The work described in this dissertation concentrates on the derivation of the reduction of the threshold current under external light injection locking condition and the application of this particular phenomenon in NRZ-to-RZ data format converter.
This dissertation is organized as follows: Chapter 2 derives the formula of the threshold current reduction. First, we will give a phenomenological approach, which is introduced by Coldren and Corzine, to diode lasers to derive the expression of the threshold current of diode lasers. Furthermore, by introducing the Li’s theory, we can derive the threshold current reduction under external light injection locking. Chapter 3 is the experimental setup and results of the FPLD-based NRZ-to-RZ converter. First, we use a continuous-wave (CW) light injection into the FPLD to observe the external light-injection-induced gain-switching. That is, the synchronously-modulated Fabry-Perot laser diode at below threshold condition becomes gain-switching when its threshold current is reduced by an external injection locking. Furthermore, we inject an incoming NRZ data stream into the FPLD, which receives the high level (“1” bit), resulting in the generation of a
RZ data, whereas receives the low-level (“0” bit), ceasing the lasing of the FPLD. Such an external injection-locking not only achieves the NRZ-to-RZ conversion, but also helps to suppress the gain of side modes and spontaneous emission, providing the FPLD output data-stream with a high side mode suppression ratio (SMSR) and low noise level. Chapter 4 is the experimental setup and results of an all-optical OR logic gate, which is realizes by using such an FPLD-based NRZ-to-RZ converter. In such an OR gate, the issue of pulse inequality is solved by slightly enlarging the amplitude of the RF modulating power. In Chapter 5, we propose a transmission configuration by using a preemphasis technique (proposed by Nakazawa et al.) to transmit the converted RZ data stream. The corresponding parameters for realizing this configuration are calculated. In addition, for evaluation of chirp of such a data format converter, a dynamic relative chirp measurement is constructed for observation of the results of chirp compensation and the chirp after transmission. Finally, Chapter 6 summarizes the dissertation and gives some future directions.
(a)
(b)
Fig. 1.1 Schematic illustration of (a) homostructure and (b) double-heterostructure semiconductor lasers with their typical physical dimensions. The dotted area represents the depletion region in the vicinity of the homojunction. The hatched area shows the thin (~0.2 μm) active layer of a semiconductor material whose band gap is slightly lower than that of the surrounding cladding layers.
[G. P. Agrawal, and N. K. Dutta, Long-wavelength Semiconductor Lasers, New York: Van Nostrand Reinhold, 1986.]
Fig. 1.2 Schematic illustration of the simultaneous confinement of the charge carriers and the optical mode to the active region occurring in a double-heterostructure semiconductor laser. The active layer has a lower band gap and a higher refractive index than those of the cladding layers.
[H. C. Casey, Jr., and M. B. Panish. Heterostructure Lasers, Parts A and B. New York:
Academic Press, 1978.]
Fig. 1.3 Basic p+nn+ laser structure showing the stripe geometry contact which confines the active portion of the junction to a narrow region. The xyz axes are also defined.
[J. C. Dyment, “Hermite-gaussian Mode Patterns in GaAs Junction Lasers,” Appl. Phys. Lett.
10, 84 (1967).]
Fig. 1.4 Two types of diode lasers with strong transverse confinement: (a) a gain-guided laser with poor lateral confinement; (b) an index-guided laser for strong lateral confinement. The current flow is laterally restricted by a strip of width w.
[Peter Vasil’ev, Ultrafast Diode Lasers: Fundamentals and Applications, Boston: Artech House Publishers, 1995]
Fig. 1.5 Energy-band diagram of a p-n junction at (a) zero bias and (b) forward bias. (c) Schematic representation of the electron and hole densities under forward bias. Radiative recombination of electrons and holes in the narrow overlapping region generates light.
[G. P. Agrawal, and N. K. Dutta, Long-wavelength Semiconductor Lasers, New York: Van Nostrand Reinhold, 1986.]
Fig. 1.6 Energy-band diagram of a double-heterostructure semiconductor laser at (a) zero bias and (b) forward bias. (c) The band-gap discontinuities at the two heterojunctions help to confine electrons and holes inside the active region, where they recombine to produce light.
[G. P. Agrawal, and N. K. Dutta, Long-wavelength Semiconductor Lasers, New York: Van Nostrand Reinhold, 1986.]
Fig. 1.7 Dielectric waveguiding in a heterostructure semiconductor laser. The relatively higher refractive index ( μ2 >μ1) of the active layer allows total internal reflection to occur at the two interfaces for angles such that sinθ μ μ> 1 2.
[G. P. Agrawal, and N. K. Dutta, Long-wavelength Semiconductor Lasers, New York: Van Nostrand Reinhold, 1986.]
Fig. 1.8 Schematic illustration of (a) spontaneous-emission and (b) stimulated-emission processes wherein an electron-hole pair recombines to generate a photon. In the case of stimulated emission the two outgoing photons match in their frequency and direction of propagation.
[G. P. Agrawal, and N. K. Dutta, Long-wavelength Semiconductor Lasers, New York: Van Nostrand Reinhold, 1986.]
Fig. 1.9 Experimental setup. PC: polarization controller; SMF: single mode fiber; APD:
avalanche photodiode; EDFA: erbium-doped fiber amplifier; BERT: bit-error tester.
[C. W. Chow, C. S. Wong, H. K. Tsang, “All-optical NRZ to RZ format and wavelength converter by dual-wavelength injection locking,” Opt. Commun. 209, 329 (2002).]
Fig. 1.10 Experimental setup for NRZ-to-PRZ converter; EDFA: erbium-doped fiber amplifier; PC: polarization controller; OBF: optical bandpass filter; PBS: polarization beam splitter.
[Y. D. Jeong, H. J. Lee, H. Yoo, and Y. H. Won, “All-optical NRZ-to-PRZ converter at 10 Gb/s based on self-phase modulation of Fabry-Perot laser diode,” IEEE Photon. Technol. Lett.
16, 1179 (2004).]
Fig. 1.11 Experimental setup. The gain-switched DFB3 is used to manipulate the gain in SOA1 and SOA2.
[J. P. R. Lacey, M. V. Chan, R. S. Tucker, A. J. Lowery and M. A. Summerfield, “All-optical WDM to TDM transmultiplexer,” Electron. Lett. 30, 1612 (1994).]
(a)
(b)
Fig. 1.12 (a) All-optical WDM-to-TDM node incorporating the modular functions of: (1) NRZ-to-RZ conversion; (2) extinction ratio enhancement; and (3) wavelength shifting. (b) Conceptual operation of the WDM-to-TDM node.
[D. Norte and A. E. Willner, “Demonstration of an all-optical data format transparent WDM-to-TDM network node with extinction ratio enhancement for reconfigurable WDM networks,” IEEE Photon. Technol. Lett. 8, 715 (1996).]