ABSTRACT
Reflectance anisotropy spectroscopy (RAS) is a highly sensitive optical probe for the real-time study of the epitaxial growth of zincblende semiconductors. Here we report on (1) non-equilibrium RAS spectra acquired in real time during the homoepitaxial growth of GaAs, and (2) RAS spectra for GaAs surfaces under equilibrium with several arsenic overpressures. We show that in both cases RAS spectra can be decomposed into two basic components, each with a characteristic line shape. We further show that both dynamic and equilibrium RAS spectra are described by the same pair of basic components. We conclude that the time evolution of non-equilibrium RAS spectra acquired during the epitaxial growth can be described in terms of RAS spectra for equilibrium surfaces. The results reported here should be useful for the interpretation of the physics underlying the rapid time evolution of dynamic RAS spectra during the first monolayer growth. Thus, we show that RAS constitutes a valuable tool for the study of epitaxial growth mechanisms.
ABSTRACT
A tunable single- and dual-wavelength thulium-doped all-fiber laser is demonstrated based on the implementation of an in-fiber acousto-optic tunable bandpass filter (AOTBF). The AOTBF is fabricated to be operated in the 1.9 µm region, and takes advantage of the intermodal coupling effect produced by traveling flexural acoustic waves in an optical fiber. It exhibits a 3-dB bandwidth of 2.04 nm with an insertion loss of 4.75 dB. The tuning properties of the AO device allows a continuous-wave operation with characteristics of wide tuning range (211.5 nm), narrow linewidth (50 pm) and high signal-to-noise ratio (60 dB). In the dual-wavelength regime, the laser is capable of independent tuning of each of the laser lines, achieving a tunable dual-wavelength emission that extends from 1802.67 to 1932.75 nm. A controllable wavelength spacing with minimum and maximum separations of 1.04 and 130.08 nm is obtained.
ABSTRACT
A long-cavity passively mode-locked thulium-doped all-fiber laser is reported incorporating a tapered acousto-optic tunable bandpass filter (AOTBF). The operation of the AOTBF relies on the intermodal coupling between core and cladding modes when a flexural acoustic wave propagates along an 80 µm tapered fiber. The filter works in transmission and exhibits a 3 dB bandwidth of 9.02 nm with an insertion loss of 3.4 dB. The laser supports ultrashort pulse generation at a low repetition rate of 784.93 kHz. Optical pulses with 2.43 nm of optical bandwidth and 2.1 ps pulse duration were obtained in a broad tuning range from 1824.77 to 1905.16 nm.
