Powerful (up to 100 W) Continuous Laser Arrays for Pumping Solid-state Lasers
A. L. Ter-Martirosyan1, firstname.lastname@example.org, M. A. Sverdlov1, C. N. Rodin1, email@example.com, N. A. Pikhtin2
JSC “Semiconductor devices”, St. Petersburg, Russia
Ioffe Institute, St. Petersburg, Russia
Powerful high-performance continuous wave (CW) laser arrays, emitting in the spectral range of 808 nm and designed for pumping solid-state lasers, have been developed. The laser arrays have high efficiency of converting electric current to light (more than 50%) and small geometrical dimensions. The chips are manufactured based on heterostructures grown by metal organic chemical vapour deposition (MOCVD) epitaxy. The parameters of the developed LA and the results of their life tests are given in the article.
Keywords: semiconductor laser, laser arrays, pumping solid-state lasers
Powerful high-performance continuous wave (CW) laser arrays (LA), emitting in the spectral range of 808 nm and designed for pumping solid-state lasers, have been developed.
The development of LA for systems of continuous optical pumping of solid-state lasers is aimed at increasing efficiency, reducing energy consumption, reducing size and increasing the service life of final products.
Diode-pumped solid-state lasers used all over the world for material processing, isotope separation, disease treatment, analytical instrumentation, atmosphere monitoring, controlled fusion and military applications are rapidly replacing outdated lamp-pumped solid-state lasers and gas lasers. Diode-pumped solid-state lasers are characterized by significantly higher efficiency, reliability, better mass-dimensional parameters, and the absence of a high supply voltage. Around the world, most high-tech production lines use diode-pumped solid-state lasers to process materials (cutting, drilling holes, hardening, fitting resistors, engraving and marking). Industrial laser systems for processing materials become an integral part of any high-tech production.
High-tech lasers can significantly increase labour productivity, qualitatively reduce energy consumption, are environmentally friendly equipment. Moreover, the laser beam allows to obtain qualitatively new results (precision machining, hardening of materials) compared with traditional equipment. The lamp-pumped solid-state lasers used to equip the production lines for the last 10–15 years, along with the above-mentioned advantages, have significant drawbacks: high power consumption (low efficiency), large dimensions and weight, and a short service life.
The parameters of the developed LA and the results of their life tests are given in the article.
The report submits the results of the development of new-generation energy-efficient injection LA for systems of continuous optical pumping of solid-state lasers with a generation wavelength of 808 nm, a maximum power conversion efficiency of 55% and an output optical power of up to 100 watts.
LAs provide, at a temperature of 25 °C, the following basic parameters, presented in Table 1.
LAs are a source of laser radiation with a narrow spectral line (3–4 nm). The main differences of the LAs are their high efficiency of converting electric current to light (more than 50%) and small geometrical dimensions. The chips are manufactured based on heterostructures grown by metal organic chemical vapour deposition (MOCVD) epitaxy. This method of growth of heterostructures allows to control the chemical composition and thickness of the grown semiconductor layers with high accuracy, providing high reproducibility of parameters, which makes it possible to significantly reduce the operating current, increase efficiency and achieve high values of optical radiation power. Based on advanced post-growth technologies, the specialists of JSC “Semiconductor devices” have developed a highly productive technological cycle of LA production.
When choosing a heterostructure to create an LA, the heterostructures used to create single laser diodes emitting at a wavelength of 808 nm were analysed. To date, there are two approaches to create separate-confinement epitaxial nanostructured heterostructures (ENHS):
double separate-confinement ENHS based on aluminium-non-containing (Al-free) waveguide layers and active region [1–6];
double separate-confinement ENHS based on aluminium-containing layers of the waveguide and the active region [1, 7–10].
The maximum achieved optical output power in single laser diodes based on double separate-confinement ENHS based on an aluminium-non-containing solid solution system is 9.9 W (radiating aperture width is 100 μm, cavity length is 3000 μm) . The maximum achieved optical output power in single laser diodes based on double separate-confinement ENHS based on an aluminium-containing system of solid solutions is 13 W (radiating aperture width is 100 μm, cavity length is 4000 μm) . It should be noted that such an output optical power was obtained on a laser diode using a double separate-confinement ENHS with an ultra-wide symmetric waveguide (the total thickness of the waveguide layer is 3 μm). In a laser diode based on a double separate-confinement ENHS of with a waveguide 1 μm thick, the maximum power achieved is 8.9 W .
From the above, it can be concluded that single laser diodes made on the basis of aluminium-non-containing and aluminium-containing solid solutions in the waveguide layers and the active region have approximately the same power characteristics. In terms of manufacturability, the most convenient solid solution system is AlGaAs. The advantage of the AlGaAs solid solutions system is the maximum reproducibility of the results, as well as the minimum mismatch over the lattice period with the GaAs substrate in the entire composition range (the maximum mismatch value is obtained for AlAs and is 1.18 ∙ 10–3), which is important for ensuring high optical characteristics of the laser diode. The disadvantages of the AlGaAs system include the high oxidative ability of aluminium. Therefore, it is not recommended to use layers of AlxGa1–xAs with a composition on aluminium (x) greater than 0.6 in the creation of double separate-confinement ENHS, since the higher the aluminium content in the layer, the more intense the oxidation process. Strong oxidation of aluminium layers can lead to deterioration of the output characteristics of laser diodes, namely: a decrease in reliability, an increase in resistance and optical losses.
Based on the above, double separate-confinement ENHS was developed based on the AlGaAs solid solution system to create LA.
When developing the design of a double separate-confinement ENHS for LA, the main objectives were: ensuring the operation of LA on a zero fundamental transverse mode of an electromagnetic wave; ensuring minimal internal optical loss in the LA waveguide; the suppression of the emission of charge carriers from the active region into the waveguide layers; ensuring minimal electrical resistance of ENHS; ensuring ENHS technological design.
For ENHS, the following layer compositions were selected: Al0,55Ga0,45As emitter layer; Al0,37Ga0,63As waveguide layer; the active region is an Al0,1Ga0,9As quantum well with a thickness of 12 nm.
To ensure the operation of the LA at the zero-transverse mode of the electromagnetic wave and the minimum internal optical loss, the distribution profiles of the electromagnetic wave in the waveguide were calculated using the model proposed in . To meet these requirements, we used an extended waveguide (1.5 μm). A feature of working with a wide waveguide is the need to calculate at least two more higher order modes. Suppression of these modes can be achieved by maximizing the optical losses and minimizing their modal amplification. To suppress higher order modes, an asymmetric waveguide is used with a shift of the active region towards the P‑emitter.
Fig. 1 shows the results of calculating the distribution of modes of different orders of an electromagnetic wave. Table 1 presents the parameters of the calculated ENHS.
For testing the LA manufacturing technology, the optimum technological scheme of the assembly was chosen, which allows to obtain LA with an output optical power up to 100 W operating in a continuous mode at the final stage as finished products.
The construction of the technological scheme for carrying out assembly operations in the manufacture of LA is based on a certain basic concept:
- high reproducibility of technological process characteristics;
- possibility of carrying out several technological processes in one reaction chamber without opening;
- possibility of easy adjustment modes when changing the parameters of technological processes;
- constant technological control over the quality of technological operations;
- ease of retrofitting and maintenance of the equipment used;
- possibility of making adjustments and improving the technological scheme;
- minimal environmental impact during the process.
The installation of LA chips on the heat sink was carried out by the method of flux-free soldering through a buffer plate (submount) made of CuW (80 / 20). The installation of the insulating plate on the heat sink was carried out to ensure electrical isolation between the LA conductive contacts. As the material of the plate, the Al2O3 ceramic plate (polycor) metallized on one side, having high insulating properties and having rather good thermal conductivity, was used.
To measure the LA output optical power, a calibrated bolometric meter (Ophir) was used.
Spectral measurements were performed using an ASP‑150TF fibre optic spectrometer.
Fig. 2 shows the appearance of LA on a water-cooled heat sink. Fig. 3 shows the dimensional drawing of LA on a water-cooled heat sink. Fig. 4 shows the typical dependence of the output optical power on the pump current and the efficiency of the LA. As the analysis of dependence shows, the maximum efficiency of LA reaches 55%, at a temperature of TO LA +25 °C.
Fig. 5 shows a typical LA spectrum. The narrow spectral emission band of ~3–4 nm indicates a high homogeneity and quality of mounting of LA chips. The distribution of optical power over the strip emitters (Fig. 6) shows a high homogeneity across the entire width of the radiating platform, which confirms the quality of the heterostructure and you low manufacturability of all post-production processes of LA.
The developed LAs were subjected to life tests in the mode of maintaining a constant pump current of 115–117A (with a nominal output power of 100 W). At a temperature of LA +25 °C for 1.000 hours, the drop of the output optical power was no more than 1.5%. Thus, by linear extrapolation of the time dependence, it is possible to estimate the expected life of the LA at T = +25 °C as 10,000 hours.
Fig. 7 shows the dependence of the LA output power on the operating time.
Thus, the powerful high-efficiency continuous LA designed for pumping solid-state lasers based on Nd+ ions were developed and investigated in this paper.
The work was carried out in accordance with the Program of the Union State of Russia and Belarus “Development of critical standard technologies for designing and manufacturing nanostructured micro and optoelectronics products, devices and relating systems, and equipment for their production and testing” – Composite Part of Research and Development Work “Development of parametric rows of high-power microwave photodiodes, as well as structurally and technologically similar new-generation energy-efficient injection lasers in the manufacture of epitaxial nanostructured heterostructures, energy-efficient injection lasers chip separation, application of protective coatings on mirrors of energy-efficient injection lasers and assembly of energy-efficient injection lasers, systems optical diode pumping for a high-energy all-solid-state laser and fully solid-state laser for industry, energy and special applications” code of the Composite Part of Research and Development Work is “Luch‑3.2.1”.
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