Patent Grant
9450673
The field of the present invention relates to generating microwave-frequency electrical signals and microwave-frequency sources utilizing a dual optical-frequency source and optical frequency division. In particular, apparatus and methods are described herein for generating microwave-frequency electrical signals exhibiting reduced phase noise relative to a dual optical-frequency reference source.
Subject matter disclosed or claimed herein may be related to subject matter disclosed in:
Pan, S., and Yao, J., “A wavelength-switchable single-longitudinal-mode dual-wavelength erbium-doped fiber laser for switchable microwave generation,” Opt. Express 17, 5414-5419 (2009);
A microwave-frequency source for generating an output electrical signal at an output frequency fM comprises: a dual optical-frequency reference source, an electro-optic sideband generator, an optical bandpass filter, an optical detector, a reference oscillator, an electrical circuit, and a voltage-controlled electrical oscillator. The dual optical-frequency reference source is arranged so as to generate (i) a first optical reference signal at a first optical reference frequency v1 and (ii) a second optical reference signal at a second optical reference frequency v2>v1. The electro-optic sideband generator is arranged so as to (i) receive the first and second optical reference signals and a sideband generator input electrical signal at the frequency fM and (ii) generate therefrom multiple sideband optical signals at respective sideband optical frequencies of the form v1±n1fM and v2±n2fM, wherein n1 and n2 are integers. The optical bandpass filter is arranged so as to transmit a subset of the multiple sideband optical signals including the sideband optical signal at a frequency v1+N1fM and the sideband optical signal at a frequency v2−N2fM, wherein N1 and N2 are integers. The optical detector is arranged so as to (i) receive the transmitted sideband optical signals and (ii) generate an optical detector electrical signal at a beat frequency fBEAT=(v2−N2fM)−(V1+N1fM). The reference oscillator is arranged so as to generate a reference oscillator electrical signal at a reference oscillator frequency fR. The electrical circuit is arranged so as to (i) receive the optical detector electrical signal and the reference oscillator electrical signal, (ii) generate therefrom, using a comparator portion of the electrical circuit, an electrical error signal dependent on relative phase of the optical detector and reference oscillator electrical signals, and (iii) process the electrical error signal using a loop-filter portion of the electrical circuit. The voltage-controlled electrical oscillator is arranged so as to (i) receive the loop-filtered electrical error signal as a VCO input electrical signal and (ii) generate a VCO output electrical signal at the frequency fM, wherein a first portion of the VCO output electrical signal is received by the electro-optic sideband generator as the sideband generator input electrical signal and a second portion of the VCO output electrical signal forms the output electrical signal of the microwave-frequency source. Reception of the first portion of the VCO output electrical signal by the electro-optic sideband generator as the sideband generator input electrical signal results in the electrical circuit and the voltage-controlled oscillator being coupled in a negative feedback arrangement so as to function as a phase-locked loop.
Using the microwave-frequency source, a method for generating a microwave-frequency output electrical signal at the output frequency fM comprises: (a) generating the first and second optical reference signals; (b) generating the multiple sideband optical signals; (c) transmitting a subset of the multiple sideband optical signals including the sideband optical signals at frequencies v1+N1fM and v2−N2fM; (d) generating the optical detector electrical signal at the beat frequency fBEAT=(v2−N2fM)−(v1+N1fM); (e) generating the reference oscillator electrical signal; (f) generating the loop-filtered; and (g) generating the VCO output electrical signal at the frequency fM.
Objects and advantages pertaining to microwave-frequency sources and dual-frequency optical sources, and methods for their use, may become apparent upon referring to the example embodiments illustrated in the drawings and disclosed in the following written description or appended claims.
This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.
The embodiments depicted are shown only schematically: all features may not be shown in full detail or in proper proportion, certain features or structures may be exaggerated relative to others for clarity, and the drawings should not be regarded as being to scale. The embodiments shown are only examples: they should not be construed as limiting the scope of the present disclosure or appended claims.
Microwave- and radio-frequency oscillators are widely used in communications, remote sensing, navigation, radar, scientific measurements, computers, clocks, time standards, and in other areas. They provide a single electrical frequency that can often be controllably tuned. Their performance is characterized using various metrics including power output, frequency tuning range, and frequency stability. The last of these metrics, frequency stability, is crucial to most applications, and the cost of an oscillator is directly related to the level of frequency stability that it provides. The very highest performance oscillators have typically been based on electrical oscillators that employ high-Q dielectric resonators to create a highly stable frequency. Optical detection of two highly coherent laser signals is another well-known approach to generate a stable radio-frequency or microwave-frequency signal; two optical signals coincident on an optical detector (i.e., a photodetector) and having a frequency difference within the detection frequency bandwidth of the optical detector produce an electrical output signal from the optical detector at the optical difference frequency of the optical signals, also referred to as a beat note or a beat frequency. A newer approach developed over the last few years employs a revolutionary, all-optical approach to microwave synthesis using a device known as a frequency-comb optical divider (or simply “optical divider”). Optical dividers accept as an input a highly coherent optical signal that oscillates at 100 s of THz. This is normally a laser signal that has been stabilized by an optical reference cavity. This very high input frequency is divided down to a rate in the radio-frequency or microwave-frequency range (100 s of MHz or 100 s of GHz) using the optical divider. This division process is accompanied by a dramatic reduction of the phase noise in the final signal relative to the initial optical signal thereby endowing the radio/microwave signal with remarkable (and record) stability. The frequency-comb optical divider employs a special mode locked laser to achieve this reduction.
Disclosed herein are novel, inventive apparatus and methods for achieving optical frequency division and stable operation of a microwave-frequency signal provided by an electrical oscillator having a frequency control input (sometimes called a voltage-controlled oscillator, or VCO). This novel approach is described herein along with preliminary measurements that demonstrate the feasibility of the method. In the inventive approach two reference optical signals, provided by lasers, are frequency-stabilized so that the relative frequency of the lasers (i.e., their difference frequency) is as stable as possible (or practicable to achieve performance necessary for a given use or application, i.e., relatively stabilized within an operationally acceptable reference bandwidth). The laser signals are then phase modulated using a cascade of phase modulators that are driven by the VCO. In the optical spectrum, this creates a spectrum of sidebands on the initial laser frequencies spaced by the modulation frequency. The phase-modulated optical signals can be spectrally broadened to further increase number of sidebands in the side-band frequency spectrum, using an intensity modulator, a dispersion compensator, an optical amplifier (if needed or desired), and a nonlinear optical medium. It is desirable for the frequency separation of the lasers be as large as practicable (for a given use or application) to provide the maximum practicable stabilization of the VCO. However, the frequency separation cannot exceed the range of sidebands generated by the phase modulation cascade (and nonlinear optical broadening, if employed). The two innermost sidebands (see
Compared to a conventional frequency-comb optical divider, the inventive method disclosed herein has not yet attained the same, record frequency stability. However, improvements in pulse-broadening methods described above should result in performance improvements beyond those already demonstrated, since phase noise is reduced by the square of the division ratio and larger difference frequency results in a larger division ratio. Also, offsetting the current performance limitation, the inventive approach does not require a mode-locked frequency comb generator, which is a sophisticated and costly device. Instead, the inventive method employs relatively simpler and less costly optical components, most of which are available commercially. Also, in many examples the reference frequency (i.e., the difference frequency of two optical sources) depends on the relative stability of two resonances within a single resonator. This is, in principle, a more robust reference as technical noise in the system (i.e., arising from instabilities in equipment, as opposed to quantum noise inherent in the physics of the system) is common to both resonances and therefore largely cancelled-out in the difference frequency. In contrast, the conventional divider approach relies upon an absolute reference frequency, which is more strongly impacted by technical noise. The principle of operation of the inventive optical frequency divider disclosed herein is also different in that the repetition frequency (i.e., sideband spacing) is set by an electrical VCO as opposed to an optical cavity. As a result it is possible to tune the microwave frequency of the output signal, which is not readily achieved with a conventional frequency-comb optical divider. Also, the conventional frequency-comb divider approach relies upon optical detection of a train of high peak power pulses of light with high bandwidth. The linearity of the photodetection process has been shown to be crucial to attaining frequency stability using this approach and greatly restricts the types of optical detectors that may be employed. In contrast the present invention can employ lower-bandwidth optical detectors or detectors with relaxed linearity requirements.
The inventive apparatus and methods disclosed herein for achieving optical frequency division for high-performance microwave-frequency signal generation employ cascaded phase modulation comprising direct phase modulation and also self-phase modulation (if needed or desired to achieve larger division ratios than can be achieved using direct phase modulation alone). In contrast to a comb of spectral lines produced by a mode-locked laser, cascaded phase modulators do not have an intrinsic repetition frequency since there is no optical cavity. While this can endow the cascade-generated comb of sidebands with an arbitrarily chosen line spacing frequency, it also means that optical frequency division must be accomplished in a fashion different from that used with conventional frequency combs.
It is interesting to contrast the inventive microwave-frequency source with a more conventional microwave-frequency source employing a frequency reference 10 and a VCO 30. In the conventional approach (see
The use of a frequency separation as opposed to an absolute frequency to derive a reference for microwave generation has been demonstrated using conventional mode-locked laser frequency combs. In those types of devices the frequency-comb optical divider is implemented by locking a comb at two frequencies of a reference cavity. As in other conventional optical dividers, detection of the comb provides a divided-down signal, in this case of the frequency separation of the reference cavity modes. This two-point lock approach has also been implemented previously in using two atomic lines and a frequency microcomb.
An example of a dual optical-frequency reference source is shown in more detail in
Dual pumping of fiber-based SBS lasers in a single, common FLC previously has been applied to generate stable microwave-frequency signals by conventional optical detection of the optical signals, with the two pump laser wavelengths being the sidebands of a single continuous-wave (CW) laser generated by electro-optic phase modulation at the desired microwave frequency. In that previous arrangement the two SBS laser frequencies are by necessity sufficiently close in frequency that their difference frequency (i.e., beat note) is directly detectable using an optical detector. In contrast, in the inventive arrangement the SBS laser frequencies v2 and v1 are separated at much larger frequency separations (e.g., 109, 198, 327, and 1612 GHz in various experiments, by dual pumping on disk resonator optical modes separated by 10, 18, 30 and 148 cavity FSR using independently tunable CW lasers). These increasing frequency separations increase the phase noise reduction by optical frequency division by increasing division factors; it is therefore desirable that the frequency separation (i.e., the difference frequency v2−v1) be made as large as possible or practicable. Such large frequency separations are typically so large that it is not possible to optically detect the difference frequency (i.e., the beat note) because it greatly exceeds the detection bandwidth of optical detectors. The maximum separation of 1612 GHz in embodiments constructed thus far is limited by one of the SBS pump lasers being at 1537 nm, near the edge of the gain bandwidth of the erbium-doped fiber amplifier employed; other pump sources can be employed to achieve greater spectral separation of the optical reference signals 120a/120b. Measured optical spectra of the dual SBS lines at some of these spectral separations is shown in
In the examples of
To further enhance the sideband spectral width, additional phase modulators can be used, or phase modulators providing larger phase modulation amplitude can be used. Alternatively, or in addition, so-called continuum generation (e.g., as described in the publications of Huang et al (2008), Morohashi et al (2009), Ishizawa et al (2013, and Suzuki et al (2013), incorporated above) can be employed (path II in
The two transmitted optical sidebands 430a/430b (at respective frequencies v1+N1fM and v2−N2fM, separated by &that is typically <<fM) are transmitted by the optical bandpass filter 410 and are detected using an amplified optical detector 420 (125 MHz bandwidth in this example). Linear superposition of those transmitted sideband signals results in a signal incident on the optical detector 420 that produces an optical detector electrical signal at the beat note frequency fBEAT=(v2−N2fM)−(V1+N1fM). A reference oscillator 510 generates a reference oscillator electrical signal at frequency fR (a quartz oscillator operating at about 10 MHz in this example; any suitable reference oscillator can be employed; frequencies fR from about 1 MHz to about 500 MHz have been employed). The phase noise contributed by the reference oscillator 510 is also reduced by the overall frequency division and does not present any limit on the phase noise reduction achievable by the optical frequency division process. An electrical circuit 520 compares the phases of the photodiode and reference electrical signals and generates an error signal (in any needed, desired, or suitable way), which is then loop-filtered by the circuit 520 (in any needed, desired, or suitable way). The loop-filtered error signal serves as a VCO electrical input signal 530 used for phase-locking the VCO 600 to a subharmonic (i.e., integer submultiple) of the frequency v2−v1±fR. In the example the VCO comprises a microwave generator operated using the external FM modulation input mode; any suitable type or implementation of a VCO can be employed.
To establish the phase noise level of the Brillouin lasers (the phase noise reference level), the uppermost curve in
To illustrate the improvement in the performance of the VCO phase noise under phase-locked conditions, the dashed curve in
In the examples shown so far the microwave frequency fM has been fixed; however, that frequency can be adjusted by varying the division ratio. Specifically, by changing the division ratio N1+N2 and VCO frequency fM for a fixed separation of the dual SBS lines v2−v1, phase locked frequency synthesis of different carrier frequencies (e.g., fM=(v2−v1±fR)/(N1+N2)) is possible. For example, in
Referring to
The reference oscillator 510 is arranged so as to generate a reference oscillator electrical signal at a reference oscillator frequency fR. The electrical circuit 520 is arranged so as to (i) receive the optical detector electrical signal 440 and the reference oscillator electrical signal, (ii) generate therefrom, using a comparator portion of the electrical circuit 520, an electrical error signal dependent on relative phase of the optical detector and reference oscillator electrical signals, and (iii) process the electrical error signal using a loop-filter portion of the electrical circuit 520. The voltage-controlled electrical oscillator 600 is arranged so as to (i) receive the loop-filtered electrical error signal 530 as a VCO input electrical signal and (ii) generate a VCO output electrical signal at the frequency fM, wherein a first portion 620 of the VCO output electrical signal is received by the electro-optic sideband generator 200/300 as the sideband generator input electrical signal and a second portion 610 of the VCO output electrical signal forms the output electrical signal of the microwave-frequency source. Reception of the first portion 620 of the VCO output electrical signal by the electro-optic sideband generator 200/300 as the sideband generator input electrical signal results in the electrical circuit 520 and the voltage-controlled oscillator 600 being coupled in a negative feedback arrangement so as to function as a phase-locked loop (PLL). Comparator and loop-filter portions of the electrical circuit 520 can be implemented in any needed, desired, or suitable way.
A method for generating the microwave-frequency output electrical signal 610 at the output frequency fM comprises: (a) generating the first and second optical reference signals 120a/120b using the dual optical-frequency reference source 100; (b) generating the multiple sideband optical signals 210/310 using the electro-optic sideband generator 200/300; (c) transmitting the sideband optical signals 430a/430b using the optical bandpass filter 420; (d) generating the optical detector electrical signal 440 using an optical detector 420; (e) generating the reference oscillator electrical signal using the reference oscillator 510; (f) generating the loop-filtered electrical error signal 530 using the electrical circuit 520; and (g) using the voltage-controlled electrical oscillator 600 to generate the VCO output electrical signal 610/620. Reception of the first portion 620 of the VCO output electrical signal by the electro-optic sideband generator 200/300 as the sideband generator input electrical signal results in the electrical circuit 520 and the voltage-controlled oscillator 600 being coupled in a negative feedback arrangement so as to function as a phase-locked loop.
The output frequency fM can lie anywhere within the so-called microwave portion of the electromagnetic spectrum, e.g., between about 0.3 GHz and about 300 GHz. In some examples the output frequency fM is between about 1 GHz and about 100 GHz.
As illustrated schematically in
It is therefore advantageous to increase the division ratio to reduce phase noise of the generated microwave-frequency electrical signal. One way to achieve a higher division ratio is to increase the reference difference frequency v2−v1. In some examples, the reference difference frequency v2−v1 is greater than about 100 GHz. In some of those examples, the reference difference frequency v2−v1 is greater than about 1 THz, greater than 10 THz, or greater than 100 THz. Still larger reference difference frequencies can be employed as needed, desired, or suitable, or as suitable optical sources are developed or become available.
Depending on the reference difference frequency vz−v1 and the desired output frequency fM, any suitable division ratio N1+N2 can be employed. In some examples the division ratio N1+N2 is greater than or equal to 10, greater than or equal to 50, greater than or equal to 100, or greater than or equal to 1000. As noted above, a greater division ratio provides greater reduction of phase noise.
The reference optical signals 120a/120b can be provided at any needed, desired, or suitable optical frequencies. In some examples the, first and second optical reference frequencies v1 and v2 are each between about 75 THz and about 750 THz (i.e., wavelengths between about 400 nm and about 4 μm), between about 120 THz and about 430 THz (i.e., wavelengths between about 700 nm and about 2.5 μm), or between about 150 THz and about 300 THz (i.e., between about 1 μm and about 2 μm). The latter two ranges can be convenient due to ready availability of fiber-optic and/or solid state sources in those wavelength regions. Other wavelengths can be employed.
In some examples, the reference oscillator frequency and the beat frequency can be between about 1 MHz and about 500 MHz, between about 5 MHz and about 100 MHz, or between about 10 MHz and about 50 MHz. In some examples, the reference oscillator 510 comprises a crystal oscillator, e.g., a quartz oscillator. In some other examples, the reference oscillator 510 comprises an electrical oscillator, e.g., a frequency-synthesized oscillator. Any suitably stable reference oscillator can be employed that provides sufficiently stable (i.e., operationally acceptable) performance of the microwave-frequency source in a given use or application.
In some examples, the dual optical-frequency reference source 100 is stabilized so as to maintain fluctuations of a reference difference frequency vz−v1 (i.e., relative frequency stability of the lasers) within an operationally acceptable optical reference bandwidth. In some examples, the operationally acceptable reference bandwidth (given as a bandwidth characterized over a given time interval) is less than about 100 Hz over about a 1 second timescale, or less than about 1 Hz over about a 1 second timescale. In some examples, the operationally acceptable reference bandwidth (given as optical phase noise at a given offset frequency relative to the optical carrier frequency) is about −40 dBc/Hz at 100 Hz offset frequency and about −80 dBc/Hz at 10 kHz offset frequency, or about −80 dBc/Hz at 100 Hz offset frequency and about −125 dBc/Hz at 10 kHz offset frequency. Generally, improved levels of relative stability of the laser sources will translate directly into improvements in the overall frequency stability of the microwave-frequency output signal. Still better stabilized references can be employed as needed, desired, or suitable, or as suitable optical sources are developed or become available.
As noted above, in some examples the dual optical-frequency reference source 100 comprises first and second pump laser sources 130a/130b and an optical resonator 110 (a disk resonator;
Instead of an optical resonator acting as a dual-pumped stimulated Brillouin laser (SBL), in some other examples the dual optical-frequency reference source 100 comprises a dual-mode laser source. In still other examples the dual optical-frequency reference source 100 comprises first and second reference laser sources that are each frequency-locked to a corresponding distinct resonant optical mode of a common optical reference cavity 180 (
As noted above, in some examples the electro-optic sideband generator comprises only two or more electro-optic phase modulators 220/230 (path I of
In other examples, the conventional technique of continuum generation is included in the portion 300 of the electro-optic sideband generator (path II of
As already noted, optical frequency division (OFD) now enables the generation of ultra-stable microwave-frequency signals with record low phase noise. A key ingredient for OFD is a stable optical frequency reference (OFR). Optical references can be of two types: an absolute frequency reference or a difference-frequency reference. The latter type of reference is well suited for application with the optical frequency divider described herein. The inventive dual-frequency optical source illustrated schematically in
Key components for microwave-frequency signal generation using the OFD approach are the stable optical frequency reference and the optical frequency divider. An inventive electro-optic frequency divider was disclosed above. Conventional, stable optical frequency references used in various OFD approaches include cavity-stabilized lasers with sub-Hertz stability, or two lasers with exceptional relative frequency stability by frequency-locking to a single, common reference optical cavity 180 (
In the inventive dual-frequency optical source 100, we transform the conventional role of the optically detected dual-mode lasers (bandwidth up to 100 GHz) to stable optical reference references, with much larger frequency separation in a range between about 300 GHz to about 100 THz or even higher. The reference difference frequency produced by the inventive dual-mode stimulated Brillouin lasers (SBL) can then be divided down to microwave frequencies using optical frequency dividers (such as the inventive electro-optic frequency divider described above, or a conventional mode-locked laser frequency comb). The phase noise of the generated microwave-frequency signal is also divided from that of the relative stability between the dual mode SBLs by 20 log10 N dB, where N is the division factor. The phase noise of the optically divided microwave-frequency signal can be greatly reduced relative to that of the dual-frequency optical-frequency reference, which as noted above has low phase noise itself. Optical frequency division is a relatively new approach to microwave generation. Frequency references, both absolute or difference-based, have typically used passive Fabry-Perot resonators, which are bulky and delicate devices.
As already described above and shown in
A method for generating first and second optical output signals 120a/120b comprises simultaneously pumping the optical resonator 110/110′ with optical pump power from first and second pump laser sources 130a/130b.
In some examples of an inventive dual-frequency optical source, the free spectral range (FSR) of the optical resonator 110/110′ is substantially equal to the Brillouin shift frequency of the optical resonator. In some examples, the optical resonator comprises silica and the Brillouin shift frequency of the optical resonator is about 10.9 GHz; other needed, desired, or suitable materials can be employed that exhibit different Brillouin shift frequencies.
The reference optical signals 120a/120b can be provided at any needed, desired, or suitable optical frequencies. In some examples the, first and second optical reference frequencies v1 and v2 are each between about 75 THz and about 750 THz (i.e., wavelengths between about 400 nm and about 4 μm), between about 120 THz and about 430 THz (i.e., wavelengths between about 700 nm and about 2.5 μm), or between about 150 THz and about 300 THz (i.e., between about 1 μm and about 2 μm). The latter two ranges can be convenient due to ready availability of fiber-optic and/or solid state sources in those wavelength regions. Other wavelengths can be employed.
In some examples, the dual optical-frequency reference source 100 is stabilized so as to maintain fluctuations of a reference difference frequency v2−v1 (i.e., relative frequency stability of the lasers) within an operationally acceptable optical reference bandwidth. In some examples, the operationally acceptable reference bandwidth (given as a bandwidth characterized over a given time interval) is less than about 100 Hz over about a 1 second timescale, or less than about 1 Hz over about a 1 second timescale. In some examples, the operationally acceptable reference bandwidth (given as optical phase noise at a given offset frequency relative to the optical carrier frequency) is about −40 dBc/Hz at 100 Hz offset frequency and about −80 dBc/Hz at 10 kHz offset frequency, or about −80 dBc/Hz at 100 Hz offset frequency and about −125 dBc/Hz at 10 kHz offset frequency. Generally, improved levels of relative stability of the laser sources will translate directly into improvements in the overall frequency stability of the microwave-frequency output signal. Still better stabilized references can be employed as needed, desired, or suitable, or as suitable optical sources are developed or become available.
In some examples, each one of the first and second pump laser sources is frequency-locked to the corresponding resonant optical mode of the resonator 110/110′ by a Pound-Drever-Hall mechanism; any suitable mechanism can be employed, e.g., a Hänsch-Couillaud mechanism. In the example of
The use of a fiber-loop optical resonator 110′ enables use of readily available fiber optic components that have been developed for telecommunications-related applications, and also enables a performance benefit in terms of a significantly lower phase noise (up to 40 dB lower) in the frequency reference itself compared with the disk resonator 110. This improvement is believed to arise for two reasons. First, the fiber-loop cavity has a much larger round trip length (on the order of 100 m to 500 m) compared with a disk resonator (round trip length on the order of 10 mm to 100 mm). Consequently, frequency fluctuation arising from random temperature fluctuations of the fiber loop cavity are greatly reduced relative to the disk resonator. Second, the longer cavity length of a fiber-loop cavity results in a correspondingly longer intracavity photon storage time compared to the disk resonator. A longer photon storage time in turn leads to correspondingly lower fundamental Schawlow-Townes frequency noise of the laser.
While dual-pumped Brillouin lasers using optical fiber have been previously demonstrated (as noted above), they have been operated as microwave sources with frequency separation typically less than about 100 GHz and not as difference frequency references with frequency separation greater than about 300 GHz, about 1 THz, about 5 THz, about 10 THz, about 100 THz, or even higher. To the inventors' knowledge, there has been no work demonstrating dual-pumped, dual-Brillouin laser line operation at difference frequencies greater than about 300 GHz; in fact, such large frequency separation would make previous dual-Brillouin lasers unsuitable for their intended purpose. Before development of the optical frequency division techniques disclosed above, there would have been no need to generate dual reference frequencies so widely spaced apart. There also has been no attempt to characterize or to understand operation of the dual-Brillouin laser with this application in mind.
The detailed layout for using the dual-mode SBLs as optical difference frequency references with optical frequency division for stable microwave generation is shown in
A series of FLCs have been constructed, consisting of a 90/10 fused coupler, and different cavity round trip path lengths of 45 meter (FSR 4.4 MHz), 200 meter (FSR 1 MHz) and 500 meter (FSR 400 kHz); any other needed, desired, or suitable fiber-loop cavity length can be employed, e.g., greater than or equal to about 40 meters long, greater than or equal to about 100 meters long, greater than or equal to about 200 meters long, or greater than or equal to about 500 meters long. A longer fiber-loop optical resonator typically exhibits less frequency and phase noise relative to a shorter fiber-loop optical resonator, all other things being equal. The measured optical finesse of these FLCs are around 40 (mainly limited by the coupler loss), and the pump threshold for SBS oscillation is on the order of a few hundred microwatts. Above SBL threshold, the pump power of each pump laser (at 1550 nm wavelength region) is increased until a point is reached (˜1 mW) such that lasing at the first-order Stokes frequency starts to excite second-order Stokes waves in the resonator.
In order to characterize the phase noise of the relative frequency stability of dual-mode SBLs (i.e., stability of the difference frequency v2−v1), the frequency separation of the dual SBLs is tuned to a frequency sufficiently low to be detected directly using a fast photodiode and processed electronically (this is essentially the case of unity division). The detected beat note is set at 20 GHz and its phase noise is measured using a phase noise analyzer (Rohde Schwartz, model number FSUP26). The measured single-sideband (SSB) phase noise spectra of the SBL beat note from the different FLCs are shown in
Furthermore, the measured single-side-band (SSB) phase noise of the difference frequency of the dual-mode SBLs (without any optical frequency division) is already improved compared with previous results. The measured phase noise of the dual-mode SBL beat note (carrier: 20 GHz, offset frequency: 1 kHz) is −95 dBc/Hz (45 m FLC), −101 dBc/Hz (200 m FLC), and −105 dBc/Hz (500 m FLC), as shown in
Finally, the predicted phase noise of the optically-divided microwave signals based on OFD from the dual-mode SBL difference frequency reference (based on a 200m FLC) is also plotted in
The predicted phase noise levels in
In addition to the preceding, the following examples fall within the scope of the present disclosure or appended claims:
A microwave-frequency source for generating an output electrical signal at an output frequency fM, the microwave-frequency source comprising: (a) a dual optical-frequency reference source arranged so as to generate (i) a first optical reference signal at a first optical reference frequency v1 and (ii) a second optical reference signal at a second optical reference frequency v2>v1; (b) an electro-optic sideband generator arranged so as to (i) receive the first and second optical reference signals and a sideband generator input electrical signal at the frequency fM and (ii) generate therefrom multiple sideband optical signals at respective sideband optical frequencies of the form v1±n1fM and v2±n2fM, wherein n1 and n2 are integers; (c) an optical bandpass filter arranged so as to transmit a subset of the multiple sideband optical signals including the sideband optical signal at a frequency v1+N1fM and the sideband optical signal at a frequency v2−N2fM, wherein N1 and N2 are integers; (d) an optical detector arranged so as to (i) receive the transmitted sideband optical signals and (ii) generate an optical detector electrical signal at a beat frequency fBEAT=(v2−N2fM)−(v1+N1fM); (e) a reference oscillator arranged so as to generate a reference oscillator electrical signal at a reference oscillator frequency fR; (f) an electrical circuit arranged so as to (i) receive the optical detector electrical signal and the reference oscillator electrical signal, (ii) generate therefrom, using a comparator portion of the electrical circuit, an electrical error signal dependent on relative phase of the optical detector and reference oscillator electrical signals, and (iii) process the electrical error signal using a loop-filter portion of the electrical circuit; and (g) a voltage-controlled electrical oscillator arranged so as to (i) receive the loop-filtered electrical error signal as a VCO input electrical signal and (ii) generate a VCO output electrical signal at the frequency fM, wherein a first portion of the VCO output electrical signal is received by the electro-optic sideband generator as the sideband generator input electrical signal and a second portion of the VCO output electrical signal forms the output electrical signal of the microwave-frequency source, (h) wherein reception of the first portion of the VCO output electrical signal by the electro-optic sideband generator as the sideband generator input electrical signal results in the electrical circuit and the voltage-controlled oscillator being coupled in a negative feedback arrangement so as to function as a phase-locked loop.
The microwave-frequency source of Example 1 wherein phase noise of the output electrical signal of the microwave-frequency source is reduced by a factor of about (N1+N2)2 relative to phase noise of a reference difference frequency signal at a reference difference frequency v2−v1 of the dual optical-frequency reference source.
The microwave-frequency source of any one of Examples 1 or 2 wherein the output frequency fM is between about 0.3 GHz and about 300 GHz.
The microwave-frequency source of any one of Examples 1 or 2 wherein the output frequency fM is between about 1 GHz and about 100 GHz.
The microwave-frequency source of any one of Examples 1-4 wherein the reference oscillator frequency and the beat frequency are between about 1 MHz and about 500 MHz.
The microwave-frequency source of any one of Examples 1-4 wherein the reference oscillator frequency and the beat frequency are between about 5 MHz and about 100 MHz.
The microwave-frequency source of any one of Examples 1-4 wherein the reference oscillator frequency and the beat frequency are between about 10 MHz and about 50 MHz.
The microwave-frequency source of any one of Examples 1-7 wherein the reference oscillator comprises a crystal oscillator, e.g., a quartz oscillator.
The microwave-frequency source of any one of Examples 1-7 wherein the reference oscillator comprises an electrical oscillator, e.g., a frequency-synthesized oscillator.
The microwave-frequency source of any one of Examples 1-9 wherein a reference difference frequency v2−v1 is greater than about 100 GHz.
The microwave-frequency source of any one of Examples 1-9 wherein a reference difference frequency v2−v1 is greater than about 1 THz.
The microwave-frequency source of any one of Examples 1-9 wherein a reference difference frequency v2−v1 is greater than about 10 THz.
The microwave-frequency source of any one of Examples 1-9 wherein a reference difference frequency v2−v1 is greater than about 100 THz.
The microwave-frequency source of any one of Examples 1-13 wherein N1+N2 is greater than or equal to 10.
The microwave-frequency source of any one of Examples 1-13 wherein N1+N2 is greater than or equal to 50.
The microwave-frequency source of any one of Examples 1-13 wherein N1+N2 is greater than or equal to 100.
The microwave-frequency source of any one of Examples 1-13 wherein N1+N2 is greater than or equal to 1000.
The microwave-frequency source of any one of Examples 1-17 wherein the first and second optical reference frequencies v1 and v2 are each between about 75 THz and about 750 THz.
The microwave-frequency source of any one of Examples 1-17 wherein the first and second optical reference frequencies v1 and v2 are each between about 120 THz and about 430 THz.
The microwave-frequency source of any one of Examples 1-17 wherein the first and second optical reference frequencies v1 and v2 are each between about 150 THz and about 300 THz.
The microwave-frequency source of any one of Examples 1-20 wherein the dual optical-frequency reference source is stabilized so as to (i) maintain fluctuations of a reference difference frequency v2−v1 within an operationally acceptable optical reference bandwidth or (ii) maintain phase noise of a reference difference frequency signal within an operationally acceptable reference phase noise level.
The microwave-frequency source of Example 21 wherein the operationally acceptable reference bandwidth is less than about 100 Hz over about a 1 second timescale.
The microwave-frequency source of Example 21 wherein the operationally acceptable reference bandwidth is less than about 1 Hz over about a 1 second timescale.
The microwave-frequency source of Example 21 wherein the operationally acceptable reference phase noise level is about −40 dBc/Hz at 100 Hz offset frequency and about −80 dBc/Hz at 10 kHz offset frequency.
The microwave-frequency source of Example 21 wherein the operationally acceptable reference phase noise level is about −80 dBc/Hz at 100 Hz offset frequency and about −125 dBc/Hz at 10 kHz offset frequency.
The microwave-frequency source of any one of Examples 1-25 wherein (i) the dual optical-frequency reference source comprises first and second pump laser sources and an optical resonator, (ii) a free spectral range of the optical resonator is substantially equal to an integer submultiple of a Brillouin shift frequency of the optical resonator, (iii) each one of the first and second pump laser sources is frequency-locked to a corresponding resonant optical mode of the optical resonator, and (iv) the first and second optical reference signals comprise stimulated Brillouin laser output generated by optical pumping of the optical resonator simultaneously by the first and second pump laser sources, respectively.
The microwave-frequency source of Example 26 wherein the free spectral range of the optical resonator is substantially equal to the Brillouin shift frequency of the optical resonator.
The microwave-frequency source of any one of Examples 26 or 27 wherein the optical resonator comprises silica and the Brillouin shift frequency of the optical resonator is about 10.9 GHz.
The microwave-frequency source of any one of Examples 26-28 wherein the optical resonator comprises a ring optical resonator.
The microwave-frequency source of Example 29 wherein the ring optical resonator comprises a disk optical resonator.
The microwave-frequency source of any one of Examples 26-28 wherein the optical resonator comprises a fiber optical resonator.
The microwave-frequency source of Example 31 wherein the fiber optical resonator comprises a fiber Fabry-Perot optical resonator.
The microwave-frequency source of Example 31 wherein the fiber optical resonator comprises a fiber-loop optical resonator.
The microwave-frequency source of any one of Examples 26-33 wherein each one of the first and second pump laser sources is frequency-locked to the corresponding resonant optical mode of the optical resonator by a Pound-Drever-Hall mechanism.
The microwave-frequency source of any one of Examples 1-25 wherein the dual optical-frequency reference source comprises a dual-mode laser source.
The microwave-frequency source of any one of Examples 1-25 wherein the dual optical-frequency reference source comprises first and second reference laser sources, wherein the first and second laser sources are each frequency-locked to a corresponding distinct resonant optical mode of a common optical reference cavity.
The microwave-frequency source of any one of Examples 1-25 wherein the dual optical-frequency reference source comprises first and second reference laser sources, wherein the first and second laser sources are each frequency-locked to a corresponding distinct atomic transition.
The microwave-frequency source of any one of Examples 1-37 wherein (i) the electro-optic sideband generator comprises one or more electro-optic phase modulators each driven by a corresponding portion of the sideband generator input electrical signal at the frequency fM, and (ii) the one or more phase modulators are arranged so as to transmit the first and second optical reference signals so as to generate the multiple optical sideband signals.
The microwave-frequency source of any one of Examples 1-37 wherein (i) the electro-optic sideband generator comprises two or more electro-optic phase modulators each driven by a corresponding portion of the sideband generator input electrical signal at the frequency fM, and (ii) the two or more phase modulators are arranged in series so as to sequentially transmit the first and second optical reference signals so as to generate the multiple optical sideband signals.
The microwave-frequency source of any one of Examples 38 or 39 wherein pairs of sideband optical signals are generated with n1+n2 ranging from 2 up to at least 30.
The microwave-frequency source of any one of Examples 38 or 39 wherein pairs of sideband optical signals are generated with n1+n2 ranging from 2 up to at least 100.
The microwave-frequency source of any one of Examples 1-41 wherein (i) the electro-optic sideband generator comprises one or more electro-optic phase modulators each driven by a corresponding portion of the sideband generator input electrical signal at the frequency fM, an intensity modulator driven by a corresponding portion of the sideband generator input electrical signal at the frequency fM, a dispersion compensator, an optical amplifier, and a nonlinear optical medium, and (ii) the one or more phase modulators, the intensity modulator, the dispersion compensator, the optical amplifier, and the nonlinear optical medium are arranged in series so as to sequentially, in order, transmit the first and second optical reference signals so as to generate the multiple optical sideband signals.
The microwave-frequency source of any one of Examples 38-42 wherein pairs of sideband optical signals are generated with n1+n2 ranging from 2 up to at least 100.
The microwave-frequency source of any one of Examples 38-42 wherein pairs of sideband optical signals are generated with n1+n2 ranging from 2 up to at least 10000.
The microwave-frequency source of any one of Examples 42-44 wherein (i) the intensity modulator comprises an electro-optic Mach-Zehnder modulator, (ii) the dispersion compensator comprises a suitably dispersive optical fiber, (iii) the optical amplifier comprises a doped optical fiber amplifier and (iv) the nonlinear optical medium comprises a nonlinear optical fiber.
A method, using the microwave-frequency source of any one of Examples 1-45, for generating a microwave-frequency output electrical signal at an output frequency fM, the method comprising: (a) using the dual optical-frequency reference source, generating (i) the first optical reference signal at the first optical reference frequency v1 and (ii) the second optical reference signal at the second optical reference frequency v2>v1; (b) using the electro-optic sideband generator, (i) receiving the first and second optical reference signals and the sideband generator input electrical signal at the frequency fM and (ii) generating therefrom the multiple sideband optical signals at respective sideband optical frequencies of the form v1±n1fM and v2±n2fM, wherein n1 and n2 are integers; (c) using the optical bandpass filter, transmitting the subset of the multiple sideband optical signals including the sideband optical signal at a frequency v1+N1fM and the sideband optical signal at a frequency v2−N2fM, wherein N1 and N2 are integers; (d) using the optical detector, (i) receiving the transmitted sideband optical signals and (ii) generating the optical detector electrical signal at the beat frequency fBEAT=(v2−N2fM)−(v1+N1fM); (e) using the reference oscillator, generating the reference oscillator electrical signal at the reference oscillator frequency fR; (f) using the electrical circuit, (i) receiving the optical detector electrical signal and the reference oscillator electrical signal, (ii) using the comparator portion of the electrical circuit, generating, from the optical detector electrical signal and the reference oscillator electrical signal, the electrical error signal dependent on relative phase of the optical detector and reference oscillator electrical signals, and (iii) using the loop-filter portion of the electrical circuit, processing the electrical error signal; and (g) using the voltage-controlled electrical oscillator, (i) receiving the loop-filtered electrical error signal as a VCO input electrical signal and (ii) generating the VCO output electrical signal at the frequency fM, wherein the first portion of the VCO output electrical signal is received by the electro-optic sideband generator as the sideband generator input electrical signal and the second portion of the VCO output electrical signal forms the microwave-frequency output electrical signal, (h) wherein reception of the first portion of the VCO output electrical signal by the electro-optic sideband generator as the sideband generator input electrical signal results in the electrical circuit and the voltage-controlled oscillator being coupled in a negative feedback arrangement so as to function as a phase-locked loop.
A dual-frequency optical source comprising: (a) first and second pump laser sources arranged to generate optical pump power at respective first and second pump laser frequencies vpump1 and vpump2; and (b) an optical resonator characterized by a Brillouin shift frequency vB and a free spectral range that is substantially equal to an integer submultiple of the Brillouin shift frequency, wherein: (c) each one of the first and second pump laser sources is frequency-locked to a corresponding resonant optical mode of the optical resonator; (d) first and second optical output signals of the dual-frequency optical reference source at respective first and second output frequencies v1=vpump1−vB and v2=vpump2−vB comprise stimulated Brillouin laser output generated by simultaneous optical pumping of the optical resonator by the first and second pump laser sources, respectively; and (e) an output difference frequency v2−v1 is greater than about 300 GHz.
The dual-frequency optical source of Example 47 wherein the free spectral range of the optical resonator is substantially equal to the Brillouin shift frequency of the optical resonator.
The dual-frequency optical source of any one of Examples 47 or 48 wherein the optical resonator comprises a ring optical resonator.
The dual-frequency optical source of Example 49 wherein the ring optical resonator comprises a disk optical resonator.
The dual-frequency optical source of any one of Examples 47 or 48 wherein the optical resonator comprises a fiber optical resonator.
The dual-frequency optical source of Example 51 wherein the fiber optical resonator comprises a fiber Fabry-Perot optical resonator.
The dual-frequency optical source of Example 51 wherein the fiber optical resonator comprises a fiber-loop optical resonator.
The dual-frequency optical source of any one of Examples 51-53 wherein the fiber optical resonator includes an optical fiber greater than or equal to about 40 meters long.
The dual-frequency optical source of any one of Examples 51-53 wherein the fiber optical resonator includes an optical fiber greater than or equal to about 100 meters long.
The dual-frequency optical source of any one of Examples 51-53 wherein the fiber optical resonator includes an optical fiber greater than or equal to about 200 meters long.
The dual-frequency optical source of any one of Examples 51-53 wherein the fiber optical resonator includes an optical fiber greater than or equal to about 500 meters long.
The dual-frequency optical source of any one of Examples 47-57 wherein the optical resonator comprises silica and the Brillouin shift frequency of the optical resonator is about 10.9 GHz.
The dual-frequency optical source of any one of Examples 47-58 wherein the output difference frequency v2−v1 is greater than about 1 THz.
The dual-frequency optical source of any one of Examples 47-58 wherein the output difference frequency v2−v1 is greater than about 10 THz.
The dual-frequency optical source of any one of Examples 47-58 wherein the output difference frequency v2−v1 is greater than about 100 THz.
The dual-frequency optical source of any one of Examples 47-61 wherein the first and second optical reference frequencies v1 and v2 are each between about 75 THz and about 750 THz.
The dual-frequency optical source of any one of Examples 47-61 wherein the first and second optical reference frequencies v1 and v2 are each between about 120 THz and about 430 THz.
The dual-frequency optical source of any one of Examples 47-61 wherein the first and second optical reference frequencies v1 and v2 are each between about 150 THz and about 300 THz.
The dual-frequency optical source of any one of Examples 47-64 wherein the dual-frequency optical source is stabilized so as to (i) maintain fluctuations of the output difference frequency v2−v1 within an operationally acceptable optical bandwidth or (ii) maintain phase noise of an optical signal at the output difference frequency within an operationally acceptable reference phase noise level.
The dual-frequency optical source of Example 65 wherein the operationally acceptable bandwidth is less than about 100 Hz over about a 1 second timescale.
The dual-frequency optical source of Example 65 wherein the operationally acceptable bandwidth is less than about 1 Hz over about a 1 second timescale.
The dual-frequency optical source of Example 65 wherein the operationally acceptable reference phase noise level is about −40 dBc/Hz at 100 Hz offset frequency and about −80 dBc/Hz at 10 kHz offset frequency.
The dual-frequency optical source of Example 65 wherein the operationally acceptable reference phase noise level is about −80 dBc/Hz at 100 Hz offset frequency and about −125 dBc/Hz at 10 kHz offset frequency.
The dual-frequency optical source of any one of Examples 65-69 wherein each one of the first and second pump laser sources is frequency-locked to the corresponding resonant optical mode of the resonator by a Pound-Drever-Hall mechanism.
A method for generating first and second optical output signals using any one of Examples 47-70, the method comprising simultaneously pumping the optical resonator with optical pump power from the first and second pump laser sources, wherein: (a) the first and second pump laser sources are arranged to generate optical pump power at respective first and second pump laser frequencies vpump1 and vpump2; (b) the optical resonator is characterized by a Brillouin shift frequency vB and a free spectral range that is substantially equal to an integer submultiple of the Brillouin shift frequency; (c) each one of the first and second pump laser sources is frequency-locked to a corresponding resonant optical mode of the optical resonator; (d) the first and second optical output signals are at respective first and second output frequencies v1=vpump1−vB and v2=vpump2−vB and comprise stimulated Brillouin laser output generated by the simultaneous optical pumping of the optical resonator by the first and second pump laser sources, respectively; and (e) an output difference frequency v2−v1 is greater than about 300 GHz.
The method of Example 71 further comprising, using an optical frequency divider, dividing the output difference frequency by a division factor N.
The method of Example 72 wherein the division factor is greater than or equal to about 10.
The method of Example 72 wherein the division factor is greater than or equal to about 100.
The method of Example 72 wherein the division factor is greater than or equal to about 1000.
The microwave-frequency source of any one of Examples 1-45 wherein the dual optical-frequency reference source comprises the dual-frequency optical source of any one of Examples 47-70.
The method of Example 46 wherein the dual optical-frequency reference source comprises the dual-frequency optical source of any one of Examples 47-70.
The method of Example 77 further comprising performing the method of any one of Examples 71-75.
It is intended that equivalents of the disclosed example embodiments and methods shall fall within the scope of the present disclosure or appended claims. It is intended that the disclosed example embodiments and methods, and equivalents thereof, may be modified while remaining within the scope of the present disclosure or appended claims.
In the foregoing Detailed Description, various features may be grouped together in several example embodiments for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that any claimed embodiment requires more features than are expressly recited in the corresponding claim. Rather, as the appended claims reflect, inventive subject matter may lie in less than all features of a single disclosed example embodiment. Thus, the appended claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate disclosed embodiment. However, the present disclosure shall also be construed as implicitly disclosing any embodiment having any suitable set of one or more disclosed or claimed features (i.e., a set of features that are neither incompatible nor mutually exclusive) that appear in the present disclosure or the appended claims, including those sets that may not be explicitly disclosed herein. In addition, for purposes of disclosure, each of the appended dependent claims shall be construed as if written in multiple dependent form and dependent upon all preceding claims with which it is not inconsistent. It should be further noted that the scope of the appended claims does not necessarily encompass the whole of the subject matter disclosed herein.
For purposes of the present disclosure and appended claims, the conjunction “or” is to be construed inclusively (e.g., “a dog or a cat” would be interpreted as “a dog, or a cat, or both”; e.g., “a dog, a cat, or a mouse” would be interpreted as “a dog, or a cat, or a mouse, or any two, or all three”), unless: (i) it is explicitly stated otherwise, e.g., by use of “either . . . or,” “only one of,” or similar language; or (ii) two or more of the listed alternatives are mutually exclusive within the particular context, in which case “or” would encompass only those combinations involving non-mutually-exclusive alternatives. For purposes of the present disclosure and appended claims, the words “comprising,” “including,” “having,” and variants thereof, wherever they appear, shall be construed as open ended terminology, with the same meaning as if the phrase “at least” were appended after each instance thereof, unless explicitly stated otherwise.
In the appended claims, if the provisions of 35 USC §112(f) are desired to be invoked in an apparatus claim, then the word “means” will appear in that apparatus claim. If those provisions are desired to be invoked in a method claim, the words “a step for” will appear in that method claim. Conversely, if the words “means” or “a step for” do not appear in a claim, then the provisions of 35 USC §112(f) are not intended to be invoked for that claim.
If any one or more disclosures are incorporated herein by reference and such incorporated disclosures conflict in part or whole with, or differ in scope from, the present disclosure, then to the extent of conflict, broader disclosure, or broader definition of terms, the present disclosure controls. If such incorporated disclosures conflict in part or whole with one another, then to the extent of conflict, the later-dated disclosure controls.
The Abstract is provided as required as an aid to those searching for specific subject matter within the patent literature. However, the Abstract is not intended to imply that any elements, features, or limitations recited therein are necessarily encompassed by any particular claim. The scope of subject matter encompassed by each claim shall be determined by the recitation of only that claim.
This application claims benefit of (i) U.S. provisional App. No. 61/931,060 entitled “Cascaded Optical Divider and Microwave Synthesizer” filed Jan. 24, 2014 in the names of Kerry Vahala, Scott Diddams, Jiang Li, Xu Yi, and Hansuek Lee, and (ii) U.S. provisional App. No. 61/982,749 entitled “Dual SBS Lasers as Frequency References for Stable Microwave Generation by Optical Frequency Division” filed Apr. 22, 2014 in the names of Jiang Li and Kerry Vahala. Both of said provisional applications are hereby incorporated by reference as if fully set forth herein.
This invention was made with government support under Grant No. FA9550-10-1-0284 awarded by the Air Force, under Grant No. PHY1225565 awarded by the National Science Foundation, and under Grant No. W31P4Q14-1-0001 awarded by the U.S. Army Aviation and Missile Research, Development, and Engineering Center. The government has certain rights in the invention.
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