As we have seen in a previous article, VCSEL are often used for NIR illumination. One of the technical reasons, apart from their low size and low weight, is that they offer high performance spectral characteristics. Among those spectral characteristics, one can notice a narrow laser spectrum, a quite stable wavelength and a low wavelength dependance to temperature.
Therefore, manufacturers of VCSELs and NIR illuminators control the spectral properties, meaning the frequency response and behaviours at different wavelengths, the peak wavelength and the full-width-at-half-maximum (FWHM) of their VCSELs.
The VCSEL emission spectrum is defined by the cavity mode. For example, the typical FWHM of a multimode VCSEL is below 1 nm and single-mode VCSEL can also be designed to reach linewidth well below 0.010 nm. Here you can see on the image 1 the laser spectrum of a multimode VCSEL. Looking at the spectral density of each mode, the central wavelength is centred at 846.682 nm and the FWHM is 0.352 nm.
One of the spectral characteristics of the VCSEL is also the spectral shift with temperature. The VCSELs have a wavelength dependence to temperature which is quite low. It is also defined as the VCSEL's change of cavity mode (0.06 nm/°C).
The wavelength of VCSELs also depend on the current applied. On the image 2, the fundamental mode of a VCSEL shift with the current applied, about 0.04 nm for 1 mA.
Some manufacturers of VCSEL Laser illuminators have implemented high resolution characterization platforms for controlling the purity of the laser spectrum, the FWHM and the peak wavelength. FLIR Systems in USA, for example, implemented the ZOOM Spectra laser spectrum analyser. The ZOOM Spectra offers ultra-high spectral resolution (6 GHz) in NIR (<1100 nm), a robust Gigabit Ethernet data communication and a long-lasting factory calibration, which are particularly useful for demanding VCSEL production environments.
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