Noise Equivalent Power (NEP)
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.