Li Ai-Zhen

Pulse Wavelength Scan of Room-Temperature Mid-Infrared Distributed Feedback Quantum Cascade Lasers for N2O Gas Detection

The tunable diode laser absorption spectroscopy under a pulse wavelength scan scheme is adapted to home-made room-temperature mid-infrared distributed feedback quantum cascade lasers; and identification of N2O spectral fingerprint is demonstrated experimentally. By driving the laser at 800 ns pulse duration, a wave number tuning of about 1.6 cm−1 is produced, which make both 1289.04 cm−1 and 1289.86 cm−1 absorption fingerprints of N2O gas to be definitely assigned. The measured relative absorption intensity is consistent with the HITRAN data precisely.

Tunable Diode Laser Absorption Spectroscopy Detection of N2O at 2.1 μm Using Antimonide Laser and InGaAs Photodiode

Tunable diode laser absorption spectroscopy detection of N2O around 2.1 μm is demonstrated by using a homemade InGaAsSb/AlGaAsSb MQW laser diode and an InGaAs wavelength extended photodiode. Details of the devices and the detection system are described. In the system, the laser is driven by low frequency pulses with long duration to form a wavelength scan around 4741 cm−1; the absorption information is obtained from the detected signal of the photodiode. By using a gas cell with 15 cm path length, a detection limit is estimated to be smaller than 0.2 Torr.

Continuous Wave Performance and Tunability of MBE Grown 2.1 μm InGaAsSb/AlGaAsSb MQW lasers

InGaAsSb/AlGaAsSb multi quantum well ridge waveguide lasers at 2.1 μm wavelength are fabricated by using molecular beam epitaxy. Continuous wave performance and tunability of the lasers are evaluated in a wide temperature range extend to 80°C. Output power of the laser at 30°C exceeds 30 mW/facet at driving current of 0.5 A, the characteristic temperature T0 is 89 K in 0–50°C range. No fast degradation is observed in accelerated aging test at 90°C for those lasers with lower Al content in cladding layers. Temperature tunability of the lasers is 1.36 nm/K. Single-mode output with side mode suppression ratios greater than 20 dB is achieved in a certain driving current region; current tunability is 8×10−3 nm/mA regardless of mode hopping.

Gas sensor using a robust approach under time multiplexing scheme with a twin laser chip for absorption and reference

A tunable diode laser absorption spectroscopy nitrous oxide gas sensor at 2.1 micrometres using one antimonide twin laser chip and one InGaAs photodiode is demonstrated, in which time multiplexing techniques are adopted to acquiring both the absorption and reference signal in a robust fluctuation tolerable scheme. Electronics in analogue modality is developed to extracting absorption information and compensating for fluctuations, resulting in a direct analogue voltage output corresponding to the target gas concentration in real time. The performance of the gas sensor is evaluated experimentally, the validity and feasibility of this scheme is also discussed.

The Self-Heating Effect of Quantum Cascade Lasers Based on a Spectroscopic Method

We investigate the self-heating effect of mid-infrared quantum cascade lasers by using a direct-based pulse injecting current and spectroscopy method. Based on the characterization system, the thermal characteristics of gas source MBE grown 8.4 μm InP-based GaInAs/AlInAs DFB-QCLs are evaluated. The method and characterization system are also useful in evaluating the thermal characteristics of other types of mid-infrared diode lasers.

An Innovative Gas Sensor with On-Chip Reference Using Monolithic Twin Laser

An innovative gas sensor with on-chip reference using a monolithic twin laser is proposed. In this sensor a monolithic twin laser generates two closer laser beams with slight different wavelengths alternatively, one photodiode is used to catch both absorption and reference signals by time division multiplexing. The detection of nitrous oxide adopting this scheme using a 2.1 μm antimonide laser and an InGaAs photodiode has been demonstrated experimentally with detection limit below 1 ppm. Using this on chip reference scheme the fluctuations from the optical path and devices can be compensated effectively; the sensor system is simplified distinctly.

High-Temperature Operation of 8.5 μm Distributed Feedback Quantum Cascade Lasers

We present a distributed feedback quantum cascade laser (DFB-QCL) emitting at a wavelength of 8.5 μm and operating up to 420 K (147°C) with a low-threshold current density in pulsed mode. The DFB-QCLs studied are based on a four-well active design; the central portion of the waveguide consists of 60 periods of lattice matched InP-based InGaAs/AlInAs. In the design of the device, an active structure with lower doping and a deep-top grating process are utilized to achieve high temperature operation with a lower-threshold current density. At 420 K, a low-threshold current density of 3.28 kA/cm2 and a single mode peak power of 15mW are achieved on an epilayer-up mounting device with ridge width of 26 μm and cavity length of 3.0 mm. A side mode suppression ratio of 25 dB at 420 K is obtained.

Low threshold distributed feedback quantum cascade lasers with widely spaced emission wavelengths

We report lasing properties of distributed feedback quantum cascade lasers (DFB QCLs) including a double-phonon-resonance active region, at wavelength of about 8.4 μm. A broad gain spectrum is generated due to the coupling between the lower laser level in the active region and the levels in the injector, and is demonstrated by the lasing spectrum of the corresponding Fabry–Perot QCLs whose width is 0.5 μm at 1.5 times of the threshold current. As a result, the DFB QCLs employing different grating periods exhibit a wavelength span of 0.18 μm at room temperature and total wavelength coverage of 0.28 μm at various heat sink temperatures. A high side mode suppression ratio of about 30 dB and a low threshold current density of 1.78 kA/cm2 are achieved as the lasers operate at room temperature in pulsed mode.

Room-temperature AlGaAsSb/InGaAsSb multi-quantum-well lasers with high characteristic temperature

Planar structure AlGaAsSb/InGaAsSb lasers operated at 2.01 µm with high characteristic temperature have been fabricated from a strained multiple quantum-well heterostructure. To decrease the free carrier induced absorption of optical mode in the mid-infrared, we design a broaden waveguide layer in the laser structures to decrease the optical mode distribution in the heavy doped cladding layer, therefore it can be absorbed easily. To enhance the characteristic temperature of laser diodes, Al constituent up to 80% was applied to the AlGaAsSb cladding layer. The laser diodes with a threshold current density of 1.8 kA/cm2 can be pulsed operating up to 340 K. The characteristic temperature T0 is 125 K and 90 K in the operating temperature ranges 170-220 K and 230-340 K, respectively. The emission spectrum shows a multiple longitudinal mode.