Improving the spectral performance of extended cavity diode lasers using laser chips with bent ridge waveguide
Fig. 1 Schematic of ECDL with laser chip (a) with straight and (b) with bent ridge waveguide.
Fig. 2 (a) false color map of the RF spectra and (b) the corresponding side mode suppression ratios of the +1st and -1st side modes of the ECDL with straight waveguide-laser chip.
Fig. 3 (a) false color map of the RF spectra and (b) the corresponding side mode suppression ratios of the +1st and -1st side modes of the ECDL with bent waveguide-laser chip.
Extended cavity diode lasers (ECDLs) usually feature optical linewidths (full-width-at-half-maximum, FWHM) in the range of several kHz for the Lorentzian linewidth and few 100 kHz for time scales of 1 ms. For this reason, ECDLs are typically used in a variety of fields such as coherent optical communication and coherent manipulation of atoms and molecules utilized, for example, in optical atomic clocks and matter-wave interferometers.
Spectral purity is one of the key parameters of the performance of ECDLs. Experiments as well as simulations show the possibility of the coexistence of multiple stable continuous-wave (cw)-modes [1, 2]. The presence of strong side modes effectively broadens the optical linewidths of ECDLs and, as experiments show, also increases frequency noise of the main mode. Here, we present one approach to improve the side mode suppression of ECDLs by using semiconductor laser chips with an advanced ridge waveguide (RW) structure.
Fig. 1 shows schematics of the investigated ECDLs. These use 2 mm long, double quantum well AlGaAs RW diode laser chips with a ridge width of 5 µm and a volume holographic Bragg grating (VHBG) with a Bragg wavelength of 1064.125 nm for optical feedback. The RW diode laser chips differ with respect to the ridge waveguide geometry. One laser chip features a straight (Fig. 1a) and the other a bent ridge waveguide (Fig. 1b). Bending of the waveguide allows for a chip layout, which features one facet at right angle to the waveguide while the other is at an angle to the normal of the chip facet. At the latter facet the effective, unwanted back-reflection of RR = 10-3…10-4 achievable with anti-reflective coating only is further reduced by several orders of magnitude [3]. This strongly suppresses parasitic Fabry-Pérot modes of the laser chip. The reflectivity of the front facets corresponds to RF = 30%. In both cases the ECDL resonator length is chosen to feature a free spectral range of approximately 4 GHz.
To measure the side mode suppression ratio of the ECDLs, we carry out a heterodyne beat note measurement with a reference laser (a DFB laser in this case) to down-convert the optical spectrum of the ECDLs into the RF domain. The side mode suppression ratio is calculated as the power ratio between the main peak and the respective side mode peak in the RF spectrum.
Fig. 2a shows the false color map of the RF spectra when tuning the injection current up from 30 mA (below threshold) for the ECDL which is operated with the laser chip that features the straight ridge waveguide. The RF spectra are normalized to the RF peak power in the map. The corresponding side mode suppression ratios of the respective +1st and -1st side modes are shown in Fig. 2b. At some current settings the side modes are only 3.3 dB below carrier. The side modes appear periodically before a mode hop. After a mode hop the side mode suppression ratio exceeds 60 dB.
Fig. 3a and b show the false color map of the RF spectra and the side mode suppression ratios of the ECDL which is operated with the laser chip that features the bent ridge waveguide. The minimum side mode suppression ratio between threshold and 220 mA corresponds to 46 dB. Compared to the ECDL with straight waveguide-laser chip, the ECDL with the bent waveguide-laser chip shows an increased side mode suppression ratio, an increased single mode tuning range and therefore features improved spectral stability and spectral purity.
Publications
[1] M. Radziunas, V.Z. Tronciu, E. Luvsandamdin, C. Kürbis, A. Wicht, H. Wenzel, “Study of Microintegrated External-Cavity Diode Lasers: Simulations, Analysis, and Experiments”, IEEE J. Quantum Electron., vol. 51, no. 2, 2000408 (2015).
[2] V.Z. Tronciu, M. Radziunas, Ch. Kürbis, H. Wenzel, A. Wicht, “Numerical and experimental investigations of micro-integrated external cavity diode lasers”, Opt. Quant. Electron., vol. 47, no. 6, pp. 1459-1464 (2015).
[3] A. Klehr, H. Wenzel, J. Fricke, F. Bugge, G. Erbert, “Generation of spectrally stable continuous-wave emission and ns pulses with peak power of 4 W using a distributed Bragg reflector laser and a ridge-waveguide power amplifier”, Opt. Express, vol. 22, no 20, pp. 23980-23989 (2014).