Depth: Research progress of high-power semiconductor lasers

introduction

Laser is another major invention of mankind since the 20th century, after atomic energy, electronic computers, and semiconductors. Semiconductor laser science and technology takes semiconductor laser devices as the core, covering the study of the law, generation method, device technology, control means and application technology of stimulated radiation amplification of light, and the required knowledge integrates geometric optics, physical optics, semiconductor electronics, thermodynamics and other disciplines.

After more than 50 years of development, semiconductor laser, as a world-class research direction, has developed by leaps and bounds along with international scientific and technological progress, and has also benefited from breakthroughs in various related technologies, materials and processes. The progress of semiconductor laser has received great attention and attention in the international scope, not only in the field of basic science and continuous research and deepening, the level of science and technology continues to improve, but also in the field of application continues to expand and innovate, the application of technology and equipment emerge in an endless stream, the application level has also been greatly improved, in the national economic development of all countries in the world, especially in the fields of information, industry, medical and national defense has been an important application.

At present, the development of semiconductor lasers in the world is in a new stage of rapid development, and China's laser science and technology has basically maintained a trend of synchronous development with the world. From the perspective of comprehensive social development, industrial economic upgrading, national defense and security application and economic structure transformation, from the perspective of national competitive development, more clear needs are put forward for the comprehensive innovation of semiconductor laser technology and the transformation and development of industrial applications. In this paper, the development history and current situation of semiconductor lasers are reviewed, and the achievements of Changchun Institute of Optics, Fine Mechanics and Physics in recent years in high-power semiconductor lasers, especially in high-power semiconductor laser laser light sources, vertical cavity surface-emitting lasers and new laser chips.

The development history of high-power semiconductor lasers

In 1962, American scientists announced the successful development of the first generation of semiconductor lasers———GaAs homogeneous structure injection semiconductor lasers. Since the threshold current density of stimulated emission of the laser of this structure is very high, it requires 5 × 10^4 ~ 1 × 10^5 A/ cm2, so it can only work in a low-frequency pulse state under liquid nitrogen refrigeration. Since then, the research and development and utilization of semiconductor lasers have become the focus of attention.

In 1963, Kroemer of the United States and Alferov of the Academy of Sciences of the former Soviet Union proposed to sandwich a narrow bandgap semiconductor material between two wide-bandgap semiconductors to form a heterostructure, in order to produce high-efficiency radiation recombination in narrow-bandgap semiconductors. With the development of heterojunction material growth processes such as vapor phase epitaxy (VPE), liquid phase epitaxy (LPE), etc., in 1967, Woodall of IMB successfully used LPE to grow AlGaAs on GaAs. From 1968 to 1970, Panish, Hayashi and Sμmski of Bell Labs in the United States successfully studied AlGaAs/GaAs single heterojunction lasers with a threshold current density of 8.6 × 10^3 A/cm2 at room temperature, which was an order of magnitude lower than that of homojunction lasers.

Just as American scholars were working on the research of single heterojunction lasers, Alferov and others from the Institute of Physics of the former Soviet Academy of Sciences announced the successful development of double heterojunction semiconductor lasers (HD-LD). In this structure, the active region of the p-GaAs semiconductor is sandwiched between the n-AlGaAs layer and the p-AlGaAs layer with a wide bandgap, so that the threshold current at room temperature is reduced to 4 × 10^3 A/cm2. The reason why the threshold current density of the double heterojunction semiconductor laser can be significantly reduced is mainly due to the two effects of the double heterojunction: (1) The band gap of the cladding material on both sides of the active region is wider than that of the active region material, which makes the carriers injected into the double heterojunction semiconductor laser effectively confined to the active region, which is conducive to generating high gain; (2) The refractive index of the material in the active region is greater than that of the cladding materials on both sides, and the optical waveguide structure formed can confine most of the light in the active region.

The advent of dual heterostructure lasers marks a new era in the development of semiconductor lasers. In 1978, semiconductor lasers were successfully used in fiber-optic communication systems. With the continuous emergence of new materials and structures, the electrical and optical properties of semiconductor lasers have been greatly improved. After entering the 80s of the 20th century, due to the introduction of new achievements in semiconductor physics research——— band engineering theory, and the new growth processes of crystal epitaxial materials, such as molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOCVD) and chemical beam epitaxy (CBE), etc., have made semiconductor lasers successfully adopt quantum well and strain subwell structures, and many laser devices with excellent performance have been prepared, such as various quantum well lasers, strain subon well lasers, Vertical cavity surface-emitting lasers and high-power semiconductor laser arrays achieve high power output.

The thickness of the material in the narrow bandgap active region of the quantum well laser is usually smaller than the de Broglie wavelength of the electrons in the material (generally less than 10 ~ 20 nm), so that the injected electrons can be effectively absorbed by the potential well. In quantum wells, the movement of electrons and holes along the direction of the vertical well wall presents the characteristics of quantization, and the density of states of electrons also becomes stepped, so only a small injection current is required to achieve particle number reversal, so quantum well lasers have a small threshold current, high differential quantum efficiency and high output power.

In 1977, Kenichi Iga of Tokyo Institute of Technology in Japan proposed the concept of a vertical-cavity surface-emitting laser (VCSEL), the working principle of which is shown in Figure 1. Because the optical resonator is perpendicular to the substrate of the semiconductor chip, this laser can realize laser emission on the surface of the chip, and has the advantages of low threshold current, stable single-wavelength operation, high-frequency modulation, easy two-dimensional integration, and no cavity surface threshold damage. With the maturity of the material growth process and the optimization of the device structure, a series of progress has been made in the low threshold current and room temperature operation of VCSEL devices, and the continuous laser injection at room temperature was realized in 1988. At present, VCSELs have been widely used in the fields of optical communication, optical interconnection, laser fuze, laser display, optical signal processing, and chip-level atomic clocks.

With the development of theoretical research and preparation technology, especially with the support of projects such as SHEDS, ADHEL in the United States and BIOLAS in Germany, the technology of semiconductor laser chip structure, epitaxial growth and device packaging has been greatly developed. Semiconductor lasers have a wide range of applications in military, industrial processing, laser medical, optical communications, optical storage and laser printing and other information fields due to their high conversion efficiency, long life, small size, light weight, high reliability, direct modulation and easy integration with other semiconductor devices.

Research status of high-power semiconductor lasers

At present, the major technical problem of semiconductor laser research in the world is: how to obtain high power, high reliability and high energy conversion efficiency at the same time, while improving the beam quality and having good spectral characteristics. With the development and progress of material growth technology and device preparation technology, new active materials are emerging, better device structures and processes are becoming more and more mature, and the power, reliability and energy conversion efficiency of semiconductor lasers have been rapidly improved. Compared with the disadvantages of other lasers in the past, such as poor beam quality and excessive spectral linewidth, the performance of semiconductor lasers has been continuously improved, and other laser light sources are gradually replacing other laser light sources in many fields, and their application prospects are becoming more and more extensive.

3.1 Output power of semiconductor lasers

Commercial high-power semiconductor lasers mainly operate in the near-infrared band, and their wavelength range is between 800 ~ 1100 nm. At present, there are two main ways to increase the output power of semiconductor lasers: one is to increase the output power of a single tube laser on a semiconductor laser chip, and the other is to increase the number of luminous points of semiconductor lasers.

To increase the output power of a single-tube laser, it is necessary to improve the chip structure of the laser and improve key technologies such as material growth, chip preparation, cavity surface coating, and packaging heat dissipation. The increase in the number of laser luminescence points is mainly manifested in laser linear array (multiple laser units are integrated with the chip in the direction of the epitaxial layer, also known as laser bar), stacked array, single-tube module, area array and other laser beam combining technologies. Based on the spot, polarization and spectral characteristics of semiconductor lasers, the traditional beam combining (TBC) technology uses spatial beam binding, polarization beam combining and wavelength beam combining to perform energy beam combining and beam shaping for single tubes, linear arrays and stacked arrays from the external optical system. External cavity feedback wavelength beam combining (ECFWBC) technology uses the grating for external optical feedback to achieve spectral beam combining, which can ensure good beam quality on the basis of increasing power.

3.1.1 Single-tube output power and single-tube combined beam light source

In recent years, the output power of diode lasers in the near-infrared band has increased significantly, and the continuous output power of single-tube lasers has now exceeded 10 W and can reach up to 25 W, as shown in Table 1.

The semiconductor laser single-tube beam combination is the smallest optical module composed of a single laser tube, which can directly combine the beam to achieve fiber output. The advantages of laser single-tube beam combination are: long life, high reliability, good quality of slow-axis beam, and can be coupled to fibers with a core diameter of ≤ 100μm; Due to the dispersion of the heat source and the small heat generation, conduction cooling or air cooling can be used, so the overall size of the light source module is small and the weight is light. There is no need for high-current drive, and the parallel connection method can be adopted, and the power supply requirements are relatively low. However, compared with linear array and stacked array, the output power of a single laser tube after beam combination is generally hundreds of watts, so it is generally used in occasions where the power demand is tens of watts to hundreds of watts, or the volume and reliability requirements are very high, such as fiber laser pumping, laser medical treatment, etc.

In recent years, the application range of single-tube coupled light sources has become more and more extensive and has developed rapidly. Fraunhofer USA uses 120 single tubes coupled into 200 μm fibers with a power output of > 700 W. nLight uses 72 single tubes with a wavelength of 940 nm, arranged into 4 units, to achieve 700 W continuous power output from the fiber.

3.1.2 The output power of the laser linear array and the combined beam light source of the linear array

With the development of technology and the improvement of the output power of a single tube, the output power of the diode laser bar has also increased significantly. The output power of the centimeter-bar bar has increased from 240 W in 2000 to about 1 000 W, a fourfold increase, as shown in Table 2.

Semiconductor laser linear array beam combination refers to several conduction cooling or large-channel heat sink packages of laser linear arrays, through optical components to achieve the superposition of laser energy in discrete spatial positions. The advantages of this laser linear array beam combining technology are that the linear array optical path is independent, the assembly and adjustment are simple, the accuracy is high, and there is no tolerance accumulation problem. The heat source is dispersed, and conduction cooling or large-channel water cooling can be used, with low heat dissipation requirements; The electrical connection between the arrays is isolated from the coolant, so ordinary purified water can be used as the coolant; The collimated linear array beam is not affected by the thickness of the heat sink, and the combined beam spot has no dark area superposition. However, due to the scattered linear array arrangement of semiconductor lasers, the volume of linear array combined beam light sources with the same power is significantly larger than that of stacked array combined beam sources. Due to the total volume and the complexity of the optical path, no more than 50 laser arrays are involved in beam combining, making this technology suitable for applications with output powers ranging from hundreds of watts to 3 000 W.

In recent years, the research on semiconductor laser linear array beam has also developed rapidly. Limo in Germany uses 38 conductive heat sink packaged laser linear arrays to form 8 linear array beam units, achieving a core diameter of 200 μm, 0. 22 numerical aperture fiber with 1 200 W power output. Dilas uses 28 laser arrays to achieve a 775 W power single wavelength laser output from a 200 μm core, 0.22 numerical aperture fiber, and then 3 835 W continuous power output from a 500 μm core, 0.12 numerical aperture fiber through wavelength combining. Trumf uses a laser linear array with a low fill factor to make a 100 W linear array module with a 100 μm core diameter and 0.12 numerical aperture fiber, and then uses 19 modules to achieve spatial beam binding through fiber bundling, and then achieves 3 000 W continuous power output for a 600 μm core diameter and 0.12 numerical aperture fiber through wavelength beam combination.

3.1.3 Laser array combined beam light source

Semiconductor laser stacking is formed by directly stacking the bars of several microchannel heat sink packages in the fast axis direction, and the laser stacking beam combination technology is the most commonly used beam combining method for high-power semiconductor laser light sources.

Under the circumstance that the bars of a single-layer laser can be guaranteed to continuously output hundreds of watts of laser power, the number of laser bars in the laser array generally cannot exceed 50 due to the water pressure drop of the microchannel in the heat sink. In this way, a single laser array is capable of continuously operating several kilowatts of laser output. By increasing the number of laser arrays for laser beam combining, tens of thousands or even hundreds of thousands of watts of semiconductor laser output can be realized. The laser array light source has the advantages of compact structure and small size (including microchannel heat sink, the volume of a single laser bar is about 0.6 cm3, and the 50-layer bar bar is no more than 30 cm3), which is the main packaging method for semiconductor laser light source to achieve high power output.

In addition, the cross-sectional diameter of the water channel in the microchannel heat sink is in the micron range, which is easy to be blocked, which requires that the coolant of the laser array must be highly insulated pure deionized water, and regularly maintained and replaced, so the requirements for coolant and heat sink are very high.

Taking into account aspects such as output power and reliability, diode laser beam combining technology based on laser arrays should be used in applications that require a continuous output of 3 000 W or more.

Based on laser stacking array, combined with parallel plate stacking method and laser beam combining technology, Laserline has developed a variety of semiconductor laser direct processing machines, and the representative parameters are shown in Table 3. The beam quality of 100 mm·mrad for 15 kW and 20 mm·mrad for 2 kW exceeds that of the Nd∶ YAG laser at the same power. The shelf life of the company's diode laser light source is up to 5 years (43 800 h), which is dozens of times longer than that of the lamp pump Nd:YAG laser (< 2 000 h), which makes it very competitive in the materials processing market. At present, the company's products have been directly used in cladding, surface strengthening, metal welding and deep penetration welding and other material processing fields.

3.2 Conversion efficiency of semiconductor lasers

The power conversion efficiency of semiconductor lasers is one of the most important indicators of semiconductor lasers. Semiconductor lasers with high conversion efficiency generate less waste heat and high energy utilization, which can greatly extend the working life of the device and improve reliability. It also means that smaller, lighter, and more economical cooling systems can be used, making the mobile platform of the diode laser system unbeatable.

With the development of technology and the support of scientific research projects in various countries (the Defense Advanced Technology Research Projects Agency (DARPA) has set up a special ultra-high efficiency laser light source (SHEDS) project with the goal of improving the electro-optical conversion efficiency of semiconductor lasers to 80%), the efficiency of high-power semiconductor laser light sources has reached a high level. The infrared band can reach more than 70%. Table 4 shows the relationship between the conversion efficiency and wavelength of high-power semiconductor laser devices in the world.

3.3 Reliability of semiconductor lasers

The reliability of semiconductor lasers is an important technical indicator in the application. In the fields of communication and optical storage, the reliability of low-power semiconductors has been basically solved, and the working life can meet the practical requirements. High-power semiconductor lasers face basic problems such as end face catastrophic damage, hole burning, electrothermal burning, filament effect, and the lifetime of microchannel heat sink when working continuously at high current. These problems are generally solved by: improving the quality of crystal growth; improve the preparation process and packaging technology; Increase the spot size; Optimize the heat transfer structure and heat dissipation method, etc.

In recent years, due to the improvement of the conversion efficiency of semiconductor lasers and the improvement of packaging and heat dissipation engineering, the longest life of a single tube of semiconductor lasers has reached more than 100,000 hours, and the reliability of linear arrays has also been significantly improved. The research progress of single tubes and bar bars is shown in Tables 5 and 6.

3. 4 Beam quality of semiconductor lasers

In laser medical, display, free-space optical communication, pumped fiber lasers, direct material processing and other applications, laser light sources are required to meet high output power and high beam quality at the same time. Although the traditional semiconductor laser with wide strip structure has the advantages of high power and high efficiency, it is easy to produce filament effect and complex multi-lobe near-field pattern, and the beam quality is not high.

In order to improve the beam quality of a single tube of a semiconductor laser, the chip structure and processing technology can usually be changed so that the outgoing laser is limited in the lateral and transverse directions, so as to keep the single and stable emitting mode. However, the Wavelength beam combining (WBC) technology can be used to improve the beam quality of the semiconductor laser beam combining light source.

3.4.1 Lateral mode limitation of a single tube of semiconductor lasers

The easiest way to improve the lateral mode of a single diode laser tube is to use a ridge waveguide (RW) to introduce a mode-selective design to the side of the laser to improve beam quality and brightness. However, the ridged waveguide has a weak limitation on the lateral mode, and the high-order mode is easy to be lasing when operating at high current and high power.

In terms of improving the quality of the side beam, the current representative device is a cone laser with a Master oscillator popwer amplifier (MOPA) structure, as shown in Figure 2. The MOPA structure refers to the injection of a single-mode master oscillator (MO) laser with low power and very high beam quality into a semiconductor amplifier (PA) for amplification, and when the entire chip is integrated as a resonator monolithically, it is called a cone laser. It has the advantage of only one epitaxial growth, easy to fabricate and compact structure, and can also integrate gratings and other structures for spectral linewidth modulation.

Cone lasers have been around for almost 20 years, and their performance has improved very quickly. The FBH Institute in Germany has successively reported a variety of wavelengths of cone lasers. Among them, the near-diffraction-limited continuous output power of the 808 nm wavelength device can reach 4.4 W with a beam quality of 1.9 mm·mrad, and the beam quality is 1.3 mm· mrad, with a brightness of 460 MW·cm^ - 2·sr^ - 1; The operating output power is up to 27 W under pulse conditions and up to 9 W near diffraction-limit. The DBR cone laser with a wavelength of 979 nm achieves a continuous output power of 12 W and a conversion efficiency of about 44%, at 11. At 4 W, the beam mass is 1.1 mm·mrad and the brightness can reach 1 100 MW·cm ^- 2 ·sr ^- 1. The output power of the DBR cone laser at a wavelength of 1 060 nm reaches 12.2 W, and the beam quality at 10 W is only 1. 2 mm·mrad, line width of only 17 pm ( FWHM), brightness of 800 MW·cm ^-2·sr ^-1.

Other methods for lateral mode limitation of laser tubes include angled grating distributed feedback lasers and plate-coupled waveguide semiconductor lasers.

3.4.2 Transverse mode limitations of a single tube of semiconductor lasers

In 2002, Ledentsov et al. proposed a novel laser structure based on a longitudinal photonic band crystal waveguide, which uses periodically growing semiconductor layers to form a bandgap photonic crystal in the vertical direction of the laser for light confinement. With the introduction of this technology, the problem of poor beam quality of traditional chips has been greatly improved. Diode lasers can achieve large mode spot size, low cavity surface damage threshold, single horizontal mode, low divergence angle, and near-circular spot operation from the chip, so it is easier to obtain high beam quality and high brightness lasers. In recent years, this new type of laser based on photonic band crystal waveguide has been rapidly developed, and the performance indicators are shown in Table 7.

3. 4. 3 Semiconductor laser external cavity feedback spectral beam combining technology

According to the TBC principle, increasing the laser power through spatial beam combination will lead to a decrease in the overall beam quality of the laser system. Polarization and wavelength combining technologies can only increase the laser power by a certain multiple while maintaining the same beam quality. The beam quality of a TBC light source is generally much larger than that of a laser unit.

ECFWBC technology combines the internal oscillation of the diode laser with the feedback of the external optical system, so that the resonant wavelength of each laser unit is matched with the external grating dispersion and external cavity feedback, so that all laser units resonate in the same direction to keep the near-field and far-field coincidence of output. The beam quality of a combined laser is the same as that of a single laser cell, and the laser power is the sum of all laser cells, as shown in Figure 3. Therefore, as long as the laser unit has a high beam quality, the diode laser beam combining light source can also achieve a high-power laser output with a close diffraction limit. This ECFWBC technique has the advantages of high diffraction efficiency, high damage threshold, multiple coupling elements, and easier output of high power.

The Massachusetts Institute of Technology (MIT) in the United States, Teradiode in the United States, Coherent in the United States, Aculight in the United States, Thales in France, and the Technical University of Denmark (DTU) have made important progress in the research of ECFWBC technology, as shown in Table 8. Teradiode has reached the level of commercialization, and in 2012, its 2 030 W diode laser beam combiner product has reached the level of a commercial all-solid-state laser at the same power.

3. 4.4 Beam quality of high-power diode laser combiner light source

Figure 4 shows the development process of beam quality of high-power semiconductor laser combined beam light sources in recent years. From 1998 to 2007, the beam quality of lasers of the same power was improved by a factor of nearly 10. From 2007 to 2012, the beam quality of light sources based on conventional laser beam combining increased by about 3 times in the order of kilowatts to 10,000 watts, approaching and partially reaching the level of lamp pump Nd:YAG lasers. The spectral beam combining technology greatly improves the beam quality of the beam combining light source, which is about 10 times higher in the order of 100 watts to kilowatts, of which the beam quality of 940 W is 3.5 mm· mrad, 2 030 W beam quality of 3.75 mm·mrad, which is the same as the beam quality level of CO2 lasers; The 360 W beam quality is 0.6 mm·mrad (2x diffraction limit), which exceeds the beam quality of CO2 lasers and is close to the beam quality level of all-solid-state lasers. At present, the semiconductor laser beam combining light source can be competent for the application fields with strict requirements for power and beam quality, including metal cutting, deep penetration welding, etc., among which the semiconductor laser beam combining light source based on traditional beam combining can be used for laser cladding, deep penetration welding, etc., and the semiconductor laser beam combining light source based on spectral beam combining meets the processing requirements of metal cutting.

3.5 Narrow spectral linewidth of semiconductor lasersNarrow linewidth

Semiconductor lasers have important applications in laser communication, optical interconnection, nonlinear frequency conversion and other fields. Generally, frequency selection is carried out by preparing a Bragg grating on a semiconductor laser, which can be placed on the cavity surface at one end of the diode laser as a wavelength reflector (distributed Bragg reflection, DBR) to select the lasing wavelength, or distributed along the entire semiconductor laser resonator (distributed feedback, DFB), or an external grating (such as a volume Bragg grating - VBG, or a volume holographic grating - VHG).

3.5.1 Distribution of Bragg reflective lasers

DBR lasers use a Bragg grating instead of one cleavage cavity surface of the laser, eliminating the need for secondary epitaxy. In 2010, the FBH Institute in Germany obtained a high-power DBR laser using a surface Bragg grating with a 90 μm strip width single tube output power of 14 W, a maximum conversion efficiency of 50%, and a wavelength shift of 0.074 nm/K. In the same year, the unit reported a narrow linewidth ridged waveguide DBR laser, which uses a sixth-order surface grating with a lasing wavelength of 974 nm, a single-mode output power of more than 1 W, and a 3 dB spectral linewidth of only 1. 4 MHz。 In 2011, the unit reported a narrow linewidth of 1 064 nm wavelength DBR exciter with a FWHM of 180 kHz, an intrinsic linewidth of only 2 kHz at 180 mW, and a wavelength shift of 0.083 nm/K.

3. 5.2 Distributed feedback lasers DFB lasers

It was first developed by H. Bell Labs. Kogelikan et al. proposed in 1972 and achieved continuous operation at room temperature in 1975, and then received attention and rapid development in the field of optical communication, and its research progress is shown in Table 9. DFB lasers are characterized by the grating distributed throughout the resonator, and the light waves gain gain and lasing at the same time as feedback, relying on the frequency selection principle of the grating to achieve wavelength selection. There are two manufacturing methods for it: one is to interrupt when a part of the p-type waveguide layer is grown, and a layer of low refractive index grating layer is epitaxized, and then the wafer is moved out of the growth reaction chamber, and photolithography and etching are used to form a unified grating, and then the epitaxial growth reaction chamber is re-epitaxized, and the growth continues on the grating, and finally a DFB laser is formed. Another method is to form a surface grating by etching after the epitaxial growth is complete, without the need for secondary epitaxial technology.

3.5.3 External cavity grating lasers

External cavity grating lasers achieve the purpose of stabilizing the wavelength through the feedback of external grating elements and the resonance of the laser cavity. In general, the external cavity laser can narrow the laser linewidth and operate at a single frequency due to the relatively long cavity length and the external cavity grating with selective reflection for a specific longitudinal mode. The research progress is shown in Table 10.

3.6 VCSEL diode lasers

VCSELs occupy an important position in semiconductor lasers due to their advantages of low threshold current, stable single-wavelength operation, high-frequency modulation, easy two-dimensional integration, and no cavity surface threshold damage. Based on GaAs substrates, VCSEL devices can achieve high-quality material growth for high material gain, and can also be used to form DBRs on monolithic blocks with epitaxial growth lattice matching, high refractive index difference, and lower resistance AlAs and GaAs materials. In terms of device performance and practicality, devices in the 850 nm and 980 nm bands consistently represent the highest level of VCSEL semiconductor laser research.

3.6.1 850 nm band VCSELs

850 nm is the first low-loss window for quartz-based fibers, and high-speed modulated 850 nm VCSELs can be used for short- to medium-distance LANs, free-space optical communications, and optical interconnects. With the huge application needs of the information age, the performance of VCSELs such as low power consumption and high-speed modulation was rapidly improved in the late 90s and early 21st centuries of the 20th century.

In 1998, ULM University produced a VCSEL device with an electro-optical conversion efficiency of 57%, which has held the record for the highest conversion efficiency for nearly 10 years. In 2004, ULM University's surface-embossed device achieved a single-mode of 6 mW and a single-mode rejection ratio of 40 dB. In 2009, the data transfer rate reached 32 Gbit/s. As a result of these advances, 850 nm VCSEL devices were the first to be commercially produced.

Subsequently, 850 nm VCSEL devices began to replace edge-emitting lasers for short-range fiber communications. In January 2002, Ulm Photonics' VC-SEL arrays and discrete devices prepared by the flip-chip method reached a rate of 10 Gbit/s and were mass-produced. At the same time, FujiXerox in Japan began mass production of VCSELs.

In 2003, Petar Pepeljugoski et al. conducted transmission tests of multimode fiber at 15.6 Gb/s, 1 km and 20 Gb/s, 200 m, and the results showed that the indicators were in line with the coarse wavelength division multiplexing (CWDM) 2 × 20 Gbit/s Ethernet standards. In 2010, Westbergh and others in Switzerland carried out error-free large-capacity communication of multimode high-speed devices with direct modulation with transmission rates of up to 40 Gbit/s. Currently, 850nm VCSELs can enable high-speed communication at up to 1000 m and 25 GHz, with communication energy consumption as low as 69 fJ/bit. In the market, companies such as Coherent, Honeywell, EMCORE, and AXT in the United States, as well as some optical communication equipment manufacturers in South Korea and Japan, have commercialized VCSEL devices and chips.

3.6. 2 980 nm band VCSELs

VCSELs in the 980 nm band have grown rapidly over the past 10 years due to the traction of applications such as fiber laser and solid-state laser pumping, laser illumination, frequency doubling, and more. In recent years, research and development have focused on improving the power and efficiency of area-emitting semiconductor lasers, achieving high power density and high beam quality.

In 2001, the University of Ulm in Germany reported a device with a continuous output of 890 mW in a single tube, and integrated a 2D area array with a continuous output of 1.4 W using 19 single tubes in parallel. In 2004, Ulm Photonics achieved an integrated cell area array with a continuous output of 6W with a total of 224 VCSELs and a slope efficiency of 0. 6 W/A with a conversion efficiency of 22%.

Princeton Optronics is a company specializing in high-power near-infrared VCSELs. The company achieved a 980 nm device with a continuous output of 3 W in 2005. In 2007, the company launched an area of 0. A 22 cm2 area array with a continuous output power of more than 230 W has a conversion efficiency of 50% and a temperature drift coefficient of less than 0.07 nm/°C. In 2010, the company introduced a 100-watt high-power array for near-infrared active laser illumination, enabling speckle-free imaging at 500 m. In 2012, the company introduced a 980 nm high-power VC-SEL area array and area array combination module for solid-state laser pumping, with a continuous output of more than l 4 kW.

Research progress of Changchun Institute of Optics, Fine Mechanics and Physics in high-power semiconductor lasers

Through the hard work of researchers, Changchun Institute of Optics, Fine Mechanics and Physics (Changchun Institute of Optics and Mechanics) has made remarkable achievements in high-power semiconductor lasers in the past few decades.

4.1 New material quantum well lasers

In 1996, Changchun Institute of Optics and Mechanics took the lead in developing an 808 nm continuous output power of 3. 6 W, Schottky barrier current-limited InGaAsP/InGaP/GaAs aluminum-free quantum well new material high-power lasers with an operating life of more than 10 000 h. The 808 nm laser linear array has a continuous output power of up to 150 W, a quasi-continuous output of more than 150 W, and a device lifetime of more than 10 000 h.

In 2000, Changchun Institute of Optics and Mechanics developed the first 808 nm laser array and laser fiber coupling module for InGaAsP/InGaP/GaAs aluminum-free quantum wells in the world. Compared with GaAlAs/GaAs semiconductor lasers, this laser has the advantages of long life and high reliability. In 2004, the laser basically met the requirements of high-power devices, achieving a continuous optical power output of 60 W/bar and a pulse output of 100 W/bar, and the emission wavelength deviation was controlled at 3 nm.

4.2 High-power laser beam combiner light source

Purely from the perspective of the external optical system, laser beam combining is divided into spatial beam combination, polarization beam combination and wavelength beam combination. Combined with the geometrical optical shaping method, we have developed a combined beam light source based on three traditional laser devices: single tube, linear array and stacked array.

4.2.1 Single-tube combined beam laser light source based on TBC technology

The laser single tube is the smallest component unit of a semiconductor laser, with good beam quality and high brightness. The single-tube beam combining light source does not need beam shaping, and after the spatial ladder arrangement and fast and slow axis collimation, the beam is directly combined by the optical path of the respective space beam combining mirrors, and then coupled into the optical fiber through beam expansion, which has the advantages of easy heat dissipation, small size, light weight, high reliability and low cost, and is an effective light source in the fields of fiber pumping, laser display and laser medical treatment. Since the output power of each laser tube is generally not more than 10 W, and it needs to be equipped with independent fast and slow axis collimators and space beam merging mirrors, the output power of the light source should not be too high, generally not more than 300 W. If the power is further increased, there are many components involved, and the assembly becomes very complex, losing the advantages of cost and size.

Based on the single-tube beam combining technology, we use multiple high-brightness laser single tubes, combined with their thermal dispersion layout, to develop a variety of beam-combining light sources with air-cooled structure: 105 μm/0. 2NA fiber continuous output 30 ~ 70 W; 200 μm/0. 2NA fiber continuous output power 80 ~120 W.

4. 2.2 Linear combined beam laser light source based on TBC technology

Laser linear beam combination is a laser linear array that uses several conduction cooling heat sink packages, separated in physical positions, superimposed by spatial beam mirrors, and then polarized wavelength combining, which can achieve power output in the range of hundreds of watts to 3 kW. The structure can be conductively cooled by the overall industrial water, which has the advantages of high reliability and easy maintenance. Due to the heat dissipation limitation of conduction heat sink, the output power of a single linear array should not be too high, generally 40 ~ 80 W. Due to the poor beam quality, the conventional centimeter linear array requires an additional beam shaping structure and a complex optical system, so the linear array is often combined with a mini linear array with a smaller bar width or a centimeter linear array with a low fill factor.

Based on the linear array beam light source, we have developed a laser with a continuous 400 W power output of 200 μm / 0.2NA fiber, which can be used for cutting thin stainless steel plates. A 200 μm/0.2NA fiber continuous 3 000 W power output laser was developed for sheet metal welding. A photo of the device is shown in Figure 5.

4. 2.3 Stacked beam laser light source based on TBC technology

The laser stacking array is a semiconductor laser linear array that is packaged with deionized water cooling by microchannel heat sink and stacked at a physical position in the fast axis direction. Due to its excellent heat dissipation characteristics, the single-layer stacked array can work at 100 ~ 300 W, which has the advantages of easy to achieve high power output and compact structure, and is the most important beam combination form for semiconductor lasers to achieve thousands of watts or even tens of thousands of watts of laser power output. Laser stacking arrays usually use centimeter-line arrays, so beam shaping is required before laser beam combining. We have developed a 6 kW ball valve surface enhanced light source and a 10,000-watt laser cladding light source using multiple sets of laser arrays, as shown in Figure 6.

Conventional TBC technology is limited by the beam combining mechanism, and the beam quality of the laser after beam merging is worse than that of the laser unit. In order to further improve the beam quality, the combination of semiconductor lasers and external optical systems has proven to be one of the effective ways to solve this problem. It uses a semiconductor laser chip coated with an anti-reflection coating on the front cavity surface and the external optical system to form a resonant cavity as a whole, and through the external grating adjustment, all laser units on the laser chip resonate in the same direction, and completely coincide in the near field and far field, so as to achieve the overall beam quality is only the laser output of the unit beam quality, and the beam quality at the same high power is dozens of times higher than that of the conventional method.

We have successively developed spectral beam combining light sources of tens of watts to hundreds of watts at 808 nm and 970 nm by using the external cavity WBC technology based on transmission grating, and the beam quality is only 3 ~ 5 mm·mrad, which is close to the beam quality of the laser unit. Figure 7 shows the experimental setup for spectral beam binding.

4.3 VCSEL Single Tube and Area Array

Changchun Institute of Optics and Mechanics broke the shackles of traditional concepts, put forward new ideas such as multi-gain region, modulation doped DBR, and large optical aperture, theoretically predicted the possibility of high-power VCSELs above the watt level, and carried out the research work of high-power 980 nm VCSELs for the first time in China, and achieved a series of breakthrough results. In 2003, we used three strain-compensated InGaAs/GaAsP quantum wells to obtain a high-performance 987 nm VCSEL for the active region: a device with a mouth diameter of 430 μm had a continuous output of more than 1.5 W at room temperature, a lasing peak width at half height of only 0.8 nm, a watt-class output device far-field divergence angle of less than 10°, and a characteristic temperature of more than 220 K. Subsequently, by optimizing the device structure and process, the output power of the 980 nm VCSEL was refreshed to 1.95 W in 2004, and the pulse output was 10. 5 W, which was the highest level in domestic and foreign reports at that time. In 2009, we developed a high-density integrated array with a continuous output of more than 2.5 W (20% efficiency) and a pulse output of kilowatts.

In 2010, we proposed and realized the monolithic integration of high-power VCSEL integrated area array and its microlens surface array for the first time in the world, and the 6 × 6 VCSEL integrated microlens array achieved 1.0 W fundamental mode laser output, and the divergence angle was increased from 14. 8° down to 6. 6°, the beam quality has been doubled, which has opened up a new direction for the development of large-scale integrated array lasers with high beam quality. Figure 8 shows a photograph of an array of integrated microlenses.

In 2010, we developed a VCSEL area array with a pulse output of 138,319,510 W (60 ns × 100 Hz) for 5×5, 10×10, 20×20. In 2011, we invented two polarization-controlled VCSEL laser structures and laser fabrication methods, and developed a high-power VC-SEL area array with the highest integration of 30 × 30 (64 cells/mm^2) (Fig. 9), with a total of 900 devices integrated in an area of 3.75 nm × 3.75 mm, and a continuous output power of 2.9 W for a single-tube device, opening up a new way for the development of megawatt and even higher power laser light sources.

In 2011, we developed a VCSEL with a peak power of 92 W driven by a current of 110 A and 60 ns, which was the best level reported internationally that year and set a world record for a single-tube laser that year.

In 2013, we proposed and designed the AlGaAs/In-GaAlAs wide barrier structure, which realizes 795 nm and 894 nm high-temperature operation (75 °C) VCSELs, which are suitable for low-power chip size integration of sensors such as miniature atomic clocks and atomic gyroscopes, and realize functions such as timing, positioning, and navigation. The chip volume is only 0.05 mm3, the device has a highly stable single-mode laser output of more than 0.2 mW, and the operating current is less than 1. 5 mA with less than 3 mW of power consumption.

In 2014, we invented a area array hybrid package structure and its preparation method to solve the problem of driving high-power VCSEL area array low voltage (4 V, high current 50 ~ 500 A), as shown in Figure 10. Four high-power VCSELs are connected in series to form a high-power quasi-array module in the 980 nm band, with a size of 2.2 mm × 2. mm, with an output power of up to 210 W. This breakthrough makes it possible for micro-miniaturized high-power VCSEL modules to be practically available in laser fuze, laser ranging, and laser area radar systems.

4.4 New laser chips

4. 4.1 High brightness Bragg reflective waveguide photonic crystal laser

We conducted research on an 808 nm Bragg reflective waveguide photonic crystal laser (Figure 11). The photonic crystal was used to manipulate the optical mode to achieve near-circular beam output, and the divergence angle of the fast axis (vertical) of the traditional diode laser was successfully compressed from 40° to less than 10° (half-height and full width), and a stable circular spot laser output was realized, in which the optimized three-quantum well laser with 95% power can have a vertical divergence angle as low as 9.8°, which is the lowest value currently reported, as shown in Figure 12. Due to the increased pattern size of this laser in the vertical direction, catastrophic light damage can be effectively suppressed. The single-tube CW and pulsed output power of the wide-strip laser can exceed 3.5 W and 11 W, respectively, the bar-strip pulse output power > 70 W, and the single-mode quasi-CW power of the 10 μm strip wide-ridged device can also exceed 1.1 W under pressure testing conditions without cavity surface passivation.

4. 4.2 Bragg Reflection Waveguide Dual-Beam Laser

Dual-beam lasers have important applications in the fields of high-speed laser scanning, high-precision laser detection, in-situ depth monitoring, and off-axis external cavity lasers. The traditional method is to split one laser into two or combine two laser beams, but this method requires precise optical alignment, is not compact, and is difficult to mass-produce.

The Bragg reflection waveguide structure is introduced in the vertical direction of the semiconductor laser, and the Bragg reflection waveguide is used to control the laser to work in the complete photonic bandgap guidance mode, which can achieve stable symmetrical double-beam laser output from the chip level, which is simple in structure, low in price, and easy to mass produce. For the first time in the world, we have developed a Bragg reflective waveguide dual-beam laser (Fig. 13), which outputs two symmetrical, near-circular lasers with a single-beam laser with a vertical divergence angle of 7.2° and a horizontal divergence angle as low as 5.4°. In addition, this laser has a significant spectral modulation effect.

The development trend of high-power semiconductor lasers

In order to meet the demand for semiconductor lasers in all walks of life, high-power semiconductor lasers must have higher power, conversion efficiency, reliability, beam quality and better spectral characteristics, and it is necessary to start from the following aspects: (1) Develop new structures and processes to improve the indicators of semiconductor laser single tubes; (2) Develop new materials and new structures of semiconductor lasers to achieve laser output from ultraviolet to far-infrared bands: (3) Develop new laser beam combining technology to improve the output power of semiconductor lasers; (4) Expand the application fields of semiconductor lasers, such as 3D printing, ultrashort pulse processing, nano-optics and other emerging fields, and promote the development of semiconductor laser application technology.