System performances of the MCU-based auto-restorable injection-locking transmitter

MCU Desired Temperature (o C)

Fig. 5.8 The recovery experience of the self-restorable unlocked FPLD and corresponding MPD current controlled by MCU.

5.5 System performances of the MCU-based auto-restorable injection-locking transmitter

To obtain the BER performance, the DC bias current of the FPLD is set as 30 mA and the amplitude of the pseudo random binary sequence (PRBS) data stream to directly modulate the FPLD is set to reach an on/off extinction ratio of 10 dB. The FPLD temperature is controlled at 25^oC. In the Fig. 5.9, the BER performance was analyzed at back-to-back condition without the use of any WDM filter between FPLD and receiver, as shown in the Fig.

5.1 of the manuscript. First of all, the BER performances of the FPLD working at free-running and manually operated to reach a perfectly injection-locking condition are measured and shown as the blue and green curves, respectively. Without perfect matching between the FPLD mode and the external injection wavelength (i.e. the loose injection-locking condition), the BER significantly degrades with its response curve located between those of the free-running and manually injection-locking cases. The BER response

obtained when detuning the injection wavelength away from the FPLD mode by only 0.03 nm

Fig. 5.9 The BER of the FPLD transmitter measured at three different conditions.

Afterwards, the MPD-MCU based self-restorable injection-locker is initiated to achieve auto-restoration injection-locking, and the measured BER response (black dashed curve) shows almost identical response with the best case obtained at manually injection-locking condition. Note that the auto-restoration speed strictly depends on the cooling/heating efficiency of the TEC and the environmental temperature. Such a long lock-in time is the worst case only when the TEC chip for controlling the temperature of the FPLD is located outside the transmitter module. Therefore, a delayed temperature compensating response is inevitably observed, which can be greatly improved by adding high-power or built-in TEC.

A commercially available FPLD TO-can module with a built-in TEC device [5.12] has been introduced recently in the market, which could greatly shorten the temperature compensation response as well as the lock-in speed through the proposed self-restoration scheme. In

addition, the measured BER of the optical transmitting eye diagram can accurately calculate from the recorded Q factor [5.13]. At 2.5 Gbit/s, the BER of the injection-locked FPLD modulated by PRBS data-steam with 2³¹-1 pattern length is 10^-12 at a receiving power of -25 dBm. After the self-restoration, the Q factor measured from the optical eye histogram is as much as 8.2, providing a reachable BER of 1.1u10^-16 at a data rate of 2.5 Gbit/s.!!

! Fig. 5.10 Left: (a) Threshold current of FPLD before and after self-restoration. Right: The Optical eye-diagrams (b) before and (c) after auto-restoration.

The optical eye-diagram and Q value of before and after auto-restoration are shown in Fig. 5.10. The lower Q value of about 7.4 caused by the significant overshoot of the FPLD transmitted data-stream is obtained before auto-restoration condition, which is mainly attributed to the lower bias point and modulation base below the FPLD threshold current as shown in Figs. 10(a) and 10(b). After auto-restorable injection-locking operation, the threshold current of FPLD is effectively decreased due to the optical injection at a proper wavelength coincident with the longitudinal mode of the FPLD [5.5], such that the overshoot problem observed on the eye-diagram is solved and the Q value can be further promoted to 8.2 or larger as shown in Fig. 5.10(c). We have also measured the BER performances by

slightly detuning the wavelength of incoming source away from that of the free-running FPLD at very beginning, then comparing the receiving sensitivity of FPLD before and after self-restoration control, as shown in Fig. 5.11. Without self-restoration scheme, the degradation on BER performance of the injected FPLD transmitter becomes serious with increasing wavelength deviation. In comparison, the power penalty for obtaining the same BER at 10^-9 from the injection-locked FPLD transmitter without a self-restoration control greatly increases from 0.8 to 1.9 dB as the deviated wavelength enlarges from 0.02 to 0.04 nm.

After self-restoration control, the mode wavelength of the FPLD eventually coincides with that of the incoming source to results in a receiving sensitivity of -30.1 and -28.2 dBm at BER of 10^-9 and 10^-12, respectively.

Fig. 5.11 The BER of the FPLD transmitter before and after self-restoration at different wavelength-deviation conditions.

The additional cost is an important issue for the implementation of the self-restorable injection-locked FPLD in practical networks. The component expense of the proposed self-restorable injection-locking FPLD scheme is briefly listed as Table I, which is compared with the reflective semiconductor optical amplifier (RSOA) based injection-locked transmitter

[5.14] and the traditional coherent wavelength injection locked FPLD without self-restorable function. As compared with the ROSA (OA-RL-OEC-1550, CIP) based injection-locked transmitter, a great reduction on the cost at the light source is reached due to the use of a commercially available FPLD in our scheme. The TEC cooler and controller are necessary for all architectures. Only the cost of an ADC/DAC integrated MCU controller is added into the proposed scheme, but the market price of a popular MCU (such as the ATMEL MEGA88V) used in commercial systems is just US$ 1.

TABLE 1. Expense comparison of the injection-locked transmitter solutions between RSOA and FPLD with/without self-restoration.

System Expense RSOA WDM-PON Injection-locked FPLD with Self-Restoration

Injection-locked FPLD without Self-Restoration

Light Source US$500~1000 US$5~20 US$5~20

TEC controller US$10 US$10 US$10

MPD-MCU - US$ ~1 -

For the future integration of optical transceiver in the proposed system, a schematic diagram was modified from a typical 2.5-Gbps small-form-factor-pluggable (SFP) optical transceiver (SI1525-40ATO, SANOC). Most of the components in the system are commercial and ready-to-use parts in a typical SFP transceiver application. For example, the laser diode driver, limiting amplifier, TEC controller, MCU, and PIN-TIA receiver were chosen as MAX3738 (MAXIM), MAX3747 (MAXIM), MAX8521 (MAXIM), ATMEGA88 (ATMEL), and TRR-1F41-320 (Truelight), respectively. In particular, the proposed system needs to control the temperature of the FPLD and a TEC cooler should be integrated in the transceiver.

A commercial laser TO-can module built-in TEC cooler [5.12] with a maximum diameter of

5.6mm and a total length of 12.7 mm was demonstrated by Sumitomo Electric Industries, Ltd.

as shown in Fig. 5.12. By implanting such a transmitter, the system could be achieved in a compact-size module such as a typical SFP optical transceiver for future access networks and LAN application.

! Fig. 5.12 Future system of the MPD-MCU based self-restoration unit.

However, some degradation on the eye-diagram and BER when detuning the polarization state of the external injection to be orthogonal to the preferred polarization state of the FPLD was observed. In practical application, such a problem relies on the self-restoration of the preferred polarization for the injection-locked FPLD, which can be achieved by feedback controlling an electronically tunable polarization controller added prior to the WRC-FPLD.

Our proposed scheme cannot eliminate the polarization sensitive problem accompanied with the injection-locked FPLD, however, and the MCU of self-restorable injection locker can concurrently deal both the deviation of FPLD wavelength and the mismatched polarization of the incoming light. The schematic Fig. 5.14 of the self-restorable injection locker modified to involve the input polarization function is shown as below. The polarization degree

directly affects the injection efficiency, which is a little bit different from the wavelength detuning effect. That is, the requested power of optical injection into the FPLD could be greatly increased due to the deviation of the injected polarization from the preferred state of the FPLD. In our self-restorable design, the concept to achieve the self-restoration of the preferred polarization for the FPLD is based on feedback controlling an electronically tunable polarization controller added prior to the FPLD to be injection-locked. The MPD current of the FPLD is monitored during operation for feedback controlling the polarization controller to obtain the maximum output power from the injection-locked FPLD. After comparing the monitored MPD current with previous value, the MCU sends a controlling signal to detune incoming polarization via a polarization controller. With the same feedback control loop, the two important issues of wavelength locking and polarization matching can be concurrently implemented for the practical self-restorable injection locker. In particular, the polarization control is the unique and inherent problem when utilizing the injection locking technique to control the laser diode wavelength. Nevertheless, such injection-locked laser diodes still exhibit a benefit on narrow linewidth, long transmission distance, and especially the enhanced modulation bandwidth when comparing with other kinds of injection-locked sources.

5.6 Summary

In this chapter, a novel in situ and self-restorable scheme has been demonstrated for the injection-locking FPLD-based WDM transmitter by an integrated MPD in connection with an MCU controlling unit. The injection-locking and self-restoration characteristics such as the locking-in wavelength range and the illuminated power dependent MPD current of the injection-locked FPLD are analyzed. The integrated MCU calculates the monitored photocurrent from the MPD and dynamically controls the FPLD temperature via a TEC controller. After self-restorable injection locking, the transmitter exhibits a Q factor of 8.2 to provide a reachable BER as low as 1.1ˢ10^-16, and an SMSR of >35 dB. The FPLD directly

modulated by a PRBS datasteam can be self-restoration injection-locked within 50 seconds, achieving a BER of 10^-12 at a data rate of 2.5 Gbit/s with a receiving power as low as -25 dBm.

With the self-restorable injection-locking scheme, the APC function that was prohibited in the injection-locked FPLDs can be added into future FPLD-based optical networks with a vastly improved stability and reliability. The proposed scheme works for most of the injection-locking FPLDs and can be cost effectively implemented using a minimum amount of redundant network resources.

References

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[5.3] Z. Xu, Y.-J. Wen, W.-D. Zhong, C.-J. Chae, X.-F. Cheng, Y. Wang, C. Lu, and J.

Shankar, “High-speed WDM-PON using CW injectionlocked Fabry-Pérot laser diodes,” Opt. Express. 15, 2953, (2007).

[5.4] N. Kashima, S. Yamaguchi, and S. Ishii, “Optical transmitter using side-mode injection locking for high-speed photonic LANs,” IEEE J. Lightwave Technol. 22, 550, (2004).

[5.5] Y.-C. Chang, Y.-H. Lin, J. H. Chen, and G.-R. Lin, "All-optical NRZ-to-PRZ format transformer with an injection-locked Fabry-Perot laser diode at unlasing condition", Opt. Express. 12, 4449, (2004).

[5.6] C. K. Chan, F. Tong, L. K. Chen, K. P. Ho, and D. Lam, “Fiber-fault identification for branched access networks using a wavelength-sweeping monitoring source,” IEEE Photon. Technol. Lett. 5, 614, (1999).

[5.7] K. Lee, S. B. Lee, J. H. Lee, Y. -G. Han, S. -G. Mun, S. -M. Lee, and C. -H. Lee, "A self-restorable architecture for bidirectional wavelength-division-multiplexed passive optical network with colorless ONUs," Opt. Express, 15, 4863, (2007).

[5.8] R. D. Esman and L. Goldberg, “Simple measurement of laser diode spectral linewidth using modulation sidebands,” Electron. Lett. 24, 1393, (1988).

[5.9] S. Bouchoule, N. Stelmakh, M. Cavelier, and J.-M. Lourtioz, “Highly attenuating external cavity for picosecond-tunable pulse generation from gain/Q-switched laser diodes,” IEEE J. Quantum Electron. 29, 1693, (1993).

[5.10] L. Li, “Static and dynamic properties of injection-locked semiconductor lasers,”

IEEE J. Quantum Electron. 30, 1701, (1994).

[5.11] K. Petermann, “Laser Diode Modulation and Noise,” Publishers Dordrecht, The Nitherlands: Kluwer Academic, (1998).

[5.12] M. Ichino, S. Yoshikawa, H. Oomori, Y. Maeda, N. Nishiyama, T. Takayama, T.

Mizue, I. Tounai, and M. Nishie, “Small Form Factor Pluggable Optical Transceiver Module with Extremely Low Power Consumption for Dense Wavelength Division

Multiplexing Applications,” Proceedings of 2005 Electronic Components and Technology Conference, 1, 1044, (2005).

[5.13] N. S. Bergano, F. W. Kerfoot, C. R. Davidsion, “Margin measurements in optical amplifier system,” IEEE Photon. Technol. Lett. 5, 304, (1993).

[5.14] W. Lee, M.-Y. Park, S.-H. Cho, J. Lee, C. Kim, G. Jeong, and B.-W. Kim,

“Bidirectional WDM-PON Based on Gain-Saturated Reflective Semiconductor Optical Amplifiers,” IEEE Photon. Technol. Lett. 17, 2460, (2005).

Chapter 6 Conclusions

6.1 Summary

In this dissertation, we investigated four novel design of weak-resonant-cavity-Fabry-Perot-laser-diode (WRC-FPLD) in the wavelength division multiplexer passive optical network (WDM-PON) systems. The side-mode injection-locked FPLD transmission diagnosis of a multi-channel selectable transmitter is introduced in chapter 2. The WRC-FPLD with enhanced injection-locking bandwidth is introduced in chapter 3. The pulsating master and injected slave weak-resonant-cavity laser diodes based quasi-color-free 2.5Gb/s WDM-PON is introduced in chapter 4. The self-restorable injection-locking monitor by integrated photodiode for FPLD WDM Transmitter is introduced in chapter 5. From these researches performed in this dissertation, some valuable feature and contribution can be briefly summarized as following.

(1) Side-mode injection-locked FPLD transmission diagnosis of a multichannel selectable

z The effect of the injection-locking power and side-longitudinal-mode order on the linewidth, SMSR, and BER characteristics of a slave FPLD injected by another spectrally sliced master FPLD are theoretically analyzed.

z The back-to-back and 25km-SMF transmission performances of the 2.5-Gbit/s directly modulated FPLD based WDM-PON transmitter under side-mode injection-locking is demonstrated.

z The maximum usable channels of the side-mode injection-locking FPLD are 22, covering a wavelength-locking range up to 24 nm.

z A BER of <10^-12 is obtained for the nearest 13 modes and a 10^-10 error rate can be achieved for all of the 22 injection-locked modes.

(2) Weak-resonant-cavity Fabry-Perot laser diode with Enhanced Injection-locking Bandwidth

z The effects of the front-facet reflectivity of the WRC-FPLD on the injection locking range, the spontaneous emission dependent SNR and Q-factor, and the BER transmission response are theoretically and experimentally investigated

z As a result, the 30-nm wavelength-locking capacity of the directly modulated AR-FPLA based ONU transmitter with 1% front0facet reflectivity under side-mode injection-locking condition is demonstrated for 2.5-Gbit/s DWDM-PON application.

z The largest SMSR up to 40 dB and a least Q factor of 9.5 is achieved when injection-locking the central mode with a seeding power level of -3 dBm, which provides a receiving sensitivity of -24.4 dBm at BER of 10^-12.

(3) Pulsating master and injected slave weak-resonant-cavity laser diodes based quasi-color-free 2.5Gb/s WDM-PON

z The quasi-color-free pulsed RZ transmission can be achieved up to 16 DWDM channels.

The acceptable tolerance of wavelength locking bandwidth can be enhanced with increasing RF power to improve gain-switching mode linewidth.

z A receiving sensitivity for 2.5Gbs/s back-to-back transmission at BER<10^-10 can be -25.6 dBm. With appropriate temperature tuning of the slave WRC-FPLDs, the variation on receiver sensitivity penalty at BER=10^-10 can be confined within 1.6 dB under a temperature shift of 15^oC.

z A well-opened eye pattern can be obtained with a relatively large dynamic range, in which the rising and falling time (defined as the duration between 20% and 80% of

on-level amplitude) are 48 ps and 52 ps.

(4) Self-Restorable Injection-Locking Monitor by Integrated Photodiode for FPLD WDM Transmitter

z A novel in situ and self-restorable scheme has been demonstrated for the injection-locking FPLD-based WDM transmitter by an integrated MPD in connection with an MCU controlling unit. The injection-locking and self-restoration characteristics such as the locking-in wavelength range and the illuminated power dependent MPD current of the injection-locked FPLD are analyzed.

z The FPLD directly modulated by a PRBS datasteam can be self-restoration injection-locked within 50 seconds, achieving a BER of 10^-12 at a data rate of 2.5 Gbit/s with a receiving power as low as -25 dBm.

z After self-restorable injection locking, the transmitter exhibits a Q factor of 8.2 to provide a reachable BER as low as 1.1u10^-16, and an SMSR of >35 dB.

z With the self-restorable injection-locking scheme, the APC function that was prohibited in the injection-locked FPLDs can be added into future FPLD-based optical networks with a vastly improved stability and reliability.

z The proposed scheme works for most of the injection-locking FPLDs and can be cost effectively implemented using a minimum amount of redundant network resources.

As our best knowledge, the commercial TO-can package could meet the request of 10-Gb/s application. However, in order to enhance the locking bandwidth and injection efficiency, the weak-resonant cavity of our proposed laser is unfavorable for high speed operation. Our proposed WRC-FPLD is specially designed for WDM-PON application, and the investigated results exhibits many benefits such as data rate of 2.5G/s, 30-nm injection range, and quasi-color-free operation. In order to take the advantages, the bandwidth of WRC-FPLD was measured at 3 GHz. Some of the remarkable researches on similar transmitters of

WDM-PON solutions are summarized as below table. In comparison with the results of ASE light source injected FPLD, the incoherent light injection usually brings a relatively high intensity noise to limit the achievable bit rate or transmission distance. The WDM-PON transmitters based on such an injection source can provide a highest data rate up to 1.25 Gbps with the minimum WDM channel spacing of 100 GHz. With the use of coherent light source injection, the WRC-FPLD based WDM-PON transmitter becomes a potential candidate for achieving 2.5-Gbps transmission in WDM-PON system with channel spacing of 50 GHz [3].

Although a 10-Gbps WDM-PON injected by DFB laser diode was previously demonstrated [4], which only allows a wavelength-detuning range of 3.6 GHz (0.029 nm) at BER of 10-9, and the wavelength matching and temperature controlling circuitry is mandatory. Since the temperature-dependent wavelength shifting slope of a regular commercial FPLD is 0.06 nm/oC, and the temperature of FPLD with locking range of 0.029 nm should be maintained within +/-0.25 ^oC. This is not an easy approach to deal with the locking range and the BER performance within such a tiny temperature tolerance. There is always a tradeoff between intensity noise and coherence of the injection-locked FPLD or WRC-FPLD based transmitters in the WDM-PON architectures. Our proposed pulsating injected WRC-FPLD system is the only quasi-color-free WDM-PON system with the data-rate over 2.5Gb/s.

Injection-locked

WDM-PON solutions Max Data Rate Distance Locking bandwidth Channel Spacing

ASE injected FPLD 1.25 Gbps Low 20km N.A. Poor 100 GHz

ASE injected RSOA 1.25 Gbps Low 20km Color-Free Best 100 GHz

QD-MLL injected FP 2.5 Gbps Good 25km 0.27nm Poor 50 GHz

DFB injected FP 10Gbps 10 Gbps Best 10km 0.029nm Very poor N.A.

ASE injected

WRC-FPLD 1.25 Gbps Low 25km Quasi-Color-Free (3.2dB penalty)

Good 200 GHz

Pulsating injected

WRC-FPLD 2.5 Gbps Good 25km Quasi-Color-Free

(1dB penalty) Good 100 GHz