Introduction

Question 1

How do lasers work? (5 marks)

Answer 1

• Need three main things: a lasing material such as a gas (CO2) or a crystal, a pump which could be electrical or optical to create a population inversion in the material, a cavity to contain the light.
• Phosphors are used which emit light when excited, they decay to a meta-stable state and give off light.

Question 2

What are the essential properties of a laser? (3 marks)

Answer 2

Monochromatic, coherent, non-diverging.

Question 3

Give two uses of lasers and the type of laser used. (6 marks)

Answer 3

• Surgical CO2 10,600 nm lasers, used to cut and coagulate at the same time. Work by essentially boiling tissue, as it is absorbed in the skin and blood.
• Diabetic retinopathy - 584 nm green Argon laser used as it travels through the front portion of the eye with little absorbtion, by has a high attenuation coefficient for blood so coagulates bleeds on the retina.
• Excimer 193 nm UV laser for laser eye surgery, high absorbtion in protien and low in water. Re-shapes the cornea.

Question 4

What are chromophores and sketch u_a versus wavelength and explain how the diabetic retinopathy and CO2 laser work. (7 marks)

Answer 4

Chromophores are the parts of tissue which absorb or scatter light. Different tissues have different chromophores, match laser to intended absorbtion and clinical purposes.

On plot of u versus wavelength:

Protien is on the far left and starts high and drops down, this is where the excimer 193 nm UV laser has high absrobtion.

Haemaglobin squiggles down in the same area, which is high for 584 nm argon laser, but low for water which gradually increases.

At the far right is the CO2 laser 10,600 nm which has a high absorption in water.

Question 5

For a laser what is meant by the penetration depth and what is the equation for intensity? (2 marks)

Answer 5

I(x) = I(0)exp(u_a+u_s)x

Penetration depth = distance taken for intensity to drop to 1/e of original value.

Question 6

What is are IPLs? (3 marks)

Answer 6

Intense pulses light sources: basically bright lamps. Not monochromatic and not coherent, just bright. Used in things such as PDT therapy.

Question 7

What is a thermopile used for and how does it work? (3 marks)

Answer 7

Used to measure power output of laser. It is essentially a thermocouple.

Two pieces of metal joined together, heating causes one to expand more than the other, change in resistance/voltage can be used to determine power.