Optical waveguides – With optical coupler – Particular coupling function
Reexamination Certificate
2000-12-15
2003-10-21
Kim, Robert H. (Department: 2882)
Optical waveguides
With optical coupler
Particular coupling function
C385S037000, C385S132000
Reexamination Certificate
active
06636662
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a planar waveguide dispersion compensator for an optical signal, and a method for compensating for dispersion in an optical signal.
BACKGROUND TO THE INVENTION
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 being subject to delays in their propagation time along the fiber which depend on their wavelength. This variable delay generates several problems in optical communications networks. As transmission rates increase in digital optical communications networks cheap, reliable and efficient means to implement dispersion compensation and to control the pulse profile of an optical signal during transmission through optical media are becoming highly desirable.
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 a system may 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+Ø
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
)
(
1
)
z
1
+
(
d
2
β
2
/
d
ω
2
)
(
2
)
z
2
+
2
(
d
β
2
/
d
ω
)
(
3
)
(
d
z
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). Inducing a sufficiently negative dispersion of group velocity to enable an optical pulse to remain unchanged as it propagates requires consideration of a number of factors, particularly in a planar waveguide environment. Although polymer materials can provide a negative 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 a sufficiently negative group delay dispersion is induced.
Another way to induce a negative group velocity 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 complex, expensive, and are subject to fatigue.
SUMMARY OF THE INVENTION
One object of the present invention seeks to obviate or mitigate the above problems by providing a dispersion compensator for an optical signal. Another object of the present invention seeks to provide a method of compensating for dispersion in an optical signal. Another object of the invention seeks to provide an optical component including a dispersion compensator. Another object of the invention seeks to provide a node for an optical network including a dispersion compensator. Another object of the invention seeks to provide an optical transmission system including a dispersion compensator. Yet another object of the invention seeks to provide a planer waveguide strip lens for use in a dispersion compensator. Yet another object of the invention seeks to provide a composite strip lens for use in a dispersion compensator.
One aspect of the invention provides a dispersion compensator for an optical signal comprising:
an arrayed waveguide grating having a number M of waveguides, the arrayed waveguide grating decomposing the optical signal into N component signals each having a separation wavelength &dgr;&lgr; from an adjacent component signal;
at least one path-length adjuster varying the path-length of at least one of the N component signals to induce a phase shift &Dgr;&phgr; between the initial phase of each component signal in the AWG waveguides and the final phase of each component signal output by the AWG waveguides; and
a recombiner to re-combine the phase-shifted component signals into a re-combined signal, wherein the phase shift &Dgr;&phgr; of each component signal is selected to adjust at least one characteristic of the optical signal in the re-combined signal.
The dispersion compensator may further include an M:N coupler, wherein the arrayed waveguide grating is connected to the M:N coupler such each of the N component signals is carried along one of N waveguides.
The component signal separation wavelength &dgr;&lgr; multiplied by the number of waveguides N preferably equals the bandwidth &Dgr;&lgr; of the optical signal.
At least one path-adjuster may comprise at least one lens having a refractive index which is capable of differing from the refractive index of a waveguide along which a component signal is propagating.
At least one path-adjuster preferably comprises at least one strip lens having a refractive index which is capable of differing from the refractive index of a waveguide along which a component signal is propagating, and wherein at least one strip lens is thicker at either end than in a middle portion.
Preferably, at least one characteristic is a group delay of the optical signal.
Preferably, the phase shift &Dgr;&phgr; of each component signal is a quadratic function of the wavelength of each component signal.
At least one characteristic of the optical signal adjusted is preferably a width of a pulse profile of the optical signal.
The phase shift &Dgr;&phgr; of each component signal is preferably determined to induce an appropriate dispersion compensating group delay for the re-combined signal.
Preferably, the recombiner comprises: a reflector capable of reflecting the phase shifted component signals; the reflector being provided so that the phase shifted component signals return along their incident paths.
For example, the reflector may be a mirror or mirror or a partially silvered mirror(s).
The recombiner may include a N:M coupler; an arrayed waveguide having a number M of waveguides, and M:1 coupler provided to combine the phase shifted component signa
Bricheno Terry
Thompson George H
Whiteaway James E
Barber Therese
Barnes & Thornburg
Kim Robert H.
Nortel Networks Limited
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