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 (R_λ) of ET-2030 at 532 nm (from graph in data sheet):  0.27 A/W
Responsivity (R_λ) 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. 

Noise Equivalent Power (NEP) Application Note