High-brightness tapered diode lasers generally work in the fundamental transverse mode. Still, under the condition of high power, the beam quality is often deteriorated by the influence of higher-order modes. At the junction of the device ridge waveguide and the conical amplifier, there is a great difference in the refractive index step △n, which is the key area for producing high-order modes and influencing each other. In this paper, the physical process of high-order side mode excitation in a large optical cavity InGaAs/AlGaAs conical LD was studied. The mode changes of the propagation interface were simulated by the eigenmode expansion method (EME). The effects of various separation distances of the beam spoilers were compared and verified by experiments. The results show that through the practical design of the separation distance of beam spoilers at the mode propagation interface, the tapered LD can maintain the high beam quality of M2=1.9 at 3.2 W.
High brightness broad area lasers with high polarization purity are highly efficient light sources for high brightness fiber coupled and direct semiconductor lasers. Effect of lateral index step on the performance of high-power broad-area 970-nm diode lasers based a large-optical-cavity waveguide structure was studied and presented here. The index step of the 80-μm wide ridge is found a key parameter to control the output power, lateral far-field angle, beam waist and polarization purity. The threshold current decreases with the increase of the etching depth while the slope efficiency increases. When gain guide lateral waveguide by very shallow etching was used, the beam waist expands to a size of more than 200 μm, which was attributed to anti-guiding effect and current spreading. When large index step is introduced by deep etching, enhanced filamentation was observed, which is attributed to an enhanced confinement of the higher order modes. What’s more, the strain introduced by the etching of the ridge can deteriorate the polarization purity. The study in this paper shows that the lateral index step should be optimized to fabricate high brightness high efficiency broad area lasers with high polarization purity.
In this research, the transversal mode of 970 nm broad area laser diode based on a large optical cavity is studied. The vertical far-field patterns were measured to be dependent on the cavity caused facet loss. High-orders modes achieve lasing with higher facet loss while they can be completely suppressed by a low facet loss. The suppression of high-order modes was attributed to be an increase in the discrimination of differential quantum efficiency between high-order modes and the fundamental mode, which matches well with simulation. When the discrimination exceeds probable fluctuations, the high-order modes will be completely suppressed and lasing of single fundamental mode will achieve. According to our experiment results, the optimal resonance cavity length is about 6 mm in order to realize fundamental mode and simultaneously a high slope efficiency. As a consequence, high efficiency 970 nm broad area laser with slope efficiency over 1.07 W/A and full wavelength half maximum (FWHM) less than 2 nm@1A is demonstrated.
KEYWORDS: Semiconductor lasers, Waveguides, Refractive index, Neodymium, Laser applications, Far-field diffraction, High power lasers, Laser energy, Optical resonators, Near field
In this research, the transversal mode of 970 nm broad area laser diode based on a large optical cavity is studied. The vertical far-field patterns were measured to be dependent on the cavity caused facet loss. High-orders modes achieve lasing with higher facet loss while they can be completely suppressed by a low facet loss. The suppression of high-order modes was attributed to be an increase in the discrimination of differential quantum efficiency between high-order modes and the fundamental mode, which matches well with simulation. When the discrimination exceeds probable fluctuations, the high-order modes will be completely suppressed and lasing of single fundamental mode will achieve. According to our experiment results, the optimal resonance cavity length is about 6 mm in order to realize fundamental mode and simultaneously a high slope efficiency. As a consequence, high efficiency 970 nm broad area laser with slope efficiency over 1.07 W/A and full wavelength half maximum (FWHM) less than 2 nm@1A is demonstrated.
KEYWORDS: Semiconductor lasers, Diffraction gratings, Diffraction, Deep ultraviolet, High power lasers, Nonlinear frequency conversion, Polarization, Laser systems engineering, Second-harmonic generation, Crystals
We reported a high-power narrowband blue diode laser which is suitable for subsequent nonlinear frequency conversion into the deep ultraviolet (DUV) spectral range. The laser is based on an external cavity diode laser (ECDL) system using a commercially available GaN-based high-power blue laser diode emitting at 448 nm. Longitudinal mode selection is realized by using a surface diffraction grating in Littrow configuration. The diffraction efficiency of the grating was optimized by controlling the polarization state of the laser beam incident on the grating. A maximum optical output power of 3.1 W in continuous-wave operation with a spectral width of 60 pm and a side-mode suppression ratio (SMSR) larger than 10 dB at 448.4 nm is achieved. Based on the experimental spectra and output powers, the theoretical efficiency and output power of the subsequent nonlinear frequency conversion were calculated according to the Boyd– Kleinman theory. The single-pass conversion efficiency and output power is expected to be 1.9×10-4 and 0.57 mW, respectively, at the 3.1 W output power of the ECDL. The high-power narrowband blue diode laser is very promising as pump source in the subsequent nonlinear frequency conversion.
Beam quality of tapered laser diodes is limited by higher order lateral mode. On purpose of optimizing the brightness of tapered laser diodes, we developed a novel design of tapered diodes. This devices based on InGaAs/AlGaAs asymmetry epitaxial structure, containing higher order lateral mode filtering schemes especially photonic crystal structures, which fabricated cost effectively by using standard photolithography and dry etch processes. Meanwhile, the effects of photonic crystal structures on mode control are also investigated theoretically by FDBPM (Finite-Difference Beam Propagation Method) calculation. We achieved a CW optical output power of 6.9W at 940nm for a single emitter with 4 mm cavity length. A nearly diffraction limited beam of M2 ≈1.9 @ 0.5W has been demonstrated, and a highest brightness of β =75MW/(cm2 ·sr) was reached.
For many applications, laser diodes with very narrow and stable emission wavelength are needed. The realization of mode selection by an external cavity system with a grating is widely used. The influence of longitudinal mode selection by external grating on the filamentation and catastrophic optical mirror damage (COMD) of 970 nm broad area single emitters is studied in this paper. The emitters worked at three configurations: free running, with 10% mirror feedback (mirror lasers), and 10% grating feedback (grating lasers). The grating lasers showed very short lifetime caused by COMD, while the free-running lasers and mirror lasers show no power degradation. The COMD was confirmed by optical microscope showing cracking and melting of the optical antireflective (AR) coatings. By observing the near-field pattern of the three lasers, the COMD of the grating lasers was attributed to the pronounced filamentation induced by the grating feedback. What’s more, the filamentations vary when the locked wavelength change which indicates that the carrier dynamics thus the refractive index profile is very sensitive to the locked lasing wavelength.
True blue nitride laser diodes (LDs) are one of the key challenges for epitaxy of nitrides due to the variety of its potential applications. The growth of high temperature p-type layers may cause thermal degradation of the InGaN-based multiple quantum wells (MQWs) active region because of the annealing effect, since thick p-AlGaN layers were introduced as upper optical cladding layer in the LDs. The degradation was found in blue LDs grown on both Si and sapphire substrate. In the degraded LD wafer samples, “Dark” non-radiative MQWs regions were observed by microscopic photoluminescence. Formation of metallic indium precipitates and voids in these regions were confirmed by transmission electron microscope. The thermal degradation is attributed to the decomposition of indium-rich InGaN materials in the MQWs. The indium-rich InGaN materials were supposed to be accumulated at dislocation related V-shaped pits according to the surface morphology by atomic force microscope. To obtain high quality InGaN-based MQWs, one of the four methods can be introduced to eliminate the degradation. A lower thermal budget can suppress the decomposition of indium-rich InGaN clusters by a lower p-cladding layer growth temperature. The use of low threading dislocation density substrates results in low density indium-rich InGaN clusters. The introducing of H2 carrier gas during the quantum barriers growth or a 2-step growth scheme with a higher quantum barrier growth temperature etches off the indium-rich InGaN clusters. The suppression of the thermal degradation in the MQWs makes it possible for lasing of blue laser diode directly grown on Si.
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