Researchers have developed a new method of fabricating compact mode-locked lasers on photonic chips, using lithium niobate for active mode locking. This technology promises to bring large-scale ultrafast laser experiments to a chip-scale format, with plans to further shorten pulse durations and increase peak powers.
Lasers have become relatively commonplace in everyday life, but they have many uses besides providing light shows at rave parties and scanning barcodes on groceries. Lasers are also important in the fields of telecommunications, computing, and biological, chemical, and physical research.
In the latter application, lasers capable of emitting ultrashort pulses of one trillionth of a second (1 picosecond) or less are particularly useful. Using lasers that operate on such small time scales, researchers can study physical and chemical phenomena that occur extremely quickly -- for example, the making or breaking of molecular bonds during chemical reactions, or the movement of electrons inside materials. These ultrashort pulses are also widely used in imaging applications because they have extremely high peak intensity but low average power, thus avoiding heating or even burning of samples such as biological tissue.
In a paper published in Science, Alireza Marandi, assistant professor of electrical engineering and applied physics at Caltech, describes a new method developed by his lab to make such lasers, called mode-locked lasers, on photonic chips. The lasers are built using nanoscale components (a nanometer is one billionth of a meter) and can be integrated into light-based circuits, similar to the electricity-based integrated circuits found in modern electronics.
"We are interested in more than just making mode-locked lasers more compact," Marandi said. "We are excited to make a well-performing mode-locked laser on a nanophotonic chip and combine it with other components. At that point, we will be able to build a complete ultrafast photonic system in an integrated circuit. This will bring the wealth of ultrafast science and technology that currently belongs to meter-scale experiments to millimeter-scale chips."
Ultrafast lasers and Nobel Prize recognition
Such ultrafast lasers are so important to research that this year's Nobel Prize in Physics was awarded to three scientists for developing lasers that can generate attosecond pulses (an attosecond is one fifth of a second). However, such lasers are currently extremely expensive and bulky, and Marandi noted that his research is exploring ways to achieve such timescales on chips that can be much cheaper and smaller, with the goal of developing affordable, deployable ultrafast photonic technologies.
"These attosecond experiments are almost always done with ultrafast mode-locked lasers," he said. "Some of these experiments can cost up to $10 million, and a big part of that is the cost of the mode-locked laser. We're excited to think about how to replicate these experiments and capabilities in nanophotonics."
At the heart of the nanophotonic mode-locked laser developed in Marandi's lab is lithium niobate, a synthetic salt with unique optical and electrical properties that allow the laser pulse to be controlled and shaped by the application of external radiofrequency electrical signals. This approach is called intracavity phase modulation active mode locking.
"About 50 years ago, researchers used intracavity phase modulation to create mode-locked lasers in desktop experiments and believed that this method was not very suitable compared with other technologies," said Guo Qiushi, the paper's first author and a former postdoc in Marandi's lab. "But we found it to be a perfect fit for our integration platform."
"In addition to being small, our laser exhibits a range of fascinating properties. For example, we can precisely tune the repetition rate of the output pulses over a wide range. We can exploit this to develop chip-scale stable frequency comb sources, which are critical for frequency metrology and precision sensing," added Guo, now an assistant professor at the City University of New York's Advanced Science Research Center.
Future goals and research implications
Marandi said his goal is to continue improving the technology so that it can operate on shorter time scales and at higher peak powers, with the goal of reaching 50 femtoseconds (a femtosecond is one trillionth of a second), which would be a 100-fold improvement over his current device, which produces pulses with a length of 4.8 picoseconds.
The paper introducing the research, titled "Ultrafast Mode-Locked Lasers in Nanophotonic Lithium Niobate," was published in the Nov. 9 issue of Science.