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The Basic Parameters of Passive Optical Network Devices

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There are many devices elementary but necessary for the Passive Optical Network (PON) applications that require the transmission, combining, or distribution of optical signals. These passive devices include the Optical Splitter/Coupler, Optical Switch, Optical Attenuator, Optical Isolator, Optical Amplifier, and WDM Filters (CWDM/DWDM Multiplexer) etc. Tips: The passive devices are components that do not require an external energy source.

When working with these passive devices it is important to have a basic understanding of common parameters. Some of the basic parameters that apply to each device are Optical Fiber Type, Connector Type, Center Wavelength, Bandwidth, Insertion Loss (IL), Excess Loss (EL), Polarization-Dependent Loss (PDL), Return Loss (RL), CrossTalk (XT), Uniformity, Power Handling, and Operating Temperature.

Connector Type and Optical Fiber Type

Many passive devices are available with receptacles or fiber optic pigtails. The pigtails may or may not be terminated with a fiber optic connector. If the device is available with a receptacle or connector, the type of receptacle or connector needs to be specified when ordered. You should also note the type of optical fiber used by the manufacturer of the device to ensure it is compatible with the optical fiber used for your application.

Center Wavelength and Bandwidth

Center Wavelength is the nominal operating wavelength of the passive device.

Bandwidth (or bandpass) is the range of wavelengths over which the manufacturer guarantees the performance of the device. Some manufacturers will list an operating wavelength range instead.

Types of Loss

  • IL is the optical power loss caused by the insertion of a component into the fiber optic system. When working with passive devices, you need to be aware of the IL for the device and the IL for an interconnection. IL as stated by the manufacturer typically takes into account all other losses, including EL and PDL. IL is the most useful parameter when designing a system.
  • EL may or may not be defined by the manufacturer. EL associated with fiber optic couplers, is the amount of light lost in the coupler in excess of the light lost from splitting the signal. In other words, when a coupler splits a signal, the sum of the power at the output ports does not equal the power at the input port; some optical energy is lost in the coupler. EL is the amount of optical energy lost in the coupler. This loss is typically measured at the specified center wavelength for the device.
  • PDL is only a concern for Single-Mode passive devices. It is often the smallest value loss, and it varies as the polarization state of the propagating light wave changes. Manufacturers typically provide a range for PDL or define a not-to-exceed number.
  • RL, short for Return Loss or Reflection Loss, is typically described as this: when a passive device is inserted, some of the optical energy from the source is going to be reflected back toward the source. RL is the negative quotient of the power received divided by the power transmitted.

Tips: IL, EL, PDL, RL are all measured in decibels(dB).

CrossTalk (XT)

XT in an optical device describes the amount of light energy that leaks from one optical conductor to another. XT is not a concern in a device where there is a single input and multiple outputs. However, it is a concern with a device that has multiple inputs and a single output, such as an optical switch. XT is also expressed in dB, where the value defines the difference between the optical power of one conductor and the amount of leakage into another conductor. In an optical switch with a minimum XT of 60 dB, there is a 60 dB difference between the optical power of one conductor and the amount of light that leaked from that conductor into another conductor.

Uniformity

Uniformity is a measure of how evenly optical power is distributed within the device, expressed in dB as well as XT. For example, if a device is splitting an optical signal evenly into four outputs, how much those outputs could vary from one another is defined by uniformity. Uniformity is typically defined over the operating wavelength range for the device.

Power Handing

Power Handling describes the maximum optical power at which the device can operate while meeting all the performance specifications defined by the manufacturer. Power handling may be defined in mW(milliwatt) or dB, where 0 dBm is equal to 1 mW.

Operating Temperature

Operating Temperature describes the range of temperatures that the device is designed to operate in. This can vary significantly between devices, because some devices are only intended for indoor applications while others may be used outdoors or in other harsh environments.

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How Much Do You Know About OADM

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The OADM, or optical add drop multiplexers, is a gateway into and out of a single mode fiber. In practice, most signals pass through the device, but some would be “dropped” by splitting them from the line. Signals originating at that point can be “added” into the line and directed to another destination. An OADM may be considered to be a specific type of optical cross-connect, widely used in wavelength division multiplexing systems for multiplexing and routing fiber optic signals. They selectively add and drop individual or sets of wavelength channels from a dense wavelength division multiplexing (DWDM) multi-channel stream. OADMs are used to cost effectively access part of the bandwidth in the optical domain being passed through the in-line amplifiers with the minimum amount of electronics.

OADMs have passive and active modes depending on the wavelength. In passive OADM, the add and drop wavelengths are fixed beforehand while in dynamic mode, OADM can be set to any wavelength after installation. Passive OADM uses fiber optic filters, fiber gratings, and planar waveguides in networks with WDM systems. Dynamic OADM can select any wavelength by provisioning on demand without changing its physical configuration. It is also less expensive and more flexible than passive OADM. Dynamic OADM is separated into two generations.

A typical OADM consists of three stages: an optical demultiplexer, an optical multiplexer, and between them a method of reconfiguring the paths between the optical demultiplexer, the optical multiplexer and a set of ports for adding and dropping signals. The optical demultiplexer separates wavelengths in an input fiber onto ports. The reconfiguration can be achieved by a cross connect patch panel or by optical switches which direct the wavelengths to the optical multiplexer or to drop ports. The optical multiplexer multiplexes the wavelength channels that are to continue on from demultipexer ports with those from the add ports, onto a single output fiber.

Physically, there are several ways to realize an OADM. There are a variety of demultiplexer and multiplexer technologies including thin film filters, fiber Bragg gratings with optical circulators, free space grating devices and integrated planar arrayed waveguide gratings. The switching or reconfiguration functions range from the manual fiber patch panel to a variety of switching technologies including microelectromechanical systems (MEMS), liquid crystal and thermo-optic switches in planar waveguide circuits.

CWDM and DWDM OADM provide data access for intermediate network devices along a shared optical media network path. Regardless of the network topology, OADM access points allow design flexibility to communicate to locations along the fiber path. CWDM OADM provides the ability to add or drop a single wavelength or multi-wavelengths from a fully multiplexed optical signal. This permits intermediate locations between remote sites to access the common, point-to-point fiber message linking them. Wavelengths not dropped, pass-through the OADM and keep on in the direction of the remote site. Additional selected wavelengths can be added or dropped by successive OADMS as needed.

FiberStore provides a wide selection of specialized OADMs for WDM system. Custom WDM solutions are also available for applications beyond the current product designs including mixed combinations of CWDM and DWDM.