In fiber optic communication, multiplexing is considered to be the principal means for the expansion of the capacity of existing fiber network engineering. Multiplexing techniques include time division multiplexing TDM (Time Division Multiplexing) technology, MIMOs SDM (Space Division Multiplexing) technology, WDM (Wavelength Division Multiplexing) technology and frequency division multiplexing FDM with (Frequency Division Multiplexing) technology. However, because of the FDM and WDM is generally believed that there is no essential difference, so that the wavelength division multiplexing is roughly divided frequency division multiplexing is a “niche”, which both included in a class. Following discussion space division multiplexing (SDM), time division multiplexing (TDM), wavelength division multiplexing (WDM), CWDM OADM multiplexing approach.
TDM technology is very mature multiplexing electronics communication. This technique is the transmission time is divided into serveral time slots, into the corresponding slot would be needed to transmit the multiplexed signal according to certain rules, in order to achieve multi-channel signal multiplexed transmission. However, this technique is the use of electronics communication, due to the electron velocity space capacity and compatibility with many aspects of the restrictions, electronic time division multiplexing rate is not too high. For example, PDH signals only reach 0.5Gbps, althought the SDH system signal the synchronized interleaved multiplexing method has reached the rate of 10Gbps (STM-64), However, to achieve 20Gbps is quite difficult. Other hand, in the optical fiber, for the optical signals generated the loss (Attnuation), the reflectance, chromatic dispersion and polarization mode disoersion PMD will seriously affect the transmission of the modulation signal of the high rate. When the signal to STM-64 or higher rate, PMD pluse spreading effect, it will cause a signal “fuzzy”, causing the receiver to produce error signal misjudgment. This is due to the different modes of polarized light will produce a slight time difference in the fiber runs, and thus general
requirements the PMD coefficient must 0.1ps/km following. In summary, the limitations of the electrical time division multiplexing technology, the electronics communication transmission rate is limited to less than 10-20Gbps.
Optical time division multiplexing (OTDM)
OTDM signal modulation with a plurality of radio channel having a different channel of the same optical frequency, after multiplexing in the same fiber transmission expansion technology. Optical time division multiplexing technology include: ultra-narrow optical pulse generation and modulation techniques, optical multiplexing / de-multiplexing, optical timing extraction techniques.
a. Ultra narrow optical pulse generation. Optical time division multiplexing requires a light source to provide a duty cycle of 5 ~ 20GHz quite small, ultra narrow optical pulse output, realized gain switch the LD mode locking method, electro-absorption strobe continuous light modulation method and fiber grating method SC (Supercontinum,) optical pulses. The gain switching method can generate a pulse width of 5 ~ 7ps, the pulse repetition frequency of the optical pulse can be arbitrarily adjusted about at 10GHz, the advantage is easily synchronized with the other signals. Pulse source in the gain switching method has been used in a variety of high-speed optical transmission experiment generated and optical measurement. SC optical pulse width greater than 1ps, and narrowest reached 0.17ps. Further, the use of the adjusted linear modulation fiber grating dispersion value on the output of the electro-absorption modulator light pulse shape is corrected, can also produce a light pulse of the pulse duration is 5.8ps 10GHz, the duty cycle of 6.3%.
b. Full optical multiplexing/de-multiplexing. Optical time division multiplexing by the optical delay line and 3dB optical directional coupler constitute. in the ultra-high-speed system, it is preferable to intergrate optical delay line and the direation of the 3dB optical coupler in a plane on a silicon substrate to form a planar waveguide circuit (PLC) as the optical multiplexer. The all-optical demultiplexer demultiplexing the light receiving end of the OTDM signal. Has developed a device forms as a demultiplexer: optical Kerr switch matrix optical demultiplexer, cross-phase modulation frequency shift optical demultiplexer, four-wave mixing switch optical demultiplexer and non-line sexual fiber loop mirror type (NOLM) optical demultiplexer. Regardless of devices, require reliable and stable control of low power optical signal, regardless of the polarization.
c. Optical timing extraction technology. Optical timing extraction requirements of ultra-high-speed operation, low phase noise, high sensitivity, and has nothing to do with the polarization. Has been developed a high-speed microwave mixer PLL Road (PLL) constituted as a phase detector, the additional use of a Fabry – Perot interferometer constituted by an optical path the light oscillation circuit (FPT) clock recovery functions can be completed .
For a general understanding of the SDM is : multiplexing of the multiplexing of the plurality of optical fibers,i.e. the cable. In some places, there are off-the-shelf optical fiber communication network pipeline, and there is a spare position. Therefore, in order to increase capacity in the pipeline pulled into more optical fiber, which is more convenient than electronically. Another understanding MIMOs: realize space division multiplexing in an optical fiber, i.e. the space of the light beam of the optical fiber core region segmentation. Because part of the core of the single-mode fiber core diameter of only 9 ~ 10mm, and the phase of each point of the wavefront of the transmitted beam to exist Fluctuations, and thus the spatial segmentation of this wave surface is extremely difficult. While recently been proposed a the theoretical segmentation method of the degree of coherence, but from the practical, still have a long road to go.
WDM is the electrical signal of the multiple sources of the respective optical carrier, after multiplexing in an optical fiber transmission, and available at the receiving end of coherent heterodyne detection communication method or tuned passive filter directly detecting the conventionalthe communication method to achieve channel selection. WDM technology can not only expand the communication capacity, and can bring huge economic benefits for communication. Thus, in recent years, research in this area is in the ascendant WDM technology is the system of carrying multiple wavelengths (channels) in a single optical fiber, an optical fiber into multiple “virtual” fiber, each virtual fiber work independently wavelength. Each channel running speeds of up to 2.5 ~ 10Gbps.
Dense Wavelength Division Multiplexing
The so-called dense wavelength division multiplexing technology, which is often said DWDM refers to a fiber optic data transmission technology, this technology is the use of the wavelength of the laser according to the bit parallel transmission the string line transmission waydata is transmitted in the optical fiber.
The DWDM first introduced in the optical signal distribution to a specific band within the specified frequency (wavelength, lambda), signal is then multiplexed into a fiber, in this way we can greatly increase the bandwidth of the already laid cable. Since the introduction of the signal is not terminated at the optical layer, the rate and format of the interface can be maintained independently, thus allowing the service provider to integrate the existing equipment in the DWDM technology and network, while existing laying cable not able to use a lot of bandwidth.
DWDM can put a plurality of optical signals with the transmission, the results of these optical signals can be compiled into the same group at the same time is amplified and transmitted through a single optical fiber, the bandwidth of the network is also greatly increased. Each bearer of the signal can be set to a different transmission rate (OC-3/12/24, etc.) and different formats (SONET, ATM, data, etc.). For example, a DWDM network rate OC-48 (2.5 Gbps) and OC-192 (10 Gbps) SONET DWDM based on mixed signals. To huge bandwidth up to 40 Gbps. Using DWDM system can still achieve the above objectives while maintaining system performance, and the same degree of the existing transmission system reliability and stability. Future DWDM terminal can carry a total of 80 wavelengths much as the OC-48 in order to achieve a transmission rate of 200 Gbps, the OC-192, or up to 40 wavelengths in order to achieve a transmission rate of 400 Gbps, this bandwidth is sufficient seconds transmission 9 rolls of Encyclopedia!
DWDM technology of choice for fiber applications today, but its expensive prices affect their wider application. Faced with the needs of the communications market, CWDM (coarse wavelength division multiplexing) came into being. CWDM low cost high access bandwidth suitable for a variety of popular peer-to-peer, Ethernet, SONET ring network structure, particularly suitable for short-distance, high-bandwidth access point intensive communication applicationsoccasions, such as building or building network communications. Is particularly worth mentioning is that with the use of CWDM and PON (passive optical network). PON is an inexpensive, a point-to-multipoint optical fiber communication mode, combined with CWDM, each single wavelength channel can be used as the the virtual optical link of the PON, the central node and a plurality of distributed nodes for broadband data transmission.
However, CWDM is a compromise between cost and performance, and inevitably there are some performance limitations. Industry experts point out, CWDM is still less than 4 points:
1. Less CWDM support the number of multiplexed wavelengths in a single fiber, resulting in higher cost of future expansion;
2. Multiplexer multiplex demodulator and so the cost of equipment should be further reduced, these devices can not only DMDM a simple modification of the corresponding device;
3. CWDM has not yet formed a standard.
People’s interest in wavelength division multiplexing (WDM) optical network field, more and more concentrated OADM. These devices in the field of optical wavelength traditional SDH OADM in the time domain function. Particular OADM can be separated from the beam of a WDM channel (ceded function), and is generally based on the same wavelength to insert new information on the optical carrier (insert function). OADM selectivity, from the transfer device to select the next channel signal or on the road signal, or simply by the signal of a certain wavelength, but does not affect the transmission of the other wavelength channels. OADM in the optical domain, OADM in the time domain, the completion of the function, and having transparency, and can handle any format and rate of the signal in SDH. Can improve the reliability of the network, reducing node cost, and improve the efficiency of network operation, key equipment is essential for the formation of all-optical network. For OADM, in the sub-outlet and inserted between the mouth and between the input and output ports must have a high degree of isolation (> 25dB) in order to minimize the same wavelength interference effects, otherwise it will seriously affect the transmission performance. Have been proposed the achieve OADM several techniques: WDMMUX / DEMUX; between optical circulator or a fiber grating in MachZehnder structure; the tandem MachZehnder implemented with integrated optics technology structure and the interference filter. The first two methods so that the isolation of the highest, but they require expensive equipment, MachZehnder structure (with fiber grating or optical integration technology) is still under development, and the need for further improvement in order to achieve the required isolation.
From the current of view, all-optical network is first applied to the LAN, MAN the internal optical routing, the technology used is based on WDM EDFA and broadband. In the long run, all-optical network inevitably points toward the waves, time division, space division three ways to combine the direction of development. Its application will be extended to the WAN. Network range can cover the entire country or several countries, and ultimately achieve a high-speed large-capacity all-optical network to meet the future demand for communication services.