Method for altering the temperature dependence of optical waveguides devices

In accordance with the invention glass waveguide devices are provided with enhanced temperature stability by incorporating within appropriate lengths of the waveguides a transparent compensating material having a refractive index variation with temperature that differs substantially from that of the waveguide. The compensating material can be a non-glass material, such as a liquid, driven into the glass by heat and pressure. In a preferred embodiment, D.sub.2 O is incorporated into waveguides for optical communications. The D.sub.2 O is transparent to the preferred communications wavelengths centered at about 1.55 .mu.m and has a dn/dT opposite in polarity to the dn/dT of glass. The resulting structure exhibits enhanced temperature stability with reduced magnitude of dn/dT. The technique is particularly useful in devices based on interference between multiple waveguides, as it is not necessary to reduce dn/dT to zero in the respective waveguides. It suffices to compensate the differences. Such compensation can be achieved by compensating materials having dn/dT of either the same polarity as the dn/dT of the waveguides or the opposite polarity. Preferred embodiments include routers, Fourier filters and Bragg filters. In single waveguide devices such as gratings, compensating materials of opposite polarity can substantially enhance the temperature stability.

FIELD OF THE INVENTION 
This invention relates to methods for making optical waveguide devices and, 
in particular, to a method for making such devices having enhanced 
temperature stability. 
BACKGROUND OF THE INVENTION 
As optical communications systems are widely deployed, there is an 
increasing need for devices capable of combining, separating, switching, 
adding and dropping optical signals. For example, broadband optical 
multiplexers are needed for delivering voice and video signals to the 
home, for combining pump and communications signals in an optical 
amplifier, and for adding monitoring signals to optical fibers. Dense 
wavelength-division multiplexing (WDM) systems need multiplexers to 
combine and separate channels of different wavelengths and need add-drop 
filters to alter the traffic. Low speed optical switches are needed for 
network reconfiguration. 
These important functions are typically performed by optical waveguide 
devices such as integrated optical silica waveguide circuits formed on 
planar silicon substrates. Such waveguides are typically formed by 
depositing base, core and cladding layers on a silicon substrate. The base 
layer can be made of undoped silica. It isolates the fundamental optical 
mode from the silicon substrate and thereby prevents optical loss at the 
silica substrate interface. The core layer is typically silica doped with 
phosphorus or germanium to increase its refractive index and thereby 
achieve optical confinement. The cladding is typically silica doped with 
both boron and phosphorus to facilitate fabrication and provide an index 
matching that of the base. Using well-known photolithographic techniques, 
the cores can be economically configured into a wide variety of compact 
configurations capable of performing useful functions. See, for example, 
Y. P. Li and C. H. Henry, "Silicon Optical Bench Waveguide Technology", 
Ch. 8, Optical Fiber Telecommunications, Vol. IIIB, p. 319-375 (Academic 
Press, 1997). 
Other waveguide devices are made of optical fiber. Optical fibers typically 
comprise a higher index core, which can be doped silica, and a surrounding 
cladding of a lower index glass. A variety of all-fiber devices are made 
by providing one or more Bragg gratings in the fiber core. Such gratings 
are conventionally made by providing the core with a photosensitive dopant 
such as germanium and side-writing a grating using ultraviolet light. 
One shortcoming of these optical waveguide devices is their sensitivity to 
temperature. Many waveguide devices are based upon optical interference 
between beams of light propagated down different paths. Depending on the 
phase relationship between the beams at the point of recombination, light 
will either be transmitted or reflected back. Spectrally narrow, high 
contrast resonances can be readily designed, enabling high performance 
wavelength division multiplexers and blocking filters. However variable 
ambient temperature has a perceptible and disadvantageous effect on the 
performance of such devices. The refractive index of the composite glass 
structure through which the light travels depends on temperature. Thus the 
spectral positions of critical resonances shift with temperature. 
Similar problems occur in fiber waveguide devices. Bragg gratings, for 
example, are critically dependent on the path lengths between successive 
index perturbations. But these path lengths change due to the temperature 
dependence of the refractive index, shifting the operating wavelength of 
the gratings. 
For many applications such variation is not acceptable, and the devices are 
placed in temperature compensating packages for stable operation. Such 
packaging is expensive and adds reliability problems. Accordingly there is 
a need for waveguide devices having enhanced temperature stability. 
SUMMARY OF THE INVENTION 
In accordance with the invention glass waveguide devices are provided with 
enhanced temperature stability by incorporating within appropriate lengths 
of the waveguides a transparent compensating material having a refractive 
index variation with temperature that differs substantially from that of 
the waveguide. The compensating material can be a non-glass material, such 
as a liquid, driven into the glass by heat and pressure. In a preferred 
embodiment, D.sub.2 O is incorporated into waveguides for optical 
communications. The D.sub.2 O is transparent to the preferred 
communications wavelengths centered at about 1.55 .mu.m and has a dn/dT 
opposite in polarity to the dn/dT of glass. The resulting structure 
exhibits enhanced temperature stability with reduced magnitude of dn/dT. 
The technique is particularly useful in devices based on interference 
between multiple waveguides, as it is not necessary to reduce dn/dT to 
zero in the respective waveguides. It suffices to compensate the 
differences. Such compensation can be achieved by compensating materials 
having dn/dT of either the same polarity as the dn/dT of the waveguides or 
the opposite polarity. Preferred embodiments include routers, Fourier 
filters and Bragg filters. In single waveguide devices such as gratings, 
compensating materials of opposite polarity can substantially enhance the 
temperature stability.

It is to be understood that these drawings are for purposes of illustrating 
the concepts of the invention and are not to scale. 
DETAILED DESCRIPTION 
Referring to the drawings, FIG. 1 is a block diagram of the steps involved 
in enhancing the thermal stability of a waveguide device. The first step, 
as illustrated in block A, is to provide a glass waveguide device to be 
improved. The waveguide device can be either a planar waveguide device, a 
fiber waveguide device or a combination of the two. Exemplary devices 
include routers, Fourier filters and Bragg gratings. 
FIG. 2A is a cross sectional view of an exemplary waveguide device (here a 
planar device) comprising a substrate 10, such as silicon, a base layer 
11, such as undoped silica, one or more waveguide defined by one or more 
cores 12, 13, 14 and a cladding 15. The cores can be P-doped or Ge-doped 
silica having a refractive index increased by a percentage (typically 
.DELTA.=0.60-0.70%) as compared with the base. The cladding can be doped 
with boron and phosphorus to achieve both a lowered flow temperature and 
an index preferably equal to the base layer. 
FIG. 2B is a plan view of the device of FIG. 2A. The cores 12, 13, 14 
define optical waveguides of different lengths that extend between a 
common input 15 and a common output 16. Variations in temperature will 
produce different absolute thermal pathlength changes in the two 
waveguides. The methods for fabricating such waveguides are well known in 
the art and are described in further detail in C. H. Henry et al. "Glass 
Waveguides on Silicon for Hybrid Optical Packaging, J. Lightwave Technol., 
1539 (1989). 
The next step, which is optional in some applications, is to mask the 
waveguide, leaving exposed those regions where the refractive index 
variation with temperature (dn/dT) is to be altered. Where dn/dT is to be 
altered for the full length of the waveguide, masking is not required. But 
in applications where it is desired to equalize the effect of temperature 
variation among plural waveguides, different length waveguides will 
generally require masking to provide exposed regions of different length. 
The masking material should be impermeable to the treatment material. 
Silicon nitride films having a thickness on the order of 1 .mu.m is 
preferred for masking devices to be treated with D.sub.2 O. Such films can 
be deposited by plasma CVD. 
The third step (FIG. 1, block C) is to incorporate into the exposed regions 
of the waveguides a thermal compensating material which is transparent to 
the operating wavelength and which has a dn/dT different from that of the 
waveguide material. Typical waveguide glasses have a positive dn/dT, so 
the material incorporated into glass should have a negative dn/dT or a 
positive dn/dT substantially different from that of glass. Suitable 
negative dn/dT compensating materials include D.sub.2 O, ethanol and 
methanol. The amount of material should exceed 1 weight percent of the 
glass and preferably should exceed 10%. D.sub.2 O is preferred for glass 
communications devices operating at 1.55 .mu.m. 
D.sub.2 O can be incorporated in glass by exposing the glass to D.sub.2 O 
steam at elevated temperature (100-300.degree. C.) and pressure (15-1500 
psi) for a period typically 1-20 hr. FIG. 2C shows the device of FIG. 2B 
after treatment with D.sub.2 O in an exposed region such as triangle 20. 
Waveguide 12 is not exposed. Longer waveguide 13 is exposed over a first 
length, and the longest waveguide 14 is exposed over a second length 
longer than the first. The resulting device has enhanced temperature 
stability. Specifically, the constitutive waveguides are processed so that 
their optical pathlengths are affected equally by changes in temperature. 
Alternatively if a compensating material having a positive dn/dT greater 
than glass were used, then the shorter waveguides would be treated over 
longer lengths to achieve compensation. 
The final steps, which are optional, are to remove the mask (block D) and 
to seal the incorporated material into the glass (block E). Sealing can be 
done by applying a thin coating of metal such as a few hundred nanometers 
of chromium or gold over the treated region 20. 
The degree of temperature compensation which can be achieved by this 
process is demonstrated by the following specific example. Sample 1 is a 2 
cm length planar waveguide treated with D.sub.2 O at 300.degree. C. for 15 
hrs. Sample 2 is a 2 cm length of similar, untreated planar waveguide. 1.5 
micrometer laser light was launched into each of the two samples and the 
temperature was raised approximately 40.degree. C. from room temperature 
to about 62.degree. C. The interference between the front (entrance face) 
and back (exit face) reflections were monitored. FIG. 3A shows the 
interference fringes plotted against temperature for the treated sample 
and FIG. 3B shows the infringes for the untreated sample. As can be seen, 
the treated sample has fewer fringes corresponding to a lower magnitude 
dn/dT. Specifically, the magnitude of dn/dT for the treated sample is 9/16 
that of the untreated sample for an enhancement factor e.apprxeq.0.56 
(56%). The length l of treated region required to compensate two 
waveguides of unequal length l.sub.1, l.sub.2 can readily be calculated 
from the difference in length .DELTA.l=.vertline.l.sub.1 -l.sub.2 
.vertline. and the enhancement factor e by the relation l=.DELTA.l/e. So, 
for example, if .DELTA.l is 0.25 mm and e=0.5, then l is 0.5 mm. 
Device Applications 
The process of FIG. 1 permits the fabrication of a wide variety of glass 
waveguide devices with enhanced temperature stability. In general, the 
device is fabricated in the usual fashion, and the process of FIG. 1 is 
then applied after fabrication to alter the temperature coefficient of 
refractive index for one or more of the glass waveguides. In 
multi-waveguide devices, this alteration can be applied in a spatially 
selective manner to equalize the temperature effects on different 
waveguides and thereby making the overall device temperature insensitive. 
In single waveguide devices the reduction in temperature sensitivity is 
proportional to minimization of dn/dT. Three important device applications 
will be illustrated: 1) temperature compensation of a multiwaveguide 
router, 2) temperature compensation of a multiwaveguide filter, and 3) 
reduction in temperature dependence of a single waveguide Bragg grating. 
A. Temperature Compensation of a Multiwaveguide Router 
FIG. 4 schematically illustrates an improved form of a device known as a 
waveguide grating router. The conventional portion of the device 40 
comprises a pair of star couplers 41, 42 connected by an array 43 of 
waveguides that act like a grating, specifically there is a constant 
pathlength difference between adjacent waveguides in the array. The two 
star couplers 41, 42 are mirror images, except the number of inputs and 
outputs can be different. 
In conventional operation, the lightwave from an input waveguide 44 couples 
into the waveguide grating array 43 by input star coupler 41. If there 
were no differential phase shift in the grating region, the lightwave 
propagation to the output coupler 42 would appear as if it were the 
reciprocal propagation in the input coupler. The input waveguide would 
thus be imaged at the interface between the output coupler and the output 
waveguides. The imaged input waveguide would be coupled to one of the 
output waveguides. But the linear length difference in the grating array 
results in a wavelength-dependent tilt of the wavefront in the grating 
waveguides and thus shifts the input waveguide image to a 
wavelength-dependent position. As the wavelength changes, the input 
waveguide image sweeps across and couples light onto different output 
waveguides. The structure and operation of the conventional device is 
described in greater detail in U.S. Pat. No. 5,467,418 issued to C. 
Dragone on Nov. 14, 1995 which is incorporated herein by reference. 
In accordance with the invention, the temperature stability of the device 
is enhanced by introducing a region of altered dn/dT in the waveguide 
grating to compensate the thermal response of the constituent waveguides. 
This may be conveniently accomplished using the process of FIG. 1 by 
introducing D.sub.2 O into a triangular region 45 of the array. The base b 
of the triangle is located so that the longer waveguides have longer 
treated segments in the triangular region. Assuming the 9/16 reduction of 
FIGS. 3A, a triangle for a typical grating array would have a base on the 
order of 1 cm. If, instead of using a negative dn/dT compensating 
material, one used a compensating material having a positive dn/dT 
substantially greater than glass, then compensation could be achieved by 
inverting the triangular region 45 so that the shortest waveguide was 
treated over the longest region. 
B. Temperature Compensated Multiwaveguide Filters 
FIG. 5 is a schematic top view of a simple form of a monolithic optical 
waveguide filter 10 known as a Fourier filter. The conventional Fourier 
filter comprises a pair of optical waveguides 51 and 52 on a substrate 53 
configured to form a plurality N of optical couplers 54, 55 and 56 
alternately connected by a plurality of N-1 delay paths 57 and 58. Each 
coupler is comprised of a region of close adjacency of the two waveguides 
where the exponential tail of light transmitted on each of waveguides 51 
and 52 interacts with the other, coupling light from one waveguide to the 
other. The amount of power coupled from one waveguide to the other is 
characterized by the effective length of the coupler. 
Each delay path comprises a pair of waveguide segments between two 
couplers, for example segments 57A and 57B between couplers 54 and 55. The 
segments are configured to provide unequal optical path lengths between 
the two couplers, thereby providing a differential delay. 
In operation, an optical input signal is presented at an input coupler, 
e.g. along waveguide 51 at coupler 54, and a filtered output is presented 
at an output coupler, e.g. along waveguide 52 at coupler 56. The sequence 
of couplers and delays provide light at the input with a plurality of 
paths to the output. In general there will be 2.sup.N-1 paths where N is 
the number of couplers. 
Each of the optical paths of the filter provide light corresponding to a 
harmonic component in a Fourier series whose summation constitutes the 
transmission function of the filter. By proper choice of parameters one 
can closely approximate a desired transmission function. The structure and 
fabrication of such filters is described in further detail in U.S. Pat. 
No. 5,596,661 issued to C. H. Henry et al. on Jan. 21, 1997 which is 
incorporated herein by reference. 
As can readily be seen, the proper operation of the Fourier filter depends 
upon precise control of the differential delay between coupled waveguides. 
Variation of this differential delay due to different effects of 
temperature change adversely affects performance of the filter. 
In accordance with the invention, this device can be temperature 
compensated by forming one or more regions 59A, 59B of altered dn/dT in 
accordance with the method of FIG. 1. Preferably the regions 59A, 59B are 
formed in the longer waveguides e.g. 57B and 58B. Assuming the level of 
alteration shown in FIG. 3A, compensation for typical Fourier filters 
could be achieved in rectangular regions having lengths of approximately 
twice the pathlength difference. 
C. Bragg Gratings With Reduced Temperature Sensitivity 
FIG. 6 is a schematic cross section of an optical waveguide Bragg grating 
device comprising a length of optical waveguide 60 (here optical fiber) 
having a core 61, a cladding 62, and a Bragg grating 63 comprising a 
plurality of index perturbations 64 in the core index substantially 
equally spaced along the waveguide. These perturbations selectively 
reflect light of the wavelength .lambda. equal to twice the spacing 
.LAMBDA. between successive perturbations, i.e. .lambda.=2.LAMBDA.. The 
remaining wavelengths pass essentially unimpeded. Such Bragg gratings have 
found use in a variety of applications including filtering, stabilization 
of semiconductor lasers, reflection of fiber amplifier pump energy and 
compensation for fiber dispersion. The temperature sensitivity of the 
Bragg resonance depends in important part on dn/dT of the waveguide in 
which it is written. In accordance with the invention, the temperature 
stability is enhanced by reducing the thermal sensitivity of the waveguide 
in accordance with the method of FIG. 1. Here optional masking could 
selectively expose a waveguide portion 65 where the grating is written. In 
the case of fiber waveguides, which often use pure silica cladding, it may 
be necessary to use a different cladding, such as P, B doped silica, to 
permit introduction of D.sub.2 O. No change in typical cladding 
composition is needed for Bragg gratings in planar waveguide Bragg 
gratings. 
It is to be understood that the above-described embodiments are 
illustrative of only a few of the many possible specific embodiments which 
can represent applications of the principles of the invention. Numerous 
and varied other arrangements can be made by those skilled in the art 
without departing from the spirit and scope of the invention.