The external differential quantum efficiency, defined as the ratio of number of photons emitted per unit time to number of carriers passing the laser diode junction, is known to be sensitive to laser diode’s operating temperature. In this paper, high-resolution spectral emissions of a commercially available GaN-based blue laser diode are measured and utilized to study the effect of temperature on the external differential quantum efficiency and over the operating temperature range of 270 – 330 °K. Upon studying the L-I curves and over the full range of laser diode’s operating current and temperature, three distinct temperature regimes of the quantum efficiency were identified with the regime of temperature range 285 -301 °K yielding the highest temperature stability. In addition to experimentally determining the characteristic temperature of the laser diode, the effect of non-radiative recombination and free carrier absorption processes on external differential quantum efficiency will be discussed.
LIDAR systems are used in many atmospheric research applications. They consist mainly of a transmitting subsystem that emits a pulsed laser beam into the atmosphere and a receiving subsystem for laser backscattered signal detection and analysis. Many of these atmospheric applications requires high accuracy LIDAR systems, which is the focus of this article. In this paper, we provide information on the basic characteristics and performance of a detection chain for a LIDAR system used in remote sensing of the atmosphere. The detection chain, which is characterized by its low-cost and low-power consumption, allows high spatial and temporal resolution, reliable operation and wide dynamic range: A fast, low noise and high efficiency photodetector used in analog mode convert the detected optical backscattered signal into an electrical signal. A wide bandwidth, low noise and low distortion analog variable gain amplifier (DC-300 MHz) amplifies the electrical signal generated by the photodetector before digitization. The variation of the gain of the amplifier makes it possible to obtain a wide dynamic range, which extends over several orders of magnitude (∼10<sup>5</sup>). A fast analog-to-digital converter (ADC 20 MHz digitization rate, 12-bit resolution sampling) that permits a spatial resolution of 7.5 m follows the amplification stage. A high-speed data interface to computer allows fast readout of the acquired signal. Noise and interference, which can affect the performance of the detection chain, will be also discussed.
Nitrogen dioxide, one of the main air pollutants, has strong light absorption cross section in the blue region of the optical spectrum. Recent availability of blue laser diodes provides possibility of detecting NO<sub>2</sub> in open-air paths with very low detection limits. However, in the blue region, the sharp features of the NO<sub>2</sub> spectrum is relatively broad with typical width of tenths of nanometer. This poses a serious challenge for implementing traditional direct or wave-modulated tunable diode laser absorption spectroscopy. In this study, we report the usage of a blue laser diode with multi longitudinal modes tuned over about one nanometer in detecting trace amount of NO<sub>2</sub> gas. Details of the setup and its optimization will be presented along with a comparison of its performance with other NO<sub>2</sub> detection optical methods.
Typical emission spectra of GaN-based blue laser diodes are known to have irregular shapes. Hence, well-resolved study of their spectra may help in understanding the origin of their spectral shapes irregularity. In this paper, the spectra of a commercial GaN-based blue laser diode are studied as a function of injection current and temperature using a spectrometer with highresolution of 0.003-nm over the spectral region 440 – 450 nm. The obtained laser spectra are used to track the longitudinal modes evolution as a function of operating currents and temperatures as well as to precisely map single mode operation. In addition, yielded laser spectra will be utilized to evaluate few parameters related to the laser diode, such as mode spacing, optical gain, slope efficiency and threshold current at certain temperature.
Preliminary measurements of profiles of aerosol/cloud in the lower atmosphere using a homemade stationary groundbased lidar system will be presented. In addition, information on basic characteristics and performance of the lidar system will be provided. Aerosol/Cloud lidar system in monostatic coaxial configuration uses the fundamental (1064 nm) and the second harmonic (532 nm) of a pulsed solid state Nd:YAG laser to provide information on the relative concentration and spatial distribution of aerosol particles and cloud water droplets. Beam expander is used to reduce the laser beam divergence before to be transmitted into the atmosphere. In this study, high-resolution vertical profiles from the near ground up to 15 km altitude are obtained. A Newtonian telescope of diameter 400 mm with an adjustable field of view (FOV) is used to collect the elastic backscattered signal. A photomultiplier tube (PMT) is used for the 532 nm wavelength detection channel, while an avalanche photodiode (APD) is used for the 1064 nm wavelength detection channel. The optoelectronic detection channels use two similar very high frequency preamplification circuit. Data are acquired with a nominal spatial resolution of 7.5 m using a 12-bit 20 MHz analog-to-digital converter (ADC) for each channel. Many functions, such as, range determination, background subtraction, digitization, and averaging are performed by the receiver subsystem. In addition, spatial resolution and linear dynamic range were optimized during signal processing.
A fast- and low-noise optical receiver using a silicon avalanche photodiode coupled to a very high-frequency preamplifier circuit is developed, characterized, and tested. The gain, bandwidth, and noise of the preamplifier are optimized to the avalanche photodiode performance to achieve detection and amplification without distortion of the fast and weak optical signal. A low noise level at the preamplifier circuit input of only 1.6 nV/Hz1/2 and a very good linearity from 1 kHz to 280 MHz are achieved. In addition, a good detection performance of the optical receiver is attained: A low-noise equivalent power of only 1.2 pW/Hz1/2 and a minimum signal-to-noise ratio requirement of only 3 are reached. Furthermore, the maximum detection range of the light detection and ranging remote sensing system, using the developed optical receiver, is estimated.
A Fast and Low-noise Optical Receiver using a Silicon Avalanche Photodiode with an internal gain of 100 connected to a Broadband Preamplifier Circuit was developed. The optical receiver and the receiving optics form the detection channel of a Cloud-Aerosol Lidar Remote Sensing System used to measure profiles of aerosol and cloud backscatter at the near-infrared wavelength of 1064 nm. While a 10 Hz repetition rate solid state pulsed Nd:YAG laser emitting at 1.06 μm and the emission optics form the transmission channel. The preamplifier circuit with a 300 MHz bandwidth and a gain of 10 is capable of accommodating laser pulses of 10 ns full width at half maximum. The preamplifier matches 50 Ω impedances at the input and the output sides. The input matching is used to reduce the Johnson noise and hence a much better sensitivity was achieved. The output matching was useful when this preamplifier is to be connected to other instrumentation requiring 50 Ω impedance matching or to be interfaced in cascade to increase the overall gain of the detection chain. These 50 Ω impedances at the input and output sides, also allows using the preamplifier coupled with a photodiode at the input in the detection of fast signals without distortion or integration. A low noise level at the preamplifier circuit input of only 1.6 nV/Hz<sup>1/2</sup> and a very good linearity from 1 KHz to 280 MHz were achieved, allowing the transmission of the backscattered signal to the acquisition system without distortion. In addition, the experimental characterization of the optical receiver coupled with the receiving optics showed good detection performance of the lidar detection channel: A low Noise Equivalent Power of 50 pW/Hz<sup>1/2</sup> and a high Signal-to- Noise Ratio of 2 were achieved. Furthermore, the maximal range of the lidar remote sensing system was estimated.
Availability of high intensity light emitting diodes in the blue region offer excellent opportunity for using them in active Differential Optical Absorption Spectroscopy (DOAS) to detect air pollution. Their smooth and relatively broad spectral emissions as well as their long life make them almost ideal light sources for active DOAS. In this study, we report the usage of a blue light emitting diode in an active DOAS setup to measure traces of NO2 gas and achieving few parts per billion detection limit for a path length of 300 m. Details of the setup will be presented along with the effects on measurement accuracy due to shifts in the measured spectra calibration and due to using theoretical instrument Gaussian function instead of the measured instrument function.
High resolution spectral lines study is performed on the emissions of a blue laser diode as a function of applied current. The range of applied current used is between the threshold current of 20 mA and 100 mA with a 0.2 mA increment. With this range of current, the observed emission spectra are between 446 and 448 nm. Typically, 21 longitudinal modes are observed with a mode spacing of 0.05 nm. This mode spacing is found to be in good agreement with the predicted values calculated using the GaN index of refraction and the length of laser cavity. The peak location of each longitudinal mode is measured to shift uniformly with a rate of 0.0045 nm/mA. The intensity and wavelength of each longitudinal mode are observed to be stable over extended period of time. Selected longitudinal modes will be employed to detect traces of pollution gases.
Temperature effects on the spectral lines of two Fabry-Perot GaN-based blue laser diodes obtained from Toptica and Roithner Laser Tech are experimentally investigated over the temperature range 5 °C to 60 °C in steps of 0.5 °C. A high resolution monochromator SPEX 1403 with a nominal resolution of 0.003 nm is used in this study. A detailed comparison on the number of modes, mode spacing, emission range and change of emission wavelength per degree Celsius will be presented in this paper. The results of this comparison are used to investigate the suitability of the employment of these laser diodes in open-path detection of NO<sub>2</sub> gas pollution.
The pulse from a transversely excited atmospheric CO2 laser consists of a sharp spike followed by a long, drawn out tail
region spanning about 2-5 μs caused by the nitrogen gas in the laser cavity. The nitrogen tail is undesirable in many
applications because it decreases the average power of the laser pulse. High stability and energy-efficient laser-induced
plasma shutter to clip the nitrogen tail of CO2-TEA based DIAL is built. Optimum shutter gases pressures and laser
breakdown intensities are reported. Clipped laser pulses are also field tested.