We report our recent progress in generating and verifying entangled photon pairs in a telecommunication-length few-mode fiber using spontaneous four-wave mixing in a non-degenerate pump configuration. We demonstrate through seeding the desired four-wave mixing effect for pair production in the presence of a strong mode-coupling when using integrated mode multiplexers known as photonic lanterns. The mode content and phase-matching requirements were validated through computations and experiments. We discuss the challenges and requirements for phase matching and photon visibility without modifying the fiber structure. This work represents our first steps toward an efficient tunable source of entangled photons for space division multiplexing applications.
We report our progress on the design of a highly sensitive photoacoustic spectroscopy sensor using an extrinsic Fabry- Perot interferometer fiber-optic microphone for detecting parts-per-billion-level trace gas concentrations. A theoretical model is set up to predict the mechanical behavior of the sensor and extended with a mathematical framework for detecting gas concentration from the generated acoustic modes in a photoacoustic gas cell. A detection limit up to 1.55ppb for Nitric oxide is predicted based on the model for a minimum detectable pressure of 2.1μpa√Hz. We also investigated different frequency response of two different gas cells with the finite element method (FEM) using COMSOL for the fiber-optic acoustic sensor.
We report a fiber-optic-based ultrafast time-stretch laser detection and ranging (Lidar) sensor with 10 MHz speed and 10 μm accuracy with 30 mm dynamic range for head motion detection under the thermoplastic mask during image-guided radiotherapy procedures. The sensor (1) is miniaturized and fits under the mask, (2) is small enough not to cause attenuation in radiation beams, (3) has a spatial resolution of a tenth of a millimeter, (4) is real-time, and (5) is immune to electromagnetic radiation.
High degrees of security have been achieved in quantum communication over fiber using entangled photons. For communications applications, it is crucial to produce high-yield entangled photon pairs (EPPs) with the capacity to transmit them over telecommunication-length fiber distances and the ability to integrate and transmit them over existing classical communication systems. Since spatial division multiplexing (SDM) is currently being demonstrated to replace single-mode communication systems, it only makes sense to demonstrate the generation and transferring EPPs in various fiber modes suitable for generation and transmission in SDM fibers. We experimentally demonstrate EPP formation in a 25-km-long graded-index few-mode fiber (three modes) via the intermodal spontaneous four-wave mixing (SFWM) effect. Mode profile and phase matching condition have been confirmed through experiments.
The experimental study of cooling by anti-Stokes fluorescence in a fiber or a radiation-balanced fiber laser necessitates the development of a sensor that can measure the temperature of the fiber core with an excellent temperature and spatial resolution, a large dynamic range, a small drift, a fast response, and a low absorptive loss. We report an in-situ slow-light fiber sensor written directly in a Yb-doped silica fiber using a femtosecond laser. The sensor has a spatial resolution of 6.5 mm, an excellent measured temperature resolution of 0.9 m°C/√Hz, and a measured drift as low as 20 m°C/min. One of the grating’s slow-light resonances is interrogated with a tunable 1.55-μm laser to measure the temperature-induced shift in the resonance wavelength when the fiber is optically pumped. The laser frequency is also modulated at 30 kHz to greatly reduce the detection noise. The sensor was pumped with 0.58 mW from a 1020-nm laser and measured a positive temperature change of 0.33 °C. The dominant source of heating is shown to be likely the photodarkening loss induced in the Yb-doped fiber when the FBG was written. The total FBG loss is predicted to be ~24 m-1 at 1020 nm and expected to reduce after annealing. Projections indicate that if the loss of the rare-earth doped FBG can be decreased to the level of the loss observed in slow-light FBGs written in SMF-28 fibers, these sensors can be used to measure ASF cooling.
We use a comprehensive model of cooling by anti-Stokes fluorescence in a single-mode fiber that includes the effects of fiber loss, concentration quenching, mode profiles, and amplified spontaneous emission to analyze the trends of cooling in single-mode Yb-doped ZBLANP fibers. Simulations demonstrate that heat extraction varies significantly along the fiber. There is an optimum pump power (58 mW at 1015 nm for the modeled fiber) for which the maximum heat extracted per unit length is at the start of the fiber. Launching more power moves the coolest point further down the fiber. At substantially higher powers, ASE has a significant heating effect, and coupled with the heating due to absorptive loss, the entire fiber warms up. For a given fiber length, the total extracted heat is maximized for a different pump power (430 mW for a 20-m length). The temperature change is then negative along the entire fiber, and the total extracted heat is 7.12 mW (1.65% cooling efficiency). When the fiber absorptive loss is negligible, this value increases to 30.5 mW for a 2-W pump, giving a 3.48% cooling efficiency, only slightly below the quantum limit (3.7%). The optimum dopant concentration has a similar trade-off: the total extracted heat is maximized for a Yb concentration of 2 wt.%, and the cooling efficiency for 0.5 wt.%. A model of ASF cooling in fiber lasers is also described and exploited to investigate how to select the fiber laser parameters to extract the most power output from a radiation-balanced fiber laser. It shows that increasing the cavity length increases cooling at the expense of laser efficiency, and that a low output coupler reflectivity enhances ASF cooling. Simulations predict that a large-mode-area fiber laser should produce 12.7 W of output power at 63% efficiency, a performance limited by the fiber’s absorptive loss, the core diameter (30 μm), and concentration quenching.
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