O. Brox

975-nm high-power broad area diode lasers optimized for narrow spectral linewidth applications

Many pumping and direct diode applications of high power diode lasers require sources that operate within a narrow (< 1nm) temperature stable spectral line. The natural linewidth of high power broad area lasers is too wide (4-5nm) and varies too quickly with temperature (0.3-0.4nm/K) for such applications. The spectrum can be narrowed by introducing gratings within the diode laser itself or by the use of an external stabilization via a Volume Bragg Grating, VBG. For optimal loss-free, low cost wavelength stabilization with a VBG, the narrowest possible far field angles are preferred, provided power and efficiency are not compromised. Devices that contain internal gratings are potentially the lowest manufacturing cost option, provided performance remains acceptable, as no external optics are required. Therefore, in order to address the need for high power with narrow linewidth, three different diode laser device designs have been developed and are discussed here. For VBG use, two options are compared: (1) devices with high conversion efficiency (68% peak) and reasonable far field (45° with 95% power content) and (2) devices with extremely small vertical far field angle (30° with 95% power content) and reasonable conversion efficiency (59% peak). Thirdly, the latest performance results from broad area devices with internal distributed feedback gratings (DFB-BA Lasers) are also presented, constructed here using buried overgrowth technology. DFB-BA lasers achieve peak conversion efficiency of 58% and operate with < 1nm linewidth operation to over 10W continuous wave at 25°C. As a result, the system developer can now select from a range of high performance diode laser designs depending on the requirements.

Reliable operation of 976nm high power DFB broad area diode lasers with over 60% power conversion efficiency

Diode lasers that deliver high continuous wave optical output powers (> 5W) within a narrow, temperature-stable spectral window are required for many applications. One technical solution is to bury Bragg-gratings within the semiconductor itself, using epitaxial overgrowth techniques to form distributed-feedback broad-area (DFB-BA) lasers. However, such stabilization is only of interest when reliability, operating power and power conversion efficiency are not compromised. Results will be presented from the ongoing optimization of such DFB-BA lasers at the Ferdinand-Braun- Institut (FBH). Our development work focused on 976nm devices with 90μm stripe width, as required for pumping Nd:YAG, as well as for direct applications. Such devices operate with a narrow spectral width of < 1nm (95% power content) to over 10W continuous wave (CW) optical output. Further optimization of epitaxial growth and device design has now largely eliminated the excess optical loss and electrical resistance typically associated with the overgrown grating layer. These developments have enabled, for the first time, DFB-BA lasers with peak CW power conversion efficiency of > 60% with < 1nm spectral width (95% power content). Reliable operation has also been demonstrated, with 90μm stripe devices operating for over 4000 hours to date without failure at 7W (CW). We detail the technological developments required to achieve these results and discuss the options for further improvements.

High-power 894 nm monolithic distributed-feedback laser

A ridge-waveguide InGaAs/GaAsP laser, emitting up to 250 mW in a single lateral and longitudinal mode at a wavelength of 894 nm, is presented. The distributed feedback is provided by a second order grating, formed into an InGaP/GaAs/InGaP multilayer structure. Owing to the stable lasing frequency, the large side mode suppression ratio (> 40 dB) and small spectral line width (< 200 kHz) the diode laser is well suited for caesium D1 spectroscopy. This was verified by the measurement of the hyperfine structure of the D1 line.

Compact sources for the generation of high-peak power wavelength-stabilized laser pulses in the picoseconds and nanoseconds ranges

Diode lasers are ideally suited for the generation of optical pulses in the nanoseconds and picoseconds ranges by gainswitching, Q-switching or mode-locking. We have developed diode-laser based light sources where the pulses are spectrally stabilized and nearly-diffraction limited as required by many applications. Diffraction limited emission is achieved by a several microns wide ridge waveguide (RW), so that only the fundamental lateral mode should lase. Spectral stabilization is realized with a Bragg grating integrated into the semiconductor chip, resulting in distributed feedback (DFB) or distributed Bragg reflector (DBR) lasers. We obtained a peak power of 3.8W for 4ns long pulses using a gain-switched DFB laser and a peak power of more than 4W for 65ps long pulses using a three-section DBR laser. Higher peak powers of several tens of Watts can be reached by an amplification of the pulses with semiconductor optical amplifiers, which can be either monolithically or hybrid integrated with the master oscillators. We developed compact modules with a footprint of 4×5cm2 combining master oscillator, tapered power amplifier, beam-shaping optical elements and high-frequency electronics. In order to diminish the generation of amplified spontaneous emission between the pulses, the amplifier is modulated with short-pulses of high amplitude, too. Beyond the amplifier, we obtained a peak power of more than 10W for 4ns long pulses, a peak power of about 35W for 80ps long pulses and a peak power of 70W for 10ps long pulses at emission wavelengths around 1064nm.

Distributed feedback lasers in the 760 to 810 nm range and epitaxial grating design

We present the results from distributed feedback (DFB) lasers with emission wavelengths ranging from 760 to 810 nm and focus on the optimization of Bragg gratings realized with a patterned GaAsP layer that is overgrown with an AlGaAs cladding layer. The impact of the thickness and material composition of the GaAsP grating lines on the DFB laser performance is theoretically and experimentally investigated. 767 nm ridge waveguide DFB lasers with optimized gratings show excellent optoelectronic characteristics in terms of slope efficiency (0.9 W A−1) and linewidth (11 kHz).

Narrow Vertical Far-Field 975-nm Broad-Area DFB Lasers for Wide Temperature Range Operation

We report on 100-mum-wide 975-nm distributed-feedback lasers with a cavity length of 2 mm, which deliver 2.4 W within a 14deg vertical far-field angle (full-width at half-maximum) and 35% power conversion efficiency in combination with a spectral linewidth of 0.27 nm (95% power content). The epitaxial design is a compromise between a narrow vertical far-field and a good power conversion efficiency, as required for application as a pump source. Narrow linewidth operation is sustained to 100degC, enabled by use of a grating with a high coupling coefficient.

Integrated 1060nm MOPA pump source for high-power green light emitters in display technology

We present a compact green light emitter for laser displays and focus on the pump source for a SHG waveguide in single-pass configuration. The developed pump source has a RW-structure consisting of three sections: a DFB, a spacer and an amplifier section. The optical output power is 305mW for currents of 120mA and 400mA in DFB and amplifier section. The control of the current in the amplifier section allows a modulation of the output power from 5mW to 305mW. Spectral characteristics as well as measured beam divergence are well suited for pumping SHG waveguide crystals. Results on the hybrid 530nm emitter are summarized.

High-power distributed-feedback tapered master-oscillator power amplifiers emitting at 1064 nm

In this work, we investigate experimentally optimized monolithic distributed-feedback (DFB) tapered master-oscillator power amplifiers (MOPA). The devices consist of three autonomously driven sections: a 1 mm long DFB ridgewaveguide (RW) laser, a 1 mm long RW pre-amplifier and 2 mm or 4 mm long tapered amplifiers. The ridge width and the full taper angle are 5 μm and 6°, respectively. Both laser facets are anti-reflection coated. The second order Bragg gratings in the DFB laser were realized by holographic photolithography, wet-chemical etching and a two-step epitaxy. The DFB tapered MOPAs emit nearly diffraction limited spectral single mode CW radiation at 1064 nm. The 6 mm long devices provide an optical power of about 12 W at DFB laser, pre-amplifier and tapered amplifier currents of 150 mA, 400 mA and 18 A, respectively. The 4 mm long devices generate more than 4 W at a tapered amplifier current of 7 A. The spectral drift versus output power is below 50 pm/W.

Dynamics of a gain-switched distributed feedback ridge waveguide laser in nanoseconds time scale under very high current injection conditions

We present detailed experimental investigations of the temporal, spectral and spatial behavior of a gain-switched distributed feedback (DFB) laser emitting at a wavelength of 1064 nm. Gain-switching is achieved by injecting nearly rectangular shaped current pulses having a length of 50 ns and a very high amplitude up to 2.5 A. The repetition frequency is 200 kHz. The laser has a ridge waveguide (RW) for lateral waveguiding with a ridge width of 3 µm and a cavity length of 1.5 mm. Time resolved investigations show, depending on the amplitude of the current pulses, that the optical power exhibits different types of oscillatory behavior during the pulses, accompanied by changes in the lateral near field intensity profiles and optical spectra. Three different types of instabilities can be distinguished: mode beating with frequencies between 25 GHz and 30 GHz, switching between different lateral intensity profiles with a frequency of 0.4 GHz and self-sustained oscillations with a frequency of 4 GHz. The investigations are of great relevance for the utilization of gain-switched DFB-RW lasers as seed lasers for fiber laser systems and in other applications, which require a high optical power.

High-power ridge waveguide DFB and DFB-MOPA lasers at 1064 nm with a vertical farfield angle of 15°

We present high-power ridge waveguide (RW) distributed feedback (DFB) lasers and DFB master optical power amplifiers (DFB-MOPAs) with high-quality beams optimized for pulsed operation and current modulation. A Bragg grating ensures stable longitudinal single-mode emission around 1064 nm. Furthermore, vertical and lateral structures of the devices were optimized for stable fundamental-mode operation. The slope efficiency of 1 mm DFB lasers slightly above threshold is as high as 0.95 W/A and the continuous wave (cw) optical output power is almost 400 mW at a current of 500 mA. For the DFB-MOPAs a cw output power of 1 W has been obtained. Due to low ellipticity, low divergence and low beam steering of the output beams the devices are well suited for efficient coupling to single mode waveguides.