Delta Developments


Pulsed Laser Energy Meters

We manufacture and supply a variety of systems that measure the energy of a pulsed laser whether it is repetitively pulsed or single shot.   We use fast photo diodes for the ultra-violet to near infra-red region (0.25 - 1.8µm).  These systems can also measure the pulsed peak power and pulse shape.   For the longer wavelengths (1-30µm) we use pyroelectric detectors.

The Heads with Photo Diodes measure both the pulse energy and pulse power. The pulse energy can be read by a slow ’Scope (1MΩ) or by our battery powered Pulse Acquisition Unit. The fast BNC output needs a fast 50Ω ’Scope to show the pulse power and pulse shape (<1.4ns rise time).   For longer wavelengths, the Pyroelectric Head can be used for energy measurements but has no output for pulse shape.

Energy Displays

Any of the Heads can be used with the battery powered Acquisition Unit (Model 40) to display the energy reading.

Alternatively, the Model 19 Data Acquisition Module has a Digital Display of the energy and an opto-isolated computer interface. This Module is powered by our Standard PSU Rack.

There is also a self contained Pulsed Laser Energy Monitor which contains a polarization compensated beam splitter and energy acquisition circuit to give readings of the pulse energy in the throughput beam. This also has an output for a fast ’Scope to give pulse shape with <1.4ns rise time.

Beam Direction

All the Heads have an integrating sphere diffusing system so that the sensitivity is almost independent of beam position, diameter or angle over the allowed range (See separate page Principles of Integrating Spheres ).

Calibration

All our Instruments have calibrations traceable to the UK National Physical Laboratory. The absolute accuracy is from 1.4% to ±3.0% depending on the instrument. Please see the separate web page on how we achieve our claimed Calibration Accuracy.

Photo Diode Heads

A Diffusing Head is the most generally useful. The geometry of the integrating sphere (with the detector facing the centre of the sphere) ensures a sensitivity essentially independent of beam position or beam angle (see separate page Principles of Integrating Spheres). for the smallest energies you will need to use the Lens Heads or Fibre Optic Heads. However, the penalty for the high sensitivity is that the tolerance of beam direction is not as good as with an integrating sphere.

The measurable power ranges given in the Specification Table below are simply those that correspond to the maximum or minimum energy sensitivities. For measurements of much lower pulsed powers please see details of our Peak Power Meters.

The bias for the photo-diode heads can come from a small battery pack or from any of the pulse acquisition units

 


Diffusing Head (Model 27)

Line drawing of Integrating Sphere Head with detector behind diffusers facing the sphere centre

With the detector facing the centre of the sphere, the signal is independent of beam angle and position.
This system attenuates the light by ×103 - ×108


Lens Head (Model 49)

Line drawing of Lens Head with diffuser and detector at focal point of lens

By collecting all the light directly onto the detector a very high sensitivity is achieved.
The diffuser spreads the light across the detector thus allowing the maximum possible linear photocurrent.


Fiber Optic Head (Model 50)

Line drawing of Fibre Optic Head with diffuser and detector behind end of fibre coupler

Any of the standard Fiber connectors can be supplied.
The diffuser ensures that the sensitivity is the same whatever the power distribution within the fibre.


Linearity and Range of Energies

Each Head is individually made for your particular 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 measure a factor of 1000:1 in energy and 100:1 in pulse power. Because no individual Head could ever cover the huge possible range of pulse energies it is important that you tell us the actual energies you wish to measure.

The specification for our standard designs are shown below with the maximum and minimum sensitivities that are possible. However, please enquire if you wish to vary any of them: special designs do not necessarily cost any more.


Specifications for Heads with Photo-diodes
Diff. Head
Model 27
Lens Head
Model 49
Fiber Head
Model 50
Max. Poss. Sens: Energy o/p (1 MΩload) 1.0V/µJ 0.2V/nJ 0.2V/nJ
Useable Energy range 10nJ - 3µJ 50pJ - 15nJ 50pJ - 15nJ
Measurable Power range 100mW - 30W 1 - 300mW 1 - 300mW
Min. Poss. Sens: Energy o/p (1 MΩ load) 5.0V/J
Useable Energy range 2.0mJ - 0.3J
Measurable Power range 40kW - 12MW
Aperture: 20mm (<2% change) 20mm (<3% change) PC/FC Coupler
Acceptance angle: (either side of axis) ±30% (<2% change) ±3.0° (<3% change)
Calibration Accuracy: ±2.5% ±3% ±4.0%
Head Size (length × diam.): 72 × 74 mm 70 × 74 mm 70 × 74 mm
Energy BNC: Signal Level: up to 3.0V (proportional to energy)
Output impedance: 1 MΩ (always use 1 MΩ load at ’Scope)
Minimum pulse length: 100ps
Maximum pulse length: 20µs
Max. Rep.rate: 200 pulses per sec.
Power BNC: Signal Level: up to 3.0V (proportional to peak power)
Output impedance: 50Ω (always use a 50Ω load at 'Scope)
Response time: Step edge gives 10 - 90% rise in <1.4ns. An impulse gives FWHM of <2.0ns
Minimum pulse length for accurate power measurements is 5ns
 
Spect. Range: 0.25-1.1µm (Si)   or 0.8-1.8µm (InGaAs)
Mounting: M6 tapped hole (= 'OBA')
(This accepts most of the bench rods currently in use)
Variants: Other apertures are available. eg 7mm as needed for safety checks.
A longer integrating time constant would allow a longer pulse duration but require a lower PRF.
There is a further range of Heads for measurements of very low power measurements with NEP down to 3.5nW/cm2 RMS. Details are on the Web Page on Peak Power Measurements.

Pyroelectric Head

To make pulsed energy measurements on a repetitively pulsed Carbon Dioxide laser at 10.6µm you have, unfortunatey, to use a Pyroelectric Head with its attendant disadvantages.
These Heads also have a spherical cavity with diffusing walls with the pyroelectric detector facing the centre of the sphere. However, because diffusers for use in the far infra-red are not very satisfactory, there is some dependence of the signal on the beam position and the angle on the sphere surface.   Also, following a short impulse, pyroelectric detectors give some piezoelectric ringing at about 10MHz. This means that pulse shape and peak power measurements are not possible with any precision. The output socket provides energy measurements but there is no pulse shape output.

Wavelength Range: 0.5 - 20µm
Sensitivity: 10V/mJ (Other values may be possible)
Output: 0 - 10V from BNC socket at 1 kΩ impedance
Response time: Signal rises with 1/e time of 60µs then decays with 1/e time of 4ms.
Input Aperture: 18mm
Max beam size: 14mm (limited by diffuser curvature)
Min beam size: 4mm (limited by speckle pattern effects)
Positional dependence: <4% change as beam is scanned horizontally
<5% change as beam is scanned vertically
Head size: 63mm diam × 93 mm long
Mounting: M6 tapped hole (= 'OBA')
(This accepts most of the bench rods currently in use)

Pulsed Laser Energy Monitor

The Pulsed Laser Energy Meter provides a complete beam sampling and pulse acquisition system to measure the output of a pulsed laser. It contains a polarisation compensated beam splitter to sample the beam and a diffusing sphere to give a constant signal for any beam position or beam angle within the input and output ports. Range plates can be inserted in front of the detector to give different sensitivities and allow for different wavelengths. Thus the same Instrument can cover a huge range of energies and wavelengths. It has outputs for pulse shape and pulse energy. The output signal can be referred to the energy leaving the Monitor or to that entering the Monitor. It can be self triggering or triggered externally from the laser.

Wavelength range: 0.35 - 1.06µm
Energy Range with maximum sensitivity: 10µJ - 1mJ
Energy Range with minimum sensitivity: 300mJ - 10J
Max rep. rate: 20kHz at 12µs duration, 500Hz at 1.5ms duration
Min rep. rate: Acquires energy reading in a single shot
Transmission: 80%
Max beam size: 38mm
Overall size: 26cm long × 18cm high × 12cm wide (Display Meter 16 × 12 × 5 cm)

Data Acquisition

All the Heads detailed in the previous pages will need some separate system to display their output (except for the beam sampling Pulsed Laser Energy Monitor which is self contained). Most needs can be met conveniently by using a ’Scope. You should use a load of 1MΩ for the signal from the energy output socket and 50Ω for the power/pulse shape socket.

For a more convenient system without ’Scope or for data logging applications you may prefer to use one of the Data Acquisition Units. They all provide the HV bias needed for the Photo-diode.

  1. Model 40 is a portable battery powered display unit. It acquires the energy value from any Head.
  2. Model 19 is powered from our standard PSU rack. It acquires the energy from any Head.
  3. Model PPM/1 is also powered from our PSU rack. It acquires the peak power from any Head

(1)   Model 40 Battery Powered Acquisition Unit

Takes the energy signal from the Head, captures the top of the decaying waveform and displays it on an analogue meter. The display is automatically updated at each successive laser pulse and the reading remains valid until the next pulse. Thus it is equally useful for repetitively pulsed or single shot lasers. It operates only on the energy output from the Head.

Range switch: The Range switch will be engraved to suit your application such as: 1, 3, 10, 30, 100µJ/CM2 or 10nJ, 30nJ, 100nJ, 300nJ, 1J
Sensitivity: Full scale on the Meter with 100mV, 300mV, 1.0V, 3.0V, 10V These correspond to different energies depending on the sensitivity of the Head. The sensitivity will be adjusted to suit the exact sensitivity of the particular Head.
Rep. rate: From a single pulse up to 100 pulses per sec. Higher rep rates may be possible.
Output Signal: 0 - 2.0V from BNC socket for DVM or ’Scope (output impedance 1KΩ)
Triggering:
  1. On “Internal” the trigger level is 17% of FSD of the Range being used.
  2. Alternatively, an external pulse of 3 - 5V into 100KΩ will trigger the unit.
  3. The Reset button provides an alternative method of setting the zero.
Size: 185mm long, × 130mm wide × 70mm high.

(2)   Model 19 Data Acquisition Module

Plugs into our standard PSU rack. It acquires the pulse from the energy socket on the Head with an accuracy of 12 bits (l/4000) and displays it on an on-board Panel Meter. An opto-isolated computer interface allows good data-logging without introducing noisy earth loops into the system. It must be triggered by a pulse within 2-3µs on either side of the laser pulse.

Specifications
DC output: 10V corresponds to FSD on Panel Meter. Output impedance is 450Ω.
Repetitive Rate: From a single pulse up to 1kHz. Higher rep rates may be possible.

(3)   Model PPM/1 Peak Power Module

Plugs into our standard PSU rack. It acquires the pulse from a power socket on the Head and displays it on an on-board Panel Meter. It is entirely self triggering but requires a stream of essentially constant pulses to capture the peak of the pulse. Normally it would display the average value of the peak power but can be switched to display the maximum peak power achieved or the minimum peak power achieved.

Specifications
Sensitivity: This will be adjusted to suit the Head so that it reads in mJ, µJ etc.
Pulse duration: 6ns to 2.5µs (less than 3% variation in reading)
Pulse rep. rate: From 20Hz up to 200kHz. (slower rates may be possible)
Output Signal: 2.0V from 1 kΩ corresponds to FSD on Meter. This can drive an external DVM.

For further details of the operation of the Peak Power Module ask for data sheet PPM/1.


Some of our Energy Measuring Systems

Picture of various Heads and Display Units used for Pulsed Laser Energy Measurements

 

The Head HL in this picture is mostly used for peak power measurements of the weak return signal from a distant target. Details of Model HL