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Low-cost and wideband frequency tunable optoelectronic oscillator based on a directly modulated distributed feedback semiconductor laser Jintian Xiong, Rong Wang, Tao Fang, Tao Pu,* Dalei Chen, Lin Lu, Peng Xiang, Jilin Zheng, and Jiyong Zhao PLA University of Science and Technology, Nanjing 210007, China *Corresponding author: [email protected] Received August 5, 2013; revised September 9, 2013; accepted September 13, 2013; posted September 13, 2013 (Doc. ID 195075); published October 9, 2013 A novel scheme to realize a low-cost and wideband frequency tunable optoelectronic oscillator based on a directly modulated distributed feedback (DFB) semiconductor laser is proposed and experimentally demonstrated. In the proposed scheme, neither an external modulator nor an electrical filter is used, and no more than 25 dB of the electrical loop gain is required due to the high modulation efficiency of the relaxation oscillation frequency of the DFB laser. Microwave signals with frequency coarsely tuned from 3.77 to 8.75 GHz are generated by changing the bias current and operation temperature of the DFB laser. The single sideband phase noise of the generated 6.97 GHz microwave signal is measured to be −103.6 dBc∕Hz at 10 kHz offset. © 2013 Optical Society of America OCIS codes: (230.4910) Oscillators; (230.0250) Optoelectronics; (140.3490) Lasers, distributed-feedback. http://dx.doi.org/10.1364/OL.38.004128
Optoelectronic oscillators (OEOs) have been of great interest since they were first proposed in the late 20th century [1]. Their ability to generate high-frequency microwave signals with ultralow phase noise makes them useful for various applications within optical and wireless communications, optical signal processing [2], modern instrumentation, microwave imaging, and microwave spectroscopy [3]. In a conventional OEO structure, an expensive high-speed external modulator is required to form the loop, and a high-Q electrical bandpass filter (EBPF) is also needed to select the desired oscillation frequency. The use of an external modulator usually results in a high RF loss due to its low modulation efficiency. Recently, Sung et al. proposed a novel structure using directly modulated semiconductor lasers under strong optical injection, in which a continuous-wave laser and an external modulator are replaced by a master and a slave laser to reduce the RF link loss of the feedback loop [4]. They achieved 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 only several or tens of megahertz [5,6]. Thanks to the development of microwave photonic bandpass filters (MPBPF) [7], wideband tunable ranges can be realized [8–10]. In [8], a MPBPF consisting of a polarization modulator and a chirped fiber Bragg grating (FBG) was proposed, and frequency tunable signal from 5.8 to 11.8 GHz was generated by adjusting the polarization state of the light wave. In [9], a MPBPF consisting of two phase modulators (PMs) and a phase-shifted FBG was incorporated into an OEO, and frequency tuning from 3 to 28 GHz was realized by tuning the wavelength of the light wave. In [10], a MPBPF consisting of a broadband PM and a tunable optical bandpass filter was used to realize a wideband tunable frequency from 4.74 to 38.38 GHz by directly tuning the bandwidth of the optical bandpass filter. However, the MPBPFs in all the examples above were composed of a high-speed modulator and a high-performance optical filter, which make the system bulky and costly. In addition, RF amplifiers with 0146-9592/13/204128-03$15.00/0
high gain (up to ∼60 dB) are necessary to compensate for the high link loss of the feedback loop. In this Letter, a novel scheme to realize a low-cost and wideband frequency tunable OEO based on a directly modulated distributed feedback (DFB) semiconductor laser is proposed and experimentally demonstrated. In the proposed scheme, neither an external modulator nor an electrical filter is used, which significantly reduces the system cost. In addition, no more than 25 dB of the electrical loop gain is required to enable the oscillation due to the high modulation efficiency of the relaxation oscillation frequency when the DFB laser is directly modulated. Moreover, the proposed OEO is tunable by directly changing the bias current and the operation temperature of the DFB laser. Microwave signals with the frequency coarsely tuned from 3.77 to 8.75 GHz are generated. The phase noise performance of the generated microwave signal is also investigated. The schematic of the proposed OEO is shown in Fig. 1. A DFB laser is used as the optical source and electricaloptical modulator simultaneously. An optical isolator (ISO) is also used to minimize any distortions to the laser signal via counterpropagating reflections within the OEO system. In order to effectively suppress the unwanted
Fig. 1. Schematic of the proposed OEO. ISO, isolator; PC, polarization controller; PBS, polarization beam splitter; PBC, polarization beam combiner; OSA, optical spectrum analyzer; PD, photodetector; LNA, low noise amplifier; Att, attenuator; ESA, electrical spectrum analyzer. © 2013 Optical Society of America
October 15, 2013 / Vol. 38, No. 20 / OPTICS LETTERS
spur modes, a double loop constructed by a polarization beam splitter (PBS) and a polarization beam combiner (PBC) is used. After the optical combination in PBC, the light is splitted into two parts by a 10 dB optical coupler. One output with 10% coupled power is connected to the optical spectrum analyzer (Anritsu MS9710C) which operates at a resolution of 0.05 nm to inspect the optical signal, while the other output with 90% coupled power is converted into an electrical signal via a photodetector, which then relays the signal back toward the laser. A low-noise amplifier (LNA) is used to provide a sufficient electrical gain in the OEO loop, and an electrical attenuator (Att) is followed to control the amount of electrical gain such that the DFB laser is under small signal modulation. The generated microwave signal is received after a 10 dB electrical coupler. The 90% part is measured by an electrical spectrum analyzer (Agilent N9030A) with 26.5 GHz bandwidth, and the 10% part is fed back to the DFB laser to close the loop. It can be seen that neither an external modulator nor an electrical filter is used in this configuration. As we know, the frequency located at the relaxation oscillation frequency (f r ) will achieve the most effective modulation efficiency when a semiconductor laser is directly modulated by an RF signal. It can be clearly seen from the frequency response of a semiconductor laser, where the f r is determined. Accordingly, when the loop is closed, the frequency located at f r will acquire the largest gain in the loop oscillation. Hence, the oscillation will occur when the loop gain is sufficient, and no external modulator or electrical filter is required within the loop to enable the oscillation. The relaxation oscillation frequency can be determined using the following equation [11]: fr
q gγ p P 0 ∕2π;
(1)
where g is the differential optical gain constant, γ p is the photon decay rate given by the reciprocal of the photon lifetime, and P 0 is the steady-state photon density in the laser cavity. Equation (1) clearly indicates that f r can be increased by suitably designing the laser parameters. For a laser given parameters, one can increase f r by increasing the pump current (I b ), which leads to an increased P 0 ; or by decreasing the laser operation temperature (T), which leads to an increased γ p . Thus, we can simply tune the f r of the DFB laser by changing either the I b or T applied, and this forms the mechanism for providing a wideband tunable oscillation frequency within the loop, accordingly. To verify the tunability of f r , we measured the frequency responses of the DFB laser at different T and I b by a vector network analyzer (Agilent N5230A). The results are shown in Fig. 2. As can be seen from Fig. 2(a), the f r and modulation bandwidth are increased as T decreases with I b fixed at 20 mA. Figure 2(b) illustrates the increase in f r and modulation bandwidth of the DFB laser with I b , while the T is fixed at 25°C. These results both agree well with Eq. (1). However, it should be noted that the peaks of the response curves vary with I b and T, which indicates that different loop gain is required for different f r to perform oscillation. Additionally, it is
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Fig. 2. Measured frequency response of the DFB laser at different operation temperature (T) and bias current (I b ). (a) I b is fixed at 20 mA. (b) T is fixed at 25°C.
obvious that changing I b at fixed T is more effective to realize the tuning of f r in a wide band. An experiment based on the configuration in Fig. 1 was carried out. The parameters of the key devices used in the experiment are as follows: the DFB laser has a threshold current of 13.65 mA, and the lengths of the SMF in the dual loops are ∼1 and ∼2.77 km, respectively. The PD is a positive intrinsic-negative diode with a transfer impedance amplifier (PIN-TIA, PTHS992-003) embodied, which has a 3 dB bandwidth of 10 GHz and a responsivity of 0.9 A∕W. It should be noted that a 16 dB gain is provided by the TIA. The LNA (A-INFOMW LA2018N4020) has a gain of about 35 dB and a bandwidth of 0.8–18 GHz. The Att has an attenuation of 16 dB. By closing the loop, the OEO starts to oscillate at the frequency around the peak response of the DFB laser. The Vernier effect formed by the dual loops would perform the fine mode selection of the OEO. By directly changing the bias current and operation temperature of the DFB laser, the frequency of the oscillating signal can be tuned in a wide range. Figure 3 shows the spectra of the generated electrical signals with the frequency coarsely tuned from 3.77 to 8.75 GHz. It can be seen that all the fundamental components are with high power output (6.5–10.5 dBm), and all the harmonic components are at least 27 dB lower than the fundamental components. However, a few things should be noted. First, the largest tunable range is limited by the modulation characteristic of the DFB laser. It has been illustrated in Fig. 2(b) that, as the bias current increases, the curve of modulation response becomes flatter. Therefore, a DFB laser with sharper modulation response can be used to extend the tunable range. It can be realized by suitably designing the laser parameters or by optical injection to enhance the resonance peak [12]. Second, the maximum tuning range is also limited by the packaging of the DFB laser diode, which is not optimized for highfrequency modulation, and thus provides a limited modulation bandwidth and low modulation efficiency of f r .
Fig. 3. Spectra of the generated electrical signals with frequency coarsely tuned from 3.77 to 8.75 GHz.
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Fig. 4. Oscillation conditions of the frequencies plotted in Fig. 3, which show the relationship between the f osc and I b and T of the laser.
Third, the oscillation frequency can’t be finely tuned due to the fact that the electrical loop gain is no more than 25 dB in the experiment while it can be inferred from Fig. 2 that a different loop gain is required for each value of f r in order to generate an adequate oscillation signal. A variable RF attenuator and higher electrical loop gain will be helpful to achieve more oscillation frequencies. For the purpose of confirming the relationship between the oscillation frequency (f osc ) and I b and T of the laser, the oscillation conditions of the frequencies plotted in Fig. 3 are recorded in the experiment. The results are shown in Fig. 4. It can be seen that the tunability of f osc is effectively realized by changing I b when T is adjusted at around 25.5°C. Moreover, f osc is approximately in proportion to the square root of I b , which agrees well with Eq. (1). The phase noise performance of the proposed OEO was also investigated, and was measured using a signal source analyzer (Agilent N9030A). As shown in Fig. 5, the single sideband (SSB) phase noise of the generated 6.97 GHz microwave signal by the proposed scheme is −103.6 dBc∕Hz at 10 kHz offset. It is the best one among the generated signals we achieved, and others are all below −90.86 dBc∕Hz at 10 kHz offset. Although there are still some peaks at around 200 kHz and its integermultiple frequencies, which correspond to the side modes of the OEO due to the fiber loops used in our experiment, the side modes have a maximal phase noise of −81 dBc∕Hz, indicating that the spectrum purity of the OEO is good. It is confirmed by the corresponding electrical spectrum of the 6.97 GHz signal in 1 MHz span, which is shown as the inset in Fig. 5. The side mode suppression ratio is only 50 dB, which is 10 dB smaller than the result reported in [13]. It is caused by the finite polarization extinction ratio of the PBS used in the experiment, so it can be further improved by using a better PBS. In conclusion, a low-cost OEO with large frequency tunability from 3.77 to 8.75 GHz is realized based on a directly modulated DFB semiconductor laser. The key significance of the proposed scheme is that neither an external modulator nor an electrical filter is used and no more than 25 dB of the electrical loop gain is required to enable the oscillation. In addition, its tunability is achieved by directly changing the bias current and the operation temperature of the DFB laser. The SSB phase
Fig. 5. Phase noise of the generated 6.97 GHz microwave signal. Inset, electrical spectrum of the generated 6.97 GHz microwave signal in 1 MHz span. Resolution bandwidth = 9.1 kHz..
noise of the generated 6.97 GHz signal is measured to be −103.6 dBc∕Hz at 10 kHz offset. Although the frequency cannot be finely tuned, a new oscillation mechanism is provided, and the tunable range of the proposed OEO could be further extended by providing larger loop gain and replacing the DFB semiconductor laser we used with one with improved high-frequency characteristics and optimized packaging. Further work will focus on realizing finely tuning and larger tunable frequency range based on this new proposed scheme. This work was supported in part by the National Basic Research Program of China (973 Program) under grant 2012CB315603; the National Natural Science Foundation of China (NSFC) under grants 61177065, 61032005, and 61174199; and the Jiangsu Province Natural Science Foundation under grant BK2012058. The authors would like to thank Agilent Inc. for lending the phase noise measurement device. References 1. X. S. Yao and L. Maleki, J. Opt. Soc. Am. B 13, 1725 (1996). 2. H. Tsuchida and M. Suzuki, IEEE Photon. Technol. Lett. 17, 211 (2005). 3. X. S. Yao and L. Maleki, in Microwave Photonics 1996 (MWP ’96), Technical Digest (IEEE, 1996), pp. 265–268. 4. H. K. Sung, X. Zhao, E. K. Lau, D. Parekh, C. J. ChangHasnain, and M. C. Wu, IEEE J. Sel. Top. Quantum Electron. 15, 572 (2009). 5. S. Poinsot, H. Porte, J. Goedgebuer, W. T. Rhodes, and B. Boussert, Opt. Lett. 27, 1300 (2002). 6. E. Shumakher, S. Ó. Dúill, and G. Eisenstein, J. Lightwave Technol. 27, 4063 (2009). 7. J. Capmany, B. Ortega, and D. Pastor, J. Lightwave Technol. 24, 201 (2006). 8. Z. Tang, S. Pan, D. Zhu, R. Guo, Y. Zhao, M. Pan, D. Ben, and J. P. Yao, IEEE Photon. Technol. Lett. 24, 1487 (2012). 9. W. Li and J. P. Yao, IEEE Trans. Microwave Theor. Tech. 60, 1735 (2012). 10. X. Xie, C. Zhang, T. Sun, P. Guo, X. Zhu, L. Zhu, W. Hu, and Z. Chen, Opt. Lett. 38, 655 (2013). 11. K. Y. Lau and A. Yariv, IEEE J. Quantum Electron. QE-21, 121 (1985). 12. H. K. Sung, E. K. Lau, and M. C. Wu, IEEE J. Sel. Top. Quantum Electron. 13, 1215 (2007). 13. Y. Jiang, J. Yu, Y. Wang, L. Zhang, and E. Yang, IEEE Photon. Technol. Lett. 19, 807 (2007).
OPTICS LETTERS / Vol. 38, No. 20 / October 15, 2013
Low-cost and wideband frequency tunable optoelectronic oscillator based on a directly modulated distributed feedback semiconductor laser Jintian Xiong, Rong Wang, Tao Fang, Tao Pu,* Dalei Chen, Lin Lu, Peng Xiang, Jilin Zheng, and Jiyong Zhao PLA University of Science and Technology, Nanjing 210007, China *Corresponding author: [email protected] Received August 5, 2013; revised September 9, 2013; accepted September 13, 2013; posted September 13, 2013 (Doc. ID 195075); published October 9, 2013 A novel scheme to realize a low-cost and wideband frequency tunable optoelectronic oscillator based on a directly modulated distributed feedback (DFB) semiconductor laser is proposed and experimentally demonstrated. In the proposed scheme, neither an external modulator nor an electrical filter is used, and no more than 25 dB of the electrical loop gain is required due to the high modulation efficiency of the relaxation oscillation frequency of the DFB laser. Microwave signals with frequency coarsely tuned from 3.77 to 8.75 GHz are generated by changing the bias current and operation temperature of the DFB laser. The single sideband phase noise of the generated 6.97 GHz microwave signal is measured to be −103.6 dBc∕Hz at 10 kHz offset. © 2013 Optical Society of America OCIS codes: (230.4910) Oscillators; (230.0250) Optoelectronics; (140.3490) Lasers, distributed-feedback. http://dx.doi.org/10.1364/OL.38.004128
Optoelectronic oscillators (OEOs) have been of great interest since they were first proposed in the late 20th century [1]. Their ability to generate high-frequency microwave signals with ultralow phase noise makes them useful for various applications within optical and wireless communications, optical signal processing [2], modern instrumentation, microwave imaging, and microwave spectroscopy [3]. In a conventional OEO structure, an expensive high-speed external modulator is required to form the loop, and a high-Q electrical bandpass filter (EBPF) is also needed to select the desired oscillation frequency. The use of an external modulator usually results in a high RF loss due to its low modulation efficiency. Recently, Sung et al. proposed a novel structure using directly modulated semiconductor lasers under strong optical injection, in which a continuous-wave laser and an external modulator are replaced by a master and a slave laser to reduce the RF link loss of the feedback loop [4]. They achieved 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 only several or tens of megahertz [5,6]. Thanks to the development of microwave photonic bandpass filters (MPBPF) [7], wideband tunable ranges can be realized [8–10]. In [8], a MPBPF consisting of a polarization modulator and a chirped fiber Bragg grating (FBG) was proposed, and frequency tunable signal from 5.8 to 11.8 GHz was generated by adjusting the polarization state of the light wave. In [9], a MPBPF consisting of two phase modulators (PMs) and a phase-shifted FBG was incorporated into an OEO, and frequency tuning from 3 to 28 GHz was realized by tuning the wavelength of the light wave. In [10], a MPBPF consisting of a broadband PM and a tunable optical bandpass filter was used to realize a wideband tunable frequency from 4.74 to 38.38 GHz by directly tuning the bandwidth of the optical bandpass filter. However, the MPBPFs in all the examples above were composed of a high-speed modulator and a high-performance optical filter, which make the system bulky and costly. In addition, RF amplifiers with 0146-9592/13/204128-03$15.00/0
high gain (up to ∼60 dB) are necessary to compensate for the high link loss of the feedback loop. In this Letter, a novel scheme to realize a low-cost and wideband frequency tunable OEO based on a directly modulated distributed feedback (DFB) semiconductor laser is proposed and experimentally demonstrated. In the proposed scheme, neither an external modulator nor an electrical filter is used, which significantly reduces the system cost. In addition, no more than 25 dB of the electrical loop gain is required to enable the oscillation due to the high modulation efficiency of the relaxation oscillation frequency when the DFB laser is directly modulated. Moreover, the proposed OEO is tunable by directly changing the bias current and the operation temperature of the DFB laser. Microwave signals with the frequency coarsely tuned from 3.77 to 8.75 GHz are generated. The phase noise performance of the generated microwave signal is also investigated. The schematic of the proposed OEO is shown in Fig. 1. A DFB laser is used as the optical source and electricaloptical modulator simultaneously. An optical isolator (ISO) is also used to minimize any distortions to the laser signal via counterpropagating reflections within the OEO system. In order to effectively suppress the unwanted
Fig. 1. Schematic of the proposed OEO. ISO, isolator; PC, polarization controller; PBS, polarization beam splitter; PBC, polarization beam combiner; OSA, optical spectrum analyzer; PD, photodetector; LNA, low noise amplifier; Att, attenuator; ESA, electrical spectrum analyzer. © 2013 Optical Society of America
October 15, 2013 / Vol. 38, No. 20 / OPTICS LETTERS
spur modes, a double loop constructed by a polarization beam splitter (PBS) and a polarization beam combiner (PBC) is used. After the optical combination in PBC, the light is splitted into two parts by a 10 dB optical coupler. One output with 10% coupled power is connected to the optical spectrum analyzer (Anritsu MS9710C) which operates at a resolution of 0.05 nm to inspect the optical signal, while the other output with 90% coupled power is converted into an electrical signal via a photodetector, which then relays the signal back toward the laser. A low-noise amplifier (LNA) is used to provide a sufficient electrical gain in the OEO loop, and an electrical attenuator (Att) is followed to control the amount of electrical gain such that the DFB laser is under small signal modulation. The generated microwave signal is received after a 10 dB electrical coupler. The 90% part is measured by an electrical spectrum analyzer (Agilent N9030A) with 26.5 GHz bandwidth, and the 10% part is fed back to the DFB laser to close the loop. It can be seen that neither an external modulator nor an electrical filter is used in this configuration. As we know, the frequency located at the relaxation oscillation frequency (f r ) will achieve the most effective modulation efficiency when a semiconductor laser is directly modulated by an RF signal. It can be clearly seen from the frequency response of a semiconductor laser, where the f r is determined. Accordingly, when the loop is closed, the frequency located at f r will acquire the largest gain in the loop oscillation. Hence, the oscillation will occur when the loop gain is sufficient, and no external modulator or electrical filter is required within the loop to enable the oscillation. The relaxation oscillation frequency can be determined using the following equation [11]: fr
q gγ p P 0 ∕2π;
(1)
where g is the differential optical gain constant, γ p is the photon decay rate given by the reciprocal of the photon lifetime, and P 0 is the steady-state photon density in the laser cavity. Equation (1) clearly indicates that f r can be increased by suitably designing the laser parameters. For a laser given parameters, one can increase f r by increasing the pump current (I b ), which leads to an increased P 0 ; or by decreasing the laser operation temperature (T), which leads to an increased γ p . Thus, we can simply tune the f r of the DFB laser by changing either the I b or T applied, and this forms the mechanism for providing a wideband tunable oscillation frequency within the loop, accordingly. To verify the tunability of f r , we measured the frequency responses of the DFB laser at different T and I b by a vector network analyzer (Agilent N5230A). The results are shown in Fig. 2. As can be seen from Fig. 2(a), the f r and modulation bandwidth are increased as T decreases with I b fixed at 20 mA. Figure 2(b) illustrates the increase in f r and modulation bandwidth of the DFB laser with I b , while the T is fixed at 25°C. These results both agree well with Eq. (1). However, it should be noted that the peaks of the response curves vary with I b and T, which indicates that different loop gain is required for different f r to perform oscillation. Additionally, it is
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Fig. 2. Measured frequency response of the DFB laser at different operation temperature (T) and bias current (I b ). (a) I b is fixed at 20 mA. (b) T is fixed at 25°C.
obvious that changing I b at fixed T is more effective to realize the tuning of f r in a wide band. An experiment based on the configuration in Fig. 1 was carried out. The parameters of the key devices used in the experiment are as follows: the DFB laser has a threshold current of 13.65 mA, and the lengths of the SMF in the dual loops are ∼1 and ∼2.77 km, respectively. The PD is a positive intrinsic-negative diode with a transfer impedance amplifier (PIN-TIA, PTHS992-003) embodied, which has a 3 dB bandwidth of 10 GHz and a responsivity of 0.9 A∕W. It should be noted that a 16 dB gain is provided by the TIA. The LNA (A-INFOMW LA2018N4020) has a gain of about 35 dB and a bandwidth of 0.8–18 GHz. The Att has an attenuation of 16 dB. By closing the loop, the OEO starts to oscillate at the frequency around the peak response of the DFB laser. The Vernier effect formed by the dual loops would perform the fine mode selection of the OEO. By directly changing the bias current and operation temperature of the DFB laser, the frequency of the oscillating signal can be tuned in a wide range. Figure 3 shows the spectra of the generated electrical signals with the frequency coarsely tuned from 3.77 to 8.75 GHz. It can be seen that all the fundamental components are with high power output (6.5–10.5 dBm), and all the harmonic components are at least 27 dB lower than the fundamental components. However, a few things should be noted. First, the largest tunable range is limited by the modulation characteristic of the DFB laser. It has been illustrated in Fig. 2(b) that, as the bias current increases, the curve of modulation response becomes flatter. Therefore, a DFB laser with sharper modulation response can be used to extend the tunable range. It can be realized by suitably designing the laser parameters or by optical injection to enhance the resonance peak [12]. Second, the maximum tuning range is also limited by the packaging of the DFB laser diode, which is not optimized for highfrequency modulation, and thus provides a limited modulation bandwidth and low modulation efficiency of f r .
Fig. 3. Spectra of the generated electrical signals with frequency coarsely tuned from 3.77 to 8.75 GHz.
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Fig. 4. Oscillation conditions of the frequencies plotted in Fig. 3, which show the relationship between the f osc and I b and T of the laser.
Third, the oscillation frequency can’t be finely tuned due to the fact that the electrical loop gain is no more than 25 dB in the experiment while it can be inferred from Fig. 2 that a different loop gain is required for each value of f r in order to generate an adequate oscillation signal. A variable RF attenuator and higher electrical loop gain will be helpful to achieve more oscillation frequencies. For the purpose of confirming the relationship between the oscillation frequency (f osc ) and I b and T of the laser, the oscillation conditions of the frequencies plotted in Fig. 3 are recorded in the experiment. The results are shown in Fig. 4. It can be seen that the tunability of f osc is effectively realized by changing I b when T is adjusted at around 25.5°C. Moreover, f osc is approximately in proportion to the square root of I b , which agrees well with Eq. (1). The phase noise performance of the proposed OEO was also investigated, and was measured using a signal source analyzer (Agilent N9030A). As shown in Fig. 5, the single sideband (SSB) phase noise of the generated 6.97 GHz microwave signal by the proposed scheme is −103.6 dBc∕Hz at 10 kHz offset. It is the best one among the generated signals we achieved, and others are all below −90.86 dBc∕Hz at 10 kHz offset. Although there are still some peaks at around 200 kHz and its integermultiple frequencies, which correspond to the side modes of the OEO due to the fiber loops used in our experiment, the side modes have a maximal phase noise of −81 dBc∕Hz, indicating that the spectrum purity of the OEO is good. It is confirmed by the corresponding electrical spectrum of the 6.97 GHz signal in 1 MHz span, which is shown as the inset in Fig. 5. The side mode suppression ratio is only 50 dB, which is 10 dB smaller than the result reported in [13]. It is caused by the finite polarization extinction ratio of the PBS used in the experiment, so it can be further improved by using a better PBS. In conclusion, a low-cost OEO with large frequency tunability from 3.77 to 8.75 GHz is realized based on a directly modulated DFB semiconductor laser. The key significance of the proposed scheme is that neither an external modulator nor an electrical filter is used and no more than 25 dB of the electrical loop gain is required to enable the oscillation. In addition, its tunability is achieved by directly changing the bias current and the operation temperature of the DFB laser. The SSB phase
Fig. 5. Phase noise of the generated 6.97 GHz microwave signal. Inset, electrical spectrum of the generated 6.97 GHz microwave signal in 1 MHz span. Resolution bandwidth = 9.1 kHz..
noise of the generated 6.97 GHz signal is measured to be −103.6 dBc∕Hz at 10 kHz offset. Although the frequency cannot be finely tuned, a new oscillation mechanism is provided, and the tunable range of the proposed OEO could be further extended by providing larger loop gain and replacing the DFB semiconductor laser we used with one with improved high-frequency characteristics and optimized packaging. Further work will focus on realizing finely tuning and larger tunable frequency range based on this new proposed scheme. This work was supported in part by the National Basic Research Program of China (973 Program) under grant 2012CB315603; the National Natural Science Foundation of China (NSFC) under grants 61177065, 61032005, and 61174199; and the Jiangsu Province Natural Science Foundation under grant BK2012058. The authors would like to thank Agilent Inc. for lending the phase noise measurement device. References 1. X. S. Yao and L. Maleki, J. Opt. Soc. Am. B 13, 1725 (1996). 2. H. Tsuchida and M. Suzuki, IEEE Photon. Technol. Lett. 17, 211 (2005). 3. X. S. Yao and L. Maleki, in Microwave Photonics 1996 (MWP ’96), Technical Digest (IEEE, 1996), pp. 265–268. 4. H. K. Sung, X. Zhao, E. K. Lau, D. Parekh, C. J. ChangHasnain, and M. C. Wu, IEEE J. Sel. Top. Quantum Electron. 15, 572 (2009). 5. S. Poinsot, H. Porte, J. Goedgebuer, W. T. Rhodes, and B. Boussert, Opt. Lett. 27, 1300 (2002). 6. E. Shumakher, S. Ó. Dúill, and G. Eisenstein, J. Lightwave Technol. 27, 4063 (2009). 7. J. Capmany, B. Ortega, and D. Pastor, J. Lightwave Technol. 24, 201 (2006). 8. Z. Tang, S. Pan, D. Zhu, R. Guo, Y. Zhao, M. Pan, D. Ben, and J. P. Yao, IEEE Photon. Technol. Lett. 24, 1487 (2012). 9. W. Li and J. P. Yao, IEEE Trans. Microwave Theor. Tech. 60, 1735 (2012). 10. X. Xie, C. Zhang, T. Sun, P. Guo, X. Zhu, L. Zhu, W. Hu, and Z. Chen, Opt. Lett. 38, 655 (2013). 11. K. Y. Lau and A. Yariv, IEEE J. Quantum Electron. QE-21, 121 (1985). 12. H. K. Sung, E. K. Lau, and M. C. Wu, IEEE J. Sel. Top. Quantum Electron. 13, 1215 (2007). 13. Y. Jiang, J. Yu, Y. Wang, L. Zhang, and E. Yang, IEEE Photon. Technol. Lett. 19, 807 (2007).
