Calculator
Example data table
Example photodiode measurements showing how responsivity changes with wavelength and power.
| Wavelength (nm) | Optical Power (mW) | Photocurrent (µA) | Responsivity (A/W) |
|---|---|---|---|
| 650 | 0.50 | 95 | 0.190 |
| 850 | 0.50 | 120 | 0.240 |
| 1310 | 0.50 | 155 | 0.310 |
| 1550 | 0.50 | 165 | 0.330 |
Formula used
- Current responsivity: R = Iph / P (A/W)
- Voltage responsivity: Rv = Vout / P (V/W)
- Quantum efficiency link: R = η q λ / (h c), and η = R h c / (q λ)
- Photon energy: E = h c / λ
- Photon flux: Φ = P / E (photons/s)
- Shot noise: in = √(2 q I B) (A rms), where I = Iph + Id
- Thermal noise: vn = √(4 k T R B) (V rms) across a load resistance
How to use this calculator
- Select a calculation mode matching your measurement setup.
- Enter known values and choose proper units for each field.
- Provide wavelength when you want quantum efficiency and photon metrics.
- Optionally add dark current, bandwidth, temperature, and load resistance for noise estimates.
- Press Calculate to show results above the form.
- Use Download CSV or Download PDF to export results.
Professional guide
1) Why responsivity matters
Detector responsivity links optical input to an electrical output, letting you compare sensors across wavelengths and operating conditions. For photodiodes, it is commonly expressed in A/W, while amplified detectors may be specified in V/W. A higher value generally means more signal for the same optical power.
2) Typical ranges and benchmarks
Silicon photodiodes often provide useful response from about 400–1100 nm, with responsivity near 0.2 A/W at 650 nm and around 0.5–0.6 A/W close to 900 nm. InGaAs devices are common for 900–1700 nm and may reach roughly 0.8–1.0 A/W near 1550 nm, depending on design and temperature.
3) Connecting responsivity to quantum efficiency
Responsivity is tied to quantum efficiency (QE), the fraction of photons converted into collected charge carriers. The relationship uses wavelength: longer wavelengths give more A/W for the same QE because each photon has lower energy. For example, a detector with 60% QE at 850 nm yields roughly 0.41 A/W, while the same QE at 1550 nm yields roughly 0.75 A/W.
4) Measurement workflow and calibration
A solid measurement starts with a calibrated optical power reference, stable coupling, and consistent spot size on the active area. Record optical power and photocurrent at each wavelength point, subtract dark current when relevant, and keep polarization and temperature stable. If you use a transimpedance amplifier, include its gain uncertainty in the final budget.
5) Bandwidth, noise, and limits
Responsivity alone does not guarantee sensitivity at high speed. Shot noise scales with the square root of current and bandwidth, while thermal noise depends on temperature, resistance, and bandwidth. If you enter bandwidth and dark current, this calculator estimates shot noise and a simple current-domain SNR. These estimates help you judge whether the measurement is detector-limited or electronics-limited.
6) Using V/W and converting outputs
When the detector provides a voltage output, V/W is convenient because it absorbs gain factors. If you measure current but want voltage, use a known transimpedance gain (V/A) or a load resistance (Ω) to compute V = I·G or V = I·R. This is useful for quick interface checks with ADC full-scale ranges.
7) Photon metrics for optical link budgets
Photon energy and photon flux translate watts into photons per second, which is useful in link-budget and quantum-limited analyses. At 1550 nm, 1 mW corresponds to about 7.8×1015 photons/s. Combining photon flux with QE provides an intuitive expectation for charge generation before any amplifier stage.
8) Common pitfalls and best practices
Watch for unit mistakes (mW vs W, µA vs A), detector saturation at high power, and wavelength drift. Ensure the beam is fully captured by the active area and avoid reflections that change the true delivered power. Re-check dark current after thermal settling, and use repeated readings to quantify stability and repeatability.
FAQs
1) What is detector responsivity in simple terms?
Responsivity is the electrical output per unit optical power. For photodiodes it is commonly A/W (photocurrent per watt). For amplified detectors it may be V/W (voltage per watt).
2) Why does responsivity depend on wavelength?
Photon energy changes with wavelength. For the same quantum efficiency, longer wavelengths produce higher A/W because each photon carries less energy, so more photons arrive per watt.
3) Should I subtract dark current before calculating R?
Yes when dark current is not negligible compared with the photocurrent. Subtracting it improves accuracy, especially at low optical power and when temperature changes during measurements.
4) How do I choose between A/W and V/W?
Use A/W when you directly measure detector current. Use V/W when the detector output is voltage or when gain is fixed and included. You can convert between them using the known transimpedance gain (V/A).
5) What bandwidth should I enter for noise estimates?
Enter your measurement bandwidth in hertz, such as the low‑pass filter cutoff or equivalent noise bandwidth. Larger bandwidth increases both shot and thermal noise, reducing SNR.
6) What if my calculated quantum efficiency is above 100%?
That usually indicates a unit mistake, wavelength mismatch, or internal gain (for example avalanche multiplication). Recheck power calibration, wavelength units, and whether your device includes gain.
7) Can this calculator handle pulsed sources?
Yes if you use average optical power and average photocurrent for steady readout. For fast pulses, ensure your bandwidth matches the detection method and interpret noise estimates as approximations.