Optical Sources and Laser Properties, Lecture notes of Optics

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Optical Sourcesw.wang

Wei-Chih WangDepartment of Mechanical EngineeringUniversity of WashingtonME

Fiber Optic Sources Two basic light sources are used for fiber optics: lasers andlight-emitting diodes (LED). Each device has its ownadvantages and disadvantages as listed in Table Characteristic w.wang

LED^

Laser Output power^

Lower^ Higher Spectral width^

Wider^ Narrower Numerical aperture^

Larger^ Smaller Speed^

Slower^ Faster Cost^

Less^ More Ease of operation^

Easier^ More difficult

A. Guenther UCONN

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LASER

LASAER = light amplification by stimulated emission of radiationInvented dated to 1958 with the publication of the scientific paper,Infrared and Optical Masers, by Arthur L. Schawlow, then a BellLabs researcher, and Charles H. Townes, a consultant to Bell Labs

Property of Laser Light• Nearly"monochromatic: consists of an extremely narrowrange of wavelengths • Highly Directional: travel in a single direction within anarrow cone of divergence • Highly Coherence: coherence is the most fundamentalproperty of laser light and distinguishes it from the lightfrom other sources w.wang

Type of Laser^ λ (nm)^21 w.wang

(^9) A. Guenther UCONN

• 2 ∆λ (Hz) σ (cm) ∆
• –1–3g(cm) I N(cm) 21 sat - HeNe 632. 2 (W/cm) - 9 –13 2 × 10 3 × - 9 –3 7 × 10 2 × - 6. - Argon 488. - 9 –12 2 × 10 2.5 × - 15 –3 1 × 10 5 × - 16. - HeCd 441. - 9 –14 2 × 10 9 × - 12 –3 4 × 10 3 × - 7. - Copper Vapor 510. - 9 –14 2 × 10 8 × - 13 –2 6 × 10 5 × - 9. - CO10,600 - 7 –18 6 × 10 3 × - 15 –3 5 × 10 8 × 10 1.6 × - – - Excimer 248. - 13 –16 1 × 10 2.6 × - 16 –2 1 × 10 2.6 × 10 3.4 × - Dye (Rh6-G) - 13 –16 5 × 10 2 × - 18 2 × 10 2.4 3.4 × - Ruby 694. - 11 –20 3 × 10 2.5 × - 19 4 × 10 1.0 3.8 × - Nd:YAG 1064. - 11 –19 1.2 × 10 6.5 × - 19 3 × 10 2.0 1.2 × - Ti:Al O^760 - 14 –19 1.5 × 10 3.4 × - 18 3 × 10 1.0 2.0 × - Semiconductor - 14 –15 1 × 10 1 × - 18 3 1 × 10 10 2.5 ×

Requirements for a laser There are three types of processes involving theinteraction of light beams with atoms that haveelectrons residing in various energy levels: SPONTANEOUS EMISSIONABSORPTIONSTIMULATED EMISSION w.wang

A. Guenther UCONN

Most excited energy levels undergo spontaneous emission. Each level has a specific lifetimew.wang

τ^ over

which it will remain in that level before decaying to a lower-lying level. That lifetime is determined bythe interactions of the electron with the other electrons and nuclei of that atom. Typical lifetimes ofelectrons residing in specific levels that decay by radiating in the visible portion of the spectrum are ofthe order of 10–100 nsec. The photon radiated during spontaneous emission has the exact wavelength λand frequency^ νcorresponding to the difference in energy^21

∆ E of the two involved energy levels^21

(1 and 2 in this case) according to the relationship

∆ E =^ h ν=^21

in which^ h^ is Planck’s constant such that

–34^ h = 6.63 × 10joule-sec and

c^ is the speed of light,^ c^ = 3

8 × 10 m/sec.Because different materials have different energy-level arrangements, they radiate at differentwavelengths and thus emit different colors or frequencies of light that are specific to the material.

The second process is^ absorption w.wang

ABSORPTION OF LIGHT shown in Figure b, which occursif the atom has its electron in level1 and a photon of light ofwavelength^ λcollides with the^21 atom. During the collision, thephoton is absorbed by the atomand the electron is moved up tothe higher energy level 2. Thisprocess is the way light interactswith practically all of matter. Itcan happen from any energy levelthat is occupied (generally theground state) and always booststhe atom to a higher-lying levelwhile eliminating the photon. Thisoften results in heating of theabsorbing material

∆ E^21 ∆ E 21 ∆ E^21 ∆ E 21 ∆ E^21 A. Guenther UCONNA. Guenther UCONN

The third process, shown in Figure cis referred to as^ stimulated emission w.wang

. It

results when an electron is in a higher-lying level, such as level 2, and aphoton of light of wavelength

λ^21

collides with the atom. During thecollision the photon stimulates theatom to radiate a second photonhaving exactly the same energy

∆ E^21

Stimulated Emissionas that of the incident photon andtraveling in exactly the same directionin order to satisfy the laws ofconservation of energy andmomentum. Hence, one photon leadsto two identical photons, which, ineffect, leads to an amplificationprocess. A photon has been gained atthe expense of the loss of energystored within the atom.

A. Guenther UCONN

∆ E^21 ∆ E^21

∆ E^21 ∆ E^21 ∆ E^21

Suppose that we were able to “pump” (excite) a significant amount of population ofthe medium from level 0 to level 2. Also, for the time being let us assume that thereis no population in level 1. (This is an unlikely scenario but we will do this as a“thought” experiment for illustrative purposes.) Then again, let us consider having abeam of photons of energy w.wang

∆ E and wavelength^ λ^21

enter the medium. According to

the earlier discussion, and considering the process that can occur is stimulatedemission, and we would expect more photons to be generated as the beamprogresses. This can be described mathematically in the equation below^ in which we now have the population density

N in the expression along with the^2

appropriate cross section

Stimulated Emission^ σ.^21

Now, if population is allowed to be in both level1 and level 2, both absorption and stimulatedemission will occur within the medium andtherefore Hence, if more population exists in level 2than in level 1,^ N will be greater than^2 w.wang

N and^1

the exponent of above equation will bepositive. The beam will grow and emerge fromthe medium with a greater intensity than whenit entered. In other words, for amplification orgain to occur, the condition must be Having^ N be larger than^ N^2

is known as

having a^ population inversion

, which is not a

POPULATION INVERSION normal, naturally occurring relationship.

A. Guenther UCONN

It is useful to describe the product of w.wang

σand^ ∆ N as the small-signal-gain coefficient^21

g or^21

g =^ σ∆ N^21 21212 By considering the units of both^ σ(length^ ) and^ ∆ N^21

3 (l/length ) we can see that^ g

21

has the units of 1/length. Hence, if

2 σis given in units of cm 21

and^ ∆ N is given in^21

3 units of (1/cm ),^ g will be given in (1/cm), more commonly expressed as cm^21

-.

Values of the cross sections

σand^ ∆ N , and the small-signal gain^21

g^21

Small-signal-gain coefficient

ATOM^ w.wang

MOLECULE^

ION

• He-Ne (Helium-Neon) Metal Vapor Lasers • Cu (Copper) Vapor • Au (Gold) Vapor Ionized vapor Lasers *He-Cd (Helium-Cadmium)

• CO(Carbon^2 Dioxide) • N^ (Nitrogen)^2 • Chemical (HF-DF) • FIR - Far Infrared • Excimer Laser

• Ar+ (Argon ion) • Kr+ (Krypton ion)

Gas Laser

Population Inversion in gas laser- Applied voltage produces an electric field accelerates the electrons within the gas.- Excited electrons collide with the gas atoms and excite the atoms to excited energylevels, some of which serve as upper laser levels.- Lower-lying levels, those to which higher-lying levels can transition, typicallydecay to the ground state faster than the higher-lying levels, thereby establishing apopulation inversion between some of the higher and lower levels.- The laser light then occurs when the higher-lying levels decay to the lower levelswhile radiating photons at the wavelengths corresponding to the energy separationbetween the levels.- In many instances the excitation is a two-step process in which the electrons* first excite a long-lived or metastable (storage) level or they ionize the atom,leaving an ion of that species and another electron. In either case, that level* then transfers its stored energy to the upper laser level via a subsequentcollision with the laser species.w.wang