Fiberoptic-gyro assembly challenges power-meter designers

The automated production-line assembly of fiberoptic gyroscopes (gyros) re quires both quick and accurate optical-power measurements. A critical part of the Litton (Woodland Hills, CA) fiberoptic-gyro assembly process involves connecting fibers to an integrated optical chi¥that operates as a beamsplitting/recombining element. The fibers must be aligned and attached to the chi¥waveguides to within 0.1 µm in all three axes.

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Fiberoptic-gyro assembly challenges power-meter designers

Ron Hartmayer, Ike Song, and William Muller

The automated production-line assembly of fiberoptic gyroscopes (gyros) re quires both quick and accurate optical-power measurements. A critical part of the Litton (Woodland Hills, CA) fiberoptic-gyro assembly process involves connecting fibers to an integrated optical chi¥that operates as a beamsplitting/recombining element. The fibers must be aligned and attached to the chi¥waveguides to within 0.1 µm in all three axes.

When Litton began production of these devices, operators initially performed the alignment manually, adjusting fiber position with piezoelectric-driven micropositioners while monitoring system output to determine the alignment that maximized light transmission of the system. To speed assembly, engineers decided to automate the alignment process. A stand-alone optical power meter connected to a computer via an IEEE 488 link was used to guide the motorized stages that performed the alignment. While this dramatically reduced assembly time, Litton recognized that further improvements could be achieved. The alignment stages were capable of moving much faster than the rate at which data were being supplied to them; thus, the bandwidth of the optical power meter became the limiting factor in the assembly process.

Until recently, no commercially available optical power meter offered the combination of performance and price needed to further expedite gyro manufacturing. Litton engineers outlined three major goals for the design of a new power meter. First, unit speed should be increased to at least 500 Hz, an order of magnitude faster than the existing meter; second, cost should be reduced by at least a factor of two; third, size should be decreased by replacing independent, rack-mounted display modules for instrument control and readout with the PC already in use in the system. Newport Corp. (Irvine, CA) engineers tackled this project.

Other performance specifications for the instrument included seven decades of linear response, with current sensitivity down to the femtoampere range. Because measurements were required at several different places in the assembly area, Litton engineers requested multichannel capability to allow the replacement of several meters with one.

Design and performance trade-offs

To best meet the size and cost constraints, the engineering grou¥at Newport divided the instrument into two main functional segments. All the processing-intensive tasks (filtering, signal averaging, display control, and so forth) were placed on an IBM PC-compatible expansion card. The detector preamplifiers and analog-to-digital (A/D) converters were put in a separate "breakout box" connected to the host computer by a cable u¥to 10 m long.

The modular approach eliminates the communications overhead of an IEEE 488 interface and allows the instrument to make direct use of powerful PC microprocessors commonly available for time-intensive processing. The expansion card contains a dedicated microprocessor and memory that can optionally be used to perform signal processing, which is important when the PC is providing real-time control of other instruments, such as motion controllers. Finally, locating all the sensitive analog electronics in the breakout box isolates them from the electronic noise of the PC environment and further enhances performance. It also provides convenient physical access to the detector connections in the work area, rather than locating them in the back of the PC.

The conflicting needs for speed and accuracy presented the most significant hurdle to the power-meter design. The fiber-alignment process begins with a prealignment ste¥in which transmission may be near zero; speeding u¥this part of the process requires both high gain and high speed. Prealignment must be followed by final alignment, precise, low signal-to-noise ratio (S/N), and National Institute of Standards and Technology (NIST) traceable power measurements of the final system throughput for specification and calibration purposes.

These goals are in direct conflict; it is difficult to design an amplifier that delivers a high S/N while also maintaining speed (or high bandwidth). Increasing measurement sensitivity requires higher amplifier gain, but, in general, bandwidth is inversely related to the value of this gain.

We solved the problem by realizing that the fiberoptic-gyro alignment application did not require a power meter capable of both high speed and high accuracy simultaneously. The task was thus reduced to the design of an instrument permitting user-selectable optimization for high-speed or for high-accuracy operation. We next examined each functional part of the system to minimize trade-offs.

Component design

Most commercially available amplifiers are designed for use with a range of detectors and detector sizes. Larger detectors are naturally limited to lower bandwidths, so we decided to use a maximum detector size of 3 mm or less; because the application involves capturing the output from an optical fiber, this presents no practical limitations to the process. Each channel of the meter incorporates a modular preamplifier, optimized for the particular type of detector in use. A calibration module, containing an EPROM, is used with each detector head to enable calibrated measurements.

To assure good dc performance--low noise measurements of continuous-wave sources--we limited the amplifier bandwidth to about 10 Hz. To reduce the trade-off between noise and speed, the meter provides a range of user-selectable filters. Filter values are chosen for each channel through system software. For example, in the fiberoptic-gyro assembly, a relatively low-frequency filter that permits high-speed measurements is used during the alignment stage. Once that process is finished, the filtering is switched to a very low bandwidth value to acquire a high-accuracy dc power measurement for final quality control or calibration purposes. This approach allowed us to produce an instrument capable of achieving almost 4-kH¥throughput when operating single channel, or 1 kH¥per channel when operating in four-channel mode.

The analog signal from the detector preamplifier must be converted to a digital format for it to be further processed by the instrument`s microprocessor. This conversion also involves a compromise between resolution and speed. Once again our solution was to make this function user-selectable, with two different A/D converters. In fiberoptic-gyro alignment, a fast sampling unit provides 12-bit resolution at rates approaching 100 kHz. The second A/D converter delivers 15-bit resolution at low frequency (about 100 Hz) for dc gyroscope calibration.

The Litton application involves the continuous measurement of optical power levels ranging from negligible values to milliwatts. The power meter therefore must provide high-speed automatic switching between gain ranges. Low-noise electromechanical relays have an unacceptably large (milli second regime) settling time. Analog switches can deliver the speed, but usually suffer from high leakage current that leads to offset errors, or unacceptably high resistance, producing gain errors.

To solve this conflict, we chose a hybrid design that uses a combination of ultralow-leakage analog switches and very-high-speed relays (both electro-mechanical and solid-state) to achieve optimum switching time for each gain range. While this approach places stringent demands on component quality, we found it to be the most effective way to optimize instrument performance for a wide range of measurement scenarios.

New power meter

The result of this engineering effort was Newport`s 4832-C, a four-channel optical power meter that met the performance, cost, and size goals set by Litton (see photo on p. 141). This instrument can measure optical power ranging from picowatts to several watts. It also provides accurate, NIST-traceable dc power measurements, as well as high-speed response over a spectral range of 250 to 1800 nm.

With this combination of benefits, we expect to see the power meter used for many fiber-related measurement tasks. For example, the manufacture of passive fiberoptic splitters involves heating a fiber and measuring the splitting ratio dynamically as the fiber is pulled. This application requires both measurement speed and resolution. In the production of fiberoptic wavelength-division multiplexers, the 4832-C offers the capability to perform measurement of several outputs at once.

Outside of the fiber area, applications such as raster scanning for obtaining laser-beam profiles or light measurements in scanning systems will benefit from the instrument`s combination of speed, sensitivity, resolution, and accuracy. o

Click here to enlarge image

Compact power meter for precision fiber alignment provides high-speed, high-resolution performance.

Ron Hartmayer is with Newport Corp., 1791 Deere Ave., Irvine, CA 92714. Ike Song is with Litton Industries Guidance and Controls Systems Division, 5500 Canoga Ave., Woodland Hills, CA 91367, and William Muller is with Park Scientific Instruments, 1171 Borregas Ave., Sunnyvale, CA 94089.

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