Laser Power Meter Heads with Control Unit
Our Laser Power Meters are designed to make the accurate measurement of continuous laser power as simple as possible. They accept wide divergence beams and give a rapid response yet still provide traceability to the UK National Standards for CW laser power measurement.
- Measures power of a CW laser or the mean power of a pulsing laser
- Measures beam in free-space or light diverging from a fibre
- Rapid response time: 0.1 - 0.3 secs
- ±1.6% overall absolute accuracy (traceable to UK NPL)
- Battery powered for low noise, portability and reduced Earth Loops
- Choice of detector type: Thermopile, Si, Ge, GaInAs
Design of Systems for Measuring CW Laser Power
Most meters for the measurement of CW laser power up to about 100W use an almost total absorption of the laser beam on a relatively massive target. Although very simple, this approach can give some or all of the following problems:-
- The response is usually inconveniently slow.
- The sensitivity may vary across the target.
- The sensitivity may drop at high powers due to non-linear cooling.
- At the highest power densities there may be damage to the target.
By using an Integrating Sphere , our design of laser power meter avoids these problems - or at least reduces them. The sphere is coated with a robust high reflectivity diffusing paint which spreads the absorption evenly over the whole sphere. Also the power on the detector is reduced to a level well within the linear range (See Principle of Integrating Spheres). With correct design, the integrating sphere gives 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 ±30 degrees).
There is a BNC output socket to send the power reading to a Chart Recorder or DVM or PC. The design of the electronics allows you to measure the mean power of a stream of short pulses.
The usual detector in our integrating sphere CW power meter is a Thin Film Thermopile . This has a fast response time of 0.2 secs combined with an almost flat spectral response from 0.4 to 1.5µm. For the very lowest powers the thermopile is replaced by a photo-diode (Si, Ge, GaInAs as appropriate); a thumb-wheel switch then gives the appropriate wavelength correction.
For pulsed lasers, the photo-diode systems can have a BNC socket to allow viewing of the pulse shape of a pulsed laser on a 'Scope (Please request this at time of ordering).
| 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 (InGaAs) | 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 within 0.5% in 0.2 secs). The bandwidth switch gives a further factor of 2 reduction in NEP but the 10-90% response time is then 400ms (settling to within 0.5% in 1.0 sec).
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, a Gaussian shaped beam must have a FWHM of at least 20mm and have no little ripples on its Profile.
Calibration Accuracy
Our Calibration of a Broadband System will normally have an absolute accuracy at your chosen wavelength of close to ±1.6%. This is traceable to the National Physical Laboratory at Teddington. Details of how we achieve this accuracy are given on our web page Calibration Accuracy. The Calibration Certificate will also give details of the small change expected at the limits of the spectral range (usually less than 6% over the range 0.4 to 1.5µm).
For a Photo-diode System we can also calibrate it to ±1.6% at your chosen wavelength. For any other wavelengths you then simply set a digital switch to the new wavelength. The sensitivity tracks the wavelength setting correctly to better than ±4% over the full spectral range allowed for that type of photo diode. All this information is given on the Calibration Certificate.
Display Unit
- Size: 16cm × 9.6cm × 5cm high.
- Weight: 550gms
- Cable to Head: 1.5m screened cable
- Batteries: re-chargeable Metal Hydride typically giving 20hr use between charges
- Battery monitor: battery state is monitored continuously (Recharge when it reaches Red zone)
- Ambient back-off: (up to 2x FSD)
- Output BNC: gives 1V FSD from 1kΩ
Analogue or Digital Readout ?
Analogue meters are normally strongly preferred because its is easier to follow a trend and they do not suggest an unjustified accuracy. However, if you really do prefer a digital readout, we can supply one. It fits into the same control unit shown in the picture above.
Rapid Pulses
The Display Units are designed to correctly integrate pulsed currents from the detector. They show the mean power of rapidly pulsing lasers of any pulse duration with any repetition rate from GHz down to 10Hz. Thus these Laser Power 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 read low by an amount depending on the duty cycle. This lower limit can be changed on request at the cost of speed of response.
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 (= '0' BA). This takes most of the standard optical posts.
Non Standard Systems
About half the systems we sell are non-standard and contain modifications to suit the particular requirements of our customers. These changes do not necessarily cost any more. Please contact us if your exact needs do not appear on this web page.