U.S. patent application number 09/778522 was filed with the patent office on 2001-09-20 for ultra-fast tunable optical filters.
This patent application is currently assigned to Ben-Gurion University of the Negev Research and Development Authority. Invention is credited to Majer, Daniel, Sadot, Dan, Shekel, Eyal.
Application Number | 20010022877 09/778522 |
Document ID | / |
Family ID | 11069155 |
Filed Date | 2001-09-20 |
United States Patent
Application |
20010022877 |
Kind Code |
A1 |
Sadot, Dan ; et al. |
September 20, 2001 |
Ultra-fast tunable optical filters
Abstract
An optical filter including at least one multiport optical
coupler formed on a gallium arsenide substrate, one connection port
of the at least one multiport optical coupler receiving an input
optical signal, and another connection port of the at least one
multiport optical coupler outputting a filtered optical signal and
at least one electrically tunable optical resonator, formed on the
gallium arsenide substrate and connected to at least one of the at
least multiport optical coupler.
Inventors: |
Sadot, Dan; (Kfar Bilu,
IL) ; Majer, Daniel; (Givat Shmuel, IL) ;
Shekel, Eyal; (Jerusalem, IL) |
Correspondence
Address: |
PATREA L. PABST
HOLLAND & KNIGHT LLP
SUITE 2000, ONE ATLANTIC CENTER
1201 WEST PEACHTREE STREET, N.E.
ATLANTA
GA
30309-3400
US
|
Assignee: |
Ben-Gurion University of the Negev
Research and Development Authority
|
Family ID: |
11069155 |
Appl. No.: |
09/778522 |
Filed: |
February 7, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09778522 |
Feb 7, 2001 |
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09230959 |
Apr 8, 1999 |
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6222964 |
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09230959 |
Apr 8, 1999 |
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PCT/IL97/00264 |
Aug 3, 1997 |
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Current U.S.
Class: |
385/27 |
Current CPC
Class: |
G02F 1/225 20130101 |
Class at
Publication: |
385/27 |
International
Class: |
G02B 006/26 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 4, 1996 |
IL |
119006 |
Claims
We claim:
1. An optical filter comprising: at least one multiport optical
coupler, one connection port of said at least one multiport optical
coupler receiving an input optical signal, and another connection
port of said at least one multiport optical coupler outputting a
filtered optical signal; and at least one tunable optical resonator
connected to at least one of said at least one multiport optical
coupler.
2. An optical filter according to claim 1 and wherein: said at
least one multiport optical coupler, has at least first, second,
third and fourth connection ports, said first connection port
receiving an optical signal, said second connection port outputting
a filtered optical signal; and said at least one tunable optical
resonator is connected across said third and fourth connection
ports.
3. An optical filter according to claim 1 and wherein: said at
least one multiport optical coupler comprises at least first and
second optical couplers, each having at least first, second and
third connection ports; said first connection port of said first
optical coupler receives an optical signal, said second and third
connection ports of said first optical coupler are coupled to said
at least one tunable optical resonator, said first and second
connection ports of said second optical coupler are coupled to said
at least one tunable optical resonator, and said third connection
port of said second optical coupler outputs a filtered optical
signal.
4. An optical filter according to claim 1 and wherein said at least
one tunable optical resonator is operative to select an optical
signal with a specific wavelength.
5. An optical filter according to claim 1 and wherein said at least
one tunable optical resonator is operative to enable the
polarization of said filtered optical output signal to be
selected.
6. An optical filter with variable finesse comprising: an optical
element with variable finesse receiving an optical signal and
providing a filtered output; and a finesse controller operative to
select the finesse of said variable finesse optical element.
7. An optical filter with variable finesse according to claim 6 and
wherein said optical element with variable finesse comprises an
optical coupler with variable power splitting ratio between its
connection ports.
8. An optical filter with variable finesse according to claim 6 and
comprising: at least one multiport optical coupler with variable
power splitting ratio, one connection port of said at least one
multiport optical coupler receiving an input optical signal, and
another connection port of said at least one multiport optical
coupler outputting a filtered optical signal; and at least one
tunable optical resonator connected to at least one of said at
least one multiport optical coupler.
9. An optical filter with variable finesse according to claim 8 and
wherein: said at least one multiport optical coupler with variable
power splitting ratio, has at least first, second, third and fourth
connection ports, said first connection port receiving an optical
signal, said second connection port outputting a filtered optical
signal; and said at least one tunable optical resonator is
connected across said third and fourth connection ports
10. An optical filter with variable finesse according to claim 8
and wherein: said at least one multiport optical coupler with
variable power splitting ratio comprises at least first and second
optical couplers, at least one of which has variable power
splitting ratio, and each having at least first, second and third
connection ports; said first connection port of said first optical
coupler receives an optical signal, said second and third
connection ports of said first optical coupler are coupled to said
at least one tunable optical resonator, said first and second
connection ports of said second optical coupler are coupled to said
at least one tunable optical resonator, and said third connection
port of said second optical coupler outputs a filtered optical
signal.
11. An optical filter with variable finesse according to claim 6
and wherein said at least one tunable optical resonator is
operative to select an optical signal with a specific wavelength
thereby providing tunability to both the wavelength and finesse of
said optical filter.
12. An optical filter according to claim 6 and wherein said at
least one tunable optical resonator is operative to enable the
polarization of said filtered optical output signal to be
selected.
13. An integrated optical filter comprising: at least one multiport
optical coupler, one connection port of said at least one multiport
optical coupler receiving an input optical signal, and another
connection port of said at least one multiport optical coupler
outputting a filtered optical signal; and at least one optical
resonator connected to at least one of said at least one multiport
optical couplers; and wherein at least one of said at least one
multiport optical coupler and said at least one optical resonator
are formed on an integrated optics substrate.
14. An integrated optical filter according to claim 13, and wherein
said at least one multiport optical coupler has at least first,
second, third and fourth connection ports, said first connection
port receiving an optical signal, said second connection port
outputting a filtered optical signal; and said at least one optical
resonator is connected across said third and fourth connection
ports, and at least one of said at least one multiport optical
coupler and said at least one optical resonator are formed on an
integrated optics substrate.
15. An integrated optical filter according to claim 13 and wherein:
said at least one multiport optical coupler comprises at least
first and second optical couplers, each having at least first,
second and third connection ports; said first connection port of
said first optical coupler receives an optical signal, said second
and third connection ports of said first optical coupler are
coupled to said at least one optical resonator, said first and
second connection ports of said second optical coupler are coupled
to said at least one optical resonator, and said third connection
port of said second optical coupler outputs a filtered optical
signal, and at least one of said at least one multiport optical
coupler and said at least one optical resonator are formed on an
integrated optics substrate.
16. An integrated optical filter according to claim 13 and wherein
said at least one optical resonator comprises a tunable optical
resonator.
17. An integrated optical filter according to claim 13 and wherein
at least one of said at least one multiport optical coupler and
said at least one optical resonator includes a discrete
non-integrated optical component.
18. An optical filter comprising: at least three optical couplers,
each having at least three connection ports; and at least two
optical resonators, at least one of which is tunable, each of said
at least two optical resonators being connected between two of said
at least three optical couplers; and wherein first connection port
of said first optical coupler receives an input optical signal, and
last connection port of said last optical coupler outputs a
filtered optical signal.
19. An optical filter according to claim 18 and comprising: at
least first, second and third optical couplers each having at least
first, second and third connection ports; and at least first and
second optical resonators, at least one of which is tunable, each
of said at least first and second optical resonators being
connected between two of said at least first, second and third
optical couplers; and wherein said first connection port of said
first optical coupler receives an input optical signal, said second
and third connection ports of said first optical coupler are
coupled to the first of said at least first and second optical
resonators, said first and second connection ports of said second
optical coupler are coupled to the first of said at least first and
second optical resonators, said third and fourth connection ports
of said second optical coupler are coupled to the second of said at
least first and second optical resonators, said first and second
connection ports of said third optical coupler are coupled to the
second of said at least first and second optical resonators, and
said third connection port of said third optical coupler outputs a
filtered optical signal.
20. An optical filter according to claim 18 and wherein said
optical resonators comprise loops of optical transmission medium
differing in length from each other by predetermined amounts.
21. An optical filter according to claim 20 and wherein said
difference in length of said loops of optical transmission medium
is controlled by means of a piezoelectric transducer operative to
stabilize the resonator length.
22. An optical filter according to claim 18 and wherein at least
one of said optical couplers is formed on an integrated optics
substrate.
23. An optical filter according to claim 18 and wherein at least
one of said optical resonators is formed on an integrated optics
substrate.
24. An integrated optical filter comprising: at least three optical
couplers, at least one of which is formed on an integrated optics
substrate, each of said at least three optical couplers having at
least three connection ports; and at least two optical resonators,
at least one of which is formed on an integrated optics substrate,
each of said at least two optical resonators being connected
between two of said at least three optical couplers; and wherein
first connection port of said first optical coupler receives an
input optical signal, and last connection port of said last optical
coupler outputs a filtered optical signal.
25. An integrated optical filter according to claim 24 and
comprising: at least first, second and third optical couplers, at
least one of which is formed on an integrated optics substrate,
each of said at least first, second and third optical couplers
having at least first, second and third connection ports; and at
least first and second optical resonators, at least one of which is
formed on an integrated optics substrate, each of said at least
first and second optical resonators being connected between two of
said at least first, second and third optical couplers; and
wherein: said first connection port of said first optical coupler
receives an input optical signal, said second and third connection
ports of said first optical coupler are coupled to first of said at
least first and second optical resonators, said first and second
connection ports of said second optical coupler are coupled to
first of said at least first and second optical resonators, said
third and fourth connection ports of said second optical coupler
are coupled to second of said at least first and second optical
resonators, said first and second connection ports of said third
optical coupler are coupled to second of said at least first and
second optical resonators, and said third connection port of said
third optical coupler outputs a filtered optical signal.
26. An integrated optical filter according to claim 24 and wherein
at least one of said optical resonators is tunable.
27. An optical filter according to claim 1 and wherein said tunable
optical resonator is tuned by altering the phase of an optical
signal traversing through it by means of a phase modulator.
28. An optical filter according to claim 6 and wherein said tunable
optical resonator is tuned by altering the phase of an optical
signal traversing through it by means of a phase modulator.
29. An optical filter according to claim 13 and wherein said
tunable optical resonator is tuned by altering the phase of an
optical signal traversing through it by means of a phase
modulator.
30. An optical filter according to claim 18 and wherein said
tunable optical resonator is tuned by altering the phase of an
optical signal traversing through it by means of a phase
modulator.
31. An optical filter according to claim 24 and wherein said
tunable optical resonator is tuned by altering the phase of an
optical signal traversing through it by means of a phase
modulator.
32. An optical filter according to claim 1 and wherein said
filtered optical output is converted to an electronic signal by
means of a photodetector.
33. An optical filter according to claim 6 and wherein said
filtered optical output is converted to an electronic signal by
means of a photodetector.
34. An optical filter according to claim 13 and wherein said
filtered optical output is converted to an electronic signal by
means of a photodetector.
35. An optical filter according to claim 18 and wherein said
filtered optical output is converted to an electronic signal by
means of a photodetector.
36. An optical filter according to claim 24 and wherein said
filtered optical output is converted to an electronic signal by
means of a photodetector.
37. An active wavelength division multiplexing system including an
optical filter according to claim 1, said filter being operative to
select a desired wavelength of an optical signal.
38. An active wavelength division multiplexing system including an
optical filter according to claim 6, said filter being operative to
select a desired wavelength of an optical signal.
39. An active wavelength division multiplexing system including an
optical filter according to claim 13, said filter being operative
to select a desired wavelength of an optical signal.
40. An active wavelength division multiplexing system including an
optical filter according to claim 18, said filter being operative
to select a desired wavelength of an optical signal.
41. An active wavelength division multiplexing system including an
optical filter according to claim 24, said filter being operative
to select a desired wavelength of an optical signal.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the field of tunable
optical filters, especially for use in optical communications
systems.
BACKGROUND OF THE INVENTION
[0002] High-speed data communications systems need to support the
aggregate bandwidth requirements of current and future applications
such as supercomputer interconnection high-quality video
conferencing, and multimedia traffic. It has long been clear that
these bandwidth requirements can only be met by using optical
transmission technologies. Many current approaches favor packet
switching and ATM (asynchronous transfer mode) technology, due to
their flexibility. The most promising candidate for the future
hardware backbone for such networks is dense optical WDM
(wavelength division multiplexing), a method of multiplexing a
large number of optical data channels on a wavelength basis, i.e.
each wavelength is regarded as a different channel, and is routed
and manipulated separately from all other wavelengths.
[0003] Dense WDM needs advanced optoelectronic components and
subsystems, capable of handling the extremely high aggregate bit
rates and traffic levels demanded by modern optical data
communications systems. One of the most critical components needed
for implementation of WDM packet-switched systems is an ultra-fast
tunable filter--a wavelength selective element in which the central
wavelength of the selected bandpass can be tuned externally and
dynamically at a very high rate.
[0004] Fast tunable filters are known and available commercially,
but the tuning speed of all currently known types falls far short
of the requirements of future and even of some current optical data
transmission systems. The most common optical filters are based on
classical interferometers, and include Fabry-Perot and Bragg
filters. Such filters are tuned by mechanically moving the
resonator structure, and the tuning speed is therefore
comparatively slow--typically of the order of milliseconds, or, for
the very fastest types, several tens of microseconds.
[0005] Another type of tunable filter is based on the
Acousto-Optical effect. Such components depend on the interaction
between an acoustic wave generated in the device, and the optical
signal inputted to the component. Tunability is achieved by
altering the frequency of the acoustic wave, which can be simply
accomplished by altering the frequency of the electronic signal
used to generate the acoustic wave. These filters are, however,
polarization dependent, which causes many practical problems.
Tuning speeds are reasonably high, of the order of
microseconds.
[0006] Yet another tunable filter is based on a micromachined
semiconductor structure, where the thickness of one of the parts of
the structure is altered electrically. Here too, tuning speeds of
the order of microseconds can be achieved.
[0007] The next generation packet-switched WDM networks are being
designed for use with traffic throughputs of the order of
Tbits/sec. Such systems therefore require switching and tuning
speeds of the order of one nanosecond, and it is evident that even
the fastest of the above mentioned filter technologies falls
woefully short of these requirements, by about three orders of
magnitude.
SUMMARY OF THE INVENTION
[0008] The present invention seeks to provide an improved high
speed tunable optical filter which overcomes disadvantages and
drawbacks of existing tunable optical filters, which provides
tuning speeds of the order of one nanosecond, and which is capable
of implementation as a low cost, high production volume monolithic
component.
[0009] There is thus provided in accordance with a preferred
embodiment of the invention, a tunable optical filter including at
least one multiport optical coupler, to one connection port of
which is inputted an optical signal, and from another one of which
is outputted an optical signal to the end user, an optical
transmission line of predetermined length configured as a
resonator, with one of its ends connected to yet another one of the
connection ports, and its other end connected to still another one
of the connection ports, and with a phase modulator inserted in the
above mentioned resonator such that the interaction of the phase
modulated signal in the resonator with the input signal allows only
signals of a preselected wavelength to be transmitted from the
output port to the end user.
[0010] In accordance with another preferred embodiment of the
invention, the output signal can be extracted from the resonator,
by means of an additional coupler inserted into the resonator. In
this case, the use of three-port couplers is sufficient.
[0011] In accordance with a further preferred embodiment of the
invention, a filter with variable finesse can be provided by the
use of couplers with variable power splitting ratios.
[0012] In accordance with yet another preferred embodiment of the
invention, there is provided a compound resonator tunable optical
filter including at least three multiport optical couplers, to one
of the ports of the first coupler is inputted an optical signal,
and from one of the ports of the last coupler is outputted a
filtered optical signal to the end user, at least two optical
resonators of predetermined length interconnecting the free ports
of the couplers, and at least one phase modulator inserted in at
least one of the above mentioned optical resonators, such that the
interaction of the phase modulated signal with the input signal to
the first coupler is operative to allow only signals of a
preselected wavelength to be transmitted from the output port of
the last coupler to the end user.
[0013] In applications where an electronic signal is required for
the end use, the optical output signal of the filter may be
converted into such a signal by means of a fast photodetector
mounted on the output port. In applications where an optical signal
is required for the end use, such as for optical spectrum analysis,
the output optical data signal may be utilized directly.
[0014] In accordance with a further preferred embodiment of the
invention, there is provided an optical filter including at least
one multiport optical coupler one connection port of the at least
one multiport optical coupler receiving an input optical signal,
and another connection port of the at least one multiport optical
coupler outputting a filtered optical signal, and at least one
tunable optical resonator connected to at least one of the at least
one multiport optical coupler.
[0015] In accordance with vet another preferred embodiment of the
invention, there is provided an optical filter as described above
and wherein the at least one multiport optical coupler has at least
first, second, third and fourth connection ports, the first
connection port receiving an optical signal, the second connection
port outputting a filtered optical signal and the at least one
tunable optical resonator being connected across the third and
fourth connection ports.
[0016] In accordance with still another preferred embodiment of the
invention, there is provided an optical filter as described above
and wherein the at least one multiport optical coupler consists of
at least first and second optical couplers, each having at least
first, second and third connection ports, the first connection port
of the first optical coupler receives an optical signal, the second
and third connection ports of the first optical coupler are coupled
to the at least one tunable optical resonator, the first and second
connection ports of the second optical coupler are coupled to the
at least one tunable optical resonator, and the third connection
port of the second optical coupler outputs a filtered optical
signal.
[0017] In accordance with another preferred embodiment of the
invention, there is provided an optical filter as described above
and wherein the at least one tunable optical resonator is operative
to select an optical signal with a specific wavelength or to enable
the polarization of the filtered optical output signal to be
selected.
[0018] In accordance with yet a further preferred embodiment of the
invention, there is provided an optical filter with variable
finesse consisting of an optical element with variable finesse
receiving an optical signal and providing a filtered output, and a
finesse controller operative to select the finesse of the variable
finesse optical element. The variable finesse optical element of
this embodiment could consist of an optical coupler with variable
power splitting ratio between its connection ports.
[0019] In accordance with still another preferred embodiment of the
invention, there provided an optical filter with variable finesse
consisting of at least one multiport optical coupler with variable
power splitting ratio, one connection port of the at least one
multiport optical coupler receiving an input optical signal, and
another connection port of the at least one multiport optical
coupler outputting a filtered optical signal, and at least one
tunable optical resonator connected to at least one of the at least
one multiport optical couplers.
[0020] There is provided in accordance with a further preferred
embodiment of the invention, an optical filter with variable
finesse wherein the at least one multiport optical coupler with
variable power splitting ratio, has at least first, second, third
and fourth connection ports, the first connection port receiving an
optical signal, and the second connection port outputting a
filtered optical signal, and the at least one tunable optical
resonator being connected across the third and fourth connection
ports.
[0021] In accordance with still another preferred embodiment of the
invention, there is provided an optical filter with variable
finesse wherein the at least one multiport optical coupler with
variable power splitting ratio consists of at least first and
second optical couplers, at least one of which has variable power
splitting ratio, and each having at least first, second and third
connection ports, the first connection port of the first optical
coupler receives an optical signal, the second and third connection
ports of the first optical coupler are coupled to the at least one
tunable optical resonator, the first and second connection ports of
the second optical coupler are coupled to the at least one tunable
optical resonator, and the third connection port of the second
optical coupler outputs a filtered optical signal.
[0022] In addition, there is provided in accordance with another
preferred embodiment of the invention, an optical filter with
variable finesse wherein the at least one tunable optical resonator
is operative to select an optical signal with a specific
wavelength, thereby providing tunability to both the wavelength and
finesse of the optical filter, or enabling the polarization of the
filtered optical output signal to be selected.
[0023] Additionally, there is provided in accordance with still
another preferred embodiment of the invention, an integrated
optical filter consisting of at least one multiport optical
coupler, one connection port of the at least one multiport optical
coupler receiving an input optical signal, and another connection
port of the at least one multiport optical coupler outputting a
filtered optical signal, and at least one optical resonator
connected to at least one of the at least one multiport optical
coupler, and wherein at least one of the at least one multiport
optical coupler and the at least one optical resonator are formed
on an integrated optical substrate.
[0024] In accordance with yet another preferred embodiment of the
invention, there is provided an integrated optical filter wherein
the at least one multiport optical coupler has at least first,
second, third and fourth connection ports, the first connection
port receiving an optical signal, the second connection port
outputting a filtered optical signal, and the at least one optical
resonator is connected across the third and fourth connection
ports, and wherein at least one of the at least one multiport
optical coupler and the at least one optical resonator are formed
on an integrated optical substrate.
[0025] Additionally, there is provided in accordance with a further
preferred embodiment of the invention, an integrated optical filter
wherein the at least one multiport optical coupler consists of at
least first and second optical couplers, each having at least
first, second and third connection ports, the first connection port
of the first optical coupler receives an optical signal, the second
and third connection ports of the first optical coupler are coupled
to the at least one optical resonator, the first and second
connection ports of the second optical coupler are coupled to the
at least one optical resonator, and the third connection port of
the second optical coupler outputs a filtered optical signal, and
wherein at least one of the at least one multiport optical coupler
and the at least one optical resonator are formed on an integrated
optical substrate.
[0026] In addition, there is provided in accordance with other
preferred embodiments of the invention, an integrated optical
filter wherein the at least one optical resonator is a tunable
optical resonator, or in which at least one of the at least one
optical couplers and the at least one optical resonators includes a
discrete non-integrated optical component.
[0027] In accordance with still another preferred embodiment of the
invention, there is provided an optical filter of the compound
resonator type, consisting of at least three optical couplers, each
having at least three connection ports, and at least two optical
resonators, at least one of which is tunable, each of the at least
two optical resonators being connected between two of the at least
three optical couplers, and wherein the first connection port of
the first optical coupler receives an input optical signal, and the
last connection port of the last optical coupler outputs a filtered
optical signal.
[0028] Additionally, there is provided in accordance with a further
preferred embodiment of the invention, an optical filter of the
compound resonator type, consisting of at least first, second and
third optical couplers each having at least first, second and third
connection ports, and at least first and second optical resonators,
at least one of which is tunable, each of the at least first and
second optical resonators being connected between two of the at
least first, second and third optical couplers, and wherein the
first connection port of the first optical coupler receives an
input optical signal, the second and third connection ports of the
first optical coupler are coupled to the first of the at least
first and second optical resonators, the first and second
connection ports of the second optical coupler are coupled to the
first of the at least first and second optical resonators, the
third and fourth connection ports of the second optical coupler are
coupled to the second of the at least first and second optical
resonators, the first and second connection ports of the third
optical coupler are coupled to the second of the at least first and
second optical resonators, and the third connection port of the
third optical coupler outputs a filtered optical signal.
[0029] In addition, there is provided in accordance with other
preferred embodiments of the invention, an optical filter of the
compound resonator type, as described in the previous two
paragraphs and wherein the optical resonators consist of loops of
optical transmission medium differing in length from each other by
predetermined amounts, or wherein this difference in length is
controlled by means of a piezoelectric transducer operative to
stabilize the length.
[0030] In accordance with still another preferred embodiment of the
invention, there is provided an optical filter, of the compound
resonator type, as described in the previous paragraphs, and
wherein at least one of the optical couplers or one of the optical
resonators is formed on an integrated optics substrate.
[0031] In accordance with yet a further preferred embodiment of the
invention, there is provided an integrated optical filter, of the
compound resonator type, consisting of at least three optical
couplers, at least one of which is formed on an integrated optics
substrate, each of the at least three optical couplers having at
least three connection ports, and at least two optical resonators,
at least one of which is formed on an integrated optics substrate,
each of the at least two optical resonators being connected between
two of the at least three optical couplers, and wherein the first
connection port of the first optical coupler receives an input
optical signal, and the last connection port of the last optical
coupler outputs a filtered optical signal.
[0032] There is further provided in accordance with yet another
preferred embodiment of the invention, an integrated optical
filter, of the compound resonator type, consisting of at least
first, second and third optical couplers, at least one of which is
formed on an integrated optics substrate, each of the at least
first, second and third optical couplers having at least first,
second and third connection ports, and at least first and second
optical resonators, at least one of which is formed on an
integrated optics substrate, each of the at least first and second
optical resonators being connected between two of the at least
first, second and third optical couplers, and wherein the first
connection port of the first optical coupler receives an input
optical signal, the second and third connection ports of the first
optical coupler are coupled to the first of the at least first and
second optical resonators, the first and second connection ports of
the second optical coupler are coupled to the first of the at least
first and second optical resonators, the third and fourth
connection ports of the second optical coupler are coupled to the
second of the at least first and second optical resonators, the
first and second connection ports of the third optical coupler are
coupled to the second of the at least first and second optical
resonators, and the third connection port of the third optical
coupler outputs a filtered optical signal.
[0033] Additionally, there is provided in accordance with a further
preferred embodiment of the invention, an integrated optical filter
of the compound resonator type wherein at least one of the optical
resonators is tunable.
[0034] In accordance with another preferred embodiment of the
invention, there is further provided an optical filter wherein the
tunable optical resonator is tuned by altering the phase of an
optical signal traversing through it by means of a phase
modulator.
[0035] In addition, there is provided in accordance with yet
another preferred embodiment of the invention, an optical filter
whose filtered optical output is converted to an electronic signal
by means of a photodetector.
[0036] There is additionally provided in accordance with yet
another preferred embodiment of the invention, an active wavelength
division multiplexing system including an optical filter as
described in this invention, the filter being operative to select a
desired wavelength of an optical signal.
[0037] When the tunable optical filter is implemented using bulk
fiber optical components, a loop of fiber acts as the tuned
resonator and a bulk electro-optical phase modulator is inserted in
the loop to provide the variable phase delay which provides the
resonator with its tunability. In order to miniaturize the filter
to make it compatible with the other integrated opto-electronic
components of an optical communications system, and in order to
provide the very short loop lengths needed to meet the required
specifications of the filter for dense WDM use, and in order to
reduce the manufacturing costs of such a filter, the filter can
also be implemented on a monolithic integrated optics chip, such as
of gallium arsenide, with all of the component parts defined by
means of standard semiconductor manufacturing techniques.
[0038] The operation of the filters is based on an interferometric
interaction between the input optical signal and its delayed
replica produced as a result of the signal traversing the
resonator. When the input signal and the delayed signal are in
phase at the output port of the last coupler, an output signal is
obtained at a specific wavelength and with the appropriate design,
the device operates as an optical narrow bandpass filter. The
output spectrum characteristics can be controlled by the phase
delay introduced by the phase modulator, so that the device can
perform dynamic interferometric processing of the optical signal,
creating a dynamic tunable filter. Since optical phase modulation
can be performed at exceedingly high rates, the result is an
ultra-fast tunable wavelength selective filter. Current technology
phase modulators are capable of operation in the 10 GHz range, so
that tuning times of the order of one nanosecond are attained. The
filter thus offers an attractive solution for the ultra-fast tuning
speeds required in Tbit/sec packet-switched WDM networks.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] The present invention will be understood and appreciated
more fully from the following detailed description, taken in
conjunction with the drawings, in which:
[0040] FIG. 1 is a schematic view of a single resonator tunable
delay-line optical filter constructed and operated according to a
preferred implementation of the present invention, showing a single
optical coupler, a loop of optical transmission line with a phase
modulator acting as a tuned resonator, and an optional
photodetector for converting the outputted optical signal to an
electronic signal if required.
[0041] FIG. 2 is a schematic view of a single resonator tunable
delay-line optical filter constructed and operated according to
another preferred implementation of the present invention, which
differs from the implementation shown in FIG. 1 in that an
additional optical coupler is inserted into the resonator loop, and
the filtered output optical signal extracted from this second
coupler
[0042] FIG. 3 is a representation of a 5-node WDM network frequency
comb, with 100 GHz channel spacing, and 20 GHz channel bandwidth,
and the frequency comb of a single resonator filter for use in that
WDM system, constructed according to the present invention.
[0043] FIG. 4 is a schematic view of a compound resonator tunable
delay-line optical filter constructed and operative according to
another preferred embodiment of the present invention.
[0044] FIG. 5 shows frequency response transmission plots for a
single resonator tunable filter, constructed and operated according
to a preferred embodiment of the present invention.
[0045] FIG. 6 presents theoretical frequency response transmission
plots for the compound resonator tunable filter shown schematically
in FIG. 3, illustrating the advantages of the compound resonator
filter over the single resonator filter.
[0046] FIG. 7 shows the frequency response transmission plots for
three compound resonator filters constructed with different
couplers, illustrating the variation in filter finesse attainable
thereby.
[0047] FIG. 8 shows a computer simulation of frequency response
transmission plots for three cases of wavelength misalignment
between the transmitter laser and a compound resonator tunable
filter used in the receiver of an optical communication link.
[0048] FIG. 9 presents plots of the BER (bit error rate) of the
optical communication system with the three wavelength misalignment
cases described in FIG. 8.
[0049] FIG. 10 shows a graph of the optical transmission as a
function of the tuning of a single resonator tunable filter,
constructed and operated according to a preferred embodiment of the
present invention, for signals of different optical
polarization.
[0050] FIG. 11A and 11B show an additional embodiment of the
present invention, in the form of a single resonator tunable delay
line optical filter implemented on a monolithic integrated optics
substrate of gallium arsenide. FIG. 11A shows a schematic layout
view of the circuit on the chip, whilst FIG. 11B is a cut-away
cross section of the chip, showing the microelectronic structure of
the chip.
[0051] FIG. 12A and 12B illustrate a more advanced embodiment of
the monolithic optical filter, including three couplers and three
gates, for providing dynamic control both of the filter center
frequency, and of the filter finesse value. FIG. 12A shows a
schematic layout view of the circuit on the chip, whilst FIG. 12B
is a cut-away cross section of the chip, showing the
microelectronic structure of the chip.
[0052] FIG. 13 shows an additional embodiment of the present
invention, in the form of a compound resonator tunable delay line
optical filter implemented on a monolithic integrated optics
substrate of gallium arsenide.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
[0053] Reference is now made to FIG. 1, which shows the
construction and operation of a single resonator tunable delay line
optical filter according to a preferred embodiment of the present
invention. It comprises an optical coupler 6 with four ports
labeled 1 to 4, a fiber loop 8, and an optical phase modulator 10.
The fiber loop 8 is connected between ports 3 and 2 of the coupler
6, and thus interacts with the coupler as a resonant element. The
optical coupler 6 is designed to optimize the filter performance in
terms of transmitted power and filter finesse.
[0054] The optical signal is inputted through port 1, and the
filtered signal is outputted through port 4. If an electronic data
signal output is required for the end use application, such as for
communications systems, the optical output signal is converted into
an electronic data signal by means of photodetector 12. For optical
signal output use, such as in an optical spectrum analyzer, the
output signal is taken directly from port 4. The embodiment shown
in FIG. 1 acts as a stop band filter, transmitting all wavelengths
except that selected by the filter.
[0055] A further embodiment of the present invention is shown in
FIG. 2, and operates as a pass band filter, transmitting only the
wavelength selected by the filter. This embodiment differs from the
embodiment shown in FIG. 1 in that the filtered output signal is
extracted from the resonator by means of an additional optical
coupler 14 inserted into the resonator loop 8. In this embodiment,
use may be made of couplers having only 3 ports, 1, 2 and 3 for the
input coupler 6, and 11, 12 and 13 for the output coupler 14. The
fourth port of each of these couplers is terminated internally with
a built-in absorber. The output signal is taken from port 11 of the
output coupler 14, either as an optical output, or converted into
an electronic signal by means of photodetector 15.
[0056] The performance of these filters can be calculated using
Fabry-Perot resonator theory, as described in a number of standard
works on Opto-electronics, such as for instance, in "Guided Wave
Optoelectronics", Chapter 3, Springer Verlag, 1990, by T Tamir.
Using the nomenclature of the simplest embodiment shown in FIG. 1,
it can be shown that the normalized light intensity l.sub.filter at
the output port 4 is described by: 1 I filter = ( R - e i ) ( R - e
- i ) ( 1 - t 23 ) 2 + 4 t 23 sin 2 ( nL ( v - v 0 ) c ) , ( 1
)
[0057] where,
[0058] .sub.23 is the complex transfer coefficient of the field
amplitude from port 2 to port 3 of the coupler 6,
[0059] .delta. is the total optical field phase delay after
propagating through loop 8,
[0060] R is the power reflection coefficient of the coupler 6, i.e.
.vertline.t.sub.23.vertline..sup.2
[0061] v is the optical frequency,
[0062] n is the refractive index of the optical medium constituting
the loop 8,
[0063] c is the speed of light,
[0064] L is the loop length, and
[0065] v.sub.0 is the resonance frequency satisfying the condition:
2 2 v 0 nL c = 2 k , ( 2 )
[0066] k being any integer.
[0067] The resulting frequency characteristic of the filter is a
comb of narrow passband frequencies, one frequency for each value
of k.
[0068] An important figure of merit for characterizing the
wavelength selectivity of filters Is the finesse. The finesse is
defined as the ratio between the free spectral range (FSR), which
is the frequency range between two resonance frequencies, and the
full width half maximum (FWHM) of the filter: 3 Finesse = FSR FWHM
. ( 3 )
[0069] The free spectral range (FSR) is a function of the loop
length, and is given by: 4 FSR = c nl . ( 4 )
[0070] The phase modulator 10 inserted within the fiber loop is
operative to control the phase delay of the signal traversing the
fiber loop. This enables external control of the phase matching
condition of the fiber loop resonator. A change of the phase
matching condition leads to a frequency shift of the resonance
frequency v.sub.0, proportional to the phase shift imposed by the
phase modulator This mechanism essentially adds dynamic tuning
capability to the filter. Currently available phase modulators,
such as those based on LiNbO.sub.3 technology, are capable of
high-speed operation in the multi-GHz range. The filter according
to the present invention can therefore be tuned in extremely short
time periods, of the order of a nanosecond. This is approximately
three orders of magnitude faster than other currently known
filters.
[0071] The phase displacement .DELTA..delta. imposed by the phase
modulator causes a shift .DELTA.v in the optical resonance
frequency v.sub.0, given by: 5 v = c nL ( 5 )
[0072] so that the normalized light intensity l.sub.filter at the
output port 4 of the phase tuned filter is described by equation
(1), but with the term v.sub.0 replaced by v.sub.0.div..DELTA.v, as
follows: 6 I filter = ( R - ) ( R - - i ) ( 1 - t 23 ) 2 + 4 t 23
sin 2 { nL [ v - ( v 0 + c nL ) ] / c } . ( 6 )
[0073] In order to design a filter for dense WDM applications, two
important conditions must be fulfilled:
[0074] (1) the filter must reject all unselected channels within
the WDM band, which is achieved by providing a sufficiently large
free spectral range (FSR), and
[0075] (2) the filter must minimize crosstalk from adjacent
unselected channels, which is achieved by constructing the filter
with a high finesse value design.
[0076] Although the single-resonator tunable filter, when
constructed using practical lengths of optical fiber, performs well
in terms of filter finesse (typically above 1250), its FSR is not
satisfactory since only 20 GHz separates between two adjacent
channels. A much more realistic requirement is an FSR of the order
of 100 GHz, equivalent to about 0.8 nm, which is a candidate for
the standard channel spacing in dense WDM systems. The filter FSR
should be slightly larger than the channel spacing:
FSR=.DELTA.+FWHM. (7)
[0077] where
[0078] .DELTA. is the channel spacing of the WDM system, and
[0079] FWHM is the filter bandwidth.
[0080] Accordingly, all N channels adjacent to the selected one
will be rejected, where 7 N = FSR FWHM - 2 = Finesse - 2 Finesse .
( 8 )
[0081] In the upper section of FIG. 3 is a representation of a
typical 5-node WDM network frequency comb, with 100 GHz channel
spacing and 20 GHz channel bandwidth, and in the lower section, the
frequency comb of a single resonator filter constructed according
to the present invention for use in that WDM system, and having an
FSR of 120 GHz typically, and a 20 GHz FWHM bandwidth.
[0082] As illustrated in FIG. 3 and shown by equation (8), such a
WDM system would be capable of supporting 5 channels at any given
time-1 selected channel and 4 rejected channels.
[0083] However, because of the performance limitations of the
currently available DFB and DBR semiconductor lasers used in
optical data communications systems, and particularly because of
the instability of the central wavelength, a more conservative
design is required. Equation (7) should be amended to:
FSR=.DELTA.+k.multidot.FWHM, (9)
[0084] where k is a factor which depends on the variance of the
central wavelength. Using this design, the number of nodes that the
tunable filter can support becomes: 8 N 1 k Finesse ( 10 )
[0085] However, according to equation (4), in order to achieve an
FSR of the order of 100 GHz with a single fiber loop, the required
loop length is about 2 mm. This length is totally unrealistic for
practical construction of single resonator filters using fiber
optical loops. Two further embodiments of the present invention are
thus proposed for providing practical solutions to this
problem.
[0086] In the next section, an embodiment of the present invention
using a compound-resonator structure implemented using optical
fiber construction is proposed. The compound resonator filter
design removes the severe loop length limitation mentioned
above
[0087] In a later section, a further embodiment of the present
invention with a single resonator structure is proposed, but the
loop length problem is overcome to a large extent by implementation
of the filter on a microscopic scale on an integrated optics
Gallium Arsenide substrate, using standard semiconductor
manufacturing techniques
[0088] FIG. 4 shows a compound resonator tunable delay line optical
filter constructed and operated according to another preferred
embodiment of the present invention. It comprises three optical
couplers 20, 21, 22, two fiber optical loops 24, 25, denoted by the
terms L1 and L2, and an optical phase modulator 30. The optical
signal is inputted through port 1 of the coupler 20, and the
filtered signal is outputted through port 4 of coupler 22. As
previously, if an electronic output signal is required, the optical
signal is converted by means of photodetector 32. The three optical
couplers need not be identical, and can be selected to optimize the
filter performance in terms of transmitted power and filter
finesse. It can be shown that the compound filter has the following
transfer function: 9 E out = E in t 1 t 2 t 3 exp [ - i ( L 1 + L 2
) ] { 1 - r 1 r 2 exp ( - 2 i L 1 ) - r 1 r 3 t 2 3 exp [ - 2 i ( L
1 + L 2 ) ] 1 - r 2 r 3 exp ( - 2 i L 2 ) } [ 1 - r 2 r 3 exp ( - 2
i L 2 ) ] ( 11 )
[0089] where
[0090] L.sub.1 and L.sub.2 are the single-pass lengths of each
resonator loop,
[0091] .beta. is the propagation constant in the fiber,
[0092] t.sub.1 and R.sub.1 are the field amplitude transmission and
reflection coefficients of optical coupler t, respectively, and are
defined by
t.sub.1=t.sub.13=t.sub.24 r.sub.1=t.sub.14=t.sub.23, (12)
[0093] where r.sub.14 is the complex transfer coefficient from port
t to port j of the coupler.
[0094] The compound resonator filter has much more design
flexibility, due to the additional loop resonator, and the
additional independent transmission and reflection coefficients of
the additional optical couplers. This allows the construction of
filters with a wide selection range of FSR, FWHM, and filter
finesse In particular, it is feasible and practical to realize a
filter with FSR in the order of hundreds of GHz, i.e., a few nm, as
required for dense WDM applications in optical communications.
[0095] FIG. 5 shows transmission plots for a single-resonator
tunable filter, constructed and operated according to a preferred
embodiment of the present invention, with a loop 8 of length 1 cm,
and a coupler 6 with power splitting ratio of 10/90. The curves
were calculated from equation (1). The different curves, plotted
for phase shifts of 0, .pi./4 and .pi./2, show the level of
tunability achievable in such a filter using phase modulation. In
this example, the filter finesse exceeds 125.
[0096] FIG. 6 presents theoretical frequency response transmission
plots for the compound resonator tunable filter shown schematically
in FIG. 4. The compound resonator configuration has major
advantages over the single resonator design. Most important is that
the FSR can be extended to the range of hundreds of Ghz (a few nm.)
and more. This is achieved using an appropriate design of the two
fiber loop resonators L1 and L2, 24 and 25, with a very slight
length difference between them, of the order of fractions of a mm.
This length difference can be accurately controlled with a PZT
(piezo-electric transducer).
[0097] A further important advantage of the compound resonator is
the large sidemode rejection ratio, which is significantly
increased in comparison with the single resonator filter. While the
single resonator filter exhibits 10 dB sidelobe suppression, that
of the compound resonator filter whose results are shown in FIG. 6
exceeds 40 dB.
[0098] Furthermore, the filter finesse can be designed more
flexibly, since there are more independent coupler reflection and
transmission parameters to use for optimization of the desired
finesse. The filter of FIG. 6 has a finesse of over 1000. This
filter is constructed with a fiber loop length of 20 cm, and a loop
length difference of 0.1 mm. The input coupler 20 has a power
splitting ratio of 2/98, the intermediate coupler 21, a ratio of
1/99, and the output coupler 22, a ratio of 10/99.
[0099] FIG. 7 shows the frequency response transmission plots for
three compound resonator filters constructed using couplers with
different power splitting ratios, illustrating the variation in
filter finesse attainable thereby. Response curve 1 is obtained
from a compound resonator filter with coupler splitting ratios of
20/80, 8/92 and 10/90 for the input, intermediate and output
couplers respectively, and the resulting filter has a finesse of
about 25. Response curve II has couplers of splitting ratios 10/90,
1/99 and 10/90 respectively, resulting in a finesse of the order of
250. Response curve III has couplers of splitting ratios 1/99,
0.1/99.9 and 10/90 respectively, and the filter a finesse of
1000.
[0100] The ability to select the filter finesse is an important
feature of the compound resonator filter, since different optical
communications systems require different wavelength selective
curves to optimize system performance One important consideration
in filter design consideration is the wavelength stability of the
transmitter laser used in the system. If the wavelength stability
of the laser is such that there could arise significant wavelength
misalignment between the laser frequency and the filter mid-band
frequency, then a filter with lower finesse is required to
compensate for this misalignment.
[0101] FIG. 8 shows a computer simulation of frequency response
transmission plots for three cases of wavelength misalignment
between the transmitter laser and the filter at the receiver of an
IM/DD (intensity-modulation/direct-detection) optical communication
link. The laser spectrum chosen was one with a Lorenzian lineshape
and a bandwidth of 1 GHz, which would represent the modulation of a
datastream running at 1 Gbit/sec. The spectral response curves of
the filter are generated from equation (11), while the filter
finesse is 25. The three different curves represent the following
cases: a perfectly aligned case (curve I), a 15 GHz wavelength
misalignment (curve II) and a 30 GHz wavelength misalignment (curve
III). As is evident, when the wavelength misalignment is large (15
or 30 GHz), a filter of low finesse is essential to compensate for
the misalignment.
[0102] FIG. 9 presents plots of the BER (bit error rate) of an
IM/DD optical communication system with the three wavelength
misalignment cases described in FIG. 8. The filter itself uses
couplers with power splitting ratios of 50/50, 1/99 and 10/90, and
has a finesse of 25. It uses a PIN photodiode for detection, and
the whole system has an NEP of 30 pW/{square root}Hz, and a
responsistivity of 4. The transmission bit rate is 1.25 Gbit/sec.
It is observed that for a system with a perfectly aligned laser, as
shown by curve 1, the receiver sensitivity, defined at a BER of
10.sup.-9, is -18.5 dBm. The same system using a laser with
wavelength misalignment of 15 Ghz, as shown in curve II, suffers a
power penalty of about 2 dBm, while a 30 GHz misalignment, as shown
in curve III, produces a loss of about 7.5 dBm compared with the
perfectly aligned case. These noise figures are obtained with a
filter of relatively low finesse, 25, specifically selected to
compensate for the laser misalignment. If a narrower band filter
with higher finesse were used, the power penalty with wavelength
misalignment becomes much higher
[0103] On the other hand, the use of a filter with a very low
finesse increases the power penalty as a result of crosstalk
between adjacent channels Therefore, an optimization procedure must
be followed to minimize the power penalties derived from both
wavelength misalignment and channel crosstalk
[0104] Because of birefringence in the active optical medium in the
phase modulator, transmission of the TE and TM polarization
components of the output of the filter will change with the
wavelength to which the filter is tuned. This effect allows yet a
further embodiment of the present invention, whereby the filter
acts as a polarization selector for separating TE and TM components
of a mixed polarization signal In this embodiment, an additional
polarizer must be used in the output or input line to remove the
polarization component not required. FIG. 10 is a graph of the
optical transmission as a function of the tuning of the compound
resonator tunable filter of FIG. 6., which illustrates how a filter
constructed and operative according to a preferred embodiment of
the present invention, can perform polarization selection.
[0105] All of the above described implementations of the present
invention, using optical fibers are comparatively expensive to
manufacture, since each component part has to be assembled in the
filter and fine tuned individually. Furthermore, the use of the
bulk fiber configuration results in a bulky component package.
[0106] Both of these disadvantage can be overcome by means of a
further embodiment of the present invention, whereby the filter is
implemented by monolithic integration on a single opto-electronic
chip, as used in integrated optics technology. The main advantage
of this integrated chip implementation is commercial, since
production using standard semiconductor industry technology enables
the filters to be manufactured at lower cost, more reproducibly and
with higher reliability. Such mass production is essential to allow
the proliferation of WDM-based optical data communication
systems.
[0107] However, besides the commercial advantages, the integrated
optics monolithic implementation also has a number of technological
advantages. Firstly, the optical loop resonator length can be made
significantly smaller, down to the order of a millimeter or less.
As a result, the resonance build-up time is considerably shorter,
thus increasing the filter tuning speed. More important, the filter
free spectral range (FSR) can be increased by an order of
magnitude, up to the order of a hundred GHz. This is sufficient for
current WDM systems using a small number of laser sources, with 10
GHz bandwidth typically. In order to achieve a monolithic filter
with an FSR of 5 Thz, as demanded by the requirements of the
currently proposed WDM networks, which will be required to cover
the whole EDFA (Erbium Doped Fiber-Optical Amplifiers) bandwidth, a
more advanced configuration must be used, such as the compound
resonator filter described previously
[0108] If the monolithic embodiment is constructed without
incorporating a phase modulation element, a fixed wavelength
monolithic filter is obtained, which can be constructed with
selected center wavelength and finesse according to the parameters
and dimensions chosen. Because of its small size and superior
properties, such filters are useful for static switched WDM optical
communication system applications.
[0109] In addition, the losses within the device can be decreased.
The fiber loop configuration includes connections to an external
phase modulator, which introduces losses of an additional pair of
connectors (about 0.2 dB each). The integrated optics configuration
incorporates the phase modulator on the same chip. Therefore,
losses are reduced because of the reduction in the use of one set
of connectors.
[0110] Furthermore, temperature dependence can be overcome quite
simply, since the whole device is integrated on a very small chip,
which can be kept at fixed temperature by means of simple and
inexpensive control methods.
[0111] Finally, the monolithic design leads to a very small device,
of size similar to that of semiconductor lasers and suitable for
integration with other OEIC (Optoelectronic integrated circuit)
components into complete integrated optics communications
systems.
[0112] FIGS. 11A and 11B show a schematic view of an additional
embodiment of the present invention, in the form of a single
resonator tunable delay line optical filter implemented on a
monolithic integrated optics substrate of gallium arsenide. Any
other suitable integrated optics substrate material could also be
used. FIG. 11A is a plan view of the chip layout, while FIG. 11B is
a cut-away cross section of the chip showing the microelectronic
structure of the chip. The filter is manufactured on an MBE
(molecular beam epitaxy) grown n-type GaAs wafer 40. Using standard
photolithographic techniques, a mesa is defined and etched by RIE
(reactive ion etching). The mesa defines the waveguide 42, and the
resonator loop 43 of the optical structure of the filter. These
waveguides are defined using one of the standard
cladding/core/cladding waveguiding structures used in integrated
optics GaAs technology. A metal gate 44 over part of the resonator
loop, and its associated bond pad 46, are defined by a further
process. The device is covered with an insulating layer 41 such as
polyimide to passivate the device, and to insulate the gate 46 from
the wafer A via 45 in the polyimide layer facilitates the
gate-to-resonator loop contact area. An Ohmic contact is evaporated
onto the back of the wafer, and is alloyed in Finally, the
substrate is cleaved to create cleaved edge facets 47 for interface
to the input and output optical signals
[0113] The gate 44 located above a section of the resonator loop
together with the section of the loop itself, act as a phase
modulation element. The signal applied to the gate creates an
electric field across that section of the resonator loop, causing a
change in the waveguide refractive index by means of a physical
effect, such as the linear electro-optic effect. The change in
refractive index introduces a phase change in the optical signal
propagating in the resonator loop, analogous to that introduced by
the Lithium Niobate phase modulator 10 of the fiber optical
implementation shown in FIG. 1. If this gate is omitted, a static
monolithic filter is obtained. The coupling between the
through-waveguide 42 and the resonator loop 43 takes place across
the gap 48, whose width and length are calculated to provide the
correct power splitting ratio for the designed filter
operation.
[0114] FIGS. 12A and 12B illustrate a schematic view of a more
advanced embodiment of the monolithic optical filter. FIG. 12A is a
plan view of the chip layout, while FIG. 12B is a cut-away cross
section of the chip showing the microelectronic structure of the
chip. This construction comprises a GaAs wafer 52 on which are
defined a first waveguide 53, and a second waveguide 63, with a
resonator loop 56 disposed between them. A first metal gate 59 is
located above a section of the resonator loop, and two further
gates 54 and 55 are located between each waveguide and the
resonator loop. Before deposition of the metallic gates the whole
GaAs structure is covered with an insulating layer 58 such as
polyimide, containing vias 57 to provide contact between these
three gates and the underlying GaAs layer. The manufacturing
technique is similar to that of the simpler embodiment filter shown
in FIGS. 11A and 11B.
[0115] This advanced embodiment acts as a bandpass filter, by
selecting, from the range of wavelengths inputted to the first
waveguide 53, the specific wavelength to be switched to the second
waveguide 63, by means of the resonance loop 56. The first gate 59
together with the section of resonator loop under it operate as a
phase modulator for tuning the filter passband. As in the
embodiment of FIG. 11A and 11B, if this embodiment is constructed
without the phase modulator gate 59, a static filter is
obtained.
[0116] The two other gates 54 and 55 are operative to change the
coupling between the first waveguide and the resonator loop, and
between the resonator loop and the second waveguide as a function
of the voltage applied to them In this way, the filter finesse can
be changed dynamically by means of the control signals applied to
gates 54 and 55
[0117] FIG. 13 shows a schematic layout view of a compound
resonator monolithic tunable filter constructed and operated
according to another preferred embodiment of the present invention,
analogous to the fiber optical implementation described in FIG. 4.
Use of the compound resonator monolithic embodiment enables filters
with an FSR of up to 5 Thz to be constructed, for use in the next
generation WDM system technology.
[0118] The embodiment shown in FIG. 13 is constructed on a
substrate 52 of Gallium Arsenide, and has a passivating layer 58 of
polyimide, as in the previously described monolithic embodiments.
It has two resonator loops 56, 70, and a variable phase modulator
gate 59 acting on one of the loops 56. The phase change is varied
by means of a voltage applied to pad 60. This embodiment of the
filter can also be constructed in a fixed wavelength form, in which
case the gate 59 and pad 60 are not fabricated. The coupling
between the two resonators can be varied by means of gate 55. Gate
54 varies the coupling between the input line 53 and the first
resonator 56, and gate 74, the coupling between the second
resonator 70 and the output port 76. In this way, the filter
finesse can be changed dynamically by means of control signals
applied to gates 54, 55 and 74. The optical signal is inputted
through line 53, and the filtered signal outputted through line 63
to the output port 76.
[0119] Some of the component parts of the monolithic
implementations described in FIGS. 10, 11 and 12 may be implemented
as discrete non-integrated components, if such hybrid construction
is necessary convenient or economical for the specific application
required.
[0120] It will be appreciated by persons skilled in the art that
the present invention is not limited by what has been particularly
shown and described hereinabove. Rather the scope of the present
invention includes both combinations and subcombinations of various
features described hereinabove as well as variations and
modifications thereto which would occur to a person of skill in the
art upon reading the above description and which are not in the
prior art.
* * * * *