Laser-based sensor for measuring an external magnetic field

1. A sensor for measuring an external magnetic field, the sensor including:

(a) an optical cavity;

(b) a laser medium which together with the optical cavity has a laser threshold;

(c) a laser pump; and

(d) a radio-frequency (RF) drive applied to the laser medium, such that the laser threshold varies with a change in the external magnetic field;

wherein, the RF drive is applied to the laser medium at or around a resonance frequency which varies depending on the external magnetic field, such that depending on the value of the external magnetic field, the RF drive induces transitions between at least two states of the laser medium, each state causing a different laser threshold in an intensity of a laser output; and

the intensity of the laser output is measured to provide a measurement of the value of the external magnetic field.

2. The sensor according to claim 1 , wherein the laser pump is a light source, in or close to the visible range, or electrical pumping applied to the laser medium.

3. The sensor according to claim 1 , wherein the laser output is directly fibre coupled.

4. The sensor according to claim 1 , wherein a laser pump signal and its fluctuations are monitored and used in combination with the laser output to reduce technical noise.

5. The sensor according to claim 1 , wherein the RF drive and any RF fluctuations are monitored and applied together with a laser output signal to reduce technical noise.

6. The sensor according to claim 1 , wherein the laser medium is a solid-state material.

7. The sensor according to claim 6 , wherein the laser medium is given by negatively charged nitrogen-vacancy (NV) centres in diamond.

8. The sensor according to claim 7 , wherein NV centre laser pumping occurs from the electronic ground state triplet 3 A 2 to the electronic excited state triplet 3 E and the states of the laser medium causing a different laser threshold in an intensity of a laser output, are at least two out of three states of the ground state triplet 3 A 2 .

9. The sensor according to claim 8 , wherein at least one of the transitions between the 3 E states and the phonon-added states of the 3 A 2 are used as a lasing transition.

10. The sensor according to claim 6 , wherein the laser medium is formed by silicon-vacancy centres in diamond.

11. The sensor according to claim 6 , wherein the laser medium is formed by cerium in yttrium-aluminium-garnet.

12. The sensor according to claim 1 , wherein the sensor achieves a sensitivity better than nT/√(Hz).

13. The sensor according to claim 1 , wherein the cavity includes a diffraction grating.

14. A method for measuring the strength of an external magnetic field, the method including the following steps:

(a) providing a sensor including a laser pump and a laser medium which together with an optical cavity forms a laser threshold;

(b) applying a radio-frequency (RF) drive to the laser medium, such that the laser threshold varies with a change in the external magnetic field; wherein, the RF drive is applied to the laser medium at or around a resonance frequency which varies depending on the external magnetic field, such that depending on the strength of the external magnetic field, the RF drive induces transitions between at least two states of the laser medium, each state causing a different laser threshold in an intensity of a laser output; and detecting the intensity of the laser output, wherein the intensity of the laser output provides a measurement of the strength of the external magnetic field.

15. The method according to claim 14 , further including the step of determining the laser output and a sensitivity of the sensor as a function of a plurality of parameters from a steady-state solution to a plurality of differential equations relating to the sensor, wherein a laser pump strength is set to an operating point which is determined from the laser output as a function of the laser pump strength and the sensitivity of the sensor is optimised as a function of a remainder of the plurality of parameters within given constraints.

16. The method according to claim 15 , further including the step of determining a sensor measurement range as a range of detuning between an RF addressed transition frequency and an RF drive frequency over which the output changes as a function of detuning.

17. The method according to claim 16 , wherein a combination of a density of optically active atoms or colour centres and a coherence time of the RF addressed transition is selected based on an optimal sensor sensitivity and/or sensor measurement range.

18. The method according to claim 16 , wherein an optical cavity loss rate is selected to achieve optimal sensor sensitivity and/or sensor measurement range.

19. The method according to claim 16 , wherein an RF drive strength is selected based on an optimal sensor sensitivity and/or sensor measurement range.

20. The method according to claim 14 , wherein a laser pump strength is set to the operating point, wherein the operating point is given by the highest laser threshold as a function of detuning.