Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-07-17
2003-07-22
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S050121, C372S045013, 43
Reexamination Certificate
active
06597718
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an electroabsorption-modulated laser (EML) device and, more particularly, to an EML operating at a wavelength of 1310 nm by using a Fabry-Perot (FP) structure as its lasing section.
TECHNICAL FIELD
Optical transmitters, transceivers, and transponders utilizing electroabsorption modulated lasers (EMLs) at bit rates of up to 10 Gb/sec are of increasing interest, particularly when compared to the performance of directly modulated lasers. The zero dispersion property of conventional optical fiber at wavelengths close to 1300 nm makes the option of using a 1300 nm EML even more attractive than a 1550 nm EML, due to the non-existent dispersion penalty at the latter wavelength. Further, the modulation speed of an EML device is limited only by the RC time constant in the modulator section of the device. EML devices with a small enough device capacitance enable system designers to realize transmitters and transponders with speeds over 10 Gb/sec without the complication of the relaxation resonance frequency effects incurred in the directly modulated distributed feedback (DFB) or Fabry Perot (FP) lasers. In additional EML devices can be designed with a very high extinction ratio and system sensitivity penalties associated with this parameter can be virtually eliminated.
At the present invention, 1500 nm EMLs are being deployed in high speed (e.g., 2.5 Gb/sec and 10 Gb/sec) fiber optic networks. One exemplary EML arrangement is discussed in the article The advantage of these devices, as compared to conventional directly modulated DFBs, is that EML lasers exhibit highly superior eye diagrams, with less pulse distortion/ringing minimal chirp characteristics, high extinction ratio, and simplified driver circuitry. However, these advantages are obtained at the expense of a more complicated chip design, requiring multiple epitaxial regrowth steps, and additional wafer processing procedures to obtain electrical isolation between the electroabsorption modulator and laser sections of the device. Moreover, EML devices require tightly confined matching of the optical characteristics between the DFB grating, the active material gain characteristics, and the modulator material absorption characteristics. The combination of these factors can result in low yields and high costs, particularly when manufactured in high volume. Therefore, any new innovations which can relax any of the above requirements of the EML design can have considerable commercial impact and lead to competitive advantages.
At the same time, there is a rapid increase in the deployment of fiber-optic-based equipment which utilizes transponder, transceiver and transmitter modules operating at 10 Gbit/sec and at wavelengths near the 1310 nm dispersion minimum of the optical fiber. Currently, directly modulated 1310 nm DFB or FP lasers are utilized in these types of applications. However, directly modulated DFB or FP lasers exhibit severe limitations due to relaxation oscillation effects and the difficulties of modulating the drive current at 10 Gb/sec. The advantages of utilizing an EML laser operating at 1310 nm are significant, for the same reasons outlined in the previous paragraph. Some development work has moved forward regarding a 1310 nm EML, as discussed in the article “10 Gbit/s penalty-free transmission over 48 km using 1.3-&mgr;m wavelength electroabsorption modulated lasers (EML) for metro-loop transmission link”, by T. Tanbun-Ek et al, appearing in OFC Proceedings, San Jose Calif., 1996, at p.207. However, the research discussed in this and other papers presumes the use of a DFB section as the lasing device, requiring a complicated fabrication sequence, which is subject to low yields and high costs.
Thus, a need remains in the art for an EML device that is less costly and easier to manufacture than DFB-EML devices, yet is capable of operating at a wavelength of approximately 1310 nm.
SUMMARY OF THE INVENTION
The need remaining in the prior art is addressed by the present invention, which relates to an electroabsorption-modulated laser (EML) device and, more particularly, to an EML operating at a wavelength of 1310 nm by using a Fabry-Perot (FP) structure as its lasing section.
In accordance with the present invention, an FP laser and electroabsorption (EA) modulator are simultaneously formed on a common InP substrate, using (for example) a selective area growth (SAG) process. A trench is formed between the top conducting layers of the two sections to provide for electrical isolation between the FP laser and the EA modulator.
It is an advantage of the present invention that even though an FP laser exhibits a wider spectral bandwidth than the DFB laser conventionally used in an EML structure, the bandwidth is not a concern when operating at 1310 nm, the zero dispersion wavelength of the transmission fiber.
Other and further advantages of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.
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Tanbun-Ek Tawee
Tsang Won-Tien
Tzeng Liang David
Ip Paul
Multiplex, Inc.
Nguyen Tuan
Testa Hurwitz & Thibeault LLP
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