Spatial coherence of electrically pumped random terahertz lasers
Eva A. A. Pogna, Alessandra Di Gaspare, Kimberly Reichel, Chiara Liberatore, Harvey E. Beere, David A. Ritchie, and Miriam S. Vitiello
Light sources with high radiance and tailored coherence properties are highly desirable for imaging applications in the mid-infrared and terahertz (THz) spectral regions, which host a large variety of molecular absorptions and distinctive fingerprints to be exploited for sensing and tomography. Here, we characterize the spatial coherence of random multimode THz quantum cascade lasers (QCLs) emitting > mW optical power per mode and showing low divergence (10°–30°), performing a modified Young’s double-slit experiment. Partial spatial coherence values ranging between 0.16 and 0.34 are retrieved, depending on the specific degree of disorder. These values are significantly lower than those (0.82) of conventional Fabry–Perot THz QCLs exploiting an identical active region quantum design. We then incorporate the devised low spatial coherence random lasers into a confocal imaging system with micrometer spatial resolution and demonstrate notable imaging performances, at THz frequencies, against spatial cross talk and speckles.
  • Jan. 26, 2022
  • Photonics Research
  • Vol.10 Issue, 2 02000524 (2022)
  • DOI:10.1364/PRJ.440463
Microstructure and Mechanical Properties of Adding Ti Powder Weld Joint in Dual-Phase Steel to Aluminum Alloy Under Process Condition of Laser Deep Fusion Welding
Xu Shaohua, Zhou Dianwu, and Liu Jinshui
DP590 dual-phase steel and 6016 aluminum alloy were used as test materials. A laser deep fusion welding test was conducted using Ti powder in the lap mode of "steel at the top and aluminum at the bottom," and the microstructure and properties of the joint before and after adding the Ti powder between them were compared. The test results show that under a laser power of 1600 W, a welding speed of 35 mm/s, a defocus amount of +1.0 mm, and back protection using the Ar protective gas of 15 L/min, the welding quality is improved by the addition of the Ti powder, and the average line load is 110 N/mm, which is 10% higher than the condition without the addition of the powder. The addition of the Ti powder can refine the grain size. During the welding process, the Ti powder will be absorbed into the liquid aluminum and react with Fe to form a ductile phase Fe2Ti, which will improve the property of weld-joint.
  • Jan. 24, 2022
  • Laser & Optoelectronics Progress
  • Vol.59 Issue, 3 0314007 (2022)
  • DOI:10.3788/LOP202259.0314007
Generation of Wideband Chaos Without Time-Delay Signature via Cascaded Chirped Fiber Bragg Grating Feedback
Wang Xiangyu, Wang Longsheng, Guo Yuanyuan, Jia Zhiwei, Wang Anbang, and Wang Yuncai
Using a semiconductor laser subjected to optical feedback from cascaded chirped fiber Bragg gratings (CFBGs), we propose and demonstrate a method for generating wideband chaos without a time-delay signature (TDS). The high dispersion of cascaded CFBGs creates irregular external-cavity modes and destroys the resonance of external-cavity mode, eliminating the TDS of the chaotic signal. In addition, the irregular external-cavity modes beat with the internal-cavity modes of the laser, introducing new high-frequency oscillations, which enhances the bandwidth of the chaotic signal. The effects of the dispersive optical feedback strength and wavelength detuning between the laser and CFBG on bandwidth and TDS in feedback systems with single CFBG and cascaded CFBGs were compared in the experiment. The obtained results show that the feedback system with cascaded CFBGs exhibits better performances in bandwidth improvement and TDS suppression under strong feedback and negative wavelength detuning conditions. Finally, we experimentally obtained a TDS-free chaotic signal with a 3 dB bandwidth of 12 GHz.
  • Jan. 24, 2022
  • Laser & Optoelectronics Progress
  • Vol.59 Issue, 3 0314006 (2022)
  • DOI:10.3788/LOP202259.0314006
Icing Characteristics of Textured Silicone Rubber Surface Based on Laser Engraving
Zhao Meiyun, Yang Fan, Tian Sen, Zhao Yan, Kang Meng, and Zhao Xinze
Because of its good mechanical and electrical insulation properties, silicone rubber has various applications. However, in a low-temperature environment, the ice coating on its surface severely limits its application. In this paper, a laser engraving machine was used to process a series of micron-sized cylindrical textures on the surface of a composite insulator umbrella skirt, resulting in a superhydrophobic surface that required no chemical modification. Three-dimensional topography instruments and a scanning electron microscope were used to observe the surface morphology of the samples. The adhesion measurement device was used to assess the ice adhesion of textured silicone rubber surfaces under various operating conditions. A high-speed camera was used to capture the condensation-melting process of droplets on the surfaces of various textured silicone rubber. The results show that under certain working conditions, the cylindrical texture sample with the processing power of 35 W and texture diameter and spacing of 350 μm exhibits the best hydrophobicity and the lowest adhesion, and the best anti-icing performance. Friction tests confirm that the surface texture of the processed silicone rubber has certain ice-repellent durability and stability.
  • Jan. 24, 2022
  • Laser & Optoelectronics Progress
  • Vol.59 Issue, 3 0314005 (2022)
  • DOI:10.3788/LOP202259.0314005
Research on Threshold Gain and Output Optical Power of Photonic Crystal Surface Emitting Lasers
Li Rusong, and Lu Huanyu
Photonic crystal surface emitting lasers (PCSELs) have the advantages of high power and high beam quality because they can achieve large area single mode lasing. They have important application prospects in optical communications, laser radar, laser printing, laser display, and laser processing. This paper focuses on the analysis of the band-edge lasing principle and threshold gain of the photonic crystal laser with in-plane multi directional distributed feedback effect, and combines the semiconductor laser rate equation to deduce the output optical power formula of PCSELs, and also gives some effective methods to increase the optical power of PCSELs. The research results can provide theoretical guidance for the development of high performance PCSELs.
  • Jan. 24, 2022
  • Laser & Optoelectronics Progress
  • Vol.59 Issue, 3 0314004 (2022)
  • DOI:10.3788/LOP202259.0314004
Bumpy Janus Film Prepared by Femtosecond Laser for Fog Water Collection
Chen Liang, and Su Yahui
Fog water collection is the process of converting gaseous fog into liquid water,which has important research value. The control of condensation efficiency is crucial factor in the collection of fog water. Improving condensation efficiency is mainly related to the thickness of the boundary layer and effective renewal on the substrate surface. In this paper, inspired by the back structure of desert beetles, a Janus film with bump structure on the surface of aluminum foil is prepared by femtosecond laser micro-nano processing and surface chemical modification, which could achieve efficient droplet condensation. The experimental results show that compared with the flat Janus film, the water collection efficiency of the bumpy Janus film is increased by 80%. More importantly, the bumpy Janus film can capture a horizontal fog flow and adapts harsh natural environment.
  • Jan. 24, 2022
  • Laser & Optoelectronics Progress
  • Vol.59 Issue, 3 0314003 (2022)
  • DOI:10.3788/LOP202259.0314003
Numerical Simulation of Laser Cladding 316L/H13+20%WC Composite Coating on H13 Steel Surface
Qiu Huanxia, Yu Wenbin, Song Jianli, Deng Jia, Li Yunyi, and Deng Qilin
H13 steel has often been used as a hot-work die material. However, it tends to undergo failure because of wear and crack generation under high-temperature and high-pressure working conditions. Laser cladding is an effective remanufacturing method for such materials. However, the temperature gradient and cooling rate of the formation process are very large owing to rapid cooling and heating characteristics, often inducing excess thermal stress and cracking of the coating. Herein, a numerical simulation of a laser cladding 316L/H13+20%WC composite coating on H13 steel surface was performed. The variation in the temperature gradient and cooling rate with time and the influence of substrate preheating on the temperature gradient and cooling rate were studied. Moreover, to verify the numerical simulation results, a laser cladding formation test of the composite coating was conducted on the H13 steel substrate. Experimental results show that substrate preheating can significantly reduce the temperature gradient and cooling rate on the top surface of the coating and exerts a certain restraining effect on the cracks of the sample surface. The findings of this study provide a reference for the laser cladding modification and repair of H13 steel dies.
  • Jan. 24, 2022
  • Laser & Optoelectronics Progress
  • Vol.59 Issue, 3 0314002 (2022)
  • DOI:10.3788/LOP202259.0314002
Design of High Power Low Loss 852 nm Fabry-Perot Laser
Li Yaobin, Li Ming, Qiu pingping, Yan Weinian, Jia Ruiwen, and Kan Qiang
The influence of internal loss and internal quantum efficiency on the output power of the laser is theoretically analyzed, and the comprehensive optimization design of the 852 nm Fabry-Perot (FP) laser is carried out by using PICS3D software. The designed device has the characteristics of small far-field divergence angle, low internal loss, and high internal quantum efficiency, which can achieve stable and high power output under large current. The basic transverse mode 852 nm FP laser was fabricated with internal loss less than 1 cm-1, divergence angle of fast axis is 42.3°, divergence angle of slow axis is 5.6°, and unilateral output power of 115 mW without coating. The theoretical and experimental results show that while increasing the thickness of the waveguide layer and non-doping the waveguide can reduce the absorption of light caused by carriers and reduce the internal loss of the laser. By increasing the component of Al and the doping concentration in the AlGaAs material, the carrier leakage can be effectively suppressed and the high internal quantum efficiency can be ensured.
  • Jan. 24, 2022
  • Laser & Optoelectronics Progress
  • Vol.59 Issue, 3 0314001 (2022)
  • DOI:10.3788/LOP2022259.0314001
All-fiber spatiotemporal mode-locking lasers with large modal dispersion
Huaiwei Zhang, Yunhong Zhang, Jiying Peng, Xinyang Su, Xiaosheng Xiao, Dongjian Xu, Junhao Chen, Tianran Sun, Kai Zheng, Jianquan Yao, and Yi Zheng
It is a challenging problem to balance the modal walk-off (modal dispersion) between multiple transverse modes and chromatic dispersion in long step-index multimode fibers (MMFs). By properly designing the oscillator, we have overcome the difficulty and successfully obtained an all-fiber spatiotemporal mode-locked laser based on step-index MMFs with large modal dispersion for the first time, to our knowledge. Various proofs of spatiotemporal mode-locking (STML) such as spatial, spectral, and temporal properties, are measured and characterized. This laser works at a fundamental frequency of 28.7 MHz, and achieves a pulse laser with single pulse energy of 8 nJ, pulse width of 20.1 ps, and signal-to-noise ratio of ∼70 dB. In addition, we observe a dynamic evolution of the transverse mode energy during the STML establishment process that has never been reported before.
  • Jan. 21, 2022
  • Photonics Research
  • Vol.10 Issue, 2 02000483 (2022)
  • DOI:10.1364/PRJ.444750
Sub-terahertz-repetition-rate frequency comb generated by filter-induced instabilities in passive driven fiber resonators
Pan Wang, Jiangyong He, Xiaosheng Xiao, Zhi Wang, and Yange Liu
Ultrahigh-repetition-rate frequency comb generation exhibits great potential in applications of optical waveform synthesis, direct comb spectroscopy, and high capacity telecommunications. Here we present the theoretical investigations of a filter-induced instability mechanism in passive driven fiber resonators with a wide range of cavity dispersion regimes. In this novel concept of modulation instability, coherent frequency combs are demonstrated numerically with rates up to sub-terahertz level. Floquet stability analysis based on the Ikeda map is utilized to understand the physical origin of the filter-induced instability. Comparison with the well-known Benjamin–Feir instability and parametric instability is performed, revealing the intrinsic distinction in the family of modulation instabilities. Our investigations might benefit the development of ultrahigh-repetition-rate frequency comb generation, providing an alternative method for the microresonators.
  • Jan. 21, 2022
  • Photonics Research
  • Vol.10 Issue, 2 02000465 (2022)
  • DOI:10.1364/PRJ.442615