Youjian Song

Timing jitter optimization of mode-locked Yb-fiber lasers toward the attosecond regime

We demonstrate ultra-low timing jitter optical pulse trains from free-running, 80 MHz repetition rate, mode-locked Yb-fiber lasers. Timing jitter of various mode-locking conditions at close-to-zero intracavity dispersion (-0.004 to +0.002 ps(2) range at 1040 nm center wavelength) is characterized using a sub-20-attosecond-resolution balanced optical cross-correlation method. The measured lowest rms timing jitter is 175 attoseconds when integrated from 10 kHz to 40 MHz (Nyquist frequency) offset frequency range, which corresponds to the record-low timing jitter from free-running mode-locked fiber lasers so far. We also experimentally found the mode-locking conditions of fiber lasers where both ultra-low timing jitter and relative intensity noise can be achieved.

Sub-100-as timing jitter optical pulse trains from mode-locked Er-fiber lasers

We demonstrate sub-100-as timing jitter optical pulse trains generated from free-running, 77.6 MHz repetition-rate, mode-locked Er-fiber lasers. At -0.002(±0.001) ps2 net cavity dispersion, the rms timing jitter is 70 as (224 as) integrated from 10 kHz (1 kHz) to 38.8 MHz offset frequency, when measured by a 24 as resolution balanced optical cross correlator. To our knowledge, this result corresponds to the lowest rms timing jitter measured from any mode-locked fiber lasers so far. The measured result also agrees fairly well with the Namiki-Haus analytic model of quantum-limited timing jitter in stretched-pulse fiber lasers.

Environmentally Stable, High Pulse Energy Yb-Doped Large-Mode-Area Photonic Crystal Fiber Laser Operating in the Soliton-Like Regime

A high pulse energy passively mode-locking fiber laser operating in the soliton-like regime is demonstrated. The laser is based on a linear cavity design. A segment of Yb-doped single-polarization large-mode-area photonic crystal fiber serves as the gain medium, and the self-starting mode-locking is achieved by a high contrast semiconductor saturable absorber mirror. The laser directly generates 600-fs pulses with 900 mW of average power at a repetition rate of 47.3 MHz, corresponding to a single pulse energy of 19 nJ. Furthermore, this fiber laser is directly used for pumping ZnTe to generate broadband terahertz radiation, resulting in a compact terahertz source.

Impact of pulse dynamics on timing jitter in mode-locked fiber lasers

We investigate the high-frequency timing jitter spectral density of mode-locked fiber lasers in different mode-locked regimes. Quantum-noise-limited timing jitter spectra of mode-locked-regime-switchable Yb fiber lasers are measured up to the Nyquist frequency with sub-100-as resolution. The integrated rms timing jitter of soliton, stretched-pulse, and self-similar Yb fiber lasers is measured to be 1.8, 1.1, and 2.9 fs, respectively, when integrated from 10 kHz to 40 MHz. The distinct behavior of jitter spectral density related to pulse formation mechanisms is revealed experimentally for the first time.

A hollow beam from a holey fiber

A high-quality spectrally isolated hollow beam is produced through a nonlinear-optical transformation of Ti: sapphire laser pulses in a higher order mode of a photonic-crystal fiber (PCF). Instead of a doughnut shape, typical of hollow beams produced by other methods, the far-field image of the hollow-beam PCF output features perfect sixth-order rotation symmetry, dictated by the symmetry of the PCF structure. The frequency of the PCF-generated hollow beam can be tuned by varying the input beam parameters, making a few-mode PCF a convenient and flexible tool for the guiding and trapping of atoms and creation of all-fiber optical tweezers.

Self-similar evolution in a short fiber amplifier through nonlinear pulse preshaping.

We report on a nonlinear preshaper that optimizes initial pulses for self-similar evolution in a next fiber amplifier. It consists of a pair of gratings and a segment of single-mode fiber (SMF). The grating pair provides negative chirp to make the pulses preshaped temporally and spectrally in the SMF. With this optimization, the self-similar amplification can be realized in a 2.2 m Yb-doped fiber in a large range of pump power. After amplification, the pulse can be dechirped to transform-limited pulses with ~60 fs pulse duration.

High pulse energy mode-locked multicore photonic crystal fiber laser.

A high pulse energy passively mode-locked fiber laser operating in the all-normal dispersion regime is demonstrated. The gain material is an Yb-doped multicore photonic crystal fiber with 18 cores in array-type geometry. Robust and self-starting mode locking is achieved using a fast semiconductor saturable absorber mirror. The laser generates 180 nJ chirped pulses at a 14.48 MHz repetition rate for an average power of 2.6 W. The 1.15 ps output pulses are compressed to 690 fs outside the cavity.

Ultralow-noise mode-locked fiber lasers and frequency combs: principles, status, and applications

We review the most recent progress in ultralow-noise mode-locked fiber lasers and fiber-based frequency-comb sources. With the rapid progress in theory, measurement, and control of noise in passively mode-locked fiber lasers, we have reached the point where the residual carrier–envelope-offset phase jitter (when stabilized) and pulse timing jitter performance of such laser sources can be fully optimized to the unprecedented levels of attoseconds regime. In this paper, first, major principles in building such low-noise passively mode-locked fiber lasers are reviewed. We then define noise in mode-locked fiber lasers and present the basic theoretical and numerical framework for analyzing the noise in mode-locked fiber lasers. More detailed discussions on theory, measurement methods, state-of-the-art performances, and stabilization methods of intensity noise, timing jitter, and comb-line frequency noise follow. Finally, we overview today’s most representative applications of such ultralow-noise mode-locked fiber lasers and frequency-comb sources. As an already powerful tool for various high-precision applications, ultralow-noise mode-locked fiber lasers will keep finding more exciting applications in optical science and photonic technology in the coming years.

Reduction of timing jitter and intensity noise in normal-dispersion passively mode-locked fiber lasers by narrow band-pass filtering.

Fiber lasers mode-locked with normal cavity dispersion have recently attracted great attention due to large output pulse energy and femtosecond pulse duration. Here we accurately characterized the timing jitter of normal-dispersion fiber lasers using a balanced cross-correlation method. The timing jitter characterization experiments show that the timing jitter of normal-dispersion mode-locked fiber lasers can be significantly reduced by using narrow band-pass filtering (e.g., 7-nm bandwidth filtering in this work). We further identify that the timing jitter of the fiber laser is confined in a limited range, which is almost independent of cavity dispersion map due to the amplifier-similariton formation by insertion of the narrow bandpass filter. The lowest observed timing jitter reaches 0.57 fs (rms) integrated from 10 kHz to 10 MHz Fourier frequency. The rms relative intensity noise (RIN) is also reduced from 0.37% to 0.02% (integrated from 1 kHz to 5 MHz Fourier frequency) by the insertion of narrow band-pass filter.

Broadband Terahertz Pulses Generated by a Compact Femtosecond Photonic Crystal Fiber Amplifier

We demonstrate a scalable, compact, high-power, and broadband terahertz (THz) source based on a cutting-edge large-mode-area photonic-crystal-fiber amplifier system. A 3-mm (110)-cut bulk GaP crystal was used as the THz emitter based on an optical rectification technique. Systematic optimization of the operation parameters allowed a THz output up to 150 μW to be achieved with an input laser power of 12 W in the GaP crystal.