High-efficiency single-frequency DBR fiber laser at 1091 nm utilizing a Yb:YAG crystal-derived silica fiber.

A single-frequency distributed Bragg Reflector (DBR) fiber laser operating at 1091 nm was demonstrated by using a Yb:YAG crystal-derived silica fiber (YDSF). The YDSF was prepared via the molten core (MC) method, with a Yb2O3 doping concentration of 5.60 wt.% in the core, resulting in a gain coefficient of 1.45 dB/cm at 1091 nm. Employing 0.8 cm of the YDSF, we attained a single-frequency laser with a maximum output power of 145 mW and a slope efficiency of 31.8%. The laser exhibited an optical signal-to-noise ratio (OSNR) exceeding 71 dB, a linewidth of ∼34 kHz, and a stabilized relative intensity noise (RIN) at -132 dB/Hz for frequencies over 4.5 MHz. The fiber laser could serve as an outstanding seed source for high-power, narrow-linewidth fiber amplifiers operating at 1091 nm.

Widely tunable NCPM-KTA OPO based on non-collinear phase-matching in a four-mirror ring cavity.

Based on non-collinear phase-matching (PM), a new method for widely tunable wavelength was proposed and demonstrated in a four-mirror ring cavity of non-critical phase-matching (NCPM) KTiOAsO4 (KTA) optical parametric oscillator (OPO). Wavelength tuning range of 141 nm from 1535.56 nm to 1676.73 nm was achieved by moving one mirror of ring cavity back and forth. The tuning theory and tuning method of non-collinear PM were analyzed in detail. The output energy and pulse width of signal were measured and compared in collinear and non-collinear PM condition. This method is also applicable to OPOs of other nonlinear crystals based on four-mirror ring cavity.

Single-frequency fiber laser of 315†mW at 1940†nm based on a Tm : YAG/Ho : YAG-co-derived silica fiber.

A 1940 nm single-frequency distributed Bragg reflector (DBR) fiber laser was demonstrated based on a Tm : YAG/Ho : YAG-co-derived silica fiber (THCDSF). The THCDSF, which had a core dopant concentration of 8.02 wt.% Tm2O3 and 1.18 wt.% Ho2O3, was prepared via the melt-in-tube (MIT) method using a Tm : YAG and a Ho : YAG as the precursor core and a silica tube as the cladding. Employing 1.8 cm of the THCDSF, we achieved a maximum single-frequency output power of 315 mW at 1940 nm when pumped by a 1610 nm fiber laser. The slope efficiency of the laser was 29.68% for the absorbed pump power. The laser linewidth was less than 23.65 kHz, and the relative intensity noise (RIN) stabilized at -145 dB/Hz after exceeding 4.8 MHz. To the best of our knowledge, this is the first demonstration of a single-frequency DBR laser with a Tm3+/Ho3+ fiber as the gain medium.

Bio-Inspired Conductive Hydrogels with High Toughness and Ultra-Stability as Wearable Human-Machine Interfaces for all Climates.

Drawing inspiration from Salicornia, a plant with the remarkable ability to thrive in harsh environments, a conductive hydrogel with high toughness and ultra-stability is reported. Specifically, the strategy of pre-cross-linking followed by secondary soaking in saturated salt solutions is introduced to prepare the PAAM-alginate conductive hydrogel with dual cross-linked dual network structure. It allows the alginate network to achieve complete cross-linking, fully leveraging the structural advantages of the PAAM-alginate conductive hydrogel. The highest tensile strength of the obtained conductive hydrogel is 697.3 kPa and the fracture energy can reach 69.59 kJ m-2 , significantly higher than human cartilage and natural rubbers. Specially, by introducing saturated salt solutions within the hydrogel, the colligative properties endow the PAAM-alginate conductive hydrogel with excellent water retention and anti-freezing properties. The prepared conductive hydrogels can work stably in an ambient environment for more than 7 days and still maintain good mechanical behavior and ionic conductivity at -50 °C. Benefiting from the excellent comprehensive performance of conductive hydrogels, wearable human-machine interfaces that can withstand large joint movements and are adapted for extreme environments are prepared to achieve precise control of robots and prostheses, respectively.

Comparison of a high-power 3.75 µm BaGa4Se7 OPO based on a plane-parallel resonator and an unstable resonator with a Gaussian reflectivity mirror.

Using a homemade Nd:YAG laser with a pulse repetition frequency of 300 Hz as a pump source, we demonstrated a tunable BaGa4Se7 (BGSe) optical parametric oscillator (OPO). Wavelength-tuning ranges of 1.42-1.59 µm and 3.21-4.22 µm were realized, and a maximum average output power of 1.03 W at 3746 nm was achieved in a plane-parallel resonator. To date, this is the highest output power at around 4 µm obtained from a BGSe OPO pumped by a 1 µm laser, to our knowledge. Moreover, the BGSe OPO employing a plano-convex Gaussian reflectivity mirror (GRM) as an output coupler (OC) was demonstrated for the first time, to our knowledge. At the same pump power, the corresponding output power and M2 value of the idler at 3746 nm were 0.86 W and 10.2 (x direction) and 11.6 (y direction), respectively. The results indicate that the beam quality factor M2 of the idler has an over 30% improvement in comparison with the BGSe OPO based on a conventional flat OC of a plane-parallel resonator. Limited by the beam quality of the pump source, there is little room for improvement in the beam quality of the BGSe OPO with a GRM OC, which could be improved in the future by optimizing the pump beam.

Modeling for extracavity-pumped terahertz parametric oscillators.

This paper presents a modeling method for extracavity-pumped terahertz parametric oscillators (TPO) based on stimulated polariton scattering, in which the pumping beam is from a different laser, and the Stokes beam oscillates in its cavity. After suitable approximations and assumptions, the average THz wave amplitude in the nonlinear crystal is expressed as a function of the fundamental and Stokes wave amplitudes. Then the rate equation for the Stokes wave is obtained based on the Stokes wave increment within a cavity roundtrip timescale. After solving the Stokes wave rate equation, the Stokes wave temporal evolution is considered as a known parameter, and the properties of the residual fundamental and terahertz waves are obtained by numerically solving the coupled wave equations. This modeling method is applied to an extracavity-pumped TPO based on MgO:LiNbO3 crystal. The simulation results are basically consistent with the experimental results. The main reasons causing the deviations of the simulation results from the experimental results are analyzed. To the best of our knowledge, this is the first time to perform the modeling for extracavity-pumped Q-switched TPOs.

High-efficiency 940- and 969-nm brightness-maintaining wavelength-multiplexed LD-pumped 240-W thin-rod Yb:YAG amplifier.

A high-efficiency ultrafast laser amplifier based on thin-rod Yb:YAG was demonstrated, featuring a 940- and 969-nm brightness-maintaining wavelength-multiplexed laser diode (LD)-pumping method. Two high-brightness LDs (940 nm and 969 nm) were spectrally combined into one beam spatially with a dichroic mirror, thus enabling twice pump power while maintaining high brightness. A maximum signal power of 240 W was obtained at a repetition rate of 1 MHz, with a power extraction efficiency (PEE) of ∼51%. To the best of the authors' knowledge, this is the first report of >50% efficiency as well as the highest average power operating at the fundamental mode for thin-rod Yb:YAG amplifiers. The beam quality factors (M2) of the amplified signal were measured to be 1.72 and 1.12 for the horizontal and vertical directions, respectively. A preliminary pulse compression was conducted at a signal power of 80 W with a chirped volume Bragg grating (CVBG) compressor. The compressed pulse duration was 744 fs with an average power of 66.5 W, corresponding to a compression efficiency of 83.1%.

Wearable Sensors Adapted to Extreme Environments Based on the Robust Ionogel Electrolytes with Dual Hydrogen Networks.

Nonvolatile ionogels are promising soft electrolyte materials for flexible electronics, but it is challenging to fabricate stable electrolytes with mechanical robustness. Here, through rationally optimizing the chemical structure of polymer matrix and ionic liquids, the high-performance ionogel electrolytes with mechanical robustness and stability were fabricated. There are double hydrogen bonding networks in the as-prepared ionogel electrolytes, one of which exists between the polymer chains while the other one existing between the polymer chains and ionic liquid molecules. By adjusting the content of the ionic liquid and the ratio of the two hydrogen bonding networks, the prepared ionogel electrolytes exhibit tunable properties with an elasticity of 1.3-30 kPa, a stretchability of more than 1800%, a fracture energy of 125.8-548.3 KJ m-3, and a coordinated self-healing efficiency of 6.2-37.9% to satisfy the needs of different application scenarios. The assembled wearable sensors based on the high-performance ionogel electrolytes can be attached to a part of the human body, detecting various motions and body temperature. Benefiting from the nonvolatile and hydrophobic properties of the ionogel electrolytes, the wearable sensors can be operated under extreme environments including high/low temperature (-15-100 °C) and high humidity (100% relative humidity). It is believed that this work provides prospects for the application of wearable electronic devices.

High-repetition-rate, 50-µJ-level, 1064-nm, CPA laser system based on a single-stage double-pass Yb-doped rod-type fiber amplifier.

A 1064-nm femtosecond fiber chirped pulse amplification (FCPA) laser system based on a single-stage double-pass Yb-doped rod-type photonic crystal fiber (PCF) amplifier was demonstrated with a pulse repetition rate of 500 kHz, which was specially designed for expected conversion efficiency enhancement of a 10.8 eV source. With a series of Yb:fiber power amplifiers, the average output power was boosted to approximately 35 W. Further, using a transmission gratings-based pulse compressor, an average output power of 27.5 W was achieved, corresponding to a pulse energy of 55 µJ and a compression efficiency of 78.6%. The shortest pulse duration was optimized to be 204 fs, which was also accompanied by obvious pedestal. A pulse duration of 336 fs was also obtained when the pulse quality was at a top priority. To the best of our knowledge, this is the first demonstration of high-repetition-rate high-pulse-energy 1064-nm, instead of 1035-nm, femtosecond laser, based on commercially available Yb-doped rod-type PCF amplifier.

Zwitterionic Hydrogel Electrolyte with Tunable Mechanical and Electrochemical Properties for a Wearable Motion and Thermal Sensor.

A high-performance zwitterionic hydrogel electrolyte was successfully fabricated, in which the polymer chains were cross-linked by multiple reversible non-covalent bonds. The mechanical and electrochemical properties of the synthesized zwitterionic hydrogel electrolyte can be facilely modulated by immersing the as-prepared zwitterionic hydrogel in saline solutions with different concentrations. The processed zwitterionic hydrogel electrolyte exhibits tunable mechanical and electrochemical properties (favorable elasticity of 3.2-202 kPa, high fracture energy of 0.34-1.15 MJ m-3, high stretchability of 1000-2880%, high self-healing efficiency of 30-70%, and coordinated ionic conductivity of 0.16-1.65 S m-1) to satisfy the needs of different application scenarios. A wearable sensor based on the zwitterionic hydrogel electrolyte is demonstrated. The wearable sensor can be attached to a part of the human body, and various human motions can be successfully monitored. Besides being functionalized as a motion monitor, the wearable sensor can also be used as a thermometer to monitor the body temperature instantaneously, showing an encouraging and strong practicability in wearable electronic device.

Consecutive 1015-1105-nm wavelength tunable "figure-of-9" mode-locked Yb:fiber oscillator.

A widely wavelength tunable mode-locked Yb-doped fiber oscillator based on nonlinear amplifier loop mirror (NALM) is reported, in which only a piece of short (∼0.5 m) single-mode polarization-maintaining (PM) Yb-doped fiber is employed, instead of the frequently used long (a few meters) double cladding (DC) fiber in previous papers. Experimentally, the center wavelength can be consecutively tuned from 1015 to 1105 nm by tilting the silver mirror, corresponding to a tuning range of 90 nm. To the best of our knowledge, this is the broadest consecutive tuning range in Yb:fiber mode-locked fiber oscillator. In addition, the mechanism of wavelength tuning is tentatively analyzed and attributed to the combined action of the spatial dispersion induced by a tilting silver mirror and the limited aperture in the system. Specific to the wavelength of 1045 nm, the output pulses with 13-nm spectral bandwidth can be compressed to 154 fs.

Stable 5-GHz fundamental repetition rate passively SESAM mode-locked Er-doped silica fiber lasers.

A stable passively mode-locked Er-doped silica fiber laser with a fundamental repetition rate of up to 5 GHz is demonstrated, which, to the best of our knowledge, is the highest repetition rate for 1.5 µm semiconductor saturable absorber mirror (SESAM) mode-locked Er-doped silica fiber (EDF) lasers. A segment of commercially available EDF with a net gain coefficient of 1 dB/cm is employed as gain medium. The compact Fabry-Pérot (FP) cavity features a fiber mirror, namely multiple-layer dielectric films (DFs) directly coated on end facet of a passive fiber ferrule, enabling a short cavity length of 2 cm configured. The mode-locked oscillator operates at 1561.0 nm with a signal-to-noise ratio (SNR) of 62.1 dB, whose average power is boosted to 27 mW by a single-mode Er-doped fiber amplifier (EDFA) and spectral bandwidth is broadened form 0.69 nm to 1.16 nm with a pulse width of 3.86 ps. The fiber laser shows excellent spectral stability without conspicuous wavelength drifting for 3 hours. Moreover, the basic guidelines of selecting SESAM for high repetition rate passively mode-locked fiber lasers is given.

Linearly polarized single-frequency fiber laser based on the Yb:YAG-crystal derived silica fiber.

A linearly polarized low-noise single-frequency fiber laser was demonstrated by using a homemade 1.2-cm-long Yb:YAG crystal derived silica fiber. A maximum output power of greater than 60 mW was obtained with a signal-to-noise ratio of ∼80dB and a polarization extinction ratio of 27.8 dB. Additionally, the relative intensity noise was measured to be -145dB/Hz above 6.5 MHz. A frequency fluctuation of less than 20 MHz was also obtained. The output power was scaled up to 14.5 W with a one-stage all-fiber amplifier scheme with a slope efficiency of 56.4%.

Distributed sparse signal sensing based on compressive sensing OFDR.

The maximum detectable vibration frequency of an optical frequency domain reflectometry (OFDR) system is limited by the tunable rate of the laser source. Unlike uniform sampling with the time-resolved method, the sampling frequency is randomly modulated so that the vibration signal applied on the interrogation fiber is sampled by a multi-frequency sub-Nyquist sampling method and reconstructed by the compressive sensing technique. First, we give a full treatment to prove that the proposed method has the same performance as the conventional method. Second, in a further proof-of-concept experiment, the measurable frequency of a sparse signal is achieved up to 200 Hz with a sweeping rate of 40 nm/s. This method can recover the vibration signal with sampling rates less than that required by the Nyquist sampling theory, which is a significant step toward a high-performance OFDR system, especially for evaluating the intrinsic frequency of the object's structural condition.

Broadening frequency response of a distributed sparse-wideband vibration sensing via a time-division multi-frequency sub-Nyquist sampling.

The maximum detectable vibration frequency response range is inversely related with the sensing fiber length in direct-detection intensity-measuring coherent optical time domain reflectometry (DI-COTDR). Unlike the conventional uniform sampling, the pulse repetition rate is modulated in a time-division manner so that a multi-frequency sub-Nyquist sampling is realized along every point of the sensing fiber. A 24-kHz vibration signal can be detected and recovered by a compressive sensing technique using sampling pulses with repetition rate lower than 5-kHz, which is ten-fold lower compared to that required in the conventional uniform sampling method. Also, a multi-frequency vibration signal can be identified and recovered by this technique. The proposed method can break through the theoretical maximum detection frequency of traditional systems without any hardware modification. Therefore, such a method is of great significance for broadening the frequency response range of the distributed sparse-wideband vibration sensing.

Modeling of intracavity-pumped Q-switched terahertz parametric oscillators based on stimulated polariton scattering.

In this paper, the rate equations describing the operation of intracavity-pumped Q-switched terahertz parametric oscillators based on stimulated polariton scattering are given for the first time. The rate equations are obtained under the plane-wave approximation, the oscillating fundamental and Stokes waves are supposed to be round uniform beam spots. Considering the fact that the terahertz wave nearly traverses the pump and Stokes beams and using the coupled wave equations, the terahertz wave intensity is expressed as the function of the fundamental and Stokes intensities. Thus, the rate equations describing the evolution processes of the fundamental and Stokes waves are obtained in the first step. The THz wave properties are then obtained. Several curves based on the rate equations are generated to illustrate the effects of the nonlinear coefficient, the THz wave absorption coefficient, and pulse repetition rate on the THz laser characteristics. Taking the intracavity-pumped Mg:LiNbO3 TPO as an example, the THz frequency tuning characteristic and the dependences of the fundamental, Stokes, and THz wave powers on the incident diode pump power are calculated. The theoretical results are in agreement with the experimental results on the whole.

Tunable Stokes laser based on the cascaded stimulated polariton scattering and stimulated Raman scattering in RbTiOPO4 crystal.

Stimulated polariton scattering (SPS) and stimulated Raman scattering (SRS) in ${{\rm RbTiOPO}_4}$RbTiOPO4 (RTP) crystal are combined in an intracavity-pumped Stokes parametric oscillator (SPO) to extend the tunable Stokes laser spectral range. The pumping laser wavelength is 1064 nm from a diode-end-pumped acousto-optically Q-switched Nd:YAG laser. By the SPS process in the SPO, the SPS-Stokes wave can be discontinuously tuned in the range of 1075.7-1076.0 nm, 1077.7-1080.4 nm, 1081.8-1082.2 nm, and 1084.8-1087.8 nm, respectively. By the following SRS process in the same RTP crystal, the laser wavelength is further shifted in the range of 1107.7-1108.1 nm, 1109.0-1112.7 nm, 1114.3-1115.1 nm, and 1117.8-1121.1 nm, respectively. A maximal average output power of 970 mW is achieved for the SRS-Stokes wave at the peak wavelength of 1118.8 nm. It is obtained when the diode power is 7.9 W, and the pulse repetition frequency (PRF) is 10 kHz.

110 mW single-frequency Yb:YAG crystal-derived silica fiber laser at 1064 nm.

A single-frequency laser based on Yb:YAG crystal-derived silica fiber (YDSF) was demonstrated. The YDSF was fabricated by a molten-core method with a doping concentration of 4.8 wt. % for Yb2O3, whose gain coefficient and transmission loss were measured to be 1.7 dB/cm and 0.005 dB/cm, respectively. An over 110 mW stable single-frequency laser at 1064 nm was obtained based on a distributed Bragg reflector setup, showing a slope efficiency of 18.5%. The signal-to-noise ratio was ∼80 dB, and the frequency fluctuation was less than 20 MHz within 10 min.

All-fiber-integrated Yb:YAG-derived silica fiber laser generating 6 W output power.

A Yb:YAG-derived silica fiber was fabricated by a molten-core fabrication method, in which a Yb:YAG crystal was used as the core material and a silica tube was used as the cladding material. The fiber's transmission loss was measured to be 0.49 dB/m at 1.55 µm, using a cut-back method. The fiber microstructure image showed that the cladding region was a uniform glass structure, while the core structure was amorphous. An all-fiber-integrated cladding-pumped laser based on Yb:YAG-derived silica fiber was demonstrated. With an incident pump power of 28 W, an output power of 6 W was obtained at 1.06 µm, with a slope efficiency of 21.7%.

Dual Path Lock-In System for Elimination of Residual Amplitude Modulation and SNR Enhancement in Photoacoustic Spectroscopy.

A technique for elimination of residual amplitude modulation (ERAM) in photoacoustic spectroscopy based on dual path lock-in was proposed and experimentally demonstrated. There are two lock-in amplifiers, one is for gas concentration demodulation and another for residual amplitude modulation (RAM) measurement by tuning the reference signal in different phases, and then a dual path lock-in technique based on subtraction is applied to RAM removal, improving the second harmonic profile significantly. In this system, the signal to noise ratio (SNR) increases about two times based on our dual path lock-in technique compared to one distributed feedback laser diode (DFB-LD). The system achieved a good linear response (R-square = 0.99887) in a concentration range from 100 ppmv to 2400 ppmv and a minimum detection limit (MDL) of 1.47 ppmv.