Generating Multiple Frequency Combs

Artist’s rendering of multiplexed optical pulses in a crystalline resonator. (Source: Woogieworks)

Ultra­short optical pulses are becoming more and more relevant in a number of appli­cations including distance measurement, molecular finger­printing and ultrafast sampling. Many of these appli­cations rely not only on a single stream of pulses of optical frequency combs but require two or even three of them. None­theless, these multi-comb approaches signi­ficantly speed up acqui­sition time over conven­tional techniques.

These trains of short optical pulses are typically produced by large pulsed laser sources. Multi-comb appli­cations therefore require several such lasers, often at prohi­bitive costs and complexity. Further­more, the relative timing of pulse trains and their phases must be very well syn­chronized, which requires active elec­tronics that synchronize the lasers. Now, the research team of Tobias J. Kippen­berg at EPFL, together with the group of Michael Gorodetsky at the Russian Quantum Centre, has developed a much simpler method to generate multiple frequency combs. The tech­nology uses small optical micro­resonators to create optical frequency combs instead of conven­tional pulsed lasers.

The micro­resonator consists of a crystal­line disk of a few milli­meters in diameter. The disk traps a continuous laser light and converts it into ultrashort soliton pulses thanks to the special nonlinear properties of the device. The solitons travel around the micro­resonator 12 billion times per second. At every round, a part of the soliton exits the resonator, producing a stream of optical pulses.

The micro­resonator the researchers used here has a special property in that it allows the light to travel in the disk in multiple spatial modes of the resonator. By launching continuous light­waves in several modes at the same time, multiple different soliton states can be obtained simul­taneously. In this way, the scientists were able to generate up to three frequency combs at the same time. The working principle is the same as spatial multi­plexing used in optical fiber communi­cation: the infor­mation can be sent in parallel on different spatial modes of a multimode fiber. Here, the combs are generated in distinct spatial modes of the micro­resonator.

The method has several advantages, but the primary one is that it does not require complex synchro­nization electronics. “All the pulses are circu­lating in the same physical object, which reduces potential timing drift, as encountered with two independent pulsed lasers,” explains Erwan Lucas. “We also derive all the continuous waves from the same initial laser by using a modulator, which removes the need for phase synchro­nization.” Using this multi­plexing scheme, the team demon­strated several appli­cations, such as dual-comb spectro­scopy, or rapid optical sampling. The acqui­sition time could be adjusted between a fraction of a millisecond to 100 nanoseconds.

The researchers are now working on developing a new demon­stration with the triple-comb source: “We had not planned for a demon­stration, as we did not expect our scheme to work so easily,” says Lucas. “We are obviously working on it.” The tech­nology can be integrated with both photonic elements and silicon micro­chips. Establishing multi-comb generation on a chip may catalyze a wide variety of appli­cations such as integrated spectro­meters or LIDAR, and could make optical sensing far more accessible. (Source: EPFL)

Reference: E. Lucas et al.: Spatial multiplexing of soliton microcombs, Nat. Phot., online 1 October 2018; DOI: 10.1038/s41566-018-0256-7

Link: Institut de Physique, École Polytechnique Fédérale de Lausanne EPFL, Lausanne, Switzerland

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