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Abstract
We have designed and fabricated a hybrid integrated laser source with full C-band wavelength tunability and high-power output. The external cavity laser is composed of a gain chip and a dual micro-ring narrowband filter integrated on the silicon nitride photonic chip to achieve a wavelength tuning range of 55 nm and a SMSR higher than 50 dB. Through the integration of the semiconductor optical amplifier in the miniaturized package, the laser exhibits an output power of 220 mW and linewidth narrower than 8 kHz over the full C-band. Such a high-power, narrow-linewidth laser diode with a compact and low-cost design could be applied whenever coherence and interferometric resolutions are needed, such as silicon optical coherent transceiver module for space laser communication, light detection and ranging (LiDAR).
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1. Introduction
Thanks to the combination of very high index contrast and the availability of CMOS fabrication technology, silicon photonics (SiPh) integrated circuits provide a widely used platform for the applications such as coherent optical communication [1–3], light detection and ranging (LiDAR) [4,5] and coherent detection fiber sensing [6]. The above mentioned coherent detection applications based on phase measurement are proposing the demand to spectral purity and low phase noise, which requires the lasers have narrow linewidth. Moreover, SiPh devices especially modulators [7–9] have large optical transmission loss, and the lack of on-chip silicon laser source brings extra coupling loss, leading to higher power requirements for laser sources. Furthermore, the sensitivity of the receiver in the space laser communication system also puts forward higher demands for the linewidth of the laser [10]. External cavity lasers (ECLs) based on Fabry-Perot (FP) etalon [11], fiber Bragg grating (FBG) [12] and planar waveguide [13,14] have achieved kHz linewidth at specific wavelength, and have been applied widely in related fields. Generally, the frequency selection elements of the above lasers are working at fixed wavelength (tuning range less than 2 nm), and the output power generally is under 20 mW. The SiPh dual micro-ring resonators (MRRs) ECLs [15,16] with a wide range of wavelength coverage and narrow linewidth, are of high importance for wide range of applications benefiting from their low cost and flexible integration with SiPh systems.
In recent years, several researchers have proposed different schemes for investigating high power and narrow linewidth semiconductor lasers, mainly focusing on improving the power of the gain chip or integrating an external amplifier [17,18]. One implementation of high-power narrow linewidth lasers was the use of a dual-parallel gain chip with laser output power exceeding 105 mW, laser linewidth up to kHz order, and a tuning range generally covering the full C-band [19,20]. Multi-mode gain is another way to achieve high-power laser output. The output power can reach 150 mW, but the linewidth of this laser is in the order of 100 kHz [21]. The more mature solution in the industry is to integrate an external amplifier, NEC Yamanashi Ltd reported an SiPh hybrid integrated ECL with a booster semiconductor optical amplifier (SOA) through passive alignment technology, achieving an output power greater than 100 mW and laser linewidth narrower than 15 kHz along the whole C-band [22]; Later, NeoPhotonics used similar structures achieved an output power of 21.5 dBm (∼141 mW) and a laser linewidth of 40 kHz - 60 kHz in a wavelength range of 65 nm, moreover an on-chip integrated sensor was developed to against the temperature instability of SiPh external cavity, a frequency stability of 1 GHz was achieved [23]. Due to the large transmission loss of silicon (Si), it is difficult to further narrow the linewidth. In addition, additional temperature control is needed to achieve higher frequency stability because of the large thermo-optic coefficient of Si. Compared to Si material, Si3N4 has lower optical transmission loss and smaller thermo-optic coefficient, which is beneficial to narrowing the linewidth of the laser.
In this paper, to further enhance the performance of the output power and narrow linewidth, we coupled the gain chip with the silicon nitride (Si3N4) dual micro-ring narrowband filter chip to form an ECL, and then integrated the ECL with the SOA through the dual-collimating lens coupling. An external SOA separates gain from amplification to avoid thermal crosstalk. Dual-collimating lens coupling requires less mode field matching between two devices, and can be easily realized in the package. By virtue of the hybrid integration method, the laser output power higher than 220 mW and the linewidth narrower than 8 kHz are obtained. Based on the Vernier effect of the double micro-ring in the narrowband filter chip, the wavelength tuning range of 55 nm in C-band is completed. The laser has SMSR higher than 50 dB and frequency drift less than 200 MHz over 2 hours. This laser has been proven to be useful in coherent laser communication systems. To the best of our knowledge, this demonstrated output power is the highest reported for the hybrid integrated silicon photonic tunable lasers with the linewidth less than 10 kHz.
2. Principle and design
2.1 Structure of ECL device
The proposed ECL device is shown in Fig. 1 (a), including a Si3N4 dual micro-ring narrowband filter chip, a gain chip, an SOA, three collimation lens and a polarization maintaining (PM) fiber collimator. The ECL is formed by butt-coupling the gain chip and the Si3N4 dual micro-ring narrowband filter chip. The Si3N4 dual micro-ring narrowband filter chip is designed with three parts: spot-size converter (SSC), phase shifter, and dual ring resonators. A Sagnac loop reflector is used in the Si3N4 chip. The dual ring resonator are inserted in the Sagnac loop acting as the narrowband filter. The SSC is used to match the mode field of the gain chip and Si3N4 waveguide, increasing the coupling efficiency. The high-Q dual micro-ring filter structure implements the function of the frequency selecting and linewidth narrowing of the laser. The phase shifter is designed to regulate the central frequency of the laser longitudinal mode generated by the gain chip and the dual micro-ring narrowband filter chip combined cavity.
Fig. 1. Hybrid integrated semiconductor laser (a) Structure diagram; (b) Photograph of the butterfly module package.
The narrow linewidth laser output from the gain chip is coupled to the SOA chip for optical power amplification. Considering the difference in mode field size of the gain chip and SOA and the influence of thermal crosstalk, the dual collimating lens structure is employed to improve the coupling efficiency. Finally, the laser is coupled to a PM fiber collimator to achieve a PM output with high power and narrow linewidth. The designed and fabricated ECL device is packaged in a butterfly shell as shown in Fig. 1(b). A TEC is mounted in the shell to control the temperature of the chip.
The InP gain chip (Thorlabs SAF1126C) we used exhibits 80 nm bandwidth gain spectrum with the central wavelength of 1550 nm. The waveguide structure of the gain chip is rib waveguide and the series resistance is ∼1 Ω. The length of the gain waveguide is 1 mm, and the size of the output optical mode field is 4 μm × 1 μm, which only supports optical single-mode transmission. Tilted output waveguide of the gain chip is to reduce the back reflection, and then the lateral beam exit angle is 19.5°. The maximum operating current of the gain chip is 350 mA. The AlGaInAs quantum well rib waveguide SOA is developed by ShiJia Photons with a series resistance of approximately 0.4 Ω. The length of the SOA chip is 2.5 mm. In order to reduce the reflection of the chip facet, the input and output waveguides are tilted and the two end faces are coated with anti-reflection film. The maximum operating current of the SOA chip is 1.5 A and the amplification factor is greater than 25 dB. SOA achieves such high gain by increasing the mode size and cavity length, while larger mode sizes have smaller divergence angles to ensure higher coupling efficiency.
The Si3N4 dual micro-ring narrowband filter chip was fabricated in Ligentec SA with standard multi-project wafer (MPW) process [24]. A strip waveguide with dimension of 1 μm × 800 nm is used for the MRR. The propagation loss of the waveguide is about 0.1 dB/cm. The heater is placed above the waveguide with a distance of 1.7 μm, which has been proven reasonable [25]. The thermo-optic coefficient of Si3N4 waveguide is 2.45 × 10−5 /K, which guarantees smaller frequency drift from the temperature fluctuation compared with Si waveguide.
2.2 Intrinsic linewidth analysis and simulation
The ECL can be simplified as an equivalent model as shown in Fig. 2. The coupling facet of the gain chip is coated with an anti-reflection film, with a reflectivity of 0.01%, and the other facet has a reflectivity of 10% to increase the output power. The external cavity can be considered as a complex wavelength-dependent effective refractive index ${r_{eff}}(w )$. ${r_{eff}}(w )$ is related to the structural design of Si3N4 dual micro-ring narrowband filter chip [26].
Since the laser is output from another facet of the gain chip instead of the coupling facet, the Lorentz linewidth of the ECL device can be expressed as [27,28]
Here, ${\alpha _H}$ is the linewidth enhancement factor of the gain medium, ${P_1}$ is the output power of the external cavity laser.${v_g} = c/{n_{g1}}$, ${v_g}$and${n_{g1}}$ are the group velocity and the group refractive index of the waveguide in gain chip, respectively. h is the Planck constant, v is the central frequency of the laser, ${n_{sp}}$ is the spontaneous emission coefficient. ${\alpha _{tot}} = {\alpha _i} + {\alpha _m}$ is the total loss, ${\alpha _i}$ is the internal loss of the active area, ${\alpha _m} ={-} 1/{L_a}\ln ({{r_1}|{{r_{eff}}(w )} |} )$ is the reflection loss, which is related to the reflectivity of gain chip and external cavity chip.
The intrinsic linewidth of the ECL is defined as
where $F = 1 + A + B$ is the linewidth narrowing factor, andThe intrinsic linewidth of the ECL is simulated. Some parameters used in the simulation are shown in Table 1, some of which are typical empirical values. Both micro-rings adopt an add-drop structure, with symmetrical coupling regions. The power coupling ratio between the ring and the bus waveguide is 0.08, indicating an over-coupled state. The perimeters of the two micro-rings are designed to be 1229 μm and 1251 μm, and the corresponding Q values are 6.12 × 104 and 6.23 × 104, respectively. The effective length of the Si3N4 external cavity is 35.96 mm, and the corresponding theoretical longitudinal mode interval is 29.0 pm. According to the Vernier effect, the amplitude of the output spectrum is the maximum when the resonant central wavelengths of dual MRRs are the same.
Table 1. Parameters used in the simulation of intrinsic linewidth
The resonance curves and cascaded spectrum of the two MRRs are shown in Fig. 3 (a). Due to Vernier effect, the laser wavelength tuning range of the cascaded micro-rings can be $FSR = \frac{{FS{R_1} \cdot FS{R_2}}}{{|{FS{R_1} - FS{R_2}} |}}$, one that $FS{R_\textrm{m}} = \frac{c}{{{n_g}L}}$ is the free spectral range of the MRR, thus FSR = 53 nm, as shown in Fig. 3 (b). The mode suppression ratio between the main mode and the side mode of the cascaded spectrum is ∼ 3 dB, which is convenient to realize the single longitudinal mode emitting of laser.
Fig. 3. Simulation results of intrinsic linewidth of the external cavity laser. (a) resonance curves and cascaded spectrum of the two micro-ring resonators; (b) cascaded spectrum of the dual micro-ring resonators in large wavelength range; (c) effective reflectivity and (d) effective phase of the external cavity; (e) linewidth narrowing factor A, B and F as a function of the optical wavelength; (f) linewidth of the external cavity laser
The effective reflectivity and phase curves of the external cavity are shown in Fig. 3 (c) and (d). The effective reflectivity of the external cavity is Lorentz-type and depends mainly on the transmission curve of the cascaded dual MRRs, while the passive waveguide and coupling efficiency only affect its amplitude. The effective phase of the external cavity changes rapidly near the resonant point and slowly elsewhere.
The curves of the linewidth narrowing factors A, B and F as a function of wavelength are shown in Fig. 3 (e). The linewidth narrowing effect appears in the long-wave direction of resonant wavelength. Figure 3 (f) shows the linewidth curve and effective reflectance curve, and marks the linewidth at resonance, the minimum linewidth and the linewidth at the maximum F in turn. The minimum linewidth appears between the resonant point and the maximum F. In practice, with the use of the phase shifter in the Si3N4 dual micro-ring narrowband filter chip, the phase of the external cavity can be adjusted to achieve the minimum linewidth.
3. Experiment and results
In this section, we characterize the performance of the ECL device. First, the transmission spectrum of the external cavity is exhibited and the P-I curve is measured, then the wavelength tuning and the SMSR characteristics are studied, as well as the linewidth and the relative intensity noise (RIN) measurement are researched, and finally the frequency stability and Allan variance are investigated.
3.1 Transmission spectrum of the external cavity
The transmission spectrum of the external cavity is measured by OSA (APEX AP2083A) with built-in tunable laser, as shown in Fig. 4. Figure 4(a) exhibits the mode difference between the main mode and the adjacent side mode is about 2 dB, which is slightly reduced compared to the simulation results (3 dB). This is mainly due to the misalignment of the resonant frequencies of the two micro-rings. The measured FSR of the external cavity is 54 nm, which is close to the designed value. The simulated teansmission spectrum and the experimental spectrum are compared in Fig. 4 (b), the simulation results are consistent with the experimental results.
3.2 P-I curve
After coupling the gain chip with the external cavity chip, we measured the output power without SOA amplification, as shown in Fig. 5(a). The gain chip is driven by an ultralow noise current source (Vescent, D2-105-500), the temperature of the baseplate is stabilized by a precision temperature controller (Thorlabs, ITC4001), and the temperature is set at 22.2 ℃. Figure 5 shows the output power as a function of the gain chip current before coupled to the SOA, and the phase in the external cavity is a fixed value during the measurement. The threshold current of the gain chip is 50 mA.
Fig. 5. (a)The output power of the gain chip coupled external cavity; (b) The output power of ECL as a function of the SOA current
Performing the measurement of the P-I curve requires the control of the current of the SOA and gain chip in the ECL device as well as the chip temperature. The SOA is driven by an industry leading current source (ILX Lightwave, LDC-3736), which allows up to 4 A of low noise current, other testing conditions are the same as above. The gain chip current is fixed at 100 mA, 150 mA and 200 mA, respectively, and the current of SOA is changed to measure the P-I curve. The test results are shown in Fig. 5(b). As the SOA current increases, the output power tends to approaches saturation, which is determined by the gain characteristics of the SOA. For different input power, SOA has different magnification and saturation value. With a gain chip current of 200 mA and an SOA current of 1.2 A, the output power of ECL device of 226.3 mW can be achieved and the power consumption of the gain chip and SOA is 330 mW and 576 mW, respectively. The SOA did not saturate until the current of the SOA up to 1.5A, we didn’t add higher current to avoide damaging the SOA as the maximum operating current of the SOA chip is 1.5A, so its saturation power was greater than 240 mW.
3.3 Tuning characteristic
Owing to the Vernier effect of the dual MRRs, the external cavity laser has a good performance of wide wavelength tuning range. By adjusting the power of the thermal electrode on phase shifter and two MRRs in Si3N4 dual micro-ring narrowband filter chip, ECL device can be tuned. During the wavelength tuning, the driving current of the gain chip and the SOA remains unchanged, only changing the power of the thermal electrode. The power of the gain chip is 88 mW, and the power of the SOA is 75 mW. At this point, the output power of the ECL device is 7 mW. Figure 6 shows the measured spectrum with the wavelength tuning range of 55 nm in C-band from 1529.37 nm to 1584.13 nm.
The corresponding relationship between the power applied on the thermal electrode in the two MRRs and phase and the laser center wavelength in the laser tuning process is shown in Fig. 7(a). For some central wavelengths, it is only necessary to adjust the power of the thermal electrode on the MRR to obtain it. The realization of the other central wavelength also requires to regulate the power of the thermal electrode on phase shifter. The electrode power on MRR 1 and MRR 2 is basically linear. We also show the total power including all thermal electrodes, gain chip and SOA. The maximum total power consumption of the ECL device is ∼460 mW.
The SMSR of the laser spectrum is also an important parameter for ECL device. The SMSR of an ECL device is determined by the gain spectrum of the gain chip and the equivalent reflection spectrum of the Si3N4 dual micro-ring narrowband filter chip. Figure 7(b) shows the SMSR of the designed ECL device within the tuning wavelength range. At different center wavelengths, the SMSR of the laser spectrum is greater than 50 dB. The inset in Fig. 6 shows the laser spectrum with the center wavelength of 1550.67 nm, and the SMSR is 51.6 dB. The measurement value of longitudinal mode interval is 24.2 pm, which is basically consistent with the design value.
3.4 Linewidth
According to the analysis in Section 2.2, the frequency noise of the ECL device is mainly determined by the characteristics of the Si3N4 external cavity chip and the coupling between the external cavity chip and the gain chip. The linewidth of the ECL device was measured by OEwaves Laser Linewidth Measurement System (OE4000) with an external isolator. We set the driving current of SOA and gain chip to 80 mA, so that the output power is less than 10 mW which meets the limit of the OE4000 input power. The measured frequency noise power spectrum density (PSD) curve at wavelength of 1550.67 nm is shown in Fig. 8 (a). The noise caused by external interference such as current source noise, temperature and vibration is mainly reflected in the low frequency band. This is the reason why the noise of 100 Hz to 1 kHz laser frequency drops quickly and there is a spike noise. In the high frequency band, the white noise $S_v^0$ of the laser is 755.4 Hz2/Hz, and the laser linewidth $\Delta v = \pi S_v^0$[30] is calculated to be 2.37 kHz.
At the same time, the laser linewidth at high power was measured. The drive current of the SOA is 1 A and the drive current of the gain chip is 200 mA. Limited by the input power of the laser linewidth measurement system, the laser output is coupled into the attenuator to meet the incident power requirements. At high power, the laser frequency noise curve is basically consistent with that at low power, and the influence of the spontaneous emission noise of SOA on the laser linewidth can be almost ignored, as shown in Fig. 8(a). We also measured the laser noise spectral density at each wavelength in Fig. 6, and obtained their corresponding intrinsic linewidths in Fig. 8(b). As shown in the Fig. 8(b), in the wavelength tuning range, the laser linewidth is between 2 kHz and 8 kHz, which mainly due to the different location of the laser longitudinal mode in the resonance curve of the dual MRRs.
According to the simulation results of laser linewidth in section 2.2, fine-tuning the laser center frequency will result in a minimum linewidth point. We achieve detuning of the laser center frequency by changing the current of the gain chip. The frequency detuning is read by a high-precision spectrometer (APEX AP2083A). The laser linewidth, output power and frequency detuning during is recorded under different current of the gain chip, as shown in Fig. 9 (a). It can be seen that the trend is basically consistent with the simulation results. When the frequency detuning is 1.31 GHz, the laser linewidth can be minimized to 995 Hz. Figure 9 (b) shows the frequency noise measured under different frequency detuning values. In order to observe the changes in intrinsic linewidth more clearly, we only present white noise at high frequencies. As the frequency detuning gradually decreases, the spectral noise shows a trend of first rapidly decreasing and then gradually increasing.
Fig. 9. (a) The spectral linewidth as function of the detuned frequency and gain chip current (b) Frequency noise spectrum of the ECL device when the laser frequency is detuned
We measured the RIN of the ECL with OEwaves low frequency (OE4000) and high frequency (OE4001) RIN measurement system. The measured RIN curves at wavelengths of 1530 nm, 1550 nm and 1580 nm in the frequency ranges of 1 Hz - 100 MHz and 100 MHz - 40 GHz are shown in Figs. 10 (a) and (b), respectively. Compared with 1550 nm, there is an obvious noise increase at 1 kHz - 100 kHz at the other two wavelengths of 1530 nm and 1580 nm, which is mainly caused by the thermal noise introduced by the driven current of the MRRs. For the frequency offset of 50 MHz as an example, the RIN is -146 dBc/Hz, -153 dBc/Hz, and -141 dBc/Hz for 1530 nm, 1550 nm and 1580 nm, respectively.
The RIN curves under different powers are shown in Fig. 11(a) and (b). The attenuator is used to insure the same input power in the RIN measurement. At high power (220 mW), the RIN is significantly reduced. Compared with the measurement under low power, there are fewer spikes in the RIN curve under high power, and the bulge between 106 and 107 Hz is also significantly reduced as shown in Fig. 11(a). For the high frequencies, as shown in Fig. 11(b), the RIN curve at high power has no relaxation oscillation process, which is due to the weak response of SOA to high frequencies.
3.5 Stability
We measured the frequency stability of the ECL device and calculated the Allan variance, as shown in Fig. 12. From Fig. 12(a) it can be seen that the short-term (5 mins) frequency drift is less than 50 MHz, and for the long-term (120 mins) the center frequency of the ECL device is drifted within 200 MHz. Based on the frequency stability data in Fig. 12(a), the Allan variance of the ECL device was calculated as shown in Fig. 12(b), with the value of 2.3 × 10−8 at 100 s.
4. Application
In this section, we demonstrated the feasibility of this high power, narrow linewidth tunable ECL for space coherent laser communications. The proof-of-concept space coherent communication scheme is shown in Fig. 13 [31]. The developed tunable ECL is used as the transmitter laser source and a commercial narrow-linewidth laser (RIO ORIONTM laser module) as local oscillator laser. The central wavelength of both lasers is tuned to 1555.732 nm. 2.5 Gbit/s pseudo-random binary signals code are applied to the IQ modulator and 5 Gbit/s quadrature phase shift keying (QPSK) modulation signal is entered into the optical 90° hybrids after attenuation. In the current space optical satellite links, a rate of 5Gbit/s can still meet the data transmission requirements.
Under ideal conditions, for an inter-satellite link of 3000 km, the total link loss is -56 dB, and the signal optical power reaching the detector is dozens of uW. In real space laser communication links, the main factors that reduce signal quality include Doppler frequency shift, power fluctuation, wavefront distortion caused by the atmosphere, platform vibration, tracking error, and coupling of space light with single-mode fibers. In the simulation experiment, in order to demonstrate whether the laser we built can meet the QPSK coherent communication requirements for linewidth and phase stability of laser source at low received power, we only consider the propagation loss in inter-satellite link, using an attenuator to simulate it. The signal optical power into the optical 90° hybrid is 20.43 μW and the local optical power is 70.19 μW. The I and Q signals are sampled by high-speed oscilloscope synchronously with the sampling rate of 20GSa/s, and then the homodyne demodulation of the QPSK signal can be realized by off-line processing. The baseband data of I and Q are shown in Fig. 14, and there is no error code compared with the transmitter original code. These obtained results show that the linewidth and frequency stability of our developed high power, narrow linewidth ECL device meet the 5 Gbit/s QPSK space coherent communications requirements.
Digital coherent transmission is the most promising candidate in high-speed optical transport fiber networks, and with the increase of communication speed, more advance multi-level modulation formats require the tunable lasers to have such characteristics as with very high output power and narrow linewidth, compact size and low power consumption [22]. The laser designed in this article has potential applications in highspeed coherent communications with high-order formats such as 16/64 QAM [23,32].
5. Discussion
We compared the performance of our fabricated laser with the published hybrid integrated external cavity semiconductor lasers, including output power, intrinsic linewidth, tuning range, and SMSR, as shown in Table 2. Our laser has superior performance in output power and linewidth. In particular, in terms of output power, the 220 mW output power exceeds the current level, and the intrinsic linewidth of the laser is also relatively narrow to meet the current applications of coherent optical communication. The wavelength tuning range and SMSR of the laser are competitive. The follow-up work can be proceeded with optimizing the frequency stability of the ECL device.
Table 2. Performance comparison with other ECLs
6. Conclusion
We designed and fabricated a high power, narrow linewidth ECL with a hybrid integration of gain chip, SOA and Si3N4 dual micro-ring narrowband filter chip. The structure and parameters of the narrowband filter chip are optimized by using the linewidth narrowing theory of the external cavity laser. By means of the optimal design of the SSC and the adjustment of the spot size with the dual collimating lens, we achieve efficient optical transmission between the external cavity, gain chip and SOA, and obtain a high output power of 220 mW. By virtue of the Vernier effect of the double micro-rings in the narrowband filter chip, the laser has a narrow linewidth <8 kHz in the wavelength tuning range of 55 nm and an intrinsic linewidth of 2.37 kHz at 1550 nm. The SMSR of the laser is higher than 50 dB and the frequency stability less than 200 MHz over 2 hours. The realization of a hybrid integrated ECL with high power and narrow linewidth provides motivation for the further development of coherent optical communication and LiDAR.
Funding
National Natural Science Foundation of China (62075026, 62275253); Natural Science Foundation of Shanghai (21ZR1472100); Strategic Pilot Science and Technology Project of CAS (Class B) (XDB43030400); Liao Ning Revitalization Talents Program (XLYC2002111); Fundamental Research Funds for the Central Universities (DUT22ZD202).
Acknowledgments
This work was supported by the National Natural Science Foundation of China (NSFC) (Grant Nos.62275253, 62075026), Shanghai Natural Science Foundation of China (21ZR1472100), Strategic Pilot Science and Technology Project of CAS (Class B) (XDB43030400). Liao Ning Revitalization Talents Program (XLYC2002111) and Fundamental Research Funds for the Central Universities (No. DUT22ZD202). The authors thanks ZhangJiang Laboratory for their support and guidance in this project.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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