Lingxiao Zhu

Spectral narrowing of chirp-free light pulses in anomalously dispersive, highly nonlinear photonic-crystal fibers

Spectral narrowing of nearly chirp-free 50-fs pulses delivered by a diode-pumped ytterbium solid-state laser (Yb DPSSL) is experimentally demonstrated using an anomalously dispersive, highly nonlinear photonic-crystal fiber (PCF). The ratio of spectral narrowing and the accompanying temporal pulse broadening are controlled by the peak power of Yb DPSSL pulses at the input of the fiber.

Pulse fidelity control in a 20-μJ sub-200-fs monolithic Yb-fiber amplifier

We discuss nonlinearity management versus energy scalability and compressibility in a three-stage monolithic 100-kHz repetition rate Yb-fiber amplifier designed as a driver source for the generation and tunable parametric amplification of a carrier-envelope phase stable white-light supercontinuum.

Performance of PAC/PDM composite coagulants for removal of algae from Lake Taihu waters in summer.

The enhanced coagulation of algae-rich raw water from Lake Taihu in summer was studied by use of composite coagulants. The composite coagulants were composed of polyaluminum chloride (PAC) and polydimethyldiallylammonium chloride (PDM) with various intrinsic viscosity values (0.55-3.99 dL/g) and different mass percentages (5-20%) in the formulation. For raw water with temperature of 28-29 degrees C and algae content of 3.60x10(4)-3.70x10(4) cells/ml, the algae-removal rates of 89.0% and 89.3-93.1% could be realized by using PAC and PAC/PDM (0.55/5%-3.99/20%) with dosages of 8.37 mg/L and 5.93-3.58 mg/L, respectively, when 2 NTU residual turbidity of treated water after sedimentation was required. Compared with using PAC only, the removal rate of CODMn using PAC/PDM increased at least 4.4% when the dosage was 8 mg/L, and increased at least 5.0% when the dosage was 10 mg/L. The composite coagulants could still function well when raw water quality deteriorated and algae content reached 8.00x10(4) cells/ml. The enhanced coagulation efficiency of PAC/PDM (0.55/5%) could be better than that of PAC combined with prechlorination process when the same dosages are used.

High-fidelity, 160 fs, 5 μj pulses from an integrated Yb-fiber laser system with a fiber stretcher matching a simple grating compressor

Although femtosecond microjoule Yb-fiber systems are attractive because of a straightforward power scalability, they inherently suffer from a lowered pulse fidelity as a result of complex dispersion and nonlinearity management. Here, we present an integrated Yb-fiber system delivering high-fidelity microjoule pulses compressible down to 160 fs. The system uses a dispersion compensating fiber stretcher that is specially designed to match the dispersion of a 1480 lines/mm grating compressor. Performance analysis suggests the further possibility of scaling the pulse energy to tens of microjoules without pulse quality deterioration using this dispersion management scheme.

Broadly tunable carrier envelope phase stable optical parametric amplifier pumped by a monolithic ytterbium fiber amplifier

In an effort to develop a robust and efficient front end for a chirped-pulse parametric amplification chain, we demonstrate a broadband difference-frequency converter driven by a monolithic femtosecond Yb-doped-fiber amplifier and emitting carrier-envelope-offset-free pulses with the energy of tens of nanojoules tunable in the wavelength range from 1200 nm to beyond 2 mum. Next to providing these seed pulses, the system enables direct optical synchronization of Nd- and Yb-doped pump lasers for subsequent parametric amplification.

Generation of high fidelity 62-fs, 7-nJ pulses at 1035 nm from a net normal-dispersion Yb-fiber laser with anomalous dispersion higher-order-mode fiber

We present a mode-locked 24 MHz Yb-doped fiber oscillator with a higher-order mode fiber for dispersion compensation. The oscillator operates in the net normal dispersion regime and generates clean 6 nJ pulses that can be dechirped down to 150 fs.

High peak-power monolithic femtosecond ytterbium fiber chirped pulse amplifier with a spliced-on hollow core fiber compressor

We demonstrate a monolithic Yb-fiber chirped pulse amplifier that uses a dispersion matched fiber stretcher and a spliced-on hollow core photonic bandgap fiber compressor. For an output energy of 77 nJ, 220 fs pulses with 92% of the energy contained in the main pulse, can be obtained with minimal nonlinearities in the system. 135 nJ pulses are obtained with 226 fs duration and 82 percent of the energy in the main pulse. Due to the good dispersion match of the stretcher to the hollow core photonic bandgap fiber compressor, the duration of the output pulses is within 10% of the Fourier limited duration.

Suppression of population-lifetime-determined energy instability in a femtosecond kHz Yb CPA

Energy saturation is achieved by seeding 1-kHz Yb:CaF2 amplifier by μJ pulses from a monolithic Yb-fiber MOPA leading to 6-mJ output. The saturation is possible at any rep-rate with appropriate seed and optical loss levels.

Optimizing the fluorescent signal for in-vivo nonlinear imaging in a completely all-fibered ytterbium chirped pulse amplifier

Multiphoton microscopy, including two-photon laser scanning microscopy and second-harmonic generation is a unique technique for non-invasive imaging in biological samples. However, clinical translation and applications outside the laser laboratory have been hampered by the need to use bulky, expensive and environmentally sensitive lasers, that often require active water cooling and have high energy consumption. Femtosecond fiber lasers, which have relaxed cooling and energy consumption requirements, are very attractive as master sources for multiphoton microscopy [1]. A challenge for applications in nonlinear imaging in living tissue is to maximize the total fluorescent yield from each fluorophore. Excitation with a transform-limited pulse significantly increases the efficiency of excitation [2]. Transform limited pulses are also advantageous to minimize the thermal load in living tissue. Multiphoton microscopy would strongly benefit from the implementation of robust all-integrated fiber based concepts that allow for truly turn-key maintenance free operation.

Efficient spectral broadening and recompression of 200 fs pulses from a monolithic Yb-FCPA to 66 fs

Ytterbium doped fiber chirped pulse amplifiers (FCPAs) are attractive sources to replace their solid-state counterparts in applications like micromachining, for example for manufacturing photonic devices in bulk glass substrates. For such applications, a pulse duration of several tens of fs is often considered to be optimal so that one can write features with a sub-wavelength precision. However, the shortest attainable pulse duration from μJ -level Yb-doped FCPAs is limited to ~160 fs because of gain narrowing in such systems. Therefore, spectral broadening of the output of the Yb-doped FCPA is needed to allow recompression to several tens of fs. Many methods to obtain spectral broadening are rather lossy (e.g. white light generation in a bulk substrate followed by spatial filtering) or produce pulses that are difficult to compress (e.g. spectral broadening in highly nonlinear or photonic crystal fibers). In a bid to obtain well-compressible pulses with a high energy-efficiency, we couple the compressed output pulses from our monolithic Yb-doped FCPA into a 20 cm long piece of polarization maintaining (PM) large mode area (LMA) fiber.