Gain Flattening Filters

The current explosion of applications of optical fiber based communications is driven by the market demand for higher bandwidth caused in turn by the rapid expansion of the Internet and the increasing demand for the rapid transfer of data. Technically, the use of optical fiber for long range transmission was greatly enhanced with the introduction of the erbium doped fiber amplifier (EDFA). The EDFA allows the optical amplification of signals modulated on a carrier light wave. The EDFA currently operates efficiently over a 1527 to 1563 nm. Advances in EDFA are allowing this window to be extended to 1617 nm.

Within this window, many separate light signals (each at a different wavelength) can be transmitted and periodically amplified to regenerate the signal. However, the gains of current EDFAs are not constant over their full window. This can cause difficulties when a light signal undergoes multiple stages of amplification as adjacent light signals can then vary tremendously in their intensity.

One solution is to couple the amplifier to a second device that has the inverse characteristics of the EDFA and flattens the gain across the band. One class of device is based on thin film gain flattening filter (GFF) technology.

Thin films have the advantage over alternative technologies in that they are passive devices, simple in operation, robust and very insensitive to changes of temperature. Every EDFA amplification system has a different gain characteristic, depending upon both the design and on the fiber itself. Designing a gain flattening thin film filter is a new challenge any time the gain spectrum is significantly changed.

Iridian Spectral Technologies, 1500 Montreal Road, M-50 Building, Ottawa, ON K1A 0R6. Tel: 613-741-4513; Fax: 613-741-9986.