Synchronization of Optically Coupled Resonant Tunneling Diode Oscillators

Synchronization of ly Coupled Resonant Tunneling Diode Oscillators Bruno Romeira a, José M. L. Figueiredo a, Charles N. Ironside b, and José M. Quintana c a Centro de Electrónica, Optoelectrónica e Telecomunicações (CEOT), Departamento de Física, Universidade do Algarve, 8005-139 Campus de Gambelas, Faro, Portugal; b School of Engineering, University of Glasgow, Glasgow G12 8QQ, United Kingdom; c Instituto de Microelectrónica de Sevilla (IMSE-CNM-CSIC), Universidad de Sevilla, 41092, Sevilla, Spain ABSTRACT We experimentally investigate the synchronous response of two fiber-optic coupled optoelectronic circuit oscillators based on resonant tunneling diodes (RTDs). The fiber-optic synchronization link employs injection of a periodic oscillating optical modulated signal generated by a master RTD-laser diode (LD) oscillator to a slave RTD-photodetector (PD) oscillator. The synchronous regimes were evaluated as a function of frequency detuning and optical injection strength. The results show the slave RTD-PD oscillator follows the frequency and noise characteristics of the master RTD-LD oscillator resulting in two oscillators with similar phase noise characteristics exhibiting single side band phase noise levels below -100 dbc/hz at 1 MHz offset from the carrier frequency. synchronization of RTD-based optoelectronic circuit oscillators have many applications spanning from sensing, to microwave generation, and data transmission. Keywords: laser diodes, photo-detectors, microwave generation, optical injection, optoelectronic devices, resonant tunneling diodes, synchronization 1. INTRODUCTION The development of optical modulation and synchronization techniques is motivated to a large point by applications in microwave and millimeter-wave generation and transmission in optical fiber links, such as phased array antenna, and indoor personal communication systems. 1, 2 In this sense various synchronization configurations based on semiconductor laser sources have been reported spanning from direct or external modulation, and optical feedback, to heterodyning techniques using two or more semiconductor laser sources, or employing dualmode semiconductor lasers. 3 In laser systems, injection locking where the optical oscillation in the slave laser is locked to a stable master signal has been applied, for example, to stabilization of lasers, wavelength conversion, improvement of coherence, reduction of noise, and enhancement of modulation bandwidth. 4, 5 Nevertheless, most laser configurations capable of synchronization are based on complex schemes that require a considerable number of linear and nonlinear optoelectronic and optical components making them not practical configurations for some low cost applications. To overcome the complexity of semiconductor laser related synchronization schemes, a number of optical injection methods have been applied using optoelectronic devices such as heterojunction phototransistor (HPT) receivers using either direct detection or heterodyning techniques. 6 Recently, great interest has been paid to direct optical injection locking of microwave, millimeter-wave, and terahertz generators employing resonant tunneling diode (RTD) devices, 7 9 due to their inherent high speed, structural simplicity, and relative easiness of fabrication. RTDs are nanoscale double barrier quantum well (DBQW) semiconductor structures which utilize wave nature of electrons, exhibiting a pronounced non-linear current-voltage (I-V) characteristic with a widebandwidth negative differential resistance (NDR) region. Since they are made of the same III-V semiconductor material alloys used in commercial available laser diodes (LDs), and photo-detectors (PDs), RTD-based light 8, 10 receiver and transmitter oscillators can be obtained. Further author information: (Send correspondence to Bruno Romeira) Bruno Romeira: E-mail: bmromeira@ualg.pt 8th Iberoamerican Optics Meeting and 11th Latin American Meeting on Optics, Lasers, and Applications, edited by Manuel Filipe P. C. Martins Costa, Proc. of SPIE Vol. 8785, 87851Y 2013 SPIE CCC code: 0277-786X/13/$18 doi: 10.1117/12.2022468 Proc. of SPIE Vol. 8785 87851Y-1

In this work, we demonstrate synchronization of optically coupled RTD optoelectronic circuit oscillators employing a master-slave optical fiber link configuration. We consider a master RTD-LD oscillator producing periodic self-oscillations that are optically inject into a slave RTD-PD that has a similar free-running frequency of the master signal. We show that in this optical synchronization scheme, the slave RTD-PD reproduces the dynamics of the master RTD-LD, and a complete synchronization state between the coupled slave and master oscillators is achieved. We evaluate the synchronous response as a function of frequency detuning and optical injection strength. The use of the RTD avoids the need of low noise and high power electrical and optical amplifiers employed in conventional synchronization schemes and networks since it exhibits an NDR region over a wide range of frequencies, which provides gigahertz bandwidth electronic gain. This allows the implementation of novel self-sustained oscillator access interfaces that can be synchronized with both electrical and optical signals, and realized in a compact single chip. Synchronization schemes employing RTDs can found various interesting applications in fiber-optic links for sensing, microwave and milimiter-wave generation, and data transmission in radio-over-fiber femtocellular networks. 11 2. EXPERIMENTAL SETUP AND DEVICES DESCRIPTION 2.1 Resonant tunneling diode optoelectronic circuit oscillators In this section we discuss the master and slave RTD-based optoelectronic circuit oscillators employed in the fiber-optic synchronization links. 2.1.1 Slave RTD-PD The RTD devices employed in this work consist of AlAs/InGaAs/AlAs double barrier quantum well (DBQW) structures grown on semi-conducting InP substrates. In Fig. 1 is presented the schematic of the cross-section of the RTD-PD device showing the most relevant semiconductor epitaxial layers. Inset is a top view microphotograph of an RTD-PD with the gold contact pads (left), and the lowest conduction energy band of the DBQW region (right). The epitaxial structure includes two 500-nm-thick n-ingaalas surrounding the DBQW layers that serves to absorb incident photons at a wavelength around 1550 nm. The RTD-PD woks as follows. Under the influence of the built-in electric field created by the applied voltage drop across the 500-nm-thick n-ingaalas layer in the collector side, when illuminated, the photo-generated electrons and holes in it are separated by the electric field, the holes are accumulated at the barriers of the resonant structure, whereas the electrons are collected by the external circuit. Therefore, the photo-detection mechanism in RTD-PD devices is strongly dependent on the applied bias voltage. Further description of RTD-based PD devices and operating principle can be found in. 8 Light Colector Emi er n+ InGaAs 400 µ m Emitter 60 µ m n InGaAlAs 40 µ m AlAs Colector InGaAs AlAs Collector n InGaAlAs n+ InP Substrate InP DBQW 2nm E 0 6nm 2nm Collector Current (ma) B A A 60 B 40 20 0 0 1 2 3 Voltage (V) DC Bias 1 µ F RTD-PD hv (c) Figure 1. Slave RTD-PD. Schematic of the cross-section of the RTD-PD device showing the DBQW epi-layer structure. Inset is the top view micro-photograph of the RTD-PD chip with an active area of 17 µm 17 µm (left), and the conduction band edge DBQW region (right); responsivity as a function of the wavelength with the bias voltage as a parameter. Inset is shown the illuminated I-V characteristic employing an optical signal at 1550 nm and 6 mw optical power; (c) circuit schematic of the implemented RTD-PD voltage and optically controlled microwave oscillator. In the optical characterization setup the RTD-PD was illuminated from the top using a lensed single-mode fiber, employing optical signals at wavelengths ranging from 1518 nm to 1570 nm. Figure 1 shows the responsivity as a function of the wavelength with the DC bias voltage as a parameter. The results show an Proc. of SPIE Vol. 8785 87851Y-2

increase of the responsivity in the peak-to-ndr transition of around 0.2 A/W at wavelengths ranging from 1518 nm to 1545 nm. In the region of interest, that is, in the NDR region, the RTD-PD devices show typical responsivities around 0.35 A/W at 1550 nm. For operation as a slave oscillator, the RTD-PD device is wire bonded to a 50 Ω transmission line in a printed circuit board (PCB). The circuitry includes a 1 µf capacitor shunting the RTD-PD transmission line, which decouples the DC bias circuit from the resonant tank formed by the transmission line and bonding wires inductances, and the RTD-PD intrinsic capacitance. The corresponding circuit schematic is shown in Fig. 1(c). This arrangement provides the appropriate resonant conditions for operation of RTD-PD device as a voltage and optically controlled microwave oscillator, as described in section 3. 2.1.2 Master RTD-LD The master RTD-LD consists of a communications laser diode operating around 1550 nm driven by a double barrier quantum well resonant tunneling diode device with similar characteristics of the RTD device employed in the slave circuit. The devices are surface-mounted on a PCB, the laser diode and the RTD are connected in series, and are fed through the transmission microstrip line. The physical layout of the RTD-LD oscillator circuit is schematically represented in Fig. 2 showing both and light and ports. A detailed description of the hybrid circuit components, implementation, and characterization of RTD-LD hybrid integrated circuits can be also found in. 12 Figure 2 shows LD, and RTD-LD I-V characteristics. The corresponding equivalent electrical circuit is illustrated in Fig. 2(c), where the parallel 1 µf capacitor corresponds to the shunt capacitance for circuit bias stabilization, as described in the previous sub-section. In the free-running operation the RTD produces current oscillations that drive the laser yielding the optical modulated used in this work as the master signal. The circuit allows bias control of and laser emission by the RTD electrical characteristics, operating as an optoelectronic voltage controlled oscillator, as described in section 3. DC + Microstrip line capacitor 1 µ F RTD Printed Circuit Board Au p Laser diode n Bias-T DC Bias 1 µ F RTD RTD-LD LD (c) Figure 2. Master RTD-LD. Physical layout of the RTD and LD chips mounted on a PCB circuit; current voltage characteristics of LD, and RTD-LD; (c) schematic configuration of the implemented circuit. 2.2 Master-slave optical synchronization experimental setup The synchronization of a transmitter-receiver system can be achieved by injection, unidirectional or mutual coupling synchronization schemes. Here we consider the situation of unidirectional driving, in which one has two coupled transmitter-receiver oscillators such that a signal from the transmitter is injected into the receiver in such a way that both systems become synchronized. The experimental setup consists of an RTD-LD free-running oscillator (master) and an RTD photo-detector oscillator (slave) optically coupled through a single mode optical fiber, Fig. 3. As detailed in the next section of experimental results, using an unidirectional synchronization and under appropriate bias conditions, the slave oscillator synchronizes to the master oscillator without requiring any additional optical or electrical amplification. We employed two techniques of synchronization, the free-running synchronization scheme, Fig. 3, using a master oscillator operating in the free-running mode, and the injection locked synchronization scheme, Fig. 3, where the master oscillator is first electrically injection locked to an external reference source. In both situations, prior to synchronization, the slave oscillator was operating in the free-running mode. Proc. of SPIE Vol. 8785 87851Y-3

RTD-LD optical fiber RTD-PD RTD-LD optical fiber RTD-PD Figure 3. Master-slave optical fiber link synchronization setups using two free-running RTD optoelectronic circuit oscillators. Free-running synchronization scheme, and injection locked synchronization scheme. 3. EXPERIMENTAL RESULTS 3.1 Master and slave voltage controlled oscillators When either the master or the slave RTD optoelectronic circuits are independently DC biased in the their NDR region, free-running relaxation oscillations in the microwave band are produced, with a frequency range determined by the bias voltage and the external resonant tank circuit. In Fig. 4 are presented the frequency tuning curves as a function of the bias voltage for both the RTD-LD free-running oscillator (master) and the RTD-PD oscillator (slave) circuits. Both master and slave operate as voltage controlled oscillators (VCOs) where the master RTD-LD produces fundamental frequency oscillations ranging from 0.917 GHz to 1.129 GHz, whereas the slave RTD-PD oscillates with fundamental frequencies ranging from 0.901 GHz to 1.095 GHz. Figure 4 presents the spectra when the master and slave RTD oscillators are independently DC bias in the NDR region slightly above the peak voltage, and the the light of the fiber-optic link was off. The spectra shows fundamental oscillations close to 0.96 GHz. Since the range of frequencies of both oscillators overlap in a wide region of operating conditions, the circuits fulfill the necessary conditions to be employed in a master-slave synchronization link. In what follows, we demonstrate two synchronization schemes, the free-running scheme and the injection locked synchronization scheme, when the master and slave oscillators are operating under the similar bias conditions shown in Fig. 4. Figure 4. Master and slave voltage controlled oscillators characteristics. Frequency tuning curves; master and slave spectra s when VCO circuits are operating independently and the light was off. 3.2 Free-running synchronization Figure 5 presents the experimental results using the free-running synchronization scheme, Fig. 3. Figure 5 and displays the unsynchronized and synchronized spectra, respectively, measured in the the of the slave RTD-PD oscillator. In the unsynchronized spectrum, Fig. 5, it is possible to identify both master and slave signal oscillation peaks plus sidebands, an indication that the oscillators are free-running independently. As shown in Fig. 5, under appropriate bias conditions, the slave free-running oscillator was synchronized at 0.959 GHz oscillation frequency by the master RTD oscillator. In the synchronous regime the Proc. of SPIE Vol. 8785 87851Y-4

slave RTD-PD oscillator follows the frequency of the master RTD-LD oscillator as a function of the optical injection strength and quality factor of the slave circuit, which follows the Adler s theory of injection locked oscillators. 13 The locking range of the slave oscillator was measured varying the DC bias voltage of the master RTD-LD, which shifted the free-running frequency and locked the slave oscillator, as a function of the injected optical power level, Fig. 5(c). A locking range of 3.5 MHz was achieved for an optical power level of around 2.5 mw. The results are limited by the optical losses due to light decoupling and coupling of the laser and photodetector using lensed fibers. In order to further evaluate the synchronous response of this scheme, we measured the single side band (SSB) phase noise characteristics of both master and slave circuits working independently, and when they were synchronized, Fig 5(d). The results show the RTD slave oscillator clearly follows the 1/f noise characteristics of the master RTD-LD oscillator at offsets below 1 MHz of the carrier frequency. As a result, we measured a phase noise reduction of the slave oscillator of about 18 db at 1 MHz offset of the 0.96 GHz carrier frequency for 2.5 mw optical power level. The phase noise measurements confirm the signals characteristics of the slave RTD-PD are mainly determined by the master RTD-LD oscillator performance. (c) (d) Figure 5. Experimental results of the free-running synchronization scheme. Unsynchronized spectrum measured in the slave when the master RTD-LD was DC biased at 1.973 V. Synchronized spectrum measured when the master RTD-LD was DC biased at 1.976 V. In and, the bias voltage of the slave RTD-PD oscillator was fixed at 2.591 V and the optical power level in the fiber link was around 1 mw. (c) Locking range as a function of optical power level. (d) SSB phase noise results. 3.3 Injection locked synchronization Although the free-running synchronization scheme described in the previous sub-section has interesting applications in the study of coupled oscillators, further techniques including injection locking, 8 and self-synchronization, can provide considerably improvements of RTD oscillators phase noise characteristics, highly desirable for applications in microwave photonics systems. Considering this important aspect, we have studied a similar synchronization scheme consisting of an RTD-LD oscillator (master) that, prior to synchronization, was injection locked with an external signal with -20 dbm power level, Fig. 3. An identical RTD photo-detector oscillator as discussed in the previous sub-section was employed as the slave device. Notice that the driving signal was injected electrically for purposes of demonstration and experimental convenience, however, since the 10, 12 Proc. of SPIE Vol. 8785 87851Y-5

RTD employed in the master circuit has photo-detection capabilities, the master circuit can be controlled also optically for applications where is necessary to receive and detect optical modulated signals transmitted through a fiber-optic link. Figures 6 and present the unsynchronized and synchronized spectra, respectively, measured in the of the slave RTD-PD oscillator. As shown in Fig. 6, when the driving reference source was set to 0.96 GHz, the slave free-running oscillator was synchronized by the injection locked master RTD-LD oscillator at the same signal frequency. The locking range of the slave oscillator as a function of the injected optical power level is presented in Fig. 6(c) showing a similar performance as demonstrated in the free-sunning synchronization scheme for the range of optical power levels under analysis. (c) (d) Figure 6. Experimental results of the injection locked synchronization scheme. Unsynchronized spectrum measured in the slave when the master RTD-LD was DC biased at 1.973 V and injection locked with a 0.9625 GHz reference source. Synchronized spectrum when the master RTD-LD was injection locked with a 0.96 GHz reference source. In and, the bias voltage of the slave RTD-PD oscillator was fixed at 2.591 V and the optical power level in the fiber link was around 1 mw. (c) Locking range as a function of optical power level. (d) SSB phase noise results. The experimental results on the SSB phase noise characteristics show the RTD slave oscillator follows the noise characteristics of the injection locked master RTD-LD oscillator. However, since in this case the master signal as a superior phase noise quality, we observe a substantial phase noise reduction in the slave. In the master-slave synchronization and using an optical power level of arounf 2.5 mw, the phase noise of the slave oscillator was reduced to -101.3 dbc/hz at 1 MHz offset of the carrier frequency, a 20 db phase noise reduction when compared with the slave RTD-PD operating independently. The injection-locking results shown here provide an interesting synchronization scheme with low phase noise and clean signals for applications in microwave photonics systems. 4. CONCLUSION We have demonstrated a unidirectional synchronization scheme using two optically coupled resonant tunneling diode optoelectronic integrated circuit oscillators in a master-slave fiber-optic link configuration. We achieved synchronization with optical power levels around 1 mw and locking ranges up to 3.5 MHz. The results show the RTD slave oscillator follows the frequency and noise characteristics of the master RTD oscillator. A phase Proc. of SPIE Vol. 8785 87851Y-6

noise reduction of the slave oscillator of more than 20 db at 1 MHz offset from the 0.96 GHz carrier frequency was achieved for injected optical levels of around 2.5 mw. RTD-based optoelectronic integrated circuit oscillators avoids the need of low noise and high power electrical and optical amplifiers employed in conventional synchronization schemes. This provides the implementation of simple, and low phase-noise self-sustained oscillator access points that can be synchronized with both electrical and optical signals, and realized in a compact single chip. Synchronization schemes employing RTDs can found various functions including generation, amplification, and distribution of carriers, for interesting applications in fiber-optic links for sensing, and data transmission in radio-over-fiber femtocellular networks. ACKNOWLEDGMENTS This work was supported by Fundação para a Ciência e a Tecnologia under Project PTDC/EEATEL/100755/2008- WOWi-Wireless-optical-wireless interfaces for picocellular access network. The authors would like to thank Dr. Kris Seunarine, University of Glasgow, for the fabrication of the RTD-photodector devices, and Dr. Horacio Cantu, University of Glasgow, for the fruitful discussions on RTD microwave oscillators. REFERENCES 1. A. Daryoush, synchronization of millimeter-wave oscillators for distributed architectures, IEEE Trans. Microwave Theory Techn. 38(5), pp. 67 476, 1990. 2. L. Johansson and A. Seeds, Generation and transmission of millimeter-wave data-modulated optical signals using and optical injection phase-lock loop, Journal of Lightwave Technol. 21(2), pp. 511 520, 2003. 3. K. Razavi and P. Davies, Semiconductor laser sources for the generation of millimetre-wave signals, IEE Proc.-Optoelectron 145(3), pp. 159 163, 1998. 4. J. Wang, M. K. Haldar, L. Li, and F. V. C. Mendis, Enhancement of modulation bandwidth of laser diodes by injection locking, IEEE Photon. Technol. Lett. 8(1), pp. 34 36, 1996. 5. T. B. Simpson, J. M. Liu, and A. Gavrielides, Bandwidth enhancement and broadband noise reduction in injection-locked semiconductor lasers, IEEE Photon. Technol. Lett. 7(7), pp. 709 711, 1995. 6. J.-Y. Kim, C.-S. Choi, W.-Y. Choi, H. Kamitsuna, M. Ida, and K. Kurishima, Characteristics of InP- InGaAs HPT-based optically injection-locked self-oscillating optoelectronic mixers and their influence on radio-over-fiber system performance, IEEE Photon. Technol. Lett. 19(3), pp. 155 157, 2007. 7. B. Romeira, J. M. L. Figueiredo, T. J. Slight, L. Wang, E. Wasige, C. N. Ironside, A. E. Kelly, and R. Green, Nonlinear dynamics of resonant tunneling optoelectronic circuits for wireless/optical interfaces, IEEE J. Quantum Electron. 45(11), pp. 1436 1445, 2009. 8. B. Romeira, J. M. L. Figueiredo, C. N. Ironside, A. E. Kelly, and T. J. Slight, control of a resonant tunneling diode microwave-photonic oscillator, IEEE Photon. Technol. Lett. 22(21), pp. 1610 1612, 2010. 9. S. Suzuki, M. Asada, A. Teranishi, H. Sugiyama, and H. Yokoyama, Fundamental oscillation of resonant tunneling diodes above 1 THz at room temperature, Appl. Phys. Lett. 97(24), p. 242102, 2010. 10. B. Romeira, K. Seunarine, C. N. Ironside, A. E. Kelly, and J. M. L. Figueiredo, A self-synchronized optoelectronic oscillator based on an RTD photo-detector and a laser diode, IEEE Photon. Technol. Lett. 23(16), pp. 1148 1150, 2011. 11. H. I. Cantu, B. Romeira, A. E. Kelly, C. N. Ironside, and J. M. L. Figueiredo, Resonant tunneling diode optoelectronic circuits applications in radio-over-fiber networks, IEEE Trans. Microwave Theory Tech. 60(9), pp. 2903 2912, 2012. 12. B. Romeira, J. Javaloyes, J. M. L. Figueiredo, C. N. Ironside, H. I. Cantu, and A. E. Kelly, Delayed feedback dynamics of Liénard-type resonant tunneling-photo-detector optoelectronic oscillators, IEEE J. Quantum Electron. 49(1), pp. 31 42, 2013. 13. R. Adler, A study of locking phenomena in oscillators, Proc. IEEE 61(10), pp. 1380 1385, 1973. Proc. of SPIE Vol. 8785 87851Y-7