Medical applications are a surgical fit for 2 µm lasers

Lasers have become essential for a wide range of industries, including medical applications. For many use cases, laser-based surgery improves accuracy compared to that of a scalpel, reduces the chance of infection, and creates clean cuts from a variety of angles. While laser sources with output wavelengths from 1 to 1.5 µm are widely available and often used, significant advancements in 2 µm lasers have led to their rapidly increasing popularity.

Evolution of 2 µm lasers

The first 2 µm laser sources were large and expensive liquid-nitrogen-cooled devices. Today, 2 µm diode lasers can be as small as 30 mm long, with even smaller 2 µm fiber lasers available. Both pulsed and continuous-wave (CW) emission are achievable.

The two rare-earth elements most used as laser gain dopants for both CW and pulsed laser operation are thulium (Tm³⁺) and holmium (Ho³⁺). Ions from these elements are used to achieve laser emission in many different host crystals and glass fiber types. Thulium lasers prove to be a better option for CW operation, while holmium better aligns with pulsed and Q-switched lasers because of the higher gain of holmium-doped crystals. Another advantage of thulium lasers is that their ions can be excited using commercially available laser diodes with wavelengths of 800 nm. Holmium requires a 1.9 µm pump source to excite the ions.

2 µm absorption in water

2 µm laser sources are well-suited for precise surgical applications because of the high absorption of 2 µm wavelengths by water molecules, which comprise most biological tissue. The stretching and bending vibrations of water’s O-H bonds match the vibrational frequency of 2 µm laser pulses.

Determining the exact amount of absorption attributed to a specific vibrational mode is difficult because all these effects are coupled—the spike in absorption is easily observable. Water’s absorption spectrum and the depth of tissue penetration for different laser types is shown in Figure 1. At 2 µm, the combination of the absorption peak and penetration depth around 0.6 mm at 2 µm make this wavelength region optimal for laser surgery. There is another absorption peak at 3 µm, but the penetration depth at that wavelength is not high enough for surgical applications.

Coagulation under 2 µm radiation

Other laser types lead to even more coagulation. However, too much clotting can cause complications after the surgical procedure. The coagulation level of 2 µm lasers is a beneficial middle-ground with a coagulation depth around 0.1 to 0.2 mm. This balances bleeding mitigation with thermal injury and the prevention of post-procedure complications. Because surgery is a very precise process, the depth of cuts needs to be finely controlled to reduce damage to the underlying tissue. The speed of cutting and vaporization of 2 µm also remain relatively constant no matter how vascular the biological tissue is, making 2 µm laser systems flexible for working with different types of tissues.

Eye safety, surgical applications

2 µm wavelengths are deemed safe for eye exposure—the typical range is around 1.4 to 2.4 µm, as these wavelengths are strongly absorbed in the vitreous body of the eye and do not reach the retina. The intensity threshold for irreversible eye damage of a 2 µm laser is also much higher than the threshold for shorter wavelengths such as 1 µm. But even with these safety benefits, 2 µm beams can still potentially damage other parts of the eye besides the retina, so all laser safety precautions should still be taken.

Laser sources in the 2 µm spectral region have proven very useful in medicine applications. Their high absorption in biological tissue, small thermal injury zone, and eye-safe wavelengths will likely continue boosting their popularity and become more widely used for laser surgery.