EP2013951A2 - Pulsed uv and visible raman laser systems - Google Patents
Pulsed uv and visible raman laser systemsInfo
- Publication number
- EP2013951A2 EP2013951A2 EP07776322A EP07776322A EP2013951A2 EP 2013951 A2 EP2013951 A2 EP 2013951A2 EP 07776322 A EP07776322 A EP 07776322A EP 07776322 A EP07776322 A EP 07776322A EP 2013951 A2 EP2013951 A2 EP 2013951A2
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- Prior art keywords
- fiber
- wavelength
- output
- laser system
- power
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/30—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range using scattering effects, e.g. stimulated Brillouin or Raman effects
- H01S3/302—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range using scattering effects, e.g. stimulated Brillouin or Raman effects in an optical fibre
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/06—Construction or shape of active medium
- H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
- H01S3/067—Fibre lasers
- H01S3/06754—Fibre amplifiers
- H01S3/06758—Tandem amplifiers
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/09—Processes or apparatus for excitation, e.g. pumping
- H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
- H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
- H01S3/0941—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light of a laser diode
- H01S3/09415—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light of a laser diode the pumping beam being parallel to the lasing mode of the pumped medium, e.g. end-pumping
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/10007—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers
- H01S3/10015—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers by monitoring or controlling, e.g. attenuating, the input signal
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/10084—Frequency control by seeding
- H01S3/10092—Coherent seed, e.g. injection locking
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
- H01S3/16—Solid materials
- H01S3/1601—Solid materials characterised by an active (lasing) ion
- H01S3/1603—Solid materials characterised by an active (lasing) ion rare earth
- H01S3/1618—Solid materials characterised by an active (lasing) ion rare earth ytterbium
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/23—Arrangements of two or more lasers not provided for in groups H01S3/02 - H01S3/22, e.g. tandem arrangements of separate active media
- H01S3/2375—Hybrid lasers
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/23—Arrangements of two or more lasers not provided for in groups H01S3/02 - H01S3/22, e.g. tandem arrangements of separate active media
- H01S3/2383—Parallel arrangements
Definitions
- the present invention relates generally to solid state lasers and more particularly to fiber lasers using nonlinear wavelength conversion to produce output in the ultra violet (UV) and/or visible wavelength ranges.
- UV ultra violet
- Coherent light sources in the visible (400-775 nm) wavelength range and in the UV or deep UV (DUV) range (150-400 nm) find a number of important applications (such as in medicine, life sciences, material processing, photolithography and metrology). Typically, a high output power is desired and different output wavelengths are required for different applications.
- the output wavelengths that are produced by this method are also limited to harmonics (e.g., 2 nd , 3 rd , 4 th ) of the fundamental or pump wavelengths.
- harmonics e.g., 2 nd , 3 rd , 4 th
- Such laser outputs are, for example, 532nm, 355nm, and 266nm that are produced by harmonic conversion of 1064 nm Nd: Y AG laser output.
- Optical Parametric Oscillator may be utilized with a DPSS laser to provide additional output wavelength tunability, provided that a nonlinear crystal with a suitable transparency range and phase matching conditions exists. This is not always possible. Furthermore, because the output wavelength from OPO is determined by phase matching conditions of the nonlinear crystal, the laser systems utilizing OPOs are generally complex, and suffer from poor stability, as compared to the laser systems that utilize harmonic converters.
- DPSS lasers are typically operated either in Q-switched (long, 30-50 ns pulses) regime where the pulse repetition frequencies are limited to several kHz, or in a mode-locked (5-10 ps pulses) regime where the spectral width of the output is significantly larger, and therefore coherence length of the laser output is shorter then that of a continuous wave or CW laser. Therefore, such DPSS lasers are not suitable for producing quasi-CW output, where optical pulses are sufficiently long to keep the high coherence, but at the same time repetition frequency is high enough so that for a particular detector the output light appears effectively CW.
- fiber lasers are perfect candidates for creating high power CW or quasi-CW sources in visible and UV ranges by harmonic conversion, hi addition, the fiber laser output wavelength is not limited to a discrete value and can be tuned in a relatively broad range, for example ⁇ 1030-1120 nm for Yb-doped devices.
- OPOs may be utilized to provide additional wavelengths, but, as stated above, laser systems utilizing OPOs are generally complex, and suffer from poor stability, as compared to the laser systems that utilize harmonic converters.
- One embodiment of the invention is a Raman laser system comprising: a pulsed light source generating a pulsed light having an optical spectrum centered at a source wavelength; (ii) a Raman conversion fiber coupled to the pulsed light source, wherein the pulsed light traverses the nonlinear Raman conversion fiber and is converted by a cascaded Stimulated Raman Scattering process into a first pulsed light output having an optical spectrum centered at a first output wavelength which is longer than the source wavelength; and (iii) a harmonic generator operatively coupled to said Raman conversion fiber to accept the first pulsed light output and to convert it to higher optical frequency such that said harmonic generator is producing the final light output at the final output wavelength situated in the 150-775 nm range.
- One embodiment of the invention is a laser system that comprises: a light source generating a light, said light source comprising a fiber laser or fiber amplifier operating in a visible spectral range; and a frequency converter operatively coupled to the light source to accept the light provided by the light source and to convert it to higher optical frequency such that the frequency converter is producing light output in the final output wavelength situated in the 150-400 run range.
- the final output wavelength is situated in the 150-300 nm range.
- FIG. 1 is a block diagram view of the pulsed cascaded Raman laser system 10 according to one embodiment of the present invention
- FIG. 2 is a plot (produced by numerical modeling) of optical power versus fiber length, for pump light, one intermediate Stokes order and output light, in accordance with some aspects of the present invention
- FIG. 3 A is a plot of conversion efficiency versus fiber length; [00017] FIG. 3B shows optical pulse shapes at different points within the fiber for conversion into the second Stokes order in accordance with some embodiments of the present invention;
- FIG. 4 is a spectrum graph of the output of the 38 m long nonlinear Raman converter fiber pumped by 400 ps long light pulses with the spectrum centered at the source wavelength of 1064 run produced by a Nd:YAG pulsed laser system;
- FIG. 5 illustrates schematically first example of the laser system according to the present invention
- FIG. 6 illustrates schematically one embodiment of the pulsed laser source (MOPA) utilized in the laser system of FIG. 5;
- MOPA pulsed laser source
- FIG. 7 illustrates schematically second example of the laser system according to the present invention
- FIG. 8 illustrates schematically third example of the laser system according to the present invention
- FIG. 9 illustrates schematically fourth example of the laser system according to the present invention.
- FIG. 10 illustrates schematically fifth example of the laser system according to the present invention.
- FIG. 11 illustrates schematically sixth example of the laser system according to the present invention
- FIG. 12 illustrates schematically seventh example of the laser system according to the present invention.
- FIGS 13A and 13B are block diagrams of two embodiments of another laser system according to the present invention.
- Sub-200 nm laser light sources are very important for metrology applications in the semi-conductor industry. As the feature sizes of integrated circuits are shrinking, shorter wavelength light is used for a photolithography. Mask and wafer inspection, as well as optics manufacturing is then in need of the same or similar DUV light wavelength.
- the systems presented used, based on solid-state laser sources, harmonic conversion and OPOs 5 typically work at very low repetition rates, are very bulky, complex, expensive and require frequent and complicated maintenance.
- the fiber laser output wavelength is not limited to a discrete value and can be tuned in a relatively broad range, for example about 570-630 nm for Sm-doped lasers or amplifiers. This tunability provides for tuning or adjustment of the final output wavelength provided by the laser system.
- SRS cascaded stimulated Raman scattering
- a harmonic generation (e.g., 2 nd , 3 rd or 4 ⁇ ) is then utilized to convert the power output of the last Stokes order to higher optical frequency (shorter wavelengths), such that the harmonic generator produces light output at the final output wavelength situated in the 150-775 nm range.
- FIG. 1 One embodiment of the laser system of the present invention is shown in FIG. 1, and is designated generally throughout by the reference numeral 10.
- laser system 10 includes a pulsed light source 102, (for example, a "master oscillator - power amplifier” (MOPA) that includes the initial "master oscillator” pulsed light source 102') that generates a pulsed light 104 having an optical spectrum centered at a source wavelength ⁇ p .
- a Raman wavelength converter 106 comprising a nonlinear Raman conversion fiber 106' is operatively coupled to the pulsed light source 102.
- N 1, 2, 3 or 4
- the pulse width an the peak pulse power provided by the pulsed light source 102 as well as the Raman conversion fiber length, and the dispersion parameter of this fiber should be preferably chosen to maximize conversion of the source power to the power output of the last Stokes order at the output wavelength in a single pass through the nonlinear Raman conversion fiber 106'.
- the Raman wavelength shifting in optical fibers is utilized to produce coherent light output at any wavelength where the glass material used to make the fiber is transparent.
- Multiple-order stimulated Raman scattering (SRS) generation where the i-th Stokes order of the initial wavelength serves as a pump for generation of the (i+1)- th Stokes order is utilized as a cascaded wavelength converter (from shorter to longer wave-lengths) for accomplishing a significant wavelength shift within a transparency window of the glass material used to make the optical fiber, resulting in producing a new output wavelength ⁇ i ou t that might be not available from other types of laser sources.
- SRS stimulated Raman scattering
- the Raman conversion fiber 106' may be a single nonlinear fiber having a uniform chromatic dispersion throughout the fiber length, such that the chromatic dispersion is normal for the source wavelength, the output wavelength, and every wavelength of a plurality of intermediate Stokes orders.
- the value of the chromatic dispersion at each of the source, output and intermediate Stokes wavelength is determined, at least in part, by the requirement to minimize nonlinear optical effects detrimental to the conversion efficiency including spectral broadening caused by four-wave-rnixing and by the requirement to minimize pulse walk-off for any two consecutive conversion orders (wavelengths).
- the fiber length of the Raman conversion fiber 106' is determined from the peak pulse power of a source and the Raman gain, attenuation and effective area of the fiber, such that the stimulated Raman scattering threshold is overcome for N consecutive Stokes orders but is not overcome for (N+l) order resulting in source power converted mostly into the N-th order.
- the Raman conversion fiber 106' may be made by a serial connection of nonlinear Raman fiber segments, each having normal chromatic dispersion for a particular subset of the input, output, and intermediate Stokes orders wavelengths that are traversing the particular fiber segment, and the value of the chromatic dispersion at each of the wavelengths of the subset is determined by the requirement to minimize nonlinear optical effects detrimental to the conversion efficiency including spectral broadening caused by four-wave-mixing and the requirement to minimize pulse walk-off for any two consecutive conversion orders (wavelengths).
- each fiber segment has a fiber segment length
- the fiber segment length is predetermined from the peak pulse power of the first Stokes order entering that segment and Raman gain, attenuation and effective area of the fiber segment such that the stimulated Raman scattering threshold is overcome for those Stokes orders that are generated and traverse that particular segment, and the last fiber segment length is just sufficient to overcome N-th Stokes threshold but not sufficient to overcome (N+l) Stokes threshold resulting in source power converted mostly into the N-th order.
- a harmonic generator 110 is operatively coupled to the nonlinear Raman conversion fiber 106' to accept the first pulsed light output signal 108 at the wavelength ⁇ i om and to convert it to higher optical frequency such that the harmonic generator 110 is producing the final pulsed light output 112 at the wavelength ⁇ out situated in the 150-775 nm range.
- the harmonic generator 110 may be, for example, 2 nd , 3 rd , or 4 th , etc. harmonic generator, which means that it is converting the first output wavelength ⁇ i ou t to the final wavelength of ⁇ ou t /2; ⁇ i ou t /3, or ⁇ i ou t /4.
- the Raman conversion of the pulsed light takes place in a single pass through the optical fiber 106' and does not require any resonant cavities or wavelength-selective elements.
- each subsequent Stokes order will have a slightly broadened spectrum, since the Raman gain spectrum in glass is relatively broad and the generation of a new Stokes order takes place by amplification of a spontaneous Raman emission. Additional spectral broadening takes place due to the influence of four- wave mixing. For these reasons, the precise selection of the output wavelength may be difficult. Therefore, we suggest that the seed light is preferably provided at the Raman- fiber input for the first pulsed light output wavelength ⁇ i ou t and (optionally) for each intermediate Stokes wavelengths.
- FIG. 1 where the seed light sources 112 (for the first pulsed light output) and 114 (for one of the intermediate Stokes orders) are shown.
- it is sufficient to provide only a few microwatt of power in each seed but providing higher power (up to 10 mW) will result in a shorter Raman fiber length required, slightly higher conversion efficiency and less noise.
- pulsed light source 102 also referred to as a pump herein
- the lasers seeding intermediate Stokes orders 114 are required to have a very narrow spectral output, due to a relatively weak wavelength dependence of Raman gain (less than 1 nm is generally sufficient), and can have multiple longitudinal mode structure.
- the seed source 112 for the first output wavelength ⁇ i ou t preferably provides a narrow spectral line, and more preferably single-frequency. Since it will be amplified by a stimulated Raman scattering, the spectral width of the output of the Raman wavelength converter 106 will be nearly the same as the spectral width of the seed source 112. Thus, having a narrow line, highly coherent seed source 112 (preferably less than 100 pm linewidth) will ensure that a high conversion efficiency can be reached in the harmonic conversion stages.
- External cavity, DFB, DBR or vertical cavity semiconductor diode lasers are most suitable, but a single- frequency DFB or DBR fiber lasers, as well as solid state lasers, can also be used as a first output seed source 112.
- the Raman process is polarization selective, it is important that all of the pump 102 and seed sources 112, 114 are polarized, and their output polarization states are aligned at the input of the Raman converter fiber 106'. It is also preferable that the Raman converter fiber 106' is polarization-preserving, otherwise, a combination of the half- wave and quarter-wave plates will be required to correct the ellipticity of the converter output and align its polarization in respect to the optical axes of the nonlinear crystals utilized in a harmonic generator (converter) 110. As is well known in the art, an optical birefringence property of the nonlinear crystals is commonly used to achieve phase matching, or ensure that light of different harmonics propagates at the same speed through the crystal.
- the Raman converter 106 has to be designed according to the type of Raman conversion fiber 106' used.
- Our numerical calculations and preliminary experiments indicate that silica-based fiber 106' has a sufficiently large Raman gain to achieve high conversion efficiency in a relatively short length, as will be described below.
- the fiber might need to be designed to have a large mode area, to avoid optical damage.
- the Raman gain for a silica-based fiber has two closely spaced peaks, approximately at 13.4 and 14.6 THz from the optical frequency of the pump (i.e., the pulsed light source 102).
- the pump wavelength ⁇ p, intermediate Stokes, and the output seed wavelengths are chosen such that the distance between any consecutive two in optical frequency domain is between 12.5 and 14.7 THz, and more preferably at exactly the maximum of the Raman gain for a given converter fiber.
- the pulse width provided by the light source 102 is 0.1 to 100ns and a duty cycle of the pulse is 1:2 to 1:1000.
- a duty cycle of the pulse is 1:2 to 1:1000.
- SBS stimulated Brillouin scattering
- the duty cycle (the ratio of pulse width and repetition period, which is also the ratio of average to peak power) is less than 1 : 100, which, for pulses longer than 10 ns, will limit the repetition frequency to values lower than 1 MHz, which is not desirable if the goal is to produce a quasi-CW source.
- the optical pulses are too short, then the Raman conversion efficiency will be affected by the walk-off in time between pump and Stokes orders caused by fiber dispersion. Therefore, unless a Raman conversion fiber is specially designed to match group velocities of pump and Stokes' orders light, it is preferable not to have pulses shorter than 300 ps (0.3ps).
- the preferred range for pulse width is 0.3-10 ns, with the corresponding repetition frequency range of 1-30 MHz. It is also preferable that the pulse shape is as close as possible to a perfect rectangle, since the presence of any pulse "wings" will cause an incomplete conversion. Another important consideration is the amount of spectral broadening imposed on the Raman converter output by a nonlinear optical effect known as self-phase modulation (SPM), which can be detrimental for the efficiency of the subsequent harmonic conversion. As is well known in the art, SPM only causes optical frequency change, resulting in spectral broadening, if the light intensity is changing in time. Therefore, a nearly perfect rectangular pulse, with the flat top and shortest possible rise and fall times, is preferred to minimize the influence of the SPM.
- SPM self-phase modulation
- the term “nonlinear fiber” refers to an optical waveguide (including a fiber) having sufficiently large Raman gain, sufficiently long length, sufficiently low attenuation and sufficiently small effective area for the fundamental waveguide mode that the optical power threshold for generation of at least one (first) Stokes order does not exceed an optical damage threshold for the material the optical waveguide is made of.
- An arbitrary large wavelength shift can be accomplished by a cascaded stimulated Raman scattering process, provided that a suitable glass material exists that is transparent (i.e., low loss, or loss that is smaller than Vz Raman gain provided but the Raman fiber) and has sufficient Raman gain at the pump and all of the required Stokes orders wavelengths. Therefore, using a pulsed laser, Raman wavelength conversion process, and the appropriate harmonic generator, in principle any desired output wavelength, including those in the visible and UV ranges can be produced.
- the nonlinear Raman conversion fiber 106' may be manufactured from an optical glass transparent in the mid-infrared wavelength range, the glass being preferably made from a member selected ' from the group consisting of sulfides, selenides, tellurides, germanates (based on GeO 2 ), aluminates (based on Al 2 O 3 ), and tellurites. (based on TeO 2 ).
- the output power of the laser system 10 is limited only by the 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 to make the Raman fiber is dictated by the output wavelength and power requirements.
- Fused silica based material is preferred in the wavelength range of 800-1900 ran due to its high transparency and optical damage thresholds and Raman gain sufficient to achieve efficient conversion in a relatively short fiber length.
- Other glass types may also be used if higher Raman gain or different wavelength range is desired.
- a high-power optical amplifier 105 is coupled between the initial pulsed light source 102'(a pulsed master oscillator in this embodiment) and the nonlinear Raman conversion fiber 106' for amplifying the pulsed light 104 such that the average power and the peak pulse power of the pulsed light source 102 can be increased, hi this way, cost-effective pump sources based on the well developed fiber amplifier technology for the amplifier 105 may be utilized.
- the high-power optical amplifier 105 in conjunction with the initial pulsed light source 102' forms the master oscillator power amplifier (MOPA).
- MOPA master oscillator power amplifier
- the method and apparatus of the present invention are especially 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 specifically, it is noted that a Yb-doped fiber based laser or MOPA can provide an optical output in the 1030 to 1120 nm range and Er, Tm and Nd-doped silica fiber based lasers or MOPAs are capable of providing an output in 1530-1610 nm, 1800-2000 nm, and 890-930 nm ranges, respectively.
- the light source 102 includes a tunable laser for tuning the source wavelength ⁇ p, wherein the tuning of the source wavelength provides fine tuning of the final output wavelength X 0111 .
- rare-earth doped fiber amplifiers Due to a very long (several ms) lifetime of the excited states, rare-earth (e.g., Er or Yb) doped fiber amplifiers essentially amplify the average power of the incoming signal, and for a very small duty cycle, an amplifier 105 with only a modest average power output can produce very large peak pulse power.
- rare-earth e.g., Er or Yb
- 1 ns long pulses from a master oscillator 102% can be amplified to a peak power of 20 kW in a multiple-stage Er or Yb-doped amplifier 105, while the average output power of power amplifier 105 is only 2 W, if the repetition rate is 100KHz (peak power is 1000Ox average power).
- a master oscillator 102% e.g., a directly modulated distributed feedback (DFB) laser diode
- DFB distributed feedback
- Directly modulating a semiconductor laser diode with an electrical pulse generator or connecting the diode output to an separate electro-optic intensity modulator for setting the pulse width can be used to make the initial pulsed light source 102' for generating the pulsed light 104, with or without further amplification (i.e. with or without the optional amplifier 105).
- a rectangular pulse is preferred for maximizing the conversion efficiency (minimizing the effect of incomplete conversion in the pulse wings) and minimizing spectral broadening (by self-phase modulation (SPM)).
- the pulsed light source 102 and to some extent the whole laser system 10 can be made wavelength tunable by using a tunable master oscillator pulse source 102' (such as an external cavity semiconductor laser, directly modulated or coupled to a separate modulator).
- a tunable master oscillator pulse source 102' such as an external cavity semiconductor laser, directly modulated or coupled to a separate modulator.
- FIG.2 shows the results of the simplified numerical modeling based on the coupled power equations presented in the paper by G.Vareille, O.Audouin, E.Desurvire, Electronics Letters, 1998, v.34, No. 7, p.675 and parameters typical of a single-mode silica based fiber.
- a pulsed light nature of the source 102 is neglected (for simplification) and instead IOOOW of continuous wave (CW) pump power at a source wavelength ⁇ p of 1056.3 run is directly provided at the input of a nonlinear Raman conversion fiber, along with the small amount (10 ⁇ 6 W) of seed power for the 1 st and 2nd Stokes orders.
- CW continuous wave
- FIG. 2 is a plot of optical power versus the length of the Raman conversion fiber 106'. As shown in FIG. 2, the output optical power at the pump wavelength ⁇ p is fully converted to the 1 st Stokes order at the wavelength of 1114 nm before the conversion step to the 2nd order starts. The plot also demonstrates that approximately.12.5 m fiber length is required for each conversion step and all power is converted into the 2 nd Stokes order (an output wavelength ⁇ lout of 1178 nm) at 25 m length, with 90% efficiency.
- the conversion is accomplished in a single pass through the fiber and, due to its non-resonant nature and the self phase-matched nature of the Stimulated Raman Scattering, is largely unaffected by the variations in fiber dispersion, attenuation or effective area along its length.
- the operation of the pulsed cascaded Raman wavelength converter 106 is advantageously robust to variations in dispersion, loss and effective area of the nonlinear Raman conversion fiber 106'.
- the main requirement for the operation of the pulsed cascaded Raman wavelength converter 106 is that the nonlinear Raman conversion fiber 106' has normal (negative) dispersion at any point along its length, at least for the wavelengths of the source ⁇ p , first output ⁇ jout and any of the intermediate Stokes orders that are traversing this point.
- Normal (negative) dispersion is needed to prevent nonlinear optical effects detrimental to the cascaded Raman conversion efficiency, such as pulse break-up caused by soliton effects, spectral line broadening caused by four-wave mixing, amplification of noise and spontaneous emission by a parametric or modulational instability effect and generation of "parasitic" spectral lines.
- the absolute value of the nonlinear Raman conversion fiber dispersion can not be allowed to be too large to avoid the "dispersion walk-off' in time . "(i.e., separation of light pulses at different wavelengths due to dispersion) of the light pulses of different Stokes orders, causing conversion efficiency decrease and pulse shape distortion.
- the allowable value depends on the pulse width and the length of fiber required for one conversion step to be completed (12.5 m in the example of FIG. 2), but in general it is preferred that the amount of walk-off is less than 10% of the pulse width.
- FIGS 3A and 3B present the modeling results for laser system 10 that comprises a pulse light source 102 providing 400 ps long optical pulses with 100 ps rise and fall times and 25 kW peak power.
- seed source 112, 114, etc. provides 1.0 mW of CW power for all Stokes orders up to the 4th.
- FIG. 3 A illustrates dependence of the average power (averaged over the duty cycle) in the 2 nd Stokes order on the position within the fiber 106'. As is evident from FIG.3A, almost 80% average power conversion efficiency to the second Stokes wavelength can be reached for the 26 m long fiber 106'. For the conversion to 3 rd Stokes order, the model predicts maximum 69% average power conversion efficiency for the 36 m fiber length (not shown).
- FIG. 3 A illustrates dependence of the average power (averaged over the duty cycle) in the 2 nd Stokes order on the position within the fiber 106'.
- the model predicts maximum 69% average power conversion efficiency for the 36 m fiber length (not shown).
- FIG.4 presents the output pulsed signal spectrum at the end of the nonlinear Raman conversion fiber 106' for the fiber length of 38 meters.
- the choice of fiber length for the given input pulse peak power uniquely determines at which Stokes order (2 nd Stokes order for the case of FIG. 4) the conversion will stop.
- FIG.4 illustrates relative powers corresponding to the wavelengths of ⁇ p, 1 st Stoke and 2 nd Stoke, at the Raman conversion fiber's output. As can be seen from FIG.
- the conversion efficiency from 1064 nm light provided by the Nd: YAG laser into the 2 nd Stokes is approximately 60% (i.e., «4/7), although no seed light was provided for either 1 st or 2 nd Stokes, and the input pulse shape was close to a double exponential, very different from the optimum rectangle, which resulted in a noticeable amount of pump light ( ⁇ p) and 1 st Stokes light still present at the output.
- the measured 2 nd Stokes pulse energy was about 10 ⁇ J, which means that the output pulse peak power was about 25 kW. For the duty cycle of 1 : 100, this would correspond to the average output power from the Raman converter 106 of 250 W.
- output wavelength range of approximately 1030- 1120nm is directly accessible for an Yb-doped fiber laser systems.
- Raman wavelength conversion by the Raman converter 106 to up to 3 rd Stokes order, this range can be effectively extended to 1030-1340 nm. Therefore, output wavelengths ⁇ out in the 515-770 nm range can be produced by 2 nd harmonic generation, 343.3-446.7 nm by 3 rd harmonic generation, 257.5-335 nm by 4 th harmonic generation and so on.
- fiber laser systems such as those including a silica based fiber which is Er-Yb co-doped for 1530-1570 nm range, Nd-doped for 890-930 nm range (working at the 3-level transition) or Tm-doped for 1800-2000 nm range, can also be utilized to pump Raman wavelength converters 106.
- Additional output wavelengths ⁇ oUt can be produced by sum frequency mixing, in a suitable nonlinear crystal, the outputs of two different fiber lasers, each one possibly followed by a Raman wavelength converter and harmonic generator.
- any desired output wavelength ⁇ oUt in the 150-775 nm range can be produced by a suitable combination of one or two pulsed fiber lasers, Raman converters, harmonic generators and sum- frequency mixing stages, as will be illustrated below.
- harmonic generators 110 in such a way that they use exclusively borate nonlinear crystals (LBO, BBO, CLBO), which are known to have the highest optical damage thresholds and are therefore capable of producing higher powers by harmonic conversion.
- LBO, BBO, CLBO exclusively borate nonlinear crystals
- Those skilled in the art will appreciate that these examples represent only a small subset of the almost infinite number of possibilities opened by using Raman wavelength conversion technology and that other non-linear crystals may also be utilized.
- Sodium beacons created by exciting atomic sodium in the upper atmosphere, can be used as "guidestars" for adaptive optical systems and are therefore of great interest for advanced laser-based defense system applications.
- a high-power CW or quasi-CW sources tuned to the sodium D2 absorption line at the wavelength of 589.16 nm are required for this purpose.
- FIG. 5 An exemplary laser system 10 for producing an output at 589.16 nm is illustrated in FIG. 5.
- the pulsed light source 102 is an Yb-doped fiber system (master oscillator - power amplifier, or MOPA).
- MOPA master oscillator - power amplifier
- the Raman converter 106 can advantageously use silica-based Raman conversion fiber 106' with low loss and high optical damage thresholds.
- the harmonic generator 110 is a 2 nd harmonic generator comprising a nonlinear LBO (LiB 3 Os, lithium triborate) crystal IIOA and optional lenses HOB focusing the first light output 108 from the Raman converter 106 and collimating the final light output from the LBO crystal HOA.
- a seed light for the Raman converter output wavelength has to be provided at the input of the Raman conversion fiber 106'.
- the corresponding seed light source 112 can be an external cavity diode laser, or a vertical cavity laser, or a solid state laser with preferably a single narrow spectral line output.
- a second seed light from a second seed light source 114 can also be optionally provided at the wavelength of the first Stokes order of 1114 nm.
- a second seed light source 114 can be an Yb-doped fiber laser with fiber Bragg grating reflectors, but of course other diode, fiber or solid-state lasers may also be utilized.
- the initial light source 102' (master oscillator) includes a laser diode 102A' and an electro-optical modulator (EOM) 102B' for creating pulsed light 104 provided to the optical amplifier 105 (power amplifier). This modulator is utilized to form optical pulses and to' set the pulse width.
- Laser diode 102A' of this embodiment is the fiber Bragg grating (FBG) stabilized CW diode laser emitting 100 mW at 1056.3 nm and EOM 102B' is a lithium niobate intensity modulator which is used to form 2 ns long pulses at the 2 MHz repetition rate (1:250 duty cycle).
- FBG fiber Bragg grating
- EOM 102B' is a lithium niobate intensity modulator which is used to form 2 ns long pulses at the 2 MHz repetition rate (1:250 duty cycle).
- the power amplifier potion of MOPA includes four fiber amplifier stages 105A-105D, two of the stages 105A and 105B utilize Yb-doped single mode polarization maintaining fiber and two of the stages 105C and 105D utilize Yb doped polarization maintaining double-clad fiber. Yb- doped fibers are pumped by 976 nm pumps to provide amplification.
- the first amplifier stage 105A provides an average power output of 30 mW
- the second stage 105B amplifies it to 400 mW
- the third stage 105C further amplifies it to 5W
- the last stage 105D provides average optical power output of 140 W and peak optical output power of 35 kW in 2ns pulses at 2MHz repetition rate.
- the modulator 102B' has greater than 5 GHz electrical bandwidth, it should be capable of producing nearly rectangular pulse shape with less than 100 ps rise and fall times.
- Narrowband optical filters 105E are inserted after each amplification stage 105A-105C except the last amplification stage 105D.
- the two filters following the single mode fiber stages 105A, 105B are constructed using an optical circulator and an FBG, and the last filter of stage 105D is a thin-film device within a free-space optical isolator, so that they simultaneously serve to isolate amplification stages from each other and to reduce the accumulation of spectrally broadband amplified spontaneous emission.
- Those skilled in the art will appreciate that many variations to this example design are possible — different fiber, different number of stages, different power, pulse width and repetition rate.
- Other types of CW light sources with external modulators or directly modulated diode lasers can be used as an initial light source for the MOPA.
- the MOPA output mixed with Raman seeds from seed sources 112, 114, is launched into a Raman converter fiber 106', as shown in FIG. 1.
- Wavelength conversion proceeds through the first Stokes order at 1114 nm to the output second Stokes order at 1178.32 ran.
- the frequency shift between pump and first Stokes, as well as the one between first and second Stokes, is chosen to be 14.69 THz, corresponding to one of the peaks of Raman gain in fused silica. Based on the modeling and preliminary experimental results described above, it is expected that at least 50% of the MOPA output power or 70 W will be converted into a single-frequency output at 1178.32 nm.
- the first output signal 108 is then provided to the harmonic generator 110 which, in this embodiment, includes one LBO crystal HOA.
- LBO crystal IIOA is the second harmonic generator.
- the LBO crystal IIOA receives the first output wavelength ⁇ iout of 1178.32 ran from the Raman converter 106 and provides the desired wavelength ⁇ o U t of 589.16 nm.
- Table I provides the summary of crystal's (LBO IIOA) parameters utilized in the laser system 10 of example 1.
- Table I Calculated phase matching parameters for the laser system 10 of FIG.5
- the first row lists the type of the nonlinear crystal (s) used and the second the type of a nonlinear process the crystal is performing.
- Rows 3-5 list the output and two input wavelengths (for the case when the nonlinear process is a second harmonic generation, the two input wavelengths are the same).
- Row 6 provides the crystal temperature and rows 7-8 provide the propagation direction angles with respect to the crystal optic axes (LBO is a so-called bi-axial crystal) required for phase matching.
- Row 9 specifies the effective nonlinearity coefficient (a measure of how efficient the conversion can be for a given input power and crystal length), and row 10 provides the input and output beam angular walk-off (slight angular separation of the input light and the harmonic light within the crystal) caused by crystal birefringence.
- NCPM non-critical phase matching
- SNLO a free software package from Sandia National Labs.
- the second harmonic generation efficiency can reach at least 50%, resulting in 35 W output at 589.16 nm wavelength.
- building laser systems that provide an average output power greater than 50 W in the sodium D2 line is feasible.
- Sub-200 nm laser light sources are very important for metrology applications in the semiconductor industry. As the feature sizes of integrated circuits are shrinking, shorter wavelength light is used for a photolithography. Mask and wafer inspection, as well as optics manufacturing is then in need of the same or similar DUV light wavelength.
- FIG.7 presents an example of a 198.7 nm laser system 10 according to one embodiment of the present invention.
- 1067.6 nm Yb- doped fiber MOPA 102 pumps the Raman wavelength converter 106 which, in accordance with the design principles disclosed above, produces a narrow linewidth output at the 2 nd Stokes wavelength ⁇ i out of 1192.2 nm.
- the 1 st Stokes order wavelength is 1126.5 nm
- the Stokes frequency shift is 14.67 THz and corresponds to one of the Raman gain peaks in fused silica.
- the first output signal 108 from the Raman wavelength converter 106 is then provided to the harmonic generator 110 which, in this embodiment, includes two LBO crystals HOA, HOB and a BBO (beta barium borate, P-BaB 2 O 4 ) crystal HOC.
- the harmonic generator 110 which, in this embodiment, includes two LBO crystals HOA, HOB and a BBO (beta barium borate, P-BaB 2 O 4 ) crystal HOC.
- LBO HOA The three nonlinear crystals, LBO HOA, LBO HOB and BBO HOC, are used to generate a 6 th harmonic of the 1192.2 nm wavelength by: (i) second harmonic generation (SHG) via LBO HOA producing wavelength of 596.1 nm, (ii) another SHG (LBO HOB producing the 298.05 nm wavelength), (iii) and sum-frequency mixing (SFM) of 596.1 and 298.05 via BBO HOC producing 198.7 nm output. More specifically, LBO crystals HOA and HOB are second harmonic generators (SGHs).
- the LBO crystal HOA receives the first output wavelength ⁇ i ou t of 1192.2 nm from the Raman converter 106 and provides 596.1 nm output to the second LBO crystal HOB. Any residual light at 1192.2 nm wavelength is filtered out of the system by filter (dichroic mirror) HOD.
- SFM sum frequency mixing
- the second LBO crystal HOB can not be non-critically phase matched.
- the phase matching temperature for the second harmonic generation of 596.1 nm is 453 K (180 Celsius)
- the birefringent walk-off is 14 milliradians.
- the BBO crystal HOC is a uni-axial crystal.
- the phase-matching crystal temperature is 453 K
- the effective nonlinearity is pm/V
- the birefringent walk-off is 29.2 milliradians, for the nonlinear process of sum frequency mixing of 596.1 and 298.05 nm.
- Table II provides the summary of crystal's parameters utilized in the laser system 10 of example 2.
- BBO is a uni-axial crystal, so only one phase matching angle is listed for it in Table II.
- FIG. 7 Also shown in FIG. 7 are (i) the two lenses Ll and L2 associated with each non linear crystal, one to focus the beam to a waist size optimal for maximizing conversion efficiency for a given crystal and another to re-collimate the output, (ii) a dichroic mirror M that removes unused 1192.5 nm light, (iii) a custom waveplate WP; and (iv) a harmonic separation optics set HOE.
- the optimal waist size for the beam focusing depends on the peak power of the light being converted, the length and the optical damage threshold of the crystal, the amount of birefringent walk-off and other parameters, and can be determined from modeling or in the experiment.
- the waveplate WP is custom designed to rotate either (but only one of) the 596.
- the harmonic separation optics set HOE is used to separate output 198.7 nm from both 596.2 and 298.1 and usually includes one or several prisms made of dispersive glass. For simplicity, those additional optical elements are not shown in optical schematics given for the remaining examples. Those skilled in the art will be able to determine where and when such elements should be used.
- the advantage of the laser system 10 of FIG. 7 is that a minimum number of nonlinear crystals (only 3) are used to produce the sub-200 nm output. However, it may exhibit a significant birefringent walk-off in the BBO crystal HOC. Large walk-off does not allow tight focusing of the laser beams and therefore results in the lower conversion efficiency, since a shorter crystal or larger beams (lower optical power density) have to be used.
- the walk-off influence can be reduced if multiple 180° rotated crystals of the same kind are used, but is likely to reduce the useful lifetime of the device, because more surfaces will be exposed to the high optical power. Diffusion or adhesive-free bonding can be utilized to eliminate additional exposed crystal surfaces by seamlessly joining the 180° rotated crystals together.
- the overall conversion efficiency can reach 0.2% for a 100 W average, 25 kW peak power MOPA operating at 1:250 duty cycle (for example, 2 ns pulses at 2 MHz repetition rate, or 0.5 ns pulses at 8 MHz repetition rate), resulting in a 200 mW average power output at 198.7 nm.
- the pulsed light source 102 is a 1041 nm Yb-doped MOPA.
- This light source 102 pumps the Raman wavelength converter 106 producing its output ⁇ i o m at the 3 rd Stokes wavelength of 1228.2 nm through intermediate 1096.7 (1 st ) and 1158.7 nm (2 nd ) Stokes orders (14.63 THz step).
- the harmonic generator 110 includes 4 stages, or 3 LBO crystals HOA, HOB, HOC and a CLBO (cesium lithium borate, CsLiB 6 O 10 ) crystal HOD.
- Two LBO crystals HOA and HOB are used to consecutively produce 2 nd (614.1 nm) and 4 th (307.05 nm) harmonics of the Raman converter output by frequency doubling or SHG.
- the third LBO crystal HOC mixes the 1228.2 nm remaining light (10 to 90%, preferably 50%) at the output of the first LBO crystal HOA with its fourth harmonic (307.05 nm) from the output of the second one HOB by SFM (sum frequency mixing) to produce the light output at 245.64 nm (5 th harmonic of 1228.2 nm).
- a second high-power amplification stage 102B is added to the MOPA to generate a second output at 1041 nm (the reason for having two separate high-power stages is to keep the peak power in each one below the nonlinear distortion and damage thresholds for the amplifier fiber), which is mixed with 245.64 nm light in the CLBO crystal to finally produce the output ⁇ ou t at 198.7 nm by SFM.
- Table III provides the summary of crystal's parameters utilized in the laser system 10 of example 3.
- nonlinear crystals can be used to perform the required conversion.
- ADA ammonium dihydrogen arsenate, NH 4 H 2 AsO 4
- ADP ammonium dihydrogen phosphate, NH 4 H 2 PO 4
- FIG. 8 is more complex than that of FIG. 7. It utilizes four instead of three crystals, and the Raman converter provides the 3 rd and not 2 nd Stokes order.
- the laser system 10 of FIG. 8 has, however, one very important advantage.
- the last crystal (CLBO) is much closer to the non-critical phase matching condition and therefore, birefringent walk-off is nearly negligible.
- a high peak IR (1041 nm) power is supplied to it directly from Yb-doped MOPA. This can result in conversion efficiency in respect to UV power approaching 80%, and therefore minimum incoming UV power into the CLBO crystal will be needed to achieve the same DUV (deep UV) power output, thus minimizing optical damage to the CLBO crystal.
- optical power values as well as temperatures, phase matching angles, effective nonlinearity coefficient and birefringent walk-off values shown in FIG. 8 are given only as a guideline. Other configurations and operating temperatures may also be utilized.
- an output of 2 W at 198.7 nm can be produced from two arms of the Yb-doped MOPA delivering 100 and 32 W average power, respectively, which translates into an overall conversion efficiency of 1.5%, at least seven times higher than that of the laser system 10 of FIG. 7.
- the laser system 10 of FIG. 9 is similar to that of FIG. 8. It utilizes the same pulsed light source 102 and Raman converter 106 providing an output at the wavelength ⁇ iout of 1228.2 nm, first and last crystals HOA, HOD.
- the 3 rd harmonic (409.4 nm) of the Raman converter's 1228.2 nm output is produced by SFM mixing 2 nd harmonic (614.1 nm) and original 1228.2 nm light in the LBO crystal HOB, and the 5 th harmonic (245.64 nm ) of the 1228.2 nm light is now produced by SFM mixing 2 nd (614.1 nm) and 3 rd (409.4 nm) harmonics in the CLBO crystal HOC. Due to higher effective nonlinearity for crystals HOB and HOC in the 2 nd and 3 rd conversion stages (values shown in Fig.
- the overall conversion efficiency (predicted by numerical modeling) for the laser system 10 of this embodiment will have even higher (more than 2.1%) conversion efficiency than that of the laser system of FIG. 8, and should result in at least 3 W of 198.7 nm DUV light at the output of the 4 th stage (CLBO HOD) of the harmonic converter 110.
- Table IV provides the summary of crystal's parameters utilized in the laser system 10 of example 4.
- FIG- 10 An embodiment of a 193 nm laser system 10 is illustrated in FIG- 10.
- the laser system 10 of this embodiment includes a pulsed light source 102 comprising one master oscillator and pre-amplifier and two high power optical amplifiers More specifically, the output of the 1049.5 nm Yb-doped pre-amplifier is split and amplified separately by two high-power amplifiers.
- the first amplifier 105i pumps a Raman converter 106 ⁇ producing 2 nd Stokes signal at 1169 nm, and the second amplifier 105 2 pumps another Raman converter IO6 2 producing 1 st Stokes signal at 1106 nm.
- a single Raman converter can be designed to produce output simultaneously at 1 st and 2 nd Stokes wavelengths, of course with lower power.
- the same narrow line 1106 nm source can be used to seed the final output wavelength for second and the intermediate Stokes order for first Raman converter 106 ⁇ .
- the 2 nd harmonic of the 1169 nm light (584.5 nm) is generated in the first LBO crystal 11OA by frequency doubling (SHG), and 3 rd harmonic (389.7 nm) is generated in the second LBO crystal HOB by SFM.
- the 2 nd and 3 rd harmonics are mixed by SFM in the CLBO crystal HOC to produce the 5 th harmonic at 233.8 nm.
- the 233.8 nm light is mixed with the output of a second Raman converter IO6 2 (1106 nm) in another CLBO crystal HOD to produce 193 nm output.
- Table V provides the summary of crystal's parameters utilized in the laser system 10 of example 5.
- FIG. 11 Another embodiment of a 193 nm laser system 10 is illustrated in FIG. 11.
- the harmonic generator 110 comprises four nonlinear crystals — LBO HOA, LBO HOB, BBO HOC and CLBO HOD generating 2 nd , 3 rd , 5 th and 6 th harmonic of the 1158 nm wavelength by SHG and SFM, respectively.
- the calculated nonlinear conversion parameters are shown in Table VI. As can be seen from the table, the BBO crystal HOC operates with a significant birefringent walkoff which, depending on the power levels used, might necessitate using a short length of the crystal or elliptical beam focusing (with major axis of the ellipse along the walkoff direction), or both. Table VI provides the summary of crystal's parameters utilized in the laser system 10 of example 6. Table VI. Calculated phase matching parameters for crystals of laser system 10 shown in FIG.11
- two pulsed fiber-based MOPA sources are used — Nd-doped 102i operating at 913 run (so-called 3-level transition for the Nd ions) and Yb-doped 102 2 operating at 1081 nm.
- the Nd-doped MOPA 102i pumps the Raman converter 106 producing the first output at 952 nm (1 st Stokes order for 913 nm pump in silica fiber).
- the harmonic generator 110 comprises three nonlinear crystals.
- the first two crystals HOA and HOB are both LBO, generating respectively second (476 nm) and 4 th (238 nm) harmonic of the Raman converter 952 nm output.
- the third crystal HOC is CLBO and it performs sum frequency mixing of the 238 and 1081 nm to produce the final output at 195.1 nm.
- the calculated nonlinear conversion parameters are specified in Table VII.
- this example embodiment uses the Nd-doped fiber MOPA system operating at the 3-level transition, which is more difficult to achieve than conventional 1064 nm 4- level transition operation, it has a significant advantage in that only three nonlinear crystals are used and they all are operated at or near the non-critical phase matching (NCPM) condition.
- NCPM non-critical phase matching
- laser system 10 do not utilize OPOs, thus producing stable outputs at the desired output wavelength. Furthermore, because the output wavelength from OPO is determined by phase matching conditions, and the embodiments of laser system 10 utilize Raman wavelength converters 106, laser systems 10 disclosed above do not need to produce phase matching prior to providing light to harmonic generator/converter 110.
- a visible light source is used in the inventive method and apparatus to provide light to the frequency converter that converts it to longer optical frequency such that the frequency converter produces light output in the final output wavelength situated in the 150-400 nm range.
- the visible light sources can be either a fiber laser or an optical amplifier (laser amplifier), and both are referred to as lasers sources or lasers herein.
- the laser system 10 of this embodiment includes a light source 102 which comprises a visible fiber amplifier or a visible fiber laser.
- the light source 102 may be, for example, a "master oscillator - power amplifier” (MOPA) that comprises the initial "master oscillator” pulsed light source 202' and an amplifier 206A.
- the laser system 10 may be a CW system (FIG 13B), or it may utilize an optional optical modulator 204A driven by an electrical pulse generator 102B that provides pulse modulation of the seed light entering the amplifier 206A (FIG. IA).
- FIG. IA In the embodiment of Figs.
- the visible light fiber amplifier 206A comprises a Sm doped silica based fiber 206A'.
- the high-power optical amplifier 206A amplifies the (optionally pulsed) light 204 from the visible seed source 212.
- the frequency converter 210 is operatively coupled to the light source 202 (which in the embodiment of FIG. 13A is a pulsed light source, and is a CW source in the embodiment of FIG. 13B) to accept the light output 208A at the wavelength ⁇ iA o ut and to convert it to higher optical frequency, such that the frequency converter 210 is producing the final pulsed light output 212 at the wavelength ⁇ out situated in the 150-400 nm range.
- the light source 102 provides more than 10 W and preferably more than 2OW of optical power.
- the frequency converter 210 may be, for example, 2 nd , 3 rd , or 4 th , etc. harmonic generator, which means that it is converting the first output wavelength ⁇ i ou t to the final wavelength of ⁇ ou t — ⁇ iout /2; ⁇ i ou t /3, or ⁇ i out IA.
- fiber 206A' is operatively coupled to the pulsed light source 202', which includes the modulator 204A and the electrical pulse generator 204B.
- the high-power optical amplifier 206A amplifies the pulsed light 204 from the seed source 212, such that the average power and the peak pulse power of the pulsed light source 202 can be increased.
- the high-power optical amplifier 206A, in conjunction with the initial pulsed light source 202% form the master oscillator power amplifier (MOPA).
- MOPA master oscillator power amplifier
- the pulse width provided by the light source 202 is 0.01 to 100ns and a duty cycle of the pulse is 1:2 to 1:1000000, for example a pulse width of 0.1 to 80ns, and a duty cycle of 1 :2 to 1 : 10000.
- stimulated Brillouin scattering (SBS) in the amplifier (or laser) fiber can limit the maximum amount of power that can be converted and one has to make sure that the pump optical spectrum is broad enough to suppress SBS.
- the duty cycle (the ratio of pulse width and repetition period, which is also the ratio of average to peak power) is less than 1 :100, which, for pulses longer than 10 ns, will limit the repetition frequency to values lower than 1 MHz 5 which is not desirable if the goal is to produce a quasi-CW source.
- rare-earth (e.g., Sm) doped fiber amplifiers Due to a very long (several ms) lifetime of the excited states, rare-earth (e.g., Sm) doped fiber amplifiers essentially amplify the average power of the incoming signal, and for a very small duty cycle, an amplifier 206A with only a modest average power output can produce very large peak pulse power.
- a master oscillator 202' e.g., a directly modulated laser diode
- the average output power of power amplifier 206A is only 2 W, if the repetition rate is 100KHz (peak power is 1000Ox average power).
- Directly modulating a semiconductor laser diode with an electrical pulse generator or connecting the diode output, as described above, to a separate electro-optic intensity modulator for setting the pulse width can be used to make the initial pulsed light source 202' for generating the pulsed light.
- a rectangular pulse is preferred for maximizing the conversion efficiency (minimizing the effect of incomplete conversion in the pulse wings) and minimizing spectral broadening (by self-phase modulation (SPM)).
- the light source 202 and to some extent the whole laser system 10 can be is wavelength tunable or adjustable.
- the light source 202 includes a tunable laser for tuning the source wavelength, wherein the tuning of the source wavelength provides fine tuning or adjustment of the final output wavelength ⁇ o Ut .
- output wavelength range of approximately 575- 625 ⁇ m is directly accessible for an Sm-doped fiber laser systems.
- Fiber laser systems such as those including a silica based fiber which is Dy doped for the output in the 550-600 nm range (and pumped by 425 inn ⁇ 25 nm pump), as well as Pr and Eu- doped fiber lasers can also be utilized.
- Frequency converter 210 utilizing second, third or fourth harmonic generators can be utilized to provide the desirable output wavelength.
- output wavelengths ⁇ out can also be produced by sum frequency mixing, in a suitable nonlinear crystal, the outputs from laser 106A and the first linear crystal 210A, or a SHG can be utilized in the crystal 210B to half the wavelength of light produced by the crystal 210A.
- the first row lists the type of the nonlinear crystal(s) used and the second the type of a nonlinear process the crystal is performing.
- Rows 3-5 list the output and two input wavelengths (for the case when the nonlinear process is a second harmonic generation, the two input wavelengths are the same).
- Row 6 provides the crystal temperature and rows 7-8 provide the propagation direction angles with respect to the crystal optic axes (LBO is a so-called bi-axial crystal) required for phase matching.
- Row 9 specifies the effective nonlinearity coefficient (a measure of how efficient the conversion can be for a given input power and crystal length), and row 10 provides the input and output beam angular walk-off (slight angular separation of the input light and the harmonic light within the crystal) caused by crystal birefringence.
- a 594 nm output can be produced from Samarium or Dysprosium doped fiber lasers, and may also be provided by Praseodymium doped fluoride glass fiber lasers.
- the light source 102 comprises a Sm-doped fiber laser, pumped by a blue broad area laser BAL(s), for example a blue GaN based laser (425 nm ⁇ 50 nm output).
- BAL(s) for example a blue GaN based laser (425 nm ⁇ 50 nm output).
- other visible light fiber lasers pumped by a shorter wavelength visible light pump sources may be utilized instead of Sm doped fiber laser.
- Dy pumped, for example by a 425 nm ⁇ 25 nm souce
- Pr, or Eu doped fiber lasers 206A may also be used.
- the light source 102 may include an initial (optional oscillator or optionally pulsed) light source 202' and laser 206B operatively coupled to this light source 202'.
- the frequency converter 210 comprises a 2 nd harmonic generator based on nonlinear LBO (LiB 3 O 5 , lithium triborate) crystal 210A and a third harmonic generator based on BBO (beta barium borate, P-BaB 2 O 4 ) crystal.
- the frequency converter would include only one nonlinear crystal, such as LBO 1 IQA, and a filter for filtering out the remaining 594nm light.
- Light source 102 advantageously has narrow line width output (1-100pm) to increase conversion efficiency. It can be done, for example, by amplifying a lower power oscillator output with the high power amplifier.
- the temperature of the BBO crystal 210B is also tuned to 433 K 3 to achieve phase matching for the sum frequency mixing of the 594 nm and 297 nm light propagating at the 82.9 degrees angle to its optic axis (BBO is a uni-axial crystal).
- this embodiment minimizes the optical damage to the nonlinear crystals by high power laser beams by minimizing the number of crystal surfaces (it utilizes no more than 2 non-linear crystals) and by "trading" short wavelength power for long wavelength power (using less of the 297 nm and more of the 594 nm light power at tiie input of the BBO crystal to produce the same amount of 198 nm output).
- the optical system 10 did not utilize OPOs, thus producing a more stable output wavelength X 0 ⁇ (since the output wavelength is a harmonic of the input one and is not determined by phase matching conditions).
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Also Published As
Publication number | Publication date |
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EP2013951A4 (en) | 2011-08-03 |
WO2007127356A2 (en) | 2007-11-08 |
JP2009535666A (en) | 2009-10-01 |
WO2007127356A3 (en) | 2009-02-12 |
JP5269764B2 (en) | 2013-08-21 |
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