New self-align method for fabrication of 980 nm ridge waveguide laser diodes

A novel self-align method has been developed for the fabrication of 980 nm ridge waveguide laser diodes. It utilizes the facts that (1) the thickness of photoresist on the ridge top is substantially less than that in its vicinity and (2) their respective exposure times differ accordingly. Except for replacing the second photolithographic step with a simple flood-exposure, the fabrication procedure is identical to that for conventional ridge waveguide laser diodes. No additional materials or processing steps are required. As a result, the laser fabrication is significantly simplified with excellent reproducibility.     

Spectroscopy of the 689 nm intercombination line of strontium using an extended-cavity InGaP/InGaAIP diode laser

A visible diode laser emitting at 690 nm at room temperature has been frequency-stabilized using a simple scheme based on optical feedback from a diffraction grating. The possibilities offered by these lasers for high-resolution spectroscopy are demonstrated by recording the sub-Doppler signal of the 689 nm intercombination line of strontium and resolving the hyperfine structure of the close P63 R70 (8, 4) transitions of iodine.     

Native oxide buried InAlAs ridge waveguide laser diode

InAlAs wet thermal oxidation process was adopted to fabricate ridge waveguide laser diodes. First, applying the oxidation process on the whole etched surface of an InAlAs upper cladding layer, we formed a current blocking and optical confining layer of ridge waveguide laser diode. This is a self-aligned structure, which makes the laser diode fabrication steps much simpler and etching-depth control less important. Assessment of the fabricated laser diode revealed that current-voltage characteristics and slope efficiency did not worsen, and threshold current was reduced by the oxidation process. PACS 85.35.Be; 42.55.Px     

Analysis of thermal performance of InGaP/InGaAlP quantum wells for high-power red laser diodes

The effect of quantum well (QW) strain on the thermal performance of InGaP/InGaAlP lasers emitting at around 635 nm is investigated theoretically. It is found that compressively-strained QWs offer superior thermal performance due to an increased barrier height for electrons, leading to a reduced electron leakage. The theoretical predictions are supported by experimental data from red-emitting laser diodes with an improved characteristic temperature T ₀.     

Numerical study on lateral mode behavior of 660-nm InGaP/AlGaInP multiple-quantum-well laser diodes

Theoretical analysis for InGaP/AlGaInP laser diodes with different ridge waveguide structures is performed to investigate the lateral mode behavior using advanced device simulation. The internal physical mechanisms including temperature-induced changes in the refractive index profile, spatial hole burning effect, lateral carrier distribution, and gain profile variation with increasing input current are discussed by theoretical calculation to analyze the effects of different ridge structures on the lateral mode behavior of 660-nm AlGaInP laser diodes. The simulation results show that the use of narrow and shallow ridge geometry is the approach to obtaining single mode operation. Furthermore, it is found that the different values of the ridge height cause the lateral carrier distribution within the active region to be varied, which is also an important factor in determining the emergence of the first order lateral mode in addition to the geometry-dependent waveguide cutoff condition.     

Transverse mode coupling in narrow-ridge waveguide laser diodes

The letter presents numerical data and gives an explanation for the spontaneous creation of spatial patterns in narrow-ridge waveguide laser diodes by including the effect of diffraction and non-linear coupling between transverse modes and the active material. In addition, experimental measurements from several laser diodes are made and directly compared to the simulation. Excellent agreement between numerical and experimental data is found.     

Mounting-induced strains in red-emitting (Al)InGaP laser diodes tuned by pressure

The hydrostatic pressure behavior of red-emitting diode lasers packaged on Si, AlN, and diamond submounts is studied in the 0–2 GPa range by emission and photocurrent spectroscopy. Photocurrent spectroscopy allows for simultaneous measurement of the InGaP quantum well and (Al_0.5Ga_0.5)_0.5In_0.5P waveguide. A broadening of the absorption edge of the waveguide is observed for all devices and explained by the pressure-induced direct-to-indirect transition in this material. For the QW resonance, distinct differences are observed for differently packaged devices. Thus, very low pressure tuning rates are demonstrated for devices packaged on diamond and AlN submounts and explained by the presence of shear strain components. Consistently we find the device packaged on Si to be least affected by the strain caused by the pressure cycling.     

Response of metal-insulator-metal point contact diodes to visible laser light

Video response and mixing behaviour of metal-insulator-metal point contact diodes have been investigated for visible laser light. Thermally enhanced tunneling is shown to dominate the dc detection behavior of those diodes, while mixing of frequencies being more than several MHz apart is a more complex phenomenon involving thermal, field-and photo-assisted tunneling. In further experiments the potential of point contact diodes for optical heterodyne spectroscopy was examined. Two green laser lines of 122 GHz frequency difference were mixed with the second harmonic of an appropriate microwave frequency, generated simultaneously on the diode. The modestS/N ratio achieved has to be assigned to the different behaviour of metal-insulator-metal diodes in the visible and rf range.     

LP MOVPE growth of AlGalnP/GalnP and its application to visible laser diodes

High-quality and uniform bulk layers of (Al _x Ga_1–x )_0.5In_0.5P (x=0–0.7) and AlGalnP/GainP quantum wells (QWs) are grown on 2°-off (100) GaAs substrates by low-pressure metal organic vapour phase epitaxy at a low growth rate of 0.3 nm s^-1. The amount of lattice mismatch and the variation of PL peak energy of (Al_0.5Ga_0.5)_0.5In_0.5P on the 50-mm substrate are less than 6×10^-4 and 2 meV, respectively. (Al_0.5Ga_0.5)_0.5In_0.5P/Ga_0.5In_0.5P SQWs show narrow PL spectra even from a 0.6 nm well measured at 20 K. The variation of PL peak energy from (Al_0.5Ga_0.5)_0.5In_0.5P/Ga_0.5In_0.5P MQWs is less than 10 meV. Also, as-cleaved AlGalnP/GalnP lasers fabricated by a three-step MOVPE show a pulsed threshold current of 82 mA at room temperature, output power of 12 mW, and the lasing wavelength at 668.2 nm.     

Multi-gigahertz laser generation based on monolithic ridge waveguide and embedded copper nanoparticles

Copper(Cu)nanoparticles(NPs)are synthesized under the near-surface region of the Nd∶Y₃Al₅O₁₂(Nd:YAG)crystal by direct Cu⁺ions implantation.Subsequently,the monolithic ridge waveguide with embedded Cu NPs is fabricated by C⁴⁺ions irradiation and diamond saw dicing.The nonlinear optical response of the sample is investigated by the Z-scan technique,and pronounced saturable absorption is observed at the 1030 nm femtosecond laser.Based on the obvious saturable absorption of Cu NPs embedded Nd:YAG crystal,1μm monolithic mode-locked pulsed waveguide laser is implemented by evanescent field interaction between NPs and waveguide modes,reaching the pulse duration of 24.8 ps and repetition rate of 7.8 GHz.The work combines waveguides with NPs,achieving pulsed laser devices based on monolithic waveguide chips.     

Performance characteristics of deep violet InGaN DQW laser diodes with InGaN/GaN superlattice waveguide layers

The effect of the indium (In) composition of In_xGa_1−xN (GaN) waveguide layers on the performance of deep violet In_0.082Ga_0.918N/GaN double quantum well (DQW) laser diodes (LDs) emitting at 390 nm output emission wavelength has been numerically investigated. Simulation results indicated that by increasing In composition of the In_xGa_1−xN waveguide layers, the threshold current decreases, the slope efficiency, and differential quantum efficiency (DQE) increase, whereas the output power decreases. The increase in the In composition of the InGaN waveguide layers increases the refractive index and consequently increases the optical confinement factor (OCF) which result in the increase in the slope efficiency and DQE and the decrease in the threshold current. The decreasing movement of electron and hole carriers from the bulk waveguide layers to the active regions also causes to decrease the output power. A new LD structure with InGaN/GaN superlattice (SL) waveguide layers has been proposed to exploit the increased OCF of InGaN waveguide structures, and the enhanced electron and hole mobilities and the tunneling effect of the periodic structure of the SL structures. The results also showed that the use of InGaN/GaN SL waveguide structures effectively improves the output power, slope efficiency and DQE and decreases the threshold current of the LD compared with (In)GaN bulk waveguide structure.     

Current spreading effects in 680-nm-band self-sustained pulsating AlGalnP visible laser diodes

We have developed 680-nm-band AlGalnP visible laser diodes having very lowintensity noise with stable self-sustained pulsation. The pulsation characteristics were affected by current spreading outside the ridge stripe. We obtained AlGalnP visible laser diodes with stable self-sustained pulsation by optimizing the ridge structure. The lasers achieved relative intensity noise (RIN) as low as-136 dB Hz^–1 in the temperature range 20–50°C at 3 mW.     

The single-longitudinal-mode operation of a ridge waveguide laser based on two-dimensional photonic crystals

An electrically driven, single-longitudinal-mode GaAs based photonic crystal (PC) ridge waveguide (RWG) laser emitting at around 850 nm is demonstrated. The single-longitudinal-mode lasing characteristic is achieved by introducing the PC to the RWG laser. The triangle PC is etched on both sides of the ridge by photolithography and inductive coupled plasma (ICP) etching. The lasing spectra of the RWG lasers with and without the PC are studied, and the result shows that the PC purifies the longitudinal mode. The power per facet versus current and current-voltage characteristics have also been studied and compared.     

The ridge waveguide fabrication with periodically poled MgO-doped lithium niobate for green laser

Ridge waveguides were fabricated using an external field, a precision lapping machine and neutron loop discharge (NLD) in magnesium-oxide-doped lithium niobate. The measured quasi-phase-matching (QPM) wavelength of the second-harmonic generation (SHG) in a 30 mm long periodically poled magnesium-doped lithium niobate (PPMgLN) ridge waveguide which has a domain period of 6.8 μm is about 532 nm. A fabricated periodically poled magnesium-doped lithium niobate ridge waveguide was duty cycle of 51.9 ± 2.83% and demonstrated second-harmonic generation. By using this periodically poled magnesium-doped lithium niobate ridge waveguide, highly effective, low-cost optical devices with high power or short wavelength can be achieved.     

Optimum design of InGaP/GaAs dual-junction solar cells with different tunnel diodes

We design the InGaP/GaAs dual-junction (DJ) solar cells by optimizing short-circuit current matching between top and bottom cells using the Silvaco ATLAS. The relatively thicker base layer of top cell exhibits a larger short-circuit current density (J _sc) while the thicker base layer of bottom cell allows for a smaller J _sc. The matched J _sc of 10.61±0.05 and 13.25±0.06 mA/cm ² under AM1.5G and AM0 illuminations, respectively, are obtained, leading to the increased conversion efficiency. The base thicknesses of top InGaP cells are optimized at 0.8 and 0.65 μm for AM1.5G and AM0 illuminations, respectively, and the base thicknesses of bottom GaAs cells are optimized at 2 μm. For the optimized solar cell structure, the maximum J _sc = 10.66 mA/cm ² (13.31 mA/cm ²), V _oc =  2.34 V (2.35 V), and fill factor =  87.84% (88.1%) are obtained under AM1.5G (AM0) illumination, exhibiting a maximum conversion efficiency of 25.78% (23.96%). The effect of tunnel diode structure, i.e, GaAs/GaAs, AlGaAs/AlGaAs, and InGaP/InGaP, on the characteristics of solar cells is investigated. The photogeneration rate in the DJ solar cell structure is also obtained by incident light of different wavelengths.     

Output light power of InGaN-based violet laser diodes improved by using a u-InGaN/GaN/AlGaN multiple upper waveguide

The upper waveguide(UWG) has direct influences on the optical and electrical characteristics of the violet laser diode(LD) by changing the optical field distribution or barrier of the electron blocking layer(EBL). In this study, a series of In GaN-based violet LDs with different UWGs are investigated systematically with LASTIP software. It is found that the output light power(OLP) under an injecting current of 120 mA or the threshold current(Ith) is deteriorated when the UWG is u-In_(0.02)Ga_(0.98)N/GaN or u-In_(0.02)Ga_(0.98)N/Al_xGa_(1-x)N(0 ≤ x ≤ 0.1), which should be attributed to small optical confinement factor(OCF) or severe electron leakage. Therefore, a new violet LD structure with u-In_(0.02)Ga_(0.98)N/GaN/Al_(0.05)Ga_(0.95)N multiple layer UWG is proposed to reduce the optical loss and increase the barrier of EBL. Finally,the output light power under an injecting current of 120 mA is improved to 176.4 mW.     

A horn ridge waveguide DFB laser with reduced spatial hole burning for high-speed modulation

In this paper, we propose a DFB laser with a horn ridge waveguide (HRW) to suppress the longitudinal spatial hole burning (LSHB) effect in the lasers cavity, thus to reduce the rolloff at low frequency. The simulation result shows that HRW DFB lasers could significantly suppress the LSHB effect and its modulation bandwidth is increased by 14% comparing with the conventional straight ridge waveguide (RW) DFB lasers when the normalized coupling coefficient (κL) is 3.0. The calculated eye diagrams of HRW DFB lasers under direct 25 Gbps modulation have clearer opening than that of the conventional RW DFB lasers. These superior properties are due to the suppression of the LSHB effect by the HRW structure.     

Graded-index ridge surface plasmon polaritons waveguide

We propose and analyze a long-range dielectric-loaded surface plasmon polariton （SPP） waveguide based on graded-index ridge over server millimeter distances. Then the influence of the dielectric thickness and the ridge refractive index on propagation length and mode width is discussed and simulated with the finite ele- ment raethod. The result shows that the SPP can propagate as long as 3.42 mm, as well as the mode width keeping as 1.64μm, a better one compared with the fixed refractive index. Considering its nanoscale dimension and outstanding performance, the structure is easily realized when connected with electrodes.     

Simultaneous acquisition of color and distance information by LiDAR with RGB visible laser diodes

We propose colors light detection and ranging (LiDAR) using visible laser diodes (LD) of red, green, and blue (RGB) colors to identify colors as well as obtain position information of surrounding objects. To verify the principle, we performed round-trip time-of-flight (TOF) measurements by irradiating a target of different colors with pulsed light from a LD of RGB colors. It is clarified that accurate distance measurements of up to 12 m can be realized in any RGB LD, and the received signal in each LD has different intensity characteristics depending on the target color. From a judgment table created from reception intensity characteristics for three types of LD and seven types of color target, the color of each target is successfully identified.