Spectral and Modal Properties of a Mid-IR Spherical Microlaser

Abstract

Whispering gallery mode (WGM) lasers are well-known for their small sizes, low thresholds and narrow linewidths, and have been the subject of research and development for more than two decades. However most of the initial reports and studies have focused on near-IR and visible WGM microlasers. Recent advances in the synthesis, doping and processing of high quality mid-IR transparent glasses have provided the means to extend the wavelengths of these microlasers to the mid-IR range. In this paper, we present a detailed characterization of a compact and low-cost mid-IR spherical WGM microlaser based on heavily doped Er:ZBLAN glass operating near 2.7 µm with an estimated linewidth less than 1 MHz. We describe experimental results on observed single-mode and multimode spectra, thermal tuning, and polarization characteristics of such microlasers. Using a simple analysis of spherical microresonator modes and including considerations of pump and laser mode overlap we provide guidelines for optimized microlaser design.

Power scaling of narrow-linewidth mid-IR spherical microlasers

“…We fabricated several Er:ZBLAN MSs (with uniform Er density of 8 mol %) of diam. between 55 and 150 µm, and characterized their lasing properties using a fiber taper coupler fabricated using a multi-mode low-OH silica fiber and a standard experimental configuration [8,9]. As expected, the largest η ext was observed with a microlaser of 55 µm diam.…”

“…As evident from the figure, the pump absorption efficiency for a 180 µm diam. microlaser is about 10 times smaller than its maximum value that occurs around D L ∼ 44 µm, implying preference for using smaller microspheres than the ones we used previously [8,9]. In our earlier experiments [8,9], fabrication of high-Q ZBLAN microspheres (MSs) was done by using a relatively simple "fiber melting technique" [10] in which the lower limit of the diam.…”

“…Moreover, since the circulating optical power is not easily accessible in ICLs and QCLs, these lasers are not easily adaptable for ultrahigh-sensitivity intracavity molecular detection. In addition, the 2.7 -2.8 µm MIR spectral range for high-sensitivity molecular detection application of several important molecular species (such as H 2 O, CO 2 , HF, HOCL) is not easily achievable by narrow linewidth ICLs or QCLs [3,4].Glass-based whispering gallery mode lasers (WGMLs) are alternative compact sources of narrow linewidth NIR [5,6] and MIR radiation [7,8,9] that are not only highly amenable for intracavity sensing applications (because of the extension of the evanescent tail into the surrounding analyte-bearing medium), but also readily lend themselves to much simpler fabrication processes [5-10], largely because high quality factor ("high-Q") glass WGML microspheres of variable dimensions are relatively easy to fabricate individually, and the doped-glass gain medium also serves as the laser cavity without the need for any external mirror or feedback…”

“…Glass-based whispering gallery mode lasers (WGMLs) are alternative compact sources of narrow linewidth NIR [5,6] and MIR radiation [7,8,9] that are not only highly amenable for intracavity sensing applications (because of the extension of the evanescent tail into the surrounding analyte-bearing medium), but also readily lend themselves to much simpler fabrication processes [5-10], largely because high quality factor ("high-Q") glass WGML microspheres of variable dimensions are relatively easy to fabricate individually, and the doped-glass gain medium also serves as the laser cavity without the need for any external mirror or feedback…”

“…Here, h ext is the ratio between P L,J and the input pump power at the coupling junction (P P,J ). For larger microspheres [8], the combination of poor phase matching with WGM modes [9,12] and the small evanescent tail at the pump wavelength results in less efficient pumping. Fig.…”