Vertical-cavity lasers for miniaturized atomic clocks

Atomic clocks are radio frequency sources that generate stable electrical signals with fixed frequencies. Interest in miniaturized atomic clocks—with volumes of a few cubic centimeters—has increased as their accuracy and stability have improved. They are capable of operating with low power consumption and long-term instabilities below ∼10⁻¹¹ per day. In contrast, comparable quartz-based clocks are much larger, less accurate, and consume more power. Miniaturized atomic clocks are used in the precision time protocol, which is an approach to distribute synchronization over next-generation Internet protocol-based packet networks. They can also be useful in civil and military applications, as well as global positioning system receivers, which lock more rapidly to satellites to provide faster acquisition of precise positioning information. Research into miniaturized atomic clocks has been primarily occurring in the United States.^{1, 2}

However, in 2008 the European Commission-funded microelectromechanical systems (MEMS) atomic clocks for timing, frequency control, and communications (MAC-TFC) initiative began. Here, we describe our advances in vertical-cavity surface-emitting lasers (VCSELs) for use in miniaturized atomic clocks.

Miniaturized atomic clocks generally comprise a laser source, rubidium or cesium vapor cell, photodiode (which receives the laser light transmitted through the vapor cell), tunable oscillator, and intelligent electronics. Miniaturized atomic clocks employ a quantum mechanical effect called coherent population trapping,³ which requires illuminating the cesium atom with two coherent laser beams with a certain frequency spacing and center wavelength. Among the allowed optical transitions in the cesium atom, the so-called D1 line has a frequency spacing of about 9.2GHz and preferred center wavelength of ∼894.6nm. Rather than employing two coherent light sources, one can harmonically modulate the current of the laser (with the center wavelength) with a frequency of 4.6GHz. One then makes use of the sidebands generated from the modulation.

VCSELs are compelling light sources for atomic clocks because they simultaneously meet the requirements of operating at 65–80 ^°C while emitting a low-noise, narrow-linewidth, single-mode, and single-polarization laser beam. Their sub-milliampere threshold currents are favorable for low power consumption. Hybrid integration with the clock microsystem is straightforward, unlike designs using regular edge-emitting laser diodes. VCSELs emitting at 894.6nm have been developed and employed in prototype atomic clocks.⁴ However, these may suffer from polarization instability during clock operation, i.e., the polarization of the light output can switch from one orientation to another.

We designed our VCSELs to employ surface gratings to guarantee stable, linearly polarized light output.⁵ We grew our VCSELs using solid-source molecular beam epitaxy on n-doped (100)-oriented gallium arsenide (GaAs) substrates. The active region contains three indium/gallium arsenide (InGaAs) quantum wells and is positioned in an optical cavity between n- and p-doped Al_0:2Ga_0:8As/Al_0:9Ga_0:1As-distributed Bragg reflectors. To maximize compactness in the clock microsystem, we soldered the laser chip upside down on a carrier that provides the electrical signal. Thus, we fabricated flip-chip-bondable VCSEL chips where both the p- and n-contacts are accessible from the epitaxial side of the chip (see Figure 1).

For polarization control, we etched a surface grating in the top GaAs layer of the upper p-doped Bragg mirror.⁶ The surface grating reflects parallel and orthogonally polarized light (relative to the grating lines) differently. Hence, there are different threshold gains for both orientations. The lower threshold gain determines the polarization of the laser light. For so-called inverted grating VCSELs,⁷ we added an extra, top GaAs quarter-wave anti-phase layer. Specifically, we used inverted gratings with quarter-wave etch depth, 0.6μm grating period, and 50% duty cycle. These grating parameters are favorable in terms of low threshold current, low diffraction loss, and high orthogonal polarization suppression ratio.⁷ The emission of these devices is always polarized orthogonally to the grating lines, independent of the etch depth. Figure 2 shows the light out-coupling facet of our inverted grating VCSEL.⁸

Having designed and fabricated our VCSELs, we tested their performance characteristics. Figure 3 shows their polarization-resolved spectra. As expected from our design, the polarization is orthogonal to the grating lines. At a current of 2.9mA, the side-mode suppression ratio is almost 30dB. Additionally, the peak-to-peak difference between the dominant and the suppressed polarization modes is as high as 40dB. These results show the presence of a single-mode, single-polarization laser beam, and are thus suitable for stable atomic clock operation.

Cesium-based MEMS atomic clocks require VCSELs with modulation bandwidths of at least 5GHz. To test whether our VCSELs meet this criterion, we measured the small-signal modulation response curves (see Figure 4). We obtained a sufficient 3dB bandwidth of 5.7GHz at only 0.5mA bias current, which was just 0.25mA above threshold. The maximum bandwidth is about 12GHz.

In summary, we have presented some static and dynamic properties of 894.6nm VCSELs for cesium-based miniaturized atomic clocks. We employed integrated inverted gratings for polarization control. Our VCSELs are polarized orthogonally to the grating lines with sufficient suppression of the competing polarization, even at elevated substrate temperatures. The devices consume very little power since the required modulation bandwidth of about 5GHz is reached above the sub-milliampere threshold currents. In future work, we will integrate our VCSELs with atomic clock microsystems with our MAC-TFC project partners.

The authors would like to thank Y. Men for performing the electron-beam lithography and M. T. Haider and T. Purtova for their technical assistance with microwave measurements. This work is funded by the European Commission within the Seventh Framework Programme (grant agreement number 224132). Marwan Bou Sanayeh is supported by the German Academic Exchange Service (DAAD).