Optical waveguides – Temporal optical modulation within an optical waveguide – Electro-optic
Utility Patent
1998-10-07
2001-01-02
Lee, John D. (Department: 2874)
Optical waveguides
Temporal optical modulation within an optical waveguide
Electro-optic
C385S039000
Utility Patent
active
06169824
ABSTRACT:
DESCRIPTION
1. Technical Domain
This invention relates to a non-linear optical device for processing an optical signal.
It is particularly applicable to optical telecommunications.
The device according to the invention is a fast non-linear optical device that can perform various types of optical signal processing.
The non-linearity of the device is a result of the fact that the optical power at the output from this device is not proportional to the optical power at its input, even under transient conditions.
Due to this non-linearity, an optical signal may be modulated by optical power variations from another optical signal.
If the wave lengths of these two signals are different, the information to be transmitted will be transferred from one channel to another channel.
This type of processing, called wave length conversion processing, is useful for transmitting information in a system based on wave length multiplexing.
A distinction is usually made between four types of non-linearities, namely inverted over-linear type, uninverted over-linear type, inverted under-linear type and uninverted under-linear type.
Depending on the type of non-linearity, the device according to the invention may be used for reshaping or demultiplexing optical signals or to extract a clock signal from an optical pulse stream in a system based on time multiplexing.
Furthermore, due to the use of an interferometer with multiple arms in the invention, new applications can be considered due to the possibility of “modeling” the response of this interferometer with multiple arms according to a shape defined in advance (hence an analogy with electric filters).
2. State of Prior Art
It is relatively difficult to design a non-linear and fast optical component for “reasonable” optical powers, such as powers used for optical telecommunications.
One known solution consists of using saturation of semiconductor optical amplifiers.
This subject is described in document (1) which, like the other documents mentioned later, is mentioned at the end of this description.
This type of solution results in two types of devices, one of which is based on saturation of the gain of semiconductor optical amplifiers and the other is based on saturation of the refraction index of these amplifiers.
Devices in the first type use simple semiconductor optical amplifiers as they were developed for amplification of optical signals.
In this case, the non-linearity obtained, which is of the inverted under-linear type, is contrary to the non-linearity required for most applications and which is of the uninverted super- or under-linear type.
Devices of the second type are based on a semiconductor optical amplifier being added into an optical interferometer made on a semiconducting substrate.
This interferometer may be:
a Michelson interferometer (see document (2))
or an asymmetric Mach-Zehnder interferometer (see document (3))
or a symmetric Mach-Zehnder interferometer (see document (4)).
FIG. 1
is a schematic view of a known non-linear optical device for processing an optical signal.
This device in
FIG. 1
comprises a Mach-Zehnder interferometer I formed on a semiconducting substrate
2
.
This interferometer comprises two Y optical junctions that are formed on substrate
2
, and that are marked with references
4
and
6
respectively in FIG.
1
.
The interferometer also comprises two parallel optical wave guides
8
and
10
of the same length, that form the arms of the interferometer and connect junctions
4
and
6
to each other, as can be seen in FIG.
1
.
The device shown in
FIG. 1
also comprises two identical semiconductor optical amplifiers
12
and
14
which are formed on substrate
2
and are inserted in interferometer arms
8
and
10
respectively, as shown in FIG.
1
.
This device is fitted with means
16
designed to power amplifiers
12
and
14
by means of biasing currents.
The concept of symmetry or asymmetry of a Mach-Zehnder type interferometer is related to the optical power.
More precisely, considering interferometer I in
FIG. 1
, this symmetry or this asymmetry concerns the two optical junctions
4
and
6
.
The Mach-Zehnder interferometer is said to be symmetric when these two junctions are 50:50 junctions (in other words when the distribution of the corresponding optical power is 50%:50%).
The Mach-Zehnder interferometer is said to be asymmetric when the optical junction
4
is of the X:(100-x) type and the optical junction
6
is of the (100-x):x type where x≠50, the corresponding optical power then being distributed in the ratio x%:(100%-x%) for junction
4
and (100%-x%):x% for junction
6
.
For example, consider a device using a symmetric Mach-Zehnder type interferometer.
In this case, the x parameter is equal to 50.
As can be seen in
FIG. 1
, the input to junction
4
(designed for control of the device) and the output from junction
6
(device output) are marked as references
18
and
20
respectively, and the interferometer has two optical junctions
22
and
24
that are formed on the substrate
2
and are coupled to the interferometer arms
8
and
10
respectively on the side of the optical junction
6
.
One of junctions
22
and
24
, namely junction
24
in the case shown in
FIG. 1
, is used to inject an optical input signal Se into the interferometer arm
10
.
The input
18
is used to inject an optical control signal Sc.
This signal Sc is a continuous optical signal, in other words a signal for which the optical power is constant with time.
An optical output signal Ss is recovered at output
20
.
Junction
22
, which is associated with the other arm
8
, is only used to balance the optical power of the Mach-Zehnder interferometer, so that the amplitudes of the optical fields E
1
and E
2
of the control signal Sc are identical when these optical fields interfere at the output.
By injecting an input signal Se with a sufficient optical power into arm
10
of the interferometer, the refraction index of the corresponding semiconductor optical amplifier
14
is modified by saturation.
The result is a variation in the phase of the optical field E
2
with respect to the optical field E
1
.
This phase variation is transformed by interference into a transmission variation in the Mach-Zehnder interferometer.
Under these conditions, when a continuous optical signal Sc is injected to input
18
of the device in
FIG. 1
, this signal Sc is modulated by input signal Se to obtain an optical output signal Ss at the output
20
from the device, which is modulated by the input signal Se.
Known non-linear optical components for processing a signal have disadvantages.
A discrete amplifier component may be used in gain saturation to convert the wave length, but this technique degrades the signal extinguishing rate for conversion to long wave lengths.
This type of configuration cannot be used for other signal processing functions.
A Mach-Zehnder or Michelson type of interferometer can also be used.
These two devices, that use semiconductor optical amplifiers, can improve the signal extinguishing rate, but the interferometer setting has to be modified if there is a variation in the input signal Se.
It is impossible to process signals for which the input extinguishing rates are large or which could “overlap” the two slopes of the interferometer transmission curve, without deformation.
Distortion may then appear on the processed signal.
This is illustrated diagrammatically in
FIG. 2
, which shows variations in the light power Ps of the output signal Ss as a function of the light power Pe of the input signal Se (curve I).
It also shows an input signal Se that “overlaps” the two slopes of the interferometer transmission curve (curve II), and the corresponding output signal Ss (curve III).
To overcome this defect, it has been proposed to servocontrol the current in one of the interferometer arms to the power of the input signal (see document (5)).
This solution gives a wider usage range compared with the variation of the input signal, but has the disadvantage that it limits the throughput due to electronic processin
Chelles Sandrine
Rigny Arnaud
Sigogne Didier
France Telecom
Lee John D.
Pearne & Gordon LLP
Song Sarah U.
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