Quasi-continuous Wave Mode Research Articles

Standoff Detection of Oil and Powder Mixtures at 12 Meters Using a Tunable Quantum Cascade Laser-Based System with a Close Focus Telescope and Uncooled Infrared Detector

We have designed and demonstrated a quantum cascade laser (QCL) based standoff system that utilizes an uncooled mercury cadmium telluride (MCT) detector with lock-in signal processing for chemical identification at a distance of 12.5meters in indoor ambient light conditions. In the system, a tunable quad-QCL operating (1MHz) in quasi-continuous wave mode between 8.45 and 10.03μm (∼1182 to 1000 cm-1) serves as the active mid-infrared source for remotely interrogating mineral, powder, and thin film oil samples including powder mixtures (6, 12.5, 25, and 50%) of crystalline quartz (SiO2) in KBr. Light as reflected from a given sample is collected using a 10-inch (25.4cm) Dall Kirkham telescope and coupled with ZnSe optics to an uncooled MCT detector. The mixture dependence of the highly transparent KBr and strongly absorbing quartz was found to fit a modified version of the Schatz reflectance model for compacted powder mixtures. All reflectance spectra reported are relative to an Au-coated diffuse reflector. A NIST traceable polystyrene standard reflector was also used to determine the QCL wavelength tuning range and calibration.

Экспериментальные исследования зажигания ракетных топлив кислород--водород, кислород--метан полупроводниковым лазером

The paper presents results of experimental studies aimed at introducing laser ignition of fuel mixtures into aero-space design practice. The source of ignition energy was a semiconductor laser featuring fibre radiation output, operating in a quasi-continuous wave mode. We carried out experiments for oxygen--hydrogen and oxygen--methane fuel types. The purpose of our research was to demonstrate the fundamental possibility of implementing fuel ignition by means of this type of laser, using a rocket engine igniter and a low-thrust rocket engine as examples. Employing semiconductor lasers directly as an ignition source for fuel mixtures in aerospace technology is attractive as it may feasibly reduce the requirements for thermal conditions during operation of the laser ignition system on board a rocket or spacecraft, as well as expand the range of permissible vibration and shock loads. The paper presents experimental results that delineate operating parameter ranges and operation cyclograms for the devices under consideration that ensured stable ignition of oxygen--hydrogen and oxygen--methane fuel mixtures; we also list the required power parameters for a semiconductor laser. The investigation revealed the specifics of using a semiconductor laser-based ignition system, which will be useful in developing laser rocket launching devices, ensuring reliable repeated on-off functionality

Pulse control of frequency and width for a real-time independently adjustable laser source

A set of semiconductor laser pulse seed sources based on an embedded chip is proposed. The greatest feature is that the optical pulse frequency and width can be independently adjusted in real time. The pulse seed sources can be switched independently and online from the gain-switched mode to the quasi-continuous wave mode to obtain optimal optical parameters for specific applications. To explore the physical mechanism of the semiconductor laser source, the rate equation that describes the carrier-photon transient change in a semiconductor laser cavity is numerically derived and solved. Subsequently, problems that need to be considered while designing the drive circuit are identified. The system evaluation indicates that the optical pulse frequency adjustment range is 250 Hz to 42 MHz, and the narrowest optical pulse output width is 80 ps. The pulse seed source can drive semiconductor lasers with different central wavelengths (1064, 1550, and 1970 nm), and can also simultaneously drive two semiconductor lasers and output dual-band optical pulses. It can be used as a seed source for general high-power optical systems, and exhibits good application value and extensive market prospects.

Effects of laser modes on Nb segregation and Laves phase formation during laser additive manufacturing of nickel-based superalloy

Nb-bearing nickel-based superalloys were fabricated by laser additive manufacturing using both continuous-wave (CW) and quasi-continuous-wave (QCW) laser modes. Effects of laser modes on Nb segregation and Laves phase morphology were investigated. The responses to aging treatments of two different fabricated samples were also investigated. CW mode tends to develop columnar dendrites, relative severe Nb segregation and continuous distributed Laves phase, while QCW mode tends to produce fine equiaxed dendrites, less Nb segregations and fine discrete Laves phase. In addition, the QCW sample responses better to aging treatment than the CW sample.

Continuous-wave and quasi-continuous wave thulium-doped all-fiber laser: implementation on kidney stone fragmentations.

A continuous-wave (CW) as well as quasi-continuous wave (QCW) thulium-doped all-fiber laser at 1.94 μm has been designed for targeting applications in urology. The thulium-doped active fiber with an octagonal-shaped inner cladding is pumped at 793 nm to achieve stable CW laser power of 10 W with 32% lasing efficiency (against launched pump power). The linear variation of laser power with pump offers a scope of further power scaling. A QCW operation with variation of duty cycle from 0.5% to 90%, repetition rate from 0.1 Hz to 1 kHz, and pulse width from 40 μs to 2 s has been presented. Laser power of 9.5 W in CW mode of operation and average power of 5.2 W with energy range of 10.4-104 mJ in QCW mode of operation has been employed to fragment calcium oxalate monohydrate kidney stones (size of 1.5-4 cm) having different colors and composition. Dependence of ablation threshold, ablation rate, and average fragmented particle size on the average power and energy has been studied. One minute of laser exposure results in fragmentation of a stone surface with ablation rate of 8 mg/min having minimum particle size of 6.54 μm with an average size of 20-100 μm ensuring the natural removal of fragmented parts through the urethra.

High temperature heat source generation with quasi-continuous wave semiconductor lasers at power levels of 6 W for medical use

We investigate a technology to create a high temperature heat source on the tip surface of the glass fiber proposed for medical surgery applications. Using 4 to 6 W power level semiconductor lasers at a wavelength of 980 nm, a laser coupled fiber tip was preprocessed to contain a certain amount of titanium oxide powder with a depth of 100 μm from the tip surface so that the irradiated low laser energy could be perfectly absorbed to be transferred to thermal energy. Thus, the laser treatment can be performed without suffering from any optical characteristic of the material. A semiconductor laser was operated quasi-continuous wave mode pulse time duration of 180 ms and >95% of the laser energy was converted to thermal energy in the fiber tip. Based on two-color thermometry, by using a gated optical multichannel analyzer with a 0.25 m spectrometer in visible wavelength region, the temperature of the fiber tip was analyzed. The temperature of the heat source was measured to be in excess 3100 K.

Influence of Package Structure on the Performance of the Single Emitter Diode Laser

The package structure critically influences the major characteristics of semiconductor lasers, such as thermal behavior, output power, wavelength, and far-field distribution. In this paper, a new single emitter package structure called F-mount is designed and compared with the conventional package structure C-mount. The influence of package structure on their performances is characterized and analyzed. The thermal resistances of lasers with different package structures are calculated through simulation, and are contrasted with experimental results. Some devices are also tested for the maximum output power level. Under the continuous wave (CW) condition, the maximum power of F-mount reaches 12.6 W at 808 nm while the output power only reaches 10.9 W for C-mount. Under the condition of 0.5% duty cycle (100 μs, 50 Hz), the catastrophic optical mirror damage level reaches 58.7 W at 74 A for F-mount, and 54.8 W at 57 A for C-mount are reported for the first time. It is experimentally found that there is an obvious wavelength difference between the two type structure lasers: about 1.37 nm in CW mode and 2.89 nm in quasi CW mode. Theoretical analysis shows that red-shift and blue-shift is a result of external strain in the package process of F-mount and C-mount, respectively. It is also found that the package structure has an effect on the divergence angle of slow axis far fields, but little impact on that of fast axis far fields. The analysis shows that package structure has a strong influence on the performance of the laser; therefore, the package should be optimized to achieve better performance for some special applications.

Wavelength modulation spectroscopy based on quasi-continuous-wave diode lasers

A modified wavelength modulation spectroscopy (WMS) based on the self-heating effect of the tunable diode laser when driven in quasi-continuous-wave (QCW) mode is investigated. A near-infrared distributed feedback (DFB) diode laser working at the QCW mode is employed as the QCW light source, and CO2 is selected as the target gas. The characteristic of the QCW second harmonic (2f) line profile is analyzed through a comparison with that of the traditional CW WMS with the same system. A noise-equivalent absorbance of 3.2×10-5 Hz1/2 for CO2 at 1.58 μm is obtained with 18-m optical path. The QCW WMS lowers the dependence on lasers and expands selectivity, thus verifying the feasibility of the method.

LEDs based on InAsSbP/InAsSb heterostructures (λ = 4.7 μm) for carbon monoxide detection

Light-emitting diodes (LEDs) operating in the 4.4–4.8-μm wavelength range have been developed for detecting the presence of carbon monoxide (CO) in air. The proposed LEDs are based on InAsSbP/InAsSb heterostructures with InAs0.85Sb0.15 active region, which were grown by metalorganic vapor-phase epitaxy. The electrolumuinescent properties of LEDs have been studied. The output power of LED operating in a pulsed mode reaches 50 μW at a current pulse amplitude of 2 A, and that in a quasi-continuous wave mode is 1 μW at a current of 200 mA. The absorption of LED radiation in a gaseous medium containing 10% CO has been studied. The proposed LEDs can be used to simultaneously detect both CO2 (absorbing at λ = 4.3 μm) and CO (λ = 4.67 μm).

LEDs based on InAs/InAsSb heterostructures for CO2 spectroscopy (λ = 4.3 μm)

Light-emitting diodes (LEDs) operating in a 4.1–4.3 μm wavelength range have been created on the basis of InAs/InAsSb heterostructures grown by metalorganic vapor-phase epitaxy. The output radiation power of LEDs is increased using flip-chip design. Investigation of the electrolumuinescent properties of LEDs with smooth and profiled output edge surface showed that the latter LEDs possess a greater efficiency, which is related to an increase in the radiation yield due to multiply repeated reflection from the curved surface. The output power of LED operating in a quasi-continuous wave mode was 30 μW at a current of 200 mA and that in a pulse mode was 0.6 mW at a current pulse amplitude of 2 A.

High-power mid-infrared light emitting diodes grown by MOVPE

Light emitting diodes (LEDs) are fabricated on the basis of MOVPE-grown N-InAsSbP/n-InAsSb/P-InAsSbP heterostructures. LEDs operating in the 3.45–4.45 µm wavelength range at room temperature are produced by employing InAsSb active layers with different Sb content. The photoluminescence of InAsSb layers and electroluminescence properties of LEDs are investigated. LED light-current characteristics in both quasi-continuous wave mode and pulsed mode are also studied. Increasing LED efficiency with improved carrier confinement is observed. When operating at 50% duty cycle, the LEDs (room temperature wavelength λ=4.27 µm) produce an average power of 0.3 mW when driving an average current of 0.25 A. With a peak current of 1.4 A (5% duty cycle), the pulse power of the diodes (room temperature wavelength λ=4.27 µm) is measured as 2.5 mW.

High power aluminium-free InGaAsP/GaAs pumping diode lasers

We have developed low pressure metal-organic chemical vapour deposition technology for the growth of aluminium-free InGaAsP compounds lattice-matched to GaAs substrate. High power laser diodes based on these materials and emitting at 808 nm and 890 nm wavelengths have been fabricated and optimized. A threshold current density of 240 A cm −2, differential efficiency of 1.3 W A −1, characteristic temperature as high as 350 K (for 980 nm) and output power as high as 67 W in quasi-continuous wave mode have been obtained for the diodes with uncoated facets. The lasers have demonstrated excellent reliability (projected lifetime in excess of 100 years) and they are suitable for a broad range of pumping applications.