The goal of this work is to find a stable injection lock setup with over 100 mW of output power. There are two diodes characterized for this aim while injection locking them. One is available at Thorlabs with a center wavelength of 660 nm and up to 120 mW of output power in the free-running state. The other diode manufactured by Ushio is not yet readily available on the market but has a center wavelength of 675 nm with up to 0,25 W of output power and can be directly operated at 671 nm by lowering the temperature. There are concerns that this temperature would be below the dew point, especially in summer.
This causes condensation on the diode from the water vapor in the air, explaining why this diode is tested only via the second approach. In Section 2 a short introduction into semiconductor laser physics is explaining the working principle of laser diodes. Additionally, the injection locking theory and the Gaussian beam model are discussed. The experimental setup and instrument control are described in Section 3. This is followed by the characterization of the Thorlabs diode with a center wavelength of 660 nm in Section 4 and the Ushio diode with a center wavelength of 675 nm at appropriate temperatures in Section 5. An outlook on further improving the setup by actively stabilizing the injection lock is given in Section 6. In cold atom experiments for laser cooling and trapping of atoms, laser powers of more than 100 mW are often required.
Furthermore, magneto-optical traps, while loading, are in need of sufficient power to reduce the number of atoms lost on the way. The output power of an external cavity diode laser does not produce enough output power for such applications. Alternatively, tapered amplifiers are a way to produce sufficient output power. These are currently not available for the targeted laser cooling wavelength for lithium of 671nm. However, injection locking of free-running, high power diodes is an alternative to tapered amplifiers for delivering enough output power. This is a technique to produce higher output power at a stable frequency. To achieve this, a weak signal of typically a few mW of an external cavity diode "seed" laser is injected into a free-running "slave" diode, which is capable of higher output power.
Table of Contents
1 Introduction
2 Theory of laser injection locking
2.1 Semiconductor laser diodes
2.1.1 p-n junction
2.1.2 Fabry-Perot laser diode and interferometer
2.1.3 External cavity diode laser
2.2 Driven oscillator model for injection locking
2.3 Mode-matching with the Gaussian beam model
2.3.1 Properties of a Gaussian beam
2.3.2 Gaussian beam propagation using ABCD-matrices
3 Experimental setup
3.1 Setup with optical parts
3.2 Instrument control
4 Characterization of a 660 nm Thorlabs diode
4.1 Free-running laser diode
4.1.1 Wavelength tuning
4.1.2 Lasing threshold
4.2 Beam shaping
4.3 Demonstration of injection locking
4.3.1 Current dependency
4.3.2 Lasing threshold in injection locked state
5 Characterization of a 675 nm Ushio laser diode
5.1 Free running diode
5.1.1 Wavelength tuning
5.1.2 Lasing threshold
5.2 Beam shape and coupling efficiency
5.3 Demonstration of injection locking
5.3.1 Current dependency
5.3.2 Current-temperature stability maps
5.3.3 Lasing threshold in injection locked state and variation of seed power
5.3.4 Minimizing seed power
6 Active stabilization of injection locking
7 Conclusion
Research Objectives and Core Topics
The primary goal of this research is to achieve a stable injection lock configuration capable of delivering over 100 mW of output power. By characterizing two specific high-power laser diodes, the work evaluates their performance in an injection-locked setup, focusing on stable frequency generation and spatial mode-matching for laser cooling applications.
- Theoretical modeling of injection locking and Gaussian beam propagation.
- Experimental validation of laser diode characterization at varying currents and temperatures.
- Optimization of fiber coupling efficiency and beam shaping techniques.
- Active stabilization strategies to maintain long-term injection lock reliability.
Excerpt from the Book
2.2 Driven oscillator model for injection locking
In injection locking, the injected laser beam from the master laser locks the phase and frequency of the slave laser diode. It can be thought of as similar to an external cavity diode laser in which the output is constrained to a certain frequency. The model of a driven oscillator can be applied to the slave laser diode.
The following discussion is based on [Sie86] and was inititally done for a ring laser to separate the input and output beam easily. Additionally, this can be expanded to a different laser setup in which light is injected into a laser diode.
The parameters for the oscillator, in this case the slave laser diode itself, are: the unconstrainted frequency of the slave laser diode ω0 and the intensity of the output beam I0. The driving frequency of the injected signal associated with the seed laser is ω1 and the intensity I1. The phase-amplitude equations for the time-varying amplitude of the cavity signal and the time varying phase inside the cavity are given by
Summary of Chapters
1 Introduction: Provides the motivation for high-power lasers in cold atom experiments and outlines the research objective of utilizing injection-locked diodes.
2 Theory of laser injection locking: Covers the physical principles of semiconductor laser diodes, the driven oscillator model for injection locking, and Gaussian beam theory.
3 Experimental setup: Details the optical layout of the injection lock, including fiber coupling, Faraday isolators, and the automated instrument control system.
4 Characterization of a 660 nm Thorlabs diode: Presents the experimental results for the Thorlabs laser diode regarding its free-running properties, beam shaping, and injection locking performance.
5 Characterization of a 675 nm Ushio laser diode: Describes the testing and characterization of the high-power Ushio diode, including current-temperature stability maps and the impact of seed power.
6 Active stabilization of injection locking: Outlines the implementation of a lock servo system designed to maintain stable injection locking over extended durations.
7 Conclusion: Summarizes the effectiveness of the injection-locked diode systems and discusses the scalability of the setup for future cold atom research.
Keywords
Injection locking, laser diodes, cold atom experiments, semiconductor physics, Gaussian beam, Fabry-Perot cavity, frequency stabilization, mode-matching, beam shaping, output power, spectral purity, current modulation, temperature control, laser cooling, active stabilization.
Frequently Asked Questions
What is the fundamental goal of this thesis?
The thesis aims to develop a stable injection lock setup capable of producing over 100 mW of output power, which is necessary for cooling and trapping atoms in cold atom experiments.
What are the central research themes?
The central themes include the theory of driven oscillators in lasers, the experimental characterization of high-power semiconductor diodes, and the optimization of fiber coupling and beam shaping for stable light injection.
What is the primary research question?
The work seeks to determine whether high-power, free-running diodes can be stabilized through injection locking to provide narrow-linewidth, high-power output suitable for atomic physics applications.
Which scientific methods were employed?
The research uses experimental characterization techniques including Fabry-Perot cavity analysis, automated data acquisition of power and spectral properties, and the implementation of active lock servo algorithms.
What does the main part of the thesis cover?
The main body focuses on the theoretical background, the design and construction of the experimental optical setup, and the detailed characterization of Thorlabs and Ushio laser diodes.
Which keywords best characterize this work?
Key terms include injection locking, laser cooling, high-power semiconductor diodes, Gaussian beam model, spectral purity, and frequency stabilization.
How does seed power affect the injection lock quality?
Increased seed power generally improves the injection lock interval and enhances stability over time, reducing the sensitivity to mechanical or thermal disturbances.
What role does the Fabry-Perot cavity play in this study?
The Fabry-Perot cavity serves as the primary diagnostic tool for monitoring the frequency spectrum, confirming single-mode operation, and measuring the spectral purity of the locked laser.
Why is active stabilization necessary?
Active stabilization helps mitigate frequency and current drifts caused by thermal or mechanical variations, allowing for reliable long-term performance even with lower injected seed power.
- Arbeit zitieren
- Anonym (Autor:in), 2021, Laser injection locking of lithium diodes. An experimental approach with a 660 nm thorlabs diode and a 675 nm Ushio laser diode, München, GRIN Verlag, https://www.grin.com/document/1145748
