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Spectrum
Spectrometer
The measurement of the radiation spectrum requires a slightly different sensor set-up. The laser light is coupled into an optical fiber that is guided to a spectrometer. Coupling all the laser light is less important than with the PI measurement since also a partial coupling into the fiber will reveal the laser spectrum. The multi-mode fiber guides the light into a tabletop spectrometer that uses diffraction gratings to separate the different wavelength components from the laser light onto a detector array. The spectrometer itself is characterized by its wavelength range and its resolution. The wavelength range is determined by the diffraction gratings and the detectors it uses similarly as it is the case for the laser photodetector itself. The resolution is the smallest spectral feature that can be resolved and it is often increased with the spectrometer size. A spectrometer the size of a large desktop PC can reach a resolution of less than 0,1nm while smaller spectrometers would range in resolutions around 1nm or more.
The optical fiber can collect the radiation directly from the integrating sphere or from a dedicated lense that is placed in front of the laser diode. In the spectrometer the light is optically spread at different angles depending on the wavelength. An array of sensors produce the spectrum of the measured light.
Wavelength Parameters
The result of the measurement is a plot of light intensity as a function of wavelength. The numerical parameters that are extracted are usually the center wavelength (also called lasing mode), the mode spacing and the side mode suppression. These parameters are best shown on an actual laser spectrum. A parameter that hardly can be determined by this kind of measurement is the laser linewidth. What can be seen in the spectrometer is most certainly limited by the resolution of the spectrometer itself. Considering a very good spectrometer having a resolution limit in the range of 0.01nm and most laser diodes having a linewidth of 10-3nm or even 10-6nm makes the point clear.
A change in temperature also has an influence on the spectral properties of the laser. Increasing temperature usually leads to increasing laser wavelength. In standard Fabry Perot lasers this shift is related to the change in the refractive index of the laser material with temperature, hence increasing the optical length of the cavity. This material dependent coefficient is often depicted in the datasheets for the different materials such as GaAs, InP and GaN.
For lasers with a distributed feedback structure to select the wavelength, the effect of temperature is more complex and needs to be discussed in relation to the specifics of the distribution grating design.
InP laser spectrum.
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