When it comes to probing the secrets of matter, few tools have been as useful to chemists as the humble photon. By using sophisticated photonic devices called optical parametric oscillators (OPOs), modern chemists are able to produce photons at a wider range of wavelengths, and at a narrower bandwidth than ever before. In spectroscopy, different wavelengths of light correspond to different types of absorption within a chemical structure. Access to a wide range of wavelengths allows the chemist to look at a wide variety of properties, and the narrower the spectral bandwidth of the light source, the higher the resolution of the readings will be.
While some standalone laser sources do allow a degree of tunability in their design, typically the range is small. Equipping a laser source with an OPO can extend this range, as Frank Mueller, product manager at Linos Photonics, explains: ‘The principal advantage [of OPO-equipped lasers] over what we might call ordinary lasers is that they offer a very broad tuning range. For example, our standard device covers a range of 1.4-4.7μm, with just a small gap at 2-2.3μm. The user can access any wavelength within this range, whereas most lasers only cover a wavelength range of a few nanometres. The powers available depend on the system set-up; they vary from approximately 10mW to above 1W on request.’
While greatly extending the wavelength range of the laser, the OPO does not necessarily impair the quality of the light produced: ‘The line-width of the OPO output can be extremely small – as small as the line-width of pumped lasers in principle,’ says Mueller. ‘We have measured a short time line-width on the timescale of 20μs in the order of 10kHz.’
Linos uses an integrated continuous wave (CW) solid state laser to pump its OPOs: ‘In our case it’s at 1,064nm, but it could also be at different wavelengths,’ explains Mueller. When describing the idea behind the OPO, Mueller states that the light from the pump lasers is passed into a non-linear crystal – lithium niobate (LiNbO3) in this case. Pairs of photons are generated, one designated ‘signal’ and the other ‘idler’, and their frequencies always add up to the pump frequency. ‘The non-linear crystal is placed inside an optical cavity that is resonant for at least one of the generated frequencies,’ says Mueller. ‘If the pump power exceeds a threshold, where the gain compensates the power losses of the resonant wave, or waves, a process called optical parametric oscillation starts. In this way, one is able to build up a single pair of signal and idler waves with macroscopic power levels.’
Choosing a colour that suits
‘Most of our customers do spectroscopy,’ says Mueller. ‘Depending of course upon the species to which they wish to apply their experiment, they need lasers in the near or mid infrared.’ Angus Henderson, a scientist and product manager at US-based Lockheed Martin Aculight, a company that produces OPO-equipped CW laser sources, confirms that the near and mid infrared wavelengths are particularly useful to spectrometrists: ‘Most of our devices operate in the 3-4μm wavelength range, in which there are a lot of interesting molecular species in terms of the wavelengths of their absorption spectra. There are lots of vibrational absorptions in particular at this range, especially those corresponding to carbon-hydrogen stretch absorptions,’ he says, referring to the covalent bonds present in all organic compounds.
‘It’s a wavelength range that you can’t obtain via many other high resolution laser sources, and as a result it’s of interest for applications such as hydrocarbon spectroscopy within the oil and gas industry,’ he says, adding that detection of trace amounts of hydrocarbon gasses is one example application. According to Henderson, techniques used in conjunction with such a narrow bandwidth light source include high sensitivity photoacoustic cavity spectroscopy, and ring-down spectroscopy, both of which are able to achieve high-sensitivity detection. The choice of wavelength range is important for these high sensitivity techniques: ‘You need the very strong vibrational absorptions in the 3-4μm range to get the high sensitivities.’
Away from the molecular vibrational absorptions at mid-IR wavelengths, Mueller says that OPO-equipped lasers can be used for more exotic types of spectroscopy: ‘On species at ultra-low temperatures, for example, the line-widths [of the absorptions] can become very small, very narrow. This is the reason the applications require such ultra-narrow laser sources,’ he says.
Outside of spectroscopy altogether, CW lasers equipped with OPOs can be used in high-sensitivity metrology, or a range of research and development applications in which a selectivity of wavelength is desirable. Henderson explains that the 3μm wavelength produced by Aculight’s OPO-equipped lasers is readily absorbed by water, and that this characteristic opens up the possibility of applying the technology to medical applications. Alongside its high spectral resolution system, Aculight offers low resolution product for R&D applications in which wavelength coverage is required without such precise and costly wavelength specificity.
‘The OPO is intrinsically narrow line-width as long as the pump laser is narrow line-width, and you typically get out a line-width which is equivalent to what you pump it with,’ he says. ‘Our single frequency version is pumped by a single frequency fibre laser; the broadband version is pumped by a broader line-width fibre laser.’
Solid advantage
Mueller acknowledges that applications for OPO-equipped lasers are mostly confined to scientific R&D: ‘Most of our customers are universities and research organisations,’ he says. ‘That said, at some point in the future a market for commercial applications may emerge, as the systems get more and more robust, as the price goes down. The first OPOs that were on the market were fairly difficult to handle – they really needed to be used by a scientist. Now they are getting more and more reliable, due in part to the development of high quality non-linear materials. The next step will always be fully computerised and automatic tuning.’
Henderson echoes this sentiment, describing the importance of a durable device: ‘OPOs can be pumped by different types of lasers, but we use a 1μm fibre laser to pump the Argos OPO. These have taken over from solid state lasers because of their ease of use and their durability; the lack of free-space optics in a fibre laser gives them the ability to maintain alignment and produce a very good beam quality. Ruggedness is absolutely essential to us; it’s the reason that we’re able to ship these systems to the UK and Germany without having to go over there and install them ourselves,’ he says, adding that no other business model could work for the company. ‘We don’t have the manpower to support international service of our lasers. Traditionally, scientific lasers sold internationally are supported by a network of service engineers and technical support staff, mainly because they have to be. They’re very tweaky instruments, with free-space aligned lasers and optics that can get contaminated with dust and so on. They can also be very difficult to maintain,’ he says, adding that the scientific lasers he has in mind are those based on Ti:sapphire architectures in particular. ‘They’re very complex, and require watercooling, which is inherently unreliable. We don’t have any water-cooling in our devices, and no free-space optics; the OPO is built as a quasi-monolithic block. It’s a very different type of architecture to the traditional type of scientific lasers that are sold into this field.’
Berlin-based Angewandte Physik und Elektronik (APE) produces OPOs to alter the wavelength range of pulsed lasers, primarily for use in the life sciences, and the thinking behind the company’s recent product releases mirrors that seen in CW lasers. A push towards ruggedness and usability has allowed the company to offer products which can be used by non-laser specialists, including the recently released Chameleon OPO – an alignment-free, fully computer-controlled device.
Pushing further
Mueller explains that Linos’ current development efforts usually go towards extending the wavelength range of the OPOs: ‘On request we can drive the wavelength range even further than 4.7μm, but this is currently a tough job. We are always limited by the materials that are available: lithium niobate, lithium tantalate, and KTP are most commonly used. The materials each have a limited transmission range – there are non-linear materials available that also have a transmission range at longer wavelengths, but they have other difficulties.’
Henderson says that Aculight is also looking at extending the products’ wavelength range, achieving this by increasing the output power of the systems: ‘We’ve already developed a system that puts out about three times as much power as other versions. We’re power scaling for applications that need more power, and we’re looking at using that power to extend the wavelength range into the near-IR, where other sources are not so readily available.
‘We’ve done a frequency doubling of one of these OPOs, down into the 700-1,000nm range. This competes with Ti:sapphire lasers, without requiring water cooling. We think that there’s potential for producing adjacent wavelength ranges without the hassle of some of the other technologies,’ he says.
According to Mueller, the wavelength range of the CW OPOs is constrained by the limits of the non-linear materials used. Pulsed systems, he says, are able to bypass some of these limitations because the high peak power in a pulse can overcome the transmission threshold of the material more easily.
Finger on the pulse
Marco Arrigoni is director of marketing at Coherent, a US-based specialist in lasers for scientific applications which produces OPOs and amplifiers for applications using ultrafast pulses of laser light (femtosecond). He explains the motivation for going to a pulsed regime: ‘With pulses of light of a few tenths or a few hundredths of a femtosecond, you can unlock a lot of new science; you can freeze in time a range of processes like the vibrations and motions in molecules, or the dynamics of molecules on a smaller scale. There is a lot of physics that can take advantage of these femtosecond pulses.’
A mode-locked laser, he explains, generates a steady stream of pulses at a repetition rate of 50-100MHz, with durations of 10-100fs, and energy per pulse typically between 5 and 20nJ. The average power is therefore around 1-2W. ‘There are many things you can do with these pulses,’ he explains, ‘but there are many things you can’t do because the energy per pulse is too low.’
Through chirped-pulse amplification, or CPA, the repetition rate can be reduced in exchange for higher peak power pulses. Arrigoni explains that a typical amplifier produced by the company emits pulses with a 1kHz repetition rate and pulse energy of 10-100mJ (a million-fold increase), with pulses of 25-100fs duration.
He adds that while an amplifier can be altered to vary the pulse duration, the wavelength of the pulses cannot be changed; an optical parametric amplifier (OPA) can be used after an amplifier in the same way that an OPO can be used after a laser oscillator in order to allow the users to gain access to a wider wavelength range. Because of the high peak power possible with pulsed lasers, the range of wavelengths accessible through OPAs is greater than that of OPOs: ‘An OPO after a laser oscillator expands the tuning range from 700-1,000nm to something as broad as 500-4,000nm, or even longer,’ says Arrigoni. ‘An OPA after an amp can expand the tuning range to 190nm-20μm.’
Applications for the spectra of pulsed wavelengths generated by an OPA-equipped laser run along similar lines to those of their continuous wave counterparts; novel types of spectroscopy are of chief importance. APE, in conjunction with High Q Laser, offers its picoEmerald OPO-equipped ultrafast system for use in coherent anti-Stokes Raman spectroscopy (CARS), as well as other research applications in the life sciences.
Arrigoni explains that many experiments using this type of equipment are ‘pump and probe’ arrangements, in which two pulsed beams of different wavelengths are used: ‘You use your pump beam to excite a system, which could be a collection of atoms and molecules, and then you use the probe beam to interrogate the system by finding out, for example, how quickly the molecule’s excitation decays to the original level, or by creating a transient absorption by exciting it further to a next level. This all gives you an idea of the dynamics of your system,’ he says. The technique is gaining importance in some biophotonic research.
As is the case with many ultrafast devices, these technologies are currently expensive and, for the most part, confined to the laboratory, but applications are already emerging for these powerful and sophisticated devices.
