arXiv subjects

Angzhen Li

Publications and source records attributed to Angzhen Li.

4 recordsLinked to original sources

arXiv2023

Evaporation characteristics of Er$^{3+}$ doped silica fiber and its application in the preparation of whispering gallery mode lasers

The fabrication of whispering gallery lasers (WGL) is used to experimentally evaluate the evaporation rate (mol/$\mu$m) and ratio (mol/mol) of erbium and silica lost from a doped fiber during heating. Fixed lengths of doped silica fiber are spliced to different lengths of undoped fiber and then evaporated by feeding into the focus of a CO$_{2}$ laser. During evaporation, erbium ions are precipitated in the doped silica fiber to control the erbium concentration in the remaining SiO$_2$, which is melted into a microsphere. By increasing the length of the undoped section, a critical point is reached where effectively no ions remain in the glass microsphere. The critical point is found using the lasing spectra of the whispering gallery modes in microspheres with equal sizes. From the critical point, it is estimated that, for a given CO$_{2}$ laser power, $6.36 \times 10^{-21}$~mol of Er$^{3+}$ is lost during the evaporation process for every cubic micron of silica fiber. This is equivalent to $1.74 \times 10^{-7}$~mol of Er$^{3+}$ lost per mol of SiO$_{2}$ evaporated. This result facilitates the control of the doping concentration in WGLs and provides insight into the kinetics of laser-induced evaporation of doped silica.

physics.optics↗

arXiv2021

Blue-band frequency comb and photodarkening in silica whispering gallery microresonators

To date there are extensive studies of optical nonlinearities in whispering gallery resonators (WGRs) in the near and mid-infrared wavelengths. Pushing this research into the visible region is equally valuable. Here, we demonstrate a Kerr frequency comb and Raman lasing at 462 nm in an SiO2 WGR. Notably, due to the high optical intensities achieved, photodarkening is unavoidable and can quickly degrade the optical quality of both the coupling optical nanofiber and the microcavity even at very low pump powers. Nonetheless, stable stimulated Raman scattering (SRS) and hyper-parametric oscillation in normally dispersed WGRs is demonstrated in the presence of photodarkening by taking advantage of in-situ thermal bleaching. These observations highlight the challenges of silica-based, short wavelength nonlinear optics in high quality, small mode volume devices. We propose a method to overcome this apparent limitation and demonstrate blue-band nonlinear optical processes in silica WGRs, thus providing a baseline for optics research in the blue region for any optical devices fabricated from SiO2.

physics.optics↗

arXiv2021

A packaged whispering gallery resonator device based on an optical nanoantenna coupler

In this work, we present the design and fabrication of a packaged whispering gallery mode (WGM) device based on an optical nanoantenna as the coupler and a glass microsphere as the resonator. The microspheres were fabricated from SiO$_2$ fiber or Er$^{3+}$-doped fiber, the latter creating a WGM laser with a threshold of 93 $\mu$W at 1531 nm. The coupler-resonator WGM device is packaged in a glass capillary. The performance of the packaged microlaser is characterized, with lasing emission both excited in and collected from the WGM cavity via the nanoantenna. The packaged system provides isolation from environmental contamination, a small size, and unidirectional coupling while maintaining a high quality (Q-) factor ($\sim$10$^8$). It opens up new possibilities for practical applications of WGM microdevices in a variety of fields such as low threshold lasers, filters, and sensors.

physics.optics↗

arXiv2020

A tellurite glass optical microbubble resonator

We present a method for making microbubble whispering gallery resonators (WGRs) from tellurite, which is a soft glass, using a CO$_2$ laser. The customized fabrication process permits us to process glasses with low melting points into microbubbles with loaded quality factors as high as $2.3 \times 10^6$. The advantage of soft glasses is that they provide a wide range of refractive index, thermo-optical and optomechanical properties. The temperature and air pressure dependent optical characteristics of both passive and active tellurite microbubbles are investigated. For passive tellurite microbubbles, the measured temperature and air pressure sensitivities are 4.9 GHz/K and 7.1 GHz/bar, respectively. The large thermal tuning rate is due to the large thermal expansion coefficient of $1.9 \times 10^{-5}$ K$^{-1}$ of the tellurite microbubble. In the active Yb$^{3+}$-Er$^{3+}$ co-doped tellurite microbubbles, C-band single-mode lasing with a threshold of 1.66 mW is observed with a 980 nm pump and a maximum wavelength tuning range of 1.53 nm is obtained. The sensitivity of the laser output frequency to pressure changes is 6.5 GHz/bar. The microbubbles fabricated by this novel method have a low eccentricity and uniform wall thickness, as determined from electron microscope images and the optical spectra. The compound glass microbubbles described herein have potential for a wide range of applications, including sensing, as tunable microcavity lasers, and for integrated photonics.

physics.optics↗