24 Wavelength DBR Laser Array with Surface Gratings for Optical Systems

Arrays with small linewidth lasers featuring defined spatial positions and emission frequencies are attractive light sources for optical systems like 3D scanners. Moreover, diode lasers are particularly well suited to miniaturize such optical systems because of their small dimensions and high conversion efficiencies. In order to meet the requirements of the applications, the integration of gratings into the cavities is inevitable, thus reducing the linewidth and enabling to precisely adjust the wavelength. We have developed an ideally suited array of ridge-waveguide (RW) distributed Bragg reflectors (DBR) lasers with Bragg reflectors based on third order surface gratings.

Fig. 1 shows the fabricated diode laser array. It has a footprint of 3 x 3 mm² and incorporates 24 DBR-RW lasers, each with a length of 1 mm for the active section and 1 mm for the DBR section. The pitch between adjacent emitters is 87 µm, and the ridges have widths of 2.5 µm to obtain fundamental lateral mode operation. The area behind the gratings serves as fan-out for the p-metal contacts. Bond pads are positioned at the rear end of the device and allow addressing each DBR-RW laser individually. The 2 µm long and 150 nm wide grating slits were defined with E-beam lithography. Gratings with periods Λ_D# ranging from Λ_D24 = 406.3 nm (diode D24) to Λ_D01=412.2 nm (diode D01) were written to define third order gratings. We took the dispersion of the effective index into account to adjust the intended wavelengths spacing of 0.5 nm between adjacent emitters. The difference of the grating periods between increases gradually, starting from Λ_D24-Λ_D23 = 0.25 nm to 0.26. After E-beam lithography we applied an etching process to obtain V-shaped grating groves tapered towards the active region. Subsequent to the implementation of the gratings, a standard RW fabrication process followed. A front facet power reflectivity of 0.6 is adjusted to obtain small linewidth operation and high endurance against optical feedback. For characterization of the DBR-RW laser array, it is mounted p-side up on C-mount (Fig. 2).

Fig. 3 summarizes optical spectra from the fabricated DBR-RW array. All spectra show single mode emission with a side mode suppression ratio > 50 dB. The peak wavelengths decrease linearly with rising emitter number. The spacing between adjacent emitters is close to 0.5 nm, as intended. However, the spectral distances reveal variations ranging from 0.41 nm to 0.56 nm in detail. The threshold currents are below 16 mA. At a current of 40 mA all 24 DBR-RW diodes emit with an optical output power > 17 mW. Linewidth measurements have been performed with a Fabry-Perot interferometer (Toptica FPI 100). Results indicate that the emission linewidths are below 5.7 MHz, which is the resolution limit of the applied interferometer. Due to the small size, high conversion efficiency, and high spectral purity we believe the array will be of interest for the development of sophisticated optical systems like 3D scanners.

Publications

O Brox, J Fricke, A Klehr, A Maaßdorf, M Matalla, H Wenzel, G Erbert “24-wavelength distributed Bragg reflector laser array with surface gratings” Electronics Letters 51 (17), 1352-1354 (2015).

O Brox, J Fricke, A Klehr, A Maaβdorf, M Matalla, H Wenzel, G Erbert „Array with 24 distributed Bragg reflector lasers for scanning applications: Fabrication and characterisation” The European Conference on Lasers and Electro-Optics, CB_11_5, Munich 2015