Category Archives: WDM Optical Network

Introduction for Optical Add-Drop Multiplexer

The evolution of single wavelength point-to-point transmission lines to wavelength division multiplexing optical networks arose the need to separate/route different wavelength channels. What kind of device can meet this need? The answer is optical add-drop multiplexer (OADM). This device is used for multiplexing and routing different channels of light into or out of a single mode fiber (SMF). “Add” means adding one or more new wavelength channels to an existing multi-wavelength WDM signal. “Drop” means dropping or removing one or more channels, passing those signals to another network path. OADM is particularly important in metropolitan WDM lightwave services where offices or sites can be connected by different add-drop channels. It can increase the network flexibility along the optical link.

Principle

A traditional OADM has three stages: an optical multiplexer, and optical demultiplexer, and between them a method of reconfiguring the paths and a set of ports for adding and dropping signals. The multiplexer is to couple two or more wavelengths into the same fiber. The reconfiguration can be achieved by a fiber patch or by optical switches which direct the wavelengths to the add ports. The demultiplexer separates wavelengths in a fiber and direct them into many fibers.

OADM-WDM-CWDM

Add-Drop Configurations

To realize the configurations performance of adding or dropping functions, planar and fiber technology could be used. No matter what kind of devices, several factors should be considered, such as low insertion loss, high isolation, polarization insensitivity, and the cost. Planar devices provide compact solutions with the possibility of adding or dropping many channels using only one integrated optical circuit with arrayed-waveguide-grating or waveguide-grating-router. But they have the main disadvantages of high insertion loss and polarisation dependence. Comparatively, all-fiber devices are more attractive because of the low insertion losses, polarisation insensitivity and ease of coupling between output and input of the optical network by using simple splices and pigtails. But these devices are sensitive to environmental variations. There is another kind of devices based on free space optics (micro mirrors and gratings) also used to perform add-drop operations well. However, these devices have high insertion losses and cost much. So thin film filter devices have been used for multiplexing/demultiplexing applications.

Types

OADM have passive modes with fixed wavelength (COADM) and active modes with dynamic wavelength (ROADM). In fixed wavelength OADM, the wavelength has been selected and remains the same until people change it. In dynamic wavelength OADM, the wavelengths could be directed selectively without changing its physical configuration. Different types have different functions. For the OADM with fixed wavelengths, the node’s routing is determined but is not flexible. For the OADM with dynamic wavelengths, it’s more flexible but cost too much.

OADM selectively drops a required wavelength from multiple wavelengths in a fiber and adds the same wavelength into the data flow but with different data content. It’s used in both CWDM systems and DWDM systems. CWDM OADM is designed for the CWDM passive optical systems. It can add/drop wavelengths from multiple fibers onto one optical fiber. And DWDM OADM is designed to add/drop one multiple DWDM channels into one or two fibers. Although OADM is good enough to use, it is still under improvement. In the future,  it will be more cost-effective.

Differences between CWDM System and DWDM System

As more bandwidth and faster data transmission rates over long distances are needed, users have to rely on more fiber optics. But laying more fibers will cost much. So WDM (Wave Division Multiplexing) technology appeared and solved this problem.

WDM is a technology which transmits multiple optical signals on a single fiber by using different wavelengths, or colors, of laser light to carry the different signals. By using bidirectional communications over a single fiber, network users can realize a multiplication effect in their available fiber’s capacity. The effect of expanding the capacity of the network without laying more fibers makes WDM systems so popular. The concept was first published in 1978. At the very beginning, only two signals could be combined by using WDM systems. But now more signals could be handled and transmission rates get faster.

Currently WDM systems are divided into two different types: CWDM (coarse wavelength division multiplexing) and DWDM (dense wavelength division multiplexing). CWDM systems typically have 8 channels with 8 wavelengths separated by 20 nm from 1470 nm to 1610 nm. But the channels could also be increased to 16. Typical DWDM systems use 40 channels and provide wavelengths up to 96 with 0.4nm spacing. The channel spacing decides the call of “coarse” and “dense”. There are many differences between these two systems.

WDM-CWDM-DWDM

Transmission Distance

CWDM systems can’t transmit data over distances as long as DWDM system because the wavelengths are not amplified. Usually CWDM can travel somewhere about 100 miles. Therefore CWDM is limited in its functionality over longer distances. While DWDM systems are set for long haul transmission by keeping the wavelengths tightly packed. They can transmit more data over longer distances with less interference than a CWDM system. So it’s suitable for data transmission over long distances.

Power Requirements

The power requirements for DWDM are significantly higher. For example, DWDM lasers are temperature-stabilized with Peltier coolers integrated into their module package. The cooler along with associated monitor and control circuitry consumes around 4 W per wavelength. But CWDM uses uncooled laser transmitter and it consumes about 0.5 W of power.

Cost

Two main factors like operating and hardware cost cause difference between CWDM and DWDM. The first example has been referred before. CWDM modulation laser is uncooled, but DWDM laser is cooling. Cooling laser using temperature tuning, uncooled laser adopts electronic tuning. The range of temperature distribution is non-uniform in a very wide wavelength, so the temperature tuning is very difficult to realize, which results in high cost. CWDM doesn’t have this problem. Second, for instance, DWDM transceivers are typically four or five times more expensive than CWDM counterparts. That’s mainly because of the lasers. Typical wavelength tolerances for DWDM lasers die are on the order of ±0.1 nm; whereas tolerances for CWDM lasers die are ±2-3 nm. Lower die yields increase the costs of DWDM lasers relative to CWDM lasers. From these two sides, CWDM system is cheaper than DWDM system.

Each kind of WDM system has its advantages and disadvantages. CWDM is very flexible and is good for campus LAN expansion. DWDM has large capacity and is ideal for long distance data transmission. CWDM and DWDM technology continue be to improved. The two have different functions and will complement not replace the other.