An advanced theoretical approach to study super-multiperiod superlattices:theory vs experiments

A new theoretical method to study super-multiperiod superlattices has been developed.The method combines the precision of the 8-band kp-method with the flexibility of the shooting method and the Monte Carlo approach.This method was applied to examine the finest quality samples of super-multiperiod Al_(0.3)Ga_(0.7)As/GaAs superlattices grown by molecular beam epitaxy.The express photoreflectance spectroscopy method was utilized to validate the proposed theoretical method.For the first time,the accurate theoretical analysis of the energy band diagram of super-multiperiod superlattices with experimental verification has been conducted.The proposed approach highly accurately determines transition peak positions and enables the calculation of the energy band diagram,transition energies,relaxation rates,and gain estimation.It has achieved a remarkably low 5%error compared to the commonly used method,which typically results in a 25%error,and allowed to recover the superlattice parameters.The retrieved intrinsic parameters of the samples aligned with XRD data and growth parameters.The proposed method also accurately predicted the escape of the second energy level for quantum well thicknesses less than 5 nm,as was observed in photoreflectance experiments.The new designs of THz light-emitting devices operating at room temperature were suggested by the developed method.

Slow and fast light in quantum-well and quantum-dot semiconductor optical amplifiers Invited Paper

Slow and fast light in quantum-well （QW） and quantum-dot （QD） semiconductor optical amplifiers （SOAs） using nonlinear quantum optical effects are presented. We demonstrate electrical and optical controls of fast light using the coherent population oscillation （CPO） and four wave mixing （FWM） in the gain regime of QW SOAs. We then consider the dependence on the wavelength and modal gain of the pump in QW SOAs. To enhance the tunable photonic delay of a single QW SOA, we explore a serial cascade of multiple amplifiers. A model for the number of QW SOAs in series with variable optical attenuation is developed and matched to the experimental data. We demonstrate the scaling law and the bandwidth control by using the serial cascade of multiple QW SOAs. Experimentally, we achieve a phase change of 160^o and a scaling factor of four at 1 GHz using the cascade of four QW SOAs. Finally, we investigate CPO and FWM slow and fast light of QD SOAs. The experiment shows that the bandwidth of the time delay as a function of the modulation frequency changes in the absorption and gain regimes due to the carrier-lifetime variation. The tunable phase shift in QD SOA is compared between the ground- and first excited-state transitions with different modal gains.

Stimulated Brillouin scattering effect on gain saturation of distributed fiber Raman amplifiers

Gain saturation is a significant phenomenon of fiber Raman amplifiers（FRAs）.Gain figures versus signal power are well explained.For the small signal,the coupled ordinary differential equations are used,and for the large signal,the Raman gain coefficient is modified.It is shown that the saturation power of FRAs decreases with the pump power,and gain saturation is easier to occur in the forward pump scheme than in the backward pump scheme.These phenomena are well explained by the stimulated Brillouin scattering （SBS） effect.This research provides a guide to the fabrication of practical FRAs.

New chemically stable Er_(3+)-Yb_(3+) co-doped phosphate glass for optical waveguide lasers and amplifiers

A set of phosphate glasses with different amounts of Al2O3, Na2O, La2O3 and AlF3 contents were prepared to search for a phosphate glass which is suitable for ion-exchange process. Based on chemical durability test results and spectroscopic properties, a glass with excellent chemical durability and good spectroscopic properties is developed for ion-exchange process. The emission cross section of Er3+ in this glass is calculated to be 0.72×10~20 cm2 by McCumber theory. The planar waveguide with three modes at 632.8 nm and one mode at 1540 nm can be realized by 335℃ 3 hours ion-exchange in 5%AgNO3+95%KNO3 molten salt.

Rhodamine 6G-doped polymer optical fiber amplifiers

Rhodamine 6G-doped step-index polymer optical fiber is fabricated. The characteristics of the amplification of rhodamine 6G-doped step-index polymer optical fiber amplifier have been studied. The high-gain optical amplification with a tunable wavelength range from 585 to 613 nm is obtained in a step-index polymer optical fiber doped with rhodamine 6G at 10-ppm level, which can be used for broadband amplifiers and tunable lasers.

Investigation on gain characteristics in non-degenerate cascaded phase sensitive parametric amplifiers

We theoretically and experimentally show the impact of the ratio between the signal and idler generated from the PIA part on the gain characteristics in the continuous wave (CW) pump non-degenerate cascaded phase-sensitive fiber optical parametric amplifier (PS-FOPA). The results show that the length of highly nonlinear fiber (HNLF) used for generating the idler can cause the variation of power ratio between the idler and signal, which significantly affects the gain characteristics of the PS-FOPA under the small signal gain condition. To obtain high gain, it is better to choose long HNLF to generate idler. In our experiment, 5.5 dB gain and 18 nm bandwidth (on/off gain>10 dBm) in PS-FOPA can be achieved when 300 m-long HNLF instead of 200 m-long HNLF is used in PIA.

Simulation of Fast-Recovery Cross-Modulation Characteristics in Semiconductor Optical Amplifier with Assist Light

We analyzed the characteristics of cross-modulations (XM) and their recovery times in a semiconductor optical amplifier by a newly-developed TMM. The calculated results suggest faster recovery of the XMs by introducing a high-power assist light.

All-optical wavelength conversion based on nonlinear polarization rotation (NPR) in SOA and AWG filtering

An all-optical wavelength converter, based on nonlinear polarization rotation （NPR） in semiconductor optical amplifier （SOA） and array waveguide grating （AWG） filtering, is experimentally demonstrated. The wavelength converter can provide excellent operation including extinction ratio and Q factor. The simultaneous two wavelength conversion outputs are successfully obtained at 20 Gb/s.

All-fiber ring-cavity based frequency swept laser source for frequency domain OCT

We develop a high-speed tunable, quasi-continuous-wave laser source for frequency domain （FD） optical coherence tomography （OCT）. The laser resonance is realized within a unidirectional all-fiber ring cavity consisting of a fiber coupler, two fiber isolators, a semiconductor optical amplifier （SOA）, and a fiber Fabry- Perot tunable filter （FFP-TF） for frequency tuning. Light output from the coupler is further amplified and spectral shaped by a booster SOA terminated at both ends with two isolators. The developed laser source provides up to 8000 sweeps per second over a full-width wavelength tuning range of 120 nm at center wavelength of 1320 nm with an average power of 9 mW, yielding an axial resolution of 13.6μm in air and a maximum sensitivity of about 112 dB for OCT imaging. The instantaneous linewidth is about 0.08 nm, enabling OCT imaging over an axial range of 3.4 mm in air. For optimization consideration based on this custom-built swept laser, experimental study on imaging quality relevant parameters of the swept laser with sine and ramp driving waveforms to the FFP-TF is conducted, and investigation of the swept laser on the cavity length is done. Implementing the laser source in our established swept source based OCT （SS-OCT） system, real-time structural imaging of biological tissue is demonstrated.

Wavelength tunability of tandem optical parametric oscillator based on single PPMgOLN crystal

The theoretical analysis and experimental results of the wavelength tunability of a tandem optical parametric oscillator （TOPO） based on a single nonlinear crystal are presented.TOPO is a configuration wherein the signal laser is used as a pump laser to generate secondary optical parametric oscillator （OPO）.The cascaded parametric interactions are achieved synchronously in a single-grating-period MgO doped periodically poled lithium niobate （PPMgOLN）.Tunable multiple-wavelength mid-infrared （mid-IR） lasers are obtained by changing the temperature of the crystal.When the PPMgOLN crystal with a grating period of 31.2 μm is operated at 148 ℃,the dual OPOs generate an identical mid-IR laser of 2.83 μm.The secondary OPO transforms into an optical parametric amplifier,in which different frequency mixing from the signal laser results in the amplification of the idler laser in the first OPO.TOPO is a useful configuration for multiple laser output,broad tuning range,and high-efficiency mid-IR lasers.

Widely tunable all-optical wavelength converter based on four-wave mixing using two orthogonally polarized pumps

The principle of broad-band orthogonal-pump (BOP) four-wave mixing in semiconductor optical amplifiers is analyzed in theory. The conversion efficiency reduces rapidly as the detuning of wavelength between the signal and pump increase which can be solved by introducing a BOP method. The constant conversion efficiency and signal-to-noise ratio are obtained over a large wavelength detuning range. The wavelength conversion efficiency with variation smaller than 3.88 dB over 52-nm range has been experimentally demonstrated by using BOP, with the 10-GHz output of distributed feedback/electro-absorption modulator as signal. Conventional single-pump scheme is also performed for comparison and the experimental results fit well with the theory.

All-optical reconfigurable multi-logic gates based on nonlinear polarization rotation effect in a single SOA

We demonstrate an all-optical reconfigurable logic gate based on dominant nonlinear polarization rotation accompanied with cross-gain modulation effect in a single semiconductor optical amplifier （SOA）.Five logic functions,including NOT,OR,NOR,AND,and NAND,are realized using 10-Gb/s on-off keying signals with flexible wavelength tunability.The operation principle is explained in detail.By adjusting polarization controllers,multiple logic functions corresponding to different input polarization states are separately achieved using a single SOA with high flexibility.

Study on the 4×10 Gb/s,400 km dispersion compensation by chirped optical fiber grating

In this paper, the dispersion compensation for 4×10 Gb/s. 400 km G.652 fiber by chirped optical fiber Bragg grating (FBG) is introduced. For the first time, we have measured and compensated the polarization mode dispersion (PMD) of FBG. which in each channel is less than 1.1 ps. When the bit error rate (BER) is 10-and the bit error is zero, the transmission power penalty of each channel is less than 2 dB, and the best result is negative which means that the receiver sensitivity is increased after transmission.

40-GHz wavelength tunable mode-locked SOA-based fiber laser with 40-nm tuning range

A 40-GHz wavelength tunable mode-locked fiber ring laser based on cross-gain modulation in a semicon- ductor optical amplifier （SOA） is presented. Pulse trains with a pulse width of 10.5 ps at 40-GHz repetition frequency are obtained. The laser operates with almost 40-nm tuning range. The relationship between the key laser parameters and the output pulse characteristics is analyzed experimentally.

Design of Raman-parametric fiber amplifier for wavelength division multiplex transmission system

We optimize the novel configuration of a hybrid fiber amplifier - Raman assisted-fiber-based optical parametric amplifier （R-FOPA）, in which the parametric gain and Raman gain profiles are combined to achieve a flat composite gain profile. The pump powers and the fiber length in the hybrid amplifier are effectively optimized by genetic algorithm （GA） scheme. The optimization results indicate that the RFOPA can achieve a 200-nm flat bandwidth spectrum with the gain of 20 dB and ripple of less than 4 dB.

Optical fiber acoustic sensing multiplexing system based on TDM/SFDM

We propose a system of time-division multiplexing （TDM） and spatial frequency-division multiplexing （SFDM）. Extrinsic Fabry Perot interferometric sensors are applied to detect weak acoustic signals. The broadband source is employed, the light from it is modulated by a pulse signal sequence and is efficiently amplified by semiconductor optical amplifiers. Experimental results show that the equivalent noise pressure spectrum level is -97.2 dB re 1 rad/√Hz below 1250 Hz, and the cross talk between two sensors in one TDM channel is -32.7 dB with a cavity length difference of 60 μm. The number of sensors in this multiplexing system can theoretically reach 160.

Analysis on the effect of nonlinear polarization evolution in nonlinear amplifying loop mirror

By considering the cross phase modulation （XPM） between the two orthogonal poparization components, the nonlinear birefringence and nonlinear polarization evolution （NPE） in highly-nonlinear fiber （HNLF）, as well as the unequal evolutions of the state of polarization （SOP） between the clockwise （CW） and counter-clockwise （CCW） waves in a nonlinear amplifying loop mirror （NALM） are analyzed. It is pointed out that the traditional cosine expression is no longer valid for the power transmission of NALM due to uncompleted interference under the high power condition. The analytical expression considering NPE effect is derived, and the experimental result is presented.

S+C band optical amplification in Er^(3+)-Tm^(3+) co-doped fiber

A new type of optical amplifier based on co-doping erbium in thulium doped fiber is proposed to realize S＋C band gain by dual-wavelength （800＋1410 nm） pumping scheme which is obtainable from laser diode. A novel model is established for the co-doped fiber considering the Er^3＋ to Tm^3＋ energy transfer process. Using appropriate fiber parameters and energy transfer parameters, the coupled rate equations are analyzed and solved, the concentrations of Er^3＋ and Tm^3＋ and fiber length were optimized to get more uniform gain. The results predicted that the S＋C band gain can be achieved at the same time by co-doping erbium in thulium doped fluoride fiber.

Photo-acoustic spectrum and detection of nitrogen dioxide

The photo-acoustic （PA） spectrum of nitrogen dioxide （NO2） in the range of 420 - 520 nm with a Nd：YAG pumped optical parametric generator and amplifier as radiation source is presented. The spectrum has a characteristic of banded structure superimposed on continuum. The banded structure of the spectrum can be assigned to NO2 B2B1← X^2A1 transition. While the continual one comes from the chaos states of the first excited state A^2B2 and the high vibration levels of the ground state X^2A1. The relationship of PA signal with buffer gas pressure and NO2 concentration is measured. The PA signal intensity increases with buffer gas pressure and almost is invariable when the buffer gas pressure is more than 3.00 × 10^4 Pa. The PA signal intensity has linearity with NO2 concentration. The detection limit is about 2×10^-5 on the basis of SNR = 1, however lower value of the detection limit can be expected by improving the apparatus properties.

10 Gb/s symmetric WDM-PON using stable multi-longitudinal mode Brillouin/SOA fiber laser as upstream colorless source

We propose a stable multi-longitudinal Brillouin/semiconductor fiber laser （BSFL） as the upstream source in a bidirectional single-fiber wavelength-division multiplexing-passive optical network （WDM-PON）. The downstream wavelength serves as the pump of the BSFL. Brillouin-frequency-shifted （-10.8 GHz） upstream carrier is generated to suppress the Rayleigh backscattering and back reflection-induced crosstalk. The stable multi-longitudinal operation of the BSFL, attributed to the four-wave mixing （FWM） effect in the semiconductor optical amplifier （SOA） reduces the difficulty of generating a stable single-longitudinal fiber laser-based upstream carrier. Bidirectional symmetric transmission at 10 Gb/s over a 12.5-km single mode fiber with less than 2-dB power penalty is demonstrated.