| Single-longitudinal-mode(SLM),narrow linewidth fiber laser is widely used in long distance optical fiber sensing,coherent light communication,radio over fiber,high precision spectrum characterization,laser radar,fiber remote sensing and other fields,due to its advantages of good coherence,high modulation rate,large gain,stable spectrum and strong anti-interference ability.In particular,the narrow linewidth Tm3+-doped fiber laser(TDFL)in 2 ?m band also possesses the characteristics of high atmospheric transmittance and human eye security,so it has very important application potential in near-surface space optical communication,laser radar and other fields.Since the laser linewidth is related to the key technical indexes such as the loadable signal modulation rate and the laser radar spatial resolution,it is of great significance to accurately measure the laser linewidth to improve the laser performance,spatial optical communication rate and the laser radar spatial resolution.In this thesis,the researches of precise characterization method of laser linewidth in 2 ?m band were carried out theoretically and experimentally.At the same time,this characterization method was used for the manufacturing of novel SLM fiber lasers with narrow linewidth in 2 ?m band.Further,the characteristics of SLM TDFL in 2 ?m band applied to fiber sensing were studied.The main research results and innovations are summarized as follows:1.The internal relationship between laser spectral linewidth and phase noise was studied,and the basic principle of linewidth measurement based on phase noise was explained in detail.The relationship between laser linewidth and power spectral density of frequency noise and phase noise was deduced.Numerical simulation and characteristic analysis were carried out on the output excitation spectral lines of linear cavity fiber laser and ring cavity fiber laser,respectively.The influence factors of laser spectral linewidth were discussed in detail,and the theoretical basis of the output linewidth narrowing was obtained.2.A laser linewidth measurement method in 2 ?m band based on phase noise demodulation was proposed,and a corresponding linewidth measurement system based on polarization maintaining(PM)3x3 coupler was established.By measuring the phase noise and frequency noise of the laser,the output laser linewidth was calculated.The all-PM structure can effectively eliminate the signal fading caused by the random change of polarization state induced by external environmental disturbance.At the same time,this structure can also simplify the optical path structure.When the denoising algorithm was added,the simulation results show that the demodulation results were in good agreement with the preset values when the phase signals were considered without noise and with noise,respectively,and the estimated errors of two cases are ±0.1 rad and-0.17 rad?0.15 rad.The experimental results show that the phase demodulation results were consistent with the simulation results.The power spectral density of the phase fluctuation in both experiment and simulation were consistent,and the correlation coefficient was 0.98.The correlation coefficients between any one of 10 groups of repeated experimental results and the simulation result were all above 0.98.The correlation coefficients among 10 pairs of randomly selected experimental results were all above 0.99.3.Theoretical derivation and simulation study of output characteristics of Fiber Bragg grating(FBG),FBG-based F-P filter and phase-shift FBG filter in 2 ?m band was carried out.A linear cavity SLM TDFL structure based on FBG-based F-P filter and saturated absorber was proposed.The laser output was in a stable SLM state at room temperature,with a central wavelength of 1942.01 nm,a 3 dB bandwidth of 0.06 nm,and an optical signal to noise ratio(OSNR)of about 50 dB.A ring cavity SML TDFL based on FBG-based F-P filter and saturable absorber was proposed.The output linewidth of the laser was measured by the self-made linewidth measuring system.The results show that the laser output was in a stable SLM state at room temperature,with the center wavelength of 1942.03 nm,and an OSNR of as high as 60 dB.The linewidth of the laser was about 47 kHz when measuring time was 0.001 s.4.An SLM TDFL with ring cavity based on phase-shift FBG filter was proposed.Without the saturated absorber,a stable SLM operation was obtained by using the narrow-band filtering performance of the phase-shift FBG and a subcavity.The output laser has a central wavelength of 1941.80 nm and an OSNR of about 60 dB.At the measurement time of 0.001 s,the output linewidth is 41 kHz by using the linewidth measurement method aforementioned.On the basis of this structure,a ring cavity SLM TDFL based on phase-shift FBG filter and saturated absorber was proposed further.The output linewidth is 18 kHz when the measuring time is 0.001 s.The effectivity of the saturated absorber to further narrow the laser linewidth had been validated.5.An SLM fiber laser with a ring cavity based on a Sagnac interferometer filter for temperature sensing in 2 ?m band was demonstrated.Based on the temperature sensitivity of the polarization maintaining fiber(PMF)in the Sagnac interferometer,temperature measurement was realized by laser wavelength under different temperatures.Both theoretical simulation and experimental results show that the resonant wavelength of the laser moves in the direction of short wavelength when the temperature increases.A sensitivity of 2.09 nm/? had experimentally been achieved.Compared with the optical fiber laser sensor in 1.55 ?m,the sensor in 2 ?m band is more eonducive to improving the sensing sensitivity.An SLM fiber laser with a ring cavity based on fiber F-P interferometer filter for micro-displacement sensing in 2 ?m band was proposed.The micro-displacement was measured according to the change of the laser wavelength.When the displacement increases,the laser wavelength shifted to the long wavelength direction,and the sensitivity of micro-displacement sensing(change in resonant wavelength of the laser generated by unit displacement)was measured as 33 nm/?m. |
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