Power Meters for CW Lasers
- Choice of detector type: Thermopile, Si, Ge, GaInAs
- Measures high PRF lasers and pure CW lasers
- Can measure diverging light from end of fibre
- ±1.6% overall absolute accuracy (traceable to UK NPL)
- Rapid response time: 0.1 - 0.3 secs
- Battery powered for low noise and portability
Most systems for laser power measurement up to about 100W CW have almost total absorption of the beam on a relatively massive target. This approach can give some or all of the following problems. (1) The response is usually inconveniently slow. (2) The sensitivity may vary across the target. (3) The sensitivity may drop at high powers due to non-linear cooling. (4) At the highest powers there may be damage to the target.
By using an Integrating Sphere , our laser power meters avoid these problems. The sphere is coated with a robust high reflectivity diffuser which spreads the absorption evenly over the whole sphere while the power on the detector is reduced to a level well within the linear range. The geometry of the integrating sphere is chosen to give a signal which is essentially independent of beam position, beam size and beam angle. This allows the measurement of the diverging output from an optical fibre (up to a 60 degree cone or +/- 30 degrees on either side of the axis).
The standard detector is a Thin Film Thermopile . This has an almost flat spectral response and yet the response time is still 0.2 sec. For the very lowest powers we offer spheres with a photo-diode (Si, Ge, GaInAs); a thumb-wheel switch then gives the wavelength correction appropriate to each type of diode. At request, the photo-diode systems can have a BNC socket added which will show the pulse shape of a repetitively pulsing laser.
The electronics is designed to measure the mean power of a stream of short pulses. There is a BNC output socket for a chart recorder or DVM.
| Model Name | Detector Type | Spectral Range | Power Ranges | N.E.P. | Aperture | Head Size L × D |
|---|---|---|---|---|---|---|
| Model 55 | Thermopile | 0.4 - 1.5µm | 1mW - 1W | 15µW | 18mm | 74mm × 80mm |
| Model 69 | Thermopile | 0.4 - 1.5µm | 10mW - 10W | 150µW | 25mm | 99mm × 92mm |
| Model 68 | Thermopile | 0.4 - 1.5µm | 0.1W - 100W | 1.5mW | 35mm | 114mm × 112mm |
| M29(Si) | Si Diode | 0.35 - 1µm | Lowest 10µW - 3mW | 200nW | 18mm | 65mm × 74mm |
| Highest 3mW - 1W | 30µW | |||||
| M29(Ge) | Ge Diode | 0.9 - 1.6µm | Lowest 30µW - 3mW | 600nW | 18mm | 65mm × 74mm |
| Highest 3mW - 1W | 100µW | |||||
| M29(GaAs) | GaInAs | 0.9 - 1.7µm | Lowest 10µW - 3mW | 600nW | 18mm | 65mm × 74mm |
| Highest 3mW - 1W | 100µW |
Power Ranges
For the photo diode systems there are 6 positions for the range switch. These ranges can be put anywhere between the sets given as ‘lowest’ and ‘highest’. For example 100µW 300µW, 1 mW, 3mW, 10mW, 30mW.
Other power ranges may be available; please enquire.
Noise/Bandwidth
For the Thermopile systems the NEP is given with the bandwidth switch in the normal position This gives 10-90% rise time of 80ms at the output BNC. (settling to 0.5% in 0.2 secs). The bandwidth switch gives a further factor of 2 reduction in NEP. The 10-90% response time is then 400ms (0.5% settling in 1.0sec).
Max. Power Density
There is a risk of damage to the diffuser if the power density exceeds 20W/cm2. This limit needs to be taken seriously. For example, with M68 receiving 100W, even a really smooth Gaussian beam must have a FWHM of at least 20mm.
Calibration Accuracy
The absolute accuracy of our own Calibration is normally better than ±1.6%. This is traceable to the UK National Physical Laboratory. The Calibration Certificate will also give details of the change expected at the limits of the spectral range.
For Photo-diode systems a digital switch has to be set to the appropriate wavelength to get the correct reading. The sensitivity then remains within ±5% as measurements are made over the full spectral range.
Analogue Display Unit
- Size: 16cm × 9.6cm × 5cm high.
- Weight: 550gms
- Cable to Head: 1.5m screened cable
- Batteries: either 2x 9V alkaline PP3 (life 60-100 hrs) or re-chargeable Metal Hydride (life 10hrs)
- Battery monitor: battery stae is monitored continuously)
- Ambient back-off: (up to 2x FSD)
- Output BNC gives 1V FSD from 1kΩ
Analogue and Digital Readouts
Analogue meters are normally strongly preferred because its is easier to follow a trend and they do not suggest an unjustified accuracy. However, if required, a digital display unit can be supplied.
Rapid Pulses
The Display Units are designed to integrate pulsed currents. They show the mean power of rapidly pulsing lasers of any pulse duration with any repetition rate from GHz down to 10Hz. Thus these CW Meters are ideal for measurements of lasers for fiber optic or free space communication or pulsed laser therapy. Below 10Hz the reading will appear noisy and will be low by an amount depending on the duty cycle.
Temperature Effects
- Heads M55, M69 and M68 have a temperature coefficient of -0.4%/°C at all λ
- Head M29 (Si) has a temperature coefficient between -0.3%/°C and +1.0%/°C depending on the wavelength.
- Storage Temp Range: -20°C to +60°C
- Operating Temp Range: +15°C to +25°C
Water Cooling
Heads M69 and M68 have fittings for water cooling. You should use water cooling if you will regularly have exposures to more than 5W for more than about 20secs.
Head Mounting
M6 thread (≡'O' BA). This takes most of the standard optical posts.
Non Standard Systems
About half the systems we sell are non-standard and have been adapted to suit the precise requirements of our customers. Please contact us if your exact needs do not appear in this brochure.