High Repetition Rate Actively Mode-locked Fiber Laser With Tunable Pulse Duration And Repetition Rate At 1.5 μM

Ultra-high repetition rate ultra-short pulse laser refers to ultra-short pulse laser with repetition rate above GHz,which plays an unmentioned role in improving optical communication transmission capacity,optical frequency comb,laser radar,optical sensing,biomedical laser processing and other fields.Mode-locking technology is a main technical means to achieve high repetition rate ultra-short pulse.By locking the phase of longitudinal mode in the cavity to achieve the laser output modulation technology,it can produce ultra-high repetition rate ultra-short pulse laser with frequency GHz and pulse width of ps or fs magnitude.Among many kinds of mode-locked lasers,active mode-locked fiber lasers have many advantages,such as low noise,high laser quality,good tunability,convenient implementation and easy coupling with fiber.In particular,the tunability of the output mode-locked pulse parameters makes the active mode-locked fiber laser meet different requirements in applications and greatly simplifies the process of production and research.In this thesis,an actively mode-locked fiber laser with controllable pulse repetition rate and tunable pulse duration was studied.In the aspect of repetition rate,this thesis studies the influence of delay line on repetition rate,and explores the realization of rational harmonic mode-locking through delay line to realize the multiplication of frequency.In terms of pulse width,the influence of SOA current and cavity length detuning on the pulse width of active mode-locked fiber laser is studied.The specific contents are as follows:1.On the basis of reviewing the research progress of high repetition rate lasers and the development status of tunable mode-locked lasers at home and abroad,this thesis,studies the generation method of high repetition rate pulses,and analyzes the active mode-locking.Factors affecting repetition rate and pulse width in lasers:The principle of the effect of cavity length and harmonic order on repetition rate is explained.According to the Kuizenga-Siegman theory,the effect of gain medium,optical fiber,modulator and other devices on the effect of pulse width were further analyzed.2.A set of high repetition rate actively mode-locked Er:fiber laser with tunable pulse duration was studied.The ring cavity is used as in the laser,the lithium niobate electro-optical intensity modulator is used as the mode-locking device,and the erbium-doped fiber is used as the gain medium.By changing the harmonic order,the laser is able to output mode-locked pulses with pulse widths of 10 ps,4 ps and 5 ps at repetition rates of 1 GHz,2.5 GHz and 6 GHz,respectively.The continuous repetition rate tuning of an active mode-locked laser has been studied experimentally.By adjusting the fiber delay line and adjusting the modulation frequency,the repetition rate tuning with step of 2 MHz has been completed in the range of 2.49～2.51 GHz.At the same time,the delay line parameters required by continuous tuning repetition rate in active harmonic mode-locked laser are studied.The rational number harmonic mode locking based on tunable fiber delay line is studied.By adjusting the cavity length,the principle of rational number harmonic mode locking is analyzed,and the output of rational number harmonic mode locking at other offsets is inferred.3.he influence of the driving current of the semiconductor optical amplifier(SOA)on the laser polarization stateand pulse width were studied in the experiment.Continuous tuning of pulse widths in the range of 50 ps,26 ps and 15 ps was achieved by controlling the SOA current at repetition retes of 1 GHz,2.5 GHz and 6 GHz,respectively.It is found that the change in the polarization state will cause the change of the modulation depth of the Mach-Zehnder Modulator(MZIM)to change the pulse width.According to this principle,the method of adjusting the PC is used to tune the pulse width.Finally,at 2.5 A pulse width tuning range of about 40 ps is achieved at a repetition rate of GHz.4.The effects of cavity length and frequency detuning on pulse width are studied.In the experiment,it has been found that increasing the delay time of the delay line positively within a certain range will lead to a linear increase in the pulse width.This method enables continuous tuning of pulse widths from 4 ps to 25 ps.The reason for analyzing this phenomenon is that the modulation frequency is slightly detuned from the cavity length.It was also experimentally studied that the negative offset delay time does not lead to a linear increase in the pulse width.But astable mode-locked state at offset-20 ps cannot be maintain.Through analysis and discussion,it is believed that the nonlinear gain and gain saturation of SOA are the reasons for the asymmetry of the influence of the positive and negative offset of the delay time on the pulse width.