Optical waveguides – Temporal optical modulation within an optical waveguide – Electro-optic
Reexamination Certificate
1999-10-14
2001-12-18
Bovernick, Rodney (Department: 2874)
Optical waveguides
Temporal optical modulation within an optical waveguide
Electro-optic
C385S008000
Reexamination Certificate
active
06332048
ABSTRACT:
This application claims priority under 35 U.S.C. §§119 and/or 365 to 9803522-3 filed in Sweden on Oct. 15, 1998; the entire content of which is hereby incorporated by reference.
TECHNICAL FIELD OF THE INVENTION
The present invention relates partly to an electro-absorption modulator, particularly with improved performance, partly to different methods for manufacturing of such an electro-absorption modulator.
DESCRIPTION OF RELATED ART
Electro-absorption modulators (EAM), particularly of the kind that includes a waveguide, and often monolithically integrated together with so-called DFB lasers (Distributed Feedback Lasers), is a very important component class for fiber optic transmission at high bit rates (typically 2,5 Gb/s and 10 Gb/s; in the future most likely 40 Gb/s) in combination with long-haul transmission.
The reason for this is that such EAM components have more ideal characteristics than, for instance, directly modulated lasers (specifically, they have less dynamic wavelength deviations, so-called chirp, during modulation). Furthermore, they are considerably simple to manufacture and require only a low driving voltage (compared to other classes of external optical modulators, such as, e.g., Mach-Zender modulators manufactured in lithium neobate). Not at least, the EAM component is very useful for applications including wavelength division multiplexing (WDM).
A conventional EAM component consists of a waveguide, with a waveguide core whose refractive index is higher than that of the surrounding, see for instance EP 0,809,129, EP 0,726,483, GB 2,281,785, and references therein. For components operating with launched light of a wavelength of 1.3 or 1.55 &mgr;m, the core consists typically of a semiconductor material, such as InGaAsP or alternating layers of InGaAsP and InP. The core is undoped or only slightly doped. Above and below the core, there are typically a p-doped InP layer and an n-doped InP layer, respectively, so that the complete structure composes a p-i-n diode.
The attenuation through the modulator depends on the difference in energy between the photons of the launched light and the band gap of the core. The band gap is affected, inter alia, by the electrical field applied. Thus, the p-i-n diode is arranged so that, when it is reverse biased, a large portion of the launched light is absorbed, whereby a photo current arises. Typically, extinction ratios of 10-30 dB may be achieved.
A problem in this respect is that the band gap is also strongly temperature dependent. The photo current that arises, causes ohmic heating, and thus a temperature rise, whereby the attenuation is affected. The absorptionen, which in a first approximation is proportional to the luminous power, is highest in the beginning of the modulator, i.e., where the light is launched into the modulator, whereby also the photo current is highest there. Thus, the strongest affection of the attenuation is achieved there.
SUMMARY OF THE INVENTION
To conceive how this temperature dependence may create problems in a digital transmission system, let us consider the following example. Suppose that one wants to send one or several consecutive “ones”, preceded by a long sequence of “zeros”. These “zeros” correspond to a reverse biased diode. According to the discussion above, this results in a large temperature increase, particularly then in the beginning of the modulator. When the voltage then is changed to a lower value, corresponding to a transmitted “one”, an optical pulse out of the diode is achieved. Instead of a sharp flank, reflecting the difference in externally applied voltage, a slow building-up process is achieved; only after a certain period of time the component has approached a constant value of the optical power level for the “one”. This clearly restricts the bandwidth of the transmission medium.
It is an object of the present invention to provide an electro-absorption modulator with improved performance.
It is a further object of the invention to provide an electro-absorption modulator in lack of problems that arise because of the strong temperature dependence of the band gap in the core of the modulator.
It is in this respect a particular object of the invention to provide an electro-absorption modulator that provides an optical pulse with short rise time when the voltage over the modulator is reduced.
It is yet another object of the invention to provide a method in manufacturing of said electro-absorption modulator.
Further objects of the present invention will be apparent from the specification below.
According to a first aspect of the present invention, an electro-absorption modulator (EAM), of the kind that includes a waveguide, for modulation of light, is provided, comprising a waveguide core, a waveguide cladding and an electrode, the modulator being arranged to modulate light launched into the modulator as a response to a voltage being applied to the electrode. The modulator is characterized in that the width and/or thickness of the waveguide core are/is varying along the length of the modulator in such a way that the width is smaller in the portion of the modulator where the light is intended to be input, for the purpose of reducing the absorption of the modulator there.
This variation is particularly arranged so that a mainly uniform photo current distribution, and thus temperature distribution, is achieved along the length of the modulator, for the purpose of reducing the thermally dependent, optical rise time of the modulator.
The electro-absorption modulator is preferably arranged to be used for intensity modulation of digital signals for fiber optic transmission. It may be monolithically integrated with a DFB laser (Distributed Feedback Laser) on a semiconductor substrate and manufactured using any of the material systems InP/InGaAsP, InP/InGaAIAs or GalnAs/AlGaAs.
The waveguide core may be of bulk or quantum well structure and the waveguide may be formed as a buried waveguide or as a, socalled, ridge waveguide.
According to a second aspect of the present invention there is provided a method in manufacturing of an electro-absorption modulator (EAM) according to the first aspect.
To manufacture a waveguide core with a varying width along the length of the modulator, so-called tapered photolithography masks, or selective etching may be used. If a waveguide core with quantum well structure is manufactured, inactive so-called SCH (Separate Confinement Heterostructure) layers in the quantum well structure may be etched selectively.
To manufacture a waveguide core with a varying thickness along the length of the modulator, SAE (Selective Area Epitaxy) or partially making the waveguide core thinner, through masking and etching, may be used.
An advantage of the present invention is that intensity modulation can be performed faster, which gives a higher transmission capacity.
REFERENCES:
patent: 4199221 (1980-04-01), Rivoallan et al.
patent: 4913506 (1990-04-01), Suzuki et al.
patent: 6167070 (2000-12-01), Sakata
patent: 0 726 483 (1996-08-01), None
patent: 0 809 129A2 (1997-11-01), None
patent: 2 281 785 A (1995-03-01), None
patent: 10-65275 (1998-03-01), None
patent: 10-163568 (1998-06-01), None
patent: 10-256669 (1998-09-01), None
patent: 10-275960 (1998-10-01), None
Bovernick Rodney
Burns Doane Swecker & Mathis L.L.P.
Connelly-Cushwa Michelle R.
Telefonaktiebolaget LM Ericsson (publ)
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