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Lasers & Sources

A rugged optical pulse source for low-timing-jitter applications

A low-jitter optical clock source that is both inherently robust to thermal effects and offers electronically-tunable-repetition-rate optical pulses has been constructed from standard telecommunications components.
10 January 2007, SPIE Newsroom. DOI: 10.1117/2.1200611.0532

Stable clocks are an essential sub-component of many high-precision systems, such as the analog-to-digital converters (ADCs). ADCs are critical building blocks in a wide range of processing systems—converting real-world analog signals like audio signals and radar returns into discrete binary sample form—because they enable subsequent analysis, manipulation, and storage of digitized analog information by today's powerful digital systems. Photonic technology can improve electronic ADC performance by providing optical pulses with extremely low-timing jitter: an optoelectronic ADC has a photonic front-end for analog sampling that benefits from many attractive properties of optics including very-high-input bandwidths, interleaved architectures with no additional jitter penalties, and very-low-timing-jitter optical pulse sources.

In order to sample very rapidly changing analog signals at exactly the correct time, a low-timing-jitter pulse source is essential (see Figure 1). Pulse rates of many gigahertz may be required, depending on the ADC application, and the repetition rate of the pulse train must satisfy the Nyquist sampling theorem. Moreover, the clock duration must be short enough to ensure that the analog signal level does not vary unacceptably during the sampling aperture.

Figure 1. Pulse timing jitter (τjitter) leads to significant error in the sampled value for rapidly varying signals. Similarly, a very short sampling pulse minimizes the amount that the unknown signal varies during the sampling aperture. ADC: analog to digital converter.

Some of the lowest published timing-jitter values for optical pulse sources relate to mode-locked laser systems locked to optical combs, which, in turn, are locked to atomic transition lines.1 The high complexity of this approach, however, limits its widespread use. More practical state-of the-art optical sampling sources include gain-switched diode lasers, with published timing jitters around 200fs,2 and fiber-ring mode-locked lasers with timing jitter values an order of magnitude lower.3,4 The most stable gain-switched laser pulses are achieved by seeding an extended cavity with a portion of the output pulse train to reduce spontaneous emission effects. But both gain-switched and fiber-ring lasers suffer from the fact that the output repetition rate of the laser is fixed by the round-trip time of the (extended) laser cavity. Thus, any variation in temperature often results in the repetition rate varying unacceptably for high-precision ADC sampling applications. Temperature variations can be compensated for—by actively stressing the fiber, for example—but frequent calibration may be required if operating such pulse sources over an extended period.

We report an alternative source of very-low-timing-jitter optical pulses that is based on soliton-effect compression of a periodic input waveform.5 Attractive features include an electronically-tunable repetition rate and an inherent robustness to thermal variations. In addition, the set-up comprises only standard telecommunications components, none of which require any precise alignment or specific environmental controls beyond those afforded to standard scientific equipment.

In this system, the initial periodic input waveform is generated by driving an optical amplitude modulator with the single-frequency output from a low-phase-noise microwave oscillator (see Figure 2). Through the technique known as soliton-effect compression, the periodic waveform is then compressed by a combination of nonlinear and dispersive effects to durations on the order of 10ps. Standard single-mode fiber has been used to compress the pulses to ∼10ps. Models have shown that sub-picosecond-duration pulses should be generated using alternating sections of single-mode and dispersion-shifted fiber.

Figure 2. A low-phase-noise oscillator drives an optical amplitude modulator to generate a periodic optical waveform, which is then amplified and launched into a compression fiber, generating an extremely stable optical pulse train.

This scheme has achieved timing jitters as low as ∼250fs over the single-sideband (SSB) frequency-offset range from 100Hz-10MHz. At present, the close-in SSB phase noise (and hence the timing jitter) is dominated by the microwave oscillator used to generate the initial periodic waveform. In the future, this oscillator will be upgraded to reduce the pulse timing jitters substantially: on the order of tens of femtoseconds.

In summary, an extremely-stable optical pulse source has been implemented from standard telecommunications components using soliton-effect compression. This new pulse source offers tunable-repetition-rate optical pulses with both the repetition rate and timing jitter inherently robust to thermal effects. Presently, this scheme offers comparable timing jitter performance to the very best gain-switched laser systems. In the future, the combination of a lower-phase-noise microwave oscillator and dispersion-tailored fiber should provide optical pulse performance that begins to rival state-of-the-art fiber-ring mode-locked lasers, but in an economic and robust package. With the characteristics of electronically-tunable repetition rates and ultra-stable sub-picosecond pulse durations, this new pulse source promises to open up new functionality in such applications as high-resolution optical sampling and distributed clock systems.

Gregor McDonald
Malvern, United Kingdom

Gregor McDonald is a research scientist in the Photonic Structures and Processing group at QinetiQ in the United Kingdom. His work has primarily been involved in the development of high-speed optoelectronic correlators and optoelectronic analog-to-digital converters. In addition, he has recently presented papers at SPIE conferences on several topics, including optical correlation, high-speed digitization, and low-jitter optical pulses.

Alwyn Seeds
University College London
London, United Kingdom

Alwyn Seeds is the head of both the Department of Electronic & Electrical Engineering and the Ultra-fast Photonics and Optical Networks Group at University College London in the United Kingdom. He has authored over 250 papers on microwave and optoelectronic devices and their systems applications.