High-energy picosecond single-pass multi-stage optical parametric generator and amplifier

We demonstrate a new management of multi-stage optical parametric generator(OPG)and amplifier(OPA)to obtain high-energy picosecond sources with high beam quality.The setup of multi-stage OPG-OPA requires mode-matching between the pump beam and the stable mode of the OPG-OPA.In a proof-of-principle experiment,the single-pass multi-stage OPG-OPA consists of three walk-off compensated KTP crystal pairs and two lenses,pumped by an 86 ps,1064 nm 10 kHz picosecond laser.The signal light at~1.77μm has an average output power of 502 mW with record energy up to 50.2μJ.The beam quality factor of the signal light can be improved toM^(2)_(x) ×M^(2)_(y)after filtering out about 40%signal power.To the best of our knowledge,it is the first picosecond single-pass multi-stage OPG-OPA pumped at kHz regime.

Efficiency-enhanced picosecond mid-infrared optical parametric downconversion based on a cascaded optical superlattice

We demonstrate an efficiency-enhanced picosecond （ps） mid-infrared radiation via optical parametric downconversion. Based on a cascaded periodically poled MgO-doped stoiehiometric lithium tantalate crystal （MgO：sPPLT）, a tandem optical parametric oscillation-optical parametric amplification （OPO-OPA） process is achieved. Compared with a single OPO process, the conversion efficiency obtains an enhancement of 71%.

All-optical NRZ-to-AMI conversion using linear filtering effect of silicon microring resonator

We experimentally demonstrate 10-Gb/s format conversion from non-return-to-zero （NRZ） to alternatemark-inversion （AMI） using the linear filtering effect of silicon microring resonator. Our discussion and analysis in simulation further show that a 10-Gb/s AMI signal with good quality can be obtained by a resonator with a notch depth larger than 25 dB when the 3-dB bandwidth is 0.4 nm.

Flat-top optical frequency comb generation based on optical fiber-loop modulation with external injection

An optical frequency comb generation scheme using time-to-frequency conversion and optical fiber-loop modulation is proposed and demonstrated experimentally. Flat-top optical pulse train from an intensity modulator is injected into a fiber loop, which includes a phase modulator and an amplifier and is cyclically modulated by the phase modulator, to generate abundant optical comb lines. A total of 110 comb lines with 3-dB spectral power variation and 10-GHz frequency spacing are obtained. The proposed scheme is relatively simple and uses only one intensity modulator and one phase modulator.

Theoretical study of all-optical NRZ to RZ format conversion with tunable pulse width based on XPM in a silicon waveguide

An all-optical format conversion from non-return-to-zero （NRZ） to return-to-zero （RZ） is presented based on cross-phase modulation （XPM） in a silicon waveguide with a detuned optical bandpass filter （OBPF）. The simulation results show that the tunable bandwidth of the OBPF leads.to RZ signals with tunable pulse width. The conversion efficiency （CE） and the pattern effect of the RZ signal are attributed to the parameters of the pump pulse and the OBPE The converted RZ signal exhibits lower timing jitter than the NRZ signal.

589-nm yellow laser generation by intra-cavity sum-frequency mixing in a T-shaped Nd:YAG laser cavity

To obtain high power 589-nm yellow laser, a T-shaped thermal-insensitive cavity is designed. The optimal power ratio of 1064- and 1319-nm beams is considered and the fundamental spot size distribution from the output mirror to the two laser rods are calculated and simulated, respectively. As a result, a 589-nm yellow laser with the average output power of 5.7 W is obtained in the experiment when the total pumping power is 695 W. The optical-to-optical conversion efficiency from the fundamental waves to the sum frequency generation is about 15.2% and the pulse width is 150 ns at the repetition rate of 18 kHz. The instability of the yellow laser is also measured, which is less than 2% within 3 h. The beam quality factors are Mx^2 = 4.96 and My^2 = 5.08.

Widely continuous-tunable 2.789-4.957 μm twin-MgO:PPLN cascaded optical parametric oscillator

An all-solid-state mid-infrared optical parametric oscillator with wide tunability by using two identical multi-grating periodically poled 5-mol.-% MgO-doped lithium niobate cascaded is reported. The pump laser is an acousto-optically Q-switched Nd：YAG laser with 150-ns pulse width at repetition rate of 10 kHz. Wide tunability from 2.789 to 4.957μm at the idler wavelength is achieved by varying the temperature from 40 to 200 ~C and translating the grating periods from 26 to 31 μm with a step of 0.5μm. When the incident pump average power is 8.15 W, the maximum idler output average power is 2.23 W at 3.344 μm and the optic-optic conversion efficiency is about 27.4%.

Experiment study of wavelength conversion in a dispersion-flattened photonic crystal fiber

Wavelength conversion based on four-wave mixing （FWM） has been demonstrated using a 40-m dispersion flattened highly nonlinear photonic crystal fiber （HNL-PCF）. A conversion efficiency of -26 dB for a pump power of 19.5 dBm and a conversion bandwidth of 28 nm have been obtained, which are limited by the continuous wave （CW） laser wavelength range and tunability of optical band pass filters （OBPFs）.

Analysis of ultra-broadband high-energy optical parametric chirped pulse amplifier based on YCOB crystal

A new type of optical parametric chirped pulse amplifier is designed and analyzed for the amplification of pulse centered at 808 nm. A novel crystal, yttrium calcium oxyborate YCa40（BO3）3 （YCOB）, is utilized in the power amplification stage of optical parametric amplification （OPA）. Noncollinear phase matching parameters in the xoz principle plane of YCOB, compared with those in BBO and DKDP, are analyzed by numerical simulation. The results show that YCOB rather than DKDP can be used in the power amplification stage of OPA to realize the amplification of chirped pulse to several joules with a gain bandwidth exceeding 100 nm. This can be used to gain a high intensity pulse of -10 fs after the compressor.

High-average-power,high-repetition-rate dual signal optical parametric oscillator based on PPMgLN

A high-average-power,high-repetition-rate dual signal optical parametric oscillator based on periodically poled MgO-doped lithium niobate（PPMgLN） with a phase-reversed grating is reported.The pump laser is an acousto-optically Q-switched Nd：YVO_4 laser with a maximum average power of 7.6 W.When the repetition rate is 50 kHz and the pulse width of the pump source is 23 ns,the maximum average dual signal output power is about 1.9 W,leading to a conversion efficiency of 25%.Over a 30-min interval,the instability of the signal power measured is less than 0.5%.

Novel high-sensitivity coherent transceiver for optical DPSK/DQPSK signals based on heterodyne detection and electrical delay interferometer

We present a novel coherent transceiver for optical differential phase-shift keying/differential quadrature phase-shift keying （DPSK/DQPSK） signals based on heterodyne detection and electrical delay interferometer. A simulation framework is provided to predict a theoretical sensitivity level for the reported scheme. High sensitivity of –45.18 dBm is achieved for 2.5-Gb/s return-to-zero （RZ）-DPSK signal, and high sensitivities of –36.83 dBm （I tributary） and –35.90 dBm （Q tributary） are observed for 2.5-GBaud/s RZ-DQPSK signal in back-to-back configuration. Transmission for both signals over 100 km is also investigated. Experimental results are discussed and analyzed.

Tunable and coherent nanosecond 7.2-12.2 μm mid-infrared generation based on difference frequency mixing in ZnGeP_2 crystal

A coherent mid-infrared laser source,which can be tuned from 7.2 μm to 12.2 μm based on the type-Ⅰ phase-matched difference frequency generation(DFG) in an uncoated ZnGeP2(ZGP) crystal,is reported.The two pump waves are from a type-Ⅱ phase-matched dual-wavelength KTP optical parametric oscillator(OPO) of which the signal and idler waves are tuned during 1.85-1.96 μm(extraordinary wave) and 2.5-2.33 μm(ordinary wave),respectively.The maximum energy of the generated mid-infrared laser is 10 μJ at 9.22 μm,corresponding to the peak power of 2.2 kW.

A robust method for frequency stabilization of 556-nm laser operating at the intercombination transition of ytterbium

A frequency-stabilized 556-nm laser is an essential tool for experimental studies associated with ^1S0-^3p1 intercombination transition of ytterbium （Yb） atoms. A 556-nm laser light using a single-pass second harmonic generation （SHG） is obtained in a periodically poled MgO：LiNbO3 （PPLN） crystal pumped by a fiber laser at 1111.6 nm. A robust frequency stabilization method which facilitates the control of laser frequency with an accuracy better than the natural linewidth （187 kHz） of the intercombination line is developed. The short-term frequency jitter is reduced to less than 100 kHz by locking the laser to a home-made reference cavity. A slow frequency drift is sensed by the 556-nm fluorescence signal of an Yb atomic beam excited by one probe beam and is reduced to less than 50-kHz by a computer-controlled servo system. The laser can be stably locked for more than 5 h. This frequency stabilization method can be extended to other alkaline-earth-like atoms with similar weak intercombination lines.

Numerical analysis for four-wave mixing based wavelength conversion of differential phase-shift keying signals

We numerically investigate the main constrains for high efficiency wavelength conversion of differential phase-shift keying （DPSK） signals based on four-wave mixing （FWM） in highly nonlinear fiber （HNLF）. Using multi-tone pump phase modulation techniques, high efficiency wavelength conversion of DPSK signals is achieved with the stimulated BriIlouin scattering （SBS） effects effectively suppressed. Our analysis shows that there is a compromise between conversion efficiency and converted idler degradation. By optimizing the pump phase modulation configuration, the converted DPSK idler＇s degradation can be dramatically decreased through balancing SBS suppression and pump phase modulation degradation. Our simulation results also show that these multi-tone pump phase modulation techniques are more appropriate for the future high bit rate systems.

Design of GaAs/Al_xGa_(1-x)As asymmetric quantum wells for THz-wave by difference frequency generation

The energy levels, wave functions and the second-order nonlinear susceptibilities are calculated in GaAs/Al0.2Ga0.8As/Al0.5Ga0.5As asymmetric quantum well （AQW） by using an asymmetric model based on the parabolic and non-parabolic band. The influence of non-parabolicity can not be neglected when analyzing the phenomena in narrow quantum wells and in higher lying subband edges in wider wells. The numerical results show that under double resonance （DR） conditions, the second- order difference frequency generation （DFG） and optical rectification （OR） generation susceptibilities in the AQW reach 2.5019 μm/V and 13.208 μm/V, respectively, which are much larger than those of the bulk GaAs. Besides, we calculate the absorption coefficient of AQW and find out the two pump wavelengths correspond to the maximum absorption, so appropriate pump beams must be selected to generate terahertz （THz） radiation by DFG.

Wavelength conversion of chaotic message through gain modulation by injection-locked Fabry-Perot laser diode

We propose a wavelength conversion scheme for chaotic optical communications（COC） based on a Fabry-Perot （FP） laser diode.The FP laser,as a wavelength converter,is injection-locked at one of longitudinal modes by an external continuous-wave（CW） light.The simulation results demonstrated that the chaos masked signal at wavelengthλ_1,which corresponds to the other longitudinal mode of FP laser,can be converted to the injection-locked mode（wavelengthλ_2） based on cross-gain modulation in a closed-loop COC link.A 1.2-GHz chaos masked sinusoidal signal is successfully decoded with signal-to-noise ratio （SNR） beyond 8 dB in 15-nm wavelength conversion range,and the effects of SNR on the signal frequency and conversion span are also investigated.

Optical properties and CO_2 laser SHG with HgGa_2S_4

Optical properties and nonlinearity on phase-matching (PM) of two (yellow and orange) phase crystals were investigated in details. Damage threshold was determined in comparison with middle infrared (IR) crystal at identical experimental conditions. Second harmonic generation (SHG) of 30 ns TEA CO2 laser pulses was realized with 5% efficiency in energy and 6.9% in peak power.

RZ-OOK to NRZ-OOK format conversion based on a single fiber Bragg grating

We propose a return-to-zero on-off keying （RZ-OOK） to non-return-to-zero （NRZ） OOK conversion scheme based on a single custom-designed fiber Bragg grating （FBG）. The custom-made FBG is designed and synthesized using discrete layer-peeling algorithm. It is shown that such a FBG can replace the combination of interferometer and the cascaded filter that are invariably employed together in the reported schemes for RZ-OOK to NRZ-OOK format conversion. Simulation results show that the input 20-Gb/s RZ-OOK signals with different duty cycles can be converted into NRZ-OOK signals with high Q-factor.

Investigation on bismuth-oxide photonic crystal fiber for optical parametric amplification

A hexagonal solid-core bismuth-oxide micro-structure fiber is developed to balance its dispersion and nonlinearity.This simulation and calculation results show that the bismuth-oxide photonic crystal fiber(Bi-PCF) has near zero dispersion around 1550 nm.Its dispersion slop in the communication wavelength range is also relatively flat.Moreover,both nonlinear coefficient and model field distribution are obtained.Compared with the experimental results by SiO2-PCF,it can be seen that the Bi-PCF shows excellent characteristics for the optical parametric amplification(OPA).

Improvement of four-wave mixing-based wavelength conversion efficiency in dispersion shifted fiber by 40-GHz clock pumping

40-GHz clock modulated signal as a pump to improve the efficiency of four-wave mixing （FWM）-based wavelength conversion in a 26.5-km dispersion shifted fiber （DSF） is investigated. The experimental results demonstrate that the conjugated FWM component has higher intensity with the clock pumping than that with the continuous-wave （CW） light pumping. The improvement of FWM-based wavelength conversion efficiency is negligible when the pump power is less than Brillouin threshold. But when the pump power is greater than Brillouin threshold, the improvement becomes significant and increases with the increment of pump power. The improvement can increase up to 9 dB if pump power reaches 17 dBm.