Optoelectronic frequency-modulated continuous-wave terahertz spectroscopy with 4 THz bandwidth

Broadband terahertz spectroscopy enables many promising applications in science and industry alike. However, the complexity of existing terahertz systems has as yet prevented the breakthrough of this technology. In particular, established terahertz time-domain spectroscopy (TDS) schemes rely on complex femtosecond lasers and optical delay lines. Here, we present a method for optoelectronic, frequency-modulated continuous-wave (FMCW) terahertz sensing, which is a powerful tool for broadband spectroscopy and industrial non-destructive testing. In our method, a frequency-swept optical beat signal generates the terahertz field, which is then coherently detected by photomixing, employing a time-delayed copy of the same beat signal. Consequently, the receiver current is inherently phase-modulated without additional modulator. Owing to this technique, our broadband terahertz spectrometer performs (200 Hz measurement rate, or 4 THz bandwidth and 117 dB peak dynamic range with averaging) comparably to state-of-the-art terahertz-TDS systems, yet with significantly reduced complexity. Thickness measurements of multilayer dielectric samples with layer-thicknesses down to 23 µm show its potential for real-world applications. Within only 0.2 s measurement time, an uncertainty of less than 2 % is achieved, the highest accuracy reported with continuous-wave terahertz spectroscopy. Hence, the optoelectronic FMCW approach paves the way towards broadband and compact terahertz spectrometers that combine fiber optics and photonic integration technologies.

Metrological characteristics of the frequency method for measuring the viscosity of magnetorheological fluids in a rotary viscometer

A frequency method for measuring the dynamic viscosity of rheological media is described using the example of a magnetorheological fluid. The method is based on the principle of operation of a rotary viscometer, in which the torsion angle depends on the characteristics of the viscoelastic medium. Keywords frequency method, dynamic viscosity, torque, beat signal, methodical error, resolution, accuracy, twist angle, torsion. [email protected]; [email protected]

Miniature Fiber-Optic Magnetic Field Sensor Based on Ampere Force and Fiber Laser

A compact fiber-optic magnetic field sensor is proposed by packaging an orthogonal dual-frequency fiber grating laser and a copper wire with alternating electrical current together inside epoxy resin. The alternating current generates Ampere force in a magnetic field, which changes the birefringence of the fiber laser and hence tunes the frequency of the beat signal after photodetection. The magnetic field magnitude can then be detected by measuring the frequency change of the beat signal. The sensitivity of the sensor can be tuned with a maximum response of 35.21 kHz/kGs demonstrated. Moreover, the sensor shows good immunity to environment interference.

Absolute Distance Measurement Using Frequency-Scanning Interferometry Based on Hilbert Phase Subdivision

In order to eliminate the influence of laser frequency nonlinearity, the frequency-scanning interferometry (FSI) often uses the beat signal of an auxiliary interferometer as the external clock. The time points at every amplitude peaks and bottoms of the auxiliary beat signal are selected as the sampling time points for the main interferometer signal. To satisfy the Nyquist sampling requirement, the optical path difference (OPD) of the delay fiber in auxiliary interferometer should be at least twice longer than the measurement distance. In this paper, we proposed a method to shorten the length of delay fiber. The Hilbert transform was used to extract the phase of the auxiliary interference signal and calculate the time points corresponding to subdivided phase intervals. Then, the main interference signal was resampled at these moments, and the fast Fourier transform was performed on the resampled signal. The experimental results showed that the target at the distance of about 5 m was measured when the OPD of the auxiliary interferometer was about 4.5 m. The standard deviation of the distance measurement results could reach 4.64 μm.