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.
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.
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.
Two-color holography is an effective solution to the volatile readout problem in volume holographic data storage based on photorefractive materials. Popular materials for two-color holography are reduced doped and nondoped near-stoichiometric lithium niobate crystals. However, the lifetime at room temperature is from several weeks to several months depending on the reduction state of the material. Moreover, reductive treatment will degrade the nonvolatility of two-color holograms. The important issue for two-color holography is how to increase the lifetime. In this contribution, lifetimes of two-color nonvolatile holograms recorded in as-grown near-stoichiometric lithium niobate and tantalate crystals were compared by extrapolating the high-temperature data. The dark-decay time constants obey an Arrhenius dependence on absolute temperature and yield activity energy of 1.06 eV around in all measured crystals. Lifetimes of holograms in nondoped and slightly doped crystals depend on the proton concentration. Lifetimes of hologram in lithium tantalate are one order of magnitude longer than those in lithium niobate at the same proton concentration. The lifetime of two-color holograms in lithium tantalite is longer than 20 years.
We observed that newly developed near-stoichiometric LiNbO3 crystals doubly doped with Tb and Fe have three different types of energy levels: UV absorption centers just above the valence band, metastable shallow electron traps slightly below the conduction band, the deep traps located about 1.9 eV below the conduction band. Irradiation with UV light induced a stable absorption band extending from (lambda) equals 650 nm to the absorption edge, which is caused by the photoinduced charge transfer from UV-sensitive absorption centers to deep traps via the conduction band. The electron lifetimes at shallow and deep traps could be controlled by doping concentrations. Based on these favorable energy states, nonvolatile two-color holographic recording has been demonstrated by use of 852-nm recording beams and UV gating light. Quasi-nonvolatile one-color recording at 532 nm has also been demonstrated in these codoped crystals. Hologram recording from the UV-exposed, colored state revealed a much improved sensitivity in comparison to that from the uncolored state.
We show that stoichiometric LiNbO3 crystal containing nonstoichiometric defects much less than congruent LiNbO3 exhibit some advantageous properties for the holographic data storage (HDS) applications. This was confirmed by the two-beam coupling experiment and digital hologram test. In order to interpret the high performance of stoichiometric LN, we compared some related parameters such as linear electro-optic constants, photoconductivity and photovoltaic constants between stoichiometric and congruent LN crystals. In all measurements, the stoichiometric crystals grown by the novel double crucible CZ method were found to be more excellent as HDS medium. This superiority was obvious at the geometry using the extraordinary polarization.
Compact solid-state blue laser have great potential for use in optical data storage, laser beam printing, particle countering, reprographics, holography, and fluorescent bioanalysis. We report recent progresses in qualities of LiB3O5 and K3Li2 (TaxNb1-x)5O15 nonlinear crystal which are essential in manufacturing bulk-type blue SHG devices. We also review newly developed violet-blue laser, 20 mW output, using intracavity frequency doubling of a diode laser pumped Cr:LiSrAlF6 laser with low loss LiB3O5 crystal as a frequency doubler.
We report on the top seeded solution growth of LBO from an excess of B2O3 solution. Parameters investigated included the Li2O/B2O3 ratio, rotation, pulling and cooling rates. Unstable crystal growth encountered, such as hopper growth and inclusions, can be attributed to the high viscosity of the solution, and methods of increasing the forced convection was examined by the melt simulation using the tracer method. With careful control of the above parameters, clear crystals of approximately 40 X 48 X 30 mm have been successfully grown. It was characterized that the grown LBO crystals have good optical homogeneity, no growth striations, no growth sector boundaries, small change of refractive index less than 1 X 10-5/cm. A bulk laser damage threshold of LBO was determined to be 45.0 GW/cm2 at 1.064 micrometers , 1.1 nsec pulse width. This is one of the highest bulk damage threshold of any inorganic nonlinear optical crystal.
Clear and transparent MgO doped LiTaO3 single crystals have been grown by the Czochralski method from a congruent melt. Their optical damage resistance has been characterized by measurement of the change of photoinduced birefringence. We compare the optical damage resistance of LiTaO3 and LiNbO3, each containing almost the same amount of Fe impurities (less than 1 ppm), and show that LiTaO3 has about 4 times lower optical damage resistance, which is not consistent with the data previously reported in the literature. On the other hand, MgO doped yielded an improvement in the optical damage resistance, as reported for MgO doped LiNbO3 crystals. The other important features of MgO doping are that: improved transparency spectra especially in a visible wavelength region, resulting in a change of color to clear and transparent from brown-yellow, and a shift in the absorption edge to shorter wavelengths of 270 nm. These are important advantages when considering shorter wavelength accessibility and high conversion efficiency in second harmonic generation devices.