Scientists develop wideband quantum-dot frequency-modulated comb lasers

Since the concept of laser frequency combs was born in the late 1990s, it has revolutionized the accurate measurement of frequency and time. In addition to its initial applications in optical clocks and precision spectroscopy, optical frequency combs (OFCs) have shown great potential in a variety of applications, including ultraviolet and infrared (IR) spectroscopy, remote sensing, optical frequency synthesis, and high-speed optical communications.

However, the intense light pulses delivered by amplitude modulated (AM) OFC are not conducive to dense wavelength division multiplexing (DWDM) systems where many microring modulators are deployed. This is because the high instantaneous power of the light pulse leads to strong thermal nonlinearity.

On the other hand, the formation of wideband OFC relies on careful design of waveguide group velocity dispersion (GVD), which is challenging for platforms where GVD is primarily determined by materials. Therefore, in order to use OFC in industry, it is necessary to improve the system size, weight, power consumption and cost (SWaP-C) of OFC.

In a new paper published in the journal Light: Science and Applications, a team of scientists led by Professor John Bowers of the Energy Efficiency Institute at the University of California, Santa Barbara, USA, has developed an advanced quantum dot (QD) based laser. A proper laser cavity design enables a record 3 dB optical bandwidth of 2.2 THz in the telecom O-band.

Channel spacing up to 60GHz helps eliminate channel crosstalk in data transmission. More interestingly, this quasi-continuous wave FM comb does not provide strong light pulses, which is conducive to integrating DWDM systems.

By utilizing the QD laser, the wideband FM comb is generated from a 1.35 mm long, 2.6 um wide laser cavity with an electro-optical conversion efficiency of more than 12%. Compared to other integrated OFC technologies, the reported QD laser-based FM combs exhibit superior SWaP-C, a solution sought by both academia and industry.

QD's excellent material properties make it a promising platform for FM comb generation. Ultrafast gain dynamics allow for huge Kerr nonlinearity and four-wave mixing, which makes QD lasers more suitable for FM brush generation in optical communication frequency bands than traditional quantum-well diode lasers.

Importantly, this reported method allows us to improve the optical bandwidth without having to carefully design waveguide dispersion. This achievement was made possible by Kerr nonlinear engineering, which can be simply controlled by applying a voltage to the saturable absorber part of the laser. As a result, this approach reduces challenges in the manufacturing process.

These scientists comment on their achievements in this work:

"It's an evolution of thinking. The first mode-locked laser was demonstrated in 1963 and great progress has been made since then. It used to be thought that a mode-locked laser had to provide a strong pulse because of its AM (amplitude) modulation) nature. However, we show that it does not have to be so. FM (frequency modulated) mode-locked lasers are experiencing a Renaissance. Its essence is to provide broadband and flat-top spectrum and quasi-modulated continuous-wave emission."

"Although FM combs have been demonstrated in mid-infrared quantum cascade lasers, the FM properties of near-infrared quantum dot mode-locked lasers have not been fully exploited. We are trying to understand it and apply FM mode-locked lasers to high speed PIC (photonic integrated circuits) for data centers, "they added.

"The proposed technology solves the problem OFC has encountered with PIC and is compatible with the mature CMOS industry. Our results provide novel insights for improving PIC's FM mode-locked lasers. This breakthrough may open up new areas for further research. A new generation of PIC for 5G/6G communication, artificial intelligence and autonomous driving, "the scientists said.

Source: Laser Network