The objective of this thesis project is the study of an innovative application of balanced detection to self-mixing interferometry. Thanks to the reintroduction of part of the laser radiation, reflected by a target into the cavity of emission, the laser becomes sensitive to the distance traveled by the beam and can be used as sensor of vibrations, distance and velocity. Firstly, applying the balanced detection and the differential reading of the signals, produced by the monitor photodiode of laser and an external detector (on which a beam partition is engraved), the correlation of noise levels, detected by the two photodiodes, is analyzed. After the discovery of a strong correlation of shot noise, the project focuses its interests on the application of balanced detection to improve SNR (with the result of an increase of about 12 dB) and obtain a greater resolution for interferometric measurements. In this horizon the reproducibility of the results is showed for different kinds of lasers, making balanced detection a general usable technique, and new prototypes have been developed which are increasingly advanced, compact, miniaturized, simple, immune to disturbances and – not a secondary aspect – at a lower cost of realization. These last features are very important to develop a versatile tool for various applications and of great interest to many companies in the measurement industry. In the final phase of the project, it was possible to produce a prototype in the laboratory with all of the aforementioned features: through its performance test, the remarkable filtering capacity of balanced detection against disturbances was further verified. Besides, by analyzing the noise reduction with balanced detection, the correlation indexes of 1/f and shot noises, superimposed on the interferometric signals detected by monitor and external photodiodes, were quantified. The demonstration of a correlation of at least 90% for 1 / f and above 80% for shot noise, in the spectrum part where they respectively dominate, is the successful conclusion of an intense work that is initiated by the design of circuits and their adaptation to the various laser sources, and then showed the excellent filtering properties of balanced detection against disturbances and noise. The consequent realization of a universal interferometer, which can be used for various self-mixing interferometry applications, is expected to have very positive prospects for the metering market thanks to the specific qualities and characteristics of the model, as better described above. The main structure of the thesis is summarized below: - in the first chapter the principles of laser operation, the self-mixing interferometry and the foundations of the optics necessary for project realization are examined; - in the second one is described in detail the technique of balanced detection and the main expectations of its use in self-mixing interferometry; - in the third one all the design choices of the circuits used for the various laser sources are illustrated and described; - in the fourth one the various phases and major problems that have been encountered and solved in the realization of the first balanced detection interferometer are reported. In addition, fringes measurements and spectral analysis of noise sources obtained through balanced detection are documented for two different laser sources (a DFB and a Fabry-Perot); - the fifth chapter describes the performance of an advanced prototype that uses a new position of the external photodiode, which is placed very close to the laser emission opening, this to simplify the structure of the system, avoiding the use of a beam splitter. Then, the final model made in the laboratory is presented, with the analysis of the correlation index of the various noises based on the part of spectrum in which they are dominant; - In the sixth section conclusions are exposed on the results obtained and the possibilities for applying the interferometer in future projects are proposed.
La seguente tesi tratta la realizzazione di interferometri che sfruttano un’innovativa applicazione della balanced detection alla tecnica di interferometria laser a retroiniezione. Quest’ultima si basa sulla reintroduzione nella cavità del laser di una parte della sua radiazione emessa e poi riflessa da un bersaglio: in tal modo il laser diventa sensibile alla distanza percorsa dal fascio riflesso e può assumere la funzione di sensore di vibrazioni, distanza e velocità. Mediante la rivelazione bilanciata, dalla lettura differenziale del fotodiodo di monitor interno al laser e di un detector esterno (sul quale viene fatta incidere una partizione del fascio laser), è stata analizzata la correlazione presente tra i fondi di rumore rivelati dai due detector. Dimostrata una elevata correlazione dello shot noise si è quindi applicata la tecnica di balanced detection per avere un SNR migliore (ottenendo un incremento di circa 12 dB) e, pertanto, misure interferometriche a maggiore risoluzione. Dopo aver documentato che le correlazioni dei fondi di rumore sono valide per varie tipologie di laser, verificando nel contempo l’applicabilità generale della tecnica, si è di seguito proceduto alla realizzazione di prototipi sempre più avanzati, compatti, miniaturizzati, semplici, immuni a disturbi e - aspetto invero non secondario - a minor costo di realizzazione. Si è cioè metodicamente ricercata e perseguita l’applicazione del modello in questione, con la maggiore versatilità possibile, ai diversi impieghi dell’interferometria a retroiniezione, rendendo così lo strumento appetibile per parecchie aziende del settore. Nella fase conclusiva del progetto si è riusciti a produrre in laboratorio un prototipo finale con tutte le caratteristiche sopra citate: attraverso il test delle sue prestazioni si è inoltre ulteriormente riscontrata la notevole capacità filtrante della balanced detection nei confronti dei disturbi. Mediante l’analisi della riduzione del fondo di rumore con la rilevazione bilanciata, si è quantificato l’indice di correlazione del rumore 1/f e dello shot noise sovrapposti ai segnali interferometrici rivelati dai fotodiodi di monitor del laser ed esterno. Dimostrata una correlazione di almeno il 90% per il rumore 1/f e superiore all’80% per lo shot noise, nella parte di spettro in cui rispettivamente essi dominano, si è di fatto concluso con successo un lavoro intenso che ha preso l’avvio dalla progettazione di circuiti e il loro adattamento alle varie sorgenti laser, per poi giungere alla verifica delle ottime proprietà filtranti della balanced detection nei confronti di rumori e disturbi. La consequenziale realizzazione di un interferometro universale, pertanto utilizzabile per varie applicazioni di interferometria a retroiniezione, si ritiene possa avere prospettive molto positive per il mercato dei misuratori, grazie anche alle specifiche qualità e caratteristiche del modello, come sopra meglio riportate. Si sintetizza di seguito la struttura portante della tesi: - nel primo capitolo sono esaminati i principi di funzionamento dei laser, l’interferometria a retroiniezione e i fondamenti di ottica necessari per la realizzazione progetto; - nel secondo è descritta nel dettaglio la tecnica della balanced detection e le principali aspettative di un suo impiego nell’interferometria a retroiniezione; - nel terzo sono illustrate e descritte tutte le scelte di progettazione dei circuiti impiegati per le varie sorgenti laser; - nel quarto sono riportate le diverse fasi e le principali problematiche, che sono state via via affrontate e risolte nella realizzazione del primo interferometro a rilevazione bilanciata. Inoltre vengono documentate le misure di frange e l’analisi spettrale e nel tempo dei fondi di rumore, ottenuti attraverso la balanced detection, per due diverse sorgenti laser (un DFB e un Fabry-Perot); - nel quinto sono progressivamente descritte le prestazioni di un prototipo avanzato che sfrutta un posizionamento del fotodiodo esterno rasente l’apertura di emissione del laser, tutto ciò per semplificarne la struttura, evitando l’uso di un beam splitter. E’ presentato inoltre il modello finale realizzato in laboratorio, con l’analisi conclusiva dell’indice di correlazione dei vari rumori in base alla parte di spettro in cui risultano dominanti; - nel sesto sono riportate le conclusioni sui risultati ottenuti e le possibilità di applicazione dell’interferometro in tal modo realizzato.
Rivelazione bilanciata per interferometria a retroiniezione
MENINI, GIACOMO
2016/2017
Abstract
The objective of this thesis project is the study of an innovative application of balanced detection to self-mixing interferometry. Thanks to the reintroduction of part of the laser radiation, reflected by a target into the cavity of emission, the laser becomes sensitive to the distance traveled by the beam and can be used as sensor of vibrations, distance and velocity. Firstly, applying the balanced detection and the differential reading of the signals, produced by the monitor photodiode of laser and an external detector (on which a beam partition is engraved), the correlation of noise levels, detected by the two photodiodes, is analyzed. After the discovery of a strong correlation of shot noise, the project focuses its interests on the application of balanced detection to improve SNR (with the result of an increase of about 12 dB) and obtain a greater resolution for interferometric measurements. In this horizon the reproducibility of the results is showed for different kinds of lasers, making balanced detection a general usable technique, and new prototypes have been developed which are increasingly advanced, compact, miniaturized, simple, immune to disturbances and – not a secondary aspect – at a lower cost of realization. These last features are very important to develop a versatile tool for various applications and of great interest to many companies in the measurement industry. In the final phase of the project, it was possible to produce a prototype in the laboratory with all of the aforementioned features: through its performance test, the remarkable filtering capacity of balanced detection against disturbances was further verified. Besides, by analyzing the noise reduction with balanced detection, the correlation indexes of 1/f and shot noises, superimposed on the interferometric signals detected by monitor and external photodiodes, were quantified. The demonstration of a correlation of at least 90% for 1 / f and above 80% for shot noise, in the spectrum part where they respectively dominate, is the successful conclusion of an intense work that is initiated by the design of circuits and their adaptation to the various laser sources, and then showed the excellent filtering properties of balanced detection against disturbances and noise. The consequent realization of a universal interferometer, which can be used for various self-mixing interferometry applications, is expected to have very positive prospects for the metering market thanks to the specific qualities and characteristics of the model, as better described above. The main structure of the thesis is summarized below: - in the first chapter the principles of laser operation, the self-mixing interferometry and the foundations of the optics necessary for project realization are examined; - in the second one is described in detail the technique of balanced detection and the main expectations of its use in self-mixing interferometry; - in the third one all the design choices of the circuits used for the various laser sources are illustrated and described; - in the fourth one the various phases and major problems that have been encountered and solved in the realization of the first balanced detection interferometer are reported. In addition, fringes measurements and spectral analysis of noise sources obtained through balanced detection are documented for two different laser sources (a DFB and a Fabry-Perot); - the fifth chapter describes the performance of an advanced prototype that uses a new position of the external photodiode, which is placed very close to the laser emission opening, this to simplify the structure of the system, avoiding the use of a beam splitter. Then, the final model made in the laboratory is presented, with the analysis of the correlation index of the various noises based on the part of spectrum in which they are dominant; - In the sixth section conclusions are exposed on the results obtained and the possibilities for applying the interferometer in future projects are proposed.| File | Dimensione | Formato | |
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Descrizione: Testo della tesi
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https://hdl.handle.net/10589/135079