Beam Profiling Aids Fiber Optics Manufacturing

Dr. Derrick Peterman, John Fleischer and Dan C. Swain

In the fiber optics industry, one constantly hears that new markets will remain elusive unless production efficiency is improved. It is a common goal, therefore, to develop better processes. We propose beam profiling as a scalable improvement for the alignment of a variety of fiber optic components.

Fiber-lens alignment is required in the production of many components, including optical switches, multiplexer/demultiplexers and filters. The light from the end of a bare fiber is highly divergent, and substantial power is lost in the fiber optic component unless a lens is used to collimate the beam. A fiber-lens pair is needed whenever the light from the fiber must pass through an optical element, such as a grating, a filter or a mirror.

A fiber-lens pair is required whenever the light from a fiber passes through an optical element. Lenses may be contact-bonded to the fiber (above) or precision-aligned in proximity to it (left). Images courtesy of Photon Inc.

After the light has interacted with the elements, it is put back into a fiber via a lens coupled to the destination fiber. The lens can be contact-bonded to the fiber or precision-aligned in proximity to it. Rotational and linear alignment of the fiber are critical prior to bonding.

Backreflection’s problems

The goal in alignment is to minimize insertion loss as the light passes through the fiber-lens pair; the backreflection technique is often employed. A mirror reflects the light from the lens into the fiber, which couples a power meter to measure the backreflected light. The coupling is adjusted, often manually, until the backreflected power is at maximum. To a degree, the insertion loss can be determined by reading the power meter.

Beam profiling quantifies orthogonal profiles, power, and beam width and position, and offers a three-dimensional representation of the beam.

The technique is common throughout the industry, but it suffers from several problems. For example, because the operator usually manually adjusts the mirror, the process is open to variation and is dependent upon operator skill.

Ideally, the process would be automated, but it is difficult to determine a robust algorithm for optimal coupling. Moreover, it is difficult to quantify the process other than simply minimizing loss. Parameters that may be critical to coupling, such as the pointing of the beam, are only inferred. In fact, one can determine if a fiber-lens pair is “lossy,” but not the source of that loss.

The backreflection technique is commonly used to minimize insertion loss. A tip-and-tilt mirror reflects the light through the fiber-lens pair. The technique suffers from several problems.

Again, at best, only one parameter is quantified, and low insertion loss does not necessarily indicate good optical performance because there are a host of optical alignment issues to consider.

Quantifying coupling

In contrast, profiling the light as it is emitted through the fiber-lens pair provides a quantifiable, repeatable means to improve and automate this process. Profiling the light yields more information than the backreflection technique. The cause of poor alignment can be identified and tracked from the beam-profile data, enabling the appropriate corrective actions to be taken before value is added to that assembly or process.

Parameters that quantify fiber-lens coupling are beam width; beam position or pointing; beam collimation or waist along the optical axis; general features of beam profile, including Gaussian fit; and beam power:

• Beam width. The beam width is a function of the distance between the fiber and the collimating lens when measured at positions along the optical axis of the beam. In some cases, beam width at a given distance from the lens quantifies coupling and correlates to the insertion loss of the lens coupling.

Profiling the light from the fiber-lens pair offers more information than the backreflection technique, enabling the development of automated alignment processes.

• Beam position/pointing. If the light emerges at an angle to the optical axis, much of its power will be lost in the device. A component that displays low insertion loss but features incorrect beam pointing often fails to deliver optimum system performance. Beam pointing is especially important for microelectromechanical optical switching applications, in which the alignment of the light on the reflecting mirror is critical.

• Collimation/beam waist. Measuring the beam width at points along the optical path indicates collimation. For a collimating lens, it directly measures the performance of the lens and its alignment with the fiber. In addition, coupling the light between two fiber-lens pairs is at a maximum when the positions of the waists overlap. Collimation measurement can be automated by mounting the beam profiler onto a motorized stage, enabling accurate and quick positioning while collecting the data.

As fiber arrays become more common, it is increasingly important to be able to analyze multiple beams simultaneously. Here, beam profiling analyzes three beams (top). For complex arrays (bottom), the profiler can be repositioned to collect data from various parts of the array and at different working distances.

• General features. A profile that deviates significantly from a Gaussian may indicate a defect in the lens or poor alignment between the elements. A non-Gaussian profile may indicate the presence of higher-order modes in the beam. Because different modes propagate at different speeds, mode dispersion will increase the bit error rate introduced by the optical component.

• Power. Some profiling instruments also measure beam power, enabling the simultaneous quantification of transmitted power and beam profile.

The profiling process

Based on our experience with beam-profiling instrumentation and on the requirements of the fiber-lens alignment process, we recommend the scanning-slit technique for this application.^1,2 It offers submicron accuracy and resolution of the beam position and width. Slits can be manufactured as small as 1 μm, and data sampling can occur at the submicron level. The single detector helps to ensure a uniform response across the detection plane. Slit-based profilers also measure beam power with high accuracy and resolution.

The scanning-slit technique is suited to alignment by beam profiling. Light emerging from the fiber-lens pair passes through the slit onto a photodetector. The photocurrent as a function of the position of the slit generates the profile.

But whatever type of profiler is selected, it is critical that the instrument have a means of communicating with the other components in the manufacturing process, such as through ActiveX, GPIB command sets or a software-development kit for automation.

The process looks like this:

• From an initial position, the fiber or array is illuminated. The beam profile, along with other relevant parameters, is acquired.

• Parameters measured from the profile are loaded into an alignment algorithm that controls the fiber or array position. The algorithm is based on previous empirical results and can include positioning formulas, look-up tables, offset calculations or various combinations of data processing and measurements.

• In this closed-loop process, the fiber or array is actively positioned until the fiber-lens alignment meets the predetermined acceptance criteria.

• Once the beam profile meets quality-assurance limits, epoxy is applied to fix the alignment. During the curing process, profile data is sent back to the positioning system to maintain accuracy. The components are cured completely before the assembly is removed from the alignment tool for the next production step.

This technique offers scalability, repeatability and feedback on the production process. It can display varying levels of automation, depending on the required throughput and the maturity of the process. The alignment can be performed manually, or it can be automated using positioning mechanisms. As the process becomes better understood, or as factors dictate improvement, additional automation can be introduced.

This method produces repeatable components because more than power loss data is used. Fiber-lens pairs or arrays may display low loss, but they also can produce different beam widths or beam pointing and, therefore, may act differently in a larger optical assembly. Several parameters can be used to quantify alignment with beam profilers so that the fiber-lens pairs or arrays can be optimized and manufactured in a much more controllable manner day by day.

Finally, if a fiber-lens pair or an array fails, analyzing the data that are collected with that part identifies the cause quickly. Contrast this with the backreflection method, which fails a part without offering further information to guide the process engineer. In addition, because alignment is monitored over time, if the process is in danger of yielding numerous product failures, the engineer can intervene before a catastrophic number of failures occur. Conversely, the parts that show unusually high quality can be analyzed for clues to optimize the process.

As the fiber optic industry evolves, production efficiency must improve. There is an ongoing industrywide debate as to the best approach to automated production. Some preach standardization; others closely guard their proprietary methods.

The path for manufacturing

Many in the investment community predict that production efficiencies in fiber optics will follow the transformation in semiconductors to high-volume manufacturing, but the jury is still out regarding whether an industry built on electrons has something to offer one built on photons. Whatever the outcome, we expect beam-profiling instrumentation to play a significant role.

References

1. T.F. Johnston and J.M. Fleischer (April 1995). Measurement of the absolute accuracy (to <0.5%) of a clip-level beam profiler using Fresnel diffraction by a wide slit. Proc. SPIE, pp. 234-240.

2. T.F. Johnston Jr. and J.M. Fleischer (April 1996). Calibration standard for laser beam profilers: method for absolute accuracy measurement with a Fresnel diffraction test pattern. Applied Optics, pp. 1719-1734.

Meet the authors

Derrick Peterman, John Fleischer and Dan C. Swain are a sales engineer, the president and an automation engineer, respectively, at Photon Inc. in San Jose, Calif.