Green Laser Diodes Research Articles

Improving photoelectric characteristics of GaN-based green laser diodes by inserting electron blocking layer with gradient Al composition

This paper proposes four new gradient composition electron blocking layer (EBL) structures to enhance the photoelectric performance of GaN-based green laser diodes (LDs). The optical and electrical properties of new structure LDs are theoretically analyzed by LASTIP. It is observed that the implementation of a gradient composition EBL structure increases the effective barrier height for electrons, thereby better inhibiting the electron current overflow from the active region. In addition, the optical field leakage is effectively suppressed, lower threshold current and higher output power are obtained. The four different new structures have obvious differences in the improvement of the hole current injection and slope efficiency of multiple quantum well (MQW) LDs, and the underlying reasons for these phenomena are explored. Simulation results indicate that adopting a gradually descending Al component EBL structure yields optimal improvements in the photoelectric performance of LDs.

Titanium:sapphire-on-insulator integrated lasers and amplifiers.

Titanium:sapphire (Ti:sapphire) lasers have been essential for advancing fundamental research and technological applications, including the development of the optical frequency comb1, two-photon microscopy2 and experimental quantum optics3,4. Ti:sapphire lasers are unmatched in bandwidth and tuning range, yet their use is restricted because of their large size, cost and need for high optical pump powers5. Here we demonstrate a monocrystalline titanium:sapphire-on-insulator (Ti:SaOI) photonics platform that enables dramatic miniaturization, cost reduction and scalability of Ti:sapphire technology. First, through the fabrication of low-loss whispering-gallery-mode resonators, we realize a Ti:sapphire laser operating with an ultralow, sub-milliwatt lasing threshold. Then, through orders-of-magnitude improvement in mode confinement in Ti:SaOI waveguides, we realize an integrated solid-state (that is, non-semiconductor) optical amplifier operating below 1 μm. We demonstrate unprecedented distortion-free amplification of picosecond pulses to peak powers reaching 1.0 kW. Finally, we demonstrate a tunable integrated Ti:sapphire laser, which can be pumped with low-cost, miniature, off-the-shelf green laser diodes. This opens the doors to new modalities of Ti:sapphire lasers, such as massively scalable Ti:sapphire laser-array systems for several applications. As a proof-of-concept demonstration, we use a Ti:SaOI laser array as the sole optical control for a cavity quantum electrodynamics experiment with artificial atoms in silicon carbide6. This work is a key step towards the democratization of Ti:sapphire technology through a three-orders-of-magnitude reduction in cost and footprint and introduces solid-state broadband amplification of sub-micron wavelength light.

47 Gbps 100 m ultra-high-speed free-space visible light tricolor laser communication system utilizing time domain hybrid Huffman coding.

The escalating demand for greater transmission capacities in the forthcoming 6 G communication landscapes necessitates the investigation of upper segments of the electromagnetic spectrum by both academic institutions and the industrial sector. This effort aims to circumvent the impending spectrum resource limitations. Against this backdrop, laser diodes (LDs) emerge as a critical technology for high-speed visible light communication (VLC), owing to their significant modulation bandwidth potential. This paper details what we believe to be a novel visible light laser communication (VLLC) system powered by red/green/blue (RGB) tricolor laser diodes. It highlights a successful 100-meter free-space transmission achieved through a time domain hybrid huffman coding (TDHHC) technique. The system's performance review unveiled impressive data transmission rates for the red, green, and blue laser diodes channels at 16.852 Gbps, 14.442 Gbps, and 15.755 Gbps, respectively, culminating in a cumulative transmission rate of 47.049 Gbps while maintaining a bit error rate (BER) beneath the stringent threshold of 3.8E-3, mandated by 7% hard decision forward error correction (HD-FEC) standards. As far as we known, this marks the highest data rate documented in a long-distance tricolor VLLC system.

Turbidity-tolerant underwater wireless optical communications using dense blue–green wavelength division multiplexing

Underwater wireless optical communication (UWOC) has demonstrated high-speed and low-latency properties in clear and coastal ocean water because of the relatively low attenuation ‘window’ for blue–green wavelengths from 450 nm to 550 nm. However, there are different attenuation coefficients for transmission in ocean water at different wavelengths, and the light transmission more seriously deteriorates with fluctuations in the water turbidity. Therefore, traditional UWOC using a single wavelength or coarse blue–green wavelengths has difficulty tolerating variations in water turbidity. Dense wavelength division multiplexing (WDM) technology provides sufficient communication channels with a narrow wavelength spacing and minimal channel crosstalk. Here, we improve the UWOC in clear and coastal ocean water using dense blue–green WDM. A cost-effective WDM emitter is proposed with directly modulated blue–green laser diodes. Dense wavelength beam combination and collimation are demonstrated in a 20-metre underwater channel from 490 nm to 520 nm. Demultiplexing with a minimum channel spacing of 2 nm is realized by an optical grating. Remarkably, our WDM results demonstrate an aggregate data rate exceeding 10 Gbit/s under diverse water turbidity conditions, with negligible crosstalk observed for each channel. This is the densest WDM implementation with a record channel spacing of 2 nm and the highest channel count for underwater blue–green light communications, providing turbidity-tolerant signal transmission in clear and coastal ocean water.

Effect of Asymmetric InAlGaN/GaN Superlattice Barrier Structure on the Optoelectronic Performance of GaN-Based Green Laser Diode

An asymmetric InAlGaN/GaN superlattice barrier structure without the first quantum barrier layer (FQB) is designed, and its effect on the optoelectronic performance of GaN-based green laser diode (LD) has been investigated based on simulation experiment and analytical results. It is found that, compared with conventional GaN barrier LD, device performance is significantly improved by using FQB-free asymmetric InAlGaN/GaN superlattice barrier structure, including low threshold current, high output power, and high photoelectric conversion efficiency. The threshold current of LD with novel structure is 16.19 mA, which is 22.46% less than GaN barrier LD. Meanwhile, the output power is 110.69 mW at an injection current of 120 mA, which is 16.20% higher compared to conventional LD, and the wall-plug efficiency has an enhancement of 9.5%, reaching 20.27%. FQB-free asymmetric InAlGaN/GaN superlattice barrier layer can reduce optical loss, suppress the polarization effect, and improve the carrier injection efficiency, which is beneficial to improve output power and photoelectric conversion efficiency. The novel epitaxial structure provides theoretical guidance and data support for improving the optoelectronic performance of GaN-based green LD.

Full-duplex wireless light communication using green laser diodes

The integration of wireless light communication into a wireless fidelity (Wi-Fi) module and gateway enables real-time integrated communication networks that satisfy practical application demands. In particular, wireless green light communication tools can operate underwater and in free-space environments. Here, we design, fabricate, and characterize a full-duplex light communication system using green laser diodes (LDs). Operating within the transmission control protocol/internet protocol (TCP/IP), full-duplex wireless data transmission is confirmed in underwater and free-space environments at a communication rate of 10 Mbps. Through connections to a Wi-Fi module and gateway, the system is accessed by the network via the TCP/IPv4 internet scheme, and real-time video transmission is demonstrated.

Evaluating Noise Reduction Methods for Raman Spectroscopy in Transmission and Reflection Configurations

This work involved comparing Raman signals obtained from two different Raman spectroscopy configurations, using two distinct noise reduction methods. The excitation light source was a laser diode with a wavelength of 532 nm. A long-pass filter and focusing lens were utilized to block the excited light from the source and concentrate the Raman signals due to their weaker nature compared to the excited light signals. Light of 532 nm wavelength was blocked during green laser diode illumination using a long-pass filter. Two configurations were studied: transmission Raman spectroscopy (TRS) and reflection Raman spectroscopy (RRS). Raman signals from both configurations were compared, and the boxcar averaging and Vancouver Raman algorithm (VRA) noise reduction methods were investigated and compared. The results showed that Raman signals from the transmission configuration were higher than those from the reflection configuration, and noise signals were effectively reduced using both the boxcar averaging and VRA methods.

Controlling GaN-Based Laser Diode Performance by Variation of the Al Content of an Inserted AlGaN Electron Blocking Layer.

The leakage of the electronic current of a laser diode (LD) has some significant influences on the performance of the LD. In this study, commercial simulation software LASTIP is used to numerically evaluate the performances of LDs by using different wavelengths and Al contents of the electron blocking layer (EBL). These LDs a adopt multilayer structure, which contains cladding layers, waveguide layers, multiple quantum well layers, contact layers and an AlxGa1-xN EBL. The influence mechanism of EBL is theoretically examined by analyzing the simulated performances. It is found that for short-wavelength violet LDs, the electrical and optical properties of the LD will reach the optimum state when the Al content (x) in the EBL is nearly 0.25. For long-wavelength green LDs, it will achieve optimum electrical and optical properties when the Al content in the EBL is as low as possible. We also compare the simulation results of LDs with emission wavelengths in the range of violet and green, including blue cyan, for a more general evaluation. According to the simulated results, it is verified that the influence of the EBL's Al content on LD performance enhances as the wavelength increases.

Measuring spatial visual loss in rats by retinotopic mapping of the superior colliculus using a novel multi-electrode array technique

BackgroundThe retinotopic map property of the superior colliculus (SC) is a reliable indicator of visual functional changes in rodents. Electrophysiological mapping of the SC using a single electrode has been employed for measuring visual function in rat and mouse disease models. Single electrode mapping is highly laborious requiring long-term exposure to the SC surface and prolonged anesthetic conditions that can adversely affect the mapping data. New methodTo avoid the above-mentioned issues, we fabricated a fifty-six (56) electrode multi-electrode array (MEA) for rapid and reliable visual functional mapping of the SC. Since SC is a dome-shaped structure, the array was made of electrodes with dissimilar tip lengths to enable simultaneous and uniform penetration of the SC. ResultsSC mapping using the new MEA was conducted in retinal degenerate (RD) Royal College of Surgeons (RCS) rats and rats with focal retinal damage induced by green diode laser. For SC mapping, the MEA was advanced into the SC surface and the visual activities were recorded during full-filed light stimulation of the eye. Based on the morphological examination, the MEA electrodes covered most of the exposed SC area and penetrated the SC surface at a relatively uniform depth. MEA mapping in RCS rats (n=9) demonstrated progressive development of a scotoma in the SC that corresponded to the degree of photoreceptor loss. MEA mapping in the laser damaged rats demonstrated the presence of a scotoma in the SC area that corresponded to the location of retinal laser injury. Comparison with existing methods and conclusionsThe use of MEA for SC mapping is advantageous over single electrode recording by enabling faster recordings and reducing anesthesia time. This study establishes the feasibility of the MEA technique for rapid and efficient SC mapping, particularly advantageous for evaluating therapeutic effects in retinal degenerate rat disease models.

Comparative evaluation of Indocyanine Green and 810nm Diode Laser Assisted Antimicrobial Photodynamic Therapy with 810nm Diode Laser Assisted New Attachment Procedure as adjuncts to Scaling and Root Planning in Supportive Periodontal Therapy: A Randomized Controlled Split-Mouth Clinical Trial

BACKGROUND: Minimally invasive nonsurgical management of recurrent periodontal pockets remains the most viable option during supportive periodontal therapy (SPT), but with inconclusive results. The purpose of this study is to evaluate and compare the adjunctive use of Indocyanine green (ICG) and 810nm Diode laser assisted photodynamic therapy (IG-PDT) with 810nm Diode laser assisted new attachment procedure (LANAP) in the management of persistent pockets in SPT. MATERIALS AND METHODS: This single blinded randomized controlled split mouth clinical trial included 20 participants with at least two adjacent teeth with PPD ≥5mm and Clinical attachment level (CAL) ≥3mm in the contralateral quadrants. IG-PDT group received application of 5mg/ml solution of ICG for one minute followed by 810±10nm diode laser with 1W power density. LANAP group received 810±10nm diode laser application at 3W power density with 100ms and 650ms pulse duration at two consecutive passes. Clinical parameters i.e., Plaque index (PI), Gingival index (GI), PPD, CAL gain were recorded at baseline, 3 and 6 months. Landry’s Healing index was assessed on the 7th day and 1 month postoperatively. Patient comfort was assessed using Visual Analog Scale (VAS) immediate post-operatively. RESULTS: Student t test and repeated measures of ANOVA were used for statistical analysis. Both site level and patient level assessments of clinical parameters showed improvement in clinical parameters in both groups. However, IG-PDT group showed significantly greater improvements in PPD (<0.001) and CAL (<0.001) at 3 and 6 months evaluation. IG-PDT showed better healing (P-0.03) at 7th post-operative day. However, LANAP showed less discomfort (2.40±0.50) compared to IG-PDT group (3.20±0.61). CONCLUSION: IG-PDT is shown to provide better clinical outcomes compared to LANAP in management of patients under SPT.

Experimental study of laser spot tracking for underwater optical wireless communication.

In this paper, a novel laser spot tracking algorithm that incorporates the Kalman filter with the continuously adaptive Meanshift algorithm (Cam-Kalm) is proposed and employed in an underwater optical wireless communication (UOWC) system. Since the Kalman filter has the advantage of predicting the state information of the target spot based on its spatial motion features, the proposed algorithm can improve the accuracy and stability of the moving laser spot tracking. A 2 m optical wireless communication experimental system with auto-tracking based on a green laser diode (LD) is built to evaluate the tracking performance of different algorithms. Experimental results verify that the proposed algorithm outperforms conventional tracking algorithms in aspects of tracking accuracy, interference resistance, and response time. With the proposed Cam-Kalm algorithm, the experimental system can establish an effective communication link, while the maximum tracking speed is 20 mm/s given the forward-error-correction (FEC) threshold.

Spontaneous Emission Studies for Blue and Green InGaN-Based Light-Emitting Diodes and Laser Diodes

We investigated the efficiency droop phenomenon in blue and green GaN-based light-emitting diodes (LEDs) and laser diodes (LDs), which poses a significant challenge in high-power LEDs and is characterized by a reduction in external quantum efficiency at higher injection currents. Utilizing identical epi-structures for blue and green LEDs and LDs, with variations only in indium composition, our experiments revealed a gradual blue shift in the emission wavelengths as the injection current increased. Notably, the blue LED demonstrated a smaller shift compared to the green LED. In addition, the full width at half maximum of emission spectra increased with increasing injection current density, indicative of efficiency droop. Significantly, LDs consistently exhibited lower junction temperatures despite operating at higher current densities. This is attributed to the enhanced heat dissipation capability of the ridge waveguide LD structure, which results in a narrower emission spectrum and reduced efficiency droop compared to mesa LED structures. These outcomes highlight the efficiency of the ridge waveguide LD structure in heat dissipation from the active layer, offering crucial insights for the advancement of high-power light-emitting devices.

The effect of rose bengal activated with green diode laser light on selected Gram-positive and Gram-negative bacterial strains

In recent years the photodynamic activity of rose bengal activated with green light against selected bacterial strains has been reported. However, according to our knowledge, the differences between the sensitivity of Gram-positive and Gram-negative bacterial strains in the presence of this photosensitizer have not been described. The aim of the conducted research was to examine the antibacterial effect of 535 nm wavelength diode laser light in the presence of rose bengal as photosensitizer on selected reference bacterial strains: Pseudomonas aeruginosa, Enterococcus faecalis, Escherichia coli, Klebsiella pneumoniae and Staphylococcus aureus. Sterile 96-well microtiter plates were used to determine the antibacterial activity of the green light and rose bengal solutions at various concentrations. The labelled bacterial suspensions were placed to each well of the 96-well microtiter plate filled with liquid medium LB and solution of rose bengal. The plates were exposed to green diode laser light. After 24 hours of incubation at 37oC, the turbidance was read in a spectrophotometer. The irradiation in the presence of photosensitizer can act in an antibacterial manner, either bacteriostatically or bactericidally. The tested strains exhibit different sensitivity to irradiation because of the structure of the cell wall, the presence of different bacterial pigments and photoreceptor proteins in some species of bacteria. Gram-positive bacteria, Staphylococcus aureus and Enterococcus faecalis were the most photosensitive strains due to the higher possibility of rose bengal penetration into the bacterial cell, leading to the bacteriostatic effect. Our results show that rose bengal may be applied in the treatment of Gram-positive infections.

Blue and Green Low Threshold Laser Diodes With InAlN Claddings

Blue and Green Low Threshold Laser Diodes With InAlN Claddings

Watt-level tunable Ti:Sapphire laser directly pumped with green laser diodes.

We demonstrate a Ti:Sapphire laser generating in excess of 1.2 W in continuous-wave operation when pumped directly with four green laser diodes eliminating the need for a complex pump laser. As a result, improvement of laser efficiency is achieved without sacrificing beam quality. Tunability within the range of 740-840 nm is attained validating the concept of a direct laser-diode pumped Ti:Sapphire laser.

The critical impact of the waveguide thickness on the optical and threshold behaviors of InGaN-based green laser diodes

We study the optical structures of green (520 nm) InGaN-based laser diodes (LDs) with the complex 1D transfer matrix method, in consideration of the anti-guiding effect and mode leakage into the substrate. Two characteristic thicknesses of the waveguide are found to play critical roles in the behavior of the LD threshold and the beam quality. When a minimum threshold is desired, there exists an optimal thickness (OT) of the waveguide, which is strongly affected by the index contrast of the structure and the anti-guiding factor of the active region. A proper waveguide thickness (WT) makes the structure more tolerant of the mode leakage and the anti-guiding effect. An intrinsic failure in the lasing mode emerges with improper WTs. When a complete elimination of the substrate mode is desired, the waveguide above a critical thickness (CT) is required. To compromise the threshold and the beam quality, an eclectic thickness, while a high composition in the waveguide is suggested.

High-brightness green laser diode that uses a multi-order half-wave plate to realize twice-combined polarized beams

In this paper, we describe a method for realizing twice-combined polarized beams by selectively changing the polarization state of lasers with different wavelengths. This improves the output power and brightness of a green laser diode system. A multi-order half-wave plate plays a significant part in combining the polarization states of the first-combined polarized beam by changing the polarization state from orthogonal to approximately parallel. We obtained a maximum output power of 2.85 W and a laser brightness of 25.36 MW/cm2·sr with an optical–optical conversion efficiency of 82.85% and a spectral width of 15.48 nm. Compared with traditional methods for combining polarized beams, the output power is improved by 22.85% and the brightness by 8.65%. This structure provides a new technology for achieving high output power and high brightness from a green laser diode system, and it breaks through the limit of the number of laser units that can be used in combining polarized beams.

Double-sided asymmetric metasurfaces achieving sub-microscale focusing from a GaN green laser diode.

We proposed and demonstrated a highly efficient sub-microscale focusing from a GaN green laser diode (LD) integrated with double-sided asymmetric metasurfaces. The metasurfaces consist of two nanostructures in a GaN substrate: nanogratings on one side and a geometric phase based metalens on the other side. When it was integrated on the edge emission facet of a GaN green LD, linearly polarized emission was firstly converted to the circularly polarized state by the nanogratings functioning as a quarter-wave plate, the phase gradient was then controlled by the metalens on the exit side. In the end, the double-sided asymmetric metasurfaces achieve a sub micro-focusing from linearly polarized states. Experimental results show the full width at half maximum of the focused spot size is about 738 nm at the wavelength 520 nm and the focusing efficiency is about 72.8%. Our results lay a foundation for the multi-functional applications in optical tweezers, laser direct writing, visible light communication, and biological chip.

Speckle- and interference fringes-free illumination system with a multi-retarder plate.

Our study proposes a novel method for obtaining speckle-free homogeneous illumination using a combination of a multi-retarder plate, a microlens array, a Fourier lens, and a diffraction optical element (DOE) based on pseudorandom binary sequences. The proof-of-concept multi-retarder plate is introduced to generate multiple uncorrelated laser beams, while a mathematical model was developed to explain the method's mechanism and evaluate its effectiveness. In the DOE passive (stationary) mode, the method was found to reduce speckle contrast to 0.167, 0.108, and 0.053 for red, green, and blue laser diodes, respectively. In active mode, the speckle contrast was further reduced to 0.011, 0.0147, and 0.008. The observed differences in speckle contrast in the stationary mode were attributed to variations in the coherence lengths of the RGB lasers. By implementing the proposed method, we successfully generated a square-shaped illumination spot that was free from interference artifacts. The spot obtained exhibited a slow, weak variation in intensity across the screen, attributable to the multi-retarder plate's suboptimal quality. However, this limitation can be readily addressed in future studies through the adoption of more advanced fabrication techniques.

Potential application of a green fiber laser in laser display systems.

This study explores the application of optical fiber lasers in display systems by integrating a P r3+-doped green all-fiber laser into a laser projection display system. As a control group to compare the results, a 520nm semiconductor green laser diode module was integrated, similar to the experimental group. The color gamut and speckle performances were studied and compared. The results showed that the experimental group performed slightly better in the color gamut volume. The speckle contrast decreased rapidly in the experimental group when power increased. To our knowledge, this is the first study to apply a fiber laser to a laser display system. The results shed light on developing laser display systems with fewer or no speckle reduction elements.