The cavity stability of solid lasers is one of the key indicators of their performance, directly affecting the quality, power stability and application reliability of the output beam. The stability of the laser cavity requires a comprehensive consideration of thermal, mechanical, optical and environmental factors.
1 Key Influencing Factors
01 Cavity Stress
- Thermal deformation and thermal stress : When the laser works, the optical cavity deforms due to heat absorption, resulting in optical path deviation or beam quality degradation. The different materials thermal expansion coefficient difference will also produce thermal stress, may damage the components.
- Residual stress : Under the thermal-mechanical coupling effect of machining, casting, welding and other processes, internal stress (also known as residual stress) exists in the material due to inhomogeneous plastic deformation, temperature field change and phase transformation. These stresses have a great impact on the machining accuracy, fatigue strength and service life of the cavity. During the long-term slow release process, the optical path will deviate due to the deformation of the cavity.
02 Environment and External Disturbances
- Cavity cleanliness : the cleanliness of the cavity surface, especially the attachment of contaminants (such as particles, oil, organic residues, etc.). This is a key factor in the performance, stability and lifetime of the laser because contaminants can affect beam quality, gas purity or cause damage to the components.
- Cavity sealing : Cavity Sealing includes structure sealing and cooling system sealing. If the cavity structure cannot be sealed well, leading to external pollutants and water vapor into the cavity, it will cause optical lens pollution, crystal deliquescence and other bad problems, which will directly lead to the laser not working properly.
- External interference : The stable operation of the laser is easily affected by external interference such as vibration and temperature change. When the temperature of the cavity is below the ambient dew point, it is easy to expose, and it needs to be prevented by thermal control or dehumidification systems to prevent water vapor condensation causing damage to the optical components. The rigidity of the mechanical structure of the cavity is not enough, which is easy to cause vibration transmission, leading to the device in the optical cavity to produce moving deformation, affecting the stable output of the laser.
2 Design Optimization Schemes
01 Material Selection
The cavity adopts a material with low thermal expansion coefficient (6061-T651 aluminum alloy) to reduce thermal deformation.
- 6061-T651 state aluminum plate on the basis of T6 increased pre-stretching treatment, effectively eliminating the internal stress, processing deformation tendency is smaller.
- High thermal conductivity (about 167 W/(m∙K)), can quickly disperse heat, reduce thermal deformation, ensure dimensional stability, maintain the accuracy of optical components.
- Tensile strength (about 240-300 MPa in T6 state) is sufficient to support conventional optical components, taking into account strength and cost.
- Easy to achieve high reflectivity by precision polishing or coating (such as 266nm band reflectivity > 90%), and simple alloy elements (Mg + Si), low light absorption.
△ Contrasting Material Properties Between 6061 and 7075 Aluminium Alloys
02 Stress Elimination
- The roughs are subjected to natural and artificial timing and vibration treatment to eliminate internal stresses in parts. The cavity is pre-processed using high-precision, high-stability machine tools and advanced machining processes, reducing the excess amount of various parts, reducing machining deformation in the after process, and heat treatment and stationary treatment are used after the pre-processing to release part of the mechanical stress.
- Using the difference in the thermal expansion coefficient of the material, the cavity is subjected to multiple high-temperature cycle treatments. This process causes the internal thermal and external stresses of the metal material to deformation, thereby improving its mechanical properties, stability and service life.
03 Structural Design
Mechanical Structure Design
- The use of a one-cavity design solution, with no splicing or forming surfaces in the cavity, can significantly improve the mechanical structural rigidity of the cavities and help to resist external interference such as vibration and temperature change. Due to the absence of seams, the formed surface is smooth and continuous, which is conducive to obtaining high-quality product surfaces and reducing the need for subsequent treatment; The overall structural design avoids leakage or wear problems that might occur at the joint, significantly improving the sealing and durability of the cavity.
- The optical-electromechanical design solution is adopted to reduce redundant components and reduce heat capacity. The internal structure of the cavity is divided into different functional areas, reducing the electromagnetic and optical stray light interference present in each functional area, and improving system integration and maintainability.
- Add a damping device at the base of the cavity to isolate the external vibration and avoid the concentrated stress when the cavity is fixed.
Cooling System Design
- The thermal field distribution is predicted through simulation simulation analysis and combined with experimental validation to optimize the thermal conduction path, reduce local overheating, and ensure that the heat dissipation scheme is compatible with the overall structure of the laser. By designing water-cooled channels within the side wall of the cavity, introducing an external cooling medium, the heat in the optical cavity is quickly removed while isolating the influence of the external environment on the ophthalmic cavity temperature, ensuring the stability of the laser working environment.
- The key heat sources (amplified crystal, laser collector, LD pump source, etc.) are thermally isolated from the whole cavity to reduce the influence of heat conduction on the stability of the optical path。The crystal module heat sink substrate adopts high thermal conductivity material, and the substrate surface is machined with micron-level flow channel by micro-manufacturing technology. The heat is taken away by forced convection heat transfer through high-speed cooling liquid (deionized water). The microchannel cooling heat transfer coefficient can reach 10000W/㎡∙K , which is more than 10 times that of traditional water cooling.
04 Environmental adaptability
- Effective cleaning : The cavity is cleaned by professional cleaning process to ensure that the surface cleanliness reaches the optical grade requirements and at the same time to avoid corrosion damage to the cavity surface.
- Cleaning and inspection : the cavity needs to be cleaned in a dust-free environment before being put into use， The cavity is cleaned by a dust removal device, and the surface cleanliness, particle count, ion residue and other key parameters are tested to ensure the cleanliness of the cavity and meet the requirements.
△ Cleaning and Inspection Steps
The stability of the laser cavity is the key factor of the stability of the laser. In the actual operation process, it is necessary to carry out comprehensive evaluation from the aspects of thermal management, structural strength, machinability, economy and optical performance. The above design method and operation technology are still being improved and iterated, aiming to continuously improve the quality and stability of the product.
Trailer For the Series
This article begins the High Reliability series on mechanical reliability. In the future, we will continue to extend and in-depth analyze key technical difficulties such as accurate regulation of the electric control system. Stay tuned.
