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IBM researchers discover servo control method for optics

Posted: 15 Dec 2004 ?? ?Print Version ?Bookmark and Share

Keywords:ibm? thomas j. watson research? wavelength-locked loop? dsp?

IBM Corp. researchers from the company's server division and Thomas J. Watson Research Center have collaborated on an unusual project to bring the advantages of phase-locked loops to wavelength domains.

Though the most evident set of applications is in optical components, the generic capability of a "wavelength-locked loop" (WLL) as a servo control active-feedback loop applies to any wavelength in an electromagnetic domain.

Lawrence Jacobowitz, principal researcher at the Watson Center, began general explorations on optical technologies for server disaster recovery with the man responsible for setting technology road maps within the server division, Casimir DeCusatis. The duo looked at IBM's work in autofocusing for optical storage and explored ways to use the technology in tunable optical networks using common DSP devices. The resulting integrated circuit will find some of its earliest uses in tunable laser transmitters and filters, but the initial development effort was not quite so focused.

"We were studying fiber optics for any type of server or storage cluster, looking at how to make distributed storage cost-effective at distances out to 100 kilometers," DeCusatis said. "The study of protocol-neutral fiber interconnect led to some general exploration on wavelength-agile tunable optical components, but Larry's group did not set out with such a broad goal in mind."

The breakthrough Jacobowitz's team achieved occurred when the researchers studied the frequency doubling of dither modulation and realized that wave-length locking could offer some of the same advantages as phase-locked loops in electronics.

In electronic signal processing, PLLs were first proposed in the 1930s as a simple servo control mechanism to replace superheterodyne receivers. They became popular in the 1940s as a means of synchronizing signals for vertical and horizontal sweeps in early TV sets. PLLs first were implemented as analog ICs in 1965 and as digital-equivalent ICs in 1970. They are now used in virtually all radios, cellular phones, pagers and other communication equipment as a means of locking and synchronizing clocks.

Current PLL designs are based on a phase detector, voltage-controlled oscillator, loop filter and associated synchronization circuits. If the extension of the servo loop concept to wavelength locking proves as popular to use and as easy to implement as IBM researchers anticipate, the future of the WLL could be as revolutionary as the 70-year history of the PLL.

The active-feedback loop created by the WLL allows mutual alignment and tracking of a laser in real-time, enabling the tuning of the center wavelength of the laser source to precise alignment with any point in the passband of a spectral filter or other wavelength-selective device.

In some of the earliest experiments of the WLL method, Jacobowitz's team sinusoidally dithered the voltage bias, and a photodetector downstream could sense the intensity of the light transmitted as a slow sine-varying reference signal. This signal is carried along with the data signal created by traditional data modulation and does not affect the ability of the laser to transmit the data signal.

This dither-modulated signal can be used to create an electronic feedback loop, which continuously compares the data-modulated signal after it passes through the filter with the original dither-modulated input reference. The WLL generates an error signal, which represents the difference between the wavelength of the feedback signal and the original dither-modulated bias reference that induced the wavelength variation in the transmitted output of the laser.

Broadening concept

Jacobowitz said the application of WLL was broadened by applying concepts of frequency doubling to the locking. The wavelengths of the laser and the filter center are aligned when the error signal waveform frequency is two times that of the dither-modulation frequency. This frequency doubling represents the signature that locks the closed-loop circuit to any selectable value. The bipolar error signal uses opposite phases of the feedback signal and provides an indication of which direction and by how much the laser needs to be tuned.

The key to exploitation by common DSP techniques is that each position on the filter wavelength function swept by the laser exhibits a unique waveform, which can then be digitized and compared with values in the lookup table of a DSP device.

If one was designing a variable optical attenuator (VOA), intentional offsets could be introduced. Similarly, a DSP device could be combined with arrays of lasers or arrays of specific filters and other wavelength-selection devices to implement optical amplifier controllers, wavelength-dependent automated gain control blocks, dynamic gain equalizers, interleavers and similar optoelectronic devices. By tightening control of the wavelengths, the WLL method could allow much finer wave-division-multiplexed grids, based on 12.5GHz spacing as compared with current ITU-approved grids of 100, 50 and 25 GHz.

"In every case we've examined, using the WLL could substantially reduce the cost of designing and manufacturing standard optical building blocks," Jacobowitz said.

IBM has demonstration circuits it has developed to showcase its techniques, but Jacobowitz pointed out that "it isn't in our business model to offer optoelectronic circuits." Instead, the teams will offer design libraries with DSP macros, which might be licensed to merchant DSP vendors or offered within IBM's own ASIC libraries.

IBM has applied for more than 20 patents covering WLLs. Among them are separate patents on such applications as VOAs, silicon oxi-nitride waveguide switches, polarization-mode dispersion and frequency-guided filters for dispersion management. Also included are patents for semiconductor optical amps; erbium-doped optical amps that use WLL for AGC and dynamic gain equalization; wavelength modulation for MPLS switching and routing; and synchronous optical clock distribution.

Jacobowitz hinted that work has only just begun applying WLLs to RF systems. Wavelength-division multiple-access, free-space broadcast techniques for optical backplanes have already been disclosed in a patent application, and more purely RF implementations will follow.

- Loring Wirbel

EE Times

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