Editor’s note: With the addition of Larry Donalds to the Fiber Optic Center team, we now offer preform fabrication consulting services to manufacturers of specialty optical preforms and fibers. You can look to Larry for guidance selecting the right MCVD fabrication system for your needs, post-installation troubleshooting to meet exacting product specs, developing custom solutions, training personnel, and writing standard operating procedures (SOPs). 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. We invite you to read this article by Larry Donalds – where he shares multiple troubleshooting tips – and review his impressive bio at the end of this article.
What keeps you awake at night?
If you manufacture specialty optical preforms and fibers, you know it can be challenging to enhance reproducibility and high yields. Believe me, you aren’t alone. It seems everyone in this business has a problem that keeps them up at night – an issue they can’t solve or a technique they aren’t able to master.
- Do you struggle with strength and deposition? Proper tube cleaning and flame polish can prevent strength and deposition issues.
- 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 burner H2/O2 ratio can prevent preform sagging and glass burn-off during collapse, which leads to variability later in the process.
- Do you have issues with stress layer (high-expansion coefficient glass) deposition bubbles or cracking later in the process? There are proven techniques to minimize these issues that can be programmed into your deposition process.
- Do you have high purity raw materials? Regarding corrosive gases, specifying the purity levels (water content and metallic impurities) that you will deliver to your preform during deposition is critical. Regarding liquid reagents, bubbling oxygen through to vaporize them adds an additional purification, but you still need to carefully specify low metallic impurities in liquid reagents to achieve low loss fibers.
Many of these issues can be solved and specialty preforms can be mass produced by adding custom solutions, standardizing procedures, and ensuring personnel are properly trained. In the 24 years I worked in 3M Company’s specialty fiber lab, we developed and manufactured high-performance optical preforms and fibers. In doing so, we created and patented techniques to manufacture Polarization Maintaining (PM) and Polarizing (PZ) fibers – techniques that offered reproducibility and high yields.
A brief comparison of the two types of MCVD preform fabrication systems
If you’re entering the optical fiber preform manufacturing market, you may be investigating the two types of systems available to make a purchase decision. If you already have a fabrication system – no matter which system you use – you can call us for technical help with troubleshooting, training personnel, writing SOPs, and developing custom solutions.
Here’s a very quick look at the pros and cons of the two types of MCVD gas delivery systems. If you’d like more advice and insight, I encourage you to call me. We can further delve into the pros and cons – and identify the system that best meets your needs.
- Stainless steel – Built entirely out of stainless steel, this gas delivery system is generally considered to be more leak-tight. However, if you have a problem with moisture in your gas streams or if you do develop a leak, the internal system surfaces can corrode. If this happens, you will most likely experience high fiber losses. To remove the contaminated delivery lines, you will be required to dismantle the system and replace components and liquid reagents. Another disadvantage of stainless steel is not being able to observe chemicals in the bubblers (color changes can indicate purity issues and poor bubble streams can impact vapor generation).
- Teflon and glass – Teflon and glass systems, manufactured by companies like SG Controls Ltd, are corrosion proof. If a gas drier should fail and a wet gas stream is introduced to the chemicals, the fiber water peak will rise. Typically merely waiting for the system to dry down will put you back in business. Disassembling the delivery system delivery tubing is not required. Potentially, there is a slightly higher chance of leaks, but this is rarely a problem. Also, a “clear” benefit is the ability to observe chemicals in the bubblers.
Teaming with SG Controls offers several additional advantages. When you look at the breadth and depth of technology they provide, they offer a total solution including preform deposition, preform stretching on the lathe, and draw tower technology. New vapor deposition systems for rare earth materials are also being developed. In terms of custom solutions – for both systems – their powerhouse engineering team is capable of tackling any issue you throw at them.
Take advantage of “Technical Troubleshooter” skills to optimize your preform design, ensure reproducibility, and get high yields.
After working 24 years with specialty preforms and fibers (and other technologies), I often sum up my career as: “Technical Troubleshooter.” I’ve spent most of my career troubleshooting problems to optimize the design, maintain purity to get good optical properties in the manufacturing process, ensure reproducibility, and achieve high yields. Much of my time was dedicated to developing, operating, and troubleshooting MCVD fabrication equipment to manufacture specialty preforms and fibers. Here are a few highlights:
Activities related to developing and troubleshooting specialty preforms and fibers:
- Developed a rotary seal
- Built a gas delivery system
- Corrosive gas handling
- Designed burners
- Improved lathe chucks
- Improved pyrometry
- Developed stress deposition techniques for PM and PZ
- Designed preform grinding fixtures
- Developed grinding techniques, along with a spreadsheet for technicians to follow
- Developed preform stretching techniques, along with a spreadsheet for technicians to follow
- Developed multiple over jacket/stretch techniques for preforms
- Developed solution doping techniques including:
- Oxide deposition
- Rare earth chloride doping
- Oxide drying
- Solution doped chlorides conversion to oxides procedure
- Specified manufacturing equipment
- Oversight of plant installation and startup
- Preform recipe transfer
Specific tasks related to deposition tube preparation:
- Fusing extension handle with consistent glass thickness
- Removing metallic impurities
- Etching the tube surface
- Fusing exhaust tube with consistent glass thickness
- Straightening and stress relieving the deposition tube
- Flame polishing the deposition tube
Specific tasks related to reproducibility:
- Valve leak detection
- Pyrometer calibration
- Temperature control
- Mass flow controller calibration
- Diameter control
- Glass burn-off control
EXAMPLES: Troubleshooting specialty preforms and fibers
Troubleshooting example #1: 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 overcollapse. I wrote active Excel programs for technicians, which calculated the overcollapse and stretch diameters to achieve the required core-to-clad ratio to meet optical specifications in drawn fibers. This process can increase fiber yield per preform from 8-10 km to 40-50 kilometers of erbium doped fiber.
Troubleshooting example #2: Certain steps are extremely important in the preform fabrication process. For example, you generate chloride vapors that are delivered to the deposition tube in the lathe. The vapors are heated inside the rotating quartz tube to ~1700 degrees C with an H2/O2 burner on the outside. The chloride vapors are converted to an oxide, which is then fused into clear glass. Exact control of this process is incredibly important. Mass flow controllers control gas flows. As an example, let’s say you want 100 CCs per minute of a particular gas. You set the flow rate at 100 CCs and make 8 or 10 preforms. Eventually you discover you’re not getting the same optical results. What’s wrong? You can change flows to compensate. If the flow controller eventually stops working you have no idea of the real flow used in the past. Repeating the preform now becomes very time consuming. A routine mfc calibration plan is critical to prevent this issue and improve reproducibility. Companies like SG Controls have developed auto-calibration systems to ensure the calibrations are performed routinely.
The same thing is true for temperature control – the pyrometer may say it’s 1700 degrees C, but is it really? I utilized handheld pyrometer standards, so we could observe the real hot zone temperature during deposition and compare it to the process pyrometer’s readout. When you carefully control your process equipment, the reproducibility of your product goes way up – and you’re not losing time from the production process.
Troubleshooting example #3: Your gas delivery system may have 100 valves that open and close. If one of those valves is leaking and you’re not getting the flow you think you are, you have a serious problem. You have to find a way to leak test your system. I had SG Controls develop an automatic leak test. We turned it on at night and received a report. The report noted if any valves were leaking and, if so, which ones. Prior to that, we would spend up to two days manually testing the system when problems occurred.
Troubleshooting example #4: As you know, you generate the gas streams with the gas delivery system. Some things are important along the way to ensure the high-purity gas stream arrives at the lathe where deposition happens. To help shore up the process, I developed a rotary seal for 3M, resulting in a record of invention. It was leak tight and corrosive resistant, with a low-flow indicator to detect o-ring leakage, prevent gas stream loss, and signal the rebuild time.
Advanced applications are driving the need for high-performance specialty fiber. Take advantage of FOC’s expertise to master the preform fabrication process.
Today, PM and PZ specialty fibers are being used in a variety of demanding applications. For example, Polarization Maintaining optical fiber is used in gyroscopes for missile navigation. Some telecom fibers require specialty fiber amplifiers to regenerate the signal. Fabricating specialty preforms and fibers to support advanced applications requires a highly specific “recipe” of rare earth elements, gas mixtures, and exacting mechanical processes.
If you’re looking to manufacture specialty fiber, you may need advice to select the right MCVD system that best meets your needs. Once the preform fabrication system is installed, you’ll need to get it up and running quickly. That is where I can assist by, training your personnel, and substantially shorten the learning curve. You cannot risk taking 1-2 years to get the system up and running before producing final product.
If you are manufacturing specialty fiber, I’m happy to help you troubleshoot all types of preform fabrication problems, train your personnel, write standard operating procedures (SOPs), and identify custom solutions. The goal, of course, is to achieve reproducibility and increase yield. If you have any questions whatsoever, I encourage you to call us – we’re here to help.
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