Current research / employment

Abstract about my IMPRS PhD project:

Whispering Gallery Mode (WGM) resonators have been of great interest in optics recently. Their compact sizes and long photon confinement due to total internal reflection in a circular geometry lead to various interesting applications.

We use them as narrow frequency filters in laser setups, to minimize and stabilize the laser linewidths. We fabricate microspheres by melting the tips of telecom fibers, as well as crystalline disks using a homebuilt turning machine. Coupling into such resonators requires the use of the evanescent field. We fabricate tapered fibers with diameters down to 1 μm, as well as angle-polished fibers, and use prisms to couple into and out of resonators. Typical Quality (Q) factors of these resonators can be around 108 in resonators with a diameter of just 100 μm.

In an erbium-doped fiber laser that we set up we achieved linewidth narrowing of a factor of 105 using a microsphere as a filtering element in a ring laser setup. Tapered fibers and angle-polished fibers were used for coupling. The resulting linewidth was measured to be 170 kHz using the heterodyne measurement technique, by analyzing the beat with a grating stabilized diode laser. In fact, the final linewidth that was determined was still limited by the resolution of the measurement.

In a more recent experiment, we set up a laser using a semiconductor optical amplifier (SOA) at 1550 nm as a gain medium, integrated into a fiber ring laser, as illustrated in Fig. 1. To stabilize the laser, we cut and polished a calcium fluoride (CaF2) disk with a diameter of 5 mm, and prism coupling was used. By coupling into one side, and out of the other as illustrated, a narrow filter resulted in the loop cavity. Various techniques were used to measure the single-mode emission. First, the heterodyne method was applied using a more stable commercial diode laser. The result was a linewidth of just 70 kHz, although the reference laser was specified to have a width of that order. Since we were at the limit of the resolution, we investigated the linewidth using the self-heterodyne technique: the laser was split into two parts, one of which was delayed by a 45 km fiber line, and the other part was offset in frequency using an acousto-optic modulator (AOM). The two were then overlapped on a detector, and the resulting beat signal showed a linewidth of just 13 kHz for the stabilized laser. Furthermore, the three-cornered hat technique – common in microwave frequency comparison of atomic clocks – was applied in the optical region using three lasers, to determine the absolute performance of our laser. The results for each laser corresponded exactly to the results obtained using the self-heterodyne technique. Finally, the stability of our laser was 10-11 at 20 μs averaging, indicating an astounding fast linewidth of just 2 kHz. Passive frequency stabilization using whispering gallery mode resonators can be more cost-effective and better than conventional techniques to stabilize lasers.

Publications B. Sprenger, H. G. L. Schwefel, Z. H. Lu, S. Svitlov, L. J. Wang, CaF2 whispering-gallery-mode resonator stabilized narrow linewidth laser, Opt. Lett., 35(17), 2870-2872, (2010)

B. Sprenger, J. Zhang, Z. H. Lu, L. J. Wang, Atmospheric transfer of optical and radio frequency clock signals, Opt. Lett., 34, 965, (2009)

B. Sprenger, H. G. L. Schwefel, L. J. Wang, Whispering-gallery-mode-resonator-stabilized narrow-linewidth fiber loop laser, Opt. Lett., 34, 3370, (2009)