KEYWORDS: Near field scanning optical microscopy, Polarization, Birefringence, Modulators, Anisotropy, Near field optics, Modulation, Signal detection, Statistical analysis, Optical components
A new method for linear birefringence measurement in micro-scale is presented. Mueller matrix is used to represent the
anisotropy properties of sample, optical fiber probe of scanning optical near-field microscopy (SNOM) and other optical
components. Two polarization modulators are used to complete the quantitative measurement. Firstly, the Mueller matrix
of the probe is calculated by analyzing the output signal without sample. Together with the matrix of the probe, the
elements of the Mueller matrix of sample could be calculated sequentially. The principle and realization method of the
scheme for linear birefringence measurement are presented. The result shows that the dual-modulator-based system can
separate the anisotropy of probe and sample, which could not be distinguished in the current scheme that using only one
polarization modulator.
An integrated Single photon detector (SPD) module that combines a single-photon avalanche photodiode (APD) cooled
by thermoelectric cooler and auxiliary circuits has been demonstrated at 1550 nm. We have employed two coaxial
cables to suppress the spike noise and introduce the timing gates to extract avalanche signals. In a typical condition of
such SPDs, the dark count probability was reduced to 4.7E-5 per pulse, without affecting the detection efficiency (11%)
at 218K.
We have demonstrated a gated-mode single-photon detector at 1550 nm using two thermoelectrically cooled InGaAs/InP
avalanche photodiodes (APDs). Balanced outputs from the two APDs were used to cancel the charge and discharge
spikes, which were attributable to capacitive behavior in a gated mode. The avalanche signals were not attenuated during
the spike cancellation, which enable one to reduce the bias voltage applied to the APDs and thus reduce the dark count
probability. We obtained a quantum efficiency of 10.5% with a dark count probability of 4.8E-5 per gate at 212 K. A
single photon detector module that integrated APD and all necessary circuits into a compact bin has been performed.
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