Biophotonics_Concepts_to_Applications

• 1550-nm erbium-doped opticalfiber lasers have been used for treatment of facial
  and nonfacial cutaneous photodamage and resurfacing of facial scars.


• Fiber-coupled Er:YAG lasers emit at 2940 nm and are effective for procedures
  such as skin resurfacing and acne treatments.


• Fiber-coupled CO 2 lasers can emit more than 10 W at 10.6μm for a wide range
  of surgical tissue removal procedures in disciplines such as cardiology, der-
  matology, gynecology, and orthopedics.

Example 6.12Consider a CO 2 laser that emits at a wavelength of 10.6μm.

This laser may be used to cut tissue by vaporizing it. (a) What is the optical

penetration depth into the tissue if the absorption coefficientμa=10^3 cm−^1?

(b) Suppose the power from a single 1.5-ms pulse is 0.5 W and that it is

delivered to the tissue through an opticalfiber with a core radius of 300μm.

Assuming that the factorρC≈4.2 J/(cm^3 /°C) holds for tissue, what is the

maximum temperature rise in the tissue due to this pulse?

Solution: (a) From Eq. (6.6) the penetration depth into the tissue is

1/μa≈ 10 μm. Thus the light energy is deposited in a small volume of tissue,

which rapidly heats up from 37 °C and will vaporize at 100 °C.

(b) The irradiance from thefiber is E 0 =P 0 /πa^2 , wherea= 300μm is the

fiber radius. From Eq. (6.24) the resultant maximum temperature riseΔTis

DT¼

laP 0 s

qCpa^2

¼

ðÞ 103 cm^1 ðÞ 0 :5WðÞ 1 :5ms

½Š 4 :2J=ðcm^3 =CÞpðÞ 3  10 ^2 cm^2

¼ 63 : 2 C

Thus the tissue temperature becomes T = 37 °C +ΔT = 100.2 °C.

6.5.4 Photoablation.

Tissue photoablation processes are based on the absorption of short pulses of
high-energy laser UV light to break molecular bonds in the tissue. This molecular
breaking up condition is calledphotodissociationand causes the tissue to volatize
[ 53 – 56 ]. The use of photoablation allows the removal of tissue in a very clean and
precise fashion. In this laser-tissue interaction mode there is no damage to adjacent
tissue, such as can arise from thermal effects in the ablation methods used in
coagulation and vaporization processes. A major use of the photoablation process is
in eye (corneal) surgery.

6.5 Light-Tissue Interaction Mechanisms 183