Beam analysers offer healthcheck for lasers

07 Jan 2003

Our new series of buyer's guides takes an in-depth look at a different piece of technical equipment each issue. This month Oliver Graydon investigates the pros and cons of the range of products available for laser beam analysis. Next month, we profile CCD cameras.

From Opto & Laser Europe January 2003

Whether you're pumping a crystal, performing a demanding R&D experiment or attempting to couple light into an optical fibre, obtaining accurate knowledge of your laser beam's size and shape is essential. Today, the most reliable and precise way to obtain this information is with the help of a beam analyser (also known as a beam profiler). But with at least a dozen beam analyser suppliers in the market, all selling several different models, how do you go about selecting the one that is most appropriate for your needs?

In the first of a series of buyer's guides comparing technical equipment, Opto & Laser Europe takes a look at beam analysers and offers some independent advice to help you make an informed purchasing decision. Our updates on the latest technology, breakdowns of selection criteria and samples of the latest products on the market should make it easier for you find the best solution for your needs.

We begin with an overview of the types of technology on the market. Beam analysis is currently carried out using one of two methods - either imaging or scanning. The former technique employs a camera - typically based on a charge-coupled device (CCD) chip or an array sensor - to capture an image of the beam. The second approach involves scanning a knife blade or a slit across a detector to generate an x-y intensity profile of the incident beam. By generating profiles of several slits in different orientations, a complete profile of the beam shape can be constructed.

Both techniques interface with the user's computer. The advent of sophisticated beam analysis software enables computers to calculate and display an impressive range of beam parameters. As well as plotting x-y and 3D beam-intensity profiles, analysers can measure the beam's diameter; ellipticity; centroid, major and minor axes; peak location; and even its M² value and power.

So which technique should you be using - imaging or scanning? The answer is that as each device has its own advantages and drawbacks, your choice should depend on the characteristics of your source. Ask yourself the following questions:

What is your source's wavelength? Scanning beam analysers can be used with a range of detectors of various wavelengths, including Si (190-1150 nm), Ge and InGaAs (800-1800 nm), InAs (1.5-4 µm) and pyroelectric (up to 20 µm). By contrast, the imaging technique is best suited to sources with wavelengths of less than 1.1 µm, owing to the wavelength response of the silicon CCD sensor. Optional extras such as a phosphor coating, intensified CCD or pyroelectric array, for example, can extend the range of such analysers, but they can be pricy and offer limited resolution.

What is your source's output power? Using beamsplitters to tap off a small portion of the beam means that both the scanning and imaging techniques can be used with high-power beams. Neutral density filters can also be used to reduce the incident power of the beam. The scanning approach tends to offer a higher dynamic range (more than 50 dB) but the dynamic range of CCD cameras is rising rapidly, with 14-bit (48 dB) cameras now available. For industrial sources such as Nd:YAG lasers, a pyroelectric-based system is probably the most appropriate.

How big is your beam? CCD cameras often make use of standard-sized silicon sensors (up to 8.8 x 6.6 mm), which are capable of measuring a beam of up to about 6 mm in diameter. Larger sensors, measuring 14 x 13 mm or more, are available at an extra cost. As for the scanning technique, a range of detector geometries is available with entrance apertures measuring up to 25 mm in diameter for both silicon and pyroelectric detectors.

What resolution do you require? The detail and accuracy of the beam profile produced varies according to the technique you use, as does the smallest size of beam that can be measured. For the CCD cameras used in imaging beam analysis, the resolution of the image is determined by the pixel size of the sensor. For standard silicon CCDs this is typically around 4.65 x 4.65 µm, although if infrared or pyroelectric arrays are used the resolution can increase to 20 x 20 µm or even 100 x 100 µm.

By contrast, the scanning approach is capable of measuring spot sizes as small as 0.5 µm with a resolution approaching 0.1 µm, if a very narrow slit is chosen. Slits are often interchangeable and can vary from 1.8 µm up to 25 µm in width.

Are you using a CW or pulsed source? If you need to analyse both continuous wave (CW) and pulsed sources, the CCD imaging technique offers a clear advantage. It can analyse lasers of almost any pulse rate (from 1 Hz repetition rate upwards). By contrast, the scanning technique often requires a minimum pulse-rate of several kilohertz.

How rapidly does your measurement need to be updated? Depending on the resolution and beam size settings, the scanning approach can deliver a measurement update rate of up to 10 Hz. A CCD-based imaging system can operate at faster rates, especially if a region of interest is selected for refreshing.

How much can you spend?

The price of beam analysis systems obviously varies according to the performance they offer. As a rough guide, the starting price of CCD-based beam analysers is currently around $4000 (€3915), while a scanning solution tends to be slightly more expensive at $5000 upwards.

The data available to Opto & Laser Europe suggest that if you need a general-purpose instrument for characterizing a wide range of sources (especially those with a wavelength of less than 1 µm) and can live with slightly lower specifications, the imaging approach to beam analysis is probably the best choice. It provides a cost-effective solution that combines ease of use and lots of flexibility with powerful analysis capabilities. The other major advantages of using an imaging-based system are that it can analyse low repetition-rate pulsed sources, and that no assumptions need to be made about the beam's shape because it is imaged.

If, on the other hand, you're working with a specific source and need a high-performance instrument with maximum dynamic range and high resolution, the scanning approach may be more suitable. This is especially true if you want to analyse beams with either very small or very large diameters. In addition, because a large range of detectors is available, this approach is likely to be the most cost-effective for the analysis of longer-wavelength (more than 1 µm) sources, such as lasers in the telecoms window (1.3 or 1.5 µm).

Recently, several manufacturers have introduced both scanning and imaging systems designed for use with industrial lasers such as Nd:YAGs. Invariably these make use of pyroelectric detectors, which can withstand far higher energy densities.