ONT (or ONU)
The Optical Network Terminal (or Optical Network Unit) resides at the subscriber’s end of the PON. It provides the interface between the network and the subscriber’s equipment. Frequently the laser used at the ONT is a Fabry Perot type and operates at 1310nm and only transmits when given permission by the OLT.

The Optical Line Terminal is the PON controller card or unit it is located at the central office. The terminal at the subscriber’s end of the network is the ONT or optical network terminal. Several OLTs may be located in a single chassis. The laser at the OLT is frequently a DFB (distributed-feedback laser) transmitting at 1490nm or 1550nm and is always on. Signals from the OLT tell the ONTs when to send upstream traffic to it.

Optical attenuator
The optical attenuator reduces the intensity of lightwaves, usually so that the power is within the capacity of the detector. There are three basic forms of attenuators, all of which fit the definition provided above under attenuator: the fixed optical attenuator, a stepwise variable optical attenuator, and the continuous variable optical attenuator. Attenuation is normally achieved either by a doped fibre or an offset or core misalignment.

Optical channel
A wavelength band, e.g. for WDM (wavelength division multiplexers) optical communications.

Optical channel spacing
Parameter used with CWDM and DWDM. The wavelength separation between adjacent WDM channels.

Optical channel width
The optical wavelength range of a channel of a WDM.

Optical Polarization
The term used to describe the orientation in space of time varying field vector of an optical signal.

Optical power budget
The amount of optical power required in a fibre optic link to overcome losses and loss margins.

Optical spectrum (or visible wavelength range)
Begins at 400nm and extends to 700nm. The earlier is the ultraviolet, the later the far infrared.

Optical Time Domain Reflectometer is a device that measures the distance to a reflectant surface by measuring the time it takes a generated lightwave pulse to leave and return. By taking different measurements, for instance reflectance from an end, backscattering etc. it can be used to predict the location and nature of a discontinuity, crack, scratch, open connector, break etc.

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