External Cavity Controls Transverse Mode in VCSEL
The fundamental transverse mode of a vertical-cavity surface-emitting laser (VCSEL) produces a high-quality Gaussian beam, as it does with any laser. As the bias current of the device is increased, however, higher-order transverse modes often begin to oscillate, increasing beam divergence and decreasing beam quality.
Researchers at Instituto Mediterráneo de Estudios Avanzados in Esporles, Spain, have demonstrated that operating a VCSEL in an external Littman-like resonator can control the transverse mode of the laser. Such a configuration could increase the utility of the devices in many applications where a high-quality Gaussian beam is required.
Figure 1. A Littman-like external resonator forces a vertical-cavity surface-emitting laser to oscillate in its fundamental transverse mode.
The resonator is a three-mirror configuration, with the back surface of the VCSEL and an external mirror serving as end mirrors and the diffraction grating acting as the turning mirror (Figure 1). The zero-order reflection from the grating is the laser output, and the first-order reflection is the intracavity beam. Adjusting the orientation of the external mirror controls the frequency of the feedback. Such a configuration often is used to control the frequency of edge-emitting diode lasers and of nondiode lasers.
The researchers, however, used it to control the transverse mode and polarization of a VCSEL. When they operated the 840-nm laser without the external resonator, the VCSEL had a threshold current of 1.60 mA and oscillated in a pure fundamental mode at threshold. As the current was increased, the first transverse mode began to oscillate at 1.64 mA.
Figure 2. Adjusting the feedback frequency forces oscillation in either polarization of the fundamental transverse mode or in a higher-order transverse mode. The optical spectra for four different angles of the external mirror were collected with a bias current of 1.7 mA.
When the VCSEL was in the external resonator, the researchers could adjust the feedback frequency to force oscillation in either polarization of the fundamental transverse mode or in a higher-order transverse mode (Figure 2). In the spectra shown, the first peak occurred when the laser was oscillating in a pure fundamental mode (TEM00) in one polarization, and the second peak occurred when the laser oscillated in fundamental mode in the orthogonal polarization. (The frequency difference between the two polarizations is the result of inherent birefringence in the VCSEL.) The third peak occurred when the laser was oscillating in the first high-order mode (TEM01), and the fourth peak occurred when the laser was oscillating in the second high-order mode (TEM11).
Seeking to maximize the fundamental-mode output, the investigators increased the bias current to 4 mA — 2 1/2 times the threshold — and obtained 2.7 mW of fundamental-mode output. When the current exceeded 4 mA, the VCSEL broke into higher-mode oscillation. The same laser, operating at a 4-mA bias current without an external resonator, produced only 1.9 mW, with four transverse modes oscillating.
- beam divergence
- Increase in the diameter of an initially collimated beam, as measured in milliradians (mrad) at specified points; i.e., where irradiance is a given fraction (often 1/e2) of peak irradiance.
- gaussian beam
- A beam of light whose electrical field amplitude distribution is Gaussian. When such a beam is circular in cross section, the amplitude is E(r) = E(0) exp [-(r/w)2], where r is the distance from beam center and w is the radius at which the amplitude is 1/e of its value on the axis; w is called the beamwidth.
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