Optical Spectral Analysis

Types of Optical Spectrum Analyzers

On an optical spectrum analyzer, incoming light passes through a wavelength-tunable optical filter (monochromator or interferometer) which resolves individual spectral components. The photodetector then converts the optical signal to an electrical current proportional to the incident optical power. An exception to this description is the Michelson interferometer, which is not actually an optical filter. The current from the photodetector is converted to a voltage by the transimpedance amplifier and then digitized. Any remaining signal processing, such as applying correction factors, is performed digitally.

The signal is then applied to the display as the vertical, or amplitude, data. A ramp generator determines the horizontal location of the trace as it sweeps from left to right. The ramp also tunes the optical filter so that its resonant wavelength is proportional to the horizontal position. A trace of optical power versus wavelength results. The displayed width of each mode of the laser is a function of the spectral resolution of the wavelength-tunable optical filter.

Interferometer-based optical spectrum analyzers

Fabry-Perot interferometers

The Fabry-Perot interferometer, consists of two highly reflective, parallel mirrors that act as a resonant cavity which filters the incoming light. The resolution of Fabry-Perot interferometer based optical spectrum analyzers, dependent on the reflection coefficient of the mirrors and their spacing, is typically fixed, and the wavelength is varied by changing the spacing between the mirrors by a very small amount.

The advantage of the Fabry-Perot interferometer is its very narrow spectral resolution, which allows it to measure laser chirp. The major disadvantage is that at any one position multiple wavelengths will be passed by the filter. The spacing between these responses is called the free spectral range. This problem can be solved by placing a monochromator in cascade with the Fabry-Perot interferometer to filter out all power outside the interferometer’s free spectral range about the wavelength of interest. 

Michelson Interferometers

The Michelson interferometer, is based on creating an interference pattern between the signal and a delayed version of itself. The power of this interference pattern is measured for a range of delay values. The resulting waveform is the autocorrelation function of the input signal. This enables the Michelson interferometer-based spectrum analyzer to make direct measurements of coherence length, as well as very accurate wavelength measurements. Other types of optical spectrum analyzers cannot make direct coherence-length measurements.

To determine the power spectra of the input signal, a Fourier transform is performed on the autocorrelation waveform. As no real filtering occurs, Michelson interferometer-based optical spectrum analyzers cannot be put in a span of zero nanometers, which would be useful for viewing the power at a given wavelength as a function of time. This type of analyzer also tends to have less dynamic range than diffraction-grating-based optical spectrum analyzers. 

Learn more, by downloading this application note on Optical Spectral Analysis.