APPARATUS WITH NON-LINEAR FILTER
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.
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This application claims the benefit of co-pending U.S. patent application Ser. No. 14/695,890, filed Apr. 24, 2015, which claims the benefit of then-co-pending 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 INVENTIONThe invention relates generally to communication and more particularly to optical communication.
BACKGROUNDOptical 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.
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 INVENTIONIn 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.
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.
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.
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.
Referring to
Referring now to the eye diagram that is shown on the left-hand side of
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.
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.
Referring now to
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.
Referring now to
Referring now to
Referring now to
Referring again to
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.
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.
Though
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
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.
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.
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.
Numerous other embodiments may be envisioned without departing from the scope of the invention.
Claims
1-39. (canceled)
40. An apparatus for pre-compensating a distortion being introduced by a laser, the apparatus comprising:
- an input circuit;
- a non-linear filter coupled to the input circuit; and
- a summer coupled to the input circuit and the non-linear filter,
- wherein the input circuit comprises a drive input node and a drive output node,
- wherein the input circuit is configured to receive a drive input signal at the drive input node and configured to provide a drive signal at the drive output node,
- wherein the non-linear filter comprises a plurality of taps arranged in parallel,
- wherein each tap of the plurality of taps comprises a tap output node,
- wherein each tap of the plurality of taps comprises a multiplexer and a multiplier,
- wherein the multiplexer together with the multiplier are configured to generate a non-linear tap output signal at the tap output node,
- wherein the multiplexer is coupled to the drive output node,
- wherein the multiplexer comprises a first multiplexer input node, a second multiplexer input node, a selector node, and a multiplexer output node,
- wherein the multiplexer is configured to receive a first weight signal at the first multiplexer input node, configured to receive a second weight signal at the second multiplexer input node, and configured to receive the drive signal at the selector node,
- wherein the multiplexer is configured to select the first weight signal or the second weight signal based on the drive signal's amplitude, and configured to produce the selected one of the first weight signal or the second weight signal, wherein: the drive signal's amplitude is to be used during an operation of the multiplexer for selecting the first weight signal or the second weight signal for the multiplexer; when the drive signal's amplitude is level one, the multiplexer is configured to select the first weight signal; when the drive signal's amplitude is level zero, the multiplexer is configured to select the second weight signal; the level one is different from the level zero; and the first weight signal is different from the second weight signal, such that the non-linear tap output signal at the tap output node has a first characteristic for the level one amplitude of the drive signal, which is different from a second characteristic for the level zero amplitude of the drive signal,
- wherein the multiplier is coupled to the drive output node and the multiplexer output node,
- wherein the multiplier comprises a first multiplier node coupled to the drive output node, a second multiplier node coupled to the multiplexer output node, and a multiplier output node,
- wherein the multiplier is configured to receive a tap signal, configured to receive the selected one of the first weight signal or the second weight signal, configured to scale the tap signal based on the selected one of the first weight signal or the second weight signal, and configured to produce the scaled tap signal at the multiplier output node,
- wherein the tap signal is scaled differently depending on the drive signal's amplitude,
- wherein the non-linear tap output signal is the scaled tap signal,
- wherein the tap output node is the multiplier output node,
- wherein the tap signal for a first tap of the plurality of taps comprises the drive signal,
- wherein each of the tap signals for other taps of the plurality of taps comprises a respective delayed version of the drive signal,
- wherein the summer is coupled to the drive output node and the tap output nodes of the plurality of taps, and
- wherein the summer is configured to sum the drive signal from the drive output node and the non-linear tap output signals from the tap output nodes of the plurality of taps, configured to produce a pre-compensated drive signal, and configured to drive the laser using the pre-compensated drive signal.
Type: Application
Filed: Apr 20, 2023
Publication Date: Aug 17, 2023
Applicant: SEMTECH CANADA CORPORATION (Burlington, ON)
Inventor: Gurpreet S. BHULLAR (Ottawa)
Application Number: 18/304,273