Tunable short-wavelength radiation

Abstract

The disclosure relates to apparatus for generating tunable short wavelength radiation, the apparatus comprising: a first cavity and a second cavity, wherein the first cavity contains: a first gain medium for providing lasing for a fundamental first wavelength in a range that includes a near-visible wavelength band, wherein the first gain medium is a vibronic solid-state gain medium, and a first nonlinear optical element that is configured for partially converting the first wavelength to a shorter second wavelength, wherein the second wavelength is in an ultraviolet, UV, wavelength band; wherein the apparatus is configured for output of a beam of radiation of the second wavelength from the first cavity to the second cavity; and wherein the second cavity contains: a second gain medium for providing lasing at a third wavelength, and a second nonlinear optical element for frequency-mixing the third wavelength with the second wavelength from the first cavity to generate a fourth wavelength, wherein the fourth wavelength is in a deep-ultraviolet, DUV, band.

Description

[0001] TUNABLE SHORT-WAVELENGTH RADIATION

[0002] Field of the Invention

[0003] The present invention relates to the generation of wavelength-tunable deepultraviolet (DUV) radiation. Merely by way of example, the invention relates to laser and nonlinear optical conversion methods and apparatus employing two or more cavities.

[0004] Background to the Invention

[0005] The ultraviolet (UV) part of the electromagnetic spectrum is defined as the 10 - 400 nm wavelength region. The UV region is often sub-divided into sub-bands of wavelength, but there are several conventions. For instance, the UV bands: UV-A (315-400 nm); UV-B (280-315 nm); and UV-C (100-280 nm) are commonly used to classify different radiological effects of UV radiation on human tissue. UV can also be sub-divided as near-UV (300-400 nm), middle-UV (200-300), far-UV (122-200 nm), and extreme-UV (10-121 nm). The region 100-200 nm has been also termed vacuum-UV (VUV) as light in this region is strongly absorbed by the oxygen in the atmosphere. The term deep-UV (DUV) is also commonly used in laser technology and does not have an exact specified wavelength range, but is often taken as wavelength less than 300 nm, for example, encompassing much of the UV-C band and where some classes of optical materials still have sufficiently low absorption necessary for the DUV laser radiation to be generated and transmitted by the laser technology.

[0006] The DUV part of the spectrum is useful in laser technology since its short wavelength enables numerous applications including photolithography, optical inspection, metrology, spectroscopy, medical, photochemistry, and material processing. Unfortunately, there are very limited laser sources available that are directly emitting in the DUV spectral region. Excimer gas lasers such as a KrF laser operating at 248 nm or an ArF laser operating at 193 nm are the main commercial laser sources directly emitting in the DUV range and can provide powerful pulsed DUV radiation, which underpins semiconductor lithography, for example, and other light-matter interactions. However, there is a problem that each excimer gas laseronly operates at a discrete wavelength with minimal capacity for wavelength-tuning. Excimer gas lasers are also bulky, have low efficiency, and use harmful gas species. There are other gas lasers (e.g. N2, He-Cd) that operate in the UV at a discrete wavelength, but at relatively low power and not in the DUV. Semiconductor (or diode) lasers based on GaN materials can also operate in the UV region with some degree of tunability, but their performance, including power and lifetime, does not extend effectively into the DUV.

[0007] Nonlinear optical frequency-shifting of ‘standard’ non-DUV lasers can be performed to generate DUV wavelengths. Standard industrial-class lasers are typically infrared solid-state lasers (e.g. Nd:YAG laser) or fibre lasers (e.g. Yb-doped fibre laser) operating at wavelengths longer than one micron in the infrared (IR) part of spectrum. Due to the long wavelength of the laser source, multiple nonlinear-optical frequency conversion steps are required, typically involving a cascaded chain of second, third, fourth or higher harmonic conversion stages, to obtain wavelengths in the DUV range. There is a problem that multiple harmonic or other nonlinear optical conversion steps, that tend to decrease in efficiency in each step, leads to low overall efficiency and therefore low output power, and for fundamental lasers with near-fixed wavelength (such as Nd:YAG) offers minimal wavelength tuning ability.

[0008] One potential method for creating tunable DUV wavelength operation from a nearfixed wavelength fundamental laser source is to introduce a nonlinear optical downfrequency conversion step using a nonlinear crystal, in a cavity arrangement known as an optical parameter oscillator (OPO), to create a tunable wavelength. However, this down-conversion step for wavelength tunability comes at some cost as the down-conversion stage, usually using a harmonic of the fundamental laser as the pump frequency, can be inefficient and produces a lower frequency that is further from the DUV range. Extra harmonic conversion or frequency-mixing stages are therefore used to reach the DUV spectral region. This further increase in the number of nonlinear steps which increases the complexity of the system and results in poorer efficiency and hence reduced DUV power, which is unacceptable when efficient, high power DUV radiation is needed.The lack of DUV laser sources, and the inability for wavelength tuning of the few available sources (such as excimer gas laser sources), means that there is a need for alternative and improved methods and apparatus for DUV radiation production.

[0009] Summary of the Invention

[0010] Aspects of the present invention are set out in the appended independent claims, while details of certain embodiments are set out in the appended dependent claims.

[0011] The present invention provides apparatus for generating tunable short wavelength radiation, as set out in claim 1 of the appended claims. Advantageously, the present invention enables tunable DUV radiation with capability for higher efficiency, and higher power scalablity, using a reduced number of steps for conversion. In other words, the apparatus and methods of the present invention enable efficient generation of high-power, wavelength-tunable, DUV radiation.

[0012] Thus, in a first aspect the invention provides apparatus for generating tunable short wavelength radiation, the apparatus comprising: a first cavity and a second cavity, wherein the first cavity contains: a first gain medium for providing lasing for a fundamental first wavelength in a range that includes a near-visible wavelength band, wherein the first gain medium is a vibronic solid-state gain medium, and a first nonlinear optical element that is configured for partially converting the first wavelength to a shorter second wavelength, wherein the second wavelength is in an ultraviolet, UV, wavelength band; wherein the apparatus is configured for output of a beam of radiation of the second wavelength from the first cavity to the second cavity; and wherein the second cavity contains: a second gain medium for providing lasing at a third wavelength, and a second nonlinear optical element for frequencymixing the third wavelength with the second wavelength from the first cavity to generate a fourth wavelength, wherein the fourth wavelength is in a deep-ultraviolet, DUV, band.

[0013] The first wavelength may be less than 800 nm, the second wavelength may be less than 400 nm, and the fourth wavelength may be less than 300 nm.The first gain medium may comprise alexandrite, CrLiCAF, CrLiSAF, or Ti:sapphire.

[0014] The first gain medium may be diode-pumped.

[0015] The first cavity may be a standing-wave cavity or a uni-directional ring cavity, and the second cavity may be a standing-wave cavity or a uni-directional ring cavity.

[0016] The first nonlinear optical element may comprise lithium triborate, LBO, beta-barium borate, BBO, or potassium dihydrogen phosphate, KDP.

[0017] The second nonlinear optical element may be configured for sum-frequency generation to generate DUV radiation, and the second nonlinear optical element may comprise lithium triborate, LBO, beta-barium borate, BBO, caesium lithium borate, CLBO, or potassium beryllium fluoroborate, KBBF.

[0018] The second gain medium may be a solid-state gain medium.

[0019] The second gain medium may be a vibronic solid-state gain medium.

[0020] The second gain medium may comprise alexandrite, CrLiCAF, CrLiSAF, or Ti:sapphire.

[0021] The second gain medium may be a fibre amplifier gain medium.

[0022] The apparatus may further comprise, in the first cavity and / or the second cavity, one or more of a birefringent tuning plate, a Fabry-Perot etalon, a diffraction grating, or a volume Bragg grating, for spectral control of the lasing wavelength and spectral narrowing.

[0023] One or both of the first cavity and the second cavity may be a uni-directional ring laser cavity, and the apparatus may comprise a Faraday rotator element and polarisation-dependent loss element for control of the uni-direction.

[0024] The uni-directional ring laser cavity may be configured for spectral control and spectral narrowing of the wavelength to produce single longitudinal mode operation.The apparatus may be configured for maintaining or varying the spectral control by adjusting the cavity length by moving a mirror.

[0025] The apparatus may comprise a Q-switching device such as a Pockels (electro-optic) cell, an acousto-optic cell, ora saturable absorber, for Q-switched pulsed operation of the first cavity and / or the second cavity.

[0026] The apparatus may comprise a modelocking modulation device based on a Pockels (electro-optic) cell, an acousto-optic cell, a saturable absorber, or a Kerr lens, for modelocking mode-locked pulsed operation of the first cavity and / or the second cavity.

[0027] The apparatus may further comprise a third cavity, wherein the apparatus is configured for output of a beam of radiation of the fourth wavelength from the second cavity to the third cavity; and wherein the third cavity contains: a third gain medium for lasing at a fifth wavelength; and a third nonlinear optical element for frequency-mixing the fifth wavelength with the fourth wavelength to generate a sixth wavelength.

[0028] In another aspect the invention provides a method of generating tunable short wavelength radiation using the apparatus according to the first aspect, wherein the method comprises: generating radiation having the first wavelength in the first cavity; partially converting the first wavelength to the second wavelength using the first nonlinear optical element; outputting the beam of radiation of the second wavelength from the first cavity to the second cavity; generating radiation having the third wavelength in the second cavity; frequency-mixing the third wavelength with the second wavelength in the second cavity to generate the fourth wavelength; and outputting the fourth wavelength from the second cavity.

[0029] The method may comprise adjusting the lasing wavelength of the first cavity using a spectral control element that is internal to, or part of, the first cavity.

[0030] The method may comprise narrowing the lasing bandwidth of the first cavity using a spectral control element that is internal to, or part of, the first cavity.

[0031] The lasing wavelength of the first cavity may be a single longitudinal mode.The method may comprise adjusting the lasing wavelength of the second cavity using a spectral control element that is internal to, or part of, the second cavity.

[0032] The method may comprise narrowing the lasing bandwidth of the second cavity using a spectral control element that is internal to, or part of, the second cavity.

[0033] The lasing wavelength of the second cavity may be a single longitudinal mode.

[0034] The method may comprise generating the second harmonic generation of the fundamental cavity lasing radiation at the first wavelength using the first nonlinear optical element.

[0035] Lasing in both the first and second cavities may be in a pulsed mode.

[0036] The pulsed mode of the first and second cavities may be by Q-switching at the same pulse rate and synchronised for temporal overlap of the output of the first cavity with the second cavity lasing in the second nonlinear optical element.

[0037] The pulsed mode of the first and second cavities may be by modelocking at the same pulse rate and synchronised for temporal overlap of the output of the first cavity with the second cavity lasing in the second nonlinear optical element.

[0038] The first wavelength may be approximately 744 nm, the second wavelength may be approximately 372 nm, the third wavelength may be approximately 744 nm, and the fourth wavelength may be approximately 248 nm.

[0039] Apparatus and methods of the present disclosure also provide methods for spectral selection of the two cavities to provide wavelength tuning of the generated DUV, and pulsed operation for increasing the peak power in the cavities to enhance the efficiency of the nonlinear frequency conversion steps.

[0040] It will be apparent that the present invention provides manifold and substantive benefits over previous methods for DUV generation. A major benefit of the apparatus and methods of the present disclosure is the ability to achieve wavelength-tunable DUV with just a two-step dual cavity system that is highly efficient due to the cavity enhancement of the nonlinear conversion steps and the power scalability that arises from the use of a vibronic solid-state gain medium inthe first cavity and the added power provided by the additional gain medium in the second cavity. The wide bandwidth tunability of a near-visible vibronic gain medium in the first cavity and the potential for tunability of the gain medium in the second cavity provides flexible options for wide wavelength selection and tuning options. The provision of an efficient, high-power, and widely wavelength-tunable DUV source fulfils a significant scientific and industrial need in this difficult DUV wavelength region that is currently poorly addressed by suitable high power tunable laser sources.

[0041] In another aspect the invention provides apparatus for generating tunable short wavelength radiation, the apparatus comprising: a first cavity and a second cavity, wherein the first cavity contains: a first gain medium for providing lasing for a fundamental first wavelength in a range that includes a near-visible wavelength band, wherein the first gain medium is a vibronic solid-state gain medium, and a first nonlinear optical element that is configured for partially converting the first wavelength to a shorter second wavelength, wherein the second wavelength is in an ultraviolet, UV, wavelength band; wherein the apparatus is configured for output of a beam of radiation of the second wavelength from the first cavity to the second cavity; and wherein the second cavity contains: a second gain medium for providing lasing at a third wavelength, and a second nonlinear optical element for frequencymixing the third wavelength with the second wavelength from the first cavity to generate a fourth wavelength, wherein the fourth wavelength is less than 300 nm.

[0042] The fourth wavelength may be greater than or equal to 10 nm and less than 300 nm.

[0043] The first wavelength may be 744 nm, the second wavelength may be 372 nm, the third wavelength may be 744 nm, and the fourth wavelength may be 248 nm.

[0044] In another aspect the invention provides apparatus for generating tunable short wavelength radiation, the apparatus comprising: a first cavity and a second cavity, wherein the first cavity contains: a first gain medium for providing lasing for a fundamental first wavelength that is less than 800 nm, wherein the first gain medium is a vibronic solid-state gain medium, and a first nonlinear optical element that is configured for partially converting the first wavelength to a shorter secondwavelength, wherein the second wavelength is in an ultraviolet, UV, wavelength band; wherein the apparatus is configured for output of a beam of radiation of the second wavelength from the first cavity to the second cavity; and wherein the second cavity contains: a second gain medium for providing lasing at a third wavelength, and a second nonlinear optical element for frequency-mixing the third wavelength with the second wavelength from the first cavity to generate a fourth wavelength, wherein the fourth wavelength is less than 300 nm.

[0045] In another aspect the invention provides apparatus for generating tunable short wavelength radiation, the apparatus comprising: a first cavity and a second cavity, wherein the first cavity contains: a first gain medium for providing lasing for a fundamental first wavelength that is less than 800 nm, wherein the first gain medium is a vibronic solid-state gain medium, and a first nonlinear optical element that is configured for partially converting the first wavelength to a shorter second wavelength, wherein the second wavelength is less than 400 nm; wherein the apparatus is configured for output of a beam of radiation of the second wavelength from the first cavity to the second cavity; and wherein the second cavity contains: a second gain medium for providing lasing at a third wavelength, and a second nonlinear optical element for frequency-mixing the third wavelength with the second wavelength from the first cavity to generate a fourth wavelength, wherein the fourth wavelength is less than 300 nm.

[0046] Brief Description of the Drawings

[0047] Embodiments of the invention will now be described, by way of example only, and with reference to the drawings in which:

[0048] Figure 1 illustrates a previous method for generating DUV laser radiation using a fundamental laser with near-fixed wavelength;

[0049] Figure 2 illustrates a previous method for generating tunable DUV laser radiation;

[0050] Figure 3 shows a simplified schematic illustration of an example comprising a first ring cavity and a second ring cavity to achieve efficient generation of DUV radiation with wavelength tunability;Figure 4 shows a simplified schematic illustration of an example comprising a first standing-wave cavity and a second standing-wave cavity to achieve efficient generation of DUV radiation with wavelength tunability;

[0051] Figure 5 shows a simplified schematic illustration of apparatus for controlling spectral and temporal form and lasing directionality;

[0052] Figure 6 shows a simplified schematic illustration of an example in which the first and second cavities are augmented by a third cavity in the manner of the second cavity to further extend the DUV wavelength tuning range;

[0053] Figure 7 shows a simplified schematic illustration of an example comprising a first cavity system producing wavelength-tunable UV output and a second cavity system that converts the output of the first cavity to produce a wavelength-tunable DUV output; and

[0054] Figure 8 shows a simplified schematic illustration of exemplary apparatus for implementing methods of the present disclosure.

[0055] In the figures, like elements are indicated by like reference numerals throughout.

[0056] Detailed Description of Preferred Embodiments

[0057] Fig. 1 shows a simplified schematic illustration of a previous implementation for DUV generation but with little to no wavelength tunability, useful for understanding the improved apparatus and methods of the present disclosure. The apparatus comprises a laser 100 having a near-fixed wavelength with severely limited wavelength tunability. The laser may be, for example, the industrial and scientific standard Nd:YAG laser operating with a near-infrared wavelength of approximately 1064 nm, with narrow gain bandwidth with less than 1 nm wavelength tuning range. A first beam 150 output from the laser 100 is incident on a nonlinear material 110, which is usually a birefringent crystal that can perform the operation of second harmonic generation (SHG) that converts some of the incident laser radiation to produce a second beam 160 at a second wavelength with twice the frequency (and half the wavelength) of the incident beam 150. For the case of Nd:YAG at 1064 nm, this second wavelength is 532 nm in the green part of the spectrum. The secondbeam 160 at this second wavelength enters a second nonlinear medium 120, which may also be a birefringent nonlinear crystal, which produces second harmonic generation of this wavelength to produce a third beam 170 which for Nd:YAG is at 266 nm in the DUV and is at the fourth harmonic (four times the frequency, quarter of the wavelength) of the fundamental laser 100. Further conversion is possible by introducing a third nonlinear optical medium 130, that produces sum-frequency mixing of the fourth harmonic wavelength in the third beam 170 with the residual part of fundamental wavelength remaining in the first beam 150 to produce a fourth beam 180 at the fifth harmonic (five times the frequency, fifth of the wavelength of the fundamental laser), which for Nd:YAG is at DUV wavelength 213 nm. Since the nonlinear optical frequency conversion process in each of these steps requires high intensity to produce efficient conversion of the order of ten(s) of percent, it is usual to run the laser 100 in a pulsed mode (e.g. Q-switched) to increase its peak output power and further to use focusing optics to produce small focal area and obtain high peak intensity in the nonlinear materials 110, 120, and 130. This implementation has two disadvantages. Firstly, the multiple nonlinear optical conversion steps, each with limited and often decreasing efficiency, compound to produce poor overall efficiency. Secondly, due to the near-fixed wavelength of the input laser 100, the generated DUV is also near-fixed in wavelength, which severely limits the application possibilities of the DUV source.

[0058] Fig. 2 shows a simplified schematic illustration of a further previous implementation that attempts to generate tunable wavelength DUV, useful for understanding the improved apparatus and methods of the present disclosure. In this example there are similar elements as in the example of Fig. 1 , with a near-fixed wavelength laser 100 and nonlinear optical elements 110, 120 and 130 for harmonic frequency conversion, but intermediate is a further nonlinear optical system 210 that is in the form of an optical parametric oscillator (OPO) where difference-frequency mixing is performed in a nonlinear crystal contained in a cavity with mirrors reflective for (at least) one of the two generated down-converted frequencies, to produce a beam 220 that is lower frequency (and longer wavelength) but can be tunable in wavelength. In one example, the input to the OPO 210 is the second beam 160 produced by second harmonic generation of the first beam 150 (the fundamentallaser beam) and is commonly referred to as the ‘pump beam’ (having a pump frequency coP) and by frequency-mixing in the nonlinear crystal the pump light is converted into a pair of beams known as the signal beam (at signal frequency cos) and the idler beam (at idler frequency Wi) and where:

[0059] COp = Ws + COi,

[0060] for photon energy conservation.

[0061] The frequency split between signal and idler is determined by a process known as phase-matching, achieved by angle or temperature adjustment of the nonlinear crystal to tune the wavelength of the output signal beam 220, that can subsequently be converted by further nonlinear crystals 120 and 130 to DUV, as is also the case in the example of Figure 1 , but now with wavelength tunability DUV. For the case of near-fixed wavelength Nd:YAG at 1064 nm for the first beam 150, its second harmonic produced by the nonlinear material 110 is at wavelength 532 nm (for the second beam 160). The second beam 160 is input to the OPO element 210 and the output 220 of the OPO can be tuned in the near-infrared wavelength range (e.g. from 700 - 900 nm), and using further second harmonic generation in the further nonlinear element 120 converted to tunable UV-blue of 350 - 450 nm (for the third beam 170), which is then converted by the further nonlinear element 130 in a further second harmonic step to produce tunable DUV of 175 -225 nm output radiation (for the final output beam 180). Whilst this method can produce tunable DUV, in practice there is a problem that the large number of conversion steps involved in this method, including the added complexities of the OPO stage 210, make this approach too inefficient and limited in DUV power.

[0062] Improved apparatus and methods for generating DUV radiation will now be described with reference to Figs. 3 to 8.

[0063] Fig. 3 shows a simplified schematic illustration of an example comprising a first ring cavity 300 and a second ring cavity 400 for achieving efficient generation of DUV radiation, with wavelength tunability. The first ring cavity 300 produces a wavelength-tunable UV output 350, and the second ring cavity 400 converts the output of the first cavity 300 to produce a wavelength-tunable DUV output 360.The first cavity 300 contains a laser gain element 310 that is a vibronic solid-state medium, which can provide gain in a widely tunable wavelength band (typically around 100 nm, or more) that encompasses the near-visible spectral region of less than 800 nm that can be directly converted to UV of less than 400 nm by second harmonic generation. The tunable vibronic solid-state laser gain medium for the laser gain element 310 may be, for example, alexandrite, chromium-doped LiCAF, chromium-doped LiSAF, or titanium-doped sapphire, or any other suitable gain medium. For high efficiency of operation, these gain media benefit from being able to be optically-pumped by efficient semiconductor diode lasers. The gain element 310 is incorporated into a cavity 300 which, by way of example, may be a ring cavity comprising mirrors 320a, 320b, 320c, and 320d that allow laser operation at a first wavelength in the band of the gain medium 310. The cavity 300 further contains a nonlinear optical material 340 that converts the first wavelength of the laser cavity radiation into a second wavelength that includes the UV band of less than 400nm. The nonlinear optical material 340 may be a single nonlinear crystal providing second harmonic generation of the fundamental laser radiation. The nonlinear crystal for second harmonic generation in this wavelength range may be, for example, lithium triborate (LBO), beta-barium borate (BBO), or potassium dihydrogen phosphate (KDP). The frequency-converted second wavelength that is produced is extracted as an output beam 350, in this example via a mirror 320d that provides transmission at the frequency-converted wavelength band. The first cavity may also contain additional elements 330, such as elements for spectral tuning and spectral narrowing of the lasing wavelength, elements for producing pulsed operation of the fundamental laser cavity that can provide higher peak power for enhancing frequency conversion in the nonlinear optical element 340, and elements for forcing uni-directional lasing in the case of a ring laser cavity.

[0064] In this example, the tunable second wavelength output beam 350 from first cavity 300 is directed into a second cavity 400. The second cavity 400 comprises, in this example, a ring cavity formed by mirrors 420a, 420b, 420c, and 420d. The second cavity 400 contains a laser gain element 410 and a nonlinear optical element 440. The gain element 410 in the second cavity 400 provides laser oscillation at a third wavelength, within the gain band of the laser gain element 410. The nonlinearoptical element 440 is used to generate frequency mixing of the laser radiation at the third wavelength of this second cavity 400 with the input tunable second wavelength beam 350 from the first cavity 300 to create a fourth wavelength tunable DUV output beam 360, in this example via mirror 420d of the second cavity 300 which has transmission at the DUV wavelength. The nonlinear optical element 440 may be lithium triborate (LBO), beta-barium borate (BBO), caesium lithium borate (CLBO), potassium beryllium fluoroborate (KBBF), or any other suitable nonlinear optical element 440 for frequency conversion to DUV. The gain element 410 in the second cavity 400 may be a tunable laser medium, such as another vibronic solid-state gain medium, and provides capacity for further wavelength tuning of the output DUV radiation 360. Alternatively, the gain element 410 may be a narrowband gain medium such as a solid-state gain medium with near-fixed laser wavelength, with the wavelength tuning of DUV output 360 provided predominantly by wavelength tuning of output beam 350 of the first cavity 300. The gain medium 410 may also be a fibre amplifier with some capacity for wavelength tuning. As with the first cavity 300, the second cavity 400 may contain further cavity elements 430 such as elements for spectral tuning and spectral narrowing, elements for pulsed laser operation, and elements for uni-directional operation in the case of the second cavity 400 being a ring laser cavity.

[0065] It will be appreciated, therefore, that the first cavity 300 contains a laser gain medium 310 and a nonlinear optical element 340, wherein the gain medium 310 is a vibronic solid-state gain medium which can provide gain for laser action at a first wavelength, in a band that encompasses the near-visible spectral region (e.g. of less than 800 nm). The intracavity nonlinear optical element 340 converts this first lasing wavelength to a second, shorter, wavelength that encompasses the UV band (e.g. of less than 400 nm). The second cavity 400 also contains a gain medium 410 and a nonlinear optical element 440, wherein the gain medium 410 provides gain for laser action at a third wavelength that frequency mixes in the intracavity nonlinear optical element 440 with the second wavelength output of the first cavity, to produce a fourth wavelength that encompasses the DUV band (e.g. less than 300 nm).The configuration illustrated in Fig. 3 results in many advantages compared to the configuration illustrated in Fig. 2. The combination of two cavities 300, 400 in this example provides tunable DUV radiation, replacing the multiple steps of the previous method of Fig. 2. By virtue of the use of a near-visible vibronic solid-state gain medium in the first cavity 300, there is less frequency multiplication required in the nonlinear conversion required to produce DUV, and the DUV can be wavelength-tunable. Due to the intensity enhancement of the laser radiation inside both the first and second laser cavities 300 and 400, the overall conversion efficiency is very high, with potential for near unity frequency conversion in the first cavity 300 compared to what the fundamental laser output would have been without the nonlinear element 340 and optimum output coupling, and high DUV nonlinear conversion in the second cavity 400. The vibronic solid-state laser element 310 in the first cavity 300 is power scalable and with the additional gain medium 410 in the second cavity 400 can provide further power scaling. Advantageously, the apparatus provides flexible and wide wavelength tunability of DUV generation by combining the wide-tunability of the vibronic gain medium 310 in the first cavity 300 and options for further tunability with the second cavity 400, as both cavities 300,400 can be independently controlled in terms of different options for gain medium and separate spectral control.

[0066] Fig. 4 shows a simplified schematic illustration of an example comprising a first standing-wave cavity and a second standing-wave cavity for achieving efficient generation of DUV radiation with wavelength tunability. As illustrated in Fig. 4, in this example the apparatus comprises a first cavity 301 producing wavelength-tunable UV output 350 and a second cavity 401 that converts the output of the first cavity 301 to produce a wavelength-tunable DUV output 360. In this example, a tunable vibronic solid-state laser gain medium 310 is provided in the first cavity 301 , which can provide gain in a widely tunable wavelength band that includes the near-visible spectral region (< 800 nm) that can be directly converted to UV (< 400 nm) by second harmonic generation. The gain element 310 is incorporated into the cavity 301, which in this example is a standing-wave cavity formed using mirrors 321a, 321b, 321c that allows laser operation at the wavelength band of the gain medium 310. It will be appreciated that a standing-wave cavity may be formed usingany suitable configuration in which the laser radiation propagates in both directions in the cavity, whether linear, folded or ring. The cavity 301 further contains a nonlinear optical material (or set of nonlinear optical materials) 340 that converts the cavity fundamental laser radiation to tunable radiation at a second wavelength that includes the UV band (<400 nm) and provides an output beam 350 via a mirror 321c that provides transmission at the frequency-converted second wavelength band. Element 340 may be a single nonlinear crystal providing second harmonic generation of the fundamental laser radiation. The cavity 301 may further contain other elements 330 such as elements for spectral tuning and spectral narrowing of the lasing wavelength, and elements for pulsed operation of the fundamental laser that can provide higher peak power for enhancing frequency conversion in nonlinear optical element 340. In this example, the second wavelength output beam 350 from cavity system 301 is directed into a second cavity 401 which is another standingwave cavity formed by mirrors 421a, 421b, 421c and contains a laser gain element 410 and a nonlinear optical element 440. The gain element 410 in this second cavity 401 produces laser oscillation at a third wavelength within the gain band of the gain element 410. The nonlinear optical element 440 is used to generate frequency mixing of the third wavelength laser radiation of the second cavity 401 with the input second wavelength beam 350 from the first cavity 301 , to create a fourth wavelength DUV output beam 360 that is output via a mirror 421b of the second cavity 401 having transmission at the DUV wavelength. As with the first cavity 301 , the second cavity 401 may contain further cavity elements 430 such as elements for elements for spectral tuning and spectral narrowing and elements for pulsed laser operation.

[0067] The benefits of the configuration illustrated in Fig. 4 of high efficiency and tunability of the generated DUV radiation are the same as for the configuration illustrated in Fig. 3, but the alternative configuration of Fig. 4 illustrates that the advantages of the present disclosure are not limited to the example shown in Fig. 3. It will be appreciated that there are other similar cavity designs and modifications that could also be used. For example the first cavity may be a uni-directional ring cavity and the second cavity be a standing-wave cavity, or vice versa, the first cavity may be a standing-wave cavity and the second cavity may be a uni-directional ring cavity.The output DUV beam 360 in Fig. 4 is output via a mirror 421b after a single pass of the nonlinear optical element 440 but mirror 421b could be reflective at the DUV wavelength, producing its double-pass through nonlinear element 440. In this case, the DUV output from the second cavity 401 would be from mirror 421c with an intermediate optic (e.g. 45° mirror) placed in path between the first cavity 301 and the second cavity 401 for transmitting the second wavelength beam 350 of first cavity 301 and reflecting fourth wavelength DUV light to extract a useable DUV beam.

[0068] Fig. 5 shows a simplified schematic illustration of apparatus for controlling spectral and temporal form and lasing directionality, illustrating the further elements 330, 430 that may be provided in the first and second cavities. For the first cavity, these further elements 330 may include one or more spectral tuning and narrowing elements 332, one or more elements for pulsed laser operation 334, and / or one or more elements for uni-directional operation 336 to provide spectral, temporal and directional control of the oscillating laser cavity radiation at the first wavelength 315. The spectral control may be performed using any suitable number of elements. For example, the spectral tuning and narrowing elements 332 may comprise one or more of a birefringent tuning plate, Fabry-Perot etalon, diffraction grating, or volume Bragg grating. The laser cavity may be a uni-directional ring cavity operating in a single longitudinal mode, and its single wavelength can be maintained or adjusted by changing the round-trip length of the cavity with active adjust of the position of one of the cavity mirrors. Uni-directional operation of a ring cavity can be performed where element 336 includes a Faraday rotation element in combination with a polarisation loss element to favour single direction of the oscillation of the cavity radiation 315. Pulsed operation can be performed where element 334 is a Q-switching device such as a Pockels (electro-optic) cell, an acousto-optic cell, or a saturable absorber to provide time-dependent switching of the loss of the cavity radiation 315. Alternatively, the element(s) for pulsed laser operation 334 could be a modelocking device providing periodic modulation of the cavity radiation 315, such as with an electro-optic or acousto-optic modulator or saturable absorber such as a semiconductor saturable-absorber mirrors (SESAM) device or configuring the cavity to use the nonlinear optical Kerr effect in the gain medium itself as the modulationto perform Kerr lens modelocking. The same set of elements 330 may be introduced into the second cavity 400 of Fig. 3, where 315 would be radiation at the lasing third wavelength of the second cavity to perform spectral, pulsed, and uni-directional operation of a ring cavity configuration.

[0069] Fig. 6 illustrates a configuration in which the first cavity 300 and second cavity 400 of the example of Fig. 3 are augmented with a third cavity 500 that is similar to the second cavity 400. The third cavity 500 may be a ring cavity formed by mirrors 520a, 520b, 520c, 520d and containing a laser gain element 510 and a nonlinear optical element 540. The gain element 510 in the third cavity 500 provides laser oscillation at a fifth wavelength within the gain band of the gain element 510. The nonlinear optical element 540 is used to generate frequency mixing of the laser radiation at the fifth wavelength of the third cavity 500 with the input tunable fourth DUV wavelength beam 360 from the second cavity 400 to create a sixth wavelength tunable DUV output beam 370 via a mirror 520d of the third cavity 500 having transmission at the DUV wavelength. As with the first and second cavities 300, 400, the third cavity 500 may contain additional cavity elements 530 such as elements for spectral tuning and spectral narrowing, elements for pulsed laser operation, and elements for uni-directional operation in the case of the third cavity being a ring laser cavity.

[0070] It will be appreciated that various modifications could be made to the examples illustrated in Figs. 3 to 6 whilst still obtaining advantages of the invention. For example, additional optical components such as lenses can be incorporated inside the cavities, and / or between the cavities. The elements forming the laser cavities may be high reflectivity mirrors and output to a coupling mirror with transmission for the frequency converted light, but other elements, that are well known to the skilled reader and so will not be described in detail here, could be used as feedback elements for the light to form the cavity. For example, the end face of one of the gain media could be made reflective to form a mirror for cavity formation without need of a separate mirror element.

[0071] Further ConfigurationFig. 7 shows a simplified schematic illustration of a further example comprising a first cavity 300 producing wavelength-tunable UV output and a second cavity 402 that converts the output of the first cavity to produce a wavelength-tunable DUV output 360.

[0072] The first cavity 300 contains a laser gain element 310 that is a vibronic solid-state medium which can provide gain in a widely tunable wavelength band that encompasses the near-visible spectral region (e.g. less than 800 nm) and is incorporated inside the cavity 300. In this example the first cavity 300 is a ring cavity formed using mirrors 320a, 320b, 320c, 320d that allows laser operation at a first wavelength in the band of the gain medium 310. The cavity further includes a nonlinear optical material 340 that converts the first wavelength of the laser cavity radiation into a second wavelength that includes the UV band (e.g. less than 400 nm) and that is extracted as an output beam 350, in this example via mirror 320d that provides transmission at the second wavelength. The first cavity 300 may benefit from containing other elements 330 such as elements for spectral tuning and spectral narrowing of the lasing wavelength, elements for producing pulsed operation of the fundamental laser cavity that can provide higher peak power for enhancing frequency conversion in nonlinear optical element 340, and elements for enforcing uni-directional lasing in the case of a ring laser cavity.

[0073] In this example, the tunable second wavelength output beam 350 from the first cavity 300 is directed into a second cavity 402 that in this example is (but need not necessarily be) a ring cavity formed using mirrors 420a, 420b, 420c, 420d and contains a nonlinear optical element 440. Different from the example illustrated in Fig. 3, in this example the second cavity 402 does not contain a gain element 410. The second cavity 401 is made resonant with the second wavelength beam 350 from the first cavity, which can be accomplished by adjustment of the cavity length, for instance by position adjustment of one of the cavity mirrors 420a, 420b, 420c, 420d. The nonlinear optical element 440 is used to generate second harmonic generation of the input tunable second wavelength beam 350 from the first cavity 300 to create a third wavelength tunable DUV output beam 360, in this example via a mirror 420d of the second cavity having transmission at the DUV wavelength.Exemplary Apparatus

[0074] Fig. 8 shows a simplified schematic illustration of exemplary apparatus 800 for performing methods of the present disclosure, for example to control the operation of the lasers and generation of the DUV radiation.

[0075] It will be appreciated that the apparatus 800 may comprise any suitable computer or server, for example. As shown, the apparatus 800 includes a communication interface 805 which is operable to transmit signals to and receive signals from other devices via a network 806. For example, the apparatus 800 may output a control signal to a laser via the network 806. It will be appreciated that when the apparatus 800 is connected to a network 806, the apparatus 800 of Fig. 8 need not necessarily be co-located with the apparatus illustrated in Figs. 3 to 7.

[0076] The apparatus 800 may also comprise a user interface 807. The user interface 807 may comprise a display, keyboard, mouse, or any other suitable form of user interface. The user interface 807 may be configured for receiving a user input for control of a laser, for example.

[0077] A controller 801 controls the overall operation of the apparatus 800 in accordance with software stored in a memory 802, for example to perform control of one or more lasers to perform any of the methods described above. The software may be preinstalled in the memory 802 and / or may be downloaded via the network 806 or from a removable data storage device (RMD), for example. The software includes, among other things, an operating system 803 and a DUV generation control module 804. DUV generation control module 804 is operable to perform control of the lasers according to any of the examples described above, to generate the DUV radiation.

[0078] The apparatus 800 has been described for ease of understanding as having a number of discrete modules. Whilst these modules may be provided in this way for certain applications, for example where an existing system has been modified to implement the invention, in other applications, for example in systems designed with the inventive features in mind from the outset, these modules may be built into the overall operating system or code and so these modules may not be discernible as discrete entities. These modules may also be implemented in software, hardware,firmware, or a mix of these. As those skilled in the art will appreciate, the software modules may be provided in compiled or un-compiled form and may be supplied to the apparatus 800 as a signal over a computer network, or on a recording medium. Further, the functionality performed by part or all of this software may be performed using one or more dedicated hardware circuits. However, the use of software modules is preferred as it facilitates the updating of the apparatus 800 in order to update the functionalities.

[0079] The controller 801 may comprise any suitable form of processing circuitry including (but not limited to), for example: one or more hardware implemented computer processors; microprocessors; central processing units (CPUs); graphics processing units (GPUs); arithmetic logic units (ALUs); input / output (IO) circuits; internal memories I caches (program and / or data); processing registers; communication buses (e.g. control, data and / or address buses); direct memory access (DMA) functions; hardware or software implemented counters, pointers and / or timers; and / or the like.

[0080] Modifications and Alternatives

[0081] Detailed embodiments and some possible alternatives have been described above. As those skilled in the art will appreciate, a number of modifications and further alternatives can be made to the above embodiments whilst still benefiting from the inventions embodied therein. It will therefore be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto.

[0082] Various other modifications will be apparent to those skilled in the art and will not be described in further detail here.

[0083] The present disclosure also includes the following numbered clauses:

[0084] 1. Apparatus for generating tunable short wavelength radiation, the apparatus comprising:

[0085] a first cavity and a second cavity,

[0086] wherein the first cavity contains:a gain medium for providing lasing for a fundamental first wavelength in a range that includes a near-visible wavelength band, wherein the gain medium is a vibronic solid-state gain medium, and

[0087] a first nonlinear optical element that is configured for partially converting the first wavelength to a shorter second wavelength, wherein the second wavelength is in an ultraviolet, UV, wavelength band;

[0088] wherein the apparatus is configured for output of a beam of radiation of the second wavelength from the first cavity to the second cavity;

[0089] wherein the second cavity is configured to be resonant with the second wavelength; and

[0090] wherein the second cavity contains a second nonlinear optical element that is configured for second harmonic generation of the beam of radiation of the second wavelength, to generate a tunable DUV output beam having a third wavelength. 2. The apparatus according to clause 1, wherein the second cavity has an adjustable length.

[0091] 3. The apparatus according to clause 2, wherein the apparatus comprises one or more mirrors, wherein second cavity is formed using the one or more mirrors, and wherein the length of the second cavity is adjustable by adjusting a position of the one or more mirrors.

[0092] 4. The apparatus according to any preceding clause, wherein the first wavelength is less than 800 nm, the second wavelength is less than 400 nm, and the third wavelength is less than 300 nm.

[0093] 5. The apparatus according to any preceding clause, wherein the gain medium comprises alexandrite, CrLiCAF, CrLiSAF, orTi:sapphire.

[0094] 6. The apparatus according to any preceding clause, wherein the gain medium is diode-pumped.

[0095] 7. The apparatus according to any preceding clause, wherein

[0096] the first cavity is a standing-wave cavity or a uni-directional ring cavity, and1

[0097] the second cavity is a standing-wave cavity or a uni-directional ring cavity.

[0098] 8. The apparatus according to any preceding clause, wherein the first nonlinear optical element comprises lithium triborate, LBO, beta-barium borate, BBO, or potassium dihydrogen phosphate, KDP.

[0099] 9. The apparatus according to any preceding clause, wherein the second nonlinear optical element comprises lithium triborate, LBO, beta-barium borate, BBO, caesium lithium borate, CLBO, or potassium beryllium fluoroborate, KBBF.

[0100] 10. The apparatus according to any preceding clause, wherein the apparatus further comprises, in the first cavity and / or the second cavity, one or more of a birefringent tuning plate, a Fabry-Perot etalon, a diffraction grating, or a volume Bragg grating, for spectral control of the lasing wavelength and spectral narrowing.

[0101] 11. The apparatus according to any preceding clause, wherein one or both of the first cavity and the second cavity is a uni-directional ring laser cavity, and wherein the apparatus comprises a Faraday rotator element and polarisation-dependent loss element for control of the uni-direction.

[0102] 12. The apparatus according to clause 11, wherein the uni-directional ring laser cavity is configured for spectral control and spectral narrowing of the wavelength to produce single longitudinal mode operation.

[0103] 13. The apparatus according to any preceding clause, wherein the apparatus comprises a Q-switching device such as a Pockels (electro-optic) cell, an acoustooptic cell, or a saturable absorber, for Q-switched pulsed operation of the first cavity and / or the second cavity.

[0104] 14. The apparatus according to any preceding clause, wherein the apparatus comprises a modelocking modulation device based on a Pockels (electro-optic) cell, an acousto-optic cell, a saturable absorber, or a Kerr lens, for modelocking mode-locked pulsed operation of the first cavity and / or the second cavity.

[0105] 15. A method of generating tunable short wavelength radiation using the apparatus of any one of clauses 1 to 14, wherein the method comprises:

[0106] generating radiation having the first wavelength in the first cavity;partially converting the first wavelength to the second wavelength using the first nonlinear optical element;

[0107] outputting the beam of radiation of the second wavelength from the first cavity to the second cavity;

[0108] generating the tunable DUV output beam having the third wavelength using the second nonlinear optical element; and

[0109] outputting the tunable DUV output beam from the second cavity.

[0110] 16. The method according to clause 15, wherein the method comprises adjusting the lasing wavelength of the first cavity using a spectral control element that is internal to, or part of, the first cavity.

[0111] 17. The method according to clause 15 or 16, wherein the method comprises narrowing the lasing bandwidth of the first cavity using a spectral control element that is internal to, or part of, the first cavity.

[0112] 18. The method according to any one of clauses 15 to 17, wherein the lasing wavelength of the first cavity is a single longitudinal mode.

[0113] 19. The method according to any preceding clause, wherein the first wavelength is approximately 744 nm, and the third wavelength is approximately 248 nm.

Claims

24Claims1. Apparatus for generating tunable short wavelength radiation, the apparatus comprising:a first cavity and a second cavity,wherein the first cavity contains:a first gain medium for providing lasing for a fundamental first wavelength in a range that includes a near-visible wavelength band, wherein the first gain medium is a vibronic solid-state gain medium, anda first nonlinear optical element that is configured for partially converting the first wavelength to a shorter second wavelength, wherein the second wavelength is in an ultraviolet, UV, wavelength band;wherein the apparatus is configured for output of a beam of radiation of the second wavelength from the first cavity to the second cavity;and wherein the second cavity contains:a second gain medium for providing lasing at a third wavelength, and a second nonlinear optical element for frequency-mixing the third wavelength with the second wavelength from the first cavity to generate a fourth wavelength, wherein the fourth wavelength is in a deep-ultraviolet, DUV, band.

2. The apparatus according to claim 1 , wherein the first wavelength is less than 800 nm, the second wavelength is less than 400 nm, and the fourth wavelength is less than 300 nm.

3. The apparatus according to claim 1 or 2, wherein the first gain medium comprises alexandrite, CrLiCAF, CrLiSAF, orTksapphire.

4. The apparatus according to any preceding claim, wherein the first gain medium is diode-pumped.

5. The apparatus according to any preceding claim, whereinthe first cavity is a standing-wave cavity or a uni-directional ring cavity, and the second cavity is a standing-wave cavity or a uni-directional ring cavity.

6. The apparatus according to any preceding claim, wherein the first nonlinear optical element comprises lithium triborate, LBO, beta-barium borate, BBO, or potassium dihydrogen phosphate, KDP.

7. The apparatus according to any preceding claim,wherein the second nonlinear optical element is configured for sumfrequency generation to generate DUV radiation, andwherein the second nonlinear optical element comprises lithium triborate, LBO, beta-barium borate, BBO, caesium lithium borate, CLBO, or potassium beryllium fluoroborate, KBBF.

8. The apparatus according to any preceding claim, wherein the second gain medium is a solid-state gain medium.

9. The apparatus according to any preceding claim, wherein the second gain medium is a vibronic solid-state gain medium.

10. The apparatus according to claim 9, wherein the second gain medium comprises alexandrite, CrLiCAF, Or: USAF, orTksapphire.

11. The apparatus according to any one of claims 1 to 7, wherein the second gain medium is a fibre amplifier gain medium.

12. The apparatus according to any preceding claim, wherein the apparatus further comprises, in the first cavity and / or the second cavity, one or more of a birefringent tuning plate, a Fabry-Perot etalon, a diffraction grating, or a volume Bragg grating, for spectral control of the lasing wavelength and spectral narrowing.

13. The apparatus according to any preceding claim, wherein one or both of the first cavity and the second cavity is a uni-directional ring laser cavity, and wherein the apparatus comprises a Faraday rotator element and polarisation-dependent loss element for control of the uni-direction.

14. The apparatus according to claim 13, wherein the uni-directional ring laser cavity is configured for spectral control and spectral narrowing of the wavelength to produce single longitudinal mode operation.

15. The apparatus according to claim 14, wherein the apparatus is configured for maintaining or varying the spectral control by adjusting the cavity length by moving a mirror.

16. The apparatus according to any preceding claim, wherein the apparatus comprises a Q-switching device such as a Pockels (electro-optic) cell, an acoustooptic cell, or a saturable absorber, for Q-switched pulsed operation of the first cavity and / or the second cavity.

17. The apparatus according to any preceding claim, wherein the apparatus comprises a modelocking modulation device based on a Pockels (electro-optic) cell, an acousto-optic cell, a saturable absorber, or a Kerr lens, for modelocking mode-locked pulsed operation of the first cavity and / or the second cavity.

18. The apparatus according to any preceding claim, wherein the apparatus further comprises a third cavity,wherein the apparatus is configured for output of a beam of radiation of the fourth wavelength from the second cavity to the third cavity;and wherein the third cavity contains:a third gain medium for lasing at a fifth wavelength; and a third nonlinear optical element for frequency-mixing the fifth wavelength with the fourth wavelength to generate a sixth wavelength.

19. A method of generating tunable short wavelength radiation using the apparatus of any one of claims 1 to 18, wherein the method comprises:generating radiation having the first wavelength in the first cavity; partially converting the first wavelength to the second wavelength using the first nonlinear optical element;27outputting the beam of radiation of the second wavelength from the first cavity to the second cavity;generating radiation having the third wavelength in the second cavity; frequency-mixing the third wavelength with the second wavelength in the second cavity to generate the fourth wavelength;andoutputting the fourth wavelength from the second cavity.

20. The method according to claim 19, wherein the method comprises adjusting the lasing wavelength of the first cavity using a spectral control element that is internal to, or part of, the first cavity.

21. The method according to claim 19 or 20, wherein the method comprises narrowing the lasing bandwidth of the first cavity using a spectral control element that is internal to, or part of, the first cavity.

22. The method according to any one of claims 19 to 21, wherein the lasing wavelength of the first cavity is a single longitudinal mode.

23. The method according to any one of claims 19 to 22, wherein the method comprises adjusting the lasing wavelength of the second cavity using a spectral control element that is internal to, or part of, the second cavity.

24. The method according to any one of claims 19 to 23, wherein the method comprises narrowing the lasing bandwidth of the second cavity using a spectral control element that is internal to, or part of, the second cavity.

25. The method according to any one of claims 19 to 24, wherein the lasing wavelength of the second cavity is a single longitudinal mode.

26. The method according to any one of claims 19 to 25, wherein the method comprises generating the second harmonic generation of the fundamental cavity lasing radiation at the first wavelength using the first nonlinear optical element.

27. The method according to any one of claims 19 to 26, wherein lasing in both the first and second cavities is in a pulsed mode.

28. The method according to claim 27, wherein the pulsed mode of the first and second cavities is by Q-switching at the same pulse rate and synchronised for temporal overlap of the output of the first cavity with the second cavity lasing in the second nonlinear optical element.

29. The method according to claim 27, wherein the pulsed mode of the first and second cavities is by modelocking at the same pulse rate and synchronised for temporal overlap of the output of the first cavity with the second cavity lasing in the second nonlinear optical element.

30. The method according to any preceding claim, wherein the first wavelength is approximately 744 nm, the second wavelength is approximately 372 nm, the third wavelength is approximately 744 nm, and the fourth wavelength is approximately 248 nm.

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