Superior performance of a single-wavelength high-power DFB laser
Fig. 1. Schematic top-view of the quarter-wave phase shifted DFB laser with 5° tilted RW and grating. The phase shift is centered in the middle of the cavity. The facets are additionally AR coated. Insets show the outline of the quarter-wave phase shift in the second order grating and the uncoated reflectivity of the facet as function of the tilt angle.
Fig. 2. Continuous-wave power-current characteristics of the DFB laser at temperatures of 15°C (solid), 25°C (dashed), 35°C (dash-dotted) and 45°C (dotted). The inset shows a typical optical spectrum at 250 mA and 20°C.
Fig. 3. Peak wavelength (a) and side mode suppression ratio (SMSR) (b) as a function of the injection current at different temperatures showing mode-hop-free tuning almost up to 400 mA.
Applications in holography, spectroscopy, and medicine need lasers that provide an ultra-small optical spectrum, which common Fabry-Perot type diode lasers cannot provide. Instead, Bragg gratings are needed – periodic variations of the refractive index along the cavity axis fixing the lasing wavelength. They can either be implemented in only a part of the cavity being electrically unbiased (distributed Bragg reflector, DBR, laser) or along the entire cavity containing both the laser-active region and the grating (distributed feedback, DFB, laser). While DBR lasers suffer from mode hops (resulting in wavelength jumps) when the current or temperature is varied, DFB lasers offer a more stable spectral behavior. To ensure single-mode emission as well as a wide mode-hop-free range of operation conditions, a quarter-wave phase shift has to be inserted into the Bragg grating within the cavity, and anti-reflection (AR) coatings have to be applied to both facets.
At FBH, we have successfully realized such a DFB laser emitting near 780 nm, a wavelength required for compact frequency references based on rubidium vapor. We achieved this by using FBH's proven buried grating technology based on a two-step epitaxy for fabricating DFB lasers. In order to reduce parasitic reflections from the facets that could deteriorate the spectral behavior, both the ridge waveguide for lateral optical confinement and the grating were tilted with respect to the facets (Fig. 1). The optical power emitted from one facet of a DFB laser with AR-coated facets, as studied here, is naturally lower compared to a DFB laser with a similar epitaxial layer structure grating but one high-reflection facet. Nevertheless, the output power achieved in this case – over 100 mW across a wide temperature range (Fig. 2) – is much higher than what other groups have reported for DFB lasers with a high-reflection coated facet which are known to exhibit mode hops depending on the phase of the grating at that facet. The optical spectrum reveals single-mode operation (inset in Fig. 2). The variation of the side-mode suppression ratio and the peak wavelength with current and temperature shows a mode-hop-free tuning for almost the entire parameter range investigated (Fig. 3).
The presented device concept based on tilted ridge waveguides and gratings with a quarter-wave shift is expected to become an enabling component for more complex systems requiring the integration of DFB laser sources which are mode-hop-free by design.