Optical pressure sensors

What are optical pressure sensors?

Optical pressure sensors detect a change in pressure through an effect on light.

In the simplest case this can be a mechanical system that blocks the light as the pressure increases. In more advanced sensors, the measurement of phase difference allows very accurate measurement of small pressure changes.
 

Working principle

A simple optical pressure sensor

In an intensity-based optical pressure sensor, an increase in pressure will cause the source of light to be progressively blocked. The sensor then measures the change in light received.

For example, in the simple mechanism shown below, the pressure moves a diaphragm and the attached opaque vane blocks more of the light from the LED. The fall in light intensity is detected by the photodiode and gives a direct measurement of pressure.

A simple optical pressure sensor like this needs a reference photodiode (as shown to the right), which is never blocked by the vane. This allows the sensor to correct for changes in the light output due to other factors, like aging of the light source, variations in supply voltage, etc.

These mechanical systems are relatively large. Much smaller versions can be constructed with a reflective membrane and two optical fibres, one as a source of light and the other to receive the reflected light. Pressure bends the membrane and changes the amount of light reflected back to the detector (see right).

Other fibre-optic sensors use interferometry to measure changes in the path length and phase of light caused by changing pressure. The rest of this section will focus on these.
 

Function

Fibre-optic pressure sensors can be classified as either extrinsic, where the sensing takes place outside the fibre, or intrinsic, where the fibre itself changes in response to pressure.

Very sensitive optical measurements can be made by exploiting interferometry: measuring the change of phase between light that has taken two different paths. This can detect changes in distance corresponding to a fraction of the wavelength of light.

There are two common types of pressure sensor that use interferometry. These are the Fabry-Pérot interferometer (FPI) and fibre Bragg grating (FBG).

The FPI is an extrinsic sensor that uses interference between multiple light rays reflected back and forth between two surfaces in a cavity. As the spacing between them changes, interference will change the amount of light received at a particular wavelength.

This is one of the best optical sensor technologies. It is simple, accurate and easily scaled for different sizes and pressure ranges.

An FBG is an intrinsic sensor that has a regular series of reflective structures in the fibre that are affected by stretching or squashing the fibre. This causes the wavelengths of the reflected light to change.
 

A cross-section of a Fabry-Pérot cavity on the tip of an optical fibre

Construction

A Fabry-Pérot cavity with two parallel reflecting surfaces can be constructed on the tip of an optical fibre (as shown, left).

A semi-reflecting surface is attached to the fibre (M1) and a reflective membrane is created at the opposite end of the cavity (M2). This membrane forms a diaphragm that is moved by pressure.

The change in spacing between the mirrors causes a difference in the path travelled by each ray of light (E1 and E2) and hence a relative phase shift between them. The resulting interference will reinforce or reduce particular wavelengths of light.

The multiple reflections and the large number of interfering rays result in a very high-resolution measurement.

A Bragg grating can be created within a fibre using a series of periodic changes in the refractive index of the fibre. This causes particular wavelengths of light to be reflected or transmitted, based on the ratio between the wavelength and the spacing. As a result, the spectrum of the reflected light changes as the fibre, and the spacing, is stretched.

The fibre can be attached to a diaphragm that stretches the fibre when pressure is applied. Compressing the fibre also changes the effects of the grating, creating two peaks in the spectrum.

A Bragg grating created within an optical fibre, where n₀, n1, n2 and n3 represent periodic changes in the refractive index of the fibre

The output from either type of sensor can be measured in two ways. If a monochromatic or narrow-band source is used, there will be a change in the amplitude of the output signal as the length of the cavity (or the spacing of the grating) modifies how much of that wavelength is reflected.

A wide-band light source, such as a white light, can also be used. In this case, the frequencies at which constructive or destructive interference occurs will change with pressure. This can be measured with a spectrum analyser.

These structures, in particular Fabry-Pérot cavities, are also suitable for silicon fabrication techniques allowing even smaller optical sensors to be made as micro-electromechanical systems (MEMS) devices.

Waveguides (equivalent to optical fibres) and mechanical components such as cantilevers and membranes can be constructed at the micrometre scale.

These sensors can respond very rapidly to pressure changes because of their small size. Light-emitting diodes, solid state lasers, photodiode detectors and electronics can all be integrated on the same device.
 

Applications

Because of their freedom from electromagnetic interference, fibre-optic sensors are very useful in harsh environments.

One example is the oil and gas industry. Conditions in a well can easily reach 20 kpsi and 185º C. Optical sensors continue to perform well under these extremes.

Their small size, flexibility, the absence of any potentially hazardous voltages, and the fact that the sensors are made of non-toxic materials makes them very well suited to medical applications.

There are many places in the body where measuring pressure can be important for diagnosis, long-term monitoring or during treatment.

As well as more obvious measurements such as pressure in blood vessels and the lungs, it is often useful to measure pressure in the digestive tract, bladder, brain, bones and joints. Fibre-optic sensors allow this to be done in a minimally invasive way.

The immunity to electromagnetic interference is valuable when pressure needs to be monitored during MRI scans or RF ablation procedures.

The requirements for a pressure sensor vary depending on the reason for the measurement, where the measurements are made, the range of values to be measured, and whether it’s for a single measurement or long-term monitoring. There are also various standards defined for medical equipment. Fibre-optic sensors can be designed to meet a wide range of these requirements.

A Fabry-Pérot sensor can be used to accurately monitor pressure at a specific location in the body and is typically introduced via a catheter.

Multiple fibre Bragg gratings can be created within a fibre, allowing pressure to be measured along its length. This has been used, for example, to measure the pressure changes throughout the colon during digestion.

The fibre can also provide measurement over a 2D area. This is useful for monitoring the pressure on the body for bed-bound patients to reduce the risk of ulcers.
 

Advantages and disadvantages

Intensity based sensors are not very sensitive to temperature change because the measurement and reference detectors are affected equally. Because the amount of movement needed is very small, hysteresis and repeatability errors are very low.

The small size and flexibility of fibre-optic sensors means they can be deployed in locations that would be hard to access with other techniques.

The fact that the sensing element itself is passive and does not need a power supply enables the sensors to be used in a wide range of applications where getting power to the sensor could be a problem. This also eliminates signal transmission problems due to parasitic capacitance, electromagnetic interference, etc.

On the other hand, their small size can mean they are not as robust as other sensor types. Their high sensitivity can also make them more sensitive to acoustic or mechanical vibration.

Want to learn more about the other core technologies used in pressure sensors? Click the links below to jump to the section you're interested in.

• Capacitive vs. piezoresistive vs. piezoelectric pressure sensors
• Capacitive pressure sensors
• Piezoresistive strain gauge sensors
• Piezoelectric pressure sensors
• MEMS pressure sensors

Looking for more on pressure sensor technology? Check out the further chapters of this guide below, or if you're pressed for time you can download it in a PDF format here.