Beam Properties

Polarization

The emission from a unipolar laser is always polarized linearly with the electric field perpendicular to the layers (and the copper sub mount) because the intersubband transition exhibits a quantum mechanical selection rule. With the copper submount on a table, the light is emitted with vertical polarization.

Beam divergence

The unipolar laser is designed around a tightly confined waveguide. For this reason, the beam diffracts strongly at the output facet and has a (full) divergence angle of about 60 degrees perpendicular to the layer and 40 degrees parallel to the layers (see figures below). A f#1 optics will typically collect about 70% of the emitted output power. Be careful that the collected output power will decrease with the square of the f-number of the collection optics.
Typical beam profile 
of an uncollimated QCL.
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Our TO3-L and HHL-packaged QCLs are collimated with a high numerical aperture (NA = 0.85) aspherical lens to maximize collection efficiency. The beam diameter is ≤ 4 mm at the output of the package and the full-angle beam divergence at 1/e^2 is ≤ 6 mrad for HHL and ≤ 10 mrad for TO-3. A beam picture of a typical collimated QCL emitting at a wavelength of 8 µm is displayed below, showing the good beam quality.


Typical beam profile 
of a collimated QCL.

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Mode


All of our stock lasers have a single spatial mode. The mode is typically the Gaussian TEM00 mode. If this is a specific requirement in your application, please be certain to ask for a mode measurement.