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A 600-μm long-cavity laser diode with a front-facet reflectance of 2% is demonstrated as a colorless OC-192 transmitter for the future DWDM-PON, which is packed in a TO-56-can package of only 4-GHz frequency bandwidth but can be over-bandwidth modulated with 10-Gbit/s non-return-to-zero data-stream. The coherent injection-locking successfully suppresses its side-mode intensity and noise floor level, which further improves its modulation throughput at higher frequencies. With increasing the coherent injection-locking power from −12 to −3 dBm, the side-mode suppression ratio significantly increases from 39 to 50 dB, which also suppresses the frequency chirp from −12 to −4 GHz within a temporal range of 150 ps. The dense but weak longitudinal modes (with 0.6-nm spacing) in the long-cavity laser diode suppresses to one single-mode in a 100-GHz wide DWDM channel for carrying the OC-192 data at 9.953 Gbit/s. Such an over-bandwidth modulated laser diode still exhibits an on/off extinction ratio of 6.68 dB and a signal-to-noise ratio of 4.96 dB, which can provide a back-to-back receiving power sensitivity of −12.2 dBm at BER of 10−9. After 25-km DSF transmission of the OOK data-stream at a bit rate up to 10 Gbit/s, the receiving power sensitivity is −10.1 dBm at a requested BER of 10−9.
© 2013 Optical Society of America
1. Introduction
To meet the future demands on bandwidth channel capacity and selectivity of next-generation broadband networks, the dense wavelength division multiplexing passive optical network (DWDM-PON) has been recognized as the most promising solution [1]. The light source in either the central office (CO) or the user-end network unit requires a broadband gain spectrum to offer each optical network unit (ONU) channel its own carrier wavelength [2]. Therefore, the universal light sources with operating wavelengths covering all DWDM channels play a pivotal role in promoting the feasibility of the next-generation DWDM-PON system. Several kinds of laser diodes have been considered so far, including the distributed feedback laser diodes (DFBLDs), the mutually injection-locked Fabry-Perot Laser Diodes (FPLDs) at central office or user end [3], the injection-locked reflective semiconductor optical amplifiers (RSOAs) [4, 5], the injection-locked Fabry-Perot laser diodes (FPLDs) [5–9] and the injection-locked long-cavity colorless laser diodes (or so-called weak-resonant-cavity FPLDs, WRC-FPLDs) [10–12] at optical network unit, etc. Among the proposed DWDM-PON transmitters, the injection-locked long-cavity colorless laser diode has also been investigated as a promising candidate because of its specific properties such as broadband colorless operation with weak longitudinal modes and partial coherence. In previous work, the 10-Gbit/s modulation of a coherently injection-locked conventional FPLD has been demonstrated [13]. The main drawback of a typical FPLD with high end-facet reflectance is that the injection-locking range is extremely small hence accurate wavelength control between the injected master and the FPLD slave is needed. Since 2009, a new class of colorless laser diode was originated from the imperfect anti-reflection (AR) coating on the front-facet of a conventional FPLD [14, 15]. Such a long-cavity colorless laser diode features a larger injection-locking range and a broader gain spectrum than a conventional FPLD, which further provides better coherence with lower noise figure than a RSOA after injection. By implementing injection-locking technique [16], the long-cavity colorless laser diode transmitters can cost-effectively offer each ONU channel its carrier wavelength. The effect of low front-facet reflectance (<2%) on the 2.5 Gbit/s data transmission performance of a coherently injection-locked long-cavity colorless laser diode has been investigated [17]. In the past work, a bias tee was used to couple the DC bias current and PRBS modulation data for driving the long-cavity colorless laser diode to achieve 2.5 Gbit/s transmission. At that time, neither the microwave connector nor the package was considered in detail. Besides, the OOK and orthogonal-frequency-division-multiplexing (OFDM) transmission performance of the injection-locked Fabry Perot laser diode with a slightly decreased front-facet reflectance (10%) has also been discussed [18]. The injection-locked FPLD with front-facet reflectance of 10% can achieve high-speed modulation by using the modulation format of OFDM [18, 19]. After 50-km SMF transmission, the BER still reaches the required 3.8 × 10−3 (forward error correction threshold) with injection power of −12 dBm [19]. Although increasing the front-facet reflectance further improves the coherence of cavity, which also suffers from a slight shrinkage on selectable channel linewidth and a severe range on the injection-locking wavelength.
In previous works, the colorless RSOA channelized by the WDM-PON itself was first proposed to be the universal slave laser transmitter [20]. However, the drawbacks of non-resonance and non-coherence still limit the direct modulation bandwidth of the RSOA. In comparison, the long-cavity colorless laser diode can be considered as a promising solution owing to the advantages of its colorless operation and partial coherence. The long-cavity colorless laser diode based WDM-PON system with channelized amplified spontaneous emission (ASE) injection-locking located at remote node was proposed. The channelized ASE-injected long-cavity colorless laser diode [21, 22] has shown colorless operation with a limited transmission data rate of 2.5 Gbit/s due to the relative intensity noise (RIN) accompanied with ASE. To upgrade the transmission capacity that can meet the inevitably increased demand of next-generation DWDM-PON, there have been several attempts to operate different light sources up to 10 Gbit/s. As early as 1995, the 10-Gbit/s long-haul transmission using a directly modulated DFBLD was proposed [23]. The DFBLD is undoubtedly the most ideal choice owing to its high-speed modulation property; however, the applicability of DFBLD in all DWDM-PON channels with different wavelengths makes it the highest cost solution. In 2007, a continuous wave (CW) injection-locked FPLD based high-speed bidirectional WDM-PON was presented, which offers 16 injection-locked channels for DWDM-PON application [9]. Even the low-bandwidth sources like RSOA [24, 25] have been demonstrated their capability for carrying NRZ data at 10 Gbit/s by using electronic equalization and FEC techniques. The operation of versatile laser diodes well beyond their intended bandwidth through careful interfacing to the RF source have been investigated [26–32]; however, most of previous works were focused on the discussion of conventional laser diodes. As the long-cavity colorless laser diode possesses the specific properties of broad spectrum and partial coherence, it is worthy to investigate the possibility of operating the long-cavity colorless laser diode transmitter at high-speed NRZ modulation.
In this work, a 600-μm long-cavity laser diode with its front-facet reflectance as low as 2% is demonstrated as a colorless and universal OC-192 transmitter for the future DWDM-PON after injection-locking, which is packed in a TO-56-can package with a modulation frequency bandwidth of only 4 GHz but can be over-bandwidth directly modulated to deliver the non-return-to-zero data-stream at 10 Gbit/s effectively. With the modified rate equations under injection-locking case, the ideal output eye-diagram of the directly NRZ-OOK encoded long-cavity colorless laser diode is simulated by using the experimentally obtained parameters. The effects of biased current and injection-locking power on the modulation throughput, bandwidth and bit-error-rate (BER) performance under OC-192 NRZ data transmission are characterized. The modulation response, side-mode suppression ratio (SMSR), extinction ratio (ER), signal-to-noise ratio (SNR) of the long-cavity colorless laser diode under coherent injection-locking are optimized to promote the receiving power sensitivity of the 25-km transmitted 10-Gbit/s NRZ data-stream with such a TO-56 packed long-cavity colorless laser diode.
2. Experimental setup
The proposed 10-Gbit/s DWDM-PON system with a coherently injection-locked and directly modulated long-cavity colorless laser diode transmitter for transmitting the OC-192 NRZ-OOK broadcast data is illustrated in Fig. 1. The long-cavity colorless laser diode is a buried heterostructure Fabry Perot laser diode with a front-facet reflectance as low as 2%. First, A 200 nm-thick n-type InP layer with dopant concentration of 5 × 1018 cm−3 was grown on InP substrate and then two n-type InP layers containing dopant concentrations of 2 × 1018 cm−3 were employed as the cladding layers. The active InGaAsP layer were composed by the 0.9% compress-strain multi-quantum wells with well thickness of 50 Å and the 0.6% tensile-strain barriers with thickness of 85 Å at 1.1 μm. Afterwards, a 500 nm-thick intrinsic InP was grown and then two p-type cladding layers with dopant concentration of 1 × 1017 cm−3 (100-nm thick) and 1 × 1018 cm−3 (1.2-μm thick) were grown on the gain region. At last, a 200 nm-thick p-type InGaAs layer with dopant concentration of 2 × 1018 cm−3 was deposited as the top contact layer. The long-cavity colorless laser diode was designed as same as the conventional structure of FPLD with its front-facet reflectance ranged between 1.5% and 2% and its resonant cavity length of 600 μm. At optical network unit of user end, the long-cavity colorless laser diode was externally injection-locked by a single-mode tunable laser (Agilent, 8164A) in central office passing through a polarization controller. The injection level was adjusted from −12 to −3 dBm to analyze the effect of injection-locking power on the SMSR, modulation bandwidth, chirp, and BER of NRZ transmission. The long-cavity colorless laser diode was packed in a conventional TO-56-can package with a ball lens and pigtailed by a SMF-fiber with FC/APC type connector [10]. Comparing to the previous work [17], the pin of the TO-56-can was cut and shortened to 1.5 mm long, and was soldered to a jacket SMA connector with a frequency bandwidth of up to 12 GHz, as shown in the right part of Fig. 1. To improve the direct modulation response, the SMA-connected input port of the long-cavity colorless laser diode was directly driven by a PRBS OC-192 NRZ-OOK pattern generator (Agilent, 70843B) at a pattern length of 223-1 without using other microwave components such as a bias tee. Such an operation releases the influence of the electrical fading circuitry in the modulation bandwidth of the long-cavity colorless laser diode module. The DC biased point was set at 2 times of the threshold current (Ith) and the peak-to-peak amplitude of the electrical PRBS digital data was set at 500 mV.
Fig. 1 Left: A DWDM-PON testing system with wavelength-tunable laser source for analyzing the coherently injection-locked and directly NRZ modulated long-cavity colorless laser diode transmitter for OC-192 transmission. Right: The illustration of the TO-56-can packed long-cavity colorless laser diode.
Afterwards, the power-current characteristic of the long-cavity colorless laser diode injection-locked at different powers were measured by a commercial optical power meter (ILX OMM-6810B). A commercial microwave spectrum analyzer (Agilent, 8565E) was used to measure the frequency modulation responses with increasing the injection power level from −12 to −3 dBm. The dynamic chirp of the transmitted data was measured by a commercial optical chirp form tester (Advantest, Q7606B). The eye-diagram of the OC-192 NR data transmitted by the injection-locked long-cavity colorless laser diode was analyzed by a digital sampling oscilloscope (Agilent, 86106A). According to the datasheet of DCA (Agilent 86106A), the CW inaccuracy is only 0.2%. The BER performances of the 10 Gbit/s NRZ-OOK data transmitted by the long-cavity colorless laser diode was analyzed via a commercial BER tester (Agilent, 70843A) after receiving by a NRZ optical receiver (Agilent, 83434A). In consideration of the future application in optical distribution network, both the 25-km single-mode fiber (SMF) and the 25-km long dispersion-shifted fiber (DSF, Corning MetroCor Optical Fiber) were applied to the DWDM-PON setup for characterizing the metropolitan transmission performance.
3. Results and discussions
Figure 2 shows the power-current characteristics of the long-cavity colorless laser diode without and with coherent injection locking varying from −15 to 0 dBm. The externally injected photons can effectively promote the stimulated emission by reducing the spontaneous emission to enhance the gain of the colorless laser diode [33, 34]. In other words, the threshold current is decreased by implementing injection locking, because the externally injected photons effectively promote the stimulated emitting photons in the long-cavity colorless laser diode cavity [34]. As the injection-locking power increases from −15 to 0 dBm, the threshold current of the injection-locked laser diode is effectively reduced from 14.5 to 9 mA. This causes the increasing output power with the enlarged injection level. The power-to-current slope (dP/dI) at bias currents larger than 20 mA keeps invariant, whereas the dP/dI at slightly above threshold region shows a decreasing trend due to the contribution of the externally injected photons. At such low bias conditions, the excited carriers fail to duplicate all of the externally injected photons such that most of the incoming photons left in the cavity but not amplified. In this case, the output response behaves somewhat like the summation of powers from the duplicated stimulated emission and the external injection but the latter one is dominated in this region. According to the lasing spectra shown in Fig. 2, the gain-spectrum with a 3-dB linewidth of 4.7 nm is provided at a bias current around two times of it threshold current. By applying the coherent injection-locking at −12 dBm, the long-cavity colorless laser diode is ensured to be operated in single-mode lasing with a spectral linewidth (FWHM) of 0.08 nm.
Fig. 2 spectra without (left) and with injection locking (middle) and power-current responses (right) of the long-cavity colorless laser diode.
Although injection locking has been verified since early years to enhance the intrinsic modulation bandwidth [35, 36] of a laser diode, the injection-locked long-cavity colorless laser diode shows a slightly extraordinary modulating response as compared to other transmitters. According to left part of Fig. 3, the relaxation resonance frequency of the long-cavity colorless laser diode is also enhanced by injection locking, which is attributed to the enhancement on both the relative strength of the coupling and gain coefficients [37]. In the meantime, the injection-locked long-cavity colorless laser diode shows a decayed throughput power with almost identical negative power to frequency slope at <5 GHz but presents a better response in high frequency region (6-10 GHz), which essentially benefits the NRZ transmission performance under high-speed modulation. The improvement on high-frequency (7-10 GHz) response becomes much apparent as the injection power increases from −12 to −6 dBm. However, as the coherent injection-locking power is larger than −6 dBm, the throughput power response in high frequency region seems to seriously decade. Injection locking with appropriate injection-locking power can effectively enhance the bandwidth of the long-cavity colorless laser diode; however, the response of the long-cavity colorless laser diode at low frequency region is reduced.
Fig. 3 Left: the frequency responses of the long-cavity colorless laser diode at different injection-locking power level. Right: the SMSR and frequency bandwidth of the coherently injection-locked long-cavity colorless laser diode with different injection powers.
At constant current, the excited carriers are limited so that only part of the injected photons are duplicated to enhance the stimulated emission, whereas other excessive photons fail to induce more stimulated emission photons and remain as the continuous-wave lasing in the colorless laser diode cavity. This results in a stronger DC component and a gradually decayed throughput power in the frequency response spectrum of the injection-locked long-cavity colorless laser diode. The intense injection-locking inevitably leads to the enlarged negative power-to-frequency slope, which eventually degrades the high-frequency modulation bandwidth. To enhance the modulation response, the bias current has to be further increased such that the abundant carriers can provide sufficient throughput power response at higher frequency region. The correlation among the SMSR, modulation bandwidth and the injection-locking power of the coherently injection-locked long-cavity colorless laser diode is depicted in the right part of Fig. 3. The externally injected photons with high coherence assist the injection-locked mode in achieving gain, which enables the SMSR to enhance from 39.6 to 50.3 dB by enlarging the injection-locking level from −12 to −3 dBm with the suppressed side-mode intensity and ASE power. Nevertheless, the effect of side-mode suppression seems to saturate at injection power larger than −3 dBm. When the injection power goes beyond −3 dBm, the SMSR can be further enhanced by less than 1 dB. The frequency bandwidth (f6dB) of the long-cavity colorless laser diode is also affected by injection power at some extent. As injection power is adjusted from −12 to −9 dBm, the 10-dB frequency bandwidth increases from 7.8 to 9.1 GHz. Apparently, implementing appropriate injection-locking power can effectively suppress the side-mode intensity but inevitably set a compromise between the modulation throughput responses in low (<5 GHz) and high (6-10 GHz) frequency regions of the long-cavity colorless laser diode.
With the modified rate equation set [38, 39] for the coherently injection-locked long-cavity colorless laser diode by using the experimentally obtained modulation response in time domain, the eye-diagram of the directly NRZ-OOK encoded long-cavity colorless laser diode can be simulated, as given by
in which N denotes the carrier number, ϕ the phase difference (ϕ slave- ϕ master), S the photon number of the long-cavity colorless laser diode, I the bias current, ηi the internal quantum efficiency, a the differential gain, Γ the optical confinement factor, νg the velocity, τp the photon lifetime, τs the spontaneous carrier lifetime, κ the coupling efficiency, α the linewidth enhancement factor, Sinj the injection photon number, and Δωinj the detuning frequency. The requested characteristic parameters of the long-cavity colorless laser diode for analytically solving the aforementioned equations are summarized in Table 1.
Table 1. Characteristic Parameters of Laser Diodes for Simulation
In Fig. 4, the output shape of the directly NRZ-OOK encoded long-cavity colorless laser diode is simulated by either using a degraded shape obtained by fast Fourier transforming the modulation response of the long-cavity colorless laser diode in frequency domain, or by simply using the original electrical OOK data shape in time domain that is output from the PRBS generator. Alternatively, such a shape can be accessed by simply seeding a TTL data into the long-cavity colorless laser diode and then functionalize the monitored output.
Fig. 4 (a) the modulation response in frequency domain, (b) the FFT of modulation response in time domain, (c) the FFT (solid) and fitting function (dashed), and (d) the simulated eye-diagram.
The analog modulation response of the long-cavity colorless laser diode in frequency domain is shown in the upper left of Fig. 4(a). For simulation, the electrical OOK data shape at the PRBS generator output in time domain, as shown in Fig. 4(b). After fitting the time-domain response with a polynomial function at a degree of 9 [see Fig. 4(c)] and sending into the MATLAB simulation program, the rate equation can numerically solved to provide a simulated eye-diagram, as shown in the lower right part of Fig. 4(d). In this simulation, the modulation rate of the injection-locked long-cavity colorless laser diode is 9.953 Gbit/s (OC-192), and the external injection-locking power is −3 dBm. According to the simulated result, even the long-cavity colorless laser diode with 3-dB bandwidth of only 4.9 GHz can present a clear eye-diagram at 10 Gbit/s without serious distortion. By directly modulating the coherently injection-locked long-cavity colorless laser diode with a NRZ-OOK PRBS data stream at 10 Gbit/s, the eye-diagrams without and with increased injection power from −12 to −3 dBm are shown in Fig. 5. According to the frequency response shown in Fig. 3, the long-cavity colorless laser diode without injection shows a flat response in low frequency region and a 3-dB bandwidth of 4.9 GHz. Therefore, the long-cavity colorless laser diode without injection is expected to demonstrate a clear eye-diagram by using full-band carrier to transmit. Without injection, the overshooting effect is observed on the leading edge of the eye-diagram owing to the frequency chirp induced when biasing the laser diode closer to the threshold. As the injection power is enlarged to effectively reduce the threshold current of the laser diode, the chirp induced overshooting phenomenon can be greatly suppressed. With coherent injection-locking, it is feasible to control the whole power of modulation signal in single mode so that the modulation throughput response can be maximized within a limited injecting level. As increasing the injection power decreases the threshold current, the modulating amplitude from pattern generator is modified for reaching the optimized ER. As enlarging the injection-locking power can suppress the side-mode intensity and the noise floor, the ER and SNR can be improved from 5.36 to 5.88 dB and 4.3 to 4.41 dB, respectively, with injection power from −12 to −6 dBm. However, the improvement on SNR seems to be confined by the imperfect injection-locking condition. That is, the long-cavity colorless laser diode without temperature controller cannot avoid the fluctuation in temperature during the process of injection locking. Therefore, the BER response of the long-cavity colorless laser diode is unable to demonstrate an oblique line. With imperfect injection-locking situation, the external photons without modulation become part of noise in the cavity. With injection-locking power of −3 dBm, the long-cavity colorless laser diode shows a receiving power of −12.2 dBm with ER of 6.68 dB and SNR of 4.96 dB.
Fig. 5 The optical eye-diagrams and BER of the directly OC-192 NRZ modulated long-cavity colorless laser diode without and with injection power from −12 to −3 dBm.
In addition, the frequency chirp plays a pivotal role in high-speed modulated transmission. The Fig. 6 shows the time-resolved intensity and chirp waveforms for the long-cavity colorless laser diode coherently injection-locked at different power levels. The dynamic chirp of the free-running long-cavity colorless laser diode can be estimated by [21, 40]
where Δνc denotes the time-dependent frequency chirp, ϕ the dynamic phase change, and SLm denotes the maximal photon number in the injection-locked mode. The rising part of the output signal from long-cavity colorless laser diode exhibits a positive chirp due to the positive frequency deviation. Similarly, a negative chirp is observed in the falling part of the output signal from the long-cavity colorless laser diode. The injection-locking technique definitely shows a remarkable improvement on suppressing the dynamic chirp [27, 40, 41] and enhance the signal-to-noise ratio [42] of the coherently injection-locked long-cavity colorless laser diode. By implementing a coherent injection locking with power of −9 dBm, the peak-to-peak chirp is suppressed from −12.1 (at free-running case) to −5.4 GHz. Moreover, it can be observed that as the injection power increases from −9 to −3 dBm, the peak-to-peak chirp further reduces to −4.1 GHz. The strong injection can reduce the change of carrier density and the corresponding variation on refractive index in the active region. Therefore, the frequency deviation in response to the changes of carrier density and refractive index is also decreased.Fig. 6 Transmitted data chirp of injection-locked long-cavity colorless laser diode without and with coherent injection at different power levels.
In consideration of practical optical distribution network, the BER performance of OC-192 NRZ data stream over 25-km transmission is discussed. Under back-to-back, 25-km SMF and 25-km DSF transmissions, the BER of the directly modulated long-cavity laser diode at 7, 8, and 10 Gbit/s are compared in Fig. 7. The injection-locking level is set as −6 dBm, because the BER response at 10 Gbit/s reveals a significant reduction on receiving power penalty by 5 dB when the long-cavity colorless laser diode is injection-locked with a power of more than −9 dBm. As the modulation rate increases from 7 to 8 Gbit/s in back-to-back transmission case, the ER slightly degrades from 6.1 to 5.8 dB to cause a power penalty of about 1 dB at BER of 10−9. The transmitted eye-diagram suffers from a chromatic dispersion to seriously distort after 25-km SMF transmission. As a result, the SNR is decreased by 2.1 dB to cause an additional power penalty of 0.7 dB at least. To overcome the chirp induced degradation, the 25-km long DSF with zero-dispersion wavelength shifting to 1550 nm is applied to suppress the chromatic dispersion. With an injection-locking power of −6 dBm, the received eye-diagram before and after OC-192 filtering and reshaping are compared in left part of Fig. 7. It is observed that the distortion can be significantly improved by filtering the received data stream with a standard OC-192 optical receiver. At a requested BER of 10−9, the receiving power is −17.8 and −16.7 dBm with data rate of 7 and 8 Gbit/s, respectively. If the transmission bit rate is further increased up to 10 Gbit/s, the receiving power sensitivity inevitably degrades to −10.1 dBm at BER of 10−9, which results in a power penalty of −6.7 and −7.7 dB as compared to the sensitivities obtained at 7 and 8 Gbit/s respectively. A significant BER floor occurs when transmitting the data stream at bit rate of up to 10 Gbit/s, which is attributed to the finite SNR and ER of the long-cavity colorless laser diode operated at such high data rate.
Fig. 7 Left: the measured eye-diagrams of (a) the optical data-stream and (b) the OC-192 filtered and reshaped electrical data-stream from the directly modulated long-cavity colorless laser diode with injection-locking power of −6 dBm at bit rate of 10 Gbit/s under 25-km DSF transmission. Right: the BER responses of the directly modulated long-cavity colorless laser diode at bit rate of (a) 7 Gbit/s, (b) 8 Gbit/s, and (c) 10 Gbit/s under back-to-back (short dashed), 25-km DSF (solid), and 25-km SMF (dashed) transmissions.
In high-speed transmission, the BER performance is seriously affected by chirp and modulation bandwidth of the long-cavity colorless laser diode. Since the coherent injection locking efficiently reduces the dynamic frequency chirp and enlarges the throughput power at frequencies beyond modulation bandwidth, the long-distance transmission performance of the directly OC-192 NRZ modulated long-cavity colorless laser diode can be significantly improved. Without considering the package bandwidth, the simulation results show less distorted eye-diagram as compared to those obtained from experiments. This also elucidates the contribution from the TO-56-can with a limited bandwidth of only 4 GHz. Therefore, it is expectable to achieve a better OC-192 NRZ transmission performance via improving the bandwidth of TO-56-can package. Several solutions are considered to facilitate the long-cavity colorless laser diode for high-speed applications at next stage. Firstly, the transmission performance of the long-cavity colorless laser diode under high-speed modulation strictly depends on the inherent modulation bandwidth of the whole package. However, the bandwidth of long-cavity colorless laser diode is almost limited by TO-56-can package (with a 3-dB bandwidth of about 4 GHz). Therefore, improving the bandwidth of the TO-can package would be straightforward to further enhance the transmission bit rate of long-cavity colorless laser diode up to 10 Gbit/s. Secondly, considering that the 3-dB bandwidth of the laser diode is also proportional to the square root of (Ibias-Ith), the increase of bias current is also expected to improve the 25-km SMF transmission performance of long-cavity colorless laser diode. At last, the BER floor shown in Fig. 5 indicates that the transmission of the directly OC-192 NRZ modulated long-cavity colorless laser diode without temperature control could be impaired by thermal fluctuations. Hence, the stabilized temperature control of the slave long-cavity colorless laser diode is also mandatory to improve the receiving power sensitivity at BER of 10−9 under 10-Gbit/s transmission. Nevertheless, this work has already demonstrated a new record on modulation bandwidth and OOK data bit-rate for such a 600-μm long-cavity colorless laser diode proposed for the future DWDM-PON applications at a bit-rate over 10 Gbit/s.
4. Conclusion
In this work, we demonstrate the possibility of 10-Gbit/s NRZ data transmission using a long-cavity colorless laser diode packed in a TO-56 can with a modulation bandwidth limited at 4 GHz. This implementation relies on the coherent injection locking and specific SMA connection of such a TO-can packed long-cavity colorless laser diode transmitter. The dense but weak longitudinal modes (with 0.6-nm spacing) in the long-cavity laser diode suppresses to one single-mode in a 100-GHz wide DWDM channel for carrying the OC-192 data at 9.953 Gbit/s. The effect of injection power on modulation throughput, SMSR, chirp, ER and SNR are also analyzed to optimize the BER performance of the beyond-bandwidth modulated laser diode. With increasing the coherent injection-locking power from −12 to −3 dBm, the side-mode suppression ratio significantly increases from 39 to 50 dB, which also suppresses the frequency chirp from −12 to −4 GHz within a temporal range of 150 ps. The coherent injection-locking shows the significant suppression on the noise floor level and frequency chirp, providing an enhanced SMSR of 42 dB and reduced chirp of −5.4 GHz even with the injection of only −9 dBm. After overcoming the limited frequency bandwidth set by the conventional TO-56-can package, the directly NRZ modulated data rate of the long-cavity colorless laser diode can be operated up to 10 Gbit/s with its ER and SNR of 6.68 dB and 4.96 dB, respectively. At back-to-back transmission case, the receiving power sensitivity of −12.2 dBm at BER of 10−9 is achieved. As the strong injection-locking effectively reduces the transient change on carrier density and corresponding refractive index in active region, the peak-to-peak chirp is further suppressed to −4 GHz by enlarging the coherent injection-locking power up to −3 dBm. For 25-km transmission, the receiving power can be improved by using DSF instead of SMF owing to the improvement on dispersion. At a requested BER of 10−9, the receiving power of the 25-km DSF transmission is −16.7 dBm with a NRZ-OOK data rate of 8 Gbit/s. By increasing the bit rate up to 10 Gbit/s, the receiving power sensitivity slightly degrades to −10.1 dBm with corresponding power penalty of 6.7 dB. For such a 600-μm long-cavity colorless laser diode, these are already the new records on both the allowable modulation bandwidth and the transmittable OOK data bit-rate, which enables its future DWDM-PON applications at a bit-rate over 10 Gbit/s.
Acknowledgments
This work was partially supported by the National Science Council, Taiwan, R.O.C. and the Excellent Research Projects of National Taiwan University, Taiwan, R. O. C., under grants NSC100-2221-E-002-156-MY3, and NSC101-2221-E-002-071-MY3.
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