How to use WDM technology to expand fiber capacity?

Overview:
Expanding fiber capacity with WDM
What’s Mux and Demux
CWDM vs DWDM
What does optical add-drop multiplexer stand for
WDM bands
WDM technology: TFF and AWG
WDM-PON in access network
WDM in 5G network

Wavelength Division Multiplexing (WDM) is one of the most common ways of using wavelengths to increase bandwidth by multiplexing various optical carrier signals onto a single optical fiber. It combines a series of optical carrier signals with different wavelengths carrying various information and coupled to the same optical fiber for transmission at the transmitting end. At the receiving end, optical signals of various wavelengths are separated by a demultiplexer. This technique of simultaneously transmitting two or many different wavelengths in the same fiber is called wavelength division multiplexing, or WDM. 

WDM provides an easy-to-implement solution for long-distance transmission of high-speed and large-capacity information, which is widely used in backbone network and optical transport network. Based on cost consideration, the current metro network is mainly based on CWDM and FOADM (fixed optical add/drop multiplexer) technologies. The DWDM and ROADM technologies for long haul network are expected to sink to metro network. The coming 5G application promotes the upgrading of AON. As the key part for AON, ROADM market is expected to increase rapidly, especially in metro network application.

Increasing Capacity with WDM
As shown in the figure below, the traditional optical transmission method is that one fiber can only transmit one wavelengths of signal in a single time. If you want different services, you need countless different and independent optical fibers for transmission. However, if there is a large amount of services, a large number of optical fibers need to be laid for transmission, which poses a great challenge to cabling space and cost. The application of a WDM system can quickly solve the above problems. The WDM system can carry multiple signals through multiplexing and demultiplexing technologies, such as ATM, IP, etc., and multiple service signals can be transmitted through a single optical fiber, which greatly reduces the amount of optical fiber. The WDM system can carry multiple signals, such as ATM, IP, etc., through multiplexing and demultiplexing technology, the multiple service signals can be transmitted through a single optical fiber, which greatly reduces the amount of optical fiber. This is an ideal technology for capacity expansion. When introducing new broadband services such as CATV, HDTV, B-ISDN, etc., only one additional wavelength needs to be added.

What’s Mux and Demux
MUX and DEMUX are two key devices in the WDM system, and we can use the functions of optical prism to help understanding.

The main function of the combiner MUX is to combine multiple signal wavelengths into one fiber for transmission.  At the transmitting end, the N optical transmitters operate on N different wavelengths respectively, and the N wavelengths are separated by appropriate intervals, which are respectively recorded as λ1, λ2, ... λn. A multiplexer combines these optical wavelengths into a single-mode fiber. Since optical carrier signals of different wavelengths can be regarded as independent of each other (regardless of fiber nonlinearity), multiplexing transmission of multiple optical signals can be realized in one optical fiber. Through multiplexing, communication carriers can avoid maintaining multiple lines and effectively save operating costs.

The main function of DEMUX is to separate the multiple wavelength signals transmitted in one fiber. In the receiving part, the optical carrier signals of different wavelengths are separated by a Demux and further processed by the optical receiver to restore the original signal. A multiplexer (Demux) is a device that reverses the processing of a multiplexer.

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

CWDM vs DWDM
WDM technology can be divided into CWDM, and DWDM according to different wavelength modes. The wavelength range specified by ITU for CWDM (ITU-T G.694.2) is 1271 to 1611 nm, but considering the relatively large attenuation of the 1270-1470nm band in the application, the 1470-1610nm band range is usually used. DWDM channels are more densely spaced, using C-band (1530nm-1565nm) and L-band (1570nm-1610nm) transmission windows. Ordinary WDM generally adopts 1310 and 1550nm wavelengths.

CWDM is Coarse Wavelength Division Multiplexing, which has 18 wavelengths from 1270nm to 1610nm with 20nm wavelength interval. CWDM is an ideal solution for short-range communications because it is compact and cost-effective, especially the filter and laser components.

DWDM is Dense Wavelength Division Multiplexer. DWDM can carry 40, 80 or up to 160 wavelengths with a narrower spacing of 1.6/0.8/0.4nm (200/100/50GHz).  Compared with CWDM, DWDM can carry 8~160 wavelengths on one optical fiber, which is more suitable for long-distance transmission. With the help of EDFA, DWDM system can work in the range of thousands of kilometers.

The most difference between CWDM and DWDM is the spacing of wavelength which causes the number of wavelength or channels that can be used. That's the difference between “Coast” and “Dense”. CWDM channels each consume 20 nm of space. Instead of the 20 nm spacing in CWDM, DWDM uses either 50, 100 or 200 GHz spacing which allows many more wavelengths to be packed onto the same fiber. The common channels of CWDM are 8 to 18, while DWDM can carry 40, 80 or even up to 160 wavelengths, which is suitable for long-distance transmission. With the help of EDFA, DWDM system can work in the distance of thousands of kilometers.

What does optical add-drop multiplexer stand for
OADM is Optical Add-Drop Multiplexer, which used in WDM systems for multiplexing and routing different channels of light into or out of a single mode fiber. Its main function is to selectively drop or add one or multiple wavelength channels without affecting the transmission of other wavelength channels.  The OADM device is one of the key devices of the all-optical network.

"Add" refers to the capability of the device to add one or more new wavelength channels to an existing multi-wavelength WDM signal while "drop" refers to drop or remove one or more channels, passing those signals to another network path.

WDM Bands
O band：1260-1360 nm
E band：1360-1460 nm
S band：1460-1530 nm
C band：1530-1565 nm
L band：1565-1625 nm

The wavelength of fiber can be divided into several bands, each band is used as an independent channel to transmit predetermined wavelength. The common WDM bands can be divided into O/E/S/C/L five bands.

The O-band is the first wavelength band used for long-distance optical communications in history, and the signal distortion (due to dispersion) is minimal. Optical fiber shows the lowest loss in the C band and L band, so DWDM systems typically use these two bands. In addition to the O band to L band, there are two other bands, the 850nm band and U band (Ultra-long band: 1625-1675nm). The 850nm band is the main wavelength for multimode fiber optic communication systems incorporating VCSELs. U-band lasers are expensive, so U-band is usually used as a monitoring band at present.

WDM Technology
There are three competing filtration technologies: Thin Film Filters (TFF), Array Waveguides (AWG), and optical comb filter. TFF and AWG are the two most commonly used WDM technologies.  AWG is more cost-effective in long-distance, high-channel-capacity DWDM applications, while TFF is more ideal in low-channel-capacity CWDM metro area network

TFF-Thin Film Filter（FWDM）
Thin film filter (TFF) technology is the first WDM commercial technology, FWDM (Filter Wavelength Division Multiplexing) is based on mature TFF technology. The main advantage of thin-film filters over other technologies is their high accuracy in small form factor devices.

Filters is the key component of TFF technology. The picture shows the structure of a three-port WDM device, which consists of a dual-fiber collimator, a single fiber collimator and a TFF filter. The TFF filter is attached on the end-face of the collimating lens of the dual-fiber collimator.

In order to de-multiplex all the wavelengths, n three-port devices are cascaded to construct a WDM module, as shown in figure. The TFF filters in each device transmit different wavelengths. The WDM module can function as a de-multiplexer or multiplexer, depending on the signal direction.

The WDM module based on cascaded three-port WDM devices has a relatively large size, which does not meet the requirement by some special applications. Compact WDM module is developed for such applications, as shown in figure. The TFF filters are fixed on a glass bench and the input/output fiber collimators are aligned one by one. The compact DWDM and CWDM modules are usually called CDWDM and CCWDM, respectively.

The principle is to use an input lens to focus the optical signals of the wavelengths λ1, λ2...λn on the first filter; the optical signal with the wavelength λ1 passes through the first filter and is coupled to the first output lens. In the first output fiber, the optical signal of wavelength λ1 is separated; the remaining optical signals are reflected by the first slide to the next slide for optical signal separation; and so on, until all signals are separated. The coupling between the wavelength channels is achieved in the form of zigzag lines of light.

Structure of a typical AWG is shown in figure. It consists of five parts: a transmitter waveguide, an input star coupler (FPR (free propagation region) in figure), arrayed waveguides, an output star coupler and dozens of receiver waveguides. The DWDM signals emit from the transmitter waveguide and are separated into the arrayed waveguides after free propagation in the input star coupler. The separation is colorless, which means that all the wavelengths are separated into the arrayed waveguides identically. The arrayed waveguides generate phase difference between the multiple optical beams. The phases of the multiple beams are in arithmetic progression, which is just like the traditional gratings. Thus, the different wavelengths are dispersed and then focused at different positions in the output star coupler. The receiver waveguides are set at the focusing positions. Different wavelengths are received by different waveguides and thus parallel demultiplexing of DWDM signals are realized.

The main advantage of AWG over TFF is that its cost does not depend on the wavelength count, which makes them cost-effective for large channels applications. Another advantage of AWG is the flexibility to choose the channel number and spacing.

WDM applications

WDM-PON in access network
In order to meet the needs of high bandwidth, multi-service, and low-cost smooth evolution of the optical fiber access network, WDM-PON technology is introduced into the access network. Based on WDM technology, the Tbit-level bandwidth capacity of optical fibers can be fully utilized, greatly expanding the number of optical fiber-carrying users, and the WDM overlay solution will not affect existing services, enabling smooth upgrades. NG-PON2 is an evolution system of the existing GPON/XG-PON.

WDM in 5G Network
In the C-RAN large concentration scenario, each wireless base station usually needs 12 high-speed optical interfaces. For this reason, China Mobile has launched a 12-wave MWDM transmission solution. The 12 wavelengths selected are shown in the figure. MWDM reuses the first 6 wavelengths of CWDM, compresses the 20nm wavelength interval of CWDM to 7nm, and uses Thermal Electronic Cooler (TEC) temperature control technology to expand 1 wave into 2 waves. In this way, an increase in capacity can be achieved while further saving optical fibers.

China Telecom chose a 12-wavelength LWDM transmission scheme with a channel spacing of 800GHz and 12 wavelengths as shown in the figure.  LWDM uses 12 wavelengths in the O-band range from 1269nm to 1332nm, with a wavelength interval of 4nm. Its channel interval is 200~800GHz, this range is between DWDM (100GHz, 50GHz) and CWDM (about 3THz). The characteristic of LWDM working wavelength is that it is located near zero dispersion, with small dispersion and good stability. At the same time, LWDM can support 12-wave 25G to increase the capacity and save fiber.