Efficient second-harmonic generation in birefringently phase-matched GaAs/Al(2)O(3) waveguides

We report efficient second-harmonic generation of femtosecond pulses in birefringently phase-matched GaAs/Al(2)O(3) waveguides pumped at 2.01mum. By use of pump pulses of ~200-fs duration and type I interaction, practical second-harmonic average powers of up to ~650muW were obtained, with an average input power of ~50muW. Waveguides of four different widths and two different lengths were investigated, and a normalized conversion efficiency of greater than 1000%W(-1)cm(-2) was obtained for a 1-mm waveguide. Measurements of pump and second-harmonic spectra provided clear evidence of phase matching and depletion of the pump spectrum. The measured bandwidth of the second harmonic was ~1.3nm. From the measurements of transmitted pump power at the phase-matching wavelength, pump depletions of more than 80% were recorded.     

Backward second-harmonic and third-harmonic generation in a periodically poled potassium titanyl phosphate waveguide

We observed backward second-harmonic and backward third-harmonic generation in a periodically poled KTiOPO(4) waveguide using nanosecond laser pulses. The highest conversion efficiency achieved for the backward second-harmonic generation, occurring at the 25th-order grating, was ~0.6% . The backward third-harmonic generation was the result of mixing the pump beam with the forward or the backward second-harmonic beam. Conversion efficiency of ~0.4% was achieved at a pump wavelength of 1233.7 nm, where the two constituent nonlinear processes are both quasi-phase matched.     

Measurement of the nonlinear coefficient of orientation-patterned GaAs and demonstration of highly efficient second-harmonic generation

Quasi-phase-matched (QPM) GaAs structures, 0.5 mm thick, 10 mm long, and with 61-mum grating periods, were grown by a combination of molecular-beam epitaxy and hydride vapor phase epitaxy. These were characterized by use of mid-IR second-harmonic generation (SHG) with a ZnGeP(2) (ZGP) optical parametric oscillator as a pump source. The SHG efficiencies of QPM GaAs and QPM LiNbO(3) were directly compared, and a ratio of nonlinear coefficients d(14)(GaAs)/d(33) (LiNbO(3))=5.01+/-0.3 was found at 4.1-mum fundamental wavelength. For input pulse energies as low as 50muJ and approximately 60-ns pulse duration, an internal SHG conversion efficiency of 33% was measured in QPM GaAs.     

Phase matching in monolithic Bragg reflection waveguides

We report what is believed to be the first observation of second-harmonic generation by type I phase matching the bulk chi(xyz)((2))(d(14)) nonlinear coefficient using Bragg reflection waveguides. Second-harmonic power of 0.7 microW was observed for a pump wavelength of 1587.8 nm with an average power of 25.2 mW and a pulse width of approximately 2 ps at a repetition rate of 75.6 MHz. An order of magnitude enhancement between the phase-matched and un-phase-matched second-harmonic conversion efficiency has been observed. Conversion efficiency at the phase-matched wavelengths was 0.001%. The bandwidth of the second harmonic was found to be equal to 0.43 nm, agreeing with the theoretical predictions.     

Efficient second-harmonic generation by scattering from porous gallium phosphide

We experimentally study second-harmonic generation by femtosecond Cr: forsterite-laser radiation scattered on the surface of porous gallium phosphide with characteristic pore sizes and distances between the pores comparable with the second-harmonic wavelength. The intensity of the second-harmonic signal from samples with initial crystallographic surface orientations (110) and (111) is more than an order of magnitude higher than the intensity of the second harmonic generated in reflection from single-crystal gallium phosphide. The efficiency of second-harmonic generation by macroporous gallium phosphide substantially increases as the pump wave-length becomes shorter. The influence of light localization and scattering effects on the enhancement of second-harmonic generation and polarization properties of the second-harmonic is discussed.     

Corrugated-enhanced second harmonic generation in metal–insulator–metal plasmonic waveguides

Nonlinear effects such as second-harmonic generation (SHG) are important for applications such as switching and wavelength conversion. In this study, the generation of second harmonic in metal–insulator–metal (MIM) plasmonic waveguides was investigated for both symmetric and asymmetric structures. Symmetric means that the metals at the top and bottom of the dielectric layer are the same and asymmetric means that the metals at the top and bottom of the dielectric layer are different. Two different structures are considered here as plasmonic waveguide for generation of second harmonic and analyzed using finite-difference time domain method. Besides the structure has grating on both sides for more coupling between photons and plasmons. The wavelength duration of grating per length unit (number of grooves) will be optimized to reach the highest second harmonic generation. To perform this optimization, the wavelength of operation λ = 458 nm is considered. It was shown that field enhancement in symmetric MIM waveguides can result in enhancement of SHG magnitude compared to the literature values and asymmetric device results in more than two orders of magnitude enhancement in SHG compared to symmetric structure. It is also shown that the electric field of second harmonic depends on the thickness of crystal (insulator). So, its thickness is optimized to achieve the highest electric field.     

Second-harmonic generation from a first-order quasi-phase-matched GaAs/AlGaAs waveguide crystal

We demonstrate, for the first time to our knowledge, the generation of second-harmonic pulses by use of a novel methodology for achieving first-order quasi-phase matching in a semiconductor waveguide crystal. This methodology is based on a periodic modulation of the susceptibility coefficient along the direction of light-beam propagation in which advantage is taken of the fact that chi((2))(GaAs)>chi((2))(Al(x)Ga(1-x)As) . Efficient second-harmonic generation at 975 nm of a pump wavelength of 1950 nm has been demonstrated for a crystal with a nonuniform domain dimension (duty cycle, ~39/61).     

Continuous-wave single-frequency 532 nm laser source emitting 130 W into the fundamental transversal mode

The nonlinear effect of second-harmonic generation is an efficient way to realize high-power green laser sources. But when scaling up the harmonic power, many setups reported in the literature have been limited by conversion efficiency degradation or the fundamental laser power. Here we report on the generation of 134 W of cw laser light at a wavelength of 532 nm from a fundamental power of 149 W by second-harmonic generation in an external optical resonator comprising a lithium triborate crystal. The external conversion efficiency was 90%. The harmonic light consisted of a single spectral line. At least 97% of it was emitted into the fundamental transversal mode.     

Cascaded second-harmonic and sum-frequency generation of a CO2 laser by use of a single quasi-phase-matched GaAs crystal

Cascaded second-harmonic and sum-frequency generation were used to obtain the third harmonic of a CO(2) laser in a single quasi-phase-matched GaAs crystal. Both continuous-wave and pulsed regimes with a single-pass configuration were studied. The continuous case confirmed the good capability of the GaAs stack to bear high average power density. In the pulsed regime a 0.66% peak power conversion efficiency was achieved for the third harmonic when the laser was pumping at 8.2 MW/cm(2) , in fair agreement with theoretical predictions.     

Second-harmonic generation of 405-nm light using periodically poled KTiOPO4 pumped by external-cavity laser diode with double grating feedback

A frequency-doubled laser diode system for generation of blue–UV light is described. The system is based on an external-cavity high-power laser diode with double feedback from the zeroth and the first orders of a diffraction grating. Light at 405 nm is generated in a single-pass configuration using periodically poled KTiOPO₄. We show that the double grating feedback improves the second harmonic conversion efficiency by several orders of magnitude as compared to the freely running laser. The conversion efficiency may be improved further such that higher second-harmonic powers may be generated.     

Quasi phase matching in GaAs--AlAs superlattice waveguides through bandgap tuning by use of quantum-well intermixing

We report the observation of second-harmonic generation by type I quasi phase matching in a GaAs-AlAs superlattice waveguide. Quasi phase matching was achieved through modulation of the nonlinear coefficient chi((2))(zxy), which we realized by periodically tuning the superlattice bandgap. Second-harmonic generation was demonstrated for fundamental wavelengths from 1480 to 1520 nm, from the third-order gratings with periods from 10.5 to 12.4microm . The second-harmonic signal spectra demonstrated narrowing owing to the finite bandwidth of the quasi-phase-matching grating. An average power of ~110 nW was obtained for the second harmonic by use of an average launched pump power of ?2.3mW .     

单到单和单到双信道皮秒脉冲全光波长转换的实验研究

基于周期极化反转铌酸锂（PPLN）光波导级联倍频和差频（SHG+DFG）的二阶非线性效应,提出并实验研究了皮秒脉冲的可调谐波长转换以及单信道到双信道的波长转换.信号光采用重复频率为40 GHz,脉宽为1.57 ps的脉冲信号.连续抽运光由光纤环形腔激光器（FRL）提供.不同于传统的SHG+DFG型波长转换,信号光固定在PPLN光波导倍频过程的准相位匹配（QPM）波长处,通过调节抽运光的波长实现了转换空闲光的可调谐输出.当使用两个抽运光时实验观察到了单信道到双信道的波长转换.     

Photonic band-gap enhanced second-harmonic generation in a planar lithium niobate waveguide

Enhanced second-harmonic generation (SHG) conversion efficiency was theoretically predicted in waveguide geometry with coupling to a one-dimensional grating photonic band gap (PBG). We report a series of experiments using samples made with lithium niobate. A waveguide was fabricated near the surface by applying the proton-exchange technique. The characteristics of waveguide modes were determined by several techniques: prism coupling, diffraction, and Cherenkov radiation. The WKB method was used to analyze the results. Ultraviolet laser lithography was applied to make PBG gratings on the sample. We further investigated Cherenkov second-harmonic generation (CSHG), i.e., SHG radiated into the substrate, under the condition of a band-edge PBG resonance in the waveguides. The SHG inside planar waveguides was also experimentally investigated. We fabricated waveguides with multiple pump modes and found that the second mode was more efficient in enhancing the second harmonic signal. This result is explained by our model. Several samples were investigated in detail; the highest conversion efficiency of CSHG with a PBG was enhanced around 50 times above the CSHG signal without a PBG. A numerical model was constructed with parameters calculated from our sample characterization data to interpret the experimental results.     

Generation of tunable green radiation in bulk periodically poled KTiOPO4

We report a characterisation of a flux-grown periodically poled KTiOPO₄ crystal used for second-harmonic generation in a diode laser source, that operates with fundamental radiation tunable around 1083 nm wavelength. The conversion efficiency is measured for the three different grating periods of the crystal, as a function of input wavelength, crystal temperature and fundamental power. The measurements were fitted to the theoretical model of Boyd and Kleinman for frequency conversion of focused Gaussian beams. This enabled us to determine the temperature and wavelength dependence of the difference in refractive index between the interacting waves.     

Tunable visible solid-state lasers based on intracavity frequency doubling of Cr:forsterite in KTP

Broadband tunability in the yellow-red region is obtained by use of intracavity type II second-harmonic generation of gain-switched Cr:forsterite lasers in KTiOPO(4) . An operating wavelength range of 67 nm, from 587 to 654 nm, is obtained. Second-harmonic energy of almost 1 mJ is achieved at a peak wavelength of 620 nm. A measured factor-of-34 enhancement in acceptance angle is obtained with broadband harmonic generation. Frequency doubling is optimized with respect to the Cr:forsterite pump and laser parameters.     

Theory of backward second-harmonic generation of short laser pulses in periodically and aperiodically poled nonlinear crystals

We study numerically and analytically the backward second-harmonic generation in periodically and aperiodically poled nonlinear crystals in the quasiphase-matched regime. In our numerical analysis, we apply the optimum control technique based on the shooting method. Due to the fast and efficient numerical technique, we are able to analyze this frequency-conversion process in detail, taking into account various factors ?C the pump intensity, group-velocity mismatch and dispersion, linear phase mismatch, and ??chirp?? of nonlinear grating. Also we derive several approximate solutions for the backward harmonic efficiency employed in the undepleted pump approximation.     

Enhanced second-harmonic generation from planar photonic crystals

Strongly enhanced second-harmonic generation is observed from a two-dimensional square lattice GaAs/AlGaAs photonic crystal waveguide when the fundamental beam, the second-harmonic beam, or both beams resonantly couple to a leaky eigenmode. P-polarized second-harmonic spectra are obtained for s-polarized, 150-fs pump pulses that are tuned from 5000 to 5600 cm(-1) and directed along the gamma-chi direction of the crystal for various angles of incidence. Compared with off-resonant conditions, enhancements of >1200x in the second-harmonic conversion are observed for resonant coupling of both the fundamental and the second-harmonic fields to leaky eigenmnodes. The angular and spectral positions of the peaks are in good agreement with simulations.     

啁啾补偿的折返点匹配二倍频

提出了一种啁啾补偿群速色散的倍频方法,通过向入射基频光引入合适的初始啁啾,让其与色散相互作用,以实现对基频光脉冲宽度的主动控制,提高转换效率。研究结果表明,这种方法能显著地提高倍频转换效率,以氘含量12.6%(摩尔百分比)的KD*P晶体对脉宽30fs,中心波长为1.053μm的宽带基频光的折返点匹配宽带二倍频过程为例,当基频光转换极限位置处于晶体中心时,能取得最佳的啁啾补偿效果,转换效率可提高近22%。进一步研究了转换效率和转换带宽与晶体长度的关系。     

Quantum-noise measurements in high-efficiency single-pass second-harmonic generation with femtosecond pulses

The quantum-noise properties of single-pass second-harmonic blue-light generation with femtosecond pulses have been measured. A conversion efficiency of as much as 63.5% of second-harmonic generation at 428.8 nm was observed in a KNbO(3) crystal with femtosecond (130-fs) pulses with wavelengths centered at 857.6 nm. The quantum noise on the generated blue light was measured to be 1.0 dB (1.4 dB of squeezing inferred) below the shot-noise limit. The noise reduction was found to be sensitive to the average power and the center wavelength of the input fundamental pulses under the condition of strong pump depletion.     

Experimental observation of all-optical non-return-to-zero-to-return-to-zero format conversion based on cascaded second-order nonlinearity assisted by active mode-locking

We report the experimental demonstration of all-optical format conversion by exploiting the cascaded second-harmonic generation and difference-frequency generation (cSHG/DFG) in a periodically poled lithium niobate (PPLN) waveguide assisted by the reflective semiconductor optical amplifier (RSOA)-based active mode-locking. 10 and 20 Gbit/s format conversions from non-return-to-zero (NRZ) to return-to-zero (RZ) are successfully observed. Two schemes with either the NRZ signal or the pump optical clock set at the quasi-phase matching (QPM) wavelength are both verified in the experiment.