Optical activity measurement by use of a balanced detector optical heterodyne interferometer

A high signal-to-noise ratio balanced detector system for 2 μm coherent wind lidar

A balanced detector system with a high signal-to-noise ratio (SNR) is the key technology for 2 μm coherent wind lidar, which can enhance the ability for the digital receiver to detect extremely weak signals. The photodiode is the main detection component of a balanced detector system. The variance of junction capacitance of different photodiodes will cause the degradation of SNR. Based on the linear relationship between the junction capacitance of a photodiode and the reverse voltage in the double logarithmic coordinates, a balanced detector system is designed to have the ability to adjust the reverse voltage by using the external digital-to-analog converter. An optimization of the circuit performance can be achieved by adjusting the junction capacitance, which significantly affects the circuit noise and signal bandwidth in the design of the high bandwidth transimpedance amplifier, which is sensitive to parasitic capacitance and component error. A simplified optical system is built to test the performance of the designed balanced detector system. The results show that the SNR of the detector system is ∼10% higher than that of the commercial product, which can further increase the detection distance of the wind lidar by ∼1.5 km for the detection limit of 10 km.

Astrobiological polarimeter

Chirality is an excellent indicator of life, but naturally occurring astrobiological (as well as terrestrial) samples nearly always exhibit massive depolarizing light scattering, which renders conventional polarimeters useless. For astrobiological applications, we instead consider a novel polarimeter originally developed for non-invasive human-glucose measurement. It involves deliberately rotating in time the plane of polarization of a linearly polarized beam and detecting the shift in the plane of the rotating linearly polarized component of the transmitted light from a chiral sample relative to the input polarization plane. We find that this polarimeter can operate in 3 orders of magnitude more depolarizing scattering than conventional polarimeters. Furthermore, it can also be designed to be lightweight, compact, and energy efficient.

Polarized photon-pairs heterodyne polarimetry for ultrasensitive optical activity detection of a chiral medium

A polarized photon-pairs heterodyne polarimetry is proposed in order to measure in an ultrasensitive manner the circular birefringence of a chiral medium via optical rotation detection. A balanced detector is integrated into this polarimeter. Thus, shot-noise-limited detection by this polarimeter can be achieved. Experimentally, the detection sensitivity for the circular birefringence of a glucose-water solution up to partial differential |n(r) - n(l)| = 2 x 10(-11) at 10 mg/dL is verified. To our knowledge, this is the highest sensitivity ever measured of a chiral liquid solution based on single traveling sample cell geometry. Finally, when compared to a fiber loop ring-resonator in the frequency domain for a chiral liquid, this polarimeter shows an order of 10(4) enhancement on the sensitivity of natural optical activity measurement.