Optical filter control

1. An optical filter assembly comprising:

a tuneable optical filter;

a beam splitter assembly configured to:

split an input beam into an output beam, a reference beam, and a probe beam,

direct the output beam and the probe beam through the tuneable optical filter such that the probe beam is at an angle relative to the output beam, and

direct the reference beam to a first detector without directing the reference beam through the tuneable optical filter;

the first detector configured to measure an intensity of the reference beam;

a second detector configured to measure an intensity of the probe beam after it has passed through the tuneable optical filter; and

a controller configured to adjust the tuneable optical filter based on the measured intensity of the reference beam and the measured intensity of the probe beam.

2. The optical filter assembly of claim 1 , wherein the beam splitter assembly, when directing the output beam and the probe beam, is configured to:

direct the probe beam and the output beam such that the probe beam and the output beam intersect within the tuneable optical filter.

3. The optical filter assembly of claim 2 , further comprising:

a beam separation assembly configured to:

separate the output beam and the probe beam after the output beam and the probe beam have passed through the tuneable optical filter, and

direct the probe beam to the second detector.

4. The optical filter assembly of claim 3 , wherein the beam separation assembly comprises a lens and an optical fibre having a core and an angled reflective surface at an end of the optical fibre,

wherein the lens is configured to focus the output beam and the probe beam onto a focal plane,

wherein the optical fibre is positioned such that the end of the optical fibre is located at the focal plane,

wherein an end of the core is located at a focal point of the output beam and the angled reflective surface is located at a focal point of the probe beam.

5. The optical filter assembly of claim 4 , wherein the probe beam is received at the second detector after the probe beam is reflected from the angled reflective surface.

6. The optical filter assembly of claim 1 , wherein the controller, when adjusting the tuneable optical filter based on the measured intensity of the reference beam and the measured intensity of the probe beam, is configured to:

adjust the tuneable optical filter based on a ratio of the measured intensity of the reference beam and the measured intensity of the probe beam.

7. The optical filter assembly of claim 1 , wherein the controller is further configured to:

determine a target value of a ratio of the measured intensity of the reference beam and the measured intensity of the probe beam; and

wherein the controller, when adjusting the tuneable optical filter, is configured to:

adjust the tuneable optical filter based on determining the target value of the ratio.

8. The optical filter assembly of claim 1 , wherein the tuneable optical filter is any one of:

an etalon;

a Mach-Zehnder interferometer; or

a Michelson interferometer.

9. The optical filter assembly of claim 1 , wherein the beam splitter assembly is further configured to:

receive the input beam polarised with a first polarisation and a secondary beam polarised with a second, orthogonal polarisation, and

wherein the beam splitter assembly comprises a polarisation combiner configured to combine the secondary beam with the output beam.

10. A method, comprising:

splitting, by an optical filter assembly, an input beam into an output beam, a reference beam, and a probe beam;

directing, by the optical filter assembly, the output beam and the probe beam through a tuneable optical filter of the optical filter assembly,

wherein the probe beam is directed through the tuneable optical filter at an angle relative to the output beam;

directing, by the optical filter assembly, the reference beam to a detector without directing the reference beam through the tuneable optical filter;

measuring, by the optical filter assembly via the detector, an intensity of the reference beam;

measuring, by the optical filter assembly, an intensity of the probe beam after it passes through the tuneable optical filter; and

adjusting, by the optical filter assembly, the tuneable optical filter based on the measured intensity of the reference beam and the measured intensity of the probe beam.

11. The method of claim 10 , wherein adjusting the tuneable optical filter based on the measured intensity of the reference beam and the measured intensity of the probe beam comprises:

adjusting the tuneable optical filter based on a ratio of the measured intensity of the reference beam and the measured intensity of the probe beam.

12. The method of claim 11 , further comprising:

determining a target value of a ratio of the measured intensity of the reference beam and the measured intensity of the probe beam; and

wherein adjusting the tuneable optical filter comprises:

adjusting the tuneable optical filter based on determining the target value of the ratio.

13. The method of claim 10 , wherein directing the output beam and the probe beam comprises:

directing the probe beam and the output beam such that the probe beam and the output beam intersect within the tuneable optical filter.

14. The method of claim 13 , further comprising:

separating the output beam and the probe beam after the output beam and the probe beam have passed through the tuneable optical filter.

15. The method of claim 14 , wherein the detector is a first detector; and

wherein the probe beam is received at a second detector after the output beam and the probe beam are separated.

16. The method of claim 10 , further comprising:

focusing the output beam and the probe beam onto a focal plane,

wherein an optical fibre is positioned such that an end of the optical fibre is located at the focal plane,

wherein an end of a core of the optical fibre is located at a focal point of the output beam, and

wherein an angled reflective surface of the optical fibre is located at a focal point of the probe beam.

17. The method of claim 16 , wherein the detector is a first detector, and

wherein the probe beam is received at a second detector after the probe beam is reflected from the angled reflective surface.

18. The method of claim 10 , wherein the detector is a first detector, and

wherein the probe beam is received at a second detector after being directed through the tuneable optical filter.

19. The method of claim 10 , further comprising:

receiving the input beam polarised with a first polarisation and a secondary beam polarised with a second, orthogonal polarization; and

combining the secondary beam with the output beam.

20. The method of claim 10 , wherein the tuneable optical filter is any one of:

an etalon;

a Mach-Zehnder interferometer; or

a Michelson interferometer.