Most of the nonlinear optical material for ultraviolet generation are borate material such as LBO, BBO, and CLBO. There are the intrinsic disadvantages of those UV devices such as walk-off and hygroscopicity. As a result, the laser system based on those devices always require a special beam shape treatment and an anti-humidity control during storage and in operation. On the other hand, quasi phase matching devices are very attractive since there are no requirement of beam shaping. We focus on periodically poled LaBGeO5 (PP-LBGO) device since it is QPM structure and transparent at UV region. In addition, even it is borate crystal, it does not have hygroscopicity. Therefore, it is easy to treat in the fabrication process and assembling phase. In this paper, fabrication of PP-LBGO for 266nm generation was demonstrated with the first order QPM structure. The periodicity of the first order QPM for 266 nm generation from 532 nm is approximately 2.1 micrometer. The thickness of 0.3 mm LBGO plates are prepared. The electrode pattern was created on the plate. Several different electrode size and poling conditions were investigated. Finally, we successfully fabricated 2.1 micrometer periodically poled structure with 0.3 mm thick device with 10 mm in length. Its aspect ratio of periodic structure was achieved approximately 300. By using fabricated device, the better conversion efficiency for 266 nm generation was confirmed than former 2nd order PP-LBGO device by using pulsed 532 nm laser.
We focused on frequency conversion of simple and compact CW PM-Yb-doped fiber laser based on FBGs with wavelength adjustable function. By using the temperature dependency of FBG’s center wavelength, it is possible to adjust the oscillation wavelength by controlling both FBG temperatures. We tested 6 μm core based FBGs with at the center wavelength of 1040 nm, 1064 nm and 1090 nm. We found that it is possible to tune approximately 500 pm by controlling FBG temperatures between 15 and 85 degree-C. The tunability of wavelength was is around 6.0~8.5 pm/degree-C. It was possible to achieve 10 W of output for each wavelength. In addition, we applied this configuration to 10 μm core fiber at center wavelength of 1064 nm. It was obtained wavelength adjustability of 8.3 pm/degree-C with output of over 35 W. Its linewidth is narrower than 50 pm and suitable for frequency conversion. The tuning range was similar to the 6 μm core fiber case and slight difference and variations might be related to wavelength, refractive index distribution, thermal expansion coefficient, and fixation condition of FBGs. By using over 35 W fiber laser, it is possible to realize around 10 W of green-light SHG laser combined with high conversion efficiency QPM devices such as PP-Mg: SLTs with wavelength tunable function.
We developed a high-power, continuous-wave (CW), single-frequency 852nm laser source, for the purpose of fourth harmonic generation at 213nm. Our approach is the doubly resonant sum-frequency mixing (DRSFM) with two fiber sources. An in-house single-frequency master oscillator at 1907nm is amplified by an in-house clad-pumped amplifier to 5W, and a commercial single-frequency master oscillator at 1540nm is amplified by a commercial amplifier to 10W. The two beams are combined via a dichroic mirror to a single beam before incident on a dual-wavelength resonator, consisting of one set of dual-wavelength mirrors. The external resonator is locked to the 1907nm laser frequency, and the frequency of the 1540nm is locked to the resonator, realizing double-resonance. With a periodically-poled stoichiometric lithium tantalate in the resonator, the sum-frequency at 852nm is efficiently generated. All 3 waves are in the same polarization (e-ray), allowing the effective use of Brewster-cut device, eliminating reflection loss for all wavelengths without any antireflection coatings. With 4.6W at 1907nm and 7.7W at 1540nm incident onto the resonator, 5.2W at 852nm was generated, representing the efficiency of greater than 40%. The experimental result indicates our current setup will be more efficient with higher input powers at 1907nm. With both fiber sources at 1540nm and 1907nm being scalable in output power, the output at 852nm is also scalable. By the forth harmonic of 852nm, 0.456 W CW 213nm was generated.
We developed the dual line fiber laser module based on FBG combination. The proposed configuration has several advantages such as compact, simple, and inexpensive. The laser was composed pump LD (40W), two HR FBGs for 1053 nm and 1058 nm, Yb-doped fiber, two OC FBGs for 1053 nm and 1058 nm, and delivery fiber. All single mode fibers were polarization maintained with approximately 6 micron core. All FBGs were mounted on holders with TECs and their temperatures were controlled independently. The center wavelengths of HR and OC FBGs were temperature dependent and their shifts are approximately 7 nm/degree-C for all integrated FBG. By adjusting the temperature, it is possible to realize the resonant condition for only 1053 nm or only for 1058 nm. Based on this configuration, we demonstrated dual line CW fiber laser module. This module was compact with the size of 200 mm X 150 mm X 23 mm. By adjusting the FBG temperatures, we obtained the output power of more than 10 W at 1053 nm and 1058 nm with linear polarization.
We focused on wavelength conversion of simple and compact CW Yb-Doped fiber laser based on FBGs with wavelength adjustable function. By controlling temperatures of FBGs in fiber laser, it was possible to tune oscillated wavelength from 1064.101 nm to 1064.414 nm with more than 20 W in CW operation mode. Based on this fundamental light, frequency converted light (SHG and THG) were generated by utilizing two PP:Mg-SLT devises. We obtained more than 3 W of SHG light with tuning range of 150 pm and more than 35 mW of THG with tuning range of 100 pm. By selecting FBG grating and QPM grating properly, we can realize adjustable wavelength laser with the same scheme from 1040 nm to 1090 nm and their SHG/THG. With this combination of FBG based fiber laser and QPM devices, it is possible to tune the wavelength just by temperature tuning without any changes of beam shape and beam pointing.
In this work the performance of annealed proton-exchanged (APE) waveguides in periodically poled stoichiometric lithium tantalate (PPSLT) for high power applications in the C-band is investigated. Two APE-PPSLT chips comprising 50 waveguides produced with different poling periods and mask width for proton-exchange (PE) were characterized. The performance of the PPSLT devices was also compared with a periodically poled lithium niobate (PPLN) waveguide. Despite lower efficiency, no photorefractive issues or deleterious green light emission were observed in the PPSLT waveguides. The experimental results suggest that the homogeneity of the PPSLT waveguides can be further improved, which will enhance their efficiency.
Several watts compact CW green laser head without any cooling is demonstrated by combining CW fiber laser and PPMgSLT. Since the conventional high power visible laser has huge heat sources at its laser head, it requires air or water cooling. In addition, the optical system, which is mounted this type of head, has sometime problems of optical stability caused by those heat sources from the head. The laser head we demonstrated has three input and output ports of the laser light; fiber input of fundamental light (1064nm) from CW fiber laser, SHG (532 nm) output into free space, and fiber output of residual fundamental laser light. The size of laser head was 110mm X 78mm X 64mm (550cc). More than 25W of CW fundamental light from single mode fiber was focused into 30mm-long PPMgSLT device operated at 40 degree C. The linewidth of the laser was 0.09 nm at FWHM. 5W of 532 nm light was generated from PPMgSLT. Because of high power durability of PPMgSLT, it could be easily realized several watts of visible light generation by simple singlepass configuration. Residual fundamental light was separated by harmonic separator and was coupled into large core multi-mode fiber. As a result, there are no remarkable heat sources at the laser head. The stable green light from this head was confirmed without any cooling at the laser head. Since this configuration doesn’t affect any thermal turbulence in surroundings, the stability of the optical system would be improved by using this laser head.
Formation and relaxation dynamics of electron polarons in lithium niobate crystals is investigated by measuring transient
absorption induced by blue femtosecond pulses. Anisotropy in absorption change distinguishes between small free
polarons and small bound polarons, revealing that the dynamics is influenced by MgO-doping and stoichiometry control.
In crystals doped with MgO at concentrations above threshold, small free polarons are generated within 100 fs and decay
at tens of nanosecond. In the presence of antisite defects, sequential formation of polaronic states occurs: electrons
initially trapped as small free polarons become trapped as small bound polarons at picosecond time scale. The results are
relevant for nonlinear optical applications of pulsed or high-power lasers.
Green-induced infrared absorption (GRIIRA) properties were characterized for LiNbO3 (LN) and LiTaO3 (LT) single
crystals. GRIIRA was measured using a photothermal common-path interferometer. Several LN and LT with different
concentrations and doping materials were investigated. Mg-doped near-stoichiometric LT (Mg-SLT) had the lowest IR
absorption. In addition, no GRIIRA was observed only in MgSLT. Therefore, Mg-SLT could be the most suitable
material for high power green generation. In LN crystals, higher GRIIRA was observed in non-doped congruent LN
(CLN) which has low photorefractive damage threshold. The 1.3 mol% Mg doped SLN had the lowest GRIIRA within
the investigated LN crystals. Even in the materials with low GRIIRA, two different characteristics were observed;
initially high IR absorption and slow relaxation time of GRIIRA. The first characteristic was observed for Mg doped LN
which has a higher number of scattering centers. The second was strongly observed in Mg doped LN which has a higher
level of absorption in the UV region.
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