High optical rejection optical spectrum analyzer/monochromator

A dual-channel, double-filtering, multi-pass optical spectrum analyzer comprising:

• (i) a diffraction grating (DG);
• (ii) first and second input ports (P1′, P1″) for directing first and second input light beams (LR, LT), respectively, onto the diffraction grating in a first plane for dispersion a first time;
• (iii) polarization-decomposing means (PDM) for decomposing a single light beam into first and second components having mutually-orthogonal linear states of polarization and supplying said first and second components to said first and second input ports as said first and second input light beams (LR, LT), respectively,
• (iv) a retroreflector means (RAM1) for receiving the first and second dispersed light beams and returning same to the diffraction grating in a second plane for dispersion a second time;
• (v) first and second intermediate output ports (P2′, P2″) for receiving the first and second twice-dispersed light beams, respectively;
• (vi) first and second secondary input ports (P3′, P3″) coupled to the first and second intermediate output ports, respectively, by polarization-maintaining waveguide means (PMF2′, PMF2″) and for directing the twice-dispersed first and second light beams onto the diffraction grating in a third plane for dispersion a third time, with their states of polarization having a predetermined orientation relative to the states of polarization of the first and second light beams when first incident upon the diffraction grating, the retroreflector means (RAM1) reflecting the three-times-dispersed first and second light beams back to the diffraction grating means in a fourth plane for dispersion a fourth time;
• (vii) first and second output ports (P4′, P4″) for receiving the first and second light beams, respectively, after dispersion the fourth time, the first, second, third and fourth planes being parallel to each other and the dispersion plane and spaced from each other in a direction perpendicular to the dispersion plane;
• (viii) means (TT) for rotating at least one of the first retroreflector and the grating to effect wavelength scanning;
• (ix) first and second detectors (D′, D″) for receiving the first and second light beams from the first and second output ports (P4′, P4″), respectively, and converting same to first and second electrical signals, respectively;
• and
• (x) microprocessor means (MP) coupled to said rotating means (TT) and to the detectors (D′, D″) for controlling rotation of the first retroreflector (RAM1) and processing the first and second electrical signals;
• the arrangement being such that, each time the first and second light beams are incident upon the diffraction grating, their linear states of polarization are substantially parallel to each other and the dispersion planes for all wavelengths within an operating band of the optical spectrum analyzer.