RZ-OOK to NRZ-OOK format conversion based on a single fiber Bragg grating

In this work we study all-optical multi-channel return-to-zero (RZ)–on-off keying (OOK) to nonreturn-to-zero (NRZ)–OOK format conversion in single uniform fiber Bragg grating (FBG) for mixed line-rate dense wavelength-division multiplexing systems using mathematical simulations. Forty and 20 Gbit/s RZ–OOK signals with 33% and 50% duty cycles are converted to NRZ–OOK signals in single uniform FBG with 21% reflectivity. Impact of amplitude noise from FBG contrast profile on modulation format conversion efficiency is also studied.

A novel sensor based on a single fiber Bragg grating is described. The sensor is configured by encapsulating half of a fiber Bragg grating in a polymer filled metal cylinder and fixing the other half to the metal cylinder. The results show that this sensor is able to measure pressure and temperature simultaneously. Using a novel tactic to transfer the applied pressure to the axial extended-strain on the fiber Bragg grating, the pressure sensitivity reaches 9.65×10 −3 MPa −1 , which is about 4700 times larger than that of the bare fiber Bragg grating. The accuracy of the pressure measuring can also be improved through introducing a fixed Bragg wavelength as a reference.

A three element, 15.3 cm, fiber Bragg grating array (FBGA) operating at 1550 nm wavelength is fabricated using a single mode photosensitive fiber. The FBGA is initially simulated using in-house developed software based on the Transfer Matrix Method, then fabricated using a double frequency Argon laser and a phase mask technique, and interrogated using Optical Frequency Domain Reflectometry. A single fiber Bragg grating (FBG) is accurately strain calibrated using a Fabry-Perot interferometer and piezoelectric actuation. The piezoelectric is linearly ramped, and the shifts in the Bragg wavelength along with the fringe count from the Fabry-Perot interferometer are recorded. The fringe count is then used to determine the strain on the FBG and compared to changes in the Bragg wavelength in-order to calculate the strain gage factor. This result is used to calibrate the FBGA for strain measurements. The FBGA is then bonded to a cantilever beam with three electric strain gages attached next to each FBG in the array. The axial strain results obtained from the electric strain gages and FBGA are compared for various displacements of the cantilever beam. The Fabry-Perot interferometer and piezoelectric calibration method is a non-destructive process that eliminates the need to bond the FBG to an external support during the calibration process, and can also be used to calibrate electric strain gages.

A vector curvature sensor based on a single fiber Bragg grating (FBG) is proposed and experimentally demonstrated. The sensor is easily fabricated by encapsulating an FBG on a thin steel plate with ultraviolet glue. When the FBG deviates from the neutral plane, its effective refractive index and grating constant are changed by bending, therefore, the sensor can realize curvature measurement. Due to the opposite stress direction on the two sides of the neutral plane during bending, the sensor can realize vector measurement of curvature. The curvature sensitivity of the sensor in convex and concave bending is 558.42 pm/m^-1 and -818.09 pm/m^-1, respectively. This sensor has the advantage of simple structure, low cost, and easy industrial production. It has potential applications in engineering health monitoring and deformation measurement.

The global analysis for an all-optical gain-clamped L-band erbium-doped fiber amplifier using a single fiber Bragg grating

We report a continuous-wave Tm-doped fiber (TDF) laser with a single fiber Bragg grating (FBG). The Fresnel effect presenting in the passive fiber end-face and high-reflectivity FBG established the resonator. The peak wavelength of the laser was 1942.25 nm with a spectral linewidth of 194 pm. The maximum output power was 37.5 W, corresponding to a slope efficiency of 63.7% with a beam quality factor of M 2 ∼ 1.51. To the best of our knowledge, this is the highest slope efficiency of a TDF laser pumped by 793 nm laser diode.

Fiber optic sensors (FOS) in long-term structural health monitoring (SHM) have drawn significant attention due to their pivotal role in detecting defects and measuring structural performance in diverse infrastructures. While using FOS, temperature variation due to environmental factors is still considered one of the major challenges to isolating sensing parameters. To address this issue, we reported a machine learning (ML)-augmented multi-parameter sensing system that enables simultaneous detection of strain and temperature effects based on one single fiber Bragg gratings (FBGs) sensor for SHM. The initial phase entailed designing, fabricating, and characterizing a novel FBG sensor in the laboratory, incorporating a set of four FBGs, each distinguished by distinct Bragg wavelengths. In the next phase, ML algorithms are employed to separate temperature effects from strain variations. As a proof of concept, mechanical loading tests are conducted on the sensor, exposing the FBG portion to various temperature conditions. In the final phase, data collected from a post-tensioned concrete bridge embedded with both strain and temperature FBG sensors are utilized, and the developed ML models are applied to observe real-environment outcomes. Despite the limited feature points of collected FBG spectrums, the developed ML models effectively address cross-sensitivity issues induced by temperature perturbations. The long-term benefit of using FOS is that it will enable a better understanding and utilization of aging infrastructure. This will potentially reduce embodied carbon of infrastructure in the future and assist in the global efforts to achieve Net-Zero.

An efficient fiber Bragg grating (FBG) sensor integration system, which utilizes a small number of standard Telecom opto-electronics components, is presented in this paper. Wavelength-swept optical pulses were generated by driving a standard telecom DFB laser diode by high amplitude (>3 A) and short duration (300 ns) current pulses. A total laser diode's wavelength sweep in excess of 10 nm was achieved, which allows for simultaneous interrogation of several FBGs within similar wavelengths. The application of short duration wavelength-swept optical pulses also allows for time division multiplexing of small FBG arrays containing FBGs with overlapping characteristic wavelengths. When a short, wavelength-swept optical pulse is launched into the fiber containing single or multiple FBGs along its length, fractions of the launched pulse are back-reflected towards the detector. Since the launched pulse wavelength changes over the time, the reflections from the FBGs occur in the time moments that depend on the FBGs' characteristic wavelengths. Measurement of the time delay among back-reflected optical pulses is then used to determine the FBGs characteristic wavelengths. The experimental system demonstrated FBG wavelength readout resolution, which exceeded 3 pm at 1 KHz sampling rate, maximum sampling rate of over 40 KHz, and capability to readout/demultiplex over 30 FBGs located down a single optical fiber.

We demonstrate a novel random laser based on a single fiber Bragg grating. A long fiber Bragg grating fabrication technique allows the insertion of a large number of randomly distributed phase errors in the structure of the grating which induces light localization. By writing such a grating in a polarisation maintaining Er-doped fiber, a random laser is demonstrated by pumping the fiber with 976 and 1480 nm pump lasers. The number of emitted modes is observed to be a function of the length of the grating and of the pump power and single-mode operation is shown to be possible. The random fiber laser shows low-threshold (approximately 3 mW) and measured approximately 0.5 pm emission linewidth at a wavelength of around 1534 nm.

Power-compensated displacement sensing based on single mode-multimode fiber Bragg grating structure

In vibration sensing using a fiber Bragg grating (FBG) based on the intensity modulation scheme, light incident on a FBG is narrow-band light tuned to the slope in a reflectance spectrum of the FBG. Since the sensor sensitivity is proportional to the slope of the spectrum curve, enhancement of the sensitivity requires the slope to be steeper. In this paper, vibration sensing by usee of an in-fiber Fabry-Perot interferometer with FBG reflectors (FBG-FPI) is presented. Because a reflectance spectrum of a FBG-EPI has much steeper slope thatn a single FBG, a FBG-FPI yields improvement in sensitivity of the sensing. High-sensitivity measurement of vibration of a piezoelectric transducer is experimentally demonstrated. Comparing to a vibration sensor using a single FBG, the use of the FPI structure has increased the sensitivity of vibration by 30dB.

In this paper, the impact of cascading optical filters on 100 Gb/s and 400 Gb/s systems has been studied using two types of filters: fiber Bragg grating (FBG) and thin-film filters (TFF). Simulation results indicate a smaller eye closure penalty (ECP) for TFF cascade relative to FBG filter cascade. At 3-dB ECP, the number of cascaded filters for single-carrier 100 Gb/s polarization-multiplexed quadrature phase shift keying (PM-QPSK) system is 4 TFF and 2 FBG with 3-dB bandwidth of 43 GHz at 50 GHz channel spacing. Also simulation results indicate that at 3-dB ECP, the number of cascaded filters for single-carrier 400 Gb/s polarization-multiplexed 16-ary quadrature amplitude modulation (PM-16QAM) system is 2 TFF with 3-dB bandwidth of 90 GHz at 100 GHz channel spacing. A single FBG with 3-dB bandwidth of 90 GHz at 100 GHz channel spacing results in an ECP of 4-dB for 400 Gb/s PM-16QAM system.

We demonstrate an optical scheme allowing for simultaneous wavelength locking and spectral filtering using a single fiber Bragg grating. We observe substantial increase of extinction ratio and reduction of the wavelength chirp. The scheme also provides stable wavelength necessary for dense WDM system applications.

This work has demonstrated the performance characteristics of single Bragg grating fiber under various filter impulse functions and grating filter profiles. Single fiber Bragg grating (FBG) power propagation, grating index change, coupling coefficient and apodization variations versus grating length variations is demonstrated at the central wavelength of 1550 nm by using both Hamming, Blackman impulse filter function and Hamming, Blackman grating profile. As well as the single FBG power spectrum, delay, dispersion, transmittivity/reflectivity are simulated and clarified against wavelength variations using both Hamming, Blackman impulse filter function and Hamming, Blackman grating profile. The Hamming impulse function filter and Hamming grating profile have clarified better performance than Blackman impulse function filter and Blackman grating profile.

We demonstrate a novel technique for tunable dispersion slope compensation that relies on a single nonchannelized chirped fiber Bragg grating (FBG). This grating is unique in that it is written such that the time delay as a function of wavelength is a third-order profile that covers several WDM channels over a continuous bandwidth of many nanometers. The dispersion range provided by the grating for the WDM channels can be up to 400 ps/nm, enabling dispersion slope compensation.