China
Africa
Explore chapters and articles related to this topic
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.
Optical Fibers: Guiding and Propagation Properties
Published in Le Nguyen Binh, Digital Processing, 2017
The spread of the group delay due to the spread of source wavelength can be in ps/km. Thus, the linewidth of the light source contributes significantly to the distortion of optical signal transmitted through the optical fiber due to the fact that the delay differences between the guided modes carried by the spectral components of the lightwaves. Hence, the narrower the source linewidth, the less dispersed the optical pulses. Typical line-width of Fabry–Perot semiconductor lasers is about 1–2.0 nm while, the DFB (distributed feedback) laser would exhibit a linewidth of 100 MHz. (How many nm is this 100 MHz optical frequency equivalent to?) Later we will see that, under the case that the source linewidth is very narrow, such as the external cavity laser (ECL), then the components of the modulated sources, the bandwidth of the channel would play the principal role in the distortion.
Review on the Developments and Potential Applications of the Fiber Optic Distributed Temperature Sensing System
Published in IETE Technical Review, 2021
Ramji Tangudu, Prasant Kumar Sahu
Y. Zhang et al. [36] proposed, and demonstrated a Brillouin OTDR-based DTS system using the combination of cuckoo search (CS) algorithm, and improved differential evolution (IDE) algorithm. As compared with the CS algorithm, the CS-IDE algorithm offered better performance. In this work, the analysis was based on the Brillouin center frequency shift. The system offered a 5 m of spatial resolution for a launch optical signal of 0.05 s of pulse width from the distributed feedback laser diode (DFBLD) device. The proposed DTS system has a sensing range of 30 km. At a distance of 25 km location, a fiber of length 1 km was kept in a heating box with 30°C, 40°C, 50°C, 60°C, 70°C, and 80°C of temperatures. For all these temperatures measurements, the CS-IDE algorithm offered approximately 0.1 times better sensing resolution as compared to the CS algorithm. The regression factor between temperature, and Brillouin center frequency was 0.9877, and 1.00297 for CS-IDE, and CS algorithms respectively [36].