For this project, we attempted to construct a laser microjet, which is a 2D cutting machine that combines a high-power pulsed laser with an extremely thin (~50um diameter) water jet that precisely guides the laser beam by means of total internal reflection in a manner similar to conventional optical fibers. The laser microjet has several advantages over both waterjets and traditional laser cutters: 1. **Precision**: The laser microjet can achieve higher precision and much smaller kerf due to the thin waterjet that maintain the collimation of the laser, ensuring minimal divergence 2. **No Abrasive**: It does not require abrasives, reducing cost, wear, mess, and part contamination 3. **Heat Management**: The water jet cools the cutting zone, minimizing thermal damage, and reducing heat-affected zones, which is beneficial for cutting heat-sensitive materials 4. **Edge Quality**: Produces smoother and cleaner edges, reducing the need for secondary finishing processes ![[Pasted image 20240520122354.png]] source: Synova AG ## Design There are essentially two requirements for building a laser microjet: 1. Creating a thin jet of water that is laminar over a distance of several cm 2. Fiber-coupling in a high-powered pulsed laser beam directly into the waterjet orifice ### Hydraulics Fortunately, commercial waterjet orifices are designed to create laminar flow streams. They are designed using hard materials like ruby, sapphire, or diamond, to reduce wear over time. Due to the lack of abrasive, using such an orifice in a laser microjet should last essentially indefinitely (except if there's a clog due to impure water). ![[Pasted image 20240520123250.png]] source: DTI However, traditional waterjets use water intensifiers with maximum pressures as high as 100,000 psi. Since the water in a microjet is not actually performing any cutting, we utilize much lower pressures – essentially as little as necessary to create laminar flow through a small orifice. There is a trade-off between pressure and flow rate requirements for different orifice sizing. In small holes, viscous forces dominate over inertial ones. For flow through a small orifice to remain laminar, the Reynolds number must be low. Since the orifice diameter is small, maintaining laminar flow requires higher pressure to ensure the velocity and the kinematic viscosity of the fluid result in a low Reynolds number. A larger orifice can be used at lower pressure, but higher flow rates are required for laminar flow. We use an [A2 ruby orifice](https://accustream.com/collections/ruby-diamond-orifices-for-accustream/products/ruby-orifice?variant=39318913155119) with 0.002'' diameter from Accustream in conjunction with the "air liquid pump system components" water intensifier from Aliexpress, which claims 8700psi to 14500psi (600-1000bar) water pressure. ![[Pasted image 20240520123720.png]] The cutting head was designed by Vineet. It clamps a 0.5'' diameter, 0.5'' length quartz window at the top and the orifice from below, with a water inlet from the side. ![[Pasted image 20240520123858.png]] ### Optics ![[Pasted image 20240520130430.png]] The laser must be aligned with and focused on the orifice with very little margin for error. If the light is too intensely focused on the ruby, it will explode as a result of thermal shock. Any light that is not coupled into the orifice will be lost; in particularly bad scenarios, it could heat & boil the water leading to an explosion through the quartz window or other components. As mentioned, the orifice is 50um in diameter. With a nice achromatic doublet lens, we can achieve a spot size of ~20um. Therefore, we cannot be off by more than 15um in either X or Y alignment even given perfect tip/tilt alignment. Assuming perfect X/Y alignment, we may not be off by more than 0.001°. To achieve this level of alignment, we actively image the spot on the orifice with a beam-splitter, a long-focus lens and a camera system. Using the WL11050 window from Thorlabs with an AR coating for 700-1100nm, we should get ~1% reflectance of 650nm at 45° AOI. Since the guiding light for the JPT M7 MOPA laser is 5mW of 650nm, we should easily be able to see 500uW on the sensor when focused. ![[Pasted image 20240520130916.png]] The achromatic doublet I chose was the AC254-075 from Thorlabs. Being made of N-BK7, this adds additional difficulty due to the dispersion of the material – the true focal point of the high-powered IR laser will be *below* that of the red light focal point, meaning that to properly focus the cutting laser we need to slightly defocus the red guiding light. A SM1ZA fine-pitch translation mount assists with this task. ## Fabrication ### Hydraulics The first attempt at setting up the hydraulic component went pretty well! We were quite quickly able to establish a 50um laminar jet: ![[NoAudiOjet.mp4]] ### Optics ![[Pasted image 20240520132000.png]] ![[Pasted image 20240520131751.png]] ![[Pasted image 20240520131732.png]] ![[Pasted image 20240520131654.png]] ![[Pasted image 20240520131223.png]] ![[Pasted image 20240520131806.png]] ![[Pasted image 20240520131303.png]] # a little bonus ![[Wavemeter.mov]] ![[MOT2.mp4]]