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| {{Refimprove|date=September 2007}}
| | If that is possible then buy your multi-room cctv packages DVR from the operator for cash. For the last two connections, run an RJ45 relating to the router port and also the computer and another RJ45 through the router port for the DVR. Cctv dvr sata hard drives cctv system Inaccuracy - A CCTV camera could be very beneficial in attaining proof crime, nevertheless the technology isn't perfect.<br><br> |
| The '''active laser medium''' (also called '''gain medium''' or '''lasing medium''') is the source of optical [[gain]] within a [[laser]]. The gain results from the [[stimulated emission]] of electronic or molecular transitions to a lower energy state from a higher energy state
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| previously populated by a [[laser pumping|pump source]].
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|
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| Examples of active laser media include:
| | A conventional CCTV camera situated indoors by way of example would stand out terribly, whereas you could easily place an IP camera on the shelf camera dvr recorders and most people simply wouldn't notice it. If you might have a wired home security system, this enables the DVR to push recording if some of contacts are tripped.<br><br>Unfair deadlines also create chaotic situations throughout the implementation phase. Often writers forget that while not everything that they write needs to become "fun," they ought to be for your most part best cctv dvr card enjoyable. Cctv dvr d1 recording [http://www.inter.rs/goto/?cid=caffe&xrl=http://cctvdvrreviews.com cctv dvr brands in india] His bulging biceps originated from lifting and working with large bits of steel. And writing just isn't like manual labour: you imagine very hard as you're posting, you concentrate. |
| *Certain [[crystal]]s, typically doped with [[rare earth element|rare-earth]] [[ion]]s (e.g. [[neodymium]], [[ytterbium]], or [[erbium]]) or [[transition metal]] ions ([[titanium]] or [[chromium]]); most often [[yttrium aluminium garnet]] (YAG), [[yttrium orthovanadate]] (YVO<sub>4</sub>), or [[sapphire]] (Al<sub>2</sub>O<sub>3</sub>);<ref>Hecht, Jeff. ''The Laser Guidebook: Second Edition.'' McGraw-Hill, 1992. (Chapter 22)</ref>
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| *[[Glass]]es, e.g. silicate or phosphate glasses, doped with laser-active ions;<ref>Hecht, Chapter 22</ref>
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| *[[Gas]]es, e.g. mixtures of [[helium]] and [[neon]] (HeNe), [[nitrogen]], [[argon]], [[carbon monoxide]], [[carbon dioxide]], or metal vapors;<ref>Hecht, Chapters 7-15</ref>
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| *[[Semiconductor]]s, e.g. [[gallium arsenide]] (GaAs), [[indium gallium arsenide]] (InGaAs), or [[gallium nitride]] (GaN).<ref>Hecht, Chapters 18-21</ref>
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| * Liquids, in the form of dye solutions as used in [[dye lasers]].<ref>[[F. J. Duarte]] and L. W. Hillman (Eds.), ''Dye Laser Principles'' (Academic, New York, 1990).</ref><ref>[[F. P. Schäfer]] (Ed.), ''Dye Lasers'', 2nd Edition (Springer-Verlag, Berlin, 1990).</ref>
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| In order to lase, the active gain medium must be in a nonthermal energy distribution known as a [[population inversion]]. The preparation of this state requires an external energy source and is known as [[laser pumping]]. Pumping may be achieved with electrical currents (e.g. semiconductors, or gases via [[glow discharge|high-voltage discharges]]) or with light, generated by [[discharge lamp]]s or by other lasers ([[semiconductor laser]]s). More exotic gain media can be pumped by [[chemical reactions]], [[nuclear fission]], or with high-energy [[electron beam]]s.<ref name="rp">[http://www.rp-photonics.com/gain_media.html Encyclopedia of laser physics and technology]</ref>
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| ==Example of a model of gain medium==
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| [[Image:LaserLevels1.png|thumb|right|200px|Fig.1. Simplified scheme of levels a gain medium.]]
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| A universal model valid for all laser types does not exist.<ref name="siegman">
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| {{cite book
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| |url=http://www.uscibooks.com/siegman.htm
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| |author=A.E.Siegman
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| |title=Lasers
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| |year=1986
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| |publisher=University Science Books
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| |isbn= 0-935702-11-3
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| }}
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| </ref>
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| The simplest model includes two systems of sub-levels: upper and lower. Within each sub-level system, the fast transitions ensure that thermal equilibrium is reached quickly, leading to the [[Maxwell–Boltzmann statistics]] of excitations among sub-levels in each system ''(fig.1)''. The upper level is assumed to be [[metastable]].
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| Also, gain and refractive index are assumed independent of a particular way of excitation.
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| For good performance of the gain medium, the separation between sub-levels should be larger than working temperature; then, at pump frequency <math>~\omega_{\rm p}~</math>, the absorption dominates.
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| In the case of [[Amplifier|amplification]] of optical signals, the lasing frequency is called ''signal frequency.'' However, the same term is used even in the laser [[oscillators]], when amplified radiation is used to transfer energy rather than information. The model below seems to work well for most optically-pumped [[solid-state laser]]s.
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| ===Cross-sections===
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| The simple medium can be characterized with [[cross section (physics)|effective cross-sections]] of [[Absorption (electromagnetic radiation)|absorption]] and [[Emission (electromagnetic radiation)|emission]] at frequencies <math>~\omega_{\rm p}~</math> and <math>~\omega_{\rm s}~</math>.
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| *Let <math>~N~</math> be concentration of active centers in the solid-state lasers.
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| *Let <math>~N_1~</math> be concentration of active centers in the ground state.
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| *Let <math>~N_2~</math> be concentration of excited centers.
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| *Let <math>~N_1+N_2=N~</math>.
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| The relative concentrations can be defined as <math>~n_1=N_1/N~</math> and <math>~n_2=N_2/N~</math>.
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| The rate of transitions of an active center from ground state to the excited state can be expressed with <math>~ W_{\rm u}=
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| \frac{I_{\rm p}\sigma_{\rm ap}}{ \hbar \omega_{\rm p} }+\frac{I_{\rm s}\sigma_{\rm as}}{ \hbar \omega_{\rm s} } ~</math> and
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| The rate of transitions back to the ground state can be expressed with <math>~W_{\rm d}=\frac{ I_{\rm p} \sigma_{\rm ep}}{ \hbar \omega_{\rm p} }+\frac{I_{\rm s}\sigma_{\rm es}}{ \hbar \omega_{\rm s} } +\frac{1}{\tau}~</math>,
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| where <math>~\sigma_{\rm as} ~</math> and <math>~\sigma_{\rm ap} ~</math> are [[Absorption cross section|effective cross-sections]] of absorption at the frequencies of the signal and the pump.
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| <math>~\sigma_{\rm es} ~</math> and <math>~\sigma_{\rm ep} ~</math> are the same for stimulated emission;
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| <math>~\frac{1}{\tau}~</math> is rate of the spontaneous decay of the upper level.
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| Then, the kinetic equation for relative populations can be written as follows:
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| <math>~
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| \frac
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| {{\rm d}n_2}
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| {{\rm d}t}
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| =
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| W_{\rm u} n_1 -
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| W_{\rm d} n_2
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| ~</math>,
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| <math>~
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| \frac{{\rm d}n_1}{{\rm d}t}=-W_{\rm u} n_1 + W_{\rm d} n_2
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| ~</math>
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| However, these equations keep <math>~ n_1+n_2=1 ~</math>.
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| The absorption <math>~ A ~</math> at the pump frequency and the gain <math>~ G ~</math> at the signal frequency can be written
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| as follows:
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| <math>~ A = N_1\sigma_{\rm pa} -N_2\sigma_{\rm pe} ~</math>,
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| <math>~ G = N_2\sigma_{\rm se} -N_1\sigma_{\rm sa} ~</math>.
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| ===Steady-state solution===
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| In many cases the gain medium works in a continuous-wave or [[quasi-continuous function|quasi-continuous]] regime, causing the time [[derivative]]s of populations to be negligible.
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| The steady-state solution can be written:
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| <math>~ n_2=\frac{W_{\rm u}}{W_{\rm u}+W_{\rm d}} ~</math>,
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| <math>~ n_1=\frac{W_{\rm d}}{W_{\rm u}+W_{\rm d}}.</math>
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| The dynamic saturation intensities can be defined:
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| <math>~ I_{\rm po}=\frac{\hbar \omega_{\rm p}}{(\sigma_{\rm ap}+\sigma_{\rm ep})\tau} ~</math>,
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| <math>~ I_{\rm so}=\frac{\hbar \omega_{\rm s}}{(\sigma_{\rm as}+\sigma_{\rm es})\tau} ~</math>.
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| The absorption at strong signal:
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| <math>~ A_0=\frac{ND}{\sigma_{\rm as}+\sigma_{\rm es}}~</math>.
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| The gain at strong pump:
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| <math>~ G_0=\frac{ND}{\sigma_{\rm ap}+\sigma_{\rm ep}}~</math>,
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| where <math>~ D=
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| \sigma_{\rm pa}
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| \sigma_{\rm se}
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| -
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| \sigma_{\rm pe}
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| \sigma_{\rm sa}
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| ~</math>
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| is determinant of cross-section.
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| Gain never exceeds value <math>~G_0~</math>, and absorption never exceeds value <math>~A_0 U~</math>.
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| At given intensities <math>~I_{\rm p}~</math>, <math>~I_{\rm s}~</math> of pump and signal, the gain and absorption
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| can be expressed as follows:
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| <math>~A=A_0\frac{U+s}{1+p+s}~</math>,
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| <math>~G=G_0\frac{p-V}{1+p+s}~</math>,
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| where
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| <math>~p=I_{\rm p}/I_{\rm po}~</math>,
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| <math>~s=I_{\rm s}/I_{\rm so}~</math>,
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| <math>~U=\frac{(\sigma_{\rm as}+\sigma_{\rm es})\sigma_{\rm ap}}{D}~</math>,
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| <math>~V=\frac{(\sigma_{\rm ap}+\sigma_{\rm ep})\sigma_{\rm as}}{D}~</math> .
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| ===Identities===
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| The following identities<ref name="uns">{{cite journal
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| | author=D.Kouznetsov
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| | coauthors=J.F.Bisson, K.Takaichi, K.Ueda
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| | title=Single-mode solid-state laser with short wide unstable cavity
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| |url=http://josab.osa.org/abstract.cfm?id=84730
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| |journal=[[JOSAB]]|volume=22| issue=8| pages=1605–1619
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| | year=2005
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| | doi=10.1364/JOSAB.22.001605
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| | bibcode=2005JOSAB..22.1605K
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| }}</ref> take place:
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| <math>U-V=1 ~ </math>, <math>~ A/A_0 +G/G_0=1~.\ </math>
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| The state of gain medium can be characterized with a single parameter, such as population of the upper level, gain or absorption.
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| ===Efficiency of the gain medium===
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| The efficiency of a '''gain medium''' can be defined as
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| <math>~ E =\frac{I_{\rm s} G}{I_{\rm p}A}~</math>.
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| Within the same model, the efficiency can be expressed as follows:
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| <math>~E =\frac{\omega_{\rm s}}{\omega_{\rm p}} \frac{1-V/p}{1+U/s}~</math>.
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| For the efficient operation both intensities, pump and signal should exceed their saturation intensities;
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| <math>~\frac{p}{V}\gg 1~</math>, and <math>~\frac{s}{U}\gg 1~</math>.
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| The estimates above are valid for a medium uniformly filled with pump and signal light. The [[spatial hole burning]] may slightly reduce the efficiency because some regions are pumped well, but the pump is not efficiently withdrawn by the signal in the nodes of
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| the interference of counter-propagating waves.
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| ==See also==
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| *[[Population inversion]]
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| *[[Laser construction]]
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| *[[Laser science]]
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| *[[List of laser articles]]
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| *[[List of laser types]]
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| ==References and notes==
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| <references/>
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| ==External links==
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| *[http://www.rp-photonics.com/gain_media.html Gain media] Encyclopedia of Laser Physics and Technology
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| {{DEFAULTSORT:Active Laser Medium}}
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| [[Category:Laser gain media| ]]
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| [[Category:Laser science]]
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Unfair deadlines also create chaotic situations throughout the implementation phase. Often writers forget that while not everything that they write needs to become "fun," they ought to be for your most part best cctv dvr card enjoyable. Cctv dvr d1 recording cctv dvr brands in india His bulging biceps originated from lifting and working with large bits of steel. And writing just isn't like manual labour: you imagine very hard as you're posting, you concentrate.