Simultaneous spectral and temporal laser pulse characterization in the nanosecond range employing an all-fiber time-delay spectrometer

Abstract

Summary form only given. The development of pulsed light sources is crucially connected to proper characterization methods especially for the spectral and temporal emission behavior. In the nanosecond regime, the characterization in either domain is a straight forward task employing standard metrology like optical spectrum analyzers (OSA) and oscilloscopes. However, in contrast to the ultrashort-pulse scale, where simultaneous characterization in the time and frequency domain is already demonstrated, there is no proper technique to achieve the same for nanosecond pulses. In contrast to the common approach of a spectrometer, where the not directly accessible frequency domain is mapped onto a spatial scale, a Time-Delay Spectrometer (TDS) decodes the spectrum into a time dependent signal easily measureable with a fast single-point detector. This principle, realized in a setup employing optical fibers as time delay lines and step-chirped fiber Bragg grating (FBG) arrays as wavelength-selective mirrors, has already proven to be applicable for the spectral characterization of pulsed light sources. However, recent progress in the inscription of FBGs during the drawing process allows the gratings to achieve a much higher reflectivity of up to 40% enabling a much simpler structure of the FBG arrays employed in a TDS. Based on this modified structure, a TDS is realized not only characterizing in the frequency domain but also measuring the temporal behavior of the spectral components. In order to demonstrate the capabilities of the proposed method, a pulsed laser diode, usually employed as a seed source in a multiple stage fiber amplifier system, is investigated exemplarily. Based on the measurement with the TDS, spectrograms of the 10 ns pulses are generated plotting the information in the wavelength and time domain simultaneously. The emission behavior is characterized by a broad and weak emission at the pulse front which directly narrows down to a sharp and stable laser line. With a spectral and temporal resolution in the range of 100pm and 500ps respectively, the TDS shows very good agreement with referential measurements in both domains. As a unique feature, the TDS is also capable of performing the measurement for single pulses. This is of particular interest for an investigation of pulse-to-pulse variations which are disturbing in many applications of pulsed light sources. Especially in the frequency domain, this effect is difficult to measure. The TDS is a novel technique for accessing pulse-to-pulse variations in both domains and hence might offer important insights for the development of pulsed light sources.