Coupled opto-electronic oscillators with low noise

Coupled opto-electronic oscillators with a whispering-gallery-mode (WGM) optical resonator inside the laser cavity to produce oscillation signals out of the optical spectral range, e.g., RF or microwave frequencies.

BACKGROUND

This application relates to signal oscillators.

Signal oscillators may be constructed by using both electronic and optical components to form opto-electronic oscillators (OEOs). See, e.g., U.S. Pat. Nos. 5,723,856 and 5,777,778, which are incorporated herein by reference as part of the specification of this application. Such an OEO may include an electrically controllable optical modulator and at least one active opto-electronic feedback loop that includes an optical part and an electrical part interconnected by a photodetector. The opto-electronic feedback loop receives the modulated optical output from the modulator and converted it into an electrical signal to control the modulator. The loop produces a desired delay and feeds the electrical signal in phase to the modulator to generate and sustain both optical modulation and electrical oscillations outside the optical in, e.g., the microwave or radio frequency spectral range when the total loop gain of the active opto-electronic loop and any other additional feedback loops exceeds the total loss. The generated oscillating signals may be tunable in frequency and can have narrow spectral linewidths and low phase noise in comparison with the signals produced by other RF and microwaves oscillators. Notably, the OEOs are optical and electronic hybrid devices.

An OEO may be configured as a coupled opto-electronic oscillator (COEO) which directly couples a laser oscillation in an optical feedback loop (e.g., a laser cavity) to an electrical oscillation in an opto-electronic feedback loop. See, e.g., U.S. Pat. Nos. 5,929,430 and 6,567,436, which are incorporated herein by reference as part of the specification of this application. An optical resonator may be used in an optical section in which the optical feedback loop and the opto-electronic feedback loop overlap. The optical resonator may be an optical whispering-gallery-mode (“WGM”) resonator which supports a special set of 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. Optical WGM resonators with high quality factors have been demonstrated at Q values greater than 109. Such hi-Q WGM resonators may be used to produce oscillation signals with high spectral purity and low noise.

SUMMARY

This application describes implementations of COEO devices with a WGM resonator inside the laser cavity to produce oscillation signals out of the optical spectral range, e.g., RF or microwave frequencies.

DETAILED DESCRIPTION

Implementations of coupled OEO devices described in this application use a high gain optical amplifier at one end of the laser cavity to boost the optical gain and an optical whispering gallery mode (WGM) resonator in the laser cavity to filter the light inside the laser cavity. In one implementation, the laser cavity includes a first semiconductor optical amplifier forming a first end of the laser cavity, a second semiconductor optical amplifier, and an electro-absorption modulator which forms a second end of the laser cavity. The modulator is integrated to the second semiconductor optical amplifier and modulates light in the laser cavity in response to a modulation signal. The optical whispering gallery mode resonator for this COEO implementation is located inside the laser cavity between the first and the second semiconductor amplifiers. A beam splitter inside the laser cavity is used to split a portion of the light in the laser cavity to produce an optical output and a photodetector is used to receive the optical output and to convert the optical output into a detector output. A circuit is further used to receive the detector output and to produce the modulation signal from the detector output. In this implementation, the beam splitter, the photodetector, the circuit, and the laser cavity forms an opto-electronic loop to support an oscillation at a frequency of the modulation of light.

The following sections first describe the exemplary geometries of the WGM resonators that may be used in COEO devices.FIGS. 1,2, and3illustrate three exemplary WGM resonators.

FIG. 1shows a spherical WGM resonator100which is a solid dielectric sphere. The sphere100has an equator in the plane102which is symmetric around the z axis101. The circumference of the plane102is a circle and the plane102is a circular cross section. A WG mode exists around the equator within the spherical exterior surface and circulates within the resonator100. The spherical curvature of the exterior surface around the equator plane102provides spatial confinement along both the z direction and its perpendicular direction to support the WG modes. The eccentricity of the sphere100generally is low.

FIG. 2shows an exemplary spheriodal microresonator200. This resonator200may be formed by revolving an ellipse (with axial lengths a and b) around the symmetric axis along the short elliptical axis101(z). Therefore, similar to the spherical resonator inFIG. 1, the plane102inFIG. 2also has a circular circumference and is a circular cross section. Different from the design inFIG. 1, the plane102inFIG. 2is a circular cross section of the non-spherical spheroid and around the short ellipsoid axis of the spheroid. The eccentricity of resonator100is (1−b2/a2)1/2and is generally high, e.g., greater than 10−1. Hence, the exterior surface is the resonator200is not part of a sphere and provides more spatial confinement on the modes along the z direction than a spherical exterior. More specifically, the geometry of the cavity in the plane in which Z lies such as the zy or zx plane is elliptical. The equator plane102at the center of the resonator200is perpendicular to the axis101(z) and the WG modes circulate near the circumference of the plane102within the resonator200.

FIG. 3shows another exemplary WGM resonator300which 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 inFIGS. 1 and 2, the exterior surface provides curvatures in both the direction in the plane102and the direction of z perpendicular to the plane102to confine and support the WG modes. Such a non-spherical, non-elliptical surface may be, among others, a parabola or hyperbola. Note that the plane102inFIG. 3is a circular cross section and a WG mode circulates around the circle in the equator.

The above three exemplary geometries inFIGS. 1,2, and3share a common geometrical feature that they are all axially or cylindrically symmetric around the axis101(z) around which the WG modes circulate in the plane102. The curved exterior surface is smooth around the plane102and provides two-dimensional confinement around the plane102to support the WG modes.

Notably, the spatial extent of the WG modes in each resonator along the z direction101is limited above and below the plane102and hence it may not be necessary to have the entirety of the sphere100, the spheroid200, or the conical shape300. Instead, only a portion of the entire shape around the plane102that is sufficiently large to support the whispering gallery modes may be used to form 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.

FIGS. 4A and 4Bshow a disk-shaped WGM resonator400and a ring-shaped WGM resonator420, respectively. InFIG. 4A, the solid disk400has a top surface401A above the center plane102and a bottom surface401B below the plane102with a distance H. The value of the distance H is sufficiently large to support the WG modes. Beyond this sufficient distance above the center plane102, the resonator may have sharp edges as illustrated inFIGS. 3,4A, and4B. The exterior curved surface402can be selected from any of the shapes shown inFIGS. 1,2, and3to achieve desired WG modes and spectral properties. The ring resonator420inFIG. 4Bmay be formed by removing a center portion410from the solid disk400inFIG. 4A. Since the WG modes are present near the exterior part of the ring420near the exterior surface402, the thickness h of the ring may be set to be sufficiently large to support the WG modes.

An optical coupler is generally used to couple optical energy into or out of the WGM resonator by evanescent coupling.FIGS. 5A and 5Bshow two exemplary optical couplers engaged to a 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. 5Ashows 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. 5Bshows 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.

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 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 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.

High Q WGM resonators with proper input and output optical couplers may be used to perform as ultra narrow bandpass optical filters inside the laser cavities of COEO devices to filter out optical noise such as noise caused by the amplified spontaneous emission (ASE). Such intracavity WGM resonator and the laser cavity are designed to be matched in their optical modes. In addition, as described in U.S. Pat. No. 6,567,436, the opto-electronic loop is configured to meet proper mode matching conditions. Notably, the presence of the WGM resonator inside the laser cavity changes the noise distribution inside the laser cavity. Light that transmits through the WGM resonator prior to interaction with an optical gain medium tends to have lower noise than light that emerges from an optical gain medium and propagates towards the WGM resonator. Hence, the optical signal to the optical portion of the opto-electronic loop may be selected from light that transmits through the WGM resonator prior to interaction with an optical gain medium in such COEO devices to reduce the noise in the opto-electronic loop and hence the generated oscillation signal.

FIG. 6illustrates an example of a COEO600with an intracavity WGM resonator630. The entire COEO600may be placed on a base601and is enclosed in a housing. The optical path between optical components may be in free space. Alternatively, optical waveguides such as fibers or waveguides formed on substrates may be used. The laser cavity in the COEO600is formed by a first optical gain medium610on one side of the WGM resonator630as the first end of the laser cavity, and a second optical gain medium620and an optical modulator622on the other side of the WGM resonator630as the second end of the laser cavity. Hence, light inside the laser cavity is reflected between the first optical gain medium610and the optical modulator622is amplified by the gain media610and620to sustain a laser oscillation. The light is optically filtered by the WGM resonator630and is modulated by the optical modulator622in response to a modulation signal623that is applied to control the modulation by the modulator622. An optical beam splitter640, e.g., an optical beam splitting cube, is placed inside the laser cavity to split a portion of the light generated inside the laser cavity to the optical portion of the opto-electronic loop of the COEO600. The beam splitter640or another optical beam splitter may be used to generate another optical output as the optical output of the COEO600. For example, a beam splitting cube may be placed between the gain medium610and the WGM resonator630to split a portion of the beam propagating from the WGM resonator630towards the gain medium610as the optical output of the COEO600.

The opto-electronic loop of the COEO600includes the beam splitter640, a photodetector650, a circuit660, and the laser cavity as a closed loop to support an oscillation at a frequency of the modulation of light. An electrical link651is used to connect the photodetector650and the circuit650to direct the detector output to the circuit650. Another electrical link623is used to connect the modulator622and the circuit660to provide the modulation signal to the modulator622. The circuit660may include a signal amplifier (e.g.,664A and664B) which amplifies the signal received from the photodetector650, and a voltage controlled phase shifter665which adjusts the phase of the modulation signal at the modulator622. The modulation signal from the circuit660at the modulator622is in phase to generate and sustain the oscillation and is different from a negative feedback loop where the feedback signal is out of phase with the modulation to avoid any oscillation.

The optical gain media610and620on two sides of the WGM resonator630may be designed to have different optical gains. For example, the gain of the medium610may be higher than that of the medium620. Under this configuration, the light in the laser cavity leaving the first gain medium has just passed through the gain medium610twice and thus is strongly amplified by the first gain medium610. The WGM resonator630subsequently filters the amplified light to remove light at frequencies outside the resonant frequency of the WGM resonator. This filtering removes various noise signals in the light including the noise caused by ASE in the optical gain media. Accordingly, with a sufficient gain in the medium610, the light coming out of the WGM resonator630towards the second gain medium620has more power and less noise than other light beams in the laser cavity. Hence, a portion of this light coming out of the WGM resonator630towards the second gain medium620may be directed out of the laser cavity to the optical portion of the opto-electronic loop. Therefore, under this configuration, the optical beam splitter640may be placed between the WGM resonator630and the second optical gain medium620to split light in the laser cavity that travels from the first gain medium610towards the second gain medium620as the optical output to the photodetector650.

The first optical gain medium610may be a semiconductor optical amplifier (SOA) with a high optical gain, e.g., 15 to 30 dB. The SOA610may also be designed to have a high saturation power, e.g., in the range from 16 dBm to 20 dBm. The second gain medium620and the modulator622may be a second semiconductor optical amplifier and an electro-absorption modulator, respectively. This second semiconductor optical amplifier and the electro-absorption modulator may be an integrated semiconductor device. The optical gain of the second SOA620is less than the first SOA610, e.g., may be at 15 dB or less and its saturation power may also be less than that of the first SOA610, e.g., in a range from 8 dBm to 12 dBm.

The COEO600may also include a narrow band optical filter612between the first gain medium610and the WGM resonator630provide additional optical filtering to further suppress any optical noise. A first optical coupler631is located between the first gain medium610and the optical whispering gallery mode resonator630to couple light into and out of the optical whispering gallery mode resonator630. A second optical coupler632is located between the second gain medium620and the optical whispering gallery mode resonator630to couple light into and out of the optical whispering gallery mode resonator630. The couplers631and632may be prisms or other optical coupling devices. In addition, optical lenses may be placed at selected locations inside the laser cavity to collimate and focus the light. A lens may be placed between the beam splitter640and the photodetector650to direct the light into the sensing area of the photodetector650.

The circuit660in the electrical portion of the opto-electronic loop may include a photodetector bias circuit663which provides an electrical bias to the photodetector650via the link651. The circuit660may also include a modulator bias circuit670which provides an electrical bias to the electro-absorption modulator622. A planar electrode strip661may be used at the input of the circuit660to receive the detector signal from the photodetector in an impedance-matched condition. As illustrated a λ/2 resonance line is used as the strip661, where λ is the wavelength of the oscillation signal in the detector output in, e.g., an RF or microwave frequency. A feed line662, separate from the line661, is used to deliver the received signal to the rest of the circuit660. At the output of the circuit660, a planar electrode strip669, e.g., a λ/2 resonance line, may be used to output the modulation signal to the modulator622in an impedance-matched condition. An receiving electrode667delivers the modulation signal to a λ/4 resonance line668which is coupled to the electrode669. Various designs may be used to provide impedance-matching conditions at the input and output of the circuit660. A signal coupler may be used in the circuit660to generate an output for the RF or microwave signal.

Only a few implementations are disclosed. However, it is understood that variations and enhancements may be made.