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 — also known as optical frequency combs — but require two or even three of them. 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 lasers, often at prohibitive cost and complexity. Furthermore, the relative timing of pulse trains and their phases must be very well synchronized, which requires active electronics to synchronize the lasers.
EPFL Researchers have developed a much simpler method to generate multiple frequency combs. The technology uses small devices called optical microresonators instead of conventional pulsed lasers.
The microresonator consists of a crystalline disk a few millimeters in diameter. The disk traps a continuous laser light and converts it into ultrashort pulses — solitons — using 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 has the special property that it allows the light to travel in the disk in multiple different ways, called spatial modes of the resonator. By launching continuous light waves 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 the spatial multiplexing used in optical fiber communication, where 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 primarily that it does not require complex synchronization electronics. All the pulses are circulating in the same physical object, which reduces the potential timing drift possible with two independent pulsed lasers. Since the continuous waves are all derived from the same initial laser by using a modulator, there is no need for phase synchronization.
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 technology can be integrated with both photonic elements and silicon microchips, which could lead to applications such as integrated spectrometers and LIDAR, and could therefore make optical sensing far more accessible.
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