Search

We have produced a sample of laser-cooled atoms in a micro-fabricated alkali vapor cell using a grating MOT to direct the beams. We show that by detecting the blue fluorescence resulting from a two-photon cascade transition, we improve the rejection of cooling light scattered from the grating.

We have developed a cold atomic beam coherent population trapping clock with the goal of creating a compact, low power device. The clock employs traditional two-zone Ramsey interrogation performed on the D2 line of an atomic beam of 87Rb generated from a 2D+-MOT. The current fractional frequency instability is $2.5 \times 10^{-11}\tau^{-1/2}$ . A preliminary evaluation of systematics and noise limiting the current performance is presented.

When performing precision measurements, the quantity being measured is often perturbed by the measurement process itself. This includes precision frequency measurements for atomic clock applications carried out with Ramsey spectroscopy. With the aim of eliminating probe-induced perturbations, a method of generalized auto-balanced Ramsey spectroscopy (GABRS) is proposed. Here, the usual local oscillator frequency control loop is supplemented with a second control loop derived from secondary Ramsey sequences interspersed with the primary sequences and with a different Ramsey period. This second loop feeds back to a secondary clock variable and ultimately compensates for the perturbation of the clock frequency caused by the measurements in the first loop. We show that such a two-loop scheme can lead to perfect compensation of measurement-induced light shifts and does not suffer from the effects of relaxation, time-dependent pulse fluctuations and phase-jump modulation errors that are typical of other hyper-Ramsey schemes.

Point source interferometry (PSI) is a promising technique that could lead to a compact, high-performance gyroscope based on atom interferometry. We consider the trade-offs in size and performance with PSI. In particular, we discuss the sensitivity and dynamic range for a simple PSI gyroscope with an evacuated volume ranging from 1 mm3 to 10 cm3. We also discuss the stability required for the initial atomic distribution in order to achieve part-per-million scale factor stability.

We demonstrate a Brillouin microresonator laser operating at 1560 nm that is frequency doubled and stabilized to Rubidium to improve the microresonator's drift by 1000×, reaching a fractional frequency stability of 4.8×10−12 at 1000 s.

We describe progress at NIST to develop a series of “chip-scale”, SI-traceable electromagnetic reference instruments based on microfabricated alkali vapor cells. We anticipate being able to realize the second, the meter, the volt, the ampere and the kelvin using microfabrication techniques and spectroscopic measurement of alkali atom energy levels. We report here on initial fabrication efforts to develop optical wavelength references in which atomic vapor cells are integrated with single-mode photonics.

An upgrade in the laser system for our cold-atom clock based on coherent population trapping is described that makes use of an electro-optic modulator to significantly improve the phase coherence of the interrogation spectrum relative to what was achieved with our previous phase-locked loop approach. With this improvement in phase coherence, we have observed a reduction in the light shift by over two orders of magnitude. We present the impact of this improvement on the performance of the clock, for which the light shift was previously dominated by the incoherent light in the laser spectrum. We also demonstrate a new optical setup to reduce the Doppler shift.

We present an atom interferometer based on an expanding cloud of laser-cooled atoms sensitive to rotations along two axes and acceleration along one axis in an effective volume of 1 cm3. We observed spatially resolved fringes created by the apparatus' rotation by imaging the expanding cloud after short free-fall durations. If the atom cloud does not start as a point source, a bias is introduced in the scale factor that differs from the simple point-source limit. We explored the scale factor deviation experimentally with different initial cloud sizes and present our understanding of this important systematic.

Over the last decade, miniature instruments based on microfabricated alkali vapor cells have emerged as a compelling technology for achieving small size and low-power operation while retaining much of the high precision afforded by the use of atomic spectroscopy. Chip-scale atomic clocks1 are now a commercial reality2 and achieve frequency instabilities below 10–11 at one hour of integration while consuming only 120 mW of power, 30 times less than any pre-vious commercial atomic clock. This improvement in the power consumption is enabled mainly by the use of a vertical-cavity surface emitting laser as the light source and by the small size and good thermal isolation of the physics package.

A compact cold-atom clock based on coherent population trapping (CPT) has been developed. The clock typically demonstrates a short-term fractional frequency stability of 4×10 -11 /√τ, limited by frequency noise on the interrogation lasers. The clock interrogates a sample of 2×10 5 atoms under free-fall with typical cycle and Ramsey periods of 50 ms and 16 ms, respectively. The largest two systematic frequency shifts that can limit the clock's long-term stability are the light shift and the Doppler shift.

A compact cold-atom clock based on coherent population trapping (CPT) is being developed. Long-term goals for the clock include achieving a fractional frequency accuracy of 1×10-13 in a package of less than 10 cm3 in volume. Here we present an overview of a prototype clock design, and a systematic evaluation of the first-order Doppler shift. We also introduce our second-generation physics package.

We demonstrate atom number enhancement in a magneto-optical trap (MOT) by use of bichromatic cooling to slow an atomic beam that is loaded into a MOT. Bichromatic cooling employs stimulated emission to apply strong cooling forces that are not limited by spontaneous emission. We demonstrate a factor of 3.5 increase in atom number captured from the atomic beam for a 1.5 cm cooling length. For a 1.5 cm effective cooling length, our technique yields MOT atom number enhancement that is about three times higher than the enhancement acheived via spontaneous emission.

We describe the progress towards the realization of a cold-atom frequency standard based on coherent population trapping (CPT). We explain our particular CPT configuration and give details on the experimental setup.

The Atomic Devices and Instrumentation Group at NIST Boulder is working with the MEMS and Nanotechnology Exchange (MNX) to develop an Atomic MEMS process technology module for Chip Scale Atomic Devices (CSADs). Such devices are of continuing research interest as a key technology in a number of U.S. Defense Advanced Research Projects Agency (DARPA) programs and are being investigated for use in advanced frequency referencing, magnetometry and inertial sensing. The pursued module is designed to reduce development time and cost in CSAD research programs by providing a proven process to fabricate the elements commonly needed in most CSAD projects at the wafer level. The integrated platform provides thermal isolation, cell heating and electromagnetic control forming a ‘smart’ package for miniature atomic vapor cells. This paper describes the design of the package made available through the MNX module. The device will able to thermally stabilize a vapor cell at 150°C with less than 5 mW of heater power. In addition, a system is currently under development at NIST Boulder designed to allow wafer-level, parallel fabrication of micromachined atomic vapor cells. The fabrication system will be suitable for use in the R&D fabrication facilities, such as those comprising the MNX network.

For cold samples of laser-cooled atoms to be useful in emerging technologies such as compact atomic clocks and sensors, it is necessary to achieve small sample sizes while retaining a large number of cold atoms. Achieving large atom numbers in a small system is a major challenge for producing miniaturized laser-cooled atomic clocks, since the number of captured atoms in a vapor-cell magneto-optical trap (MOT) scales as the fourth power of the laser beam diameter [1]. This strong dependence on size is fundamentally set by the maximum spontaneous light force ħkγ/2, where ħk is the photon momentum and γ/2 is the maximum spontaneous photon scatter rate of a saturated transition of linewidth γ. We are attempting to surmount the limit imposed by spontaneous emission by using bichromatic cooling [2] — a technique that uses stimulated emission to slow the atoms. We have built a table-top experiment that uses stimulated-emission bichromatic cooling to pre-cool rubidium atoms and dramatically enhance the trappable atom number in a small MOT. The apparatus lets us test how bichromatic cooling scales with miniaturization. Here we report on our first experimental results of cooling a thermal beam of rubidium atoms down to MOT capture velocities.

Atomic magnetometers based on absorption or polarization rotation of light in an alkali vapor have recently demonstrated sensitivities rivaling those of superconducting quantum interference devices (SQUIDs) [1]. Miniaturization of such devices containing vapor cells fabricated with micro—electro—mechanical (MEMS) technology has been the focus of development for the better part of the last decade. In this paper, we describe a portable magnetometry system with a sensitivity below 50 fT/√Hz at 100 Hz. The atomic magnetometer consists of a microfabricated sensor head that is fiber coupled to a control module consisting of a laser and electronics. We describe the construction of this system and present the results of sensitivity measurements with an emphasis on identifying and characterizing the source of 1/f (flicker) noise. This portable magnetometer system was developed to measure magnetocardiograms (MCG) of human subjects inside a shielded environment [2].

We describe the main features of a cold atom frequency standard based on coherent population trapping (CPT). We explain our particular CPT configuration and our experimental setup, and present simulations of the expected performance (stability and major systematics).

We summarize recent results at NIST to develop high-performance yet highly miniaturized magnetic sensors based on atomic vapors confined in microfabricated alkali vapor cells. These sensors currently achieve sensitivities in the range of a few tens of femtotesla in a 1 Hz bandwidth in detection volumes of about 1 mm3. They require no cooling and only a small amount of heating, making them ideal for deployment in remote, power starved locations.

We present initial measurements of an atomic magnetometer in which the microfabricated sensor head is coupled to the control system by optical fibers. Because there are no electrical connections, the sensor head is largely non-magnetic, and degradation of the magnetometer performance due to internal magnetic fields is avoided. The sensor head is fabricated by use of methods derived from micro-electromechanical (MEMS) systems and integrated with off-the-shelf micro-optical components. Light is coupled through fibers to and from the sensor head for interrogation of the atoms as well as temperature stabilization of the vapor cell.

We describe recent work at NIST to develop compact, sensitive atomic magnetometers using a combination of precision optical spectroscopy, atomic physics and techniques of micro-electro-mechanical systems (MEMS). These instruments have sensor head volumes in the range of a few cubic millimeters but are capable of sensing magnetic fields of a few picotesla per root-hertz in the earthpsilas field and as weak as 70 femtotesla per root-hertz in a shielded, low-field environment. We discuss the design, fabrication and testing of several of these devices and propose several applications for which such instruments might be of use.

This paper reports on a method for improving the optical efficiency of micromachined reflectors integrated in rubidium vapor cells. A hybrid bulk micromachining and multilayer thin film process is used to form the reflectors on angled sidewalls, which redirect laser light through the vapor cell and back toward the plane of the source with reduced optical power loss. The optical return efficiency of two paired dielectric reflectors is shown to be improved by as much as eight times over silicon reflectors alone. The D1 absorption line in a one cubic millimeter miniature alkali 87 Kb vapor cell by use of two integrated thin film reflectors is experimentally demonstrated.

We describe recent progress in the development of millimeter-scale instruments based on alkali atom vapor cells implemented with microfabrication techniques. Because of their small size and correspondingly low power requirements, these "chip-scale" atomic clocks and magnetometers have the potential to bring atomically precise instrumentation to portable, battery-operated systems such as GPS receivers, remote sensors and wireless communication devices. In addition, wafer-level processing and assembly potentially allows for very low cost per instrument if high volumes are produced.

We demonstrate a simple setup for observing very high contrast coherent population trapping resonances in rubidium - 87. The high contrast is achieved by eliminating the 52 P 1/2 DC-light background through polarization and spectral filtering.

We demonstrate self-injection locking of a low-power, compact microwave oscillator by use of feedback generated from four-wave mixing in an atomic vapor. The four-wave mixing process creates an effective microwave filter with extremely high off-resonant signal suppression. This type of locking results in a shorter locking time, increased short-term stability, improved close-in phase noise, and reduced spurious signals as compared to current techniques used in miniature atomic clocks. Other possible benefits and drawbacks are mentioned.

This paper presents an application of micro glass blowing, in which multiple glass spheres are simultaneously shaped on top of a silicon wafer and subsequently filled with rubidium. The fabrication process is based on etching cavities in silicon, followed by anodic bonding of a thin glass wafer to the etched silicon wafer. The bonded wafers are then heated inside a furnace at a temperature above the softening point of the glass, and due to expansion of the heated trapped gas in the cavities, the glass is blown into three-dimensional spherical cells. Microscopic alkali vapor cells are achieved by evaporation of 87Rb through a small glass nozzle into the cell cavities. The cells are then sealed by anodic bonding. The results of the cell fabrication and characterization are presented.

This paper demonstrates a simple technique to significantly improve the long-term frequency stability in atomic clocks based on coherent population trapping (CPT). In this technique, a servo is used to control the local oscillator power level in such a way that the optical spectrum generates no net light shift. This ensures that the clock frequency is always given by the atomic resonance frequency that is not perturbed by the incident light fields

We describe the first local oscillator (LO) that demonstrates viability in terms of performance, size, and power, for chip-scale atomic clocks (CSAC) and has been integrated with the physics package at the National Institute of Standards and Technology (NIST) in Boulder, CO. This voltage-controlled oscillator (VCO) achieves the lowest combined size, DC power consumption, phase noise, and thermal frequency drift among those previously reported, while achieving a tuning range large enough to compensate for part tolerances but small enough to permit precision locking to an atomic resonance. We discuss the design of the LO and the integration with the NIST physics package.

An optically pumped magnetometer was drastically miniaturized, by taking advantage of MEMS techniques, producing the chip-scale atomic magnetometers (CSAM) physics package. The key component of the package is an alkali vapor cell. To probe the magnetic field experienced by the atoms, the injection current to the VCSEL was modulated at 3.4 GHz near half the hyperfine frequency of 87Rb. Lock-in detection is used to determine the center of the resonance, and the magnetic field is determined by counting the modulation frequency, which is related to the Larmor precession frequency under these locked conditions. The ITO heaters create a large magnetic field gradient, which broadens the resonance and reduces the sensitivity.

We demonstrate component-level functionality of the three critical subsystems for a miniature atomic clock based on microfabrication techniques: the physics package, the local oscillator and the control electronics. In addition, we demonstrate that these three components operating together achieve a short-term frequency instability of 6times10-10/radictau, with a total volume below 10 cm 3 and a power dissipation below 200 mW

We report on our progress in developing a prototype chip-scale atomic clock and describe the integration of the local oscillator and the physics package. A discussion of the design and results is given with a focus on frequency stability, size, and power consumption. Finally, we suggest methods to improve reliability.

We report recent improvements on our microfabricated atom-optical magnetometer. Using a semiconductor laser and 1 mm3 alkali vapor cell, our magnetometer detects a magnetic field at 50 pT / Hz1/2 sensitivity in a 12 mm3 package.

We compare the performance of two chip-scale atomic clock physics packages, one based on excitation using the D/sub 2/ line of Cs and the other using the D/sub 1/ line of /sup 87/Rb.

MEMS technology is used to shrink atomic clocks from their present-day table-top sizes down to only 1 cm/sup 3/ while at the same time retaining timing stability on the order of 30 ns over one hour. The power consumption of the system is less than 30 mW.

We summarize the development of microfabricated atomic frequency references at NIST. The physics packages of these devices have volumes near 10 mm/sup 3/ and power dissipation below 150 mW, and can potentially achieve a fractional frequency instability in the range of 10/sup -11/ over long periods. A fully integrated frequency reference could find application in portable, battery operated devices such as global positioning system (GPS) receivers and cellular telephones.

The physics package of a vertically integrated chip-scale atomic clock, based on cesium, has recently been demonstrated at NIST. This device requires 69 mW of electrical power to maintain the vapor cell 34 K above the temperature of the baseplate. The physics package structure is analyzed by use of analytical thermal modeling and finite-element calculation. Improvements to the design are proposed to reduce the power consumption of the physics package alone to near 15 mW and of a full chip-scale atomic clock to below approximately 30 mW. Power consumption at this level will open the door to the use of atomic frequency references in portable, battery-operated applications such as wireless communications and global positioning.

We report on the fabrication of millimeter-sized vapor cells and their performance in atomic clocks based on coherent population trapping (CPT). We discuss two fabrication techniques. The first one is based on hollow-core pyrex fibers, fused with a CO/sub 2/ laser or micro-torch, and the second one involves anodic bonding of micro-machined silicon wafers to pyrex. Key aspects of the discussion are the performance of the cell in frequency references, the potential for further miniaturization of the cells and the ability to manufacture them on a large scale with reproducible performance.

We consider theoretically the expected performance of ultra-small vapor-cell frequency references with cell volumes below 1 mm/sup 3/. The short-term stability is found to degrade as the size is reduced. The amount of degradation depends on whether a buffer gas or wall coating is used for the atomic confinement. Preliminary experimental results with all-optical excitation indicate that the atomic Q-factor behaves as expected.

Experimental results are presented on a hybrid optical/electronic oscillator which uses an optical delay line to generate high spectral purity microwave signals. At Fourier frequencies above 10 kHz, the single sideband (SSB) phase noise spectrum decreases as roughly 1/f/sup 2/ attaining a value of -138 dB below the carrier in a 1 Hz bandwidth (dBc/Hz) at 20 kHz offset. The origin of this noise is in part the fundamental shot noise on the light itself, although other optical noise sources such as double Rayleigh scattering and stimulated Brillouin scattering also appear to be important under certain operating conditions. The frequency stability over several hours is dominated by changes in the fiber ambient temperature with a coefficient of about 10/sup -5//K originating mostly from changes in the fiber refractive index with temperature.

The Primary Atomic Reference Clock in Space (PARCS) project is a joint NIST-JPL-University of Colorado venture aimed at placing a Cs atomic clock aboard the International Space Station (ISS). This orbiting clock will achieve high accuracy, in part due to the long Ramsey times afforded by the microgravity environment, and allow for precision tests of fundamental physics including relativity theory. As part of this effort, we are evaluating the characteristics of a prototype cold Cs source based on launching atoms from an optical molasses. Experimental results, in conjunction with theoretical modeling of atom flux requirements, will be applied to the design and construction of a robust, space-qualified device. The apparatus described here will be used to develop other PARCS components such as the microwave cavity structure and detection systems, and to investigate two-dimensional cooling schemes for future Cs fountains and space clocks.

Dark-line resonances in Cs vapor are studied for potential use in a compact, low-power microwave frequency reference. The resonance is excited using light from a VCSEL and detected using the change in DC absorption, resulting in an extremely simple physics package with very high tolerance to misalignment and potentially low acceleration sensitivity. We anticipate engineering the package developed into a device with a volume less than 1 cm/spl times/1 cm/spl times/2 cm and a power dissipation of much less than 100 mW, depending on the thermal environment.