Coherent optical techniques to promote the next generation of optical transmission - amplifiers, telecommunications, cable - Communications Industry

Due to the EDFA optical amplifiers, wavelength division multiplexing

, dispersion compensation and forward error correction (FEC), ultra-long distance optical transmission system capacity increased rapidly in recent years. Widely used amplitude shift keying (OOK) intensity modulation and direct detection receiver technology in the channel rate is less than 10Gbits / s for WDM system applications to navigate.

However, in the "post-bubble era," people lower unit transmission costs accelerated pace. In order to effectively extend the existing cable network transmission distance and increase transmission capacity, need to address the many technical problems. These technical challenges include: improving the system to overcome the noise and deterioration of the ability to transfer to higher spectral efficiency. To achieve this goal, the need to consider the use of new modulation techniques, such as: differential phase shift keying (DPSK) modulation, while also using electronic equalization technology.

To further improve the transmission capacity and transmission distance without repeaters, we need to break through the existing mechanism of the direct detection receiver using coherent optical detection techniques. Coherent optical detection techniques, in addition to optical signal detection range, can detect the optical signal phase and polarization state (polarization state). Using coherent light technology, in the same spectral bandwidth can transmit greater amounts of data. In addition, coherent systems, optical signal phase information can be used to efficiently compensate optical signal caused by the deterioration in the electrical domain.

Figure: Coherent optical receiver can use the "homodyne" receiver technology and "heterodyne" receiver technology. "Homodyne" receiver when the signal frequency of the vibration frequency and consistency; "heterodyne" receiver, the difference between the two frequencies for a fixed IF frequency.

Coherent optical receiver configuration Figure 1 shows the basic structure of coherent optical receiver, the received signal light with the optical local oscillator (LO) signal mixing. The signal light from the high-frequency (1550nm band, equivalent to 200THz) down to the microwave frequency band (frequency in GHz magnitude). Photodetector produce travel beat signal, the signal is called intermediate frequency signal (IF), the frequency set fIF, the frequency of the signal light frequency and the vibration frequency of the poor light.

If signal light frequency and the vibration frequency of the same light that the technique is called "zero gap" to receive. Signal light frequency and the vibration frequency of light is not the same, that the technique is called "heterodyne" receiver, fIF = fC-fLO, which fC, fLO are receiving optical signals, optical signals of the oscillator center frequency.

For heterodyne receiver technology, in order to form a complete bilateral band, the IF must be more than twice the rate of data signals. Homodyne receiver technology requires local oscillator Laser Devices and receive optical signals must be phase-locked; contrast, the heterodyne receiver requires two frequency locked, the frequency difference constant.

Coherent optical receiver components, including the main components: local oscillator light source, optical coupler, balanced optical detector, phase / frequency locking device, polarization control loop and Telecommunications No. processors. In order to improve receiver sensitivity, the following essential components:

(1) local oscillator light source, optical power requirements of high relative intensity noise (RIN) is low, narrow line width. 1

(2) balanced photodetector with high responsivity, input power dynamic range, common mode rejection ratio (CMRR) high. 2

(3) To reduce the IF signal frequency, phase noise, system requirements for the LO laser frequency, phase locked.

Coherent optical receiver performance and benefits

Coherent receiver has many advantages, can increase the light Network Performance. Benefits include upgrading the existing Optical fiber The data capacity, can support a variety of modulation, receiver sensitivity, but also compensation for optical transmission in the electric field caused by deterioration.

, For example, have proven multi-level modulation, such as differential quadrature phase shift keying technology to upgrade the existing capacity of optical data links. In other words, in the same spectral bandwidth, compared to commonly used amplitude shift keying modulation, phase shift keying technology can transmit more data.

Wree and others have reported using DQPSK modulation transmission experiment through the use of polarization multiplexing, transfer rate can reach 40Gbits / s, while the amplitude modulation technique used often in the same spectrum bandwidth only can transfer 10Gbits / s of data 3. This applies in the usual spectrum of optical amplifier bandwidth data capacity can be raised to 4 times the original. The signal can be through the use of four Mach-Zehnder interference delay device, using its associated data is demodulated, but the four Mach-Zehnder interferometer their need for a control circuit.

This receiver than the coherent receiver are much more complex. Transmission distance is limited, and only several hundred kilometers. This is mainly due to the transmission process, the polarization mode coupling between the signal distortion caused.

Coherent optical receiver technology used, the received signal phase and polarization can be more efficient in the detection of electric fields. The distortion caused by optical transmission can be a good equalizer to compensate by electricity, thus achieving a longer transmission distance. This receiver can also be eliminated and the side of the dispersion compensation fiber optical amplifiers. Coherent optical receiver

Another advantage is that can support multiple modulation. At present, the optical transponder manufacturers are developing new modulation methods such as: DPSK, DQPSK, and multi-band encoding. In the practical application of these technologies, the optical network related equipment should also be gradually adapt to these modulations. Detection technique based on coherent optical receiver to have the ability to deal with only slight changes in electrical links, optical components without having to change the system can be flexibly applied to a variety of modulation. 3

Using phase shift keying (FSK) modulation, in order to reduce the frequency jitter, phase jitter caused by noise, phase-locked loop (PLL) for use is of particular importance. Shown in Figure 2, coherent system of IF signal line width (spectral width) is usually a few MHz before, using existing optimum design of DFB laser and phase-locked loop can now be reduced to only 100Hz, the gap up to 104 times (Figure 2). 4

Phase-locked loop (PLL) is optimized, the coherent optical detection system output IF signal line width is only 100Hz.

In addition, if we neglect the relative intensity of the vibration noise source, the received signal after mixing with the local oscillator signal can be

Coherent optical techniques to promote the next generation of optical transmission - amplifiers, telecommunications, cable - Communications Industry

By: yoyo

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