Patent ID: 12189221

DETAILED DESCRIPTION

As discussed above, as the speed of optical communications continues to increase, there is an increasing need to move more of the signal processing into the optical domain. Therefore, the ability to perform matched filtering on optical signals prior to converting them to electrical signals is very desirable. Further, it is desirable to use electrical domain techniques to control the transfer function of the optical filter. Although some optical low-pass filters, band-pass filters, and high-pass filters have been implemented with tunable wavelengths, conventional filters do not allow for electrically programming the filter coefficients.

Aspects and embodiments are directed to providing optical analogues of an IIR filter and a FIR filter, where the filter coefficients determine the tap weights. In particular, certain aspects and embodiments provide a method for implementing an electrically controllable optical IIR filter. Certain other aspects and embodiments provide a method for implementing an electrically controllable optical FIR filter. As discussed in more detail below, according to certain embodiments, an IIR filter architecture and an FIR filter architecture are each replicated in the analog optical domain. Further, the bit duration and the filter shape are both made electrically configurable.

It is to be appreciated that embodiments of the methods and apparatuses discussed herein are not limited in application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The methods and apparatuses are capable of implementation in other embodiments and of being practiced or of being carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting.

Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use herein of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. References to “or” may be construed as inclusive so that any terms described using “or” may indicate any of a single, more than one, and all of the described terms. Any references to front and back, left and right, top and bottom, upper and lower, and vertical and horizontal are intended for convenience of description, not to limit the present systems and methods or their components to any one positional or spatial orientation. The terms light, light signal, and optical signal may be used interchangeably herein and refer generally to an electromagnetic signal that propagates through a given medium, which may be empty space, e.g., a vacuum, or may be an atmospheric, e.g., air, or other medium, such as fiber or other optics components. The terms “light,” “light signal,” and “optical signal” are not meant to imply any particular characteristic of the light, such as frequency or wavelength, band, coherency, spectral density, quality factor, etc., and may include radio waves, microwaves, infrared, visible, and/or ultraviolet electromagnetic radiation, or other non-ionizing electromagnetic radiation conventionally processed in the field of optics.

Referring toFIG.3, there is illustrated a block diagram of an example on an optical IIR filter300according to certain embodiments. The optical input signal supplied at an input302of the filter300is represented by X(t) and the optical output signal supplied at an output304of the filter300is represented by Y(t). The optical input signal may be a phase modulated or intensity modulated optical signal. In certain examples, both the input302and the output304can be fiber coupled, although that is not a requirement. The filter300includes a plurality of delays310. As with a digital IIR filter, the delays310may be fixed as a function of a bit duration, and therefore, all the delays310may be the same duration. However, in certain examples, variable delay may be needed to accommodate different bit rates. According to certain embodiments, the variable delay can be implemented in two ways depending on the amount of delay required. For high speed optical processing, a material such as Lithium Niobate (LiNbO3) may be used. Thus, the delays310may be implemented on a Lithium Niobate (or other piezoelectric material) substrate306that is coupled to a second substrate308, which may be silicon for example, on which other components of the filter300are implemented. The delay introduced by the Lithium Niobate substrate306is a function of an applied control voltage312(Vπ). Thus, by adjusting the control voltage312, the amount of delay can be varied to adjust to a given bit rate of the optical input signal X(t). The control voltage312provides phase control of the optical output signal Y(t).

According to certain embodiments and applications, for example, for processing of optical signals modulated at radio frequency (RF) data rates, a combination of the Lithium Niobate substrate306and delay lines may be used to implemented variable delays310. Referring toFIGS.3and4, each or at least some of the delays310may be implemented using a combination of a variable delay410provided by the Lithium Niobate substrate306and one or more delay lines420. In such examples, the delay attributed to the Lithium Niobate substrate306(variable delay410) is again variable (adjusted by the control voltage312), while the delay attributed to each delay line420may be fixed. In certain examples, multiple switched delay lines420can be used to accommodate multiple bit rates at lower speeds. Thus, one or more switches430can be included in at least some of the delays310, and controlled via a control signal432to open or close the switches430as need to connect (or disconnect) the delay line(s)420to achieve a desired delay value. The control signal432may be a current or voltage and may be produced by the same or a different source as the control voltage312. In implementations using both the Lithium Niobate substrate306and switchable delay lines420, phase control of the output optical signal Y(t) may be provided by a combination of the control voltage312and control signal432.

The values of the delays310are thus controlled by one or more external control signals/voltages, and therefore may be highly variable and flexibly altered. Accordingly, a wide range of bit rates may be accommodated by the filter300with only an adjustment of the control voltage(s)312,432, without requiring any other changes to the filter design or implementation.

Referring again toFIG.3, as shown, the coefficients for weighting on the forward path are B0through Bn. The coefficients for weighting on the return path are A1through An. According to certain embodiments, rather than being implemented using gain elements, such as amplifiers, the coefficients may be implemented using programmable attenuators320. The use of attenuators320rather than gain elements may reduce the complexity of the optical device. In certain examples, the attenuators320are connected to a control bus330. Each attenuator320may be individually addressed and programmed via coefficient control words supplied over the control bus330. Thus, the weighting coefficients can be dynamically altered by changing the control signal(s) supplied on the control bus to change the value(s) of any one or more of the programmable attenuators320. Thus, the control signal(s) supplied on the control bus330provide amplitude control of the output optical signal Y(t). In certain examples, a gain element340may be connected on the input302to compensate for the losses due to the use of the attenuators320instead of gain elements.

The optical output signal Y(t) at the output304is generated by summing together multiple weighted delayed versions of the input signal X(t) and weighted delayed versions of the output signal Y(t), as shown inFIG.3, using summers350. According to certain embodiments, the filter300may be implemented on the substrate308as a photonic integrated circuit.

Further aspects and embodiments are directed to an optical FIR filter, which may also be implemented as a photonic integrated circuit, for example.FIG.5is a block diagram of one example of an optical FIR filter500according to certain aspects. As inFIG.3, the optical input signal supplied at an input502of the filter500is represented by X(t), and the optical output signal supplied at an output504of the filter500is represented by Y(t). The input optical signal X(t) may be a phase modulated or intensity modulated optical signal. Both the input502and the output504may be fiber coupled, although that is not a requirement.

The optical FIR filter500includes a plurality of delays510. As with a digital FIR filter, the delays510may be fixed as a function of a bit duration, and therefore, all the delays510may be the same duration. However, as with the IIR filter300discussed above, in certain examples, variable delay may be needed to accommodate different bit rates. As discussed above, according to certain embodiments, (for example, as may be used in applications performing high speed optical processing) the variable delays510may be implemented on a piezoelectric substrate506, which may be made of a material such as Lithium Niobate (LiNbO3), for example. As discussed above, the delay introduced by the Lithium Niobate substrate506is a function of an applied control voltage512(Vπ). Thus, by adjusting the control voltage512, the amount of delay can be varied to adjust to a given bit rate of the optical input signal X(t). Further, as discussed above with reference toFIG.4, according to certain embodiments and applications (for example, for processing of optical signals modulated at radio frequency (RF) data rates), a combination of the Lithium Niobate substrate506and delay lines420may be used to implement the variable delays510in the same manner as in examples of the IIR filter300. The control voltage512, optionally in combination with the control signal432, provides phase control of the optical output signal Y(t) by adjusting the variable delays510.

Referring again toFIG.5, the coefficients for weighting are B0 through Bn. According to certain embodiments, rather than being implemented using gain elements, such as amplifiers, the coefficients may be implemented using programmable attenuators520. The use of attenuators520rather than gain elements may reduce the complexity of the optical device. In certain examples, the attenuators520are connected to a control bus530. Each attenuator520may be individually addressed and programmed via a control signal (e.g., coefficient control words) supplied over the control bus530. Thus, the weighting coefficients can be dynamically altered by changing the control signal(s) supplied on the control bus530to change the value(s) of any one or more of the programmable attenuators520. Thus, the control signal(s) supplied on the control bus530provide amplitude control of the output optical signal Y(t). In certain examples, a gain element540may be connected on the input502to compensate for the losses due to the use of the attenuators520instead of gain elements. The weighted delayed version of the input signal X(t) are summed together using summers550to generate the output optical signal Y(t). According to certain embodiments, the filter500may be implemented on a substrate508, which may be silicon, for example, as a photonic integrated circuit.

Thus, aspects and embodiments provide an approach to implement and externally configure, through the use of one or more control signals, optical IIR and optical FIR filters. For applications using high bit rates, and therefore relatively short delay values, the delays310/510can be implemented using a piezoelectric substrate306/506, such as Lithium Niobate, for example. For lower data rates, where delays values greater than what may be provided by the piezoelectric substrate306/506alone, delay lines can be used in combination with the variable delay provided by the piezoelectric substrate306/506. Multiple switchable fixed delay lines420can be used to support different delay values as may be needed for multiple different bit rates. The phase delay of the Lithium Niobate substrate306/506is controlled from an external voltage source. Similarly, switching of the delay line420may be controlled from an external source as well. Attenuators320/520may be used to weight each tap of the respective IIR filter300or FIR filter500, as discussed above. Each attenuator320/520may be individually controlled from an external port via a control bus330/530. An amplifier or other gain element340/540may be used to compensate for the loss introduced by the use of the attenuators320/520. Coupling of the optical input signal X(t) to the optical filters300/500and coupling of the optical output signal from the optical filters300/500may be through a fiber optic cable or free space. As discussed above, the optical filters300/500are applicable for using with both phase and intensity modulated signals.

Having described above several aspects of at least one embodiment, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the invention. Accordingly, the foregoing description and drawings are by way of example only, and the scope of the invention should be determined from proper construction of the appended claims, and their equivalents.