Alan Brannon

A Local Oscillator for Chip-Scale Atomic Clocks at NIST

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

HBAR-Based 3.6 GHz oscillator with low power consumption and low phase noise

We have designed and built 2 oscillators at 1.2 and 3.6 GHz based on high-overtone bulk acoustic resonators (HBARs) for application in chip-scale atomic clocks (CSACs). The measured phase noise of the 3.6 GHz oscillator is -67 dBc/Hz at 300 Hz offset and -100 dBc/Hz at 10 kHz offset. The Allan deviation of the free-running oscillator is 1.5 times 10-9 at one second integration time and the power consumption is 3.2 mW. The low phase noise allows the oscillator to be locked to a CSAC physics package without significantly degrading the clock performance.

Design method for low-power, low phase noise voltage-controlled oscillators

This paper presents a design method for voltage controlled oscillators (VCOs) with simultaneous small size, low phase noise, DC power consumption and thermal drift. We show design steps to give good prediction of VCO phase noise and power consumption behavior: (1) measured resonator frequency-dependent parameters; (2) transistor additive phase noise/ noise figure characterization; (3) accurate tuning element model; and (4) bias-dependent model in case of an active load. As an illustration, the design of a 3.4-GHz bipolar transistor VCO with varactor tuning is presented Oscillator measurements demonstrate low phase noise (-40dBc@ 100Hz and better than -lOOdBcfflOkHz) with power consumption on the order of a few milliwatts with a circuit footprint smaller than 0.6cm2. The temperature stability is found to be better than +/-10ppm/degC from -40degC to +30degC. The oscillators are implemented using low-cost off-the-shelf surface-mountable components, including a micro-coaxial resonator. The VCO directly modulates the current of a laser diode and demonstrates a short-term stability 2-10/radictau Bias of clock. when locked to a miniature Rubidium atomic clock.

Advances in chip-scale atomic frequency references at NIST

Coherent population trapping (CPT) resonances usually exhibit contrasts below 10% when interrogated with frequency modulated lasers. We discuss a relatively simple way to increase the resonance contrast to nearly 100% generating an additional light field through a nonlinear four-wave mixing interaction in the atomic vapor.1 A similar method can also be used to create a beat signal at the CPT resonance frequency that can injection-lock a low-power microwave oscillator at 3.4 GHz directly to the atomic resonance.2 This could lead to chip-scale atomic clocks (CSACs) with improved performance. Furthermore, we introduce a miniature microfabricated saturated absorption spectrometer3 that produces a signal for locking a laser frequency to optical transitions in alkali atoms. The Rb absorption spectra are comparable to signals obtained with standard table-top setups, although the rubidium vapor cell has an interior volume of only 1 mm3 and the volume of the entire spectrometer is around 0.1 cm3.

Component-level demonstration of a microfabricated atomic frequency reference

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

Measuring Transistor Large-Signal Noise Figure for Low-Power and Low Phase-Noise Oscillator Design

This paper presents an experimental method for determining additive phase noise of an unmatched transistor in a stable 50-Omega environment. The measured single-sideband phase noise is used to determine the large-signal noise figure of the device. From the Leeson-Cutler formula and a known oscillator circuit with the characterized transistor, the phase noise of the oscillator can be predicted. The method is applied to characterization of several bipolar devices around 3.4 GHz, the frequency of interest for miniature rubidium-based atomic clock voltage-controlled oscillators.

Long-term stability of NIST chip-scale atomic clock physics packages

We discuss the long-term stability of the NIST chip-scale atomic clock (CSAC) physics packages. We identify the major factors that currently limit the frequency stability of our CSAC packages after 100 s. The requirements for the stability of the vapor cell and laser temperature, local magnetic field, and local oscillator output power are evaluated. Due to the small size of CSAC physics package assemblies, advances MEMS packaging techniques for vacuum sealing and thermal isolation can be used to achieve the temperature stability goals. We discuss various ideas on how to aid temperature control solutions over wide variations in ambient temperature by implementing atom-based stabilization schemes. Control of environment-related frequency instabilities will be critical for successful insertion of CSACs into portable instruments in the areas of navigation and communication.

System-level integration of a chip-scale atomic clock: microwave oscillator and physics package

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.

Chip-scale atomic devices at NIST

We provide an overview of our research on chip-scale atomic devices. By miniaturizing optical setups based on precision spectroscopy, we have developed small atomic sensors and atomic references such as atomic clocks, atomic magnetometers, and optical wavelength references. We have integrated microfabricated alkali vapor cells with small low-power lasers, micro-optics, and low-power microwave oscillators. As a result, we anticipate that atomic stability can be achieved with small size, low cost, battery-operated devices. Advances in fabrication methods and performance are presented.