"Trial and error." These two words sum up my 24 years developing and manufacturing specialty preforms and fibers where I operated an MCVD gas delivery system. Our team was constantly testing new ideas to improve fiber strength and yield as well as the reproducibility of our manufacturing process. In fact, I viewed my role there as “technical troubleshooter,” thanks to the extensive time spent troubleshooting problems to optimize the design and achieve optical properties in the manufacturing process.
If you operate an MCVD system, you’ve probably experienced your fair share of trial and error. As you know, sometimes a problem is a really tough nut to crack. Perhaps it’s a process issue that’s challenging to solve or a technique that’s difficult to master. I’ve compiled a few tips that, I hope, will help you optimize the design and enhance your manufacturing process. Don’t hesitate to contact me if you’d like to dig deeper into any of these quick tips.
Tip #1: Looking to purchase an MCVD? Start by comparing the 2 types of gas delivery systems
If you’re entering this market, you may be investigating the two types of systems available: stainless steel and Teflon/glass. Comparing the 2 systems is a fairly hefty subject, so here’s my quick tip: A good way to ensure high yields and reproducibility is to initially select the system that best meets your needs. A good resource is my in-depth article, which looks at the pros and cons of the two types of MCVD gas delivery systems. If you’d like more insight, I encourage you to call me. We can delve into the details and identify the system that supports your design goals. Read more: “Comparing the 2 types of MCVD Gas Delivery Systems.”
Tip #2: Try this approach for your tube-cleaning process
The tube-cleaning process is a critical part of preparation. I find that using the first cleaning step to remove metallic impurities works well. If you were to HF etch first, you may etch the glass around the metallic impurity, but not necessarily remove it. If you only HF etch, some metallic impurities may remain. Here is my approach to the tube-cleaning process:
- For the first step, aqua regia can be used (an acid mix of hydrochloric acid, nitric acid, and water). A typical acid cleaning cycle is 30-60 minutes with aqua regia to remove metallic impurities, followed by a DI water rinse. You may want to adjust the time depending on your process.
- The next step is to etch the glass surfaces, cleaning the inside and outside. Most MCVD operators use hydrofluoric acid. A 15% concentration for 30-60 minutes is a good starting point. You may want to adjust the time and concentration for your process.
- Follow that etch cycle with a final DI rinse and dry with nitrogen to prevent water spotting.
Tip #3: Leak test the rotary seal to ensure it is leak tight
Before you set up the preform, I recommend leak testing the rotary seal. I do this by flowing oxygen through the rotary seal. Start by plugging the end of the rotary seal with a solid quartz tube. Next, lower the regulated O2 pressure to ~ 5 psi, slightly above what the system will see during deposition. Observing the O2 carrier mass flow controller, you will see it go to zero if the system is leak tight. This procedure will leak test the oxygen carrier line throughout the gas system, from input to rotary seal. Taking time to conduct this leak test is worthwhile. If a leak is present in the rotary seal, you can potentially add moisture and/or lose part of the gas stream, which would reduce layer thickness and potentially refractive index. If the leak rate changes during the run, it can result in axial preform variability.
Tip #4: Modify your system with a high-quality rotary seal
It takes a lot of work to generate gas streams that are uniform and reproducible, with no variation. But if your rotary seal leaks, this can introduce moisture and potential corrosion, and you may see a higher OH peak in measured fibers. Common rotary seal designs, where the shaft slips into a housing and seals on the inside diameter of the rubber O-ring, typically wear quickly. This, of course, impacts the quality of the seal and may force frequent rebuilds to prevent contamination. A better solution is available. The rotary seal by SG Controls seals with the O-ring side against an adjustable glass pressure plate. In this design, there is extremely low wear on the O-ring. This is a reliable rotary seal that works with both types of gas delivery systems. Over the years, I have seen no corrosion issues and minimal leakage issues with this type of rotary seal.
Tip #5: Install a glass bubbler to observe chemical color and level
Over the years, I have found chemical visibility to be a real advantage. In a glass bubbler, you can confirm proper chemical level, confirm a uniform bubble stream, and view the chemical color. (A color change may indicate delivery system corrosion.) Glass bubblers offer these additional benefits:
- The bubble stream can be observed and frit plugging can be detected, rather than assuming all is well.
- POCl3 is not compatible with stainless steel for long durations. If you use this liquid and have a stainless steel gas delivery system, you may want to install a glass bubbler to create a hybrid system.
- On rare occasions, you may want to increase the chemical temperature in the bubblers above 40 degrees C. This practice can pull metal ions from stainless steel and into your bubble stream. Clearly, this can degrade the fiber’s optical properties and should be considered when choosing an MCVD system.
Tip #6: Implement automated calibration procedures for your mass flow controllers
Calibration is critical. Let’s say you think your flow rate is 200 CC per minute, but you have not calibrated the mass flow controllers. You manufacture a large quantity of preforms, and one of your mass flow controllers fails. You buy and install a new controller and set the flow for what you thought it was. However, it turns out your actual flow rate was not 200 CC per minute. The preform cannot be reproduced. You need to start over, experiment, and change flows until you achieve the desired optical properties. Meanwhile, time and materials are lost. Knowing your absolute flow rate at all times is critical. I recommend SG Controls’ automated calibration software interface to ensure a high level of assurance for this critical step in the process.
Tip #7: Keep the temperature low to prevent tube shrinkage
Keeping your deposition temperature as low as possible prevents tube shrinkage AND prevents water from moving toward the core, which will cause absorptions in the final fiber measurements. Adding dopants such as phosphorous and fluorine reduce the required deposition temperature.
Tip #8: Use a hand-held pyrometer calibrated with a blackbody standard to check deposition tube temperature accuracy
Deposition tube temperature is critical, which means your process pyrometer must be calibrated. To ensure accuracy, I use a handheld calibration standard that is independent of the process pyrometer. Look through the pyrometer viewfinder, moving through the hot zone until the maximum temperature is detected. This maximum temperature is the actual temperature. Compare this reading to the process pyrometer and check for calibration shifts. You do not need to purchase an expensive blackbody unit. Instead, purchase a hand-held standard that is calibrated with a blackbody standard. In addition, you could temporarily mount the hand-held pyrometer on the fire carriage to spot-check the process pyrometer calibration. Read more: “Critical Design Goals to Manufacture Optical Fiber Preforms.”
Tip #9: Avoid “undershoot/overshoot” during the lathe deposition process
It’s essential to program the PID to maintain tube temperature to the desired set-point while avoiding undershoot/overshoot. Here’s an example of an undershoot/overshoot oscillation. Let’s say you set the temperature at 1700 degrees C, but the PID is not optimized. When the temperature drops to 1699 degrees C, the PID increases hydrogen flow. The temperature climbs to 1710 degrees. The PID reacts by reducing hydrogen. Now the temperature plummets to 1690 degrees. The differential gap has widened, with severe temperature swings. A quality PID that is properly programmed will tightly control the deposition temperature along the entire preform length. To achieve consistency and reproducibility along the preform length, I recommend a tight control of +/-1 to 2 degrees C.
Tip #10: Identify the optimum H2/O2 level to help achieve straight preforms
Do you have difficulty achieving straight preforms? This is definitely an art and requires some practice. I have found procedures that deliver a reproducible technique. For example, setting the optimum H2/O2 ratio can prevent preform sagging and glass burn-off during collapse, which leads to variability later in the process.
Tip #11: Master tube straightening techniques
Operating an MCVD is a hands-on activity. This is particularly true for the initial deposition and final tube straightening processes. Over the years, I’ve developed very specific techniques to control the process – and the outcome. Here are some quick tips:
- During the initial deposition tube straightening step, it’s important to take time to properly form joints to prevent lost fabrication runs. If you have a thickened joint area, unfused oxides can condense, potentially plugging the exhaust tube and/or forming cracks.
- In the final tube straightening process, there is one final step, which I call stress relief. Some stresses may have developed during the straightening process. When your burner initially heats up at the process start point the tube could sag. To prevent this possibility I heat the headstock end very warm and look for tube sagging. If sagging occurs, the exhaust end is lightly heated while supporting the headstock end to eliminate runout.
- Read more tips on tube straightening: “Preparing to manufacture an optical fiber preform.”
Tip #12: Use this flame polish tip
Fluorine gas can be introduced to assist with the glass vaporization (internal surface layer removal) to provide a very clean surface for deposition. In addition to the oxygen flow for pressure control, fluorine gas will accelerate glass vaporization at high polish temperatures. This final preparation step will provide a pristine surface to prevent deposition issues, such as bubbles, and potential weak final fiber.
Tip #13: Implement this preform stretching/over-collapse technique to increase yield for erbium doped fiber
Solution doping adds erbium and other rare earths into a glass core. If the ratio of the doped core to the preform outside diameter is too large, undesirable optical properties will be obtained. Correcting the incorrect ratio will also yield more drawn fiber. Collapsing another glass tube onto the preform can correct the core-to-clad ratio. If more ratio change is needed and a larger diameter is not drawable, preform stretching can be used to reduce the core diameter, followed by an additional over-collapse. I wrote active Excel programs for technicians, which calculated the over-collapse and stretch diameters to achieve the required core-to-clad ratio to meet optical specifications in drawn fibers. This process can increase erbium fiber yield per preform by a factor of 5 or better.
Tip #14: Add MCVD options to optimize preform design, ensure reproducibility, and get high yields
During my MCVD years, I actively sought out various add-on control devices to improve our process and fiber quality. In fact, some features were specifically designed for my needs. Here’s a brief overview of recommended options. Read more: “MCVD options: These add-on features can enhance your optical fiber preform strength, yield, and reproducibility.”
- Multichannel gas drier and dew point monitor – Removes moisture from the input gas streams and includes a dew point monitor to track moisture levels.
- Automatic valve leak testing – Cycles each valve in the system in the open and closed position. This reduced our leak test time from 2 days of manual testing to 2 hours of automatic testing.
- Auto-calibration software interface – Gives you absolute clarity on the operation of your mass flow controllers.
- Atmospheric pressure compensation – Modifies the preform recipe to compensate for atmospheric pressure changes.
- Lathe enclosure – Offers a combination of positive pressure and filtered air to reduce particulates, which can cause weak spots in the drawn fiber.
- Supplementary isothermal bubbler heater – Adds heat to the bubbler chemical to compensate for cooling due to the bubbling effect, re-stabilizing the temperature in less than 5 minutes.
- Scanning pyrometer – Ensures the pyrometer is reading the maximum tube temperature to prevent overheating the deposition tube, which can reduce the efficiency of your depositions and potentially cause premature tube shrinkage.
- Diameter control – Gives you a greater ability to control deposition tube diameter from run to run, and within a run.
- Extended bed lathe with motorized tail stock – Allows you to stretch preforms to a smaller diameter, which can have a direct impact on fiber yields.
- Vertical stretch over jacket lathe – Eliminates the gravity issue that causes tubes to sag when you heat them and allows you to process longer preforms.
- Manual lathe control box – Allows you to manually control the burner temperature during setup.
- Automatic chemical refill system – Mostly takes the operator out of the process, which significantly reduces the chance for errors and introducing moisture into the system.
In summary …
Whether you currently have an MCVD gas delivery system or are looking to purchase one, I’m here to help you identify and install the equipment you need. In addition to being a “technical troubleshooter,” I view myself as an advocate for FOC customers. I’m here to help you get the precise system or add-on feature that supports your preform manufacturing goals.
A unique and particularly valuable service is Larry’s troubleshooting expertise to overcome the many preform fabrication and fiber draw issues manufacturers face when producing specialty optical fibers.
