Title: Fabry-Perot interferometer including membrane supported reflector
Abstract: An interferometer comprises a membrane, a substrate, and a support structure. The membrane comprises a first reflector. The substrate comprises a second reflector. The support structure circumferentially couples the membrane to the substrate and orients the first reflector parallel to and facing the second reflector.
Patent Number: 6,958,818 Issued on 10/25/2005 to Payne
| Inventors:
|
Payne; Alexander (Ben Lomond, CA)
|
| Assignee:
|
Silicon Light Machines Corporation (Sunnyvale, CA)
|
| Appl. No.:
|
323560 |
| Filed:
|
December 18, 2002 |
| Current U.S. Class: |
356/519 |
| Intern'l Class: |
G01B 009/02 |
| Field of Search: |
356/454,480,519
372/32
359/577,578,585,586,589
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Toatley, Jr.; Gregory J.
Assistant Examiner: Lyons; Michael A.
Attorney, Agent or Firm: Okamoto & Benedicto LLP
Claims
1. An interferometer comprising:
a) a membrane comprising a first reflector;
b) a substrate comprising a second reflector; and
c) a support structure circumferentially coupling the membrane to the substrate
and orienting the first reflector of the membrane parallel to and facing the second
reflector of the substrate,
wherein the support structure maintains the membrane in bi-axial tension using
first and second couplings to the substrate, the second coupling being further
distant from the first reflector than the first coupling.
2. The interferometer of claim 1 wherein the first coupling comprises a support
mechanism arranged around the first reflector, and the second coupling comprises
a surrounding connection to the substrate outside of the support mechanism.
3. The interferometer of claim 2 wherein the membrane comprises a resilient material.
4. The interferometer of claim 3 wherein the support mechanism comprises a plurality
of posts arranged in a proximately circular pattern around the first reflector.
5. The interferometer of claim 4 wherein the surrounding connection comprises
a continuation of the membrane which couples to the substrate outside of the plurality
of posts.
6. The interferometer of claim 3 wherein the support structure comprises a support layer.
7. The interferometer of claim 3 wherein the first reflector comprises a first
multilayer reflector.
8. The interferometer of claim 7 wherein the second reflector comprises a second
multilayer reflector.
9. The interferometer of claim 8 wherein the substrate further comprises a transparent
bulk material.
10. The interferometer of claim 9 wherein a through optical path comprises a
transparent bulk material.
11. The interferometer of claim 10 wherein the trough optical path further comprises
an anti-reflective coating coupled to a surface of the substrate opposite to the
second multilayer reflector.
12. The interferometer of claim 8 wherein the resilient material transmits light.
13. The interferometer of claim 12 wherein the resilient material encapsulates
the first multilayer reflector.
14. The interferometer of claim 13 wherein the first multilayer reflector comprises
a first multilayer stack of silicon and silicon dioxide, and further wherein the
resilient material comprises silicon nitride.
15. The interferometer of claim 14 wherein the second multilayer reflector comprises
a second multilayer stack of silicon and silicon dioxide.
16. The interferometer of claim 15 wherein at least a first silicon layer of
the first multilayer stack of silicon and silicon dioxide includes sufficient doping
to provide electrical conductivity, thereby forming a first conductive layer, and
further wherein at least one silicon layer of the second multilayer stack of silicon
and silicon dioxide includes sufficient doping to provide electrical conductivity,
thereby forming a second conductive layer.
17. The interferometer of claim 16 wherein a space between the first and second
multilayer stacks of silicon and silicon dioxide forms an interferometric cavity
and further wherein an electrical bias applied between the first and second conductive
layers adjusts a cavity length of the interferometric cavity.
18. The interferometer of claim 17 wherein the membrane further comprises access
holes, the access holes providing access to the interferometric cavity for an etchant
during fabrication.
19. An interferometer comprising:
a) a membrane maintained in biaxial tension comprising an annular member and
a first multilayer stack of silicon and silicon dioxide encapsulated by first and
second silicon nitride layers;
b) a substrate comprising a silicon bulk material and a second multilayer stack
of silicon and silicon dioxide; and
c) a support structure coupling the membrane to the substrate and orienting the
first multilayer stack of silicon and silicon dioxide parallel to and facing the
second multilayer stack of silicon and silicon dioxide.
20. An interferometer comprising:
a) first means for reflecting maintained in biaxial tension;
b) second means for reflecting coupled to the first means for reflecting, the
first means for reflecting oriented parallel to and facing the second means for
reflecting; and
c) means for electrically biasing the first means for reflecting relative to
the second means for reflecting.
Description
FIELD OF THE INVENTION
This invention relates to the field of Fabry-Perot interferometers. More particularly,
this invention relates to the field of Fabry-Perot interferometers having a micro-electro-mechanical structure.
BACKGROUND OF THE INVENTION
Charles Fabry and Alfred Perot invented the Fabry-Perot interferometer in
the late 1800's. The Fabry-Perot interferometer includes two glass plates that
have been lightly silvered on facing surfaces. The glass plates are arranged parallel
to each other so that the lightly silvered surfaces produce an interference cavity
defined by a separation distance between the glass plates. If the separation distance
is fixed, the Fabry-Perot interferometer is referred to as a Fabry-Perot etalon.
In either the Fabry-Perot interferometer or the Fabry-Perot etalon, the interference
cavity causes multiple beam interference. The multiple beam interference occurs
when first and second partially reflecting surfaces are oriented parallel to each
other and illuminated by light. Provided that reflection coefficients for the first
and second partially reflecting surfaces are not small, the light reflects between
the two partially reflecting surfaces multiple times. This produces a transmitted
multiple beam interference for the light exiting the second surface in a forward
direction and a reflected multiple beam interference for the light exiting the
first surface in a reverse direction.
If the Fabry-Perot interferometer is illuminated by a broad light source and
the
transmitted multiple beam interference is collected by a focusing lens, a circular
interference pattern is produced on a screen at a focal length of the focusing
lens. The circular interference pattern exhibits bright narrow rings of light separated
by larger dark rings.
Goossen et al. in "Silicon modulator based on mechanically-active anti-reflection
layer with 1 Mbit/sec capability for fiber-in-the-loop applications,"
IEEE Phtonics
Technology Letters, Vol. 6, No. 9, September 1994, pp. 1119-1121, teach a mechanical
anti-reflection optical switch. The optical switch consists of a SiN
x
membrane suspended over a Si substrate. The SiN
x membrane has a square
shape and is suspended from corners by arms. The SiN
x layer has a thickness
of a quarter wavelength of incident light. A SiN
x index of refraction
for the SiN
x layer is a square root of a Si index of refraction for
the Si substrate. When an air gap separating the SiN
x membrane from
the Si substrate is an even multiple of a quarter wavelength, an antireflection
condition exists. When the air gap is an odd multiple of a quarter wavelength of
the incident light, a high reflection condition exists. The optical switch is in
an off-state when the anti-reflection condition exists and is an on-state when
the high reflection condition exists.
Fabricating the SiN
x membrane so that the SiN
x index
of refraction is the square root of the Si index of refraction is difficult. Further,
fabricating the arms and the SiN
x membrane in a reproducible manner
so that production devices operate in a similar manner is difficult. Moreover,
it is desirable to have an optical switch which is more economical to produce than
the optical switch taught by Goossen et al.
Miles, in U.S. Pat. No. 5,835,255 issued on Nov. 10, 1998 and entitled, "Visible
Spectrum Modulator Arrays," teaches a micro-fabricated interferometric light modulator.
The micro-fabricated interferometric light modulator includes a transparent substrate
and a rectangular membrane suspended above the substrate. The transparent substrate
includes first and second surfaces, and also includes a transparent film on the
second surface. The transparent film is conductive. A mirror, either a metal or
dielectric mirror, lies on the transparent film. The membrane is suspended above
the mirror by parallel support structures, which support two edges of the rectangular
membrane. The membrane is both reflective and conductive. The membrane and the
mirror form an interferometric cavity which is modulated by biasing the membrane
relative to the transparent film. In operation, the micro-fabricated light modulator
modulates light incident upon the first surface of the transparent substrate by
interferometrically causing the incident light to exit the first surface or by
interferometrically causing the incident light to not exit the first surface.
Miles further teaches an alternative micro-fabricated interferometric light
modulator in which the membrane is a square membrane. The square membrane is suspended
by arms from centers of each of four lengths defining the square membrane.
Fabricating the transparent and conducting film of the micro-fabricated
light modulators is difficult. Further, keeping a separation distance defining
the interferometric cavity of the micro-fabricated light modulators constant across
the interferometric cavity is difficult. Additionally, the combination of the rectangular
membrane and the parallel support structures gives rise to a tendency for the rectangular
membrane to deform cylindrically. The cylindrical deformation of the rectangular
membrane reduces the effectiveness of the interferometric cavity. Moreover, it
is desirable to have an interferometric light modulator which is less costly to
manufacture and which is more reproducible than the micro-fabricated interferometric
light modulators taught by Miles.
What is needed is an interferometric light modulator which is economical to
fabricate, which is more easily reproducible in a production setting, which does
not rely on a rectangular membrane supported by parallel support structures, and
which does not rely on arms to support a moving surface.
SUMMARY OF THE INVENTION
The present invention is an interferometer. The interferometer comprises a membrane,
a substrate, and a support structure. The membrane comprises a first reflector.
The substrate comprises a second reflector. The support structure circumferentially
couples the membrane to the substrate and orients the first reflector parallel
to and facing the second reflector.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 isometrically illustrates the preferred interferometer of the present invention.
FIG. 2 illustrates a first and second multilayer reflector and a substrate of
the preferred interferometer of the present invention.
FIG. 3 graphically illustrates a transmitted wavelength of the preferred interferometer
of the present invention.
FIG. 4 graphically illustrates a transmitted wavelengths of the preferred interferometer
of the present invention over a telecommunications C band.
FIG. 5 illustrates a WDM (wavelength division multiplex) channel monitor employing
the preferred interferometer of the present invention.
FIG. 6 illustrates a first alternative interferometer of the present invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
The preferred interferometer of the present invention is illustrated in FIG.
1. The preferred interferometer
10 comprises a membrane
12, a plurality
of posts
14, and a substrate
16. The membrane
12 comprises
a first reflector
18 and a flexible annular member
20. Preferably,
the membrane
12 further comprises release slots
21. Alternatively,
the membrane
12 does not include the release slots
21. The substrate
16 preferably comprises a second reflector
22, a transparent bulk
material
24, and an anti-reflective coating
26. Alternatively, the
substrate
16 does not include the anti-reflective coating
26.
In the preferred interferometer
10, the membrane
12 couples to
the
substrate
16 via the plurality of posts
14 and a membrane extension
28. The plurality of posts
14 and the membrane extension
28
hold the membrane
12 in bi-axial tension. The bi-axial tension in the membrane
10 maintains the first reflector
18 parallel to the second reflector
22. Preferably, the posts
14, the flexible annular member
20,
the membrane extension
28 comprise a resilient material. Alternatively,
only the flexible annular member
20 comprises the resilient material. Preferably,
the resilient material comprises Si
3N
4. Alternatively, the
resilient material comprises another material with resilient properties.
An advantage of the preferred interferometer
10 is that the biaxial tension
in the membrane
10 results in the first reflector having a highly flat surface,
which promotes parallelism of the first and second reflectors,
18 and
28.
The first reflector
18 includes a first conducting layer. An electrical
conductor
32 couples to the first conducting layer, which provides an electrical
biasing path for the first conducting layer. The second reflector
28 includes
a second conducting layer. In the preferred interferometer
10, the first
and second reflectors,
18 and
28, form an interferometric cavity
30. The interferometric cavity
30 is adjusted by electrically biasing
the first conducting layer relative to the second conducting layer. This causes
the first reflector
18 to move relative to the second reflector
22
adjusting a cavity length for the interferometric cavity
30.
The preferred interferometer
10 transmits light when the cavity length
is an integral multiple of a half wavelength of the light. Otherwise the preferred
interferometer
10 reflects light. If first, second, and third light wavelengths,
λ
1, λ
2, and λ
3, are incident
upon the preferred interferometer
10 and the cavity length is an integral
multiple of only the second light wavelength λ
2, the second wavelength
λ
2 transmits and the first and third light wavelegnths, λ
1
and λ
3, reflect.
A partial cross-section of the preferred interferometer
10 is further
illustrated
in FIG. 2. The partial cross section
40 comprises the first reflector
18
and the substrate
16. The substrate
16 comprises the second reflector
22, the transparent bulk material
24, and the anti-reflective coating
26. The first reflector
18 and the second reflector
22 form
the interferometric cavity
30.
Preferably, the preferred interferometer
10 operates over telecom
C and L bands, which comprises light of wavelength within the range of 1,520 to
1,620 nm. Alternatively, the preferred interferometer operates over a different
light wavelength band.
It will be readily apparent to one skilled in the art that the preferred interferometer
is appropriate for operation over wavelength bands other than the telecom C and
L bands.
For operation over an infra-red telecommunications band, the first reflector
18 preferably comprises a first multilayer reflector. The first multilayer
reflector preferably comprises first and second encapsulating layers,
42
and
43, of Si
3N
4 and alternating layers of low refractive
index material
44 comprising SiO
2 and high refractive index material
46 comprising poly-Si (polycrystalline Si). Preferably, the first multilayer
reflector comprises four pairs of the alternating layers plus an extra layer of
the low refractive index material
44. Alternatively, the first multilayer
reflector comprises more or less than four pairs of the alternating layers.
For operation over the infra-red telecommunications band, the second reflector
22 preferably comprises a second multilayer reflector. The second multilayer
preferably comprises a third encapsulating layer
48 and the alternating
layers of the low refractive index material
44 comprising SiO
2
and the high refractive index material
46 comprising poly-Si. Preferably,
the second multilayer reflector comprises four pairs of the alternating layers
plus an extra layer of the low refractive index material
44. Alternatively,
the second multilayer reflector comprises more or less than four pairs of the alternating layers.
The first, second, and third encapsulating layers,
42,
43 and
48,
and the alternating layers of the low refractive index material
44 and high
index of refraction material
46 preferably comprise quarter wavelength films.
Alternatively, the alternating layers comprise a range of thicknesses about a quarter
wavelength, which broadens an operational wavelength band. The quarter wavelength
films have optical path lengths of a quarter wavelength of an intermediate wavelength
within the infra-red telecommunications band. Choosing the intermediate wavelength
as 1,550 nm gives the qurater wavelength as 387.5 nm. Layer thicknesses are determined
by dividing the quarter wavelength by the refractive index. Table 1 provides the
refractive indexes and the layer thicknesses for Si
3N
4, SiO
2,
and poly-Si.
| |
TABLE 1 |
| |
|
| |
Material |
Refractive index |
Layer Thickness |
| |
|
| |
Si3N4 |
2.00 |
193.8 nm |
| |
SiO2 |
1.44 |
269.1 |
| |
Poly-Si |
3.63 |
106.7 |
| |
|
For operation over the infra-red telecommunications band, the transparent bulk
material preferably comprises Si. Alternatively, the transparent bulk material
comprises SiO
2. The anti-reflective coating
26 preferably comprises
a quarter wavelength film of Si
3N
4. Alternatively, the anti-reflective
coating
26 comprises a quarter wavelength film of another suitable optical
coating material.
Si
3N
4 has low optical absorption
for wavelengths below 4,350 nm. Poly-Si and single crystal Si have low optical
absorption for wavelengths above 850 nm. SiO
2 has low optical absorption
over a wavelength range from 159 to 7,700 nm.
Referring to FIGS. 1 and 2, the preferred interferometer
10 is fabricated
using semiconductor processing techniques of film deposition and etching. Fabrication
begins with a Si wafer, which forms the transparent bulk material
24. The
second reflector
22 is deposited on the Si wafer by depositing the layers
of the second multilayer reflector. Next, a sacrificial layer of poly-Si is deposited
onto the second reflector. Alternatively, the sacrificial layer comprises a material
other than poly-Si such as SiO
2. (Note that in a later step, the sacrificial
layer will be etched away through the release holes
21, hence it is called
the "sacrificial" layer.)
Following deposition of the sacrificial layer, the sacrificial layer is
etched to form an inverse of the plurality of posts
14 and to form edges
of the sacrificial layer where the membrane extension
28 will couple to
the substrate
16. A first layer of Si
3N
4 is then deposited
over the sacrificial layer forming a first layer of the membrane, a first layer
of the plurality of posts, and a first encapsulating layer
42 of the first
multilayer reflector. Following this, the alternating layers of the low refractive
index material
44 and the high refractive index material
46 are deposited
on a center region of the membrane
12. A second layer of Si
3N
4
is deposited over the first layer of Si
3N
4 and over
the alternating layers of the first multilayer reflector, which completes fabrication
of the membrane
12, the plurality of posts
14, and the first reflector
18. The release slots
21 are then etched through the first and second
layers of Si
3N
4 to the sacrificial layer. Preferably, XeF
2
gas is then used to etch the sacrificial layer through the release slots
21. The XeF
2 gas etches the sacrificial layer to completion,
which releases the membrane
12 and forms the interferometric cavity
30.
Alternatively, another selective etchant is used to etch the sacrificial layer.
An advantage of employing the sacrificial layer in the fabrication of the preferred
interferometer
10 is that, because the sacrificial layer is formed with
a uniform thickness, the sacrificial layer assures parallelism of the first and
second reflectors,
18 and
28.
It will be readily apparent to one skilled in the art that, since the release
slots
21 function to provide access to the sacrificial layer for the XeF
2
gas, the release slots
21 can be replaced by other access entries to the
sacrificial layer such as release holes in the membrane extension
28.
FIG. 3 graphically illustrates intensities of first, second, and third VDM (wavelength
division multiplex) channels,
50,
52, and
54, on a channel
spacing
55 of 0.2 nm and also illustrates an interferometer transmission
56 of the preferred interferometer
10 configured for the telecom
C band. The interferometer transmission
56 has a maximum transmission at
1544.5 nm, which is the wavelength of the second WDM channel
52, and has
a full width half maximum
58 of 0.0300 nm. This is accomplished by adjusting
the cavity length of the preferred interferometer
10 to an integral multiple
of half of 1544.5 nm. Thus, directing the first, second, and third WDM channels,
50,
52, and
54, onto the preferred interferometer
10
with the cavity length adjusted to integral multiple of half of 1544.5 nm causes
the first and third WDM channels,
50 and
54, to reflect from the
preferred interferometer
10 and also causes the second WDM channel
52
to transmit through the preferred interferometer
10.
FIG. 4 graphically illustrates the interferometer transmission
56 of
the preferred interferometer
10 of the present invention tuned over the
telecom C band. The preferred interferometer
10 is tuned by adjusting the
cavity length of the interferometric cavity
30. The interferometric cavity
30 preferably has a non-deflected cavity length of 3,126 nm. With a 0.0
nm relative deflection of the first reflector
18, the interferometer transmission
56 occurs at 1,563.0 nm. With a 37.0 nm relative deflection
60 of
the first reflector
18 towards the second reflector
22, the interferometric
transmission occurs at 1,544.5 nm. With a 74.0 nm relative deflection
62
of the first reflector
18 towards the second reflector
22, the interferometric
transmission occurs at 1,526.0 nm.
It will be readily apparent to one skilled in the art that the non-deflected
cavity
length of 3,126 nm can be replaced by a longer or shorter cavity length. If the
non-deflected cavity length is replaced by a significantly longer cavity length,
a larger relative deflection is needed to tune the preferred interferometer
10
to a specific wavelength. For example, if the non-deflected cavity length is 6,252
nm, twice the 37.0 nm relative deflection
60 of the first reflector towards
the second reflector is needed to move the interferometric transmission to 1,544.5
nm. If the non-deflected cavity length is replaced by a significantly shorter cavity
length, a smaller relative deflection is needed to tune the preferred interferometer
to the specific wavelength. For example, if the non-deflected cavity length is
1,563 nm, half the 37.0 nm relative deflection
60 is needed to tune the
preferred interferometer to 1,544.5 nm.
A WDM channel monitor employing the preferred interferometer
10 of the
present
invention is illustrated in FIG. 5. The WDM channel monitor
70 comprises
an input optical fiber
72, a first collimating lens
74, the preferred
interferometer
10, a second collimating lens
76, and an output optical
fiber
78. The first ball lens
74 couples the input optical fiber
72 to the preferred interferometer
10. The second ball lens
76
couples the preferred interferometer
10 to the output optical fiber
78.
Preferably, the output optical fiber
78 couples to a photodetector (not
shown), which is coupled to electronics (not shown). Alternatively, the output
optical fiber
78 couples to downstream optical network components.
In operation of the WDM channel monitor
70, the first reflector
18
is resonated to cause an interferometric transmission to sweep across a wavelength
band. The photodetector detects the interferometric transmission and outputs a
photodetector signal to the electronics. The electronics process the photodetector
signal to provide individual channel power for WDM channels across the wavelength band.
A first alternative interferometer of the present invention is illustrated in
FIG.
6. The first alternative interferometer
80 comprises an alternative membrane
82, a spacer layer
84, and an alternative substrate
86. The
alternative membrane
82 comprises a third reflector
86. The alternative
substrate comprises a fourth reflector
88 and a transparent bulk material
90. In the alternative interferometer
80, the third and fourth reflectors,
86 and
88, form an alternative interferometric cavity
92.
In the alternative interferometer
80, the spacer layer
84 couples
the alternative membrane
82 to the substrate
86.
It will be readily apparent to one skilled in the art that other various modifications
may be made to the embodiments without departing from the spirit and scope of the
invention as defined by the appended claims.
*
