1.5-$\mu$ m Frequency-Stabilized $\lambda/4$ -Shifted DFB LD Employing an External Fiber Ring Cavity With a Linewidth of 2.6 kHz and an RIN of $-$ 135 dB/Hz

We present a
¹³
C
₂
H
₂
frequency-stabilized λ/4-shifted distributed feedback laser diode (DFB LD) with an external fiber ring cavity. By controlling the λ/4-shifted DFB passband to track one Fabry-Pérot mode associated with the fiber ring cavity, the oscillation frequency was continuously tuned over 1.2 GHz without mode-hopping. The optical frequency was stabilized to a
¹³
C
₂
H
₂
absorption line with an external frequency modulation scheme that enabled us to realize an output beam without additional modulation. As a result, we obtained a frequency stability as high as 1.4 × 10
^-10
for τ = 1 s and 1.3 × 10
^-11
for τ = 100 s. In addition, we simultaneously achieved a linewidth of 2.6 kHz and a relative intensity noise of less than -135 dB/Hz.