JP2010140037A - Tunable optical filter having electro-optical whispering-gallery-mode resonator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a tunable optical filter using a whispering-gallery-mode (WGM) optical resonator. <P>SOLUTION: The tunable optical filter using the whispering-gallery-mode (WGM) optical resonator is described. The WGM optical resonator in the filter exhibits an electro-optical effect and thereby is tunable by being applied with a control electrical signal. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical filter based on an optical resonator and a cavity.

  Optical filters have a wide range of applications. A commonly used type of optical filter is a bandpass optical filter, in which optical spectral components within the spectral window are transmitted through the filter, while other spectral components outside the spectral window are blocked. An optical resonator such as a Fabry-Perot resonator can be used as such a bandpass filter.

  An optical whispering gallery mode (WGM) resonator is a special optical resonator that supports a special set of resonator modes known as whispering gallery (WG) modes. These WG modes represent an optical field confined in an internal region near the resonator surface due to total internal reflection at the boundary. A small optical WGM resonator is formed using microspheres having a diameter of several tens to several hundreds of microns. Such a spherical resonator includes at least a part of a sphere including the equator of the sphere. Since the size of the resonator is generally much larger than the wavelength of light, the loss of light due to the finite curvature of the resonator is small. As a result, a high quality factor, Q, can be achieved with such a resonator. Some of the submillimeter-sized microspheres have been demonstrated to exhibit very high quality factors for light waves, for example in the range of 10 3 to 10 9 for quartz microspheres. Therefore, when coupled to the whispering gallery mode, light energy can circulate in the WGM resonator while maintaining a long photon lifetime. Such high Q WGM resonators are used in many optical applications including optical filters.

  This application describes various embodiments of tunable optical filters that use WGM resonators that exhibit electro-optic effects. In one embodiment, the input optical signal is directed to an optical resonator that is configured to support a whispering gallery mode and that includes a portion where the whispering gallery mode exists. At least a part of the optical resonator exhibits an electro-optic effect. The light is coupled and extracted from the optical resonator to generate a filtered optical output from the input optical signal. An electrical control signal is applied to at least a portion of the optical resonator to tune the transmission spectral peak of the optical resonator, thereby selecting a spectral component of the input optical signal at the filtered light output.

  In the above embodiment, the unmodulated light beam may be branched into the first and second beams. The first beam modulates as an input optical signal that carries the signal. The second beam may be guided through an optical delay path. The filtered optical output and the second beam after passing through the optical delay path are combined to generate a combined optical signal. Next, the combined optical signal is converted into an electrical signal. The signal is then extracted from the electrical signal.

  An implementation of a tuned filter including an optical resonator, at least one electrode, and a control unit is also disclosed. The optical resonator is configured to support a whispering gallery mode, and includes at least a portion where the whispering gallery mode exists. At least a part of the optical resonator exhibits an electro-optic effect. An electrode is formed on the optical resonator, and an electric control signal is guided to the optical resonator and spatially overlapped with the whispering gallery mode. The control unit is connected to at least one electrode, and an electric control signal is supplied to a part of the control unit to tune the refractive index, and the transmission peak of the optical resonator is tuned by the electro-optic effect.

  One use of the tunable filter is for use in a receiver that receives a radiated signal carrying multiple signal channels and extracts a selected channel from the received signal channel. The receiver may include an optical modulator that modulates the light beam in response to the radiation signal to generate an optical modulation signal that carries the signal channel. An optical filter is disposed to receive the optical modulation signal and filter to produce a filtered optical output that carries only the selected signal channel. A photodetector is provided to convert the filtered light output into an electrical signal. The receiver also includes a mixer that mixes the electrical signal with the reference signal to extract the selected signal channel.

  These and other embodiments are described in further detail in the following drawings, detailed description, and claims.

FIG. 1 illustrates various example resonator configurations that support whispering gallery modes and are formed of an electromagnetic wave sensitive material for spectral tuning. FIG. 2 illustrates various example resonator configurations that support whispering gallery modes and are formed of an electromagnetic wave sensitive material for spectral tuning. FIG. 3 illustrates various example resonator configurations that support whispering gallery modes and are formed of an electromagnetic wave sensitive material for spectral tuning. FIG. 4A shows various example resonator configurations that support whispering gallery modes and are formed from electromagnetic wave sensitive materials for spectral tuning. FIG. 4B illustrates various example resonator configurations that support whispering gallery modes and are formed of an electromagnetic wave sensitive material for spectral tuning. FIG. 5A shows an example of evanescent coupling. FIG. 5B shows an example of evanescent coupling. FIG. 6A illustrates one embodiment of a tunable WGM resonator filter based on the electro-optic effect. FIG. 6B illustrates one embodiment of a tunable WGM resonator filter based on the electro-optic effect. FIG. 7 shows another embodiment of a tunable WGM resonator filter based on the electro-optic effect. FIG. 8 shows the measured transmission spectrum of a filter based on the design of FIG. 7, with the maximum transmission corresponding to 12 dB attenuation of the input signal. FIG. 9 shows a signal transmission system using a tunable WGM filter based on the design of FIG. FIG. 10 illustrates one embodiment of a microwave or RF transceiver system based on the design of FIG.

  The WGM resonator transmits light at a wavelength that resonates in the WGM mode. Therefore, the resonant state of the WGM resonator creates a spectral transmission window with a narrow bandwidth due to the high quality factor Q of the resonator. The WGM resonator may generate a Lorentz type filter function. The transmission peak of the WG resonator can be tuned by the refractive index change that affects the WG mode. Accordingly, when the entire WGM resonator, or at least the region where the WG mode exists, exhibits an electro-optic effect, an electric control signal such as a DC voltage can be applied to the resonator to tune the filter function. As described below, such a tunable WGM resonator filter structure is a compact design that has a wide tunable spectral range on the order of 10 9 Hz with low optical loss (eg, less than about 20 dB), and about tens of tens. A high tuning speed of less than μ seconds can be provided.

  Such a tunable WGM resonator filter may use WGM resonators with different resonator shapes. 1, 2 and 3 show three exemplary shapes for implementing such a WGM resonator.

  FIG. 1 shows a spherical WGM resonator 100 that is a solid dielectric sphere. The sphere 100 has an equator on a plane 102 symmetric about the z-axis 101. The outer periphery of the surface 102 is circular, and the surface 102 has a circular cross section. The WG mode exists around the equator in the outer surface of the sphere and circulates in the resonator 100. The spherical curvature of the outer surface around the equatorial plane 102 provides spatial confinement along both the z direction and its vertical direction to support the WG mode. The eccentricity of the sphere 100 is generally small.

  FIG. 2 shows an exemplary spheroid microresonator 200. The resonator 200 may be formed by rotating an ellipse (having axial lengths a and b) around an axis of symmetry along the elliptical short axis 101 (z). Accordingly, like the spherical resonator of FIG. 1, the surface 102 of FIG. 2 also has a circular outer periphery and a circular cross section. Unlike the design of FIG. 1, the surface 102 of FIG. 2 is a circular cross section of a non-spherical elliptical rotator and a circular cross section around the elliptical minor axis of the spheroid. The eccentricity of the resonator 100 is (1-b2 / a2) 1/2 and is generally large, for example, greater than 10-1. Therefore, the outer surface of the resonator 200 is not part of the sphere, and the effect of spatially confining the mode along the z direction is greater than the outer surface of the sphere. More specifically, the shape of the cavity in the plane containing z, such as the zy or zx plane, is elliptical. The equator plane 102 at the center of the resonator 200 is perpendicular to the axis 101 (z), and the WG mode circulates near the periphery of the plane 102 inside the resonator 200.

  FIG. 3 shows another exemplary WGM resonator 300 having a non-spherical outer surface whose outer surface can be mathematically represented by a quadratic equation in Cartesian coordinates. Similar to the shape of FIGS. 1 and 2, the outer surface provides bending in both the direction within the plane 102 and the z-direction perpendicular to the plane 102 to confine and support the WG mode. The surface of such a non-spherical body or non-elliptical rotating body may be a parabola or a hyperbola, for example. Note that surface 102 in FIG. 3 has a circular cross section and the WG mode circulates around the equator circle.

  The above three exemplary shapes of FIGS. 1, 2, and 3 are common geometric features that are all axisymmetric or cylindrically symmetric about an axis 101 (z) in which the WG mode circulates about the axis in the plane 102. Share The curved surface of the outer surface is smooth around the surface 102, provides two-dimensional confinement around the surface 102, and supports the WG mode.

  It should be noted that the spatial extension along the z-direction 101 of the WG mode in each resonator is limited above and below the surface 102, and therefore it is necessary to include the entire sphere 100, spheroid 200, or cone 300. There is no. Alternatively, only a portion of the overall shape around surface 102 that is large enough to support whispering gallery modes may be used for the WGM resonator. For example, the shape of a ring, a disk, or the like formed from an appropriate cross section of a sphere may be used as the sphere WGM resonator.

  4A and 4B show a disk type WGM resonator 400 and a ring type WGM resonator 420, respectively. In FIG. 4A, the solid disk 400 has a top surface 401A above the central surface 102 and a bottom surface 401B below the surface 102 at a distance H. The value of distance H is large enough to support the WG mode. Beyond this sufficient distance above the center plane 102, the resonator may have sharp edges as shown in FIGS. 3, 4A and 4B. The curved surface 402 of the outer surface can be selected from any of the shapes shown in FIGS. 1, 2, and 3 to achieve the desired WG mode and spectral characteristics. The ring resonator 420 of FIG. 4B may be formed by removing the central portion 410 from the solid disk 400 of FIG. 4A. Since the WG mode exists near the outer portion of the ring 420 near the outer surface 402, the ring thickness h may be set sufficiently large to support the WG mode.

  In general, optical energy is coupled to or extracted from the WGM resonator by evanescent coupling using an optical coupler. 5A and 5B show two examples of an optical coupler joined to a WGM resonator. The optical coupler can be brought into direct contact with the outer surface of the resonator or separated to a certain gap to achieve the desired critical coupling. FIG. 5A shows the tip of an angle-polished fiber as a coupler for a WGM resonator. A waveguide with an angled end face, such as a planar waveguide or other type of waveguide, may be used as a coupler. FIG. 5B shows a microprism as a coupler for a WGM resonator. Other evanescent couplers such as couplers made of optical band gap materials may be used.

  In the WGM resonator having a uniform refractive index, a part of the electromagnetic field of the WG mode is located on the outer surface of the resonator. The gap between the optical coupler and the WGM resonator with uniform refractive index is generally necessary to achieve proper optical coupling. This gap is used to properly “unload” the WG mode. The Q factor of the WG mode is determined by the dielectric material characteristics of the WGM resonator, the shape of the resonator, the external conditions, and the coupling strength by the coupler (for example, prism). The maximum Q factor can be achieved if all parameters are properly balanced and a critical coupling state is achieved. In a WGM resonator having a uniform refractive index, the coupling is strong when a coupler such as a prism is in contact with the outer surface of the resonator, and the Q factor can be reduced by applying this load. Therefore, the gap between the surface and the coupler is used to weaken the coupling and increase the Q factor. In general, this gap is very small, for example, less than one wavelength of light coupled to the WG mode. This gap may be maintained at an appropriate value using a precision positioning device such as a piezoelectric element.

  The tunable WGM resonator filter may be made at least in part of a material whose refractive index changes in response to an applied stimulus such as a radiation field or an electric field. Such a tuning mechanism can be used to tune the transmission peak of the filter, particularly in certain applications, providing dynamic tuning capability. Furthermore, tuning can be used to avoid certain complications associated with changes in resonator shape or dimensions, and to compensate for certain variations during filter operation. For example, the whole of the WGM resonator in which the WG mode exists or a part of the WGM resonator can be constructed using an electro-optic material. An external electric field can be applied to change the refractive index of the resonator that tunes the resonator.

  FIGS. 6A and 6B show an example of a tunable electro-optic WGM resonator filter 600. The electro-optic material for all or part of the resonator 610 may be any suitable material, including electro-optic crystals such as lithium niobate and semiconductor multiple quantum well structures. One or more electrodes 611 and 612 are formed on the resonator 610 to control at least the region where the WG mode is present to control the refractive index of the electro-optic material and change the filter function of the resonator. An electric field can be applied. Assuming that the resonator 610 has the disk or ring shape of FIG. 4A or 4B, the electrode 611 may be formed on top of the resonator 610 as shown in the side view of the device of FIG. May be formed at the bottom of the resonator 610. In one embodiment, the electrodes 611 and 612 may constitute an RF or microwave resonator and may be applied to co-propagate the RF or microwave signal in conjunction with the desired optical WG mode. For example, the electrodes 611 and 612 may be microstrip line electrodes. Electrodes 611 and 612 may form an electrical waveguide to guide electrical control signals to propagate along the WG mode optical path. An electrical control signal may be supplied to the electrodes 611 and 612 using a filter control unit 630 such as a control circuit.

  In the operation of the filter 600, the filter control unit 630 may supply a voltage as an electrical control signal to the electrodes 611 and 612. In some operations, the control voltage may bias the transmission peak of the filter 600 at a desired spectral position as a DC voltage. If such tuning is required, the DC voltage may be adjusted by the control unit 630 to tune the spectral position of the transmission peak. For dynamic tuning operation, the control unit 630 adjusts the control voltage in response to the control signal, for example, to maintain the transmission peak at the desired spectral position or frequency, or to set the frequency of the transmission peak to the target position. You may change to Depending on other operations, the control unit 630 may adjust the control voltage by varying the time, for example, scanning the transmission peak at a fixed or variable rate, or changing the transmission peak constant in a predetermined manner. May be.

  A tunable WGM resonator filter 600 is shown including two optical couplers 621 and 622. The coupler 621 is an input coupler that couples the input optical signal 601 to the resonator 610 for filtering. A coupler 622 that is generally at a different location from the input coupler 621 couples the filtered light output from the resonator 610 as a filtered output signal 602. Couplers 621 and 622 may be implemented using tilted fibers and prisms. Other implementations for the coupler are possible. For example, an optical gap material may be used as the optical coupler.

  FIG. 7 shows another embodiment of a tunable WGM resonator filter 700. This WGM resonator is a microdisk WGM resonator 710 made of an electro-optic material wafer such as a commercially available lithium niobate wafer. In one embodiment, a Z-cut LiNbO 3 disk cavity with a diameter d = 4.8 mm and a thickness of 170 μm may be used. The periphery of the cavity is made in a donut shape with a radius of curvature of 100 μm. Several substantially identical disks were made and compared. The reproducible value of the quality factor of the main series cavity mode is Q = 5 × 10 6 (the maximum observed value is Q = 5 × 10 7), which corresponds to the 30 MHz bandwidth of the mode. The light is sent to the cavity and extracted through a diamond prism that is coupled. The repeatable value of fiber-to-fiber insertion loss with this technique is 20 dB (the measured minimum insertion loss is about 12 dB). Maximum transmission is obtained when the light resonates in the cavity mode.

  The top and bottom of the disk resonator 710 were coated with conductive layers 711 and 712, respectively, to receive external electrical control signals. The conductive films 711 and 712 may be formed using a metal such as indium. Tuning of the filter 700 is obtained by applying a voltage to the top and bottom conductive coatings. Each conductive film may not be present at the center of the resonator, and is present at the periphery of the resonator where the WGM is located. The conductive coating of this design can reduce the overall impedance of the electrical path and thus shorten the filter 700 tuning time.

  The maximum frequency deviation in the TE and TM modes can be written as:

  Here, .nu.0 = 2.times.10 @ 14 Hz is the carrier frequency of the input optical signal and the oscillation frequency of the laser that generates the input signal. r33 = 31 pm / V and r13 = 10 pm / v are electro-optic constants of Z-cut LiNbO3. n0 = 2.28 and ne = 2.2 are the refractive indices of LiNbO3 along two orthogonal birefringence axes.

  It should be noted that TE and TM modes can be selected during operation of such a filter, depending on the needs of a particular application. For example, the TM mode produces a better quality factor than the TE mode in some applications, so it can be used when a high quality factor or a narrow filter linewidth is desired. The TE mode is used when the quality factor is not so important because the electro-optic effect deviation amount is three times larger than that of the TM mode for the same applied voltage value. The required power can be reduced by using the TE mode.

FIG. 8 shows the experimentally measured electro-optic tuning of the filter spectral response and the tuning of the center wavelength with applied voltage to the LiNbO 3 WGM filter based on the design of FIG. When the tuning voltage is changed from zero to 10 V, the spectrum of the filter shifts by 0.42 GHz with respect to the TM polarized wave, which matches the theoretical value. This special filter exhibits a linear voltage dependence in the tuning range of ± 150 V, and the total tuning span is the free spectral spacing (FSR) of the WGM cavity.
spectral range).

  The dependency Δν (Ez) has a hysteresis characteristic when a large DC electric field (Ez> 2MV / m) is applied to the cavity. A sudden change in the applied voltage leads to incomplete mode shift compensation, ie Δν (Ez = 0) ≠ 0, and the resonator frequency is in the first position in seconds after the electric field is switched off. Return. The maximum frequency tuning of this non-linear filter was about 40 GHz.

  It can be seen that the insertion loss of the exemplary filter is mainly due to the inefficiency of the coupling technique with the diamond prism configuration. In this connection, an anti-reflection coating is applied to the coupling prism to reduce such losses. Similarly, a special diffraction grating may be arranged in a high refractive index fiber as an optical coupler to significantly reduce the loss.

  FIG. 9 shows a signal transmission system 900 that transmits a video signal using a tunable WGM filter 910 on an optical fiber line. Such transmission lines would be important for the development of portable optical domain microwave navigation and communication devices that can provide significantly higher capabilities for applications such as NASA planetary exploration. A video signal having a half wave height full width value of about 20 MHz and a zero carrier frequency is sent from the CCD camera 901 to the mixer 903 and mixed with the 10 GHz microwave carrier generated from the microwave source 905. The obtained modulated microwave signal is filtered by a filter 920 to suppress and amplify a higher-order harmonic signal component, and the optical signal 932 is increased using an optical modulator 930 such as a Mach-Zehnder electro-optic modulator. Convert.

  A laser 960 is used to generate an unmodulated laser beam, for example, 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. Beam 962A is sent to optical modulator 930 and modulated to generate modulated signal 932. Modulator signal 932 is then sent through an optical filter transmission line with tunable WGM filter 910. The other unmodulated beam 962B is sent, for example, through a fiber loop optical delay line 940 to an optical splitter 964 that operates as a combiner to combine the unmodulated beam 962B and the filtered modulated signal 932. This synthesis provides a heterodyne-type detection mechanism and can reduce the noise effects of the laser 960. Next, the combined signal from the optical splitter 964 is received and detected using a photodetector 950 such as a high-speed photodiode. The optical delay line 940 and the combined beam splitter 964 may be removed from the system 900 if the laser 960 can generate a stable laser output. The filtered optical signal 932 generated by the filter 910 may be sent directly to the detector 950.

  The photodiode output is mixed with the microwave carrier by mixer 970 to restore the original signal. The microwave carrier wave here operates as a local oscillator. In the example shown in FIG. 9, the microwave carrier wave is branched from the microwave output from the microwave source 905. The restored video signal is displayed using a display unit 980 such as a TV.

  In this example, the filter output from filter 910 is mixed by light field 962B and measured by photodiode 950 to characterize the filtered signal and read the encoded information. Filter 910 is a high Q WGM cavity that adds group delay to the signal. If the laser 960 used in the experiment has a wide line width, this group delay may cause frequency-to-amplitude laser noise conversion unless the mechanism is balanced. In order to avoid this conversion, a WGM filter 910 is inserted in the Mach-Zehnder configuration having the fiber delay line Lf to compensate for the group delay. The length of the delay line is equal to Lf = dn0F / 2nf = 1.2 m, where nf = 1.5 is the refractive index of the fiber material and F = 300 is the cavity finesse. Such compensation is not necessary if the laser linewidth is much smaller than the cavity resonance width. In testing the system 900, the optical characteristics of the filter were achieved using a semiconductor diode laser, such as laser 960, with a very large 30 MHz half-wave full width line. The laser power in the fiber was about 2.5 mW.

  Using the basic layout of the system 900 of FIG. 9, a microwave or RF transceiver system can be constructed. FIG. 10 shows an embodiment having a microwave transmitter 1010 and a tunable optical microwave filter 1030. The transmitter 1010 has a transmission antenna that transmits a microwave signal into the air. The receiver antenna 1020 receives the signal from the airborne transmitter 1010 and sends the received signal to the optical filter 1030. As in system 900, tunable WGM filter 910 is used to selectively transmit light modulation signal 932 from light modulator 930. Filter 910 is tuned by control unit 630. The microwave signal transmitted by the transmitter 1020 may include a number of signal channels at different frequencies, eg, another video signal from another video source, such as another CCD camera. When the bandwidth of each channel is equal to or less than the bandwidth of the optical filter 910 and separate channels are sufficiently separated by the optical modulation signal 932, the optical filter 910 is tuned and one channel in the received signal displayed on the TV 980 is displayed. While the other channels carried by the optical signal 932 are optically blocked. In this situation, the system 1000 may be used in a broadcast system in which each receiver can be operated to select any one channel in the broadcast signal. A local RF or microwave generator 1040 is implemented to provide a local oscillator signal to the mixer 970 in restoring the desired channel signal.

  Tunable optical filters are important elements for various optical devices and systems. Examples of such devices and systems include wavelength division multiplexing (WDM) for building reconfigurable networks, and analog RF optical communication links. Desired characteristics of the filter include fast tuning, small size, wide tuning range, low power consumption, and low cost. Wavelength demultiplexing and channel sections of WDM systems require tunable narrowband optical filters that are compatible with single mode fiber.

  Fabry-Perot and fiber Fabry-Perot tunable filters are one type of a wide variety of tunable filters. A Fabry-Perot filter is characterized by a useful figure of merit called finesse equal to the ratio of the free spectral spacing (FSR) of the filter to the bandwidth. Finesse indicates how many channels fit in one span FSR. A Fabry-Perot filter typically has a finesse of about 100, a bandwidth of about 125 GHz, and a tuning speed in the millisecond range. These filters also meet the -20 dB channel-to-channel blocking condition for 50 GHz channel spacing.

  The tunable WGM filter described herein can be characterized by parameters similar to the Fabry-Perot filter. Comparing this tunable WGM filter with the Fabry-Perot filter, it can be seen that the tunable WGM filter is superior to the Fabry-Perot filter. For example, a tunable WGM filter can be designed to operate over a wide spectral range. Using lithium niobate as the electro-optic material, the tunable WGM filter can only operate at wavelengths limited by the absorption loss of lithium niobate, and the operating wavelength will be in the range of about 1.0 to 1.7 μm. . It should be noted that this range includes the communication C band around 1.55 μm wavelength. The reproducible value (F) of the finesse of the filter can exceed F = 300 and can be as high as F = 1000. The tuning speed of a tunable WGM filter can be approximately 10 ns, but in some implementations the actual spectral shift time is determined by the 30 MHz bandwidth of the filter and does not exceed 30 μs. A suppression ratio of at least −20 dB of channel crosstalk for 50 MHz channel spacing was observed.

  Although only a few embodiments are disclosed, it will be appreciated that modifications and enhancements may be made.

Claims (19)

A method for filtering an optical signal comprising:
Directing an input optical signal to an optical resonator configured to support a whispering gallery mode and having a portion in which the whispering gallery mode exists, wherein at least a portion of the optical resonator is electro-optic A step to show the effect,
Combining light from the optical resonator and generating a filtered optical output from the input optical signal;
Applying an electrical control signal to at least a portion of the optical resonator to tune a transmission spectral peak of the optical resonator, thereby selecting a spectral component of the input optical signal at the filtered optical output; ,
Including methods.   The method of claim 1, further comprising using a TM mode in a whispering gallery mode when the input optical signal is coupled to the optical resonator and light is coupled out of the optical resonator.   The method of claim 1, further comprising using at least a portion of a non-spherical shape as the optical resonator supporting the whispering gallery mode.   4. The method of claim 3, wherein the non-spherical shape is a spheroid.   4. The method of claim 3, further comprising using at least a portion of a sphere as the optical resonator.   The method of claim 1 further comprising using a disk resonator as the optical resonator. Receiving an input electrical signal carrying a number of signal channels;
Optically modulating a light beam with the input electrical signal to generate an optical modulation signal as the input optical signal carrying the multiple signal changes as the signal;
Tuning the spectral transmission peak of the optical resonator to transmit a selected signal channel to the filtered light output while optically blocking other signal channels;
Converting the filtered light output into an electrical signal;
Extracting the selected channel from the electrical signal;
The method of claim 1 comprising: Furthermore,
Branching the unmodulated light beam into first and second beams;
Modulating the first beam as the input optical signal;
Directing the second beam through an optical delay path;
Combining the filtered light output and the second beam after passing through the optical delay path to generate a combined optical signal;
Converting the combined optical signal into an electrical signal;
Extracting the signal from the electrical signal;
The method of claim 1 comprising:   The method of claim 1, further comprising using a TE mode in a whispering gallery mode when the input optical signal is coupled to the optical resonator and light is coupled out of the optical resonator. A tunable optical filter comprising:
An optical resonator configured to support a whispering gallery mode, the optical resonator comprising at least a portion where the whispering gallery mode exists, wherein at least a portion of the optical resonator exhibits an electrooptic effect When,
At least one electrode formed on the optical resonator for guiding an electrical control signal to the optical resonator and spatially overlapping the whispering gallery mode;
A control unit connected to the at least one electrode and supplying an electrical control signal to the part to tune the refractive index, thereby tuning the transmission peak of the optical resonator by the electro-optic effect;
With a filter.   11. The filter of claim 10, wherein the tunable optical resonator comprises a lithium niobate crystal.   The filter according to claim 10, further comprising an optical coupler that is evanescently coupled to the optical resonator.   The filter of claim 12, wherein the optical coupler is a fiber coupler.   The filter of claim 12, wherein the optical coupler includes a waveguide.   The filter of claim 12, wherein the optical coupler comprises an optical gap material.   The filter of claim 12, wherein the optical coupler includes a prism. An apparatus comprising a receiver for receiving a radiated signal carrying a plurality of signal channels and extracting a selected channel from the received signal channel, the receiver comprising:
An optical modulator that modulates a light beam in response to the radiation signal to generate an optical modulation signal that carries the signal channel;
(1) An optical resonator configured to support a whispering gallery mode and including at least a portion where the whispering gallery mode exists, wherein at least a portion of the optical resonator exhibits an electro-optic effect. An optical resonator, and (2) at least one electrode formed on the optical resonator for guiding an electrical control signal to the optical resonator and spatially overlapping the whispering gallery mode; A control unit connected to the at least one electrode and supplying an electrical control signal to the part to tune the refractive index, thereby tuning the transmission peak of the optical resonator by the electro-optic effect; A tunable optical filter, wherein the optical filter is arranged to receive the light modulation signal and to filter the selected signal channel Generates a filtered optical output which conveys and a tunable optical filter,
A photodetector for converting the filtered light output into an electrical signal;
A mixer for extracting the selected signal channel by mixing the electrical signal with a reference signal;
An apparatus comprising:   The apparatus of claim 17, wherein the tunable optical resonator comprises a crystal of lithium niobate.   The apparatus of claim 17, further comprising an optical coupler that is evanescently coupled to the optical resonator.

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