Wavelength stabilization

For many applications in high-resolution laser spectroscopy, it is essential that the laser wavelength stays as stable as possible at a preselected value Aq. This 

In this section we discuss some methods of wavelength stabilization with their advantages and drawbacks. Since the laser frequency v = c/A is directly related to the wavelength, one often speaks about frequency stabilization, although for most methods in the visible spectral region, it is not the frequency but the wavelength that is directly measured and compared with a reference standard. There are, however, new stabilization methods that rely directly on absolute frequency measurements (Vol.2, Sect. 9.7). 

In Sect. 5.3 we saw that the wavelength A or the frequency v of a longitudinal mode in the active resonator is determined by the mirror separation d and the refractive indices 2 of the active medium with length L and n outside the amplifying region. The resonance condition is 

For simplicity, we shall assume that the active medium fills the whole region between the mirrors. Thus (5.83) reduces, with L = d and 2 = i = . to 

Any fluctuation of n or causes a corresponding change of A and v. We obtain from (5.84) 

To illustrate the demands of frequency stabilization, let us assume that we want to keep the frequency i/ = 6-10 Hz of an argon laser constant within 1 MHz. This means a relative stability of Ap/p = 1.6-10, and implies that the mirror separation of d = 1 m has to be kept constant within 1.6 nm  

From this example it is evident that the requirements for such stabilisation are by no means trivial. Before we discuss possible experimental solutions, let us consider the causes of fluctuations or drifts in the resonator length d or the refractive index n. If we were able to reduce or even to eliminate these causes, we would already be well on the way to achieving a stable laser frequency. We shall distinguish between long-term drifts of d and n, which are mainly caused by temperature drifts or slow pressure changes and short-term fluctuations, caused, for example, by acoustic vibrations of mirrors, by acoustic pressure waves which modulate the refractive index, or by fluctuations of the discharge in gas lasers or of the jet flow in dye lasers. 

To illustrate the demands of frequency stabilization, let us assume that we 

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Obviously, such a high-resolution monochromator requires active wavelength stabilization in order to avoid drift problems. This has been accomplished through an internal neon lamp, mounted on an adjustable stand in front of the intermediate slit between the pre- and echelle-monochromator, so that it can be moved into the beam automatically if necessary. The neon lamp emits many relatively narrow lines in the 580-720 nm range, and, in the absence of any pre-selection, these are separated by the echelle grating into various superimposed orders. This means that without pre-dispersion at least two neon lines for every grating position surely fall on the detector, and can be used for stabilization. The precision of this stabilization is only limited by the stepper motor for grating adjustment, and is better than one-tenth of a pixel width (see Welz et al. [10]). 

We present material and deviee eharaeterization of 280 nm semiconductor ultraviolet light emitting diodes. These devices exhibit low series resistance, wavelength stability with increasing current, and have a half-life in excess of 570hrs, depending upon the injection current. Time-resolved photoluminescence studies of these materials prior to fabrication have been correlated with the device performance. We also discuss the potential for use in water purification. 

Up to now the quality of our measurements still suffers from the thermal drift of this birefringence as well as the amplitude and wavelength stability of the laser. Therefore, the measuring time for one run was restricted to a few minutes and by this the maximum delay time limited to 20 ps. 

This amplifier is necessary to bring the signal, which is proportional to the wavelength deviation (Uin AA.), 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 [5.62,5.63], which is widely used for intensity stabilization and wavelength stabilization of lasers. 

Fig. 5.47. Experimental realization of an acoustically isolated table for a wavelength-stabilized laser system...

The wavelength stabilization system consists essentially of three elements (Fig. 5.49) ... 

Fig. 5.50. Laser wavelength stabilization onto the transmission peak of a stable Fabry-Perot interferometer as reference...

Fig. 5.51. Wavelength stabilization onto the slope of the transmission T(i) of a stable reference FPI...

Since the accuracy of wavelength stabilization increases with decreasing molecular linewidth, spectroscopists have looked for particularly narrow lines that could be used for extremely well-stabilized lasers. It is very common to stabilize onto a hyperfine component of a visible transition in the h molecule... 

A simple technique for wavelength stabilization uses the orthogonal polarization of two adjacent axial modes in a HeNe laser [5.80]. The two-mode output is split by a polarization beam splitter BSl in the two orthogonally polarized modes, which are monitored by the photodetectors PDl and PD2. The difference amplification delivers a signal that is used to heat the laser tube, which expands until the two modes have equal intensities (Fig. 5.53). They are then kept at the frequencies = vq Av/2 = vo c/(4nd). Only one of the modes is transmitted to the experiment. 

For a more complete survey of wavelength stabilization, the reader is referred to the reviews by Baird and Hanes [379], Ikegami [380], Hall et al. [381], Bergquist et al. [383] and Ohtsu [384] and the SPIE volume [385]. 

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