Xiaoping Xie

Generation of 10-GHz ultra-short pulses with low time jitter in an actively mode-locked fiber laser

We have experimentally achieved the 8.3-ps ultra-short pulse at 10 GHz repetition rate with the time jitter as low as 590 fs in an actively mode-locked fiber ring laser. The ring-cavity laser is mode-locked by a semiconductor optical amplifier based on cross-gain modulation. The external CW source is modulated with radio frequency signal by an amplitude modulator as the external optical pulses and, then, injected into the fiber ring cavity to achieve active mode locking. Further investigating the laser output characteristics, it indicates that the linewidth of employed CW source affects properties of the generated ultra-short pulse, such as phase noise and time jitter. Ultra-short pulse at high repetition rate with low time jitter can be generated by the optimization of CW laser source.

Generation of 10 GHz, 1.9 ps optical pulse train using semiconductor optical amplifier and silica-based highly nonlinear fiber

We propose a simple approach to generate a high quality 10 GHz 1.9 ps optical pulse train using a semiconductor optical amplifier and silica-based highly nonlinear fiber. An optical pulse generator based on our proposed scheme is easy to set up with commercially available optical components. A 10 GHz, 1.9 ps optical pulse train is obtained with timing jitter as low as 60 fs over the frequency range 10 Hz-1 MHz. With a wavelength tunable CW laser, a wide wavelength tunable span can be achieved over the entire C band. The proposed optical pulse generator also can operate at different repetition rates from 3 to 10 GHz.

Generation of 10-GHz duty-cycle tunable square optical pulse in an SOA-based mode-locked fiber laser

We demonstrate the generation of 10-GHz optical square pulses by injecting a picosecond pulse train into an SOA-based mode-locked fiber laser. The novel scheme exploits nonlinear effects and gain saturation phenomenon in the semiconductor optical amplifier (SOA). This technique uses gain-compression dynamics between the input pulses and the generated ones in gain-saturated SOA to form square pulses. The center wavelength of the generated optical square pulse can be tuned from 1530 to 1570 nm by adjusting the center wavelength of the optical band pass filter (OBPF) in the SOA-based mode-locked fiber ring laser. The duty cycle of the output pulse can be tuned from 12.7 to 88.4%, which strongly depends on the input power and intra-cavity power.

Influence of laser linewidth and polarization modulator length on polarization shift keying for free space optical communication

Modulating signal with polarization modulator (PolM) is the simplest method for polarization shift keying (PolSK) in free space optical communication. However, this method has an intrinsic drawback on degree of polarization (DOP) reduction for the existence of polarization mode dispersion (PMD) in PolM. In this work, we analyze this change of DOP and its influence on PolSK using coherency matrix. We demonstrate that the decrease of DOP after PolM will generate extra loss and bit error ratio (BER) for PolSK communication, while this loss and BER will aggravate with the increase of laser linewidth and PolM length. For a practical PolSK system, laser linewidth should be less than 0.008nm.

A high-efficiency tunable polarization-insensitive wavelength convertor based on degenerate four-wave mixing in a highly nonlinear photonic crystal fiber

A high-efficiency tunable polarization-insensitive all-optical wavelength convertor for 10 Gb/s return-to-zero on-off keying (RZ-OOK) signals using degenerate four-wave mixing (FWM) in a highly nonlinear photonic crystal fiber (PCF) is demonstrated. The residual birefringence in the 50 m dispersion-flattened PCF with 11 W-1mkm-1 nonlinear coefficient guarantees the FWM-based wavelength conversion to be polarization-insensitive when the pump polarization is exactly at 45d to the birefringent axes of the PCF. Experimental results show that with 45d pump launch, the polarization dependence of FWM in the PCF can be decreased to less than 0.6 dB over the entire 25 nm conversion bandwidth. The optical signal-to-noise ratios (OSNR) of the converted signals are better than 40 dB and the conversion efficiencies are better than -15 dB over the conversion range with 10 Gb/s signal polarization-scrambled.

Flexible optical clock recovery utilizing a multi-function semiconductor fiber laser

We demonstrate a multi-function fiber laser based on cross-gain modulation in a semiconductor optical amplifier (SOA). Depending on the input signals, the fiber cavity can emit a continuous wave (CW) laser, mode-locked pulses, or act as a clock recovery device. With an extra CW light overcoming the pattern effect in the clock recovery process, a 10-GHz synchronous clock sequence with <0.1 power fluctuation and <120-fs timing jitter is extracted from the transmission return-to-zero data stream. We further analyze the recovered clock properties as a function of the input signal, and find that the clock recovery system presents good stability over a large range of input signal characteristics. The multi-function fiber laser exhibits the advantages of compact configuration and low cost, which is very convenient and attractive for optical communications and signal processing.

Experimental research of optical preamplifier receiver for RZ-DPSK based on delay interferometer

We present simulation results for a 5 Gb/s optically pre-amplified, differential phase shift keying communication system achieving -48.1 dBm (about 24.7 photons/bit) receiver sensitivity at 10-9 bit-error-rate and using an optical delay line interferometer made by ourselves with 1 bit delay as the demodulator. The system is also experimental tested and sensitivity of the receiver is -36.5 dBm (about 106.6 photons/bit). The experimental results have some penalty in sensitivity comparing to the results in the simulation because the 1 bit delay interferometer is sensitive to the environment such as temperature, tremble and so on. To our knowledge if the interferometer is well designed and optimized, the experimental results can correspond with the simulative results and an optically pre-amplified direct detection DPSK receiver with high sensitivity can be realized.

The influence of atmospheric turbulence on IM/DD space optical communication system

Lots of researches have simulated performance of optical system based on plane wave or spherical wave model, but optical field is Gauss distribution in real optical communication system. So we derive the relationships between the signal-to-noise ratio (SNR), bit error rate (BER), channel capacity (C) and the transmission distance of the space laser communication system, which is obtained by the Gaussian beam propagation model for on-off keying (OOK) modulated signal under horizontal link. We numerically study the influence of atmospheric turbulence on system performance. we get the conclusions: Under weak turbulence intensity, as the turbulence intensity increases, the C and SNR for OOK modulated space laser communication system decrease while the BER increase; In the case of strong turbulence intensity, the intensity scintillation and the BER dramatically increase with the increase of transmission distance, and then become saturated; In the condition of turbulence intensity and transmission distance are both constant, the longer the wavelength results in greater C, higher SNR and lower BER. Selecting longer communication wavelength can mitigate the impact of the atmosphere turbulence on the communication system in some extent.

A 2.5 Gbit/s free space transmission link over 1km

We demonstrate error free transmission of a single 2.5Gbit/s NRZ data channel at 1550nm over 1 km of free space, and the results of 3Gbit/s experiment are given at the same time. The transmitted beam is detected by avalanche photodiode (APD), Bit Error Rate (BER), Signal-to-Noise Ratio (SNR) and Jitter of the detected signals are measured and compared.