噪声等效功率(Noise Equivalent Power,NEP)
光电探测器灵敏度是一个方便的，甚至是必要的度量标准，通过它可以量化特定光电探测器的性能并与其他探测器进行比较。但是，定义和验证可能很困难。噪声等效功率 (NEP) 是量化光电探测器灵敏度或噪声源产生的功率的常用指标。但鉴于用于描述它的不同定义和计算方法，即使是这个通用指标也会引起混淆。 NEP 最常用的定义如下:在 1 Hz 输出带宽中导致信噪比 (S/R) 为 1 的输入信号功率。对于探测器，例如光电探测器，NEP 表示设备的灵敏度，单位为瓦特/赫兹的平方根 (W/√Hz)。
本质上，NEP 表示给定探测器每平方根带宽的最小可探测功率；换句话说，它是可以检测到的最弱光信号的量度。因此，希望 NEP 尽可能低，因为低 NEP 值对应于较低的本底噪声，因此检测器更灵敏。即使在更高的输入强度下，低 NEP 也是有益的，因为它会导致输出信号中的噪声特性更低。即使阻挡光检测器的光输入，也会产生一定量的输出噪声（例如热噪声或散粒噪声），从而导致特定平均输出噪声功率进入连接的负载。该噪声功率以及由此产生的噪声等效功率都取决于相关的测量带宽。该带宽通常归一化为 1 Hz，通常远低于探测带宽，以允许直接比较具有不同带宽规格的探测器。
NEP有多种定义公式，一般Observational Astrophysics书中的定义比较好理解一些，探测器光电流的起伏可以表示为，
是截至频率为fc的理想矩形传递函数，η是量子效率，探测器可以作为一个滤波器看待。δ(t)是狄拉克函数，对于入射功率为
，探测器的信噪比为，
然后将此计算应用于探测器同时从微弱光源和主要背景接收光子的情况。最小可探测信号 (S/N=1) 然后受到背景波动的限制，因此 iS = σB，对应的探测器的入射功率，
其中σB=σi=sqrt(2η*/n*fc)= sqrt(2η*PB/hν*fc)，PS以单位带通fc=1Hz为单位，单位为 W/Hz^1/2 ，它是来自探测器背景的噪声等效功率。
对于NEP的测量，可以参照下面的方法进行，
Electronic circuits output noise as well as the desired signal; an example would be the hiss heard in a stereo when no music is playing. The noise spectrum has a relatively flat response, and the noise level changes with the square root of the frequency range; for example, if the frequency range doubles, the noise component increases by √2 (1.414). The optical input has to be large enough to overcome the noise component of the detector in order to have a measurable signal output. Noise equivalent power (NEP) is a specification that allows a customer to determine the noise component of a detector for their particular wavelength and measurement bandwidth. NEP is the minimum optical power required for an output signal-to-noise ratio (SNR) of 1. This means the signal level and the noise level are the same; an SNR of 10 or larger is recommended. NEP has units of W/√Hz or W, depending on the whether or not the measurement bandwidth is included.
NEP is calculated from measurement or photodiode parameters such as dark current (the current that flows through a reverse biased photodiode when light is not present). Typical values and noise sources are:
Non-amplified standard detector: 0.1 pW/√Hz (dark current)
High speed detector: 30 pW/√Hz (noise from internal 50 Ω impedance matching resistor)
Amplified detector: 25 pW/√Hz (amplifier noise)
EOT’s data sheets provide NEP at a specific wavelength – this value is needed to determine NEP for other wavelengths. An example is shown below for the ET-2030:
Measurement Wavelength: 532 nm
Measurement Bandwidth: 800 MHz
Responsivity (Rλ) of ET-2030 at 532 nm (from graph in data sheet): 0.27 A/W
Responsivity (Rλ) of ET-2030 at 830 nm (from table in data sheet): 0.47 A/W
Datasheet NEP: 0.01 pW/√Hz
Including the bandwidth:
For an SNR of 10 the optical power would need to be 5.66 nW (-52.5 dBm). This value would be fine for a spectrum analyzer, but oscilloscopes typically require an output of 1 mV or higher for a minimum value. In practice, this is too low of a value: 10 mV to 20 mV would be an acceptable minimum reading. Using the ET-2030 as an example, and working backward from the oscilloscope output of 1 mV:
To calculate your NEP, enter the specifications below for your product.
文章转载请注明出处：http://www.lunmtech.com/newsinfo/1675321.html
