Classically, an external cavity diode laser is built by feeding the -1st order reflection of a grating back into the laser diode. This results in a frequency filtering, as the grating angle determines the frequency of the retro-reflected light. The operating principle is illustrated in the figure on the right. The grating based lasers offer large mode-hop free tuning, especially in the similar Littman-Metcalf configuration. An inherent disadvantage is the movable grating, which reduces the long term stability of the laser cavity. The wavelength dependent output angle can be compensated by an additional mirror, which is fixed to the grating.

Interference filter based diode lasers have emerged in the last years as narrow tunable interference filters with high transmission became available. They have transmissions up to 98% with a spectral FWHM of 0.35nm at 780nm laser wavelength. Their transmission frequency can be tuned by tilting the filter. The per-angle-tuning is lower compared to a grating and drifts or vibrations of the filter do not influence the total cavity length. Therefore, the laser cavity can be constructed as a monolithic system with the piezo at the so called cat-eye end reflector as a fine-tuning element. The cat-eye construction consists of a lens that focuses the laser beam onto the endmirror, thus ensuring a 180° reflection angle. However, the transverse position of the element has to be aligned to ensure an optimal mode overlap with the incident beam. 

The benefits of this construction are the less sensitive alignment and resulting better long term stability as well as the insensitivity of the output mode to coarse wavelength tuning. In addition, depending on the output mirror reflectivity and cavity length, this laser can offer significantly lower noise. In the following two laser constructions are introduced where one focuses on compactness and large mode-hop free tuning as a general purpose laser and the other on lowest possible noise, which results in narrow linewidth and low spectral noise, comparable to Ti:Sa lasers.

This laser was designed for optomechanics experiments, which are extremely sensitive to laser noise. There, the noise power at the frequency of the mechanical oscillator is determining the optomechanical cooling performance. This noise has been reduced by extending the laser cavity to about 2.5m while keeping a small footprint of the cavity housing. The total linewidth also benefits from this and becomes very narrow. Remaining fluctuations are dominated by low frequency mechanical modes and could be compensated by locking to a stable cavity, which should result in linewidths <<1kHz. A picture of the laser is shown below, next to the specifications.

All properties are typical values only and depend on the laser diode and optics properties.

The second version is a general purpose laser with reduced cavity length and focus on usability and modulation/tuning capabilities. It has integrated cylindrical lens pair for beam shaping and single stage isolator, coarse wavelength tuning form outside, large mode-hop free tuning and fast modulation. The specifications are shown below next to a picture of the laser.

All properties are typical values only and depend on the laser diode and optics properties.