Laser noise spectrum

The observed noise spectrum of a fluorescence signal excited by a laser with flie beat frequency pattern of Eq. 52 is then the product of (52) and (50), or... 

The frequency noise power spectral density of a SL typically exhibits a 1/f dependence below 100 kHz and is flat from 1 MHz to well above 100 MHz. Relaxation oscillations will induce a pronounced peak in the spectrum above 1 GHz. The "white" spectral component represents the phase fluctuations that are responsible for the Lorentzian linewidth and its intensity is equal to IT times the Lorentzian FWHM.20 xhe 1/f component represents a random walk of the center frequency of the field. This phase noise is responsible for a slight Gaussian rounding at the peak of the laser field spectrum and results in a power independent component in the linewidth. Figure 3 shows typical frequency noise spectra for a TJS laser at two power levels. 

The high-frequency part of the noise spectrum is mainly caused by fast fluctuations of the refractive index in the discharge region of gas lasers or in the liquid jet of cw dye lasers. These perturbations can only be reduced partly by choosing optimum discharge conditions in gas lasers. In jet-stream dye lasers, density fluctuations in the free jet, caused by small air bubbles or by pressure fluctuations of the jet pump and by surface waves along the jet surfaces, are the main causes of fast laser frequency fluctuations. Careful fabrication of the jet nozzle and filtering of the dye solution are essential to minimize these fluctuations. 

The high-frequency part of the noise spectrum is mainly caused by fast fluctuations of the refractive index in the discharge region of gas lasers or in the liquid jet of cw dye lasers. These perturbations can be only partly reduced by choosing the optimum discharge conditions in gas lasers. 

The Raman spectrum was measured at room temperature. The light souree was green laser (= 532 nm). The diameter of the foeused laser beam was 1 ocm. All spectra were recorded by 120 time integrations after 0.5 second irradiation to eliminate laser noise. Fig. 2 shows a sehematie diagram of irradiation of a polarized laser beam onto a specimen. 

A connnon teclmique used to enliance the signal-to-noise ratio for weak modes is to inject a local oscillator field polarized parallel to the RIKE field at the detector. This local oscillator field is derived from the probe laser and will add coherently to the RIKE field [96]. The relative phase of the local oscillator and the RIKE field is an important parameter in describing the optical heterodyne detected (OHD)-RIKES spectrum. If the local oscillator at the detector is in phase with the probe wave, the heterodyne mtensity is proportional to... 

Note MALDI spectra are acquired just above the threshold laser fluence for ion formation. Thus, single-shot spectra normally show a low signal-to-noise ratio (Chap. 5.2.3) due to poor ion statistics. Therefore, 50-200 single-shot spectra are usually accumulated to produce the final spectrum. [47]... 

Common to all narrow-bandwidth excitation schemes is sequential scanning of an experimental parameter in order to adjust the Raman shift in CRS detection. In order to obtain an entire CRS spectrum, this is not only time consuming but also prone to sources of noise induced by fluctuations in laser pulse parameters. As a consequence, dynamical changes in a CRS spectrum are difficult to follow. This problem can be circumvented by use of multiplex CRS spectroscopies [48, 49], which will be discussed in combination with CARS and SRS microscopy in Sects. 6.3 and 6.4, respectively. 

When a pump and a Stokes laser beam coincide on the sample and their difference frequency matches a particular molecular vibrational frequency, then SRS appears in the form of a gain of the Stokes pulse intensity and a loss of the pump pulse intensity, as first observed by Woodbury and Ng in 1962 [170] and by Jones and Stoicheff in 1964 [171], respectively (see Fig. 6.1). SRS has long been recognized as a highly sensitive spectroscopic tool for chemical analyses in the condensed and gas phases [172, 173, 29, 174]. For example, a shot-noise limited SRS spectrum of a single molecular monolayer was demonstrated by Heritage and Allara in 1980 [175]. In this section, we discuss the fundamental properties and applications of SRS microscopy, as was first successfully demonstrated by Nandakumar et al. [20] and subsequently reported by several research teams [21, 12, 13, 22]. 

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