An optical isolator and optical isolation method based on a traveling wave Mach-Zehnder modulator

Abstract

The application provides an optical isolator based on a traveling wave Mach-Zehnder modulator and an optical isolation method, comprising: a first traveling wave Mach-Zehnder modulator and a second traveling wave Mach-Zehnder modulator connected in series; and an optical delay structure arranged between the first traveling wave Mach-Zehnder modulator and the second traveling wave Mach-Zehnder modulator. The application is an optical isolation structure based on a series TWMZM modulator, which is used for realizing an integrated optical isolator capable of effectively improving reverse optical isolation, stable performance, simple operation and low requirement on the performance of the modulator. The application has a simple structure, and only needs to add a passive optical waveguide between the series TWMZM. The application is simple to operate, and can realize optical isolation by using only one driving signal.

Description

Technical Field

[0001] This invention relates to the field of optical isolator technology, specifically to an optical isolator and optical isolation method based on a traveling-wave Mach-Zehnder modulator. Background Technology

[0002] In recent years, with the rapid development of emerging network applications such as the Internet of Things, autonomous driving, telemedicine, and distance education, higher demands have been placed on high-speed, high-capacity communication technologies. Optical communication, due to its large bandwidth, high reliability, low cost, and strong anti-interference capabilities, has achieved rapid development in the field of high-speed, high-capacity communication. In analog optical links, due to the widespread application of laser sources and the increasing complexity of optical links, optical isolators are needed to prevent reverse light transmission. An optical isolator is a non-reciprocal device in an optical path system that only allows unidirectional light transmission; it is commonly referred to as an optical unidirectional device. In an optical path, it can suppress or eliminate the adverse effects of reflected light generated at discontinuities on light sources and other systems—self-coupling effects and reflection noise—which can degrade the stability of light sources and other systems, or even render them inoperable. The most effective way to solve self-coupling effects and reflection noise is usually to use optical isolators in the optical path system.

[0003] To meet the requirements of low cost, low power consumption, and high speed in next-generation communication systems, integrated photonic devices represent a promising development direction. Currently, integrated photonic devices based primarily on silicon and III-V materials have realized crucial components for future communication systems, such as high-speed modulators, narrow-linewidth lasers, and other important passive devices. However, easily implementable, low-cost integrated optical isolators remain lacking. In commercial silicon photonic devices, optical isolators are often external miniature optical devices based on Faraday rotation, requiring precise alignment and incurring additional packaging costs. With the increasing development of integrated photonic platforms, both silicon-based platforms and the recently popular lithium niobate platform have seen the emergence of semiconductor light sources using heterogeneous integration of III-V materials or optical amplifiers using doped materials. This makes the demand for integrated optical isolators increasingly urgent, driving research into this area. The lack of integrated optical isolators will hinder the research of highly integrated coherent optical transceivers.

[0004] Optical isolation is a key performance indicator for optical isolators, defined as the power difference between light from two directions after passing through the isolator. A necessary condition for realizing an optical isolator is an asymmetric transmission matrix, or a transmission matrix capable of breaking Lorentz reciprocity. Existing methods for realizing optical isolators mainly fall into three categories: Faraday effect, nonlinear effect, and time-domain modulation of dielectric properties. Integrated optical isolators utilizing the Faraday effect primarily achieve optical isolation of up to 30 dB by depositing or bonding magneto-optical layers on SOI microrings or interference structures. However, existing methods require post-processing of the optical isolator, which is incompatible with standard fabrication processes, and this method introduces high insertion loss. While nonlinear SOI optical isolators are compatible with CMOS processes, they require extremely high pump light power or can only isolate light above a certain power threshold. Optical isolators based on refractive index modulation have been implemented on III-V materials, lithium niobate, heterogeneous integrated SiP, and SOI platforms. Among these, an optical isolator design based on a tandem SiP phase modulator has been proposed, but it can only achieve 3 dB of optical isolation. There are also designs that utilize directional couplers on phase modulators, implemented on lithium niobate platforms. Designs based on cascaded SiP modulators and directional couplers require careful control of the optical path length and corresponding driving phase. Currently, there is also a simpler method for implementing optical isolators based on electro-optic modulators. This method utilizes a traditional traveling-wave Mach-Zehnder modulator (TWMZM) to achieve different modulation effects on the forward and reverse light in the optical waveguide through the propagation of microwave signals on the traveling-wave electrodes, i.e., creating different phase differences, and using an interference structure to achieve optical isolation. However, this method using a single TWMZM involves the modulation switching between different output states, resulting in periodic light leakage in the output light, which limits the improvement of its optical isolation. Summary of the Invention

[0005] The purpose of this invention is to propose an optical isolator and optical isolation method based on a traveling wave Mach-Zehnder modulator. It is an optical isolation structure based on a series TWMZM modulator, which can effectively improve the reverse optical isolation, has stable performance, is simple to operate, and does not have high requirements for modulator performance.

[0006] An optical isolator based on a traveling-wave Mach-Zehnder modulator includes:

[0007] A first-row Mach-Zehnder modulator and a second-row Mach-Zehnder modulator connected in series.

[0008] And an optical delay structure disposed in the first traveling-wave Mach-Zehnder modulator and the second traveling-wave Mach-Zehnder modulator.

[0009] The first traveling-wave Mach-Zehnder modulator includes:

[0010] The first optical waveguide used for transmitting optical signals;

[0011] The first optical coupler is connected to the output terminal of the optical waveguide;

[0012] The first phase-shifting waveguide and the second phase-shifting waveguide are connected to the optical coupler;

[0013] The first traveling wave electrode group is used to modulate the optical signal within the optical waveguide;

[0014] A second optical coupler connected to the phase-shift waveguide;

[0015] The second traveling wave Mach-Zehnder modulator includes:

[0016] The second optical waveguide is used to transmit optical signals;

[0017] A third optical coupler connected to the output end of the optical waveguide;

[0018] The third and fourth phase-shifting waveguides are connected to the optical coupler;

[0019] The second traveling wave electrode group is used to modulate the optical signal within the optical waveguide;

[0020] A fourth optical coupler connected to the phase-shift waveguide;

[0021] The first traveling wave electrode group and the second traveling wave electrode group are connected in series.

[0022] The first traveling wave electrode group has a contact for applying an electrical signal at one end near the first optical waveguide, and the first traveling wave electrode group is connected in series with the second traveling wave electrode group at one end near the optical delay structure.

[0023] The second traveling-wave electrode group is connected in series with the first traveling-wave electrode at one end near the optical delay structure; the second traveling-wave electrode has a contact for applying an electrical signal at one end near the second optical waveguide.

[0024] The first and second phase-shifting waveguides are parallel to the first traveling-wave electrode group, and the third and fourth phase-shifting waveguides are parallel to the second traveling-wave electrode group.

[0025] The operation method of the optical isolator based on the traveling-wave Mach-Zehnder modulator includes the following steps:

[0026] 1) When the optical signal transmission direction is the same as that of the electrical signal: When the optical signal is input into the first optical waveguide of the first traveling-wave Mach-Zehnder modulator, it is split into two paths after passing through the first optical coupler and input into the first and second phase-shift waveguides respectively; the electrical signal is also applied to the first traveling-wave electrode group of the first traveling-wave Mach-Zehnder modulator. After the optical signal is modulated in the same direction as the electrical signal, it is input into the optical delay structure and delayed by a quarter of the electrical signal period. Then it is input into the second traveling-wave Mach-Zehnder modulator. After passing through the fourth optical coupler, it is split into two paths and input into the third and fourth phase-shift waveguides respectively; the electrical signal is applied to the second traveling-wave electrode group without delay through the series connection of the first and second traveling-wave electrode groups. After the optical signal is modulated in the same direction as the electrical signal, it is input into the third optical coupler, combined into one path, and output through the second optical waveguide.

[0027] 2) When the optical signal transmission direction is opposite to that of the electrical signal: When the optical signal is input into the first optical waveguide of the first traveling-wave Mach-Zehnder modulator, it is split into two paths after passing through the first optical coupler and input into the first and second phase-shift waveguides respectively; the electrical signal is applied to the second traveling-wave electrode group of the second traveling-wave Mach-Zehnder modulator. The electrical signal is applied to the first traveling-wave electrode group without delay through the series connection of the first and second traveling-wave electrode groups. After the optical signal is inversely modulated by the electrical signal, it is input into the optical delay structure for a quarter-cycle optical delay. Then it is input into the second traveling-wave Mach-Zehnder modulator, and after passing through the fourth optical coupler, it is split into two paths and input into the third and fourth phase-shift waveguides respectively; after the optical signal is inversely modulated by the electrical signal, it is input into the third optical coupler, combined into one path, and output through the second optical waveguide.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] First, the co-directional light does not have periodic light leakage, which is beneficial for the application of optical isolators, protecting lasers and reducing multipath interference in optical links.

[0030] Second, the structure is simple, requiring only the addition of a passive optical waveguide between the series TWMZM.

[0031] Third, it is easy to operate and can achieve optical isolation using only one drive signal. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of an optical isolator structure implemented using tandem TWMZM;

[0033] Figure 2 Schematic diagram of an implementation method for an optical isolator;

[0034] Figure 3 This invention presents a comparison of optical leakage in the same direction with and without optical delay when the optical isolator is driven by a periodic square wave signal. Figure 3(a) Comparison of the output light of the first traveling wave Mach-Zehnder modulator and the output light of the second traveling wave Mach-Zehnder modulator under the condition of no optical delay; Figure 3 (b) Comparison of the output light of the first traveling wave Mach-Zehnder modulator and the output light of the second traveling wave Mach-Zehnder modulator under the condition of light delay.

[0035] Figure 4 This invention presents a comparison of optical leakage in the same direction with and without optical delay when the optical isolator proposed in this invention is driven by a periodic sine wave. Figure 4 (a) Comparison of the output light of the first traveling wave Mach-Zehnder modulator and the output light of the second traveling wave Mach-Zehnder modulator under the condition of no optical delay; Figure 4 (b) Comparison of the output light of the first traveling wave Mach-Zehnder modulator and the output light of the second traveling wave Mach-Zehnder modulator under the condition of light delay. Detailed Implementation

[0036] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0037] In the technical solution of this invention, an optical isolator implemented by connecting two TWMZMs in series can effectively eliminate periodic optical leakage, and its structure is as follows: Figure 1 As shown. The optical isolator includes a first optical waveguide 1, a first optical coupler 2, a first optical phase-shifting waveguide 3, a second optical phase-shifting waveguide 4, a first traveling-wave electrode group 5, a second optical coupler 6, an optical delay structure 7, a fourth optical coupler 8, a third optical phase-shifting waveguide 9, a fourth optical phase-shifting waveguide 11, a second traveling-wave electrode group 10, a third optical coupler 12, and a second optical waveguide 13.

[0038] The structure comprises two traveling-wave Mach-Zehnder modulators (hereinafter referred to as TWMZMs) with an adjustable (or fixed) optical delay structure (7) between them. The two TWMZMs include a traveling-wave electrode assembly, a coupler, and a phase-shifting waveguide. The traveling-wave electrode assembly and the phase-shifting waveguide are collectively referred to as the phase shifter, and the coupler is used to achieve beam splitting and interference effects. The traveling-wave electrodes of the two TWMZMs can be, for example,... Figure 1The push-pull drive of the GSG configuration shown can also be a dual drive of a separate GSSG configuration or a single drive of a GS configuration. The traveling wave electrodes of the two TWMZMs are connected in series through wiring to ensure the consistency of the microwave signal applied to the two TWMZMs. The two TWMZMs are connected by an optical delay structure (2). The waveguide length of the optical delay structure is designed so that the difference between the propagation time of the optical signal passing through this part and the microwave signal delay passing through the connection part of the traveling wave electrodes of the two TWMZMs is an integer multiple of one-quarter of the microwave signal period when the integral of the microwave signal modulation effect is zero during reverse propagation. The principle of this device is that when the propagation direction of the microwave signal on the traveling wave electrode is the same as the propagation direction of the light in the optical waveguide of the TWMZM, the optical phase shift generated by one of the two phase shifters of the TWMZM can be expressed as

[0039]

[0040] Wherein, the voltage applied to the traveling wave electrode is denoted as V(t), where t is time; c o c is the speed of light propagation in an optical waveguide. e Let be the propagation speed of the microwave signal on the traveling wave electrode. This formula represents the phase shift of the optical signal output by the phase shifter at time t. This phase shift is related to the electrical signal before time t, and since the propagation speed of light and the propagation speed of electrical signal may be different, the speed difference needs to be considered.

[0041] When the propagation direction of the microwave signal on the traveling wave electrode is opposite to the propagation direction of the light in the phase shifter's optical waveguide, the phase shift can be expressed as:

[0042]

[0043] The integral part represents the electrical signal that modulates the light at that moment, including... All electrical signals within the time interval. However, for co-propagation, the modulated electrical signal originates only from... or Within the range.

[0044] When the propagation speed of microwaves is similar to that of light, equation (1) can be written as follows:

[0045]

[0046] Equation (2) can be written as

[0047]

[0048] Using a peak-to-peak value of 2V π The cycle is A square wave n∈Z can satisfy

[0049]

[0050]

[0051] That is, when a periodic square wave is used as the modulation signal, the integral of the modulation effect on the reverse-propagating optical signal is zero, and its phase difference after passing through the two phase shifters is also zero; while the phase difference of the co-propagating optical signal after passing through the two phase shifters is ±π. Because the modulator's operating state inevitably passes through a static bias point when propagating in the same direction, switching between two adjacent maximum / minimum operating points, the output light intensity of the co-propagating and reverse-propagating light becomes the same, failing to meet the requirements for an optical isolator, a phenomenon known as periodic optical leakage. Furthermore, due to the bandwidth limitations of the driving signal from devices such as the signal generator and modulator in practical applications, this optical leakage can have a significant duration.

[0052] To eliminate this light leakage, consider the following: Figure 1 The cascaded TWMZM structure is shown. Let the transfer function of this structure under the condition that the optical signal and microwave signal propagate in the same direction be...

[0053] T(t)=T1(t)*T2(t) (7)

[0054] Here, T1(t) and T2(t) are the transfer functions of the two TWMZMs, respectively. Considering that both TWMZMs are biased at their maximum operating point, i.e., the static output is at maximum power, we use 1 to represent the maximum output power and 0 to represent the minimum output power.

[0055] When the light wave and the microwave signal are in the same direction, let the period of the square wave driving signal be P. Since the peak-to-peak value of the square wave is 2V... π T1(t) can be expressed as

[0056]

[0057] The optical signal output from the first TWMZM is delayed using an optical delay line. T2(t) can be expressed as

[0058]

[0059] Substituting (8) and (9) into (7) yields...

[0060]

[0061] That is, by cascading two TWMZMs, reverse periodic optical leakage can be completely eliminated, improving the optical isolation of TWMZMs used as optical isolators, such as... Figure 2 As shown.

[0062] When the light wave and the microwave signal are in opposite directions, the integral of the modulation effect on the light signal is 0. Since the static bias point is the maximum operating point, the output of the reverse-propagating light signal is always 1.

[0063] The cascaded TWMZM optical isolator was verified by using Ansys Lumerical INTERCONNECT software to build a simulation model. In the model, square waves and sine waves were used as driving signals, and the unidirectional output light intensity in the parts with and without optical delay was compared.

[0064] like Figure 2 As shown, when the driving signal is a square wave signal with a frequency of 1 GHz, the periodic optical leakage in the output optical signal manifests as a spike signal. After adding a quarter-cycle optical delay, this spike can be effectively suppressed.

[0065] like Figure 3 As shown, when the driving signal is a sinusoidal signal with a frequency of 1 GHz, the optical leakage in the output optical signal manifests as a raised cosine-shaped optical pulse. Compared to the optical leakage when a square wave signal is used as the driving signal, this pulse has a wider width and a longer duration. Similarly, by adding an optical delay, this optical leakage can be effectively suppressed.

[0066] Furthermore, considering the average output power of the co-directional light under a certain driving voltage amplitude and the bandwidth limitation of the traveling wave electrode, as the driving signal frequency increases, its loss on the traveling wave electrode increases due to bandwidth limitation, the modulation effect weakens, and the optical isolation of a single TWMZM gradually decreases to zero. However, it can achieve a maximum value at discrete octave points, that is, with the optical delay length remaining constant, the optical isolation reaches a maximum value when the optical signal delay is an integer multiple of the microwave signal delay.

[0067] Detailed operation steps:

[0068] 1. In the optical signal generation module, the laser source emits light, which is then polarized by a three-ring polarization controller before being input into the modulator and the optical isolator.

[0069] 2. The electrical signal generation module outputs a periodic electrical signal with a peak-to-peak value twice the half-wave voltage of the TWMZM and a period four times the propagation time difference between the light wave and the microwave signal caused by the optical delay. This periodic electrical signal is applied to the traveling wave electrode of the TWMZM through a high-frequency circuit.

[0070] 3. The signal receiving section receives the optical signal transmitted through the series-connected TWMZM. An optical oscilloscope is used to observe the co-directional optical waveform and average power of the optical isolator.

[0071] 4. Switch the input and output optical ports of the optical isolator while keeping the microwave signal loading position unchanged, and observe the waveform and average optical power of the reverse light to obtain the optical isolation of the optical isolator.

Claims

1. An optical isolator based on a traveling-wave Mach-Zehnder modulator, characterized in that, include: A first-row Mach-Zehnder modulator and a second-row Mach-Zehnder modulator connected in series. And the optical delay structure disposed in the first traveling wave Mach-Zehnder modulator and the second traveling wave Mach-Zehnder modulator; The first traveling-wave Mach-Zehnder modulator includes: The first optical waveguide used for transmitting optical signals; The first optical coupler is connected to the output terminal of the optical waveguide; The first phase-shifting waveguide and the second phase-shifting waveguide are connected to the optical coupler; The first traveling wave electrode group is used to modulate the optical signal within the optical waveguide; A second optical coupler connected to the phase-shift waveguide; The second traveling wave Mach-Zehnder modulator includes: The second optical waveguide is used to transmit optical signals; A third optical coupler connected to the output end of the optical waveguide; The third and fourth phase-shifting waveguides are connected to the optical coupler; The second traveling-wave electrode group is used to modulate the optical signal within the optical waveguide; A fourth optical coupler connected to the phase-shift waveguide; The first traveling wave electrode group and the second traveling wave electrode group are connected in series; The first traveling wave electrode group has a contact for applying an electrical signal at one end near the first optical waveguide, and the first traveling wave electrode group is connected in series with the second traveling wave electrode group at one end near the optical delay structure. The second traveling-wave electrode group is connected in series with the first traveling-wave electrode group at one end near the optical delay structure; the second traveling-wave electrode group has a contact for applying an electrical signal at one end near the second optical waveguide. The first and second phase-shifting waveguides are parallel to the first traveling-wave electrode group, and the third and fourth phase-shifting waveguides are parallel to the second traveling-wave electrode group.

2. An optical isolation method, characterized in that, The optical isolator based on a traveling-wave Mach-Zehnder modulator as described in claim 1 comprises the following steps: 1) When the direction of optical signal transmission is the same as that of electrical signal: When the optical signal is input into the first optical waveguide of the first traveling wave Mach-Zehnder modulator, it is split into two paths by the first optical coupler and input into the first phase-shifting waveguide and the second phase-shifting waveguide, respectively. The electrical signal is also applied to the first traveling wave electrode group of the first traveling wave Mach-Zehnder modulator. After the optical signal is modulated in the same direction as the electrical signal, it is input into the optical delay structure and delayed by a quarter of the electrical signal period. Then it is input into the second traveling wave Mach-Zehnder modulator. After passing through the fourth optical coupler, it is split into two paths and input into the third phase-shifting waveguide and the fourth phase-shifting waveguide respectively. The electrical signal is applied to the second traveling wave electrode group without delay through the series connection of the first traveling wave electrode group and the second traveling wave electrode group. The optical signal is input to the third optical coupler after being modulated in the same direction by the electrical signal, and then combined into one channel and output through the second optical waveguide. 2) When the direction of optical signal transmission is opposite to that of electrical signal: When the optical signal is input into the first optical waveguide of the first traveling wave Mach-Zehnder modulator, it is split into two paths by the first optical coupler and input into the first phase-shifting waveguide and the second phase-shifting waveguide, respectively. An electrical signal is applied to the second traveling wave electrode group of the second traveling wave Mach-Zehnder modulator. The electrical signal is applied to the first traveling wave electrode group without delay through the series connection of the first and second traveling wave electrode groups. The optical signal is input into the optical delay structure after being inversely modulated by the electrical signal. After a quarter-signal period of optical delay, it is input into the second traveling wave Mach-Zehnder modulator. After passing through the fourth optical coupler, it is split into two paths and input into the third and fourth phase-shift waveguides respectively. The optical signal is input to the third optical coupler after being inverted and modulated by the electrical signal, and then output through the second optical waveguide.

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