激光原理及激光器简介-英文PPT

Brief introduction of laser
By Jiang Wei and Zeng Xiangze Physics College Shandong University 2011.4.21
Principal of laser light
The interaction between electrons consists of three basic process:spontaneous emission, stimulated absorption and stimulated emission.
• Einstein has proved that B21 is equal to C12. • For an atomic system,stimulated absorption and emission both happen if there exists optical signal with energy density u(µ21). • From equation 1 and 2, we know: the net stimulated emission energy E=B21u(µ21) N2hµ21dtdv-C12u(µ21) N1hµ12dtdv =(N2-N1)B21u(µ21) hµ21dtdv Therefore, the laser light emits only when N2>N1
ቤተ መጻሕፍቲ ባይዱ
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• spontaneous emission is the process in which excited atoms can experience spontaneous transition from high energy level to low energy level. Simultaneously, certain photon is emitted .
• We use "Pumping" method to achieve "population inversion".In terms of different energy levels, there are 3-level system,4level system and so on.
• stationary state • excited state • metastable state(one of the excited states in which atoms can stay longer)
3.Optical resonator It is made of two mirrors ,of which one is total reflecting ,the other is partial reflecting. Light of which direction is parallel to the axis of the optical resonator reflects between the two mirrors . In this process, light intensity goes up . When it reaches a certain level ,the laser light cross the partial reflecting mirror.
How can we get laser light ?
He-Ni laser
1.Active material It’s the material which receive energy from pump resource and then generate laser light. Its atomic system must has the metastable state . It can be solid, liquid or gas. 2.Pump resource It is used to stimulate the atomic system(active material) and promote the coming of population inversion. It can be optical energy , electric energy, chemical energy, etc.
• 3-level system Particles in the stationary state absorb energy given by the pump and transfer from E1 to E3.Since particles can't stay long in E3, they tend to move to E2 and release some heat.This achieve the goal "population inversion"
• However, under normal circumstance, N satisfies the Boltzmann Distribution: N2/N1=exp[-(E2-E1)/kT] since E2>E1, N2<N1 Now, we know that the key point to generate laser light is "population inversion" which means we must do something to make N2>N1 to meet the required condition.We will discuss this later in detail.
• stimulated emission • When there is an atom at its high energy level and an photon with energy hµ coming inside, the atom is likely to transfer to the low energy level and emit a photon if hµ=E2-E1.The photon is same as the source photon in frequency, emitting direction, phase and polarization. • E=B21u(µ21) N2hµ21dtdv where B21 is Einstein Coeffient
And the optical resonator must be stable cavity， it means that paraxial ray can reflect many times in it . It satisfy this condition :
L is the distance between the two mirrors of which curvature radiuses are R1,R2. For a certain optical resonator ,not all the electromagnetic oscillation with any frequency can exist in it. For example ,in a plane parallel cavity (平面平行腔) ,only when the light form standing wave , can it become laser light . Now its frequency satisfy the condition : namely,
where ž is the refractive index of the active material.
• Of course , it require some other conditions for different optical resonators. • After all, the laser light has better characteristics superior to ordinary light in brightness, monochrome , direction and coherence which relate to its physical creation process .
• E2-E1=hµ where µ is the frequency of the photon
• stimulated absorption: • When there is an atom at its low energy level and an photon with energy hµ coming inside, the atom is likely to absorb the photon and transfer to the high energy level if hµ=E2-E1 • The transition does not happen spontaneously but occurs due to the photon stimulating • E=C12u(µ21) N1hµ12dtdv where C12 is Einstein Coeffient