An Ultrafast Yellow Laser for Biomedical Applications

Experiment with a two-step nonlinear frequency conversion to convert mid-infrared laser light into yellow light that can be tuned from 570 nm to 596 nm. (Source: V. Sharma)

Researchers have developed a new compact and ultrafast, high-power yellow laser. The tunable laser exhibits excellent beam quality and helps fill the need for a practical yellow light source emitting ultrafast pulses of light. “The yellow-orange spectral range is highly absorbed by hemo­globin in the blood, making lasers with these wave­lengths parti­cularly useful for biomedical appli­cations, derma­tology treatments and eye surgery,” said research team member Anirban Ghosh from the Photonic Sciences Lab at the Physical Research Labora­tory in India. “A femtosecond, tunable yellow laser source might one day offer medical treat­ments that produce less thermal damage and are more selective.”

Although studies have shown that laser emitting in the yellow spectral range are optimal for various medical treatments, such wave­lengths are usually created using bulky and ineffi­cient copper vapor lasers, dye lasers and optical para­metric oscilla­tors. These sources have been used success­fully for various applications, but they suffer from one or more drawbacks such as low average power, lack of good spatial beam profile, limited or no wavelength tuna­bility and broad output pulses. “Femto­second lasers are important for many appli­cations because they emit a large number of photons in a short period to provide a very high intensity and extremely high precision without causing any thermal damage,” said Ghosh. “However, there is no commer­cially available femto­second yellow laser that can provide all the desired parameters needed for the appli­cations that would benefit from this wavelength range.”

To address these limitations in a single experi­mental confi­guration, the researchers used a recently developed ultraf­ast solid-state Cr2+:ZnS laser emitting in the mid-infrared range along with a two-stage frequency-doubling process. Frequency doubling an ultrafast laser is not an easy process and requires identifying the right crystal to produce a quality laser output with the desired properties. “We frequency-doubled the ultrafast mid-infrared laser with a peak wave­length at 2360 nm in two different nonlinear crystals and used simple optical components available in any standard optics labora­tory to achieve high power, tunable, ultrafast yellow laser source,” said Ghosh. “As a byproduct, our source provides tunable ultrafast near-infrared radiation with sub­stantial average power useful for various fields, including spectro­scopy, material processing and imaging.”

Tests of the new laser showed that it can provide a maximum output average power over 1 Watt with 130 femto­second pulses at a repetition rate of 80 MHz with an out­standing spatial beam profile. The researchers also observed excellent power stabi­lity over a long duration. The researchers plan to further improve the laser’s pulse duration, effi­ciency and compact­ness. They are also working to optimize the laser so that it can operate at room tempera­ture to make it more practical for long-term use. (Source: OSA)

Reference: D. Yadav et al.: Watt-level, ultrafast, tunable yellow source based on single-pass, fourth-harmonic generation of Cr2+:ZnS laser at 2360  nm, Opt. Lett. 45, 5109 (2020); DOI: 10.1364/OL.404358

Link: Photonic Sciences Lab, Physical Research Laboratory Ahmedabad, Navrangpura, India

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