Linewidth Measurement of Distributed-Feedback Semiconductor Lasers Operating Near 2.05 $\mu \text{m}$

The first unambiguous linewidth measurements are reported for fiber-pigtailed high-power distributed feedback (DFB) semiconductor lasers operating near 2.05 μm wavelength range. The DFB lasers are packaged inside standard 14-pin butterfly package with polarization- maintaining output fiber and an integrated optical isolator. The Schawlow-Townes linewidth was measured using the coherent frequency-discriminator technique with a fiber Mach-Zehnder interferometer and the total linewidth was characterized using a heterodyne setup. Our measurements indicate less than 100 KHz Schawlow-Townes linewidth and a total linewidth of less than 200 KHz over 50 ms measurement time.