Second-harmonic Power Research Articles

Improvement of second-harmonic generation in quantum-cascade lasers with true phase matching

About a 100-fold improvement of the second-harmonic generation in a quantum-cascade laser with integrated optical nonlinearity was obtained by including phase-matching considerations in the design of the deep-etched ridge waveguide. The waveguide layer structure was optimized to minimize the phase mismatch of the zero-order mode of the fundamental light with the second-order transverse mode of the second-harmonic light. Exact phase matching is made possible by the faster decrease of the modal refractive index of the fundamental light with decreasing ridge width relative to the refractive index of the second-harmonic light. Up to 240 μW of the second-harmonic power and a nonlinear power conversion efficiency of up to 36 mW/W2 were achieved.

Efficient frequency doubling by a phase-compensated crystal in a semimonolithic cavity.

In multiple-pass nonlinear frequency conversion devices, interacting waves may accumulate different phases, owing to dispersive elements in the system. Phase compensation is therefore necessary for efficient frequency conversion. We experimentally demonstrate phase compensation in a compact semimonolithic frequency-doubling cavity by using a periodically poled KTP crystal. The conversion efficiency of the crystal was found to decrease at high pump powers, owing to power-dependent thermal lensing. This experimental observation was supported by a theoretical calculation of the conversion efficiency in a cavity, considering the mismatch between the mode's thermally loaded and unloaded cavities. A design procedure was also presented to compensate for the thermal lensing effect. The highest conversion efficiency of 56.5%, corresponding to a second-harmonic power of 117.5 mW at 532 nm, was achieved with a cw Nd:YAG pump power of 208 mW.

Second-harmonic generation in periodically poled GaN

We report the experimental demonstration of second-harmonic generation in periodically poled GaN by first-order quasiphase matching. The periodically poled structure was grown by plasma-assisted molecular-beam epitaxy. We observed about 9 μW of second-harmonic power from a fundamental input laser wavelength of 1658.6 nm with a normalized conversion efficiency of 12.76% W−1 cm−2. The ability to perform nonlinear wavelength conversion by periodic poling and the fact that GaN has a very wide window of transparency, pave the way for GaN to be used for nonlinear optical devices in telecommunications as well as a nonlinear light source for biochemical detection in the far infrared and deep ultraviolet.

Diode-pumped quasi-three-level 456-nm Nd:GdVO4 laser

A diode-pumped quasi-three-level Nd:GdVO4 laser emitting at a wavelength of 912 nm corresponding to the 4F3/2 — 4I9/2 transition of the neodymium ion and at the second-harmonic linewidth of 456 nm after intracavity frequency doubling in a KNbO3 crystal is studied. The maximum output power at the fundamental frequency was 2.96 W and the conversion efficiency amounted to 48 %. The second-harmonic output power achieved 220 mW for the conversion efficiency of 15 %.

Efficient frequency doubling of 1-W continuous-wave Ti:sapphire laser with a robust high-finesse external cavity.

We demonstrated an intrinsic conversion efficiency of 56% from the input fundamental power to the generated second-harmonic power. The second-harmonic power of 581 mW was obtained from the external cavity with a LiB3O5 crystal through the frequency doubling of a 1.17-W Ti:sapphire laser at 746 nm, when the finesse of the robust external cavity was 260.

Self-organized parametric down conversion and photoinduced reflectivity in optical fibers

We report new experimental results concerning self-organized nonlinear optical interactions in optical fibers. A self-organized parametric down conversion was observed if the second-harmonic seeding radiation had been above the saturation level. If furthermore increasing the second-harmonic input power a nearly total photoinduced reflectivity for the second-harmonic green light and about 50% reflectivity for the fundamental infrared radiation were observed.

Simultaneous second-harmonic generation with two orthogonal polarization states in periodically poled KTP

Simultaneous generation of second-harmonic light with orthogonal polarizations in the blue spectral region is demonstrated by use of typeII and typeI quasi-phase matching for the nonlinear coefficients d(33) , d(24) , and d(32) in periodically poled KTP. The second-harmonic power ratio in both polarization states can be tuned by the fundamental polarization and (or) the periodically poled KTP temperature. Femtosecond pulse frequency-doubling efficiencies of as much as 39.5% W (-1) and 8.1% W (-1) are demonstrated in 7-mm periodically poled KTP for typeI and typeII processes, respectively. The efficiency limitations caused by various effects of the group-velocity mismatch are discussed.

Theoretical analysis of enhanced second-harmonic generation based on a self-formed antiguide structure

An antiguide structure having a core of lower refractive index confines lightwaves owing to Fresnel multiple reflection. Within a planar waveguide of a nonlinear-optical photoconductive polymer, the second-harmonic wave generated under the Cerenkov-radiation type phase matching can shape the antiguide structure to increase the second-harmonic power itself due to the optical confinement. Here, we theoretically analyze the enhancement.

Positive phase-velocity tapering of broadband helix traveling-wave tubes for efficiency enhancement

A positive phase-velocity tapering of 1.5 octave broadband helix traveling-wave tubes for efficiency enhancement, where the phase velocity is linearly increased in the output section, was studied by using the one-dimensional nonlinear theory. At high frequencies, the electromagnetic wave in the positively tapered section traps the fastest electrons in the decelerating electric field, extracting more energy from the electron beam. At low frequencies, a decreased velocity difference between the electron beam and the electromagnetic wave destroys the phase condition for second-harmonic generation, retaining fundamental wave efficiency as well as reducing its second-harmonic power.

Synthesis, structure, thermal and non-linear optical properties of L-argininium hydrogen selenite.

L-Argininium hydrogen selenite (C(6)H(15)N(4)O(2)HSeO(3)) is a new semiorganic compound of the hydrogen selenite family with non-linear optical properties. The crystal lattice is monoclinic with unit-cell parameters a = 22.493 (5), b = 5.1624 (13), c = 9.730 (4) A, beta = 95.68 (3) degrees, V = 1124.3 (6) A(3), Z = 4, space group C2. Second-harmonic generation measurements performed on powder samples, using a Q-switched Nd:Yag laser (lambda = 1064 nm), showed the second-harmonic power to be about twice that of urea. Differential scanning calorimetry measurements revealed the existence of a phase transition with onset at 289 K.

Active formation of an antiguide structure in a photorefractive polymer for enhanced second-harmonic generation

An antiguide structure for enhanced second-harmonic generation was actively constructed in a photorefractive polymer by use of a pump beam. Irradiation of a pump beam enhanced second-harmonic power, and blocking the pump returned the power to the initial value. The electric-field dependence of the degree of enhancement of the second-harmonic power confirmed that the antiguide structure was constructed through a photorefractive-index change in the medium. The photorefractive-index change accompanied molecular reorientation induced by the pump-generated space charges. The thermo-optic effect on formation of the structure is also discussed.

Efficient and compact intracavity-frequency-doubled Nd:GdVO4/KTP laser end-pumped by a fiber-coupled laser diode

A compact and efficient diode-pumped intracavity-frequency-doubled Nd:GdVO4/KTP green laser is demonstrated with a flat–flat cavity design. With a 1.3 at. % Nd3+-doped GdVO4 crystal and pumped at the weak-absorption peak of 806 nm, the second-harmonic output power at 532 nm was measured to be 1.95 W at an incident pump power of 8.4 W, corresponding to an optical conversion efficiency of 23.2%. The output characteristic at the fundamental wavelength of 1.063 μm was investigated with two different pump wavelengths. More than 4.5-W output power was generated when the laser was pumped at 806.2 nm.

Time decay of the nonlinear microwave response of superconductors in the critical state

We report experimental results on the second-order response of superconductors in the critical state exposed to intense pulsed microwave (mw) fields. The second-harmonic power radiated by the samples is investigated during the time interval in which the mw pulse endures, for mw pulses ≈1 ms long. The second-harmonic signal exhibits a time decay whose features depend on the investigated sample, the input power level, and the way in which the value of the static magnetic field has been reached. The transient is ascribed to processes of magnetic flux redistribution, which come into play when fluxons in the critical state are driven by electromagnetic (em) fields of high frequency.

Development of a high-power deep-ultraviolet continuous-wave coherent light source for laser cooling of silicon atoms

We developed a deep-UV single-mode coherent light source through two-stage highly efficient frequency conversions by use of external cavities. In the first stage, second-harmonic power of 500 mW was obtained by frequency doubling of a 746-nm Ti:sapphire laser with a conversion efficiency of 40%. In the second stage, 50-mW power at ~252nm was obtained by doubly resonant sum-frequency mixing of 373-nm light from the first-stage conversion and 780-nm light from a diode laser. The output performance of this deep-UV light source is sufficient for laser cooling of neutral silicon atoms.

High beam quality of ultraviolet radiation generated through resonant enhanced frequency doubling of a diode laser

The infrared emission from an external-cavity-tapered diode laser is frequency doubled in a 5-mm β-BaB2O4 crystal that is placed in a resonant-enhancement cavity. For a coupled diode power of 535 mW, 100 mW of continuous-wave, second-harmonic power is generated at λ=392 nm. The ultraviolet (UV) beam is measured to have a nearly ideal beam-quality parameter, M2≃1, in both meridians even though substantial double refraction is present in the nonlinear crystal. Printing of an array of submicron dots on a phase-change film is demonstrated with this UV beam. Also, by printing on photochemical film we demonstrate that improved film sensitivity is possible when a UV source is used.

A numerical study of optical second-harmonic generation in a one-dimensional photonic structure

We present a band-gap engineering approach for obtaining a finite photonic-band-edge-resonant stratified periodic dielectric structure in the presence of material dispersion. When the structure contains second-order nonlinear optical materials, enhanced phase-matched optical second-harmonic (SH) generation may be obtained. Using a transfer-matrix technique, we computed the transmitted and reflected SH power for a GaAs/AlAs Bragg grating and found that a substantial enhancement of the SH power is obtained for a modest number of periods. Asymptotically for a large number of periods, we found the SH power to scale as the eighth power of the total structure thickness.

Efficient quasi-phase-matched frequency doubling with phase compensation by a wedged crystal in a standing-wave external cavity

In standing-wave enhancement cavities for frequency doubling, second-harmonic fields are generated in both directions of propagation. To add the fields coherently, one should compensate for the phase shifts introduced by dispersive elements in the cavity. We experimentally demonstrate phase compensation in a compact standing-wave frequency-doubling cavity by use of a wedged periodically poled KTP crystal. The highest conversion efficiency and second-harmonic power obtained by pumping with a 1064-nm cw Nd:YAG laser were 69.4% and 268 mW, respectively.

Efficient operation of an intracavity-doubled nd:yvo_4/ktp laser end pumped by a high-brightness laser diode

Using a 1-W high-brightness laser diode as the pump source, we investigated the output characteristics of an intracavity frequency-doubled Nd:YVO(4)/KTP laser. Highly efficient operation was realized based on an optimized cavity design. The second-harmonic output power at 532 nm was measured to be 286.5 mW at an incident pump power of 881.4 mW, corresponding to an optical-to-optical efficiency of 32.5%.

Analytical and Experimental Studies on Nd:YVO4/KTP Lasers Pumped by High-brightness Laser Diodes

We report analytical and experimental studies on the characteristics of a high-brightness laser diode endpumped Nd:YVO4/KTP laser. A simple model was developed to optimize the cavity parameters and estimate the green output power of intracavity frequency doubled lasers. Using a 1 W high-brightness laser diode as the pump source, high efficiency operation was realized. The second-harmonic output power at 532 nm was measured to be 286.5 mW at an incident pump power of 881.4 mW, corresponding to an optical to optical efficiency of 32.5%.

Effects of reorientation of molecules in a nonlinear-optical photoconducting polymer on formation of an antiguide structure

When the poled nonlinear-optical photoconducting polymer is irradiated by the pump light, the generated space charges screen the poling electric field and induce reorientation of the nonlinear-optical molecules. It results in reduction of refractive indices and of a nonlinear-optical coefficient of the polymer. Due to the former reduction, the antiguide structure is so made as to confine the leaky lightwaves. We confirmed the reduction by observing the second-harmonic generation from the pump-irradiated polymer and compared the self enhancement of the Cerenkov-radiation type second-harmonic power generated from a thin-film waveguide.