Fiber Optic Depolarizers Information

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Selecting fiber optic depolarizers

 

Fiber optic depolarizers are devices that randomize the polarization of an input light beam. Optical fibers often carry the highly polarized light created by lasers or laser diodes. However many instruments and sensors are sensitive to a polarized light source. An important application of fiber optic depolarizers includes eliminating the effects of polarization sensitivity of fiber sensors, Raman amplifiers, and other optical instruments. They are also useful for reducing the polarization dependent loss of optical components.

 

Operation

 
Fiber optic depolarizers function by splitting an input light beam into multiple beams with a differential time delay and then recombining the beams to produce an output beam with pseudo-random polarization. The fixed phase relationship between the split beams is eliminated when the delay between them is larger than the coherence length of the source beam.

 

Types

 
Fiber optic depolarizers can be either active or passive devices. Active depolarizers often use piezoelectric elements to modulate the length of a birefringent fiber, altering the state of polarization. A commonly used type of depolarizer for use with fiber optic applications is a passive Lyot type. A Lyot depolarizer has two plates, the second twice the thickness of the first, with optic axes aligned at 45° to each other.

 

Specifications

 
The following specifications determine the functionality of fiber optic depolarizers:

 

  • Center wavelength is the nominal wavelength the depolarizer is designed to operate at.
  • Output degree of polarization describes the magnitude of polarization of the output beam.
  • Coherence length of light source refers to the distance within which the beam is considered to have well defined amplitude and phase.
  • Insertion loss is the loss of optical power that results from inserting a device along an optical fiber.
  • Return loss is the ratio of reflected power to incident power; it is a measure of the amount of reflected power on a transmission line when it is terminated or connected to any passive or active device.
  • Residual extinction ratio is the ratio of the optical power of a beam transmitted through a device parallel to the polarizing axis compared to the power of a beam travelling perpendicular to the polarizing axis.
  • Maximum optical power is the highest output power the depolarizer can handle.
  • Operating temperature defines the ambient temperature the device is designed to operate at.

 
Resources

 
Phoenix Photonics - Technical Brief: Depolarizers [pdf]

 
Newport Corporation - Fiber Optic Depolarizers

 
Image credit:

Newport Corporation