Abstract
A simple and robust analog feedforward and digital feedback compound control system is presented to lock the frequency of a slave continuous wave (CW) laser to an optical frequency comb. The beat frequency between CW laser and the adjacent comb mode was fed to an acousto-optical frequency shifter (AOFS) to compensate the frequency dithering of the CW laser. A digital feedback loop was achieved to expand the operation bandwidth limitation of the AOFS by over an order of magnitude. The signal-to-noise ratio of the interference signal was optimized using a grating-based spectral filtering detection unit. The complete system achieved an ultrastable offset-locking of the slave CW laser to the frequency comb with a relative stability of ±3.62 × 10−14. The Allan deviations of the beat frequency were 8.01 × 10−16 and 2.19 × 10−16 for a gate time of 10 s and 1000 s, respectively. The findings of this study may further improve laser interferometry by providing a simple and robust method for ultrastable frequency control.
Highlights
The accuracy of laser interferometry is highly dependent on the stability of the laser wavelength.In some ultra-precision measurement applications (e.g., Laser Interferometer Gravitational-WaveObservatory (LIGO) and semiconductor manufacturing equipment), the laser interferometer works in a vacuum environment [1,2,3]
The range of offset frequency v is limited to the linewidth of the continuous wave (CW) laser, which is approximately 100 kHz in system time delay ∆t is a result of the acousto-optical frequency shifter (AOFS) module time delay, the photodiode conversion delay, and our system, the capability of phase noise suppression is mainly determined by time delay Δt
In order ratio (SNR), the frequency comb was preselected by reducing the noise from the unneeded comb to extract the beat signal between the CW laser and the reference comb with a high signal-to-noise modes with a grating and a pinhole [19]
Summary
The accuracy of laser interferometry is highly dependent on the stability of the laser wavelength. Frequency or the phase difference between the CW laser and the adjacent reference comb mode is detected and feedback is controlled via the operation temperature, current, and/or the piezo cavity length regulator of the CW laser [10,11,12,13,14] In these locking systems, dynamic properties of the control loop should be carefully considered, especially for the response of the laser oscillator and servo actuators. Many commercial CW lasers cannot be precisely controlled with a bandwidth over the kHz level Another promising solution known as the acousto-optic frequency shifter (AOFS)-based feedforward method for laser frequency control was first proposed for direct carrier-envelope phase stabilization of frequency combs [15]. The proposed method may be further applied to precision laser interferometry with ultra-high frequency stability and easy feasibility
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