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Diode lasers are additionally stabilized due to their long line length and fluctuations in the emitted wavelengths. The line width of the laser diodes in the magnitude of 10 MHz is insufficient for many applications in quantum optics and nuclear physics. The targeted excitation of atomic states via optical transitions in particular requires a high frequency resolution. A diffraction grating which reflects part of the light emitted by the laser diode back to it can be used to create an external resonator. Via the resonance condition a particular wavelength of the light emitted by the diode is selected and the line width of the laser considerably reduced. The diode emits a light beam horizontally and on the opposite side the beam hits a diffraction grating, whose first diffraction order is reflected back to the diode. The zero order beam is deflected by 90° and decoupled from the laser. The reflection grating and the exit surface of the laser diode act as an external resonator of length l with a resonance wavelength
λres = n · 2l
By changing the angle &alpha between the grating plane and the beam axis, you can precisely adjust the wavelength of the first order reflected back to the diode. The pivot point of the grating must be positioned so that the resonator length adjusts accordingly and it is possible to fulfill the resonance condition with the selected wavelength. The line width of the lasers is thereby reduced by up to several hundred kilohertz.
The angle &alpha between the grating and the diode beam can also be adjusted. When the laser is optimally adjusted, an applied alternating current can be used to tune a frequency range of several gigahertz continuously and without mode hopping. This procedure is used in spectroscopy. Furthermore, a control signal can be applied to stabilize the laser on a spectral line.
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