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Journal of Applied Physics

Optical characterization of a vertical cavity surface emitting laser for a coherent population trapping frequency reference

Christopher M. Long, Kent D. Choquette

Published Feb 1, 2008• 18 citations
Publisher

Abstract

Characterization of a vertical cavity surface emitting laser source for a frequency reference that uses coherent population trapping with atomic rubidium is reported. The frequency reference requires a low-noise laser that operates in a single transverse mode at 794.7nm. We show that biasing the laser at low output power and modulating it at higher frequency than its low relaxation oscillation frequency are necessary to obtain symmetric modulation sidebands and improve the long-term performance of the frequency reference.

DOI
10.1063/1.2838175

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Characterization of a vertical cavity surface emitting laser source for a frequency reference that uses coherent population trapping with atomic rubidium is reported. The frequency reference requires a low-noise laser that operates in a single transverse mode at 794.7nm. We show that biasing the laser at low output power and modulating it at higher frequency than its low relaxation oscillation frequency are necessary to obtain symmetric modulation sidebands and improve the long-term performance of the frequency reference.
The spectroscopic technique of coherent population trapping (CPT) enables an all-optical interrogation of the groundstate hyperfine splitting of cesium (or rubidium), compared to the optical-microwave double resonance technique conventionally employed in atomic frequency standards. All-optical interrogation enables the reduction of the size and power consumption of an atomic clock by two orders of magnitude, and vertical-cavity surface-emitting lasers (VCSELs) are preferred optical sources due to their low power consumption and circular output beam. Several research teams are currently using VCSELs for DARPA's chip-scale atomic clock (CSAC) program with the goal of producing an atomic clock having a volume < 1 cm^3, a power consumption < 30 mW, and an instability (Allan deviation) < 1x10^-11 during a 1-hour averaging interval. This paper describes the VCSEL requirements for CPT-based atomic clocks, which include single mode operation, single polarization operation, modulation bandwidth > 4 GHz, low power consumption (for the CSAC), narrow linewidth, and low relative intensity noise (RIN). A significant manufacturing challenge is to reproducibly obtain the required wavelength at the specified VCSEL operating temperature and drive current. Data are presented that show the advantage of operating at the D1 (rather than D2) resonance of the alkali atoms. Measurements of VCSEL linewidth will be discussed in particular, since atomic clock performance is especially sensitive to this parameter.
We investigate a versatile vertical cavity surface-emitting laser (VCSEL) with a slight optical feedback from a simple wedge positioned at a well determined distance from the VCSEL. We use such a laser in conjunction with a miniature Rb atomic cell to detect coherent population trapping (CPT) when the external cavity mode spacing frequency corresponds to half the hyperfine frequency of Rb. The feedback greatly enhances the rf modulation efficiency of the VCSEL and as a result we obtain high contrast CPT signals at an input of −16dBm of rf modulation power compared to +1.25dBm needed in the absence of the external cavity.
A low-cost, potentially compact and robust microwave frequency reference can be constructed by use of vertical-cavity surface-emitting lasers and coherent population-trapping resonances in Cs vapor cells. Fractional frequency instabilities of 2×10-11/τ/s have been achieved with a minimum of 7×10-13 at τ=1000 s. The performance of this device as a function of external parameters such as light intensity, optical detuning, and cell temperature is discussed. The dependence of the dark-line resonance signal on these parameters can be understood largely by means of a simple, three-level model. The short-term stability depends critically on the optical detuning, whereas the long-term stability is limited currently by line shifts due to drifts in cell temperature.
We report system-level evaluation of manufacturable, quantum-optimized 795-nm vertical-cavity surface-emitting lasers (Q-VCSELs) for rubidium (Rb) atomic sensors, with emphasis on integration into an 87Rb coherent population trapping (CPT) clock prototype. Operation at 795 nm (Rb D1) is constrained by increased internal loss/absorption, polarization instability, and stringent requirements on spectral purity. At the component level, we employ epitaxial and layout engineering to improve mode discrimination, polarization robustness, and high-temperature efficiency, achieving stable single-transverse-mode emission and linear polarization from 20–80 °C with side-mode suppression ratio (SMSR) >30 dB and polarization extinction ratio (PER) >20 dB. The devices exhibit low threshold current (<400 µA at 80 °C) and slope efficiency >0.5 mW/mA, enabling >1 mW single-mode, single-polarization output power in chip-scale form factors. Quantum-relevant characterization shows linewidth on the order of ~30–35 MHz and relative intensity noise below −130 dB/Hz at frequencies greater than 1KHz. We then integrate the Q-VCSEL into an 87Rb CPT clock setup and demonstrate a clear CPT resonance, achieving ~2.5% contrast and ~3 kHz CPT linewidth (FWHM), corresponding to a measured figure of merit of ~8.2 ppm. We further map key operating tradeoffs relevant to clock optimization, including optical power broadening (contrast peaking near ~50–60 µW while CPT linewidth increases with optical power) and RF-drive dependence across representative modulation powers. These results connect device-level laser characteristics to clock-level observables and provide a manufacturable component pathway for scalable Rb-based timing modules.
With two vertical-cavity surface-emitting lasers working under the master-slave sideband injection-locking configuration, we have realized a quasi-bichromatic laser beam with residual phase noise Δφ(2) < 0.282 rad(2). The two wanted frequency components share more than 96% power of the beam. With the realized beam, we have carried out coherent population trapping (CPT) resonance experiment with (87)Rb in the lin[perpendicular]lin CPT scheme, and recorded CPT resonance signal with contrast of 60%. Such laser system is promising to realize a lin[perpendicular]lin CPT clock with high performance and low power consumption.
We have performed a series of experiments demonstrating the use of a vertical-cavity surface-emitting laser for nonlinear spectroscopy on the cesium D2 transition at 852 nm wavelength. Due to the high modulation efficiency of the VCSEL sufficiently strong modulation sidebands at 9.2 GHz frequency can be produced by direct modulation of the laser injection current. Using, the carrier and one of the modulation sidebands coherent population trapping (CPT) resonances in a buffered cesium vapor can be prepared with linewidths of only 60 Hz. For application of these narrow resonances to precision magnetometry we find a sensitivity of 10 pT in 0.3 s integration time. For the use of our setup as a frequency reference a relative stability of 1 • 10 − 11 / τ is predicted. In both applications of CPT resonances the use of VCSELs as light sources helps to build compact and reliable devices.