Mechanical profilers go digital

Built around a micromirror-based spatial light modulator, a mechanical beam profiler takes advantage of the micromirrors' digital qualities, making measurements that are intrinsically repeatable. Even fluctuating beams can be measured.

The optical-power spatial profile of a laser beam at a given beam-propagation plane is a critical design parameter for numerous optical-system applications, including laser radar, optical inspection and nondestructive testing, laser surgery, fiberoptic and free-space optical communications, optical tweezers for molecular manipulations, biomedical imaging, and semiconductor processing.

Currently, commercial optical profilers can be divided into two categories. The first is the charge-coupled-device (CCD)-based profiler, in which a 2-D pixelated imaging device takes a direct time-integrated snapshot of the optical-power 2-D profile. Typically, these profilers are effective for low-power (milliwatts) visible-beam measurements. Manufacturers have modified these CCD profilers for IR applications by using IR-sensitive phosphor films deposited over a visible-band CCD, thus producing indirect measurement of the IR beam. Such profilers suffer from spatial-resolution limits that result from charge leakage in the phosphor films. In addition, the films have a nonlinear output response to the incident light, requiring constant device calibration and additional software processing, which leads to a limited dynamic range for the profiler.

Conventional mechanical profilers

A more effective approach to profiling, although more costly (by a factor of four or more), is the mechanical profiler. In this case, the profiler operates by scanning a mechanical component, such as a knife-edge across the laser beam, and measuring the throughput power with a calibrated large-area photodetector or power meter. Optical-power readings are recorded for given knife-edge positions, then a software analysis converts measured knife-edge power data into a spatial-power plot. The knife-edge can move first in the x direction of the beam spatial coordinates and then in the orthogonal y direction of the spatial coordinates, leading to a complete 2-D spatial beam map.

Hence, mechanical profilers work with analog motion of the chosen mechanical sampling component. Because analog motion of the knife-edge can be controlled to micron-scale precision using sophisticated motion mechanics and electronics, a mechanical profiler can deliver high spatial resolution. In addition, a mechanical profiler can work with low and high optical beam powers over a large (for example, 40-dB) dynamic range.

Nevertheless, mechanical profilers require precise, repeatable motion of the mechanical element over a very large scan zone corresponding to the entire beam zone. For example, for a 10 × 10-mm scan zone with a 10-µm scan step, a total of a million scan-step positions are required to complete the mechanical profiling operation. This analog-motion operation implemented in 2-D must be extremely precise and completely repeatable over years of profiler operation. In reality, these analog-motion mechanical profilers, although excellent in performance, require constant calibration throughout their operating lives.

Today's CCD- and mechanical-based profilers cannot operate with laser beams that fluctuate in optical power during the profiling measurement. Although it is possible to tap optical power before making the profiling power measurement, one then must place a beam-spoiling component in the beam path—particularly disadvantageous at high powers where thermal lensing in the inserted tap component can modify the original beam profile of the laser beam being tested.

It would be desirable to have a mechanical beam profiler that could measure the profile of fluctuating beams and that would also have 100% measurement repeatability, eliminating the need for constant calibration. In addition, it would be highly attractive if the profiler had a cost-effective design to enable wide-scale, low-maintenance use in all industries that use lasers, including laser fabrication and testing.¹

Going digital

At Nuonics, a new type of mechanical profiler has been developed that, for the first time, takes advantage of the digital paradigm for beam profiling. Here, "digital" does not refer to the traditional use of analog-to-digital (A-D) electrical-signal converters that are used on the electrical signal produced by an analog-domain profiler, such as a CCD or a mechanical profiler with an analog power meter. Instead, digital refers to an intrinsically digital optical operation of the mechanical profiling process.

FIGURE 1. A power-independent optical beam profiler developed by Nuonics contains a Texas Instruments digital micromirror device (DMD), two large-area photodetectors (PD1 and PD2), and a processor, such as a computer.Click here to enlarge image

An implemented version of this digital profiler uses an all-digital 2-D reflective-mode optical spatial light modulator, such as the digital micromirror device (DMD) developed by Texas Instruments (TI; Dallas, TX). The DMD is a high-space-bandwidth device with nearly 1 million square micromirrors, each 13.8 µm per side for TI's IR design. The micromirrors are digital in operation, with two tilt states (for example, ±9.2° tilt angles), thus providing two symmetric beam-deflection ports for placement of large-area photodetectors (see Fig.1). In effect, the DMD serves as an all-digital software-programmable 2-D moving mechanical element that can implement beam-profiling methods such as knife-edge and pinhole measurements.

Because the locations of the micromirrors are always the same over the incident beam cross section, the software-implemented moving knife-edge has the same position each time the measurement steps are repeated, thus providing reliable data. The all-digital nature of the spatial light modulator (SLM) is what gives the proposed profiler its 100% profile-measurement repeatability. In addition, the profiler uses a pair of optical power meters, enabling instant normalization of total optical-power data. Hence, even if the beam being tested suffers optical power variations, the time-integrated detector pair provides an instantaneous power-referencing level for each profiler measurement.

FIGURE 2. A 2-D beam profile was generated by the digital profiler for a 1550-nm test laser beam from a semiconductor laser coupled to a fiber gradient-index lens. The beam was measured to be 0.48 mm in diameter (full-width half-maximum).Click here to enlarge image

Initial experiments have been performed with the proposed profiler for near-IR (a band centered on 1550 nm) and visible wavelengths. Beam profiles have been successfully obtained using the DMD-profiler 2-D knife-edge method (see Fig. 2). In addition, the profiler has also been successfully tested on a laser beam that had an intentionally time-varying optical-output power.²

Promising outlook

The digital approach results in a unique chip-scale mechanical profiler that holds traditional mechanical profilers' excellent attributes, such as high-power operation, high dynamic range, and broadband (UV, visible, and IR) functionality. More important, for the first time the proposed profiler takes advantage of the digital realm of optical-beam characterization, leading to intrinsic data-measurement repeatability and profiling flexibility via software implementation of various mechanical profiling elements within one compact unit.

In addition, the chip-scale design leads to cost-effectiveness for large-scale volume manufacturing of profilers (see Laser Focus World, April 2004, p. 152). The profiler also exhibits a laser-power-independent mode of operation and, via advanced options, is capable of providing very high-resolution (for example, submicron-scale) spatial-power maps over large 1 × 1-cm optical regions.³