laser system 10 disclosed above uses a Raman wavelength converter 106, so that the laser system 10 disclosed above converts the light into a harmonic generator / converter. There is no need to perform phase matching before feeding to 110.

  In accordance with another embodiment of the present invention, a frequency converter that converts an optical frequency to a higher frequency so that the frequency converter generates an optical output at a final output wavelength that is in the range of 150-400 nm. In order to provide light, a visible light source is used in the method and apparatus of the present invention. The visible light source can be a fiber laser or an optical amplifier (laser amplifier), both referred to herein as a laser source or laser.

  13A and 13B, the laser system 10 of the present embodiment includes a light source 102 having a visible light fiber amplifier or a visible light fiber laser. The light source 102 may be, for example, a “parent oscillator-power amplifier” (MOPA) having an initial “parent oscillator” pulsed light source 202 ′ and an amplifier 206A. The laser system 10 can be a CW system (FIG. 13B), or an optical modulator 204A driven by an electrical pulse generator 204B that provides pulse modulation of seed light entering the amplifier 206A, as needed. Can be used (FIG. 13A).

In the embodiment of FIGS. 13A and 13B, visible optical fiber amplifier 206A has Sm-doped silica-based fiber 206A ′. The high power optical amplifier 206A amplifies the light (pulsed light if necessary) 204 from the visible seed light source 212. The optical converter 208 receives the optical output 208A with a wavelength of λ _1A and converts it to a higher optical frequency so that the frequency converter 210 generates a final pulsed optical output with a wavelength λ _output in the range of 150-400 nm. In order to do so, it is operably coupled to a light source 202 (which is a pulsed light source in the embodiment of FIG. 13A and a CW light source in the embodiment of FIG. 13B). The light source 202 preferably supplies optical power exceeding 10 W, and more preferably supplies optical power exceeding 20 W. The frequency converter 210 can be, for example, a second-order, third-order, or fourth-order harmonic generator, such that the frequency converter 210 produces a first output wavelength λ _{1A output} , λ _output = λ _This means conversion to a final wavelength of _{1A output} / 2, λ _{1A output} / 3, or λ _{1A output} / 4, or the like.

If the laser system 10 is a pulsed laser system, the fiber 206A ′ is operably coupled to a pulsed light source 202 ′ having a modulator 204A and an electrical pulse generator 204B. The high power optical amplifier 206A amplifies the pulsed light 204 from the seed light source 212 so that the average power and peak pulse power of the pulsed light source 202 ′ can be increased. The high power optical amplifier 206A, together with the initial pulse light source 202 ′, forms a parent oscillator-power amplifier (MOPA). The modulated pulsed light from the 594 nm seed light source 212 enters the amplifier 206A, which generates an amplified pulsed light output signal 208A having an optical spectrum centered at the output wavelength λ _{1A output} . According to the embodiment of the laser system of FIG. 13A, the pulse width provided by the light source 202 is 0.01-100 ns, the duty cycle is 1: 2-1: 1000000, for example, the pulse width is 0.1. ˜80 ns, duty cycle is 1: 2 to 10000. To optimize efficiency, it is important to consider the requirements on the width and repetition rate of the light pulse. In principle, there is no upper limit to the optical pulse width. However, for pulses longer than a few nanoseconds, stimulated Brillouin scattering (SBS) in the amplifier (ie, laser) fiber can limit the maximum amount of power that can be converted, so that the pump light spectrum is sufficient to suppress SBS. It must be guaranteed that it is wide. In order to increase the nonlinear conversion efficiency in the crystal, it is desirable that the duty cycle (which is the ratio of the pulse width to the repetition time and the ratio of the average power to the peak power) is smaller than 1: 100. As a result, 10 ns For longer pulses, the repetition frequency will be limited to values below 1 MHz, which is undesirable if the goal is to create a pseudo CW source.

  Since the lifetime of the excited state is very long (several ms), rare earth (eg Sm) doped fiber amplifiers essentially amplify the average power of the incoming signal, and for very small duty cycles, the average power output is Even the amplifier 206A, which is only moderate, can generate a very large peak pulse power. For example, a 1 ns long pulse from a parent oscillator 202 ′ (for example, a direct modulation laser diode) can be used if the repetition rate is 100 kHz (peak power is 10,000) even if the average output of the multistage Sm-doped (fiber) amplifier 206A is only 2 W. × average power) and can be amplified to a peak power of 20 kW by the power amplifier 206A.

  Direct modulation by semiconductor laser diode electrical pulse generator or diode output for setting pulse width, connection to individual electro-optic intensity modulator as described above, initial pulse light source for pulsed light generation 202 ′ can be used to create. As described above, rectangular pulses are preferably formed to maximize conversion efficiency (minimize the effect of incomplete conversion in pulse wing) and minimize spectral broadening (by self-phase modulation (SPM)). .

Since the Sm, Pr or Eu doped fiber amplifier 206A has a relatively wide spectral gain bandwidth (several tens of nanometers), the light source 202 and, to some extent, the entire laser system 10 may be tunable or tunable. it can. That is, according to some embodiments of the present invention, the light source 202 has a tunable laser for tuning the original wavelength, and tuning of the original wavelength provides a fine or fine adjustment of the final output wavelength λ _output. .

As already mentioned above, the Sm-doped fiber laser system can directly handle an output wavelength range of approximately 575-625 nm. Others, such as laser systems, including Dy-doped silica-based fiber lasers (pumped with 425 nm ± 25 nm pump light) for outputs in the range of 550-600 nm, as well as Pr-doped and Eu-doped fiber lasers A type of fiber laser system can also be used. A frequency converter 210 using a second, third, or fourth harmonic generator can be used to provide the desired output wavelength. For example, the output from the laser 106A and the first nonlinear crystal 210A can be sum frequency mixed in an appropriate nonlinear crystal to generate an output wavelength λ _output , or the wavelength of the light generated by the crystal 210A can be changed. The crystal 210B can be halved by SHG.

  In an exemplary embodiment, the frequency conversion is limited to the non-linear crystal used is the borate (LBO, BBO, CLBO), which is known to have the highest photodamage threshold and therefore can generate higher power through harmonic conversion. The inventors have chosen to design the vessel 210. These laser systems 10 are advantageous in that they achieve relatively high conversion efficiencies while at the same time avoiding or minimizing crystal damage that can be caused by short wavelength (UV) high power beam injection. This is done by minimizing the number of nonlinear crystals, at least in part. One skilled in the art will recognize that these examples are just a few of the many possibilities and that other non-linear crystals can be used. As shown in FIGS. 13A and 13B, no OPO is used in these embodiments of the laser system 10. Table VIII gives a summary of the parameters of the crystals used in the laser system 10 of FIGS. 13A and 13B. It is preferred that none of the crystals exhibit a birefringence walk-off greater than 30 mrad, and it is even more preferred that the birefringence walk-off of any crystal is 25 mrad or less.

In Table VIII, as in all previous examples, the first row shows the type of nonlinear crystal used, and the second row shows the nonlinear process performed by the crystal. The third to fifth rows show the output wavelength and the two input wavelengths (if the nonlinear process is second harmonic generation, the two input wavelengths are the same). The sixth row gives the crystal temperature, and the seventh to eighth rows give the propagation direction angle with respect to the crystal optical axis necessary for phase matching (LBO is a so-called biaxial crystal). Line 9 presents the effective nonlinear coefficient (a measure of how much conversion efficiency can be for a given input power and crystal length), and line 10 is the input and output beams produced by the birefringence of the crystal. Angle walk-off (a slight angle shift between the input light and the harmonic light in the crystal).

  Note that the 594 nm light output can be generated from a samarium doped or dysprosium doped fiber laser and can also be provided by a praseodymium doped fluoride glass fiber laser.

In the embodiment shown in FIGS. 13A and 13B, the light source 102 comprises a Sm-doped fiber laser pumped with a blue broad area laser (BAL), eg, a blue GaN-based laser (425 nm ± 25 nm light output). As described above, other visible fiber lasers pumped with a shorter wavelength visible light pump source can be used in place of the Sm-doped fiber laser. A Dy-doped fiber laser (eg, pumped with a 425 nm ± 25 nm light source) and a Pr-doped or Eu-doped fiber laser 206A may also be used, for example. In the present embodiment, the light source 202 supplies pulsed light having a wavelength λ _p = 594 nm to the frequency converter 210. More particularly, the light source 202 can include an initial light source 202 '(optionally an oscillator or optionally pulsed) and a laser 206A operatively coupled to the light source 202'. The frequency converter 210 is based on a second harmonic generator based on a non-linear LBO (lithium triborate (LiB ₃ O ₅ )) crystal 210A and a BBO (beta-barium borate (β-BaB ₂ O ₄ )) crystal 210B. Has a third harmonic generator. The second harmonic generator 210A of the present embodiment converts the first output wavelength λ _{1A output} = 594 nm to a wavelength of 297 nm. Next, the BBO crystal 210B converts the 297 nm light and the remaining 595 nm light into light having a final output wavelength λ _output = 198 nm by SFM (sum frequency mixing). Of course, if a 297 nm light output is desired, a BBO crystal is not needed and the frequency converter has a single non-linear crystal and a filter for filtering out residual 594 nm light, such as LBO 110A. It will be.

  The light source 202 advantageously has a pin width (1-100 pm) output to increase the conversion efficiency. This can be achieved, for example, by amplifying the low power oscillator output with a high power amplifier.

  More specifically, the LBO crystal 210A has an effective nonlinear coefficient value of 0.39 pm / V for the second harmonic generation at 594 nm, and between the pump light beam and the second harmonic light beam (long crystal For light propagating at 90 ° and 65.4 ° angles to the optical axis with a relatively small (14 mrad) birefringence walk-off (allowing high conversion efficiency to be achieved at the medium level peak power used) Adjusted to 433K to achieve phase matching. The temperature of the BBO crystal 210B is also to achieve phase matching for the sum frequency mixing of 594nm light and 297nm light propagating at an angle of 82.9 ° to the optical axis (BBO is a uniaxial crystal) 433K. For this SFM process, the birefringence walk-off between the incoming 594 nm wavelength light and the incoming 297 nm wavelength light and the 198 nm third harmonic output light is relatively small (22 mrad), and a medium level peak power using a long crystal. High conversion efficiency can be achieved. In this embodiment, the number of crystal surfaces is kept to a minimum (up to two nonlinear crystals are used in this embodiment), and short wavelength power is “exchanged” with long wavelength power (BBO crystal). By using less 297 nm light and increasing 594 nm light to generate the same amount of 198 nm output, the optical damage to the nonlinear crystal by the high power laser beam can be minimized. .

In the above embodiment, no OPO is used in the optical system 10, and therefore a more stable output wavelength λ _output is generated (because the output wavelength is a harmonic of the input wavelength and is not determined by phase matching conditions). Please note that.

  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. Accordingly, 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 block diagram of a pulsed multi-stage Raman laser system 10 according to one embodiment of the present invention. FIG. 6 is a graph (by numerical model calculation) of optical power versus fiber length for pump light, one of the intermediate order Stokes lights, and output light in accordance with some aspects of the present invention. It is a graph of conversion efficiency versus fiber length. FIG. 6 shows optical pulse shapes at different points in the fiber for conversion to second-order Stokes light according to some embodiments of the present invention. FIG. 7 is a spectrum diagram of the output of a 38 m long nonlinear Raman conversion fiber pumped with a 400 ps long optical pulse having a spectrum centered at the original wavelength of 1064 nm generated by an Nd: YAG pulsed laser system. FIG. 6 shows a first example of a laser system according to the invention in a simplified manner. FIG. 6 illustrates schematically an embodiment of a pulsed laser source (MOPA) used in the laser system of FIG. A second example of a laser system according to the invention is shown in a simplified manner. A third example of a laser system according to the invention is shown in a simplified manner. 4 shows a fourth example of a laser system according to the invention in a simplified manner. A fifth example of a laser system according to the invention is shown in a simplified manner. 6 schematically shows a sixth example of a laser system according to the invention. 7 schematically shows a seventh example of a laser system according to the invention. FIG. 3 is a block diagram of two embodiments of another laser system in accordance with the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Laser system 102 Pulse light source 102 'Parent oscillator pulse light source 104 Pulse light 105 Optical amplifier 106 Raman wavelength converter 106' Raman conversion optical fiber 108 First pulse light output signal 110 Harmonic converter 110A, 110B Nonlinear crystal 112, 114 Seed light source

Claims (6)

In the laser system,
A pulsed light source that generates pulsed light having an optical spectrum centered on the original wavelength;
A nonlinear Raman conversion fiber coupled to the pulse light source, wherein the pulsed light passes through the nonlinear Raman conversion fiber, corresponds to final order Stokes light by a multistage stimulated Raman scattering process, and is longer than the original wavelength. A nonlinear Raman conversion fiber having an optical spectrum centered on the output wavelength of
A seed light source for injecting a small amount of CW light or pulsed light into the nonlinear Raman conversion fiber, wherein the small amount of CW light or pulsed light propagates through the nonlinear Raman conversion fiber together with the pulsed light from the pulsed light source A seed light source that seeds the Raman conversion to increase the Raman conversion efficiency, and
A harmonic generator operatively coupled to the nonlinear Raman conversion fiber, such that the harmonic generator generates an optical output at a final output wavelength within a range of 150-775 nm; A harmonic generator for receiving the first pulsed light output and converting the received first pulsed light output to a higher optical frequency;
With
The nonlinear Raman conversion fiber has a uniform normal chromatic dispersion over the fiber length, the normally chromatic dispersion the source wavelength, at normal chromatic dispersion with respect to all the wavelengths of the output wavelength and a plurality of intermediate-order Stokes beam A laser system characterized by comprising a single nonlinear fiber. In a method for controlling wavelength conversion and multistage stimulated Raman scattering for wavelength conversion, the method comprises:
Generating a pulsed light having an optical spectrum centered on the original wavelength and having a pulse width and a peak pulse power;
Generating seed light having a seed wavelength;
A non-linear Raman conversion fiber having a fiber length and a dispersion parameter is used to convert the pulse light and the seed light into a power output having an output wavelength longer than the original wavelength by a multistage stimulated Raman scattering process. Combining and propagating through the nonlinear Raman conversion fiber;
In order to maximize the conversion of the original power into the power output of the final Stokes light of the output wavelength in a single pass of the nonlinear Raman conversion fiber, the pulse width, the peak pulse power, the seed wavelength, the fiber Setting a length and the dispersion parameter; and
Higher the light supplied by the Raman conversion fiber to at least one harmonic generator such that the harmonic generator generates a light output of a final output wavelength that is in the range of 150-775 nm Combining the light to convert to optical frequency;
Including
The step of setting includes providing a series connection of nonlinear fiber segments;
Each of the nonlinear fiber segments has parachromatic dispersion for a particular combination of input wavelength, output wavelength, and intermediate order Stokes light wavelength passing through a particular fiber segment;
The requirement for minimizing non-linear light effects detrimental to conversion efficiency, including spectral broadening caused by four-wave mixing, where the value of the parachromatic dispersion at each of the wavelengths of the combination and any two consecutive conversion orders (wavelengths) ) Is also determined by the requirements to minimize pulse walk-off,
Each of the fiber segments has a fiber segment length;
The peak of the first order Stokes light entering the segment so that the fiber segment length generates the threshold for the stimulated Raman scattering to generate the order of Stokes light generated in the particular segment and passing through the particular segment. Determined from the pulse power and the Raman gain, attenuation and effective area of the fiber segment;
The final fiber segment length has a light intensity threshold value at which the Stokes light is converted to the Nth order, but the (N + 1) th order Stokes light is not converted.
As a result, most of the original power is converted to Nth-order Stokes light. In the laser system,
A pulsed light source that generates pulsed light having an optical spectrum centered on the original wavelength;
A nonlinear Raman conversion fiber coupled to the pulse light source, wherein the pulsed light passes through the nonlinear Raman conversion fiber, corresponds to final order Stokes light by a multistage stimulated Raman scattering process, and is longer than the original wavelength. A nonlinear Raman conversion fiber having an optical spectrum centered on the output wavelength of
A seed light source for injecting a small amount of CW light or pulsed light into the nonlinear Raman conversion fiber, wherein the small amount of CW light or pulsed light propagates through the nonlinear Raman conversion fiber together with the pulsed light from the pulsed light source A seed light source that seeds the Raman conversion to increase the Raman conversion efficiency, and
A harmonic generator operatively coupled to the nonlinear Raman conversion fiber, such that the harmonic generator generates an optical output at a final output wavelength within a range of 150-775 nm; A harmonic generator for receiving the first pulsed light output and converting the received first pulsed light output to a higher optical frequency;
With
The seed light source comprises a first seed light source and a second seed light source, the first seed light source provides seed light to the output wavelength, and the second seed light source has the original wavelength. The laser system according to claim 1, wherein seed light is provided with respect to the intermediate order Stokes light wavelength. In the laser system,
A pulsed light source that generates pulsed light having an optical spectrum centered on the original wavelength;
A nonlinear Raman conversion fiber coupled to the pulse light source, wherein the pulsed light passes through the nonlinear Raman conversion fiber, corresponds to final order Stokes light by a multistage stimulated Raman scattering process, and is longer than the original wavelength. A nonlinear Raman conversion fiber having an optical spectrum centered on the output wavelength of
A seed light source for injecting a small amount of CW light or pulsed light into the nonlinear Raman conversion fiber, wherein the small amount of CW light or pulsed light propagates through the nonlinear Raman conversion fiber together with the pulsed light from the pulsed light source A seed light source that seeds the Raman conversion to increase the Raman conversion efficiency, and
A harmonic generator operatively coupled to the nonlinear Raman conversion fiber, such that the harmonic generator generates an optical output at a final output wavelength within a range of 150-775 nm; A harmonic generator for receiving the first pulsed light output and converting the received first pulsed light output to a higher optical frequency;
With
The laser system, wherein the seed light source provides seed light with respect to the output wavelength. In the laser system,
A pulsed light source that generates pulsed light having an optical spectrum centered on the original wavelength;
A nonlinear Raman conversion fiber coupled to the pulse light source, wherein the pulsed light passes through the nonlinear Raman conversion fiber, corresponds to final order Stokes light by a multistage stimulated Raman scattering process, and is longer than the original wavelength. A nonlinear Raman conversion fiber having an optical spectrum centered on the output wavelength of
A seed light source for injecting a small amount of CW light or pulsed light into the nonlinear Raman conversion fiber, wherein the small amount of CW light or pulsed light propagates through the nonlinear Raman conversion fiber together with the pulsed light from the pulsed light source A seed light source that seeds the Raman conversion to increase the Raman conversion efficiency, and
A harmonic generator operatively coupled to the nonlinear Raman conversion fiber, such that the harmonic generator generates an optical output at a final output wavelength within a range of 150-775 nm; A harmonic generator for receiving the first pulsed light output and converting the received first pulsed light output to a higher optical frequency;
With
The laser system, wherein the seed light source provides a Stokes wavelength seed light having an intermediate order of the original wavelength. In the laser system,
A pulsed light source that generates pulsed light having an optical spectrum centered on the original wavelength;
A nonlinear Raman conversion fiber coupled to the pulse light source, wherein the pulsed light passes through the nonlinear Raman conversion fiber, corresponds to final order Stokes light by a multistage stimulated Raman scattering process, and is longer than the original wavelength. A nonlinear Raman conversion fiber having an optical spectrum centered on the output wavelength of
A seed light source for injecting a small amount of CW light or pulsed light into the nonlinear Raman conversion fiber, wherein the small amount of CW light or pulsed light propagates through the nonlinear Raman conversion fiber together with the pulsed light from the pulsed light source A seed light source that seeds the Raman conversion to increase the Raman conversion efficiency, and
A harmonic generator operatively coupled to the nonlinear Raman conversion fiber, such that the harmonic generator generates an optical output at a final output wavelength within a range of 150-775 nm; A harmonic generator for receiving the first pulsed light output and converting the received first pulsed light output to a higher optical frequency;
With
The seed light source comprises a first seed light source and a first group of seed light sources, wherein the first seed light source provides seed light for the output wavelength, and the first group of seed light sources The laser system, wherein the light source provides seed light with respect to the intermediate-order Stokes light wavelength of the original wavelength.

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