Systems and methods for low noise frequency multiplication, division, and synchronization
Low phase noise radio frequency (RF) sources generated by voltage controlled oscillators (VCOs) are described. Optical modulators driven by a VCO may be used to generate optical side-bands to cw lasers. The spectral extent of said side-bands can be increased via frequency broadening in highly nonlinear waveguides. Free running mode locked low phase noise comb oscillators can be used as reference oscillators to generate beat signals between those side-bands and individual comb modes at distal spectral regions, thereby creating an error signal used to reduce the phase noise of VCOs and the generation of low phase noise RF signals. VCO phase noise may be reduced by using free-running modelocked comb lasers phase locked to external frequency references, by omitting a reference comb and using a nonlinear interferometer for generating an error signal, or by locking a slave comb to the modulation frequency of an intra-cavity modulator driven by the VCO.
1. A source of low phase noise microwave frequencies, said source comprising:
a continuous wave (cw) laser;
said cw laser configured to be modulated with an optical modulator;
said optical modulator configured to be driven by a microwave frequency,
said optical modulator configured to generate side-bands to the cw laser in a frequency domain with a frequency spacing given by a modulation frequency of the optical modulator;
a nonlinear medium to nonlinearly broaden the spectral extent of said side-bands and to generate a supercontinuum spectrum;
a nonlinear interferometer configured to generate a beat signal; and
a detector to detect said beat signal;
wherein said beat signal comprises amplified phase noise from said microwave frequency, and
wherein said source is configured to use feedback from said beat signal to reduce the amplified phase noise from said microwave frequency.
2. The source of low phase noise microwave frequencies according to claim 1 , wherein said nonlinear interferometer comprises an f-2f interferometer.
3. The source of low phase noise microwave frequencies according to claim 2 , wherein said nonlinear interferometer comprises a nonlinear waveguide.
4. The source of low phase noise microwave frequencies according to claim 1 , wherein said low phase noise is obtainable with microwave frequencies greater than about 25 GHz and up to about 1000 GHz, and wherein said phase noise is lower than −120 dBc/Hz at a frequency offset of about 100 kHz.
5. The source of low phase noise microwave frequencies according to claim 1 , further comprising:
a radio frequency (RF) reference configured to provide an RF reference signal which, together with an output of said detector, provides for generation of an error signal and reduction of phase noise via said feedback.
6. The source of low phase noise microwave frequencies according to claim 5 , wherein said feedback comprises a feedback loop.
7. The source of low phase noise microwave frequencies according to claim 5 , said source comprising: a controllable microwave generator configured to receive said error signal and to generate a phase-noise reduced output thereof.
8. The source of low phase noise microwave frequencies according to claim 7 , wherein said phase-noised reduced output of said controllable microwave generator is coupled to said optical modulator via a feedback arrangement.
9. The source of low phase noise microwave frequencies according to claim 7 , wherein said controllable microwave generator comprises a voltage controlled oscillator (VCO).
10. The source of low phase noise microwave frequencies according to claim 1 , wherein said nonlinear interferometer comprises a 2f-3f interferometer or a 3f-4f interferometer.
11. The source of low phase noise microwave frequencies according to claim 1 , wherein said nonlinear interferometer comprises a waveguide interferometer.
12. The source of low phase noise microwave frequencies according to claim 11 , wherein said waveguide interferometer comprises a periodically poled Lithium Niobate (PPLN) waveguide arranged in a 2f-3f interferometer configuration in which a 2f-3f beat signal at a red wavelength is generated.
13. The source of low phase noise microwave frequencies according to claim 1 , wherein said optical modulator comprises: one or more cascaded phase modulators disposed downstream from said cw laser and arranged such that an output of said cw laser, including the spectral content of said side-bands, is frequency broadened via phase modulation.
14. The source of low phase noise microwave frequencies according to claim 1 , wherein said source comprises at least one fiber amplifier disposed between said optical modulator and said nonlinear medium.
15. The source of low phase noise microwave frequencies according to claim 14 , wherein said at least one fiber amplifier comprises an Erbium-doped amplifier.
16. The source of low phase noise microwave frequencies according to claim 1 , wherein said optical modulator is configured to generate a pulse train at a repetition rate corresponding to said modulation frequency, said source further comprising an optical amplitude modulator configured to reduce the repetition rate of said pulse train to a reduced repetition rate.
17. The source of low phase noise microwave frequencies according to claim 16 , further configured to inject the pulse train at said reduced repetition rate into said nonlinear medium.
18. The source of low phase noise microwave frequencies according to claim 16 , wherein said optical modulator is configured to drive a frequency near 25 GHz.
19. The source of low phase noise microwave frequencies according to claim 16 , wherein said reduced repetition rate is near 1 GHz.