Optical waveguides – With optical coupler – Particular coupling function
Reexamination Certificate
2001-07-10
2004-02-10
Bruce, David V. (Department: 2882)
Optical waveguides
With optical coupler
Particular coupling function
C385S028000, C385S129000
Reexamination Certificate
active
06690855
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a planar waveguide dispersion compensator for an optical signal, and to a method for compensating for dispersion in an optical signal. Particularly, but not exclusively, the invention further relates to a thermally responsive lens providing dispersion compensation in a planar waveguide device, and to a method of tuning a thermally responsive dispersion compensator.
BACKGROUND TO THE INVENTION
Digital optical transmission systems such as glass fiber pulse code modulation (PCM) transmission systems are known to suffer from chromatic (wavelength dependent) dispersion. Such dispersion leads to optical signals propagating along a fiber or within a planar waveguide being subject to delays in their propagation time which depend on their wavelength. In this document, the term planar waveguide refers to an optical waveguide which is provided in a substantially integrated form such as in a planar light circuit, and which comprises a light-guiding region supported by a suitable substrate for example, a silicon or silica type substrate. In particular, the term planar waveguide encompasses a thin strip or film of material having a relatively higher refractive index which is embedded in the surface of a planar or laminar substrate.
The variable delay which chromatic dispersion generates in optical communications networks creates several problems, especially in digital optical transmission systems. As transmission rates increase in digital optical communications networks, providing cheap, reliable and efficient means to implement dispersion compensation and to control the pulse profile of an optical signal during transmission through optical media is highly desirable. By reducing the amount of dispersion in an optical signal higher bit rates can be more reliably accommodated.
The theoretical approach to preventing spread in a digital signal during transmission involves compensating for the variations in phase that arise from a frequency dependent group velocity in the transmission system.
Two ways in which a system may be constituted to compensate for dispersion are adding a length of line, for example an additional length of optical waveguide, of opposite dispersion characteristics to the previous portion of the line or applying a suitable phase-versus-frequency characteristic to the signal. Consider the case where a spectral component of a signal propagating along line
1
of length z
1
has angular frequency &ohgr;. The spectral component has a propagation constant &bgr;
1
along line
1
. Along an additional length of line, line
2
of length z
2
, the spectral component has a propagation constant &bgr;
2
. Either propagation constant &bgr;
1
, &bgr;2 may be frequency dependent. If the initial, arbitrary, phase is &phgr;
0
, then the phase at output is &phgr;
1
=&ohgr;t+&phgr;
0
−&bgr;
1
z
1
−&bgr;
2
z
2
.
The change of phase at a given frequency deviation &dgr;&ohgr; from the center frequency is given by {t-(d&bgr;
1
/d&ohgr;)z
1
−(d&bgr;
2
/d&ohgr;)z
2
−&bgr;
2
(dz
2
/d&ohgr;)}&dgr;&ohgr;. To prevent distortion of the signal, the phase variation should remain zero over the whole range of frequencies contained within it. As the d&bgr;/d&ohgr; and dz/d&ohgr; terms can vary over the frequency range, it is necessary that the second derivative with respect to frequency is also zero giving:
(
d
2
β
1
/
d
ω
2
)
Z
1
(
1
)
+
(
d
2
β
2
/
d
ω
2
)
z
2
(
2
)
+
2
(
d
β
2
/
d
ω
)
(
3
)
(
dz
2
/
d
ω
)
+
β
2
(
d
2
z
2
/
d
ω
2
)
(
4
)
=
0
The above equation shows three ways that are available for compensating group delay distortion in a fixed length z
1
of line
1
represented by term (1). Firstly, term (2), can provide compensation by adding line
2
of length z
2
of opposite group velocity dispersion. Secondly, term (3) can provide compensation when the length z
2
of line
2
is linearly dependent on the frequency. Thirdly, term (4) can provide compensation when the length z
2
of line
2
is a strongly quadratic function of frequency and dominates term (3).
A number of factors influence dispersion and delay and it is not easy to compensate in a planar waveguide device for the group velocity dispersion of the transmission system through which an optical pulse propagates. Although polymer materials can provide a compensating dispersion of group velocity, such materials are generally considered unsuitable for pulse reforming due to size constraints in a planar waveguide device. An optical pulse needs to have a relatively long propagation path within the polymer material to ensure that a sufficiently large compensating group delay dispersion is induced.
Another way to induce a compensating group delay dispersion for a signal is to linearly change the path-length of each component signal of a pulse to induce a sufficient relative change in phase with respect to the relative wavelength difference between the component signals. This is described by term (3) in the equation and can be achieved in non-planar optical environments for example, by using an adjustable chirped grating.
Conventional dispersion compensators using techniques such as stretchable chirped fibre gratings to alter the refractive index of the fibres implementing the grating are generally complex, expensive, and are subject to fatigue.
SUMMARY OF THE INVENTION
The invention seeks to obviate or mitigate the above problems by providing a dispersion compensator for an optical signal.
A first aspect of the invention seeks to provide a dispersion compensator for an optical signal comprising: optical signal input means to receive said optical signal as input; optical signal decomposing means connected to said input means and arranged to decompose the optical signal into a plurality of component signals, each component signal having a different passband from an adjacent component signal; optical dispersion means having an optical path-length adjuster arranged to receive each said component signal with an initial phase and configured to adjust the optical path length of at least one said component signal to induce a phase shift in said component signal on output; and an optical signal combiner arranged to re-combine the component signals output by said path-length adjuster into a re-combined signal, wherein the phase shift of each component signal is selected to correct in the recombined signal any dispersion present in the inputted optical signal.
Advantageously, the optical signal decomposition means is able to resolve said component signals sufficiently for said induced relative phase shift to provide a satisfactory level of dispersion compensation.
Advantageously, the invention enables a digital optical signal to receive compensation for any dispersion.
Preferably, said optical signal decomposing means comprises a first array of M waveguides and said optical dispersion means comprises a second array of N waveguides and said compensator further includes: a first 1:M coupler connected to said signal input means and splitting said inputted optical signal along said first array of waveguides; and a second M:N coupler connected to said first array of waveguides and to said second array of waveguides and arranged to decompose optical signals from said first array of waveguides into said component signals.
Preferably, said path-adjuster comprises at least one lens having a refractive index which is capable of differing from the refractive index of the waveguide along which a component signal is propagating.
Preferably, said dispersion compensator according to said first aspect is provided as a planar waveguide device, wherein the path-adjuster comprises at least one strip lens embedded in a first layer of said waveguide device, wherein each said strip lens has a refractive index which is capable of differing from the refractive index of the waveguide along w
Bricheno Terry
Thompson George H B
Whiteaway James
Barber Therese
Barnes & Thornburg
Bruce David V.
Nortel Networks Limited
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