1. Introduction

Generation of high performance photonic microwave subcarrier with narrow-linewidth, low phase noise and wideband tunability has received considerable attention, due to its potential applications in radio-over-fiver (RoF) [1, 2] and optical signal processing [3]. Photonic microwave technology enables the processing of microwave signals through photon technology and optical fiber transmission, which shows unique advantages in low propagation loss and free of electromagnetic interference. Traditional techniques for photonic microwave generation include optical injection locking (OIL) [4], optical phase-lock loop (OPLL) [5], monolithic dual-wavelength laser [6], and external or direct modulation on lasers [7, 8]. Besides, nonlinear dynamics in optically injected semiconductor lasers, especially period-one oscillation and four-wave mixing, have also drawn great attention in microwave photonics due to wide tunable frequency band and no need for electronic microwave sources [9–17]. Period-one oscillation has been widely investigated in distributed feedback (DFB) lasers [11–14], vertical-cavity surface-emitting lasers [15], as well as quantum-dot DFB lasers [16]. Monolithically integrated DFB lasers were experimentally demonstrated for tunable and narrow linewidth millimeter-wave generation based on four-wave mixing [17].

Whispering-gallery mode (WGM) microresonators, with the merits of small footprint, high Q-factor and capability for integration, have also been employed to investigate optically injection nonlinear dynamics for their potential applications in tunable photonic microwave generation and optical frequency combs [18–23]. To realize high efficiency unidirectional emission from circular microdisk lasers, we proposed the microdisk lasers directly connecting with an output waveguide, which can be fabricated by standard contacting photolithography technique. The output waveguide breaks the symmetry of the microdisk and results in a unidirectional emission from the coupled modes, which have polygonal shaped mode patterns and high-Q factors [24]. Recently, the dynamical states in AlGaInAs/InP microdisk lasers connected to an output waveguide have been demonstrated in detail, including injection locking, four-wave mixing, and period-one and period-two oscillations, in company with photonic microwaves generations [21]. Integrated semiconductor twin-microdisk lasers under mutually optical injection have been proposed and demonstrated experimentally with microwave generation obtained from the electrode of the microlasers [22, 23]. However, due to the intrinsic lasing linewidth restriction of semiconductor microdisk lasers, and the fluctuations of temperature and injection current, the generated microwave signals have the 3-dB linewidth of tens of MHz.

To obtain high performance photonic microwave output, approaches such as optoelectronic oscillators (OEOs) [8, 25–29], optical feedback [12, 30, 31], and side-band injection locking [32] have been applied for narrowing and stabilizing the microwave signal effectively. OEO configuration was proposed by utilizing an E/O modulator and electrical feedback [8], and can be applied for generating high performance microwave signal in a wide tunable range with high purity and low phase noise, due to the doubly locking to optical injection and current modulation [26]. Broadly tunable microwave subcarrier with a linewidth below 1-kHz was realized using an optically injected DFB laser in period-one state under optoelectronic feedback loop [27]. The conventional OEO using an external modulator usually needs high-gain radio-frequency (RF) amplifiers to compensate the RF link loss and an electrical filter to select the desired oscillation. In contrast, direct modulated semiconductor lasers under optical injection can be attractive as a novel type of OEO, because of the relatively simple architecture. A 20-GHz RF signal with the phase noise of −123dBc/Hz was achieved by using a direct modulated DFB laser under strong optical injection and OEO system with an electrical filter [29], with the tunability of the OEO limited by the electrical filter.

In this paper, wideband frequency tunable, narrow-linewidth and low phase noise photonic microwave generation is experimentally demonstrated using a microdisk laser subject to optical injection and optoelectronic feedback. Direct modulated microdisk laser subject to external optical injection provides the microwave subcarrier at period-one state. An optoelectronic feedback loop is used to doubly lock and stabilize the generated microwave signal without using the electrical filter, which makes it much low-cost and of wideband frequency tunability. Frequency tunable range from 8.8 to 17 GHz is obtained, and 3-dB linewidth of the generated microwave signal is reduced to less than 1 kHz, with a phase noise of –101 dBc/Hz at offset of 10 kHz. This configuration of photonic microwave source based on the microdisk laser with the optical injection is potential to be realized by monolithic optoelectronic integrated circuits, fabricated using efficient large-scale manufacturing techniques. This paper is organized as follows. In Section 2, the device fabrication and static lasing characteristics are presented. In Section 3, the experimental setup is described and the small signal modulation characteristics are presented under free-running state and optical injection. In Section 4, detailed investigations of linewidth narrowing and phase noise reduction by the feedback loop are presented. Finally, a conclusion is drawn in Section 5.

2. Device fabrication and static characteristics

Microdisk lasers with a radius of 8 μm connected with a 2-μm-wide output waveguide are fabricated using an AlGaInAs/InP epitaxial wafer as in [33]. The schematic diagram of the microdisk laser connected with an output waveguide is shown in Fig. 1(a). The active region of the laser wafer consists of six compressively strained 6-nm-thick quantum wells and 9-nm-thick barriers. Contacting photolithography is employed to pattern the etching mask SiO₂ layer, and the microresonator with an etching depth of 4 μm is fabricated by inductively coupled plasma (ICP) etching technique. After that, a silicon nitride (SiN_x) layer is deposited on the wafer to provide the active layer from oxidation and the microresonator is then laterally surrounded by the benzocyclobutene (BCB) layer, which is spin-coated and thermo-cured to create a flat surface for the following lift-off process. Subsequently, the reactive ion etching (RIE) process is utilized to expose the top of the microdisk laser. Then, a SiO₂ layer is deposited for insulating purpose and annular contact window is opened. Finally, a Ti/Pt/Au layer is evaporated for the electrical injection. Figure 1(b) shows the scanning electron microscope (SEM) image of an 8-μm-radius microdisk resonator after ICP etching process, and Fig. 1(c) shows the microscopic image of the fabricated microdisk laser after the deposition of p-type and n-type electrodes. Annular p-type electrode is used to obtain a larger spatial overlap between the lasing mode intensity distribution and the injected carrier distribution [34].

 

Fig. 1 (a) Schematic diagram of the microdisk laser connected to an output waveguide. (b) SEM image of an 8-μm-radius microdisk resonator after ICP etching process. (c) The microscopic image of the microdisk laser after the deposition of the p-type and n-type electrodes.

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After cleaving the output waveguide to a length of about 20 μm, the microdisk laser is bonded on an AlN submount with the stage temperature at 287 K controlled by a thermoelectric cooler (TEC). Output powers (L-I) coupled into a multi-mode fiber (MMF) and a single-mode fiber (SMF) are measured and plotted in Fig. 2(a) as functions of continuous-wave (CW) bias current. The threshold current is estimated to be 4.5 mA from the clear inflection point of the MMF L-I curve, indicating evolution from spontaneous to stimulated emission. The output powers are 86 and 17 μW from the MMF and SMF at 40 mA, respectively. A series resistance of 24 Ω is estimated from the voltage-current (V-I) curve above the threshold. The lasing spectra at 4.5 and 30 mA are shown in Fig. 2(b), which are measured by an optical spectrum analyzer (OSA) at the resolution of 0.02 nm. Single mode operation at 1557.11 nm is obtained at injection current of 30 mA, with a side mode suppression ratio of 40 dB and a free spectral range (FSR) of 14.6 nm. The dominate mode Q factor can be estimated to be 7.9 × 10³ at 1554.15 nm from the full width at half maximum (FWHM) around the threshold current of 4.5 mA.

 

Fig. 2 (a) Output powers and voltage versus the applied current, and (b) lasing spectrum at 4.5 and 30 mA, for an 8-μm-radius microdisk laser at 287 K. MMF: multi-mode fiber; SMF: single-mode fiber.

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3. Experimental setup and small signal modulation

The experimental setup as shown in Fig. 3 is used for photonic microwave generation based on the microdisk laser under optical injection and optoelectronic feedback loop. A tunable laser with the linewidth of 100 kHz is used as a master laser to inject light into the microdisk laser, through an optical circulator and a tapered SMF. The microdisk laser output coupled into the SMF is amplified by an erbium-doped fiber amplifier (EDFA) and filtered by a tunable band-pass filter (BPF), then split up into two parts with 1% of the light for monitoring the optical spectra and 99% of the light detected by an 18-GHz-bandwidth high-speed photodetector (PD). The generated electrical signal is amplified by a RF amplifier with a magnification of 35 dB, then adjusted by a variable RF attenuator from 2 to 25 dB to control the feedback strength, and finally applied to the microdisk laser through a bias-Tee to form an optoelectronic feedback loop. Direct current modulation on the microdisk laser is provided by the RF signal driven by the optoelectronic feedback. The generated microwave signal is measured by an electrical spectrum analyzer (ESA) after splitting by a RF splitter with the bandwidth of 6-18 GHz. In addition, a vector network analyzer (VNA) with a bandwidth of 20 GHz is used to measure the small signal modulation response.

 

Fig. 3 Experimental setup for photonic microwave generation based on the microdisk laser under optical injection and optoelectronic feedback loop. SMF: single mode fiber; EDFA: erbium-doped fiber amplifier; BPF: band-pass filter; PD: photodetector; BC: blocking capacitor; RF Attn.: variable RF attenuator; OSA: optical spectrum analyzer; ESA: electrical spectrum analyzer; VNA: vector network analyzer.

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The small signal modulation responses for the 8-μm-radius microdisk laser under free-running state and optical injection are measured and plotted in Figs. 4(a) and 4(b) at 287 K, respectively. In the free-running state, the 3-dB bandwidths of 14.6, 15.3, and 16.4 GHz are obtained for the microdisk laser from the fitted solid lines in Fig. 4(a) at CW bias currents of 20, 25, and 30 mA, respectively. Relaxation resonant peak is gradually suppressed and the corresponding resonance frequency increases from 10.5 to 12.3 GHz as the bias current increases. Under certain optical injection conditions, periodic oscillations can be observed with the red shift of the lasing mode wavelength due to the variation of carrier concentration [29]. A photocurrent of 53 μA is measured from the microdisk laser at zero bias voltage under the injection optical power of 3 mW. Assuming a responsivity of 0.5 A/W, the actual optical power injected to the microdisk laser is estimated to be 106 μW inside the microdisk laser, indicating an optical injection efficiency of 3.5%. In the following sections, the injection optical power P_i measured at the tunable laser output facet is still used to represent the injection level. With the injection optical power of 3 mW and the bias current of 30 mA, the small signal modulation responses at period-one state are shown in Fig. 4(b) as the detuning frequency △f = 10.4, 15.3, and 19.1 GHz, which is defined as the lasing frequency difference between the master laser and the free-running microdisk laser. Additional resonance peaks at high frequency side are observed at the practical beating frequency of the master laser and the microdisk laser, which can be several times of the free-running relaxation oscillation frequency.

 

Fig. 4 Small-signal modulation responses (a) at the free-running state and bias currents of 20, 25, and 30 mA, and (b) under optical injection at the bias current of 30 mA, the injection optical power of 3 mW, and △f = 10.4, 15.3 and 19.1 GHz, for the 8-μm-radius microdisk laser at 287K.

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4. Narrow-linewidth microwave generation

In this section, we characterize the generated microwave signal at period-one state under the optical injection and optoelectronic feedback. The microdisk laser is biased at CW current of 30 mA with free-running lasing mode at 1557.11 nm in the following investigations. Under the injection optical power P_i = 4.5 mW and the detuning frequency △f = 5.1 GHz, the lasing spectra and corresponding photonic generation microwave spectra are measured and plotted in Figs. 5(a) and 5(b) without the optoelectronic feedback, and in Figs. 5(c) and 5(d) with the optoelectronic feedback, respectively. The arrow, vertical dashed line, and circle marks in Figs. 5(a) and (c) indicate the injected mode, lasing mode under free-running state and optical injection state, respectively. In Figs. 5(a) and 5(b), the injected mode intensity is 7.1 dB larger than the lasing mode with a frequency difference of 13.2 GHz, and the generated microwave signal has a 3-dB linewidth of 26 MHz according to the Lorentz fitting, which is usually limited by the lasing mode linewidth, because the tunable laser has the linewidth of ~100kHz..

 

Fig. 5 Optical and electrical spectra of the microdisk laser under optical injection, (a) (b) without and (c) (d) with the optoelectronic feedback situation. The injection parameters are P = 4.5 mW and △f = 5.1 GHz, and the feedback strength is optimized by adjusting the variable RF attenuator. Inset in (b) and (d): Zoom-in electrical spectra.

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When the optoelectronic feedback is turned on, a clean and narrow-linewidth microwave signal is observed at the center frequency of 14.7 GHz, as shown in Figs. 5(c) and 5(d), by adjusting the microwave amplifier and attenuator to give an optimized feedback loop gain of 25 dB. The optoelectronic feedback modulation level can be deduced from the obtained electrical spectra and the setting of RF attenuator, which is estimated to be about −10 dBm, considering the given operating conditions and additional insertion loss of the RF attenuator. The injected mode intensity is only 5.3 dB larger than the lasing mode due to sideband enhancement under the feedback RF modulation. However, the generated peaks in the lasing mode side are higher than that in the injected mode side in the lasing spectra. So we can expect that the injected mode light is weaker than the lasing mode inside the microdisk. The strong peak for the injected mode in the lasing spectra is mainly caused by the reflection of the injected light at the cleaved facet of the microdisk laser. In addition, zoom-in electrical spectra are shown in the inset of Figs. 5(b) and 5(d) for the feedback off and on situations, with the resolution bandwidth (RBW) of 100 kHz and 1 kHz, respectively. The 3-dB linewidth of the generated microwave signal in Fig. 5(d) is less than 1 kHz, which is less than one 20000th of that without the optoelectronic feedback loop. Furthermore, a frequency shift of 1.5 GHz in the generated microwave signal can be observed between the optoelectronic feedback on and off states. This is because the relatively strong feedback strength will lead to a rise in laser temperature and thus cause the red shift of cavity lasing mode due to heating effect under the RF feedback modulation. The remarkable linewidth narrowing effect of the generated microwave signal can be characterized by the sideband-injection locking scheme, where the multiple modulated sidebands are yielded in the microdisk laser modulated by the feedback RF signal under period-one oscillation. Subsequently, the lasing mode overlaps with the generated 1st order sideband or within the locking range of the 1st order sideband of the injected mode, and then the microdisk laser locks to the sideband of the injected mode. In turn, the phase-correlated modes ultimately generate a high-quality, narrow-linewidth photonic microwave signal. In addition, the optoelectronic feedback loop, which is essentially a microwave cavity, will oscillate as the closed-loop gain is up to the oscillation threshold and also ensures the dramatic linewidth narrowing of the output microwave signal [18].

To further demonstrate the phase noise properties of the generated microwave signal, we measure the single sideband (SSB) phase noise spectra of the above microwave signals, with the carrier frequencies centered at 13.2 and 14.7 GHz for the optoelectronic feedback turned off and on, respectively. As shown in Fig. 6, the SSB phase noise of the generated 14.7 GHz microwave signal under the optoelectronic feedback is −101 dBc/Hz at the frequency offset of 10 kHz from the center frequency. The lowest phase noise is about −125 dBc/Hz at an offset of 1.57 MHz, which is 53 dB lower than that in the open loop state. The additional peaks at about 3.6 MHz and its integer multiple frequencies can be observed in phase noise curve under feedback on state, which is due to the spurious oscillation of the 80 m fiber loop in our experiment. The results verify that the microdisk laser subject to optical injection and optoelectronic feedback can generate narrow-linewidth and low phase noise microwave signals, because the feedback signal of the period-one oscillation serves as a microwave reference to doubly lock the lasing mode to the external optical injection and the current modulation. Forming a closed loop feedback can notably alleviate the frequency jitters caused by temperature and bias current fluctuations. It should be noted that the remarkable low phase noise level microwave signal is obtained by the microdisk laser without extra frequency and temperature stabilization technique.

 

Fig. 6 SSB phase noise spectra for the generated microwave signal using an optically injected microdisk laser with optoelectronic feedback loop turned off and on, respectively. Measurements are taken with carrier frequencies centered at 13.2 and 14.7 GHz, respectively.

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Furthermore, tunable high-quality photonic microwave signals are demonstrated under the optoelectronic feedback by simply varying the injection parameters, such as the detuning frequency △f and the injection optical power P_i. As shown in Fig. 7, the microwave signals from 8.8 to 17 GHz are achieved with a coarse tuning step of 0.5 GHz by varying △f from 3.16 to 13.55 GHz with P = 4 mW, for the microdisk laser with the bias current of 30 mA at 287 K. The feedback strength is adjusted to realize narrow-linewidth signals by tuning the variable RF attenuator for each detuning frequency. The generated microwave signal can have the linewidth in kHz scale over a wide frequency range. The lowest tunable frequency is limited by the occurrence of injection locking and chaos state at low detuning frequency, while the highest tunable frequency is limited by the response bandwidth of the high-speed PD, microdisk laser and the RF splitter. We believe that wider tunable frequency range can be realized by using a PD and RF splitter with higher response bandwidths.

 

Fig. 7 Generated frequency tunable microwave signals with the detuning frequency △f varying from 3.16 to 13.55 GHz and the injection optical power P_i of 4 mW, for the microdisk laser at the bias current of 30 mA at 287 K.

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Under stable detuning frequency △f and the feedback strength of the microwave attenuator, evolution of the lasing spectra and the generated microwave signal spectra versus the injection optical power P_i are recorded and shown in Figs. 8(a) and 8(b) for the microdisk laser with the optoelectronic feedback turned on state. The optical injection power P_i varies from 0.1 to 4.9 mW, while the microdisk laser is still biased at 30 mA with a free-running lasing wavelength of 1557.11 nm and the detuning frequency △f of 3.16 GHz. At a low injection optical power, the microwave signal generated by the mode beating is very weak, which yields unstable modulation from the optoelectronic feedback and the generated microwave signal exhibits a wide peak around 10 GHz at P_i = 0.1 mW, as shown in Fig. 9(b). With the increase of the injection optical power, the lasing mode exhibits linewidth narrowing and slight wavelength red shift as shown in Fig. 8(a), so the corresponding microwave spectra gradually move to higher frequencies in Fig. 8(b), accordingly. As P_i is increased to 3.8 mW, stable optoelectronic feedback loop establishes with the lasing mode doubly locked to the optical injection and the RF modulation. The linewidth of generated microwave signal narrows sharply with the 3-dB linewidth at kHz-scale, as shown in the enlarge view of Fig. 8(b). However, as P_i is larger than 4.7 mW, the generated microwave signal peak becomes wide again. The detailed lasing spectra and the microwave signal spectra are plotted in Figs. 9(a) and 9(b) at P_i = 0.1, 4.0, and 4.9 mW, respectively.

 

Fig. 8 Evolution of the (a) lasing spectra and (b) microwave signal spectra versus the injection optical power P_i, for the microdisk laser biased at 30 mA with the optoelectronic feedback turned on. Zoom in is the enlarged view of the doubly locked region.

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Fig. 9 Detailed (a) lasing spectra and (b) microwave signal spectra at P_i = 0.1, 4.0 and 4.9 mW, respectively, for the microdisk laser biased at 30 mA with the optoelectronic feedback turned on.

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Finally, we further investigate the influence of the optoelectronic feedback strength on the generated microwave signals by varying the bias voltage of the RF amplifier. The generated microwave signals and lasing spectra at the period-one state under optoelectronic feedback are plotted in Figs. 10(a) and 10(b) with the feedback loop gain of 10, 17, 20, and 23 dB, respectively, while the microdisk laser is biased at 30 mA with the injection optical power P_i = 4.5 mW and △f = 3.16 GHz. The microwave signal linewidth gradually decreases from hundreds of kHz to 1-kHz with the feedback strength increasing from 10 to 23 dB. Meanwhile, the generated microwave frequency increases due to the red shift of the lasing mode, which can be attributed to the refractive index variation of the microresonator with the increase of the feedback strength. Thus, efficient feedback strength is indispensable for stably locking the microwave oscillation to guarantee a narrow-linewidth, high-performance RF signal output.

 

Fig. 10 (a) Generated microwave signals and (b) lasing spectra at period-one state under optoelectronic feedback with the feedback loop gain of 10, 17, 20, and 23 dB, respectively, for the microdisk laser at bias current of 30 mA, the injection optical power P_i = 4.5 mW and △f = 3.16 GHz.

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5. Conclusion

In conclusion, we have investigated the photonic microwave generation characteristics with an AlGaInAs/InP microdisk laser under both optical injection and optoelectronic feedback. Period-one dynamical state is utilized to generate the frequency tunable microwave subcarrier and optoelectronic feedback loop is used to stabilize and lock the oscillation. 3-dB linewidth of the generated microwave signal is reduced from 23 MHz with no feedback loop to less than 1 kHz with the optoelectronic feedback turned on. SSB phase noise of −101 dBc/Hz is obtained at a frequency offset of 10 kHz and the lowest phase noise is about −125 dBc/Hz at an offset of 1.57 MHz. Frequency tunable range from 8.8 to 17 GHz is obtained for narrow-linewidth microwave generation by adjusting the injection and feedback parameters. The performance of the microwave generation is still limited by the output power and direct modulation bandwidth of microdisk laser, and the optoelectronic feedback system. Higher frequency tunable range with better signal quality is expected by further optimizing the microdisk laser design, feedback system and using RF device components with wider bandwidth. Moreover, considering the advantages of compact size and high-Q factor of WGM microresonator, we believe that the photonic microwave generation system based on WGM microresonator lasers can provide potential compact and integrated solutions for the next generation wireless technology.