阅读说明：本技术 Mz调制器中的rf啁啾降低 (RF chirp reduction in MZ modulators ) 是由 N·D·威特布莱德 S·琼斯 于 2018-09-24 设计创作，主要内容包括：一种用于平衡的推挽式马赫-曾德尔调变器的波导结构。该波导结构包括多个层。所述多个层依次包括：绝缘或半绝缘衬底；下包覆层；波导芯层；以及上包覆层。所述下包覆层、波导芯层和上包覆层被蚀刻以形成：信号波导和接地波导,他们通过所述下包覆层连接；以及信号线和接地线,每个都位于相应的波导附近,并且每个都经由在所述下包覆层的平面中连接的一个或多个相应的电阻结构而被连接到所述相应的波导。(A waveguide structure for a balanced push-pull Mach-Zehnder modulator. The waveguide structure includes a plurality of layers. The plurality of layers sequentially comprises: an insulating or semi-insulating substrate; a lower cladding layer; a waveguide core layer; and an upper cladding layer. The lower cladding layer, waveguide core layer, and upper cladding layer are etched to form: a signal waveguide and a ground waveguide connected by the lower cladding layer; and signal and ground lines, each located near a respective waveguide, and each connected to the respective waveguide via one or more respective resistive structures connected in the plane of the lower cladding layer.)

1. A waveguide structure for a balanced push-pull mach-zehnder modulator, the waveguide structure comprising:

a plurality of layers comprising, in order:

an insulating or semi-insulating substrate 501;

a lower cladding layer 502;

a waveguide core layer 503; and

an upper cladding layer 504; wherein the lower cladding layer, waveguide core layer, and upper cladding layer are etched to form:

a signal waveguide 512 and a ground waveguide 513 connected by the lower cladding layer; and

signal line 511 and ground line 514, each located near a respective waveguide, and each connected to the respective waveguide by one or more respective resistive structures 530 connected in the plane of the lower cladding layer.

2. The waveguide of claim 1, wherein each of the signal, ground, signal and ground waveguides includes a respective electrode 521, 522, 523, 524, wherein the electrodes of the signal and signal waveguides are electrically connected to each other and the electrodes of the ground and ground waveguides are electrically connected to each other.

3. The waveguide structure of claim 1 or 2, wherein the resistance of each of the resistive structures is between 10 ohms and 10 kiloohms.

4. The waveguide structure of any one of the preceding claims, wherein the resistive structure is part of the lower cladding layer.

5. The waveguide structure of claim 4, wherein the resistive structures each comprise an elongated portion of the lower cladding layer that is connected at one end to the corresponding waveguide and at another end to the corresponding line by respective connection portions of the lower cladding layer.

6. A waveguide structure according to any one of the preceding claims, wherein each line electrode is connected to the respective waveguide electrode via a plurality of conductive elements spanning the gap between the line and waveguide, and wherein the resistive structure is provided for each conductive element.

7. A balanced push-pull mach-zehnder interferometer comprising a waveguide structure according to any preceding claim.

8. A method of fabricating a waveguide structure for a mach-zehnder interferometer, the method comprising:

providing a layered structure comprising:

an insulating or semi-insulating substrate 501;

a lower cladding layer 502;

a waveguide core layer 503; and

an upper cladding layer 504; etching the lower cladding layer, waveguide core layer, and upper cladding layer to form:

a signal waveguide 512 and a ground waveguide 513 connected by the lower cladding layer; and

signal line 511 and ground line 514, each located near a respective waveguide, and each connected to the respective waveguide by one or more respective resistive structures 530 in the plane of the lower cladding layer.

9. The method of claim 9, and comprising depositing an electrode 521, 522, 523, 524 on each of the signal, ground, signal and ground waveguides, wherein the electrodes of the signal and ground waveguides are electrically connected to each other and the electrodes of the ground and ground waveguides are electrically connected to each other.

Technical Field

The present invention relates to a mach-zehnder interferometer. In particular, the present invention relates to a waveguide structure for a mach-zehnder interferometer having reduced chirp at low frequencies and a method of fabricating the same.

Background

A mach-zehnder modulator, as schematically shown in fig. 1, splits an input optical beam 102 along two optical paths 103, 104 by using a beam splitter 101, and then recombines the optical beams from each optical path at an optical combiner 105 to form an output optical beam 107 (and a complementary output 106). Each optical path 103, 104 comprises an electro-optic material such that by providing a voltage across the electro-optic material a phase change of the light beam travelling along the optical path may be induced.

In case the mach-zehnder modulator is used for intensity modulation, pure intensity modulation may be obtained by providing each of the optical paths 103, 104 with an equal and opposite phase shift (i.e. voltage). Any difference between the amplitude of the phase shifts on each optical path 103, 104 will result in an undesirable phase modulation of the output, referred to as "chirp". This mode of operation is referred to as "push-pull" operation.

The push-pull operation may be implemented in various ways. FIG. 2 shows an example of a so-called "series" or "single drive" push-pull modulator, which is a cross-sectional view of a Mach-Zehnder interferometer taken along line A-A in FIG. 1. The interferometer includes a substrate 201 made of a semiconductor such as InP or GaAs. The substrate is insulating or "semi-insulating," i.e., doped to provide a high resistivity, such that it effectively acts as an insulator (e.g., having a resistivity greater than 105 ohm-cm). On top of the substrate are several layers, including in order a lower cladding layer 202, a waveguide core layer 203, and an upper cladding layer 204. The upper and lower cladding layers are doped semiconductors, e.g., the upper cladding layer may be p-doped and the lower cladding layer may be n-doped, or vice versa. Each layer 202-204 may have a composite structure depending on the desired waveguide characteristics. These layers are etched to form four parallel structures (presented in order): a signal line 211, a signal waveguide 212, a ground waveguide 213, and a ground line 214. The gap between each line 211, 214 and the respective waveguide 212, 213 is etched down to the substrate 201. The gaps between the waveguides are etched down to the lower cladding layer 202, i.e. the waveguides 212, 213 are connected via the lower cladding layer 202.

Each waveguide 212, 213 includes a dielectric cladding layer 221 on either side of the waveguide to protect the sidewalls. The signal waveguide 212 includes a signal waveguide electrode 222, and the ground waveguide 213 includes a ground waveguide electrode 223. Each waveguide electrode is made of metal and is located on top of the upper cladding layer 204. The waveguide core layer 203 forms the core 224 of each waveguide 212, 213.

Each line 211, 214 includes a dielectric layer 231 on top of the upper cladding layer 204 (this is not strictly required, however, it is advantageous to reduce the capacitance of the line and eliminate parasitic circuit paths). The signal line 211 includes a signal line electrode 232, and the ground line 214 includes a ground line electrode 233. Each line electrode is made of metal and is located on top of the dielectric layer 231.

The signal line electrode 232 and the ground line electrode 233 together form a transmission line carrying an AC modulated signal. The ground line electrode 233 is connected to ground, and the signal line electrode 232 is connected to an AC voltage source 241. Each line electrode 232, 233 is connected to a respective waveguide electrode 222, 223 by conductive air bridges 242, 243 spaced along the length of the modulator. The central portion of the lower cladding layer 202 connecting the waveguides 212, 213 is connected to a DC voltage source 244 to provide a bias voltage.

When an AC signal voltage V is supplied to the signal line electrode 232 (and due to the air bridge 242 to the signal waveguide electrode 222), the voltage is split to provide a voltage V/2 across each waveguide 212, 213 in opposite directions. The signal line 212 and ground line 213 each act as a capacitor that forms a voltage divider when subjected to an AC load (with the midpoint at the center portion of the lower cladding layer 202 connecting the waveguides 212, 213).

However, it has been found that at low frequencies (below about 1GHz), the voltage division is not uniform and excessive "chirp" (i.e., undesirable phase modulation for the output) is experienced at the output of the modulator.

Disclosure of Invention

According to a first aspect of the present invention, there is provided a waveguide structure for use in a balanced push-pull mach-zehnder modulator. The waveguide structure includes a plurality of layers. The plurality of layers includes in order:

an insulating or semi-insulating substrate;

a lower cladding layer;

a waveguide core layer; and

and (6) coating the layer.

The lower cladding layer, waveguide core layer, and upper cladding layer are etched to form:

a signal waveguide and a ground waveguide connected by the lower cladding layer; and

a signal line and a ground line, each located near a respective waveguide, and each connected to the respective waveguide via one or more respective resistive structures connected in the plane of the lower cladding layer.

According to a second aspect, there is provided a balanced push-pull mach-zehnder interferometer comprising a waveguide structure according to the first aspect.

According to a third aspect, a method of manufacturing a waveguide structure for a mach-zehnder interferometer is provided. There is provided a layered structure comprising:

an insulating or semi-insulating substrate;

a lower cladding layer;

a waveguide core layer; and

and (6) coating the layer.

The lower cladding layer, waveguide core layer, and upper cladding layer are etched to form:

a signal waveguide and a ground waveguide connected by the lower cladding layer; and

signal and ground lines, each located near a respective waveguide, and each connected to the respective waveguide by one or more respective resistive structures in the plane of the lower cladding layer.

Other embodiments are defined in claim 2 and the like.

Drawings

FIG. 1 is a schematic diagram of a Mach-Zehnder interferometer;

FIG. 2 is a cross-section of the interferometer of FIG. 1;

FIG. 3 is a circuit diagram of an equivalent circuit of the interferometer of FIG. 1;

FIG. 4 is a circuit diagram of an equivalent circuit of an exemplary interferometer;

FIG. 5A is a plan view and FIG. 5B is a cross-section of an exemplary interferometer;

figure 6 is a graph of chirp parameter versus frequency for a prior art interferometer and an exemplary interferometer.

Detailed Description

At low frequencies, the series push-pull modulator will experience unacceptable levels of chirp due to voltage imbalance between the two waveguides. The reason for this can be seen by considering the full equivalent circuit of the modulator of fig. 1, as shown in fig. 3. Each of the waveguides 212, 213 has an associated capacitance C_wHaving a reactance of 1/(2 pi fC)_w) (where f is the frequency of the AC signal). There is also a capacitance C on each of the lines 211, 214_LAnd a capacitance C exists between each line and the corresponding waveguide_TBut due to C_TRatio C_WMuch smaller so that these capacitances have negligible effect on the circuit. There is a resistance R between the central portion of the lower cladding layer connecting waveguides 212, 213 and the DC voltage source 244_DCWhich provides a ground path. Other alternative ground paths having various different resistances may also be present within the circuit.

At high frequencies, the reactance of the waveguides 212, 213 is much smaller than the resistance to ground via the DC voltage source. Thus, leakage of the RF signal to ground through the DC voltage source is not significant because of the resistor R_DCHas only a negligible effect on the total impedance between the central portion of the lower cladding layer and ground. However, at lower frequencies, the reactance of the waveguide will increase, and the total impedance between the central portion of the lower cladding layer and ground will be substantially magnitude-dependentSubstantially lower than the impedance between the signal electrode and the lower cladding layer, resulting in a difference in voltage across the two waveguides.

An improved waveguide structure is described below which allows the voltage across each waveguide to be balanced at a much lower frequency than in prior art structures.

Fig. 4 shows an equivalent circuit of the waveguide structure. Respective resistive structures having reactances R are used to connect the central portion of the lower cladding layer of the waveguide to respective portions of the lower cladding layer located below each of the lines 211, 214. This effectively bypasses the capacitor C_T. The area of these lines is much larger than the waveguide, so the capacitance C_LSpecific capacitance C_WMuch larger. This means that the reactance R and the capacitance C_LSpecific capacitance C at lower frequencies_WA lower impedance path is provided so that the voltage divider can remain substantially balanced at lower frequencies.

The lower the reactance R, the lower the frequency at which the circuit remains balanced. However, if the reactance R is too low, a short circuit will form between the signal line electrode and the ground line electrode via the signal line, the reactance R and the ground line, effectively excluding the waveguide from the circuit. Therefore, the reactance R must be chosen to balance these effects. Reference to

In fig. 4, the value of R reduces the impedance between the points labeled x and y corresponding to the signal line electrode and the ground line electrode. This impedance should typically be 10 times lower than the resistance Rdc between the central portion of the lower cladding layer of the waveguide and the DC bias source. Depending on the value of Rdc, typical values may be greater than 10 ohms, greater than 100 ohms, greater than 1 kiloohm, etc., but are typically less than 100 kiloohms, less than 10 kiloohms, etc.

Fig. 5A shows a plan view of a portion of an exemplary waveguide structure, and fig. 5B is a cross-section along line B-B. The waveguide structure includes an insulating or semi-insulating substrate 501, which may be made of a semiconductor doped such as InP or GaAs, to have semi-insulating properties (e.g., having a resistivity greater than 105Ohm cm). On top of the substrate 501 are several semiconductor layers, including in sequence a lower cladding layer 502, a waveguide core layer 503, and an upper cladding layer 504. The upper and lower cladding layers 502, 504 comprise doped semiconductors, e.g., the upper cladding layer may be a p-doped semiconductor layer and the lower cladding layer may be an n-doped semiconductor layer, or vice versa, or any other combination that may form a waveguide structure. These layers 502, 503, 504 are selectively etched to form a signal line 511, a signal waveguide 512, a ground waveguide 513, and a ground line 514. The waveguides 512, 513 are connected by a central portion of the lower cladding layer. Each of the waveguides 512, 513 and the lines 511, 514 has a respective electrode 521, 522, 523, 524, and the electrode 521, 524 of each line 511, 514 is connected to a respective waveguide electrode 533, 523 by a conductive air bridge 525. The air bridge may be periodically provided in a length direction of the waveguide 512, 513. For simplicity, only two such bridges are shown in fig. 5A, but in other exemplary waveguide structures, the number of bridges is greater than this number.

The respective portions of lower cladding layer 502 within each waveguide 512, 514 are connected to the respective portions of lower cladding layer 502 within the respective line 511, 514 by a respective resistive structure 530. The resistive structure 530 is formed as part of the lower cladding layer 502, which includes an elongated portion 531 extending in the direction of extension of the waveguides 512, 513. Each elongated portion 531 is connected at one end to a portion of the lower cladding layer within the respective line 511, 514 and at the other end to a portion of the lower cladding layer 502 within the respective waveguide 512, 513 by a connecting portion 532. The length and width of the resistive structure 503 are selected to provide an appropriate reactance R.

Other arrangements of resistive structures providing appropriate reactance R may also be used. However, as in the example above, the resistive structure is in the plane of the lower cladding layer. The resistive structure may be part of the lower cladding layer 502 (as in the example above) or a separate body (e.g., formed during different processing steps).

The number and location of the resistive structures 530 may vary. For example, there may be one resistive structure at each end of the interferometer, or there may be multiple resistive structures along the length of the interferometer. As another example, one resistive structure may be provided for each air bridge in the electrode. This has the advantage that the waveguide structure can be designed in a regular repeating pattern, making manufacture easier.

Figure 6 shows the effect of frequency (x-axis) on the chirp parameter (y-axis, higher values meaning greater chirp, which is undesirable) for a prior art interferometer (chirp parameter represented by line 61) and an interferometer according to the structure of figures 5A and 5B (chirp parameter represented by line 62). As can be seen from fig. 6, the interferometers of fig. 5A and 5B maintain low chirp at frequencies several orders of magnitude below the frequency at which the chirp of the prior art interferometers becomes excessive.

The waveguide structure may include a dielectric on the sides of the signal and ground waveguides between the upper cladding layer and the electrodes of the signal and ground lines filling the area between each line and each waveguide and/or filling the area between the waveguides.

The waveguide structure may be fabricated by: providing a layered structure comprising an insulating or semi-insulating substrate, a lower cladding layer, a waveguide core layer and an upper cladding layer, etching these layers to form the above structure, and applying the electrodes to the wires and waveguides formed by the etching.