Patent Description:
Electronic oscillators are used in many microelectronics systems in the art today. The best microwave oscillators, although capable of extraordinarily low phase noise, are substantial in size. Radio Frequency (RF) signals have been generated using miniaturized oscillator architectures by exploiting optical frequency combs. In these devices, the RF frequency has been fixed at a value corresponding to the free-spectral range of the resonator used to generate it. Such miniaturized oscillator architectures, however, do not provide variable frequency tuning having comparable extraordinarily low phase noise to the larger electronic oscillators.

For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the specification, there is a need in the art for a systems and methods for a tunable radio frequency (RF) synthesizer utilizing optical frequency combs. Document <CIT> discloses a laser system to emit stimulated Brillouin scattering light. Document <CIT> discloses optical devices extending the free spectral range and tunability of, and enabling hitless switching of, integrated optical filters suitable for add-drop filters. Document <CIT> discloses a silicon-based narrow-linewidth high-power external cavity laser based on a transverse magnetic mode.

Document <CIT> discloses generating microwave or radio-frequency signals derived from dual optical-frequency references and considers producing stable microwave outputs by controlling and locking two optical frequencies and then using electro-optic modulation and filtering to create sidebands. Such sidebands, when combined and detected, yield a stable RF output whose frequency corresponds to the difference between optical frequencies after division.

The Embodiments of the present disclosure provide methods and systems for a tunable radio frequency synthesizer utilizing optical frequency combs and will be understood by reading and studying the following specification.

In one embodiment, an optical radio frequency (RF) signal generator comprises: a Stimulated Brillouin Scattering (SBS) pump laser segment that includes a first SBS pump laser and a second SBS pump laser each generating SBS laser light at different respective frequencies; a TE/TM dual comb resonator segment comprising a comb optical resonator coupled to the first SBS pump laser and the second SBS pump laser, wherein the comb optical resonator generates a pair of counter-propagating optical frequency combs of different polarities from the SBS laser light; and a filter resonator segment configured to provide feedback to the TE/TM dual comb resonator segment to lock a relative position of the pair of counter-propagating optical frequency combs, the filter resonator segment comprising a tunable optical filter, the filter resonator segment configured to output a discrete tuned RF signal output based on a comb line pair in the pair of counter-propagating optical frequency combs that includes a single comb line from each of the pair of counter-propagating optical frequency combs, wherein a frequency of the output RF signal is varied as a function of the difference between the frequencies of the comb line pair passed by the tunable optical filter to an optical detector.

Embodiments of the present disclosure can be more easily understood and further advantages and uses thereof more readily apparent, when considered in view of the description of the preferred embodiments and the following figures in which:.

In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize features relevant to the present disclosure. Reference characters denote like elements throughout figures and text.

In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of specific illustrative embodiments in which the embodiments may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the scope of the present disclosure. The following detailed description is, therefore, not to be taken in a limiting sense.

Currently in the art, quality oscillator having low phase noise is achievable by relatively large, heavy, bulky devices of scale in the order of a square foot or larger. The embodiments described herein provide at least equivalent levels of performance, but in a small form factor. The tunable optical RF signal generator <NUM> disclosed herein can be used in applications where a small form factor high quality oscillator with low phase noise is needed, such as for small wearable RF transceivers, or any application that could make use of a small optical RF signal generator or oscillator.

In the present disclosure, embodiments for a radio frequency (RF) synthesizer that utilizes optical frequency combs are realized by generating two optical frequency combs in a single resonator. Utilization of this dual-comb resonator comprises pumping one comb of the resonator with a first optical mode (for example, the fundamental transverse electric (TE) mode) and pumping the second comb of the resonator in the counter-propagating direction with a second optical mode (for example, the fundamental transverse magnetic (TM) mode). Due to geometric dispersion, these two mode families will form combs with unequal repetition rates, or equivalently, unequal spectral separations among their lines. When two lines, one from each comb, are then stabilized (i.e., locked) with respect to each other, the combs will yield a range of beat frequencies (for example, increasing from <NUM> or specified a frequency off-set) corresponding to the locked lines in steps of frep1-frep2, where frep1 and frep2 are the repetition rates of the first and second combs, respectively. The two combs are combined in a shared physical path and filtered such that a selected beat frequency is produced by a high-speed detector the light is eventually directed into, resulting from the beat between one line of each comb. The present architecture described herein yields a stable beat frequency exhibiting lower phase noise than produced by optical frequency combs controlled via resonance splitting. As discussed below, embodiments of the present disclosure also make use of injection-locked Stimulated Brillouin scattering (SBS) lasers to reduce the phase noise of the optical pump used to generate the combs, and thus obtain extremely low phase noise. It should also be noted that in alternate implementations two combs may optionally be generated from the same mode family to produce a single RF frequency that cannot be tuned, but that may potentially have lower phase noise.

<FIG> is a diagram of an example optical RF variable signal generator embodiment, also referred to herein as a tunable optical RF synthesizer, at <NUM>. The tunable optical RF signal generator <NUM> is described herein in terms of: a low-noise SBS pump laser segment <NUM>, a TE/TM dual comb optical resonator segment <NUM>, and a filter resonator segment <NUM>. It should be understood that although the tunable optical RF synthesizer is described herein in terms of these segments <NUM>, <NUM> and <NUM>, this is for the purpose of illustration. It should be understood that the features and elements described for each segment may be combined or integrated in various combinations to implement hardware realization of the embodiments.

Before going into further detail about the composition of the three segments of the tunable optical RF signal generator <NUM>, the general principle of operation for producing optical combs is described.

When the tunable optical RF synthesizer <NUM> is in operation, light exists within its TE/TM dual comb resonator segment <NUM> in the time domain as a series of pulses, and in the frequency domain as a series of lines (also referred to as spikes or teeth) where the spacing between the lines is uniform. An optical frequency comb is generated in the TE/TM dual comb resonator segment <NUM> by injecting via a coupler a single frequency laser light into its optical resonator <NUM> that propagates in a given direction (for example, in the clockwise (CW) direction). For a low loss (i.e. high quality) resonator that has a third order optical nonlinearity, the non-linear coefficient of the resonator <NUM> compresses the propagating light into a soliton pulse that travels around the resonator <NUM> in the propagation direction. That is, laser light propagating in the CW direction produces a soliton pulse that travels around the resonator in the CW direction. Similarly, laser light propagating in the counter-clockwise (CCW) direction produces a soliton pulse that travels around the resonator in the CCW direction. Each time a soliton pulse passes through a coupler, a portion of the soliton pulse will periodically exit the resonator <NUM> at a rate corresponding to the round trip time of the resonator <NUM>. In the frequency domain, this corresponds to an output where the lines of the comb being separated by the free spectral range (FSR) of the resonator <NUM>. When that high frequency comb output is sent to an optical sensor, the sensor will measure the pulse repetition or FSR and produce a very narrow low phase noise electrical signal in the RF domain with a frequency corresponding to the FSR of the optical resonator. With embodiments of the present disclosure, the tunable optical RF signal generator <NUM> utilizes dual counterpropagating optical frequency combs to produce an RF electrical signal output that is not fixed at the FSR of the optical resonator <NUM>, but that instead can be controlled to dynamically scan a range of frequencies (for example, from <NUM> to <NUM>).

The optical RF signal generator <NUM> includes a low-noise SBS pump lasers segment <NUM> that comprising a pair of SBS pump lasers (shown at <NUM> and <NUM>) each generating SBS laser light (SBS1 and SBS2) at different respective frequencies. As illustrated in <FIG>, each of the SBS pump lasers <NUM>, <NUM> comprises a laser light source <NUM>, a pump optical resonator <NUM>, a Bragg grating add-drop filter <NUM> coupled to the laser light source <NUM>, and a pump optical resonator <NUM> couple to the Bragg grating add-drop filter <NUM>. Laser light source <NUM> may comprise a laser diode, such as but not limited to distributed feedback (DFB) laser diodes, or other laser light generating devices. The laser light source <NUM> for each of the SBS pump lasers <NUM>, <NUM> establishes an injection lock where the transmission spectrum of the pump optical resonator <NUM> provides optical feedback that propagates back into the laser light source <NUM> so that the emission wavelength of the laser light source <NUM> naturally locks to one of the resonance frequencies of the pump optical resonator <NUM>.

Upon startup, the laser light sources <NUM> are not yet functioning as lasers, but instead are producing a spontaneous emission. That spontaneous emission propagates out from the respective laser light source <NUM> into the add-drop filter <NUM>. The spontaneous emission will hit that filter and a portion (within the filter stop band) will diffract and come out of the filter and coupled by coupler <NUM> into the optical resonator <NUM>. The balance will pass through the add-drop filter <NUM>. Of the diffracted portion of the spontaneous emission, at least a portion of that light will be of a frequency band aligned with a resonance frequency of the optical resonator <NUM>. This will result in the portion of the light on-residence to propagate (for example, CCW), which will produce more backscattering within the optical resonator <NUM> in the opposite direction (for example, CW) that will come out of the optical resonator <NUM> at the coupler <NUM> and back to the add-drop filter <NUM>, and again diffract from the filter's Bragg grating, creating backscattering back into the light source <NUM>. As a result, light re-entering the laser light source <NUM> corresponds to a single resonance of its respective ring resonator <NUM>. This backscattering feedback into the light source <NUM> provides a feedback that sets the lasing frequency for that SBS pump laser. Moreover, within the waveguide of the pump optical resonator <NUM> a counter propagating Brillouin scatting gain occurs that creates optical gain at a set frequency offset (a slightly longer wavelength) from the established laser light source lasing frequency. Because this light is outside of the stop-band of the add-drop filter <NUM> Bragg grating, it will pass through the add-drop filter <NUM> to provide the SBS laser outputs (shown in <FIG> as SBS1 and SBS2).

The resulting SBS laser outputs (SBS1 and SBS2) from the low-noise SBS pump laser component <NUM> are two SBS lasers each already comprising a very narrow line width a very stable in power. The SBS lasers act as two separate pumps for two separate frequency combs generated by the TE/TM dual comb resonator <NUM>. The SBS1 and SBS2 are also of substantially different frequencies. As an example, in one embodiment the SBS1 laser pump has a wavelength of <NUM> while the SBS2 laser pump has a wavelength of <NUM>. In other embodiments, other sets of SBS1 and SBS2 lasing frequencies may be produced by the SBS pump laser component <NUM>.

As shown in <FIG>, the TE/TM dual comb resonator component <NUM> comprises a comb optical resonator <NUM> (such as a ring resonator or microdisk, for example) through which the output of the two SBS laser pumps (SBS1 and SBS2) are coupled into, with one propagating in the respective opposite direction than the other. In <FIG>, SBS1 is coupled into the comb optical resonator <NUM> by a first coupler <NUM> to propagate in a clockwise direction, while SBS2 is coupled into the comb optical resonator <NUM> by a second coupler <NUM> to propagate in a counter-clockwise direction. In some embodiments, the SBS <NUM> and SBS2 may be coupled into their respective coupler <NUM> and <NUM> via an add-drop filter <NUM>.

Prior to being coupled into the comb optical resonator <NUM>, one of the generated SBS lasers (either SBS1 or SBS2) passes through a polarization rotator <NUM> so that the lines of the counterpropagating laser combs within the comb optical resonator <NUM> are also of opposing polarizations. As an example, in <FIG> the SBS1 laser light passes through polarization rotator <NUM> prior to being coupled into the comb optical resonator <NUM>, while the SBS2 laser does not. Although the SBS1 and SBS2 are originally generated with the same (e.g., TE) polarization, then the polarization rotator with rotate the SBS1 laser to the opposite (e.g., TM) polarization. As a result, a first propagating comb (shown as Comb <NUM>) generated in the comb optical resonator <NUM> from the SBS1 laser light will have a TM polarization, while the counterpropagating second comb (shown as Comb <NUM>) generated in the comb optical resonator <NUM> from the SBS2 laser in the CCW direction will have a TE polarization. The two resulting combs (Comb <NUM> and Comb <NUM>) will have a slightly different repetition rates because TE polarized light and TM polarized light have different group velocities, so the first and second combs they take different amounts of time to take a round trip around the comb optical resonator <NUM>. In <FIG>, the second comb (the CCW propagating Comb <NUM>) exits the comb optical resonator <NUM> via the coupler <NUM>, which may be the same coupler used to introduces the SBS1 light into the comb optical resonator <NUM>. The first comb (the CW propagating Comb <NUM>) exits the comb optical resonator <NUM> via the coupler <NUM>, which may be the same coupler used to input light into the comb optical resonator <NUM>. In this implementation, because the SBS1 laser light was the light beam shifted in polarization by the polarization rotator <NUM>, the resulting Comb <NUM> output from the comb optical resonator <NUM> is applied to another polarization rotator <NUM> to place the Comb <NUM> and Comb <NUM> back into the same polarization. In other embodiments, the resulting Comb <NUM> output from the comb optical resonator <NUM> could instead be applied to a polarization rotator to place the Comb <NUM> and Comb <NUM> back into the same polarization.

For example, the TM Comb1 exiting the comb optical resonator <NUM> is rotated to TE so that both Comb <NUM> and Comb <NUM> are TE polarized entering the filter resonator segment <NUM>. Note that in other embodiments, either of the polarization rotators <NUM> or <NUM> may be placed in the waveguides to shift the polarization of either SBS1 or SBS2, or of Comb <NUM> or Comb <NUM>, so long as the counter-propagating beams in the comb optical resonator <NUM> are of opposite polarity, and the output of the Combs from the TE/TM dual comb resonator segment <NUM> applied to the filter resonator segment <NUM> (as discussed below) are of the same polarity.

As illustrated in <FIG>, in the filter resonator segment <NUM>, Comb <NUM> and Comb <NUM> are combined into a shared waveguide using a codirectional coupler <NUM>, so that the spectra of the two combs occupy the same spatial volume. Comb <NUM> and Comb <NUM> now combined to form a dual optical comb signal <NUM>, one output from the codirectional coupler <NUM> is fed to a tunable optical filter <NUM> and then applied to an optical detector <NUM>. The optical filter <NUM>, which may comprise a vernier optical filter, is adjusted to select two lines from the dual optical comb signal <NUM>, one from each of the Combs <NUM> and <NUM> that are closest together.

In the embodiment shown in <FIG>, filter resonator segment <NUM> utilizes thermal feedback to the comb optical resonator <NUM> to lock the selected two lines together, either the same frequency (such that the respective selected lines are in alignment) or to a fixed frequency offset with respect to each other. In some embodiments, a controller <NUM> (such as a proportional-integral-derivative (PID) controller, for example) is configured to output a voltage to a microheater <NUM> that applies heat to the comb optical resonator <NUM>. The dual optical comb signal <NUM> optical filter <NUM> (as measured from a output of the codirectional coupler <NUM>) is measured an optical detector <NUM> that feeds a measurement of the dual optical comb signal <NUM> to the controller <NUM>. The controller <NUM> determines and outputs the voltage applied the microheater <NUM> that locks the selected two comb lines together (i.e., either to the same frequency, or at a selected fixed frequency offset). By locking the selected two lines together, the comb optical resonator <NUM> fixes the relative positions of the Comb <NUM> and Comb <NUM>.

The optical filter <NUM> comprises a pair of add-drop optical ring filter resonators <NUM>, that each receive from the codirectional coupler <NUM> the dual optical comb signal <NUM>. Each of the add-drop optical ring filter resonators <NUM> is thermally adjustable to pick-out a single line from each of the two combs present in the dual optical comb signal <NUM>. Adjustment of the passband formed by the add-drop optical ring filter resonators <NUM> is controlled using one or more microheaters <NUM>, <NUM> that are adjustable either through a common voltage controller (such as the controller <NUM>), or through separate voltage controllers.

The thermal adjustment of the add-drop optical ring filter resonators <NUM> creates a passband that passes a pair of adjacent lines from the dual optical comb signal <NUM> that includes a single line from each of the Combs <NUM> and <NUM>. The RF signal output <NUM> (shown as Vout) from the tunable optical RF signal generator <NUM> will vary as a function of the difference between the frequencies of the two comb lines passed by the add-drop optical ring filter resonators <NUM> to the optical detector <NUM>.

Frequency tuning of the RF signal output <NUM> (Vout) from tunable optical RF signal generator <NUM> is illustrated more particularly in <FIG>. Shown generally at <NUM> the Comb <NUM> generated from SBS laser pump <NUM> (shown as by solid lines) and the Comb <NUM> generated from the SBS laser pump <NUM> (shown by dashed lines) are superimposed. A selected line of Comb <NUM> is locked to a lines of Comb <NUM> as illustrated by the "locked comb lines" at <NUM>. Through thermal feedback to the comb optical resonator <NUM>, those two lines are locked together to either the same frequency, or to a fixed frequency offset with respect to each other. As discussed above, the FSR for light beams having TE and TM polarizations within the comb optical resonators <NUM> are not the same. The two combs, Comb <NUM> and Comb <NUM>, therefore have slightly different line spacing in the frequency domain.

Each sequential pairing of lines <NUM> of the Combs <NUM> and <NUM>, to either the left or right of the locked comb lines <NUM>, comprises a uniformly increasing frequency difference between the two lines of the respective comb line pair <NUM>. That is, the delta in frequency of the lines comprising the neighboring comb line pairs <NUM> will increase as there distance from the locked comb lines <NUM> increases (as shown at f<NUM>, f<NUM>, f<NUM>, f<NUM>, f<NUM>,.

Each of the ring filter resonators <NUM> are adjusted to pass one comb line of the adjacent comb lines pairs <NUM>. That is, the temperatures of each of the ring filter resonators <NUM> is adjusted by the controller <NUM> to shift the effective passbands of the ring filter resonators <NUM> to pass the comb lines pairs <NUM> having a frequency difference corresponding to a desired frequency output from the optical RF signal generator <NUM>, as shown by example in <FIG> at <NUM> where the ring filter resonators <NUM> are adjusted to pass the comb line pairs <NUM> spaced at a frequency difference of f<NUM>.

For example, to obtain the frequency delta of f<NUM>, one of the ring filter resonators <NUM> would be adjusted to pass the comb line of that comb line pair <NUM> produced from Comb <NUM>. The other ring filter <NUM> would be adjusted to pass the line of that comb line pair <NUM> produced from Comb <NUM>. The comb lines of that comb line pair <NUM> having a frequency delta of f1 are the only comb lines passed from the ring filter resonators <NUM> on to the optical detector <NUM> of the filter resonator segment <NUM> that produces the Vout electrical RS signal output for the optical RF signal generator <NUM>.

The light incident on that optical detector <NUM> comprises the comb lines of that passed comb line pair <NUM> (which may also be called a comb "doublet"). The output Vout from the optical detector is an osculating RF electrical signal having a frequency that is a function of the difference in frequency of the light incident on the optical detector <NUM>.

To obtain discrete tuning of the RF signal output Vout, the ring filter resonators <NUM> are adjusted (thermally) to control which comb line pair or doublet <NUM> is passed to the optical detector <NUM>, which thus dictates corresponding difference in optical frequency observed by the optical detector <NUM>. In some embodiments, each frequency delta of a comb line pair <NUM> comprises a uniform step change in frequency from the frequency delta of its neighboring comb line pair <NUM>. For example, if the frequency delta of f<NUM> is <NUM>, then the delta of f<NUM> will have a further uniform step of <NUM> (i.e., f<NUM> = <NUM>) and the delta of f<NUM> another further <NUM> of <NUM> (i.e., f<NUM> = <NUM>) and so forth. When the locked comb lines <NUM> are locked with an offset or bias in frequency, then the f<NUM>, f<NUM>, f<NUM>. will comprise the respective step plus that initial bias. In some embodiments, the filter resonator segment <NUM> may include a reference signal utilized to introduce the offset frequency between the comb lines of the locked comb lines <NUM> when it is desired for the locked comb lines to be locked with an offset.

The resulting Vout from the tunable optical RF signal generator <NUM> is an electrical oscillating frequency signal having a very low phase noise and very low comb line width. Narrow line width optical sources <NUM>, <NUM> are used by the <NUM> the SBS pump laser segment <NUM> to produce SBS laser outputs SBS1 and SBS2 that pump the TE/TM dual comb resonator <NUM> to generate the narrow line width uniform spacing set of combs, Comb <NUM> and Comb <NUM>. Comb <NUM> and Comb <NUM> are in turn used to produce a narrow line width stable RF frequency that is discretely tunable.

It should be understood that the larger the difference in the frequencies between the SBS1 and SBS2 laser pumps are, the larger the maximum delta-frequency between lines comprising the comb line pairs <NUM> can be, and according, the larger the largest comb line pair <NUM> frequency delta Fn can be. For example, when the step size is <NUM> and it is desired for the Vout to reach <NUM>, <NUM> comb line pairs <NUM> can be formed (i.e., n=<NUM>), from <NUM> comb lines of both Comb <NUM> and Comb <NUM> in the spectral region between the Locked Comb Lines <NUM> and frequency of the SBS1 pump laser.

Another benefit is that when the two combs, Comb <NUM> and Comb <NUM>, are locked together as described above (either with or without an offset), there is a multiplicative benefit in the stability of the repetition rate of the two combs. The farther away the locked position is from the frequencies of the two pump lasers SBS1 and SBS2, the more stable the repetition rate. The more comb lines produced between either SBS pump laser frequency and the locked comb lines <NUM>, the more stable the repetition rate will be. For example, if there are <NUM> comb lines produced by each comb between the SBS1 pump laser frequency and the frequency of the locked comb lines <NUM>, and <NUM> comb lines produced by each comb between the SBS2 pump laser and the locked comb lines <NUM> frequency, then the total beneficial stability enhancement factor in the repetition rate is proportional to that sum of comb lines.

As illustrated in <FIG>, in some embodiments, each of the two SBS pump lasers <NUM> an <NUM> optionally utilize phase modulators <NUM>, <NUM> to apply a modulation to the SBS1/SBS2 laser light (at <NUM> for example) to create a PDH loop to lock the SBS pump lasers to the comb optical resonator <NUM> that creates the combs. A first optical sensor <NUM> measures SBS1. In some embodiments, the SBS1 light may be coupled to the first optical sensor <NUM> from a reflection port of the coupler <NUM>. A lock in amplifier (LIA) <NUM> receives the output from that first optical sensor <NUM> and demodulates the signal using the voltage that drove the phase modulator <NUM>. That demodulated signal can be used by controller <NUM> (or other controller) to produce a first SBS control signal to control a frequency of SBS1 to keep it on a resonance with the comb optical resonator <NUM>.

The same type of control loop may be optionally implemented to control a frequency of SBS2. That is, a second optical sensor <NUM> measures SBS2. In some embodiments, the SBS2 light may be coupled to the second optical sensor <NUM> from a reflection port of the coupler <NUM>. A lock in amplifier (LIA) <NUM> receives the output from that second optical sensor <NUM> and demodulates the signal using the voltage that drove the phase modulator <NUM>. That demodulated signal can be used by controller <NUM> (or other controller) as a second SBS control signal to control a frequency of SBS2 to keep it on a resonance with the comb optical resonator <NUM>.

In the embodiment shown in <FIG>, a singe reference signal <NUM> is supplied to drive the phase modulator <NUM> for SBS1 and the phase modulator <NUM> for SBS2. In other embodiments, separate reference signals can instead be used for each phase modulator.

With regards to manufacturing, in some embodiments the tunable optical RF signal generator <NUM> is fabricated on or from a substrate wafer. The substrate wafer may be comprised of, for example, silicon as its topmost layer. A layer of silicon dioxide is thermally grown on the wafer, for example <NUM>-<NUM> microns thick, followed by the deposition through Plasma-enhanced chemical vapor deposition (PECVD) or low pressure chemical vapor deposition (LPCVD) of some thickness of silicon nitride, for example <NUM>. The device architecture is etched into this guiding layer, for example through conventional electron-beam lithography- or photolithography-based procedures in combination with reactive ion etching. Following this, an upper cladding layer, for example silicon dioxide, may be deposited through PECVD or LPCVD. Additional steps may be included to reduce loss in the waveguide, for example annealing the sample at high temperatures after the deposition of the upper cladding, or removal of the residual resist layer following the reactive ion etch step. If different nitride thicknesses are desired for the SBS lasers as compared to the comb generating resonator, these devices may be fabricated in different layers and coupled vertically to one another.

In the fabrication of the radio frequency synthesizer, the design may include piezoelectric material (PZT) elements or other form of microheaters to control the temperature of the SBS resonators <NUM>, the temperature of the comb-generating resonator <NUM> and/or the temperature of the ring filter resonators <NUM> of the add-drop optical ring filter resonators <NUM>.

In some embodiments, the low-noise SBS pump laser segment <NUM>, the TE/TM dual comb resonator <NUM>, and the filter resonator segment <NUM>, are each realized in a Silicon Nitride waveguide platform. However the laser light sources <NUM>, <NUM> may utilize a different Silicon Nitride thickness for the waveguides than the TE/TM dual comb resonator segment <NUM> or the filter resonator segment <NUM>. Accordingly, in the fabrication of the optical RF signal generator <NUM>, a transition block may be included between waveguides of different thickness. Examples of such transition optional blocks are shown in <FIG> as the "vertical couplers" <NUM>. For example the low-noise SBS pump laser segment <NUM> may be positioned in a layer below the TE/TM dual comb resonator <NUM> layer and the vertical couplers <NUM> function to handoff the light between the layers.

In some embodiments, the phase modulators <NUM>, <NUM> are realized in a different material platform than other elements of the SBS pump laser segment <NUM>, such as Lithium Niobate. As such, the phase modulators <NUM>, <NUM> may be implemented on a different chip than used for the other components of the SBS pump laser segment <NUM> and the light routed between the two.

<FIG> is a flow chart diagram illustrating a method <NUM> of one embodiment of the present disclosure for generating a variable radio frequency (RF) signal. It should be understood that method <NUM> may be implemented using any one of the embodiments described above. As such, elements of method <NUM> may be used in conjunction with, in combination with, or substituted for elements of any of the embodiments described herein. Further, the functions, structures, features and other description of elements for such embodiments described herein may apply to like named elements of method <NUM> and vice versa. The method <NUM> beings at <NUM> with generating in a Stimulated Brillouin Scattering (SBS) pump laser segment a first SBS laser light and a second SBS laser light. These SBS laser light beams are generated by SBS laser sources (such as laser diodes) as previously discussed above. The method proceeds to <NUM> with applying the first SBS laser light and the second SBS laser light to a comb optical resonator of a TE/TM dual comb resonator segment, wherein the comb optical resonator generates a pair of counter-propagating optical frequency combs of different polarities from the first SBS laser light and second SBS laser light. The method further proceeds to <NUM> with generating feedback from a filter resonator segment to the comb optical resonator to lock a relative position of the pair of counter-propagating optical frequency combs. The first and second combs may be controlled to lock a selected pair of comb lines together, either at the same frequency (such that the respective selected lines are in alignment) or to a fixed frequency offset with respect to each other.

The method proceeds to <NUM> with filtering, with a tunable optical filter of the filter resonator segment, a comb line pair of the locked optical frequency combs, wherein the comb line pair includes single comb line from each of the pair of counter-propagating optical frequency combs. As discussed above, the filter resonator segment combines the first comb of the pair of counter-propagating optical frequency combs and the second comb of the pair of counter-propagating optical frequency combs to form a dual optical comb signal applied to the tunable optical filter. The filtered signal, now comprising a single comb line pair passed by the tunable optical filter, is fed to an optical detector that produces an electrical signal that varies as a function of the difference in frequency between the lines of the comb line pair (e.g., the associated beat frequency). The method then then proceeds to <NUM> with outputting a discrete tuned RF signal output based on the comb line pair.

In various alternative embodiments, system and/or device elements, method steps, or example implementations described throughout this disclosure (such as any of the SBS pump laser segment, TE/TM dual comb optical resonator segment, Filter resonator segment, or any controllers, processors, circuits, or sub-parts thereof, for example) may be implemented at least in part using one or more computer systems, field programmable gate arrays (FPGAs), or similar devices comprising a processor coupled to a memory and executing code to realize those elements, processes, or examples, said code stored on a non-transient hardware data storage device. Therefore, other embodiments of the present disclosure may include elements comprising program instructions resident on computer readable media which when implemented by such computer systems, enable them to implement the embodiments described herein. As used herein, the term "computer readable media" refers to tangible memory storage devices having non-transient physical forms. Such non-transient physical forms may include computer memory devices, such as but not limited to punch cards, magnetic disk or tape, any optical data storage system, flash read only memory (ROM), nonvolatile ROM, programmable ROM (PROM), erasable-programmable ROM (E-PROM), random access memory (RAM), or any other form of permanent, semi-permanent, or temporary memory storage system or device having a physical, tangible form. Program instructions include, but are not limited to computer-executable instructions executed by computer system processors and hardware description languages such as Very High Speed Integrated Circuit (VHSIC) Hardware Description Language (VHDL).

As used herein, terms such as SBS pump laser segment, TE/TM dual comb optical resonator segment, Filter resonator segment, lasers, laser source, resonator, ring resonator, optical detector, controller, waveguide, coupler, add-drop filter, optical comb, amplifier, refer to the names of elements that would be understood by those of skill in the art.

Claim 1:
An optical radio frequency (RF) signal generator (<NUM>), the signal generator (<NUM>) comprising:
a Stimulated Brillouin Scattering (SBS) pump laser segment (<NUM>) that includes a first SBS pump laser (<NUM>) and a second SBS pump laser (<NUM>) each generating SBS laser light at different respective frequencies;
a TE/TM dual comb resonator segment (<NUM>) comprising a comb optical resonator (<NUM>) coupled to the first SBS pump laser (<NUM>) and the second SBS pump laser (<NUM>), wherein the comb optical resonator segment (<NUM>) generates a pair of counter-propagating optical frequency combs, wherein a first comb of the pair of counter-propagating optical frequency combs is transverse electric (TE) mode polarized and a second comb of the pair of counter-propagating optical frequency combs is transverse magnetic (TM) mode polarized; and
a filter resonator segment (<NUM>) configured to provide feedback to the TE/TM dual comb resonator segment (<NUM>) to lock a the pair of counter-propagating optical frequency combs at the same frequency or to a fixed frequency offset with respect to each other, the filter resonator segment (<NUM>) comprising a tunable optical filter (<NUM>), wherein the optical filter (<NUM>) comprises a pair of add-drop optical ring filter resonators (<NUM>) that each receive the dual optical comb signal (<NUM>) from a codirectional coupler (<NUM>), wherein each of the add-drop optical ring filter resonators (<NUM>) is thermally adjustable to pick-out a single line from each of the two combs present in the dual optical comb signal (<NUM>), wherein the filter resonator segment is configured to output a discrete tuned RF signal output based on a comb line pair in the pair of counter-propagating optical frequency combs that includes a single comb line from each of the pair of counter-propagating optical frequency combs, wherein a frequency of the output RF signal is varied as a function of the difference between the frequencies of the comb line pair passed by the tunable optical filter to an optical detector (<NUM>).