Patent Description:
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.

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.

The copending US patent application No. <CIT> discloses a method for fabricating a patterned non-ferromagnetic nonlinear SBO or PBO.

This sub-group of borates has some remarkable properties. First, it has a uniquely large (even among borates) bandgap of ~<NUM> eV and its UV cut-off is about <NUM>. 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 nonhydroscopic. It is easy to grow this crystal by the known conventional techniques.

In addition, these crystals have a very high (for borate) thermal conductivity of ~<NUM> 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 (TPA) at <NUM> - 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 <NUM> with fluencies typical for non-linear conversion regimes (-<NUM>-<NUM> MW/cm<NUM>). Clearly with the periodic phase matching structure method of fabrication disclosed in the copending application <CIT>, this group of borates is an ideal material for nonlinear interactions.

<NPL> and <NPL> both disclose exemplary SBO crystals having a multi-domain structure.

It is, therefore, desirable to provide a laser based on SBO or PBO.

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 <NUM>-photon absorption is these crystals.

The above and other aspects and feature will become more readily apparent in conjunction with the following drawings, in which:.

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.

<FIG> illustrates a general optical schematic <NUM> of the inventive laser system. The schematic <NUM> is configured as a source of electromagnetic (EM) radiation <NUM> incident on a frequency converter <NUM> which is based, at least in part, on patterned SBO or PBO nonlinear crystal <NUM> and configured to convert a fundamental frequency into a higher harmonic. Typically, the converters are placed in a single-pass or multi-pass resonator.

The EM source <NUM> 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 <NUM> is a high power source with the output of at least <NUM> kW and as high as of MWs. However, laser systems operating under a kW power level are also part of the disclosed subject matter.

The configuration of source <NUM> is not limited to any particular lasing medium. Preferably, it is a solid state laser system including fiber and yttrium aluminum garnet (YAG) lasing media, with the disk lasers being a subclass of YAGs. The light emitting ions doped in the lasing media care 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 <NUM> - <NUM> range, light emitters may include ions of ytterbium (Yb), erbium (Er), neodymium (Nd), and Thulium. The mentioned elements are however do not represent the exclusive list of all rare earth elements that may be used for light generation.

The architecture of laser source <NUM> may be represented by a variety of specific configurations. For example, the illustrated exemplary schematic of laser source includes a combination of oscillator <NUM> and power amplifier <NUM> representing a well-known master oscillator (MO) power amplifier (PA) scheme (MOPA). The MO <NUM> may include semiconductors or fibers preferably operating at a single frequency. For example, MO <NUM> can be configured in accordance with the schematics disclosed in <CIT> and <CIT> which are owned by the assignee of the current applications.

Considering that modern power levels of known oscillators have reached a kW level, the architecture of source <NUM> may be represented only by lasers omitting thus the amplifier. As to the amplifier, its examples can be found in <CIT> disclosing an Yb/YAG system or <CIT> disclosing a fiber amplifier and many others owned by the assignee of the current application.

Regardless of its configuration, laser source <NUM> preferably outputs a single frequency, single transverse mode sub-nanosecond output in the QCW and pulsed regime. However, a beam quality factor M<NUM> may be higher than <NUM>, for example <NUM>.

Referring to <FIG>, frequency converter <NUM> 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 <NUM> is configured with a periodic structure <NUM> of domains <NUM> and <NUM> having respective opposite polarities +/- which alternate one another. These domains have highly parallel walls. The periodic structure <NUM> allows the use of a QPM technique to generate high harmonic wavelength of the fundamental wave which includes second harmonic generation, third and higher harmonic generation, and optical parametric interactions. Recent experiments conducted by the Applicants resulted in crystal <NUM> provided with a volume periodic pattern which includes a sequence of uniformly dimensioned 3D-domains <NUM>, <NUM> 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 l and ranging from about <NUM> to about <NUM> and a clear aperture which has a diameter varying from about <NUM> to about <NUM>. The crystal <NUM> 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 <NUM>, configured to convert a fundamental frequency of laser source <NUM> to a DUV range, has a coherence length l ranging between <NUM> to about <NUM>. The volume pattern may extend through the entire thickness of crystal block <NUM> between faces +C and -C, or terminate at a distance from one of these faces. The crystal <NUM> is based on the discussed above unique qualities and disclosed in copending, co-owned <CIT>.

The SBO/PBO <NUM> is characterized by a relatively short UV absorption cut-off (λcutoff) 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.

Accordingly, SBO/PBO crystal 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 <NUM> has been utilized as an external seed of a free-electron laser with outputs as short as about <NUM> 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.

Referring to <FIG>, an exemplary schematic setup of system <NUM> includes converter <NUM> configured with SHG <NUM> and FHG <NUM> stages. The SHG <NUM> doubles the frequency of the pump wave in a <NUM> wavelength range to Green light and the latter continuing frequency conversion to obtain ultraviolet/deep UV (UV/DUV <NUM>) light at or lower than a 2xx nm wavelength. For example, a pump wavelength at a <NUM> output by source <NUM> (fundamental frequency ω), is converted into a second harmonic 2ω (<NUM> wavelength) in SHG <NUM> which, in turn, is converted into the fourth harmonic 4ω (<NUM> wavelength. ) The SHG <NUM> may be based on BBO, LBO CLBO, SBO, PBO and other nonlinear crystals. The FHG <NUM> includes SBO/PBO crystal <NUM>.

<FIG> exemplifies a schematic configured to generate a TH (THG) <NUM>. The system <NUM> includes source <NUM> outputting light at fundamental frequency ω which is incident on SHG <NUM>. The latter <NUM> converts the fundamental frequency into second harmonic 2ω. The THG <NUM> receives a remaining portion of light at the fundament frequency and second harmonic and combines these frequencies to create the third harmonic. The SHG <NUM> may have the configuration of <FIG>, while THG <NUM> includes SBO/PBO crystal <NUM>. A non-inclusive example can be illustrated by a fundamental wavelength of <NUM> which eventually is converted into the TH of about <NUM>. The system <NUM> may be further provided with a FiHG <NUM> combining the unused SH and generated TH.

<FIG> illustrates still another example of system <NUM> with converter <NUM> configured to generate the fifth harmonic (FiHG). The converter <NUM> operates by initially generating the SH in SHG <NUM>. The unused light at the fundamental (pump) is tapped off the SH at the output of SHG <NUM> and further guided to FiHG <NUM> 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 <NUM>.

Based on the foregoing, SBO/PBP quasi phase matched crystal <NUM> 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>, light at a signal wavelength propagates through crystal <NUM> 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.

As known to one of ordinary skill, it is irrational to use standard crystals, such as PPKTP or PIPLIN for generating the FH because this harmonic of <NUM>- <NUM>" fundamental wavelength coincides with (or even falls beyond) the cutoff wavelength of these materials. The crystals that may generate the FH have very low nonlinearity. The SBO/PBO, however, is highly nonlinear and has a cutoff wavelength around <NUM> which obviously extends its conversion abilities to this wavelength allowing thus inventive laser system <NUM> operate way below <NUM> and even below <NUM>, which is not possible to realize with the known crystals.

<FIG> illustrates another configuration of system <NUM> including laser source <NUM> which is a diode laser and SBO/PBO <NUM>. In light of the characteristics of the latter, SBO/PBO10 is configured from a monolithic slab with can sequentially double the fundamental frequency and further generate a higher harmonic at for example <NUM> and <NUM>. For this reason, the domain period along a path of light at the fundamental frequency varies from the one for SHG and, then, for example, the FHG. Such a configuration can be used in a microchip of no longer than <NUM>-<NUM> and including a laser diode on vanadate and SBO <NUM> to produce a mW output.

Claim 1:
A laser system (<NUM>), comprising:
a laser source (<NUM>) outputting light at a fundamental frequency; and
a frequency converter (<NUM>) operative to convert the fundamental frequency into a higher harmonic and including at least one frequency converting stage which is based on a SrB<NUM>O<NUM> (SBO) or PbB<NUM>O<NUM> (PBO) crystal (<NUM>),
wherein the SBO/PBO crystal (<NUM>) is configured with a plurality of domains (<NUM>, <NUM>) with respective periodically alternating polarity of the crystal axis,
wherein the domains (<NUM>, <NUM>) in the SBO/PBO crystal (<NUM>) are configured with a periodic structure (<NUM>) enabling use of quasi phase matching, QPM, and
wherein the domains (<NUM>, <NUM>) have highly parallel walls deviating from one another less than <NUM> over a <NUM> distance.