Compact optoelectronic oscillator based on a Fabry Perot resonant electro-optic modulator

Transcription

1 Compat optoeletroni osillator based on a Fabry Perot resonant eletro-opti modulator Jian Dai ( 戴键 ) 1,2, *, Yitang Dai ( 戴一堂 ) 1, Feifei Yin ( 尹飞飞 ) 1, Yue Zhou ( 周月 ) 1, Jianqiang Li ( 李建强 ) 1, Yuting Fan ( 樊宇婷 ) 1, and Kun Xu ( 徐坤 ) 1,3 1 State Key Laboratory of Information Photonis and Optial Communiations, Beijing University of Posts and Teleommuniations, Beijing , China 2 State Key Laboratory of Millimeter Waves, Southeast University, Nanjing , China 3 Shool of Siene, Beijing University of Posts and Teleommuniations, Beijing , China *Corresponding author: daijian@bupt.edu.n Reeived July 14, 2016 aepted September 9, 2016 posted online Otober 27, 2016 A novel ompat optoeletroni osillator (OEO) employing a Fabry Perot (FP) resonant eletro-opti (EO) modulator is proposed and experimentally demonstrated. The resonant modulator is used as the optial storage element as well as the mode seletion element, whih an greatly redue the system omplexity and make the system more portable. Moreover, the optial resonane and eletrial transmission response for the FP resonant EO modulator are theoretially and experimentally studied. The proposed OEO osillates at 10 and 20 GHz in the proof-of-onept experiment, and the orresponding single-sideband phase noise an reah below 118 and 108 db Hz at 1 MHz offset frequeny, respetively. OCIS odes: , doi: /COL Mirowave osillators have been widely used in the field of radar [1], ommuniations [2], navigation [3], eletroni warfare [4], and modern physis experiments [5]. However, as wireless ommuniation tehnology rapidly develops, the traditional mirowave osillators an no longer meet the high performane requirements, suh as high output frequeny and low phase noise. As an exellent low phase noise radio frequeny (RF) soure, the optoeletroni osillator (OEO) has been intensively investigated and has attrated great interest in reent years [6 9]. The low phase noise OEO normally requires a long optial fiber in order to reah a high Q value. However, using long optial fiber auses small mode spaing. Many approahes have been applied to suppress the spurious modes, suh as ultra-narrow RF bandpass filters (BPFs) [10,11], optial BPFs [12], injetion loking [13,14], multi-loop osillation [15,16], or photoni resonators [17 20]. Yet these methods also have their own drawbaks. The onventional ultra-narrow BPF is not narrow enough to filter out all spurious modes. The injetion loking tehnique tends to limit the phase noise of the OEO due to the injeted RF soure. Although the multi-loop osillation an suppress the spurious modes, it is very diffiult to stritly math the loops, whih ould ause the instability of the system. High-Q photoni resonators, inluding the Fabry Perot (FP) resonator and the whispering gallery mode (WGM) resonator, are of great value to the mode seletion of the OEO. In addition, the high-q resonator an also funtion as an optial energy storage element by trapping light in the resonator. In Refs. [17,18], a highfinesse FP etalon is used as the mode seletor, and it an improve the frequeny stability of the OEO due to its ultralow temperature dependeny. However, these shemes are based on the intensity modulated optial link, whih performs just like a finite impulse response mirowave photoni filter. The bias voltage drift and disrete devies would make the osillator inompatible with the temperature ontrol module. In Ref. [16], a WGM miroresonator is used to suppress the spurious modes. The frequeny seletivity of the phasemodulation-based optial storage loop is not a singlet urve per free spetral range (FSR), whih would be too diffiult for single frequeny osillation to filter two adjaent modes. Moreover, the resonator with an external modulator would make the osillator diffiult to integrate. The same problem exists for the onfiguration with an eletro-opti (EO) modulator plaed in front of an FP resonator. In this Letter, we proposed and experimentally demonstrated a novel ompat OEO employing an FP resonant EO modulator. A high-quality FP resonator not only has the apability of storing optial energy, but also serves as a mode seletor. It is relatively easy to manufature an FP resonator and to ouple the light into it when ompared with a WGM resonator [21]. In addition, the integration of an FP resonator and an EO modulator helps to redue the system omplexity signifiantly, whih also makes the osillator more portable and ompatible with a temperature ontrol module. In the proof-of-onept experiment, the harateristis of the novel optial storage link have been theoretially and experimentally investigated. Based on the proposed ompat OEO, 10 and 20 GHz signals osillate respetively with orresponding single-sideband (SSB) phase noise below 118 and 108 db Hz at 1 MHz offset frequeny. The phase noise performane an be further improved with the enhanement of the FP resonator s quality fator. Figure 1 shows the shemati diagram of the proposed OEO based on an FP resonant EO modulator. The light /2016/110701(5) Chinese Optis Letters

2 the driven signal, and is the half-wave voltage of the internal phase modulator. After several round trips, the total output optial field is E out ¼ XN k¼1 E k (2) where N is the number of round-trip times. Assume that the refletivity of the FP resonator s end faes is the same (R ¼ R 1 ¼ R 2 ), the orresponding eletrial output signal an be expressed as Fig. 1. Shemati diagram of the proposed ompat OEO. LD: laser diode PC: polarization ontroller OSA: optial spetrum analyzer PM: phase modulator PD: photodiode EA: eletrial amplifier PS: phase shifter ESA: eletrial spetrum analyzer. wave generated from the tunable laser was first sent into the built-in phase modulator. The polarization ontroller is used to align the light s polarization state to the main axis of the LiNbO 3 modulator driven by the osillating RF signal. After the optial signal is onverted bak into the eletrial domain, it will be amplified, phase shifted, and fed bak into the osillation loop via the RF hain. When the modulation frequeny is a multiple of the FSR of the FP resonator, the output optial signal after osillating in the FP EO modulator for k times an be expressed as I out ¼ γje out j 2 ¼ 2γRð1 RÞE2 in 1 þ R 2ωo n os 0 L X þ þ πv RF osðω RF tþ ¼ 2γRð1 RÞP in J 1 þ R n ðβþ n¼ os 2 ω on 0 L þ nω V RF t þ nπ (3) π 2 where γ is the responsivity of the photodiode, V RF and ω RF are the amplitude and angular frequeny of the driving RF signal, respetively, and P in is the optial output power of the laser. Considering the fundamental omponent of the output signal, the eletrial transmission response of the optial storage link an be derived as G o ¼ ¼ h γrð1 RÞβP in 1þR h 2γRð1 RÞβP in 1þR sin ω RF t þ 2ω on 0 L sinðω RF tþ sin ¼ 2γ2 R 2 ð1 RÞ 2 β 2 P 2 in ð1 þ RÞ 2 sin 2 2ωo n 0 L V 2 RF R load 2ωo n 0 L þ sin V 2 RF R load i 2Rload ω RF t 2ω on 0 L i πv DC 2Rload þ πv DC R 2 load V : (4) π p p k 1 E k ¼ E in ð1 R 1 Þð1 R 2 Þ R 1 R 2 ωo n exp j 0 L þ πv 2k 1 DC exp ðjβþ k (1) where E in is the output optial field from the laser, R 1 and R 2 are the refletivity of the FP resonator s end faes, k is the number of round-trip times in the resonator, ω o is the angular frequenies of the optial arrier, n 0 and V DC are the inherent refrative index and the bias voltage of the travelling waveguide EO modulator, respetively, L is the length of the resonator, is the veloity of light, β ¼ πv RF ðtþ is the modulation index in whih V RF ðtþ is If the laser wavelength was set at a proper position, the gain of the optial segment would ahieve maximum value, as shown in 2ω o n 0 L ¼ mπ þ π 2 (5) where m is an integer number. To form the osillation state, the feedbak RF segment is neessary to satisfy the gain and phase-mathing onditions of the hybrid optoeletroni loop. The two stage amplifiers are used to make the loop gain exeed the loss. Theoretially, the gain of the amplifiers should satisfy

3 G e > 1 ð1 þ RÞ 2 ¼ G o 2γ 2 R 2 ð1 RÞ 2 β 2 P 2 : (6) in R2 load The loop length an be adjusted via the phase shifter in the eletrial hain, whih would help to math the osillation frequeny with the frequeny seletion mode of the optial storage element. Assisted by the oupler, part of the osillating signal an be sent out, while the rest would be fed bak into the loop. Thanks to the frequeny seletion and high quality harateristi of the FP EO modulator, the ultra-narrow RF BPF and long optial fiber oil are not required. In addition, the proposed OEO an be more ompat via the integration of the phase modulator and FP resonator. As mentioned above, the FP EO modulator is the key part of our OEO. Both the optial resonane and eletrial transmission harateristis play important roles for the energy storage and frequeny seletion. The orresponding measurement setups for resonane and filtering harateristis are in an open loop onfiguration. First, for measuring the optial power resonane, the setup onsists of a frequeny fine-tuning laser soure with a narrow linewidth, an eletrial funtion generator, a photodiode, and a high-speed resolution osillosope. The light wave generated from a 1550 nm tunable diode laser (New Fous TLB-6728) at a power level of 30 mw is injeted into the FP EO modulator (Opto Comb WTEC-01). The narrow linewidth and wide tuning resolution of the laser are 200 khz and 0.01 nm, respetively. In addition, the tuning parameter of the laser is about 0.04 nm/v. Then the laser wavelength is sanned by driving the laser urrent with a 60 Hz triangular signal generated from the funtion generator (Tektronix AFG3151 C). As the blue line shows in Fig. 2, the orresponding period and peak-to-peak value of the ontrol signal are 1/60 s and 2 V, respetively. Taking the tuning parameter of the laser into onsideration, the tuning range and speed are 10 GHz and 9.6 nm/s, respetively. In order to haraterize the FP resonator, the output optial signal oming from the resonator will be onverted bak to the eletrial domain via a photodiode before the fast digital real time osillosope (Tektronix MDO4024C), whih permits the analysis of the power peak lines at the FP resonane. In Fig. 2, we present the measurement results of the resonane peaks. As previously analyzed, the red urve learly shows the 10 GHz wavelength sweep range and 2.5 GHz FSR. It is obvious that the wavelength span is broad enough to san four full FSRs of the resonator. Seond, for measuring the eletrial transmission response of the optial storage element employing the FP EO modulator, the measuring setup onsists of a tunable laser diode, a photodiode, and a vetor network analyzer. A narrow linewidth laser with a 40 khz linewidth and at a power level of 14 dbm is used as the laser soure. The light wave is fed into the FP EO modulator driven by the RF signal generated from the output port of the vetor network analyzer (Agilent N5244A). In addition, both the wavelength of the laser and the bias voltage of the modulator are finely tuned to meet Eq. (5) in the experiment. A polarization ontroller is used to align the polarization Fig. 2. Laser wavelength sanning voltage and orresponding optial resonane spetrum of the FP resonator. Fig. 3. Measured transmission response of the optial link employing an FP resonant EO modulator. (a) Transmission response of the proposed optial storage link from 4.5 to 23 GHz. (b) Zoomed-in view of the response entered at 10 GHz. () Zoomed-in view of the response entered at 20 GHz

4 state of the light to the main axis of the EO modulator. After the optial signal is onverted bak into the eletrial domain via the high-speed photodiode with a 3 db bandwidth of 20 GHz, it will return to the input port of the vetor network analyzer. Figure 3(a) gives the normalized eletrial transmission response of the optial storage link from 4.5 to 23 GHz, and reflets that the transmission response is a periodi filter with 2.5 GHz FSR, whih is equal to the optial FSR of the FP resonator. As is learly shown, we an see that the 3 db bandwidth of the responses entered at 10 and 20 GHz are 47 and 54 MHz, respetively. In addition, the orresponding out-of-band rejetion an reah 38 and 32 db. With the enhanement of the Q value of the FP resonator, the frequeny seletion harateristis would be muh better. The system operation priniple has been theoretially desribed and the harateristis of the optial segment have been measured earlier. Then a proof-of-onept experiment based on the onfiguration in Fig. 1 is arried out to verify the proposed sheme. Based on the previous optial energy storage and frequeny seletion segment, an eletroni feedbak hain is neessary to form the osillation loop. The RF hain mainly omprises two stage RF amplifiers, a phase shifter, and a mirowave oupler. The amplifiers are used to provide suffiient eletrial gain in the loop. The gain of the first stage amplifier is about 30 db from DC to 26 GHz while for the seond amplifier it depends on the osillating frequeny. For the 10 GHz osillation frequeny, a mirowave amplifier from 9 to 12 GHz with a 25 db gain is used, while it was replaed by another power amplifier from 18 to 22 GHz with a 30 db gain in the 20 GHz osillation loop. For the RF phase shifter, it is operated manually with a 900/GHz tuning parameter from DC to 26.5 GHz. By finely tuning the phase shifter and the laser wavelength, 10 and 20 GHz mirowave signals an be generated, respetively. Then, for measuring the optial spetra of the proposed OEO, 10% of the optial signal from the 10 db optial oupler is injeted into the optial spetrum analyzer (Yokogawa AQ6370C). Just as shown in Fig. 4, the optial spetrum is measured both with a 5 nm span and a 0.02 nm resolution bandwidth. Figure 4 illustrates that the pump light at nm is the most powerful, and the optial spetra are frequeny ombs with 10 and 20 GHz intervals, whih are equal to the osillation frequeny, respetively. Finally, the RF signal is produed by the frequeny omb on a high-speed photodiode. The orresponding eletrial spetrum and phase noise are taken by a 40 GHz RF spetrum analyzer (Agilent 9030A) via a fration of the generated RF signal output from the 10 db mirowave oupler. Meanwhile, the rest of the mirowave signal is fed bak into the optoeletroni hybrid osillating loop. In Fig. 5, the power spetrum of the osillating signal is Fig. 4. Normalized optial power spetrum of the proposed ompat OEO. (a) Optial spetra of (a) 10 and (b) 20 GHz osillation loop. Fig. 5. (a) 10 and (b) 20 GHz Eletrial power spetra of the proposed ompat OEO

5 In onlusion, we propose and experimentally demonstrate a novel ompat OEO employing an FP EO modulator, whih serves as both the energy storage and frequeny seletion elements. Due to the integration of a modulator and an FP resonator, the novel ompat OEO an be realized without a long optial fiber and a large RF narrow BPF. A proof-of-onept experiment is investigated to prove the feasibility of the proposed OEO. Based on the frequeny seletion harateristis of the optial storage link, the OEO osillates at 10 and 20 GHz, respetively, and the orresponding SSB noise an reah 108 and 118 db Hz at an offset of 1 MHz. Moreover, its performane an be greatly improved with the enhanement of the FP resonator s quality fator. This work was supported in part by the National Natural Siene Foundation of China (No ) and the Postdotoral Siene Foundation of China (No. 2016M590067). Fig. 6. SSB phase noise of the osillation frequenies at (a) 10 and (b) 20 GHz generated by the proposed ompat OEO. measured with different resolution bandwidths. As shown in the figure, the spetrum of the 10 GHz osillating signal is measured with a 450 MHz frequeny span and 100 khz resolution, while the 20 GHz RF signal with a 320 MHz span and 10 khz resolution. The output powers are 16.8 and 24.4 dbm, respetively. A major ause of the power redution is the different eletrial transmission harateristi of the optial segment. The SSB phase noise performane of the proposed OEO has also been investigated. As shown in Fig. 6(a), the SSB phase noise of the 10 GHz generated signal is about 118 db Hz at an offset of 1 MHz while, for the 20 GHz generated signal, the SSB phase noise is about 108 db Hz at an offset of 1 MHz, as shown in Fig. 6(b). The feasibility of the novel ompat RF photoni osillator has been verified. The phase noise performane is mainly limited by the Q value of the FP resonator and the noise figure of the optial storage element. By inreasing the resonator s quality fator, it an help to improve the phase noise performane of the final osillating signal. The insertion loss of the embedded phase modulator is the main limitation of the FP resonator s high quality fator. Finally, a stable FP resonator is useful for improving the stability of the OEO, as desribed in Ref. [18]. Referenes 1. C. Wang and J. P. Yao, IEEE Trans. Mirowave Theory Teh. 61, 4275 (2013). 2. G. Puerto, J. Mora, B. Ortega, and J. Capmany, Opt. Express 18, (2010). 3. H. Emami and M. Ashourian, IEEE Trans. Mirowave Theory Teh. 62, 2462 (2014). 4. V. Moreno, M. Rius, J. Mora, M. A. Muriel, and J. Company, IEEE Photonis J. 5, (2013). 5. J. Weber, Phys. Rev. 90, 977 (1953). 6. X. S. Yao and L. Maleki, J. Opt. So. Am. B 13, 1725 (1996). 7. Y. Ji, X. Jia, Y. Li, J. Wu, and J. Jin, Chin. Opt. Lett. 11, (2013). 8. H. C. Yu, M. H. Chen, H. B. Gao, C. Lei, H. Zhang, S. G. Yang, H. W. Chen, and S. Z. Xie, Photon. Res. 2, B1 (2014). 9. L. Huo, Y. Yang, C. Lou, and Y. Gao, Chin. Opt. Lett. 3, 140 (2005). 10. T. Wang, W. Li, and N. H. Zhu, Opt. Commun. 318, 162 (2014). 11. L. X. Wang, N. H. Zhu, W. Li, and J. G. Liu, IEEE Photonis Tehnol. Lett. 23, 1688 (2011). 12. W. Li, J. G. Liu, and N. H. Zhu, IEEE Photonis Tehnol. Lett. 27, 1461 (2015). 13. W. M. Zhou and G. Blashe, IEEE Trans. Mirowave Theory Teh. 53, 929 (2005). 14. K. H. Lee, J. Y. Kim, W. Y. Choi, H. Kamitsuna, M. Ida, and K. Kurishima, IEEE Photonis Tehnol. Lett. 20, 1151 (2008). 15. X. S. Yao and L. Maleki, IEEE J. Quantum Eletron. 36, 79 (2000). 16. E. C. Levy, O. Okusaga, M. Horowitz, C. R. Menyuk, W. Zhou, and G. M. Carter, Opt. Express 18, (2010). 17. I. Ozdur, D. Mandridis, N. Hoghooghi, and P. J. Delfyett, J. Lightwave Tehnol. 28, 3100 (2010). 18. M. Bagnell, J. D. Rodriguez, and P. J. Delfyett, J. Lightwave Tehnol. 32, 1063 (2014). 19. K. Volyanskiy, P. Salzenstein, H. Tavernier, M. Pogurmirskiy, Y. K. Chembo, and L. Larger, Opt. Express 18, (2010). 20. A. A. Savhenkov, V. S. Ilhenko, W. Liang, D. Eliyahu, A. B. Matsko, D. Seidel, and L. Maleki, Opt. Lett. 35, 1572 (2010). 21. H. Tavernier, P. Salzenstein, K. Volyanskiy, Y. K. Chembo, and L. Larger, IEEE Photonis. Tehnol. Lett. 22, 1629 (2010)