Heewon Yang

Sub-20-Attosecond Timing Jitter Mode-Locked Fiber Lasers

We demonstrate 14.3-attosecond timing jitter [integrated from 10 kHz to 94 MHz offset frequency] optical pulse trains from 188-MHz repetition-rate mode-locked Yb-fiber lasers. In order to minimize the timing jitter, we shorten the non-gain fiber length to shorten the pulsewidth and reduce excessive higher-order nonlinearity and nonlinear chirp in the fiber laser. The measured jitter spectrum is limited by the amplified spontaneous emission limited quantum noise in the 100 kHz-1 MHz offset frequency range, while it was limited by the relative intensity noise-converted jitter in the lower offset frequency range. This intrinsically low timing jitter enables sub-100-attosecond synchronization between the two mode-locked Yb-fiber lasers over the full Nyquist frequency with a modest 10-kHz locking bandwidth. The demonstrated performance is the lowest timing jitter measured from any free-running mode-locked fiber lasers, comparable to the performance of the lowest-jitter Ti:sapphire solid-state lasers.

Gigahertz repetition rate, sub-femtosecond timing jitter optical pulse train directly generated from a mode-locked Yb:KYW laser

We show that a 1.13 GHz repetition rate optical pulse train with 0.70 fs high-frequency timing jitter (integration bandwidth of 17.5 kHz-10 MHz, where the measurement instrument-limited noise floor contributes 0.41 fs in 10 MHz bandwidth) can be directly generated from a free-running, single-mode diode-pumped Yb:KYW laser mode-locked by single-wall carbon nanotube-coated mirrors. To our knowledge, this is the lowest-timing-jitter optical pulse train with gigahertz repetition rate ever measured. If this pulse train is used for direct sampling of 565 MHz signals (Nyquist frequency of the pulse train), the jitter level demonstrated would correspond to the projected effective-number-of-bit of 17.8, which is much higher than the thermal noise limit of 50 Ω load resistance (~14 bits).

1.2-GHz repetition rate, diode-pumped femtosecond Yb:KYW laser mode-locked by a carbon nanotube saturable absorber mirror

We demonstrate a 1.2-GHz repetition rate, diode-pumped, self-starting, 168-fs (FWHM) pulsewidth Yb:KYW laser mode-locked by a carbon nanotube (CNT) saturable absorber mirror. To our knowledge, this result corresponds to the highest repetition rate from CNT-mode-locked femtosecond bulk solid-state lasers, reaching the GHz regime for the first time.

Remote Laser-Microwave Synchronization Over Kilometer-Scale Fiber Link With Few-Femtosecond Drift

We show the remote synchronization between a mode-locked laser and a microwave source, separated by a hundreds meter- to kilometer-scale fiber link, with few-femtosecond rms timing drift maintained over several hours. In a laboratory test, the measured timing drift between a mode-locked laser and a 2.856-GHz microwave source, separated by a 610-m fiber link, is 2.7 fs rms (0.048 mrad phase drift) over 7 h. The corresponding relative instability in remote laser-microwave synchronization is 7.2 × 10 -19 in 6300 s averaging time, in terms of overlapping Allan deviation. We further installed 1.15-km long fiber links in an accelerator building and measured the relative phase drift at the link outputs in a klystron gallery, which resulted in 6.6-fs and 31-fs rms timing drift maintained over 7 and 62 h, respectively. To achieve this performance, we combined a balanced optical cross-correlator (BOC)-based stabilized fiber link for remote timing transfer and an optical-microwave phase detector (OM-PD) for local optical-to-microwave synchronization. We identified the impact of power and polarization-state drift in the fiber link and amplitude-to-phase conversion in the OM-PD on the link stability. Based on this analysis, possible technical improvements enabling even higher timing precision and stability are identified.

10-fs-level synchronization of photocathode laser with RF-oscillator for ultrafast electron and X-ray sources.

Ultrafast electron-based coherent radiation sources, such as free-electron lasers (FELs), ultrafast electron diffraction (UED) and Thomson-scattering sources, are becoming more important sources in today’s ultrafast science. Photocathode laser is an indispensable common subsystem in these sources that generates ultrafast electron pulses. To fully exploit the potentials of these sources, especially for pump-probe experiments, it is important to achieve high-precision synchronization between the photocathode laser and radio-frequency (RF) sources that manipulate electron pulses. So far, most of precision laser-RF synchronization has been achieved by using specially designed low-noise Er-fibre lasers at telecommunication wavelength. Here we show a modular method that achieves long-term (>1 day) stable 10-fs-level synchronization between a commercial 79.33-MHz Ti:sapphire laser oscillator and an S-band (2.856-GHz) RF oscillator. This is an important first step toward a photocathode laser-based femtosecond RF timing and synchronization system that is suitable for various small- to mid-scale ultrafast X-ray and electron sources.

1.13-GHz Repetition Rate, Sub-Femtosecond Timing Jitter, CNT-Mode-Locked Ultrafast Yb:KYW Laser

We show 1.13-GHz repetition rate, 0.70-fs timing jitter optical pulse train directly generated from diode-pumped, CNT-mode-locked Yb:KYW laser. The measured jitter is the lowest for GHz pulse trains, and is suitable for high-resolution analog-to-digital conversion.

Progress in ultrafast fiber lasers for ultralow-jitter signal sources

We introduce the most recent progress in the optimization of ultrafast fiber lasers for building ultralow timing jitter signal sources. Using a sub-20-attosecond-resolution timing jitter measurement technique, we optimize the timing jitter of optical pulse trains from mode-locked Er-fiber and Yb-fiber lasers to 70 attoseconds and 175 attoseconds, respectively, when integrated from 10 kHz to 40 MHz offset frequency. To our knowledge, these results correspond to the lowest rms timing jitter demonstrated from fiber lasers so far, the equivalent phase noise of which is comparable to that of the best microwave sources available, with much reduced cost and engineering complexity.