resonant tunneling diode optoelectronics. C. N. Ironsidea, Mohsin Hajia, Lianping Houa, Jehan Akbara, Anthony E. Kellya, K. Seunarinea,. Bruno Romeirab ...
Review of optoelectronic oscillators based on modelocked lasers and resonant tunneling diode optoelectronics C. N. Ironsidea, Mohsin Hajia, Lianping Houa, Jehan Akbara, Anthony E. Kellya, K. Seunarinea, Bruno Romeirab, José M. L. Figueiredob a School of Engineering, University of Glasgow, G12 8LT Glasgow, UK; b Department of Physics, Center of Electronics, Optoelecronics and Telecommunications (CEOT), University of Algarve, 8005-139 Faro, Portugal ABSTRACT Optoelectronic oscillators can provide low noise oscillators at radio frequencies in the 0.5-40 GHz range and in this paper we review two recently introduced approaches to optoelectronic oscillators. Both approaches use an optical fibre feedback loop. One approach is based on passively modelocked laser diodes and in a 40 GHz oscillator achieves up to 30 dB noise reduction. The other approach is based on resonant tunneling diode optoelectronic devices and in a 1.4 GHz oscillator can achieve up to 30 dB noise reduction. Keywords: Delay line, modelocked laser diode, microwave photonics, optoelectronic oscillator, resonant tunneling diode
1. INTRODUCTION Optoelectronic oscillators (OEO) consist of an optical and/or electronic gain component coupled with a component that can store an optical signal, preserve its phase and act as a high-Q resonator - these OEOs can be used as RF oscillators that have very low noise and narrow linewidths. The first implementations of these oscillators were reported in 1996 and used optical fibre as the storage component of the oscillator1. In the work reported here we still use optical fibre as the storage element but we have considerably simplified the complexity of OEOs and extended them to much higher RF frequency up to 40GHz has been obtained - although this limit has essentially been set by the bandwidth of test equipment that we have available. We discuss two types of OEOs, one based on monolithically modelocked laser diodes – the MLLD, and the other based on an optoelectronic integrated circuit (OEIC) a resonant tunneling diode integrated with a photodiode – the RTD-PD.
2. MODELOCKED LASER OPTOELECTRONIC OSCILLATORS 2.1 The monolithically modelocked laser diode optoelectronic oscillator (MLLD-OEO) Reversed biased sections of a laser diode act as saturable absorbers and this makes passive modelocking of laser diodes relatively easy to implement. Although easy to implement, passive modelocking suffers from the disadvantage of relatively large pulse jitter. However, it is a promising candidate for compact and low cost optical pulse sources for use in optical clocking, clock recovery, and optoelectronic oscillators (OEOs)2. Quantum well (QW) materials in particular are excellent platforms for fabricating mode locked laser diodes (MLLD), however, the susceptibility of spontaneous emission noise and intercavity losses makes passive MLLDs prone to broad linewidths and therefore substantial phase noise. Currently, various methods are used to reduce the phase noise by synchronizing the pulses to an external RF electrical clock via hybrid3 or synchronous mode locking4, however, these require high frequency electronic driving oscillators and subsequent device probing technicalities. Here we discuss a simplified and inexpensive method of reducing the linewidth and jitter using an all optical regenerative mode locking (AO-RML) technique. A length of optical fiber is used to relay the output pulse stream of the laser back into the SA facet of the MLLD to induce a self-intensity beating, and thus, self-phase referencing of the pulses. The memory of the phase is then extended and is relative to the length of the fiber loop. The main advantage of
this technique is that it is not limited to bandwidth restrictions imposed by electrical components and can therefore be scaled to operate at much higher repetition rate frequencies. Figure 1 shows a schematic of the experiment setup used to carry out the AO-RML technique. A 40 GHz two-section MLLD, fabricated on a five-quantum-well AlGaInAs/InP epitaxial structure was used for this experiment. Further details about the device structure and fabrication procedures can be found in5. The output of the laser was around 1550 nm and was coupled into a lensed fiber, which was fed through a circulator to minimize back reflections from component interfaces. The signal was fed into a dispersion compensating erbium doped fiber amplifier (EDFA) before being coupled back into the SA end of the MLL cavity. A polarization controller was positioned to ensure the injected signal was in TE mode in order to promote better carrier coupling effects in the QWs. A 3 dB fiber splitter was used to form an output arm for subsequent signal and spectrum analysis.
Figure 1. Schematic drawing of experimental setup: SA saturable absorber section of the laser diode reverse biased; Gain section of the laser forwarded biased, coupled to optical fibre, EDFA erbium doped fibre amplifier, polarization control used to make it TE mode.
The laser was passively mode locked when the gain section was forward biased with 53 mA and a reverse bias of -2.3 V was applied to the SA section. The resulting output as viewed on a RF spectrum analyzer via a high speed photodetector (matched up to 45 GHz) is shown in Fig. 2 (red dashed trace). The linewidth at -3 dB from the peak was 1.6 MHz (10 kHz RBW, 1 kHz VBW), with a phase noise of -68 dBc/Hz at a 1 MHz offset.
Figure 2. Comparison of RF spectrum of the AO- RML operation and free running MLLD modelocked with a repetition rate close to 40GHz: Insert corresponding phase noise measurements.
The corresponding RMS timing jitter was 5.1 ps (integrated from 4 – 80 MHz). For a comparison, the linewidth at the output of the RML loop is also shown in Fig. 2 (blue trace). The MLL was operated under the same conditions. The EDFA was used to supply enough gain to operate above threshold (i.e. it was the dispersion compensating element of the EDFA that was favored for this experiment, and not gain amplification). A noticeable reduction of the linewidth was observed on the RF spectrum analyzer. The linewidth was reduced to less than 20 kHz, with a phase noise of -96 dBc/Hz at a 1 MHz offset, and an RMS jitter of 650 fs. The pulse width, measured using a second harmonic generation (SHG) autocorrelator, was 2 ps assuming a sech2 pulse shape. The pulse width and optical spectra remained unchanged under the AO-RML technique, which implies that the dispersion in the optical fiber had minimal effect. The total average output power was 4 mW. Although the linewidth was significantly enhanced, some additional supermode noise spurs were apparent due to harmonic mode locking effects. These were formed by the modal interactions of the fiber cavity resonance, which gave rise to large amplitude and phase fluctuations separated in frequency at ~10 MHz intervals, determined by the fiber cavity length. Increasing the EDFA gain by more than 3 dB would further increase and extend the bandwidth of supermodes generated as a function of pump power, due to the increasing signal propagation around the fiber loop. A substantial increase in the EDFA gain (> 5 dB) would eventually degrade the signal such that the autocorrelation trace of the emitted pulse train was distorted. There are currently a number of methods used to suppress the supermode noise, such as those used in harmonically mode locked fiber lasers6.In this work, the composite cavity loop (CCL) technique was used since it is, in comparison, considered the most adaptable method for this experiment. The CCL is formed using a sub cavity within the fiber loop, with a length that can also be arbitrarily long providing its modes coincide with the fundamental fiber cavity at large multiples of the free spectral range (FSR). This reduces the number of modes in the fiber cavity, thereby lessening the effect of supermode noise. The modified experimental setup for the AO-RML using a CCL is shown in Fig. 3.
Figure 3. Compare with figure(1) an extra branch of the external optical fibre loop has been inserted to suppress the supermodes –this arrangement is called the composite cavity loop (CCL).
For this configuration, a 20 GHz mode locked laser fabricated on a three-quantum-well based AlGaInAs/InP material structure was used to test the repeatability of the AO-RML technique, while being observable on the RF spectrum analyzer, which was limited to 40 GHz. Further details regarding the device structure can be found in7. The laser was passively mode locked when 80 mA was supplied to the gain section, and -2.8 V was applied to the SA. The pulse width was measured at 1.8 ps, and the total average output power was 5 mW. Two 3 dB couplers were used to form a CCL, with an optical delay line positioned in between to enable fine tuning of the cavity length, such that the modal spacing could be accurately defined. Here, the CCL was tuned at ~13 m, which corresponds to an overlap of every third mode of the fundamental fiber cavity loop. Figure 4 shows a comparison of the level of supermode noise as the CCL length was tuned. When the modes of the CCL corresponded with every third mode of the fundamental (outer) cavity loop, the supermodes were considerably reduced from -70 dBm to below -115 dBm. The 3 dB linewidth of the free running 20 GHz MLL was 230 kHz (20 Hz RBW, 10 Hz VBW) (Fig. 5, red dashed trace), with a phase noise of -72 dBc/Hz at a 1 MHz offset (Fig. 6, red dashed trace). The resultant 3 dB linewidth of the signal
after AO-RML via CCL was measured at 427 Hz (Fig. 5, blue trace), with a phase noise of -108 dBc/Hz at a 1 MHz offset (Fig. 6, blue trace). The RMS jitter was 363 fs, which is an improvement from that of the free running MLLD (1.5 ps). The pulse width was 2.1 ps, which remained somewhat unaffected, with only a 0.3 ps increase in pulse width possibly as a result of dispersion in the additional fiber forming the CCL.
Figure 4. The top figure shows the system running without the CCL and clearly show the supermode; the bottom figure show the suppression of the supermeodes after CCL has been included.
Figure 5. A detail of the RF spectrum of the CCL system showing the linewidth of the OEO operating in the AO-RML configuration.
Figure 6. Single sideband power spectral density of the free running MLL (red dashed), and while under AO-RML operation with minimal supermode noise.
To summarize, a novel method of reducing phase noise and linewidth of a PMLLD has been described, resulting in low linewidths (427 Hz) and sub-picosecond RMS jitter values (363 fs).A CCL structure was incorporated to the experimental setup to reduce the effects of supermode noise, and further improve the jitter reduction. The technique, known as AO-RML, was shown to work successfully on a 40 GHz and 20 GHz MLL and can theoretically be scaled up to terahertz frequencies. We expect further improvement of the linewidth and phase noise by incorporating a longer fiber cavity length, thereby extending the memory of the phase. Further work will also include monolithically integrating the loop design for on-chip linewidth enhancements, which may be promising for the development of compact, low cost and low noise OEOs.
3. RESONANT TUNNELLING DIODE OPTOELECTRONIC OSCILLATORS 3.1 The Resonant Tunnelling Diode Photodetector Optoelectronic Oscillator (RTD-PD-OEO) Resonant tunnelling diodes can be relatively easily integrated with optoelectronic components and provides electronic gain in the same chip as optical components such a photodetector, laser or modulator – see8 for a review. In this section we discuss how a resonant tunneling diode photodetector (RTD-PD) can provide enough electronic gain to drive a laser diode and thus an optoelectronic oscillator. Figure 7 shows the layout of the RTD-PD-OEO. The RTD-PD is configured as a waveguide photodiode that uses the negative differential resistance (NDR) of the DC biased RTD to provide broadband electrical gain to the chip9. The electrical output from the chip is sufficient to drive the laser diode that is coupled into the optical fibre. The optical fibre loops back to the optical input of the RTD-PD and thus completes the OEO. Fundamentally, the RTD has a very broad bandwidth response and in this set-up the frequency of oscillation is determined by the circuit parasitics – in this case the bond wire between the RTD-PD chip and the laser diode chip.
Figure 7. (a) Shows the layout of the RTD-PD chip and (b) shows how the RTD-PD is included in the OEO system. (c) Current-voltage curve of the device with the NDR region.
The RF spectra of the electrical output from the RTD-PD-OEO operating in the free-running mode and with selfinjection are shown in Fig. 8 for various lengths of optical fibre. In the self-injection mode, supermodes between 35 and 40 dBc are observed due to optical fiber.
Figure 8. The RF spectra of the electrical output of RTD-PD-OEO with various lengths (b), (c) and (d) of optical fibre compared to the free running RTD-PD with no optical fibre (a) – the reduction of the linewidth of the oscillator is apparent.
The phase noise characteristics of the RTD-PD-OEO are shown in Fig. 9 kHz as a function of in-fiber optical power and varying the optical fibre length. The phase noise can be enhanced by increasing either the fiber optical loop length or the optical power level. The results show phase noise reduction below -100 dBc/Hz at 10 KHz offset from the carrier frequency which corresponds to about 30 dB phase noise reduction when compared with RTD-PD free-running oscillations.
Figure 9. (a) RTD-OEO SSB phase noise at 10-kHz as a function of in-fiber optical power for L=0.814 km and L=1.219 km. (b) Measured SSB phase noises for P~8.6 dBm.
From these RTD-PD-OEOs, the results show that longer fibre and more power coupled in the fibre results in lower phase noise and thus narrow linewidths and there are considerable improvement compared to the free running RTD-PD. The system presented here is very simple, providing an OEO configuration without the need of extra RF or optical amplification.
4. CONCLUSION: COMPARISON OF THE MLLD-OEO AND RTD-OEO The MLLD-OEO and RTD-PD-OEO are both of similar complexity and both considerably simpler than other OEOs. The MLLD-OEO includes an EDFA not required by the RTD-PD-OEO which has sufficient electronic gain to operate without requiring the optical gain supplied by an EDFA. Presently the RTD-PD-OEO operates at much lower frequency but both have approximately the same low phase noise performance with a floor at around -120dBc/Hz. Neither has yet reached the low noise performance of the best commercially available OEO – they have noise floors of around -160 dBc/Hz but have moved away from optical fibre as the high-Q optical signal storage component10. However, at 40 GHz the MLLD-OEO is the highest frequency OEO yet reported and furthermore by using shorter LDs or harmonically modelocked LDs and low dispersion optical fibre their operating frequency could be extended 100s of GHz and perhaps as far as 2 THz.
REFERENCES [1] Yao, X. S. and Maleki, L., “Optoelectronic oscillator for photonic systems,” IEEE J. Quantum Electron. 32(7), 1141-1149 (1996). [2] Hou, L., Stolarz, P., Dylewicz, R., Haji, M., Javaloyes, J., Qiu, B. and Bryce, A. C. “160-GHz Passively Mode-Locked AlGaInAs 1.55-μm Strained Quantum-Well Compound Cavity Laser ”, IEEE Photon. Technol. Lett. 22(10), 727-729 (2010). [3] Martijn, J.R. Heck, Edcel J. Salumbides, Amandine Renault, Erwin A.J.M. Bente, Yok-Siang Oei, Meint K. Smit, René van Veldhoven, Richard Nötzel, Kjeld S. E. Eikema, and Wim Ubachs, “Analysis of hybrid modelocking of two-section quantum dot lasers operating at 1.5 μm,” Opt. Expr. 17(20), 18063-18075 (2009). [4] Arahira, S. and Ogawa, Y., “Synchronous mode-locking in passively mode-locked semiconductor laser diodes using optical short pulses repeated at subharmonics of the cavity round-trip frequency,” IEEE Photon. Technol. Lett. 8(2), 191-193 (1996). [5] Hou, L., Staler, P., Javaloyes, J., Green, R.P., Ironside, C.N., Sorel, M. and Bryce, A.C., “Subpicosecond pulse generation at quasi-40-GHz using a passively mode-locked AlGaInAs-InP 1.55-μm strained quantum-well laser,” IEEE Photon. Technol. Lett. 21(23), 1731-1733 (2009). [6] Quinlan, F., Ozharar, S., Gee, S. and Delfyett, P. J., “Harmonically mode-locked semiconductor-based lasers as high repetition rate ultralow noise pulse train and optical frequency comb sources,” J. Opt. A. 11(10), 103001 (2009). [7] Hou, L., Haji, M., Akbar, J., Qui, B. and Bryce, A. C., “Low divergence angle and low jitter 40 GHz AlGaInAs/InP 1.55 μm mode-locked lasers,” Opt. Lett., 36(6), 966-968 (2011).
[8] Figueiredo, J. M. L., Romeira B., Slight, T. J. and Ironside, C. N., “Resonant Tunnelling Optoelectronic Circuits,” in Advances in Optical and Photonic Devices Edited by Ki Young Kim, ISBN 978-953-7619-763, 352 pages, http://sciyo.com/articles/show/title/resonant-tunnelling-optoelectronic-circuits (2010) [9] 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). [10] http://www.oewaves.com/.
