Modal phase-matching second harmonic generation in Bragg fiber

Modal phase-matching second harmonic generation in uniformly poled Bragg fiber is theoretically proposed. The very low group velocity of the modes in Bragg fiber near the in-band cutoff frequencies leads to high nonlinear conversion efficiency comparable to that of the periodically poled conversional fiber. The subsequence phase-matched bandwidth reduction by slow light can be retrieved in a certain degree through structure parameters optimization.     

Detection of higher nonlinear harmonics of volume photorefractive gratings in reflection geometry

A comprehensive technique has been used to detect diffraction from the second and the third nonlinear spatial harmonic components, along with the fundamental harmonic of volume photorefractive gratings recorded in a channel waveguide in a LiNbO₃ substrate. For the first time to our knowledge, efficient diffraction from the higher spatial harmonic components of a photorefractive grating has been detected in reflection geometry. The dependence of a Bragg wavelength shift on the order of nonlinearity is also reported.     

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.     

Blue‐green light generation using high brilliance edge emitting diode lasers

A review about second harmonic generation using edge emitting diode lasers and nonlinear crystals to obtain laser radiation in the blue‐green spectral range is presented. Therefore, pump laser radiation with high brightness and narrow bandwidth is necessary. Thus, this review gives an overview of the advances made with distributed feedback and Bragg reflector lasers, tapered lasers and amplifiers as well as external cavity diode lasers and master oscillator power amplifier schemes to achieve high brilliance emission. Since periodically poled materials have enabled high second harmonic conversion efficiencies with low and moderate pump powers, the review is focused on frequency doubling using those materials. The most commonly used materials, their properties and limitations are discussed briefly. Single pass and resonant SHG setups with waveguide and bulk nonlinear crystals are discussed and an emphasis on building compact and integrated devices is made.     

Broadening of the phase-matching bandwidth in quasi-phase-matched second-harmonic generation using GaN-based Bragg reflection waveguide

We present an analysis of a high index core symmetric Bragg reflection waveguide (BRW) design based on a GaN/AlxGa1-xN system for efficient quasi-phase-matched second-harmonic generation for broadband applications. By choosing the fundamental frequency to be a BRW mode and suitably tailoring the overall dispersion characteristics, the strong dispersion of the second-harmonic mode is partially canceled, leading to phase matching between the fundamental and second-harmonic over a broad range of wavelengths. The crucial interplay between the dispersive behavior of the fundamental and second-harmonic wave manifests as a broad acceptance bandwidth of approximately 33 nm accompanied with appreciable conversion efficiency (22.8%/W) for a 10 mm long waveguide. The impact of tailoring the dispersion characteristics on the conversion efficiency is also discussed.     

Frequency Doubling of a Laser Diode Using a Domain-Inverted LiTaO3Waveguide with a Monolithic Bragg Reflector

We report wavelength stabilization of a laser diode using a highly efficient distributed Bragg reflector (DBR) grating formed on a LiTaO₃ quasi-phase-matched second harmonic generation (QPM-SHG) device. Fabricating the second-order DBR on the LiTaO₃ waveguide, the reflectivity of 90% and FWHM wavelength bandwidth of 0.2 nm were obtained. By stabilizing the oscillation wavelength of the laser diode, 3.1 mW of blue light was generated in the QPM-SHG device with the monolithic grating.     

Enhanced third harmonic reflection and diffraction in Silicon on Insulator photonic waveguides

Enhanced third harmonic (TH) generation from Silicon-On-Insulator (SOI) planar waveguides as well as SOI photonic crystal (PhC) slabs is studied in different angular configurations, both in the visible and infrared energy ranges. In the SOI planar waveguide, the multilayer structure causes the optical properties such as TH reflection to be different from those of bulk silicon samples. This behavior is well reproduced by calculations of TH reflectance.Measurements of third-harmonic reflection and diffraction from one-dimensional PhC slabs etched in the SOI waveguide are also reported. The angular positions of TH peaks at various diffraction orders agree well with those calculated from a nonlinear grating equation. Both reflection and diffraction processes contribute to enhanced TH generation efficiency in the PhC slabs.TH reflectance measurements performed on PhC slabs in the near infrared show a resonant interaction between the incident beam and the photonic structure, dependent on the angle of incidence. This leads to a nonlinear conversion efficiency which is strongly enhanced with respect to that of the SOI waveguide, due to the excitation of strong local fields associated with the presence of photonic modes in the PhC slab.     

Broadband second harmonic generation in a tapered isotropic semiconductor slab using total internal reflection quasi phase matching

This paper analytically describes the concept of enhancing the bandwidth of second-harmonic generation in the mid-infrared region in an isotropic tapered semiconductor slab configuration. In this slab geometry, the lengths between successive reflection points due to total internal reflection of the incident laser radiation increase when it propagates through the slab. A computer aided simulation has been carried out to determine the possibility of generating broadband second harmonic intensity for broadband fundamental laser radiations as they are allowed to undergo total internal reflection inside the tapered isotropic semiconductor crystal slab made of either gallium arsenide (GaAs) or zinc selenide (ZnSe). The simulated results indicate wide 3 dB bandwidths of 187 nm and 196 nm in a 30 mm long tapered slab of GaAs and ZnSe respectively. The conversion efficiency, after considering the absorption and reflection losses, is quite satisfactory (≈ 1%). The effects of variations in temperature, incident angle of the fundamental laser radiations at the air-slab interface, length and tapering angle of the semiconductor slab have been studied for generated second harmonic radiations in both GaAs and ZnSe crystals. Optimising these parameters a wider broadband frequency converter with appreciable conversion efficiency can be designed.     

Ge0.05Si0.95/Si异质结单模共面布拉格反射波导光栅的设计与分析

GeSi/Si异质结布拉格反射光栅是硅基光电集成领域一种重要的集成光学器件,分析GeSi/Si异质结的传光特性和布拉格条件,通过求解布拉格光栅方程,得出耦合系数和耦合效率。利用上述原理设计出入射角为66°,波导层的厚度为2μm,光栅长度为4252μm,槽深为0.05μm,光栅周期为0.456μm,滤波带宽为0.214nm,耦合效率为84.1%的1.3μm Ge_0.05Si_0.95/Si异质结单模共面布拉格反射光栅,并用数值模拟了入射光波电场和反射光波电场的分布。     

Wideband wavelength conversion using double-pass cascaded χ:χ interaction in lossy waveguides

We report the wavelength conversion based on double-pass cascaded nonlinear interaction (χ⁽²⁾:χ⁽²⁾) of sum and difference frequency generation in quasi-phase matched lithium niobate waveguides and compare it with double-pass cascaded second harmonic generation and difference frequency generation with and without waveguide loss. It is shown that the efficiency decreases considerably even for the low-loss waveguide compared to the lossless one especially for long waveguides and to achieve the higher efficiency for the same length, the amount of the extra power to compensate the loss increases. Also, an increased detuning of pump wavelength is proposed to flatten the response with a small efficiency penalty. The detuning- and loss-compensating pump powers can be found using the design diagrams in which the criteria for the design of waveguide length and the assignment of pumps power to obtain the desired efficiency, ripple and bandwidth are presented assuming a 75-nm pump wavelength difference.     

带宽调制型单光纤光栅温变无补偿位移传感

报道了利用反射谱带宽调制和光强差分探测技术实现单一光纤光栅温变无补偿位移精确测量的新方法。设计了一种结构新颖的曲臂梁位移传感装置,结合光波导理论与材料力学原理分析了光纤光栅在高斯应变作用下光栅反射谱侧向梯度展宽的成因,理论推导了特殊结构梁在外力作用下光栅反射谱带宽/反射光强与压力之间的响应关系。光栅反射谱侧向梯度展宽的同时反射光强线性增加,利用光强差分检测方法消除光源出光抖动的影响,提高了位移测量精度。基于带宽调制的光纤光栅位移传感方法免受温度变化的影响,在-10℃~80℃的温度变化范围内,测量误差小于1.2%,实现了单光纤光栅温变无补偿位移测量。     

Temperature-dependent second harmonic generation process based on an MgO-doped periodically poled lithium niobate waveguide

The temperature dependency of a 5-mol% MgO-doped periodically poled lithium niobate waveguide was investigated in this paper. We started with the temperature-dependent refractive index equation for the waveguide. Secondly, the temperature dependency of the second harmonic generation effect was experimentally researched under different temperatures and pump powers. The quasi-phase matched wavelengths, efficiency bandwidths and peak efficiencies of the waveguide were measured. The experimental results agreed with theoretical simulations, which are indispensable in the following all-optical sampling studies based on the cascaded second harmonic generation/difference-frequency generation process in the current device.     

Theoretical analysis of the TE mode Cerenkov type second harmonic generation in ion-implanted X-cut lithium niobate channel waveguides

Optical second harmonic generation (SHG) in the form of Cerenkov radiation from ion-implanted lithium niobate (LiNbO₃) channel waveguides is analyzed by directly resolving the wave equations. Useful formula of the SHG efficiency is derived and expressed in terms of waveguide parameters. Numerical examples are plotted for LiNbO₃ crystals. The results enable the optimization of waveguide design for efficient second harmonic generation in the Cerenkov configuration.     

Ultra-narrow bandwidth resonant reflection grating filters using the second diffracted orders

In this paper, we design resonant reflection grating filters employing the second diffracted orders as the leaky modes, then analyze the bandwidth of the reflection peak and the electric field distributions inside the waveguide under resonance. The numeric calculation confirms that ultra-narrow resonant reflection peaks can be observed in these structures. At the same time, strong electric field enhancement appears under resonance. It provides a new approach to diversify the resonant reflection filters and may open a new way to the realization of ultra-narrow bandwidth filters.     

铌酸锂质子交换波导中切伦科夫倍频实验研究

报道了质子交换铌酸锂平板波导中切伦科夫倍频蓝绿光输出研究。理论上计算了质子交换波导参数对不同基波波长倍频转换效率的影响。制备出适合于蓝绿光输出的铌酸锂质子交换波导并观测到较高转换效率的倍频光产生,实验中我们测量了倍频转换效率与抽运动率以及抽运波长的关系,实验结果与理论预测吻合。     

Phase-matched second harmonic blue light generation in ion implanted KNbO3 planar waveguides with 29% conversion efficiency

We report for the first time to our knowledge, noncritical phase-matched second harmonic generation in an ion-implanted KNbO₃ planar waveguide. The guided TM₀ mode of the fundamental wave (868 nm) is converted into the second harmonic TE₁ mode (434 nm). From 1.3 kW of internal fundamental peak power 385 W second harmonic blue light is generated giving a conversion efficiency of 29%. A comparison of the measured and the theoretical efficiencies implies that phase-matching is achieved over the full waveguide length. Based on these first experimental results we estimate that 100 mW of blue light can be generated from 400 mW of input power using an optimized KNbO₃ planar waveguide of 1 cm length.     

Integrated optical Bragg filter with fast electrically controllable transfer function

We demonstrate an integrated optical Bragg filter based on lithium niobate, with a fast electrically controllable transfer function. The technique is based on phase-shift keying of the reflection Bragg grating in a waveguide. The transfer function can be tuned or favourably reconfigured by applying different combinations of electrical voltage to the electrode pairs on both sides of the waveguide. The demonstrated time of reconfiguration is less than 1 μs.     

采用光栅角色散的宽带二倍频及聚焦特性分析

针对带宽为20 nm左右、中心波长为1 053 nm的线性啁啾宽带激光,分析了采用光栅角色散方式宽带二倍频的原理,讨论了光栅角色散性能及光栅加工误差引入的基频光位相扰动对二倍频光转换效率及聚焦特性的影响.研究结果表明:采用光栅角色散方式的宽带二倍频光的转换效率、脉冲宽度和带宽均明显增大;在0～22 nm基频光带宽范围内,二倍频光均可保持80％左右的转换效率;对于入射基频光带宽为22 nm的情况,光栅角色散率在30～80 μrad/nm范围内变化时,宽带二倍频转换效率均可保持在70％以上;基频光有位相扰动时,二倍频光带宽对二倍频聚焦光斑的主瓣影响不大,但对旁瓣有一定的匀滑作用,采用光栅角色散方式宽带二倍频的聚焦光斑旁瓣的匀滑效果更为明显.     

Bandwidth enhancement and response flattening of cascaded sum- and difference-frequency generation-based wavelength conversion

We investigate bandwidth enhancement and response flattening of wavelength conversion based on cascaded sum-frequency generation and difference-frequency generation (SFG/DFG) in quasi-phase-matched periodically poled lithium niobate waveguides by use of two pump lights. Analysis shows that the conversion properties deeply depend on the pump settings. The bandwidth is efficiently enhanced by augmenting the pump wavelength difference. The critical bandwidth corresponding to the response fluctuation of 1 dB in a 3-cm-long waveguide reaches 80-90 nm, which is enough to cover the entire conventional-band and long-wavelength-band. The cascaded SFG/DFG bandwidth is about 24% broader than that of the cascaded second-harmonic generation and difference-frequency generation-based configuration with the same waveguide length. However, the response fluctuation is also enhanced together with the bandwidth enhancement. The method of pump detuning is presented to flatten the response fluctuation. The fluctuation is reduced by more than 1 dB and it can be further reduced by increasing the pump detuning value with a little efficiency penalty.     

Sub-millimeter third harmonic generation in n-InSb waveguide

Resonant third harmonic generation of a sub-millimeter wave in n-InSb waveguide embedded with a density ripple is investigated. The non-linearity arises through the modulation of free electron mass while the ripple accounts for the phase mismatch. The efficiency of the third harmonic generation is large. However, as the plasma frequency increases the attenuation rate of the third harmonic increases and the third harmonic efficiency decreases as (ω_pb/c) is raised.