Integrated-optic waveguide devices and method

It is possible to utilize changes in the complex effective refractive index caused by bringing a metal membrane in close proximity to the core of an optical waveguide to form many devices. The invention described herein provides structures and processes that do so, the structure comprising a substrate, a lower cladding, a waveguide core, a removable upper cladding which supports a metal membrane, and an electrode for deflecting said membrane. Switchable devices using this structure include, but are not limited to, polarizers, mode converters, optical switches, Bragg devices, directional couplers and channel waveguides.

FIELD OF THE INVENTION 
This invention deals with optical waveguide devices, more particularly with 
integrated-optic waveguide devices. 
BACKGROUND OF THE INVENTION 
Integrated-optic waveguides normally consist of a layer of lower cladding 
deposited upon a substrate, a layer of a core material deposited upon the 
layer of cladding, and another layer of cladding deposited on top of the 
core. The two layers of cladding need not be of the same material. Indeed, 
the top layer is sometimes air. Also, the lower cladding may be the 
substrate. The index of refraction, n.sub.core, for the core must be 
significantly greater than the index of refraction for both claddings. 
This difference causes the peak intensity of the optical mode to be 
contained in the core while the "tails" of the mode extend into the 
cladding. 
The complex index of refraction for a material may be written as follows: 
EQU n=n+ik 
where k is the absorption variable, which, as shown above, is the imaginary 
part of the complex refractive index. In most cladding materials, it is 
desired that the k be very small, so the refractive index approximately 
equals n. The effective index for a guided mode is a function of both the 
core and cladding indices since the mode extends into the claddings, and 
is thus also complex-valued. 
Metals, which can be used as a cladding, possess a relatively large value 
for k. This larger value results in more of the light being absorbed than 
with a dielectric cladding, making a metal-clad waveguide very lossy. The 
amount of loss in the waveguide is a function if interaction length of the 
metal, the proximity of metal to the core, and the direction of 
polarization of the mode. This apparently undesirable property can make 
metal in optical waveguides very useful. It is possible to effectively 
change the imaginary part, k.sub.eff, of the effective index of the guided 
mode by bringing metal into close range or contact with the core, the 
metal acting like a switch or attenuator. Additionally, metal can be used 
to alter the real part of the effective index, n.sub.eff, to create 
devices such as directional coupler switches, switchable Bragg deflectors, 
and the like. 
Using the processing techniques developed in deformable mirror device (DMD) 
technology, it is possible to form a waveguide with a metal membrane 
supported upon the upper cladding. The membrane can be controlled to bring 
it into proximity with the core to alter the effective index of the guided 
mode. 
SUMMARY OF THE INVENTION 
Other objects and advantages will be obvious, and will in part appear 
hereinafter and will be accomplished by the present invention which 
provides structure for the control of light in integrated-optic 
waveguides. The device is fabricated by placing at least one electrode in 
a substrate, forming a lower cladding layer upon said substrate, laying 
down a core upon said lower cladding, covering said core with a layer of 
spacer, covering said spacer with a metal membrane with holes. After the 
device is thus formed, the spacer is removed via an etch through the holes 
to form a deflectable metal membrane suspended over an air gap under which 
lies an electrode. 
Having fabricated such a device, it is possible to control the deflection 
of the membrane so as to produce various effects in the guided mode of 
light. These effects can be embodied in various devices which include, but 
are not limited to, ON/OFF switches, switchable polarizers, switchable 
Bragg devices, switchable mode converters, and switchable directional 
couplers.

DESCRIPTIONS OF THE PREFERRED EMBODIMENTS 
FIG. 1a shows a cross-section of an integrated-optic waveguide device. 
Substrate 10 is doped with material to make it conductive and to form an 
electrode 12. This electrode could also be deposited. Lower dielectric 
cladding 14, possibly silicon dioxide (SiO.sub.2), is formed on the 
substrate 10 with its doped or deposited electrode 12. The dielectric core 
16, possibly silicon nitride (Si.sub.3 N.sub.4), is then put down onto the 
lower cladding 14. Upper cladding 18, which may or may not be the same 
material as the lower cladding 14, must be of a material which can be 
selectively and isotropically etched. Optional dielectric buffer layer 17, 
which will not be removed by the upper cladding etch, can be formed on the 
core before the upper cladding so as to enhance or reduce absorption of 
the guided mode during operation of the device. The metal membrane 22, in 
this case aluminum (Al), is then put down upon the upper cladding. 
Metal membrane 22 has minute holes in it, shown in the perspective drawing 
FIG. 2, for etching out the upper cladding 18. In this case the upper 
cladding is an organic polymer spacer. The holes in the membrane 22 are 
placed so as to allow the center of the spacer 18 to be etched out, 
leaving the edges of the membrane supported by the remaining spacer, with 
an air gap 20 between the core 16 and the membrane 22, or between the 
optional layer 17 and the membrane 22. 
FIG. 1b demonstrates what happens when a potential difference is applied 
between membrane 22 and electrode 12, causing an electrostatic attraction 
between them. The membrane 22, which is supported by spacer 18, is 
attracted to the electrode 12 electrostatically, causing the membrane to 
deform towards the electrode. The membrane 22 does not have to be 
supported on all four sides, nor is it limited to a square or rectangular 
shape. The air gap 20 becomes smaller in volume because of the membrane's 
movement. The air in the gap can evacuate from the gap by the etch access 
holes in the membrane. In both of these cross-sections the light is shown 
moving in the direction of the arrow 26b or arrow 26a. When the membrane 
deflects it in general causes a change in both the real and imaginary 
parts of the effective index of the guided mode, offering the possibility 
of creating various devices. 
The direction of propagation of the light becomes easier to distinguish 
when viewing the perspective drawing of the planar waveguide device in 
FIG. 2. Arrows 26a and 26b show that light could travel through the 
waveguide from right to left or left to right. Because of the manner in 
which the membrane 22 rests on the spacer layer 18, the light could also 
travel in the directions of the arrows 28a and 28b, or from front to back 
or back to front. 
This drawing does not show the implanted region 12 in the substrate 10, 
because lower cladding layer 14 and core layer 16 are covering it. The 
open area inside of the edges formed by spacer layer 18 is the air gap 20. 
The air gap 20 is formed by an etch that enters the spacer layer through 
the etch access hole array, one of which is 24. For ease of viewing the 
air gap 20 and the dimension of the spacer layer 18, the membrane 22 is 
shown as if it were lifted off of the spacer layer. Because these devices 
are monolithically manufactured, this really is not true. The membrane is 
actually an integral part of the device, not a removable component as 
shown. 
FIG. 3 shows a structure for limiting the direction the light can travel 
through the device. In the cross-sectional view, a ridge 30 is placed in 
the core 16. The ridge's width runs along the axis defined by the arrows 
26a and 26b, and its length runs the length of the device. The ridge can 
be made of the same material, as implied in the drawing, or of a different 
material deposited before the spacer layer is laid down. The ridge causes 
a difference in the effective index of refraction and thus creates a 
channel that confines light in the lateral direction, inside the box 
formed by the ridge. 
In FIG. 4 the ridge 30 is shown as it would appear if the membrane were 
removed. In this embodiment, the etch access holes 24 have been set closer 
to the front and back edges of the membrane. This causes the spacer layer 
18 to be transformed from a hollow box as in FIG. 2, to a set of pontoons 
that support the membrane along two opposite edges that are parallel to 
the axis of the channel waveguide. This type of support structure could be 
used for all devices of this type, including planar waveguides. The air 
can now flow out the ends when the membrane 22 is drawn downward towards 
the core 16. One advantage of using only two edges for support is that 
transitions from cladded to uncladded regions for the light are 
eliminated, thereby reducing scattering losses. 
The basic structures above can be used to change the imaginary parts of the 
refractive index, creating many devices some of which are a switchable 
polarizer, an ON/OFF switch, or a variable attenuator. 
An ON/OFF switch can be effected by creating sufficient loss in the 
waveguide so that essentially no light is transmitted. The loss is created 
by the membrane when it comes into proximity or contact with the core. The 
light is then absorbed by the metal, turning it "OFF". 
The device could be operated as a switchable polarizer as follows. When 
light of random polarization enters the waveguide, it could be polarized 
into transverse electric, or TE polarized, light by changing the relative 
proximity of the membrane. The metal membrane absorbs more of the 
transverse magnetic, or TM polarized, light, causing the resultant light 
to be substantially TE polarized. The buffer layer 17 in FIG. 1a can be 
formed at an optimum thickness to maximize the extinction ratio. 
FIG. 5 shows a perspective view of a variable attenuator. In this 
embodiment, the distance 19 is considerably smaller than distance 21. This 
causes the metal to be brought into proximity with the waveguide core at 
the wider end of the device with lower voltages than at the narrower end. 
The difference in the two distances 19 and 21 may be caused by a change in 
the positions of the access holes 24 in the membrane. The spacer 18 is 
then formed in such a manner as to cause the different membrane widths. At 
a given voltage, the membrane will be in proximity to the core starting 
from the wider end and going to an intermediate point along the device. 
The location of this intermediate point is dependent on the value of the 
applied voltage. The variation in the amount of membrane brought into 
proximity with the core along the length of the channel causes a 
controllable variable attenuation of the light, where the amount of 
attenuation is called by the amount of the membrane in contact with the 
device, which, in turn, is controlled by the value of the voltage applied 
to the electrode. Just as the ON/OFF switch can be operated as a 
switchable polarizer, this variable attenuator could be operated as a 
variable polarizer. 
Devices can also be created that operate by changing the real part of the 
refractive index. Some of these devices are directional coupler switches, 
switchable Bragg reflectors, deflectors and filters, and mode converters. 
Note that in all devices which operate primarily by changing the real part 
of the refractive index, it may be desirable to use a dielectric buffer 
layer 17 on top of the core to reduce the absorption losses due to the 
metal membrane. 
A directional coupler device, shown in FIG. 6, can be made by bringing two 
channel waveguides 32 and 34 into close lateral proximity for a length 
known as the coupling region, indicated by dashed box 33. The evanescent 
tail of the lateral optical mode profile of each waveguide extends into 
the other and results in a coupling of the two modes. The local coupling 
strength depends on the separation of the two waveguides, and determines 
the required length of the coupling region. In addition to the coupling 
strength, the transfer efficiency of light from one waveguide to the other 
depends on the difference in propagation constants of the modes of the two 
guides. Complete transfer of light from one waveguide to the other is 
possible only if the difference in propagation constants is zero. However, 
for any value of the coupling strength, the transfer efficiency can be 
made zero, resulting in all of the output light remaining in the same 
waveguide as the input, by causing an appropriate mismatch in propagation 
constants. Such a mismatch can be imposed, for example, by deflecting the 
membrane in such a way as to cause it to come into closer proximity with 
one of the coupled waveguides than the other, as discussed below. Of 
course, alteration of the transfer efficiency over ranges other than 100% 
to 0% is possible. With this choice of transfer efficiencies, however, the 
device constitutes a useful routing type of switch for optical signals. 
Because directional couplers are also wavelength- and 
polarization-sensitive, variations of this device can be used for 
switchable wavelength or polarization filtering. 
A means for implementing a switchable directional coupler is illustrated in 
FIG. 6, in which a non-etching auxiliary spacer layer 19 is deposited of 
thickness and patterned in such a way as to prevent the metal membrane 
from coming into as close proximity with the left coupled waveguide as 
with the right. An alternative means of imbalancing the membrane proximity 
might be to offset the center of the etched-out area under the membrane 
from the centerline of the coupled waveguide pair. Depending on the length 
of the coupling region and the required length of contact of the membrane, 
in these devices the membrane may or may not completely cover the coupling 
region. 
A possible structure for the Bragg devices previously mentioned is shown in 
FIG. 7. The waveguide effective index can be periodically modulated under 
the deflectable portion of the membrane by some means such as a deposited 
or etched corrugation on the core layer, forming a grating 46. Various 
types of grating are possible, which can be configured to act as mirrors, 
deflectors or couplers. Because the diffraction properties of such a 
grating are also wavelength- and polarization-sensitive, these structures 
can be used in a number of wavelength- or polarization-sensitive devices. 
The wavelength at which a Bragg grating reflects depends on the 
periodicity of the grating. Since the effective periodicity is altered by 
a change in the real part of the effective index of the grating structure, 
a deflection of the membrane which changes the effective index can induce 
a change in the diffraction properties of the grating device. Thus a 
structure like the one shown in FIG. 7 can be used to make switchable 
filters, deflectors, or couplers, in which the wavelength or polarization 
of peak reflection, or the angle of deflection of guided beams is 
controlled by the state of deflection of the membrane. There are also 
devices in which the periodicity of the grating may be quite long, because 
the grating couples modes of nearly-equal effective index, such as in 
TE-TM mode converters. For these cases, instead of fabricating a fixed 
grating on the core, it may suffice to form a series of membrane devices 
of the type of FIGS. 2 or 4 along the waveguide length, which, when the 
membranes are deflected, themselves constitute a switchable grating. 
A process flow such as that shown in FIG. 8 is used to produce membrane 
devices. The process begins at step 48 with an implantation or diffusion 
to dope the substrate. This doping, which may be blanket or patterned, 
defines the lower electrode 12 shown in FIG. 1. As part of this step, a 
drive-in diffusion may be necessary to ensure that the dopant extends deep 
enough so as not to be consumed by the oxidation step which is used to 
form the lower cladding. Next, the lower cladding 14, possibly SiO.sub.2, 
is formed in step 50 for example, by a thermal oxidation of the silicon 
substrate. The planar waveguide core 16 is formed in step 52 possibly by a 
low pressure chemical vapor deposition of Si.sub.3 N.sub.4. Membrane DMD 
fabrication begins with the deposition of a polymer spacer 18 in step 54 
which may double as the upper cladding layer of the waveguide. This spacer 
is hardened by baking or UV curing, and the DMD membrane is deposited on 
the spacer in step 56 by sputtering or evaporating a thin tensile film 22 
of aluminum alloy. This film is patterned and etched in step 58 with an 
array of tiny access holes 20 through which the polymer can be etched. By 
using an isotropic selective etch, such as a plasma etch, the polymer 
spacer is removed in step 60 in the region of the membrane containing the 
access holes, but it remains elsewhere to support the membrane. Thus we 
are left with a thin metal membrane supported over an exposed portion of 
the waveguide. This membrane can be electrostatically deflected by 
application of a voltage between the membrane and the underlying 
electrode. 
An alternative method of building the membrane DMD makes it possible to 
completely remove the spacer, an example of which is shown in FIG. 9a. 
After the spacer 18 is deposited, vias 62a and 62b are patterned in it. 
When the membrane film is deposited it contacts the layer under the spacer 
in the vias, forming supports 64a and 64b. The membrane is patterned as 
described above. When the spacer is etched, it can be completely removed, 
leaving the membrane supported by metal supports 64a and 64b over air gap 
20 shown in FIG. 9b. This method eliminates the need for a controlled 
spacer etch. 
Several variations on this flow are possible, corresponding to the various 
device configurations discussed previously. For example, the implanted 
electrode may be replaced by a metal electrode. A method of achieving this 
would be to etch recesses into the core and lower cladding after the 
waveguide core layer is deposited. An electrode metal film is deposited 
and etched to form electrodes in the recesses. The recesses allow the 
membrane, when deflected, to intimately contact the waveguide core. A thin 
insulating layer must also be deposited over the electrode to prevent it 
from shorting to the deflected membrane. DMD membrane fabrication would 
then follow as described above. 
Additionally, the array of plasma access holes may be extended to the edge 
of the membrane along the axis parallel to the direction of light 
propagation. When the spacer is etched, polymer spacer will remain only 
along the sides of the membrane, so that no polymer is in contact with the 
waveguide core along the path of light propagation, as in FIG. 4. 
Furthermore, the planar waveguide in FIG. 1 may be replaced by the channel 
waveguide in FIG. 3. A ridge can be patterned and etched into the core, or 
dielectric loading strips can be deposited and patterned onto the core. An 
alternative means of forming a channel waveguide could be by diffusion of 
a species which raises the refractive index into the core or lower 
cladding, forming a partially buried waveguide. 
A dielectric buffer layer 17 in FIG. 1 may be deposited on the core to 
enhance the device operation. This layer should be of a material that will 
not etch during the etching of the spacer. Also, an auxiliary spacer layer 
19 from FIG. 6 could be deposited and patterned after the core layer. This 
material should also not etch during the etching of the spacer. After any 
of the above modifications, the DMD process continues as initially 
discussed. 
These devices can all be manufactured monolithically, lowering defects and 
improving yield. The process is relatively inexpensive, providing 
reasonably priced, reliable devices that have many uses. The structure can 
be used in device which rely on changes in either or both of the real or 
imaginary parts of the effective refractive index. 
Thus, although there has been described to this point a particular 
embodiment for an integrated-optic waveguide device, it is not intended 
that such specific references be considered as limitations upon the scope 
of this invention except in-so-far as set forth in the following claims.