Choosing a Laser Power Meter: Photodiode, Thermal or Pyroelectric Sensor

Scope

This article explains how to choose the sensor and meter for measuring a laser's power or pulse energy, and how to avoid the errors that come from choosing badly. It complements the optical power meter entry, which describes how the instruments work.

In short: use a photodiode sensor for powers below about tens of milliwatts and wherever speed or sensitivity matters, including fiber work; a thermal (thermopile) sensor for higher continuous or average power and for broadband or unknown wavelengths; and a pyroelectric sensor to measure the energy of individual pulses. Then check four things against the beam: wavelength coverage, power or energy range, aperture against beam size, and the sensor's damage limits.

The three sensor types

Photodiode sensors use silicon for roughly 400 to 1100 nm and InGaAs or germanium in the near infrared. They measure a photocurrent and convert it to power through the responsivity at the wavelength the meter is set to. Their sensitivity and speed make them the choice for low powers, fiber-coupled measurements and alignment, where a fast display matters. At high power they saturate, and many have a removable attenuator that extends the range at the cost of a separate calibration.

Thermopile sensors absorb the beam on a black coating and measure the temperature rise. The reading depends only on absorbed power, so it is nearly independent of wavelength, which suits multi-line, broadband and unknown sources. Large heads with water or fan cooling reach kilowatts. They respond in seconds, drift with the room temperature, and cannot resolve small powers.

Pyroelectric sensors produce a charge proportional to the change in temperature, so they respond to each pulse and give its energy rather than an average. They do not respond to continuous light. They suit pulsed lasers at repetition rates up to the sensor's specified maximum; above it, a thermal sensor reading average power is used and the energy is found by dividing by the repetition rate.

Wavelength

A photodiode meter's reading is correct only at the wavelength it is set to. Responsivity at constant quantum efficiency is proportional to wavelength, so a silicon sensor set to 633 nm but measuring a 532 nm laser reads low by the ratio 532/633: 16%, or 0.75 dB. Near the edges of a detector's range the responsivity changes faster and the errors are larger. Thermal and pyroelectric sensors avoid this, but their absorber coatings still have a calibrated range, and a coating that reflects part of a far-infrared or ultraviolet beam reads low.

Power and energy range

Pick a sensor whose range puts the expected reading well inside it, not at either end. Two conversions are worth keeping in mind:

(See decibels.) For pulsed lasers, the average power is the pulse energy times the repetition rate: 10 mJ pulses at 10 Hz carry 0.1 W on average, within the range of a small thermal head, while each pulse's peak power may be megawatts and decide the damage risk.

Aperture and beam size

The whole beam must fall on the active area. For a Gaussian beam of 1/e² radius $w$ centered on a circular aperture of radius $a$, the fraction collected is

$$1 - e^{-2a^2/w^2}.$$

An aperture equal to the beam radius collects 86.5%; one 1.5 times the beam radius, 98.9%; twice the beam radius, 99.97%. A sensor whose aperture is at least twice the 1/e² beam diameter therefore loses nothing measurable, while one matched to the 1/e² diameter reads 13.5% low. Divergent beams, such as those from bare laser diodes or cleaved fibers, need the sensor close to the source or an integrating sphere head that accepts light from a wide angle; fiber measurements use a fiber adapter that places the fiber end at the sensor.

Damage limits

Every sensor has a maximum power or energy and a maximum density: average irradiance in W/cm² for continuous beams, and fluence in J/cm² for pulses, quoted for a stated wavelength and pulse length. Compare the beam with both.

Worked examples. A 5 W beam 8 mm in diameter has an average irradiance of $5\ \text{W}/(\pi\,(0.4\ \text{cm})^2)$ = 9.9 W/cm². A 1 mJ, 10 ns pulse focused to 1 mm diameter has a fluence of 0.127 J/cm² and an average irradiance during the pulse of 1.3 × 10⁷ W/cm². The second beam carries far less energy but can damage a coating that the first would not. The usual remedy is to measure where the beam is larger, before the focus, or to use a sensor with a diffuser or volume absorber rated for pulses.

Checklist

1. Wavelength: within the sensor's calibrated range; set the meter's wavelength for photodiode sensors.
2. Power or energy: expected value well inside the range; average power for continuous and high-repetition-rate lasers, pulse energy for low-repetition-rate pulsed lasers.
3. Aperture: at least twice the 1/e² beam diameter, or an integrating sphere or fiber adapter for divergent light.
4. Damage: irradiance and fluence below the sensor's limits at the actual wavelength and pulse length.
5. Speed: a fast photodiode readout for alignment; patience with a thermal head, letting it settle and zeroing it with the beam blocked.
6. Calibration: traceable, current, and valid at the wavelengths used.

Common errors

Wrong wavelength setting on a photodiode meter, the most common error, typically several percent and more near the band edges.

Saturation of a photodiode sensor, which reads too low without warning on some meters; check by adding a known attenuator and seeing whether the reading falls by the expected amount.

Back-reflection from the sensor face into the laser, which can destabilize diode lasers; tilt the sensor slightly.

Ambient light and heat on sensitive heads: room light on a photodiode, a warm hand or air currents on a thermopile. Zero with the beam blocked, under the same conditions.

Polarization and angle: sensors are calibrated at normal incidence; strongly tilted beams reflect more from the window or coating.

References: International Organization for Standardization, ISO 11554:2017, Optics and photonics: Lasers and laser-related equipment: Test methods for laser beam power, energy and temporal characteristics; E. L. Dereniak and G. D. Boreman, Infrared Detectors and Systems (Wiley, 1996).