Yizhen Wei

Performance Analysis of Space-Diversity Free-Space Optical Links Over Exponentiated Weibull Channels

We propose a simple analytical method for evaluating the average bit-error rate (BER) performance of multiple input multiple output (MIMO) free-space optical (FSO) communication systems over independent exponentiated Weibull (EW) channels. We first present an accurate approximation to the probability density function (PDF) of the sum of independent EW random variables corresponding to the EW-distributed FSO channel fading. Then, a closed-form expression for the BER of MIMO FSO systems using ON-OFF keying is derived based on the obtained PDF. The proposed method is validated through comparing the BER results obtained by the analytical calculation and the Monte Carlo simulation.

Optical logic gates based on electro-optic modulation with Sagnac interferometer

In this work, we present a new structure to realize optical logic operation in a Sagnac interferometer with electro-optical modulation. In the scheme, we divide two counterpropagation signals in a Sagnac loop to two different arms with the electro-optical crystal by using two circulators. Lithium niobate materials whose electro-optical coefficient can be as large as 32.2×10(-12)  m/V make up the arms of the waveguides. Using the transfer matrix of the fiber coupler, we analyze the propagation of signals in this system and obtain the transmission characteristic curves and the extinction ratio. The results indicate that this optical switching has a high extinction ratio of about 60 dB and an ultrafast response time of 2.036 ns. In addition, the results reveal that the change of the dephasing between the two input signals and the modification of the modulation voltage added to the electro-optical crystal leads to the change of the extinction ratio. We also conclude that, in cases of the dephasing of two initial input signals Δφ=0, we can obtain the various logical operations, such as the logical operations D=A¯·B, D=A·B¯, C=A+B, and D=A⊕B in ports C and D of the system by adjusting the modulation voltage. When Δφ≠0, we obtain the arithmetic operations D=A+B, C=A⊕B, D=A·B¯, and C=A¯·B in ports C and D. This study is significant for the design of all optical networks by adjusting the modulation voltage.

Multi-Wavelength Brillouin Fiber Laser With Triple Brillouin Frequency Spacing

We demonstrate a triple Brillouin frequency spacing, multi-wavelength Brillouin laser combined with two cavities: a single Brillouin frequency spacing cavity and a double Brillouin frequency spacing cavity. In this simple structure, the optimization of multi-wavelength lasing is done with proper adjustments of Brillouin pump (BP) power and 980-nm pump power. When setting the BP power to 3-dBm and 980-nm pump power to 27.78 dBm, up to ten channels with frequency spacing of 0.26 nm are generated. The output wavelength can be tuned in a wide range of 45 nm, from 1525 to 1570 nm. We also compared and analyzed this scheme under the condition of single linear cavity 1 and double ring cavity 2. The proposed multi-wavelength laser has the potential applications in the areas of space optical communication and optical communications and microwave signal source.

A Smooth Evolution to Next Generation PON Based on Pulse Position Modulation (PPM)

A smooth evolution of passive optical networks (PONs) is proposed, which realizes an upgrade link coexisting with the existing link by pulse position modulation (PPM). In the evolution, the optical distribution network and existing optical network units remain with no changes. The coexisting brings in crosstalk from the upgrade PON link to the existing PON link. The crosstalk is reduced by PPM because PPM provides good suppression at low frequency and high receiving sensitivity. The coexisting scheme is experimentally demonstrated and error free operation is achieved for both the existing signal and the upgrade signal. The tested power penalty of the existing signal coexisting with the upgrade signal in PPM is ~0.5 dB, better than Manchester code, which has been considered as the best candidate for upgrade signal modulation. Taking code efficiency into consideration, the PPM with four pulse positions (4PPM) shows the best performance in the coexisting evolution.

A Smooth and Gradual Evolution to Next Generation PON Based on Simple Line-Coding

We propose a smooth and gradual evolution method to next generation passive optical network (PON) based on simple line-coding. At the beginning, a new OLT and some new ONUs are added into the existing PON without changing the existing ONUs. The evolution is gradual. The existing ONUs can be upgraded with demand. The coding is operated by software and requires no extra equipment. During different evolution stages, different line-codes are applied to reduce the crosstalk induced by the upgrade service to the existing service. The crosstalk is tested and shown not serious in the experiment.

Wave-optics simulation of the channel fading in modulating retro-reflector free-space optical link

The channel characteristics of modulating retro-reflector (MRR) free-space optical (FSO) link are studied by using wave-optics (WO) simulation method. The optical wave launched from the transceiver is reflected back in the same direction to the transceiver by the use of cube-corner reflector (CCR). This propagation process is divided into three parts: forward propagation through atmospheric turbulence, reflection of CCR, and backward propagation through the turbulence. For WO simulation under turbulence conditions, the split-step beam propagation method is implemented, and the influence of atmospheric turbulence along the propagation path is described by a set of random thin phase screens. The reflection effect of CCR is taken into account by using ray-tracing approach to calculate the intensity and phase of the optical field output from CCR. Integrating the reflection effect into WO simulations, we study the statistics of the intensity fluctuations in the forward, backward and double-pass links, and the correlation of fluctuations in the forward and backward links. The histograms of the normalized intensity for three links could be approximated by Gamma-Gamma (GG) distributions except for relatively large receiver apertures. There exists a relatively large correlation between the forward and backward links.

Microchip dual-frequency laser with well-balanced intensity utilizing temperature control.

A continuous-wave microchip dual-frequency laser (DFL) with well balanced intensity was presented. In order to obtain such a balanced intensity distribution of the two frequency components, the DFL wavelengths were precisely tuned and spectrally matched with the emission cross section (ECS) spectrum of the gain medium by employing a temperature controller. Finally, when the heat sink temperature was controlled at -5.6°C, a 264 mW DFL signal was achieved with frequency separation at 67.52 GHz and intensity balance ratio (IBR) at 0.991.