Phase-modulation laser diode

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

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).

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). 

It will be seen that, as in the case of the LED, control of the bias voltage gives simple modulation of the laser output intensity. This is particularly useful in phase-modulation fluorometry. However, a measure of the late awareness of the advantages of IR techniques in fluorescence is that only recently has this approach been applied to the study of aromatic fluorophores. Thompson et al.(51) have combined modulated diode laser excitation at 670 and 791 nm with a commercial fluorimeter in order to measure the fluorescence lifetimes of some common carbocyanine dyes. Modulation frequencies up to 300 MHz were used in conjunction with a Hamamatsu R928 photomultipler for detecting the fluorescence. Figure 12.18 shows typical phase-modulation data taken from their work, the form of the frequency response curves is as shown in Figure 12.2 which describes the response to a monoexponential fluorescence decay. 

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

However, unlike diode lasers, an external acousto-optic or electro-optic modulator is required for phase-modulation lifetime fluorometry to be performed with a HeNe laser. 

The technical noise, which represents the major limitation, decreases with increasing frequency. It is therefore advantageous to choose the modulation frequency as high as possible. For diode lasers this can be achieved by modulation of the diode current. For other lasers, often electro-optical modulators outside the laser resonator are used as phase modulators (Fig. 1.6), resulting in a frequency modulation of the transmitted laser beam [3],... 

Fig. 1.9 (a) Water overtone absorption line measured with unmodulated (b) and with phase-modulated single-mode diode laser... 

R. Grosskloss, P. Kersten, W. Demtroder, Sensitive amplitude and phase-modulated absorption spectroscopy with a continuously tunable diode laser. Appl. Phys. B 58, 137 (1994)... 

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