Electro-optic whispering-gallery-mode resonator devices

Description

This patent document claims the benefits of U.S. Provisional Application No. 61/053,404 entitled “CPW On-Chip Excitation of Receiver” and filed May 15, 2008, and U.S. Provisional Application No. 61/114,146 entitled “WGMR Based Tunable SSB Modulator” and filed Nov. 13, 2008. The disclosures of the above applications are incorporated by reference as part of the disclosure of this document.

BACKGROUND

This document relates to optical resonators and devices using optical resonators.

Optical resonators can be configured in various configurations. Examples of well-known optical resonator designs includes Fabry-Perot optical resonators and optical ring resonators. As another example, an optical material such as a dielectric material may be shaped to construct an optical whispering-gallery-mode (“WGM”) resonator which supports one or more 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 equator of the sphere. 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 small. As a result, a high quality factor, Q, e.g., greater than 10⁹, may be achieved in such resonators. Hence, optical energy, once coupled into a 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, optical delay, optical sensing, lasers, and opto-electronic oscillators.

SUMMARY

Various implementations are described in greater detail in the attached drawings, the detailed description and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-8 show various features of examples.

FIGS. 1A and 1B show an example of an electro-optic WGM resonator.

FIGS. 2A and 2B show an example of spatially modulated electrodes for a WGM resonator that can provide efficient interaction between microwave and optical fields in the WGM resonator.

FIG. 2C shows an example of the spatial distribution of a spatially modulated RF field in a shaped waveguide on top of a whispering gallery mode resonator.

FIGS. 3A and 3B show two configurations of broadband excitation waveguides for input and output in a WGM resonator having spatially modulated electrodes.

FIG. 4 shows an example of the RF field distribution in an on-chip resonator based on the spatially modulated electrodes in FIGS. 2A and 2B.

FIG. 5 shows a design where the electrode couples RF signal in a polarization that is in the plane of the optical loop in which the light is confined in a WGM.

FIG. 6 and FIG. 7 show examples of single sideband modulation.

FIG. 8 shows a schematic of a tunable opto-electronic oscillator (OEO) based on single sideband modulation in an electro-optic WGM resonator.

DETAILED DESCRIPTION

Many WGM resonators are axially or cylindrically symmetric around a symmetry axis around which the WG modes circulate in a circular path or the equator. The exterior surface of such a resonator is smooth and provides spatial confinement to light around the circular plane to support one or more WG modes. The exterior surface may be curved toward the symmetry axis to spatially confine the light along the symmetry axis. A WGM resonator may be shaped symmetrically around a symmetry axis and has a protruded belt region to form a circular path to confine light in one or more WG modes. The exterior surface of the protruded belt region may be any suitable geometrical shape such as a flat surface or a curved surface. Such a WGM resonator may be configured in any suitable physical size for a given wavelength of light. Various materials can be used for WGM resonators and include, for example, crystal materials and non-crystal materials. Some examples of suitable dielectric materials include fused silica materials, glass materials, lithium niobate materials, and calcium fluoride materials.

A whispering gallery mode resonator can be made of a material exhibiting an electro-optic effect and can include electrodes on the optical resonator to apply an RF or microwave signal to the optical resonator to effectuate the electro-optic effect to control the one or more optical whispering gallery modes circulating along a circular optical loop near a rim of the optical resonator. The electro-optic effect in such a WGM resonator can be used to tune the resonator and to modulate light for a wide range of applications.

FIGS. 1A and 1B show an example of an electro-optic WGM resonator 100 having a WGM resonator 110. The electro-optic material for the entire or part of the resonator 610 may be any suitable material, including an electro-optic crystal such as Lithium Niobate (“Lithium Niobate resonator”) and semiconductor multiple quantum well structures. One or more electrodes 111 and 112 may be formed on the resonator 110 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 110 has disk or ring geometry, the electrode 111 may be formed on the top of the resonator 110 and the electrode 112 may be formed on the bottom of the resonator 110 as illustrated in the side view of the device in FIG. 1B. In one implementation, the electrodes 111 and 112 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 111 and 112 may be microstrip line electrodes. The electrodes 111 and 112 may also form an electrical waveguide to direct the electrical control signal to propagate along the paths of the WG modes. An RF or microwave circuit 130 such as a control circuit may be used to supply the electrical control signal to the electrodes 111 and 112.

In operating the tunable resonator 100, the control unit 130 may supply a voltage as the electrical control signal to the electrodes 111 and 112. The control voltage may be a DC voltage to set the resonance peak of the resonator 100 at a desired spectral location. The DC voltage may be adjusted by the control unit 130 to tune the spectral position of the transmission peak when such tuning is needed. For dynamic tuning operations, the control unit 130 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 130 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 or to produce signal modulation. In some applications, both a modulation electrical signal and a DC electrical signal can be applied to the electrodes on the resonator 100.

For example, a Z-cut LiNbO₃ disk cavity with a diameter of d=4.8 mm and a thickness of 170 μm may be used as the resonator 110. The cavity perimeter edge may be prepared in the toroidal shape with a 100 μm radius of curvature. As an alternative to the strip electrodes shown in FIG. 1A, the top and bottom surfaces of the disk resonator may be coated with conductive layers for receiving the external electrical control signal. A metal such as indium may be used to form the conductive coatings. Tuning is achieved by applying and adjusting a voltage to the top and bottom conductive coatings. Each conductive coating may be absent on the central part of the resonator and are present at the perimeter edge of the resonator where WGMs are localized.

One technical feature of such an electro-optic WGM resonator is the phase matching between the applied RF and microwave signal and the light in a WGM inside the resonator to provide efficient interaction between the light and the applied RF and microwave signal. The geometry of the electrodes on the WGM resonator can be designed to facilitate this phase matching. As an example, in a WGM optical resonator made of a material exhibiting an electro-optic effect and shaped to support one or more optical whispering gallery modes circulating along a circular optical loop near a rim of the optical resonator, electrodes can be formed on the optical resonator to apply an RF or microwave signal to the optical resonator to effectuate the electro-optic effect and, notably, the electrodes can be designed to include electrode segments located on one surface of the optical resonator connected in series along part of the circular optical loop and two adjacent electrode segments are at different radial distances from a center of the circular optical loop to cause a spatial modulation in the applied RF or microwave signal in the optical resonator to facilitate the phase matching.

This design can be used in various applications. For example, lithium niobate whispering gallery mode receiver has high sensitivity to microwave pulses. This makes it very useful as a microwave sensor for many applications. Manufacturing of such a receiver includes multileveled microwave assembling procedure which requires very experienced personnel and has low yield. Planar technologies on the other hand promise much easier and cheaper manufacturing of receiver. Narrowband microwave excitation of the receiver requires RF tune-up of the microwave resonator. This is risky time consuming part of the assembly process with low yield. It also requires very experienced operator. Broadband excitation removes these risks. The present design can be used in manufacture of on-chip specific set of broadband planar microwave components, microwave components on the resonator and to couple these two structures together by bonding.

The present design of spatially modulated electrodes for WGM resonators can provide efficient interaction between microwave and optical field in the WGM resonator. FIGS. 2A and 2B show an example.

Referring to FIG. 2A, the electro-optical WGM resonator 110 includes electrode segments 122 and 121 that are spatially interleaved and connected around the circular rim of the resonator 110 to form an arc electrode structure with its two terminals connected to RF feeds 131 and 132. The distances of the electrode segments 121 and 122 are at different radial distances from a center of the circular optical loop in which the light circulates inside the resonator 110. The electrode segments 121 and 122 may have different segment lengths and different radial distances to provide a spatial modulation in the applied RF or microwave signal in the optical resonator 110 to facilitate the phase matching.

In the illustrated example, the electrode segments 121 and 122 are arc segments with an equal length which is estimated as:

L = λ rf 2 ⁢ ( N - n )
where N is refractive index of RF field in resonator, n is the refractive index of optical field of working polarization, lambda is operating RF wavelength in vacuum. The actual length of section is a result of a computer simulation. The electrode segments 121 are at one radial distance from the center of the resonator 110 and the electrode segments 122 are at a different radial distance.

FIG. 2C shows an example of the spatial distribution of the spatially modulated RF field in a shaped waveguide on top of the whispering gallery mode resonator. The length of each section of the spatially modulated electrode determines phase matching. The RF field is color coded linearly from green to blue.

To achieve broadband excitation waveguides for input and output coupling must be impedance matched with resonator based waveguide. We propose two configurations for low and high frequencies as depicted at FIGS. 3A and 3B. Advantage of this particular approach is better manufacturability. All RF component is prepared before installation of the sensitive element—microdisk. This approach can increase the yield and reduce the cost of the assembly.

FIG. 4 shows an example of the RF field distribution in on-chip resonator. RF field at 35 GHz in this particular configuration is phase matched to optical modes of lithium niobate optical resonator.

Electro-optic WGM resonator can also be used to provide single sideband (SSB) modulation. In this regard, a method for operating a whispering gallery mode resonator device achieve SSB operation can include coupling light into an optical resonator made of a ferroelectric crystal and structured to support optical whispering gallery modes in two orthogonally polarized TE and TM modes circulating along a circular optical loop near a rim of the optical resonator; and applying an RF or microwave signal to the optical resonator in an electric field polarization oriented relative to a crystal axis of the ferroelectric crystal to effectuate coupling between light in an optical whispering gallery mode in the TE mode and light in another optical whispering gallery mode in the TM mode to produce a tunable optical single sideband modulation at a modulation frequency equal to a difference in optical frequencies of the optical whispering gallery modes in the TE and TM modes.

We demonstrate a broadly tunable and highly efficient resonant electro-optical modulator based on a whispering gallery mode resonator made with an electro optic crystal. This device is used to engineer a tunable opto-electronic oscillator.

The growing demand for more bandwidth in wireless broadband communication networks can be satisfied with the radio over fiber (RoF) technology, which allows reusing frequency bands within local wireless cells. Electro-optical modulators, high stability RF oscillators, and fast photodiodes are the key players in the game, and one of the basic challenges is the development of the high frequency elements to reduce the unwanted interference between the neighboring cells. It is expected that the local wireless cells will be operating at frequencies approaching 60 GHz, so modulators and oscillators must operate at such frequencies. A challenge of fiber based networks associated with increasing frequencies is the nonlinear and dispersive characteristics of the fiber that impact RoF links. One approach addressing these challenges is the use of single sideband modulators. Here we introduce a new class of whispering gallery mode-based single sideband modulators and oscillators that will be important in current and future applications of RoF networks. These devices are unique in that they are fundamentally single sideband, as opposed to previous realizations where cancelation schemes remove the undesired sideband. Our modulator is narrowband, yet widely tunable, in contrast to conventional wide band modulators with fixed band frequency. This property is expected to lead to new architectures for RoF applications.

Whispering gallery mode (WGM) resonators made of LiNbO₃ and LiTaO₃ have been used to create coupling between light and RF fields, achieved by engineering the shape of a micro-strip RF resonator coupled to a WGM resonator. Based on this interaction resonant electro-optic modulators (EOMs) have been developed. The modulators generally operate either at the baseband or at bands detuned from the baseband by several free spectral ranges (FSRs) of the WGM resonator. The later versions of the modulator have the highest efficiency at higher RF frequencies, but is narrowband and barely tunable, in contrast with proposed here tunable EOM.

FIG. 5 shows a design where the electrode couples RF signal in a polarization that is in the plane of the optical loop in which the light is confined in a WGM.

FIG. 6 and FIG. 7 show examples of single sideband modulation.

The modulation is achieved between the WGM modes separated not by the free spectral range of the resonator, but rather by some frequency given by the resonator shape, temperature, and the bias voltage. Tests were conducted to demonstrate the single sideband modulation using heterodyne technique. The modulated light was mixed with an optical local oscillator, demodulated at a fast PD, and sent to an RF spectrum analyzer. The measured value of the second sideband rejection (SSR) was about 70 dBc and was limited by the maximum sensitivity of our measurement system. Conducted tests demonstrated that the electro-optical tunability of the modulator was over 50 V span of the DC bias.

The light confined in two optical WGMs characterized with electric field operators {right arrow over (E)}₁ and {right arrow over (E)}₂ is coupled with the RF field {right arrow over (E)}_M in the case of nonzero interaction energy

E = 1 8 ⁢ ⁢ π ⁢ ∫ V ⁢ ∑ i , j , k ⁢ r ijk ⁢ D i ⁢ D j ⁢ E Mk ⁢ ⅆ v , ( 1 )

where r_ijk describes the space dependent electro-optic nonlinearity of the resonator host material, D_i=Σ_l^∈_ilE_l, electric field E_l is presented as a sum of two counter-propagating waves, and V is the volume of the WGM resonator. We have studied the interaction between two WGM mode families having different polarizations. Because the mode families are shifted one with respect to the other in the frequency space it is possible to realize single sideband tunable modulation in the system. The interaction is achieved due to the non-diagonal elements of the linear electro-optic tensor of the material (e.g. r₅₁). Those elements introduce coupling between TE and TM WGMs in a resonator fabricated from a z-cut LiTaO₃ preform if the RF field has a radial component so that

E = n e 2 ⁢ n o 2 4 ⁢ ⁢ π ⁢ ∫ V ⁢ r 51 ⁡ ( E → TM · E → RF ) ⁢ E TE * ⁢ ⅆ v , ( 2 )
where we take into account that {right arrow over (E)}_TE={right arrow over (z)}E_TE. The averaged interaction energy is generally zero because n_e≠n_o. However, either periodical poling of the material or creating a special electrode shape for the RF mode phase matches the interaction such that E≠0. Finally, a composite resonator can be fabricated from, say, x-cut and z-cut cylindrical segments attached together so that the nonzero interaction again becomes possible.

We have achieved interaction of the light confined in TE and TM mode families experimentally and realized single sideband modulation in a WGM resonator having 48 GHz free spectral range. The second sideband rejection (SSR) is greater than 70 dBc. The modulation frequency is tuned from 10.5 GHz to 14.5 GHz with +50 V DC bias change, and can be readily extended to 20 GHz and beyond. Wider band thermal tunability is also demonstrated. The possibility of increasing the modulation bandwidth of the EOMs without significant efficiency deterioration is a great advantage of this technology. The RF bandwidth of our modulator is approximately 20 MHz, however it can be significantly larger (1 GHz and more) if one uses smaller WGM resonators and takes advantage of the interaction of different WGM families.

An example of an application for the tunable modulator is the tunable opto-electronic oscillator (OEO). The CEO is based on an electro-optic feedback loop that directly converts light energy to spectrally pure RF radiation. The CEO performance does not degrade with RF frequency, and an CEO operating at, say, 5 GHz has the same noise performance as one operating at 60 GHz, assuming the same amplifier noise for the two examples. This is a useful feature for the RoF applications.

We have studied the CEO shown in (FIG. 8). A semiconductor laser is injection locked to a lithium tantalate resonator arranged as a tunable single-sideband modulator; the output of the modulator is fed to a photodetector, which produces an electric signal that is amplified before being fed back to the modulator to complete the CEO loop. The single-sideband modulator is ideally suited for the CEO because it produces an RF signal on a photodiode with the highest possible power for a given optical power. The narrow bandwidth of the resonator provides the filter function required for the CEO loop, and the change in the mode spacing produced by the applied to the WGM resonator DC voltage bias tunes the CEO frequency. We expect that the oscillator can be tuned from 2 to 18 GHz (the demonstrated tunability currently corresponds to the tunability of our modulator. The spectral purity goal corresponds to the phase noise of −120 dBc at 10 kHz. The entire size of the packaged oscillator should not exceed an inch cubed.

FIG. 8 shows a schematic of the tunable OEO. Light is sent from the laser to the electro-optic WGM resonator (WGMR) through the polarization control device and the coupling diamond prism. The distributed feedback (DFB) [DFB] semiconductor laser is injection (self-) locked to the WGM (say, TM) due to the backward Rayleigh scattering within the resonator. The lock keeps the laser carrier frequency at the WGM even though the resonant WGM frequency drifts in time because of thermal variations. The polarization of the light entering the resonator is selected such that some light enters the TM WGM and the rest is reflected from the prism-resonator interface. In the case of the electro-optical modulation the optical sideband is generated in the selected TE mode. The light leaving this mode has the same polarization as the light reflected from the coupler. Interfering on the photodiode the light creates RF signal at the frequency coinciding with the frequency difference between the TM and TE modes. The preamplified RF signal is returned to the WGM resonator via an RF electrode. The system oscillates if the optical power as well as RF amplification is high enough. The oscillation frequency can be tuned with a DC bias applied to the WGM resonator as the bias changes the frequency difference between the TM and TE modes.

A demonstration of a tunable resonant electro-optical modulator based on a high-Q whispering gallery mode resonator made out of lithium tantalate is achieved. The demonstrated tunability range approaches 5 GHz and potentially can exceed 20 GHz. The modulation frequency is determined by the properties of the RF electrode and can potentially exceed 60 GHz. The modulation bandwidth can scale from approximately 1 MHz to 1 GHz, depending on the loading of the resonator. A compact tunable opto-electronic oscillator is one of immediate applications of the modulator.

While this document contains many specifics, these should not be construed as limitations on the scope of an invention or of what may be claimed, but rather as descriptions of features specific to particular embodiments of the invention. Certain features that are described in this document in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or a variation of a subcombination.

Only a few implementations are disclosed. Variations and enhancements to the described implementations and other implementations can be made based on what is disclosed.