Research On Mid-infrared GHz High Repetition Rate Ultrafast Laser

High repetition-rate ultrashort pulsed lasers operating in the mid-infrared range(particularly 2?5?m)are essential for several cutting-edge technology applications,such as high-speed free-space optical communications,astronomical optical frequency comb and precision spectroscopy.Ultrashort pulsed lasers with repetition rates of tens of Gigahertz are urgently needed to facilitate the development of laser technology and its cross-fields.Mid-infrared lasers operating at 2?5?m can be produced by direct las-ing(such as thulium-doped fiber lasers and quantum cascade lasers)or by nonlinear frequency conversion technology.Thulium-doped silica fiber technologies,with its huge gain bandwidth and ease of processing,are currently the most established gain material at 2?m,while optical parametric oscillation is the dominant technology in the3 to 5?m region.In this thesis,we propose to develop an ultrashort pulsed laser system that covers the entire 2?5?m range and feature GHz repetition rate.A variety of high repetition-rate pulsing techniques such as extra-cavity modulation,active mode-locking and opti-cal parametric amplification have been investigated.The research can be summarized as follows:1.We have for the first time experimentally demonstrated a high repetition-rate picosecond fiber laser source at 2?m by a spectrally masked phase modulation tech-nique,where a tunable seed diode laser and an FBG can be combined to yield effective pulse generation.The repetition-rate of this laser source can be continuously and flexi-bly tuned from 1 to 10 GHz by simply changing the RF signal.We achieved the shortest pulse width of 50 ps and a high SNR of 55 d B at the repetition rate of 10 GHz.Since SMPM directly modulates the phase of laser,there is no need for a DC bias.The sim-plicity and robustness of such a picosecond laser as well as the ability to synchronize with an external trigger make it a highly useful source for 2?m high-speed optical data processing,communications,and metrology.2.Two GHz repetition-rate actively mode-locked thulium fiber lasers were demon-strated by employing a lithium niobate phase modulator and a lithium niobate intensity modulator,which can produce<30 picosecond pulses at a maximum repetition rate of19 GHz.We have improved the repetition-rate of actively mode-locked Tm-doped fiber laser by more than one order of magnitude.The repetition rate of the mode-locked pulses can be fine controlled from 14.6 MHz to 19 GHz by synchronizing the RF fre-quency with an integer multiple of the FSR.The 19 GHz pulses exhibit a super-mode suppression ratio of 46 d B and a pulse width of 26.5 ps.We have also experimentally observed rational harmonic mode-locking in the laser and obtained 14 GHz and 21 GHz repetition rate pulses using a 7 GHz modulating signal.Active mode-locked thulium-doped laser has proven to be an excellent solution for gigahertz clock signal generators since its relative ease in configuration as well as its compatibility with fiber communi-cation systems.3.We have studied the detuning effect in active mode-locked lasers.The detuning effect can be eliminated by either locking the cavity length with a feedback circuit or by feeding back the longitudinal self-beat signal to drive the modulator.A SESAM mode-locked erbium-doped fiber laser with PZT-PLL stabilization was first built to ob-tain 100 MHz pulses with a frequency drift of less than 3 m Hz over 72 hours.The Allan deviation was 5.3*10^-14@1000 s.A regenerative mode-locked thulium fiber laser with PLL operation at 10 GHz was then demonstrated.The system shows long-term(>16hours)repetition-rate frequency stability with excellent picosecond jitter performance.The Allan deviation of the system with an integration time of 1000s was 2*10^-12at 10GHz.4.High repetition-rate ultrafast mid-IR pulses have been demonstrated by nonlin-ear optical modulation of CW mid-IR lasers,utilizing a pump laser with high pulse energy.We have demonstrated high-repetition-rate femtosecond pulses by direct optical modulation of an ICL and an external-cavity quantum cascade laser(EC-QCL).A peri-odically poled lithium niobite crystal is used as the nonlinear material.The first ampli-fied 78 MHz,140 fs pulses were generated at 3.6?m with an average power of 300m W and a peak power of 27.4 k W.A 9*10⁶-fold magnification of the peak power was achieved compared to the injecting CW signal.A tunable optical parametric amplifier based on EC-QCL beyond 4?m was then demonstrated.A 78 MHz,337 fs pulses were generated with an average power of 200 m W and a peak power of 7.6 k W.A 1.2*10⁶-fold magnification of the peak power was achieved compared to the injecting CW signal.The wavelength tuning range of the mid-IR pulses reached 300 nm(from 4400 nm to4700 nm).Such a wavelength-agile pulsed source is expected to enable more laser in-strumentations for mid-IR spectroscopy and sensing.