Chongqing Wu

All-optical differentiator and high-speed pulse generation based on cross-polarization modulation in a semiconductor optical amplifier

We propose an all-optical intensity differentiation scheme based on cross-polarization modulation (XPolM) in a semiconductor optical amplifier (SOA) while demonstrating the absolute value of differential signal that can be obtained by the SOA-based XPolM of two parts with relative delay from the input signal and well extracted by the polarization filter. The differentiation errors and eye diagrams versus sampling time Delta are investigated for data rate at 12.5 Gbits/s, and the minimal error approximately 0.06 is achieved at Delta=0. Owing to a much faster polarization response, our scheme bears great potential for all-optical signal processing over 100 Gbits/s. By application of the differentiator, we further obtain the 20 GHz short pulse train with a pulse width of approximately 10 ps.

Design of SOA-based dual-loop optical buffer with a 3 /spl times/ 3 collinear coupler: guideline and optimizations

In this paper, theoretical and experimental analyses of the semiconductor optical amplifier (SOA)-based dual-loop all optical buffer (DLOB) with a 3 /spl times/ 3 collinear coupler are presented. Two kinds of DLOB configurations and their choices of nonlinear elements are compared. After that, a novel SOA-based DLOB with Sagnac configuration is proposed. The proposed DLOB is simple, compact, and inherently stable. The required control signal power of this optical buffer (OB) is on the order of 1 mW, and no extra optical amplifier is needed to compensate the power loss. How the linewidth enhancement factor of SOA, the asymmetry power ratio, and the polarization characteristic of 3 /spl times/ 3 collinear coupler, as well as SOA directional gain difference, may hamper the performance is examined systematically. Finally, the loading and reading operations with DLOB for packet data at the bit rate of 150 Mb/s and 2.5 Gb/s are demonstrated.

An Enhanced SOA-Based Double-Loop Optical Buffer for Storage of Variable-Length Packet

To extend the buffer depth of a fiber loop optical buffer, we have experimentally demonstrated an enhanced semiconductor optical amplifier (SOA)-based dual-loop optical buffer (DLOB) for storing variable-length optical packets. We have theoretically derived constraints governing the buffer depth of the DLOB, in which the SOA not only provides a nonlinear phase shift in the loop to implement the buffer function but also compensates for the fiber loop attenuation during long-time storage. It is found that the maximum allowable length of a stored packet to avoid the counter-propagation packet collision inside the SOA depends on the SOA bias position as well as the length of the fiber loop. To demonstrate the effectiveness of the proposed enhanced configuration, we have successfully demonstrated the storage of 2.5-Gbps variable-length packets even when the length of the input packet exceeds the corresponding length of the fiber loop. Another unique advantage of the proposed enhanced DLOB configuration is that it can also overcome the problem of power leakage of the stored packet due to a directional gain difference of single SOA and gain saturation. Index

Dual-Wavelength Packets Buffering in Dual-Loop Optical Buffer

We demonstrated 10-Gb/s dual-wavelength packets buffering in a dual-loop optical buffer with a semiconductor optical amplifier. Confirmed by experiments, the crosstalk between lambda1 and lambda2 signals, which is the main reason of restraining the buffering time, is decreased by using power equalization. The buffered data packets are retrieved after they are buffered for one, two, and three cycles with the maximum buffering time up to 3.75 mus. The power penalty, the extinction ratio, and the signal-to-noise ratio are 2.1, 7.5, and 4 dB, respectively, after three cycles. We obtain the eye diagrams for both wavelengths with good results, and the buffer is transparent for either wavelength.

Spectral-resolved backreflection measurement of polarization mode dispersion in optical fibers

An improved backreflection technique is proposed to perform the spectral-resolved measurement of polarization mode dispersion (PMD) in optical fibers. This technique is based on the PMD dynamical equation and realized by measuring the polarization state evolutions of the reflected signal in both frequency and time domains. Two experimental setups, employing the far-end Fresnel reflection, are constructed to verify this technique. The agreement between the results of the proposed backreflection technique and the conventional forward technique is observed.

Optical Methane Sensor Based on a Fiber Loop at 1665 nm

A fiber loop method for methane sensing using a 50 mm gas gap at 1665 nm is introduced for the first time, to our knowledge. A pulse of light is coupled into the loop and goes through the gas chamber four times. The absorption loss and the methane concentration are obtained by figuring out the decreasing ratio of the output pulse train with a curve fitting process. The advantage of this method is its insensitivity to power fluctuations of the laser. The best split ratio of the coupler for the fiber loop method was put forward and investigated, and the absorption spectrum, including the wavelengths and the strengths of the absorption lines of methane around 1665 nm, were measured with a Super-Luminescent Diode. This system was used to measure two gas samples and the errors are found to be less than 3%.

Generalized frequency dependence of output Stokes parameters in an optical fiber system with PMD and PDL/PDG

Dependence of output optical power, Stokes vector and degree of polarization on optical frequency is presented for an optical fiber system with both polarization mode dispersion and polarization-dependent loss or gain. The newly formulated equations are generalized for input light with arbitrary degree of polarization. The spectral resolved measurements of polarization mode dispersion using partially polarized light agree well with our theory.

Poincare Sphere Method for Optimizing the Wavelength Converter Based on Nonlinear Polarization Rotation in Semiconductor Optical Amplifiers

We present a Poincare sphere (PS) model to optimize the wavelength converter (WC) based on nonlinear polarization rotation (NPR) in semiconductor optical amplifiers (SOA). The PS method is applied to describe the variations of ellipticipy angle and azimuth of SOA output state of polarization (SOP) induced by polarization-dependent gain (PDG) and polarization-dependent phase shift, respectively. The theoretical results reveal that the polarization dependence of the linewidth enhancement factor (LEF) and the variation of SOA gain are significant factors for the NPR. The evolutions of output SOP on the PS under quasi-continuum condition have been demonstrated experimentally. The experimental results agree well with our analysis.

Measurement of SOA Linewidth Enhancement Factor With a Sagnac Fiber Loop

We demonstrate a novel method to measure the linewidth enhancement factor (alpha-factor) of the semiconductor optical amplifier (SOA). We derived theoretically the quantitative relationships among the alpha-factor, the cross-gain modulation, the cross-phase modulation, and the contrast ratio of an SOA-based Sagnac fiber loop. We found that the alpha-factor value can be calculated directly from the maximum power contrast ratio measurement. Our experiment results show that the obtained alpha-factor fluctuates within a small range of 5.23 to 6.83 when the bias current varies from 130 to 240 mA. Compared with existing measurement methods, our method is more attractive because of its simple configuration and better stability

Power Equalization for SOA-Based Dual-Loop Optical Buffer by Optical Control Pulse Optimization

In this paper, a theoretical study and experimental demonstration are applied to achieve power equalization for semiconductor optical amplifier (SOA)-based dual-loop optical buffers (DLOBs). It is found that, due to the gain saturation and limited linewidth-enhancement factor of the SOA, the peak power of a packet pulse with an optically controlled delay of 9.9 mus is 4.83 dB lower than that of a packet pulse without storage. In order to eliminate the 4.83-dB output power fluctuation of the DLOB, a simple power-equalization method based on the optimization of an optical control pulse is proposed. By injecting a negative optical control pulse, the output power fluctuation of a packet pulse can be effectively reduced to zero. We have also investigated the peak power level of the optical control pulse required to fulfil the buffer function. It is found that the SOA with larger linewidth-enhancement factor and larger small-signal gain should be used to reduce the peak power of the optical control pulse. It is also theoretically found that, due to the negative optical-control pulse injection, the packet signal with Gaussian profile has some distortion after storage. However, the distortion effect is mitigated when the shape of the input pulse is more similar to the square profile. Finally, the proposed method for achieving power equalization in an SOA-based optical buffer has been justified by carrying out a 2.5-Gb/s 2times2 exchange-bypass optical switch experiment. We believe that this power-equalization method can be also applied to other SOA cross-phase modulation-based applications