After electronic integrated circuits (Eics), silicon (Si) photonics technology is expected to achieve photonic integrated circuits (PIC) with high density, advanced functions and portability. Although various silicon photonics fountifiers are rapidly developing PIC capabilities to enable mass production of modulators, photodetectors and, more recently, lasers, silicon PIC has not yet met the stringent requirements for laser noise and overall system stability for many applications such as microwave oscillators, atomic physics and precision metrology. Semiconductor lasers must strongly suppress amplified-spontaneous emission noise to achieve narrow linewidths in these applications. They also need to be isolated from the rest of the optical system, otherwise the laser source will be very sensitive to backreflection from downstream optical elements, which is beyond the control of the PIC designer. In many integrated photonic solutions, bulk optical isolators must be inserted between the laser chip and the rest of the system, which greatly increases the complexity and cost of assembly and packaging.
Here, the John E. Bowers research group at the University of California, Santa Barbara, used three-dimensional integration technology to solve this problem, bringing ultra-low noise lasers without isolators to silicon photonics. Through multiple monolithic and heterogeneous processing sequences, the authors demonstrate direct on-chip integration of III-V gain media and ultra-low loss silicon nitride waveguides with optical losses of about 0.5 dB per meter. Thus, thanks to the use of ultra-high quality factor cavities, the photonic integrated circuits demonstrated enter the stage where ultra-low noise lasers and microwave synthesizers can be generated without optical isolators. This photonic integrated circuit also provides excellent scalability for complex functions and volume production, and improves stability and reliability over time. Therefore, three-dimensional integration on ultra-low loss photonic integrated circuits marks a key step toward complex systems and networks on silicon. The results are published in the journal Nature under the title "3D integration enables ultralow-noise isolator-free lasers in silicon photonics." Doctoral students Chao Xiang are co-authors of the program, and Warren Jin, Osama Terra, and Bozhang Dong are co-authors.
3D integration of laser and ultra-low loss PIC
As shown in Figure 1a, the authors effectively separate the three-dimensional Si PIC into layers with their own photonic functions. The designed device consists of four main functional layers, including III-V gain layer, silicon PIC layer, silicon nitride redistribution layer (RDL) and silicon nitride ULL layer. The gap between the silicon layer and the ULL silicon nitride layer is about 4.8 μm, so the ULL silicon nitride layer can be effectively isolated from subsequent silicon and indium phosphide (InP) processing steps, thereby maintaining the performance of the ULL silicon nitride layer. The authors also introduce photonic RDL between the Si layer and the ULL SiN layer to control the coupling between the top active layer and the bottom ULL passive layer. When necessary, RDL can provide efficient active layer to passive layer conversion. Figure 1a also shows a cross-section of the 3D integrated circuit, showing its compatibility with fountion-supplied silicon photonic components. In addition, this PIC can be further heterothetically integrated with the EIC to achieve high-density 3D E-PIC. In the three-dimensional photonic integrated structure, the thick oxide spacer layer forms an effective barrier to the origin of the back-end loss, thus fully integrating the ultra-high Q-value resonator with the high-performance III-V/Si distributed feedback (DFB) laser.
Figure 1:3D integrated Si PIC chip
Single-chip self-injection locking laser
The authors use the self-injection locking function of the InP/Si DFB laser to implement a thermally tunable SiN ultra-high Q resonator on a 3D Si PIC to create an ultra-low noise laser (FIG. 2a). To set the device to the appropriate operating conditions, the InP/Si laser wavelength is adjusted by the applied gain current, the SiN ring resonance is adjusted by the thermal heater, and the front and rear phases are adjusted by a thermal phase regulator placed on the Si waveguide. Once the wavelength and phase matching conditions are reached, the free-running laser is locked onto the ultra-high Q-value resonator due to Rayleigh backscattering, resulting in a variety of resonator-defined laser characteristics.
The authors studied the dynamics and performance of a self-injection locked (SIL) laser using the measuring device shown in Figure 2b. Since there is an on-chip phase tuner between the laser and the ring resonator, the phase-dependent locking dynamics can be clearly revealed. Figure 2c shows the dependence of laser coherence on the power of the phase tuner that causes the phase shift. The laser wavelength is preset to match one of the ring resonances. It can be observed that the laser coherence changes periodically when the phase of the laser to the resonator is tuned to several unidirectional π cycles. During each cycle, the laser undergoes low phase-noise locking, chaotic coherent collapse, and high phase-noise free-running states.
Figure 2: Laser self-injection locking and phase noise
Cavity mediated feedback sensitivity
In the current SIL configuration, the output of the laser can be output from either a pass-through port or a drop port (Figure 3a). The ring resonator itself is both an intensity filter for the forward output and an intensity filter for the reverse reflection. This provides another degree of freedom for controlling the feedback sensitivity by changing the load factor of the ring resonator. The experimental setup shown in Figure 3b illustrates the dependence on feedback. Depending on the strength of the feedback, the laser can operate in several different states. The authors calculated the critical feedback level as a function of the cavity load Q value (Figure 3c). Under different Rayleigh backscattering intensity (R), the tolerance degree of the laser to the downstream reflection is different. In general, large feedback with high Q value is beneficial to improve the downstream reflection tolerance. When the phase response provided by the resonator cannot compensate for the greater reflected power outside the resonator, this effect will reach saturation at a certain loading Q value.
Figure 3: Feedback insensitivity of SIL lasers
Tunable microwave frequency generation
The ability to integrate ultra-low noise lasers at the wafer level opens up the possibility of realizing photonic devices that could not be integrated before, as shown in Figure 4a. In this heterodyne jump scheme, the generated microwave signal phase noise is the sum of the phase noise of the heterodyne jump laser. To verify the feasibility of our laser for heterodyne microwave synthesis, the authors performed a tunable microwave synthesis experiment (Figure 4b). As shown in the bottom illustration, an optical phase-locked loop that drives the laser current can be used to improve long-term stability. Chip packaging can further improve stability. Microwave frequency tuning is achieved by tuning the ring resonance of one ring resonator while keeping the resonance of the other ring resonator fixed.
Figure 4c summarizes the resulting microwave signal, whose frequency can be tuned from 0 GHZ to 50 GHZ at 1 GHZ pitch. The frequency tuning is continuous and determined by the thermal phase tuner control on the ring resonator. The phase noise of microwave signals generated at different frequencies is characterized. The results show that the phase noise of the microwave signal is determined by the laser phase noise and remains constant at different microwave carrier frequencies.
Source: NetEase
