Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-06-05
2004-11-23
Healy, Brian (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S003000, C385S014000, C356S477000, C356S478000, C065S385000, C065S425000, C430S290000, C430S321000
Reexamination Certificate
active
06823110
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for adjusting a photosensitive waveguide to have a desired stabilized optical path length. More specifically, the present invention provides a commercially feasible process for adjusting the optical path length of interferometer waveguide devices.
Optical path length is the distance light travels in a medium scaled by the refractive index n of the medium. The refractive index determines the speed v of an optical signal in the medium by the following equation,
v
=
c
n
(
1
)
where c is the speed of light in vacuum. Optical path length (OPL) or &Dgr; is defined by the following equation:
&Dgr;=nL (2)
where L is the physical length of the medium. As may be appreciated from equations (1) and (2) above, the OPL of an optical waveguide may be lengthened by increasing the index of refraction of the medium or shortened by decreasing the index of refraction. Precise control of the OPL of an optical waveguide segment or an optical waveguide device becomes a crucial issue when precise timing and synchronization of signals is needed or adjustment of the phase of an optical signal in relation to another is required.
The relative phase difference, &PHgr;, between light beams at two locations, expressed as a fraction of an optical cycle, is
&PHgr;=&thgr;
1
−&thgr;
2
(mod 2&pgr;) (3)
where &thgr;
1
is the wave phase of the first beam at the first location and &thgr;
2
is the phase of the second beam at the second location. In some devices such as interferometers, one is interested in the phase difference between two beams at the same location. The phase difference is related to the difference in optical path length that the two beams have traveled. If the two light beams, beam
1
and beam
2
, started at the same location with the same phase (as in interferometers), then
θ
1
-
θ
2
=
2
π
λ
(
Δ
1
-
Δ
2
)
(
4
)
where &lgr; is the light wavelength in a vacuum and &Dgr;
1
and &Dgr;
2
are the optical path lengths that beam
1
and beam
2
traveled, respectively.
An interferometric device may be defined as an optical instrument that splits and then recombines a light beam, causing the recombined beams to interfere with one another.
FIG. 1
illustrates the structure of a common interferometric device, a fiber Mach-Zehnder interferometer
10
. The fiber Mach-Zehnder interferometer
10
includes a first input port
11
, a second input port
12
, a first leg
14
, a second leg
16
, a first output port
22
, a second output port
24
, a first coupler
26
, and a second coupler
28
. The terms input port and output port are relative, depending on the optical path length of each leg and the use of the device. Also, since the device is symmetric, the orientation of the device may be reversed.
A light signal enters either one of the two input ports, in the illustrated example of FIG.
1
through port
12
. The light signal is then split into two component beams at the first coupler
26
. The split beams travel independently through the two legs
14
and
16
of the interferometer. The two beams are recombined at the second coupler
28
.
In most cases, it is desired to control the phase difference between the two beams at the recombination point, coupler
28
. By making the phase difference equal to m*&pgr; (where m is an integer) at this point, the input power may be made to exit at mostly either one or the other output port. As shown in equation 4, this phase difference is related to the OPL difference between the interferometer legs, which may be adjusted by changing the refractive index along a portion of a leg.
One way of using the interferometer as an optical add/drop multiplexer is to add Bragg gratings into the legs of the device. The interferometer
10
may include, as illustrated in
FIGS. 2 and 3
, an optional first Bragg grating
18
in the first leg and an optional second Bragg grating
20
in the second leg. As illustrated in
FIGS. 2 and 3
, a Mach-Zehnder add/drop device may be used to insert or remove a specific wavelength from an optical signal.
FIG. 2
illustrates a Mach-Zehnder having gratings
18
and
20
that reflect a signal of a specific wavelength, &lgr;
4
, which is removed or dropped out of port
12
. The remaining wavelengths exit through the first output port
22
.
FIG. 3
illustrates the opposite function, where a signal of a certain wavelength, &lgr;
4
, is inserted or added through port
24
and the recombined multiple wavelength signal exits through port
22
. A description of the manufacture and use of couplers and of wavelength selective optical devices may be found in U.S. Pat. No. 4,900,119, relevant portions of which are incorporated herein by reference. The OPL difference between the legs in this add/drop device is to be properly set such that the two beams propagating through each leg of the device will recombine at the couplers
26
and
28
with the desired phase difference.
It is apparent that even a relatively small difference in optical path lengths between the two legs of the interferometer may change the performance of the device. For instance, a~5° error in phase difference between the two interferometer beams at the recombination coupler
28
may cause ~5% of the input energy to exit ports that it shouldn't exit, severely degrading device performance. Accordingly, in applications such as the above-described Mach-Zehnder device
10
, it is important to adjust precisely the OPL difference between the device legs to control the manner in which the energy exits from the device. This is known as “optical trimming” or “trimming”.
FIG. 4
illustrates common fabrication steps for creating optical fiber Mach-Zehnder devices. As seen in Step
1
, the basic structure of a Mach-Zehnder interferometer is accomplished by fusing at two locations two lengths of optical fiber together until the cores are in close proximity. For some devices, it is considered important that the OPL of the two middle sections, the legs of the device, be about the same after the device is fused.
The resulting device, as seen in Step
2
, may then be hydrogen loaded to increase the photosensitivity of the optical fibers. Methods for hydrogen loading optical fibers are discussed, for example, in U.S. Pat. Nos. 5,235,659 and 5,287,427 and in co-pending commonly assigned U.S. Application entitled “ACCELERATED METHOD FOR INCREASING THE PHOTOSENSITIVITY OF A GLASSY MATERIAL”, U.S. Ser. No. 09/616,117, filed Jul. 14, 2000, which is hereby incorporated by reference. Other methods for hydrogen loading an optical fiber are discussed in the relevant literature. Alternatively, increasing the photosensitivity of the fiber using doping or other methods known in the art may help eliminate Step
2
.
Step
3
comprises writing a grating
18
and
20
into each one of the legs of the Mach-Zehnder interferometer. The step of writing a grating is usually achieved by exposing the photosensitive fiber to a pattern of actinic radiation. The pattern may be achieved in several ways, such as with a phase mask or a holographic approach known in the art. At this point of the manufacturing process, the OPL of the Mach-Zehnder interferometer legs will likely need adjusting, for even if the original OPLs were set properly, minute differences between the inscriptions of each grating into the legs would generally cause the device performance to degrade from the desired parameters. Device performance may degrade because the optical phase difference between the beams may change with the OPL difference and cause the balance of the input energy that exits from ports to change. Accordingly, one must adjust the OPL of at least one of the legs of the device to achieve a desired device operation. Traditionally this is attempted, as illustrated in Step
4
, by changing the refractive index of regions
40
of the legs by exposing them to localized fringeless ultraviolet radiation. The UV exposure increases the refractive index of the exposed portions, lengthening the O
Battiato James M.
Brennan, III James F.
Healy Brian
Petkovsek Daniel
Rosenblatt Gregg H.
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