Optical parametric oscillator (OPO) lasers have long been utilized in sophisticated test and measurement applications such as mass spectrometry, photoacoustic imaging and spectroscopy. Now, these “tunable” pulsed lasers are being utilized 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 utilized 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 products perform as expected. The more precise these tests, the higher the quality of the product – a factor 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 given optical materials that 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. This allows more flexibility in the types of tests that can be performed and decreases complexity so manufacturers can ensure optical products perform as expected,” said Dr. Mark Little, Technical and Scientific Marketing Consultant for OPOTEK, LLC, a global manufacturer of tunable lasers for research and diagnostics, with solutions for photoacoustic, spectroscopy, diagnostics, hyperspectral imaging and medical research.
Little adds that there can be significant advantages to using pulse-based lasers. 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 if the transmission properties of optical materials or coatings are affected. Optical component manufacturers may want to test for this to ascertain if high intensity light will cause damage such as non-linear effects [unwanted wavelength generation] or solarization or photobleaching across a spectrum of wavelengths, including down to ‘deep’ UV,” explained Little, adding that 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. The 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 between 1064 nm to 532 nm is significant,” said Dr. Little, adding that each of those harmonics increases the cost. “Optical component manufacturers will want to know how their products perform at the wavelengths between those harmonics.”
According to Little, a more versatile, high-resolution option are OPO lasers that can be “tuned” to specific wavelengths across a wide spectrum. In this approach, optical parametric oscillators (OPO) convert the fundamental wavelength of pulsed mode Nd:YAGs to the selected frequency. Leading manufacturers like Carlsbad, CA-based OPOTEK have developed a diverse array of OPO technologies that ensures that many wavelengths from the deep UV to the mid-infrared can easily be produced.
“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 nanometers,” explained Little. “Some tests require high-resolution wavelengths and with a broadband light source, you may not be able to achieve it.”
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 if 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.
“You can ‘fire’ an OPO continuously for hours or days to determine if solarization will occur,” said Little.
The effects of solarization are even more pronounced in the “Deep UV” (Deep Ultraviolet) 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.
According to Little, OPO lasers can be designed to generate wavelengths down to 190 nm through multiple stages of optical conversion. Unlike typical fixed wavelength deep ultraviolet (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 the optical material will transmit without degradation at shorter UV wavelengths,” said Little.
Given the potential variety of tests at various wavelengths, optical component manufacturers would be wise to 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 ensure optical products perform as expected and over time, for the ultimate competitive edge.