MOEMS technologies for high-power laser focusing
Precise Ablation in Endoscopic Laser Surgery
Endoscopic laser surgery (ELS) is a class of minimally invasive surgical procedures used for treating head and neck conditions, such as laryngeal cancer. The main characteristic of ELS is the use of a fiber laser tool, which allows performing incisions on difficult to reach surgical sites. This avoids the need for highly invasive open surgeries.
To achieve high precision, traditional laser systems use optics to focus the laser beam into a sub-millimeter laser spot. However, this is not the case with fiber laser tools, since the limited space of endoscopic systems makes difficult to integrate advanced optics. In fiber laser tools, the fiber tip is placed very close to the tissue to be ablated to minimize the laser spot size. Nonetheless, due to beam divergence, the laser spot is slightly larger than the core of the fiber. This reduces the precision and quality of the ablation, often leading to undesired thermal damage to surround tissue.
To avoid thermal damages and tissue carbonization, we seek to develop a fiber laser tool capable of performing focused ablation. The major challenge of this is ensuring that the laser spot will be focused on the target tissue while the surgeon manipulates the tool. Without the visual feedback of near-contact ablation, adjusting the distance between the tool and the tissue becomes difficult, making the operation of the tool less intuitive and increasing the mental workload of the surgeon. To solve these problems, we are developing a compact optical system capable of automatically adjusting its focal length to match the distance to the target tissue.
Concept of a compact optical system with adjustable focal length based on a MEMS varifocal mirror
To allow miniaturization and integration with an endoscopic laser tool, our focusing system is based on MOEMS (Micro-Opto-Electro-Mechanical Systems) devices, such as a varifocal mirror. A varifocal mirror is a thin membrane mirror, which can be mechanically deformed in order to change its curvature and consequently its focal length. Similar devices have been developed for imaging applications, but not for high-power lasers. Therefore, the key component of this project is the development of novel MOEMS devices specifically designed for high-power laser focusing.
MEMS Varifocal Mirror with Hydraulic Actuation
To achieve large focal length range and make the mirror compatible with the high-power laser, we used two main strategies. The first one was to create a large usable aperture (4mm) to allow expanding the diameter of the surgical laser to prevent damage to the mirror. The second one was to use hydraulic actuation to deform the mirror, since this actuation mechanism can provide large deflection even for mirrors with large aperture. Additionally, the hydraulic actuation system helps cooling the mirror under laser irradiation and does not create spherical aberration, improving the quality of the focusing.
Using MEMS fabrication technology, we were able to successfully develop our varifocal mirrors. Experimental results showed that the curvature of our mirror can be changed from -30m-1 to 30m-1 with 9000 Pa of actuation and that the settling time of the mirror can be lower than 200 ms. This allows us to control the focal length of the surgical laser in the range from 14 mm to 140 mm. Preliminary results with CO2 laser also showed that our mirrors are able to withstand a continuous wave beam of 3W without suffering thermal damage.
Prototype of MEMS varifocal mirror
Experimental characterization of the fabricated MEMS varifocal mirrors: deflection of the mirror and focal length of the system as a function of the applied hydraulic pressure
-  A. Geraldes, A. Jacassi, P. Fiorini and L. S. Mattos, "Large-Stroke Varifocal Mirror with Hydraulic Actuation for Endoscopic Laser Surgery," in 7th IEEE International Conference on Biomedical Robotics and Biomechatronics (BioRob), 2018.
-  A. Geraldes, P. Fiorini and L. S. Mattos, "Design and Fabrication of a Hydraulic Deformable Membrane Mirror for High-Power Laser Focusing," in International Conference on Optical MEMS and Nanophotonics (OMN), 2018.
-  A. Geraldes, L. Geretti, D. Bresolin, R. Muradore, P. Fiorini, L. S. Mattos and T. Villa, "Formal Verification of Medical CPS: a Laser Incision Case Study," ACM Transactions on Cyber-Physical Systems, vol. 2, no. 4, p. 35, 2018.