1.8-THz-wide optical frequency comb emitted from monolithic passively mode-locked semiconductor quantum-well laser

Mode-locking is a technique applied in optics by which the optical output is not a continuous emission but in the form of  pulses of light, which can have extremely short duration, of the order of pico-/femto-seconds. Recently, integrated passively mode-locked semiconductor lasers have become of particular interest to scientists as they are robust and compact sources for generating coherent optical frequency combs, which are of great interests in spectroscopy, metrology, communications and millimeter wave/terahertz generation.  This technique is unique in that it does not require an external radio frequency source or optical pump like other optical frequency comb generation methods. However, recent studies have shown that the bandwidth of a comb generated from a passively mode-locked semiconductor laser reveal the performance of quantum-dash and quantum-dot active regions better than those of the quantum-well active regions.

The research was conducted by Mu-Chieh Lo within the team of researchers led by professor Guillermo Carpintero at Carlos III University of Madrid in the framework of a EU funded Marie Curie ITN “FIber-Wireless Integrated Networks for 5th Generation delivery” (FIWIN5G), , in collaboration with Rui Santos and Luc Augustin from SMART Photonics. It presented an integrated mode-locked quantum-well laser developed through a generic InP photonic integration platform with a new record bandwidth of 14 nm that was comparably wider than that of a passively mode-locked quantum-dot-based laser. In their study, the researchers designed a colliding-pulse mode-locked laser in a symmetric arrangement. Moreover, they introduced two new features that included; symmetrically positioned intracavity electro-optic phase modulators and an extracavity semiconductor optical amplifier. Their work is now published in the research journal, Optics Letters.

The research team commenced their experiments by developing a photonic integrated circuit through a generic approach and fabricated within a multi-project wafer run by SMART Photonics. The photonic integrated circuit was composed of one saturable absorber, two semi-conductor optical amplifiers, two electro-optic phase modulators, two multimode interference couplers/ reflectors, and straight/bent passive waveguides in a symmetric geometry with respect to the saturable absorber. In order to electrically drive the photonic integrated circuit, the researchers wire-bonded the metal pads of the photonic integrated circuit and connected them to an external direct current source.

The authors observed that a 21.5-GHz colliding-pulse mode-locked laser with intracavity phase modulators was developed using the InP generic photonic integration technology platform. Moreover, they noted that the autocorrelation trace and radio frequency spectrum agreed well with the mode-locking scheme by exhibiting a narrow pulse width of 0.35 picoseconds on a pedestal as well as a radio frequency linewidth of 450 kHz and a 35-dB signal-to-noise ratio.

The study successfully presented a record broad optical frequency comb generated from a passively mode-locked quantum well laser. It has been seen that by inserting a booster semiconductor optical amplifier, higher power greater than a thousandth of a watt has been achieved. The optical comb has a record 3-dB bandwidth of 14 nm (1.8 THz) that is even wider than that of a quantum dot-based passively mode-locked laser.