OPO Lasers: Putting Optical Components to the Test

Mark Little, Opotek, LLC

Optical parametric oscillator (OPO) lasers test optical fibers and components to characterize the spectral response of optical components. OPO lasers are common in sophisticated test and measurement applications such as mass spectrometry, photoacoustic imaging, and spectroscopy. Now, these tunable pulsed lasers are being used to facilitate a range of tests at different wavelengths to qualify and quantify the performance of optical components such as fiber optic strands, filters, lenses, and coated mirrors.

Lasers, in general, have long been used in the test and measurement of optical materials. By design, most optical components reflect, filter, or transmit specific wavelengths, or ranges of wavelength. Therefore, it is critical to perform tests of component materials and coatings to ensure that products perform as expected. The more precise these tests, the higher the quality of the product — a factor that manufacturers can turn into a competitive advantage.

Because testing conditions should replicate or simulate the actual operational environment, lasers can be used to deliver a narrow wavelength band, pulse duration (if applicable), and power level to determine the spectral response of optical components.

These tests deliver critical information to optical component manufacturers related to factors such as absorption, scattering, and other optical properties. They can also be used to assess how coatings on optical surfaces will perform. Damage testing has become even more important to identify whether given optical materials can be damaged at different wavelengths. Coatings can also become compromised at specific wavelengths, leading to performance issues.

Because there is such a range of tests, there is an advantage if the laser can be tuned to any required wavelength. Tuning the laser allows more flexibility in the types of tests that can be performed and decreases complexity enabling manufacturers to ensure that optical products perform as expected.

Although continuous wavelength lasers are an inexpensive solution for testing optical materials, they don’t provide a broad range of high-resolution wavelengths, and the peak power they can generate is limited. Pulse-based lasers produce high-intensity light bursts that can be used to determine whether the transmission properties of optical materials or coatings are affected. Optical component manufacturers may want to test for this to ascertain whether high-intensity light will cause damage such as nonlinear effects [unwanted wavelength generation] or solarization or photobleaching across a spectrum of wavelengths, including down to deep ultraviolet (UV). Continuous wave lasers are not powerful enough for this level of damage testing.

When single wavelength pulse-based lasers are required, Nd:YAG lasers are an ideal option because they are relatively inexpensive and simple to use. A 1064-nm laser can also be modified using additional hardware to operate at its other harmonic frequencies: 213, 266, 355, and 532 nm. While this provides five defined wavelengths for testing, each modification adds to the cost. There are gaps between the wavelengths, and the jump from 1064 to 532 nm is significant. Each of those harmonics increases the cost. Optical component manufacturers will want to know how their products perform at the wavelengths between those harmonics.

OPO lasers provide a more versatile, high-resolution option. These lasers can be tuned to specific wavelengths across a wide spectrum. In this approach, OPO lasers convert the fundamental wavelength of pulsed mode Nd:YAGs to the selected frequency. An OPO laser can be tuned to a very specific wavelength resolution by simply punching in a number like 410, 410.1, or 410.2 nm. Some tests require high-resolution wavelengths and with a broadband light source, which may not be able achievable.

Testing the Limits of Optical Components

Many optical components are sensitive to certain wavelengths, and destructive damage testing determines the limits of what the material can withstand. Laser-induced damage threshold testing (LIDT) is one example.

Certain wavelengths can trigger photochemical reactions in optical materials, changing their molecular structure or chemical composition and making them less effective. Some materials can absorb specific wavelengths of light, leading to localized heating and potential thermal damage. When the intensity of the light exceeds the damage threshold of the material, it can lead to melting, evaporation, cracking, or other forms of physical damage.

Optical fibers and components often have protective coatings that are also vulnerable to damage from certain wavelengths. For instance, UV light can cause photodegradation of coatings, reducing their protective properties. One of the most common applications is fiber optics, where prolonged exposure to high-intensity laser light can cause various forms of damage, including photodarkening, photobleaching, coating degradation, and thermal effects. To test fiber optic strands, laser light is transmitted from one end to the other to assess the performance and characteristics of the fiber.

To determine peak power, for example, pulse-based OPO lasers can deliver concentrated bursts of energy in short durations measured in nanoseconds. Because peak power is calculated by dividing the energy of a single pulse by the pulse duration, OPO lasers can deliver megawatts of energy, versus milliwatts for continuous wave lasers.

Some manufacturers may also want to perform continuous testing over time to ascertain whether an optical material may change over time. One concern is solarization, or photobleaching, which can occur due to prolonged exposure to UV or other forms of radiation. Solarization causes a gradual increase in the absorption of light, leading to a decrease in fiber performance — a concern with fiber optic materials.

The effects of solarization are even more pronounced in the deep UV range, which generally refers to wavelengths below 210 nm. To mitigate UV effects, fiber optics providers apply special chemistry treatments and utilize unique optical materials to prevent light absorption and UV damage in deep UV wavelengths.

OPO lasers can be designed to generate wavelengths down to 190 nm through multiple stages of optical conversion. Unlike typical fixed wavelength deep UV lasers, OPO lasers are solid-state and so do not require expensive consumables such as specialized gas or chemical mixtures as the lasing medium. To qualify fiber optics for deep UV and validate the chemistry and coatings for the optical material, manufacturers must be able to test the product to ensure that the optical material will transmit without degradation at shorter UV wavelengths.

Given the potential variety of tests at various wavelengths, optical component manufactures should consider the merits of pulse-based OPO lasers. The flexibility and resolution provided are ideal for determining the absorption, transmission, and reflection characteristics of materials and coatings, as well as damage testing. In doing so, manufacturers can ensure that optical products perform as expected and over time.

This article was written by Mark Little, PhD, Technical and Scientific Marketing Consultant for Opotek, LLC, Carlsbad, CA. For more information, call 760-929-0770 or visit here .

---