Narrow Linewidth Lasers And Narrow Linewidth Optical Frequency Comb

Ultra-narrow-linewidth lasers have merits of high frequency stability, low frequency noise. They have already been widely used in optical atomic clock, high resolution spectroscopy, low noise microwave generation, measurements on fundamental constants and tests of physics.In one of its important applications, optical atomic clocks, a narrow linewidth laser with high frequency stability, called local oscillator (LO), probes the clock transition of cold atoms in optical lattice sites or a trapped single ion, and it is also the output signal of the optical atomic clock. Since the natural linewidth of the clock transition of the neutral atoms or the single ion is generally a few mHz to a few Hz, it requires the linewidth of the LO reaches to the hertz level or below. When phase-locking to the clock transition line, the frequency of the LO is calibrated by the neutral atoms or single ion. Therefore, narrow linewidth lasers are the core of optical atomic clocks. Here we constructed two ultra-narrow-linewidth lasers at578nm for probing the clock transition of ytterbium (Yb) optical clock and an ultra-narrow-linewidth laser at266nm for probing the clock transition of mercury (Hg) optical clock.To reduce the laser frequency noise, a laser is often frequency-stabilized to the resonance of an ultra-stable optical Fabry-Perot (FP) cavity using the Pound-Drever-Hall (PDH) technique. In the limit of high signal to noise ratio (SNR) and tight lock, the performance of the frequency-stabilized laser is determined by the length stability of the reference cavity. Therefore, it is critical to make the cavity length insensitive to environmental vibrations and thermal fluctuation. Besides vibration isolation, the reference cavity is often specially designed to be insensitive to vibrations. To improve the laser frequency stability, reference cavities are usually made of ULE with ultralow coefficient of thermal expansion (CTE), and they are precisely temperature-controlled at their zero expansion temperature. Moreover, the reference cavities are kept within vacuum chambers for less pressure fluctuation and less thermal conduction through air. To meet the application to Yb optical clock, we constructed578nm narrow linewidth lasers for probing the clock transition of Yb clock. The578nm laser light is the summing of a1319nm Nd:YAG laser and a1030nm fiber laser. With both slow servo signal feedback to a PZT attached on the cavity inside the1319nm laser and a fast servo feedback to an AOM located at the output of the summing device, we realized a servo bandwidth as high as150kHz. The linewidth and frequency stability of each578nm ultra-stable laser are measured by beating against a narrow-linewidth optical frequency comb. The linewidths of two578nm lasers are8.73Hz and1.18Hz, respectively. The frequency stabilities are3.68×10-15and1.24×10-15at the average time of1s, respectively. In the experiment, we measured the absolute frequencies of two578lasers to be518295680512.7(1.8) kHz and518295563923.9(1.9) kHz with an fs optical frequency comb referenced to an Rb clock. Finally, we observed the clock transition of the171Yb with one of the578nm narrow-linewidth lasers.To meet the application to Hg optical clocks, we constructed a1062nm narrow linewidth diode laser frequency stabilized to a high-finesses F-P cavity with the PDH technique. The quadruple output (266nm) from another fiber laser at1062nm probed the clock transition of Hg atoms. The1062nm fiber laser inherits the phase coherence and frequency stability of the frequency-stabilized diode laser with the precise phase locking technique. In the experiment, we observed the clock transition of199Hg in MOT with the narrow-linewidth266nm laser.To meet the applications of measurements on above narrow-linewidth lasers at578nm, precision control and coherence transfer and optical frequency synthesizer, we constructed a narrow-linewidth optical frequency comb based on a Ti:sapphire mode-locked laser phase-locked to a1064nm subhertz-linewidth laser. Every single tooth of the optical frequency comb inherits the frequency stability of the1064nm subhertz-linewidth laser. By beating against a second independent1064nm subhertz-linewidth laser both at1064nm and at532nm and a third independent narrow-linewidth laser at578nm,, we measured the absolute linewidth of the comb teeth to be0.6～1.2Hz over an octave spectrum.As a demonstration, one of the578nm (named as ’5781#’) laser systems is precisely phase-locked to the narrow-linewidth optical frequency comb, and the other (named as ’5782#’)is frequency-stabilized to a high-finesse F-P cavity. Measurements of the beating note between these two578nm lasers show the linewidth of5781#is1.13Hz and its frequency stability is1.39×10-15at the average time of1s. These results indicate that this narrow linewidth comb can transfer the coherence and frequency stability of the subhertz-linewidth laser from1064nm to578nm. The above experiment is a demonstration of optical frequency synthesizers.