Narrow Linewidth External Cavity Semiconductor Laser Based On Fiber Bragg Grating

Narrow line-width laser is the core element of optical communications, optical detection and other areas. Existing narrow linewidth lasers such as solid-state lasers are quite expensive. In this condition, the external cavity lasers have gotten widely attention because of their low prices and superior performance. In order to fulfil the needs of optical communication, optical detection field, we set a goal of 10 kHz.Some research has been done in this article including simulation, device process and so on.As for the theory, an innovative way to simulate combined static theory and dynamic theory is presented. The static characteristic is simulated by MATLAB according to a brief model which use the effective reflection coefficient Reff to simplify the effect of external cavity, while the dynamic characteristic is simulated according to the modified rate equation. In this model, the simulation results showed that: The threshold current can be reduced from 9.04 mA to 4.01 mA when the external cavity grating reflectivity increased from 0.1 to 0.9. At the same time,the line width can be reduced to 95.27 kHz 1.34 kHz when it operate in 60 mA. The laser linewidth decreases from 3.20 kHz to 0.36 kHz when the external cavity length increased from 2cm to 6cm. when the length of gain chip decrease from 600μm to200μm, the laser linewidth decreases from 1.69 kHz to 1.22 kHz. Conclusions can be made that the external cavity feedback enhancement can reduce the threshold current and the laser linewidth. Increase the length of the external cavity can reduce the linewidth as well. Besides that, reducing the gain chip size can also contribute to the linewidth narrowing.As for the experiment, we have a three-stage device fabrication process. First of all, a InP gain chip is made because of its gain spectrum. Secondly, both PLC and FBG are made for the external cavity chip. At last, after screening the PLC and FBG,a external cavity semiconductor laser which has a extremely narrow linewidth based on FBG is packaged. There are three crucial problems to be solved. To fulfil the single factor a narrow reflectivity peak grating is made. To increase the coupling efficiency an optical lens is used for beam shaping. To make it work steadily current source and TEC which have high precision are used. According to the measurement results, we can see that the laser threshold current is about 12.37 mA, the minimum line width is 100.281 Hz, while the linewidth which shares the same structures made by William et al. is 1 kHz.