Abstract
This thesis introduces a novel external cavity tunable laser which is pivot-point-independent with a large mode-hop-free tuning range and fast tuning speed. The research focuses on the laser cavity design that achieves mode-hop-free tuning.Among the diverse categories of tunable lasers, external cavity tunable lasers demonstrate superior performance, offering an exceptional combination of a broad tuning range and narrow linewidth, which is unmatchable by other tunable laser configurations. The Littrow/Littman configuration, which employs a diffraction grating to achieve precise control of the lasing wavelength, is highly regarded for its superior tuning accuracy and stability. This approach continues to serve as a foundational methodology in the development of external cavity tunable lasers, underpinning advancements in both academic research and industrial applications. Nonetheless, external cavity lasers employing gratings as mode selectors are inherently limited by pivot-point-based mechanical movements, which hinder the advancement and practical utility of these tunable lasers. Therefore, the development and investigation of alternative grating-free tuning mechanisms for external cavity lasers are imperative.
Mode hopping is a phenomenon wherein a laser undergoes discontinuous transitions between distinct longitudinal modes within its gain spectrum. This behaviour is particularly undesirable during wavelength tuning, as it compromises the stability and precision of the laser output. Avoiding mode hopping is critical in high-precision applications such as spectroscopy, telecommunications, and interferometry, where consistent laser performance and spectral purity are of paramount importance. In order to prevent mode hopping during the tuning process, it is essential that the laser's output wavelength undergoes a continuous variation, with the central wavelength of the mode selector also adjusting in synchrony with the cavity wavelength. Grating-based external cavity tunable lasers are particularly effective in this regard, as they utilize mechanical movement around a pivot point to simultaneously modulate both the cavity length and the incident angle of the grating. Through a carefully engineered external cavity design, these adjustments ensure that both the cavity wavelength of the laser output and the reflection wavelength on the grating change in a continuous and synchronized manner. Meanwhile, rapid mode-hop-free tuning necessitates the continuous and repetitive mechanical movement upon the pivot point, while simultaneously ensuring that the pivot point retains adequate stability and mechanical precision under the repeated dynamic forces.
In this thesis, the etalon is employed as a mode selector in the laser system, and the narrow band filter is also employed to ensuring the single wavelength output. To avoid mode hopping, we need to find a cavity design that allows the cavity mode/frequency changes synchronously with the etalon mode/frequency. Since that the light path difference of the etalon is mathematically related to the wavelength in a cosine function dependent on the incident angle, it is essential to design an external cavity such that the cavity length also follows a similar cosine dependence on the same incident angle. This would enable the synchronization of both modes in the wavelength tuning operation. The cavity length calculation is performed using Mathematica, and Taylor Expansion is utilized to determine the rate of synchrony.
The first cavity design introduced in this thesis is corotating the periscope and the etalon together on a rotating stage which is pivot-point-free. This design was proposed (via patent) prior to the initiation of this research. However, it has yet to be experimentally validated. This thesis repeats the calculation of the cavity design and laser system buildup, conducts simulations and experiments based on the groundwork, and thereby validates the feasibility of this design. This thesis further explores the effects and influence of different types of etalons (air gap etalon and solid etalon) on the laser system's performance. Although the air gap etalon is theoretically capable of achieving near-perfect synchronous tuning, its practical implementation remains challenging. In contrast, while the solid etalon does not achieve perfect synchronization, it enables mode-hop-free tuning within a certain operational range. The research also investigates the impact of various parameters on the mode-hop-free tuning range of the laser system under consideration.
The improved cavity design (case 2) is based on fixed double periscope sets (right-angle mirror sets) and rotatable double-sided reflector. In this design, the only movable component is the double-sided reflector which need to reflect the laser on its both surfaces. The functions and positioning of each component have been comprehensively analysed and detailed. Furthermore, it has been mathematically validated that the wavelength tuning of this design is independent of the rotating mirror's pivot point, thus confirming its pivot-point-independence. Different positioning methods of the right-angle mirror sets will have varying effects on the tuning process. The tunability and capability are recorded and discussed in the simulations. This thesis further provides an analysis of the manufacturing and installation tolerances associated with the various components of the tunable laser, and offers detailed guidance on the assembly and experimental setup of the laser system.
Although the design and simulation of the tunable laser have been finalized, yielding highly favourable results, there has been insufficient time to assemble the laser system and conduct the subsequent experimental investigations. Regarding the cavity design based on the double periscope and a rotatable double-sided reflector, a CN patent application has already been submitted. Additionally, a further design concept (case 3) is discussed in the section on future work, with simulations demonstrating the feasibility of this design. While the performance of design case 3, as shown in the simulation results, does not offer the same level of mode-hop-free tuning range as design case 2, it suggests the potential for a larger tuning angle. This is because the change in the etalon's incident angle in design case 3 is half that in case 2, thus ensuring the etalon is less likely to exceed its "walk-off" limit compared to case 2. To further extend the tuning range of the tunable laser, an approach can be adopted where the rotation of the reflecting mirror and the etalon are independently controlled, ensuring synchronization between the cavity mode and the etalon mode. This requires the integration of an additional actuator specifically for controlling the etalon's rotation, in addition to the actuator responsible for the rotation of the reflecting mirror. Continuous rotation of the etalon is not necessary, and it should only be adjusted when mode hopping is imminent, realigning the etalon mode with the cavity mode. As a result, design case 3, which accommodates a larger angular limit for the rotating mirror, allows for the maximization of the laser system's tuning range. Moreover, a new laser system needs to be established to conduct additional experiments and to identify the optimal design configuration.
| Date of Award | 2025 |
|---|---|
| Original language | English |
| Supervisor | Kang Li (Supervisor) & Nigel Copner (Supervisor) |