Passive Atomic Frequency Standards

1
Passive Atomic Frequency Standards

• W.J. Riley

2006 IEEE International Frequency Control
Symposium Miami, Florida USA Short Course June
4, 2006
Rev B 06/10/06
2
Introduction
A passive atomic frequency standard (AFS) is the
most common type of atomic clock. It uses a
crystal oscillator or other frequency source to
excite a passive atomic discriminator that
produces a correction signal in a frequency
control loop to lock the oscillator to the atomic
reference. The crystal oscillator provides the
stable output frequency.
3
Basic Block Diagram
Basic Block Diagram of a Passive Atomic
Frequency Standard
4
Passive AFS Types

• Passive atomic frequency standards are usually
  categorized by their physics package type
• Rubidium Gas Cell
• Cesium Beam Tube (Classic or Laser
  Pumped/Detected)
• Passive Hydrogen Maser
• Trapped Mercury Ion
• Cs or Rb Fountain
• Optical Standard (Several Species)
• Coherent Population Trapping (CPT)
• Chip-Scale Atomic Clock (CSAC)
• The first two are by far the most widely used,
  and are emphasized in this tutorial.

5
Atomic Resonances
Atomic frequency standards use atomic resonances
that are based on fundamental properties of
nature.
MF
3
2
Electron spin and dipole
? Energy
1
0
2
Nucleus Electron
-1
Closed electronic shell
1
N
Magnetic Field ?
F2
S
?W
2
3
4
X
F1
S
Electron
-1
N
Nuclear spin and dipole
MF
-2
-2
-1
0
1
-3
Hydrogen-like (or alkali) atoms
Hyperfine structure of 87Rb, with nuclear spin
I3/2, ?0?W/h6,834,682,605 Hz
Credit Cutler FCS 2002 Tutorial
6
Physical Methods
 
Microwave Pulses

1. 1.               Confinement refers to the way
   the atoms are located, free of Doppler broadening
   and external disturbances.
2. 2.               Preparation/State Selection
   refers to the way the atoms are prepared for
   interrogation by putting them into a particular
   atomic state.
3. 3.               Interrogation refers to the
   method used to probe the atoms so that a
   discriminator signal is produced.
4. 4.               Detection refers to the way the
   result of the interrogation is observed.
5. Note The term atom is used broadly to also
   include molecules, ions and other particles.

7
Commercial Freq Standards
Technology Intrinsic Accuracy Stability (1s) Stability (floor) Aging (/day) initial to ultimate Cost
Hydrogen Maser (Active) 10-11 10-13 10-15 10-15 to 10-16 150X
Cesium Beam 10-13 10-11 10-14 nil 20X
Passive H Maser 10-10 10-12 5x10-15 10-15 40X
Rb Gas Cell 10-9 10-11 10-13 10-11 to 10-13 X
Hi-Quality Qz 10-6 to 10-8 10-12 10-12 10-9 to 10-11 0.5X
Passive AFS covered in this tutorial
Credit M. Garvey/Symmetricom
8
Rubidium Frequency Standards
Rubidium Gas Cell Passive Atomic Frequency
Standards
9
Physics Package
The physics package is the atomic discriminator
in the AFS frequency lock loop. It most commonly
contains alkali atoms (Cs or Rb) within a
physical structure that allows the atoms to be
confined, put into a particular atomic state,
interrogated by resonant radiation, and the
resulting interaction detected. This interaction
is based on the atoms undergoing transitions
between two atomic energy levels, which
correspond to a particular frequency. The classic
physics package designs are the rubidium gas
cell and the cesium beam
tube
Size D Battery
Size Shoe Box
Symmetricom DRCBT Cs Beam Tube
PerkinElmer RFS-10 Rb Physics Package
10
Rb Physics Package
Rubidium Gas Cell Physics Package
11
Optical Pumping
Rubidium frequency standards use optical pumping
by a Rb spectral lamp to create a non-equilibrium
population difference between the two ground
state hyperfine energy levels. This allows the
hyperfine frequency to be measured by
interrogating the atoms with microwave radiation
and observing the change in light transmission
through the cell.
12
Hyperfine Filtration
The efficiency of the optical pumping is enhanced
by a fortuitous overlap between the optical
absorption lines of the two naturally-occurring
isotopes, 85Rb and 87Rb. This is the main reason
that rubidium is used in most gas cell atomic
frequency standards. The filter cell can be
separate or integrated with the absorption
(resonance) cell.
13
Rb Lamp
The Rb electrodeless discharge lamp and its RF
exciter are important parts of a rubidium
frequency standard. The lamp materials and
processing is critical for long life under the
conditions associated with its hot glass envelope
and plasma. The exciter must provide reliable
starting and stable running conditions.
Mounted PerkinElmer RAFS Rb Lamp
Rb Lamp Operating in Temex Neuchatel Time
Rubidium Frequency Standard
14
Rb Gas Cell
A rubidium gas cell is a glass enclosure
containing Rb-87 or natural Rb (72 Rb-85, 28
Rb-87) , and an inert buffer gas (e.g. N2, Ar) or
a mixture thereof. The cell may be used alone (a
resonance cell) or as an absorption cell with a
separate Rb-85 filter cell. The cell is operated
at a stable elevated temperature to establish
sufficient Rb vapor pressure, and the buffer gas
prevents wall collisions that would broaden the
Symmetricom 8130A Resonance Cell. Size 1
diameter x 1 long
resonance line. The cell is inside a microwave
cavity and is surrounded by a coil to produce a
static DC magnetic field (C-field). The rubidium
inside the cell is not consumed, and lasts
infinitely.
15
Microwave Cavity
The 6.8 GHz microwave cavity of a RFS, more than
any other item, determines the size of the
physics package, and much effort has been devoted
toward devising small microwave
cavities/resonators having a suitable field
H-field distribution. The classic TE011 has an
ideal field pattern, but is quite large (the size
of a coffee mug) even if dielectrically loaded,
and various smaller TE111 and magnetron
configurations have been used. More recently, a
significantly smaller capacitively-tuned
resonator has been developed for a very small
commercial RFS.
Credit Ref 2.
TE011 ? 80 cm3
TE111 ? 17 cm3
Jin Resonator ? 1 cm3 U.S Patent No.6,133,800
16
Rb Discriminator Signal
The Rb signal is generated as a change in the
light transmission through the absorption cell in
response to the application of resonant ?W
energy. Optical pumping by the
hyperfine-filtered Rb lamp excites atoms from the
lower ground state to an optical state, from
which they immediately decay to one of the ground
states with equal probability, thus creating a
higher population in the upper ground state.
Equilibrium is restored by the resonant RF, which
allows more light to be absorbed, reducing the
light transmission.
17
Rb Resonance Line
The light transmission is sensed by a
photodetector whose response varies as a
Lorentzian function of the applied microwave
frequency. This resonance line has a width of a
few 100 Hz (Q ? 107). The line slope corresponds
to the amplitude of the fundamental discriminator
signal vs. frequency.
18
Servo Modulation
The Rb resonance is interrogated by applying low
frequency (?150 Hz) FM to the ?W excitation and
observing the resulting AC recovered signal. The
sense of the fundamental component varies
depending on whether the frequency is below or
above the center of the line. At resonance, the
fundamental component is a null, and a 2nd
harmonic component is present.
19
Servo Amplifiers
Analog Servo
Hybrid Servo
Digital Servo
Numeric Servo
20
RFS RF Chain
Modern Symmetricom 8130A RF Chain
Classic Efratom M-100 RF Chain
There are many ways to implement an RFS RF chain.
These two tactical RFS designs are typical of
the classic and modern approaches. They
synthesize the same nominal Rb frequency, but the
newer design has a DDS for high resolution
digital tuning and servo FM, and requires no
tuned circuits or critical adjustments.
21
Rubidium Gas Cell Clocks

• Commercial
• Small Size and Low Cost
• Moderate Performance
• Military/Aerospace
• Environmental Hardening
• Full Performance
• Trend Toward COTS and PEMs
• Space
• High Performance
• High Reliability

Symmetricom X72
Symmetricom 8130
PerkinElmer RAFS
22
RFS Stability
RFS stabilities span 1-2 decades depending on
type of unit and typical versus spec values
23
RFS Design Trends

• Digital Techniques
• DDS Frequency Synthesis (Simplify RF Chain,
  Allow Digital Tuning, Improved Servo
  Modulation and Zeeman Interrogation)
• Microprocessor Control (User Interface,
  Improved Performance)
• Digital Servos (Simplify hardware, Reduce
  Analog Errors)
• RF Microcircuits
• PLL and SSB Mixer Chips (Replace SRD
  Multiplier)
• Emphasis on Small Size and Low Cost
• Commercial Telecom Applications
• Less Emphasis on High Performance
• Used with GPS Syntonization
• New Technologies
• Laser Pumping (Replace Lamp Filter Cell)
• CPT Interrogation (Eliminate Microwave Cavity,
  Use Cs)

24
Cesium Frequency Standards
Cesium Beam Tube Passive Atomic Frequency
Standards
25
Cs Hyperfine Energy Levels
(F, mF) (4,4) (4,3) (4,2) (4,1) (4,0)
(4,-1) (4,-2) (4,-3) (4,-4)
Ground State Hyperfine Energy Levels of Cs-133
9.2
Energy (Frequency) (GHz)
9.192,631,770 GHz
(3,-3) (3,-2) (3,-1) (3,0) (3,1) (3,2)
(3,3)
0
Magnetic Field HO
Energy states at H HO
Credit Cutler FCS 2002 Tutorial
26
Cs Beam Tube
Schematic of a Classic Magnetically Selected
Cesium Beam Tube (CBT)
DC
C-FIELD POWER SUPPLY
MAGNETIC SHIELD
HOT WIRE IONIZER
C-FIELD
B-MAGNET
GETTER
Cs-BEAM
A-MAGNET
CAVITY
GETTER
ION COLLECTOR
PUMP
DETECTOR SIGNAL
VACUUM ENVELOPE
FREQUENCY INPUT 9,192,631,770 Hz
OVEN HEATER POWER SUPPLY
PUMP POWER SUPPLY
DETECTOR POWER SUPPLY
Credit Cutler FCS 2002 Tutorial
27
Magnetic State Selection
Atomic state selection is accomplished in a
cesium beam tube by means of a strong
inhomogeneous magnetic field that deflects the
atoms in the lower and upper hyperfine states
differently.
Cs VAPOR, CONTAINING AN EQUAL AMOUNT OF THE
TWO KINDS OF Cs ATOMS
KIND 1 - ATOMS (LOWER STATE)
S
ATOMIC BEAM
N
MAGNET (STATE SELECTOR)
ATOMIC BEAM SOURCE
Credit HP 5062C Training Manual
KIND 2 - ATOMS (UPPER STATE)
VACUUM CHAMBER
Credit Cutler FCS 2002 Tutorial
28
Cs Atom Detection
Magnetic deflection is also used after the
microwave cavity to detect those atoms that have
undergone a transition in response to their
interaction with the microwave signal. Detection
is then accomplished by a hot wire ionizer and
electron multiplier to produce a current
proportional to the detected signal.
DETECTOR
NO SIGNAL
NO SIGNAL
S
STATE SELECTED ATOMIC BEAM
N
MICROWAVE CAVITY
MAGNET
MICROWAVE SIGNAL (OF ATOMIC
RESONANCE FREQUENCY)
DETECTOR
MAXIMUM SIGNAL
S
STATE SELECTED ATOMIC BEAM
N
MICROWAVE CAVITY
MAGNET
Credit Cutler FCS 2002 Tutorial
29
CBT Resonance Pattern
A wide frequency sweep shows the central
resonance and three Zeeman lines on each side
Linewidth ? 450 Hz Atomic line Q ? 2x107
The central response pattern shows the Ramsey
fringe on top of the Rabi pedestal.
?0 9 192 631 770 Hz
Credit M. Garvey/Symmetricom
30
CBT Cesium Instrument
Cesium Beam Tube
Cesium Instrument
Symmetricom 5071A
Symmetricom 7610
Credit M. Garvey/Symmetricom
Cut-Away View of 5171A CBT and ?W Cavity
CreditZ Tom Van Baak www.leapsecond.com
31
Optically Pumped/Detected CBT
Credit R. Lutwak/Symmetricom
32
Other Atomic Freq Standards

• Other Passive
• Atomic Frequency Standards
• Passive Hydrogen Maser
• Trapped Mercury Ion Standard
• Cesium and Rubidium Fountains
• Chip-Scale Atomic Clock

33
Passive H-Maser
Teflon coated storage bulb
Microwave cavity
Microwave output
Microwave input
The passive H-Maser is a passive version of the
active (oscillating) H-Maser. It acts as a
resonant filter between its microwave input and
output ports.
State selector
Hydrogen atoms
Credit Cutler FCS 2002 Tutorial
34
Trapped Hg Ion Standard
Trappe Mercury 199 In Standard
spherical cloud
linear cloud
Credit Cutler FCS 2002 Tutorial
35
Fountain Standards
NIST F1 Cesium Fountain Standard
Cesium fountain clocks are the current basis for
international standards of time and frequency.
Credit Cutler FCS 2002 Tutorial
36
Laser Cooling of Atoms
1
Direction of motion
Light
Light
Atom
Credit Cutler FCS 2002 Tutorial
Laser cooling can create atoms that move very
slowly (equivalent to mK temperatures). This
virtually eliminates Doppler shifts, and allows
long observation times for high accuracy.
Consider two rays of light slightly lower in
frequency than the atom readily absorbs. One ray
travels in the same direction as the atom, the
other moves in the opposite direction. The atom
is more likely to absorb the photon that is
moving toward it whose frequency is shifted
upward. The atom absorbs the photons momentum,
which opposes its motion and therefore slows
(cools) it.
37
Chip-Scale Atomic Clock (CSAC)
Statement of The Problem Current atomic clocks
are too big, too heavy, and consume too much
power for portable applications. Proposed
Solution DARPA program to develop a 1 cm3, 30 mW
chip-scale atomic clock (CSAC) with a stability
of 1x10-11 at 1-hour. Status Several promising
designs are currently under development. Most
are based on ultra-miniature Cs gas cells using
VCSEL laser excitation and coherent population
trapping (CPT).
Atomic Wristwatch Credit www.leapsecond.com
38
CSAC (Cont)
NIST CSAC Physics Package
Symmetricom CSAC Physics Package
Symmetricom Phase II CSAC Prototype 10 cm3, 100 mW
The NIST and Symmetricom CSAC designs use
microfabricated cesium gas cells, VCSEL laser
diodes, and CPT interrogation. With basic
physical principles verified, the biggest
remaining challenge is realizing a full-featured
electronic design within the size and power goals.
39
Coherent Population Trapping
                                                                                                                                                                              
Dr. John Kim of the U.S Office of Naval Research
holding a 40 cm3, 1 watt Rb Kernco, Inc. atomic
clock based on a CPT physics package
CPT excites a coherence between the hyperfine
ground states with a pair of optical fields
40
Acknowledgments and Thanks

• Len Cutler/Aglient for material from FSC 2002
  Tutorial (note that the original sources for his
  material includes organizations like NIST and
  JPL)
• Material from the NIST web site
• Material from the Symmetricom web site
• Mike Garvey/Symmetricom for material from his
  presentations
• Robert Lutwak/Symmetricom for optical CAFS
  material
• John Vaccaro/PerkinElmer for RFS/RAFS material
• Pascal Rochat/Temex Neuchatel Time for RFS
  picture
• Symmetricom for material from my previous
  presentations
• Miscellaneous credits are shown on slides as
  organization and/or individual names

41
References

1. C. Audoin and B. Guinot, The Measurement of Time,
   Cambridge University Press, 2001, ISBN
   0-521-00397-0 An excellent middle technical
   level book on atomic clocks and timekeeping get
   it from Amazon.com.
2. J. Vanier and C. Audoin, The Quantum Physics of
   Atomic Frequency Standards, Adam Hilger, 1989,
   ISBN 0-85274-433-1 The in-depth Bible for this
   subject.
3. L.S Cutler, Passive Atomic Frequency Standards,
   http//www.ieee-uffc.org/freqcontrol/tutorials/
   Cutler_2002.htm Tutorial at 2002 Frequency
   Control Symposium highly recommended as a
   complement to this tutorial.