1-D ELECTRO-OPTIC BEAM STEERING DEVICE

Abstract

In this paper, we present the design and fabrication of a 1D beam steering device based on planar electro-optic thermal-plastic prisms and a collimator lens array. With the elimination of moving parts, the proposed device is able to overcome the mechanical limitations of present scanning devices, such as fatigue and low operating frequency, while maintaining a small system footprint (~0.5mm×0.5mm). From experimental data, our prototype device is able to achieve a maximum deflection angle of 5.6° for a single stage prism design and 29.2° for a cascaded three prisms stage design. The lens array shows a 4µm collimated beam diameter.

1. INTRODUCTION

Optical switches are used in communication networks to direct optical signals from the transmitter to the desired receiver to complete data transfers. Conventional optical switches rely on beam steering mechanisms to guide a light beam in a desired path. Current beam steering mechanisms found in optical switches consist of both mechanical and non-mechanical systems. Mechanical systems are commonly utilize mechanisms driven by various physical effects, such as electrostatic[1], electromagnetic[2],, or piezoelectric effect[3]. However, these mechanisms generally suffer from problems related to the motion of physical components, such as mechanical fatigue, stiction, hysteresis, and low operating frequencies. Non-mechanical (i.e. acousto-optic, thermal optic and electro-optical) systems avoid the issues that plague mechanical systems, but each presents their own limitations: acousto-optic driven systems require a large driving power in order to create good efficiency on the diffraction effects produced by acoustic waves [4]; thermo-optic systems have slow switching times, thermally induced stress during operations[5], and operations susceptible to ambient temperature fluctuations. Thus, electro-optic-based systems are more prominent for beam steering inside optical switches, providing high speed beam steering and with less susceptibility to environmental noise and thermally-induced stresses, providing the fast and accurate switching crucial in modern communication networks. Several electro-optic devices for beam steering have been reported by using bulk crystals [6][7][8][9], yet these devices tend to have large scales, and thus require relatively high driving voltages.

To overcome the high driving voltage requirement, thin-film polymer electro-optic devices are used. Thin-film polymer based devices allow the reduction of system size, thereby reducing the operating voltage. Moreover, thin-film polymers are compatible with current micro-machining processes, making them relatively easy to fabricate. However, these polymers produce a relatively small deflection angle compared to bulk devices due to the inherently small change in the refractive index when a voltage is applied [10].

In this paper, we propose an electro-optic beam steering system based on thin-film polymers. Using a single or three cascaded deflector stage and a micro-machined collimator, the system is capable of producing a relatively large angle of deflection. A confined output beam profile at a relatively low operating voltage (compared to current bulk crystal based devices) is also presented [7]. To achieve the specifications, we investigated different materials and device geometries in order to optimize the deflection. We present the design concept and the fabrication of the device, and discuss the deflection result of the tested device.

2 .DESIGN

The proposed electro-optic scanner consists of two primary stages, a beam collimating stage and a deflecting stage. The beam collimating stage is designed to collimate and reduce the beam size from the input light source. The collimated light beam then enters the deflecting stage where it is then deflected according to the voltage applied.

2.1 Collimating Stage

Beam collimation and spot size reduction is crucial in beam steering applications. For this device, a ray-tracing program was used to model various lens combinations and placements to help collimate and reduce the diverging beam from the input fiber. The resulting lens array (Figure 3) can be broken down to two stages: collimation and beam-reduction. The light collimation is achieved by the convex-concave lens combination (Lens 1 and 2), while the beam reduction is provided by the two cascaded double convex lenses (Lens 3 and 4).

Figure 3.

Beam path inside prism

2.2 Deflecting Stage

The deflecting stage is made of a free-standing triangular-shaped electro-optic device. Our proposed design makes use of a free-standing prism based on thermal-plastic material that exhibits an electro-optic (EO) effect.

The EO effect is the index of refraction change in the material under an applied electric field [8]. When light passes through two mediums with mismatched refractive indices, it will have a different exit angle than its entry angle, in accordance with Snell’s Law:

$${\mathrm{n}}_1\text{ sin }{\mathrm{\theta }}_1={\mathrm{n}}_2\text{ sin }{\mathrm{\theta }}_2$$ (1)

where n₁ and n₂ are the index of refraction of the two mediums, and θ₁ and θ₂ are the incident angle and the exit angle of the beam, respectively. A larger refractive angle can be achieved if a light beam is incident within the critical angle from a medium with a much higher refractive index to a lower one or vice versa. To maximize this effect, we choose a baseline index of the EO polymer (AJTB141/APC) [11] that has a relatively high refractive index (1.60) compared to air (1.0) as the material for the deflection stage. Furthermore, by optimizing the relative position and incident angle of the input beam and the prism geometry, a larger deflection angle due to a relatively small refractive index change in the EO polymer can be achieved.

For scanning or beam deflecting applications, the deflection angle needs to be controlled. Rewriting Equation (1), the output angle can be expressed as a function of the ratio between the index of refraction of the two mediums:

$$\text{sin }{\mathrm{\theta }}_2=({\mathrm{n}}_1/{\mathrm{n}}_2)\text{ sin }{\mathrm{\theta }}_1$$ (2)

Thus, being able to control the index allows us to control the exit angle. The electro-optical property of the EO polymer is governed by:

$$\mathrm{\Delta }\mathrm{n}=\frac{1}{2}{n}_p^3{r}_{31}{E}_z$$ (3)

where r₃₁ is EO coefficient for beam incident (direction ‘1’) perpendicular to the applied electric field direction. E_z is the applied electric field along z-direction where is the same as subscript “3” in EO coefficient. r₃₁ of the AJTB141/APC polymer is found to be 45pm/v at wavelength equals 1.3µm, a relatively good value for an EO polymer. This effect allows us to vary the index of refraction when a voltage is applied across the material via the electrodes (shown in Figure 2). The effect of index change on the beam path (red line) can be observed in Figure 2. Equation 2 can be applied to the 2 side walls of the prism shape where incident beam enters and exits the EO polymer (the effect is shown in Figure 3). The following sets of equations are derived to predict the output angle of the beam, based on the input angle of the beam for a constant index of refraction for a single prism.

$${\mathrm{\varphi }}_1={\text{sin}}^{-1}{\mathrm{n}}_{\text{in}}\text{ sin }{\mathrm{\varphi }}_{\text{in}}/{\mathrm{n}}_{\text{prism}}$$ (4)

$${\mathrm{\varphi }}_2=\alpha -{\mathrm{\varphi }}_1$$ (5)

$${\mathrm{\varphi }}_3={\text{sin}}^{-1}{\mathrm{n}}_{\text{prism}}\text{ sin }{\mathrm{\varphi }}_2/{\mathrm{n}}_{\text{out}}$$ (6)

$${\mathrm{\varphi }}_{\text{out}}=\mathit{\text{abs }}(\alpha -{\mathrm{\varphi }}_3)$$ (7)

Figure 2.

Prism deflector beam deflection

To amplify the deflection angle, two more cascading stages of prism are added to form a three-prism array (Figure 4). The same set of equations (4 to 7) applies to the cascading stages, and the three prisms can be combined by using the of the ϕ_out first two prisms as the ϕ_in of the subsequent prism. These sets of equations are calculated twice, once for the EO polymer under normal conditions and once more when there is an applied voltage, to calculate the difference in the output angles (Δϕ) between the two states. Finally, Δϕ is optimized by adjusting the other parameters in the sets of equations. The parameters are optimized by an exhaustive parametric study using MATLAB, the algorithm applies a given voltage (100V) and substitutes all possible values for the parameters ϕ_in, and α_n for all three prisms in a cascading configuration to search for the maximum value of Δϕ. Once the maximum Δϕ is found, the algorithm returns those parameters and the optimal prism design is obtained. These parameters are incorporated in our design of the prism array. The resulting parameters obtained from the algorithm are: α₁= 72.7°, α₂= 27.6°, α₃= 5.9°, ϕ_in=66.1°, Δϕ = 42.81°. The calculation as mentioned before is based on an applied voltage 100V, here film thickness is 4.7 µm, and R₃₁= 45 pm/V. If film thickness is larger, the required input voltage to achieve the deflection angle will be lower. This is also true if R₃₁ value is higher. The optimally designed collimator and prism are then placed according to the angle ϕ_in to form the EO beam steering device. The arrangement allows the deflection of a collimated and reduced size beam, with electronically controlled scanning. Additionally, with the use of thermal-plastic material in the design, the deflection stage can be fabricated using a simple hot embossing molding process [12]. The steps for the device fabrication are discussed in Section 3.

Figure 4.

Cascading three-prism array amplifying output deflection angle. The incident angle and base angle for the prism must be chosen carefully to optimize the output deflection angle.

3. FABRICATION

The fabrication of the device is divided into two major parts. The first part is the fabrication of the collimator lens array and the fiber groove using dry etching and solvent assisted micro molding (SAMIM) techniques. The second part is the formation of the free-standing prism using hot-embossing molding technique.

3.1 Collimator Lens Array and Fiber Groove

For the collimating stage of the device, we first create a fiber groove that allows light coupling from an optical fiber into the collimator lens array. The fiber groove is created on a silicon substrate using a Deep Reactive Ion Etching (DRIE) process. As shown in Figure 5a, the silicon substrate is etched down ~63µm for the core of a single mode optical fiber (~9µm)to be aligned with the surface of the substrate, where the collimator lens array will be sit.

Figure 5.

Visualization of fiber groove and collimator lens array fabrication: (a) DRIE of fiber groove, (b) spin-coating of SU-8, (c)PDMS mold bonding for SAMIM process, (d)Apply shadow mask for photolithography, (e) development of unexposed SU-8, (f) fitting of optical fiber in fiber groove.

After the fiber groove is etched, we begin to fabricate the lens array on the surface of the silicon substrate. A 5µm-layer of SU-8 5 photoresist is spin-coated on top of the substrate at 3000 rpm for 30 seconds (Figure 5b). The SU-8 is then baked at 95°C for 1 minute and 30 seconds. After baking, a PDMS mold with the lens pattern is aligned with the fiber groove (Figure 5c), and ethanol is applied for SAMIM to obtain the shape of the lens array on the SU-8. After mold is removed, a shadow mask is placed on top of the lens array (Figure 5d), and the structure is exposed to UV light to cross link the SU-8 in the lens array area and remove excess SU8 outside the lens area. The exposed substrate is then developed (Figure 5e), so that only the lens array remains on the substrate. Finally, an optical fiber (SMF-28 with 10µm core and 125µm diameter) is placed in the fiber groove and aligned with the lens array (Figure 5f).

3.2 Deflecting Prism

To create the free-standing prism structure for the deflecting stage, we employ the hot embossing molding process to for the thermal-plastic EO material.

A silicon (Si) mold is first fabricated for the hot embossing procedure. In our device, the beam impinges from the side of theprism, thus the surface quality of the sidewalls is important and it must be vertical to achieve best optical performance. Therefore, anisotropic etching is used to create the hi-aspect ratio structure. The standard batch process for Deep Reactive Ion Etching (DRIE) is shown in Figure 6, the process itself is a series of repeating loops, and each loop contains two steps: depositing and etching. To create the desired depth for our mold, the process was run for 40 loops. It can be observed (Figure 6g) that the iterations of the etching process results in scalloped sidewalls, though our application requires smooth sidewalls for the best performance. Therefore, additional processing steps are required to produce smoother sidewalls.

Figure 6.

Stand botch process in DRIE of the silicon mold

To smooth out the sidewall, the scallop structures are thermally oxidized to form SiO₂ inside an oxidation furnace (parameters shown in Table 1). The SiO₂ is then removed by dipping the mold into a 49% HF solution for several seconds. This process take advantage of the faster growth rate of thermal silicon oxide on the tips of the scallop structure compared to the bases, thus during the removal of SiO₂, a higher proportion of the scalloped edges are removed compared to the bases, gradually producing a smoother sidewall. The geometry change of the sidewall can be visualized in Figure 7.

Table 1.

Oxide Furnace Parameter

	Temperature (°C)	Hold time (min)	Ramp rate(°C /min)	Target Temperature	O2
Temperature ramp up	20	XXX	10	1100°C	close
Dwell	1100°C	140	XXX	XXX	10 slpm
Temperature ramp down	1100°C	XXX	10	20	close

Figure 7.

Concept of reducing roughness of sidewalls.

The oxide-growth process is then repeated with a higher furnace dwelling time (280 minutes). When desired roughness is observed on the mold under the microscope, the mold is ready for the hot-embossing process.

A 200nm gold layer is sputtered on top of a silicon (Si) substrate to form the bottom electrode of the prism deflector. Then, the polymer solution is prepared and drop-casted onto the substrate. The substrate is then placed into vacuum oven with vacuum switched on for 12 hours at room temperature, and then the temperature is raised to 85°C for the next 12 hours. Once the polymer is prepared, we begin the molding process. First, the surface of the mold is then treated with (Heptadecafluoro -1,1,2,2-Tetrahydrodecyl) Trichlorosilane from Gelest Inc. to prevent the EO polymer adhering to the mold after the molding process, and then it is placed on top of the polymer-coated substrate (Figure 8a). Next, it is put into the hot-embossing machine, where the molding process is taking place. The upper stage and the lower stage of hot embossing machine is first heated up to above the glass transition temperature of the polymer (about 150°C in our experiment), and then they are brought into contact with the silicon mold and the polymer substrate (Figure 8b). Approximately 11MPa pressure is applied, and it is hold for roughly 1 to 2 hours to allow the polymer to conform to the silicon mold (Figure 8c). Finally, we bring the mold and substrate to room temperature and release the pressure. The mold pattern is replicated on the polymer (Figure 8d). After the pattern is got, a gold layer is then deposited on top of the molded polymer to serve as the top electrode for the deflector stage.

Figure 8.

Hot-Embossing Molding Process: (a) place fabricated mold, (b) Apply pressure and heat in the embosser, (c)Embossing by holding the heat and pressure, (d) removal of mold.

The device then undergoes a poling process that will activate the electro-optic property of the EO polymer by inducing a preferred orientation to the EO chromophore inside the polymer using high voltages. During the process, the device is placed inside a 80°C vacuum oven overnight to reduce its moisture, as moisture can cause damage to the device when poling. Then, the device is placed on hot plate and raised to 130°C at 5°C/min, which is close to the polymer’s glass transition temperature (T_g). And then, a variable voltage from 0–600V is applied across the top and bottom electrode of the device. The current across the device is monitored while we raise the voltage in a step profile the to ensure the device is not damaged during the process. The device is placed at maximum voltage for 20 minutes and then it is bought down to room temperature rapidly to ensure the chromophore remains in the induced orientation. Finally, voltage is removed from the device and the poling process is completed.

4. RESULTS AND DISCUSSION

Figure 9 shows a Scanning Electron Microscope (SEM) picture of collimator lens arrays aligned with the fiber groove; showing that the SAMIM process has successfully produced the lens array pattern aligned to the fiber groove. However, as seen from the picture, the height of the lens are not very tall (approximately 4µm), due to the limitations of the SAMIM process. The short lenses result in a vertical loss of light, and cropping the beam profile into an oval (as shown in Figure 9).

Figure 9.

SEM picture of the micro-fabricated collimating lens and fiber groove

In Figure 10, we show the image of the beam output observed with a CCD camera (1st vision Inc., 1280×1024 resolution) at 15cm (Figure 10a), 20cm (Figure 10b), and 25cm (Figure 10c) from the collimator output. The device has a 4 µm collimated beam diameter and a diverging angle of less than 1° 25 cm from the lens array.

Figure 10.

Collimator lens beam output profile at observed at different distances (a) 15cm, (b) 20cm, (c)25cm. The result shows beam diverges less than 1o and maintain 4µm beam profile up to 25cm distance

Figure 11 shows the surface of the silicon mold under SEM after DRIE (Figure 11a), and after the smoothing process (Figure 11b). Undesirable scalloped structures can be observed on the sidewall of the mold, which will be replicated on the final device if uncorrected. To reduce the sidewall roughness, thermal oxide is grown and etched away after the DRIE process. The effectiveness of the thermal oxide growth and etching process can be seen in Figure 11b, where the sidewall is visibly smoother by SEM observation than the to post-DRIE mold (Figure 11a).

Figure 11.

Sidewall surface condition after (a) DRIE, (b) Oxide growth-etch process

Figure 12 is an SEM picture of part of a deflecting prism formed by the hot-embossing technique with the smoothed silicon mold. Figure 12a shows that the prism structure is successfully replicated on the EO polymer material with a vertical sidewall. Figure 12b shows the roughness of the sidewall is minimal, allowing the prism deflector to have greater optical performance. We can conclude that the hot-embossing technique can successfully create the prism structure with desirable results for our device. Figure 12c shows a photograph of the cascaded three prism deflector stage.

Figure 12.

SEM pictures of the hot embossed sidewall of prism deflector: (a) overview, (b) zoom in view

Finally, the optical performance and the functionality of the device are tested. We applied different voltages to the device to vary deflections of the output light beam. The EO device is secured on a high precision X-Y-Z moving stage (Thorlab, 0.1 µm step resolution), and connected to a DC amplifier and a power supply (Agilent E3630A). A 40 mW 1310 nm light source (Lucent, analog laser 2000L) is used as input light. An objective lens is placed near the edge of the EO device where light beam exits, to magnify the output beam, and the image is captured using a CCD camera (100× magnification) with an IR adaptor. The results of the output deflected beam with different applied voltages are shown in Figure 13. Testing the system shows a deflection angle of 5.6° at 300V with an incident light angle of 70°. For the cascaded three prisms deflector, a deflection angle of 29.2° at 60V with the same incident angle is observed (Figure 14).

Figure 13.

(a) Images of the deflected optical beam resulting from different applied voltages for a single stage deflector, (b) deflection angles as a function of applied voltage. Theory (dotted) Experiment (circle).

Figure 14.

Images of the output deflected optical beam from the three prism stage deflector.

5. CONCLUSION

In this paper, we have presented the design and fabrication of a 1D Electro-optic polymer based beam steering device. The device is able to overcome the limitation of existing mechanical scanner devices while maintaining a small system footprint. The device does not require a complex manufacturing process, due to the use of soft-lithography compatible material and thermal plastic composite, which can be shaped by hot-embossing molding. The simple fabrication process is not heavily reliant on specialized equipment or resources, reducing production costs. Our prototype is capable of delivering a collimated beam (beam spot size: 4µm) with the help of the collimating lens array, and can produce a 5.6° deflection angle under an applied voltage of 300V for single prism deflector and 29.2° deflection angle under an applied voltage of 60V for three prism deflector. Due to the limitations of the SAMIM process, the beam profile is clipped into a non-ideal oval shape. In our future work on this project, we will explore the possibility of fabricating the lens array using the thermal plastic material, which will allow us to reduce the clipping of the beam output, as well as combining the two stages on a single substrate to further reduce the system size. Furthermore, additional stages can be added to the device for scanning of beam in alternative directions to produce a device with 2D scanning capabilities, allowing the device to be used in applications such as micro-displays or micro-imagers.

Figure 1.

Ray-tracing simulation of collimator lens array design

Biographies

Chi leung Tsui is currently a graduate student in mechanical engineering at University of Washington. His current research interests are in sensors and actuators design for optomechatronics and optical system control.

Wei-Chih Wang graduated with a Ph.D. in electrical engineering from University of Washington in 1996. He is currently a research associate professor at Department of Mechanical Engineering, University of Washington. Dr. Wang's principle research is in the area of optical MEMS, fiberoptic sensors, and biomedical instrumentation. His recent research has been devoted to develop polymer micro sensors and actuators for noninvasive industrial and biomedical applications. More recently, his work has been expanded to metamaterial, and artificial electromagnetic polymer material study.