Optical waveguides – Temporal optical modulation within an optical waveguide – Electro-optic
Reexamination Certificate
1999-03-30
2003-02-11
Healy, Brian (Department: 2874)
Optical waveguides
Temporal optical modulation within an optical waveguide
Electro-optic
C385S001000, C385S014000, C385S024000, C385S122000, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06519374
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to predistortion compensation in nonlinear electro-optical applications and, more particularly, to a predistortion arrangement which includes at least one mixer for producing second or higher order intermodulation products which are useful in distortion compensation. The invention is particularly well suited for production in integrated circuit form.
Nonlinear distortion is problematic in the use of nonlinear electro-optical devices such as, for example, lasers and modulators. One prior art approach, as described in U.S. Pat. No. 4,992,754, issued to Blauvelt et al., relies on predistorting an input signal in an attempt to counteract the nonlinearity of the electrooptical device in a way which is intended to provide an overall linear transfer function. As described by the '754 patent, prior art techniques generally divide an input signal along two or more electrical paths and generate predistortion along one or more of the paths.
FIGS. 1 and 2
correspond to
FIGS. 1 and 3
, respectively, taken from the '754 patent with reference numbers applied to the FIGS. consistent with the present description.
FIG. 1
illustrates a prior art predistortion arrangement generally indicated by the reference number
10
. Arrangement
10
includes input and output directional couplers
12
a
and
12
b,
respectively, a time delay circuit
16
in a signal path
18
and a distortion path
20
. The latter includes a distortion generator
22
, an amplitude adjust section
24
, a frequency tilt adjust section
26
and a phase adjust section
28
arranged in series along distortion path
20
.
Referring to
FIG. 2
, the '754 patent contemplates an implementation
30
of the block diagram of
FIG. 1
, however, no provision is made for distortion generator
22
(see FIG.
1
). Implementation
30
does provide distortion path
20
including amplitude adjust section
24
, frequency tilt adjust section
26
and phase adjust section
28
all of which are indicated within dashed lines. A detailed accounting of the components in
FIG. 2
is not necessary for reasons which will become evident, however, the presence of certain components should be noted. Specifically, a number of magnetic/inductive components are present. In particular, directional couplers
12
a
and
12
b
are normally implemented using transformers. Also, distortion path
20
includes a transformer
30
serving as a signal splitter, inductors L
1
and L
2
following frequency tilt adjust section
26
and a transformer T
2
connected to directional coupler
12
b.
As will be seen, the presence of these inductive components is considered to be particularly disadvantageous in accordance with the teachings of the present invention.
Referring to
FIG. 3
, which corresponds to
FIG. 2
of the '754 patent having reference numbers applied consistent with the present description, one implementation of a distortion amplifier (see item
22
in
FIG. 1
) is illustrated in the form of a push-push amplifier generally indicated by reference number
32
. Once again, it is of interest here that amplifier
32
includes aforementioned splitter transformer
30
. It is submitted that the specific implementations used in the '754 patent, and in the prior art generally, share at least one disadvantage related to requirements for magnetic components which, in turn, imposes limitations on the adaptability of predistortion in compensating for the nonlinearity of electro-optical devices, as will be further described.
The present invention provides a highly advantageous drive arrangement utilizing a predistortion for driving nonlinear electro-optical devices in a heretofore unseen manner which resolves the foregoing limitations.
SUMMARY OF THE INVENTION
As will be described in more detail hereinafter, there is disclosed herein a predistortion arrangement. This arrangement, like the prior art circuits illustrated in
FIGS. 1-3
, develops predistortion for using an input signal to drive a nonlinear device having a drive input. However, the present invention includes first means for sampling a portion of the input signal. The sampled portion of the input signal is introduced to at least one predistortion path including a mixer responsive to the sampled portion of the input signal for producing at least second order intermodulation products of the input signal. The second order intermodulation products are then conditioned. Combining means then combine the conditioned second order intermodulation products with the sampled input signal in a way which produces a predistorted drive signal compensated for second order distortion for use in driving the drive input of the nonlinear device.
In one aspect of the invention, the sampling means and the combining means are provided using resistive networks. In one feature, the use of the resistive networks eliminates the need for directional couplers incorporating magnetic components. Accordingly, in one feature, the predistortion arrangement including the mixer and the resistive networks may be provided in the form of an integrated circuit.
In another aspect of the invention, higher order intermodulation products may be generated using one or more additional predistortion paths, each of which relies on the use of mixers to generate the higher order intermodulation products.
REFERENCES:
patent: 4992754 (1991-02-01), Blauvelt et al.
patent: 5252930 (1993-10-01), Blauvelt
patent: 5327279 (1994-07-01), Farina et al.
patent: 5570063 (1996-10-01), Eisenberg
patent: 2306294 (1974-05-01), None
patent: 3307309 (1984-09-01), None
patent: 4120029 (1992-12-01), None
patent: 0594344 (1994-04-01), None
patent: 2713851 (1995-06-01), None
Hewlett Packard, IAM-8 Series Active Mixers, Application Note S013, Hewlett Packard Corporate Product Publication, (No date).
Hewlett Packard, Silicon Bipolar MMIC 5 GHz Active Double Balanced Mixer/IF Amp, Technical Data, Hewlwtt Packard Corporate Product Publication, (No date).
Manfred Bertelsmeier and Willmut Zschunke, Linearization of Broadband Optical Transmission Systems by Adaptive Predistortion, Sep. 1984, Frequenz.
Krasulick Stephen Bradley
Stook Christopher Robert
Healy Brian
Uniphase Corporation
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