Narrow Linewidth Lasers:Application To Optical Clocks

Spectrally narrow, ultrastable lasers have a variety of important applications such as optical atomic clocks, high-resolution laser spectroscopy, generation of low phase noise microwave signals, measurements of fundamental physical constants and tests of fundamental physics.In one of its important applications, optical atomic clocks, a narrow linewidth laser source with high frequency stability, called the local oscillator (LO), probes cold atoms in optical lattice sites or a trapped single ion to resolve an ultra-narrow highly-stable transition (clock transition), which is used as a feedback signal to control the frequency of the LO. The frequency stability of optical atomic clocks depends on the frequency noise and frequency stability of the LO, which enables high frequency resolution and a reduced Dick effect, resulting partly from longer interaction time with atoms. In fact the performance of state-of-the-art optical clocks based on neutral atoms is usually limited by an imperfect local oscillator. Improving local oscillator thus plays a critical role in optical atomic clocks as well as its other applications.To suppress the frequency noise and improve the frequency stability, a laser is usually frequency-stabilized to an ultra-stable optical reference cavity by the Pound-Drever-Hall (PDH) technique. This thesis first gives a brief introduction to the PDH technique, including a variety of noise sources that might limit the performance of ultrastable lasers. Assuming a good signal-to-noise ratio (SNR) and a tight phase lock, we find that the laser frequency stability depends on the length stability of the reference cavities. Here I discuss how we improve the length stability of reference cavities, including the shape and support configuration of the reference cavities for vibration insensitivity and isolation from environmental vibration and acoustic noise. Experimentally, I show how we realized ultrastable reference cavities (two1064nm cavities and two578nm cavities) based on vacuum chambers, precision temperature control and acoustic isolation. Lasers stabilized to these cavities have almost reached to the thermal noise limit for these reference cavities. Two1064nm lasers have achieved a linewidth of1Hz (RBW=0.25Hz) and fractional frequency instability of1.7×10-15at an averaging time of1s. The resulting stabilized578nm laser is measured to have a linewidth of0.25Hz (RBW=85mHz) and fractional frequency instability of≤3×10-16at an averaging time of1-10s, a result that advances the state-of-the-art for laser stabilization.To further characterize the performance of the ultra-stable narrow-linewidth lasers, the578nm laser was used to probe the clock transition of cold ytterbium (Yb) atoms trapped in optical lattice sites. We resolved an atomic spectrum with spectral linewidth1Hz, corresponding to a line quality factor of>5×1014at a transition frequency of518THz. With the stable laser source and the signal to noise ratio afforded by the Yb optical clock, we dramatically reduced the key instability limitations of the clock, and made measurements consistent with a clock instability of5×10-16/√τ, the lowest recorded for an atomic clock.Further improvements of ultrastable lasers, especially reducing the thermal noise limit of the reference cavities, are discussed. Alternative methods to generate narrow linewidth laser light are also introduced. As an application to optical clocks, improvements on the LO directly reduce the Dick-effect of optical atomic clocks, which is the main limitation of the frequency stability of state-of-the-art optical atomic clocks. Therefore, I also consider ways to reduce the Dick effect limitation as a means toward even more stable optical clocks.