The invention relates to the field of optical communication links. More specifically it relates to optical modulators for these optical communication links.
The still exponentially growing internet data traffic continues to drive the need for higher bandwidth optical communication links. An exemplary block diagram of such a communication link is shown in
The aforementioned rapidly growing need for more bandwidth may outpace the bandwidths that can be achieved by either the driver electronics, the electro-optic modulator, the photodetector or the receiver. Moreover, signal distortion introduced by the transmission channel, for example due to chromatic dispersion of an optical fiber, may corrupt the received signal, again effectively limiting the useful bandwidth.
Today, several methods exist to overcome such bandwidth limitations. Using suitable analog filtering in the electrical domain, for example peaking in the frequency domain can be added at either the driver or receiver side which helps to enhance bandwidth (known as continuous-time linear equalization). While relatively simple to implement, the disadvantage of this method are limitations in the freedom of realizable filter shapes, required to invert the distortion introduced by the optical communication channel. Alternatively, one can use finite impulse response filters realized as e.g. feedforward equalizers (FFEs, in which the output consists of a linear combination of delayed versions of the input signals, with adjustable weights or tap coefficients), possibly in combination with decision feedback equalizers. These can be implemented in either the analog domain, or in the digital domain. In the digital domain, even more sophisticated techniques such as maximum likelihood sequence estimation can be used. The advantage of these techniques is their significantly higher capacity to overcome bandwidth limitations or other forms of signal distortion. However, the implementation complexity can be considerable. In addition, for high baudrate optical communication links power-hungry high sampling rate analog-to-digital and/or digital-to-analog converters may be required.
There is therefore a need for good building blocks, systems and methods which allow to increase the bandwidth of optical communication links.
It is an object of embodiments of the present invention to provide a good Mach-Zehnder modulator.
The above objective is accomplished by a method and device according to the present invention.
In a first aspect embodiments of the present invention relate to an electro-optic Mach-Zehnder modulator. The modulator comprises:
a first and a second optical waveguide,
an optical splitter configured for splitting an incoming optical signal in a first optical signal over the first optical waveguide and a second optical signal over the second optical waveguide and an optical combiner configured for combining the optical signals from the optical waveguides,
a plurality of pairs of electro-optic phase shifters, for each pair one phase shifter per optical waveguide, distributed over the length of the optical waveguides, each pair forming a segment of the modulator, wherein the electro-optic phase shifters are configured for phase-modulating the optical signals by means of an electrical signal,
at least one crossing element configured for crossing the optical waveguides between two segments.
In embodiments of the present invention the Mach-Zehnder modulation may comprise at least one delay element configured for delaying the optical signals between two segments.
In embodiments of the present invention the Mach-Zehnder modulator may comprise at least one transmission line connected with inputs of the phase shifters such that phase-modulating the optical signals can be done by the electrical signal which travels over the at least one transmission line.
In embodiments of the present invention the combiner or splitter may comprise a 90° phase shifter for one of the optical waveguides.
In embodiments of the present invention a distance between adjacent segments may be the same for the different adjacent segments.
An electro-optic Mach-Zehnder modulator, according to embodiments of the present invention, may comprise an optical network which is configured for switching between a direct connection of optical waveguides between adjacent segments and/or a crossing element between the adjacent segments and/or a delay element between the adjacent segments.
In embodiments of the present invention the at least one delay element comprises an optical building block configured for introducing optical delay.
In embodiments of the present invention the segments are equal.
Alternatively a length of the phase shifter may be varying between the different pairs of phase shifters.
An electro-optic Mach-Zehnder modulator, according to embodiments of the present invention, may be configured such that, in operation, the electrical and optical signal are propagating in the same direction.
An electro-optic Mach-Zehnder modulator, according to embodiments of the present invention, may be configured such that, in operation, the electrical and optical wave are counterpropagating.
An electro-optic Mach-Zehnder modulator, according to embodiments of the present invention, may comprising one or more biasing circuits configured for separately biasing at least one of the phase shifters of the phase shifter pairs.
In embodiments of the present invention a crossing element or a delay element may be present between each of the adjacent segments.
In embodiments of the present invention the Mach-Zehnder modulator may comprise a first and a second transmission line wherein the first transmission line is connected with inputs of the first phase shifters and the second transmission line is connected with inputs of the second phase shifters.
In embodiments of the present invention the Mach-Zehnder modulator may be configured for applying the electrical signal between inputs of the first and the second transmission line at first ends of the transmission lines and opposite second ends of transmission lines may be terminated with a predefined impedance between them.
In a second aspect, embodiments of the present invention relate to a communication link which comprises a transmitter, a receiver, and an optical link between the transmitter and receiver. The transmitter comprises an electro-optic Mach-Zehnder modulator according to any of the previous claims.
In a third aspect embodiments of the present invention relate to a method for designing a Mach-Zehnder modulator according to embodiments of the present invention. The method comprises introducing at least one crossing element and/or at least one delay element between segments of the modulator in order to obtain a predefined transfer function.
Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.
Any reference signs in the claims shall not be construed as limiting the scope.
In the different drawings, the same reference signs refer to the same or analogous elements.
The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.
The terms first, second and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.
Moreover, the terms top, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.
It is to be noticed that the term “comprising”, used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression “a device comprising means A and B” should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.
Similarly it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.
Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.
In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.
When focusing on the field of MZMs, the following approaches are taken in most practical prior art to improve the bandwidth or to introduce peaking in the modulator frequency response. In most materials systems, the MZMs are relative long (a few millimeter to a few centimeter) in order to have a sufficiently large interaction between the electrical and optical signal. At high speeds, the wavelength of the electrical signal becomes small compared to the dimensions of the MZM and transmission line effects start to become apparent. These effects may be used to improve the bandwidth or to introduce peaking in the modulator response. Such techniques include (but are not limited to):
In a first aspect embodiments of the present invention relate to an electro-optic Mach-Zehnder modulator 100, more specifically a periodically loaded travelling wave MZM. A schematic block diagram of such a modulator is shown in
The Mach-Zehnder modulator, moreover, comprises an optical splitter 112 configured for splitting an incoming optical signal in a first optical signal over the first optical waveguide 114a and a second optical signal over the second optical waveguide 114b and an optical combiner 116 configured for combining the optical signals from the optical waveguides 114a, 114b.
The Mach-Zehnder modulator, moreover, comprises a plurality of pairs of electro-optic phase shifters 122a, 122b. Each pair comprises one phase shifter 122a, 122b per optical waveguide 114a, 114b. The pairs of electro-optic phase shifters are distributed over the length of the optical waveguides 114a, 114b. Each pair is forming a segment 118 of the modulator. The electro-optic phase shifters are configured for phase-modulating the optical signals. They are controlled by means of an electrical signal.
The Mach-Zehnder modulator, moreover, comprises at least one crossing element 140 configured for crossing the optical waveguides between two segments 118. In embodiments of the present invention there is no interaction between the optical signals in a crossing. The Mach-Zehnder modulator may comprise one or more delay elements 130 configured for delaying the optical signals between two segments 118.
In embodiments of the present invention both the crossing element and the delay element may be present.
In embodiments of the present invention the delay element provides an additional optical delay between two adjacent segments compared to the delay between two other segments between which no such delay element is present.
In
In embodiments of the present invention shaping may be achieved by only using a delay element 130.
In embodiments of the present invention shaping may be achieved by only providing a crossing element 140 between two segments, wherein the optical delay of the crossing element is different from zero.
When a delay element and a crossing element are inserted the optical delay of the crossing element may be the same as the optical delay of the delay element. This is, however, not strictly required.
If, in embodiments of the present invention, multiple crossing elements or delay elements are inserted, these should not necessarily have the same delay.
It is an advantage of embodiments of the present invention that by adding a crossing element between segments, the EO frequency response can be altered to extend the modulator bandwidth or induce peaking to overcome other bandwidth limitations in the optical link. Additionally an optical delay element may be added for altering the EO frequency response or to extend the modulator bandwidth or induce peaking to overcome other bandwidth limitations in the optical link.
It is an advantage of embodiments of the present invention that a passive structure may be obtained that does not require any additional power consumption to achieve frequency response shaping.
It is an advantage of embodiments of the present invention that no additional electric circuits for shaping the frequency response are required, thus saving chip area for the electronics.
It is an advantage that a MZM 100 according to embodiments of the present invention still behaves as a normal MZM (optically broadband, low chirp, same DC-characteristics, same insertion loss, but at a lower extinction ratio).
It is an advantage that the manufacturing process for the photonic modulator, according to embodiments of the present invention does not need any alterations to realize the MZM structure (assuming the initial process could manufacture MZMs). Indeed, in embodiments of the present invention there are only changes to the routing of the optical waveguides, not to the phase shifters or the transmission lines. For example the standard PN-junctions of the said process may be used for the phase shifters.
It is an advantage of embodiments of the present invention that when non-return-to-zero (NRZ) signaling is used, the electrical driver should not have a linear output stage.
A block diagram of a travelling wave Mach-Zehnder modulator (TW-MZM) is shown in
The MZM of
The voltage on the transmission lines induces a phase difference between the top and bottom arm of the Mach Zehnder. In combination with the 90° phase shift, this results in the following transmission characteristic in DC (neglecting all losses and assuming ideal linear phase shifters):
With Pout and Pin the optical power at the out- and input of the modulator, PSi.V the phase difference induced by the ith segment due to the applied voltage V and V=Vin+−Vin−. The inventors made an equivalent block diagram of the modulator of
For this equation an ideal transmission line and linear phase shifters are assumed. In embodiments of the present invention the velocity of the (modulating) electrical signal traveling on the transmission line and the optical signal (undergoing modulation) in the optical waveguide may be matched, such that TE=TO. According to the equation for the phase shift shown above the different phase shifts add perfectly together yielding the maximal modulation efficiency and bandwidth, if TE=TO.
The invention is, however, not limited to MZMs wherein the optical delay over the optical waveguides is equal to an electrical delay between the adjacent segments (if no delay element or crossing element is present between the segments).
In embodiments of the present invention equalization is achieved by replacing certain optical connections between consecutive segments by a delay element 130 or a crossing 140. This is the key idea of this invention. If a delay element is inserted between two consecutive segments (in this case segment 1 and 2), the structure of
In embodiments of the present invention the delay element may be an optical building block configured for introducing delay. This may for example be a delay line or a ring resonator. Assuming the optical delay line (ODL) introduces an additional delay TODL, the following response can be derived:
Only the optical signal travelling in the segments before the optical delay line undergo an extra delay. If TE=TO the following equation can be obtained (see also the simplification above):
Converting this equation to the z-domain results in the following equation:
In the equation in the z-domain, a transfer function from V to PStot of the form a.z−1+b is observed (with a,b>0), indicating that frequency response of the MZM can be shaped to a lowpass characteristic by introducing delay. Note that delay can be added between any two segments and that multiple delays can be added to obtain more complex transfer functions. The maximal filter order is limited by the number of segments.
By introducing delay, all terms in the z-domain transfer function are positive. As a result, introducing delay will decrease the bandwidth if TE=TO. By introducing a crossing 140 between 2 segments, a negative coefficient can be generated. The corresponding structure and block diagram are shown in
In the first equation, a transfer function from V to PStot of the form −a.z−1+b is observed (with a,b>0). A FIR filter with negative tap is obtained. With this kind of filters, peaking at higher frequencies can be introduced resulting in a higher modulator bandwidth.
A MZM 100 according to embodiments of the present invention may comprise multiple delay elements (e.g. optical delay lines) and crossing elements to generate more complex transfer functions in order to optimize the bandwidth or generate sufficient peaking to mitigate other bandwidth limitations (i.e. losses on the electrodes of the transmission line). In some embodiments of the present invention, even passband responses can be generated.
A Mach-Zehnder modulator according to embodiments of the present invention may comprise a first and a second transmission line 124a, 124b. The first transmission line 124a is connected with inputs of the first phase shifters 122a and the second transmission line 124b is connected with inputs of the second phase shifters 122b, such that phase-modulating the optical signals can be done by an electrical signal over the respective transmission lines.
There are also single-arm (push-pull) periodically loaded TW-MZM implementations possible (with slight rearrangements in the connections of the PN-junctions to the electrodes). In this case, a single transmission line is sufficient to make an MZM, in accordance with embodiments of the present invention. Both phase shifters of the pairs of phase shifters are connected to the single transmission line such that phase-modulating of the optical signals can be done by an electrical signal on the single transmission line.
An exemplary embodiment of a MZM 100 according to the present invention is schematically drawn in
PS
tot=−3PS(t−2TODL)+PS(t−TODL)+6PS(t)
In the z-domain this becomes:
The peaking of the exemplary MZM of
The graph in
In this example the modulator is 2.5 mm long. In this example a 56 Gb/s transmission is targeted and the delay is optimized using simulations to have maximal peaking at 25-30 GHz. In this example the optimum delay is 7 ps, resulting in a 500 μm delay line.
It is not required to connect the PN-junctions in the way demonstrated here. Each PN junction can also be connected between the G and the S-line. The depletion PN junction phase shifters are placed in series with the signal lines (i.e. the transmission lines 124a, 124b) and are biased through an inductive line. Termination resistors are present on-chip, a thermo-optic heater may be used to bias the MZM at quadrature. The standard MZM uses exactly the same design but with direct connections between all segments.
It is also possible to use different ways of terminating the transmission lines. The operation of the MZM is unaffected by this. An example thereof is illustrated in
Possible variants with GSSG structure (dual arm) are illustrated in
The PN junctions are now biased by applying a DC-voltage between the S- and G-lines. This DC-voltage should be added to the S-pins together with the data signal, so a bias-T should be added to avoid issues with applying both an AC and DC-signal to the same line.
In this case, the P- and N-regions can be switched, but care should be taken that they are biased in the their correct operating regions (both should have the same reverse bias voltage), and that the phase shifters introduce opposite phase shifts.
The currently drawn example is the P-N/N-P configuration, but the N-P/P-N configuration is also possible. Both require a differential voltage at the GSSG pins to operate.
However, if one choses to use the P-N/P-N configuration or the N-P/N-P configuration, differential signaling on the GSSG pins will result in identical phase shifts in both arms. In that case, an identical voltage should be applied to both S-pins.
Hence, in embodiments of the present invention the pairs of electro-optic phase shifters which are configured for phase-modulating the optical signals may be PN-junctions. These may be connected in different ways with one or two transmission lines. Electro-optic phase shifters as known by the person skilled in the art may be used and they may be electrically connected in accordance with electrical connection schemes known by the person skilled in the art.
In the different variants, there are always pairs of electro-optic phase shifters with for each pair one phase shifter per optical waveguide. They are connected or driven in such a way that the data signal introduces a positive phase shift in one arm and a negative phase shift in the other arm of the MZM. In this way, the optical crossings can be inserted together with optical delays between segments to obtain frequency response shaping.
The bundle of graphs indicated by SH in
As can be seen the measurement results and the theoretical transfer function are close to each other.
Measurements on these examples show that at DC, the Vπ (defined as the voltage that should be applied to the input of the modulator to obtain a 180 degree phase shift between the outputs of the phase shifters in both arms) of the standard and shaped modulator is respectively 11.8V and 29.6V (PN reverse bias 1V). The reason therefore is that in this example only 4 of the 10 segments are actually contributing to the DC phase shift. The insertion loss at a reverse bias of 1V coming from the PN junctions is in both designs very similar, 2.6 dB and 3.1 dB for the standard and shaped modulator respectively. The small deviation is caused by 0.3 dB loss from the crossing and 2 times 0.1 dB from the additional waveguide. The transfer functions were measured using a vector network analyzer and a 70 GHz photodiode, the results are shown in
In
In embodiments of the present invention one or more biasing circuits 126 may be present for separately biasing at least one of the phase shifters 122a, 122b of the phase shifter pairs. An example of such a phase shifter pair is shown in
In embodiments of the present invention a length of the phase shifter 122a, 122b (along the length direction of the waveguide) may be varying between the different pairs of phase shifters. An example thereof is schematically illustrated in
In embodiments of the present invention the structure may be completely defined in layout. Thus the response is fixed once these devices are manufactured.
Alternatively, in embodiments of the present invention an optical network may be inserted that allows switching between a direct connection of optical waveguides between adjacent segments and/or a crossing element 140 between the adjacent segments and/or a delay element 130 between the adjacent segments. Switching may be done between a direct connection, an optical delay line or an optical delay line and a crossing to tune the response after manufacturing. The switching may be implemented using optical switch elements.
In embodiments of the present invention the optical delay line may be followed by a crossing or vice versa. Different configurations are possible between two segments. The crossing may for example be inserted between 2 optical delay lines.
In embodiments of the present invention the optical delays of the delay lines may be chosen equal. In other embodiments this may not be the case in order to optimize the performance.
In embodiments of the present invention the segments may be chosen equal. In other embodiments the segments may differ to optimize performance.
In embodiments of the present invention the phase shifters may comprise different electrode structures. The electrode structures may for example be electrode structures with a single transmission line. The electrode structures not necessarily require an additional bias line. The only requirement is that phase shifters should be present in both arms (both optical waveguides 114a, 114b) of the MZM structure 100. A phase shifter typically may be implemented using a PN junction which behaves as a capacitor which loads the transmission line. Typically pairs of phase shifters are PN-junctions with abutted N- or P-regions. The invention is, however, not limited thereto. Any other phase shifter known by a person skilled in the art may be used (e.g. lateral PN, n-i-p-n). Also other materials like III-V compounds or more exotic materials such as polymers or thin films may be used for the phase shifter.
In embodiments of the present invention additional elements may be added in the optical delay line to tune the delay (and as such optimize the transfer function).
In embodiments of the present invention the electrical and optical wave may be propagating in the same direction. The invention is, however, not limited thereto. Equalization is also possible when both are counterpropagating.
A MZM 100 according to embodiments of the present invention may be used as an intensity modulator. In such embodiments a fixed DC-phase difference of 90° may be present between both arms (the first optical waveguide 114a and the second optical waveguide 114b). This is, however, not strictly required. Other operation modes are also possible.
In some embodiments of the present invention equalization may be obtained by an MZM which is not driven by a differential signal.
It can be seen that the modulation efficiency can be traded for bandwidth. Decreasing the modulator length does exactly the same. It can be seen that for shaped MZMs higher bandwidths are possible for the same Vπ. Shaping is, however, not limited thereto, as discussed before it can do much more than bandwidth enhancement.
In embodiments of the present invention the termination impedance of the modulator may be tuned to trade peaking for modulation efficiency.
In embodiments of the present invention at least some of the electro-optic phase shifters 122a, 122b are configured to operate as traveling-wave segments. It is an advantage of embodiments of the present invention that more phase shift per unit length is obtained since the number of intermediate interruptions is reduced. It is an advantage of embodiments of the present invention that a high tap accuracy can be obtained. When at least some of the electro-optic phase shifters are configured to operate as traveling-wave segments, the tap accuracy is determined by the length of the travelling wave segments, instead of the number of segments.
In embodiments of the present invention a plurality of electro-optic phase shifters may be connected to a transmission line. The resolution to realize a FIR filter tap in that case is determined by 1/N, with N the number of segments. However, as illustrated in
In a second aspect embodiments of the present invention relate to a communication link 200 which comprises a transmitter 210, a receiver 220, and an optical link 230 between the transmitter 210 and the receiver 220. An example of such a communication link is schematically drawn in
It is an advantage of embodiments of the present invention that the modulator according to embodiments of the present invention can be designed to have a specific transfer function and hence to compensate for bandwidth deterioration in the optical link.
In a third aspect embodiments of the present invention relate to a method for designing a Mach-Zehnder modulator 100 according to embodiments of the present invention. The method comprises introducing at least one crossing element 140 between two segments 118 in order to obtain a predefined transfer function. Additionally one or more delay elements 130 may be introduced. The desired transfer function may for example be defined in the z-domain and the positions of the at least one delay element and/or the at least one crossing element may be obtained therefrom as is illustrated in the description above. By introducing the at least one crossing element and/or the at least one delay element between the segments, according to embodiments of the present invention, the shaped MZM may act as a FIR filter.
Number | Date | Country | Kind |
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20158779.7 | Feb 2020 | EP | regional |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2021/054173 | 2/19/2021 | WO |