Is there something specific in VCSEL spectrum? Vertical Cavity Surface Emitting Lasers (VCSEL) are semiconductor laser diodes with laser beam emission perpendicular to the top surface of the chip. Tens of thousands of VCSEL can be processed simultaneously on a wafer and functionally tested on-waver before they are cleaved into individual chip. The output aperture of VCSEL is characterized by a small divergence angle (typically 0.2 to 0.8 N.A.) associated with a diameter which can be typically between 6 and 150 µm.
Depending on the size of the cavity, VCSEL are naturally single-mode or multimode, but spatial filtering can be added at the output of the cavity in order to only emit the longitudinal mode. This is a way to increase the power of single-mode VCSEL.
VCSEL spectrum stability and coupling
VCSEL make possible high coupling efficiency with optical fibers, contrary to many standard laser-edge diodes with larger apertures. However, in the case of multimode VCSEL, the resulting optical etendue of the fiber has to be carefully compared with the fiber to be used. Lower optical etendue of the fiber, compared to the etendue of the VCSEL, generally results in unequal coupling between spatial modes. Indeed, in the case of a multimode VCSEL the different spatial modes (with different emission angles) correspond to different spectral modes i.e. different spectral peaks.
A multimode VCSEL, considered as a simple packaged chip, is intrinsically unstable. A very small change in the driver current or temperature shift the central wavelength and induce transfer of energy between modes. Moreover, speckle effect has to be taken into account when considering coupling with an optical fiber or a detector and has not to be mistaken for VCSEL instability.
VCSEL spectrum depending on the application
The distance between spectral modes is typically around 15 pm at the minimum and the global linewidth of the spectrum variable from few pm to more than 1 nm. For instance, the VCSEL used for telecom optical link are submitted to the 1000 BASE-SX Gigabit Ethernet specification requiring a spectral linewidth (rms) of 0,85 nm at 850 nm.
Single-mode VCSEL can also be slightly tuned in wavelength. But, one may be careful that many of them, sold as single-mode, are in fact multimode for most of the temperature and current combinations and it is sometimes very difficult to find a stable single-mode regime.
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