Compact littrow-type scanning spectrometer

An optical instrument for diffracting light into a selected wavelength, the optical instrument comprising:

• a) a collimating mirror to reflect light originating from a focal point of the collimating mirror into a collimated beam;
• b) a light inlet optical fiber located substantially at the focal point of the collimating mirror;
• c) a light outlet optical fiber also located substantially at the focal point of the collimating mirror;
• d) a substantially planar reflector angled across the collimated light beam and having a light aperture for receiving the light inlet optical fiber and the light outlet optical fiber, the substantially planar reflector at least partially surrounding the aperture;
• e) a substantially planar diffraction grating positioned to receive collimated light reflected by the substantially planar reflector;
• wherein the planar reflector can receive a selected wavelength of light diffracted by the planar diffraction grating and reflect the selected wavelength to the concave mirror to be reflected by the concave mirror to the outlet optical fiber and wherein the diffraction grating is mounted for rotation to permit selection of the light output wavelength or spectral band.

A method of optically selecting a spectral band from a sample light received through a light aperture located in and surrounded by a planar mirror, the method comprising;

• a) collimating divergent sample light received from the input aperture with a concave mirror;
• b) reflecting the collimated sample light with the planar mirror to a diffraction grating at the Littrow angle for the spectral band;
• c) reflecting the diffracted spectral band with the planar mirror to the concave mirror;
• d) focusing the diffracted spectral band to the light aperture;
• and
• e) rotating the diffraction grating to select the light output wavelength or spectral band.

A spectrometer providing a light output in a selected wavelength or spectral band from a sample light input, the spectrometer comprising:

• a) a planar diffraction grating to receive a collimated beam of sample light from the sample light input on a grating incident path at an incident angle and of diffracting the incident light to provide a diffracted light output in the selected wavelength or spectral band along the incident path in the opposite direction to the incident light the diffraction grating being mounted for rotation to permit selection of the light output wavelength or spectral band;
• and
• b) a planar mirror disposed to provide the collimated light beam to the diffraction grating on the grating incident path, to receive the diffracted light from the diffraction grating also on the grating incident path,
• c) a concave mirror to provide the collimated light beam to the planar mirror along a planar mirror incident path and receive the diffracted light output along the same planar mirror incident path wherein the planar mirror can reflect the collimated light beam from the concave mirror on the planar mirror to the diffraction grating on the diffraction grating incident path and can reflect the diffracted light output received on the grating incident path to the concave mirror on the planar mirror incident path;
• d) a light input aperture, comprising an optical input opening through the planar mirror, to receive into the spectrometer light from the sample source and provide the sample light to the concave mirror for collimation;
• and
• e) a light output aperture, also comprising an optical output opening through the planar mirror, to receive the diffracted light output from the concave mirror and output the diffracted light output from the spectrometer, wherein the concave mirror focuses the diffracted light output to the light output aperture.