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USB DAQ devices ensure optical fibre integrity

25 November 2010

Two USB I/O devices for data acquisition are helping researchers at Bath University to perfect the production of photonic crystal optical fibre, which has interesting and potentially revolutionary applications for light signal transmission

Photonic Crystal Fibre (PCF) technology has found a wide variety of applications in many fields of science and medicine, ranging from a source for broadband white light to an imaging tool for biomedical scanning and a highly versatile platform for environmental sensing. PCFs are potentially much more efficient at transmitting light than conventional optical fibres and can operate over greater distances without signal boosting.

Unlike conventional optical fibres, which are composed of two different types of glass, PCF is made entirely from pure silica glass with a ‘honeycomb-like’ hexagonal array of air holes that run the entire length of the fibre. These air holes have diameters in the order of microns and must be uniform over many hundreds of metres.

Precise control over the pitch and diameter of these air holes is crucial for managing the fibres’ optical properties such as which wavelengths of light are transmitted and how ultra-short optical pulses can be transmitted without destroying their integrity.

Control over the hole parameters is achieved by applying different pressures to the fibre whilst the glass is at a high temperature (in excess of 1,800oC) within the furnace. This changes the size, pitch and wall thickness of the honeycomb-type holes which, in turn, alter the fibre properties. Regulating parameters such as temperature and fibre thickness is critical to the successful application of the fibres.

Feeding a glass pre-form into the top of the furnace, the research team uses nitrogen to 'inflate' different regions of the fibre as it is drawn at a constant speed from below. Steve Renshaw, a member of the Department of Physics at Bath, takes up the story:

“The pressure of the nitrogen is determined by electronic regulators, controlled by 0-10V signals provided via a Measurement Computing USB-3103 device. This is connected to a PC running a LabVIEW application that works with a touch-screen user interface.”

Another device, a USB-1408FS, monitors feedback signals from the regulators, providing control for three positive pressure lines and two vacuum lines. The USB-3103 is a 16-bit, 8-channel voltage output device and the USB-1408FS is a DAQ module capable of 14-bit resolution for 8 input channels.

“The big advantage for us,” says Steve “is that both devices can be programmed quickly and easily in LabVIEW, using the Universal Library software.” The fibre drawing process takes place on a pulling tower over three floors. “Previously, to control pressure somebody had to be stationed at the top of the tower and listen to instructions shouted up from somebody below monitoring the fibre as it was drawn. Now this can be done by one person without the delay of having to run up several flights of stairs.”

With the new system, all controls are in one place on one monitor. Another significant benefit is that the compact USB devices can be mounted adjacent to the regulators rather than relying on long lengths of signal leads. Steve Renshaw again:

“Pressures of up to 68kPa are provided by the regulators but if we see a significant difference in voltage returned, we know something has gone wrong which might be a pressure leak from a tube or pressure leaking within different regions of the pre-form. So we now know to stop the draw process immediately and fix the problem, which saves time, money and effort.”

Measurement Computing products are available in the UK from Adept Scientific. Steve chose the USB devices after identifying what he needed from Adept’s website. He has been impressed with the robust nature of the products and the competence and apparent experience of Adept's staff when purchasing the devices.

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