Compact Semiconductor-Based Laser Sources with Narrow Linewidth and High Output Power
Compact Semiconductor-Based Laser Sources with Narrow Linewidth and High Output Power

Author: Stefan Spießberger

Publisher: Cuvillier Verlag

Published: 2012-11-28

Total Pages: 140

ISBN-13: 3736942613

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This work presents a fully semiconductor-based laser source that meets the requirements of coherent optical communication links in space and further allows to benefit from the numerous advantages of semiconductor lasers. A hybrid master oscillator power amplifier (MOPA) concept enables narrow-linewidth high-power operation. The experimental part of this thesis describes how the linewidth of the laser sources is characterized with a beat note measurement technique. A novel heterodyne linewidth measurement setup with a weak frequency lock of the two lasers features a virtually unlimited resolution. The analysis of the beat note signal is carried out by evaluating radio frequency beat note spectra as well as frequency noise spectra. Further, the characteristics of DFB and DBR lasers optimized for narrow linewidth operation are compared in terms of spectral stability, linewidth, and performance in a master oscillator power amplifier system. DBR lasers with an overall length of 4mm and a front facet reflectivity of 30% feature an intrinsic linewidth of 2 kHz at 180mW and a FWHM linewidth as small as 180 kHz up to an output power of 180mW. An intrinsic linewidth of 2 kHz is, to our knowledge, the smallest linewidth that has been reported in the literature so far. The concept of a hybrid integrated MOPA has been developed and the assembly procedure and the characteristics of the micro-integrated MOPA system are presented. The MOPA is realized on a footprint of 10 x 5mm2. The MOPA system features an output power of more than 1W, a FWHM linewidth of 100 kHz and an intrinsic linewidth of 3.6 kHz. To our knowledge, this is the first micro-integrated MOPA concept that simultaneously features narrow linewidth and addresses the requirements of spaceborne laser sources like mechanical stability, compactness, and efficiency.

A compact mode-locked diode laser system for high precision frequency comparison experiments (Band 64)
A compact mode-locked diode laser system for high precision frequency comparison experiments (Band 64)

Author: Heike Christopher

Publisher: Cuvillier Verlag

Published: 2021-04-09

Total Pages: 206

ISBN-13: 3736963998

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Optical frequency combs (OFC) have revolutionized various applications in applied and fundamental sciences that rely on the determination of absolute optical frequencies and frequency differences. The latter requires only stabilization of the spectral distance between the individual comb lines of the OFC, allowing to tailor and reduce system complexity of the OFC generator (OFCG). One such application is the quantum test of the universality of free fall within the QUANTUS experimental series. Within the test, the rate of free fall of two atomic species, Rb and K, in micro-gravity will be compared. The aim of this thesis was the development of a highly compact, robust, and space-suitable diode laser-based OFCG with a mode-locked optical spectrum in the wavelength range around 780 nm. A diode laser-based OFCG was developed, which exceeds the requirements with a spectral bandwidth > 16 nm at 20 dBc, a comb line optical power > 650 nW (at 20 dBc), a pulse repetition rate of 3.4 GHz, and an RF linewidth of the free-running pulse repetition rate < 10 kHz. To realize a proof-of-concept demonstrator module, the diode laser-based OFCG was hybrid-integrated into a space-suitable technology platform that has been developed for future QUANTUS experiments. Proof of sufficient RF stability of the OFCG was provided by stabilizing the pulse repetition rate to an external RF reference. This resulted in a stabilized pulse repetition rate with an RF linewidth smaller than 1.4 Hz (resolution limited), thus exceeding the requirement. The developed diode laser-based OFCG represents an important step towards an improved comparison of the rate of free fall of Rb and K quantum gases within the QUANTUS experiments in micro-gravity.

Epitaxial Design Optimizations for Increased Efficiency in GaAs-Based High Power Diode Lasers
Epitaxial Design Optimizations for Increased Efficiency in GaAs-Based High Power Diode Lasers

Author: Thorben Kaul

Publisher: Cuvillier Verlag

Published: 2021-04-09

Total Pages: 136

ISBN-13: 3736963963

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This work presents progress in the root-cause analysis of power saturation mechanisms in continuous wave (CW) driven GaAs-based high-power broad area diode lasers operated at 935 nm. Target is to increase efficiency at high optical CW powers by epitaxial design. The novel extreme triple asymmetric (ETAS) design was developed and patented within this work to equip diode lasers that use an extremely thin p-waveguide with a high modal gain. An iterative variation of diode lasers employing ETAS designs was used to experimentally clarify the impact of modal gain on the temperature dependence of internal differential quantum efficiency (IDQE) and optical loss. High modal gain leads to increased free carrier absorption from the active region. However, less power saturation is observed, which must then be attributed to an improved temperature sensitivity of the IDQE. The effect of longitudinal spatial hole burning (LSHB) leads to above average non-linear carrier loss at the back facet of the device. At high CW currents the junction temperature rises. Therefore, not only the asymmetry of the carrier profile increases but also the average carrier density in order to compensate for the decreased material gain and increased threshold gain. This carrier non-pinning effect above threshold is found in this work to enhance the impact of LSHB already at low currents, leading to rapid degradation of IDQE with temperature. This finding puts LSHB into a new context for CW-driven devices as it emphasizes the importance of low carrier densities at threshold. The carrier density was effectively reduced by applying the novel ETAS design. This enabled diode lasers to be realized that show minimized degradation of IDQE with temperature and therefore improved performance in CW operation.

A deep ultraviolet laser light source by frequency doubling of GaN based external cavity diode laser radiation
A deep ultraviolet laser light source by frequency doubling of GaN based external cavity diode laser radiation

Author: Norman Ruhnke

Publisher: Cuvillier Verlag

Published: 2022-05-13

Total Pages: 130

ISBN-13: 373696613X

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A compact and portable laser light source emitting in the wavelength range between 210 nm and 230 nm would enable numerous applications outside of laboratory environments, such as sterilization and disinfection of medical equipment, water purification or gas and air analysis using absorption spectroscopy. Such a source is also highly attractive for the identification and quantification of proteins and biomolecules by means of laser-induced fluorescence or Raman spectroscopy. In this thesis, a novel concept to realize such a compact and portable laser light source with low power consumption and an emission around 222 nm is investigated. The developed concept is based on single-pass frequency doubling of a commercially available high-power GaN laser diode emitting in the blue spectral range. Due to the low frequency doubling conversion efficiencies in this wavelength range of about 10-4 W-1, a laser diode with high optical output power above 1 W is required as pump source. Moreover, it has to exhibit narrowband emission in the range of the acceptance bandwidth of the applied nonlinear BBO crystal. Since GaN-based high-power laser diodes typically show broad emission spectra of Δλ = 1…2 nm, stabilizing and narrowing their wavelength by using external wavelength-selective elements is investigated and presented for the first time. With the understanding for the novel concept gained in this work, a compact ultraviolet laser light source was realized. It has a power consumption of less than 10 W and is exceptionally robust due to its immoveable components. The demonstrated output power of 160 μW enables numerous industrial and everyday applications for which previous laser systems have been too complex and overly cost- and energy-intensive.

Numerical Modeling of Narrow-linewidth Quantum Dot Lasers
Numerical Modeling of Narrow-linewidth Quantum Dot Lasers

Author: Bjelica, Marko

Publisher: kassel university press GmbH

Published: 2017-01-01

Total Pages: 137

ISBN-13: 3737602840

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The quantization of the active laser medium has enabled numerous advances in fiber-optic communications, e.g., higher efficiency of laser diodes, higher modulation bandwidth, lower spectral linewidth of the emitted signal. In recent years the quantum dot lasers have demonstrated a strong potential to continue this trend, therefore, by progressing from standard quantum well to quantum dot designs, it can be expected that the quantum dot lasers will play an increasingly important role in future fiber-optic communications. The research work presented in this dissertation seeks to further develop the quantum dot laser designs and improve the understanding of complex operating conditions affecting the laser linewidth. This is achieved by developing a comprehensive laser simulator, that was applied to design and simulation of edge-emitting lasers and laser arrays. As a result, the optimized laser diodes have demonstrated a significantly lower linewidth compared to equivalent quantum well designs. Due to their narrow linewidth, the realized photonic devices can be a viable solution for high bit rate fiber-optic networks.

Development and analysis of diode laser ns-MOPA systems for high peak power application
Development and analysis of diode laser ns-MOPA systems for high peak power application

Author: Thi Nghiem Vu

Publisher: Cuvillier Verlag

Published: 2017-02-14

Total Pages: 138

ISBN-13: 3736984804

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This work aims at designing and characterizing diode laser based master oscillator power amplifier (MOPA) systems, which are targeted to be implemented into micro light detection and ranging (LIDAR) or differential absorption LIDAR (DIAL) systems for water vapor and aerosol detections. These light sources operate in the ns-pulse regime at a repetition rate of 25 kHz, leading to a resolution in the meter range in an altitude of 6 km. The monolithic MOPA, where Master Oscillator (MO) and Power Amplifier (PA) are integrated on one single chip, operates at 1064 nm wavelength. A peak power of 16.3 W with a pulse width of 3 ns was obtained. A spectral linewidth of about 150 pm and a side mode suppression ratio (SMSR) of 30 dB was observed. A ratio of 9% between the amplified spontaneous emission (ASE) and the laser was estimated. These spectral properties fulfill the requirements for aerosol detection. The hybrid MOPA systems have separate chips for MO and PA. Different hybrid MOPA systems provide a stabilized wavelength at 1064 nm, a tunable wavelength around 975 nm and a dual wavelength around 964 nm. They therefore enable to detect a well-defined absorption line, scan over absorption line and switch between on/off line in DIAL applications, respectively. Their spectral linewidth is below 10 pm, limited by the resolution of the spectrum analyzer. An SMSR of more than 50 dB for the MO and of more than 37 dB for the whole MOPA was reached. A ratio between ASE and laser below 1% was estimated. These spectral properties meet the requirements for water vapor absorption lines detection at atmospheric condition. Diode laser based MOPA systems were therefore proven to be potential light sources for micro-pulse-LIDAR systems – the basis for a new generation of ultra-compact, low-cost systems.

Analysis of Spatio-Temporal Phenomena in High-Brightness Diode Lasers using Numerical Simulations
Analysis of Spatio-Temporal Phenomena in High-Brightness Diode Lasers using Numerical Simulations

Author: Anissa Zeghuzi

Publisher: Cuvillier Verlag

Published: 2020-10-22

Total Pages: 176

ISBN-13: 3736962894

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Broad-area lasers are edge-emitting semiconductor lasers with a wide lateral emission aperture. This feature enables high output powers but also diminishes the lateral beam quality and results in their inherently non-stationary behavior. Research in the area is driven by application, and the main objective is to increase the brightness, which includes both output power and lateral beam quality. To understand the underlying spatio-temporal phenomena and to apply this knowledge in order to reduce costs for brightness optimization, a self-consistent simulation tool taking all essential processes into account is vital. Firstly, in this work a quasi-three-dimensional opto-electronic and thermal model is presented that describes essential qualitative characteristics of real devices well. Time-dependent traveling-wave equations are utilized to characterize the inherently non-stationary optical fields, which are coupled to dynamic rate equations for the excess carriers in the active region. This model is extended by an injection-current-density model to accurately include lateral current spreading and spatial hole burning. Furthermore, a temperature model is presented that includes short-time local heating near the active region as well as the formation of a stationary temperature profile. Secondly, the reasons of brightness degradation, i.e. the origins of power saturation and the spatially modulated field profile, are investigated. And lastly, designs that mitigate those effects limiting the lateral brightness under pulsed and continuous-wave operation are discussed. Amongst those designs a novel “chessboard laser” is presented that utilizes longitudinal-lateral gain-loss modulation and an additional phase tailoring to obtain a very low far-field divergence.

Broad-Area Laser Bars for 1 kW-Emission
Broad-Area Laser Bars for 1 kW-Emission

Author: Matthias M. Karow

Publisher: Cuvillier Verlag

Published: 2022-06-27

Total Pages: 143

ISBN-13: 3736966261

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ndustrial laser systems for material processing applications rely on the availability of highly efficient, high-brightness diode lasers. GaAs-based broad-area laser bars play a vital role in such applications as pump sources for high-beam-quality solid-state lasers and, increasingly, as direct processing tools. This work studies 940 nm-laser bars emitting 1 kW optical power at room temperature, identifying those physical mechanisms that are currently limiting electrical-to-optical conversion efficiency as well as lateral beam quality. In the process, several diagnostic studies on bars with varied lateral-longitudinal design were carried out. The effects of technological measures for performance optimization were analyzed, yielding a new benchmark in efficiency and lateral divergence. The studies into altered resonator lengths of 4 and 6 mm as well as fill factors between 69 and 87 % successfully reduce both the voltage dropping across the device and power saturation at high currents, enabling 66 % efficiency at the operation point. Concrete measures how to reach efficiencies ≥70 % are presented thereafter, showing that doubling the efficiency value of the first 1 kW-demonstration in 2007 – amounting to 35 % – is in near reach. Investigation of the beam quality bases on a herein proposed and realized concept, in which the far field is resolved for each individual bar emitter. In this way, it is possible to determine how far-field profiles vary along the bar width and how much these variations affect the overall bar far-field. Further, such effects specific to bar structures can be separated into non-thermal and thermal influences. The effect of mechanical chip deformation (bar smile) as well as neighboring-emitter interaction has been investigated for the first time in active kW-class devices, yielding a lateral divergence as low as 8.8° at the operation point.

Two-step MOVPE, in-situ etching and buried implantation: applications to the realization of GaAs laser diodes
Two-step MOVPE, in-situ etching and buried implantation: applications to the realization of GaAs laser diodes

Author: Pietro della Casa

Publisher: Cuvillier Verlag

Published: 2021-03-25

Total Pages: 250

ISBN-13: 3736963971

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This work is about two-step epitaxial growth using metalorganic vapor-phase epitaxy (MOVPE) for the realization of edge-emitting near-infrared laser diodes. The fabricated gallium arsenide-based devices fall into two categories: high-power lasers (watt range, multimodal) and tunable lasers (milliwatt range, monomodal). Common to both cases is that surface contamination – particularly that due to oxygen – needs to be removed before regrowth. Thus, in-situ etching with carbon tetrabromide (CBr4) is first studied. The experimental results include kinetic data, the effects of different etching conditions as well as substrate characteristics, and the effectiveness in reducing surface contamination. These investigations pave the way to devices based on 2-step epitaxy combined with in-situ etching. Correspondingly, thermally-tuned SG-DBR lasers operating around 975 nm have been successfully realized, obtaining a tuning range of 21 nm. In addition, the possibility of using electronic tuning in similar devices has been explored. High-power broad-area lasers have also been realized, using two-step epitaxy combined with ex-situ and in-situ etching, to create a buried, shallow “mesa” containing the active zone. This approach allows introducing lateral electrical and optical confinement, and – simultaneously – non-absorbing mirrors at the laser facets. Additionally, a different strategy to create a buried current aperture is presented, which is based on ion implantation followed by epitaxial regrowth. This enables to improve device performance and simultaneously introduce non-absorbing mirrors at the facets with correspondingly increased reliability.

AlN base layers for UV LEDs
AlN base layers for UV LEDs

Author: Sebastian Walde

Publisher: Cuvillier Verlag

Published: 2021-06-22

Total Pages: 156

ISBN-13: 373696451X

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To enable the fabrication of high performance ultraviolet (UV) light-emitting diodes (LEDs) this work aims at improving the quality of AlN base layers on sapphire substrates. The main issues for UV LEDs are still a limited internal quantum efficiency due to a high amount of threading dislocations along with a limited light extraction efficiency due to total internal reflection at the AlN/sapphire interface. Therefore, high-temperature annealing of AlN/sapphire layers and growth on nanopatterned sapphire substrates were comprehensively investigated. High-temperature annealing was applied to AlN layers of different strain and thickness grown by metalorganic vapour phase epitaxy (MOVPE). The threading dislocation density could be successfully reduced by more than one order of magnitude down to 6 × 108 cm-2. Wave optical simulations of UV LEDs on nanopatterned sapphire substrates (NPSS) were conducted and showed a potential increase in light extraction efficiency compared to a planar substrate. The optimized MOVPE growth process on sapphire nanopillars and sapphire nanoholes resulted in a fully coalesced and atomically smooth AlN surface. The threading dislocation density was reduced to 1 ×109 cm-2 for AlN on both nanopillars and nanoholes. UVC LEDs emitting at 265 nm wavelength were grown on top of the developed templates. Increased internal efficiency was obtained by reduced dislocation density and more efficient light extraction was achieved on NPSS in case of a transparent heterostructure and reflective contacts. Thus, the developed templates yield considerable improvement in light output compared to conventional templates.