Linearized modulator

Abstract

An electro-optical modulator. The electro-optical modulator includes an input waveguide configured to direct light into a modulation region of the electro-optical modulator, a plurality of sub-modulators within the modulation region, each sub-modulator having a transfer function between an applied voltage and an optical phase shift, and an output waveguide configured to direct light out of the modulation region. The combination of the transfer functions of each sub-modulator is such that the overall transfer function between the applied voltage and the optical phase shift of the modulation region is substantially linear over a range of operating voltages.

Description

Technical Field

[0001] The present invention relates to an electro-optic modulator, and in particular to an electro-optic modulator that has been linearized with respect to an operating voltage range. Background Art

[0002] Various types of electro-optic modulators have an inherently nonlinear transfer function between the applied operating voltage and the resulting optical phase shift. This nonlinearity is a relatively severe limitation on systems using such modulators.

[0003] For example, in analog applications such as fiber-optic radio, nonlinearities limit the dynamic range and maximum power of RF signals. This can result in poor link gain and a high signal-to-noise ratio. In LIDAR applications, nonlinear phase modulators require more bits in the digital-to-analog converter to compensate, increasing system size and power. In general communications applications, because pulse amplitude modulation 4 (PAM-4) modulators are nonlinear, they require nonlinear electrical drivers to compensate, which increases driver complexity and power consumption.

[0004] Figure 1 and Figure 2 The transfer functions of the PN (depletion) modulator and the MOSCAP modulator are shown respectively. The graph is the refractive index Δn as a function of the operating voltage V eff The transfer function of a PN modulator follows a square root characteristic, where the slope decreases with the operating voltage. In contrast, the transfer function of a MOSCAP modulator has a "knee point" characteristic, where the slope decreases at the "turn-on" voltage, V th Sudden changes. th The slope of the gradient after the modulator capacitance C ox Directly proportional. Summary of the Invention

[0005] At a general level, embodiments of the present invention are directed to linearized modulators formed by cascading multiple sub-modulators having different transfer functions.

[0006] Thus, in a first aspect, embodiments of the present invention are directed to an electro-optical modulator comprising:

[0007] an input waveguide configured to guide light into a modulation region of the electro-optic modulator;

[0008] a plurality of sub-modulators within the modulation region, each sub-modulator having a transfer function between an applied voltage and an optical phase shift; and

[0009] an output waveguide configured to guide light out of the modulation region;

[0010] The combination of the transfer functions of each modulator is such that the overall transfer function between the applied voltage and the optical phase shift of the modulating region is substantially linear over an operating voltage range.

[0011] Thus, such electro-optic modulators are linearized over the operating voltage range, thereby overcoming the drawbacks discussed previously. In some embodiments, the transfer function of each submodulator is controlled by adjusting the geometry or doping level of the corresponding junction (PN or MOSCAP). In some embodiments, the total phase shift of the modulator is the sum of the phase shifts of each submodulator (i.e., the superposition of the independent transfer functions) multiplied by the optical length of each submodulator. The optical length can refer to the path length of light through the submodulator.

[0012] The electro-optical modulator may have any one of the following optional features or any combination of the following optional features to the extent they are compatible.

[0013] At least two sub-modulators of the plurality of sub-modulators may have different transfer functions from each other.In some embodiments, each sub-modulator may have a different transfer function from the other sub-modulators.

[0014] The submodulators can be carrier depletion-based modulators. For example, each submodulator can include a PIN or PN junction connected to a pair of electrodes. The transfer function of each submodulator can be determined by varying one or more of the following: optical length; depletion width; and doping level.

[0015] The submodulators can be modulators based on metal oxide semiconductor capacitors (MOSCAPs). For example, each submodulator can include an oxide barrier between two doped regions so that the resulting structure can function as a capacitor. The transfer function of each submodulator can be determined by varying one or more of the following: optical length; oxide thickness; and doping level.

[0016] The submodulators can share common electrodes. That is, each submodulator can be connected to the same electrode pair as the other submodulators. The submodulators can be driven by a common driver. That is, each submodulator can experience the same operating voltage as the other submodulators during use.

[0017] A modulation region may be disposed within a first arm of a Mach-Zehnder modulator (MZM). A second modulation region may be disposed within a second arm of the MZM and may include a second plurality of sub-modulators, each of which has a transfer function between an applied voltage and an optical phase shift that differs from other sub-modulators in the second plurality of sub-modulators. The sub-modulators of the second modulation region may be substantially identical to those of the first modulation region. That is, each sub-modulator of the first modulation region may have a corresponding sub-modulator in the second modulation region. The second modulation region may be driven by a different driver than the first modulation region. An input waveguide may be coupled to a splitter that divides input light between each arm of the MZM. The input light may be split 50:50 between each arm of the MZM. An output waveguide may be coupled to a combiner that combines light received from each of the MZMs.

[0018] The modulation region may be within a ring modulator. The ring modulator may be a micro ring modulator (MRM). The input waveguide and the output waveguide may be regions of a bus waveguide, and the bus waveguide may include a coupling region coupled to the ring modulator.

[0019] The sub-modulators may be based on silicon. Alternatively, the sub-modulators may be based on III-V semiconductors.

[0020] The electro-optic modulator may further include a plurality of modulation regions, each modulation region including a corresponding plurality of sub-modulators, the combination of the transfer functions of each corresponding plurality of sub-modulators being such that the total transfer function of each modulation region between an applied voltage and an optical phase shift of the corresponding modulation region is substantially linear within an operating voltage range. The sub-modulators of each modulation region may be driven by a common driver. The modulation regions may share a common ground electrode. The plurality of modulation regions may be located within one arm of the Mach-Zehnder modulator. The electro-optic modulator may further include a second plurality of modulation regions in a second arm of the Mach-Zehnder modulator, each modulation region in the second arm including a corresponding plurality of sub-modulators, the combination of the transfer functions of each corresponding plurality of sub-modulators being such that the total transfer function of each modulation region in the second arm between an applied voltage and an optical phase shift of the corresponding modulation region is substantially linear within an operating voltage range.

[0021] The electro-optic modulator may be a silicon-based modulator.The electro-optic modulator may be a III-V semiconductor-based modulator.

[0022] In a second aspect, an embodiment of the present invention provides a method for modulating an optical signal using the electro-optical modulator of the first aspect, the method comprising:

[0023] providing an input signal to the input waveguide;

[0024] modulating the input signal using the modulation region to form a modulated output signal; and

[0025] The modulated output signal is output from the output waveguide. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

[0027] Figure 1 shows the transfer function of the PN modulator;

[0028] Figure 2 shows the transfer function of the MOSCAP modulator;

[0029] Figure 3 shows a Mach-Zehnder depletion modulator according to one embodiment of the present invention;

[0030] Figure 4 A microring depletion modulator according to one embodiment of the present invention is shown;

[0031] Figure 5 shows the total transfer function of an electro-optical modulator according to one embodiment of the present invention, which is formed by the sum of the transfer functions of a plurality of sub-modulators;

[0032] Figure 6 shows a Mach-Zehnder MOSCAP modulator according to one embodiment of the present invention;

[0033] Figure 7 A micro-ring MOSCAP modulator according to one embodiment of the present invention is shown; and

[0034] Figure 8 A multi-modulation region Mach-Zehnder depletion modulator according to one embodiment of the present invention is shown. DETAILED DESCRIPTION

[0035] Various aspects and embodiments of the present invention will now be discussed with reference to the accompanying drawings. Other aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

[0036] Figure 3A Mach-Zehnder depletion modulator 300 is shown in accordance with one embodiment of the present invention. Light in the form of an optical signal to be modulated enters the modulator via an input waveguide 301 and reaches a splitter 302, which then divides the received signal between a first arm 303 and a second arm 304 of the Mach-Zehnder depletion modulator. In some embodiments, the signal is divided equally between each arm of the Mach-Zehnder.

[0037] Taking the first arm 303 as an example, it is formed by a waveguide extending parallel to the second arm toward the output waveguide 312 of the modulator. Set along the first arm and along the waveguide is a modulation region, which includes three sub-modulators: 305a, 305b and 305c. Each of the sub-modulators is formed by a first doped region 306a / 306b / 306c and a second doped region 307a / 307b / 307c. The first doped region is doped with a first dopant and the second doped region is doped with a second dopant. For example, the first doped region can be n-doped and the second doped region can be p-doped (or vice versa). In some embodiments, the intrinsic region is retained between the doped regions to provide a PIN junction. In other embodiments, the intrinsic region is not retained to provide a PN junction. The waveguide in each arm can be a ridge waveguide or a rib waveguide, and the doped region can extend up to the opposite sidewalls of the waveguide. Alternatively, the waveguide may be a ridge waveguide or a rib waveguide, and the doped sidewalls may extend across both the upper and lower surfaces of the waveguide.

[0038] The first doped regions 306a-306c are all connected to a common electrode 308 extending along the first arm 303 of the modulator. The second doped regions 307a-307c are also connected to a common electrode 309. In this example, the common electrode 308 is a drive electrode or signal electrode and provides an operating voltage V, while the common electrode 309 is a ground electrode or body ground. Each of the sub-modulators 305a, 305b, and 305c has a different optical length: L1, L2, and L3. The first doped region 306a, 306b, and 306c of each sub-modulator has a different doping concentration than the other sub-modulators, as indicated by the degree of shading. In this example, the first sub-modulator 305a has the shortest optical length L1, but the highest dopant concentration. The third sub-modulator 305c has the longest optical length L3, but the lowest dopant concentration. The second sub-modulator 305b is located between the first and second sub-modulators in terms of both optical length and dopant concentration. Each sub-modulator also has a depletion region whose width can be adjusted to tune the modulation profile.

[0039] As a result of having the modulation region thus configured, each sub-modulator is sensitive to the applied voltage V and the phase shift φ imparted to the light passing through it. n There are different transfer functions between them. Figure 5 As discussed in detail, this can result in an overall transfer function in the modulation region that is substantially linear over the operating voltage range.

[0040] The structure in the first arm 303 is essentially mirrored in the second arm 304. The second arm has another modulation region that also includes three sub-modulators: 310a, 310b, and 310c. The first sub-modulator 310a of the second arm 304 is essentially identical to the first sub-modulator 305a of the first arm 303, as are the second and third modulators of each arm. The ground electrode 309 in the first arm can be shared with the second arm.

[0041] At the ends of the two arms 303 and 304, remote from the splitter 302, the two arms converge in an optical combiner 311. Here, any phase shift imparted to the optical signal within any given arm is used to modulate the amplitude of the output signal in a manner known per se for Mach-Zehnder modulators. The output signal is then sent through an output waveguide 312 for further processing, transmission, or detection.

[0042] Figure 4 A microring depletion modulator 400 is shown in accordance with one embodiment of the present invention. The modulator comprises a bus waveguide 401 and a coupling region 402 of the bus waveguide, which couples light transmitted along the bus waveguide into a microring resonator waveguide 403. In this embodiment, the left-hand side of the bus waveguide 401 is considered the input waveguide, and the right-hand side of the bus waveguide is considered the output waveguide. Disposed along the microring resonator is a modulation region comprising three sub-modulators: 404a, 404b, and 404c, as indicated by the dashed regions. As with the Mach-Zehnder modulation discussed previously, each sub-modulator comprises a first doped region 405a / 405b / 405c and a second doped region 406a / 406b / 406c. The first doped region is connected to a common electrode 407 (in this example, a driver electrode), which extends around the outer circumference of the microring resonator. Similarly, the second doped region is connected to a common electrode 408 (in this example, a ground electrode), which extends around the inner circumference of the microring resonator. and Figure 3 Compared to the modulators shown, the first sub-modulator 404a in this example has the longest optical length and the lightest doping concentration, and the third sub-modulator 404c has the shortest optical length and the heaviest doping concentration.

[0043] In use, unmodulated light enters the modulator from the left-hand side of the bus waveguide 401, as indicated by the dashed arrows. The light is then coupled into the ring resonator at the coupling region 402 and passes through each of the sub-modulators, where it is modulated to produce a modulated output (indicated by the dashed arrows). The modulated output is then coupled back into the bus waveguide at the coupling region 402 and exits the modulator at the right-hand side of the bus waveguide.

[0044] and Figure 3 Like the modulators in

[15] , the transfer function of the modulation region as a whole can be linearized over the operating voltage range by adjusting the optical length, doping concentration, or depletion width of each sub-modulator relative to the other.

[0045] This principle is Figure 5 , which is a graph of the phase change V as a function of the applied voltage φ. It can be seen that each of the sub-modulators has a different transfer function n1(V)L1, n2(V)L2 and n3(V)L3. The superposition of these transfer functions results in a total transfer function φ tot , the total transfer function is essentially linear over a wider voltage range than any transfer function of the individual submodulators. The total transfer function can take the following form:

[0046]

[0047] where φ n is the phase change of the nth sub-modulator, λ is the operating wavelength, n n (V) is the refractive index of the nth sub-modulator when voltage V is applied, and L n is the optical length of the nth submodulator. Generally, such a total transfer function can be derived by simulating a range of doping levels or intrinsic region widths (or both) and extracting the nonlinear transfer function for each. An optimization function (such as least squares) can be used for the combination of N transfer functions to achieve substantial linearity (i.e., the most linear response). This operation can then be repeated for different numbers of regions N until the desired level of linearity is achieved.

[0048] Figure 6 A Mach-Zehnder MOSCAP modulator 600 is shown in accordance with one embodiment of the present invention. Figure 6 The modulator 600 is Figure 3 The modulator 300 of FIG. 3 shares roughly the same structure in that it has two arms, each with a modulation region. Each modulation region includes three sub-modulators: 601a / 602a, 601b / 602b, and 601c / 602c. However, unlike Figure 3In contrast to the modulators in the

[0045] , oxide regions 604 / 605 are present in and along each arm of the Mach-Zehnder circuit. The oxide regions allow each modulator to function as a capacitor, thus eliminating the need for a constant voltage to be applied to the electrodes to modulate the signal.

[0049] In addition, with Figure 3 In contrast to modulator 300 , the transfer functions of the sub-modulators of modulator 600 are adjusted by varying any one or more of: optical length; oxide region thickness; and doping concentration.

[0050] Figure 7 A micro-ring MOSCAP modulator 700 is shown according to one embodiment of the present invention. As before, the modulator 700 is coupled to Figure 4 The modulator 400 in FIG. 4 shares roughly the same structure in that it includes a bus waveguide, a ring resonator / waveguide with a modulation region, and three sub-modulators within the modulation region: 701a, 701b, and 701c. However, unlike Figure 4 In contrast to the modulator in , the oxide region 704 exists within the ring resonator so that the sub-modulator can operate as a capacitor and thus does not apply a constant voltage to the electrodes to modulate the signal.

[0051] Figure 8 FIG. 8 shows a multi-modulation region Mach-Zehnder depletion modulator 800 according to an embodiment of the present invention. The wide-type structure of the modulator 800 is similar to Figure 3 The broad structure of the modulator 300 shown matches that of the input waveguide, the splitter, the two arms, the combiner, and the output waveguide. Figure 8 In each arm of the Mach-Zehnder modulator, there are multiple modulation regions 801a / 802a, 801b / 802b and 801c / 802c.

[0052] As before, each modulation region includes multiple sub-modulators. In this example, the first modulation region 801a in the first arm includes three sub-modulators: 803a(i), 803a(ii), and 803a(iii). Each modulation region has a voltage V n The modulation region of the first arm shares a common ground electrode. This common ground electrode can also be shared with the modulation region of the second arm.

[0053] Although Figure 8 The modulator 800 shown uses a depletion junction structure, but it can certainly use a modulator with respect to Figure 6 and Figure 7 The MOSCAP structure discussed.

[0054] Figure 8The structure shown allows for finer control over the linearization of the transfer function.

[0055] While the modulators discussed above utilize the Kerr effect, the present invention is equally applicable to other modulators, such as those utilizing the Franz-Keldysh effect or the quantum confined Stark effect (so-called QCSE modulators). The present invention is applicable to any modulator having parameters that can be adjusted to tune the transfer function.

[0056] Although the present invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when this disclosure is given. Therefore, the exemplary embodiments of the present invention set forth above are considered to be illustrative rather than restrictive. Various changes may be made to the embodiments without departing from the spirit and scope of the present invention.

Claims

1. An electro-optic modulator, comprising: an input waveguide configured to guide light into a modulation region of the electro-optic modulator; a plurality of sub-modulators within the modulation region, each sub-modulator having a transfer function between an applied voltage and an optical phase shift; as well as an output waveguide configured to guide light out of the modulation region; wherein the combination of the transfer functions of each sub-modulator is such that the overall transfer function between the applied voltage and the optical phase shift of the modulating region is substantially linear over an operating voltage range, and Wherein a first transfer function of a first sub-modulator of the plurality of sub-modulators is different from a second transfer function of a second sub-modulator of the plurality of sub-modulators.

2. The electro-optic modulator of claim 1, wherein the sub-modulator is a carrier depletion based modulator.

3. The electro-optic modulator of claim 2, wherein the transfer function of each sub-modulator is determined based on one or more of: optical length; depletion width; or doping level.

4. The electro-optic modulator of claim 1, wherein the sub-modulator is a metal oxide semiconductor capacitor based modulator.

5. The electro-optic modulator of claim 4, wherein the transfer function of each sub-modulator is determined based on one or more of: optical length; oxide thickness; or doping level.

6. The electro-optic modulator of claim 1, wherein the sub-modulators share a common electrode.

7. The electro-optic modulator of claim 1, wherein the sub-modulators are driven by a common driver.

8. The electro-optic modulator of claim 1, wherein the modulation region is disposed within a first arm of a Mach-Zehnder modulator.

9. An electro-optical modulator as described in claim 8, wherein the modulation region is a first modulation region and a second modulation region is arranged in a second arm of the Mach-Zehnder modulator, the second modulation region includes a second plurality of sub-modulators, each sub-modulator in the second plurality of sub-modulators has a transfer function between an applied voltage and an optical phase shift, and the transfer function is different from other sub-modulators in the second plurality of sub-modulators.

10. The electro-optic modulator of claim 9, wherein the sub-modulators of the second modulation region are substantially identical to the sub-modulators of the first modulation region.

11. The electro-optic modulator of claim 8, wherein the input waveguide is coupled to a splitter that divides the input light between each arm of the Mach-Zehnder modulator.

12. The electro-optic modulator of claim 8, wherein the output waveguide is coupled to a combiner that combines light received from each arm of the Mach-Zehnder modulator.

13. The electro-optic modulator of claim 1, wherein the modulation region is located within a ring modulator.

14. The electro-optic modulator of claim 13, wherein the input waveguide and the output waveguide are regions of a bus waveguide, and the bus waveguide includes a region coupled to the ring modulator.

15. The electro-optical modulator of claim 1 , further comprising a plurality of modulation regions, each modulation region comprising a corresponding plurality of sub-modulators, the combination of the transfer functions of each corresponding plurality of sub-modulators making the total transfer function of each modulation region between the applied voltage and the optical phase shift of the corresponding modulation region substantially linear within the operating voltage range.

16. The electro-optic modulator of claim 15, wherein the sub-modulators of each modulation region are driven by a common driver.

17. The electro-optic modulator of claim 15, wherein the modulating regions share a common ground electrode.

18. The electro-optic modulator of claim 15, wherein the plurality of modulation regions are located within a first arm of a Mach-Zehnder modulator.

19. The electro-optic modulator of claim 18 , further comprising a second plurality of modulation regions in a second arm of the Mach-Zehnder modulator, each modulation region in the second arm comprising a corresponding plurality of sub-modulators, the combination of transfer functions of each corresponding plurality of sub-modulators causing the total transfer function of each modulation region of the second arm between an applied voltage and an optical phase shift of the corresponding modulation region to be substantially linear within the operating voltage range.

20. A method for modulating an optical signal using the electro-optical modulator of claim 1, the method comprising: providing an input signal to the input waveguide; modulating an input signal using a modulation region to form a modulated output signal; as well as The modulated output signal is output from an output waveguide.

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