Mamyshev oscillator (MO) represents a powerful mode-locking technique to produce high energy and ultrashort pulses from a fiber system. However, it is challenging to achieve low repetition rates directly from an oscillator while fully exploiting the outstanding characteristics of MOs due to the constraint of large dispersion and high nonlinearity of conventional solid-core fibers. Here, a new method of combining a low-dispersion and low-nonlinearity hollow-core fiber (HCF) with the MO is proposed to overcome the problem, achieving a reduction in repetition rate from 20 to 1 MHz and realizing sub-50 fs pulses. Furthermore, the HCF provides anomalous dispersion and offers chirp management in the 1 MHz MO, effectively improving the pulse energy from 381 nJ (20 MHz all-normal-dispersion MO) to 514 nJ. The influence of intracavity chirp management is investigated by numerical simulations. To the best of our knowledge, it is the first time of realizing high-energy sub-50 fs pulses at 1 MHz repetition rate directly from an oscillator. The maximum peak power of the output pulses is more than 100 times higher than the previously reported low-repetition ultrafast fiber lasers. This high-performance laser has good potential for applications such as high-precision micromachining, high-order nonlinear microscopy imaging, and femtosecond-laser-assisted chemical ionization mass spectroscopy.

Hollow-core fiber (HCF), in which >99.99% of the light is guided in a central air (or vacuum) filled core, is a radically new fiber technology offering the potential to overcome the nonlinear limits associated with the delivery of high-brightness laser pulses over long distances in conventional solid-core fiber. Overcoming these limits is particularly challenging at visible wavelengths where the core sizes of single-mode fibers (SMFs) are reduced. In this work, the delivery of near-diffraction-limited, kilowatt-peak-power, sub-nanosecond laser pulses in the green wavelength range over hundred-meter scale lengths of a hollow-core anti-resonant fiber (HC-ARF) which offers broadband low-loss guidance in the visible is experimentally demonstrated. Substantially reduced nonlinearity-induced spectral broadening is observed relative to silica-core SMF. The simulation further confirms that the broadening observed (in the HC-ARF) is entirely due to the interaction of the light with the air in the core and thus can effectively be eliminated by evacuating the fiber. Moreover, access to lower-loss is noted, and visible guiding HC-ARFs (that are now becoming available) will improve the throughput efficiency and extend power delivery to kilometer distance scales. The results demonstrated here pave the way for future long-distance HCF pulse delivery applications, such as remote industrial e-mobility manufacturing.

Label-free and multiphoton micro-endoscopy can transform clinical histopathology by providing an in situ tool for diagnostic imaging and surgical treatment in diseases such as cancer. Key to a multiphoton imaging-based micro-endoscopic device is the optical fiber, for distortion-free and efficient delivery of ultra-short laser pulses to the sample and effective signal collection. In this work, we study a new hollow-core (air-filled) double-clad anti-resonant fiber (DC-ARF) as a high-performance candidate for multiphoton micro-endoscopy. We compare the fiber characteristics of the DC-ARF with a single-clad anti-resonant fiber (SC-ARF) and a solid core fiber (SCF). In this work, while the DC-ARF and the SC-ARF enable low-loss (<0.2 dBm−1), close to dispersion-free excitation pulse delivery (<10% pulse width increase at 900 nm per 1 m fiber) without any induced non-linearities, the SCF resulted in spectral broadening and pulse-stretching (>2000% of pulse width increase at 900 nm per 1 m fiber). An ideal optical fiber endoscope needs to be several meters long and should enable both excitation and collection through the fiber. Therefore, we performed multiphoton imaging on endoscopy-compatible 1 m and 3 m lengths of fiber in the back-scattered geometry, wherein the signals were collected either directly (non-descanned detection) or through the fiber (descanned detection). Second harmonic images were collected from barium titanate crystals as well as from biological samples (mouse tail tendon). In non-descanned detection conditions, the ARFs outperformed the SCF by up to 10 times in terms of signal-to-noise ratio of images. Significantly, only the DC-ARF, due to its high numerical aperture (NA) of 0.45 and wide-collection bandwidth (>1 µm), could provide images in the de-scanned detection configuration desirable for endoscopy. Thus, our systematic characterization and comparison of different optical fibers under different image collection configurations, confirms and establishes the utility of DC-ARFs for high-performing label-free multiphoton imaging-based micro-endoscopy.

We present the development of a Mamyshev oscillator (MO) operating at 1040 nm with a low repetition rate of 1.18 MHz enabled by a hollow core fiber. The MO produces ultrashort pulses with a maximum pulse energy of 514 nJ and a minimum de-chirped pulse width of 46 fs.

We show the potential of combining a hollow-core NANF with a wideband YDFA for 1-μm transmission through a conceptual demonstration. Penalty-free transmission over a 2.24-km NANF at >100Gb/s is reported across a 16.3-THz bandwidth (1020-1080nm).

We report the first double-nested antiresonant hollow core fiber. The fiber matches the loss of commercial solid core fibers in the C-band (0.174 dB/km) and fundamentally improves it (0.22 dB/km) in the O-band.

High-power laser delivery with near-diffraction-limited beam quality is typically limited to tens of metres distances by nonlinearity-induced spectral broadening inside the glass core of delivery fibres. Anti-resonant hollow-core fibres offer not only orders-of-magnitude lower nonlinearity but also loss and modal purity comparable to conventional beam-delivery fibres. Using a single-mode hollow-core nested anti-resonant nodeless fibre with 0.74 dB km−1 loss, we demonstrate the delivery of 1 kW of near-diffraction-limited continuous-wave laser light over a 1 km distance, with a total throughput efficiency of ~80%. From simulations, a further improvement in transmitted power or length of more than one order of magnitude should be possible in such air-filled fibres, and considerably more if the core is evacuated. This paves the way to multi-kilometre, kilowatt-scale power delivery that is potentially useful not only for future manufacturing and subsurface drilling but also for new scientific possibilities in sensing, particle acceleration and gravitational wave detection. Microstructured optical fibre is shown to be able transmit high-power laser light over long distances with high throughput efficiency.

Over 94 % coupling efficiency is presented experimentally for a Gaussian-beam into an antiresonant hollow-core fiber optimized for 2 nd order antiresonance guidance at 1064 nm , which demonstrates approximately 1.5 % (0.06 dB ) improvement over the 1 st window counterpart.

High power laser delivery with near-diffraction-limited beam quality, widely used in industry for precision manufacturing, is typically limited to tens of metres distances by nonlinearity-induced spectral broadening inside the glass-core delivery fibres. Anti-resonant hollow-core fibres offer not only orders-of-magnitude lower non-linearity, but also loss and modal purity comparable to conventional beam-delivery fibres. Using a single-mode hollow-core nested anti-resonant nodeless fibre (NANF) with 0.74-dB/km loss, we demonstrate delivery of 1 kW of near-diffraction-limited continuous wave laser light over an unprecedented 1-km distance, with a total throughput efficiency of ~80%. From simulations, more than one order of magnitude further improvement in transmitted power or length should be possible in such air-filled fibres, and considerably more if the core is evacuated. This paves the way to multi-kilometre, kW-scale power delivery – not only for future manufacturing and subsurface drilling, but also for new scientific possibilities in sensing, particle acceleration and gravitational wave detection.

For over 50 years, pure or doped silica glass optical fibres have been an unrivalled platform for the transmission of laser light and optical data at wavelengths from the visible to the near infra-red. Rayleigh scattering, arising from frozen-in density fluctuations in the glass, fundamentally limits the minimum attenuation of these fibres and hence restricts their application, especially at shorter wavelengths. Guiding light in hollow (air) core fibres offers a potential way to overcome this insurmountable attenuation limit set by the glass's scattering, but requires reduction of all the other loss-inducing mechanisms. Here we report hollow core fibres, of nested antiresonant design, with losses comparable or lower than achievable in solid glass fibres around technologically relevant wavelengths of 660, 850, and 1060 nm. Their lower than Rayleigh scattering loss in an air-guiding structure offers the potential for advances in quantum communications, data transmission, and laser power delivery.

Hollow core (HC) antiresonant fibers (ARFs) offer potential for low optical loss, low glass-mode overlap, and wide transmission bandwidth that could cover regions from the UV to the Mid-IR, and be optimized to work in the 1μm and 1.55μm bands [1,2]. The optical properties of ARFs depend mainly on the arrangement and thickness of the core surrounding membranes. To date, the state-of-the-art optical loss in a 1μm-guiding HCF is reported by Maurel et al. [3] in a Kagome fiber, showing 8.5dB/km at the Nd-Yb:YAG laser wavelengths. Other low loss results at the 1μm region have been reported by Wheeler et al. [4] showing 12.3dB/km at 1010nm in a Kagome HCF; Chen et al. [5] showing 12.3dB/km at 1047nm in a 37-cell photonic bandgap fiber (PBGF); and Debord et al. [6] showing a loss of 8-20dB/km in the 800-1200nm region in tubular lattice HCF. The addition of smaller nested tubes to the known `tubular' hollow core ARFs can considerably reduce their optical loss, allowing in principle a HC-Nested Antiresonant Nodeless Fiber (NANF) to achieve total loss values lower than conventional solid fibers [7]. Recent NANF results showed loss of 1.3dB/km at 1450nm (Bradley et al. [8]).

We report on the longest hollow-core-fiber transmission experiments to date. The enabling fiber is a low-loss (1.18dB/km @1550nm) and record-length (4.8km) nested-antiresonant nodeless fiber (NANF). We reached 125km and 340km in this NANF with 32GBaud PM-16QAM and PM-QPSK transmission, respectively, with 61 WDM channels loading.

We report a hollow core Nested Antiresonant Nodeless Fibre (NANF) with a loss of 0.65dB/km across the full C and L telecommunication bands. The fabricated fibre is 1.23km long, it is effectively single moded over sufficiently long lengths, and is able to transmit data.

Significant progress has been made in recent years in driving down the loss of hollow core fibres (HCFs), such as photonic bandgap fibres (PBGF), antiresonant nodeless fibres (ANF), and nested antiresonant nodeless fibres (NANF) towards that of their conventional solid single mode counterparts. In order to achieve the ultimate performance of HCFs, an accurate control of the dimensions and symmetry of the internal constituent elements is key in both the assembled preforms and drawn canes and fibres. Non-destructive optical methods to monitor and assess the internal features of HCFs over the entire fabrication process are therefore of great interest. Previously, morphology dependent resonances [1] and whispering gallery modes (WGMs) [2], excited by side illumination of the fibre, have been used to enable the precise measurement of cladding diameter variations with an accuracy greater than that of competing interferometric approaches. Motivated by this earlier work we have started studies into the opportunities provided by side illumination of HCF canes and fibres being drawn, and report some of our initial observations, which we hope will ultimately provide useful information on the internal features of HCFs.

Atmospheric air-filled hollow core (HC) fibers, representing the simplest yet reliable form of gas-filled hollow core fiber, show remarkable nonlinear properties and have several interesting applications such as pulse compression, frequency conversion and supercontinuum generation.Although the propagation of sub-picosecond and few hundred picosecond pulses are well-studied in air-filled fibers, the nonlinear response of air to pulses with a duration of a few picoseconds has interesting features that have not yet been explored fully.Here, we experimentally and theoretically study the nonlinear propagation of ~6 ps pulses in three different types of atmospheric air-filled HC fiber.With this pulse length, we were able to explore different nonlinear characteristics of air at different power levels.Using in-house-fabricated, state-of-the-art HC photonic bandgap, HC tubular and HC Kagomé fibers, we were able to associate the origin of the initial pulse broadening process in these fibers to rotational Raman scattering (RRS) at low power levels.Due to the broadband and low loss transmission window of the HC Kagomé fiber we used, we observed the transition from initial pulse broadening (by RRS) at lower powers, through long-range frequency conversion (2330 cm -1 ) with the help of vibrational Raman scattering, to broadband (~700 nm) supercontinuum generation at high power levels.To model such a wide range of nonlinear processes in a unified approach, we have implemented a semi-quantum model for air into the generalized nonlinear Schrodinger equation, which surpasses the limits of the common single damping oscillator model in this pulse length regime.The model has been validated by comparison with experimental results and provides a powerful tool for the design, modeling and optimization of nonlinear processes in air-filled HC fibers.

We report on the development of the first multi-lane all-optical switch with directly integrated multi-core fibers. A 3-port single-sided beam-steering switch connecting 4-core fibers shows core-to-core losses below 2.2 dB with less than 1-dB variation.

We review and compare recent hollow core photonic crystal fibers, both bandgap-guiding and anti-resonant, which were designed and fabricated for high power laser delivery applications.

We have demonstrated the generation of a 320 Gb/s NyquistOTDM signal by rectangular filtering on an RZ-OTDM signal with the filter bandwidth (320 GHz) equal to the baud rate (320 Gbaud) and the reception of such a Nyquist-OTDM signal using polarization-insensitive time-domain optical Fourier transformation (TD-OFT) followed by passive filtering. After the time-to-frequency mapping in the TD-OFT, the NyquistOTDM signal with its characteristic sinc-shaped time-domain trace is converted into an orthogonal frequency division multiplexing (OFDM) signal with sinc-shaped spectra for each subcarrier. The subcarrier frequency spacing of the converted OFDM signal is designed to be larger than the transform-limited case, here 10 times greater than the symbol rate of each subcarrier. Therefore, only passive filtering is needed to extract the subcarriers of the converted OFDM signal. 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