Monolithic passively mode-locked (PML) semiconductor lasers emitting at 1070 nm are promising ultra-fast sources for new prospective photonic transport systems in a 1 µm transmission window waveband [1,2]. As residual optical feedback (OFB) can deteriorate the emitted optical pulse trains, experimental studies exploring the immunity of the laser against perturbations by unintentional OFB with nanometric precision are demanded, but however yet lacking. We study experimentally the impact of sub-wavelength scale fine-delay controlled short-cavity OFB on the optical spectra, repetition rate (RR) and average optical output power of a monolithic PML semiconductor quantum-well (QW) laser. The laser emits picosecond short optical pulses at 1070 nm with a free-running RR of 13.6 GHz corresponding to an internal pulse round-trip time of 75 ps. A similar device has been investigated in . OFB is provided by a free-space external cavity exhibiting a macroscopic delay length Tmac ranging from 145 ps up to 275 ps round-trip time. A wedged glass on a high-precision linear translation stage provides delay with nanometric precision. We first study the optical pulsed emission in dependence of changing Tmac from 145 ps to 275 ps. The color-coded measured nonlinear auto-correlation (AC) signals and RRs are depicted in Fig. 1(a) and Fig. 1(b), respectively.
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