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| '''Stimulated emission''' is the process by which an atomic electron (or an excited molecular state) interacting with an electromagnetic wave of a certain frequency may drop to a lower [[energy]] level, transferring its energy to that field. A new photon created in this manner has the same [[phase (waves)|phase]], [[frequency]], [[Polarization (waves)|polarization]], and [[Direction (geometry)|direction]] of travel as the photons of the incident wave. This is in contrast to [[spontaneous emission]] which occurs without regard to the ambient electromagnetic field.
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| However, the process is identical in form to atomic [[Absorption (electromagnetic radiation)|absorption]] in which the energy of an absorbed photon causes an identical but opposite atomic transition: from the lower level to a higher energy level. In normal media at thermal equilibrium, absorption exceeds stimulated emission because there are more electrons in the lower energy states than in the higher energy states. However, when a [[population inversion]] is present the rate of stimulated emission exceeds that of absorption, and a net [[optical amplification]] can be achieved. Such a [[gain medium]], along with an optical resonator, is at the heart of a [[laser]] or [[maser]].
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| Lacking a feedback mechanism, [[laser amplifier]]s and [[Amplified spontaneous emission|superluminescent]] sources also function on the basis of stimulated emission.
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| Stimulated emission was a theoretical discovery by [[Einstein]] <ref name=Einstein1916>{{cite journal|last=Einstein|first=A|title=Strahlungs-emission und -absorption nach der Quantentheorie|journal=Verhandlungen der Deutschen Physikalischen Gesellschaft|year=1916|volume=18|pages=318–323|bibcode = 1916DPhyG..18..318E }}</ref> within the framework of the [[old quantum theory]], wherein the emission is described in terms of photons that are the quanta of the EM field. Stimulated emission can also be described classically, however, without reference to either photons, or the quantum-mechanics of matter.<ref name="Fain & Milonni">{{Cite doi|10.1364/JOSAB.4.000078}}</ref>
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| ==Overview==
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| [[Electron]]s and how they interact with [[electromagnetic field]]s are important in our understanding of [[chemistry]] and [[physics]].
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| In the [[Classical electromagnetism|classical view]], the energy of an electron orbiting an atomic nucleus is larger for orbits further from the [[atomic nucleus|nucleus]] of an [[atom]]. However, quantum mechanical effects force electrons to take on discrete positions in [[Atomic orbital|orbitals]]. Thus, electrons are found in specific energy levels of an atom, two of which are shown below:
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| [[Image:Stimulated Emission.svg|center|550px]]
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| When an electron absorbs energy either from [[light]] (photons) or [[heat]] ([[phonon]]s), it receives that incident quanta of energy. But transitions are only allowed between discrete energy levels such as the two shown above.
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| This leads to [[emission line]]s and [[Spectral line|absorption line]]s.
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| When an electron is [[Excited state|excited]] from a lower to a higher energy level, it will not stay that way forever.
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| An electron in an excited state may decay to a lower energy state which is not occupied, according to a particular time constant characterizing that transition. When such an electron decays without external influence, emitting a photon, that is called "[[spontaneous emission]]". The phase associated with the photon that is emitted is random. A material with many atoms in such an excited state may thus result in [[radiation]] which is very spectrally limited (centered around one [[wavelength]] of light), but the individual photons would have no common phase relationship and would emanate in random directions. This is the mechanism of [[fluorescence]] and [[thermal emission]].
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| An external electromagnetic field at a frequency associated with a transition can affect the quantum mechanical state of the atom. As the electron in the atom makes a transition between two stationary states (neither of which shows a dipole field), it enters a transition state which does have a dipole field, and which acts like a small electric [[dipole]], and this dipole oscillates at a characteristic frequency. In response to the external electric field at this frequency, the probability of the atom entering this transition state is greatly increased. Thus, the rate of transitions between two stationary states is enhanced beyond that due to spontaneous emission. Such a transition to the higher state is called [[Absorption (electromagnetic radiation)|absorption]], and it destroys an incident photon (the photon's energy goes into powering the increased energy of the higher state). A transition from the higher to a lower energy state, however, produces an additional photon; this is the process of '''stimulated emission'''.
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| ==Mathematical model==
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| Stimulated emission can be modelled mathematically by considering an atom that may be in one of two electronic energy states, a lower level state (possibly the ground state) (1) and an ''excited state'' (2), with energies ''E''<sub>1</sub> and ''E''<sub>2</sub> respectively.
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| If the atom is in the excited state, it may decay into the lower state by the process of [[spontaneous emission]], releasing the difference in energies between the two states as a photon. The photon will have [[frequency]] ν and energy ''h''ν, given by:
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| :<math>E_2 - E_1 = h \, \nu_0</math>
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| where ''h'' is [[Planck constant|Planck's constant]].
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| Alternatively, if the excited-state atom is perturbed by an electric field of frequency <math>\nu_0</math>, it may emit an additional photon of the same frequency and in phase, thus augmenting the external field, leaving the atom in the lower energy state. This process is known as '''stimulated emission'''.
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| In a group of such atoms, if the number of atoms in the excited state is given by N<sub>2</sub>, the rate at which stimulated emission occurs is given by:
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| :<math>\frac{\partial N_2}{\partial t} =
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| -\frac{\partial N_1}{\partial t} =
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| - B_{21} \ \rho (\nu) N_2 </math>
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| where the [[proportionality constant]] ''B''<sub>21</sub> is known as the ''[[Einstein coefficients|Einstein B coefficient]]'' for that particular transition, and ρ(ν) is the radiation density of the incident field at frequency ν. The rate of emission is thus proportional to the number of atoms in the excited state N<sub>2</sub>, and to the density of incident photons.
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| At the same time, there will be a process of atomic absorption which ''removes'' energy from the field while raising electrons from the lower state to the upper state. Its rate is given by an essentially identical equation:
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| :<math>\frac{\partial N_2}{\partial t} =
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| -\frac{\partial N_1}{\partial t} =
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| B_{12} \ \rho (\nu) N_1 </math> .
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| The rate of absorption is thus proportional to the number of atoms in the lower state, N<sub>1</sub>. Einstein showed that the coefficient for this transition must be identical to that for stimulated emission:
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| :<math> B_{12} = B_{21}</math> .
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| Thus absorption and stimulated emission are reverse processes proceeding at somewhat different rates. Another way of viewing this is to look at the ''net'' stimulated emission or absorption viewing it as a single process. The net rate of transitions from E<sub>2</sub> to E<sub>1</sub> due to this combined process can be found by adding their respective rates, given above:
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| :<math>\frac{\partial N_1 \ (net)}{\partial t} =
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| - \frac{\partial N_2 \ (net)}{\partial t} =
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| B_{21} \ \rho (\nu) (N_2-N_1) =
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| B_{21} \ \rho (\nu) \ \Delta N </math>.
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| Thus a net power is released into the electric field equal to the photon energy ''h''ν times this net transition rate. In order for this to be a positive number, indicating net stimulated emission, there must be more atoms in the excited state than in the lower level: <math>\Delta N > 0 </math>. Otherwise there is net absorption and the power of the wave is reduced during passage through the medium. The special condition <math>N_2 > N_1 </math> is known as a [[population inversion]], a rather unusual condition that must be effected in the [[gain medium]] of a laser.
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| The notable characteristic of stimulated emission compared to everyday light sources (which depend on spontaneous emission) is that the emitted photons have the same frequency, phase, polarization, and direction of propagation as the incident photons. The photons involved are thus mutually [[coherence (physics)|coherent]]. When a population inversion (<math>\Delta N > 0 </math>) is present, therefore, [[optical amplification]] of incident radiation will take place.
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| Although energy generated by stimulated emission is always at the exact frequency of the field which has stimulated it, the above rate equation refers only to excitation at the particular optical frequency <math>\nu_0</math> corresponding to the energy of the transition. At frequencies offset from <math>\nu_0</math> the strength of stimulated (or spontaneous) emission will be decreased according to the so-called [[spectroscopic line shape|line shape]].
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| Considering only [[homogeneous broadening]] affecting an atomic or molecular resonance, the [[Atomic spectral line|spectral line shape function]] is described as a [[Cauchy distribution|Lorentzian distribution]]:
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| :<math> g'(\nu) = {1 \over \pi } { (\Gamma / 2) \over (\nu - \nu_0)^2 + (\Gamma /2 )^2 }</math>
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| where <math> \Gamma \, </math> is the [[full width at half maximum]] or FWHM bandwidth.
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| The peak value of the Lorentzian line shape occurs at the line center, <math> \nu = \nu_0</math>. A line shape function can be normalized so that its value at <math>\nu_0</math> is unity; in the case of a Lorentzian we obtain:
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| :<math>g(\nu) = { g'(\nu) \over g'(\nu_0) } = { (\Gamma / 2)^2 \over (\nu - \nu_0)^2 + (\Gamma /2 )^2 } </math>.
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| Thus stimulated emission at frequencies away from <math>\nu_0</math> is reduced by this factor. In practice there may also be broadening of the line shape due to [[doppler broadening|inhomogeneous broadening]], most notably due to the [[Doppler effect]] resulting from the distribution of velocities in a gas at a certain temperature. This has a [[Gaussian]] shape and reduces the peak strength of the line shape function. In a practical problem the full line shape function can be computed through a [[convolution]] of the individual line shape functions involved. Therefore optical amplification will add power to an incident optical field at frequency <math>\nu</math> at a rate given by:
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| :<math>P =h \nu \ g(\nu) B_{21} \ \rho (\nu) \ \Delta N </math>.
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| ==Stimulated emission cross section==
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| The stimulated emission cross section (in square meters) is
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| :<math>\sigma_{21}(\nu) = A_{21} { \lambda^2 \over 8 \pi n^2} g(\nu)</math>
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| where
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| :''A''<sub>21</sub> is the [[Einstein coefficients|Einstein ''A'' coefficient]] (in radians per second),
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| :λ is the wavelength (in meters),
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| :''n'' is the [[refractive index]] of the medium (dimensionless), and
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| :''g''(ν) is the spectral line shape function (in seconds).
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| ==Optical amplification==
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| Under certain conditions, stimulated emission can provide a physical mechanism for [[optical amplifier|optical amplification]]. An external source of energy stimulates atoms in the ground state to transition to the excited state, creating what is called a [[population inversion]]. When light of the appropriate frequency passes through the inverted medium, the photons stimulate the excited atoms to emit additional photons of the same frequency, phase, and direction, resulting in an amplification of the input [[irradiance|intensity]].
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| The population inversion, in units of atoms per cubic meter, is
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| :<math>\Delta N_{21} = \left( N_2 - {g_2 \over g_1} N_1 \right)</math>
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| where ''g''<sub>1</sub> and ''g''<sub>2</sub> are the [[degenerate energy level|degeneracies]] of energy levels 1 and 2, respectively.
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| ===Small signal gain equation===
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| The intensity (in [[watt]]s per square meter) of the stimulated emission is governed by the following differential equation:
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| :<math>{ dI \over dz} = \sigma_{21}(\nu) \cdot \Delta N_{21} \cdot I(z) </math>
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| as long as the intensity ''I''(''z'') is small enough so that it does not have a significant effect on the magnitude of the population inversion. Grouping the first two factors together, this equation simplifies as
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| :<math>{ dI \over dz} = \gamma_0(\nu) \cdot I(z) </math>
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| where
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| :<math> \gamma_0(\nu) = \sigma_{21}(\nu) \cdot \Delta N_{21} </math>
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| is the ''small-signal gain coefficient'' (in units of radians per meter). We can solve the differential equation using [[separation of variables]]:
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| :<math>{ dI \over I(z)} = \gamma_0(\nu) \cdot dz </math>
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| Integrating, we find:
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| :<math>\ln \left( {I(z) \over I_{in}} \right) = \gamma_0(\nu) \cdot z </math>
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| or
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| :<math> I(z) = I_{in}e^{\gamma_0(\nu) z} </math>
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| where
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| :<math> I_{in} = I(z=0) \, </math> is the optical intensity of the input signal (in watts per square meter).
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| ===Saturation intensity===
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| The saturation intensity ''I''<sub>S</sub> is defined as the input intensity at which the gain of the optical amplifier drops to exactly half of the small-signal gain. We can compute the saturation intensity as
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| :<math>I_S = {h \nu \over \sigma(\nu) \cdot \tau_S }</math>
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| where
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| :''h'' is [[Planck's constant]], and
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| :τ<sub>S</sub> is the saturation time constant, which depends {{Citation needed|date=November 2010}} on the spontaneous emission lifetimes of the various transitions between the energy levels related to the amplification.
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| :
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| :<math>\nu</math> is the frequency in Hz
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| ===General gain equation===
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| The general form of the gain equation, which applies regardless of the input intensity, derives from the general differential equation for the intensity ''I'' as a function of position ''z'' in the [[gain medium]]:
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| :<math>{ dI \over dz} = { \gamma_0(\nu) \over 1 + \bar{g}(\nu) { I(z) \over I_S } } \cdot I(z) </math>
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| where <math>I_S</math> is saturation intensity. To solve, we first rearrange the equation in order to separate the variables, intensity ''I'' and position ''z'':
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| :<math>{ dI \over I(z)} \left[ 1 + \bar{g}(\nu) { I(z) \over I_S } \right] = \gamma_0(\nu)\cdot dz </math>
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| Integrating both sides, we obtain
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| :<math>\ln \left( { I(z) \over I_{in} } \right) + \bar{g}(\nu) { I(z) - I_{in} \over I_S} = \gamma_0(\nu) \cdot z</math>
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| or
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| :<math>\ln \left( { I(z) \over I_{in} } \right) + \bar{g}(\nu) { I_{in} \over I_S } \left( { I(z) \over I_{in} } - 1 \right) = \gamma_0(\nu) \cdot z</math>
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| The gain ''G'' of the amplifier is defined as the optical intensity ''I'' at position ''z'' divided by the input intensity:
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| :<math>G = G(z) = { I(z) \over I_{in} } </math>
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| Substituting this definition into the prior equation, we find the '''general gain equation''':
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| :<math>\ln \left( G \right) + \bar{g}(\nu) { I_{in} \over I_S } \left( G - 1 \right) = \gamma_0(\nu) \cdot z</math>
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| ===Small signal approximation===
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| In the special case where the input signal is small compared to the saturation intensity, in other words,
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| :<math>I_{in} \ll I_S \, </math>
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| then the general gain equation gives the small signal gain as
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| :<math> \ln(G) = \ln(G_0) = \gamma_0(\nu) \cdot z</math>
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| or
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| :<math> G = G_0 = e^{\gamma_0(\nu) z}</math>
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| which is identical to the small signal gain equation (see above).
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| ===Large signal asymptotic behavior===
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| For large input signals, where
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| :<math>I_{in} \gg I_S \, </math>
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| the gain approaches unity
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| :<math>G \rightarrow 1 </math>
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| and the general gain equation approaches a linear [[asymptote]]:
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| :<math>I(z) = I_{in} + { \gamma_0(\nu) \cdot z \over \bar{g}(\nu) } I_S</math>
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| ==References==
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| {{Reflist}}
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| *{{cite book | title = Fundamentals of Photonics | author = Saleh, Bahaa E. A. and Teich, Malvin Carl | publisher = John Wiley & Sons | location = New York | year = 1991 | isbn= 0-471-83965-5 }}
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| *{{cite book | title = Atomic and Laser Spectroscopy | author = Alan Corney | publisher = Oxford Uni. Press | location = Oxford | year = 1977 | isbn= 0-19-921145-0}} ISBN 978-0-19-921145-6.
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| ==See also==
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| *[[Absorption (optics)|Absorption]]
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| *[[Active laser medium]]
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| *[[Laser]] (includes a [[Laser#History|history]] section)
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| *[[Laser science]]
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| *[[Rabi cycle]]
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| *[[Spontaneous emission]]
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| {{DEFAULTSORT:Stimulated Emission}}
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| [[Category:Electromagnetic radiation]]
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| [[Category:Concepts in physics]]
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| [[Category:Laser science]]
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