Optical waveguides – Accessories – Attenuator
Reexamination Certificate
1998-09-30
2001-01-23
Lee, John D. (Department: 2874)
Optical waveguides
Accessories
Attenuator
C359S846000, C359S850000
Reexamination Certificate
active
06178284
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a variable attenuator for attenuating an optical signal transmitted between an optical signal source and an optical signal receiver. More specifically, the present invention relates to the reflection of a transmitted optical signal off of divided surfaces for variably attenuating the optical signal.
2. Description of the Related Art
In optical data communications, signals are typically transmitted from a signal source to a signal receiver over an optical fiber network.
FIG. 1
illustrates the general concept of optical signal transmission between an optical signal source
5
and an optical signal receiver
10
, using a high reflectivity (HR) coated surface
15
. For the sake of simplicity the various light beams illustrated in the figures are all shown as arcs to help in distinguishing their direction of travel; this illustration should not be considered as indicating any particular characteristic of the light beams themselves.
Suppose light is introduced into the system through the optical signal source
5
(e.g., a single mode optical fiber). As the light exits the end of the optical signal source
5
it starts to spread out to form the “sending beam”
7
. Sending beam
7
is illustrated as a series of solid arcs moving from the top of FIG.
1
to the bottom of FIG.
1
. Sending beam
7
is collimated by a lens
20
(or other focusing means) and then it falls upon the HR coated surface
15
.
The reflection of the sending beam
7
by the HR coated surface
15
is a “returning beam”
12
that travels to optical signal receiver
10
(e.g., a single mode fiber). The returning beam
12
is illustrated as a series of dotted-line arcs moving from the bottom of
FIG. 1
to the top of FIG.
1
. Returning beam
12
is refocused (by the same lens
20
as used for sending beam
7
or by a different focusing means, such as a separate lens) to be collected by optical signal receiver
10
.
It is well known that if the HR coated surface
15
is a nearly flat, highly reflecting surface, the optical coupling from the optical signal source
5
to the optical signal receiver
10
will be very good, less than 0.5 dB loss in typical implementations using active alignment in manufacture. Further, it is well understood that if the reflecting surface of HR coated surface
15
is translated left or right by a few microns, the optical coupling will be changed negligibly.
Optical signal systems have a signal intensity range in which they function best. If a signal falls below the operational range, the system will either incorrectly detect the signal or will not detect the signal at all. If the signal is above the operational range, the system will saturate and may result in a false reading of the data in the optical signal. Thus, optical signal levels which are too high or too low result in unreliable transmission of data or can interfere with other data-carrying signals.
The path attenuation of a fiber is a function of fiber length and the fiber attenuation coefficient. Further, the sensitivity of the receiver and the emitter output may exhibit changes due to aging. Thus, many optical transmission lines are designed with built-in attenuators which attenuate the optical signals within the waveguide to be within the optimal functional range of the optical system.
There are several known ways of providing attenuation of an optical signal. One method involves the use Faraday rotation in suitable doped Garnet films. By varying the applied magnetic field from an electromagnet, the polarization of transmitted light is changed and by using polarization selective optical elements, the attenuation can be varied. A problem with this attenuation method is that the electromagnet dissipates large amounts of electrical power and is quite large.
Another known method of attenuation involves the use of motorized variable attenuators where, for example, an opaque attenuating wedge is driven into the beam path to block a portion of the optical signal beam. In addition to being bulky, however, this method also is costly and slow-acting.
An additional attenuation method involves the use of liquid crystal designs which can work at very low electrical power levels and which function in a manner similar to Faraday rotation, but with liquid crystal rotation of polarization. Such systems are temperature and polarization sensitive and organic material in the beam path can be chemically unstable, causing shortened device life.
Attenuation using Micro Electro Mechanical Systems (MEMS) technology has been accomplished using a Mechanical AntiReflection Switch (MARS) modulator, an example of which is illustrated in
FIGS. 2 and 3
. These devices operate on the principle that varying the phase between two portions of a light beam allows the attenuation of the optical signal to be controlled, as described in more detail below.
FIG. 2
shows a cross-section of a typical MARS modulator, and
FIG. 3
is a top view of the MARS modulator depicted in
FIG. 2. A
typical MARS modulator
50
has a conductive or semi-conductive based substrate
52
that is transparent to the operating optical band width of the modulator.
A membrane
54
is suspended above the substrate
52
, thereby defining an air gap
56
in between the substrate
52
and the membrane
54
. A membrane
54
is typically fabricated from a silicon nitride film which is a dielectric. A metal film
58
is deposited around the top periphery of the membrane
54
. Since the metal film
58
is optically opaque, only the center
60
of the membrane
54
remains optically active. When an electrostatic potential is applied in between the metal film
58
and the below lying substrate
52
, the metal film
58
becomes charged and is deflected by electrostatic forces toward the substrate
52
. The result is that the membrane
58
deflects dowardwardly in the direction of arrows
59
and the size of the air gap
56
is reduced. By applying a potential difference of about 40 volts to electrical connections coupled to the membrane
54
and the substrate
52
, large electric fields are developed between the substrate
52
and metal film
58
causing an electrostatic force between the membrane
54
and the underlying silicon large enough to bow the membrane
54
closer to the underlying silicon. By increasing the applied voltage, the cavity width is decreased. By varying the cavity width, the relative phase between light reflected by the membrane
54
and light reflected by the underlying substrate
52
is also varied, thereby allowing control of the attenuation.
In order to assemble the device and in order to equalize the gas pressure on each side of the membrane
54
, and allow quick response time, it is necessary to perforate the membrane
54
with very small holes. In
FIG. 3
the perforation of the membrane
54
with very small holes
62
is depicted. The membrane
54
has a natural mechanical resonance; the resonance is damped by the gas viscosity passing through the holes
62
. The inclusion of the holes
62
in the membrane
54
results in an optical loss, but the size and number of the holes
62
is selected to minimize this optical loss to a negligible level. Typically such holes
62
are approximately 3-5 &mgr;M in diameter and are provided merely to minimize vibration, i.e., they do not provide any optical functions.
FIG. 4
is a partial cross-sectional view of the prior art MARS modulator of FIG.
2
. Light traveling from top to bottom, identified as
64
in
FIG. 4
, will be partially reflected by the membrane
54
and partially transmitted beyond the membrane
54
. The partially reflected light is identified as
66
in FIG.
4
. The light transmitted beyond the membrane
54
is reflected by the floor of the cavity; this reflected light is identified as
68
in FIG.
4
. Depending upon the cavity width and the wavelength of light used, the reflections will interfere constructively or destructively when they are received by an optical receiver (not shown). Constructive interference occurs when
Bergmann Ernest E.
Ford Joseph E.
Walker James A.
Lee John D.
Lucent Technologies - Inc.
Synnestvedt & Lechner LLP
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