Reduksi radiasi, penguatan laser dan mode rongga
GRE Subject Test · GRE Physics Topik 44 5:20 Narasi bahasa Inggris · Subtitle bahasa Inggris + 中文 disematkan langsung
Bab-bab
Transkrip
A photon beam has no definite range, but a heavy charged particle does, and shielding calculations fail if the two are swapped.
光子束没有确定的射程，而重带电粒子有；如果把两者弄混，屏蔽计算就会出错。
This topic separates stopping from exponential attenuation, counts what a detector records, and explains population inversion and the modes of a laser cavity.
本主题区分阻止与指数衰减，计算探测器记录的计数，并解释粒子数反转和激光谐振腔的模式。
Heavy charged particles lose energy continuously through many small collisions, so they have an approximate range.
重带电粒子通过多次小碰撞连续损失能量，因此具有近似射程。
Under a constant-loss model, a particle with initial energy E0 and loss rate dE by dx stops after E0 divided by dE by dx.
在恒定损耗模型下，初始能量为 E0、损耗率为 dE/dx 的粒子在行进 E0 除以 dE/dx 的距离后停止。
Electrons straggle more and radiate.
电子的径迹更分散且会辐射光子。
Photons instead interact in single events, so a narrow beam attenuates exponentially as I equals I0 times e to the minus mu x, with no definite maximum depth.
光子则以单次相互作用为主，窄束强度按指数规律衰减，即 I = I0 e^(−μx)，没有确定的最大穿透深度。
The half-value layer is ln 2 divided by mu.
半值层为 ln 2 除以 μ。
Apply the exponential only to the stated narrow-beam geometry.
仅将指数公式应用于所指的窄束几何条件。
A linear attenuation coefficient of 0.2 per centimetre gives a half-value layer of 3.47 centimetres, and the ratio I over I zero equals e to the minus 2, which is approximately 0.135 after 10 centimetres.
线性衰减系数为 0.2 /cm 时，半值层为 3.47 cm，经过 10 cm 后，I/I₀ = e^(−2) ≈ 0.135。
Multiply by detector efficiency for recorded counts. With mu x equal to natural log of 4 and epsilon equal to 0.25, the recorded fraction is 0.25 times 1 over 4, which equals 1 over 16.
记录计数需乘以探测器效率：当 μx = ln4 且 ε = 0.25 时，记录到的分数为 0.25 × 1/4 = 1/16。
Build-up from scattered photons makes broad-beam shielding transmit more than the narrow-beam exponential. State the geometry.
散射光子引起的累积效应会使宽束屏蔽的透射率高于窄束指数模型；请明确说明几何条件。
Stimulated emission produces a photon matching the stimulating photon in frequency, direction and phase, giving coherent amplification.
受激辐射产生的光子与激发光子在频率、方向和相位上均一致，从而实现相干放大。 该过程与吸收相互竞争。
It competes with absorption; net gain needs population inversion N2/g2>N1/g1 (N2>N1 for equal degeneracies).
实现净增益需要粒子数反转，即N2除以g2大于N1除以g1；若简并度相等，则要求N2大于N1。
Positive-temperature equilibrium has N2/g2<N1/g1 by the Boltzmann factor, so a two-level system in equilibrium cannot lase continuously; practical lasers pump a third level or a metastable state.
根据玻尔兹曼因子，正温度平衡态下N2除以g2小于N1除以g2，因此处于平衡态的两能级系统无法实现连续激光振荡。 实际激光器通常泵浦第三能级或亚稳态。
A linear cavity of length L supports standing modes with L equals m lambda over 2, that is, frequencies nu sub m equals m c over 2 L, with spacing c over 2 L.
长度为L的线性腔体支持驻波模式，满足L等于mλ除以2，即频率ν_m等于mc除以2L，模式间隔为c除以2L。
For L equals 30 centimetres, the spacing is 500 megahertz.
当L等于30厘米时，间隔为500兆赫。
Gain must exceed the round-trip losses for oscillation.
增益必须超过往返损耗才能产生振荡。
Interferometers such as Michelson use the same coherence. Fringe counts track optical path changes, as in the gas-cell measurement.
迈克尔逊干涉仪等利用相同的相干性，条纹计数追踪光程变化，如气室测量所示。
A narrow beam falls as e to the minus mu x, so its half-value layer is ln 2 over mu.
窄束光的强度按 e 的负 μx 次方衰减，所以半值层等于 ln 2 除以 μ。
With mu equal to 0.2 per centimetre, that is 3.47 centimetres.
μ 为每厘米 0.2 时，半值层为 3.47 厘米。
Three half-value layers transmit one eighth, ignoring scattered build-up.
三个半值层透过八分之一，这里忽略了散射累积。
In a laser cavity the modes sit c over 2 L apart, and gain needs N 2 over g 2 to exceed N 1 over g 1.
在激光谐振腔中，相邻模式相隔 c/2L，而增益要求 N2/g2 大于 N1/g1。
A cavity of length L contains a gain medium over its entire length with intensity gain coefficient g, distributed intensity loss coefficient alpha, and mirror intensity reflectivities R1 and R2.
长度为L的谐振腔内充满增益介质，其强度增益系数为g，分布强度损耗系数为alpha，镜面强度反射率分别为R1和R2。
Derive the threshold gain.
推导阈值增益。
State the intensity amplitude convention.
说明强度与振幅的约定。
One round trip multiplies intensity by R sub 1, times R sub 2, times e to the power of 2 g minus alpha L.
一次往返使强度乘以R下标1，乘以R下标2，乘以e的2g减alphaL次方。
Threshold requires that factor equal one, so g sub th equals alpha plus natural log of 1 over R sub 1 R sub 2, all over 2 L.
阈值要求该因子等于1，因此g下标th等于alpha加上以e为底1除以R下标1R下标2的对数，再除以2L。
Above threshold, unsaturated gain must exceed this loss; steady lasing involves saturation.
超过阈值后，未饱和增益必须大于此损耗；稳态激光涉及饱和效应。
Award round trip, threshold condition, logarithmic result and convention or saturation.
评分点包括：往返、阈值条件、对数结果以及约定或饱和。
An amplitude coefficient convention would introduce a factor of two and must be declared.
若采用振幅系数约定，则会引入因子2，且必须予以声明。
Assigning photons a definite range or charged particles a single exponential law, and forgetting that equilibrium two-level populations cannot invert.
将光子赋予确定的射程，或带电粒子赋予单一指数定律，并忘记平衡态二能级布居无法实现反转。
Check which interaction model the beam species requires.
请检查光束种类所需的相互作用模型。
half-value layer: The material thickness that halves an exponentially attenuated beam, equal to ln2/μ.
半值层：使指数衰减光束强度减半的材料厚度，等于 ln2 除以 μ。
population inversion: A non-equilibrium state with more population in the upper laser level than the lower, enabling net stimulated emission.
粒子数反转：一种非平衡态，其中上能级的粒子数多于下能级，从而实现净受激辐射。
Retrieve the meanings without reading the definitions, then change a parameter in the original transfer and test the limiting case.
不阅读定义，凭记忆回顾各术语含义，然后改变原始传递函数中的一个参数，并检验极限情况。