Page 174 - An Introduction to Microelectromechanical Systems Engineering

Fiber-Optic Communication Devices 153

The wavelength locker from Digital Optics Corporation miniaturizes the ele-
ments of a standard locker to result in a device that measures 5.5 × 5.5 × 4.0 mm 3
(see Figure 5.13). The miniature beam splitter is a diffractive optical element con-
sisting of a multiple-phase grating [29, 30]. Relief structures etched in an optically
transparent substrate, such as fused silica or silicon in the infrared portion of the
spectrum, form regions of varying phases and thus diffract the incident light in a
specific pattern or orientation. The efficiency of the grating (the intensity that is
available in the higher order diffraction orders) is tailored by adjusting the planar
topography and phase of the relief structures. A thorough analysis typically involves
using a fast Fourier transform of the output light intensity to calculate the phase
pattern of the diffractive element. In this particular case, a high-efficiency phase
grating divides the incident light beam into two higher order beams noted as +1 and
–1 orders. A second set of similar diffractive optics corrects the path of the two
diffractive beams (and possibly collimates them) for normal incidence on the
detecting photodiodes.
The etalon is a cube of fused silica (n = 1.444) whose polished facets have been
coated with a partially reflective dielectric mirror. For the wavelength range of 1,550
to 1,650 nm, the dielectric mirror typically consists of alternating layer pairs, each
one quarter of a wavelength in thickness, of Al O and SiO ,orTiO and SiO . The
2 3 2 2 2
optical thickness of the glass cube determines the free spectral range. For the locker
to work properly on the ITU grid, the FSR must equal the ITU channel separation of
50 GHz ± 0.01 GHz. This means that the smallest thickness of the etalon must equal
to approximately 2.076 mm. The fine calibration of the FSR to the 50-GHz ITU
spacing relies on the dependence of the index of refraction, n, on temperature to
adjust the optical length of the etalon. For fused silica, the temperature coefficient of

Figure 5.13 A schematic illustration of the wavelength locker from Digital Optics Corporation. A
first diffractive element divides the light from the laser into two beams. The first one travels
through a calibrated etalon and is detected by a photodetector. The second is detected directly by
another photodetector and serves as an intensity reference.