Erbium-doped Fiber Gain Random Fiber Lasers:Theory And Experiments

The study of random fiber laser has significant theoretical and practical value for laser physics, random scattering and single-mode narrow linewidth fiber laser. Making use of the various advantages of a random fiber laser, such as simple setup, low cost, narrow linewidth and low frequency noise, to improve and upgrade the laser design and performance of existing fiber lasers will provide important guarantee to application fields like high speed large capacity optical fiber communication system, long-distance optical fiber sensor and laser imaging, et al. This paper focuses on the theoretical and experimental research of Er-doped gain random fiber lasers, and the main research direction is the improvement of random fiber laser performances. At the same time, the theoretical analysis and experiment results further perfect our understanding of the random fiber lasers. Most of the works are at the forefront of this field. In this paper, the main research results and innovation points are as follows:(1) On the basis of fully understanding of the origin of Rayleigh scattering in optical fiber waveguides and the lasing principle of Rayleigh random fiber lasers, we proposed an enhanced Rayleigh scattering scheme to achieve comparable random lasing performances but more compact setup. The proposed laser simply applied 500 m enhanced Rayleigh scattering fiber and 3 dB linewidth of 3.5 kHz wavelength tunable random fiber laser was successfully achieved. The latest reported compact random fiber laser is 560 m. The forward scattering loss of the homemade enhanced Rayleigh fiber is 0.54 dB/km.(2) Random feedback induced by CO2 laser refractive index modulation on single-mode fiber was proposed for the first time to mitigate the high intensity noise in Rayleigh random fiber lasers due to chaotic gain competition among huge density of random modes. The achieved narrow linewidth random fiber laser has an extinction ratio of 59 dB,3 dB linewidth of 2.4 kHz and wavelength tuning range of 1549.722 to 1550.226 nm. The scheme retained the random feedback characteristics and greatly reduced the feedback fiber length and random mode number. The reduced mode number, to some extent, mitigates the gain competition and lowers the noise strength of the random fiber lasers. The random feedback fiber length is as short as?10 cm and the relative intensity noise on average is?20 dB lower than Rayleigh random fiber lasers. And laser frequency jitter is of the same magnitude as a commercial narrow linewidth fiber laser.(3) Random feedback was realized by using femtosecond laser micro aching technique, producing random feedback fibers of simply 1 cm long for single section, and the random index modulations spaced from 10 to 50 microns. Single-mode random fiber laser with 3 dB linewidth of ?2.1 kHz, extinction ratio of ?60 dB was realized using 8 sections cascaded random feedback fibers fabricated with different writing parameters. The frequency noise above 1 kHz is at?1 Hz/Hz1/2, Allen variance within 100 s is?1.8 x 10"12. The proposed laser is completely different from traditional single-mode lasers in term of the mode selection mechanism. This scheme takes both the frequency noise and intensity noise into consideration; the lasing parameters reach and even exceed a commercial narrow linewidth laser with electrical stabilization circuit. In addition, the proposed random fiber laser owes the advantages of simple setup, low cost, etc.(4) Due to the birefringence effect existing in optical fiber waveguides, the state of polarization of a random fiber laser changes randomly with random polarization modes gain competition. A single polarization random fiber laser scheme was proposed and obtained for the first time employing the idea that suppression of polarization modes gain competition through birefringence. In experiments, low gain polarization modes were effectively suppressed by increased birefringence in random feedback fiber, resulting in single polarization lasing emission. And the observed laser linewidth can be further compressed due to the suppressed polarization modes competition. The lasing linewith could be as narrow as 2 kHz and even narrower in single polarization condition. And both the intensity and frequency noise are at low level state. This work introduces polarization-dependence into the field of random fiber laser for the first time and puts forward the linewith compression of random fiber laser through suppressing the polarization modes competition.(5) The essence of the Rayleigh random fiber laser was revealed for the first time. Random fiber laser is generally recognized as a one-dimensional random laser, but due to the birefringence effect in the optical fiber waveguide, random fiber laser is essentially three-dimensional random laser with transverse confinement. A set of polarization-dependent rate equations were established for EDF random fiber laser, this model fully considers the time-dependent Rayleigh random feedback and polarization dependent loss, and the calculation results essentially confirm the random output characteristic which proves the accuracy of the model together with experiments. According to the fluctuation output of a random fiber laser, the idea by using random fiber laser as physical entropy for random number generation was put forward for the first time. A preliminary study has been carried on and quasi-random bits of 2 kbps were achieved based on EDF random fiber laser. The proposed random number generator may find applications in secure communication and cryptography.