Numerous imaging applications require IR illumination. IR illuminators are used for night vision in both defence and civilian use, as street illumination, high-speed video, 3-D imaging, hunting, fishing and rescue. And in recent years, high-power Vertical-Cavity Surface-Emitting Lasers (VCSEL) showed that they ruled infrared (IR) and near infrared (NIR) illumination.
For most consumer applications, it is preferable that the illumination be unobtrusive to the human eye, and therefore in the infrared region. On the other hand, low cost silicon photodetectors or cameras limit the wavelength on the long end of the spectrum. Therefore, for these applications, the most desirable range is between 800 and 900 nm. Most common wavelength are 830, 976, 1064 nm. But many others between 630 and 1064 nm are available.
Some of these NIR illuminators are on Size Weight and Power (SWaP)-sensitive platforms. In such cases, VCSEL are very suitable. VCSEL-based compact Laser illuminators are high power laser illuminators capable of up to tens or even hundreds of watts of peak powers. VCSEL are available in arrays and the array size can be increased to achieve hundreds of watts. And VCSELs are available for CW, quasi-CW or pulsed illumination.
For NIR illumination, the presence of background radiation such as sunlight can degrade the signal to noise ratio (SNR) of a sensor or camera. This can be compensated with a spectral filter on the detector or camera, but implementing this without a loss of efficiency requires a light source with a narrow and stable spectrum. And once again, VCSEL are compelling with those requirements.
When comparing VCSELs to different IR sources for illumination, like high-power LED and edge-emitting laser diodes (EELD), it is clear that VCSEL have the best spectral properties. They have a narrow laser spectrum, a quite stable wavelength and are much lower wavelength dependant to temperature (5 times less than EELDs).
The VCSEL emission spectrum is defined by the cavity mode. For example, the typical full-width-at-half-maximum (FWHM) of a multimode VCSEL is below 1 nm, whereas a LED has about 30 nm FWHM and an EELD has about 3-5 nm FWHM. Narrower-band filters can also be used in combination with VCSELs to improve signal-to-noise ratio of the system through better rejection of parasitic light. In addition, spectral shift with temperature is also defined by the VCSEL’s change of cavity mode (0.06 nm/°C).
Now, you know why VCSEL are often used for NIR illumination and the importance of their spectral properties.
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