2.0-μm emission and energy transfer of Ho~(3+)/Yb~(3+) co-doped LiYF_4 single crystal excited by 980 nm

Ho3+/Yb3+co-doped Li YF4 single crystals with various Yb3+concentrations and ～ 0.98 mol% Ho3+concentration are grown by the Bridgman method under the conditions of taking Li F and YF3 as raw materials and a temperature gradient(40°C/cm–50°C/cm) for the solid–liquid interface.The luminescent performances of the crystals are investigated through emission spectra,infrared transmittance spectrum,emission cross section,and decay curves under excitation by 980 nm.Compared with the Ho3+single-doped Li YF4 crystal,the Ho3+/Yb3+co-doped Li YF4 single crystal has an obviously enhanced emission band from 1850 nm to 2150 nm observed when excited by a 980-nm diode laser.The energy transfer from Yb3+to Ho3+and the optimum fluorescence emission around 2.0 μm of Ho3+ions are investigated.The maximum emission cross section of the above sample at 2.0 μm is calculated to be 1.08×10-20cm2 for the Li YF4 single crystal of1-mol% Ho3+and 6-mol% Yb3+according to the measured absorption spectrum.The high energy transfer efficiency of88.9% from Yb3+to Ho3+ion in the sample co-doped by Ho3+(1 mol%) and Yb3+(8 mol%) demonstrates that the Yb3+ions can efficiently sensitize the Ho3+ions.     

Cascaded dual-channel fiber SPR temperature sensor based on liquid and solid encapsulations

In order to control the working wavelength range of the fiber surface plasmon resonance(SPR) temperature sensor and realize the wavelength division multiplexing type multi-channel fiber SPR temperature sensor, by comprehensively investigating the influence of liquids with different thermal-optical coefficients and solid packaging materials on the performance of fiber SPR temperature sensor, a dual-channel fiber SPR temperature sensor based on liquid–solid cascade encapsulation was designed and fabricated. The liquid temperature sensing stage encapsulated in capillary worked in 616.03 nm–639.05 nm band, the solid sensing stage coated with pouring sealant worked in 719.37 nm–825.27 nm band, and the two stages were cascaded to form a fiber dual-channel temperature sensor. The testing results indicated that when the temperature range was 35℃–95℃, the sensitivity of two-stage temperature detection was -0.384 nm/℃and -1.765 nm/℃ respectively. The proposed fiber sensor has simple fabrication and excellent performance which can be widely used in various fields of dual-channel temperature measurement and temperature compensation.     

Spectroscopic properties in Er³⁺/Yb³⁺ Co-doped fluorophosphate glass

Using the technique of high-temperature melting, a new Er 3+ /Yb 3+ co-doped fluorophosphate glass was prepared. The absorption and fluorescence spectra were investigated in depth. The effect of Er 3+ and Yb 3+ concentration on the spectroscopic properties of the glass sample was also discussed. According to the Judd–Ofelt theory, the oscillator strength was computed. The lifetime of 4 I 13/2 level (τ m ) of Er 3+ ions was 8.23 ms, and the full width at half maximum of the dominating emission peak was 68 nm at 1.53 μm. The large stimulated emission cross section of the Er 3+ was calculated by the McCumber theory. The spectroscopic properties of Er 3+ ion were compared with those in different glasses. The full width at half maximum and σ e are larger than those of other glass hosts, indicating this studied glass may be a potentially useful candidate for high-gain erbium-doped fiber amplifier.     

Noise-like rectangular pulses in a mode-locked double-clad Er:Yb laser with a record pulse energy

Generation of noise-like rectangular pulse was investigated systematically in an Er–Yb co-doped fiber laser based on an intra-cavity coupler with different coupling ratios. When the coupling ratio was 5/95, stable mode-locked pulses could be obtained with the pulse packet duration tunable from 4.86 ns to 80 ns. The repetition frequency was 1.186 MHz with the output spectrum centered at 1.6 μm. The average output power and single pulse energy reached a record 1.43 W and1.21 μJ, respectively. Pulse characteristics under different coupling ratios(5/95, 10/90, 20/80, 30/70, 40/60) were also presented and discussed.     

Simultaneous measurement of strain and temperature using Fabry–Pérot interferometry and antiresonant mechanism in a hollow-core fiber

A fiber-optic sensor for the simultaneous measurement of strain and temperature is proposed and experimentally demonstrated based on Fabry–Pérot(FP) interference and the antiresonance(AR) mechanism. The sensor was implemented using a single-mode fiber(SMF)–hollow-core fiber–SMF structure. A temperature sensitivity of 21.11 pm/℃ was achieved by tracing the troughs of the envelope caused by the AR mechanism, and a strain sensitivity of 2 pm/με was achieved by detecting the fine fringes caused by the FP cavity. The results indicate that the dual-parameter sensor is stable and reliable.     

Energy storage and heat deposition in Cr,Yb,Er co-doped phosphate glass

Energy storage and heat deposition in Cr,Yb,Er co-doped phosphate glass were reported. A model based on rate equations was used to determine the energy storage from the free-oscillating output energy characteristics. The heat deposition was calculated by measuring the temperature rise of the glass rod. The results provided important information for the glass operating in Q-switched mode, and also for calculating the temperature profiles and cooling requirements of the glass under single shot and repetitive pulsed conditions.     

Asymmetrical tapered SMS fiber coupler for simultaneous measurement of temperature and refractive index and its application for biosensing

An asymmetrical tapered singlemode–multimode–singlemode(SMS) fiber coupler based on two parallel physical contact SMS fiber structures was proposed. Since the coupler includes modes both from fiber core and cladding,two dips of the transmission spectrum exhibit different sensing characteristics to the surrounding temperature and refractive index(RI) change, which allows the use of the standard matrix inversion method to determine temperature and RI simultaneously. The temperature sensitivities of 0.0498 and 0.0324 nm/℃ and RI sensitivities of 1151.76 and 1325.66 nm/RIU have been achieved, respectively. For biosensing application, with the functionalized fiber coupler sensor, a human chorionic gonadotropin concentration of 0.05 mIU/mL has been detected for a wavelength shift of 0.2 nm with good stability and excellent selectivity. The developed tapered SMS fiber coupler structure has advantages of simultaneous measurement of two independent parameters,simple configuration, low cost, and good repeatability that offer a great potential for medical diagnostics.     

Temperature dependence of radiation-induced attenuation of optical fibers

We investigate the temperature dependence of radiation-induced attenuation (RIA) at 1 310 nm for a Ge/P co-doped fiber after a steady-state γ-ray irradiation.A γ irradiation facility 60Co source is used to irradiate the fiber at a dose rate of 0.5 Gy/min,satisfying a total dose of 100 Gy.The test temperature ranges from-40 to 60℃ by 20℃,and the RIA of the fiber is obtained using a power measuring device.The experimental result demonstrates that RIA exhibits a steady,monotonic,and remarkable temperature dependence after approximately 48h of accelerated annealing at 70℃.The optical fiber irradiated with a high dose and annealed sufficiently can be used as a temperature sensor.     

Spectroscopic properties in Er3＋/yb3＋ Co-doped fluorophosphate glass

Using the technique of high-temperature melting, a new Er3＋/Yb3＋ co-doped fluorophosphate glass was prepared. The absorption and fluorescence spectra were investigated in depth. The effect of Er3＋ and Yb3＋ concentration on the spectroscopic properties of the glass sample was also discussed. According to the Judd Ofelt theory, the oscillator strength was computed. The lifetime of 4113/2 level （t-m） of Er3＋ ions was 8.23 ms, and the full width at half maximum of the dominating emission peak was 68 nm at 1.53 μm. The large stimulated emission cross section of the Er3＋ was calculated by the McCumher theory. The spectroscopic properties of Er3＋ ion were compared with those in different glasses. The full width at half maximum and σe are larger than those of other glass hosts, indicating this studied glass may be a potentially useful candidate for high-gain erbium-doped fiber amplifier.     

Effect of sintering temperature on luminescence properties of borosilicate matrix blue–green emitting color conversion glass ceramics

The color conversion glass ceramics which were made of borosilicate matrix co-doped(SrBaSm)Si_2O_2N_2:(Eu~(3+)Ce~(3+)) blue–green phosphors were prepared by two-step method in co-sintering. The change in luminescence properties and the drift of chromaticity coordinates(CIE) of the(SrBaSm)Si_2O_2N_2:(Eu~(3+)Ce~(3+)) blue–green phosphors and the color conversion glass ceramics were studied in the sintering temperature range from 600℃ to 800℃. The luminous intensity and internal quantum yield(QY) of the blue–green phosphors and glass ceramics decreased with the sintering temperature increasing. When the sintering temperature increased beyond 750℃, the phosphors and the color conversion glass ceramics almost had no peak in photoluminescence(PL) and excitation(PLE) spectra. The results showed that the blue–green phosphors had poor thermal stability at higher temperature. The lattice structure of the phosphors was destroyed by the glass matrix and the Ce~(3+) in the phosphors was oxidized to Ce~(4+), which further caused a decrease in luminescent properties of the color conversion glass ceramics.     

High temperature-sensitivity sensor based on long period fiber grating inscribed with femtosecond laser transversal-scanning method

We propose a high temperature-sensitive long period fiber grating(LPFG) sensor fabricated by using the femtosecond laser transversal-scanning method. The femtosecond pulses scan over the whole fiber core and some part of the cladding region; the modified regions are more extended. It is found that the LPFG-I fabricated by the transversal-scanning method shows higher temperature sensitivity and better temperature uniformity than that of LPFG-II written by the femtosecond laser point-by-point method. The LPFG-I with a temperature sensitivity of 75.96 pm/°C in the range of 25°C–400°C is measured. Moreover, in the range from 400°C to 800°C, a higher temperature sensitivity of 148.64 pm/°C and good linearity of 0.99 are achieved, while the temperature sensitivity of LPFG-II is only 95.55 pm/°C. LPFG-I exhibits better temperature characteristics, which, to the best of our knowledge, has the highest sensitivity in silica fiber temperature sensors.     

A Novel Mach–Zehnder Interferometer Based on Hybrid Liquid Crystal–Photonic Crystal Fiber

We propose a novel all fiber Mach–Zehnder interferometer(MZI) based on photonic crystal fiber(PCF) filled with liquid crystal(LC). The interference between the core mode and the cladding modes of a PCF is utilized.To excite the cladding modes, a region is formed using fiber fusion splicer. Due to the fact that varying effective index difference between the core region and the LC-filled cladding region can cause different transmission spectra,we mainly study the MZIs with different LC-filled structures and different lengths of LC filling. The measured results demonstrate that quite clear interference spectra can be obtained. Through analysis spatial frequency spectrum and temperature spectrum of two MZIs with different LC-filled structures, we can obtain that the MZI with adjacent two LC-filled holes has clearer interference spectrum and higher temperature sensitivity. Thus we choose this MZI to measure the temperature sensitivity with different lengths of LC filling. When the length of LC filling is 2 cm, the temperature sensitivities can be enlarged to 1.59 nm/C. The interferometer shows a good temperature tunability and sensitivity, which can be a good candidate for a highly tunable optical filtering and temperature sensing applications.     

Up-conversion luminescence properties and energy transfer of Er~(3+)/Yb~(3+)co-doped oxyfluoride glass ceramic containing CaF_2 nano-crystals

The Er3+/Yb3+co-doped transparent oxyfluoride glass-ceramics containing CaF2nano-crystals were successfully prepared. After heat treatments, transmission electron microscopy(TEM) images showed that CaF2nano-crystals of 20–30 nm in diameter precipitated uniformly in the glass matrix. Comparing with the host glass, high efficiency upconversion luminescence of Er3+at 540 nm and 658 nm was observed in the glass ceramics under the excitation of 980 nm. Moreover,the size of the precipitated nano-crystals can be controlled by heat-treatment temperature and time. With the increase of the nano-crystal size, the intensity of the red emission increased more rapidly than that of the green emission. The energy transfer process of Er3+and Yb3+was convinced and the possible mechanism of Er3+up-conversion was discussed.     

Thermal characteristics of Fabry–Perot cavity based on regenerated fiber Bragg gratings

The Letter reports the thermal stability and strain response of Fabry–Perot(FP) cavity under different high temperatures. The FP cavity was made by thermal regeneration of two identical cascaded fiber Bragg gratings(FBGs). It is demonstrated that the FP cavity is capable of measuring temperatures from 300℃ to 900℃ with a temperature sensitivity of 15.97 pm/℃. The elongation of the fiber was observed through the drifted Bragg wavelength at 700℃ or above when weight was loaded. The elongation was further inferred by the slight change in the interference spectra of the FP cavity at 900℃.     

An all-fiber type Er~(3+)/Yb~(3+) co-doped fiber laser

In this paper, a distributed Bragg reflection (DBR) type Er~(3+)/Yb~(3+) co-doped fiber laser of high output power and high slope efficiency was developed. Its gain medium was a 4.45-m-long Er3+/Yb3+ co-doped fiber. When it was pumped by a 1064-nm Nd:YAG, the linewidth of output laser was measured as 0.072 nm by 3 dB and 0.192 nm by 25 dB at 1552.08 nm. The maximum output power was measured as 69 mW. Its power stability was < 5%, side mode suppression ratio was 59 dB, and the output wavelength stability was ±0.01 nm. The laser had a threshold of 12 mW and a slope efficiency of 22%.     

FFPI-FBG hybrid sensor to measure the thermal expansion and thermo-optical coefficient of a silica-based fiber at cryogenic temperatures

In this Letter, a sensor consisting of a fiber Bragg grating(FBG) and a fiber Fabry–Perot interferometer(FFPI)sensor is developed to measure the coefficient of thermal expansion(CTE) and the thermo-optical coefficient(TOC) of a silica-based optical fiber at cryogenic temperatures. The FFPI is fabricated by welding together two acid-etched fibers. The temperature performance of the FFPI-FBG hybrid sensor is studied in the temperature range of 30–273 K. The CTE and TOC of the optical fiber at cryogenic temperatures are derived analytically and verified by experiments.     

Optical temperature sensor based on up-conversion fluorescence emission in Yb3+:Er3+ co-doped ceramics glass

Yb3+:Er3+co-doped oxy-fluoride ceramics glass has been prepared.The mechanism of up-conversion emissions about Er3+was discussed,and the temperature properties of green up-conversion fluorescence between 303 and 823 K were investigated.The results show that the sensitivity of this sample reaches its maximum value,about 0.0047 K 1,when the temperature is 383 K,indicating that this kind of sample can be used as high temperature and high sensitivity optical temperature sensor.     

Bismuth-doped germanate glass fiber fabricated by the rod-in-tube technique

Bismuth(Bi)-doped laser glasses and fiber devices have aroused wide attentions due to their unique potential to work in the new spectral range of 1 to 1.8 μm traditional laser ions, such as rare earth, cannot reach. Current Bi-doped silica glass fibers have to be made by modified chemical vapor deposition at a temperature higher than2000°C. This unavoidably leads to the tremendous loss of Bi by evaporation, since the temperature is several hundred degrees Celsius higher than the Bi boiling temperature, and, therefore, trace Bi(～50 ppm) resides within the final product of silica fiber. So, the gain of such fiber is usually extremely low. One of the solutions is to make the fibers at a temperature much lower than the boiling temperature of Bi. The challenge for this is to find a lower melting point glass, which can stabilize Bi in the near infrared emission center and, meanwhile, does not lose glass transparency during fiber fabrication. None of previously reported Bi-doped multicomponent glasses can meet the prerequisite. Here, we, after hundreds of trials on optimization over glass components,activator content, melting temperature, etc., find a novel Bi-doped gallogermanate glass, which shows good tolerance to thermal impact and can accommodate a higher content of Bi. Consequently, we successfully manufacture the germanate fiber by a rod-in-tube technique at 850°C. The fiber exhibits similar luminescence to the bulk glass, and it shows saturated absorption at 808 nm rather than 980 nm as the incident power becomes higher than 4 W. Amplified spontaneous emissions are observed upon the pumps of either 980 or 1064 nm from germanate fiber.     

FP cavity and FBG cascaded optical fiber temperature and pressure sensor

A fiber Bragg grating(FBG) and Fabry–Perot(FP) cavity cascaded fiber sensing system was manufactured for temperature and pressure sensing. Temperature sensing as high as 175°C was performed by an FBG for the linear variation of an FBG wavelength with temperature. After the temperature was sensed, the demodulation system can find the original FP cavity length and its pressure and cavity length correlation coefficient; thus, the ambient pressure would be calculated. The sensing pressure can be as high as 100 MPa with a repeatability of 1/10,000 and high stability. This kind of fiber sensor has been used in the Shengli Oil Field.     

All-fiber laser seeded femtosecond Yb:KGW solid state regenerative amplifier

A high efficiency compact Yb:KGW regenerative amplifier using an all-fiber laser seed source was comprehensively studied. With thermal lensing effect compensated by the cavity design, the compressed pulses with energy of 1 m J at 1 k Hz and 0.4 m J at 10 k Hz in sub-400-fs pulse duration using chirped fiber Bragg grating(CFBG) stretcher were demonstrated.A modified Frantz–Nodvik equation was developed to emulate the dynamic behavior of the regenerative amplifier. The simulation results were in good agreement with the experiment. Numerical simulations and experimental results show that the scheme can be scalable to higher energy of multi-m J, sub-300 fs pulses.