Intensity stabilization

The intensity 7(t) of a cw laser is not completely constant, but shows periodic and random fluctuations and also, in general, long-term drifts. The reasons for these fluctuations are manifold and may, for example, be due to an insufficiently filtered power supply, which results in a ripple on the discharge current of the gas laser and a corresponding intensity modulation. Other noise sources are instabilities of the gas discharge, dust particles diffusing through the laser beam inside the resonator, and vibrations of the resonator mirrors. In multimode lasers, internal effects, such as mode competition, also contribute to noise. In cw dye lasers, density fluctuations in the dye jet stream and air bubbles are the main cause of intensity fluctuations. 

Of the various possible methods, we shall discuss two that are often used for intensity stabilization. They are schematically depicted in Fig. 5.46. In the first method, a small fraction of the output power is split by the beam splitter BS to a detector (Fig. 5.46a). The detector output Vd is compared with a reference voltage Fji and the difference AV = Fp — Fr is amplified and fed to the power supply of the laser, where it controls the discharge current. The servo loop is effective in a range where the laser intensity increases with inaeasing current. 

The upper frequency limit of this stabilization loop is determined by the capacitances and inductances in the power supply and by the time lag between the current increase and the resulting increase of the laser intensity. The lower limit for this time delay is given by the time required by the gas discharge to reach a new equilibrium after the current has been changed. It is therefore not possible with this method to stabilize the system against fluctuations of the gas discharge. For most applications, however, this stabilization technique is sufficient it provides an intensity stability where the fluctuations are less than 0.5 %. 

To compensate fast intensity fluctuations, another technique, illustrated in Fig. 5.46b, is more suitable. The output from the laser is sent through a Pock- 

This amplifier is necessary to bring the signal, which is proportional to the deviation of the intensity from its nominal value, really back to zero. This cannot be performed with a proportional amplifier. The third amplifier is a differentiating device that takes care of fast peaks in the perturbations. All three functions can be combined in a system called PID control [352, 353], which is widely used for intensity stabilization and wavelength stabilization of lasers. 

Of the various possible methods, we shall discuss two that are often used for intensity stabilization. They are schematically depicted in Fig. 5.44. In the first method, a small fraction of the output power is split by the beam splitter 

The intensity I(t) of a cw laser is not completely constant but shows periodic and random fluctuations and also, in general, long-term drifts. 

For spectroscopic applications of dye lasers, where the dye laser has to be tuned through a large spectral range, the intensity change, caused by the 

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In principle the viscosity of nitrostarch is affected by the same factors which cause a change of viscosity in nitrocellulose. Thus a high nitrogen content in the product, an elevated nitration temperature, or an intensive stabilization boiling of the nitrated product lowers the viscosity of the nitrated substance. However, in the case of nitrostarch the changes in viscosity under the influence of these factors are insignificant. 

Figure 2 Schematic diagram of the time-resolved IR spectrometer. CLS, cavity length stabilizer SHG, second harmonic generator IS, intensity stabilizer BPS, beam pointing stabilizer CS, coherent seeder PR, polarization rotator.

Studies on complex systems, such as metalloen-zymes, increasingly require the measurement of a minor spectral component in a far larger assembly. Basic aspects of optical spectroscopic techniques that make them particularly effective are the speed, sensitivity, and linearity of light detectors as well as the intensity, stability, and precision of conventional and laser light sources. 

With a similar setup as used by Ippen et al. for pump-probe experiments, except for an intensity stabilizer in both beams, we performed experiments on the electronic origin at 6027 A and vibronic transitions at 5933 and 5767 A. The results of these experiments are shown in Fig. 22. Except for minor details, the transient on the purely electronic transition is in agreement with our expectation that the singlet excited state is long lived (19.5 ns) on a picosecond time scale. The transient on the 261 cm vibration confirms what was already known from the optical absorption spectrum, namely, that it is very short lived. From the near Lorentzian lineshape at low temperature we calculate a 3.3 ps relaxation time in... 

Example 1.3 With an intensity-stabilized light source and lock-in detection the minimum relative absorption that may be safely detected is about AP/P > 10 , which yields the minimum measurable absorption coefficient cKmin for an absorption pathlength L,... 

The sensitivity of the absorption measurement procedure depicted in Figure 6.4 depends critically on the light intensity stability of the laser source. Typical tuneable laser sources with output intensity Iq exhibit short-term fluctuation, or amplitude noise, of the order 6I/I0 = 10 .Thismeansthatspecieswhichgenerate... 

Fig. 5.44a,b. Intensity stabilization of lasers (a) by controlling the power supply, and (b) by controlling the transmission of a Pockels cell... 

Fig. 5.46a,b. Intensity stabilization of a cw dye laser by control of the argon laser power (a) experimental arrangement (b) stabilized and unstabilized dye laser output P X) when the dye laser is tuned across its spectral gain profile... 

Assume that the output power of a laser shows random fluctuations of about 5%. Intensity stabilization is accomplished by a Pockels cell with a halfwave voltage of 600 V. Estimate the ac output voltage of the amplifier driving the Pockels cell that is necessary to stabilize the transmitted intensity if the Pockels cell is operated around the maximum slope of the transmission curve. 

Because of the high spectral power density of many lasers, the detector noise is generally negligible. Intensity fluctuations of the laser, which limit the detection sensitivity, may essentially be suppressed by intensity stabilization (Sect. 5.4). This furthermore increases the signal-to-noise ratio and therefore enhances the sensitivity. 

The laser power was measured with a Spectra Physics power meter (Model 404), which had a calibration accuracy of 15%. Our power values refer always to the plane of the NLC sample i.e., the reflection losses on the lens and windows are taken into account. The intensity stability of the laser was better than 0.5%. 

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