Ultrashort pulses at 920 nm are a highly desired light source in two-photon microscopy for the efficient excitation of green fluorescence protein. Although Nd3 + -doped fibers have been utilized for 920-nm ultrashort pulse generation, the competitive amplified spontaneous emission (ASE) at 1.06 μm remains a significant challenge in improving their performance. Here, we demonstrate a coordination engineering strategy to tailor the properties of Nd3 + -doped silica glass and fiber. By elevating the covalency between Nd3 + and bonded anions via sulfur incorporation, the fiber gain performance at 920 nm is enhanced, and 1.06-μm ASE intensity is suppressed simultaneously. As a result, the continuous-wave laser efficiencies and signal-to-noise ratio at 920 nm by this fiber are significantly enhanced. Importantly, the stable picosecond pulses at 920 nm are produced by a passive mode-locking technique with a fundamental repetition rate up to 207 MHz, which, to the best of our knowledge, is the highest reported repetition rate realized by Nd3 + -doped silica fibers. The presented strategy enriches the capacity of Nd3 + -doped silica fiber in generating 920-nm ultrashort pulses for application in biophotonics, and it also provides a promising way to tune the properties of rare-earth ion-doped silica glasses and fibers toward ultrafast lasers.
We demonstrate a method of Photonic Crystal Fiber (PCF) inverse design for nonlinear wavelength conversion based on Four-Wave Mixing (FWM), where Deep learning Neural Networks (DNN) is applied to predict PCF structure parameters. By applying empirical formula of PCF dispersion instead of numerical simulation, a large dataset of phase-matching curves is generated of various PCF designs. The average running time of DNN prediction is 0.2s. With the help of DNN, we design and fabricate a PCF for wavelength conversion via FWM from 1064 nm to 770 nm. Pumped by a microchip laser at 1064 nm, signal wavelength is detected by optical spectrum analyzer at 770.2nm
Quartz Enhanced Photothermal Spectroscopy (QEPTS) is a technique, which enables developing gas sensors characterized by a broadband operational range and superb sensitivity. QEPTS relies on the thermoelastic effect induced by the illumination of the Quartz Tunning Fork (QTF) with a modulated laser radiation, which generates a piezoelectric signal. QTF excitation can occur at any wavelength, which is impossible to observe in e.g. semiconductor detectors. The sub-dollar cost of a QTF delivers simultaneously broadband and sensitive detection capability and significantly reduced costs of the sensor. Furthermore, as in majority of laser-based sensors, the sensitivity of QEPTS-based systems can be easily enhanced by increasing laser-gas interaction path length. This is typically realized by using multipass cells (MPCs), which significantly increase the sensor’s complexity and decrease its robustness. Instead of using MPCs, an Antiresonant Hollow-Core Fiber (ARHCF), designed for light transmission in more than one spectral band can be used as a long gas absorption cell, leading to the increase in the sensor’s performance while keeping its design simple. Here, we present a sensor utilizing a combination of an ARHCF-based absorption cell and the QEPTS. In the developed system the gas-filled ARHCF substitutes an MPC. The spectroscopic signal analysis relies on the use of a simple QTF with a resonance frequency of 32.744 kHz connected with a self-made, low-noise amplifier and an addition of a wavelength modulation spectroscopy – based signal retrieval scheme for sensor’s performance enhancement. The sensor enables simultaneous detection of acetylene and methane at parts-per-million by volume level sensitivity, targeting their absorption lines in the near- and mid-infrared. The results confirm excellent suitability of the ARHCF-aided QEPTS sensors for being employed as a versatile gas detectors.
This research was funded by Narodowe Centrum Nauki (NCN), grant number UMO-2018/30/Q/ST3/00809.
We present the results of modelling of pressure-driven gas flow in a 14.7 meters long nodeless Antiresonant Hollow Core Fiber (ARHCF) for predicting the gas exchange time in the ARHCF-based laser absorption spectroscopy measurement systems. The implemented physical model is based on the Navier-Stokes equations for laminar flow. The tunable diode laser absorption spectroscopy has been used for determining experimentally the ARHCF gas filling time. The obtained results confirmed the requirement for more complex geometric models to properly predict the core filling time of nodeless ARHCFs than a simple, single-channel approach, which can be used effectively for gap-less ARHCFs.
Visible laser of Pr3+ ions doped in fluoride glass fibers has found numerous applications in display techniques, visible light communication and scientific research. Unfortunately, the poor mechanical strength, degraded chemical stability and high cost limit the further development of fluoride glasses in practice. Instead, oxide glass is good host candidate for fiber lasers due to its advantages of mature processing technique, stable chemical properties, high mechanical strength and low cost. But it is difficult to achieve laser output at visible wavelengths in Pr3+ doped oxide glass fiber because of photodarkening after exposure to 488 nm laser. In this work, we study such photodarkening effect and relevant mechanisms in Pr3+ doped silicate glasses and fibers by using X-ray and 488 nm laser as irradiation sources, respectively. Results indicate that due to relatively low energy of Pr3+: 4f5d, electrons of Pr3+ ions will be trapped to form defect (trapped hole center) through excited state absorption under exposure to both 488 nm laser and X-ray radiation.
In this work, we present simultaneous and sensitive detection of methane and ethane at ~3.34 µm using a 15-meteres long self-fabricated silica ARHCF and Wavelength Modulation Spectroscopy technique. The ARHCF was filled with a mixture of 10 ppmv and 20 ppmv ethane and methane, respectively via air-tight housings placed at both fiber end-facets. The gas molecules were excited using a self-built continuous wave Difference Frequency Generation source which radiation was coupled into the gas-filled ARHCF. The ARHCF-aided gas sensor reached a minimum detection limit at parts-per-billion by volume level, confirming the suitability of the proposed approach for trace-gas sensing.
One of the main challenges of laser-based gas sensingis the development of sensors delivering simultaneously high sensitivity, selectivity, fast-response time and non-complex design. Mostly, the detection capability of such sensors depends on the interaction path length between the laser light and the measured gas. Hence, long optical paths are highly desired for e.g. low-concentration gas sensing. Our proposal is to use Antiresonant Hollow-Core Fibers (ARHCFs), which filled with the target gas mixture form absorption cells with potentially any length, delivering low-volume, long and versatile optical paths within the sensor configuration. Currently, the ARHCF core is filled with the target gas via specially designed bulk-optics-based cells placed at the fiber’s ends. This solution provides relatively fast fiber core filling time, however being only efficient while an overpressure is used to force the gas flow through the core, not the diffusion. Therefore, searching for alternative ways of fiber filling with the target gas is necessary. We propose a method of processing the fiber structure using a femtosecond laser allowing for non-invasive accessing the fiber core for more efficient and faster gas diffusion into it through the fabricated microchannels. The fiber structure modification was optimized in a way that does not introduce any unwanted damage of the fiber e.g. cracks on the glass parts or cladding capillaries. The performed experiments have indicated that the laser-processing of the ARHCF structure introduces negligible transmission loss regardless of the number of fabricated microchannels and their length (0.2dB loss for 25 microchannels), confirming the proposed method suitability.
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