Optically-Injected-Semiconductor-Laser-Based Tunable Dual-Frequency Optoelectronic Oscillator under Subharmonic Microwave Modulation

: To obtain low-phase-noise microwave signals with a widely tunable frequency range, an optoelectronic oscillator (OEO) was constructed on the basis of an optically injected semiconductor laser (OISL) for the generation of high-quality microwave signals. Our OEO relied on the effect of wavelength-selective ampliﬁcation and the period-one (P1) oscillation under optical injection. The signal’s frequency stability, side-mode-suppression ratio (SMSR) and linewidth were optimized by the subharmonic microwave modulation technique in the OEO loop. The experimental results showed that the frequency of the signal obtained by the proposed OEO could be tuned up to 18 GHz. Using the dual-loop OEO structure, the SMSR was increased to 55 dB. Moreover, the phase noise of the obtained microwave signal was lower than − 81 dBc/Hz at 1 kHz frequency offset and − 119 dBc/Hz at 10 kHz frequency offset. This was achieved by introducing subharmonic microwave modulation in the OEO loop, respectively. Furthermore, via the utilization of a Fabry–Perot laser diode (FP-LD) in the proposed structure, a dual-loop OEO with different dual-frequency conﬁgurations (which could be tuned up to 12 and 18 GHz) was obtained.


Introduction
With the rapid development of information technology, the generation of microwave signals with low phase noise and a widely tunable frequency range plays a crucial role in wireless communication, radar and optical communication [1][2][3][4][5]. The microwave signals generated by traditional electrical methods traditionally demonstrate poor phase noise. This fails to satisfy the requirements of modern communication and radar systems, especially in high-frequency regimes. Recently, OEOs have been proposed as effective solutions to replace traditional electrical methods, allowing for a higher central frequency and lower phase noise [6][7][8][9]. OEOs can generate microwave signals (based on the microwave photonic technique) to overcome the limitations of microwave signal sources realized by traditional electrical methods in terms of tunable frequency range and phase noise performance. In addition, the phase noise of the microwave signals generated by OEOs is usually independent from oscillation frequency. Therefore, OEOs have broad prospects in microwave systems with high frequency and large bandwidth. Amongst the proposed OEO techniques, electrical band-pass filters (EBPFs) have usually been the ones adopted. The resultant microwave signals thus show poor tunability, as limited by the EBPFs [10]. To improve the frequency tunability of the obtained microwave signal, EBPFs were replaced by using microwave photonic filters (MPFs). In Ref. [11], an MPF-based widely tunable OEO was proposed. The MPF was used based on a polarization beam splitter (PBS), a chirped fiber Bragg grating (CFBG) and a polarization modulator present in the structure. By changing the polarization state via a polarization controller (PC) in the link, the signal's frequency was varied from 5.8 to 11.8 GHz. In Ref. [12], a MPF was realized via cascading of the phase modulator (PM) and the optical band-pass filter (OBPF) to complete the conversion from phase modulation to intensity modulation. This could be used in the OEO structure to generate tunable microwave signals from 4.74 to 38.38 GHz. Unfortunately, the MPF usually has a relatively large 3-dB bandwidth. Thus, it is difficult to suppress the side modes of the microwave signals when the OEO has a long cavity that will reduce the spectral purity of the generated signals. It is also difficult to suppress other spurious signals by utilizing the common MPF in the OEO loop. Furthermore, the structures of the MPFs are usually complex. This will deteriorate the phase noise of the oscillation signals. Therefore, a multiple-loop OEO has been considered as an effective way to solve these problems.
In Ref. [13], a dual-loop OEO relying on a polarization-multiplexed structure was advanced by utilizing the phase-shifted fiber Bragg grating (PS-FBG), PBS and polarization beam combiner (PBC) in the structure. This allowed for the generation of microwave signals ranging from 1 to 4 GHz. Additionally, the SMSR of the generated signal could be improved by this scheme. In this configuration, the PS-FBG could be utilized as an OBPF to choose the oscillation frequency. The PBS and PBC could be used to obtain the proposed dual-loop OEO. Then, the side modes could be effectively suppressed due to the Vernier effect. It should be noted that the PS-FBG is very sensitive to environmental fluctuations. Thus, the generated microwave signal's frequency of the PS-FBG-based OEO will drift. This decreases the frequency stability of the microwave signals. In Ref. [14], a dual-loop OEO was proposed on the basis of OISL to construct an MPF in the system, with a tunable frequency range from 9.63 to 19.11 GHz. In this method, the obtained 13.74 GHz microwave signal's phase noise was −103.2 dBc/Hz at 10 kHz frequency offset. To simplify the structure of the OEO loop, a directly modulated semiconductor laser was utilized to replace the PM in [15]. However, the signal's maximum frequency was restricted by the operating bandwidth of the directly modulated semiconductor laser, which was usually lower than the bandwidth of external modulator. In Ref. [16], a dual-loop OEO with a large tunable frequency range of 60 GHz, which was constructed by utilizing stimulated Brillouin scattering (SBS), was proposed. However, the SMSR was only 35 dB. The performance of the oscillation signal was mainly restricted by the stability of its frequency and power. In Ref. [17], a coupled dual-loop SBS-based OEO was investigated to improve the stability of the system. The obtained 15 GHz signal had a phase noise of only −95 dBc/Hz at 10 kHz frequency offset, while the performance of the near end phase noise (<1 kHz) was poor. In Ref. [18], a new type of OEO (known as a parity-time symmetric OEO) was proposed to suppress the side modes. The OEO could generate a 4.07 GHz single-mode microwave signal, with phase noise of −108 dBc/Hz at 10 kHz frequency offset when there is no filter in the OEO loop. In Ref. [19], an OEO with higher frequency up to W-band was implemented. In this scheme, the side modes could be suppressed by introducing electrical injection locking technique. On the other hand, an integrated OEO based on micro-disk resonator, which was fabricated on silicon-based photonic platform, was proposed [20]. The research made a huge contribution to the development of integrated frequency tunable OEO.
Furthermore, with the recent progress in multiband microwave systems (such as multifunctional modern radar and next-generation (such as 5G) wireless and mobile radio communication [21]), the demand for tunable dual-frequency OEOs has been increasing. In Ref. [22], a dual-frequency OEO was employed on the basis of a PS-FBG, so as to realize the conversion similarly to [12]. In this scheme, the frequencies of two generated signals could be defined by the differential in frequency between the two light sources and the frequency of a notch in the PS-FBG. By tuning the wavelengths of different light sources, the central frequencies of the two generated signals could be independently adjusted. However, the quality of the oscillation signal was influenced by the fluctuation of the notch frequency in the grating reflection spectrum of the PS-FBG. In Ref. [23], a dual-frequency OEO based on the dual-parallel Mach-Zehnder modulator (DPMZM) and CFBG was proposed. The DPMZM was utilized to implement carrier phase-shifted double sideband (CPS-DSB) modulation so that the frequency-doubled microwave signal could be obtained. The CFBG was utilized to ensure that another signal oscillated at base frequency by adjusting the phase between the sideband signals and the optical carrier signal. Finally, the two oscillation signals with frequencies of 10 and 20 GHz were generated. Although the system was simple to construct, bias drifts deteriorated the stability of the proposed dual-frequency DPMZM-based OEO.
In this article, we proposed a tunable dual-frequency OEO based on an OISL for microwave signal generation with low phase noise and wide tunability. By introducing the subharmonic microwave signal applied to a PM in the proposed system, the linewidth and SMSR could be optimized. The optical signal after optical injection was modulated via a dual-output Mach-Zehnder modulator (DOMZM) to achieve complementary intensity modulation. Then, the output of the two optical paths from the DOMZM was connected with different fiber lengths in the loop, respectively. Hence, the side mode and the phase noise could be rejected by the Vernier effect. After the optical signals beating in the balanced photodetector (BPD), the microwave signal was fed back to the DOMZM to construct the OEO loop. The experimental results on the OEO showed that the high-quality microwave signals (with the tunable frequency up to 18 GHz) could be obtained. The phase noise was lower than −81 dBc/Hz at 1 kHz frequency offset when the frequency of the generated signal was 15 GHz. Moreover, the phase noise was superior to −117.6 dBc/Hz at 10 kHz frequency offset when the generated signal frequency was 15 GHz. The phase noise value was 13.7 dB lower than that obtained in the open-loop structure and 8.9 dB lower than that obtained in the single-loop OEO. Furthermore, by utilizing an FP-LD in the proposed OEO structure, a dual-frequency dual-loop OEO with different frequency configurations could be tuned up to 12 and 18 GHz. It should be noted that the tunability for the proposed microwave signal generation is only limited by the electrical bandwidths of the amplifier, electro-optic modulator and photodetector. Thus, higher microwave frequencies could be obtained using the devices with larger bandwidths in the OEO loop.

Operating Principle
The schematic diagram of the dual-loop dual-frequency OEO is shown in Figure 1. The OEO loop is made up of two tunable laser sources, a 3-dB optical coupler (OC), a DOMZM, a PM, three PCs, an optical circulator (CIR), a spool of single-mode fiber (SMF), a variable optical delay line (VODL), a distributed-feedback semiconductor laser diode (DFB-LD) or an FP-LD, a BPD, an EBPF and an electrical amplifier (EA). The generated microwave signals of the proposed OEO can be tested using the electrical spectrum analyzer (ESA) and signal source analyzer (SSA), respectively.
As shown in Figure 1, two laser sources were utilized as master lasers (MLs). An FP-LD without an optical isolator was used as a slave laser (SL). Two optical signals emitted from two MLs with the frequencies of f m1 and f m2 were combined by an OC. Then, the optical signals were modulated by the microwave signal with the frequency of f 0 /N (N is an integer) via a PM to realize phase modulation. The frequency f 0 /N was used to implement subharmonic microwave modulation, which could improve the frequency stability and spur the suppression ratio in the OEO loop [24,25]. In addition, under subharmonic microwave modulation, the 3-dB bandwidth of the oscillated signal would be optimized. The modulated optical signals output from the PM were injected into the SL through a CIR to achieve optical injection. The PCs were utilized to modify the polarization state between the MLs and SL to obtain proper injection efficiency in the system. Under proper optical injection, the cavity mode of the SL was red-shifted. At the same time, the optical gain spectrum could be produced when one of the modulated Nth-order sideband signals fell into the locking area of the SL. Thus, the sideband signal of interest could be selected and amplified. Meanwhile, through the influence of subharmonic microwave modulation, one of the Nth-order modulation sidebands after modulation would locate nearby the red-shifted light and lock it to ameliorate the signal's quality. Then, the optical signals from the SL were sent to the DOMZM through the CIR again. The DOMZM was set at the quadrature bias point in the OEO loop to realize complementary intensity modulation. The two optical outputs of the DOMZM passed through a spool of SMF and the VODL individually. The VODL was used to adjust the phase of the optical signals, so that balanced differential detection could be accomplished to improve the quality of the generated microwave signal. Afterwards, the two paths of the optical signals were coupled into the BPD. After beating in the BPD, the signals were processed by the EA and EBPF. Then, two microwave signals with different frequencies would be brought out. Finally, by using the electrical divider, the microwave signals were divided into two parts: one was returned to the DOMZM's RF port to achieve a closed loop, and the other was measured via the ESA and SSA. The operating principle of the OISL is shown in Figure 2. We assumed that the system was under single optical injection. The detuning frequency could be expressed as ∆f = f m − f s , which was the differential in wavelength between the ML and the freerunning SL. Before the CIR, a PC could be utilized to modify the polarization state of the injected signal so that the SL could work at the P1 oscillation state under proper optical injection. Meanwhile, the cavity mode of the SL red-shifted to the frequency f cav through the changing effect in carrier concentration under P1 oscillation [26]. In this way, a new frequency f cav was produced because of the disappearance of the frequency f s when the SL entered the P1 state. When the output signal from the SL was sent to a photodetector through the CIR again, the two dominant wavelengths with the frequencies of f m and f cav would generate a beating signal, which had a frequency of f c , as shown in Figure 2. f c could be tuned from a few GHz to tens of GHz by changing the detuning frequency ∆f, and the injection ratio (R), respectively or together. Here, the parameter R is expressed as the square root of the power ratio between the injected light and the optical signal generated by the free-running SL. Furthermore, the injected light was modulated in a PM by the subharmonic microwave signal with frequency f 0 /N, where the frequency f 0 was approximately equal to the frequency f c . One of the Nth-order sidebands of the modulated signal was located near the locking area of the SL and locked it. Therefore, the P1 oscillation frequency was locked. At the same time, only the OEO mode that was close to the Nth-order sideband would oscillate steadily. Finally, the OEO could oscillate stably to generate a signal with high quality. Furthermore, the free-running frequencies of ML1 and ML2 were f m1 and f m2 in the proposed OEO system, respectively. However, f m1 was different from f m2 in our structure. The microwave signal with a frequency of f 0 /N modulated the PM, which was used to implement subharmonic microwave modulation. Then, the modulated optical signals were injected into the FP-LD. As far as we know, FP-LD is a multilongitudinal mode semiconductor laser. The different longitudinal mode of the FP-LD would be red-shifted and work at the P1 oscillation state via dual-beam optical injection, so that two microwave signals with different frequencies could be obtained when the optical signals were beaten in the BPD.

Experiment and Results
An experiment based on the proposed scheme as shown in Figure 1 was displayed to verify the proposed approach. First, the performance of the dual-loop OEO with a single frequency was demonstrated. The experimental setup was shown in Figure 3. The ML in the OEO loop was a tunable laser source (NKT E15). The power of light generated by the ML was set as 13 dBm, and the wavelength was set as 1549.505 nm. The optical signal from the ML was transmitted to a PM (EOSPACE) with 3-dB bandwidth of 40 GHz to be modulated. Then, the modulated optical signals were injected into the DFB laser with a free-running power of 10.5 dBm through the PC and CIR. Then, the optical signals from the SL were modulated by the DOMZM with 3-dB bandwidth of 20 GHz to construct the OEO loop. In the OEO cavity, the fiber length of the long-loop was approximately 10 km when using a spool of SMF, and that of the short-loop was approximately 10 m. The BPD (U2T) in the OEO loop with a 3-dB bandwidth of 43 GHz was utilized to realize the optical-to-electrical conversion as well as the Vernier effect to improve SMSR. The EA with an operational bandwidth from 0.3 to 18 GHz and a gain of more than 34 dB was used for signal amplification. The EBPF with an operational bandwidth from 10 to 18 GHz and an insertion loss of less than 2 dB was utilized to filter the oscillation signal. Finally, the generated signals were analyzed using the ESA and SSA.
The frequencies of the generated microwave signals could be modified by changing the detuning frequency ∆f when R was fixed. In the experiment, ∆f was adjusted by changing the free-running wavelength of the SL when the wavelength of the light output from the ML was fixed. The free-running wavelength of the SL could be changed from 1549.580 to 1549.615 nm by adjusting the semiconductor laser driver. In this way, the detuning frequency ∆f could be tuned from 9.375 to 13.75 GHz. To demonstrate the performance of the proposed OEO structure, we studied the generated microwave signal's spectrum, SMSR and phase noise. As shown in Figure 4, when the detuning frequency ∆f was modified from 9.375 to 13.75 GHz and the frequency of the 1/2 subharmonic microwave signal was adjusted from 6.25 to 8.75 GHz, the generated signal would be tuned from 12.5 to 17.5 GHz. In addition, the zoom-in spectra of the generated 14, 15 and 16 GHz microwave signals are displayed in Figure 5, at the time when the span of ESA was set as 4 GHz. It is important to note that the tunable range of the generated signal was mainly restricted by the operating frequencies of the amplifier, electro-optic modulator and photodetector. Thus, the maximum frequency of the generated signal would be increased via the utilization of the devices with higher frequency ranges.   The side mode near the central frequency of the oscillation microwave signal would induce far end phase noise degradation, so we tested three different kinds of OEOs, known as the open-, single short-, and dual-loop OEOs. In the experiments, it was found that the phase noise properties of single short-and single long-loop OEOs were comparable. For simplicity, only the results for the single-short loop case were shown in the following part of this paper. When the generated signal's frequency was 15 GHz, the subharmonic microwave signal's frequency was 7.5 GHz, with the phase noise of −116 dBc/Hz at 10 kHz frequency offset and −87 dBc/Hz at 1 kHz frequency offset. The SMSR of the signals generated by three different kinds of structures were investigated. The generated signal spectra with a span of 2 MHz were shown in Figure 6. The open-loop structure was constituted when there was no feedback in the dual-loop OEO. The SMSR of the open-loop system was 47 dB. The fiber length in the single-loop structure was approximately 10 m, which comprised the pigtail fibers of the optic-electronic devices employed in the system. The SMSR was 50 dB in the single-loop OEO, which was 3 dB lower than that in the open-loop system. The SMSR of the proposed dual-loop OEO was 55 dB, demonstrating an increase of 5 dB and 8 dB compared with that of the single-loop and open-loop system, respectively. Furthermore, the experimental result of the measured 15 GHz microwave power spectrum with a span of 1 GHz, as shown in Figure 7, had a spur suppression ratio of 68 dB and a 3-dB bandwidth of <100 kHz. The generated 15 GHz signal could almost be considered as a single-mode microwave signal. When the 1/2 subharmonic microwave signal failed to be applied to PM, the spur suppression ratio was only 27 dB with a large 3-dB bandwidth of 12.5 MHz, as shown in Figure 8. The stability of the generated microwave signals between the OEO with subharmonic microwave modulation and the OEO without subharmonic microwave modulation were illustrated in Figure 9. It could be seen that the generated microwave signal of the OEO with subharmonic microwave modulation showed high stability for more than 30 min. Thus, the frequency stability, SMSR and spectral purity of the generated signal were effectively improved by subharmonic microwave modulation.    As can be seen in Figure 10, we made a comparison between the three kinds of OEOs to further analyze the signal's phase noise at 15 GHz. The generated 15 GHz microwave signal's phase noise in the open-loop system was −103.9 dBc/Hz at 10 kHz frequency offset, whereas those of the single-and dual-loop systems were −108.7 and −117.6 dBc/Hz, respectively. The phase noise of the dual-loop structure was 13.7 dB lower than that in the open-loop system and 8.9 dB lower than that in the single-loop OEO. According to the experimental results shown in Figure 10, the phase noise performance was greatly optimized because of the dual loop with feedback. The phase noises of the microwave signals with the frequencies of 14, 15 and 16 GHz were also probed, as can be seen in Figure 11. The phase noises of the proposed dual-loop OEO were −114.8, −117.6 and −119.5 dBc/Hz at 10 kHz frequency offset for different frequencies. At the same time, the phase noise was lower than −81 dBc/Hz when the frequency offset was 1 kHz.  According to the nonlinear dynamic theory of semiconductor lasers, an FP-LD was used as the SL. A DFB laser and a tunable laser source (NKT E15) were individually used as ML1 and ML2. Then, a dual-frequency dual-loop OEO was realized. When the frequencies of 1/2 or 1/3 subharmonic microwave signals applied to the PM were tuned to 5.5, 5.75 and 6 GHz, the wavelength and output power of ML1 and ML2 were fixed. The wavelength difference between ML1 and ML2 was approximately 1 nm so that the frequency difference between the two lasers could be much greater than the bandwidth of the BPD. The output wavelength and power of the free-running FP-LD were fixed by controlling the laser driver. The optical power of light injected into the SL could be tuned by adjusting the PCs in the OEO link. As can be seen in Figure 12, when the frequency of the subharmonic microwave signal was 6 GHz, the two frequencies of the microwave signals generated from the proposed dual-frequency dual-loop OEO could be tuned up to 12 and 18 GHz. They were also restricted by the maximum frequencies of the amplifier, electro-optic modulator and photodetector.

Conclusions
A tunable dual-frequency dual-loop OEO was investigated based on OISL in the P1 oscillation state. The experimental results indicated that the frequency stability and SMSR of the generated signal were improved by introducing the subharmonic microwave signal via a PM connected to the ML in the OEO loop. Moreover, the generated signal's frequency was tuned up to 18 GHz by changing the detuning frequency ∆f or the power of the injected light. Meanwhile, the frequency of the subharmonic microwave signal was changed. The tunable frequency range of the microwave signal was restricted by the maximum frequencies of the amplifier, the electro-optic modulator, and the photodetector. By using larger-bandwidth optic-electronic devices, the maximum frequency of the generated signal was further increased. Due to the advantage of using a dual-loop configuration, the SMSR was measured up to 55 dB. Meanwhile, the obtained phase noise was lower than −81 dBc/Hz at 1 kHz frequency offset and −119 dBc/Hz at 10 kHz frequency offset. Finally, by using an FP-LD in the OEO structure, a dual-frequency dualloop OEO with different frequencies which could be tuned up to 12 and 18 GHz was experimentally demonstrated.