Page 172 - An Introduction to Microelectromechanical Systems Engineering

Fiber-Optic Communication Devices 151

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reflective element is a layer of gold measuring 1.4 × 1.5 mm . The large but finite
reflectivity (~ 98%) of gold results in some coupling losses measuring about 0.2 dB.
Four gold electrodes on a glass substrate directly underneath the mirror actuate the
mirror and cause rotation. The dimensions of the torsional flexures and electrode-
mirror gap are such that full angular rotation is obtained at about 120V. The overall
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die measures 3 × 3mm .
Santur Corporation has disclosed sufficient details to recreate a fabrication
process. One starts with a SOI wafer that is approximately 155 µm thick composed
of a crystalline top silicon layer (15 µm thick) on top of a 0.25-µm thick silicon
oxide layer over a silicon handle substrate. A gold layer (20 to 50 nm thick) is depos-
ited either using evaporation or sputtering. Because gold deposited in either method
tends to be under stress, it is desirable for the SOI silicon layer to be as thick as possi-
ble. The torsion flexures and mirrors are then lithographically delineated and etched
into the SOI silicon layer using standard dry etch methods. The front side is pro-
tected, and the bulk of the wafer is etched from the back side using KOH or TMAH.
The etch stops at the intermediate silicon oxide layer. An etch of the oxide layer then
releases the suspended mirror structure. A flash sputter deposition or evaporation of
gold on the back side of the mirror can greatly alleviate any bowing of the mirror
due to stresses from the gold film on the front side by adding a balanced stress on the
back side. Finally, the silicon wafer is anodically bonded to a glass substrate upon
which gold electrodes were previously deposited and lithographically delineated.

Wavelength Locker

The function of a wavelength locker is to measure the difference between the actual
and desired wavelengths of a laser and provide an error signal that can be used by
the laser’s electronic control loop to correct for the deviation. This function is key to
all lasers used in fiber-optic telecommunication in order to “lock” the laser output
to an assigned wavelength on the ITU grid and to offset drift due to aging and envi-
ronmental conditions. Wavelength lockers are available as stand-alone products
external to the laser or can be integrated within the laser resonant cavity. This sec-
tion selects for description a micromachined wavelength locker from Digital Optics
Corporation of Charlotte, North Carolina, which operates in the wavelength range
from 1,525 to 1,625 nm.
A wavelength locker is a special implementation of a one-dimensional planar
resonant cavity known as a Fabry-Perot etalon [27]. For an etalon of physical length
d and index of refraction n bounded by two partially reflective surfaces of reflectiv-
ity r, the frequency spacing between adjacent longitudinal modes is known as the
free spectral range (FSR) and equals c/2d under normal incidence. The transmis-
sion transfer function of the etalon, therefore, consists of periodic peaks whose
sharpness is a function of the finesse, a measure of the loss in the resonator’s mir-
rors. A low mirror reflectivity results in a low finesse and broad transmission peaks
[see Figure 5.12(a)]. The utility of the etalon is that once its transmission peaks are
calibrated, it forms a wavelength “ruler” against which an incident beam of
unknown wavelength can be measured. The calibration process entails fixing the
frequency position of the transmission peak relative to the ITU grid and ensuring
that the FSR matches the periodicity of the ITU grid, typically 50 GHz. An etalon is
a simple structure, but to obtain a precise and accurate measurement of the