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Programmable Optical Filter Uses Digital Mirror Array

Photonics Spectra
Oct 2005
Breck Hitz

Investigators at Texas Tech University in Lubbock have demonstrated a programmable optical filter whose underlying technology may be applicable to numerous telecommunications devices, such as variable optical attenuators, dynamic optical add/drop multiplexers and dispersion compensators.

The technology involves spatially dispersing the spectral components of a signal across an array of individually adjustable mirrors so that the components can be reflected in different directions. Depending on the design of the device and how the reflected beams are handled, the various functionalities can be implemented.

The scientists designed the device to be a multiwavelength pass filter. They collimated the light from a fiber onto a grating and recollimated the diffracted light onto the mirror array (Figure 1). The mirrors in the array could reflect the spectral components back toward the original fiber or into a beam stop. A λ/4 plate between the grating and the mirror array minimized polarization effects.

Programmable Optical Filter Uses Digital Mirror Array
Figure 1. The spectrum of light from the fiber was dispersed across the surface of the mirror array so that selected spectral components could be reflected back into the fiber. Images ©IEEE.

The light from the fiber was incident on the 600-lines-per-millimeter grating from Edmund Industrial Optics of Barrington, N.J., at a 35°; angle, and the first-order diffraction occurred at approximately 21°. The mirror array from Texas Instruments Inc. of Plano had ~750,000 individual mirrors, each 13 µm square and on a 14-µm pitch. Each mirror was independently addressable and could be tilted ±12° from its rest position with a switching speed of less than 15 µs.

The scientists calculated that the reciprocal of the spatial wavelength dispersion at the mirror array was approximately 6.49 nm/mm. They programmed the array so that the entire device functioned as a comb filter, transmitting 40 channels within the C-band with a channel spacing of 100 GHz, which corresponds to the standard ITU grid (Figure 2).

Programmable Optical Filter Uses Digital Mirror Array
Figure 2. When the mirror array was programmed appropriately, the comb filter transmitted 40 C-band channels with a channel separation of 100 GHz.
To achieve this, they programmed the mirror array with a repetitive pattern of eight micromirror lines reflecting back toward the grating (“on”), and four reflecting toward the beam dump (“off”). The 3-dB bandwidths of the individual transmission lines were ~0.4 nm, and the adjacent channel crosstalk was better than N35 dB. The measured deviation of the individual channel peak positions from the ITU grid was less than 0.033 nm.

The spacing between transmission channels could be changed by reprogramming the mirror array. A 50-GHz spacing could be obtained by programming repetitive patterns of four micromirror lines “on” and two “off,” or a 200-GHz spacing by programming 16 “on” and eight “off.”

Programmable Optical Filter Uses Digital Mirror Array
Figure 3. The shape of individual transmission channels can be adjusted by changing the number of micromirror lines that reflect that channel. These data show the shape of a channel when the number of micromirror lines is increased from one to 21 in steps of two micromirror lines.

Alternatively, the bandwidth of the individual channels could be adjusted by changing the number of micromirror lines that reflected that channel back toward the grating; that is, the number of “on” micromirror lines. With only one micromirror line “on,” a channel’s bandwidth was ~0.23 nm.

As the scientists increased the number of “on” micromirror lines to 21, they observed the bandwidth of the channel increase to ~1.22 nm (Figure 3). The total insertion loss decreased from ~18 to ~6 dB as the number of “on” micromirror lines increased from one to nine, because the spot size on the mirror array was about 80 µm, corresponding to nine micromirror lines.

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