Greg Blackman on the latest developments in supercontinuum sources, lasers delivering broadband light for a whole range of applications
The advantage of a laser is its brightness; the drawback, or what could be considered a drawback in some circumstances, is it only emits light at one wavelength. Supercontinuum sources are sold as having the brightness of a laser with the spectral coverage of a lamp, meaning the user can pick and choose the wavelength required for their application. These sources have been around for a while and are reasonably well established as a research tool, but are now becoming reliable enough for industrial use, says Ross Hodder, global sales manager at Fianium, a manufacturer of supercontinuum lasers.
‘We’re now seeing the technology become ultra-reliable to several thousand hours’ operation and the cost is coming down. It starts to become a really attractive lamp and LED replacement technology,’ Hodder comments. ‘Imaging and industrial inspection and medical imaging applications that are currently using lamp technology are now looking to supercontinuum either to provide enhanced performance, because they need the high brightness, wider wavelength ranges or better stability, or even to reduce the maintenance and cost of the system.’
One of the developments all the supercontinuum source providers are working on is extending the supercontinua further into the UV, which mainly depends on the design of the photonic crystal fibre (PCF) generating the supercontinua. ‘Containing UV radiation in a single-mode fibre is very difficult, as the shorter the wavelength the more challenging it is for this wavelength to travel down the fibre as single-mode,’ explains Husain Imam, regional sales manager at NKT Photonics.
Each of the wavelengths in a spectrum ranging from 350nm to 2.4µm will have different mode sizes, with the UV light having a very small mode-field diameter compared to the longer wavelengths. Engineering them all to travel down the fibre in single-mode with high reliability is a challenge, says Imam.
NKT Photonics’ core technology is the photonic crystal fibre, the nonlinear medium that generates broadband supercontinua. NKT Photonics also provides its SuperK supercontinuum laser, which operates over various wavelength ranges.
Extending the wavelength range into the UV has potential benefits in spectroscopy, where the more wavelengths emitted by a light source the more useful it is, as well as confocal and fluorescence imaging. Hodder comments: ‘There are fluorescent dyes at 400nm, 405nm and 355nm that you wouldn’t have been able to use with supercontinuum lasers five years ago, but which the new sources are suitable for.’ A third area where these light sources could be useful is process inspection in areas like semiconductor manufacture. ‘There’s a massive improvement in inspection capability for semiconductor manufacturers at shorter wavelengths,’ states Hodder.
Fianium’s supercontinuum lasers are based on an ytterbium fibre laser linked to a PCF. The first generation of systems Fianium built in 2004 had a short wavelength cut in of around 500nm, which went out to beyond 2µm. The company is now releasing UV-enhanced systems with a cut-in as low as 320nm. This is achieved largely through optimising the supercontinuum fibre design to access these shorter wavelengths – Fianium is working with the University of Bath to develop new fibre designs for different broadband emission spectra.
A PCF is generally made of a solid silica core and a cladding region containing air tubes. Manipulating the size of the core or the ratio of air to silica in the cladding, among other parameters, alters the wavelength range produced by the laser. The nonlinearity is invoked by using the peak power of a pulsed laser – to produce effective supercontinuum light, firstly, the laser has to have a high peak power, typically a picosecond-pulsed laser, and secondly, there needs to be a highly nonlinear medium, which is the photonic crystal fibre. The fibre has to be designed to handle the high peak powers without degradation and still provide high power supercontinuum output, as Imam explains: ‘As the spectrum gets broader, balancing the amount of peak power to generate bluer wavelengths, for example, at high power, while maintaining high reliability requires a combination of good fibre design and efficient system design of the light source.’
As well as designing supercontinuum lasers that emit further into the UV, there is also work to improve the range in the infrared. NKT Photonics is working on fibre technology for supercontinuum generation in the mid-infrared for an EU project called Minerva, led by Gooch and Housego. The project is developing a mid-IR imaging spectroscopy platform for the early detection of cancer, including fibres for supercontinuum generation and delivery.
A supercontinuum source is ideal as a replacement for a bank of multiple single-wavelength lasers in a device like a spectrometer. PicoQuant has developed its Solea supercontinuum white light laser source, with a spectral range of 480 to 700nm, for just such an application. ‘One of PicoQuant’s main markets is spectroscopy,’ explains Uwe Ortmann, head of sales and marketing at the company. ‘In the past, we worked with single wavelength diode lasers. If you want to change from one excitation to another, you have to change the laser heads. We have customers that buy 10 to 15 laser heads to cover the whole spectrum, which then becomes clumsy to change wavelengths. In these cases there is a demand for a supercontinuum laser that’s integrated into our software architecture and whereby the beam is tuneable over a wide range of wavelengths.’
PicoQuant provides picosecond pulsed lasers based on diode lasers. Its supercontinuum laser is also based on diode laser technology, which allows it to be triggered electronically and synchronised with other fixed pulse rate lasers.
To replace mid-power conventional laser systems requires a relatively high-power supercontinuum source to cover all the wavelengths after filtering. For example, flow cytometry systems might have up to eight lasers at 100mW average power, so to replace each of these the broadband source has to be reasonably high-power.
Hodder says that one of Fianium’s areas of development is in terms of power scaling. Fianium’s current range of products runs from 200mW, which might be used for general-purpose spectroscopy applications, to a 10W system. ‘We are working on a 25W system, which is still in the R&D phase,’ he says, ‘and want to potentially increase that to several hundred watts in the future.’
The other area for high-power supercontinuum sources is in military and sensing applications, particularly for operating over long distances. In hyperspectral imaging, for example, a supercontinuum laser can be used to build up information at any wavelength. The light can be filtered at the emission side, or some systems will collect the whole light emission into a spectrometer and build up a mass of spectral information and then pick the relevant data from it.
There is also potential for the lasers to replace light sources from different pieces of apparatus in, say, the medical field, by basing it all on a supercontinuum laser. Imam at NKT describes, as an example, the diagnostic equipment used in ophthalmology: ‘OCT based on specific wavelengths of light is used to generate cross-sectional images of the retina; there are other imaging techniques used to examine the eye based on other wavelengths of light; and clinicians might also want to probe the eye with spectroscopy which uses yet further wavelengths.’ The tests are run on three different pieces of equipment using three different laser technologies. ‘A supercontinuum source incorporates all those wavelengths in one box and potentially clinicians could have one instrument based on a supercontinuum laser to carry out all these tests,’ he says.
Imam adds: ‘A supercontinuum source may not have the highest performance in OCT, retinal imaging or spectroscopy, but it can do all of them to a level of quality that if you put these three methods together you get more information than from each of the individual instruments on their own.’
Supercontinuum sources are typically based on picosecond or femtosecond pulsed lasers, but French firm Leukos has developed microchip and diode laser-based sources that have around 500ps up to 4ns pulse widths. ‘The advantage of a longer pulse width is that it provides higher energy than short picosecond pulses,’ says Karine Weck, director of sales. Leukos’ microchip-based lasers deliver more than 3µJ per pulse and operate down to 320nm. ‘There are a lot of applications that don’t require a lot of power but where high energy is beneficial,’ she says.
Weck says that CARS (coherent anti-Stokes Raman scattering) microscopy, for example, could benefit from nanosecond pulse durations (the technique usually uses femtosecond lasers). ‘We have demonstrated a CARS supercontinuum system based on nanosecond pulses, which offers dual output in a compact profitable configuration,’ she states.
Leukos uses several technologies for the seed laser, including microchip lasers, diode lasers and fibre lasers. Its supercontinuum sources based on diode laser technology can reach up to 2W output power, while its mode-locked fibre supercontinuum laser provide up to 6W of power.
Extending the wavelength range and increasing the output power, as well as reducing the cost of these systems, will ultimately make supercontinuum sources applicable for use in more areas. Imam also states that reliability is an important issue, and he says that NKT Photonics’ main motivation is to provide supercontinuum light sources that are reliable and high-performance with a long lifetime. Subjecting the fibres to too much peak power, for instance, can cause them to degrade and shorten their lifetime. However, supercontinuum sources are now robust enough to be used in industrial applications and, by providing broadband spectral coverage, can reduce the complexity of a system by replacing a number of single-wavelength lasers, or give high brightness illumination as a replacement for LEDs.