在周期性极化铌酸锂波导中的波长转换与光谱分析

实验研究了在准相位匹配LiNbO₃波导中级联的倍频和差频波长转换,通过对转换频谱分析和波形对比分析,讨论了在脉冲泵浦情况下走离效应对光脉冲之间波长转换的影响：由于波导的色散性能,不同波长的光脉冲在波导中传播具有不同的群速度,处在0.8 μm光波段的倍频光脉冲比处在1.55 μm光波段的泵浦光脉冲的传播速度慢,导致了倍频光脉冲与泵浦光脉冲在传播的过程中发生走离;因此倍频脉冲在频域被压缩而在时域被展宽,并遗传给差频过程使得转换脉冲的谱宽变窄、脉宽增加、占空比变大,表明脉冲光泵浦波长转换对信号频谱及波形不是严格透明的转换。实验同时实现了一对多通道的波长转换,表明连续控制光的线宽对转换信号的信噪比有较大的影响。     

LiInSe2 nanosecond optical parametric oscillator

Optical parametric oscillation using the new lithium selenoindate nonlinear crystal is reported for what is to our knowledge the first time. A 17 mm long, type II phase-matched sample is pumped by a 10 ns Nd:YAG laser. The minimum pump energy threshold is 3 mJ for a signal-resonant configuration. The signal and idler waves are tunable between 1.47 and 1.57 microm, and 3.3 and 3.78 microm, with a total output energy of 170 microJ corresponding to a 2.4% energy conversion at 8 mJ pump, only limited by the AR coatings damage. With optimized crystal quality and coatings, lithium selenoindate should show superior performance as compared with AgGaS(e)2 crystals, owing to its 4x larger thermal conductivity.     

Generation of high-contrast millijoule pulses by optical parametric chirped-pulse amplification in periodically poled KTiOPO4

A new high-contrast, high-gain optical parametric chirped-pulse amplifier (OPCPA) architecture is demonstrated in periodically poled KTiOPO4 (PPKTP). This architecture overcomes parametric fluorescence contrast limitations of the OPCPA in periodically poled materials. The scheme is based on two passes of a single relay-imaged pump pulse and a free-propagating signal pulse through a 1.5 mm x 5 mm x 7.5 mm PPKTP crystal. The output energy of 1.2 mJ is generated at a center wavelength of 1053 nm by 24 mJ of pump energy. A prepulse contrast level of > 3 x 10(7) was measured with > 10(6) saturated gain in the amplifier. Amplified pulses were compressed to 200 fs. This simple and versatile concept requires only a modest pump energy from a commercial pump laser and represents a possible high-contrast front end for high-energy Nd:glass-based petawatt-class lasers.     

Wavelength-agile mid-infrared (5-10 microm) generation using a galvano-controlled KTiOPO4 optical parametric oscillator

A wavelength-agile mid-infrared (IR) ZnGeP2 (ZGP) optical parametric oscillator (OPO) using a galvano-controlled double-crystal KTiOPO4 (KTP) OPO was demonstrated. The mid-IR wavelength was tuned by varying the KTP OPO pump wavelength while the ZGP crystal angle remained fixed. Rapid tuning of the KTP OPO was achieved by changing the crystal angle by using the galvano scanner. Our mid-IR source can jump to a different wavelength without scanning through the intermediate wavelengths while also permitting continuous-wavelength scanning. The mid-IR source can be tuned from approximately 5to10 microm at a phase-matching angle of 51 degrees , while the pump wavelength is controlled in the 1.95-2.2 microm range.     

Experimental observation of a 1.5 microm band wavelength conversion and logic NOT gate at 40 Gbit/s based on sum-frequency generation

We have experimentally verified a new idea for 40 Gbit/s wavelength conversion within the 1.5 microm band based on sum-frequency generation (SFG) in a periodically poled LiNbO3 waveguide. The spectrum and the temporal waveform of the output pump reveal that the input cw pump is converted to an optical pulse during SFG. Not only wavelength conversion but also a logic NOT gate at 40 Gbit/s are experimentally observed.     

Actively Q-switched Nd:YVO4 laser using an electro-optic periodically poled lithium niobate crystal as a laser Q-switch

We demonstrate a low-voltage and fast laser Q-switching by using an electro-optic periodically poled lithium niobate (EO PPLN) crystal. The half-wave voltage measured from the EO PPLN crystal was 0.36 V x d (microm)/L (cm), where d is the electrode separation and L is the electrode length. When a 13-mm-long EO PPLN was used as a laser Q switch at 7-kHz switching rate, we measured an approximately 12-ns pulse width and approximately 0.74-kW laser pulses at 1064-nm wavelength from a diode-pumped Nd:YVO4 laser with continuous 1.2-W pump power at 809-nm wavelength.     

Optical parametric oscillation in quasi-phase-matched GaAs

We demonstrate an optical parametric oscillator (OPO) based on GaAs. The OPO utilizes an all-epitaxially-grown orientation-patterned GaAs crystal that is 0.5 mm thick, 5 mm wide, and 11 mm long, with a domain reversal period of 61.2 microm. Tuning either the near-IR pump wavelength between 1.8 and 2 microm or the temperature of the GaAs crystal allows the mid-IR output to be tuned between 2.28 and 9.14 microm, which is limited only by the spectral range of the OPO mirrors. The pump threshold of the singly resonant OPO is 16 microJ for the 6-ns pump pulses, and the photon conversion slope efficiency reaches 54%. We also show experimentally the possibility of pump-polarization-independent frequency conversion in GaAs.     

Degenerate parametric frequency conversion of phase-modulated femtosecond laser pulses in crystals with a regular and chirped domain structure

A systematic theoretical analysis of the degenerate parametric frequency conversion in a LiNbO₃ crystal with a regular domain structure and a linearly varying domain thickness (chirped crystal) is presented for the pulses of a titanium-sapphire laser with a wavelength of 0.8 μm and durations of 100 and 50 fs in the presence and in the absence of phase modulation. The results are obtained with regard to the difference in the group velocities of interacting pulses and the group velocity dispersion. For an effective frequency conversion of the phase-modulated (PM) pump pulse, it is expedient to employ chirped crystals in which the domain thickness decreases from the entrance to the exit of the crystal. The pump energy is effectively converted into subharmonic energy when the pump carrier frequency decreases with time. It is demonstrated that the efficiency of the energy conversion to a subharmonic of 80% can be realized for PM pumping with a pulse duration of 100 fs in the chirped LiNbO₃ crystal. The efficiency of the parametric frequency conversion depends on the pump intensity as well as on the phase modulation of the pulse and the chirp of the crystal. Note that a variation in one of these parameters causes variations in the remaining parameters needed for the maximum efficiency of the parametric frequency conversion.     

Continuous-wave and pulsed laser performance of Nd:LuVO4 crystal

Received August 11, 2003 We report continuous-wave and actively Q-switched laser performance achieved with Nd:LuVO4 crystal for the 4F(3/2) --> 4I(1/2) transition (corresponding wavelength of 1065.8 nm) under high-power diode pumping. Continuous-wave output power of 12.55 W is obtained with an optical conversion efficiency of 50.2%. In actively Q-switched operation the average output power reaches 5.42 W at a pulse repetition frequency of 40 kHz with 18 W of pump power incident upon the crystal, yielding an optical conversion efficiency of 30.1%. The pulse energy and peak power reach 138 microJ and 16.2 kW, respectively, at a pulse repetition frequency of 25 kHz under a pump power of 14.2 W; the pulse duration is 8.5 ns.     

Doubly resonant continuous-wave optical parametric oscillator pumped by a single-mode diode laser

The performance characteristics of a doubly (signal and idler) resonant continuous-wave optical parametric oscillator based on periodically poled lithium niobate and pumped by a 100-mW single-mode laser diode at 810 nm are reported. Pump power thresholds as low as 16 mW and wavelength tuning over the range 1.15-1.25 microm at the signal and 2.31-2.66 microm at the idler were achieved through variation of crystal temperature, pump wavelength, and grating period. Up to 5 mW of signal output was obtained with the single-mode diode pump, and signal powers of up to 39 mW were obtained when pumping with a 400-mW injection-locked broad-area diode laser.     

Picosecond pulse amplification in a core-pumped large-mode-area erbium fiber

Amplification in a single-clad, large-mode-area erbium fiber as an alternative to double-clad Er-Yb amplifiers is presented. Both signal and pump are coupled through a mode-matched splice into the fundamental mode, which ensures preferential gain in the fundamental mode while minimizing the amplified spontaneous emission (ASE). The 875 microm(2) effective area of the Er fiber enables amplification of 6 ps pulses at 1.55 microm wavelength by approximately 33 dB in a single stage to >25 kW peak power with low nonlinear pulse distortion and a diffraction-limited output beam with M(2)<1.1.     

Optimization of the idler wavelength tunable cascaded optical parametric oscillator based on chirp-assisted aperiodically poled lithium niobate crystal

We present the numerical results for the optimization of the pump-to-idler conversion efficiencies of nanosecond idler wavelength tunable cascaded optical parametric oscillators(OPO) in different wavelength tuning ranges, where the primary signals from the OPO process are recycled to enhance the pump-to-idler conversion efficiencies via the simultaneous difference frequency generation(DFG) process by monolithic aperiodically poled, magnesium oxide doped lithium niobate(APMg LN) crystals. The APMg LN crystals are designed with different chirp parameters for the DFG process to broaden their thermal acceptance bandwidths to different extents. The idler wavelength tuning of the cascaded OPO is realized by changing the temperature of the designed APMg LN crystal and the cascaded oscillation is achieved in a single pump pass singly resonant linear cavity. The pump-to-idler conversion efficiencies with respect to the pump pulse duration and ratio of OPO coefficient to DFG coefficient are calculated by numerically solving the coupled wave equations. The optimal working conditions of the tunable cascaded OPOs pumped by pulses with energies of 350 μJ and 700 μJ are compared to obtain the general rules of optimization. It is concluded that the optimization becomes the interplay between the ratio of OPO coefficient to DFG coefficient and the pump pulse duration when the idler wavelength tuning range and the pump pulse energy are fixed. Besides, higher pump pulse energy is beneficial for reaching higher optimal pump-to-idler conversion efficiency as long as the APMg LN crystal is optimized according to this pump condition. To the best of our knowledge,this is the first numerical analysis of idler wavelength tunable cascaded OPOs based on chirp-assisted APMg LN crystals.     

Characteristics of Wavelength Tunable Femtosecond Soliton Pulse Generation Using Femtosecond Pump Laser and Polarization Maintaining Fiber

We have investigated both experimentally and numerically the characteristics of wavelength tunable femtosecond soliton pulse generation using a pulse width variable fiber laser and two different types of polarization maintaining fibers. The generated soliton shows the feature of the pulsewidth becoming almost constant at - 250 fs under any conditions of the pump pulse for 220m fiber. High conversion efficiency from pump pulse to a generated soliton pulse accounting for as much as 73% can be obtained. This efficiency decreases with increase in the input power or wavelength shift. A superior conversion efficiency and broad wavelength shift can be obtained by using a more shortened pump pulse. In the numerical calculations, it is predicted that under a condition of constant power of pump pulse, the maximum wavelength shift is achieved when the soliton number N is -1.4. The difference of mode field diameter and the group velocity dispersion (GVD) coefficient β₂ affect the wavelength shift and conversion efficiency. Using the fiber with small mode field diameter and small absolute value of GVD coefficient β₂, a high conversion efficiency and large wavelength shift can be obtained.     

Yb3+-doped YVO4 crystal for efficient Kerr-lens mode locking in solid-state lasers

We report the first demonstration, to our knowledge, of soft-aperture Kerr-lens mode locking in a diode-pumped femtosecond Yb3+:YVO4 laser. Near-transform-limited pulses as short as 61 fs are generated around a center wavelength of 1050 nm with an output power of 54 mW and a pulse repetition frequency of 104.5 MHz. This is, to our knowledge, the shortest pulse generated directly from an Yb laser having a crystalline host material. The femtosecond operation has a mode-locking threshold at an absorbed pump power of 190 mW. The nonlinear refractive indexes of the Yb3+:YVO4 crystal have been measured to be 19 x 10(-16) cm2/W and 15 x 10(-16) cm2/W for the sigma and pi polarizations, respectively, at 1080 nm.     

Modeling the terahertz radiation generation process,obtained by filtration of spectral supercontinuum,formed during propagation of femtosecond laser pulse in GaAs crystal

We present the results of theoretical studies of the process of generation of terahertz radiation arising via interaction of few-cycle laser pulses propagating in an isotropic nonlinear medium. Numerical time-integration, by the finite-difference method, of the system of nonlinear Maxwell equations has been performed. We consider the interaction of mutually-orthogonal linearly polarized pulses, both having the central wavelength of 1.98 μm, durations of 30 fs, and the energies of 30 nJ, propagating along the normal to the 〈110〉 plane in a 1 mm-thickness GaAs crystal. In the nonlinear part of the medium polarization the without-inertia nonlinear second-order susceptibility is taken into account. The process of formation of a terahertz pulse arising via spectral filtration of supercontinuum formed in the spectra of pump pulses at the output of nonlinear crystal is studied. The dependences of both the current frequency of the pump pulses on time for different lengths of nonlinear crystal and of pump pulse durations on the crystal length are obtained. Also the dependences of the current frequency of the terahertz pulse on time at different crystal lengths, as well as of the efficiency of generation of terahertz radiation on the length of nonlinear crystal and on the energy of pump pulses are obtained.     

Optical parametric generation of a mid-infrared continuum in orientation-patterned GaAs

We have generated an ultrabroad mid-infrared continuum by using single-pass optical parametric generation (OPG) in orientation-patterned GaAs (OP-GaAs). The spectrum spans more than an octave, from 4.5 to 10.7 microm, measured 20 dB down from the peak. The 17.5 mm long, 0.5 mm thick, all-epitaxially-grown OP-GaAs sample with a 166.6-microm quasi-phase-matching period was pumped with 3.1-3.3 microm wavelength, 1 ps pulses up to 2 microJ in energy. The OPG threshold was observed at 55 nJ pump energy with the pump polarized along the [111] crystal direction. The slope efficiency near threshold was 51%, and the external conversion efficiency was as high as 15%.     

Low-pump-threshold continuous-wave singly resonant optical parametric oscillator

We describe a compact all-solid-state continuous-wave singly resonant optical parametric oscillator (SRO) with a minimal pump-power requirement. The SRO is based on periodically poled LiNbO(3) as the nonlinear material and is pumped by a 1-W diode-pumped Nd:YVO(4) minilaser at 1.064 microm . By exploiting the intracavity pumping technique in a 50-mm crystal, we have achieved SRO operation threshold at a diode pump power of only 310 mW.At 1 W of input diode power, the SRO delivers 70 mW of output power in the nonresonant idler at 3.66 microm , at a photon conversion efficiency of 55%. Multiparameter tuning of the SRO yields a signal wavelength range from 1.45 to 1.60 microm and an idler wavelength range from 3.16 to 4.02 microm in the mid infrared. The device is characterized by robust turnkey operation and long-term amplitude-stable performance.     

Watt-level, high-repetition-rate, mid-infrared pulses generated by wavelength conversion of an eye-safe fiber source

We report on a mid-infrared (mid-IR) source consisting of an approximately 10 W average-power, linearly polarized 1.54 microm wavelength pulsed fiber source pumping an optical parametric oscillator. From this source, we obtained average power in excess of 1 W in the 3.8-4.0 microm wavelength range at a pulse repetition frequency of 100 kHz. With a slightly different setup, we also achieved an average power of 0.25 W at 4.5 microm wavelength. To our knowledge, these values represent the highest mid-IR power obtained through wavelength conversion of an eye-safe fiber source.     

An efficient intracavity-pumped KTP optical parametric oscillator at 1572 nm

We demonstrate an efficient and eye-safe wavelength intracavity optical parametric oscillator (OPO),based on a KTP crystal inside a Q-switched Nd:YVO4 laser end pumped by a fiber-coupled diode laser. Inthe acousto-optic Q-switched operation with the pulse repetition rate of 10 kHz, a 1572-nm eye-safe laser with the average power of 237 mW at the incident pump power of 5.64 W is obtained. Under the pulse repetition rate of 5 kHz, the signal light with pulse width of 2 ns and peak power of 18.5 kW is achieved.The conversion efficiency of the average power is 4.2% from pump diode to OPO signal output and thesignal pulse duration is about 13 times shorter than that of the depleted pump light.     

Generation of femtosecond anti-stokes pulses through phase-matched parametric four-wave mixing in a photonic crystal fiber

Phase-matched parametric four-wave mixing in higher-order guided modes of a photonic crystal fiber is shown to result in an efficient decay of 40-fs 800-nm Ti:sapphire laser pump pulses into an anti-Stokes signal with a central wavelength around 590-600 nm and a Stokes signal centered at 1.25 microm. The photonic crystal fiber is designed in such a way as to minimize the group-velocity dispersion at the pump wavelength, phase match the parametric four-wave-mixing process, and reduce the group delay between the pump and the anti-Stokes pulses. The duration of the anti-Stokes pulse under these conditions, as shown by cross-correlation frequency-resolved optical gating measurements, is less than 200 fs.