Simon Lefrancois

Scaling of dissipative soliton fiber lasers to megawatt peak powers by use of large-area photonic crystal fiber

We report an all-normal dispersion femtosecond laser based on large-mode-area Yb-doped photonic crystal fiber. Self-starting mode-locked pulses are obtained with an average power of 12 W at 84 MHz repetition rate, corresponding to 140 nJ of chirped pulse energy. These are dechirped to a near transform-limited duration of 115 fs. Experimental results are consistent with numerical simulations of dissipative soliton intra-cavity pulse evolution, and demonstrate scaling of 100 fs pulses to megawatt peak powers.

Fiber Four-Wave Mixing Source for Coherent Anti-Stokes Raman Scattering Microscopy

We present a fiber-format picosecond light source for coherent anti-Stokes Raman scattering microscopy. Pulses from a Yb-doped fiber amplifier are frequency converted by four-wave mixing (FWM) in normal-dispersion photonic crystal fiber to produce a synchronized two-color picosecond pulse train. We show that seeding the FWM process overcomes the deleterious effects of group-velocity mismatch and allows efficient conversion into narrow frequency bands. The source generates more than 160 mW of nearly transform-limited pulses tunable from 775 to 815 nm. High-quality coherent Raman images of animal tissues and cells acquired with this source are presented.

Generation of megawatt peak power picosecond pulses from a divided-pulse fiber amplifier

Control of nonlinearity is a challenge in fiber amplifiers designed to generate pulses of a few picoseconds duration, and as a result, picosecond fiber amplifiers have failed to reach peak power of 1MW. Divided-pulse amplification, combined with the use of circular polarization, allows the generation of 2.2 ps pulses with energy as high as 2.5 μJ and peak power of 1 MW.

Energy scaling of mode-locked fiber lasers with chirally-coupled core fiber.

We report a mode-locked dissipative soliton laser based on large-mode-area chirally-coupled-core Yb-doped fiber. This demonstrates scaling of a fiber oscillator to large mode area in a format that directly holds the lowest-order mode and that is also compatible with standard fiber integration. With an all-normal-dispersion cavity design, chirped pulse energies above 40 nJ are obtained with dechirped durations below 200 fs. Using a shorter fiber, dechirped durations close to 100 fs are achieved at pump-limited energies. The achievement of correct energy scaling is evidence of single-transverse-mode operation, which is confirmed by beam-quality and spectral-interference measurements.

Controlling free-carrier temporal effects in silicon by dispersion engineering

Nonlinear silicon photonics will play an important role in future integrated opto-electronic circuits. Here we report temporal pulse broadening induced by the dynamic interplay of nonlinear free-carrier dispersion coupled with group-velocity dispersion in nanostructured silicon waveguides for the first time, to the best of our knowledge. Further, we demonstrate that the nonlinear temporal dynamics are supported or countered by third-order dispersion, depending on the sign. Our time-domain measurements of the subpicojoule pulse dynamics are supported by strong agreement with numerical modeling. In addition to the fundamental nonlinear optical processes unveiled here, these results highlight dispersion engineering as a powerful tool for controlling free-carrier temporal effects.

Scaling Fiber Lasers to Large Mode Area: An Investigation of Passive Mode-Locking Using a Multi-Mode Fiber

The mode-locking of dissipative soliton fiber lasers using large mode area fiber supporting multiple transverse modes is studied experimentally and theoretically. The averaged mode-locking dynamics in a multi-mode fiber are studied using a distributed model. The co-propagation of multiple transverse modes is governed by a system of coupled Ginzburg-Landau equations. Simulations show that stable and robust mode-locked pulses can be produced. However, the mode-locking can be destabilized by excessive higher-order mode content. Experiments using large core step-index fiber, photonic crystal fiber, and chirally-coupled core fiber show that mode-locking can be significantly disturbed in the presence of higher-order modes, resulting in lower maximum single-pulse energies. In practice, spatial mode content must be carefully controlled to achieve full pulse energy scaling. This paper demonstrates that mode-locking performance is very sensitive to the presence of multiple waveguide modes when compared to systems such as amplifiers and continuous-wave lasers.

Nonlinear silicon photonics analyzed with the moment method

We apply the moment method to nonlinear pulse propagation in silicon waveguides in the presence of two-photon absorption (TPA), free-carrier dispersion, and free-carrier absorption (FCA). The evolution equations for pulse energy, temporal position, duration, frequency shift, and chirp are obtained. We derive analytic expressions for the free-carrier induced blueshift and acceleration and show that they depend only on the pulse peak power. Importantly, these effects are independent of the temporal duration. The moment equations are then numerically solved to provide fast estimates of pulse evolution trends in silicon photonic waveguides. We find that group-velocity and free-carrier dispersion dominate the pulse dynamics in photonic crystal waveguides. In contrast, two-photon and FCA dominate the temporal dynamics in silicon nanowires. To the best of our knowledge, this is the first time the moment method is used to provide a concise picture of free-carrier effects in silicon photonics. The treatment and conclusions apply to any semiconductor waveguide exhibiting TPA.

Low-threshold Brillouin laser at 2 μm based on suspended-core chalcogenide fiber.

We present, to the best of our knowledge, the first demonstration of a 2 μm Brillouin laser based on a thulium-doped fiber pump and a chalcogenide fiber. A short 1.5 m piece of suspended-core chalcogenide As38Se62 fiber is employed as a gain medium, taking advantage of its small effective mode area and high Brillouin gain coefficient. A record-low lasing threshold of 52 mW is achieved, which is about 10 times lower than previously demonstrated in silica fiber cavities.

Non-degenerate two-photon absorption in silicon waveguides: analytical and experimental study

We theoretically and experimentally investigate the nonlinear evolution of two optical pulses in a silicon waveguide. We provide an analytic solution for the weak probe wave undergoing non-degenerate two-photon absorption (TPA) from the strong pump. At larger pump intensities, we employ a numerical solution to study the interplay between TPA and photo-generated free carriers. We develop a simple and powerful approach to extract and separate out the distinct loss contributions of TPA and free-carrier absorption from readily available experimental data. Our analysis accounts accurately for experimental results in silicon photonic crystal waveguides.

Hybrid Brillouin/thulium multiwavelength fiber laser with switchable single- and double-Brillouin-frequency spacing

We demonstrate a multiwavelength laser at 2 µm based on a hybrid gain scheme consisting of a Brillouin gain medium and a thulium-doped fiber. The laser has switchable frequency spacing, corresponding to the single and double Brillouin frequency shifts. In the 20 dB bandwidth, seven lasing channels with a frequency spacing of 0.1 nm (7.62 GHz) and eleven channels with a double-spacing of 0.2 nm (15.24 GHz) are obtained. A wavelength tunability of 1.3 nm is realized for both laser configurations by shifting the pump wavelength. Strong four wave mixing is observed in the double-spacing laser resulting in an improved performance: larger number of channels and better temporal stability.