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Laser Laser 레이저 laser light amplification by stimulated emission of radiation

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Laser Laser 레이저 laser light amplification by stimulated emission of radiation Powered By Docstoc
					                                  Laser

레이저(laser, light amplification by stimulated emission of radiation)는 더 이상 우리에게
생소한 말이 아니다. 1960 년 Dr. T. H. Maiman 에 의해 Ruby 레이저가 처음 시현된 이후
이산화탄소 레이저, 염료(dye) 레이저, 반도체(diode) 레이저로 확산된 레이저 시대의 도래는
현재에 이르러 CD player 와 광통신에서부터 군사용 거리 측정기에 이르기까지 사회의
곳곳에서 사용되지 않고 있는 곳이 없을 정도이다. 화학은 레이저의 개발과 생산에
선도적인 역할을 담당해왔으며 개발된 다양한 레이저는 화학의 여러 분야에 다시 적용되어
화학의 많은 발전을 이루게 했다. 특히 분자나 원자 내의 에너지 상태와 에너지의 흐름을
규명하고, 화학반응이 발생하는 근본적인 경로를 밝히는데 다양하게 사용되어왔다.




     그림 설명: 루비 레이저의 구조와 최초의 발명자 Dr. T. H. Maiman. Flash
     tube 에서 발생되는 매우 강한 빛에 의해 루비봉(ruby bar) 내의 불순물인 Cr3+
     ion 이 여기 됨 (excited)으로 높은 에너지 레벨에 위치해서 population
     inversion 이 발생된다. 루비 봉의 양쪽 끝은 금속 코팅이 되어 있어 거울로서
     작용하게 되어 유발방출(stimulated emission)을 일으키게 되고 결과적으로
     레이저 빔이 루비봉의 한쪽 끝으로 나오게 된다.


레이저는 빛의 한 형태이다. 그러나, 우리가 통상 접하게 되는 자연광(自然光)과 달리
레이저는 높은 순도의 한가지 파장만을 갖고 있으며 거의 평행광선(highly collimated)이고
매우 높은 광도(光度, light intensity)를 갖고 있다. 동시에 레이저를 구성하는 광자(光子)들은
모두 같은 위상을 갖는다(coherence). 이 같은 성질들은 매우 고밀도로 빛에너지를 밀집해
금속을 절단하거나 핵융합을 유발하는데 사용될 수 있으며, 분자 내의 특정한 운동이나 그
변화를 관찰하거나 분자 내 특정한 운동들을 선택적으로 일어나게 할 수도 있다. 이 같은
레이저의 특성을 살려 수많은 분석장비들이 개발되어 현재 사용되고 있다. 최근 들어
눈부시게 개발되고 있는 반도체 레이저들은 크기가 작고 다루기가 쉬운데다 저렴해서 더욱
많은 활용도(예: 광통신, CD player, 먼지 측정기, pointer 등)를 갖고 있으며 장차
computer 의 발달과 함께 전자 산업에도 그 적용 예가 급속히 증가할 것으로 예상된다.


Laser 의 발생(lasing): Laser 가 발생(lasing)되기 위해서는 크게 세 가지 조건이 작용한다.
첫째 조건: Light amplification. 특정한 media 를 지나갈 때 일반적으로 빛은 해당 media 에
흡수되어 media 를 지나간 후에는 지나가기 전에 비해 빛의 세기는 약해진다. 그러나
특정한 조건 하에서는 오히려 media 를 지나가는 과정에서 빛은 증폭되어 여러 번 반복해서
media 를 지나갈수록 더욱 빛의 세기는 강해지는 경우가 있다. 이 같은 경우 증폭되는
정도를 gain(G)이라 부르는데 통과 후 빛의 세기(I)는 통과 전 빛의 세기(Io)에 gain 을
곱해준 값이 된다.


                             I = Io ․ G


그 같은 증폭의 조건은 소위 population inversion 상황에서 만들어진다. 두개의 에너지
레벨이 있는 상황에서 통상적으로는 낮은 에너지 레벨에 더 많은 수의 분자들이 있게
마련인데 그 같은 현상을 우리는 Boltzmann distribution 으로 설명한다. 그러나 Boltzmann
distribution 은 외부의 간섭이 없는 평형상태를 가정하는 것으로, 만일 전류, 전기 방전, 빛의
조사, 가열, 화학 반응 등의 간섭(perturbation)이 발생하면 에너지 레벨 사이에는 더 이상
Boltzmann distribution 에 의한 분자 분포가 형성되지 않는다. 특정한 경우에는 오히려 위의
에너지 레벨에 상대적으로 더 많은 분자가 분포되는 일도 발생하는데, 그 것이 레이저
발생의 가장 기본적인 조건인 population inversion 이다. 여기서 말하는 두개의 에너지 레벨
중에서 아래 에너지 레벨은 해당 분자가 가질 수 있는 가장 낮은 에너지 상태 곧 ground
state 는 아니다. 왜냐하면, 아무리 분자분포를 바꾼다 하더라도 ground state 가 아닌 곳에
ground state 보다 더 분자를 분포하게 하는 것은 거의 불가능에 가깝기 때문이다.
                                      Energy Transfer




               Pumping                           Lasing




                                           Depopulation



                            Ground State
     그림 설명: Energy-level scheme for lasing. 바닥 상태에서 여기된 분자들은 특정
     에너지 레벨을 거쳐 사이에서 레이저가 발생하는 두 개의 에너지 레벨들 중
     위쪽 레벨에 도달하게 된다. 결과적으로 두개의 레벨 사이에는 population
     inversion 이 형성된다. 이 때 해당 media 를 두개의 거울 사이에 위치하면
     stimulated emission 이 발생(laser)하게 되고, 결과적으로 아래 레벨에 도달한
     분자들은 depopulation mechanism 에 의해 바닥상태로 다시 돌아온다.


어떻게 population inversion 을 형성할 것인가 하는 것은 새로운 레이저를 개발하는
과정에서 가장 어려운 부분에 해당한다. Population inversion 이 형성되기 위해서는 에너지
레벨들의 구조나 에너지 전달들이 모두 잘 조화되어 있어야 하지만 동시에 조금은
아이러니칼 하게도 두개의 레벨 사이에서는 쉽게 emission 이 발생하지 말아야 한다. 이는
쉽게 emission 이 발생하는 경우, population inversion 이 형성될 여지가 없이 분자들이 빛을
사방으로 방출하며 낮은 레벨로 이동해버릴 것이기 때문이다. 오히려 lasing 은 많은 경우
쉽게 emission 이 발생하지 않는 레벨들 사이에서 발생한다. 여기서 기억할 것은 lasing 은
그냥 emission 이 아니라 stimulated emission 이란 사실이다.
% 결과적으로 짧은 파장의 레이저를 개발하는 일은 매우 어렵다. 이는 커다란 에너지
갭(짧은 파장) 사이일수록 빛의 방출이 훨씬 잘 발생하기 때문이다. 따라서 아직도 인간은
진동수 배수화(frequency doubling) 말고는 쓸만한 UV 레이저를 만들어내지 못했다. Star
wars program 에 사용을 계획 중인 X-ray 레이저는 아직도 개발이 요원해서 결국 접근하는
상대 핵탄두의 파괴를 위해 레이저 사격보다는 발사체의 직접 충돌 방법을 채택했다.


두번째 조건: Lasing 은 두개의 에너지 레벨 사이에서 발생하기 때문에 Bohr 의 frequency
condition 이 성립한다.


                                h = Eb - Ea


Lasing 은 stimulated emission 의 결과이기 때문에 모든 광자는 같은 에너지(같은 진동수)를
갖게 된다. 결과적으로 laser 는 같은 파장을 가진 매우 강력한 빛에 해당되게 되는 것이다.


세번째 조건: 이는 lasing 발생되기 위한 레이저의 구조를 말한다. 앞에서 기술한 것처럼
레이저는 반복적인 stimulated emission 에 의한 빛의 증폭 결과이다. 따라서, 빛을 여러 번
레이저 media 속으로 통과시킬 필요가 있는데, 이를 위해 레이저는 두개의 거울을 사용한다.
두개의      거울을   사용하는    이유는   두개의     거울       사이에서   빛이    반복적으로        왕복하게
만듦으로써 여러 번의 증폭 기회를 갖고 결과적으로 매우 강력한 빛(레이저 빔)을 형성하는
것이다. 그러나, 두개의 거울 사이에서만 빛이 계속 왕복한다면 그 용도는 매우 제한적일
것이다. 이를 극복하기 위해 두개의 거울 중 한 개의 거울은 대부분 반사-일부 통과하는
특성을 가진 거울(output coupler)을 사용한다. 결과적으로 두개의 거울 사이에서 형성된
강력한 빛(레이저 빔)의 일부는 레이저를 빠져 나와 공간으로 진행하게 된다.



                                                                   Laser Beam

Mirror                Laser Medium             Partially-transmitting
                                               Mirror


      그림 설명: 레이저의 구조. 레이저는 두개의 거울 사이에서 발생한다. 두개의
      거울 사이에서 왕복하는 빛은 medium 을 통과할 때마다 계속 증폭되게 되고
      결과적으로 매우 강력한 빛은 두개의 거울 사이에 형성한다. 구개의 거울 중
      하나는 일부 통과(partially transmitting) 특성을 가진 거울로 일부의 빛은
      레이저 cavity(두개 거울 사이의 공간)를 떠나 공간으로 진행한다.
여기서 우리가 알 수 있는 것은 지금 말하는 laser medium 은 일종의 빛 증폭기 구실을
한다는 것이다. 동시에 stimulated emission 은 방향과 phase 가 일치하기 때문에 발생되는
레이저는 정확히 같은 방향을 갖게 되고(highly collimated) phase 가 일치(coherent)하는
특성을 갖게 된다. 그러나, 그 같은 특성은 단순히 두개의 거울을 마주 보게 한다고 해서
이루어지는 것은 아니고 정확한 평행광과 coherency 를 갖게 하기 위해서는 거울 사이의
거리와 각을 미세 조정할 필요가 있다. 다른 한편으로 평면 거울을 사용하는 경우 빛의
평행성(collimation) 자체는 최고로 유지할 수 있으나 빛의 유실이 많아 lasing 자체가
어려워질 수 있다. 따라서, 일반적으로 평면 거울 보다는 오목 거울을 사용하고 두개 거울
사이의 거리가 거울의 초점 거리(f)보다 크고 radius of curvature (r, 2․f) 보다 짧으면 되는
것으로   알려져       있다.   또   전반사용   거울에   대해서는   많은   경우    단순한   거울보다는
회절발(grating)을   사용해서      특정   파장의   빛만   반사하도록    한다.   이는   실제   레이저
미디움에서 형성되는 레이저의 파장이 하나가 아니라 여럿이 될 수 있기 때문인데(한 개
이상의 에너지 갭에서 lasing 이 발생할 수 있기 때문에), 회절발을 사용함으로써 오직 한
파장 만 발생하도록 유도한다. 이외에도 두 거울의 위치와 각에 의해 몇 개의 레이저
모드들이 있어 아주 미세하나마 파장의 차이가 발생할 수 있는데 그 같은 분야는 우리
수업의 범주을 벗어나는 탓에 다루지 않도록 한다.


레이저의 예(CO2 laser): 레이저의 전형적인 예로 CO2 laser 를 살펴보기로 하자. CO2 laser 는
약 1000 cm-1(10 μm) 지역에 두 개의 band center(9.4 와 10.4 μm)를 갖고 있다. 각각의
band 는 두 개의 다른 진동전이들(0001 → 1000, 0001 → 0200)에 기인한 것들로 각 진동
band 들의 P 와 Q-branch 들의 진동-회전 line 들은 보통 1 - 2 cm-1 정도씩 떨어져 있다. CO2
분자의 대칭성에 기인해 각 line 들은 모두 짝수 회전양자수(J)을 갖고 홀수 회전양자수
line 들은 missing 되어 있다. 여기서 lasing 이 발생하는 두 가지 진동 전이에 대해 살펴보자.
우선 낯설은 세자리 숫자는 CO2 분자가 갖는 세가지 진동 형식들(symmetric stretch,
bending, asymmetric stretch)이 각각 어떤 진동양자수를 갖는가를 보여준다. 0001 진동
상태란 ν1 진동(symmetric stretch) mode 는 바닥 상태(v1 = 0)이고, ν2 진동(bending) mode
역시 바닥 상태(v2 = 0)에 있으나, ν3 진동(asymmetric stretch)는 첫 번째 진동 여기 상태(v3 =
1)에 있음을 보여준다. 따라서, 0001 → 1000 전이의 경우 v1 = 0 → 1, v2 = 0 → 0, v3 = 1 →
0 으로 바뀌는 전이를 의미한다. 0001 → 0200 전이의 경우는 v1 = 0 → 0, v2 = 0 → 2, v3 = 1
→ 0 으로 바뀌는 전이에 해당한다.
                   N2                                        CO2
                                             u             g+          u


                  v=1
                                                      0001


2000 cm-1


                                                        10.4 m
                                             9.4 m




                                                                   1000
                        Electric Discharge                         0200

1000 cm-1


                                                                               0110




                                                      0000
                             Ground State
             그림 설명: 이산화탄소 레이저 발생에 관련된 에너지 레벨들.


그렇다면 진동 상태를 보여주는 기호 중 가운데 숫자의 오른쪽 어깨에 표시된 숫자는
무엇을 말하는 것일까? 여러분들이 알다시피 CO2 분자의 bending 은 사실은 두 개의 진동
mode 가 중첩되어 있다. 같은 진동수를 갖는 두 개의 진동 mode 들이 공존하는 것이다.
분광학에서는 같은 진동수를 가져 구별할 수 없는 두 개의 진동 mode 들에 대해 한 가지
진동양자수(이 경우 v2)를 부여하고 대신 ℓ양자수(이 경우 ℓ = +1 or -1)로 어떤 진동 mode 로
전이되었는가를 표시한다. 굳이 그렇게 표시하는 데는 나름대로 이유가 있는데 자세한
설명은 피하고, 기본적으로 미시세계에서는 두 개의 degenerate 된 진동은 사실상 구별
불가능하고 대신 두 진동 mode 들 간의 가능한 상호작용의 차이가 있게 되기 때문이다.
0200 진동 상태의 경우 20 란 bending 의 두 번째 진동여기 상태(v2 = 2)인데, 사실상은 두
개의 bending mode 들(ℓ = +1 과 ℓ = -1)이 각각 v = 1 인 상태에 있음을 의미한다. 아무튼
CO2 laser 는 v3 = 1 인 상태(asymmetric stretch 상태)가 v1 = 1 인 상태(symmetric stretch
상태)나 v2 = 2 인 상태(bending first overtone)상태로 바뀌면서 그 차이에 해당하는 에너지가
stimulated emission 의 형태로 방출되는 것이다.


CO2 laser 의 laser medium 은 기체 상태로 CO2, N2, He 이 1 : 1 : 8 정도 비율로 채워지며,
전체 압력은 laser 의 용도에 따라 다르지만 보통 수 torr 에서 수백 torr 의 압력을 갖는다.
CO2 가 사용되는 것은 실제 lasing 전이가 CO2 분자의 진동 에너지 레벨들 사이에서
발생하니까 당연한 것으로 받아들여지는데, N2 와 He 의 용도는 무엇일까? CO2 laser
medium 인 gas mixture 에는 에너지 공급 방법으로 전기 방전이 가해진다. 이 때 전기
에너지를 가장 쉽사리 흡수(전자와의 충돌을 통해)하는 분자는 특성상 N2 이다. 전기 에너지
흡수의 결과로 N2 분자는 진동을 시작하는데 N2 분자의 진동은 분자 dipole moment 의
변화를 유발하지 않기 때문에 스스로 전자기파(적외선)를 방출하며 바닥 상태로 떨어지지
못한다. 대신, N2 의 v = 1 상태와 매우 유사한 에너지 레벨을 가진 CO2 분자의 0001 상태에
분자 충돌 과정을 거쳐 에너지를 전달하고 N2 분자 자신은 바닥 상태로 내려간다. 이렇게
0001 상태에 이른 CO2 분자는 위에 기술한 두 가지의 진동전이들을 통해 lasing 을
일으키고 결과적으로 laser 를 발생한다.


진동전이들 후 CO2 분자는 1000 상태와 0200 상태에 이르게 되는데 CO2 분자들은 이들
상태로부터 0110 상태를 거쳐 바닥상태에 이르게 된다. 그런데 이 과정에서 분자들은 주로
자신들이    가진    에너지를    분자   충돌을   통해   해소하게        되는데      He   분자들은   다수가
존재하면서 충돌을 통해 이들 에너지 레벨들에 있는 CO2 분자들의 에너지를 흡수한 후
높은 병진운동 에너지를 갖게 되고, 그 같은 병진운동 에너지는 용기벽과의 충돌 등을 통해
자신의    에너지를    해소하게    된다.   따라서,   CO2   laser   medium   의    벽은   지속적으로
냉각시켜주어야 한다. 바닥 상태에 도달한 CO2 분자는 다시 전기 에너지를 흡수한 N2 와
충돌해 0001 상태로 되돌려지게 된다(pumping).


CO2 laser 는 간편하게 만들 수 있는 laser 로 각광 받고 있다. 여러분들도 전원과 냉각이
되는 유리관, 기체 혼합물, 그리고 두 개의 거울 만 있으면 별 어려움 없이 만들 수 있다.
게다가 오랜 사용으로 기술이 축적되어 고출력이며 소형인 CO2 laser 들이 상품화되어
시판되고 있다. 치과에서 충치 치료에 사용하는 laser, 피부의 점 빼는 laser, 땅콩 껍질
까는 laser, 핵반응을 유발하는 laser 등으로 많이 사용되고 있다. CO2 laser 의 장점을
들자면 우선 잘 optimize 하면 에너지의 전환효율이 매우 높다는 것이다. 약 20%의
전기에너지를 laser 에너지로 바꿀 수 있다고 한다. 여러분 생각에는 그 것이 그리 높지
않다고 생각될지도 모르지만 20% 효율은 laser 의 세계에서는 상당히 높은 것이다. 동시에
CO2 laser 의 laser line 들(진동-회전 전이에 의한)은 그 진동수가 정확히 잘 알려져 있고
활용이 용이해서 다양한 고 에너지 분광학의 에너지원으로 많이 사용된다.
Laser
                                   .




Lasers range in size from microscopic diode lasers (top) with numerous
applications, to football field sized neodymium glass lasers (bottom) used for
inertial confinement fusion, nuclear weapons research and other high energy
density physics experiments.

A LASER (Light Amplification by Stimulated Emission of Radiation) is an optical
source that emits photons in a coherent beam. Laser light is typically near-
monochromatic, i.e. consisting of a single wavelength or hue, and emitted in a
narrow beam. This is in contrast to common light sources, such as the
incandescent light bulb, which emit incoherent photons in almost all directions,
usually over a wide spectrum of wavelengths. Laser action is understood by
application of quantum mechanics and thermodynamics theory (see laser
science).

The verb "to lase" means "to produce coherent light" or possibly "to cut or
otherwise treat with coherent light", and is a back-formation of the term laser.


Physics




Principal components:
1. Active laser medium
2. Laser pumping energy
3. Mirror
4. Partial mirror
5. Laser beam

A laser is composed of an active laser medium (or 'gain medium') and a resonant
optical cavity.

The gain medium transfers external energy into the laser beam. It is a material of
controlled purity, size and shape, which uses a quantum mechanical effect called
stimulated emission (discovered by Albert Einstein while researching the
photoelectric effect) to amplify the beam. The gain medium is pumped by an
external energy source, such as electricity or light (from a classical source such
as a flash lamp, or another laser). The pump energy is absorbed by the medium,
producing excited states. When the number of particles in one excited state
exceeds the number of particles in some lower state, population inversion is
achieved. In this condition, an optical beam passing through the medium
produces more stimulated emission than the stimulated absorption so the beam is
amplified. An excited laser medium can also function as an optical amplifier.

The light generated by stimulated emission is very similar to the input signal in
terms of wavelength, phase, and polarization. This gives laser light its
characteristic coherence, and allows it to maintain the uniform polarization and
monochromaticity established by the optical cavity design.

The resonant cavity (see also cavity resonator) contains a coherent beam of light
between reflective surfaces so that each photon passes through the gain medium
multiple times before being emitted from the output aperture or lost to diffraction
or absorption. As light circulates through the cavity, passing through the gain
medium, if the gain (amplification) in the medium is stronger than the resonator
losses, the power of the circulating light can rise exponentially. However, each
stimulated emission event returns a particle from its excited state to the ground
state, reducing the capacity of the gain medium for further amplification. When
this effect becomes strong, the gain is said to be saturated. The balance of pump
power against gain saturation and cavity losses produces an equilibrium value of
the intracavity laser power which determines the operating point of the laser. If
the pump power is chosen too small (below the "laser threshold"), the gain is not
sufficient to overcome the resonator losses, and the laser will emit only very small
light powers. Note that the gain medium will amplify any photons passing through
it, regardless of direction, however it is only the ones that happen to be aligned
with the cavity that manage to make multiple passes through the medium and so
have significant amplification.
Experiment using a (likely argon) laser. (US military)

The beam in the cavity and the output beam of the laser, if they occur in free
space rather than waveguides (as in an optical fiber laser), are often Gaussian
beams. If the beam is not a pure Gaussian shape, the transverse modes of the
beam may be analyzed as a superposition of Hermite-Gaussian or Laguerre-
Gaussian beams. The beam often has a very small divergence (highly collimated),
but a perfectly collimated beam cannot be created, due to the effect of diffraction.
Nonetheless, a laser beam will spread much less than a beam of incoherent light.
The distance over which the beam remains collimated increases with the square
of the beam diameter, and the angle at which the beam eventually diverges varies
inversely with the diameter. Thus, a beam generated by a small laboratory laser
such as a helium-neon (HeNe) laser spreads to approximately 1 mile (1.6
kilometres) in diameter if shone from the Earth's surface to the Moon. By
comparison, the output of a typical semiconductor laser, due to its small diameter,
diverges almost immediately on exiting the aperture, at an angle that may be as
high as 50°. However, such a divergent beam can be transformed into a
collimated beam by means of a lens. In contrast, the light from non-laser light
sources cannot be collimated by optics as well or much.




A HeNe laser demonstration at the Kastler-Brossel Laboratory at Univ. Paris 6.
The glowing ray in the middle is an electric discharge producing light in much the
same way as a neon light; though it is the gain medium through which the laser
passes, it is not the laser beam itself which is visible there. The laser beam
crosses the air and marks a red point on the screen to the right.

The output of a laser may be a continuous, constant-amplitude output (known as
CW or continuous wave), or pulsed, by using the techniques of Q-switching,
modelocking, or gain-switching. In pulsed operation, much higher peak powers
can be achieved.

Some types of lasers, such as dye lasers and vibronic solid-state lasers can
produce light over a broad range of wavelengths; this property makes them
suitable for the generation of extremely short pulses of light, on the order of a
femtosecond (10-15 s).

Though the laser phenomenon was discovered with the help of quantum physics,
it is not essentially more quantum mechanical than are other sources of light. In
fact the operation of a free electron laser can be explained without reference to
quantum mechanics.

It should be understood that the word light in the acronym Light Amplification by
Stimulated Emission of Radiation is typically used in the expansive sense, as
photons of any energy; it is not limited to photons in the visible spectrum. Hence
there are X-ray lasers, infrared lasers, ultraviolet lasers, etc. Because the
microwave equivalent of the laser, the maser, was developed first, devices that
emit microwave and radio frequencies are usually called masers. In early literature,
particularly from researchers at Bell Telephone Laboratories, the laser was often
called the optical maser. This usage has since become uncommon, and as of
1998 even Bell Labs uses the term laser[1].


History

In 1916, Albert Einstein laid the foundation for the invention of the laser and its
predecessor, the maser, in a ground-breaking rederivation of Max Planck's law of
radiation based on the concepts of spontaneous and induced emission. The
theory was forgotten until after World War II.
In 1953, Charles H. Townes and graduate students James P. Gordon and Herbert
J. Zeiger produced the first maser, a device operating on similar principles to the
laser, but producing microwave rather than optical radiation. Townes' maser was
incapable of continuous output. Nikolay Basov and Aleksandr Prokhorov of the
Soviet Union worked independently on the quantum oscillator and solved the
problem of continuous output systems by using more than two energy levels.
These systems could release stimulated emission without falling to the ground
state, thus maintaining a population inversion. Townes, Basov and Prokhorov
shared the Nobel Prize in Physics in 1964 "for fundamental work in the field of
quantum electronics, which has led to the construction of oscillators and
amplifiers based on the maser-laser principle."

In 1957 Charles Hard Townes and Arthur Leonard Schawlow, then at Bell Labs,
began a serious study of the infrared maser. As ideas were developed, infrared
frequencies were abandoned with focus on visible light instead. The concept was
originally known as an "optical maser". Bell Labs filed a patent application for their
proposed optical maser a year later. Schawlow and Townes sent a manuscript of
their theoretical calculations to Physical Review, which published their paper that
year (Volume 112, Issue 6).

Simultaneously, Gordon Gould, a graduate student at Columbia University, was
working on a doctoral thesis on the energy levels of excited thallium. Gould and
Townes met and had conversations on the general subject of radiation emission.
After that meeting, Gould made notes about his ideas for a "laser" in November
1957. In 1958, Prokhorov proposed an open resonator which became an
important ingredient of future lasers. The first introduction of the term "laser" to
the public was in Gould's 1959 paper "The LASER, Light Amplification by
Stimulated Emission of Radiation". Gould intended "aser" to be a suffix, to be
used with an appropriate prefix for the spectra of light emitted by the device (e.g.
X-ray laser = xaser, UltraViolet laser = uvaser). None of the other terms became
popular, although "raser" is sometimes used for radio-frequency emitting devices.

Gould's notes included possible applications for a laser, such as spectrometry,
interferometry, radar, and nuclear fusion. He continued working on his idea and
filed a patent application in April 1959. The U.S. Patent Office denied his
application and awarded it to Bell Labs in 1960. This sparked a legal battle that
spanned three decades, with scientific prestige and much money at stake. Gould
won his first minor patent in 1977, but it was not until 1987 that he could claim his
first significant patent victory when a federal judge ordered the government to
issue a patent to him for each of the optically pumped and the gas discharge
laser.

The first working laser was made by Theodore H. Maiman in 1960[2] at Hughes
Research Laboratories in Malibu, California, beating several research teams
including those of Townes at Columbia University, and Arthur L. Schawlow at Bell
Labs. Maiman used a solid-state flashlamp-pumped synthetic ruby crystal to
produce red laser light at 694 nanometres wavelength. Maiman's laser, however,
was only capable of pulsed operation due to its three energy level transitions.
Later in the same year the Iranian physicist Ali Javan, together with William Bennet
and Donald Herriot, made the first gas laser using helium and neon. Javan later
received the Albert Einstein Award in 1993.

The concept of the semiconductor laser diode was proposed by Basov and
Javan; and the first laser diode was demonstrated by Robert N. Hall in 1962.
Hall's device was constructed of gallium arsenide and produced emission at 850
nm, in the near-infrared region of the spectrum. The first semiconductor laser
with visible emission was demonstrated later the same year by Nick Holonyak, Jr.
As with the first gas lasers, these early semiconductor lasers could be used only
in pulsed operation, and indeed only when cooled to liquid nitrogen temperatures
(77 K).

In 1970, Zhores Alferov in the Soviet Union and Izuo Hayashi and Morton Panish
of Bell Telephone Laboratories independently developed continuously operating
laser diodes at room temperature, using the heterojunction structure.

The first application of lasers visible in the daily lives of the general population
was the supermarket barcode scanner, introduced in 1974. The laserdisc player,
introduced in 1978, was the first successful consumer product to include a laser,
but the compact disc player was the first laser-equipped device to become truly
common in consumers' homes, beginning in 1982.

Recent innovations
Graph showing the history of maximum laser pulse intensity throughout the past
40 years.

Since the early period of laser history, laser research has produced a variety of
improved and specialized laser types, optimized for different performance goals,
including

               new wavelength bands
               maximum average output power
               maximum peak output power
               minimum output pulse duration
               maximum power efficiency

and this research continues to this day.

Lasing without maintaining the medium excited into a population inversion, was
discovered in 1992 in sodium gas and again in 1995 in rubidium gas by various
international teams. This was accomplished by using an external maser to induce
"optical transparency" in the medium by introducing and destructively interfering
the ground electron transitions between two paths, so that the likelihood for the
ground electrons to absorb any energy has been cancelled.
In 1985 at the University of Rochester's Laboratory for Laser Energetics a
breakthrough in creating ultrashort-pulse, very high-intensity (terawatts) laser
pulses became available using a technique called chirped pulse amplification, or
CPA, discovered by Gérard Mourou. These high intensity pulses can produce
filament propagation in the atmosphere.




Uses

At the time of their invention in 1960, lasers were called "a solution looking for a
problem". Since then, they have become ubiquitous, finding utility in thousands of
highly varied applications in every section of modern society, including consumer
electronics, information technology, science, medicine, industry, law enforcement
and the military.

In 2004, excluding diode lasers, approximately 131,000 lasers were sold world-
                                        [3]
wide, with a value of US$2.19 billion     . In the same year, approximately 733
                                                           [4]
million diode lasers, valued at $3.20 billion, were sold     .




A laser harp.

The benefits of lasers in various applications stems from their properties such as
coherency, high monochromaticity, and capability for reaching extremely high
powers. For instance, a highly coherent laser beam can be focused down to its
diffraction limit, which at visible wavelengths corresponds to only a few hundred
nanometers. This property allows a laser to record gigabytes of information in the
microscopic pits of a DVD. It also allows a laser of modest power to be focused
to very high intensities and used for cutting, burning or even vaporizing materials.
For example, a frequency doubled neodymium yttrium aluminum garnet (Nd:YAG)
laser emitting 532 nanometer (green) light at 10 watts output power is
theoretically capable of achieving a focused intensity of megawatts per square
centimeter. In reality however, perfect focusing of a beam to its diffraction limit is
somewhat difficult.




Lasers used for visual effects during a musical performance. (A laser light show.)

Consumer electronics and communication In consumer electronics,
telecommunications, and data communications, lasers are used as the
transmitters in optical communications over optical fiber and free space. They are
used to store and retrieve data from compact discs and DVDs, as well as
magneto-optical discs. Laser lighting displays (pictured) accompany many music
concerts.

Science In science, lasers are employed in a wide variety of interferometric
techniques, and for Raman spectroscopy and laser induced breakdown
spectroscopy. Other uses include atmospheric remote sensing, and investigation
of nonlinear optics phenomena. Holographic techniques employing lasers also
contribute to a number of measurement techniques. Laser (LIDAR) technology
has application in geology, seismology, remote sensing and atmospheric physics.
Lasers have also been used aboard spacecraft such as in the Cassini-Huygens
mission. In astronomy, lasers have been used to create artificial laser guide stars,
used as reference objects for adaptive optics telescopes.

Medicine In medicine, the laser scalpel is used for laser vision correction and
other surgical techniques. Lasers are also used for dermatological procedures
including removal of tattoos, birthmarks, and hair; laser types used in
dermatology include ruby (694 nm), alexandrite (755 nm), pulsed diode array (810
nm), Nd:YAG (1064 nm), Ho:YAG (2090 nm), and Er:YAG (2940 nm). Lasers are
also used in photobiomodulation (laser therapy) and in acupuncture.

Industry In industry, laser cutting is used to cut metals and other materials. Laser
line levels are used in surveying and construction. Lasers are also used for
guidance for aircraft. Lasers are used in certain types of thermonuclear fusion
reactors. Lasers are also used extensively in both consumer and industrial
imaging equipment. The name laser printer speaks for itself but both gas and
diode lasers play a key role in manufacturing high resolution printing plates and in
image scanning equipment.

Law enforcement and road safety In law enforcement the most widely known use
of lasers is for lidar, to detect the speed of vehicles.




The surface of a test target is instantly vaporized and bursts into flame upon
irradiation by a high power continuous wave carbon dioxide laser emitting tens of
kilowatts of far infrared light. Note the operator is standing behind sheets of
plexiglass which is naturally opaque in the far infrared.

Military Military uses of lasers include use as target designators for other
weapons; their use as directed-energy weapons is currently under research.
Laser weapon systems under development include the airborne laser, the
advanced tactical laser, the Tactical High Energy Laser, the High Energy Liquid
Laser Area Defense System, and the MIRACL, or Mid-Infrared Advanced Chemical
Laser.
Popular misconceptions

The representation of lasers in popular culture, especially in science fiction and
action movies, is generally very misleading. Contrary to their portrayal in many
science fiction movies, such as Star Wars, a laser beam is never visible in the
vacuum of space, and even in air, beams or rays of laser light are not necessarily
any more visible than rays from any other light source. In air the beam can hit
dust and other particles in its path and scatter producing a glowing "ray", in much
the same way that a sunbeam glows in dusty air, an effect which can be used to
make the beam more visible by increasing the number of particles suspended in
the air using, for instance, a theatrical fog machine.

Moderate intensity (greater than ~10 milliwatts) laser beams of shorter green and
blue wavelengths and high intensity beams of longer orange and red wavelengths
can be visible in air due to Rayleigh scattering or at very high intensities possibly
Raman scattering. With even higher intensity pulsed beams, the air can be heated
to the point where it becomes a plasma, which would also be visible. This would
also cause a rapid heating and explosive expansion of the surrounding air which
would produce a popping noise analogous to the thunder which acompanies
lightning. This phenomenon is also capable of causing a retroreflection of the
laser beam back into the laser source possibly damaging its optics. When this
phenomenon occurs in certain scientific experiments it is variously referred to as
a "plasma mirror" or "plasma shutter".

Science fiction films special effects often depict laser beams propagating at only
a few metres per second—i.e., slowly enough to see their progress, in a manner
reminiscent of conventional tracer ammunition—whereas in reality a laser beam
travels at the speed of light, and would seem to appear instantly to the naked eye
from start to end.

Some action movies depict security systems using lasers of visible light (and their
foiling by the hero, typically using mirrors); the hero may see the path of the beam
by sprinkling some dust in the air. It is actually far easier and cheaper to build
infrared laser diodes rather than visible light laser diodes and such systems
almost never use visible light lasers.
In action movies laser weaponry is commonly portrayed as generating a 'zapping'
sound when fired. The only sounds emitted by real-world lasers are the sounds of
the equipment used to generate them, which is typically a low-pitched hum.

Several of these misconceptions can be found in the James Bond film Goldfinger,
the first film to feature a laser. In one of the most famous scenes in the Bond
films, Bond, played by Sean Connery faces a laser beam approaching his groin
while melting the solid gold table to which he is strapped. The director Guy
Hamilton found that a real laser beam would not show up on camera so it was
added as an optical effect. The melting effect on the table was achieved by a
man underneath the table holding an oxyacetylene torch, while a real laser would
have produced a fairly heat-free and silent cut. [1]

"LASER"

Even though the word laser comes from an initialism (Light Amplification by the
Stimulated Emission of Radiation), it has been incorporated into the language as
a separate word—an acronym—and as such it is written in lower case letters. By
back-formation, the verb "to lase" has also been created, meaning "to produce
coherent light through stimulated emission".
A dye laser used at the Starfire Optical Range for LIDAR and laser guide star
experiments is tuned to the sodium D line and used to excite sodium atoms in the
upper atmosphere.

Scientific misconceptions

Besides in movies and popular culture, laser misconceptions are present in many
science texts. For example, laser light is not inherently parallel light as is usually
claimed. All laser beams spread out as they propagate, due to diffraction. For a
good quality "singlemode" beam, the divergence (cone angle) of the beam is
inversely proportional to the width of the beam at its narrowest point, so a beam
can be made more parallel by increasing its minimum diameter. All beams
eventually spread out, however, since the beam cannot be infinitely wide at its
narrowest point. Note that poor-quality laser beams spread much faster with
distance than singlemode beams do.
Laser safety

Even the first laser was recognized as being potentially dangerous. Theodore
Maiman characterized the first laser as one Gillette; as it could burn through one
Gillette razor blade. Today, it is accepted that even low-power lasers with only a
few milliwatts of output power can be hazardous to a person's eyesight.

At wavelengths which the cornea and the lens can focus well, the coherence and
low divergence of laser light means that it can be focused by the eye into an
extremely small spot on the retina, resulting in localized burning and permanent
damage in seconds or even faster. Lasers are classified into safety classes
numbered I (inherently safe) to IV (even scattered light can cause eye and/or skin
damage). Laser products available for consumers, such as CD players and laser
pointers are usually in class I, II, or III. Certain infrared lasers with wavelengths
beyond about 1.4 microns are often referred to as being "eye-safe". This is due
to the fact that the intrinsic molecular vibrations of water molecules very strongly
absorb light in this part of the spectrum and thus a laser beam at these
wavelengths is attenuated so completely upon its passage through the eye's
cornea that no light remains to be focused by the lens onto the retina. The label
eye-safe can be misleading however, as it only applies to relatively low power
continuous wave beams and any high power or q-switched laser at these long
wavelengths will still obviously burn the cornea, causing severe eye damage.


Categories

By type

                                      For a more complete list of laser types see
                                      this list of laser types.
Spectral output of several types of lasers.
Gas lasers
         The Helium-neon laser (HeNe) emits 543 nm and 633 nm and is very
         common in education because of its low cost. Carbon dioxide lasers
         emit up to 100 kW at 9.6 µm and 10.6 µm, and are used in industry for
         cutting and welding. Argon-Ion lasers emit 458 nm, 488 nm or 514.5 nm.
         Carbon monoxide lasers must be cooled but can produce up to 500 kW.
         The Transverse Electrical discharge in gas at Atmospheric pressure
         (TEA) laser is an inexpensive gas laser producing UV Light at 337.1 nm.
         Metal ion lasers are gas lasers that generate deep ultraviolet
         wavelengths. Helium-Silver (HeAg) 224 nm and Neon-Copper (NeCu)
         248 nm are two examples. These lasers have particularly narrow
         oscillation linewidths of less than 3 GHz (0.5 picometers) [2] making
         them candidates for use in fluorescence suppressed Raman
         spectroscopy.
Chemical lasers
         Chemical lasers are powered by a chemical reaction, and can achieve
         high powers in continuous operation. For example, in the Hydrogen
         fluoride laser (2700-2900 nm) and the Deuterium fluoride laser (3800
         nm) the reaction is the combination of hydrogen or deuterium gas with
         combustion products of ethylene in nitrogen trifluoride.
Excimer lasers
         Excimer lasers produce ultraviolet light, and are used in semiconductor
         manufacturing and in LASIK eye surgery. Commonly used excimer
         molecules include F2 (emitting at 157 nm), ArF (193 nm), KrCl (222 nm),
         KrF (248 nm), XeCl (308 nm), and XeF (351 nm).
Solid-state lasers
         Solid state laser materials are commonly made by doping a crystalline
         solid host with ions that provide the required energy states. For example,
         the first working laser was made from ruby, or chromium-doped
         sapphire. Another common type is made from Neodymium-doped
         yttrium aluminium garnet (YAG), known as Nd:YAG. Nd:YAG lasers can
         produce high powers in the infrared spectrum at 1064 nm. They are
         used for cutting, welding and marking of metals and other materials,
         and also in spectroscopy and for pumping dye lasers. Nd:YAG lasers
         are also commonly frequency doubled to produce 532 nm when a
         visible (green) coherent source is required.
         Ytterbium, holmium, thulium and erbium are other common dopants in
         solid state lasers. Ytterbium is used in crystals such as Yb:YAG,
         Yb:KGW, Yb:KYW, Yb:SYS, Yb:BOYS, Yb:CaF2, typically operating
         around 1020-1050 nm. They are potentially very efficient and high
         powered due to a small quantum defect. Extremely high powers in
         ultrashort pulses can be achieved with Yb:YAG. Holmium-doped YAG
         crystals emit at 2097 nm and form an efficient laser operating at infrared
         wavelengths strongly absorbed by water-bearing tissues. The Ho-YAG
         is usually operated in a pulsed mode, and passed through optical fiber
         surgical devices to resurface joints, remove rot from teeth, vaporize
         cancers, and pulverize kidney and gall stones.
         Titanium-doped sapphire (Ti:sapphire) produces a highly tunable
         infrared laser, used for spectroscopy.
         Solid state lasers also include glass or optical fiber hosted lasers, for
         example, with erbium or ytterbium ions as the active species. These
         allow extremely long gain regions, and can support very high output
         powers because the fiber's high surface area to volume ratio allows
         efficient cooling, and its waveguiding properties reduce thermal
         distortion of the beam.
Semiconductor lasers
         Commercial laser diodes emit at wavelengths from 375 nm to 1800 nm,
         and wavelengths of over 3 μm have been demonstrated. Low power
         laser diodes are used in laser pointers, laser printers, and CD/DVD
         players. More powerful laser diodes are frequently used to optically
         pump other lasers with high efficiency. The highest power industrial
         laser diodes, with power up to 10 kW, are used in industry for cutting
         and welding. External-cavity semiconductor lasers have a
         semiconductor active medium in a larger cavity. These devices can
         generate high power outputs with good beam quality, wavelength-
         tunable narrow-linewidth radiation, or ultrashort laser pulses.
         Vertical cavity surface-emitting lasers (VCSELs) are semiconductor
         lasers whose emission direction is perpendicular to the surface of the
         wafer. VCSEL devices typically have a more circular output beam than
         conventional laser diodes, and potentially could be much cheaper to
         manufacture. As of 2005, only 850 nm VCSELs are widely available, with
         1300 nm VCSELs beginning to be commercialized [3], and 1550 nm
         devices an area of research. VECSELs are external-cavity VCSELs.
         Quantum cascade lasers are semiconductor lasers that have an active
         transition between energy sub-bands of an electron in a structure
         containing several quantum wells.
Dye lasers
         Dye lasers use an organic dye as the gain medium. The wide gain
         spectrum of available dyes allows these lasers to be highly tunable, or
         to produce very short-duration pulses (on the order of a few
         femtoseconds).

By output power

Note that the significance of these figures varies; they represent peak power
output. Many lasers are designed for a high peak output with an extremely short
pulse, and this is technically very different from the technology behind a steady
beam such as a communication, data, or cutting laser. Also note that usually,
output power is a small fraction of the input power needed to generate the laser.

            5 mW - laser in a CD-ROM drive
            5-10 mW - laser in a DVD player
            100 mW - laser in a CD-R drive
   250 mW - output power of Sony SLD253VL red laser diode, used in
    consumer 48-52 speed CD-R burner. link (PDF)
   1 W - output power of green laser in current Holographic Versatile
    Disc prototype development.
   100 to 500 Watt (peak output 1.5 kW) - typical sealed CO2 lasers
    used in industrial Beam Laser Machines (cutting lasers). These are
    usually compact, extremely reliable, inexpensive to run and can
    provide over 20,000 hours of cutting before requiring service. [4]
   As of 2005 the National Ignition Facility is working on a system that,
    when complete, will contain a 192-beam, 1.8-megajoule, 700-
    terawatt laser system adjoining a 10-meter-diameter target chamber.
    [5]
   As of 2006, manufacturers have expressed confidence that "no
    fundamental barriers stand in the way of squeezing 1 kW out of a
    single 1 cm diode laser bar" [6]
   1.25 PW - world's most powerful laser (claimed on 23 May 1996 by
    Lawrence Livermore Laboratory).

				
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