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Ultrafast Coherent Optical Signal Processing In spite of tremendous advances in optical communications, the methods most often used to encode and decode data on an optical carrier remain primitive: most systems still rely only on the presence or absence of an optical pulse to represent a binary "1" or "0" without controlling the phase or frequency of the optical signal. The prevailing strategy for increasing the data capacity has been to simply add more wavelength channels, or (to a lesser extent) to increase the speed of each channel. Coherent optical communication opens the possibility of communicating more efficiently by utilizing both the amplitude and phase of the optical signal to convey information. Unfortunately, even with coherent modulation formats that transmit multiple bits per symbol, the symbol rate is still limited by the speed of electronic receiver circuits. One recent approach to coherent optical detection is to replace the local oscillator with a train of ultrashort optical pulses. These optical pulses can serve as gating pulses for the coherent detection in much the same way that short electrical pulses are used in a sampling oscilloscope. The temporal resolution of such a system is determined not by the electronic speed by instead by the optical pulsewidth, which can be an order of magnitude shorter than the fastest electrical pulses. Most existing techniques for optical gating rely on nonlinear optical processes such as sum-frequency generation, two-photon absorption, or four-wave mixing to produce an intensity cross-correlation between the two optical signals. At least two companies now sell commercial products that are capable of recording optical signals as fast as 320 Gb/s by using short-pulses and non-linear optical sampling. One key shortcoming of nonlinear techniques is that they require a significant optical power in order to achieve a measurable nonlinear interaction. For this reason, nonlinear optical sampling systems are not very useful for measuring extremely weak optical signals. Furthermore, nonlinear processes like two-photon absorption or sum-frequency generation yield no information about the phase of the optical signal, hence they can only be used to diagnose amplitude-modulated signals. The idea of using coherent linear techniques for ultrafast signal characterization was demonstrated only recently by C. Dorrer and others at Lucent Technologies, who showed that by using phase-diversity coherent optical detection it is possible to optically sample on-off-keyed optical signals as fast as 640 Gb/s and phase-shift-keyed signals up to 40 Gb/s. In the few implementations reported so far, the optical sampling pulses were produced at low rates (below 100 MHz) from a passively-modelocked fiber laser that is not synchronized or phase-locked to the data. Because the fiber laser and data source are not phase-locked, this system requires off-line computation to subtract out any relative phase drift between the signal and LO.
Fig. 1. (a) Optical spectrum of a coherent system in which the local oscillator is replaced with a short-pulse laser. The broad spectrum of the laser overlaps with the signal to be sampled. (b) The signal and local oscillator are mixed in a optical hybrid. Because the Hybrid detector uses balanced detection, a non-zero output signal can be produced only if both signal and LO inputs are present. The ultrafast optical pulses therefore act as a gating signal for the coherent detection. (c) The resulting electrical signals I(t) and Q(t) each contain an electrical pulse whose height is proportional to the instantaneous in-phase and quadrature components of the signal at the sampling instant. The scope of potential applications for this technique could be greatly expanded if one were to instead use ultrashort pulses that are synchronized and phase-locked with the data signal to be diagnosed. Under these conditions, one can imagine for example using coherent ultrafast detection to extract a 10 Gb/s tributary data stream from a 400 Gb/s optically encoded QPSK signal. We envision several potential applications of this technology including: coherent serial-to-parallel conversion, ultra-high speed A/D conversion, high-speed phase-sensitive temporal demultiplexing, and ultrafast optical sampling and constellation visualization. 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.
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