High-Performance Acousto-Optic Modulators for Improving the Recognition Accuracy of Weak Microwave Signals by Radio Frequency Identification Systems

Abstract

By leveraging the high sensitivity of acousto-optic modulators to microwave signals, their integration into RFID systems can significantly improve the detection of faint microwave signals. Traditional acousto-optic modulators, which use bidirectional uniform comb transducers (IDTs), suffer from a 50% microwave signal loss due to the diffraction efficiency being less than 50%, complicating the accurate identification of already weak signals. This paper presents a solution to the 50% loss issue by replacing the IDT with a floating electrode unidirectional transducer (FEUDT), a key structure in detecting microwave signals. The resulting samples exhibited an optical waveguide insertion loss of 8.73 dB, a mode field diameter of 6.48 $\mathsf{\mu }$m, an operating frequency of 141.2 MHz, and a diffraction efficiency of 61.72%, and required only 0.57 W of driving power. Comparative empirical tests with traditional transducers under identical conditions demonstrated that the FEUDT-based acousto-optic modulator surpassed the 50% diffraction efficiency limit of conventional modulators, which facilitated the detection of weak microwave signals.

1. Introduction

A microwave signal captured by an RFID system and input into a transducer serves as a modulation signal. The transducer converts the modulation signal into a surface acoustic wave and causes changes in the refractive index within the optical waveguide to modulate the optical wave. When the optical wave carrying the information of the modulation signal is demodulated, all the information contained in the original microwave signal can be obtained [1].

With the development of the times, radio frequency identification (RFID) technology plays a crucial role in multiple fields, including the Internet of Things, intelligent systems, automation, and medical monitoring [2]. Society’s requirements for the accuracy of RFID systems are getting higher and higher, necessitating an enhanced ability to detect weak microwave signals. However, the performance limitations of signal receivers in traditional RFID systems often result in the inability to detect weak microwave signals in many application scenarios. To overcome this limitation, the industry standard practice is to integrate amplifiers into the system to enhance the strength of the microwave signals, thereby improving the recognition accuracy of the system. Nevertheless, this approach significantly increases the complexity of the system and may also undermine the accuracy of the recognition signals [3].

As a key component for signal modulation and processing, the acousto-optic modulator can detect tiny changes in the microwave electric field due to its remarkable advantage of being extremely sensitive to changes in microwave signals [4]. This precise physical measurement capability makes the acousto-optic modulator a valuable resource in radio frequency identification (RFID) systems [5]. In such systems, it can enhance the detection of weak microwave electric fields, thereby improving the accuracy of radio frequency identification according to contemporary requirements.

Since L. Kuhn’s pioneering design of a component leveraging surface acoustic waves (SAWs) to modulate optical signals [6], the 1970s saw researchers propose the development of acousto-optic devices that capitalize on the interaction between SAWs and optical waveguides, spurred by advancements in both optical waveguide and SAW technologies [7,8]. As SAW technology has evolved and matured, a proliferation of waveguide-type acousto-optic devices has emerged [9,10,11], leading to the development of numerous sophisticated acousto-optic devices, including modulators [12,13], filters [14,15], and frequency shifters [16,17].

Currently, only a few manufacturers around the world possess the professional skills to design and manufacture acousto-optic devices that meet high standards. In particular, the strict requirements in the design and manufacturing of acousto-optic modulators have brought significant production challenges. A high-performance acousto-optic modulator requires a high operating frequency, minimal driving power, and superior diffraction efficiency. However, the diffraction efficiency of acousto-optic modulators has historically been limited to below 50% and awaits revolutionary progress [18].

In the development and research of acousto-optic modulators, researchers made several significant attempts to address the challenges associated with these devices

Table 1. State of research.

References	Electrode Structure	Drive Power	Diffractive Efficiency
1969 [19]	IDT	1 W	50%
1974 [20]	Bent IDT	1.4 W	50%
1997 [21]	Buried-type IDT	-	40%
2009 [22]	Dual-electrode IDT	-	0–8%

. These advancements illustrate the ongoing efforts to enhance the performance and efficiency of acousto-optic modulators in various applications.

Drawing from the aforementioned studies and additional related research on acousto-optic modulators [23], it is evident that the diffraction efficiency of bidirectional uniform interdigital transducer (IDT)-based acousto-optic modulators (AOMs) is capped at a maximum of 50% [24,25]. Presently, a central research focus and challenge is enhancing the diffraction efficiency of these modulators while concurrently reducing the driving power. This challenge can be tackled through the optimization of the transducer design, choice of metallic materials, process flow, and packaging [26].

The transducer structure, as a core component of the acousto-optic modulator, plays a crucial role in the performance of the device and is the key to reducing the driving power, increasing the operating frequency, and enhancing the diffraction efficiency. Therefore, optimizing the design of this structure is of vital importance for improving the performance of acousto-optic modulators (AOMs) and overcoming the current limitations in terms of efficiency and power consumption.

Based on this, this paper reports an acousto-optic modulator with an FEUDT structure, which was designed to enhance the recognition accuracy of weak microwave signals in radio frequency identification (RFID) systems. The obtained samples broke through the inherent 50% energy loss limitation, which allowed for more microwave signals to be converted into usable surface acoustic waves. As the carrier passes through the waveguide, it produces a more significant diffraction effect, thereby increasing the device’s diffraction efficiency. This research makes the acousto-optic modulator more sensitive to changes in microwave signals, making it more suitable for detecting the weak microwave signals captured by RFID systems, and thus, opening up more application scenarios for RFID systems.

2. Materials and Methods

2.1. Theoretical

The acousto-optic modulator is a precision device mainly composed of three key components: the substrate, the optical waveguide, and the transducer [4]. Due to the piezoelectric effect in lithium niobate materials, upon receiving an electrical signal, the transducer can convert microwaves into surface acoustic waves (SAWs). This process causes elastic deformation of the medium, resulting in periodic changes in the density distribution and the formation of regions with alternating high and low densities. Correspondingly, the refractive index of the medium also experiences periodic fluctuations, effectively forming an optical “phase grating”, whose period is equivalent to the wavelength of the mechanical wave. When an optical wave passes through this medium, it will undergo diffraction, and the intensity, frequency, and direction of the diffracted light is dynamically modulated according to the changes in the wave field.

As the key part of the acousto-optic modulator, the transducer structure plays a vital role in the overall performance of the device. Traditionally, the bidirectional uniform interdigital transducer (IDT) has been a commonly used transducer type in acousto-optic modulators [27]. The surface acoustic waves (SAWs) excited by the IDT propagate evenly in two directions; only the waves propagating in one direction are used for optical modulation.

K. Yamanouchi proposed the floating electrode unidirectional transducer (FEUDT) [28,29,30]. In this design, one of the electrodes in each pair of interdigital electrodes is separated from the bus bar to form a floating electrode. This innovation breaks the symmetry of the interdigital transducer within a half-wavelength range, thus endowing the excited surface acoustic waves with directivity. The floating electrode can reflect the surface acoustic waves because it has a higher acoustic impedance relative to the free surface and its reflection point is different from the excitation point. This design is conducive to the unidirectional propagation of surface acoustic waves, enabling the forward-propagating surface acoustic waves to be enhanced while the reverse-propagating ones are attenuated. The schematic diagram of its basic structure is shown in the attached Figure 1.

Each unit of the cycle is composed of 6 finger electrodes, with short- and open-circuit electrodes forming a distributed reflector. David P. Morgan used quasi-static analysis to provide an FEUDT analysis [31]:

$$\varphi \left(x\right)=[G_e\left(x\right)+G_s\left(x\right)]{\sigma }_e\left(x\right)+G_e\left(x\right){\sigma }_a\left(x\right),$$

(1)

where $\varphi \left(x\right)$ is the surface potential, and the electrostatic Green’s function is represented by

$$G_e\left(x\right)=-{\displaystyle \frac{ln\left|x\right|}{\pi \epsilon }}$$

(2)

The Green’s function for surface acoustic waves (SAWs) is represented by

$$G_s\left(x\right)=j{\Gamma }_{s}exp(-jk|x\left|\right)$$

(3)

$$({\epsilon }_0+{\epsilon }_p^T)\approx {\epsilon }_S(\infty )$$

(4)

$${\Gamma }_s={\displaystyle \frac{\Delta \nu }{\nu \epsilon }}$$

(5)

${\sigma }_e\left(x\right)$ is the electrostatic charge density obtained by using the electrostatic Green’s function with the electrode voltage, and ${\sigma }_a\left(x\right)$ is the acoustic charge density caused by the SAW.

This theoretical analysis proves Yamanouchi’s early prediction of the FEUDT performance characteristics.

Based on the principle of interference, the surface acoustic waves (SAWs) excited by the transducer only achieve maximum intensity when the distance from the center of the acoustic wave excitation is an odd multiple of half the wavelength of the SAW. Consequently, the frequency at which the maximum SAW amplitude is observed is the operating frequency $f_0$ of the transducer, and it adheres to the corresponding relationship formula [32]:

$$f_0={\displaystyle \frac{v}{2p}}$$

(6)

Here, v denotes the wave velocity of the SAW on the surface of the free piezoelectric material. Utilizing an FEUDT (floating electrode unidirectional transducer) enhances the forward propagation of SAWs and diminishes the backward propagation, thus enabling the design of acousto-optic modulators that surpass the diffraction efficiency bottleneck of conventional IDT-based acousto-optic modulators, which are limited to a maximum efficiency of 50%.

2.2. Simulation

Subsequently, this paper proposes a series of simulation and experimental tests to verify the feasibility of the acousto-optic modulator scheme based on the FEUDT electrode structure. Through COMSOL theoretical simulation and micro-nano process research, an $128YX-LiNb{O}_3$ crystal was selected as the initial material for the acousto-optic modulator, with a substrate thickness of 3$\lambda$. The Mach–Zehnder (MZ) optical waveguide was fabricated on the crystal by using the buffer proton exchange process [33]. The floating electrode unidirectional transducer (FEUDT) made of gold (Au) and other parameters identical to those of the control group’s bidirectional uniform comb transducer (IDT) were etched on the waveguide chip through electron beam vacuum deposition. Assuming that the electrode mass load could be neglected, with a film thickness of 0.5 $\mathsf{\mu }$m, the acousto-optic modulator was constructed. The feasibility of this structure was confirmed through model simulation, and a test system was established to evaluate the transducers and modulators. The waveguide insertion loss, mode field, operating frequency, diffraction efficiency, and driving power of the acousto-optic modulator were analyzed.

To ensure that variables other than the electrode configuration were consistent with those of the IDT (bidirectional uniform interdigital transducer) and to mitigate the impact of extraneous factors, the parameters for the FEUDT (floating electrode unidirectional transducer) were carefully matched. Given the structural attributes of the FEUDT, to match the wavelength of the surface acoustic wave excited by the IDT, the center-to-center distance of the FEUDT was established as p = $\lambda$/6 = 4.5, with the finger width a and gap b both set to $\lambda$/6 = 2.25. The finite element method (FEM) is a computational technique employed for solving intricate partial differential equations. In this research, COMSOL Multiphysics was utilized to simulate the FEUDT by employing FEM. To avert acoustic wave reflections from the bottom surface of the piezoelectric substrate during the simulation, a perfectly matched layer (PML) with a thickness of 2$\lambda$/3 was incorporated at the model’s base. The PML progressively absorbs mechanical and electrical disturbances before they reach the outer boundary, thus preventing reflections from disrupting the surface acoustic wave propagation and enabling finite domain simulation within an effectively infinite domain.

According to the above data, the two-dimensional geometric modeling of the FEUDT and IDT was established through a COMSOL finite element analysis.

By examining the periodic nature of the FEUDT, a representative cycle was selected for simulation modeling. The FEUDT electrode comprised six finger electrodes in a periodic array, as depicted in Figure 2a, with the control group IDT shown in Figure 2b. Simulation calculations revealed that the operating frequency of the IDT structure under these parameters was 134.06 MHz, whereas the FEUDT structure operated at 141.45 MHz.

In the FEUDT structure, finger electrode 3 was designated as the terminal voltage for excitation, while finger electrodes 1 and 4 were connected and set with an open-circuit floating voltage to create a short-circuit floating electrode. Similarly, finger electrodes 2 and 5 were set with a floating voltage to form an open-circuit floating electrode. In the absence of external electric fields, their charge densities remained constant. The increased distance between the transduction center and the reflection center of the floating electrode, coupled with the stronger effect of the excitation electrode’s amplitude, significantly amplified the propagation amplitude of the SAW.

The simulation extracted the SAW propagation intensity for both the IDT (bidirectional uniform interdigital transducer) and FEUDT (floating electrode unidirectional transducer), as illustrated in Figure 2c. The particle vibration displacement intensity decayed exponentially within two wavelengths from the surface of the piezoelectric substrate, with the SAW energy concentrated on the surface of the lithium niobate medium. The maximum particle vibration intensity at a distance of 1.5 $\mathsf{\mu }$m from the surface for the FEUDT was $1.93\times {10}^{-14}$ $\mathsf{\mu }$m, while the IDT achieved a maximum intensity of $1.74597\times {10}^{-14}$ $\mathsf{\mu }$m at a distance of 1.6 $\mathsf{\mu }$m from the surface. The particle vibration intensity of the FEUDT exceeded that of the IDT at the same depth, with the energy being more concentrated on the surface of the piezoelectric medium, which reduced the generation of bulk acoustic waves. This was attributed to the greater number of finger electrodes on the FEUDT, which, due to the mass-loading effect, stimulated a more robust surface acoustic wave [34,35].

To highlight and compare the unidirectional propagation performance of the FEUDT, a surface acoustic wave resonator was constructed using both the IDT and FEUDT, as shown in Figure 3a. The resonator, which consisted of 8 periodic units, was positioned at the geometric center of the device to simulate the SAW propagation. The blue region designates the boundary material as the PML. The particle vibration displacement intensity of the SAW for the IDT and FEUDT at their respective center frequencies of 134.06 MHz and 141.45 MHz is displayed in Figure 3b.

The surface acoustic wave (SAW) initiated at the geometric center of the IDT device’s input terminal propagated uniformly in both directions, as depicted in the figure. The average particle vibration intensity was $1.66\times {10}^{-3}$ $\mathsf{\mu }$m when moving horizontally to the left and $1.67\times {10}^{-3}$ $\mathsf{\mu }$m when moving to the right. The particle vibration displacement image exhibits symmetrical characteristics on both sides of the excitation source.

In contrast, the SAWs excited at the geometric center of the FEUDT device’s input terminal demonstrated a pronounced difference in propagation at both ends, as shown in the figure. By analyzing the average particle vibration intensity in both directions before and after the excitation source, it was found that the average vibration intensity of the particles that moved horizontally to the right was 1.87 times that of those that moved to the left. This highlights the FEUDT’s superior unidirectional propagation capability. Consequently, the FEUDT electrode structure was selected for the acousto-optic modulator’s electrode component, which significantly enhanced the energy conversion efficiency; reduced the device’s driving power; improved diffraction efficiency; and thus, effectively doubled the effectiveness while halving the effort.

Theoretical calculations and simulation results both indicate that the floating electrode unidirectional transducer (FEUDT) structure, by leveraging the asymmetry of the electrode structure to alter the reflection characteristics of surface acoustic waves (SAWs) and thereby manifesting unidirectional propagation, effectively enhanced the forward propagation of the SAWs while reducing the energy of its backward propagation. This mechanism could effectively avoid the inherent 50% energy loss associated with traditional bidirectional uniform interdigital transducers (IDTs). As a result, it broke through the traditional diffraction efficiency limit of 50%. Under the same conditions, more microwave signal information can be loaded onto the carrier and subsequently read out.

The above two electrode structures were realized through elaborate manufacturing processes to collect experimental data. High-quality Mach–Zehnder (MZ) waveguides with excellent optical properties were fabricated on $128YX-LiNb{O}_3$ by utilizing the buffer proton exchange technique, which is renowned for its ability to maintain the optical integrity of the lithium niobate substrate.

2.3. Fabrication

This paper details the fabrication of an optical waveguide on a 3-inch, 0.5 mm thick $128YX-LiNb{O}_3$ substrate. The acousto-optic interaction region within the waveguide spans 1.8 cm in length, with a waveguide line width of 5 $\mathsf{\mu }$m. A 3% solution of benzoic acid and lithium benzoate served as the exchange medium. The specific fabrication process is outlined below:

1. Coating: A CJGP-450A high-vacuum three-target magnetron sputtering coating machine was employed. The sample, thoroughly cleaned with acetone and deionized water, was placed in an oven at 100 °C for approximately 8 min. After cooling, the sample was positioned in the coating machine. The ${\mathrm{SiO}}_2$ coating process commenced under a vacuum of less than $3\times {10}^{-4}$ Pa and concluded when the ${\mathrm{SiO}}_2$ film reached a thickness of 100 nm.

2. Photolithography: A URE-2000 photolithography machine was utilized with a positive photoresist applied to the sample surface by spin-coating, followed by baking at 100 °C for 15 min and cooling to room temperature. The masked sample underwent contact exposure for 6 s. The exposed sample was then developed in a solution that contained 1 g NaOH and 200 mL deionized water, with the ultraviolet-exposed photoresist dissolved in 30 s. Subsequently, a ${\mathrm{SiO}}_2$ etchant was applied to the 100 nm ${\mathrm{SiO}}_2$ layer for 70 s, followed by the removal of the remaining photoresist with acetone and thorough cleaning with deionized water.

3. Buffer proton exchange: The photolithographically processed sample was placed in a clean graphite crucible, into which precisely measured benzoic acid and lithium benzoate powder were added. The crucible was sealed and placed in a proton exchange furnace to heat to melting point. The exchange treatment was conducted at 245 °C with a 3% exchange liquid concentration for 5 h, and the sample was retrieved once cooled to room temperature.

4. Annealing: post-buffer proton exchange, the sample was subjected to a high-temperature annealing treatment at 360 °C for 3 h and 30 min, followed by cooling to room temperature before removal.

Upon the completion of the waveguide fabrication, it was inspected under a microscope. The waveguide edge was smooth, and the sample surface was clear and defect-free, which met the criteria for a high-quality waveguide.

5. Photolithography: The electrode mask structure for the acousto-optic modulator was designed using L-Edit 15.0 software. In the design, auxiliary corner patterns were added to the transition areas of the finger edges to mitigate the input impedance and reflection loss of the electrical signal in areas with right-angle turns. A negative photoresist was applied to the sample surface with the prepared optical waveguide using a URE-2000 photolithography machine. Process conditions similar to those used for photolithography of the optical waveguide were employed to etch the transducer electrodes onto the chip surface with the fabricated waveguide.

6. Coating: A QHV-D86 high-vacuum single-gun electron-beam coating machine was used. The lithium niobate wafer samples were first cleaned with acetone and then thoroughly washed with deionized water to ensure cleanliness. The samples were treated in a dryer at 100 °C for 6–8 min and then transferred to the highly cleaned vacuum chamber of the coating machine. A crucible that contained an appropriate amount of Cr particles was placed in the crucible slot. When the vacuum of the coating machine reached $3\times {10}^{-4}$ Pa, the proportional factor was adjusted, the electron gun was preheated for 5 min, the X-deflection current was set to 0.47, the high voltage was turned on, and the beam current was adjusted to 13 to start coating the Cr film. The real-time thickness of the Cr was determined by a thickness meter. If sparks were found in the crucible during the coating process, the beam current was reduced to 12 to ensure the flatness of the new film layer on the sample. The coating was stopped when the thickness reached 50 nm. The proportional factor and metal source were switched, and the source current was adjusted to about 131. When the gold particles began to melt, the coating was started. The coating was stopped when the thickness meter showed 500 nm, and then nitrogen was injected to take out the sample.

7. Stripping: The coated samples were placed in an acetone solution, and the remaining Au was removed by separation to manufacture the acousto-optic modulator. The specific detailed process flow is shown in Figure 4.

The finally prepared acousto-optic modulator sample is shown in Figure 5 below.

3. Results

3.1. Optical Testing

3.1.1. Insertion Loss

We established a complete optical testing platform to conduct comprehensive optical tests on the obtained samples. In the acousto-optic modulator, insertion loss originated from various sources, which primarily included the overall losses composed of a series of factors, such as the optical waveguide loss, end-face reflection loss, and coupling loss. After measuring the overall loss of the device in this step, we proceeded to package the device. All values obtained in subsequent tests took into account the loss situation of this test sample.

Since the surface acoustic wave of the transducer on the acousto-optic modulator chip was bidirectional, each electrode corresponded to left and right optical waveguides, denoted as IDT-L, IDT-R, FEUDT-L, and FEUDT-R, and the insertion loss of each waveguide was tested separately with an input power of 700 $\mathsf{\mu }$W, as shown in

Table 2. Device geometry parameters.

Waveguide Number	Output Power	Insertion Loss	Extinction Ratio
IDT-L	53.64 $\mathsf{\mu }$W	11.16 dB	38.41 dB
IDT-R	62.17 $\mathsf{\mu }$W	10.52 dB	38.57 dB
FEUDT-L	93.06 $\mathsf{\mu }$W	8.76 dB	37.53 dB
FEUDT-R	93.67 $\mathsf{\mu }$W	8.73 dB	38.60 dB

. The average insertion loss of the four optical waveguides was 9.79 dB. The insertion loss of the optical waveguide in the acousto-optic modulator samples fabricated in this paper met the basic standard compared with the current process standard and could be further tested.

3.1.2. Near-Field Mode

The near-field mode of the MZ waveguide acousto-optic modulator, also known as the mode field distribution, is an important parameter to describe the propagation characteristics of light inside the waveguide, which can be used to characterize the propagation characteristics of light. By combining the fiber mode field diameter used in the test, the mode field diameters of the four waveguides were calculated, as shown in Figure 6. Both the insertion loss and mode field test met the basic fabrication requirements of the acousto-optic modulator.

3.2. Comprehensive Testing of Acousto-Optic Modulator Performance

3.2.1. S Parameter Testing of IDT and FEUDT

Before testing, it was necessary to package and solder the chip. The chip was fixed on a carrier and silicone was applied on both ends as sound-absorbing material to absorb sound waves and reduce the standing waves caused by reflections from the ends of the chip. Subsequently, the pins of the gold wire and the left and right finger electrodes were connected to the corresponding circuit boards. The entire device was placed in a drying oven at 60 °C for 1–2 h to allow all the adhesive layers to solidify. After both the IDT and FEUDT electrodes were processed, the SMA RF head and the RF coaxial transmission line of the RG-316 model were connected to the vector network analyzer. The S11 curve is shown in Figure 7. In order to effectively suppress unnecessary signal reflections, a certain method of impedance matching was adopted in the connection between the vector network analyzer and the device. The frequency range of the vector network analyzer was 50 MHz–232 MHz.

Since the IDT had radiation admittance, the strongest response could be observed at the center frequency of 133.2 MHz, with an intensity of about −26.18507 dB. After the impedance matching, the S parameters of the FEUDT electrode were observed using the vector network analyzer, as shown in Figure 7. The operating frequency was 141.2 MHz, significantly higher than the 133.2 MHz of the bidirectional IDT. The response was strong at the center frequency, with a reflection loss of −32.14913 dB, which was also lower than the approximately −26 dB loss of the bidirectional IDT. This indicates that under the same geometric parameters, compared with the bidirectional IDT, the FEUDT had a higher operating frequency and a lower reflection loss, which was in good agreement with the FEM simulation results.

Finally, the measured IDT device operating frequency was compared and analyzed with the frequency obtained from the simulation. As shown in

Table 3. Comparison of operating frequency between simulation and measurement.

Electrode Structure	${\mathit{f}}_0$ Simulation	${\mathit{f}}_0$ Actual Measurement	$\mathit{\Delta }$${\mathit{f}}_0$
IDT	134.06 MHz	133.2 MHz	0.86 MHz
FEUDT	141.45 MHz	141.2 MHz	0.25 MHz

, the actual center operating frequency errors of the IDT and FEUDT were −0.86 MHz and −0.25 MHz, respectively, and thus, exhibited good consistency.

3.2.2. Acousto-Optic Diffraction Efficiency Testing of FEUDT and IDT

The performance of an acousto-optic modulator depends on its diffraction efficiency [36]. In this study, we investigated the performance of the IDT and FEUDT acousto-optic modulators and verified their reliability and feasibility by establishing a testing system and implementing tests. The diffraction efficiency testing system was set up as shown in Figure 8.

The test platform was set up according to the test schematic diagram, using a 1550 nm single-mode fiber laser and a benchtop power meter for measurement. In the absence of a radio frequency (RF) signal source, the output power of the optical waveguide was measured. By repeatedly adjusting, the maximum output power of the waveguide, denoted as P1, was obtained. Under unchanged conditions, when an RF signal was applied, the transducer generated surface acoustic waves under the influence of the RF signal to modulate the optical wave, which resulted in the maximum output optical power of the waveguide under this RF signal modulation, denoted as P2. Therefore, by measuring the change in the output optical power of the waveguide when applying and not applying the RF signal to the modulator, the diffraction efficiency of the sample could be determined.

Since the display value of the test output power meter was not zero when no RF signal was applied, the drive source was used to apply power signals to each of the IDT and FEUDT lead wires. Based on the preliminary test results, the RF signal frequencies applied were 133.2 MHz and 141.2 MHz, and the RF signal power was adjustable between 0 and 5 W.

The test results show that the display value of the power meter only changed after the signal source was applied. After the reading stabilized, it was different from the reading before the signal source was applied, indicating that the signal had been successfully modulated. First, the driver switch was turned on to test the acousto-optic modulator of the IDT structure. The frequency was adjusted to 133.2 MHz, and the drive power was set to 1 W. The input electrical signal power was adjusted, and the change in the optical power meter display value on the other side was observed. The waveguide on the left of the modulated IDT was denoted as IDT-L, and the waveguide on the right was denoted as IDT-R. The waveguide with the left side of the FEUDT modulated was FEUDT-L, and the waveguide with the right side modulated was FEUDT-R. The test results are shown in

Table 4. The parameters of the acousto-optic modulator investigated in this study.

Serial Number	IDT-L	IDT-R	FEUDT-L	FEUDT-R
Drive power	0.84 W	0.84 W	0.57 W	0.57 W
Po-off	53.64 $\mathsf{\mu }$W	70.66 $\mathsf{\mu }$W	92.40 $\mathsf{\mu }$W	88.38 $\mathsf{\mu }$W
Po-on	24.55 $\mathsf{\mu }$W	37.40 $\mathsf{\mu }$W	75.98 $\mathsf{\mu }$W	37.71 $\mathsf{\mu }$W
Diffractive efficiency $\eta$	45.90%	47.07%	17.78%	61.72%

.

According to the experiment, when the input optical power of the IDT-R was 700 $\mathsf{\mu }$W and the RF driver source switch was turned off, the output power measured by the power meter was 70.66 $\mathsf{\mu }$W. By setting the signal frequency to 133.2 MHz and slowly adjusting the driving power to the maximum diffraction efficiency, the reading at the diffraction end decreased from 70.66 $\mathsf{\mu }$W to 37.40 $\mathsf{\mu }$W, with a driving power of 0.84 W. Similarly, using the FEUDT-R as the electrode, the output power decreased from 88.38 $\mathsf{\mu }$W to 37.71 $\mathsf{\mu }$W under a 141.2 MHz RF signal with a driving power of 0.57 W.

4. Discussion

By comparing and analyzing the results of this experiment, it can be concluded that the acousto-optic modulator with the FEUDT as the electrode could effectively reduce the driving power compared with the bidirectional IDT. The diffraction efficiency of the two-side waveguide modulated by the bidirectional IDT and FEUDT showed a significant difference, with a 43.94% higher diffraction efficiency on the right side of the FEUDT. This indicates that the surface acoustic wave excited by the FEUDT has a unidirectional propagation characteristic.

The output optical power with the driving power turned on was lower than that measured with the driving power turned off, indicating that when the acousto-optic switch was turned on, some of the light that should have been normally output was lost due to interaction, which demonstrated that the acousto-optic modulator we made had a diffraction effect on the incident light.

Based on the above experimental data, the output optical power of the IDT-R and FEUDT-R at different driving frequencies were measured multiple times, and the output power and diffraction efficiency were plotted against the driving frequency, as shown in Figure 9.

According to Figure 9, the optimal diffraction efficiencies of the acousto-optic modulator with a working length L of 18000 $\mathsf{\mu }$m were achieved at around 133 MHz and 141 MHz, with values of 47.07% and 61.72%, respectively, which corresponded to the measurements of the S11 parameter and simulation results, indicating that the acousto-optic modulator had a strong response to the surface acoustic wave at the center frequency of IDT and would achieve the highest diffraction efficiency at this frequency.

For the driving power, the acousto-optic modulator we made with the FEUDT as the transducer could be driven as low as 0.57 W, which is significantly lower than the driving power of various acousto-optic modulators listed in

Table 5. Comparison of technical specifications of acousto-optic modulators with different electrode structures.

References	Electrode Structure	Drive Power	Diffractive Efficiency
1969 [19]	IDT	1 W	50%
1974 [20]	Bent IDT	1.4 W	50%
1997 [21]	Buried-type IDT	-	40%
2009 [22]	Dual-electrode IDT	-	0–8%
Design of this article 1	IDT	0.84 W	47.07%
Design of this article 2	FEUDT	0.57 W	61.72%

. Although the finger width of the FEUDT was smaller than that of the bidirectional IDT, the driving power of the FEUDT (0.57 W) was significantly lower than that of the bidirectional IDT (0.84 W), indicating that the FEUDT had a stronger suppression effect on the sideband levels. This was because the floating electrode structure in the FEUDT broke the structural symmetry of the bidirectional IDT within a half-wavelength, and the acoustic impedance of the floating electrode was greater than that of the free material surface. The excited acoustic surface wave was reflected by the floating electrode, which made the amplitude of the forward-propagating acoustic surface wave greater than that of the backward-propagating acoustic surface wave, and more energy was transmitted forward, which reduced the driving power of the modulator. This proved that using the FEUDT as the IDT of the acousto-optic modulator could effectively reduce the driving power of the device.

In addition, the diffraction efficiencies of the different acousto-optic modulators were compared. The acousto-optic modulator using the FEUDT (floating electrode unidirectional transducer) as the electrode had a diffraction efficiency of 61.72%, which is superior to the data given in Table 5 and 14.65% higher than that of the IDT (bidirectional uniform interdigital transducer) with the same period. The modulation efficiency of 61.72% indicates that the acousto-optic modulator that employed the FEUDT structure had successfully surpassed the traditional limitation of a maximum modulation efficiency of 50% when using the IDT structure as the transducer. The test results demonstrate that employing the FEUDT structure can effectively enhance the diffraction efficiency of the acousto-optic modulator.

5. Conclusions

The samples successfully prepared in this study had an optical waveguide insertion loss of 8.73 dB, a mode field diameter of 6.48 $\mathsf{\mu }$m, an operating frequency of 141.2 MHz, and a diffraction efficiency of 61.72%, and required only 0.57 W of driving power. Among these, the diffraction efficiency of 61.72% was the key performance indicator that this study focused on. A high diffraction efficiency means that more microwave signals can effectively modulate light and transmit information, which enhances the sensitivity of the acousto-optic modulator in detecting weak microwave electric fields and significantly improves the recognition accuracy of the RFID system. Moreover, since there is less loss of optical signals during transmission, the efficiency and intensity of the signal transmission in the RFID system are also increased, enabling the RFID system to identify signals more accurately when reading tags, reducing the occurrences of misreads and missed reads, and further strengthening the accuracy and efficiency of the system. In addition, the acousto-optic modulator provided in this paper, due to its characteristics of high diffraction efficiency and low driving power, can provide a more stable optical signal output and enhance the performance and reliability of the RFID system in complex environments.