Narrowband Visible Light: The OPO Reigns

Abstract

Fundamental research often requires narrowband light to accurately map out fluorescence spectra, reaction energetics, or other key questions in chemical, biological, and physical systems. Tunable narrowband sources are often key to this endeavor. Pulsed or continuous wave, the optical parametric oscillator (OPO) has come a long way, dropping in price while improving in performance. We explore the landscape and some common uses of this essential tool for spectroscopists.

Introduction

When it comes to spectroscopy, the combination of narrowband light sources with broadband detectors (or the converse, narrowband detection with broadband light sources) are the equivalent of the surgeon’s scalpel. They allow us to see deeply into the spectroscopic fine structure, providing experimental data to feed models, probing atomic and molecular structure, and allowing the science of spectroscopy to move forward.

For nearly a century before the advent of the laser, spectroscopists only had narrowband detectors. Kirchhoff and Bunsen’s work with their “spectroscope” led to the discovery of rubidium and cesium, and the mapping out of many of the fundamental emission lines in atomic spectroscopy, as the pair feverishly fed element after element into their “Bunsen Burner” and recorded spectra. Of course, Kirchhoff would go on to use the spectroscope to study radiative equilibrium (leading to the eponymous “Kirchoff’s Laws”) and make contributions across the landscape of physics, including thermodynamics and fluid mechanics.

Even then, while Kirchhoff and Bunsen could get reasonable resolution and sensitivity on their detectors - Kirchhoff dispersed light from the sun over nearly three meters, for example, and at least for steady-state experiments in flames or studying solar emission, photographic plates could be exposed for long periods – it was hard to obtain sufficient intensity of monochromatic light to do the inverse experiment. In other parlance, while it was easy to obtain fluorescence or absorption spectra, it was difficult to obtain excitation spectra.

This situation changed with the advent of the laser. Early solid state and gas lasers emitted one or more narrowband lines, which had some limited spectroscopic usefulness. It was the development of the organic dye laser in 1966 that provided revolutionary access to narrowband light in the visible and near-infrared. This resulted in an explosion of scientific and spectroscopic work, as nicely summarized by Frank Duarte in 2003.[1]  However, as Duarte notes, “…over the years dye lasers have acquired a reputation in some quarters as being ‘user unfriendly.’” 

Duarte does a good job of defending the many contributions and the unique attributes of dye lasers, including the available high pulse energy, femtosecond, and narrow linewidth, among several. However, the reputational aspect of dye lasers is hardly addressed, except though technological advances, and your author’s opinion and experience is that while dye lasers are a certain amount of “fun” and great tools for certain problems, they are also more difficult to maintain and operate than might be optimal.

 

Optical Parametric Oscillator (OPO) Operation

OPOs were invented in advance of the dye laser, in 1965, by Giordmaine and Miller of Bell Labs.[2] Their work utilized Lithium Niobate, which continues to be an important crystal in many OPOs on the market today. They could convert incoming 529 nm light from a CaWO₄:Nd³⁺ laser to tunable light over the range 970-1150 nm by varying the temperature of the oscillator crystal. The duo also observed conversion of only some of the pump pulses and stated that the conversion appeared highly dependent on the mode structure of the pump beam. These observations would prove prescient; while research continued, development of the OPO into a practical device would depend strongly on the availability of very high-quality crystals and appropriate laser sources and coatings, which were not available until the late 1980s and 1990s.

I was fortunate to witness some of the explosion in OPO research as a post-doc and staff member at Sandia National Labs in the late 1990s. Colleagues Tom Kulp, Scott Bisson, and the late Peter Powers at the Combustion Research Facility were among those at the forefront of the developments, and I had a ringside seat. The race was on to develop wavelength coverage and set new power and pulse-length milestones. Many of the applications of OPOs have been focused around spectroscopy, and OPOs have opened new vistas for optical sensing.

Harmonic processes for frequency conversion in crystals, including second harmonic generation, sum and difference frequency generation, and nonlinear processes such as OPO operation have two fundamental constraints.

The first, naturally, is energy conservation – the sum of the energy of photons produced must equal that of the input photons. The second requirement is a phase matching requirement; briefly put, the crystal structure must support the generation of photons in phase, or the photons destructively interfere and there is no gain built up in the crystal. Changing the angle or the temperature of the crystal changes the effective periodicity of the crystal, which can change the wavelength(s) generated.

The difficulty with the phase requirement is that there are few natural crystals that accommodate certain laser wavelengths, and of course those crystals would then only accommodate a single wavelength or its harmonics. The optimum condition (from a developer’s point of view) would be a crystal that could accommodate multiple wavelengths, or indeed any input wavelength.

Quasi-phase matching

This is where periodically-poled lithium niobate (PPLN) reenters the picture. For an incident wave E, a generated wave P will become out-of-phase and destructively interfere after a certain number of periods. The method of quasi-phase matching, which was developed independently by Armstrong, et al.[3] and by Franken and Ward[4] in 1962 and 1963 (before OPOs were invented), can correct for this destructive interference. If the phase of the generated wave is inverted periodically, after a set “interaction length” L when E and P begin to get out of phase, the phase relationship is effectively reset: if the two waves were φ out of phase at the point of the inversion, they will be – φ out of phase after inversion. If φ is relatively small, this allows gain to build up during each interaction length and minimizes destructive interference. 

Physically, quasi-phase matching is accomplished by periodic poling in the crystal. This is done in PPLN by periodically (in space) imposing a very strong electric field on the crystal, which permanently switches the electric dipole (dependent on the position of the Li and Nb ions in the crystal). This periodic poling allows quasi-phase matching for certain sets of wavelengths, and in fact different poling spacings can be introduced at different points in the crystal. This allows a single crystal to be used for different wavelength regions. Figure 1 provides a conceptual illustration of the periodic poling on the generated wave, in the case in which the generated wave has a slightly longer period than the incident wave. Figure 2 provides an illustration of the gain build-up in the generated wave with and without poling.  Without poling, destructive interference eliminates gain build-up.

Figure 1: Effect of periodic poling on the phase between the incident and generated waves.

 

Figure 2. The gain build-up in the generated wave with and without poling.

Typical PPLN is doped with magnesium oxide to increase its optical damage threshold. Effective anti-reflective (AR) coatings have also been key, because PPLN has a high refractive index (>2), and without AR coatings, losses in OPOs would be extreme. Several other crystals are also used in OPOs, for example BBO (Barium Borate), KTP (Potassium Titanyl Phosphate), and ZGP (Zinc Germanium Phosphide). Different crystals are used in various wavelength regions, some more suited for IR generation and some for the visible and UV.

OPOs in Practice

With the principle of energy conservation and quasi-phase matching, it is possible to understand not only straight harmonic generation, but nonlinear sum frequency and difference frequency generation. In sum frequency generation, two input photons are converted by the crystal to a single photon with the combined energy of the two input photons. Second harmonic generation is a special case of sum frequency generation, in which the two incoming photons are the same wavelength. 

Difference frequency generation may be considered to have several forms: 1) Two input frequencies can go into a crystal to produce multiple output photons at the difference frequency; 2) A single input frequency can produce two lower frequency beams (where the higher frequency is called the “signal” and the lower is called the “idler”); 3) two input frequencies can be input into a crystal, and one of them can be amplified, with the amplified output at one of the input frequencies, and a lower frequency output, in an Optical Parametric Amplifier (OPA).

With these options it is not surprising that OPOs and their cousins take many forms. In the early days OPOs were quite temperamental, had limited tuning ranges with gaps in tuning, and were relatively low in power. OPOs available on the market today have considerably improved in range and ease of use, while also coming down in price as crystals and coatings have become less expensive. The ability to grow very spatially-precise poled crystals in multiple materials, combined with precision movements, has contributed to this improvement, to large extent, as well as the increasing availability of stable pump lasers (both pulsed and CW) at numerous wavelengths. To achieve reliable tuning, modern OPOs are all computer-controlled.

An example of the range of product offerings can be seen in the plethora of OPO and OPA products from Spectra-Physics. The Spirit-OPA® is an ultrafast amplifier that is designed for 350 fs pumping at up to 30W, and outputs 630-1020 nm on the signal and 1040-2600 nm on the idler beams. Either can be modified with harmonics to extend the range further. The TOPAS Prime OPO is meant for pumping from 770-830 nm with a sub-150 fs beam and has an even wider range. The Spectra-Physics Inspire™ has a smaller range but uses only a single set of optics and crystals. Spectra-Physics also offers the continuous wave MixTrain laser sum and difference frequency mixers among their many laser offerings.

There are also companies that specialize in OPOs, foremost among them Opotek, headquartered in Carlsbad, Calif. They focus on the nanosecond market, with OPOs ranging from 210-3100 nm, typically 10-20 Hz repetition rate and relatively high energy. Their Radiant and Opolette models are among their leading offerings; with 4-7 cm^-1 linewidth over the entire tuning range of each device. InnoLas Laser, from Austria, has similar nanosecond-class offerings of OPOs, which operate from about 400 to 2100 nm, tailored either to diode-pumped or flashlamp-pumped Nd:YAG, and which reach 150 mJ in pulse energy. 

Other companies have niche offerings, like APE, which offers an OPO solution able to reach below 200 nm, if deep-UV coverage is required. APE also offers picosecond and femtosecond OPOs to be pumped with either NIR lasers around 1 µm, or Ti:Sapphire. The latter offerings include GHz repetition rates.  Coherent markets and sells the Chameleon MPX and Compact OPO systems produced by APE, which are Ti:Sapphire-pumped and cover wavelengths as broad as 340-4000 nm. Toptica offers the TOPO CW OPO laser for NIR and MIR applications; the system covers 1450-4000 nm.

Finally, Eksplsa has a notable range of picosecond and nanosecond-class OPOs built in their factory in Vilnius, Lithuania, with possibly the broadest range of repetition rates and energies among the 15 models of OPO that they sell worldwide. Ekspla systems include models with an integrated pump laser and OPO, and purpose-built systems like the PhotoSonus, with 150 mJ of pulse energy for photoacoustic imaging. Ekspla also has systems with very narrow, transform-limited linewidths.

OPOs Enabling Science

As one would expect, OPOs are used in a variety of ways to facilitate discoveries in science.  They have largely replaced dye lasers as tunable sources due to their convenience and relatively recent reliability gains.

For example, in biophotonics OPOs are a popular tool for fluorescence microscopy. Tissue typically scatters strongly under visible excitation, with the result that visible light penetration is limited to about 100 microns, unless the tissue is specially treated by a “clearing” technique to minimize the scattering. One way to circumvent this problem is to use near-infrared (NIR) light, which penetrates much more deeply – often millimeters. Intense, ultrafast pulses of NIR light can generate multiphoton absorbance and excitation in fluorescent tags and nanoparticles, as discussed in the January 2020 Lasers and Optics Interface column in Spectroscopy Magazine.[5]

Likewise, OPOs are an indispensable tool in physical chemistry, where the tunable source can be used to probe rotational, vibrational, and electronic states of molecules and atoms. This is useful, for example, for probing energy transfer in molecular beam experiments, or to precisely understand energy levels and transitions.

In my own home field of combustion science, planar laser-induced fluorescence is used to determine the distribution of radical species (e.g., OH and CH) as well as pollutants such as NO as they form in prototypical laboratory flames and in actual devices. OPOs are indispensable in such studies; their high repetition rates allow flame dynamics to be captured with fidelity, and statistical/spatial distributions can be easily developed.

OPOs have evolved from the temperamental creatures that they were in the relatively recent past, to reliable and versatile tools today. The variety of pump lasers, repetition rates, and wavelength ranges available on the market today makes the OPO the reigning king of tunable laser sources.   

[1] Frank J. Duarte, "Organic Dye Lasers: Brief History and Recent Developments," Optics & Photonics News 14(10), 20-25 (2003).

[2] Giordmaine, J. and Miller, R. “Tunable Coherent Parametric Oscillation in LiNbO₃ at Optical Frequencies". Phys. Rev. Lett. 14 (24): 973 (1965).

[3] J.A. Armstrong, N. Bloembergen, J. Ducuing, P.S. Pershan, “Interactions between light waves in a nonlinear dielectric,” Phys. Rev., 127 (1962), pp. 1918-1939.

[4] P.A. Franken, J.F. Ward, “Optical harmonics and nonlinear phenomena,” Rev. Mod. Phys., 35 (1963), pp. 23-39.

[5] S.G. Buckley, “The Rise of the Upconversion Materials,” Spectroscopy 35(1), pp16-21, (2020).