By up-shifting the relaxation oscillation peak and suppressing its relative intensity noise in a weak-resonant-cavity Fabry-Perot laser diode (WRC-FPLD) under intense injection-locking, the directly modulated transmission of optical 16 quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) data-stream is demonstrated. The total bit rate of up to 20 Gbit/s within 5-GHz bandwidth is achieved by using the OFDM subcarrier pre-leveling technique. With increasing the injection-locking power from −12 to −3 dBm, the effective reduction on threshold current of the WRC-FPLD significantly shifts its relaxation oscillation frequency from 5 to 7.5 GHz. This concurrently induces an up-shift of the peak relative intensity noise (RIN) of the WRC-FPLD, and effectively suppresses the background RIN level to −104 dBc/Hz within the OFDM band between 3 and 6 GHz. The enhanced signal-to-noise ratio from 16 to 20 dB leads to a significant reduction of bit-error-rate (BER) of the back-to-back transmitted 16-QAM-OFDM data from 1.3 × 10−3 to 5 × 10−5, which slightly degrades to 1.1 × 10−4 after 25-km single-mode fiber (SMF) transmission. However, the enlarged injection-locking power from −12 to −3 dBm inevitably declines the modulation throughput and increases its negative throughput slope from −0.8 to −1.9 dBm/GHz. After pre-leveling the peak amplitude of the OFDM subcarriers to compensate the throughput degradation of the directly modulated WRC-FPLD, the BER under 25-km SMF transmission can be further improved to 3 × 10−5 under a receiving power of −3 dBm.
© 2014 Optical Society of America
Nowadays, the wavelength division multiplexed passive optical network (WDM-PON) is considered for the next-generation fiber-optic access service because of its unique features including dense channel with high capacity, strong security, easy upgradability, and high flexibility [1–3]. Many directly modulated light sources have been proposed for the use in either the central office (CO) or the optical network unit (ONU) of WDM-PON, such as the distributed feedback laser diodes (DFBLDs), reflective semiconductor amplifiers (RSOAs), and Fabry-Perot laser diode (FPLDs). Figure 1 schematically illustrates the end-facet reflectance dependent spectral characteristics of the aforementioned transmitters without and with injection-locking. These light sources can be either free-running or wavelength injection-locking to form the non-coherent [4–6] or coherent [7–9] transmitters, which effectively upgrade the transmission performance of optical data in the WDM-PON. Typically, the DFBLD with highest coherence is indeed the best choice for carrying the optical data for transmission [10, 11]. However, the DFBLD is also a non-universal transmitter with preset and almost non-variable wavelength for discrete WDM-PON channel, which consists of the lowest temperature dependent tunable range and makes it the most expensive solution for WDM-PON. To meet the demand of a colorless transmitter applicable for all WDM-PON channels, some works focused on using the wavelength injection-locked broadband light source have been proposed to release the cost issue of the traditional DFBLD transmitters used in WDM-PON [12–14]. For example, the injection-locked RSOA  was proposed at early stage, which is able to cover more WDM-PON channels due to its wide tunable range. However, the non-coherence and low modulation bandwidth (<2.5 Gbit/s) of the RSOA transmitter limited the transmission capability for WDM-PON. Later on, Park et al. proposed the externally single-mode injection-locked RSOA with reduced mode beating noise and dispersion during long distance transmission .
Alternatively, the injection-locked FPLD has been proposed because of its higher coherence than the RSOA [15–17]. On the other hand, a new class of injection-locked weak-resonant-cavity Fabry-Perot laser diode (WRC-FPLD) [18–20] has emerged with imperfect anti-reflection (AR) coating on the front-facet of a long-cavity FPLD. In contrast to the conventional FPLD, the longer cavity length of the WRC-FPLD effectively reduces its longitudinal mode spacing to increase dense WDM channels with sufficient mode number. The WRC-FPLD with lower front-facet reflectance provides higher injection efficiency and broader gain spectrum to cover more WDM channels. Recently, the coherent optical orthogonal frequency division multiplexing (OFDM) format has been proposed for next-generation PON to enable the transmitter with high data capacity and high spectral usage efficiency within a finite bandwidth [21–23]. To provide a potential subscriber network with both the higher channel capacity and the data-format flexibility for fiber-to-the-home applications, it is necessary to fuse such a new-class data format into the WDM-PON in the near future. At present, the optical OFDM transmissions based on different transmitters for dense WDM-PON application have been successively demonstrated [24–33]. The direct modulation of a single-mode DFBLD was considered as the most common candidate to carry the OFDM data to achieve the 30-Gbit/s transmission [24, 25], and the DFBLD based QAM-OFDM transmission was demonstrated for the WDM extended reach PON . In addition, the optical OFDM was used for wavelength reused down-stream transmission at 10 Gbit/s , and the 7.5-Gbit/s OFDM transmission carried by a directly modulated injection-locked RSOA was demonstrated with a bandwidth usage of only 1 GHz . Later on, the 40-Gbit/s OFDM PON was achieved by using an electroabsorption modulator (EAM) with the aid of subcarrier-adaptive modulation format and pre-emphasis . Another approach using the dual-band Optical OFDM signal transmitted over 25-km SMF at a total bit rate of up to 19.125 Gbit/s was demonstrated by using the IMDD system with an integrated EAM and laser diode link . More recently, the directly modulated and injection-locked FPLD for 16-QAM OFDM transmission at 10 Gbit/s was proposed as a colorless transmitter for the DWDM-PON [31, 32]. By externally modulating a mode-locked laser comb based multi-carrier transmitter, a novel WDM-OFDM PON scheme with a bit rate of 12.75 Gbit/s per channel was also demonstrated . To increase the data bandwidth, the injection-locked WRC-FPLD under direct modulation was employed to transmit the OFDM data , and the OFDM with 3-GHz transmission bandwidth was carried by the WRC-FPLD packed in a common TO-can with a limited frequency response . However, this kind of low-endface-reflectance WRC-FPLD transmitter usually accompanies with relaxation oscillation dependent relative intensity noise (RIN) band to restrict the OFDM modulation over 5 GHz.
In this work, we study the effect of RIN band shift in the WRC-FPLD modulation response, which can ameliorate the back-to-back and 25-km single mode fiber (SMF) 16-QAM OFDM transmission of up to 20 Gbit/s. The employment of the external injection-locking plays an important role to suppress and up-shift the relaxation oscillation induced RIN peak, so as to upgrade the directly modulated WRC-FPLD with 16-QAM OFDM transmission performance. By setting the OFDM data with a total bandwidth of 5 GHz band at central frequency of 3 GHz, and by using the subcarrier pre-leveling technique to compensate the declined frequency response of the directly modulated WRC-FPLD, the impaired the 16-QAM OFDM transmission delivered with the injection-locked WRC-FPLD based universal WDM-PON transmitter is demonstrated.
2. Experimental setup
Figure 2 illustrates the testing bench for the directly modulated and coherently injection-locked long-cavity WRC-FPLD with a limited bandwidth of 5 GHz for the OFDM transmission at a total bit rate of 20 Gbit/s. The gray region shows the device and bonding structures of the WRC-FPLD packed in a TO-56-can. In order to remain the dense and weak longitudinal modes with partial coherence for DWDM-PON application, the front-end facet reflectance of the WRC-FPLD was reduced to 1% and the cavity length of the WRC-FPLD was lengthened to 600 um as compared to a conventional FPLD with a front-end facet reflectance of 30% and a cavity length of 200 um. Such a design not only greatly reduces the power budget under injection-locking operation, but also provides an enlarged density of cavity modes to fit with the DWDM-PON channels. The WRC-FPLD exhibits numerous longitudinal modes spaced by 0.5 nm and ranged between 1570 and 1590 nm. To enhance the modulation bandwidth with suppressed the RIN level, a tunable laser with its wavelength optimized at 1580.65 nm serves as the coherent master to injection-lock the slave WRC-FPLD. The WRC-FPLD was biased at 35 mA (twice the threshold current of 17 mA) and injection-locked by adjusting the tunable laser power from −12 to 0 dBm. The temperature was controlled at 25°C to prevent wavelength drift. With the WRC-FPLD based transmitted in the central office (CO), the pseudo random binary sequence (PRBS) data was serial-to-parallel mapped into the 16-QAM formatted data-stream.
In our home-made MATLAB program for generating a QAM-OFDM formatted data-stream under a preset FFT size, the raw bit rate (D in unit of bit/sec) can be calculated by multiplying the requested subcarrier spacing (Δf in unit of 1/sec) with the subcarrier number (N, also means the number of QAM symbols) and the QAM level (m in unit of bit/symbol). The requested subcarrier spacing is defined as Δf = Sampling rate (samples/sec) / FFT size (samples). That is, the total raw bit rate D (bit/sec) = Δf (1/sec) × N (symbol) × m (bit/symbol). By substituting with the AWG sampling rate of 24 GS/s, the fast-Fourier transform (FFT) size of 512, the subcarrier number of 115 and QAM-level of 4, the exactly raw bit rate of the 16-QAM OFDM format used in our work is 21.56 Gbit/s. Since the HD-FEC is employed as the performance threshold, the net bit rate is modified as 20.05 Gbit/s after subtracting the FEC overhead of 7%. The amplitudes of the 16-QAM OFDM subcarriers were pre-leveled by a rising exponential function before converting into an analog waveform by an arbitrary waveform generator (Tektronix, AWG 7122B) with sampling rate of 24 GS/s. The WRC-FPLD was directly modulated by the AWG generated 16-QAM OFDM data in combination with a DC bias current via a bias tee. A tunable laser (Agilent, 8568B) simulated as a purely coherent master was employed to externally injection-lock the directly modulated WRC-FPLD after passing through a polarization controller. In the optical network units (ONUs), the injection-locked WRC-FPLD transmitter and its delivered OFDM data-stream transmitted through 25-km standard single mode fiber (SMF) was measured by an optical spectrum analyzer (Advantest, Q8384) and an optical receiver (Nortel, pp-10G) with a linear amplifier. The amplified analog data-stream was sampled by a real-time oscilloscope (Tektronix, DSO 71254) with a sampling rate of 100 GS/s. The time-domain OFDM waveform was then fast Fourier-transformed back to the frequency domain and decoded with homemade MATLAB demodulation software after resampling. The constellation plot, EVM and BER analyses of the back-to-back and 25-km SMF transmitted 16-QAM OFDM data carried by the coherently injection-locked and directly modulated long-cavity WRC-FPLD were compared each other. The RIN was measured by an optical spectrum analyzer (Agilent, 70001A + 70810B).
3. Results and discussions
To discuss the effect of relaxation oscillation induced noise in the WRC-FPLD on the signal-to-noise ratio (SNR) disturbance of the 16-QAM OFDM data, the RIN spectra of the WRC-FPLD injection-locked at different power levels are shown in Fig. 3(a). The RIN peak is gradually reduced and up-shifted by enlarging the injection power from −12 to 0 dBm. The separation the OFDM and RIN bands effectively enhanced the SNR of the QAM data carried by higher frequency OFDM subcarriers.
Figure 3(b) shows that the SNR of the OFDM data stream transmission within the modulation bandwidth has a decreasing trend at higher frequencies due to the rising edge of the RIN band. Obviously, the SNR at higher offset frequencies can be improved from 16 to 20 dB, while the RIN concurrently suppresses its average level from −99 to −103 dBc/Hz at offset frequency below 5 GHz, associated with its peak up-shifted from 5 to 7.5 GHz by enlarging the injection level from −12 to −3 dBm. The RIN of the WRC-FPLD keeps almost unchanged with the injection power at offset frequency lower than 3 GHz, whereas it exponentially arises at beyond 3.5 GHz and needs to be suppressed with enlarging the injection level. As a result, the SNR of the 16-QAM data between the 81st and the 115th OFDM subcarriers can be improved by 3 dB at least. This essentially promote all of the WRC-FPLD delivered 16-QAM OFDM data beyond the detection criterion set with the forward error correction (requested a minimal SNR of 15.5 dB for corresponding BER of 3.8 × 10−3).
In principle, the threshold current of the injection-locked WRC-FPLD can be derived from the rate equations, which is a function of the ratio of injected photon number to the output photon number [36, 37]38], where Rfront is the front facet reflectivity of the WRC-FPLD, L the cavity length, α the linewidth enhancement factor, Sinj the externally injected photon number, SB the output photon number of the WRC-FPLD, and Ntr the transparency photon number. When the WRC-FPLD is externally injection-locked, the effective threshold current (Ith') is decreased due to the increasing of second term of the Eq. (1). The output photon number of the injection-locked WRC-FPLD can be rewritten as39, 40]Eq. (1) and Eq. (3) into the Eq. (4), the relaxation oscillation induced peak in the RIN spectrum of the WRC-FPLD can effectively be up-shifted and suppressed by employing the injection-locking technique, which is mainly attributed to the decreased effective threshold current and up-shifted relaxation oscillation frequency.
At free-running case, the filtered single-mode output exhibits an output of −3 dBm (as compared to the full-band output of 0 dBm). The output power of the WRCFPLD at bias current of 35 mA (twice the threshold condition) varies from 0.32 to 0.6 dBm as the injection-locking power enlarges from −12 to 0 dBm. Figure 4 shows the shifted RIN peak frequency at different Sinj/SB ratios (equivalent to the ratio of injection power to output power), which is in good agreement with a simulation curve modeled by using Eq. (3) with the inset parameters.
Figure 5(a) indicates the BER performance of the back-to-back 20-Gbit/s OFDM data stream delivered by the WRC-FPLD biased at twice the threshold current and injection-locked at different powers. The related constellation plots of the optical 16-QAM OFDM data carried by WRC-FPLD at free-running and injection-locked cases with different injection-locking powers are shown in the inset, indicating that the error vector magnitude (EVM) of the optical OFDM data is reduced from 8.3% (BER = 2.5 × 10−3) at free-running to a minimum of 6.2% (BER = 4.98 × 10−5) when injection-locking at −3dBm. The EVM of the OFDM data is calculated from the constellation plot by using the following equation :
where N denotes the number of measured symbols, Sr(n) the normalized received nth symbol which is corrupted by Gaussian noise, St(n) the ideally transmitted value of the nth symbol x(n), and P0 the maximum normalized power of ideal symbol. In the meantime, the EVM calculated from the measured SNR  is also used to check the consistency. As the injection level enlarges from −12 to −3 dBm, the bit-error-rate (BER) is improved by more than one order of magnitude (from 1.3 × 10−3 to 5 × 10−5). These results correlate well with the RIN improvement shown in Fig. 3(b), in which the RIN peak of the WRC-FPLD is exactly separated from the modulation band under an injection power of up to −3 dBm. Such an operation essentially leads to an enhancement on the signal to noise ratio (SNR) of the OFDM data stream within the modulation bandwidth. The intense injection-locking helps to increase the side-mode suppression ratio (SMSR) of the WRC-FPLD, however, which also causes the degraded high-frequency modulation response of the WRC-FPLD. The BER is increased when the injection level enlarges more than −3 dBm, while the degraded OFDM transmission is mainly impaired by the declined frequency response (associated with a negative slope of throughput power to frequency response) of the WRC-FPLD caused by intense injection, as shown in the frequency region smaller than the arrow indicated in Fig. 5(b).
The performance of OFDM transmission over 25-km single-mode fiber (SMF) is further characterized. Figure 6(a) shows the BER response of the 25-km SMF transmitted 16-QAM OFDM data with a total bandwidth of 5 GHz when delivered by using the directly modulated WRC-FPLD. Even after 25-km SMF transmission, the BER can be monotonically decreased to 1.1 × 10−4 by increasing the injection-locking power up to 3 dBm. The received OFDM spectra delivered by the WRC-FPLD at different injection-locking powers are shown in Fig. 6(b). Apparently, the negative throughput slope is significantly increased due to the intense of injection-locking and declined frequency response of the WRC-FPLD, which seriously distorts the OFDM spectra modulated on the WRC-FPLD carrier at intense injection condition. This is the reason to cause the reduced BER of the WRC-FPLD gradually saturated at 10−4 as the injection power enlarges from −9 to 3 dBm, in addition to the contribution of the relaxation oscillation noise peak up-shifted away from the OFDM modulation band. In Fig. 6(b), the green doc curves depict the RIN band of the WRC-FPLD, which are gradually up-shifted and attenuated to separate from the OFDM band. In contrast to the increasing BER of 16-QAM OFDM data carried by the WRC-FPLD at intense injection-locking power larger than −3 dBm in the back-to-back case (see Fig. 5(a)), the BER obtained at same condition after 25-km transmission becomes three times larger but still in a decreasing trend. This is mainly attributed to the dispersion effect during long-distance transmission, which eventually dominates over the declined modulation effect of the WRC-FPLD to degrade the BER of the 16-QAM OFDM data after 25-km transmission.
Owing to the declined frequency response of the WRC-FPLD under strong injection locking, the BER performance of the OFDM data stream transmission was limited. In previous works, the fine adjustment on the subcarrier amplitude to compensate the negative throughput slope (with a definition of R = dP/df in unit of dBm/GHz) of the modulation spectral response has been employed to improve the degraded BER. In this work, we also demonstrate such a subcarrier amplitude pre-leveling approach to flatten the modulation response of the injection-locked and directly modulated long-cavity WRC-FPLD. Figure 7(a) shows the pre-compensated 16-QAM OFDM spectra generated from the arbitrary waveform generator (AWG 7122B) with gradually enlarged positive throughput slopes prior to modulate the WRC-FPLD. Each subcarrier amplitude of the 16-QAM OFDM data was pre-leveled and several throughput slopes of R = 1.3, 1.9, 2.5 and 3.2 were preset for compensation. After 25-km SMF transmission, the spectra of these pre-compensated OFDM data stream with different throughput slopes directly modulated on the WRC-FPLD are received and shown in Fig. 7(b). To equivalently recover the declined frequency response of the WRC-FPLD under large bias and strong injection, it is observed from the modulated spectra that a positive throughput slope ranged between 1.9 and 2.5 can achieve the optimized compensation.
As a result, Fig. 8(a) illustrates the optimized BER response as a function of the spectral pre-leveling slope for the 16-QAM OFDM data stream carried by the directly modulated long-cavity WRC-FPLD. The pre-leveling effectively improves the BER performance from 1.1 × 10−4 to 3 × 10−5 by setting the positive throughput slope as dP/df = 1.9 dBm/GHz for the subcarrier amplitude of the 16-QAM OFDM to be transmitted by the WRC-FPLD under injection-locking power of 0 dBm. The BER performance is inversely degraded if the spectral slope of the pre-leveled 16-QAM OFDM data is less or over compensated, as shown in Fig. 8(a). With a pre-leveled throughput slope of 1.9, the 16-QAM OFDM data reveal a better constellation plot as compared to that transmitted without pre-leveling, as shown in Fig. 8(b). As expected, the directly modulated WRC-FPLD greatly improves its transmission capability and performance via the coherent injection-locking and the OFDM subcarrier pre-leveling processes. Figure 8(c) reveals the receiving power sensitivities of the 16-QAM OFDM data stream transmitted back-to-back or over 25-km long SMF by using the directly modulated and injection-locked WRC-FPLD without and with pre-leveling. Under back-to-back transmission, the directly 16-QAM-OFDM modulated WRC-FPLD without pre-leveling can achieve a BER of 3.8 × 10−3 (the criterion set by forward error correction FEC) at a receiving power of −10 dBm, and the receiving power penalty of −1 dB can be obtained by pre-leveling the 16-QAM OFDM subcarrier amplitude prior to modulate the WRC-FPLD.
After 25-km SMF transmission, the receiving power sensitivity is degraded to −8 dBm. The receiving power penalty further enlarged by 2 dB when comparing the back-to-back transmission with the 25-km transmission case. The receiving power penalty between the 25-km SMF and back-to-back transmitted OFDM data streams can be decreased to 0.75 dB after pre-leveling the subcarrier amplitudes. These results elucidate that the fusion of spectral pre-leveling techniques into the wavelength injection-locking WRC-FPLD based DWDM-PON transmitter can essentially compensate the modulated power declination, thus providing the BER of the 16-QAM OFDM data improved by almost one order of magnitude when transmitting over 25-km SMF by using the directly modulated and coherently injection-locked long-cavity WRC-FPLD.
The improved transmission of 16-QAM OFDM data with 5-GHz bandwidth by up-shifting and suppressing the relaxation oscillation induced relative intensity noise peak of the injection-locked and directly modulated WRC-FPLD is demonstrated. With injection-locking level enlarged from −12 to −3 dBm, the up-shifted RIN bands from 5 to 7.5 GHz effectively suppresses the noise level from −99 to −104 dBc/Hz at offset frequencies between 3 and 6 GHz, thus enhancing the SNR from 16 to 20 dB for the 16-QAM data carried by the OFDM subcarriers at higher offset frequencies. This improves the back-to-back transmission BER by more than one order of magnitude from 1.3 × 10−3 to 5 × 10−5. After 25-km transmission, the BER of the OFDM data can only be improved to 1.1 × 10−4 even with enlarging the injection-locking level up to 3 dBm. The degradation of BER by almost three times originates from the dispersion during long-distance transmission. The OFDM transmission is slightly distorted by the declined high-frequency response of the WRC-FPLD under intense injection-locking. The extremely large injection-locking power inevitably degrades the modulation throughput at high OFDM subcarrier frequencies and by enlarges the negative throughput slope from −0.8 to −1.9 dBm/GHz with injection level increased from −12 to −3 dBm. The OFDM data-stream pre-leveling technique effectively compensates the declined frequency response set by intense injection-locking, which improves the BER from 1.1 × 10−4 to 3 × 10−5 by setting the positive throughput slope as dP/df = 1.9 dBm/GHz for the subcarrier amplitude of the 16-QAM OFDM to be transmitted by the WRC-FPLD under injection-locking power of 0 dBm. After 25-km transmission, the fusion of pre-leveling and injection-locking promotes the receiving power sensitivity from −8 to −10 dBm at FEC limit, and reduces the power penalty between back-to-back and 25-km transmissions to only 2 dB. The up-shifted relaxation oscillation peak associated with the attenuated RIN level of the injection-locked WRC-FPLD facilitates the directly modulated 16-QAM OFDM transmission up to 20 Gbit/s.
This work was supported by the National Science Council, Taiwan, R.O.C., under grants 100-2623-E-002-002-ET and 101-2221-E-002-071-MY3, and Excellent Research Projects of National Taiwan University under grants 103R89081 and 103R89083.
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