Miniature Ultrafast Laser Ideal for Nanophotonic Applications

Ultrafast mode-locked lasers have many industrial and manufacturing applications, including computing and telecommunications, as well as R&D. Their high speeds and pulse-peak intensities are useful for critical functions such as medical imaging, optical atomic clocks, and metrology instrumentation that measures and processes data.   

Because the pulses of light are extremely short and well-focused—for example, just a few femtoseconds (1 fs = 10^-15s)—they are typically preferred for imaging devices because they can have extremely large peak intensities but low average power. Also, in micromanufacturing applications, a wide variety of materials can be cut into highly precise, complex shapes without transferring heat beyond the material being removed, greatly reducing the occurrence of heat-affected zones in the surrounding material—essentially a “cold” ablation process. 

However, because these mode-locked lasers are expensive, bulky, and power-demanding bench-top systems that are largely impractical for general use, they are mostly employed in laboratory research.  

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To make these laser systems smaller, easier to handle, and more accessible for a wider range of applications, researchers at the City University of New York (CUNY) have developed a miniature mode-locked laser system that is so small it can fit on a fingertip or a nanophotonic chip.  

“Our goal is to revolutionize the field of ultrafast photonics by transforming large lab-based systems into chip-sized ones that can be mass produced and field deployed,” said lead researcher Qiushi Guo, a physics professor and faculty member with the CUNY Advance Science Research Center. 
 

Unique properties 

Mode-locked lasers generate ultrashort pulses with peak powers substantially exceeding their average powers.  

“However, integrated mode-locked lasers that drive ultrafast nanophotonic circuits have remained elusive because of their typically low peak powers, lack of controllability, and challenges when integrating with nanophotonic platforms,” Guo said.   

Guo and his research team turned to thin-film lithium niobate—a well-known crystalline material with unique optical, electro-optic, and piezoelectric properties—to create their prototype mode-locked laser. Lithium niobate allows the laser beam to be precisely controlled and shaped using an external radio-frequency electrical signal.  

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In addition to its small size, the prototype laser has other useful properties. In their experiments, the researchers were able to combine the high laser gain of III-V semiconductors and the efficient pulse-shaping capability of thin-film lithium niobate nanoscale photonic waveguides to generate a high output peak power of 0.5 watts. Also, by adjusting the pump current of the laser, the team was able to tune the repetition frequencies of out pulses in a very wide range of 200 MHz. By employing the strong reconfigurability of the laser, they hope to enable chip-scale, frequency-stabilized comb sources (essential for precision sensing). 
 

Applications abound 

Guo plans to improve this technology so it can successfully operate at even shorter timescales and higher peak powers, with a goal of 50 femtoseconds—a 100-fold improvement over his current prototype. His ultimate goal is to create a scalable, integrated, ultrafast photonic system that can be translated for use in portable and handheld devices.

“This achievement paves the way for eventually using cell phones to diagnose eye diseases or analyze food and environments for things like E. coli and dangerous viruses,” Guo said. “It could also enable futuristic chip-scale atomic clocks, which allow navigation when GPS is compromised or unavailable.” 

Mark Crawford is a technology writer in Corrales, N.M.