| The explosive requirement of information demands that the transmission capacity must be enlarged Wavelength division multiplexing (WDM) has been regarded as a powerful method to meet this requirement because of its realization and low cost. Generally, the designed channel spacing of ITU-T standard in WDM systems is from 0.8nm to 1.6nm, which is much larger than the spacing required by actual communication bandwidth. For example in lightwave transmission systems, a SGHz channel occupies only 0.04nm at the wavelength band of 1.55 n m. Thus, most of the bandwidth in the actual transmission systems may be wasted. From a demand for further amount of aggregate transmission capacity, not only expanding an available wavelength range but also cramming frequency-spacing have been investigated. In such dense wavelength division multiplexing (DWDM) systems a light source having narrower linewidth and further stability of the wavelength has become a requirement. In present commercial WDM systems, distributed feedback lasers (DFB-LD) are widely used as a single-mode tight source. However, the emission wavelength of a DFB-LD needs to be carefully turned on the predetermined wavelength grid by adjusting both laser temperature and bias current. Furthermore, DFB-LD has such drawbacks as a relatively large variation of the lasing wavelength with ambient temperature (~0.1nm/C) which restricts its application in DWDM systems. In recent years, the fiber grating external cavity semiconductor laser (FGESL) regarded as one of the candidate light sources of DWDM systems, has received considerable attention. Usually, it is thought that the lasing wavelength of a FGESL is locked at the Bragg reflection wavelength of fiber grating (FG). Owing to a relatively small dependence of thereflection spectrum of FG on ambient temperature (-0.01nm/C), the lasing wavelength of a FGESL is thought to behave more stable than that of a DFB-LD. Even so, the instability sometimes accompanied by mode hoping of the lasing wavelength of the FGESL, and the instability of output power have been experimentally observed. The objective of this work is just to explain these experimental phenomena and explore the physical mechanism hidden in these phenomena.In present studies of FGESL, the theoretical models usually neglected the contribution of external cavity and fiber grating to the phase condition, and considered the mode distribution of FGESL is in accordance with the mode distribution of LD without FG external cavity. Obviously, such process method deviates from the physical facts. In this work, Based on the threshold condition of the fiber grating external cavity semiconductor laser (FGESL) which the phase of the fiber grating has been included, the effect of the temperature variation and the FG external cavity length on the lasing wavelength of the FGESL has been investigated theoretically. In addition, we have used the phase condition of FGESL to get the longitudinal mode distribution of FGESL, then the influence of the injected current on the output characteristics of FGESL has been studied. Numerical simulations show that: (1)owing to the temperature variation resulting in the change of the refractive index of both the semiconductor medium and the fiber, and further inducing the shift of the longitudinal modes of the FGESL, the lasing wavelength of the FGESL has a shift tendency to longer wavelengths with the increase of the temperature; the mode hopping happens for a short external cavity and disappears for a long external cavity. (2)Under certain temperature, for a short external cavity, a slight change of the external cavity length will result in an obvious variation of the lasing wavelength; and for a long external cavity (>10cm), the change of the external cavity length gives almost no effect on the lasing wavelength. (3)With the increase of the injected current, the lasing wavelength of FGESL appears fluctuation, the side mode suppression ratio (SMSR) increases accompanied by an oscillation, and the P-I curve distorts slightly.
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