Planar waveguide and a process for its fabrication

A planar waveguide and a process for making a planar waveguide is disclosed. The waveguide has a layer of doped host material formed on a substrate. The host material is a trivalent material such as a metal fluoride, wherein the metal is selected from the Group III B metals and the lanthanide series rare earth metals of the Mendeleevian Periodic Table. The dopant is a rare earth metal such as erbium. The waveguide has an emission spectrum with a bandwidth of about 60 nm for amplification of an optical signal at a wavelength of about 1.51 .mu.m to about 1.57 .mu.m. The waveguide is made by forming the layer of doped host material on a substrate. The film is formed by evaporating materials from two separate sources, one source for the dopant material and a separate source for the host material and forming a film of the evaporated materials on a substrate.

FIELD OF THE INVENTION 
This invention relates to optical amplifiers and, in particular, planar 
optical amplifiers involving rare earth doped materials. 
ART BACKGROUND 
Considerable recent research has involved the development of optical 
amplifiers useful in optical communications. Typically, these amplifiers 
involve a waveguide formed in a glassy material (a material that has no 
long-range ordering and is characterized by an absence of Bragg peaks in 
X-ray diffraction and/or a glass transition observed in differential 
scanning calorimetry) with a rare earth dopant present in the waveguide 
core and with a region of lower refractive index surrounding the core. 
Generally, the glassy host material does not substantially affect the 
emission spectrum of the dopant, and the rare earth dopant material is 
chosen to have a spectral emission line corresponding to a wavelength at 
which optical communication is to be performed. For example, most 
long-haul optical communication is performed either at 1.3 .mu.m or 1.55 
.mu.m. Optical devices that amplify signals at 1.3 .mu.m are described in 
U.S. Pat. No. 5,140,658 to Sunshine et al. 
Optical amplifiers at 1.55 .mu.m have been demonstrated and are described 
in U.S. Pat. No. 5,119,460 to Bruce et al. These amplifiers involve a 
waveguide fiber having erbium, that emits at 1.52 to 1.56 .mu.m, present 
in the core at concentrations typically in the range 10 to 1000 parts per 
million. During operation of the amplifier, optical power at a wavelength 
0.975 or 1.48 .mu.m is introduced into the waveguide core along with a 
signal at the 1.55 .mu.m wavelength. The optical power induces a 
transition in the erbium that populates a state, the .sup.4 I.sub.13/2 
state, capable of stimulated emission around 1.55 .mu.m, and the signal 
induces this transition from the populated state. Thus, the output from 
the amplifier involves a signal at 1.55 .mu.m that has an intensity 
approaching that of the combined power and signal inputs. In this manner, 
an optical signal is amplified, in contrast to electrical amplification 
involving conversion of the optical signal to an electrical signal, 
followed by electrical amplification and another conversion back to an 
optical signal. 
The concentration of the dopant affects the efficiency of the amplifier. 
Since the properties of the amplifier depend upon the absolute number of 
dopant atoms in the host material, the dopant concentration that is 
necessary for adequate performance depends upon the length of the device. 
For example, the dopant concentration in fiber amplifiers is much less 
than the dopant concentration in planar optical amplifiers, because fiber 
amplifiers are much longer than planar optical amplifiers. However, high 
dopant concentrations lead to concentration quenching of the luminescence 
from the dopant. If such quenching occurs, the amplifier gain is reduced 
and the amplifier performance is consequently degraded. Therefore, planar 
optical amplifiers that amplify signals at 1.55 .mu.m and that overcome 
the problems associated with high dopant concentration, and a process for 
making such planar optical waveguides, are sought. 
SUMMARY OF THE INVENTION 
The present invention contemplates a planar optical waveguide that 
amplifies an optical signal at a wavelength from about 1.51 .mu.m to about 
1.57 .mu.m. The planar optical waveguide contains a region suited for 
guiding the signal comprising a doped, waveguide host material. The 
waveguide host material is either a polycrystalline or a single 
crystalline material with trivalent cations. It is advantageous if the 
host material is a trivalent material such as a fluoride of a IIIB metal 
or a rare earth metal (lanthanide series) from the Mendeleevian Periodic 
Table. For example, the material is lanthanum fluoride (LaF.sub.3), 
yttrium fluoride (YF.sub.3), or lutetium fluoride (LuF.sub.3). The 
waveguide host material is doped with other rare earth ions. It is 
advantageous if the dopant is erbium (Er). 
It is advantageous if the dopant concentration in the host material is 
about 0.05 atomic percent to about 12 atomic percent in planar waveguides 
that are about 1 cm to about 20 cm in length. Since the concentration of 
dopant may vary through the thickness of the host material, it is 
advantageous if the maximum dopant concentration is within this range. In 
a preferred embodiment, the maximum dopant concentration is about 4 to 
about 5 atomic percent. 
The dopant concentration through the thickness of the host material is 
either constant or varied. If varied, it is advantageous if the dopant 
profile matches the intensity profile of the light transmitted through the 
waveguide. In this regard, it is also advantageous if the maximum dopant 
concentration is at or near the center of the waveguide. 
The amplifier of the present invention significantly amplifies a signal 
over a broad band. Significant amplification means that the intensity of 
the signal throughout the entire bandwidth is not less than one-third of 
the peak intensity. For example, the planar waveguide doped with erbium as 
previously described has an emission spectrum spanning a range from 1.51 
.mu.m to 1.57 .mu.m in wavelength, giving a 60 nm bandwidth. Furthermore, 
the amplifier of the present invention provides an environment in which 
the lifetime of the spontaneous luminescent emission from the host 
material, as measured according to the description given below, is at 
least about 1 ms. Since longer lifetimes provide a better environment for 
signal amplification, it is advantageous if this lifetime is at least 
about 10 ms. 
To fabricate the planar optical waveguide, a layer of waveguiding host 
material is formed on a substrate. The substrate has a refractive index 
that is lower than the refractive index of the waveguiding host material. 
If the substrate does not have a refractive index that is lower than the 
refractive index of the waveguiding host material, a buffer layer of a 
material with a suitable refractive index is formed between the substrate 
and the waveguiding host material. The planar waveguide is adapted to 
receive an optical signal and to receive power to amplify the optical 
signal. The waveguide is further adapted to output a signal that is an 
amplified input signal. 
Examples of suitable substrates on which the host material is formed 
include single crystalline quartz substrates, fused quartz substrates, 
aluminum oxide substrates, calcium fluoride substrates or silicon 
substrates. Since it is advantageous if the substrate's coefficient of 
thermal expansion matches that of the waveguiding host material, single 
crystalline quartz substrates and aluminum oxide substrates are 
advantageous in this regard. If the substrate has a higher refractive 
index than the waveguide, a film that forms an optical buffer layer is 
formed on the substrate before the film of the host material is formed 
thereon. For example, if the substrate is a silicon substrate, the buffer 
layer is a material that has a refractive index lower than the refractive 
index of the waveguide. Silicon dioxide is an example of a suitable buffer 
layer material. The silicon dioxide layer is formed on the silicon 
substrate using conventional techniques. 
The layer of doped host material is then formed on the substrate. It is 
advantageous if the trivalent host material and the dopant are deposited 
from separate sources, using conventional apparatus such as evaporation 
ovens, electron beam evaporators, and the like. In one embodiment, the 
trivalent host material is formed using LaF.sub.3, YF.sub.3 or LuF.sub.3 
as source material and the dopant is introduced using ErF.sub.3 as source 
material. Sources and techniques for depositing films of the host and 
dopant materials specified above onto substrates are well known to those 
skilled in the art. It is advantageous if the temperature of the substrate 
during the formation of the doped, host material layer is about 
300.degree. C. to about 600.degree. C. 
The thickness of the host material layer so formed is a matter of design 
choice. Film thicknesses of about 0.8 .mu.m to about 2 .mu.m are 
contemplated as suitable. Film thicknesses greater than 2 .mu.m are also 
contemplated to reduce coupling losses to fibers with larger cores. 
Because independent sources are used for the layer of host material and the 
dopant material, the concentration of the dopant in the host material is 
widely variable. For example, it is contemplated that the maximum 
concentration of ErF.sub.3 in LaF.sub.3 host material is from about 0.05 
atomic percent to about 20 atomic percent in the optical devices of the 
present invention. The concentration of the dopant is variable throughout 
the thickness of the layer of the host material in a particular device. As 
previously mentioned, if the dopant concentration varies through the 
thickness of the host material, then the maximum dopant concentration in 
the layer is found near the center of the waveguiding layer. Such a dopant 
profile is advantageous because the maximum mode intensity of the light 
occurs also at the center of the waveguide.

DETAILED DESCRIPTION 
As discussed, the invention is directed to a planar waveguide which 
amplifies an optical signal at a wavelength of about 1.51 .mu.m to about 
1.57 .mu.m. The present invention also contemplates a process for making 
such a waveguide. 
The planar optical waveguide contains a region suited for guiding the 
signal comprising a layer of a doped host material. Er is an example of a 
suitable dopant. 
The host material is chosen to match the valence state of the dopant, such 
that the dopant substitutes for a host cation rather than occupying 
interstitial sites. The symmetry of the site at which dopant atoms are 
introduced as well as the size and oxidation state of the 
host-constituent-atom being replaced by the dopant determines the allowed 
oxidation states of the dopant atoms. Site symmetries, exemplary host 
materials having those symmetries, and the other properties that lead to a 
desired oxidation state are well known and are tabulated in compendia such 
as Laser Crystals, Alexander Kaminskii, Springer-Verlag, 1981, and the 
Major Ternary Structural Families, Miller & Roy, Springer-Verlag, 1974. 
Thus, for example, Er.sup.3 + are maintained in this valence state by 
introduction into a film of a trivalent host material that is, for 
example, a fluoride of a IIIB metal or a rare earth metal from the 
Mendeleevian Periodic Table. Examples of these host materials include 
YF.sub.3, LaF.sub.3, and LuF.sub.3. The structure of the host material is 
either single crystalline, polycrystalline or amorphous. Polycrystalline 
materials have certain processing advantages because they are more easily 
produced. 
The gain of the amplifier is proportional to the concentration of the 
dopant in the host. Dopants are introduced in the concentration range of 
about 0.05 to about 12 atomic percent. Concentrations of less than about 
0.05 atomic percent typically lead to undesirably low gain in the 
amplifier. Although concentrations above about 2 atomic percent are 
typically avoided because the possibility of concentration quenching is 
enhanced at these concentrations, the present invention contemplates 
dopant concentrations of up to about 12 atomic percent in the waveguiding 
host material. The problems associated with concentration quenching are 
reduced because the dopant is introduced substitutionally into the host 
material. Consequently, fewer charge compensating defects arise in these 
materials than in materials in which the dopant occupies interstitial 
sites in the host material. This in turn reduces the ion clustering, which 
would otherwise occur at such high dopant concentrations and which would 
deteriorate amplifier performance. In a preferred embodiment the dopant 
concentration is about 4 atomic percent to about 5 atomic percent. 
The waveguides of the present invention are illustrated in FIGS. 1-4. FIG. 
1 illustrates a planar waveguide 10 formed on a substrate 20. As indicated 
by the shaded area 30 the layer 10 is doped through its entire thickness. 
The layer 10 is formed directly on a suitable substrate 20. Quartz, fused 
quartz, aluminum oxide, silicon and calcium fluoride are examples of 
suitable substrate materials. 
In the alternate embodiment pictured in FIG. 2, the waveguide has a buffer 
layer 40 formed between the substrate layer 80 and the waveguide material 
70. The buffer layer 40 provides optical separation between the substrate 
80 and the waveguide layer 70 when the refractive index of the substrate 
80 is higher than that of the waveguide material 70. In such an instance, 
the buffer layer 40 is made of a material with an index of refraction that 
is lower than the index of refraction of the waveguide material 70. One 
skilled in the art will recognize that, to achieve optical separation, the 
index of refraction of the buffer layer need be only nominally lower than 
the index of refraction of the waveguide material. For example, if the 
substrate 80 is made of a material with a high index of refraction such as 
silicon, it is advantageous if the waveguide has a buffer layer. Silicon 
dioxide, which has an index of refraction that is lower than that of the 
trivalent host materials of the present invention, is an example of a 
suitable buffer layer material. FIG. 2 also depicts a waveguide in which 
the doping profile of the waveguiding material 70, indicated by the shaded 
area 60, varies through the thickness of the waveguiding layer. 
The waveguide layer (10 in FIG. 1) is formed in one embodiment by placing 
the substrate in a chamber. The chamber is evacuated. Separate sources for 
the host and dopant materials are provided so that the amount of material 
from each source that is incorporated into the waveguide layer 10 is 
controlled. For example, the host and dopant material are each placed in a 
separate Knudsen oven that is commercially obtained from EPI of Saint 
Paul, Minn. The composition of the layer is controlled by controlling the 
amount of material that is evaporated from each source. Increasing or 
decreasing the temperature of the oven in which the dopant source is 
placed will correspondingly increase or decrease the dopant concentration 
in the waveguide layer. The dopant concentration is varied through the 
thickness of the waveguide layer by modulating the temperature of this 
oven as the waveguiding layer is formed. A similar effect is obtained by 
individually controlling the oven shutters which controls the amount of 
material flowing from the oven and into the chamber. 
In one embodiment wherein a waveguide layer with a composition of 5 atomic 
percent ErF.sub.3 and 95 atomic percent LaF.sub.3 is desired, the 
temperatures of the Knudsen ovens are adjusted so that for 5 units of 
ErF.sub.3 evaporated, 95 units of LaF.sub.3 are evaporated. To form a 
waveguide layer with this composition, the oven temperature for LaF.sub.3 
is set at about 1360.degree. C. and the oven temperature for ErF.sub.3 is 
set at about 1150.degree. C. 
In an alternate embodiment, after the waveguide layer with the desired 
composition is formed on the substrate, the layer is patterned to form a 
ridge waveguide. Such a waveguide is illustrated in FIG. 3. To pattern the 
waveguide layer, a material such as a photoresist (not shown) is first 
formed on the waveguide layer. The photoresist is patterned by 
conventional techniques such as lithography in conjunction with etching to 
form the configuration shown in FIGS. 3 and 4. (Lithographic and etching 
techniques are described respectively in Nishihara et at., Optical 
Integrated Circuits, McGraw-Hill 1985). The ridge waveguide illustrated in 
FIG. 3 results when the waveguide layer is formed directly over the 
substrate 200 and patterned as described above to form a ridge 220. The 
ridge waveguide illustrated in FIG. 4 results when the ridge 330 is formed 
over a buffer layer 310, which is formed over a substrate 300. Illustrated 
schematically in FIG. 3 are a coupling 230 for introducing optical power 
for amplification and for introducing a signal into the ridge waveguide 
220 and a coupling for 240 for signal output. 
In one embodiment of the present invention, the dopant concentration in the 
waveguide layer is uniform through the thickness of the waveguide layer. 
In an alternate embodiment, the dopant profile is matched to the intensity 
profile of the light that travels through the waveguide. In the latter 
embodiment the dopant profile through the waveguide is such that the 
maximum dopant concentration in the host material is found at a point 
about equidistant from the top and the bottom of the planar waveguide. 
This dopant profile is depicted as 60 in FIG. 2 and 210 in FIG. 3. 
The resulting amplifiers are useful for amplification of signals associated 
with optical communications. Nevertheless, other applications such as high 
power optical amplifiers contemplated for use in cable television systems 
are possible and are not precluded. Insertion of signal and amplification 
power is accomplished by expedients such as described in Integrated Optics 
Theory and Technology by Hunsperger, Springer-Verlag, 1982. Output signals 
are coupled to the waveguide amplifier by methods such as those described 
in Hunsperger. Typical approaches for input and output coupling include 
the use of an input and output silica optical fiber butted to the 
waveguide region of the amplifier. 
The following examples are illustrations of specific embodiments of the 
claimed invention. 
EXAMPLE 1 
A silicon wafer with a 10 .mu.m thick layer of silicon dioxide formed 
thereon was placed in a vacuum chamber. The chamber was evacuated to a 
pressure of about 10.sup.-9 Torr. Sources of LaF.sub.3 and ErF.sub.3 were 
each placed in a separate Knudsen ovens (EPI, St. Paul, Minn.). The ovens 
were heated to a temperature of about 1360.degree. C. for LaF.sub.3 and 
1150.degree. C. for ErF.sub.3. The pressure during the deposition was 
maintained at or below 10.sup.-7 Torr. During deposition the temperature 
of the ovens was controlled by a closed loop feedback system, using a 
tungsten/rhenium thermocouple. For a uniform composition throughout the 
film, the temperatures of the Knudsen cell ovens were kept constant during 
the formation of the layer. The resulting film had a composition that was 
about five mole percent ErF.sub.3 and 95 mole percent LaF.sub.3 uniformly 
throughout the layer. 
The substrate temperature was held at about 550.degree. C. The resulting 
film thickness was about 0.8 .mu.m. The film thickness was measured using 
a DEKTAK.TM. instrument. The composition and film thickness were 
independently determined using Rutherford Backscattering Spectrometry. The 
resulting films were polycrystalline. 
The resulting waveguides transmitted signals with wavelengths of about 0.20 
.mu.m to about 20 .mu.m. A photoluminescence spectrum of a planar 
waveguide was measured using the 514.5 nm line of an argon laser, with a 
power of about 400 mW and a beam diameter of about 0.5 mm. The 
luminescence was spectrally analyzed with a single-grating monochromator, 
and the signal was detected with a liquid-nitrogen cooled germanium 
detector. The pump beam was chopped at a frequency of 11 Hz, and the 
signal was amplified using a lock-in amplifier. The photoluminescence is 
depicted in FIG. 5 as a function of wavelength. As demonstrated by FIG. 5, 
the planar amplifier significantly amplified signals in the band of about 
1.51 .mu.m to about 1.57 .mu.m. Thus the amplifier demonstrated a broad 
(about 60 nm) bandwidth of significant amplification. 
The lifetime of the luminescence as a function of the concentration of the 
dopant in the film was measured by monitoring the decay of the 
luminescence of the waveguide layer made according to the above example 
after switching off the light source used to photo-excite the films. These 
lifetimes as a function of Er concentration in the film are illustrated in 
FIG. 5. The measured lifetime of the luminescence in a film that contained 
5 atomic percent Er decayed exponentially with a time constant of 12.8 ms. 
This relatively long lifetime illustrates that the waveguide layer 
provides a good environment for amplification.