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Lasers have widespread applications as a light source in a variety of fields, including manufacturing, medicine, high-speed communications, electronics, and scientific research.

In recent years, the demand for lasers with increased control over their output has grown significantly. In particular, ultranarrow bandwidth mode-locked lasers, which can produce extremely short laser pulses (short bursts of light) ranging from picoseconds to nanoseconds, have received considerable attention. Such short laser pulses are extremely beneficial for many applications—from diamond cutting to semiconductor manufacture. However, these applications can be further improved with the incorporation of lasers with tunable pulse duration.

A laser works by reflecting light back and forth between a highly reflective and a selective reflective mirror inside a cavity, and then amplifying it using a material called the gain medium. Conventional continuous-wave lasers emit a continuous beam of light waves (modes) with different wavelengths and random phases.

Mode-locked lasers, on the other hand, lock the phases of the different modes together and use a material called saturable absorber (SA) to produce a stream of extremely short, powerful light pulses. The mirrors in these lasers, also known as filters, control the bandwidth—and therefore the number of modes of the laser output—which directly influences the pulse duration (duration of light emission by the laser).

Currently, practical applications of mode-locked lasers are hindered by narrow-bandwidth filters that usually have a fixed bandwidth for a rigid pulse-width that cannot be modulated.

In a recent study in Advanced Photonics Nexus, researchers have presented an innovative ultranarrow bandwidth mode-locked laser with a tunable pulse bandwidth.

"We have successfully developed an ultranarrow bandwidth mode-locked laser with a widely tunable pulse duration using a novel filtering mechanism and a single-wall carbon nanotube SA," explains Weixi Li, one of the lead authors.

The researchers chose single-wall carbon nanotubes (SWCNTs) as SAs due to their ultrafast recovery time (in the femtosecond range), cost-effective fabrication, and ability to generate a stable pulse duration in the femtosecond to picosecond range. Additionally, they used a unique filter configuration, where the two filters—made from fiber Bragg gratings, a type of selective reflector—have an extremely narrow overlap between the range of wavelengths they can each reflect, creating an ultranarrow bandwidth.

To achieve tunable pulse-width, the researchers integrated a mechanism to apply mechanical stress to one of the gratings. By altering the applied stress, they changed the range of reflected wavelengths, which in turn changed the overlap between the filters. This altered the number of modes, thus controlling the pulse-width. Through this strategy, they achieved a wide tunable range from 481 picoseconds to 1.38 nanoseconds.

This nearly 1 nanosecond tuning range is the largest ever reported for a narrow-bandwidth passively mode-locked laser. Furthermore, if the stress is applied to both filters, the tunable range can be widened even further, resulting in even shorter pulse widths.

The laser also features a long-cavity structure, which helps lower power consumption and achieve a less than one megahertz repetition frequency, making it suitable for a broad range of applications. Numerical simulations supported the experimental results.

"We have not only designed a simple, flexible, and tunable scheme for narrow-bandwidth mode-locked lasers, but have also developed an ideal light source with robust output for important fields such as cutting of single-crystal diamonds and laser stealth cutting of semiconductor wafers," says the principal investigator, Professor Chengbo Mou of Shanghai University.

This breakthrough marks a significant step forward in laser technology and paves the way for new and diverse applications.