High frequency radio frequency (RF) signals with low phase noise are highly desirable in various communication and radio detection and ranging (radar) systems [1–3]. The key for an oscillator to generate a low phase noise and high frequency oscillation is to have a high quality factor (Q) or low loss energy storage element in the oscillator loop at the desired frequency band [4]. Unfortunately, such a high Q element is difficult to find at RF frequencies beyond 10 GHz in general, although the Q value of sapphire resonators can be sufficiently high at cryogenic temperatures in a narrow frequency band around 9 GHz [5,6]. On the other hand, optoelectronic oscillators (OEOs) [7], which take the advantage of low loss and high bandwidth of optical components, have been shown to be able to generate such high spectral purity RF oscillations of more than 75 GHz [8,9], due to the fact that a long length of low loss optical fiber [10–13] or optical resonators [14–16] having extremely high Q in an ultra-wide bandwidth can be readily used as such energy storage elements for an optical signal carrying RF sidebands up to hundreds of GHz.

An OEO typically includes the following essential optical and RF components: a laser, a modulator, an optical energy storage element (such as a length of optical fiber or an optical resonator), a photodetector (PD) for converting the modulated light into RF, an RF bandpass filter (BPF) for single mode selection, and an RF low noise amplifier (LNA) for providing sufficient gain for the OEO loop to start oscillation [7]. An optical resonator can be used to replace the RF BPF for single mode selection, however, with the requirement to lock the laser frequency to one of the resonant peaks of the resonator [17]. Conventionally, discrete components have been used to fabricate an OEO, making it bulky, costly, and cumbersome to use [18–31]. To fully unlock the potential of the OEOs, monolithically integrating most of the photonic components in an OEO on a tiny chip is highly attractive because it offers the advantages of miniaturization, low cost, easy fabrication, high reliability, and low power consumption compared with that made with bulk optics [32–34]. Recently, several integrated OEO devices have been demonstrated in silicon on insulator (SOI) and indium phosphide (InP) platforms [35–39]. Unfortunately, these platforms have fundamental material limitations that are difficult to overcome. For example, both Si and InP modulators rely on switching mechanisms (carrier injection and quantum-confinement Stark effect respectively) that are intrinsically nonlinear, absorptive, and sensitive to temperature fluctuations. On the other hand, lithium niobate (LiNbO3, LN) has been commonly used to fabricate high-performance electro-optic modulators, thanks to its wide bandgap (high transparency) and large second order (χ2) electro-optic coefficient (30 pm/V) [40]. In contrast to Si and InP, the χ2 process in LN changes its index of refraction linearly with an applied electrical field, on femtosecond timescale. The recent advances in thin film lithium niobate (TFLN) technologies [41–46] make it possible to integrate complicated photonic systems on a chip. However, to the best of the authors’ knowledge, the TFLN photonic integrated OEO has not been reported, despite the attractive characteristics it offers.

Here, we experimentally demonstrate the feasibility of fabricating photonic integrated OEOs on the TFLN platform. Two types of photonic integrated chips (PICs) are designed and fabricated. One PIC is composed of an MZM and an add-drop microring resonator (MRR) for realizing a fixed-frequency OEO. The MZM is for intensity modulation (IM) while the add-drop MRR acts both as an energy storage component and a filter for single mode selection. The other PIC includes a phase modulator (PM) and a notch MRR for implementing an ultra-wide range frequency-tunable OEO. In the fixed-frequency OEO, the RF frequency is fixed at 30 GHz, which is determined by the free spectral range (FSR) of the add-drop MRR. The phase noise obtained is −76dBc/Hz at 1 kHz and −102dBc/Hz at 10 kHz, respectively, which is the best among the reported photonic integrated OEO without using long optical fibers and is much lower than that of the X-band OEO integrated on the InP platform (−60dBc/Hz at 10 kHz from the 8.87 GHz carrier) [35]. In the ultra-wide range frequency-tunable OEO, the oscillation frequency is determined by the frequency spacing between the laser carrier and one of resonances of the notch MRR, while the frequency-tuning range is limited by the FSR of the notch MRR. Wide range tunable signals from 20 to 35 GHz have been obtained by tuning the notch of the notch MRR. Even wider frequency tuning range is feasible if the RF components, such as the low noise amplifier (LNA) with wider bandwidths, are available. The phase noise remains the same at around −87dBc/Hz at 10 kHz at different oscillation frequencies, which confirms the key advantage of the phase noise of OEOs independent of oscillation frequency. More impressively, the phase noise of our Ka-band OEO is even lower than that of the X-band OEO integrated on the SOI platform (−80dBc/Hz at 10 kHz for 3–7.4 GHz) [36]. Our work marks the first step of fabricating integrated OEO chips on the TFLN platform for achieving low cost, small size, low power consumption, high frequency, and wide tunability, which also shows certain advantages over the OEOs integrated on other platforms.

Figure 1(a) depicts the schematic of the fixed-frequency OEO constructed with a TFLN PIC incorporating an MZM and an add-drop MRR (left), and the relationships between the FSR of the MRR and the optical carrier with its modulations sidebands, as well as the RF oscillation frequency of the OEO (right). In operation, a CW light used as an optical carrier is modulated by the MZM to produce a set of sidebands before entering the add-drop MRR, which acts as a comb filter. Only the optical carrier and its modulation sidebands aligned with the transmission peaks of the MRR are allowed to pass with a minimum loss and enter into the erbium-doped optical fiber amplifier (EDFA) before beating in the PD to produce an RF signal with a frequency equal to the FSR of the MRR. In operation, the EDFA is set to work in automatic power control (APC) mode for minimizing fluctuations of the output optical power caused by the mechanical vibrations of the chip-fiber coupling fixture so that the optical power into the PD remains constant around 9 dBm, which can be eliminated if the TFLN PIC is well packaged with a stable output. This RF frequency is then amplified by the LNA to provide the required gain for the RF signal, before feeding back to the MZM to close the OEO loop via an electrical coupler (EC) for establishing the OEO oscillation, provided that the small signal open loop (SSOL) gain of the system exceeds unity [7]. In fact, the MRR also functions as an RF bandpass filter to select a single RF oscillation mode, with the RF center frequency determined by its FSR [47].

Figure 1(b) shows the schematic of the ultra-wide range frequency-tunable OEO utilizing a TFLN PIC containing a PM and a notch MRR (left), and the diagram depicts the relationships between the optical carrier with its modulation sidebands and the transmission dips of the notch MRR, as well as the relationship between the OEO oscillation frequency and the optical carrier, together with its sidebands (right). In operation, the optical carrier is injected into the PM for generating a set of modulation sidebands before entering the MRR. If the optical carrier and its modulation sidebands directly enter the PD without going through the notch MRR to beat, no RF signal can be produced because of the perfect cancellation of the beating signals between the carrier and the sidebands [48,49]. However, if one of the phase modulation sidebands is suppressed by falling into a notch of the MRR, the perfect cancellation is voided such that the phase modulated signal can be converted into an RF signal by the PD, with a frequency equal to the spacing between the carrier and the notch. This RF frequency is then amplified by the LNA before feeding back to the PM to close the OEO loop and establish the OEO oscillation. By tuning the resonances of MRR, the oscillation frequency of the OEO can be tuned accordingly.

The PICs for the fixed-frequency and the ultra-wide range frequency-tunable OEOs were designed and fabricated in an X-cut TFLN platform with a device layer of 360 nm, as shown in Figs. 2(a) and 2(b). First, we patterned and defined all optical waveguides through e-beam lithography (EBL) and inductively coupled plasma (ICP) etching processes. Figure 2(c) (left) shows the SEM image of the etched TFLN waveguide and its sidewalls. Subsequently, a 1-μm-thick SiO2 layer was deposited by plasma enhanced chemical vapor deposition (PECVD). Next, we used e-beam evaporation to fabricate the NiCr load with a thickness of 0.2 μm and the gold layer with a thickness of 0.9 μm. A lift-off process was followed to fabricate the capacitance-loaded traveling-wave electrodes (CL-TWEs). Figure 2(c) (right) shows the SEM image of a zoom-in view of CL-TWEs. Here, CL-TWEs with air bridges were used in TFLN modulators to reduce the geometric length of the device while maintaining a low driving voltage as shown in Figs. 2(a) and 2(b) [50]. The thicknesses of the silicon oxide above and below the electrode were both 1 μm. Figure 2(d) shows the SEM image of the zoom-in view of the coupling region of the MRR. In both fixed-frequency and frequency-tunable OEOs, the ring waveguide width (W1), the bus waveguide width (W2), and the gap (W3) between them were 2.2 μm, 1.38 μm, and 0.8 μm, respectively.

We first demonstrate the fixed-frequency OEO constructed with a TFLN PIC incorporating an MZM with an add-drop MRR shown in Fig. 2(a), in reference to the fixed-frequency OEO implemented using a silicon photonic spiral shape resonator reported by Do et al. [47]. In our photonic integrated fixed-frequency OEO, the most critical component is the MZM, which should have a low half-wave voltage (Vπ) and a large modulation bandwidth. The low Vπ helps to relax the required LNA gain for reducing the heat, power consumption, and cost. In fact, it may even help to completely eliminate the need for an LNA if the optical power entering into the PD is sufficiently high and therefore eliminate the LNA flicker noise contribution to the OEO phase noise (Appendix A) [7]. The TFLN MZM shown in Fig. 2(a) is equipped with a folded CL-TWE with a total modulation length of 2.5 cm. The measured 3 dB electro-optic (EO) bandwidth and the extinction ratio (ER) are 38 GHz and 29 dB, respectively, as shown in Fig. 3(a). This large 3 dB EO bandwidth ensures that the OEO will generate the high frequency signals at Ka-band. Figure 3(b) shows that the measured Vπ is only 1.2 V, which is significantly lower compared to the typical Vπ of >4V with commercial MZMs fabricated with bulk LN [51].

The add-drop MRR is another key component for the fixed-frequency OEO, which serves both as an energy storage component and a filter for OEO single mode selection. The MRR in the fixed-frequency OEO should be featured with a narrow bandpass bandwidth and low insertion loss (IL) to generate a low phase noise RF signal. The higher the Q factor, the better the selectivity of the filter, resulting in superior phase noise of the OEO [47]. Figure 3(c) shows the measured transmission spectra of both the through (red curve) and drop (blue curve) ports of the add-drop MRR, as labeled in Fig. 1(a). Note that the transmission data are obtained with a testing MRR identical to the one in Fig. 2(a), which was purposely fabricated on the same chip, to eliminate the influence of the MZM in testing. Specifically, the output of a tunable laser controlled by a computer is coupled into the testing MRR via a polarization controller, and the output from the MRR is coupled into a PD to measure the output power as the laser wavelength is scanned [52]. The FSR is measured to be 30 GHz, corresponding to the circumference of 4.5 mm, for realizing 30 GHz OEO oscillation and filtering out other oscillation modes. An ER of 25 dB is obtained at the MRR drop port, as shown in the inset of Fig. 3(c). The measured IL and the 3 dB bandwidth of the drop port at a wavelength of 1550.14 nm are 1.5 dB and 742 MHz (corresponding to a Q value of 2.8×105), respectively. Compared with the Q value of 2.2×105 and IL of >5dB of the add-drop MRR fabricated with the silicon platform used in the OEO of Ref. [47], our TFLN MRR has much lower IL and slightly better Q. Reducing the waveguide scattering loss caused by sidewall roughness can further increase the Q and minimize the IL.

The SSOL gain of the fixed-frequency OEO is expected to have a sharp peak, with its center frequency and bandwidth determined by the FSR and the 3 dB bandwidth of the add-drop MRR, respectively. An EDFA (Amonics: AEDFA-PKT-DWDM-15-B-FA) operating in the constant power mode with a nominal gain of 2 dB is used mainly for stabilizing the optical output from the TFLN PIC, and an LNA (Talent Microwave) with a power gain of 8 dB is used to ensure the SSOL gain above the oscillation threshold. A vector network analyzer (VNA) (Agilent Technologies: N5227A) is used to characterize the frequency response of the SSOL gain by measuring the transmission coefficient S21 (Appendix B). In particular, the scanning RF signal coming from the VNA is fed into the MZM to linearly modulate the optical carrier provided by the laser (HAN’S RAYPRO) with a wavelength of 1550.14 nm and a power of 18 dBm. Due to the low Vπ (1.2 V) of our MZM, a small LNA power gain of only 8 dB at Ka-band is required. By electrically tuning the add-drop MRR, the optical carrier frequency can be aligned with one of the resonance peaks of the MRR, with its modulation sidebands aligned with the adjacent resonance peaks. This modulation induced optical comb passing through the add-drop MRR is finally converted back into an RF signal by beating in the PD before being amplified by the LNA. Figure 3(d) shows the measured frequency response of the SSOL gain of the fixed-frequency OEO, which has a center frequency of 30 GHz and a 3 dB bandwidth of 743 MHz, consistent with the FSR and the 3 dB bandwidth of the MRR, respectively, and a peak gain of 1.1 dB. It is important to notice that the 3 dB optical bandwidth of the MRR is effectively preserved in the microwave frequency domain.

One of the most important figures of merit for an OEO is its spectral purity, which can be quantified by measuring its phase noise using an RF spectrum analyzer (RFSA) (Agilent Technologies: PXA N9030A). Figure 4(a) is the photograph of the measurement setup while Fig. 4(b) shows the zoom-in view of the chip test bench. Optical fibers are used to couple light into and out of the chip, while high frequency RF probes are used to feed the RF signal back to the modulator and DC probes are deployed to adjust the resonances of the MRR. The phase noise of our Ka-band fixed-frequency OEO (blue curve) is shown in Fig. 4(c), as compared to that of a commercial RF source (Keysight: E8257D). It can be seen that the phase noise in our Ka-band OEO has a high flat region at frequency offsets below 400 Hz from the 30 GHz center frequency, which can be attributed to the frequency fluctuations of the laser [47], as well as fluctuations of the fiber-chip coupling due to the random mechanical vibrations. The former can be minimized by stabilizing the laser frequency to the resonance peak of the MRR while the latter can be addressed by permanently fixing the fiber-chip interface with adhesive during chip packaging. Nevertheless, at frequency offsets beyond 400 Hz from the center frequency, the phase noise starts to drop rapidly, probably because the impact of laser fluctuation and the mechanical vibration on the phase noise is reduced at higher frequencies [47]. The measured phase noises at 1 kHz and 10 kHz offsets from the 30 GHz center frequency are −76dBc/Hz and −102dBc/Hz, respectively, which are the lowest among the reported photonic integrated OEOs without using long optical fibers and are significantly lower in particular than those of the X-band OEO integrated on the InP platform (−60dBc/Hz at 10 kHz from the 8.87 GHz carrier) [35]. The phase noise can be further reduced by increasing the Q factor of the MRR, stabilizing the chip-fiber coupling via proper packaging and eliminating the EDFA, as well as using high-performance optical and electrical components, such as lasers with low frequency jitter and relative intensity noise (RIN), and LNAs with low phase noise [47].

Note that when the fiber length is increased to 500 m and a narrow bandwidth BPF at Ka-band is used, the OEO phase noise is reduced to around −128dBc/Hz at 10 kHz, agreeing well with the theoretical result (Appendix C) [7]. This result is comparable to the phase noise of the hybrid-integrated OEO with a 2-km-long well-wound polarization-maintaining fiber coil having a phase noise of −128dBc/Hz at 10 kHz at oscillation frequencies of 3–18 GHz [38].

Figure 4(d) is the RF spectrum of the OEO oscillation measured with an RFSA operating at a frequency span of 40 MHz and a resolution bandwidth (RBW) of 40 kHz, showing the center oscillation frequency of 30 GHz, side-mode spacing of 8 MHz, and side mode suppression ratio (SMSR) of 50 dB. As discussed previously, the 30 GHz is defined by the FSR of the MRR. From the 8 MHz mode spacing, one can estimate that the total loop delay is approximately 25 m, mainly due to the fibers in the EDFA, the fiber pigtail on the off-chip PD (Finisar: XPDV2320R), and the fiber pigtail for coupling light out of the TFLN PIC. Note that 30 GHz is 3750 times the 8 MHz, agreeing with the expectation that the FSR of the MRR should be an integer multiple of the mode spacing defined by the fiber loop. The SMSR of the OEO may be further improved by carefully adjusting the fiber length so that the modes defined by the total fiber length are more precisely aligned with the resonance peaks of the MRR.

To evaluate the frequency stability of the Ka-band signal generated by the OEO, the RFSA is set to operate at max-hold mode to record the history of the oscillation frequency for 8 min. The inset in Fig. 4(d) shows a flat signal spectrum with a width of 28 kHz around the 30 GHz carrier, indicating that the frequency fluctuation range of the 30 GHz signal is about 28 kHz, or a frequency stability of 10−6 over a period of 8 min. The poor frequency stability mainly results from the following four reasons: (1) the power fluctuation of the fiber-chip coupling due to the random mechanical vibrations; (2) the change of the oscillation frequency due to the shift of the MRR wavelength caused by the photorefractive and thermal effects of TFLN; (3) the fluctuation of the laser wavelength due to the frequency jitter; (4) the noise of the EDFA or the RF LNA. We can adopt the following methods to address the above issues separately: (1) addressing the power fluctuation by permanently fixing the fiber-chip interface with adhesive during chip packaging; (2) using the self-injection-locking method to lock the laser carrier frequency onto the center frequency of one of resonances of the add-drop MRR in the fixed OEO, while the frequency stabilized feedback controller can be deployed to stabilize the resonances of notch MRR in the tunable OEO; (3) using a laser with low frequency jitter and RIN; (4) eliminating the EDFA via proper packaging and utilizing an LNA with low phase noise.

As shown in Fig. 2(b), the PM used in the ultra-wide range frequency-tunable OEO is also fabricated with a folded CL-TWE design, with a total modulation length of 3.8 cm. The Vπ and the 3 dB EO bandwidth of the PM are estimated to be 1.6 V and 17 GHz, respectively, based on the corresponding parameters of the MZM in the fixed-frequency OEO discussed in the last section.

The notch MRR is a key component serving as a notch optical filter for the ultra-wide range frequency-tunable OEO, which should be featured with a narrow bandpass bandwidth and large FSR for generating RF signals with high SMSR and wide range tunability. The measured FSR and the ER of the MRR are 70.6 GHz (corresponding to the circumference of 2 mm) and 23 dB, respectively, as shown in Fig. 5(a). The 3 dB bandwidth of the notch with a wavelength of 1549.816 nm is measured to be 160 MHz, corresponding to the Q value of 1.3×106, which is about an order of magnitude higher than that of the MRR used in the photonic integrated OEO (1.4×105) fabricated on the silicon platform [36]. The Q value of MRR can be further increased by reducing the waveguide scattering loss caused by sidewall roughness. As can be seen from Fig. 5(b), the notch of MRR can be shifted from 1549.816 to 1549.965 nm by tuning the applied voltage to the electrode of the MRR from 0 to 30 V, corresponding to an electrical tuning efficiency of 5 pm/V or 0.62 GHz/V.

In the frequency-tunable OEO, the 3 dB bandwidth and the tuning range of the notch MRR determine the 3 dB bandwidth and tuning range of the SSOL gain of the OEO. Similar to the fixed frequency OEO discussed in the last section, an EDFA (Amonics AEDFA-PKT-DWDM-15-B-FA) operating in the constant power mode is used mainly for stabilizing the optical output into the PD, an LNA (Talent Microwave) is used to provide the required loop gain to satisfy the oscillation condition (Appendix B), and a vector network analyzer (VNA) (Agilent Technologies: N5227A) is used to characterize the SSOL gain of the OEO by measuring the transmission coefficient S21. Thanks to the low Vπ (1.6 V) of our PM, an LNA power gain of 29 dB at Ka-band is sufficient for the OEO to sustain oscillation at Ka-band, which is noticeably lower than the 46 dB gain required for the SOI photonic integrated OEO to oscillate at X-band [37], but larger than the LNA gain (8 dB) required for the fixed-frequency OEO to oscillate, mainly because the Vπ of the PM is larger and the 3 dB EO bandwidth of the PM is smaller than those of the MZM [7], respectively. In the measurement, a scanning RF signal from the VNA is fed into the PM to linearly modulate the optical carrier from the laser (HAN’S RAYPRO) with a wavelength of 1550.14 nm and an output power of 18 dBm. Whenever one of the phase modulation sidebands falls into the notch of the MRR, the phase modulated signal is converted to an intensity modulated signal. Figure 5(c) shows the measured frequency response of the SSOL gain of the frequency-tunable OEO, with the center frequency tuned from 20 to 35 GHz by tuning the applied voltage to the electrode of the notch MRR from 0.8 to 21.4 V. Even wider frequency tuning range is feasible if microwave components, such as the LNA with wider bandwidths, are available. It can be seen from Fig. 5(c) that the SSOL gain of the OEO is 2 dB, which is well above the oscillation threshold, and its 3 dB bandwidth is 161 MHz, the same as that of the notch MRR, indicating that the optical bandwidth of the MRR is effectively preserved in the microwave frequency domain.

The generation of high frequency RF signals with wide range tunability and high SMSR is important for many practical applications. The RF signal generated by our frequency-tunable OEO can be tuned from 20 to 35 GHz by varying the voltage applied to the MRR electrode from 0.8 to 21.4 V, as shown in Fig. 5(d). Even wider frequency tuning range is feasible if microwave components, such as the LNA with wider bandwidths, are available. Compared to the tuning range (3–7.4 GHz) of the OEO implemented with the SOI PIC [36], the tuning range of our OEO implemented with the TFLN PIC is much wider (15 GHz versus 4.4 GHz). Figure 5(e) shows the frequency spectrum of the generated RF signal when the OEO is tuned to 30 GHz. It can be seen that the mode spacing determined by the total signal path length of all the optical and RF components used in the OEO loop is 8 MHz, with an SMSR of 49 dB. The SMSR remains constant when the RF frequency is tuned in the entire tuning range, an important advantage of the photonically tuned OEO.

The phase noise of the frequency-tunable OEO is measured by using the experimental setup in Fig. 4, with the results shown in Fig. 5(f). It can be seen that the phase noise curves of the signals generated by the OEO at 20, 30, and 35 GHz are practically the same, which remain high and flat at offset frequencies below 10 kHz from their perspective carrier frequencies and can be attributed to the fluctuations of the laser frequency [47], as well as the random mechanical vibrations of the fiber-chip coupling, similar to the case of the fixed frequency OEO discussed in the last section. Nevertheless, when the frequency offsets from the carrier are beyond 10 kHz, the influences of laser fluctuations and the system mechanical vibration are reduced. In particular the phase noises at 10 kHz frequency offset are all rapidly dropped to −87dBc/Hz, as shown in Fig. 5(g), which are even lower than that of the X-band OEO integrated on the SOI platform (−80dBc/Hz at 10 kHz for 3–7.4 GHz) [36]. For the tunable OEO based on the PM and notch MRR, the oscillation frequency is determined by the frequency difference between the laser and the notch, and therefore is sensitive to the fluctuations of the laser frequency. The sharp jump of the OEO’s phase noise below a corner offset frequency fc=6.5kHz in Fig. 5(f) is probably due to the rapid increase of the laser linewidth or the range of laser fluctuation with the measurement time because the laser’s linewidth is a Sigmoid function of measurement duration [53], which may have a sharp transition around a measurement duration of 1.5×10−4s (=1/fc). Such an explanation needs to be further confirmed experimentally, which is beyond the scope of this paper. The phase noise can be further improved by increasing the Q factor of the notch MRR and using high-performance optical and electrical components, such as lasers with low frequency jitter and RIN, and LNAs with low phase noise [47]. Additionally, the proper packaging to minimize the power fluctuation due to vibration of the chip-fiber coupling and eliminate the use of the EDFA can also help to reduce the phase noise close to the carrier frequency.

Figure 5(h) shows the optical spectrum measured by an optical spectrum analyzer (Anritsu MS9740A) when the frequency-tunable OEO is operating at 30 GHz. The optical carrier is generated by the laser (HAN’S RAYPRO) with a wavelength of 1550.14 nm and a power of 18 dBm. As can be seen, the power of the lower first-order sideband with a wavelength of 1549.9 nm is 7 dB lower than that of the upper first-order sideband with a wavelength of 1550.38 nm due to the filtering action of the notch MRR. Although the lower first-order sideband is not completely suppressed, the difference between the two sidebands is sufficient to enable the phase-modulation to intensity-modulation conversion.

In this paper, the first photonic integrated OEOs on the TFLN platform for achieving compact footprint, low cost, and low power consumption are proposed and experimentally demonstrated. Two types of OEO chips are designed and fabricated: one is the fixed frequency OEO operating at 30 GHz and the other is the tunable frequency OEO with a tuning range of 15 GHz at Ka-band. The fixed frequency OEO is realized with a TFLN PIC consisting of an MZM and an add-drop MRR. The MZM is fabricated using a folded CL-TWE design with a total modulation length of 2.5 cm for realizing a measured Vπ of 1.2 V and a 3 dB EO bandwidth of 38 GHz. This Vπ is significantly lower than the typical values of >4V for the conventional MZMs fabricated with bulk LN [51], which enables the significant reduction of the LNA gain required. The FSR of the add-drop MRR is designed to be 30 GHz, which ensures the OEO oscillation at 30 GHz and filters out other oscillation modes. Finally, the phase noises of the OEO chip are measured to be −76dBc/Hz at 1 kHz and −102dBc/Hz at 10 kHz, respectively, which are the lowest among photonic integrated OEOs without using long optical fibers, as shown in Table 1 for comparison.Table 1.

Comparison of Our Work with the Previous Photonic Integrated OEOs^a

The frequency-tunable OEO includes a TFLN PIC containing a PM and a notch MRR. The PM is also fabricated using folded CL-TWE design with a total modulation length of 3.8 cm for realizing a Vπ of 1.6 V and a 3 dB EO bandwidth of 17 GHz, estimated with the design parameters of the MZM. Thanks to the low Vπ of our PM, an LNA power gain of 29 dB is sufficient to enable the oscillation of the frequency-tunable OEO, which is much lower than the 46 dB LNA power gain required for the X-band OEO implemented with the SOI chip [37]. The Q value of notch MRR is 1.3×106, which is the highest among the optical resonators (1.4×105) in the photonic integrated OEOs [36]. By tuning the notch of the MRR, wide range tunable microwave signals from 20 to 35 GHz are generated, with a phase noise of −87dBc/Hz at 10 kHz independent of oscillation frequencies, which is even lower than that of the tunable frequency OEO chip of much lower frequency fabricated on the SOI platform (−80dBc/Hz at 10 kHz for 3–7.4 GHz) [36], as shown in Table 1 for comparison. In addition, the tuning range of our TFLN PIC based OEO is much wider than that of the SOI PIC based OEO (15 GHz versus 4.4 GHz). Even wider frequency tuning range could also be achieved with our TFLN PIC based OEO if microwave components, such as LNAs with wider bandwidths, are available in our laboratory.

It is important to compare the characteristics, particularly the oscillation frequencies and the associated phase noises, between our two types of TFLN PIC OEOs. The fixed frequency OEO is constructed with a PIC consisting of an MZM and an add-drop MRR, with its oscillation frequency determined by the FSR of the add-drop MRR, which cannot be changed once fabricated; while the tunable frequency OEO is constructed with a PIC consisting of a PM and a notch MRR, with its oscillation frequency determined by the frequency spacing between the laser and one of the resonances of the notch MRR, and the tuning range determined by its FSR. By tuning the notch of the notch MRR or the frequency of the laser, the oscillation frequency can be tuned, although in our experiment, the frequency tuning is enabled by tuning the notch of the MRR with an applied voltage. Therefore, the phase noise of this tunable OEO is expected to be higher than that of the fixed frequency OEO because both the fluctuations of the laser frequency and the noise of the voltage applied onto the notch MRR can significantly be converted to the phase noise of the tunable OEO. For increasing signal stability, the self-injection locking method can be used to lock the laser carrier frequency onto the center frequency of one of the resonances of the add-drop MRR in the fixed OEO [54], or the frequency stabilized feedback controller can be deployed to stabilize the resonances of notch MRR in the tunable OEO [55]. Finally, as mentioned in previous sections, the phase noises of both OEOs can be reduced by increasing the Q factor value of the MRR and using high-performance optical and electrical components, such as lasers with low frequency jitter and RIN, and LNAs with low phase noise [47]. In addition, the proper packaging to minimize the fluctuation of the chip-fiber coupling and eliminate the use of the EDFA can also help to reduce the phase noise.

It is also important to compare our work with previously reported photonic integrated OEOs in recent years, as shown in Table 1, which indicates that not only is the frequency of our OEO based on the TFLN platform much higher than those of OEOs based on the SOI and InP platforms but also the measured phase noise (including the contribution of the optical fiber in the loop) is notably lower. Note that the mode spacing in our TFLN OEO is 8 MHz, corresponding to an equivalent loop delay of approximately 25 m, mostly from the EDFA and the pigtail of the PD used in the loop. On the other hand, the equivalent length (LQ) of each MRR in our work (as well as in the previous works of others), which can be obtained from the corresponding Q values, is around 0.22 m or less, as also shown in Table 1. Therefore, the phase noise of our OEO is primarily determined by the fiber length in the loop, not the MRR.

For fair comparisons, the fiber loop lengths for each OEO, as well as the fiber-free equivalent phase noise (defined as the phase noise corresponding to the LQ of each optical resonator) derived from the measured phase noise and the corresponding Q-value equivalent length of the MRR in each OEO (see Appendix D), are also provided in Table 1. It can be seen that the fiber-free equivalent phase noises of our Ka-band PIC OEOs are much lower than that of the SOI OEO in Ref. [39], but somewhat higher than those of the X-band InP OEO in Ref. [35] and the X-band SOI OEO in Ref. [36], respectively.

In order to obtain an equivalent fiber length of 2 km, an MRR Q value of 1.2×1010 is required, which is difficult to achieve with TFLN, SOI, or SiN platforms. Therefore, it may be more feasible to make a compact TFLN PIC OEO with much lower phase noise by using a fiber coil made with a length of small-diameter (100 µm or less) fibers, considering that coin sized fiber coils with hundreds of meters of fiber can be made with such small-diameter fibers. To make fair comparisons with other PIC OEOs, we included the fiber length used in each OEO, the Q-value, the equivalent length of each MRR, and the fiber-free equivalent phase noise in Table 1.

Although the phase noise of our photonic integrated TFLN OEO is not to the performance level of a typical OEO with a long fiber delay [12,38], it is nevertheless a first step toward using the TFLN platform for fabricating integrated OEO chips, which have been shown to be superior to the OEOs made with the SOI and InP PICs in terms of the operation frequency and phase noise. The successful demonstration of semiconductor lasers and PDs on the TFLN platform [43,44] opens up promising opportunities for achieving heterogeneous integration of all photonic components on a single chip. Further studies will be conducted to fabricate such a fully photonic integrated OEO by putting all necessary photonic components on a single TFLN chip, including the distributed feedback (DFB) laser, modulator, MRR, and PD. Further integration of electronics, such as the trans-impedance amplifier and LNA, on the TFLN platform will be challenging due to the technical difficulties in compatibility between the TFLN and the mature complementary metal-oxide-semiconductor (CMOS) technologies. Alternatively, a hybrid-integrated OEO can be achieved by connecting a TFLN photonic chip with an electronic chip including an electrical amplifier and other components through wire-bonding to microstrip lines.