Studies On The Waveguide Bragg Grating Of Weak Refractive Index

As an important communication technology in today’s information society, photonic communication technology develops rapidly. The requirements for optical network transmission capacity now are tens thousands times than past, as a result of which, discrete photonic devices can’t meet the capacity requirements.The photonic integrated circuit has opened a new window in the field of optical communication with the emergence of Wavelength Divison Multiplexing(WDM) technology. Among so many photonic devices, the waveguide Bragg grating plays an important role. It can be used in the Distributed Feedback (DFB) lasers, grating couplers and so on. This thesis focuses on the structures and properties of waveguide grating filter, and proposes a special property for the first time, namely integral invariance of reflective spectrum for weak-coupled waveguide Bragg grating, that is if only the grating phase is changed along the cavity and index modulation strength is uniform, the integral of the reflective spectrum of arbitrary grating profile nearly maintains a constant.In the first chapter, we give a brief introduction of the trend of the photonic communications development by telling a story from the original discrete device to the photonic integrated circuit of modern communication. We introduce the waveguide grating in details, and show the basic structures and properties of refractive index modulation of several common waveguide gratings. Then combined with the actual fabrication process of two grating structures, we proposed the existence of waveguide gratings’manufactural errors. In fact, this thesis is written in order to have a deeper understanding of the fabrication errors in the waveguide gratings.In chapter two, we derive the basic principle of waveguide grating. First we make a basic model of the transmission in the grating, and use the phase matching principle to confirm a relationship between the reflective wavelength and grating period, which is called "Bragg condition". Then we use the coupled-mode theory to analyze the relationships between the forward wave and backward wave. And to solve the equation of Coupled Mode Theory(CMT), we choose a way called transfer matrix method. By this method, we can get some basic transmission characteristics of the waveguide grating, such as the reflection spectrum, transmission spectrum and time delay. Finally, we carry out a simulation of several examples of common gratings. The sampling structure mentioned in this chapter is very important for the reconstruction equivalent chirp technology, especially in the design of DFB lasers.In chapter three, we utilize the CMT to analyze the reflection spectrum of the weak-coupled waveguide gratings. Also we raise a special property which has never been proposed before, which is the integral invariance of reflective spectrum for weak-coupled waveguide Bragg grating. To demonstrate this characteristic, we design five different structures with the same coupling coefficient and calculate their areas of reflection spectrum. Then we change the coupling coefficient to verify the scope of application and find that if only the refractive index is weak and grating phase is changed along the cavity, the integral invariance works well. At last, we apply this feature to analyze two cases, and give a reasonable explanation to the problems of the forbidden band width and the manufactural errors in the waveguide grating. And we put forward a method to reconstruct the coupling coefficient of the chirp grating though the estimation of the band width. In the last chapter, the error analysis method is applied to the sampling waveguide grating fabrication in the DFB lasers. Due to the existed errors in the process of waveguide grating manufacture of lasers, the judgment of the peak position of laser is always affected by the inevitable errors. In this part, we build a mode of error confirmed to gaussian distribution, and then by controlling the expectations and standard error of this gaussian distribution function, we can calculate the transmission spectrum of different error range. The final simulation results tell us that even if the sampling grating has errors, the lasing wavelength can keep constant. But as the error range drifts from the ideal grating period, the reflectivity of the peak decays quickly.