Helica™ Sensors, fabricated from chiral long period gratings, can be used to optically sense environmental factors, such as chemical composition, temperature, and pressure.

Chiral gratings are structurally formed into fiber. As such, Helica™ Sensors do not require the use of photosensitive fibers or rely on applying or relieving stress in the fiber. As a result, they are highly stable at high temperature and in other environmental conditions that can cause conventional long period gratings to degrade. Since photosensitive fibers are not needed, Helica™ Sensors can also be fabricated from refractory or radiation resistive glasses, according to the application. Such sensors can be used to probe harsh environments with high levels of radiation, high temperature, or corrosive chemicals such as might exist in nuclear power plants or in bore holes in oil fields and in deep mines.

Helica™ Sensors come in two varieties: single helix or double helix, as shown below. In both, the optical fiber’s refractive index is modulated by twisting a fiber with noncircular or nonconcentric cross section as it is passed through a miniature oven to produce the highly stable grating. A concentric core is used to produce a double helix structure and a non-concentric core is used to produce a single helix structure. While the double helix structures are polarization sensitive, single helix structures are polarization insensitive. The second polarization in double helix Helica™ Sensors can be used for added sensing channels, or as a reference channel. Both Helica™ Sensor types can be used for a myriad of sensor applications

Above, left, is an exemplary double helix Helica™ Sensor spectrum which illustrates the sensitivity to the handedness of light. In this example, the Helica™ Sensor is right handed and interacts only with right circularly polarized light. The inset shows the shift of a transmission dip when the fiber is surrounded by gasoline.

Single helix Helica™ Sensors are polarization insensitive, as shown above on the right, and their spectra are similar to conventional long period gratings. The primary differences between single and double helix CLPGs are given in the table below

A wide variety of sensors as well as input and output fibers are available to meet your needs. The wavelength of the dips can be tailored to suit the application. Please call us to discuss your specific requirements and receive a prompt quotation.

Performance of a fuel level sensor based on a double helix Helica™ Sensor is shown below.

The dip wavelength in transmission of a single helix Helica™ Sensor is shown below left as a function of temperature. On the right is the dip position of a single helix CLPG as it was cycled continuously for 24 hours around 400 °C after being annealed at 800 °C for 2 hours.

All information contained herein is believed to be accurate and is subject to change without notice. No responsibility is assumed for its use. Chiral Photonics, Inc., its subsidiaries and affiliates, or manufacturer, reserve the right to make changes, without notice, to product design, product components, and product manufacturing methods. Some specific combinations of options may not be available.

A fiber-optic sensor element is now available for applications requiring accurate measurements to 1000°C.

	This product is based on Chiral Photonics' chiral grating, fabricated by twisting a fiber as it is passed through a miniature heat zone to produce a distinct dip in the transmission spectrum. The spectral position of the dip in this chiral fiber changes with temperature allowing it to be used as a temperature sensor.
Temperature testing was carried out in a computer-controlled high-temperature oven in which the temperature was monitored by a thermocouple. Both long-term temperature stability and temperature sensitivity were tested using a Micron Optics fiber interrogator to monitor sensors as they were cycled from room temperature to 1000 °C. Use of the interrogator reduces the characterization and testing time and increases the accuracy with which the dip position can be measured. In addition, the dip position can be traced in real time. A micro-mirror may be attached to each chiral fiber so that they can be characterized in reflection, as required by the interrogator. We find that the dip wavelength shifts to the red by approximately 1.3 nm as the temperature is raised by 100 °C. The figure shows the wavelength of transmission dip of a chiral fiber versus temperature. The temperature was cycled five times from 700 °C to 1000 °C in the course of 24 hours, dwelling for 3 hours at these temperatures. The inset shows the temperature variations. As seen in the figure, the chiral fiber is capable of reliably measuring temperature up to 1000 °C with better than +/- 1% accuracy.

All information contained herein is believed to be accurate and is subject to change without notice. No responsibility is assumed for its use. Chiral Photonics, Inc., its subsidiaries and affiliates, or manufacturer, reserve the right to make changes, without notice, to product design, product components, and product manufacturing methods. Some specific combinations of options may not be available.