III-V on silicon microdisk lasers

Research Area: Heterogeneous integration technology for silicon photonics , Integrated lasers and LEDs , Silicon photonics for telecom, datacom and interconnect

Main Researcher: Pauline Mechet

Microdisk/ring lasers typically have two lasing modes, one lasing in clockwise (CW) and another lasing in counter-clockwise (CCW) direction. In an ideal disk/ring lasers these two modes lase at the same wavelength and laser light from the CW mode does not couple in to CCW mode and vice-versa. These lasers have special LI (light-current) characteristics. Generally there are three different lasing regimes which are known as bi-directional, alternating oscillating and unidirectional (bi-stable) regimes. These operating regimes of disk/ring lasers occur in the increasing order of the injection current. In the unidirectional regime, the output of one of the modes (CW or CCW) is high while that of another is suppressed due to the non-linear cross-gain saturation in the active medium. The output power of the dominant mode can be suppressed while that of the suppressed mode can be enhanced by injecting an external optical pulse in the direction of the suppressed mode. This way, depending on the direction of injection of the external optical pulse, the random switching between CW and CCW modes is possible and microdisk laser behaves as an all-optical set-reset flip-flop(SR-FF).
We have realized an SR-FF working at the telecom wavelength (1.55µm) with an ultra-small (7.5µm diameter) III-V microdisk laser heterogeneously integrated onto SOI chip and working in a continuous-wave regime with an electrical power consumption of few milliwatts. Switching between CW and CCW modes is demonstrated with an external optical pulse of record low energy of only 1.8fJ and the switching time obtained is 60ps [12].

We also demonstrated low power all-optical gating in a microdisk of 10µm diameter at a speed of 10GHz with the probe and pump power of 170µW and 4mW, respectively, inside the SOI waveguide. Measured transient responses (rise and fall time) show that the 20GHz speed is well within the reach [7].

As the demand for bandwidth increases, optical interconnects are coming closer and closer to the chip. Optical interconnects on silicon-on-insulator (SOI) are desirable as this allows for integration with CMOS and the mature processing can be used for photonic integrated circuits. A heterogeneous integration process can be used to include III-V active optical components on SOI. For dense integration compact sources and detectors are required, but they typically need different epitaxial structures to be efficient which limits the integration density. We demonstrated the very compact integration of sources and detectors thanks to the use of an epitaxial structure which contains both the layers for a laser and for a detector. Microdisk lasers and waveguide detectors were completely fabricated in a 200mm CMOS pilot line [6].
We report a compact optical interconnect on a silicon-on-insulator platform consisting of a directly modulated microdisk laser and detector connected via a silicon waveguide. All fabrication steps for the laser and detector, except for the detector mesa, are carried out simultaneously in the clean-room of the Photonics Research Group. The microdisk laser has a threshold current of 0.45mA and a slope efficiency of 57μW/mA. The responsivity of the detectors is 0.69A/W. The full optical link has a static efficiency of 3% and a bandwidth of 7.6GHz. A large signal modulation is applied and a 10Gb/s bit pattern could be resolved [5].

We demonstrate unidirectional bistability in microdisk lasers electrically pumped and heterogeneously integrated on SOI [4]. The lasers operate in continuous wave regime at room temperature and are single mode. Integrating a passive distributed Bragg reflector (DBR) on the waveguide to which the microdisk is coupled feeds laser emission back into the laser cavity. This introduces an extra unidirectional gain and results in unidirectional emission of the laser, as demonstrated in simulations as well as in experiment. We theoretically and numerically analyze the unidirectional behavior of ring or disk lasers coupled to bus waveguide with a relatively strong reflector on one side, and find analytical expressions for the ratio of the powers in clockwise and counter clockwise modes. At low bias levels this ratio depends on the coupling coefficients that determine the coupling between clockwise and counterclockwise modes, while at high bias levels it also depends strongly on the gain suppression. We also theoretically and numerically invest the feedback sensitivity of such lasers and come to the conclusion that ring or disk lasers are generally much more sensitive to external reflections than traditional Fabry-Perot, DFB or DBR lasers [2]. At high bias levels, also the feedback sensitivity strongly depends on the gain suppression and at high enough power levels it can be better than that of traditional edge-emitting lasers.
We demonstrate an all-optical low-power 2R regenerator of 10Gb/s NRZ data based on a 10μm diameter electrically pumped microdisk laser, heterogeneously integrated onto Silicon-On-Insulator and processed in a CMOS pilot-line. The scheme results in BER improvement and works for sub-milliWatt level input signals. The laser operates in continuous wave regime, is single mode at room temperature and consumes 6mW of electrical power. Its regeneration capability is investigated in simulations and experimentally demonstrated [1].

Other people involved:

• Pauline Mechet

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