4.1 Introduction and motivation
Versatile DWDM-PON architectures with acceptable capacity and flexibility have emerged as the subscriber networks to meet the demand of the next-generation fiber-to-the-home application [4.1]. With the amplified spontaneous emission (ASE) injection-locking of the Fabry-Perot laser diode (FPLD) [4.2], or the reflective semiconductor optical amplifiers (RSOA) [4.3-4.5], or the WRC-FPLD [4.6] several kinds of the quasi-color-free transmitters were considered to overcome the mode-selection problem happened in conventional laser diode based transmitters. However, the spectrally sliced incoherent ASE suffers from large intensity noise to limit the transmission bit-rate below 2.5 Gb/s. Alternatively, the coherent injection-locking schemes by using the new master broad-band light sources (BLSs) such as a mutually injection-locked FPLD [4.7] or a quantum-dash passively mode-locked laser (QD-MLL) [4.8] have been proposed recently. The mutually injected antireflection-coated FPLDs has been a employed as master BLS to injection-lock the slave FPLDs for 125-Mb/s transmission with 50-GHz channel spacing. Later on, a highly coherent CW light injection-locked FPLD based 10-Gb/s bi-directional WDM-PON architectures was reported [4.9]. Nevertheless, all these coherence-injection applications require a precisely controlling wavelength match between the master and the slave lasers such that the shortcomings on the wavelength maintenance and stability are accompanied for practical WDM-PON systems.
Currently, the proposed approaches are limited issues of their colorless transmitter, low
operation bandwidth, power budget and high intensity noise when applying to the WDM-PON link. In comparison with the results of PRZ injection-locking and CW injection locking, the CW injection locking need incoherent light injection (sliced ASE) for colorless operation, and 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-Gbit/s transmission in WDM-PON system with channel spacing of 50 GHz. 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. Nowadays, most of the proposed WDM-PON works on a FPLD-FPLD injection-locked scheme, which CW injection locking still faces problems of wavelength discontinuity and finite injection-locking wavelength range due to the limitation on lasing mode selected by the resonant cavity of the FPLD.
Nevertheless, the coherent master BLS injection-locked slave laser diode has oriented a new solution towards high-bit-rate WDM-PONs. In this chapter, we demonstrate a novel bi-directional WDM-PON with RZ data-format at 2.5 Gb/s by using the slave WRC-FPLDs coherently injection-locked by a pulsated master WRC-FPLD based quasi-colorless source.
In particular, the on-off-keying RZ encoding is achieved by reducing the biased current of slave WRC-FPLD at below threshold current that is a nonlinear function of the external injection power or photon density [4.10-4.11]. Without using any data-format transformer circuit, both the down- and up-stream slave WRC-FPLDs are directly modulated by the PRBS NRZ data and coherently injection-locked by the gain-switched master WRC-FPLD. After 200-GHz AWG channelization, both the back-to-back and 25-km transmitted performances bi-directional RZ transmission at 2.5 Gb/s are analyzed.
4.2 Concept of coherent injection light source and quasi-color-free injection locking
The bi-directional 2.5-Gb/s RZ WDM-PON is contrasted quasi-color-free with the slave WRC-FPLDs which are coherently injection-locked by the pulsating master WRC-FPLD, as illustrated in Fig. 4.1. The WRC-FPLD is a general buried heterostructure FPLD with different end-face reflectivity. First of all, the WRC-FPLD was fabricated by growing a 2000-$-thick n-type InP layer with dopant concentration of 5x1018 cm-3 on substrate, and two n-type InP films containing with dopant concentrations of N=2x1018 cm-3 (2000-A thick) and N=2x1018 cm-3 (1000-A thick) were employed as the cladding layers. The active InGaAsP layer composed of the 0.9% compress-strain multi-quantum wells with thickness of 50 A at emission wavelength of 1.55 Pm and the 0.6% tensile-strain barriers with thickness of 85 A at 1.1 Pm. The design of such a WRC-FPLD can greatly suppress the temperature-dependent wavelength shift with a 'O/'T slope as low as 0.08 nm/oC, which is very close with that obtained from common quantum-well-based LDs. Afterwards, the p-type cladding layers including a 5000-A-thick intrinsic InP and a 1000-A-thick / 1.2Pm-thick p-type InP layers with corresponding doped concentrations of 1x1017 / 1x1018 cm-3, respectively were grown upon the gain region. Finally, a 2000-A-thick p-type InGaAs layer with dopant concentration of 2x1018 cm-3 was deposited as a contact layer.
WRC-FPLD
Fig. 4.1 The WRC-FPLD based bi-directional quasi-color-free 2.5-Gb/s RZ WDM-PON with a pulsed WRC-FPLD coherent injection-locker
The master WRC-FPLD with an integrated optical isolator is gain-switched for injection-locking the down- and up-stream slave WRC-FPLDs. The master WRC-FPLD gain-switched at nearly threshold condition exhibits a relative-intensity-noise (RIN) of -130 dB/Hz, which is amplified then by EDFA and channelized by a 200-GHz AWG to function as the multi-wavelength coherent seeding source. The longitudinal-mode wavelength of the master WRC-FPLD is temperature-detuned to match the ITU-T defined DWDM channel. The separation/combination is performed by the band separator/combiner (BS/BC). The design and fabrication of the WRC-FPLDs were modified from a conventional buried heterostructure FPLD without significantly increasing the production cost. Under free-running case, such a WRC-FPLD shows a threshold current up to 27 mA, as shown in Fig. 4.2. The resonant cavity length of all WRC-FPLDs is 600 Pm with corresponding longitudinal mode spacing of 0.6 nm. The back and front facet reflectivity of WRC-FPLD are 94% and 1%, respectively.
Such a highly asymmetric coating design allows the efficient external injection from the front facet to reduce the power budget and to avoid the power consumption at rear-facet of the WRC-FPLD. The fiber-pigtailing for the TO-56-can packed WRC-FPLD is also supported
by the laser diode manufacturer using laser fusing-splicing technology. For reducing the surface reflectivity, the end face of single-mode fiber (Corning, SMF28) is polished to tilt by 8 degree with respect to its surface normal. The WRC-FPLD is directly modulated by 2.5 Gbit/s pseudorandom binary sequences (PRBS) data stream with pattern length of 223-1 for transmission performance diagnosis. The Vp-p of the electrical PRBS digital data is 1 V (from -500 mV to 500 mV). All of the BER data were taken by using a commercial receiver (Sanway Optoelectronics tech. Corp. Ltd, SI1525-80ATOS-S) in connection with an error detector (Hewlett Packard, 70842B). Afterward, the temperature of the WRC-FPLD is varied from 20oC to 40oC to adjust the number of the injection-locked modes involved within a single AWG-channelized spectral window.
Fig. 4.2. Configuration and band structure of the 1% front-facet AR-coated WRC FPLD.
4.3 Performances and discussions of quasi-color-free 2.5Gb/s RZ WDM-PON
The phenomena of threshold current reduction have been demonstrated under external injection locking [4.10-11]. Nevertheless, the applications of threshold current reduction are usually limited by insufficient injection power. A commercial FPLD exhibits the threshold current reduction of 2 mA, while the injection-locked power needs a further amplification to overcome the end-face transmission loss of the FPLD. In contrast, the power-current
characteristics of the WRC-FPLD under different injection levels are shown in Fig. 4.3(a), which reveals a threshold current reduction by 10 mA with the external injection power increasing up to 9 dB. The related high threshold current of WRC-FPLD is caused by 1-%
reflectivity on the front face of the resonance cavity.
10 15 20 25 30 35 40
Fig. 4.3(a) The P-I curve of the slave WRC-FPLD under master WRC-FPLD injection-locking with different power levels.
Under such a low end-face reflectivity condition, the external injection compensates the reflection of the resonance cavity, and the lasing phenomenon is easier to be built-up at a lower driving current of WRC-FPLD. The reduction on threshold can be theoretically derived as [4.1]. Note that the power-current slope remains almost constant within the injection power ranged between -12 and +3 dBm, indicating that the reshaping on rising and falling edge of the generated RZ data shape with changing injection level can be negligible in this case. Figure 3(b) illustrates the operating principles of the RZ data-stream generation from slave WRC-FPLD initiated by the external injection from the gain-switched master WRC-FPLD.