High-power Broad-area Laser Diodes Research Articles

Thermal Lensing Effect Reduction on High-Power Multi-Mode Diode Lasers

High-power broad-area diode lasers are needed for a variety of applications. However, such lasers suffer from deteriorated beam quality due to multimode behavior in the lateral direction at high power. We propose an approach to reduce the thermal lensing effect by introducing a thermal compensation layer. As a result of reducing the thermal lensing effect, we demonstrated that the lateral divergence angle was reduced by 16.2%, the lateral beam parameter product was reduced by 12.8%, and the lateral brightness was increased by 18.4% for 230 μm wide diode operating at 50 A current while maintaining a good output power.

A Composite Cavity for Mode Control in Broad-Area Diode Laser

A unstable cavity structure composed of the arc-shaped cavity, Fabry-Perot cavity, and sidewall saw-tooth microstructure is proposed to achieve mode selection in high-power broad-area diode laser with 980nm wavelength. In this cavity, the loss difference between the higher-order modes and the fundamental mode is increased to suppress the lasing of higher-order modes. The lateral divergence is reduced by about 28%. The number of modes in the near-field spot is reduced and the light intensity is concentrated towards the center of the Near-field profile. Also, the lateral beam parameter product decreases by 15%.

High-power diode lasers with in-situ-structured lateral current blocking for improved threshold, efficiency and brightness

We present high-power GaAs-based broad-area diode lasers with a novel variant of the enhanced self-aligned lateral structure ‘eSAS’, having a strongly reduced lasing threshold and improved peak conversion efficiency and beam quality in comparison to their standard gain-guided counterparts. To realize this new variant (eSAS-V2), a two-step epitaxial growth process involving in situ etching is used to integrate current-blocking layers, optimized for tunnel current suppression, within the p-Al0.8GaAs cladding layer of an extreme-triple-asymmetric epitaxial structure with a thin p-side waveguide. The blocking layers are thus in close proximity to the active zone, resulting in strong suppression of current spreading and lateral carrier accumulation. eSAS-V2 devices with 4 mm resonator length and varying stripe widths are characterized and compared to previous eSAS variant (eSAS-V1) as well as gain-guided reference devices, all having the same dimensions and epitaxial structure. Measurement results show that the new eSAS-V2 variant eliminates an estimated 89% of lateral current spreading, resulting in a strong threshold current reduction of 29% at 90 μm stripe width, while slope and series resistance are broadly unchanged. The novel eSAS-V2 devices also maintain high conversion efficiency up to high continuous-wave optical power, with an exemplary 90 μm device having 51.5% at 20 W. Near-field width is significantly narrowed in both eSAS variants, but eSAS-V2 exhibits a wider far-field angle, consistent with the presence of index guiding. Nonetheless, eSAS-V2 achieves higher beam quality and lateral brightness than gain-guided reference devices, but the index guiding in this realization prevents it from surpassing eSAS-V1. Overall, the different performance benefits of the eSAS approach are clearly demonstrated.

Spectral linewidth narrowing of broad-area blue diode bar in V-shape external Talbot cavity.

A 1-D linear array of 23 high-power broad-area laser diode (BALD) beams in the blue spectral region (447 nm) is combined employing a V-shape external Talbot cavity in Littrow configuration. A surface grating provides optical feedback via self-imaged diffractive coupling to the diode bar and induces all the emitters to lase at a common central wavelength. The external cavity reduces the spectral linewidth of the free-running laser diode bar from several nm to 20-50 pm (FWHM) with the power level of 11.8 W. The narrow spectrum of the external cavity stabilized laser can be tuned in the range of 3-4 nm by adjusting the tilt angle of the grating while the laser diode bar is operated in constant current mode at a temperature of 20°C.

Selective loss tailoring of broad-area diode lasers

High-power high-brightness broad-area diode lasers have poor lateral beam quality, which has limited the application range of this kind of device. To improve the lateral beam quality, a sawtooth microstructure laser was proposed and the microstructures were carefully designed according to the profiles of each lateral mode and their dependence on the self-heating induced thermal lens effect. As a result of selective mode loss adjustment, a 42% improvement in lateral beam quality was achieved under the power-maintained condition. This technology enables us to develop high-brightness direct-diode laser systems.

Slow degradation of a multiemitter 638-nm high-power broad-area laser diode during long-term aging

Laser-based displays have attracted much attention because they are the only displays that can express the full color gamut of Ultra-High Definition Television (U-HDTV), International Telecommunication Union Radio Communication Sector Broadcasting Service (ITU-R BT).2020. We have developed 638-nm broad-area laser diode (LD) with 75-μm-wide dual emitters and achieved wall plug efficiency of over 40.5% at 25°C. But slow degradation is still a factor limiting the lifetime due to the requirement for a high temperature and high-power operation. We performed the long-term aging test of 638-nm LDs under the different conditions, including a high-power operation such as 1.3 to 2.5 W, and investigated the behavior of output power characteristics, especially the dependence of its output characteristics on temperature. After long-term aging, the threshold current of the LD increased and its slope efficiency (SE) decreased. The measured dependence of the output characteristics of the LD on temperature was converted into that on the junction temperature using the thermal resistance between the junction and package. The latter dependence showed an increase of threshold current and no change of SE. This result indicates that the slow degradation of the red broad-area LD was caused by the increase of the nonradiative recombination rate in the active layer during aging.

Beam waist shrinkage of high-power broad-area diode lasers by mode tailoring.

A new approach was proposed and its role in improvement of the beam quality of high-power broad-area diode lasers was demonstrated, in which a composite arrow array and trench microstructure was used to suppress the beam waist and tailor the high order lateral modes. The beam waist shows a special shrinkage with increasing injection current resulting from the combined effect of mode tailoring and the thermal lens effect. A 58% improvement in lateral beam parameter product was realized compared with conventional broad-area diode lasers.

Multiemitter 638-nm high-power broad area laser diodes for display application

Laser-based displays have gathered much attention because only the displays can express full color gamut of Ultra-HDTV, ITU-R BT.2020. There are many types of laser-based displays. A projector with a single spatial light modulator is mainstream in the displays so far. This projector uses the laser light sources under pulse with duty of 20% to 50%. We improve a triple-emitter red color 638-nm high-power broad area laser diode (LD) for this application. Countermeasures are adopted to improve the output power characteristics and strengthen the facet to the fatal degradation. The improved triple emitter shows the output of 5.0 W at the injection current of 5 A under pulse with duty of 30%, case temperature of 25°C. The output is increased by 4.1% compared to our former one. The peak wall plug efficiency reaches to 41.6% under 25°C. The long-term aging test is also performed. Mean time to failure of the improved LD is estimated 72,000 h under the output of 3.5 W, pulse condition with duty of 30%. The value is ∼3.3 times longer than that of the former.

Loss tailoring of high-power broad-area diode lasers.

Broad-area diode lasers (BALs) with high power are highly desirable for a variety of applications. However, such lasers suffer from strongly deteriorated beam quality due to multimode behavior in the lateral direction. In this Letter, we present an approach to flexibly tailor the optical loss of different-order lateral modes by etching micro-holes on the laser mesa with controlled position and numbers. Through arranging the micro-holes at the peak positions of high-order lateral modes with an increasing number from the mesa center to both edges, high-order modes are suppressed due to a larger propagation loss than the fundamental mode. As a result of enhanced mode discrimination, we demonstrate that this technique provides a greatly improved beam quality and about two times higher brightness for 100μm wide BAL, while maintaining high power and slope efficiency output.

High-power broad-area laser diode performance improvement with a double pedestal structure

We propose a novel heat-sinking structure for a high-power broad-area laser diode. In our laser diode, the pedestal structure is introduced to both top and bottom sides of the laser diode so that the heat generated within the device can be more efficiently released. The characteristics of high-power broad-area laser diodes containing the double pedestal structure are numerically analyzed by self-consistent electro-thermal-optical simulation. It is demonstrated that our laser diode has narrower lateral far-field angle and lower active region temperature compared to previously reported high-power laser diode structures.

Suppression of higher-order modes in a large-optical-cavity waveguide structure for high-power high-efficiency 976-nm diode lasers

High-power high-efficiency broad-area diode lasers are highly efficient light sources. A study on the design of large-optical-cavity 976-nm diode lasers is presented here. Efforts were made to suppress the higher-order modes and to explore new design methods for high-power high-efficiency diode lasers. The mode distribution, confinement factor, internal optical loss and far-field pattern were numerically calculated to analyze the waveguide design. To suppress the higher-order transverse modes, both gain and loss discrimination mechanisms were experimentally investigated. By adjusting the position of the active region, successful suppression of higher-order modes can be achieved. Alternatively, higher-order modes can be suppressed by increasing the cavity length, which is attributed to the discrimination of modal loss between the higher-order modes and the fundamental mode. A high-efficiency 4-mm-long broad-area single emitter with an ultralow internal optical loss of 0.27 cm–1 and an internal quantum efficiency of 93.5% was designed and fabricated. A high slope efficiency of 1.1 W/A and >66% wall-plug efficiency was obtained at 25 °C under continuous current injection. The far-field divergence angles with 95% power content were 38° and 9.7° for the fast and slow axes, respectively, at an output power of 10.0 W.

Highly reliable operation under high case temperature in 638-nm BA-LD

We developed two types of 638-nm high-power broad area-laser diodes (BA-LDs). One was dual-emitter LD mainly for CW operation and the others triple one for pulse. Total emitter width of dual-emitter LD was 150 µm and that of triple one 180 µm. Each LD was studied in terms of optical power-current and wall plug efficiency (WPE)-current dependence on case temperature and reliability under the pulse condition with frequency of 120 Hz, and duty of 40%. The dual-emitter LD emitted 2.5 W at case temperature of 55 °C, whereas triple one did 2.1 W at the same condition. This was mainly originated from low heat generation of dual-emitter LD because of the narrow emitter width. On the contrary, the narrow emitter width increased the optical power density and shortened the lifetime. We discussed how to estimate the lifetime of light sources using red LDs which have multiple degradation modes under pulse operation. And then, we proposed a dulled drive current waveform for dual-emitter LD to improve a lifetime, resulting in MTTF exceeding 20,000 h at the output over 2 W under the pulse condition.

Dynamics of a green high-power tunable external-cavity broad-area GaN diode laser

Although external-cavity feedback with a grating is widely used to achieve a tunable high-power narrow-linewidth broad-area diode laser (BAL) system, the dynamics of such a system is seldom studied. In this paper, the temporal dynamics of a tunable high-power green external-cavity diode laser system based on a GaN BAL and Littrow external-cavity is investigated experimentally. The regular pulse package oscillation (PPO) is observed just above the threshold. The oscillating period of the pulse package decreases with the increasing injected current. As the current increases further, the pulse package oscillates irregularly, and finally changes to a chaotic state. The PPO is observed, for the first time to our knowledge, in a BAL with an external-cavity grating feedback.

Coherent addition of high power broad-area laser diodes with a compact VBG V-shaped external Talbot cavity

We introduced a compact V-shaped external Talbot cavity for phase locking of high power broad-area laser diodes. The length of compact cavity is ∼25 mm. Near diffraction-limit coherent addition of 10 broad-area laser diodes indicated that high quality phase locking was achieved. We measured the near-field emission mode of each individual broad-area laser diode with different feedback, such as a volume Bragg grating and a high reflection mirror. We found out that the best result of phase locking broad-area laser diodes was achieved by the compact V-shaped external Talbot cavity with volume Bragg grating feedback.

Impact of longitudinal refractive index change on the near-field width of high-power broad-area diode lasers

Typical for broad-area laser (BAL) diodes operating in a continuous-wave mode is a narrowing of the near-field (NF) width at the output facet for high injection currents (output powers). This phenomenon increases the facet load of BALs, resulting in a reduction in the level of catastrophic optical mirror damage. In this letter, we demonstrate theoretically that thermally induced changes in the refractive index in both lateral and longitudinal directions not only cause the NF narrowing at the front facet but also a broadening of the NF at the back facet. In contrast, a sole lateral self-heating induced variation in the refractive index (commonly referred to as thermal lensing) does not result in a NF narrowing. Our theoretical findings are confirmed by measurements of the current-dependent profiles of the NF at the back and front facets of a BAL with a stripe width of 120 μm emitting at 960 nm. Furthermore, our quasi three-dimensional thermo-electro-optic simulations indicate that a longitudinally homogeneous device temperature can reduce the front-facet load while keeping the beam quality unchanged compared with the experimental results.

785 nm grating-coupled external-cavity laser for shifted-excitation Raman difference spectroscopy

A 785 nm grating-coupled external-cavity (EC) laser was fabricated and applied as a light source for shifted-excitation Raman difference spectroscopy (SERDS) to extract Raman signal from the fluorescence background. A simple Littrow-type EC configuration with the grating lines parallel to the p–n junction was applied to narrow the linewidth of a commercially available high-power broad-area laser diode. The characteristics of the EC laser, including wavelength tuning range, output power, and emission spectra, were investigated in detail. A wavelength tuning range of more than 6 nm was realized at an injection current of 900 mA by simply changing the grating rotation angle. The output power after Raman probe coupling exceeded 40 mW and the spectral linewidth was narrowed significantly down to less than 0.1 nm from 0.9 nm (the free-running width). Then, five shifted emission lines were used for the Raman measurement with sesame oil as the target, whose Raman signal was overwhelmed in the fluorescence background. Due to the constraint relationship between the signal intensity and the spectral resolution, the best reproducibility of the Raman signal without fluorescence background was obtained when the excitation wavelength interval was fixed at around the linewidth of the Raman peak. In conclusion, the grating-coupled EC laser with flexible wavelength tunability, high output power, and narrow spectral linewidth was demonstrated to be an ideal light source for SERDS application.

Suppression of far-field blooming in high-power broad-area diode lasers by optimizing gain distribution

Far-field blooming, a serious far-field dependence on driving current, affects the stability of beam quality and applications of broad-area (BA) diode lasers. In this Letter, the lateral ridge waveguide (LRW) is introduced to BA lasers by a simple and cost-effective approach to control the far-field stability and beam divergence. The influences of LRW length on output power, near- and far-field, are investigated and it is found that the optimized LRW length is able to improve both the far-field blooming and output power. The mechanism behind this is analyzed and a 0.13°/A dependence of lateral divergence angle on the injection current is achieved.

Injection-insensitive lateral divergence in broad-area diode lasers achieved by spatial current modulation

High-power broad-area (BA) diode lasers often suffer from low beam quality, broad linewidth, and a widened slow-axis far field with increasing current. In this paper, a two-dimensional current-modulated structure is proposed and it is demonstrated that it can reduce not only the far-field sensitivity to the injection current but also the linewidth of the lasing spectra. Injection-insensitive lateral divergence was realized, and the beam parameter product (BPP) was improved by 36.5%. At the same time, the linewidth was decreased by about 45% without significant degradations of emission power and conversion efficiency.

Spectrum Improvement of a High-Power Broad-Area Laser Diode Based on a Grating Semi-Feedback External Cavity Scheme

We propose a grating semi-feedback external cavity (GSFEC) scheme for high-power broad-area laser diodes for improving the spectrum characteristics of high-power semiconductor lasers. We design this scheme as two crossed cylindrical resonators employing a holographic diffraction grating and a rectangular highly reflecting mirror. In our experiments, we obtain stable spectrum characteristics over a tuning range of 5 nm, with maximum output power up to 764 mW at an operating current of 3.5 A. The wavelength drift rate with injection current decreases from 0.59 nm/A of free-running condition to 0.2 nm/A, and the laser bandwidth is compressed to 21.5% at 2 A.

Long-Term Reliability of 915-nm Broad-Area Laser Diodes Under 20-W CW Operation

We report on the long-term reliable continuous wave (CW) operation of high-power and high-efficiency 915-nm broad-area laser diodes (BA-LDs). An output power of over 20 W and a maximum power-conversion efficiency of over 65% have been marked at a heat-sink temperature of 20 °C. Stable operations of over 5000 h have also been achieved for the 915-nm and 976-nm BA-LDs with CW output powers of 20 W, respectively. The window structure with a bandgap difference of 100 meV provides the reliable operation at the high-output power range.