Method Of Minimizing The Short-Term Frequency Instability Of Laser-Pumped Atomic Clocks
Original Publication Date: 2005-Oct-24
Included in the Prior Art Database: 2005-Oct-24
National Institute of Standards and Technology
John Kitching: INVENTOR [+4]
The present invention optimizes the performance of laser-pumped atomic frequency references by minimizing the noise source originating from optical pumping. This method is based on a new understanding of FM-AM conversion and optical pumping process wherein the frequency reference short-term instability is minimized when (a) the laser frequency is turned nominally a few tens of MHz away from the center of the atomic absorption line, and (b) the modulation frequency of the servo used to lock the external oscillator is set either far below or far above the inverse of the optical pumping time of the atoms. The exact parameters for the optimization depend on the particular experimental situation. In one embodiment these parameters can be approximately calculated using a model simulating the clock performance. The model is described in J. Kitching, L Hollberg, S. Knappe and R. Wynands, Opt. Lett, 26, 1507, 2001 and J. Kitching, H.G. Robinson, L. Hollberg, S. Knappe and R. Wynands, J. Opt. Soc. Am. B 18. 1676, 2001), the entire contents on both of which are hereby incorporated by reference as if fully set forther herein.
A METHOD OF MINIMZING THE SHORT -TERM FREQUENCY INSTABILITY OF LASER-PUMED ATOMIC CLOCKS
CROSS REFERENCE TO RELATED APPLICATIONS
5 This application is a non-provisional application of provisional application
number 60/303,911 filed on July 7,2001, which is hereby incorporated by reference as if fully set fort herein.
BACKGROUND OF THE INENTION
1. Field of the Invention
10 This invention relates to laser-pumped atomic clocks, and more paricularly, to a
method of optimizing the performance of laser-pumped atomic frequency references with
respect to optical pumping noise by detung the laser frequency and adjustig other
controllable operating parameters.
2. Background of the Invention
15 Frequency references with high stabilty are required for modem, high-speed
communcations systems and similar applications. Atomic frequency references or stadards are based on the energy difference between two levels of a quatu system. In an atom, for example, quatu mechancs requies that the electrons can only exist in certain states with specific, discrete energies. Differences between the energies of these 20 states define correspondingly specific frequencies that are, to a high degree, similar for every atom, and therefore atoms make good frequency references.
A dipole moment, oscilating at one of these frequencies, can be excited by an electromagnetic wave propagating in the same space as the atom. Frequency references are available that employ an excitation scheme in which microwave fields excite the
atoms of a sample. When the microwave frequency exactly corresponds to the atomic
oscillation frequency, a change in the atomic state occurs which can be detected by measurg the absorption of an optical field propagating simultaeously through the
s All-optical excitation technques have been developed, in which no microwaves are applied directly to the atoms. Instead the injection curent of a laser is modulated with an external oscilator to produce two optical fields separated by the atomic oscilation frequency, and ths laser field is passed though the atomic system. When the difference frequency' of the two optical fields is near the atomic oscilation frequency, a 10 change in the absorption of the field by the atoms occurs. Ths change in absorption, due to a phenomenon called coherent population trapping (CPT), can be used to lock the external oscilator frequency to the atomic transition. This locked frequency provides the output of the frequency reference and has the long-term stability and repeatabilty
inherent to the.
is Hence, all available frequency standards generate a detection signal that
quatifies a resonant interaction between an incident electro-magnetic radiation and a
The shift in atomic energy levels due to applied electrc fields is well known as the Stark effect, but for electric dipole transitions between states of well defin...