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Abstract
As a photonic-based microwave signal generation method, the optoelectronic oscillator (OEO) has the potential of meeting the increasing demand of practical applications for high frequency, broadband tunability and ultra-low phase noise. However, conventional OEO systems implemented with discrete optoelectronic devices have a bulky size and low reliability, which extremely limits their practical applications. In this paper, a hybrid-integrated wideband tunable OEO with low phase noise is proposed and experimentally demonstrated. The proposed hybrid integrated OEO achieves a high integration level by first integrating a laser chip with a silicon photonic chip, and then connecting the silicon photonic chip with electronic chips through wire-bonding to microstrip lines. A compact fiber ring and an yttrium iron garnet filter are also adopted for high-Q factor and frequency tuning, respectively. The integrated OEO exhibits a low phase noise of -128.04 dBc/Hz @ 10 kHz for an oscillation frequency of 10 GHz. A wideband tuning range from 3 GHz to 18 GHz is also obtained, covering the entire C, X, and Ku bands. Our work demonstrates an effective way to achieve compact high-performance OEO based on hybrid integration, and has great potential in a wide range of applications such as modern radar, wireless communication, and electronic warfare systems.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Microwave sources have been widely used in various applications like modern radar, wireless communication, and electronic warfare systems [1–3]. With the rapid development of information and communications technologies, the available payload and spectrum are becoming increasingly scarce, which calls for compact and high-frequency microwave sources. For electronic oscillators, it is challenging to generate high-frequency signals with low phase noise [4]. Generally, the electronic oscillator only has a low phase noise at low frequencies. Although a high frequency signal can be obtained by the frequency multiplication process, the phase noise performance is also degraded by a factor of 20log10N, where N is the frequency multiplication factor. On the other hand, an optoelectronic oscillator (OEO) [5–9] is an attractive candidate to generate high-frequency microwave signals with ultra-low phase noise and frequency tunability, owing to the advantages of high Q-factor and large bandwidth brought by photonic technology [10–14]. By incorporating high-Q optical energy storage elements such as a long fiber delay line [14] or a high-Q optical resonator [15,16], an ultra-low noise of -163 dBc/Hz @ 6 kHz has been obtained [14], showing significant advantage over its electrical counterparts. However, a typical OEO system uses various discrete optoelectronic devices, including a light source, an electro-optic modulator, a photodetector (PD), and an electrical amplifier. As a result, the system is large, bulky, and expensive.
The rapid development of photonic integrated circuits (PICs) has made it possible to achieve integrated OEOs with compact size and light weight. The overall cost of the OEO system can also be reduced due to the high potential for mass production, for example based on the complementary metal-oxide-semiconductor (CMOS) compatible fabrication process. Several integrated OEOs have been proposed and demonstrated in recent years. For instance, Zhang et al. [17] proposed a partially integrated OEO based on the phase-modulation to intensity-modulation conversion in a silicon photonic chip. Key optoelectronic devices, including a high-speed phase modulator, a tunable high-selectivity microdisk resonator, and a high-speed PD are integrated. Frequency tunable oscillations are realized from 3 GHz to 7.4 GHz with the help of other discrete devices like the light source and electrical amplifiers. The phase noise is around -80 dBc/Hz @ 10 kHz. In [18], Gunn et al. proposed a coupled OEO using CMOS photonics technology. Every component of the OEO loop is integrated on chip except for a long fiber ring and a semiconductor optical amplifier. The phase noise of the generated microwave signal is as low as -112 dBc/Hz @ 10 kHz, but the oscillation frequency is fixed at 10 GHz. In addition, although the above two solutions show the great potential of silicon photonics, the integration of light sources or optical amplifiers is still missing. In our previous work, an InP-based PIC that consists of a directly modulated laser (DML), an optical delay line, and a PD is fabricated to form an integrated OEO [19], and the PIC is wire-bonded to other electronic devices on a printed circuit board. Nevertheless, due to the relatively high noise of the DML and the lack of high-Q energy storage element in the OEO cavity, the phase noise is only about -91 dBc/Hz at a 1 MHz offset. The frequency tuning range of the InP integrated OEO is also very limited (20 MHz). To sum up, current studies on integrated OEOs still exhibit a relatively low integration level, a poor phase noise performance, or a limited frequency tuning range.
In this work, a hybrid integrated wideband tunable OEO with low phase noise is proposed and demonstrated. The laser chip, the silicon photonic chip and a compact fiber loop are first hybrid integrated together, and the silicon photonic chip is then wire-bonded to microstrip lines to connect with the electronic chips. The phase noise performance of the hybrid integrated OEO is evaluated under different loop lengths, and a low phase noise of -128.04 dBc/Hz @ 10 kHz is demonstrated for an oscillation frequency of 10 GHz. Wideband frequency tuning is also achieved from 3 GHz to 18 GHz with the help of a tunable yttrium iron garnet (YIG) filter. This work demonstrates an effective method to achieve compact high-performance OEO based on hybrid integration, and it has great potential in a wide range of practical applications that require compact high-performance microwave sources.
2. Design and principle
Figure 1(a) shows the schematic of the proposed integrated OEO. An optical carrier from a high-power distributed feedback (DFB) laser chip is first routed to a Mach-Zehnder modulator (MZM) at the silicon photonic chip. A radiofrequency (RF) signal is loaded onto the optical carrier at the MZM to realize intensity modulation. The intensity-modulated optical signal is then coupled out of the silicon photonic chip to experience optical delay by a well-wound polarization-maintaining (PM) fiber ring. The optical signal at the output of the fiber ring is coupled back to an on-chip PD to realize optical-to-electrical conversion. The converted microwave signal is then amplified and filtered by RF amplifiers and a narrow bandpass YIG filter, respectively. A digital attenuator is also adopted to precisely control the loop gain. The oscillating microwave signal is split into two channels by a 90:10 electrical coupler. One of the two channels is used to guide the microwave signal back to the MZM to close the OEO loop, and the other channel is for real-time monitoring of the frequency spectrum and phase noise performance. A stable single-mode oscillation would occur when the overall gain of the opto-electronic loop exceeds the loss.
Fig. 1. Schematic and micrographs of the hybrid integrated OEO. (a) Schematic of the proposed integrated OEO. A DFB laser is hybrid integrated with a silicon photonic chip by microlens. A high-speed MZM and PD are fabricated on the silicon photonic chip. The silicon photonic chip is wire-bonded to microstrip lines to connect with the electronic chips. All the optical and electrical components are packaged on an aluminum alloy cavity. MZM: Mach-Zehnder modulator; PD: photodetector; EA: electrical amplifier; YIG: yttrium iron garnet; ATT: attenuator. (b) Micrograph of the entire optical and electrical chips. Optical chips are placed on the ceramic substrate. (c) Micrograph of the integration of the DFB laser chip and the silicon photonic chip. The hybrid integration is realized by two lenses and an isolator to match the mode profile. (d) Micrograph of the silicon photonic chip. Size of the chip is 4 mm × 2.1 mm. Cantilever couplers are used to realize edge coupling with the fiber array.
The micrograph of all optical and electrical chips is shown in Fig. 1(b). All the optical and electrical chips are placed at an aluminum alloy cavity, which has a total size of only 3 cm × 7 cm × 1.4 cm. The electrical chips are micro-assembled directly on the cavity surface for heat dissipation. Several key electrical building blocks, such as a bias tee, electrical amplifiers, attenuators (ATT), and a coupler, are integrated on electrical chips. The hybrid integrated optical chips are then wire-bonded to microstrip lines to connect with the electrical chips. The PM fiber ring and YIG filter are packaged together with the optical and electrical chips to form a closed OEO cavity, and they are placed at different layers to minimize the total module footprint. The YIG filtering response is tuned by varying the applied current to control the magnetic field surrounding the YIG sphere [20]. The filter in our packaged OEO module has a tuning range of 3-18 GHz.
A photograph of the hybrid integrated optical chips is shown in Fig. 1(c). The high-power DFB laser chip and the silicon photonic chip, both working in O band, are hybrid integrated by two lenses and an isolator on a ceramic substrate. To stabilize the working temperature of the optical chips, a thermistor is placed near the photonic chips to monitor the temperature, and a thermoelectric cooler is placed under the ceramic substrate for temperature control. The details of the silicon photonic chip are shown in Fig. 1(d). The silicon photonic chip mainly consists of an MZM, a PD, and cantilever couplers. The cantilever couplers are used to couple with single-mode fibers and the laser chip, and the corresponding coupling losses are about 2 dB and 3.5 dB, respectively.
3. Experiment
The silicon photonic chip is fabricated at a CMOS-compatible foundry. The MZM in the silicon photonic chip is designed based on the free-carrier plasma dispersion effect in silicon, and a travelling-wave electrode structure for differential signal is used for high-speed RF signal modulation. The 3-dB modulation bandwidth of the MZM is measured as 33.68 GHz under a reverse bias voltage of $- $6 V, as shown in Fig. 2(a). The high-speed PD in the silicon photonic chip is achieved by a germanium-doped PIN structure, and its electrical pads are placed away from the MZM for enough space to connect the electrical chips. The measured RF S21 response of the PD at a reverse voltage of -2 V is shown in Fig. 2(b). The 3-dB bandwidth is about 35.45 GHz.
Fig. 2. Measured RF S21 response of the MZM and PD on the silicon photonic chip. (a) Measured RF S21 response of the MZM under different reverse voltages. (b) Measured RF S21 response of the PD at a reverse voltage of $- $2 V.
During the experiment, the gain condition of the stable single-mode oscillation is realized by precisely adjusting the gain and loss of the electrical chips with the help of the ATT. Stable single-mode oscillation is obtained when the overall gain exceeds the loss of the OEO loop. Figure 3(a) shows the measured 10-GHz single-mode-oscillation of the hybrid integrated OEO when a 500-m PM fiber ring is adopted. The resolution bandwidth (RBW) is 1 MHz. In this case, the currents of the DFB laser chip and the YIG filter are 100 mA and 187 mA, respectively. In addition to the 10-GHz frequency component, a second harmonic frequency component at about 20 GHz can also be observed, which is occurred mainly by the nonlinearity of the electro-optical modulator. It can be reduced by tuning the bias voltage of the modulator [8] or placing an electrical filter at the output port. The total gain of the electrical amplifier is 40 dB, and the open loop gain is about 10 dB. From the zoom-in view of Fig. 3(a), a high side-mode suppression ratio (SMSR) of 70 dB is achieved. Figure 3(b) shows the measured single-sideband (SSB) phase noise of the generated signal, which is roughly -115.83 dBc/Hz @ 10 kHz. As a comparison, the phase noise of our previous InP integrated OEO is only about -60 dBc/Hz @ 10 kHz [19], which is 380,000 times higher than this work. Thus, a low phase noise is achieved for the hybrid integrated OEO due to the enhanced Q-factor using the 500-m-long PM fiber ring.
Fig. 3. Experimental results of the hybrid integrated OEO under a single-mode oscillation of 10 GHz. (a) Electrical spectrum of the produced 10-GHz microwave signal. The RBW is 1 MHz. Inset: zoom-in-view of the spectrum around 10 GHz with an RBW of 0.5 kHz. A SMSR of 70 dB is observed. (b) SSB phase noise of the generated 10 GHz signal. The phase noise is measured as -115.83 dBc/Hz@10 kHz.
Frequency tunability of the hybrid integrated OEO is also demonstrated. The oscillation frequency of the hybrid integrated OEO is tuned by varying the operating current of the YIG filter to tune its center frequency. Figure 4(a) shows the superimposed spectrum of the different oscillation frequencies with a frequency tuning step of about 1 GHz. A wideband frequency range from roughly 3 GHz to 18 GHz is obtained, which covers half of the S-band and the entire C, X, and Ku bands. The power flatness within the full tuning range is less than 1 dB, which is achieved by precisely controlling the cavity gain using the ATT. The tuning range is mainly limited by the YIG in the current integrated OEO module. As can be inferred from Fig. 2(a-b), a wider frequency tuning range of more than 30 GHz is possible by using a YIG filter with higher frequency tunability. The phase noise of the generated microwave signal in the entire frequency tuning range is also investigated, and the results are shown in Fig. 4(b). The phase noise is below -110 dBc/Hz at a 10-kHz offset frequency in the entire tuning range. The deterioration of the phase noise performance at high center frequencies is mainly caused by the reduced performance of the electrical devices in the OEO loop.
Fig. 4. Frequency tunability of the hybrid integrated OEO. (a) Superimposed spectrum with a frequency tuning step of 1 GHz. RBW: 1 MHz. A wideband frequency tuning range from 3 GHz to 18 GHz is achieved by altering the central frequency of the YIG filter. (b) Measured phase noise at a 10-kHz offset frequency of different oscillation frequencies. All the phase noise values are below -110 dBc/Hz@10 kHz.
4. Discussion
We can expect a higher performance of the proposed integrated OEO by replacing with better components: The relative intensity noise (RIN) of the DFB laser influences the input noise density injected into the oscillator. In our design, the RIN is -135 dB/Hz, it can be replaced by a laser better than -160 dB/Hz. The E/O conversion efficiency of the modulator and photodetector used in the proposed OEO can be replaced by higher performance ones to lower the needed electrical gain of the OEO loop, so that the caused noise by the amplification would be reduced.
Frequency stability is measured under the oscillating frequency of 9.34 GHz, the overlapping Allen deviation is 7.67 × 10−8@10s, corresponding to an averaging frequency shift of about 716 Hz. There is no mode hopping during the measurement (with a total measurement time of about 10 minutes). We think the relatively large frequency shifts comes from two aspects: (1) Heat fluctuation of the electrical amplifier: The power consumption of the electrical amplifier is approximately 2 W. Even with an aluminum cavity with good heat dissipation, heat fluctuations still influence the stability of the OEO circuit. The available solution is to reduce the needed gain of the electrical amplifier by using components with better performance (e.g., modulator with higher modulation efficiency, photodetector with higher responsibility) to improve the E/O conversion efficiency, so the heat of the electrical amplifier would also be reduced. (2) Phase changes caused by the long fiber: There is a long fiber in the OEO loop to increase the Q value of the OEO. However, the long fiber is sensitive to changes in the environment, thus unwanted phase changes would occur and influence the frequency stability of the OEO. Schemes such as a phase locked loop could be utilized to reduce the unwanted phase disturbances.
Further investigation on the phase noise performance of the hybrid integrated OEO is carried out. According to the Yao-Maleki model [8,9], the SSB phase noise density of an OEO can be simplified as the power spectral density (PSD) of the optoelectronic loop:
where f′ is the frequency offset, δ is the input noise-to-signal ratio, and τ is the loop delay. Thus, the SSB phase noise is inversely proportional to the square of the loop delay τ and can be easily improved by using a spool of long fiber. The SSB phase noise spectra of the hybrid integrated OEO measured under different fiber lengths of 250 m, 1 km, 2 km are shown in Fig. 5(a). The corresponding optical delays are 1.25 µs, 5 µs, and 10 µs, respectively. The oscillation frequency is kept at 10 GHz. The measured phase noise values at a 10 kHz offset are -109.16 dBc/Hz, -121.97 dBc/Hz, and -128.04 dBc/Hz, respectively, corresponding to a difference of about +6 dB, -6 dB, and -12 dB compared to the case of 500-m optical fiber, which agrees well with the theoretical model.Fig. 5. Phase noise performance of the integrated OEO under different delay methods. (a) Phase noise of the proposed hybrid integrated OEO under different loop lengths. The phase noise is -109.16 dBc/Hz@10 kHz, -121.97 dBc/Hz@10 kHz, and -128.04 dBc/Hz@10 kHz for fiber lengths of 250 m, 1 km, and 2 km, respectively. (b) Estimated phase noise and propagation loss of a fully integrated OEO with different time delays. Three CMOS-compatible platforms (Si, SiN, SiO2) are selected. Typical value of the propagation loss coefficients of the three platforms are adopted, which are 1 dB/cm, 0.1 dB/cm, and 0.01 dB/cm, respectively. For an optical delay of 100 ns, propagation losses of all the three platforms exceed 20 dB and the phase noise is only about -88 dBc/Hz@10 kHz.
Although the phase noise performance of the proposed hybrid integrated OEO is considerably enhanced due to the use of a long optical fiber, a fully integrated OEO without any optical fiber may still be desired. An integrated optical delay line can be used instead of the optical fiber as the energy storage element in the fully integrated OEO. To investigate the feasibility of the fully integrated OEO with integrated optical delay line, a simple calculation about the relationship between the phase noise of the fully integrated OEO and the propagation loss of optical delay-line waveguides on chip is carried out. The input noise-to-signal ratio δ used in the calculation is 6.45 × 10−14 Hz-1, which is derived from the measured phase noise value and loop delay in our experiment. Three CMOS-compatible platforms have been selected: silicon (Si), silicon nitride (SiN), and silica (SiO2). Typical values of the propagation loss coefficient are adopted, which are 1 dB/cm, 0.1 dB/cm, and 0.01 dB/cm, respectively [21–23]. The group indexes are 4, 2, and 1.5, respectively. As shown in Fig. 5(b), the phase noise is higher than -90 dBc/Hz at a 10 kHz offset with an optical delay of 100 ns, but the propagation losses of all the three platforms exceed 20 dB, which is difficult for on-chip light sources or semiconductor optical amplifiers to counteract. Thus, a well-wound fiber ring is more practical for an integrated OEO system to generate low phase noise signals with compact size. Ultra-high-Q integrated microring resonator [24–28] is another promising element for large on-chip optical delay. With the rapid development of PICs, super compact low phase noise integrated OEOs can be expected in the near future with an optical delay beyond the µs level based on an ultra-high-Q integrated microring resonator with mature fabrication process. A more compact OEO with good phase noise performance can also be expected by utilizing an Integrated Laser Mach Zehnder (ILMZ) with low noise.
The limiting factor for developing high performance OEO systems for on-chip integration is the monolithic integration of all the best optoelectronic components based on different material platforms. For example, the best modulator with low half-wave voltage is based on the lithium niobate platform, the semiconductor laser with low RIN is based on the III-V platform, the ultrahigh Q integrated MRR is based on the SiN platform. It is a challenge to realize monolithic integration of these components from different material platform. With the development of the photonic integration technology, it can be solved by mature wafer bonding process.
In addition, we have made a comparison of the proposed hybrid integrated OEO with other compact OEO systems, as shown in Tab. 1. Compared to other integrated OEO schemes, the proposed hybrid integrated OEO shows advantages in terms of wide tuning range, low phase noise and high integration level. In addition, compact OEO composed of discrete optoelectronic components has also been reported in the literature such as in [33], whose phase noise is as good as −130 dBc/Hz@10 kHz. The size of the packaged system is in the order of one liter. Although the phase noise of our work is not better than this compact discrete system, our work shows much higher integration level, where most of the optical and electrical components are integrated on-chip, and the size of the packaged module including the optical and electrical chips is only about 3 cm × 7 cm × 1.4 cm.
Table 1. Comparison of different compact OEOs
5. Conclusions
In this work, we have proposed a hybrid integrated wideband tunable OEO with low phase noise, where hybrid integration among a laser chip, a silicon photonic chip, and electronic chips has been demonstrated. The size of the hybrid integrated chips is only 3 cm × 7 cm × 1.4 cm. A wideband frequency tuning range from 3 GHz to 18 GHz is achieved, and the phase noise of the generated microwave signal can be as low as -128.04 dBc/Hz @ 10 kHz. A higher integration level and more compact size can be expected by using an integrated microwave photonic filter based on phase-modulation to intensity-modulation conversion [29,30] using an on-chip phase modulator and high-Q microring resonator. It is also promising to realize monolithic integration of all the possible optical and electrical components on a CMOS-compatible silicon photonic chip, and heterogeneous integration of the laser onto the silicon photonic chip can also be expected [31,32]. Together with a well-wound fiber ring or ultra-high-Q integrated microring resonator, super-compact integrated OEOs with a wideband tunable range and ultra-low phase noise are possible to meet the urgent needs of a wide range of applications including modern radar, wireless communication, and electronic warfare systems. The uncertainty and precision in the measurement of the oscillating frequency and phase noise should also be considered in the future work to estimate precisely the performance of the integrated OEO [33,34].
Funding
National Natural Science Foundation of China (61925505, 62001043, 62135014, 62205329, 62235015); Beijing Municipal Natural Science Foundation (Z210005); China Postdoctoral Science Foundation (2022M723072); Chinese Academy of Sciences through Strategic Priority Research Program (XDB43030000, XDB43030200).
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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