Narrow-band single mode emission of blue-violet lasers based on GaN

Laser diodes (LDs) based on GaN with a stable narrow-band emission in the blue-violet spectral region are considered as promising light sources for spectroscopy and atomic clocks. FBH successfully realized such an emission with distributed Bragg reflector (DBR) and distributed feedback (DFB) LDs, respectively, using a Bragg grating as an optical wavelength filter. This filter consists of a periodic modulation of the effective index of a waveguide. While the strong optical absorption in the un-pumped active region of a GaN-based laser structure favors the concept of a DFB LD, the short wavelength suggests gratings of high order to keep the chip processing technology simple. On the way to fabricate a DFB LD based on GaN we have successfully fabricated an undoped laser structure which contains a 10^th order surface grating with V-shaped grooves. The grating was laterally coupled (LC) to a ridge waveguide (RW), and the effect of the grating was demonstrated in optically pumped lasing experiments.

The grating was defined alongside a 2 µm wide ridge (W_R) using i-line wafer stepper lithography and inductively coupled plasma (ICP) etching. The resulting grating structure had a period of 802 nm and consisted of 600 nm deep V-shaped grooves with an upper opening of 450 nm. The etching depth and the shape of the grooves were precisely adjusted by the ICP etching parameters such as the etching gas (BCl₃) and the pressure. The gratings were covered with a 250 nm thick plasma-enhanced chemical-vapor deposited SiN_x, and a 1.5 µm wide opening (W_O) was structured on the ridge to define the narrow optically pumped area, as shown in Fig. 1. Finally, the wafer was cleaved into 1.6 mm long bars whose facets were antireflection-coated (front facet, R < 0.2%) and high-reflection-coated (back facet, R ~ 97%) using layer pairs of Ta₂O₅/SiO₂.

The laser bars were excited using an ArF laser emitting at a wavelength of 193 nm which operated at a pulse repetition rate of 50 Hz. A typical high-resolution emission spectrum of a LC-DFB laser operated at 18% above laser threshold showed a single peak emission at 404.2 nm with a full width at half maximum of 0.06 nm and a side mode suppression ratio of 24 dB, as shown in Fig. 2(a). In contrast, RW lasers without gratings which were fabricated in parallel showed multiple peaks in the emission spectrum (not shown here), which confirms the coupling of the grating with the laser mode in the LC-DFB laser. Moreover, the temperature-dependent wavelength shifts of the RW and LC-DFB laser were determined to be 0.058 and 0.019 nm/K, respectively in the temperature range from 30°C to 70°C, as shown in Fig. 2(b). The emission wavelength of the LC-DFB laser shifts much less than those of the RW laser because it only depends on the change of the effective refractive index with temperature, which is much smaller than the temperature-dependent shift of the maximum in the material gain spectrum.

In conclusion, we have demonstrated the operation of optically pumped LC-DFB lasers based on GaN using 10^th order surface gratings. The effect of the grating was verified by the small linewidths in the emission spectrum and the temperature stability of the lasers. Experiments are underway to realize laser diodes with the same concept for the first time.

Publication

J.H. Kang, M. Martens, H. Wenzel, V. Hoffmann, W. John, S. Einfeldt, T. Wernicke, and M. Kneissl, "Optically pumped distributed feedback lasers based on GaN with 10^th-order laterally coupled surface gratings", will be presented at International Workshop on Nitride Semiconductors in October 2016.