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Nov 17, 2015 - 1. Introduction. The optoelectronic oscillator (OEO) has received ... Fabry-Perot (FP) laser diode under optical injection has been implemented. ... A phase shift fiber Bragg grating (PS-FBG) [4] or ... DC-40 GHz RF generation has ..... Phase noise (@10-kHz) (a), carrier frequency (b), and carrier power (c) drift ...
A Widely Tunable Optoelectronic Oscillator Based on Directly Modulated Dual-Mode Laser Volume 7, Number 6, December 2015 Biwei Pan Dan Lu, Member, IEEE Limeng Zhang Lingjuan Zhao

DOI: 10.1109/JPHOT.2015.2498906 1943-0655 Ó 2015 IEEE

IEEE Photonics Journal

Tunable OEO Based on Dual-Mode Laser

A Widely Tunable Optoelectronic Oscillator Based on Directly Modulated Dual-Mode Laser Biwei Pan, Dan Lu, Member, IEEE, Limeng Zhang, and Lingjuan Zhao Key Laboratory of Semiconductor Materials Science, Beijing Key Laboratory of Low Dimensional Semiconductor Materials and Devices, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China DOI: 10.1109/JPHOT.2015.2498906 1943-0655 Ó 2015 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Manuscript received September 23, 2015; revised November 2, 2015; accepted November 3, 2015. Date of publication November 9, 2015; date of current version November 17, 2015. This work was supported in part by the National 973 Project under Grant 2011CB301702, by the 863 Project under Grant 2013AA014202, and by the National Natural Science Foundation of China under Grant 61201103, Grant 61335009, and Grant 61321063. Corresponding author: D. Lu (e-mail: [email protected]).

Abstract: A widely tunable optoelectronic oscillator (OEO) based on a directly modulated monolithic integrated dual-mode amplified feedback laser (AFL) is proposed and experimentally demonstrated. In this OEO scheme, the functions of the laser source, the intensity modulator, and the tunable high-Q RF filter are all realized through an AFL chip. On one hand, the beating frequency of the two modes defines the central oscillating frequency, which functions as an active tunable microwave photonic filter. On the other hand, by taking advantage of the photon–photon resonance, the laser has high modulation efficiency around the beating frequency that is far beyond the intrinsic modulation bandwidth of the lasers. It eliminates the use of high-speed external modulators and dramatically reduces the threshold gain of the oscillator. By tuning the injection currents of the laser sections, stable microwave signals that are continuously tunable from 28 to 41 GHz with signal-sideband phase noise below −106 dBc/Hz (at a 10-kHz offset) and threshold electronic gain no more than 27 dB are experimentally realized. Index Terms: Microwave photonics, semiconductor lasers.

1. Introduction The optoelectronic oscillator (OEO) has received intense investigation for its outstanding phase noise performance and capability of generating high-frequency pure microwave signals [1]. It can find numerous applications in wireless communications, radar systems, signal processing, sensors, etc. For an OEO loop, a long optical fiber is usually employed to improve the quality of the output microwave signal [2], but it inevitably introduces a dense mode-spacing ranging from kilohertz to megahertz. To define the desired oscillating frequency and guarantee the spectrum purity, a high-Q electronic band-pass filter (EBPF) is required, but for high frequencies, it is not easy to find an EBPF with sufficiently narrow bandwidth and wide frequency tuning range. To overcome the tunability limitation, several OEO structures based on tunable microwave photonic filters (MPF) have been presented in recent years. For the MPF proposed in [3], a Fabry-Perot (FP) laser diode under optical injection has been implemented. A frequency tuning range of 6.41–10.85 GHz is realized by either adjusting the wavelength of the pump laser or the

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longitudinal modes of the F-P laser. In [4] and [5], tunable MPFs have been achieved by phasemodulation to intensity-modulation conversion. A phase shift fiber Bragg grating (PS-FBG) [4] or a tunable optical filter [5] have been used to select the carrier and one of the phase modulated sideband. In [6], a tunable MPF has also been demonstrated through the combination of a polarization modulator and a chirped FBG. An OEO scheme with a frequency tuning range from 5.8 to 11.8 GHz is realized. In 2013, a MPF consisting a finite impulse filter and an infinite impulse response filter has been proposed to realize a tunable microwave output from 6.88 to 12.79 GHz [7]. More recently, an MPF using a broadband light source with a MZI has been reported. A quasi-continuous tuning range from 1 to 12 GHz is achieved [8]. DC-40 GHz RF generation has also been reported by using stimulated Brillouin scattering in high nonlinear fibers [9]. Although some excellent results have been obtained, the reported MPFs are based on tunable laser sources, bulky optical filters or special fibers, requiring mechanical tuning or high pump power, which are complicated and costly. Besides, electrical amplifiers with considerably high gain (∼50–60 dB) are required to drive the modulators in these OEO systems, resulting in considerable power consumption and high costs. Recently, simple OEO structures using tunable dualmode lasers as the MPF have been presented. J. Maxin et al. [10] have demonstrated a tunable OEO scheme based on an Er, Yb: glass dual-mode laser with a frequency-locked loop. Although a very low phase noise was achieved, the laser source was formed by discrete components, which would suffer problems in both footprint and stability. Wang et al. [11] have presented an OEO structure based on monolithic dual-mode laser, but an additional RF reference was needed in the loop. In our previous work [12], an autonomous OEO scheme based on monolithic dualmode laser has been proposed. However, external modulator and high gain electronic amplifier were still needed. To achieve high modulation efficiency and low RF threshold gain, direct modulated semiconductor lasers are very attractive. Sung et al. have proposed an OEO scheme based on directly modulated lasers under strong optical injection, in which a 7 dBm RF threshold gain is attained [13]. However, an EBPF with fixed central frequency is still used, which limits its tunability. Xiong et al. have demonstrated a simple OEO based on direct modulated DFB laser [14]. However, the tuning range is very limited due to the falling of the resonance peak when the bias current or the temperature increase. For an improved scheme presented in [15], the frequency tuning range is enhanced through additional optical injection, but another tunable laser source is needed to form the optical injection system, which results in a complicated system. In this paper, we propose a compact and widely tunable OEO scheme based on a directly modulated AFL. In this scheme, the functions of the laser source, intensity modulator and tunable MPF are realized through a directly modulated dual-mode AFL, which significantly simplifies the system configuration and reduces the footprint, power consumption and cost. The beating-frequency of the two modes defines the central oscillating frequency, which functions as an active microwave photonic filter. Moreover, the laser has a high modulation efficiency around the beating frequency through taking advantage of the photon-photon resonance [16], which eliminates the use of the external modulator and dramatically reduces the threshold electronic gain of the oscillator. By tuning the injection currents of the laser sections, stable microwave signals continuously tunable from 28 to 41 GHz are realized with signal-side-band (SSB) phase noises below −106 dBc/Hz (@ 10-kHz offset) and threshold electronic gain no more than 27 dB. The lowest SSB phase noise of −115.93 dBc/Hz is achieved at 33.5 GHz. Within a measurement time of 100 seconds, the drift of phase noise (@10-kHz offset), carrier frequency, and power level of this signal are less than ±3 dB, 1.5 kHz, and 1 dB, respectively. This simple and compact structure gives a promising direction to realizing an integrated OEO chip.

2. Experimental Setup Fig. 1 shows the experimental setup of the AFL-based OEO configuration, which is formed by two coupled oscillation loops. The dual-mode emission of the AFL passes through a circulator and is divided into two part through a 10/90 optical coupler. One part (90%) of the optical power

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Fig. 1. Experimental setup of proposed directly-modulated-AFL-based OEO. AFL: Amplified feedback laser; VOA: Variable optical attenuator; PC: Polarization controller; PD: Photodetector; LNA: Low noise electronic amplifier; EC: Electronic coupler.

is delivered to a photodiode (u2t BPD-02 with 50 GHz-bandwidth) to be converted into an electric signal. It is then amplified through a low noise electric amplifier (LNA) and directly modulates the DFB section of the AFL through a Bias-T to accomplish the optoelectronic loop. Another part (10%) of the optical power is feedback to the AFL to form an all-optical loop. The introduction of the all-optical fiber loop is to suppress the unwanted side modes in conjunction with the optoelectronic loop. One should be noted that the purpose of using a circulator instead of an optical coupler in the all-optical loop is to avoid the interference of the lights on the PD from the two loops, which are usually realized by using dual-electrical loop [17] or polarization orthogonal dual-optical loop methods [18]. Besides, the circulator also directs the light in the all-optical loop feedback to the AFL to realize a self injection locking, so as to narrow the linewidth of the laser as well as the beating frequency [19] and increase the Q value of this MPF. The total length of the all-optical and optoelectronic loops are about 5 km and 6 km (5 km þ 1 km), respectively. A 27-dB LNA is used in the optoelectronic loop to provide sufficient electrical gain. In addition, a variable optical attenuator (VOA) and a polarization controller (PC) are used before port 1 of the circulator to control the injection strength and polarization state of the optical feedback signal, respectively. The output signal is monitored through an RF spectrum analyzer (ESA) (Agilent PXA N9030A) and an optical spectrum analyzer (OSA) (Advantest Q8384). In the proposed OEO structure, the key device is the AFL. It is a monolithic integrated dualmode laser consisting of a DFB section and a short integrated feedback cavity, as shown in Fig. 2. The 220-m-long DFB section functions as a laser source, where a complex-coupled grating is applied to guarantee the single mode operation of this section. The short integrated feedback cavity consists of a 240-m phase section and a 320-m amplifier section, allowing the adjustment of the feedback phase and strength through injection currents. Under proper bias condition, the laser can work in dual-mode state due to the two external cavity modes getting comparable threshold gain. Detailed fabrication and performance of the device is presented in [20]. As shown in Fig. 1, the dual-mode AFL first servers as the laser source for the whole oscillator. Second, the beating of the two laser modes will greatly facilitate the initial oscillation of the OEO, which traditional relies on the transition from noise to oscillation in the normal OEO loop. The beating-frequency defines the central oscillating frequency, which functions as an active MPF. Tuning of this MPF can be easily realized through adjusting the DC currents of the laser sections. Third, the AFL can be direct modulated with high efficiency around the beating-frequencies due to the photon-photon resonance. The requirement for high electrical gain in the loop can thus be relaxed. In the experiment, the AFL was an unpackaged chip mounted on a Cu heat-sink. The temperature of the device was maintained at 25 °C by a thermoelectric cooler (TEC). The DFB section

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Fig. 2. Schematic view of the AFL.

Fig. 3. (a) Optical spectrum of the dual-mode AFL. (b) Corresponding RF beating-note. (RBW: 3 MHz; VBW: 10-kHz). (c) Measured frequency response of the dual-mode AFL at 33.5 GHz modespacing. (d) Zoom-in frequency response in the scanning range of 100 MHz.

was pumped through an RF probe with a DC bias Tee. The DFB section, the phase section, and the amplifier section were independently driven by DC currents, as I DFB , I P , and I A , respectively. Optical output was coupled to a tapered single-mode fiber from the facet of DFB side.

3. Experimental Results When I DFB , I P , and I A were biased at 70 mA, 0 mA, and 55 mA, respectively, the laser exhibited a dual-mode emission with 1.2 mW output power. The optical spectrum of this state is shown in Fig. 3(a). Wavelength difference ðÞ between the two main modes is 0.27 nm, corresponding to a beating frequency of 33.5 GHz. The strength difference of them is about 3.5 dB. The two side-mode with the same mode-spacing are generated due to the four-wave mixing (FWM) of the two coexisting main modes [21], which indicates a high phase correlation and an efficient mode-beating of the them. Fig. 3(b) shows the RF spectrum of the corresponding beating signal, where a carrier-to-noise ratio larger than 25 dB is obtained. The frequency response of this

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Fig. 4. (a) Measured beating-frequencies as a function of I A . (b) Frequency response of the device under different beating-frequencies.

Fig. 5. (a) RF spectrum of the generated 33.5-GHz microwave signal under 1-GHz span (RBW: 3 MHz VBW: 3 MHz). (b) Zoom-in view of this signal with 1-MHz span and 9.1-kHz RBW. (c) Phase noise spectrum of the generated 33.5-GHz signal.

state was measured by a vector network analyzer (VNA) (HP 8510c), as shown in Fig. 3(c). As can be seen, the modulation response exhibits strong resonance at 33.5 GHz due to the photonphoton oscillation, which results from the dual-mode emission with 33.5-GHz mode-spacing. Zoom-in frequency response in a scanning range of 100 MHz with linear scale is plotted Fig. 3(d), which shows a −3-dB linewidth of 5.8 MHz through Lorentz fitting. This frequency response peak defines the oscillation frequency, as well as providing strong modulation response, which dramatically decreases the RF threshold gain of the whole loop. By adjusting the bias current of I A from 30 to 130 mA with I DFB fixed at 70 mA and I P biased at 0 mA, respectively, the beating-frequency of the device can be continuously tuned from 28 to 41 GHz, as shown in Fig. 4(a). The dual-mode state is chosen with the criteria that the intensity difference between the two modes is smaller than 10 dB. The overlapped frequency responses under different beating frequencies are shown in Fig. 4(b). The frequency response becomes weaker with the increasing of the beating frequency, which is due to the damping of the deliverable driving RF signal to the DFB section, since the contact pad of DFB section was not specially designed for high-frequency operation. Further increase of the frequency response can be expected if high frequency design of the DFB section is adopted. When the bias current of amplifier section exceeded 130 mA, three or more modes appeared simultaneously, which limited the further tuning of the beating-frequency. By closing the OEO loop with an optical injection power of −16 dBm measured at port 1 of the circulator, the OEO starts to oscillate at the beating-frequency of the AFL. The two lasing modes will be automatically locked to the FSR of the feedback loop due to the frequency pulling effect happed in the AFL. The dual-loop configuration performs a fine mode selection. Fig. 5(a) shows the RF spectrum of the generated 33.5-GHz signal in a 1-GHz span. The zoom-in view of this signal in 1-MHz span and 9.1-kHz RBW is also plotted in Fig. 5(b), showing a > 60 dB sidemode suppression ratio. The single-sideband (SSB) phase noise spectrum of the obtained signal is measured using the build-in phase noise module of the RF spectrum analyzer. As

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Fig. 6. Phase noise (@10-kHz) (a), carrier frequency (b), and carrier power (c) drift within 100 seconds continuous measurement.

Fig. 7. (a) Spectra of the generated microwave signal at a frequency tuning range of 32 to 41 GHz with a tuning step of 1-GHz. (b) 10-kHz phase noise performance as functions of oscillationfrequency.

shown in Fig. 5(c), the phase noise of the 33.5-GHz signal is −115.93 dBc/Hz at 10-kHz frequency offset, which is close to the measurement limit of the spectrum analyzer. A better phase noise value will be expected with a higher precision measurement. Other side-modes have a maximal phase noise of −76 dBc/Hz, indicating a good side-mode suppression. The frequency stability of the obtained signal was measured by continuously recording the phase noise, carrier frequency, and carrier power using the “spot frequency” mode of the RF spectrum analyzer. As shown in Fig. 6, within 100 seconds of continuously measurement, the phase noise drift at 10-kHz offset is < ±3 dB, the carrier frequency drift is 1.5 kHz, and the carrier power drift is < 1 dB. By tuning the bias currents of I A , the oscillation-frequency can be continuously tuned in a wide range. Fig. 7(a) shows the overlapped RF spectra of the generated microwave signal over the tuning range of 28–41 GHz with a tuning step of 1 GHz. Phase noise performance at different working frequencies is summarized in Fig. 7(b). As we can see, in all the frequency tuning range, the SSB phase noises are below −106 dBc/Hz@10-kHz. The tuning range of this OEO scheme is determined by the frequency tuning range of the AFL. Since the AFL-structure can work in other frequency ranges, like 10-GHz, 20-GHz, and 60-GHz, through choosing appropriate cavity length [22]–[24], the proposed OEO scheme can be easily extended to many other frequency-bands. Moreover, it is also technologically possible to integrate the optical coupler, resonator and the PD with the AFL to form a single chip OEO.

4. Conclusion In conclusion, a compact and widely tunable OEO configuration based on directly-modulated dual-mode laser has been proposed and experimentally demonstrated. The functions of the laser source, intensity modulator, and tunable narrow-band RF filter are realized by a single AFL chip, which significantly simplifies the configuration and reduces the power consumption and cost. Stable microwave signals continuously tunable from 28 to 41GHz with signal-side-band (SSB) phase noise below −106 dBc/Hz (@ 10-kHz frequency offset) and threshold electronic gain

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< 27 dB were experimentally realized. The minimum phase noise of −115.93 dBc/Hz was achieved at 33.5 GHz, and the stability of the signal was also investigated. This scheme shows a promising direction to realizing an integrated OEO chip in the InP material system.

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