Coherent Optical Frequency Domain Reflectometry as Characterisation Tool for S-Band TDFAs

Additive manufacturing has already found broad acceptance in rapid prototyping of machinery and is an emerging technology in many other fields such as radio frequency (RF) engineering, where the advantages of the so-called 3D printing technology overcome limitations of established processes and allow entirely new designs. The ability to create almost arbitrary shapes with high precision has proven very useful for antenna design, for example. Using conductive and dielectric ink, RF transmission lines can be 3D printed directly on uneven surfaces. As for RF structures geometrical dimensions are crucial for the resulting RF properties such as impedance, a technique to measure the distance between the printing nozzle and the substrate is necessary. This turns out to be a challenging task since a small spot size is required and transparent (dielectric) as well as reflective (conductor) materials must be detected while maintaining a mechanically flexible and robust system. We propose a distance measurement system based on coherent optical frequency domain reflectometry to accurately measure this distance. The proposed miniaturized coupling optic uses a gradient-index (GRIN) lens with a diameter of less than 3 mm, can be integrated into a printing head easily and is compatible to standard single-mode fibers. In first experiments, we have achieved very promising results that show a good agreement with (destructive) microscopic measurements. Reflective and transparent surfaces can be detected with μm-accuracy.

Recent progress on novel long-range coherent optical frequency domain reflectometry is reviewed along with its applications to diagnosing problems with optical fiber cables. The measurement range of a conventional C-OFDR is limited to the coherence length of the laser used as the light source, since the phase noise of the laser degrades the sharpness of the beat spectrum. We have developed phase-noise-compensated optical frequency domain reflectometry (PNC-OFDR) to overcome this limitation by introducing a novel phase-noise compensation technique, and we achieved a very high-resolution measurement over the fiber link length. We describe the principle of PNC-OFDR and recent related developments, and discuss its use in diagnosing issues with fiber networks.

We report a noble coherent optical frequency domain reflectometry (OFDR) system that simply enables measurement range enhancement up to full coherence length of a laser source. The proposed technique is based on bidirectional determination of Rayleigh backscattered signal along a fiber. To do this, complex OFDR signals are acquired with the help of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pi / 2$</tex-math></inline-formula> phase-shifting interferometry in a detection part and an additional delay fiber in a reference arm. Through a bidirectional determination process using the complex OFDR signals, mirrored signals appearing at both sides of the spatial domain were completely discriminated, so that ambiguity arising due to a positional origin can be removed. Bidirectional distributed strain and temperature sensing is successfully performed for the first time. Spectral shifts by applying strain to and heating of the test fiber were found to be independent on both sides of the reference position, indicating that twofold enhancement of the measurement range can be obtained compared to previous systems.

Coherent optical frequency domain reflectometry for interrogation of bend-based fiber optic hydrocarbon sensors

This letter describes a novel configuration and theory for the degree of coherence measurement based on the statistical speckle analysis in coherent optical frequency domain reflectometry. The difference from the previous letter is using a standard phase modulator instead of a single-sideband modulator, which not only simplifies the system configuration, but also yields modulation efficiency, resulting in the operation with lower light source power. A novel theory is presented to describe the operation of the proposed configuration, and experimental results show the validity of the new configuration and theory.

Coherent optical frequency domain reflectometry is used to multiplex a serial array of tapered optical fibre sensors. By monitoring changes in the Rayleigh backscattered signal from tapered regions of a single mode optical fibre, the refractive index of the medium surrounding each taper can be determined. The approach is used to demonstrate a liquid flow sensor.

Summary form only given. We demonstrated coherent optical frequency domain reflectometry (C-OFDR) for a long optical fiber transmission line. We simulated the measurement of a reflection at a one-way distance of >1000 km, and achieved a spatial resolution of about 130 m.

Fig. 1: Map of the REFIMEVE network (green links) and its connection to European links.In recent years, significant technological progress has demonstrated the feasibility of using the long distance fiber optic links as large scale distributed networks for environmental sensing [1]. Optical fibers are inherently sensitive to external perturbations: their mechanical structure responds to strain, while the light propagating within them undergoes measurable intensity and phase variation when subjected to vibration or seismic waves. A notable example is the French national research infrastructure REFIMEVE [2], which distributes ultrastable time and frequency references across more than 9000 km of fiber links connecting laboratories throughout France and Europe (see Fig. 1). The infrastructure has demonstrated strong potential for geophysical studies [3]. Applications such as earthquake detection, volcano monitoring, and environmental hazard surveillance are attracting increasing interest worldwide, particularly because they can leverage already existing fiber networks. In this context, the European project SENSEI (Smart European Networks for Sensing the Environment and Internet Quality) [4] aims to harness this potential by developing the next generation photonic technologies for detecting both natural phenomena, such as earthquakes, volcano activity, and anthropogenic events including construction activity or vehicular traffic.Within this framework, one of our objectives is to develop a coherent optical frequency domain reflectometry (C-OFDR) [5]. Current systems are limited to approximately 100 km by the coherence length of the laser source. Here, we take benefit from the low frequency noise laser source generated by REFIMEVE frequency reference in order to extend the sensing range. In our setup, the output of a low noise laser is frequency modulated and a fiber under test is studied in a Michelson interferometer configuration. By analyzing the Rayleigh backscattered signal along the fiber, the system enables detailed diagnostics of the fiber under test including the detection of localized fiber deformations, faulty connectors, attenuation variations, and disturbances induced by environmental vibrations. As a first demonstration, we tested a prototype over a long range fiber link made of laboratory spools extending up to 335 km. The system successfully identified the position of the optical amplifier and a PC connector placed at the end of the fiber with km scale spatial resolution. In addition, vibration induced perturbation was observed and is under study, highlighting the potential of this technique for seismic applications. In future work, we plan to deploy the C-OFDR system on the operational REFIMEVE fiber network to evaluate its performance under real field conditions. This approach positions C-OFDR as a powerful tool for telecommunication infrastructure monitoring and distributed geophysical sensing. References :[1] G. Marra et al., Science 361 (2018), https://doi.org/10.1126/science.aat4458[2] REFIMEVE, https://www.refimeve.fr/en/homepage/[3] M. B. K. Tønnes, PhD Thesis (2022), https://hal.science/tel-03984045v1[4] SENSEI, https://senseiproject.eu/[5] C. Liang et al., IEEE Access. 9 (2021), DOI: 10.1109/ACCESS.2021.306125

This paper investigates for the first time the distributed gain of the silica-based thulium-doped fiber amplifiers (TDFA) using coherent optical frequency domain reflectometry (COFDR). Gain spectra and distributed gain are reported for the various pump schemes. Results show that the 805 nm pump has a more pronounced effect when added to 1050 nm than when added to 1426 nm, with a peak gain increase of 0.9 dB (~4% pump power increase gives -20% more gain). This paper also observes that for the 1426-nm upconversion pump, the 805-nm auxiliary pump resulted in a slight gain shift, increasing gain at longer wavelengths (>1470 nm) while reducing at lower wavelengths.

We demonstrate a wafer-bonded silica-on-silicon planar waveguide platform with record low total propagation loss of (0.045 ± 0.04) dB/m near the free space wavelength of 1580 nm. Using coherent optical frequency domain reflectometry, we characterize the group index, fiber-to-chip coupling loss, critical bend radius, and propagation loss of these waveguides.

Coherent optical frequency domain reflectometry (C-OFDR) is demonstrated for a long single-mode optical fiber at a wavelength of 1.55 /spl mu/m by using a highly coherent lightwave source and an electrooptical (EO) external phase modulator. Rayleigh backscattering and Fresnel reflection from a 16.4-km optical fiber are observed when the modulation frequency is swept from 4 GHz to 8 GHz for 40 ms. The insertion of an optical fiber delay line in a probe lightwave path makes it possible to distinguish the Rayleigh backscattering profile due to the first-order modulation sideband in the phase-modulated lightwave from those due to the higher order sidebands. A spatial resolution of better than 100 m is achieved for two neighboring Fresnel reflectors located near the far end of the test fiber. >

Optical-fiber self-referred refractometer based on Fresnel reflection at the fiber tip

In modern society, the amount of data being transferred is growing at an exponential rate, about 2.5 quintillion bytes data created per day. Such data can be realized in the form of include the audio, video, or text. Furthermore, due to the many advantages of the optical fiber, including the low-cost, immunity to electromagnetic interface, light weight, and high carrier frequency, it is taking over the industry. The question then arises as how to diagnose the inner fault and maintain the integrity becomes the crux for the telecommunication system based on optical fiber. There are two main high-precision measurement methodologies to complete the task mentioned above: one is the optical time-domain reflectometry (OTDR), which has been widely applied in technologies such as light detection and ranging (LiDAR) in faraway situations. The other one is optical frequency-domain reflectometry (OFDR), which is also referred to as optical frequency modulated continuous wave (OFMCW) or chirped frequency measurement. Compared with the OTDR, the OFDR measures distance by comparing the phase change of the chirped incident optical signal and reflected signals to capture the distance information. Even though the OFDR has been proven to have many advantages over OTDR in terms of the high spatial resolution at μm level, low power consumption, and eye-friendliness, both the high complexity and cost significantly limit its applications. To overcome the above shortages in the OFDR system and expand this technology in the various applications, the following research was conducted. Firstly, since the optical tuning source utilized in OFDR is too expensive and complex to be implemented in-situ measurements, there are some researches completed in the tuning laser source control methods proposed to reduce the cost and complexity of this technology and fit the various applications. Additionally, to enhance the accuracy of the OFDR measuring, some tuning rate error compensating algorithms were introduced. Benefiting from the improved spatial resolution, the physical unclonable function (PUF) in the optical fiber can be achieved as its unique identification to sustain the data transferring security. Furthermore, even though the initial optical frequency (IOF) in OFDR has been disregarded in the distance measuring, it does influence the PUF reader precision. To fully analyze such effect, some research and IOF mitigating methods were proposed, in which a fast processing method for the PUF reader has been introduced by decreasing the calculating complexity. From the above research, a low-cost, accurate, and effective OFDR system has been

The distributed-gain along erbium doped fibres was measured with coherent optical frequency-domain reflectometry. The optimum length for different amplifier specifications were measured and compared to the cutback technique.

Optical frequency domain reflectometry (OFDR) measurements are performed by recording the interference pattern of light backscattered by density fluctuations or Bragg gratings along the length of an optical fiber. Changes in local temperature or strain in the fiber cause shifts in the backscattered light spectrum, which can be calibrated to the applied temperature or strain. Comparing the backscattered optical spectra from each reflection site with a reference (unstrained) spectrum allows for quantification of local spectral shifts. While mature OFDR-based technologies can provide high-precision spatially distributed measurements over kilometer lengths, the post-processing approaches used to recover spectral shifts from raw optical intensity data can limit the use of OFDR in harsh environments. This paper presents a novel approach to post-processing OFDR data which extends the usability of optical fibers exposed to harsh environments. This approach is the first to use the spectral shift quality, calculated for two OFDR measurements, to identify an appropriate reference scan from which the spectral shift can be determined. Three experimental data sets are used to test the algorithm: (1) an optical fiber heated to >950°C, (2) an aluminum-embedded optical fiber under strain from differential thermal expansion, and (3) an optical fiber exposed to a total neutron flux of $2\times 10^{13}$ n/cm2/s over 40 hours in a nuclear test reactor. Results show that using a metric of quality to select an appropriate reference measurement extends the functional range of OFDR strain and temperature sensors. Furthermore, this algorithm can be applied to existing OFDR data.