Theoretical and experimental investigations of dispersion-managed, polarization-maintaining 1-GHz mode-locked fiber lasers.

High-repetition-rate (up to GHz) femtosecond mode-locked lasers have attracted significant attention in many applications, such as broadband spectroscopy, high-speed optical sampling, and so on. In this paper, the characteristics of dispersion-managed, polarization-maintaining (PM) 1-GHz mode-locked fiber lasers were investigated both experimentally and numerically. Three compact and robust 1-GHz fiber lasers operating at anomalous, normal, and near-zero dispersion regimes were demonstrated, respectively. The net dispersion of the linear cavity is adjusted by changing types of PM erbium-doped fibers (EDFs) and semiconductor saturable absorber mirrors (SESAMs) in the cavity. Moreover, the long-term stability of the three mode-locked fiber lasers is proved without external control. In order to better understand the mode-locking dynamics of lasers, a numerical model was constructed for analysis of the 1-GHz fiber laser. Pulse evolution simulations have been carried out for soliton, dissipative-soliton, and stretched-pulse mode-locking regimes under different net dispersion conditions. Experimental results are basically in agreement with the numerical simulations.

Distribution and altitudinal patterns of carbon and nitrogen storage in various forest ecosystems in the central Yunnan Plateau, China.

The carbon (C) pool in forest ecosystems plays a long-term and sustained role in mitigating the impacts of global warming, and the sequestration of C is closely linked to the nitrogen (N) cycle. Accurate estimates C and N storage (SC, SN) of forest can improve our understanding of C and N cycles and help develop sustainable forest management policies in the content of climate change. In this study, the SC and SN of various forest ecosystems dominated respectively by Castanopsis carlesii and Lithocarpus mairei (EB), Pinus yunnanensis (PY), Pinus armandii (PA), Keteleeria evelyniana (KE), and Quercus semecarpifolia (QS) in the central Yunnan Plateau of China, were estimated on the basis of a field inventory to determine the distribution and altitudinal patterns of SC and SN among various forest ecosystems. The results showed that (1) the forest SC ranged from 179.58 ± 20.57 t hm-1 in QS to 365.89 ± 35.03 t hm-1 in EB. Soil, living biomass and litter contributed an average of 64.73%, 31.72% and 2.86% to forest SC, respectively; (2) the forest SN ranged from 4.47 ± 0.94 t ha-1 in PY to 8.91 ± 1.83 t ha-1 in PA. Soil, plants and litter contributed an average of 86.88%, 10.27% and 2.85% to forest SN, respectively; (3) the forest SC and SN decreased apparently with increasing altitude. The result demonstrates that changes in forest types can strongly affect the forest SC and SN. This study provides baseline information for forestland managers regarding forest resource utilization and C management.