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Coherence and Interference of Light
Published in Lazo M. Manojlović, Fiber-Optic-Based Sensing Systems, 2022
Spectral interferometry is a very powerful interrogation tool especially in the case of a large number of quasidistributed extrinsic low-finesse Fabry–Pérot interferometric sensors aimed, for example, for distributed measurements of temperature, pressure, force, and strain. This technique allows us multiplexing a large number of sensors placed along a single optical fiber. As shown in Figure 4.18, a white-light source spectrum is modulated, where the modulation “frequency” depends on the optical path difference. So, in the case of several sensing interferometers, we will have more complex modulation scheme with the same number as the sensor number of modulation “frequencies”. Therefore, by locking on a single “frequency” of the captured channeled spectrum, one can track the changes in “frequency” of the particular sensor and perform the necessary measurements on this particular sensor.
The Measurement of Ultrashort Laser Pulses
Published in Chunlei Guo, Subhash Chandra Singh, Handbook of Laser Technology and Applications, 2021
Rick Trebino, Rana Jafari, Peeter Piksarv, Pamela Bowlan, Heli Valtna-Lukner, Peeter Saari, Zhe Guang, Günter Steinmeyer
The SPIDER phase ΔφSPIDER(ω) between the two pulses can be analytically retrieved from the measured SSPIDER(ω) interferogram by extracting the phase of its IFT (with a specified finite frequency range). This is known as the Takeda algorithm [114] for numerical phase demodulation. As in standard spectral interferometry, it relies on a Fourier filtering approach. Then, it isolates one of the modulation side-bands, e.g., the positive modulation sideband, from which one can simply extract Δφ(ω) = ωT by applying the complex logarithm (or arg function) to this modulation term. An alternative method for reconstructing the complex-valued sideband is given by the Hilbert transform [115], and a third method is the use of wavelets [116]. All approaches ultimately isolate the underlying spectral phase difference, removing the constant group delay corresponding to the interference of identical pulses, and any deviation from a linear relation vs. ω indicates the SI of two pulses with different phases.
High-precision Ramsey-comb spectroscopy on molecular deuterium for tests of molecular quantum theory
Published in Molecular Physics, 2023
Charlaine Roth, Andrés Martínez de Velasco, Elmer L. Gründeman, Mathieu Collombon, Maximilian Beyer, Vincent Barbé, Kjeld S. E. Eikema
We use an interferometric phase measurement setup to measure and account for the NOPCPA-induced differential amplification phase shifts for all Ramsey fringes. It is depicted in Figure 6. The principle is based on spectral interferometry. Before the NOPCPA, a little bit of power of the original frequency comb pulses is split-off and later recombined with the amplified pulses. Transmission through a single-mode fibre is used to ensure perfect spatial overlap. A small delay of about 1 ps is applied between the amplified and the original pulses and this produces a spectral interference pattern in the frequency domain. A diffraction grating and optics are then used as a high-resolution spectrometer to monitor this pattern. Two interference patterns are produced, one for each of the two Ramsey-comb pulses. With a Pockels cell and polarisation optics we project them onto a camera above each other–see the camera picture in Figure 6. The phase of the patterns depends on the time delay and phase difference of the two interfering pulses. A comparison of the two interference patterns yields the phase shift between the two amplified pulses and thus to the differential amplification phase shift,3 equal to . The measured phase difference between the two patterns is in part due to the geometry and alignment of the two interference patterns. By exchanging the projection of both pulses (using Pockels cell PC3 in Figure 6), this geometrical phase shift can be eliminated and the pure differential phase shift of the amplified pulses relative to the original frequency comb pulses can be obtained. The outcome of a typical phase measurement is shown in Figure 7. For more details of the procedure, see the Appendix 2.