Optical transmitter

Abstract

To provide an optical transmitter for wavelength division multiplexing for optical fiber including one or a plurality of drivers that drives a graphene electro-absorption modulator (EAM) with respect to each optical input that modulates a set of optical input and light from the optical input for each wavelength of a wavelength division multiplexed (WDM) optical signal to be transmitted and each graphene electro-absorption modulator.SOLUTION: In a pulse amplitude modulation (PAM) 4 optical modulation system, a driver 710 has data input, and output that drives a low-pass filter 712 that performs lowpass filtering of data from the data input to provide lowpass filter data, and each graphene EAM706 by a combination of lowpass filter data and a bias voltage. With the bias voltage, for example, when transmission of the EAM706 increases, an effective index of refraction for dimming decreases, and vice versa. Dispersion in fiber is compensated by biasing the EAM in a certain region and pre-charging dimmer.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] The present invention relates generally to optical transmitters, for example for CWDM (Coarse Wavelength Division Multiplexing). [Background technology]

[0002] Coarse wavelength division multiplexing (CWDM) is a technology that allows the combining of multiple wavelength optical signals in a single single-mode fiber. In CWDM, "coarse" refers to an inter-channel wavelength separation of 20 nm. Thus, four wavelengths occupy 80 nm, and eight wavelengths occupy 160 nm. Using CWDM, it is possible to aggregate wavelengths that each carry a signal at 25 Gb / s. Thus, combining four wavelengths is equivalent to a 100 Gb / s (4 x 25 Gb / s) optical link.

[0003] In practical systems, each wavelength signal can be modulated according to the PAM4 format (four levels of pulse amplitude encoding two bits), so that the data rate per wavelength can be 50 Gb / s. For example, if there are four wavelengths, the effective data rate can therefore be 200 Gb / s (4 x 50 Gb / s).

[0004] Increasing bandwidth requirements make transmission at 400 Gb / s, 800 Gb / s, and 1600 Gb / s desirable. The first stage, up to 400 Gb / s, can be achieved with eight wavelengths of CWDM, known as CWDM8. The CWDM8 specification is described in two documents (Non-Patent Document 1 and Non-Patent Document 2) published by the CWDM8 Multi-Source Agreement Consortium (https: / / www.cwdm8-msa.org / ). One important aspect of the CWDM8 specification is the large spectral range covered by eight wavelengths from 1271 to 1411 nm. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] 400G CWDM8 MSA 2km Technical Specifications [Non-patent document 2] 400G CWDM8 MSA 10km Technical Specification [Non-patent document 3] "Chirp management in silicon-graphene electro absorption modulators", Opt. Express 25, Sorianello et al., 19371~19381 (2017) Summary of the Invention [Means for solving the problem]

[0006] In one embodiment, a WDM optical transmitter for transmitting wavelength division multiplexed (WDM) signals along an optical fiber is described. The WDM optical transmitter may have a set of optical inputs (which may be internal to the transmitter), one for each wavelength of the WDM optical signal to be transmitted. The WDM optical transmitter may further include a graphene electroabsorption modulator (EAM) for each optical input that modulates light from the optical input. The WDM optical transmitter may further include one or more drivers for driving each graphene electroabsorption modulator, although drivers may be shared between EAMs or each EAM may have its own driver. The driver or drivers may have a data input, a low-pass filter that low-pass filters data from the data input to provide low-pass filtered data (which may include band-pass filtered data), and an output that drives a graphene electroabsorption modulator. The dimmed light from the optical inputs may be combined at an optical multiplexer to provide a combined WDM multiplexed optical output.

[0007] The driver is configured to drive the EAM with a combination, e.g., a sum, of the low pass filter data, i.e., a varying voltage drive derived from the low pass filter data, and a bias voltage, which can be configured to bias the graphene electroabsorption modulator into a region where a change in transmission of the graphene electroabsorption modulator in a first direction causes a change in the effective refractive index for dimming in a second, opposite direction, and vice versa.

[0008] In this way, a chirp having a first sign is added to the dimming, which can compensate for a second, opposite-sign chirp due to transmission of the dimming through the optical fiber.

[0009] In some implementations, the bias voltage is configured to bias the graphene of the graphene electroabsorption modulator into a region where increasing the transmission of the graphene electroabsorption modulator decreases the effective refractive index for dimming light passing through the graphene electroabsorption modulator, and vice versa, thus adding a positive chirp to the dimming light to compensate for the negative chirp due to the transmission of the dimming light through the optical fiber.

[0010] The modulation of the light typically consists of a form of pulse modulation, where data is encoded into pulses of light by any of a range of types of codes, for example NRZ (non-return to zero) codes. The modulation may also consist of PAM (pulse amplitude modulation), for example PAM4, where two bits encode four amplitude levels.

[0011] As described below, a pulse traveling through an EAM acquires a positive chirp (instantaneous frequency increasing with time). In some implementations, the amount of chirp applied can be controlled by controlling the peak-to-peak amplitude of the drive. Additionally or alternatively, the bias voltage to the EAM may be controlled to control the amount of chirp applied. In implementations, the optoelectronic properties of graphene can result in a large positive and nearly linear chirp. Positive pre-chirping of the signal compensates for dispersion, e.g., anomalous dispersion, of the optical fiber, typically but not necessarily composed of silica (see below). In effect, the pulse can be focused in time to a distance where dispersion is compensated.

[0012] Prechirping also reduces intersymbol interference (ISI), which synergistically helps reduce ISI by low-pass filtering the data before applying it to the modulator.

[0013] The broadband response of graphene EAMs facilitates multiplexing a wide range of wavelengths for applications in, for example, CWDM8 or CWDM16 systems.

[0014] In some implementations, a graphene phase modulator is optically coupled in series with a graphene electroabsorption modulator in the signal path for one or more of the wavelengths, which can increase the amount of positive chirp and therefore the length of fiber over which dispersion is compensated.

[0015] To phase-modulate an optical signal, a bias voltage is selected to bias the graphene phase modulator into a region where it remains transparent with respect to a varying applied voltage. Here, "remain substantially transparent" may be taken to mean that the transparency should not vary by more than 3 dB, preferably no more than 1 dB, with the phase modulation drive voltage. Preferably, the bias voltage should bias the graphene phase modulator so that the portion of the modulator made of graphene has a loss at the relevant wavelength of the optical signal of less than 3 dB, preferably less than 1 dB. A driver for the graphene phase modulator can be configured to drive the modulator with a combination of a bias voltage and a varying voltage drive derived from low-pass filter data. The varying voltage drive derived from low-pass filter data changes the effective refractive index of the graphene phase modulator with respect to dimming, while the bias voltage maintains the modulator in a region where transparency is substantially constant.

[0016] In some implementations, a graphene phase modulator is optically coupled in series with a graphene electro-absorption modulator to counterintuitively add negative chirp to the dimming. This can be achieved by biasing the graphene phase modulator as described above but driving the phase modulator with a varying voltage drive consisting of the inverse of the low-pass filtered data. This can be derived, for example, by inverting the data and then low-pass filtering the inverse data, or by inverting the low-pass filtered data for the EAM. It will be recognized that the peak-to-peak level of this varying drive (for either configuration of phase modulator) will typically be different from that used for the EAM. By driving the graphene phase modulator with the inverse of the data, when the effective phase applied by the EAM is increasing, that applied by the graphene phase modulator will be decreasing, and vice versa. Thus, the graphene phase modulator will add chirp opposite to that of the EAM. This can be used to compensate for the excess positive chirp contribution added by the EAM.

[0017] In some implementations, excess positive chirp compensation is wavelength dependent. Anomalous dispersion in optical fibers is typically wavelength dependent, and therefore different pre-chirps may be applied to different wavelengths (i.e., wavelength bands) of a WDM signal. The amount of pre-chirp may be adjusted by adjusting the peak-to-peak (Vpp) drive voltage of the EAM, and thus, in some implementations, different Vpp may be applied to different wavelengths. Additionally or alternatively, an additional negative chirp may be added to some of the wavelengths (bands). For example, a graphene phase modulator may be provided that adds negative chirp to only selected ones of the optical inputs.

[0018] In some implementations, graphene phase modulators that add negative chirp are applied to wavelengths below the limit wavelength at which the anomalous dispersion of an optical fiber (the refractive index increases with wavelength) changes to normal dispersion (the refractive index decreases with wavelength). In the case of CWDM16, graphene phase modulators that add negative chirp may be used for wavelengths below about 1300 nm, such as 1291 nm and 1271 nm.

[0019] In some implementations, the optical path from each optical input includes an optical splitter that splits the optical input into two or more branches, a graphene electroabsorption modulator for each branch that modulates the light in the branch, and an optical combiner that combines the dimmed light from the branches. This can be used to implement PAM4, PAM8, or higher modulation. Thus, one or more drivers can be configured to drive the graphene electroabsorption modulators in the branches with groups of two or more bits of data from the data input, for example, MSB (Most Significant Bit) and LSB (Least Significant Bit) bits.

[0020] One or each of the branches may have a phase shifter (such as a 90-degree phase delay) or other element that rotates the polarization of one branch (or set of branches) with respect to the other. In this way, the light in the two branches can be given orthogonal linear or circular polarizations. This is advantageous because it allows the phase and chirp characteristics of each branch / polarization to be preserved after their combination. The polarization rotation element, i.e., the function that rotates the polarization of one branch (or set of branches) with respect to the other, can be part of / implemented by the combiner.

[0021] Generally, each optical input may have its own optical splitter, however optical polarization rotators and optical combiners may be implemented separately for each WDM wavelength or for a set or all of the WDM wavelengths.

[0022] In the former case, each of the optical inputs may have a respective optical polarization rotator and optical combiner, and the transmitter may further comprise an optical wavelength multiplexer that multiplexes the optical outputs from the optical combiners.

[0023] In the latter case, branches corresponding to the same polarization or bit significance (e.g., LSB, MSB) may first be combined by optical WDM multiplexing, and then one (or both) of the combined wavelengths may be polarization rotated before the two (or more) sets of multiplexed, orthogonally polarized signals are combined to provide a dimmed output.

[0024] Thus, the transmitter may comprise a first optical coupler that combines a first branch of the branches from each optical input and a second optical coupler that combines a second branch of the branches from each optical input. The first and second optical couplers may comprise respective first and second optical wavelength multiplexers. A shared optical polarization rotator may be coupled to the output of the first optical wavelength multiplexer. The transmitter may further comprise a third optical coupler that combines the polarization-rotated output of the first optical wavelength multiplexer with the output of the second optical wavelength multiplexer to provide a combined wavelength division multiplexed output.

[0025] In some implementations, graphene electroabsorption modulators and graphene phase modulators may comprise one or more graphene layers. These may be integrated with a waveguide, for example, adjacent to the waveguide core, so that evanescent waves of light propagating through the waveguide couple to the graphene. In some implementations, the waveguide may comprise a longitudinal structure on a substrate, e.g., having a rectangular cross-section, and graphene layers may be provided above and / or below this structure, or between a pair of such waveguide structures, potentially widely spaced, similar to a slot waveguide, or just narrow enough, e.g., for the graphene to effectively form an integral waveguide structure. The structure may be fabricated from, for example, silicon, silica, silicon nitride, or a polymer, but the waveguide may be electrically conductive (e.g., Si), and the graphene may be electrically insulated from the waveguide by a thin oxide layer. One or two (or more) graphene layers may be utilized, although the graphene may be single-layered or multilayered and optionally doped.

[0026] In some implementations, the graphene electroabsorption modulator and the graphene phase modulator may be implemented in different longitudinal regions of the same waveguide, for example, by graphene provided over or underneath, and the graphene in these two (or more) regions may be provided with respective electrode connections implementing the graphene electroabsorption modulator and the graphene phase modulator.

[0027] In some implementations, graphene electroabsorption modulators and graphene phase modulators may further comprise a drive electrical connection to one of the one or more graphene layers and a second or counter electrode connection. The counter electrode connection may consist of an electrical connection to a waveguide (e.g., to a silicon waveguide) and / or an electrical connection to a second of the one or more graphene layers and / or an electrical connection to an additional metal layer of the device. For example, in one implementation, a silicon waveguide (S), a 10 nm SiO2 insulating layer (I), and a graphene layer (G) form a SIG capacitor with connections to S and G.

[0028] In some implementations, a wavelength division multiplexed (WDM) optical transmitter includes an optical signal source such as a DFB (distributed feedback) laser, and the set of optical inputs may consist of an internal input that receives the signal from the laser.

[0029] In a related aspect, a graphene-based modulator is provided that is configured to provide a pre-chirp to normal dispersion. Accordingly, an optical modulator is provided that includes a graphene electroabsorption modulator optically coupled in series with a graphene phase modulator, the graphene electroabsorption modulator configured to apply a positive chirp to a modulated optical signal, and the graphene phase modulator configured to apply a negative chirp to the modulated optical signal.

[0030] In a further related aspect, a method of transmitting a wavelength division multiplexed (WDM) signal along an optical fiber is provided. The method may include inputting data for transmission. Optionally, the method may also include low-pass filtering the data, which may include band-pass filtering the data. The method may then modulate each wavelength (band) of the WDM optical signal with a portion of the low-pass filtered data using a respective graphene electro-absorption modulator (EAM). The modulating step may include biasing each graphene electro-absorption modulator to a region where the graphene electro-absorption modulator applies a (positive) pre-chirp to the modulation wavelength to compensate for a (negative) chirp in the optical fiber.

[0031] The method may further include phase modulating one or more of the wavelengths using a graphene phase modulator to add additional chirp to the wavelengths. The additional chirp may be positive to extend the length of the dispersion-compensated fiber, or negative to reduce excess positive chirp from the EAM and / or compensate for normal dispersion of the fiber at one or more of the wavelengths. The method may also adjust the drive of each EAM according to the wavelength being modulated. Thus, in general, the method may include providing different pre-chirps, positive or negative and / or varying in amount, for different wavelengths of the WDM signal to, among other things, compensate for different dispersions of the fiber at different wavelengths.

[0032] The method may further include splitting each wavelength of the WDM optical signal into two or more branches. The modulating step may include modulating each branch using a respective graphene electroabsorption modulator and combining the dimming light in each branch. The method may further include rotating the polarization of the light in (or from) one of the branches (for each wavelength) with respect to the other so that the combined dimming light from the branches consists of light of two orthogonal polarizations. The polarization rotation may be performed before or after combining the wavelengths of the WDM signal using one or more optical multiplexers.

[0033] In a further related aspect, there is provided an optical transmission system comprising means for implementing the above-described method.

[0034] In some implementations of the above devices and methods, the optical fiber may comprise a photonic crystal optical fiber, which can help extend the operating wavelength range, for example, to wavelengths greater than 1610 nm, especially when the material comprising the fiber operates in a large spectral region where absorption is large.

[0035] These and other aspects of the present invention will now be further described, by way of example only, with reference to the accompanying figures. [Brief explanation of the drawings]

[0036] [Figure 1]FIG. 1 illustrates the transmission properties of a graphene EAM and an EAM according to the prior art. [Figure 2a] 2 illustrates chirp-related characteristics of the EAM of FIG. 1 according to the prior art. [Figure 2b] 2 illustrates chirp-related characteristics of the EAM of FIG. 1 according to the prior art. [Figure 2c] 2 illustrates chirp-related characteristics of the EAM of FIG. 1 according to the prior art. [Figure 3] FIG. 1 illustrates an example of a CWDM optical transmitter. [Figure 4] FIG. 4 is a diagram showing details of an optical modulation system in the transmitter of FIG. [Figure 5] FIG. 5 shows a version of the optical modulation system of FIG. 4 for longer fiber lengths. [Figure 6] FIG. 5 shows a version of the optical modulation system of FIG. 4 for compensating for both anomalous and normal dispersion. [Figure 7] FIG. 1 illustrates a PAM4 optical modulation system. [Figure 8] FIG. 8 is a diagram illustrating a detailed example of the PAM4 optical modulation system of FIG. 7. [Figure 9] 9 illustrates an example version of the PAM4 optical modulation system of FIG. 8 based on the optical modulation system of FIG. 5. [Figure 10] 9 illustrates an example version of the PAM4 optical modulation system of FIG. 8 based on the optical modulation system of FIG. 6. [Figure 11] FIG. 11 is a diagram showing an example of a CWDM optical transmitter for implementing the optical modulation systems of FIGS. [Figure 12] FIG. 12 illustrates a modification of the CWDM optical transmitter of FIG. DETAILED DESCRIPTION OF THE INVENTION

[0037] In the drawings, like elements are designated by like reference numerals.

[0038] Graphene can be used to realize electroabsorption modulators. Non-Patent Document 3 describes the compensation of fiber dispersion at 1550 nm using a graphene-based electroabsorption modulator. The dispersion compensation was as high as 840 ps / nm, which far exceeded the requirements for 2 km and 10 km long links.

[0039] Figure 1, taken from the same paper, shows the simulated effective refractive index change (left-hand axis) and transmission (right-hand axis) at 1550 nm for a single-layer graphene (SLG)-on-silicon waveguide EAM (see the paper for fabrication details) with a 100 μm-long SLG region.

[0040] The inset shows a vertical cross section through the EAM 100, comprising a silicon-on-insulator substrate defining a region 102 of silica in which a silicon waveguide 104 is formed. A thin oxide layer 106 is provided on top of the waveguide, and a monolayer 108 of graphene is provided on top of the oxide. Electrical connections 110a,b are made to the graphene and silicon. Viewed from above, the graphene extends longitudinally along the waveguide for 100 μm in this example.

[0041] The same structure can be used to implement the phase modulator described below.

[0042] In use, the graphene layer of the EAM is driven so that its Fermi level is above the Pauli blocking state, i.e., above 0.4 eV (in this example), as illustrated by the shaded area in Figure 1. The horizontal voltage axis in Figure 1 is relative to the Dirac point voltage, about -7 V in this example, so the EAM can be biased at, for example, about -1 V. (The Dirac point voltage depends on the doping: there is typically some internal doping, and there can also be external doping, and the effective doping can be varied with the applied bias, but at the Dirac point the carrier density is near zero and the resistivity is at a maximum.)

[0043] As can be seen from Figure 1, when operating in this regime, when a voltage is applied to the graphene EAM, the transmission increases while the effective refractive index of the optical mode decreases. Therefore, when a pulse is created, the phase of the optical beam at the modulator's output is smaller at the pulse peak than at the pulse tail. By differentiating the phase, the instantaneous frequency shift can be determined; Figure 2a illustrates an example pulse, showing amplitude (power) on the left-hand axis and instantaneous frequency shift on the right-hand axis (the continuous curve is experimental data, and the dots represent simulations). It can be seen that there is a positive (upward) frequency chirp in the pulse that is approximately linear.

[0044] The chirp can be defined by the chirp parameter C given by the following equation:

[0045]

number

[0046] In the formula, L is the element length (100 μm in this example). The change in refractive index Δn max is the maximum variation in the effective refractive index obtained when driving the modulator to minimum absorption, which can be determined, for example, from Figure 1 or by fitting the curve to a theoretical version of Figure 2a (as described in Sorianello et al., ibid.), and λ is the central wavelength. In this example, the EAM is driven with a peak-to-peak voltage of 2.7 V, with a chirp parameter C = 0.27.

[0047] Figure 2b illustrates another pulse, showing amplitude (left-hand axis) and phase (right-hand axis) in the left-hand graph, and amplitude (power) and chirp (instantaneous frequency shift) in the right-hand graph. It can be seen that the phase time profile has the same shape and period as the amplitude envelope, but with the opposite sign.

[0048] Figure 2c illustrates the magnitude of the chirp (in units of GHz change in frequency) on the left-hand axis versus the peak-to-peak applied voltage for the EAM. Referring back to Figure 1, it can be seen that above about 0.45 eV the transmission of graphene remains nearly constant while the phase decreases approximately linearly. Thus, by varying the peak-to-peak voltage, the phase change and hence the degree of chirp can be varied.

[0049] 3, which shows an example of a CWDM16 optical transmitter 300 comprising a set of optical inputs 302, one for each wavelength, a graphene EAM 304 for each optical input, a multiplexer that combines the modulator light from the EAMs, and an optical output 308 that may be suitable for coupling into an optical fiber, such as a single-mode optical fiber. The multiplexer 306 may be implemented using integrated optics or free-space micro-optics.

[0050] Substantially the same graphene EAM design can be used for each EAM, thus facilitating fabrication. This is because graphene EAMs can operate over a wide range of wavelengths. Voltage-tunable chirp can similarly compensate for optical fiber chromatic dispersion over a wide range of wavelengths. For example, for CWDM8, fiber dispersion ranges from -5 ps / (nm km) to +7 ps / (nm km), and for CWDM16, the maximum dispersion at 1610 nm is approximately +20 ps / (nm km). The design in Figure 3 can accommodate wavelengths above 1610 nm, for example, well beyond 2000 nm or even greater than 2100 nm. The zero-dispersion wavelength of standard single-mode fiber is approximately 1310 nm, and the dispersion penalty increases with increasing distance from this wavelength.

[0051] For reference, two example sets of center wavelengths for CWDM16 are given in Table 1 and Table 2 below:

[0052] [Table 1]

[0053] [Table 2]

[0054] The asterisks in Table 1 indicate wavelengths where there is normal rather than anomalous dispersion.

[0055] FIG. 4 illustrates details of the optical modulation of the system of FIG. 3. Thus, the graphene EAM 304 is driven by a data input 400 via an electrical low-pass filter 402 (LPB = low-pass band). The data 400 is provided by a modulator 404, which provides, for example, PAM4 or NRZ data. The low-pass filter 402 reduces inter-symbol interference (ISI). A DC bias voltage generator, which provides a bias voltage to the EAM 304, is not shown in FIG. 4. The bias voltage may be applied, for example, via a bias tee (not shown) between the low-pass filter 402 and the EAM 304.

[0056] The low-pass filter 402 may have a cutoff frequency in the range of 0.4 to 0.8 times the baud rate of transmission. The length of the graphene EAM and the amplitude (or peak-to-peak voltage) of the drive signal may be adjusted according to the length of the optical fiber to be compensated. The design goal is that the sum of the positive chirp imposed by the graphene EAM and the negative chirp imposed by the fiber at the design length of the fiber should be zero at the wavelength of the modulator light.

[0057] Systems such as those described above can be used for optical fiber lengths of 2 km or 10 km, for example, with either CWDM8 or CWDM16, and NRZ or PAM4 modulation can be utilized. For example, a CWDM16 transmitter can operate each optical channel (wavelength) at 100 Gb / s, thus providing an aggregate raw data rate of 1.6 Tb / s.

[0058] Figure 5 shows an optical modulation system 500 that is a variation of the system of Figure 4 that can accommodate additional lengths of fiber. Elements similar to those previously described are designated by similar reference numerals.

[0059] In the arrangement of FIG. 5, a graphene phase modulator 502 is coupled in series with the graphene EAM 304, either before or after the EAM, to provide an additional positive chirp. The phase modulator is driven by the same low-pass filtered data signal as in FIG. 4, but is biased differently. More specifically, the phase modulator is biased beyond the shaded region into the right-hand portion of FIG. 1. This is where the transmission through the graphene integrated waveguide varies little, but where the effective refractive index (i.e., phase) still varies significantly, e.g., approximately linearly. The bias arrangement is not shown in FIG. 5, but can be achieved by setting appropriate bias voltages as shown by FIG. 1. For example, a large negative bias voltage, i.e., a bias voltage that is more negative than the bias voltage used for the graphene EAM, can be utilized.

[0060] For example, in some implementations, a graphene EAM may be biased at −1 V as described above, whereas data modulation may be applied such that the voltage to the EAM varies between −1 V and some positive voltage depending on the desired degree of positive chirp. In contrast, a graphene phase modulator may be biased at a bias voltage much less than −1 V such that when a data signal is applied to the bias, the voltage to the graphene phase modulator remains less than −1 V and the graphene remains in its transparent region. While the same signal is provided to both the EAM and the phase modulator as illustrated in FIG. 5, the signals provided for these devices may have different peak-to-peak amplitudes (as well as different bias voltages applied to the devices).

[0061] The arrangement of Figure 5 can add additional positive chirp to the dimming without substantially affecting the amplitude modulation and therefore compensate for the anomalous dispersion of the optical fiber over a greater length, i.e., it can compensate for a greater degree of negative chirp imparted by the fiber.

[0062] Figure 6 shows a version of an optical modulation system 600 suitable for compensating for both anomalous and normal dispersion. Again, similar elements to those previously described are indicated by similar reference numerals. Thus, as in the arrangement of Figure 5, a graphene phase modulator 502 is optically coupled in series with the graphene EAM 304 and biased as previously described.

[0063] However, in Figure 6, the phase modulator 502 is driven by an inverted version of the data 602 via a second low-pass filter 604. Referring back to Figure 1, the slope of the refractive index change (phase) with data voltage is therefore effectively reversed compared to the graphene EAM. The graphene phase modulator 502 therefore adds a negative chirp to the signal. This negative chirp can be used to compensate for excess positive chirp introduced by normal dispersion in the graphene EAM and / or optical fiber. Again, the design goal is that the total chirp, i.e., positive chirp from the EAM, negative chirp from the phase modulator, and positive or negative chirp from the optical fiber, should be zero over the design length of the fiber.

[0064] 7 shows a set of PAM4 modulators 701, in this example 16 modulators, for a CWDM16 transmitter, each comprising a pair of graphene EAM devices as described above. These can be used in the general arrangement of FIG. 3, one per wavelength (band).

[0065] More specifically, each optical modulation system includes an optical power splitter 702 that splits the optical signal into two branches, e.g., in a ratio of x:(1-x), e.g., 1:2 (for PAM4) in this implementation. Each branch includes a respective graphene EAM 706a,b, and the optical power from the two branches is combined in an equal ratio in an optical power combiner 708, e.g., for PAM4 (the MSB branch carries twice the signal level of the LSB branch). In a modified implementation, one of the branches includes a 90-degree polarization rotation element 704 (e.g., an optical delay or waveplate), which may be part of the combiner 708. The combined data rate is twice the data rate of each individual EAM. The electrical signal may be sorted into bit pairs, where the least significant bit of the pair may modulate one EAM and the most significant bit of the pair may modulate the other EAM.

[0066] In some implementations, the polarization rotator and combiner is realized with an integrated polarization rotator and combiner having two input waveguides supporting at least orthogonal optical polarizations and one output waveguide supporting at least two orthogonal optical polarization modes. The polarization rotator and combiner takes light at two inputs of the same optical polarization and provides two combinations at the output onto the same waveguide but each of different orthogonal polarizations.

[0067] For example, in some implementations, polarization rotation and combiner functionality can be achieved on the output optical fiber using a dual polarization grating coupler. A dual polarization grating coupler can have two input waveguides and optical outputs from the chip oriented at an angle with respect to the normal to the plane of the chip (the combiner). The outputs can comprise gratings formed on the waveguides. The light in the input waveguides can then be split into two orthogonal polarizations leaving the chip.

[0068] Figure 8 shows a more detailed example of the arrangement of Figure 7 to illustrate a pair of polarization shifters 704a,b. Figure 8 also illustrates MSB and LSB drivers 710a,b (in practice these are generally part of a combined driver) and their respective low-pass filters 712a,b, providing a dimmed output 714. Other aspects of the system are as described above, and in particular the graphene EAM is designed with a chirp that compensates for the chirp introduced by the fiber (over the length of the fiber).

[0069] FIG. 9 shows a version of the system of FIG. 8 that incorporates a pair of phase shifters 900a,b as described above with reference to FIG. 5, to provide an additional positive chirp over wavelength (band).

[0070] Figure 10 shows a version of the PAM4 optical modulation system of Figure 8 including a pair of similar graphene-based phase shifters driven from the inverse MSB and LSB data through respective electrical low-pass filters 1000a,b as described above with reference to Figure 6. This provides negative chirp to compensate for excess positive chirp from EAM and / or normal dispersion at the relevant wavelength (band) for the optical fiber.

[0071] 7-10, in some implementations the dimming at each wavelength may be combined in a wavelength multiplexer. In other implementations, the optical system is modified so that the LSB and MSB branches are kept separate and then multiplexed together separately in their respective wavelength multiplexers before combining and optionally rotating the polarization of one branch with respect to the other.

[0072] 11, which shows an example of a CWDM optical transmitter 1100 in which the MSB and LSB branches for each wavelength are combined before wavelength division multiplexing. Thus, the CWDM optical transmitter 1100 comprises a set of continuous wave (CW) light sources 1102, e.g., diode lasers, at different wavelengths, each coupled to a respective PAM4 modulator 701, e.g., comprising an optical power splitter, EAM, and optical power combiner, as shown in FIGS. 7-10. Light from each modulator 701 is provided to a wavelength combiner 1104, which provides a wavelength division multiplexed modulated output 1106. In an example implementation, the MSB and LSB polarizations are orthogonal, and the wavelength combiner 1104 supports two orthogonal polarizations.

[0073] Figure 12 shows an example of a CWDM optical transmitter 1200 in which the MSB and LSB branches rotate the polarization of one branch with respect to the other and are multiplexed separately before combining. This avoids the need for a wavelength multiplexer that supports orthogonal polarizations. Any of the optical modulation / compensation systems of Figures 7-10 can be used in the optical transmitter of Figure 12, but can be modified to multiplex the wavelengths of each branch before combining.

[0074] The CWDM optical transmitter 1200 includes a pair of continuous-wave (CW) light sources 1202, e.g., diode lasers, each coupled to a respective optical power splitter 1204 that provides two outputs in an x:(1-x) ratio, e.g., 2:1 (MSB:LSB). The MSB output (solid line) from each power splitter is provided to a graphene EAM device 1206, as described above, and the LSB output (dashed line) from each power splitter is similarly provided to a graphene EAM device 1210. The graphene EAM devices 1206 and 1210 may be referred to as an MSB driver and an LSB driver, respectively, in conjunction with optional compensation and electrical filtering, as described above. The modulated MSB wavelength is provided to a first wavelength multiplexer 1208, and the modulated LSB wavelength is provided to a second wavelength multiplexer 1212. Each optical multiplexer combines the same polarization components of each wavelength—e.g., assuming two orthogonal polarizations for each wavelength, the transmitter would have one multiplexer for each polarization mode. After wavelength multiplexing, the dimmed output from each of the two multiplexers 1208, 1212 is provided to a polarization rotator and combiner 1214 (which may be two devices or a combining device) as described above, which has a dimmed output 1216. Thus, the polarization of one set of multiple wavelengths can be rotated with respect to the others, for example, so that the modulated MSB and LSB wavelength components have orthogonal polarizations.

[0075] The optical modulation systems of Figures 4 through 10 may be used with CWDM8 or CWDM16 optical transmitters that utilize PAM4 modulation to double the data rate while allowing compensation for anomalous and / or normal dispersion, as described above. Thus, the optical modulation systems of Figures 8 through 10 may be implemented for each wavelength (band) of the transmitter, or only at selected wavelengths.

[0076] In some implementations, compensation of normal dispersion by adding negative chirp is utilized only for short wavelengths, e.g., wavelengths below 1310 nm, such as the 1270 nm and 1290 nm wavelengths of CWDM 16. Thus, for example, the optical modulation systems of Figures 6 and 10 may be implemented for only a subset of wavelengths, e.g., only two wavelengths below 1310 nm.

[0077] In some implementations, the graphene electroabsorption array comprises an array of linear waveguides, each of which may comprise a length of, for example, 0.1 mm, e.g., in the range of 0.05 to 0.150 mm, and which is covered with graphene, electrically modulated, or otherwise includes graphene in the evanescent wave region of the waveguide. Each waveguide may be provided with a wavelength-specific bias voltage and drive signal, although the drive signals may be equal in amplitude for all modulators in the array. Because graphene EAMs can operate beyond 1610 nm, the array may be configured for, for example, 16 CWDM wavelengths or more. The total range of operation may extend beyond 2100 nm.

[0078] One portion of the electroabsorption modulator may operate at wavelengths corresponding to the fiber's negative chromatic dispersion (e.g., 1270 nm and 1290 nm). Two waveguide lengths of the same waveguide covered with graphene and two drive voltage and bias combinations on two lengths of graphene may be applied. While the first length serves to operate the electroabsorption modulator, the second length is also an electroabsorption modulator, but is operated near or in the transparency region of the graphene to minimize the additional contribution of absorption modulation. The role of the second electroabsorption modulator may be to induce a negative chirp in the signal. To achieve the negative chirp, the second modulator may be driven with a driver signal that is inverted relative to the driver of the first modulator.

[0079] It is generally desirable to increase the bandwidth of the described systems. One approach would be to increase the bandwidth of each transmitter, but this is difficult in practice and may result in shorter propagation distances. Another approach would be to increase the number of different wavelengths and combine them into a single fiber, as discussed above, but silica fiber has increasing losses beyond the L-band (1560-1610 nm). Therefore, in some implementations, the system utilizes a combination of graphene modulators and photonic crystal optical fiber to provide useful performance above 1600 nm, e.g., 2000 nm and beyond.

[0080] Many other useful alternatives will no doubt occur to those skilled in the art, and it will be understood that the invention is not limited to the described embodiments, but encompasses modifications apparent to those skilled in the art that are within the spirit and scope of the claims appended hereto. [Explanation of symbols]

[0081] 100 EAM 102 Silica Domain 104 Silicon Waveguide 106 Thin oxide layer 108 graphene monolayer 110a, 110b Electrical connections 300 CWDM16 Optical Transmitter 302 Optical Input 304 Graphene EAM 306 Multiplexer 308 Optical Output 400 Data Entry 402 Low-pass filter 404, 404' driver 500 Optical Modulation System 502 Graphene Phase Modulator 600 Optical Modulation System 602 Reverse Data 604 Low-pass filter 701 PAM4 Modulator 702 Optical power splitter 704 90-degree polarization rotation element 704a, 704b Polarization shifter 706a, 706b Graphene EAM 708 Optical power combiner 710a, 710b MSB / LSB Drivers 712a, 712b Low-pass filters 714 Dimming Output 900a, 900b phase shifter 1000a, 1000b Electrical Low-Pass Filter 1100 CWDM optical transmitter 1102 Continuous wave light source 1104 Wavelength multiplexer 1106 Wavelength division multiplexing dimming output 1200 CWDM optical transmitter 1202 Continuous Wave Light Source 1204 Optical Power Splitter 1206 Graphene EAM Device 1208 First Wavelength Multiplexer 1210 Graphene EAM Device 1212 Second Wavelength Multiplexer 1214 Polarization Rotators and Combiners 1216 Dimming Output

Claims

1. 1. A wavelength division multiplexed (WDM) optical transmitter for transmitting a WDM signal along an optical fiber, comprising: a set of optical inputs, one for each wavelength of a wavelength division multiplexed (WDM) optical signal to be transmitted; a graphene electro-absorption modulator (EAM) for each optical input that modulates light from the optical input; one or more drivers for driving each graphene electroabsorption modulator; at least one graphene phase modulator optically coupled in series with the graphene electroabsorption modulator; Equipped with the one or more drivers have a data input, a low pass filter that low pass filters data from the data input to provide low pass filtered data, and an output that drives each graphene electroabsorption modulator with a combination of the low pass filtered data and a bias voltage configured to bias the graphene electroabsorption modulator into a region where a change in transmittance of the graphene electroabsorption modulator in a first direction causes a change in effective refractive index for dimming in a second, opposite direction, and vice versa; providing a chirp having a first sign to the dimming to compensate for a chirp having an opposite second sign due to transmission of the dimming through the optical fiber; and at least one graphene phase modulator optically coupled in series with the graphene electro-absorption modulator, wherein the one or more drivers are configured to drive the at least one graphene phase modulator with a combination of inverse low-pass filtered data from the data input and a bias voltage configured to bias the graphene phase modulator into a region where the low-pass filtered data changes the effective refractive index of the graphene phase modulator for the dimming while the graphene phase modulator remains transparent. WDM optical transmitter.

2. 2. The transmitter of claim 1, wherein an increase in the transmittance of the graphene electroabsorption modulator decreases the effective refractive index for the dimming, and vice versa, and wherein the first sign is positive and the second sign is negative, providing a positive chirp for the dimming to compensate for a negative chirp due to transmission of the dimming through the optical fiber.

3. 10. The transmitter of claim 1, comprising a graphene electro-absorption modulator (EAM) for each optical input that modulates light from the optical inputs, and a graphene phase modulator optically coupled in series with the graphene electro-absorption modulator for only selected ones of the optical inputs.

4. 4. The transmitter of claim 1, wherein the one or more drivers are configured to drive the graphene electroabsorption modulator for different wavelengths with different drive voltages and provide different positive pre-chirps to the dimming of different wavelengths to match dispersion variations in the optical fiber with wavelength.

5. 5. The transmitter of claim 1, comprising, for each of the optical inputs, an optical splitter that splits the optical input into two or more branches, a graphene electro-absorption modulator for each branch that modulates light in the branch, and an optical combiner that combines the dimmed light from the branches.

6. 6. The transmitter of claim 5, wherein the one or more drivers are configured to drive the graphene electro-absorption modulators in the branches with groups of two or more bits of data from the data input.

7. 7. The transmitter of claim 5, further comprising an optical polarization rotator that rotates the polarization of light in one or more of the branches so that the combined dimming light from the branches consists of light of two orthogonal polarizations.

8. 8. The transmitter of claim 7, wherein each of the optical inputs has a respective optical polarization rotator and optical coupler, the transmitter further comprising an optical wavelength multiplexer that multiplexes the optical outputs from the optical couplers.

9. 8. The transmitter of claim 7, wherein the optical inputs have a shared optical polarization rotator, the transmitter further comprising a first optical coupler combining a first branch of the branches from each optical input and a second optical coupler combining a second branch of the branches from each optical input, the first and second optical couplers comprising respective first and second optical wavelength multiplexers, the shared optical polarization rotator coupled to an output of the first optical wavelength multiplexer, and the transmitter further comprising a third optical coupler combining a polarization-rotated output of the first optical wavelength multiplexer with an output of the second optical wavelength multiplexer to provide a combined wavelength division multiplexed output.

10. 4. The transmitter of claim 3, wherein one or both of the graphene electroabsorption modulator and graphene phase modulator comprises one or more graphene layers integrated with a waveguide, a drive electrical connection and a counter electrode connection to one of the one or more graphene layers.

11. 4. The transmitter of claim 3, wherein the WDM optical signal is a CWDM8 or CWDM16 optical signal.

12. 12. The transmitter of claim 1, wherein the optical fiber comprises a photonic crystal optical fiber.

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