Injection locking characteristics of indium arsenide quantum dash lasers

The study of injection locking characteristics was performed on an InAs Quantum Dash (QDash) semiconductor laser for the first time. The linewidth enhancement factor(α-parameter) of a QDash laser was measured using an injection locking technique that takes advantage of the asymmetry of the injection range. Studies were performed as functions of injecesed photon density, wavelength, and output power. To understand the behavior of the α-parameter versus wavelength, the Hakki-Paoli method, a technique that utilized the below threshold amplified spontaneous emission spectrum, was used to measure the modal gain over 1550 nm to 1573 nm. The α-parameter was found to have changed dramatically with power, indicating a large nonlinear gain coefficient, ε. Using a curve fit of the α versus power curve taken from the injection locking data, ε was measured to be 1.4*10-14 cm3, 1000 times larger than the typical ε of quantum well lasers, changing the dynamics of the laser. The small α-parameter and giant ε dramatically change the dynamics of the laser. To study the effects of the small α-parameter and giant ε further, an operational map was created using an Agilent Technologies High Resolution Spectrometer (HRS) with a resolution of 1 MHz. The new operational map of the InAs QDash laser has features never before seen with other devices, such as the avoidance of coherence collapse with optical feedback

State-of-the-art InAs/GaAs quantum dot material for optical telecommunication

This thesis reports on the characterization of the state-of-the-art In(Ga)As/GaAs quantum dot (QD) material grown by molecular beam epitaxy for optical telecommunication applications. A wide variety of characterization methods are employed to investigate the material properties and characteristics of a number of QD-based devices enabling future device optimization. The motivation that prompted this study was predicated mainly upon two technological advantages. First, that the QDs gain spectra exhibits a symmetric gain shape and thus the change of refractive index with respect to gain is negligible at the lasing wavelength. This is therefore expected to result in a zero or a very small linewidth enhancement factor (LEF), which is desirable for instance, for high-speed modulation purposes where frequency chirp under modulation, which is directly proportional to the LEF, may be substantially reduced. Second, the fact that not only QDs exhibit a damped frequency response attributed to the carrier relaxation dynamics but also as the resilience of a laser to optical feedback is inversely proportional to the fourth power of the LEF, QD lasers are expected to demonstrate a relatively higher feedback insensitivity. This bodes well for operating these devices isolator free, which would be greatly cost-effective. The absorption and gain spectra of the QD active material are investigated in chapters 2 and 3, respectively. The LEF of QD lasers at a range of temperatures is studied in chapter 3, which confirms the expectation for the first time for In(Ga)As/GaAs QD lasers from -10 oC to 85 oC. Subsequently, the findings of chapters 2 and 3 are employed in chapter 4 with an electro absorption modulator device in mind which would be able to operate with chirp control. In chapter 5, the modulation response of QD lasers is investigated through examining the relative intensity noise (RIN) spectra in the electrical domain. The resilience of the devices to optical feedback is subsequently studied through the RIN characteristics at a range of temperatures. Chapter 6 provides a summary of the thesis findings and possible future works that may be carried out as continuation to this project, which fell outside of the remit of this work

Quantum Well and Quantum Dot broadband Optical Devices for Optical Coherence Tomography Applications

In this thesis quantum well (QW) and quantum dot (QD) based devices are investigated with the aim of obtaining broad bandwidth light sources for optical coherence tomography (OCT) applications. QD based structures have many possible advantages for broadband applications due to their inhomogeneous broadening. However, more investigation is required in order to fulfill this potential. Firstly, in chapter one, an introduction to the fundamental principles of semiconductor heterostructures is provided followed by basic concepts of OCT. The experimental techniques used in this thesis are outlined and briefly discussed. Brief reviews of the gain measurement techniques which have been used throughout this thesis are presented. Free carrier effects have been highlighted as a source of line-width broadening in QD structures. However, to date the effects of free carriers have mostly been experimentally determined at comparatively high carrier densities. In chapter 2 I develop a model for the gain and spontaneous emission spectra of QD active elements and show that not only are free carrier effects important at high QD occupancies, but also at much lower carrier densities where QD lasers would normally operate. Furthermore, it is shown that the choice of carrier distribution function is far less important than was previously thought in describing the experimentally observed gain and spontaneous emission spectra. The literature has suggested that incorporating QW layers in hybrid QW/QD structures changes the behaviour of the QDs. Optical pumping of the QD active element by emission from the QW active element is investigated experimentally in chapter 3. Analysis of a QD laser, a hybrid QW/QD super luminescence diode (SLD) and mesa diodes with different active element designs show that emission from the quantum well layer does indeed modify the QD spontaneous emission, suggesting optical pumping of the QD states and the prospect for enhanced gain from the QD ground-state. Finally in chapter 4, different configurations of swept light sources (SLSs) are implemented with the aim of obtaining broader spectral bandwidth. It is demonstrated that increasing the gain of the QD-SOA is important in enhancing the sweep range. The use of complimentary SOAs is then explored. InP QW-SOAs and GaAs based QD-SOAs have overlapping gain and SE spectra which is utilised in a swept source laser (SSL) and filtered ASE configuration SLS. The results suggest that such sources may be able to achieve ~220nm sweep bandwidths. Chapter 5 summarizes the whole thesis and provides an overview of future work

Studies of (GaAI)As injection lasers operating with an optical fiber resonator

The characteristics of an optical fiber external resonator in conjunction with (GaAl)As stripe geometry lasers are described. We have observed a 6–10% reduction in the threshold current and have obtained 150 ps pulses at gigahertz repetition rates. The fiber resonator has also been used to quench self‐pulsations in a (GaAl)As injection laser. In order to explain many of our results we have used a model that uses the conventional semiconductor rate equations modified by the addition of saturable electron traps and the effects of the external cavity. Our results predict many of the self‐locking effects observed in injection lasers operating in an external cavity. Furthermore, the degree of self‐locking will be a strong function of the external cavity length and the density of saturable absorbers

Measurement and analysis of temperature-dependent optical modal gain in single-layer InAs/InP(100) quantum-dot amplifiers in the 1.6- to 1.8-µm wavelength range

In this paper, measurements and analysis of the small-signal net modal gain of single-layer InAs/InP(100) quantum-dot (QD) optical amplifiers are presented. The amplifiers use only a single layer of InAs QDs on top of a thin InAs quantum well. The devices have been fabricated using a layer stack that is compatible with active–passive integration scheme, which makes further integration possible. The measurement results show sufficient optical gain in the amplifiers and can thus be used in applications such as lasers for long-wavelength optical coherence tomography and gas detection. The temperature dependence of the modal gain is also characterized. An existing rate-equation model was adapted and has been applied to analyze the measured gain spectra. The current injection efficiency has been introduced in the model to obtain a good fit with the measurement. It is found that only a small portion ($sim$1.7%) of the injected carriers is actually captured by the QDs. The temperature dependence of several parameters describing the QDs is also discovered. The mechanisms causing the blue shift of peak gain as the current density increases and the temperature changes are analyzed and discussed in detail

Mode-locking and frequency mixing at THz pulse repetition rates in a sampled-grating DBR mode-locked laser

We report a sampled grating distributed Bragg reflector (SGDBR) laser with two different gratings which mode-lock independently at respective pulse repetition frequencies of 640 and 700 GHz. The device operates in distinct regimes depending on the bias conditions, with stable pulse trains observed at 640 GHz, 700 GHz, the mean repetition frequency of 666 GHz, and the sum frequency of 1.34 THz (due to nonlinear mixing). Performance is consistent and highly reproducible with exceptional stability observed over wide ranges of drive bias conditions. Furthermore, a monolithically integrated semiconductor optical amplifier is used to amplify the pulse trains, providing an average output power of 46 mW at 666 GHz

Development of Advanced GaAs Based Quantum Dot Devices

This thesis details research on the development of ~1.3μm quantum dot (QD) devices. QD devices which are theoretically ideal for the realisation of temperature insensitive lasers. A method to measure the recombination coefficients in a semiconductor laser is developed, and the role of Auger recombination in the realisation of temperature insensitive lasers is discussed. Moreover, due to a broad spectral linewidth and strong state-filling effects, QD structures are promising for application as broadband light sources. It is reported that the Auger recombination coefficient decreases with increasing device temperature, as measured by several complicated experimental techniques. In chapter 2, a simple analysis method (small signal modulation) to measure all of the recombination coefficients is introduced and discussed. In chapter 3, experimental data based on the small signal modulation technique is analysed. Which shows that all of the recombination coefficients, including the Auger coefficient, are a function of temperature and modulation doping in QD lasers. Following on from chapter 3, in chapter 4 the dynamic characteristic (differential carrier lifetime) of a 3μm-ridge QD laser device fabricated from commercial QD material is investigated. The modelled GS peak gain as a function of current density is determined based on the recombination coefficients, the random population model and the measured gain (via the iv Hakki-Paoli method). Then, by comparing the modelled GS gain to the experimental results, the carrier thermal escape parameter is determined. Finally in chapter 4, the variation of the Auger coefficient is explored to investigate the possibility of a temperature independent current density. The selective intermixing technique can be used in order to achieve broadband light source devices. In chapter 5, the intermixing method is introduced based on both quantum well and quantum dot structures. Then, a number of different capping materials on samples with different active region structures are discussed based on photoluminescence measurements from intermixed structures. The potential for selective area intermixing of an integrated device with a TiO2 and SiO2 cap annealed on a p-doped sample is demonstrated at the end of the chapter. Finally, in chapter 6, two integrated devices are fabricated based on this TiO2 and SiO2 cap. These devices demonstrate a broad emission bandwidth, and by applying a fast Fourier transform to the spectra in order to determine the point spread function of the instrument, and application of the Rayleigh criterion for resolution, an estimation of the resolution in an OCT system is made

Characterization of mid-infrared quantum cascade lasers

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2009.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (p. 97-99).Quantum cascade lasers provide some of the highest output powers available for light in the mid-infrared range (from 3 to 8 m). As many of their applications require portability, designs that have a high wall-plug efficiency are essential, and were designed and grown by others to achieve this goal. However, because a large fraction of these devices did not operate at all, very few of the standard laser measurements could be performed to determine their properties. Therefore, measurements needed to be performed that could non-destructively probe the behavior of QCLs while still providing useful information. This thesis explores these types of measurements, all of which fall into the category of device spectroscopy. Through polarization-dependent transmission and photovoltaic spectroscopy, a large portion of the quantum mechanical bandstructure could be determined, along with many of the parameters characterizing crystal growth quality. In addition, high-resolution transmission spectroscopy was used to find the properties of the QCL waveguide. In order to find the correspondence between theory and experiment, bandstructure simulations were performed using a three-band p model, and two-dimensional electromagnetic simulations were performed to describe the laser's optical properties. These simulations were found to be in relatively good agreement with the device measurements, and any discrepancies were found to be consistent with problems in the growth and fabrication.by David Patrick Burghoff.S.M

Some issues relevant to propagation of lightwave signals in optical fibers

Fiber optics is a promising technology that can enable the high bit rates and long spans that are on increasing demand. Although the fiber bandwidth is as large as several terahertz, there are several phenomena, related to both intrinsic fiber properties and characteristics of the state-of-the-art transmitters and receivers, which seriously degrade the performance of fiber communication systems, imposing limits on the transmission bandwidths and distances that can be achieved. In this thesis, some of the issues affecting linear and nonlinear propagation in optical fiber will be theoretically and experimentally studied. Schemes for compensation of some of these phenomena or amelioration of their effects will be presented