U.S. patent application number 11/294965 was filed with the patent office on 2006-04-20 for tunable laser modules incorporating micromachined pellicle splitters.
This patent application is currently assigned to INTEL CORPORATION. Invention is credited to William J. Kozlovsky, Mark McDonald.
Application Number | 20060082889 11/294965 |
Document ID | / |
Family ID | 33489659 |
Filed Date | 2006-04-20 |
United States Patent
Application |
20060082889 |
Kind Code |
A1 |
Kozlovsky; William J. ; et
al. |
April 20, 2006 |
Tunable laser modules incorporating micromachined pellicle
splitters
Abstract
A micromachined pellicle beam splitter and method of manufacture
thereof are disclosed. In one embodiment, the beam splitter
includes a silicon frame with a silicon nitride membrane attached
to the frame and covering an opening through the frame. Other
materials may be utilized, however, the coefficient of thermal
expansion (CTE) of the membrane should be greater than that of the
frame. The beam splitter may be manufactured by coating a silicon
substrate with a layer of silicon nitride, patterning an opposite
side of the silicon substrate with a photoresist or a metallic
layer to define an opening an etching an opening through the
substrate to the silicon nitride with either a dry etch or wet etch
technique. An improved tunable laser module incorporating the
micromachined pellicle beam splitter and a method of tuning a laser
diode are also disclosed.
Inventors: |
Kozlovsky; William J.;
(Sunnyvale, CA) ; McDonald; Mark; (Mill Valley,
CA) |
Correspondence
Address: |
MARSHALL, GERSTEIN & BORUN LLP (INTEL)
233 S. WACKER DRIVE
6300 SEARS TOWER
CHICAGO
IL
60606
US
|
Assignee: |
INTEL CORPORATION
Santa Clara
CA
|
Family ID: |
33489659 |
Appl. No.: |
11/294965 |
Filed: |
December 6, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10980057 |
Nov 3, 2004 |
6972907 |
|
|
11294965 |
Dec 6, 2005 |
|
|
|
10454071 |
Jun 4, 2003 |
6859330 |
|
|
10980057 |
Nov 3, 2004 |
|
|
|
Current U.S.
Class: |
359/639 |
Current CPC
Class: |
G02B 27/142 20130101;
G02B 27/108 20130101; G02B 27/1073 20130101 |
Class at
Publication: |
359/639 |
International
Class: |
G02B 27/12 20060101
G02B027/12 |
Claims
1. A method of fabricating a pellicle beam splitter, the method
comprising: depositing a silicon nitride film on a first side of
silicon substrate, depositing a mask on a second side of the
silicon substrate, the mask defining an opening, etching the
silicon substrate through the opening of the mask to produce an
opening in the substrate extending from the second side to the
first side thereof and up to the silicon nitride film.
2. The method of claim 1 wherein the etching is a wet etch
process.
3. A method of fabricating a pellicle beam splitter, the method
comprising: depositing a silicon nitride film on a first side of a
first silicon substrate, depositing a second silicon substrate on
the silicon nitride film so that the silicon nitride film is
sandwiched between a first side of the second silicon substrate and
the first side of the first silicon substrate, depositing masks on
second sides of the first and second silicon substrates, the masks
each defining openings that are aligned with each other, etching
the first and second silicon substrates through the openings of the
masks to produce openings in the first and second substrate
extending from the second side to the first side of each substrate
and up to the silicon nitride film.
4. The method of claim 3 wherein the etchings of the first and
second silicon substrates are wet etch processes carried out
simultaneously and the etching of the first silicon substrate is a
wet etch carried out before or after the etching of the second
silicon substrate which is a dry etch.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation of application Ser. No. 10/980,057
filed on Nov. 3, 2004, now U.S. Pat. No. 6,972,907, which is a
divisional of application Ser. No. 10/454,071, filed on Jun. 4,
2003, now U.S. Pat. No. 6,859,330, which are both incorporated
herein by reference.
TECHNICAL FIELD
[0002] Micromachined pellicle optical beam splitters are disclosed.
More specifically, pellicle beam splitters are disclosed which
comprise a silicon frame and a silicon nitride membrane. Methods of
manufacturing the disclosed beam splitters using wet and dry etch
techniques are also disclosed. Tunable laser modules including a
disclosed pellicle beam splitter are also disclosed.
BACKGROUND OF THE RELATED ART
[0003] Pellicle bean splitters are known. Currently available
pellicle beam splitters are relatively large in size and consist of
a nitrocellulose membrane or pellicle stretched over a rigid frame.
The frames are often fabricated from metal, such as aluminum.
[0004] A light source is directed at the membrane and a known
fraction of the optical amplitude is reflected while a majority of
the optical amplitude is transmitted through the membrane. Pellicle
beam splitters are useful in monitoring the amplitude of the light
transmitted through the beam splitter. The known fraction of the
optical amplitude that is reflected can be transmitted to a monitor
photodiode where a determination can be made as to whether an
adjustment to the optical amplitude is necessary.
[0005] As noted above, optical beam splitters are relatively large
in size and cannot be used in smaller applications such as
telecommunication modules and other applications that use
semiconductor lasers as the light source. Accordingly, there is a
need for a beam splitter that is as effective as a pellicle beam
splitter in transmitting a majority of the optical amplitude while
reflecting a known fraction of the amplitude for monitoring
purposes and that further is small enough for the telecommunication
modules and other laser applications.
[0006] There is an increasing demand for tunable lasers given the
advent of wavelength-division multilplexing (WDM) which has become
widespread in fiber optic communication systems. WDM transponders
include a laser, a modulator, a receiver and associated
electronics. One WDM transponder operates a fixed laser in the
near-infrared spectrum at around 1550 nm. A 176 wavelength system
uses one laser per wavelength and therefore such a system typically
must store a 176 additional WDM transponders as spares to deal with
failures. This high inventory requirement contributes to the high
cost of these systems.
[0007] In response, tunable lasers have been developed. A single
tunable laser can serve as a back-up for multiple channels or
wavelengths so that fewer WDM transponders need to be stocked for
spare part purposes. Tunable lasers can also provide flexibility at
multiplexing locations, where wavelengths can be added and dropped
from fibers as needed. Accordingly, tunable lasers can help
carriers effectively manage wavelengths throughout a fiber optics
network.
[0008] Two currently available tunable lasers are distributed
feedback (DFB) lasers and distributed brag reflector (DBR) lasers.
A conventional tunable laser module 10 is illustrated in FIG. 1. In
tunable lasers, the output power is most often measured from the
front of the laser diode gain chip 12 of the laser 11, and not from
a rear facet as is done with non-tunable lasers. The output of the
laser diode gain chip 12 is directed through a collimating lens 13
and isolator 14. The optical output then engages the cubicle power
tap 15 at an angle of about 45.degree. where a fraction of the
light is reflected toward a detector shown at 16 and the remaining
output passes through the lens 17 to the fiber 18. The detector 16
and diode gain chip 12 are linked by various circuitry shown at 19
for tuning the laser or laser diode shown at 12.
[0009] A cube power tap 15 is typically a solid, coated optical
element assembled into a standard beam splitter cube that reflects
a small portion of the light and sends it to the detector 16 as
discussed above. However, one difficulty with the standard beam
splitter cube 15 is that it has many surfaces that can provide
stray reflections. Although the amplitude of the stray reflections
may be relatively small due to anti-reflection coatings applied to
the surfaces of the cube 15, the presence of the reflected light
can interfere with small signals that are typical of servo signal
inputs used by the control mechanism 19 and diode gain chip 12 to
adjust the wavelength of the laser 12.
[0010] As a result, there is a need for an improved power tap
device which can eliminate the stray reflective rays.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The disclosed apparatuses and methods are illustrated more
or less diagrammatically in the accompanying drawing wherein:
[0012] FIG. 1 is a schematic illustration of a tunable DFB or DBR
laser module in accordance with the prior art;
[0013] FIG. 2 is a sectional view of a micromachined pellicle beam
splitter made in accordance with this disclosure;
[0014] FIG. 3 illustrates, graphically, the reflectivity of a
silicon nitride film versus film thickness for a P polarization, C
band light wave directed at a silicon nitride film at a 45.degree.
angle of incidence;
[0015] FIG. 4 illustrates, graphically, the spectral performance of
five pellicle membranes set in P polarization at a 45.degree. angle
of incidence wherein the membranes have thicknesses of about 25 nm,
426 nm, 827 nm, 1228 nm and 1529 nm.
[0016] FIG. 5 is a schematic illustration of a tunable laser module
incorporating a disclosed micromachined pellicle beam splitter as
shown in FIG. 1;
[0017] FIG. 6 illustrates, graphically, detector sensitivities or
responsivities versus wavelength for InGaAs, Ge and Si detectors
that can be used to design an appropriate micromachined pellicle
beam splitter for a tunable laser module incorporating one of said
detectors;
[0018] FIG. 7 illustrates, graphically, the spectral performance of
a silicon nitride pellicle film set in P polarization at a
45.degree. angle, at near zero wave solution, for three membranes,
all at about a half wavelength thickness for near zero wave
solution, i.e., about 34, 30 and 26 nm or about 30 nm and
.+-.12%;
[0019] FIG. 8 illustrates, graphically, the spectral performance of
three pellicle membranes set in P polarization at a 45.degree.
angle wherein one membrane has a thickness less than a half
wavelength for the C band (.about.370 nm) and the two other films
shown are about 2% thicker and about 2% thinner than the initial
films, i.e., about 362 and 378 nm; and
[0020] FIG. 9 illustrates, graphically, the spectral performance of
three pellicle membranes set in P polarization at a 45.degree.
angle wherein the first membrane has a thickness greater than a
half wave for the C band (.about.430 nm) and the other two
membranes have thicknesses that are about 2% greater and 2% thinner
than the first membrane, i.e., about 438 and 422 nm.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
[0021] A silicon micromachined pellicle beam splitter is disclosed.
As shown in FIG. 2, a beam splitter 30 includes a silicon frame 31
that is coated with a silicon nitride membrane 32. The frame 31 is
fabricated from a silicon substrate using dry or wet etch
processes. For example, if a draft angle is desired as indicated by
the tapered wall 33 shown in phantom in FIG. 2, a wet etch process
may be required. If no draft angle is desired, then a dry etch
process can be used.
[0022] The substrate 31 is coated with the silicon nitride layer
32. Then, a photoresist, metal or other protective layer (not
shown) is coated onto the underside 34 of the substrate 31 leaving
an uncoated area that eventually defines the etched volume shown at
35. An etch process is carried out to create the etch volume 35
without damaging the silicon nitride layer 32. If a wet etch
process is utilized, potassium hydroxide is a suitable etchant.
[0023] It may also be desirable to include a protective support
shown in phantom in FIG. 2 at 36. If this is the case, then the
silicon nitride film 32 is sandwiched between the silicon substrate
31 that becomes the frame 31 and an additional silicon substrate
36. Again, another protective layer such as a photoresist or
metallic layer is coated onto the top side 37 of the substrate 36
and the etching process is carried out through the substrate 31 and
through the substrate 36 leaving the silicon nitride membrane 32
intact. If a draft angle is desired for the substrate or frame 31,
a wet etch process may be carried out through the substrate 31 and
if no draft angle is warranted for the protective frame 36, a dry
etch process may be carried out for the substrate 36.
[0024] If a draft angle is desired for one substrate 31 but not the
other substrate 36, or vice versa, then the wet and dry etchings
are carried out separately. Otherwise, if the same etching
technique is used for both substrates 31, 36, the etchings may be
carried out concurrently.
[0025] For tunable laser applications, the thickness of the silicon
nitride film or pellicle 32 should be on the order of about 20-60
nm because such a thickness results in a reflectivity of about 1%
in the P polarization in the C band at a 45.degree. angle of
incidence as shown in FIG. 3. This approximately 1% reflectivity is
a convenient level for power monitoring.
[0026] Further, thicknesses for the membrane 32 of approximately
one-half of the optical wave for C band light can also be achieved.
As shown in FIG. 3, in addition to low reflectivities for thin
silicon nitride films with thickness less than 60 nm, low
reflectivities are also exhibited for silicon nitride films having
thicknesses of about one-half of the optical wave for C band light.
Films of these thicknesses may also provide low reflectivity at the
design wavelength. It may be convenient for a power monitoring
application that the calibration curve never encounters a zero in
reflectivity. Thus, it may be desirable for the thickness of the
membrane 32 of the pellicle beam splitter 30 to be more or less
than one half of a wavelength thickness optically.
[0027] FIG. 4 illustrates, graphically, the reflectivity that
results from various selected silicon nitride film thicknesses, 25,
426, 827, 1228 and 1629 nm, as a function of wavelength. It will be
noted that the reflectivity at a 1550 mm wavelength using a thin,
25 nm film thickness remains relatively constant. Therefore, thin
silicon nitride films (20-40 nm) may prove to be more convenient
for the wavelength range shown in FIG. 4 because of the constant
reflectivity or relatively flat slopes of the reflectivity
curves.
[0028] Further, more complex film stacks may be utilized depending
upon the spectral property desired. Thus, FIG. 2 also shows an
optional layer 38 may be used to protect the silicon nitride layer
32 or vary the spectral property of the beam splitter 30. One
suitable material for the additional layer 38 is silicon dioxide.
However, other materials will be apparent to those skilled in the
art who desire to vary the spectral properties of the beam splitter
30. Film stacks of three or more films are contemplated and may be
desirable for a variety of applications.
[0029] The combination of silicon for the substrate or frame 31 and
silicon nitride for the membrane 32 is advantageous because silicon
has a coefficient of thermal expansion on the order of about 2.6
while silicon nitride has a coefficient of thermal expansion on the
order of about 4. As a result, the silicon nitride membrane 42 will
remain in a state of tension which results in the low reflectivity
of the beam splitter 30. Because silicon dioxide has a CTE of about
0.5, it would not a suitable material for the membrane layer 38
when silicon is used for the frame 31. Materials other than silicon
nitride could be used for the membrane layer 32, however, the
coefficient of thermal expansion of the membrane layer 32 should be
greater than that of the material used for the substrate or frame
31.
[0030] The draft angle provided by the wall shown in phantom at 33
in FIG. 2 is useful if an angle of incidence of about 45.degree. is
utilized. The draft angle provided by the wall 33 reduces the
amount of clipping caused by the frame 31.
[0031] Another advantage to the beam splitter 30 is the very small
beam displacement upon transmission. Specifically, the amount of
the beam displacement is less than the thickness of the membrane
layer 32 and, as a result, the use of very small beams with the
beam splitter 30 is possible and therefore the beam splitter 30
will be useful in telecom modules and other devices requiring the
use of very small beams.
[0032] While an approximately 30 nm thickness has been suggested
for the membrane layer 32, particularly if silicon nitride is
chosen as the material for the membrane 32, the 30 nm thickness is
suggested for small beam applications, such as telecom modules. The
thickness of the membrane 32 can vary greatly, depending upon the
particular application. The use of a thin film, however, permits a
wide range of convergence with minimal affect on interference
properties. Further, thin films are typically very parallel, which
avoids substantial angular displacement of the beam upon
transmission through the beam splitter 30.
[0033] FIG. 5 illustrates a tunable laser module 10a equipped with
a pellicle beam splitter 30 as disclosed in FIG. 2. The components
of the module 10a that are the same as those shown in FIG. 1 will
be referenced with like reference numerals with the suffix "a". For
the reasons set forth above, the beam splitter 30 is superior to
the cube 15 (FIG. 1) because of its ability to eliminate stray
reflections.
[0034] Specifically, the components of the laser 11a include a back
cavity mirror 22 with a reflective coating. Between the diode gain
chip 12a and the back cavity mirror 22 are one or more thermally
tuned filters shown at 20, 21 and a diode intracavity collimating
lens 25 or laser cavity lens 25. Light reflected off of the back
cavity mirror 22 passes through the filters 20, 21 and through the
lens before passing through the diode gain chip 12a where it again
passes through a diode output collimating lens 13a before passing
through the isolator 14a to the pellicle beam splitter 30. A small
fraction of light is reflected off of the pellicle beam splitter
30, which as shown in FIG. 5 is preferably disposed at an angle of
about 45.degree. to the light path. The small fraction of light is
detected at the detector 16a and a signal is transmitted to the
adjustment circuitries shown at 19a for tuning the laser diode gain
chip 12a. The majority of the light passes through the pellicle
beam splitter 30 and through the fiber focusing lens 17a to the
polarization preserving fiber 18a.
[0035] FIGS. 4 and 6-9 illustrate various methodologies for
designing the micromachined pellicle beam splitter 30 for use in a
tunable laser module 10a.
[0036] Referring to FIG. 6, it is well known that the
responsivities of the detector shown at 15a will vary depending
upon the wavelength detected and therefore the tuning range of the
module 10a. Variances in detector sensitivity can decrease the
effective resolution of the detector circuitry (16a, 19a and 12a)
and may require the use of extensive look-up tables for power
calibration.
[0037] As shown in FIG. 6, silicon detectors in the visible,
germanium in the S-band, C-band and L-band and
indium/gallium/arsenic detectors in the L-band all exhibit
variations and detectors sensitivity across relatively wide tuning
ranges. Design of the pellicle beam splitter 30 incorporated into a
tunable laser module such as that shown at 10a in FIG. 5 can
compensate for at least some variations in detector sensitivity as
illustrated in FIGS. 7-10.
[0038] Turning to FIG. 7, the spectral performance of three
pellicle membranes of a disclosed beam splitter 30 is shown where
the line 41 represents a silicon nitride film having a thickness of
34 nm, the line 42 represents a silicon nitride film having a
thickness of about 30 nm and the line 43 represents silicon nitride
film having a thickness of about 26 nm. These three films represent
thicknesses approaching a near zero wave solution as shown in the
graph of FIG. 3. The film represented by the line 41 is
approximately 12% thicker than the film represented by the line 42
and the film represented by the line 43 is about 12% thinner than
the film represented by the line 42. The response of these silicon
nitride films is nearly flat over the wavelength range of interest.
Hence, choosing these thicknesses will provide little or no
compensation for the germanium or indium/gallium/arsenic detectors
illustrated in FIG. 6. However, for the flat portion of the
indium/gallium/arsenic curve (1500 to 1600 nm), the thin silicon
nitride membranes would be suitable. These membranes would not
provide a compensating effect for a germanium detector over the
same wavelength and thus, a different compensation scheme, if
desired, would need to be investigated as shown below.
[0039] Turning to FIG. 8, the spectral performance of a pellicle
membrane 32 of a beam splitter 30 set in P polarization at a
45.degree. angle is presented. The line 44 represents a silicon
nitride film having a thickness of about 370 nm, the line 45
represents a silicon nitride film having a thickness of about 362
nm and the line 46 represents silicon nitride film having a
thickness of about 378 nm. As can been seen in FIG. 8, the
reflectivity increases as wavelength increases for these three
films which would be useful in compensating for the drop in
responsivity of a germanium detector at wavelengths exceeding 1500
nm or in the C-band. In other words, using the disclosed pellicle
beam splitters, with an appropriate silicon nitride thickness, can
greatly assist in flattening out the responsivity curve for a
germanium detector and the C-band (see FIG. 6). Thus, referring to
FIGS. 8 and 3 together, films having wavelengths approaching the
half-way thicknesses shown in FIG. 3 (i.e., 300 to 390 nm, 700 to
790 nm or 1100 to 1190 nm) would prove useful in flattening out the
responsivity curve for a germanium detector in the C-band.
[0040] Turning to FIG. 9, the spectral performance of a pellicle
beam splitter 30 with a silicon nitride membrane 32 is illustrated
in P polarization at a 45.degree. angle of incidence wherein the
thicknesses of the silicon nitride membrane 32 are longer than a
C-band wavelength. Specifically, the line 47 represents a membrane
32 with a thickness of about 430 nm, the line 48 represents a
silicon nitride membrane with a thickness of about 438 nm and the
line 49 represents a silicon nitride membrane 32 with a thickness
of about 422 nm. Thus, the membranes represented by the lines 48
and 49 are slightly thicker and thinner (.+-.2%) than the membrane
represented by the line 47. As can be seen in FIG. 9, the
reflectivity decreases as the wavelength increases. These membranes
could be useful for an indium/gallium arsenic detector in the
C-band or the S-band range to compensate for the downward slope of
the responsivity curve for an indium/gallium/arsenic detector as
the wavelength increases (see FIG. 6). Referring to FIGS. 3 and 9,
membranes having thicknesses greater than the half-way thickness
for the C-band as shown in FIG. 3, i.e., 10 to 80 nm, 410 to 480
nm, 810 to 880 nm, 1210 to 1280 nm or 1610 to 1680 nm would prove
useful for flattening out the downward slope of the responsivity
curve for an indium/gallium/arsenic detector as shown in FIG.
6.
[0041] Similarly, returning to FIG. 4, the spectral performance of
5 silicon nitride membranes 32 set in P polarization at a
45.degree. angle of incidence is presented wherein the line 51
represents a membrane having a thickness of about 25 nm, the line
52 represents a membrane having a thickness of about 426 nm, the
line 53 represents a membrane having a thickness of about 827 nm,
the line 54 represents a membrane having a thickness of about 1228
nm and the line 55 represents a membrane having a thickness of
about 1629 nm. The downward slope of these lines as wavelength
increases could be used to compensate for the upward slope of the
responsivity curve of an indium/gallium/arsenic detector in the
C-band range or the S-band range (see FIG. 6). Referring to FIGS. 9
and 3, membranes having thicknesses at or close to the half-wave
wavelengths shown in FIG. 3, i.e., 10 to 50 nm, 410 to 450 nm, 810
to 850 nm, 1210 to 1250 nm and 1610 to 1650 nm would be useful in
compensating for the upward slope of the responsivity curve of an
indium/gallium/arsenic detector in the C-band range r the S-band
range as shown in FIG. 6.
[0042] Thus, an improved tunable laser module 10a is disclosed
whereby a pellicle beam splitter 30 as disclosed herein, with an
appropriately selected silicon nitride membrane 32 thickness that
can compensate for variances in responsivity of the detector 16a
over the tunable wavelength range.
[0043] Specifically, referring to FIG. 5, light is generated by the
laser 11a and reflected off of the back cavity mirror 22, through
one or more filters shown at 20, 21 and through the diode
intracavity collimating lens 25 to the diode gain chip 12a. Light
emerges from the diode gain chip (or other suitable gain media) 12a
and passes through another collimating lens 13a before passing
through an isolator 14a. Light emerging from the isolator 14a
engages the pellicle beam splitter 30 where a fraction is reflected
to the detector 16a and an adjustment to the diode gain chip 12a
output is made either directly or by way of a control circuitry
19a. Thus, light passing through the beam splitter 30 and through
the fiber focusing lens 17a to the polarization preserving fiber
18a is constantly monitored by way of the beam splitter 30 and
detector 16a and tuned or adjusted by way of the circuitry 19a and
diode gain chip 12a are other suitable gain media. Various other
control loops will be apparent to those skilled in the art. Thus,
the wavelength of the output from the laser 11a can be adjusted by
modifications to the one or more thermally tuned filters shown at
20, 21.
[0044] In the foregoing detailed description, the disclosed
structures and manufacturing methods have been described with
reference exemplary embodiments. It will, however, be evident that
various modifications and changes may be made thereto without
departing from the broader spirit and scope of this disclosure. The
above specification and figures accordingly are to be regarded as
illustrated rather than restrictive. Particular materials selected
herein can be easily substituted for other materials that will be
apparent to those skilled in the art and would nevertheless remain
equivalent embodiments of the disclosed devices and manufacturing
methods.
* * * * *