Current Research and Development in Optical Fiber Communications in China

The following sections are included:

• INTRODUCTION

• CONTENTS

The optical soliton communication research in China, which began in 1984, is reviewed briefly in this paper. The main theoretical works include: optical soliton transmission characteristics, optimal design of optical soliton communication system and theories related to the key components such as soliton sources, EDFA, etc. An experimental system with 2.5 Gb/s, 21 km has been developed. Possible developments in the near future is viewed.

This paper is an introduction of high speed optical communication systems used in China, especially those deployed by the Ministry of Post and Telecommunication.

Practical high speed systems, such as 565 Mb/s system and the optical devices used in these systems manufactured in China are described.

Further development of high speed systems and devices such as SDH systems at 622 Mb/s and 2.5 Gb/s, EDFA, MQW laser diodes and a comprehensive test bed are discussed.

A 151 km “WDM + EDFA” optical fiber transmission system with 8 channels and 2 EDFAs has been built for the first time in China. Some formulas are given to estimate the effect of optical signal power and ASE noise on BER, the power penalty caused by the Stimulated Raman Scattering (SRS) and the crosstalk due to the grating demultiplexer in optical amplified WDM systems with EDFAs. The calculation results are well agreed with experimental data.

In addition, an 8-channel, optically amplified WDM system with regenerated transmission distance of 1208 km, an 8-channel, 240 km optically amplified WDM system and a 12-channel, 240 km optically amplified WDM system have been developed successively.

A general expression is derived to calculate the power penalty due to Stimulated Raman Scattering (SRS) in a WDM system with EDFAs. It is concluded that the damage to the “WDM + EDFA” system caused by SRS depends greatly on the disposal of the EDFAs, and an easy way has been provided to minimize the power penalty. Finally, expressions are given to estimate the transmission distance and the number of EDFAs limited by SRS in the “WDM + EDFA” system.

Quantum well lasers have attracted a great deal of attention by their many advantages such as low threshold current density, excellent temperature feature, high modulation rate and wavelength adjustability etc. The investigation on quantum well laser in mainland China started in the early 80s. AlGaAs/GaAs QW laser diode and InGaAs/GaAs strained layer QW laser diode have been developed using MBE technology with extremely low threshold current and high T₀. Now the growth technologies for QW structure have been expanded to MOCVD technology. Emission wavelengths, on longer wavelength sides have been expanded up to 1.3 μm and 1.55 μm with InGaAsP/InP material system for application in optical fiber communication. On shorter wavelength sides, the emission wavelength has been expanded to lower than 670 nm, for applications in optical information processing. The characteristics of these devices will be demonstrated in this paper.

The QW-DFB LD and low-dimension quantum wire and quantum dot lasers are under investigation.

The co-cavity two-section (CCTS) bistable semiconductor laser with DH and MQW structures has been investigated experimentally and theoretically in this laboratory. Many attractive functions, such as optical bistable switching, optical memory effect, high stability of single longitudinal mode emission spectrum in steady on state, high gain optical amplification, mode conversion effect on injecting signal light, long delay and abnormal relaxation oscillation, ultrafast light pulse generation are investigated in the device. As a high speed, single wavelength, novel laser light source, a monolithic MQW CCTS bistable laser with MQW EA modulator has been fabricated.

GaAs/GaAlAs MQW reflection modulators and SEED have been investigated. The analysis is emphasized on the combined behaviors of quantum confined Stark effect (QCSE), distributed Bragg reflector (DBR), and different asymmetric Fabry–Perot cavities (ASFP). Experimental results include the fabrication and characterization of a modulator array with a contrast ratio of about 10 dB and S-SEED array with optical switch energy less than 10 fJ/(μm)². An application of a modulator array in microoptical interconnection module is also demonstrated.

Distributed feedback (DFB) semiconductor lasers, especially those with gain-coupled (GC) mechanisms, are studied. A GaAlAs/GaAs multi-quantum well GC-DFB laser with a loss grating is fabricated using MBE for the first time. A 1.3 μm InGaAsP/InP DFB laser with a loss grating and one with a gain grating formed by injected carriers are developed by LPE and MOVPE, respectively.

GC-DFB lasers monolithically integrated with electroabsorption modulator is studied systematically for the first time. A novel integrated device structure is proposed and fabricated successfully.

The recent progress on the research of Er³⁺-doped fiber amplifier (EDFA) and laser (EDFL) in our laboratory is reported in this paper. A novel tandem two-stage EDFA configuration was achieved with a small signal gain of 41 dB, output power of 14.5 dBm (both at 1550 nm) and a bandwidth of more than 40 nm. In a novel coupled-ring EDFL, bi-directional simultaneous single longitudinal mode oscillation was accomplished with a laser linewidth of less than 1.0 GHz. By introducing polarization-evolution self-loss modulation in a LD pumped figure-8 all-fiber EDFL, correlated self Q-switching with variable period and mode-locking with low threshold were observed, respectively.

A simple analytical expression is derived to calculate the Bit-Error Rate in lightwave systems with optical amplifiers. According to this expression the Bit-Error Rate can be estimated by the signal power and the ASE spectrum density before the receiver. The theoretical calculation agrees with the experiment very well, so it can provide a method to judge the quality of the optical channel in the optical communication system with optical amplifiers. Futhermore, the impact of the ASE noise on the receiver sensitivity is discussed. It is important to select proper receivers in practical systems.

This paper presents the R&D of GaAs-based high speed devices and circuits at the Hebei Semiconductor Research Institute (HSRI) in China. It is concerned with low noise and medium power GaAs-MESFET MMIC and monolithic laser diode driver and preamplifier for 2.4 Gb/s fiber communication application, GaAs-based high speed digital circuits like single and dual modulus frequency divider, digital/analog convertor, shift register, HEMT and HBT integrated circuits.

This paper describes a practical electro-optic sampler of the measuring-microscope mode for noninvasive measurements of voltage waveforms at points internal to a GaAs integrated circuit. A 360° linear sweep phase shifter is used to provide the scanning delay of the sampling point in time, and was typically demonstrated in measuring the dynamic frequency divider circuit. In addition to the characterization internal to GaAs ICs, the overlap of the electrical signal field in an integrated circuit and the interference effect on the signal voltage calibration are also investigated.