Sensing optical cavity mismatch with a mode-converter and quadrant photodiode

Abstract

We present a new technique for sensing optical cavity mode mismatch and alignment by using a cylindrical lens mode converting telescope, radio-frequency quadrant photodiodes, and a heterodyne detection scheme. The telescope allows the conversion of the Laguerre-Gauss bullseye mode (LG01) into the 45 • rotated Hermite-Gauss ("pringle") mode (HG11), which can be easily measured with quadrant photodiodes. We show that we can convert to the HG basis optically, measure mode mismatched and alignment signals using widely produced radio-frequency quadrant photodiodes, and obtain a feedback error signal with heterodyne detection. arXiv:1909.08084v2 [physics.optics]

High Frame-Rate Phase Camera for High-Resolution Wavefront Sensing in Gravitational-Wave Detectors

“…However the phase camera can also be useful for controlling alignment and mode-matching in an interferometer. While quadrant-photodiode and bullseye-photodiode-based schemes, with [10] and without [11] optical mode converters, have the advantage of higher signal-to-noise, they offer only four "pixels" across the beam. In particular, the 320×240 pixel resolution of the CMOS phase camera provides the sensing capabilities to operate and control interferometers with higher-order Laguerre-Gauss or Hermite-Gauss modes as the operating resonant mode, a scheme that was proposed to reduce the coupling of thermal noise to the gravitational readout [34,35].…”

“…Similarly, beam waist position and size sensing can be done using bulls eye segment diodes [9], mode converters [10] and aperture-based schemes [11]. Alternatives to this readout scheme using jitter modulation techniques have also been proposed [12].…”

Resonant modes analysis of a FP cavity under different laser alignment conditions

“…A theoretical description of misalignment and mode mismatch was done by Anderson. [7,8] In this paper, the influence of varying lasers' alignment conditions on FP cavities' resonant modes was analyzed using OSCAR [9] codes in MATLAB. The relationship between different mismatch conditions and resonant modes energies in the cavity was revealed.…”

Power coupling losses for misaligned and mode-mismatched higher-order Hermite–Gauss modes

“…We consider the traditional heterodyne wavefront sensing (WFS) scheme [23,24] and the more recently proposed radio-frequency jitter modulation sensing (RFJ) scheme [25] for the alignment sensing, and the mode-converter sensing (MCS) scheme [26] and radio-frequency lens modulation sensing (RFL) scheme [25,[27][28][29] for mode mismatch sensing, as shown in Fig. 1.…”