Catalytic Atroposelective Synthesis of N-N Axially Chiral Indolylamides.

The catalytic atroposelective synthesis of N-N axially chiral indolylamides was established via dynamic kinetic resolution, which makes use of chiral Lewis base-catalyzed asymmetric acylation of N-acylaminoindoles as a new type of platform molecule with anhydrides. By this strategy, a series of N-N axially chiral indolylamides were synthesized in overall good yields (up to 98%) with excellent enantioselectivities (up to 99% ee). Moreover, some of these N-N axially chiral indolylamides display some extent of anticancer activity, which demonstrates their potential application in medicinal chemistry. Therefore, this work has not only provided a new strategy for the synthesis of N-N axially chiral monoaryl indoles but also offered a new member of N-N axially chiral monoaryl indoles with configurational stability and promising application, thereby solving the challenges in atroposelective synthesis and application of N-N axially chiral monoaryl indoles.

Hundred-watt level linearly polarized visible supercontinuum generation.

An all-fiber linearly polarized supercontinuum (SC) laser source with 93 W average output power and spectrum ranging from 520 nm to 2300 nm is experimentally demonstrated. The linearly-polarized SC is generated in a piece of 2.6 m long polarization-maintaining photonic crystal fiber (PM-PCF), pumped by a polarization-maintaining picosecond Yb-doped master oscillator power amplifier (PM-MOPA). The source exhibits a flat spectrum from 600 nm to 1880 nm at -10 dB level except for the residual pump peak. A new method is proposed to measure the polarization extinction ratio (PER) of each single wavelength of the broadband supercontinuum at a high-power level, resulting in larger than 16 dB PER from 900 nm to 1600 nm and larger than 15 dB PER from 540 nm to 650 nm. To our knowledge, this is the first demonstration of hundred-watt level linearly polarized visible SC and the first demonstration of PER measurement of each single wavelength within such a wide spectrum range.

High power LP11 mode supercontinuum generation from an all-fiber MOPA.

High power LP11 mode supercontinuum is generated from 25/250 large mode area (LMA) fiber. A mechanical long period grating (LPG) is utilized to control the transverse modes in the LMA fiber to realize the LP11 mode supercontinuum generation in a master oscillator power amplifier (MOPA) configuration. The generated LP11 mode supercontinuum covers the spectral range from ~900 nm to ~2100 nm with a -30-dB spectral width of ~1200 nm and 50% optical to optical conversion efficiency. The seed laser produces picosecond pulses with 1 MHz repetition rate at the wavelength of 1060 nm. After multi-stage amplification in ytterbium-doped fibers, the average output power is scaled to 54.51 W and 56.79 W respectively for LP11 and LP01 mode, accompanying supercontinuum generation. Obvious difference of supercontinuum generation between the LP01 and LP11 modes is experimentally observed due to the different dispersion characters.

Hyperspectral imaging for the spectral measurement of far-field beam divergence angle and beam uniformity of a supercontinuum laser.

A new novel method, hyperspectral imaging (HSI), is presented in this work to measure the beam divergence angle and beam profile uniformity of supercontinuum lasers. The obtained results of divergence angles are consistent with theoretically calculated values. The uniformity of different-size projected Gaussian beams was measured through referencing the data sets provided by HSI camera under the wavelength variation. HSI, compared with traditional methods, is much faster and capable of providing critical reference to supercontinuum output parameters measurements and practical application in far-field situation.

Ultraviolet-enhanced supercontinuum generation with a mode-locked Yb-doped fiber laser operating in dissipative-soliton-resonance region.

We experimentally demonstrate an all-fiber, ultraviolet-enhanced, supercontinuum generation in a specifically designed seven-core photonic crystal fiber pumped by a picosecond Yb-doped master oscillator power amplifier (MOPA). The MOPA source is seeded by a giant-chirped Yb-doped mode-locked fiber laser operating in the dissipative-soliton-resonance (DSR) region. The DSR is achieved by using a nonlinear optical loop mirror (NOLM) with a fundamental repetition rate of 4.5 MHz and a central wavelength of 1035 nm. An extremely wide optical spectrum spanning from 350 nm to 2400 nm is obtained with a total output power of 6.86 W.

High-power visible-enhanced all-fiber supercontinuum generation in a seven-core photonic crystal fiber pumped at 1016 nm.

An 80 W 350-2400 nm monolithic supercontinuum (SC) source is reported. The high-power SC is generated in a uniform multi-core photonic crystal fiber (PCF) pumped by a 1016 nm pulsed fiber laser. The specially designed PCF has seven 4.5 µm diameter cores, a 0.85 air-filling fraction, and a zero dispersion wavelength (ZDW) of 991 nm. The 1016 nm pulsed laser delivers up to 114 W average power, which is believed to be the highest currently reported for picosecond fiber lasers working below 1020 nm. In order to ensure a robust and compact all-fiber structure, the pump laser is fusion spliced to the PCF using a selective air-hole collapse technique, achieving an ultra-low splicing loss of 0.2 dB despite the severe mode field mismatch. The proximity of the pump wavelength to the ZDW of PCF leads to enhanced visible generation. The output SC has a high spectral density of up to 108 mW/nm (at 580 nm) and over 50 mW/nm across the entire visible waveband. The achieved short-wavelength edge and high-spectral-power density in the visible region, to the best of our knowledge, are the best results reported for high-power visible SC sources.

State distributions in two-dimensional parameter spaces of a nonlinear optical loop mirror-based, mode-locked, all-normal-dispersion fiber laser.

We present the results of numerical simulations of dissipative soliton generation using nonlinear Schrödinger equations in an all-normal-dispersion (ANDi) mode-locked fiber laser based on a nonlinear optical loop mirror (NOLM). Firstly, systematic and computationally intensive analysis of the pulse state distributions in two-dimensional parameter spaces of an ANDi fiber laser was conducted. In addition, we determined that unstable non-vanishing regions including pulsation and noise-like pulses are directly related to the saturable absorptions of NOLMs and that two critical filter bandwidths separate those regions from stable ones. Finally, we found that the multi-pulsing power threshold can be maximized by using an optimal optical filter bandwidth.

High-power transverse-mode-switchable all-fiber picosecond MOPA.

A high-power transverse-mode-switchable all-fiber picosecond laser in a master-oscillator power-amplifier (MOPA) configuration is demonstrated. The master oscillator is a gain-switched laser diode delivering picosecond pulses with 25 MHz repetition rate at the wavelength of 1.06 µm. After multi-stage amplification in ytterbium-doped fibers, the average output power is scaled to 117 W. A mechanical long-period grating is employed as a fiber mode convertor to achieve controllable conversion from the fundamental (LP01) to the second-order (LP11) mode. Efficient mode conversion is demonstrated and the output characteristics for both modes are investigated. It is shown that LP01 and LP11 modes have nearly identical optical-to-optical conversion efficiency during amplification, but the nonlinear spectral degradation is significantly alleviated for LP11 mode operation. Owing to the compact all-fiber architecture, this high-power transverse-mode-switchable fiber laser is reliable during long-term operation and thus promising for many practical applications, e.g. high-resolution laser micro-processing.

1016nm all fiber picosecond MOPA laser with 50W output.

This paper presents an all fiber high power picosecond laser at 1016 nm in master oscillator power amplifier (MOPA) configuration. A direct amplification of this seed source encounters obvious gain competition with amplified spontaneous emission (ASE) at ~1030 nm, leading to a seriously reduced amplification efficiency. To suppress the ASE and improve the amplification efficiency, we experimentally investigate the influence of the gain fiber length and the residual ASE on the perforemance of the 1016 nm amplifier. The optimized 1016 nm MOPA laser exhibits an average power of 50 W and an optical conversion efficiency of 53%.

Ultrahigh-brightness, spectrally-flat, short-wave infrared supercontinuum source for long-range atmospheric applications.

Fiber based supercontinuum (SC) sources with output spectra covering the infrared atmospheric window are very useful in long-range atmospheric applications. It is proven that silica fibers can support the generation of broadband SC sources ranging from the visible to the short-wave infrared region. In this paper, we present the generation of an ultrahigh-brightness spectrally-flat 2-2.5 µm SC source in a cladding pumped thulium-doped fiber amplifier (TDFA) numerically and experimentally. The underlying physical mechanisms behind the SC generation process are investigated firstly with a numerical model which includes the fiber gain and loss, the dispersive and nonlinear effects. Simulation results show that abundant soliton pulses are generated in the TDFA, and they are shifted towards the long wavelength side very quickly with the nonlinearity of Raman soliton self-frequency shift (SSFS), and eventually the Raman SSFS process is halted due to the silica fiber's infrared loss. A spectrally-flat 2-2.5 µm SC source could be generated as the result of the spectral superposition of these abundant soliton pulses. These simulation results correspond qualitatively well to the following experimental results. Then, in the experiment, a cladding pumped large-mode-area TDFA is built for pursuing a high-power 2-2.5 µm SC source. By enhancing the pump strength, the output SC spectrum broadens to the long wavelength side gradually. At the highest pump power, the obtained SC source has a maximum average power of 203.4 W with a power conversion efficiency of 38.7%. It has a 3 dB spectral bandwidth of 545 nm ranging from 1990 to 2535 nm, indicating a power spectral density in excess of 370 mW/nm. Meanwhile, the output SC source has a good beam profile. This SC source, to the best of our knowledge, is the brightest spectrally-flat 2-2.5 µm light source ever reported. It will be highly desirable in a lot of long-range atmospheric applications, such as broad-spectrum LIDAR, free space communication and hyper-spectral imaging.

80 nJ ultrafast dissipative soliton generation in dumbbell-shaped mode-locked fiber laser.

A novel all-fiberized dumbbell-shaped mode-locked fiber laser was developed to directly generate 80 nJ dissipative solitons, which can be linearly compressed from 85 to 1.2 ps externally with a diffraction grating pair. The pulse peak power reached 42 kW after compression. With the most available pump power, stable dissipative soliton bundles with up to 628 nJ bundle energy were obtained. The corresponding average output power reached 2.2 W. The employment of dual-nonlinear-optical-loop mirrors and large-mode-area fibers in the cavity played an essential role in improving structural compactness and producing high-energy ultrafast pulses. To the best of our knowledge, these are the most energetic compressible dissipative solitons generated from a strictly all-fiber cavity.

Selective transverse mode operation of an all-fiber laser with a mode-selective fiber Bragg grating pair.

We propose a novel approach for achieving selective transverse mode operation of few-mode all-fiber lasers. Lasing in a specific transverse mode is enabled by employing a pair of few-mode fiber Bragg gratings as an efficient transverse mode selector. As a proof-of-principle, we have implemented a ytterbium-doped all-fiber laser operating in the second-order (LP11) transverse mode. The achieved output power is 81 mW, and the slope efficiency is as high as 54%. This simple and compact all-fiber laser is also capable of delivering cylindrical vector beams through appropriate polarization control. Furthermore, the approach has great scalability and can be applied to other working modes and spectral regimes.

Highly stable, monolithic, single-mode mid-infrared supercontinuum source based on low-loss fusion spliced silica and fluoride fibers.

A 0.8 to 4.5 µm highly stable all-fiber spliced mid-infrared supercontinuum (SC) source was presented. The joint between the single-mode (SM) pump silica fiber and the ZBLAN fiber (ZrF4 - BaF2 - LaF3 - AlF3 - NaF, a type of fluoride fiber) was fusion spliced, which greatly improved the SC's stability. The low-loss splicing was guaranteed by the similar mode field areas of the fundamental mode LP(01) of the silica and ZBLAN fibers. At the splicing joint the ZBLAN fiber enveloped the silica fiber, thus increasing the robustness of the splice. A low splicing loss of less than 0.1 dB was calculated, which ensured that the whole SC source was very reliable. The SC had a maximal average power of 550.8 mW with a 1.5 dB spectral bandwidth ranging from 2642 to 4065 nm. In particular, the SC power for λ>3.8 µm was measured to be 116.1 mW with a power ratio of ∼21.1% of the total SC power. Perfect Gaussian beam profiles of the SC source demonstrated its SM operation. Over 12 h of continuous operation of this SC source showed its outstanding power stability with a root mean square variation of 0.59%, which also demonstrated the high quality of the fusion spliced joint.

Mode-locked all-fiber dumbbell-shaped laser based on a nonlinear amplifying optical loop mirror.

We report a hybrid passively mode-locked dumbbell-shaped fiber laser based on a nonlinear optical loop mirror and a nonlinear amplifying optical fiber-loop mirror. The laser produced noise-like pulses with repetition rate of 8.85 MHz and pulse energy of 16.2 and 26.4 nJ from the two output ports, respectively. Several interesting phenomena are observed and briefly discussed in the paper.

High-power near-infrared linearly-polarized supercontinuum generation in a polarization-maintaining Yb-doped fiber amplifier.

We report an all-fiber linearly-polarized (LP) supercontinuum (SC) source with high average power generated in a polarization-maintaining (PM) master-oscillation power-amplifier (MOPA). The experimental configuration comprises an LP picosecond pulsed laser and three PM Yd-doped fiber amplifiers (YDFA). The output has the average power of 124.8 W with the spectrum covering from 850 to 1900 nm. The measured polarization extinction ratio (PER) of the whole SC source is about 85% which verifies the SC an LP source. This work is, to our best knowledge, the highest output average power of an LP SC source that ever reported. The influence of PM fiber splicing method on the LP SC property is investigated by splicing the PM fibers with slow axis parallel or perpendicularly aligned, and also an LP SC with low output power is demonstrated.

Versatile long cavity widely tunable pulsed Yb-doped fiber laser with up to 27655th harmonic mode locking order.

We report flexible dissipative soliton generation from an all-fiberized all-normal-dispersion (ANDi) long cavity actively mode locked ytterbium doped fiber laser based on improved harmonic mode locking technique. The laser is featured with unusually wide and fine tunabilities in repetition rate and operating wavelength, meanwhile superior stability is maintained. The repetition rate of the laser can be flexibly controlled from 226 kHz to 6.25 GHz (corresponding to the highest 27655th order harmonic mode locking) in the interval of 226 kHz, while supermode suppression is confined above 50 dB and the pulse duration is retained in the range of 38 ps~80 ps. As high as 4.3 nJ pulse energy can be achieved with a low pump power of 160 mW when operating at the fundamental mode locking regime. The operating wavelength of the laser can be tuned in the wide range of 1005 nm~1100 nm. As far as we know, it is the first demonstration of up to ten thousands order stable harmonic mode locking in ANDi fiber laser, which manifests the capability of generating both high energy pulse and ultra-high repetition rate pulse in a single ANDi cavity. The destabilization of dissipative soliton under strong pump is also demonstrated.

Ultra-compact Watt-level flat supercontinuum source pumped by noise-like pulse from an all-fiber oscillator.

We demonstrate Watt-level flat visible supercontinuum (SC) generation in photonic crystal fibers, which is directly pumped by broadband noise-like pulses from an Yb-doped all-fiber oscillator. The novel SC generator is featured with elegant all-fiber-integrated architecture, high spectral flatness and high efficiency. Wide optical spectrum spanning from 500 nm to 2300 nm with 1.02 W optical power is obtained under the pump of 1.4 W noise-like pulse. The flatness of the spectrum in the range of 700 nm~1600 nm is less than 5 dB (including the pump residue). The exceptional simplicity, economical efficiency and the comparable performances make the noise-like pulse oscillator a competitive candidate to the widely used cascade amplified coherent pulse as the pump source of broadband SC. To the best of our knowledge, this is the first demonstration of SC generation which is directly pumped by an all-fiber noise-like pulse oscillator.

All-fiberized synchronously pumped 1120 nm picosecond Raman laser with flexible output dynamics.

A largely simplified and highly efficient all-fiber-based synchronously pumping scheme is proposed. The synchronization between pump light and the cavity round-trip can be achieved by adjusting the repetition rate of pumping light without the requirement of altering the cavity length. Based on this scheme, we achieved generating narrow linewidth highly efficient 1120 nm pulse directly from an all-fiber Raman cavity. By pump repetition rate detuning and pump duration adjustment, the duration of the 1120 nm pulse can be widely tuned from 18 ps to ~1 ns, and the repetition rate can be adjusted from 12.41 MHz to 99.28 MHz by harmonic pumping. Up to 4.3 W high power operation is verified based on this scheme. Owing to the compact all-fiber configuration, the conversion efficiency of the 1066 nm pump light to the 1120 nm Stokes light exceeds 80% and the overall conversion efficiency (976 nm-1066 nm-1120 nm) is as high as 53.7%. The nonlinear output dynamics of the Raman laser are comprehensively explored. Two distinct operation regimes are investigated and characterized.

0.4 µJ, 7 kW ultrabroadband noise-like pulse direct generation from an all-fiber dumbbell-shaped laser.

We report the direct generation of 0.4 µJ, 7 kW ultrabroadband picosecond noise-like pulses from an Yb-doped all-fiber oscillator based on dual nonlinear optical loop mirrors (NOLMs). Under the highest pump power, the average power of the main output port reached 1.4 W, and the 3 dB spectral bandwidths reached 76 nm and 165 nm from the two output ports, respectively. The design of dual-NOLMs shows both exceptional compactness in construction and distinct flexibility on the engineering of the mode-locking behaviors. To the best of our knowledge, this is the first demonstration of a watt-level dual-NOLM-based fiber laser. Based on this laser, the pulse energy and peak power of picosecond noise-like pulse from an all-fiber oscillator have been elevated by an order of magnitude.