The CO2 Laser Spectrometer uses an array of PVF2 Pyroelectric detectors in the focal plane of a grating spectrometer. The detectors are located by computer ray tracing in the precise positions of the various lines in the P and R branches of the CO2 spectrum. Following one shot of the laser, the detectors are accessed sequentially to give a bar chart display in the spectrum. The NEP of 50nJ or 30µW is very low compared to phosphor screen instruments.
With a special type of pyroelectric plastic a speed of response of about 100ns can be achieved so that the time dependence of each of the spectral lines can be output to a multichannel analyser system.
Different arrays can be used such as linear pyroelectric or linear Silicon thus giving a conventional spectrometer or spectrograph readout.
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The Delta Developments CW Power Meters provide a fast speed of response while still maintaining accuracy over a broad spectral band. This is a feature normally available only with slow thermal instruments.
Beam Angle. The laser beam strikes the back of an Integrating Sphere. The geometry is arranged so that the signal is independent of the beam position or angle. This large acceptance angle allows accurate measurement even with the rapidly diverging output from the end of an optical fibre.
Rapid Pulses. All the instruments correctly measure the mean power of rapidly pulsing lasers from GHz down to 10Hz or less. Thus they can be used on lasers for Fibre-Optic Communication or Free-Space Communication or Pulsed Laser Therapy.
Calibration. Normally we calibrate at a single wavelength in the middle of the spectral range you will be using. This provides an absolute accuracy of ±1.8% traceable to the Standards held at the UK national Physical Laboratory. The Calibration Certificate will give details of the change expected at the limits of the spectral range (typically the sensitivity stays within ±5% relative to the central point).
| Model No | Detector Type | Power Ranges | Aperture | Wavelength Range | |
|---|---|---|---|---|---|
| M 55 | Thin Film Thermopile | x1 Range | 1mW - 1W | 18mm | 300nm - 2µm, |
| M 69 | Thin Film Thermopile | x10 Range | 10mW - 10W | 25mm | 300nm - 2µm, |
| M 68 | Thin Film Thermopile | x100 Range | 100mW - 100W | 35mm | 300nm - 2µm, |
| M 29 (Si) | Silicon Photo-Diode | Min Power Max Power |
10µW - 3mW 3mW - 1W |
18mm | 350nm - 1µm, |
| M 29 (Ge) | Germanium Diode | Min Power Max Power |
30µW - 3mW 10mW - 1W |
18mm | 800nm - 1.8µm |
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Types of Laser Measured. We offer several systems which measure the energy of repetitive and single shot pulsed lasers such as Nd:YAG, Ruby, TEA etc. Fast photo-diodes are usually used in the visible to near infra-red region (0.3 - 1.8µm) and pyroelectric detectors for the longer wavelengths (up to 30µm).
Pulse shape and power. The photo-diode systems can also measure the pulse shape and pulse power with a rise time of <1.4ns. This BNC output needs a fast 50 Ohm ’Scope.
Beam Direction. All the Heads have an integrating sphere diffusing system so that the sensitivity is almost independent of beam position and beam angle over the allowed range.
Linearity. A Head is individually made for each application with the appropriate diffusers and attenuators in front of the detector to keep it within the linear range. Heads can be made to cover anywhere from a few µJ to 100J,with peak pulsed powers from a few mW to 1GW. Any given head can usually measure a factor of 2000:1 in energy and 100:1 in pulse power.
Powering of Heads. The smaller Heads need a simple battery pack to provide a high voltage bias for the fast photo-diode. The battery powered Model 40 Acquisition Unit can also provide this bias. The larger more complex heads have their own PSU (See Price List for details).
Data Acquisition. For all the Heads the pulse energy can be read by a slow ’Scope (1M Ohm). Alternatively, you can use the Model 40 Pulse Acquistion Unit (battery powered), the Model 19 Acquisition Module (digital display of energy, powered from our standard PSU Rack) or the Model 8 Peak Power Module (digital display of peak power, also Rack powered). There is also a Module for the IEEE interface.
Pulsed Laser Energy Monitor. This is a self contained Instrument (seen at back, right in the picture above). It has its own polarisation compensated beam splitter and on-board acquisition circuitry. It gives shot-by-shot readings of pulse energy. There is also a BNC output giving pulse shape with <1.4ns rise time for use with a fast 50 Ohm ’Scope. To allow for the transmission of the beam splitter, the readings can be referred to either the energy entering or that leaving the Instrument.
Calibration. All our Instruments have calibrations traceable to UK National Physical Laboratory. Typical absolute accuracy is ±3.0%.
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Laser Types Measured. All our Peak Power Measuring Heads are aimed at measuring high repetition rate pulsed lasers, emitting about 10W in 50ns but there are options for shorter pulses and higher or lower powers. The Standard Head (Model SD) has 37mm aperture but there is a larger head (Model SD60) with a 60mm aperture.
Beam Direction. The Heads contain an integrating sphere diffusing system to eliminate variations of sensitivity with beam position, beam direction, or beam polarisation. This system is so tolerant that it will even measure accurately the widely diverging light from the end of an optical fibre.
Calibration. All the Heads have built in Calibration facilities which we use to provide an accurate link between the power in a short high power pulse and the UK National Standards of CW power at about 1mW. The final calibration has an absolute accuracy of typically better than ±2.5%.
Signal Acquisition. All the Heads have an output impedance of 50 Ohms and are intended for use with either a fast 50 Ohm ’Scope or our Digital Readout Module in the Power Rack.
Powering Heads. Most of the heads can be powered by a simple battery pack. A few need a stand alone PSU or one of our supply racks which can take other Modules. Details are on the Price List.
Digital Readout. The optional Digital Readout Module is particularly useful when the pulses from the laser are slightly inconsistent because it has modes to display either the average peak power or the maximum peak power or the minimum peak power.
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The Laser Beam Profile Monitor Mk4A has a receiving head with an array of detectors, each with its own amplifier. The signal from each detector/amplifier pair is corrected for its individual sensitivity and zero offset (called "equalisation").
The system can be used for lasers which are single shot or repetitively pulsed or CW. After each laser shot the new profile is shown on the Display Screen until the next pulse, when it is updated automatically.
For most purposes Pyroelectric detectors are to be preferred. However, for some applications an array of Silicon offers a higher sensitivity and allows a higher repetition rate. For CW lasers the pyroelectric detectors need a chopper.
The Equalisation Scheme guarantees elements matched in sensitivity to better than 0.9% RMS. Point by point subtraction of zeroes gives a dynamic range between 20,000:1 and 25,000:1 and also allows a high repetition rate. In addition to the working profile, extra profiles can be stored within the Unit.
Because the system is built up from plug-in Modules, extra features can be added either at the time of ordering or retrospectively. Examples are:- an IEEE or RS232 interface for computerised data collection or a Profile Averaging Module to allow point-by-point averaging over 2 or 4 or 8 ...128 laser pulses.
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The Boxcar Detectors (also called Chopped Signal Integrators or Phase Sensitive Detectors) are designed to process the signals obtained from a chopped light beam detected by a pyroelectric, photoconductive, photovoltaic detector or a photomultiplier. However, they can also be used on any signal which has a constant phase relationship to a synchronizing pulse.
The system provides Synchronous Detection (or Boxcar Integration) to give a steady output even from low level signals by averaging the input during periods of signal 'on' and subtracting this from the averaged input during periods of signal 'off'.
The different response times of different detectors are accommodated by varying the duration and timing of the time windows accepting 'on' values or 'off' values. This provides a degree of flexibility not normally available even in a costly lock-in system.
We make two broad types of system – using either one or two stages.
| The simple system achieves its integration of the signal in one stage of capacitor storage. | |
| The advanced system has first a stage of analogue storage on a capacitor and then digital storage of that result; this allows for very short window times at low repetition rates. |
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The Driver Module for Lasers or LEDs can be used for any industry standard 14 pin DIL device with a fibre output. BNC sockets allow modulation of the laser current in two bands: DC to 50kHz and 10kHz to 500MHz.
There are three control Modes selected by a front panel switch:
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These Amplifiers were originally designed specially for Photon-Drag Detectors for CO2 TEA Lasers, but can, of course, be used for any general purpose amplification requirement. They have been carefully designed to eliminate noise from all sources and are mounted in an ultra screened box thus minimising pick up from the laser HV discharge or from supply transients.
The battery state is monitored continuously and on board regulation ensures that the performance remains constant as it discharges. Any reflections back from the oscilloscope are absorbed because the amplifiers are reverse terminated (that is, the output impedance is exactly 50 Ohms right up to the maximum frequency).
Faster Amplifiers are also available. For example, 0.7ns rise time at a gain of x10.
We also make ultra low-noise amplifiers (DC - 10Hz) for use with slow thermal detectors such as calorimeters or thermopiles. These are usually battery powered. Noise level is typically 200nV P-P at G = x100.
For any special amplifier, please contact Delta and give details of your requirement.
As well as its own standard instruments for laser measurements, Delta Developments carries out Consultancy and does Special Designs to suit the exact needs of individual customers. We have many years experience in these special projects and work closely with the customer to achieve their objective.We regard the writing of a comprehensive Report or an Instruction Manual as almost the most important part of each job.
For a Consultancy we usually first visit your site to assess the problem. We would then do some analysis and possibly some measurements before writing a full report which would include our recommendations for the next stage.
At the end of any Special Design Project the customer receives an Instruction Manual with full details of the design, calibration details, circuits, circuit layouts and the specification with constraints to be observed in use.
