Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2001-03-02
2003-06-10
Ullah, Akm E. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C439S290000, C359S627000
Reexamination Certificate
active
06577799
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the production of planar lightwave circuits (PLC's). More particularly, the invention relates to a method for defining the light wave circuit or components of the circuit by direct photoinduced changes in refractive index of a thin film forming the basis of the PLC.
BACKGROUND ART
There is an increasing demand for planar lightwave circuits for advanced optical communications networks, optical sensing and also as the basis of other photonic devices, such as high frequency signal processors for use in military and other applications.
The basis of most PLC's is a trilayer of optically transparent thin films deposited on a substrate of generally silicon or silica as shown in FIG.
1
. The central, or core layer
1
of the sandwich structure normally has higher refractive index n
c0
than the refractive index n
c1
of outer cladding layers
2
,
3
, and this simple system is known as a planar waveguide. Light injected into the core layer undergoes total internal reflection at both core/cladding boundaries
4
,
5
and is confined in this transverse dimension, resulting in 1-dimensional light guidance. However as a consequence of the constant refractive index in the plane of the film total internal reflection is not possible, and light spreads or diffracts laterally in the guiding layer. To impart useful functionality to a planar waveguide, 2-dimensional light guidance is required, and planar diffraction must be overcome by introducing local changes in the core layer refractive index. The light guides so formed are known as channel waveguides, the basic elements of PLC'S, and the final product is a planar lightwave circuit which can exhibit a wide range of optical functions. An example of a simple device is a concatenated Y-junction splitter where the signals from a single input channel are split evenly into a larger number of output channels independent of wavelength.
Currently, there are several classes of materials and processing methods which can be used to produce planar waveguides. These include silica glass (using plasma enhanced chemical vapour deposition: PECVD, or flame hydrolysis: FHD); organically modified silicate glasses (ORMOSILs) and plastics produced via wet chemical synthesis and spin coating; and III-V semiconductors produced by MOCVD or MBE growth.
The waveguides in the PLC are defined by structuring or patterning of the refractive index in the plane of the film. In the microelectronics industry, the standard patterning technique is known as mask photolithography. The first step in this process is to deposit an additional thin film of photo resist onto the planar waveguide core, usually by spin coating. The photo resist film is then preferentially exposed to a broadband extended UV source through an amplitude mask such that a photochemical reaction is initiated below the high transmission areas of the mask. The photochemical reaction changes the solubility of the photo resist enabling it to be removed by agitation in a suitable solvent. Depending on whether negative or positive tone resist is used, the irradiated regions will remain or be removed respectively. The photo resist pattern may then be transferred to the waveguide core layer by removing core material from the unwanted regions by a process such as reactive ion etching. Removal of the remaining resist and over cladding with a low refractive index film completes the standard processing of the PLC. Two dimensional waveguiding is therefore achieved through selective removal of the high index core material.
As an alternative to the photo resist technology, materials such as plastics, ormosils and some glasses can allow refractive index patterning to be achieved without the use of an additional photo resist layer. In this class of materials, direct exposure generally to UV radiation initiates a photochemical reaction that raises the refractive index of the core material, enabling channel waveguides to be formed. The materials are generically described as photosensitive. The use of mask photolithography on these films therefore results in the production of PLC'S without the requirement for the deposition of an additional photo resist layer or any reactive ion etching or wet development.
The production of a suitable mask is however costly and time consuming and, particularly in the prototyping stage, limits the number of device designs that can be tested. Recently it has been demonstrated that the requirement for a mask can be eliminated by using the laser direct writing (LDW) technique. This is described in M. Svalgaard, ‘Direct writing of planar waveguide power splitters and directional couplers using a focused ultraviolet laser beam’,
Electronics Letters,
Vol. 33, No. 20, pp. 1694-1695, 1997. In contrast to standard mask photolithography, in the LDW process the photosensitive planar waveguiding film is accurately traversed under a focused laser beam to locally expose the material and directly delineate the channel waveguides without the use of a mask. The LDW process is very versatile and permits a wider range of structures than is generally possible by exposure through a mask. For example, using a mask the exposure will be uniform across the whole wafer, and hence the refractive index change obtained cannot vary from one part of the waveguide structure to the next. Using LDW, on the other hand, permits the exposure, and hence the induced change of the refractive index, to be adjusted even over short distances permitting a wider range of waveguide structures to be written. Furthermore, the LDW process permits the waveguide pattern to be changed from wafer to wafer because the generated pattern can be under direct software control.
As with mask photolithography, depending on the waveguide material used, the laser induced photochemical reaction can either directly induce a refractive index change in the waveguiding material without further processing, the material system can be locally exposed and then subjected to wet development, or a secondary layer of photo resist may be used and the photo resist pattern transferred to the waveguiding core layer. Therefore depending on the laser source, the process is applicable to many different material systems e.g. glass, polymer, sol-gel, Ti:LiNbO
3
. In addition, the use of high power lasers and/or different laser wavelengths can access photosensitive mechanisms that cannot be attained with mask photolithography.
In general the refractive index, n, of a photosensitive material is dependent on it's exposure, F, to electromagnetic radiation, and as shown in
FIG. 2
the material response function, n(F), typically exhibits a saturation behaviour up to a maximum exposure, F
sat
. A plan view of the laser writing process is shown in
FIG. 3
, where we consider delineation of a channel waveguide of width
2
a
in the z direction. For a laser irradiance distribution, I(y,z), the lateral exposure of the material in the y direction, F(y), is given by;
F
(
y
)
=
1
v
∫
-
a
(
a
-
y
)
a
(
a
-
y
)
I
(
y
,
z
)
ⅆ
z
(
1
)
where v is the writing velocity. The lateral exposure, F(y), for a truncated (beam waist, w=a) TEM
00
beam with irradiance distribution I(y,z)=I
00
exp(−2(y
2
+z
2
)/w
2
), is shown by the dashed line in FIG.
4
. Clearly the exposure is peaked around y=0. For optimum use of the available refractive index change the maximum lateral exposure, F
max
should equal the material saturation exposure, F
sat
. Taking into account the material response n(F), the corresponding refractive index distribution n(y) also shows a maximum at y=0 as shown by the dashed line in FIG.
5
. Laser direct write systems so far have employed the focusing or imaging of weakly truncated Gaussian (TEM
00
) beams, and operating under this scenario therefore produce channel waveguides with laterally graded refractive index profiles. Indeed, graded thickness profiles are often observed in wet developed polymer waveguides. T
Charters Robbie
Ladouceur Francois
Luther-Davies Barry
Nixon & Peabody LLP
Studebaker Donald R.
The Australian National University
Ullah Akm E.
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