WTD develop high-speed polarization multiplexing coherent detection optical module technology considerations

In order to achieve accurate, effective, and reliable coherent optical communication, WTD in the coherent optical communication technology to develop high-speed polarization multiplexing coherent detection optical module products mainly consider the following several technical points.

1. Using high-standard light source technology

Coherent optical fiber communication system for signal light and local oscillator light source is relatively high, it requires a narrow spectral lines, high frequency stability. Linewidth light source itself determines the system can achieve the lowest error rate, should be minimized, while the frequency of the semiconductor laser to change the operating temperature and injection current is very sensitive to the variation amount is generally in the tens of gigahertz per degree Celsius and the tens of gigahertz per mA, therefore, for the frequency stability, in addition to the injection current and temperature stability, we must also take other frequency stabilization measures to maintain a stable optical frequency.

2. To achieve a breakthrough in terms of access technology

Coherent detection receiver technology consists of two parts, one is the light receiver technology, the other is a variety of standard intermediate frequency after demodulation techniques.

Balanced receiver method: frequency shift keying (FSK) standard, since the semiconductor laser modulation process will inevitably with additional amplitude modulation noise, using a balanced approach reduces AM reception noise. The main idea of ​​balancing law when the optical signal from the optical fiber into the local oscillator light by polarization control to ensure that the polarization state of the signal to adapt the local oscillator and signal lights through the polarizing beam splitter into two paths, and receives two identical PIN photodetector, such that the output of the two photodetectors is equal amplitude and inverted envelope signal, then the two signals after synthesis, so that doubling the FM signal, the AM noise and spurious cancel each other The DC component also counteract, to eliminate the influence of noise amplitude modulation requirements.

Polarization control technology: coherent optical communication system receiver must request signal light and the LO light polarized with biased in order to achieve good mixing effect and improve reception quality. Signal light after the single-mode fiber for long distance transmission, the polarization is random fluctuation, in order to solve this problem, a lot of ways, such as using polarization maintaining fiber, polarization controller and polarization diversity reception and other methods. When the light transmission in the general fiber, phase and polarization plane will randomly change the polarization maintaining fiber it is through the selection process and material makes the light phase and polarization unchanged specialty optical fiber, but the fiber loss, the price is very expensive; polarization The controller is mainly the signal light and the LO light with bias, this method is relatively slow response speed, loop control requirements are relatively high; polarization diversity receiver is the use of signal light and the LO light After mixing, the polarization The mixed light beam components into two mutually perpendicular polarization components, the LO light in two perpendicular polarization components by the polarization controller, so that the two components of equal power, so that when the signal light of random polarization fluctuations may cause one of the intermediate frequency signal branch decline, but another branch of the IF signal still exists, so the demodulated signal of the system and the resulting polarization of the signal light is almost independent of the technical response faster, more practical, but to achieve more complex.

3. The use of external optical modulation technology

Because when a parameter of the semiconductor laser direct modulation of the optical carrier, always incidental to other parasitic oscillation parameters, such as amplitude shift keying (ASK) modulation directly associated with the phase change, and the modulation depth will be limited. In addition, the frequency characteristics will encounter uneven and relaxation oscillation problems. Thus, in the coherent optical communication system, in addition to direct injection current can FSK frequency modulation, the other is the use of external light modulation.

4. The rational use of non-linear crosstalk control technology

Since the coherent optical communication, often using DWDM technology. Thus, the optical fiber nonlinear effects may make a coherent optical communication channel signal strength and phase affected by other channel signals to form non-linear crosstalk.

Coherent optical communication technology features and benefits

Coherent optical communication full use of coherent communication has the mixer gain, excellent channel selectivity and adjustable features. Compared with IM/DD system, coherent optical communication system has the following unique advantages:

1. High sensitivity, long distance relay

Coherent optical communications is a major advantage of enabling coherent detection, thereby improving the sensitivity of the receiver. In the coherent optical communication system, the coherent mixing light output current size of the signal light power and the local oscillator optical power proportional to the product. Under the same conditions, the coherent receiver to improve receiver sensitivity than the average of about 18dB, can achieve close to the shot noise limit performance, and therefore increase the unrepeated transmission distance of optical signals.

2. selectivity, large communication capacity

Another major advantage of coherent optical communication is that it can improve the selectivity of the receiver. In direct detection, the reception band is wider, to suppress interference noise, usually takes place before the detector narrowband filter, but the band is still very wide. Coherent heterodyne detection, the probe is a signal light and LO light of the mixed light, so only in the frequency band noise in the system before they can enter, while others are relatively narrow bandwidth microwave noise IF amplifier filtered out. Visible, heterodyne detection has good filtering properties, which will play a major role in the coherent optical communication applications. Furthermore, since the coherent detection excellent wavelength selectivity, coherent receiver can make the frequency spacing WDM system greatly reduced, that is, dense wavelength division multiplexing (DWDM), to replace the traditional light multiplexing of large frequency interval, having a wave division multiplexing potential advantages to achieve higher transfer rates.

3. You can use the equalization electronics to compensate for fiber dispersion effect of light pulses

If heterodyne detection of coherent optical communication transmission function of the IF filter transfer function coincided with the opposite optical fiber, optical fiber can reduce the influence of the dispersion of the system.