New Technology of Optical Fiber Communication

1. Concept of WDM

WDM (wavelength division multiplexing) technology is a technology that transmits multiple wavelength optical signals in one optical fiber at the same time.

The basic principle of optical wavelength division multiplexing (WDM) is that the optical signals of different wavelengths are combined (multiplexed) at the transmitting end and coupled to the same optical fiber on the optical cable line for transmission, and the optical signals of combined wavelengths are separated (demultiplexed) at the receiving end for further processing, and the original signals are recovered and sent to different terminals. Therefore, this technology is called optical wavelength division multiplexing, Referred to as optical wavelength division multiplexing technology.

How wide is the bandwidth of optical fiber?

As shown in Fig. 7.6, there are two low loss transmission windows in the optical fiber: the wavelength is 1.31 μ m(1.25~1.35 μ m) Window, corresponding bandwidth (| Δ f|=|- Δλ c/ λ 2|, λ and Δλ Are the central wavelength and corresponding band width respectively, C is the speed of light in vacuum) is 17700 GHz; Wavelength 1.55 μ m(1.50~1.60 μ m) The corresponding bandwidth is 12500 GHz.

When the two windows are combined, the total bandwidth exceeds 30thz. If the channel frequency interval is 10GHz, ideally, one fiber can accommodate 3000 channels.

At present, some optical devices and technologies are not very mature, so it is difficult to realize optical frequency division multiplexing (OFDM) with dense optical channels. In this case, wavelength division multiplexing with small channel spacing in the same window is called dense wavelength division multiplexing (DWDM).

At present, the system is an optical communication system composed of 8, 16 or more wavelengths on a pair of optical fibers (single optical fiber can also be used) in the 1550nm wavelength range, in which the spacing between each wavelength is 1.6nm, 0.8nm or less, which corresponds to the band width of 200ghz, 100GHz or narrower.

WDM, DWDM and OFDM are not much different in essence

In the past, technicians used to use WDM and DWDM to distinguish between 1310 / 1550nm simple multiplexing and 1550nm intensive multiplexing, but at present, DWDM technology is used in telecom applications.

Since the multiplexing of 1310 / 1550nm exceeds the gain range of EDFA and is only used in some special occasions, WDM is often used to replace DWDM.

WDM technology is of great significance for network upgrading, developing broadband services (such as CATV, HDTV and IP over WDM), fully tapping the potential of optical fiber bandwidth and realizing ultra-high speed optical fiber communication. Especially WDM and EDFA have a strong attraction to modern information network.

At present, "erbium doped fiber amplifier (EDFA) dense wavelength division multiplexing (WDM) non-zero dispersion optical fiber (NZDSF, i.e. G.655 fiber) photonic integration (PIC)" is becoming the main technical direction of long-distance high-speed optical fiber communication lines in the world.

If all optical fiber transmission links in an area are upgraded to WDM transmission, we can set an optical cross connection device (OXC) for cross connection of optical signals in wavelength units or an optical add drop multiplexer (OADM) for optical uplink and downlink at the intersection (node) of these WDM links, and a new optical layer will be formed on the physical layer originally composed of optical fiber links.

In this optical layer, the wavelength channels in adjacent optical fiber links can be connected to form an optical path across multiple OXC and OADM to complete end-to-end information transmission, and this optical path can be flexibly and dynamically established and released according to needs, which is the eye-catching and new generation WDM all-optical network.

2. Basic form of WDM system

Optical wavelength division multiplexer and demultiplexer are the key components of WDM technology. The device that combines signals of different wavelengths and outputs them through an optical fiber is called multiplexer (also known as multiplexer).

On the contrary, the device that decomposes the multi wavelength signal sent by the same transmission fiber into each wavelength and outputs it respectively is called demultiplexer (also known as demultiplexer).

In principle, this device is reciprocal (bidirectional reversible), that is, as long as the output and input of the demultiplexer are used in reverse, it is a multiplexer.

Therefore, the multiplexer and demultiplexer are the same (unless there are special requirements).

The basic composition of WDM system mainly includes the following two forms: dual fiber unidirectional transmission and single fiber bidirectional transmission.

(1) Dual fiber unidirectional transmission.

Unidirectional WDM transmission means that all optical paths are transmitted in the same direction on one optical fiber at the same time.

As shown in Figure 7.7, dimmed signals with different wavelengths containing various information will be sent at the transmitter λ 1, λ 2,…, λ N is combined by optical multiplexers and transmitted unidirectionally in one optical fiber.

Since the signals are carried through different light wavelengths, they will not be confused with each other.

At the receiving end, signals of different wavelengths are separated by optical demultiplexer to complete the task of multi-channel optical signal transmission.

The principle of transmission through another optical fiber in the opposite direction is the same.

(2) Single fiber bidirectional transmission. Bidirectional WDM transmission means that the optical path transmits to two different directions on one optical fiber at the same time. As shown in Figure 7.8, the wavelengths used are separated from each other to realize bidirectional full duplex communication.

Several key system factors must be considered in the design and application of bidirectional WDM system:

For example, in order to suppress multi-channel interference (MPI), we must pay attention to the influence of light reflection, isolation between bidirectional paths, type and value of crosstalk, power level value and mutual dependence of two-way transmission, optical monitoring channel (OSC) transmission and automatic power shutdown, and use bidirectional fiber amplifier at the same time.

Therefore, the development and application of bidirectional WDM system are relatively demanding, but compared with unidirectional WDM system, bidirectional WDM system can reduce the number of optical fibers and line amplifiers.

In addition, by setting an optical add drop multiplexer (OADM) or optical cross connector (OXC) in the middle, the optical signals of each wavelength can be merged and shunted, and the wavelength add / drop and route allocation can be realized. In this way, the inserted or separated signals can be reasonably arranged according to the traffic distribution of optical fiber communication lines and optical networks.

3. Performance parameters of optical wavelength division multiplexer

Optical wavelength division multiplexer is an important part of wavelength division multiplexing system. In order to ensure the performance of wavelength division multiplexing system, the basic requirements for wavelength division multiplexer are:

• low insertion loss

• high isolation

• flat in band and steep out of band insertion loss

• good temperature stability

• multiple channels

• small size, etc

(1) Insertion loss insertion loss refers to the additional loss caused by the addition of optical wavelength division multiplexer / demultiplexer, which is defined as the ratio of optical power between the input and output ports of the passive device, i.e

(7.1)

Where Pi is the optical power sent into the input port; Po is the optical power received from the output port.

(2) Crosstalk suppression system crosstalk refers to the influence degree that the signals of other channels are coupled into a channel and degrade the transmission quality of the channel. Sometimes, this degree can also be expressed by isolation. For demultiplexer

(7.2)

Where Pi is the wavelength λ I is the input optical power of the optical signal, and PIJ is the wavelength λ The optical signal of I is connected in series to the wavelength of λ Optical power of channel J.

(3) Return loss return loss refers to the ratio of the optical power returned from the input port of the passive device to the input optical power, i.e

(7.3)

Where PJ is the optical power sent into the input port, and PR is the return optical power received from the same input port.

(4) Reflection coefficient reflection coefficient refers to the ratio of reflected light power PR to incident light power PJ at a given port of WDM device, i.e

(7.4)

(5) Working wavelength range working wavelength range refers to the wavelength range in which WDM devices can work according to the specified performance requirements( λ Min to λ max)。

(6) Channel width channel width refers to the wavelength interval between light sources to avoid crosstalk.

(7) Polarization dependent loss (PDL) refers to the maximum change of insertion loss caused by the change of polarization state.

New Technology of Optical Fiber Communication 1

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