Pulsed laser diodes and pulsed LEDs used in range finders, environmental scanners and image capture emit pulses with a few nanoseconds length of very high peak power. To measure the temporally resolved pulse shape fast detectors are required. These are usually small-area photodiodes with diameters of sometimes significantly less than 1 mm. The small detector area of the photodiodes results in metrological limitations:
- The extent of the laser spot is larger than the active area of the photodiode and thus does not allow measurement of the radiant power (W).
- The position of the photodiode in the laser spot is critical because of possible modes (inhomogeneous laser spot).
- Very small photodiodes cannot be calibrated absolutely.
- Attachment optics used to focus the laser spot on the photodiode surface cannot be calibrated.
- The electronic wiring of the photodiodes required for short pulse lengths further limits the calibration capability.
With the ISD-1.6-SP-series of detectors in combination with the P-9710-2 and P-9710-4 optometers, Gigahertz-Optik provides a way to determine the
absolute peak performance of pulsed lasers and pulsed LEDs.
Function and structure:
The detector incorporates two photodiodes within a compact integrating sphere assembly. The first photodiode has a short rise time and hence, in conjunction with a sufficiently fast optional oscilloscope, allows the measurement of the relative time resolved pulse shape(pulse length, half-width, peak power). The second photodiode measures the absolute pulse energy (in joules) of a single pulse or pulse train. The evaluation is carried out by an optometer of the P-9710-series according to the pulse-stretching method. The absolute peak power can be calculated from the pulse energy and the relative pulse shape. Thus, the short duration light signal can be completely characterized.
The integrating sphere with a diameter of 16 mm offers a measuring aperture of 5 mm, alternatively 7, mm diameter and can be calibrated to measure the absolute radiant power (W). Because of the very small diameter of the integrating sphere, the temporal pulse deformations (pulse-stretching-effect of integrating spheres) are small compared to integrating spheres with larger diameters. As a result, pulses of a few nanoseconds pulse length are hardly deformed and can be measured in a time-resolved manner. The sphere itself, the photodiodes and the electrical circuit are housed in a CNC-machined aluminium housing.
The optional oscilloscope is connected via a BNC connector. The optometer is connected via a 2 m cable with a multi-pin connector in which the calibration data are stored.
The integrating sphere also offers two SMA-fiber connectors. For example, a spectrometer for measuring the wavelength and an auxiliary lamp for compensating for possible influences of the back reflection through the sample at the measurment port(self-absorption correction) can be connected.
Because of its small diameter, the sphere factor of the integrating sphere is relatively small. As a result, the permissible beam divergence in the version with a 7 mm measuring opening is additionally limited compared to the 5 mm version..
Gigahertz-Optik offers various optometers with the required "pulse energy" measurement function for measuring the pulse energy of short pulse
P-9710-2: Single-channel optometer with manual triggering of the measurement
P-9710-4: Single-channel optometer with TTL trigger input for triggering the measurement
P-2000-2: two-channel optometer
P-9801-V02: Eight-channel optometer
To evaluate the time resolved pulse shape, the user must provide a sufficiently fast oscilloscope.
The factory calibration of the spectral sensitivity of the pulse Energy detector is performed by the calibration laboratory for optical Radiation measurements at Gigahertz-Optik. The principle of the pulse-stretch-method allows the calibration of the detector in CW operation. The CW-calibration is fully traceable.
The application areas of the detector can be found, for example, in the development and quality assurance (on- and in-line) of pulsed laser diodes and pulsed LEDs as well as in the end application of such light sources.
Figure 1: ISD-1.6-SP-V02 detector with single channel optometer P-9710-2
Figure 2: System illustration, oscilloscope, P-9710-2, ISD-1.6-SP-V02
Figure 2: Schematic representation (1: integrating sphere 2: measuring aperture 3: spherical surface of the first reflection 4: pulse energy photodiode 5: pulse progression photodiode 6: 2 x SMA connectors 7: cable for optometer 8: female BNC connector oscilloscope 9: bias voltage)
Figure 3: Schematic measuring arrangement (1: ISD-1.6-SP-Vxx 2: oscilloscope 3: P-9710-4 4: bias voltage 5: TTL signal trigger Input
Picture 4: Typical spectral sensitivity
Detector for the measurement of the temporal intensity course and the radiant power of pulse lasers and pulse LEDs. In conjunction with the optometers P-9710-2 and P-9710-4 and a fast oscilloscope, the absolute peak performance of pulse lasers and pulse LEDs can be determined.
Compact measuring head with 16 mm diameter integrating sphere. Si photodiodes for radiant power and temporal intensity curve for pulse lengths in the ns range.
Peak power up to typ. 200 W. Spectral sensitivity range 400 to 1100 nm.
The application areas of the detector can be found, for example, in the development and quality assurance (on- and in-line) of pulse laser diodes and pulse LEDs. In addition, in measurement tasks in the context of the application of the aforementioned pulse laser diodes and pulse LEDs.
With the 7 mm measuring aperture, the detector is also suitable for measuring tasks in the field of laser safety (ISD-1.6-SP-V02 with 7 mm aperture for the detection of eye safety).
Factory calibration of the spectral sensitivity of the photodiode for radiant power. Traceable to PTB calibration standards
|Spectral radiant flux||
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typical pulse measurement:
V01: 5 mm
V02: 7 mm
|max. Radiant Power (Peak)||
V01: typically 200 W (@ 950 nm)
V02: typically 300 W (@ 950 nm)
Application: (10 to 30) ° C
Storage: (-10 to 50) ° C
The device must not be exposed to high humidity. Range 20% ~ 70% RH not condensing.
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