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Abstract
We demonstrate hybridly integrated narrow-linewidth, tunable diode lasers in the InP/GaAs-Si3N4 platform. Silicon nitride photonic integrated circuits, instead of silicon waveguides that suffer from high optical loss near 1 µm, are chosen to build a tunable external cavity for both InP and GaAs gain chips at the same time. Single frequency lasing at 1.55 µm and 1 µm is simultaneously obtained on a single chip with spectral linewidths of 18-kHz and 70-kHz, a side mode suppression ratio of 52 dB and 46 dB, and tuning range of 46 nm and 38 nm, respectively. The resulting dual-band narrow-linewidth diode lasers have potential for use in a variety of novel applications such as integrated difference-frequency generation, quantum photonics, and nonlinear optics.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Narrow linewidth, tunable lasers are well suited for numerous applications in coherent optical communications, optical sensing, Light Detection and Ranging (LiDAR) and spectroscopy [1]. Narrow linewidth fiber and solid-state lasers have been demonstrated before by using a self-feedback circuit [2,3] or external cavity with narrow-band distributed feedback Bragg reflector [4,5]. However, these lasers are bulky and complex. Semiconductor lasers are promising for emerging applications where size, weight and power consumption (SWaP) are important operational parameters [6–9]. Since it is difficult to reduce the linewidth of conventional distributed feedback (DFB) semiconductor diode lasers below ~100 kHz, hybridly integrated diode lasers have attracted considerable interest due to their potential to achieve ultra-narrow linewidth and tunable operation [10,11]. The Schawlow-Townes linewidth of semiconductor lasers is broadened by the refractive index and phase changes and the gain-index coupling, which resulted from the spatial variation of the internal intensity and carries density affected by spontaneous emission events [12]. An external cavity can reduce the broadening due to its insensitivity to the spatial variation and phase/gain changes. In addition, the low loss microring resonators in the passive platform effectively extend the optical length of the laser cavity due to the multiple roundtrips in the microring resonators [13]. The cavity photon lifetime for the hybridly integrated diode laser is increased, which leads to the greatly reduced Schawlow-Townes linewidth [14]. The two microring resonators also act as a wavelength filter. Through the thermo-optic effect, i.e., heating of the resonators resulting in the increment of the effective waveguide index and thereby the optical length of the resonator, the wavelength tuning is obtained [15]. The wavelength tuning range can be enhanced through Vernier effect [16].
There are a few methods that can be used for hybrid integration, e.g., lensed coupling [17], edge coupling [18–21], vertical coupling [22,23], die/wafer bonding [24–28] or heteroepitaxy [29]. We choose edge-coupling for our hybrid integration platform, since the active and passive chip can be optimized and fabricated independently [30–33]. Although silicon waveguides are capable of providing tight optical confinement and large thermal-optical effects, they are not suitable for the 1 µm wavelength band due to high absorption loss. For the 1.55 µm wavelength band, it is also not an ideal platform for the applications requiring high power continuous waveform (CW) light due to the two-photon absorption that limits the maximum light intensity. Silicon nitride (Si3N4) has been widely utilized for integrated photonics to create passive optical components with high performance as a result of its low nonlinearity, high index contract with silica, very large transparency window and low linear propagation loss [34]. Narrow-linewidth hybrid lasers operating around 1.55 µm have been demonstrated using Si3N4 ring resonators and Indium Phosphide (InP) quantum-well reflective semiconductor optical amplifiers (RSOAs) [35–39]. Besides the InP gain chip, the Gallium Arsenide (GaAs) gain chip is also a good candidate for hybrid laser integration due to its higher wall plug efficiency, which is important for high power operation [40]. Low water absorption near 1 µm is considered important for applications in sensing, free space communications, and biomedical systems. Diode lasers near 1 µm are also important for LiDAR, high power lasers [41] and seed sources to fiber lasers [42]. The emitting wavelength range of GaAs gain medium is around 750-1100 nm, where the silicon waveguide has high propagation loss. However, because of very large transparency window of Si3N4 (from 300 nm to several microns), the InP gain chip operating at 1.55 µm and GaAs gain chip operating at 1 µm can be hybridly integrated together into the same Si3N4-based platform with low loss.
In this work, we demonstrate chip-scale, narrow linewidth, hybridly integrated, dual-band diode lasers based on the InP/GaAs RSOA and Si3N4 external cavity. This design can provide on-chip narrow linewidth laser sources with wide wavelength tunability around 1.55 µm and 1 µm simultaneously for passive photonic integrated circuits. The performance of the InP-Si3N4 hybrid laser (i.e., output power, side mode suppression ratio, laser linewidth, wavelength tuning range) is comparable to the others demonstrated before [43–45]. To the best of our knowledge, the III-V/Si3N4 hybrid Fabry–Perot laser working near 1 µm was first demonstrated in [46] through wafer bonding. But we demonstrate the stable single frequency lasing at 1 µm with narrow spectral linewidths and wavelength tunability in the hybrid platform for the first time. Dual-band single frequency lasing on a single chip is potential to enable new applications in integrated nonlinear optics and provides a highly effective approach to difference frequency generation of 3-5 µm radiation for use in compact optical gas sensors [47]. Besides, high-power broadband laser sources enable technologies such as integrated spectroscopy systems [48] and wide band wavelength division multiplexing [49,50].
2. Laser design and fabrication
Figure 1 shows the schematic plot of the hybridly integrated diode laser. It consists of two RSOAs (i.e., InP and GaAs gain chips) and a Si3N4/SiO2/Si chip. The buried oxide (BOX) layer is thick enough (4 µm) to prevent optical leakage from the Si3N4 waveguide layer to the Si substrate. The RSOA has a high reflection (HR) coated back-facet with a 90% reflectivity and an anti-reflection (AR) coated front facet. Due to the large mode mismatch between the waveguides in the RSOA and Si3N4 chips, we use a well-designed spot size converter to efficiently couple the light between active and passive chips [39]. The waveguide width at the input end of the converter is 5.9 µm, so the diode laser mode can be matched well with the input end of the converter. Then the waveguide is narrowed down to the width of the single mode waveguide. The total converter length is 50 µm. The detailed design is given in [51]. Figure 2(a) shows the profile evolution of the optical mode propagating from the single-mode ridge waveguide in the gain chip to the silicon nitride waveguide. The mode profiles are obtained through the finite-element method (FEM) analysis. The minimal coupling loss of < 0.3 dB between the two chips is obtained through the 3D FDTD simulation. In our best samples, we achieve the experimentally measured coupling loss of less than 2 dB. Besides, the RSOA and Si3N4 waveguide are both angle-cleaved to eliminate the reflection at the interface between the RSOA and passive chip. The Si3N4-based external cavity is composed of double-ring micro-resonators with slightly different free spectral ranges (FSRs) acting as a wavelength filter and extended cavity. Figure 2(b) shows the simulated and measured transmission spectra of the double-ring filter around the 1.55 µm wavelength range. The radii for the two microrings are 51 and 54 µm, respectively. We observe excellent agreement between the simulation and experiment. A simple cleaved facet at the Si3N4 waveguide output port is utilized here to reflect the light back into the laser cavity, which can help to obtain the wide wavelength tunability due to its broadband back reflection. Figure 2(c) shows the SEM image of the cleaved facet indicating a smooth facet surface. The measured reflectivity of the cleaved facet is around 7.5%. The light is coupled into a lensed fiber for measuring the output of the laser at the cleaved facet through a well-designed spot size converter. The width of the single mode silicon nitride waveguide for the lasing wavelength of 1.55 µm is set to be 900 nm and the height is 300 nm. The propagation loss of the silicon nitride waveguide is ~0.50 dB/cm for the wavelength of 1.55 µm. The hybrid composite laser cavity consists of the RSOA, double-ring resonators, and input/output waveguides. The longitudinal modes of the entire hybrid cavity are defined by the total effective cavity length, while the mode selection is obtained by the two microring resonators with slightly different FSRs. The resonance of ring resonators is thermally tuned by micro-heaters to obtain wavelength tunability.
Fig. 2 (a) Spot size converter and the profile evolution of the optical mode propagating from the single-mode ridge waveguide in the gain chip to the silicon nitride waveguide. (b) Simulated and measured transmission spectra of the double-ring filter. (c) SEM image of the Si3N4 cleaved waveguide facet.
The passive chip fabrication process starts with a SiO2/Si wafer where the SiO2 cladding layer is about 4 μm thick. We deposit Si3N4 using Tystar Nitride low-press chemical vapor deposition (LPCVD) tool. The Si3N4 waveguide are patterned and etched by electron beam lithography (EBL) and reactive ion etching (RIE) with a fluorine-based plasma chemistry, respectively. Then a SiO2 cladding layer is deposited. In order to thermally tune the ring resonators, Chromium/Platinum (Cr/Pt) heaters are deposited and patterned over the resonators.
3. Experimental results of InP-Si3N4 hybrid laser
Here we use active alignment to demonstrate the hybrid integration of the two RSOAs and passive chips for simplicity. To obtain the efficient control of the two RSOAs, they are placed at the two different sides of the Si3N4 passive chip, as shown in [11]. This configuration allows us to put each RSOA on an independent stage for the accurate control of coupling with the passive cavity. The InP-Si3N4 and GaAs-Si3N4 hybrid lasers can operate at the same time. The laser light output from the InP and GaAs gain chips are detected from two different output ports through a lensed fiber separately. All the measurement results are obtained at room temperature with uncooled lasers. Figure 3 shows the experimental results of the hybridly integrated laser based on the InP RSOA gain chip. The light intensity-current (L-I) curve and current-voltage (I-V) for the laser is shown in Fig. 3(a). The threshold current is around 60 mA. The slope efficiency is 68 mW/A. The normalized output optical spectrum is shown in Fig. 3(b) (the pump current is set at around 100 mA). The single frequency lasing with ~52 dB side mode suppression ratio (SMSR) is obtained by taking advantage of the Vernier effect between the two microrings. Since the resolution of optical spectrum analyzer is inadequate to resolve the laser spectral linewidth, we use a delayed self-heterodyne (DSH) interferometer with a 10-km delay line to measure the laser spectral linewidth as shown in Fig. 3(c). Compared with the optical heterodyne detection method, the DSH approach does not require a reference laser with a narrower linewidth at a nearby frequency [11,33]. The red dots in Fig. 3(d) shows the measured RF-beat spectrum. A Lorentzian fit is shown by the blue line. An 18-kHz full width at half maximum (FWHM) laser linewidth is obtained. The linewidth of the hybrid laser is reduced mainly due to the increased cavity length and microring based delay-line filter. Figure 3(e) shows the superimposed spectra. The tuning range of ~46 nm is obtained. Coarsely tuning is obtained here when we thermally tune only one of the two microring resonators.
Fig. 3 Experimental results of the InP-Si3N4 hybrid laser. (a) L-I curve (blue) and I-V curve (red). (b) Normalized output optical spectrum with the single frequency operation. (c) Delayed self-heterodyne experimental setup; OSA: optical spectrum analyzer; VOA: variable optical attenuator; FPC: fiber polarization controller; PD: photodiode; ESA: electrical spectrum analyzer; EOM: electro-optic modulator. (d) Recorded RF beat spectrum (red dots), the blue line shows a Lorentzian fit corresponding to a laser linewidth of 18-kHz. (e) Superimposed spectra when we thermally tune one of the two microresonators (the tuning range is ~46 nm).
4. Experimental results of GaAs-Si3N4 hybrid laser
In addition to the InP gain chip, GaAs gain chip is also integrated into the same hybrid platform due to the large transparency window of Si3N4. Figure 4 shows the experimental results of the GaAs-Si3N4 hybrid laser in the same chip. The radii of the two microrings in the Si3N4 external cavity are 50 and 52 nm. Figure 4(a) shows the L-I curve and I-V curve for the laser. The threshold current is around 85 mA. The slope efficiency is 220 mW/A, which is higher than that of the InP-Si3N4 hybrid laser due to the higher photon energy for 1 µm, the higher internal quantum efficiency and the lower internal loss [40]. The output power of above 20 mW can be obtained from the output port. From the normalized output optical spectrum shown in Fig. 4(b) (the pump current is set at around 150 mA), it is found that the single frequency lasing is also obtained here. The SMSR is ~46 dB. As shown in Fig. 4(c), the FWHM laser linewidth is 70-kHz which is broader than that of the InP-Si3N4 hybrid laser. The possible reasons include the higher loss of the Si3N4 waveguide at the wavelength of 1 µm and the larger linewidth enhancement factor of GaAs gain medium. For the 1.55 µm laser linewidth measurement, SMF-28e is used as the fiber delay line. All other components are telecom compatible products. For the 1 µm laser linewidth measurement, it is best to use HI1060 as the fiber delay line. The other components (i.e., isolator, couplers, VOA, FPC, EOM) should be designed for the 1 µm band. Figure 4(d) shows the superimposed spectra when one of the two microresonators are thermally tuned. The tuning range is 38 nm. When we thermally tune the microring resonator, the resonance peaks of the two microring resonators may not be completely overlapped, which results in the extra loss in the laser cavity, thereby the power fluctuation.
Fig. 4 Experimental results of the GaAs-Si3N4 hybrid laser. (a) L-I curve (blue) and I-V curve (red). (b) Normalized output optical spectrum. (c) Recorded RF beat spectrum (red dots), the blue line shows a Lorentzian fit corresponding to a laser linewidth of 70-kHz. (d) Superimposed spectra when we thermally tune one of the two microresonators.
5. Conclusion
In conclusion, we have demonstrated the hybrid integration of a low-loss, passive Si3N4 external cavity with two RSOAs simultaneously in silicon photonics platform to obtain the ultra-narrow laser linewidth and wide wavelength tunability. The hybrid integration allows multiple active chips with different types of gain media to be integrated easily in the platform at the same time. In addition to the InP chip operating around 1.55 µm, a GaAs chip at 1 µm is also integrated in the same hybrid platform. Single frequency lasing at 1.55 µm and 1 µm has been demonstrated simultaneously on a single chip with the spectral linewidths of 18-kHz and 70-kHz, side mode suppression ratio of 52 dB and 46 dB, and tuning range of 46 nm and 38 nm, respectively. The dual-band single frequency diode lasers have a great potential for a wide range of applications in integrated nonlinear optics and quantum optics.
Funding
Army Research Office (W911NF-14-1-0640) and Office of Naval Research (N00014-17-1-2556).
Acknowledgments
The authors acknowledge the use of the Gatech Nanotechnology Research Center Facility and associated support services in the completion of this work. This work was also performed in part at the Cornell NanoScale Facility, a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (Grant ECCS-1542081).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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