Two-slot coiled coaxial cable resonator: reaching critical coupling at a reduced number of coils

This paper reports the experimental demonstration of a coiled coaxial cable resonator capable of meeting the critical coupling condition using a reduced number of coils relative to previously reported coiled resonators. By introducing a second slot along the length of the device, a two-slot coiled coaxial cable resonator was fabricated and critical coupling observed at 22 turns. An additional device with one-slot, but otherwise identically constructed, was also fabricated. After 44 turns, the one-slot device had yet to reach critical coupling. An ultrahigh signal-to-noise ratio (greater than 70 dB) was observed at critical coupling of the two-slot device. This reduction in number of slots necessary to reach critical coupling, and the corresponding reduction of physical length of the device, makes this demonstration of the control of critical coupling a potentially important step towards the successful application of coiled coaxial cable resonators to microwave communication and robust sensing applications

Multiplexed Displacement Fiber Sensor Using Thin Core Fiber Exciter

This letter reports a multiplexed optical displacement sensor using a thin core fiber (TCF) exciter. The TCF exciter is followed by a stripped single mode optical fiber. A small section of buffer is used as the movable component along the single mode fiber. Ultra-weak cladding mode reflection (\u3c − 75 dB) was employed to probe the refractive index discontinuity between the air and buffer coating boundary. The position change of the movable buffer segment results in a delay change of the cladding mode reflection. Thus, it is a measure of the displacement of the buffer segment with respect to the glass fiber. The insertion loss of one sensor was measured to be less than 3 dB. A linear relationship was evaluated between the measurement position and absolute position of the moving actuator. Multiplexed capability was demonstrated and no cross talk was found between the sensors

Field-programmable gate array-controlled sweep velocity-locked laser pulse generator

This manuscript reports a FPGA-controlled sweep velocity-locked laser pulse generator (SV-LLPG) design based on an all-digital phase-locked loop (ADPLL). A distributed feedback (DFB) laser with modulated injection current was used as a swept-frequency laser source. An open loop pre-distortion modulation waveform was calibrated using a feedback iteration method to initially improve frequency sweep linearity. An ADPLL control system was then implemented using a field programmed gate array (FPGA) to lock the output of a Mach–Zehnder interferometer that was directly proportional to laser sweep velocity to an on-board system clock. Using this system, linearly chirped laser pulses with a sweep bandwidth of 111.16 GHz were demonstrated. Further testing evaluating the sensing utility of the system was conducted. In this test, the SV-LLPG served as the swept laser source of an optical frequency domain reflectometry (OFDR) system was used to interrogate a sub-terahertz range fiber structure (sub-THz-FS) array. A static strain test was then conducted and linear sensor results were observed

Ultra-weak intrinsic FP cavity array for distributed sensing

This Letter reports on an ultraweak intrinsic Fabry–Perot interferometer (IFPI) array fabricated by a femtosecond (fs) laser for distributed sensing applications. Ultralow reflectors (\u3c - 60dB) were obtained. IFPIs with different physical lengths showed identical temperature sensitivity (-1.5GHz/°C). A distributed temperature sensing test was conducted. No crosstalk between IFPS elements in the array was observed, implying the device\u27s utility as a distributed sensing system. The possibility of using smaller bandwiths for sensor interrogation was experimentally proven. A small-scale temperature distribution test was conducted on a continuously cascaded ultraweak IFPI array, demonstrating its high spatial resolution. The temperature detection limit of this system was measured to be less than 0.667°C

THz Fiber Bragg Grating for Distributed Sensing

This letter reports a fiber Bragg grating for distributed sensing applications fabricated using single-mode optical fiber and a femtosecond laser and interrogated in the terahertz range. A theoretical model of device behavior was derived, which agreed well with experimentally observed device behavior. In order to investigate the utility of terahertz fiber Bragg gratings (THz FBGs) as a sensing modality, temperature tests were conducted. The results demonstrated a sensitivity of -1.32 GHz/ºC and a detection resolution of less than 0.0017 ºC. A temperature distribution test was also conducted using a THz FBG, demonstrating its potential as a distributed sensing platform with high spatial resolution. The feasibility of interrogating THz FBGs using narrow interrogation bandwidths was also experimentally shown

TEF, Vol. 1 No. 1

https://scholarworks.sfasu.edu/tef/1000/thumbnail.jp

A Sweep Velocity-Controlled VCSEL Pulse Laser to Interrogate Sub-THz-Range Fiber Sensors

This letter reports an all-digital sweep velocity-controlled pulse laser with enhanced sweep bandwidth. A vertical cavity surface emitting laser was employed as the optical source. An all-digital phase locked loop was designed and implemented using a field programmable logic array chip to lock the laser sweep velocity within each chirped pulse. A laser temperature controller was used to tune the operation temperature to enhance the laser mode-hop-free bandwidth, resulting in a total frequency excursion of 212.12 GHz and a constant sweep velocity of 30.28 GHz/ms. The system was further adopted to interrogate a sub-terahertz-range fiber sensor array. A series of static strain tests were conducted to demonstrate the system sensing capability. Highly linear results with a sensitivity of -0.1426 GHz/μϵ were observed

Extended-bandwidth frequency sweeps of a distributed feedback laser using combined injection current and temperature modulation

This article details the generation of an extended-bandwidth frequency sweep using a single, communication grade distributed feedback (DFB) laser. The frequency sweep is generated using a two-step technique. In the first step, injection current modulation is employed as a means of varying the output frequency of a DFB laser over a bandwidth of 99.26 GHz. A digital optical phase lock loop is used to lock the frequency sweep speed during current modulation, resulting in a linear frequency chirp. In the second step, the temperature of the DFB laser is modulated, resulting in a shifted starting laser output frequency. A laser frequency chirp is again generated beginning at this shifted starting frequency, resulting in a frequency-shifted spectrum relative to the first recorded data. This process is then repeated across a range of starting temperatures, resulting in a series of partially overlapping, frequency-shifted spectra. These spectra are then aligned using cross-correlation and combined using averaging to form a single, broadband spectrum with a total bandwidth of 510.9 GHz. In order to investigate the utility of this technique, experimental testing was performed in which the approach was used as the swept-frequency source of a coherent optical frequency domain reflectometry system. This system was used to interrogate an optical fiber containing a 20 point, 1-mm pitch length fiber Bragg grating, corresponding to a period of 100 GHz. Using this technique, both the periodicity of the grating in the frequency domain and the individual reflector elements of the structure in the time domain were resolved, demonstrating the technique\u27s potential as a method of extending the sweeping bandwidth of semiconductor lasers for frequency-based sensing applications