Abstract
Passively mode-locked (PML) quantum dot lasers (QDL) are promising pulse sources for time-critical applications, such as ultra-fast optical sampling and frequency comb generation. However, the widespread applicability of these devices is hindered by their large timing jitter (TJ). The most efficient TJ reduction techniques include hybrid mode-locking (HML) and optical feedback (OFB) [1]. Single-cavity OFB has already been extensively studied and has been shown to lead to significant TJ reductions. First investigations on dual-cavity OFB have also shown promising TJ improvement, as well as a suppression of OFB induced sidebands in the power spectrum as compared to single-cavity resonant OFB [2]. Resonant FB is achieved if the delay time is exactly an integer multiple of the pulse repetition period T [3]. However, the TJ, the repetition rate (RR) and ML dynamics of a PML laser subject to OFB critically depend on the strength and delay time of the OFB. Thus, in our work we experimentally and numerically investigate the RR and TJ dependence of a PML QDL subject to dual-cavity OFB as a function of both delay times. The two-section QDL is 3 mm long and emits an optical pulse train with a RR of 13.3 GHz and a pulse width in the order of 15 ps. OFB is realized by two external free-space cavities with delay times corresponding approximately to 60 · T and 120 · T (T = 1/RR = 75 ps) (Fig. 1). The length of both cavities can be fine-tuned with µm resolution. The individual OFB strengths are in the order of 5 % and can be adjusted by neutral density filters (NDF). For each measurement point the OFB is turned off and on to avoid hysteresis effects. Analysis is performed by a fast photo-diode together with an electrical spectrum analyzer and an optical auto-correlator. The numerical simulations base on the delay differential equations model presented in [3]. For the simulations, a 3 mm long PML two-section laser was considered with a pulse width in the order of 10 ps. The two cavity lengths amount to τ1 ≈ 120·T and τ2 ≈ 60·T (T = 76 ps). The FB ratio is 5 % for each cavity. Fig. 2 shows the RR as a function of the two delay times of the two OFB cavities in experiment. A two-fold periodical structure of the RR can be observed. The increase or decrease of the RR is attributed to a leading or trailing re-injection of the OFB pulse back into the laser cavity. In particular the RR is influenced and changed in the whole delay region.
© 2015 IEEE
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