High frequency Pound-Drever-Hall sensing of ring resonator cavities

摘要

This paper establishes a procedure for increasing the sensitivity of measurements in integrated ring resonators beyond what has been previously accomplished. This is achieved by a high-frequency phase modulation lock to the ring cavities. A prototyped fiber Fabry-Perot cavity is used for comparison of the method to a similar cavity. The Pound-Drever-Hall (PDH) method is chosen as a proven, ultra-sensitive method with the exploration of a much higher frequency modulation than has been previously discussed to overcome comparatively low finesse of the ring resonator cavities. The high frequency facilitates the use of the same modulation signal to separately probe the phase information of different, integrated ring resonators with quality factors of 5.6 ×10~5 and 2.4 ×10~5.The large free spectral range of small cavities and low finesse provide a challenge to sensing and locking the stability of diode lasers due to the small dynamic range and signal-to-noise ratios (S/N). This can be offset by a calculated increase in modulation frequency using the PDH approach. A distributed feedback (DFB) laser is locked to a ring resonator cavity to demonstrate this sensitivity. This approach using integrated ring resonators is measured to have a refractive index resolution of 1.9 ×10~(-8)that can be compared to other fiber and integrated sensors.The relationship between the signal-to-noise ratio and dynamic frequency range of the cavity error signal is explored with an algorithm to optimize this relationship. The free spectral range and the loss of the cavity provide input parameters to this relationship to determine the optimum S/N and range of the respective cavities used for locking and sensing. The purpose is to show how future contributions to the measurements and experiments of micro-cavities, specifically ring resonators, is well-served by the PDH method with high-frequency modulation.