A Laser Spectrum Analyzer (LSA) can be defined as a high resolution spectrometer aimed at the characterization of narrow linewidth light sources. In other words it measures the spectral density of a laser as a function of the wavelength. These instruments are typically used to check the spectral purity of laser sources, or to measure their Full Width Half Maximum.
It exists two main methods to perform spectral analysis:
- Grating based Laser Spectrum Analyzers:
Using one or several diffraction grating elements (which have wavelength-depending refraction angle), the grating based Laser Spectrum Analyzer will disperse the light and the wavelengths will propagate in different directions as shown in figure 1. Then thanks to a set of optical elements, the dispersed light will get to a photodetector and read out. This technic offers the advantage of covering a very large wavelength range and a really good Signal to Noise Ratio.
On the other hand, the acquisition time of the full spectrum can be long (about a second) therefore it is not suitable for real-time analysis or single pulse characterization. The device also needs to be very regularly recalibrated due to a quick loss of absolute accuracy coming from the slight misalignment of the mechanical parts that can occur in time.
Figure 1: Grating spectrometer working principle
- Interferometric Laser Spectrum Analyzers:
Based on Michelson, Fizeau or Fabry-Perrot interferometers as presented on figure 2, these instruments offer much higher spectral resolution than a grating Laser Spectrum Analyzer (in the order of a few GHz) and better absolute accuracy but only in a limited spectral range. They generally use opto-mechanic elements to mechanically scan the incoming signal and the read-out is performed thanks to a camera.
Figure 2: Interferometric spectrometers working principle
Recent advances in the semiconductor technologies have allowed the fabrication of a whole new range of integrated Laser Spectrum Analyzer using interferences. The SWIFTS technology is a good example of how a Laser Spectrum Analyzer can be integrated into a compact design while keeping high spectral resolution, and high measurement rate.
Figure 3: SWIFTS technology working principle
To conclude, there are several existing solutions to perform the characterization of a laser and choosing one requires a tradeoff between speed, accuracy, size, accessible spectral range and price. Also, it is worth noticing that there is a simplified version of the interferometric Laser Spectrum Analyzer specialized in the single wavelength tracking: the wavelength meter. On the contrary of a regular LSA which reads-out the spectrum, the wavelength meter has a single output: the wavelength of a laser, generally with a much better precision than a Laser Spectrum Analyzer.