Compact and efficient high-power mid-infrared supercontinuum fiber laser source based on a noise-like pulse and germania fiber.

Here, we demonstrate a compact and efficient high-power mid-infrared supercontinuum (MIR-SC) laser source based on a tunable noise-like pulse (NLP) fiber laser system and a short section of single-mode germania-core fiber (GCF). The NLP all-polarization-maintaining fiber laser system can deliver the maximum output power of ∼30.6 W and a broadband spectrum (∼1.8-2.7 µm) with a compact single-stage fiber amplifier. By directly pumping only ∼6.5 cm-long GCF with a core diameter of ∼3.5 µm, a MIR-SC (spectral coverage of ∼1.5-3.3 µm) with a maximum power of ∼25.2 W and a power conversion efficiency ∼81.2% is obtained, which represent the highest power and efficiency in any single-mode GCF-based MIR-SCs, to the best of our knowledge. Our study contributes to the high-power MIR-SC laser source with compact all-fiber configuration, and will prompt its practical applications.

Over 3.8 W, 3.4†µm picosecond mid-infrared parametric conversion based on a simplified one-to-many scheme.

In this paper, we demonstrate a simplified one-to-many scheme for efficient mid-infrared (MIR) parametric conversion. Such a scheme is based on a continuous wave (CW) single longitudinal mode master oscillator power-amplifier (MOPA) fiber system as the signal source and a picosecond pulsed MOPA fiber system, exhibiting multiple longitudinal modes, as the pump source. The signal and pump beams are combined and co-coupled into a piece of 50-mm long 5% MgO-doped PPLN crystal for the parametric conversion. As high as ∼3.82 W average power at a central idler wavelength of ∼3.4 µm is achieved when the launched pump and signal powers are ∼41.73 and ∼11.45 W, respectively. Above some threshold value, the delivered idler power shows a roll-over effect against the signal power and saturation-like effect against the pump power. Consequently, the highest conversion efficiency is observed at such a threshold pump power. To the best of our knowledge, our result represents the highest average power produced from any single-pass parametric conversion source with >3 µm idler wavelength feeding with a CW signal. Moreover, our proposed scheme can simplify the design of parametric conversion system significantly and meanwhile make the system more robust in applications. This is attributed to two main aspects. Firstly, the scheme's one-to-many feature can reduce wavelength sensitivity remarkably in the realization of quasi-phase-matching. Secondly, for moderate power requirement it does not always require a high peak power synchronized pulsed signal source; a CW one can be an alternative, thereby making the system free from complex time synchronization and the related time jitter.

5.4 W, 2.35†µm cascaded Raman fiber laser pumped by dissipative soliton resonance-like pulses.

We present a nonlinear amplifying loop mirror-based mode-locked fiber laser. By adjusting the pump power, the proposed laser exhibits a dissipative soliton resonance (DSR)-like pulse operation with a maximum pulse width of 150 ns. Subsequently, a three-stage Tm3+-doped fiber amplifier is implemented using a single-mode double-cladding Tm3+-doped fiber to increase the DSR-like pulse output power to 52.5 W, achieving a pump slope efficiency of 47.1% in the main amplifier. A 25 m first-order Raman-gain fiber (UHNA7) is pumped by a DSR-like pulse, and 16.3 W of pure 2.135 µm first-order Raman light with a spectral purity of 73.4% is obtained. Finally, 5.4 W of 2.35 µm second-order Raman light with a spectral purity of 66% is obtained using a 10 m 98% germania-core fiber as a second-order Raman-gain fiber cascaded after UHNA7 fiber. To the best of our knowledge, this is the highest output power ever obtained from a 2.3 µm laser.

In-fiber chirped Fabry-Perot cavity for temperature sensing.

Measurement resolution and dynamic range of conventional optical fiber sensors are often mutually restricted. In this work, an in-fiber chirped Fabry-Perot cavity (interferometer) is proposed, for the first time to our knowledge, to resolve the conflict between the resolution and dynamic range. The chirped Fabry-Perot interferometer is constructed by two chirped fiber Bragg gratings inscribed in the opposite directions, resulting in a gradually varied (i.e., chirp) cavity length for different reflection wavelengths. As such, the interference spectrum exhibits high figure of merit (FOM) and large free spectrum range (FSR) at long and short wavelength regions, respectively, enabling high-resolution and large-dynamic-range measurement simultaneously. Temperature tests are then carried out to confirm the validity of the solution. The proposed sensing schema may be developed further and find vital applications in biomedicine fields such as endosomatic temperature monitoring of living bodies. The proposed concept of chirped Fabry-Perot interferometer can provide breakout ideas for other sensing scenarios where high-resolution and large-dynamic range are demanded and can be further generalized to other measurands or even free-space interference metrologies.

Wavelength-tunable spatiotemporal mode-locking in a large-mode-area Er:ZBLAN fiber laser at 2.8†µm.

We report a tunable spatiotemporally mode-locked large-mode-area Er:ZBLAN fiber laser based on the nonlinear polarization rotation technique. A diffraction grating is introduced to select the operating wavelength. Under the spectral and spatial filtering effects provided by the grating and spatial coupling respectively, stable ps-level spatiotemporally mode-locked pulses around 2.8 µm with a repetition rate of 43.4 MHz are generated. Through a careful adjustment of the grating, a broad wavelength tuning range from 2747 to 2797 nm is realized. To the best of our knowledge, this is the first wavelength-tunable spatiotemporally mode-locked fiber laser in the mid-infrared region.

High-Resolution and Large-Dynamic Range Fiber-Optic Sensors Based on Dual-Mode Direct Spectrum Interrogation Method.

Conventional optical fiber temperature/strain sensors often have to make compromises between the resolution and the dynamic range. Here we present a new method that meets the measurement requirements for both high resolution and large dynamic range. A high-quality optical fiber Fabry-Perot Interferometer (FPI) constructed using a pair of chirped fiber Bragg gratings is employed as the sensor and a dual-mode direct spectrum interrogation method is proposed to identify the small drift of external temperature or strain. As a proof-of-concept illustration, a temperature resolution of 0.2 °C within 30-130 °C is demonstrated. For strain sensing, the resolution can be 10 µÎµ within 0-1000 µÎµ. The measurement resolution can be improved further by routinely increasing the reflectivity of the CFBG and the cavity length and the sensor can also be mass-produced. This new sensing schema not only resolves the conflict between the resolution and the dynamic range of fiber-optic temperature/strain sensors but can also be extended to other sensors and measurands.

Temperature-immune Fabry-Perot cavity sensor based on an opened hollow-core anti-resonant fiber.

A new approach to conquer the thermal phase drift of an optical fiber Fabry-Perot interferometer (FPI) sensor is proposed and experimentally demonstrated. By employing a hollow-core anti-resonant fiber (HC-ARF) and optimizing the fusion splicing (includes mode field adaptation) between the lead-in single-mode fiber (SMF) and the HC-ARF, a high spectral resolution (λ/Δλ ≈ 3.8 × 104) optical fiber air-cavity FPI sensor with a fringe visibility higher than 7 dB is constructed. To eliminate the thermal phase drift (i.e. temperature crosstalk) of the sensor that originates from the intrinsic thermal expansion effect of the silica material of the HC-ARF, the FPI air cavity is connected to the external environments, by which the effect of air expelling from the cavity with temperature increasing can well compensate the temperature-induced cavity elongation. As a result, the thermal phase drift of the FPI is reduced to zero at a temperature range of ∼ 80-110 °C and within the temperature range of 40-80 °C, the thermal phase drift is still halved compared with the sealed FPI cavity. The nearly zero thermal phase drift of a FPI at such a temperature range has never been achieved before, to our best knowledge. As a proof of concept, a temperature-immune fiber-optic strain sensor is demonstrated. This work offers a new and efficient approach to eliminate the thermal phase drift (i.e. temperature crosstalk) of a fiber-optic device, which may significantly improve the measurement accuracy and detection limit of fiber-optic FPI sensors. Furthermore, the principle and schema can be generalized to a wide variety of fiber-optic devices.

Non-uniform droplet deposition on femtosecond laser patterned superhydrophobic/superhydrophilic SERS substrates for high-sensitive detection.

Surface-enhanced Raman scattering (SERS) sensors combined with superhydrophobic/superhydrophilic (SH/SHL) surfaces have shown the ability to detect ultra-low concentrations. In this study, femtosecond laser fabricated hybrid SH/SHL surfaces with designed patterns are successfully applied to improve the SERS performances. The shape of SHL patterns can be regulated to determine the droplet evaporation process and deposition characteristics. The experimental results show that the uneven droplet evaporation along the edges of non-circular SHL patterns facilitates the enrichment of analyte molecules, thereby enhancing the SERS performance. The highly identifiable corners of SHL patterns are beneficial for capturing the enrichment area during Raman tests. The optimized 3-pointed star SH/SHL SERS substrate shows a detection limit concentration as low as 10-15 M by using only 5 µL R6G solutions, corresponding to an enhancement factor of 9.73 × 1011. Meanwhile, a relative standard deviation of 8.20% can be achieved at a concentration of 10-7 M. The research results suggest that the SH/SHL surfaces with designed patterns could be a practical approach in ultratrace molecular detections.

Soliton rain in all-polarization-maintaining mode locked fiber laser.

For the first time the phenomenon of soliton rain is observed in a mode-locked fiber laser with all-polarization-maintaining (all-PM) architecture. The laser is mode-locked using a semiconductor saturable absorber mirror (SESAM) and operates in the all-normal dispersion (ANDi) regime. The operation state of the laser can be switched from dissipative soliton to soliton rain by simply raising the pump power, without any manipulation of the intracavity polarization state given that all components of the resonator are made of PM fibers. The soliton rain generated in the laser is self-starting and replicable, since it occurs in every individual operation of the laser as the pump power is increased to an approximately invariant value.

Frequency-domain-resolved investigation of unitary h-shaped pulses.

We experimentally investigate the generation of h-shaped pulse in an all-polarization-maintaining (PM) and all-normal-dispersion (ANDi) mode-locked fiber laser. The generated pulse is demonstrated to be a unitary pulse, instead of a noise-like pulse (NLP). Furthermore, by employing an external filtering system, the obtained h-shaped pulse can be resolved into rectangular-shaped pulses, chair-like pulses, and Gaussian pulses. The authentic AC traces with a double-scale structure of unitary h-shaped pulses and chair-like pulses are observed on the autocorrelator. The chirp of h-shaped pulses is also proved similar to that of DSR pulses. To the best of our knowledge, this is the first time that the existence of unitary h-shaped pulse generation has been confirmed. Moreover, our experimental results reveal the close relationship of formation mechanisms of dissipative soliton resonance (DSR) pulses, h-shaped pulses, and chair-like pulses, which helps to unify the essences of such "DSR-like" pulses.

High performance on-chip polarization beam splitter at visible wavelengths based on a silicon nitride small-sized ridge waveguide.

Due to sensitive scaling of the wavelength and the visible-light absorption properties with the device dimension, traditional passive silicon photonic devices with asymmetric waveguide structures cannot achieve polarization control at the visible wavelengths. In this work, a simple and small polarization beam splitter (PBS) for a broad visible-light band, using a tailored silicon nitride (Si3N4) ridge waveguide, is presented, which is based on the distinct optical distribution of two fundamental orthogonal polarized modes in the ridge waveguide. The bending loss for different bending radii and the optical coupling properties of the fundamental modes for different Si3N4 ridge waveguide configurations are analyzed. A PBS composed of a bending ridge waveguide structure and a triple-waveguide directional coupler was fabricated on the Si3N4 thin film. The TM excitation of the device based on a bending ridge waveguide structure shows a polarization extinction ratio (PER) of ≥ 20 dB with 33 nm bandwidth (624-657 nm) and insertion loss (IL) ≤ 1 dB at the through port. The TE excitation of the device, based on a triple-waveguide directional coupler with coupling efficiency distinction between the TE0 and TM0 modes, shows a PER of ≥ 18 dB with 50 nm bandwidth (580-630 nm) and insertion loss (IL) ≤ 1 dB at the cross port. The on-chip Si3N4 PBS device is found to possess the highest known PER at a visible broadband range and small (43 µm) footprint. It should be useful for novel photonic circuit designs and further exploration of Si3N4 PBSs.

High-responsivity and high-speed black phosphorus photodetectors integrated with proton exchanged thin-film lithium niobate waveguides.

We present a high-performance broadband (450-1550 nm) black phosphorus photodetector based on a thin-film lithium niobate waveguide. The waveguides are fabricated by the proton exchange method with flat surfaces, which reduces the stress and deformation of two-dimensional materials. At a wavelength of 1550 nm, the photodetector simultaneously achieves a high responsivity and wide bandwidth, with a responsivity as high as 147 A/W (at an optical power of 17 nW), a 3-dB bandwidth of 0.86 GHz, and a detectivity of 3.04 × 1013 Jones. Our photodetector exhibits one of the highest responsivity values among 2D material-integrated waveguide photodetectors.

Mid-infrared ultrashort pulses generated from a hybrid mode-locked Er:ZBLAN fiber laser.

By combining nonlinear polarization rotation (NPR) and semiconductor saturable absorber, we report a hybrid mode-locked Er:ZBLAN fiber oscillator at 2.8 µm. Stable 325-fs mode-locked pulses with an average power of 131 mW and a record signal-to-noise ratio of 79 dB at the fundamental frequency of 55.4 MHz are generated. Numerical simulations are carried out based on the modified coupled nonlinear Schrödinger equations, and offer new insights into the underlying dynamics of pulse generation. The simulations indicate that compared with Er:ZBLAN fiber lasers mode-locked by NPR alone, the hybrid mode-locked Er:ZBLAN fiber oscillator allows a wider range and a lower threshold of the pump power while maintaining the ultrashort pulse width. Moreover, we numerically demonstrate that the hybrid mode-locked oscillator is less sensitive to the variation of polarization states, which will increase its robustness against environmental disturbance. This is the first time that the hybrid mode-locking technique is applied in the mid-infrared, opening up new opportunities for the development of stable ultrafast mid-infrared laser sources and practical applications outside the laboratory.

976†nm all-polarization-maintaining mode-locked fiber laser based on nonlinear polarization evolution.

An all-polarization-maintaining (PM) mode-locked fiber laser based upon nonlinear polarization evolution (NPE) that operates around 976 nm is presented. The NPE-based mode-locking is realized using a special section of the laser which comprises three pieces of PM fibers with specific deviation angles between the polarization axes and a polarization-dependent isolator. By optimizing the NPE section and adjusting the pump power, dissipative soliton (DS) pulses with a pulse duration of ∼6 ps, a spectral bandwidth of >10 nm and a maximum pulse energy of 0.54 nJ are generated. Self-starting, steady mode-locking operation is achievable within a pump power range of ∼2 W. Moreover, by incorporating a segment of passive fiber into the appropriate location in the laser resonator, an intermediate regime between stable single-pulse mode-locking and noise-like pulse (NLP) is realized in the laser. Our work expands the dimension of the research on the mode-locked Yb-doped fiber laser operating around 976 nm.

All-fiber 3.4-W 2.8-µm ultra-short pulse MOPA system seeded by the soliton self-frequency shift of 2-µm pulses.

We report an all-fiber 2.8-µm ultra-short pulse master oscillator power amplifier (MOPA) system seeded by a soliton self-frequency shift from a mode-locked thulium-doped fiber laser. This all-fiber laser source delivers 2.8-µm pulses with an average power of 3.42 W, a pulse width of 115 fs, and a pulse energy of 45.4 nJ. We demonstrate, to the best of our knowledge, the first femtosecond watt-level all-fiber 2.8-µm laser system. A 2.8-µm pulse seed was obtained via the soliton self-frequency shift of 2-µm ultra-short pulses in a cascaded silica and passive fluoride fiber. A novel, to the best of our knowledge, high-efficiency and compact home-made end-pump silica-fluoride fiber combiner was fabricated and used in this MOPA system. Nonlinear amplification of the 2.8-µm pulse was realized, and soliton self-compression was observed accompanied by spectral broadening.

High-energy femtosecond pulse generation from a hybrid mode-locked large-mode-area Er:ZBLAN fiber laser.

We report a hybrid mode-locked fiber laser at 2.8 µm based on a large-mode-area Er:ZBLAN fiber. Reliable self-starting mode-locking is achieved via the combination of nonlinear polarization rotation and a semiconductor saturable absorber. Stable mode-locked pulses with a pulse energy of 9.4 nJ and a pulse duration of 325 fs are generated. To the best of our knowledge, this is the highest pulse energy directly generated from a femtosecond mode-locked fluoride fiber laser (MLFFL) to date. The measured M2 factors are below 1.13, indicating a nearly diffraction-limited beam quality. Demonstration of this laser provides a feasible scheme for the pulse energy scaling of mid-infrared MLFFLs. Moreover, a peculiar multi-soliton mode-locking state is also observed, in which the time interval between the solitons varies irregularly from tens of picoseconds to several nanoseconds.

High-Sensitivity Temperature Sensor Based on Fiber Fabry-Pérot Interferometer with UV Glue-Filled Hollow Capillary Fiber.

Optical fiber Fabry-Pérot (FP) interferometer sensors have long been the focus of researchers in sensing applications because of their simple light path, low cost, compact size and convenient manufacturing methods. A miniature and highly sensitive optic fiber temperature sensor using an ultraviolet glue-filled FP cavity in a hollow capillary fiber is proposed. The sensor is fabricated by fusion splicing a single-mode fiber with a hollow capillary fiber, which is filled with ultraviolet glue to form a FP cavity. The sensor has a good linear response in the temperature testing and high-temperature sensitivity, which can be increased with the length of the FP cavity. The experimental results show that the temperature sensitivity reaches 1.174 nm/°C with a high linear response in the range of 30-60 °C. In addition, this sensor is insensitive to pressure and can be highly suitable for real-time water temperature monitoring for ocean research. The proposed ultraviolet glue-filled structure has the advantages of easy fabrication, high-temperature sensitivity, low cost and an arbitrary length of capillary, which has broad application prospects for marine survey technology, biological diagnostics and environmental monitoring.

Proposal for complete characterization of attosecond pulses from relativistic plasmas.

In this study, we propose two full-optical-setup and single-shot measurable approaches for complete characterization of attosecond pulses from surface high harmonic generation (SHHG): SHHG-SPIDER (spectral phase interferometry for direct electric field reconstruction) and SHHG-SEA-SPIDER (spatially encoded arrangement for SPIDER). 1D- and 2D-EPOCH PIC (particle-in-cell) simulations were performed to generate the attosecond pulses from relativistic plasmas under different conditions. Pulse trains dominated by single isolated peak as well as complex pulse train structures are extensively discussed for both methods, which showed excellent accuracy in the complete reconstruction of the attosecond field with respect to the direct Fourier transformed result. Kirchhoff integral theorem has been used for the near-to-far-field transformation. This far-field propagation method allows us to relate these results to potential experimental implementations of the scheme. The impact of comprehensive experimental parameters for both apparatus, such as spectral shear, spatial shear, cross-angle, time delay, and intensity ratio between the two replicas has been investigated thoroughly. These methods are applicable to complete characterization for SHHG attosecond pulses driven by a few to hundreds of terawatts femtosecond laser systems.

Tunable mode-locked Tm-doped fiber laser based upon cross-phase modulation.

We demonstrate the generation of soliton and dissipative soliton in an ultrafast thulium (Tm) doped fiber laser based upon cross-phase modulation (XPM) induced mode-locking. The mode-locking is realized by periodically modulating the 2-µm signal through XPM that is activated by an injected 1.5-µm pulsed laser. Such a mechanism enables the laser to be mode-locked in various operation regimes without any real or artificial saturable absorbers. Thanks to the XPM pulling effect, the wavelength of the Tm-doped fiber laser can be tuned by adjusting the repetition frequency of the 1.5-µm pulsed laser. The maximum tuning ranges achieved in this work for the soliton and dissipative soliton regimes are respectively 11 nm and 15 nm. The outcomes of this work not only provide a continuously and controllably wavelength-tunable ultrafast laser but also offer a passively synchronized dual-color fiber laser system, which is promised for many important applications such as Raman spectroscopy, nonlinear frequency conversion systems, and multi-color pump-probe systems.

Tunable thulium-doped mode-locked fiber laser with watt-level average power.

We demonstrate a wavelength-tunable, sub-200 fs, and watt-level thulium-doped ultrafast fiber oscillator with a fundamental frequency repetition rate of 509.7 MHz. The wavelength can be tuned between 1918.5 nm and 2031 nm by adjusting the intra-cavity waveplates. When the wavelength is tuned to below 2000 nm, the average output power exceeds 1 W. The oscillator provides a maximum average power of 1.314 W (corresponding to a pulse energy of 2.58 nJ) and a highest peak power of 12.5 kW at 1940 nm. Such a high-power, tunable 2-µm mode-locked fiber laser is an ideal light source candidate for a variety of applications, such as frequency metrology, molecular spectroscopy, and ultrafast pump-probe spectroscopy.