Optical frequency combs (OFCs) have been revolutionizing numerous fields in metrology and spectroscopy. So far, self-referenced OFCs have been mainly based on modelocked fiber or solid-state lasers at wavelengths imposed by the respective gain materials. VECSELs have a large flexibility in their emission wavelength offered by bandgap engineering, making them ideally suited for applications in spectroscopy and sensing. In addition, VECSELs can easily operate at GHz repetition rates, thus enabling a high power per comb mode and the ease of access to individual optical lines. Multi-GHz OFCs are also advantageous for low-noise RF generation by optical-to-microwave frequency division, because it enables operation at lower noise levels in the photo-detection of the comb pulse train.
In this presentation, we will first review the initial work on the detection, noise characterization and stabilization of the carrier envelope offset (CEO) frequency of VECSELs. Then we discuss future application areas, such as their use in low noise microwave generation. In the traditional approach of locking the OFC to an ultrastable reference laser, the achieved phase noise of the generated microwave directly depends on the properties of the optical lock. This is a challenge for VECSEL combs, which currently exhibit higher noise than state-of-the-art ultrafast fiber or bulk lasers. A new RF-generation scheme appears promising for this task, in which a free-running or RF-locked OFC acts as a transfer oscillator. The method does not require any optical lock of the OFC and circumvents the need for high bandwidth actuators.
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