| Free-space optical communication(FSO)has a broad application prospect in earth observation,navigation,5G network,intelligent city,environmental monitoring and so on due to its large bandwidth,good confidentiality,low power consumption and flexible erection.The mode-locked fiber laser has the characteristics of short pulse duration,high peak power and good atmospheric transmission performance,which can avoid the accumulation of phase noise of modulated signals caused by environmental factors,and has great potential for high-speed laser communication systems.With the improvement of integrated fiber devices and fiber doping technology,the research of 2?m band fiber lasers is increasing day by day.The 2?m band is in the atmosphere and smoke transmission window.According to the relationship between scattering intensity and wavelength,it can be seen that 2?m laser carrier are less affected by atmospheric scattering compared with1.55?m.At present,the FSO mainly uses 1.55?m fiber communication band,and the laser source is generally a narrow line wide semiconductor laser,which can be easily affected by atmospheric turbulence.The waveform phase distortion and beam drift caused by turbulence will reduce the communication quality.In rainy and foggy weather,especially haze weather,the communication link can be even interrupted.In order to study the application of fiber laser in the field of FSO and to explore the new technology of high-speed FSO,a series of studies on the generation,noise suppression and digital modulation of high repetition rate holmium doped actively mode-locked fiber laser are carried out.Furthermore,the characteristics of 2?m fiber laser in the atmospheric turbulent channel are studied.The specific researches are as follows:1.Multi-pulse dynamics of 2?m mode-locked fiber laserWe experimentally demonstrated a holmium-doped passively mode-locked fiber laser based on the non-linear polarization rotation(NPR)structure.The gain medium is a 1.2m holmium-doped fiber,the total cavity length is 7.95m,corresponding to a fundamental frequency of 25.52MHz.As the pump power increased to 1.2W,the traditional soliton can be obtained.Due to the pump conversion efficiency of holmium doped fiber is low and the threshold of the cavity is high,we have realized fs-order fundamental-frequency dispersion-managed soliton pulses in a near-zero dispersion region thulium-doped fiber laser based on the NPR structure.Under the action of soliton quantization,the modulation instability breaks the balance of the interaction of the solitons by increasing the intracavity continuous wave component.On this basis,the instability of the continuous wave can be suppressed by changing the linear phase delay point in the cavity and generate global soliton interaction.We experimentally obtain 2,3,4 and 5 dispersion-managed soliton outputs at a stable fundamental frequency of 27.38MHz.The position of solitons with different group dispersions is random after the dispersion management cavity cycles,but the position of each period is fixed and the intensity is the same.With the increase of the soliton energy in the cavity,multi-soliton bunch state can be obtained by decreasing the critical value of peak power clamping and increasing the number of solitons in the cavity.The durations of the solitons decrease from several ns dominated by global soliton interaction to 300ps dominated by local soliton forces.2.Generation and modulation/demodulation of holmium-doped mode-locked pulseWe experimentally demonstrate a holmium-doped actively mode-locked fiber laser for2?m high-speed digital modulation.The NPR structure is added into the cavity,which can suppress the super-mode noise while realizing wavelength tunable.Five wavelength channels can be obtained from 2058.4nm to 2078.6nm with a side-mode suppression ratio(SMSR)greater than 40d B.The total cavity length is 9.52m,corresponding a fundamental frequency of 21MHz.Up to 1.008GHz repetition rate mode-locked pulse can be obtained by adjusting the driven signal and bias voltage of the MZM,the corresponding SNR is greater than 49.66d B.A part of the driven signal is sent to the arbitrary waveform generator as the clock reference to generate the synchronous pseudo-random digital signal for modulating the mode-locked pulse.The phase matching between the modulated signal and the mode-locked pulse is realized by adjusting the delay of the digital signal.The optical signal-to-noise ratio(OSNR)of the modulated mode-locked pulse is 13.72d B.The bit error rate(BER)can be obtained by a MATLAB program that calculates the offline data from OSC.The minimum BER is 2×10-7,which is limited by the hardware conditions of the computer.3.Reaserch of repetition rate controllable mode-locked fiber laser and generation of multi-wavelength mode-locked fiber laserWe have added a Lyot filter and a polarization dependent isolator in the cavity to suppress the super-mode noise and ambient noise.A high precision variable optical delay length is inserted into the cavity to achieve repetition rate controllable.The SNR of the mode-locked pulse is increased from 42.64d B to 49.45d B with a repetition rate about680MHz.The accuracy of cavity length can be controlled from several hundred fs to tens fs by adding a high precision optical delay line in the cavity.Therefore,the fundamental frequency accuracy of the mode-locked fiber laser with cavity length greater than 8m can be controlled to several tens Hz.The cavity fundamental frequency tuning range is18.21MHz?18.13MHz with a tuning accuracy of 73.52Hz,and the stable mode-locked pulse with a repetition rate of 1,3 and 6 GHz is obtained.The repetition rate and pulse width of the mode-locked pulse can be independently tunable by changing the optical delay line and the type of driven signal.Multi-wavelength mode-locked pulses with 2,3,4,and 5wavelength channels can be obtained as the VODL remove from the cavity.The maximum repetition rate is 2.412GHz,and the mode-locked pulse sequence can be modulated into digital signal with a SNR of 17.5d B.The experimental study of 2?m multi-wavelength mode-locked fiber laser can be applied to WDM system in laser communication4.Propagation characteristics of holmium-doped laser carrier in complex channelWe have experimentally studied the propagation characteristics of holmium-doped mode-locked fiber laser carriers in turbulent and smoke channels with a data rate of2.02Gb/s.The pulse sequence can be modulated by a digital signal and transmitted in a tunable simulated atmospheric turbulent channel.The optical SNR are 15.04d B,11.94d B,8.51d B and 6.67d B at the condition of back-to-back,Cn2=1.01×10-16m-2/3,Cn2=2.79×10-16m-2/3 and Cn2=5.71×10-16m-2/3,respectively.Moreover,we have obtained the power jitter and BER of the 1.55?m and 2?m laser carrier at the same transmission conditions.The minimum sensitively of the 2?m and 1.55?m laser carrier is-19.52d Bm and-14.45d Bm respectively at the data transmission rate of 2.02Gb/s.Under the turbulence condition of Cn2=5.71×10-16m-2/3,the standard deviation of the 1.55-?m laser is larger than that of the 2-?m laser.In addition,we experimentally demonstrate an indoor simulated smoke chamber.The transmission characteristics of 0.85,1.06,1.55,and 2.04?m at the condition of BTB,V=0.5,0.05,0.005km had been studied.Two actively mode-locked fiber laser at wavelength of 1.55 and 2.04?m had been built and modulated in digital signal at a data rate of 4Gb/s.The eye diagram and bit error rate(BER)of two wavelength laser carriers at four transmission condition can be detected.The measured optical signal-to-noise ratios of the 1.55 and 2.04?m laser carriers for V=0.005 km are 4.83 and8.62d B,respectively.The corresponding link sensitivities are 14.59 and 17.74d Bm respectively,indicating that the 2.04?m data transmission system is more reliable under a complex channel. |
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