Continuous Tuning Techniques

Since the laser wavelength Al of a single-mode laser is determined by the optical path length n d between the resonator mirrors, 

Equation (5.89) reveals that the wavelength shift A A is proportional to 0 bnt is 

Unfortunately, the reflection losses of an etalon increase with increasing tilting angle d (Sect. 4.2 and [340, 387]). This is due to the finite beam radius w of the laser beam, which prevents a complete overlap of the partial beams reflected from the front and back surfaces of the etalon. These walk-off losses increases with the square of the tilting angle 0, see (4.64a), (4.64b) and Fig. 4.42. 

For wider tuning ranges interferometers with a variable air gap can be used at a fixed tilting angle 9 (Fig. 5.44a). The thickness t of the interferometer and with it the transmitted wavelength = 2nt cos 9/ m can be tuned with a piezocylinder. This keeps the walk-off losses small. However, the extra two surfaces have to be antireflection-coated in order to minimize the reflection losses. 

An elegant solution is shown in Fig. 5.44b, where the interferometer is formed by two prisms with coated backsides and inner Brewster surfaces. The air gap between these surfaces is very small in order to minimize shifts of the transmission peaks due to changes of air pressure. 

If the plate is tilted by the angle Aa, the optical path length changes by 

Similar but smaller mode hops occur when the wavelength-selecting elements are continuously tuned but the resonator length is kept constant. 

---

For a very precise measurement of small spectral invervals between lines a sideband technique is very useful. In this technique part of the laser beam is sent through a Pockels cell (Fig. 5.64), which modulates the transmitted intensity and generates sidebands at the frequencies vr = vl /. When = vl + / is stabilized onto an external FPI, the laser frequency ul = — / can be continuously tuned by... 

At t) these integral and derivative approximations are accurate and the conventional tuning techniques used for continuous controllers discussed in Section... 

The possibility of continuously tuning the wavelength of such narrow-band lasers has certainly opened a new area of spectroscopy. A single-mode tunable laser represents a device which is a combination of an intense light source and an ultrahigh resolution spectrometer. Tunable lasers and tuning techniques are therefore covered in a separate chapter. 

He-Ne laser oscillating at A = 0.63 ym is selected by a Fox-Smith cavity (see Sect.6.5) with a frequency which can be continuously tuned through the gain profile of the laser transition. The output from a 1.15 ym He-Ne laser is sent col linear with the 0.63 ym beam through the gas discharge and is separated from the 0.63 ym by a prism. The attenuation or amplification of the 1.15 ym beam is detected as a function of the laser frequency and of the inversion n(3 2) - n(2P ), which can be altered by controlling the discharge conditions. The authors call this spectroscopic technique "tuned differential sepctroscopy". 

In AFS, the analyte is introduced into an atomiser (flame, plasma, glow discharge, furnace) and excited by monochromatic radiation emitted by a primary source. The latter can be a continuous source (xenon lamp) or a line source (HCL, EDL, or tuned laser). Subsequently, the fluorescence radiation is measured. In the past, AFS has been used for elemental analysis. It has better sensitivity than many atomic absorption techniques, and offers a substantially longer linear range. However, despite these advantages, it has not gained the widespread usage of atomic absorption or emission techniques. The problem in AFS has been to obtain a... 

Organic dye lasers are especially useful for nanosecond absorption spectroscopy since their wavelength can be tuned continuously over a range of several hundred angstroms and their output pulses reach several megawatt at pulsewidths down to 10" sec with mode-locking techniques. 

Direct synthesis of metal-substituted zeolites has long been sought. However, since the post-synthesis modifications can be made under wide-ranging conditions (temperature, solvent, atmosphere, pH, etc.) far from those for the zeolite synthesis, the modifications of zeolites present us with powerful indirect methods for manipulating the properties of zeolites. Therefore, the fine-tuning of the properties of zeolites will continue to be achieved by developing various post-synthesis modification procedures as well as direct synthetic techniques. 

The use of self-assembly techniques in molecular electronics has proven to be useful, as shown by the many publications cited. We expect the field to continue to develop and mature as researchers fine-tune their procedures and new methods are developed. Processes refined for the molecular electronics field will find applications in other nanotechnology areas the reverse will also be true. Thus, as it will be beneficial for those in the solid-state microelectronics field to look toward molecular electronics for solutions to their problems, it will also be beneficial for those in the field of self-assembled molecular electronics to look outside that narrow range of technology for potential solutions to their problems. The coming years will surely see many exciting developments. 

---