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tEchnology opos


Lightin tune I


greg Blackman on the technology surrounding optical parametric oscillators


n most cases, lasers emit at a specific wavelength of light, typically dependent on their gain material. The Nd:YAG laser emits at 1064nm and this is suitable for many areas, but there are times when alternative wavelengths are required. In spectroscopy, the analyst will often want to illuminate under subtly different wavelengths. This is where optical parametric oscillators (OPOs) come in, as these devices allow the analyst to tune their laser system to any wavelength within a relatively broad range, which is critical to many applications.


‘The main advantage with all OPOs is that new wavelengths can be generated outside of standard laser wavelengths; the user is not limited in the same way as with typical laser gain materials,’ explains Edlef Büttner, R&D director at Berlin-based Angewandte Physik und Elektronik (APE). ‘Standard lasers are restricted by the gain material and with high- energy lasers there are a few, mostly fixed frequencies available only. An OPO can fill gaps in between the different laser gain materials and its harmonics. The ability to cover a wide range of wavelengths is the big advantage of OPOs.’ APE manufactures its


synchronously-pumped, quasi- CW OPO, which generates a continuous train of identical pulses


20 ElEctro opticS l June 2011


all interconnected by the cavity as a feedback mechanism.


An optical parametric oscillator consists essentially of an optical resonator and a non-linear optical crystal. Using a 1064nm fibre laser as the pump source, the photons are frequency converted in an OPO cavity into a short wavelength (the signal wavelength) and a long wavelength (the idler wavelength). The share of the energy between the signal and idler photons is defined by physics and always adds up to the photon energy of the source wavelength. The ratio between signal and idler changes depending on the poling period of the non-linear crystal, which allows continuous tuning over a wide wavelength range.


Spectroscopy One of the core markets for OPO technology is spectroscopy. Chemists investigating hydrocarbons, for example, as part of basic scientific research or in the oil and gas industry use infrared laser spectroscopy to classify organic compounds. The region between around 3 and 4µm contains virtually all the characteristic absorption frequencies of molecules with carbon-hydrogen bonds. ‘There is a lot of science involved in studying hydrocarbon molecules


and how they behave under different circumstances,’ explains Angus Henderson, principal scientist at Lockheed Martin Aculight. ‘Different hydrocarbons have different features, and very precise wavelengths are scattered anywhere between 3-4µm. To have a laser that covers that range with good beam quality and Watt- level power is an enabling capability. There’s no other source that can come anywhere near that.’


Lockheed Martin Aculight’s Argos C-module covers almost the entire hydrocarbon absorption region. Henderson says that there are other lasers in the 3-4µm range but nothing that provides the requisite power levels. ‘In the mid-IR, there have recently been some demonstrations of interband cascade lasers, but these produce milliwatts of power and have a fairly restricted tuning range,’ Henderson says. ‘This is compared to 3-10W produced by [Lockheed Martin Aculight’s] OPOs with excellent beam quality.’ Other applications may have


different requirements. Detecting methane leaks along gas pipelines from the air is another area where OPOs are used. High power is more important in these types of applications, Henderson says, while it might be less important to have a narrow spectrum.


Within an opo cavity, the pump beam is frequency converted into signal and idler wavelengths. image courtesy of Angewandte physik und Elektronik (ApE)


Lockheed Martin Aculight Argos OPOs are continuous wave (CW) devices pumped by fibre lasers, with three standard modules covering 1.46-2.06µm for the signal wavelength and 2.2-3.9µm in the idler wavelength. The company has also designed a lower power module (D-module) for longer wavelengths up to 4.6µm. ‘Between these four modules, 1064nm can be converted to any wavelength between 1.38


lockheed Martin Aculight’s Argos opos are continuous wave devices pumped by fibre lasers


and 4.6µm – that’s a tremendous wavelength range,’ Henderson states.


periodical poling There have been gradual advances in power and tuneability in OPO technology over recent years. However, one of the most important advances, according to Büttner, has been the use of periodically poled crystals as gain material, which has added new flexibility to OPO tuning. ‘Previous OPOs were limited to the parameters of the birefringent crystal materials,’ he notes. ‘Periodical poling allows materials to be modified according to the needs of the application. The flexibility of the instrument is therefore broadened and users can select a material that covers a specific wavelength range or that matches a specific pair of input and output wavelengths.’ Within limits, Henderson says that the non-linear crystal can have arbitrary poling periods. ‘There’s no limitation for the OPO in terms of being able to pole the right poling periods,’ he says. But he adds that the transmission of the material ends up being the limiting factor, because at around 4µm, lithium niobate, a common non-linear material used in OPOs, starts to absorb. Therefore, at 4.6µm, as in the case of Lockheed Martin Aculight’s D-module, much lower power is generated than at, say, 3µm because a lot of the photons are absorbed within the crystal. The


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