Optoelectronic Oscillator Topologies based on Resonant Tunneling Diode Fiber Optic Links Bruno Romeira* a, José M. L Figueiredo a, Kris Seunarine b, Charles N. Ironside b, a Department of Physics, CEOT, University of Algarve, 8005-139 Faro, Portugal; b School of Engineering, University of Glasgow, G12 8LT Glasgow, UK ABSTRACT We investigate optoelectronic oscillator (OEO) configurations based on a laser diode driven by resonant tunnelling diode (RTD) optical waveguide photo-detector (PD) oscillators, with an optical fiber feedback loop carrying a fraction of the laser diode optical output that is re-injected into the OEO through the optical waveguide of the RTD-PD. In the configurations reported here, we take advantage of the RTD negative differential resistance to provide electrical highbandwidth. The optical fiber loop acts as a high quality optical energy storage element with low transmission loss. The RTD based OEO can produces stable and low-phase noise microwave signals with attractive applications in photonics and communication systems, mainly in fiber-optic based communication links since the RTD-OEO can be accessed both optically and electrically. Keywords: Delay line, laser diode, microwave photonics, optoelectronic oscillator, resonant tunneling diode 1. INTRODUCTION Optoelectronic oscillators (OEO) utilize high-quality (Q) factor optical storage elements (~10 10 ) to provide levels of frequency stability that cannot be achieved using traditional electronic oscillators. OEOs are nowadays considered to be excellent ultra-pure microwave generators, and they are expected to play an important role in several applications such as lightwave communications and radar technology 1. The basic OEO architecture consists of an externally intensity modulated fiber link that incorporates at least one high-q optical energy storage element 1. The photo-detected output is then amplified and fed back to the external modulator input. To produce stable oscillations the OEO must satisfy the Barkhausen criteria: the loop phase shift must be an integer multiple of 2π and the loop gain must be equal to unity. For typical OEOs, the last criterion means that a significant amount of electrical gain is necessary to compensate the high insertion losses: typically greater than 40 db of RF power amplification subsequent to the photo-detector is necessary in order to satisfy the Barkhausen criteria. In this paper, we present a simple OEO configurations consisting of a laser diode (LD) driven by a resonant tunnelling diode (RTD) self-sustained oscillator monolithic integrated with a waveguide photo-detector (PD) circuit 2 where a fraction of the LD modulated output is re-injected into the OEO through the PD waveguide via an optical fiber. The RTD negative differential resistance (NDR) provides the RF power gain required to achieve the oscillations without the need of extra electrical or optical amplification 2,3. Unlike with typical OEOs, here the optical loop works as a passive feedback route allowing the self-injection of a fraction of OEO optical output signal. The presence of the RTD waveguide PD reduces significantly the number of components required to obtain an OEO system. When the circuit is DC biased in the RTD NDR region, high-frequency self-sustained oscillations are produced in electrical domain, which combined with photo-detection allows the control of the laser diode emission characteristics, leading to the implementation of low power consumption optoelectronic circuits capable of producing high-frequency electrical and optical signals whose operations are controllable both electrically and optically 2,3. Moreover, RTD self-sustained oscillation frequency can be tunable by a DC bias voltage 2,3. *bmromeira@ualg.pt; phone +351 289 800 987; fax +351 289 800 066;
We investigate two RTD-OEO configurations where an RTD-PD oscillator drives a semiconductor external modulator or a laser diode. The first configuration works as follows: the RTD-PD self-sustained electrical oscillations drive both a laser diode connected in series with the RTD-PD and an external modulator whose optical output is injected through a high-q optical fiber delay line into the RTD-PD; the modulator optical input is provided by a continuous wave (CW) laser diode source. In the former configuration the RTD-PD self-sustained oscillations drive the laser diode connected in series and a fraction of the corresponding laser optical output is re-injected into the system via the waveguide PD using an optical fiber time-delay line. In both RTD-OEO configurations the re-injection of optical signals through the waveguide PD leads to the generation of highly stable electrical and optical signals. The optoelectronic oscillator configurations reported here can be accessed both optically and electronically and generate low-phase noise microwave signals with applications in photonics and communication applications 4. 2. OPTOELECTRONIC OSCILLATOR DESCRIPTION 2.1 Experimental circuit Figure 1 presents a typical schematic of the RTD-OEO core: a RTD-PD optical waveguide connected in series through Au wire-bonding with a laser diode with operation wavelength ~1.55 um both assembled on a printed circuit board 2,3.The RTD-PD corresponds to an optical waveguide (OW) photo-detector (PD) embedding in the core a double barrier quantum well (DBQW) 3. The measured RTD-PD responsivity at 1550 nm was ~0.28 A/W. As shown in Fig. 1(b) the RTD-PD in series with the LD preserves the RTD-PD NDR current-voltage characteristic. The RTD-PD-LD circuit combines the LD emission, the waveguide photo-detection and NDR functionalities in a single hybrid circuit with both optical and electrical input and output ports. Additionally, since the circuit is an optoelectronic voltage controlled oscillator, e.g., the free-running frequency of the RTD oscillator can be controlled by adjusting the DC bias voltage. The diagram of Fig. 1(c) summarizes all the five port functions realizable with the circuit discussed here. Figure 1. (a) Schematic diagram (not scaled) of the RTD-PD and LD hybrid integrated circuit on a printed circuit board. (b) RTD-PD-LD I-V characteristic showing the NDR region. (c) RTD-PD-LD circuit functional diagram. When DC biased in the NDR region, the circuit used shown in Fig. 1(a) oscillates with a natural frequency ranging from 1.05 GHz to 1.41 GHz. In this investigation, the bias was set close to the valley region of the RTD-PD-LD I-V curve, Fig. 1, to oscillate around 1.4 GHz. The bias condition where set to operate the RTD-PD in the NDR close to the valley region where the responsivity has maximized values 3. In what follows, we present and discuss the optical self-injection locking results showing stable frequency operation with low-phase noise. 2.2 RTD-OEO using an external modulator The RTD-OEO configuration employing an external modulator, shown schematically in Fig. 2, consists of an RTD-PD in series with a single mode laser diode, as described in subsection 2.1, operating as a self-sustained optoelectronic voltage controlled oscillator (OVCO) and a feedback circuit including an electro-optic amplitude modulator, an optical fiber delay line (405 m optical fiber roll), an erbium doped fiber amplifier (EDFA), an electrical amplifier. The proposed self-injection locking scheme is achieved by driving the external modulator with the RF signal produced by the OVCO. The external modulator optical out is amplified using an EDFA and injected into the RTD optical waveguide via single
mode optical fiber delay line with hundreds of meters. The optical injection scheme provides a delayed replica of the RTD-OEO electrical output that is re-injected into the RTD-PD oscillator through the external modulated optical signal. Figure 2. (a) RTD optoelectronic oscillator architecture employing external modulation of RTD RF output. 2.3 RTD-OEO using direct modulation of a laser diode The compact RTD based OEO configuration is shown in Fig. 3 and corresponds to a much simpler arrangement consisting of a hybrid circuit of a monolithic RTD waveguide photo-detector in series with a laser diode. Due to the NDR, the gold wire-bonding inductance and the RTD-PD intrinsic capacitance the RTD-PD-LD circuit works as an optoelectronic voltage controlled oscillator producing both electrical and optical output signals at a frequency determined by the wire-bonding inductance and RTD-PD capacitance. As in the previous configuration the RTD-PD acts as an O/E converter, and the laser diode performs the E/O conversion. The delay line corresponds to an optical fiber roll 405 m long acting as an optical feedback loop. The fiber loop allows the re-injection into the oscillator of a delayed replica of a fraction of the laser diode optical output via the RTD-PD, improving substantially the stability of free-running oscillations. Figure 3. Simple and lower cost RTD-OEO configuration employing direct modulation of laser diode with RTD RF current oscillations. The arrangement of Fig. 3 is a much simpler and low cost configuration when compared with the topology of Fig. 2. However, its operation frequency is limited by laser maximum achievable laser modulation frequency, which in this experiment was around 11 GHz, since the RTD-PD works a considerable higher frequency. In what follows, we present and discuss the self-injection locking results of the two RTD-OEO configurations introduced and evaluate their performance in terms of frequency stability and signals phase noise levels.
3. EXPERIMENTAL RESULTS AND DISCUSSION 3.1 Results of RTD-OEO using external modulation Figure 4(a) shows the free-running OEO spectrum for the RTD-PD-LD oscillator biased in the NDR region (~1.4 V) and without feedback (the optical loop is open). The oscillation frequency is around 1.3969 GHz. Due to the low quality factor of the RTD oscillator the frequency stability and phase noise are poor, as expected. Closing the loop (schematic of Fig. 2) leads to a much more stable behavior. Figure 4(b) shows the oscillator RF spectrum when the in-fiber power level incident in the waveguide RTD-PD is 4.4 mw it is important to mention that only a small fraction of this power is effectively coupled into the waveguide PD because the modal mismatch between the fiber and the semiconductor optical waveguide. The figure shows that the self-injection configuration reduces the phase noise and enhances substantially frequency stability. However, unwanted side modes separated by about 406 khz from the center frequency can be observed because the delay line creates spurious microwave cavity modes, and they increase as in-fiber optical power increases. The side modes separation is called the free spectral range (FSR) of the RTD-OEO and is due to the electrical and optical delays of the RTD-OEO configuration under consideration. The optical delay is a function of the light velocity, the refractive index of the fiber, and of the length the loop. For optical fiber delay lines above 1 km the global delay of the RTD-OEO topology is mainly determined by the equivalent optical delay. In Fig. 4(b) the FSR single mode suppression ratio (SMSR) was about 45 dbc and can be improved by using additional delay lines 5. In order to verify phase noise reduction performance, single-sideband (SSB) phase noises of RTD-OEO output signals were measured. The SSB Phase noise at 100 khz from the center carrier frequency as a function of in-fiber optical power is presented in Fig. 4(c). The phase noise of the open loop free running oscillator was around -78 dbc/hz. Thus, we obtain up to 29 db phase noise reduction with relatively long term frequency stability at moderate optical power levels. Figure 4. (a) Spectrum of free-running oscillation and (b) spectrum trace of self-injection using 4.4 mw of in-fiber optical power level (the frequency span and resolution bandwidth were 1 MHz and 10 khz, respectively). The top part of figures (a) and (b) also show the respective spectrogram of the spectral density recorded over approximately 30 s. (c) SSB Phase noise at 100 khz from the center carrier frequency as a function of in-fiber optical power. The investigation shows that an increase of the in-fiber optical power produces stronger phase noise reduction. We also observed the frequency stability improves with the length of the delay line. However, increasing the fiber length leads to the increasing of unwanted sidemodes closer to the carrier frequency. This can impose some practical limitation to this method of increasing the stability and phase noise performance. 3.2 Results of RTD-OEO using direct modulation of a laser diode The open loop self-sustained oscillation spectrum of the compact RTD-OEO configuration presented above (section 2.3) is shown in Fig. 5(a). The RTD-PD-LD oscillator was biased in the NDR region. There is no great difference between spectrum of Fig. 4(a) and Fig. 5(a), with the compact and the more complex configuration showing similar frequency instability and low phase noise. Figure 5(b) shows the output RF spectrum when the loop is closed (schematic of Fig. 3)
and for 5.4 mw in-fiber power before the coupling to the waveguide RTD-PD. The sidemodes are now separated by about 387 khz with single mode suppression ratio of about 48.6 dbc. Figure 5(c) presents the SSB phase noise of both free-running and self-injection locked signals. From the results, we obtain up to 19 db phase noise reduction at 100 khz offset from the carrier frequency with relatively long term frequency stability at moderate optical power levels: free running -78 dbc/hz @100 khz, and self-injection locked -96.6 dbc/hz. Figure 5. (a) Spectrum of free-running oscillation and (b) spectrum trace of self-injection using 5.4 mw of in-fiber optical power level (the frequency span and resolution bandwidth were 1 MHz and 10 khz, respectively). The top part of figures (a) and (b) also show the respective spectrogram of the spectral density recorded over approximately 30 s. (c) SSB phase noise at 100 khz from the center carrier frequency as a function of in-fiber optical power. 4. CONCLUSION We have investigated optoelectronic oscillator (OEO) topologies based on resonant tunnelling diode (RTD) photodetector oscillators and using an optical fiber delay line energy storage element with low transmission loss carrying the intensity-modulated laser light. This new concept of OEO provides stable and low-phase noise microwave signals taking advantage of the RTD negative differential resistance that provides high-bandwidth gain with attractive applications in photonics and communication systems. The RTD-based OEO topologies presented here can be accessed both optically and electrically, and thus can be easily interfaced with fiber-optic communication links. ACKNOWLEDGEMENT This work was supported by the Fundação para a Ciência e a Tecnologia (FCT) under the project WOWi (PTDC/EEA- TEL/100755/2008). B. Romeira also acknowledges support from FCT through a PhD grant (SFRH/BD/43433/2008) and Fundação Calouste Gulbenkian, Portugal, through the Programa Estímulo à Criatividade e à Qualidade de Investigação 2009. REFERENCES [1] Yao, X. S. and Maleki, L., Optoelectronic oscillator for photonic systems, IEEE J. Quantum Electron. 32(7), 1141-1149 (1996). [2] Romeira, B., Figueiredo, J. M. L., Slight, T. J., Wang, L., Wasige, E., Ironside, C. N., Kelly, A. E. and Green, R., Nonlinear dynamics of resonant tunneling optoelectronic circuits for wireless/optical interfaces, IEEE J. Quantum Electron. 45(11), 1436 1445 (2009). [3] Romeira, B., Figueiredo, J. M. L., Ironside, C. N., Kelly, A. E. and Slight, T. J., Optical Control of a Resonant Tunneling Diode Microwave-Photonic Oscillator, IEEE Photon. Technol. Lett. 22(21), 1610-1612 (2010). [4] Seeds, A. J. and Williams, K. J., Microwave Photonics, J. Lightw.Technol. 24(12), 4628 4641 (2006).
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