Widely tunable monolithically integrated GaAs lasers with sampled gratings
Fig.1: Top down photography of the realized SG-DBR laser
Fig.2: Measured laser wavelengths in dependence of the currents applied to the rear and front heater (current gain section 150 mA, C-mount temperature 20°C)
Fig.3: Spectra of the realized SG-DBR laser for different rear heater currents (current gain section 150 mA, C-mount temperature 20°C)
Widely tunable diode lasers are key components for optical systems for industrial and biomedical sensor applications. Preferred devices should be robust without any moving parts and allow wavelength tuning only with control of the driving currents. Nowadays, sampled grating (SG) distributed Bragg reflector (DBR) lasers based on InP are well established and offer wide wavelength tuning around 1.3 µm and 1.55 µm. This is in contrast to the short wavelength GaAs material system where the integration of gratings with reduced periods and very high coupling coefficient is challenging.
Recently we have realized a GaAs-based SG-DBR laser (Fig. 1). The device has a novel vertical layer structure which is grown in two steps by metal-organic vapor phase epitaxy. The structure allows the integration of gratings with very high coupling coefficient while keeping the oxygen incorporation at the regrowth interface at a low level. An InGaAs quantum well (QW) is asymmetrically embedded in GaAs confinement and AlGaAs cladding layers. The first epitaxy stops after the growth of the first part of the p-confinement layer above the QW. The n-confinement layer beneath the QW contains two InGaP layers acting as etch stop and grating layers. Placing the grating layer near the intensity peak of the vertical mode ensures a high coupling coefficient. Two lithographical steps, each followed by selective wet etching, are performed before overgrowth. Grating bursts with a length of 5 µm were defined with Ebeam (Vistec SB251), repeated with periods of 50 µm and 45 µm for the rear and front grating, respectively. Within a burst, the gratings have a period of 140 nm and a duty cycle of 50%. After completing the second epitaxy a standard ridge waveguide laser fabrication process follows. Fig. 2 summarizes measured wavelengths in dependence of the currents applied to the front and rear heater sections. Stripes of similar colors indicate areas of the same Vernier mode. The SG-DBR emits ten Vernier modes around 970 nm and continuous tuning of a single mode can be achieved if both heater currents are controlled simultaneously. The optical spectra for a fixed front heater current of 250 mA are summarized in Fig. 3. The spacing between two Vernier modes is 2.3 nm, and the spectral distance between the first and the tenth mode is 20.6 nm (side mode suppression > 40 dB).
Due to the thermal wavelength control the developed laser is interesting for a number of industrial and biomedical sensor applications which do require a wavelength tuning speed within microseconds.
This work has been partly carried out within the MidTECH project funded by the European Union’s Horizon 2020 program under grant agreement No. 642661.
Publications
O. Brox, M. Tawfieq, P. Della Casa, P. Ressel, B. Sumpf, G. Erbert, A. Knigge, M. Weyers, H. Wenzel "Realisation of a widely tunable sampled grating DBR laser emitting around 970 nm", IET Electronics Letters (2017), DOI: 10.1049/el.2017.0521
M. Tawfieq, H. Wenzel, O. Brox, P. Della Casa, B. Sumpf, G. Tränkle, "Concept and numerical simulations of widely tunable GaAs-based sampled-grating diode laser emitting at 976 nm", IET Optoelectronics (2017), DOI: 10.1049/iet-opt.2016.0068