Wavelength dispersive device with temperature compensation
View Patent ↗The invention relates to fiber-optic wavelength dispersive devices incorporating a wavelength dispersive reflector that provides auto-compensation of variations of output spectral characteristic with temperature and includes a transmissive dispersion that is followed by a beam-folding reflecting surface in a double-pass configuration grating and is coupled to a wedged shaped prism.
1. A fiber optic wavelength dispersive device comprising:
a front-end unit comprising a first port for launching an input optical signal;
a wavelength dispersive reflector for receiving the input optical signal, for dispersing the input optical signal into a plurality of sub-beams of light with different central wavelengths, and for directing said sub-beams back at first angles along first optical paths in a dispersion plane;
a spherical reflector for redirecting the input optical signal launched from the first port to the wavelength dispersive reflector, and for redirecting the sub-beams from the wavelength dispersive reflector back along second spatially separated optical paths; and,
an array of beam receiving elements for receiving each of the sub-beams at a different location along the array according to the central wavelength thereof;
wherein the wavelength dispersive reflector comprises:
a transmissive dispersion grating (TDG) for receiving the input optical signal and for transmitting each of the plurality of sub-beams of light at a different angle of diffraction,
a reflective surface spaced from the TDG for reflecting the plurality of sub-beams of light back through the TDG, and
a first wedged prism of an optically transparent material for refracting each of the sub-beams of light in the dispersion plane at an angle of refraction dependent on the temperature of the device, the first wedged prism having an apex angle;
wherein a shift in the angles of refraction of the sub-beams provided by the first wedged prism due to a change in the temperature of the device at least partially compensates for a shift in the angles of diffraction caused by the change in the temperature of the device.
2. The device of claim 1 , wherein the apex angle of the first wedged prism is such that the first wedged prism is effective to cause the first angles to vary with the temperature of the device at a rate of at least 1 μrad/C so as to counteract temperature-induced variations of the second optical paths that occur outside of the wavelength dispersive reflector.
3. The device of claim 2 , wherein the first wedged prism is disposed between the diffraction grating and the reflective surface.
4. The device of claim 3 , wherein the first wedged prism has a first surface facing the diffraction grating and a second surface defining the apex angle with the first surface, and wherein the first wedged prism is oriented with an apex side thereof away from the TDG.
5. The device of claim 4 , wherein the second surface comprises the reflective surface.
6. The device of claim 3 , further comprising a second wedged prism disposed between the spherical reflector and the wavelength dispersive reflector for refracting the sub-beams at angles of refraction that depend on temperature so as to further counteract temperature-induced variations of the second optical paths that occur outside of the wavelength dispersive reflector.
7. The device of claim 2 , wherein the first wedged prism is disposed between the spherical mirror and the TDG.
8. The device of claim 5 , wherein the first wedged prism is integral with the TDG.
9. The device of claim 2 , further comprising an aberration correction prism of optically transparent material disposed in the second optical paths between the spherical mirror and the beam receiving elements.
10. The device of claim 1 , wherein the array of beam receiving elements comprises an actuation array for selectively redirecting one or more of the sub-beams back to the spherical reflector for reflecting said one or more of the sub-beams therefrom back to the wavelength dispersive reflector for recombination into an output beam, whereupon the output beam is redirected by the spherical reflector to the front-end unit.
11. The device of claim 10 , further comprising an aberration correction prism of optically transparent material disposed in the optical paths of the input and output beams between the spherical mirror and the actuation array.
12. The device of claim 11 , wherein the first wedged prism compensates for a combined effect of a temperature dependence of dispersion properties of the TDG and a temperature dependence of refractive properties of the aberration correction prism.
13. The device of claim 10 , wherein the actuation array comprises a micro-electro-mechanical system (MEMS) device having a plurality of reflectors movable about an axis parallel to the dispersion plane for receiving each sub-beam at a different reflector.
14. The device of claim 10 , wherein the front-end unit includes:
a plurality of ports comprising the first port and output ports for receiving the output beam, and
a switching lens having an optical axis for converting an angular displacement of the output beam into a lateral displacement corresponding to a selected one of the output ports.
15. The device of claim 10 , wherein the actuation array comprises a liquid crystal deflection array having a plurality of pixels, each of the pixels having an optical phase array acting as an electro writable diffraction grating.
16. The device of claim 1 , wherein the array of beam receiving elements comprises photodetectors.
17. The device of claim 1 , wherein the array of beam receiving elements comprises reflective LC cells for selectively modifying the sub-beams and for directing the modified sub-beams for coupling into the input port.
18. The device of claim 1 , further comprising a heating or cooling element and a temperature sensor thermally coupled to the first wedged prism for maintaining a temperature difference between the first prism and the rest of the device.