Inspired by the works in progress in the field of optical fiber communication, a team of researchers has developed an optical sensing system based on spatial multiplexing — the simplest of its kind yet. The system is implemented through the use of a technique which uses a single continuous-wave laser and a single optical microresonator to generate different independent streams of ultrashort optical pulses.
Ultrashort optical pulses, of which a single stream of pulses is quantifiable in terms of an “optical frequency comb”, are gaining wider applicability as researchers are finding ways to deploy the technology with multi-comb approaches. Some of the most advanced applications include distance measurement, molecular fingerprinting and low-noise microwave generation. The multi-comb approach is the main driving force behind the advances in these applications and is preferred over the traditional techniques because it fast-tracks acquisition time.
However, a major limitation has been the prohibitive costs and complexities which the acquisition of multiple large pulsed laser sources for the implementation of multi-comb approaches entail. To boot, the relative timing of the lasers’ pulse streams and their phases must be synchronized by active electronics that are highly precise.
The Inroads Made by Researchers
The latest breakthrough in the sphere of optical sensing comes as an outcome of the collaboration between research teams from EPFL and Russian Quantum Centre. The fruits of the research simplify the generation of multiple frequency combs using a small device named “optical microresonators” in place of traditional pulsed lasers.
A microresonator is made with a few-millimeters-wide crystalline disk which has distinctive non-linear properties that trap a stream of laser light and transform it into ultrashort pulses, known as solitons. And as a soliton travels around the microresonator at a rate of about 12 billion times per second, a part of it breaks away from the resonator, letting off a stream of optical pulses.
The distinctive properties of the microresonator used in this research imbue it with multiple spatial modes. The spatial modes are the ways in which light can travel in the disk. When continuous lightwaves travel through the diverse spatial modes of the resonator, solitons emerge in various states simultaneously. As a result, the microresonator provides the researchers with up to three frequency combs simultaneously.
The underlying mechanism of the microresonator is akin to that of optical fiber communication. Different combs emerge from each active spatial mode of the resonator in the same way that information passes through various parallel spatial modes of a multimode fiber.
However, the advantage of simplicity which the microresonator comes with is based particularly in the fact that it functions optimally without the need for active electronics to synchronize it. Since all the pulses are traveling within the same physical object, the time drifts which attend the use of two independent pulsed lasers is significantly averted. Phase synchronization is irrelevant in this technology since a modulator can be used to generate different streams of continuous waves in the same laser.
What This Latest Innovation in Optical Sensing Systems Holds for Distance Detection
Distance detection is a highly critical element in technologies such as robotics, autonomous vehicles, and satellites. Optical frequency comb technology brought about ground-breaking absolute distance measurements, in addition to larger ambiguity range and faster updates. But although it out-performed the traditional ranging technologies in terms of various performance matrices, its cost and bulky form factor prohibited its widespread application.
However, the latest advancements in optical sensing systems have brought about cheap, portable miniature frequency combs (microcombs), together with capabilities for soliton mode-locking that further harnesses the special properties of the microcombs.
As super-sharp, multi-wavelength laser sources, the microcombs can imbue LIDAR systems with unprecedented capabilities for the highly precise measurement of longer-range absolute distances. To further enhance the precise distance measurement of these comb-based LIDAR systems, researchers have deployed the fractional frequency instability of rf time base and optical frequency to stabilize the comb’s frequencies.
Researchers have also found that by integrating an optical clock that has a top-draw fractional frequency instability of < 10-17 with the comb-based LIDAR, an nm-precise measurement of long-range distances of over 105 km (about the distance between earth and moon) can be made possible.
The latest innovation in the field of optical sensing systems based on spatial multiplexing has been shown to have an unprecedented range of applicability and to also make for acquisition times ranging from a fraction of a millisecond to 100 nanoseconds.
And as researchers are looking to integrate this innovation into silicon microchips, the prospects that this innovation brings will significantly boost the widespread use of optical sensing technologies such as LIDAR, spectrometers, etc.