Orientation-enhanced nonlinear optical properties and phase-conjugate reflective system of a novel Azobenzene doped polymer film

This paper reports that the nonlinear refractive index of a novel organic optical storage film doped azodiphenylamine polymer is measured by using the Z-scan technique. The nonlinear refractive index up to 3.7× 10^-6 cm^2/W induced by thermo-optical effect is obtained. It indicates that the sample has excellent optical non- linear properties. The physical mechanism of the great nonlinear optical effect is analysed and the optical conjugate characteristic is also discussed with degenerate four-wave-mixing. The phase conjugate wave diffracted from the formative refractive index grating in the sample is acquired and its equivalent reflectivity reaches about 22%. On this basis, the reflective wave phase-conjugated mirror system was designed, and the image aberration experienced in propagation in the storage experiment is corrected by using the system.

Theoretical study on phase conjugation in weakly injected vertical-cavity surface-emitting lasers

This paper gives a detailed theoretical investigation on phase conjugation induced by nearly degenerate fourwave mixing in single mode vertical-cavity surface-emitting lasers （VCSELs） with weak optical injection. Considering VCSELs that can work in linearly polarized or elliptically polarized states, it derives the theoretical expression of the conjugated field by small signal analysis based on the vectoral rate equations-the spin-flip model. For linearly polarized state, VCSELs show similar conjugate spectra to edge-emitting semiconductor lasers. For the elliptically polarized state, dichroism and birefringence parameters as well as the spin-flip rate can change the conjugate spectra. Especially, when frequency detuning of the probe and pump waves is between the positive and negative relaxation oscillation frequency, changes are evident. For specific values of parameters, conjugate efficiency between 20 dB to 40 dB are obtained.

OPTICAL PHASE CONJUGATION RESPONSE OF PHOTOINDUCED POLYMER FILMS CONTAINING AZOBENZENE MOIETIES WITH CHIRAL GROUP

An optically active monomer containing azobenzene moieties with chiral group (s-2-methyl-butyl), 4-[2-(methacryloyloxy)ethyloxy] -4'-(s-2-methyl-1-butyloxycarbonyl) azobenzene (M1) was synthesized. Polymer (PM1) possessing optical phase conjugated response was obtained by homopolymerization of the optically active monomer (M1) using free radical polymerization. The polymer was very soluble in common solvents and good optical quality films could be easily fabricated by spin coating. The optical phase conjugated responses of the polymer PM1 were measured by degenerate four-wave mixing (DFWM). In comparison with polymer containing no chiral group, it was found from the preliminary measurement of photoisomeric change that optical phase conjugated response of the PM1 in the long-range order hexagonal symmetry microstructure could be easily controlled by choosing the appropriate polarization direction of the irradiating beams (514.5 nm) and the irradiating number, presumably due to the chiral group in the PM1 molecular structure. For the case of the polymer investigated here, a chiral group side chain was introduced to increase optical phase conjugated response intensity with different polarization directions of the irradiating beams, which aims originally at searching for a new photoactive material.

Wavefront correction of Ti:sapphire terawatt laser with varying precision of phase conjugation between deformable mirror and wavefront sensor

The phase conjugation between the deformable mirror and the wavefront sensor in the aberration correction of a terawatt Ti：sapphire laser is studied experimentally and theoretically in this paper. At varying values of phase- conjugation precision, we focus the corresponding beams into spots of the same size of 5.1 μm × 5.3 μm with a f/4 parabola in the 32 TW/36 fs Ti：sapphire laser system. The results show that the precision of conjugation can induce an intensity modulation but does not significantly affect the wavefront correction.

SYNTHESIS AND PROPERTIES OF A NEW AZOBENZENE SIDE-CHAIN POLYMER CONTAINING A TEMPO RADICAL

To allow anisotropies of optical properties in a magnetic field, nitroxide radical is introduced into the ortho-position of the phenylene ring in the side chain. A new azobenzene side-chain polymer (TEMPO-PAZ) containing TEMPO radical was synthesized. The polymer has a good solubility in organic solvents. The ESR spectrum of the polymer indicated three absorption lines characteristic of TEMPO radical. The optical phase conjugated responses (I-4) of the polymer films were investigated by degenerate four-wave mixing (DFWM). The experimental results showed that optical phase conjugated response of the TEMPO-PAZ could be easily controlled by choosing the appropriate direction of magnetic field presumably due to the nitroxide radical in the TEMPO-PAZ molecular structure. For the polymer investigated here, the nitroxide radical was introduced to increase optical phase conjugated response intensity in a magnetic field, aiming originally at searching for a new photo-active organic magnetic multifunctional materials.

Sparse Planar Retrodirective Antenna Array Using Improved Adaptive Genetic Algorithm

An improved adaptive genetic algorithm is presented in this paper. It primarily includes two modified methods： one is novel adaptive probabilities of crossover and mutation, the other is truncated selection approach. This algorithm has been validated to be superior to the simple genetic algorithm （SGA） by a complicated binary testing function. Then the proposed algorithm is applied to optimizing the planar retrodirective array to reduce the cost of the hardware. The fitness function is discussed in the optimization example. After optimization, the sparse planar retrodirective antenna array keeps excellent retrodirectivity, while the array architecture has been simplified by 34%. The optimized antenna array can replace uniform full array effectively. Results show that this work will gain more engineering benefits in practice.

Joint frequency offset tracking and PAPR reduction algorithm in OFDM systems

This paper presents an algorithm that aims to reduce the peak-to-average power ratio（PAPR） of orthogonal frequency division multiplexing（OFDM） communication systems while maintaining frequency tracking.The algorithm achieves PAPR reduction by applying the complex conjugates of the data symbol obtained from the frequency domain to cancel the phase of the data symbol.A likelihood estimator is used to obtain the sub-carrier phase error due to the residual carrier frequency offset（RCFO） using the same complex conjugates as a pilot signal.Furthermore,a joint time and frequency domain multicarrier phase locked loop（MPLL） is developed to compensate additional frequency offset.Simulation results show that this algorithm is capable of reducing PAPR without impacting the frequency tracking performance.

Fast optical wavefront engineering for controlling light propagation in dynamic turbid media

While propagating inside the strongly scattering biological tssue,photons lose their incident directions beyond one transport mean free path(TMFP,~1 millimeter(mm)),which makes it challenging to achieve optical focusing or clear imaging deep inside tissue.By manipulating many degrees of the incident optical wavefront,the latest optical wavefront engineering(WFE)technology compensates the wavelfront distortions caused by the scattering media and thus is toward breaking this physical limit,bringing bright perspective to many applications deep inside tissue,eg,high resolution functional/molecular imaging,optical excitation(optogenetics)and optical tweezers.However,inside the dynamic turbid media such as the biological tissue,the wavefront distortion is a fast and continuously changing process whose decorrelation rate is on timescales from milliseconds(ms)to microseconds(μs),or even faster.This requires that the WFE technology should be capable of beating this rapid process.In this review,we discuss the major challenges faced by the WFE technology due to the fast decorrelation of dynamic turbid media such as living tissue when achieving light focusing/imaging and summarize the research progress achieved to date to overcome these challenges.

Study of Temperature Enhancement of the Photorefractive Effect in Ce：Fe：LiNbO_3 Crystal

in the range of 20 to 120 ℃, the two-beam coupling exponential gain coefficient and the four-wave mixing phase conjugation reflectivity have been investigated. It is shown that the values such as the gain, the phase conjugation reflectivity and the response speed increase as the temperature increases. At about 55, 71 and 110℃, extraordinary enhancement of the gain and the phase conjugation reflectivity were observed.The mechanism is analyzed by the phase change in the crystal at these temperatures.

A new circulating two-cell structure for stimulated Brillouin scattering phase conjugation mirrors with 1-J load and 10-Hz repetition rate

A new approach to realize high-energy and high-power stimulated Brillouin scattering phase conjugation mirrors （SBS-PCMs） is described.The reflectivity of SBS-PCM is investigated under a 10-Hz repetition rate and a high energy load.The relationship between reflectivity and input energy is examined experimentally with different PCM structures,focus lengths,and medium cell structures.A medium cell with a circulating structure is designed,and its advantage is demonstrated through an experimental comparison with traditional PCM structures.The 30-cm focus lens and 150-cm collimation lens are optimized when the input energy reaches 1010 mJ at 10-Hz repetition rate.Therefore,a reflectivity of 84.7% and a higher energy load using the circulating two-cell structure are achieved.

Optimized optical phase conjugation conf iguration for f iber nonlinearity compensation in CO-OFDM systems

The optimized optical phase conjugation （OPC） configuration is proposed for the 40-Gb/s CO-OFDM system. The proposed configuration for nonlinear cancellation is systematically depicted in transmission links with lumped amplification. Numerical simulations are performed to demonstrate effectiveness. Simulation results show that mid-span spectral inversion （MSSI） can partially compensate for nonlinear distortions. Moreover, its optimized configuration can further improve system performances and increase nonlinear compensation effectiveness. Compared with MSSI, the maximal Q factor, nonlinear threshold, and transmission distance of optimized OPC configuration increase by over 1.6 dB, 2 dB, and 2 times, respectively.

Effects of Chirped Super-Gaussian Pulses on Optical Communication System Using Midway Optical Phase Conjugation

Relationship between the initial chirp of super-Gaussian pulse and dispersion and nonlinearity effects of a single-mode fiber in the optical communication system using midway optical phase conjugation is analyzed. The results of numerical simulations are useful for improving compensation for pulse distortion.

Frequency-dispersive method for improving broad-band SBS phase conjugation of Cr:LiSAF laser

After passing through four dispersive-prisms, the Q-switched Cr.LiSAF laser with broad frequency band is focused into carbon disulfide (CS2) to produce backward stimulated Brillouin scattering (SBS). Our experimental results and illustrative analysis have shown that this frequency-dispersive method can efficiently reduce the broad-band SBS intensity threshold, compress its pulsewidth, and improve the beam quality.

Effect of focal pumping conditions on the phase-conjugation characteristics of STS and SBS

The phase-conjugation characteristics of stimulated thermal scattering (STS) in absorbing liquid and stimulated Brillouin scattering (SBS) separately excited in pure acetone were compared, and the effect of focal pumping conditions on them was studied. It is shown that high-quality phase conjugation can be obtained by STS in absorbing liquid under stronger pumping conditions.

Compensation for the self-steepening effects inoptical fiber communication system usingmidway optical phase conjugation

Effects of self-steepening (SS) of chirped Gaussian pulses on optical fiber communication system using midway optical phase conjugation (OPC) are analyzed. Dynamic evolution of the ultrashort pulses is simulated numerically. It is found that OPC cannot compensate for pulse waveform distortion due to SS. The initial chirp of pulses and dispersion can counteract SS and improve the compensation performance for the distortion.

Enhancement of phase conjugation degenerate four-wave mixing using a Bessel beam

We report on the enhancement of phase conjugation degenerate four-wave mixing(DFWM) in hot atomic Rb vapor by using a Bessel beam as the probe beam. The Bessel beam was generated using cross-phase modulation based on the thermal nonlinear optical effect. Our results demonstrated that the DFWM signal generated by the Bessel beam is about twice as large as that generated by the Gaussian beam, which can be attributed to the extended depth and tight focusing features of the Bessel beam. We also found that a DFWM signal with reasonable intensity can be detected even when the Bessel beam encounters an obstruction on its way, thanks to the selfhealing property of the Bessel beam. This work not only indicates that DFWM using a Bessel beam would be of great potential in the fields of high-fidelity communication, adaptive optics, and so on, but also suggests that a Bessel beam would be of significance to enhance the nonlinear process, especially in thick and scattering media.

122-W high-power single-frequency MOPA fiber laser in all-fiber format

We demonstrate a high-power single-frequency master oscillator power amplifier （MOPA） fiber laser.The central wavelength of the single-frequency fiber laser seed is 1 063.8 nm,with a linewidth narrower than 20 kHz and output power of 120 mW.By using two-stage amplification,a single-frequency fiber laser with an output power of 122 W is obtained,and the optical-optical conversion efficiency is 72%.No significant amplified spontaneous emission （ASE） or stimulated Brillouin scattering （SBS） is observed.The output power can be further increased by launching more pump power.

Coaxial combination of coherent laser beams

Based on polarization state conversion, a technique for coaxially coherent combination of laser beams is introduced. Laser beams can be coaxially coupled into one beam with high combination efficiency and perfect beam quality. A polarized laser beam combination system based on master oscillator power amplifier （MOPA） configuration is developed and the efficiencies of both unit combination and the whole system are investigated. In the experiment of combining four beams with single longitudinal mode, a combination efficiency of 85.3% is achieved. It can be further enhanced by improving the stability of experimental environment and the quality of optical and mechanical components.

All-fiber cascaded ytterbium-doped nanosecond pulsed amplifier

A cascaded ytterbium-doped all-fiber master oscillator power amplifier（MOPA） with 1064-nm pulsed laser is demonstrated,and it can produce up to 20-W average power and 0.1-mJ pulse energy at tunable repetition rates from 10 to 200 kHz.Two main difficulties of the all-fiber configuration are overcome：all fiber isolator and mode field adapter between single mode fiber（SMF） and double cladding fiber（DCF）. Gain saturation and nonlinear effect are analyzed theoretically,and the possibility of further power-scaling of this cascade scheme is predicted.

All-fiberized Tm-doped fiber MOPA with 30-W output power

We demonstrate a 30-W all-fiberized laser in a master oscillator power amplifier （MOPA） structure with a central wavelength of 1 950 nm. The laser is a continuous-wave （CW） operation with double-clad Tm-doped silica fiber pumped by pigtailed laser diodes with a central wavelength of 793 nm. The optical-to-optical efficiency of the MOPA system is 37.5% with respect to the total 80-W pump power. The maximal slope efficiency of the system is approximately 43%, which belongs to the main amplifier and exceeds the Stoke limit. No obvious amplified spontaneous emitting （ASE） is observed in the experiment, and the power can be scaled straightforwardly by imDroving the pump power.