Power Measurement and LASER Delivery Systems

Question 1

When will the power of a LASER need to be measured?

Answer 1

• When the unit is serviced.

- When novel techniques are employed.

Question 2

What are two devices commonly employed in measuring LASER power?

Answer 2

• Thermopile.

- Photodiode.

Question 3

Explain how a thermopile measure LASER power.

Answer 3

• Thermopile is made from a bank of thermocouples.
• Use the Seebeck effect to measure temperature.
• Two dissimilar metals joined together.
• Voltage is induced across them as they are
  heated/cooled.
• Voltage indicates temperature.
• Temperature indicates power.
• Cool end kept at ambient temperature.

Question 4

Explain how a Photodiode measures LASER power.

Answer 4

• Si Photodiode.
• Current changes upon irradiation.
• Measures current, indicate power.
• Voltage indicates temperature.
• Wavelength specific.
• Quick response time.

Question 5

What are advantages of a thermopile?

Answer 5

• Large diameter (~10mm).
• Good absorption of light (>98%).
• Good wavelength response (200-20,000nm).
• Good power handling (up to 10W max, 10kw/cm^2 average power density, 10GW?cm^2 damage threshold).
• A response time of 1-2s.

Question 6

Why might an integrating sphere be used in power measurement?

Answer 6

Multiple diffuse reflections fro the inner surface mean the power of the LASER of broad beam source, measured at the output is free from beam-shape/geometry influences.

Question 7

When would a spectroradiometer be used to assess the power output of a source?

Answer 7

• When assessing an unknown or novel source, as measurement with other devices may give inconsistent and unsatisfactory readings.
• When a higher level of precision is needed (e.g. PDT).

Question 8

How are LASERs guided using TIR?

Answer 8

• An optical fibre is made from a highly transparent core, surrounded by a cladding of a slightly different refractive index.
• LASER light incident below the angle of acceptance at the aperture will be incident on the cladding above the brewster angle.

Question 9

What is the cone of acceptance, and when will it differ fro the output cone?

Answer 9

• Cone of acceptance is the solid angle defined by the numerical aperture of the fibre.
• This differs from the output cone if the input and output of the fibre are in different media. Though the Numerical Aperture remains the same for both cases.

Question 10

What is the advantage of using multimodal fibres in surgery?

Answer 10

Increase in illumination from range of harmonics.

Question 11

What type of light source is silica glass used for and why?

Answer 11

• Visible and near IR.

- Very low loss.

Question 12

What type of light source ZBLAN glass used for?

Answer 12

Holmium and Erbium LASERs.

Question 13

What determines the fibre used for CO2 LASERs?

Answer 13

• Cost.
• Manufacturing difficulties.
• Other losses.

Question 14

What problems occur with far IR transmission?

Answer 14

Fresnel loss within fibre core if very high.

Question 15

What two conditions need to be met for coupling onto a fibre?

Answer 15

1. Matching acceptance cone.

2. Focus to roughly size f fibre core.

Question 16

What considerations go into the choice of mirrors for an articulated arm?

Answer 16

• Reflectivity.

- Ability to form a flat surface.

Question 17

What is the only material transparent to both the treatment and aiming beam for a CO2 LASER?

Answer 17

Zinc Selenide.

Question 18

What are some parameters of fibres used in endoscopy?

Answer 18

• Crosstalk between fibres in the bundle.
• The illumination level depends on core diameters.
• Image resolution depends on core radius.
• Contrast depends on light intensity.
• The ratio of core areas to total bundle area ~ 50%.
• Defects and ordering errors limit image clarity.

Question 19

Describe the construction of a LASER fibre.

Answer 19

• 2 part core with higher n central section.
• LASER initiating doping agents and reflectors at each end.
• Next concentric section is illuminated as a pump source only.
• LASER power depends on the length of the fibre.