1. Introduction

Optoelectronic oscillator (OEO) has attracted much attention since Yao and Maleki proposed the concept in 1996 [1–4]. Owing to its low phase noise and superior performances at high frequency, OEO can find its broad applications in photonics and RF systems such as communication systems, radar, optical signal-processing and microwave photonics. During the recent two decades, persistent efforts have been made to pursue high-performance OEOs based on various optoelectronic devices. In [5], Yao employed a dual-drive MZ modulator to further reduce the close-to-carrier phase noise of the oscillator by at least 20 dB with the carrier suppression technique. Zhou et al. reported a new injection-locked dual optoelectronic oscillator that used a long optical fiber loop master oscillator to injection lock into a short-loop signal-mode slave oscillator to improve the phase noise in [6]. All of the electrical devices used in the master loop also appeared in the slave loop, which increased complexity to the system [7]. demonstrated the generation of 10 GHz microwave signal with the phase noise of −140 dBc/Hz at the frequency offset of 10 kHz in a fiber mode-locked laser setup. Generally speaking, there are two deficiencies in traditional OEO schemes: First, a large threshold gain that is used to compensate the loop loss and guarantee single-mode oscillation is required when employing an external modulator. Second, because of the need for long optical fibers with high Q value (10 kilometers correspond to 20 kHz mode-spacing), an electrical bandpass filter (EBPF) with a high quality factor is needed inevitably. Moreover, the use of EBPF debases the frequency tunability of generated microwave signal. The YIG filter solves the problem of tunability by replacing the EBPF. In [8], the oscillator can be tuned from 6 to 12 GHz in steps of 3 MHz, and exhibits a phase noise of −128 dBc/Hz at 10 KHz away from the carrier. While, the external modulator is still used which makes the needed electrical threshold gain remain at a high level.

Recently, the OEOs based on optoelectronic characteristics of semiconductor lasers themselves have gained extensive attention [9–13], which are expected to be an effective option to overcome some existing problems in OEOs. In [9], Xiong et al. realized a low-cost and wideband frequency-tunable OEO based on single directly modulated distributed feedback (DFB) semiconductor laser with the tuning frequency from 3.77 GHz to 8.75 GHz, which benefitted from the gain of relaxation peak. Another method that takes advantage of lasers’ optoelectronic responses is the optical injection technique. It is found that under the condition of optical injection, the increase of resonant frequency can enlarge the tuning range of OEO, and the high modulation efficiency at the resonant frequency can decrease the required threshold electrical gain of the loop [10]. Sung et al. adopted optical injection to reduce the threshold RF gain by 7dB without using any external modulator [11]. However, an extra EBPF was still employed to select the oscillating frequency. The built-in microwave photonic filter(MPF) provides an effective solution to this problem [12,13]. In [12], through external injection, the FP-LD functioned as a tunable high-Q MPF, and the frequency tuning was realized simultaneously. In [13], microwave signals with frequency tuned from 5.98 GHz to 15.22 GHz were generated without any EBPF. To further improve the performance of OEO, a scheme with an external low phase noise microwave signal source injecting to the slave laser was proposed to reduce the phase noise at the frequency offset of 1 kHz by 18 dB [14]. But this also adds a bit complexity to the system. In [15], synchronized dual-wavelength narrow-linewidth laser and high-quality microwave signal generation were demonstrated using mutual-injection-locked DFB lasers with optoelectronic feedback. The above systems based on discrete semiconductor lasers, have to employ optical circulator, polarization controller (PC) and/or external intensity modulator to align the states between the two lasers and up-convert the radio frequency (RF) signals to the optical carrier. Therefore, the entire system suffers from bulky configuration, complexity and instability. Fortunately, the integration of discrete lasers opens a new door to simplify the system configuration. Sung et al. proposed a simple scheme to obtain a high-quality microwave signal with an integrated dual-section DFB-LD [16]. In [17], Pan et al. obtained stable microwave signals that were continuously tunable from 28 GHz to 41 GHz with single-sideband (SSB) phase noise below −106 dBc/Hz (at 10 kHz) based on a monolithic integrated dual-mode amplified feedback laser (AFL). However, the system in [16] still use an extra EBPF, which increases the realization cost and limits the frequency tunability, and the monolithically integrated AFL needs complicated active-passive integration.

In this paper, we demonstrate a simple frequency-tunable optoelectronic oscillator using an integrated multi-section (IMS) DFB laser. The method of direct modulation and a built-in MPF are adopted to make the structure simpler and more flexible taking the place of an external modulator and an EBPF. In the proposed scheme, the key device is IMS-DFB laser which can overcome the drawbacks of discrete lasers such as fussy polarization controlling and complex structure. The two DFB sections which are fabricated by Reconstruction Equivalent Chirp (REC) technique with high controlling accuracy and low manufacturing cost are injected mutually. The sidemode suppression ratio (SMSR) of obtained signal is 60.94 dB which improves over 40 dB compared with a single loop. The SSB phase noise of the generated signal at the frequency of 20.3 GHz is −115.3 dBc/Hz@10kHz and −92.9 dBc/Hz@1kHz. What’s more, we demonstrate that the phase noise has an improvement about 8 dB at the frequency offset of 1 kHz when employing 13 km and 5.4 km fibers as the dual loop. The frequency tuning range of the MPF is from 16.9 GHz to 34.7 GHz based on our integrated laser, while the tuning range of the OEO is from 17.3 GHz to 21.7 GHz on account of the limited bandwidth of the electrical amplifier. This simple and compact structure consisting of IMS-DFB laser fabricated by REC technology gives a prospect to develop towards the integration of OEO based on optical injection.

2. Principle and experimental setup

Figure 1 illustrates the experimental diagram of proposed optoelectronic oscillator. In this system, the traditional bulky optical injection subsystem that is based on discrete optoelectronic devices is replaced by a compact IMS-DFB laser which is similar to what has been described in [18–20]. The integrated multi-section laser consists of a rear DFB section, a phase section and a front DFB section, which are electrically isolated from each other. All the three sections have the same active layer and can be controlled by different current sources separately. The RF signal can be loaded at the front section to modulate the front DFB laser. The gratings in all sections are fabricated by REC Technique by means of sampled grating pattern which is formed by a conventional holographic exposure combining with a conventional photolithography [19]. As is known, it is indispensable to control the detuning frequency between two lasers to ensure the required working states in optical-injection lasers. Compared with the traditional discrete laser sources that are used for optical injection, it is more challenging to control the wavelengths of the monolithically integrated multi-section lasers since all the integrated lasers share the same heat sink and a single Thermo Electric Cooler (TEC). In this work, fortunately, the wavelengths of the two section lasers and the detuning frequency between the two sections can be controlled accurately through changing the sampling periods [21]. Benefitting from the high wavelength-controlling accuracy of REC technique, the largest wavelength deviation is less than 0.2nm [22], which is sufficient for the process of integrated optical injection. The two section lasers are injected mutually because there is no optical isolator between them.

 

Fig. 1 Schematic diagram (a) of the proposed OEO based on the integrated multi-section laser (b) (OSA: optical spectrum analyzer, PC: polarization controller, PBS: polarization-beam splitter, PBC: polarization-beam coupler, PD: photodetector, EA: electrical amplifier, EC: electrical coupler, ESA: electrical spectrum analyzer).

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The bias currents of the rear section and the front section lasers are labeled as I_DC1 and I_DC2, which are provided by two THORlabs (LDC205C) respectively. To ensure the stability of laser, the temperature control circuit is controlled by the current-temperature controller (ILX-Lightwave, LDC-3724) which remains the temperature at 21 °C. The output of IMS-DFB laser is connected to an optical splitter. 1% of the light is extracted for signal monitoring using an optical spectrum analyzer (FINISAR Wave-Analyzer 1500s) with a resolution of 1.2 pm, meanwhile, the other 99% part is sent to the loop. Before the polarization beam splitter (PBS), a polarization controller (PC1) is used to make sure the polarization is parallel with that of PBS. The two orthogonal polarized light respectively pass through two standard single-mode fibers (SMFs) (1.1 km and 2.9 km) to obtain a high Q value. The light from the output combination of PBC is detected by a photo-detector (FINISAR u2t XPDV2120RA) with the bandwidth of 50 GHz and responsivity of 0.65 A/W. The generated RF signal is amplified by two cascaded electrical amplifiers (EAs) with the total gain of 49 dB (EA1, bandwidth is 40 GHz and noise figure is 6 dB; EA2, bandwidth is 20 GHz and noise figure is 7 dB). The amplified RF signal whose frequency equals to the beat frequency between the front and rear section lasers is split by an electrical coupler, the 50% part is measured by an electrical spectrum analyzer (ROHDE&SCHWARZ FSW-SIGNAL&ANALYZER), while the other 50% part is fed back to the RF port of the integrated laser to modulate the front section laser. Obviously, neither a high-speed external modulator nor an EBPF is required in the system.

The operation principle is based on the wavelength-selective amplification of the rear and front section lasers under mutual injection. As is shown in Fig. 2(a), the cavity modes of the free-running rear and front section lasers are labeled as f_r and f_f. They will red-shift to f_r^’ and f_f^’ respectively under mutual injection. There will appear an ultra-narrow gain spectrum which is centered at the cavity mode f_r^’ as has been verified in our previous work [23]. The front section laser is modulated by the RF signal f_m as Fig. 2(b) shows. When adjusting I_DC2, the weak + 1st sidemode of the modulated signal will fall into the range of the gain spectrum and get amplified in Fig. 2(c). The amplified sidemode will lock the rear section cavity mode f_r^’ if the sidemode is around the center of the spectrum. Since it will only happen for those frequencies falling in the gain spectrum, a single passband MPF with its amplitude response shape following the gain spectrum can be generated consequently, as Fig. 2(d) shows.

 

Fig. 2 Illustration of the IMS-DFB laser under mutual injection: (a) four cavity modes under different conditions: free-running rear laser mode f_r and red-shift mode f_r^’ with a gain spectrum under injection, free-running front laser mode f_f and red-shift mode f_f^’ under injection (b) optical signal of the modulated front section laser with the modulation frequency f_m. (c) the red-shift front laser mode with amplified + 1st sidemode locking the red-shifted rear laser mode. (d) the frequency response of the front section laser under injection and no injection.

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As is known, there is a peak in the frequency response curve named resonance peak which comes from the coupling between carrier and photons when the front section laser is directly modulated. The power of the frequency centered at the resonance peak is highest. When the loop cavity is closed, the resonance frequency will obtain the highest modulation efficiency which will decrease the RF threshold gain necessary for loop oscillation. The undesired sidemodes outside the bandwidth of the MPF are filtered by the MPF, the main mode which locks the rear laser mode will be selected and amplified through numerous feedback. Moreover, optical injection technology has been proved to enhance the frequency response and modulation bandwidth of the semiconductor laser [24]. With the help of optical injection, the resonance peak is enhanced evidently, making the threshold gain decreased and the tuning range enlarged. The complexity of the system is greatly reduced without using an external modulator or an EBPF.

3. Experimental results

In the proof of experiment, when I_DC1 increases, the mode of the rear section laser will red-shift, and also the mode emitted from the front section laser because $\xi$ becomes bigger. Here $\xi$ is defined as the power ratio of light between the rear and front section lasers. As Fig. 3 shows, the black line is optical spectrum of the free-running front section laser without any injection from the rear section laser when I_DC1 = 0 mA, I_DC2 = 76 mA, the blue one is the mode of the free-running rear laser when I_DC1 = 86 mA, I_DC2 = 29 mA (which is called transparent current) and the red line represents the optical spectrum when the integrated laser works in the condition of mutual injection but not injection-locked. What should be noted is that the power of free-running rear laser mode is low because the light is absorbed partly when passing through the phase and front sections. In this experiment, we set the current imposed to the phase section at 0 mA. Two strong peaks are plotted in the red curve. The left one is the red-shifted mode generated from the rear section laser with the frequency f_r^’ and the right one is the red-shifted cavity mode generated by the front section laser with the frequency f_f^’, which is changed with $\xi$ and the I_DC2. Therefore, RF signals with different frequencies that are modulated on the front laser will receive different gains. Especially, the RF signal with the frequency f_m corresponding to the difference between f_r^’ and f_f^’ will obtain the largest gain, which forms the pass band of the corresponding MPF. In addition, the central frequency of MPF can be tuned by adjusting the injection ratio and the currents injected to the two section lasers.

 

Fig. 3 Optical spectrum of the integrated laser at different currents: black curve is the free-running mode of front section laser when I_DC1 = 0 mA, I_DC2 = 76 mA; the blue curve is free-running mode of rear section laser when I_DC1 = 86 mA, I_DC2 = 29 mA ; the red curve is spectrum of mutual injection when I_DC1 = 86 mA, I_DC2 = 76 mA.

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At the same time, the frequency response of the front section laser changes under optical injection. Figure 4 shows the modulation response curves when the driving currents are set as different combinations. From the curves in Fig. 4(b), we can see, the frequency response has a peak when the front laser is under optical injection compared with that of no optical injection as Fig. 4(a) shows. And the modulation frequency where the peak appears varies from 16.9 GHz to 34.7 GHz under different bias currents. However, it should be noted that there is a deep notch around 15 GHz because of the package as is explained in [18,19]. At the same time, the peak acts as the built-in MPF which filtrates the superfluous sidemodes outside the peak. The details of the peak is shown in Fig. 4(c) from which we can see the 3-dB bandwidth of the MPF at the frequency of 26.31 GHz is 6 MHz. The bandwidth of the MPF at different frequencies varies from 6 to 18 MHz as is plotted in Fig. 4(d). So, it is possible to obtain a low phase noise and wide tunable microwave signal under the condition of optical injection and direct modulation.

 

Fig. 4 The frequency response of the IMS-DFB laser: (a) no injection with I_DC1 = 0 mA, I_DC2 = 75 mA ; (b) under injection when adjusting I_DC1 and I_DC2 with the frequency of the oscillation peak varied from 16.9 GHz to 34.7 GHz. There exists a deep notch around 15 GHz which is caused by the package. (c) details of the MPF at 26.31 GHz. (d) bandwidth of the MPF at different frequencies.

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Once the loop cavity is closed, an OEO can be obtained. Initially, a 1.1 km SMF connects the PC1 and PD directly. The electrical spectrum of the RF signal has many sidemodes and the SMSR is only 19.78 dB when I_DC1 = 92.5 mA and I_DC2 = 79.8 mA, as shown in Fig. 5(a). According to theoretical calculation, the mode interval is approximately 200 kHz ~1 km which is also consistent with experimental result. To further suppress the undesired sidemodes, a dual-loop structure is employed. The short loop determines the mode interval and long loop affects the phase noise of the OEO because of its high Q factor. Thanks to the Vernier Effect, the unwanted sidemodes are suppressed, which contributes to the improved SMSR. Herein, we use PBS and PBC to avoid the interference and beat noise caused by the coherent light when detected on one PD. Figure 5(b) shows the obtained electrical spectrum of the 19.78 GHz RF signal and its SMSR is 60.94 dB, which achieves an improvement by 41.16 dB compared with that of the single loop structure at the same bias currents of the two section lasers. Besides, The long-term frequency stability is investigated by measuring frequency drifts over a period of 20 min by using the “max hold” function on the ESA as is shown in Fig. 5(b). The center frequency of the microwave signal drifts 6.8 kHz in 20 min without the mode-hopping phenomenon.

 

Fig. 5 Measured electrical spectrum of the generated 19.78 GHz beat signal. (a) single loop OEO, (b) dual-loop OEO. (Span, 2MHz; RBW, 10kHz). The inset is the long-term frequency stability that drifts 6.8 kHz in 20 min.

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In order to realize frequency tunability of OEO, we adjust the electrical current imposed on the front DFB section. In this work, I_DC1 is fixed at 91 mA, while I_DC2 is changed from 69.86 mA to 96.68 mA. The measured electrical spectrums of OEO are shown in Fig. 6(a). The tuning frequency varies from 17.3 GHz to 21.7 GHz with the change of I_DC2. The relationship between the OEO frequency and the current is plotted in Fig. 6(b). Therefore, tunable OEO is successfully realized by adjusting the bias current. The corresponding optical spectrum is shown in Fig. 7. Although a relatively moderate tunability has been achieved, the tuning range is much narrower than that of the built-in microwave photonic filter, and the powers of the RF signals decline at the higher frequencies. This phenomenon is attributed to the imperfect performance of EA2. As is shown in Fig. 6(a), the bandwidth of EA2 is only 20 GHz and the its gain factor reduces to 10 dB at the frequency of 22 GHz. As a result, the decline of electrical gain affects the power of the signal higher than 20 GHz. At the same time, the total electrical gain can’t reach the threshold gain as a result of the huge drop after 22 GHz leading to the restriction in the tuning bandwidth of our OEO compared with the frequency response of the integrated laser shown in Fig. 4(b). The upper tuning range can reach up to 34.7 GHz if a wide-bandwidth amplifier is used.

 

Fig. 6 Microwave signals of different frequencies (a) electrical spectrum of the generated microwave signal at different frequencies (Span: 26 GHz, RBW: 10kHz, VBW: 100Hz), and the blue curve is the measured frequency response of EA2 (Mini-Circults ZX60-24-S + ). (b) measured output frequency as a function of I_DC2 when I_DC1 is fixed at 91 mA.

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Fig. 7 The corresponding optical spectrum of IMS-DFB laser when generating microwave signals at different frequencies.

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To further investigate the quality of the generated microwave signals, we measured the phase noise data by an ESA. Figure 8(a) shows the phase noise performance of OEO at different frequencies from 17.3 GHz to 21.7 GHz. When I_DC1 is 92.92 mA and I_DC2 is 79.28 mA, the phase noise curve of the 20.3 GHz RF signal is plotted in red line shown in Fig. 8(a). As can be seen, the free-running OEO has a phase noise of −92.9 dBc/Hz at 1 kHz and −115.3 dBc/Hz at 10 kHz. According to the measured data in Fig. 8(a), the values of phase noise at the frequency offset of 1kHz and 10 kHz are recorded and plotted in Fig. 8(b). Although there exists a little fluctuations at different frequencies, the phase noises at the frequency offset of 1 kHz are all lower than −85.7 dBc/Hz and that of 10 kHz are all below −108.7 dBc/Hz. Therefore, the phase noise performance of our OEO based on such a compact IMS-DFB laser is superior or at least comparable to the previous work that are based on discrete injection lasers [12–14].

 

Fig. 8 (a) measured phase noise of the OEO at different frequencies. (b) phase noises at 1 kHz (red) and 10kHz (black) of the stabilized OEO for different frequencies.

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4. Discussion and conclusion

What we need to pay attention to is that, the phase noise of OEO will decrease if the noise of the system is lower or the time delay of the loop is longer according to [25]. First, the flicker noise of the system degrades the phase noise at the frequency offset lower than 10 kHz [26] and the limited polarization extinction ratio of the PBS which makes part of the fields couple with the same polarization will add additional noise [27]. So, the phase noise will get further improved if the low-noise-figure electrical amplifier and high-polarization-extinction PBS are used. As to the longer time delay, we have measured its effect on phase noise when employing the 13 km and 5.4 km SMFs to substitute the 1.1 km and 2.9 km ones as the dual loop. As is shown in Fig. 9, the phase noise approximately equals to before at the offset of 10kHz while it is only −100.51 dBc/Hz at the offset of 1 kHz which gets improved by 8 dB. However, the frequency stability of the OEO needs further improvement. New configurations such as employing the optoelectronic feedback loop to the IMS-DFB laser will be investigated in the future work to enhance the performance of stability.

 

Fig. 9 Comparison of the SSB phase noise spectrum with different length of SMFs: black one is 2.9 km and 1.1 km, red one is 13 km and 5.4 km.

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To conclude, in this paper, we have proposed and experimentally demonstrated a novel approach to realize an OEO based on an IMS-DFB laser of which the front and rear sections are mutually injected. The OEO is realized based on the optoelectronic characteristics of the IMS-DFB laser itself. In the experiment, high performance microwave signals of different frequencies are obtained by adjusting the bias currents of two section lasers. The SSB phase noise of generated signal at the frequency of 20.3 GHz is −115.3 dBc/Hz@10kHz and −92.9 dBc/Hz@1kHz. The sidemode suppression ratio is 60.94 dB which gets improved over 40 dB compared with a single loop. What’s more, we discussed the effect on phase noise at the frequency of 1 kHz which improves 8 dB when employing 13 km and 5.4 km fibers as the dual loop. The compact integrated mutual-injection laser fabricated by REC technique not only acts as the light source but also as a built-in MPF and a modulator simultaneously, which simplifies the structure greatly. This work provides a promising direction of development to the integration of OEO.