Bent electro-absorption modulator

Details Bent electro-absorption modulator Bent electro-absorption modulator

2001-08-17

2003-01-28

Ngo, Hung N. (Department: 2874)

Optical waveguides

Temporal optical modulation within an optical waveguide

Electro-optic

C385S008000, C385S132000, C385S032000

Reexamination Certificate

active

06512860

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method and a device for modulating optical signals based on modulation of the absorption and of the bending losses in bend, quantum well semiconductor waveguide sections. The complex refractive index of the optical active semiconducting components of the waveguide section is modulated through the Quantum Confined Stark Effect (QCSE), by applying a variable electric or electromagnetic (EM) field. The modulation results in a modulation of the effective refractive index contrast and the absorption coefficient for the waveguide at the frequency of theoretical signal.
BACKGROUND OF THE INVENTION
In optical communication it is often of interest to obtain a high bit rate in the optical signals, the improvement of the present standards of 10 Gbit/s being restrained by the modulation speed of optical modulators. Typically, two classes of optical modulators are used, interferometric devices such as Mach-Zehnder type modulators and Electro-Absorption Modulators (EAMs).
Mach-Zehnder modulators utilise optic active materials to control a phase shift between two arms in an interferometer whereby the resulting signal may be modulated. Mach-Zehnder modulators presently provide modulation speeds up to 40 Gbit/s, however, 100 Gbit/s have been reported. It is a disadvantage of Mach-Zehnder modulators that they are typically large, expensive, and require a large voltage amplitude to produce the required phase shift.
In EAMs, a modulated absorption coefficient is induced in active semiconductor materials using a modulated electric field, i.e. utilising QCSE. There are two characteristic energy regimes for a semiconductor material, being denoted as below bandgap and above bandgap, where the absorption coefficient (proportional to the imaginary part of the refractive index) of the semiconductor material is zero or non-zero, respectively. This is shown schematically in
FIG. 1
where the curves show no absorption at low energies/frequencies below bandgap and high absorption at high energies/frequencies above bandgap. The boundary between these two regimes, i.e. the bandgap region where the curves rise steeply, can be shifted due to the Quantum Confined Stark Effect (QCSE) when the material comprises a Quantum Well semiconductor structure. This is also shown in
FIG. 1
, where the absorption (i.e. the imaginary part of refractive index) are shifted to lower energies/lower frequencies when a reverse bias is applied. The QCSE in bulk structures is denoted the Franz-Keldysh Effect (FKE). The QCSE is observed when reverse biasing the semiconductor structure. The amount of absorption near the bandgap is thereby increased for increasing reverse bias. Thereby, the optical absorption may be modulated between a low and a high value for light in a narrow energy/frequency region as indicated by the shadowed region
2
in FIG.
1
. The change in the absorption due to the QCSE or FKE is the mechanism used in EAMs. Presently, EAMs can provide modulations speeds up to 40 Gbit/s.
In an article by Veldhuis et al, Optics Communications, 168 (1999) 481, an optic intensity modulator based on a bent channel waveguide is disclosed. The bent channel waveguide has a fixed bending radius . When the lateral refractive index contrast between the core and the cladding material is high enough, all the light in the waveguide will be guided. If the lateral refractive index contrast is lowered sufficiently, part of the light will be radiated out of the waveguide, the exact fraction depending on the value of the contrast. By adjusting the contrast, the precise transmitted power may be controlled.
FIG. 6
summarises the length and changes
n
act
in the refractive index of the core, assuming constant cladding index, required to achieve a 30 dB extinction. Veldhuis et al proposes the use of thermo-optic or electro-optic actuation for controlling the refractive index in thermo-optic or electro-optic polymers applied in NxM matrix switches to decrease cross-talk and increase compactness.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a device and a method for modulating electromagnetic (EM) radiation, which provide an enhanced extinction ratio and faster operation than conventional modulators.
It is another object of the present invention to provide a device and a method for modulating EM radiation, which provides more compact modulators than conventional modulators.
It is a further object of the present invention to provide a device and a method for modulating EM radiation, which requires smaller voltage swings than conventional modulators.
The response of an optical active semiconductor material to an EM field (light) is governed by the complex refractive index of the semiconductor material, generally denoted as n=Re(n)+i Im(n). The imaginary part Im(n) determines the amount of light, which will be absorbed in the semiconductor material, while the real part Re(n) of the refractive index determines the speed of light in the medium. The refractive index is a function of frequency and the amount of absorption hence depends on the frequency (or wavelength) of the light.
The lateral confinement of light in typical waveguides is based on total internal reflection. Total internal reflection is the reflection of EM radiation from the interface of a medium with larger index of refraction n
1
with a medium of smaller index of refraction n
2
<n
1
when making an angle of incidence T! sin
1
n
2
n
1
to normal. Thus, the lateral confinement of light depends upon the index contrast between the waveguide core and the surrounding material as well as upon the angle of incidence of light on the boundaries between the waveguide core and the surrounding material. Hence a change in the index contrast may, depending on the angle of incidence, introduce losses du to lack of total internal reflection. Also, varying the direction of the lateral confinement parts or side walls will change the angle of incidence and may, depending on the index contrast, introduce large losses due to lack of total internal reflection. Variations in the direction of the lateral confinement parts may be a bent waveguide if both sides of the lateral confinement vary identically. It may also be a variation of only one of the sides such as a narrowing of the waveguide. Alternatively, the width of the waveguide may vary in that both sides performs repeated change of directions, such as a wobbling. All these different scenarios will introduce losses since they change the angle of incidence on at least one side of the lateral confinement boundary of the waveguide, the collective term bending losses will be used for simplicity.
The present invention provides an optical intensity modulation by introducing a modulated loss governed by a modulation of the real part of the refractive index. The invention may be implemented as an optical modulator based on these modulated losses alone, or may be used to improve the performance of existing optical modulators by introducing an extra loss for improving the extinction ratio. A modulation of the real part of the complex refractive index can be accomplished in different ways, however, in order to obtain modulation speeds fast enough for industrial application in optical communication and related fields, the working principle and material composition of devices must be carefully considered. The present invention will provide more compact optical intensity modulators with improved performance (extinction) and speed.
It is known from prior art electro-absorption modulators to use QCSE to obtain a modulation in the absorption (or equivalently the imaginary part of the refractive index). The modification of the refractive index, shifted due to the QCSE in case of a quantum well semiconductor structure comprising an optical active semiconducting material core, is not only restricted to the imaginary part of the refractive index. Also the real part of the refractive index will be modified. The change of the real part can be calculated from the ch

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