Ultrafast Optical Clock Recovery

Clock recovery is one of the most important and sometimes overlooked functions that is required of any optical transciever or regenerator. Many published papers on optical switching and all-optical logic tacitly assume that you begin with optical signals that are synchronized with one another. The goal of this project is to build a system for achieving this synchronization, even when one or both of the two optical signals are too fast to be seen with a conventional photodetector.

Most current clock recovery systems use a fast photo-detector followed by an electrical clock recovery circuit to synchronize clock with the incoming data. The limited speed of photo-detectors and electrical circuits prevents these systems from being used in future optical time division multiplexed (OTDM) systems where the single-channel data rate could exceed 40 Gb/s. Optical clock recovery systems can overcome this limitation by using ultrafast nonlinearities to replace high-speed electronic components. Several nonlinear processes have been exploited for this purpose including four-wave mixing in fiber or semiconductor waveguides, cross-absorption modulation in an electroabsorption modulator, and phase-modulation in semiconductor amplifiers. Other approaches utilize injection locking or self-seeding in laser cavities.

We are investigating clock recovery systems that utilize two-photon absorption (TPA): a process in which two photons are simultaneously absorbed in a photodiode to generate a single electron-hole pair. Unlike conventional photodetection, two-photon absorption produces a photocurrent proportional to the square of the optical power. Because of this quadratic nonlinearity, two-photon absorption can be used to measure the correlation between high-speed optical signals, in much the same way that sum-frequency generation is used in optical autocorrelators to diagnose pulses that are too short to be measured electrically.

Contrary to popular believe, two-photon absorption can be a relatively efficient nonlinear process, compared to other nonlinear effects. We routinely observe two-photon absorption at (CW) power levels in the milliWatt range. Some of the most sensitive autocorrelation measurements reported to date have used two-photon absorption. Two-photon absorption (TPA) has several features that make it attractive for clock recovery including simplicity, wide optical bandwidth, ultrafast response time, and polarization insensitivity. One real benefit of this work is that you can perform ultrafast clock recovery using an inexpensive silicon photodetector.

Fig. 1. Cross-correlation between 80 Gb/s NRZ data and 10 GHz clock, measured using two-photon absorption in a silicon avalanche photodiode. The two curves shown here represent the minimum and maximum signals obtained by adjusting the data polarization state.

Fig. 2. Schematic of 80 Gb/s to 10 GHz sub-harmonic optical clock recovery system based on two-photon absorption.

Fig. 3. (a) Measured RF spectrum and (b) single-side-band phase noise of original clock signal and recovered clock.

References

Note: This is not a comprehensive list of references such as you might find in a well-written journal article; it is provided only as a suggested starting point for visitors interested in learning more about this subject.