Asymmetric emission spectrum of an injection laser with an external optical feedback

A study was made of the feasibility of reducing the width of the emission spectrum of a single-frequency injection laser of the ILPN-202 type (λ = 1.3 μm) by establishing an external optical feedback. The temperature of the laser was stabilized near 18 °C to within 5 X 10 ^–3 °C and the current was kept constant to within 10 μA. The emission spectrum was recorded using a Fabry–Perot scanning interferometer. The experimental results were interpreted on the basis of a theory allowing for the amplitude–phase coupling. Criteria of stable single-frequency emission in the presence of an external optical feedback were established. The following quantities were determined: the amplitude–phase coupling constant α = 4 ± 0.5; the width of the emission line of an isolated injection laser Δν₀ = 60 MHz obtained when the injection current exceeded the threshold by the factor J/J_th = 2; the coherence length ofthe radiation of an isolated laser (236 cm) in the case when J/J_th = 1.3. An estimate was obtained of the width of a laser line in the presence of an external optical feedback and the value obtained was Δν = 600 kHz. The spectrum of an optical signal frequency modulated at 30 MHz was determined when the depth of modulation ofthe pump current was less than 1%. The slope ofthe modulation characteristic of the injection laser in the presence of an optical feedback was 200 MHz/mA.