Absorbing waveguides absorbed power

Waveguide absorbers (in waveguides for power transmission above 1000 Me) for the separation of part or the complete absorption of the incoming power... 

When the applied magnetic field is swept to bring the sample into resonance, MW power is absorbed by the sample. This changes the matching of the cavity to the waveguide and some power is now reflected and passes via the circulator to the detector. This reflected radiation is thus the EPR signal. 

Schottky barrier varactors do not store charge at all. When driven into forward conduction the Schottky diode absorbs energy and the multiplier efficiency is increased, as long as it is not overdriven to a point where rectification occurs, that is when the MMW power is converted into DC. For a discussion of these devices and their application the text by Maas is recommended. The Schottky barrier varactor multiplier is thus the preferred choice for doublers and triplers at MMW frequencies. Like the other devices they would normally be bought in their waveguide structures with the tuning stubs to match the device impedance to the source and load. 

In a termination, power is absorbed by a length of lossy material at the end of a shorted piece of transmission Hne (Fig. 4.24(a) and Fig. 4.23(c)). This type of termination is called a matched load as power is absorbed and reflections are very small irrespective of the characteristic impedance of the transmission line. This is generally preferred as the characteristic impedance of transmission lines varies with frequency, particularly so for waveguides. When the characteristic impedance of a hne does not vary much with frequency, as is the case with a coaxial hne, a simpler smaller termination can be reahzed by placing a resistor to ground (Fig. 4.24(b)). 

The optical power limits in other materials have not been as extensively studied. In GaAs, a MZM has been operated with 10 mW (Roussell, 1997), but the upper limit is unknown. It has been reported that more than 40 mW can propogate in an InGaAsP/InP waveguide (Yu, 1995). In this case also the upper limit is unknown. However, the upper limit for the EA modulator is expected to be lower than the MZM because when off, the EA modulator is absorbing aU of the optical power, whereas the MZM disperses the optical power out of the guide and into the substrate when off. 

Fig. 6-2 Squiggly lines denote the fraction T of ray power lost to the absorbing cladding from a bound ray at (a) the reflection points on a step-profile waveguide and (b) the turning points of a graded-profile waveguide.

When the waveguide is only slightly absorbing, as is normally the case in practice, we can obtain approximations to the power attenuation coefficient in explicit form. In one approach, the imaginary part of the propagation constant is determined by setting i) j in the eigenvalue equation for the... 

If the core and cladding materials of the waveguide are slightly absorbing, the ray power attenuation coefficient of Eq. (6-3) is identical to the modal power attenuation coefficient of Table 11-2, page 232, provided the lateral shift is included. Details are presented in Section 36-10. 

In this section we compare the attenuation of modal power due to slightly absorbing media with the corresponding attenuation of ray power. For simplicity we consider a step-profile planar waveguide of core width 2p, and, if superscripts r and i denote real and imaginary parts, the core and cladding refractive indices are = njj, + ini and ci = "cl "ci> respectively, where The modal propagation constant... 

The planar material to be processed 1s Inserted In a slot at the centre of the broad dimension along the waveguide length and absorbs an amount of energy from the travelling microwave field dependent upon the effective loss factor of the Insertion. Any remaining power Is absorbed by the matched water load, with very little energy reflected back towards the source. 

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