Second-harmonic Conversion Efficiency Research Articles

Design and Operation of High-Energy and High-Average-Power Diode-Pumped Single Nd:YAG Amplifier with Stimulated-Brillouion-Scattering Phase Conjugate Mirror

We describe a compact laser-diode-pumped, phase conjugate Nd:YAG master oscillator power amplifier system with a reduced number of components in the single slab amplifier geometry. This system is readily suited for pumping a Ti:sapphire amplifier, making it possible to construct a compact, high-repetition-rate, terawatt-peak-power chirped-pulse amplification system. An average infrared power of 362 W at a repetition rate of 1 kHz in a 29 ns pulse has been produced with an optical-to-optical efficiency of 14%. With a KTiOPO4 frequency-doubling crystal, an average green output power of 132 W at a repetition rate of 1 kHz has also been generated when pumped at an input incident power of 222 W, corresponding to a second-harmonic energy conversion efficiency of 60%. The average power at both infrared and green wavelengths represents a record performance for a single-amplifier system. We discuss in detail the design, performance and operation of the system including output power, optical efficiency, beam quality and stability.

0-dB wavelength conversion using direct-bonded QPM-Zn : LiNbO/sub 3/ ridge waveguide

We fabricate a 50-mm-long ZnO-doped quasi-phase matched LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ridge waveguide by employing the direct bonding method. A second-harmonic generation conversion efficiency of 560%/W and an output power of 110 mW with a pump power of 190 mW are achieved. We demonstrate 1.55-μm band wavelength conversion based on a /spl chi//sup (2)/ cascade scheme. An internal wavelength conversion efficiency of -0.3 dB is successfully achieved.

Reshaping of a second-harmonic curve in periodically poled Ti:LiNbO3 channel waveguide by a local-temperature-control technique

We have demonstrated reshaping and bandwidth control of a second-harmonic (SH) curve in a periodically poled Ti:LiNbO3 (Ti:PPLN) waveguide (Λ=16.6μm) by using a local-temperature-control technique. With this technique, we have achieved several useful shapes of the SH curve such as an almost ideal sinc function, and double and multipeaks in a 74-mm-long Ti:PPLN waveguide, which has a prechirped SH curve at room temperature. More than 60% improvement of SH conversion efficiency and a 5nm broadening of SH phase-matching bandwidth were achieved.

Efficient frequency doubling in a transversely diode-pumped Nd:YAG laser

A simple scheme is proposed for efficient frequency doubling in a Nd:YAG laser transversely pumped by diode lasers. The average output TEM00-mode power of the 1064-nm Q-switched laser at the Q-switching rate exceeding 20 kHz is 15 W. The radiation power at the second harmonic frequency (λ=532 nm) is found to be 12 and 8.3 W for KTP and LBO crystals for Q-switching rates f≥20 kHz and 10 kHz, respectively. The maximum second-harmonic conversion efficiency is ∼80%.

Configuration to improve second-harmonic-generation conversion efficiency.

The dependence of second-harmonic-generation (SHG) conversion efficiency on the intercrystal phase shift between two nonlinear crystals is analyzed with heuristic theory, and a novel scheme for SHG with two cascaded nonlinear crystals and a polarization modulator is proposed. More than 30 W output power is obtained experimentally at 532 nm with 70% external doubling efficiency. To the best of our knowledge, this is the highest conversion efficiency obtained with LiB3O5 crystal external frequency doubling. This scheme provides a simple and effective method for improving second-harmonic conversion efficiency.

Second-harmonic generation in one-dimensional photonic edge waveguides.

Diffraction losses in one-dimensional photonic crystal (PC) waveguides are the primary limitation on second-harmonic (SH) conversion efficiency. By using a finite difference time domain (FDTD) code taking into account second-order nonlinear polarization, we investigated these losses numerically, particularly at the SH wavelength. We propose an efficient SH conversion scheme in Al(x)Ga(1-x)As/air-etched waveguides. An analytical model is used to extrapolate the conversion efficiency to a number of periods for which time consumption makes the FDTD codes unsuitable.

Highly efficient blue-light generation from a compact, diode-pumped femtosecond laser by use of a periodically poled KTP waveguide crystal.

We present a simplified, potentially portable, and highly efficient blue-light source from a periodically poled KTP waveguide crystal with a compact femtosecond Cr:LiSAF laser. This light source generates 5.6 mW of blue average output power at 424 nm with 27 mW of incident fundamental in a single-pass extracavity arrangement at room temperature. The overall system efficiency of electrical power to blue light is 0.5%, and the internal second-harmonic generation conversion efficiency is as high as 37%. The slope efficiency of 5.5% pJ(-1) at low pulse energies is, to our knowledge, the highest slope efficiency yet reported for frequency conversion into the blue spectral region.

Temperature and wavelength tuning of second-, third-, and fourth-harmonic generation in a two-dimensional hexagonally poled nonlinear crystal

Using high-power nanosecond pulses, we measured the second-harmonic conversion efficiency of two-dimensional hexagonally poled lithium niobate as a function of temperature and wavelength. These results were compared with theoretical estimates and with measurements in one-dimensional periodically poled lithium niobate. We found that for a substantial range of parameters a two-dimensional noncollinear interaction has a broader tuning response than a one-dimensional collinear interaction. We also observed and characterized third- and fourth-harmonic generation processes in the same crystal.

Performance optimization of an external enhancement resonator for optical second-harmonic generation

We study the factors that ultimately limit the performance of an external enhancement resonator for optical second-harmonic generation (SHG). To describe the resonant SHG process we introduce a theoretical model that accounts for the intensity-dependent cavity loss that is due to harmonic generation and that also includes a realistic assumption about the shape and the frequency width of the laser mode. With the help of this model we optimized the performance of a doubling cavity based on a lithium triborate (LBO) crystal. This cavity was used for frequency doubling the output of a single-frequency titanium-doped sapphire laser at 850 nm. We were able to push the total second-harmonic conversion efficiency to 53% (a 1.54-W pump resulted in 820 mW of second-harmonic light), which to our knowledge is the best result ever reported for a LBO-based doubling cavity.

Characterization of the room-temperature phase of Tl2MoO4: crystal structure, symmetry mode analysis and second-harmonic generation measurements.

The crystal structure of the glaserite-related compound dithallium(I)-molybdate(VI), which at 293 K crystallizes monoclinic, space group C121 with lattice parameters a = 10.565 (3), b = 6.418 (1), c = 8.039 (2) A, beta = 91.05 (4) degrees, has been determined. The structure was refined as an inversion twin to a final R(F(all)) value of 0.0611 for 1006 unique reflections [R(F(obs)) = 0.0285 for 644 observed reflections]. Second-harmonic generation measurements led to a value of d(eff) = 5.5 +/- 0.5 pm V(-1) as an estimation of the second-harmonic conversion efficiency at phase matching. Symmetry mode analysis shows that, in general, primary modes have the highest amplitudes, yet surprisingly some of the secondary modes assume amplitudes of comparable magnitude. A comparison of the phase at 293 K with that at 350 K (space group P3m1) shows that the main change can be described as a rotation of the molybdate tetrahedra around the trigonal a(b) axis. The molybdate tetrahedra as well as the octahedra around one of the symmetry-independent Tl atoms are more strongly distorted in the monoclinic phase. The coordination number for the other two Tl atoms is decreased from 12 and 10 in the high-symmetry phase to 10 and 9 in the monoclinic phase. Furthermore, the number of common edges between the Tl and Mo coordination polyhedra is reduced and the common face which is observed between them in the high-temperature phase is changed to a common edge in the low-temperature phase. The contribution of the primary symmetry modes leads exactly to this change in the coordination spheres of the atoms.

Backward second-harmonic generation in submicron-period ion-exchanged KTiOPO 4 waveguide

For the first time, we demonstrated backward second-harmonic generation (SHG) in a submicron-period ion-exchanged KTiOPO 4 (KTP) waveguide. Such a period is the shortest ever achieved in ferroelectric materials. The characteristics of backward quasi-phase-matching at the 6th and 7th orders were investigated. Our analyses on the backward SHG show that the second-harmonic linewidth and conversion efficiency are very sensitive to the grating period error. According to our measurement on the backward SH linewidth, we determined the period error to be ∼4.5×10 −3 μm for our sample, which is close to the value of ∼7×10 −3 μm measured by other techniques.

Pulsed blue-light generation by the frequency doubling of the 4 F 3/2 to 4 I 9/2 transition in Nd:YAG and Nd:YAlO 3

We report on the frequency doubling of Q-switchedNd:YAG and Nd:YAlO3 lasers emitting at 946 and 930 nm, respectively (4F3/2 to 4I9/2 transition). The neodymium-doped laser host crystals were excited with a flashlamp-pumped Cr:LiSAF laser operating in a free-running mode. Blue-light pulses were obtained at both 473 nm (9 mJ, 25 ns FWHM) and 465 nm (4.4 mJ, 35 ns FWHM) by using a potassium niobate crystal as an extra-cavity frequency doubler. The second-harmonic generation conversion efficiencies reached 53% and 31%, respectively.

Thermal effects on second-harmonic generation in biaxial molecular crystals

The theory required for description of the effect of the temperature rise that is due to self-induced heating on second-harmonic generation is developed and applied to the biaxial molecular crystal (-)2-α-(methylbenzylamino)-5-nitropyridine (MBANP). It is shown that for the range of parameters considered there is an optimum input power of the fundamental beam for highest second-harmonic conversion efficiency. A maximum efficiency of 61% was calculated for the optimum input power of 19.6 kW with a beam radius of 0.25 mm and an interaction length of 1.6 cm.

Self-Frequency Doubling Laser Performance of Nd:GdCOB Crystal with a Phase-Matching Angle Out of the Principal Plane

The self-frequency doubling laser performance of Nd:GdCOB crystal cut in phase-matching direction (θ = 66.3°, ϕ = 134.4°) was compared with the crystal cut in the direction of (θ = 90°, ϕ = 46°). Results show that the former has a higher second-harmonic conversion efficiency than the latter. Using Ti:sapphire laser as pumping source, a maximum of 11.3 mW of green light has been obtained with an incident power of 520 mW. This corresponds to an optical-optical conversion efficiency of 2.2%. Laser threshold for self-frequency doubling operation is measured to be 1 mW. To our knowledge, this is the lowest threshold achieved so far.

Influences of Microcavity Confinement on OpticalSecond-harmonic Generation in a Single Quantum Well Embedded in anAsymmetric Fabry–Perot Microcavity

A theoretical analysis of the optical second-harmonic generationarising from intersubband transitions in a quantum well embedded in anasymmetric Fabry–Perot microcavity is presented. Starting fromthe Maxwell–Lorentz equations, and thereafter matching theboundary conditions, the field at the second-harmonic frequency in thequantum-well microcavity system is determined. Following the samescheme the field at the fundamental frequency in the quantum-wellmicrocavity system is also obtained. By using the transfer-matrixapproach the influences of the distributed Bragg reflector are alsoincluded. The second-harmonic generation conversion efficiency in themicrocavity structure is thus determined. By carrying out detailednumerical calculations for different confinements of the cavity, theinfluences of the microcavity on optical second-harmonic generationare shown.

Tunable high-repetition-rate visible solid-state lasers based on intracavity frequency doubling of Cr:forsterite

High-repetition-rate solid-state lasers broadly tunable in the red-orange and infrared spectral regions are reported. Wavelengths between 590 and 670 nm and between 1170 and 1360 nm have been generated by gain-switched Cr:forsterite lasers incorporating intracavity frequency doubling in KTP. Broadband, tuned, and narrow-band systems operating at repetition rates between 1 and 20 kHz have been developed. Maximum visible output power of 40 mW was achieved by frequency doubling of a narrow-band infrared laser, corresponding to a fundamental to second-harmonic conversion efficiency of more than 30%.

Improved second-harmonic generation by selective Yb ion doping in a new nonlinear optical crystal YCa/sub 4/O(BO/sub 3/)/sub 3/

We describe experiments characterizing a new nonlinear optical crystal, YCa/sub 4/O(BO/sub 3/)/sub 3/ (YCOB). This crystal has a number of advantages over other commonly available nonlinear optical crystals. It has a higher nonlinear coefficient than KDP, can be fabricated to large sizes (/spl sim/3-in diameter, 8-in length), and has a high damage threshold. Moreover, this new nonlinear optical crystal is nonhygroscopic, has good optical quality and mechanical properties, allowing easy optical polishing. This crystal, YCa/sub 4/O(BO/sub 3/)/sub 3/, commonly termed YCOB, is one of a family of new nonlinear crystals, the oxyborates, that include RECa/sub 4/O(BO/sub 3/)/sub 3/ (RE=La/sup 3+/, La/sup 3+/, Y/sup 3+/, Sm/sup 3+/, Gd/sup 3+/, Er/sup 3+/, and Nd/sup 3+/). In this paper, we also successfully demonstrate a technique for improving the nonlinear optical properties of this crystal. This technique, ion substitution, has previously had limited success with other crystal hosts. However, the inclusion of yttrium in YCOB provides the opportunity to exploit this technique. Yb/sup 3+/, which has larger mass, but approximately the same atomic size as Y/sup 3+/ can be substituted into the crystal structure without introducing stress and nonuniformities. A systematic investigation of the linear and nonlinear characteristics of several crystals doped with various levels of Yb demonstrate that selective substitution of Yb in YCa/sub 4/O(BO/sub 3/)/sub 3/ improves the second-harmonic conversion efficiency by increasing the optical nonlinearity.

Large enhancement in quasiphase matched second-harmonic generation efficiency by an external temperature gradient

We show that for continuous wave second-harmonic generation using quasiphase matching in periodically segmented waveguides, thermal loading caused by the fundamental and the generated beams degrades the quasiphase matching condition which corrupts the spectral quality of the second-harmonic field. We also show that a carefully designed external temperature gradient leads to an enhancement of the conversion efficiency and control of the spectral line shape of the output field. Compared to earlier work, our simple scheme yields an enhancement of the second-harmonic conversion efficiency by a factor of 2–4 depending on the input fundamental power.

Nonlinear contrawave mixing devices in poled-polymer waveguides

We investigated efficient surface-emitting second-harmonic-generation devices in poled polymers, using a 4-dimethylamino-4′-nitrostilbene side-chain polymer. The investigation included characterization of the linear and nonlinear optical properties of the polymer, design of efficient surface-emitting second-harmonic-generation devices based on poled polymers, development of an efficient in-plane poling technique, and demonstration of surface-emitting second-harmonic generation in poled polymers. As a result, strong field in-plane parallel poling was successfully performed with poling fields over 300 V/μm, which led to a large nonlinearity of 150 pm/V at 1064 nm (near resonance). A thick cover layer and a highly resistive substrate were found to be essential for efficient in-plane poling without breakdown at relatively small fields and significant charge injection. We achieved quasi-phase matching in the transverse direction by fabricating nonlinear–linear multilayer waveguides. Each layer had approximately a 150-nm thickness. The largest second-harmonic power conversion efficiency to date in the poled-polymer devices is 0.6%/W cm, which is comparable with those of semiconductor multilayer devices.

Optical second-harmonic generation in two-level quantum wells embedded in a planar microcavity

A theoretical analysis of the optical second-harmonic generation arising from intersubband transitions in quantum wells embedded in a planar microcavity is presented. Starting from the Maxwell-Lorentz equations, an expression for the field in each layer and an integral equation for the local field in a quantum-well structure are obtained. Then, matching the boundary condition, the field in the quantum-well microcavity system is determined. Following the same scheme, the field at second-harmonic frequency in the quantum-well microcavity system is obtained and the second-harmonic generation conversion efficiency is thus determined. By carrying out detailed numerical calculations, the influence of the microcavity on optical second-harmonic generation is shown.