1. Introduction

Photonic generation of microwave signals has been widely investigated owing to its merits of broadband and low loss capability [1–3]. Various photonic techniques, including optical frequency combs [4,5], optical phase-locked loops [6,7], sideband injection locking [8,9], periodic oscillations under optical injection [10,11], and optoelectronic oscillators (OEOs) [12], have been reported for generating photonic microwave signals. The OEO, which is an optoelectronic feedback loop, has received considerable attention for generating low phase noise and tunable microwave signals [13,14].

A conventional OEO consists of a laser, an external modulator, a long optical fiber, a photodetector (PD), a narrowband electrical bandpass filter (EBPF), and an electrical amplifier (EA). The introduction of an external modulator usually leads to high radio frequency (RF) loss owing to its low modulation efficiency. To reduce the link RF loss and simplify the OEO system, the external modulator was replaced by a directly modulated semiconductor laser, which acted as both the laser source and the modulator [15]. In addition, because of the high modulation response at the relaxation oscillation frequency of the directly modulated semiconductor laser, the oscillating frequency of the OEO could be selected by directly changing the bias current and operation temperature without using EBPF [16]. However, the tuning range of the generated microwave signal is limited by the relaxation oscillation frequency of the laser. To overcome the tuning limitation, OEOs with external optical injection have been studied. An optically injected Fabry-Pérot (FP) laser was demonstrated to generate frequency-tunable microwave signals ranging from 6.41 to 10.85 GHz [17]. A frequency tunable OEO with a tuning range from 5.95 to 15.22 GHz based on a directly modulated distributed-feedback (DFB) semiconductor laser under external optical injection was also demonstrated experimentally [18]. Recently, wideband tunable OEO was demonstrated with an ultra-narrow passband microwave photonics filter based on stimulated Brillouin scattering and an infinite impulse response [19]. A frequency-tunable OEO was proposed using a dual-polarization quadrature phase shift-keying for multi-format signal generation [20]. Silicon photonic integrated OEOs, which include a high-speed phase modulator, a thermally tunable micro-disk resonator, and a high-speed PD, were also demonstrated for tunable microwave signal generation [21].

With the advantages of small mode volume and low power consumption, whispering-gallery mode (WGM) microlasers are attractive as potential light sources for photonic integration. We have already demonstrated low phase-noise microwave generation by OEOs based on the directly modulated WGM microcavity semiconductor lasers [22,23]. We realized tunable photonic microwave generation under both external optical injection and optoelectronic feedback [24]. To extend the frequency tunable range without the external optical injection, here we demonstrate the wideband frequency-tunable microwave generation by the OEO with a directly modulated AlGaInAs/InP integrated twin-square microlaser. The mutual optical injection between two square cavities enables a modulation response peak around the beating frequency of the resonances in the two square cavities through the photon-photon resonance, in addition to the peak created around the relaxation oscillation frequency [25,26]. Therefore, a self-sustaining oscillation can be generated around the modulation response peak at the relaxation oscillation frequency or the beating frequency owing to the relatively low electrical-optical conversion loss. By tuning the injection currents of the two square cavities separately to control the working state of the twin-square microlaser, wideband frequency-tunable microwave signals ranging from 2.22 to 19.52 GHz are generated. The single sideband (SSB) phase noises are below −110 dBc/Hz at the 10 kHz offset frequency, and the side-mode suppression ratios (SMSRs) are approximately 40 dB for the generated microwave signals. Neither an external modulator nor an EBPF is needed for the OEO, which simplifies the system and reduces the cost.

2. Device fabrication

Figure 1(a) shows the schematic diagram of an integrated twin-square microlaser with an AlGaInAs multiple-quantum-well (MQW) active layer. A bridge waveguide between the two square cavities is used to achieve mutual optical injection. The twin-square microcavity is laterally surrounded by the benzocyclobutene (BCB) layer, which is spin-coated and thermo-cured to create a flat surface. Two separate p-electrodes are deposited to achieve independent current injection to the two square cavities, and an isolation trench on the waveguide is accomplished by inductively coupled plasma (ICP) etching of the p-InGaAs ohmic contact layer to improve the isolation resistance [27]. The twin-square microlaser is fabricated on the AlGaInAs/InP compressively-strained MQW epitaxial wafer using similar fabrication techniques as in [28]. Figure 1(b) shows the top-view microscope image of a fabricated AlGaInAs/InP twin-square microlaser, where two square cavities with the same side length of 16 μm are connected by a bridge waveguide with a length of 40 μm and a width of 1.5 μm.

 

Fig. 1 (a) Schematic of an integrated twin-square microlaser. (b) Top-view microscope image of a fabricated AlGaInAs/InP integrated twin-square microlaser.

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The fabricated AlGaInAs/InP integrated twin-square microlaser is cleaved parallel to the bridge waveguide close to the p-electrode, and is then bonded p-side up on an AlN sub-mount. The p-electrode of cavity A is connected to the microstrip line for performing high-speed direct modulation tests. The device is mounted on a thermoelectric cooler (TEC) to maintain a temperature of approximately 290 K in the following experiments. The output light is collected by coupling a single-mode fiber (SMF) close to the vertex of one square cavity. For the twin-square microlaser working in the free-running state achieved by applying a continuous wave (CW) injection current to cavity A or cavity B separately, the twin-square microlaser works as two individual square microcavity lasers, with lasing behavior similar to that in [28]. The two square cavities will have a slight difference in their mode wavelengths owing to fabrication-induced deviations in cavity geometry. The wavelength of a square cavity can be tuned by adjusting the injection current through the current-induced heating effect [29]. The mutual optical injection between the two square cavities will result in different dynamic states because the wavelengths are similar, which can improve the direct modulation characteristics [25,26].

3. Small-signal modulation response

The small-signal modulation properties are measured by utilizing a 20-GHz network analyzer. The SMF coupled light from the integrated twin-square microlaser is amplified by 20 dB with an erbium-doped fiber amplifier (EDFA), and then passes through a tunable optical bandpass filter (OBPF) to filter out the background noise before it is detected by a high-speed photodetector (PD) [28,29]. The biased current and modulated signals are combined using a high frequency bias-T and are then fed to the microstrip of cavity A through a radio frequency probe. In the experiment for evaluating the mutual optical injection state of the twin-square microlaser, cavity B is biased with a CW injection current.

Figure 2(a) shows the measured small-signal modulation responses of the integrated twin-square microlaser working at the free-running state (I_B = 0). Relaxation oscillations with frequencies (f_r) of 3.2, 5.4, 6.6, and 8.3 GHz, and the peak heights of 9.2, 8.9, 7.7, and 6.3 dB relative to the response at the low frequency of ~1 GHz, are observed with the injection current of cavity A (I_A) equaling 5, 7, 9, and 11 mA, respectively. The practical value of the resonance peak decreases from −6.8 to −20.1 dB as I_A is increased from 5 to 11 mA, which shows a degradation of 13.3 dB in the modulation response, indicating increased loss in electrical-optical conversion.

 

Fig. 2 Small-signal modulation responses of the integrated twin-square microlaser working at (a) the free-running state with I_A = 5, 7, 9, and 11 mA, and I_B = 0, and (b) the mutual optical injection state with I_B = 26.2, 26, 25.8, and 25.6 mA, and I_A = 20 mA.

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By applying injection currents to both square cavities, a mutual optical injection through the bridge waveguide can be realized by precise control of the currents to match the mode wavelengths. The two square cavities have almost the same mode wavelength for the microlaser when I_A = 20 mA and I_B = 26.5 mA. Figure 2(b) shows the measured small-signal modulation responses of the integrated twin-square microlaser working at the mutual optical injection state. The injection current of cavity A is fixed at 20 mA, and that of cavity B is varied from 26.2 to 25.6 mA to tune the wavelength difference between the lasing modes in the two square cavities. The modulation responses exhibit strong resonance peaks at 9.8, 12.7, 15.4, and 18.6 GHz, with corresponding peak values of −14.7, −21.9, −23.9, and −26.6 dB caused by the photon-photon resonance. The resonance peak can be tuned over a wide range by adjusting the injection current of cavity B, which can be much higher than that of the free-running relaxation oscillation frequency. However, the resonance peak becomes weaker with the increase of the beating frequency because of the lower modulation response at high frequency, which is mainly limited by the carrier life time. It also should be noted that the twin-square microlaser will enter optical injection locking states similar to [26] as the beating frequency is further reduced.

4. Experimental OEO setup

Figure 3 shows the experimental setup of the proposed wideband frequency-tunable OEO. The output light from the integrated twin-square microlaser is collected by a tapered SMF coupled to one vertex of cavity A, and passes through a circulator and is then amplified by 10 dB with an EDFA. A tunable OBPF is used to filter out the amplified background noise and the undesired minor modes. The optical power is divided into two parts through a 1:1 beam splitter.

 

Fig. 3 Schematic diagram of the proposed OEO. EDFA: erbium-doped fiber amplifier; PC: polarization controller; SMF: single mode fiber; OBPF: optical bandpass filter; VOA: variable optical attenuator; PD: photodetector; BC: blocking capacitor; EA: electrical amplifier; RF Attn.: RF attenuator; OSA: optical spectrum analyzer; ESA: electrical spectrum analyzer.

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One part (~50%) of the optical power is fed back to cavity A through another 1:9 beam splitter, a variable optical attenuator (VOA), a 3 km SMF (SMF 1), a polarization controller (PC), and a circulator, which forms a long all-optical feedback loop (Loop 1) to reduce the linewidth of the microlaser and to improve the frequency stability of generated microwave [18,30]. The VOA and PC can control the injection strength and polarization state of the optical feedback signal, respectively. Ten percent of the light after the 1:9 beam splitter is sent to an optical spectrum analyzer (OSA) for monitoring the optical spectrum.

Another part (~50%) of the optical power is delivered to a 50 GHz photodiode with a responsivity of 0.7 A/W to be converted into an electric signal after 2.5 km of SMF (SMF 2). After blocking the direct current (DC) electrical signal with a blocking capacitor (BC), the generated RF electrical signal is amplified by an electrical amplifier (EA) with a gain of 40 dB, and is then attenuated by a variable RF attenuator with a tuning range of 0 to 30 dB and a tuning step of 1 dB to control the feedback power. An electrical spectrum analyzer (ESA) is used to measure the electrical spectra of the generated microwave signals. Finally, the feedback RF signal is applied to cavity A through a 26.5 GHz bias-Tee and a 40 GHz RF probe, forming an optoelectronic feedback loop (Loop 2).

The dual-loop structure with different delay times can suppress spur modes based on the Vernier effect [31,32]. The OEO will experience the lowest electrical-optical conversion loss around the relaxation oscillation frequency or the beating frequency of the photon-photon resonance owing to the modulation response peak, as shown in Fig. 2, and then forms a self-sustaining oscillation. By tuning the bias current of cavity A at the free-running state, or tuning the frequency difference between the lasing modes in the two square cavities at the mutual optical injection state, the oscillation frequency of the proposed OEO can be controlled.

5. Experimental results

In this section, we characterize the performance of the microwave signals generated by the OEO shown in Fig. 3. In the following experiments, we combine the free-running and mutual optical injection states of the twin-square microlaser to achieve broadband frequency tuning.

For the OEO with the twin-square microlaser working at the free-running state (with I_B = 0), frequency-tunable microwave generation can be achieved at the relaxation oscillation frequency by tuning the injection current I_A [16].Fig. 4(a) shows the microwave spectra of the OEO measured by the ESA with a resolution bandwidth (RBW) of 100 kHz as I_A is increased from 5 to 12 mA. The microwave signals of the first harmonics ranging from 2.22 to 9.01 GHz are obtained. However, because of the uneven frequency responses of the electrical components in the OEO, the generated microwave frequency is not strictly equal to the relaxation oscillation frequency of the microlaser. As shown in Fig. 2(a), the small-signal modulation response curve becomes flatter and the peak value drops with the increase of I_A, which indicates an increased loop loss. Thus microwave frequency is limited because the RF gain cannot compensate for the total loop loss at a relatively high injection current. In addition, we find that the RF power of the second harmonic increases with the signal frequency.

 

Fig. 4 Experimental spectra of the OEO with the twin-square microlaser working at the free-running state (I_B = 0). (a) Microwave spectra with I_A increased from 5 to 12 mA (RBW = 100 kHz). (b) Microwave spectrum with I_A = 9 mA (RBW = 100 kHz). Inset in (b): microwave spectrum around 6.78 GHz (RBW = 1 kHz, span = 4 MHz). (c) Phase noise spectrum of the generated first harmonic of 6.78 GHz. Inset in (c): microwave spectrum around 6.78 GHz (RBW = 10 Hz, span = 200 Hz). (d) Optical spectrum with I_A = 9 mA.

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Figure 4(b) shows the electrical spectrum of a microwave signal with the first harmonic frequency of 6.78 GHz. The microwave spectrum is measured with I_A = 9 mA and RF gain of 30 dB. The inset of Fig. 4(b) shows the zoomed-in view of the microwave spectrum around 6.78 GHz measured with a span of 4 MHz and an RBW of 1 kHz. The SMSR is 38 dB. Figure 4(c) shows the SSB phase noise of the first harmonic of 6.78 GHz. The SSB phase noise at the 10 kHz offset frequency obtained is as low as −115 dBc/Hz. The 3-dB linewidth is less than 20 Hz, as shown in the inset of Fig. 4(c). Figure 4(d) shows the optical spectrum measured by the OSA with a resolution of 0.02 nm. The microlaser is modulated under a strong electrical signal resulting in a comb structure, which is consistent with the existence of high-order harmonics in the electrical spectrum.

To further extend the tunable range of the microwave signals generated by the OEO, we switch the twin-square microlaser into the mutual optical injection state. Figures 5(a) and 5(b) show the lasing spectrum and the corresponding microwave spectrum without the feedback loops, respectively. The spectra are measured with I_A = 20 mA and I_B = 25.7 mA. The lasing spectrum with multiple peaks corresponds to period-one state similar to that observed in a microcavity laser with external optical injection [24]. The lasing modes A and B correspond to the coupled modes mainly distributed in cavities A and B, respectively. The intensity of lasing mode A is 3 dB higher than that of lasing mode B, with a frequency difference of 15.57 GHz. The generated microwave signal has a 3-dB linewidth of 6.2 MHz according to the Lorentz fitting, as shown in Fig. 5(b). The linewidth of the microwave signal can be narrower than the optical linewidth of the lasing mode in the twin-square microlaser owing to the strong mode correlation induced by the mutual optical injection [33], but it is still limited by the lasing mode linewidth.

 

Fig. 5 (a) Optical spectrum and (b) corresponding microwave spectrum of the twin-square microlasers with I_A = 20 mA and I_B = 25.7 mA.

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To reduce the linewidth, the all-optical feedback loop (loop 1) and the optoelectronic feedback loop (loop 2) are introduced, as shown in Fig. 3. Figure 6(a) shows the microwave spectra measured by the ESA with an RBW of 100 kHz. The frequency of the first harmonic increases from 9.06 to 19.52 GHz as I_B is decreased from 26 to 25.4 mA with I_A = 20 mA. As shown in Fig. 2(b), a modulation response peak around the beating frequency is obtained owing to the photon-photon resonance. Thus, a self-sustaining oscillation can be formed around the beating frequency (with the lowest loop loss) [32]. The first harmonic signals with frequencies up to 19.52 GHz can be generated by the OEO with the twin-square microlaser working at the mutual optical injection state. The RF power of the second harmonic in Fig. 6(a) with high frequency is found to be much higher than those shown in Fig. 4(a).

 

Fig. 6 Experimental spectra of the OEO with the twin-square microlaser working at the mutual optical injection state. (a) Microwave spectra with I_B decreased from 26 to 25.4 mA, and I_A = 20 mA (RBW = 100 kHz). (b) Microwave spectrum with I_A = 20 mA and I_B = 25.7 mA (RBW = 100 kHz). Inset in (b): microwave spectrum around 16.78 GHz (RBW = 1 kHz, span = 4 MHz). (c) Phase noise spectrum of the first harmonic of 16.78 GHz. Inset in (c): Microwave spectrum around 16.78 GHz (RBW = 100 Hz, span = 200 Hz). (d) Optical spectrum with I_A = 20 mA and I_B = 25.7 mA.

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Figure 6(b) shows the microwave spectrum of the OEO with I_A = 20 mA and I_B = 25.7 mA. The RF gain is 30 dB. The frequency of the first harmonic is 16.78 GHz. The inset of Fig. 6(b) shows the zoomed-in view of the microwave spectrum around 16.78 GHz with a span of 4 MHz and an RBW of 1 kHz. The SMSR is 40 dB. Figure 6(c) shows the SSB phase noise spectrum of the first harmonic of 16.78 GHz. The phase noise is −113 dBc/Hz at the 10 kHz offset frequency, which is similar to that measured with the twin-square microlaser working at the free-running state, as shown in Fig. 4(c). The 3-dB linewidth is also less than 20 Hz, as shown in the inset of Fig. 5(c). Figure 6(d) shows the corresponding optical spectrum with I_A = 20 mA and I_B = 25.7 mA measured by the OSA with a resolution of 0.02 nm. The mutual optical injection produces the period-one oscillation of the twin-square microlaser, which provides the microwave subcarrier seed for the generation of microwave signals.

By tuning the injection currents of the two square cavities separately, wideband frequency-tunable microwave signals are generated by the OEO with the twin-square microlaser. Figure 7(a) shows the microwave spectra measured by the ESA with an RBW of 100 kHz, which presents the wideband tunability from 2.22 to 19.52 GHz for the generated microwave signals.

 

Fig. 7 (a) Microwave spectra of the microwave signals generated by the OEO showing wideband frequency tunability (RBW = 100 kHz). (b) RF threshold gain and the SSB phase noise at 10 kHz offset frequency versus the microwave frequency for the first harmonics.

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Figure 7(b) shows the RF threshold gain and the SSB phase noise at the 10 kHz offset frequency as functions of the microwave frequency for the first harmonics. The lowest threshold gain around 9.06 GHz benefits from the modulation response peak of the twin-square microlaser owing to the photon-photon resonance. Within the entire tuning range, the SSB phase noises at the 10 kHz offset frequency are kept within the range from −110 to −117 dBc/Hz. Thus, the experimental results show that wideband frequency-tunable microwave signals with low phase-noise can be generated by the OEO based on the directly modulated AlGaInAs/InP twin-square microlaser by tuning the injection currents of the two square cavities.

6. Conclusion

In conclusion, a simple and cost-effective OEO based on a directly modulated AlGaInAs/InP integrated twin-square microlaser have been proposed and demonstrated. Wideband frequency-tunable microwave signals are generated by the OEO without requiring an external modulator, EBPF, or external optical injection. Owing to the enhancement of the direct modulation response by the relaxation oscillation or the photon-photon resonance for the twin-square microlaser, high-quality microwave signals with linewidths below 20 Hz ranging from 2.22 to 19.52 GHz are generated. Within the entire tuning range, the SMSRs are approximately 40 dB, and the SSB phase noises at the 10 kHz offset frequency are below −110 dBc/Hz. Taking into account the advantages of compact size and the simple fabrication process for the WGM semiconductor microlaser, our approach provides a potential solution for a cost-effective, small-footprint, ultrapure microwave source.