High Power Amplification And Precise Control Of Optical Fiber Frequency Comb

Optical frequency combs with high average power and broadband spectral range provide significant experimental tools for precision spectroscopy, coherent anti-Stokes Raman scattering spectrum imaging, and high-accuracy long-distance measurement. Especially for the application of ultraviolet or mid-infrared frequency comb generation, high-power high accurate frequency comb is essential to acting as the driving source. To study the power scaling and precise control of frequency combs, my works are focused on the improvement of high-power fiber amplifiers, synchronization of multi-color femtosecond lasers, generation of cascade high-power multi-color fiber frequency comb, and suppression of carrier-envelope (CE) phase noise for coherent frequency comb combination. In this dissertation, we demonstrated a high-power low-noise broadband frequency comb stabilized by feedback control scheme. Meanwhile, an active feed-forward method was employed for compensating the relative carrier-envelope drifts of fiber optical amplifiers, paving a novel way to generate high-power, high-accuracy optical frequency combs by coherently combining a large number of fiber amplifiers seeded by the same comb oscillator.The works demonstrated in the dissertation include:high-power fiber amplifier improvement, multi-color laser synchronization,100-W frequency comb stabilization, and carrier-envelope phase noise compensation for comb combination. The details are summarized as follows:1. We optimized the temporal and spectral performance of high-power fiber amplifiers by changing the laser oscillator, the gain medium and the amplification structure. 1) Ultrashort pulses with an average power of303W at1031nm were produced by four-stage large-mode-area photonic crystal fiber amplifiers seeded by a diode-pumped Yb:YAG ceramic laser oscillator.2) A20-W all fiber picosecond laser at1064nm was built via nonlinear polarization rotation mode-locking and cascade all fiber amplifiers, electronic control and mechanical protection were integrated for practical use.3) We observed the16%increase of the slope efficiency and restraint of amplified spontaneous emission at1530nm in polarized separated erbium-doped single mode amplifier, which has the potential for ultra low noise frequency comb amplification.2. We achieved high-power synchronized multi-color ultrashort lasers by spectral fraction amplification and master-slave laser configuration, which could be used to generate ultra broadband optical frequency by spectral beam combination.1) Passive synchronization of three femtosecond mode-locked lasers at different central wavelengths was achieved. The timing jitter was7.7fs between800-nm Ti:sapphire and high-power1030-nm pulses, and56.5fs between1550-nm and1030-nm pulses, respectively.2) Synchronized frequency-doubled laser pulses at515nm with an average power of16.7W was obtained, corresponding to a nonlinear frequency conversion efficiency of33%.3. High-power infrared frequency comb was generated with carrier-envelope offset frequency locked to several millihertz via feedback scheme, which was incident on nonlinear crystal to obtain ultraviolet frequency comb.1) A frequency comb with100-W average power was achieved based on a Ti:sapphire femtosecond laser oscillator and large-mode-area fiber amplifiers, the line-width of locked offset frequency was2.25mHz.2) Experimental comparison between self-and cross-referenced f-2f measurements for carrier-envelope phase detection was implemented, the phase noise of locked beat signal were0.41and0.49rad, revealing a line-width of1.86and2.06mHz, respectively.3) By frequency quadrupling femtosecond pulse train from high-power large-mode-area fiber chirped-pulse amplifier at1030nm, we obtained ultraviolet pulse at258nm with an average power of1.62W, corresponding to an optical-to-optical efficiency of3.85%.4. Successful carrier-envelope drift noise suppression during comb amplification was achieved for demonstration of frequency comb combining with two amplifier branches.1) A Mach-Zender interferometer was used to characterize the relative CE drifts of an optical frequency comb before and after power scaling. The frequency noise of the relative CE drifts was well controlled in a variation range from±15Hz of free-running to approximately±1.5Hz via an active feed-forward compensation method, corresponding to an accumulated phase noise reduced from0.23rad to0.14rad.2) We controlled the CE drifts of two10-W Yb-doped fiber amplifiers to demonstrated coherent optical comb combination, opening up a way to scale the average power of optical frequency comb.