FBH research: 08.06.2021

Superior output power – up to 7 W from 783 nm wavelength-stabilized DBR tapered laser diodes

Raman spectroscopy as well as pump sources for Tm:YAG lasers require wavelength-stabilized laser diodes emitting around 785 nm. For such applications, a narrow spectral emission width as well as a good beam quality is needed. Distributed Bragg reflector (DBR) tapered laser diodes are a cost-efficient and robust solution to meet these requirements.

Promising tools for medical in-vivo applications are based on Raman spectroscopic techniques. Raman spectroscopy offers fast, label-free and non-destructive analysis and can, for example, be used to identify cancerous tissue. Within the research project HypeRam an innovative imaging technique which combines spectral and lateral information has been developed. It enables tissue classification and, for example, identification of tumor boundaries. FBH developed suitable laser diodes as excitation light sources. The requirement for this imaging technique was a laser source with narrow linewidth emission at around 785 nm and up to 4 W output power.

Based on a well-established vertical structure at 783 nm, tapered laser diodes with an optical output power of up to 7 W were manufactured [1,2]. The overall length of the devices is 4 mm (Fig. 1). The DBR section consists of a tapered 500 µm long 10th-order surface grating which was etched using e-beam lithography. The ridge waveguide (RW) section of 1 mm length and the tapered section of 2.5 mm length have separate electrical contacts. The RW width is 2.2 µm, which widens in the tapered section to a width of 264 µm at the front facet.

Up to 6.4 W, the spectral emission is dominantly single mode. Some mode hops occur as expected for DBR laser diodes. At 6 W output power the measured spectrum shows a full width at half maximum (FWHM) of about 19 pm, limited by the resolution of the used spectrometer (Fig. 2). At 1/e2-level the lateral beam propagation ration M2 is 2.5 at 6 W. The central lobe contains 70 % of the emitted power.

To the best of our knowledge this is the highest reported output power for monolithic DBR tapered laser diodes with wavelength stabilization at 783 nm.

This work has been supported by the Leibniz Association within the project HypeRam and was partly funded by the German BMBF within the framework of "Research Fab Microelectronics (FMD)" under ref. 16FMD02

Publications:

[1] B. Sumpf, L. S. Theurer, M. Maiwald, A. Müller, A. Maassdorf, J. Fricke, P. Ressel, G. Tränkle, „Narrow spectral line-width 785 nm DBR tapered lasers with 7 W output power“, Novel In-Plane Semiconductor Lasers XX (2021) doi: 10.1117/12.2578354

[2] B. Sumpf, L. S. Theurer, M. Maiwald, A. Müller, A. Maassdorf, J. Fricke, P. Ressel, G. Tränkle, „783 nm wavelength stabilized DBR tapered diode lasers with 7 W output power”, Applied Optics (2021), doi: 10.1364/AO.422688