U.S. patent application number 11/433554 was filed with the patent office on 2008-01-10 for integrated optical filters utilizing resonators.
This patent application is currently assigned to LAMBDA CROSSING LTD.. Invention is credited to Daphna Bortman-Arbiv, Moti Margalit, Jacob Scheuer.
Application Number | 20080008423 11/433554 |
Document ID | / |
Family ID | 33130493 |
Filed Date | 2008-01-10 |
United States Patent
Application |
20080008423 |
Kind Code |
A1 |
Scheuer; Jacob ; et
al. |
January 10, 2008 |
Integrated optical filters utilizing resonators
Abstract
A filtering method and optical filter structure are presented.
The structure comprises an input waveguide, an output waveguide,
and a filter stage formed by at least one closed loop resonator
optically coupled to the input and output waveguides. A level of
the coupling from each of the waveguides to the resonator is at
least 5 times greater than a loss-per-revolution of the resonator.
The filter structure thus provides for reducing a bandwidth and
insertion loss while filtering at least one optical channel from a
multi-channel light signal.
Inventors: |
Scheuer; Jacob; (Petach
Tikva, IL) ; Margalit; Moti; (Zichron Yaacov, IL)
; Bortman-Arbiv; Daphna; (Zichron Yaacov, IL) |
Correspondence
Address: |
BROWDY AND NEIMARK, P.L.L.C.;624 NINTH STREET, NW
SUITE 300
WASHINGTON
DC
20001-5303
US
|
Assignee: |
LAMBDA CROSSING LTD.
Caesaria
IL
|
Family ID: |
33130493 |
Appl. No.: |
11/433554 |
Filed: |
May 15, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10406794 |
Apr 3, 2003 |
7065276 |
|
|
11433554 |
May 15, 2006 |
|
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Current U.S.
Class: |
385/50 |
Current CPC
Class: |
G02B 6/29343 20130101;
G02B 2006/12109 20130101; G02B 6/12007 20130101 |
Class at
Publication: |
385/050 |
International
Class: |
G02B 6/26 20060101
G02B006/26; G02B 6/42 20060101 G02B006/42 |
Claims
1. An optical filter structure comprising an input waveguide, an
output waveguide, and a filter stage formed by at least one closed
loop resonator optically coupled to the input and output
waveguides, wherein a level of the coupling from each of the
waveguides to the resonator is at least 5 times greater than a
loss-per-revolution of the resonator, the filter structure
providing for reducing a bandwidth and insertion loss while
filtering at least one optical channel from a multi-channel light
signal.
2. The structure according to claim 1, wherein said at least one
resonator and the waveguides are made of at least one dielectric
material with a refractive index different from a refractive index
of a surrounding medium.
3. The structure according to claim 1, wherein the at least one
closed loop resonator has a free spectral range of about 200-1000
GHz.
4. The structure according to claim 1, comprising at least one
additional closed loop resonator, the at least two closed loop
resonators being optically coupled to each other.
5. The structure according to claim 4, wherein the at least two
closed loop resonators are arranged in a serial-cascaded
relationship between the input and output waveguides and are
directly optically coupled to each other.
6. The structure according to claim 4, wherein the at least two
closed loop resonators are arranged in a spaced-apart relationship
between the input and output waveguides, each of the resonators
being optically coupled to said waveguides, and the resonators
being optically coupled to each other via segments of said
waveguides between the resonators, thereby forming a compound
closed loop resonator.
7. The structure according to claim 4, wherein the at least two
closed loop resonators are arranged in a serial-cascaded
relationship and are optically coupled to each other via at least
one additional waveguide, the resonators at opposite sides of the
additional waveguides thereby forming first and second filter
stages, respectively.
8. The structure according to claim 5, comprising an additional
filter stage formed by one of said two waveguides, an additional
waveguide spaced-apart therefrom and at least one additional closed
loop resonator accommodated between said one of said two waveguides
and said additional waveguide.
9. The structure according to claim 8, wherein the closed loop
resonators between said one of said two waveguides and said
additional waveguide are arranged in a serial-cascaded relationship
being directly optically coupled to each other.
10. The structure according to claim 8, wherein the closed loop
resonators between said one of said two waveguides and said
additional waveguide are arranged in a spaced-apart relationship,
each being optically coupled to these waveguides and optically
coupled to the adjacent resonator via segments of these waveguides
between the adjacent resonators.
11. The structure according to claim 6, comprising an additional
filter stage formed by one of said two waveguides, an additional
waveguide spaced-apart therefrom and at least one additional closed
loop resonator accommodated between said one of said two waveguides
and said additional waveguide.
12. The structure according to claim 11, wherein the closed loop
resonators between said one of said two waveguides and said
additional waveguide are arranged in a serial-cascaded relationship
being directly optically coupled to each other.
13. The structure according to claim 11, wherein the closed loop
resonators between said one of said two waveguides and said
additional waveguide are arranged in a spaced-apart relationship,
each being optically coupled to these waveguides and optically
coupled to the adjacent resonator via segments of these waveguides
between the adjacent resonators.
14. The structure according to claim 7, wherein the resonators of
the first filter stage are arranged in a serial-cascaded
relationship being directly optically coupled to each other.
15. The structure according to claim 7, wherein the resonators of
the second filter stage are arranged in a serial-cascaded
relationship being directly optically coupled to each other.
16. The structure according to claim 7, wherein the resonators of
the first filter stage are arranged in a spaced-apart parallel
relationship, each being optically coupled to said input and said
additional waveguides and optically coupled to the adjacent
resonator via segments of these waveguides between the adjacent
resonators.
17. The structure according to claim 7, wherein the resonators of
the second filter stage are arranged in a spaced-apart parallel
relationship, each being optically coupled to said additional and
said output waveguides and optically coupled to the adjacent
resonator via segments of these waveguides between the adjacent
resonators.
18. The structure according to claim 4, wherein the closed loop
resonators are characterized by the at least one of the following:
the resonators have equal free spectral ranges; the resonators have
different free spectral ranges; a free spectral range of the closed
loop resonator is about 200-1000 GHz; each of the resonators is
wavelength tunable at least across its own free spectral range; a
ratio between the largest free spectral range and a bandwidth of
the filter structure substantially does not exceed 30; the coupling
level between the waveguides and the resonators is higher than
12%.
19. A tunable optical filter structure comprising at least two
waveguides and at least two closed loop resonator optically coupled
to the waveguides and to each other, wherein a level of the
coupling from each of the waveguides to the resonator is at least 5
times greater than a loss-per-revolution of the resonator.
20. The structure according to claim 19, wherein the closed loop
resonators are characterized by the at least one of the following:
the resonators have equal free spectral ranges; the resonators have
different free spectral ranges; a free spectral range of the closed
loop resonator is about 200-1000 GHZ; each of the resonators is
wavelength tunable at least across its own free spectral range; a
ratio between the largest free spectral range and a bandwidth of
the filter structure substantially does not exceed 30; the coupling
level between the waveguides and the resonators is higher than
12%.
21. An optical filter structure for filtering at least one optical
channel from a multi-channel light signal, the filter structure
comprising an input waveguide, an output waveguide, and a filter
stage formed by at least one closed loop resonator optically
coupled to the input and output waveguides, wherein a level of the
coupling from each of the waveguides to the resonator is at least 5
times greater than a loss-per-revolution of the resonator, the
filter structure thereby reducing a bandwidth and insertion loss of
the filtering.
22. A method for reducing a bandwidth and insertion loss while
filtering at least one optical channel from a multi-channel light
signal, the method comprising inputting the light signal into an
input waveguide of an optical filter structure that comprises at
least one closed loop resonator optically coupled to said input
waveguide and at least one output waveguide with a level of the
coupling from each of the waveguides to the resonator being at
least 5 times greater than a loss-per-revolution of the
resonator.
23. The method according to claim 22, wherein a free spectral range
of the closed loop resonator is about 200-1000 GHz.
24. A method for reducing a bandwidth and insertion loss while
filtering at least one optical channel from a multi-channel light
signal, the method comprising passing the light signal through an
input waveguide of an optical filter structure, that comprises at
least two closed loop resonators optically coupled to said input
waveguide and at least one output waveguide and to each other, with
a level of the coupling from each of the waveguides to the
resonator being at least 5 times greater than a loss-per-revolution
of the resonator.
25. The method according to claim 24, wherein the resonators have
equal free spectral ranges.
26. The method according to claim 24, wherein at least some of the
resonators have different free spectral ranges.
27. The method according to claim 24, wherein a free spectral range
of the closed loop resonator is about 200-1000 GHz.
28. The method according to claim 24, wherein each of the
resonators is wavelength tunable at least across its own free
spectral range.
29. The method according to claim 24, wherein a ratio between the
largest free spectral range and a bandwidth of the filter structure
substantially does not exceed 30.
30. The method according to claim 24, wherein the coupling level
between the waveguides and the resonators is higher than 12%.
Description
FIELD OF THE INVENTION
[0001] The present invention is generally in the field of optical
devices and relates to a filtering device and method, utilizing
optical resonators.
BACKGROUND OF THE INVENTION
[0002] Optical filters play an important role in wavelength
division multiplexing (WDM) communication systems. WDM systems
achieve high bandwidth transmission by combining multiple optical
channels, each of a different wavelength range, in an optical
fiber. A filter is utilized to extract a specific optical channel
from a multi-channel signal at a receiver side, and can be either
fixed to a given wavelength range or tunable across a range of
wavelengths.
[0003] Integrated optics provides for a compact method to realize
an optical filter, and especially a tunable optical filter. One
method of realizing filters in integrated optics technology is to
combine multiple optical resonators [B. E. Little et al, "Microring
Resonator Channel Dropping Filters", IEEE J. Lightwave Tech. 15,
998-1005 (1997)].
[0004] Generally, a tunable filter is characterized by such key
parameters as bandwidth, insertion loss, attenuation (rejection) of
out of band signal, free spectral range (FSR), and tuning
range.
[0005] An important feature, characterizing all optical resonators
and resonator based devices, is the periodicity of their spectral
response, i.e., the spectral response repeats itself with a period
known as the Free Spectral Range (FSR). FIG. 1A illustrates the
spectral response (transfer function) of a resonator coupled to
input and output ports. The FSR of such a device is the spectral
spacing between the peaks of the transfer function. In optics, such
a device can be realized, for example by a Fabry-Perot (FP)
resonator comprised of a pair of partially reflecting mirrors (FIG.
1B), or by a ring resonator coupled to two waveguides which serve
as input/output ports (FIG. 1C). The geometrical structure and
constituent materials of the resonator device determine the total
roundtrip delay of the device, which is the inverse of the FSR.
[0006] A resonator is characterized by such parameters as FSR, loss
per roundtrip and coupling to input/output ports. The FSR indicates
the spectral period of the resonator, and the coupling indicates
the fraction of the light intensity in the input/output ports that
is coupled into the resonator (and vice versa). All these
parameters affect the filter profile. For example, a filter
bandwidth can be narrowed by (1) decreasing the coupling, or (2) by
decreasing the FSR (increasing the resonator roundtrip) and keeping
the coupling level constant. Decreasing the coupling also results
in an increase of the out of band signal attenuation and the input
to filtered output ratio (insertion loss) of the filter.
[0007] Generally, the requirements for filters in optical
communications involve a narrow bandwidth and a wide FSR.
Therefore, the known resonator based filters (e.g., WO 00/72065)
were designed accordingly (i.e., large FSR and small coupling in
order to achieve narrow bandwidth). In principal, this design
approach exhibits superior filter performance. However, when
accounting for the resonators' loss per roundtrip, the situation
becomes more complex since for ring resonators, a large FSR implies
small radii, which in turn implies higher radiation related losses.
Hence, it is clear that not every filter shape or FSR may be
achieved within a given loss budget.
SUMMARY OF THE INVENTION
[0008] There is accordingly a need in the art to facilitate
filtering of one or more optical channels from a multi-channel
light signal by providing a novel resonator-based filter method and
structure that provides for simultaneously achieving narrow
bandwidth and low insertion loss of the filtering process.
[0009] The main idea of the present invention is associated with
the following: The input to output insertion loss of a resonator
filter is determined by the resonator loss per roundtrip and by the
coupling coefficient. Generally, the insertion loss decreases as
the coupling is increased. The filter bandwidth depends primarily
on the coupling coefficient and the FSR. Narrowing the bandwidth is
possible by decreasing either the coupling coefficient or the FSR.
However, the FSR of the resonator is required to be as large as
possible, or at least as large as the spectral band in which the
filter is operating. The demand for a large FSR leads to a small
roundtrip resonator. For a ring or closed loop resonator, a small
roundtrip requires a small curvature of the resonator, which in
turn introduces large radiation losses and, hence, high loss per
roundtrip. In order to realize a narrow bandwidth filter structure
using a large FSR resonator, a small coupling coefficient is
needed. Accordingly, the insertion loss of such a filter structure
would be high due to both the inherent high loss of the resonator
and the required small coupling.
[0010] The present invention solves the high insertion loss problem
of a narrow band filter structure by optically coupling at least
one small-FSR closed loop resonator to input/output waveguides with
high coupling coefficients, namely, with the coupling level at
least 5 times higher than the loss-per-revolution of the resonator.
For example, the construction may be such that a waveguide with a
0.5.times.1.4 .mu.m core and a refractive index of about 2 is used
being surrounded by a medium with a refractive index of 1.5 and
coupled to a ring resonator with a coupling gap of about 1 .mu.m,
such that the interaction region of about 50 .mu.m between the
waveguide and ring resonator provides a 20% coupling. A ring
resonator with no more than 4% loss satisfies this requirement.
[0011] The small FSR problem can be solved by utilizing several
resonators with different FSRs (Vernier effect). Such an approach
is disclosed in "Integrated-Optic Double-Ring Resonators with a
Wide Free Spectral Range of 100 GHz", Senichi Suzuki et al.,
Journal of Lightwave Technology, Vol. 13, pp. 1766-1771 (1995).
[0012] The implementation of the filter structure with several
resonators also improves the out of band rejection ratio of the
structure, which deteriorates for large coupling levels.
[0013] It should be understood that a multiple-resonator structure
would exhibit insertion loss higher than that of a single-resonator
structure with the same resonator parameters, but not necessarily
higher than the insertion loss of a structure comprised of less
resonators with larger FSR and lower coupling level.
[0014] There is thus provided according to one aspect of the
invention, an optical filter structure comprising an input
waveguide, an output waveguide, and a filter stage formed by at
least one closed loop resonator optically coupled to the input and
output waveguides, wherein a level of the coupling from each of the
waveguides to the resonator is at least 5 times greater than a
loss-per-revolution of the resonator.
[0015] Preferably, said at least one resonator and the waveguides
are made of at least one dielectric material with a refractive
index different from a refractive index of a surrounding
medium.
[0016] A reasonable narrow bandwidth filter for DWDM communication
systems requires out of band rejection ratio of at least 30 dB,
minimal insertion loss and an appropriate bandwidth. The bandwidth
depends on the data rate, for example, a data rate of 10 GBs
requires a bandwidth of approximately 20 GHz. These parameters
depend on the architecture of the filter structure.
[0017] The filter structure of the present invention may comprise
the single closed loop resonator having a free spectral range of
about 200-1000 GHz.
[0018] Preferably, the filter structure comprises more than one
closed loop resonator, which are optically coupled to each other.
The closed loop resonators may be arranged in a serial-cascaded
relationship between the input and output waveguides and be
directly optically coupled to each other. The closed loop
resonators may be arranged in a spaced-apart relationship between
the input and output waveguides, each of the resonators being
optically coupled to said waveguides, and the resonators being
optically coupled to each other via segments of said waveguides
between the resonators, thereby forming a compound closed loop
resonator. The closed loop resonators may be arranged in a
serial-cascaded relationship and be optically coupled to each other
via an additional waveguide, such that the resonators at opposite
sides of the additional waveguides form first and second filter
stages, respectively. Various combinations of these configurations
are possible.
[0019] In the multiple-resonator structure, the resonators may have
the same or different free spectral ranges, wherein a free spectral
range of the resonator is preferably about 200-1000 GHz. Each of
the resonators may be wavelength tunable at least across its own
free spectral range. Preferably, a ratio between the largest free
spectral range and a bandwidth of the entire filter structure
substantially does not exceed 30. The coupling level between the
waveguides and the resonators is preferably higher than 12%.
[0020] According to another broad aspect of the present invention,
there is provided a tunable optical filter structure comprising at
least two waveguides and at least two closed loop resonator
optically coupled to the waveguides and to each other, wherein a
level of the coupling from the waveguides to the resonator is at
least 5 times greater than a loss-per-revolution of the
resonator.
[0021] According to yet another broad aspect of the present
invention, there is provided a method for reducing a bandwidth and
insertion loss while filtering at least one optical channel from a
multi-channel light signal, the method comprising inputting the
light signal into an input waveguide of an optical filter structure
that comprises at least one closed loop resonator optically coupled
to said input waveguide and at least one output waveguide with a
level of the coupling from the waveguides to the resonator being at
least 5 times greater than a loss-per-revolution of the
resonator.
[0022] The present invention according to its yet another aspect
provides a method for reducing a bandwidth and insertion loss while
filtering at least one optical channel from a multi-channel light
signal, the method comprising inputting the light signal into an
input waveguide of an optical filter structure that comprises at
least two closed loop resonators optically coupled to said input
waveguide and at least one output waveguide and to each other, a
level of the coupling from the waveguides to the resonator being at
least 5 times greater than a loss-per-revolution of the
resonator.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] In order to understand the invention and to see how it may
be carried out in practice, a preferred embodiment will now be
described, by way of non-limiting example only, with reference to
the accompanying drawings, in which:
[0024] FIG. 1A illustrates the spectral response of a filter device
utilizing a resonator coupled to an input and output ports;
[0025] FIG. 1B exemplifies the implementation of a resonator-based
filter device utilizing a Fabry-Perot (FP) resonator;
[0026] FIG. 1C exemplifies a single resonator based filter
structure, that is suitable for realizing the principles of the
present invention;
[0027] FIG. 2 exemplifies the insertion loss as a function of loss
per roundtrip, for the filter structure of FIG. 1C with a 20%
coupling coefficient and loss per roundtrip varying between 2% and
30%;
[0028] FIG. 3A exemplifies a multi-stage filter structure suitable
for realizing the principles of the present invention;
[0029] FIG. 3B illustrates yet another example of filter structure
suitable for realizing the principles of the present invention, in
the form of a single-stage multi-resonator structure where the
resonators are directly optically coupled to each other;
[0030] FIG. 3C schematically illustrates an example of a filter
structure suitable for realizing the principles of the present
invention, in the form of double-stage multiple-resonator structure
where the resonators of each stage are directly optically coupled
to each other and the two stages are coupled to each other via an
intermediate waveguide;
[0031] FIG. 4A exemplifies a single-stage compound resonator
structure suitable for realizing the principles of the present
invention;
[0032] FIG. 4B illustrates one more example of a filter structure
suitable for realizing the principles of the present invention,
where a combination of parallel and serial-cascaded arrangements of
the resonators is used;
[0033] FIGS. 5A and 5B illustrate, respectively, the insertion loss
as a function of the coupling coefficient, and the insertion loss
as a function of the ratio between the FSR and a required
bandwidth, for the filter structure of FIG. 3A;
[0034] FIGS. 6A and 6B illustrate the insertion loss and the
rejection ratio as a function of FSR for the filter structure
generally similar to that of FIG. 3B;
[0035] FIGS. 7A and 7B illustrate the insertion loss and the
rejection ratio as a function of the (external) coupling level, for
the filter structure generally similar to that of FIG. 3B;
[0036] FIG. 8 illustrates the technique of tuning the frequency
response of a high-FSR resonator-based filter structure;
[0037] FIGS. 9A-9B and 10A-10B illustrate the enhanced tuning
capabilities of a two-resonator based filter structure of FIG.
3B.
DETAILED DESCRIPTION OF THE INVENTION
[0038] FIG. 1A illustrates the spectral response of a filter
structure utilizing a resonator coupled to an input and output
ports. FIG. 1B exemplifies the implementation of the
resonator-based filter structure utilizing a Fabry-Perot resonator
comprised of a pair of partially reflecting mirrors. FIG. 1C
illustrates a filter structure utilizing a single closed-loop
resonator coupled to two waveguides which serve as input/output
ports.
[0039] As indicated above, a reasonable narrow bandwidth filter for
DWDM communication systems requires out of band rejection ratio of
at least 30 dB, minimal insertion loss and an appropriate
bandwidth. These parameters depend on the architecture of the
filter structure.
[0040] Turning back to FIG. 1C, this filter structure can be
configured in accordance with the principles of the present
invention, namely, the closed loop resonator can be selected with a
relatively small FSR (but not less than the spectral band in which
the filter is operating), and the coupling level from the
input/output waveguides to the resonator is at least 5 times
greater than the loss-per-revolution of the resonator. The
resonator and the waveguides are preferably made of a dielectric
material with refractive index n different from the refractive
index of surrounding media.
[0041] FIG. 2 exemplifies the insertion loss as a function of loss
per roundtrip for a single ring-resonator based filter structure
with a 20% coupling coefficient and loss per roundtrip varying
between 2% and 30%. As shown, the input to output insertion loss of
the filter structure decreases with the increase in the loss per
rounding.
[0042] To solve the small FSR problem, a filter structure can be
formed by several resonators with different FSRs (Vernier effect).
The following are some more examples of a multi-resonator filter
structure architecture utilizing the principles of the present
invention.
[0043] FIG. 3A illustrates a filter structure 10 utilizing an
input/throughput waveguide W.sub.1, a drop waveguide W.sub.2,
intermediate waveguides W.sub.3 and W.sub.4; and serially cascaded
closed loop (ring) resonators R.sub.1, R.sub.2 and R.sub.3 coupled
to each other indirectly, via the intermediate waveguides W.sub.3
and W.sub.4 between them. The resonators and the waveguides are
made of dielectric material(s) with refractive index or indices
different from the refractive index of surrounding media. The
resonators R.sub.1, R.sub.2 and R.sub.3 have either identical or
different FSRs. Each of the resonators can be tuned at least across
its own FSR. The ratio between the largest FSR and the bandwidth is
preferably lower than 30. The coupling level between the
input/output waveguides and the resonators is at least 5 times
greater than the loss-per-revolution of the resonator, and is
preferably higher than 12%.
[0044] FIG. 3B illustrates a filter structure 100 utilizing an
input/throughput waveguide W.sub.1, a drop waveguide W.sub.2, and
closed loop resonators R.sub.1, R.sub.2, . . . , R.sub.N serially
cascaded between these waveguides and directly coupled to each
other.
[0045] Similarly to the previous example, as well to all other
examples described below, the resonators and the waveguides are
preferably made of dielectric material(s) with refractive index or
indices different from that of surrounding media; the resonators
have either identical or different FSRs; each of the resonators can
be tuned at least across its own FSR. The preferred conditions in
this example, as well as in all the other multiple-resonator
structures described below, is that the ratio between the largest
FSR and the bandwidth of the filter structure is lower than 30,
and/or the coupling level between the input/output waveguides and
the resonators is at least 5 times greater than the
loss-per-revolution of the resonator, and is preferably higher than
12%.
[0046] FIG. 3C illustrates a filter structure 200 including
multiple stages (two such stages I and II being shown in the
figure) associated with input/throughput and drop (output)
waveguides W.sub.1 and W.sub.2 and at least one intermediate
waveguide--one such waveguide W.sub.3 being used in the two-stage
structure of the present example. In the present example, each
stage is designed as the filter structure of FIG. 3B, and the two
stages are optically coupled to each other via the intermediate
waveguide W.sub.3. The filter stage I is composed of serially
cascaded directly coupled ring resonators R.sub.1, R.sub.2, R.sub.3
(generally n such resonators), and the filter stage II is composed
of serially cascaded directly coupled ring resonators R.sub.4 and
R.sub.5 (generally m such resonators, wherein m may and may not be
equal to n). The ring resonators R.sub.3 and R.sub.4 are coupled to
each other via the intermediate waveguide W.sub.3.
[0047] In the above examples, the serial-cascaded arrangement of
ring resonators is used. FIG. 4A illustrates a filter structure 300
formed by input/throughput and drop waveguides W.sub.1 and W.sub.2,
and closed loop resonators R.sub.1-R.sub.N arranged in a
parallel-cascaded manner. The resonators R.sub.1-R.sub.N are
arranged in a spaced apart relationship between the waveguides
W.sub.1 and W.sub.2, each resonator being optically coupled to the
waveguides W.sub.1 and W.sub.2, and each two locally adjacent
resonators being coupled to each other via linear segments of the
waveguides W.sub.1 and W.sub.2 between these resonators. This is
the so-called "closed loop compound resonator" for storing optical
energy of a predetermined frequency range, disclosed in WO 01/27692
assigned to the assignee of the present application.
[0048] FIG. 4B exemplifies a filter structure 500 utilizing
combinations of the serial and parallel approaches of arranging the
closed loop resonators. Here, a ring resonator R.sub.1 is
accommodated between and optically coupled to an input/throughput
waveguide W.sub.1 and an intermediate waveguide W.sub.3, ring
resonators R.sub.2 and R.sub.3 are arranged in a spaced-apart
parallel relationship between and optically coupled to the
waveguides W.sub.3 and a further intermediate waveguide W.sub.4,
and ring resonators R.sub.4 and R.sub.5 are directly coupled to
each other and arranged in a serial-cascaded relationship between
the waveguide W.sub.4 and a drop waveguide W.sub.5.
[0049] In all the above examples, the mechanisms influencing the
insertion loss and the bandwidth are essentially the same. The
correct method to compare between the filter architectures is to
introduce a set of requirements such as bandwidth and rejection
ratio, and compare the insertion loss of the various designs with
different FSRs and coupling levels, which meet the set of
requirements.
[0050] FIGS. 5A and 5B illustrate, respectively, the insertion loss
as a function of the coupling coefficient, and the insertion loss
as a function of the ratio between the FSR and a required
bandwidth, for indirectly coupled ring resonators, as exemplified
in FIG. 3A. Four graphs G.sub.1-G.sub.4 are shown corresponding to
the loss per roundtrip levels L of, respectively, 2%, 3%, 4% and
5%. The required out of band rejection ratio used to generate these
graphs was 30 dB.
[0051] The FWHM and the out of band rejection ratio (RR) in dB of a
single closed loop resonator is given by: FWHM = FSR .pi. .times.
cos - 1 .function. [ 1 - ( 1 - x ) 2 2 .times. x ] ; .times. x = (
1 - k ) .alpha. ##EQU1## RR = 10 log 10 .function. ( 1 + x 1 - x )
2 ##EQU1.2## where k is the coupling coefficient and .alpha.=
{square root over (1-L)}, where L is the loss per roundtrip in the
resonator.
[0052] The insertion loss (IL) of a single ring resonator is given
by: IL = 10 log 10 .function. [ .alpha. k 2 ( 1 - x ) 2 ]
##EQU2##
[0053] The insertion loss IL and out of band rejection ratio RR of
serially cascaded resonators, which are indirectly coupled, can be
found by summing over the IL and RR (in dB) of all the
resonators.
[0054] As can be seen in graphs G.sub.1-G.sub.4 of FIGS. 5A-5B, the
insertion loss decreases for higher coupling level and smaller FSR
(keeping the required bandwidth constant). Because of the higher
coupling levels, more resonators are needed in the filter
structure, in order to maintain the same level of out of band
rejection ratio. For example, in order to achieve insertion loss
smaller that 2.5 dB, the coupling level is to be larger than about
14%, and the FSR (largest) is to be smaller than approximately 30
times the required bandwidth. Although the insertion loss continues
to decrease as the coupling level is increased, for coupling levels
higher than 35-40%, the improvement in the insertion loss becomes
negligible.
[0055] The reason for the improvement in the insertion loss is
associated with the relation between the coupling coefficient and
the loss per revolution in the resonators. The insertion loss
increases with higher loss per revolution, decreases with higher
coupling levels, and is independent of the FSR. Nevertheless, in
order to maintain the filter bandwidth while decreasing the FSR,
higher coupling levels are needed. In addition, the loss per
revolution generally decreases for smaller FSR, and as a result the
insertion loss improves. According to the dependencies in FIG. 5A,
in order to achieve low insertion loss (for example less than 1.5
dB), the coupling should be at least 5 times larger than the loss
per revolution. The required ratio even increases for
higher-loss-per revolution levels.
[0056] Reference is now made to FIGS. 6A and 6B showing the
insertion loss and the rejection ratio as a function of FSR, and
FIGS. 7A and 7B showing the insertion loss and the rejection ratio
as a function of the (external) coupling level. Each of these
figures includes four graphs H.sub.1-H.sub.4 corresponding to the
loss per roundtrip levels L of 2%, 3%, 4% and 5%, respectively, in
a filter structure generally similar to that of FIG. 3B, including
two directly coupled serially cascaded ring resonators, for a FWHM
of approximately 20 GHz. The drop function of such a structure is
given by: D = - k ext .times. k i .times. .times. n ( 1 - L ) exp
.function. ( j .times. .phi. 1 + .phi. 2 2 ) 1 - ( 1 - k ext )
.times. ( 1 - L ) { ( 1 - k i .times. .times. n ) .times. exp
.times. ( j .times. .times. .phi. 2 ) - exp .times. ( j .times.
.times. .phi. 1 ) [ ( 1 - k i .times. .times. n ) - ( 1 - k ext )
.times. exp .function. ( j .times. .times. .phi. 2 ) ] } ##EQU3##
where k.sub.in and k.sub.ext are the waveguide-resonator and
resonator-resonator coupling coefficients, respectively, and
.phi..sub.1,2=2.pi.f/FSR.sub.1,2, where FSR.sub.1,2 is the FSR of
the first and second resonators, respectively.
[0057] As for the previous structure (shown in FIG. 3B), the
insertion loss decreases as the coupling level and FSR increase. In
order to achieve a reasonable level of the insertion loss, for
example, lower than 3 dB, the resonator-waveguide coupling level
should be higher than about 12% (FIG. 7A), and the FSR is smaller
than about 600 GHz (which means that the FSR to bandwidth ratio is
smaller than 30 as shown in FIG. 7B). On the other hand, increasing
the coupling levels above 35% (FIG. 6A) and decreasing the FSR
below 200 GHz, yields a poor out of band rejection ratio (less than
30 dB--FIG. 6B).
[0058] The reason for the above is the same as that for the
indirectly coupled resonators architecture, namely, increasing the
coupling to loss-per-revolution ratio results in a decrease of the
insertion loss. Similarly, in order to achieve low insertion loss
(for example less than 1.5 dB), the coupling should be at least 5
times larger than the loss per revolution (FIG. 7A).
[0059] For both configurations (directly and indirectly coupled
serially cascaded resonators), it appears that good insertion loss
(sufficiently low) and out of band rejection ratio (sufficiently
high) can be achieved, if the coupling levels are at least 5 times
greater than the loss-per-revolution (approximately 10% for 2%
loss-per-revolution) and the FSR to bandwidth ratio is decreased
approximately below 30.
[0060] In the above-described filter structures (FIGS. 3A-3C and
4A-4B), a resonator-based filter structure can be easily tuned in
wavelength by changing the refractive index of each resonator and
by that, changing its resonance frequencies. The change of the
refractive index can be achieved by various methods such as heating
the resonator (thermo-optic effect), subjecting it to electric
field (electro-optic effect), mechanical pressure, free carrier
injection change of refractive index, piezo electric effect,
etc.
[0061] The utilization of several resonators with small and
different FSRs (Vernier effect) instead of resonators with large
FSR also introduces improvement in the tuning characteristics of
the device. The tuning range of the device depends on the maximal
shift in the resonator resonance frequency that can be induced by
changing its refractive index. This shift is given by: .DELTA.
.times. .times. f = f n eff .times. .DELTA. .times. .times. n eff
##EQU4## wherein f is the resonance frequency, n.sub.eff is the
effective index and d is the .DELTA.n.sub.eff is the induced change
in the effective index which is approximately equal to the change
in the resonator refractive index.
[0062] It is important to understand that the maximal resonance
shift depends mainly on the material properties and that the
influence of the resonator structure on this effect is practically
negligible.
[0063] If the device is comprised of resonators with high FSR (for
example, small rings), the possible tuning is defined by the
possible shift .DELTA.f of the resonance frequency of the
resonator. FIG. 8 illustrates the tuning of the frequency response
(transfer function) of a high-FSR resonator-based filter structure.
If, however, the FSRs of the multiple resonators are different and
relatively small, the transmission peak of the device can be
shifted substantially more than each resonator tuning range. If
each resonator can be tuned across its own FSR, the device tuning
range is practically unlimited.
[0064] FIGS. 9A-9B and 10A-10B illustrate the enhanced tuning
capabilities of a two-resonator based filter structure (as that
shown in FIG. 3B, namely including two directly coupled serially
cascaded ring resonators), where the maximal FSR of the resonator
is smaller than the maximal possible resonance shift denoted by
.DELTA.f.sub.max. FIGS. 9A-9B illustrate a situation where no
resonance shift is applied on the resonators, FIG. 9A showing the
resonance frequencies of the first (curve H.sub.1) and second
(curve H.sub.2) resonators, and FIG. 9B showing the complete
transfer function. FIGS. 10A-10B illustrate a situation where the
first resonator (curve H.sub.1) is shifted by 0.4 .DELTA.f.sub.max
and, as a result, the transmission peak is shifted by 1.8
.DELTA.f.sub.max. This demonstrates another benefit of using small
FSR resonators, consisting in a substantial increase in the
capability to tune the filter structure in wavelength.
[0065] The filter structure according to the present invention may
be used in several key devices in WDM systems. The low loss and
high extinction ratio are important factors in providing optical
monitoring functionality of the information channels. In this case,
the filter structure of the present invention provides for scanning
across the band of channels and monitoring the power and frequency
of each channel, as well as the noise between the channels. For
accurate readings, a high out of band extinction ratio is critical.
Another possible application for the filter structure of the
present invention is in an optical receiver. The filter structure
can be used to isolate a given channel to be detected from a
multitude of other channels and optical noise. The technique of the
present invention provides for optimizing such critical parameters
of this filter structure as the out of band extinction ratio and
the filter width and filter insertion loss.
[0066] Those skilled in the art will readily appreciate that
various modifications and changes can be applied to the embodiments
of the invention as hereinbefore exemplified without departing from
its scope defined in and by the appended claims.
* * * * *