Rapidly frequency-tuneable, in-vacuum, and magnetic levitation chopper for fast modulation of infrared light

Abstract

We present an in-vacuum mechanical chopper running at high speed and integrated into a magnetic levitating motor for modulating optical beams up to 200 kHz. The compact chopper rotor allows fast acceleration (10 kHz s−1 as standard) for rapid tuning of the modulation frequency, while 1 mm diameter slots provide high optical throughput for larger infrared beams. The modulation performances are assessed using a reference visible laser and the high brightness, broadband, infrared (IR) beam of synchrotron radiation at the MIRIAM beamline B22 at Diamond Light Source, UK. For our application of IR nanospectroscopy, minimizing the temporal jitter on the modulated beam due to chopper manufacturing and control tolerances is essential to limit the noise level in measurements via lock-in detection, while high modulation frequencies are needed to achieve high spatial resolution in photothermal nanospectroscopy. When reaching the maximum chopping frequency of 200 kHz, the jitter was found to be 0.9% peak-to-peak. The described chopper now replaces the standard ball-bearing chopper in our synchrotron-based FTIR photothermal nanospectroscopy system, and we demonstrate improved spectroscopy results on a 200 nm thickness polymer film.

Optical choppers: novel approaches for higher chop frequencies

“…Therefore, we have first proposed and patented [17] a configuration of chopper disk with non-linear margins, outward or inward oriented. We have demonstrated that such devices, in their simpler configuration, with semi-circular margins of the windows do allow for a more flexible design of the profile of the generated laser impulses [16,18], as a supplemental degree-of-freedom (DOF) is added to the analysis and design process, i.e. the radius ρ of the semi-circular margins.…”

“…Choppers with rotational elements are one of the most utilized optomechanical modulators [1], together with optical scanners of different types [2][3][4][5][6]. Most common choppers are built with rotational disks [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24], but we have introduced, patented [25], and developed [26][27][28][29][30] a novel type of choppers, with rotational shafts of different shapes (for example cylindrical, conical, or spherical), with slits of different profiles (i.e., with linear margins or as cylindrical holes). Such devices have the potential of surpassing the well-known 10 kHz limit of the chop frequency fc of choppers with disks.…”

VO2 Thin Film Enabled Free Space Modulation of Infrared Light Using Pulsed Electric Field

“…To utilize broadband IR light emitters, which are cost-effective, external modulation or chopping of the optical beam is necessary to achieve the desired modulation depth and frequency. Various types of optical beam choppers have been reported in the literature, including mechanical choppers, microelectromechanical systems (MEMS) choppers or in-vacuum choppers. − However, mechanical choppers are bulky systems with a limited frequency modulation range of ∼10 kHz, while infrared lasers with built-in pulsing circuits are expensive as well as bulky . Therefore, these IR modulation platforms are not suitable for applications in portable and miniaturized systems, especially together with a broad-wavelength source of IR light.…”