Metrological characterization of custom-designed 8946 nm VCSELs for miniature atomic clocks

F Gruet,E Kroemer,R Boudot,R Michalzik,D Wahl,D Miletic,A Al-Samaneh,L Bimboes,C Affolderbach,G Mileti

doi:10.1364/oe.21.005781

Abstract

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 μW. The VCSEL frequency noise is 10(13) · f(-1) Hz(2)/Hz in the 10 Hz < f < 10(5) 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 τ = 1 s and few 10(-12) at τ = 10(4) s integration time..

Highlights

Over recent years, coherent population trapping (CPT) physics [1] combined with micro-electromechanical systems (MEMS) fabrication techniques and semiconductor lasers has allowed the development of miniature atomic clocks exhibiting a low power consumption (≈ 150 mW), a volume of 10–15 cm3 and frequency stability performances better than 10−11 at 1 hour to 1 day of integration time [2]
We report on the characterization and validation of customdesigned 894.6 nm vertical-cavity surface-emitting lasers (VCSELs), for use in miniature Cs atomic clocks based on coherent population trapping (CPT)
We have studied the spectral properties of custom-designed VCSELs emitting at 894.6 nm, in view of their suitability for the development of miniature atomic clocks based on CPT

Summary

Introduction

CPT physics [1] combined with micro-electromechanical systems (MEMS) fabrication techniques and semiconductor lasers has allowed the development of miniature atomic clocks exhibiting a low power consumption (≈ 150 mW), a volume of 10–15 cm and frequency stability performances better than 10−11 at 1 hour to 1 day of integration time [2]. By etching a mesa down to the first layer pairs of the n-type DBR and applying a polyimide passivation layer that contains a circular hole for a Au-electroplated n-via, the n-contact becomes accessible from the epitaxial side of the chip This chip design applies the so-called inverted grating relief [10] by which favorable single-mode and polarization-stable laser emission is achieved.

Standard VCSELs

Grating relief VCSELs

Laser noise and dynamics

Conclusions