Parity-time Symmetric Single-longitudinal-mode Narrow Linewidth Fiber Laser

Compared with ordinary light sources,laser has advantages of high orientation,high coherence and high brightness.Since the laser first appeared in 1960 s,it has been widely concerned and constantly developed.Over the past few decades,the single-longitudinal-mode(SLM)fiber laser,which is characterized by small size,light weight and low cost,has been extensively applied in medical,communication,military,transportation and other industries.With the continuous innovation of scientific research,the generation technology of SLM laser with wider wavelength tunable range,larger extinction ratio,narrower linewidth and higher stability has been deeply discussed by many research groups at home and abroad.In recent years,a new concept,parity-time(PT)symmetry,has been gradually introduced from the field of quantum mechanics to the field of optics for mode selection of lasers.PT symmetry,as a more effective SLM laser generation technique,shows superior side mode suppression ability and has obtained preliminary results in practical operation.In this thesis,the mode selection theory of PT symmetry is simply numerically simulated,a Sagnac loop with nonreciprocal light transmission effect is proposed to experimentally investigate the generation method of SLM narrow linewidth fiber laser with tunable wavelength.Then,assisted by the inherent narrow linewidth characteristic of stimulatedBrillouin scattering(SBS),the linewidth of SLM laser is further compressed by effective combination with PT symmetry.Generally speaking,Hamiltonians for non-Hermitian systems have complex eigenvalues,whereas Hamiltonians subject to the special form of PT symmetry have real eigenvalues.In the field of optics,a PT-symmetric system is usually constructed in two geometrically identical and coupled optical loops,where one loop represents gain and the other represents loss.When the gain coefficient is greater than the coupling coefficient,the primary mode satisfying PT symmetry breaking condition continues to oscillate in resonator,whereas the other side modes are greatly inhibited.The advantage of mode selection based on PT symmetry is that once PT breaking condition is reached,the gain contrast between the primary mode and the next competing mode increases rapidly,and the SLM oscillation is easily satisfied without the control of other optical elements.Firstly,the mode selection capability of PT symmetry is simulated numerically.It is found that when PT symmetry is applied for 500 times,the gain contrast between the primary mode and the next competing mode is as high as 38.28 dB,which creates a very favorable condition for the realization of SLM laser.Then,a stable and tunable PT-symmetric SLM fiber laser using a nonreciprocal Sagnac loop is demonstrated experimentally.The nonreciprocal Sagnac loop only containing a 3-dB optical coupler(OC)and a polarization controller(PC)is incorporated into a fiber ring cavity,where the nonreciprocal light transmission between frequency-degenerate clockwise(CW)and counterclockwise(CCW)resonator modes is induced to suppress the MLM oscillation of laser.Due to a controlled birefringence introduced by PC,the two optical paths along the CW and CCW directions in the Sagnac loop are defined as the gain and loss loops of PT-symmetric system,respectively.An optical tunable filter(OTF)with a fixed bandwidth is used to roughly select longitudinal modes in the laser.After that,by adjusting the PC to control the polarization state of two light waves in the Sagnac loop,PT symmetry is broken when the gain coefficient is greater than the coupling coefficient,the SLM oscillation of laser is thus realized.The experimental results show that the optical signal to noise ratio(OSNR)of SLM laser at 1550 nm is 43.0 dB,demonstrating the great superiority of PT symmetry in mode selection.By tuning the central wavelength of OTF,the wavelength of SLM laser varies from 1530 to 1560 nm,and the 3-dB Lorentzian linewidth varies within a range from 529 to 687 Hz.During a 30-minute observation period,the variations of wavelength drift and OSNR of SLM laser are less than 6 pm and 2.42 dB,respectively.Compared with other SLM fiber lasers based on PT symmetry,the advantages of the proposed scheme are obvious,which include simple structure,low cost,convenient operation,and good stability.Finally,aided by the effect of SBS on linewidth compression and the effective combination with PT symmetry,a tunable SLM narrow linewidth Brillouin fiber laser(BFL)based on PT symmetry is further demonstrated experimentally.A narrow-band gain curve induced by SBS can be acted as a filter element for selecting mode preliminary in a fiber ring cavity.After that,a PT-symmetric structure is still formed by the bidirectional transmission path within the Sagnac loop,in which PT symmetry can be broken by adjusting the PC to change the polarization state of light waves,resulting in the SLM oscillation.Meanwhile,the SBS process is beneficial to linewidth narrowing.The experimental results show that the SLM laser with an OSNR of 50.1 dB and an ultra-narrow linewidth of 300 Hz at 1547.90 nm is obtained.Compared with the original linewidth of pump laser,the linewidth-reduction ratio can be up to 680 times.By tuning the wavelength of pump laser,the available wavelength range of BFL continuously varies from 1547.59 to 1548.42 nm,covering 0.83 nm,and the OSNRs remain above 47.5 dB.The key advantage of the proposed scheme is that by constructing a simple Sagnac loop,the SLM selection based on PT symmetry and the linewidth narrowing benefiting from SBS effect can be accomplished simultaneously without the use of additional filters.Moreover,the wavelength tunability of BFL is virtually unrestricted.By extending the wavelength range of pump laser,the available wavelength of BFL can vary from 1528.4 nm to 1563.4 nm,with a range of 35 nm.