WO2014197780A1 - Low-noise microwave-frequency generator - Google Patents

Abstract

A microwave-frequency generator comprising silicon-photonic devices and a resonant cavity integrated on a single substrate is disclosed. Integration of silicon-photonic devices and waveguides on a single monolithic substrate enables lower phase noise in the microwave-frequency output signal. Further, the integration of low-loss waveguides that define the resonant cavity, low-noise operation can be achieved without introducing significant optical loss, thereby enabling high Q. In some embodiments, RF electronics devices are also integrated on the same substrate.

Description

LOW-NOISE MICROWAVE-FREQUENCY GENERATOR

Field of the Invention

[oooi] The present invention relates to integrated photonic electronic circuits, specifically, to architectures of optoelectronic and coupled optoelectronic oscillators for microwave frequency generation.

Background of the Invention

[0002] Microwave-frequency generators that can generate a multi-gigahertz frequency signal having high spectral purity and high frequency stability have applications in many areas such as communication, radar and metrology. Different approaches can be used to obtain high spectral purity and high repetition rate signals. Conventional electronic approaches rely on using a high quality factor (Q) resonator in order to get high spectral purity.

[0003] There has been a great effort to develop compact microwave frequency sources with low noise. To this end, the use of photonic elements for microwave-frequency generation is attractive - particularly for frequencies of 1 GHz or higher. Frequency multiplication of low-frequency, low noise electronic oscillators multiplies phase noise.

Additionally, the performance of conventional electronics degrades at higher frequencies, hence optical based sources have great value at higher frequencies.

[0004] The essence of a low noise microwave source lies within the highly stable, high-Q resonant cavity. One well-known approach for providing such a cavity is based on the use of an optical carrier and a low-loss optical-delay line, such as is described by Yao, et al., in "Optoelectronic microwave oscillator," J. Opt. Soc. Amer. B, Vol. 13, No. 8, pp. 1725- 1735 (1996). Such delay lines can achieve equivalent radio-frequency (RF) Q's as high as 10⁶ at 10GHz. Unfortunately, these lines are typically a few kilometers long and, as a result, are bulky and thermally sensitive. Further, their thermal stabilization can be difficult and, as a result, they can produce spurious modes that require bulky and complex systems to mitigate.

[0005] In some other prior-art approaches, the fiber delay line is replaced by an optical resonator, such as a whispering gallery mode resonator. Examples of such systems are described by Volyanskiy, et a/., in "Compact optoelectronic microwave oscillators using ultra-high Q whispering gallery mode disk-resonators and phase modulation," Optics Express, Vol. 18, pp. 22358-22363 (2010) and Savchenkov, et al., in "Phase noise of whispering gallery photonic hyper-parametric microwave oscillators," Optics Express, Vol. 16, pp. 4130-4144 (2008), as well as by Delfyett, et al., in U.S. Patent 8,717,657, issued May 6. 2014. Resonator-based systems having similar Q's as those of delay-line

approaches have been demonstrated; however, the resonator-based systems require much shorter fiber lengths due to the resonance effect. In addition, optical resonators can provide augmented filtering to remove noise from the optical carrier by virtue of their wavelength- dependent resonance-enhancement factor. Unfortunately, resonator-based approaches still require long lengths of optical fiber coupling into and out of their optical elements (i.e., lasers, modulators, resonators and detectors), which leads to additional loss at each fiber interconnect.

[0006] Still another prior-art approach is based on a combination of polarization- maintaining optical fiber links and directional couplers, which collectively form a fiber-ring resonator. An example of a fiber-ring resonator oscillator is described by Saleh, et al., in "Optoelectronic oscillator based on fiber ring resonator: overall system optimization and phase noise reduction," Proc. of the IEEE Int. Freq. Control Symp. (IFCS 2012), Baltimore, MD, USA, (2012). While these resonators are considerably smaller than the kilometer-length fibers required in the aforementioned delay-line approaches, they still require several meters of optical fiber to form the high-Q resonant cavity. This introduces system

complexity and cost, as well as significant temperature sensitivity, that make the

manufacture and use of such systems problematic. In addition, quality factor is strongly tied to the coupling factors of the directional couplers.

Summary of the Invention

[0007] The present invention enables the generation of low-noise microwave signals without some of the costs and disadvantages of the prior art. Embodiments of the present invention include integrated monolithic platforms comprising electronic circuit elements, photonics devices, and planar-lightwave-circuit-based low-loss waveguide elements that at least partially define a high-Q, low-loss resonant cavity. Embodiments of the present invention include optoelectronic oscillators and coupled optoelectronic oscillators that can be more compact, less expensive, lower noise, and/or simpler to use than microwave- frequency generators known in the prior art. [0008] An embodiment of the present invention is an optoelectronic oscillator comprising active-photonics devices and RF-electronic circuitry formed on the same substrate as a photonic lightwave circuit-based ring resonator. Integrated active-photonics elements include an optical source, amplitude modulator, and phase modulator;

conventional integrated-circuit-based elements include an oscillator, mixer, amplifiers, and RF filter; and the resonant cavity includes a planar-lightwave-circuit-based ring resonator. These elements are optically and electrically coupled to collectively define a frequency- stabilized, low-noise, sinusoidal output signal. In some embodiments, the output signal is a square-wave signal.

[0009] Another embodiment of the present invention is a coupled optoelectronic oscillator comprising complex photonic and electronic circuits disposed on the same substrate as a photonic lightwave circuit-based ring resonator. Integrated active-photonics elements include a semiconductor optical amplifier-based optical source, phase-control elements, an amplitude modulator, and a photodetector; conventional integrated-circuit- based elements including an oscillator, mixer, amplifiers, a frequency filter, and RF filter; and the resonant cavity includes a ring resonator having an output that is optically coupled with the input of the semiconductor optical amplifier to form the coupled optoelectronic oscillator.

[ooio] Some embodiments of the present invention include additional features, including:

i. a phase modulator for synthesizing an RF frequency output signal; or ii. a time-domain optical isolator for removing co-propagating modes from counter-propagating modes; or

iii. an RF filter that operates in the optical domain to enable generation of a square-wave output signal; or

iv. a saturable absorber for narrowing optical pulses propagating through the high-Q waveguide ring; or

v. a laser, a Brillouin laser, a semiconductor optical amplifier, or an

Erbium-doped waveguide amplifier that is used as the optical source; or vi. a phase-locking circuit; or

vii. a linear resonant cavity, wherein the linear resonant cavity can be tunable; or

viii. a resonant cavity that is defined by a combination of active-photonics devices and waveguides; or

ix. any combination of i, ii, iii, iv, v, vi, vii and viii.

[ooii] In some embodiments, photonic elements and waveguide structures are integrated on a first chip, while RF electronic devices are disposed on a second chip. The two chips are operatively coupled to collectively define an optoelectronic oscillator. In some embodiments, the two chips are operatively coupled to collectively define a coupled optoelectronic oscillator.

[0012] In some embodiments, at least a portion of the resonant cavity of a coupled optoelectronic oscillator or optoelectronic oscillator is formed using ultra-low-loss waveguides.

[0013] In some optoelectronic oscillator structures in accordance with the present invention, the low-loss, high-Q waveguide ring is optically coupled with a mode-locked laser having a pulsation ratio that matches the output frequency of the optoelectronic oscillator.

[0014] Some embodiments include active dispersion management circuitry to mitigate phase-noise generation due to dispersion associated with optical devices in the optical path.

[0015] An embodiment of the present invention is a microwave-frequency generator, the microwave-frequency generator comprising: a resonant cavity , the resonant cavity being characterized by a quality factor equal to or greater than lxlO⁶; a first active- photonics device that is optically coupled with the resonant cavity; and a first RF-electronics device that is operatively coupled with the first active-photonics device; wherein the microwave-frequency generator is characterized by the resonant cavity including a first waveguide. Brief Description of the Drawings

[0016] FIG. 1 depicts a representative block diagram of an integrated system in accordance with the present invention.

[0017] FIG. 2 depicts a schematic drawing of an OEO in accordance with a first illustrative embodiment of the present invention.

[0018] FIG. 3 depicts a schematic drawing of a side view of OEO 200.

[0019] FIG. 4 depicts operations of a method for forming a microwave-frequency generator in accordance with the first illustrative embodiment of the present invention.

[0020] FIG. 5A depicts a cross-sectional view of substrate 500.

[0021] FIG. 5B depicts a top view of region A of substrate 500.

[0022] FIG. 5C depicts a side view of substrate 510 after formation of cladding portion 516.

[0023] FIG. 5D depicts a side view of OEO 200 after removal of substrate 512 and BOX layer 514.

[0024] FIG. 5E depicts a top view of OEO 200 after patterning of silicon layer 310.

[0025] FIG. 5F depicts a schematic drawing of a side view of OEO 200 after quantum-well intermixing and proton implantation.

[0026] FIG. 6 depicts a schematic drawing of an OEO in accordance with another embodiment of the present invention.

[0027] FIG. 7 depicts a schematic drawing of an OEO in accordance with another embodiment of the present invention.

[0028] FIG. 8 depicts a schematic drawing of an OEO in accordance with a fourth embodiment of the present invention.

[0029] FIG. 9 depicts a schematic drawing of an OEO in accordance with a fifth embodiment of the present invention.

[0030] FIG. 10 depicts a schematic drawing of a COEO in accordance with another embodiment of the present invention. [0031] FIG. 11 depicts a schematic drawing of a COEO in accordance with another embodiment of the present invention.

[0032] FIG. 12 depicts a schematic drawing of a COEO in accordance with another embodiment of the present invention.

[0033] FIG. 13 depicts a schematic drawing of a COEO in accordance with another embodiment of the present invention.

[0034] FIG. 14 depicts a schematic drawing of a COEO in accordance with another embodiment of the present invention.

[0035] FIG. 15 depicts a schematic drawing of a portion of a COEO in accordance with another embodiment of the present invention.

[0036] FIG. 16 depicts a schematic drawing of a portion of a COEO in accordance with another embodiment of the present invention.

[0037] FIG. 17 depicts a schematic drawing of a portion of a COEO in accordance with another embodiment of the present invention.

[0038] FIG. 18A depicts a schematic drawing of a portion of an OEO in accordance with another embodiment of the present invention.

[0039] FIG. 18B depicts a schematic drawing of a first embodiment of a mode-locked laser in accordance with the present invention.

[0040] FIG. 18C depicts a schematic drawing of a second embodiment of a mode- locked laser in accordance with the present invention.

[0041] FIG. 18D depicts a schematic drawing of a third embodiment of a mode- locked laser in accordance with the present invention.

[0042] FIG. 19 depicts a schematic drawing of a phase-locking circuit in accordance with the present invention.

[0043] FIG. 20 depicts a schematic drawing of a COEO in accordance with another embodiment of the present invention.

[0044] FIG. 21 depicts a schematic drawing of a stimulated Brillouin laser as applied to an OEO in accordance with another embodiment of the present invention. [0045] FIG. 22 depicts a first dispersion-managed microwave frequency generation approach in accordance with the present invention.

[0046] FIG. 23 depicts a second dispersion-managed microwave frequency generation approach in accordance with the present invention.

[0047] FIG. 24 depicts the optoelectronic feedback path of a feed-forward version of system 2300.

[0048] FIG. 25 depicts a third dispersion-managed microwave frequency generation approach in accordance with the present invention.

[0049] FIG. 26 depicts a schematic drawing of a first high-Q resonant cavity in accordance with the present invention.

[0050] FIG. 27 depicts a schematic drawing of a second high-Q resonant cavity in accordance with the present invention.

[0051] FIGS. 28A-B depict schematic drawings of basic implementations of an optical microwave pulse isolator based on timed gating.

[0052] FIGS. 28C-D depict schematic drawings of isolator configurations in which no external RF driver is required.

[0053] FIG. 28E depicts a schematic drawing of an isolator configuration that contains a narrow-band microwave filter.

[0054] FIG. 28F depicts a schematic drawing of an isolator configuration wherein the saturable absorber is replaced by a photodetector.

Detailed Description

[0055] The following terms are defined for use in this Specification, including the appended claims:

• "Ultra-low-loss waveguide" is defined as a waveguide having optical

propagation loss less than or equal to 1 dB/meter;

• "waveguide" is defined as a planar-lightwave-circuit-based waveguide

formed on a substrate. It should be noted that the definition of the term "waveguide" is meant to exclude a conventional optical fiber; • "hybrid-photonics device" is defined as a photonic device comprising a compound-semiconductor layer structure, wherein the compound- semiconductor layer structure is bonded to an underlying semiconductor layer comprising a waveguide, such that the compound-semiconductor layer and the waveguide are optically coupled. Although the semiconductor layer typically comprises silicon, it should be noted that the definition of the term "hybrid-photonics device" is meant to include devices wherein the semiconductor layer comprises a material other that silicon, such as compound semiconductors, silicon compounds, germanium, silicon germanium, and the like. It should be noted that the definition of hybrid- photonics device is meant to include heterogeneous-photonic devices.

[0056] It is an aspect of the present invention that photonics devices and waveguide elements that define a resonant cavity can be integrated on a single substrate to enable a microwave-frequency generator having lower phase noise in its output signal than can be achieved in the prior art. Further, by forming the waveguides using a low-loss planar- lightwave-circuit (PLC) technology, low-noise operation can be achieved without introducing significant optical loss, thereby enabling a resonant cavity having a high quality factor.

[0057] FIG. 1 depicts a representative block diagram of an integrated system in accordance with the present invention. System 100 comprises substrate 102, RF electronics 104, photonics integrated circuit 106, and PLC 108.

[0058] RF electronics 104 includes one or more electronic devices suited for operation at microwave frequencies.

[0059] Photonics integrated circuit (PIC) 106 is a combination of one or more active photonic devices and one or more connecting waveguides. In some embodiments, PIC 106 includes:

i. one or more hybrid-photonics devices formed on substrate 102 and optically coupled with one or more waveguides disposed on substrate 102; or

ii. one or more hybrid-photonic devices formed on a separate substrate and integrated with substrate 102 using conventional hybrid integration techniques and optically coupled with one or more waveguides disposed on substrate 102; or

iii. one or more hybrid-photonic devices formed on separate substrate and co-packaged with substrate 102 such that they optically coupled with one or more waveguides disposed on substrate 102; or

iv. one or more conventional photonic devices formed on a separate

substrate and integrated with substrate 102 using conventional hybrid integration techniques and optically coupled with one or more waveguides disposed on substrate 102; or

v. one or more conventional photonic devices formed on a separate substrate and co-packaged with substrate 102 such that they optically coupled with one or more waveguides disposed on substrate 102; or vi. any combination of i, ii, iii, iv, and v.

[0060] PLC 108 comprises one or more waveguides formed on substrate 102.

[0061] RF electronics 104, PIC 106, and PLC 108 are operatively coupled to collectively define a microwave-frequency generator having resonant cavity 110, and each is formed on substrate 102 to collectively define a monolithically integrated system, as discussed below. One skilled in the art will recognize that numerous system architectures for forming microwave-frequency generators, such as optoelectronic oscillators (OEOs) and coupled optoelectronic oscillators (COEOs), are within the scope of the present invention. Further, it will be clear to one skilled in the art, after reading this Specification, how to specify, make, and use alternative embodiments of the present invention wherein at least some of the components of RF electronics 104 and PIC 106 are formed on a separate substrate or disposed on substrate 102 via another integration process, such as conventional hybrid integration, etc.

[0062] In some embodiments, resonant cavity 110 is formed primarily of waveguide elements of PLC 108. In some embodiments, resonant cavity 110 includes one or more devices of PIC 106, as well as waveguide elements of PLC 108.

[0063] FIG. 2 depicts a schematic drawing of an OEO in accordance with a first illustrative embodiment of the present invention. OEO 200 includes oscillator 202, mixer 204, RF filter 206, amplifiers 208, source 210, phase modulator 212, amplitude modulator 214, photodetectors 216-1 and 216-2, bus waveguides 220 and 222, and waveguide ring 224.

[0064] FIG. 3 depicts a schematic drawing of a side view of OEO 200. OEO 200 includes a layer structure formed on substrate 102, where the layer structure comprises lower cladding 302, cladding 304, core layer 306, silicon layer 310, compound

semiconductor structure 314, and metallization layer 316. For clarity, metal traces between devices is not shown in FIG. 3.

[0065] FIG. 4 depicts operations of a method for forming a microwave-frequency generator in accordance with the first illustrative embodiment of the present invention. Exemplary methods suitable for integration of silicon-photonic devices with low-loss, PLC- based waveguides (hereinafter, referred to as "waveguides") are described by J. Bauters, et al., in "Ultra-low-loss high-aspect-ratio S13N4 waveguides," Optics Express, Vol. 19, pp. 3163-3174 (2011), as well as PCT Patent Application PCT/US2013/060927.

[0066] Method 400 begins with operation 401, wherein partially formed PLC 108 is provided. Method 400 is described with continuing reference to FIGS. 2 and 3, as well as reference to FIGS. 5A-F.

[0067] Partially formed PLC 108 is provided on substrate 500, which comprises substrate 102, lower cladding 302, core layer 306, and cladding portion 502.

[0068] FIG. 5A depicts a cross-sectional view of substrate 500.

[0069] FIG. 5B depicts a top view of region A of substrate 500. Region A includes the waveguide components of PLC 108.

[0070] Substrate 102 is a conventional silicon substrate. One skilled in the art will recognize, after reading this Specification, that substrate 102 can include any substrate material suitable for use with planar processing technologies and, specifically, the fabrication of PLC components. In some embodiments, substrate 102 comprises a material other than silicon. Materials suitable for use in substrate 102 include, without limitation, glass, fused silica, germanium, a silicon compound (e.g., silicon germanium, silicon carbide, etc.), a compound semiconductor, and the like. [0071] Lower cladding 302 is a layer of silicon dioxide formed on substrate 102 via thermal oxidation. Lower cladding 302 can have any suitable thickness and is generally within the range of 6 microns to 30 microns. In this exemplary embodiment, lower cladding 302 has a thickness of approximately 15 microns. In some embodiments, lower cladding 302 is formed by a process other than thermal oxidation, such as low-pressure chemical- vapor deposition (LPCVD), sputtering, spin-on techniques, sintering, and the like.

[0072] Core layer 306 is a layer of stoichiometric silicon nitride formed on lower cladding 302 via LPCVD and patterned in conventional fashion to define waveguide cores 506-1, 506-2, and 506-3 (referred to, collectively, as cores 506) of bus waveguides 220 and 222 and waveguide ring 224, respectively. Each of cores 506-1 and 506-2 is separated from core 506-3 by gap, gl.

[0073] Core layer 306 is a layer of stoichiometric silicon nitride having a thickness of approximately 40 nanometers (nm) . Each of cores 506 has a width that is typically within the range of approximately 1 micron to approximately 20 microns, such that they operate as ultra-low-loss waveguides.

[0074] One skilled in the art will recognize that the material and dimensions of core layer 306 and cores 506 are matters of design choice and that many alternative materials and dimensions can be used without departing from the scope of the present invention. For example, materials suitable for use in core layer 306 include, without limitation, silicon oxides (e.g., doped or undoped silicon oxide, SiOx, etc.), silicon nitride, silicon oxynitride, silicon carbide, silicon germanium, hafnium oxide, aluminum oxide, silica, and the like.

[0075] Typically, core layer 306 is annealed (e.g., at 950° C for several hours) after its formation to reduce its hydrogen concentration and increase its density. This facilitates ultra-low-loss operation of waveguides 220, 222, and ring 224.

[0076] Cladding portion 502 is a layer of silicon dioxide formed over cores 310 by TEOS-based LPCVD. Cladding portion 502 has thickness t2 (neglecting the thickness of core layer 306) and top surface 504. In some embodiments, surface 504 is planarized to facilitating wafer bonding .

[0077] In some embodiments, cladding portion 502 comprises a material other than TEOS-based LPCVD-deposited silicon dioxide. Suitable materials for use in cladding portion 502 include, without limitation, low-temperature deposited silicon dioxide, spin-on glass, plasma-enhanced-chemical-vapor-deposition (PECVD) dielectrics, glasses (e.g., borophosphosilicate glass, phosphosilicate glass, etc.), silicon oxides and silicon oxynitride.

[0078] At operation 402, silicon layer 310 is provided on substrate 510, which comprises handle substrate 512, buried oxide (BOX) layer 514, silicon layer 310, and cladding portion 516.

[0079] Substrate 510 is a conventional silicon-on-insulator substrate originally comprising active layer 520 disposed on BOX layer 514. In some embodiments, substrate 510 includes a handle substrate other than silicon, such as a compound semiconductor substrate. In some embodiments, substrate 510 includes a separation layer, an ion- implanted layer (analogous to the layer employed in the SmartCut process used for SOI substrate fabrication), or a stop-etch layer in place of BOX layer 514.

[0080] Active layer 520 is a layer of single-crystal silicon, typically having a thickness of approximately 0.2 - 1.0 microns. In some embodiments, active layer 520 is a material other than silicon but that is suitable for guiding an optical signal at the wavelength of operation for OEO 200. Materials suitable for use in active layer 520 include, without limitation, compound semiconductors, silicon compounds, germanium, silicon germanium, and the like.

[0081] Silicon layer 310 and cladding portion 516 are formed from active layer 520 by thermally oxidizing the active layer to form a silicon dioxide layer (i.e., cladding portion 516) having thickness t3 and top surface 518. Although the value of t3 is a matter of design choice, exemplary values for t3 are within the range of a few nm to several microns. The non-oxidized portion of active layer 520 defines silicon layer 310. In some

embodiments, cladding portion 514 is a layer of dielectric material formed on active layer 520, where the dielectric material is suitable for use as cladding material, as well as for bonding with cladding portion 502.

[0082] FIG. 5C depicts a portion of substrate 510 after formation of cladding portion

516.

[0083] At operation 403, substrates 500 and 510 are joined at bonded interface 522 via wafer bonding (preferably oxygen-plasma-assisted bonding). The bonding process begins by exposing surfaces 504 and 518 in an oxygen plasma to activate them and provide appropriate surface termination. Surfaces 504 and 518 are then brought into physical contact at room temperature and annealed at an elevated temperature (e.g., 950° C) for a suitable period of time (e.g., 3 hours) to improve its strength and quality.

[0084] Although oxygen-plasma-assisted bonding is the preferred method for joining substrates 500 and 510, it will be clear to one skilled in the art, after reading this

Specification, how to specify, make, and use alternative embodiments of the present invention wherein a different wafer bonding technique is used to join the substrates.

Exemplary bonding methods suitable for joining substrates 500 and 510 include, without limitation, fusion bonding, thermo-anodic bonding, argon-plasma-assisted bonding, adhesive bonding (e.g., using BenzoCycloButene (BCB), etc.), and the like.

[0085] Once the two substrates are joined, cladding portions 502 and 516 form a substantially continuous layer of silicon dioxide that collectively defines cladding layer 304. Cladding layer 304 serves as an upper cladding layer for bus waveguides 220 and 222 and ring 224 (thus completing these waveguide structures) and also as an inter-waveguide cladding between cores 506 and silicon layer 310. Cladding layer 304 has thickness tl, which is approximately equal to the combined thicknesses t2 and t3.

[0086] Once cladding layer 304 is fully formed, the structures of ring 224 and bus waveguides 220 and 222 are complete and collectively define ring resonator 226. In this example, ring resonator 226 has a free-spectral range (FSR) of 20 GHz/n, where n is an integer, and is characterized by a high Q (typically > 10⁶). In some embodiments, resonant cavity 110 comprises a structure other than a waveguide ring, such as a racetrack, coupled rings, etc.

[0087] As discussed above, the quality factor for a resonator depends on numerous factors, including the overall losses in the resonator architecture. It should be noted, therefore, that providing bus waveguides 220 and 222, and waveguide ring 224 as ultra- low-loss waveguides facilitates the formation of a high-Q resonant cavity. It will be clear to one skilled in the art, however, after reading this Specification, how to specify, make, and use alternative embodiments that comprise waveguides other than an ultra-low-loss waveguides - for example, waveguides having optical propagation loss that is greater than 1 dB/meter. It should be noted, however, that the use of ultra-low-loss waveguides provides embodiments of the present invention significant advantage over the prior art.

[0088] At operation 404, handle substrate 512 and BOX layer 514 are removed. [0089] FIG. 5D depicts a side view of OEO 200 after removal of substrate 512 and BOX layer 514.

[0090] At operation 405, silicon layer 310 patterned to define electronics region 524 and silicon waveguides 526-1, 526-2, and 526-3.

[0091] FIG. 5E depicts a top view of OEO 200 after patterning of silicon layer 310.

[0092] Electronics region 524 is an area of silicon layer 310 suitable for the formation of RF electronics 104.

[0093] Silicon waveguides 526-1, 526-2, and 526-3 (referred to, collectively, as waveguides 526) are formed to facilitate their integration into hybrid-photonics devices, as well as for conveying optical signals between those devices and vertical couplers 312, as described below.

[0094] It should be noted that in some embodiments, cladding layer 304 does not include bonded interface 522 and, instead is formed as a single layer having thickness tl after the definition of waveguide cores 506. In such embodiments, active layer 520 is provided already having a thickness already suitable for use as silicon layer 310. In such embodiments, active layer 520 is bonded directly to cladding layer 304 at operation 403. In some embodiments, a thin oxide layer is formed on active layer 520 prior to wafer bonding to facilitate the bonding process.

[0095] Further, in some embodiments, electronics region 524 is not provided as an area of silicon layer 310. Instead, electronic region 524 is provided by adding a region of semiconductor material, such as silicon-germanium, that is particularly well suited for supporting RF electronics 104. Materials suitable for use in electronic region 524 include, without limitation, silicon-germanium, germanium, compound semiconductors, and the like. Such a region of semiconductor material can be disposed on cladding layer 304 by processes such as the wafer bonding approach described above, sputtering and zone-melt recrystallization, or another suitable process. In some embodiments, RF electronics 104 is added to system 200 via hybrid bonding, bump bonding or integrating a conventional CMOS IC via flip-chip bonding or another conventional integration technology.

[0096] At operation 406, compound semiconductor structure 316 (hereinafter referred to as "structure 316") is bonded to waveguides 526. Structure 316 comprises an MQW layer, as well as additional layer structure formed on a conventional compound semiconductor substrate, where the layer structure is suitable for supporting the formation of the photonic components included in photonic integrated circuitry 106. In some embodiments, structure 316 includes a different quantum-well structure, such as a single quantum-well, quantum dots, and the like. Examples of silicon-photonic devices and their formation are described by J.E. Bowers, et a/., in "Silicon Evanescent Lasers and Amplifiers," Proc. of the Group IV Photonics Conference (GFP), ThBl, Ottawa, Canada, (2006).

[0097] In operation 406, structure 316 is bonded to waveguides 526 such that each of the silicon waveguides and a region of the MQW layer form a hybrid waveguide suitable for supporting propagation of an optical mode.

[0098] Once structure 316 and waveguides 526 are bonded, the compound semiconductor handle substrate is removed.

[0099] In some embodiments, operation 406 is repeated one or more times to add different compound semiconductor structures to the substrate. As a result, different regions of material having different layer structures, bandgaps, quantum-well designs, etc., can be integrated on substrate 102.

[00100] At operation 407, quantum-well intermixing is performed on structure

316 to alter the bandgaps, as necessary, in the regions of source 210, phase modulator 212, amplitude modulator 214, and photodetectors 216-1 and 216-2. Quantum-well- intermixed devices suitable for use in the present invention are disclosed by Skogen, et a/., in "Postgrowth control of the quantum-well band edge for the monolithic integration of widely tunable lasers and electroabsorption modulators," IEEE Journal of Selected Topics in Quantum Electronics, Vol. 9, pp. 1183-1190 (2003), and U.S. Patent Publication

2009/0246298-A1. Once quantum-well intermixing is complete, these regions are then electrically isolated via proton implantation as necessary. It should be noted that, in some embodiments, quantum-well intermixing is not required. In some embodiments, one or more of the elements of PIC 106 is joined with substrate 102 via a conventional hybrid bonding process, such as die bonding, bump bonding, etc.

[OOioi] FIG. 5F depicts a schematic drawing of a side view of OEO 200 after quantum-well intermixing and proton implantation.

[00102] At operation 408, oscillator 202, mixer 204, RF filter 206, and amplifiers 208 are formed in conventional fashion in electronics region 524. [00103] At operation 409, metallization layer 316 is formed and patterned to electrically interconnect the devices of RF electronics 104 and the silicon-photonic devices of photonic integrated circuitry 106.

[00104] System 200 is an example of an OEO architecture that takes advantage of the advantages provided by a high-functionality photonic-integrated circuit (PIC) technology, such as a hybrid-photonics, that can monolithically integrate

photodetectors, optical modulators, and an optical source with waveguides.

[00105] The base function of system 200 is provided by coupling source 202 through amplitude modulator 214. Amplitude modulator 214 is an optical intensity modulator, such as a Mach-Zehnder modulator (MZM). The output from modulator 214 is optically coupled to ring resonator 226.

[00106] The laser wavelength of source 210 is kept centered on the ring- resonance frequency by incorporating a Pound-Drever-Hall (PDH) feedback loop into the architecture. The PDH feedback loop modulates phase modulator 212 with an RF tone, detects the output from ring resonator 226 at photodetector 216-1, and identifies the phase of RF tone, which then used to provide a feedback signal to source 210.

[00107] The drop signal from ring resonator 226 is detected at photodetector

216-2 and amplified. The synthesized RF frequency is filtered away (a harmonic of the ring FSR) at RF filter 206, amplified and used to drive amplitude modulator 214. Output signal 228 is a substantially sinusoidal signal provided by tapping the RF frequency from RF electronics 104, while an optical output modulated by the RF frequency is formed by tapping photonic integrated circuitry 106.

[00108] Although, in system 200, source 210 is a silicon-photonics laser, in some embodiments, source 210 is a different light source. Sources suitable for use in embodiments of the present invention include, without limitation, lasers, pumped amplifiers (e.g., semiconductor optical amplifiers (SOAs), Erbium-doped waveguide amplifiers (EDWAs), etc.), and the like. In some embodiments, source 210 is external to substrate 102 or includes a pump source that is external to substrate 102.

[00109] System 200 is an example of an OEO architecture that is constructed in monolithic fashion to exploit all of the advantages afforded by integrating the disparate technologies of RF circuitry 104, PIC 106, and PLC 108 on a single substrate. As discussed above, however, in some embodiments, at least one of RF circuitry 104, PIC 106, and PLC 108 is formed on a separate substrate. In some embodiments, at least one of RF circuitry 104, PIC 106, and PLC 108 is integrated using hybrid-integration technology.

[ooiio] It should be noted that the system architecture depicted in FIG. 2 is merely one example of a microwave-frequency generator architecture in accordance with the present invention. Examples of alternative architectures in accordance with the present invention are provided below. It will be clear to one skilled in the art, after reading this Specification, how to alter method 400 to realize these alternative architectures.

System-level Architectures

[OOiii] FIG. 6 depicts a schematic drawing of an OEO in accordance with another embodiment of the present invention. OEO 600 includes oscillator 202, mixer 204, amplifiers 208, source 210, phase modulator 212, amplitude modulator 214,

photodetectors 216-1 and 216-2, bus waveguides 220 and 222, waveguide ring 224, and optical RF filter 602.

[00112] System 600 is analogous to system 200; however, in system 600, RF filter 206 is replaced by optical RF filter 602, which is included in photonic integrated circuitry 106. As a result, system 600 provides output signal 608 as a substantially square- wave signal. One skilled in the art will recognize that additive white noise can result in a smaller phase error in a square-wave OEO system.

[00113] RF filter 602 includes waveguide ring 604 and phase control element

606. Ring 604 has an FSR (20 GHz) that corresponds to the synthesized RF frequency and is a harmonic of the FSR of ring resonator 226. It should be noted that RF filter 602

enables filtering of harmonics of the RF frequency required to form square-wave output signal 608.

[00114] In addition, in system 600, RF electronics 104 includes limiting amplifier 610, which is connected to amplitude filter 214. By driving amplitude filter 214 with a peak-to-peak voltage equal to the MZM V-π, output signal 608 is further enhanced. [00115] FIG.7 depicts a schematic drawing of an OEO in accordance with another embodiment of the present invention. System 700 includes amplifiers 208, source 210, amplitude modulator 214, photodetector 216, bus waveguides 222 and 220, waveguide ring 224, balanced detector 704, coupler 706, and low-pass filter 708. System 700 is a variation of system 200 wherein source 210 is directly locked to ring resonator 226 using a coherent receiver.

[00116] In system 700, the PDH stabilization loop of system 200 is replaced by direct optical-phase locking of the laser of source 210 to ring resonator 226. In addition, a portion (typically < 10%) of each of the output power from source 210 and the optical signal in bus waveguide 222 (i.e., the drop port of ring resonator 226) is tapped off and provided to 2x2 coupler 706, which is optically coupled with balanced detector 704. As a result, the two optical signals are combined and the transmission phase through waveguide ring 224 is detected. This signal is amplified, filtered at low-pass filter 708, and used to tune the laser of source 210 to the resonance frequency of ring resonator 226.

[00117] System 700 has the advantage that the feedback loop can have a wide bandwidth - sufficient to reduce the linewidth of source 210.

[00118] FIG. 8 depicts a schematic drawing of an OEO in accordance with a fourth embodiment of the present invention. System 800 includes amplifiers 208, source 210, phase modulator 212, bus waveguides 222 and 220, waveguide ring 224, balanced detector 704, coupler 706, and low-pass filter 708.

[00119] System 800 is a phase-modulated OEO architecture, combining direct locking of source 210 to waveguide ring 224 with optical phase modulation for the synthesized RF frequency. The phase-modulated optical signal is detected in a coherent receiver comprising balanced detector 704 and coupler 706, using a tapped output from source 210 as a local oscillator signal. The detected RF signal is filtered at RF filter 206 and used to drive the phase modulator. The low-frequency component of the detected signal is used to provide laser control. System 800 has improved amplitude-noise suppression as compared to system 200; however, this improved noise suppression comes at the expense of potentially increased sensitivity to optical-phase noise. [00120] FIG. 9 depicts a schematic drawing of an OEO in accordance with a fifth embodiment of the present invention. System 900 includes amplifiers 208, source 210, phase modulator 212, bus waveguides 222 and 220, optical-domain RF filter 602, waveguide ring 224, integrator 902, coupler 904, and I/Q detector 906. System 900 is a square-wave version of phase-modulated system 800.

[00121] As in intensity-modulated system 200, RF filter 602, which includes waveguide ring 604 and phase controller 606, operates in the optical domain to filter harmonics of the RF frequency required to form a square-wave output signal.

[00122] In system 900, a 90° optical hybrid is integrated to form a coherent receiver, which is defined by optical I/Q detector 906. The in-phase component carries the square-wave that drives phase modulator 212. The quadrature component is mixed with the square wave to form a feedback signal that keeps source 210 on resonance of ring resonator 226.

[00123] In some embodiments, the detected in-phase square wave is detected to generate a feedback signal provided to a limiting-amplifier cross-over control. In such embodiments, the duty cycle can be kept to 50%.

[00124] FIG. 10 depicts a schematic drawing of a COEO in accordance with another embodiment of the present invention. System 1000 includes: substrate 1002, which comprises oscillator 202, mixer 204, amplifiers 208, RF filter 206, LP filter 708; and substrate 1004, which comprises phase modulator 212, MZM-based amplitude modulator 214, photodetectors 216-1 and 216-2, source 1002, optical tap 1004, phase controller 1006, and PLC 108, which includes ring resonator 226.

[00125] COEO 1000 comprises two separate substrates - substrate 1002, which includes RF electronics 104, and substrate 1004, which includes monolithically integrated PIC 106 and PLC 108, which collectively define resonant cavity 110. In some embodiments, PLC 108 is formed on a third substrate. Source 1002 comprises a semiconductor optical amplifier (SOA). The optical output from the drop-port of ring resonator 226 is coupled back to the input of source 1002 to form the COEO architecture. In addition, optical tap 1008 directs part of the optical signal to photodetector 216-2, which detects it and provides it to an amplifier 208. This signal is amplified, filtered at RF filter 206, and used to drive amplitude modulator 214, which includes an MZM structure. [00126] It should be noted that system 1000 includes a PDH feedback loop, which tunes the phase of the COEO such that the cavity modes line up with the resonance of low-loss ring resonator 226.

[00127] FIG. 11 depicts a schematic drawing of a COEO in accordance with another embodiment of the present invention. System 1100 is a variation of system 1000 that includes time-domain optical isolator (TISO) 1102.

[00128] TISO 1102 comprises a pair of optical modulators (e.g., Mach-

Zehnder modulators) that are sequentially modulated to filter out co-propagating modes from counter-propagating modes in the COEO.

[00129] In system 1100, the detected and filtered RF signal is used to drive the dual optical modulators of TISO 1102, where the physical separation of the optical modulators is carefully controlled . As a result, and by applying an appropriate RF delay for each part, preferential optical modulation can be achieved for co-propagating COEO optical modes while RF-modulation in counter-propagating modes is suppressed .

[00130] FIG. 12 depicts a schematic drawing of a COEO in accordance with another embodiment of the present invention. System 1200 is another variation of system 1000; however, system 1200 employs an optical implementation of a time-domain optical isolator - TISO 1202.

[00131] TISO 1202 includes a pair of saturable absorbers, which receive a filtered signal derived by splitting off the optical signal of ring resonator 226, which is filtered at RF filter 602 in the optical domain, amplified and injected into the saturable absorbers (SA) pair of TISO 1202 such that there is high optical isolation between the injected optical pulses and the main COEO cavity. In some embodiments, this is achieved through optical injection into the main waveguide orientation of the SA in the COEO.

[00132] The bleaching of the SA pair of TISO 1202 functions in analogous fashion to the operation of TISO 1102 described above and with respect to FIG. 11.

[00133] By providing a gating window that is very narrow, limited by the optical pulse-width, system 1200 offers improved performance over prior-art microwave- frequency generators. [00134] It should be noted that, although applied to a microwave-frequency generator in systems 1100 and 1200, the use of time-domain optical isolation and sequential modulation to give rise to unidirectional lasing is applicable to systems other than microwave-frequency generators, such as mode-locked lasers, etc.

[00135] FIG. 13 depicts a schematic drawing of a COEO in accordance with another embodiment of the present invention. System 1300 is a variation of system 1200, wherein tapped optical pulses are frequency shifted.

[00136] In system 1300, the generated RF frequency is detected, divided by two at divider circuit 1302, and used to drive MZM-based amplitude modulator 214, which is biased around zero transmission. As a result, the output of amplitude modulator 214 includes sidebands located at +/- half-RF frequency from the main optical lines generated by the COEO.

[00137] Further, optical RF filter 602 can be tuned to filter the sidebands from any remaining fundamental, as well as re-couple these frequency-shifted pulses into the COEO. As in system 1200, the bleaching of SA pair 1204 functions in analogous fashion to the operation of TISO 1102. The injected frequency-shifted pulses, therefore, are outside the resonance of ring resonator 226 and, as a result, do not form part of the COEO output signal.

[00138] FIG. 14 depicts a schematic drawing of a COEO in accordance with another embodiment of the present invention. System 1400 is analogous to other COEO architectures disclosed above; however, in system 1400, the RF feedback path and amplitude modulator arrangement is replaced by an integrated waveguide ring that functions as an RF filter.

[00139] System 1400 includes RF filter 1402, which includes waveguide ring

1404. Waveguide ring 1404 is characterized by an FSR that substantially matches the synthesized RF frequency.

[00140] It should be noted that system 1400 includes a PDH feedback loop for tuning the cavity phase of the COEO.

[00141] FIG. 15 depicts a schematic drawing of a portion of a COEO in accordance with another embodiment of the present invention. System 1500 is another variation of system 1000, wherein saturable absorber 1502 is used to narrow the optical pulses in waveguide ring 224, thereby defining a bi-directional, d ual-ring, mode-locked laser.

[00142] System 1500 is typically operated in a bi-directional mode, wherein the two pulse trains collide at the location of saturable absorber 1502.

[00143] FIG. 16 depicts a schematic drawing of a portion of a COEO in accordance with another embodiment of the present invention . System 1600 is a variation of system 1500, wherein waveguide ring 1602 acts as an isolator. System 1600 operates as a uni-directional, dual-ring, mode-locked laser.

[00144] Waveguide ring 1600 is analogous to ring 604, described above and with respect to FIG. 6; however, waveguide ring 1602 is formed with magneto-optic materials. Waveguide ring 1602 functions to separate co-propagating and counter- propagating resonance wavelengths. The ring phase of waveguide ring 1602 is tunable such that only the co-propagating resonance overlaps with the resonance wavelength of waveguide ring 224.

[00145] In some embodiments, a PDH feedback loop, as described above, is included in system 1600.

[00146] FIG. 17 depicts a schematic drawing of a portion of a COEO in accordance with another embodiment of the present invention . System 1700 is a variation of COEOs described above, where the main cavity of the COEO is implemented in a low-loss PLC platform. System 1700 is analogous to system 1300 described above; however, system 1700 includes a source comprising an Erbium-doped waveguide amplifier rather than an SOA-based source.

[00147] Source 1220 is a conventional EDWA, which is pumped by pump lasers 1704.

[00148] In some embodiments, saturable absorbers 1706 comprise sections of

Erbium-doped waveguides.

[00149] It should be noted that, in some embodiments, pump lasers 1704, as well as some other components of system 1700 (e.g., optical tap signals, optical phase shifter 1708, optical frequency shifter 1710, and optical-domain RF filter 1712) are not implemented in the same low-loss waveguide platform as ring resonator 226. [00150] FIG. 18A depicts a schematic drawing of a portion of an OEO in accordance with another embodiment of the present invention . System 1800 includes a mode-locked laser (M LL) having a pulsation ratio that substantially matches the synthesized RF frequency of the system .

[00151] MLL 1802 comprises waveguide ring 1804, a portion of bus waveguide 222, saturable absorber 1502, source 1002, and phase modulator 212. MLL 1802 provides an output signal that is coupled to ring resonator 226 via bus waveguide 222. The FSR of ring resonator 226 is equal to (or, in some embodiments, is a sub- harmonic of) the RF frequency.

[00152] It is an aspect of the present invention that mode-locked lasers that exploit the use of a time-domain optical isolator, as discussed above and with respect to FIGS. 11 and 12, can operate unidirectionally when the distance between the two modulators (or saturable absorbers) is approximately T/4, where T is the pulse periodicity. This is also the case for harmonically mode-locked lasers, wherein the distance between the modulators should be (T/4 + k-T), where k is an integer. Under proper conditions the propagating lasing modes are amplified, whereas the counter-propagating modes (or noise) experience net loss. In some embodiments, a pair of saturable absorbers is used, wherein at least one of them is driven with an RF signal.

[00153] Reflections inside the laser cavity and/or from further downstream the circuit couple into modes that propagate in the opposite direction of the lasing mode(s) . This means that reflections do not couple into the lasing modes and hence do not distort or destabilize the lasing performance. Moreover these reflections couple into the 'lossy' or isolated direction, and hence are further suppressed . Such lasers, therefore, are suitable for use as feedback-insensitive-pulse sources or comb sources for on-chip applications.

[00154] FIG. 18B depicts a schematic drawing of a first embodiment of a mode-locked laser in accordance with the present invention . MLL 1806 comprises waveguide ring 1808, bus waveguides 1810, modulators 1812-1 and 1812-2, and gain element 1814. MLL 1806 is analogous to M LL 1802 and is suitable for use in system

1800, as well as other embodiments of the present invention .

[00155] MLL 1806 is a laser implementation that is based on an external RF driver that drives both of modulators 812-1 and 1812-2 with a T/4 time delay difference. In some embodiments, the laser is passively mode-locked by a saturable absorber whose output signal drives a modulator (or another saturable absorber) in the same cavity at the proper position and with the proper delay.

[00156] Depending on the phase difference between the RF drive on modulators 812-1 and 1812-2, MLL 1806 can lase unidirectionally in the A direction or B direction, as shown.

[00157] FIG. 18C depicts a schematic drawing of a second embodiment of a mode-locked laser in accordance with the present invention. MLL 1816 comprises waveguide ring 1818, SOAs 1820-1 and 1820-2, saturable absorber 1822, splitter 1824, and mirror 1826. MLL 1816 is analogous to MLL 1802 and is suitable for use in system 1800, as well as other embodiments of the present invention.

[00158] MLL 1816 is a self-colliding pulse mode-locked laser with a loop mirror. An optical pulse that resonates inside the cavity of MLL 1816 is, on one side, fed- back by mirror 1826 (e.g. an etched, cleaved or polished facet, a Bragg mirror, another loop mirror, etc.). The returning pulse is split over two paths by splitter 1824 (e.g. an MMI, directional coupler, Y-branch, etc.). The pulses traverse waveguide ring 1818 in counter- propagating directions and recombine again at splitter 1824.

[00159] Saturable absorber 1822 is located in the center of the loop mirror to ensure that the pulses collide at that point. Waveguide ring 1818 is preferably symmetric and SOAs 1820-1 and 1820-2 are preferably matched to facilitate low-loss coherent pulse combination at splitter 1824, thereby avoiding non-symmetric nonlinearities.

[00160] MLL 1816 provides the advantage that the position of saturable absorber 1822 can be lithographically designed, unlike designs in which an absorber is located at a cleaved or polished facet. Bragg gratings can also be lithographically defined, but these have a finite effective length, thereby increasing the absorber length, which generally prevents the formation of short optical pulses.

[00161] Colliding-pulse configurations, such as ring lasers or Fabry-Perot-type cavities with an absorber placed in the center also enable fast saturation of the absorber, but their stability suffers from the fact that the two pulses are, in principle, independent and not coherent with each other. [00162] MLL 1816, being a self-colliding configuration with loop mirror, has the advantage of a short absorber, a lithographically defined absorber placement and has a single pulse travelling inside the cavity, whose parts, when split in the mirror, remain coherent.

[00163] FIG. 18D depicts a schematic drawing of a third embodiment of a mode-locked laser in accordance with the present invention . MLL 1828 comprises bus waveguide 1830, SOAs 1820-1 and 1820-2, saturable absorber 1822, and Bragg mirrors 1832-1, 1832-2, and 1832-3. MLL 1828 is analogous to MLL 1802 and is suitable for use in system 1800, as well as other embodiments of the present invention.

[00164] MLL 1828 is a colliding-pulse laser with optical feedback, wherein two pulses inside the cavity are coupled together by means of external feedback. When the laser is fundamentally locked, two pulses, separated by a time interval T, inside the cavity such that they collide inside saturable absorber 1822. The roundtrip time inside the laser cavity is 2-T. When the feedback delay (2- L, as indicated in FIG. 18D) equals (2- k + 1) -T, where k is an integer, the two independent fields (pulses) in the laser cavity are coupled and timing jitter is reduced .

[00165] Proper choice of the laser cavity length and the feedback delay length enables the design of a harmonically mode-locked laser such that all the pulses are coupled together. Preferably, the lengths of the laser cavity and feedback delay are selected such that they do not have a common denominator.

[00166] Returning now to FIG. 18A, the drop port signal from ring resonator

226 is detected using a coherent receiver where a tapped output of MLL 1802 forms the LO signal . The low-frequency part of the detected signal tunes the wavelength of M LL 1802 to the ring resonance of ring resonator 226. The detected RF signal is used to drive saturable absorber 1502 of a modulator in the MLL cavity. This locks the pulse repetition ratio to the FSR of ring resonator 226. In some embodiments, a pair of saturable absorbers or modulators is used to form a TISO.

[00167] It is an additional aspect of the present invention that an additional phase-locking function can be included in any of the OEO and CEOE embodiments disclosed herein. [00168] FIG. 19 depicts a schematic drawing of a phase-locking circuit in accordance with the present invention . Circuit 1900 includes phase modulator 212, filter/amplifier 1902, and balanced receiver 1904.

[00169] Fig . 19 depicts a phase-locking circuit in accordance with the present invention applied to a COEO structure. In this configuration, the transmitted signal past ring resonator 226 (i.e., signal 1906) is recombined with tapped output signal 1908 from the COEO. Balanced receiver 1904 detects the interference signal between the transmitted signal and the tapped signal and generates differential photocurrent signal 1910, which is amplified and filtered at filter/amplifier 1902. The filtered and amplified signal is then used to tune a phase section (i.e., phase modulator 212) within the COEO, thereby tuning the optical frequency such that it matches that of ring resonator 226.

[00170] FIG. 20 depicts a schematic drawing of a COEO in accordance with another embodiment of the present invention. System 2000 comprises linear resonant cavity 2006, phase shifter 2008, electro-absorption modulator 2010, source 1002, RF amplifier 2014 and photodiode 2016. System 2000 is an example of a COEO comprising a linear resonant cavity.

[00171] System 2000 comprises substrates 2002 and 2004, as well as some additional electronics. Substrate 2002 comprises PIC 106, which includes phase shifter 2008, electro-absorption modulator 2010, and source 1002. Substrate 2004 comprises low-loss PLC structures including splitter 2018, waveguide ring 224, and bus waveguides 2026 and 2028, which are analogous to low-loss PLC-based waveguide structures described above and with respect to system 200. Waveguide ring 224 and bus waveguides 2026 and 2028 collectively define ring resonator 2030 having drop port 2032.

[00172] Substrate 2002 includes a cleaved facet 2024 on which a reflective coating is disposed to form a first mirror of linear resonant cavity 2006.

[00173] The second mirror of linear resonant cavity 2006 results from cycling the optical power at drop port 2032 back onto the ring in the counter-clockwise direction such that it returns (i.e., is "reflected") to PIC 106. In some embodiments, the drop port of waveguide ring 224 is terminated at a facet of substrate 2004 and provided a high- reflection coating to form a second mirror. [00174] Substrate 2004 also includes integrated delay element 2022 (e.g., one or more spiral delay loops, etc.) to provide control over the cavity length of linear resonant cavity 2006 and heater 2020 to control the resonance of waveguide ring 224. In some embodiments, an additional optical fiber is inserted between substrates 2002 and 2004 to increase the cavity length, thereby increasing laser cavity Q and further reducing the phase noise of the output RF signal.

[00175] Photodiode 2016 is a surface-normal photodiode that collects light from facet 2024. In some embodiments, this light is collected by another device, such as a waveguide photodetector integrated on substrate 2002.

[00176] The electrical output from photodiode 2016 is amplified at amplifier 2014 and fed back to the MLL cavity through the electro- absorption modulator 2010. In some embodiments, a Mach-Zehnder modulator is used in place of an electro-absorption modulator.

[00177] System 2000 offers several advantages over microwave-frequency generators of the prior art. For example, system 2000 is easier to assemble and therefore can be lower cost. The ease of assembly arises from the fact that there is a single coupling point between substrates 2002 and 2004.

[00178] In addition, the cavity length of the resonant cavity in system 2000 can be controlled by either thermal tuning of delay element 2022 or by employing an optical phase shifter on substrate 2002, where the phase shifter uses either carrier injection for on-chip integrated compound semiconductor structures or carrier depletion in the waveguides.

[00179] Further, by coating the right facet (as shown) of substrate 2004, the linear laser cavity includes the ring resonator, thereby doubling the optical group delay in one roundtrip compared to that of a ring laser cavity that contains a high Q resonator. This provides larger net finesse and improved RF side-mode suppression.

[00180] Still further, system 2000 does not require an optical isolator, thereby reducing its overall cost.

[00181] FIG. 21 depicts a schematic drawing of a stimulated Brillouin laser as applied to an OEO in accordance with another embodiment of the present invention. System 2100 comprises pump laser 2102, SOA 2104, modulator 2106, photodetector 2108, CMOS electronics 2110, and PLC substrate 2112, which includes waveguide rings 224-1 and 224-2, and bus waveguides 2114, 2116, 2118, and 2120. In this embodiment, each of pump laser 2102, SOA 2104, modulator 2106, and photodetector 2108 is integrated with the PLC substrate using conventional hybrid bonding technology.

[00182] System 2100 exploits the low-frequency noise characteristics of a stimulated Brillouin laser. One skilled in the art will recognize that there is an inherent amplitude-to-phase (AM-PM) noise conversion in an OEO at the photodetector (i.e., photodetector 2108). As a result, amplitude noise generated from the coupling of laser frequency noise and the frequency transfer function of the high-Q resonator appears as phase noise of the microwave signal.

[00183] It is yet another aspect of the present invention that low phase noise microwave frequency signals can be achieved for an OEO via active dispersion. Several examples of dispersion compensation approaches in the context of an OEO are disclosed here.

Dispersion Management Approaches

[00184] FIG. 22 depicts a first dispersion-managed microwave frequency generation approach in accordance with the present invention. System 2200 comprises pump laser 2102, modulator 2106, photodetector 2108, CMOS electronics 2110, dispersion element 2102, and PLC substrate 2204, which includes waveguide ring 224, and bus waveguides 220 and 222 (which collectively define ring resonator 226).

[00185] In an OEO, optical phase noise can bleed into phase noise of the generated microwave signal due to dispersion of the elements in the optical path - namely, laser 2102, modulator 2106, ring resonator 226, delay lines and photodetector 2108. Resonance enhanced delay elements like ring resonator 226 are highly frequency selective around their resonant frequencies. Additionally, the value of group delay, which is proportional to the second derivative of the ring's frequency transfer function, obtained for the optical carrier, is also frequency selective. Therefore, any perturbation in laser frequency with respect to the ring's frequency response can lead to large changes in group delay from one microwave cycle to another.

[00186] FIG. 23 depicts a second dispersion-managed microwave frequency generation approach in accordance with the present invention. System 2300 comprises source 1002, modulator 2302, photodetector 216, and dispersion element 2304. System 2300 represents an OEO coupled with an integrated feedback timing jitter "eater." System 2300 operates like a conventional OEO, wherein a single-linewidth laser injects light into modulator 2302, which includes a pair of phase modulators 212. Modulator 2302 is driven by the optical feedback at the loop resonance. Modulator 2302 modulates the CW light into pulse train 2306. These pulses are detected by photodetector 216 which in turn drives the modulator, thereby closing the optoelectronic feedback loop. Optical and/or electronic bandpass filters can be used to suppress supermode noise when the loop works at a higher harmonic.

[00187] This OEO is improved by adding a timing jitter "eater," wherein the derivative of the modulation is imposed on pulse train 2306 by driving phase modulator 212-3 (located after modulator 2302) with the output of photodetector 216. The amplitude is a function of the frequency - and hence spacing - of the pulse train . Moreover, phase modulator 212-3 shifts the carrier frequency of the pulse as a function of the amplitude. As a result, the carrier frequency of each pulse is a function of the spacing between that pulse and its neighbors. Position variations, therefore, are "color coded ." Dispersion element 2304 is inserted into the system to decrease the variations in pulse spacing, which effectively decreases timing jitter.

[00188] FIG . 24 depicts the optoelectronic feedback path of a feed-forward version of system 2300. System 2400 comprises feedback path 2402, which includes phase modulator 212, amplifier 208, photodetectors 216-1 and 216-2, and dispersion element 2304.

[00189] As in system 2300, changes in timing between the pulses of pulse train 2306 manifest as a change in the derivative of the applied phase to the pulse train by phase modulator 212. Phase modulator 212 shifts the carrier frequency of each pulse as a function of its amplitude. As discussed above, the carrier frequency of each pulse is a function of the spacing between itself and its neighbors, giving rise to position variations that are color coded . Dispersion element 2304 decreases the variations in pulse spacing, effectively decreasing timing jitter.

[00190] FIG . 25 depicts a third dispersion-managed microwave frequency generation approach in accordance with the present invention. System 2500 comprises oscillator 202, source 2502, dispersion element 2304, and interleaver 2504. System

2500 is an example of system for reducing timing jitter via a pulse compression approach.

[00191] The operating principle of system 2500 is based on the concept that a pulse can be compressed by a compression factor, A. With this compression factor, timing jitter, At, is also reduced by the same compression factor because random position variations are a function of the compression factor.

[00192] Oscillator 202 is a low-noise oscillator that provides a signal at an oscillation frequency, f_osc, of (20/ A) GHz to source 2502. In exemplary system 2500, fosc is equal to 78 MHz, which is 20 GHz/(28), resulting in a compression factor of 256. One skilled in the art will recognize that the frequency of oscillator 202 is a matter of design choice and that system 2500 can operate at any suitable oscillation frequency.

[00193] Source 2502 is a Fourier-domain mode-locked laser, which provides heavily chirped pulses 2506 to dispersion element 2304.

[00194] Dispersion element 2304 is a chirped grating that compresses pulses

2506 and provides them to interleaver 2504. In some embodiments, dispersion element 2304 is an optical fiber.

[00195] At interleaver 2504, is multiplied by the compression factor (i.e.,

256) giving rise to a pulse-repetition rate of 20 GHz for pulse train 2510.

[00196] Using : At ~ 1/ {f ^■ [ ri_(f) ]⁰-⁵}, where L(f) is the single side-band phase noise, dictates that a phase-noise profile for an electrical signal with a root-mean- square (rms) cycle jitter, At, at frequency fo, is the same as that of a signal with an rms cycle jitter At/A at frequency A*fo. The phase noise of the RF carrier at fRF = 20 GHz, therefore, is the same as for the oscillator frequency, fosc.

[00197] Since lower phase noise oscillators are generally achieved at lower frequencies, this approach significantly reduces phase noise for electrical signals in the microwave frequency range.

[00198] As discussed above and with respect to Fig . 2, the present invention is afforded significant advantages by the use of a resonant cavity that has a quality factor. One skilled in the art will recognize, after reading this Specification, that there are many potential designs for a low-noise, high-Q resonant cavity suitable for use in embodiments of the present invention, several of which are provided below.

High-0 Resonant Cavity Designs

[00199] FIG. 26 depicts a schematic drawing of a first high-Q resonant cavity in accordance with the present invention. Cavity 2600 includes waveguide rings 2602-1 and 2602-2 and bus waveguides 2604 and 2606.

[00200] Waveguide rings 2602-1 and 2602-2 collectively define a compound two-ring structure, which is particularly well suited for use inside a laser cavity as a comb filter (e.g., the linear resonant cavity discussed above and with respect to system 2000).

[00201] The design of cavity 2600 affords embodiments of the present invention with significant advantages over the prior art. For example, cavity 2600 provides a large envelope FSR while maintaining the high quality factor resonance peaks normally achieved using large ring diameters. The envelope FSR for two coupled rings is determined by the expression: FSR_env = -^, where c is the light speed in vacuum, n is the group refractive index of the ring waveguide (assuming a uniform waveguide width for the two rings) and ΔΙ_ is the difference in the perimeters of the two rings. The FSR corresponding to the output RF frequency in COEO architectures, such as system 2000, can be controlled between processing cycles using lithography instead of monitoring material

quality/uniformity.

[00202] In addition, cavity 2600 provides a large tuning range for FSR. As compared to a single-ring resonator having an FSR equal to c/nL, the compound ring has a tunable range that is larger by a factor of L/AL. This enhancement alleviates a required change in refractive index. Common tuning approaches, such as thermal and electrical tuning, do not provide a large change of refractive index on the conventional silica-based PLC platform.

[00203] Further, active alignment of cavity 2600 is not necessary when it is included in a COEO architecture because its resonance peaks are self-aligned and do not require tuning. [00204] FIG. 27 depicts a schematic drawing of a second high-Q resonant cavity in accordance with the present invention. Cavity 2700 includes waveguide rings 2702-1 and 2702-2 and bus waveguides 2704 and 2706.

[00205] Waveguide rings 2702-1 and 2702-2 are identical ring resonators coupled to each other through bus waveguides 2704 and 2706. By matching the path length of the waveguide between the resonators, with respect to the ring, destructive interference is achieved at the drop port. This resonance can be made significantly more narrow than that attainable using a single ring and two bus waveguides.

[00206] It is yet another aspect of the present invention that, although electromagnetic-induced transparency (EIT)-like extinction phenomenon in coupled ring resonators would normally lead one away from the use of coupled rings in such a manner, the use of cavity 2700 in OEO architectures provides significant advantages over microwave-frequency generators known in the prior art. First, quality-factor enhancement derived from the two-ring resonator structure provides a reduction in noise (o V ₂)_"

Second, the drop port can be simultaneously utilized for enhanced frequency stability using the Pound-Drever-Hall (PDH) technique - in contrast to single-ring resonator designs with through and drop ports.

Integrated Microwave Isolator Designs

[00207] Some embodiments of the present invention, including several described above, include ring-based configurations that exploit the use of timed gating to ensure unidirectional operation. Although myriad implementations of this approach can be contemplated within the scope of the present invention, several exemplary designs are provided here.

[00208] FIGS. 28A-B depict schematic drawings of basic implementations of an optical microwave pulse isolator based on timed gating.

[00209] In each of isolator designs 2800 and 2802, two gates (e.g. two saturable absorbers, two modulators, or a combination thereof) are driven by an external RF source, synchronized to the photonic microwave or pulse frequency. The phase of the two driving signals and the relative position of the modulators are substantially optimized to give rise to unidirectional transmission. In some embodiments, the RF driving signal is generated by a feedback signal in, for example, a COEO architecture. [00210] FIGS. 28C-D depict schematic drawings of isolator configurations in which no external RF driver is required. Each of isolator designs 2804 and 2806 exploits the fact that an absorber acts as a photodetector. This signal can be amplified if necessary and then be fed back to a properly placed modulator (or other saturable absorber). It should be noted that electrical isolation is required between the two gating elements.

[00211] FIG. 28E depicts a schematic drawing of an isolator configuration that contains a narrow-band microwave filter. Isolator design 2808 is particularly well suited for suppression of super-mode noise.

[00212] FIG. 28F depicts a schematic drawing of an isolator configuration wherein the saturable absorber is replaced by a photodetector. The conceptual difference is that the saturable absorber is designed to transmit part of the field, whereas the

photodetector is assumed to absorb all incoming light. Isolator 2810 preferably includes a tap waveguide. In principle isolator 2810 is equivalent to an OEO having a very short feedback loop.

[00213] It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.

Claims

What is claimed is:

1. A microwave-frequency generator, the microwave-frequency generator comprising :

a resonant cavity (110), the resonant cavity being characterized by a quality factor equal to or greater than lxlO⁶;

a first active-photonics device (210); and

a first RF-electronics device (208) that is operatively coupled with the first active- photonics device;

wherein the microwave-frequency generator is characterized by the resonant cavity including a first waveguide (224), and wherein the first silicon-photonics device is optically coupled with the first waveguide.

2. The microwave-frequency generator of claim 1, wherein the first silicon-photonics device and the first waveguide are monolithically integrated on a first substrate.

3. The microwave frequency generation of claim 2, wherein the first RF-electronics device is disposed on the first substrate.

4. The microwave-frequency generator of claim 1, wherein the first waveguide is an ultra-low-loss waveguide.

5. The microwave-frequency generator of claim 1, wherein the resonant cavity comprises a resonant device selected from the group consisting of a ring resonator (226) and a linear resonant cavity (2006).

6. The microwave-frequency generator of claim 1 further comprising an RF filter (602) that includes a waveguide ring (604), wherein the microwave-frequency generator provides a substantially square-wave output signal.

7. The microwave-frequency generator of claim 6 further comprising a coherent receiver (906) that is optically coupled with the RF filter, the coherent receiver being operative for providing a square wave signal.

8. The microwave-frequency generator of claim 6 further comprising a limiting amplifier that is operatively coupled with the RF filter.

9. The microwave-frequency generator of claim 1, wherein the microwave- frequency generator is characterized by an architecture selected from the group consisting of an OEO architecture and a COEO architecture.

10. The microwave-frequency generator of claim 1 further comprising a source, wherein the source includes a device selected from the group consisting of a laser (210), a semiconductor optical amplifier (1002), an Erbium-doped waveguide amplifier (1702), and a Brillouin laser (.

11. The microwave-frequency generator of claim 1 further comprising a first waveguide ring (1404) having a first free-spectral range that substantially matches the frequency of the output signal (1406) of the microwave-frequency generator.

12. The microwave-frequency generator of claim 1, wherein the resonant cavity includes a first waveguide ring (224) and a second waveguide ring (604).

13. The microwave-frequency generator of claim 12, wherein the resonant cavity includes a TISO (1102) comprising a first modulator and a second modulator, and wherein the TISO is dimensioned and arranged such that the first modulator and a second modulator are driven sequentially and the resonant cavity lases in only one direction.

14. The microwave-frequency generator of claim 12 further comprising a saturable absorber (1502) and an SOA (1002), wherein the first waveguide ring, second waveguide ring, SOA, and saturable absorber are arranged such that optical pulses provided by the SOA collide in the saturable absorber.

15. The microwave-frequency generator of claim 12, wherein the second waveguide ring (604) is operative as a ring-based isolator.

16. The microwave-frequency generator of claim 1, further comprising :

a first substrate (2002) that includes the first silicon-photonics device; and a second substrate (2004) that includes the first waveguide;

wherein the resonant cavity is a linear resonant cavity (2006), and wherein the linear resonant cavity comprises;

a first mirror (2024) that is defined by a first facet of a first substrate, the first facet being coated with a reflective coating; and

a second mirror comprising a ring resonator (2030), the ring resonator having a drop port (2032) that is optically coupled with the first substrate such that optical power at the drop port is directed to the first mirror.

17. A method for forming a microwave-frequency signal generator, the method comprising :

providing a resonant cavity ( 110) having a quality factor equal to or greater than lxlO⁶, wherein the resonant cavity includes a first waveguide (224) ;

providing a first silicon-photonics device (210) that is optically coupled with the first waveguide; and

providing a first RF-electronics device (208) that is operative coupled with the first silicon-photonics device;

wherein the first waveguide and the first silicon-photonics device are monolithically integrated on a first substrate ( 102) .

18. The method of claim 17, wherein the first RF-electronics device is disposed on the first substrate.

19. The method of claim 17, wherein the resonant cavity is provided such that it comprises an RF filter (602) that includes a waveguide ring (604) such that the microwave- frequency generator provides a substantially square-wave output signal.

20. The method of claim 17, wherein the resonant cavity is provided such that it comprises a first waveguide ring (224), a second waveguide ring (604), and a TISO ( 1102) comprising a first modulator and a second modulator, and wherein the TISO is dimensioned and arranged such that the first modulator and a second modulator are driven sequentially such that the resonant cavity lases in only one direction.