Broadband optical phase detection and phase noise removal with an optical resonator

• US 9,178,520 B2
• Filed: 10/13/2011
• Issued: 11/03/2015
• Est. Priority Date: 10/14/2010
• Status: Active Grant

• Current Assignee: RWTH Aachen University
• Original Assignee: RWTH Aachen University
• Inventors: Witzens, Jeremy
• Primary Examiner(s): Epps, Georgia Y
• Assistant Examiner(s): Moore, Carolynn A
• Application Number:
  US13/879,170
• Publication Number:
  US 20130221211A1
• Time in Patent Office: 1,482 Days
• Field of Search: 250/227.11, 250/216, 250/227.27, 250/227.19, 250/214.R, 359/10, 359/11, 359/245, 359/259, 385/1, 385/2, 372/12, 372/13, 372/29.014, 372/29.016
• US Class Current: 1/1
• CPC Class Codes: H01S 3/005 Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping shaping laser beam for working metal or other materials B23K26/06; optical elements, systems or apparatus in general G02B
• CPC Class Codes: H01S 3/0085 Modulating the output, i.e. the laser beam is modulated outside the laser cavity
• CPC Class Codes: H03L 7/04 wherein the frequency-determining element comprises distributed inductance and capacitance
• CPC Class Codes: H04B 10/58 Compensation for non-linear transmitter output
• CPC Class Codes: H04B 10/60 Receivers
• CPC Class Codes: H04B 10/615 Arrangements affecting the optical part of the receiver
• CPC Class Codes: H04B 10/67 Optical arrangements in the receiver
• CPC Class Codes: H04B 10/697 Arrangements for reducing noise and distortion

Abstract

A phase noise detection apparatus comprises a laser beam, an optical resonator coupled thereto at a coupling point and a photodetector receiving light from the laser beam. The laser beam and the resonator coupled thereto convert phase noise of light transported by the laser beam prior to the coupling point into intensity noise of light transported by the laser beam thereafter. Intensity noise is converted into an electrical signal by the photodetector. The electrical signal is sent through a first signal path and a second signal path such that the first signal path transports a signal substantially proportional to the intensity noise, which is integrated in an integrator in the second path. Relative gain of the two signal paths can be adjusted and the overall gain of the signal path is preferably such that the optical phase modulator at least partially cancels said phase noise in the optical domain.

Claims

1. A phase noise detection apparatus comprising:

at least one laser beam, an optical resonator coupled to the at least one laser beam at a coupling point, and a photodetector arranged to receive light from the laser beam, wherein

the at least one laser beam and the resonator coupled thereto convert phase noise of light transported by the at least one laser beam prior to the coupling point into intensity noise of light transported by the at least one laser beam after the coupling point, wherein the intensity noise after the coupling point is generated as a function of the phase noise prior to the coupling point, and

said photodetector is adapted to convert the intensity noise into an electrical signal; and

at least one electrical signal path and at least one optical phase modulator, wherein

the at least one electrical signal path is adapted to apply the electrical signal to the at least one optical phase modulator, and

the at least one electrical signal path includes first and second electrical signal paths adapted for the electrical signal to be sent thereto, to the first electrical signal path with a signal substantially proportional to the intensity noise, and to the second electrical signal path where a signal substantially proportional to the intensity noise is integrated, wherein the first and the second electrical signal paths are connected to the at least one phase modulator compensating phase noise in an optical domain, and said at least one phase modulator is (i) one out of several optical phase modulators or (ii) a unique optical phase modulator wherein both electrical signal paths are combined in an electrical domain prior to being applied to said unique optical modulator.

2. An apparatus as defined in claim 1, configured such that the phase noise is converted to intensity noise with a conversion efficiency that is substantially independent of phase noise frequency when said phase noise frequency is outside a linewidth of the resonator, and further configured such that phase noise with a frequency outside of the linewidth of the resonator is first converted into intensity noise, then converted into an electrical signal by the photodetector and thereafter applied to an optical phase modulator that at least partially cancels said phase noise with a frequency outside the linewidth of the resonator in an optical domain.

3. An apparatus as defined in claim 1, wherein the resonator is one of (i) a whispering gallery resonator, (ii) a disk resonator or (iii) a microtoroid resonator.

4. An apparatus as defined in claim 3, wherein the resonator is a ring resonator.

5. An apparatus as defined in claim 1, wherein the photodetector comprises a Germanium waveguide photodetector in which the light is at least partially absorbed by the Germanium and wherein the apparatus includes silicon on insulator material.

6. An apparatus as defined in claim 1, configured such that the first and second electrical signal paths are combined in the optical domain by applying them to separate of said several optical phase modulators optically connected such that light passes through said separate modulators and the combined signal is the sum of phase modulations applied to said light, wherein the first and second signals are combined with relative weights such that a resulting signal is modulated according to the phase noise of the light with a transfer function that is substantially constant for phase noise both inside and outside of the linewidth of the resonator for frequencies that are within passbands of the photodetector and of electronic circuits of the first and second signal paths.

7. An apparatus as defined in claim 6 wherein the transfer function is a flatband within 30% of a maximum of the transfer function for a phase noise frequency range spanning from ⅛

to ¾

of the full width at half maximum of the resonator linewidth.

8. An apparatus as defined in claim 6, wherein (i) the first electrical signal path and the second electrical signal path are combined with a relative summation weight, or (ii) a relative signal strength of the first and second electrical signal paths is adjusted with a summation weight prior to summation, wherein the summation weight is determined according to at least one of (i) quality factors of the resonator, (ii) a loaded quality factor of the resonator, (iii) a coupling strength between the waveguide and the resonator, (iv) relative signal strengths of the first and second electrical signal paths, (v) relative signal strengths of the first and second electrical signal paths measured for an artificially induced phase noise with a well-defined frequency or well-defined frequency range, (vi) relative signal strengths of the first and second electrical signal paths measured after filtering the first and second electrical signal paths with a notch filter, and (vii) residual phase noise, measured by an additional apparatus, of light having passed through at least one optical phase modulator driven by a signal generated according to the intensity noise.

9. An apparatus as defined in claim 1, wherein the at least one electrical signal path is further adapted to produce an overall gain such that the optical phase modulator is adapted to at least partially cancel said phase noise in an optical domain.

10. An apparatus as defined in claim 1, further comprising a waveguide adapted to guide the at least one laser beam therein.

11. An apparatus as defined in claim 10, further comprising a control system adapted to adjust at least one of a resonance frequency of the resonator and a center frequency of the light in the waveguide such that the center frequency falls within a predetermined region of the linewidth of the resonator.

12. An apparatus as defined in claim 11, wherein the photodetector is adapted to record the intensity noise and generates a control signal used to adjust at least one of the resonance frequency of the resonator and the center frequency of the light in the waveguide.

13. An apparatus as defined in claim 10, wherein the resonator is coupled to the waveguide, which are adapted to convert said phase noise to said intensity noise, and further includes one of (i) no additional optical elements, (ii) optical elements coupling the waveguide to the optical resonator, or (iii) optical elements adapted to adjust a resonance of the resonator and a center frequency of the light transported by the waveguide to one another with a control system.

14. An apparatus as defined in claim 13, further comprising an offset between a center frequency of the light and a resonant frequency of the resonator such that transmission through the waveguide and coupled resonator is between 20% and 80%, where 0% corresponds to a minimum transmission corresponding to where the center frequency of the light is equal to the resonant frequency of the resonator, and 100% corresponds to a transmission when the center frequency of the light is detuned from the resonant frequency by two full width at half maximum of the resonant frequency.

15. An apparatus as defined in claim 14, wherein the resonant frequency of the resonator lies within plus or minus thirty percent of the full width at half maximum of the resonance linewidth from a frequency detuned from the laser beam frequency by one of plus or minus one half of the full width at half maximum of the resonance linewidth.

16. An apparatus as defined in claim 10, configured so that at least a portion of the light transported by the waveguide is coupled to the resonator and at least a portion of the light transported by the waveguide is not coupled to the resonator, wherein at least a portion of the light coupled to the resonator is coupled back to the waveguide and interferes with the portion of the light not coupled to the resonator, resulting in the intensity noise, wherein the portion of the light coupled to the resonator and coupled back to the waveguide substantially corresponds to a time averaged version of the light transported in the waveguide such that phase noise components with a frequency outside of a linewidth of the resonator are substantially averaged out, wherein the interference between the time averaged portion of the light corresponding to the portion of the light coupled to the resonator and coupled back to the waveguide and the non-time averaged portion of the light corresponding to the portion of the light not coupled to the resonator results in a conversion of phase noise into intensity noise with a conversion efficiency that is substantially independent of the phase noise frequency for phase noise frequencies outside of the linewidth of the resonator, and further configured to apply the electrical signal to an optical phase modulator so as to at least partially cancel said phase noise in an optical domain.

17. An apparatus as defined in claim 10, wherein the waveguide is coupled to the resonator and one of (i) operatively connects directly to the photodetector after the coupling point with the resonator without being routed through waveguide combiners not coupling the waveguide to the resonator, (ii) operatively connects to the photodetector via additional waveguide combiners not coupling the waveguide to the resonator such that additional optical power added by the additional waveguide combiners to optical power routed from the resonator coupling point to the photodetector is less than 30% of said optical power.