D-module, providing up to 100mW, has still high enough power to be useful compared to many laser sources in that wavelength range, says Henderson. By comparison, the company has generated up to 10W at 3.5µm and 25W at 1.5µm. ‘There is a big variation in power, which is primarily defined by the energy of the photon. But as you head above 4µm, the power starts to be effected by absorption within the crystal,’ Henderson comments.
Other developments within OPO technology that Frank Mueller, product manager EOM/CW-OPO at Qioptiq, identifies are those aiming to address totally different wavelength ranges (further into the UV or IR) by, for example, combining the OPO with sum-, difference- or second- harmonic- frequency generation. Various research groups have also demonstrated CW OPOs addressing the terahertz region. The group is conducting research in collaboration with the Forensic Alliance into using its terahertz source for identifying and characterising illegal drugs concealed about the person or in packages. The advantage of terahertz waves are that they can penetrate various materials such as paper, wood, plastic, ceramics and human tissue in a non-invasive way (unlike X-rays) and discriminate between different heavy organic molecules. A spectral fingerprint can therefore be characterised for different drugs. The group’s terahertz source based on parametric generation delivers narrow linewidth terahertz radiation that can be tuned to target specific features.
Spectral resolution In order to begin the process of optical parametric oscillation the pump-power has to overcome a particular threshold. For pulsed lasers, the energy of a single pulse is typically high enough to overcome this threshold. In addition, the average power of pulsed systems is usually comparatively low, so thermal effects are less problematic. Therefore, according to Mueller, there are more pulsed OPOs available on the market than continuous wave versions.
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Mueller comments: ‘If narrow linewidth radiation is required, CW OPOs are superior [to pulsed systems].’ The narrower the linewidth the greater the spectral resolution. For researchers using highly accurate spectroscopy, such as for studies on ultra-cold molecules, an ultra-narrow linewidth is required with wavelength stability and accuracy. Qioptiq has custom- designed one of its OPOs so that it could be stabilised with the customer’s frequency comb. As a result, the system achieved an extremely small linewidth and very good long-term frequency stability – both parameters were measured to be sub-kilohertz. The system also offered output power, depending on wavelength, higher than 1W and it can be tuned from 1.4-4.6µm, with only a small gap of 2-2.3µm near degeneracy. APE’s sync-pumped OPOs, on the other hand, are mostly used in non- linear imaging, a type of microscopy in which non-linear effects are used to elicit fluorescence or other interactions from a sample. In this application, a high repetition rate is required to obtain a high enough frame rate, as well as high peak power to excite the non-linear effect. ‘This can’t be achieved with a continuous wave OPO because peak power is too low, and neither can it be done with the lower repetition rate of the high-energy, single pulse systems,’ says Büttner. According to Mueller, one of
the areas OPO manufacturers are working on is building more compact devices that can be integrated easier into a customer’s apparatus. Other developments include moving toward higher power levels, which in turn require higher quality non-linear crystals and more accurate polishing and coating capabilities. In addition, there are also alternative non-linear materials on the market, other than lithium niobate and lithium tantalite, which, Mueller says, address longer wavelengths and are promising candidates for use in OPOs. However, he adds, they are not yet at a stage to be used in commercial systems. l
June 2011 l ElEctro opticS 21
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