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07 Feb 2005
If you want to frequency-double a laser or build an optical parametric oscillator, you'll need a nonlinear crystal. Marc-André Herrmann guides you through the most popular materials on the market.
What exactly is a nonlinear crystal? Put simply, it's a material that exhibits interesting optical effects when it is exposed to intense laser radiation under a set of carefully controlled conditions such as phase-matching (see box "jargon buster"). Today, nonlinear crystals play a vital role in many laser-based devices. For example, they provide a convenient way to convert the emission of a laser from one wavelength to another (frequency conversion) and are found at the heart of tunable sources called optical parametric oscillators (OPOs).
This article will describe the crystals commonly encountered with solid-state lasers emitting in the near infrared (from around 1 to 2 μm). The most common applications are:
• frequency-doubling of a laser beam (also known as second harmonic generation or SHG). This is often used with Nd:YAG lasers operating at 1064 nm to create a powerful source of green (532 nm) light.
• mixing two different laser beams to create a third (also known as sum frequency generation or SFG).
• building an OPO. An OPO consists of a nonlinear crystal within a cavity. Optically pumping the crystal generates signal and idler beams that can be tuned.
In addition, some nonlinear crystals exhibit a strong electro-optic effect (change in refractive index when an electric field is applied) which can be harnessed for such tasks as electro-optic Q-switching (see OLE January p31). In this case, the crystal's ability to rotate the polarization of an incoming laser beam is used to create a controllable optical loss (switch) that prevents the laser from firing. By turning this switch on and off repeatedly, it is possible to create a train of high-energy pulses.
Fabrication Nonlinear crystals are synthetic. This means that they are obtained from a raw material (a "boule") which is grown from a melt at a high temperature (often several hundreds of degrees) using "growth stations". The growing time of a boule is usually between several weeks and a few months, depending on the growth conditions, growing techniques and the material. The growth process needs to be precisely monitored throughout to yield a raw material of high optical quality. The size and shape of a boule varies from one material to another, but boules are generally a few inches wide and weigh 100-500 g. Some materials, however, like KDP or lithium niobate, are easier to obtain and can be grown to much larger sizes.
After it has been grown, the boule is sliced and diced into crystals with the cut angle, aperture and length determined by their particular application. The crystal's optical surfaces (input and output apertures) are then laser-grade polished and, most of the time, thin-film coated before use.
For a summary of the most common types of nonlinear crystals along with their potential applications, advantages and disadvantages, see table "comparing crystals". This is not an exhaustive list and does not go into the details of individual applications. This table does not include materials like lithium tantalate or bismuth borate, which are promising but still under development.
How to buy a nonlinear crystal First of all: do not hesitate to ask for advice. Anyone who has ever purchased a nonlinear crystal knows that, despite their small size, they are quite expensive. Prices range from a few hundred US dollars to more than $1000, even for standard-sized pieces. Do not hesitate to get in touch with crystal manufacturers for guidance.
Second: quality is not a luxury. In most cases, nonlinear crystals are an essential component of the laser device they go into. In addition, they are always used in operating conditions which are very demanding and involve high-power densities of laser radiation. Any surface or bulk defect may not only result in crystal damage, but could also cause the whole system containing the crystal to malfunction. When purchasing a nonlinear crystal, bear in mind that price should not be the top criterion for making a choice - a cheaper crystal may not be up to it.
Third: a nonlinear crystal is not a raw material. Each crystal is the product of a manufacturing process (including crystal growth, processing and thin-film deposition) which is long, complex and difficult to fully control. A crystal-user wanting the production of, for example, tens of devices using a crystal should carefully choose their supplier in order to secure deliveries. Most of the laser manufacturers hedge their supply sources in order to avoid shortages; the answer to this concern is to develop and maintain a close relationship with a proven and reliable crystal manufacturer. This partnership should lead both manufacturer and user to work together to define adequate specifications and crystal manufacturing processes, in order to ensure reliable deliveries at a fair price.
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