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Abstract
Inspired by the concept of system-in-a-package (SiP) in electronics, here we report a hybrid microwave photonic receiver prototype by integrating lithium niobate (LiNbO3) dual-parallel phase modulators with silicon nitride (Si3N4) integrated tunable microring filters. In particular, we experimentally characterize these employed key elements and evaluate the down-conversion performance of RF signals from 4-20 GHz to the intermediate frequency. With the advantages of the tunable microwave photonic signal filtering, uniform system performance within a broad operation bandwidth, and low SWaP, the demonstrated hybrid microwave photonic receiver module shows a potential setup to satisfy the requirements of wireless communication systems, phased-array radar systems, and electronic warfare.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Radio-frequency (RF) receivers are required to respond to a growing number of received signals over a wider operation bandwidth up to millimeter waves for adapting to the continuous development of civil and military applications, such as mobile communication [1], satellite communication [2], unmanned aerial vehicles (UAVs) [3], phased array radar [4], and electronic warfare (EW) [5]. In recent years, software-defined RF receivers [6,7] with tunability and reconfigurability are preferred in complicated and changing scenarios. However, it is challenging for traditional microwave devices to operate with such broad bandwidth, and their frequency range that can be flexibly processed is typically limited to several GHz [8]. Besides, it is hard to realize the large-range reconfigurable signal processing with electrical methods [9].
Microwave photonic technologies [10] have been providing promising options for overcoming these limitations of traditional microwave solutions. Utilizing the abundant bandwidth in the optical domain and the unique advantages of photonic components, microwave photonic systems exhibit remarkable superiority in large operation bandwidth, broadband tunability, immunity to electromagnetic interference (EMI) [11,12]. However, most current microwave photonic systems are based on discrete photonic devices, resulting in disadvantages of large size, high power consumption, and unstable system performance. Although the maturity of silicon-based photonic integration technologies has encouraged the high-volume integrated microwave photonic system [13–18], the single material platform can not provide all the required functions and target performance so far. For example, silicon is a poor material for reliable light sources. Due to the high nonlinear loss induced by the two-photon absorption (TPA) effect [19], the standard 220 nm thick silicon waveguide can not support the high-power optical transmission, which is essential for increasing the RF gain in microwave photonic receivers. Besides, the relatively high waveguide propagation loss [13] limits the performance of high-resolution filtering and optical delay lines. Hence, inspired by the concept of system-in-a-package (SiP) in electronics [20], we try to combine the strengths of different material platforms through hybrid integration and packing technologies to build a proof-of-concept SiP prototype.
In this paper, we report a hybrid microwave photonic receiver for the down-conversion of received RF signals from 4-20 GHz to the intermediate frequency (IF). With system-level hybrid packaging technology, we establish a proof-of-concept demonstration of the receiver module by integrating lithium niobate (${\rm LiNbO_3}$) dual-parallel phase modulators with silicon nitride (${\rm Si_3N_4}$) integrated tunable bandpass filters. We experimentally characterize employed key components and measure system performance metrics of the demonstrated receiver module. This demonstrated hybrid microwave photonic receiver shows the advantages of broad operation bandwidth, tunable microwave photonic signal filtering, and superiority in size, weight, and power efficiency (SWaP).
2. Operation principle of the hybrid microwave photonic receiver
Figure 1 illustrates the schematic structure of the hybrid microwave photonic receiver, and its operation principle can be understood by considering the evolution of optical spectra at several key points within the system. The light from a continuous-wave laser is split equally into two branches as optical carriers, as shown in spectrum A. One phase modulator in the upper branch is driven by the RF input signal, while the other one in the lower branch is driven by the local oscillator (LO). Assuming the RF input signal and the LO are both single-tone signals for simplifying the calculation, and their center angular frequencies are $\omega _{RF}$ and $\omega _{LO}$, the optical fields at the outputs of two modulators can be expressed as
These extracted optical signals from the two TBPFs are then combined with at a ${\rm 50}:50$ optical coupler (see spectrum F) and detected by a balanced photodetector (BPD) for down-converting received RF signals to the IF. The BPD can reject the common-mode noise induced by the relative intensity noise (RIN) of the employed laser source [22]. The obtained electrical signal is given by
3. Module construction and experimental results
Figure 2(a) shows the experimental construction of the hybrid microwave photonic receiver module. In our demonstrated receiver prototype, the key elements are bulk-${\rm LiNbO_3}$-based dual-parallel phase modulators and ${\rm Si_3N_4}$-waveguide-based integrated TBPFs. A continuous-wave light around 1550 nm is fiber-coupled into the bulk ${\rm LiNbO_3}$ chip with a fiber coupling loss of 2 dB. This ${\rm LiNbO_3}$ chip (customized from PANWOO Integrated Optoelectronics Inc.), fabricated on the mature bulk-X-cut ${\rm LiNbO_3}$-based annealed proton exchange (APE) waveguide platform [23], consists of a ${\rm 50}:50$ optical splitter and two parallel phase modulators. Each phase modulator, with a 30 mm effective modulation arm, is simulated to support an electro-optic (EO) 3-dB bandwidth of 20 GHz and induce an insertion loss of about 2 dB. The output beams exiting the two modulators are then directly butt-coupled into the adjacent ${\rm Si_3N_4}$ chip. The modulator output ports are in line with the input ports of the ${\rm Si_3N_4}$ chip, and the two optical ports on the ${\rm LiNbO_3}$ chip and the ${\rm Si_3N_4}$ chip are designed to have the same separation of 508 ${\mu} \rm{m}$. Besides, with spot-size converters [24] fabricated on the edge of the ${\rm Si_3N_4}$ chip, this butt-coupling loss is measured to be 2 dB. As shown in Fig. 2(c), the ${\rm Si_3N_4}$ chip consists of two racetrack microring resonators (MRRs) serving as bandpass filters and a ${\rm 50}:50$ optical coupler for mixing the target filtered signals. In our demonstration, the 3-dB bandwidths of these two MRRs are designed to be 4 GHz, with coupling gaps of 0.7 ${\mu} \rm{m}$ and coupling radii of 125 ${\mu} \rm{m}$, for achieving an instantaneous bandwidth of 4 GHz in the receiver. The free spectral ranges (FSRs) of the MRRs are designed to be 55 GHz, with MRR perimeters of 3165 ${\mu} \rm{m}$, for processing the double-sideband modulated signals up to 20 GHz. These passive components are fabricated on the TriPleX asymmetric double-strip (ADS) ${\rm Si_3N_4}$ waveguide platform, which features a low propagation loss of 0.1 dB/cm [25]. Furthermore, with Cr-Au metal heaters deposited on top of two MRRs, the center frequencies can be tuned based on the thermo-optic effect. The outputs from the ${\rm Si_3N_4}$ chip are then fiber-coupled into an external BPD with a coupling loss of 2 dB for performing the optical-to-electrical conversion. The obtained IF electrical signal is then sent into a low-noise amplifier (LNA).
Fig. 1. Schematic block diagram of the proposed hybrid microwave photonic receiver module. CWL, continuous-wave laser; PM, phase modulator; TBPF, tunable bandpass filter; BPD, balanced photodetector; LNA, low-noise amplifier. Insets A-F indicate the evolution of optical power spectra (higher-order modulation sidebands in insets B and C are neglected) at several key points within the receiver system. The inset G shows the output IF spectrum.
Fig. 2. (a) Experimental construction of the hybrid microwave photonic receiver module (not to scale). SSC, spot-size converter; MRR, microring resonator. (b) The photograph of the packaged hybrid microwave photonic receiver module. This packaged module occupies a volume of ${\rm 90} \textrm{mm}\times 20 \textrm{mm}\times 10\textrm{mm}$. PMF, polarization-maintaining fiber; PMFA, polarization-maintaining fiber array. (c) The micrograph of the fabricated ${\rm Si_3N_4}$ chip. Inset is the schematic cross-section of the standard TriPleX asymmetric double-strip (ADS) ${\rm Si_3N_4}$ waveguide, with top strip thickness $h_{g1}$ = 175 nm, intermediate ${\rm SiO_2}$ thickness $h_t$ = 100 nm, bottom strip thickness $h_{g2}$ = 75 nm, a waveguide width $w$ = 1.1 ${\mu} \rm{m}$, and an etching angle $\alpha$ = $82^\circ$.
As shown by the photograph of the packaged receiver module in Fig. 2(b), except for the aforementioned optical package, we further employ RF transmission lines mounted on ceramics to carry the driving signals from SMA connectors to modulators and from modulators to off-chip matching resistors (50 ${\rm \Omega}$). These RF components are designed to be as compact as possible to minimize RF losses at high frequencies in the package. Moreover, we place a thermo-electric cooler (TEC) under the ${\rm Si_3N_4}$ chip for stabilizing the center frequencies of two TBPFs in fluctuating environmental conditions. In summary, the packaged receiver module prototype [see Fig. 2(b), external laser source and BPD are not included] occupies a volume of ${\rm 90} mm\times 20 mm\times 10mm$. The light is coupled in and out of the module with polarization-maintaining fiber and fiber array, and the driving signals are sent into the packaged module via SMA connectors.
In subsection 3.1, we first experimentally describe the employed key elements in the packaged microwave photonic receiver module, including bulk-${\rm LiNbO_3}$-based dual-parallel phase modulators and ${\rm Si_3N_4}$-waveguide-based integrated TBPFs. In subsection 3.2, we demonstrate the down-conversion of RF signals from 4-20 GHz to the IF using the hybrid microwave photonic receiver module, and experimentally measure several critical system performance metrics [26,27], including RF gain, noise figure (NF), and spurious-free dynamic range (SFDR).
3.1 Experimental characterization of key elements
We characterize the EO responses of packaged phase modulators by measuring the modulation spectrum at the output of each modulator using a high-resolution (20MHz) optical spectrum analyzer (OSA, APEX, AP2081B) and recording the -1st-order modulation sideband power, with the driving signal frequency varying from 2 to 20 GHz. As shown by the normalized measured modulation responses in Fig. 3(a), each packaged phase modulator (including the RF loss induced by RF transmission lines mounted on ceramics and SMA connectors) can support a 3-dB EO bandwidth of larger than 20 GHz, leading to a broad operating bandwidth of the microwave photonic receiver.
Fig. 3. (a) Normalized measured modulation responses of two phase modulators. (b) Half-wave voltages of two phase modulators at different frequencies.
We evaluate the half-wave voltage ($V_{\pi }$) of the phase modulator by calculating the ratio of -1st-order and -2nd-order modulation sideband powers [28] based on the measured modulation spectrum, which can be expressed by $J_{1}^2(\beta )/J_{2}^2(\beta )$, and $\beta =\sqrt {2PR}\cdot \pi /V_{\pi }$ is the modulation index of the driving signal. Here, P is the actual effective RF power loaded to the modulator and R is the input impedance (50 ${\rm \Omega}$) of the modulator. As shown in Fig. 3(b), the equivalent $V_{\pi }$ of each packaged phase modulator is determined to be around 9 V, and thus the modulation efficiency is $V_{\pi }\cdot L=27\ {\textrm{V}\cdot \textrm{cm}}$. Besides, with the driving signal of 20 dBm, $\beta$ is calculated to be about 1.1.
We characterize transmission responses of two fabricated ${\rm Si_3N_4}$ MRRs by employing the aforementioned advanced OSA with a built-in tunable laser. Figure 4(a) shows normalized measured transmission spectra of the two MRRs around 1550.3 nm, including bandpass and notch responses. The bandpass response of ${\rm MRR_1}$ exhibits a 3-dB bandwidth of 3.56 GHz and an extinction ratio (ER) of 20.4 dB, and the bandpass response of ${\rm MRR_2}$ exhibits a 3-dB bandwidth of 3.54 GHz and an ER of 21 dB. Thus, the channel bandwidth of the receiver system is determined to be 3.56 GHz. These measured 3-dB bandwidths are slightly narrower than design values due to fabrication errors. Besides, as shown in Fig. 4(a), the intrinsic insertion loss of each MRR bandpass filter is estimated to be 2 dB because of the 3 dB loss induced by the 50:50 coupler. We further measure the center wavelength tuning of each MRR by increasing the heating power applied on the micro heater (the resistance is measured to be 560 ${\rm \Omega}$) and observe a linear relationship of about 0.28 GHz/mW, as shown in Fig. 4(b). Therefore, a tuning power up to 100 mW needs to be applied on the micro heater for tuning each bandpass filter in the receiver module.
Fig. 4. (a) Measured transmission spectra [including bandpass (solid lines) and notch (dotted lines) responses] of two ${\rm Si_3N_4}$ MRRs. (b) Center wavelengths of two MRRs are tuned with increasing heating powers.
3.2 System performance of the hybrid microwave photonic receiver
With the aforementioned key elements, we have built the hybrid microwave photonic receiver module [see Figs. 2(a) and (b)]. In the experimental setup, we employ an external narrow-linewidth fiber laser (NKT photonics, Koheras ADJUSTIK, E15) to provide an optical carrier (1549.93 nm) of 100 mW with a RIN of < -135 dBc/Hz. The measured optical insertion loss from this laser source to the external BPD through the packaged module amounts to 10 dB. Here, an insertion loss of 6 dB is expected from the fiber coupling and the butt-coupling within the module, and a 4 dB loss is attributed to the modulator and the MRR bandpass filter. The external BPD (Finisar BPDV2120R) has a responsivity of 0.62 A/W and an electrical bandwidth of 40 GHz. The LNA after the BPD can provide a gain of 25 dB (NF: 4 dB) for the signal at frequencies 0.1-40 GHz. The overall power consumption of the entire microwave photonic module is estimated to be about 5 W, which can be further reduced to 3 W with the full integration of all the devices, including laser, modulators, TBPFs, BPD, and LNA, in a single package. Besides, since the TEC stability reaches 0.01 $^{\circ }$, the wavelength drifts of the employed TBPFs can be reduced to 0.15 pm based on our previous studies [29].
We perform the down-conversion function of this microwave photonic receiver in the experiment and investigate its RF link performance using the method of two-tone test [13,18,26–28]. With the two-tone test signals centered at different frequency bands sent into the receiver module, we measure output IF powers of fundamental tones and intermodulation distortion tones with different RF input powers. As an example, a two-tone test signal centered at 19 GHz with a frequency interval of 2 MHz (i.e., 18.999 GHz and 19.001 GHz) is generated by two synthesized signal generators (HP 83731B) and sent into the RF signal modulator, while the LO modulator is driven by a single-tone signal at 20 GHz of 20 dBm. Figures 5(a) and (b) exhibit optical spectra measured after the RF signal modulator and the LO modulator, respectively. Due to the low driving power, the RF input signal diverts significant optical power towards $\pm$1st-order modulation sidebands only. However, the high-power LO generates multiple modulation sidebands and causes the crosstalk with an isolation of 40 dB in the adjacent RF signal modulator. This microwave crosstalk is caused by the close distance (508 $\mu m$) between the two PM modulators fabricated on the same ${\rm LiNbO_3}$ chip. The -1st-order modulation sidebands of the RF signal and the LO are then filtered by MRR bandpass filters, while the other sidebands and the carrier are suppressed by >15 dB, as shown by the measured optical spectrum in Fig. 5(c). After the photo-detection, the two-tone RF input signal is down-converted to the IF at 0.999 GHz and 1.001 GHz. However, due to the nonlinearity in the modulation and detection, the third-order intermodulation distortion (${\rm IMD3}$) located at 0.997 GHz and 1.003 GHz can be also observed in the electrical spectrum [see Fig. 5(d)] acquired by a signal analyzer (Agilent PXA N9030A). The IMD3 is regarded as the main limiting distortion factor in the microwave photonic receiver, because the IMD2 most of the time can easily be filtered out for narrowband systems [27]. Therefore, we can measure the output IF powers of fundamental tones and IMD3 tones as functions of the RF input power to evaluate the RF gain, NF, and SFDR of the receiver. Furthermore, we perform the down-conversion experiment from the C-band to the K-band with the two-tone test signals at different frequencies. Figures 5(e)-(h) show the experimental results in the C-band (RF signal: 3.999 GHz and 4.001 GHz, LO: 5 GHz), X-band (RF signal: 8.999 GHz and 9.001 GHz, LO: 10 GHz), Ku-band (RF signal: 13.999 GHz and 14.001 GHz, LO: 15 GHz), and K-band (RF signal: 18.999 GHz and 19.001 GHz, LO: 20 GHz), respectively. Within such a broad operation bandwidth (4-20 GHz), this microwave photonic receiver module (including the LNA) maintains a good uniformity in the performance: the RF gain is measured to be -10.1$-$-13.2 dB, the NF is measured to be 44.4$-$48.7 dB, the SFDR is measured to be 102.1$-$105.1 ${\rm dB}\cdot Hz^{2/3}$, and the measured noise floor is approximately -140 dBm/Hz.
Figure 6 presents the RF gain, NF, and SFDR measured as functions of the IF with the LO centered at 15 GHz, the RF input signal centered at 15-IF GHz, and a laser output power of 100 mW. Therefore, we can expect that this microwave photonic receiver module has the capability of down-converting the broadband RF input signal with a stable performance.
Fig. 5. (a) Measured optical spectra after the RF signal modulator. (b) Measured optical spectra after the LO modulator. (c) Measured optical spectra after MRR bandpass filters. (d) Measured electrical spectrum (RBW: 10 KHz, VBW: 10 KHz) of the down-converted IF signal. (e) Measured RF gain, NF, and SFDR with the two-tone test signal centered at 4 GHz and the LO centered at 5 GHz. (f) Measured RF gain, NF, and SFDR with the two-tone test signal centered at 9 GHz and the LO centered at 10 GHz. (g) Measured RF gain, NF, and SFDR with the two-tone test signal centered at 14 GHz and the LO centered at 15 GHz. (h) Measured RF gain, NF, and SFDR with the two-tone test signal centered at 19 GHz and the LO centered at 20 GHz. OIP3, output intercept point for IMD3.
Fig. 6. RF gain, NF, and SFDR are measured as functions of the IF with the LO centered at 15 GHz and the RF input signal centered at 15-IF GHz (LO: 20 dBm, laser source power: 100 mW).
4. Discussion and outlook
To the best of our knowledge, a record-high RF link performance of chip-based microwave photonic systems has been achieved in [26], featuring a positive RF link gain of 8 dB, a minimum NF of 15.6 dB, and an overall SFDR of 116 ${\rm dB}\cdot Hz^{2/3}$. This study presents a high-performance chip-based microwave photonic notch filter scheme by optimizing the usage of the optical amplifier. Therefore, although the demonstrated microwave photonic receiver module has shown the advantages of the broad operation bandwidth, tunable microwave photonic signal filtering, uniform system performance, and low SWaP, its RF link performance is expected to be further improved in several aspects. To reduce the NF, we can apply the broadband LNA with a high gain before the RF signal modulator instead of after the PD. In this case, the NF of the entire receiver system is dominated by this low-noise pre-amplifier. For example, if another broadband RF LNA (gain: 25 dB; NF: 4 dB) is applied before the signal modulator, the NF can be reduced significantly to less than 20 dB. However, this pre-amplifier before the modulator will limit the dynamic range of the entire receiver module [18]. Moreover, because the relation between the RF loss and the optical loss in the microwave photonic system is quadratic [27], both the RF gain and the SFDR can be improved by optimizing the optical loss within the module or increasing the laser source power. For instance, the coupling loss per ${\rm Si_3N_4}$ coupler can be decreased to below 0.2 dB [24] and the laser power can be increased up to 25 dBm (the modulator chip can hold a maximum optical power up to 23 dBm). Thus, based on relevant studies [18], the RF gain is expected to be improved by larger than 10 dB and the SFDR is expected to be enlarged by about 5 dB. However, it is worthwhile to mention that although the back-end microwave amplifier (LNA after the BPD) can also increase the RF gain, it worsens the NF at the same time.
With the recent development and commercialization of lithium niobate on insulator (LNOI) technology [30], monolithic high-speed electro-optic modulators on LNOI (3-dB EO bandwidth: 100 GHz, ${\rm V}_{\pi }$:4.4 V, length: 5 mm) have been demonstrated [31]. Thus, the operation bandwidth of the microwave photonic receiver can be broadened by employing such LNOI-based modulators. Moreover, thanks to the low optical propagation loss (<0.3 dB/cm) of the LNOI waveguide [32], the TBPFs can be fabricated on the same LNOI chip with modulators, which can significantly reduce the SWaP and optical loss of the microwave photonic receiver module. Besides, compared with employed MRRs, reconfigurable integrated optical filters [33] can be utilized to realize a reconfigurable microwave signal processing in the receiver. In the future, we can employ a DFB semiconductor chip instead of the bulk fiber laser to provide the optical carrier [28]. Besides, a micro-isolator needs to be placed after the semiconductor laser chip to isolate the reflection caused by the butt-coupling between different chips within the packaged system. Therefore, we can further reduce the SWaP of the whole system by packaging the laser source and the PD chip into the module via the cost-effective and compact hybrid integration technology. For example, the semiconductor laser chip and BPD can be interconnected with our demonstrated receiver module using free space micro-optics consisting of lens and mirrors in a single package [28,34].
5. Conclusions
In conclusion, we have demonstrated a packaged microwave photonic receiver module by hybrid integrating bulk-${\rm LiNbO_3}$ dual-parallel phase modulators with ${\rm Si_3N_4}$ integrated TBPFs, aiming to filter the modulated RF input signal in the optical domain and down-convert it from 4-20 GHz to the IF. In experiment, we perform the down-conversion function of this module and demonstrate a uniform system performance from the C-band to the K-band. With the advantages of broad operation bandwidth, tunable microwave photonic signal filtering, and low SWaP, the proposed hybrid module would provide an effective path to realize a practical microwave photonic receiver. In future research, we will focus on demonstrating the performance of the hybrid microwave photonic receiver in receiving broadband linear frequency modulation (LFM) radar signals or other broadband RF signals.
Funding
National Key Research and Development Program of China (2018YFB2201802); National Natural Science Foundation of China (61771285).
Acknowledgments
The authors thank LioniX B.V. for offering the TriPleX waveguide manufacturing technology.
Disclosures
The authors declare no conflicts of interest.
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