Dohyeon Kwon

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.

Reference-free, high-resolution measurement method of timing jitter spectra of optical frequency combs

Timing jitter is one of the most important properties of femtosecond mode-locked lasers and optical frequency combs. Accurate measurement of timing jitter power spectral density (PSD) is a critical prerequisite for optimizing overall noise performance and further advancing comb applications both in the time and frequency domains. Commonly used jitter measurement methods require a reference mode-locked laser with timing jitter similar to or lower than that of the laser-under-test, which is a demanding requirement for many laser laboratories, and/or have limited measurement resolution. Here we show a high-resolution and reference-source-free measurement method of timing jitter spectra of optical frequency combs using an optical fibre delay line and optical carrier interference. The demonstrated method works well for both mode-locked oscillators and supercontinua, with 2 × 10−9 fs2/Hz (equivalent to −174 dBc/Hz at 10-GHz carrier frequency) measurement noise floor. The demonstrated method can serve as a simple and powerful characterization tool for timing jitter PSDs of various comb sources including mode-locked oscillators, supercontinua and recently emerging Kerr-frequency combs; the jitter measurement results enabled by our method will provide new insights for understanding and optimizing timing noise in such comb sources.

300-MHz-repetition-rate, all-fiber, femtosecond laser mode-locked by planar lightwave circuit-based saturable absorber

We show the implementation of fiber-pigtailed, evanescent-field-interacting, single-walled carbon nanotube (CNT)-based saturable absorbers (SAs) using standard planar lightwave circuit (PLC) fabrication processes. The implemented PLC-CNT-SA device is employed to realize self-starting, high-repetition-rate, all-fiber ring oscillators at telecommunication wavelength. We demonstrate all-fiber Er ring lasers operating at 303-MHz (soliton regime) and 274-MHz (stretched-pulse regime) repetition-rates. The 303-MHz (274-MHz) laser centered at 1555 nm (1550 nm) provides 7.5 nm (19 nm) spectral bandwidth. After extra-cavity amplilfication, the amplified pulse train of the 303-MHz (274-MHz) laser delivers 209 fs (178 fs) pulses. To our knowledge, this corresponds to the highest repetition-rates achieved for femtosecond lasers employing evanescent-field-interacting SAs. The demonstrated SA fabrication method, which is based on well-established PLC processes, also shows a potential way for mass-producible and lower-cost waveguide-type SA devices suitable for all-fiber and waveguide lasers.

Femtosecond Laser-Based Microwave Signal Generation and Distribution

We review our most recent progress in microwave photonic applications of ultralow timing jitter femtosecond mode-locked fiber lasers. Sub-femtosecond timing jitter (integrated from 10 kHz to >10 MHz offset frequency) optical pulse trains can be generated from various types of mode-locked fiber lasers using dispersion engineering and intracavity filtering. To fully utilize such ultralow-jitter lasers for microwave photonic applications, we demonstrate a sub-femtosecond-resolution (-159 dBc/Hz phase noise floor) phase detection method between optical pulse trains and microwave signals using a Sagnac-fiber-loop-based device named the fiber-loop optical-microwave phase detector (FLOM-PD). Using ultralow-jitter mode-locked Er-fiber lasers and FLOM-PDs, we generate 10-GHz microwave signals with -142 dBc/Hz absolute single-sideband phase noise at 10-kHz offset frequency. When an all-fiber Michelson interferometer-based repetition-rate stabilization method is further employed, the phase noise is suppressed to -90 dBc/Hz at 10-Hz offset frequency, which results in 3-fs absolute rms timing jitter integrated from 10 Hz to 10 MHz offset frequency. Long-distance microwave phase transfer via optical fiber links is also demonstrated using the FLOM-PD as a means for stabilizing pulse time-of-flight in fiber transfer. Relative frequency instability of 6.5 × 10-19 is demonstrated for 2.856-GHz microwave signals transferred over a 2.3-km-long fiber link. We anticipate that the capability of generation, characterization, stabilization, and transfer of ultralow-noise microwave signals using ultralow-jitter femtosecond mode-locked lasers will find more applications in microwave photonics in the near future.

All-polarization-maintaining mode-locked fiber laser based on planar lightwave circuit (PLC) device

Femtosecond mode-locked fiber lasers have been actively employed in many practical applications such as micro-machining, corneal surgery and multi-photon imaging, thanks to their compactness, alignment-free operation, and low cost. To enhance the long-term stability and self-starting operation of mode-locked lasers, saturable absorber (SA) devices, such as semiconductor saturable absorber mirrors (SESAMs), have been employed. Recently, planar waveguide-type SA device based on standard planar lightwave circuit (PLC) wafer fabrication process [1] has been developed and applied to high-repetition-rate all-fiber mode-locked lasers [2].

Ultrasensitive, high-dynamic-range and broadband strain sensing by time-of-flight detection with femtosecond-laser frequency combs

Ultrahigh-resolution optical strain sensors provide powerful tools in various scientific and engineering fields, ranging from long-baseline interferometers to civil and aerospace industries. Here we demonstrate an ultrahigh-resolution fibre strain sensing method by directly detecting the time-of-flight (TOF) change of the optical pulse train generated from a free-running passively mode-locked laser (MLL) frequency comb. We achieved a local strain resolution of 18 pε/Hz1/2 and 1.9 pε/Hz1/2 at 1 Hz and 3 kHz, respectively, with large dynamic range of >154 dB at 3 kHz. For remote-point sensing at 1-km distance, 80 pε/Hz1/2 (at 1 Hz) and 2.2 pε/Hz1/2 (at 3 kHz) resolution is demonstrated. While attaining both ultrahigh resolution and large dynamic range, the demonstrated method can be readily extended for multiple-point sensing as well by taking advantage of the broad optical comb spectra. These advantages may allow various applications of this sensor in geophysical science, structural health monitoring, and underwater science.

Robust, low-noise, polarization-maintaining mode-locked Er-fiber laser with a planar lightwave circuit (PLC) device as a multi-functional element

We demonstrate a new planar lightwave circuit (PLC)-based device, integrated with a 980/1550 wavelength division multiplexer, an evanescent-field-interaction-based saturable absorber, and an output tap coupler, which can be employed as a multi-functional element in mode-locked fiber lasers. Using this multi-functional PLC device, we demonstrate a simple, robust, low-noise, and polarization-maintaining mode-locked Er-fiber laser. The measured full-width at half-maximum bandwidth is 6 nm centered at 1555 nm, corresponding to 217 fs transform-limited pulse duration. The measured RIN and timing jitter are 0.22% [10 Hz-10 MHz] and 6.6 fs [10 kHz-1 MHz], respectively. Our results show that the non-gain section of mode-locked fiber lasers can be easily implemented as a single PLC chip that can be manufactured by a wafer-scale fabrication process. The use of PLC processes in mode-locked lasers has the potential for higher manufacturability of low-cost and robust fiber and waveguide lasers.

All-fiber repetition-rate stabilization of mode-locked lasers to 1.7×10 −13 instability with sub-fs integrated jitter

We demonstrate all-fiber-based repetition-rate stabilization of mode-locked lasers using fiber delay-line. The repetition-rate of femtosecond Er-fiber laser is stabilized to 1.7×10⁻¹³ Allan deviation at 0.1-s averaging time with 0.98-fs integrated jitter [100 Hz–10 MHz].

Timing jitter reduction in self-similar Yb-fiber lasers via narrow intra-cavity bandpass filtering

Timing jitter reduction in self-similar Yb-fiber lasers is achieved by intra-cavity bandpass filtering. The laser changes from self-similar to dissipative soliton regime, while the timing jitter is reduced from 14.6-fs to 2.8-fs [10 kHz-30 MHz].