English:
[0001] The disclosure relates to a high power solid state laser provided with at least one nonlinear converter based on patterned Strontium Tetraborate SrB4B07 (SBO) and Lead
[0002] The demand for laser tools in the ultraviolet (UV) and particularly high power deep UV (DUV) range is growing enormously to address the needs experienced by heavy industries, medicine, data storage, optical communication, entertainment and others Advances in semiconductor photolithography, micromachining and material-processing applications, for example, are driving demand for coherent light sources operating in UV and DUV spectral regions.
[0003] Although some gas lasers, such as excimer lasers can emit isolated wavelengths of coherent light in the UV and DUV spectral regions with a high average output power, compact and efficient solid-state lasers with nonlinear optical (NLO) crystals in this spectral range are still needed due to their well-known high efficiency, low maintenance, small footprint and overall low cost. The performance of solid-state lasers in the UV and DUV spectral regions depends mostly on advances in the fabrication of efficient and reliable NLO crystals discovered over the last two decades.
[0004] The copending US patent application No. discloses a method for fabricating a patterned non-ferromagnetic nonlinear SBO or PBO fully incorporated herein by reference. This subgroup of borates has some remarkable properties. First, it has a uniquely large (even among borates) bandgap of ~9 eV and its UV cut-off is about 130 nm. There is no literature data, but very likely (as many other borates) the SBO should be very transparent in VIS near infrared (IR). Its absorption should be in a single ppm/cm range. It is mechanically stable and non- hydroscopic. It is easy to grow this crystal by the known conventional techniques.
[0005] In addition, these crystals have a very high (for borate) thermal conductivity of ~16 W/m*K, It is an order of magnitude higher than that of BBO and LBO. Last but not least, the SBO crystal is one of a very few non-linear materials (if not the only one) which does not have two-photon absorption (TP A) at 266 nm - a nonlinear effect increasing the power loss and light- induced damage. Combined with the unique optical transparency and high LIDT, the SBO/PbBO crystal is probably the only non-linear material capable of withstanding sustainable multi-watt operation (pulsed and CW) at 266 nm with fluencies typical for non-linear conversion regimes (-100-500 MW/cm2). Clearly with the periodic phase matching structure method of fabrication disclosed in the copending application this group of borates is an ideal material for nonlinear interactions.
[0006] It is, therefore, desirable to provide a laser based on SBO or PBO.
[0007] This need is satisfied by a group of high power laser systems capable of operating in a UV frequency range. All of the disclosed systems have a common general optical schematic. The latter is provided with a laser source and at least one frequency converter so as to output sub-nanosecond, preferably picosecond pulses in a UV spectral region. As one of ordinary skill readily knows, ps fiber lasers participating in generating higher harmonics, such as UV light, are advantageous over ns fiber lasers because the nonlinear crystals in the ps pulsed regime have longer useful life than that of crystals irradiated by ns pulses. This advantage becomes even more prominent when the SBO or PBO is used since there is no 2 -photon absorption is these crystals.
[0009] FIG. 1 is a general optical schematic of the inventive laser system;
[0010] FIG. 2 shows a patterned SBO/PBO crystal of the inventive system;
[0011] FIG. 3 is an exemplary schematic of the inventive system of FIG. 1 used for generation of the FH.
[0012] FIG. 4 is an exemplary schematic of the inventive system of FIG. 1 used for generation of the third and higher harmonics.
[0013] FIG. 5 is an exemplary schematic of the inventive system of FIG. 1 used for generation of the fifth harmonic.
[0015] FIG. 7 is the SBO/PBO crystal of FIG. 2 configured from a single slab to provide a frequency conversion of the fundamental frequency into a plurality of successive harmonics.
[0016] Reference will now be made in detail to the disclosed inventive concepts. Wherever possible, same or similar reference numerals are used in the drawings and the description to refer to the same or like parts or steps. The drawings are in simplified form being far from precise scale.
[0017] FIG. 1 illustrates a general optical schematic 40 of the inventive laser system. The schematic 40 is configured as a source of electromagnetic (EM) radiation 42 incident on a frequency converter 44 which is based, at least in part, on patterned SBO or PBO nonlinear crystal 10 and configured to convert a fundamental frequency into a higher harmonic. Typically, the converters are placed in a single-pass or multi-pass resonator.
[0018] The EM source 42 is a laser system operating in various regimes which includes continuous a wave (CW) mode, quasi-continuous wave (QCW) mode and pulsed modes. For many applications, source 42 is a high power source with the output of at least 1 kW and as high as of MWs. However, laser systems operating under a kW power level are also part of the disclosed subject matter.
[0019] The configuration of source 42 is not limited to any particular lasing medium. Preferably, it is a solid state laser system including fiber and yttrium aluminum glass (Y AG) lasing media, with the disk lasers being a subclass of YAGs. The light emitting ions doped in the lasing media include various rare-earth metals. Since an industrial range of fundamental wavelengths and their higher harmonics is mostly associated with laser sources emitting light in a 1 - 2 mm range, ions of ytterbium (Yb), erbium (Er), neodymium (Nd), and Thulium perhaps are most frequently used. The mentioned elements are however do not represent the exclusive list of all rare earth elements that may be used for light generation in the inventive system.
[0020] The architecture of laser source 42 may be represented by a variety of specific configurations. For example, the laser source may have a MOP A configuration including a combination of master oscillator (MO) 43 and power amplifier (PA) 44. The MO 43 may include semiconductors or fibers preferably operating at a single frequency. For example, MO 43 can be configured in accordance with the schematics disclosed in PCT/US 15/65798 and PCT/US 15/52893 which are owned by the assignee of the current applications and incorporated here by reference in their entirety. Considering that modem power levels of known oscillators have reached a kW level, the architecture of source 42 may be represented only by lasers omitting thus the amplifier. As to the amplifier, its examples can be found in PCT/US2017/064297 disclosing an Yb/YAG system or USP 8068705 disclosing a fiber amplifier and many others owned by the assignee of the current application and fully incorporated herein by reference. Regardless of its configuration, laser source 42 preferably outputs a single frequency, single transverse mode sub-nanosecond output in the QCW and pulsed regime. However, a beam quality factor M2may be higher than 1, for example 1.5.
[0021] Referring to FIGs. 1 and 2, frequency converter 44 operates to generate a second harmonic (SH), third harmonic (TH), fourth harmonic (FH), and other higher harmonics as well as to perform optical parametric interactions. The crystal SBO or PBO 10 is configured with a periodic structure 12 of domains 30 and 32 having respective opposite polarities +/- which alternate one another. These domains have highly parallel walls. The periodic structure 12 allows the use of a QPM technique to generate high harmonic of the pump light. Recent experiments conducted by the Applicants resulted in crystal 10 provided with a volume periodic pattern which includes a sequence of uniformly dimensioned 3D-domains 30, 32 having respective positive and negative polarities which alternate one another and provide the crystal with a clear aperture having a diameter of up to a few centimeters. The domains each are configured with a uniform thickness corresponding to the desired coherence length 1 and ranging from about 0.2 mm to about 20 mm and a clear aperture which has a dimeter varying from about 1 mm to about 5 cm. The crystal 10 can be utilized as an optical element, such as a frequency converter incorporated in a laser which operates in a variety of frequency ranges. For example, crystal 10, configured to convert a fundamental frequency of laser source 42 to a DUV range, has a coherence length 1 ranging between 0.2 to about 5 nm. The volume pattern may extend through the entire thickness of crystal block 10 between faces +C and -C, or terminate at a distance from one of these faces. The crystal 10 is based on the discussed above unique qualities and disclosed in copending, co-owned US application 62781371which is filed concurrently with the subject matter application which incorporates it by reference in its entirety.
[0022] The SBO/PBO 10 is characterized by a short UV absorption cut-off (lcutoff) or wide energy bandgap (Eg) which guarantee the transmittance in the UV and DUV spectra. Moreover, the large bandgap significantly decreases the two-photon absorption or multi-photon absorption, and thus, in turn, increases the laser-induced damage threshold in a crystal and results in reduced non-desirable thermo-optical effects. Linear absorption of borates is typically very low as well.
[0023] Accordingly, SBO/PBO crystal 10 is particularly attractive when used in laser systems operating in ultraviolet/deep ultraviolet (UV/DUV) frequency ranges. UV/DUV lasers are widely employed in various applications. For instance, a DUV at 266 nm has been utilized as an external seed of a free-electron laser with outputs as short as about 4 nm so useful in the scientific research beyond the carbon K-edge. The industrial applications, laser machining of wide bandgap materials, microelectronics and many other are direct beneficiaries of the DUV lasers owing to their high photon energy. The conversion schemes are numerous and examples thereof are disclosed hereinbelow.
[0024] Referring to FIG. 3, an exemplary schematic setup of system 40 includes converter 46 configured with SHG 46 and FHG 48 stages. The SHG 46 doubles the frequency of the pump wave in a 1 mm wavelength range to Green light and the latter continuing frequency conversion to obtain ultraviolet/deep UV (UV/DUV 50) light at or lower than a 2xx nm wavelength. For example, a pump wavelength at a 1060 nm output by source 42 (fundamental frequency w), is converted into a second harmonic 2w (532 nm wavelength) in SHG 46 which, in turn, is converted into the fourth harmonic 4w (266 nm wavelength.) The SHG 46 may be based on BBO, LBO CLBO, SBO, PBO and other nonlinear crystals. The FHG 48 includes SBO/PBO crystal 10.
[0025] FIG. 4 exemplifies a schematic configured to generate the third harmonic (THG) 50. The system 40 includes source 42 outputting light at fundamental frequency w which is incident on SHG 46. The latter 46 converts the fundamental frequency into second harmonic 2w. The THG 50 receives a remaining portion of light at the fundament frequency and second harmonic and combines these frequencies to create the third harmonic. The SHG 46 may have the configuration of FIG. 3, and so does THG 50 including SBO/PBO crystal 10. A non- inclusive example can be illustrated by a fundamental wavelength of 1064 nm which eventually is converted into the TH of about 355 nm. The system 40 may be further provided with a FiHG 52 combining the unused SH and generated TH.
[0026] FIG. 5 illustrates still another example of system 40 with converter 44 configured to generate the fifth harmonic (FiHG). The converter 44 operates by initially generating the SH in SHG 46. The unused light at the fundamental (pump) is separated from the SH at the output of SHG 46 and further guided to the fifth harmonic generator (FiHG) 52 along a path defined by reflective elements, such as mirrors or prisms. If desired, the unconverted light at the fundamental frequency can be guided through FHG 48.
[0027] Based on the foregoing, SBO/PBP quasi phase matched crystal 10 can be used for frequency doubling, tripling etc, as well as for sum and difference frequency generation. It also can be used for parametric amplification. Referring to FIG. 6, light at a signal wavelength propagates through crystal 10 together with a pump beam of shorter wavelength resulting in several outputs which include an idler, residual pump beam and signal separate outputs, as well known to one of ordinary skill.
[0028] As known to one of ordinary skill, it is impractical to use standard crystals, such as PPKTP or PIPLIN for generating the FH because this harmonic of 1- 2” fundamental wavelength coincides with (or even falls beyond) the cutoff wavelength of these materials. The crystals that may generate the mentioned FH have very low nonlinearity. The SBO/PBO, however, is highly nonlinear and has a cutoff wavelength around 130 nm which obviously extends its conversion abilities way into the DUV frequency range allowing thus inventive laser system 40 operate way below 200 nm and even below 160 nm.
[0029] FIG. 7 illustrates another configuration of system 40 including laser source 42 of FIG. 1, which is a diode laser in this schematic, and SBO/PBO 10. The latter is configured from a monolithic slab which can first double the fundamental frequency and further generate a higher harmonic at for example 355 nm and 266 nm. For this reason, the domain period along a path of light at the fundamental frequency through the slab varies from one for SHG and, then, for example, the FHG. Such a configuration can be used in a microchip of no longer than 5 - 10 mm and including a laser diode on vanadate and SBO/PBO 10 to produce a mW output. The configuration of crystal 10 may include more than two periods.
[0030] It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. For example, the pulsed regime of the disclosed systems can be implemented by utilizing a chirp pulse amplification technique. The pulse laser sources further may be based on a passively mode locked or actively mode locked lasers outputting nanosecond, and sub nanosecond, i.e., femtosecond and picosecond pulses. The average power of the output of the disclosed pulsed systems may vary between milliwatts (mW) and about 100 W in UV/DUV frequency ranges. The disclosed schematic can operate generating harmonics higher than the fifth harmonic. Accordingly, other aspects, advantages, and modifications are within the scope of the following claims.

中文:
[0001]本公开涉及一种高功率固态激光器,其设置有基于图案化四硼酸锶SrB4BO7(SBO)和四硼酸铅PbB4O7(PBO)晶体的至少一个非线性转换器。
[0002]为满足重工业、医药、数据存储、光通信、娱乐等领域的需求,对紫外(UV)和特别是高功率深UV(DUV)范围内的激光工具的需求正在大幅增长。例如,半导体光刻、微加工和材料加工应用的进步正在推动对在UV和DUV光谱区域操作的相干光源的需求。
[0003]尽管一些气体激光器(例如,准分子激光器)可以发射具有高平均输出功率在UV和DUV光谱区域内的孤立波长的相干光,但在该光谱范围内具有非线性光学(NLO)晶体的紧凑且高效的固态激光器由于其众所周知的高效率、低维护、小占地面积和总体低成本仍然被需要。在UV和DUV光谱区域内固态激光器的性能主要取决于过去二十年发现的高效且可靠的NLO晶体制造方面的进步。
[0004]共同未决美国专利申请No.公开了一种用于制造图案化的非铁磁非线性SBO或PBO的方法,通过引用将其全部并入本文。该子组硼酸盐具有一些显著的特性。首先,它具有的独特的~9eV的大(甚至在硼酸盐中)带隙,并且其UV截止波长约为130nm。没有文献数据,但很可能(与许多其他硼酸盐一样)SBO在VIS近红外(IR)中应该非常透明。其吸收应在单个ppm/cm范围内。它是机械稳定的和非吸湿的。通过已知的常规技术很容易生长这种晶体。
[0005]此外,这些晶体具有非常高的(对于硼酸盐)~16W/m*K的热导率,比BBO和LBO高一个数量级。最后但并非最不重要的,SBO晶体是极少数在266nm处不具有双光子吸收(TPA)的非线性材料之一(如果不是唯一的)——非线性效应会增加功率损耗和光致损伤。结合独特的光学透明性和高LIDT,SBO/PbBO晶体可能是唯一能够承受266nm的可持续多瓦操作(脉冲和CW)的非线性材料,并且具有针对非线性转换机制的典型流畅性(~100-500MW/cm2)。显然,通过使用共同未决申请中公开的周期性相位匹配结构制造方法,该子组硼酸盐是非线性交互的理想材料。
[0006]因此,希望提供基于SBO或PBO的激光器。
[0007]一组能够在UV频率范围内操作的高功率激光系统满足了这一需求。所有公开的系统都具有共同的通用光学示意图。后者设置有激光源和至少一个频率转换器,以便在UV光谱区域内输出亚纳秒脉冲,优选地为皮秒脉冲。如一名普通技术人员所知,参与产生高次谐波(例如,UV光)的ps光纤激光器优于ns光纤激光器,因为ps脉冲机制下的非线性晶体比由ns脉冲照射的晶体具有更长的使用寿命。当使用SBO或PBO时,这种优势变得更加突出,因为这些晶体没有2光子吸收。
[0009]图1是本发明的激光系统的一般光学示意图;
[0010]图2示出了本发明系统的图案化的SBO/PBO晶体;
[0011]图3是图1的本发明系统用于产生FH的示例性示意图。
[0012]图4是图1的本发明系统用于产生三次和更高次谐波的示例性示意图。
[0013]图5是图1的本发明系统用于产生五次谐波的示例性示意图。
[0015]图7是图2的由单个板件(slab)配置以提供基频到多个连续谐波的频率转换的SBO/PBO晶体。
[0016]现在将详细参考所公开的发明构思。在可能的情况下,在附图和说明书中使用相同或相似的附图标记来表示相同或相似的部件或步骤。附图为简化形式,且完全不是按精确比例绘制的。
[0017]图1示出了本发明的激光系统的一般光学示意图40。示意图40被配置为电磁(EM)辐射源42入射在频率转换器44上,该频率转换器44至少部分地基于图案化的SBO或PBO非线性晶体10,并且被配置为将基频转换为高次谐波。通常,转换器被放置在单通或多通谐振器中。
[0018]EM源42是在包括连续波(CW)模式、准连续波(QCW)模式和脉冲模式的各种机制下操作的激光系统。对于许多应用,源42是具有至少1kW和高达MW的输出的高功率源。然而,在kW功率水平下操作的激光系统也是所公开主题的一部分。
[0019]源42的配置不限于任何特定的激光介质。优选地,它是包括光纤和钇铝玻璃(YAG)激光介质的固态激光器系统,并且盘式激光器是YAG的子类。掺杂在激光介质中的发光离子包括各种稀土金属。由于工业范围的基本波长及其高次谐波主要与发射1-2μm范围内的光的激光源相关联,因此镱(Yb)、铒(Er)、钕(Nd)和铥离子可能是最常用的。然而,所提及的元素并不表示可以用于本发明系统中的光产生的所有稀土元素的排他性列表。
[0020]激光源42的架构可以由多种特定配置表示。例如,激光源可以具有MOP A配置,其包括主振荡器(MO)43和功率放大器(PA)44的组合。MO 43可以包括优选地以单一频率操作的半导体或光纤。例如,MO 43可以根据由本申请的受让人拥有的PCT/US 15/65798和PCT/US 15/52893中公开的示意图进行配置,并且通过引用整体并入本文。考虑到已知振荡器的现代功率水平已经达到kW水平,源42的架构可以仅由激光器来表示,因此省略放大器。对于放大器,其示例可以在公开了Yb/YAG系统的PCT/US2017/064297或公开了光纤放大器的USP8068705和本申请的受让人拥有的许多其他专利中找到,它们通过引用完全并入本文。不管其配置如何,激光源42优选地在QCW和脉冲机制下输出单频、单横模亚纳秒输出。然而,光束品质因数M2可以高于1,例如,1.5。
[0021]参照图1和图2,频率转换器44操作以产生二次谐波(SH)、三次谐波(TH)、四次谐波(FH)和其他高次谐波,以便执行光参量交互。晶体SBO或PBO 10被配置有域30和32的周期性结构12,域30和32具有各自的彼此交替的相反极性+/-。这些域具有高度平行的壁(wall)。周期性结构12允许使用QPM技术来产生泵浦光的高次谐波。申请人最近进行的实验得到设置有体积周期图案的晶体10,该图案包括一系列均匀尺寸的3D域30、32,这些域具有各自的正极性和负极性,这些极性彼此交替,并且为晶体提供具有高达几厘米直径的通光孔径。每个域都配置有对应于所需相干长度1并且范围从约0.2μm到约20μm的均匀厚度,以及具有从约1mm至约5cm变化的直径的通光孔径。晶体10可以用作光学元件,例如,在各种频率范围内操作的激光器中包含的频率转换器。例如,被配置为将激光源42的基频转换到DUV范围的晶体10具有范围在0.2到约5nm之间的相干长度1。体积图案可以延伸穿过晶体块10的在面+C和-C之间的整个厚度,或者终止于与这些面之一相距一定距离处。晶体10基于以上讨论的独特品质并在共同未决的、共同拥有的美国申请62781371中公开,该申请与主题申请同时提交,该主题申请通过引用将其整体并入本文。
[0022]SBO/PBO 10的特征在于短UV吸收截止(λcutoff)或宽能带隙(Eg),其保证在UV和DUV光谱中的透射率。此外,大带隙显著降低了双光子吸收或多光子吸收,并且因此,反过来,增加了晶体中激光致损阈值,并且导致不希望的热光效应降低。硼酸盐的线性吸收通常也非常低。
[0023]因此,SBO/PBO晶体10当在紫外/深紫外(UV/DUV)频率范围内操作的激光系统中使用时特别有吸引力。UV/DUV激光器广泛用于各种应用。例如,266nm的DUV已被用作自由电子激光器的外部种子,其输出短至约4nm,因此在碳K边(K-edge)以外的科学研究中非常有用。由于DUV激光器的高光子能量,工业应用、宽带隙材料的激光加工、微电子和许多其他领域都是其直接受益者。转换方案很多并且其示例在下文中公开。
[0024]参照图3,系统40的示例性示意性装置包括配置有SHG 46和FHG 48级的转换器46。SHG 46将1mm波长范围内的泵浦波频率加倍为绿光,并且后者继续进行频率转换以获得2倍nm或低于2倍nm波长的紫外/深UV(UV/DUV 50)光。例如,光源42输出的1060nm的泵浦波长(基频ω)在SHG 46中被转换为二次谐波2ω(532nm波长),继而,该二次谐波被转换为四次谐波4ω(266nm波长)。SHG 46可以基于BBO、LBO CLBO、SBO、PBO和其他非线性晶体。FHG 48包括SBO/PBO晶体10。
[0025]图4举例说明了被配置为产生三次谐波(THG)50的示意图。系统40包括输出基频ω的光的光源42,该光入射在SHG 46上。后者46将基频转换为二次谐波2ω。THG 50接收二次谐波和基频的剩余部分光,并将这些频率组合以产生三次谐波。SHG 46可以具有图3的配置,并且包括SBO/PBO晶体10的THG 50也是如此。非包容性示例可以用1064nm的基本波长来说明,该基本波长最终被转换为约355mm的TH。系统40还可以设置有组合未使用的SH和所产生的TH的FiHG52。
[0026]图5示出了系统40的又一示例,其中,转换器44被配置为产生五次谐波(FiHG)。转换器44通过最初在SHG 46中产生SH来操作。基本的(泵浦)未使用的光在SHG 46的输出处与SH分离,并进一步沿着由反射元件(例如,镜子或棱镜)限定的路径引导到五次谐波发生器(FiHG)52。如果需要,可以引导基频的未转换光通过FHG 48。
[0027]基于以上所述,SBO/PBP准相位匹配晶体10可以用于倍频、三倍频等,以及用于产生和频和差频。它还可以用于参数放大。参照图6,信号波长的光与较短波长的泵浦光束一起传播通过晶体10,如一名普通技术人员所知,这得到包括闲频(idler)、剩余泵浦光束和信号分离输出的若干输出。
[0028]正如一名普通技术人员所知,使用标准晶体(例如,PPKTP或PIPLIN)来产生FH是不切实际的,因为这种1-2”基本波长的谐波与这些材料的截止波长一致(或甚至超出)。可以产生上述FH的晶体具有非常低的非线性。然而,SBO/PBO是高度非线性的,并且具有约130nm的截止波长,这显然将其转换能力扩展到DUV频率范围,从而允许本发明的激光系统40在远低于200nm和甚至低于160nm的情况下操作。
[0029]图7示出了包括图1的激光源42(其是该示意图中的二极管激光器)以及SBO/PBO10的系统40的另一种配置。后者由单片板件(slab)配置,该板件可以首先将基频加倍,并进一步产生例如355nm和266nm的高次谐波。出于这个原因,沿着基频光通过板件的路径的域周期从SHG的域周期变化,然后是例如FHG的域周期。这种配置可以用于不长于5-10mm并且包括钒酸盐激光二极管和SBO/PBO 10的微芯片,以产生mW输出。晶体10的配置可以包括多于两个周期。
[0030]应当理解的是,尽管已经结合本发明的详细描述对本发明进行了描述,但是上述描述旨在说明而非限制本发明的范围,本发明的范围由所附权利要求限定。例如,所公开系统的脉冲机制可以通过利用啁啾脉冲放大技术来实现。脉冲激光源还可以基于输出纳秒和亚纳秒(即,飞秒和皮秒脉冲)的被动锁模或主动锁模激光器。在UV/DUV频率范围内,所公开的脉冲系统的输出的平均功率可以在毫瓦(mW)和约100W之间变化。所公开的示意图可以操作以产生高于五次谐波的谐波。因此,其他方面、优点和修改在所附权利要求的范围内。