InP-based semiconductor lasers with novel sampled Bragg gratings and applications

Semiconductor diode lasers have a wide variety of applications in optical communications, spectroscopy and imaging. This thesis reports on work to produce novel semiconductor diode lasers, and expand their application areas. Semiconductor lasers designed and fabricated in this work are based on quantum well structures operating at 1550 nm. Three main findings are reported in this thesis. The first finding is about distributed feedback semiconductor lasers and laser arrays based on novel sampled Bragg grating structures. These semiconductor lasers have enhanced effective coupling coefficients compared to distributed feedback lasers based on the conventional sampled Bragg grating structure. The effective coupling coefficient of the conventional sampled Bragg grating is only about 0.32 times of a uniform Bragg grating. However, based on novel sampled Bragg grating structures, the effective coupling coefficients can be up to 0.9 times of a uniform Bragg grating. An eight-wavelength distributed feedback semiconductor laser array is reported, which has a precise wavelength separation of 100 GHz at 1550 nm. 10 mW of output optical power was achieved by a single diode laser. The second finding regards a novel dual-mode semiconductor diode laser based on a single cavity. The dual wavelengths of this laser are lasing stably with a wavelength separation of 4.45 nm. Based on this dual-mode diode laser, a THz frequency of 560 GHz is generated based on the photomixing technique. The third finding of this thesis is about THz repetition frequency mode-locked semiconductor lasers. The mode-locked semiconductor lasers are based on novel sampled grating distributed Bragg reflector structures. Compared to the mode-locked laser based on the conventional sampled grating distributed Bragg reflector structure, the effective coupling coefficient is increased by more than a factor of three in the new mode-locked laser. Optical pulses at repetition frequencies of 620 GHz and 1 THz are generated based on the mode-locked diode laser. Design, fabrication and characterisation of these semiconductor lasers are introduced in detail in this thesis

Distributed feedback lasers and integrated laser arrays for wavelength-division multiplexing systems

Distributed Feedback (DFB) lasers and integrated laser arrays are of great importance in Wavelength-Division Multiplexing (WDM) systems in fiber optic communication systems. High-performance, low-cost DFB lasers and laser arrays are highly desirable for applications in intra-datacenter transport and in local access networks. This dissertation is focused on the design, fabrication and achievement of high-performance, low-cost DFB Lasers and Integrated DFB Arrays for WDM Systems. It investigates the use of a novel sampled grating approach, called the equivalent phase shift method, to achieve integrated DFB laser arrays with single-mode lasing at uniformly-spaced and precisely-positioned wavelengths. First, laterally-Coupled DFB (LC-DFB) lasers with first-order sidewall gratings are realized, with gratings fabricated by optical interference lithography instead of e-beam. Then, LC-DFB lasers and LC-DFB laser arrays with sampled gratings and equivalent phase shifts are proposed, numerically analyzed and experimentally demonstrated. Each LC-DFB laser with an equivalent quarter-wave phase shift is shown to lase at the pre-specified wavelength in a single longitudinal mode, with good side-mode suppression ratio (SMSR) over a very wide range of injection currents. Integrated LC-DFB laser arrays with five uniformly-spaced wavelength channels are demonstrated, in close agreement with the design. For better performance, buried heterostructure (BH)-DFB laser and laser arrays are also demonstrated using the same sampled-grating technology. A 6-wavelenth laser array with a 300 μm cavity length and a 8-wavelength laser array with 250 μm cavity length are successively demonstrated, each showing precisely positioned lasing wavelengths, good SMSR, and uniformly good lasing characteristics under a wide range of operating currents and temperatures. Finally, it is demonstrated that the wavelength of a monolithic WDM laser array can be continuously tuned over a very wide wavelength range of nearly 40 nm. The proposed method offers a practical and cost-effective solution for the manufacture of high-performance, monolithic multi-wavelength DFB laser arrays as well as widely wavelength-tunable DFB lasers for integrated WDM systems.Electrical and Computer Engineerin

Single- and Dual-Frequency Laser Diodes based on Surface Gratings

The thesis covers the development of single and dual longitudinal mode distributed feedback (DFB) lasers with surface gratings. These gratings were studied since they enable regrowth-free fabrication and easy implementation of arbitrary longitudinal effective refractive index variations. Advanced transverse and longitudinal surface grating structures were modeled, simulated, and experimentally tested. Procedures for optimizing the laser structures across a wide range of parameter variations were developed. The main targets were narrow linewidth emission for optical pumping of Rubidium atomic clocks, increased direct amplitude modulation for optical communications, and photonic generation of widely tunable electrical signals in the millimeter wave band for the next generation of wireless communications.Laterally-coupled ridge-waveguide (LC-RWG) surface gratings with alternating lateral protrusions were developed to circumvent the etching aspect ratio limitation that prevents the fabrication of low-order short-wavelength surface gratings. The single longitudinal mode DFB lasers with alternating LC-RWG gratings emitting around 780 nm demonstrated improved performances, particularly important for the space borne target applications.The dual longitudinal mode emission was achieved by periodically placing phase shifts along the grating. The experiments showed that the direct amplitude modulation bandwidth is extended when the photon-photon resonance associated with dual-mode emission is properly placed with respect to the carrier-photon resonance and when both resonances are adequately damped. Difference frequency range controlled by structural variations and 40 GHz difference frequency tuning by bias were demonstrated, as well as high speed difference frequency modulation. Linearly apodized LC-RWG surface gratings enabled a lower threshold current, a higher output power, and a broader range of difference frequency tunability by bias. Also the apodization and the complex-coupling of the surface gratings enabled the use of higher facet reflectivities, leading to narrower intrinsic short timescale linewidths. The achieved performance improvements indicate that the studied laser types are promising solutions for their target applications

Laser micromachining of active and passive photonic integrated circuits

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, February 2004.Includes bibliographical references (leaves 149-158).This thesis describes the development of advanced laser resonators and applications of laser-induced micromachining for photonic circuit fabrication. Two major advantages of laser-induced micromachining are direct patterning and writing on large areas of substrates at high speed following the exposure of laser light, without using complicated photomask steps. For passive photonic devices fabrication, a novel femtosecond laser with unprecedented low repetition rates of 4 MHz is demonstrated to generate high intensity pulses, as high as 1.25 MW with 100 nJ pulse energies and 80 fs pulse durations directly from this laser resonator, without using any active devices or amplifiers. These high intensity pulses are applied to transparent glass materials to demonstrate micromachining of waveguides, gratings, couplers, and three dimensional waveguides and their beam couplings. Active and passive semiconductor devices can be monolithically integrated by employing high energy laser pulses to locally disorder quantum well regions. The 45 nm bandgap shifts at 1.55 ptm with a standard Q-switched Nd:YAG laser at 535 nm are realized. Finally, unidirectional semiconductor ring lasers for high-density integration are developed as a potential application to photonic integrated circuits. Hybrid semiconductor S-crossover and retro-reflected ring lasers, as prototypes for unidirectional operation, are built and result in up to 21.5 dB and 24.5 dB of counter-mode suppression ratio, respectively, which is in good agreement with theoretical predictions.by Seong-Ho Cho.Ph.D

Laser Micromachining of Active and Passive Photonic Integrated Circuits

Thesis Supervisor: Rajeev J. Ram Title: ProfessorThis thesis describes the development of advanced laser resonators and applications of laserinduced micromachining for photonic circuit fabrication. Two major advantages of laserinduced micromachining are direct patterning and writing on large areas of substrates at high speed following the exposure of laser light, without using complicated photomask steps. For passive photonic devices fabrication, a novel femtosecond laser with unprecedented low repetition rates of 4 MHz is demonstrated to generate high intensity pulses, as high as 1.25 MW with 100 nJ pulse energies and 80 fs pulse durations directly from this laser resonator, without using any active devices or amplifiers. These high intensity pulses are applied to transparent glass materials to demonstrate micromachining of waveguides, gratings, couplers, and three dimensional waveguides and their beam couplings. Active and passive semiconductor devices can be monolithically integrated by employing high energy laser pulses to locally disorder quantum well regions. The 45 nm bandgap shifts at 1.55 µm with a standard Q-switched Nd:YAG laser at 535 nm are realized. Finally, unidirectional semiconductor ring lasers for high-density integration are developed as a potential application to photonic integrated circuits. Hybrid semiconductor S-crossover and retroreflected ring lasers, as prototypes for unidirectional operation, are built and result in up to 21.5 dB and 24.5 dB of counter-mode suppression ratio, respectively, which is in good agreement with theoretical predictions

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

3D mapping of nanoscale physical properties of VCSEL devices

There is clear lack of methods that allows studies of the nanoscale structure of the VCSEL devices1 that are mainly focused on the roughness of the DBR, or using FIB cross-sectioning and TEM analysis of failed devices to observe the mechanism of the degradation. Here we present a recently developed advanced approach that combines Ar-ion nano-cross-sectioning with material sensitive SPM2 to reveal the internal structure of the VCSEL across the whole stack of top and bottom DBR including active area. We report for the first time the direct observation of local mechanical properties, electric potential and conductance through the 3D VCSEL stack. In order to achieve this, we use beam exit cross-section polishing that creates an oblique section with sub-nm surface roughness through the whole VCSEL structure that is fully suitable for the subsequent cross-sectional SPM (xSPM) studies. We used three different SPM measurement modes – nanomechanical local elastic moduli mapping via Ultrasonic Force Microscopy (UFM) 3, surface potential mapping via Kelvin Probe Force Microscopy (KPFM) and mapping of injected current (local conductivity) via Scanning Spreading Resistance Microscopy (SSRM). xSPM allowed to observe the resulting geometry of the whole device, including active cavity multiple quantum wells (MQW), to obtain profiles of differential doping of the DBR stack, profile of electric potential in the active cavity, and spatial variation of current injection in the individual QW in MQW area. Moreover, by applying forward bias to the VCSEL to initiate laser emission, we were able to observe distribution of the potential in the working regime, paving the way to understanding the 3D current flow in the complete device. Finally, we use finite element modelling (FEM) that confirm the experimental results that of the measurements of the local doping profiles and charge distribution in the active area of the VCSEL around the oxide current confinement aperture. While we show that the new xSPM methodology allowed advanced in-situ studies of VCSELs, it establishes a highly efficient characterisation platform for much broader area of compound semiconductor materials and devices. REFERENCES. 1. D. T. Mathes, R. Hull, K. Choquette, K. Geib, A. Allerman, J. Guenter, B. Hawkins and B. Hawthorne, in Vertical-Cavity Surface-Emitting Lasers Vii, edited by C. Lei and S. P. Kilcoyne (2003), Vol. 4994, pp. 67-82. 2. A. J. Robson, I. Grishin, R. J. Young, A. M. Sanchez, O. V. Kolosov and M. Hayne, Acs Applied Materials & Interfaces 5 (8), 3241-3245 (2013). 3. J. L. Bosse, P. D. Tovee, B. D. Huey and O. V. Kolosov, Journal of Applied Physics 115 (14), 144304 (2014)

Optical Communication

Optical communication is very much useful in telecommunication systems, data processing and networking. It consists of a transmitter that encodes a message into an optical signal, a channel that carries the signal to its desired destination, and a receiver that reproduces the message from the received optical signal. It presents up to date results on communication systems, along with the explanations of their relevance, from leading researchers in this field. The chapters cover general concepts of optical communication, components, systems, networks, signal processing and MIMO systems. In recent years, optical components and other enhanced signal processing functions are also considered in depth for optical communications systems. The researcher has also concentrated on optical devices, networking, signal processing, and MIMO systems and other enhanced functions for optical communication. This book is targeted at research, development and design engineers from the teams in manufacturing industry, academia and telecommunication industries