Dec 20 2019
A novel chip-integrated light source designed by scientists can convert infrared wavelengths into visible wavelengths. Until now, it has been difficult to generate visible wavelengths with silicon chip-based technology.
The new, flexible method for producing on-chip light will lead to the development of highly compact photonic instruments. Such instruments can be easily manufactured and would be sufficiently strong to use beyond the laboratory settings.
Researchers from the University of Maryland, the National Institute of Standards and Technology (NIST), and the University of Colorado have elucidated the novel optical parametric oscillator (OPO) light source in Optica—The Optical Society’s (OSA) journal dedicated for high-impact research.
The researchers have also shown that the OPO light source can generate output light that has a highly different wavelength, or color, when compared to the input light.
Apart from generating light at visible wavelengths, the OPO light source produces near-infrared wavelengths at the same time. These near-infrared wavelengths can be utilized in telecommunication applications.
Our power-efficient and flexible approach generates coherent laser light across a range of wavelengths wider than what is accessible from direct chip-integrated lasers. The on-chip creation of visible light can be used as part of highly functional compact devices such as chip-based atomic clocks or devices for portable biochemical analyses.
Kartik Srinivasan, Research Team Leader, National Institute of Standards and Technology
Srinivasan continued, “Developing the OPO in a silicon photonics platform creates the potential for scalable manufacturing of these devices in commercial fabrication foundries, which could make this approach very cost-effective.”
Exploiting Nonlinear Processes
While a material’s response to light usually scales in a linear fashion, the properties of a material can change more quickly in response to high-power light, producing numerous nonlinear effects. OPO light sources are types of lasers that generate a very wide range of output wavelengths using nonlinear optical effects.
The scientists wanted to find out a method to take laser emission at a readily available wavelength using compact chip lasers and then integrate it with nonlinear nanophotonics to produce laser light at wavelengths that are usually difficult to achieve using silicon photonics platforms.
Nonlinear optical technologies are already used as integral components of lasers in the world’s best atomic clocks and many laboratory spectroscopy systems. Being able to access different types of nonlinear optical functionality, including OPOs, within integrated photonics is important for transitioning technologies currently based in laboratories into platforms that are portable and can be deployed in the field.
Xiyuan Lu, Study First Author and Postdoctoral Scholar, NIST-University of Maryland
In the latest study, the scientists developed an OPO chip that was based on a microring created from silicon nitride. This kind of optical component is powered by about 1 mW of infrared laser power—approximately the same level of power present in a laser pointer.
As the light passes around the microring, its optical intensity increases until it becomes sufficiently strong to produce a non-linear optical response in silicon nitride. This phenomenon allows frequency conversion, a type of nonlinear process that can be used for creating an output frequency, or wavelength, that differs from that of the light entering the system.
“Recent progress in nanophotonic engineering has made this method of frequency conversion very efficient,” added Lu. “A key advance in our work was figuring out how to promote the specific nonlinear interaction of interest while suppressing potential competing nonlinear processes that can arise in this system.”
Testing the Light Source
The scientists used in-depth electromagnetic simulations to design the novel on-chip light source. They subsequently created the device and utilized it to transform 900-nm input light to 1300-nm wavelength (telecommunications) and 700-nm wavelength (visible) bands.
The OPO chip achieved this feat using less than 2% of the pump laser power needed by formerly reported microresonator OPOs designed for producing broadly separated output colors.
In the earlier cases, both generated colors were in the infrared wavelength. When a few simple modifications were made to the microring dimensions, the OPO chip also generated light in the 1500-nm telecommunication and 780-nm visible bands.
According to the scientists, the latest OPO chip can be used for making an entire system. This can be done by integrating an OPO chip that also includes components like detectors, filters, and a spectroscopy section, with a low-cost commercially available near-infrared diode laser. The researchers are now exploring ways to boost the output power produced from the OPO chip.
This work demonstrates that nonlinear nanophotonics is reaching a level of maturity where we can create a design that connects widely separated wavelengths and then achieve enough fabrication control to realize that design, and the predicted performance, in practice.
Kartik Srinivasan, Research Team Leader, National Institute of Standards and Technology
“Going forward, it should be possible to generate a wide range of desired wavelengths using a small number of compact chip lasers combined with flexible and versatile nonlinear nanophotonics,” concluded Srinivasan.