Metrological characterization of custom-designed 894.6 nm VCSELs for miniature atomic clocks

We report on the characterization and validation of custom-designed 894.6 nm vertical-cavity surface-emitting lasers (VCSELs), for use in miniature Cs atomic clocks based on coherent population trapping (CPT). The laser relative intensity noise (RIN) is measured to be 1×10⁻¹¹ Hz⁻¹ at 10 Hz Fourier frequency, for a laser power of 700 <i>μ</i>W. The VCSEL frequency noise is 10¹³ <i>ƒ </i>⁻¹ Hz²/Hz in the 10 Hz < <i>ƒ </i> < 10⁵ Hz range, which is in good agreement with the VCSEL’s measured fractional frequency instability (Allan deviation) of ≈ 1 × 10⁻⁸ at 1 s, and also is consistent with the VCSEL’s typical optical linewidth of 20–25 MHz. The VCSEL bias current can be directly modulated at 4.596 GHz with a microwave power of −6 to +6 dBm to generate optical sidebands for CPT excitation. With such a VCSEL, a 1.04 kHz linewidth CPT clock resonance signal is detected in a microfabricated Cs cell filled with Ne buffer gas. These results are compatible with state-of-the-art CPT-based miniature atomic clocks exhibiting a short-term frequency instability of 2–3×10⁻¹¹ at <i>τ</i> = 1 s and few 10⁻¹² at <i>τ</i> = 10⁴ s integration time.