Optical fiber end for application in an optical isolator and a method of manufacture thereof

An optical fiber having an improved input fiber end for an optical isolator. The end of the fiber has a flat end surface substantially perpendicular to the longitudinal axis of the fiber. An optical barrier layer, formed by a layer of chromium and a layer of gold, covers the end surface of the fiber with an aperture exposing the core and covering the cladding of the fiber. Light transmission into the end is substantially reduced to increase the reflection loss of the optical isolator.

BACKGROUND OF THE INVENTION 
The present invention is related to optical fibers and, more particularly, 
to techniques for reducing reflection back into input optical fibers. Such 
techniques find applications in many fields, including optical devices 
such as isolators. 
In present day optical fiber technology, semiconductor lasers are typically 
used to generate and relay light signals on optical fibers. These lasers 
are particularly susceptible to light signal reflections, which cause a 
laser to become unstable and noisy. Optical isolators are used to block 
these reflected signals from reaching the laser. Ideally, these optical 
isolators transmit all of the light signals in the forward direction and 
block all of the signals in the reverse direction. Thus an optical 
isolator's isolation, which is a measure of the amount of light lost 
travelling from an output fiber to an input fiber of the optical isolator, 
is much larger than its insertion loss, which is a measure of light lost 
travelling in the forward direction. 
The present invention allows at least one type of optical isolator to raise 
its reflection loss near the ideal, i.e., that no light at all from the 
output fiber is transmitted back into the input fiber. 
The central elements of this type of optical isolator are illustrated in 
FIG. 1A. The isolator has two birefringent crystal polarizers 10A and 10B, 
between which is placed a Faraday rotator 11. The rotator 11 is formed 
typically from doped garnet or YIG, and is placed in a permanent magnet 
12. On one side of these central elements is placed the input optical 
fiber and on the other side is placed the output optical fiber. A 
collimating element, such as an ordinary lens or a graded index lens, is 
placed between each optical fiber and the central elements. (The optical 
fibers and collimating elements are not shown in this drawing.) 
In the forward direction, collimated light from the input fiber is directed 
toward the slanted front face of the polarizer 10A. Each of the 
birefringent polarizers 10A and 10B have two indexes of refraction, one 
for the light polarized perpendicularly to the optical axis and another 
for the light polarized parallel to the optical axis of the birefringent 
polarizer 10A. The light, represented by a ray 14 in FIG. 1A, is split 
into two rays, an ordinary ray 14B polarized perpendicularly to the 
crystal's optical axis and an extraordinary ray 14A polarized parallel to 
the optical axis, in accordance with the polarization modes of the 
incoming light. The light from the polarizer 10A is rotated by the Faraday 
rotator by 45.degree.. Due to the orientation of the optical axis of the 
second polarizer 10B, the two rays 14A and 14B leave the second polarizer 
10B in parallel and in a direction so that the second collimating element 
combines and refocuses the light into the core of the output fiber. 
A slightly different operation occurs when light is sent back in the 
reverse direction, as illustrated in FIG. 1B. The light from the output 
fiber and second collimating element represented by a ray 15 is split into 
two rays 15A and 15B by the second polarizer 10B and rotated by the 
Faraday rotator 11. This rotation is nonreciprocal with the rotation of 
light in the forward direction, however, so that the ordinary ray 15B from 
the second polarizer 10B is polarized perpendicularly with the optical 
axis of the first polarizer 10A and the extraordinary ray 15A from the 
second polarizer 10B is polarized with the optical axis of the first 
polarizer 10A. The ordinary and extraordinary rays from the second 
polarizer 10B have swapped places incident upon the first polarizer 10A. 
Because of this exchange, the light, having passed through the first 
polarizer 10A, leaves the polarizer 10A in directions which are not 
parallel. The non-parallel light from the polarizer 10A is focused by the 
collimating element before the input fiber at points which are not located 
at the end of the input fiber. More precisely stated, the light is not 
focussed on the core of the input fiber and is theoretically not 
transmitted back into the fiber. A more detailed description of this type 
of optical isolator may be found in U.S. Pat. No. 5,208,876, entitled, 
"OPTICAL ISOLATOR," which issued May 4, 1993 to J. J. Pan. 
Nonetheless, while the described optical isolator operates admirably with 
high reflecting losses, there is still room for improvement. The present 
invention achieves this improved performance so that an optical isolator 
can be built with low transmission loss and very high reflection loss. 
SUMMARY OF THE INVENTION 
The present invention provides for an improved end of an optical fiber, 
which has a core and a cladding around the core parallel to a longitudinal 
axis of the fiber. The fiber end comprises a flat end surface 
substantially perpendicular to the longitudinal axis, the end surface 
intersecting the core and cladding; and an optical barrier layer on the 
end surface. The optical barrier layer, formed by a chromium layer and a 
gold layer, has an aperture exposing the core and covering the cladding. 
Light transmission into the end is substantially reduced. 
The present invention also provides for method of manufacturing an optical 
fiber having a core and a cladding around the core parallel to a 
longitudinal axis of the fiber. The method comprises forming a flat 
surface at the end of the fiber, the surface substantially perpendicular 
to the longitudinal axis; forming a reflecting layer over the end surface; 
and forming an aperture through the reflecting layer to expose the core. 
Light transmission into the fiber end is substantially reduced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A more thorough understanding of the present invention may be achieved by a 
closer examination of the operation of the previously described optical 
isolator in the reverse direction. As illustrated in FIG. 2, the rays 15A 
and 15B leave the first polarizer 10A in nonparallel rays toward a 
collimating element, here shown as an ordinary lens 13. From the direction 
of the rays 15A and 15B, the light is focussed by the lens 13 on a 
location on the end surface 24 of an input fiber 20. Specifically, the 
location is the cladding 22 and not the central core 21, the region in 
which light is transmitted in an optical fiber. Thus light transmission 
from the output fiber into the input fiber is effectively blocked. 
Nonetheless, the present invention recognizes that there is some coupling 
between the cladding and core of an optical fiber. Some of the light which 
has entered the cladding 22 of the input fiber 20 will leak back into the 
core 21. To prevent any light from entering the cladding 22, the present 
invention places an optical barrier 30 over the cladding 22 of the end 
surface 24 of the input fiber 20 as shown in FIGS. 3A and 3B. The barrier 
30 has an opening 35 which exposes the core 21 of the end surface 24, as 
shown in an end view of the fiber end in FIG. 3B. The opening 35 also 
exposes some portion of the cladding 22 around the core 21 to allows light 
to be transmitted from the core 21 in the forward direction with some 
allowance of deviation from the longitudinal axis 26 of input fiber 20. 
Thus the end of the fiber 20 has a mask over the cladding 22 and the core 
21 is exposed. 
FIG. 4A-4G illustrate a process for manufacturing the end of the input 
optical fiber according to one embodiment of the present invention. 
FIG. 4A illustrates a glass cylinder into which the end of the input fiber 
is to be mounted. The glass cylinder 31 has a central channel 32 having a 
diameter sufficient to accommodate the core 21 and cladding 22 of the 
input fiber 20. The channel 32 opens into a wide mouth portion 32A at one 
end of the cylinder 31. As shown in FIG. 4B, the cylinder 31 is coated 
with an optical barrier 33 formed by a layer of chromium, having a 
thickness greater than 500.ANG., over which is coated a layer of gold 
having a thickness greater than 1000.ANG.. The chromium and gold layers 
may be deposited on the cylinder 31 by metal sputtering processes, which 
are well known in the semiconductor industry. 
At the end of the input fiber 20, a protective jacket 23 of the fiber 20 is 
removed to expose the cladding 22 which surrounds the core 21 (not shown). 
The core 21 and cladding 22 are inserted into the channel 32 of the glass 
cylinder 31 from the wide mouth portion 32A, and set in place by a 
heat-cured epoxy, such as epoxy 353ND from Epoxy Technology, Inc. of 
Bellerica, Mass. As shown in FIG. 4C, there is a 1 to 2 millimeter gap 
from the opposite end of the cylinder 31 which has no epoxy. The assembly 
is baked to set the epoxy, and the cladding 22 and core 21 of the input 
fiber 20 which extends beyond the cylinder 31 is trimmed away. 
Then, as shown in FIG. 4D, the end of the cylinder 31 and input fiber 20 is 
polished at an angle, approximately 10.degree. from the perpendicular to 
the longitudinal axis 26 of the cylinder 31 and input fiber 20. The 
cylinder 31 has a resulting angled surface 36 with a corresponding 
coplanar end surface 24 of the input fiber 20. 
Chromium is sputtered onto the surfaces 36 and 24 of the cylinder 31 to a 
thickness of 300.ANG. to 500.ANG., as shown in FIG. 4E. The chromium layer 
is followed by a sputtering of gold to a thickness from 1000.ANG. to 
2000.ANG. to form the optical barrier 30. It should be noted that the 
optical barrier 30 is formed from chromium and gold layers because of the 
properties of each metal. Chromium adheres very well to glass, the 
material of the fiber 21 and the cylinder 31. Titanium also serves this 
purpose. Gold, being highly reflective, blocks light transmission 
effectively. Platinum, silver and aluminum are also good reflective 
metals. 
Besides metal layers, dielectric coatings of silicon dioxide and magnesium 
oxide may also be used for the barrier 30. Multiple layers of these 
coatings further decrease the wavelength selectivity of the optical 
barrier 30 so that effectiveness of the barrier 30 is increased and the 
performance of the optical isolator is further enhanced. 
To remove the layers forming the optical barrier 30 over the core 21 and 
the portions with cladding 22 around the core 21, a photoresist layer is 
deposited to a thickness of 1000.ANG. to 2000.ANG. by carefully placing a 
droplet of photoresist over the barrier 30 formed by the chromium and gold 
layers in this embodiment of the present invention. The photoresist layer 
over the core 21 is exposed by directing light from the input fiber 20 
through the chromium and gold layers. The photoresist layer is then 
developed and the portion over the core 21 is removed. With standard 
etching techniques well known in the semiconductor industry, a wet etching 
operation is performed to remove the gold and chromium layers of the 
barrier 30. Moreover, over-etching is performed on the gold and chromium 
layers so that portions of these layers over the cladding 22 and around 
the core 21 are also removed. For example, if the diameter of the core of 
the input fiber 20 is 10 microns, then over-etching of the metal layers to 
a diameter of 20 microns for the aperture 35 is suitable. The over-etching 
of the metal layers around the core 21 avoids increasing the insertion 
loss. The result of this operation is shown in FIG. 4F. 
The operation above was described for positive photoresist. Alternatively, 
if negative photoresist is used, the photoresist layer is deposited over 
the fiber end surface 24 before the deposition of the metal layers. The 
photoresist layer over the core 21 is exposed by directing light from the 
input fiber 20 and the photoresist layer is developed. Only the portion of 
the photoresist layer over the core 21 remains. Then the chromium and gold 
layer for the optical barrier 30 are deposited and a "lift-off" technique, 
well-known in semiconductor processing, is used to remove the photoresist, 
chromium and gold layers over the core 21. 
Finally, the surface of the optical barrier 30 and the end surface 34 of 
the input fiber 20 in the opening 35 are covered with antireflection 
coatings, such as layers of TiO.sub.2 and SiO.sub.2. Other antireflection 
coatings include ZrO.sub.2 and SiO.sub.2, and a combination of TiO.sub.2 
+ZrO.sub.2 and SiO.sub.2. These antireflection coatings and the techniques 
for applying them have long been practiced in the optics field. The 
antireflection coatings enhance the transmission of light in the forward 
direction through the input fiber 20. For the optical isolator, the 
insertion loss is decreased, as desired. 
Thus, with this embodiment of the present invention, an improvement of the 
reflection loss in the described optical isolator by 5 dB has been 
observed. The reflection loss has been found to reach values up to 99.99 
percent. 
The present invention has been described in the context of optical 
isolators. However, there are many other applications for the present 
invention. For example, the present invention reduces the generation and 
reception of evanescent modes of input and output light signals and may be 
used effectively in fiber optic sensors, image transmission, medical fiber 
optic endoscopy. Constant optical power transmission may be achieved 
through short haul fibers with minimal cladding mode fluctuation. 
Furthermore, the present invention can improve the following: the output 
uniformity of 1.times.N optical couplers; the coupling uniformity between 
a single-mode optical fiber and optical elements, such as a collimator, 
diffraction grating or a GRIN lens (making the GRIN lens operate as a 
quasi-point source); the performance of an optical fiber grating and 
related elements using the fiber grating, such as a filter and stabilized 
laser; the performance of high frequency photodetectors (small active 
areas) and the reduction of back reflection; the stability of 
fiber-coupled surface-emitting laser diodes; and the extinction ratio of 
fiber pigtailed Integrated Optic Circuit (IOC) devices by the reduction of 
surface mode generation and reception. 
While the above is a complete description of the preferred embodiments of 
the present invention, various alternatives, modifications and equivalents 
may be used. It should be evident that the present invention is equally 
applicable by making appropriate modifications to the embodiment described 
above. Therefore, the above description should not be taken as limiting 
the scope of invention which is defined by the metes and bounds of the 
appended claims.