Note: Descriptions are shown in the official language in which they were submitted.
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TUNABhE OPTICAh FIhTERS HAVING EhECTRO-OPTIC WHISPERING-GAhhERY
MODE RESONATORS
[0001] This application claims the benefit of U.S.
Provisional Application No. 60/444,423 entitled "TUNABLE FILTER
BASED ON TnIHISPERING GALLERY MODES" and filed on February 3,
2003.
[0002] This application also claims the benefit of U.S.
Patent Application No. 10/702,201 entitled "OPTICAL FILTER
l0 HAVING COUPLED WHISPERING-GALLERY-MODE RESONATORS" and filed on
November 4, 2003.
[OOOS] The entire disclosures of the above two patent
applications are incorporated herein by reference as part of
this application.
Statefnent Regarding Federally Sponsored Research
[0004] The systems and techniques described herein were made in
the performance of work under a NASA contract, and are subject
to the provisions of Public Law 96-517 (35 USC 202) in which the
2o Contractor has elected to retain title.
Eao~.~s~r~~uxad
[0~~~] This application relates to optical filters based on
optical resonators and cavities.
~1~
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[0006] Optical filters have a wide range of applications. One
type of commonly used optical filters is optical bandpass
filters where optical spectral components within a spectral
window transmit through the filter while other spectral
components outside the spectral window are rejected. Optical
resonators such as Fabry-Perot resonators may be used as such
bandpass filters.
[0007] An optical whispering-gallery-mode ("WGM").resonator is a
special optical resonator and supports a special set of
1o resonator modes known as whispering gallery ("WG") modes. These
WG modes represent optical fields confined in an interior region
close to the surface of the resonator due to the total internal
reflection at the boundary. Microspheres with diameters from
few tens of microns to several hundreds of microns have been
used to form compact optical WGM resonators. Such spherical
resonators include at least a portion of the sphere that
comprises the sphere°s equator. The resonator dimension is
generally much larger than the wavelength of light so that the
optical loss due to the finite curvature of the resonators is
2o small. As a result, a high quality factor, Q, may be achieved
in such resonators. Some microspheres with sub-millimeter
dimensions have been demonstrated to e~hilait very high quality
factors for light waves, e.g., ranging from 103 to 109 for quart
microspheres. Hence, optical energy, once coupled into a
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whispering gallery mode, can circulate within the WGM resonator
with a long photon life time. Such hi-Q WGM resonators may be
used in many optical applications, including optical filtering.
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Summary
[0008] This application describes various implementations of
tunable optical filters using wGM resonators exhibiting electro-
optic effects. In one implementation, an input optical signal
is directed into an optical resonator configured to support
whispering gallery modes and comprising a portion where the
whispering gallery modes are present. At least the portion of
the optical resonator exhibits an electro-optical effect. Light
is coupled out of the optical resonator to produce a filtered
optical output from the input optical signal. An electrical
control signal is applied to at least the portion in the optical
resonator to tune a spectral transmission peak of the optical
resonator and thus to select spectral components in the input
optical signal in the filtered optical output.
i5 [0009] In the above implementation, a unmodulated optical beam
may be split into first and second beams. The first beam is
modulated as the input optical signal which carries a signal.
The second beam may be directed through an optical delay path.
The filtered optical output and the second beam after the
optical delay path are combined to produce a combined optical
signal. Next, the combined optical signal is converted into an
electrical signal. The signal is then extracted from the
electrical signal.
~q~
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[0010 One implementation of the tunable filters is also
disclosed to include an optical resonator, at least one
electrode, and a control unit. The optical resonator is
configured to support whispering gallery modes and comprising at
least a portion where the whispering gallery modes are present.
At least the portion of the optical resonator exhibits an
electro-optical effect. The electrode is formed on the optical
resonator to guide an electrical control signal into the optical
resonator to spatially overlap with the whispering gallery
1o modes. The control unit is coupled to the at least one
electrode to supply an electrical control signal to the one
portion to tune a refractive index and thus a transmission peak
of the optical resonator via the electro-optical effect.
[0011 One of the application of the above tunable filter is
to use it in a receiver which receives a radiation signal
carrying a plurality of signal channels and extracts a selected
channel from the received signal channels. This receiver may
include an optical modulator to modulate an optical beam in
response to the radiation signal to produce a modulated optical
2o signal carrying the signal channels. The optical filter is
located to receive and filter the modulated optical signal to
produce a filtered optical output that carries only the selected
signal channel. An optical detector is provided to convert the
filtered optical output into an electrical signal. The receiver
-5~
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also includes a mixer that mixes the electrical signal with a
reference signal to extract the selected signal channel.
[0012] These and other implementations are now described in
greater details in the following drawings, the detailed
description, and the claims.
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Brief Description of the Drawings
[0013] FIGS. 1, 2, 3, 4A, and 4B illustrate various exemplary
resonator configurations that support whispering gallery modes
and are formed of radiation-sensitive materials for spectral
tuning.
[0014] FIGS. 5A and 5B illustrate two evanescent coupling
examples.
[0015] FIGS. 6A and 6B show one implementation of a tunable WGM
resonator filter based on an electro-optic effect.
l0 (001] FIG. 7 shows another implementation of a tunable WGM
resonator filter based on an electro-optic effect.
[0017] FIG. 8 shows a measured transmission spectrum of a
filter based on the design in FIG. 7, where the maximum
transmission corresponds to an attenuation of 12 dB of the input
signal.
[0013] FIG. 9 illustrates a signal transmission system using
a tunable WGM filter based on the design in FIG. 7.
[0019] FIG. 10 shows one implementation of a microwave or RF
transmitter-receiver system based on the design in FIG. 9.
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Detailed Description
[0020] A WGM resonator transmits light at a wavelength that is
resonant with a WGM mode. The resonance condition of the WGM
resonator, hence, produces a spectral transmission window with a
a narrow bandwidth due to the high quality factor Q of the
resonator. A WGM resonator may produce a Zorentzian-shaped
filter function. The transmission peak of the WG resonator may
be tuned by changing the refractive index experienced by the WG
modes. Therefore, when the entire WGM resonator or at least the
1o region where WG modes are present exhibits an electro-optic
effect, an electrical control signal, such as a DC voltage, may
be applied to the resonator to tune the filter function. As
described below, such a tunable WGM resonator filter can be
designed in a compact structure to have a wide tunable spectral
range on the order of 109 Hz with a low optical loss (e. g.,
around 20 dB or less) and a high tuning speed at about tens of
microseconds or less.
[0021] Such tunable WGM resonator filters may use WGM resonators
in different resonator geometries. FIGS. 1, 2, and 3 illustrate
2o three exemplary geometries for implementing such WGM resonators.
[~~22] FIG. 1 shows a spherical WGM resonator 100 which is a
solid dielectric sphere. The sphere 100 has an equator in the
plane 102 which is symmetric around the z axis 101. The
circumference of the plane 102 is a circle and the plane 102 is
~g~
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a circular cross section. A WG mode exists around the equator
within the spherical exterior surface and circulates within the
resonator 100. The spherical curvature of the exterior surface
around the equator plane 102 provides spatial confinement along
both the z direction and its perpendicular direction to support
the WG modes. The eccentricity of the sphere 100 generally is
low.
[0023] FIG. 2 shows an exemplary spheriodal microresonator 200.
This resonator 200 may be formed by revolving an ellipse (with
1o axial lengths a and b) around the symmetric axis along the short
elliptical axis 101 (~). Therefore, similar to the spherical
resonator in FIG. 1, the plane 102 in FIG. 2 also has a circular
circumference and is a circular cross section. Different from
the design in FIG. 1, the plane 102 in FIG. 2 is a circular
s5 cross section of the non-spherical spheroid and around the short
ellipsoid axis of the spheroid. The eccentricity of resonator
100 is ( 1-b2/a2) i/2 and is generally high, a . g. , greater than 10-1.
Hence, the exterior surface is the resonator 200 is not part of
a sphere and provides more spatial confinement on the modes
2o along the ~ direction than a spherical exterior. More
specifically, the geometry of the cavity in the plane in which
lies such as the ~y or ~x plane is elliptical. The equator
plane 102 at the center of the resonator 200 is perpendicular to
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the axis 101 (z) and the WG modes circulate near the
circumference of the plane 102 within the resonator 200.
[0024] FIG. 3 shows another exemplary WGM resonator 300 which
has a non-spherical exterior where the exterior profile is a
general conic shape which can be mathematically represented by a
quadratic equation of the Cartesian coordinates. Similar to the
geometries in FIGS. 1 and 2, the exterior surface provides
curvatures in both the direction in the plane 102 and the
direction of z perpendicular to the plane 102 to confine and
1o support the WG modes. Such a non-spherical, non-elliptical
surface may be, among others, a parabola or hyperbola. dote
that the plane 102 in FIG. 3 is a circular cross section and a
WG mode circulates around the circle in the equator.
[0025] The above three exemplary geonietries in FIGS. 1, 2, and 3
share a common geometrical feature that they are all axially or
cylindrically symmetric around the axis 101 (z) around which the
WG modes circulate in the plane 102. The,curved exterior
surface is smooth around the plane 102 and provides two-
dimensional confinement around the plane 102 to support the WG
modes.
[~~2~] Notably, the spatial extent of the WG modes in each
resonator along the z direction 101 is limited above and below
the plane 102 and hence it may not be necessary to have the
entirety of the sphere 100, the spheroid 200, or the conical
~10~
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shape 300. Instead, only a portion of the entire shape around
the plane 102 that is sufficiently large to support the
whispering gallery modes may be used to for the WGM resonator.
For example, rings, disks and other geometries formed from a
proper section of a sphere may be used as a spherical WGM
resonator.
[0027] FIGS. 4A and 4B show a disk-shaped WGM resonator 400 and
a ring-shaped WGM resonator 420, respectively. In FIG. 4A, the
solid disk 400 has a top surface 401A above the center plane 102
1o and a bottom surface 401B below the plane 102 with a distance H.
The value of the distance H is sufficiently large to support the
WG modes. Beyond this sufficient distance alcove the center
plane 102, the resonator may have sharp edges as illustrated in
FIG. 3, 4A, and 4B. The exterior curved surface 402 can be
selected from any of the shapes shown in FIGS. 1, 2, and 3 to
achieve desired WG modes and spectral properties. The ring
resonator 420 in FIG. 4B may be formed by removing a center
portion 410 from the solid disk 400 in FIG. 4A. Since the WG
modes are present near the exterior part of the ring 420 near
2o the exterior surface 402~ the thickness h of the ring may be set
to be sufficiently large to support the WG modes.
[~~2~] An optical coupler is generally used to couple optical
energy into or out of the WGM resonator by evanescent coupling.
FIGS. 5A and 5B show two exemplary optical couplers engaged to a
~11~
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WGM resonator. The optical coupler may be in direct contact
with or separated by a gap from the exterior surface of the
resonator to effectuate the desired critical coupling. FIG. 5A
shows an angle-polished fiber tip as a coupler for the WGM
resonator. A waveguide with an angled end facet, such as a
planar waveguide or other waveguide, may also be used as the
coupler. FIG. 5B shows a micro prism as a coupler for the WGM
resonator. Other evanescent couplers may also be used, such as
a coupler formed from a photonic bandgap material.
to [00~~] In WGM resonators with uniform indices, a part of the
electromagnetic field of the WG modes is located at the exterior
surface of the resonators. A gap between the optical coupler
and the WGM resonator with a uniform index is generally needed
to achieve a proper optical coupling. This gap is used to
properly "unload" the WG mode. The Q-factor of a WG mode is
determined by properties of the dielectric material of the WGM
resonator, the shape of the resonator, the external conditions,
and strength of the coupling through the coupler (e. g. prism).
The highest Q-factor may be achieved when all the parameters are
2o properly balanced to achieve a critical coupling condition. In
WGM resonators with uniform indices, if the coupler such as a
prism touches the exterior surface of the resonator, the
coupling is strong and this loading can render the Q factor to
be small. Hence, the gap between the surface and the coupler is
~12~
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used to reduce the coupling and to increase the Q factor. In
general, this gap is very small, e.g., less than one wavelength
of the light to be coupled into a WG mode. Precise positioning
devices such as piezo elements may be used to control and
maintain this gap at a proper value.
[0030] A tunable WGM resonator filter may be, at least in part,
made of a material whose index changes in response to an applied
stimulus such as a radiation field or an electric field. Such a
tuning mechanism may be used to tune the transmission peak of
to the filter and in particular to provide dynamic tuning
capability in certain applications. In addition, the tuning may
be used to avoid certain complications associated with a change
in the shape or dimension of the resonator and may be further
used to compensate for certain variations during operation of
the filter. For exa~le, an electro-optic material may be used
to construct the entire WGM resonator or the portion of the WGM
resonator where the WG modes are present. An external electric
field may be applied to change the refractive index of the
resonator in tuning the resonator.
[0031] FIGS. 6A and 6B show an example of a tunable electro-
optic WGM resonator filter 600. The electro-optic material for
the entirety or part of the resonator 610 may be any suitable
material, including an electro-optic crystal such as Lithium
Niobate and semiconductor multiple quantum well structures. One
-13~
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or more electrodes 611 and 612 may be formed on the resonator
610 to apply a control electrical field in at least the region
where the WG modes are present to control the index of the
electro-optical material and to change the filter function of
the resonator. Assuming the resonator 610 has disk or ring
geometry as in FIG. 4A or 4B, the electrode 611 may be formed on
the top of the resonator 610 and the electrode 612 may be formed
on the bottom of the resonator 610 as illustrated in the side
view of the device in FIG. 6B. In one implementation, the
electrodes 611 and 612 may constitute an RF or microwave
resonator to apply the RF or microwave signal to co-propagate
along with the desired optical WG mode. For example, the
electrodes 611 and 612 may be microstrip line electrodes. The
electrodes 611 and 612 may also form an electrical waveguide to
direct the electrical control signal to propagate along the
paths of the WG modes. A filter control unit 630 such as a
control circuit may be used to supply the electrical control
signal to the electrodes 611 and 612.
[0~~~] In operating the filter 600~ the filter control unit 630
2o may supply a voltage as the electrical control signal to the
electrodes 611 and 612. In some operations, the control voltage
may be a DG voltage to bias the transmission peak of the filter
600 at a desired spectral location. The DC voltage may be
adjusted by the control unit 630 to tune the spectral position
~14~
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of the transmission peak when such tuning is needed. For
dynamic tuning operations, the control unit 630 adjusts the
control voltage in response to a control signal to, e.g.,
maintain the transmission peak at a desired spectral position or
frequency or to change the frequency of the transmission peak to
a target position. In some other operations, the control unit
630 may adjust the control voltage in a time varying manner,
e.g., scanning the transmission peak at a fixed or varying speed
or constantly changing the transmission peak in a predetermined
manner.
[~03~) The tunable WGM resonator filter 600 is shown to include
two optical couplers 621 and 622. The coupler 621 is the input
coupler which couples an input optical signal 601 into the
resonator 610 for filtering. The coupler 622, generally located
at a location different from the input coupler X21, couples the
filtered light out of the resonator~610 as the filtered output
signal 602. Tapered fibers and prisms may be used to implement
the couplers 621 and 622. Other implementations for the
couplers may also be possible. For example, a photonic gap
2o material may be used as an optical coupler.
[~~~a~ FIG. 7 shows another example of a tunable WGM resonator
filter 700. The WGM resonator is a micro disk WGM resonator 710
fabricated from a electro-optic material wafer such as
commercial lithium niobate wafers. In one example, a Z-cut
~15~
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LiNb03 disk cavity with a diameter of d=4.~mm and a thickness of
170,um may be used. The cavity perimeter edge may be prepared in
the toroidal shape with a 100,ccm radius of curvature. Several
nearly identical disks were fabricated and compared. The
repeatable value of the quality factor of the main sequence of
the cavity modes is Q=5x106 (the observed maximum is Q=5x10' ) ,
which corresponds to the 30 MFIzbandwidth of the mode. Light is
sent into and retrieved out of the cavity via coupling diamond
prisms. The repeatable value of fiber-to-fiber insertion loss
1o with this technique is 20d~ (the minimum measured insertion loss
is approximately 12 d~). The maximum transmission is achieved
when light is resonant with the cavity modes.
(0035] The top and bottom surfaces of the disk resonator 710 are
coated with conductive layers 711 and 712, respectively, for
receiving the external electrical control signal. A metal such
as indium may be used to form the conductive coatings 711 and
712. Tuning of the filter 700 is achieved by applying a voltage
to the top and bottom conductive coatings. Each conductive
coating may be absent on the central part of the resonator and
2o are present at the perimeter edge of the resonator where WGMs
are localised. This design of the conductive coatings can
reduce the overall impedance of the electrical path and hence
reduce the tuning time of the filter 700.
~16~
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[0036] The maximum frequency shifts of the TE and TM modes may
be respectively written as follows:
z
~Vq.E = V~ 2 f33Ez a CllZl~
~2
_ 0
~VTM - VO ,~ ~13~'Z a
where vo =2a~1014Hz is the carrier frequency of the input optical
signal and is the lasing frequency of a laser that generates the
input signal, r33 =31 panlT~ and X13 =10 pmlT~ are the electro-optic
constants of the Z-cut LiNb03, ho = 2.2~ and ne = 2.2 are the
refractive indices of LiNb~3 along two orthogonal birefringent
axes.
[0037] Notably, TE and TM modes may be selected in operating
such filters according the needs of specific applications. For
example, the TM modes may be used because they produce better
quality factors than the TE modes in some applications where a
high quality factor or a narrow filter linewidth is desirable.
If the quality factor is not very important, the TE modes may be
used because their electro-optic shifts are three times as much
as those of TM modes for the same values of the applied voltage.
The use of TE modes may also reduce the needed electrical power.
~17~
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[0038] FIG. 8 shows experimentally measured electro-optic
tuning of the filter spectral response and tuning of the center
wavelength with the applied voltage for a ZiNb03 WGM filter
based on the design in FIG. 7. Changing the tuning voltage from
zero to 10h shifts the spectrum of the filter by 0.42 GHz for the
TM polarization, in agreement with the theoretical value. This
particular filter exhibits a linear voltage dependence in a
tuning range of ~150h and the total tuning span exceeds the
free spectral range (FSR) of the WGM cavity.
1o [003] The dependence ~v(E~) has a hysteresis feature when a
large DC electric field ( EZ > 2 M~l rn ) is applied to the cavity. A
rapid change in the applied voltage results in an incomplete
compensation of the mode shift, i . a . Ov(E~ = 0) ~ 0 , and the
resonance frequency returns to its initial position several
seconds after the electric field is switched off. The maximum
frequency tuning of the filter in this nonlinear regime was
approximately 40 GHz .
[0040] The insertion losses in the above exemplary filter are
found to be primarily due to the inefficient coupling technique
with the diamond prism configuration. In this regards, an
antireflection coating may be applied t~ the coupling prisms t~
reduce such losses. Also, a special grating may be placed on a
~lg~
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high-index fiber as the optical coupler to significantly reduce
the losses.
[0041] FIG. 9 shows a signal transmission system 900 that uses a
tunable WGM filter 910 in an optical fiber line to transmit a
video signal. Such transmission lines might be important for
the development of portable optical domain microwave navigation
and communication devices that can provide significantly higher
capability in applications such as NASA planetary explorations.
A video signal with an approximately 20111FIz FWHM bandwidth and
zero carrier frequency is sent from a CCD camera 901 to a mixer
903, where it is mixed with a lOCaFIz microwave carrier generated
from a microwave source 905. The resulting modulated microwave
signal is filtered by a filter 920 to suppress the higher
harmonic signal components, and is amplified and upconverted
into an optical signal 932 using an optical modulator 930, such
as a Mach-Zehnder electro-optic modulator.
[0042] A laser 960 is used to produce a unmodulated laser beam,
e.g., at 1550 nm. An optical splitter 962 is used to split the
laser beam into a first laser beam 962A and a second laser beam
962B. The beam 962A is sent into the optical modulator 930 and
is modulated to produce the modulated signal 932. The modulated
signal 932 is then sent through an optical filter transmission
line having the tunable WGM filter 910. The other unmodulated
beam 962B is sent through an optical delay line 940, e.g., a
~19~
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fiber loop, to an optical splitter 964 which operates as a
combiner to combine the unmodulated beam 962B and the filtered
modulated signal 932. This combination provides a heterodyned
detection mechanism and can reduce the effect of the noise in
the laser 960. An optical detector 950 such as a fast
photodiode, is then used to receive and detect the combined
signal from the optical splitter 964. If the laser 960 can
produce a stabilized laser output, the optical delay line 940
and the combining beam splitter 964 may be removed from the
to system 900. The filtered optical signal 932 produced by the
filter 910 may be directly sent to the detector 950.
[0043] The photodiode output is mixed with a microwave carrier
by a mixer 970 to restore the initial signal. The microwave
carrier here operates as a local oscillator. In the example
shown in FIG. 9, this microwave carrier is split off from the
microwave output from the microwave source 905. A display unit
980 such as a TV may be used to display the restored video
signal.
[0044] In this example, in order to characterize the filtered
signal and to retrieve the encoded information, the filter
output from the filter 910 is mixed with the light field 9628
and measured with a photodiode 950. The filter 910 is a high-Q
WGM cavity that adds a group delay to the signal. If the laser
960 used in the experiment has a large linewidth, this group
~2p~
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delay can result in a frequency-to-amplitude laser noise
conversion, unless the scheme is balanced. To avoid this
conversion, the WGM filter 910 is inserted into a Mach-Zehnder
configuration with a fiber delay line Lf to compensate for the
group delay. The delay line length is equal to
Lf =drzoFl2hf =1.2m, where of =1.5 is the refractive index of the
fiber material and F =300 is the cavity finesse. Such a
compensation may not be needed if the laser linewidth is much
smaller than the width of the cavity resonance. In testing the
1o system 900, the optical characterization of the filter was
achieved using a semiconductor diode laser as the laser 960 with
a 3011~11IIz FWHM line, which is quite large. The laser power in the
fiber was approximately 2.5 mW .
[0045] The basic layout in the system 900 in FIG. 9 may be used
to construct a microwave or RF transmitter-receiver system.
FIG. 10 illustrates one implementation 1000 having a microwave
transmitter 1010 and a tunable photonic microwave filter 1030.
The transmitter 1010 has a transmitter antenna to send out a
microwave signal through the air. A receiver antenna 1020
receives the signal from the transmitter 1010 in the air and
sends the received signal to the photonic filter 1030. As in
the system 900, the tunable WGM filter 910 is used to
selectively transmit the modulated optical signal 932 from the
~21~
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optical modulator 930. The filter 910 is tuned by the control
unit 630. The microwave signal transmitted by the transmitter
1020 may include multiple channels of signals at different
channel frequencies, e.g., different video signals from
different video sources such as different CCD cameras. If the
bandwidth of each channel is equal to or less than the bandwidth
of the optical filter 910 and different channels are
sufficiently spaced in the modulated optical signal 932, the
optical filter 910 may be tuned to select one channel in the
received signal to be displaced at the TV 930 while optically
rejecting other channels carried by the optical signal 932. In
this context, the system 1000 may be used in a broadcast system
where each receiver can be operated to select any channel in the
broadcast signal. A local RF or microwave generator 1040 is
implemented to provide the local oscillator signal to the mixer
970 in restoring the desired channel signal.
[0046] Tunable optical filters are the important elements for in
various optical devices and systems. Examples of such devices
and systems include reconfigurable networking wavelength
division multiplexing (WDM), and analog RF photonics
communication links. Desirable characteristics for the filters
include fast tuning speed, small size, wide tuning range, low
power consumption, and low cost. Wavelength demultiplexing and
~22~
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channel sections in WDM systems may require tunable narrow-band
optical filters that are compatible with single mode fibers.
[0047] Fabry-Perot and fiber Fabry-Perot tunable filters are
among the vast variety of tunable optical filters. Fabry-Perot
filters are characterized by the finesse, a useful figure of
merit, which is equal to the ratio of the filter free spectral
range (FSR) and the bandwidth. Finesse indicates how many
channels can fit in one span of the FSR. A Fabry-Perot filter
typically has a finesse of about 100, a bandwidth of about
125 f~I~z, and a tuning speed in the millisecond range. These
filters also meet -20d~ channel-to-channel isolation condition
for 50 GIIz channel spacing.
[004] Tunable WGM filters described in this application may be
characterized by similar parameters as with Fabry-Perot filters.
A comparison between the present tunable WGM filters and the
Fabry-Perot filters shows that the tunable WGM filters are
superior to Fabry-Perot filters. For example, tunable WGM
filters can be designed to operate in a wide spectral range.
Using the lithium niobate as the electro-optical material,
tunable WGM filters may operate at wavelengths only limited by
the absorption loss of lithium niobate and the operating
wavelength may range from about 1.0 to 1.7 ~.m. Notably, this
range includes the communication C band around the 1.55,um
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wavelength. The reproducible value of finesse of the filter (F)
exceeds F = 300 and may be as high as F =1000 . The tuning speed
of the tunable WGM filters may be approximately 10 hs, while the
actual spectrum shifting time in some implementations is
determined by the filter's 30 MHz bandwidth and does not exceed
30,us. At least - 20dB suppression of the channel cross-talk for
a 50 MHz channel spacing has been observed.
[0049] Only a few implementations are disclosed. However, it is
understood that variations and enhancements may be made.
~24~
