Laser-wake-field acceleration (LWFA) is an important means of acceleration that distinguishes it from conventional RF acceleration of particles.LWFA can generate acceleration gradients of GeV/cm, which is expected to significantly reduce the size and cost of gas pedals and enable brachytherapy.LWFA converts LWFA converts the light energy of a laser pulse into the kinetic energy of the accelerated electrons. The electron energy required for medical applications is tens to hundreds of keV, corresponding to a laser focusing intensity of 1014 W/cm2 or more, and if the focusing diameter is 20 μm, the peak laser power needs to reach the gigawatt level.
In this issue, a review article published in 2022 [1] is presented to discuss the ultrafast fiber laser technology and strong pulse delivery technology required for LWFA for cancer treatment. In endoscopic LWFA as shown in Figure 1, high-power femtosecond pulses excite solid-state carbon nanotubes, and the laser wake field is at a high density of LWFA, where electrons can be accelerated to tens to hundreds of keV, which is sufficient to destroy cancer cells without damaging healthy tissue.
In ultrafast fiber lasers, the signal light and pump light are transmitted in the fiber as guided waves with long action distances, which, together with the high surface-to-volume ratio of the fiber, makes it easier to boost the average power of the fiber laser. High peak power ultrafast fiber laser systems typically employ a master-oscillator-power-amplifier (MOPA) architecture, in which an ultrafast fiber oscillator provides stable seed pulses in the femtosecond or picosecond range. To address the challenges of nonlinearity and material damage, high-power ultrafast fiber lasers typically employ chirped-pulse amplification (CPA) technology and large mode area (LMA) fibers.
The peak power of the laser is increased from tens or hundreds of megawatts to gigawatt levels. The blue triangular data points surround the intensity line near 10 GW/cm2, reflecting the limitations imposed by nonlinear phase accumulation. To achieve higher peak power, coherent beam combining (CBC) of multiple ultrafast fiber exciters can be used.
In endoscopic applications, a hollow-core fiber can be used as a flexible channel to deliver laser pulses of gigawatt peak power to the LWFA device near the treatment site. In this case, the optical pulse propagates mainly in the air core, mitigating the problem of material damage, while both nonlinearity and dispersion are greatly reduced. Air-core fibers with a mode-field diameter (MFD) of 40 μm and a bend radius of about 25 cm have been shown to transmit pulses with a width of 500 fs, an energy of 500 μJ, and a peak power of 1 GW.In 2016, Mattia Michieletto et al. proposed a novel anti-resonant air-core fiber (length of 5 m, with an MFD of about 22 μm and a bend radius of 16 cm), capable of transmitting picosecond pulses with an average power of up to 70 W at low loss.
In conclusion, the rapidly developing ultrafast fiber laser technology is capable of delivering femtosecond pulses with gigawatt peak power for LWFA, and hollow-core fibers are capable of transmitting such ultrashort and ultra-powerful pulses. The combination of these two technologies is expected to enable high-density LWFA-based endoscopic cancer treatment in the future.
