For the first time achieved – CW operation from 405 nm GaN-based DBR laser diodes using high-order surface gratings
Fig. 1. Bird´s eye-view scanning electron microscope image of a DBR laser chip before the contact pad deposition.
Fig. 2. Optical power-current-voltage characteristics of a DBR laser diode for emission from the front facet.
Fig. 3. (a) 2D color map of emission spectra at different currents of a DBR laser diode measured at the front facet. (b) High-resolution emission spectra of a DBR laser diode operated at 105 mA.
Gallium nitride (GaN) based laser diodes with narrow-band emission in the blue-violet spectral region are interesting light sources for many applications. These includes atom spectroscopy, atomic clocks, medical diagnostics as well as optical communication. Compact single-mode laser diodes can be realized as distributed feedback (DFB) or distributed Bragg reflector (DBR) laser diodes.
Recently, FBH has successfully developed the technology for 10th-order V-shaped surface Bragg gratings on GaN. The successful demonstration of DFB laser diodes operating under continuous-wave (CW) condition has qualified the feasibility of these surface gratings. The work has been continued to realize GaN-based DBR laser diodes, taking advantage of the fact that a high reflectivity can be achieved via only a short grating section. Moreover, a DBR laser allows to spatially separate lateral mode confinement and light amplification from mode selection via grating dispersion.
The 600 µm long DBR laser diode consists of a 200 µm long unpumped DBR section and a 400 µm long pumped gain section. The 10th-order V-shaped surface gratings with a period of 802 nm, a depth of (650 ± 10) nm, a slant angle of 20° and a tip width of ~10 nm were defined on the DBR section of the 2 µm-wide ridge using electron-beam lithography and plasma etching (see Fig. 1). 1.5 µm-wide Pd/Pt p-contact stripes were only fabricated on the gain section. The DBR section was passivated using 300 nm-thick SiO2. The laser chips with an antireflection (AR) coating (RAR < 0.2 %) on the rear facet (DBR section) and a low-reflection coating (RLR = 25 %) on the front facet (gain section) were soldered p-side up on C-mounts.
Under CW operation, DBR laser diodes show a threshold current of (85 ± 15) mA with a slope efficiency of (0.42 ± 0.07) W/A at the front facet (see Fig. 2). When increasing the current, periodic steps of the output power indicating mode hops are observed which are attributed to a change of the difference in temperature and, therefore, in refractive index between gain section and DBR section. I.e., the gain section heats up stronger than the DBR section, and the modal gain spectrum shifts to the longer wavelength while the reflectivity spectrum of the grating is rather fixed. Thus, the lasing mode is increasingly shifted away from the maximum of the reflectivity spectrum of the DBR section until lasing hops to the adjacent shorter wavelength mode (see Fig. 3(a)). Between the mode hops, the DBR laser diode operates in single longitudinal mode at around 405 nm with a full width at half maximum of the emissions peak of 0.03 nm and a side-mode suppression ratio of 40 dB (see Fig. 3(a)). Furthermore, temperature dependent measurements show that single longitudinal mode operation is maintained over a temperature range of 20 °C to 40 °C with a peak shift of only 0.019 nm/K.
In conclusion, GaN-based DBR laser diodes capable of operating in continuous wave mode have been demonstrated for the first time using high-order surface gratings.
This work was supported by the European Fund for Regional Development of the European Union in the framework of the Berlin-Brandenburg-Polish joint project “From UV to blue – Reliable laser sources for environmental monitoring (RelyLa)”.