Wavelength-Switchable Fiber Laser Based on Long-Period Fiber Grating Written on Polarization-Maintaining Photonic Crystal Fiber.

Here we propose a wavelength-switchable erbium-doped fiber ring laser using a temperatureinsensitive spectral polarization-dependent loss (PDL) element and two fiber Bragg gratings (FBGs). The fiber PDL element was fabricated by inscribing a long-period grating (LPG) on polarizationmaintaining photonic crystal fiber (PMPCF) with a 10.6 µm CO2 laser. The LPG fabricated on PMPCF, referred to as PMPCF-LPG, has the characteristics of a fiber polarizer at two specific wavelengths due to the birefringence of PMPCF and the co-directional mode coupling of the LPG. The two wavelengths at which the fabricated PMPCF-LPG acts as a polarizer are two resonance wavelengths (~1528.58 and ~1555.90 nm) of the PMPCF-LPG, obtained for orthogonal input polarization states. By considering these two resonance wavelengths of the PMPCF-LPG, the Bragg wavelengths of two FBGs, which determine lasing wavelengths in our wavelength-switchable laser, were selected as ~1527.71 and ~1554.74 nm. As the temperature sensitivity of the PMPCF birefringence is 30 times lower than that of the birefringence of conventional polarization-maintaining fiber (PMF), the fabricated PMPCF-LPG could facilitate more stable switching operation between the two lasing wavelengths in comparison with a previous fiber laser employing an LPG inscribed on conventional PMF as a wavelength-switching filter. The lasing wavelengths of our laser could be switched by controlling input polarization of the PMPCF-LPG with a polarization controller, and temperature-insensitive wavelength switching operation was experimentally demonstrated over a temperature range of 25-100 °C.

Simultaneous Measurement of Strain and Temperature Using Long-Period Fiber Grating Written on Polarization-Maintaining Photonic Crystal Fiber.

Here we propose a novel optical fiber sensor capable of simultaneous measurement of strain and temperature by utilizing a long-period fiber grating (LPFG) inscribed on polarization-maintaining photonic crystal fiber (PMPCF) as a sensor head. The sensor head was fabricated by irradiating CO2 laser pulses to one side of PMPCF with line-by-line technique. The LPFG written on PMPCF (referred to as the PMPC-LPFG) exhibits two different wavelength-dependent loss bands, obtained at two orthogonal input polarization states. For two resonance wavelengths of these two wavelength-dependent loss bands, designated as Dips A and B, strain and temperature responses were investigated in a strain range of 0 to 2058 µÉ› with a step of 98 µÉ› and a temperature range of 30 to 85 °C with a step of 5 °C. Strain sensitivities of Dips A and B were measured and found to be approximately -0.82 and -1.43 pm/µÉ›, respectively, at room temperature (25 °C). Similarly, temperature sensitivities of Dips A and B were measured and found to be ~7.89 and ~4.76 pm/°C without applied strain (0 µÉ›), respectively. Owing to their linear and independent responses to strain and temperature, strain and temperature changes applied to the PMPC-LPFG can be simultaneously estimated from the measured wavelength shifts of the two resonance dips (Dips A and B) using their premeasured strain and temperature sensitivities. The experimental results prove that the PMPC-LPFG can be used as a sensor head for simultaneous measurement of strain and temperature.

Simultaneous Measurement of Bending and Temperature Using Long-Period Fiber Grating Inscribed on Polarization-Maintaining Fiber with CO2 Laser.

Here we report on the simultaneous measurement of bending and temperature, carried out using a long-period fiber grating (LPFG) inscribed on polarization-maintaining fiber (PMF) with a CO2 laser at ~10.6 µm. An LPFG written on PMF, referred to as a PM-LPFG, has an input-polarizationdependent resonance dip, and two separated resonance dips, designated as Dips A and B, are obtained with respect to orthogonal input polarization. At the resonance wavelengths of Dips A and B, the core mode is coupled into two different cladding modes that have different bending and temperature sensitivities. The fabricated PM-LPFG whose grating period and length are ~505 µm and ~14.65 mm, respectively, has two resonance wavelengths, i.e., λA= ~1479.98 nm and λB = ~1568.78 nm, measured with respect to two orthogonal input polarization states. The bending sensitivities of this PM-LPFG were measured as ~22.23 and ~33.38 nm/m-1 (adjusted R² values: ~0.9916 and ~0.9810) at λA and λB, respectively, in a curvature range of 1.41-2.30 m-1. The temperature sensitivities of the PM-LPFG were measured as ~0.132 and ~0.039 nm/°C (adjusted R² values: ~0.9929 and ~0.9980) at λA and λB, respectively, in a temperature range of 30-90 °C. These linear bending and temperature responses of the PM-LPFG at two different resonance wavelengths enable simultaneous measurement of bending and temperature variations applied to the PM-LPFG.

High Sensitivity Polarimetric Optical Fiber Pressure Sensor Based on Tapered Polarization-Maintaining and Fiber Bragg Grating.

In this paper, we propose a high sensitivity polarimetric optical fiber pressure sensor (OFPS) using a polarization-diversity loop composed of a polarization beam splitter, polarization controllers, and a sensor head. The sensor head consists of 8-cm-long tapered panda-type polarization-maintaining fiber (PMF) and a fiber Bragg grating (FBG) directly spliced with PMF, and the sensor head is located inside a pressure chamber. A pressure-induced birefringence change due to the photoelastic effect can be greatly enhanced at the tapered section of PMF, thereby increasing the pressure sensitivity of the sensor head. The tapered PMF was fabricated using a fusion splicer, and the tapered length and center waist diameter of the tapered PMF segment were ~350 and ~56.82 µm, respectively. At the polarization-diversity loop, PMF is used as a birefringent element to create an interference spectrum due to polarization interference. A pressure-induced birefringence change of PMF results in a wavelength shift of the interference spectrum. Because the PMF birefringence also has a cross sensitivity to temperature, the FBG is utilized for the compensation of the temperature effect on it. The resonance wavelength of the FBG is sensitive to ambient temperature changes but insensitive to changes in pressure. This spectral response of the FBG can be used to compensate additional ambient temperature changes occurred at the sensor head. The pressure sensitivity of our sensor was measured as approximately -27.70 nm/MPa, and an adjusted R² value representing the sensor linearity was measured as ~0.9903 in a measurement range of 0-0.5 MPa. Our fabricated sensor exhibits the highest pressure sensitivity among previously reported polarimetric OFPS.

60 mA Bidirectional Current Gating in Vanadium-Dioxide-Based Planar Device Using CO2 Laser.

Here we show 60 mA bidirectional current gating in a two-terminal planar device based on a highly resistive vanadium dioxide (VO2) thin film by harnessing photothermally induced phase transition occurred in VO2 when irradiating the VO2 film with a CO2 laser oscillating at 10.6 µm. The VO2 thin film was grown by a pulsed laser deposition method, and the two-terminal planar device was fabricated using the VO2 film isolated with sub-millimeter dimensions. The bidirectional current gating between 0 and 60 mA was accomplished by irradiating the VO2-based device with repetitive pulses of the CO2 laser. In terms of laser modulation parameters such as the pulse width and repetition rate, their effect on the transient responses of laser-gated currents was also investigated. With a minimum energy per pulse of ~766 mJ, a stable bidirectional current gating of up to 60 mA could be successfully implemented for the repetition rates of 0.5-3.0 Hz in a VO2 device biased at ~5.4 V, showing a switching contrast between off- and on-state currents of ~11089. This maximum onstate current (60 mA) and switching contrast are the highest values among previous gating results attained in VO2 devices with a CO2 laser.

Polarization-Controlled Switching of Dual-Wavelength Lines with Orthogonal Polarization in Erbium-Doped Fiber Ring Laser.

By incorporating an inline switching filter, which is comprised of a polarization-diversified loop (PDL), two fiber Bragg gratings (FBGs) with different resonances, and three quarter-wave plates (QWPs), we propose a dual-wavelength-switchable erbium-doped fiber (EDF) ring laser that can select and switch between two lasing lines with orthogonal polarization. The proposed laser is composed of EDF, a 980 nm laser diode, a wavelength-division-multiplexing coupler, a rotatable linear polarizer, an optical isolator, a 3 dB optical coupler, and the inline switching filter. At a special combination of the orientation angles of the three QWPs, the inline filter can offer a different transmittance according to input polarization, e.g., reflection spectra of one and the other of the two FBGs for linear horizontally and vertically polarized input light, respectively. At this special combination of the QWPs, one of two different resonances of the two FBGs can be selected by varying laser cavity polarization through the adjustment of the orientation angle of the rotatable linear polarizer. Consequently, switching operation between two laser lines with orthogonal polarization at the two FBG resonance wavelengths could be obtained by properly controlling cavity polarization. The polarization extinction ratio of each lasing line was measured as more than 19.9 dB.

Laser-Regulated 60 mA Current Switching in VO2-Based Two-Terminal Device Using 976 nm Laser Diode.

By incorporating a high-power 976 nm laser diode (LD), we demonstrated laser-regulated current switching in a two-terminal planar device based on a vanadium dioxide (VO2) thin film. The VO2 thin film was grown by pulsed laser deposition method and etched to sub-millimeter dimension for the fabrication of a two-terminal device. The reversible current switching was implemented by controlling the on/off state of the LD, which illuminates the VO2-based device. The transient responses of the device currents were analyzed when the device was excited with laser pulses of various repetition rates of up to 5.0 Hz with a pulse width fixed as 75 ms. A switching contrast between off- and on-state currents was calculated as ~9530, and average rising and falling times were measured as ~31 and ~21 ms, respectively.

Active directional switching of surface plasmon polaritons using a phase transition material.

Active switching of near-field directivity, which is an essential functionality for compact integrated photonics and small optoelectronic elements, has been challenging due to small modulation depth and complicated fabrication methods for devices including active optical materials. Here, we theoretically and experimentally realize a nanoscale active directional switching of surface plasmon polaritons (SPPs) using a phase transition material for the first time. The SPP switching device with noticeable distinction is demonstrated based on the phase transition of vanadium dioxide (VO2) at the telecom wavelength. As the insulator-to-metal phase transition (IMT) of VO2 induces the large change of VO2 permittivity at telecom wavelengths, the plasmonic response of a nanoantenna made of VO2 can be largely tuned by external thermal stimuli. The VO2-insulator-metal (VIM) nanoantenna and its periodic array, the VIM metagrating, are suggested as optical switches. The directional power distinction ratio is designed to change from 8.13:1 to 1:10.56 by the IMT and it is experimentally verified that the ratio changes from 3.725:1 to 1:3.132 as the VIM metagratings are heated up to 90 °C. With an electro-thermally controllable configuration and an optimized resonant design, we expect potential applications of the active switching mechanism for integrable active plasmonic elements and reconfigurable imaging.