使用一般環形多模雷射為主軸，將其共振腔加入由兩個不同光路差所構成的雙Mach-Zehnder干涉儀於其中，利用雙Mach-Zehnder干涉儀的free spectral range(FSR)來達到抑止除了中心波長(1550.12 nm)外的其他模態，並以模擬演算的結果來設計此種多模的消除達到單模雷射的運作。
而後利用延遲光纖100 km，使訊號頻率移動50 MHz的聲光調製器所構成的自延遲外差式量測法(delay self-heterodyne measurement)的Mach-Zehnder干涉儀光纖光路量測其雷射光源的線寬，得出我們已製作出線寬5.05 KHz的窄線寬單模雷射，其同調長度約為40 km。

In this paper, we add two Mach-Zehnder interferometers with different optical path differences in the ring resonator of a fiber laser. Next, we control the free spectral range (FSR) of these two Mach-Zehnder interferometers to suppress the modes other than the mode of central wavelength(1550.12 nm ). Using the simulation results, we are able to eliminate multiple modes of the fiber laser and obtain a single mode operation.
Then, we use a delay self-heterodyne system including a 100 km delay fiber and an acousto-optic modulator (AOM) in a Mach-Zehnder interferometer to measure the laser linewidth. From the measurement result, we confirm that we have produce a single mode laser with a linewidth of 5.05 KHz.The coherence length of the proposed fiber laser is larger than 40 km.

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