Hung-Wen Chen

3 GHz, fundamentally mode-locked, femtosecond Yb-fiber laser.

We demonstrate a fundamentally mode-locked Yb-fiber laser with 3 GHz repetition rate and ∼206  fs pulse duration. The laser incorporates two enabling technologies: a 1 cm heavily Yb-doped phosphate glass fiber as the gain medium and a high-dispersion (-1300  fs2) output coupler to manage cavity dispersion. The oscillator self-starts and generates up to 53 mW average power.

Optimization of femtosecond Yb-doped fiber amplifiers for high-quality pulse compression

We both theoretically and experimentally investigate the optimization of femtosecond Yb-doped fiber amplifiers (YDFAs) to achieve high-quality, high-power, compressed pulses. Ultrashort pulses amplified inside YDFAs are modeled by the generalized nonlinear Schrödinger equation coupled to the steady-state propagation-rate equations. We use this model to study the dependence of compressed-pulse quality on the YDFA parameters, such as the gain fibers doping concentration and length, and input pulse pre-chirp, duration, and power. The modeling results confirmed by experiments show that an optimum negative pre-chirp for the input pulse exists to achieve the best compression.

3 GHz, Yb-fiber laser-based, few-cycle ultrafast source at the Ti:sapphire laser wavelength

We demonstrate a compact ultrafast source centered at 850 nm with >200 nm bandwidth (full width at half-maximum) based on a 3 GHz Yb-fiber master-oscillator-power-amplifier system. The output pulses (with up to 13 W average power) from the laser system are coupled into short (<50 mm) pieces of photonic crystal fibers to excite broadband fiber-optic Cherenkov radiation; the resulting broad phase-matching bandwidth due to short fiber length produces an upconverted spectrum spanning in the wavelength range of 750-950 nm with average power of 94, 184, and 380 mW for fiber length of 28, 37, and 48 mm, respectively. The spectrum generated from the 37 mm fiber is then dechirped by eight double-chirped mirrors, leading to compressed pulses ~14 fs in duration. Such an ultrafast source is a promising substitute of multigigahertz mode-locked Ti:sapphire lasers for applications in optical frequency metrology and multiphoton coherent microscopy.

3 GHz, watt-level femtosecond Raman soliton source.

We demonstrate a 3 GHz repetition rate, femtosecond Raman soliton source with its wavelength tunable from 1.15 to 1.35 μm. We investigate the dependence of Raman soliton formation on different photonic-crystal fibers (PCFs), input powers, and fiber lengths. To produce a Raman soliton peaking at the same wavelength, shorter PCFs demand higher input average powers and consequently generate stronger Raman soliton pulses. Using 30 cm PCF NL-3.2-945, the resulting Raman soliton pulse at 1.35 μm has 0.9 W average power. The integrated relative intensity noise of the Raman soliton pulse at 1.35 μm generated from the 54-cm PCF NL-3.2-945 is as low as 0.33% from 100 Hz to 10 MHz.

Frequency comb based on a narrowband Yb-fiber oscillator: pre-chirp management for self-referenced carrier envelope offset frequency stabilization.

Laser frequency combs are normally based on mode-locked oscillators emitting ultrashort pulses of ~100-fs or shorter. In this paper, we present a self-referenced frequency comb based on a narrowband (5-nm bandwidth corresponding to 415-fs transform-limited pulses) Yb-fiber oscillator with a repetition rate of 280 MHz. We employ a nonlinear Yb-fiber amplifier to both amplify the narrowband pulses and broaden their optical spectrum. To optimize the carrier envelope offset frequency (fCEO), we optimize the nonlinear pulse amplification by pre-chirping the pulses at the amplifier input. An optimum negative pre-chirp exists, which produces a signal-to-noise ratio of 35 dB (100 kHz resolution bandwidth) for the detected fCEO. We phase stabilize the fCEO using a feed-forward method, resulting in 0.64-rad (integrated from 1 Hz to 10 MHz) phase noise for the in-loop error signal. This work demonstrates the feasibility of implementing frequency combs from a narrowband oscillator, which is of particular importance for realizing large line-spacing frequency combs based on multi-GHz oscillators usually emitting long (>200 fs) pulses.

Green astro-comb for HARPS-N

We report the design, installation and testing of a broadband green astro-comb on the HARPS-N spectrograph at the TNG telescope. The astro-comb consists of over 7000 narrow lines (<10-6 nm width) spaced by 16 GHz (0.02 nm at 550 nm) with wavelengths stabilized to the Global Positioning System (GPS) and with flat power from 500 to 620 nm. The narrow lines are used to calibrate the spectrograph and measure its line profile. The short term sensitivity of HARPS-N is measured to be less than 2 cm/s and the long-term drift of the spectrograph is approximately 10 cm/s/day. The astrocomb has been partially automated with future work planned to turn the astro-comb into a fully automated, push button instrument.

Spectrally flat, broadband visible-wavelength astro-comb

We demonstrate a broadband visible-wavelength astro-comb enabled by two key technologies: dispersion-managed, fiber-optic Cherenkov radiation for green-to-red source-comb generation and complementary chirped-mirror pairs for constructing broadband Fabry-Perot filtering cavities.

High-quality pulse-compression of pre-chirped pulses in fiber-amplifiers

We combine steady-state propagation-rate equation and generalized nonlinear Schrödinger equation to accurately model high-repetition rate femtosecond Yb-fiber amplifiers. Such modeling reveals the nonlinear-evolution dynamics of the amplified pulse and allows the optimization of the compressed-pulse quality.

Relative intensity noise of Raman solitons: which one is more noisy?

We experimentally study the relative intensity noise of Raman solitons and find that earlier ejected Raman soliton exhibits lower noise. We also observe the bound soliton pair existing in a large range of excitation power.

Fiber-optic demonstration of optical frequency division for erbium silicon photonics integrated oscillator

Using fiber-optic components, we demonstrate an optical frequency division scheme for a proposed erbium silicon photonics integrated oscillator. An 80-dB division ratio from 192 THz to 1 GHz is achieved without an f-2f interferometer and carrier-envelope-phase locking.