Improved common-path optical heterodyne interferometer for measuring small optical rotation angle of chiral medium

Precise and rapid detection of optical activity for accumulative femtosecond spectroscopy

“…This value of 0.10 mdeg is smaller than the value of 0.16 mdeg determined by the resolution of the lock-in amplifier because of data averaging. On the other hand it is somewhat larger than the purely theoretical value of 0.035 mdeg by Lee and Su [17] because we include all experimental noise sources. Theoretically, the angle amplification k should get infinitesimally large when approaching a retardation setting of δ = 0 • according to Eq.…”

“…The common-path optical heterodyne interferometry principle has been shown in detail by Lee and Su [17]. In contrast to their original setup, we omit the quarter-wave plate in front of the sample, since our implementation lacks a reference beam and we use the internal reference of the lock-in amplifier instead for higher precision.…”

“…Theoretically, it is given by the angular phase resolution of the lock-in amplifier, in our case 8 mdeg [30], which would lead to a resolution of the optical rotation of 0.16 mdeg for a retardation setting of δ = 2.3 • (which will be shown to be optimal below). Lee and Su have calculated a theoretical limit of 0.035 mdeg [17] considering besides the angular resolution of the lock-in amplifier also the second-harmonic error and the polarization-mixing error. However, this value is not reached in an actual experiment due to electronic noise, scattering in the sample solution or intensity fluctuations of the laser diode, to mention just a few effects.…”

“…Polarimeters based on heterodyne detection translate the optical rotation into a phase difference, which can be detected faster. We use the heterodyne detection method of Lee and Su [17] as basis for our polarimeter since it is common-path and therefore does not require the interferometric stability of related setups [18][19][20]. We show the integration of the polarimeter and a probe for linear absorption spectroscopy into an accumulative scheme for femtosecond spectroscopy [21].…”

Precise and Rapid Detection of Optical Activity for Accumulative Femtosecond Spectroscopy

“…We modify the heterodyne detection method of Lee and Su [6] and combine it with the recently developed accumulative technique that further enhances sensitivity [7,8]. To demonstrate the device's capabilities, we employ chiral methyl p-tolyl sulfoxide which undergoes a bond cleavage at the stereogenic center after UV irradiation [9], leading to non-chiral fragments.…”