Laser diodes modulation

The DLAAS devices are now commercially available from Atomica Instruments (Munich, Germany). The first system was prepared for the measurement of ultra-trace levels of A1 in the semiconductor industry. The commercial laser diode module includes the DL with a heat sink for temperature tuning, the microoptics, and the non-linear crystal for SHG. The module has the size of a HCL. Since delivery, the module has successfully worked routinely under the conditions of an industrial analytical laboratory without any repair or maintenance. The semiconductor industry in particular is interested in the measurements of light elements, such as Al, K, Ca, Na, in ultra pure water or chemicals. For example, the module designed for the detection of K provides an LOD of 0.5 pg/mL (10 pL aliquot). Similar compact and sensitive devices with electrothermal atomizers, flames or micro-plasma are currently... 

Peterman, K. (1991) Laser Diode Modulation and Noise, Kluwer Academic Publishers, Dordrecht. 

Kastner J., Tacke M., Katzir A., Edl-Mizaikoff B., Gobel R. and Kellner R., Optimizing the modulation for evanescent-wave analysis with laser diodes (EWALD) for monitoring chlorinated hydrocarbons in wat, Sensors Actuators B 1997 38 163-170. 

Prior to describing the possible applications of laser-diode fluorometry, it is important to understand the two methods now used to measure fluorescence lifetimes these being the time-domain (Tl)/4 5 24 and frequency-domain (FD) or phase-modulation methods.(25) In TD fluorometry, the sample is excited by a pulse of light followed by measurement of the time-dependent intensity. In FD fluorometry, the sample is excited with amplitude-modulated light. The lifetime can be found from the phase angle delay and demodulation of the emission relative to the modulated incident light. We do not wish to fuel the debate of TD versus FD methods, but it is clear that phase and modulation measurements can be performed with simple and low cost instrumentation, and can provide excellent accuracy with short data acquisition times. 

How can phase-modulation fluorometry contribute to this health-care need It now seems possible to construct a lifetime-based blood gas catheter (Figure 1.3), or alternatively, an apparatus to read the blood gas in the freshly drawn blood at the patient s bedside. To be specific, fluorophores are presently known to accomplish the task using a 543-nm Green Helium-Neon laser,(18 19) and it seems likely that the chemistries will be identified for a laser diode source. The use of longer wavelengths should minimize the problems of light absorption and autofluorescence of the samples, and the use of phase or modulation sensing should provide the robustness needed in a clinical environment. For the more technically oriented researcher, we note that the... 

Figure 1.3. Phase-modulation blood gas instrumentation based on a laser diode light source. Blood gas catheter (lop) direct reading blood gas syringe (bottom).

R. B. Thompson, J. K. Frisoli, and J. R. Lakowicz, Phase fluorometry using a continuously modulated laser diode, Anal. Chem. 64, 2075-2078 (1992). 

K. W. Bemdt, I. Gryczynski, andJ. R. Lakowicz, Phase-modulation fluorometry using afrequency-doubled pulsed laser diode light source, Rev. Sci. Instrum. 61, 2331-2337 (1990). 

Figure 6,9. Effective modulation of loser output as a function of radio frequency power applied to a Sharp LTQ24 laser diode at 30 MHz. Data are shown for DC bias of 50 mA ( , 7 mW laser output), 60 mA (O, 14 mW), and 70 mA (a, 19 mW). Reproduced from Ref. 25 with permission,...

Figure 11.15. Schematics of the optical arrangement and temperature probes for the Cr+ fluorescence lifetime-based fiber optic thermometers. F = short-pass optical filter Fa = bandpass or long-pass optical filter LD = laser diode LED = light emitting diode S = the fluorescence material used as sensing element vm = signal to modulate the output intensity of the excitation light source v/= the detected fluorescence response from the sensing element.

Figure 12.18. The lower portion of the figure shows diode laser phase-modulation fluorescence lifetime...

Chaos was also investigated in solid-state lasers, and the important role of a pump nonuniformity leading to a chaotic lasing was pointed out [42]. A modulation of pump of a solid-state NdPsOn laser leads to period doubling route to chaos [43]. The same phenomenon was observed in the case of laser diodes with modulated currents [44,45]. Also a chaotic dynamics of outputs in Nd YAG lasers was also discovered [46 18]. In semiconductor lasers a period doubling route to chaos was found experimentally and theoretically in 1993 [49]. 

Previously we have shown that the repetition rate of a mode locked laser equals the mode spacing to within the experimental uncertainty of a few parts in 1016 [26] by comparing it with a second frequency comb generated by an efficient electro-optic modulator [41]. Furthermore the uniform spacing of the modes was verified [26] even after further spectral broadening in a standard single mode fiber on the level of a few parts in 1018 [13]. To check the integrity of the femtosecond approach we compared the / 2/ interval frequency chain as sketched in Fig. 3 with the more complex version of Fig.4 [19]. We used the 848 nm laser diode of Fig. 4 and a second 848 nm laser diode locked to the frequency comb of the / 2/ chain. The frequencies of these two laser diodes measured relative to a quartz oscillator, that was used as a radio frequency reference for the frequency combs, are 353 504 624 750 000 Hz and 353 504 494 400 000 Hz for the / 2/ and the 3.5/ 4/ chain respectively. We expect a beat note between the two 848 nm laser diodes of 130.35 MHz which was measured with a radio frequency... 

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