External cavity diode laser setup with two interference filters

diode lasers (ECDLs) are an excellent choice in many cases [1] and are specifically used because of their low cost and the broad wavelength availability, which covers almost the entire spectrum from the near-infrared to near-ultraviolet. The first compact ECDL setup for single-frequency operation using a grating for the external cavity was described by Ted Hänsch in 1995 [2] and has remained an important configuration ever since. Twenty years later, ECDLs with the external feedback realized by a narrow-bandwidth interference filter and a semitransparent mirror [3] improved the stability against external perturbations. In recent years, the development of blue laser diodes based on gallium nitride and indium gallium nitride [4] extended the range of ECDLs to shorter wavelength. Nevertheless, due to the specific characteristics of these laser diodes, stable singlefrequency operation in the blue wavelengths region remains a challenge. In this work, we present an ECDL setup for stable single-frequency operation in the wavelength range around 450 nm (Fig. 1). We combine the design of an interference filter-based ECDL [3] in the specific realization of Ref. [5], adding a second filter and the intra-cavity stabilization technique presented by Führer et al. [6, 7]. The latter is based on the polarization-dependent locking scheme introduced by Hänsch and Couillaud [8] for locking lasers to an external cavity. We use a PL-450B indium gallium nitride (InGaN) laser diode manufactured by OSRAM [9] as active element. This single transverse mode diode has an optical output power of up to 80 mW in free-running mode. The high availability in volume and the low price make it an interesting candidate for ECDL developments. On the other hand, the spectral characteristics are less favorable for single-frequency operation than for more expensive diode variants as used in grating stabilized ECDLs in the blue so far [10, 11]. Abstract We present an external cavity diode laser setup using two identical, commercially available interference filters operated in the blue wavelength range around 450 nm. The combination of the two filters decreases the transmission width, while increasing the edge steepness without a significant reduction in peak transmittance. Due to the broad spectral transmission of these interference filters compared to the internal mode spacing of blue laser diodes, an additional locking scheme, based on Hänsch–Couillaud locking to a cavity, has been added to improve the stability. The laser is stabilized to a line in the tellurium spectrum via saturation spectroscopy, and single-frequency operation for a duration of two days is demonstrated by monitoring the error signal of the lock and the piezo drive compensating the length change of the external resonator due to air pressure variations. Additionally, transmission curves of the filters and the spectra of a sample of diodes are given.