Optical spectrum analyzer using a diffraction grating and multi-pass optics

An optical spectrum analyzer apparatus comprising:

• a transmissive diffraction grating element for angularly dispersing light incident thereupon according to its constituent wavelengths;
• a polarization control module for receiving input light for analysis, decomposing the received input light into its ordinary and extraordinary components to produce corresponding first and second light beams having respective mutually orthogonal linear states of polarization and, using at least one twisted polarization-maintaining fiber, rotating the state of polarization of at least one of the first and second light beams relative to the state of polarization of the other of the first and second light beams until the two states of polarization are parallel,
• input slit means for receiving said first and second light beams from the polarization control module and directing said first and second light beams to said diffraction grating element such that their states of polarization are parallel to each other and perpendicular to the dispersion plane of the diffraction grating element;
• a pair of non-planar optical reflection means for defining paths whereby each of said first and second light beams is directed to traverse the diffraction grating element a predetermined plurality of times each traversal occurring in a different dispersion plane from the previous traversal;
• output slit means for selectively receiving at least a portion of each of the first and second light beams following diffraction said predetermined plurality of times;
• detection means for detecting the first and second light beams leaving the output slit means and detecting energy at each of said different wavelengths without first recombining the light beams optically;
• and
• control means for adjusting one or both of the diffraction grating element and the non-planar optical means so as to select different wavelengths of the dispersed first and second light beams for output by the output slits and processing the detected energy values.