Wavemeters

Wellen-messer, m. wavemeter, ondometer, cymometer, -optik, /. wave optics. 

We measured and analyzed the vertical emission from the resonators under pulsed optical pumping. The experimental setup is illustrated in Fig. 12.8a A Ti/sapphire mode-locked laser was used to optically pump the devices at a center wavelength of 980 nm, repetition rate of 76.6 MHz, and pulse duration of approximately 150 fs. A variable attenuator was used to control the pump power. The average pump power and center wavelength were monitored by a wavemeter, through a 50/50 beamsplitter. The pump beam is focused on the back side of the sample with a 50 x objective lens. A 20 x objective lens is used to collect the vertical emission from the sample and to focus it on an IR camera to obtain the NF intensity pattern and to... 

The microwave frequency can measured with great precision using the variable-length cavity of a wavemeter Its resonant absorption is detected electronically by the extra "cavity dip," and its frequency is calculated from the mechanically calibrated dimensions of the cavity. The microwave frequency (say 10 GHz) can be read with a precision of even 1 Hz by digital counting and by using a built-in transfer oscillator in a microwave frequency counter. 

A Fourier transform spectrometer is also a scanning wavemeter. It transfers the absolute frequency calibration of a reference laser to the frequencies of all lines in the FT spectrum. The I2, Te2, Br2, and H20 line atlases, with absolute accuracy on the order of 0.001cm-1, are measured by FT spectroscopy relative to the more accurately known frequency of the He-Ne laser. 

A high resolution study of the Vj band of HCN at 3 )un has been recently reported using a standard small volume cell of non-resonant design . The 25 lowest rotational levels of the V3 band were excited with a tunable < lor center laser at an HCN pressure of 20 m Torr. The pressure of the N2 buffer gas was varied between 20 and 400 Torr to investigate pressure broadening and to detect possible frequency shifts induced by the buffer gas. In intracavity experiments performed at an HCN pressure of only 0,02 mTorr no shift of the transition frequency could be detected, to the accuracy of the wavemeter readout. The results of this photoacoustic analysis of the V3 band of HCN are ... 

In the heterodyne wavemeter, a variable-frequency local oscUlator is adjusted to give a predetermined beat frequency with the incoming wave, enabling the frequency of the incoming wave to be determined. 

The laser wavelength was determined to an accuracy of 0.01 cro using a home built He-Ne referenced wavemeter. Frequency markers were obtained by monitoring the FM sidebands imposed on a fraction of the beam by a LiTiO electro-optic modulator operating at 100 MHz. ... 

The highest accuray (i.e., the lowest uncertainty) can be achieved with modem wavemeters, which we shall discuss in Sect. 4.4.2. 

Figure 4.64 illustrates the principle of a traveling-wave Michelson-type interferometer as used in our laboratory. Such a wavemeter was first demonstrated in a slightly different version by Hall and Lee [4.67] and by Kowalski et al. [4.72]. The beams Sr of a reference laser and of a laser with unknown wavelength traverse the interferometer on identical paths, but in opposite directions. Both incoming beams are split into two partial beams by... 

Fig, 4,65, Signal sequences in the two detection channels of the traveling Michelson wavemeter. The grey signal pulses are not counted... 

There are several different designs of Michelson wavemeters that are commercially available and are described in [4.78-4.80]. 

Fig. 4.67. Wavemeter for pulsed and cw lasers, based on a combination of a small poly-chromator and three FPI with widely differing free spectral ranges [4.77]...

The device is calibrated with different lines from a cw dye laser that are simultaneously measured with the traveling Michelson wavemeter (see above). This calibration allows ... 

Fig.4.69a-e. Output signals at the poly-chromator and the three diode arrays of the FPI wavemeter, which had been illuminated by a cw HeNe laser oscillating on two axial modes (a-d). The lowest figure shows the ring intensity pattern of an excimer-pumped single-mode dye laser measured behind a FPI with 3.3 GHz free spectral range [4.70]... 

The Fizeau wavemeter constructed by Snyder [4.82] can be used for pulsed and cw lasers. While its optical design is simpler than that of the sigmame-ter and the FPI wavemeter, its accuracy is slightly lower. Its basic principle is shown in Fig. 4.70b. The incident laser beam is focused by an achromatic... 

Fig. 4,70a,b. Fizeau wavemeter (a) interference at a wedge (the wedge angle 0 is greatly exagerated) (b) schematic design A, aperture as spatial filter P, parabolic mirror C, distance holder of cerodur D, diode array... 

For absolute wavelength measurements of spectral lines the laser is stabilized onto the center of the line and its wavelength X is measured with one of the wavemeters described in Sect. 4.4. For Doppler-free lines (Chaps. 7-11), one may reach absolute wavelength determinations with an uncertainty of smaller than 10 cm" (= 20pm at A, = 500 (xm). 

If no wavemeter is available, two FPIs with slightly different mirror separations d and J2 can be used for wavelength determination (Fig. 5.58b). Assume d /d2 = p/q equals the ratio of two rather large integers p and q with no common divisor and both interferometers have a transmission peak at Ai ... 

Fig.5.90a-c. Linewidth of a single-mode pulsed laser measured with a Fabry-Perot wavemeter (a) experimental setup (b) single shot and (c) signal averaged over 500 pulses... 

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