Tunable semiconductor laser with two acousto-optic tunable filters in its external cavity

Abstract

We report a laser with a linear external cavity containing a near-IR injection semiconductor optical amplifier as a gain element and two acousto-optic tunable filters (AOTFs) with quasi-collinear interaction of acoustic and light waves. At identical frequencies of RF signals that control the filters and ensure compensation for the Doppler frequency shift of light passing through the AOTFs, the steady-state laser emission linewidth can be reduced to about 25 MHz, which is almost three orders of magnitude smaller than that in the case of one filter (20 GHz). The position of the emission line, which remains narrow, slowly fluctuates within a spectral range about 3 GHz wide, which seems to be due to the large length of the external cavity and insufficient thermal stabilization of its components. This does not prevent us from obtaining light with a large coherence length. In emission wavelength sweep mode, the instantaneous emission linewidth increases with tuning rate, reaching 0.022 nm (8.8 GHz) at the highest tuning rate: 104 nm s−1. In the case of automatic control over the output optical power at a level of 3 mW, the tuning range is 815 – 875 nm. Such instruments are of practical interest for optical coherence tomography, spectroscopy, optical metrology, and other application areas.

Advances in wide-tuning and narrow-linewidth external-cavity diode lasers

“…The basic structure is shown in Figure 5. Commonly used optical filter components include the Fabry-Perot (FP) cavity etalon [28], acoustooptic tunable filter (AOTF) [29,30], electro-optic tunable filter [31], birefringent filter (BRF) [32,33], interference filter (IF) [34,35], micro-electro-mechanical systems (MEMS) tunable optic filter [36], alldielectric thin film Fabry-Perot filter (AFPF) [37], guided-mode resonator filter (GMRF) [38], and liquid crystal film [39]. By choosing different optical filters, the wavelength-tuning and linewidth-narrowing laser mechanisms also change accordingly.…”

“…In addition, there is ample room for the optimization of tuning speed, output power, system size, and compactness. The Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences [37], National Time Service Center, Chinese Academy of Sciences [35], Hong Kong Polytechnic University in China [28], Institute of Electron Technology in Poland [83], Australian Defense Science and Technology Group [29], Axsun Technology in the USA [84], Stel'makh Polyus Research Institute in Russia [30], among other institutions [85][86][87][88][89] have conducted in-depth research on filter-ECDLs.…”

“…In 2020, Russia's Stel'makh Polyus Research Institute [30] reported a study on an ECDL with two AOTFs used as frequency-selective components. The laser has a tuning range of 60 nm (815-875 nm) with an output power of 3 mW.…”