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Lasers & Sources

Meter Readers

How to choose the right laser measurement system

From oemagazine November/December 2004
31 November 2004, SPIE Newsroom. DOI: 10.1117/2.5200411.0007

There is a diverse selection of laser power and energy measurement instruments and accessories currently available on the market. The first step in choosing the correct meter for your application is to understand what types of sensors are available. Two sets of parameters drive sensor choice: properties inherent to the laser itself (power or energy output, peak power or energy density, wavelength, repetition rate, beam diameter, etc.) and the actual measurements desired (average power, energy per pulse, rate, etc.).

There are three different sensor types typically offered, each of which is optimized for a different range of laser parameters and measurements (see table). Thermal sensors cover an extremely broad spectral range and a wide range of input powers. They offer very uniform spatial response and are less affected by changes in beam size, position, or uniformity. Their relatively slow response makes them best suited for measuring continuous-wave laser power, average power in pulsed lasers, or the energy of pulses greater than 1 ms.

Semiconductor photodiodes offer high-sensitivity and low-noise operation, enabling them to detect very low light levels; attenuating filters must be used for high powers. Photodiodes have a fast response time, allowing them to measure pulse shape. On the downside, photodiodes also have a much more limited spectral range and lower spatial uniformity than thermal sensors. This lower uniformity can affect the measurement repeatability of non-uniform beams or beams that wander between measurements.

Pyroelectric sensors measure only pulsed or modulated laser beams. They produce a signal proportional to the total pulse energy. Pyroelectric sensors feature fast response and can be used to measure pulse energy at multi-kilohertz repetition rates. Their sensitivity also allows measurement of very low energies.

Once you have chosen a sensor, you need to select a meter that is compatible with that sensor and that provides the necessary capabilities and functionality. Sensor compatibility for particular meters is vendor-specific, and can be determined from manufacturer literature. Certain sensors can also be used with oscilloscopes.

In terms of capabilities, some meters measure only power, some measure only energy, and some measure both. Power-only and energy-only meters offer simplicity and economy. Meters that measure both power and energy offer more flexibility and the opportunity to expand into additional types of measurements, such as pulse energy, at a future time.

Meter display types include analog (needle and dial) and digital. Digital meters can typically be read with greater precision than analog meters and are well suited to calibration applications; however, digital displays can be difficult to read when values change rapidly, such as during laser output power adjustments. For this reason, analog display meters are often preferred for use in tweaking laser output.

Meters are often used in conjunction with other instrumentation, or as part of an automated data acquisition system. In this case, look for an RS-232, USB, or IEEE-488 (general-purpose interface bus) interface to enable communication with a host computer.

The performance of power meters is typically characterized by the instrument's noise floor. This determines the ability to make low-light-level measurements and sets the absolute accuracy and resolution of the device. For energy meters, the deciding performance characteristic is typically the maximum repetition rate.

In summary, once you decide upon a sensor, you can then select a meter, resulting in a complete measurement solution based upon your application needs. For more complex situations, an applications engineer can typically specify the appropriate detector and meter. oe

Sean Bergman
Sean Bergman is product-line manager of power and energy products, laser measurement and control group, Coherent Inc., Portland, OR.