Photonics sensing at the thermodynamic limit

Abstract

We report a slow-light fiber Bragg grating strain sensor with a resolution limited by the extremely low thermodynamic phase fluctuations of the fiber. This was accomplished by using a short grating (4.5 mm) to enhance the thermal phase noise, an ultra-stable interrogation laser to lower the laser frequency noise, and a slow-light mode with a high group index (∼533) to suppress all other noise sources. We demonstrate that in a similar but longer grating (21 mm), the phase noise is suppressed in inverse proportion to the square root of the length, in accordance with theory, leading to a strain resolution as low as 130 fε/√Hz and a minimum detectable length of ∼3×10^-15 m at 1.5 kHz.

Fully Distributed Optical Fiber Strain Sensor With 10−12 ϵ/√Hz Sensitivity

“…The attained sensitivity is approximately only three orders of magnitude higher than the noise induced due to thermal agitations in the optical fiber over a distance equivalent to the pulse resolution [1]. Furthermore, our result was also achieved without the use of ultra-stable lasers unlike the current record-holding attempt for a single short-gauge sensor [1], while simultaneously interrogating as many as ∼1000 sensing positions with the same setup, in a time-domain multiplexing technique. We believe that this performance sets a milestone in distributed sensing research.…”

“…source and a lock-in amplifier to reduce electrical noise [1], while previously a 130 mm Fabry-Perot fiber resonator reached sensitivities of ∼0.35 × 10 −12 ε/ÝHz in the low frequency (Hz) range and ∼0.22 × 10 −12 ε/ÝHz in the kHz range, by using a laser source stabilized against a quartz-oscillator phase-locked optical frequency comb [2]. Though highly encouraging, these record-breaking sensor designs present critical implementation shortcomings and stability requirements that restrain their use in most practical environments.…”

“…The result obtained reports an improvement of 2 orders of magnitude in strain sensitivity and one order of magnitude in distance range over the performance reported using other techniques, while ensuring a constant value of sensitivity along the length (unlike phase-measuring schemes [22]). It is important to point out that the presented performance has been achieved with significantly less cumbersome stabilization loops than previous short-gauge point-sensor implementations [1], [2] while achieving the simultaneous interrogation of ∼1000 consecutive points along the fiber cable (∼10 km of fiber), with a direct detection scheme in a conventional optical fiber.…”

Reaching pε/√Hz sensitivity in a distributed optical fiber strain sensor

“…>100 points are interrogated simultaneously over the whole fiber length). Besides, we do not make use of a ultra-stable laser, unlike current record-holding attempts for a single short-gauge sensor [10]. We believe that this result may set a milestone in distributed sensing research.…”

Femtosecond FBG Written through the Coating for Sensing Applications

“…Our group showed recently the possibility to write a type I FBG through the acrylate and polyimide coating of unloaded silica fiber without any degradation of mechanical strength [ 23 ]. Loading the fiber with deuterium to reach large refractive index modulations, up to 5 × 10 −3 , and post-annealing such FBG to reduce its internal losses demonstrated recently promising perspectives for sensing at very high resolution [ 24 ]. This article now focuses on the potential of these FBGs with high-mechanical strength for sensing applications.…”