Broad-area Laser Diodes Research Articles

Influence of pump beam shaping and noise on performance of a direct diode-pumped ultrafast Ti:sapphire laser.

One of the major factors that limits the widespread use of ultrafast titanium sapphire (Ti:S) lasers in life science is its expensive and complex pump source. Broad area diode laser (BA-DL) based pump sources have high potential to solve this problem, since they are compact, inexpensive and very efficient. However, their non-diffraction limited beam profile makes it challenging to achieve high powers in Kerr-lens mode-locked (KLM) operation of Ti:S lasers. In this work, we show that the ideal way to beam shape two spectrally combined BA-DLs with different beam qualities is to aim for a compromise between matching to the cavity mode diameter and the cavity mode Rayleigh range. We furthermore conclude that the relative intensity noise (RIN) of the BA-DL pumped Ti:S laser, another important parameter for imaging applications, is sufficiently low for a wide range of life science applications. However, for applications that are highly sensitive to noise, new laser diode designs are likely necessary to reduce inherent noise originating within the laser diode.

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.

Extending the spectral region of a sub-terahertz spectroscopic system by mixing broad-area and single-mode laser beams

The photomixing technique is applied to a sub-THz spectroscopic system with a continuous wave multimode laser diode (CW-MLD) to extend its spectral range. For the MLD, a broad-area laser diode (BLD) is employed to produce broad continuous spectrum signals. By mixing the light from the BLD with light from a continuous wave single-mode laser diode, a new broad continuous component is generated in the higher frequency region (0.4–1.0 THz) of the Fourier spectrum of the signal waveform. This is achieved via a simple and low-cost experimental setup using laser diodes and fiber optics. Spectroscopic measurement of a Si wafer in the frequency range below 1 THz is demonstrated.

High-brightness broad-area diode lasers with enhanced self-aligned lateral structure

Broad-area diode lasers with increased brightness and efficiency are presented, which are fabricated using an enhanced self-aligned lateral structure by means of a two-step epitaxial growth process with an intermediate etching step. In this structure, current-blocking layers in the device edges ensure current confinement under the central stripe, which can limit the detrimental effects of current spreading and lateral carrier accumulation on beam quality. It also minimizes losses at stripe edges, thus lowering the lasing threshold and increasing conversion efficiency, while maintaining high polarization purity. In the first realization of this structure, the current block is integrated within an extreme-triple-asymmetric epitaxial design with a thin p-doped side, meaning that the distance between the current block and the active zone can be minimized without added process complexity. Using this configuration, enhanced self-aligned structure devices with 90 µm stripe width and 4 mm resonator length show up to 20% lower threshold current, 21% narrower beam waist, and slightly higher (1.03×) peak efficiency in comparison to reference devices with the same dimensions, while slope, divergence angle and polarization purity remain almost unchanged. These results correspond to an increase in brightness by up to 25%, and measurement results of devices with varying stripe widths follow the same trend.

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.

Efficient and High‐Brightness Broad Area Laser Diodes Designed for High‐Temperature Operation

AbstractSemiconductor laser diodes, manufactured as single emitters or laser bars, are highly desired light sources for direct material processing as well as optical pumping of fiber and solid‐state lasers. Laser diodes feature high optical output power and efficiency, long lifetimes, low maintenance and consequently low cost of ownership. To improve the usability and extend the application spectrum of high‐power laser diodes, relaxed cooling requirements ‐‐ without compromise in laser performance and lifetime ‐‐ are required. Therefore, great development efforts are made, both in externally and internally funded research projects, to push the maximum permissible operating temperature of such semiconductor laser diodes to higher levels.

Impact of Carrier Nonpinning Effect on Thermal Power Saturation in GaAs-Based High Power Diode Lasers

A root-cause analysis of thermal power saturation in broad area diode lasers is presented. Thermal power saturation limits in largest parts the optical power in continuous wave (CW) driven diode lasers, as junction temperature increases with bias. Accordingly, gain decreases and, hence, the carrier density in the quantum well increases (nonpinning) to compensate the low gain and the additional photon loss. A systematic variation of epitaxial designs was used to experimentally clarify the impact of modal gain on the temperature dependence of differential internal efficiency and optical loss. To account for this effect in simulation, a rate equation model was applied to determine temperature dependent slope efficiency. Longitudinal spatial hole burning (LSHB) causes an asymmetry of the carrier density profile, leading to high nonlinear carrier loss at the back facet, but also the average carrier density increases with temperature, enhancing the impact of LSHB. This enhancement effect leads to rapid degradation of internal differential efficiency with temperature and, hence, early power saturation. The extreme triple asymmetric vertical design is introduced, that enables devices to be realized that show minimized degradation of internal differential efficiency with temperature and therefore increased performance in CW operation.

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.

Beam-combining scheme of high-power broad-area semiconductor lasers with Lyot-filtered reinjection: modeling, simulations, and experiments

A brightness- and power-scalable polarization beam-combining scheme for high-power, broad-area semiconductor laser diodes is investigated numerically and experimentally. To achieve the beam combining, we employ Lyot-filtered optical reinjection from an external cavity, which forces lasing of the individual diodes on interleaved frequency combs with overlapping envelopes and enables a high optical coupling efficiency. Unlike conventional spectral beam-combining schemes with diffraction gratings, the optical coupling efficiency is insensitive to thermal drifts of laser wavelengths. This scheme can be used for efficient coupling of a large number of laser diodes and paves the way toward using broad-area laser diode arrays for cost-efficient material processing, which requires high-brilliance emission and optical powers in the kW regime.

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.

Beam shaping of a broad-area laser diode using 3D integrated optics.

Typical high power broad-area semiconductor lasers exhibit a highly astigmatic beam profile. However, many applications require a homogenous and circular symmetric beam. Thus coupling into circular multimode optical fibers is often employed. The strip-like astigmatic output of the diode laser underfills the circular multimode fiber, thus a decrease in beam quality occurs after fiber coupling due to mode mixing inside the optical fiber. This Letter presents a 3D integrated optics approach to shape the output of a broad-area laser diode. Ultrafast laser inscription is utilized to create a pair of photonic lanterns connected back to back inside a glass chip that captures and shapes the output of a commercial 976nm wavelength broad-area laser diode with 95μm emitter width. Compared to coupling to a 105μm diameter, 0.15 numerical aperture step-index multimode fiber, the photonic chip-based approach results in a 13× higher beam quality and 7× greater brightness.

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.

High beam quality broad-area diode lasers by spectral beam combining with double filters

A modified spectral beam combining (SBC) approach based on double asymmetrical filters was proposed. By using this scheme, the high-order lateral modes at the edge of the far-field pattern can be suppressed in the external cavity, and the beam quality in the slow-axis direction was improved from 16.1 to 13.4 compared to the conventional SBC. In the meanwhile, the electrical-to-optical efficiency from the modified SBC was more than 40% with an output power of 34.1 W, which is similar to that of the conventional SBC.

High-Power 1.5-$\mu$ m Broad Area Laser Diodes Wavelength Stabilized by Surface Gratings

Wavelength stabilization against temperature variation of high-power broad area 1.5- $\mu \text{m}$ InGaAsP/InP laser diodes is demonstrated by employing surface gratings. The development targets application in eye-safe automotive LIDAR systems, which would benefit from deploying narrowband receiver filters to block ambient solar radiation for improved signal-to-noise ratio. The surface grating is monolithically integrated on the laser chip using nanoimprint lithography. The peak power of the lasers exceeded 6 W in pulsed mode, for an FWHM spectral width of 0.3 nm and a peak wavelength drift of only 0.1 nm/°C. The wavelength shift with temperature is reduced by five times compared to broad area high-power Fabry–Perot laser diodes typically employed in LIDAR systems.

Generation of a broad-area laser diode in an asymmetrical V-cavity possessing a spectrally nonselective feedback mirror

Generation of a broad-area laser diode is experimentally studied in an external asymmetrical V-cavity comprising an inclined output feedback mirror (FBM) that is nonselective with respect to spectrum and phase. The study is aimed at finding the ranges of a pump current and FBM inclination angles, in which the laser beam is narrow-directed. Laser radiation spectra are measured and FBM angles are juxtaposed with the angles of the output laser beam and the average period of filament positions in the laser diode.

Catastrophic Optical Damage in 950-nm Broad-Area Laser Diodes Due to Misaligned Optical Feedback and Injection

Improper back-coupling of optical feedback is a known cause for accelerated gradual degradation as well as catastrophic optical mirror damage of GaAs-based high-power semiconductor lasers. For those material systems emitting below the GaAs bandgap (870 nm at room temperature), such damage is induced by feedback-light absorption in the non-transparent cap layers and substrate, which intensifies the heating of the area close to the emission facet. For InGaAs/AlGaAs-based diode lasers in the spectral range from 880 to 1060 nm, however, all passive layers as well as the substrate are, with respect to interband absorption, transparent for the lasing wavelength so that, in this case, the degrading mechanism of misaligned optical feedback could not be explained entirely. By use of an experimental multi-device study of 950-nm broad-area lasers, we demonstrate that the threshold of catastrophic optical damage is decreased the most by positioning the feedback return spot such that it covers the p-side solder and the highly p-doped semiconductor layers. By means of thermoreflectance microscopy in conjunction with numerical simulations we find that such low damage thresholds are caused by a strong localization of the absorption of feedback light close to the laser front facet.

Going beyond the beam quality limit of spectral beam combining of diode lasers in a V-shaped external cavity.

A novel spectral beam combining (SBC) approach based on off-axis feedback in a V-shaped external cavity (VSBC) was proposed and demonstrated. A highly reflecting mirror was used to supply the optical feedback by partial overlapping the beam. The advantages of simple setup, output coupler free, tunable beam quality and emission power over traditional SBC were presented. The beam quality exceeds single emitter with the similar energy conversion efficiency to the traditional SBC. The M2 factors of 2.31 × 3.76 in fast and slow axes and a brightness of 122 MWcm-2sr-1 were realized at 30 A based on a commercial broad-area diode laser array.

Chip-carrier thermal barrier and its impact on lateral thermal lens profile and beam parameter product in high power broad area lasers

High power broad area diode lasers with high optical power density in a small focus spot are in strong commercial demand. For this purpose, the beam quality, quantified via the beam parameter product (BPP), has to be improved. Previous studies have shown that the BPP is strongly affected by current-induced heating and the associated thermal lens formed within the laser stripe. However, the chip structure and module-assembly related factors that regulate the size and the shape of the thermal lens are not well known. An experimental infrared thermographic technique is used to quantify the thermal lens profile in diode lasers operating at an emission wavelength of 910 nm, and the results are compared with finite element method simulations. The analysis indicates that the measured thermal profiles can best be explained when a thermal barrier is introduced between the chip and the carrier, which is shown to have a substantial impact on the BPP and the thermal resistance. Comparable results are observed in further measurements of samples from multiple vendors, and the barrier is only observed for junction-down (p-down) mounting, consistent with the barrier being associated with the GaAs-metal transition.