System and method for generating an optical frequency standard

• US 10,353,270 B2
• Filed: 07/01/2016
• Issued: 07/16/2019
• Est. Priority Date: 07/03/2015
• Status: Active Grant

• Current Assignee: Quantx Labs Pty Limited
• Original Assignee: University of Adelaide
• Inventors: Perrella, Christopher, Light, Philip Stephen, Luiten, Andre Nicholas
• Primary Examiner(s): Petkovsek, Daniel
• Application Number:
  US15/741,481
• Publication Number:
  US 20180329274A1
• Time in Patent Office: 1,110 Days
• CPC Class Codes: G02F 1/3501 Constructional details or arrangements of non-linear optical devices, e.g. shape of non-linear crystals
• CPC Class Codes: G02F 1/3507 Arrangements comprising two or more nonlinear optical devices
• CPC Class Codes: G02F 1/3532 Arrangements of plural nonlinear devices for generating multi-colour light beams, e.g. arrangements of SHG, SFG, OPO devices for generating RGB light beams
• CPC Class Codes: G02F 1/37 for second-harmonic generation G02F1/3532 takes precedence
• CPC Class Codes: H01S 3/0085 Modulating the output, i.e. the laser beam is modulated outside the laser cavity
• CPC Class Codes: H01S 3/0092 Nonlinear frequency conversion, e.g. second harmonic generation [SHG] or sum- or difference-frequency generation outside the laser cavity nonlinear frequency conversion per se G02F1/35
• CPC Class Codes: H01S 3/067 Fibre lasers
• CPC Class Codes: H01S 3/1303 by using a passive reference, e.g. absorption cell H01S3/139 takes precedence
• CPC Class Codes: H01S 3/1305 Feedback control systems
• CPC Class Codes: H01S 3/1306 Stabilisation of the amplitude
• CPC Class Codes: H01S 3/2383 Parallel arrangements

Abstract

A system for generating an optical frequency standard is described. The system is based on a two-color approach and includes a first laser source that generates a first laser output at a first frequency and a second laser source that generates a second laser output at a second frequency corresponding. The first and second laser outputs are then respectively input into first and second harmonic generators to form frequency-doubled first and second laser outputs. The system also includes a two-color stabilization arrangement to stabilize the sum of the frequencies generated by first and second laser sources, including, for example, an interaction region incorporating a laser active material. The interaction region can be a gas reference cell and the laser active material can be Rubidium (in vapor form) having a two-photon transition.

Claims

1. A system for generating an optical frequency standard comprising:

a first laser source configured to generate a first laser output at a first frequency;

a first second harmonic generator configured to receive as an input the first laser output to generate a frequency-doubled first laser output at a doubled first frequency;

a second laser source configured to generate a second laser output at a second frequency different from the first frequency;

a second second harmonic generator configured to receive as an input the second laser output to form a frequency-doubled second laser output at a doubled second frequency;

a two-colour stabilisation arrangement configured to stabilise a sum of the doubled first and the doubled second frequencies including;

an interaction region incorporating a laser active material, the laser active material having a two-photon transition from a first energy level to a second energy level, the interaction region configured to receive as an input the frequency-doubled first laser output and the frequency-doubled second laser output, the frequency-doubled first laser output and the frequency-doubled second laser output selected to together cause the two-photon transition from the first energy level to the second energy level using an intermediate energy level to enhance the two-photon transition rate; and

a detector configured to detect an indicator of the two-photon transition occurring in the interaction region, the detector generating a frequency stabilisation signal for modifying either the first laser output or the second laser output to stabilise the sum of the doubled first frequency and the doubled second frequency originating from the first and second laser sources to the two-photon transition based on the indicator;

a stabilised optical output generator including a sum frequency generator configured to receive as an input the first laser output and the second laser output to generate a stabilised optical output with a frequency corresponding to the optical frequency standard.

2. The system for generating an optical frequency standard as claimed in claim 1, wherein the first and/or second frequency is in the telecommunications wavelength band.

3. The system for generating an optical frequency standard as claimed in claim 1, wherein the interaction region is a gas reference cell.

4. The system for generating an optical frequency standard as claimed in claim 3, wherein the gas reference cell is based on an isotopically pure gas.

5. The system for generating an optical frequency standard as claimed in claim 3, wherein the gas reference cell is based on a Group I element.

6. The system for generating an optical frequency standard as claimed in claim 5, wherein the gas reference cell is a Rubidium cell.

7. The system for generating an optical frequency standard as claimed in claim 1, wherein the indicator of the two-photon transition occurring in the interaction region is fluorescence emitted from the interaction region as atoms transition either directly or indirectly from the second energy level to the first energy level.

8. The system for generating an optical frequency standard as claimed in claim 1, wherein the indicator of the two-photon transition occurring in the interaction region is the absorption of the frequency-doubled first and/or second laser output associated with the two-photon transition.

9. The system for generating an optical frequency standard as claimed in claim 1, further including a single-photon stabilisation arrangement for stabilising the first or second laser output, or the doubled first frequency or the doubled second frequency of the first or second frequency-doubled laser output respectively.

10. The system for generating an optical frequency standard as claimed in claim 9, wherein the single-photon stabilisation arrangement is to stabilise detuning from the intermediate energy level.

11. The system for generating an optical frequency standard as claimed in claim 1, wherein the two-colour stabilisation arrangement further includes a frequency modulator configured to frequency modulate the first and/or second laser output at a modulation frequency for detection by the detector following the two-photon transition to generate the frequency stabilisation signal.

12. The system for generating an optical frequency standard as claimed in claim 1, further including an optical amplification stage configured to increase the optical power of the first and/or second laser output.

13. The system for generating an optical frequency standard as claimed in claim 1, further including a power control arrangement configured to modulate and/or stabilise the amplitude of the first and/or second laser output.

14. The system for generating an optical frequency standard as claimed in claim 13, wherein the power control arrangement includes an amplitude modulator configured to modulate the amplitude of the first and/or second laser output and a detector to detect the amplitude modulation on the first and/or second laser output following passage through the interaction region to provide a measure of the optical power from the first and/or second laser output interacting with the interaction region to generate a power control signal to stabilise the optical power in the interaction region.

15. The system for generating an optical frequency standard as claimed in claim 1, wherein the first and/or second laser is selected from any one of:

a fibre laser;

a diode laser;

a distributed feedback (DFB) laser;

a distributed Bragg reflector laser;

or a vertical external cavity surface emitting laser (VECSEL).

16. An optical frequency standard generated by the system claimed in claim 1.

17. A method for generating an optical frequency standard comprising:

generating a first laser output at a first frequency;

generating a frequency-doubled first laser output from the first laser output at a doubled first frequency;

generating a second laser output at a second frequency different from the first frequency;

generating a frequency-doubled second laser output from the second laser output at a doubled second frequency;

stabilising a sum of the doubled first and the doubled second frequencies by

generating a two-photon transition from a first energy level to a second energy level in an interaction region, the interaction region configured to receive as an input the frequency-doubled first laser output and the frequency-doubled second laser output, the frequency-doubled first laser output and the frequency-doubled second laser output selected to together cause the two-photon transition from the first energy level to the second energy level using an intermediate energy level to enhance the two-photon transition rate; and

detecting an indicator of the two-photon transition occurring in the interaction region to generate a frequency stabilisation signal;

modifying either the first laser output or the second laser output in accordance with the frequency stabilisation signal to stabilise the sum of the doubled first frequency and the doubled second frequency originating from the first and second laser sources to the two-photon transition based on the indicator; and

generating a stabilised optical output with a frequency corresponding to the optical frequency standard by frequency summing the first laser output and the second laser output.

18. The method of claim 17, including generating the first and/or second laser outputs in the telecommunications wavelength band.

19. An optical frequency standard system comprising:

first and second lasers and respective frequency doublers to generate frequency doubled outputs of the first and second lasers;

a gas cell based two-colour stabilisation arrangement to frequency stabilise the sum of the frequency doubled outputs of the first and second lasers;

a frequency summer to sum the frequencies of the first and second lasers to generate the optical frequency standard.