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International Conference on Optical Fiber Sensors (OFS)

Improved Closed-Loop Slow-Light Temperature Sensor With Millidegree Resolution for Laser Cooling

Enkeleda Balliu, Lauris Talbot, Chun‐Wei Chen, Benjamin N. Frey, Bailey Meehan, Thomas Wade Hawkins, John M. Ballato, Tommy Boilard, Martin Bernier, Michel J. F. Digonnet

Published Jan 1, 2023• 4 citations
Publisher

Abstract

For applications such as laser cooling of doped fibers, where it is critical to measure accurately the temperature of a cooled fiber that is very close to room temperature, it is paramount to develop a reliable, very short (mm) fiber temperature sensor with millikelvin resolution and very little drift. We report a second generation of a unique slow-light fiber Bragg grating (FBG) temperature sensor that meets these stringent requirements. Experiments and modeling establish that its temperature response depends only on well-known material constants of the silica FBG and the response of the probe laser’s wavelength controller. The response is independent of the linewidth of the slow-light resonance, hence different FBGs and/or resonances have the exact same response. Examples of measured cooling in optically pumped Yb-doped fibers show that more reliable thermal contact with the cooled fiber is obtained by wrapping the FBG and the cooled fiber.

DOI
10.1364/ofs.2023.w2.5

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For applications such as laser cooling of doped fibers, where it is critical to measure accurately the temperature of a cooled fiber that is very close to room temperature, it is paramount to develop a reliable, very short (mm) fiber temperature sensor with millikelvin resolution and very little drift. We report a second generation of a unique slow-light fiber Bragg grating (FBG) temperature sensor that meets these stringent requirements. Experiments and modeling establish that its temperature response depends only on well-known material constants of the silica FBG and the response of the probe laser’s wavelength controller. The response is independent of the linewidth of the slow-light resonance, hence different FBGs and/or resonances have the exact same response. Examples of measured cooling in optically pumped Yb-doped fibers show that more reliable thermal contact with the cooled fiber is obtained by wrapping the FBG and the cooled fiber.
For the first time, to the best of our knowledge, optical cooling is demonstrated in a fiber at atmospheric pressure. Using a specialized slow-light fiber Bragg grating temperature sensor, −5.2 mK and −0.65 K were measured in a single-mode (1% YbF3) and multimode (3% YbF3) ZBLAN fiber with respective cooling efficiencies of 2.2% and 0.90%. Fitting a recently reported quantitative model of optical cooling in fibers to the measured temperature change dependence on the pump power per unit length validates the model and allows us to infer the fibers’ absorptive loss and quenching lifetime, key parameters that are scarce in literature. These values are necessary for accurate cooling predictions and will aid in the development of fibers for application in optical coolers and radiation-balanced lasers.
This Letter reports the behavior of the slow-light resonances of a strong apodized fiber Bragg grating (FBG) in which the intrinsic loss is compensated for by a small internal gain. The 6.5-mm FBG, written with a femtosecond laser in an Er-doped single-mode fiber, was pumped at ∼ 1475 n m just below the lasing threshold to offset most of its intrinsic loss, thereby narrowing its resonances. The fundamental slow-light resonance was measured to have a linewidth of 8.5 fm, or a record group velocity of ∼ 22 k m / s , and a peak transmission near unity ( − 0.2 d B ). The measured dependencies of the linewidth and peak transmission on pump power agree well with a new model that predicts the transmission spectrum of loss-compensated FBGs in the presence of pump and signal saturation.
We proposed and experimentally demonstrated an extremely simple and feasible slow-light technique to achieve tunable optical delay by using the Er/Yb codoped fiber Bragg grating (FBG). The signal light experiences strong dispersion when it is launched into the reflection edge of FBG, and the group delay value is determined by the signal wavelength and the pump power. In the experiment, a controllable delay of 0.9 ns can be obtained through changing the 980 nm pump power. The group velocity can be slowed down to 5.6x10(7) m/s, which is 19% of the speed of light in free space. It provides a very simple approach to control the light group delay, which is likely to have important implications for practical applications.
Fiber Bragg gratings designed specifically for controlling the group velocity of light have been extensively studied in recent years to explore and improve their properties. Use of slow-light fiber Bragg gratings (FBGs) is increasingly recognized as an important component in the optical telecommunication networks, and plays a key role in resolving a problem of buffering of optical signals. This paper analyzes the slow light mechanism in FBG theoretically, explores the influences of different parameters of FBG on the characteristics of slow light by numerical simulation, and discusses the requirements for achieving large group delays.
Quantum sensing has now become one of the most advanced, rapidly growing areas of research, which permits precise scale of measurement. We have experimentally demonstrated a novel method of probing quantum phenomena of lightmatter interaction especially the fluorescence by utilizing matured fiber Bragg grating (FBG) technology. In response to the incident excitation, quantum electrodynamic response of fluorophores coated on the FBG causes a shift in the Bragg wavelength. The Bragg wavelength shift is dependent on excitation wavelength and intensity, where the increased lightmatter interaction leads to a higher shift in the Bragg wavelength. Our sensor could probe the fluorescence at microscopic scale showing Bragg wavelength shift of 10 pm per 400 μW of incident excitation power. Capturing fluorescence at this small scale with microwatt of excitation power can be potentially helpful for biosensing applications where the photodamage limit of excitation power is crucial for the dynamic measurement of biological specimens.
A novel scheme for improving the resolution of temperature sensor using fast and slow light in a fiber resonator is proposed and demonstrated experimentally. In the measurement system, the shift of resonance wavelength produced by the temperature change can be converted into strong variations of delay time. By measuring the delay time, this sensor we proposed has high-temperature sensitivity. Different from the traditional method of tracing spectrum, the temperature detection is carried out using an oscilloscope, which results in the sensor with fast response and high resolution. Moreover, the sensitivity, resolution, and measurement range of the sensor can be easily tuned by changing theQ{Q}Q-factor of the resonator. Using this approach, we demonstrate temperature measurements at fiber resonators with low-Q{Q}Qand high-Q{Q}Q, respectively, as well as a3.76×10−4  ∘C3.76\times 10^{-{4}}\,\,^{\circ }\text{C}3.76×10−4∘Cresolution and a 2706 ns/°C sensitivity of temperature sensing are obtained. The scheme is simple in structure and easy to implement, which is particularly suitable for detecting very small temperature changes.