As we all know, high-purity raw materials are critical to achieve design goals. Low metal impurities prevent reflection loss and dry raw material prevents absorption. Additionally, there are many design strategies to meet cutoff wavelength, mode field diameter (MFD) and numerical aperture (NA) requirements. Additional waveguide design fabrication goals include high purity, low OH, uniformity and reproducibility.
Over the years that 3M has operated and maintained MCVD manufacturing systems, I have worked closely with designers and have been closely involved in the design process to ensure that designs are manufacturable in terms of expansion factors, etc. In effect, the designer/operator team for preform fabrication is similar to how architects and home builders work together.
In addition to resolving design issues with designers, MCVD operators must tightly control multiple equipment and process variables throughout the fabrication process. The following paragraphs provide a high-level overview of key design goals for fiber preforms and provide MCVD operators with tips for fabricating specific glass waveguides and achieving repeatable designs.
Fiber Preform
Preform profilers are critical to achieving design goals
As an MCVD operator, you rely on a profiler to measure the refractive index and dimensions of finished preforms, such as the thickness of glass layers. This is critical information for analyzing the finished preform and predicting the ability to meet design goals. Typically, analyzers have software that predicts the cutoff wavelength, MFD and NA of the pulled fiber. As an operator, this will provide you with important information to plan MCVD process changes that will meet specified design goals.
Additional Design Considerations
The barrier layer slows OH migration, provides a high-purity glass substrate for critical deposition layers, and yields pure glass for extended mode fields – as you know, glass barrier layers are usually index-matched to the starting tube, but higher purity. Typically, you deposit several layers of high-purity glass within the base tube to form the pristine surface of the critical deposited layers that guide light. Pure glass is important because, depending on the waveguide design, the mode field propagating through the fiber core will expand according to wavelength. Depending on the optical design, some light may be guided in the blocking layer. If the barrier layer is not thick enough, the mode field may encounter the deposition tube glass and may absorb or reflect, resulting in increased fiber loss.
A recessed well (refractive index < silicon dioxide) barrier (to minimize the GeO2 core) can be used to minimize losses in the glass core – MCVD operators know that germanium in the core increases the index of refraction. However, the higher the amount added, the higher the loss of the fiber. One strategy is to lower the refractive index of the high-purity barrier around the core. The recessed well layer may be formed of a dopant such as fluorine. This design increases the refractive index difference between the barrier and core layers and increases the NA without adding core germanium. The recessed well wall thickness must be carefully designed to prevent cutting off the master die.
Important Device Considerations for Achieving Design Goals
As an operator, what can you do to ensure that your design goals are met? As mentioned above, you have to tightly control the barrier and recessed trap layers. Accurate control of airflow is critical.
Mass Flow Control Calibration – As you know, you need good calibration equipment and procedures because you must know the exact mass flow.
Chemical Vaporization Control – I talked about bubblers in a previous post (see link at the end of this post). Additionally, I plan to discuss this key feature in more detail in a future article.
Leak testing to ensure consistent flow and steam ratios – Obviously, you need a good leak testing program/system and make sure the equipment is functioning properly for consistent flow and good steam ratios. This facilitates repeatable designs.
Important Process Considerations for Achieving Design Goals
For preform manufacturers, certain process controls can help ensure that design specifications are consistently met. Along the way, there are 4 key points to help you achieve your design goals.
Carefully control the diameter of the tube with a consistent layer thickness every pass – when depositing barrier and core layers, your goal is to have a consistent thickness every time. However, the flame pressure of the burner is known to cause the deposition tube to shrink. Without diameter control, this shrinkage reduces the internal surface area. With constant vapor flow, the thickness of the deposited layer will decrease. If the diameter of the deposition tube varies between runs or within a run, the layer thickness will not replicate. Careful control of the diameter of the tube is absolutely critical. For deposition, consistency is key.
Here's another way to think about this: tube shrinkage increases the tube cross-sectional area (CSA), which lowers the internal reaction temperature (affecting deposition), and reduces the surface area for oxide deposition. In an ideal situation, if you use the exact same shrinkage to make 10 preforms with the same deposition tube CSA, then you can live with that tube shrinking. Assuming other variables are constant, these preforms will be identical. But this rarely happens because starting tubes often have different CSAs and each process involves some variability.
Tips for improving reproducibility: The internal temperature of the deposition tube is critical because the reaction takes place inside. However, the process pyrometer is reading the outside temperature. SG Controls offers a diameter control system that controls and provides optical measurement of pipe diameters. The system is a closed loop system, and the booster system controls the back pressure of the tubes through diameter measurement and feedback from the set point. It is very important to have an automatic system to control the pipe diameter. While I've had some success with manually controlling the diameter, this approach is not nearly as repeatable as an automated system.
Fiber Preform
Lower deposition temperatures reduce the likelihood of OH diffusing from the tube and burner to the wick and prevent tube shrinkage – as we discussed, a lack of diameter control results in thicker tube walls. With thicker tube walls, more burner heat must be applied to maintain consistent deposition temperatures. The deposition tube and burner are the source of water, which can diffuse through the glass layer towards the core. Keeping the deposition temperature as low as possible prevents tube shrinkage and prevents water from flowing to the core, which can cause absorption in the final fiber measurement.
Tips for keeping cool: As you probably know, adding dopants like phosphorous and fluorine lowers the deposition temperature.
Deposition tube temperature is critical in this process, which means calibration is critical to reproducible design goals – calibrating process pyrometers is absolutely critical. In fact, to ensure accuracy, I use a handheld calibration standard that is separate from the process pyrometer. I walked through the pyrometer viewfinder and through the hot zone until the highest temperature was detected. This maximum temperature is the actual temperature. I would compare the reading to the process pyrometer and check for calibration deviation. Calibration is critical – a repeatable process depends on accurate temperatures.
A tip about a handheld pyrometer: you don't need to buy an expensive blackbody instrument. Instead, purchase hand-held standards calibrated with blackbody standards. Additionally, you can temporarily mount a handheld pyrometer on a fire truck to spot check the calibration of the process pyrometer.
Control the H2/O2 ratio of the burner during tube collapse to achieve a reproducible core/cladding ratio and minimize glass burning – do this in chronological order, you’ve done your deposition, and now you have to put the The hollow tube collapses into a solid preform. For this, the H2/O2 flow of the burner must be increased to produce higher flame pressure and temperature. Flame pressure applies force and heat to the tube, forcing it to collapse. With sufficient internal tube pressure, the tube remains round. The H2/O2 ratio of the burner is critical because it directly affects the amount of glass you burn off. At crash temperatures you are evaporating the glass and will notice the white oxides condense before the burner. It is important to control the gas ratio of the burner and the burning of the glass during collapse. During fiber drawing, the preform is just scaled down.
Tips for keeping the preform straight when slumping: Companies like SG Controls offer stainless steel water-cooled burners with nitrogen curtains to reduce the hot zone width, preventing the preform from sagging during slumps. Adjustable nitrogen flow on the outside edge of the burner is blown towards the burner tube to uniformly reduce the hot zone width. This allows the operator to adjust the width of the hot zone during the folding process. This equipment can improve uniformity and fiber yield.
