Micro laser opens the door to chip size sensors

The new device is a frequency comb - a special type of laser that can generate multiple wavelengths of light, each with a fixed frequency interval. On the spectrogram, it looks a bit like the teeth of a comb. In approximately a quarter century since their first development, these "cursor rulers" have completely transformed various high-precision measurements from timing to molecular detection. In addition, each line of the comb can be isolated and has characteristics such as amplitude modulation to transmit data through optical fibers.

However, frequency combs typically require bulky, expensive, and power consuming equipment. This greatly limits their use in laboratory environments.

Now, scientists at Stanford University have adopted two different methods to create microchip level frequency combs. A strategy called optical parametric oscillation involves reflecting a laser beam within a crystal to organize the light itself into coherent and stable wave pulses. Another method is called phase modulation, which sends the laser into the cavity and applies radio frequency signals to control the phase of the light, generating frequency repetition for the comb. However, both strategies have drawbacks, such as low energy efficiency and limited ability to adjust optical parameters.

To overcome these challenges, scientists experimented with a material called thin film lithium niobate, which has many advantages over the industry standard material silicon. Two of these characteristics include how light of various wavelengths passes through it, and how it allows beams of different wavelengths to interact to produce new wavelengths.

This new material supports both optical parametric amplification and phase modulation in a single cavity. The resulting "micro comb" size is only 1 x 10 millimeters. Researchers say that such a compact size indicates that it can be used in mobile phones or smaller personal devices. They added that it can also be easily manufactured in traditional microchip factories.

"The most surprising aspect of this comb is how it performs in terms of bandwidth, spectrum, and efficiency," said Amir Safavi Noeini, Associate Professor of Applied Physics at Stanford University.

The new micro comb did not generate light pulses as expected by the researchers, but unexpectedly produced continuous output. Other combs waste power between pulses. Therefore, scientists can reduce the input power required by the device by about an order of magnitude.
The new device converts the light pumped into the cavity into a comb with an efficiency of over 93%. It can generate 200 comb lines with intervals of approximately 5.8 GHz at frequencies exceeding 1 THz. It has been proven that it is highly adjustable by simply adjusting the wireless signal applied to it. Safavi Noeini said that all of these features make it highly attractive for emerging ideas of chip level sensors that require detecting a wide range of spectra.

In addition, the device produces a flat comb, which means that the strength of comb lines with frequencies far from the center will not weaken. This flat feature helps to improve accuracy and makes micro combs available for a wider range of measurement applications.
Scientists have pointed out that the spacing between comb lines can reach 50 to 100 GHz, and the device may operate under blue to mid infrared light. This indicates that micro combs can be used for applications such as medical diagnosis, fiber optic telecommunications, LiDAR, and spectroscopy.

"We have recently started researching very lightweight, low-cost, and low-power greenhouse gas detection applications," said Safavi Noeini. Other fields such as biosensing are also very interesting.
Safavi Noeini said that in the future, scientists hope to improve the performance of devices and expand their bandwidth and operating wavelength range.
Scientists detailed their findings in the journal Nature on March 6th.

Source: Laser Net