Coherently injection-locked and directly modulated weak-resonant-cavity laser diode (WRC-FPLD) for back-to-back optical 16-quadrature-amplitude-modulation (QAM) and 52-subcarrier orthogonal frequency division multiplexing (OFDM) transmission with maximum bit rate up to 4 Gbit/s at carrier frequency of 2.5 GHz is demonstrated. The WRC-FPLD transmitter source is a specific design with very weak-resonant longitudinal modes to preserve its broadband gain spectral characteristics for serving as a colorless WDM-PON transmitter. Under coherent injection-locking, the relative-intensity noise (RIN) of the injection-locked WRC-FPLD can be suppressed to −105 dBc/Hz and the error vector magnitude of the received optical OFDM data is greatly reduced with the amplitude error suppressed down 5.5%. Such a coherently injection-locked single-mode WRC-FPLD can perform both the back-to-back and the 25-km-SMF 16-QAM-52-OFDM transmissions with a symbol rate of 20-MSa/s in each OFDM subcarrier. After coherent injection locking, the BER of the back-to-back transmitted 16-QAM-52-OFDM data is reduced to 2.5 × 10−5 at receiving power of −10 dBm. After propagating along a 25-km-long SMF, a receiving power sensitivity of −7.5 dBm is required to obtain a lowest BER of 2.5 × 10−5, and a power penalty of 2.7 dB is observed when comparing with the back-to-back transmission.
© 2012 OSA
In nowadays fiber-optics communication network, the down- and up-stream data transmission wavelengths set and controlled by central office are mandatory due to the need of unified transmitters applicable to all channel users. This has become a driving force for developing the wavelength injection-locked broadband optical transmitters with relatively high modulation bandwidth. Instead of exploring the possible transmitter candidates, the high-capacity data carrying with efficient spectral usage has recently been an intriguing topic to be achieved. To meet this demand, the transmission of optical orthogonal frequency division multiplexing (OFDM) formats using versatile transmitters has recently emerged for next-generation optical networks with light sources of limited modulation bandwidth [1-5]. For the long-term development, it is necessary to fuse such a data format into the dense wavelength-division-multiplexed passive optical networks (DWDM-PONs), thus providing a potential subscriber network with both the higher channel capacity and the data-format flexibility for fiber-to-the-home applications. Previously, one of these specific laser diode sources with a weak resonant cavity (WRC) feature as compared to the conventional Fabry-Perot laser diode (FPLD) was designed to meet the demand of colorless operation for DWDM-PON applications . By using a broadband amplified spontaneous emission (ASE) based master source to perform the wavelength injection-locking at different DWDM-PON channels, such a WRC-FPLD can be employed as a quasi-color-free DWDM-PON up-stream transmitter with direct modulation at bit rate of up to 2.5 Gbit/s [7, 8]. Even though the mode-selection problem for the 50-200GHz channelized DWDM-PON transmitters has been solved by lengthening the WRC-FPLD cavity to obtain a denser mode spectrum with a reduced mode spacing, the spectrally sliced incoherent ASE still suffers from a larger intensity noise to limit the transmission bit-rate at 2.5 Gbit/s. Alternatively, the coherent injection-locking scheme using such kind of WRC-FPLD transmitter for optical OFDM transmission has never been discussed. In this work, the M- quadrature amplitude modulation (QAM)-N-OFDM data-format transmission with the coherently wavelength injection-locked WRC-FPLD is demonstrated with a QAM level of M = 16 and a subcarrier number of N = 52. The constellation plots, error vector magnitude (EVM) and bit-error-rate (BER) performances of the coherently injection-locked and directly modulated WRC-FPLD at carrier frequency of 2.5 GHz are characterized for optical 16-QAM-52-OFDM transmission with a total bit rate at 4 Gbit/s.
2. Experimental setup
Figure 1 illustrates the testing bench for the optical 16-QAM-52-OFDM transmission at a total bit rate of 4 Gbit/s by using a coherently injection-locked WRC-FPLD transmitter. The slave WRC-FPLD is injection-locked by single-mode wavelength-tunable laser for broadband tuning its wavelength. The design and fabrication procedures of the WRC-FPLDs are modified from those of a conventional FPLD without significantly increasing the production cost. The main differences between the WRC-FPLD with the conventional FPLD are their end-face reflectance and cavity length. To assure the sufficient mode number transmitted within one DWDM-PON channelized spectral window, the WRC-FPLD cavity length is lengthened from 250 to 600 μm so as to enable 2-3 modes injection-locked in one DWDM channel. In addition, the front end-facet reflectance is reduced to 1% such that the WRC-FPLD remains a partial coherence and the power budget of the injection-locking operation can be greatly reduced. In principle, the power budget reduction depends on the end-face coating reflectance of the commercially available FPLD. In comparison with a common FPLD with a front-facet reflectance of R = 30%, the required power for injected into the WRC-FPLD with a front-facet reflectance of R = 1% is reduced by 10log[(1-1%)/(1-30%)] = 1.5 dB. Alternatively, the external coupling efficiency used in the injection-locked rate equation can also be quoted to characterize the power budget, as described by κ = (1-Rfront)1/2/τld,, where τld denotes the round-trip time.
The WRC-FPLD exhibits a threshold current of about 25 mA, a longitudinal mode spacing of 0.6 nm, the back and front facet reflectivity of 94% and 1%. Such a design of highly asymmetric coating allows the efficient injection-locking from the front facet and avoids the power consumption from rear-facet. During experiments, the optical injection power from the tunable laser is set as low as −6 dBm, and the DC biased current of the WRC-FPLD is controlled at 40 mA. The injection-locked WRC-FPLD exhibits a relative-intensity noise (RIN) of −105 dBc/Hz. Subsequently, the injection-locked WRC-FPLD is directly modulated by a linearly amplified 16-QAM/52-subcarrier OFDM data-stream generated from an arbitrary waveform generator (Tektronix 7102C) with an output power of −3.5 dBm at carrier frequency of 2.5 GHz. The frequency division between OFDM subcarriers is set as 20 MHz and the total RF bandwidth of the OFDM data-stream is set as 1 GHz to ensure the total transmission bit rate of up to 4 Gbit/s. The total OFDM bit rate of 4 Gbit/s is calculated by 52 (subcarrier number) × 20 MHz (subcarrier spacing) × 1 (symbol per Hz) × 4 (bit per symbol). The constellation plot, EVM, and BER analyses of the optical OFDM data by the coherently injection-locked and directly modulated WRC-FPLD are performed by using a real-time digital oscilloscope with associated software (Tektronix 7404A and RF Xpress). The detailed descriptions for all devices used in the system is shown in Table 1 .
3. Results and discussions
Figure 2(a) shows the frequency responses of the directly modulated WRC-FPLD without and with external injection-locking. The free-running multi-mode WRC-FPLD exhibits an original modulation bandwidth of 3.1 GHz. The usable frequency bandwidth for the optical M-QAM-N-OFDM transmission with the WRC-FPLD is strictly affected by its flatness of the 3dB direct modulation response. Under the injection-locking condition, the frequency response curve becomes more flattened and significantly extends to 4 GHz. Besides, the broadband WRC-FPLD transmitter requires injection-locking to preserve its wavelength at a desired DWDM-PON channel. This flattened modulation response is particularly important for the M-QAM-N-OFDM data-format transmission when comparing the injection-locked WRC-FPLD with the same device at free-running condition. If the response is not flattened, the received signal will suffer from the fluctuated signal to noise ratios at different OFDM subcarriers (usually a degraded SNR with the decayed response at higher frequencies). This inevitably leads to a degraded EVM for those M-QAM data carried at higher OFDM subcarrier frequencies. In addition, the power-current characteristics of the WRC-FPLD under different injection-locking power levels are shown in Fig. 2(b). In comparison with the effective threshold current of the WRC-FPLD injection-locked with a power level of −12 dBm, the threshold current is reduced from 26 to 15 mA by enlarging the injection level from −12 to −3 dBm, which reveals a threshold current reduction effect with a maximum threshold current shift up to ΔI = −8 mA as the external injection-locking power increases by ΔP = 9 dB. The unchanged power-current slope for the response curves among all injection-locking levels reveals that the reshaping effect of the directly modulated M-QAM-N-OFDM data-stream is negligible, which also remains the external quantum efficiency almost constant during the injection power adjustment. This feature helps to prevent the requirement of additional data-format pre-shaping procedure at the transmitter end.
The relatively high threshold current of WRC-FPLD is caused by its unique design of 1-% reflectivity on the front facet of the resonance cavity. Even though a trade-off between the power budget and the coherence under injection-locking condition might be set by the adjusting the end-facet reflectance of the WRC-FPLD, it is preferred to have a slightly higher front-facet reflectance for the WRC-FPLD since the coherence may play a more important role on the QAM-OFDM transmission. Although the reduced reflectivity of the facet allows an efficient external injection into the WRC-FPLD, the WRC-FPLD cavity cannot hold most of the external injection after round-trip circulation in the WRC-FPLD. That is, the injection power level needs to be increased for the WRC-FPLD with a lower front-facet reflectance, as the higher free-running power is also dissipated from this facet with a lower reflection when comparing with a facet with normal reflectance. Under such a low end-face reflectance condition, the external injection effectively contributes to the reflection of the resonance cavity, which leads to an easier lasing phenomenon of WRC-FPLD built up at a lower driving current. Figure 3(a) illustrates the free-running and single-mode injection-locking spectra of the WRC-FPLD, in which the WRC-FPLD reveals a broadband wavelength-selectivity (with a 3-dB bandwidth of nearly 40 nm) and exhibits a side-mode-extinction ratio (SMSR) of up to 45 dB after coherent injection-locking.
To discuss the noise suppression capability of the WRC-FPLD under coherently injection-locking case, the relative intensity noise (RIN) spectra of the WRC-FPLD at free-running and injection-locking cases are compared in Fig. 3(b). Originally, the relaxation oscillation peak is observed at about 2.5 GHz for the WRC-FPLD at free-running operation, indicating that the free-running WRC-FPLD has a modulation bandwidth at around this frequency. Due to the externally coherent injection-locking, the WRC-FPLD reduces its threshold current and simultaneously enhances its direct modulation bandwidth with an evidence of the enlarging relaxation oscillation frequency up to 4.9 GHz, as shown in the RIN spectrum. Note that the relative intensity noise level within the modulation bandwidth is greatly improved by 5-8 dB at 1-5 GHz. Subsequently, the Fig. 4 depicts the constellation plots and corresponding RF spectra of the optical 16-QAM-52- OFDM data-stream carried by WRC-FPLD at free-running and injection-locked cases. Without injection, the WRC-FPLD carried QAM-OFDM data shows a relatively large EVM and both the amplitude/phase errors are up to 25%, which is due to the extremely large spontaneous emission noise at free-running case cause by the special design of low end-face reflectance for the WRC-FPLD. Under coherent injection-locking, the EVM of the received optical 16-QAM-52-OFDM data is greatly reduced with its amplitude error significantly suppressed down to 7.7%. However, there is still a larger phase error (around 10%) caused by the chirp and the transient bandwidth fluctuation of the WRC-FPLD under injection-locking condition. The chirp as well as transient phase error is inevitably caused by the direct modulation of the WRC-FPLD with high-level M-QAM data. Even though the chirp of the WRC-FPLD output can be slightly reduced under the injection-locking, there is still a residual phase error accompanied with the transmitted M-QAM-N-OFDM data-stream. It is straightforward to further reduce the chirp and phase error by increasing the power of injection; however, the slope of modulation frequency response concurrently enlarges to significantly differentiate the OFDM subcarrier amplitudes. This eventually leads to an increasing amplitude error among the QAM data carrier by different OFDM subcarriers to cause the degradation of EVM. In addition, the WRC-FPLD has low end-face coating reflectance which cannot bear high injection power. Furthermore, the coherently injection-locking operation also benefits the slave WRC-FPLD from a wider injected-locking bandwidth and an enlarged side-mode suppressing ratio that is comparable with a typical laser diode source.
The Fig. 5(a) presents a relatively broadband injection-locking flexibility of the WRC-FPLD even under weak-power seeding case. It is evident from the contour mapping that the WRC-FPLD can perform a side-mode suppressing ratio larger than 35 dB within ± 0.2 nm wavelength detuning range. Within a 0.1-nm wavelength deviation between the master and slave injection-locking mode, the required injection level can be as low as −9 dBm. In this experiment, we have performed that the directly modulated WRC-FPLD can provide a qualified 16-QAM and 52-subcarrier OFDM data with relatively low EVM and BER under the external coherent injection of −7.6 dBm. At such a low-level injection, the acceptable SMSR of >30 dB can still be obtained even the injecting master wavelength is slightly detuned away from the slave WRC-FPLD’s longitudinal mode by ± 0.3 nm. Such a broadened injection-locking range of the WRC-FPLD is already 6-8 times larger than that of a conventional FPLD with standard end-facet coating reflectance.
As a result, the larger tolerance on the injection-locking range essentially benefits the WRC-FPLD from easier injection-locking even at a worse wavelength mismatching condition. Therefore, the power budget of the WRC-FPLD based transmitter is no longer a problem to limit the practical 16-QAM-52-OFDM applications. With the coherently injection-locking and direct modulation, the WRC-FPLD greatly improves its capability on carrying the 16-QAM-52-OFDM data. Using 52 OFDM subcarriers with 20 MSa/s per channel to deliver a total bit rate up to 4 Gbit/s with an RF bandwidth covering from 2 to 3 GHz, both the free-running and the coherently injection-locked WRC-FPLD based back-to-back 16-QAM-OFDM transmissions are performed. After coherent injection locking, the BER of the received 16-QAM-52-OFDM data is reduced from ~7 × 10−3 to ~3 × 10−5 at receiving power of −10 dBm and from ~1 × 10−2 to ~6 × 10−4 at receiving power of −14 dBm, as shown in Fig. 5(b). In addition, the BER curve of the injection-locked WRC-FPLD based 16-QAM-52-OFDM transmission after propagating along the 25-km long SMF is also characterized. The BER degradation by approximately one order of magnitude is observed at the same receiving power after 25-km transmission. The receiving power sensitivity of −7.5 dBm is required to obtain a lowest BER of 2.5 × 10−5. At BER of smaller than 1.5 × 10−4, a power penalty of 2.7 dB is observed when comparing with the back-to-back transmitted data carried by the injection-locked WRC-FPLD at same operating condition. Such a BER reduction by more than two orders of magnitude apparently supports the potential application of the quasi-color-free WRC-FPLD for the optical M-QAM-OFDM data-stream in next-generation DWDM-PONs
A coherently injection-locked weak-resonant-cavity Fabry-Perot laser diode under direct modulation for 16-QAM Optical OFDM transmission with maximum bit rate up to 4 Gbit/s at carrier frequency of 2.5 GHz is demonstrated. The constellation plots and RF spectra of the optical 16-QAM/52-subcarrier OFDM data carried by WRC-FPLD at free-running and injection-locked cases are compared experimentally. Without injection, the WRC-FPLD carried OFDM data shows relatively large EVM up to 25% and both the amplitude/phase errors are up to 17% due to the incorporation of mode-beating and spontaneous emission noises in the cavity. The reduction of EVM strictly depends on the suppression of the aforementioned noise sources by coherent injection-locking. Under coherent injection-locking, the EVM of the received optical OFDM data is greatly reduced with the amplitude error suppressed down to 7.7%. However, there is still a larger phase error (around 10%) caused by the chirp and the transient bandwidth fluctuation of the WRC-FPLD under injection-locking condition. Such a coherently injection-locked single-mode WRC-FPLD provides the back-to-back transmission with a symbol rate of 20 MSa/s each subcarrier for 52 subcarriers under 16-QAM modulation. After coherent injection locking, the BER of the received 16-QAM-52-OFDM data is reduced from 7 × 10−3 to 2.5 × 10−5 at receiving power of −10 dBm and from 1 × 10−2 to 6 × 10−4 at receiving power of −14 dBm. The BER degradation by approximately one order of magnitude is observed at the same receiving power after 25-km transmission. The receiving power sensitivity of −7.5 dBm is required to obtain a lowest BER of 2.5 × 10−5. At BER of smaller than 1.5 × 10−4, a power penalty of 2.7 dB is observed when comparing with the back-to-back transmitted data carried by the injection-locked WRC-FPLD at same operating condition.
The authors thank the National Science Council of Republic of China and the Chunghwa Telecom Co., Ltd. for financially supporting this research under grants NSC98-2221-E-002-023-MY3, NSC100-2221-E-002-156-MY3, and 100-SC-23, respectively.
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