It’s all in the alignment (of fiber-optic waveguide modulators)

Fiber-optic waveguide modulators used in communications networks. The modulators, which often require special connectors, work in conjunction with polarization-maintaining (PM) fibers, mostly Panda fibers.

Replace standard angled physical contact (APC) fiber-optic connectors with those specified by customers, typically physical-contact (PC) connectors. Properly align the replacement connector for maximum on/off ratio.

• Agilent Technologies: optical power meter. www.tm.agilent.com.
• Lightel Technologies: polarization-maintaining laser light source. www.lighteltech.com.
• Tektronix: function generator. www.tektronix.com.

Lightel Technologies (Kent, WA; www.lighteltech.com) manufactures fiber-optic components that use polarization-maintaining (PM) fibers and PM connector terminations. The company typically uses angled physical contact (APC) connectors, but because some applications require waveguide modulators with physical contact (PC) connectors, customers often send their modulators to Lightel for a replacement connector.

A power meter measures optical power while an operator aligns a modulator’s connector to its fiber.

Fiber-optic connectors have alignment keys, and Lightel's operators visually align the polarization axis of the PM fiber to the key. Known as Panda fibers, the PM fibers look like the face of a panda when viewed as a cross section (Ref. 1, 2).

The alignment key provides a passive visual alignment, but there’s still room for error. To optimize on/off ratio, operators apply power to the modulator and rotate the connector about 1° to 2° while monitoring its output. This active alignment improves the ratio from 15 dB to 25 dB.

An automated system (figure) produces PM light, controls the modulation, and measures output power swing at the connector. Operators use the power meter’s min/max measurements to monitor the on/off ratio while they manually align the connectors to the fiber.

A PM laser light source produces light for the modulator, and a function generator produces the modulating signal—a 10-Hz sine wave. Operators rotate the connector and look for the largest on/off ratio, which occurs when the modulator’s transfer function is at its peak as opposed to its trough.

“A sine wave provides continuous modulation voltage so the power meter can catch all possible modulator output levels,” said Shyh-Chung Lin, Lightel’s VP of business development. “We match the frequency of the modulation signal to best match the sample rate of the power meter.

“The power meter takes 10 ms to 20 ms per sample, so the operator must wait until the meter samples the maximum and minimum light levels. When alignment is achieved, the operator glues the connector to the fiber. An alignment takes about 30 min.”

Lin added, “Active alignment and in-situ monitoring is crucial to achieve the best performance in PM fiber with a waveguide modulator. With some engineering creativity, we were able tackle this measurement challenge using common test equipment.”