Optical spectrum analyzer

Jon Titus

-- Test & Measurement World, 6/1/2001

Figure 1

Figure 2

Engineers who work with LEDs, laser diodes, and other types of lasers need to know about the radiation produced at various wavelengths. These optical sources may look monochromatic to the human eye, but they include many wavelengths. To determine the wavelengths and the intensity of light at each one, engineers use an optical spectrum analyzer (OSA). The results show side peaks, wavelength separations, relative intensities, peak broadening, and other characteristics.
  Anyone who has seen a rainbow or colors in a film of oil on water has seen how two effects-refraction and diffraction-separate light into wavelengths. The optical part of an OSA, called a monochromator, works in similar ways. You could use a prism in a monochromator, but a prism attenuates too much light for many measurements, and it doesn't separate wavelengths as well as a diffraction grating (Figure 1).

How does it work?
A grating provides a series of closely spaced parallel grooves on a surface. The spacing causes constructive interference of light that effectively separates the light into a series of reflected beams. (The surfaces of an extremely thin oil film on water produce the same type of constructive interference.) The zero-order beam from a grating reflects the light source, but the 1st-order, 2nd-order, and higher-order beams separate light into wavelengths. A monochromator may scan multiple orders to cover a range of wavelengths.
  One unique monochromator design, featured in the Agilent 86140B OSA, provides a single grating, but it uses the grating twice. As shown in Figure 2, light (path 1) enters the monochromator and gets diffracted by the grating (path 2). A band of wavelengths pass through an adjustable aperture-in effect a filter-that lets an OSA user select bandwidths from 0.06 nm to 10 nm in several steps. Now comes the clever part: A mirror reflects light in the selected bandwidth back to the grating that recombines the wavelengths into a single beam that duplicates the geometry of the input light beam. An optical fiber gathers the light and transports it to a photodetector.
  The OSA provides the electronics and mechanical components that "sweep" the monochromator's diffraction grating across the wavelength span of interest while an ADC measures light intensity. The instrument plots intensity vs. wavelength data. Not all OSAs rely on diffraction gratings, however. Some employ Fabry-Perot or Michelson interferometers. An application usually dictates which type of OSA best suits the measurements of interest.