Low-Noise Operation of Mid-Infrared Quantum Cascade Lasers Using Injection Locking

Quantum cascade lasers are the most promising optical source for emission in the mid-infrared and THz region, and they are already used in a large number of applications such as free-space communications, absorption spectroscopy, sensing and so on. In all these applications, the noise properties of the optical sources are critical for the system performance. In this work, the authors present a theoretical study on the intensity noise characteristics of quantum cascade lasers (QCLs) under external non-coherent optical injection. The injection locking has been proven in the past beneficial for noise properties of bipolar lasers, and thus this technique is utilized here in quantum cascade lasers. With the help of various analytical and numerical models, it is shown that intensity noise reduction can be achieved in the operation of the so-called locked slave laser compared to its free-running values. The detailed analysis reveals the contribution of the various noise sources to the intensity noise of the laser and how they affect the injection locking process. Using different numerical models, two distinct schemes are investigated, analysed and discussed, injection on the lasing mode or on non-lasing residual modes of the slave laser cavity

Numerical Analysis of Passively Mode-Locked Quantum-Dot Lasers With Absorber Section at the Low-Reflectivity Output Facet

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Study of all-optical wavelength conversion and regeneration subsystems for use in wavelength division multiplexing (WDM) telecommunication networks

In this thesis, we study all-optical processing techniques based on non-linear semiconductor optical amplifiers for the functionalities of second and third generation optical networks. In particular, devices for all-optical wavelength conversion with regenerative properties have been investigated. The configurations under investigation are based on the non-linear four-wave mixing process which arises when fields with proper spectral and power arrangement are coupled into the active medium. The performance characteristics of the devices are determined by the properties of the optical amplifier as well as by the operating conditions like the gain of the optical amplifier, the power levels and the frequency detuning of the pump and the information signal. In the first part of this thesis, the noise properties of the converted signal generated by the four-wave mixing based wavelength conversion process were investigated. The investigation of the noise characteristics was carried out by a theoretical analysis and the corresponding experimental confirmation, including the separate study of noise induced by the four wave mixing process itself and the amplified spontaneous emission noise from the amplifier. The study of the noise properties at the new wavelength was performed for the intensity as well as the phase noise of the converted signal. The useful conclusions from this study were used for the investigation of the regenerative properties of the mixing process. The second part of this thesis starts with the experimental investigation and the confirmation of the regenerative properties of the four wave mixing process in a semiconductor optical amplifier. The regeneration and simultaneous wavelength conversion of 2.5 Gbps optical signals is experimentally demonstrated. Furthermore, the second part includes the design and optimization by numerical simulation of a wavelength conversion system with regenerative properties, for use in nodes of wavelength division multiplexing optical networks. In this thesis, an alternative configuration for the pump and data signal is proposed for the first time, in order to obtain non-linear response from the optical amplifier. Based on this approach the new signal exhibits improved noise characteristics and lower bit error rate. The numerical investigation showed successful regenerative operation at 40 Gbps.Σε αυτή τη διατριβή μελετούμε νέες τεχνικές αμιγώς οπτικής επεξεργασίας σήματος για οπτικά δίκτυα δεύτερης και τρίτης γενιάς, οι οποίες βασίζονται σε μη γραμμικούς ημιαγωγικούς οπτικούς ενισχυτές. Συγκεκριμένα, διερευνήθηκαν διατάξεις οι οποίες επιτελούν λειτουργίες μετατροπής μήκους κύματος με αναγεννητικά χαρακτηριστικά. Οι διατάξεις εκμεταλλεύονται το μη γραμμικό φαινόμενο της μίξης τεσσάρων κυμάτων που εγείρεται εφόσον κύματα κατάλληλης ισχύος και φασματικής απόστασης εισαχθούν σε ενεργό μέσο. Οι επιδόσεις και οι ιδιότητες των διατάξεων επεξεργασίας σήματος καθορίζονται από τα χαρακτηριστικά των οπτικών ενισχυτών, καθώς και από τις συνθήκες λειτουργίας όπως η απολαβή του οπτικού ενισχυτή, η ισχύς και η φασματική απόσταση των κυμάτων άντλησης και πληροφορίας που χρησιμοποιούνται. Στο πρώτο μέρος της εργασίας αυτής παρέχεται μια θεωρητική ανάλυση και η πειραματική επιβεβαίωση των ιδιοτήτων θορύβου της μετατροπής μήκους κύματος μέσω της διαδικασίας της μίξης τεσσάρων κυμάτων. Μελετώνται ξεχωριστά η επίδραση της διαδικασίας της μίξης τεσσάρων κυμάτων και του θορύβου αυθόρμητης εκπομπής από τον οπτικό ενισχυτή στα χαρακτηριστικά θορύβου του σήματος στο νέο μήκος κύματος, για δύο περιπτώσεις. Η πρώτη περίπτωση αφορά τον θόρυβο πλάτους και η δεύτερη τον θόρυβο φάσης του σήματος στην έξοδο. Από την μελέτη εξάγονται χρήσιμα συμπεράσματα τα οποία χρησιμοποιούνται για την διερεύνηση των αναγεννητικών χαρακτηριστικών της διαδικασίας της μίξης. Στο δεύτερο μέρος της εργασίας αυτής πραγματοποιείται η πειραματική επιβεβαίωση των αναγεννητικών χαρακτηριστικών της διαδικασίας της μείξης τεσσάρων κυμάτων σε οπτικό ημιαγωγικό ενισχυτή. Επιτυγχάνεται πειραματικά η αναγέννηση διαμορφωμένων οπτικών σημάτων ρυθμού 2.5 Gbps με ταυτόχρονη μετατροπή μήκους κύματος. Στην συνέχεια πραγματοποιείται η σχεδίαση και η βελτιστοποίηση μέσω αριθμητικής προσομοίωσης ενός συστήματος μετατροπής μήκους κύματος με ιδιότητες αναγέννησης διαμορφωμένων οπτικών σημάτων, για χρήση σε κόμβους δικτύων πολυπλεξίας μήκους κύματος. Στην εργασία αυτή προτείνεται για πρώτη φορά μια εναλλακτική διάταξη των κυμάτων άντλησης και πληροφορίας για την εκμετάλλευση της μη γραμμικής απόκρισης του οπτικού ενισχυτή και προκειμένου το νέο κύμα να φέρει βελτιωμένα χαρακτηριστικά θορύβου και άρα μικρότερο ρυθμό σφαλμάτων. Η θεωρητική διερεύνηση έδειξε επιτυχή λειτουργία σε ρυθμούς μέχρι 40 Gbps

Compact optical displacement sensing by detection of microwave signals generated from a monolithic passively mode-locked laser under feedback

A monolithic passively mode-locked laser is proposed as a compact optical sensor for displacements and vibrations of a reflecting object. The sensing principle relies on the change of the laser repetition frequency that is induced by optical feedback from the object under measurement. It has been previously observed that, when a semiconductor passively mode locked laser receives a sufficient level of optical feedback from an external reflecting surface it exhibits a repetition frequency that is no more determined by the mode-locking rule of the free-running operation but is imposed by the length of the external cavity. Therefore measurement of the resulting laser repetition frequency under self-injection permits the accurate and straightforward determination of the relative position of the reflecting object. The system has an inherent wireless capability since the repetition rate of the laser can be wirelessly detected by means of a simple antenna which captures the microwave signal generated by the saturable absorber and is emitted through the wiring of the laser. The sensor setup is very simple as it requires few optical components besides the laser itself. Furthermore, the deduction of the relative position of the reflecting object is straightforward and does not require any processing of the detected signal. The proposed sensor has a theoretical sub-wavelength resolution and its performance depends on the RF linewidth of the laser and the resolution of the repetition frequency measurement. Other physical parameters that induce phase changes of the external cavity could also be quantified

Pulse width narrowing due to dual ground state emission in quantum dot passively mode locked lasers

We present an experimental investigation of the emission properties of a multisection InGaAs quantum dot passively mode locked laser under dual waveband emission from the ground state (GS). A mode locking regime directly related to the GS splitting has been depicted. It is related to significant pulse width decrease with increasing injection current under dual peak emission from the GS, leading to generation of pulses with increased peak power with respect to the usual device operation. (C) 2010 American Institute of Physics. [doi:10.1063/1.3432076

Microring based devices for telecommunication applications

Semiconductor microring resonators are excellent fundamental building blocks for the development of opto-electronic integrated circuits (OEICs), due to their large third-order nonlinearities enhanced by the resonance effect and compactness which leads to monolithic integration capability. A wide range of potential applications is presented and analyzed and the applicability of microring based devices as low-cost fundamental units in access/metro optical networks is discussed

All-optical regeneration based on pump-depletion effect in fiber parametric amplification

A novel all-optical regeneration approach based on fiber-optic parametric amplification operating under the strong pump depletion regime is numerically demonstrated. The key point for the regenerator’s operation is related to the fact that a high-power intensity modulated signal depletes the continuous wave pump in such a manner that the latter becomes intensity modulated and inverted compared to the signal. Using a cascade of two such devices a non-inverted, either wavelength converted or not, amplitude modulated output, which exhibits regenerative characteristics with respect to the input signal can be provided. Extended numerical simulations have been carried out on one hand to estimate the transfer characteristics of the regenerator and on the other hand to assess the cascadability performance on a typical 40Gb/s transmission system. Through the latter analysis very promising results have been derived proving the potential of the proposed device for all-optical regeneration in cascaded operation for both return to zero and non-return to zero data transmitted along 1Mm of optical fiber at 40Gb/s