[Frontier] Potential applications of femtosecond laser in the field of vision correction

Ever since humans first used glasses for vision correction in the mid-13th century, scientists have been looking for better ways to correct human vision. With the invention of laser, people quickly realized that laser technology could become an effective means of vision correction.

The generation and progress of ultrashort pulse lasers have greatly promoted the development of laser micromachining and nonlinear optics. The interaction time between extremely short laser pulses and materials is very short. This unique cold processing process avoids thermal melting of the material surface and eliminates many negative impacts caused by thermal effects in long pulse laser processing.

In addition, the multi-photon absorption process of femtosecond laser in transparent materials has been specially designed and controlled, and can be accurately used to modify the local refractive index of the material. This method has become a potential means for human vision correction.

early application

As early as 1999, people were able to focus femtosecond laser with pulse energy of 2-4 microjoules onto the corneal stroma area to perform micromachining of corneal flaps. After the femtosecond laser creates an incision on the cornea, there are two usual treatment methods:

1. Make a corneal flap, lift the corneal flap, use an excimer laser to vaporize the tissue, and cut the corneal stroma. This process is called laser-assisted in situ keratomileusis (LASIK).

2. Another method is to directly process a lens-like structure in the middle of the cornea without making a corneal flap, and remove the lens through a small incision. This method is called (Small incision lenticule extraction, SMILE).

Figure 1. LASIK surgery process

Femtosecond laser refractive surgery has been rapidly adopted and promoted in the field of ophthalmology due to its high precision and controllability, and has become the most advanced refractive surgery method currently after Lasik surgery using traditional mechanical blades.

It is important to note that despite the great success of LASIK, only about 2% of patients with refractive errors have actually had LASIK to date. Obviously, people still have a conservative attitude towards ablative correction methods. Correction methods that are more "minimally" invasive and do not involve tissue cutting have a huge potential market.

When performing industrial micromachining, femtosecond lasers usually use peak power well above the material damage threshold to vaporize the material to achieve the purpose of shaping. And if the peak power of the femtosecond laser is controlled to a value below the material damage threshold, especially for transparent materials, it can cause changes in the local refractive index. Experiments have proven that only nanojoule-scale femtosecond lasers can achieve this effect.

Therefore, based on this material modification process, minimally invasive or even non-invasive vision correction research has made good progress in the field of basic research.

contact lenses

Contact lens manufacturers are always looking for more economical ways to produce more versatile and functional products. These manufacturers have to maintain an inventory of more than 20,000 different lenses (combinations of different curvatures, astigmatism and cylinder axes) for a long time to meet the needs of different groups of people. Even so, in order to produce contact lenses suitable for users, manufacturers still need customized special tools and a long customization cycle.

The introduction of femtosecond laser material modification methods can "process" phase-cut Fresnel-type refractive index correction structures in hydrogels (materials used to produce contact lenses). The advantage of this method is that no special custom tooling is required to produce high-quality contact lenses suitable for the user, and the diopter range can reach -9.5 to +5D.

Figure 2. Photo of a contact lens using a femtosecond laser to process a Fresnel lens (placed on an artificial cornea made of glass material)

The study found that this modification effect is almost permanent, and high-quality refractive effects can still be maintained within 5 years, see Figure 3.

Figure 3. The modification process of femtosecond laser in hydrogel still maintains almost the same effect after 5 years.

intraocular lens

As we age, the human lens undergoes long-term chemical and physical changes, resulting in blurred vision in many cases. This condition is commonly known as cataracts.

Currently developed medical methods can surgically remove the old cloudy lens and replace it with a new polymer lens. Cataract patients can quickly and effectively regain their vision. Although this method is effective, common optical correction errors may be as high as ±0.5D, which may be due to misalignment of the lens during impaction, contraction of the posterior lens capsule, or simply due to surgical mishandling.

The above-mentioned optical phase shifting effect of femtosecond laser in hydrogel can also be applied to the design and processing of intraocular lenses. Research has found that femtosecond laser can cause regular movement of water within the lens, thereby obtaining a phase-shifted structure. At present, experiments have successfully implemented this operation in an intraocular lens outside the body, and also achieved good results in an intraocular lens implanted in a rabbit eye. It is believed that this technology can be applied to human eye correction surgery in the near future.

Figure 4. (a) Structural characterization in intraocular lens; (b) Measurement of the corrected wavefront. The results are in line with expectations.

Corneal directional direct writing

With experience in intraocular lenses, the researchers also plan to use femtosecond lasers to directly generate Fresnel-type lens structures in human lenses. The initial experiment was performed on a cat's eye, which required pretreatment of the cornea like LASIK surgery. After direct writing with a femtosecond laser, a thin bubble layer visible to the naked eye appeared in the cat's eye, marking the writing position of the Fresnel lens (Figure 5). This bubble layer dissipated within 10 to 20 minutes, leaving a clear refractive index structure. By measuring the induced refraction correction of the reflected wavefront, the researchers found that the method's correction effect remained stable and effective in living cats for more than 18 months.

Figure 5. Using 405nm, 80MHz femtosecond laser to write the Fresnel lens structure in the cat's eye.

(a) A thin layer of bubbles will be visible immediately after direct writing, and will be cleared after 10 to 20 minutes;

(b) After the bubble has cleared, the eye shows a clear refractive structure and this refractive correction is stable in the living cat for at least 18 months

Further study of the direct writing mechanism revealed that after the femtosecond laser beam excites the tissue, the collagen fibers in the corneal stroma mix with the extracellular matrix, forming a new structure in a dense area with a higher refractive index. Obviously, it is difficult for a living subject to restore the corneal structure to its original form, so this change is stable. Fortunately, the researchers obtained equally effective results in human cadaver tissue experiments, and the next step will be to develop the device for human clinical testing.

References

[1] https://www.laserfocusworld.com/articles/print/volume-54/issue-03/features/nonlinear-optics-femtosecond-lasers-and-nonlinear-optics-new-approaches-solve-old-problems-in-ophthalmology.html

[2] T. Juhasz et al., IEEE J. Select Topics Quantum Electron., 5, 4, 902–910 (1999).

[3] G. A. Gandara-Montano et al., J. Vis., 17, 7, 38 (Jun. 2017).

[4] G. A. Gandara-Montano et al., Opt. Mater. Express, 7, 9, 3162–3180 (2017).