Photonic integrated circuit (PIC) radio frequency oscillator

Abstract

Techniques for a photonic integrated circuit (PIC) radio frequency (RF) oscillator are described. The PIC RF oscillator can include an optical gain medium coupled to a first mirror and configured to be coupled to the PIC. The PIC can include a first optical cavity disposed within the PIC, an adjustable mirror forming a first optical path between the first mirror and a first adjustable mirror in the gain medium, and a frequency tunable intracavity dual tone resonator disposed within the first optical cavity and causing the first optical cavity having a common optical path to generate two primary laser tones with an adjustable frequency interval. A photodetector is optically coupled to the PIC and configured to mix the two primary laser tones to form an RF output signal having a frequency selected by the adjustable frequency interval of the two primary tones.

Description

Technical Field

[0001] Cross - reference to Related Applications This application claims the benefit of U.S. Provisional Patent Application No. 63 / 213,137, having the docket number 3721 - 21.15290.US.PSP, filed on June 21, 2021, the entire specification of which is hereby incorporated by reference in its entirety for all purposes.

Background Art

[0002] The phase noise of radio frequency (RF) signals generated by radio frequency oscillators can limit the accuracy of measurements performed using the RF signals. Phase noise is the result of random variations in the phase of a waveform corresponding to deviations in the time domain from perfect periodicity.

[0003] In radar applications, an RF signal generated by an RF oscillator can be used as a clock reference signal. The clock reference signal is used to determine the time delay of radar signals. The time delay is used to determine the range of the reflected RF signal by counting clock cycles from the edge of the clock signal.

[0004] The random variations in the waveform of an RF oscillator due to phase noise introduce randomness into the intended timing measurements, potentially affecting the detection and resolution of feedback signals, and thus the target detection probability and measurement accuracy. Therefore, an RF signal having a low level of phase noise is desirable for use in radar operations, which include, but are not limited to, range - Doppler radar systems such as synthetic aperture radar (SAR) and ground moving target indication radar.

[0005] The features and advantages of the present disclosure will become apparent from the following detailed description in conjunction with the accompanying drawings that illustrate the features of the present disclosure by way of example.

Brief Description of the Drawings

[0006]

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DETAILED DESCRIPTION OF THE INVENTION

[0007] Next, with reference to the exemplary embodiments illustrated, specific language is used herein to describe them. Nevertheless, it should be understood that no limitation of the scope of the invention is thereby intended.

[0008] Before disclosing and describing the present invention, it is to be understood that the invention is not limited to the specific structures, process steps, or materials disclosed herein, but extends to equivalents thereof as would be recognized by those of ordinary skill in the relevant art. Also, it is to be understood that the terms used herein are for the purpose of describing particular embodiments only and are not intended to be limiting. The same reference numbers in different drawings represent the same elements. The numbers described in the flowcharts and processes are provided for clarity in indicating steps and operations and do not necessarily indicate a particular order or sequence.

[0009] Exemplary embodiments A first overview of the embodiments of the technology is presented below, and then specific embodiments of the technology will be described in more detail later. This initial overview is intended to assist the reader in understanding the technology more quickly, but is not intended to identify important or essential features of the technology, nor is it intended to limit the scope of the subject matter recited in the claims.

[0010] A radio frequency signal source having low phase noise can be useful in many applications, including but not limited to range Doppler radio detection and ranging (radar) systems such as synthetic aperture radar (SAR) and ground moving target indication radar, satellite communication links, navigation systems, accurate measurement time and frequency measurements, reference clock distribution, and communication radio links.

[0011] A photonic oscillator can be used to generate repetitive electronic sine waves or other types of continuous wave signals. The photonic oscillator can have a very high quality factor (Q) as well as high frequency and phase stability. The quality factor or Q factor is a dimensionless parameter that describes how much the resonator is damped. It is defined as the ratio of the initial energy stored in the resonator to the energy lost in one radian of the oscillation period. A higher Q indicates a lower energy loss rate, which is an important indicator of efficiency and stability.

[0012] In a photonic oscillator, the phase noise of the oscillator does not increase with frequency as can occur in different types of electronic oscillators such as crystal oscillators or air dielectric resonators.

[0013] The accuracy and stability of a photonic oscillator are determined by its phase noise, which itself can be limited by many factors such as thermal refractive noise. Ultimately, the phase stability of the output (and thus the frequency linewidth) is physically limited by the Schawlow–Townes limit. Schawlow and Townes calculated the fundamental (quantum) limit of the linewidth of a laser. The Schawlow–Townes linewidth can be determined by the intensity of the optical phase fluctuations caused by quantum noise. The phase noise level corresponding to this Schawlow–Townes linewidth is, in part, a result of spontaneous emission into the laser mode.

[0014] According to one embodiment of the present invention, there is disclosed a photonic dual-resonant RF oscillator that combines a prior art photonic integrated circuit (PIC) technology using an ultra-low loss optical resonator with low-noise electronics and advanced chip-scale packaging to provide a substantially low-phase-noise RF oscillator. The RF oscillator can be packaged in a small thermally controlled system that can be used in demanding physical environments.

[0015] One embodiment of this architecture is configured to generate a continuously adjustable low-phase-noise RF signal by interfering with the optical output of an innovative PIC-based two-tone common cavity laser. The laser reduces the uncorrelated phase noise of each laser oscillation mode by constraining the modes to a common optical path. The common optical path and the mixed output effectively eliminate the influence of the most classical noise sources in the laser, such as temperature refractive noise (TRN).

[0016] The PIC-based two-tone common cavity laser can include a first optical cavity that includes a frequency-adjustable compound cavity resonator. The compound cavity resonator can be formed from silicon nitride (SiN) or another desired material used to form the PIC. The frequency-adjustable compound cavity resonator is configured to constrain the first optical cavity to generate two primary tones with adjustable frequency spacing. When these two primary tones are mixed, their correlated phase noise is canceled, and the resulting RF tone approaches the Schawlow–Townes limit noise floor.

[0017] For further phase noise reduction, the PIC can include a second optical cavity that is optically coupled to the first optical cavity. The two primary tones from the first optical cavity can be used as injection seeds for the second optical cavity. In a semiconductor gain medium that uses the two primary tones to generate two secondary tones in addition to the two primary tones, a phase-maintaining four-wave mixing (FWM) process in the cavity can be driven. The second optical cavity can be designed such that the secondary tones generate feedback to effectively limit the phase of the two primary injection tones.

[0018] Four-wave mixing (FWM) is the generation of intermodulation in a non-linear optical system, where interaction between two or three wavelengths generates two or more new wavelengths. It is a parametric non-linear process in that the energy of the incoming photons is conserved. FWM is a phase-sensitive process, and in this case, the efficiency of the process is strongly affected by the phase-matching conditions.

[0019] In one embodiment, the cavity feedback and gain are configured to increase the intensity of the secondary tone to drive an additional FWM process. This results in both a cascade sequence of additional FWM tones and direct feedback to the primary tones. The feedback constrains the phases of the primary tones, enhances the correlation of their phase noise, thereby enabling the uncorrelated phase noise to be kept below the Schawlow-Townes limit. RF signals with phase noise below the Schawlow-Townes limit can be used to significantly improve radar, communication, navigation, measurement, and clock references.

[0020] FIG. 1 shows an exemplary embodiment of a photonic dual-resonant radio frequency (RF) oscillator system 100. The system 100 includes a photonic integrated circuit (PIC) 102 having a first optical cavity 104 configured to be coupled to a gain medium 108 having a mirror 106. In one example, the PIC can be formed on a silicon dioxide (SiO2) base, which carries a silicon nitride (SiN) waveguide. This is not intended to be limiting. The waveguide can be formed from any low-loss optical material that can be used to convey an optical signal.

[0021] The gain medium 108 in FIG. 1 may be disposed external to the PIC 102, integrated with the PIC 102, or formed within the PIC 102. The gain medium 108 can be a linear gain medium or a non-linear gain medium. In one embodiment, the gain medium may be a reflective semiconductor optical amplifier or another desired type of laser gain medium. The PIC 102 can further include a first adjustable mirror 110 to form a first optical cavity 104 between the mirror 106 in the gain medium 108 and the first adjustable mirror 110.

[0022] The PIC 102 can further include a frequency-adjustable intracavity dual-tone resonator 112 disposed within the first optical cavity 104 and configured to generate two primary laser tones 105 having an adjustable frequency spacing within the first cavity. The two primary laser tones 105 have a substantially common optical path within the first optical cavity 104. The common optical path length can be adjusted to have a substantially similar length using a phase modulator on the waveguide within the optical path. The constraint of the common optical path for the two primary laser tones 105 cancels many of the common-mode noise sources reported in the literature when the two primary tones 105 interfere to create a tone at a frequency representing the difference between the two primary laser tones 105. The phase noise of the difference tone can approach the classical Schawlow-Townes limit.

[0023] PIC102 can further include a photodetector 114 optically coupled to the output of PIC102. This photodetector 114 receives two primary laser tones 105 from the first adjustable mirror 110, mixes the two primary laser tones 105, and is configured to form an RF output signal 115 at a frequency determined by the difference in the adjustable frequency spacing of the two primary laser tones 105. The photodetector 114 can be directly coupled to the output of the first adjustable mirror 110. The photodetector 114 can be provided on the PIC102 or outside the PIC102, and the output signal transmitted by the first adjustable mirror 110 can be relayed to the photodetector 114 via either free space or waveguide coupling.

[0024] In another embodiment, the PIC102 of the photonic dual-resonance RF oscillator system 100 can further include a second optical cavity 120 within the PIC102 and an intracavity element that provides a nonlinear optical (NLO) gain 122. Depending on the material and wavelength selection, the NLO gain can be provided by the laser medium 108, or by a separate material element optimized to obtain the NLO gain, or by the waveguide material within the PIC102.

[0025] PIC102 further includes a second adjustable mirror 124 optically coupled to define the second optical cavity 120, and can form a second optical path between the mirror 106 in the gain medium 108 and the second adjustable mirror 124. The second optical cavity 120 can be optically coupled to the first optical cavity 104 such that the two primary laser tones 105 are directed into the second optical path.

[0026] In one embodiment, two primary laser tones 105 are injected from a first optical cavity 104 into a second optical cavity 120, which is configured to provide four-wave mixing (FWM) 121. The injection can be realized by using any component that supports partial reflectivity and / or transmission (i.e., beam splitting) with minimal absorption loss, such as a fixed or adjustable multimode interference coupler. In the four-wave mixing process, two primary tones 105 are used to generate two secondary tones via an NLO gain 122. The physical constraints of energy and momentum conservation constrain the phase relationship of the secondary tones. Next, the presence of the secondary tones constrains the phase of the primary tones. The result of imposing these constraints on the phases of the various tones is referred to as phase locking. The phase locking of the two primary laser tones has the effect of reducing phase jitter and drift (phase noise) in the two primary laser tones. This process can be cascaded to generate tertiary or additional tones. When used, these additional tones can provide additional phase constraints for further reducing the phase noise in the two primary laser tones. By adding a second optical cavity 120 coupled to the first optical cavity 104 in the PIC 102 and an intracavity element that provides a non-linear optical (NLO) gain 122, the system phase noise can be improved compared to the classical Schawlow-Townes limit.

[0027] In one example, the photonic RF oscillator system 100 is initiated as a free-running laser in a first optical cavity 104 and can then transition to a phase-locking operation to reduce the level of phase noise using FWM in a second optical cavity 120. While multiple tones are present in a common optical path shared by the first optical cavity 104, only two primary laser tones 105 pass through a frequency-tunable intracavity dual-tone resonator 112. This generates positive feedback to these two primary tones 105, and these two tones can be used to generate a low-phase-noise RF signal at the output of the photodetector 114.

[0028] FIG. 2a provides an exemplary view of a photonic dual-resonance RF oscillator system 200. The system 200 consists of two coupled cavities, namely a first optical cavity 204 and a second optical cavity 220. The two cavities can share common elements such as a laser gain medium 208. The laser gain medium 208 can be provided on the PIC 202, or the laser gain medium 208 can be disposed outside the PIC 202, and the output signal transmitted from the laser gain medium 208 can be relayed to the first optical cavity 204 via the PIC coupling 207 through either free space or waveguide coupling. The common elements shared between the two cavities 210, 220 can provide frequency tuning and strong phase noise reduction. In one embodiment, the PIC 202 can use ultra-low loss SiN waveguides 209 to route the laser light. Controllable phase delays (phase shifters) φ1 to φ7 can be used to adjust the waveguide coupling so that the system 200 can operate in a stable region.

[0029] The first optical cavity 204 is configured to generate two primary laser tones 205 having strong correlated noise. The two primary laser tones are frequency-separated by Ω Hertz 226. The output RF signal has the same frequency as the primary laser tone separation and has an RF signal output 215 of Sin(Ωt). The RF signal output 215 can be amplified using an amplifier such as a low-noise amplifier 241. The design of the first optical cavity 204 restricts all modes to a common path to reduce phase noise. The first optical cavity provides a common optical path including a mirror 206 within the laser gain medium 208. The laser gain medium 208 and the mirror 206 can be shared with the second optical cavity 220. In one embodiment, the laser gain medium 208 can include a semiconductor gain medium for providing gain to the two primary laser tones 205 and the secondary FWM tone 221 within the second optical cavity 220. The mirror 206 can be configured as a Sagnac loop mirror and can be a high-reflectivity mirror configured to reflect both the two primary laser tones 205 in the common optical path with an adjustable reflectivity mirror 210 used to form the two primary laser tones along the common optical path, and the secondary FWM tone 221 on the second optical path between the mirror 206 and the FWM cavity mirror 224. The FWM cavity mirror 224 can be an adjustable mirror based on a Sagnac loop. The laser gain medium 208 can also include a non-linear optical element as described above. Alternatively, the non-linear optical element can be included within the second cavity 222 or the FWM cavity mirror 224, or can be included within a waveguide along the second optical path between the mirror 206 and the FWM cavity mirror 224. The non-linear optical element can provide FWM of the two primary laser tones 205 together with the gain medium 208 to form the secondary FWM tone 221. In one embodiment, the output of the FWM cavity mirror can be a calibration output 237 that can be output from the PIC 202 to an optical detector 239 available for monitoring. A monitoring optical detector can be used to analyze the optical signal within the PIC and also to adjust various phase shifters to generate the two primary laser tones 205 and form the secondary FWM tone 221.

[0030] The output of PIC202 can be coupled to the optical isolator 232, which is disposed between PIC202 and the photodetector 214 to optically isolate the PIC from the photodetector. The optical isolator can substantially reduce the re-incidence of the reflected light from the photodetector 214 onto PIC202. Phase noise can increase due to residual back-reflection to the oscillator. In addition, to reduce the back-reflection from the photodetector to PIC202, the waveguide 234 can be angled with respect to the coupling 236 at the exit of PIC202 using a waveguide coupling with a baseline angle. In one embodiment, the angled coupling may be sufficient and the optical isolator 232 may not be necessary. However, if an undesirable amount of phase noise is generated due to back-reflection, the optical isolator 232 can be used to reduce the phase noise caused by the back-reflection. A similar coupling 235 can be used to couple the photodetector 234 to the calibration output 237.

[0031] In one embodiment, the photonic dual-resonance RF oscillator system 200 can include an intra-cavity dual-tone resonator 212 disposed within the first optical cavity 204. The intra-cavity dual-tone resonator 212 can be an ultra-high Q coupled resonator formed from SiN having an adjustable doublet resonance that functions as an etalon within the cavity. The doublet frequency spacing provides the frequency of the RF signal output of the photodetector 214. The doublet frequency spacing can be selected by adjusting φ2 and -φ2. The first cavity 204 also includes phase shifters φ1 and φ3 to φ7.

[0032] The phase shifters φ2 and -φ2 can be used to adjust the cavity free spectral range (FSR). The FSR is the optical frequency interval between two primary tones. The phase shifters φ1 and φ3 to φ7 can also be used to adjust waveguide coupling, compensate for manufacturing errors in the dual resonator, and adjust the optical coupling between the first optical cavity 210 (two-tone cavity) and the second optical cavity 220 (FWM cavity). The phase shifters φ1 and φ3 to φ7 will be described in more detail in the following paragraphs.

[0033] FIG. 2b shows an example of a functional block diagram of an electronic subsystem 300 configured to be coupled to the photonic dual-resonant RF oscillator system 200. The electronic subsystem can include a high-pass filter and / or a low-pass filter 330 that filters the RF output signal of the photodetector 214 in the photonic dual-resonant RF oscillator system 200. The filtered RF output signal can be sent to a low-noise amplifier 341 to generate a low-phase-noise output RF tone having a frequency of Ω Hz.

[0034] According to one embodiment, an RF synthesizer 334 can be used to send a reference RF tone at a frequency of Ω REF to the output frequency control unit 336. The output frequency control unit 336 can also receive an input of the low-phase-noise output RF tone from the low-noise amplifier 341. The output frequency control unit 336 can send a signal V = F(Ω DIF ) = F(Ω - Ω REF ) to the resonance adjustment control unit 338, and this resonance adjustment control unit 338 is used to adjust the adjustable composite resonator 212 to generate two primary tones 205 separated at a frequency of Ω.

[0035] The output of the photonic dual-resonance RF oscillator system 200's photodetector 214 can be the signal V = A(t) + B(t)sin(Ωt). The high-pass / low-pass filter 330 can send the signal V = A(t) to the device controller 340. The feedback signal V = A(t) can be used by the device controller to send the feedback signal to the photodetector drive 342, the adjustable mirror control 344, the resonator coupling control 346, and the stable current source 348. The stable current source 348 can use the feedback signal to supply a selected amount of current to the gain medium 208 to control the amplitude of the laser light in the PIC 202, the corresponding amplitudes of the two primary tones 205, and the corresponding amplitude of the FWM tone 221. The resonance coupling control 346 can adjust the frequency separation Ω of the two primary tones by sending a control signal to the adjustable composite resonator 212 using the feedback signal. The amount of laser light allowed to pass through the FWM cavity mirror 224 (FIG. 2) and / or the two-tone cavity mirror 210 can be adjusted using the adjustable mirror control.

[0036] The electronic subsystem 300 is configured to perform four main functions. It operates a phase-locked loop (PLL) that controls the RF frequency output of the photonic dual-resonance RF oscillator system 200, provides sufficient internal isolation to prevent self-interference between the various subsystems within the photonic dual-resonance RF oscillator system 200, supports an integration flow that maintains the operation of the components within a safe manufacturing exposure limit to temperature during pre-integration manufacturing and testing of the subsystems, and can sufficiently attenuate environmental fluctuations such as heat, vibration, and electromagnetic so that the internal components are maintained within a predetermined operating range.

[0037] Figures 3a - 3c provide additional details of the laser cavity within the PIC202 (Figure 2). Figure 3a shows that the PIC202 can include two laser structures sharing one gain medium 308 and one mirror 306. One laser includes a first optical cavity 304 for dual - tone laser oscillation that generates two primary tones 205 (Figure 2) having a frequency difference of Ω. The second optical cavity 320 is for FWM and phase - locking and is subjected to dual - tone injection from the first optical cavity 304.

[0038] Figure 3b shows an example of the first optical cavity 304 for dual - tone laser oscillation. Figure 3b further shows a simpler embodiment of a dual - resonance oscillator that does not use four - wave mixing components. The simpler embodiment is significantly simpler and less expensive to construct than the systems shown in Figures 1 and 2a. This would be an excellent configuration for use in cost - sensitive short - range systems such as unmanned aerial vehicles or handheld units.

[0039] The laser cavity includes the mirror 306 of the gain medium 308 with respect to the adjustable mirror 310. The adjustable compound resonator 312 has a coupling coefficient μ between the resonators. This dual - tone resonance can act as a coupled etalon having two resonances and can provide dual - tone laser oscillation.

[0040] The first optical cavity 304 can be a high - finesse cavity. The finesse of an optical resonator (cavity) is a measure of how narrow the resonance is in relation to its frequency separation, and high finesse means a sharp resonance. It is defined as the free - spectral range (FSR) (i.e., the fundamental mode spacing) divided by the full - width at half - maximum (FWHM) bandwidth of the resonance. This is completely determined by the resonator losses and is independent of the resonator length.

[0041] FIG. 3c shows an example of a second optical cavity 320 for FWM and phase locking of two primary tones, as shown in this embodiment having the respective resonance frequencies of ω1 and ω2. The dual tone can be injected from any configuration of a dual tone oscillator. The first primary tone having the resonance frequency ω1 has an intracavity photon number S1 and a phase φ1 within the second optical cavity 320. The second primary tone having the resonance frequency ω2 has an intracavity photon number S2 and a phase φ2 within the second optical cavity 320. The dual laser oscillation block 350 operates as a dual injection source for the lower laser cavity at a speed of τ c for FWM 321 and phase locking. The dual laser oscillation block 350 shown as a conceptual 45-degree angle mirror merely represents the coupling between the first optical cavity 304 and the second optical cavity 320. The coupling between the cavities 304, 320 is selective for the two primary tones that are the dual resonances of the first optical cavity 304.

[0042] The second optical cavity 320 of FIG. 3c is seeded by dual-tone injection from the first optical cavity 304 of FIG. 3b. The second optical cavity 320 uses strong non-linearity to perform FWM mixing between the injected laser tones to generate the FWM sideband. This non-linearity significantly relaxes the desired Q for such an FWM process, resulting in a phase lock between the two primary tones and correlating and reducing their relative quantum phase noise. Thus, the second optical cavity may have a low finesse. In practice, the second optical cavity 320 functions as a phase-locked cavity. The frequency of its tones is controlled by the dual-tone injection seeding of this cavity. By injecting sufficient power for dual-tone injection from the first optical cavity 304, the desired frequencies ω1 and ω2 and the associated phases φ1 and φ2 are effectively given a forced head start in the amplification process and become the main output frequency and phase of the laser in the second optical cavity 320 between mirror 306 and adjustable mirror 324. As described above, the non-linear material can be included in the gain medium 308, the adjustable mirror 324, the waveguide, or another desired location within the second optical cavity 320.

[0043] Figures 4a - 4c are exemplary embodiments used to illustrate the dual-tone laser oscillation conditions for a coupled resonator structure according to one embodiment. In FIG. 4a, four-wave mixing is not included. The gain medium 408 has a gain γ, a length L1, and a mirror 406 with a reflectivity r0. The adjustable composite resonator 412 consists of a first resonator 454 and a second resonator 456 within the first optical cavity 404. The first resonator 454 and the second resonator 456 have a natural resonance frequency ω0, a decay rate τ0, and a coupling coefficient μ between the resonators 454 and 456. The waveguide 452 coupled to the gain medium 408 has a waveguide-resonator coupling decay rate τ c , and a length L2 to the first resonator 454. The waveguide 458 has a length L3 from the second resonator 456 to the adjustable mirror 410. The adjustable mirror 410 has a reflectivity r1.

[0044] The transfer function of the coupled resonator 412 shown in Fig. 4a is as follows:

Equation

[0045] In order to perform laser oscillation, Equation 2 must be satisfied: r0r1|H(ω)| 2 =e 2α(L 1 +L 2 +L 3 )-2γL 1cos[2arg(H(ω))] (Equation 2) Here, α is the gain coefficient.

[0046] Fig. 4c is a graph showing arg(H(π)). In order to perform dual-tone laser oscillation, Equation 3 must be satisfied: cos[2arg(H(ω1))]=cos[2arg(H(ω2))] (Equation 3) Equation 3 is satisfied when there is a phase difference of π / 2 between ω1 and ω2. Therefore, by configuring the first optical cavity using the adjustable composite resonator 412 to provide a phase difference of π between ω1 and ω2, the two primary tones ω1 and ω2 are essentially enhanced and other tones are attenuated.

[0047] Figure 5a shows an exemplary graph depicting the quantum-limited phase noise of a mode-locked laser for different pulse widths for an average output of 1 milliwatt (mW) and an average output of 10 mW. Figure 5b shows an exemplary graph depicting a model of the time waveforms of four locked laser modes. The modeled time behavior of a system having a 4-mode lock tone predicts a pulse width of 2 picoseconds (ps). Such a pulse width in Figure 5a implies a phase noise limit of less than -160 dBc / Hz. The term dBc is the relative decibel with respect to the carrier, which indicates the power ratio of the signal expressed in decibels to the carrier signal. Phase noise is expressed in dBc / Hz at a given frequency offset from the carrier. A phase noise limit of less than -160 dBc / Hz indicates that the locking of the four modes in the second optical cavity is sufficient to generate strong quantum noise correlations. The FSR for the simulations shown in Figures 5a and 5b is 100 gigahertz (GHz). The examples of Figures 5a and 5b are not intended to be limiting. Depending on the system specifications, different 4-mode lock tones with different pulse widths and different phase noise levels are possible. In one embodiment, the phase noise in the RF output signal (215, Figure 2a) can be between -120 dBc / Hz and -160 dBc / Hz. Alternatively, the phase noise in the RF output signal 215 may be as low as about -165 dBc / Hz.

[0048] Based on the results shown in the graph of Figure 5b, it can be concluded that ultra-low phase noise can be achieved using at least four tones (i.e., the dual injection tones and the two FWM sidebands) to participate in the FWM process. Additional constraints arise based on the finesse of the second optical cavity, resulting in two potential operating configurations, referred to herein as Configuration 1 and Configuration 2.

[0049] In Configuration 1, the second optical cavity is configured as a low-finesse cavity. In this case, the injected dual tone and the generated sidebands of FWM fall within the FWHM of the second optical cavity. By using this configuration, the interval between the dual tones can be continuously adjusted, thereby generating a voltage-controlled oscillator (VCO) in which the differential output frequency is electronically controlled. By reducing the reflectivity of the adjustable mirror 324 (Fig. 3c), a low finesse can be achieved. As a result of the reduced reflectivity, the net system power efficiency is reduced compared to Configuration 2.

[0050] In Configuration 2, the second optical cavity is configured as a high-finesse cavity. In the optical injection seed, the frequency separation of the two primary tones injected by the first optical cavity is approximately equal to a multiple of the FSR of the second optical cavity. The RF oscillator generated by this configuration can be a stepwise adjustable oscillator having a step size proportional to the FSR of the second optical cavity. In order to provide adjustability in a small range near each step, precise thermal adjustment of the first optical cavity path length can be carried out so that the FSR of the first optical cavity matches the injected dual tone frequency interval. The second configuration is more power efficient than Configuration 1 but may have more noise. The second configuration also uses a higher control accuracy than Configuration 1. The second configuration can also benefit from a relatively long cavity that reduces the FSR and provides more adjustment steps.

[0051] Equations 4 to 7 shown below provide a set of dynamic non-linear forms for controlling the dual tone injection seed of a semiconductor laser cavity. These equations are normalized with respect to the steady-state free-running laser parameters before external injection. The three dynamic parameters are the normalized intracavity photon number

Number

Number

Number

Number

Number

Number

[0052] The intracavity photon number rate and phase equations in the cavity are shown in Equation 4 and Equation 5, respectively.

Number

Number

Number

Number

[0053] FIGS. 6a-6c show preliminary simulation results for an injection-seeded low-finesse cavity (second optical cavity) involving non-linear mixing and phase locking in a semiconductor. FIG. 6a shows the generation of cascaded FWM sidebands resulting from the injection of two tones separated by 10 GHz. As can be seen, the cascaded tones have a frequency spacing of 10 GHz and also have well-behaved time signals. The generation of harmonics and their distinct phase locking are evident in the closed-loop limit cycle shown in the plot shown in FIG. 6b. The profile shown in FIG. 6b represents a profile such that an exemplary design follows. The details of this profile depend on the design. FIG. 6c tracks the trajectories to stability for two different starting points (indicated by stars), which represent different intracavity phases of the second optical cavity. Note that the system stabilizes to the same phase-amplitude limit cycle regardless of the initial phase relationship. This means that the system has a stable operating point. A properly designed system can start operating at multiple points in the phase-amplitude space shown in the plot, but ultimately converges to a stable and well-defined operating point for any design.

[0054] In one exemplary embodiment, an output level of 10 nanowatts (nW) per mode of the output coupled in the mode (10 nW / mode) may be sufficient to effectively injection-seed the second optical cavity. Thus, by non-linearly mixing the secondary tones in the second optical cavity, a primary tone power of about 10 nW can be generated. This is an estimated value of the lower limit of the intracavity power and is based on the assumed spontaneous emission rate, losses, and other cavity parameters.

[0055] Figure 7 shows preliminary calculations indicating that a stable limit cycle was obtained for frequencies in the range of 8 - 11 GHz when the composite resonator was tuned over 3 GHz. By adjusting the internal control, the differential output frequency can be controlled over a wide frequency range. In one embodiment, the composite resonator can be tuned over 40 GHz to provide an RF output signal in the range of 1 - 40 GHz.

[0056] Figure 8a shows a block diagram of a coupled resonator structure having an adjustable doublet resonance controlled by the doublet phase shifters φ2 and -φ2. The coupled resonator regions of devices having different phase shifters are considered for device tuning. All splitters in this exemplary architecture are of 50:50 design. Figure 8b shows a two - tone signal that can be generated by a structure conforming to the architecture shown in Figure 8a having a 10 GHz doublet spacing. The coupled resonator transmission spectrum is shown for the 10 GHz doublet enabled by φ2 = 0.1π. Figure 8c shows the expected frequency difference for the range of values of the doublet phase shifter of Figure 8a. The change in the frequency doublet spacing for different values (φ2) is shown. These values provide the level of phase shift for adjusting the splitting within the range of 1 - 40 GHz. In one example, the phase shifter is configured to consume minimal power and cause minimal optical loss in the resonator.

[0057] Figures 9a and 9b show exemplary diagrams of various phase shifters that can be used in PIC202 (Figure 2a) to provide various functions. For example, using φ1 shown in Figure 9a and described in Figure 9b, the coupling between the first resonator 954 and the waveguide 960 can be adjusted. Similarly, φ3 can be used to adjust the coupling between the second resonator 956 and the waveguide 962. φ2 and -φ2 can be used to adjust the doublet spacing and provide a level of phase shift for adjusting the frequency of the output RF oscillator signal from the photodetector 914. In one embodiment, φ2 and -φ2 can adjust PIC202 to provide an output between 1 GHz and 40 GHz as described above. However, this is not intended to be limiting. The doublet spacing and FSR for the first optical cavity and the second optical cavity can be configured to provide a wide range of outputs from 100 MHz to over 200 GHz.

[0058] The resonance misalignment in the first resonator 954 and the second resonator 956 can be adjusted using φ4 and φ5. The resonance misalignment may be caused by manufacturing errors in the manufacture of the PIC. The in-cavity phase of the lower path 960 can be adjusted using φ6. Similarly, the in-cavity phase of the upper path 962 can be adjusted using φ7. The reflectivity of the adjustable mirror 924 can be adjusted using φ8. The reflectivity of the adjustable mirror 910 can be adjusted using φ9. An example of the power consumption for each phase shifter is shown in Figure 9b. In this example, a total thermoelectric power consumption of about 1.4 watts is used to adjust the phase shifter.

[0059] Overall, the set of phase shifters shown in Figure 9a can be used for (1) trimming imbalances or incompleteness resulting from manufacturing, (2) optimizing device performance by adjusting internal parameters, and (3) controlling the output frequency and phase noise. The control elements reflecting this are listed in the table of Figure 9b.

[0060] The exemplary power consumptions listed in the table of FIG. 9b are associated with the phase shifters shown in the example of FIG. 9a and represent only one of many possible controlled power distributions. Each phase shifter can use more or less power depending on the overall system design specifications and architecture, such as the exemplary systems shown in FIGS. 2a and 2b.

[0061] In one embodiment, the phase shifter can be implemented using a heater disposed on a waveguide within the PIC202 (FIG. 2a). For example, a metal microheater using a thermally conductive metal with a high heat capacity, such as platinum, titanium, beryllium, or another desired metal, can be used to drive the change in the effective path length induced by thermo-optic effects.

[0062] FIG. 10a shows an exemplary diagram of a microheater 1066 used to heat a waveguide 1068 to drive a change in the effective path length induced by thermo-optics. In this example, the waveguide is formed using silicon nitride (SiN). An exemplary diagram of the waveguide 1068 is shown in FIG. 10c. The waveguide 1068 has a height of about 550 nanometers (nm) and a width of about 1500 - 2000 nm as shown in FIG. 10c and described in FIG. 10d. The waveguide 1068 is placed within a layer of silicon dioxide (SiO2) such that the microheater 1066 is separated from the top layer of the waveguide 1068 by about 1500 - 2000 nm. The microheater 1066 has a length along the waveguide of about 1000 - 4000 nm. The microheater 1066 used for the doublet tuning of φ2 and -φ2 may have a length along the waveguide 1068 of about 1000 - 2000 nm. The resonator absorption Q due to metal absorption from the metal microheater 1066 is shown in FIG. 10b. The microheater can be used to cause a temperature change in the silicon dioxide layer 1070, which in turn causes a temperature change in the waveguide 1068. The exemplary values listed in FIG. 10d are for one instance of an exemplary system and are not intended to be limiting. Various waveguide dimensions, quality factors, bend radii, FSRs, waveguide splitter lengths, heater spacings, and phase shifter lengths can be used to achieve specific system design parameters.

[0063] As shown in the examples of FIGS. 11a and 11b, a temperature change of 100 degrees Celsius can occur in the waveguide 1068 in about 50 microseconds, and a phase change of about 0.25π occurs at a power level of 80 mW applied to the microheater. A temperature change of 200 degrees Celsius in the waveguide 1068 can cause a phase change of about 0.4π. Thus, within the power budget listed in FIG. 9b, a microheater can be used to provide the desired amount of phase change in the phase shifters φ1 to φ9 (FIG. 9a). The power budgets are not intended to be limiting. They are shown as examples. The temperature shift caused by the microheater 1066 in the silicon dioxide layer 1070 and the SiN waveguide 1068 shown in FIG. 10a has been shown to occur without damage. To calibrate and adjust the optical system within the PIC202 and provide the desired optical or RF output from the PIC202, tens of watts of power can be used to drive the thermo-optically induced change in the effective path length at dozens of different locations.

[0064] According to one embodiment of the present invention, a photonic dual - resonance radio - frequency (RF) oscillator is disclosed. The photonic dual - resonance RF oscillator has an optical gain medium coupled to a first mirror, and the gain medium is configured to be coupled to a photonic integrated circuit (PIC). The PIC includes a first optical cavity disposed within the PIC, a first adjustable mirror that forms the first optical cavity between the first mirror within the gain medium and the first adjustable mirror, and a frequency - adjustable intracavity dual - tone resonator disposed within the first optical cavity and generating two primary laser tones having an adjustable frequency spacing within the first optical cavity, wherein the two primary laser tones have a common optical path within the first optical cavity, and one or more phase shifters providing an adjustable frequency spacing. The photonic dual - resonance RF oscillator can further include a photodetector optically coupled to the PIC, and the photodetector is configured to receive the two primary laser tones from the first adjustable mirror, mix the two primary laser tones, and form an RF output signal having a frequency selected by the adjustable frequency spacing of the two primary laser tones.

[0065] In one embodiment, the PIC can further include a second optical cavity within the PIC optically coupled to the first optical cavity, an intracavity element providing non - linear optical gain, a second adjustable mirror that forms a second optical path passing through the intracavity element between the first mirror within the gain medium and the second adjustable mirror, and one or more phase shifters dynamically adjusting the phase delay within one or more of the first or second cavities. The two primary laser tones can be injected from the first optical cavity into the second optical cavity, and the second optical cavity can be configured to use the non - linear optical gain of the intracavity element to generate four - wave mixing (FWM), create at least two side tones from the two primary tones, and reduce the phase jitter and drift in the two primary tones to reduce the phase noise in the RF output signal.

[0066] In one embodiment, the cavity element is disposed in one or more of a gain medium, a waveguide optically coupled to the second adjustable mirror, or a separate element optically coupled to the second adjustable mirror.

[0067] The photonic dual resonance RF oscillator can further include an optical coupler configured to couple two primary tones from a first optical cavity to a second optical cavity.

[0068] In one embodiment, the photodetector can be optically coupled to the output of the photonic dual resonance RF oscillator via free space coupling or waveguide coupling. Alternatively, the photodetector can be provided on the PIC and optically coupled to the first adjustable mirror.

[0069] In one embodiment, the photonic dual resonance RF oscillator further includes a first waveguide implemented in the PIC and optically coupled to the first mirror and the second adjustable mirror in the gain medium; a first resonant loop waveguide implemented in the PIC and configured to couple an optical laser signal from the first waveguide to the first resonant loop waveguide at a selected distance from the first waveguide; a second resonant loop waveguide implemented in the PIC and configured to couple the optical laser signal from the first resonant loop waveguide to the second waveguide at a selected distance from the first resonant loop waveguide; and a second waveguide implemented in the PIC and configured to couple the optical laser signal from the second resonant loop waveguide to the second waveguide at a selected distance from the second resonant loop waveguide, the second waveguide being optically coupled to the first adjustable mirror.

[0070] One or more phase shifters, also referred to as phase regulators, are connected to the first waveguide and are configured to adjust the optical coupling between the first waveguide and the first resonant loop waveguide, or are connected to the second waveguide and are configured to adjust the optical coupling between the second waveguide and the second resonant loop waveguide, or are connected to the first resonant loop waveguide and are configured to adjust the resonance misalignment within the first resonant loop waveguide, or are connected to the second resonant loop waveguide and are configured to adjust the resonance misalignment within the second resonant loop waveguide, or are connected to the first waveguide and are configured to adjust the in-cavity phase of the second optical path, or are connected to the second waveguide and are configured to adjust the in-cavity phase of the first optical path, or are connected to the second adjustable mirror and are configured to adjust the reflectivity of the second adjustable mirror, or are connected to the first adjustable mirror and are configured to adjust the reflectivity of the first adjustable mirror, and can include one or more of them.

[0071] The first, third, fourth, fifth, sixth, seventh, eighth, and ninth phase shifters can consist of metal microheaters that are thermally coupled to one or more of the first waveguide, the first resonant loop waveguide, the second resonant loop waveguide, or the second waveguide using a thermally conductive metal with a high heat capacity to drive the change in the effective path length induced by thermo-optics in their respective waveguides. Examples of microheaters used to provide phase regulation by thermo-optics are not intended to be limiting. Other types of phase regulators can also be used. For example, when a photonic material such as a waveguide is an electro-optic material, the phase regulator or phase shifter can be implemented as an electro-optic phase shifter.

[0072] In one embodiment, one or more phase shifters can consist of one or more of a first doublet phase shifter coupled to a second resonant loop waveguide and a second doublet phase shifter coupled to a first resonant loop waveguide. The first doublet phase shifter and the second doublet phase shifter can be configured to adjust the optical frequency interval between two primary laser tones to select the frequency of the RF output.

[0073] In one embodiment, the first doublet phase shifter and the second doublet phase shifter can consist of metal microheaters thermally coupled to one or more of the first resonator loop waveguide or the second resonator loop waveguide, using a thermally conductive metal with a high heat capacity to drive the change in the effective path length induced by thermo-optics in their respective waveguides. Alternatively, the first and second doublet phase shifters can be implemented using an electro-optic phase shifter, or another desired type of phase shifter that can shift the phase of the optical signal in the first and second resonant loop waveguides by a desired amount within a time selected based on system parameters. A typical period can be from 25 microseconds to 200 microseconds.

[0074] According to one embodiment, the frequency-tunable intracavity dual-tone resonator can further include a first optical resonator optically coupled to a second optical path, the first optical resonator including a first phase shifter and a second optical resonator optically coupled to the first resonator and the first optical path. The second optical resonator can include a second phase shifter. The first optical resonator and the second optical resonator can be configured to be frequency-tunable using the first and second phase shifters to provide a doublet interval for selecting the frequency of the RF output signal.

[0075] Various techniques, or specific aspects or portions thereof, may take the form of program code (i.e., instructions) embodied in a tangible medium, such as a floppy disk, a compact disc read-only memory (CD-ROM), a flash drive, a hard drive, a non-transitory computer-readable storage medium, or any other machine-readable storage medium on which program code is loaded and executed by a machine, such as a computer, to cause the machine to become an apparatus for practicing the various techniques. A circuit may include hardware, firmware, program code, executable code, computer instructions, and / or software. A non-transitory computer-readable storage medium may be a computer-readable storage medium that does not include a signal. In the case of program code execution on a programmable computer, the computing device may include a processor, a storage medium readable by the processor (including volatile and non-volatile memory and / or storage elements), at least one input device, and at least one output device. The volatile and non-volatile memory and / or storage elements may be random access memory (RAM), erasable programmable read-only memory (EPROM), flash drive, optical drive, magnetic hard drive, solid state drive, or other media for storing electronic data. A low-energy fixed location node, a wireless device, and a location server may also include a transceiver module (i.e., a transceiver), a counter module (i.e., a counter), a processing module (i.e., a processor), and / or a clock module (i.e., a clock) or a timer module (i.e., a timer). One or more programs capable of implementing or utilizing the various techniques described herein may use an application programming interface (API), reusable control, etc. Such programs may be implemented in a high-level procedural or object-oriented programming language for communicating with a computer system. However, if desired, the program(s) may also be implemented in assembly language or machine language.In any case, the language is a compiled or interpreted language and can be combined with a hardware implementation.

[0076] As used herein, the term processor can include a general-purpose processor, a dedicated processor such as a VLSI, an FPGA, or other types of dedicated processors, and a baseband processor used in a transceiver that transmits, receives, and processes wireless communications.

[0077] It should be understood that many of the functional units described herein are labeled as modules to more specifically emphasize their implementation independence. For example, a module can be implemented as a hardware circuit including a custom very large scale (VLSI) circuit or a gate array, an off-the-shelf semiconductor such as a logic chip, a transistor, or other individual components. A module can also be implemented in a programmable hardware device such as a field programmable gate array, a programmable array logic, or a programmable logic device.

[0078] In one embodiment, multiple hardware circuits or multiple processors can be used to implement the functional units described herein. For example, a first hardware circuit or a first processor can be used to execute processing operations, and a second hardware circuit or a second processor (e.g., a transceiver or a baseband processor) can be used to communicate with other entities. The first hardware circuit and the second hardware circuit may be incorporated into a single hardware circuit, or alternatively, the first hardware circuit and the second hardware circuit may be separate hardware circuits.

[0079] The module can also be implemented in software for execution by various types of processors. The identified modules of executable code can include, for example, one or more physical or logical blocks of computer instructions that can be organized, for example, as objects, procedures, or functions. Nevertheless, the executable files of the identified modules need not be physically located together, but can include different instructions stored in different locations that make up the module, which, when logically combined, can achieve the defined purpose of the module.

[0080] In fact, the modules of executable code can be a single instruction, or many instructions, and can even be distributed across several different code segments, between different programs, and across several memory devices. Similarly, the operational data can be identified and shown herein within the module, embodied in any suitable form, and organized within any suitable type of data structure. The operational data can be collected as a single data set, or distributed to different locations including different storage devices, and can exist, at least in part, simply as electronic signals on a system or network. The module can be passive or active, including an operable agent for performing the desired function.

[0081] Throughout this specification, references to "an example" or "exemplary" mean that the particular feature, structure, or characteristic described in connection with the example is included in at least one embodiment of the invention. Thus, the appearances of the phrases "in an example" or the word "exemplary" in various places throughout this specification are not necessarily all referring to the same embodiment.

[0082] As used herein, for convenience, a plurality of items, structural elements, components, and / or materials may be presented in one common list. However, these lists should be interpreted as if each member of the list were individually identified as a separate and unique member. Thus, no individual member of such a list should be construed as a de facto equivalent of any other member of the same list solely on the basis that these members are presented as one common group, unless an opposite indication is given. In addition, various embodiments and examples of the present invention may be referred to herein together with alternatives for their various components. It is understood that such embodiments, examples, and alternatives should not be construed as de facto equivalents of one another, but rather as separate and autonomous representations of the present invention.

[0083] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as layouts, distances, network examples, etc., to fully understand the embodiments of the present invention. However, those skilled in the art will recognize that the present technology may be practiced without one or more of these specific details, or using other methods, components, layouts, etc. In other examples, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the present technology.

[0084] The above examples are illustrative of the principles of the present invention in one or more specific applications, but it will be apparent to those skilled in the art that many changes can be made in the form of implementation, application, and details without departing from the principles and concepts of the invention, without a prototype of the functions of the invention. Therefore, it is not intended to limit the present invention except as defined by the claims set forth below.

Claims

1. A photonic dual-resonance radio frequency (RF) oscillator, comprising: An optical gain medium coupled to a first mirror, configured to be coupled to a photonic integrated circuit (PIC), wherein the PIC comprises: A first optical cavity disposed within the PIC; A second optical cavity within the PIC optically coupled to the first optical cavity; An intra-cavity element providing non-linear optical gain; A first adjustable mirror forming a first optical cavity between the first mirror and the first adjustable mirror within the optical gain medium; A frequency-adjustable intra-cavity dual-tone resonator disposed within the first optical cavity, configured to generate two primary laser tones having an adjustable frequency interval within the first optical cavity, wherein the two primary laser tones have a common optical path within the first optical cavity; One or more phase modulators providing the adjustable frequency interval; The optical gain medium including; A photodetector optically coupled to the PIC, configured to receive the two primary laser tones from the first adjustable mirror, mix the two primary laser tones, and form an RF output signal having a frequency selected by the adjustable frequency interval of the two primary laser tones; The two primary laser tones are injected from the first optical cavity into the second optical cavity, and the second optical cavity uses the non-linear optical gain of the intra-cavity element to generate four-wave mixing (FWM), create at least two side tones from the two primary laser tones, and reduce phase jitter and drift in the two primary laser tones to reduce phase noise in the RF output signal. A photonic dual-resonance RF oscillator.

2. The PIC further comprises: A second adjustable mirror for forming a second optical path between the first mirror and the second adjustable mirror within the optical gain medium, wherein the second optical path passes through the intra-cavity element; One or more phase modulators for dynamically adjusting the phase delay within one or more of the first optical cavity or the second optical cavity. The photonic dual-resonance RF oscillator according to claim 1, further comprising

3. The photonic dual-resonance RF oscillator according to claim 2, wherein the phase noise in the RF output signal is from -120 decibels carrier / hertz (dBc / Hz) to less than -160 dBc / Hz.

4. The photonic dual-resonance RF oscillator according to claim 2, wherein the intra-cavity element is disposed in one or more of the optical gain medium, a waveguide optically coupled to the second adjustable mirror, or a separate element optically coupled to the second adjustable mirror.

5. The photonic dual-resonance RF oscillator according to claim 2, further comprising an optical coupler configured to connect the two primary laser tones from the first optical cavity to the second optical cavity.

6. The photodetector is optically coupled to the output of the photonic dual-resonance RF oscillator via free-space coupling or waveguide coupling, or The photodetector is on the PIC and is optically coupled to the first adjustable mirror. The photonic dual-resonance RF oscillator according to claim 1.

7. A first waveguide mounted in the PIC and optically coupled to the first mirror in the optical gain medium and the second adjustable mirror; A first resonant loop waveguide mounted in the PIC, spaced a selected distance from the first waveguide, and configured to connect an optical laser signal from the first waveguide to the first resonant loop waveguide; A second resonant loop waveguide mounted in the PIC, spaced a selected distance from the first resonant loop waveguide, and configured to connect the optical laser signal from the first resonant loop waveguide to the second resonant loop waveguide; A second waveguide mounted in the PIC, spaced a selected distance from the second resonant loop waveguide, and configured to connect the optical laser signal from the second resonant loop waveguide to the second waveguide, the second waveguide being optically coupled to the first adjustable mirror; The photonic dual-resonance RF oscillator according to claim 2, further comprising

8. The one or more phase regulators are A first phase adjuster coupled to the first waveguide and configured to adjust the optical coupling between the first waveguide and the first resonant loop waveguide, or A third phase adjuster coupled to the second waveguide and configured to adjust the optical coupling between the second waveguide and the second resonant loop waveguide, or A fourth phase adjuster coupled to the first resonant loop waveguide and configured to adjust the resonant misalignment within the first resonant loop waveguide, or A fifth phase adjuster coupled to the second resonant loop waveguide and configured to adjust the resonant misalignment within the second resonant loop waveguide, or A sixth phase adjuster coupled to the first waveguide and configured to adjust the in-cavity phase of the second optical path, or A seventh phase adjuster coupled to the second waveguide and configured to adjust the in-cavity phase of the first optical path in the common optical path, or An eighth phase adjuster coupled to the second adjustable mirror and configured to adjust the reflectivity of the second adjustable mirror, or A ninth phase adjuster coupled to the first adjustable mirror and configured to adjust the reflectivity of the first adjustable mirror, The photonic dual-resonance RF oscillator according to claim 7, comprising one or more of the above.

9. One or more of the first, third, fourth, fifth, sixth, seventh, eighth, or ninth phase adjusters use a thermally conductive metal with a high heat capacity to cause a change in the effective path length induced by thermo-optics in their respective waveguides, and are thermally coupled to one or more of the first waveguide, the first resonant loop waveguide, the second resonant loop waveguide, or the second waveguide, and are composed of a metal microheater, or One or more of the first, third, fourth, fifth, sixth, seventh, eighth, or ninth phase adjusters are composed of an electro-optic phase shifter used to adjust the phase in one or more of the first waveguide, the first resonant loop waveguide, the second resonant loop waveguide, or the second waveguide, The photonic dual-resonance RF oscillator according to claim 8.

10. wherein the one or more phase shifters include a first doublet phase shifter coupled to the second resonant loop waveguide, a second doublet phase shifter coupled to the first resonant loop waveguide, one or more of which are included, wherein the first doublet phase shifter and the second doublet phase shifter are configured to adjust the optical frequency spacing between the two primary laser tones to select the frequency of the RF output. The photonic dual-resonance RF oscillator according to claim 7.

11. wherein the first doublet phase shifter and the second doublet phase shifter each comprise a metal microheater having a high heat capacity and thermally coupled to one or more of the first resonant loop waveguide or the second resonant loop waveguide to cause a change in the effective path length induced by thermo-optics in their respective waveguides, or wherein each of the first doublet phase shifter and the second doublet phase shifter is composed of an electro-optic phase shifter that changes the phase in each of the first resonant loop waveguide or the second resonant loop waveguide. The photonic dual-resonance RF oscillator according to claim 10.

12. wherein the frequency-adjustable intracavity dual-tone resonator is optically coupled to the second optical path and includes a first optical resonator including a first phase shifter, and a second optical resonator optically coupled to the first optical resonator and to the first optical path in the common optical path and including a second phase shifter, further comprising, wherein the first optical resonator and the second optical resonator are frequency-adjustable using the first and second phase shifters to provide a doublet spacing for selecting the frequency of the RF output signal. The photonic dual-resonance RF oscillator according to claim 2.

13. A photonic dual-resonance radio frequency (RF) injection oscillator, comprising an optical gain medium coupled to a first mirror and configured to be coupled to a photonic integrated circuit (PIC), the PIC including a first optical cavity disposed within the PIC, a first adjustable mirror that forms a first optical cavity between the first mirror and the first adjustable mirror within the optical gain medium. A frequency-adjustable intracavity dual-tone resonator that is disposed within the first optical cavity and generates two primary laser tones having an adjustable frequency interval in the first optical cavity, wherein the two primary laser tones have a common optical path within the first optical cavity; the frequency-adjustable intracavity dual-tone resonator, A second optical cavity within the PIC that is optically coupled to the first optical cavity and is configured to receive the two primary laser tones; An intracavity element that provides a non-linear optical gain; A second adjustable mirror for forming a second optical path between the first mirror and the second adjustable mirror within the optical gain medium, wherein the second optical path passes through the intracavity element; the second adjustable mirror, One or more phase adjusters that provide the adjustable frequency interval; The optical gain medium including; A photodetector optically coupled to the PIC, which receives the two primary laser tones from the first adjustable mirror, mixes the two primary laser tones, and is configured to form an RF output signal having a frequency selected by the adjustable frequency interval of the two primary laser tones; the photodetector, Including; The two primary laser tones are injected from the first optical cavity into the second optical cavity, and the second optical cavity generates four-wave mixing (FWM) using the non-linear optical gain of the intracavity element, creates at least two side tones from the two primary laser tones, and is configured to reduce phase jitter and drift in the two primary laser tones to reduce phase noise in the RF output signal. The photonic dual-resonance RF oscillator.

14. The PIC further includes one or more phase adjusters that dynamically adjust the phase delay within one or more of the first optical cavity or the second optical cavity The photonic dual-resonance RF oscillator according to claim 13.

15. The photonic dual-resonance RF oscillator according to claim 13, wherein the phase noise in the RF output signal is from -120 decibels carrier / hertz (dBc / Hz) to less than -160 dBc / Hz.

16. The photonic dual-resonance RF oscillator according to claim 13, wherein the cavity internal element is arranged in one or more of the optical gain medium, a waveguide optically coupled to the second adjustable mirror, or a separate element optically coupled to the second adjustable mirror.

17. The photonic dual-resonance RF oscillator according to claim 13, further comprising an optical coupler configured to connect the two primary laser tones from the first optical cavity to the second optical cavity.

18. The photodetector is optically coupled to the output of the photonic dual-resonance RF oscillator via free-space coupling or waveguide coupling, or The photodetector is on the PIC and is optically coupled to the first adjustable mirror. The photonic dual-resonance RF oscillator according to claim 13.

19. A first waveguide mounted in the PIC and optically coupled to the first mirror in the optical gain medium and the second adjustable mirror; A first resonant loop waveguide mounted in the PIC, spaced by a distance selected from the first waveguide, and configured to connect an optical laser signal from the first waveguide to the first resonant loop waveguide; A second resonant loop waveguide mounted in the PIC, spaced by a distance selected from the first resonant loop waveguide, and configured to connect the optical laser signal from the first resonant loop waveguide to the second resonant loop waveguide; A second waveguide mounted in the PIC, spaced by a distance selected from the second resonant loop waveguide, and configured to connect the optical laser signal from the second resonant loop waveguide to the second waveguide, the second waveguide being optically coupled to the first adjustable mirror; The photonic dual-resonance RF oscillator according to claim 13, further comprising.

20. The one or more phase adjusters are A first phase adjuster coupled to the first waveguide and configured to adjust the optical coupling between the first waveguide and the first resonant loop waveguide, or A third phase adjuster coupled to the second waveguide and configured to adjust the optical coupling between the second waveguide and the second resonant loop waveguide, or A fourth phase adjuster coupled to the first resonant loop waveguide and configured to adjust resonance misalignment within the first resonant loop waveguide, or, A fifth phase adjuster coupled to the second resonant loop waveguide and configured to adjust resonance misalignment within the second resonant loop waveguide, or, A sixth phase adjuster coupled to the first waveguide and configured to adjust the in-cavity phase of the second optical path, or, A seventh phase adjuster coupled to the second waveguide and configured to adjust the in-cavity phase of the first optical path in the common optical path, or, An eighth phase adjuster coupled to the second adjustable mirror and configured to adjust the reflectivity of the second adjustable mirror, or, A ninth phase adjuster coupled to the first adjustable mirror and configured to adjust the reflectivity of the first adjustable mirror, The photonic dual-resonance RF oscillator according to claim 19, comprising one or more of the above.

21. One or more of the first, third, fourth, fifth, sixth, seventh, eighth, and ninth phase adjusters use a thermally conductive metal having a high heat capacity to cause a change in the effective path length induced by thermo-optics in their respective waveguides, and are thermally coupled to one or more of the first waveguide, the first resonant loop waveguide, the second resonant loop waveguide, or the second waveguide, and are composed of a metal microheater, or One or more of the first, third, fourth, fifth, sixth, seventh, eighth, or ninth phase adjusters are composed of an electro-optic phase shifter used to adjust the phase in one or more of the first waveguide, the first resonant loop waveguide, the second resonant loop waveguide, or the second waveguide, The photonic dual-resonance RF oscillator according to claim 20.

22. The one or more phase adjusters include A first doublet phase adjuster coupled to the second resonant loop waveguide, and A second doublet phase adjuster coupled to the first resonant loop waveguide, One or more of the above. The first doublet phase adjuster and the second doublet phase adjuster are configured to adjust the optical frequency interval between the two primary laser tones in order to select the frequency of the RF output. The photonic dual resonance RF oscillator according to claim 19.

23. The first doublet phase adjuster and the second doublet phase adjuster are each composed of a metal microheater thermally coupled to one or more of the first resonant loop waveguide or the second resonant loop waveguide using a thermally conductive metal having a high heat capacity to cause a change in the effective path length induced by thermo-optics in each waveguide, or each of the first doublet phase adjuster and the second doublet phase adjuster is composed of an electro-optic phase shifter that changes the phase in each of the first resonant loop waveguide or the second resonant loop waveguide. The photonic dual resonance RF oscillator according to claim 22.

24. The frequency-adjustable intracavity dual-tone resonator is optically coupled to the second optical path and includes a first optical resonator including a first phase shifter, and a second optical resonator optically coupled to the first optical resonator and to the first optical path in the common optical path and including a second phase shifter, and further includes the first optical resonator and the second optical resonator are frequency-adjustable using the first and second phase shifters to provide a doublet interval for selecting the frequency of the RF output signal. The photonic dual resonance RF oscillator according to claim 13.

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