Optical filters fill many roles

Duwayne Anderson

A key technology for controlling light in WDM systems is the optical filter, which performs functions from simple filtering in multiplexers and demultiplexers, to more sophisticated functions in optical amplifiers, modulators, and test equipment.

Mention the words "optical filters," and different things are likely to come to mind. You might recall the colors of a stained-glass window, or the brilliant hues of butterfly wings. Perhaps you envision the spectrum of a rainbow after a summer thunderstorm. Stained glass is an example of an optical filter that transmits selective wavelengths while absorbing all others, while some butterfly wings are examples of nanostructures that form photonic bandgaps.1

Rainbows are examples of a type of filter in which the spectrum of light is dispersed, or spread out. Ideally these filters don't throw away any light. Instead, they use multipath interference and/or material dispersion to direct light to different points in space, depending on the light's wavelength. This type of filter—one that spatially redistributes light based on its wavelength—is particularly useful in telecommunications systems.

Perhaps the most common example of a dispersive filter is the grating (see Fig. 1a). A typical reflective grating consists of a mirrored surface with tiny grooves. When illuminated, the light reflected from one groove interferes with the light reflected from other grooves, and at certain places in space these multiple reflections interfere constructively (they add in phase), while in other regions they interfere destructively (they add out of phase).

For some angles all the reflections add in phase, and the optical field is strong. For other angles the reflections add out of phase, and they cancel. The net result is that the angle at which the light reflects off the grating is wavelength dependent, making it possible to separate (filter) different wavelengths of light using a detector or fiber array at the focal plane of a lens.2

Another common filter is the Fabry-Perot interferometer (see Fig. 1b). This filter consists of two highly reflective mirrors, separated by a small distance. Most of the light encountering the first mirror reflects, but some of it transmits, travels through cavity (the space between the mirrors), and strikes the second mirror. At the second mirror most of the light reflects, while some transmits. The reflected light travels backwards, hitting the first mirror, where some of it again reflects and some transmits; an infinite number of increasingly dimmer reflections and transmissions result.

Depending on the spacing and index of refraction between the mirrors, at some wavelengths the multiple reflections all cancel, and all the transmitted waves reinforce. At these wavelengths the overall transmission is high, in spite of the fact that the individual mirrors are highly reflective. At these wavelengths we say that the cavity "resonates," so that light passes through in seeming violation of the fact that both mirrors are individually highly reflective. For other wavelengths the transmitted waves add out of phase and the reflected waves add in phase. At these wavelengths the interferometer's overall transmission is low, and the overall reflectivity is high.

Closely related to the Fabry-Perot interferometer is the fiber Bragg grating (FBG; see Fig. 1c). Fiber Bragg gratings consist of a region in which the index of the fiber varies periodically between high and low, and they are formed in optical fibers by exposing the fiber to interferometric patterns from a UV laser. As in the Fabry-Perot interferometer, multiple reflected and transmitted waves result. For a specific wavelength the reflected waves all add in phase, and at this wavelength the grating appears to be highly reflective, while transmitting all the others.

BENEFICIAL FILTERS
One of the most useful filters in optical telecommunications is the arrayed waveguide grating (AWG, see Fig. 1d). As its name implies, the AWG uses an array of optical waveguides in which the lengths of adjacent waveguides differ by a fixed amount. The input light from a single fiber illuminates all these waveguides, and because the waveguides are of different lengths, the phase of the light (at the output end of the array of waveguides) varies by a fixed amount, from one waveguide to the next. This variation results in a wavelength-dependent diffraction pattern that is similar to the one from a plane grating (thus the name "arrayed waveguide grating"). This diffraction pattern is then arranged so that different wavelengths illuminate different output fibers. So the AWG serves as a wavelength demultiplexer by taking multicolored light from a single fiber and sending different colors to different output fibers (it can also work in reverse as a wavelength multiplexer).3

Another useful filter is the Mach-Zehnder interferometer (see Fig. 1e), which consists of a pair of couplers connected by two paths of unequal length. The unequal length results in some wavelengths being output to the top port, and other wavelengths being output to the bottom port. This filter finds applications in interleaving the signals from two fibers, each of which carries information that has already been less finely multiplexed in the wavelength domain.

WDM FILTER USES
Let's look at how some of these optical filters might be used in a wavelength-multiplexed telecommunications system (see Fig. 2). First, time-domain data from different channels is amplitude modulated onto lasers (not shown) of different wavelengths, and the light from these lasers is multiplexed onto a single optical fiber, using a multiplexing filter. An erbium-doped fiber amplifier (EDFA) boosts the multiplexed optical signal, and the EDFA itself makes use of several optical filters to couple light from the pump lasers onto the erbium-doped fiber.

An FBG and circulator are used to drop one of the multiplexed channels. The FBG is tuned to reflect l5, and the circulator directs the reflected light to a different fiber, while all the other wavelengths continue on the fiber backbone. This hypothetical network also uses a Raman amplifier, which (like the EDFA) uses optical filters to multiplex the pump wavelengths onto the fiber, without dropping the signal wavelengths off.

At the end of the transmission line the optical wavelengths from each channel enter a demultiplexer (an AWG, in this case) that demultiplexes different wavelengths to individual output fibers, where they proceed to their respective receivers.

Optical filters are frequently used in telecommunications to separate different wavelengths. Often, this ability to separate wavelengths leads to the definition of key specifications used to categorize the filter, and classify its usefulness and value in the optical network.

Results from testing an AWG with a swept-laser system can be graphed (see Fig. 3). Sweeping the laser wavelength at a common input fiber, and measuring the transmitted optical power at each output port (using different synchronized, broadband optical detectors) resulted in each colored curve. This test is particularly powerful because of the vast amount of information it provides about the filter's characteristics. Using this sort of test data, you can determine any number of measures that define the filter's performance characteristics, including the center wavelength, 3-dB bandwidth, channel offset for each demultiplexed output port, channel spacing, and crosstalk.4

One of the most important performance characteristics is the filter's transmission loss at the center of its passband. This loss affects the overall system attenuation for that channel, and so it enters into considerations about the number of required amplifiers, the optical signal-to-noise ratio, and the bit-error rate (BER). But it's not enough to simply have high transmission at a single wavelength, because there is always variability in the filter's center wavelength and the center wavelength of the laser transmitter. The variability means the laser transmitter might actually be operating at a wavelength that is slightly off center relative to the peak transmission point of the filter.

To make the system more robust, designers usually try to design optical filters with square response curves. The ideal response is one with a flat top, infinitely steep sides, and complete rejection of light outside the passband. This ideal filter shape is never fully realized (as Fig. 3 illustrates), although some filters have impressive capabilities. Of particular importance is the amount of attenuation that the filter presents to out-of-band wavelengths. If the filter allows even small amounts of optical power from adjacent channels to leak through, it will result in added crosstalk, which will increase the BER.

Other important features include small ripples and other filter artifacts. These may be of relatively little interest to end-users, but the engineers who design the filters can often learn a great deal about the stability of their manufacturing process by observing these artifacts.

REFERENCES

1. H. Chiradella, Optics and Photonics News (March 1999).
2. J. Laude, Wavelength Division Multiplexing, Prentice Hall (1993).
3. K. Okamoto, Fundamentals of Optical Waveguides, Academic Press (2000).
4. C. Narayanan, G. Bogert, Symposium on Optical Fiber Measurements, NIST Special Publication 953 (2000).

Duwayne Anderson is an optical engineer for Tektronix, working with The Light Brigade, 7691 S 180th St., Kent, WA 98032. The Light Brigade develops fiberoptic training material and teaches fiberoptic training courses. For Contact www.lightbrigade.com or 425-251-1240