1. Introduction

Acousto-optic devices are a large class of new devices that are developing rapidly. They can rapidly and effectively control various properties of the laser, such as frequency, intensity and propagation direction^{[1, 2]}. They can also rapidly complete transfer and conversion among electric, acoustic and light information to achieve automatic frequency selection, spectroscopy and scanning of the light beam. Nowadays they are widely used in mobile communication, radar, satellite and remote sensing and control systems^[3]. An acousto-optic frequency shifter (AOFS) is an acousto-optic device to modulate laser frequency^{[4, 5]}. It can be divided into Raman-type and Bragg-type. Most of the reported Raman AOFSs are developed based on bulk wave, while devices based on SAW have been rarely reported. The combination of acousto-optic devices with SAW technology has many advantages, such as small size, light weight, and easy integration^[6-8].

In this paper, an SAW Raman AOFS is presented, which consists of a double-ended SAW resonator and an optical waveguide system. The structure parameters of the SAW resonator are optimized by simulation. The optical waveguide system, including a single-mode rectangular waveguide, tapered optical waveguide and waveguide reflector, is designed. The principle prototype is fabricated and detected. The frequency shift of 78 MHz related to the 1st-order diffracted beams and 156 MHz related to the 2nd-order diffracted beams are obtained respectively.

2. Principles

According to the characteristic of the Raman-Nath diffraction, the incident beam must be perpendicular to the propagation direction of SAW. The zero-order diffracted beam doesn't change propagation direction while the $\pm$$m$th-order diffracted beams distribute symmetrically at both sides of the zero-order diffracted beam^[9]. Taking $\theta _m$ and $f_m$ as the diffraction angle and the frequency of $m$th-order diffracted beams respectively, they can be expressed as follows^[10]:

$\begin{equation} \begin{split} \sin \theta _m =\pm m\frac{k_{\rm a} }{k_{\rm i} }=\pm m\frac{\lambda _{\rm i} }{\lambda _{\rm a} }, \quad {}& {m=0, \pm 1, \pm 2, \cdots }, \\& {k=2\pi/\lambda }, \end{split} \end{equation}$

(1)

$\begin{equation} f_m =f_{\rm i} +mf_{\rm a}, \quad {m=0, \pm 1, \pm 2, \cdots}. \end{equation}$

(2)

In Eq. (1), $k_{\rm a}$ and $k_{\rm i}$ are acoustic wave vector and incident light wave vector respectively. $\lambda _{\rm a}$ and $\lambda _{\rm i}$ are acoustic wavelength and incident light wavelength respectively. In Eq. (2), $f_{\rm i}$ and $f_{\rm a}$ are the frequency of incident light and acoustic wave respectively. Equation (2) shows that the frequency of the diffracted light is shifted $m$ times of the acoustic wave frequency. When the $\pm$$m$th-order diffracted beams are superimposed to generate optical beat signals, the optical beat frequency will be $2mf_{\rm a}$. Direct detection of the optical beat signal can determine whether the Raman AOFS works and get the frequency shift of the diffracted light.

The structure of the SAW Raman AOFS is shown in Fig. 1. It mainly consists of a double-ended SAW resonator and an optical waveguide system. The double-ended SAW resonator consists of two interdigital transducers (IDT) and two groups of reflector gratings deposited on quartz substrate. The optical waveguide system includes a tapered optical waveguide, single-mode rectangular waveguides and the waveguide reflector. The incident beam is guided perpendicularly along the propagation direction of SAW into the acoustic field generated by the SAW resonator. The Raman-Nath acousto-optic diffraction occurs in the region of the tapered waveguide^[4]. To facilitate the detection of the AOFS, two rectangular waveguides are designed at the end of the tapered waveguide. Thus the $\pm$$m$th-order diffracted beams can be extracted respectively and superimposed to generate optical beat signals at the output. The optical beat signal is detected by the photoelectric detector and the optical beat frequency can be obtained. The frequency shift of the AOFS can be translated by converting the optical beat frequency.

3. Designs

3.1 Design of the SAW resonator

The Klein-Cook parameter $Q_{\rm C}$ is used to distinguish the two acousto-optic interaction regimes and it can be expressed as^[11]:

$\begin{equation} Q_{\rm C} =2\pi W_{\rm I} \frac{\lambda _{\rm i} }{n\lambda _{\rm a}^2}, \end{equation}$

(3)

where $n$ is the reflection index of medium, and $W_{\rm I}$ is the acousto-optic interaction length. When the acoustic wave is excited by the SAW resonator, $W_{\rm I}$ is the IDT aperture and $\lambda _{\rm a}$ is equivalent to the IDT period. The requirement of the Raman-Nath diffraction is $Q_{\rm C} <1$^[12]. The incident light wavelength $\lambda _{\rm i}$ is determined as 808 nm by the semiconductor laser. The reflection index $n$ of medium is 1.67 determined by the SiO$_{2}$ waveguide. To meet the requirement of the Raman-Nath diffraction, the IDT period and aperture are designed to be 40 $\mu$m and 500 $\mu$m respectively.

Using COM software provided by Professor Hashimoto in Chiba University, $S_{12}$ curve of the SAW resonator can be simulated and the structure parameters can be optimized^[13]. Figure 2 shows the simulated relationships of the insertion loss and the $Q$ value with the IDT pair number. Considering the insertion loss, the $Q$ value and the overall size of the device, the IDT pair number is designed to be 100. A series of $S_{12}$ curves are simulated when the other structure parameters vary, including the Al electrode thickness, the grating period, the IDT gap and the gap between IDT and grating, as shown in Figs. 3-6 respectively. Comparing the $S_{12}$ curves, the structure parameters of the SAW resonator can be determined in Table 1 by minimizing the insertion loss. The simulated center frequency, $Q$ value and the insertion loss are 78.028 MHz, 10428 and -6.153 dB respectively.

3.2 Design of the optical waveguide system

The incident beam guided by the single-mode rectangular optical waveguide propagates into the SAW resonator and the acousto-optic interaction occurs in the resonant cavity. According to the characteristics of the Raman-Nath diffraction, the diffracted beams will be symmetrical at both sides of the incident axial^[14]. Thus the waveguide in the acousto-optic interaction area should be tapered, as shown in Fig. 7. Because of the extremely weak intensity, the $\pm$$m$th-order ($m \geqslant$ 3) diffracted beams are ignored in this paper. So the cone angle of the tapered waveguide is designed as the diffraction angle of $+2$nd-order diffracted beams. The $\theta _2 =2.3^\circ$ can be obtained according to Eq. (1). To separate the diffracted beams from each other for easy extraction, the length of the tapered optical waveguide is designed to be 1000 $\mu$m and the separation between the spot centers of different order diffracted beams is about 20 $\mu$m. In this case, the $\pm$$m$th-order diffracted beams can be easily extracted by the two single-mode rectangular waveguides designed at the end of the tapered waveguide.

To avoid unexpected diffracted beams propagating into the single-mode rectangular waveguide, the waveguide reflector is designed between the two single-mode rectangular waveguides at the end of the tapered waveguide, as shown in Fig. 7(b). A magnetron sputtering Al thin film is grown in the triangular shaded region to cover the step. The interface between the step profile and the Al thin film works as the reflector. For the extraction of the $\pm$1st-order diffracted beams, the 0-order diffracted beam is reflected by the waveguide reflector while the $\pm$2-order diffracted beams propagate directly out the waveguide, as shown in Fig. 7(a). For the extraction of the $\pm$2nd-order diffracted beams, only the $\pm$2nd-order diffracted beams can enter into the single-mode rectangular waveguide while the 0-order and $\pm$1st-order diffracted beams are reflected by the reflector.

The single-mode rectangular waveguide is formed by SiO$_{2}$ films with different reflection indexes^[15]. By using an effective index method, the thickness and the width of the waveguide core layer are designed as 0.8 $\mu$m and 2 $\mu$m respectively. To make sure that the light in the waveguide has the same polarization state, an Al film with a thickness of 0.4 $\mu$m and length of 400 $\mu$m is designed to coat the upper surface of the core layer to form the $E_{00}^y$ mode polarizer according to the large difference transmission loss between the different polarization modes^[16].

4. Fabrications

The fabrication process of the SAW Raman AOFS is presented as shown in Fig. 8. Firstly, a 0.8 $\mu$m-thickness Al film is deposited on a 500 $\mu$m-thickness ST-cut quartz wafer by evaporation coating method. Then the IDTs are fabricated by wet etching with buffered phosphate solution, as shown in Fig. 8(a). Secondly, 1 $\mu$m-thickness SiO$_{2}$ film with a reflection index of 1.65 and another 0.8 $\mu$m-thickness SiO$_{2}$ film with a reflection index of 1.67 are deposited as the waveguide substrate and the waveguide core respectively by using the plasma-enhanced chemical vapor deposition (PECVD) method, as shown in Fig. 8(b). Thirdly, by using the lift-off process, an Al film is deposited and patterned as the masks for dry etching in the next step, as shown in Fig. 8(c). Fourthly, a 0.8 $\mu$m-deep reactive ion etching (RIE) is applied to fabricate the SiO$_{2}$ waveguide core layer. Most of the Al masks are removed by wet etching with phosphate buffer solution, but some of them are retained as the polarizer, as shown in Fig. 8(d). Fifthly, the waveguide reflector is fabricated by using the lift-off process, as shown in Fig. 8(e). Sixthly, 1 $\mu$m-thickness SiO$_{2}$ thin film with a reflection index of 1.65 is deposited as the waveguide cladding using PECVD and the RIE process is applied to expose the electrodes, as shown in Fig. 8(f). The SEM picture of the chip package is shown in Fig. 9. The overall size of the device is 30 $\times$ 15 $\times$ 0.2 mm$^{3}$ and the quality is 238.59 mg.

5. Experiments

The experiment system, as shown in Figs. 10 and 11, is built to detect the performance of the SAW Raman AOFS. The laser beams are focused by the telephoto lens and coupled into the waveguide of the SAW Raman AOFS. The SAW is excited when the AOFS is driven and the acousto-optic interaction occurs in the resonant cavity. The $\pm m$th-order diffracted beams are superimposed to generate optical beat signals and output. The output beams are collimated by the short-focus lens and focused by the telephoto lens. The optical beat signals are converted to electric signals by the photoelectric detector, and finally the electric signals are detected by the spectrum analyzer.

6. Results

The SAW resonator is detected firstly with a vector network analyzer before the detection of the AOFS, as shown in Fig. 12. The detected results show that the center frequency of the SAW resonator is 78.3480 MHz, insert loss is -11.273 dB and the $Q$ value is 6465.7. Compared with the simulation results, the insertion loss significantly increases and the $Q$ value significantly reduces. The measured value of the center frequency is close to the design value and the relative error is 0.4%. The insertion loss tested is approximately twice that of the design value and the relative error is 61%. The relative error of the $Q$ value is 45%. This is mainly because of material defects and processing errors. These impact factors on the performance have been fully taken into account when we design the device. The insertion loss of the SAW resonator is designed to be sufficiently small and the $Q$ value is designed to be large enough. Therefore, the SAW resonator fabricated can meet the performance requirements of practical use.

The output signals of the photoelectric detector are shown in Figs. 13 and 14. Figure 13 is the electric signal converted from the optical beat signal superimposed by the $\pm$1st-order diffracted beams. The peak frequency is 156.4402 MHz. Figure 14 is the electric signal converted from the optical beat signal superimposed by the $\pm$2nd-order diffracted beams. The peak frequency is 312.4612 MHz. The results show that the SAW Raman AOFS works normally and the optical beat signals superimposed by $\pm$1st-order and $\pm$2nd-order diffracted beams can be extracted. So the design of the AOFS is feasible. When the drive power is 100 MW, the frequency shift related to the 1st-order diffracted beams is 78.2201 MHz and the relative error with the theoretical value is 0.1635%, as shown in Table 1. The frequency shift related to the 2nd-order diffracted beams is 156.2306 MHz and the relative error with the theoretical value is 0.2979%, as shown in Table 1. The maximum diffraction efficiency tested of the AOFS is 3.02%.

Compared with the reported Raman AOFS based on bulk wave, this Raman AOFS is developed utilizing the SAW technology and has advantages of smaller size, lighter weight and lower driving power. It provides some potential applications in the fields of integrated optics, laser communication, optic gyroscope, and so on.

7. Conclusions

In this paper, an SAW Raman AOFS is developed. It is constituted by a double-ended SAW resonator and optical waveguide system. The structure parameters of the SAW resonator are optimized using COM software. This design method includes the effect of the reflection of SAW, so it can simulate the performance of the SAW device more accurately. The optical waveguide system includes a single-mode rectangular waveguide, tapered optical waveguide and waveguide reflector. The tapered optical waveguide is designed according to the characteristic of Raman-Nath diffraction. The single-mode rectangular waveguide is designed using the effective index method. The principle prototype of the AOFS is fabricated using MEMS processing technology and the optical beat signals superimposed by $\pm$1st-order and $\pm$2nd-order diffracted beams are detected respectively. The frequency shift related to the 1st-order diffracted beams is 78.2201 MHz and the relative error with the theoretical value is 0.1635%. The frequency shift related to the 2nd-order diffracted beams is 156.2306 MHz and the relative error with the theoretical value is 0.2979%. The tested data shows that the error of the SAW AOFS designed is relatively small compared to the theoretical value. Thereby the design of the AOFS is feasible and this paper provides a reference for the development of the SAW Raman-type acousto-optic device.