Optical spectrum analyzer having double-pass monochromator

A scanning monochromator comprising:

• means for providing an input light beam;
• a diffraction grating for diffracting the input light beam along a dispersion axis to produce a spatially dispersed light beam;
• means for rotating said diffraction grating such that said dispersed light beam is scanned along the dispersion axis;
• means for converting said dispersed light beam into an output light beam;
• an output aperture for receiving said output light beam;
• and
• means for causing movement of said output aperture such that said output light beam and said output aperture remain in alignment during rotation of said diffraction grating.

A scanning monochromator comprising:

• means for providing an input light beam;
• a diffraction grating for diffracting the input light beam along a dispersion axis to produce a spatially dispersed light beam;
• means for rotating said diffraction grating such that said dispersed light beam is scanned along the dispersion axis;
• means for converting said dispersed light beam into an output light beam;
• an output aperture for receiving said output light beam;
• an optical detector coupled to said output aperture for converting said output light beam to an output electrical signal;
• and
• means for causing displacement of said output aperture such that the output electrical signal of said optical detector is representative only of stray light in the monochromator and electrical noise generated by the optical detector.

A scanning monochromator comprising:

• means for providing an input light beam;
• a first stage monochromator including a first diffraction grating for diffracting the input light beam and mean for rotating said first diffraction grating, said first stage monochromator providing a first stage output light beam;
• a second stage monochromator including a second diffraction grating for diffracting the first stage output light beam and means for rotating said second diffraction grating, said second stage monochromator providing a second stage output light beam;
• and
• means for rotating the polarization components of said first stage output light beam by 90.degree. prior to incidence on said second diffraction grating such that the second stage output light beam is substantially independent of the polarization of said input light beam.

A method for detecting the output light beam of a monochromator having an output aperture, said method comprising the steps of:

• detecting an output of the monochromator with the output aperture accurately aligned with an output light beam to provide a first measurement;
• displacing the output aperture so that the output light beam is no longer incident on the output aperture;
• detecting the output of the monochromator with the output aperture displaced from the output light beam to provide a second measurement;
• and
• subtracting the second measurement from the first measurement to provide an accurate measurement of the output light beam.

A double-pass scanning monochromator comprising:

• means for providing an input light beam;
• a diffraction grating for diffracting the input light beam along a dispersion axis to produce a spatially dispersed light beam;
• a slit for passing a selected portion of said dispersed light beam to produce a filtered light beam;
• means for rotating said diffraction grating such that said dispersed light beam is scanned along said dispersion axis, said means for rotating said diffraction grating comprising a drive shaft rigidly connected to said diffraction grating and defining an axis of rotation, a motor for rotating said drive shaft about said axis of rotation, a shaft angle encoder directly connected to said drive shaft for sensing the rotational position of said diffraction grating, and feedback means responsive to the sensed rotational position and to a sweep signal for controlling said motor;
• means for directing said filtered light beam to said diffraction grating such that said filtered light beam is recombined by said diffraction grating to produce an output beam;
• an output aperture for receiving said output light beam;
• an optical detector coupled to said output aperture for converting said output light beam to an output electrical signal;
• means for translating said output aperture in a plane perpendicular to said output light beam to track said output light beam during rotation of said diffraction grating, said means for translating said output aperture including means for displacing said output aperture relative to said output light beam in an optical chopping mode such that the output electrical signal of said optical detector is representative of stray light in the monochromator and electrical noise generated by the optical detector;
• and
• means for rotating the polarization components of said dispersed light beam or said filtered light beam by about 90.degree..

An optical spectrum analyzer comprising:

• means for providing an input light beam;
• a diffraction grating for diffracting the input light beam along a dispersion axis to produce a spatially dispersed light beam;
• a slit for passing a selected portion of said dispersed light beam to produce a filtered light beam;
• means for rotating said diffraction grating such that said dispersed light beam is scanned along said dispersion axis;
• means for sensing the position of said diffraction grating and providing a diffraction grating position signal;
• means for directing said filtered light beam is recombined by said diffraction grating to produce an output light beam;
• an output aperture for passing a selected portion of said output light beam to provide increased selectivity, whereby a high close-in dynamic range is achieved;
• an optical detector coupled to said output aperture for converting said output light beam to an output electrical signal;
• and
• means for displaying said output electrical signal as a function of said diffraction grating position signal to provide a display of the optical spectrum of said input light beam.

A method for analyzing a light beam with a scanning monochromator, comprising the steps of:

• providing an input light beam;
• diffracting the input light beam along a dispersion axis with a diffraction grating to produce a spatially dispersed light beam;
• passing a selected portion of said dispersed light beam through a slit to produce a filtered light beam;
• rotating said diffraction grating such that said dispersed light beam is scanned along said dispersion axis;
• directing said filtered light beam to said diffraction grating such that said filtered light beam is recombined by said diffraction grating to produce an output light beam;
• and
• directing said output light beam through an output aperture for passing a selected portion of said output light beam to provide increased sensitivity, thereby achieving a high close-in dynamic range.

A scanning monochromator comprising:

• means for providing an input light beam;
• a first stage monochromator including a first diffraction grating for diffracting the input light beam and means for rotating said first diffraction grating, said first stage monochromator providing a first stage output light beam;
• a second stage monochromator including a second diffraction grating for diffracting the first stage output light beam and means for rotating said second diffraction grating, said second diffraction grating diffracting said first stage output light beam such that said first stage output light beam is recombined by said second diffraction grating to provide a second stage output light beam;
• and
• an output aperture for passing a selected portion of said second stage output light beam to provide increased selectivity, whereby a high close-in dynamic range is achieved.