Drift calibration method of Fabry-Perot filters using two-stage decomposition and hybrid modeling.

Although tunable Fabry-Perot (F-P) filters are widely acknowledged as fiber Bragg grating (FBG) demodulators, F-P filters exhibit drift error when subjected to ambient temperature and piezo-electrical transducer (PZT) hysteresis. To address the drift issue, the majority of the existing literature makes use of additional devices like the F-P etalon and gas chamber. In this study, a novel drift calibration method based on two-stage decomposition and hybrid modeling is proposed. The initial drift error sequences are broken down into three frequency components using the variational mode decomposition (VMD), and the medium-frequency components are further broken down using the secondary VMD. The initial drift error sequences are significantly simplified by the two-stage VMD. On this foundation, the long short-term memory (LSTM) network and polynomial fitting (PF) are used to forecast the low-frequency and high-frequency drift errors, respectively. The LSTM enables the prediction of intricate nonlinear local behaviors, while the PF method predicts the overall trend. The benefits of LSTM and PF can be effectively utilized in this manner. Compared to the single-stage decomposition, two-stage decomposition achieves superior results. The suggested method is an affordable and effective alternative to the current drift calibration techniques.

Hysteresis and temperature drift compensation for FBG demodulation by utilizing adaptive weight least square support vector regression.

Hysteresis and temperature drift deteriorate the demodulation performance of tunable Fabry-Perot (F-P) filters. This study addresses a novel adaptive weight least square support vector regression (AWLSSVR) to compensate for the hysteresis and temperature drift of F-P filters. The temperature drift of a referent fiber Bragg grating(FBG) is used to estimate the temperature drifts of other three sensing FBGs, and a novel adaptive weighting strategy with an asymmetric noise interval is proposed, to eliminate the effects of noise in the training dataset. The experimental results show that when the temperature-changing modes of the training and testing datasets were close to each other, the error of the proposed method is reduced to 8.7 pm, while the errors of the other three conventional methods based on LSSVR are more than 10.8 pm. Further, when the temperature-changing modes of the training and testing datasets were partly different, the error of the proposed method was reduced to 5.4 pm, while the errors of other methods were more than 11.9 pm. It was verified experimentally that the proposed AWLSSVR method is more accurate and robust than other versions of WLSSVR for training samples with noise, requires no additional hardware, and covers the entire C band.

Gamma radiation and thermal-induced effects on the spectral performance of BACs in Bi/Er codoped aluminosilicate fibers.

The results of γ-radiation (2-72 kGy) and thermal-induced effects on BACs in Bi/Er codoped aluminosilicate fibers (BEDF) have been presented first in this paper. We observed that the radiation effect on on-off gain and optical absorption associated with BAC-Al and BAC-Si was insignificant, while the effect on luminescence was considerable. However, the effect on luminescence is caused by the radiation-induced darkening, which is likely linked to thermal bleachable Al-OHC point defects generated by γ-radiation. We carried out the thermal experiment and observed thermal bleaching of the γ-irradiated fiber at a low temperature of 300 °C. The observations indicate that, while γ-radiation could introduce significant background loss, BAC-Al and BAC-Si are fairly radiation resistant. This is the first time that BACs show good radiation resistance in irradiated BEDFs.

Graphene Q-switched distributed feedback fiber lasers with narrow linewidth approaching the transform limit.

A compact all-in-line graphene-based distributed feedback Bragg-grating fiber laser (GDFB-FL) with narrow linewidth of hundreds kHz is demonstrated and investigated in this study. Performing as an optical saturable absorber, graphene oscillates the initially kHz linewidth DFB-FL, and generates high-quality passively Q-switched pulses. Pumped with a 980 nm continuous-wave laser, the Q-switched GDFB-FL observes ~1 µs pulse durations, with pulse energies up to ~10 nJ and approaching the transform limit. The peak power is ~600 times higher than the original DFB-FL laser. By optimizing the cavity design and the graphene material, it is predicted that fast Q-switched pulses with more than MHz repetition rates and sub-100 ns pulse durations are achievable. Such transform-limited Q-switched GDFB-FLs with narrow linewidth of sub-MHz have long coherence length, good tunability, stability, compactness and robustness, with potential impact in optical coherent communications, metrology and sensing.

Polarization mode coupling and related effects in fiber Bragg grating inscribed in polarization maintaining fiber.

Polarization mode coupling (PMC) and related effects from writing fiber Bragg gratings in polarization maintaining fiber (FBGs-in-PMF) are observed experimentally for the first time by optical fiber coherence domain polarimetry (OCDP) using a broadband light source. PMC is another useful aspect of FBG-in-PMF besides Bragg wavelength and its possible potential is evaluated and discussed. A localized and long range temperature measurement based on the PMC and Bragg wavelength is given as an example.

Long-period gratings for selective monitoring of loads on a wind turbine blade.

An optical fiber sensor based on long-period gratings (LPG) for selective measurements of flap- and edge-wise bending of a wind turbine blade is presented. Two consecutive LPGs separated by 40 mm interfere to improve resolution and reduce noise in a D-shaped fiber. The mode profile of the device was characterized experimentally to provide a model describing the mode couplings. The sensor was tested on a wind turbine blade.

Transition of lasing modes in disordered active photonic crystals.

The transition from a photonic band-edge laser to a random laser in two-dimensional active photonic crystals is described. The lasing modes in the active photonic crystals shift from the edge of the photonic bandgap to the bulk of the gap when a certain amount of position and size disorder is introduced. The shift of lasing modes is determined with various gain profiles. The results show that the modulation of lasing modes is significant when the lasing transition wavelength overlaps the photonic bandgap.

Composite-cavity-based Fabry-Perot interferometric strain sensors.

A composite-cavity-based Fabry-Perot interferometric strain sensor system is proposed to gain the minimum cross sensitivity to temperature and a high multiplexing capability at the same time. The interrogation of the sensor system is based on a white-light interferometric technology, and the demodulation is achieved by analyzing the coherence spectra. A demonstration system with two sensors is presented and tested.

Electrically tunable Bragg gratings in single-mode polymer optical fiber.

We present what is to our knowledge the first demonstration of a tunable fiber Bragg grating device in polymer optical fiber that utilizes a thin-film resistive heater deposited on the surface of the fiber. The polymer fiber was coated via photochemical deposition of a Pd/Cu metallic layer with a procedure induced by vacuum-ultraviolet radiation at room temperature. The resulting device, when wavelength tuned via joule heating, underwent a wavelength shift of 2 nm for a moderate input power of 160 mW, a wavelength to input power coefficient of -13.4 pm/mW, and a time constant of 1.7 s(-1).

Enhanced backscattering from organic laser gain media bounded with rough gold films.

The enhanced backscattering from organic laser gain media that are bounded with one-dimensional (1-D) and two-dimensional (2-D) rough metal films was investigated. We prepared several organic optical gain materials by doping laser-active dyes in matrices of acrylic polymers. These materials produced efficient and broadband fluorescence emission in the visible wavelengths during the pumping of a pulsed YAG or cw argon laser. These gain materials were sliced and coupled with 1-D or 2-D randomly rough gold films with large slopes. An experimental investigation was carried out with a He-Ne laser as the scattering source and the optical gain provided by a cw argon laser. The enhanced backscattering and the satellite peaks located about the enhanced-backscattering peak were obviously amplified, with their widths narrowed. These experimental results agree well with previous theoretical predictions.

Amplification of enhanced backscattering from a dye-doped polymer bounded by a rough surface.

We report the experimental study of the enhanced backscattering from a random rough surface through a laser dye-doped polymer. The sample is a slice of pyrromethene-doped polymer coupled with a two-dimensional rough gold layer with a large slope. When the sample is illuminated with an s-polarized He-Ne laser and pumped by a cw argon-ion laser, amplified backscattering is observed. The enhanced backscattering peak increases sharply and its width narrows for a sample with low dielectric constant |?(2)|.

Soliton wavelength-division multiplexing system with channel-isolating notch filters.

We propose a wavelength-division multiplexing system in which transmission of solitons is stabilized by fixed- or sliding-frequency notch filters (a soliton rail), providing channel isolation. We demonstrate analytically and numerically that a soliton trapped in a channel between two notches is very robust. We also predict an optimum ratio between the channel separation and the soliton's spectral width. The effects of interchannel collisions are considered, and it is demonstrated that these effects can be largely eliminated by notch filters, which require a compensatory gain that is comparable with the basic gain balancing the fiber loss.

Nonlinear-optical amplifier based on a dual-core fiber.

We propose a scheme for all-optical amplification based on a nonlinear-optical coupler in which one core is amplifying while another is attenuating. In this scheme the input signal is fed into the amplifying core, from where the output is also obtained. A weak input easily couples to the lossy core and gets dissipated in it, whereas a stronger signal stays and undergoes a nearly linear amplification in the active core. When it is used for reshaping pulses in a long transmission line, this scheme should allow the pulses to be amplified while simultaneously suppressing the noises between them. Simulating equations for the cw signal in this model, we are able to find a regime that provides for a strong contrast between the suppression of weak signals and amplification of strong ones as well as a steep transition from suppression to amplification at a certainthreshold.

Time-domain soliton filter based on a semidissipative dual-core fiber.

We propose a time-domain dual-core fiber filter that greatly reduces soliton jitters and noise power in its application to ultrahigh-speed soliton communication systems. This filter is simply a dual-core fiber that has one core with negligible loss and the other with large loss.

Accurate elasto-optic probe method for measurement of coupling length in twin-core optical fiber.

The theoretical principle and experimental procedure of a new method of measuring the coupling length of a twin-core fiber are presented. This method is simple, nondestructive, and gives direct results without extensive data processing. It is based on the elasto-optic technique and had a very good spatial resolution so that coupling lengths of the order of a few millimeters can be measured. We show how this method can be applied to the measurement of the polarization coupling of birefringent noncircular twin-core fibers.

Analytical solution to soliton switching in nonlinear twin-core fibers.

We give an approximate analytical solution to the coupled nonlinear Schrödinger equations that govern the soliton switching in a nonlinear fiber coupler. We have derived, in simple analytical form, the switching condition by which solitons can be switched from one core to another. The analytical result has been checked against the numerical results, and it is found that there is close agreement.

Couplings in spun twin-core optical fibers.

A new effect of spun twin-core fiber is observed in which the coupling between the two cores is increased with the spin rate. In addition, the cladding-mode attenuation is also increased. These effects can find many useful applications in fiber devices for optical communication, sensing, or signal processing.

Twin-core optical fiber with large core ellipticity.

The first (we believe) twin-core optical fiber with large core ellipticity has been fabricated, and its significant polarization dependent coupling properties have been demonstrated.

Modified Gaussian approach for the design of optical fiber couplers of arbitrary core shapes.

Many fiber devices, e.g. fiber couplers, rely on the cladding evanescent field for their operation. We present a new method for determining the approximate fundamental mode fields, with particular reference to cladding field applications. In our method, by introducing a field which is accurate both near to and far from the core axis, coupling parameters in fiber couplers can be obtained in a simple form. We apply this to studies of couplers consisting of two identical cores of arbitrary shape. We have compared the results from the new approximation with those from the usual Gaussian in circular core cases where exact solutions can be found and have shown that the new approximation can give more accurate propagation constants and, especially, fundamental mode fields. The evanescent behavior of these fields is directly related to the coupling features of a fiber coupler. In particular, we apply this new method to twin elliptic core fibers to quantify their polarization transmitting and polarization coupling characteristics. Up to now, as far as we know, no method has seemed appropriate to handle this system. Design features of birefringent and polarization coupling of these fibers for device applications are also discussed.

Intensity noise characteristics of lasers in fiber-optic gyroscopes.

We report the first theoretical analysis and experimental results to our knowledge of the intensity noise of lasers due to signal-wave feedback in fiber-optic gyroscopes. It has been demonstrated that signal-wave feedback would cause a large amount of intensity noise in lasers and would thus limit the system performance. The relation of the intensity noise to various factors in the gyroscope has been clarified, and measures to reduce the noise are presented.