A recirculating delayed self-heterodyne method using a Mach–Zehnder modulator for kHz-linewidth measurement

• A laser linewidth measurement system using a Mach–Zehnder modulator is proposed.
• The mechanism relies on the carrier-suppressed double sideband modulation of Mach–Zehnder modulator.
• The proposed measurement system can obtain a frequency shift range reaching GHz.
• The linewidth of distributed feedback Bragg laser broadens with the rise of working temperature.

A laser linewidth measurement method which uses a Mach–Zehnder electro-optic modulator (MZM) is proposed in a loss-compensated recirculating delayed self-heterodyne interferometer (LC-RDSHI). Compared with the traditional acousto-optic modulator (AOM), the electro-optic modulator has the merits of broader bandwidth, lower insertion loss, higher extinction ratio and thus, a wider application. A theoretical analysis shows that the power spectrum curve of the novel measurement system is a Lorentzian line, which fits well with experiment. The linewidth is measured to be 137 ± 7 kHz at a frequency shift of 4 GHz. Measurement of a distributed feedback Bragg (DFB) laser has manifested that the linewidth broadens from 98.5 kHz to 137.4 kHz as the operating temperature changes by 16 °C. This work will allow investigation of narrow linewidth semiconductor and fiber laser stability.