Laser radiation measurements
Gigahertz-Optik GmbH produces instruments for measuring optical radiation from the lasers and laser diodes that are widely used in measurement, analytical and telecommunication equipment as well as in sensor technologies. The product range includes instruments for measuring continuous, modulated and pulsed radiation.
Example laser radiation measurements from different application fields are presented on this page.
Laser diodes used in distance measuring devices have a diverging elliptical beam profile with typical peak powers of up to 100 W. They are operated in a short pulse width, low pulse frequency mode with low average power. For quality assurance, the peak power and the pulse waveform are of primary interest. These two optical parameters cannot be measured with a single detector. Therefore, pulse energy and pulse waveform must each be measured separately.
The pulse waveform is measured using fast, small-area photodiodes terminated with a low-impedance shunt resistor. The temporal voltage curve across the resistor is measured and recorded with a digital oscilloscope. Rise times of less than a nanosecond are possible. The responsivity of this type of detector is very low because of the low-resistance circuitry. Additionally, the small area of the photodiode can only detect a portion of the entire laser beam profile. The photodiode and laser beam must be carefully aligned due to non-uniformity in the laser beam profile. This method is entirely suitable for the measurement of the relative pulse waveform, which is needed together with the pulse energy to calculate the peak power.
Pyroelectric detectors are generally used to measure the energy of pulsed lasers with high-powers up into the multi-digit KW range. However, these are less suitable for the low peak power and short pulse lengths of semiconductor lasers. Therefore, photodiodes are used to measure the energy of pulsed laser diodes. To increase the light-sensitive measuring area and to compensate for the beam divergence, the photodiodes are combined with a compact integrating sphere  which also provides the required signal attenuation. The response time of an integrating sphere-based detector is too slow to directly measure the pulse shape of short laser pulses. By contrast, the pulse energy of individual pulses or pulse chains can be measured very precisely with suitable detection electronics.
Since pulse energy is defined as the integral of the measurement signal over the measurement time it therefore equates to the area of the pulse.
X: Pulse Energy
x(t): Measurement signal as a function of time
T: Pulse duration
Relatively simple techniques enable the time constant of a transimpedance amplifier to be stretched by a multiple of the expected pulse length. The resulting ‘stretched pulse’ becomes flatter, but remains constant in its area.
Figure 1: Conversion of short pulses into stretched pulses with the same energy (area)
A1: original signal input
A2: signal output transimpedance amplifier
Area A1 = area A2
The pulse waveform of the stretched pulse can be recorded in a time-resolved manner using a data logger with sufficiently high sampling rate. The pulse energy is thereby calculated from the pulse waveform.
Gigahertz-Optik GmbH produces a range of Optometers suitable for measuring the pulse energy of laser diodes using the pulse stretching method. Models P-9710-2, P-9710-4, P-2000-2 and P-9801 have signal amplifiers with 20ms time constants in all gain ranges. Therefore, regardless of actual pulse length, pulses are converted to a pulse length of 20ms or a multiple of. The waveform of the stretched pulse is recorded by means of an analog-to-digital converter with a sampling time of 100 μs. Due to this high readout rate, electronic measurement uncertainty of less than +/- 1% is possible with careful adjustment of electronic offsets.
These Optometers enable the measurement of both single pulses and pulse sequences. The measurement period can be selected from 20 ms to 200 s during which time all the detected optical radiation is accumulated. If the number of pulses within the measurement window is known, e.g. based on the electronic drive circuity, then the energy of a single pulse can be calculated. If the period of a single pulse is known then its peak power can be calculated by assuming either a rectangular or triangular pulse profile. A more precise calculation of the peak power is possible if the actual pulse waveform was measured.
Photodiodes provide excellent linearity over their full dynamic range from 0.1 pA to 1 mA when operated in short-circuit mode. In order to be able to use this wide dynamic range, the transimpedance amplifiers within the Optometers are each equipped with eight decades of amplifier gain. This enables the pulse energy measurement of laser diodes with peak powers in the range of 1mW to 100W when used in conjunction with the integrating sphere detector ISD-5P-Si-SMA-V1 for example. These instruments are calibrated in terms of radiant power (in Watts) within Gigahertz-Optik GmbH’s accredited calibration laboratory for optical radiation measurements. Pulse energy is measured in units of Joules (or Watts x seconds) and is therefore determined with respect to the selected measuring time.
UV laser scanning confocal microscopes provide sample illumination from a UV laser via a microscope objective which results in a highly convergent beam. Therefore, in order to measure the total radiated power in the beam a detector with a large acceptance angle is required.
The photodiodes routinely used for radiometric measurements have a two-dimensional sensor surface. At shallow angles a portion of the incident radiation is reflected from the surface thereby reducing the measured radiation power. This can result in a significant measurement error when measuring the highly convergent beam of a confocal microscope. By appropriately combining the photodiode with a compact integrating sphere this source of error can be minimized. This design of detector offers a large acceptance angle and can also withstand higher laser power levels. Due to the size and shape of an integrating sphere detector it must be possible to rotate the microscope’s sample table to enable positioning of the detector.
Microscope objective with highly convergent beam coupled into integrating sphere.
The compact ISD-5P-SiUV integrating sphere detector combines a 50mm diameter integrating sphere with a UV-enhanced Si photodiode. Gigahertz-Optik’s Calibration Laboratory for optical radiation quantities provides calibration of spectral radiant power sensitivity (in W) over the 250 to 1100nm spectral range. Therefore, it is suitable for measuring a wide range of UV, visible and NIR lasers.Read less
The measurement of laser power is not always possible with off-the-shelf instruments due to the design and specifications of the laser. For such cases, the ability to configure systems from modular components is a good approach.
In the following example, a laser power meter was required to measure the intensity of a combined laser rangefinder and laser pointer system. For the laser rangefinder the energy of single pulses and pulse sequences (at 1550nm) need to be measured and for the laser pointer the average power is specified (at 830nm). The maximum beam diameter is 45 mm. The system must be traceable to a National Metrology Institute in order to obtain the certificate of conformity for Class 1 according to EN 60825-1 classification.
Picture: Schematic representation of the measuring device
A convenient solution was configured from Gigahertz-Optik’s portfolio of optical radiation measurement modules. A 300mm diameter integrating sphere with 50mm entrance port was used due to the large beam diameter to be measured. To cover the required spectral range, the sphere was configured with both a Si photodiode and an InGaAs photodiode. An internal baffle prevented any direct irradiation of the photodiodes from the incident beam as well as indirect irradiation by the first reflection from the back face of the sphere. The large sphere diameter and the design of the two detectors with diffuse windows ensure the best possible level of optical integration. A dual-channel Optometer was used to measure the average laser power or the laser energy of either a single pulse or pulse train. The laser energy was measured according to the pulse-stretching method.
The integrating sphere was configured with components from the UM-Series of modular integrating spheres from Gigahertz-Optik GmbH. The two photodiode detector heads were standard Gigahertz-Optik GmbH devices from the PD-11 Series connected to the two-channel P-2000 Optometer. The calibration of the spectral sensitivity from 400 nm to 1800 nm takes place in the calibration laboratory for optical radiation measurements of Gigahertz-Optik GmbH. The calibration laboratory was also responsable for the conformity test of the measurement device by the use of pulse frequency and pulse width modulated lasers.Read less