Compact and Efficient Thin‐Film Lithium Niobate Modulators

Thin‐film lithium niobate (TFLN) modulators have garnered significant attention in the field of integrated photonics due to their ability to manipulate light. Numerous TFLN modulators have been demonstrated over the past few years, with speed records consistently being broken. However, due to the low refractive index contrast and anisotropic properties of TFLN, modulators based on this material feature larger device sizes and lower efficiency. A more compact and efficient modulator is important, as it is required for future dense integration and low power consumption applications. There have been some reports on how to reduce device size, and research is ongoing. A comprehensive summary can help individuals understand the current state of research and existing problems. In this review, we provide an overview of recent advancements in TFLN modulator technology, focusing on compactness and efficiency. Herein, various types of modulators, such as the Mach–Zehnder interferometer, Michelson interferometer, resonator cavity, and Z‐cut type modulators, are discussed. Moreover, potential improvement strategies and applications for compact and efficient TFLN modulators in advanced photonic systems are explored. In conclusion, in this review, the recent achievements in compact and efficient TFLN modulators are summarized and their significance in various optoelectronic applications is emphasized.


Introduction
6] A lithium niobate modulator is a device that utilizes the electro-optic (EO) property of lithium niobate.It is widely used in optical communication systems and microwave photonics applications due to its exceptional EO property.Traditionally, the lithium niobate modulator is fabricated based on ion diffused bulk crystal. [7]Due to the low refractive index contrast, the modulation efficiency is low, and the device size is large.With the development of thin-film lithium niobate (TFLN), the modulator can be made with a more compact size and higher efficiency, and offer greater integration with other optical components. [4]Over the past few years, the TFLN modulators have experienced great development.TFLN modulator technology has seen significant advancements.Numerous TFLN modulators based on various structures have been demonstrated, such as the Mach-Zehnder interferometer (MZI), [5,6,8] Michelson interferometer (MI), [9,10] resonator cavity, [11,12] and Z-cut structure. [13,14]oreover, the speed record is consistently being broken.For instance, the modulation data rate of the TFLN modulator has reached a record 1.96 Tb s À1 , [15] which shows great potential of TFLN modulators to play big role in high speed and large capacity communications.[18][19] All of these make the TFLN modulator an attractive device.
However, the large footprint and relatively low efficiency of TFLN-based modulators remain a challenge for dense integration.The reasons behind this are mainly related to the moderate EO coefficient of LN and relatively low refractive index contrast. [4,20,21]The anisotropic property of TFLN makes it troublesome to have waveguide bends as easily as in other materials, which also contributes to typical large device sizes. [8]For example, the most widely used MZI type modulator ordinarily requires centimeter-scale waveguide lengths for the EO effect to reach sufficient accumulation to realize high modulation efficiency with low bias voltage. [5,6]Even though resonator structures can help to reduce the device size, the improvement is not as obvious as in other material platforms as it still needs a certain path length for sufficient phase accumulation. [12]This is why racetrack ring resonators are used for most TFLN-based ring resonator modulators.Although there have been some attempts to create new structures, such as photonic crystal (PC)-, [22] metasurface-, [23] and plasmonic-based [24] device structures to reduce the size, the achievement typically comes at the expense of some kind of performance metric degradation.With device integration densities becoming higher and higher, how to reduce the overall size of a TFLN modulator while maintaining high speed is an urgent problem to be solved.Until now, most researchers have concentrated on improving the speed of TFLN modulators.However, creating a compact and efficient TFLN modulator is a meaningful objective, especially with the increasing demands for integration density.Therefore, a systematic review focusing on the device size and modulation efficiency of TFLN modulators will be greatly beneficial.
We therefore summarize the key results of TFLN modulators demonstrated over the past years.Different TFLN modulators are analyzed and compared from aspects of size and efficiency.A schematic of TFLN modulators discussed in this manuscript is shown in Figure 1. [14,22,24,25]We divide TFLN modulators into several categories: MZI, MI, resonator cavity-based modulators, Z-cut modulators, and other types.We will first discuss the details of all types.Following this, we will provide some summaries and outlooks.The results presented in this manuscript aim to shed light on the current challenges and opportunities in developing highly compact and efficient TFLN modulators.We hope that these findings can inspire further contributions to the development of this field.

MZI-Based TFLN Modulator
A TFLN modulator typically functions, ultimately, from the EO effect of the material, and utilizes its unique properties to control the transmission of light.A modulator consists of a thin film of lithium niobate in which a waveguide exists that lies between two electrodes.When a voltage is applied to the electrodes, an electric field is generated within the lithium niobate film, causing a change in its refractive index.This change modifies the phase of the passing light wave.By constructing interference structures, the phase variation can be further converted into changes in intensity, achieving modulation of the intensity.Researchers have developed various sorts of such interference structures, and the MZI structure is one of them.
An MZI is a type of interferometer used to measure subtle phase differences between two light beams.It consists of two optical paths (arms) that are split and then recombined to produce an interference pattern.By adjusting the relative phase of the two arms, the interference pattern can be manipulated, allowing for the measurement of phase changes.The MZI structure is the most widely used to construct a modulator.For a TFLN-based modulator, the LN can be either non-etched or etched.In the non-etched scheme, the LN film is heterogeneously integrated with other index-matched materials, such as silicon or silicon nitride.The field of the transmitted light partially overlaps the LN material in cross section.In the etched scheme, the TFLN is directly etched to form the waveguide.Compared with the etched LN method, the heterogeneous approach avoids the etching of LN but may result in a relatively larger size due to weak mode confinement.Nonetheless, both methods have been utilized in TFLN modulator design and fabrication.In a TFLN-based modulator with an MZI structure, the phase difference between the two arms is controlled by applying an electric field to the waveguide.Thanks to the exceptional second-order EO coefficient of LN, high-speed modulation rates have been achieved with this structure.
Table 1 and 2 summarize the progress of different TFLN modulators with the MZI structure. [5,6,12,15, Figure2 summarizes the half-wave voltage, device length, and bandwidth of etched and non-etched TFLN modulators.Both these two kinds of modulators can realize a very high bandwidth.For the etched TFLN modulator, the V π L is typically below 4 V • cm with a bandwidth around 100 GHz, while for the non-etched TLFN modulator, a slightly higher V π L is observed, which is probably due to the relatively weak mode confinement compared with the directly etched structure.Such a conclusion is clearer from the comparative results shown in Figure 2c,d.As can be seen from Figure 2c, d, the required half wave voltage for a non-etched TFLN modulator is higher under the same modulation length.
For the MZI type modulator, the V π L is typically within a range from 2 to 4 V • cm.If it works under low voltage, the modulation length is usually at the centimeter level.As the combined width of an LN waveguide and co-planar travelling wave (CPW) electrodes is usually around a few hundred microns, what people see is typically a narrow and slender structure for a TLFN modulator.Compared with other materials, such a long device size is mainly due to the weak refractive index change coming from the EO effect of LN.And such a slender structure also imposes difficulties in integration with other components on the same chip. .The center picture is the cross section of the TFLN.In the Mach-Zehnder part, the inset shows a schematic of a TFLN modulator with MZI structure.In the Michelson part, the inset shows a 3D schematic of the TFLN Michelson interferometer (MI) modulator.Adapted with permission. [25]Copyright 2021, Optical Society of America.In the resonator part, the inset shows a schematic of a 1D photonic crystal (PC) resonator cavity TFLN modulator.Adapted under the terms of a CC-BY license. [22]Copyright 2020, The Author(s), published by Springer Nature.In the Z-cut part, the inset shows a schematic of a spiral waveguide Bragg grating Z-cut TFLN modulator.Adapted with permission. [14]Copyright 2023, Optica Publishing Group.In the other parts, the inset shows a schematic of a plasmonic TFLN modulator.Adapted with permission. [24]Copyright 2022, American Chemical Society.
Reducing device length is a challenging problem faced by researchers and engineers.A straightforward solution might be to fold the MZI structure.However, for X/Y cut TFLN, the optic Z axis remains the same when the MZI is folded 180°.But as the direction of the electric field then can become inverted without careful electrode placement, the accumulation of phase change is cancelled, rendering the MZI modulator nonfunctional.
To solve such a problem, three kinds of structures have been developed.The first one is based on poling, as shown in Figure 3a,b.By poling the folded waveguide, its local crystal domain is inverted, which makes the optic Z axis align with the electrical field direction.Based on such a method, Hu et al. demonstrated a kind of folded TFLN MZI modulator with a reduced modulation section length of 0.5 cm, and realized a 2.74 V cm modulation efficiency at a speed of 55 GHz. [26]he second method is based on waveguide crossing, as shown in the structure in Figure 3c.By crossing the two arms of the MZI, the effect of the electric field flipping is avoided.Based on such a method, Sun et al. have realized a compact MZI modulator with two-fifths length compared with an equivalent unfolded structure. [27]As the poling process needs a stronger electrical field, the waveguide crossing method is more widely used in both etched structures [28,29] and non-etched structures. [30]The third method is based on interdigitated T-rails CPW electrodes, as shown in Figure 3d.In the inverted waveguide area, the electrical field direction is inverted by reversing the interdigitated T-rails.Such a method adds no extra process flexibility.Based on such a method, Liu et al. realized an 0.8 cm long folded TFLN MZI modulator with a 43 GHz  Table 1.Continued.Extracted; b) Estimated.
bandwidth. [31]The key idea of folded TFLN MZI modulators is to bend the waveguide to reduce device size and maintain high speed operation.The associated optic axis direction inversion can thus be solved by either changing the optic axis direction or by changing the electrical field direction.

MI-Based TFLN Modulator
The EO modulator can also be designed as an MI.In an MI, the light will pass the modulation path twice and therefore the device length can be halved in principle.In 2019, Jian et al. demonstrated a type of TFLN modulator with the MI structure by using a multimode interference (MMI) based loop, [9] as shown in Figure 4a.With a 0.1 cm modulation length, they demonstrated 1.4 V • cm modulation efficiency and 12 GHz bandwidth.By engineering the velocity matching condition between optical and microwave modes, they then improved its speed to 17.5 GHz soon after that. [10]In addition to an MMI-based loop, an MI structure can also be implemented by using a Bragg grating, [25] as shown in Figure 4b

Resonator Cavity-Based TFLN Modulator
Resonator cavities are typically used to confine and control light.
A cavity can enhance interaction between light and matter and thus can be used to reduce device size.Therefore, resonator cavities are widely used to construct compact and efficient EO modulators in other material platforms. [17]Various types of resonator cavity modulators have also been developed for TFLN.In these kinds of modulators, the light resonates within the device, which effectively increases the modulation length.As a result, the modulation efficiency can be improved, and the device size can be reduced.So far, the primary kinds of resonator cavities used in TFLN modulators include ring cavities, Bragg grating cavities, Fabry-Perot (FP) cavities, and PC cavities.The details of these types will be discussed in detail later.

Ring Cavity
In a ring cavity, a straight waveguide guides light around a ring, and the ring waveguide acts as a resonator, allowing the light to pass around the ring multiple times.By applying an electric field on the ring waveguide, the refractive index of the lithium niobate is changed along the ring, which alters the phase of the light passing through it.Similar to the MZI type modulator, there are also two methods of forming a ring modulator in TFLN.One is by heterogeneously bonding other kinds of material with LN, [11] as shown in Figure 5a.In such a structure, the LN is not etched, and the optical mode is only partially overlapping the LN.The other one is by directly etching the LN, [12] as shown in Figure 5b.Tuning efficiency of the directly etched method is higher due to the better optical mode confinement.For example, Wang et al. demonstrated a directly etched TFLN ring modulator with a tuning efficiency of around 7 pm V À1 . [12]In contrast, the tuning efficiency for hybrid TFLN ring modulator reaches 3 pm V À1 only. [11,108]ompared with MZI or MI type modulators, the device size of a ring cavity-based TFLN modulator is more compact.][110] In contrast, due to the anisotropy of LN, the ring curvature will affect the EO efficiency of a ring modulator in X/Y cut LN. [111]A racetrack type ring modulator is adopted instead to improve the tuning efficiency, for example, as shown in Figure 5b.In TFLN ring modulators, there is a trade-off between bandwidth and Q factor.The Q factor, also known as the quality factor, is a metric of the resonant behavior of the ring resonator, and it is defined as the ratio of the energy stored in the resonator to the energy lost per cycle.A higher Q factor means higher energy storage and narrower bandwidth, compared with wider bandwidth but less energy storage character of low Q factor ring resonators.In TFLN ring modulators, a higher Q factor results in a higher modulation efficiency and lower power consumption, but it also ultimately leads to a lower response speed.A lower Q factor ring should be designed if a high response speed is desired.To summarize, there is a trade-off between bandwidth and Q factor in TFLN ring modulators, and the optimal choice depends on specific application   [26] Copyright 2021, Optical Society of America.c) Schematic of the folded TFLN MZI modulator based on waveguide crossing.Adapted under the terms of a CC-BY license. [27]Copyright 2021, The Author(s), published by MDPI.d) Schematic of folded TFLN modulator based on interdigitated T-rails co-planar travelling wave (CPW) electrodes.Adapted with permission. [31]Copyright 2022, IEEE.
requirements.Such a trade-off is experimentally validated by Wang et al. [12] According to their results, the bandwidth of a TFLN ring modulator can reach 40 GHz under a Q factor of 5700, but only 11 GHz for an 18 000 Q factor. [12]It is worth mentioning that such a trade-off exists for all kinds of resonator-cavity-based modulators and can be overcome with other structures or mechanisms.

Bragg Grating
A Bragg grating is a periodic structure that reflects specific wavelengths of light, while allowing others to pass through.In a TFLN modulator, a Bragg grating can be used to create a resonant cavity that enhances the interaction between light and the material, leading to more efficient modulation and thus a more compact size.Figure 6a shows one example of a Bragg grating modulator demonstrated on TFLN. [112]The fishbone-like grating structure results in a periodic change of the effective refractive index and acts as a wavelength-dependent filter.The applied external electrical field will cause a shift of the stopband and thus can transform a modulated electrical signal into an optical signal. [14,112,113]ased on such a structure, Pohl et al. demonstrated >100 Gbps data transmission with a device footprint of only 10 by 400 μm 2 . [112]The Bragg grating can also be combined with an  [9] Copyright 2019, Optical Society of America under the terms of the OSA Open Access Publishing Agreement.b) Adapted with permission. [25]Copyright 2014, Optical Society of America.b) Adapted with permission under the terms of the OSA Open Access Publishing Agreement. [12]Copyright 2018, Optical Society of America.
MZI structure to reduce its size. [114,115]Figure 6b shows an example of an MZI modulator using a Bragg grating as the phase shifter. [114]The slow light in the Bragg gratings is used to improve modulation efficiency.The measured V π L was 0.67 V • cm with a device length of only 0.82 mm.

FP
A Fabry-Perot (FP) resonator can also be used to design a modulator.An FP cavity is typically created by sandwiching a waveguide between two partially reflective mirrors.The light is reflected inside the resonator multiple times and thus enhances the interaction between light and TFLN.In principle, higher EO modulation efficiency is expected in an FP TLFN modulator.Figure 7 shows two examples of FP modulators based on TFLN. [116]In Figure 7a, the FP cavity is constructed by inserting a waveguide between two Bragg grating mirrors. [116]Such a device shows a low loss and 15.7 pm V À1 tuning efficiency.The measured speed is around 24 GHz.By optimizing the reflecting mirror and straight waveguide structure, Pan et al. further improved the FP modulator with a bandwidth >110 GHz. [117]As shown in Figure 7b, the authors designed a 2-by-2 FP cavity with an optimal Q factor.The effective cavity length is only about 50 μm, and its total footprint is around 4 by 500 μm 2 .Copyright 2021, IEEE.

Photonic Crystal
The slow light effect in PCs is widely used to enhance the interaction between light and optical structures.Such an idea can be applied to the design of EO modulators and has already been validated in other material platforms. [118]The PC structure creates a periodic variation in the refractive index of a material, allowing efficient modulation of light.[121][122][123][124][125][126][127][128][129][130][131] In 2020, Li et al. demonstrated a type of 1D PC EO modulator on X-cut LN, [22] as shown in Figure 8a-d.The input light is injected into a 1D PC and then modulated by it.Benefitting from the slow light effect in the 1D PC, a high modulation efficiency (16 pm V À1 ) was realized with an ultrasmall EO modal volume of only 0.58 μm 3 .Its measured bandwidth was around 17.5 GHz and is mainly restricted by the Q factor of the PC cavity.The theoretical bandwidth of a 1D PC modulator can be as high as 600 GHz with 0.0874 V • cm according to simulation results demonstrated by Qi et al. [130] The Q factor restriction is relaxed in 2D PC structures.By combining a 2D PC structure with an MZI structure, compact and high speed EO modulators are expected to be realized. [128,129]

Z-Cut TFLN-Based Modulator
Up to now, most of the reported TFLN modulators are based on X/Y-cut LN.There are several advantages of Z-cut LN based devices compared with X/Y-cut LN. [8,13,14] First, the Z-cut TFLN offers the flexibility to modulate light perpendicular to the direction of light propagation, which can be advantageous in certain applications where modulation in the plane is not feasible or desired, [8,14,21,132,133] usually to take advantage of the fact that LN is isotropic about the Z axis but not about the X/Y axis.Second, the electrical field in Z-cut TFLN is in the vertical direction and thus will have a large overlap with the optical field along a waveguide cross section.These features allow for the design of compact and efficient modulators based on Z-cut TFLN.
Figure 9a shows one example of a compact TFLN modulator based on Z-cut TFLN.In such a device, the electrodes are above  [116] Copyright 2021, Chinese Optics Letters.b) Adapted under the terms of a CC-BY license. [117]Copyright 2022, The Author(s), published by Elsevier.and below the waveguide to accommodate the crystal orientation. [13]Such a configuration makes the electrical field direction perpendicular to the optical transmission direction.Therefore, the optical waveguide can be bent in any direction without considering the electric field direction as X/Y-cut LN. Figure 9b shows an example of a spiral waveguide based on Z-cut TFLN.Such a spiral waveguide can be combined with a typical MZI structure.Figure 9c shows an example of a compact  -d) Adapted under the terms of a CC-BY license. [22]Copyright 2020, The Author(s), published by Springer Nature.c) Adapted with permission under the terms of the Optica Open Access Publishing Agreement. [13]Copyright 2022, Optica Publishing Group.d,e) Adapted with permission. [14]Copyright 2023, Optica Publishing Group.
MZI modulator with spiral arms demonstrated in Z-cut LN. [13] By bending the waveguide, its overall device size is greatly reduced compared with a typical X/Y-cut LN-based modulator.In a total footprint of 110 by 110 μm 2 area, the authors achieved a modulator that fits a 0.286 cm long optical waveguide and a V π L of about 7.4 V • cm.Its measured bandwidth is around 9.3 GHz.
It is worth mentioning that the MZI structure is not the only configuration that can be adopted in Z-cut LN.The structures used in X/Y-cut LN can also be applied to Z-cut LN. Figure 9d,e show another kind of spiral waveguide modulator with Bragg grating demonstrated on Z-cut LN. [14] Its working principle is similar to that of the Bragg grating modulator on X/Y-cut LN. [116,117,134] The difference is that the waveguide can be bent in a spiral configuration to achieve a more compact size.In such a device, the total 2.2 mm long waveguide is wrapped into a 120 by 120 μm 2 area.Its measured tuning rate is around 8.36 pm V À1 with a 25 GHz bandwidth.The Z-cut modulator, in general, allows more flexible device configurations and is expected to continue to realize compact devices in the future.

Others
In addition to the aforementioned types of modulators, there are a few other types of modulators which have been demonstrated to realize compact devices.One is based on a metasurface.A metasurface is a thin layer of subwavelength-sized structures that can manipulate light in a highly controllable way.In a metasurface-based TFLN modulator, the metasurface is fabricated on top of a lithium niobate waveguide to create a highly compact and efficient modulator.For example, Hoblos et al. demonstrated a type of 2D metasurface-based EO modulator with a record low voltage-length product of about 2:925 Â 10 À4 V ⋅ cm, [135] as shown in Figure 10a.This is possible because the interaction length is in the vertical direction.Such an ultralow value shows the potential of using metasurface for realizing compact and efficient EO modulators.
The second one is based on plasmonic structures.The extreme strong mode confinement in plasmonic structures is utilized.In 2022, Thomaschewski et al. demonstrated a kind of plasmonic TFLN MZI modulator with a low voltage-length product of about 0.23 V • cm, [24] as shown in Figure 10b.The device footprint is less than 1 mm 2 , and measured speed is above 10 GHz.The low speed is mainly due to the presence of large parasitic parameters and is expected to realize 900 GHz by optimizing the device structure to alleviate these.
The third one is a newly reported coupling modulation-type modulator demonstrated in TFLN.As discussed earlier, there exists a trade-off between a high Q factor and high bandwidth for ring modulators.By using the coupling modulation method, the problem can be ameliorated.The coupling modulation configuration has long been explored and demonstrated in other material platforms. [136,137]In 2022, Xue et al. first demonstrated coupling modulation in TFLN, [138] as shown in Figure 10c.Its working principle is based on the tuning of the coupling ratio between the ring and the bus waveguides.At the working wavelength, the ring is designed to work under the critical state (Figure 10d).When an external voltage is applied, the ring will work under the zero state (Figure 10e).Therefore, the intensity is modulated with respect to the applied voltage.Such a device shows over 67 GHz bandwidth with a high Q factor of about These are just a few examples of the many types of TFLN modulators that can be fabricated using various structures and techniques.Each type has its own unique advantages and disadvantages, depending on the specific application requirements.

Summary and Outlook
In this manuscript, we have provided an overview of compact and efficient TFLN modulators, which are an important class of optical devices used in manipulating light in various photonic applications.This review discussed recent advancements in TFLN modulator technology that have enabled smaller device footprints and higher modulation efficiencies.The review began by introducing the basic principles of TFLN modulators, including the EO effect and the fabrication techniques for TFLN waveguides.After that, different types of modulators were discussed, including MZI, MI, resonator cavity, Z-cut, and so on.The review highlighted recent progress in improving the compactness of TFLN modulators.This includes advances in fabrication techniques, material engineering, and device design.The use of novel structures and materials, such as PCs and metasurfaces, has enabled the development of highly compact modulators with small footprints, making them suitable for integration into densely integrated photonic circuits.
TFLN modulators have shown ultrahigh speed and will play a great role in applications of optical communications, microwave photonics, and quantum information.Different types of modulators serve varying applications.Among them, the MZI type stands out as the most prevalent, renowned for its ultrahigh speed and consistent performance.Nonetheless, it bears a larger device size in comparison to other types.This limitation can be mitigated by introducing bends in the MZI arms.Another alternative is adopting the Michelson structure, wherein the light traverses the path twice.Both MZI and Michelson type modulators find utility in fiber optic communication and optical signal processing.However, when confronted with the challenge of dense photonics integrated circuits requiring many cascaded EO modulators, the necessity for a more compact modulator becomes paramount.In this context, the resonator type proves to be an excellent choice.Furthermore, the Z-cut TFLN modulator can effectively lower the bias voltage required by typical X/Y cut modulators while simultaneously reducing device size, thanks to its flexible waveguide bending capabilities.
Reducing the device size and improving modulation efficiency of TFLN modulators are vital jobs to do in the expected future.
The key problems which need to be addressed in the future include the following:

Waveguide Engineering
By designing and optimizing the waveguide structure of the modulator, the interaction between light and the TFLN can be enhanced.This includes carefully selecting waveguide dimensions and geometries to achieve efficient light coupling and propagation.Waveguide engineering techniques such as PC structures, tapered waveguides, or subwavelength grating structures can be utilized to achieve higher modulation efficiencies and reduce device footprint.

Electrode Engineering
Optimizing the electrode design is crucial for improving the modulation efficiency of modulators.This includes reducing the resistance of the electrodes, minimizing electrode spacing, and increasing the overlap between the electric field and the active region of the TFLN.These factors can enhance the electric field strength and improve the modulation efficiency.One example is the previously discussed Z-cut modulator, which operates by applying the electrical field vertically and thus improving modulation efficiency.

New Structures or Principles
Exploring new structures or principles to enhance EO properties can open up possibilities for improved modulation efficiency.Researchers are continuously investigating new structures, such as metasurfaces or plasmonic structures, to achieve higher modulation performance while maintaining a small footprint.In the meantime, new principles which can break typical trade-offs can also provide solutions for more compact and efficient TFLN modulators.

Integrated Photonics
Integration of TFLN modulators with other photonic components can lead to compact and efficient systems.By integrating modulators with other functional elements like couplers, splitters, or multiplexers on a single chip, the overall size of a system can be reduced.Additionally, on-chip integration can minimize optical losses and increase overall efficiencies of a modulator.

Efficient Electrical Packaging
The electrical packaging of a modulator can also ultimately impact its size and performance.Advanced packaging techniques, such as flip-chip bonding or wafer-level packaging, can help minimize the size of a modulator while ensuring efficient electrical connections between the modulator and the driving electronics.This can reduce parasitic capacitance and inductance, resulting in improved modulation efficiency.The packaging will be very important in the development of TFLN modulators.
These strategies, when combined, can help in reducing the size and improving the modulation efficiency of TFLN modulators, enabling their integration into compact and highperformance optical communication systems and photonics applications.It is noteworthy that the realm of TFLN modulators is expansive and encompasses various aspects.Beyond the pivotal challenges associated with achieving compactness and high-speed performance, there are additional considerations to heed, such as fiber coupling and direct current (DC) drifting.Regarding fiber coupling, there are two primary structural configurations utilized in TFLN modulators.The first is vertical coupling, which shows a little bit large coupling loss but can be used for wafer-scale testing.The second is edge coupling, renowned for its ability to achieve remarkably low coupling loss.The crux of successful coupling lies in meticulously matching the mode size between the optical fiber and the waveguide.
Concerning DC drifting, this phenomenon pertains to the gradual shift or drift in the electrical characteristics of the device, including its voltage response and bias point, when a continuous DC voltage is applied.Such drift can lead to alterations in the device's performance and behavior, which are undesirable across various applications.The issue of DC drift in TFLN devices can be attributed to an array of factors, encompassing material properties, impurities, and environmental conditions.To effectively mitigate the challenges posed by DC drifting in TFLN devices, a range of techniques and strategies are employed.These include device structure optimization, process improvement, annealing, material doping, etc. [139] In conclusion, this review has provided a comprehensive overview of recent advancements in compact and high-speed TFLN modulators, highlighting their importance in various photonic applications.The review may serve as a resource for researchers and engineers working in the field of optoelectronics, photonics, and related areas.

Figure 1 .
Figure1.Schematic of different kinds of modulators based on thin-film lithium niobate (TFLN).The center picture is the cross section of the TFLN.In the Mach-Zehnder part, the inset shows a schematic of a TFLN modulator with MZI structure.In the Michelson part, the inset shows a 3D schematic of the TFLN Michelson interferometer (MI) modulator.Adapted with permission.[25]Copyright 2021, Optical Society of America.In the resonator part, the inset shows a schematic of a 1D photonic crystal (PC) resonator cavity TFLN modulator.Adapted under the terms of a CC-BY license.[22]Copyright 2020, The Author(s), published by Springer Nature.In the Z-cut part, the inset shows a schematic of a spiral waveguide Bragg grating Z-cut TFLN modulator.Adapted with permission.[14]Copyright 2023, Optica Publishing Group.In the other parts, the inset shows a schematic of a plasmonic TFLN modulator.Adapted with permission.[24]Copyright 2022, American Chemical Society.
. The high-reflectivity Bragg gratings are placed at the end of the modulation area and serve as reflectors.Compared with the MMI-based loop structure, the Bragg grating is more compact.Based on such an idea, Huang et al. demonstrated a 40 GHz modulation bandwidth with a device size of only 0.06 cm, which shows the great potential of ultra-compact modulators based on the MI structure.

Figure 2 .
Figure 2. V π L versus bandwidth map for a) etched and b) non-etched TFLN Mach-Zehnder interferometer (MZI) type modulators.V π versus modulator length map for c) etched and d) non-etched TFLN MZI type modulators.Reference numbers are marked in the figure, correspondingly.In Figure 2c, the inset figure denotes for V π from 1.5 to 4 V (dashed-line area).

Figure 3 .
Figure 3. Schematic of applied electric field direction for an MZI waveguide a) before and b) after poling.a,b) Adapted with permission.[26]Copyright 2021, Optical Society of America.c) Schematic of the folded TFLN MZI modulator based on waveguide crossing.Adapted under the terms of a CC-BY license.[27]Copyright 2021, The Author(s), published by MDPI.d) Schematic of folded TFLN modulator based on interdigitated T-rails co-planar travelling wave (CPW) electrodes.Adapted with permission.[31]Copyright 2022, IEEE.

Figure 4 .
Figure 4. Schematic of MI modulators based on a) multimode interference (MMI) and b) Bragg grating.a) Adapted with permission.[9]Copyright 2019, Optical Society of America under the terms of the OSA Open Access Publishing Agreement.b) Adapted with permission.[25]Copyright 2021, Optical Society of America.

Figure 5 .
Figure 5. a) Schematic of a heterogeneously integrated TFLN ring modulator.b) Microscope images of directly etched ring modulators.a) Adapted with permission.[11]Copyright 2014, Optical Society of America.b) Adapted with permission under the terms of the OSA Open Access Publishing Agreement.[12]Copyright 2018, Optical Society of America.

Figure 6 .
Figure 6.a) Scanning electron microscope (SEM) image of an integrated TFLN Bragg grating modulator.b) Schematic of the MZI modulator with Bragg grating phase arms.Zoomed-in are an SEM image and simulated mode profile.a,b) Adapted with permission.[112,114]Copyright 2021, IEEE.

Figure 7 .
Figure 7. a) Schematic of a Fabry-Perot (FP) modulator with two distributed Bragg reflectors.b) Schematic of a 2 by 2 FP modulator.Zoomed-in are the details of the adiabatic dual-core taper coupler and asymmetric multimode waveguide grating.a) Adapted with permission.[116]Copyright 2021, Chinese Optics Letters.b) Adapted under the terms of a CC-BY license.[117]Copyright 2022, The Author(s), published by Elsevier.

Figure 8 .
Figure 8. a) Schematic of a 1D PC electro-optic (EO) modulator in X-cut TFLN.b) Full SEM image of the whole device structure.c,d) Zoomed-in images of the 1D PC EO modulator.a-d) Adapted under the terms of a CC-BY license.[22]Copyright 2020, The Author(s), published by Springer Nature.

Figure 9 .
Figure 9. a) Cross section of a Z-cut TFLN modulator with upper and lower electrodes.b) SEM image of a spiral waveguide in Z-cut TFLN.c) Microscope image of an MZI modulator in Z-cut lithium niobate (LN) with spiral arms.d) Schematic of a spiral waveguide Bragg grating modulator on Z-cut LN, and an overview of its structure.a-c) Adapted with permission under the terms of the Optica Open Access Publishing Agreement.[13]Copyright 2022, Optica Publishing Group.d,e) Adapted with permission.[14]Copyright 2023, Optica Publishing Group.

Figure 10 .
Figure 10.a) Schematic of a 2D-metasurface-based EO modulator.b) Schematic image of a plasmonic TFLN EO modulator.c) Schematic of the coupling modulated TFLN modulator.Zoom-ins are the cross section and electrode details.Also shown are when the coupling modulation type modulator works under d) critical coupled and e) zero coupled states.a) Adapted with permission under the terms of the Optica Open Access Publishing Agreement.[135]Copyright 2022, Optica Publishing Group.b) Adapted with permission.[24]Copyright 2022, American Chemical Society.c-e) Adapted with permission under the terms of the Optica Open Access Publishing Agreement.[138]Copyright 2022, Optica Publishing Group.

Table 1 .
Comparison results of different MZI type modulators based on directly etched TLFN.

Table 2 .
Comparison results of different MZI type modulators based on non-etched TLFN.