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Optical filter basics

Optical filter is basically a two port device. It basically passes signal of desired wavelength and rejects all the other wavelengths as designed. Few of the filters are provided with third port to take out filtered signals. The same is depicted in the figure-1.

Optical filter

Optical filters are used in WDM for multiplexing and de-multiplexing. They are also used in optical amplifier to equalize the gain response and to filter the noise components.

Optical filter types:

There are three main types of optical filter based on the wavelengths they pass through viz. long pass, short pass and band pass. Long pass type transmits wavelengths above a certain cut off wavelength. Short pass type transmits wavelengths below a certain cut off wavelength and band pass type transmits wavelengths in a narrow band.

Other optical filter types include Dichroic, Dielectric, reflective, absorptive filters.

Optical Filter Specifications

Following table provides performance obtained by using different filtering technologies.

Property of the filter Fiber Bragg Grating AWG (arrayed waveguide grating) TFMF(thin-film resonant multicavity filter)
1dB bandwidth(nm) 0.3 0.22 0.4
Isolation (dB) 25 25 25
Loss(dB) 0.2 5.5 7
PDL (dB) 0 0.5 0.2
Temperature coeff. (nm/degreeC) 0.01 0.01 0.0005

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