What are the types of optical multiplexers?

WDM (Wavelength Division Multiplexing) technology is the preferred solution for 5G fronthaul networks. According to different wavelengths, it can be divided into DWDM, CWDM, FWDM, MWDM and LWDM. Among them, CWDM and DWDM are the most common.

First, let’s take a brief look at the main optical wavelengths.

The CWDM wavelength range specified by ITU (ITU-T G.694.2) is 1271 to 1611 nm, but in applications, the attenuation of the 1270-1470 nm band is relatively large, so the 1470-1610 nm band is usually used. DWDM channels are more densely spaced, using the C-band (1530 nm-1565 nm) and L-band (1570 nm-1610 nm) transmission windows. Ordinary WDM generally uses 1310 and 1550 nm wavelengths.

CWDM (Coarse Wavelength Division Multiplexer)

CWDM short for coarse wavelength division multiplexer, use for the access layer of the metropolitan area network (WAN network). CWDM has 18 different wavelength channels, and the different wavelengths of each channel are separated by 20nm. It uses wavelengths from 1270 nm to 1610 nm, and the bands it belongs to cover the five bands of O, E, S, C, and L of the single-mode fiber system.

The biggest advantage of the CWDM system is its low cost, and the device cost is mainly reflected in the filter and laser. The wide wavelength interval of 20nm also brings CWDM the advantages of low technical indicators requirements for lasers, simplified structure of optical multiplexers/demultiplexers, and improved yield. CWDM supports fewer channels than DWDM, and because it is compact and cost-effective, it makes it an ideal solution for short-distance communications.

DWDM（Dense Wavelength Division Multiplexer）

DWDM is Dense Wavelength Division Multiplexer. The channel spacing of DWDM is 1.6/0.8/0.4nm (200GHz/100 GHz/50 GHz), which is much smaller than CWDM. Compared with CWDM channels, which consume 20nm (15 million GHz) of space per channel, DWDM with tighter wavelength spacing can carry 8 to 160 wavelengths on one optical fiber, and can multiplex more wavelengths onto the same optical fiber, making it more suitable for long-distance transmission. With the help of EDFA, DWDM systems can operate within a range of thousands of kilometers.

The current DWDM system can provide a single-fiber transmission capacity of 16/20 or 32/40 waves, up to 160 waves, which increases the transmission capacity of a single optical fiber by several to dozens of times compared with single-wavelength transmission, greatly saving optical fiber resources and reducing line construction costs.

FWDM filter wavelength division multiplexer

FWDM filter wavelength division multiplexer is based on mature thin film filter technology. It can mix or separate light of different wavelengths in a wide wavelength range. It has the characteristics of wide channel bandwidth, low insertion loss, high channel isolation, high environmental stability and reliability. It is widely used in erbium-doped optical amplifiers EDFA, Raman amplifiers RFA and single-mode fiber WDM optical fiber networks.

MWDM medium wavelength division multiplexing

MWDM is a medium wavelength division multiplexing technology that reuses the first 6 waves of CWDM, compresses the 20nm wavelength interval of CWDM to 7nm, and uses TEC (Thermal Electronic Cooler, semiconductor refrigerator) temperature control technology to expand 1 wave into 2 waves, that is, the left and right offsets are expanded by 3.5nm to 12 waves (1267.5, 1274.5, 1287.5, 1294.5, 1307.5, 1314.5, 1327.5, 1334.5, 1347.5, 1354.5, 1367.5, 1374.5nm), which not only reuses the CWDM industry chain, but also meets the 10km fronthaul distance requirement, achieving capacity improvement while further saving optical fiber.

MWDM is mainly proposed in the 5G fronthaul network environment in China.

LWDM

LWDM (LAN WDM) is a fine wavelength division multiplexing technology, usually referring to a wavelength range dense wavelength division multiplexing technology using four wavelength division multiplexing technology in 100G optical modules. According to the LAN WDM wavelength defined by IEEE 802.3, its channel spacing is 200-800GHz, which is between DWDM (100GHz, 50GHz) and CWDM (about 3THz). It uses 12 wavelengths (1269.23, 1273.54, 1277.89, 1282.26, 1286.66, 1291.1, 1295.56, 1300.05, 1304.58, 1309.14, 1313.73, 1318.35nm) in the 1269nm to 1332nm band in the O-band (1260nm-1360nm) range, with a wavelength spacing of 4nm.

The characteristics of LWDM working wavelength are that it is located near zero dispersion, has small dispersion, good stability, and mainly covers 10km. At the same time, LWDM can support 12 wavelengths and 25G, which increases capacity and can further save optical fibers.

The following table provides a summary analysis of the centralized WDM products for reference only.

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