Laser intensity fluctuations

There are two detection sensitivities that are of interest for characterizing the BioCD performance. The first is a metrology sensitivity that includes experimental noise sources such as the laser intensity fluctuations and detector noise, but does not include variability of the protein spots on the disc. The second detection sensitivity is under actual assay conditions in which antibody spot variability plays a dominant role. For both of these sensitivities, it is important to define a scale-free sensitivity that is an intrinsic property of the detection platform. 

One of the shortcomings of LIBS, particularly in relation to quantitative elemental analysis, arises from the instability of the laser-induced plasma emission resulting from laser intensity fluctuations (1-5%) the amount of scattered light present depends on local matrix effects and on physical and chemical properties of the target material. The most common way of compensating for signal fluctuations in LIBS is by calculating the ratio of the spectral peak intensity to that of a reference intensity. However, this internal calibration method provides relative rather than absolute concentrations. 

The superscriptsp and r in Eq. (28) refer to polymer thin film and reference harmonic intensities. The reference, with simultaneous measurements of SHG intensity from a poled, stabilized thin film, was used to correct for eventual laser intensity fluctuations. The fits of the measured temporal growth of SHG intensity with mono-, bi-, and triexponential functions are shown in Fig. 31. The best agreement is obtained when using a triexponential function (Eq. (28)). The time constants T2, and T3 depend on the polymer, temperature, and the chromophore itself. 

Another nonlinear technique that is potentially applicable to thermometric measurements is DPWM [7,9]. Por instance, a Boltzmann plot constructed out of the relative line intensities of a DPWM spectrum can lead to temperature predictions that can be as accurate as CARS in some cases. An alternative method is to fit theoretical simulations to the experimental spectrum. Nonetheless, the versatility of CARS is not equaled by DPWM. In effect, single pulse measurements seems to be limited to some radical species and mode fluctuations of conventional lasers perturb the data severely. To avoid troubles with such laser intensity fluctuations, saturated DPWM is often employed, but the difficulties of spectral S5mthe-sis remain a serious hindrance to a major role of DPWM thermometry. 

Reflection noise degrades the transmitted signal through laser intensity fluctuations due to optical feedback from some interfaces such as the fiber end faces and the photodiode (PD) [41 -43]. This can be eliminated by using optical isolators and optical fibers with obliquely polished end faces. However, they cannot be applied to in-home and in-building networks because of their high installation cost. 

A laser beam highly focused by a microscope into a solution of fluorescent molecules defines the open illuminated sample volume in a typical FCS experiment. The microscope collects the fluorescence emitted by the molecules in the small illuminated region and transmits it to a sensitive detector such as a photomultiplier or an avalanche photodiode. The detected intensity fluctuates as molecules diffuse into or out of the illuminated volume or as the molecules within the volume undergo chemical reactions that enhance or diminish their fluorescence (Fig. 1). The measured fluorescence at time t,F(t), is proportional to the number of molecules in the illuminated volume weighted by the... 

B) Calculated Ed (solid circles left axis) is seen to fluctuate significantly in time-lapse experiments. After 30 min a large intensity fluctuation in acceptor excitation was simulated by manually diminishing laser power with 60%. The open circles depict the correction factor y, calculated according to Eq. (7.6) from cells expressing acceptors only. Calculating Ed with the online-updated y-factor (solid squares) abolished the effects of excitation fluctuations. 

This effective Q,t-range overlaps with that of DLS. DLS measures the dynamics of density or concentration fluctuations by autocorrelation of the scattered laser light intensity in time. The intensity fluctuations result from a change of the random interference pattern (speckle) from a small observation volume. The size of the observation volume and the width of the detector opening determine the contrast factor C of the fluctuations (coherence factor). The normalized intensity autocorrelation function g Q,t) relates to the field amplitude correlation function g (Q,t) in a simple way g t)=l+C g t) if Gaussian statistics holds [30]. g Q,t) represents the correlation function of the fluctuat-... 

If there is no fluctuation of laser intensity, we have to measure /q only once. Actually, the envelope of laser pulses changes in a relatively long time range (typically from several minutes to a few tens of minutes) because of the change of environmental factors such as room temperature and coolant temperature. There is also an intensity jitter caused by factors such as the mechanical vibration of mirrors and the timing jitter of electronics. Furthermore, in our system, the laser system is located about 15 m from the beam port to prevent radiation damage to the laser system. (Later, it was moved into a clean room, which was installed in the control room to keep the room temperature constant and to keep the laser system clean. The distance is about 10 m.) Therefore it is predicted that a slight tilt of a mirror placed upstream will cause a displacement of the laser pulse at the downstream position where the photodetector is placed. 

The laser source in our spectrometer is an amplified femtosecond dye laser with a much larger repetition rate than many of the existing amplified laser systems used for femtosecond spectroscopy. The amplification is necessary to improve the signal intensity which actually depends on roughly the third power of the laser intensity. The large repetition rate helps average over pulse-to-pulse fluctuations of the laser. 

Raman parent spectrum is obtained simply by adding all the spectra together. However, the ROA spectrum is obtained by either adding or subtracting from the current coadded spectrum depending on the position of quarter-wave plate. After several cycles, the Raman and ROA spectra are saved on the hard disk as one data set. After several data sets, all the data are added and saved into either the Raman parent spectrum or the ROA raw data spectrum and stored for the data manipulation. Occasionally, the raw data contains parental bias or first derivative contributions due to the fluctuation of the laser intensity or the quality of the optics, and these are reduced or removed digitally. 

Figure 5. Visible laser stability. The laser power fluctuations using a 488 nm (blue) and 568 nm (red) lasers were determined using a 10x objective and a Chroma red slide. The fluorescence was sequentially measured every 30 sec (400 times) for total time duration of 3.33 hrs. The variation of the peak to peak using 488 nm or 568 nm excitation was approximately 25%. The fluctuating power intensity line suggests that the system scanning and detection devices are yielding large power fluctuations that will affect the illumination of the sample. The Acousto Optical Transmission Filter (AOTF) is probably contributing to this 488-568 nm sinusoidal pattern.

The OHD-RIKES signal is quadratic in the laser intensity, so any laser fluctuations are magnified in the data. To compensate for such fluctuations, the probe beam is sent into a doubling crystal after the sample, and the resultant second-harmonic signal is detected by a separate lock-in amplifier. Second-harmonic generation has the same quadratic intensity dependence as the OHD-RIKES signal, so dividing the RIKES data by the second-harmonic data acts to normalize any intensity fluctuations. 

Figure 1. Intensity fluctuations in a CdSe-ZnS NC under continuous laser illumination at room temperature. Dotted horizontal line was selected as a threshold to divide off and on states.

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