Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-02-22
2002-12-31
Mack, Ricky (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S291000
Reexamination Certificate
active
06501589
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to opto-electronic devices, and more particularly, to an apparatus for absorbing stray light that is generated in or received by opto-electronic devices.
BACKGROUND OF THE INVENTION
The performance of light-emitting and light-receiving opto-electronic devices is compromised by stray light. Simply stated, stray light is light that, from the perspective of opto-electronic device performance, is in the wrong place at the wrong time. In some cases (e.g., lasers, etc.), stray light is generated by the opto-electronic device itself, in others (e.g., optical modulators, etc.), stray light originates from an external source. In either case, stray light causes problems, as the following examples illustrate.
FIG. 1
depicts an illustration of opto-electronic device
100
, which comprises vertical cavity surface emitting laser (“VCSEL”)
102
and photodetector
104
that are co-located on first major surface
108
of substrate
106
. The substrate comprises electronic driver circuitry (not depicted) for energizing VCSEL
102
. Opto-electronic device
100
also incorporates a heat sink (not depicted) that removes the heat that is generated by the electronic driver circuitry and VCSEL
102
. Metal backing
112
, which is disposed on second major surface
110
of substrate
106
, is used to attach the heat sink to substrate
106
.
VCSEL
102
is configured to emit output light
114
A away from semiconductor circuitry chip
106
in direction
116
. But as a result of design and manufacturing compromises, a portion of the output light from VCSEL
102
, ray of stray light
114
B, is typically emitted toward substrate
106
in direction
118
.
The substrate of an opto-electronic device is often transparent to light. For example, substrate
106
is transparent to light having a wavelength of 1.3 microns, an important telecommunications wavelength, when the substrate is made of silicon. In such a case, stray light
114
B that is emitted in direction
118
passes through substrate
106
, reflects off of metal backing
112
(e.g., at location
120
) and is redirected toward first major surface
108
.
At first major surface
108
, stray light
114
B might be received by parts of opto-electronic device
100
that are light sensitive, such as photodetector
104
. If stray light
114
B is received and absorbed by photodetector
104
, cross-talk between input signal
122
and output signal
114
A occurs.
FIG. 2
depicts an illustration of opto-electronic device
200
. This opto-electronic device comprises micro-mechanical optical modulator
224
, the design and operation of which are well-known in the art. The modulator includes membrane
226
that is supported by supports
228
over first major surface
108
of substrate
106
. Cavity
230
is formed in the region between membrane
226
and first major surface
108
. In micro-mechanical optical modulator
224
, substrate
106
is not an active device.
When actuated, such as by an applied voltage, membrane
226
moves toward substrate
230
. As membrane
226
moves, the size of cavity
234
(i.e., the distance or gap between the membrane and first major surface
108
) changes. This change alters the reflectivity of modulator
224
and, as such, modulator
224
is capable of modulating reflected light. See, for example, U.S. Pat. No. 5,500,761.
In many of the applications for modulator
224
, substrate
106
is transparent to light. For example, substrate
106
is transparent to light having a wavelength of 1.55 microns, another important telecommunications wavelength, when the substrate is made of silicon. To prevent stray light from passing out of substrate
106
and into, for example, an output port (not depicted), metal-backing
112
is advantageously disposed on second major surface
110
of substrate
106
. Consequently, when modulator
224
is in a low-reflectivity state, most of light
232
A that is received by modulator
224
passes through substrate
106
and is reflected off metal backing
112
(e.g., at location
234
). Reflected (ie., stray) light
232
B adds to the overall reflected signal thereby degrading the contrast (i.e., the ratio of maximum reflectivity to minimum reflectivity) of modulator
224
.
The problems caused by stray light in two different types of opto-electronic devices have been discussed above. And it will be understood that stray light causes similar problems in other types of opto-electronic devices as well. Therefore, incorporating a means to capture stray light in opto-electronic devices would improve such devices and, more generally, benefit this art.
SUMMARY OF THE INVENTION
In accordance with the present invention, the performance of opto-electronic devices is improved by a metallic anti-mirror. The metallic anti-mirror, which is disposed on a substrate of an opto-electronic device, substantially absorbs stray light that is generated by or received by the opto-electronic device.
In accordance with the illustrative embodiment of the present invention, metallic anti-reflection mirror comprises a first metal layer that is disposed on the substrate of an opto-electronic device, a dielectric layer that is disposed on the first metal layer, and a second metal layer that is disposed on the dielectric layer. This arrangement of layers, when of suitable thickness, creates a cavity that enhances the optical field in second metal layer. While the metal layers can comprise virtually any metal, those that adhere well to the substrate and dielectric layer (e.g., aluminum, chromium, etc.) are advantageously used.
Specific values of the thickness of the first metal layer and the dielectric layer will produce a metallic anti-reflection mirror that completely absorbs light (i.e., has zero reflectivity). These values are dependent on the wavelength of the light and the composition of the materials comprising the various layers. Deviations in thickness will result in an increase in the reflectivity of the anti-reflection mirror. Typically, the first metal layer has a thickness that is in a range between about 100 angstroms and about 400 angstroms and the dielectric layer has a thickness that is in a range between about 750 angstroms to about 4500 angstroms.
In one variation of a metallic anti-reflection mirror in accordance with the illustrative embodiment of the present invention, the second metal layer is partitioned into a first sub-layer and a second sub-layer. The first sub-layer is disposed on the dielectric layer,.and the second sub-layer is disposed on the first sub-layer. The second sub-layer comprises a metal, such as gold or aluminum, that advantageously protects the first sub-layer from oxidation.
REFERENCES:
patent: 4782477 (1988-11-01), Ichihara
patent: 5241520 (1993-08-01), Ohta et al.
patent: 5452282 (1995-09-01), Abraham
patent: 5665520 (1997-09-01), Yoshioka
patent: 5843626 (1998-12-01), Ohta et al.
patent: 5972461 (1999-10-01), Sandstorm
patent: 6128274 (2000-10-01), Mori et al.
Aralight
DeMont & Breyer LLC
Hindi Omar
Mack Ricky
LandOfFree
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