Diode lasers multiplexing

Farrens and Song<40) have replaced the original spark source with a picosecond diode laser in a multiplexed dual wavelength T-formatfluorometer.(41)With an overall instrumental response width of ca. 300 psec full-width half-maximum (FWHM), near-IR fluorescence lifetimes as low as 75 psec in the case of l,l -diethyl-4,4 carbo-cyanine iodide (DCI) (excitation 660 nm) and decay components as low as 48 psec in the case of 124 kDa oat phytochrome (excitation 752 nm) were reported. 

For the last decade, semiconductor diode-laser sensors have been developed at Stanford University for measurements of important parameters in laboratory-and industrial-scale gaseous flowfields. For example, a mass flux sensor was developed based on rapid measurements of O2 absorption near 760 nm in supersonic flowfields [1] and a multiplexed sensor was developed for the simultaneous measurement of various pollutants representing unburned hydrocarbons (CH4, CH3CI) near 1.65 pm [2]. 

An application for multiplexed diode-laser sensors with a potentially large impact is for measurements of important parameters at several locations in a gas turbine combustion system. In this example, illustrated schematically in Fig. 24.1, the multiplexed diode lasers are applied for simultaneous absorption measurements in the inlet, combustion, afterburner, and exhaust regions. For example, measurements of O2 mass flux at the inlet may be determined at the inlet from Doppler-shifted O2 absorption lineshapes near 760 nm. Measurements of gas temperature and H2O concentrations in the combustion and afterburner regions may be determined from H2O lineshape measurements near 1.4 pm. Finally, measurements of velocity, temperature, and species concentrations (e.g., CO, CO2, unburned hydrocarbons) may be recorded in the exhaust for the determination of momentum flux (component of thrust) and combustor emissions. 

Figure 24.1 Schematic diagram for a potential application of multiplexed diode-laser sensors for measurements of gas temperature, species concentrations, velocity, mass flux, and thrust at several locations in military- and industrial-scale gas turbines (e.g., aeropropulsion, incineration, power generation appheations)...

The general arrangement of the multiplexed diode-laser sensors for measurements in the forced combustor at Stanford University is shown in Fig. 24.2. The primary air flow (65 1/min) through the central jet (d = 2.1 cm) was acoustically forced (up to 30% RMS of the flow rate) to create coherent vortices at... 

Figure 24.2 Schematic diagram of the setup used to measure and control H2O concentration and gas temperature in the combustion region (in situ) of a forced 5-kilowatt combustor at Stanford University 1 — steel duct 2 — quartz duct 3 — A1 duct 4 — multiplexed beam 5 — tunable diode lasers 6 — data acquisition and control computer 7 — control signals 8 — primary air driver Aair sin(27r/of) 9 — fuel drivers Afuei sin(27r/of-f dfuei) 10 — demultiplexing box 11 — Si detector (ND filter) and 12 — laser beam...

Figure 24.8 Experimental schematic of the multiplexed diode-laser sensor system used to measure CO, CO2, CH4, and H2O absorption by sampling hot combustion gases 1 ECDL 1.49-1.58 pm 2 optical isolator 3 — fiber coupler 4 — 1x2 fiber splitter 5 — etalon 6 — InGaAs detector 7 — DEB 1.65 pm 8 — 2 x 1 fiber combiner 9 optical fiber 10 fiber pitch 11 — concave mirror 12 — multipass...

Multiplexed diode-laser sensors were applied for measurement and control of gas temperature and species concentrations in a large-scale (50-kilowatt) forced-vortex combustor at NAWC to prove the viability of the techniques and the robustness of the equipment for realistic combustion and process-control applications [11]. The scheme employed was similar to that for measurements and control in the forced combustor and for fast extractive sampling of exhaust gases above a flat-flame burner at Stanford University (described previously). 

These results are consistent with previous measurements which showed that CO concentration was lowest at the combustor operating conditions that most efficiently reduced the overall emission of toxic gases. Thus a measurement of CO concentration can serve as an effective indicator of combustor performance. The results demonstrate the applicability of multiplexed diode laser sensors for rapid, continuous measurements and control of multiple flowfield parameters, including trace species concentrations, in high-temperature combustion environments. 

Multiplexed diode laser sensors have also been applied for measurements of gas temperature, velocity, and H2O partial pressures in hypervelocity air flows at the Calspan University of Buffalo Research Center s (CUBRC) Large Energy National Shock Tunnel (LENS Tunnel) in Buffalo, New York [12]. The sensors were developed to provide quantitative characterization of the facility operation and, in particular, the freestream flow properties as a function of time. The measurements were recorded using a hardened probe, which contained critical optical components and photodetectors, that was installed directly into the hypersonic shock-tunnel near the nozzle exit to minimize complications due to boundary layers and facility vibration. 

Baer, D. S., M. E. Newfield, N. Gopaul, and R. K. Hanson. 1994. Multiplexed diode-laser sensor system for simultaneous H2O, O2 and temperature measurements. Optics Letters 19(22) 1900-2. 

Spiropyrans show promise for optical recording, three-dimensional optical memories,214 and holography.215 The dyes currently under study for these applications very probably will not be used merely dissolved in a bulk polymer matrix, but will be oriented in films and membranes, or adsorbed or vapor deposited on solid substrates to take advantage of the nonlinear optical properties of the colored forms. For example, thick (0.5 mm) PMMA films of 6-nitro-thiaBIPS can be used to record wavelength-multiplexed volume holograms with an infrared diode laser. This system is impractical at present because of fatigue and poor diffraction efficiencies.216... 

Furlong, E. R., D.S. Baer, and R. K. Hanson. 1996. Combustion control and monitoring using a multiplexed diode-laser sensor system. AIAA Paper No. 96-2763. 

In a typical ZoloBOSS installation, a control rack located in or near the plant control room contains multiple laser sources and detectors, diode laser drivers, and a system computer. The light from the individual lasers, ranging in nominal wavelengths between 0.67 and 1.7 pm and tuned to CO, CO2, O2, and H2O absorption features are multiplexed into one single-mode fiber using Zolo s Zmux multiplexer/demultiplexer device. 

K. L. M. "Multiplexed Diode-Laser Sensor System for Simultaneous HjO, Oj, and Temperature Measurements." Applied Optics 33 (1994) 3296-3306. 

Baer, D. S., Nagali, V, Furlong, E. R., Hanson, R. K., and Newfield, M. E. "Scanned- and Fixed-Wavelength Absorption Diagnostics for Combustion Measurements Using a Multiplexed Diode-Laser Sensor System." AIAA Journal 34 (1996) 489-93. 

Sappey, A. D. "Wavelength-Multiplexed Diode Laser Spectroscopy for Closed Loop Combustion Control and Optimization." Paperpresented atthe American-Japanese Flame Research Committees International Symposium, Waikaloa, HI, 2007. 

Several optical signals are multiplexed on the microsecond or millisecond time scale. Multiplexing of signals can be accomplished by switching several diode lasers, either electronically or by fibre switches, or by rotating elements in an optical system. The channel signal indicates the current state of the multiplexing... 

An important application of multiplexed multidetector systems is diffuse opti-eal tomography (DOT). In DOT several pieoseeond diode lasers are multiplexed into the input of a fibre switeh. The multiplexed lasers are switched conseeutively into a large number of optieal fibres whieh deliver the light to the sample. The... 

For a long time the most difficult problem of pulse-group multiplexing has been the lasers. Conventional high-repetition rate lasers cannot be turned off and on at sub-ps speed, and simple optical switches of the required speed and on-off ratio do not exist. Picosecond diode lasers, however, can be switehed on within less than a microsecond and switched off within one or two pulse periods. Diode lasers with multiplexing capability are currently available for 375 nm, 405 nm,... 

Picosecond diode lasers are by far the most economic light sources for DOT. Fibre lasers [223] and Ti Sapphire lasers [222, 443] have been used as well. These lasers have high power and short pulse width, but a high price. Wavelength multiplexing can be achieved only by synchronising several lasers and at a pulse-by-pulse basis. 

Recently Liebert et al. have demonstrated that advanced TCSPC is able to record effects of brain activity with 50 ms time resolution, clear separation of scattering and absorption, and probably better depth resolution than CW or frequency-domain techniques [324, 327, 328]. A system of four parallel TCSPC modules with four individual detectors and several multiplexed laser diode lasers is used. A fast sequence of time-of-flight distributions is recorded in consecutive time intervals of 50 to 100 ms. Variations of the optical properties in the brain are derived from the intensity and the first and second moments of the time-of-flight distributions [325]. 

Of course, a TCSPC system works effieiently only with a high-repetition-rate excitation source. Diode lasers ean be built with any repetition rate up to about 100 MHz and are available with 375 nm, 405 nm, 440 nm, and 473 nm emission wavelength. Diode lasers are cost-effieient and ean be multiplexed at ps rates see Excitation Wavelength Multiplexing, page 87. For shorter wavelengths, frequency-doubled or frequency-tripled titanium-sapphire or neodymium-YAG lasers can be used. 

Recent achievements in the design of near-infrared diode lasers, of volume-phase transmission multiplexed holographic gratings and of sensitive CCD arrays allow... 

Lieberwirth, U., Arden-Jacob, J., Drexhage, K.H., Herten, D.P., MUller, R., Neumann, M., Schulz, A., Siebert, S., Sagner, G, Klingel, S., Sauer, M., and Wolfrum, J., Multiplex dye DNA sequencing in capillary gel electrophoresis by diode laser-based time-resolyed fluorescence detection. Anal. Chem., 70, 4771, 1998. 

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