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Single-mode quasi-PT symmetric laser with high transmit power


Recently, scientists Abdullah Demir and Ramy El-Ganainy and others published a scientific and technological article entitled “Single-mode quasi PT-symmetric laser with high power emission”. Large area lasers can be used to generate high output power lasers. However, this comes at the cost of introducing a higher-order mode that reduces the beam quality. Through experiments, the authors demonstrate a new electrically pumped large-area edge-emitting laser with high emission power (∼0.4 W) and high-quality beam (M2~1.25). The laser is achieved by establishing quasi-cosmic time (PT) symmetry between the second-order mode of the large-area dual-mode laser cavity and the second-order mode of the single-mode auxiliary companion cavity, that is, by achieving partial isospectra between the two coupled cavities. This expands the effective volume of the higher-order mode. And the selective pumping of the current injection into the main laser cavity provides stronger mode gain for the base mode, so as to achieve laser emission in single mode after filtering out the high-order transverse mode. The article has been published in the journal Light: Science & Applications.

Research background

The invention of semiconductor lasers has revolutionized modern optical technology, with numerous applications in industry, telecommunications, biology, and space exploration. Although the basic principles behind laser emission in semiconductor platforms are well known, researchers have been working to optimize the performance of semiconductor lasers, including power conversion efficiency, output power, beam quality, laser energy level, spectral characteristics, size, robustness to undesirable noise and thermal management, reliability, and more. At present, the manufacture of semiconductor lasers is mainly divided into two types: vertical cavity surface-emitting lasers (VCSEL) and edge-emitting laser diodes (EE-LD). In contrast, VCSELs have near-perfect Gaussian patterns, while EE-LDs boast higher power levels. However, the output power of EE-LD is mainly limited by the total injection current, but increasing the pump current leads to an increase in current density per unit cross-sectional area, which in turn triggers output limiting effects such as nonlinear losses and optical catastrophe damage. Higher power can be obtained by enlarging the cross-section while keeping the current density below the damage threshold. However, this introduces a higher-order optical mode to reduce the quality of the emitted beam. Therefore, how to achieve large-area single-mode lasers with high power operation is very important.

Currently, the concept of non-Ermi photonics is also being used to design and improve the performance of on-chip laser systems. Theoretically, the symmetry of cosmic time (PT) is proposed and experimentally verified, and the higher-order transverse mode can be suppressed by passing field filtering. A PT symmetric laser with a single longitudinal mode laser was also exhibited. In addition, it has been reported that the non-Ermi effect and other symmetry concepts, such as the interaction between supersymmetry and topological invariants, can be used to build novel lasers. However, until now, it is unclear whether these design concepts can be extended to actual devices, as higher carrier concentrations and optical power may cause thermal effects and resonant frequency shifts that can degrade device performance.

Research innovation

Figure 1 Quasi-PT symmetric laser.

Figure 1 depicts a schematic of a quasi-PT symmetric laser. It consists of two coupled asymmetric laser waveguides. β is the propagation constant, and κ represents the coupling between modes. Among them, the high-order mode of the main potential well and the first mode of the partner potential are coupled to each other. The main potential (waveguide) has a larger cross-sectional area in comparison to provide higher output power. In the design, the companion potential supports only one mode that has the same propagation constant as the second-order mode of the main waveguide. Therefore, resonant interaction can be achieved between the second-order mode of the principal potential and the fundamental mode of the partner potential, thereby establishing quasi-or modal PT symmetry. The interaction between resonant modes expands the effective volume of higher-order modes of the combined system while keeping the volume of the base mode constant. Therefore, selective pumping by current injection in the main waveguide can provide higher modal gain for the base mode, which can act as a modal filter to effectively filter out high-order modes and achieve laser emission in a single-mode state.

Figure 2 Equipment manufacturing.

A quasi-PT symmetric laser is implemented based on the edge-emitting platform shown in Figure 2, where the substrate and quantum well materials are GaAs and InGaAs, respectively, with Al-GaAs optical cavities. where R and S are 1.10 μm and 3.2 μm, respectively, and the center wavelength is 975 nm.

Figure 3 Parameter optimization. Wm= 7.5 µm。 Design I: Wp= 1.9 µm, Design II: Wp= 2.1 µm, Design III: Wp= 1.7 µm。

To achieve quasi-PT symmetry between the fundamental mode of the companion waveguide and the second-order mode of the main waveguide, the authors performed a search for the optimal width, as shown in Figure 3. The main waveguide is approximately Wm = 7.5 μm wide and only TE0 and TE1 modes are supported. The corresponding optimal value for Wp is 1.9 μm (design I), which only supports TE0 mode. The electric field profile shows that Design I has better coupling characteristics.

Figure 4 Photocurrent curve and M2 parameter curve of the device.

Figure 4a shows the total output power of the four devices, which have nearly the same photon conversion efficiency. On the other hand, Figure 4b plots the measurement of the M2 parameter, which quantifies the beam quality as a function of the total output power. At low power, single waveguides and Design III have the worst beam quality. As power increases above 150 mW to 500 mW, the beam quality factor drops rapidly for all but optimal device I. It is worth noting that even at the 400 mW output power level, the beam quality factor M2 associated with Device I is still close to one. In comparison, the M2 of a single-waveguide device is about 1.75. As the power level increases to approximately 500 mW or higher, the beam quality of the single waveguide becomes comparable to that of Device I, both superior to Design II and III.

The near-field and far-field distributions of the emitted beams of several devices at different laser output powers are shown in Figure 5. At low power close to the laser threshold (35 mW), the near-field in the single-waveguide case shows a large deviation from the ideal Gaussian beam distribution. In addition, all PT symmetrical lasers generate a single-mode near-field at an output power of 35 mW. At a power of 400 mW, a secondary emission from the companion waveguide is detected. It can be seen that the deviation of the optimal design I from the Gaussian distribution is minimal.

Figure 5 Near-field and far-field emission profiles.

The article was published in Light: Science & Applications in the top international academic journal “Single-mode quasi PT-symmetric laser with high power emission”. (Source: LightScience Applications WeChat public account)

Related paper information:https://www.nature.com/articles/s41377‍-023-0‍1175-6

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