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Switchable metamaterial absorbers/reflectors (MAs/MRs) are important bifunctional electromagnetic devices and have been the subject of numerous scientific studies. However, there is a lack of bifunctional devices that operate in the terahertz band. Here, we theoretically propose a broadband switchable MA with many excellent properties, such as good thermal stability, high insensitivity to inferior film quality of the graphene, excitation polarization and wide incident angles, and outstanding structural parameter tolerance. The bandwidth of the proposed broadband MA is 3.4 THz with an absorptivity over 90% in the frequency band of 1.6-5 THz. The proposed absorber can switch to a reflector with a reflectivity over 93% by tuning the chemical potential of the graphene and reducing the temperature. Therefore, the switching intensity of the proposed MA exceeds 83%. The physical mechanisms of the broadband absorption of the proposed structure are investigated using the impedance matching theory and the multiple reflection interference theory. The reflection mechanism of the proposed broadband reflector is discussed by analyzing the effective parameters. The absorption and switching mechanism are theoretically investigated by performing detailed numerical calculations to analyze the surface loss intensity, electric field, and magnetic field. These findings can accelerate the development of terahertz broadband switchable devices.

A modified polarization-maintaining Sagnac loop mirror is proposed, and the simulated transmission spectrum of the proposed filter is in accordance with experimental transmission spectrum. Based on the proposed filter, a multi-wavelength thulium-doped fiber laser is constructed and experimentally demonstrated. The stable multi-wavelength oscillation within 60 minutes is achieved through the nonlinear polarization rotation effect enhanced by the highly nonlinear fiber. By fine tuning the polarization state, up to 35 lasing lines are obtained within a 10-dB bandwidth with wavelength separation of ∼1.38 nm at room temperature. The wavelength shift and the output power fluctuation are less than 0.04 nm and 1.69 dB, respectively. In addition, by changing the optical coupling ratio in the Sagnac loop mirror, different outputs with 7, 8, 16 lasing lines are also obtained.

A wavelength-switchable single-longitudinal-mode thulium-doped fiber laser using a sampled fiber Bragg grating is proposed in this study. The laser can be switched between three single-wavelength and three dual-wavelength modes of operation. For the single-wavelength mode of operation, the maximum power and wavelength fluctuations in 50 min were ±0.404 dB and ±0.01 nm, and the linewidths at three wavelengths were 0.39, 1.88, and 0.89 kHz, respectively. For the dual-wavelength mode of operation, the maximum power and wavelength fluctuations in 20 min were ±0.926 dB and ±0.03 nm, respectively. The proposed thulium-doped fiber laser will find useful application in optical communication and free-space optical detection and sensing.

We propose and experimentally demonstrate a multi-wavelength thulium-doped fiber laser using two cascaded single-mode-four-mode-single-mode (SFS) fiber interferometers and a 150-m-long highly nonlinear fiber. The transmission properties of SFS fiber interferometers with two different four-mode fiber (FMF) lengths are analyzed theoretically. The interferometer with a longer FMF is used to select specific wavelengths, and the one with a shorter FMF is used to stimulate more laser lines. By adjusting the polarization controller and increasing the pump power, the number of output laser channels and the wavelength spacings can be changed and switched. By selecting the proper length of the FMFs, 31 lasing lines are achieved with covering a wavelength range of $\sim 22$ nm. The power fluctuation and the wavelength drift of the 31-wavelength operation for the proposed fiber laser are within 0.706 dB and 0.02 nm, respectively.

A switchable multi-wavelength thulium-doped fiber laser (TDFL) using a four-mode fiber based Sagnac loop filter was proposed and demonstrated. The Sagnac loop, incorporating a 4.8 m long four-mode step-index fiber, acting as a comb filter was analyzed theoretically and experimentally. By adjusting two polarization controllers, eight stable single-wavelength operations and five stable dual-wavelength operations were obtained with easy switching between the different operation modes. The wavelength range covered during the single-wavelength switchable operation with a signal-to-noise ratio of up to 55 dB was ∼28.51 nm. In dual-wavelength switchable operation, the maximum and minimum wavelength spacings were ∼21.78 nm and ∼1.50 nm, respectively. Due to the abundant wavelength resources, the proposed TDFL has great potential in wavelength division multiplexing free-space optical communications, optical fiber sensing, and photonic generation of microwave signals.

A single longitudinal mode (SLM) fiber laser with a ring cavity based on Sagnac interferometer (SI) for temperature sensing in the 2-μm band is proposed and demonstrated. The simulation and experimental results show that the transmission spectrum of the SI filter is sensitive to temperature change. Based on this distinguishing feature, the correlation between the laser wavelength drift and temperature variation can be established. Experimental results show that the proposed laser operates stably at the resonant wavelength of 1988.21 nm and the optical signal-to-noise ratio is approximately 55 dB at ambient temperature. Through continuous measurement for 100 min, the fluctuation of the output laser power is lower than 0.74 dB, and the resonant wavelength shift is less than the minimum resolution of the spectrometer of 0.05 nm. This indicates that it has good stability for a certain period of time. At the same time, the laser operates stably in SLM state. A temperature sensitivity of 2.09 nm/°C is obtained in experiment. Notably, the proposed fiber laser sensor possesses the merits including high sensitivity, low cost and simple structure, which are beneficial to its practical application.

For the first time, to the best of the authors’ knowledge, the transmission characteristics of sampled fiber Bragg grating (SFBG) and phase-shifted SFBG (PS-SFBG) in the 2 μ m band were investigated theoretically and experimentally in detail. Using the transmission matrix method, the dependence of the transmission spectra of the SFBG and PS-SFBG on different grating parameters was simulated; the grade-zero bandwidths of the SFBG transmission peak and the PS-SFBG phase-shifted peak were as narrow as 0.076 and 0.022 nm respectively. Based on the theoretical analysis results, we experimentally fabricated the gratings with three narrow peaks. The results showed that the grade-0 transmission peak bandwidth of the SFBG and the phase-shifted-peak bandwidth of the PS-SFBG were narrower than 0.06 and 0.09 nm, respectively, indicating that the gratings are excellent filters for 2 μ m band multi-wavelength narrow-linewidth fiber lasers.

A continuous wave, switchable multi-wavelength, thulium-doped fiber laser (TDFL) with a polarization-maintaining sampled fiber Bragg grating (PM-SFBG) is proposed and demonstrated for the first time. A length of 150 m highly-nonlinear fiber (HNLF) was used to introduce the four-wave mixing effect for the efficient suppression of the wavelength competition existing in the gain medium of thulium-doped fiber. By adjusting the state of polarization (SOP) of light in the laser cavity, two six-wavelength operations at two orthogonal SOPs and a ten-wavelength operation were obtained. When the TDFL ran at the six-wavelength operation mode with a pump power of 4.11 W, there were at least four lasing wavelengths with an optical signal-to-noise ratio (OSNR) higher than 30 dB, and when it ran at the ten-wavelength operation mode with a pump power of 4.60 W, there were nine lasing wavelengths with an OSNR higher than 30 dB. Regardless of the operation mode, the TDFL exhibited high stability.

A broadband metamaterial absorber based on the novel design of the combination of cross and slotted-square graphene sheets is presented and analyzed from the perspectives: perfect absorptivity, absorption bandwidth, tunability and insensitivity.

Graphene plasmons have attracted great interest for constructing high-performance functional metamaterial devices, among which graphene-based narrowband metamaterial absorbers are valuable in numerous applications. In this paper, a graphene-dielectric-metal hybrid structure is proposed to achieve dual-band excellent absorption and high-performance refractive index sensing. Numerical simulations show that the designed structure exhibits a high Q factor of 277.8 at 1.945 THz, which is higher than most previously-reported results. The sensitivity of the proposed structure can be up to 1.84 THz/RIU when the analyte thickness is 80 μm, and it exhibits promise for sensing thin-film biomolecule and thick biologic tissues. The electromagnetic field distributions show that the electromagnetic energy is highly confined in the structure at the higher order plasmon resonance, which plays an important role in achieving the excellent characteristics of the proposed structure. The tunability of the proposed structure is realized via changing the gate voltage applied to the graphene and the angles of the incident THz wave. In addition, we reveal that the proposed structure can be switched between single-band, dual-band, and 3-band absorption by tuning the incident angle under TM and TE polarization. Further results demonstrate that the proposed device possesses good thermal stability, polarization insensitivity, and outstanding fabrication tolerance. We believe that the proposed dual-band narrowband absorber can serve as a good example for other devices based on plasmons, such as selective sensors, modulators, and optical detectors.

A novel approach for ultrafast and temperature-insensitive strain interrogation using a polarization-maintaining photonic crystal fiber (PM-PCF) based Sagnac loop interferometer (SLI) and linear wavelength-to-time (WTT) mapping is proposed and experimentally demonstrated. The PM-PCF incorporated in the SLI is used as the sensing element to achieve stable strain sensing with ultra-low temperature-dependence due to its intrinsic thermal insensitivity, which can be used to eliminate the cross-sensitivity effect and increase the measurement accuracy. A dispersive element is employed to realize the WTT mapping and real-time strain interrogation is obtained by converting the strain-encoded wavelength shift to time shift in the temporal domain, which can be directly monitored by a real-time oscilloscope. The proposed system offers an ultrafast interrogation speed of 100 MHz and a strain sensitivity of -0.17 ps/με.

A compound-cavity-based six-wavelength-switchable single-longitudinal-mode (SLM) thulium-doped fiber laser (TDFL) with a homemade polarization-maintaining sampled fiber Bragg grating (PM-SFBG) is proposed and demonstrated. The PM-SFBG in the 2 μm band is studied theoretically and experimentally, and is utilized as a polarization-dependent multi-channel reflecting filter in a multi-wavelength-switchable fiber laser, both for the first time. The SLM lasing in each channel is guaranteed by using a tri-ring sub-cavity. 6 single-wavelength operations are easily obtained and switched each other, and the maximum power and wavelength fluctuations are as low as 0.71 dB and 0.02 nm, respectively. The frequency noise of each lasing wavelength is measured through an unbalanced Michelson interferometry system, and the linewidths for all six lasing wavelengths are among 0.60–1.08 kHz, calculated by the β-separation line method on the basis of the measured frequency noise spectra. Benefiting from the enhanced polarization hole burning effect formed in the gain fiber, 9 switchable dual-wavelength operations with different wavelength intervals and orthogonal polarizations at two lasing wavelengths are obtained with the maximum power and wavelength fluctuations of only 0.95 dB and 0.03 nm, respectively. We believe the proposed TDFL will find great applications as an ideal light source in free space optical communication and optical sensing.

Principles of single- and multi-phase-shifted chirped fiber Bragg grating (CFBG) in 2 μm band are analyzed with transmission matrix method, and transmission characteristics, including transmissivity, wavelength and bandwidth performance, of single- and multi-phase-shifted CFBG in 2 μm band with different grating parameters are discussed by simulation. It is found that phase-shifted position, grating length, chirped coefficient and refractive index modulation amplitude all have impacts on bandwidth of phase-shifted transmission peak. The transmission peak bandwidths of single-, dual- and tri-phase-shifted CFBG can reach to 0.0270, 0.0172 and 0.0112 nm respectively. The experiment of phase-shifted CFBG verifies the correctness of the simulation, and the bandwidths of 0.0901 and 0.0480 nm are acquired for single- and dual-phase-shifted CFBG respectively, which are narrower than previous filters based on fiber Bragg grating in 2 μm band multi-wavelength fiber laser system.

A switchable single-polarization (SP) dual-wavelength mode-locked thulium-doped fiber laser using the polarization maintaining fiber Bragg grating and semiconductor saturable absorber mirror is proposed in 2- $\mu \text{m}$ band. By adjusting the polarization controller, stable dual-wavelength pulse lasing operation and SP single-wavelength pulse lasing operation are obtained. The dual-wavelength pulse lasing operation is obtained at 1940.97 and 1941.73 nm. In the SP single-wavelength pulse lasing state, the polarization extinction ratios are higher than 30 dB, the SP pulse duration are 3.75 and 3.73 ps.

A switchable, thulium-doped, isolator-free fiber laser is proposed and demonstrated. The isolator-free configuration, wavelength selective element and stabilization mechanism were achieved by using a theta cavity design, a two-mode fiber filter and nonlinear polarization rotation, respectively. By properly adjusting the polarization controllers to induce polarization-dependent loss, single- and dual-wavelength outputs can be obtained, and the stability of single- and dual-wavelength operations was measured for 30 min in each case. The single wavelength can be switched from 1959.17 to 1994.86 nm in a lasing range of ~35.7 nm with an optical signal-to-noise ratio (OSNR) of up to ~60 dB. When lasing dual-wavelength, the wavelength interval can be changed from ~4 to 27.36 nm with an OSNR higher than 42 dB, and the maximal lasing range of the dual-wavelength is 23.38 nm. In addition, the wavelength drift and power fluctuation of the proposed fiber laser are less than 0.14 nm and 1.976 dB, respectively, at room temperature, which demonstrates that the thulium-doped fiber laser has favorable wavelength and power stability.

The emerging material of graphene has resulted in new options for the design of metamaterial devices with excellent performances, offering an attractive platform for electromagnetic absorption applications. To date, researches on patterned graphene-based metamaterial absorbers (MAs), have focused on achieving broadband absorption while maintaining many excellent properties. Here, we propose a broadband tunable MA with a perfect absorption based on a novel design of the combination of cross- and square-shaped graphene sheets. Numerical simulation results show that the bandwidth of the proposed broadband MA reaches 2.76 THz with the absorptivity greater than 90% for both the transverse electric (TE) and transverse magnetic (TM) modes, which exceeds most of broadband absorption results achieved by MAs in previous studies. A numerical analysis employing the equivalent circuit model for the designed structure is conducted to determine the influence of the structural parameters on the absorptivity and optimize the parameters. Results show that impedance matching is achieved between the input impedance and the free-space impedance at the absorption band. Detailed numerical simulations are conducted to calculate the surface loss density, electric field, and surface currents to obtain an understanding of the high absorption mechanism. Also, the designed MA exhibits excellent properties, such as tunability, polarization independence, incident angle insensitivity and outstanding tolerance to changes in the structural parameters, suggesting that the proposed absorber is a promising candidate for various applications in the terahertz band.

A switchable single-polarization (SP), dual-wavelength thulium-doped fiber laser using polarization maintaining (PM) Fabry–Perot (F–P) filter is proposed. A combination of the PM F–P filter, a polarization controller (PC) and a polarizer is used to ensure the SP lasing operation. A stable dual-wavelength lasing operation is obtained at 1941.82 nm and 1942.21 nm. By adjusting the PCs, the proposed laser can achieve SP single-wavelength lasing operation; the polarization extinction ratios are higher than 33 dB. When the pump power is higher than 2.98 W, the optical signal-to-noise ratios of the SP single-wavelength operation can reach 60 dB, and the output power variations are less than 0.32 dB ( X -polarization) and 0.30 dB ( Y -polarization). The slope efficiencies of SP lasing operation are 6.26% ( X -polarization) and 8.79% ( Y -polarization), respectively.

A multi-wavelength thulium-doped fiber laser using a fiber-based Lyot filter is proposed in 2- $\mu \text{m}$ band for the first time. In the laser cavity, the fiber-based Lyot filter is used as the comblike wavelength selection component. By adjusting two polarization controllers, a stable multi-wavelength operation is obtained at room temperature, 17 output channels within 10-dB bandwidth from 1982 to 1998 nm are achieved. The flattened spectrum of multi-wavelength operation can obtained by using the 10% output coupling ratio. The different wavelength spacing of the fiber laser can be controlled by adjusting the polarization maintaining fiber lengths of the Lyot filters.

Metamaterial absorbers (MAs) serve as important electromagnetic wave-absorbing devices that have captured the attention of researchers for a long term. Functioning as sensitive detectors to determine perturbations in an ambient environment is another significant subsidiary function. Here, we theoretically propose an optimized fabrication method to implement terahertz MAs with fewer steps and also evaluate both absorption and sensing performances of such MAs realized by the new method. Simulation findings demonstrate that such MAs can basically maintain the original absorption features perfectly, including near-complete absorption at resonance as well as strong robustness to wide incident angles. Specifically, the full width at half-maximum and quality factor of the absorption resonances attenuate less than 26% and 8% with this new method, remaining in the ranges of ∼ 0.03 – 0.04 THz and ∼ 20 – 27 for two selected example MAs. More significantly, sensing capacities of this type of MA, in terms of maximum detection range (enhancing at least 9%), observable spectral modulation (increasing at least 6.3%), and refractive index sensitivity, are improved to a large extent because of more intense coupling between resonant field and matter in the case of surface-relief MAs. This stronger coupling results from exposing more spots of the resonantly high field to direct contact with an approaching analyte, which is illustrated by field profiles of the MAs at resonance in this work. Additionally, other desirable absorber features are also explored with such MAs, like functioning as building blocks to configure multiband MAs and strong robustness against fabrication errors. Such new-style terahertz MAs shown in the paper, acting as good examples, not only prove that terahertz MAs can be fabricated by the proposed time- and cost-saving route in contrast to the traditional MA fabrication process, but also can serve as novel platforms to explore other intriguing terahertz photonic effects, such as the field enhancement effect.

We propose and demonstrate a four-wavelength-switchable erbium-doped fiber laser (4WS-EDFL) with a four-channel superimposed high-birefringence fiber Bragg grating (SI-HBFBG) and a dual-coupler ring based compound-cavity (DCR-CC) filter. Both for the first time, a SI-HBFBG as a four-channel reflective filter is used in a multi-wavelength switchable fiber laser to define wavelength channels and a DCR-CC filter is used to select a single mode from dense longitudinal-modes in a fiber laser. We present in detail how to design, fabricate, and characterize the DCR-CC filter with both theoretical analysis and experimental results, which we believe is the first systematic approach for making a compound-cavity based filter used for selecting single-longitudinal mode (SLM) in a fiber laser. The enhanced polarization hole burning effect in a 2.9 m long erbium-doped fiber, coiled inside a three-loop polarization controller, and the polarization-mismatch-induced losses are introduced into the laser cavity to achieve wavelength-switching operations. We show that the 4WS-EDFL can be switched among fifteen lasing states, including four single-wavelength operations, six dual-wavelength operations, four three-wavelength operations and one four-wavelength operation, all with high stability. For demonstration, in switchable single-wavelength operations, the four SLM lasing outputs measured are all with an optical signal to noise ratio of >80 dB, a linewidth of

A continuous wave (CW) wavelength-tunable Tm3+-doped fiber laser with two sampled fiber Bragg gratings (SFBGs) has been proposed and demonstrated. This proposed laser is stable operating with a significant 14.44 nm wavelength range from 1942.09 nm to 1956.53 nm due to the Vernier effect. The laser OSNRs of different wavelengths ranged from 45 dB to 58 dB at a launched pump power of 3.67 W. The wavelength shift and power fluctuation were less than 0.03 nm and 0.46 dB, respectively. An 8.62% slope efficiency was achieved using a grating for laser extracting. To the authors’ knowledge, this is the first time that a wavelength-tunable Tm3+-doped fiber laser with two SFBGs has been demonstrated.

Linewidth measurement based on phase noise analysis using a 3 × 3 coupler has been applied in the 1550 nm band. One of the obvious advantages of this method compared with the common delayed self-heterodyne method is the low loss caused by the short delay line used. Taking full advantage of this particular characteristic, the linewidth of the single frequency (SF) fiber laser in the 2 μm band can also be measured. However, a specifically experimental demonstration of this method in the 2 μm band is absent to date. In this paper, a high-performance SF thulium-doped fiber laser (TDFL) was proposed and employed for the demonstration. The TDFL operates at the center wavelength of 1942.03 nm with high stability. By means of the phase noise analysis-based linewidth measurement system with a fiber delay line of only 50 m, the linewidth of the proposed TDFL is successfully measured. When the measuring time is 0.001 s, the linewidth is ~47 kHz. Furthermore, the noise characteristics of the SF laser were simultaneously studied by this method.

Optical fiber devices and applications in the 2-μm band have been investigated extensively due to its unique advantages, such as eye safety. A fiber sensor based on tilted fiber Bragg grating with a grating plane angle of 2 deg is fabricated and experimentally tested. To the best of the authors’ knowledge, this is the first tilted fiber Bragg grating sensor to realize simultaneous measurements of temperature, axial strain, and a certain range of surrounding refractive index (SRI) at the 2-μm band. This paper uses the wavelength detection method and selects three independent resonant wavelengths as the investigated parameters. Results show that perturbations of temperature, axial strain, and SRI can shift the wavelengths of the core mode resonance and cladding mode resonance to some degree. The temperature sensitivities of the core mode and cladding mode are nearly the same, but their axial-strain sensitivities are different. Furthermore, the core mode is insensitive to the change in SRI. The sensitivities of SRI, temperature, and strain can thus be obtained by experimentally determining the wavelength shifts of the three independent resonance peaks. A 3 × 3 matrix containing the relationship coefficients between the disturbances of temperature, axial strain, and SRI and wavelength shifts is constructed. By reversely solving the matrix equation, variations in temperature, strain, and SRI can be obtained using the experimental determination of wavelength drifts.

The linewidth performance of all-fiber, linear-cavity Fabry–Perot structures based on fiber Bragggratings operating at 2-μm band has been investigated numerically. The output linewidth performance oftwo symmetrical and asymmetrical cavities has been theoretically studied and comprehensively compared.The numerical analysis is based on the transmission matrix method with the simplified parameters. The sim-ulationresultsshowthatcavitylengths,cavitylengthsratio,gratinglengths,gratinglengthsratio,aswellasindexmodulationdepths, affectthe outputlinewidthperformance. Thetoleranceability ofthe asymmetricalstructure isfirst proposed and investigated under 1 mm accuracy, and single-frequency output can be realized by properlyadjusting the properties of the proposed composite linear cavity structure. © 2017 Society of Photo-Optical InstrumentationEngineers (SPIE) [DOI: 10.1117/1.OE.56.5.056107] Keywords: 2-μm band; fiber Bragg grating; linear cavity; narrow linewidth.Paper 170162 received Feb. 4, 2017; accepted for publication May 8, 2017; published online May 23, 2017.

This work aimed to evaluate pharmacokinetics, biodistribution, toxicity, and antitumor activities of a highly stable long-/medium-chain triglycerides (LCT/MCT)-based etoposide parenteral emulsion (EPE) in comparison to etoposide parenteral solution (EPS).Using high-pressure homogenization method, EPE was prepared and sterilized at 121°C for 10 min by autoclaving. The biological samples were analyzed using the UPLC-ESI-MS/MS method.Superior stability of EPE was verified with no significant changes in physicochemical properties in the accelerating and long-term stability tests. Similar pharmacokinetic behavior in beagle dogs was obtained and the AUC 0 - 12h values were 1196.73 ± 320.85 and 1505.56 ± 617.93 µg.h/L for EPE and EPS (p0.5), respectively. Likewise, no remarkable difference in biodistribution profiles in mice was found for both formulations. Safety assessment studies including hemolysis test, rabbit ear vein test and injection anaphylaxis were undertaken and the EPE was proven to be safe for intravenous administration. Specifically, after consecutive 12 weeks administration in rats, systematic and local toxicity induced by EPE were alleviated relative to that of EPS. Furthermore, significant and comparable antitumor activities to EPS were also demonstrated by EPE with tumor suppression rate (TSR) of 66.63, 55.94, and 60.16% against H460, Hep G2, and BCAP-37 human cancer cell lines in nude mice at the dose of 15 mg/kg, respectively.These results suggest that this LCT/MCT-based lipid emulsion is a promising alternative intravenous carrier for etoposide with high stability, improved convenience, alleviated toxicity, and noncompromised antitumor efficacy.

A passively mode-locked thulium-doped fiber laser (TDFL) based on a nonlinear amplifying loop mirror (NALM) is presented. By adjusting the polarization controllers, stable noise-like (NL) mode-locked femtosecond pulse operation is obtained at the 2 μm band. In the experimental period of 200 min, the output power fluctuation is less than 0.06 dB and the 3 dB spectral bandwidth variation is less than 0.02 nm, indicating that the pulsed TDFL possesses good long-term stability. To the best of our knowledge, this is the first 2 μm band NALM-based TDFL with small net anomalous dispersion for a NL femtosecond pulse. At the maximum pump power of 3.52 W, the emitting laser has a NL pulse width of 460 fs, the repetition rate of 9.1 MHz, and the NL pulse energy of 32.72 nJ.