Pulse shortening of gain switched single mode semiconductor lasers using a variable delay interferometer

We propose a pulse shaping and shortening technique for pulses generated from gain switched single mode semiconductor lasers, based on a Mach Zehnder interferometer with variable delay. The spectral and temporal characteristics of the pulses obtained with the proposed technique are investigated with numerical simulations. Experiments are performed with a Distributed Feedback laser and a Vertical Cavity Surface Emitting Laser, emitting at 1.5 µm, obtaining pulse duration reduction of 25-30%. The main asset of the proposed technique is that it can be applied to different devices and pulses, taking advantage of the flexibility of the gain switching technique

Short pulse generation from semiconductor lasers: characterization, modeling and applications

Esta Tesis describe el trabajo de investigación realizado sobre pulsos ópticos generados por láseres de semiconductor con duración de decenas de picosegundos. El trabajo se ha enfocado sobre diodos láser de cavidad vertical a 1550 nm, transmisores ópticos muy prometedores por sus ventajosas características en el entorno de las comunicaciones ópticas en fibra con modulación directa. El elevado ancho de banda previsto para las futuras redes ópticas requiere el conocimiento detallado de las propiedades eléctricas de los transmisores, así como de las propiedades de los pulsos generados por éstos que transportan la información binaria. En la presente Tesis, se describen los diferentes logros alcanzados en la caracterización de dicho tipo de láseres y de los pulsos generados por ellos, así como sus aplicaciones en un entorno de comunicaciones ópticas. Se han caracterizado láseres de cavidad vertical con emisión a 1550 nm, basados en pozos cuánticos y unión túnel, a través de medi¬das estáticas y dinámicas de la impedancia eléctrica y de la respuesta en modulación entre 0 y 10 GHz. Se han modelado los parásitos eléctricos y el circuito eléctrico equivalente de la estructura del dispositivo teniendo en cuanta los efectos de captura y escape de los portadores en los pozos cuánticos. Se han calculado los parámetros intrínsecos de los dispositivos a partir de las medidas realizadas. Se han empleado los dispositivos así caracterizados para la generación de pulsos con duraciones tan pequeñas como 55 ps a diferentes frecuencias de repetición, utilizando la técnica de conmutación de ganancia. Se ha medido la duración, la amplitud del pico, el "jitter" y el ancho espectral de los pulsos en función de los parámetros de conmutación de ganancia. Se ha investigado el efecto de la inyección óptica sobre los pulsos generados con dichos láseres de cavidad vertical conmutados en ganancia, obteniéndose una reducción del "jitter" en un amplio rango de los parámetros de inyección. Se ha diseñado e implementado un codificador de acceso múltiple por división de códigos ópticos (OCDMA) basado en líneas ópticas de retardo que emplea los pulsos generados por los dispositivos conmutados en ganancia. Finalmente, se ha propuesto y demostrado una novedosa implementación de la técnica de reconstrucción de fase por medio de la diferenciación óptica ultra rápida (PROUD), para la caracterización en amplitud y fase de pulsos ópticos. El diferenciador óptico necesario para la técnica ha sido realizado con un interferómetro birrefringente basado en fibra mantenedora de la polarización. Se ha medido la variación instantánea de la frecuencia ("time resolved chirp") de pulsos de diferente duración y forma, obtenidos con un laser conmutado en ganancia, y el factor de ensanchamiento de linea del laser utilizaso. Abstract This Thesis describes the research work that has been carried out on the generation of optical pulses, with duration of tens of picoseconds, from semiconductor lasers. The work is focused on 1550 nm Vertical Cavity Surface Emitting Lasers (VCSEL), which are promising optical transmitters due to their advantageous characteristics in the context of fiber optical communications with directly modulated sources. The high bandwidth expected for future optical networks requires the accurate knowledge of the transmitter electrical properties and of the laser generated optical pulses which carry the binary information. This Thesis describes the various achievements obtained in the characterization of these devices and the generated pulses, as well as their applications to an optical communications environment. VCSELs emitting at 1550 nm, based on quantum wells and tunnel junction, have been characterized by static and dynamic impedance measurements and modulation response between 0 and 10 GHz. The electrical parasitics and the equivalent circuit of the device have been modeled, taking into account the effects of capture and escape of carriers in quantum wells, and the laser intrinsic parameters have been calculated from the measurements. The VCSELs have been used for pulse generation using the gain switching technique, obtaining the shortest duration of 55 ps at different repetition frequencies. The duration, peak amplitude, jitter and spectral width of the pulses have been measured as a function of the gain switching conditions. The effect of optical injection on the pulses generated by gain switched VCSELs has been investigated, obtaining a jitter reduction over a wide range of injection parameters. An Optical Code Division Multiple Access (OCDMA) encoder based on optical delay lines has been designed and implemented, using the optical pulses generated by the gain switched devices. Finally, a novel implementation of the Phase Reconstruction using Optical Ultrafast Differentiation (PROUD) technique has been proposed and demonstrated for optical pulse characterization in amplitude and phase. The optical differentiator required in the PROUD technique has been realized with an interferometer based on birefringent polarization maintaining fiber. The instantaneous frequency, i.e. the time resolved chirp, of pulses with different durations and shapes obtained from a gain-switched laser, and the laser linewidth enhancement factor have been measured

Generación de pulsos ópticos a 1,5 μm mediante computación de ganancia en láseres de cavidad vertical

Optical pulses with duration below 60 ps were generated by gain-switching a Vertical Cavity Surface Emitting Laser (VCSEL) at 1535 nm. We used a radio frequency (RF) sinusoidal signal to modulate the device near threshold. Relaxation oscillations or pulse train generation were observed depending on the gain switching conditions. Repetition rates between 100 MHz and 2 GHz were tested. The dependence of the pulse width and amplitude was investigated in relation with the bias current and excitation frequency and amplitude. The optimal configuration of the current bias and RF modulating signal provided the shortest pulse duration of 57 ps at repetition rate of 800 MH

Estimación del error en la medida del factor de ensanchamiento de línea en láseres de semiconductor

En el presente trabajo se propone un método para la medida y la estimación del error de la misma en la caracterización del factor de ensanchamiento de línea (parámetro α) de los láseres de semiconductor. La técnica propuesta se basa en el cálculo del parámetro α a partir de la medida de la intensidad y de la frecuencia instantánea de los pulsos generados por un laser de semiconductor conmutado en ganancia. El error de medida se estima mediante la comparación entre el espectro medido y el reconstruido utilizando los perfiles temporales de amplitud y fase de los pulsos generados. El método se ha aplicado a un laser DFB, obteniendo un error de medida menor del 5 %

Web based interactive educational software introducing semiconductor laser dynamics: Sound Of Lasers (SOL)

In this work, educational software for intuitive understanding of the basic dynamic processes of semiconductor lasers is presented. The proposed tool is addressed to the students of optical communication courses, encouraging self consolidation of the subjects learned in lectures. The semiconductor laser model is based on the well known rate equations for the carrier density, photon density and optical phase. The direct modulation of the laser is considered with input parameters which can be selected by the user. Different options for the waveform, amplitude and frequency of thpoint. Simulation results are plotted for carrier density and output power versus time. Instantaneous frequency variations of the laser output are numerically shifted to the audible frequency range and sent to the computer loudspeakers. This results in an intuitive description of the “chirp” phenomenon due to amplitude-phase coupling, typical of directly modulated semiconductor lasers. In this way, the student can actually listen to the time resolved spectral content of the laser output. By changing the laser parameters and/or the modulation parameters,consequent variation of the laser output can be appreciated in intuitive manner. The proposed educational tool has been previously implemented by the same authors with locally executable software. In the present manuscript, we extend our previous work to a web based platform, offering improved distribution and allowing its use to the wide audience of the web

Sound of lasers (SOL) - An audiovisual approach to semiconductor laser dynamics

We present an educational software addressed to the students of optical communication courses, for a simple visualization of the basic dynamic processes of semiconductor lasers. The graphic interface allows the user to choose the laser and the modulation parameters and it plots the laser power output and instantaneous frequency versus time. Additionally, the optical frequency variations are numerically shifted into the audible frequency range in order to produce a sound wave from the computer loudspeakers. Using the proposed software, the student can simultaneously see and hear how the laser intensity and frequency change, depending on the modulation and device parameters

Self-validating technique for the measurement of the linewidth enhancement factor in semiconductor lasers

A new method for measuring the linewidth enhancement factor (α-parameter) of semiconductor lasers is proposed and discussed. The method itself provides an estimation of the measurement error, thus self-validating the entire procedure. The α-parameter is obtained from the temporal profile and the instantaneous frequency (chirp) of the pulses generated by gain switching. The time resolved chirp is measured with a polarization based optical differentiator. The accuracy of the obtained values of the α-parameter is estimated from the comparison between the directly measured pulse spectrum and the spectrum reconstructed from the chirp and the temporal profile of the pulse. The method is applied to a VCSEL and to a DFB laser emitting around 1550 nm at different temperatures, obtaining a measurement error lower than ± 8%

Characterization of Gain-Switched Pulses from 1.55 µm VCSEL

We report on short optical pulse generation by gain-switching (GS) a low-cost commercial vertical-cavity surface-emitting laser emitting at 1.55 μm. The dependence of pulse characteristics on GS parameters is investigated and analyzed. Pulses with duration of 55 ps and time-bandwidth product between 0.91 and 2.2 are obtained at repetition rates between 1 and 3 GHz

Temporally flat top pulse generation from gain switched semiconductor lasers based on a polarization interferometer with variable transfer function

We propose the use of a polarization based interferometer with variable transfer function for the generation of temporally flat top pulses from gain switched single mode semiconductor lasers. The main advantage of the presented technique is its flexibility in terms of input pulse characteristics, as pulse duration, spectral bandwidth and operating wavelength. Theoretical predictions and experimental demonstrations are presented and the proposed technique is applied to two different semiconductor laser sources emitting in the 1550 nm region. Flat top pulses are successfully obtained with input seed pulses with duration ranging from 40 ps to 100 ps