Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
2000-06-05
2002-03-19
Scott, Jr., Leon (Department: 2881)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S026000, C372S028000, C372S039000
Reexamination Certificate
active
06359916
ABSTRACT:
RELATED DISCLOSURES
This disclosure is related to the following simultaneously-filed disclosures that are incorporated herein by reference:
Coherent Population Trapping-Based Method for Generating a Frequency Standard Having a Reduced Magnitude of Total a.c. Stark Shift of inventors Miao Zhu and Leonard S. Cutler; Ser. No. 09/588,045,
Coherent Population Trapping-Based Frequency Standard Having a Reduced Magnitude of Total a.c. Stark Shift of inventors Miao Zhu and Leonard S. Cutler 09/587,719; and
Detection Method and Detector for Generating a Detection Signal that Quantifies a Resonant Interaction Between a Quantum Absorber and Incident Electro-Magnetic Radiation of inventors Leonard S. Cutler and Miao Zhu Ser. No. 09/588,032.
FIELD OF THE INVENTION
The invention relates to high-precision frequency standards and, in particular, to atomic frequency standards based on coherent population trapping (CPT).
BACKGROUND OF THE INVENTION
The proliferation of telecommunications based on optical fibers and other high-speed links that employ very high modulation frequencies has led to an increased demand for highly-precise and stable local frequency standards capable of operating outside the standards laboratory. Quartz crystals are the most commonly-used local frequency standard, but in many cases are not sufficiently stable to meet the stability requirements of modern, high-speed communications applications and other similar applications.
To achieve the stability currently required, a frequency standard requires a frequency reference that is substantially independent of external factors such as temperature and magnetic field strength. Also required is a way to couple the frequency reference to an electrical signal that serves as the electrical output of the frequency standard. Potential frequency references include transitions between quantum states in atoms, ions and molecules. However, many such transitions correspond to optical frequencies, which makes the transition difficult to couple to an electrical signal.
Transitions between the levels of certain ions and molecules and between the hyperfine levels of certain atoms have energies that correspond to microwave frequencies in the 1 GHz to 45 GHz range. Electrical signals in this frequency range can be generated, amplified, filtered, detected and otherwise processed using conventional semiconductor circuits.
An early example of a portable frequency standard based on an atomic frequency reference is the model 5060A frequency standard introduced by the Hewlett-Packard Company in 1964. This frequency standard used a transition between two hyperfine levels of the cesium-133 atom as its frequency reference, and had a frequency accuracy of about two parts in 10
11
. Current versions of this frequency standard have an accuracy of about five parts in 10
13
and a stability of a few parts in 10
14
.
Less accurate but smaller frequency standards have been built that use a transition between the hyperfine states of a suitable quantum absorber as their frequency reference. The quantum absorber is confined in a cell located in a microwave cavity.
FIG. 1
is an energy diagram of a simplified quantum absorber. The quantum absorber has a ground state that is split into two groups of sub-states by the hyperfine interaction. At room temperature, all the sub-states in the two groups are approximately equally populated. For convenience, the two groups of sub-states into which the ground state S of the quantum absorber is split by hyperfine interaction will be called the lower ground state |g
1
> and the upper ground state |g
2
>. The upper ground state and the lower ground state are separated by an energy corresponding to an angular frequency &ohgr;
0
in the microwave frequency range. References in this disclosure to frequency should be taken to denote angular frequency.
The quantum absorber additionally has an excited state |e) that is also split by hyperfine interaction into groups of sub-states. The energy differences between the groups into which the excited state is split are small, so the excited state will be treated as a single state in this discussion. The excited state is essentially unpopulated at room temperature.
The quantum absorber is illuminated with monochromatic light having a frequency that corresponds to the energy of the transition between one of the ground states and the excited state. The monochromatic light is conventionally generated by a lamp whose output is filtered to remove all but the desired frequency. For example, consider the transition between the lower ground state |g
1
> and the excited state |e>. These states have energies of E
g0
and E
e0
, respectively. The transition frequency &ohgr;
1
corresponding to the energy of this transition is:
&ohgr;
1
=(
E
g0
−E
e0
)/
h,
where h is Planck's constant divided by 2&pgr;.
When the monochromatic light has a frequency &ohgr;
1
, the quantum absorber can absorb a quantum of the light, which causes the quantum absorber to move from the lower ground state |g
1
> to the excited state |e>. The quantum absorber shown can return from the excited state to either one of the ground states, emitting a quantum of fluorescent light. When the quantum absorber returns to the lower ground state, the monochromatic light can move it back to the excited state. However, when the quantum absorber returns to the upper ground state |g
2
>, the monochromatic light is incapable of moving it back to the excited state. Thus, after one or more absorption/emission cycles, absorption of the incident light and emission of fluorescent light cease because the quantum absorber becomes trapped in the upper ground state. Thus, the monochromatic light creates a population imbalance between the ground states.
Feeding microwave energy into the microwave cavity at a frequency corresponding to the energy difference between the two ground states tends to equalize the populations of the ground states. The change of population causes the absorption of the light transmitted through the cell to increase. The increase can be detected and the resulting detection signal can be used to control the microwave frequency to a frequency at which the absorption of the light transmitted through the quantum absorber is a maximum. When this condition is met, the microwave frequency corresponds to, and is determined by, the energy difference between the ground states. The microwave signal, or a signal derived from the microwave signal, is used as the frequency standard.
The energy difference between the ground states is relatively insensitive to external influences such as electric field strength, magnetic field strength, temperature, etc., and corresponds to a frequency that can be handled relatively conveniently by electronic circuits. This makes the energy difference between the ground states a relatively ideal frequency reference for use in a frequency standard.
More recently, frequency standards have been proposed that use as their frequency reference coherent population trapping (CPT) in the ground states of a quantum absorber. For example, a CPT-based frequency standard is described by Normand Cyr, Michel Têtu and Marc Breton in
All
-
Optical Microwave Frequency Standard: a Proposal,
42 IEEE T
RANS. ON
I
NSTRUMENTATION
& M
EASUREMENT,
640 (April 1993). The structure of a CPT-based frequency standard can be similar to that of the frequency standard described above, but the CPT-based frequency standard uses a semiconductor laser as its light source, and only includes a microwave cavity if coherent emission, to be described below, is detected. The quantum absorber is illuminated with incident light having two main frequency components. Each of the main frequency components has a frequency that corresponds to the energy of the transition between one of the ground states |g
1
> and |g
2
> and the excited state |e> of the quantum absorber. The incident light can be generated using two phase-locked la
Agilent Technologie,s Inc.
Hardcastle Ian
Jr. Leon Scott
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