University of California, Santa Barbara (UCSB) researchers led by John Bowers have developed a quantum-dot (QD) mode-locked laser that allows amplitude-modulated (AM) and frequency-modulated (FM) combs to be generated independently from the same device. The broadband, dual-mode laser could enable opportunities for small-footprint, energy-efficient frequency combs for silicon PICs in data centers and other applications. The UCSB QD platform enables devices to be fabricated with a bandwidth that is comparable to the best QD mode-locked lasers reported to date, the researchers said. Both the AM and the FM pulse widths generated in the UCSB devices meet the state-of-art for QD mode-locked lasers. Despite optical frequency combs’ widespread utility in remote sensing, spectroscopy, and optical communications applications, the optical pulses delivered by amplitude-modulated frequency combs are not favorable to dense wavelength-division multiplexing (DWDM) systems. These systems use many micro-ring modulators, and the high, instantaneous power of the optical pulses creates strong thermal nonlinearities. On the other hand, according to the UCSB researchers, the formation of a broadband optical frequency comb relies on careful engineering of the group velocity dispersion (GVD) of the waveguide. This is a challenge for platforms where the GVD is determined by the material. As a result, the system size, weight, power consumption, and cost (SWaP-C) of optical frequency combs must be improved to increase the likelihood for their use in industry. The researchers used a colliding-pulse structure to give the QD mode-locked laser a fast repetition rate of 60 GHz. This allows the QD laser to provide support for DWDM systems and reduces channel crosstalk in data transmission. The design of the laser cavity enables a 3-dB optical bandwidth as high as 2.2 THz in the telecom O-band. The broadband FM comb is generated from a 1.35-mm-long and 2.6-μm-wide laser cavity, with a high wall-plug efficiency of over 12%. In addition to the group velocity dispersion (GVD) of the waveguide, the generation of FM combs relies on the nonlinear properties of the laser’s active region, including spatial hole burning, Kerr nonlinearities, and four-wave mixing. The QD mode-locked laser exhibits a high four-wave mixing efficiency of −5 dB, which enables it to generate FM combs efficiently. The quantum-dot (QD) laser is a promising platform for the generation of both frequency-modulated (FM) and amplitude-modulated (AM) combs. The mechanisms of these combs are different and determined by the gain dynamics of the laser. The AM comb formation requires a slow gain, which can be achieved by applying a low injection current to the QD laser’s gain section. The FM comb formation relies on a fast gain to generate giant Kerr nonlinearity and four-wave mixing. This can be realized by simply controlling the biases on the gain and the saturable absorber. Engineering of the Kerr nonlinearity contributes to a large improvement in the 3-dB optical bandwidth to 2.2 THz. Courtesy of B. Dong et al. The researchers also showed how the Kerr nonlinearity can be engineered to improve the FM comb bandwidth in the QD laser without the need for GVD engineering. This was achieved by instead applying a voltage to the saturable absorber section of the laser. This approach could also make the fabrication process less challenging, the researchers said. The giant Kerr nonlinearity and four-wave mixing in QD lasers make them more suitable for FM comb generation in the optical communication band than conventional quantum-well diode lasers. Compared to FM combs generated by other integrated optical frequency comb (OFC) technologies, the researchers determined that FM combs based on QD lasers exhibit superior SWaP-C. The broadband nature of FM combs makes them more desirable for high-capacity optical communication systems than conventional AM combs. The researchers said that the developed technique is CMOS compatible. The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-023-01225-z).
