Optical comb injection for optical demultiplexing and harmonic frequency locking

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Shortiss, Kevin
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University College Cork
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With the continued growth of internet traffic, new optical communication infrastructures capable of dramatically increasing network bandwidth are being considered. Optical superchannels consisting of densely packed channels will be required for future networks, which could potentially be implemented using optical frequency combs - optical sources which consists of a series of discrete, equally spaced frequency lines. Optical combs can increase the spectral efficiency of these superchannels by allowing the channels to be more densely packed, while simultaneously reducing the number of components required (decreasing the energy consumption), and simplifying the digital signal processing required. Despite these advantages, the trade-off between cost and performance must be favourable in order for optical combs to become feasible for use in future communication networks. Photonic integrated circuits integrate several components together on a single semiconductor chip. These photonic circuits reduce both the cost and power consumption of devices, and hence recent research has been focused on creating suitable on-chip coherent optical comb transmitters. This thesis investigates an approach which is being used to demultiplex narrowly spaced optical combs on a photonic integrated circuit. By injection locking a laser to one of the lines in the optical comb (i.e, forcing a laser to lase with the frequency of that comb line), the comb line can be amplified and demultiplexed. This work investigates the physics of these active demultiplexers, both experimentally and numerically. It is found that the optimal side mode suppression ratio is obtained when the ratio of the comb's power to the injected laser's power is small, which also indicates optimal performance occurs when the locking range of the injected laser is at its smallest. The relaxation oscillations of the injected laser affect how well the comb can be demultiplexed, and as a result better side mode suppression ratios can be achieved at larger comb spacings. Further, it is shown that the relaxation oscillations within the injected laser can become undamped due to the comb injection, and frequency lock to fractions of the optical comb spacing. The injected laser can even become locked at detunings between the comb lines, creating a new output optical comb through nonlinear processes. The above phenomena are investigated numerically using two dimensional maps, and it is found that Arnol'd tongues appear in the injected laser's locking map.
Integrated photonics , Optical injection , Optical combs , Optical filter , Laser dynamics
Shortiss, K. 2020. Optical comb injection for optical demultiplexing and harmonic frequency locking. PhD Thesis, University College Cork.