Accurate profiling requires an appropriate choice of equipment
Generic instruments can provide good results, but certain types of applications require sophisticated instruments.
Determining beam intensity as a function of position in space is known as beam profiling. Guiding light through delicate optical fibers and carving through pieces of steel are diverse applications in which beam profiling is used to achieve optimal performance. In these and other applications, light is finessed through a system of lenses to produce a beam of optimal size.
"Beam profiling" is a somewhat generic term and sometimes generic instruments are sufficient for good results (see Fig. 1). Sometimes, however, effective beam profiling requires more sophistication and care must be taken to select the right equipment for the beam-profiling application and to correctly interpret the results (see table).
Cameras that consist of some type of two-dimensional detector have become fairly ubiquitous tools in optics labs. Silicon-based charged-coupled-device (CCD) cameras, which measure wavelengths from 350 to 1100 nm, are fairly inexpensive, the results are fairly intuitive, and software packages are readily available to analyze the data. For many beam-profiling applications, especially at visible wavelengths, cameras provide a good, multipurpose solution.
There are however, applications for which cameras do not provide very useful measurements. For instance, the beam under analysis can saturate the camera detector and beam attenuation, if not performed carefully, can affect the beam profile. This is especially true for beam powers of more than a few hundred milliwatts, for which large amounts of attenuation are required.
Beams smaller than 50 µm in diameter are usually not well characterized by cameras because the pixel dimensions of most silicon-based cameras—usually 5 to 10 µm—do not allow measurements of good resolution at these spot sizes. A lens can be used for smaller spot sizes to increase the beam size, but this adds to the complexity of the measurement and the lens can introduce distortions into the profile.
At wavelengths above 1100 nm, cameras can still be used, but they provide less quantitative results. Indium gallium arsenide (InGaAs) arrays, which cover wavelengths from 700 to 1700 nm, are quite expensive and because the pixel sizes are around 30 µm or larger, they do not always provide better results than other low-cost techniques.
Vidicon cameras are used above 1100 nm, but our experience has shown that these lower-cost detectors tend to have nonuniformities across the detector area and a nonlinear response, limiting their effectiveness for beam profiling. Hence the typical beam-width accuracy with a vidicon camera is in the 5% to 8% range. A vidicon camera does provide good qualitative results when a limited region of the detector is used, roughly 1 × 1-mm square.
Recently, very low-cost cameras that use a phosphor to convert light between 1100 and 1700 nm into visible light have become available. The visible light is then imaged with a silicon CCD camera. These phosphor-based cameras are fine for locating near-infrared beams and providing similar simple qualitative information, but they suffer from problems of accuracy and linearity as well. A similar technique is used for ultraviolet (UV)-light profiling, with much better results because the phosphors that convert UV light to visible light are of higher quality and uniformity than those available for the near infrared.
FIGURE 2: The high resolution of the slit-profile technique allows for the beam separation of two adjacent beams to be determined to a few tens of nanometers.
Slit-based scanners are commonly used for high-resolution beam profiling. In a slit scanner, a beam is directed at the scanner and a slit moves into the path of the beam. Behind the moving slit is a single element photo detector. The light that passes through the slit induces a voltage in the detector. By resolving this photo-induced voltage as a function of slit position, a high-resolution profile is obtained (see Fig. 2). This technique has proven to be quite accurate.1, 2
Slit scanners are preferable to knife-edge scanners because knife-edge scanners diminish intensity variations in the beam profile. Dual-axis slit scanners are achieved by mounting two slits at 90° angles on a rotating drum. An attractive feature of slit scanners is that the slit acts as an attenuator, and therefore, higher-power beams can be measured directly without the need for additional attenuating optics. In addition, the high dynamic range of the slit scanner makes it possible to vary the intensity of the beam, as usually occurs during a focusing operation, without constantly having to adjust the level of attenuation.
Slit scanners are valuable for accurate and high-resolution beam size and position measurements at visible wavelengths, for beams with dimensions of less than 100 µm. In addition, when submicron resolution for beam size and position are required, slit scanners have proven to be the superior solution. Using state-of-the-art electronics, we have recently achieved measurement resolution of beam size and position on the order of a few tens of nanometers.
Slit scanners have also proven to be cost-effective solutions for beam profiling in fiberoptic communication applications, over the wavelength range of 800 to 1700 nm. These applications often require high measurement precision to achieve optimal optical alignment for low-loss optical components. Cameras used at these wavelengths are either more expensive than slit scanners for the same results, or simply technically inadequate to provide the data required.
Measurements at high beam powers (more that 1 W) are yet another application for which slit scanners excel. Using pyroelectric detectors in the slit scanners, these instruments provide a turnkey solution for the analysis of high-power beams. Beams at 10.6 µm produced by CO2 lasers can be characterized up to 100 W using slit scanners. For applications at wavelengths above 1.5 µm, slit scanners also provide a cost-effective alternative to cameras designed for these longer wavelengths.
Slit scanners provide two-dimensional information about the beam structure that is useful to identify characteristics of a beam. Since this information is generated through orthogonal one-dimensional profiles, the two-dimensional information, especially if it is complex, may not be completely resolved.
Divergent sources provide several challenges to beam profiling. A divergent beam in this case is defined as any beam with a full divergence angle of 15° or more. Common examples include bare (unlensed) edge-emitting laser diodes, some vertical-cavity surface-emitting lasers (VCSELs), light-emitting diodes (LEDs), waveguides, and many optical fibers.
An important characteristic of these sources is the divergence angle, because knowing this angle is important in determining how to couple these sources to other optics. This becomes especially challenging for very large divergence angles (>20°, full angle) when the beam size grows very quickly and is usually larger than any single detector when the beam reaches the profiler. The divergence angle of a bare edge-emitting laser diode or LED is often measured using a camera, a lens to collect the light, and an attenuator to prevent the camera detector from saturating. If the divergence angle needs to be known only to about 15% accuracy, this is an acceptable technique. For most applications, however, 15% accuracy is insufficient.
This camera technique fails to provide a high level of accuracy for two reasons: first, the numerical aperture of the lens is often not sufficient to capture all of the light from the divergent source, and second, the attenuation also tends to narrow the divergent profile because light at the edge of the profile travels through more attenuating material than the light in the middle. This is true for any type of camera-lens combination and cameras are therefore not well suited for beam profiles of highly divergent sources.
FIGURE 3: At the lower right corner of this high-resolution, high-dynamic-range goniometric profile of a VCSEL, a shadow of a bonding wire indicates the level of high-resolution detail.
A goniometric radiometer, or angular profiler provides the best way to characterize these sources. In a typical goniometric profile, the test source is held fixed and a detector is rotated at a uniform radius to determine the beam intensity as a function of angle. Recently, a technique for fast goniometric profiles was deveoped in which neither the test source nor the instrument detector moves.3, 4 Because goniometric scans do not use a lens or collecting optic, they produce highly accurate results of beam divergence and far-field angular profiles (see Fig. 3).
Optical fibers present another application for which goniometric scans have been shown to provide the most accurate results. Although cameras or slit scanners can adequately measure some optical fibers, for mode-field-diameter (MFD) measurements of single-mode and specialty fibers, goniometric profiles are necessary to thoroughly characterize these divergence sources.5, 6
As with any technical measurement, of course, the results produced are a function of the effort and care used to obtain them. A fully comprehensive review of beam profiling techniques would require more space than we have here, but a more comprehensive review of beam profiling is available online.7
- T. F. Johnston Jr. and J. M. Fleischer, presented at the Laser Beam Control, Diagnostics and Standards Conf., Society Photo-Optical Instrum. Engineers, San Jose, CA, paper 237SB-29 (February 1995).
- T. F. Johnston Jr. and J. M. Fleischer, Appl. Optics 35(10) 1719 (April 1996).
- D. Peterman, J. Guttman, Laser Focus World 37(7) 149 (July 2001).
- J. L. Guttman, J. M. Fleischer, A. M. Cary, "Real-time goniometric radiometer for rapid characterization of laser diodes and VCSELs," presented at Laser Beam Optics Characterization VI, Munich, Germany (June 2001).
- J. L. Guttman, NIST Special Publication 953: Technical Digest of the Symp. Optical Fiber Measurements 2000, 69.
- J. L. Guttman, NIST Special Publication 988: Tech. Digest Symp. Optical Fiber Measurements 2002, 33.
DERRICK PETERMAN is a sales engineer with Photon-Inc., 6860 Santa Teresa Blvd., San Jose, CA 95119-1205; e-mail: firstname.lastname@example.org.