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Laser Diode Characterization

Beam Pattern

Near Field Pattern

Beam profile parameters have a great influence in the performance of a laser diode and are usually part of a measurement routine as well. The spatial distribution of radiation will have a major influence in how well (or how hard) most of this radiation can be utilized in an optics setup or if a considerable part of the laser light will go aside of the optic setup.

The latter case would not only mean a very low system efficiency, but also poses additional problems created by the stray laser light due to eye safety concerns or by the heat that is created when the stray light is absorbed by neighboring apparatus.

The near field can be measured with an imaging microscope objective onto a CCD imager. Astigmatism effects can make the imaging a bit tricky since the sides of the emitter can have their foci within the laser cavity. This is specially true for gain guided lasers and it leads to give the wrong impression that the near-field image is elliptical.

Near Field measurement of a laser bar with multple emitters.

Far Field Measurement

Whereas the near-field distribution is interesting to understand new laser designs, the more practical far-field (Fraunhofer regime, d > a²/λ0) regime has more empirical consequences and is therefore measured instead in the context of laser diode industrial production. For a typical edge emitting “single mode” laser diode with an maximum aperture of just 5µm this regime is reached at just 50µm off the laser emitter facet. On the other side, the regime threshold for a multimode laser diode with a maximum 100µm emitter size starts at 1cm distance from the emitter facet. The optical intensity needs to be measured as a function of angular position about a complete hemisphere. For edge-emitting laser diodes with an elliptic beam profile, just the two axes vertical and horizontal are measured. The laser is turned on at a low power that avoids over heating and a sensor at sufficient distance from the laser diode is scanned across an entire half circle having the laser facet at its center.

Example for the far field radiation pattern of a laser diode.

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