The optical frequency comb, which is a series of equidistant coherent optical lines in the frequency domain, has been greatly developed in the past two decades[
Chinese Optics Letters, Volume. 20, Issue 3, 032201(2022)
Fabrication of the high-
The microresonator-based soliton microcomb has shown a promising future in many applications. In this work, we report the fabrication of high quality (Q)
The optical frequency comb, which is a series of equidistant coherent optical lines in the frequency domain, has been greatly developed in the past two decades[
Compared with the conventional optical frequency comb generated by the mode-locked laser, the performance of the soliton microcomb has the advantages of low-power consumption, small footprint, simple structure, and integrability[
In recent years, because of the possibility to achieve ultra-low linear and nonlinear optical losses and engineer the dispersion of waveguides and microresonators precisely[
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In this work, we adopt the traditional subtractive process to fabricate dispersion-engineered microring resonators. By introducing the crack isolation trenches and performing the annealing process to reduce the tensile stress, we successfully deposit thick stoichiometric film without cracks in the central area. With our optimized processing technology, the intrinsic Q is demonstrated to be as high as , corresponding to a propagation loss as low as 0.058 dBm/cm. Furthermore, in our dispersion-engineered microring resonators with a cross section of , we also obtain an intrinsic Q of about with a free spectral range (FSR) of about 230 GHz, and the four-wave mixing (FWM) threshold of about 13.4 mW. In such a high Q microring, we have demonstrated the generation of soliton microcombs with on-chip pump power of about 100 mW.
2. Device Fabrication
To generate the Kerr soliton in the microresonator, anomalous group velocity dispersion (GVD) is required for permitting the phase and energy matching of the nonlinear optical interactions. The material GVD of is normal in the communication band, and therefore, the geometric dispersion has to be engineered by precisely tailoring the cross section of the waveguide to make the overall waveguide GVD anomalous. Figures 1(a) and 1(b) show the typical simulated dispersion trends with varied waveguide cross-section dimensions. The waveguide dispersion is computed based on COMSOL Multiphysics, and the material dispersion is included by expressing the refractive index in the Sellmeier equations[
Figure 1.(a) Simulated dispersion curves for different heights of the Si3N4 waveguide, with the waveguide width fixed at 1.8 µm. (b) Simulated dispersion curves for different widths of the Si3N4 waveguide, with the waveguide height fixed at 800 nm. (c) Using a diamond scriber on the 4 in. wafer to draw a square area. (d) Using lithography to define the patterns of the trenches. (e), (f) The relevant partial magnifications show that the cracks are successfully blocked by the trenches. (g) The surface range of 1 µm × 1 µm is scanned by atomic force microscopy (AFM), and the corresponding root-mean-square (RMS) roughness is 0.26 nm. (h) Scanning electron microscope (SEM) image of a crack passing through the etched waveguide.
From the above simulation results, for microcomb generation, the thickness of the device layer should be around 800 nm. Our fabrication process starts from the preparation of high Q film to match the anomalous dispersion. The original 4 in. (1 in. = 2.54 cm) substrate is made up of 500 µm silicon and 4 µm wet oxidation silicon dioxide (). Since low losses are very important for nonlinear optics, we use low-pressure chemical vapor deposition (LPCVD) combined with post-annealing to deposit stoichiometric film with minimal absorption loss. However, due to the high tensile stress, the stoichiometric film with thickness exceeding 400 nm easily cracks, as shown in Figs. 1(c)–1(f). These cracks typically start from the edge of the wafer and propagate into the central area[
The subtractive fabrication flow is illustrated in Fig. 2. The device is patterned by e-beam lithography with hydrogen-silsesquioxane (HSQ) resist. Following the development of the pattern, the film is etched with a -based gas in an inductively coupled plasma (ICP) etcher. Figure 2(b) shows the SEM image of the microring resonator after the etching process. As shown in Fig. 2(c), at the bottom of each sidewall of the waveguide, one microtrench is formed, which is also reported in Ref. . We attribute this phenomenon to the formation of charged polymer after the increase of the oxygen ratio[
Figure 2.(a) Fabrication process of the Si3N4 device. (b) SEM image of the Si3N4 microring resonator and bus waveguide. (c) SEM image of the cross-section view of the bus waveguide with photoresist, which is removed before SiO2 depositing. The microtrench is at the bottom of each sidewall of the waveguide. (d) SEM image of the cross-section view of the bus waveguide after depositing the SiO2 for protection. The Si3N4 waveguide is painted with green color.
3. Optical Characterization of Microring
To realize soliton microcombs, we fabricate the microring resonator with a cross section of according to the numerical prediction and a radius of 100 µm, as shown in Figs. 2(c) and 2(d). The bus waveguide has the same cross section as the microring to get a high coupling ideality[
Figure 3.(a) Schematic of the experimental setup. FPC, fiber polarization controller; EDFA, erbium-doped fiber amplifier; FG, function generator; OSA, optical spectrum analyzer; OSC, oscilloscope; PD, photon detector. (b) The typical transmission of fundamental TE (red curve) and TM (blue curve) modes from 1550 nm to 1630 nm. The envelope of the transmission line decreasing with the increase of wavelength is due to the power variation of the laser itself. Total insertion losses of fundamental TE modes and TM modes are about −3.6 dB and −4.5 dB, respectively.
The loaded Q of all TM and TE resonances within the measurement spectrum (blue and red cross) are extracted and shown in Fig. 4(a), and the loaded Q of the microring resonator fabricated from the film without the annealing process at fabrication flow (green and purple triangle) are also illustrated for comparison. It is clear that loaded Q is approximately doubly increased with annealing process, which can not only release the tensile stress inside the film, but also remarkably reduce the absorption loss[
Figure 4.(a) Loaded Q of the resonances in the microring with and without the annealing process of the Si3N4 film. The diameter of the microring is 200 µm with the cross section of 1.8 µm × 800 nm. The inset shows the histogram of the linewidth of fundamental TM (blue) and TE (red) modes. (b) The typical TM optical mode with the Lorentz fitting linewidth of about 128 MHz. (c) The typical transmission spectrum of the optical mode with the linewidth of 65 MHz in another microring with the cross section of 3 µm × 800 nm.
To explore the narrowest linewidth available with our current processing technology, we fabricate the microring resonator with the same radius and height, but with a larger ring width of 3 µm. Because of the relatively large ring width, scattering loss from etched sidewalls is reduced. As shown in Fig. 4(c), the linewidth of the optical mode at 1552.6 nm is about 65 MHz, corresponding to a loaded factor of , and the intrinsic factor can be estimated as . The propagation loss is estimated as [
FWM is the basis of frequency comb generation, and the threshold is inversely proportional to squared Q. Therefore, the next step is to characterize the FWM threshold of our device, which has a cross section of and a radius of 100 µm for optimized dispersion engineering. To obtain the threshold, a blue detuned laser is scanned to approach the resonance mode and stabilized at a blue detuning position, while the output light of the chip is measured by an optical spectrum analyzer (OSA). When the pump power exceeds the threshold, FWM can occur, which plays an important role in comb generation. As shown in Fig. 5, the output power of the primary FWM sidebands is recorded for different pump powers. The on-chip threshold power is for a fundamental TM mode near 1562 nm with a loaded Q of about . The inset of Fig. 5 shows the optical spectrum with pump power , which is slightly above the threshold. It is obvious that the initial spacing of this primary comb state is not single but multiple FSRs, which is related to the mode linewidth and the dispersion of the device.
Figure 5.Relationship between the output power of the primary FWM sidebands and the on-chip pump power of the microring resonator with a cross section of 1.8 µm × 800 nm and a radius of 100 µm, showing the FWM threshold of about 13.4 mW. Inset: optical spectrum with on-chip pump power of 13.9 mW.
4. Soliton Microcombs
Further increasing the pump power and tuning the frequency of the pump laser, a series of frequency comb states and final soliton state can be generated from our device. To investigate the evolution of the comb states, the pump laser amplified by the erbium-doped fiber amplifier (EDFA) with an input power is launched to the chip. The on-chip pump power is about 100 mW. The microring resonator used in this experiment is the same as the above threshold measurement, and the pumped resonance mode is the fundamental TM mode around 1562.7 nm. For soliton comb generation, the pump laser needs to be scanned across the resonance mode from the blue side to the red side. However, because of the existence of the thermal effect[
Figure 6.(a) Evolution of the intracavity power as the laser frequency is scanning across the resonance mode. (b) Schematic of auxiliary-laser-assisted thermal response control method. The optical spectra of (c) multi-soliton and (d) single soliton states with the smooth envelope fitted by the sech2 function (red curve).
To compensate the influence of the thermal effect in the high Q microring, as shown in Fig. 3(a), another auxiliary laser (red) is coupled to another optical mode in the opposite direction of pump laser[
In conclusion, we have presented the fabrication process of crack-free stoichiometric LPCVD film and ultra-low-loss waveguides and microring resonators. With the crack isolation trenches and the annealing process, cracks can be blocked out of the central area of the wafer. Based on the homemade LPCVD film, the highest intrinsic Q of about has been observed in our fabricated microring resonator. Moreover, by designing the proper waveguide shape to engineer the dispersion, we have also demonstrated the FWM threshold of our device with and the soliton microcomb generation with 100 mW on-chip pump power in the same device. Here, the Q factor could be further optimized, for example, by annealing the LPCVD film with a higher temperature to 1200°C[
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Shuai Wan, Rui Niu, Jin-Lan Peng, Jin Li, Guang-Can Guo, Chang-Ling Zou, Chun-Hua Dong. Fabrication of the high-
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Category: Optical Design and Fabrication
Received: Nov. 18, 2021
Accepted: Dec. 13, 2021
Published Online: Jan. 11, 2022
The Author Email: Chun-Hua Dong (firstname.lastname@example.org)