| Narrow linewidth laser has advantages of ultra-long coherent length and low phase noise,which has be widely used in coherent optical systems like coherent telecommunication,distributed fiber sensing,laser radar and remote sensing.Since the linewidth of the laser source plays a key role in the coherent detection systems to strongly influence the measurement range,precision,sensitivity,and the noise.Therefore,achieving narrow linewidth laser under normal conditions has become a matter of great interest.Accurate measurement of the laser linewidth is a prerequisite for the study of narrow linewidth laser.However,the conventional method,delayed self-heterodyne/homodyne interferometry,inevitably suffers the broadening of the Gaussian spectrum,which will hide the laser’s intrinsic linewidth.Therefore,the conventional method cannot accurately measure such narrow linewidths(kHz or even Hz).In view of the limitations of the conventional linewidth measurement methods,we proposed a new method to measure the laser linewidth precisely by using the amplitude difference comparison of coherent envelope based on a short delayed self-heterodyne interferometry.This method provides a basis for quantitatively analyzing the evolution of laser linewidth compression with different mechanisms.The conventional linewidth compression methods,such as electrical feedback control or optical self-injection feedback,are difficult to obtain several hundred Hz or Hz level narrow linewidth laser.Aiming at the bottleneck of laser linewidth compression under normal conditions,we proposed two kinds of ultra-narrow linewidth lasers.One is based on Rayleigh scattering and self-injection structure and the other is based on stimulated Brillouin and Rayleigh feedback.Through theoretical analysis and experimental study,we investigated the effects of these two mechanisms on characteristics of the output lasers,and achieved two kinds of lasers with arrow linewidth of 130 Hz and 75 Hz,respectively.Besides,in order to reduce the cost of the pumped laser for stimulated Brillouin,a linewidth compression technique based on dual-cavity feedback is proposed,which can reduce the output linewidth of the DFB laser to about 1 kHz.The core contents of this thesis include:(1)We introduce a new amplitude difference comparison of coherent envelope(ADCCE)method to measure the laser linewidth precisely.This method can measure the linewidth of single longitudinal mode laser with arbitrary linewidth,especially for narrow linewidth detection.The feasibility and accuracy of this method are confirmed theoretically and experimentally.This method can eliminate the effect of the broadened Gaussian spectrum induced by the 1/f frequency noise,and the length of delaying fiber can be greatly reduced at the same time.We analyzed,in detail,the relationship between intrinsic laser linewidth,contrast difference with the second peak and the second trough(CDSPST)of the strong coherent envelope,and the length of the delaying fiber.The correct length for the delaying fiber can be chosen by combining the estimated laser linewidth with a specific CDSPST to obtain the accurate laser linewidth.Our results indicate that this method can be used as an accurate detection tool for measurements of narrow(kHz)or ultra-narrow(Hz)linewidths.(2)Laser linewidth compression mechanism based on Rayleigh scattering is further studied.The compression mechanism based on Rayleigh backscattering lies in the fact that the linewidth of scattering light is narrower than that of the incident light in high Rayleigh scattering structure.Then after cyclic amplification of the narrower linewidth of the scattering light,the linewidth of the output laser would be compressed.Rayleigh backscattering can be collected in any waveguide structure and all wave bands,which could have a revolutionary impact on the field of laser.Combined with the self-rejection feedback structure,more stable output laser can be achieved by expanding the free spectra range of the laser cavity.The ultra-narrow linewidth of 130 Hz laser output was obtained at room temperature by optimizing the Rayleigh feedback and self-injected laser intensity.(3)We proposed and demonstrated a method of laser linewidth compression based on stimulated Brillouin gain and Rayleigh feedback.The relationship between the output laser linewidth and the input pump linewidth was studied theoretically and experimentally.It is shown that narrower linewidth of the pump laser leads to narrower bandwidth of the stimulated Brillouin gain,and finally the bandwidth of the stimulated Brillouin will tend to a fixed value.We used a simplified dual-cavity feedback structure(DCFS)to compress the linewidth of DFB laser from MHz to kHz,then this narrower linewidth laser was used to excite the stimulated Brillouin scattering ring cavity to achieve the narrow linewidth laser of ~200 Hz with a side mode suppression ratio of ~60 dB.By combining with Rayleigh feedback structure,the ultra-narrow linewidth laser of ~75 Hz with a side mode suppression ratio of ~70 dB can be obtained.(4)In order to reduce the cost of stimulated Brillouin pump source for ultra-narrow fiber laser,we proposed and experimentally demonstrated a novel method to compress DFB laser linewidth by utilizing a dual-cavity feedback structure(DCFS).For comparison,both single and dual-cavity feedback are analyzed to investigate the effects of external cavity length and external feedback rate on DFB laser linewidth compression and their longitudinal mode characteristics.The DCFS first provides optical self-injection feedback to compress the laser linewidth,and then the two feedback lengths are carefully optimized to achieve single longitudinal mode(SLM)output via the Vernier principle and the suppression of modes overlapping between two cavities.The linewidthes of 1 MHz and ~200 kHz were successfully compressed to ~2.7 and 1.5 kHz with a side mode suppression ratio of 38 dB and 45 dB,respectively.The stability of the DCFS output power can be controlled within ~0.21% percentage by using the high isolation of DFB(30 d B).Our method provides a simple,effective,low cost way to achieve DFB linewidth compression,which will greatly improve the performance of coherent detection systems using DFB laser as sources. |
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