Laser source technology for optical fiber sensing Part Two
2.2 Single wavelength sweep laser source
The realization of laser single wavelength sweep is essentially to control the physical properties of the device in the laser cavity (usually the center wavelength of the operating bandwidth), so as to achieve the control and selection of the oscillating longitudinal mode in the cavity, so as to achieve the purpose of tuning the output wavelength. Based on this principle, as early as the 1980s, the realization of tunable fiber lasers was mainly achieved by replacing a reflective end face of the laser with a reflective diffraction grating, and selecting the laser cavity mode by manually rotating and tuning the diffraction grating. In 2011, Zhu et al. used tunable filters to achieve single-wavelength tunable laser output with narrow linewidth. In 2016, Rayleigh linewidth compression mechanism was applied to dual-wavelength compression, that is, stress was applied to FBG to achieve dual-wavelength laser tuning, and the output laser linewidth was monitored at the same time, obtaining a wavelength tuning range of 3 nm. Dual-wavelength stable output with a line width of approximately 700 Hz. In 2017, Zhu et al. used graphene and micro-nano fiber Bragg grating to make an all-optical tunable filter, and combined with Brillouin laser narrowing technology, used the photothermal effect of graphene near 1550 nm to achieve a laser linewidth as low as 750 Hz and a photocontrolled fast and accurate scanning of 700 MHz/ms in the wavelength range of 3.67 nm. As shown in Figure 5. The above wavelength control method basically realizes the laser mode selection by directly or indirectly changing the passband center wavelength of the device in the laser cavity.
Fig. 5 (a) Experimental setup of the optical-controllable wavelength-tunable fiber laser and the measurement system;
(b) Output spectra at output 2 with the enhancement of the controlling pump
2.3 White laser light source
The development of white light source has experienced various stages such as halogen tungsten lamp, deuterium lamp, semiconductor laser and supercontinuum light source. In particular, the supercontinuum light source, under the excitation of femtosecond or picosecond pulses with super transient power, produces nonlinear effects of various orders in the waveguide, and the spectrum is greatly broadened, which can cover the band from visible light to near infrared, and has strong coherence. In addition, by adjusting the dispersion and nonlinearity of the special fiber, its spectrum can even be extended to the mid-infrared band. This kind of laser source has been greatly applied in many fields, such as optical coherence tomography, gas detection, biological imaging and so on. Due to the limitation of light source and nonlinear medium, the early supercontinuum spectrum was mainly produced by solid-state laser pumping optical glass to produce the supercontinuum spectrum in the visible range. Since then, optical fiber has gradually become an excellent medium for generating wideband supercontinuum because of its large nonlinear coefficient and small transmission mode field. The main nonlinear effects include four-wave mixing, modulation instability, self-phase modulation, cross-phase modulation, soliton splitting, Raman scattering, soliton self-frequency shift, etc., and the proportion of each effect is also different according to the pulse width of the excitation pulse and the dispersion of the fiber. In general, now the supercontinuum light source is mainly towards improving the laser power and expanding the spectral range, and pay attention to its coherence control.
3 Summary
This paper summarizes and reviews the laser sources used to support fiber sensing technology, including narrow linewidth laser, single frequency tunable laser and broadband white laser. The application requirements and development status of these lasers in the field of fiber sensing are introduced in detail. By analyzing their requirements and development status, it is concluded that the ideal laser source for fiber sensing can achieve ultra-narrow and ultra-stable laser output at any band and any time. Therefore, we start with narrow line width laser, tunable narrow line width laser and white light laser with wide gain bandwidth, and find out an effective way to realize the ideal laser source for fiber sensing by analyzing their development.
Post time: Nov-21-2023