noise-equivalent power (original) (raw)
Acronym: NEP
Definition: the input power to a detector which produces the same signal output power as the internal noise of the device
Categories: 
light detection and characterization, 
fluctuations and noise
- properties of photodetectors
- responsivity
- gain
- spectral response of a photodetector
- quantum efficiency
- photon detection efficiency
- dark current
- noise-equivalent power
- detectivity
- bandwidth
- dynamic range
- linearity
- (more topics)
Related: photodetectorssignal-to-noise ratiodetectivityresponsivity
Units: W or W/Hz1/2
Formula symbol: NEP
Page views in 12 months: 3152
DOI: 10.61835/n3z Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
Content quality and neutrality are maintained according to our editorial policy.
📦 For purchasing photodetectors, use the RP Photonics Buyer's Guide — an expert-curated directory for finding all relevant suppliers, which also offers advanced purchasing assistance.
Contents
What is a Noise-equivalent Power?
When a photodetector does not get any input light, it nevertheless produces some noise output with a certain average power, which is proportional to the square of the r.m.s. voltage or current amplitude. The noise-equivalent power (NEP) of the device is the optical input power which produces an additional output power identical to that noise power for a given bandwidth (see below). If the input is interpreted as a signal, the output signal and noise powers are then identical, i.e., the signal-to-noise ratio would be 1.
The inverse of the noise-equivalent power is called the detectivity.
The possible signal-to-noise ratio of a measurement (for a 1-Hz bandwidth) can be estimated simply as the available input power divided by the noise-equivalent power. For that purpose, one does not need to know the detector's responsivity.
Note that the noise-equivalent power depend on the optical wavelength, since that influences the responsivity of the detector. The lowest NEP is achieved for those wavelengths where the responsivity is the highest.
Influence of the Bandwidth
The noise power and thus also the noise-equivalent power depends on the measurement bandwidth. (For white noise, it is proportional to that bandwidth.) At first glance, one may find it most natural to use the full detection bandwidth of the device. Then, however, the NEP would not allow a fair comparison of detectors with different bandwidth; it would be reduced if additional electronic filtering, reducing the detection bandwidth, would be applied. Therefore, it is common to assume a bandwidth of 1 Hz, which is usually far below the detection bandwidth.
Some authors specify the NEP in units of W / Hz1/2 rather than W (watts), as would be the usual units for a power. Effectively, they base the NEP on the square root of a power spectral density (PSD) rather than on a power. The numerical results are the same as when assuming a bandwidth of 1 Hz.
Measurement of Noise-equivalent Power
For experimentally obtaining the noise-equivalent power, one first needs to measure the noise amplitude of the instrument output in the given noise bandwidth (e.g. 1 Hz) without any optical input. That result has to be divided by the responsivity.
For example, consider a photodetector which in the dark produces a photocurrent (dark current or current generated by its electronics) with a noise amplitude of 1 nA / Hz1/2. If its responsivity is 0.5 A/W, and we consider a bandwidth of 1 Hz, the NEP is 1 nA / 0.5 A/W = 2 nW. For a larger bandwidth of 10 kHz, the noise amplitude would rise to 100 nA — not to 10,000 nA, as this scales with the square root of the bandwidth –, and the NEP would rise to 200 nW.
The square root dependence on the bandwidth may be surprising; it is related to the fact that the noise power scales linearly with the bandwidth, and is proportional to the square of the noise amplitude. In our example case, the noise power in a 1-Hz bandwidth generated by the detector current in a 50-Ω-resistor would be 50 Ω · (1 nA)2, and in a 10-kHz bandwidth it would be 10,000 times larger, i.e., 50 Ω · (100 nA)2.
Optimization
Obviously, a low noise-equivalent power is desirable because that power level is about the minimum input power level which can be detected easily when averaging the signal over a time of the order of one second. Using advanced methods such as lock-in detection, one can actually detect much weaker signals, provided that these have a bandwidth far below the detection bandwidth. In effect, one restricts the detection bandwidth to a value far below 1 Hz, which also reduces the noise power with which the signal has to compete. The required averaging time is correspondingly longer.
If the responsivity of a photodetector (e.g. a photodiode) can be increased without increasing the delivered noise power, the noise-equivalent power can be reduced. However, by using an avalanche photodiode, for example, where the responsivity can be greatly enhanced due to an internal amplification mechanism, one also obtains substantially more noise, so that the noise-equivalent power may even be increased.
Frequently Asked Questions
This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).
What is the noise-equivalent power (NEP) of a photodetector?
The noise-equivalent power (NEP) is the optical input power that generates an output signal power equal to the detector's intrinsic noise power. At this input power level, the signal-to-noise ratio is exactly 1.
Is a lower or higher NEP better for a photodetector?
A low NEP is desirable because it represents the approximate minimum optical power that can be easily detected. Therefore, photodetectors with a lower NEP are more sensitive and can measure weaker light signals.
How does the measurement bandwidth influence the NEP?
Since noise power is proportional to the bandwidth, the NEP, which is related to the noise amplitude, scales with the square root of the measurement bandwidth. For fair comparison between detectors, it is typically specified for a standard bandwidth of 1 Hz.
Why are the units for NEP sometimes given as W/Hz1/2?
Specifying the NEP in units of W/Hz1/2 is numerically equivalent to stating the value for a 1-Hz measurement bandwidth. This unit reflects that the NEP is based on a noise spectral density rather than a total noise power integrated over a wider band.
Suppliers
Sponsored content: The RP Photonics Buyer's Guide contains 124 suppliers for photodetectors. Among them:
âš™ hardware
The 710 Series are high sensitivity, low noise photodetector-amplifier modules that offer the flexibility of incorporating a variety of silicon or InGaAs, PIN or APD photodetectors. AMI also offers the capability of integrating other detectors on a custom basis.
âš™ hardware
Hamamatsu Photonics is a leading manufacturer of a very wide range of photodetectors, essential components in a vast array of modern scientific and commercial instruments and devices.
âš™ hardware
FEMTO offers a wide range of very low-noise photodetectors with bandwidths up to 2 GHz and gains up to 1012 V/W. Intensities ranging from fW to mW can be measured with an NEP down to 0.7 fW/√Hz. There is a choice of Si or InGaAs photodiodes covering a wavelength range from 190 nm to 1700 nm, with either a free space or fiber input. Customized models are available upon request.
âš™ hardware
Gentec Electro-Optics offers a great range of power detectors based on silicon or germanium photodiodes for powers up to 750 mW.
âš™ hardware
The ULN-PDB module is a plug-and-play ultralow noise balanced photodetector in a compact and user-friendly package. It offers the best performances in terms of signal-to-noise ratio. ULN-PDB is proposed with InGaAs or Si photodiodes (with FC connectors) and offers a bandwidth of 100 MHz (adjustable on demand) with a high gain of 39 kV/A (adjustable on demand) in a DC- or AC-coupled configuration.
âš™ hardware
Ultrafast photodetectors from ALPHALAS for measurement of optical waveforms with rise times starting from 10 ps and total spectral coverage from 170 to 2600 nm (VUV to IR) have bandwidths from DC up to 30 GHz. Configurations include free-space, fiber receptacle or SM-fiber-pigtailed options and have compact metal housings for noise immunity. The UV-extended versions of the Si photodiodes are the only commercial products that cover the spectral range from 170 to 1100 nm with a rise time < 50 ps. For maximum flexibility, most models are not internally terminated. A 50 Ohm external termination supports the highest speed operation, while a high impedance load generates large amplitude signals. Applications include pulse form and duration measurement, mode beating monitoring and heterodyne measurements. Balanced photodiodes complement the large selection of more than 70 unique models.
âš™ hardware
Menlo Systems offers a series of photodetectors for lowest light level signals. From avalanche to PIN photodiodes, you can find the detector that is best for your specific application.
âš™ hardware
CSRayzer CR2000AH-1550-70M includes a 200 μm InGaAs avalanche photodiode and a hybrid preamplifier for the use in high speed, ultra-low light detection, in laser range finding, LIDAR and free space communications.
Questions and Comments from Users
Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.
Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.
By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.