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s direct modulation optical OFDM by external optical injection
Optics Communications 285 (2012) 136–139
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Performance enhancement of 10 Gb/s direct modulation optical OFDM by external optical injection Colm Browning ⁎, Kai Shi, Frank Smyth, Barry Cardiff, Prince M. Anandarajah, Liam P. Barry The Rince Institute, Dublin City University, Glasnevin, Dublin 9, Ireland
a r t i c l e
i n f o
Article history: Received 9 June 2011 Received in revised form 2 August 2011 Accepted 3 September 2011 Available online 20 September 2011 Keywords: Orthogonal Frequency Division Multiplexing (OFDM) Optical injection Direct modulation
a b s t r a c t We show experimentally and by simulation a performance enhancement of a directly modulated 10 Gb/s optical Orthogonal Frequency Division Multiplexing (OFDM) system due to external optical injection. The experiment is performed back to back and over 12 km of single mode fiber. The injection extends the range of linear operation of the laser and therefore extends the usable bandwidth for direct modulation formats which are susceptible to nonlinearity, such as OFDM. Nonlinearity in the system and its reduction due to injection are estimated by means of a two tone test. Additionally the performance enhancement on OFDM systems was verified in both simulation and experimentally by the comparisons of the average Bit Error Rate (BER) and Error Vector Magnitude (EVM). © 2011 Elsevier B.V. All rights reserved.
1. Introduction Orthogonal Frequency Division Multiplexing (OFDM) has recently been the subject of much research as a multicarrier modulation technique in optical communications systems [1]. It is widely used in both wired and wireless broadband communications systems and is specified for in WPAN, WLAN, ADSL and WiMAX standards. OFDM's popularity stems from its high bandwidth efficiency due to the compact arrangement of overlapping, but orthogonal, subcarriers; additionally the inclusion of a cyclic prefix (CP) converts the time dispersive channel into a circular convolution facilitating the construction of a simple maximum likelihood (ML) equalizer in the frequency domain. This frequency domain ML equalizer is implemented as a bank of one-tap equalizers and can compensate for any linear impairments introduced by the channel. Its simple construction relative to other time domain equalizers, and the fact that it facilitates good performance in the presence of dispersion contribute to OFDM's suitability for use in optical communications systems. Provided the physical channel is linear and the receiver is correctly synchronized, the orthogonal subcarrier property is preserved ensuring that there is zero Inter Symbol (i.e. subcarrier) Interference (ISI). The cost effectiveness and network flexibility that is offered by OFDM, coupled with its ability to facilitate speeds of up to 100 Gb/s, make it a promising candidate for future broadband optical access networks (≥10 Gb/s) [2]. In these types of networks, for which dynamic bandwidth allocation and network reconfigurability will be key attributes, it is important to maintain cost efficiency due to high market volume. OFDM will make it feasible to realize these attributes by allowing the ⁎ Corresponding author. E-mail address: [email protected] (C. Browning). 0030-4018/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.09.009
assignment of different subcarriers to different users. As this will be implemented electronically using advanced digital signal processing (DSP) expensive optical components are avoided and greater network flexibility is offered. Furthermore, linear impairments are compensated for in a computationally efficient way [1] in the frequency domain unlike other multiplexing techniques such as Time Division Multiplexing (TDM) which employ complex time domain equalizers. In designing a cost effective broadband optical access network it is desirable to make use of directly modulated lasers. This is because of their low cost and small size relative to transmitters that employ external optical modulators. Furthermore, the problems of polarization dependence and high insertion loss associated with such transmitters are avoided. OFDM is a candidate for use in future access networks and indeed recent research has looked at direct modulation optical OFDM (DMO-OFDM) in optical access networks [3] [4]. One limiting factor in these systems is due to nonlinearity introduced when directly modulating a laser at frequencies near its resonant peak — this coupled with the aforementioned property of OFDM where it requires a linear transmission system means that the usable bandwidth is limited. This paper shows the utilization of external optical injection into the directly modulated laser's cavity to shift its nonlinear range of operation, and hence improve the overall system performance. This technique may be applied to directly modulated lasers of various bandwidths to expand their linear range of operation. Furthermore, photonic integration would make this technique suitable for use in low cost optical communications systems. External optical injection has been performed previously to enhance the system performance with different data formats [5] [6] but holds particular importance for OFDM. This paper demonstrates the first use of optical injection to improve linearity, and thus overall system performance in a direct modulation OFDM system.
C. Browning et al. / Optics Communications 285 (2012) 136–139
2. Nonlinearity and external optical injection Information about the channel is transmitted to an OFDM receiver by means of a known training sequence which contains all subcarrier frequencies. The training sequence along with the cyclic prefix (CP), which ensures a full copy of each subcarrier is obtained at the receiver (thereby maintaining orthogonality), allows channel estimation and equalization at the receiver. This is because the effect of any linear impairments introduced by the channel (most importantly dispersion), on any subcarrier frequency can be estimated by analyzing the received training sequence. The subsequent data carrying portion of each subcarrier is adjusted accordingly. However this equalization method cannot be applied to compensate for nonlinear distortions as the loss of subcarrier orthogonality due to the nonlinear nature of the channel occurs before the signal is downconverted to the electrical domain by photodetection (itself a nonlinear process), and this results in Inter Symbol (i.e. subcarrier) Interference. When directly modulating a laser, nonlinear interactions between the carriers and the photons in the laser cavity occur. This gives rise to nonlinear operation at certain frequencies. This nonlinear range of operation is indicated by a resonant peak, or relaxation oscillation, in the laser's modulation response [7]. It has been shown that the linear range of operation can be expanded by coupling the light from an external laser into the cavity of the modulated laser (master–slave configuration) [8]. This is useful for direct modulation OFDM systems, which are highly intolerant to nonlinearities, because it extends the system bandwidth limitation imposed by nonlinearity due to direct modulation or simply improves the performance of fixed bandwidth OFDM transmission. 3. Direct modulation OFDM system Two 10 Gb/s OFDM signals with 32 and 64-QAM respectively on all subcarriers were generated. The real and imaginary components of these complex baseband signals were then digitally mixed with the In-Phase (I) and Quadrature (Q) components of an RF carrier to give real valued OFDM signals centered on a given RF frequency above DC. An OFDM symbol rate of 39.06 MHz was used and the bandwidths of the 32 and 64QAM signals were 2.265 GHz and 1.916 GHz respectively. One OFDM symbol (0.7% of the entire OFDM signal) was set to be the training sequence so as to facilitate channel estimation and equalization at the receiver. A Cyclic Prefix (CP) of 6.25% was used. This resulted in a CP length of 1.6 ns which is significantly longer than the maximum delay spread caused by dispersion over 12 km of SSMF for the given signal bandwidths, ensuring subcarrier orthogonality at the receiver. These overheads along with 0.7 Gb/s, reserved for forward error correction (FEC), meant that the raw data rate was 11.395 Gb/s. The experiment was performed for the back to back case and also over 12 km of SSMF, with and without optical injection of a single mode laser. 3.1. Simulation Firstly, this complete OFDM system was simulated in Matlab. The signals were generated in Matlab as per the above specifications. Direct modulation of the laser and external injection were modeled using the laser rate equations, including the injection terms, derived in [9]. The simulated laser device parameters [10] are presented in Table 1 and were used, along with the input electrical drive signal, to solve the extended laser rate equations with injection terms. As shown in the table, the injected photon density was 5 ×1021cm− 3 and this corresponded to an optical injection power of 1.5 dBm. The effects introduced by propagating over standard single mode fiber (SSMF), namely nonlinearity, dispersion and loss, were simulated by modeling the propagation of the electric field through the fiber using the nonlinear Schrödinger equation (NLSE). The equation was solved using the split step Fourier method for a number of steps over a length
137
Table 1 Simulated laser parameters. Parameter
Value
Wavelength, λ Group Refractive Index, η Optical Confinement Factor, Γ Differential Gain Coefficient, g0 Transparency Density, n0 Facet Reflectivity Coefficient, R Linewidth Enhancement Factor, α Area of Active Region, A Photon Lifetime, τp Carrier Lifetime, τn Injected Photon Density, P0 Coupling Coefficient, κ Detuning, Δf
1540 nm 3.63 0.35 1 × 10− 12cm2 1 × 1018cm− 3 0.32 6.8 0.03 × 10− 12m2 2 ps 0.3 ns 5 × 1021cm− 3 2.5 × 1011s− 1 5 GHz
of 12 km. First and second order terms only were included in the calculation of dispersion. The modeled receiver comprised of a 10 GHz bandwidth PIN photodetector with an integrated trans-impedance amplifier (TIA), including shot noise and thermal noise associated with such receivers, a digital RF mixer, low pass filtering and digital signal processing (DSP) normally associated with OFDM systems. This includes serial to parallel conversion, FFT, channel estimation and equalization, and error vector magnitude (EVM) measurement. Results were taken for both the injected and non-injected cases. In the latter case the injection level was set to zero. 3.2. Experimental Setup As Fig. 1 below shows, the OFDM signals were output from a 10 GSa/ s Arbitrary Waveform Generator (AWG). The slave laser's threshold current was 10 mA and for this experiment was biased at 21 mA — giving a 3 dB bandwidth of about 5 GHz and optical output power of 0 dBm. The slave emitted light at 1540 nm. By mixing the 32-QAM and 64-QAM OFDM signals with intermediate RF frequencies of 2.867 GHz and 3.043 GHz respectively, the highest frequency subcarrier in each case was placed at4 GHz which was inside the slave laser's region of nonlinear operation under free-running conditions. Using an optical circulator and polarization controller, light from the master laser – a tunable External Cavity Laser (ECL) – was coupled into the cavity of the slave laser. The injected optical power was kept constant at 2.8 dBm throughout (measured at circulator port 2). The master was tuned to 1540.05 nm — this corresponds to a detuning of roughly 5 GHz which is well within the locking range for this injection power. The signals were detected using a 10 Gb/s PIN photodetector with an integrated TIA and captured with a Real Time Oscilloscope (RTS). The addition of the Erbium Doped Fiber Amplifier (EDFA) and optical bandpass filter at the receiver provided the necessary sensitivity to allow BER versus received power curves to be measured. All processing was done using Matlab offline. BER was measured experimentally by comparing the transmitted bit sequences (input to the transmitter QAM modulator) and received bit sequences obtained after QAM demodulation. Multiple captures of the OFDM signal by the RTS were aggregated to provide the required number of observed errors for BER measurement. Measurements of EVM from the received constellations were taken for both the injected and non-injected cases. To characterize nonlinearity in the laser, Third Order Intermodulation Distortion (IMD3) was also measured using a two tone test [11], for various frequencies, for both the injected and non-injected scenarios. 4. Results and discussion Table 2 summarizes the simulated and experimental results. Strong agreement exists between the simulated and experimental improvement in EVM. Fig. 2 shows the experimental BER versus received optical
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C. Browning et al. / Optics Communications 285 (2012) 136–139
Fig. 1. The direct modulation optical OFDM experimental setup. Insets show the transmitted and received electrical spectra.
power for the back to back and 12 km cases; injected and free-running. It can be seen in Fig. 2, for both back to back and 12 km scenarios, that the injection enhances the system performance by eliminating the BER floor which appears in the non-injected cases at values close to 10− 3. The fact that there appears to be less of an improvement due to injection after transmission over 12 km is attributed to the performance being limited by non-linearities introduced by the fiber. Fig. 3 shows the average value of EVM per subcarrier number for the 32-QAM OFDM signal for injected and non-injected regimes; experimental and simulated. The effect incurred by modulating close to the resonant peak of the laser and the subsequent improvement on each subcarrier due to injection can clearly be seen in the figure. Average EVM increases with subcarrier number as the subcarrier frequencies increase to within the laser's region of nonlinear operation but this effect is highly reduced by the injection; in this case only a small rise in average EVM is observed due to the extension of the laser's linear range. Fig. 4 shows the result of the two tone test, at 4 GHz, performed on the system under normal and injection conditions. Used to characterize nonlinearity in the optical system, this test indicates the extent to which nonlinearity is reduced due to external injection [11]. The IMD3 seen in Fig. 4 in the non-injected case was the highest value measured and its reduction by optical injection was the highest reduction recorded. Here, IMD3 was measured to be −22.03 dB, relative to the fundamental, in the non-injected case. Injection yielded an IMD3 value of −34.97 dB relative to the fundamental, giving an improvement of 12.94 dB. The tones are separated by the OFDM subcarrier spacing.
Fig. 2. BER vs received optical power, under non-injection and injection conditions, for back to back and 12 km cases.
Table 2 Average EVM for the simulated and experimental received constellations and the improvements due to injection. Order (Simulated — 0 km) 32-QAM 64-QAM (Simulated — 12 km) 32-QAM 64-QAM (Experimental — 0 km) 32-QAM 64-QAM (Experimental — 12 km) 32-QAM 64-QAM
Non-injected
Injected
Improvement
6.96% 7.56%
5.15% 5.02%
1.81% 2.54%
7.9% 7.89%
6.% 5.09%
1.9% 2.8%
7.62% 7.95%
6.07% 6.3%
1.55% 1.65%
8.32% 8.77%
7.28% 6.9%
1.04% 1.87%
Fig. 3. Average EVM per subcarrier for the non-injected (dashed) and injected 32-QAM cases; both experimental (a) and simulated (b). The non-injected case displays increasing average EVM with subcarrier number. The rise in EVM with increasing subcarrier frequency is stemmed in the injected case.
C. Browning et al. / Optics Communications 285 (2012) 136–139
139
Table 3 BER for all cases (experimental). Order (back to back) 32-QAM 64-QAM (12 km) 32-QAM 64-QAM
Fig. 4. Example two tone test result at 4 GHz showing IMD3 for the non-injected (dashed) and injected (solid) cases. The zoomed inset shows the reduction due to injection of the lower frequency IMD3 more clearly.
Non-injected
Injected
2.85 × 10− 4 4 × 10− 3
1.91 × 10− 5 7.2 × 10− 4
7.9 × 10− 4 8.4 × 10− 3
8.5 × 10− 5 1.7 × 10− 3
Fig. 5 shows the received back-to-back 32-QAM constellations of all OFDM symbols under injected and non-injected regimes. The figure shows how as system nonlinearity is decreased under injection conditions each constellation point becomes more well defined. In other words phase and amplitude noise on the received RF constellation, due to nonlinearity, is decreased as the EVM improves from 7.62% (a) to 6.07% (b). How BER relates to EVM depends upon constellation size. This is intuitive when one considers the improvement in signal to noise ratio (SNR) required when moving to higher order QAM. Table 3 tabulates the experimentally measured BER of the signals under both conditions. These performances were achieved at a received optical power of − 3.5 dBm (back to back) and −5.5 dBm (12 km). These results indicate that external optical injection can improve the performance of a DMO-OFDM system and extend the usable bandwidth for the modulating OFDM signals. This is particularly important in the case of cost effective direct modulation systems where cheap moderate bandwidth lasers are employed. It also can potentially enhance system performance in directly modulated OFDM systems operating well beyond 10 Gb/s by reducing nonlinearities. 5. Conclusion Direct Modulation Optical OFDM is a promising technique, particularly for cost effective next generation optical access networks due to its low cost, flexibility and its capability to provide high speed transmission. But OFDM's intolerance to nonlinearities places a performance limitation on a DMO-OFDM system as signal bandwidths increase to the point of entering the directly modulated laser's region of nonlinear operation. Results presented in this paper show how directly modulated OFDM signals are affected by nonlinear interactions in the laser cavity and how utilizing external optical injection to extend the laser's range of linear operation can improve the performance of DMO-OFDM systems. The development of photonic integration will allow for the restrictions of polarization dependence and higher cost imposed by external optical injection to be overcome and hence make this technique feasible for such low cost systems. References
Fig. 5. Experimental estimated 32-QAM constellations of all received OFDM symbols for free running (a) and injected (b) cases.
[1] J. Armstrong, Journal of Lightwave Technology 27 (3) (2009) 189 http://jlt.osa. org/abstract.cfm?URI=JLT-27-3-189. [2] N. Cvijetic, D. Qian, J. Hu, IEEE Communications Magazine 48 (7) (2010) 70, doi:10.1109/MCOM.2010.5496880. [3] D. Qian, J. Yu, J. Hu, N.J. Ji, T. Wang, ECOC 2008. 34th European Conference on, 2008, 2008. [4] 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, J. Le Masson, Optical Communication, 2008, ECOC 2008. 34th European Conference on, 2008, , 2008, p. 1, doi:10.1109/ECOC.2008.4729361. [5] F. Smyth, L.P. Barry, SPIE 2002, 2002, p. 159. [6] A. Kaszubowska, P.M. Anandarajah, L.P. Barry, IEEE Photonics Technology Letters 7 (2) (2002) 233, doi:10.1109/68.980532. [7] G.H.M. van Tartwijk, D. Lenstra, Quantum and Semiclassical Optics 33 (1995) 90. [8] G. Yabre, J.L. Bihan, Journal of Quantum Electronics 33 (7) (1997) 1132. [9] G. Agrawal, N.K. Dutta, Long-Wavelength Semiconductor Lasers, Van Nostrand Reinhold Company, New York, 1986. [10] J. Helms, Journal of Lightwave Technology 9 (11) (1991) 1567, doi:10.1109/50.97648. [11] X.J. Meng, T. Chau, M. Wu, IEEE Transactions on Microwave Theory and Techniques 47 (7) (1999) 1172, doi:10.1109/22.775454.
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Performance enhancement of 10 Gb/s direct modulation optical OFDM by external optical injection Colm Browning ⁎, Kai Shi, Frank Smyth, Barry Cardiff, Prince M. Anandarajah, Liam P. Barry The Rince Institute, Dublin City University, Glasnevin, Dublin 9, Ireland
a r t i c l e
i n f o
Article history: Received 9 June 2011 Received in revised form 2 August 2011 Accepted 3 September 2011 Available online 20 September 2011 Keywords: Orthogonal Frequency Division Multiplexing (OFDM) Optical injection Direct modulation
a b s t r a c t We show experimentally and by simulation a performance enhancement of a directly modulated 10 Gb/s optical Orthogonal Frequency Division Multiplexing (OFDM) system due to external optical injection. The experiment is performed back to back and over 12 km of single mode fiber. The injection extends the range of linear operation of the laser and therefore extends the usable bandwidth for direct modulation formats which are susceptible to nonlinearity, such as OFDM. Nonlinearity in the system and its reduction due to injection are estimated by means of a two tone test. Additionally the performance enhancement on OFDM systems was verified in both simulation and experimentally by the comparisons of the average Bit Error Rate (BER) and Error Vector Magnitude (EVM). © 2011 Elsevier B.V. All rights reserved.
1. Introduction Orthogonal Frequency Division Multiplexing (OFDM) has recently been the subject of much research as a multicarrier modulation technique in optical communications systems [1]. It is widely used in both wired and wireless broadband communications systems and is specified for in WPAN, WLAN, ADSL and WiMAX standards. OFDM's popularity stems from its high bandwidth efficiency due to the compact arrangement of overlapping, but orthogonal, subcarriers; additionally the inclusion of a cyclic prefix (CP) converts the time dispersive channel into a circular convolution facilitating the construction of a simple maximum likelihood (ML) equalizer in the frequency domain. This frequency domain ML equalizer is implemented as a bank of one-tap equalizers and can compensate for any linear impairments introduced by the channel. Its simple construction relative to other time domain equalizers, and the fact that it facilitates good performance in the presence of dispersion contribute to OFDM's suitability for use in optical communications systems. Provided the physical channel is linear and the receiver is correctly synchronized, the orthogonal subcarrier property is preserved ensuring that there is zero Inter Symbol (i.e. subcarrier) Interference (ISI). The cost effectiveness and network flexibility that is offered by OFDM, coupled with its ability to facilitate speeds of up to 100 Gb/s, make it a promising candidate for future broadband optical access networks (≥10 Gb/s) [2]. In these types of networks, for which dynamic bandwidth allocation and network reconfigurability will be key attributes, it is important to maintain cost efficiency due to high market volume. OFDM will make it feasible to realize these attributes by allowing the ⁎ Corresponding author. E-mail address: [email protected] (C. Browning). 0030-4018/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.09.009
assignment of different subcarriers to different users. As this will be implemented electronically using advanced digital signal processing (DSP) expensive optical components are avoided and greater network flexibility is offered. Furthermore, linear impairments are compensated for in a computationally efficient way [1] in the frequency domain unlike other multiplexing techniques such as Time Division Multiplexing (TDM) which employ complex time domain equalizers. In designing a cost effective broadband optical access network it is desirable to make use of directly modulated lasers. This is because of their low cost and small size relative to transmitters that employ external optical modulators. Furthermore, the problems of polarization dependence and high insertion loss associated with such transmitters are avoided. OFDM is a candidate for use in future access networks and indeed recent research has looked at direct modulation optical OFDM (DMO-OFDM) in optical access networks [3] [4]. One limiting factor in these systems is due to nonlinearity introduced when directly modulating a laser at frequencies near its resonant peak — this coupled with the aforementioned property of OFDM where it requires a linear transmission system means that the usable bandwidth is limited. This paper shows the utilization of external optical injection into the directly modulated laser's cavity to shift its nonlinear range of operation, and hence improve the overall system performance. This technique may be applied to directly modulated lasers of various bandwidths to expand their linear range of operation. Furthermore, photonic integration would make this technique suitable for use in low cost optical communications systems. External optical injection has been performed previously to enhance the system performance with different data formats [5] [6] but holds particular importance for OFDM. This paper demonstrates the first use of optical injection to improve linearity, and thus overall system performance in a direct modulation OFDM system.
C. Browning et al. / Optics Communications 285 (2012) 136–139
2. Nonlinearity and external optical injection Information about the channel is transmitted to an OFDM receiver by means of a known training sequence which contains all subcarrier frequencies. The training sequence along with the cyclic prefix (CP), which ensures a full copy of each subcarrier is obtained at the receiver (thereby maintaining orthogonality), allows channel estimation and equalization at the receiver. This is because the effect of any linear impairments introduced by the channel (most importantly dispersion), on any subcarrier frequency can be estimated by analyzing the received training sequence. The subsequent data carrying portion of each subcarrier is adjusted accordingly. However this equalization method cannot be applied to compensate for nonlinear distortions as the loss of subcarrier orthogonality due to the nonlinear nature of the channel occurs before the signal is downconverted to the electrical domain by photodetection (itself a nonlinear process), and this results in Inter Symbol (i.e. subcarrier) Interference. When directly modulating a laser, nonlinear interactions between the carriers and the photons in the laser cavity occur. This gives rise to nonlinear operation at certain frequencies. This nonlinear range of operation is indicated by a resonant peak, or relaxation oscillation, in the laser's modulation response [7]. It has been shown that the linear range of operation can be expanded by coupling the light from an external laser into the cavity of the modulated laser (master–slave configuration) [8]. This is useful for direct modulation OFDM systems, which are highly intolerant to nonlinearities, because it extends the system bandwidth limitation imposed by nonlinearity due to direct modulation or simply improves the performance of fixed bandwidth OFDM transmission. 3. Direct modulation OFDM system Two 10 Gb/s OFDM signals with 32 and 64-QAM respectively on all subcarriers were generated. The real and imaginary components of these complex baseband signals were then digitally mixed with the In-Phase (I) and Quadrature (Q) components of an RF carrier to give real valued OFDM signals centered on a given RF frequency above DC. An OFDM symbol rate of 39.06 MHz was used and the bandwidths of the 32 and 64QAM signals were 2.265 GHz and 1.916 GHz respectively. One OFDM symbol (0.7% of the entire OFDM signal) was set to be the training sequence so as to facilitate channel estimation and equalization at the receiver. A Cyclic Prefix (CP) of 6.25% was used. This resulted in a CP length of 1.6 ns which is significantly longer than the maximum delay spread caused by dispersion over 12 km of SSMF for the given signal bandwidths, ensuring subcarrier orthogonality at the receiver. These overheads along with 0.7 Gb/s, reserved for forward error correction (FEC), meant that the raw data rate was 11.395 Gb/s. The experiment was performed for the back to back case and also over 12 km of SSMF, with and without optical injection of a single mode laser. 3.1. Simulation Firstly, this complete OFDM system was simulated in Matlab. The signals were generated in Matlab as per the above specifications. Direct modulation of the laser and external injection were modeled using the laser rate equations, including the injection terms, derived in [9]. The simulated laser device parameters [10] are presented in Table 1 and were used, along with the input electrical drive signal, to solve the extended laser rate equations with injection terms. As shown in the table, the injected photon density was 5 ×1021cm− 3 and this corresponded to an optical injection power of 1.5 dBm. The effects introduced by propagating over standard single mode fiber (SSMF), namely nonlinearity, dispersion and loss, were simulated by modeling the propagation of the electric field through the fiber using the nonlinear Schrödinger equation (NLSE). The equation was solved using the split step Fourier method for a number of steps over a length
137
Table 1 Simulated laser parameters. Parameter
Value
Wavelength, λ Group Refractive Index, η Optical Confinement Factor, Γ Differential Gain Coefficient, g0 Transparency Density, n0 Facet Reflectivity Coefficient, R Linewidth Enhancement Factor, α Area of Active Region, A Photon Lifetime, τp Carrier Lifetime, τn Injected Photon Density, P0 Coupling Coefficient, κ Detuning, Δf
1540 nm 3.63 0.35 1 × 10− 12cm2 1 × 1018cm− 3 0.32 6.8 0.03 × 10− 12m2 2 ps 0.3 ns 5 × 1021cm− 3 2.5 × 1011s− 1 5 GHz
of 12 km. First and second order terms only were included in the calculation of dispersion. The modeled receiver comprised of a 10 GHz bandwidth PIN photodetector with an integrated trans-impedance amplifier (TIA), including shot noise and thermal noise associated with such receivers, a digital RF mixer, low pass filtering and digital signal processing (DSP) normally associated with OFDM systems. This includes serial to parallel conversion, FFT, channel estimation and equalization, and error vector magnitude (EVM) measurement. Results were taken for both the injected and non-injected cases. In the latter case the injection level was set to zero. 3.2. Experimental Setup As Fig. 1 below shows, the OFDM signals were output from a 10 GSa/ s Arbitrary Waveform Generator (AWG). The slave laser's threshold current was 10 mA and for this experiment was biased at 21 mA — giving a 3 dB bandwidth of about 5 GHz and optical output power of 0 dBm. The slave emitted light at 1540 nm. By mixing the 32-QAM and 64-QAM OFDM signals with intermediate RF frequencies of 2.867 GHz and 3.043 GHz respectively, the highest frequency subcarrier in each case was placed at4 GHz which was inside the slave laser's region of nonlinear operation under free-running conditions. Using an optical circulator and polarization controller, light from the master laser – a tunable External Cavity Laser (ECL) – was coupled into the cavity of the slave laser. The injected optical power was kept constant at 2.8 dBm throughout (measured at circulator port 2). The master was tuned to 1540.05 nm — this corresponds to a detuning of roughly 5 GHz which is well within the locking range for this injection power. The signals were detected using a 10 Gb/s PIN photodetector with an integrated TIA and captured with a Real Time Oscilloscope (RTS). The addition of the Erbium Doped Fiber Amplifier (EDFA) and optical bandpass filter at the receiver provided the necessary sensitivity to allow BER versus received power curves to be measured. All processing was done using Matlab offline. BER was measured experimentally by comparing the transmitted bit sequences (input to the transmitter QAM modulator) and received bit sequences obtained after QAM demodulation. Multiple captures of the OFDM signal by the RTS were aggregated to provide the required number of observed errors for BER measurement. Measurements of EVM from the received constellations were taken for both the injected and non-injected cases. To characterize nonlinearity in the laser, Third Order Intermodulation Distortion (IMD3) was also measured using a two tone test [11], for various frequencies, for both the injected and non-injected scenarios. 4. Results and discussion Table 2 summarizes the simulated and experimental results. Strong agreement exists between the simulated and experimental improvement in EVM. Fig. 2 shows the experimental BER versus received optical
138
C. Browning et al. / Optics Communications 285 (2012) 136–139
Fig. 1. The direct modulation optical OFDM experimental setup. Insets show the transmitted and received electrical spectra.
power for the back to back and 12 km cases; injected and free-running. It can be seen in Fig. 2, for both back to back and 12 km scenarios, that the injection enhances the system performance by eliminating the BER floor which appears in the non-injected cases at values close to 10− 3. The fact that there appears to be less of an improvement due to injection after transmission over 12 km is attributed to the performance being limited by non-linearities introduced by the fiber. Fig. 3 shows the average value of EVM per subcarrier number for the 32-QAM OFDM signal for injected and non-injected regimes; experimental and simulated. The effect incurred by modulating close to the resonant peak of the laser and the subsequent improvement on each subcarrier due to injection can clearly be seen in the figure. Average EVM increases with subcarrier number as the subcarrier frequencies increase to within the laser's region of nonlinear operation but this effect is highly reduced by the injection; in this case only a small rise in average EVM is observed due to the extension of the laser's linear range. Fig. 4 shows the result of the two tone test, at 4 GHz, performed on the system under normal and injection conditions. Used to characterize nonlinearity in the optical system, this test indicates the extent to which nonlinearity is reduced due to external injection [11]. The IMD3 seen in Fig. 4 in the non-injected case was the highest value measured and its reduction by optical injection was the highest reduction recorded. Here, IMD3 was measured to be −22.03 dB, relative to the fundamental, in the non-injected case. Injection yielded an IMD3 value of −34.97 dB relative to the fundamental, giving an improvement of 12.94 dB. The tones are separated by the OFDM subcarrier spacing.
Fig. 2. BER vs received optical power, under non-injection and injection conditions, for back to back and 12 km cases.
Table 2 Average EVM for the simulated and experimental received constellations and the improvements due to injection. Order (Simulated — 0 km) 32-QAM 64-QAM (Simulated — 12 km) 32-QAM 64-QAM (Experimental — 0 km) 32-QAM 64-QAM (Experimental — 12 km) 32-QAM 64-QAM
Non-injected
Injected
Improvement
6.96% 7.56%
5.15% 5.02%
1.81% 2.54%
7.9% 7.89%
6.% 5.09%
1.9% 2.8%
7.62% 7.95%
6.07% 6.3%
1.55% 1.65%
8.32% 8.77%
7.28% 6.9%
1.04% 1.87%
Fig. 3. Average EVM per subcarrier for the non-injected (dashed) and injected 32-QAM cases; both experimental (a) and simulated (b). The non-injected case displays increasing average EVM with subcarrier number. The rise in EVM with increasing subcarrier frequency is stemmed in the injected case.
C. Browning et al. / Optics Communications 285 (2012) 136–139
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Table 3 BER for all cases (experimental). Order (back to back) 32-QAM 64-QAM (12 km) 32-QAM 64-QAM
Fig. 4. Example two tone test result at 4 GHz showing IMD3 for the non-injected (dashed) and injected (solid) cases. The zoomed inset shows the reduction due to injection of the lower frequency IMD3 more clearly.
Non-injected
Injected
2.85 × 10− 4 4 × 10− 3
1.91 × 10− 5 7.2 × 10− 4
7.9 × 10− 4 8.4 × 10− 3
8.5 × 10− 5 1.7 × 10− 3
Fig. 5 shows the received back-to-back 32-QAM constellations of all OFDM symbols under injected and non-injected regimes. The figure shows how as system nonlinearity is decreased under injection conditions each constellation point becomes more well defined. In other words phase and amplitude noise on the received RF constellation, due to nonlinearity, is decreased as the EVM improves from 7.62% (a) to 6.07% (b). How BER relates to EVM depends upon constellation size. This is intuitive when one considers the improvement in signal to noise ratio (SNR) required when moving to higher order QAM. Table 3 tabulates the experimentally measured BER of the signals under both conditions. These performances were achieved at a received optical power of − 3.5 dBm (back to back) and −5.5 dBm (12 km). These results indicate that external optical injection can improve the performance of a DMO-OFDM system and extend the usable bandwidth for the modulating OFDM signals. This is particularly important in the case of cost effective direct modulation systems where cheap moderate bandwidth lasers are employed. It also can potentially enhance system performance in directly modulated OFDM systems operating well beyond 10 Gb/s by reducing nonlinearities. 5. Conclusion Direct Modulation Optical OFDM is a promising technique, particularly for cost effective next generation optical access networks due to its low cost, flexibility and its capability to provide high speed transmission. But OFDM's intolerance to nonlinearities places a performance limitation on a DMO-OFDM system as signal bandwidths increase to the point of entering the directly modulated laser's region of nonlinear operation. Results presented in this paper show how directly modulated OFDM signals are affected by nonlinear interactions in the laser cavity and how utilizing external optical injection to extend the laser's range of linear operation can improve the performance of DMO-OFDM systems. The development of photonic integration will allow for the restrictions of polarization dependence and higher cost imposed by external optical injection to be overcome and hence make this technique feasible for such low cost systems. References
Fig. 5. Experimental estimated 32-QAM constellations of all received OFDM symbols for free running (a) and injected (b) cases.
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