The basics of OPO technology in tunable laser design

Tunable lasers are lasers designed to emit light at various wavelengths. This enables users to tune the output for specific requirements as well as optimize their performance, thus making them valuable in diverse applications. Dye lasers tend to cover a broad wavelength range, however not with a single dye. This limitation can pose great inconvenience for applications demanding extensive tunability. Moreover, attaining and maintaining precise and reliable tunability can be challenging.

The alternative approach from it to leverage optic parametric conversion to transform readily available single-frequency laser light into the visible spectrum.

Optical Parametric Oscillators: flexible tunability

State-of-the-art OPO technology delivers visible, single frequency, tunable laser light from 450 nm – 650 nm

Optical parametric oscillators (OPOs) might be considered as light sources that deliver coherent radiation very similar to lasers – but with two main differences between the devices [1]: First, the OPO principle relies on a process referred to as parametric amplification in a so-called nonlinear optical material, rather than on stimulated emission in a laser gain medium. Second, OPOs require a coherent source of radiation as a pump source, unlike lasers, which might be pumped with either incoherent light sources or sources other than light.

In practice, the OPO concept has been experimentally demonstrated already more than half a century ago [2], but the progress in development and commercialization of turn-key devices has been stalled substantially by several technical obstacles [3]. Simply speaking, these obstacles have been easier to overcome at the high peak powers of pulsed devices, so that tunable OPOs operating in pulsed mode have become readily available from a variety of suppliers. Only relatively recently, there have been comparable advances in continuous-wave (cw) OPO technology [3], which have spurred the development of commercial systems.

This progress has been mainly driven, on the one hand, by the increasing availability of cost-effective high performance cw pump lasers, and, on the other hand, by the advent and increasingly sophisticated design of new nonlinear crystals. As to pump lasers, the operation of cw OPOs puts stringent requirements on potential light sources in terms of preferential single mode operation, noise characteristics, beam quality, and beam pointing stability. Depending on power requirements of the end-user, either high performance diode pumped solid state (DPSS) lasers (for lower powers), or fiber laser based solutions (for higher powers) are typically utilized. As to nonlinear materials and novel crystal design techniques, it should be noted that the emergence of so-called quasi-phase-matched nonlinear materials like periodically poled Lithium Niobate (whose crystal structure alters with a certain periodicity) has been of great utility for the design of practical optical parametric devices.

Practical design considerations

While OPO technology appears to be ideally suited for generating tunable cw laser light across arbitrary wavelength ranges, one must keep in mind that the OPO process itself will always generate output at wavelengths that are longer than those used for pumping. Consequently, OPO devices operating across the visible spectral range do either require UV pump sources, or, alternatively, need to employ additional frequency conversion stages. As of today, only the latter approach has been proven to be technically practicable and operationally stable in commercial turn-key systems.

For illustration, the essential building blocks of a tunable cw OPO [4], designed to cover the visible range, are shown in Figure 1. In essence, the operational principle relies on a cascaded sequence of nonlinear optical processes within two cavities, referred to as OPO and SHG cavity, respectively. As outlined above, pump laser photons are first split into pairs of photons of lower energy (signal and idler). The particular OPO scheme employed is commonly referred to as singly resonant OPO cavity design: For a certain operational wavelength of the entire system, the cavity is operated “on resonance” at either a particular signal wavelength, or a particular idler wavelength. Thereby, a precisely moveable stack of periodically poled nonlinear crystals allows for broad wavelength coverage.

Fig 1: Schematic beam path inside a commercial cw OPO system [4].

At a particular wavelength selection, a crystal layer with a suitable poling is automatically selected and its poling period fine-adjusted through a temperature control loop. At the same time, the effective OPO cavity length is actively stabilized to a multiple integer of the selected operational wavelength. While circulating one of the generated (signal or idler) waves resonantly inside the OPO cavity, its counterpart can be extracted for wavelength conversion into the visible spectral range by another nonlinear process. As illustrated in Figure 1, this wavelength conversion takes place in a second, separate cavity by frequency doubling of the primary OPO cavity output, a process widely known as second harmonic generation (SHG). Though this configuration is technically practicable and provides favorable operational stability, it should be mentioned that alternative designs, like intra-cavity frequency doubling, have been successfully demonstrated in the lab.

The performance characteristics of commercial OPOs make them competitive alternatives to conventional lasers and related technologies for the generation of widely tunable cw radiation. Applications for such OPO technology range from nanophotonics and quantum optics, where high performance, single frequency, tunable visible (or NIR) laser light is required.

The mechanisms behind wavelength tuning

There are essentially 3 techniques of wavelength tuning. The process of tuning across the entire wavelength range, termed “coarse tuning,” involves precise adjustment through atomized selection of optics and crystal temperature. This meticulous control modifies the phase-matching condition, achieving a typical resolution on the order of one nanometer. However, due to the intricacies of temperature tuning, wavelength changes require between 30 seconds and a few minutes, presenting a marginally slower process compared to alternative schemes.

For “stepwise tuning,” the adjustment is facilitated by the movement of the intra-cavity etalon. This method typically provides a tuning range on the order of 100 gigahertz, characterized by step sizes in the range of a few gigahertz and a speed of approximately 1 second per step.

Finally, the pursuit of “continuous tuning” entails Piezo scanning of the OPO cavity length. This advanced technique results in tuning intervals on the order of 10 to 50 gigahertz, with a remarkably swift tuning speed of approximately 10 gigahertz per second. The achieved resolution is well below one megahertz. (FIG 2)

This nuanced approach to tuning in tunable OPOs enables seamless and precise adjustments across a wide frequency spectrum.

Fig 2: Schematic design of 3 techniques of wavelength tuning

How to convert an OPO signal output into the visible range

Now equipped with a fully tunable OPO cavity, we can delve into the conversion process leading to visible wavelengths.

The initial step involves separating the remaining pump and splitting the primary OPO output. Utilizing the OPO signal, we can then channel it into a second bow-tie cavity specially designed for resonant frequency doubling or second harmonic generation. To simplify, this process is, in a sense, the reverse of parametric conversion, where two photons of lower energy are brought together to form one photon with higher energy (FIG 3.1). This technique enables the conversion of the OPO signal output to wavelengths within the visible spectrum.

It’s worth noting that instead of circulating the OPO idler, an alternative approach is to circulate the OPO signal and employ the idler part for frequency doubling (FIG 3.2). The resulting output in this configuration covers a range from 1,080 to 1,300 nanometers, translating to 540 to 650 nanometers after frequency doubling. Consequently, with this design, we achieve a wavelength coverage from 450 to 650 nanometers, with a small gap around 532 nanometers, effectively spanning almost the entire visible range. This intricate design proves instrumental in extending the tunable OPO capabilities into the visible spectrum.

Fig 3.1: Conceptual design of the process of converting OPO signal output to wavelength in the visible.

Fig 3.2: Alternative process by using the idler part for frequency doubling.

References

[1] R. Paschotta, Optical Parametric Oscillators, Encyclopedia of Laser Physics and Technology Ed. 1, Wiley-VCH (2008)[2] J. A. Giordmaine and R. C. Mills, Tunable coherent parametric oscillation in LiNbO¬3 at optical frequencies, Phys. Rev. Lett. 14, 973 (1965)[3] M. Ebrahim-Zadeh, Optical Parametric Oscillators, in Handbook of Optics Ed. 2, McGraw-Hill, Ed. 2 (2001)[4] J. Sperling and K. Hens, Made Easy: CW laser light widely tunable across the visible, Optik & Photonik 13, 22 (2018)