Mostafa Peysokhan

Strategies for laser cooling of Yb-doped ZBLAN and silica single-mode optical fiber (Conference Presentation)

Radiation-balanced lasers are lasers where the heat generation in the gain medium is compensated by optical refrigeration due to anti-Stokes fluorescence emission. To investigate the feasibility of RBL operation in an optical fiber, a diagnostic test and a comprehensive model are essential. The model presented here is based on a two-level system and includes the intensity saturation effect, which has been usually neglected in bulk materials. We will show that for a material with a very low dopant area such as a single mode fiber (SMF) in which the saturation power is easily attainable, there is an optimum power at which the best cooling efficiency is obtained. The effect of the dopant density on the cooling power is investigated to find the maximum cooling efficiency which can be extracted for the material. We also present data for the extraction efficiency and other parameters of commercial Yb:ZBLAN glass and Yb:Silicate SMFs to discuss their cooling feasibility. Due to the structural defects, a double exponential behavior is usually observed in the fluorescence decay of the fibers that includes an slow and a fast decay channels. Some of the ions usually reside in the fast decay side and cause a large decrease in the heat extraction efficiency. Using our model, we will first analytically show that there is a maximum limit for the fast decay lifetime below which the cooling can still be functional and secondly discuss the effect of the measured decay lifetimes on the cooling efficiency.

Temperature measurement of rare-earth-doped optical fibers using a variant of the differential luminescence thermometry (Conference Presentation)

Measurement of cooling efficiency and temperature of the doped optical fiber is critical for the development of optical refrigerators and radiation balanced lasers. Measuring the optical fiber temperature, especially for single mode fibers, is challenging. Non-contact thermometry is required because a temperature sensor which is in thermal contact with the fiber can potentially be a heat load when exposed to the scattered pump power and fiber luminescence and can lead to inaccuracies in thermometry. One of the best non-contact methods is using differential luminescence thermometry (DLT). DLT works based on the fact that the 4f electrons in rare-earths are shielded from the surroundings and host field transitions; therefore, the temperature-induced intensity changes in rare-earth material luminescence are mainly caused by changes in Boltzmann population of emitting states. We propose a variant of DLT for finding a relation between the spontaneous emission of the fiber and the temperature. Our method is based on the normalized correlation between the spontaneous emission spectrum at each temperature and the reference spontaneous emission. In this method, we chose a section of the spontaneous emission spectrum as the reference and calculate the normalized correlation factor of the spontaneous spectrum at each temperature with the reference spontaneous spectrum. We make a calibration curve, and based on the calibration curve we estimate the temperature difference from the reference. Comparisons with the conventional DLT will be presented.

A numerical study of laser cooling in a large mode area single-mode photonic crystal Yb3+:ZBLAN glass fiber (Conference Presentation)

A numerical study of laser cooling in a large mode area Single Mode Photonic Crystal Yb3+:ZBLAN glass fiber is presented. In a recent analyses on conventional single mode fibers (SMFs), strategies to maximize the cooling efficiency were highlighted and it was shown that the cooling scales quadratically with the core and inversely with the cladding radius. For conventional SMFs, heat source density can hardly be increased due to limitations in the total Yb doping concentration and the fact that the system easily operates in the pump saturation regime due to very low saturation intensities in small core single mode fibers. Therefore, it is essential to use the largest possible core radius and smallest cladding radius to obtain detectable cooling. A trivial design approach to obtain large mode areas is to decrease the numerical aperture (NA). However, there are several difficulties in applying this concept to rare-earth-doped fibers. Here, we propose large mode area single mode Yb3+:ZBLAN photonic crystal fibers as a robust alternative design. The radial distribution of the pump and laser mode intensities are numerically calculated using Finite Element Method and the heat source density is directly calculated using the pump and laser intensity distributions. The heat source density is fed back to the heat transfer module of COMSOL and radial temperature distribution across the large core of the fiber and its surrounding photonic crystal structure is calculated. Our results show that a much higher laser cooling efficiency is achievable in large mode area single mode photonic crystal fibers.

Spectral selectivity in optical fiber capillary dye lasers

We explore the spectral properties of a capillary dye laser in the highly multimode regime. Our experiments indicate that the spectral behavior of the laser does not conform to a simple Fabry-Perot (FP) analysis; rather, it is strongly dictated by a Vernier resonant mechanism involving multiple modes, which propagate with different group velocities. The laser operates over a very broad spectral range and the Vernier effect gives rise to a free spectral range, which is orders of magnitude larger than that expected from a simple FP mechanism. The theoretical calculations presented confirm the experimental results. Propagating modes of the capillary fiber are calculated using the finite-element method and it is shown that the optical path lengths resulting from simultaneous beatings of these modes are in close agreement with the optical path lengths directly extracted from the Fourier transform of the experimentally measured laser emission spectra.