Semiconductor lasers with resonant optical feedback reach intrinsic linewidths below 20 Hz
Fig. 1. Schematic of a DFB laser diode with resonant optical feedback from external Fabry-Pérot resonator. Right: compact monolithic confocal FP resonator implemented in this work
Fig. 2. RF spectrum of self-delayed heterodyne beat signal of a free-running DFB diode laser (red) and with (blue) resonant optical feedback
Fig. 3. Frequency noise power spectral density: free running DFB diode laser (red), the same laser with resonant optical feedback (blue) and grating-stabilized diode laser – ECDL – (geen)
High-precision atomic clocks, atom-interferometry experiments for extremely accurate measurements of the fundamental constants, and coherent optical free-space communication protocols are prominent examples of application fields of narrow-linewidth laser sources. Spectral properties of semiconductor lasers are often not sufficient for these applications. A common approach to reduce the frequency noise of a semiconductor laser is utilizing optical feedback from an external reflector e.g. a semi-transparent mirror, a Bragg grating or a Fabry-Pérot (FP) resonator. Compared to the other methods, the unique combination a DFB-laser diode with a resonant optical feedback from a high-finesse external FP-resonator allows for the best linewidth-reduction factor.
The idea of the resonant optical feedback is presented in Fig. 1. If the frequency of the free running DFB-diode is close enough to a resonance frequency of the confocal cavity, the resonant feedback is re-injected into the DFB-laser, while the non-resonant light is reflected. At the same time the laser locks its frequency to the cavity resonance.
Under the influence of the resonant optical feedback the linewidth of the laser is significantly reduced. Fig. 2 shows the RF-spectrum of a self-delayed heterodyne beat measurement. In this measurement the laser light is coupled to a Mach-Zehnder-interferometer with the optical path difference of 3 km (fiber delay line) and frequency shift of 78 MHz in one interferometer arm. Both beams are superimposed at the output of the interferometer and the resulting beat note is recorded with a fast photo-detector. Fig. 2 show the Fourier spectrum of the photo-detector signal.
The full information about the spectral noise of the laser is contained in the frequency noise power spectral density (PSD), shown in Fig. 3. The noise level of the laser with resonant optical feedback is more than five orders of magnitude smaller than the noise level of the same solitary diode laser without feedback, and it is three orders of magnitude smaller than the noise level of a narrow linewidth, grating-based, extended-cavity diode laser. The white frequency noise level is S0 = 5 Hz2/Hz, which corresponds to a Lorentzian linewidth of Δ∨ = S0∏ = 15.7 Hz.
In summary, resonant optical feedback allows for a substantial improvement of the spectral properties of semiconductor lasers. The resulting level of the frequency noise PSD is very close to that of the best performing low noise solid state lasers. Finally, the laser presented here can be assembled on a footprint of only 1×5 cm2 (diode, coupling lenses and the external cavity) which is important to ensure high mechanical stability.
Publication
W. Lewoczko-Adamczyk, C. Pyrlik, J. Häger, S. Schwertfeger, A. Wicht, A. Peters, G. Erbert, and G. Tränkle, "Ultra-narrow linewidth DFB-laser with optical feedback from a monolithic confocal Fabry-Pérot cavity", Opt. Express, 23, 9705 (2015)