New Dual-Mode Laser Produces Independent AM And FM Combs On Same Device

Recently, a team of researchers at the University of California, Santa Barbara (UCSB), led by John Bowers, announced that they have developed a quantum dot (QD) mode-locked laser that is capable of independently generating amplitude modulation (AM) and frequency modulation (FM) combs on the same device. In the future, such broadband dual-mode lasers could provide compact, energy-efficient frequency combs for silicon PICs in data centers and other applications.
The researchers say that the quantum dot (QD) platform enables the bandwidth of the above devices to rival the most superior QD mode-locked lasers disclosed to date. Both the AM and FM pulse widths generated in the UCSB devices meet the latest technical requirements for QD mode-locked lasers.
Although optical frequency combs have a wide range of applications in remote sensing, spectroscopy, and optical communications applications, optical pulses delivered by AM frequency combs are not favorable for dense wavelength division multiplexing (DWDM) systems. These systems use many micro-ring modulators, and the high instantaneous power of the optical pulse produces strong thermal nonlinearities.
On the other hand, according to researchers at the University of California, Santa Barbara (UCSB), the formation of broadband optical frequency combs relies on the careful design of the group velocity dispersion (GVD) of the waveguide.
This is a challenge for platforms where the group velocity dispersion (GVD) is determined by the material. Therefore, the system size, weight, power consumption, and cost (SWaP-C) of optical frequency combs must be improved to increase the likelihood of their use in industry.
The researchers used a collision pulse structure to give the quantum dot mode-locked laser a fast repetition frequency of 60 GHz. This allows the QD laser to provide support for DWDM systems and reduce channel crosstalk in data transmission. The laser cavity has been designed to enable a 3 db optical bandwidth of up to 2.2 terahertz in the telecom O-band. The broadband FM comb is generated by a laser cavity that is 1.35 mm long and 2.6 μm wide, and has a high wall insertion efficiency of more than 12 percent.
In addition to the group velocity dispersion (GVD) of the waveguide, the generation of the FM comb relies on the nonlinear properties of the active region of the laser, including spatial hole-burning, Kerr nonlinearities, and four-wave mixing.The QD mode-locked laser, with a high four-wave mixing efficiency of ?5 dB, is able to efficiently generate the FM comb.
Lasers are a promising platform for generating amplitude-modulated (AM) and frequency-modulated (FM) comb quantum dots (QDs). The mechanisms of these combs are different and are determined by the gain dynamics of the laser. AM comb formation requires a slow gain, which can be achieved by applying a low injection current to the gain section of the QD laser.
FM comb formation relies on fast gain to produce large Kerr nonlinearities and four-wave mixing. This can be achieved by simply controlling the gain and the deviation on the saturable absorber. The engineering of the Kerr nonlinearity helps to dramatically increase the 3 db optical bandwidth to 2.2 terahertz.
The researchers also showed how the frequency modulation (FM) comb bandwidth in a QD laser can be increased by engineering the Kerr nonlinearity without the need for GVD engineering. This was achieved by applying a voltage to the saturable absorption part of the laser. This approach also reduces the challenge of the fabrication process, the researchers say. The huge Kerr nonlinearity and four-wave mixing properties of quantum dot lasers make them more suitable for FM comb generation in the optical communications band than conventional quantum well diode lasers.
Compared to FM combs generated by other integrated optical frequency comb (OFC) technologies, the researchers determined that quantum-dot (QD) laser-based FM combs have superior SWaP-C. The broadband characteristics of FM combs make them more suitable for high-capacity optical communication systems than conventional FM combs. In addition, the technology is capable of CMOS compatibility.