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Optical injection locking (OIL) is an effective approach for significantly enhancing the modulation bandwidths of VCSELs. The frequency responses of OIL-VCSELs are, however, very sensitive to the applied OIL conditions. This brings about strong difficulties in practically utilizing the OIL-enhanced modulation bandwidths to achieve highly robust transmission performances of directly modulated OIL-VCSEL-based multi-mode fibre (MMF) links for cost-sensitive application scenarios such as data-centers. In this paper, directly modulated OIL-VCSEL-based real-time dual-band optical OFDM (OOFDM) transceivers with tunability in both the electrical and optical domains are experimentally demonstrated, for the first time, utilizing DACs/ADCs at sampling speeds as low as 4GS/s. The transceivers can support 15.125Gb/s adaptive OOFDM transmissions over 100m OM2 MMF links based on intensity modulation and direct detection. More importantly, the adaptability and tunability of the demonstrated transceivers enable the achievement of excellent robustness of the aggregated OOFDM transmission capacity to OIL condition variations. It is shown that, over a large diversity of OIL conditions that give rise to significantly different system frequency responses, the aggregated OOFDM transmission capacity only vary by <11% in the aforementioned transmission link.
© 2014 Optical Society of America
1. Introduction
The fiber-optic technology plays a dominant role in satisfying the unprecedentedly exponential growth of data traffic in data-centers, where multi-mode fiber (MMF) links incorporating directly modulated vertical cavity surface emitting lasers (VCSELs) are widely employed as a cost-effective, high-capacity, scalable and low-power consumption technical strategy [1]. However, the existing directly modulated VCSEL-based MMF links face great challenges to be upgraded to 10Gb/s/λ and far beyond [1]. Low-modulation bandwidths of commercially available VCSELs become one of the most significant obstacles to practically achieving the desired transmission performance of the aforementioned transmission links.
To address the above challenge, two main technical approaches can be considered including: a) adopting a highly spectral efficient transmission technique such as optical orthogonal frequency division multiplexing (OOFDM) [2,3], and b) optically enhancing the VCSEL modulation bandwidths. For the first approach, experimental investigations have confirmed that it is feasible to transmit directly modulated VCSEL-based real-time 11.25Gb/s single-band OOFDM signals over 2000m OM1/OM2 legacy MMFs [4]. Whilst for the second approach, optical injection locking (OIL) has been shown to be very effective in considerably broadening VCSEL modulation bandwidths [5,6]. However, the 3-dB modulation bandwidths of the directly modulated OIL-VCSELs are very sensitive to the applied OIL conditions including, for example, optical injection power, wavelength, polarization and temperature. More importantly, their frequency response profiles within the broadened 3-dB modulation bandwidth regions also vary considerably with the OIL conditions. For the cost-sensitive application scenarios of interest of the paper, it is, however, prohibitive to maintain all these OIL conditions sufficiently stable over a long period of time. To achieve the desired transmission performance with sufficiently high flexibility and robustness against the OIL-induced frequency response variations, full use of the entire OIL-enhanced dynamic frequency responses has to be made in a cost-effective manner.
In this paper, for the first time, use is made of the OOFDM technique and directly modulated and uncooled OIL-VCSELs in real-time OOFDM OM2 MMF links employing intensity-modulation and direct-detection (IMDD). The thrust of the paper is to experimentally demonstrate the excellent performance robustness and transceiver flexibility in both the RF and optical domains for practical implementations in data-centers. Over such transmission links, record-high 15.125Gb/s real-time end-to-end dual-band OOFDM transmissions over 100m OM2 MMFs are reported utilizing low-cost digital-to-analogue converters (DACs) and analogue-to-digital converters (ADCs) operating at sampling speeds as low as 4GS/s. In the adopted dual-band OOFDM transceiver [7], the non-RF-upshifted OFDM band locating in the vicinity of the optical carrier is termed baseband, and the RF-upshifted independent OFDM band is referred to as passband. Adaptive bit and power loading on each subcarrier involved in each individual OFDM sub-band [7] is conducted together with appropriate adjustments of the electrical signal powers of these two sub-bands and the bias conditions of the OIL-VCSEL. In addition, the RF frequency of the passband is also tunable according to the OIL conditions to ensure that the passband always occupies the optimum spectral regions for any OIL conditions. The combination of all the above-mentioned OOFDM transceiver design features ensures the full use of the available system frequency responses, thus leading to the successful experimental demonstrations of <11% variations in aggregated OOFDM signal capacities over a wide diversity of the OIL conditions.
Here it is worth addressing, in particular, that making use of very high sampling speed DACs/ADCs capable of covering the entire spectral regions enhanced by the OIL-VCSELs, single-band OOFDM transceivers are also envisaged to offer the robust high-capacity transmission performance [8]. However, at present such DACs/ADCs are prohibitive for practical implementations in the cost-sensitive data-centre scenarios.
2. Real-time directly modulated OIL-VCSEL-based dual-band OOFDM MMF systems
Figure 1 shows the considered end-to-end real-time directly modulated OIL-VCSEL-based dual-band OOFDM MMF system, whose configuration, transceiver architectures and corresponding major transceiver digital signal processing (DSP) functions are similar to those reported in [7], except that in Fig. 1 an uncooled OIL-VCSEL is employed as an intensity modulator and the transmission medium is replaced by a 100m OM2 MMF. The adopted key transceiver/system parameter values are listed in Table 1. For each OFDM sub-band in the transmitter, the real-time DSP functionalities include on-line adaptive bit and power loading of 15 data-carrying subcarriers, on-line adjustable clipping level and a 32-point inverse fast Fourier transform (IFFT) with input data being organized to satisfy the Hermitian symmetry. On the other hand, for each sub-band in the receiver, the corresponding real-time DSP functions consist of automatic symbol alignment, a 32-point FFT, channel equalization and adaptive demodulation.
Fig. 1 Real-time dual-band OOFDM OM2 MMF system using a directly modulated uncooled OIL-VCSEL. TLS: tunable laser source; VOA: variable optical attenuator; MMF: multi-mode fiber; PC: polarization controller; OC: optical circulator; OI: optical isolator; MCPC: mode conditioning patch-cord; LO: local oscillator.
Table 1. OFDM Transceiver and System Parameters
In the transmitter, to simultaneously generate two separate OFDM sub-bands, independent digital and RF electronics are adopted to produce each OFDM sub-band signal. The baseband occupies a spectral region from 0 to 2GHz, whilst the passband is produced by modulating a tunable RF carrier with another 0-2GHz OFDM baseband signal. After appropriately adjusting both the relative sub-band power levels and the absolute dual-band power level, these two sub-bands are combined in a resistive coupler and then an optimal 5.01mA DC bias is also added in a bias-tee. The resulting dual-band OFDM signal is employed to drive the uncooled single-mode VCSEL, which is optically injection-locked by an external master laser. After passing through an erbium-doped fiber amplifier (EDFA) followed by an optical filter, the OOFDM signal is launched into a 100m OM2 MMF link via a commercially available mode-conditioning patch-cord. The EDFA is used only to enlarge the optical power dynamic range, and it can be removed completely in the measurements without affecting the results presented in this paper.
In the receiver, a MMF-pigtailed 12GHz PIN + TIA is utilized to convert the received dual-band OOFDM signal into the electrical domain. To recover the baseband (passband) signal, an electrical down-conversion circuit identical to that reported in [7] is removed (included), as illustrated in Fig. 1. The received baseband signal and/or the down-converted passband signal passes to the receiver block to recover the data. For both mixers in the passband transmitter and receiver sides, the local oscillator (LO) signals are derived from the same RF signal source. A variable delay line is employed to correctly align the phase of the receiver’s LO with the phase of the received RF carrier.
Here it is worth mentioning the following three aspects: a) To maximize the signal transmission capacity, adaptive bit and power loading using 16-QAM, 32-QAM and 64-QAM [9] is taken on each subcarrier within each individual sub-band; b) The real-time bit error rate (BER) analysis of individual subcarriers, total sub-band BER and sub-band frequency response measurements in the receiver enable rapid optimization of the overall system performance; And c) An external tunable laser is employed as the master laser in Fig. 1, for practical applications, the external tunable laser may be replaced by another VCSEL embedded in the same VCSEL laser array.
3. Frequency response characteristics of OIL-VCSELs
The frequency response of an OIL-VCSEL, , is governed by [5, 10],
where f0 is the parasitic pole frequency, fres is the resonance frequency, γ is the damping frequency, is the frequency difference between the free-running VCSEL and the master laser, is the linewidth enhancement factor, G is the optical gain and is the carrier density variation relative to the threshold carrier density . The second term in Eq. (2) is the cavity resonance frequency resulting from the OIL-induced change in the active medium. If the polarization mode detuning, , is considered, the resonance frequency should be modified as [6]Generally speaking, is also a function of G and . From Eq. (1) to Eq. (3), it is apparent that the frequency response of an OIL-VCSEL is affected by a large number of parameters, most of which are also closely related. This brings about strong difficulties in predicting and maintaining the OIL-VCSEL frequency responses under practically acceptable operating environments. Furthermore, apart from the aforementioned uncertainty in the OIL-VCSEL frequency response, the OIL-induced noise also plays an important role in determining the signal-to-noise ratio (SNR) of the directly modulated OOFDM signals. It has been shown [11] that the relative intensity noise (RIN) associated with an OIL-VCSEL is strongly frequency-dependent, and alters significantly with the OIL conditions. In addition to that, the occurrence of the residual VCSEL side-modes further complicates the noise spectrum.Having discussed the physical origin of the highly dynamic nature of the OIL-VCSEL frequency characteristics, experimental explorations are then undertaken to identify the VCSEL optical injection locking conditions. For the OIL-VCSEL operating at a bias current of 7.18mA, Fig. 2 shows the experimentally measured wavelength locking range (between the free-running VCSEL and the master laser wavelength) against optical injection power ratio. As expected, the wavelength locking range increases with increasing optical injection power ratio, this agrees well with experimental results reported in [12, 13]. It can also be seen in Fig. 2 that, for an optical injection power ratio of 7.2 dB, the wavelength locking range can be as large as 0.3nm. This indicates that the OIL-VCSEL-based dual-band OOFDM transceiver can have a wavelength tunability of at least 0.3nm without compromising its direct modulation performance.
Fig. 2 VCSEL injection locking conditions: wavelength detuning range against optical injection power ratio for a 7.18mA bias current.
In the OIL-VCSEL-based optical back-to-back system, the sensitivity of the frequency responses to variations in OIL conditions is plotted in Fig. 3. It can be seen from these figures that, compared with the free running VCSEL case, OIL considerably lifts up the frequency responses at the high frequency regime, thus giving rise to the enhanced modulation bandwidths. However, the frequency responses in the vicinity of the optical carriers may be lowered slightly. In addition, by comparing Fig. 3(b) with Fig. 3(c), it is clear that the OIL condition-dependent variations in frequency response become more severer for relatively small wavelength locking ranges. For various OIL conditions in the optical back-to-back system, Fig. 4 presents the OIL-VCSEL noise spectra (measured at a resolution of 16MHz after the PIN), whose dynamic behaviours are mainly underpinned by the OIL condition-dependent RIN noise [11]. The aforementioned results indicate that, to achieve the robust transmission performance of the directly modulated OIL-VCSEL-based MMF system, it is extremely critical if use is made of the dual-band OOFDM transceivers with flexibility and adaptability to the dynamic channel spectral characteristics.
Fig. 3 (a) OIL-VCSEL frequency responses for different wavelength detuning values at a fixed optical injection power ratio of 5.7dB. (b) OIL-VCSEL frequency responses for different injection power ratios at a fixed wavelength detuning value of 0.29 nm, (c) OIL-VCSEL frequency responses for different optical injection power ratios at a fixed wavelength detuning value of 0.03 nm.
Fig. 4 OIL-VCSEL noise spectra measured after the PIN. (a) for different wavelength detuning at an optical injection power ratio of 5.3 dB, and (b) for different optical injection power ratios at a wavelength detuning of 0.27 nm.
4. Robust transmission performances
Figure 5 shows the representative spectra of the dual-band OFDM signals measured at the output of the PIN + TIA for the free running VCSEL and the corresponding OIL-VCSEL. In Fig. 5(a), the passband signal is almost invisible because of the limited modulation bandwidth of the free-running VCSEL. However, when the VCSEL is optical injection-locked and the modulation bandwidth is broadened considerably, the passband is thus observed clearly in Fig. 5(b) for a RF carrier frequency of 6.125GHz. By tuning the RF carrier frequency to 6.5GHz, the resulting signal spectrum is shown in Fig. 5(c), where, in comparison with Fig. 5(b), the passband signal spectrum becomes asymmetrical with respect to the RF carrier frequency because of the large overall system frequency response roll-off effect [7]. As demonstrated in Fig. 5(c), the transceiver tunability in the RF domain always allows the passband to be allocated on an optimum position of the frequency response of the OIL-VCSEL regardless of the OIL conditions changed. In the dual-band OOFDM transceiver configuration, the practically achievable lower boundary of the RF detuning range is mainly determined by the bandwidth of the baseband in order to minimize the cross-talk effect between these two sub-bands. Whilst the practically achievable upper boundary of the RF detuning range is mainly limited by the OIL-VCSEL frequency characteristics and the bandwidths of the involved RF components such as the bandwidth of the PIN detector.
Fig. 5 Spectra of the dual-band OFDM RF signals measured after PIN + TIA: (a) free-running VCSEL, (b) OIL-VCSEL with a 6.125GHz passband; (c) OIL-VCSEL with a 6.5GHz passband.
The robustness of the signal transmission capacity to different OIL conditions is shown in Fig. 6, where the signal transmission capacity of each sub-band measured after transmitting through the 100m OM2 MMF is plotted as a function of wavelength locking range (optical power injection ratio) in Fig. 6(a) (Fig. 6(b)). The corresponding aggregated signal transmission capacities are also presented in the same figures. In obtaining Fig. 6(a) (Fig. 6(b)), the optical injection power ratio (wavelength detuning range) is taken to be 7.2 dB (0.27nm), and for each locking wavelength (optical power injection), adaptive bit and power loading together with appropriate adjustments of the electrical sub-band powers and the passband RF frequency is applied to maximize the signal bit rates of both the baseband and passband at the FEC limit of 2.3 × 10−3 [4].
Fig. 6 Measured robustness of the aggregated signal transmission capacity. (a) Signal transmission capacities versus wavelength detuning. (b) Signal transmission capacities versus optical power injection ratio.
It is very interesting to note that, over a large wavelength locking range of 0.3 nm and an optical injection power ratio of 5 dB, which bring about significant changes to the OIL-VCSEL frequency response characteristics shown in Fig. 3 and Fig. 4, the relative aggregated signal transmission capacity variations as low as 11% are achievable, the vast majority of which is contributed by the passband signal. In Fig. 6(a), the reduction in passband signal transmission capacity with increasing wavelength locking range is a direct result of the large wavelength detuning-induced decrease in resonance frequency, as indicated by Eq. (3) and observed in Fig. 3(a). The limited detuning range of the electrical filters adopted in the transceiver also affect the observed passband signal capacity reduction. This suggests that the employment of better electrical filters with wider detuning ranges may improve the flatness of the passband capacity developing trend, thus the performance robustness of the transceivers. In Fig. 6(b), the slight reduction in passband signal capacity at the low optical injection power ratio region is due to the decreased frequency responses at the relatively high frequency region, as shown in Fig. 3(b) and Fig. 3(c). It can be also seen in Fig. 6 that aggregated 15.125Gb/s real-time dual-band OOFDM transmissions over the 100m OM2 MMF are feasible, which may be further improved if the IQ modulated passband is adopted [14].
To explore the main physical mechanisms underpinning the observed system performances, in Fig. 7(a) the measured BER versus received optical power for each sub-band is presented for the optical back-to-back and 100m OM2 MMF system configurations. To ensure that Fig. 7(a) is capable of representing all the OIL conditions explored in the paper, an optical injection power ratio of 5.7 dB and a wavelength locking range of 0.26 nm are chosen, under which the corresponding online optimized adaptive bit loading profiles are presented in Fig. 7(b) for both sub-bands. This gives rise to an aggregated signal transmission capacity of 14.25Gb/s (8.75Gb/s for the baseband and 5.5Gb/s for the passband). It is shown in Fig. 7(a) that the power penalty for the baseband is negligible, whereas the power penalty for the passband is approximately 1.2dB. The relatively large passband power penalty is mainly due to the wide passband spectrum-induced pronounced effect of differential-mode-delay (DMD) associated with the MMF [4]. The observed difference of the received optical power at the FEC limit of 2.3 × 10−3 between the baseband and passband arises due to the adaptive modulation-associated linear trade-off between the sub-band signal transmission capacity and the receiver sensitivity (in dBm) required for achieving the FEC limit [15]: for a specific transmission system, a high transmission capacity corresponds to a high minimum received optical power.
Fig. 7 (a) Representative BER performance of the back-to-back system and 100m OM2 MMF systems. The aggregated signal transmission capacity is 14.25 Gb/s. (b) Optimized bit loading profiles for both sub-bands.
Here it is worth mentioning that the use of the mode-conditioning patch-cord in Fig. 1 results in the occurrence of a relatively large number of excited optical modes including low-order modes and high-order modes. The low-order modes having large time delays between each other mainly contribute to the frequency response region occupied by the baseband. Whilst the high-order modes having small time delays between each other mainly contribute to the frequency response region occupied by the passband. In addition, the overall system frequency roll-offs are also significantly attributed by the electrical transceiver components.
5. Conclusions
Directly modulated uncooled OIL-VCSEL-based real-time dual-band OOFDM transceivers with tunability in both the electrical and optical domains have been experimentally demonstrated, for the first time, utilizing DACs/ADCs at sampling speeds as low as 4GS/s. 15.125Gb/s adaptive OOFDM transmissions have been achieved experimentally in IMDD 100m OM2 MMFs. Experimental results have also shown that, over various OIL conditions that bring about significant variations in frequency response, the aggregated transceiver transmission capacities only vary by <11% in the aforementioned system. The reported technique has potential for practical use in data-centers.
Acknowledgments
This work was supported in part by the PIANO + under the European Commission’s ERA-NET Plus Scheme within the project OCEAN under Grant Agreement 620029, in part by China Scholarship Council, National High Technology Research and Development Program of China (863 Program) (2012AA011302, 2012AA011304, 2013AA010503), NSFC (No. 61071097, No. 61107060, No. 61101095).
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