Researchers have developed a new type of frequency comb that is expected to further improve the accuracy of timing

The chip based device, known as the frequency comb, measures the frequency of light waves with unparalleled accuracy, completely changing timing, detection of exoplanets, and high-speed optical communication.

Now, scientists and collaborators from the National Institute of Standards and Technology in the United States have developed a new method for manufacturing combs, which is expected to improve their already sophisticated accuracy and allow them to measure light in frequency ranges that were previously unattainable. The expanded range will enable the frequency comb to detect cells and other biological materials.

The researchers described their work on Nature Photonics. The team includes Fran ç ois Leo and colleagues from the Free University of Brussels in Belgium, Julien Fatome from the University of Dijon Burgundy in France, and scientists from the Joint Institute of Quantum Research, a research partner at NIST and the University of Maryland.

These new devices are manufactured on small glass chips, and their operation is fundamentally different from previous chip based frequency combs.
The frequency comb acts as a ruler of light. Just like the evenly spaced scale lines on a regular ruler measure the length of an object, the frequency spikes on a micro comb measure the oscillation or frequency of light waves.

Researchers typically use three elements to construct micro combs: a single laser, called a pump laser; A tiny ring resonator, the most important component; And a micro waveguide that transmits light between the two. The laser injected into the waveguide enters the resonator and competes in a loop. By carefully adjusting the frequency of the laser, the light inside the ring can become solitons - a solitary wave pulse that maintains its shape while moving.

Whenever the soliton completes a loop, a portion of the pulse will split and enter the waveguide. Quickly, a whole row of narrow pulses filled the waveguide, with each spike separated at the same fixed interval in time, which is the time required for the soliton to complete one cycle. The peak corresponds to a set of uniformly distributed frequencies and forms the scale lines or "teeth" of the frequency comb.

Although this method of generating micro combs is effective, it can only generate combs within the frequency range centered on the pump laser frequency. To overcome this limitation, NIST researchers Gr é gory Moille and Kartik Srinivasan collaborated with an international research team led by Miro Erkintalo from the University of Auckland in New Zealand and Miro Erkintalo from the Dodd Walls Center for Photonics and Quantum Technology to theoretically predict this new process, and then demonstrated the new process of generating soliton micro combs through experiments.

The new method does not use a single laser, but two pump lasers, each emitting light at different frequencies. The complex interaction between two frequencies produces a soliton with a central frequency located precisely between the two laser colors.

This method allows scientists to generate combs with new characteristics within a frequency range that is no longer limited by the pump laser. For example, by generating combs that span different frequencies from the injection pump laser, these devices can enable scientists to study the composition of biological compounds.

In addition to this practical advantage, the physical foundation of this new micro comb may bring other important advances. An example is the potential improvement in noise associated with a single tooth of a micro comb.

In a comb generated by a single laser, the pump laser only directly carves the center tooth. As a result, the farther the teeth are from the center of the comb, the wider the teeth will be. This is not advisable because wider teeth cannot accurately measure frequency like narrower teeth.
In the new comb system, two pump lasers shape each tooth. According to theory, this should result in a set of teeth that are equally narrow, thereby improving measurement accuracy. Researchers are currently testing whether this theoretical prediction is applicable to the micro combs they manufacture.

The dual laser system provides another potential advantage: it generates two types of solitons, which can be compared to having a positive or negative sign. Whether a specific soliton is negative or positive is entirely random, as it is caused by the quantum properties of the interaction between two lasers.

This may enable solitons to form a perfect random number generator, which plays a crucial role in creating secure encryption codes and solving statistical and quantum problems that would otherwise be impossible for ordinary non quantum computers to solve.

Source: Laser Net