Laser Diodes

Laser Diodes are semiconductor lasers and come in many different shapes and sizes with laser powers ranging from a few mW to hundreds of watts. The emitted wavelength depends mainly on the semiconductor material of the laser diode cavity and laser diodes are available to cover the full visible spectrum range from blue to red and beyond, even emitting in the infrared. The laser diodes distributed by Schäfter+Kirchhoff cover the whole wavelength range from 370 nm to 2300 nm.

Divergence and Polarization

The microscopic cross-section of the laser diode active area of 1×3 μm results in emitted radiation that is divergent. Most laser diodes have a divergent radiation cone of elliptical cross-section and an approximately Gaussian intensity distribution. The ellipticity can be overcome with the help of anamorphic optics.

Some diodes (e.g. VCSEL or Circular Laser) produce a circular beam profile.

The polarization of the emitted radiation is linear and is parallel to the active area of the diode. The degree of polarization varies with the diode current and is lowest at the threshold.

Temperature and Power Dependence

The emitted spectrum is influenced by diode temperature and diode current, as well as the geometry of the laser cavity. The front face and the end face serve as a Fabry-Perot cavity allowing multiple longitudinal modes. When operated just over the threshold, the diodes have a wavelength spectrum with equidistant peaks (longitudinally multimode). On increasing the diode current (to produce a higher power output), one of the longitudinal modes is usually favored and the diode emits in (longitudinally) singlemode. However, the gain profile and the refractive index of the material are temperature dependent and, so, other longitudinal modes can be amplified and the output wavelength changes rapidly by a few nm, resulting in mode hopping

For non-stabilized singlemode diodes, mode hopping occurs stochastically and the emitted wavelength and output power can change erratically by as much as 3%. For a temperature range of 20–30°C, the center wavelength can drift by 2.5–3 nm (GaAs). Since changing the diode current changes the diode temperature, the current/power output dependence of the laser diode is only nominal. When the laser power is increased from the threshold up to the nominal power then the wavelength increases by 2–4 nm.

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