We demonstrated a data-processing method based on multiple beam interference and Fresnel equations that simultaneously gave the refraction index and absorption coefficient from the raw data of terahertz (THz) time-domain spectroscopy (TDS), which laid the foundation for obtaining the dielectric parameters. This method was independent of phase processing, and complete material information was reserved without having to cut the time-domain signal. The optical coefficients including refractive indices and absorption coefficients of white polyethylene and quartz samples at different thicknesses were obtained. The applicability and accuracy of this method were discussed and verified by comparison with the traditional data-processing method of THz TDS.
852nm Narrow linewidth lasers can be widely used in the field of ultra-fine spectrum measurement, Cs atomic clock control, satellite and optical fiber communication and so on. Furthermore, the stability of the single lateral mode is a very important condition to guarantee the narrow linewidth lasers. Here we investigate experimentally the influence of the narrow ridge structure and asymmetrical waveguide design on the stability single lateral mode of an 852nm diode laser. According to the waveguide theoretical analysis, ridge mesa etch depth (Δη , related to the refractive index difference of parallel to the junction) and ridge mesa width (the narrower the more control force to low order mode) are the main elements for lateral modes. In this paper, we designed different structures to investigate and verify major factors for lateral mode by experiment, and to confirm our thought. Finally, the 5μm mesa ridge laser, 800nm etch depth, with groove structure obtains excellent steady single lateral mode output by 150mA operating current and 30°C temperature. The optical spectrum FWHM is 0.5nm and side mode suppression ratio is 27dBm with uncoated. The laser with 1mm cavity length showed the threshold current of 50mA, a lasing wavelength of λ = 852.6nm, slope efficiency of above 0.7mW/mA. We accomplished single lateral mode of ridge waveguide edge-emitting lasers which can also be used as a laser source in the ultra-narrow linewidth external cavity laser system.
We present a data processing method based on multiple beam interference and Fresnel's formula that extract simultaneously the refraction index and the extinction coefficient from terahertz time domain spectra, and the dielectric coefficient can also be calculated. Typical THz-TDS system working in transmission mode was utilized for direct measurement of the transmission spectra with a frequency accuracy of 7.6 GHz and range from 0.3 THz to 4 THz at room temperature. This method is verified with a double-faced polished 350-μm 100-cut GaAs wafer, and the reasonable average relative error for refractive index in the whole range is less than 0.83% comparing with conventional method, which provides a new approach to process the transmission spectra with oscillations.