A quantum cascade laser based on an optical phased array

Abstract

The present invention provides a quantum cascade laser based on an optical phased array, the quantum cascade laser comprising: a substrate on which a seed laser and an optical phased array are integrated, the optical phased array dividing the laser emitted by the seed laser into multiple lasers; a phase-adjustable optical amplifier is arranged at the front end of the optical phased array to individually phase-adjust the multiple lasers. The present invention can amplify the output power of the seed laser through the array structure and improve the beam quality.

Description

Technical Field

[0001] The invention relates to the field of optoelectronic technology, and in particular to a quantum cascade laser based on an optical phased array. Background Art

[0002] Since the birth of lasers in the 1960s, they have rapidly become an important tool in production and life due to their high monochromaticity, high brightness and high directionality. Among them, mid- and long-wave infrared lasers are one of the important laser bands, which contain the fingerprint absorption characteristics of many chemicals and two atmospheric transmission windows. Quantum cascade lasers (QCLs) have become the main laser source in the mid- and long-wave infrared field due to their compact structure, ability to work at room temperature and high wavelength tunability. Quantum cascade lasers (QCLs) use quantum band engineering to adjust the energy level structure between conduction band subbands, and achieve radiative luminescence through transmission transitions between subbands by electrons instead of holes, breaking the shackles of semiconductor lasers expanding to the infrared band.

[0003] With the development of quantum cascade laser (QCL) materials and device technology, domestic and foreign research institutions have conducted a lot of research on the application of quantum cascade lasers (QCL) in the fields of laser spectroscopy detection of material components, free-space optical communications, directional infrared countermeasures, etc., and have realized the practical application of quantum cascade lasers (QCL) in some sub-sectors. In the military field, it is mainly used in directional infrared countermeasures, infrared laser guidance, laser radar and other fields. In the civilian field, the main applications are hazardous chemicals detection, gas sensors, and respiratory gas detection in medicine. In addition, the wavelength band of the quantum cascade laser (QCL) is designed to be located within the atmospheric window, which can well avoid the loss problem in the process of light propagation. Using quantum cascade lasers (QCL) as light sources for free-space optical communications greatly increases the communication distance.

[0004] As the output power of devices running at room temperature becomes higher and higher, the single-frequency characteristics become better and better, and the output wavelength continues to expand, quantum cascade lasers (QCL) are increasingly widely used in many fields, and the ever-increasing technical indicators they face are also driving the progress of quantum cascade laser (QCL) technology. High-brightness quantum cascade lasers are an urgent need, that is, a combination of high optical power and good beam quality. Major research teams have improved quantum cascade lasers (QCLs) in terms of band design, device structure, and process to increase their output power. Usually, simply to increase the optical output power, increasing the active area is the most common method, which then causes problems such as heat dissipation difficulties and poor beam quality. With the development of strip laser and optical waveguide theory and technology, some researchers have integrated a large number of single high-performance strip lasers into a single chip, enabling effective coupling between the lasers and achieving coherent output of multiple lasers, resulting in the emergence of coherent array semiconductor lasers. The most representative coherent array laser was first developed by the Spectra Diode Lab, a joint venture between Xerox and Spectra Physics, which developed a series of high-power semiconductor coherent array lasers. The emergence of coherent array semiconductor lasers has greatly improved the performance of semiconductor lasers and has achieved rapid development.

[0005] After 25 years of development, quantum cascade lasers have achieved continuous operation at room temperature of 3.4 to 17um, and the power has reached the watt level in some bands. At present, major research institutions and academic teams are constantly exploring ways to improve the performance of lasers. The world's highest output power single-tube transverse mode quantum cascade laser (QCL) has a lasing wavelength of 4.6um, a continuous wave output power of 5.1W at room temperature, and an electro-optical conversion efficiency of up to 27% in pulsed working mode. The Capasso team of Harvard University has improved the quantum cascade laser (QCL) in terms of device structure. In 2007, it first demonstrated the use of 32 DFB QCL arrays on a chip to achieve multi-wavelength switching. All array elements can operate in single mode, with wavelength tuning of 8.71 to 9.47μm. In 2012, a tapered waveguide structure was used to reduce the gain saturation effect in the quantum cascade laser (QCL) material, achieving a continuous wave output power of more than 4.5W at 4.5um. In 2018, M. Razeghi's team studied a phase-locked quantum cascade laser (QCL) array based on a multimode interference coupler MMI. The efficiency of the multimode interference coupler MMI was estimated to be 94%, and an output power of up to 15W was obtained from a laser array. In the same year, the team proposed a single-mode, high-power, mid-infrared, quantum cascade laser phased array, and obtained a single-mode peak output power of up to 30w. In China, research on quantum cascade lasers is mainly carried out by research institutions such as the Chinese Academy of Sciences and Changchun University of Science and Technology. In 2013, a ridge surface-emitting quantum cascade laser was realized for the first time to operate continuously at room temperature. In 2020, a semi-insulating buried heterostructure was used to increase the output power of the device by 85% compared with the traditional structure, and the maximum power at room temperature was increased from 280mW to 520mW. Their other work related to the array includes optimizing the structure of single-tube devices in 2019 and preparing the world's first room-temperature continuous wide-spectrum mid-infrared quantum cascade superluminescent diode array. The output power under room-temperature continuous current injection reaches 2.4 mW, the corresponding spectral width is about 200 cm-1, and the output beam quality is excellent.

[0006] There has been some development in quantum cascade laser (QCL) array structures. DFB-type QCLs are suitable as multi-wavelength array components for spectroscopy systems with relaxed power requirements, but their choice as array elements limits the peak output power of such arrays. In order to target applications like delayed detection, a single-mode QCL device that reliably provides high output power with good beam quality is required as an array element. A single-lobe far-field intensity distribution in the face-normal direction for all array elements is a key requirement for many spectroscopy and sensing applications, combined with strict power requirements for sensitive and reliable detection. This can be solved by using MOPA-type QCLs as array components. The master oscillator power amplifier (MOPA) operation is based on generating a single longitudinal and transverse mode in a low-power laser device and coupling this mode into a single-pass amplifier, where it is amplified while maintaining its spectral purity and mode profile.

[0007] For both DFB-type QCL arrays and MOPA-type QCLs, operating at higher duty cycles is challenging. With the continuous development of technology, a new type of QCL array element is needed to provide more efficient operation.

[0008] Achieving high-brightness beams through a quantum cascade laser array structure has proven to be an effective method. However, the phase of each path in the currently reported structure cannot be controlled, making it difficult to meet the phase fluctuations caused by process errors and structural errors. In addition, factors such as heat dissipation and process limit the array spacing, making it difficult for the beam quality after beam combining to meet ideal requirements, and it is difficult to eliminate the grating lobes and side lobes after beam combining. Summary of the invention

[0009] In order to solve the technical problems in the prior art that the beam quality of the quantum cascade laser of the optical phased array after beam combining is difficult to meet the ideal requirements, and the grating lobe and side lobe after beam combining are difficult to eliminate, an object of the present invention is to provide a quantum cascade laser based on an optical phased array, the quantum cascade laser comprising:

[0010] A substrate, on which a seed laser and an optical phased array are integrated, wherein the optical phased array divides the laser light emitted by the seed laser into multiple laser light paths;

[0011] A phase-adjustable optical amplifier is arranged at the front end of the optical phased array to individually phase-adjust multiple lasers.

[0012] Furthermore, the coupling strength of the seed laser should satisfy:

[0013]

[0014] Where ∧ is the period of the grating, Δn is the refractive index step, and neff is the effective refractive index.

[0015] Furthermore, the product of the length L of the seed laser and the coupling strength k is: kL=5.

[0016] Furthermore, the optical phased array is a tree-shaped MMI beam splitter.

[0017] The invention provides a quantum cascade laser based on an optical phased array. Aiming at the problems of low power and poor beam quality of quantum cascade lasers, the seed laser is divided into multiple laser paths through an optical phased array, and each path is amplified and phase-locked by a phase-adjustable optical amplifier, so as to achieve phased array laser coherent beam combining. By designing a structure combining a single-mode output seed laser with a phased array, the problem of low output power and poor beam quality of a single laser structure is solved.

[0018] The invention provides a quantum cascade laser based on an optical phased array. First, the seed DFB laser part is designed, and a grating structure is used for single-mode output. Then, the phased array part is designed to realize mutual locking of array units so that coherent output can be achieved. Finally, the seed laser is combined with the array structure. Compared with the previous single laser structure, the array-combined structure can better achieve power amplification and beam quality improvement, laying the foundation for the development of high-power, high-performance, beam-scanning quantum cascade lasers.

[0019] The present invention provides a quantum cascade laser based on an optical phased array. Based on the development status of quantum cascade laser arrays and in combination with optical phased array (OPA) technology, a quantum cascade laser (QCL) combining a seed laser and an optical phased array (OPA) is designed to achieve amplification of the laser output power, efficient beam combining and improvement of beam quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0021] Figure 1 The schematic diagram shows the structure of a quantum cascade laser based on an optical phased array in one embodiment of the present invention.

[0022] Figure 2 A schematic structural diagram of a seed laser in one embodiment of the present invention is shown.

[0023] Figure 3 A schematic structural diagram of a multimode interference coupler in one embodiment of the present invention is shown.

[0024] Figure 4 A schematic structural diagram of a phase-adjustable light emitting device according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0025] In order to make the above and other features and advantages of the present invention more clear, the present invention is further described below in conjunction with the accompanying drawings. It should be understood that the specific embodiments given herein are for the purpose of explaining to those skilled in the art and are only exemplary and not restrictive.

[0026] In order to solve the technical problems of low power and poor beam quality of quantum cascade lasers in the prior art, a quantum cascade laser based on an optical phased array is proposed.

[0027] Principle

[0028] Although the working principle of quantum cascade lasers is different from that of conventional semiconductor lasers, the coherent array semiconductor lasers also provide a new idea for the development of quantum cascade lasers. Starting from the device structure, the present invention improves the output power of quantum cascade lasers and improves the beam quality by designing a structure combining a seed laser and an optical phased array structure.

[0029] Optical phased arrays are emerging as an important source for non-mechanical beam steering. In the past few years, scholars have conducted extensive research on near-infrared optical phased arrays (OPAs), mainly motivated by light detection and ranging (LIDAR). An optical phased array (OPA) is a discrete array system consisting of distributed coherent light emitters (also known as phase shifters). Under the premise that there is no phase difference between the optical signals, the light arrives at the same time at the phase plane, and the light propagates forward without interference. For the mid-infrared (3–12μm) spectral region, there are limited demonstrations of optical phased arrays (OPAs), which are critical for chemical and biological sensing, free-space communications, and infrared countermeasures applications. For all these applications, beam controllable and high-power laser sources are very ideal.

[0030] The main goal of structural design and fabrication of coherent array lasers is to achieve high-power output of the device while being able to achieve phase locking between array units, thereby obtaining fundamental transverse mode lasing. Coherent beam combining adopts a tree-structured beam combining, and the seed laser locks the phase of the cluster laser array to achieve coherent beam combining and power amplification. The output power of a coherent laser array varies with the number of lasers in the array. For adjacent strip lasers, when the spatial spacing of the device is small (3 to 6um), the light field in the laser waveguide will overlap. This overlapping of the vanishing field leads to the mutual coupling of the light field between the waveguides, thereby determining the phase dependence between the devices of adjacent lasers, that is, achieving phase locking, which leads to the coherence of the laser output from each discrete strip laser. Although there are multiple lasers in the array, they can still oscillate at the same wavelength. The total output light field distribution is uniform in the near field, and the far field is determined by its phase relationship, and there is still good beam directivity in the direction parallel to the junction plane. For incoherent laser arrays, it is difficult to combine the laser beams emitted from multiple surfaces of the array, resulting in the brightness of the actual array beam being even lower than that of a single laser beam.

[0031] Phase-locked laser arrays are the optical counterpart of phased array radars. High beam quality can be achieved with high fill factor arrays or using external optics such as microlenses. The present invention achieves monolithic beam synthesis and steering by actively controlling the phase relationship of the lasers within the array.

[0032] Structural design

[0033] like Figure 1 The schematic diagram of the structure of a quantum cascade laser based on an optical phased array in one embodiment of the present invention is shown. According to an embodiment of the present invention, a quantum cascade laser based on an optical phased array includes: a substrate 100, on which a seed laser 101 and an optical phased array 102 are integrated. The optical phased array 102 divides a single laser emitted by the seed laser 101 into multiple lasers.

[0034] Seed Laser Selection

[0035] When selecting a seed laser, a laser with narrow bandwidth or single mode is more advantageous, and the embodiment of the present invention uses a DFB laser as the seed laser 101. In some embodiments, an SGDFB laser can be selected as the seed laser 101 to achieve wide tuning.

[0036] Figure 2 The schematic diagram of the structure of the seed laser in one embodiment of the present invention is shown. The design of the seed laser 101 needs to consider the overall laser length L, the period Λ of the grating, and the coupling strength k.

[0037] The coupling strength of the seed laser 101 should satisfy:

[0038]

[0039] Where ∧ is the period of the grating, Δn is the refractive index step, and n eff is the effective refractive index. The product of the length L of the seed laser 101 and the coupling strength k is: kL=5, which can achieve good output power.

[0040] Optical Phased Array Design

[0041] In order to split the output of the seed laser 101 into a large format array, a set of low-loss beam splitters and curved waveguides are required. In a single-mode device, a multimode interference coupler (MMI) is a good choice for beam splitting, which has been demonstrated to have a beam splitting efficiency of up to 94% in the mid-infrared laser (MIR) region.

[0042] The present invention utilizes a tree-like MMI beam splitter as an optical phased array 102, a multimode interference coupler (MMI) 103 and an on-chip integration of a phase-adjustable optical amplifier array to realize an optical phased array (OPA) array of a single-mode 16 / 32 channel 3-5um quantum cascade laser (QCL).

[0043] In order to achieve a smaller loss, the width and length of the multimode interference coupler (MMI) need to be designed. Figure 3 As shown in the structural schematic diagram of a multimode interference coupler in one embodiment of the present invention, after determining the width of the multimode interference coupler (MMI) 103, considering that the transverse mode penetrates the surrounding cladding, it is necessary to calculate the effective width value (Weff) W. Based on the effective width value (Weff) W, the optimal multimode interference coupler (MMI) length value H can be calculated.

[0044] Secondly, it is necessary to consider the input and output cone angles of the multimode interference coupler (MMI) 103 and set a suitable angle value to suppress the total reflection of the out-of-phase supermode. In order to optimize the array spacing and improve the heat dissipation efficiency of the array, the finite element method can be used to simulate the variation of the laser core temperature with the spacing width.

[0045] Phase-adjustable optical amplifier

[0046] Phase-locking techniques for narrow-ridge laser arrays are being extensively developed to achieve beam-quality coherent output. Continuous operation via tree-array branch coupling is considered a better alternative, where the unit spacing is sufficient for thermal management and all laser waveguide elements are narrow width. Y-branch coupled laser arrays suffer from mode competition between in-phase and anti-phase supermodes, resulting in poor power performance (lower brightness). The far-field distribution is a mixture of in-phase and anti-phase modes. Although mode competition can be alleviated by increasing the loss of the anti-phase mode, this requires a narrow coupling node for the Y branch, which also increases the loss of the in-phase mode.

[0047] like Figure 4 The structure diagram of the phase-adjustable optical device in one embodiment of the present invention is shown in FIG. 1 , where the optical phased array 102 divides the single laser of the seed laser 101 into multiple lasers through multiple branches 104. A phase-adjustable optical amplifier 105 is provided at the front end of the optical phased array 102 to individually phase-adjust the multiple lasers of the multiple branches 104 of the optical phased array 102 ( Figure 4 FIG. 1 schematically shows that the phase-adjustable optical amplifier 105 performs phase adjustment on the four branches 104 separately.

[0048] The present invention adopts a multimode interference coupler (MMI) 103 to replace the Y-branch coupling, and an adjustable phase optical amplifier 105 is arranged at the front end of the optical phased array 102, so as to realize a coherent laser array of 1x16 and 1x32 arrays.

[0049] In the reverse process of beam combination, the output of the seed laser 101 is divided into a laser array to form an optical phased array, which can be used to increase power output and realize non-mechanical beam scanning. At the same time, in order to compensate for process errors and facilitate the adjustment of array synthesis, a phase-adjustable optical amplifier 105 is designed in the structural design for thermal tuning, and each branch 104 can be phase-adjusted separately. TiN is selected as the heating material of the phase-adjustable optical amplifier 105.

[0050] Reflective coating design

[0051] In order to achieve single-mode emission of the seed laser 101, it is necessary to suppress the self-excited laser of the optical phased array 102, which requires that the front end face of the seed laser 101 has an ultra-low reflectivity. The anti-reflection coating is usually formed by depositing a quarter-wavelength dielectric film. In a preferred embodiment, a quarter-wavelength dielectric film Y2O3 is deposited on the front end face of the seed laser 101.

[0052] The present invention provides a quantum cascade laser based on an optical phased array array. The structural design adopts methods such as buried ridge waveguides, increased array spacing, electroplated gold cooling, etc. to optimize the MMI coupled phased array laser array structure for continuous wave operation. The specific design parameters of the optical phased array 102 can be calculated according to the relevant design parameters of the device. The present invention can amplify the output power of the seed laser through the array structure and improve the beam quality.

[0053] The present invention provides a quantum cascade laser based on an optical phased array, in which monolithic beam control can be achieved by active (electrical) phase control of a single array output, and the array output can be coupled to a silicon photonic chip for passive beam control or combination, opening up new possibilities for mid-infrared photonics.

[0054] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A quantum cascade laser based on an optical phased array, characterized in that: The quantum cascade laser comprises: A substrate, on which a seed laser and an optical phased array are integrated, wherein the optical phased array divides the laser light emitted by the seed laser into multiple laser light paths; A phase-adjustable optical amplifier is arranged at the front end of the optical phased array to individually phase-adjust the multiple lasers, and the phase-adjustable optical amplifier is thermally tuned; The seed laser is a DFB laser or a SGDFB laser, and the coupling strength of the seed laser should satisfy: Where ∧ is the period of the grating, Δn is the refractive index step, and n eff is the effective refractive index; The product of the length L of the seed laser and the coupling strength k is: kL=5; A dielectric film Y2O3 with a quarter wavelength is deposited on the front face of the seed laser; The optical phased array is a tree-shaped MMI beam splitter.

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