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 9.04mA to 4.01mA when
the external cavity grating reflectivity increased from 0.1 to 0.9. At the same time,
the line width can be reduced to 95.27kHz 1.34kHz when it operate in 60mA. The
laser linewidth decreases from 3.20kHz to 0.36kHz when the external cavity length
increased from 2cm to 6cm. when the length of gain chip decrease from 600μm to
200μm, the laser linewidth decreases from 1.69kHz to 1.22kHz. 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 12.37mA, the minimum
line width is 100.281Hz, while the linewidth which shares the same structures made
by William et al. is 1 kHz .
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