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Making sense of optics

Greg Blackman touches on some of the technology and applications surrounding optical sensing

If you use optics in any way, then the chances are there will be an optical sensor involved. From photodiodes and phototransistors to photon counting devices, and from infrared through to UV detectors, there is a sensor suited to most optical applications that you care to name.

For instance, to profile a laser beam in order to tune it to gain maximum efficiency, a sensor is needed to measure the outputted energy. Sean Bergman, product line manager of power and energy products at Coherent, cites three commonly used technologies to measure the power and energy of lasers: pyroelectrics, semiconductors in the form of photodiodes, and thermopiles.

Ophir-Spiricon has developed its Pyrocam array camera for laser-beam profiling. Greg Slobodzian, vice president of engineering, also lists microbolometers among those sensors that can profile laser beams. Each of these sensing devices will have criteria where one sensor outperforms the others, so the argument for using one over another will depend upon the properties of the laser system being measured. At the same time, the properties of these sensors make them suitable for a whole host of other applications. The breadth and scale of sensing technology is immense and cannot possibly be covered in its entirety in these pages.

Therefore, for the purpose of this article the main focus will be on pyroelectric detectors.

Pyroelectric

Pyroelectric sensors detect changes in temperature. Materials such as lithium tantalate, lithium niobate, and lead zirconate titanate (PZT) have the ability to generate an electrical potential when heated or cooled, and therefore any changes in temperature will result in a shift in polarisation of the material and the production of an electric current.

The pyroelectric material is only sensitive to pulses of radiation. This is a key difference between pyroelectric detectors and thermopile detectors, which also detect heat, but can monitor steady changes in temperature, something pyroelectric sensors are unable to do. Bergman suggests that pulses must be in the tens of kHz range, which covers a large portion of the laser market. Higher frequency pulses, however, cannot be measured using pyroelectric sensors.

Alan Liddle, product manager for the Perkin Elmer product range at Pacer, notes that the primary application of pyroelectric sensors is in motion detectors. Pacer provides a wide range of sensors and detectors, including pyroelectric detectors from Perkin Elmer. The company also supplies Perkin Elmer detectors for security and military applications, such as providing surveillance along a perimeter fence. Motion detectors with a range of several hundred metres can be positioned so that an alarm is triggered if something hot moves across the fence line.

‘The pyroelectric crystals do not saturate, unlike photodiodes, so they can be used across a wide range of energy levels, from roughly the 100nJ level to multiple joules,’ says Bergman of Coherent.

Ophir-Spiricon’s Pyrocam array camera is designed for laser beam profiling.

With regards to laser beam measurements, this is particularly advantageous as it covers the range of the majority of pulsed lasers. In addition, the coating applied to the pyroelectric crystal typically absorbs wavelengths from deep ultraviolet to far infrared, which easily covers the entire range of wavelengths utilised by commonly manufactured lasers.

‘The benefit of pyroelectric sensors over thermopiles is mainly the ability to resolve the energy of individual pulses, and secondarily their speed,’ explains Bergman. ‘Pyroelectrics respond to each laser pulse, whereas thermopiles respond in the order of seconds, so if someone wants to tune or peak a pulsed laser, they can get very fast feedback when using a pyroelectric. This can be very useful when optimising the output of ultrafast kHz lasers, for example.’

Photodiodes can also be used to resolve the energy of individual pulses. ‘One benefit of photodiodes is their speed,’ continues Bergman. ‘They are even faster than pyroelectric sensors and can easily measure extremely fast lasers or even track the output of the laser so someone can see the shape of an individual pulse. However, photodiodes saturate when exposed to laser pulses above the microjoule level. They have extremely low damage threshold and are highly wavelength dependent.

‘Typically, photodiodes are only employed for measuring below approximately 100nJ, where it can be difficult to measure with a pyroelectric sensor, because there is not enough electrical output to make accurate measurements.

‘In summary, pyroelectric sensors are used in almost all instances in which the laser is repeatedly pulsed, except when the energy is below about 100nJ or when the pulses are extremely long.’

Ophir-Spiricon’s Pyrocam array camera is designed for laser beam profiling. ‘High sensitivity is not considered important for this camera,’ says Slobodzian of Ophir-Spiricon. He explains that a highly sensitive sensor would be detrimental to measuring the properties of laser beams, as the source is extremely bright. A sensor with low sensitivity is preferred, and even then some attenuation of the signal might be needed to record the beam profile.

The Pyrocam has applications ranging from use in the medical industry, such as ensuring excimer lasers used in eye surgery are correctly focused, to the telecommunications industry, where beams passing through fibre optic cables are analysed. The camera works at room temperature and essentially covers all infrared and UV wavelengths. Pulsed and continuous laser beams can be measured, with a chopper attached to break the beam up when profiling CW lasers.

‘The fundamental technology [of pyroelectric sensors] is decades old and has not changed. What has changed recently is the approach used to integrate the pyroelectric crystal into a more accurate measurement system,’ says Bergman.

‘One important effect of pyroelectrics that is not often dealt with is their change in output as the temperature of the crystalline material changes. The sensor accuracy is affected by changes in both room temperature and in the temperature of the crystal due to the laser energy it absorbs. One of the recent advances made in the Coherent Energy Max line is a temperature compensation network contained inside the sensor.

Liddle, of Pacer, notes that automated methods of production have led to advances in the reproducibility of sensors. ‘Early pyrodetectors were hand built in discrete components,’ he says, which led to variations between sensors. ‘Using an automated method ensures high consistency and reliability.’

Liddle feels that improvements in the pyroelectric material, as well as in amplifying the signal to increase the responsiveness of the sensor and reduce noise, will continue. But it’s customer-driven innovations that Liddle puts most emphasis on, with regards to advances in the technology. Advanced electronics inside the sensor body, which can carry out a degree of amplification, are being increasingly utilised to make amplifying the signal easier for the user. In addition, technology to convert the voltage pulses into a digital sequence that the customer can integrate onto a PC is being developed. The conversion takes place within the sensor and has the advantage of improving noise immunity. ‘In two to three years we’ll be moving from analogue systems to digital systems,’ Liddle predicts.

Perkin Elmer’s pyroelectric sensors, supplied by Pacer, are used in applications such as motion detection and gas sensing. Image courtesy of Pacer.

Gas sensing

Pyroelectric sensors can also be used in gas detection, for instance to detect the presence of dangerous gases on oil platforms. Traditionally, a pellistor is used – a device in which the electrical resistance of the detecting element changes in the presence of gas. These detectors, however, can be poisoned by the gas they are detecting, rendering them useless.

Certain gases absorb IR radiation, with specific gases absorbing specific wavelengths. A filter can be fitted to the sensor to detect those wavelengths. An IR source (usually a tiny filament bulb that gives off heat) sends out pulses of radiation that the detector picks up. Any gas present will absorb IR at a specific wavelength, which therefore won’t reach the sensor, raising the alarm to a possible gas leak.

Liddle views this application as a growing market for pyroelectric detectors. ‘It is a long-lifetime sensor that doesn’t have to be replaced, as it’s not poisoned by the target gas,’ he says. In addition, pyroelectric sensors are easier and cheaper to produce in large numbers than pellistor detectors.

Cascade Technologies integrates Quantum Cascade Lasers (QCL) into gas measurement and detection instrumentation. The lasers operate on the principle that electrons cascade down a series of 20-100 semiconductor quantum wells, producing a photon at each step. This is unlike typical diode lasers, which emit only one photon over a similar cycle. This means that QCLs can outperform diode lasers operating at the same wavelength in terms of power, due to the cascading effect and the ability to carry large currents. The resultant laser beam is used as a spectroscopic tool for gas sensing, in a technique known as intra-pulse spectroscopy.

QCL-based sensors measure gases absorbing in the mid-infrared region of the electromagnetic spectrum. ‘Very few laser sources operate at midinfrared and most detection devices that do, need to be cryogenically cooled to –100-200°C,’ explains Dr Iain Howieson, managing director at Cascade. ‘Alternatively, you can use a conventional light source, which has low resolution and high noise, making it difficult to identify trace gases. It is for these reasons that conventional gas detection methods are limited to resolving concentrations greater than 100ppb. In comparison, QCL sensors have inherently high optical resolution, low noise and can conduct trace analytics at concentrations of parts per trillion.’

He adds: ‘For many applications, it is not so much a question of high sensitivity as the speed of the response that is important. The laser can be pulsed for 1μs, which creates a high frequency chirp. The energy emitted travels through a gas sample and is picked up by a high-speed mid-infrared detector to provide a near instantaneous gas spectrum, with the device capable of making one billion readings a second. This provides a snapshot of the gases being analysed, freezing any turbulence in the gas cloud.

‘Incumbent technologies based on low resolution optical sources need to apply corrective formulae to the data to make allowances for properties such as temperature and pressure,’ explains Howieson. ‘A zeroing span is also required to ensure the response doesn’t vary over time.’

Cascade’s QCL-based gas sensors are used in a number of industrial applications, such as emissions monitoring. The company supplies BP Marine, a provider of fuels, lubricants, and technical services to the marine industry, with gas sensors for measuring nitrogen oxide (NOx) and sulphur oxide (SOx) emission levels on ships’ exhausts.

Cascade’s gas analysers are fitted onto the exhaust stacks of ships to carry out emissions monitoring in situ.

‘Regulations on emissions of NOx and SOx have led to onboard ship monitoring,’ Howieson says. ‘Sensors are fitted onto each exhaust stack to carry out measurements in situ. Vessels must show compliance with the regulations and one of the ways to do that is monitoring of exhaust emissions.’

Engine efficiency has a bearing on NOx levels – increasing efficiency increases NOx emissions – as well as sulphur emissions. It can be fine-tuned through continuous monitoring to gain maximum fuel efficiency while still complying with the regulations.

Cascade also provides gas sensors for the security and medical markets, the latter being mainly pointof-care diagnostics, where real-time measurements are made of a patient's breath. The technology is only beginning to find a use in this type of application and is one area where Howieson expects to see QCL gas sensors being used increasingly in the future.

UV detection

The need to measure UV is apparent in various applications, not just for monitoring the level of UV emitted by the sun that reaches Earth, but also in water purification, curing of lacquers and glues, surface treatment of metals and plastics and combustion control in heaters. All utilise UV radiation in some form and all require detectors to measure UV levels.

Scitec Instruments supplies UV photodiodes from Sglux, a manufacturer of UV measurement and detection devices.

There are three forms of UV radiation: UVA (320-400nm); UVB (280-320nm); and UVC (180-280nm). UVC is an extremely highenergy form of radiation that kills virtually all living organisms and is used to purify drinking water and destroy microbes in waste water.

‘Using short wavelength UVC, it is possible to kill all bacteria and microbes in the water,’ says Gabriel Hopfenmueller, development engineer at Sglux. ‘UV probes are required to ensure the performance of the [UV] lamp is optimised so that the water is properly purified.’

When using UVC, the sensor material needs to be particularly robust. ‘UVC destroys many materials, not only living organisms, and the sensor itself needs to be manufactured from radiation-hardened material, such as silicon carbide, to avoid being damaged,’ explains Hopfenmueller.

Silicon carbide has been used for a long time in UV sensitive chips, due to its resistance to UV radiation. Even though it is a difficult material to construct, it remains popular for UV detectors because of this property. ‘It is a question of degradation,’ says Hopfenmueller. ‘Other materials, such as titanium dioxides, can be used in UV sensors, but they currently lack the radiation hardness of silicon carbide. Sglux is carrying out work to improve the radiation hardness of these materials to produce UV photodiodes with higher performance.’

Photon counting

There are many applications that utilise single photon sources and therefore require single photon detectors, such as spectroscopy, quantum cryptography, and quantum optics. Metrology also requires photon counting devices – backscattered laser light is used to measure the distance of an object. Applications involving fluorescence, that emit single photons of light, are also used in metrology.

ID Quantique bases its business on the technology surrounding quantum photonics and, within its optical instrumentation branch, manufactures photon counters – devices that can detect single photons of light. The company, based in Geneva, Switzerland, produces three photon counter families, which detect light in the visible and near infrared (NIR) regions of the spectrum. The id400 detector family is optimised for photon counting at 1064nm, with 30 per cent detection efficiency.

Leonard Widmer, vice president of sales at ID Quantique, expects this product to be used increasingly in LIDAR (light detection and ranging) applications, which uses the properties of scattered light to position distant objects. ‘1064nm is eye safe,’ he says – an aspect that is important for this application.

ID Quantique’s id400 single photon detector operates at 1064nm and is suitable for applications such as LIDAR (light detection and ranging).

ID Quantique's photon detectors are avalanche photodiodes with reverse bias voltage. Photodiodes are semiconductor devices (the semiconductor material used in id400 detectors is InGaAsP/InP) designed to transform light into electrical current. A voltage is applied above breakdown making it unstable, the socalled Geiger mode, and when a photon hits the active area it creates an electronhole pair and an avalanche of electrons flows through the diode.

This avalanche is then quenched and the bias returned to above breakdown to allow subsequent photons to be detected. ‘It is a cycle of events,’ says Widmer, ‘but only when a photon hits the detector.’

Widmer explains that ID Quantique’s single photon detectors are digital devices, whereby the output signal is either one or zero. The single photons are detected but the output signal is not proportional to the intensity of light.

Photon counters operate in either the free running lined or gated mode. Free running mode is where the bias voltage is always above breakdown, whereas gated detectors are only active for a certain period of time. Gated detectors have the advantage of reducing ‘dark counts’ or false counts caused by impurities in the semiconductor material, but the exact arrival time of light must be known to record a measurement.

Widmer notes that advances in photon detector technology have led to improvements in the timing resolution and the accuracy of measurements. On top of this ‘the quenching circuit in the id400 detectors is positioned very close to the active area’, explains Widmer.

The faster quenching can occur and the shorter the avalanche, the less trapped charge there is in the device. Trapped charge can produce false counts and reduce the accuracy of the reading. ‘This fast and active quenching has the key advantage of reducing false counts,’ he adds.