Monolithic dual-wavelength diode laser for generating THz radiation in spectroscopic applications

FBH research: 29.04.2025

Schematic illustration of two ridge-waveguide laser cavities with a common output at the front, obtained via a Y-shaped coupling section. Each cavity has an individual grating for wavelength stabilization at the rear side. — Schematic illustration of a Y-branch distributed Bragg reflector diode laser.

The plot shows the optical output power as a function of the injection current, reaching about 200 mW at 0.4 A. Emission spectra recorded at 200 mW show two narrowband laser emissions at around 781.4 nm and 783.4 nm, resulting in a spectral distance of 2 nm. — Fig. 2: Electro-optical characteristics (left) and spectral characteristics at 0.2 W (right) of a Y-branch distributed Bragg reflector diode laser.

The plot shows the spectral distance and the resulting frequency difference as a function of a heater current. The latter is applied individually to resistors implemented next to each grating. Heater currents as high as 0.65 A enable a span of 1 nm to 3 nm, i.e., a span of 0.5 THz to 1.5 THz. — Fig. 3: Spectral characteristics of a Y-branch distributed Bragg reflector diode laser when applying a heater current to the individual resistors implemented close to the gratings.

THz spectroscopy is a powerful tool to provide molecular-level structural information about the target under study. Low temperature grown gallium arsenide antennas enable the generation of continuous-wave THz radiation by photomixing of two laser emissions from light sources in the near-infrared spectral range.

Suitable monolithic dual-wavelength Y-branch diode lasers developed at FBH fulfill the application requirements at chip level. These lasers are based on two ridge-waveguide laser cavities with individual grating-based intrinsic wavelength stabilization and a single output (Fig. 1).

Using a well-established vertical layer structure, we have designed and manufactured Y-branch diode lasers for dual-wavelength laser emission around 783 nm [1]. The overall length of these one-chip devices is 3 mm. Two tapered 500 µm-long, 10^th-order distributed Bragg reflector (DBR) gratings are realized as rear-side cavity mirrors using e-beam lithography. They are laterally separated by 80 µm and designed for a spectral distance of 2.0 nm, corresponding to a beat frequency of 1.0 THz. S-bend-shaped ridge waveguides with a length of 2 mm couple the individual laser emissions into a 500 µm-long single output waveguide. The ridge widths along the device are 2.2 µm.

The lasers are operated with a single injection current and provide narrowband laser emissions with spectral widths of 20 pm and 200 mW of optical output power (Fig. 2). Heater currents applied to resistors implemented close to the gratings allow to adjust the available spectral distance within a span of about 1.0 nm to 3.0 nm. This adjustment enables generating tunable THz radiation within a range of 0.5 THz to 1.5 THz by photomixing (Fig. 3).

First experimental investigations using these near-infrared Y-branch diode lasers as sources for tunable continuous-wave THz radiation have been carried out, and their potential towards compact THz spectroscopy is currently evaluated further.

This work has been supported by the Leibniz Association within the project MiB under grant number 667933-2, the German Research Foundation within the project FLEMUSA under grant number 427211214 and the Federal Ministry of Education and Research within the framework of "Research Fab Microelectronics (FMD)" under ref. 16FMD02.

Publication

[1] L. S. Theurer, J.-P. Koester, A. Müller, M. Maiwald, A. Knigge, B. Sumpf, G. Tränkle, “Spectrally narrowband simultaneous dual‑wavelength emission from Y‑branch DBR diode lasers at 785 nm”, Applied Physics B 130, 205 (2024), doi: 10.1007/s00340-024-08338-2