Optical Power Measurement

Tutorial

Optical Power Meters and Detectors

Introduction

This tutorial overviews the fundamentals of optical power meters and detectors. Newport offers three different types of calibrated detectors: photodiode detectors for low power measurements, thermopile detectors for high power measurements, and pyroelectric detectors for pulse energy measurements. Basic principles of each detector type are provided. An integrating sphere and a detector can be combined together to create a detection system with unique characteristics. Several applications of integrating sphere detectors are explained. Finally, optical power meter basics are described to help the reader understand how the sensing from a detector eventually translates into an accurate calibrated optical power or energy level measurement, along with an account of spectral calibration of a photodiode detector.

Photodiode Basics

When a photon hits the photodiode material, it may generate an electron-hole pair depending on the quantum efficiency of the device. Quantum efficiency is dependent on many factors, but in general if the energy of the photon, E = hv, is greater than the energy gap of the device, these photons will be absorbed very near the surface where the recombination rate is high and will contribute to the photocurrent. It is the quantum efficiency that is responsible for the wavelength dependency of the photodiode’s spectral response. Semiconductor materials such as silicon and InGaAs possess different energy gaps; consequently, they exhibit different quantum efficiencies at different wavelengths, resulting in spectral responsivity profiles unique to the specific material type.
Semiconductor photodiodes are ideal for making measurements of low-level light due to their high sensitivity and low noise characteristics. Most photodiode manufacturers specifically design their diodes to be used in either the photoconductive (reverse biased) or the photovoltaic (no bias) mode, both having advantages and disadvantages. Newport's Low-Power 818 (see Calibrated Photodiode Sensors) and 918D Series (see High Performance Photodiode Sensors) Photodiode detectors are used in the photovoltaic mode to take advantage of the reduced noise performance.
The two primary noise sources from the diode alone are Johnson Noise and shot noise. In the photovoltaic mode with no light striking the photodiode surface, the photodiode is in thermal equilibrium producing random thermal noise known as Johnson Current Noise, given by

Opt meter equa 1-S

where k is Boltzman’s Constant, T is the temperature in Kelvin, B is the bandwidth of the detector/amplifier, and Rsh is the shunt resistance of the photodiode. It can also be seen from this equation that a photodiode with a high shunt resistance is desired to reduce the Johnson Noise.
Shot noise is the noise produced by the flow of current in the diode and is given by,

Opt meter equa 2-S

where q is the charge of an electron, Idark is the dark current, and Iphoto is the photocurrent. When a photodiode is used in the photovoltaic mode the voltage across the diode is kept at zero volts. Consequently, this almost eliminates the dark current altogether. Thus, the shot noise contributed by the dark current is also eliminated. To put these effects in perspective, if a detector were biased as in the photoconductive mode, the dark current would be about three decades larger than the noise equivalent current of an unbiased detector.
The photocurrent produced by the photodiode is measured directly by the power meter using an operational amplifier circuit known as a transimpedance amplifier. Typically, measurements can be made down to the sub-picoampere regime with good reproducibility, even at room temperatures. An exception to this rule is when the shunt resistance of the photodiode is small as with the Germanium photodiode (818-IR and 918D-IR). Because of its low shunt resistance (50 kΩ typical), tens of picoamperes can be resolved at best.

Photodiode Spectral Calibration

All Newport optical detectors are recommended for a 12 month recalibration interval. Newport maintains an advanced calibration facility to meet all of your National Institute of Science and Technology (NIST) traceability needs. In case of the 818 and 918D series photodiode detectors, computer-automated testing allows us to include a complete calibration report with every detector and matching attenuator, if included in the product. In-house reference standards are directly recertified at every wavelength to the NIST. Our comprehensive statistical testing, tight control of the measurement environment, and direct traceability gives you the highest-accuracy calibrations with results you can trust at every 10 nm step. For more details about the photodiode detector calibration, see NIST Traceable Spectral Responsivity Calibration of Photodiode Detectors.

Thermopile Detector Basics

The basic laser high-power (>1 Watt) detector is essentially a thermopile. The more familiar application for thermopiles, in fact where the common name thermo electric cooler comes from, is when a voltage is applied to cool one side of the thermopile and whatever it is bonded to. However, thermopiles for laser power measurement are used in the opposite fashion. That is, a temperature difference is used to create a voltage. One side of the material is heated by the laser and the other side is a heat sink. The laser energy absorbed by that material is converted to heat. There is a temperature difference across the thermo electric device as the heat flows through it. This temperature difference causes the thermopile to generate a voltage. That voltage is proportional to the temperature difference, which is proportional to the laser power. The monitor measures this voltage to provide the laser power reading in Watts.

For more about thermopile technologies, see Thermopile Laser Power Sensor Technology Tutorial. Also, see How to Measure Accurately Using a Thermopile Detector for a step by step instruction.

Pyroelectric Detector Basics

Pyroelectric detectors are designed to measure the energy of short optical pulses that have a maximum width of 5 µs–400 µs, depending on the detector design. These detectors are made of a ferroelectric crystal that has a permanent dipole moment. When subjected to an optical pulse, the crystal is heated and causes the dipole moment to change. The changing of this dipole moment causes a current to flow, which is converted to a voltage in the detector head that can be measured by the optical power meter or oscilloscope.

Tutorial figure 1-SFigure 1—Typical signal behavior of a Pyroelectric detector is shown above.

As shown in Figure 1, the resultant thermal pulse is broadened relative to the short optical pulse. During this thermal pulse, the current flows through the ferroelectric crystal, creating a voltage that increases in amplitude. The optical power meter has circuitry that measures the difference in voltage between when the output voltage just starts to increase and when the output voltage reaches its peak amplitude. This voltage difference is then numerically multiplied by the detector responsivity, which is in units of Joule/Volt, resulting in the energy of the pulse in units of Joules.
When using pyroelectrics, care must be taken not to exceed the maximum pulse width or the maximum repetition rate. If either of these specifications is exceeded, your measurement accuracy will degrade due to the electrical bandwidth limitation of the detector.

Integrating Spheres

Newport’s general-purpose integrating spheres (see Integrating Spheres) can be used to make a variety of measurements. Optional sphere accessories (see Integrating Sphere Accessories) are also available to enhance their utility.

Beam Power

819-IS_Beam Power-SFigure 2—Beam Power

Measuring total collimated or uncollimated beam power (Figure 2), independent of polarization or beam alignment, is straightforward. The beam is admitted into the sphere and a detector, baffled from directly reflected radiation, that measures the spatially integrated beam power. Integrating spheres are ideal for measuring the output power of divergent beams from laser diodes, lensed LEDs and lensed lamps.

Transmittance

819-IS_Diffuse Transm-SFigure 3—Diffuse Transmittance

Transmittance (Figure 3) can be measured by using the integrating sphere to collect transmitted radiation from a sample held in one of the ports. The sample is irradiated, and then compared with a direct source measurement made outside the sphere. A baffle is used to shield the detector from non-integrated transmission, and a light trap can be used to remove the unscattered component. Measurements of total integrated scatter, fluorescence, bulk scatter and forward and back scatter can also be made.

Reflectance

819-ISSpec-Dif Re-SFigure 4—Specular + Diffuse Reflectance

To measure reflectance, a sample is held in one of the ports and irradiated by an incident beam. Total reflected radiation is spatially integrated by the sphere and measured by a baffled detector. Using the normal-incidence sample holder, which reflects the specular beam back out of the input port, can eliminate the specular component of the reflective radiation. An 8°-incidence sample holder allows measurement of the “specular plus diffuse” reflectance (Figure 4). The reflectance of a sample relative to a known standard can be calculated by measuring both and taking their ratio. The sample and standard should have a similar reflectance to avoid errors caused by sample reflectivity. A dual-beam system can be used to eliminate this potential source of measurement error.

Fiber Optic Power Output

819-ISFiberScaplePow-SFigure 5—Fiber Scalpel Power

An integrating sphere is also ideal for measuring the output of optical fibers. In particular, this approach avoids the sensitivity of thermopiles to air currents and provides reliable NIST-traceable calibration of high-power, air-cooled fiber scalpels for surgical or ophthalmic applications (Figure 5).

Laser Diode Power

819-ISLaserDiodePow-SFigure 6—Laser Diode Power

An integrating sphere and calibrated detector setup is suitable for accurate, absolute value light power measurement of laser diodes. Your measurements will be insensitive to problems associated with overfilling, or saturation, of the active area of the detector. A baffle, positioned between the input port and the detector port prevents the detector from directly viewing the emitting aperture of the laser or the direct area of illumination. In an integrating sphere, the detected flux is always a small fraction of the incident flux. This attenuation, caused by light reflecting many times before reaching the detector, makes the integrating sphere an ideal tool for measurement of output light power of high-power lasers (Figure 6).

Optical Power Meter Basics

Although most people want to make measurement in units of dBm or Watts, an optical power meter is only capable of measuring either the current or the voltage generated by a photodetector.
When interfacing with a photodiode, the quantity that must be measured is current. There are numerous techniques in measuring this current, but only one will yield the detectivity, signal-to-noise, and accuracy that is expected from a semiconductor photodiode. A circuit known as a transimpedance amplifier is the circuit of choice when using a photodiode (Figure 7).

Transimped Amp-SFigure 7—Transimpedance Amplifier

The advantage that the transimpedance amplifier has over almost any other amplifier configuration is that it does not bias the photodiode with a voltage as the current starts to flow from the photodiode. Typically, one lead of the photodiode is tied to the ground and the other lead is kept at virtual ground by means of the minus input of the transimpedance amplifier. The resultant bias across the photodiode is then kept at virtually zero volts, a condition that helps minimize dark current and noise, and helps increase linearity and detectivity.
Effectively the transimpedance amplifier causes the photocurrent to flow through the feedback resistor, which creates a voltage, V = iR, at the output of the amplifier. Since the meter knows the value of the precision feedback resistor, the current can be calculated with very good accuracy.
When interfacing with a thermopile or pyroelectric detector, voltage is the quantity that the optical meter must measure. There is, however, a considerable difference in how the measurement must be made between the two types of detectors. The optical meter’s circuitry must be designed and configured to accommodate the two different types of voltage sources.
Thermopile detectors produce very slow bandwidth voltages (≈1 Hz) that can be measured in the sub-millivolt levels. One of the main concerns when trying to resolve such low voltages is to compensate for, or eliminate, thermoelectric voltages caused by dissimilar metals, which are generated in the connections and printed circuit board. It is somewhat ironic that the desirable physical effect that generates the voltage in a thermopile detector is similar to the undesirable effects that are present in the connections and printed circuit board. Precautions must be taken when choosing the electrical components to help minimize the unwanted thermoelectric voltages. Additionally, to accurately resolve small voltages, the optical meter must be able to zero any offset voltage due to temperature drift of the components and the thermopile.
Pyroelectric detectors, in contrast, produce relatively fast rise-time signals in the microsecond regime (see the figure in the Pyroelectric Basics Section). The circuitry in the optical meter must sample-and-hold both the baseline voltage and the peak amplitude of the pulse. These two voltages are then put into a differential amplifier; and it is this voltage difference that determines the amount of energy in the optical pulse by way of the responsivity of the detector. Precautions must be taken to avoid accidental triggering of the sample-and-hold circuit since these circuits are sensitive to noise. Because the faster pyroelectric detectors have narrow upper peaks, it is crucial that the bandwidth of the circuit is fast enough to capture the level of the upper peak without degradation of amplitude accuracy.

Understanding Product Specifications

Calibration Uncertainty

This value indicates the percentage measurement uncertainty associated with a detector. For more details about how we obtain the calibration uncertainties of photodiode detectors, refer to the Photodiode Basics section.

Linearity

An optical signal absorbed by the detector is converted to an electrical signal, either current or voltage, and the optical power meter translated it into an optical power. A good detector has a good linearity relation between the optical input and the electrical output. Linearity specifications of certain detectors indicate the deviation from this linear relationship within the linear region.

Uniformity

Uniformity considers the spatial variations of the detector response. For photodiode detectors, a laser with the 1 mm diameter is scanned across a typical detector to calculate the value.

Estimating Total Measurement Error

Calculating the total measurement error entails finding all the possible sources of error, estimating how much error each source will have, and then adding everything up.

For the overall measurement uncertainty in case of a cw beam, refer to the product specifications of both the power meter and the detector, and add all the sources of errors. The sources of detector errors includes the calibration uncertainty (varies depending on the detector model used and the wavelength selected), nonlinearity (+/- 2 %) , spatial nonuniformity (+/- 0.5%). The major source of an error for the power meter is given by the accuracy specifications (+/- 0.2% for 1936-R/2936-R power meter).

Total error = √((source1)2+(source2)2+ (source3)2+⋯)

Low Power Measurement

In the low power measurement, estimating the signal to noise ratio (SNR) is critical. Measure the baseline noise level, by turning off the laser beam. Especially in the low power measurement, eliminate all possible ambient light (especially for photodiode detectors), heat sources (especially for thermopile detectors), and vibration sources (especially for pyroelectric detectors) that can affect the readings. Press zero to eliminate DC offset error. Note the final noise level. Then turn on the laser and make sure that you have enough high enough power level. Newport typically defines the minimum measurable power level as 20 times higher than the noise level, to ensure that the error due to the noise is no more than 5 %.