Amplifiers – With maser-type amplifying device
Reexamination Certificate
2000-04-12
2003-02-04
Lee, Benny T. (Department: 2817)
Amplifiers
With maser-type amplifying device
C333S017200, C333S156000
Reexamination Certificate
active
06515539
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to device capable of interacting with and modifying at least one characteristic of incident microwave radiation. More particularly this invention is directed to an optically pumped multi-component chemical system capable of producing high electron spin polarization at room temperature and the use of such system in maser construction.
BACKGROUND OF THE INVENTION
This invention relates to the use of a special class of chemical systems, which, upon light excitation in an external magnetic field, significantly change their magnetic permeability in the microwave region. The invention makes possible the construction of new classes of microwave devices, including ultra low noise microwave amplifiers, phase shifters with very low losses and electromagnetic devices for the protection of sensitive receivers from strong microwave pulses.
The generation of high controllable magnetic permeability in paramagnetic materials was long ago recognized as being important for variety of microwave devices (Wittke). The original treatment of Wittke dealt with the change in the electron spin population of the Zeeman levels as a mean to perturb the electromagnetic (EM) radiation in the matter, mainly to amplify it. The population difference in the Zeeman levels (&Dgr;N) can be related directly to the material's magnetic permeability, which is more naturally used in relation to various EM applications by the following expressions:
The magnetic permeability is defined in cgs units by:
&mgr;=1=4&pgr;&kgr; (1)
where &kgr;=&khgr;/&rgr; is the volume magnetic susceptibility &khgr; is the mass magnetic susceptibility per gram, and &rgr; is the material density. It is also known that for a magnetic transition in 2S+1 energy levels of a spin system (McMillian):
P
(
m
S
)=&Dgr;
N·h
&ngr;·&rgr;(
m
S
)=&Dgr;
N·h
&ngr;·(&pgr;/4)&ggr;
2
H
1
2
(
S+m
S
)(
S−m
S
+1)
f
(&ngr;−&ngr;
0
) (2)
where P(m
s
) is the power absorbed by spin system (m
S−1
to m
S
), &ngr; is the microwave frequency, &ggr; is the electron gyromagnetic ratio, H
1
is the magnetic part of the microwave field, f(v−v
0
) is the normalized absorption/emission line shape function, which depends on the frequency v and attains its maximum at &ngr;
0
and p(m
S
) is the probability of transition per time unit. The power at frequency &ngr;, absorbed by a magnetic system with an imaginary part of the volume magnetic susceptibility, &kgr;″, which is the physical parameter important for practical applications is:
P
(
m
s
)=&pgr;&ngr;&kgr;″
H
1
2
(3)
This expression can be either positive or negative, depending on the sign of &kgr;″, implying the ability to absorb or amplify microwave radiation. Thus, in terms of eq. 2, &kgr;″ is expressed as a function of &Dgr;N between the magnetic levels:
κ
″
=
Δ
N
4
·
h
γ
2
(
S
+
m
s
)
·
(
S
-
m
s
+
1
)
f
(
v
-
v
0
)
(
4
)
The permeability (&mgr;) of common paramagnetic systems is very close to unity, implying that &kgr;″~0, with a negligible effect due to microwave excitation. For applied and practical purposes, &kgr;″ must be of the order of 0.001-0.01 in the microwave frequencies relevant for the present applications as determined by the external magnetic field. These values ensure that for applications such as phase shifters or microwave amplifiers, the change in the microwave power, i.e., absorption or emission (cf eq. 2) due to interaction with the paramagnetic material is large enough. For example, in the case of the microwave amplifier, the amplification due to &kgr;″, must be much larger than the dielectric losses in the material which always exist. The “threshold values” for &kgr;″ listed above, depend upon the specific microwave structure in which the material is inserted and the material dielectric properties. The values of &kgr;″ (0.01-0.001) are typical representative figures known for microwave amplifiers (Yariv) which are similar to the measured and calculated values of the present systems (see below).
Since paramagnetic materials at room temperature have relatively very low magnetic permeability (&Dgr;N is very small), they can not be exploited for practical purposes. Thus one must either go to very low temperatures, or use some pumping mechanism to increase the population difference in the magnetic levels, and by that, to increase the magnetic permeability. The solid state microwave masers, which were common in the 1950's and 1960's used both cooling and microwave pumping to achieve relatively high permeability (Orton et al). These masers can operate only at very low temperatures, typically in the order of 2 K (−271 ° C.) or even less, a restriction that precluded a widespread use of these devices as amplifiers. In the 1970's, a new type of solid state amplifier based on the field effect transistor (FET), followed in the 1980's by the high electron mobility transistor (HEMT) appeared. Amplifiers based on these transistors can operate at cryogenic temperatures with similar noise performance achieved by solid state maser amplifiers, at least up to frequencies of several GHz. With these transistors, and the improved technology of solid state electronics, masers were gradually removed from the scene. However, it is important to note that even today, masers are used in some specific applications where noise performance is crucial, such as radio astronomy (Glass) and in astronomic radars (for example, the Arecibo Planetary Radar for radio astronomy).
The restriction of low temperature operation in the solid state masers of the 1960's was mainly due to two reasons. First, the pumping mechanism was in the microwave region. Thus, in order to create high population inversion, k
b
T (Boltzmann constant multiplied by the temperature) must be as small as possible, compared to h&ngr; (Planck constant multiplied by the frequency of radiation). The second reason was that the solid state materials, which were used as the active material in the masers, have very steep dependence of the spin relaxation time of the magnetic levels upon temperature. Thus at higher temperatures, the fast relaxation can not permit efficient pumping of the magnetic levels, and the population of the levels does not change much. Using optical excitation to pump the magnetic levels can solve the first problem. These are called solid state optically pumped masers (Anderson et al U.S. Pat. No. 3,736,518). With optical excitation one can, in principle, operate the maser at higher temperatures, as the pumping is done with a much larger h&ngr;. However, again, the second constrain of short relaxation time of the magnetic levels begins to be of importance and the improvement in the temperature of operation is very small (operation in about 10 K instead of 2 K).
In parallel to these efforts, in the 1960's and 1970's, there were some new discoveries which showed how one can produce high population inversion in paramagnetic materials by chemically induced process, named CIDEP (chemically induced dynamic electron polarization (Muus et al)). These processes were related to the production of paramagnetic species in chemical reactions with non-Boltzmann spin populations, known as electron spin polarization. Although the initial discoveries were made in relation to non-reversible chemical reactions, later studies demonstrated this phenomenon in photophysical reversible light induced processes. In the late 1980's, a new observation of electron spin polarization generated through the interaction of photoexcited triplets and stable radicals in solution. This reversible physical process is called RTPM (radical triplet pair mechanism) and results in very high population inversion in the stable radical (Blättler et al). The reversibility of the process and the ability to control the radical polarization by m
Blank Aharon
Levanon Haim
Barnes & Thornburg
Lee Benny T.
Yissum Research Development Company of The Hebrew University of
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