Source backward wave oscillator

An extremely sensitive MODR scheme, microwave optical polarization spectroscopy (MOPS), was introduced by Ernst and Torring (1982). The most important features of MOPS are that it requires respectively 100 and 10 times lower laser and microwave intensities than MODR and results in 10 times narrower lines. This means that it will be possible to take full advantage of differential power broadening effects (Section 6.5.1) and to utilize low-power, frequency-doubled dye lasers and low-power, broadly tunable microwave sources (backward wave oscillators) in order to gain access to and systematically study perturbations. 

A family of vacuum-tube MMW sources is based on the propagation of an electron beam through a so-called slow-wave or periodic structure. Radiation propagates on the slow-wave structure at the speed of the electron beam, allowing the beam and radiation field to interact. Devices in this category are the traveling-wave tube (TWT), the backward-wave oscillator (BWO) and the extended interaction oscillator (EIO) klystron. TWTs are characterized by wide bandwidths and intermediate power output. These devices operate well at frequencies up to 100 GHz. BWOs, so called because the radiation within the vacuum tube travels in a direction opposite to that of the electron beam, have very wide bandwidths and low output powers. These sources operate at frequencies up to 1.3 THz and are extensively used in THZ spectroscopic applications [10] [11] [12]. The EIO is a high-power, narrow band tube that has an output power of 1 kW at 95 GHz and about 100 W at 230 GHz. It is available in both oscillator and amplifier, CW and pulsed versions. This source has been extensively used in MMW radar applications with some success [13]. 

Here we present results of high-field tunable-frequency ESR studies of CuGe03, which were done at the National High Magnetic Field Laboratory, Tallahassee, FL. A key feature of the employed technique is a combination of a 25 T high homogeneity resistive magnet and a set of easily-tunable sources of mm- and submm- wave radiation, Backward Wave Oscillators. 

We shall develop the theory necessary to understand quasioptics, but before that, it will be useful to consider factors that influence the choice of spectrometer components such as the magnet, the source, and the detector. In Section II we will give a brief review of the performance and characteristics of homodyne detectors. In our discussion of sources, we will discuss vacuum oscillators, such as the reflex klystron and backward wave oscillator, and solid-state sources, such as the Gunn diode. We will also discuss useful criteria for selecting a magnet. 

In this configuration, the duplexer also isolates the source from the deleterious effects of back-reflected power. Such a form of protection is crucial for high powered sources such as extended interaction oscillators (Wong, 1989) or backward wave oscillators. We see, then, that our polarization-coding techniques have a number of advantages over conventional methods of duplexing. 

Krzystek et al.245 described a methodology based on backward wave oscillator sources in high frequency and field EPR which has been applied to the study of the complex [Co(N3)(Tp Bu)]. 

The most commonly encountered MMW sources are now the Gunn and Impatt devices although the Backward Wave Oscillator (BWO) is still used for wideband spectroscopic studies. The Gunn and Impatt devices exhibit the property of negative resistance that makes them well suited as MMW oscillators. 

Without question, the optically pumped far-infrared laser has provided the main technological impetus for this phase of maturity. Notwithstanding the importance of backward-wave oscillators and the gradual development of high-frequency solid-state sources, the lasers have made possible a plethora of investigations otherwise too difficult or too costly to perform. 

Microwave spectroscopy uses tunable coherent sources of radiation such as microwave synthetizers, solid state oscillators (Gunn diodes) or electronic tubes (klystrons). These oscillators can be operated in their fundamental mode (up to 120 GHz) but harmonic generation is commonly realized with frequency multipliers up to 500 GHz, and has been used to reach 1 THz on occasions. Backward wave oscillators are available up to 1.2 THz in their fundamental mode. Figures 1 and 2 show typical rotational spectra recorded with this type of sources. Different techniques can be used to work in the THz region ... 

A typical microwave spectrometer is illustrated in Fig. 2. The essential elements of a microwave spectrometer are a microwave source, absorption cell, detection system, and a system for measuring the source frequency. Microwave sources—the klystron and, more recently, the backward-wave oscillator (BWO)—generate a very narrow band of frequencies so that the source is essentially monochromatic. Furthermore, the source frequency can be conveniently varied and is often phase stabilized to give good frequency stability. 

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