The laser beam used in SMILE surgery is engineered to interact with the corneal tissue in a specific and controlled manner, allowing it to avoid significant reactions with the top layer (epithelium) while creating the lenticule at a deeper layer. This selectivity is achieved through a combination of laser properties, tissue characteristics, and advanced surgical planning. Here's how it works:

Laser wavelength: The femtosecond laser used in SMILE surgery operates at a specific wavelength that is carefully chosen based on the optical properties of the corneal tissue. Corneal tissue is relatively transparent to the specific wavelength used in SMILE surgery, which means that the laser light can pass through the top layer (epithelium) with minimal absorption or damage.

Short pulse duration: The femtosecond laser emits pulses with an incredibly short duration, typically in the range of femtoseconds (one quadrillionth of a second). This ultra-short pulse duration allows the laser to interact with the corneal tissue at a very high peak power, which leads to optical breakdown and photodisruption. However, since the pulses are so brief, the energy is absorbed and confined to the target layer without significant thermal effects on the surrounding tissue, including the top layer.

Precise focusing and imaging: Before the surgery, the eye surgeon uses advanced imaging technology, such as optical coherence tomography (OCT), to obtain detailed cross-sectional images of the cornea. These images help the surgeon accurately plan the location, depth, and shape of the lenticule to be created. The femtosecond laser is then precisely focused on the specific depth where the lenticule needs to be formed, avoiding interaction with the top layer.

Tissue selectivity: As mentioned earlier, the femtosecond laser's wavelength and pulse duration are carefully chosen to be absorbed predominantly by the corneal tissue at the targeted depth. The corneal tissue has unique optical properties that make it more susceptible to the laser's effects compared to other structures of the eye, such as the epithelium. This tissue selectivity ensures that the majority of the laser's energy is confined to the intended layer, leaving the top layer relatively untouched.

In summary, the femtosecond laser's specific wavelength, short pulse duration, precise focusing, advanced imaging, and tissue selectivity all work together to ensure that the laser interacts primarily with the corneal tissue at the desired depth, enabling safe and effective creation of the lenticule for vision correction while minimizing disturbance to the top layers of the cornea.