All-fiber-referenced ultra-narrow-linewidth laser with broad wavelength agility

We demonstrate an ultra-narrow-linewidth laser by locking a commercial external-cavity diode laser (ECDL) to a cascaded, all-fiber reference. To circumvent the challenge of directly locking broad-linewidth sources (>100 kHz) to a high-finesse fiber reference, we employ a two-stage scheme, where a fiber ring resonator provides initial pre-stabilization, followed by a fiber delay line interferometer for further spectral purification. The stabilized laser achieves a 100-ms 1/f integrated linewidth of 28 Hz, a fundamental linewidth of 0.1 Hz, and frequency-noise suppression exceeding 80 dB at 30 Hz-1 kHz offset frequencies. Moreover, the scheme remains effective across the ECDL tuning range of 1520-1580 nm, hence offering both ultra-low phase noise and broad wavelength agility for precision metrology applications.