On-Chip Light Source Produces Range of Wavelengths

Efficient optical parametric oscillation occurs in a microring on a silicon chip, so that an infrared laser coupled into the ring is transformed into light in both the visible and telecom wavelength. (Source: X. Lu, NIST / U. Maryland)

Researchers have designed a new chip-integrated light source that can transform infrared wave­lengths into visible wavelengths, which have been difficult to produce with tech­nology based on silicon chips. This flexible approach to on-chip light generation is poised to enable highly minia­turized photonic instru­mentation that is easy to manu­facture and rugged enough to use outside the lab.

Now, investigators from the National Institute of Standards and Tech­nology NIST, University of Maryland, and University of Colorado describe their new optical parametric oscil­lator (OPO) light source and show that it can produce output light that is a very different color than the input light. In addition to creating light at visible wavelengths, the OPO simul­taneously generates near-infrared wavelengths that can be used for 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,” said research team leader Kartik Srinivasan. “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. 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.”

Although the response of a material to light typi­cally scales linearly, material properties can change more rapidly in response to light at high power, which creates various nonlinear effects. OPOs are a type of laser that use nonlinear optical effects to create a very broad range of output wave­lengths. The researchers wanted to figure out how to take laser emission at a wavelength readily available with compact chip lasers and combine it with nonlinear nano­photonics to generate laser light at wavelengths that are other­wise hard to reach with 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,” said Xiyuan Lu, a NIST-University of Maryland post­doctoral scholar. “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.”

In the new work, the researchers designed an OPO based on a microring made from silicon nitride. This optical component is fed by approxi­mately 1 milliwatt of infrared laser power – about the same amount of power found in a laser pointer. As the light travels around the microring it increases in optical intensity until powerful enough to create a non-linear optical response in silicon nitride. This enables frequency conversion, a nonlinear process that can be used to produce an output wavelength, or frequency, that is different from that of the light going into the system.

“Recent progress in nano­photonic engi­neering has made this method of frequency conversion very efficient,” said 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.”

The researchers designed the new on-chip light source using detailed electro­magnetic simulations. They then made the device and used it to convert 900-nanometer input light to 700-nanometer-wavelength (visible) and 1300-nanometer-wavelength (telecommunications) bands. The OPO accomplished this using less than 2 percent of the pump laser power required by previously reported micro­resonator OPOs developed for generating widely separated output colors. In the previous cases, both colors generated were in the infrared. With a few simple changes to the microring dimensions, the OPO also produced light in the 780-nm visible and 1500-nanometer tele­communication bands.

The researchers say that the new OPO could be used to make a complete system by combining an inex­pensive commercial near-infrared diode laser with an OPO chip that also inte­grates components such as filters, detectors and a spectro­scopy section. They are continuing to look for ways to increase the output power generated from the OPO. “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,” said Srinivasan. “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.” (Source: OSA)

Reference: X. Lu et al.: Milliwatt-threshold visible–telecom optical parametric oscillation using silicon nanophotonics, Optica 6, 1535 (2019); DOI: 10.1364/OPTICA.6.001535

Link: Microsystems and Nanotechnology Division, Physical Measurement Laboratory, National Institute of Standards and Technology NIST, Gaithersburg, USA

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