A novel approach to processing electronics design allows a laser meter to measure very fast pulsed beams and to display real-time waveforms. The meter adapts automatically to the head, optimizing the processing and adapting the display.
Paul Tozer, Lasermet Ltd., Bournemouth, UK
The ability to take accurate measurements of the power and energy emitted from a laser source is an essential requirement for any laser scientist, not only to verify the characteristics of the laser but also to ensure the safety of all personnel involved.
Conventionally, laser power meters consist of a meter used with a detector head. The meter contains all the electronics, functions and displays and can be used with a selection of detector heads, the choice of which depends on the laser power and wavelength of operation. Although there have been many advances over the years, there is a continual quest to develop meters that will take measurements with increasing accuracy, greater stability and faster responsiveness. There is also a desire for the display to be operable with an ever-increasing variety of detector heads.
With a sampling rate of up to 1 MHz,the hand-held power meter displays real-time waveforms with variable time base down to 10-μs per division.
The design team at Lasermet Ltd. has addressed these challenges through a novel approach to the design of the processing electronics. This patented technology, known as the Advanced Detector Interface (ADI), forms the heart of the ADM-1000 hand-held power meter and is the result of more than two years of intensive development. In designing the meter to respond to customer needs, the team realized that the conventional electronic design for a power meter would not achieve the necessary performance.
A specification for the new design included providing a response time well in excess that of any meter currently on the market — on the order of 1 μs. The team devised an alternative approach that achieves the response time and general characteristics sought and which enables the company to develop new heads — possibly detecting quite different parameters — that will connect to existing displays in the field by means of a simple firmware upgrade in the display.
The Advanced Detector Interface automatically optimizes the processing electronics for the particular detector,including the photodiode integrating sphere head (top) and the thermal head (bottom) shown here.
This approach offers a number of advantages over conventional meters when measuring laser power and energy, chiefly:
• An increased response rate that is effectively instantaneous. This facilitates the capture of scanning and fast pulsed laser beams with greater confidence and accuracy.
• A frequency response of 400 kHz allows the detailed analysis of complex waveforms to a level not previously possible. The measurement of peak power, average power, pulse width and pulse repetition rate can be achieved with a single hand-held device.
In numeric mode, the display can show average or peak power.
• The interface enables optimization of the electronics for each detector head, thereby achieving maximum performance every time. A simple software upgrade allows the incorporation of new heads.
Leading response times
The interface has an analogue sample rate of up to 1 million samples per second, particularly benefiting pulsed laser users and offering the precision to measure and display waveforms up to 400 kHz. The numeric and graphical oscilloscope display modes on the meter work in real time, offering the ability to display 10 μs per division and making it possible to see a continual waveform on the screen. Accurate measurements of peak and average power and energy per pulse are thus achievable.
An especially useful application of this technology is examining scanning beams such as a live raster image. By freezing the image on the screen, a single pulse or pulse burst is captured and displayed. The technology makes it possible to see complex optical pulse waveforms with greater clarity and detail. Cursors on the screen can be used to identify any area of the waveform that is of interest and to calculate the energy contained within it.
Conventionally, the processing electronics must work with a variety of detector heads (photodiode, thermal etc.), which means that the electronics cannot be optimized for any particular head. An important advantage of the new technology is that it allows configuration of the electronics to work optimally for each head. When a head is plugged into the meter, the display identifies which one is connected and adapts itself to its use.
As new types of heads are brought out, there is no need to purchase a complete new meter. Most new heads will plug straight into the display, with the processor determining the appropriate operating values from the head. If a new type of head is developed that measures a parameter or that operates over ranges outside those for which the display was designed, the new detector head can be connected to existing meters with only a firmware upgrade — no hardware changes are required. The display has a USB port so that customers can upgrade the firmware from a PC.
The ADI and firmware upgrade facility make it possible to develop an unlimited variety of detector heads, even specials and “one-offs” for measuring almost any parameter that can be connected by customers to existing displays.
Contact: Paul Tozer, Lasermet Ltd.; fax: +44 12 02 77 07 30; firstname.lastname@example.org.