Open Access
Add to BibSonomyAbstract
Performance improvement of a directly modulated 10Gb/s OFDM system by optical injection of monolithically integrated lasers is shown experimentally over differing fibre lengths. The modulation and optical injection is performed using monolithically integrated Discrete Mode lasers. It is shown that optical injection with this device reduces third order inter-modulation distortion by up to 10dB and this results in an improvement in system performance from above a forward error correction BER threshold of 1 × 10−3 to significantly below it.
© 2011 Optical Society of America
1. Introduction
Recently Orthogonal Frequency Division Multiplexing (OFDM) has attracted much research interest as a modulation technique for use in optical communication systems [1]. OFDM’s popularity is due to its inherent compact arrangement of overlapping, but orthogonal, subcarriers which offers high spectral efficiency. Perhaps OFDM’s most important property, particularly when considering it for use in optical communication systems, is its ability to convert a time dispersive channel into a circular convolution - facilitating the construction of a simple maximum likelihood equalizer in the frequency domain. Provided the channel remains linear the orthogonal subcarrier property is preserved ensuring there is zero Inter Symbol (i.e. subcarrier) Interference (ISI). Recent research has explored the potential use of OFDM in optical access networks [2, 3]. In these types of networks it is desirable to make use of directly modulated lasers because of their low cost and small footprint relative to transmitters which employ external modulators. Moreover, directly modulated lasers avoid the problems of polarization dependence and high insertion loss associated with external modulators. A limiting factor of this technique is the nonlinearity introduced while directly modulating a laser. This stems from nonlinear interactions between photons and carriers in the modulated laser’s cavity and is introduced at frequencies close to the resonant peak (or relaxation oscillation) of the laser’s modulation response [4]. As the performance of OFDM depends greatly on the linearity of the channel, this nonlinear effect places a strict limit on the usable direct modulation bandwidth. It is thus proposed to use optical injection of a modulated laser (slave) by another laser (master), with which it is monolithically integrated, to extend the usable bandwidth of the system by reducing the laser nonlinearity in the frequency range of interest. In this work we make use of single mode lasers designed using slotted waveguides (discrete mode laser), which can reduce the cost compared to conventional DFB laser structures [5]. The use of discrete mode (DM) lasers and monolithic integration makes this technique viable for cost effective optical communication systems.
2. Optical injection
It has been shown that by injecting light from an external laser (the master) into the cavity of a modulated laser (the slave), the range of linear operation can be expanded [6]. This is indicated by the shift in frequency of the resonant peak of the modulation response under injection conditions. Optical injection is achieved by coupling light from the master laser into the cavity of the slave laser. The resultant effect is to significantly reduce nonlinearities introduced in the directly modulated slave laser. Although the use of optical injection into directly modulated lasers has been employed in systems using other modulation formats before [7, 8], this technique holds particular importance for OFDM whose performance is highly degraded upon the introduction of nonlinearity to the optical system.
3. Dual section laser device
The particular device used for this work consisted of two monolithically integrated DM lasers. The fabrication of the monolithic integrated circuit involves building the devices into a common substrate so that all photonic couplings occur within the substrate and all functions are consolidated into a single, physically unique device. This makes the optical injection process simple, cost effective and polarization independent compared to external optical injection from a separate laser. The two sections were monolithically integrated in a master-slave configuration and the resultant single chip was encased in a temperature controlled hermetically sealed high-speed butterfly package. The lasers are electrically isolated by a deeply etched trench between the two sections so that each one can be biased independently. The threshold current of the slave laser is 20mA whereas the threshold current of the master is 12mA. Light from the master section can be injected into the slave section and thus optical injection can be achieved.
A schematic of the device structure, along with an output optical spectrum when optical injection is employed, is shown in Fig. 1 where HR is a high reflective coating. The basic structure of both sections is that of a Fabry-Pérot (FP) laser but with additional slots which facilitate single wavelength lasing by suppressing all but one of the optical modes [5].
Fig. 1 Monolithically integrated dual section device and output optical spectrum from master-slave configuration.
4. Experiment
The experimental setup is shown in Fig. 2. A 10Gb/s OFDM signal with 16-QAM on all subcarriers was created. Initially a complex baseband signal, the real and imaginary components were then digitally mixed with the In-Phase (I) and Quadrature (Q) components of a 1.7GHz RF carrier resulting in a real OFDM signal centred at 1.7GHz. The incorporation of a 12.5% cyclic prefix (CP) and the 0.7Gb/s needed for forward error correction (FEC) brought the raw data rate to 11.95Gb/s. The OFDM signal therefore comprised of 77 subcarriers and its total bandwidth was 3GHz. An OFDM symbol rate of 39.06MHz was used. The signal was generated using a 10GSa/s AWG and used to drive the slave laser which was biased at 30mA (1.5 times threshold) and emitted light at 1538.59nm. For optical injection the master was biased at 24mA (twice threshold) and this resulted in the slave output wavelength changing to 1538.98nm. The transmission performance over 0km, 25km and 50km of standard single mode fibre (SSMF) was measured. The optical signal was detected using an Avalanche Photodetector (APD) with an integrated TIA and captured with a Real Time Oscilloscope. Channel estimation, equalisation and Bit Error Ratio (BER) calculation were completed offline using Matlab. The performance of the 10Gb/s OFDM signal was recorded for all fibre lengths under non-injected and injected conditions. System performance was also evaluated at various received optical powers. Nonlinearity levels due to direct modulation of the slave section and its reduction due to optical injection from the master are estimated by inspecting resultant Third Order Inter-Modulation Distortion (IMD3) from a two tone test [10].
Fig. 2 The experimental setup including offline signal generation, an Arbitrary Waveform Generator (AWG), integrated DM lasers (master and slave), Variable Optical Attenuator (VOA), Avalanche Photodetector (APD), Transimpedance Amplifier (TIA), Real Time Scope and offline channel estimation, equalization and bit error ratio measurement. The insets show transmitted and received electrical spectra.
5. Results and discussion
Figure 3 shows the superimposed results of two tone tests performed on the device for injection and non-injection conditions. The slave section is directly modulated with two tones and the output optical signal is detected and then recorded while the slave alone is lasing, and also while both the master and slave are biased to achieve optical injection. By showing the level of the IMD3 (which occurs only in the presence of nonlinearity) in the received signal, the test provides an indication of the level of nonlinearity introduced to the system by direct modulation. By performing this test for both operating conditions, the reduction in nonlinearity due to optical injection can be estimated. Shown in Fig. 3 is a decrease of 10dB, from −15dB to −25dB, in IMD3 when optical injection is employed. Here the tone spacing is set to the OFDM subcarrier spacing used for transmission: 39.06MHz. Shown in Fig. 4 is the reduction in IMD3 gained by using optical injection (measured from the two tone test) at various frequencies across the frequency band of interest. The graph clearly shows how nonlinearity is reduced to a greater extent as the test was performed at frequencies approaching the slave laser’s range of nonlinear operation. The reduction in IMD3 decreases after 3GHz as test frequencies surpass this nonlinear region.
Table 1 summarises the performance in terms of BER of the system, as well as showing the system improvement attained by employing injection. All measurements were taken at a received optical power of −17dBm. Higher optical powers resulted in saturation of the receiver. Major improvements due to optical injection are displayed in all cases. The level of improvement between the injected and non-injected case over 50km is slightly reduced due to dispersive fading which begins to dominate system performance. Dispersive fading occurs due to the double side band nature of the signal and can degrade performance [9]. Figure 5 shows the received OFDM spectra, under injection conditions, over 25km and 50km. The effect of dispersive fading is more evident over 50km as higher frequency powers are reduced to close to 10dB below the maximum.
Table 1. BER of Received 10Gb/s OFDM Signals under Injection and Non-Injection Conditions
Figure 6a shows the modulation response of the integrated laser device under non-injected and injected conditions. With injection the resonant peak value moves from 2.58GHz to 4.27GHz indicating the expansion of the linear range of operation. The insets show the received constellations of all channels of the OFDM signals received under both conditions. In this case, over 25km of SSMF, the BER decreases from 1.3×10−3 to 3.76×10−6 when optical injection is used. This corresponds to a reduction in average Error Vector Magnitude (EVM), across all subcarriers of 3.83%. This expansion of the linear region of operation by injection can be attained for high and low bandwidth lasers and is not limited to integrated devices. However the use of low cost, moderate bandwidth DM lasers with integration improves the cost effectiveness, making this approach especially attractive for low cost optical communication systems.
Fig. 6 (a) shows the modulation responses of the integrated device under injection and without injection. The insets show the respective received constellation diagrams (all OFDM channels). (b) shows the received optical power versus log10(BER) for back to back and 50km of SSMF.
Figure 6b shows the improvement in performance in the back to back and 50km cases gained by employing optical injection. The difference in receiver sensitivity between the two injected cases can be attributed to the dispersive fading of the received RF signal which is evident over 50km of fibre but not in the back to back scenario. The free running cases displayed an error floor close to a BER of 1 × 10−3, caused by the nonlinear nature of the channel when injection is not used. By injecting light into the modulating laser, this nonlinearity is reduced and the error floor is improved with the system no longer being limited by nonlinearity but rather by receiver saturation.
6. Conclusion
Due to its cost effectiveness, and high spectral efficiency, direct modulation optical OFDM is a promising technology for use in next generation optical access networks. The use of optical injection with integrated DM lasers can be used to overcome the limitations imposed by the nonlinear properties of the modulating laser. Indeed, employing this technique with the integrated device under test resulted in a large reduction (≤ 10dB) in IMD3.
Optical injection is shown to significantly improve the performance of a direct modulation optical OFDM system. Results presented show how over 25km and 50km system performance improves from a BER floor above the FEC limit of 1 × 10−3 to substantially below it, thus rendering transmission viable. Expanding the linear range of operation of a directly modulated laser by optical injection is a technique which can improve the performance of direct modulation OFDM systems employing all laser transmitters. The integration of two DM lasers operating in master-slave configuration makes optical injection feasible for use in low cost optical OFDM communication systems because of its small size and cost.
Acknowledgments
The authors would like to acknowledge Achray Photonics for the high speed packaging of the monolithically integrated discrete mode lasers. The work presented in this paper has been supported by the Science Foundation Ireland (SFI) Principal Investigator, SFI PIFAS cluster and HEA PRTLI4 funding agencies.
References and links
1. J. Armstrong, “OFDM for optical communications,” J. Lightwave. Technol. 27, 189–204 (2009). [CrossRef]
2. P. Chanclou, Z. Belfqih, B. Charbonnier, T. Duong, F. Frank, N. Genay, M. Huchard, P. Guignard, L. Guillo, B. Landousies, A. Pizzinat, H. Ramanitra, F. Saliou, S. Durel, A. Othmani, P. Urvoas, M. Ouzzif, and J. Le Masson, “Optical access evolutions and their impact on the metropolitan and home networks,” in Proceedings of the 34th European Conference on Optical Communication, ECOC 2008 , 1–4 (2008).
3. N. Cvijetic, D. Qian, and J. Hu, “100 Gb/s optical access based on optical Orthogonal Frequency Division Multiplexing (OFDM),” IEEE Commun. Mag. 48, 70–77 (2010). [CrossRef]
4. G. H. M. van Tartwijk and D. Lenstra, “Semiconductor lasers with optical injection and feedback,” Quantum Semiclassic. Opt. 7, 87–143 (1995). [CrossRef]
5. C. Herbert, D. Jones, A. Kaszubowska-Anandarajah, B. Kelly, M. Rensing, J. O’Carroll, R. Phelan, P. Anandarajah, P. Perry, L. P. Barry, and J. O’Gorman, “Discrete mode lasers for communication applications,” (Invited paper) IET Optoelectron. 3, 1–17 (2009). [CrossRef]
6. G. Yabre and J. L. Bihan, “Reduction of nonlinear distortion in directly modulated semiconductor lasers by coherent light injection,” J. Quantum Electron. 33, 1132–1140 (1997). [CrossRef]
7. F. Smyth and L. P. Barry, “Effects of laser diode nonlinearities in hybrid fibre/radio Systems,” Proc. SPIE 4876, 159–167 (2002). [CrossRef]
8. A. Kaszubowska, P. M. Anandarajah, and L. P. Barry, “Improved performance of a hybrid radio/fiber system using a directly modulated laser transmitter with external injection,” IEEE Photon. Technol. Lett. 7(2), 233–235 (2002). [CrossRef]
9. J. Han, B. J. Seo, Y. Han, B. Jalali, and H. R. Fetterman, “Reduction of fiber chromatic dispersion effects in fiber-wireless and photonic time-stretching system using polymer modulators,” J. Lightwave Technol. 21, 1504–1509 (2009).
10. X. J. Meng, T. Chau, and M. C. Wu, “Improved intrinsic dynamic distortions in directly modulated semiconductor lasers by optical injection locking,” IEEE Trans. Microwave Theory Tech. 47, 1172–1176 (1999). [CrossRef]
