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# Characteristics

By Joe Ramos,2014-12-24 11:43
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Characteristics

Lasers Construction

Here we briefly describe how to construct a laser.

RI. Laser Cavity

A schematic diagram of a typical laser cavity is shown in Fig. 6.

R，; ~ 98 95% ~ 99.5%21

Gain medium

Fig. 1

M1, M2 mirrors, intensity reflectivity of M1 ~ 99.5% or higher. M2 is also called output coupler (OC), intensity reflectivity typically 98% 90%.

Characteristics: Laser output

(1) M1, M2 very flat ~/20 ( - laser wavelength).

Pop. Inv. (2) M1, M2 are aligned such that they are “very” parallel. (see exercise).

M1M2 or OC

Remarks:

(1) M1, M2 - multi-layer dielectric coating mirrors.

(2) Mirror in your bedroom, reflectivity?

(3) Sometimes concave mirrors are used. (Why?)

II. Gain (Amplified)

Laser output

p1

M1OC Fig. 2

Represents an atom with an excited electron in level A. Light inside the cavity has gain (i.e.

number of photons increases in time) if there is population inversion and the reflectivity of OC

is high enough. Several round trips after photon p1 is emitted, this process results in a lot of

photons (see exercise).

Remark: Stimulated emissions shown in Fig. 8 will be lost through the walls of cavity.

1

M1OC

Fig. 3

III. Characteristics of Laser Light

Single” frequency (or wavelength), also

known as monochromatic.

Coherent: photons are “vibrating” in phase.

Beam profile: Gaussian, i.e. Fig. 4 Z

Fig. 5

light intensity as a function of z is 22I (z)z/2 , where I is a constant, I(z)Ie0o

and is the width of the profile.

0 ZAnother way to visualize this property is depicted

in Fig. 11 showing the electric field of light wave as

Za function of z.

E(Z) M1M2=OC Fig. 6

Remark: Laser beam intensity could have other distributions, an example is depicted in Fig. 12.

Again the electric field of the light wave (at a certain instant) is shown as a function of z.

E(Z)Z

0Z

M2M1E(Z)

Fig. 7

Small beam divergence

2

3 Typical value of is ~ 0.5 10 rad.

Example: 5 Mean earth-moon distance = 3.8 10 km 35 Beam radius on moon = 0.5 10 3.8 10 km 2 = 1.9 10 km

~ 200 km (quite small!)

IV. Gain (Active) Medium

Gain means that signal got amplified (the “A” in laser). Substance, inside the laser cavity, that achieves population inversion and thus leads to laser action. It could be: gas

liquid

solid.

V. Classification of Lasers

Lasers can be classified according the following different schemes.

(A) By active media + Gas laser, e.g. He-Ne laser, Ar laser, CO laser, N laser, HCN laser. 22

Dye laser - active medium: dye molecules in liquid solvent (sometimes in solids also).

Solid state laser - crystal, or glass, doped with impurities, e.g. ruby laser, Ti:sapphire laser,

semiconductor laser.

(B) By mode of operation

CW

pulsed (see exercise)

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(C ) By pumping and

laser levels fast fast

3-level laser pump3transition 4-level laser pump

laser

laser2Ground state transitiontransitionGround state

gr. state Fig. 8 gr. state

VI. Examples 3-level4-level

In this Section, we give examples of specific lasers and briefly describe their characteristics.

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VI.1 He-Ne laser

He+Neglass tubeLaser output

M1OC

High voltage ~ 1000V Fig. 9

He pressure ~ 1 Torr (1 Torr is a pressure of 1 mm Hg).

Ne pressure ~ 0.1 Torr; wavelength 632.8 nm (red) is the most common output. 33 Output power 0.5 10 W to 25 10 W.

Remark: Recently, orange, yellow, and green outputs of wavelengths 612.0, 594.0, and 543.5 nm, 3respectively, are also available, typical output power ~ 0.2 10 W.

VI.2 Pulsed CO laser [Transverse Excited Atmospheric (TEA) CO laser] 22

OC~20kV

M1spark plugsFig. 10

Laser cavity filled with CO gas at atmospheric pressure. 2

Spark plugs create charges to facilitate discharge between the two flat electrodes.

The electrodes have to be very smooth - avoid local discharge. 9 Pulse width ~1 ns = 10 sec.

Wavelength 10.6 (m, that is far in the infrared.

Remark: similarly, a pulsed N laser (quite common) can be constructed. A N laser lases at 22

337.1 nm (in the utra-violet spectrum).

Exercise: How long does it take for an electron to travel a distance of ~1 cm under 20,000 V?

VI.3 Dye laser

4

Dye cell

laser

outputTunning plate Blue-green

(wedge)laserDye out

Fig. 11

Pump sources - usually blue, green, violet, ultraviolet lasers.

Dichroic mirror - allow ~90% of pumping laser light to pass through. But its reflectivity

for the lasing wavelength (mostly yellow or read) is ~99.5%.

Dye continuously flowing - avoid heating up.

Tunable insert, into the cavity, a plate (or wedge) that is transparent, changes cavity

property slightly, and thus changes the laser wavelength.

has to be changed ~ every 2 weeks, which is very laborious. 14 Short pulse generation, ~10 sec. pulse width can be achieved (see exercise). Remark:

For the generation of this type of short pulses, the dye cell has to be replaced by a dye jet.

Fig. 12

The reason for using a dye jet is as follows:

A short pulse becomes boarder after passing a piece of thick (~ cm) material (say glass, water,

etc).

VI.4 Ti:sapphire laser

+ Ti:sapphire means sapphire crystal doped with Ti impurities. A Ti:sapphire laser is a dye laser with the dye cell replaced by a piece of Ti:sapphire.

Focused pump beam focusedTi:sapphire crystal (red in color) Ti: sapphire crystal (red in color)Fig. 13 pumping light

The reason for choosing sapphire is that its thermal conductivity is good compared to other non-

metallic solids.

Can be pumped “harder”, and hence higher output. By “harder” we mean that the crystal

is pumped by a higher blue-green laser power.

Tuning range ~700 1000 nm.

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VI.8 X-ray laser

In recent year, there is intense interest in x-ray lasers. However, it is not easy of make a x-ray laser. One of the key problems is that there is no mirror that can reflect x-ray efficiently.

Note: Brewster’s Angle (Window)

BMedium 1

n1Interfacen2

Medium 2

Fig. 17 n and n are refractive indices of media 1 and 2, respectively. Double-headed arrows 12

indicate direction of the electric field (E) of the optical radiation. indicates that E is pointing into

the paper.

active medium

polarized

M1outputOC

Fig. 18

Reasons for using Brewster windows in a laser cavity are:

Low loss in each trip through the window.

Output laser light is polarized, which is convenient for many optical applications.

Keywords:

Gain or Amplified, population inversion, stimulated emissions, excitation, polarization.

Related Topics in the Syllabus:

X-rays, Energy levels, emission and absorption, Wave, polarization

Extensions (from the syllabus):

Bring out ideas: The Brewsters Angle

The angle of incidence at which the refracted light and reflected light are at right angles to each other is called the Brewsters Angle.

Warm-up discussion

; What type of Ruby Laser and He-Ne laser belong to? (Solid laser and gaseous laser resp.)

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; How population inversion is obtained in Ruby Laser and He-Ne laser?

; What is the application of laser? (welding, surgery, holography, printing, optical

Points for further discussion:

1. Suppose that in every trip, stimulated emissions occur 4 times as indicated by the diagram for explaining gain in the lecture notes. The intensity reflectivities of M1 and OC are 100 % and th87.5 %, respectively. What will be the number of photons at the end of the 5 trip? Assuming that

we start with one photon traveling along a direction perpendicular to the mirrors. Repeat this

exercise with the intensity reflectivity of the OC changed to 6.25 % while other parameters unchanged.

2. A laser is operated in a pulsed mode. Each pulse has an energy of 0.2 J and a pulse width of -810 sec. Calculate the instantaneous power of each laser pulse. You can assume that the pulse is rectangular in shape. Suppose the repetition rate is 10 pulses/sec, what is the average output power?

-143. A laser is producing utra-short laser pulses of 10 sec pulse width. Assume that the laser

wavelength is 790 nm. How many cycles does the optical field oscillate within the duration of the pulses. (Remark: when the laser pulse is so short, the laser output will no longer

monochromatic but have a distribution of wavelengths. For simplicity, we have assumed that it is monochromatic.)

4. Consider a light beam of 2 mm diameter travelling between

two flat mirrors whose reflecting surfaces are not exactly parallel

so that after 5 round trips, the position of the light beam deviates

1 mm from its original position as shown Fig. 14 (solid line starting

position, dashed line position after 5 round trips). The distance

between the two reflecting surfaces of the mirrors is 1.0 meter. Fig. 14 Find the angle between the two mirrors. Assume that the angle

between the two mirrors is much smaller than one degree and

during the first single trip, the light ray is perpendicular to one of

the mirrors.

5. (a) Consider a laser for measuring the distant between Hong Kong and the a spot on the moon where a set of corner cubes are located. The laser is operated in pulsed mode at a -9wavelength of 532 nm. The pulse width of the output pulses is 10 sec. What is the resolution of

the distance measurement? (Resolution is just speed of light times pulse width). Suppose that the 7-3peak power of the laser pulses is 10 W and the beam divergence is 0.5x10 radian. What would

be the power density (per unit area) of the laser pulse on the surface of the moon. Assume that the laser light does not suffer attenuation after passing through the atmosphere.

2 (b) Suppose that the surface area of each corner cube is 4 cm(and there are 100 corner

cubes), and assume that the intensity reflectivity of the corner cube is 90%, find the power density of the portion the light that get back to the earth for (i) neglect diffraction of light wave and (ii) consider also diffraction effect.

(c) A light bulb is consuming 100 W of electrical power. Suppose that 20% of the power that it consumes is converted to light power. The light bulb is at a distance of 10 km from you.

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Compare the power density that you observe due to the light bulb and the power density of part (b). Again you may assume that the light passing through the air does not attenuate. 5Given: The distance between the moon and the earth is 3.80x10 km.

6. A Nd:YAG laser is operated in a pulsed mode. The peak power of the laser radiation of 81.064 (m wavelength is 3x10 W. The laser beam diameter is 2 mm. That is, you can assume the laser energy is uniformly distributed inside a beam of diameter 2 mm. Furthermore, you can -8assume that the laser pulse is rectangular in shape and has a width of 0.5x10 sec. Compute the

electric field of the EM wave at peak power. Repeat the calculation if the beam diameter is 0.25 mm. Compare these values with a typical lab situation: the electric field between the plates of a parallel plate capacitor with plate separation of 1 mm and an applied voltage of 1500 V is likely to cause electrical breakdown. You will find the following information useful:

2-1222 The energy density of EM wave is (1/2)E. = 8.85x10 C/(N.m). You need not use the oo7wavelength in this calculation. Usually, an electric field that exceeds 10 V/m can readily create

plasma.

Activities:

; In medical uses, X-ray photography is made with the human body between X ray source and

a film as close to his body as possible. Try to find the information about General Principles of

using X ray in medical purpose. It can be used to observe bone structure and soft organs like

stomach, liver, brain, blood clearly. Is there any instruction and preparation? ; Three acronyms, CAT, MRI, and PET, are now commonly heard in hospital. Try to discuss

Related web sites:

o St. Paul’s Lasik Centre

This site contains information about LASIK service.

http://www.stpaullasik.com.hk/index2_bak.htm

o Laser Refractive Centre (Chinese version only)

This site provides a brief introduction on LASIK.

http://www.lasik.com.hk/lasik.htm

o Demonstration, The Laser

This site introduces the laser principle with simple animation.

o 物理實驗

This site contains many hyperlinks of animations which demonstrate some

principles of physics. (Although the topics in the site are in Chinese, the contents of

o Molecular Expressions Electricity and Magnetism Interactive Java

Tutorials How a Compact Disc Works

This site introduces how laser is applied in reading Compact Disc (CD).

http://micro.magnet.fsu.edu/electromag/java/cd/

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