introduction
With the beginning of large-scale deployment of 40Gb/s, a variety of new 100G/s modulation and coding formats have emerged in the industry. Faced with a variety of different transmission patterns, based on the comprehensive consideration of other system design parameters, the industry mainly from transmission distance, path spacing, compatibility with 40Gb/s and 10Gb/s systems, module cost and transmission performance. Comprehensive selection of balance and other aspects.
With the advancement of high-speed digital signal processing technology (DSP) and analog-to-digital conversion (ADC), coherent optical communication has become a research hotspot. Coherent detection combined with DSP technology can perform carrier phase synchronization and polarization tracking in the electrical domain, eliminating the two major obstacles of traditional coherent reception. The DSP-based coherent receiver has simple structure and hardware transparency; it can compensate various transmission impairments in the electrical domain, simplify the transmission link, and reduce the transmission cost; support multi-ary modulation format and polarization multiplexing to achieve high spectral efficiency transmission. . Through the industry's research and development of 100Gb/s modules in the past one or two years, the 100G/s polarization-multiplexed quadrature phase shift keying coherent module (Coherent PM-QPSK) is becoming the main choice in the industry.
Basic principles of coherent optical communication
The coherent optical communication system can divide the optical frequency band into a plurality of channels, so that the optical frequency band is fully utilized, that is, multi-channel optical fiber communication. Coherent optical communication technology has the advantage of high receiving sensitivity, and the receiving sensitivity using coherent detection technology can be 18 dB higher than the direct detection technology.
Figure 1 shows the transmitter using polarization multiplexing. The laser signal as a carrier is divided into X/Y channels by PBS (Polarization Beam Splitter), and each signal is in an I/Q modulator composed of 2 MZ modulators (I-channel). And the Q channel is 90° out of phase by modulating the signal of 10.7/27.5 Gb/s to the carrier, and then transmitting the X-axis and Y-axis optical signals together by polarization multiplexing through the optical multiplexer, thereby transmitting A 40/100 Gb/s transmission over a single fiber is achieved.
At the receiving end, unlike the intensity modulation-one direct detection system, the coherent optical fiber communication system adds a local oscillating light source (LO) required for heterodyne or homodyne reception in the optical receiver, and the light wave output from the light source is received. The dimmed wave is subjected to photoelectric mixing under the condition that the wavefront matching and the polarization matching are satisfied. By slightly changing the optical frequency of the local oscillator laser, the selected channel can be changed, so the linewidth of the local oscillator laser is very high. The frequency field of the signal light wave after the mixing is proportional to the square of the sum of the field strengths of the local oscillator, and the difference frequency signal of the local oscillator light wave and the signal light wave can be selected. Since the variation law of the difference frequency signal is the same as the variation law of the signal light wave, unlike the direct detection wave communication method, the detection current only reflects the intensity of the light wave, and thus various modulation methods such as amplitude, frequency, phase, and polarization can be realized.
In the receiver coherent detection mode of FIG. 2, since the polarization multiplexed signal is to be detected, the received signal is decomposed into two orthogonal signals by a polarization beam splitter PBS (Polariza TIon Beam Splitter), and each orthogonal signal is combined with a local light source. LO mixing, the local frequency source has a carrier frequency control accuracy of several hundred KHz. After mixing, four optical signals with polarization and phase quadrature are obtained, which are respectively detected by PIN, and then converted into four digital electrical signals by A/D circuit after being electrically amplified and filtered. Digital electrical signals are digitally balanced by digital signal processing (DSP) chips: timing recovery, signal recovery, polarization and PMD tracking, and dispersion compensation.
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