Orientation patterned GaAs waveguides for parametric conversion from near to mid-infrared have been fabricated by
MOCVD growth on OPGaAs templates. A monolithic OPO cavity was formed by dielectric facet coating. Parametric
oscillation characteristics were investigated using a pulsed source tunable in the range of 1.98-2.05μm. Type I and II
parametric interactions have been observed, differing in QPM wavelength. OPO threshold power of 7W, using a pulsed
pump, and 5.7W using a CW laser was obtained in a 13mm long waveguide of 39μm period. Overall Parametric peak
power of 0.6W at pulsed pump peak power of 11.6W was generated at signal & idler wavelengths of 3.6μm & 4.5μm
respectively and pump wavelength of 2.015μm. Tuning curves for Type I and type II parametric operation in OPGaAs
WGs have been calculated and verified by the measured signal and idler wavelengths.
In this paper we present for the first time to our knowledge an efficient and rugged light source in the visible, based on a
gain switched Yb3+ doped fiber laser source, frequency doubled by a non-critically phase matched LiB3O5 crystal. The
simple setup proves to be robust and durable against back reflections, which in turn remove the requirement for optical
isolators along most of the system.
Gain switched fiber lasers typically produce long pulses with low peak power, which are not optimal for frequency
conversion applications. However, as opposed to MOPA laser configurations, based on a semiconductor laser diode as a
seed source, the narrow spectral line width and chirp free operation of gain switched lasers render them suitable for
efficient frequency doubling.
Near-stoichiometric lithium tantalate (SLT) crystals were produced from congruent lithium tantalate by a vapor-transport
equilibration (VTE) process. The VTE'ed SLT (VSLT) crystals exhibited very low coercive field of 60-120-V/mm and
improved high intensity damage resistance. The ~603°C Curie temperature of the congruent raw material climbed to
693±0.1°C in the VSLT crystal regardless of the congruent crystal stoichiometric composition or of the exact VTE
process temperature profile. Stable, high power near room temperature second harmonic generation (SHG) and optical
parametric oscillation (OPO) were demonstrated with these crystals.
SHG measurements of a 1064-nm pulsed laser were realized by an 8-μm period grating, 21-mm-long sample. With 29-ns
long pulses at 20-kHz repetition rate, 7.3-W (average power), of 532-nm radiation was generated with 55% conversion
efficiency. With 25-ns long pulses at 10-kHz repetition rate in the same sample, 6.3-W of average-power with 61%
conversion efficiency was obtained.
1.41-W average-power, at 4.013-μm idler wavelength was generated in an OPO configuration. The 28.65-μm period
grating, 24-mm-long sample, was pumped by 25-ns pulses, 10-kHz repetition rate, 1.064-μm Nd:YVO4 laser. In addition
to the original (intended) signal and idler wavelengths, 0.27-W was obtained at 4.686-μm idler wavelength, by a
secondary OPO process which developed in the system. Primary and secondary OPO operation was also observed in a
27.15-μm period grating, 38-mm-long sample. 0.67-W at 4.48-μm and 0.15-W at 5.03-μm were generated in this
experiment.
Material preparation methods and device fabrication technologies for realization of low loss periodically oriented GaAs
waveguides are reported. Planar waveguide structures were grown by MOCVD on periodically patterned templates
prepared by wafer bonding and selective layer removal. Ridge waveguides were designed and fabricated from the planar
structures with emphasis on waveguide loss minimization. Record low losses of 2.0db/cm in periodically oriented
waveguides and 0.95db/cm in single domain waveguides were measured. Routes for further loss reduction in patterned
GaAs waveguides are discussed and initial results from further work to reduce waveguide corrugation are presented.
Optical parametric generator (OPG) is a very attractive optical down-conversion configuration since it is a single pass
process and no cavity mirror's alignment is required. Thus the system configuration is much more simple and robust.
Traditionally, OPG processes were demonstrated using a pump source with a pulse length of the order of picoseconds or
less. This is because GW/cm2 order of magnitude pump irradiance was required to excite an OPG process, and such
irradiance in nanosecond long pulses commonly damages the non-linear crystal. The introduction of periodically poled
crystals with high non-linear coefficients has significantly lowered the threshold for parametric processes. This progress
in non-linear crystals enables exciting OPG processes at less than 100MW/cm2 irradiance, using nanoseconds long
pulses from Q-switched lasers. We present an OPG with a threshold of less than 10 MW/cm2 using an 80 mm long
Periodically Poled Lithium Niobate (PPLN) non-linear crystal. High signal conversion efficiency and high power were
obtained at 25 nanosecond pulse length, 10 kHz repetition rate pumping without damaging the crystal. Theoretical
approaches for explaining this OPG regime are discussed.
We report experimental realization of a new type of optical parametric oscillator, in which oscillation is achieved by
polarization rotation in a linear retarder, followed by nonlinear mixing of the two polarized beams. The mixing is
performed by a type II degenerate parametric down conversion in a PPKTP crystal, pumped at 1064nm by a pulsed
Nd:YAG laser. A single, linearly polarized beam is generated at the degenerate wavelength. The output spectrum has a
narrow linewidth (below the instrumentation bandwidth of 1nm), and is highly stable at degeneracy with respect to
variations in the crystal temperature. The frequency locking is explained in terms of balanced roundtrip phase-matching
condition.
The introduction of periodically poled crystals with high non-linear coefficients has lowered significantly the threshold for parametric processes. This progress enables pumping frequency conversion devices with low pulse energy, Q-switched, diode-pumped, solid-state lasers.
New non-linear optical ferroelectric materials, such as KTP and Stoichiometric Lithium Tantalate (SLT) were proven to exhibit adequate deff, higher optical damage resistance and lower photo-refractivity in comparison to well-known periodically poled Lithium Niobate. Advances in poling technology have enabled the production of relatively thick periodically poled crystals from those materials. Thus, in principal much higher average power levels can be converted.
We have investigated the effects that limit frequency conversion efficiency as power levels are increased. Average power induced thermal lensing and thermal phase mismatching were considered. The resulting power limitations are discussed, and under some assumptions quantitative expressions for these limits were formulated.
Thermal lensing imposes a limit on the local power density. Thermal phase mismatching imposes a limit on the overall power.
Near stoichiometric LiTaO3 is investigated for high power quasi-phase-matched optical frequency conversion applications due to its high optical damage threshold and low coercive field. In this work, near stoichiometric undoped and MgO-doped LiTaO3 wafers were characterized by transmission through cross-polarizes, x-ray diffraction and microscope inspection. Periodically inverted domain structures were fabricated in 1 to 4mm thick wafers, by electric field poling in vacuum. Efficiency values between 10% and 16% were obtained for direct frequency conversion of 1 μm to 4 μm light using optical parametric oscillations scheme. The resulting periodically poled structure quality and frequency conversion efficiency seems to be limited by crystalline imperfections of the wafers.
We present a temperature-dependent Sellmeier equation for congruent and stoichiometric LiTaO3. The refractive indices in the range of 0.39-4.1 micrometers were determined by scanning the pump lasers wavelength and finding the QPM second harmonic wavelength for a series of period- temperature pairs, and from QPM OPO measurements. The obtained new Sellmeier coefficients were used to calculate the QPM wavelengths of the idler and signal for a PPSLT OPO pumped at 1064 nm. An OPO based on PPSLT was demonstrated. The idler-tuning range was 4.1828-3.9898 micrometers for temperatures of 40-200 degree(s)C. The obtained wavelengths were in good agreement with our predictions.
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