The miniature microwave clocks being developed at NIST are passive, vapor-cell references based on coherent population trapping (CPT). In most vapor-cell frequency references, which do not use CPT, the minimum size of the clock physics package is determined in part by the cavity that confines the microwaves used to excite the atoms. In order to be resonant, this cavity is usually larger than one-half the wavelength of the microwave radiation used to excite the atomic resonance. For hydrogen, cesium and rubidium, this wavelength is of the order of several centimeters, clearly posing a problem for the development of vapor-cell references for portable applications. Another, possibly more important, implication of the large physics package size is the power required to maintain the cell and cavity at the required temperature. It is desirable for frequency references to be useful over a reasonably wide range of temperatures. In addition, the atomic transition frequency typically has a modest dependence on the temperature of the cell. As a result, the cell temperature must be stabilized to some fixed value. If a large difference in temperature exists between the cell and the environment, a significant amount of power may be required to maintain the cell at its fixed temperature. The power depends on the size of the cell and can be as high of several watts for cells with volumes of the order of 1 cm3. Cell assemblies with smaller sizes are therefore advantageous from the point of view of power dissipation.
Because the CPT technique does not require a microwave field to be applied to the atoms in the cell, it is free from the limitations due to the wavelength of the microwave radiation. The performance of the clock therefore scales uniformly as the size is reduced from above the wavelength of the microwave radiation to below. As a result, it is possible to design and build frequency references far smaller than would ordinarily be possible with direct microwave excitation. In addition, vapor-cell frequency references based on CPT are in general simpler to implement because of the absence of a microwave cavity. Clocks based on CPT have been shown to have a stability comparable to that achieved in conventional, microwave-excited references, and some advantages in long-term stability may be gained through reduced Stark shifts. For these reasons, CPT appears to be the method of choice for small atomic clocks.
The overall design of our atomic clocks is shown in the figure above. The injection current of a diode laser is modulated at a frequency equal to one-half of the ground-state hyperfine splitting of the atoms. This modulation (a combination of FM and AM) produces sidebands on the optical carrier separated from the carrier by the modulation frequency. The two first-order sidebands are therefore separated by a frequency equal roughly to the atomic resonance frequency. These two first-order sidebands form the L optical configuration needed to excite the CPT resonance. The modulated light is passed through the atomic vapor, and the transmitted power is detected with a photodiode. As the modulation frequency is swept over the first subharmonic of the atomic resonance, a change in the transmission through the cell is observed. This change in transmission can then be used to determine how far the modulation frequency is from the atomic oscillation frequency, and to correct the modulation frequency as needed.
Physical design of an atomic frequency reference based on CPT
excitation of atomic hyperfine transitions.
The components required to excite the resonance are shown in the figure at left. The light is produced with a vertical-cavity surface-emitting laser (VCSEL), lasing near one of the optical transitions in an alkali atom. A VCSEL is used for several reasons. First, it has low threshold and operating currents, leading to low power dissipation. Second, VCSELs are usually designed to have high modulation bandwidths (because of the very small cavity size), which means that the optical sidebands on the carrier created by the modulation can contain a substantial amount of optical power even with a relatively small amount of RF modulating power. Finally, the vertical emission is advantageous with regard to our vertically integrated clock design.
Light from the VCSEL is conditioned by the optics subassembly, which consists of a collimating lens, a neutral-density filter and a quarter-wave plate. At the output of the optics assembly is a collimated light beam with circular polarization and an intensity of roughly 0.1 mW/cm2. The light then passes into the cell sub-assembly, which contains the vapor cell and two heater units to heat the cell to the required operating temperature (typically near 80 °C). The vapor cell is a sealed container, fabricated from glass and Si, that holds the alkali atoms along with a buffer gas to narrow the atomic hyperfine transition. Finally, a Si PIN photodiode detects the transmitted radiation.
For more information on the fabrication of individual components, the assembly of components into a frequency reference or the performance of the final device, please refer to the links provided.
Contact: Svenja Knappe
References:
CPT excitation of atomic hyperfine transitions by means of a modulated semiconductor laser.