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Abstract
In this paper, we present a compact size polarization-independent reflection-type potassium tantalate niobate (KTN) beam deflector, including (1) a single KTN crystal, (2) a quarter-wave plate, and (3) a retroreflector. Unlike the previously reported two KTN crystal based transmission-type polarization-independent deflector, our proposed deflector offers the advantage of compact size because only a single KTN crystal is needed. Furthermore, the reflection architecture allows more surfaces of the deflector to be cooled, enhancing the laser power handling capability and the deflection speed of continuous scanning. The experimental results were consistent with the theoretical model. Our proposed beam deflector can be beneficial for a variety of applications such as high speed, high resolution 2D/3D scanning, printings, and displays.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The electro-optic (EO) effect-based beam deflectors have been gathering growing interest due to their high speed, sustainable, and simple operation [1,2]. Among all candidates of EO materials, potassium tantalate niobate (KTa1-xNxO3, KTN) outstands for its extraordinarily large quadratic EO (QEO) effect, broadband transmission spectrum, and fast response time [3–5]. With these critical advantages, KTN based EO deflector has broadened its applications, such as rapid tunable lasers and medical imaging, high speed laser beam modulation, and high-resolution three-dimensional (3D) printing [6–8].
With the advent of space-charge controlled (SCC) KTN deflector, a large enough deflection angle could be realized with a moderate applied electric field, which enhanced the feasibility of KTN deflector for real-world applications [1]. However, the conventional KTN based EO deflectors require a polarizer in front of the deflector because the amount of deflection depends on the polarization direction of the light. The use of polarizers is not preferred for many applications because it can drop the incoming light intensity [9], and the performance is polarization dependent. To overcome this limitation, a polarization independent KTN deflector was proposed [10,11], which used two KTN crystals and a half-wave plate. This not only increases the cost but also creates a bulky system, which limits practical application.
To overcome the limitation of prior art KTN deflector technologies, in this paper, we present a compact size polarization-independent reflection-type KTN beam deflector, which only requires a single KTN crystal. This offers the advantage of compact size and lower cost while maintaining the polarization independent operation, in addition to the increase of effective optical length of KTN deflector [19].
Furthermore, the reflection-architecture allows the five side surfaces of the deflector to be cooled, which significantly enhances the heat dissipation capability of the deflector. This is particularly important for the high speed operation because the dielectric heating is proportional to the scanning frequency [12], as given by
where ${Q_d}$ is the dielectric loss, ${f_0}$ is the beam scanning frequency, $\varepsilon ^{\prime\prime}$ is the imaginary part of dielectric constant, ${\varepsilon _0}$ is the permittivity of free space, and E is the electric field. As frequency, ${f_0}$, getting higher, the importance of cooling also becomes critical since the dielectric constant is temperature dependent and directly related to deflection angle (${\theta _d}$) of KTN deflectors [13]. High sensitivity of dielectric constant on its working temperature has been investigated in our previous studies [13,17,18].To get an intuitive understanding of the heating effect of continuous scanning, we conducted the following quantitative simulation by using the COMSOL software. In the simulation, we assumed that the size of the KTN deflector was $1 \times 1 \times 1{\; }m{m^3}$. Then, the temperature increases as a function of the applied electric field at different scanning frequencies was computed. The left figure of Fig. 1(a) shows the set-up of a conventional transmission-type KTN deflector, in which only two surfaces (top and bottom surfaces) were usually cooled. The right figure of Fig. 1(a) shows the set-up of a reflection-type KTN deflector proposed in this paper, in which five surfaces were cooled. Table 1 describes the parameters that are used for the computation. We further assumed that the ambient temperature was at 300K, which is a typical room temperature. For the cooling condition, the contact-surface cooling condition is applied, which can mimic the temperature control by the contacting Peltier devices.
Fig. 1. (a) Illustration of two different surface cooling conditions; (1) surface cooling of conventional transmission-type deflector(left) and (2) surface cooling of reflection-type deflector (right). (b) Corresponding temperature distributions of KTN deflectors as a function of applied electric field and scanning frequency for the conventional transmission-type deflector and the reflection-type deflector, respectively [18].
Table 1. Parameters of dielectric heating simulation
The left figure of Fig. 1(b) shows the computed temperature distribution for the conventional transmission-type KTN deflector. It can be seen that the maximum temperature of the KTN deflector is increased from 300 K to 315 K at an applied electric field of 250 V/mm and a scanning frequency of 500 kHz. The right figure of Fig. 1(b) shows the computed temperature distribution for the reflection-type KTN deflector. In this case, the maximum temperature of the KTN deflector is only increased from 300 K to 307 K at the same applied electric field of 250 V/mm and the same scanning frequency of 500 kHz. According to the reported temperature-dependent relative permittivity curve of KTN, this 8-degree temperature difference can result in a significant change in relative permittivity [20]. From the deflection angle equation, which will be discussed in the following section, one can tell that the deflection angle is proportional to the square of relative permittivity. Thus, the reflection-architecture could indeed reduce the temperature increase caused by the continuous scanning at high scanning frequency and prevent the performance drop in scanning angle.
2. Results and Discussion
Next, we conducted the following theoretical and experimental investigations to confirm that the proposed reflection-type deflector is indeed polarization independent. Figures 2(a) and 2(b) show the schematics of the set-ups and the corresponding picture of beam scanning with reflection-type reflector, respectively. We assumed that (1) electric field was applied in the x-direction, (2) the crystal axis was in the z-direction, (3) the KTN crystal was at the paraelectric cubic phase, and (4) both x-directional side surfaces of KTN were coated with Ti/Au electrodes. Each electrode includes a layer of Ti with a thickness of 15 nm and a layer of Au with a thickness of 35 nm so that space charges could be injected into the crystal. Note that the light propagates parallel to the crystal growing direction (c-axis) to maintain a high beam profile [18,19]. The dimensions of the KTN crystal were $4 \times 4 \times 8\; m{m^3}$ in the x, y, and z directions, respectively. The Curie temperature of the KTN crystal was $24^{\circ}{\textrm{C}}$. In this case, the electric field induced refractive index modulation in the x-direction, $\mathrm{\Delta }n(x )$, could be expressed in the polar form as
where ${n_0}$ was the original refractive index of KTN without any external electric field, ɛ was the dielectric constant, ${g_{ij}}$ was the QEO coefficient in the polar form, and $E(x )$ was the electric field when a voltage was applied along the x-direction. From this refractive index modulation, we could derive the deflection angle, ${\theta _d}$, asFig. 2. (a) Schematic of reflective-type polarization-independent KTN deflectors. (b) Top-view of the experimental set-up for testing reflective-type polarization-independent KTN deflector.
When a non-polarized light hit the KTN deflector, both the x- and y-polarized light beams were deflected. So, the deflection angle equations for each polarized light beam after passing through the deflector for the first time could be expressed as
After the first deflection, both x- and y-polarized beams experienced the following steps; first, they passed through a quarter-wave plate, and next, they hit and reflected back by a retroreflector, and finally, they passed through a quarter-wave plate again but from the opposite direction. Due to the quarter-wave plate and the retroreflector, now the x-polarized light became y-polarized light and vice versa. When these reflected lights finally passed through the KTN deflector for the second time, but from the opposite direction, the deflection angle for the original x- and y-polarized lights could be expressed as
Hence, the total deflection angle for the round trip of the original x-polarized light (${\theta _{xT}}$) became
Similarly, the total deflection angle for the round trip of original y-polarized light (${\theta _{yT}}$) became
Comparing Eq. (8) and Eq. (9), one could tell that both x- and y-polarized light experienced the same deflection angle. Thus, the above theoretical analysis indicated that the proposed reflection-type reflector was polarization independent.
Then, we experimentally measured the deflection angles for the x and y polarized light beams, respectively under the following experimental conditions. We applied an 1000 V (corresponding to an applied electric field of 250 V/mm) on the KTN crystal and the operational temperature was at $28^{\circ}\textrm{C}$. A CCD camera was also used to capture the beam deflection. Figures 3(a) and 3(b) show the original location and the deflected location for the x-polarized light, respectively. On the other hand, Figs. 3(c) and 3(d) show the original location and the deflection location for the y-polarized light, respectively. Comparing these four figures, one can clearly see that there is no difference in terms of beam deflection for the x and y polarized light. This experimental result confirmed that the proposed reflection-type deflector was polarization independent.
Fig. 3. (a) The original location of x-polarized light, (b) the deflected location of x-polarized light, (c) the original location of y-polarized light, and (d) the deflected location of y-polarized light.
Furthermore, to ensure that the proposed reflection-type beam deflector could generate enough deflection angle, we also measured the deflection angle as a function of applied voltage at different operational temperatures, as shown in Fig. 4. It can be seen that a deflection angle of 15 mrad is achieved at an applied voltage of 1000V (corresponding to an applied electric field of 250 $V/mm$) at $28^{\circ}\textrm{C}$, which is $4^{\circ}\textrm{C}$ above the Curie temperature of the KTN crystal. A deflection angle of 15 mrad is large enough for some applications such as high precision beam steering. Note that the deflection angle above 1000 V at $28^{\circ}\textrm{C}$ is not measured due to the electric field induced phase transition.
Fig. 4. The experimentally measured deflection angle of the reflection-type deflector as a function of applied voltage at different operation temperatures.
Finally, to demonstrate high speed beam deflection could be achieved by the proposed reflection-type deflector, we also measured the deflection speed. In the speed measuring experiment [4], first, a photodetector connected to an oscilloscope was placed at the original location of the beam without deflection. Then, a 1000 V was applied on the deflector so that the beam can be moved away from the location of the photodetector. In this case, no light signal was detected, and a zero red-line was shown in the oscilloscope, as shown by the red-line of Fig. 5. Then, the applied voltage was quickly switched off at a speed of a few nanoseconds by triggering a parallelly connected photoconductive semiconductor switch (PCSS) with a nanosecond pulse width pulsed green laser. The voltage drop was depicted by the blue-line of Fig. 5. Since the voltage was switched off, the deflected beam moved back to the original location so that it could be detected by the photodetector. One then could see an increased detected light signal in this case, as shown by the red-line of Fig. 5. The measured deflection speed was around 200 ns, as shown by the red-line of Fig. 5, which was primarily determined by the RC time constant of the circuit. The capacitance of the KTN deflector was around ${C_{KTN}} \approx 1.8\; nF$, and the impedance of the transmission line of the circuit was $R = 50\mathrm{\Omega }$. Then, the corresponding rise time of the circuit was ${t_r} = 2.2R \cdot {C_{KTN}} \approx 200ns$, which was consistent with the measured deflection speed. Such a 200ns deflection speed is fast enough for most deflection applications.
Fig. 5. The experimentally measured deflection speed. Red-line: detected photo signal. Blue-line: Applied voltage signal across the KTN deflector.
3. Conclusion
In conclusion, a reflection-type polarization-independent KTN deflector was developed. In addition to having the same advantages as conventional non-mechanical KTN deflectors, such as high speed and high precision, it also provided the benefits of (1) compact size, (2) higher heat dissipation capability that was very helpful for increasing the continuous beam scanning frequency and the laser power handling capability, and (3) polarization independent operation. Both theoretical analysis and experimental investigation were conducted. It was confirmed that the proposed reflection-type deflector was indeed polarization independent. It also had a deflection angle of around 15 mrad which is large enough for some applications such as high precision beam steering and a fast 200 ns deflection speed, which was only limited by the RC time constant of the circuit. Such a ns range deflection speed is fast enough for most beam deflection applications such as broadband free space optical communications, fast speed 2D/3D laser beam scanning/printing, high speed large-capacity medical imaging, large size, high speed, high-resolution display.
Funding
Joint Directed Energy Transition Office (DE-JTO) (Office of Naval Research (N00014-17-1-2571)).
Acknowledgments
This research was sponsored and partially supported by the Office of Naval Research (ONR) under Grant Number N00014-17-1-2571. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the US Government. The US Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation hereon.
Disclosures
The authors declare no conflicts of interest.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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