High resolution tunable optical filter
View Patent ↗A high-resolution tunable optical filter uses a dispersive element in a double pass configuration. The double pass configuration is provided by a reflective quarter-wave plate that conveniently reduces polarization dispersion loss (PDL) of the filter. The filter also includes a fibre tube for supporting an input optical fibre and an output optical fibre, and a lens having an optical axis, which is disposed such that a focal point thereof is substantially at one of the input and output optical fibres. To provide increase resolution the filter is tuned by rotating the reflective quarter-wave plate. To provide high power capabilities the fibre jackets of the input and output fibres are optionally stripped back. To provide a symmetrical spectral response the fibre tube is optionally mounted at an angle to the optical axis of the lens.
1. An optical filter comprising:
an input port for launching an optical signal into the optical filter;
a collimator disposed for collimating light transmitted from the input port;
a dispersive element disposed for dispersing the collimated light according to wavelength in a dispersion plane;
a reflective retarder disposed for reflecting at least a portion of the dispersed light back to the dispersive element;
a focussing element disposed for receiving the light transmitted from the dispersive element and substantially focussing it to a point; and
an output port disposed at the point for receiving the focussed light, wherein the reflective retarder is rotatable about an axis substantially perpendicular to the dispersion plane for tuning the optical filter.
2. An optical filter according to claim 1 , wherein the collimator and the focussing element comprise a same lens.
3. An optical filter according to claim 2 , comprising an input optical fibre and an output optical fibre coupled to the input port and the output port, respectively.
4. An optical filter according to claim 3 , comprising a fibre tube for supporting the input and output optical fibres.
5. An optical filter according to claim 4 , wherein the fibre tube comprises an end face that is angled to reduce back reflection.
6. An optical filter according to claim 5 , comprising a fibre tube holder for supporting the fibre tube.
7. An optical filter according to claim 6 , comprising a lens holder having a first end for supporting the lens and a second end for mounting the fibre tube holder thereon.
8. An optical filter according to claim 7 , wherein the fibre tube is disposed within the fibre tube holder such that the longitudinal axis of the fibre tube is at an angle relative to the optical axis of the lens.
9. An optical filter according to claim 8 , wherein the angle is selected to compensate for angular refraction of a beam of light exiting the angled end face.
10. An optical filter according to claim 8 , wherein a fibre jacket of at least one of the input and output fibres is stripped away past the fibre tube such that a region of the at least one of the input and output fibres is exposed.
11. An optical filter according to claim 1 , wherein the output port comprises means for handling high power light that is not coupled into the output optical port.
12. An optical filter according to claim 11 , wherein the means for handling high power comprises an input optical fibre coupled to the input port and an output optical fibre coupled to the output port, at least one of the input and output optical fibres having a portion of their fibre jacket removed to a length selected to prevent burning from the high power light.
13. An optical filter according to claim 11 , wherein the means for handling high power comprises an input optical fibre coupled to the input port and an output optical fibre coupled to the output port, at least one of the input and output optical fibres having a fibre jacket fabricated from a high power resistive material.
14. An optical filter according to claim 11 , wherein the means for handling high power comprises a fibre tube holder for mounting an input optical fibre that is coupled to the input port and an output optical fibre that is coupled to the output port, the fibre tube holder having a length selected to disperse the high power light such that a fibre jacket of at least one of the input and output optical fibres is other than burned by the light.
15. An optical filter according to claim 1 , wherein the input port comprises means for making a spectral shape of the optical filter symmetrical.
16. An optical filter according to claim 15 , wherein the means for making a symmetrical spectral shape comprises a fibre tube holder for mounting input and output optical fibres at an angle to the optical axis of the collimator, the input and output optical fibres coupled to the input and output ports, respectively.
17. An optical filter according to claim 15 , wherein the means for making a symmetrical spectral shape comprises a glass rod coupled to an angled end of a fibre tube supporting input and output optical fibres coupled to input and output ports, respectively, the glass rod having an angled end for mating to the angled end of the fibre tube.
18. An optical filter according to claim 1 , wherein the reflective retarder comprises a quarter-wave plate having an anti-reflection coating on one side and a highly reflective coating on an opposing side.
19. An optical filter according to claim 1 , wherein the reflective retarder comprises a quarter-wave plate and a reflector.
20. An optical filter according to claim 1 , wherein the dispersive element is fixedly mounted within the filter.
21. An optical filter according to claim 3 , comprising a fibre strain relief tube for protecting the input and output optical fibres.
22. An optical filter according to claim 4 , wherein the fibre tube is a double bore tube.