Optical waveguides – Temporal optical modulation within an optical waveguide – Electro-optic
Reexamination Certificate
1998-12-21
2001-11-13
Font, Frank G. (Department: 2877)
Optical waveguides
Temporal optical modulation within an optical waveguide
Electro-optic
C385S002000, C356S477000
Reexamination Certificate
active
06317526
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to optical switches, and, more particularly to switching input light between output paths in an optical switch using a heater.
2. Description of the Related Art
An optical switch is a device which switches input light between output optical paths. For example, when the input light enters the optical switch from an input fiber, the light exits the optical switch through one of two output fibers. This is a 1-input 2-output switch or 1 by 2 switch, which is the most fundamental switch. There are more complicated switches, such as 2 by 2, 1 by N, N by N switches, which are realized by combining several 1 by 2 switches.
The most common method of switching the light path (the path which the input light follows through the optical switch) is a mechanical one. The switch has a moving part, such as a prism, a mirror, or a piece of fiber, that is mechanically moved to switch the light path. The mechanical switches have a drawback of slow switching and being less reliable in switching operations. For this reason, researchers have attempted to develop a non-mechanical switch. One practical non-mechanical switch is a magneto-optical switch, which is an optical switch using Faraday rotation and which was commercialized by FDK. Another type of non-mechanical switch uses a liquid crystal, which has a problem in the reliability of the liquid crystal itself.
FIG. 1
shows an example of a bulk switching device
10
of the prior art based upon a Mach-Zehnder interferometer. Mach-Zehnder interferometers are well-known in the art. The bulk switching device
10
receives input light A. Input light A is then split between paths A
1
and A
2
using conventional 50/50 coupler (or splitter)
12
, which directs half of the power of input light A to travel along path A
1
and the other half to travel along path A
2
. The 50/50 coupler is also referred to as a 3-dB coupler.
The portion of the input light A following path A
1
is reflected off of mirror
14
to 50/50 coupler (or splitter)
22
. The portion of the input light A following path A
2
travels through glass block
16
(on top of which heater
18
is situated) and is reflected off of mirror
20
to 50/50 coupler (or splitter)
22
. Then, 50/50 coupler
22
recombines the light arriving from path A
1
and the light arriving from path A
2
into output light B and C. The power included in output light B and in output light C is dependent upon the relative phase between the light arriving from path A
1
and the light arriving from path A
2
, that is, whether the light from path A
1
is in-phase (the phases of the light traveling in A
1
and A
2
differ by 0 radians or an integer multiple of 2&pgr; radians), out-of-phase (the phases of the light traveling in A
1
and A
2
differ by &pgr;/2 radians or an odd-number multiple of &pgr;/2 radians), opposite phase (the phases of the light traveling in A
1
and A
2
differ by &pgr; radians or an odd-number multiple of &pgr; radians), etc., with the light from path A
2
. Accordingly, the input light A is switched between output path B and output path C.
The bulk switching device
10
is an optical switch which switches output paths based upon temperature change. More particularly, the relative phase between the light arriving at 50/50 coupler
22
from paths A
1
and A
2
can be changed by glass block
16
by using heater
18
to heat the glass block
16
and change the refractive index of the glass block
16
, thus switching the output light path between output path B and C. However, the temperature changes are very slow because the glass block
16
is relatively large, on the order of 5 mm×5 mm or 1 cm. Therefore, minutes are required to switch the output light between paths B and C, which is too slow to be practical in lightwave systems.
FIG. 2
shows a cross-section of glass block
16
with heater
18
in the prior art. Glass block
16
includes a light beam
24
through which light traveling along path A
2
passes.
Because of its slow speed, the bulk switching device
10
is not of practical use as an optical switch.
Currently, there is work taking place on non-mechanical switches using optical waveguides. But the optical waveguide switches are not yet in use because of their large insertion loss and crosstalk.
An example of a waveguide switch of the related art is shown in
FIG. 3A
, which shows a Mach-Zehnder interferometer switch
26
. As shown in
FIG. 3A
, when input light A traveling in fiber
27
enters an input waveguide
28
formed on the surface of a glass (or another crystal) substrate
30
, the input light A is split into two arms A
1
and A
2
of the waveguide
28
. The light traveling in arms (or paths) A
1
and A
2
is then combined into one of the two output waveguides B or C, each of which is coupled to a respective fiber. Here, the output light can travel into one of the two waveguides B or C, depending on the optical phase difference between the light traveling through the two arms A
1
and A
2
. If one of two arms, for example A
2
, is heated with a heater
32
, the temperature change of the waveguide A
2
changes the refractive index of path A
2
which causes a change in optical phase of the light traveling through A
2
. Thus, the output light path is switched between B and C by an electric current input into the heater
32
. The heater
32
is, for example, a metal coating on the glass surface
30
, and is attached to the waveguide A
2
.
A cross section
34
of the substrate
30
which includes heater
32
is shown in FIG.
3
B. As shown in
FIG. 3B
, the heater
32
is placed on the surface S of the substrate
30
and to the side of waveguide A
2
, and heats an area
36
which includes waveguide A
2
. The area
36
is typically 20-30 micrometers in diameter, while the diameter of the waveguide A
2
is typically 10 micrometers, which means that the temperature change in the waveguide A
2
effected by the heater
32
is very quick, and, accordingly, the switching speed of the switch
26
is very fast. However, since the waveguide A
2
is formed on the surface of the substrate
30
and is not enclosed by the substrate
30
, the waveguide A
2
is asymmetric, which affects the polarization of the light traveling within the waveguide A
2
. The polarization of the light traveling in the waveguide A
2
is affected in that the horizontal component of the light and the vertical component of the light may each be traveling at different speeds, and may, therefore, have different optical phases when they reach the end of the waveguide A
2
. Accordingly, the horizontal oscillation of the light traveling in waveguide A
2
may be different than the vertical oscillation of the light traveling in waveguide A
2
, and the proper interference between the light traveling in arm A
1
may not occur with the light traveling in arm A
2
, thus producing incorrect output from the interferometer (or switch)
26
.
Since the switch
26
is made on a glass or other crystalline substrate, it is very expensive to make. The switch
26
is also difficult to make. In addition, the switch
26
is polarization dependent, as discussed herein above, with the polarization state of the light traveling through a fiber being unable to be maintained along the fiber for a long distance, such as ½-1 meter or longer.
The switch
26
shown in
FIG. 3A
is explained in “Low-Power Compact 2×2 Thermooptic Silica-on-Silicon Waveguide Switch with Fast Response”, by Q. Lai, W. Hunziker, and H. Melchior, IEEE Photonics Technology Letters, Vol. 10, No. 5, pp. 681-683, May 1998.
Further, and generally, optical waveguide devices of the related art also experience problems coupling to input and output fibers.
Also known in the art is a Mach-Zehnder interferometer using optical fibers and 3-dB couplers. The maintenance of the polarization state of the light traveling in an optical fiber is one of the most critical problems in most fiber interferometers using normal (conventional) fibers, not birefringent (polarization maintaining) fibers
Cao Simon
Shirasaki Masataka
Font Frank G.
Fujitsu Limited
Lauchman Layla
Staas & Halsey , LLP
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