Question

5.
a) Discuss three important practical applications of lasers.

b) Calculate the ratio of stimulated emission and spontaneous emission at an operating
temperature of 1100 K, if the wavelength of emitted light is 550 nm. Do these conditions
correspond to a laser?

c) State the characteristics of different types of optical fibres. Depict their refractive index
profiles and propagation of light through them.

Answer 1

Answer on Question #50697, Physics, Optics

a) Discuss three important practical applications of lasers.

Answer:

1. **Medical Uses of Lasers**

The highly collimated beam of a laser can be further focused to a microscopic dot of extremely high energy density. This makes it useful as a cutting and cauterizing instrument.

2. **Welding and Cutting**

The highly collimated beam of a laser can be further focused to a microscopic dot of extremely high energy density for welding and cutting.

3. **Lasers in Communication**

Fiber optic cables are a major mode of communication partly because multiple signals can be sent with high quality and low loss by light propagating along the fibers

b) Calculate the ratio of stimulated emission and spontaneous emission at an operating temperature of $1100\,\mathrm{K}$, if the wavelength of emitted light is $550\,\mathrm{nm}$. Do these conditions correspond to a laser?

Answer:

$$\frac{\text{Simulated emission rate}}{\text{Spontaneous emission rate}} = \frac{1}{\exp\left[\frac{hc}{\lambda kT}\right] - 1} = \frac{1}{\exp\left[\frac{6.63 \cdot 10^{-34} \cdot 3 \cdot 10^{8}}{550 \cdot 10^{-9} \cdot 1.38 \cdot 10^{-23} \cdot 1100}\right] - 1} = 4.5 \cdot 10^{-11} \ll 1$$

where $h = 6.63 \cdot 10^{-34} J \cdot s$ is the Planck constant; $c = 3 \cdot 10^{8} \, \text{m/s}$ is the speed of light; $k = 1.38 \cdot 10^{-23} J / K$ is the Boltzmann constant

These conditions doesn't correspond to a laser.

c) State the characteristics of different types of optical fibres. Depict their refractive index profiles and propagation of light through them.

Answer:

Optical fibres may be classified according to the following three:

1. Multimode

1.1. Step Index

1.1.1. Plastic and liquid

1.1.2. Silica high purity

1.1.3. Silica Doped

1.1.4. Fluoride

1.1.5. Incoherent and imaging bundles

1.1.6. Tapered fibres

1.2. Gradient Index

2. Single Mode

3. Photonics

3.1. Silica cavity

3.2. Hollow cavity

A Graphic Representation of How Light Rays Travel in Three Fiber Types

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