U.S. patent application number 14/695890 was filed with the patent office on 2015-10-29 for non-linear filter for dml.
The applicant listed for this patent is SEMTECH CANADA CORPORATION. Invention is credited to Gurpreet S. BHULLAR.
Application Number | 20150311671 14/695890 |
Document ID | / |
Family ID | 54139668 |
Filed Date | 2015-10-29 |
United States Patent
Application |
20150311671 |
Kind Code |
A1 |
BHULLAR; Gurpreet S. |
October 29, 2015 |
NON-LINEAR FILTER FOR DML
Abstract
A circuit is disclosed having a component having repeatable
distortion characteristics; and a drive circuit for providing a
drive signal and comprising a non-linear filter for
pre-compensating for distortion introduced by the component having
repeatable distortion characteristics in response to the drive
signal, the distortion having a non-linear response to the drive
signal.
Inventors: |
BHULLAR; Gurpreet S.;
(Ottawa, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SEMTECH CANADA CORPORATION |
Burlington |
|
CA |
|
|
Family ID: |
54139668 |
Appl. No.: |
14/695890 |
Filed: |
April 24, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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61984621 |
Apr 25, 2014 |
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Current U.S.
Class: |
372/38.02 |
Current CPC
Class: |
H01S 5/183 20130101;
H01S 5/0427 20130101 |
International
Class: |
H01S 5/042 20060101
H01S005/042; H01S 5/183 20060101 H01S005/183 |
Claims
1. A circuit comprising: a component having repeatable distortion
characteristics; and a drive circuit for providing a drive signal
and comprising a non-linear filter having at least a tap for
pre-compensating for distortion introduced by the component having
repeatable distortion characteristics in response to the drive
signal, the distortion having a non-linear response to the drive
signal.
2. A circuit according to claim 1 wherein the component comprises a
Directly Modulated Laser (DML).
3. A circuit according to claim 2 wherein the DML comprises a
Vertical Cavity Surface Emitting Laser (VCSEL).
4. A circuit according to claim 2 wherein the DML comprises a
Distributed FeedBack (DFB) laser.
5. A circuit according to claim 3 wherein the DML is operated at at
least 25 Gbps.
6. A circuit according to claim 4 wherein the non-linear filter
comprises a non-linear Finite Impulse Response (FIR) filter having
at least 2 weights for application at each delayed tap and
supporting at least one delayed tap.
7. A circuit according to claim 4 wherein the non-linear filter
comprises a non-linear Finite Impulse Response (FIR) filter having
at least 2 weights for application at each delayed tap and
supporting at least 3 delayed taps.
8. A circuit according to claim 1 wherein the non-linear filter
comprises a non-linear Finite Impulse Response (FIR) filter having
at least 2 weights for application at each delayed tap and
supporting filtering of both a rising edge, low to high signal
level response and a falling edge, high to low signal level
response.
9. A circuit according to claim 8 comprising: for each tap a first
input port for receiving a first weight, a second input port for
receiving a second other weight, a switch for switching between the
first weight and the second weight, and a weighting circuit for
weighting of a signal within the tap to produce a tap output, tap
output signals from different taps combined to form the drive
signal.
10. A circuit according to claim 8 comprising: for each tap a first
input port for receiving a first weight, a second input port for
receiving a second other weight, a scaling circuit for scaling the
first weight and the second weight, and a weighting circuit for
weighting of a signal within the tap to produce a tap output, tap
output signals from different taps combined to form the drive
signal.
11. A circuit according to claim 1 wherein the non-linear filter
comprises a non-linear Finite Impulse Response (FIR) filter having
greater than 2 weights at each delayed tap supporting filtering of
a complex amplitude dependent non-linear distortion for a signal
with a modulation scheme having greater than 2 amplitude levels of
consequence for a given data symbol, such as PAM4 or 4-Level Pulse
Amplitude Modulation.
12. A circuit according to claim 11 consisting of an analogue
filter circuit.
13. A circuit according to claim 12 wherein the circuit is
implemented in an integrated semiconductor.
14. A circuit according to claim 11 comprising: for each tap a
first input port for receiving a first weight, a second input port
for receiving a second other weight, a scaling circuit for scaling
the first weight and the second weight, and a weighting circuit for
weighting of a signal within the tap to produce a tap output, tap
output signals from different taps combined to form the drive
signal.
15. A method comprising: providing a drive current for driving a
component; filtering the drive current with a non-linear filter to
provide pre-compensated drive current pre-compensated for
distortion in a signal resulting from driving the component with
the drive current, wherein an output signal from the component in
response to the pre-compensated drive current has reduced
distortion and better approximates an ideal transmit signal for an
intended modulation.
16. A method according to claim 15 wherein the component comprises
a Directly Modulated Laser (DML).
17. A method according to claim 16 wherein the directly modulated
laser comprises a Vertical Cavity Surface Emitting Laser
(VCSEL).
18. A method according to claim 16 wherein the directly modulated
laser comprises a Distributed FeedBack (DFB) laser.
19. A method according to claim 18 wherein filtering is performed
with an analogue filter.
20. A method according to claim 15 wherein the analogue filter is
implemented in semiconductor.
21. A method according to claim 15 wherein the non-linear filter
comprises a non-linear FIR filter.
22. A method according to claim 15 wherein filtering corrects for
both a rising edge, low to high signal level response, and a
falling edge, high to low signal level response.
23. A circuit comprising: an input port for receiving a first
signal; a plurality of taps, each tap comprising an input port for
receiving a tap input signal, a first input port for receiving a
first weight, a second input port for receiving a second other
weight, and a scaling circuit for scaling an applied weighting
based on the first weight and the second weight to scale the tap
signal, the scaled tap signal for modifying the first signal.
24. A circuit according to claim 23 wherein the scaling circuit
comprises a switching circuit for switching between the different
weights to select one weight for application at a first time and
another weight for application at another time within a same signal
to be filtered.
25. A circuit according to claim 23 wherein the scaling circuit
comprises a switching circuit for switching between the different
weights to select one weight for application at a first time and
another weight for application at another time in dependence upon a
content of the signal to be filtered.
26. A circuit according to claim 23 comprising a summer for summing
an output of each of the plurality of taps.
27. A circuit comprising: an input port for receiving a first
signal; a plurality of taps, each tap comprising an input port for
receiving a tap input signal, a first input port for receiving a
first weight, a second input port for receiving a second other
weight, and a scaling circuit for scaling an applied weighting
between the first weight and the second weight to scale the tap
signal, the scaled tap signal for modifying the first signal.
28. A circuit comprising: an input port for receiving a first
signal; a plurality of taps, each tap comprising an input port for
receiving a tap input signal, a plurality of input ports each for
receiving a weight, and a scaling circuit for scaling an applied
weighting based on the received weights to scale the tap signal,
the scaled tap signal for modifying the first signal.
29. A circuit according to claim 28 wherein the scaling circuit
comprises a switching circuit for switching between the different
weights to select one weight for application at a first time and
another weight for application at another time within a same signal
to be filtered.
30. A circuit according to claim 28 wherein the scaling circuit
comprises a switching circuit for switching between the different
weights to select one weight for application at a first time and
another weight for application at another time in dependence upon a
content of the signal to be filtered.
31. A circuit according to claim 28 comprising a summer for summing
an output of each of the plurality of taps.
32. A circuit comprising: an input port for receiving a first
signal, the first signal received at a receiver via a communication
interface and from a remote location; a plurality of taps, each tap
comprising an input port for receiving a tap input signal, a
plurality of weight input ports each for receiving a weight, and a
scaling circuit for scaling an applied weighting based on the
received weights to scale the tap signal, the scaled tap signal for
modifying the first signal.
33. A circuit according to claim 32 wherein the scaling circuit
comprises a switching circuit for switching between the different
weights to select one weight for application at a first time and
another weight for application at another time within a same signal
to be filtered.
34. A circuit according to claim 32 wherein the scaling circuit
comprises a switching circuit for switching between the different
weights to select one weight for application at a first time and
another weight for application at another time in dependence upon a
content of the signal to be filtered.
35. A circuit according to claim 32 comprising a summer for summing
an output of each of the plurality of taps.
36. A method comprising providing a receiver for receiving a signal
transmitted across an optical fibre and for providing an electrical
first signal; using a filter, filtering the first signal with a
non-linear filter to provide compensation to the first signal for
distortion in the signal when transmitted resulting from driving a
transmitter at a transmit end, wherein an output signal from the
filter better approximates an ideal transmit signal for an intended
modulation.
37. A method comprising: manufacturing a circuit comprising: an
input port for receiving a first signal; a plurality of taps, each
tap comprising an input port for receiving a tap input signal, a
plurality of input ports each for receiving a weight, and a scaling
circuit for scaling an applied weighting based on the received
weights to scale the tap signal, the scaled tap signal for
modifying the first signal; testing the circuit and determining
each of the plurality of weights based on testing thereof; and
setting each of the plurality of weights based on a result of the
testing thereof and fixing each of the plurality of weights.
38. A circuit comprising: a non-linear FIR filter comprising a
plurality of taps, each tap having multiple weights and a scaling
circuit for scaling the multiple weights to affect a signal
propagating within the tap for nonlinear filtering of a first
signal.
39. A circuit according to claim 38 wherein the non-linear filter
is implemented as an analogue component within an integrated
circuit.
Description
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 61/984,621, filed on Apr. 25, 2014, the
entire contents of which are incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The invention relates generally to communication and more
particularly to optical communication.
BACKGROUND
[0003] Optical transmitters employing Directly Modulated Lasers
(DML) such as Vertical Cavity Surface Emitting Lasers (VCSELs) are
rated to operate up to a predetermined data rate. Problematically,
when operating at higher data rates, distortion from the DML itself
limits performance of the device and thus the data link. The DML
transmits an optical signal that differs from the drive signal
provided thereto such that signal reception is substantially
affected beyond short transmission distances. Added jitter and
vertical eye closure from distortion introduced by VCSEL can cause
significant reduction in signal-to-noise ratio (SNR). These
limitations on performance place a limit on the transmission
distances for higher data rates.
[0004] Linear filters are used conventionally to partially
compensate for the distortion due to the DML itself However, linear
filters fail to achieve optimal compensation for the distortion. It
would be advantageous to overcome some of the shortcomings of the
prior art.
SUMMARY OF EMBODIMENTS OF THE INVENTION
[0005] In accordance with an aspect of at least one embodiment
there is provided a component having repeatable distortion
characteristics; and a drive circuit for providing a drive signal
and comprising a non-linear filter for pre-compensating for
distortion introduced by the component having repeatable distortion
characteristics in response to the drive signal, the error having a
non-linear response to the drive signal.
[0006] In accordance with an aspect of at least one embodiment
there is provided a method comprising: providing a drive current
for driving a Directly Modulated Laser (DML); filtering the drive
current with a non-linear filter to provide pre-compensated drive
current pre-compensated for errors in a signal resulting from
driving the DML with the drive current, wherein an output signal
from the DML in response to the pre-compensated drive current
better approximates the drive current to incur reduced errors.
[0007] In accordance with an aspect of at least one embodiment of
the invention there is provided a circuit comprising: an input port
for receiving a first signal; a plurality of taps, each tap
comprising an input port for receiving a tap input signal, a first
input port for receiving a first weight, a second input port for
receiving a second other weight, and a biasing circuit for biasing
an applied weighting between the first weight and the second weight
to bias the tap signal, the biased tap signal for modifying the
first signal.
BRIEF DESCRIPTION OF THE FIGURES
[0008] FIG. 1 illustrates a typical optical output signal amplitude
of a DML (VCSEL) in response to a direct driving input signal.
[0009] FIG. 2 is a simplified block diagram of a linear finite
impulse response (FIR) filter.
[0010] FIG. 3 is a logic diagram of non-linear FIR filter.
[0011] FIG. 4 is a diagram of a non-linear FIR filter
implementation.
[0012] FIG. 5 is a diagram of another non-linear FIR filter
implementation optimized for performance.
[0013] FIG. 6 is an eye diagram of an unfiltered drive signal
alongside an eye diagram of an output signal corrected with a
non-linear FIR filter such as that of FIG. 4 or FIG. 5.
[0014] FIG. 7 is a graphical representation of the transmit signal
before and after filtering with a 4 tap non-linear FIR filter.
[0015] FIG. 8A shows a sample circuit for implementing a non-linear
filter for pre-compensating a drive signal for driving a directly
modulated laser (DML).
[0016] FIG. 8B shows another sample circuit for implementing a
non-linear filter for pre-compensating a drive signal for driving a
directly modulated laser (DML).
[0017] FIG. 8C shows another sample circuit for implementing a
non-linear filter for pre-compensating a drive signal for driving a
directly modulated laser (DML).
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0018] The following description is presented to enable a person
skilled in the art to make and use the invention, and is provided
in the context of a particular application and its requirements.
Various modifications to the disclosed embodiments will be readily
apparent to those skilled in the art, and the general principles
defined herein may be applied to other embodiments and applications
without departing from the scope of the invention. Thus, the
present invention is not intended to be limited to the embodiments
disclosed, but is to be accorded the widest scope consistent with
the principles and features disclosed herein.
[0019] Referring to FIG. 1, shown is a typical optical output
signal amplitude of a Directly Modulated Laser (DML) in the form of
a Vertical Cavity Surface Emitting Laser (VCSEL) in response to a
direct driving input signal. As is evident, the optical signal
generated (thin line) fails to follow accurately the signal
provided (thick line). The resulting overshoots and undershoots add
distortion to the signal. The distortion appears as both jitter
affecting the width of the eye and amplitude variations affecting
the opening of the eye.
[0020] Referring now to the eye diagram that is shown on the
left-hand side of FIG. 6, the signals are distorted
horizontally--i.e., increased jitter--and vertically--i.e.,
degraded SNR. As is evident, the inner eye opening is significantly
smaller than it would be if the signal was undistorted. Correction
of these distortion artifacts within the transmit signal are
important to enable transmission of higher data rates over longer
distances.
[0021] The distortion artifacts resulting from DML optical response
are amplitude dependent and thus non-linear in nature. The rising
edge and falling edge responses are different and they each need to
be compensated differently. Further, compensating one edge response
may adversely affect the other edge or may fail to achieve
significant improvement without compensating for the other edge as
well. Thus, conventional approaches using linear filters for
compensating for the distortion from the DML response are not
optimal.
[0022] A second problem is implementation efficiency. If the
distortion is repeatable and calculable, it may be possible using a
DSP to reduce the nonlinear distortion within the DML signal; that
said, such an implementation would be costly and would not lend
itself to inexpensive, low power and compact implementation. A more
simple non-linear distortion reduction method would be
preferred.
[0023] Referring now to FIG. 2, shown is a simplified block diagram
of a typical linear finite impulse response (FIR) filter. A signal
is provided to the filter and is then summed with a weighted
delayed version of the signal, termed as a delayed tap, or a
plurality of weighted sequentially delayed versions of the signal
or delayed taps. Linear FIR filters are well known and well
studied.
[0024] Because the distortion is non-linear in nature, a linear
filter is not suitable to addressing the distortion concerns
completely. In fact, such a linear filter, will fail to
substantially correct the problems disclosed above, reducing
distortion in one of the rising or falling edge response while
compounding the distortion in the other.
[0025] Referring now to FIG. 3, shown is a non-linear FIR filter
design for providing pre-compensation for some of the
non-linearities shown in FIG. 1. Here, a signal is provided to the
filter consisting of multiple delayed taps and each tap's
contribution is weighted by two different factors dependent on
input signal's instantaneous amplitude, resulting in an
amplitude-dependent, non-linear filter response. The filter
generates a different response to each of the rising and falling
edges and approximately compensates for the non-linear response of
the DML in response to a driver signal. The use of two weights per
tap, combined with scaling the tap contributions with instantaneous
input signal amplitude, allows for non-linear filter response.
While the use of the FIR architecture supports compact and
efficient implementation. For example, the non-linear FIR filter
shown is implementable as an analogue circuit within a
semiconductor, for example, without relying on complex processing
circuitry such as a DSP.
[0026] Referring now to FIG. 4, shown is a simplified diagram of a
practical implementation of non-linear FIR filter for Non-Return to
Zero (NRZ) signaling. Again, a signal provided is delayed and
tapped, and each tap contribution is weighted differently,
depending on input signal level of one or zero, to provide level
dependent non-linear operation. The tap contributions are added
back into the signal to provide filtering thereof. Signal scalars
are reduced to gates in the filter shown, as suits an integrated
hardware implementation.
[0027] Referring now to FIG. 5, shown is a diagram of another
non-linear FIR filter implementation optimized for performance in
the present embodiment. Here again, each tap signal is acted on by
two different weights. A multiplexer is used to select the
weighting for multiplication. The input signal level is used to
select one of the multiplexer's input weights, and the multiplier
scales the tap signal as per selected weight for each stage. Since
the weights for each tap switch as per the input signal's level,
the filter output need not follow a linear contour. The resulting
non-linear filtering pre-compensates for distortion in the DML.
[0028] Referring again to FIG. 6, shown is an eye diagram of an
uncorrected output signal (left-hand side) alongside an eye diagram
of an output signal corrected with a non-linear FIR filter such as
that of FIG. 4 or FIG. 5 (right-hand side), sand according to the
present embodiment. As is shown, the eye has opened up considerably
with reduced jitter and improved SNR. An improved eye diagram is
typically reflective of improved ability to transmit over greater
distances and reduced error in signal reception.
[0029] FIG. 7 shows a graphical representation of the transmit
signal before (left-hand side) and after filtering with a 4 tap
non-linear FIR filter (right-hand side). Most noteworthy, signal
distortion is greatly reduced after a short time reducing
distortion central to the eye. At the rising edge and falling edge,
distortion remains, but it is significantly reduced. Thus, the eye
opening in an eye diagram is improved. Further, other frequency
components resulting from the distortion are reduced with reduced
distortion.
[0030] Just looking to the falling edge, it is seen that whereas
without filtering, the signal bounces at the bottom down and up,
with filtering the signal remains substantially in alignment with
the desired signal contour. On the rising edge, two notable bounces
are reduced to one smaller bounce, thereby limiting the effect of
the bounce on the top of the eye.
[0031] FIGS. 8A-C shows three sample circuits for implementing a
non-linear filter according to the embodiment. Each circuit has
different drawbacks and advantages, but effectively, the filter
design allows not only for analogue hardware implementation, but
for varied implementation to take advantage of different power
sources, power levels, and other design criteria. Architectures
supporting implementation flexibility are typically desirable as
they are useful in many different applications and well suited to
implementation in many different devices.
[0032] As is seen in each of the circuit diagrams, two currents
proportional to weights are shown designated with "w" (w_0 and w_1)
being multiplexed into the scaling circuit for each tap determined
by level of the input signal (Dp, Dn). Alternatively, the currents
proportional to weights are applied to a scaling circuit such that
they are first scaled by the input signal (Dp, Dn) followed by the
tap signal (Tnp, Tnn). Further alternatively, currents proportional
to the weights are applied to a scaling circuit where the signals
being scaled are a logical combination of input signal (Dp, Dn) and
the tap signal (Tnp, Tnn). The logical combinations include input
signal (Dp, Dn) logically OR'd with tap signal (Tnp, Tnn)
designated as "Dp+Tnp"; and input signal (Dp, Dn) logically AND'd
with tap signal (Tnp, Tnn) designated as "Dp.Tnp". The scaled
version of these logically combined signals in current form is then
summed through a wire OR to produce a single tap contribution that
is dependent on the weights and the input signal amplitude.
Multiple tap contributions are summed to generate a resulting
signal that has an amplitude dependent non-linear
characteristic.
[0033] Though FIGS. 8A-C show one tap for each architecture, it is
understood by those of skill in the art that any number of taps is
supported and selection of a number of taps is dependent upon the
circuit design requirements. Further, though two weights are shown,
the filter architecture described above may be implemented with
additional weights to correct for more complex amplitude dependent
non-linear effects requiring higher granularity or resolution in
amplitude levels.
[0034] Though the above embodiments are directed to
pre-compensating the drive current, filtering of received signals
to improve data detection is also supported. The general
architecture for non-linear filter as shown in FIG. 3 can be used
for a received signal. In end-to-end fibre optic communications
such as fibre optic cables for communicating, the transmitter and
receiver pairing is known and the weights within the receiver are
tuned for use with a specific receiver or are adjusted based on a
transmitter from which a signal is received.
[0035] In use, a circuit is designed and manufactured. Once
manufactured, the circuit is tested with a representative DML
component and based on the combined circuit and DML transmit signal
characteristics, the non-linear FIR filter weights are adjusted to
pre-compensate the drive current for the DML. Thus, each product is
compensated individually, accounting for known DML response issues
as well as circuit specific response issues for a given DML. Once
compensated, the circuit operates in compensated mode. Optionally,
the circuit's operating parameters are readjusted to re-compute the
weights for the non-linear FIR filter at intervals.
[0036] In another embodiment, the optical output signal is tapped
and provided as feedback to the transmit circuit where the
non-linear FIR filter is adjusted in response to changes in
performance of the DML output signal. Further optionally, the
circuit is designed and manufactured with fixed weighting for the
non-linear FIR filter.
[0037] In another embodiment, the manufactured devices are tested,
the non-linear FIR filter is tuned--weights are set--and the
circuit is tested again. Based on its performance, the circuit is
assigned a quality level. Thus, some manufactured drive circuits
support 25 GHz while others support only 15 GHz--determined after
tuning in the manufacturing stage. This allows for a more coarse
tuning process with the performance assignment then dividing
between circuits with best tuning and those with less effective
tuning results.
[0038] Numerous other embodiments may be envisioned without
departing from the scope of the invention.
* * * * *