Page 183 - An Introduction to Microelectromechanical Systems Engineering

162 MEM Structures and Systems in Photonic Applications

to shift energy away (and thus attenuate) from the main undiffracted beam into
higher order beams (see Figure 5.18), attenuating the incident beam (attenuation is
equivalent to creating a continuum of gray shades). The closely spaced suspended
reflective ribbons used for the GLV form the elements of an adjustable-phase grat-
ing. When the ribbons are coplanar, incident light is reflected back into the aperture
without attenuation. When alternating ribbons are pulled down using electrostatic
actuation by one quarter of a wavelength (λ/4) relative to their adjacent ribbons, the
incident energy diffracts into higher orders that are directed outside the aperture,
and the incident beam is completely attenuated. When the separation is less than λ/4,
the incident beam is partially attenuated, as some energy is shifted into the higher
diffracted orders.
While the VOA derives its basic principle of operation from the GLV, it must
also address a number of specifications that are particular to fiber-optical telecom-
munications. The first one relates to the chromatic dependence of the diffraction
grating. Displays have to manipulate only three basic colors: red, green, and blue.
But VOAs must manipulate a nearly continuous spectrum of wavelengths from
1,528 nm to 1,610 nm without a chromatic dependence. The second specification is
polarization-dependent loss. A difference in attenuation between the two polariza-
tions that is larger than 0.5 dB greatly increases the risk of data errors during trans-
mission. The design from Lightconnect adapts the GLV diffractive technology with
two key modifications to applications in fiber-optical telecommunications.
In order to understand the basic operation of the achromatic design, one needs
to refer to the use of phasors for time-varying electric fields [43]. In the case of the
GLV, two phasors—one for each of the fixed and moveable ribbons—affect the

Aperture Zeroth order

Higher
orders
λ
</4 λ/4

Undeflected Partial deflection Full deflection

Intensity
Zeroth order First order

Diffraction
angle
No attenuation Partial attenuation Full attenuation
Figure 5.18 An illustration of the basic principle of operation of the variable optical attenuator
from Lightconnect, Inc. A set of suspended ribbons act as an adjustable grating. When alternating
ribbons are pulled down by λ/4, the structure becomes a phase grating and diverts the incident
energy into higher diffraction orders, thus providing full attenuation of the incident beam. When
all of the ribbons are coplanar or separated by a half wavelength, the surface acts as a reflector.
When the separation between adjacent ribbons is less than λ/4, there is light in all orders and the
incident beam is only partially attenuated.