Miniaturized External Cavity Diode Lasers emitting at 633 nm are best suited for application in absolute distance interferometry and atomic spectroscopy

Interferometric measurement of absolute distances require single-mode, tunable light sources and, depending on the measurement range and/or the frequency stability, a narrow linewidth. For instance, range finding with nm precision can be achieved when using red lasers featuring a wavelength tuning range of 25 GHz and a linewidth of 10 MHz. Visible wavelengths in the vicinity of 635 nm are particularly interesting for distance measurements due to eye safety regulations. Lasers with the above-named properties also fulfill the requirements for atomic spectroscopy (¹²⁷I₂ spectroscopy for instance).

We have developed a micro-integrated, tunable external cavity diode laser (ECDL) emitting at 633 nm with a reflection Bragg grating (RBG) as external mirror. The gain medium of the device is a laser diode (LD) emitting in the vicinity of 633 nm. The optical resonator is formed between the front facet of the laser diode and the surface of the RBG. Two gradient-index lenses, GRIN 1 and 2 collimate the beam inside the resonator and at the output of the device, respectively. The whole device is mounted on an AlN base plate having a footprint of 10 mm x 5 mm. Thermal management is carried out on-board with a micro-Peltier element. A model of the micro bench is shown in Fig. 1. The device is mounted partly by passive alignment and bonding using a flip-chip bonder (rails, Peltier element, sub-mounts, laser, thermistor, glass blocks) and partly by active alignment and gluing with UV-cured adhesive (lenses, RBG).

For single longitudinal mode operation, the free spectral range of the optical resonator (Δl_res) has to be larger than the bandwidth of the RBG (Δl_RBG). The principle is sketched in Fig. 2. The reflection spectrum of the RBG is constant, whereas the Fabry-Pérot modes are tuned by varying the current through the diode.

As seen in Fig. 3(a), the ECDL has a threshold current of 43 mA and a slope efficiency of 0.30 W/A averaged over the whole current range above threshold. A maximum output power of 10.1 mW was demonstrated at an injection current of 80 mA. From Fig. 3(b) it can be seen that the wavelength lies centered around 633 nm and is tunable without mode hops over a span of approximately 34 pm, corresponding to a frequency scanning range of 25 GHz. The laser linewidth of the ECDL is measured using a self-delayed heterodyne technique. As shown in Fig. 3(c), the laser linewidth at an output power ≤ 8 mW in the regions where no mode hops occur is smaller than 10 MHz.

The ECDL concept and the mounting technology developed in this work have been performed in view of a commercial product development. Moreover, the concept can be extended to other wavelengths and the tuning range can be adapted simply by using shorter laser chips and lenses. In order to quantify the linewidth of the ECDL, a linewidth measurement bench for red-emitting lasers has been built for the first time at the Ferdinand-Braun-Institute.

Publications

A. Bawamia, G. Blume, B. Eppich, A. Ginolas, S. Spießberger, M. Thomas, B. Sumpf, G. Erbert, "Miniaturized Tunable External Cavity Diode Laser with Single-Mode Operation and a Narrow Linewidth at 633 nm", accepted for publication in IEEE Photonics Technology Letters (2011).

B. Sumpf, A. Bawamia, G. Blume, B. Eppich, A. Ginolas, S. Spießberger, M. Thomas, G. Erbert, "Continuously current-tunable, narrow line-width miniaturized external cavity diode laser at 633 nm", submitted to SPIE Photonics West Conference 2012, San Francisco, California, US.

FBH research: 30.08.2011