Biasing method for InP Mach-Zehnder modulators directly coupled to RF driver circuits

Abstract

An optical transmitter comprises a directly coupled MZ interferometer and driver circuit. The MZ interferometer comprises a pair of differentially driven MZ electrodes configured to impart RF signals to light travelling through respective arms of the interferometer, and to receive DC bias as a positive voltage via lower n-type cladding of the MZ interferometer. The lower n-type cladding is at a different positive DC potential to an upper plane RF ground of the MZ interferometer, but the lower n-type cladding and the upper plane RF ground have similar AC potential. The MZ interferometer also comprises a pair of resistors in series configured to provide differential RF termination of the MZ electrodes; and a capacitive coupling between a virtual ground formed at a centre point between the pair of resistors and an RF ground configured to provide common-mode RF termination. The DC supply for the driver circuit is applied to the centre point of the RF termination.

Claims

1. A Mach-Zehnder (MZ) interferometer, comprising: a first coupling configured to receive a direct current (DC) supply; a pair of differentially driven MZ electrodes configured to: impart radio frequency (RF) signals to light travelling through respective arms of the MZ interferometer, and receive a DC bias as a positive voltage via lower n-type cladding of the MZ interferometer, the lower n-type cladding being at a different positive DC potential than an upper plane RF ground of the MZ interferometer and having a similar alternating current (AC) potential to the upper plane RF ground; a second coupling configured to transmit the DC supply received from the first coupling through the pair of differentially driven MZ electrodes; a pair of resistors to provide differential RF termination of the pair of differentially driven MZ electrodes; and a capacitive coupling located between a virtual ground and an RF ground, the virtual ground being formed at a center point between the pair of resistors.

2. The MZ interferometer of claim 1, wherein the capacitive coupling is configured to: provide common-mode RF termination.

3. The MZ interferometer of claim 2, wherein the capacitive coupling is provided by a 10 picofarad (pF) capacitor.

4. The MZ interferometer of claim 1, wherein the DC supply is applied to the center point via an inductive wirebond connection.

5. The MZ interferometer of claim 1, wherein the capacitive coupling is provided by one or more on-chip capacitors.

6. The MZ interferometer of claim 5, wherein the capacitive coupling is further provided by an off-chip capacitor.

7. The MZ interferometer of claim 1, wherein the pair of resistors are configured in series.

8. The MZ interferometer of claim 1, wherein the pair of resistors provide a total of 50 to 100 ohms of differential termination impedance.

9. The MZ interferometer of claim 1, wherein the second coupling is further configured to directly couple the MZ interferometer and a driver circuit.

10. A method, comprising: applying a direct current (DC) supply to a first coupling of a Mach-Zehnder (MZ) interferometer; imparting, via a pair of differentially driven MZ electrodes of the MZ interferometer, radio frequency (RF) signals to light travelling through respective arms of the MZ interferometer; applying, via lower n-type cladding of the MZ interferometer, a DC bias as a positive voltage to the pair of differentially driven MZ electrodes, the lower n-type cladding being at a different positive DC potential than an upper plane RF ground of the MZ interferometer and having a similar alternating current (AC) potential to the upper plane RF ground; transmitting, via a second coupling of the MZ interferometer, the DC supply received from the first coupling through the pair of differentially driven MZ electrodes; and providing, via a capacitive coupling, common-mode RF termination, the capacitive coupling being located between a virtual ground and an RF ground, and the virtual ground being formed at a center point between a pair of resistors of the MZ interferometer.

11. The method of claim 10, further comprising: providing, via the pair of resistors of the MZ interferometer, differential RF termination of the pair of differentially driven MZ electrodes.

12. The method of claim 10, wherein applying the DC supply comprises: applying the DC supply to the center point via an inductive wirebond connection.

13. The method of claim 10, wherein the capacitive coupling is provided via one or more on-chip capacitors.

14. The method of claim 13, wherein the capacitive coupling is further provided via an off-chip capacitor.

15. The method of claim 10, wherein the pair of resistors are configured in series.

16. The method of claim 10, further comprising: directly coupling, via the second coupling, the MZ interferometer and a driver circuit.

17. The MZ interferometer of claim 1, wherein the differential RF termination is provided on-chip.

18. The MZ interferometer of claim 1, wherein the pair of differentially driven MZ electrodes are microstrip-type MZ electrodes.

19. The method of claim 11, wherein the differential RF termination is provided on-chip.

20. The method of claim 10, wherein the pair of differentially driven MZ electrodes are microstrip-type MZ electrodes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram illustrating traditional biasing in an optical transmitter comprising an MZ interferometer and an AC coupled driver circuit;

(2) FIG. 2 is a schematic diagram illustrating biasing in an optical transmitter comprising directly coupled MZ interferometer and driver circuit;

(3) FIG. 3 illustrates a biasing and capacitive coupling scheme for use in the transmitter of FIG. 2, wherein the electrodes are of microstrip type;

(4) FIG. 4A is a cross sectional view through line AA′ of FIG. 3;

(5) FIG. 4B is an alternative cross sectional view through line AA′ of FIG. 3;

(6) FIG. 4C is another alternative cross sectional view through line AA′ of FIG. 3;

(7) FIG. 5 illustrates a an alternative biasing and capacitive coupling scheme for use in the transmitter of FIG. 2, wherein the electrodes are of segmented type;

(8) FIG. 6 illustrates an alternative biasing and capacitive coupling scheme for use in the transmitter of FIG. 2, wherein the electrodes are of segmented type;

(9) FIG. 7A is a cross sectional view through line BB′ of FIG. 6;

(10) FIG. 7B is a cross sectional view through line CC′ of FIG. 6; and

(11) FIG. 8 illustrates in-plane (a, b, c) and vertical (d) on-chip resistor geometry.

DETAILED DESCRIPTION

(12) An optical transmitter comprising an MZ interferometer and a driver circuit, where the MZ interferometer and driver circuit are directly coupled, is described herein with reference to FIGS. 2 to 8. A method of biasing an optical transmitter comprising a directly-coupled MZ interferometer and driver circuit is also described. For convenience, the following discussion refers to a MZ interferometer and driver circuit (driver chip), but it will be appreciated that these need not be separate entities and may be co-located.

(13) The inventors have realised that the concatenated bandwidth performance of an MZ modulator can be significantly improved by co-locating the RF driver circuit with the MZ interferometer(s), within the optical sub-assembly. In this arrangement, the driver circuit and MZ interferometer are directly coupled, rather than being AC coupled. Co-location shortens the length of the RF component train and reduces RF losses and distortion. The absence of a bias tee arrangement also reduces the package size.

(14) However, this co-located arrangement requires significant modifications to the biasing arrangement of the MZ interferometer.

(15) For example, the driver circuit DC supply V.sub.DD can no longer be applied at the driver output owing to the absence of a bias tee arrangement. Instead, the driver device (chip) DC supply V.sub.DD must be applied to the termination end of the MZ modulation arm. As a result, the negative MZ DC bias V.sub.cm cannot be applied at the termination end of the MZ modulation arm and must instead be provided as an opposite positive voltage +V.sub.cm to the lower n-type cladding side of the MZ interferometer device.

(16) The RF feed lines or waveguides that connect the RF signal input pads to the modulation electrodes are of a coplanar waveguide (CPW) type and/or a coplanar stripline (CPS) type. The RF signal electric field exists in a horizontal plane between a central signal track and peripheral ground tracks, or balanced differential signal tracks, respectively. However, in order to modulate the optical waveguide phase, the RF signal electric field must exist in a vertical plane between the modulation electrodes and the underlying n-type lower cladding, with the electric field passing through the optical core. The RF signal electric field must therefore transition smoothly from the horizontal plane to the vertical plane. There must be sufficient high speed coupling between RF ground on the upper plane and the RF ground in the n-type lower cladding to enable this.

(17) In the known arrangement illustrated by FIG. 1, with an AC coupled driver circuit in an off-chip location, the upper plane RF ground and n-type lower cladding RF ground are directly coupled, i.e. they are both at the same DC and AC potential.

(18) However, as discussed above, in the arrangement wherein the driver circuit and MZ interferometer(s) are co-located and directly coupled, the lower n-type cladding must be DC biased using a positive voltage +V.sub.cm relative to the RF ground on the upper plane. This arrangement requires capacitive coupling between the lower n-type cladding and the RF ground on the upper plane, such that they are at similar AC, but different DC, potentials. In other words, the lower n-type cladding must be at a different positive DC potential to the upper plane RF ground, but the n-type cladding and the upper plane RF ground must be AC coupled, or have similar AC potential to ensure successful delivery of the RF signals onto the MZ electrodes. Coupling between the upper plane RF ground, V.sub.cm and driver circuit DC supply V.sub.DD must be at the same RF potential.

(19) As will be described below, this capacitive coupling is provided both on and off-chip at low and high capacitance respectively, therefore enabling AC coupling over a wide frequency range.

(20) FIG. 2 illustrates an optical transmitter 100 in which an MZ interferometer 120 and driver circuit 130 (driver chip) are co-located and directly coupled. As previously discussed, a configuration of interferometers comprises a modulator chip, and as described herein, the driver circuit and modulator chip will be directly coupled. The driver circuit 130 is close-coupled (i.e. directly coupled) with no bias tee arrangement present. Additionally, there is no RF component train between the driver circuit 130 and MZ 120 interferometer apart from short wirebonds with a low inductance.

(21) In contrast to the traditional biasing arrangement illustrated in FIG. 1, in the exemplary transmitter 100 of FIG. 2 the RF termination 160 between the differentially driven MZ electrodes is provided on-chip. Differential termination is provided as two resistors 161, 162 in series. In this example each resistor is approximately 50 Ohms, providing a total of 100 Ohm differential termination impedance. Alternatively, each resistor may provide between approximately 25 to 50 Ohms, in combination providing a total of approximately 50 to 100 Ohms. The virtual ground (VG) formed at the centre point of the two resistors 161, 162 must be capacitively coupled to RF ground 170 to provide common-mode termination, ensuring that any common-mode excitation of the MZ electrodes 121, 122 is also terminated. Therefore, in this example both differential and common-mode RF termination is provided on—rather than off—chip. Alternatively or additionally, the resistors may be provided off-chip, although on-chip resistors provide a significant improvement in simplicity in packaging in relationship to a directly coupled driver circuit.

(22) Optimal termination geometry, which aims to provide purely resistive impedance, tends to minimize parasitic capacitance and inductance of the resistors 161, 162. Therefore, the resistors 161, 162 are kept short by use of highly resistive tracks and by being laid over an isolating stack of implanted p-cladding plus intrinsic InP above the RF grounded n-cladding. Optimal resistor geometries will be further discussed below, with reference to FIG. 8.

(23) In this example, the driver circuit DC supply V.sub.DD is applied to the centre point of the on-chip RF termination 160 via an inductive wirebond connection L.sub.WB. Typically, a 25 μm diameter gold wirebond will have an inductance of approximately 0.1 nH/mm. As previously discussed, the MZ negative DC bias V.sub.cm cannot therefore be applied at this location and is instead applied as an opposite, positive voltage +V.sub.cm to the lower n-type cladding, which must be capacitively coupled only to the upper plane RF ground to ensure delivery of signals from the RF feed lines to the MZ electrodes 121, 122. This capacitive coupling is provided by capacitors 125, 126 on- and off-chip (in this example of approximately 50 pF and 100 nF respectively), enabling AC coupling over a wide frequency range. Common-mode termination is provided by a 10 pF capacitor 127 to V.sub.cm, which in turn couples to RF ground 170.

(24) FIG. 3 illustrates a biasing and capacitive coupling scheme for use in the optical transmitter of FIG. 2 in which an optical waveguide 150 splits into two branches 151, 152. In this illustrated example, the MZ electrodes 180, 181 of the optical transmitter are of microstrip type. Differential signals S+ and S− are delivered to the MZ electrodes through GSG CPW RF waveguides W.sub.1, W.sub.2. The RF signal electric field transitions from the horizontal to the vertical plane at the locations where the CPW RF waveguides meet the MZ electrodes.

(25) In this example, the differential signals S+, S− are terminated differentially though two approximately 50 Ohm resistors 161, 162, and common-mode termination is provided by the VG capacitor coupled to the MZ DC bias V.sub.cm. The MZ DC bias V.sub.cm is in turn capacitively coupled to RF ground which additionally ensures the smooth delivery of the differential signals S+, S− onto the microstrip MZ electrodes.

(26) In the scheme illustrated in FIG. 3, positive MZ DC bias V.sub.cm is provided to both the peripheral p-type cladding and the lower n-type cladding.

(27) FIGS. 4A, 4B and 4C are alternative cross sectional views through line AA′ in the scheme shown in FIG. 3. In all three views the MZ DC bias V.sub.cm contacts the peripheral p-type cladding, and the capacitor to RF ground, in the same way. The capacitor to RF ground is formed from a layer of metal and the p-type cladding, with an insulating dielectric layer in between. However, FIGS. 4A, 4B and 4C illustrate alternative ways in which the MZ DC bias V.sub.cm may contact the lower n-type cladding.

(28) FIG. 4A illustrates a semi-insulating process, in which the n-type cladding contact for positive DC bias connection must be provided on a top (i.e. upper, as shown in FIG. 4A) face of the device.

(29) FIGS. 4B and 4C illustrate alternatives for an n+ process, in which the contact to the lower n-type cladding (and the substrate) is provided through a back face (i.e. lower, as shown in the Figures) contact (FIG. 4C), or both a top face and a back face contact (FIG. 4B).

(30) FIG. 5 illustrates an alternative biasing and capacitive coupling scheme for use in the optical transmitter of FIG. 2. In this illustrated example, the MZ electrodes 182,183 of the optical transmitter are of segmented, travelling wave type. Differential signals S+ and S− are delivered to the segmented MZ electrodes through GSG CPW and CPS RF waveguides W.sub.3, W.sub.4. The RF signal electric field transitions from the horizontal to the vertical plane at the locations where the CPS RF waveguides meet the segmented MZ electrodes.

(31) The differential signals S+, S− are terminated differentially though two approximately 50 Ohm resistors 161, 162, and common-mode termination is provided by the VG capacitor, coupled to the MZ DC bias V.sub.cm. The MZ DC bias V.sub.cm is in turn capacitively coupled to RF ground which additionally ensures the smooth delivery of the differential signals onto the segmented MZ electrodes.

(32) In common with the scheme illustrated in FIG. 3, the scheme illustrated in FIG. 5 provides positive MZ DC bias V.sub.cm to both the peripheral p-type cladding and the n-type cladding.

(33) FIG. 6 illustrates a further alternative biasing and capacitive coupling scheme for use in the optical transmitter of FIG. 2. In this further illustrated example, the MZ electrodes 182, 183 are of segmented, travelling wave type. Differential signals S+ and S− are delivered to the segmented MZ electrodes through GSG CPW and CPS RF waveguides W.sub.5, W.sub.6. The RF signal electric field transitions from the horizontal to the vertical plane at the locations where the CPS RF waveguides meet the segmented MZ electrodes.

(34) The differential signals S+, S− are terminated differentially though two approximately 50 Ohm resistors 161, 162, and common-mode termination is provided by the VG capacitor, coupled to the MZ DC bias V.sub.cm. The MZ DC bias V.sub.cm is in turn capacitively coupled to RF ground which additionally ensures the smooth delivery of the differential signals onto the segmented MZ electrodes.

(35) In contrast to the first and second biasing and capacitive coupling schemes illustrated in FIGS. 3 and 5 respectively, in the alternative scheme of FIG. 6 all capacitors are provided on the lower n-type cladding layer. This removes the requirement to provide MZ DC bias V.sub.cm to the peripheral p-type cladding, which may be more space efficient with consideration to the total chip area.

(36) FIGS. 7A and 7B are cross sectional views through lines BB″ and CC′ respectively in the scheme shown in FIG. 6. In contrast to the cross sectional views 4A, 4B and 4C described above, positive MZ DC bias V.sub.cm is not provided to the peripheral p-type cladding. Instead, in FIGS. 7A and 7B the required capacitive coupling is carried out entirely on a large area of exposed n-type cladding, located in a large n-well.

(37) As discussed above (with reference to FIGS. 3, 5 and 6), alternative biasing and capacitive coupling schemes may be used with the optical transmitter of FIG. 2. In these biasing and coupling schemes, positive MZ DC bias V.sub.cm is provided either to the peripheral p-type cladding and the n-type cladding (FIGS. 3, 4 and 5), or to the n-type cladding only (FIGS. 6 and 7).

(38) As discussed above with reference to FIGS. 2, 3, 5 and 6, differential termination of signals S+, S− is provided by two on-chip approximately 50 Ohm resistors in series. FIGS. 8A to 8D illustrate three alternative geometries for these on-chip 50 Ohm resistors 161, 162.

(39) As discussed above, these three geometries are all generally optimised by keeping the resistors 161, 162 narrow and short, by using highly resistive tracks and by laying the resistors 161, 162 over an isolating stack of implanted p-cladding plus intrinsic optical waveguide core above the RF grounded n-cladding.

(40) FIG. 8 illustrates various geometries for on-chip geometries suitable for the resistors 160,161. FIGS. 8a to 8c show in-plane descriptions of possible geometries, and FIG. 8d shows two possible vertical descriptions of the geometry of FIG. 8a.

(41) In the in-plane, on-chip resistor geometry shown in in FIG. 8a, RF differential lines are terminated using two approximately 50 Ohm metallic tracks connected to a single capacitor at virtual RF ground. In-plane geometrical choice aims to minimise the resistors' length, to reduce parasitic inductance and capacitance. Therefore, narrow and highly resistive metallic tracks are preferred. Alternative in-plane configurations are illustrated in FIGS. 8b and 8c.

(42) FIGS. 8a and 8b naturally provide an equipotential single resistors' point contact, whereas the geometry of FIG. 8c relies on good grounding homogeneity across the capacitor, and low capacitor resistance between resistors' contact points to achieve the same performance. As discussed above, V.sub.DD is applied at the VG point through an inductive wirebond connection. Since there is no RF ground connection within proximity, the VG is capacitively coupled to V.sub.cm, which in turn is capacitively coupled to RF ground, as illustrated in FIG. 2.

(43) FIG. 8d illustrates the vertical arrangement, in which parasitic capacitance is minimised by way of a thick, neutral implanted p-layer beneath the resistor metallic tracks. FIG. 8d illustrates both N-doped and Si substrates using the same implanted p-layer arrangement. In the case of a Si substrate, the back metal layer is optional.

(44) Each of the resistor geometries illustrated in FIG. 8 can be utilised with the transmitter of FIG. 2 and with the biasing and coupling schemes described with reference to FIGS. 3 to 7 above.

(45) It will be appreciated by the person skilled in the art that various modifications may be made to the above described embodiments, without departing from the scope of the present invention.

(46) The optical transmitters described above comprise one or more MZ interferometers, each MZ interferometer comprising a pair of differentially driven MZ electrodes, a pair of resistors in series configured to provide differential RF termination of the MZ electrodes; and a capacitive coupling between a virtual ground formed at a centre point between the pair of resistors, and an RF ground configured to provide common-mode RF termination.