Page 169 - An Introduction to Microelectromechanical Systems Engineering

148 MEM Structures and Systems in Photonic Applications

makes the design of closed-loop electronic circuits rather complex. Instead, it is
desirable to design a mechanical system whose force (and hence angular displace-
ment) is linear with applied voltage. A close examination of the actuator reveals that
the length of the individual comb teeth varies, becoming shorter towards the outer
periphery of the rotary actuator. As the two comb actuators in the push-pull con-
figuration are driven differentially and rotate in opposite directions, additional teeth
engage in one actuator and disengage in the other [18]. The rate at which the total
number of engaged teeth changes with angle of rotation (and applied voltage) is
determined by the geometry and layout of the comb teeth. If the total number of
engaged teeth is inversely proportional to the square of the voltage, then the nonlin-
ear dependence is eliminated. In practice, this dynamic tailoring of the number of
engaged comb teeth with angle greatly reduces the overall nonlinear dependence but
does not eliminate it. Experimental analysis shows that the behavior is generally lin-
ear with high-order ripples [20]. For the particular design used by Iolon, a differen-
tial voltage drive of 150V results in an angular rotation of ±2.5º.
Once packaged in a standard 18-pin butterfly package (see Chapter 8) with all
of the components optically aligned, the product meets all of the requirements of a
tunable laser for long-distance transmission. The power is 13 ± 0.1 dBm from 1,529
to 1,561 nm; the RIN measures –145 dB/Hz from 10 MHz to 22 GHz; the SMSR is
55 dB; and the spectral linewidth, typical of external cavity lasers with long cavities,
is very narrow, measuring 2 MHz [21]. The tuning speed of the laser is only limited
by the actuator’s mechanical response time and the bandwidth of the closed-loop
servo. A maximum tuning speed of approximately 10 ms has been reported.

The DFB Tunable Laser from Santur Corporation
The laser design used by Santur [11] bears no resemblance to the previous design,
other than achieving a similar performance. It is based on a family of integrated
semiconductor lasers called distributed-feedback (DFB) lasers [22]. These lasers are
ubiquitous as transmission sources in fiber-optic telecommunications owing to their
excellent spectral performance and proven reliability. They have been manufactured
in volume for many years and thus are cost effective. They provide a stable output
power, typically between 10 and 50 mW, and are frequency stabilized by a
Bragg grating guaranteeing no wavelength drift. It is common to obtain in a
communication-grade DFB a RIN of better than –145 dB/Hz from 50 kHz to 2.5
GHz, a SMSR higher than 45 dB, and a spectral linewidth narrower than 2 MHz
(e.g., [23]).
The details of the DFB laser are beyond the scope of this book and can be found
in [24]. In summary, the basic structure consists of a gain medium made of multiple
quantum wells in an InGaAsP/InP semiconductor crystal (see Figure 5.10). Light is
confined within the crystal to a waveguide that is made by the difference in index of
refraction between InP and InGaAsP. A periodic Bragg grating delineated immedi-
ately above the waveguide provides a wavelength filter as well as a resonant cavity.
The Bragg grating reflects light continuously over its entire length, thus behaving as
a distributed reflector and resulting in a distributed resonant cavity—hence the
name distributed feedback. This coupled role of the Bragg grating makes a full
analysis numerically complex and intensive. The grating shape, periodicity, and
index of refraction determine the center wavelength of the filter, as well as its