Stabilisation of self mode-locked quantum dash semiconductor lasers
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Full Text E-thesis
Date
2018
Authors
Asghar, Haroon
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Publisher
University College Cork
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Abstract
Semiconductor mode-locked lasers are compact pulsed sources which produce high quality optical pulses with high repetition rates and subpicosecond pulse duration. In order to use these sources in real applications, low timing jitter and robust feedback control stabilisation is highly desirable. In this thesis, a series of experimental studies have been performed to achieve stabilisation of two-section self mode-locked quantum-dash laser emitting at ∼ 1.55 µm and operating at 21 GHz repetition rate. First, stabilisation of self mode-locked quantum-dash laser over a wide range of delay tuning was achieved using symmetric dual-loop feedback. Optimum levels were determined for narrowest RF linewidth and reduced timing jitter for single- and symmetric dual-loop feedback. Two symmetric dual-loop configurations, with balanced and unbalanced feedback ratios, were studied. We have demonstrated unbalanced symmetric dual-loop feedback, with the inner cavity resonant and fine delay tuning of the outer loop, produced narrowest RF linewidth and reduced timing jitter over a wide range of delay, unlike single and balanced symmetric dual-loop configurations. This configuration with feedback lengths 80 and 140 m reduced the RF linewidth by ∼ 4-67x (∼ 2-9x timing jitter reduction) and ∼ 10-100x (∼ 2.5-10x timing jitter reduction), respectively, across the widest delay range, compared to free-running. For symmetric dual-loop feedback, the influence of different power split ratios through the feedback loops was also determined. We achieved the optimum stabilisation of self mode-locked quantum-dash laser over a wide range of delay tuning using asymmetric dual-loop feedback. Various feedback schemes were investigated and feedback levels far narrowest RF linewidth and low timing jitter were identified, for single- and asymmetric dual-loop feedback. We demonstrated that asymmetric dual-loop feedback, with the shorter feedback cavity tuned to be fully resonant, followed by fine-tuning of the phase of the longer feedback cavity, gave stable narrow RF spectra across the widest delay range, unlike single-loop feedback and free-running conditions. This asymmetric dual-loop scheme reduced the RF linewidth ∼ 2.5-4x compared to single-loop and ∼ 4-100x relative to free-running conditions. In addition, for asymmetric dual-loop feedback, significant suppression in fundamental side-mode was achieved relative to single-loop feedback. In addition, we have demonstrated an asymmetric dual-loop feedback scheme to suppress external cavity side-modes induced in self mode-locked quantum-dash lasers with conventional single- and dual-loop feedback. We reported optimal suppression of spurious tones by optimising the delay in the second loop. We observed that asymmetric dual-loop feedback, with large (∼ 8x) disparity in loop lengths, produced significant suppression in external-cavity side-modes and yielded flat RF spectra close to the main peak with low timing jitter, compared to single-loop feedback. Significant reduction in RF linewidth and reduced timing jitter was also produced by optimising delay time in the second feedback loop. Experimental results based on this feedback configuration validate predictions of recently published numerical simulations. Finally, we reported stabilisation of our self mode-locked quantum-dash laser on the widest range of delay tuning using simultaneous continuous-wave optical injection and optical feedback. With optical injection, various wavelength detuning ranges (1568 to 1578 nm) and optimum wavelengths (1571.725 to 1572.710) were determined which yielded narrowest RF linewidth and reduced timing jitter. We demonstrated that under double resonance, with both optical feedback and continuous-wave injection, a minimum RF linewidth of < 1 kHz (instrument limited) was achieved which was 2x lower than external optical feedback and > 100x lower than the free-running condition.
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Keywords
Mode-locked lasers , Semiconductor lasers
Citation
Asghar, H. 2018. Stabilisation of self mode-locked quantum dash semiconductor lasers. PhD Thesis, University College Cork.