Oxidative [3+2]Cycloaddition of Alkynylphosphonates with Heterocyclic N-Imines: Synthesis of Pyrazolo[1,5-a]Pyridine-3-phosphonates

A series of pyrazolo[1,5-a]pyridine-3-ylphosphonates were prepared with moderate to good yields by the oxidative [3+2]cycloaddition of 2-subtituted ethynylphosphonates with in situ generated pyridinium-N-imines and their annulated analogs. 2-Aliphatic and 2-Ph acetylenes demonstrate low activity, and the corresponding pyrazolopyridines were achieved with a moderate yield in the presence of 10 mol% Fe(NO3)3·9H2O. At the same time, tetraethyl ethynylbisphosphonate, diethyl 2-TMS- and 2-OPh-ethynylphosphonates possess much greater reactivity and the corresponding pyrazolo[1,5-a]pyridines, and their annulated derivatives were obtained with good to excellent yields without any catalyst. 2-Halogenated ethynylphosphonates also readily reacted with pyridinium-N-imines, forming complex mixtures containing poor amounts of 2-halogenated pyrazolopyridines

Active tapered double-clad fiber with low birefringence

We addressed the problem of a state of polarization (SOP) drift caused by heating under intense clad pumping in different types of active tapered double-clad fibers. We investigated experimentally the variations of the SOP and degree of polarization (DOP) under clad pumping in polarization-maintaining (PANDA type) and regular (non-PM) Yb-doped double-clad large mode area tapered fibers. We discovered that the birefringence of active fibers is highly dependent on the launched pump power. To solve the problem of the SOP drift in active large mode are fibers, we, for the first time to the best of our knowledge, presented an active double-clad fiber with low intrinsic birefringence as a gain medium. An Yb-doped spun tapered double-clad fiber (sT-DCF) with intrinsic birefringence as low as 1.45×10−8 was manufactured and experimentally studied. We have proved experimentally that the DOP and SOP remains more stable in sT-DCF with increasing pump power compared to PM PANDA-type and regular non-PM tapered double-clad fibers. In particular, the SOP drift in sT-DCF is almost one order of magnitude less than in other tapered fibers, while the DOP drift in sT-DCF is comparable with the drift in PANDA-type fiber and one order of magnitude less than in the non-PM tapered fiber. An active sT-DCF showing efficient amplification was demonstrated in an all-fiber-based picosecond master-oscillator power-amplifier scheme. The system delivered 50 ps pulses at 1040 nm with an average power of 50 W, 34 dB gain, 26 µm MFD and perfect beam quality.publishedVersionPeer reviewe

Random Laser Based on Ytterbium-Doped Fiber with a Bragg Grating Array as the Source of Continuous-Wave 976 nm Wavelength Radiation

A random narrow-linewidth lasing at a wavelength of 976 nm was obtained in an ytterbium-doped germanophosphosilicate fiber with an array of weakly reflecting fiber Bragg gratings (FBGs). A random laser cavity was formed by implementing the standard phase mask method of FBG inscription directly during the fiber drawing process. The UV radiation pulses of a KrF excimer laser (248 nm wavelength) synchronized with the fiber drawing speed were used to fabricate the in-fiber array of hundreds of similar FBGs. The developed laser’s slope efficiency in the backward-pumping scheme was measured as high as 33%. The stable continuous-wave operation mode of the laser was detected. The magnitude of the laser power fluctuations depends linearly on the cavity length. The random laser cavity modified with a single highlyreflected (90%) FBG demonstrates significantly better power stability and higher slope efficiency than the same one without an FBG

Random Laser Based on Ytterbium-Doped Fiber with a Bragg Grating Array as the Source of Continuous-Wave 976 nm Wavelength Radiation

A random narrow-linewidth lasing at a wavelength of 976 nm was obtained in an ytterbium-doped germanophosphosilicate fiber with an array of weakly reflecting fiber Bragg gratings (FBGs). A random laser cavity was formed by implementing the standard phase mask method of FBG inscription directly during the fiber drawing process. The UV radiation pulses of a KrF excimer laser (248 nm wavelength) synchronized with the fiber drawing speed were used to fabricate the in-fiber array of hundreds of similar FBGs. The developed laser’s slope efficiency in the backward-pumping scheme was measured as high as 33%. The stable continuous-wave operation mode of the laser was detected. The magnitude of the laser power fluctuations depends linearly on the cavity length. The random laser cavity modified with a single highlyreflected (90%) FBG demonstrates significantly better power stability and higher slope efficiency than the same one without an FBG