An ultra-low frequency noise agile laser

Abstract

Very low frequency noise laser sources are key elements of many applications, such as: atom or ion optical clocks, ultra stable microwave (MW) frequency generation, gravitational wave detection , ultra stable optical frequency transfer, and so on.The laser frequency locking technique developed by Pound, Drever and Hall (PDH) is widespread as the commonly used method of laser frequency stabilization on optical cavities, and it has been successfully demonstrated on lasers of various wavelengths. It led to a fractional frequency instability lower than 10 -15 for 1 s averaging times and subhertz line width. This approach has intrinsically two weaknesses. First, it requires fine alignment of free space optical components, tight polarization adjustment, and spatial mode matching. In addition, the cavity has to be housed in a high vacuum enclosure with thermal radiation shielding. This makes the system relatively expensive, bulky and fragile. The second weakness is that the PDH method does not allow tuning the laser frequency. An alternative method is to use a two arm (Michelson or Mach-Zehnder) interferometer to measure the frequency fluctuations during a fixed time delay. This method requires a relatively large arm imbalance to obtain sufficient frequency discriminator sensitivity. Indeed, with a Michelson interferometer, the quality factor is proportional to the fiber delay. For example, using a 5 km fiber delay line the quality factor of the interferometer is about 30 billions for a 1.55 μm wavelength laser, which is equivalent to the quality factor of a 10 c m Fabry Perot cavity with finesse about 230 thousands. This stabilization technique allows a more robust, simpler, cheaper, transportable and frequency tunable laser with low frequency noise. In the experiment, the author use the frequency shifted heterodyne Michelson interferometer to stabilize laser frequency. Since the laser frequency can be chirped by setting a frequency offset onto the demodulation signal, the laser was called an “agile laser”.