Deep ultraviolet laser source

Abstract

A method and system for generating deep ultraviolet (DUV) laser light is disclosed. In one embodiment, the DUV laser system includes a fiber laser source configured to emit a near-infrared pulsed fundamental laser beam having a pulse duration of less than 400 femtoseconds (fs), a nonlinear crystal assembly including first, second, and third nonlinear crystals configured to convert the fundamental laser beam to generate a fifth harmonic laser beam having a wavelength in a range of 200 nanometers (nm) to 230 nm, and at least one compensating plate disposed at at least one position preceding at least one of the first, second, and third nonlinear crystals, wherein a pair of pulsed laser beams transmitted through the at least one compensating plate are configured to spatially and temporally overlap within at least one of the first, second, and third nonlinear crystals.

Description

Technical Field

[0001] Cross - Reference to Related Applications This application claims priority under 35 U.S.C.§119(e) to U.S. Provisional Application No. 63 / 131,877, entitled "DEEP ULTRAVIOLET LASER SOURCE", filed on December 30, 2020, the entire disclosure of which is incorporated herein by reference.

[0002] The technical field generally relates to laser systems, and more particularly to laser systems that can generate laser light in the deep ultraviolet (DUV) wavelength range based on an ultrafast fiber laser for disinfection and sterilization applications.

Background Art

[0003] Disinfection and sterilization are necessary to limit the spread of viruses in the community and human - to - human infections. When viruses and infectious diseases are lethal and there are no vaccinations or treatment methods, special needs arise. Many of these viruses spread from person to person via aerosols or surfaces containing viral or microbial pathogens. Chemical disinfectants are a good way to kill pathogens, but it is necessary to use non - chemical disinfection continuously and invisibly to prevent the spread of viruses.

[0004] Ultraviolet (UV) light is very effective in killing microorganisms. Unfortunately, the various wavelengths or wavelength bands of ultraviolet light can have an adverse effect on normal human cells. These harmful effects include cell damage and DNA mutations, which can cause cancer and other fatal diseases. Recently, one band of ultraviolet light with a wavelength range of 200 - 230 nanometers (nm) within the ultraviolet C (UVC) band has been found to be safe for humans because its penetration length is very short, less than 1 micron (μm). Microorganisms and viral pathogens are effectively destroyed by this light, but human cells are not.

[0005] To kill pathogens, UV lamps and UV light-emitting diodes (UV-LEDs) in the 200-230nm range are currently under development. These incoherent light sources have several drawbacks. First, the power density decreases significantly with distance from the light source, requiring the light source to be placed close to the disinfection area. This limits the applications and effectiveness of these light sources. Furthermore, the lifespan of these light sources is very short, requiring continuous replacement of UV lamps or LEDs. This is not only inconvenient but also raises safety concerns if the lamps or LEDs degrade and become ineffective.

[0006] UV laser sources have high power density and directional light. The laser source can be scanned at high speed to deliver the appropriate power density for destroying pathogens. Due to its inherently superior beam quality and low beam divergence, the laser light propagates efficiently over long distances, affecting pathogens on surfaces or in volumes tens or hundreds of meters away from the laser source. Furthermore, with proper laser design, the laser source becomes more robust, resulting in a longer expected lifespan. Unfortunately, DUV laser sources do not achieve the same long operating lifespans as lasers of other wavelengths. Additionally, special precautions must be considered, such as the materials used near the laser beam to avoid damage to components. Many materials absorb in the UV wavelength range, and the emission of one or more gases from these materials can coat the optics, potentially leading to catastrophic damage to the laser components. One way to avoid this problem is to limit the types of materials used to avoid damage and to isolate the laser crystal optics. This is extremely difficult to achieve, even with precautions such as continuous purging with gases like dry air, nitrogen, argon, and helium. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] U.S. Patent Application No. 16 / 496,828 [Patent Document 2] PCT Patent Application No. PCT / US20 / 16121 [Patent Document 3] U.S. Patent Application No. 15 / 536,170 (currently U.S. Patent No. 10 / 193,296) [Overview of the project] [Means for solving the problem]

[0008] The aspects and embodiments relate to methods and systems for generating DUV laser light.

[0009] According to one embodiment, a deep ultraviolet (DUV) laser system comprises a fiber laser source configured to emit a laser beam at a fundamental wavelength in the near-infrared, wherein the fundamental laser beam is composed of a plurality of pulses having pulse durations of less than 400 femtoseconds (fs); a nonlinear crystal assembly including first, second, and third nonlinear crystals, configured to convert the fundamental laser beam to produce a fifth harmonic laser beam having wavelengths in the range of 200 nanometers (nm) to 230 nm; and at least one compensation plate positioned at least one location preceding at least one of the first, second, and third nonlinear crystals, wherein a pair of pulsed laser beams transmitted through the at least one compensation plate are configured to overlap spatially and temporally inside at least one of the first, second, and third nonlinear crystals.

[0010] In one embodiment, the DUV laser system further comprises at least one oven, each oven configured to regulate the temperature of at least one compensation plate. In a further embodiment, the oven temperature is regulated to compensate for a time delay between a pair of pulsed laser beams. In a further embodiment, the DUV laser system further comprises a controller configured to control the temperature based on the intensity value of the laser beam emitted from a fiber laser source.

[0011] In one embodiment, a first nonlinear crystal is configured to receive a fundamental laser beam and convert it to emit a second harmonic laser beam and a fundamental laser beam; a second nonlinear crystal is configured to receive the fundamental laser beam and the second harmonic laser beam and perform sum-frequency mixing of the fundamental laser beam and the second harmonic laser beam to generate a third harmonic laser beam and a second harmonic laser beam; and a third nonlinear crystal is configured to receive the second harmonic laser beam and the third harmonic laser beam and perform sum-frequency mixing of the second harmonic laser beam and the third harmonic laser beam to generate a fifth harmonic laser beam.

[0012] In one embodiment, at least one compensation plate comprises a first compensation plate positioned between first and second nonlinear crystals and a second compensation plate positioned between second and third nonlinear crystals.

[0013] In one embodiment, the DUV laser system further comprises a half-wave plate positioned between a first compensation plate and a second nonlinear crystal.

[0014] In one embodiment, the second nonlinear crystal is a type I crystal of LBO.

[0015] In one embodiment, the second nonlinear crystal is a type II crystal of LBO.

[0016] In one embodiment, the DUV laser system further comprises a half-wave plate positioned between a second compensation plate and a third nonlinear crystal.

[0017] In one embodiment, the first, second, and third nonlinear crystals include LBO, LBO, and BBO, respectively.

[0018] In one embodiment, at least one compensation plate includes a first compensation plate positioned in front of the first nonlinear crystal and a second correction plate positioned between the second and third nonlinear crystals.

[0019] In one embodiment, the DUV laser system further includes a half-wave plate disposed at a position before the first compensation plate.

[0020] In one embodiment, the second nonlinear crystal is a type-I crystal of LBO.

[0021] In one embodiment, the DUV laser system further includes at least one telescopic lens disposed upstream from the first nonlinear crystal, and the at least one telescopic lens is configured such that a light beam incident on the at least one telescopic lens enters the at least one telescopic lens as a light beam with a first diameter and exits the at least one telescopic lens as a light beam with a second diameter. In a further embodiment, the at least one telescopic lens includes a pair of telescopic lenses.

[0022] In one embodiment, the first nonlinear crystal is configured to receive a fundamental wave laser beam and convert the fundamental wave laser beam to emit a second harmonic laser beam and the fundamental wave laser beam, the second nonlinear crystal is configured to convert the second harmonic laser beam to generate a fourth harmonic laser beam, and the third nonlinear crystal is configured to receive the fundamental wave laser beam and the fourth harmonic laser beam and perform sum-frequency mixing of the fundamental wave laser beam and the fourth harmonic laser beam to generate a fifth harmonic laser beam.

[0023] In one embodiment, at least one compensation plate is disposed between the first and second nonlinear crystals.

[0024] In one embodiment, the first, second, and third nonlinear crystals each include LBO, BBO, and BBO, respectively.

[0025] In one embodiment, the DUV laser system further comprises at least one oven for adjusting the temperature of the non-linear crystal of the non-linear crystal assembly. In one embodiment, the temperature of the non-linear crystal is adjusted such that the non-linear crystal is at an optimal temperature at which non-linear multi-photon absorption by the crystal material of at least one non-linear crystal is minimized. In a further embodiment, the at least one oven is configured to heat to a temperature in the range of 10 °C to 500 °C.

[0026] In one embodiment, the wavelength of the fifth harmonic laser beam is about 206 nm.

[0027] In one embodiment, the fundamental laser beam is a broadband laser beam. In a further embodiment, the fundamental laser beam has a bandwidth of at least 2.8 nm.

[0028] In one embodiment, at least one compensating plate is made of LBO.

[0029] In one embodiment, the average output power of the fifth harmonic laser beam is at least 1 watt (W).

[0030] In one embodiment, the fiber laser source includes a mode-locked fiber laser, and a chirped pulse amplifier configured for chirped pulse amplification and including a pulse stretcher and a pulse compressor.

[0031] According to another embodiment, a method for generating deep ultraviolet (DUV) laser light includes the steps of generating a fundamental wavelength laser beam having a pulse duration of less than 400 femtoseconds (fs) in the near infrared region in a fiber laser source; guiding the fundamental wavelength laser beam through a nonlinear crystal assembly comprising first, second, and third nonlinear crystals, configured to convert the fundamental wavelength laser beam into a fifth harmonic laser beam having wavelengths in the range of 200 nanometers (nm) to 230 nm; and positioning at least one compensation plate in front of at least one of the first, second, and third nonlinear crystals, wherein the at least one compensation plate is configured such that pairs of pulsed laser beams transmitted through the at least one compensation plate spatially and temporally overlap in at least one of the first, second, and third nonlinear crystals.

[0032] In one embodiment, the method further includes the step of placing at least one compensation plate in an oven, wherein the oven is configured to regulate the temperature of at least one compensation plate.

[0033] In one embodiment, the method further includes the step of providing an oven.

[0034] In one embodiment, the method further includes the step of controlling an oven such that the temperature of at least one compensation plate compensates for the time delay between pairs of pulsed laser beams.

[0035] In one embodiment, the method further includes the step of positioning a half-wave plate in front of at least one of the first, second, and third crystals of a nonlinear crystal assembly.

[0036] In one embodiment, the method further includes the step of positioning a pair of telescope lenses in front of a first nonlinear crystal.

[0037] In one embodiment, the wavelength of the fifth harmonic laser beam is 206 nm, and the average output power is at least 1 watt (W).

[0038] In one embodiment, the method further includes the step of providing at least one compensation plate.

[0039] In one embodiment, at least one compensation plate is made from LBO.

[0040] In one embodiment, the method further includes the step of providing a nonlinear crystal assembly.

[0041] In one embodiment, the method provides a fiber laser source, further comprising the steps of: providing a mode-locked fiber laser; and a chirp pulse amplifier including a pulse extender and a pulse compressor configured for chirp pulse amplification.

[0042] In one embodiment, the method further includes the step of placing at least one of a first, second, and third nonlinear crystal in an oven configured to control the temperature of at least one nonlinear crystal.

[0043] In one embodiment, the method further includes the step of controlling the oven so that the temperature of at least one nonlinear crystal is at an optimal temperature that minimizes nonlinear multiphoton absorption by the crystalline material of at least one nonlinear crystal.

[0044] In one embodiment, the method further includes the step of controlling the oven to heat to a temperature in the range of 10°C to 500°C.

[0045] In one embodiment, the method further includes the step of irradiating at least one microbial or viral pathogen with a fifth harmonic laser beam.

[0046] According to another embodiment, the deep ultraviolet (DUV) laser system comprises a fiber laser source configured to emit a laser beam at a fundamental wavelength in the near-infrared, wherein the fundamental laser beam is a broadband laser beam and is composed of a plurality of pulses having pulse durations of less than 400 femtoseconds (fs); and a nonlinear crystal assembly including first, second, and third nonlinear crystals, configured to convert the fundamental laser beam to generate a fifth harmonic laser beam having wavelengths in the range of 200 nanometers (nm) to 230 nm.

[0047] In one embodiment, the fundamental wave laser beam has a bandwidth of at least 2.8 nm.

[0048] In one embodiment, the average output power of the fifth harmonic laser beam is at least 1 watt (W).

[0049] In one embodiment, the wavelength of the fifth harmonic laser beam is approximately 206 nm.

[0050] In one embodiment, the fiber laser source includes a mode-locked fiber laser and a chirp pulse amplifier including a pulse extender and a pulse compressor configured for chirp pulse amplification.

[0051] Further aspects, embodiments, and advantages of these examples and embodiments are described in detail below. Furthermore, it should be understood that both the information stated herein and the detailed description below are merely illustrative examples of various aspects and embodiments, intended to provide an overview or framework for understanding the nature and character of the claimed aspects and embodiments. The embodiments disclosed herein can be combined with other embodiments, and references such as “embodiment,” “example,” “several embodiments,” “several examples,” “alternative embodiments,” “various embodiments,” “one embodiment,” “at least one embodiment,” “other embodiments of this embodiment,” and “a particular embodiment” are not necessarily mutually exclusive and are intended to indicate that a particular feature, structure, or characteristic described is included in at least one embodiment. The form of such terminology in this specification does not necessarily refer to the same embodiment.

[0052] Various aspects of at least one embodiment are described below with reference to accompanying drawings, which are not intended to be shown in proportion to the actual size. The drawings are included to illustrate and further understand the various aspects and embodiments and are incorporated and constitute parts of this specification, but are not intended as definitions of limitations on any particular embodiment. The drawings, together with the rest of the specification, help to illustrate the principles and operations of the aspects and embodiments described and asserted. In the drawings, each identical or nearly identical component shown in different drawings is represented in the same drawing. For clarity, not all components are labeled in all drawings. [Brief explanation of the drawing]

[0053] [Figure 1] This is a block diagram of one embodiment of a DUV laser system according to an aspect of the present invention. [Figure 2] This is a block diagram of another embodiment of a DUV laser system according to an aspect of the present invention. [Figure 3a]This is a block diagram of a first embodiment of a DUV laser system using third harmonic generation according to an aspect of the present invention. [Figure 3b] This is a block diagram of a second embodiment of a DUV laser system using third harmonic generation according to an aspect of the present invention. [Figure 3c] This is a block diagram of a third embodiment of a DUV laser system using third harmonic generation according to an aspect of the present invention. [Figure 4] This is a block diagram of one embodiment of a DUV laser system using fourth harmonic generation according to an aspect of the present invention. [Figure 5] This is a block diagram of one embodiment of a laser source according to at least one aspect of the present invention. [Figure 6] This is a block diagram of another embodiment of a laser source according to at least one aspect of the present invention. [Figure 7] This is a schematic diagram of an active fiber used in an amplifier according to an aspect of the present invention. [Figure 8] This is a schematic diagram of an embodiment of a passively mode-locked fiber laser source according to an aspect of the present invention. [Figure 9] This table shows the parameters and results of experiments conducted according to various aspects of the present invention. [Figure 10a] This table shows the parameters and results of another experiment conducted according to various aspects of the present invention. [Figure 10b] This table shows the parameters and results for another part of the experiment, conducted according to the table in Figure 10a. [Figure 11] This is a block diagram of yet another embodiment of a DUV laser system according to an aspect of the present invention. [Modes for carrying out the invention]

[0054] One approach to address the limited operational life and material absorption problems posed by DUV laser sources is to implement the use of ultrafast optical pulses. This specification describes a DUV laser system that implements ultrafast optical pulses, mitigating the aforementioned degradation mechanisms and exhibiting a long lifetime without requiring an extremely clean environment. As will be discussed in more detail below, in one example, a DUV laser system outputs laser pulses of less than 400 femtoseconds (FS), which have a long duration.

[0055] Configurations involving ultrafast lasers with pulse durations of less than 400 fs are limited. Titanium-doped sapphire (TI: sapphire) is one option, as it has sufficient bandwidth to produce pulses much shorter than 400 fs. However, these lasers suffer from various drawbacks that hinder their use in disinfection and sterilization applications. For example, they are not very robust, are large in size, and cannot produce sufficient average power output. DUV lasers outputting in the 200-230 nm wavelength range using picosecond (PS) pulse durations have been demonstrated using disk lasers. However, the bandwidth of such lasers limits the pulse duration to picoseconds, which is not optimal for self-cleaning effects.

[0056] Alternatively, ultrafast fiber lasers based on ytterbium (Yb) doped fibers have sufficient bandwidth to support pulses shorter than 400 fs. Fiber lasers are extremely robust, compact, high-power, and efficient, offering an ideal solution for disinfection and sterilization applications. These lasers may have frequencies that are converted to 343 nm using a nonlinear optical (NLO) crystal via third harmonic generation by mixing the fundamental wavelength with a second harmonic wavelength. The systems and methods disclosed herein demonstrate high average power femtosecond fiber lasers at a wavelength of 206 nm with good length and efficiency.

[0057] In addition to disinfection and sterilization, another possible application of the disclosed laser source is ophthalmic surgery. Conventional lasers used in this application include excimer lasers or ultrafast IR lasers, which have high inherent drawbacks such as low beam quality and high average power requirements. These drawbacks limit the duration of the procedure due to the limit of heating that the eye can tolerate. The 206 nm ultrafast DUV pulse exhibited by the laser described herein has the ability to perform such surgical procedures at a much lower average power, thus potentially increasing the speed and improving the quality of ophthalmic surgical procedures.

[0058] Similarly, the disclosed lasers, when used in other applications such as smartphones and other display glasses, and micromachines for highly transparent materials such as sapphire, can dramatically improve quality and production speed. The disclosed lasers can also be implemented in wafer inspection applications such as lithography post-processing, where short wavelengths can be beneficial for the rapid localization of very small defects in silicon wafers.

[0059] As will be explained in detail below, a laser system is provided that includes a fiber laser source that outputs near-infrared (e.g., 1030 nm) pulses having a duration of less than 400 fs (including boundary values). According to one embodiment, the fiber laser source outputs pulses having a duration of about 300 fs, and in another embodiment, the output pulse has a duration of about 200 fs. The optical pulse is a frequency that is converted to a second harmonic using a first nonlinear crystal, i.e., a SHG such as lithium tribo(I) (LBO). The generation of a fifth harmonic to a wavelength less than 210 nm includes two different methods. The first method includes generating a third harmonic by sum-frequency mixing of the second harmonic and the fundamental wavelength radiation of the second nonlinear crystal, i.e., THG. The fifth harmonic is then generated by mixing the second and third harmonics in the third nonlinear crystal. In the second method, the second harmonic is frequency multiplied to generate a fourth harmonic in the second nonlinear crystal. Next, the fourth harmonic is mixed with the fundamental wave to generate the fifth harmonic in the third nonlinear crystal.

[0060] According to several embodiments, to improve the spatial and temporal overlap of mixed-frequency optical pulses in various frequency-converting NLO crystals, one or more of the above approaches may include the use of one or more time-delay compensation (TDC) crystals (also referred to herein as “compensation plates” or “TDC plates”). As has been highly appreciated, achieving optimal pulse overlap (spatially and temporally) in an NLO crystal results in higher conversion efficiency. Furthermore, without the use of TDC crystals, the time delay between pulses must be corrected with a delay line consisting of beam splitters and mirrors, which need to spatially divide, delay, and then recombine the frequencies to achieve the same effect. This latter approach has several drawbacks, including an increase in overall size and the number of optics required, and the mirrors and beam splitters result in significant losses. The mirrors and beam splitters in conventional delay line systems are also difficult to align and sensitive. In general, conventional delay line techniques reduce the overall reliability of the system and the stability of inter-pulse energy by, for example, increasing the impact of beam Poynting instability.

[0061] Referring to the drawings, Figure 1 is a block diagram showing an embodiment of a DUV laser system, which is shown overall as 100, and comprises a laser source 110 (also referred to herein as the “pump laser” and “fiber laser source”) configured to emit a laser beam 102 at a near-infrared fundamental wavelength having a pulse duration of less than 400 fs; a nonlinear crystal assembly 120 that generates a fifth harmonic laser beam 105 (also referred to herein as the DUV laser beam) having a wavelength in the range of 200 nm to 230 nm, comprising a first nonlinear crystal 122 (also referred herein as the second harmonic generation (SHG) crystal), a second nonlinear crystal 124, and a third nonlinear crystal 126 (also referred herein as the fifth harmonic generation (FIHG) crystal); and at least one compensation plate 130. As will be explained in detail below, at least one compensation plate 130 is positioned at least one location preceding at least one of the first nonlinear crystal 122, the second nonlinear crystal 124, and the third nonlinear crystal 126, and pairs of pulsed laser beams transmitted through at least one compensation plate are configured to spatially and temporally overlap with at least one of the nonlinear crystals of the first nonlinear crystal 122, the second nonlinear crystal 124, and the third nonlinear crystal 126.

[0062] As described above, according to at least one embodiment, DUV laser emission is achieved via third harmonic generation. Referring now to Figures 3a–3c, block diagrams of three separate embodiments are shown, 300a, 300b, and 300c, which each generate a fifth harmonic DUV laser emission using third harmonic generation. In each example, the first nonlinear crystal 122 receives the fundamental laser beam 102, converts the fundamental laser beam 102 to emit the second harmonic laser beam 304 and the fundamental laser beam 102; the second nonlinear crystal 124 receives the fundamental laser beam 102 and the second harmonic laser beam 304, performs sum-frequency mixing of the fundamental laser beam 102 and the second harmonic laser beam 304 to generate the third harmonic laser beam 306 and the second harmonic laser beam 304; the third nonlinear crystal 126 receives the second harmonic laser beam 304 and the third harmonic laser beam 306, performs sum-frequency mixing of the second nonlinear crystal 304 and the third nonlinear crystal 306 to generate the fifth harmonic laser beam 105.

[0063] Referring to Figure 3a, according to one embodiment, the DUV laser system 300a has a first compensation plate 330a and a second compensation plate 330b. The first compensation plate 330a is positioned between a first nonlinear crystal 122 and a second nonlinear crystal 124, and the second compensation plate 330b is positioned between the second nonlinear crystal 124 and a third nonlinear crystal 126.

[0064] As described above, the compensation plate 130 functions to improve the spatial and temporal overlap of mixed-frequency optical pulses in various frequency-converting NLO crystals. As is highly valued, the conversion rate or efficiency of an optically nonlinear process is determined by this spatial and temporal overlap, i.e., optimal interaction, in the NLO. This is a particular problem with ultrashort pulses of less than 1 ps, for example, where only a portion of the pulse of one beam overlaps with a portion of the pulse of another beam in the NLO. Temporal walk-off in nonlinear crystals is caused by the dependence of refractive index on wavelength, i.e., dispersion. When two ultrashort pulses of different wavelengths or polarizations pass through a dispersion medium, the pulses move apart in time. Different group velocities of laser pulses with different wavelengths and / or polarizations cause different temporal delays compared to propagation in a vacuum, and thus lead to non-optimal temporal overlap of pulses in nonlinear crystals. Spatial walk-off in nonlinear crystals is caused by the birefringence property of the crystal, where the walk-off occurs due to the difference in the direction of energy flow (i.e., the direction of the Poynting vector) relative to the direction of the wave vector k. At the edge of the crystal, both laser beams are separated by a distance known as the spatial walk-off angle. Both the actual spatial and temporal walk-off shorten the interaction length between the laser beams in the NLO, negatively impacting conversion efficiency.

[0065] In the embodiment of the DUV laser system 300a shown in Figure 3a, the first nonlinear crystal 122 generates spatial and temporal walk-offs between the fundamental laser beam 102 and the second harmonic laser beam 304 exiting the first nonlinear crystal 122, which are compensated by the first compensation plate 330a. The second nonlinear crystal 124 also generates spatial and temporal walk-offs between the second harmonic laser beam 304 and the third harmonic laser beam 306 exiting the second nonlinear crystal 124, which are compensated by the second compensation plate 330b.

[0066] The compensation plate 130 has different refractive indices along different axes. For example, in the case of a biaxial crystal, the refractive indices differ along each of the three axes, and in the case of a uniaxial crystal, the refractive indices differ along only two axes. The compensation plate 130 compensates for temporal discrepancies in the NLO by effectively implementing different group velocities of propagation of the two pulsed laser beams of interest. Spatial discrepancies are addressed by replacing one of the beams with the beam propagation axis via a spatial walk-off. In practice, the compensation plate 130 is configured such that an anomalous beam walks off at an angle opposite to that of a nonlinear crystal (122, 124, or 126) located downstream of the compensation plate 130 (e.g., cut and directionality, specific length / thickness). The crystal is configured such that normal waves do not displace, but anomalous waves do. The amount of displacement depends on the length of the crystal.

[0067] According to at least one embodiment, the compensation plate 130 is made from a birefringent material, which is a non-limiting example including LBO. Experiments conducted by the inventors have shown that the use of an appropriate type of birefringent material as the TDC crystal is crucial for long-term performance. When the LBO crystal was used as the TDC material, it was found to suppress long-term damage compared to other materials used. When other materials were used, various optical damages were introduced into the subsequent optical components, reducing the reliability of the system. The experimental results also show that using LBO as the TDC material provides the high temporal dynamic range required for the proper operation of the disclosed DUV system and method. According to at least one embodiment, the LBO used as the TDC crystal is cut along its optical Z-axis. As will be described in more detail below, the LBO cut along its Z-axis has been found to exhibit the best temporal dynamic range and the lowest absorption and hot spot formation.

[0068] A block diagram of another embodiment of the DUV laser system 200 is shown in Figure 2. The DUV laser system 200 is similar to that shown in Figure 1 in that it includes a laser source 110, a nonlinear crystal assembly 120 comprising a first nonlinear crystal 122, a second nonlinear crystal 124, and a third nonlinear crystal 126, configured to generate a fifth harmonic laser beam 105, and at least one compensation plate 130. Furthermore, the DUV laser system 200 also includes a half-wave plate 235 used in combination with the compensation plate 130, according to another aspect of the present invention. The laser system 300a in Figure 3a implements the use of a half-wave plate 335 positioned between the first compensation plate 330a and the second nonlinear crystal 124. In this embodiment, the second nonlinear crystal 124 is a type I nonlinear crystal, and its non-limiting embodiment includes an LBO. In type I phase matching, the fundamental wave 102 and the second harmonic 304 are biased perpendicular to each other. The addition of the half-wave plate 235 ensures that the two laser beams are polarized in the same direction in the nonlinear crystal.

[0069] Figure 3b is a block diagram of another embodiment of a DUV laser system 300b that generates a fifth harmonic DUV laser emission via a third harmonic generation. The DUV laser system 300b includes a first compensation plate 330a positioned in front of the first nonlinear crystal 122 and a second compensation plate 330b positioned between the second nonlinear crystal 124 and the third nonlinear crystal 126. The latter of these functions in a similar manner to the second compensation plate 330b described above in relation to Figure 3a.

[0070] The DUV laser system 300b also includes a half-wave plate 335 positioned before the first compensation plate 330a. In this configuration, both the half-wave plate 335 and the TDC crystal 330a are positioned upstream of the SHG crystal 122 with the aim of introducing two orthogonal polarizations at a fundamental frequency 102 that are appropriately delayed from each other based on the TDC crystal 330a design, so that the unconverted fundamental frequency can be mixed with the signal that has already been frequency-converted. As an example, the SHG signal 304 (generated from one of the fundamental frequencies of one polarization) is mixed with the fundamental frequency (unconverted) of the THG crystal 124. A similar configuration can be applied to other harmonic generation. The advantage of introducing two polarizations and a designed time delay is that the frequency conversion efficiency is improved by using a fundamental frequency that has not yet experienced frequency conversion. Furthermore, this design provides additional degrees of freedom for optimization and greater reliability, as the delay and time delay can be performed with much lower intensity. In another embodiment, the second nonlinear crystal 124 is a type I nonlinear crystal.

[0071] Based on another example relating to the generation of a fifth-harmonic DUV laser emission using a third-harmonic generation, a block diagram of the DUV laser system 300c is shown in Figure 3c. In this example, a first compensation plate 330a is placed between the first nonlinear crystal 122 and the second nonlinear crystal 124, and a second compensation plate 330b is placed between the second nonlinear crystal 124 and the third nonlinear crystal 126. According to one embodiment, the second nonlinear crystal 124 is a type II nonlinear crystal such as a type II LBO. Advantages associated with the use of type I nonlinear crystals include higher conversion efficiency due to greater nonlinearity and broader spectral acceptance. Type II nonlinear crystals can be used when the beam needs to be tightly focused on the nonlinear crystal, as they have about three times the acceptance angle and about twice as much spatial walk-off as type I. However, the nonlinearity d eff This is approximately 30% lower than that of type I. The function of the first compensation plate 330a is similar to the function described above in relation to Figure 3a. As shown in Figure 3c, this example configuration also includes the use of a half-wave plate 335.

[0072] According to one embodiment, the first nonlinear crystal 122, the second nonlinear crystal 124, and the third nonlinear crystal 126 of the DUV laser systems 300a-300c in Figures 3a-3c include LBO, LBO, and Barium Borate (BBO), respectively. However, it should be understood that other nonlinear crystalline materials are also within the scope of this disclosure.

[0073] Returning to Figure 2, according to at least one embodiment, the DUV laser system may further include at least one oven 240, each oven 240 configured to regulate the temperature of at least one compensation plate 130. According to this embodiment, the temperature of the TDC crystal 130 is modulated to adjust for the time delay in the birefringent material. That is, the oven temperature is adjusted to compensate for the time delay between a pair of pulsed laser beams introduced downstream of the nonlinear crystal. The TDC varies with laser conditions such as power and intensity level. This is due to the intensity-dependent nature of group velocity mismatch in type I SHGs.

[0074] When laser conditions change, active dynamic compensation by heating the crystal is important. For this purpose, a controller 250 is implemented to control one or more operating parameters of the oven 240 (e.g., oven temperature). According to at least one embodiment, the controller 250 is configured to control the temperature of the oven 240 based on the intensity value of the laser beam 102 emitted from the laser source 110. In one embodiment, the oven 240 is heated to a temperature of at least 400°C, and optionally to about 500°C. According to some embodiments, the oven 240 may be heated to a temperature in the range of 10°C to 500°C. The inventors have found that such a wide temperature range allows for a greater range of adjustment of the time delay between short pulses of any pair of pulsed laser beams.

[0075] In one embodiment, a calibration routine can be performed to determine the temperature of the oven 240 housing the TDC plate 130. This can be achieved by measuring the temporal delay based on the temperature and intensity / power (of the compensation plate 130). For example, for a given power of the laser source 110 and the emitted basic beam 102, the temperature of the oven 245 can be raised to the point where the temporal and spatial delays are minimized, i.e., the point where the maximum DUV output power is realized. If additional compensation is required that cannot be achieved by the temperature of the TDC plate 130, the thickness of the TDC plate can be increased (or decreased) by changing the TDC plate to a thicker or thinner plate to achieve the optimal output power.

[0076] In certain embodiments, the DUV laser system 200 may also include at least one telescopic lens 237 positioned upstream of the first nonlinear crystal 122. The telescopic lens 237 has the function of adjusting the beam size of the beam incident on the telescopic lens 237. For example, a ray incident on the telescopic lens 237 enters the telescopic lens 237 as a ray of a first diameter and exits the telescopic lens 237 as a ray of a second diameter. As seen in Figures 3a-3c, in some embodiments, at least one telescopic lens includes a pair of telescopic lenses 337. In one embodiment, the optical beam size is modified using a telescopic lens made of magnesium fluoride (MgF2) material, which the inventors have found to optimize output power and conversion efficiency and significantly improve the reliability of the system. The beam size can be increased or decreased to achieve an optimal diameter and good conversion efficiency without saturation. Compared to other widely used optical materials, MgF2 exhibits significantly lower optical nonlinearity, so high-peak power pulses do not generate hot spots that would often damage components located downstream of the telescope lens, such as the nonlinear crystals in the nonlinear crystal assembly 120. This design aspect contributes to the improved lifetime of the disclosed DUV laser.

[0077] One or two telephoto lenses 237 may also have other functions. According to another embodiment, one or more lenses are used to form a beam waist with a sufficiently long Rayleigh length so that the beam maintains the same diameter over a wide area. This helps to ensure good beam characteristics and spatial overlap between the two beams with different wavelengths in the THG and FiHG crystals.

[0078] Figure 4 is a block diagram of one embodiment of a DUV laser system 400 that generates a fifth harmonic DUV laser radiation via fourth harmonic generation. In particular, the fifth harmonic DUV laser radiation is achieved by the generation of the fourth harmonic in the second nonlinear crystal 124. The first nonlinear crystal 122 receives the fundamental laser beam 102 and converts the fundamental laser beam 102 to emit the second harmonic laser beam 304 and the fundamental laser beam 102 as described above. The second nonlinear crystal 124 converts the second harmonic laser beam 304 to generate the fourth harmonic laser beam 408. The third nonlinear crystal 126 receives the fundamental laser beam 102 and the fourth harmonic laser beam 408 and performs sum-frequency mixing of the fundamental laser beam 102 and the fourth harmonic laser beam 408 to generate the fifth harmonic laser beam 105. The first compensation plate 330a is positioned between the first nonlinear crystal 122 and the second nonlinear crystal 124. In one embodiment, the first, second, and third nonlinear crystals include LBO, BBO, and BBO, respectively.

[0079] Returning to Figure 2, in some embodiments, the DUV laser system 200 further includes at least one oven 245 for adjusting the temperature of the nonlinear crystals in the nonlinear crystal assembly 120. For example, each of the first nonlinear crystal 122, the second nonlinear crystal 124, and the third nonlinear crystal 126 may be housed in their respective ovens 245a, 245b, and 245c. As can be understood, the nonlinear crystals can be placed in a temperature-controlled oven (e.g., by a controller 250) to allow for thermal adjustment, resulting in improved conversion efficiency and higher average output power. According to one embodiment, the oven 245 adjusts the temperature of the nonlinear crystals (e.g., 122, 124, and / or 126) to an optimal or target temperature so that nonlinear multiphoton absorption by the material of the nonlinear crystals is minimized or otherwise reduced. For example, if the temperature of the NLO during operation increases (e.g., temperatures above 200°C, including 400°C, 450°C, and 500°C, which do not adversely affect the function of the NLO), two-photon absorption decreases. In one embodiment, the oven 245 is configured to heat to a temperature of at least 200°C, including a boundary value, and in another embodiment, the oven 245 is configured to heat to a temperature in the range of 10°C to 500°C. Two-photon absorption reduces both the conversion efficiency and lifetime of the FiHG crystal 126, and in some cases the lifetime of the THG crystal as well. As can be understood, the operating parameters of the oven 245 can be controlled by the controller 250. For example, the controller 250 can increase the temperature of the oven 245, leading to improved conversion efficiency and reliability of the NLO crystal.

[0080] The laser source 110 is configured as a fiber laser source. According to at least one embodiment, the wavelength of the fundamental laser beam 102 emitted by the laser source 110 is in the near-infrared range, for example, 750 to 1400 nm, and in some embodiments, it is emitted in a wavelength range of 1 μm. For example, the wavelength of the fundamental laser beam 102 may be in the range of 1030 to 1080 nm, and in one embodiment, the fundamental wavelength is 1030 nm. According to one embodiment, the fundamental laser beam 102 is a broadband laser beam, having a bandwidth of at least 2.8 nm in some embodiments and a bandwidth of as much as 16 nm in other embodiments. The inventors have found that using such a broadband light source improves the long-term reliability of the DUV system.

[0081] According to at least one embodiment, the laser source 110 is configured such that pulses of the fundamental laser beam 102 have an average power of at least 50 watts, and possibly at least 60 watts. Depending on the application, the laser source 110 may also be configured to output a fundamental laser beam 102 with an average power of less than 50 watts, for example, 5-10 watts, 10-20 watts, 20-30 watts, or 30-40 watts. In some cases, the peak power of the fundamental laser beam 102 may be on the order of megawatts (MW), for example, 1-1000 megawatts (MW). In other embodiments, the peak power of the fundamental laser beam 102 is on the order of gigawatts (GW).

[0082] In various embodiments, the overall conversion efficiency of the nonlinear crystals 122, 124, and 126 of the nonlinear crystal assembly 120 may be approximately 5%, and in other embodiments, approximately 2%. For example, with a 1030 nm and 40 W output from the laser source 110, the average output power from the DUV laser beam 105 may be approximately 800 milliwatts (mW). For the SHG crystal 122, the conversion efficiency is up to 80%, and in some embodiments, it has a conversion efficiency of approximately 50%. For the THG crystal 124 in Figures 3a-3c, the conversion efficiency is up to 40%, in some embodiments, approximately 35%, and in other embodiments, it may range from 20% to 35%. In some embodiments, the THG crystal can be configured to have a conversion efficiency that enables optimal FiHG conversion, although this may not be the maximum efficiency value of the THG crystal. For FiHG, the conversion efficiency is approximately 5%, in some embodiments, approximately 2%, and in other embodiments, approximately 1%.

[0083] The wavelength of the DUV laser beam 105 is in the range of 200–230 nm, and in some cases, in the range of 206–216 nm, including boundary values. According to at least one embodiment, the DUV laser beam 105 is approximately 206 nm, and shorter wavelengths are also within the scope of this disclosure. In certain embodiments, the fifth harmonic laser beam 105 has an average output power of at least 1 watt (W) and at least 2 W, and can range from several hundred mW (e.g., at least 100 mW, at least 200 mW) to 5 W. Higher powers are also within the scope of this disclosure. Depending on the material to be disinfected, some may be able to accommodate higher spatial power densities (and shorter required exposure times), while others require lower spatial power densities (and longer exposure times), so the output power can be adjusted or otherwise specified depending on the application.

[0084] In one embodiment, the lifetime of a DUV laser source described herein can be characterized by having a lifetime of at least 1,000 hours. Longer lifetimes are also within the scope of this disclosure. It should be understood that the absolute lifetime of a DUV laser system may vary depending on the application and specific components, such as average power output and fiber type. The term “lifetime” refers to the time during which the output power and / or other characteristics of a DUV laser system remain at or near the nominal value (such as the system’s rated power output).

[0085] While the examples of DUV laser systems disclosed so far have included at least one compensation plate 130, embodiments of the DUV system that do not include the compensation plate 130 are also within the scope of this disclosure, and in fact, the inventors have found that for certain applications, adequate DUV conversion can be achieved without using a compensation plate. An example of such a system 1100 is shown in Figure 11, which is substantially identical to the DUV system 100 in Figure 1, but without the compensation plate 130. The DUV laser system 1100 comprises a laser source 110 and a nonlinear crystal assembly 120, as described above. Furthermore, the system 1100 may include at least one oven 245 for adjusting the temperature of the nonlinear crystals in the nonlinear crystal assembly 120, as previously described with reference to the DUV system 200 in Figure 2. It should be understood that other features described above, such as one or more telephoto lenses and controllers, may also be included in the system 1100.

[0086] Laser source In certain embodiments, the laser source 110 comprises a mode-locked fiber laser and a chirp pulse amplifier with a pulse extender and pulse compressor configured for chirp pulse amplification (CPA). Such a system helps generate laser light with high and ultra-high pulse repetition rates and damage-free peak power, while having a high average power. For ultrafast pulses (e.g., shorter than 20 ps, ​​on the order of a few fs), pulse distortion increases due to optical nonlinearity that occurs as the optical pulse propagates through the optical component / material. The pulse degrades and begins to change shape, and / or forms pre-pulses or post-pulses, ultimately extending the total duration of the time envelope. This is problematic because many applications require ultrashort pulses with high peak power and high pulse energy without a temporal pedestal. A temporal pedestal can be created by higher-order dispersion introduced through the optical component or intensity-dependent optical nonlinearity (most often self-phase modulation (SPM)).

[0087] One common method for extracting more pulse energy and raising the SPM threshold is to use CPA. In this technique, the pulse is stretched in time by linearly adjusting the phase of each longitudinal mode in the spectral envelope. Bulk gratings, prisms, fibers, chirp fiber Bragg gratings, or chirp volume Bragg gratings can be used to stretch the pulse by introducing this dispersion. The pulse is then amplified through a gain material to achieve higher pulse energy before reaching a peak output that induces SPM. Finally, the pulse is compressed with a matching dispersion element, recompressing the pulse to a picosecond or femtosecond pulse duration to achieve the desired pulse energy and ultrashort pulse.

[0088] A non-limiting example of such a system is shown in Figure 5, generally referred to as 510, which is jointly owned by the applicant and whose contents are fully incorporated herein by reference and also described herein in Patent Document 1. As shown in Figure 5, the laser source 510 includes a main oscillator 512 (which may be a mode-locked laser source, an example of which is described below with reference to Figure 8) and a chirp pulse amplifier including a pulse extender 516 and a pulse compressor 518. An input laser pulse from the main oscillator 512 is stretched in time using the pulse extender 516, amplified in an amplification stage including a fiber power amplifier 515b and an optional preamplifier 515a, and compressed using the pulse compressor 518.

[0089] As is understood, the temporal stretching and compression of pulses are based on delaying different wavelengths within a pulse by different amounts of time. In the stretcher 516, short-wavelength pulses may be delayed relative to long-wavelength pulses, and vice versa, and in the compressor 518, this effect is reversed again. Bulk gratings, prisms, fibers, fiber Bragg gratings (FBGs), chirp fiber Bragg gratings (CFBGs), or chirp volume Bragg gratings (CVBGs) are examples of strongly dispersed elements that function to stretch pulses. The pulse stretcher 516 is configured to stretch the pulse duration to produce a stretched pulse with reduced peak output. In some embodiments, the pulse stretcher 516 is configured as a CFBG, as shown in Figure 5.

[0090] The chirp-amplified pulses are compressed by a pulse compressor 518, which in some embodiments is configured as a chirp-volume Bragg grating (CVBG). In some embodiments, the compressor 518 is configured using a transmission diffraction grating capable of handling high average power. For example, the transmission diffraction grating can be formed from silica using a holographic procedure and etching process that is tuned to minimize defects and imperfections.

[0091] As described in Patent Document 1, conventional CPA systems compensate for the linear portion of chirps, but compensating for nonlinear chirps requires higher-order dispersion techniques. For example, the conversion-limited sub-nanosecond pulses output by the main oscillator 512 each have a spectral bandwidth, and the spectral phase of the stretched pulse deviates from the spectral phase of the conversion-limited pulse, becoming particularly evident after compression by the compressor 518. According to various embodiments, the laser source 510 is configured to suppress pulse pedestal or pulse distortion caused by ultrafast pulses propagating through optical components or materials by correcting the phase across chirp optical pulses. To achieve this objective, the laser source 510 is configured with an adjustable pulse stretcher or compressor that is suited to controllable dispersion compensation in order to provide near-conversion-limited sub-nanosecond pulses at the output of the CPA system. This is achieved by providing a pulse shaper configured as a compact adjustable Bragg grating with a number of adjustable segments for manipulating the phase of the input optical pulse. Either or both of the pulse stretcher 516 or the pulse compressor 518 can be configured with this adjustable Bragg grating. Adjustable components are achieved by configuring a Bragg grating with selectively adjustable segments controlled by actuators. The actuators induce spectral phase changes in each segment to adjust the spectral phase to the spectral phase of a conversion-limited pulse. The actuators are controlled by correction signals output from a controller. Adjustment is performed by adjusting the selected segments to a predetermined temperature or voltage determined during the input or calibration routine.

[0092] According to at least one embodiment, the optical pulse is pre-chirpened using an FBG pulse extender or pulse shaper to improve temporal overlap and beam intensity in the NLO crystal, resulting in improved conversion efficiency and reliability. Temporary pre-chirpening is performed so that the pulse duration does not exceed 400 fs, with the aim of improving the operating lifetime of the laser.

[0093] The preamplifiers 515a and 515b in the CPA configuration are pumped by their respective pumps (not shown), which operate in the 1–2 μm range and can be driven by one or more pump drivers using a controller (such as controller 250 in Figure 2). The controller includes hardware (such as a general-purpose computer) and software that can be used to control the components of the system, such as the pumps. The pumps can be implemented by SM or MM laser diodes operating in CW mode, or by fiber laser pumps, and can be arranged in side-pumping or end-pumping configurations. According to some embodiments, laser pulses of SM light are delivered via an SM passive fiber to an active fiber of amplifier 515b having an MM core doped with one or more rare-earth ions such as ytterbium, erbium, and / or thulium, and surrounded by at least one cladding. In some embodiments, as will be described in more detail below with reference to Figure 7, the core has a double-bottleneck cross-section, which serves to increase the threshold of optical nonlinear effects.

[0094] The pulse energy can be increased by coupling an optional acousto-optic modulator or electro-optic modulator (EOM) 514 between the preamplifier stage 515a and the booster stage 515b. As understood, the optional EOM 514 can function as a pulse picker.

[0095] According to one or more embodiments, the laser source 510 comprises an ultrafast seed laser, a pulse extender based on a CFBG, a pulse shaper, a fiber preamplifier, an optional pulse picker, a fiber amplifier, and a pulse compressor based on a volume Bragg grating (VBG).

[0096] Another non-limiting example of a CPA configuration in a laser system is shown in Figure 6, which is shown overall in 610, and is jointly owned by the applicant and whose contents are fully incorporated herein by reference and also described herein in Patent Document 2. As shown in Figure 6, the laser source 610 includes a mode-locked fs laser 612 and a chirp pulse amplifier including a pulse expander 616 and a pulse compressor 618, which function in a similar manner to those described above with reference to the laser system 510 in Figure 5, except that the expanded pulses are duplicated using a pulse duplication module 619 before amplification.

[0097] As described in Patent Document 2, the pulse duplication module 619 is a full-fiber device comprising input and output optical fiber couplers with a fiber delay line positioned between them, and is configured to increase the repetition rate of the stretched laser pulse to generate a modified pulse having a desired peak-to-average power ratio. These modified laser pulses complete the remaining CPA process by being amplified in amplifiers (615a and 615b), after which the pulses are compressed to a pulse duration in the sub-nanosecond range (e.g., less than 400 fs). This process increases the peak power for efficient frequency conversion in the NLO assembly. The stretched pulse is duplicated using the pulse duplication module 619 to a pulse duration and repetition rate that simulates a nearly continuous wave (CW) configuration. This reduces the peak power and mitigates problems associated with optical nonlinearity such as self-phase modulation (SPM), simulated Raman scattering (SRS), and four-wave mixing (FWM).

[0098] As described above with respect to the laser source 510, and as applicable to several embodiments with respect to the laser source 610, the laser pulse of SM light is delivered via an SM passive fiber to an active fiber of an amplifier 515b or 615b having an MM core doped with one or more rare earth ions such as ytterbium, erbium, and / or thulium and surrounded by at least one cladding. Referring to Figure 7, the fiber power amplifier 515b or 615b may consist of a monolithic (integrated) MM core 1 extending between both ends of the amplifier, supporting multiple transverse modes, and surrounded by at least one cladding 3. The core 1 is configured to support only a single fundamental mode at the required fundamental wavelength. This is achieved by matching the mode field diameter (MFD) of the MM core 1 to the mode field diameter (MFD) of both the SM passive fiber 2 and the output passive SM fiber 9 that guide the modified laser light 148 along the core 4. In the case of side pumping, the pump light from the pump is coupled to the central core region 5.

[0099] To further increase the threshold for optical nonlinear effects, core 1 has a double bottleneck cross-section, as shown in Figure 7. The uniformly sized input core end 6 can have a geometric diameter equal to the geometric diameter of the SM core 4 of the passive fiber 2. When SM light of the fundamental wavelength is coupled to the input end 6 of the core, only fundamental modes are excited, whose intensity profile substantially matches the Gaussian intensity profile of pure SM. Core 1 further includes a large-diameter, uniformly sized mode-conversion core section 5 that receives the fundamental modes guided through the adiabatic expansion mode-conversion core region 7A. Due to the large diameter of the central core region 5, a larger amplified pump output can be received without increasing the power density within this section. This increases the threshold for optical nonlinear effects such as SPM, SRS, and FWM. The output mode-conversion core region 7B can be configured identically to core region 7A to adiabatically reduce the mode field diameter of the amplified pump light at the fundamental frequency. The amplified SM light is coupled to the output SM passive fiber 9.

[0100] The mode-locked fs laser 612 (also referred to herein as the main oscillator with reference to 512 of the laser source 510 in Figure 5, or as an ultrafast seed laser or pulse generator, or simply a mode-locked laser source) may include a passively mode-locked fiber laser source. In one embodiment, the mode-locked fs laser 512 or 612 is configured as a passively mode-locked fiber ring cavity. Such a passively mode-locked configuration relies on the presence of at least one component within the ring cavity that exhibits a nonlinear response to an increase in peak intensity.

[0101] According to at least one embodiment, the mode-locked laser sources 512, 612 are configured as passive mode-locked fiber ring cavities configured to generate sub-nanosecond giant chirp pulses. The ring fiber waveguide or cavity includes a plurality of fiber amplifiers, chirped fiber components, and spectral filters configured with spectral bandpasses centered at different central wavelengths to provide light leakage along the ring cavity in response to a nonlinear process induced within the ring. The filters work together to generate a nonlinear response, enabling stable mode-locked mode operation. An example of such a configuration is described in Patent Document 3 (now U.S. Patent 10 / 193,296), which is incorporated herein by reference.

[0102] Figure 8 is a schematic diagram of a pulse generator described in Patent Document 3, which is an example of a mode-locked fs laser source 512, 612 suitable for one or more embodiments of the present invention. The all-fiber architecture improves the environmental stability of the laser source and is configured as a ring fiber waveguide or cavity that directs light in one direction. A fiber isolator 28 provides the desired directivity of light propagation within the ring fiber waveguide. The ring cavity is configured such that the output of one of the first fiber amplifiers 12 and the second fiber amplifier 20 seeds the other fiber amplifier. Between the first amplifier 12 and the second amplifier 20, two or more identical groups or chains of fiber elements are coupled together to define the ring cavity. In addition to the fiber amplifiers, each chain includes fiber coils 16, 22 that provide periodic spectral and temporal expansion of the signal, respectively, and nanowire filters 18, 24 that operate to spectrally filter the expanded signal. Thus, the entire ring laser cavity contains two cavities, i.e., linear subcavities, which provide very weak seeding to each other. There are no longitudinal modes throughout the ring laser cavity because the signal is significantly attenuated within the transmittance range of both filters necessary to distinguish spontaneous CW laser oscillations.

[0103] This specification describes the entire architecture in general terms. One of the fiber amplifiers 12, 20 is configured to provide a much higher gain than the other amplifier. The high-level excitation amplifier creates the conditions for strong pulse broadening by SPM, positively chirpening the pulse and giving it a broad, smooth spectrum. This spectrum completely fills the passband of the downstream filter, after which a replica of it evolves within the cavity. The other, low-level excitation amplifier ensures stable performance; that is, it locks the laser into a stable equilibrium state in case a slight deviation from this state causes an action that returns the laser to the target state. The spectrum reaching the downstream filter from the low-level excitation amplifier does not completely fill the passband of this filter, thus generating a force that returns the laser to the target state when a deviation occurs. For the laser pulse to circulate and evolve within the ring cavity, the pulse must experience nonlinear spectral broadening and have sufficient intensity to recover each time it passes along the cavity. The combination of the two filters 18, 24 with weak spectral overlap acts as an effective saturable absorber. Due to the weak spectral overlap, CW pulses can be distinguished by prioritizing pulses with sufficient intensity to broaden the spectrum. When the peak intensity reaches a level sufficient to spectrally broaden the pulse, the loss of newly acquired spectral components decreases because these components spread towards the center of the filter's passband. It should be understood that stable and reproducible circulation of pulses along the cavity can occur even without spectral overlap of filters 18 and 24, but the overlap can facilitate the initiation of laser pulses.

[0104] Filters 18 and 24 are configured to allow only the desired spectral range to pass through, and to introduce either normal or anomalous dispersion as needed. One of the filters can have a bandpass up to 5 times wider than the other filters' bandpasses. Furthermore, the bandwidth of each filter can be 2 to 10 times narrower than the bandwidth of the output pulse 55. However, in some cases, the required pulse width may be narrower than the filter bandwidth. A series of spectral expansions and filters generate a pulse with a huge chirp that has the desired spectral width, pulse duration, and energy.

[0105] The ring waveguide further includes an output coupler 30 positioned immediately downstream of the fiber coil 16 that guides the chirp pulses 55 to the outside of the ring waveguide. One or two CW pumps 26 are optically coupled to each amplifier to generate the desired inversion distribution in the gain medium of the amplifiers, i.e., to initiate the operation of the pulse generators. All the components described above are interconnected by a single transverse mode (SM) fiber. Both laser sources 510 and 610 are all-fiber configurations.

[0106] experiment The functionality and advantages of the embodiments of the systems and methods disclosed herein can be better understood based on the experiments described below. The experiments are intended to illustrate various aspects of the disclosed DUV laser system.

[0107] Time delay occurring in experimental 1-NLO crystal Figure 9 is a table showing experimental parameters and the results of experiments conducted to investigate the time delays occurring in Type I and Type II SHG and THGNLO crystals. The SHG and THG nonlinear crystals were made from LBO material. The results showed that the time delay between harmonic pulses in SHG is intensity-dependent, as it changes with the beam intensity. The table in Figure 9 shows how much time delay can be compensated within the temperature range of 35°C to 190°C. The inventors found that greater compensation could be achieved by increasing the oven temperature of the TDC plate. For example, a wider range of time delays can be achieved when the oven temperature is up to 500°C. Furthermore, as the intensity increased, a wider range of time delays was needed from the crystal and / or second TDC plate of different thicknesses.

[0108] Experiment 2 - Time delay compensation in LBO cut along the Z and Y axes The experiment was conducted using the Z-axis (TDC). 0 / 0 ) and Y-axis (TDC) 90 / 90 This was done to investigate time delay compensation in LBO material cut along the z-axis. The experimental parameters and results are shown in Figures 10a (z-axis results) and 10b (y-axis results). The results of these experiments showed that the LBO cut along the z-axis exhibited the highest TDC dynamic range due to less hotspot formation and absorption. The LBO cut along the y-axis showed a much lower TDC dynamic range than the LBO cut along the z-axis, but showed a higher absolute TD value than the LBO along the z-axis, and exhibited the lowest hotspot formation and absorption.

[0109] The DUV laser system described herein has many potential applications. The output wavelength and power range are sufficient to destroy microbial and viral pathogens, but the penetration depth is very small (i.e., less than 1 micron), so it does not harm the human body. This means the system can be run continuously in indoor environments where many people gather, such as schools, airplanes, other modes of transport (subways, trains, buses, etc.), shops and malls, convention centers, and restaurants. Output power and power density can also be adjusted to suit specific environments and / or applications. The coherent laser source allows for precise (and instantaneous) irradiation of the entire space to be disinfected with light energy and quantity. This precision cannot be achieved with conventional lamps. According to at least one embodiment, the systems and methods disclosed herein may include the step of irradiating at least one microbial or viral pathogen with a fifth-harmonic laser beam.

[0110] The embodiments disclosed herein in accordance with the present invention are not limited in their application to the configuration details and arrangement of components described below or shown in the accompanying drawings. These embodiments may envision other embodiments and may be practiced or implemented in various ways. Specific implementation examples are provided herein for illustrative purposes only and are not intended to limit the scope. In particular, the operations, components, elements, and features described in relation to any one or more embodiments are not intended to be excluded from similar roles in any other embodiments.

[0111] Furthermore, the expressions and terminology used herein are for illustrative purposes only and should not be considered limiting. Singular references to examples, embodiments, components, elements, or the operation of systems and methods may include plural embodiments, and plural references to any embodiment, component, element, or act herein may include singular embodiments. Singular or plural references are not intended to limit the systems or methods, their components, acts, or elements currently disclosed. The use of “includes,” “equips,” “has,” “contains,” “involves,” and variations thereof herein means that the following items and their equivalents, as well as additional items, are included. References to “or” are construed as inclusive, and terms using “or” may refer to any single, plural, or all of the terms listed. Furthermore, if there is any inconsistency in the use of terminology between this document and documents incorporated herein by reference, the use of terminology in the incorporated references shall supplement the terminology in this document and, in the event of irreconcilable inconsistencies, the use of terminology in this document shall be governed by the use of terminology in this document. Furthermore, titles or subtitles may be used in this specification for the convenience of the reader, but they do not affect the scope of the invention.

[0112] Thus, while several aspects of at least one example have been described, it should be understood that those skilled in the art will readily come up with a variety of changes, modifications, and improvements. For example, the examples disclosed herein can be used in other circumstances. Such changes, modifications, and improvements are intended to be part of this disclosure and within the scope of the examples described herein. Accordingly, the foregoing description and drawings are illustrative only. [Explanation of Symbols]

[0113] 100, 200, 300a, 300b, 400, 1100 DUV laser systems 102 Basic Laser Beam 105 Fifth harmonic laser beam 110, 510, 610 laser sources 120 Nonlinear Crystal Assembly 122 The first nonlinear crystal 124 The second nonlinear crystal 126 The Third Nonlinear Crystal 130 Compensation Plate 235, 335 semi-corrugated plate 237, 337 telephoto lenses 240, 245, 245a, 245b, 245c Oven 250 Controllers 304 Second harmonic laser beam 306 Third harmonic laser beam 330a First compensation plate 330b Second compensation plate 512 Main Oscillator 515a Preamplifier 515b Fiber Power Amplifier 516 Pulse extender 518 Pulse Compressor 612 Mode-Locked FS Laser Source

Claims

1. A deep ultraviolet (DUV) laser system, A fiber laser source configured to emit a fundamental wave laser beam at the fundamental wavelength in the near-infrared region, wherein the fundamental wave laser beam is configured as a pulsed laser beam in which each pulse has a pulse duration of less than 400 femtoseconds (fs), A nonlinear crystal assembly comprising first, second, and third nonlinear crystals, configured to convert the fundamental wave laser beam to generate a fifth harmonic laser beam having a wavelength in the range of 200 nanometers (nm) to 230 nm, At least one compensation plate positioned at least one location preceding at least one of the first, second, and third nonlinear crystals, wherein a pair of pulsed laser beams transmitted through the at least one compensation plate are configured to overlap spatially and temporally within at least one of the first, second, and third nonlinear crystals, One or more ovens, each of which is configured to adjust the temperature of at least one compensation plate to compensate for the time delay between the pair of pulsed laser beams, A deep ultraviolet (DUV) laser system equipped with [specific features / features].

2. The DUV laser system according to claim 1, further comprising a controller configured to control the temperature based on the intensity value of the fundamental wave laser beam emitted from the fiber laser source.

3. The first nonlinear crystal is configured to receive the fundamental wave laser beam, convert the fundamental wave laser beam, and emit a second harmonic laser beam and the fundamental wave laser beam. The second nonlinear crystal is configured to receive the fundamental laser beam and the second harmonic laser beam, and to perform sum-frequency mixing of the fundamental laser beam and the second harmonic laser beam to generate a third harmonic laser beam and the second harmonic laser beam. The third nonlinear crystal is configured to receive the second harmonic laser beam and the third harmonic laser beam, and to perform sum-frequency mixing of the second harmonic laser beam and the third harmonic laser beam to generate a fifth harmonic laser beam. The DUV laser system according to claim 1.

4. The system further comprises at least one half-wave plate and at least two compensation plates, wherein the at least two compensation plates are made from LBO and comprise a first compensation plate located upstream of the first nonlinear crystal or between the first and second nonlinear crystals, and a second compensation plate located between the second and third nonlinear crystals. The DUV laser system according to claim 3, wherein the half-wave plate is disposed upstream of the first compensation plate, between the first compensation plate and the second nonlinear crystal, or between the second compensation plate and the third nonlinear crystal.

5. The DUV laser system according to claim 4, wherein the second nonlinear crystal is a type I crystal of LBO or a type II crystal of LBO.

6. The DUV laser system according to claim 3, wherein the first, second, and third nonlinear crystals each include LBO, LBO, and BBO, respectively.

7. The DUV laser system according to claim 3, further comprising at least one telescope lens located upstream of the first nonlinear crystal, wherein the at least one telescope lens is configured such that an incident light beam to the at least one telescope lens enters the at least one telescope lens as a light beam of a first diameter and exits the at least one telescope lens as a light beam of a second diameter.

8. The first nonlinear crystal is configured to receive the fundamental wave laser beam, convert the fundamental wave laser beam, and emit a second harmonic laser beam and the fundamental wave laser beam. The second nonlinear crystal is configured to convert the second harmonic laser beam to generate a fourth harmonic laser beam. The third nonlinear crystal is configured to receive the fundamental wave laser beam and the fourth harmonic laser beam, and to perform sum-frequency mixing of the fundamental wave laser beam and the fourth harmonic laser beam to generate a fifth harmonic laser beam. The DUV laser system according to claim 1.

9. The DUV laser system according to claim 1, further comprising at least one oven for adjusting the temperature of the nonlinear crystals of the nonlinear crystal assembly, thereby adjusting at least one of the first, second, and third nonlinear crystals to an optimal temperature at which nonlinear multiphoton absorption by the crystalline material of at least one of the first, second, and third nonlinear crystals is minimized.

10. The DUV laser system according to claim 1, wherein the wavelength of the fundamental laser beam is in the range of 1030 to 1080 nm and has an average output of at least 60 watts, while the wavelength of the fifth harmonic laser beam is 206 to 216 nm and has an average output of at least 1 watt (W).

11. The DUV laser system according to claim 1, wherein the fiber laser source includes a mode-locked fiber laser, a pulse amplifier, a pulse extender, and a pulse compressor configured for chirp pulse amplification.

12. A method for generating deep ultraviolet (DUV) laser light, A laser source is used to generate a fundamental wave laser beam of a near-infrared fundamental wavelength, wherein the fundamental wave laser beam is configured as a pulsed laser beam in which each pulse has a pulse duration of less than 400 femtoseconds (fs). A nonlinear crystal assembly comprising first, second, and third nonlinear crystals, wherein the nonlinear crystal assembly is configured to convert the fundamental wave laser beam into a fifth harmonic laser beam having a wavelength in the range of 200 nanometers (nm) to 230 nm, and the steps of guiding the fundamental wave laser beam through the nonlinear crystal assembly, The steps include: positioning at least one compensation plate at at least one location in front of at least one of the first, second, and third nonlinear crystals, wherein the at least one compensation plate is configured such that pairs of pulsed laser beams transmitted through the at least one compensation plate spatially and temporally overlap within at least one of the first, second, and third nonlinear crystals; A step of placing the at least one compensation plate in an oven, wherein the oven is configured to adjust the temperature of the at least one compensation plate; Methods that include...

13. The method according to claim 12, further comprising the step of controlling the oven such that the temperature of the at least one compensation plate compensates for the time delay between the pairs of pulsed laser beams.

14. The steps include: positioning a half-wave plate in front of at least one of the first, second, and third nonlinear crystals of the nonlinear crystal assembly; The steps include: positioning a pair of telescope lenses in front of the first nonlinear crystal; The method according to claim 12, further comprising:

15. The method according to claim 12, wherein the wavelength of the fifth harmonic laser beam is 206 nm or less, and the average output power is at least 1 watt (W).

16. The method according to claim 12, further comprising the step of providing the at least one compensation plate made from the LBO.

17. The method according to claim 12, further comprising the step of providing the laser source, wherein the laser source includes a mode-locked fiber laser and a chirp pulse amplifier including a pulse extender and a pulse compressor configured for chirp pulse amplification.

18. A step of placing at least one of the first, second, and third nonlinear crystals in an oven, thereby adjusting the temperature of at least one of the first, second, and third nonlinear crystals, The steps include controlling the oven so that the temperature of at least one of the first, second, and third nonlinear crystals is the optimal temperature at which nonlinear multiphoton absorption by the crystalline material of at least one of the first, second, and third nonlinear crystals is minimized, The method according to claim 12, further comprising:

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