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Last Updated: September 20, 2022

Larry Donalds You can manufacture optical fiber preforms with a very basic system, essentially a gas delivery system in a room with an MCVD lathe. Or you can enhance your system by adding many accessories that will improve fiber strength and yield as well as the reproducibility of your preform manufacturing process.

Over the years developing and manufacturing specialty fiber– where I operated and maintained an SG Controls gas delivery system – I actively sought out various add-on control devices to improve our process and fiber quality. In fact, some features such as automatic valve leak testing and auto-calibration were specifically designed at my request and to meet my needs. The following information briefly describes add-on features for MCVD systems that will enhance your ability to produce high-quality specialty fiber preforms that meet your design goals.

Multichannel gas drier and dew point monitor

Without gas drying, you can have serious problems with water peaks in your fiber and delivery line corrosion leading to contamination of chemicals. I am familiar with the SG Controls multichannel gas drier, which removes moisture from the input gas streams. This gas drier also includes a dew point monitor, so you can track the moisture levels in the gas streams. The multichannel gas drier has two drying columns. While one column dries and delivers process gas to the system, the other column is in standby mode. Depending on how you program the unit, you can instruct your MCVD system to switch to the standby column before the column in use is saturated with moisture. Also, for oxygen delivery, a “cracking” column is used prior to the drying column. This cracking column breaks the H2O in the O2 stream down to H2, H, O, and O2 before it flows through the standard drying column.

The multichannel gas drier and dew point monitor are important features, because they are totally automated. If you choose not to install an automatic gas drier, your other option is to purchase fixed drying columns with replaceable cartridges that can be mounted on a wall in your laboratory. These fixed drying columns need to be manually changed and cross-purged by an operator. In this case, you risk the chance of introducing moisture into the system during that process. Also, the gas drier in an SG Controls MCVD delivers dry nitrogen to the dry box, which encloses all the Teflon block valves and tubing to keep the system dry. It also provides dry gas to transfer chemicals, which is a critical device.

Automatic valve leak testing

The valves in your gas delivery system determine which chemicals are being delivered to the deposition tube and when. If the valves are not leak tight, you don’t get the expected gas flow or you can get cross-mixing with other chemicals. Knowing you have a leak-tight system before manufacturing your preform is very valuable. However, manually leak testing about 100 valves is extremely time-consuming. For the MCVD system I operated, it would take up to 2 days to manually leak test the entire system and track down leaks. Keep in mind, valves need to be leak tested in the open position (gas is being delivered to the system), because gas can still leak out to the atmosphere. Of course, they also need to be tested in the closed position (gas is not being delivered to the system).

Alternatively, you can purchase an automatic leak testing system as an add-on option. This computerized leak testing system automatically cycles each valve in the system in the open and closed position in all combinations. This time-saving option pressurizes each valve with low-pressure oxygen and looks for a zero-flow condition on the delivery line. This is done with each valve in the system – without an operator – and the computer program documents all results and provides a report. Based on the report, you can then determine which valve is leaking and quickly repair it.

This is a time-saving feature: Instead of taking 2 days to manually leak test the system, it took 2 hours to run the test. If you’ve conducted leak valve tests manually, you’re familiar with a key problem: If more than one valve is leaking, it’s very difficult to pinpoint which valves are causing the problem. This automatic leak testing feature provides a report that points you to the specific valves needing attention, so repairs can be quickly made.

Auto-calibration software interface

I have found that calibration is a critical factor in producing quality preforms that meet design goals. In this article, I’ll keep my comments on calibration brief. I encourage you to read one of my previous articles where I discuss the importance of calibrating your H2/O2 mass flow controllers and offer numerous tips. (Read more: “Critical Design Goals to Manufacture Optical Fiber Preforms.”)

At my request, SG Controls developed an auto-calibration system to compare the mass flow coming through each device in the gas delivery system to a mass flow standard. The mass flow standard was on a cart, positioned next to the gas delivery equipment and connected to the MCVD. The gas flow traveling through the control device would then exit the machine and flow through the calibration standard. A computer program automatically cycled the mass flow controller under test through 10 different set-points, which are compared to the mass flow control standard. This was done for each mass flow controller in the system. The MCVD I operated had 12 mass flow controllers. The mass flow controllers were automatically switched in-line with the standard, by the computer, and each one was tested without an operator present.

This option was also a time-saving feature. You can hook up the calibration cart in the evening, and go home. In the morning, you are presented with a report for each mass flow controller as compared to the standard. If one is out of spec, you have the option to correct the mass flow controller, replace it, or simply have the control recipe software automatically compensate for the incorrect values that particular mass flow controller is delivering.

A key benefit of auto-calibration software is that you have absolute clarity on the operation of your mass flow controllers. If you do not keep track of your calibrations – for example, you ask for 100 CC and you’re actually getting 110 CC – you’re constantly “chasing the design.” I have spoken with seasoned industry professionals who recognize that they got caught in this trap. Rather than keeping the recipe under control, they would get feedback from the profiler or optical fiber measurement. That result would be fed back into the MCVD to “correct” the recipe. Instead, with calibration, you’re in the driver’s seat, and you have a clear view of the road ahead. You have the ability to determine how to best move forward efficiently and effectively.

It’s helpful to think about what can happen if you do not calibrate the mass flow controllers. If you simply feed the optical results back into the MCVD system recipe, you are chasing the design. This can work for a while, but when a mass flow controller fails and must be replaced you have no idea what the actual flow was. The new, calibrated unit you install will deliver the actual flow. Unfortunately, you will need to start over and re-establish the flow rate set-points to meet specifications. Chasing the mass flow controller drift can be very time consuming. On the other hand, an auto-calibration system will provide actual flow curves of all system mass flow controllers to ensure your MCVD recipes provide reproducible results.

Atmospheric pressure compensation

When you think about it, atmospheric pressure does impact your gas delivery system. As you bubble oxygen through the chemicals, the atmospheric pressure above the liquid impacts the rate at which you can vaporize the chemical. A lower atmospheric pressure will increase the vaporization rate of your chemical. If you have a constant-temperature bubbler and very good control of your vaporization process – but if the atmospheric pressure drops – then, theoretically, you will be able to get a higher level of vapors at the same flows and temperatures. This can impact your preform results. There are features available to compensate for atmospheric pressure changes in MCVD systems.

Lathe enclosure

Some preform manufacturers install their MCVD equipment in a Class 10,000 clean room. However, it is possible to manufacture preforms with a very basic system: a lathe and gas delivery system in an open laboratory. If you choose to do this and want a higher level of purity, you can add a lathe enclosure (the lathe is encased in a glass enclosure with sliding doors). Within this enclosure, you typically would add HEPA filters to filter incoming air to Class 10,000 cleanliness or better. The enclosure creates positive pressure relative to the outside room. This combination – positive pressure with clean air – helps to prevent particulates from entering the clean environment, becoming fused to the preform, and causing weak spots in the drawn fiber.

In addition, a lathe enclosure is a safety feature, because it protects operators from the flame and moving components. If you can’t afford to install a Class 10,000 clean room, you can add a lathe enclosure in your laboratory setting. This small investment can offer ROI in terms of increased preform yields.

Supplementary isothermal bubbler heater

This immersion heater adds heat to the bubbler chemical to compensate for cooling due to the bubbling effect. This is a great feature to add to your MCVD, because it re-stabilizes the bubbler chemical temperature in less than 5 minutes. If you’ve read my previous articles, you’ll know that I consider the bubbler to be an extremely critical piece of equipment when manufacturing optical fiber preforms. In fact, I prefer a glass bubbler, because this allows you to view chemicals, confirm proper level, and confirm a uniform bubble stream. Over the years, I have found chemical visibility to be a real advantage. (Read more on isothermal bubbler heaters: “Comparing the 2 types of MCVD gas delivery systems to manufacture optical preforms: Stainless steel and Teflon/glass.”)

Scanning pyrometer

SG Controls offers a scanning pyrometer that moves to point at the hottest spot on the deposition tube. Predetermined pyrometer aiming positions versus speed are developed and used in programming. Without this option, the pyrometer aiming is adjusted for one speed. If the burner speed increases, the hot zone can lag behind the pyrometer, causing a colder than actual temperature input to the controller. Your temperature controller will compensate, bringing the actual temperature higher than the programmed value. Without this critical piece of equipment, you can overcompensate for temperature, which can reduce the efficiency of your depositions and potentially cause premature tube shrinkage. (Read more about this scanning pyrometer, including a tip regarding hand-held pyrometers: “Critical design goals to manufacture optical fiber preforms.”

Diameter control

To prevent tube shrinkage, you must carefully control the diameter of the tube with a consistent layer thickness for each pass. As your preform shrinks, the wall thickness increases, causing the internal deposition temperature to cool. This changes the layer thickness that you achieve per pass and the refractive index of that layer. Let’s say you’ve programmed the recipe to give 20 layers at X thickness. If each layer gets thinner and thinner, then the resulting preform won’t meet your specifications. SG Controls’ diameter control option gives you a greater ability to control deposition tube diameter from run to run, and within a run. (Read more about diameter control: “Critical design goals to manufacture optical fiber preforms.”)

Extended bed lathe with motorized tail stock

This add-on feature allows you to stretch preforms to a smaller diameter. You can program the lathe, fire carriage, and motorized tail stock to run at a specific speed to give you a specific preform diameter. (The program will automatically calculate this.) This is a very helpful approach to increase preform yields. In fact, you can manufacture a larger-than-desired core in your preform, stretch it to a smaller diameter, then over jacket it with quartz tubes to achieve the desired core-to-clad ratio for your fiber design. This process can yield significantly more fiber. In fact, this is becoming a popular approach to manufacture telecom fiber. The core rods are manufactured to a very large diameter, then stretched into long lengths and cut into short sections. Next, each section is over jacketed to get many preforms out of one core. This is a cost-effective way to manufacture preforms. Clearly, this add-on feature can have a direct impact on your fiber yields.

A small, related option is double jaw chucks. Typically, lathes come equipped with one set of jaws to grab the tube. If the deposition tube is not perfectly circular, you can experience movement in the jaws. The additional set of jaws offers another set of contact points, so the deposition tube is held steady. Also, the double jaw chucks can be used for over jacketing: one set of jaws holds the preform while the second set holds the over-collapse tube.

Vertical stretch over jacket lathe

As discussed above, an extended bed horizontal lathe can be used for stretching or over jacketing. Alternatively, companies like SG Controls offer a vertical stretching and over jacketing lathe. There are 2 advantages:

  1. The vertical lathe eliminates the gravity issue that causes tubes to sag when you heat them, an all-too-common situation with a horizontal lathe.
  2. Vertical stretch lathes typically are much longer, and you can process longer preforms.

Manual lathe control box

One of the many add-on options that SG Controls offers is a manual lathe control box for setup. When you’re loading deposition tubes, over collapse tubes, or preforms into the lathe – and straightening and fusing glass – you can control the burner flame and position independently of the automatic system. This feature allows you to move the burner fire carriage up and down the lathe while sitting in front of the lathe. Also, the manual lathe control box has control potentiometers for the H2/O2 flows to the burner. This allows you to manually control the burner temperature during setup.

In addition, the pilot/main burner foot toggle allows you to have both hands free to focus on straightening and fusing the preform with graphite paddles. To turn the burner on/off, you merely push the foot-control pedal. The foot pedal has a soft-start feature to slowly ramp the mass flow controller. Without this feature, if you were to depress the pedal and open a hydrogen valve to allow the mass flow controller to send flow to the burner, you would get a big surge of hydrogen. These are very nice, user-friendly features for the MCVD operator.

Chemical refill system: SiCl4, GeCl4, POCl3, and spare

As I’ve mentioned in previous articles, when you manufacture preforms and bubble gasses through the chemical reagents, the gasses vaporize and the chemical level in the bubbler drops. Certain components in the MCVD system help to compensate for that chemical drop. However, at some point the bubbler needs to be filled to maintain reproducible vapor generation. The semi-automatic chemical refill system removes the operator from the key parts of the process. This significantly reduces the chance for errors and the introduction of moisture into the system.

Typically, automatic chemical refill systems are standalone cabinets that are piped to the gas delivery system and connected to each bubbler. (These systems can be stainless steel or Teflon/glass.) Control valves are in-line with the piping to turn on/off at the appropriate time. The chemical refill cabinet also houses the bulk containers purchased from chemical suppliers. To start a chemical refill system, the delivery line is purged with a dry gas all the way through the gas delivery system to vent, bypassing the bubbler. Once complete, the bulk chemical vessel is pressurized and chemical is forced into the MCVD bubbler until the correct level is achieved. Next, the remaining chemical is purged from the refill line with dry gas for a programmed amount of time, which completes the refill process.

Concluding thoughts

As mentioned above, I worked with SG Controls to develop many of these add-on features, which I found incredibly useful. If you have questions about any of these features, I encourage you to call me. We can look at various options that will enhance your company’s fiber strength and yield as well as the reproducibility of your preform manufacturing process.

You may have a unique need. I can offer insight about whether an existing feature is available for purchase that will support your need. Alternatively, I can guide you through the process to develop a highly specific feature for your system. It’s helpful to know that SG Controls has been great to work with when it comes to creating unique add-ons to meet a specific need. In fact, think of the options in this article as examples of SG Controls’ capabilities rather than an exhaustive list.

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 an earlier article, I noted that I view myself as a “technical troubleshooter,” thanks to the extensive time spent troubleshooting problems to optimize the design and achieve desired optical properties in the manufacturing process. In addition to being a technical troubleshooter, I’m 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.

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