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We demonstrate the fabrication of fiber-optic Fabry–Perot interferometer (FPI) temperature sensors by bonding a small silicon diaphragm to the tip of an optical fiber using low melting point glass powders heated by a 980 nm laser on an aerogel substrate. The heating laser is delivered to the silicon FPI using an optical fiber, while the silicon temperature is being monitored using a 1550 nm white-light system, providing localized heating with precise temperature control. The use of an aerogel substrate greatly improves the heating efficiency by reducing the thermal loss of the bonding parts to the ambient environment. A desirable temperature for bonding can be achieved with relatively small heating laser power. The bonding process is carried out in an open space at room temperature for convenient optical alignment. The precise temperature control ensures minimum perturbation to the optical alignment and no induced thermal damage to the optical parts during the bonding process. For demonstration, we fabricated a low-finesse and high-finesse silicon FPI sensor and characterized their measurement resolution and temperature capability. The results show that the fabrication method has a good potential for high-precision fabrication of fiber-optic sensors.

Reinforcement learning (RL) is applied to improve the performance of the polarization modulator (PolM)-based W-band radio-over-fiber (RoF) system in this Letter. By controlling the polarization angle of the dual-wavelength laser source in the PolM-based scheme, the RF response can be easily modified and therefore it hugely increases the available bandwidth in the RoF system. In the proposed RL scheme, the state is described as the value of the angle from the polarization controller (PC). We use changing the angle of the polarizer (P) as the actions of the RL agent to optimize the frequency response. The agent also receives a reward from the system and learns from the environment and previous experience. Moreover, the reward is the value of error vector magnitude at each state. Therefore, the proposed scheme of RL is implemented and demonstrated in a multi-channel RoF system, and the results show that an RL agent provides an effective intelligent technique to obtain the best quality of data transmission.

We present a method to suppress the wavelength drift of a semiconductor laser with filtered optical feedback from a long fiber-optic loop. The laser wavelength is stabilized to the filter peak through actively controlling the phase delay of the feedback light. Steady-state analysis of the laser wavelength is performed to illustrate the method. Experimentally, the wavelength drift was reduced by 75% compared to the case without phase delay control. The active phase delay control had negligible effect on the line narrowing performance of the filtered optical feedback to the limit of the measurement resolution.

In this Letter, we propose a quadrature phase-stabilized three-wavelength demodulation technique for the interrogation of fiber-optic Fabry-Perot acoustic sensors. It is based on accurate and fast tuning of a monolithic modulated grating Y-branch laser. Three quadrature wavelengths are chosen to perform high-speed cavity length demodulation by wavelength switching, thereby avoiding imbalances and disturbances between the three optical paths in conventional three-wavelength quadrature phase-demodulation systems. A feedback-stabilization scheme for maintaining the quadrature phase condition is proposed for the first time, to the best of our knowledge, providing potential for long-term monitoring in harsh environments.

We propose and demonstrate the use of laser intensity modulation for the multiplexing and demultiplexing of fiber Bragg grating-based ultrasound sensors. The method utilizes an intensity modulator to modulate the output power of the laser with a frequency much higher than that of the ultrasounds. The high-frequency-modulated optical signal serves as a carrier signal. Ultrasonic signals impinged onto the sensor appear as the envelope of the carrier signal. In the frequency domain, the carrier signal and the sidebands encoded with the ultrasonic signal are separated from those of other channels and, thus, can be isolated using an electronic bandpass filter for crosstalk-free ultrasound detection. Each laser can be tuned to demodulate any sensor covered by the wavelength range of the laser, and a common photodetector is used for all channels. Both a theoretical analysis and experimental verification are provided to demonstrate the concept.

We report a fiber-optic silicon Fabry–Perot temperature sensor with high speed by considering the end conduction effect, which refers to the unwanted heat transfer between the sensing element and the fiber stub delaying the sensor from reaching thermal equilibrium with the ambient environment. The sensor is constructed by connecting the narrow edge surface of a thin silicon plate to the edge of the microtube attached to the fiber tip. Compared to the traditional design where the silicon plate is attached to the fiber end face on its large plate surface, the new sensor design minimizes the heat transfer path to the fiber stub for improved sensor speed. It has the additional benefit of increased cavity length for improved resolution. We show that, compared with the sensor of traditional design, the sensor of the new design shortened the characteristic response time in still air from 83 ms to 13 ms and improved the sensor resolution by a factor of 12, from 0.15 K to 0.012 K.

We report an ultrasonic sensor system based on a low-finesse Fabry–Perot interferometer (FPI) formed by two weak chirped fiber Bragg gratings (CFBGs) on a coiled single-mode fiber. The sensor system has several desirable features for practical applications in detecting ultrasound on a solid surface. By controlling the birefringence of the fiber coil during the sensor fabrication, the sensor is made insensitive to the polarization variations of the laser source. The circular symmetric structure of the fiber coil also renders the omnidirectional response of the sensor to ultrasound. While the fiber coil is bonded directly to the structure, the CFBGs are suspended from the structure and free from large background strains with little reduction to the sensitivity of the sensor. The low-finesse FPI features a sinusoidal reflection spectrum. Like the conventional phase-generated carried technique, a phase modulator is utilized to implement quadrature demodulation. Therefore, the sensing system is adaptive to large background perturbations experienced by the fiber coil.

We demonstrate a passive quadrature demodulation method using polarized light and a fiber-optic sensor with a birefringent low-finesse Fabry–Perot (FP) cavity. With precisely controlled birefringence in the FP cavity, the fringes probed by light polarized along the two principal axes can have a quadrature phase shift and can be separately detected. We demonstrated the concept for ultrasound detection using a sensor with an FP cavity formed by two low-reflectivity chirped fiber Bragg gratings on a coiled polarization-maintaining (PM) fiber. By controlling the bending radius, bending length, and twist of the coil structure, we obtained an optimized total phase delay of 109° between the fringes of the two polarizations along the principal axes of the PM fiber. Using a linearly polarized laser source and a polarization beam splitter, the signals at the two polarizations were separated and measured by two photodetectors. The experimental results show that the sensor can detect ultrasonic signal when the sensor spectra experience environmental drifts using a laser at a fixed wavelength.

We report a high-sensitivity fiber-optic ultrasonic sensor system using a self-injection-locked distributed-feedback (DFB) diode laser where a π-phase-shifted fiber Bragg grating (πFBG) serves as both the locking resonator and the sensing element in a fiber ring feedback loop. By controlling the delay time of the feedback light through a fiber stretcher, the laser wavelength is locked to an external cavity mode on the spectral slope of the πFBG, and the ultrasound-induced wavelength shifts of the πFBG are converted to laser intensity variation. The ultrasonic sensing scheme simplifies the feedback control because the self-injection locking automatically pulls the laser wavelength to the πFBG resonant wavelength. In addition, it improves the detection sensitivity because the frequency noise of the DFB laser is drastically reduced. We show that the sensor system achieves a strain sensitivity of 78 fe/Hz1/2 at around 200 kHz.

We report a fiber-optic micro-heater based on a miniature crystalline silicon Fabry-Perot interferometer (FPI) fusion spliced to the endface of a single-mode fiber. The silicon FPI, having a diameter of 100 μm and a length of 10 or 200 μm, is heated by a 980 nm laser diode guided through the lead-in fiber, leading to a localized hot spot with a temperature that can be conveniently tuned from the ambient temperature to >1000°C in air. In the meantime, using a white light system operating in the 1550 nm wavelength window where the silicon is transparent, the silicon FPI itself also serves as a thermometer with high resolution and high speed for convenient monitoring and precise control of the heater temperature. Due to its small size, high temperature capability, and easy operation, the micro-heater is attractive for applications in a variety of fields, such as biology, microfluidics system, mechanical engineering, and high-temperature optical sensing. As an example, the application of this micro-heater as a micro-boiler and micro-bubble generator has been demonstrated.

We demonstrate a fiber-optic acoustic emission (AE) sensor system that is capable of performing AE detection, even when the sensor is experiencing large quasi-static strains. The sensor is a Fabry-Perot interferometer formed by cascaded chirped fiber-Bragg gratings (CFBGs). The reflection spectrum of the sensor features a number of narrow spectral notches equally spaced within the reflection bandwidth of the CFBG. A semiconductor laser whose wavelength can be fast tuned through current injection is used to lock the laser line to the center of a slope of a spectral notch. When the notch is knocked out of the tuning range of the laser, a neighboring notch moves into the range. Through a smart feedback control scheme, the laser is unlocked from the current spectral lock and relocked to the desired point of the new notch. The fast speed of the unlocking/relocking process (

The paradox between a large dynamic range and a high resolution commonly exists in nearly all kinds of sensors. Here, we propose a fiber-optic thermometer based on dual Fabry-Perot interferometers (FPIs) made from the same material (silicon), but with different cavity lengths, which enables unambiguous recognition of the dense fringes associated with the thick FPI over the free-spectral range determined by the thin FPI. Therefore, the sensor combines the large dynamic range of the thin FPI and the high resolution of the thick FPI. To verify this new concept, a sensor with one 200 μm thick silicon FPI cascaded by another 10 μm thick silicon FPI was fabricated. A temperature range of -50°C to 130°C and a resolution of 6.8×10sup-3/sup°C were demonstrated using a simple average wavelength tracking demodulation. Compared to a sensor with only the thick silicon FPI, the dynamic range of the hybrid sensor was more than 10 times larger. Compared to a sensor with only the thin silicon FPI, the resolution of the hybrid sensor was more than 18 times higher.

We propose and demonstrate the use of a single-frequency laser and an acousto-optic modulator (AOM) for quadrature demodulation of fiber-optic ultrasonic sensors whose spectrum features sinusoidal fringes. The light from the laser is split into two channels, before it is combined into the fiber leading to the sensor, with an AOM in one of the channels introducing a frequency shift to the light. Thus, the light in the fiber contains two wavelengths whose difference is designed to be an odd number times a quarter of the free spectral range of the sensor, so that at least one of them is located on the spectral slope of the fringes for sensitive ultrasonic detection without the need to tune the laser wavelength. The intensities of the light in the two channels are sinusoidally modulated at two different frequencies much higher than the ultrasonic frequency, and the ultrasound signal is encoded into the amplitude of the intensity modulations. The optical signals from the two channels are separated in the frequency domain, and the ultrasound signals are detected by simple envelope detectors. Using a low-finesse Fabry–Perot interferometric sensor formed by two weak chirped fiber Bragg gratings written on a coiled bend-insensitive fiber, we demonstrate that this method can perform ultrasound detection, even when the spectrum of the sensor experiences large environmental drifts.

We demonstrate the constant temperature (CT) operation of a fiber-optic anemometer based on a laser-heated silicon Fabry-Perot interferometer (FPI), where the temperature of the FPI is kept constant by adjusting the heating laser power through a feedback control loop and the output signal is the heating laser power. We show that the CT operation can dramatically improve the frequency response over the commonly used constant power (CP) operation, where the laser heating power is kept constant and the output signal is the temperature of the FPI. For demonstration, we used a 100-μm-diameter, 200-μm-thick silicon FPI attached to the tip of a single-mode fiber as the anemometer. The FPI was heated by a 980-nm diode laser, and the temperature was measured using a 1550-nm diode laser. The effect of flow changes was simulated by exposing the silicon FPI to radiation from an external intensity-modulated laser beam. We show that the 10%-90% rise time of the step response in air was reduced from 625 ms for CP operation to 1.8 ms for CT operation, and the 3-dB bandwidth was increased from 0.5 Hz for CP operation to 2 kHz for CT operation. The response of the anemometer also shows good linearity to the radiation power.

We propose and demonstrate an intensity-demodulated fiber-optic ultrasonic sensor system that can be self-adaptive to large quasi-static background strain perturbations. The sensor system is based on a fiber ring laser (FRL) whose laser cavity includes a pair of fiber Bragg gratings (FBGs). Self-adaptive ultrasonic detection is achieved by a tandem design where the two FBGs are engineered to have differential spectral responses to ultrasonic waves and are installed side-by-side at the same location on a structure. As a result, ultrasonic waves lead to relative spectral shifts of the FBGs and modulations to the cold-cavity loss of the FRL. Ultrasonic waves can then be detected directly from the laser intensity variations in response to the cold-cavity loss modulation. The sensor system is insensitive to quasi-static background strains because they lead to identical responses of the tandem FBGs. Based on the principle, a FRL sensor system was demonstrated and tested for adaptive ultrasonic detection when large static strains as well as dynamic sinusoidal vibrations were applied to the sensor.

We present a series of sub-wavelength annular aperture (SAA) structures with annular width equal to the tip of a tapered hollow tube, which was fabricated using a heat-pulled method. The light beams emitted from the SAA-like structures created by the tapered hollow tube produced light beams characteristic of Bessel beams. We obtained a sub-micrometer focal spot with a depth-of-focus larger than 7 μm and identified the proper structure parameters needed to generate Bessel-like light beams. Our new design has potential application to areas such as optical lithography, optical trapping, and the fabrication of high aspect ratio structures.

We proposed a dual-period phase grating using a polymer-stabilized blue phase liquid crystal (PS-BPLC). High efficiencies of 38% for the short-period phase grating and 30% for the long-period phase grating have been achieved because of the electric-field-induced rectangular-like phase profile of a PS-BPLC, which agrees quite well with the simulation results. The diffraction angle and the diffraction efficiency can be alternatively tuned by a bias voltage and intensity of applied voltage. The diffracted light of each diffraction order is elliptically polarized, and its ellipticity is larger than that induced by using the conventional nematic LCs. Such a device also shows sub-millisecond response time and holds great potential for photonics applications.

In this Letter we present a high temperature multipoint sensing method using sapphire fiber air gap-based extrinsic Fabry-Perot interferometers. Three sensors are fabricated and tested in a single sensing link. Experimental results show that the air gap-based high temperature sensors have a very high temperature sensitivity (20 nm/ degrees C) and resolution (0.3 degrees C) and are capable of operating at temperatures well above 1000 degrees C. The multiplexed sapphire sensors present a significant advancement over traditional single-point sensors for critical high temperature applications.

A fiber-optic sensing platform based on a transient and traveling long-period grating (LPG) in a single-mode optical fiber has been proposed and demonstrated. The LPG is generated by pulsed acoustic waves that propagate along the fiber. First, we demonstrate the LPG for temperature measurement along the fiber. By coating the fiber with ultrathin ionically self-assembled multilayers, we then show that the LPG is capable of detecting nanometer thickness variations of the fiber. A temperature compensation method is also proposed and demonstrated. Because the acoustically generated LPG travels along the fiber, this advance is expected to yield a highly sensitive fully distributed fiber-optic biochemical sensor.

We theoretically investigate the feasibility of using a surface layer with a negative thermo-optic coefficient to compensate the thermal drift of a resonant frequency in an optical microresonator. Taking a fused-silica microsphere as an example, our analysis has shown that the thermal drift of a whisper-gallery mode can be fully compensated by such a surface layer. We analyze and compare the compensation performances by using different materials as the surface layer.

We report a novel fiber-optic sensor for measurement of static gas pressure based on the natural convection of a heated silicon pillar attached to a fiber tip functioning as a Fabry-Perot interferometer (FPI). A visible laser beam is guided by the fiber to efficiently heat the silicon pillar, while an infrared whitelight source, also guided by the fiber, is used to measure the temperature of the FPI, which is influenced both by the laser power and the pressure through natural convection. We theoretically and experimentally show that, by monitoring the fringe shift caused by the laser heating, air pressure sensing with little temperature cross-sensitivity can be achieved. The pressure sensitivity can be easily tuned by adjusting the heating laser power. In our experiment, the sensor performance within the temperature range from 20°C to 50°C and the pressure range from 0 to 1400 psi has been characterized, showing an average sensitivity of -0.52 pm/psi. Compared to the passive version of the sensor, the pressure sensitivity was ∼15 times larger, and the temperature cross-sensitivity was ∼100 times smaller.

Theoretical and experimental results have shown that mode power distribution (MPD) variations could significantly vary the phase of spectral fringes from multimode fiber extrinsic Fabry-Perot interferometric (MMF-EFPI) sensor systems, owing to the fact that different modes introduce different extra phase shifts resulting from the coupling of modes reflected at the second surface to the lead-in fiber end. This dependence of fringe pattern on MPD could cause measurement errors in signal demodulation methods of white-light MMF-EFPI sensors that implement the phase information of the fringes.

We present a novel signal-processing algorithm for single-mode optical fiber extrinsic Fabry–Perot interferometric sensors that can achieve both high-resolution, absolute measurement of the cavity length and a large dynamic measurement range simultaneously. The algorithm is based on an accurate model of the characteristics of a fiber-optic sensor that takes into account the phase shift that is due to the coupling of light reflected at the second surface to the lead-in fiber end.

Intensity-based demodulation of extrinsic Fabry-Perot interferometric (EFPI) fiber-optic sensors requires the light wavelength to be on the quadrature point of the interferometric fringes for maximum sensitivity. In this Letter, we propose a novel and remote operating-point tuning method for EFPI fiber-optic sensors using microstructured fibers (MFs) and gas pressure. We demonstrated the method using a diaphragm-based EFPI sensor with a microstructured lead-in fiber. The holes in the MF were used as gas channels to remotely control the gas pressure inside the Fabry-Perot cavity. Because of the deformation of the diaphragm with gas pressure, the cavity length and consequently the operating point can be remotely tuned for maximum sensitivity. The proposed operating-point tuning method has the advantage of reduced complexity and cost compared to previously reported methods.

We propose and demonstrate a fiber-tip sensor based on an ultra-thin silver diaphragm for highly sensitive and high frequency ultrasonic detection. The diaphragm is prepared by the vacuum thermal deposition method and then transferred to the fiber tip. The sensor demonstrated in this letter has a 300 nm thick diaphragm with an inner diameter of 75 μm, leading to a static pressure sensitivity of 1.6 nm/kPa and a resonant frequency of 1.44 MHz. This sensor has potential applications in many fields such as structural health monitoring and medical ultrasonography.

We demonstrate the advantages of using a single-crystal sapphire fiber to perform Raman spectroscopy over using a standard glass fiber. The narrow Raman peaks and the low background signal of a single-crystal sapphire fiber allow for the detection of weaker Raman signals or Raman signals much closer to the incident laser wavelength than with a standard glass fiber, which has a strong broadband Raman spectrum of its own. The Raman spectra of calcite, single-walled carbon nanotubes, and aqueous sodium carbonate solution were collected using a 50 cm long, 200 μm diameter sapphire fiber, and then they were compared with the results using a 50 cm long, 100 μm diameter glass fiber. The Raman spectrum of each tested material becomes indistinguishable due to the background Raman signal of the glass fiber but can easily be seen using the sapphire fiber, which has only a few narrow Raman peaks of its own.

We demonstrate an optical fiber refractometer based on a cladding-mode Bragg grating. It consists of a long-period grating (LPG) followed by a fiber Bragg grating (FBG). The LPG partially couples light from the core mode to a cladding mode, both of which are reflected by the FBG. Part of the cladding mode reflection is coupled back to the core mode through the original LPG and used for refractive index sensing. The core mode reflection is used to compensate for the temperature cross sensitivity of the refractometer. The sensors operate in the reflection mode and can be multiplexed on a fiber in wavelength domain.

We report a novel type of polarization optical time-domain reflectometry (POTDR) for fully distributed fiber-optic sensing, in which the reflected optical signal is from a series of fiber Bragg gratings that are uniformly distributed along the fiber. Compared with a conventional POTDR that uses the Rayleigh backscattering, this grating-assisted POTDR can have a much better signal-to-noise ratio and consequently a better measurement resolution and a larger measurement range of the fiber birefringence. Experimental results have shown that the measurement resolution of the grating-assisted POTDR is almost an order of magnitude better than that of a conventional POTDR.