A frequency tunable optoelectronic oscillator based on a directly modulated distributed-feedback (DFB) semiconductor laser under optical injection is proposed and experimentally demonstrated. Through optical injection, the relaxation oscillation frequency of the DFB laser is enhanced and its high modulation efficiency can enable the loop oscillation with a RF threshold gain of less than 20 dB. The DFB laser is a commercial semiconductor laser with a package of 10 GHz, and its packaging limitation can be overcome by optical injection. In our scheme, neither a high-speed external modulator nor an electrical bandpass filter is required, making the system simple and low-cost. Microwave signals with a frequency tuning range from 5.98 to 15.22 GHz are generated by adjusting the injection ratio and frequency detuning between the master and slave lasers. The phase noise of the generated 9.75 GHz microwave signal is measured to be −104.8 dBc/Hz @ 10 kHz frequency offset.
© 2015 Optical Society of America
Optoelectronic oscillators (OEOs) have received much attention since it was firstly proposed by Yao and Maleki in 1996 . Due to the distinguished phase noise performance and the ability to generate a high-frequency signal, OEOs are useful in wireless communications, radar, optical signal processing  and modern instrumentations . Conventionally, an expensive external modulator is required to form the feedback loop in an OEO structure, and a high-Q electrical band-pass filter (EBPF) is needed to ensure a single frequency oscillation. The use of an external modulator usually leads to high RF loss due to its low modulation efficiency, and RF amplifiers with high gain are necessary to compensate the link loss in the loop. A simple OEO based on a directly modulated wafer-fused VCSEL (vertical cavity surface emitting laser) was demonstrated, in which the VCSEL served as an external modulator . However, an expensive electrical tunable band pass YIG-filter was used and the total RF gain was near 50 dB, making the system costly. Sung et al. proposed a new structure using directly modulated semiconductor lasers under strong optical injection . In the system, an external modulator is replaced by a distributed-feedback (DFB) semiconductor laser to reduce the RF link loss of the feedback loop. They realized a low RF threshold gain of 7 dB to obtain an oscillation, but an EBPF with a fixed central frequency was still used, which limits the tunability of the OEO to tens of megahertz [6,7]. Microwave photonic filter (MPF) provides an effective solution to an OEO with a large tuning range [8–11]. In , an optical-injected Fabry-Perot laser diode (FP-LD) was implemented to act as a MPF. When the wavelength of the injected light wave changes or the longitudinal modes of the FP-LD are adjusted, the output signal frequency can be tuned from 6.41 to 10.85 GHz. However, the stability of the OEO is poor due to the wavelength drift of the LD and an external modulator is still used. In , the tunable MPF was composed of a polarization modulator and a chirped fiber Bragg grating (CFBG). By adjusting the polarization state of the signal, microwave signal with a tunable frequency within 5.8 to 11.8 GHz was generated. In , two phase modulators (PMs) and a phase-shifted FBG (PSFBG) served as a MPF, realizing a frequency tuning range from 3 to 28 GHz by adjusting the wavelength of the light wave. In , a MPF consisting of a PM and a tunable optical filter was used to realize an OEO with a wideband frequency tuning range from 4.74 to 38.38 GHz by tuning the bandwidth of the optical filter. However, all the structures mentioned in [9–11] employ a high-speed external modulator and a high-performance optical filter, making the system bulky and costly. Recently, a compact optoelectronic oscillator based on phase modulation and ultra-high Q disk resonator is demonstrated . The ultra-high Q whispering gallery-mode (WGM) optical resonator serves as an energy storage element and realize phase-to-intensity conversion as well. However, a phase modulator was still used and the tunability of the frequency is limited.
In this paper, a novel scheme to realize a frequency tunable OEO using a directly modulated DFB semiconductor laser under optical injection is proposed and experimentally demonstrated. In the proposed scheme, neither a high-speed external modulator nor an electrical bandpass filter is needed, which makes the system simple and low-cost. Besides that, due to the high modulation efficiency of the DFB laser at the relaxation oscillation frequency, a less than 20 dB gain of the electrical loop is enough for the system to oscillate. In our system, the directly modulated slave DFB laser is a commercial semiconductor laser with a peak modulation frequency of 10 GHz. However, a high relaxation oscillation peak can still exist at the frequency over 15 GHz with the help of optical injection . By adjusting the injection ratio and frequency detuning between the master and slave lasers, microwave signals with a frequency tuning range from 5.98 to 15.22 GHz are generated. The phase noise of the generated 9.75 GHz microwave signal is −104.8 dBc/Hz @ 10 kHz frequency offset. The phase noise of the other RF frequencies are all below −101 dBc/Hz at 10 kHz offset.
2. Experimental setup and principles
The schematic diagram of the proposed OEO is shown in Fig. 1. A tunable laser source (Agilent 81989A) is used as the master laser. An optical circulator is employed to realize optical injection, and it can also prevent coupling from the slave to the master laser. A DFB laser, biased at 20.38 mA (~2Ith) with an output power of −0.2 dBm, is used as the slave laser. Polarization controller 1 is employed to match the polarization states of the master and slave lasers. In order to suppress the unwanted spur modes, two optical fiber delay lines are implemented based on the Vernier effect . The dual loop structure results in a larger mode spacing and a lower threshold gain for the oscillation. Moreover, the phase noise of the oscillator, which is inversely proportional to the square of the loop length, is determined by the longer loop . Since there is no thermal isolation, the optical delay has a variation of about 33.3 ps/°C (for 1 km fiber) and 92.3 ps/°C (for 2.77 km fiber) respectively. After the output combination at a polarization beam splitter (PBS), 1% of the light is extracted for signal monitoring using an optical spectrum analyzer (Yokogawa AQ6370) with a resolution of 0.02 nm, while the other 99% of the light is detected by a photo-detector (ET3500F) with a bandwidth of 15 GHz. The generated RF signal is amplified by a low noise amplifier (LA2018N4020) with a RF gain of 25 dB and a 3-dB bandwidth of 0.8-18 GHz. Since the power of the feedback RF signal determines whether the DFB laser working under small signal modulation, an electrical attenuator is unnecessary before the laser. The amplified RF signal is split by an electrical coupler, and the 10% part is measured by an electrical spectrum analyzer (Agilent N9030A), while the 90% part is fed back to the DFB laser to close the loop. Obviously, neither a high-speed external modulator nor an electrical bandpass filter is necessary in the configuration, and an electrical amplifier with a RF gain of 25 dB is enough for the loop to oscillate.
It is well known that there is a peak in the frequency response of a directly modulated DFB laser named relaxation oscillation peak, as shown in Fig. 2. The relaxation oscillation frequency (ωR0), which centers at relaxation oscillation peak, is given as 17].
The slave laser used in our experiment is a commercial DFB semiconductor laser with a package bandwidth of 10 GHz. We measured the frequency response of the free-running DFB laser using a vector network analyzer (Anritsu MS4647A). The results are shown in Fig. 2. The temperature is fixed at 23 °C and the bias current is adjusted from 15 mA to 90 mA with a step of 15 mA. The notches in the response curve are resulted from the package of the DFB laser because their positions don’t change under different bias current. The modulation bandwidth is increased but the relaxation oscillation peak gets suppressed as the bias current increases. In this case, the tuning range of the OEO proposed in  is limited.
In order to extend the tuning range of the relaxation oscillation frequency, we apply optical injection. During optical injection, the DFB laser exhibits an injection locking frequency finj, which is the emission frequency of the DFB laser when it is injection-locked, and a red-shifted cavity mode fcav, which is the new cavity resonance frequency of the DFB laser under injection locking , as shown in Fig. 3(b). When the injection-locked laser is modulated by a RF signal at the frequency around fm, which is the frequency difference of the finj and fcav, fm = finj-fcav, the lower sideband is resonantly amplified by the cavity mode, while the upper sideband remains unchanged, which is depicted in Fig. 3(c). Therefore, thefrequencies around the lower sideband get gained from the red-shifted cavity mode and have a higher response, corresponding to an enhanced relaxation oscillation peak in the RF response curve shown as Fig. 3(d).
Under a strong optical injection, the relaxation oscillation frequency is induced by the coupling between the photon amplitude and phase, corresponding to the frequency difference between the main locking mode and the red-shifted cavity mode of the slave laser . Erwin K. L. et al. developed a small-signal, linearized theoretical model to analyze the optical injection laser system , from which the relaxation oscillation frequency of the injected DFB laser can be approximated as20,21], κ is the coupling coefficient that determines the efficiency of the injection process, R is the external injection ratio which is defined as the power ratio between the injected power and the lasing power of the free-running slave laser measured outside the cavity, r is the power reflectivity at the injectionfacet, and φ0 is the phase difference between the fields of the master and slave lasers. According to Eq. (3), the frequency enhancement term ΔωR is related to the injection ratio R and phase difference φ0 for a given slave laser, which can be changed by adjusting the master laser. The region of stable locking is shown in Fig. 4 as a function of the injection ratio and frequency detuning . At various injection parameters, the DFB laser exhibits unlocked, unstable locking and stable locking state. Our research focuses on the stable locking region, which is between the red line and blue line in Fig. 4. Figure 5 shows the calculated frequency response under various injection ratios and frequency detuning values. As the injection power increases, the injection ratio R is enhanced. However, the carriers of the slave laser is further depleted with the gradual increase of the injected power, as a result the peak of frequency response due to the relaxation oscillation is reduced, as depicted in Fig. 5(a). The phase difference between the master and slave fields φ0 is approximately changing from tan−1α of the negative frequency detuning edge (blue line in Fig. 4) to -π/2 of the positive frequency detuning edge (red line in Fig. 4), where α is the linewidth enhancement factor , which leads to an increased |sinφ| and thus a greater ΔωR2. As a result, according to the Eq. (3), the relaxation oscillation frequency, ωR, is increased, which is shown as Fig. 5(b). Here, the frequency detuning is defined as the frequency difference between the master and the free-running slave lasers (Δf = fmaster-fslave). In this way, the relaxation oscillation frequency (ωR) can be tuned over a wide range by simply adjusting the wavelength and output power of the master laser.
3. Measured results
By using the optical injection, the relaxation oscillation peak can be enhanced, which makes the RF threshold gain further reduced and the tuning range enlarged. As is shown in Fig. 6(a), the injection ratio varies from −4.39 to 0.86 dB at a fixed frequency detuning of 0 GHz. Withincreasing of the injection ratio, the relaxation oscillation frequency can be enlarged dramatically. In Fig. 6(b), the frequency detuning is increased from −16.38 to 2.50 GHz while the injection ratio is fixed at −0.01 dB. Towards positive frequency detuning, the slave laser shows a high narrow-band response peak, resulting from the resonant amplification of the modulation sideband by the red-shifted cavity mode. In our experiment, the frequency response at 14.77 GHz is improved by a 34.7 dB when the frequency detuning changes from −16.38 to 2.5 GHz. The measured results agree with the calculated frequency response in Fig. 5.
By adjusting the injection parameters, we can obtain an optimized response for every frequency, as shown in Fig. 7. The free-running DFB laser (biased at 2Ith) has a 3-dB bandwidth of 7.15 GHz and an unapparent relaxation oscillation peak. After the optical injection, the relaxation oscillation peak is enhanced and its frequency can be tuned over a much wider range. It should be noted that the bandwidth of the PD is 15 GHz, which limits the response at higher frequency. We believe the tuning range can be further improved by using a PD with larger bandwidth .
An experiment based on the setup shown in Fig. 1 is carried out. By closing the loop, the OEO starts to oscillate at the frequency around the relaxation oscillation peak. Figure 8(a) shows the measured optical spectra with and without an optoelectronic feedback when the frequency detuning and injection ratio are set as −3.86 GHz and 8.12 dB, respectively. In Fig. 8(a), the violet line displays the injection locking mode and the cavity mode under the injection locking when the loop is open. When the optoelectronic feedback is turned on, the modulation sidebands are enhanced due to the loop oscillation. Figure 8(b) shows the RF spectrum when the loop is closed. It can be seen that a clean microwave signal at 11.72 GHz is generated. Changing the frequency detuning and injection ratio by adjusting the TLS, the generated microwave signals can be tuned over a wide range. Figure 9 shows the generated microwave signals over a frequency tuning range from 5.98 to 15.22 GHz. Due to the uneven frequency response of the slave DFB laser under the optical injection, the output power does not remain constant. The tuning range is mainly limited by the bandwidth of the PD. Besides that, the frequency response of the low noise amplifier (LNA) is also not flat, of which the RF gain at 15 GHz is 5.6 dB less than that of 9 GHz, as shown in Fig. 10(a). Furthermore, the frequency response of the power splitter has a power fluctuation of 1.3 dB, as shown in Fig. 10(b). Therefore, the power spectra of the generated microwave signals shows ripples over the tuning range and a quick decrease at the frequency around 15 GHz. We believe this problem can be solved by using a PD with a broader bandwidth and an electrical amplifier with a flatter gain response.
The performance of the phase noise of the generated microwave signals is also investigated. Figure 11 shows the phase noise curve of the generated 9.75 GHz electrical signal, which is measured by an Agilent N9030A. It is measured to be −104.8 dBc/Hz at 10 kHz frequency offset. We investigate the Vernier effect by measuring the phase noise with the structure of a single loop (2.77 km) and with the structure of dual loops (1 km and 2.77 km) respectively. As shown in Fig. 11, the phase noise of the single loop structure shows many densely spaced (~72 kHz) spur modes, which correspond to the free spectral range of the loop. By implementing a dual loop structure, the spur modes are greatly suppressed and the FSR is improved to ~215 kHz. The peak of the sidemodes is reduced from −49.25 dBc/Hz to −82.7 dBc/Hz, indicating the good spectrum purity of the OEO. As we can see from Fig. 12, the phase noise of the other RF frequencies are all below −101 dBc/Hz at 10 kHz offset, which indicates the advantage of generating low phase noise at high frequency. The phase noise is degraded by the strong optical beating noise between the main locking mode and cavity mode . Using a DFB laser with a narrower linewidth, the phase noise can be improved (the linewidth of our DFB laser is 1.7 MHz). The stability of the OEO is also investigated in a laboratory environment without mechanical or thermal isolation. During half an hour observation, no mode hopping is found and the frequency drift is less than 20 kHz.
A simple and cost-effective OEO structure to generate wideband and frequency tunable microwave signals based on a directly modulated DFB semiconductor laser under optical injection has been proposed. Although the experiment for demonstrating the concept is kind of bulky, the size of the system can be further reduced by using a monolithic two-section injection locking DFB laser  and replacing the optical fibers with optical resonators . We believe the system has the potential to be integrated as a compact OEO. By changing the injection ratio and frequency detuning, microwave signals with a tuning range from 5.98 to 15.22 GHz were generated. Using a photo-detector with higher bandwidth and an electrical amplifier with flatter gain response, the maximum tunable frequency can be further increased. The highlight of our scheme is that neither a high-speed external modulator nor an electrical filter is needed and a less than 20 dB gain of electrical is enough to enable the oscillation. Therefore, the proposed OEO is simple and cost-effective, which can be implemented in modern wireless communications and radar systems.
This work was partly supported by the National Natural Science Foundation of China (NSFC) under grants 61475193, 61032005 and 61177065; and the Jiangsu Province Natural Science Foundation under grant BK2012058, BK20140414 and BK20140069. The authors would like to thank Agilent Inc. for lending the phase noise measurement device.
References and links
1. X. S. Yao and L. Maleki, “Optoelectronic microwave oscillator,” J. Opt. Soc. Am. B 13(8), 1725–1735 (1996). [CrossRef]
2. H. Tsuchida and M. Suzuki, “40-Gb/s optical clock recovery using an injection-locked optoelectronic oscillator,” IEEE Photon. Technol. Lett. 17(1), 211–213 (2005). [CrossRef]
3. X. S. Yao and L. Maleki, “Opto-electronic oscillator and its applications,” Proc. SPIE 3038, 97–107 (1997).
4. M. E. Belkin, L. Belkin, A. Loparev, A. S. Sigov, G. Suruceanu, A. Sirbu, A. Mereuta, A. Caliman, D. Ellafi, V. Iakovlev, and E. Kapon, “VCSEL-based processing of microwave signals,” in Proceedings of International Topical Meeting on Microwave Photonics and the Asia-Pacific Microwave Photonics Conference (IEEE, 2014), pp. 284–287.
5. H. K. Sung, X. X. Zhao, E. K. Lau, D. Parekh, C. J. Chang-Hasnain, and M. C. Wu, “Optoelectronic oscillators using direct-modulated semiconductor lasers under strong optical injection,” IEEE J. Sel. Top. Quantum Electron. 15(3), 572–577 (2009). [CrossRef]
6. S. Poinsot, H. Porte, J.-P. Goedgebuer, W. T. Rhodes, and B. Boussert, “Continuous radio-frequency tuning of an optoelectronic oscillator with dispersive feedback,” Opt. Lett. 27(15), 1300–1302 (2002). [CrossRef] [PubMed]
7. E. Shumakher, S. Ó. Dúill, and G. Eisenstein, “Optoelectronic oscillator tunable by an SOA based slow light element,” J. Lightwave Technol. 27(18), 4063–4068 (2009). [CrossRef]
8. S. Pan and J. Yao, “Wideband and frequency-tunable microwave generation using an optoelectronic oscillator incorporating a Fabry-Perot laser diode with external optical injection,” Opt. Lett. 35(11), 1911–1913 (2010). [CrossRef] [PubMed]
9. Z. Tang, S. Pan, D. Zhu, R. Guo, Y. Zhao, M. Pan, D. Ben, and J. Yao, “Tunable optoelectronic oscillator based on a polarization modulator and a chirped FBG,” IEEE Photon. Technol. Lett. 24(17), 1487–1489 (2012). [CrossRef]
10. W. Li and J. Yao, “A wideband frequency tunable optoelectronic oscillator incorporating a tunable microwave photonic filter based on phase-modulation to intensity-modulation conversion using a phase-shifted fiber Bragg grating,” IEEE Trans. Microw. Theory Tech. 60(6), 1735–1742 (2012). [CrossRef]
11. X. Xie, C. Zhang, T. Sun, P. Guo, X. Zhu, L. Zhu, W. Hu, and Z. Chen, “Wideband tunable optoelectronic oscillator based on a phase modulator and a tunable optical filter,” Opt. Lett. 38(5), 655–657 (2013). [CrossRef] [PubMed]
12. K. Volyanskiy, P. Salzenstein, H. Tavernier, M. Pogurmirskiy, Y. K. Chembo, and L. Larger, “Compact optoelectronic microwave oscillators using ultra-high Q whispering gallery mode disk-resonators and phase modulation,” Opt. Express 18(21), 22358–22363 (2010). [CrossRef] [PubMed]
13. P. Wang, J. Xiong, T. Zhang, J. Zheng, T. Pu, and X. Chen, “Widely Tunable OEO Based on a Directly Modulated DFB Laser under Optical Injection,” in Optical Fiber Communication Conference, OSA Technical Digest Series (Optical Society of America, 2015), paper M3E.1. [CrossRef]
14. J. Yang, J. Yu, Y. Wang, L. Zhang, and E. Yang, “An optical domain combined dual-loop optoelectronic oscillator,” IEEE Photon. Technol. Lett. 19(11), 807–809 (2007). [CrossRef]
15. X. S. Yao and L. Maleki, “Multiloop optoelectronic oscillator,” IEEE J. Quantum Electron. 36(1), 79–84 (2000). [CrossRef]
16. A. Murakami, K. Kawashima, and K. Atsuki, “Cavity resonance shift and bandwidth enhancement in semiconductor lasers with strong light injection,” IEEE J. Quantum Electron. 39(10), 1196–1204 (2003). [CrossRef]
17. J. Xiong, R. Wang, T. Fang, T. Pu, D. Chen, L. Lu, P. Xiang, J. Zheng, and J. Zhao, “Low-cost and wideband frequency tunable optoelectronic oscillator based on a directly modulated distributed feedback semiconductor laser,” Opt. Lett. 38(20), 4128–4130 (2013). [PubMed]
18. H. K. Sung, E. K. Lau, and M. C. Wu, “Optical single sideband modulation using strong optical injection-locked semiconductor lasers,” IEEE Photon. Technol. Lett. 19(13), 1005–1007 (2007). [CrossRef]
19. E. K. Lau, H. K. Sung, and M. C. Wu, “Frequency response enhancement of optical injection-locked lasers,” IEEE J. Quantum Electron. 44(1), 90–99 (2008). [CrossRef]
20. C. Henry, N. Olsson, and N. Dutta, “Locking range and stability of injection locked 1.54 µm InGaAsP semiconductor lasers,” IEEE J. Quantum Electron. 21(8), 1152–1156 (1985). [CrossRef]
22. F. Mogensen, H. Olesen, and G. Jacobsen, “Locking conditions and stability properties for a semiconductor laser with external light injection,” IEEE J. Quantum Electron. 21(7), 784–793 (1985). [CrossRef]
23. H. K. Sung, T. Jung, M. C. Wu, D. Tishinin, T. Tanbun-Ek, K. Y. Liou, and W. T. Tsang, “Modulation bandwidth enhancement and nonlinear distortion suppression in directly modulated monolithic injection-locked DFB lasers,” in Proceedings of International Topical Meeting on Microwave Photonics (IEEE, 2003), pp. 27–30. [CrossRef]
24. K. Saleh, R. Henriet, S. Diallo, G. Lin, R. Martinenghi, I. V. Balakireva, P. Salzenstein, A. Coillet, and Y. K. Chembo, “Phase noise performance comparison between optoelectronic oscillators based on optical delay lines and whispering gallery mode resonators,” Opt. Express 22(26), 32158–32173 (2014). [CrossRef] [PubMed]