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Coherence and Interference of Light
Published in Lazo M. Manojlović, Fiber-Optic-Based Sensing Systems, 2022
If the interferometer has been illuminated with a monochromatic light, one can notice a distinctive interferometric pattern on the observation screen or if an optical detector has been used to capture the fringes when the optical path difference in the interferometer changes, one can notice a very large number of fringes, which in the case of a linear optical path difference sweep produces a periodic fringe pattern. In theory, we will have an unlimited number of fringes. However, typical light source has a finite spectrum width thus limiting the number of observed/detected fringes. According to the Wiener–Khinchin theorem, the broader the spectrum, the shorter the interferometric pattern. One of the most common light sources used in the white-light interferometer or also called low-coherence interferometer is the superluminescent diode. Typical superluminescent diode emits infrared light, which has Gaussian spectrum shape given as: S(v)=2Δvln2πexp[−4ln2(v−vΔv)2],
Light Sources
Published in Toru Yoshizawa, Handbook of Optical Metrology, 2015
Superluminescent diodes (SLDs) are edge-emitting LEDs, which operate at such high current levels that stimulated emission occurs. The current densities in SLD are similar to that of a laser diode (~kA/cm2). The emission in a SLD begins with a spontaneous emission of a photon because of radiative electron–hole recombination. Sufficiently, strong current injection creates conditions for stimulated emission. Then, the spontaneously emitted photon stimulates the recombination of electron–hole pair and the emission of another photon, which has the same energy, propagation direction, and phase as the first photon. Thus, both photons are coherent. In contrast to LEDs, the light emitted from an SLD is coherent, but the degree of coherence is not so high compared to laser diodes and lasers. SLDs are a cross between conventional LEDs and semiconductor laser diodes. At low current levels, SLDs operate like LEDs, but their output power increases super linearly at high currents. The optical output power and the bandwidth of SLDs are intermediate between that of an LED and a laser diode. The narrower emission spectrum of the SLDs results from the increased coherence caused by the stimulated emission. The FWHM is typically about 7% of the central wavelength.
Depth-resolved dispersion compensation method for optical coherence tomography imaging based on rectangular window function optimization
Published in Journal of Modern Optics, 2022
Xi Zhang, Zhongliang Li, Nan Nan, Xiangzhao Wang
In order to verify the optimization effect of the proposed method, a frequency domain OCT system was used to image the sample. The light source of the system is a superluminescent diode with a power of 20 mW, centre wavelength of 840nm, and bandwidth of 50 nm. And the theoretical axial resolution of the system is 6.23 μm. The OCT system is used to image a phantom which is composed of six layers of tape and four layers of cover glass, as shown in Figure 5(a). The original A-line signal of the sample is obtained by IFFT after removing the background of the interference spectrum signal in the frequency domain, as shown in Figure 5(b). The signal intensity of the sample without dispersion compensation in the figure is low due to dispersion broadening. The FWHM of the peak at each depth is 13.6–28.9 μm, which differs significantly from the theoretical resolution.