US research team creates continuous laser light converter

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Researchers have created a device small enough to fit on a computer chip that converts continuous laser light into numerous ultrashort pulses.

It is claimed the development, from Purdue University and the US National Institute of Standards and Technology (NIST), could have applications in more advanced sensors, communications systems and laboratory instruments.

‘These pulses repeat at very high rates, corresponding to hundreds of billions of pulses per second,’ said Andrew Weiner, the scifres family distinguished professor of electrical and computer engineering.

According to a statement, the ‘microring resonator’ is fabricated from silicon nitride, which is compatible with silicon material widely used for electronics. Infrared light from a laser enters the chip through a single optical fibre and is directed by a waveguide into the microring.

The pulses have many segments corresponding to different frequencies, which are called ‘comb lines’.

By precisely controlling the frequency combs, researchers hope to create advanced optical sensors that detect and measure hazardous materials or pollutants, ultrasensitive spectroscopy for laboratory research, and optics-based communications systems that transmit greater volumes of information with better quality while increasing bandwidth.

The comb technology also has potential for a generation of high-bandwidth electrical signals with possible applications in wireless communications and radar.

The light originates from a continuous-wave laser, also called a single-frequency laser.

‘The intensity of this type of laser is constant, not pulsed,’ said Weiner. But in the microring the light is converted into a comb consisting of many frequencies with very nice equal spacing. The microring comb generator may serve as a competing technology to a… mode-locked laser, which generates many frequencies and short pulses. One advantage of the microrings is that they can be very small.’

The laser light undergoes ‘non-linear interaction’ while inside the microring, generating a comb of new frequencies that is emitted out of the device through another optical fibre.

‘The non-linearity is critical to the generation of the comb,’ said doctoral student Fahmida Ferdous. ‘With the non-linearity we obtain a comb of many frequencies, including the original one, and the rest are new ones generated in the microring.’

Although other researchers have previously demonstrated the comb-generation technique, the team is the first to process the frequencies using ‘optical arbitrary waveform technology’, pioneered by Purdue researchers led by Weiner. The researchers were able to control the amplitude and phase of each spectral line, learning that there are two types of combs — ‘highly coherent’ and ‘partially coherent’ — opening up new avenues to study the physics of the process.

Findings are detailed in a research paper appearing online this month in the journal Nature Photonics.