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Semiconductor Light Sources and Detectors
Published in Shyamal Bhadra, Ajoy Ghatak, Guided Wave Optics and Photonic Devices, 2017
where Λ is the period of sinusoidal grating and neff is the effective index of the guided mode in the laser cavity. The guided mode suffers back-reflection all along the length of the grating, which can be viewed as distributed feedback into the laser cavity containing the gain medium – hence the name distributed feedback laser. The antireflection coatings at the end-faces of the laser chip ensure that there is no reflection from the cleaved ends, thereby suppressing feedback at all other wavelengths. The lasing action then builds up only at the Bragg wavelength, forming a single longitudinal mode of the structure. Figure 8.29 schematically shows a typical spectrum of the output from a normal FP laser diode and a DFB laser. The output from the DFB laser predominantly oscillates in one longitudinal mode while the FP laser oscillates in about 10 modes.
Synchronised flexoelectric instability in an LC cell with periodic planar-homeotropic alignment
Published in Liquid Crystals, 2023
Ivan Vladimirovich Simdyankin, Artur Rubenovich Geivandov, Boris Aleksandrovich Umanskii, Serguei Petrovich Palto
Discovered long ago, the flexoelectric instability still grasps the attention of researchers. Due to the high optical anisotropy of LCs, the flexoelectric gratings can be applied for diffraction optics, light beam steering, lasing and other applications. Speaking of alternative methods of creating field-driven gratings in LC cells, there are a few of them. For instance, it can be made using patterned electrodes [17,18], periodic refractive index profile hologram record [19], patterned polymer-stabilised LC [20], or polymer layer patterned with an ion beam [21]. Although the gratings made using the aforementioned methods are electrically switchable, the grating period remains fixed, while namely electrical controlling the grating period is extremely important for many optical and photonic applications. For instance, the unique possibility of an electrically controlled period of modulation of the refractive index is desirable for tuning the lasing wavelength in distributed-feedback lasers [22, 23].