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US-12627304-B1 - Stabilized microwave-frequency source incorporating a tunable voltage-controlled oscillator

US12627304B1US 12627304 B1US12627304 B1US 12627304B1US-12627304-B1

Abstract

A tunable voltage-controlled oscillator (VCO), a dual optical-frequency reference source, an electrical reference oscillator, and an electro-optic frequency divider (eOFD) form a microwave-frequency source. The VCO generates output and modulating signals at frequency f M that is tunable over at least ±5% or more of a nominal microwave frequency. From the VCO signal at f M and output from the dual optical-frequency reference source at frequencies v 1 and v 2 , the eOFD generates an eOFD signal at δf=|v 2 −v 1 −n·f M | from which a loop-filtered error signal is derived, dependent on relative phase of the eOFD signal and a reference oscillator signal. The error signal is applied to the VCO to function as a phase-locked loop to stabilize the frequency f M , which can exhibit phase noise across the tuning range that is −120 dBc/Hz+10*log 10 (f M /10 GHZ), or less, at 10 kHz offset frequency.

Inventors

  • Jiang Li
  • Kerry VAHALA

Assignees

  • HQPHOTONICS INC.

Dates

Publication Date
20260512
Application Date
20240318

Claims (20)

  1. 1 . A microwave-frequency source comprising: (a) a tunable voltage-controlled electrical oscillator (VCO) arranged so as to generate a VCO output electrical signal at a frequency f M , a first portion of which forms an output electrical signal of the microwave-frequency source, the VCO being tunable over a tuning range for the frequency f M that spans at least ±5% of a nominal microwave frequency, the nominal microwave frequency being between 1 GHz and 100 GHz; (b) a dual optical-frequency reference source arranged so as to generate (i) a first optical reference signal at a first optical reference frequency v 1 and (ii) a second optical reference signal at a second optical reference frequency v 2 that is greater than v 1 ; (c) an electrical reference oscillator arranged so as to generate a reference oscillator electrical signal at a reference oscillator frequency δf REF ; and (d) an electro-optic frequency divider (eOFD) structured and connected so as to (i) receive the first and second optical reference signals, a second portion of the VCO output electrical signal, and the reference oscillator electrical signal, (ii) generate from the first and second optical reference signals and the second portion of the VCO output electrical signal an eOFD electrical signal at a frequency δf=|v 2 −v 1 −n·f M | where n is an integer, (iii) generating a loop-filtered electrical error signal dependent on relative phase of the reference oscillator and eOFD electrical signals, and (iv) applying the loop-filtered electrical error signal to the VCO to couple the VCO and the eOFD in a negative feedback arrangement so as to function as a phase-locked loop to stabilize the frequency f M .
  2. 2 . The microwave-frequency source of claim 1 , the tuning range for the frequency f M spanning at least ±10% of a nominal microwave frequency.
  3. 3 . The microwave-frequency source of claim 1 , the tuning range for the frequency f M spanning at least ±25% of a nominal microwave frequency.
  4. 4 . The microwave-frequency source of claim 1 , the tuning range for the frequency f M spanning at least ±33% of a nominal microwave frequency.
  5. 5 . The microwave-frequency source of claim 1 , the tuning range for the frequency f M spanning at least from 8 GHz to 40 GHz.
  6. 6 . The microwave-frequency source of claim 1 , the VCO including a yttrium iron garnet (YIG) VCO.
  7. 7 . The microwave-frequency source of claim 1 , the VCO including an integrated-circuit (IC) VCO.
  8. 8 . The microwave-frequency source of claim 1 , the VCO including a fundamental VCO and a frequency multiplier coupled to the fundamental VCO, the VCO being arranged so that the fundamental VCO provides the VCO output electrical signal over a lower-frequency portion of the tuning range for the frequency f M and the frequency multiplier provides the VCO output electrical signal over a higher-frequency portion of the tuning range for the frequency f M .
  9. 9 . The microwave-frequency source of claim 8 , the frequency multiplier including a frequency doubler.
  10. 10 . The microwave-frequency source of claim 8 , the fundamental VCO being tunable over a tuning range from 8 GHz to 20 GHz.
  11. 11 . The microwave-frequency source of claim 1 , the dual optical-frequency reference source including first and second frequency-stabilized reference laser sources operating at respective laser frequencies v L1 and v L2 .
  12. 12 . The microwave-frequency source of claim 11 , the dual optical-frequency reference source including an optical reference cavity, the first and second reference laser sources each being frequency-locked to a corresponding distinct resonant optical mode of the optical reference cavity.
  13. 13 . The microwave-frequency source of claim 12 wherein the first and second optical reference signals include laser outputs of the first and second reference laser sources so that v 1 =v L1 and v 2 =v L2 .
  14. 14 . The microwave-frequency source of claim 12 wherein (i) the optical reference cavity is characterized by a Brillouin frequency v B , (ii) the first and second reference laser sources act as pump lasers to produce corresponding first and second stimulated Brillouin laser (SBL) outputs from the optical reference cavity at respective SBL frequencies v 1L −v B and v 2L −v B , and (iii) the first and second optical reference signals include the first and second SBL outputs so that v 1 =v 1L -v B and v 2 =v 2L -v B .
  15. 15 . The microwave-frequency source of claim 12 , the optical reference cavity being arranged as a whispering-gallery-mode resonator, a ring waveguide resonator on a substrate, a disk resonator, a spiral waveguide resonator on a substrate, or a fiber-ring resonator.
  16. 16 . The microwave-frequency source of claim 12 , the first and second reference laser sources being frequency locked to the optical reference cavity by a Pound-Drever-Hall locking mechanism, a Hansch-Couillaud locking mechanism, or self-injection locking.
  17. 17 . The microwave-frequency source of claim 11 , the eOFD being structured and arranged so as to (i) generate from the first or second optical reference signals corresponding first or second optical frequency combs, or both, with respective frequency-comb components at frequencies v 1 ±n 1 ·f M and v 2 ±n 2 ·f M where n 1 and n 2 are integers, and (ii) the generate the eOFD electrical signal at the frequency δf by receiving at a photodetector a frequency-comb component of the first frequency comb and the second optical reference signal, a frequency comb component of the second frequency comb and the first optical reference signal, or frequency-comb components of both the first and second optical frequency combs, the eOFD electrical signal comprising a beat note produced by the photodetector from the received optical reference signals or frequency-comb components.
  18. 18 . The microwave-frequency source of claim 17 , the first or second optical frequency combs being generated by one or more optical modulators driven by the second portion of the VCO output electrical signal at the frequency f.
  19. 19 . The microwave-frequency source of claim 1 , the reference oscillator frequency δf REF being between 1 MHz and 40 GHz.
  20. 20 . The microwave-frequency source of claim 1 , the output electrical signal of the microwave-frequency source exhibiting, over the entire tuning range for the frequency f M , phase noise less than −120 dBc/Hz+10*log 10 (f M /10 GHZ) at 10 KHz offset frequency.

Description

BENEFIT CLAIM This application claims benefit of U.S. provisional App. No. 63/453,088 entitled “Stabilized microwave-frequency source incorporating a tunable voltage-controlled oscillator” filed 18 Mar. 2023 in the names of Li et al, said provisional application being incorporated herein by reference in its entirety. STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT This invention was made with government support under (i) Contract No. W912CG20C0034 awarded by the United States Army, and (ii) Contract No. HR001122C0019 awarded by the Defense Advanced Research Projects Agency (DARPA). The government has certain rights in the invention. FIELD OF THE INVENTION The field of the present invention relates to microwave-frequency sources. BACKGROUND Some relevant background information relevant to stabilized microwave frequency sources can be found in: [1] T. M. Fortier, M. S. Kirchner, F. Quinlan, J. Taylor, J. Bergquist, T. Rosen-band, N. Lemke, A. Ludlow, Y. Jiang, C. Oates et al., “Generation of ultrastable microwaves via optical frequency division,” Nature Photonics, vol. 5, no. 7, pp. 425-429, 2011;[2] X. Xie, R. Bouchand, D. Nicolodi, M. Giunta, W. Hänsel, M. Lezius, A. Joshi, S. Datta, C. Alexandre, M. Lours et al., “Photonic microwave signals with zeptosecond-level absolute timing noise,” nature photonics, vol. 11, no. 1, pp. 44-47, 2017;[3] J. Li, X. Yi, H. Lee, S. A. Diddams, and K. J. Vahala, “Electro-optical frequency division and stable microwave synthesis,” Science, vol. 345, no. 6194, pp. 309-313, 2014;[4] J. Li and K. Vahala, “A 30 Ghz ultra-low-phase-noise oscillator using electro-optical frequency division,” in 2017 IEEE Photonics Conference (IPC). IEEE, 2017, pp. 455-456;[5] J. Li and K. Vahala, “Small-sized, ultra-low phase noise photonic microwave oscillators at X-Ka bands,” Optica, vol. 10, no. 1, pp. 33-34, 2023;[6] M. Fujiwara, et al, “Flattened optical multicarrier generation of 12.5 GHZ spaced 256 channels based on sinusoidal amplitude and phase hybrid modulation.” Electron. Lett. 37, 967-968 (2001);[7] C. B. Huang, et al, Nonlinearly broadened phase-modulated continuous-wave laser frequency combs characterized using DPSK decoding. Opt. Express 16, 2520-2527 (2008);[8] U.S. Pat. No. 9,450,673 entitled “Stabilized microwave frequency source” issued Sep. 20, 2016 to Vahala et al; and[9] U.S. Pat. No. 9,905,999 entitled “Optical frequency divider based on an electro-optical-modulator frequency comb” issued Feb. 27, 2018 to Li et al. Each of the preceding references in incorporated by reference herein in its entirety. Low phase noise microwave oscillators and synthesizers are fundamental building blocks in communications, laboratory instrumentation, and defense applications. In the last decade, optical frequency division (OFD) has revolutionized low phase noise microwave generation and has generated ultra-low phase noise microwaves near X band (10 GHz carrier frequency) [1,2]. Conventional OFD systems transfer the fractional frequency stability from an optical reference to the microwave domain via a self-referenced, mode-locked frequency comb [1,2]. These systems require octave-spanning mode-locked lasers and a bulk cavity-stabilized reference laser. Also, highly linear photodetectors are needed to suppress amplitude to phase noise conversion caused by detection of high-peak power pulse trains [1,2]. Electro-optical frequency division (eOFD) provides a simpler way to achieve OFD using dual laser references and an electro-optical (EO) frequency comb (built from telecomm phase/amplitude modulators) [3,4]. eOFD oscillators from X to Ka band have been demonstrated with small form factors and ultra-low-phase-noise (ULPN) performances [4,5]. A ULPN eOFD oscillator at 30 GHz with phase noise of −151 dBc/Hz at 10 KHz offset has been reported [4]. At 40 GHZ, a record-low phase noise of −153 dBc/Hz has been achieved (10 kHz offset, 40 GHz carrier) in a compact modular form factor (10″×9″×2″) [5]. Other examples of ULPN microwave sources have been demonstrated [8,9]. SUMMARY An inventive microwave-frequency source comprises a tunable voltage-controlled electrical oscillator (VCO), a dual optical-frequency reference source, an electrical reference oscillator, and an electro-optic frequency divider (eOFD). The VCO generates a VCO output electrical signal at a frequency fM, a first portion of which forms an output electrical signal of the microwave-frequency source. The VCO is tunable over a tuning range for the frequency fM that spans at least ±5% of a nominal microwave frequency, the nominal microwave frequency being between 1 GHz and 100 GHz. In some examples the tuning range for the frequency fM spans at least ±10%, ±25%, or ±33% of the nominal microwave frequency; in some examples the tuning range for the frequency fM spans at least from 8 GHz to 40 GHz. The dual optical-frequency reference source generates a first optical reference signal at a first optical reference frequency v1 and a