Ruby Maser:

As was know ferrites of certain materials have atomic systems that can be made to resonate magnetically at frequencies dependent on the atomic structure of the material and the strength of the applied magnetic field. When such a resonance is stimulated by the application of a signal at that frequency, absorption will take place, as in the Resonant Absorption ferrite isolator. Alternatively, emission will occur, if the material is suitably excited, or pumped, from another source. It is upon this behavior that the Ruby Maser is based.

The material itself may be gaseous, such as ammonia, or solid-state, such as Ruby Maser. Ammonia was the original material used, and it is still used for some applica­tions, notably in the so-called atomic clock frequency standards. Extreme is the correct word to use in describing the stability of such an oscillator. The atomic clock built at Harvard University in 1960 has a cumulative error which would cause it to be incorrect by only 1 second after more than 30,000 years! From the point of view of microwave amplification, ammonia gas suffered from the disadvantage of yielding amplifiers that worked at only one frequency and whose bandwidth was very narrow. This description will therefore be aimed mainly at the ruby maser.

Fundamentals of Operation:

The electrons belonging to the atoms of a substance can exist in various energy levels, corresponding to different orbit shells for the individual atoms. At a very low temperature, most of the electrons exist in the lowest energy level, but they may be raised by the addition of specific amounts of energy. Quantum theory shows that a quantum, or bundle of energy, may provide the required energy to raise the level of an electron, provided that

where

• E = energy difference, joules
• f = photon frequency, Hz
• h = Planck’s constant = 6.626 x 10^-34 joule . s

Having been excited by the absorption of a quantum, the atom may remain in the excited state, but this is most unlikely to last for more than perhaps a microsecond. It is far more likely that the photon of energy will be re-emitted, at the same frequency at which it was received, and the atom will thus return to its original, or ground, state. The foregoing assumes, incidentally, that the re-emission of energy has been stimulated of the expense of absorption. This may be done by such measures as the provision of a structure resonant at the desired frequency and the removal of absorbing atoms, as was done in the original gas maser.

It is also possible to supply energy to these atoms in such quantities and at such a frequency that they are raised to an energy level which is much higher than the ground state, rather than immediately above it. This being the case, iris then possible to make the atoms emit energy at a frequency corresponding to the difference between the top level and a level intermediate between the top level and the ground state. This is achieved by the combination of the previously mentioned techniques (the cavity now resonates at this new frequency) and the application of an input signal at the desired frequency. Pumping thus occurs at the frequency corresponding to the energy differ­ence between the ground and the top energy levels. Reemission of energy is stimulated at the desired frequency, and the signal at this frequency is thus amplified. Practically no noise is added to the amplified signal. This is because there is no resistance in­volved and no electron stream to produce shot noise. The material that is being stimu­lated has been cooled to a temperature only a few degrees above absolute zero. It now only remains to find a substance capable of being stimulated into radiating at the frequency which it is required to amplify, and low-noise amplification will be ob­tained.

The original substance was the gas ammonia, while hydrogen and cesium fea­tured prominently among the materials used subsequently. The gaseous substance had the advantage of allowing absorbing atoms to be removed easily. Since the operating frequency was determined very rigidly by the energy levels in ammonia, the range of frequencies over which the system operated, i e its bandwidth, was extremely narrow (of the order of 3 kHz at a frequency of approximately 24 GHz). There was no method whatsoever of tuning the maser, so that signals at other frequencies just could not be amplified. To overcome these difficulties, the traveling-wave ruby maser was invented. This explanation was greatly simplified, especially that of the solid-state maser. Also, some slight liberties with the truth had to be taken in order to present an overall picture that is essentially correct and understandable.

The Ruby Maser:

A gaseous material is inconvenient in a maser amplifier, as can be appreciated. The search for more suitable materials revealed Ruby Maser, which is a crystal­line form of silica (Al₂O₃) with a slight natural doping of chromium. Ruby Maser has the advantages of being solid, having suitably arranged energy levels, and being paramag­netic, which virtually means “slightly magnetic.” This last property is due to the presence of chromium atoms, which have unpaired electron spins. These are capable of being aligned with a dc magnetic field, and this permits not only reradiation of energy from atoms in the desired direction but also some tuning facilities.

Figure 12-36 shows the energy-level situation in a three-level maser, introduced in the previous section. Energy at the correct pump frequency is added to the atoms in the crystal lattice of ruby, raising them to the uppermost of the levels shown (there are many other levels, but they are of no interest here). Normally, the number of electrons in the third energy level is smaller than the number in the ground level. However, as pumping is continued, the number of electrons in level 3 increases until it is about equal to the number in the first level. At this point the crystal saturates, and so-called Population Inversion has been accomplished.

Since conditions have been made suitable for re-radiation (rather than absorption) of this excess energy, electrons in the third level may give off energy at the original pump frequency and thus return to the ground level. On the other hand, they may give off smaller energy quanta at the frequency corresponding to the difference between the third and second levels and thus return to the intermediate level. A large number of them take the latter Course, which is stimulated by the presence of the cavity surrounding the Ruby Maser, which is resonant at this frequency. This course is further aided by the presence of the input signal at this frequency. Since the amount of energy radiated or emitted by the excited Ruby Maser atoms at the signal frequency exceeds the energy applied at the input (it does not, of course, exceed the pumping energy), ampli­fication results.

The presence of the strong magnetic field (typically about 4 kA/m) has the effect of providing a difference between the three desired energy levels that Corre­sponds to the required output frequency.. Any adjustment of this magnetic field will alter the energy levels of the ferrous chromium atoms and therefore provide a form of tuning. This is similar to the situation in ferrites, where it was found that a change in the dc magnetic field changed the frequency of Paramagnetic Resonance. This field strength can be altered to permit the ruby maser to be operated over a frequency range, from below 1 to above 6 GHz. For frequencies as high as 10 GHz and above, other materials are often used. Rutile is a very common alternative; this is titanium oxide (TiO₂) with a light doping by iron. At the higher frequencies, the required magnetic fields tend to be rather strong, so that the magnet is very often cooled also, to take advantage of superconductivity and therefore to give a reduction in the power required to maintain the magnetic field.

In order to consider the effect of cooling the Ruby Maser with liquid helium (which is almost invariably done) it is helpful to consider Figure 12-37. Figure 12-37a shows the situation at room temperature. Cooling with liquid nitrogen down to only 77 K can also be used, but it results in an increase in noise and a reduction in gain. It is seen that because of the relatively high energy possessed by the electrons at this temperature, quite a number of electrons normally exist in the fourth level, apart from the three so far mentioned. This has the undesirable effect of reducing the number of electrons in the ground level. There are fewer electrons whose energy level can be raised from the first to the third, and consequently fewer electrons that can re-radiate their excess energy at the correct frequency. The high temperature is said to mask the maser effect. If cooling is applied, the overall energy possessed by the electrons is reduced, as is the number of electrons at the fourth level. As seen in Figure 12-37b there are now an adequate number of electrons that can be jumped from the ground to the third level and then down again to the intermediate level. Maser action is maintained. Note that no maser has operated satisfactorily at room temperature. Even if such operation were possible, the noise level would be raised sufficiently to make the noise figure of the maser a very poor second to that of the parametric amplifier.

The noise figure of the cooled ruby maser is governed by the same factors as that of the ammonia maser and is therefore equally low. There is the slight noise due to the random motion of electrons in the Ruby Maser (caused by the fact that the temperature of the crystal is above absolute zero). However, most of the noise is due to the associated components, such as the waveguide leading from the antenna, and the noise created at the input to the following amplifier. The first of these problems may be reduced by making the waveguide. run as short as possible. This involves mounting the maser at the prime focus of the antenna. Such a solution is practicable only if a Cassegrain or folded horn antenna is used, and in fact that is done in practice. The problem of noise from succeeding stages is alleviated in a number of ways. One involves cooling the circulator (which must sometimes be used), in the same way as in a parametric amplifier. It is also possible to increase the gain of the maser, thereby reducing noise reflected from succeeding stages, by making it a two-stage amplifier. The amplifier following the maser can be made a relatively low-noise one, by the use of tunnel diodes or FETs.