- Email: [email protected]
Design and analysis of a novel tunable optical filter
Microelectronics Journal 37 (2006) 302–307 www.elsevier.com/locate/mejo
Design and analysis of a novel tunable optical filter S. Eliahou-Niv*, R. Dahan, G. Golan Electrical and Electronic Engineering Department, Holon Academic Institute of Technology, 52 Golomb Street, Holon 58102, Israel Received 20 March 2005; received in revised form 9 May 2005; accepted 16 May 2005 Available online 29 June 2005
Abstract In this paper, an improved design of a tunable optical filter device which is driven by a piezoelectric actuator is proposed. The device can be used either as a tunable optical filter for discrete wavelength alignment or as a dynamic optical filter. The tunable filter is electrostatically driven and consists of three main parts: The electromechanical stage, the suspension and the thin film optical filter. The electromechanical stage and the suspension were designed using graph presentation methods, studied numerically using the finite element method (FEM). The thin film optical filter was designed by a thin film design software. The electromechanical stage was integrated with the suspension and tested as an angular driver of thin-film tilt interference filter for dense-wavelength division demultiplexing system applications. q 2005 Elsevier Ltd. All rights reserved. Keywords: Optical tunable-filter; Thin-film tilt interference filter; DWDM demultiplexer
1. Introduction
2. Thin-film tilt interference filter principles
In dense-wavelength division demultiplexing systems (DWDM), hundreds of channels, closely spaced in wavelength, are sent simultaneously over the same optical fiber. Spectral filters are required for demultiplexing all these wavelengths at the receiving end. Tunable filters are in high demand in telecommunication since classical wavelength interferometers are a collection of individual assembled etalons, which are extremely expensive. The spectral window at the 1550 nm wavelength is approximately 40 nm and the channel spacing must be 0.4 nm for 100 channels. The future DWDM systems will have significantly more channels and correspondingly significantly narrower bandwidths. This make the tunable filters an attractive and crucial alternative to the discrete approach. Tunable band pass filters are used to dynamically select among different wavelength channels at receiver side of DWDM systems.
The known technology for tunable filters includes Fabry– Perot etalon and thin-film tilt interference filters. The narrowband thin-film interference filter operates with the same principles as the Fabry–Perot interferometer, and they can be considered Fabry–Perot interferometer since they usually operate in the first order. The Fabry–Perot is a simple interferometer, which relies on the interference of multiple reflected beams. Each transmitted wavefront has undergone an even number of reflections. Whenever there is no phase difference between emerging wavefronts, interference between these wavefronts produces a transmission maximum. The constructive wavelength resonates, and the phase condition is satisfied when the following expression holds:
* Corresponding author. Fax: C972 3 502 6643. E-mail address: [email protected] (S. Eliahou-Niv).
0026-2692/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2005.05.018
ml Z 2top cos q
(1)
Here, q is the incident light angle. l is the wavelength, top is the optical thickness, and m is an arbitrary integer. At other wavelengths, destructive interference of transmitted wavefronts reduces transmitted intensity towards zero. Schematic of thin-film tilt interference filter illustrates in Fig. 1. Thinfilm tilt interference filter mechanism and optical filter design are the focus of this paper. While exploring available methods suitable for micro-scaling and MEMS technology characterized by wide tuning capability, low polarization,
S. Eliahou-Niv et al. / Microelectronics Journal 37 (2006) 302–307
Thin-film interference filter Incident beam Selected wavelength
θ
top Fig. 1. Rotating etalon schematic.
and reduced processing steps, it was found that the thin-film tunable filter most attractive [1]. In this paper, we introduce a step-by-step procedure to design a tunable filter device including modeling approach, simulation.
3. Buckling beam thin-film tilt interference filter analysis The main object of the device design is to provide a large angular deflection of an optical filter support via mechanical amplification. At the first step we analyzed the buckling beam spring model. The proposed idea is to use a thin clamped–clamped beam, subjected to buckling conditions with a filter attached to its surface close to one of the supports. The characteristics of a commercial piezoelectric multilayer bender actuator (Physik Instrumente GmbH, PL140.251) were chosen as a driver input for the FEA simulations. FEM simulation was done in ANSYS. The reason for choosing electrostatic driver is the scaling down considerations for MEMS applications [2] and the author experience with deflected tips cantilever actuators [3,4].
303
In the device design the following requirements were taken into consideration: (a) A blocking force of 0.5 N of the piezoelectric driver. (b) A maximum displacement range of G1 mm. (c) Repeatability and reliability of the suspension over many Megacycles. (d) Suspension dimensions to be of the same order of the driver dimensions (millimeters). (e) Pure angular movement with minimum linear displacement of the optical filter. These considerations should meet the following material requirements: (a) Elastic module, which determines the necessary applied force on the suspension. (b) Stress should be below the fatigue stress of the material. (c) Creep resistance. (d) Temperature limitations. FEM analysis was carried out for plastic flexible member with dimension of 10!2!0.13 mm drived by the PZT actuator. Fig. 2(a) and (b) illustrates the results of the FEM analysis of a clamped–clamped buckling beam model at a compressed mode. The results have shown that for plastic with elastic modulus of 3 GPa the maximal stress is 910 kg/ cm2 double than the allowed material stress (500 kg/cm2). Diminishing the flexible member thickness by factor 2 will overcome the problem causing another severe problem of creeping while static stress is applied. One cannot use metal materials as a bending beam, since metal flexible member would increase the needed input force (steel elastic modulus is 200 GPa).
4. An ‘L shaped’ suspension thin-film tilt interference filter analysis In order to achieve large coupled planner motion suspension employing ‘L shape’ elastic member was proposed. The chosen suspension took definite shape as a result of analytical analysis for different types of flexible beams [5] and optimized by using graph representation methods [6]. The deformation of an ‘L’ shaped springs when an external force is applied horizontally was investigated for the following boundary conditions: AxZAyZBxZqBZ0 are
Fig. 2. (a) FEM analysis of a clamped–clamped buckling beam model at a compressed mode. (b) Side view.
304
S. Eliahou-Niv et al. / Microelectronics Journal 37 (2006) 302–307
Fig. 3. (a) Deformation of ‘L’ shaped spring when an external force is applied horizontally. (b) Deformation of ‘L’ shaped spring when an external force is applied vertically.
illustrated in Fig. 3(a). (1) Represents the free body diagram, (2) represents the deformed shape due to the horizontal force F, (3) represents the internal force and momentum in the vertical element, (4) represents the deformed shape of the vertical element due to the horizontal force in (3), (5) represents the internal force and momentum in the horizontal element, (6) represents the deformed shape by the force in (5). The ‘L’ shaped spring deformation due to external vertical force is illustrated in Fig. 3(b) for the following boundary conditions: AxZAyZBxZqBZ0. (1) Represents the free body diagram, (2) represents the deformed shape due to the vertical force F, (3) represents the internal force and momentum in the vertical element, (4) represents the deformed shape of the vertical element due to the vertical force in (3), (5) represents the internal force and momentum in the horizontal element, (6) represents the deformed shape by the force in (5). From the deformation analysis, the horizontal and the vertical force–deformation relation, were obtained as follows: Cx Z
Fa3 ða C bÞ 3ð4a C bÞEI
(2)
Cy Z
Fb3 ða C bÞ 3ða C 4bÞEI
(3)
Cx and Cy are the horizontal and the vertical displacements of point C, respectively. F is the applied force. a and b are the lengths of segments AB and BC, respectively. E is the elastic modulus and I is the inertial moment. The horizontal and the vertical stiffness can be determined from Eqs. (2) and (3) as follow: kx Z
3ð4a C bÞEI a3 ða C bÞ
(4)
ky Z
3ða C 4bÞEI b3 ða C bÞ
(5)
It was found that the horizontal and the vertical stiffness of the ‘L’ shaped spring are equal for the same length of the two elements. Laboratory experiments were conducted on possible shapes of the suspension and FEM analysis was carried out on the dynamic characteristics of the device in order to check deformation for external force applied vertically and horizontally. The FEM results showed that an improved suspension exhibiting reduced stress can be found in the form of flexure [7]. Eq. (6) estimates the applied bending moment for the special flexure configuration when the center of the cutting radius lies on the edge of the flexure MZ
2Eqbt2:5 9pR0:5
(6)
where M is the applied bending moment, R is the flexure-cutting radius, E is the elastic modulus, b is the flexure width, and t is the flexure thickness. The use of Eq. (6) for analytical calculations for the moment and stress have shown that Delrinw is the most suitable material with optimal dimensions of RZ2 mm and tZ50 mm. FEM simulation was done on three rightcircular flexures model. It was assumed that the actuator tip deflection of one side in the simulation is the same as 1 ⁄2 of the total displacement stroke. The simulation results showed that the maximum stress in the suspension is about 752 kPa and much smaller than the yield stress. Fig. 4(a) and (b) shows the stress analysis of the ‘L shaped’ suspension when up and down stroke is applied, respectively. The simulation results have shown maximum angular movement of G148 of the ‘L shaped’ suspension. The above results occur for initial conditions of G1 mm tip deflection of the PZT actuator. The estimated movement was more than 1.08 for ratio of 12.5% between the actuator displacement in one axis direction and the suspension typical dimension.
S. Eliahou-Niv et al. / Microelectronics Journal 37 (2006) 302–307
305
Fig. 4. Stress analysis results for ‘L shaped’ suspension: (a) Actuator up stroke. (b) Actuator down stroke.
The modified suspension using three right-circular flexures was designed to be implemented in the tunable filter device as described schematically in Fig. 5(a). A 3D model of the ‘L shaped’ suspension is illustrated in Fig. 5(b).
5. Multilayer thin-film optical filter design The approach discussed in this section is based on the design of thin-film multilayer structure. In the design of this structure we are required to find an arrangement of layers, which will give the performance of the Fabry–Perot interferometer specified in Section 2. As was described in advance, the Fabry–Perot interferometer consists of a spacer cavity layer, which is usually half a wavelength thick, bounded by two high-reflectance coatings. Multi-beam interference in the spacer cavity layer causes the transmission of the filter to be extremely high over a narrow band of wavelengths around that for which the spacer is a multiple of one half wavelength thick. It is possible to couple two or more Fabry–Perot filters in series to give a more rectangular pass band [8]. Thus one basic type of thin film structure is a stack of alternate high and low refracting index films.
To understand in a qualitative way, the performance of the suggested multilayer Fabry–Perot interferometer, it is necessary to accept several simple design statements. The first is that the free spectral width should match the possible range of angels of the incident light, which is produced by angular movement of the filter mechanism. The second is the trade off between the necessity to work at large incident angles and the large variation of the s-polarized component of the incident radiation at large incident angels. Since the incident radiation can be represented as a superposition of two plane-polarized beams, p-polarized and s-polarized the transmittance and the reflectance coefficients can be determined from Fresnel equations. The variation of both coefficients with the incidence angle can be calculated and graph plotted [9]. The reflectance coefficients of both polarizations remain constant and equal for small incidence angles. For larger incidence angles, the s-component raises to higher values while the p-component reduces and vanishes at Brewster’s angle, representing a non-linear dependence of the transmitted beam with the incident angle. From the above discussion, one can conclude that it is advised to activate the tunable tilt filter in the scanning window of small incident angles. Such an activation will minimize the filter output sensitivity to incident angle variations since the output signal is the sum of the total
Fig. 5. (a) Thin-film tilt interference filter device principle; (b) 3D model of the three right-circular flexures ‘L shaped’ suspension.
306
S. Eliahou-Niv et al. / Microelectronics Journal 37 (2006) 302–307 100
2.5
90
Incident Angle [deg.]
Transmission (%)
70 60
refraction index
0 5 10 15 20
80
50 40 30
2
1.5
20 10 0 1520
1525
1530
1535
1540
1545
1550
1555
1560
1565
1570
1
0
0.5
Wavelenght (nm)
energy of both polarized components. On the other hand, from Eq. (1) one sees that the diffraction order for a given wavelength of Fabry–Perot interferometers and bandpass interference filters is governed by cos q. Hence, to achieve a large scanning spectrum one should choose a small optical thickness and large tilting angles, which may increase the output discrepancy standing in contrast with the demand for scanning window of small incident angles. Several design configurations of thin film optical filter and simulations were carried out by the demo version of the TFCalc thin film design software, by Software Spectra, Inc. using the ‘needle’ option. Fig. 6 represents the dependence of typical transmittance profile of thin film optical filter, which was designed for perpendicular incident radiation, in the incident angle. The filter consists of two materials: TiO2 and SiO2. From the simulation results, it can be seen that the optical response for short wavelengths are wider and lower in amplitude than the optical response for longer wavelengths on the spectral window. These results can be explained by realizing that the intensity of reflected and transmitted beams is a function of the angle of incidence due to refraction effects.
Incident Angle [deg.]
90
Transmission (%)
80 70 60 50
1.5
2
thickness (nm)
Fig. 6. Thin film filter transmittance dependence on the incident radiation angle.
100
1
10 11 12 13 14 15 16 17 18 19 20
2.5 x 104
Fig. 8. Typical design of TiO2 and SiO2 thin layers filter.
Thin film optical filter was designed while two important considerations were taken into account in order to achieve good line spacing in the transmittance profile. The first is 108 range of the incident angle. This range characterized by 108 in the lower incident angle and 208 in the higher incident angle. The incident angle range is determined by the filter mechanism as was described above. The second is the spectral width, which varies between 1530 and 1560 nm, in order to cover all the C-band range. Fig. 7 represents the thin film filter transmittance that was designed according to the consideration limits which are described above. The filter materials are TiO2 and SiO2. From Fig. 7 one can realized that the spectral width is characterized by an upper and lower wavelengths of 1530 and 1560 nm, respectively. The simulation was carried out for 11 different wavelengths, which are corresponding to 11 different incident angels, it can be seen that the 11 wavelengths are almost linearly distributed in the filter transmittance profile. These results are well suitable for the C-band range DWDM applications as was required from the design considerations. Fig. 8 represents the TiO2 and SiO2 layers design of the filter, which is described above, and its transmittance is illustrated in Fig. 8. Looking at the TiO2 and SiO2 layers distribution, one can clearly observe that the design consists of two different structures: one side consists of relatively thick layer of TiO2 which is characterized by high index of refraction containing thin layers of SiO2 which is characterized by low index of refraction. The other side consists of relatively thick layer of SiO2 containing thin layers of TiO2.
40
6. Conclusion
30 20 10 0 1520
1525
1530
1535
1540
1545
1550
1555
1560
Wavelength (nm)
Fig. 7. TiO2 and SiO2 filter transmittance.
1565
1570
It was shown that the developed optical scanning device is capable of a sufficiently large angular displacement with a large planar-coupled motion. It was demonstrated that a thin-film optical filter could be designed to match this mechanism output. Finally, it was shown that the proposed
S. Eliahou-Niv et al. / Microelectronics Journal 37 (2006) 302–307
device can serve as an optical tunable filter in the C-band range of fiber optics communications.
References [1] S. Eliahou-Niv, Overview on the next generation of electro-optical components based on MEMS technology. Communication 2002 Conference, Tel-Aviv, May 2002. [2] M. Madou, Fundamentals of microfabrication, CRC Press, Boca Raton, FL, 1997. [3] A. Seifert, S. Eliahou-Niv, D. Greenblatt, I. Wygnanski, On the use of piezoelectric actuators for airfoil separation control, AIAA J 36 (8) (1998).
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[4] S. Eliahou-Niv, Active actuators for aerodynamic flow control. Microelectro-mechanical-systems, The Israel Academy of Science & Humanities, Zichron Yaacov, Israel, 1999. [5] S.D. Senturia, Microsystem Design. Kluwer Academic Publishers, Dordrecht, The Netherlands, 2005. [6] O. Shay, S. Eliahou-Niv, D. Rubin, M. Sluvatin, A. Andrusier, Multidimensional graph representations for MEMS and their applications, ISRAMEMS, the First National Conference of the Israeli MOEMS Society, 2002, Haifa, Israel, 2002. [7] S. Eliahou-Niv, Y. Artstein, R. Dahan, Buckling beam spring design & analysis, Mems Day in Israel, Technion-Israel Institute of Technology, Haifa, Israel, 2001. p. 5. [8] A. Macleod, Thin Film Optical Filters, third ed., 2001 (chapter 1). [9] B.E.A. Saleh, M.C. Teich, Fundamentals of Photonics, Wiley, New York, 1991. pp. 193–235.
Design and analysis of a novel tunable optical filter S. Eliahou-Niv*, R. Dahan, G. Golan Electrical and Electronic Engineering Department, Holon Academic Institute of Technology, 52 Golomb Street, Holon 58102, Israel Received 20 March 2005; received in revised form 9 May 2005; accepted 16 May 2005 Available online 29 June 2005
Abstract In this paper, an improved design of a tunable optical filter device which is driven by a piezoelectric actuator is proposed. The device can be used either as a tunable optical filter for discrete wavelength alignment or as a dynamic optical filter. The tunable filter is electrostatically driven and consists of three main parts: The electromechanical stage, the suspension and the thin film optical filter. The electromechanical stage and the suspension were designed using graph presentation methods, studied numerically using the finite element method (FEM). The thin film optical filter was designed by a thin film design software. The electromechanical stage was integrated with the suspension and tested as an angular driver of thin-film tilt interference filter for dense-wavelength division demultiplexing system applications. q 2005 Elsevier Ltd. All rights reserved. Keywords: Optical tunable-filter; Thin-film tilt interference filter; DWDM demultiplexer
1. Introduction
2. Thin-film tilt interference filter principles
In dense-wavelength division demultiplexing systems (DWDM), hundreds of channels, closely spaced in wavelength, are sent simultaneously over the same optical fiber. Spectral filters are required for demultiplexing all these wavelengths at the receiving end. Tunable filters are in high demand in telecommunication since classical wavelength interferometers are a collection of individual assembled etalons, which are extremely expensive. The spectral window at the 1550 nm wavelength is approximately 40 nm and the channel spacing must be 0.4 nm for 100 channels. The future DWDM systems will have significantly more channels and correspondingly significantly narrower bandwidths. This make the tunable filters an attractive and crucial alternative to the discrete approach. Tunable band pass filters are used to dynamically select among different wavelength channels at receiver side of DWDM systems.
The known technology for tunable filters includes Fabry– Perot etalon and thin-film tilt interference filters. The narrowband thin-film interference filter operates with the same principles as the Fabry–Perot interferometer, and they can be considered Fabry–Perot interferometer since they usually operate in the first order. The Fabry–Perot is a simple interferometer, which relies on the interference of multiple reflected beams. Each transmitted wavefront has undergone an even number of reflections. Whenever there is no phase difference between emerging wavefronts, interference between these wavefronts produces a transmission maximum. The constructive wavelength resonates, and the phase condition is satisfied when the following expression holds:
* Corresponding author. Fax: C972 3 502 6643. E-mail address: [email protected] (S. Eliahou-Niv).
0026-2692/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2005.05.018
ml Z 2top cos q
(1)
Here, q is the incident light angle. l is the wavelength, top is the optical thickness, and m is an arbitrary integer. At other wavelengths, destructive interference of transmitted wavefronts reduces transmitted intensity towards zero. Schematic of thin-film tilt interference filter illustrates in Fig. 1. Thinfilm tilt interference filter mechanism and optical filter design are the focus of this paper. While exploring available methods suitable for micro-scaling and MEMS technology characterized by wide tuning capability, low polarization,
S. Eliahou-Niv et al. / Microelectronics Journal 37 (2006) 302–307
Thin-film interference filter Incident beam Selected wavelength
θ
top Fig. 1. Rotating etalon schematic.
and reduced processing steps, it was found that the thin-film tunable filter most attractive [1]. In this paper, we introduce a step-by-step procedure to design a tunable filter device including modeling approach, simulation.
3. Buckling beam thin-film tilt interference filter analysis The main object of the device design is to provide a large angular deflection of an optical filter support via mechanical amplification. At the first step we analyzed the buckling beam spring model. The proposed idea is to use a thin clamped–clamped beam, subjected to buckling conditions with a filter attached to its surface close to one of the supports. The characteristics of a commercial piezoelectric multilayer bender actuator (Physik Instrumente GmbH, PL140.251) were chosen as a driver input for the FEA simulations. FEM simulation was done in ANSYS. The reason for choosing electrostatic driver is the scaling down considerations for MEMS applications [2] and the author experience with deflected tips cantilever actuators [3,4].
303
In the device design the following requirements were taken into consideration: (a) A blocking force of 0.5 N of the piezoelectric driver. (b) A maximum displacement range of G1 mm. (c) Repeatability and reliability of the suspension over many Megacycles. (d) Suspension dimensions to be of the same order of the driver dimensions (millimeters). (e) Pure angular movement with minimum linear displacement of the optical filter. These considerations should meet the following material requirements: (a) Elastic module, which determines the necessary applied force on the suspension. (b) Stress should be below the fatigue stress of the material. (c) Creep resistance. (d) Temperature limitations. FEM analysis was carried out for plastic flexible member with dimension of 10!2!0.13 mm drived by the PZT actuator. Fig. 2(a) and (b) illustrates the results of the FEM analysis of a clamped–clamped buckling beam model at a compressed mode. The results have shown that for plastic with elastic modulus of 3 GPa the maximal stress is 910 kg/ cm2 double than the allowed material stress (500 kg/cm2). Diminishing the flexible member thickness by factor 2 will overcome the problem causing another severe problem of creeping while static stress is applied. One cannot use metal materials as a bending beam, since metal flexible member would increase the needed input force (steel elastic modulus is 200 GPa).
4. An ‘L shaped’ suspension thin-film tilt interference filter analysis In order to achieve large coupled planner motion suspension employing ‘L shape’ elastic member was proposed. The chosen suspension took definite shape as a result of analytical analysis for different types of flexible beams [5] and optimized by using graph representation methods [6]. The deformation of an ‘L’ shaped springs when an external force is applied horizontally was investigated for the following boundary conditions: AxZAyZBxZqBZ0 are
Fig. 2. (a) FEM analysis of a clamped–clamped buckling beam model at a compressed mode. (b) Side view.
304
S. Eliahou-Niv et al. / Microelectronics Journal 37 (2006) 302–307
Fig. 3. (a) Deformation of ‘L’ shaped spring when an external force is applied horizontally. (b) Deformation of ‘L’ shaped spring when an external force is applied vertically.
illustrated in Fig. 3(a). (1) Represents the free body diagram, (2) represents the deformed shape due to the horizontal force F, (3) represents the internal force and momentum in the vertical element, (4) represents the deformed shape of the vertical element due to the horizontal force in (3), (5) represents the internal force and momentum in the horizontal element, (6) represents the deformed shape by the force in (5). The ‘L’ shaped spring deformation due to external vertical force is illustrated in Fig. 3(b) for the following boundary conditions: AxZAyZBxZqBZ0. (1) Represents the free body diagram, (2) represents the deformed shape due to the vertical force F, (3) represents the internal force and momentum in the vertical element, (4) represents the deformed shape of the vertical element due to the vertical force in (3), (5) represents the internal force and momentum in the horizontal element, (6) represents the deformed shape by the force in (5). From the deformation analysis, the horizontal and the vertical force–deformation relation, were obtained as follows: Cx Z
Fa3 ða C bÞ 3ð4a C bÞEI
(2)
Cy Z
Fb3 ða C bÞ 3ða C 4bÞEI
(3)
Cx and Cy are the horizontal and the vertical displacements of point C, respectively. F is the applied force. a and b are the lengths of segments AB and BC, respectively. E is the elastic modulus and I is the inertial moment. The horizontal and the vertical stiffness can be determined from Eqs. (2) and (3) as follow: kx Z
3ð4a C bÞEI a3 ða C bÞ
(4)
ky Z
3ða C 4bÞEI b3 ða C bÞ
(5)
It was found that the horizontal and the vertical stiffness of the ‘L’ shaped spring are equal for the same length of the two elements. Laboratory experiments were conducted on possible shapes of the suspension and FEM analysis was carried out on the dynamic characteristics of the device in order to check deformation for external force applied vertically and horizontally. The FEM results showed that an improved suspension exhibiting reduced stress can be found in the form of flexure [7]. Eq. (6) estimates the applied bending moment for the special flexure configuration when the center of the cutting radius lies on the edge of the flexure MZ
2Eqbt2:5 9pR0:5
(6)
where M is the applied bending moment, R is the flexure-cutting radius, E is the elastic modulus, b is the flexure width, and t is the flexure thickness. The use of Eq. (6) for analytical calculations for the moment and stress have shown that Delrinw is the most suitable material with optimal dimensions of RZ2 mm and tZ50 mm. FEM simulation was done on three rightcircular flexures model. It was assumed that the actuator tip deflection of one side in the simulation is the same as 1 ⁄2 of the total displacement stroke. The simulation results showed that the maximum stress in the suspension is about 752 kPa and much smaller than the yield stress. Fig. 4(a) and (b) shows the stress analysis of the ‘L shaped’ suspension when up and down stroke is applied, respectively. The simulation results have shown maximum angular movement of G148 of the ‘L shaped’ suspension. The above results occur for initial conditions of G1 mm tip deflection of the PZT actuator. The estimated movement was more than 1.08 for ratio of 12.5% between the actuator displacement in one axis direction and the suspension typical dimension.
S. Eliahou-Niv et al. / Microelectronics Journal 37 (2006) 302–307
305
Fig. 4. Stress analysis results for ‘L shaped’ suspension: (a) Actuator up stroke. (b) Actuator down stroke.
The modified suspension using three right-circular flexures was designed to be implemented in the tunable filter device as described schematically in Fig. 5(a). A 3D model of the ‘L shaped’ suspension is illustrated in Fig. 5(b).
5. Multilayer thin-film optical filter design The approach discussed in this section is based on the design of thin-film multilayer structure. In the design of this structure we are required to find an arrangement of layers, which will give the performance of the Fabry–Perot interferometer specified in Section 2. As was described in advance, the Fabry–Perot interferometer consists of a spacer cavity layer, which is usually half a wavelength thick, bounded by two high-reflectance coatings. Multi-beam interference in the spacer cavity layer causes the transmission of the filter to be extremely high over a narrow band of wavelengths around that for which the spacer is a multiple of one half wavelength thick. It is possible to couple two or more Fabry–Perot filters in series to give a more rectangular pass band [8]. Thus one basic type of thin film structure is a stack of alternate high and low refracting index films.
To understand in a qualitative way, the performance of the suggested multilayer Fabry–Perot interferometer, it is necessary to accept several simple design statements. The first is that the free spectral width should match the possible range of angels of the incident light, which is produced by angular movement of the filter mechanism. The second is the trade off between the necessity to work at large incident angles and the large variation of the s-polarized component of the incident radiation at large incident angels. Since the incident radiation can be represented as a superposition of two plane-polarized beams, p-polarized and s-polarized the transmittance and the reflectance coefficients can be determined from Fresnel equations. The variation of both coefficients with the incidence angle can be calculated and graph plotted [9]. The reflectance coefficients of both polarizations remain constant and equal for small incidence angles. For larger incidence angles, the s-component raises to higher values while the p-component reduces and vanishes at Brewster’s angle, representing a non-linear dependence of the transmitted beam with the incident angle. From the above discussion, one can conclude that it is advised to activate the tunable tilt filter in the scanning window of small incident angles. Such an activation will minimize the filter output sensitivity to incident angle variations since the output signal is the sum of the total
Fig. 5. (a) Thin-film tilt interference filter device principle; (b) 3D model of the three right-circular flexures ‘L shaped’ suspension.
306
S. Eliahou-Niv et al. / Microelectronics Journal 37 (2006) 302–307 100
2.5
90
Incident Angle [deg.]
Transmission (%)
70 60
refraction index
0 5 10 15 20
80
50 40 30
2
1.5
20 10 0 1520
1525
1530
1535
1540
1545
1550
1555
1560
1565
1570
1
0
0.5
Wavelenght (nm)
energy of both polarized components. On the other hand, from Eq. (1) one sees that the diffraction order for a given wavelength of Fabry–Perot interferometers and bandpass interference filters is governed by cos q. Hence, to achieve a large scanning spectrum one should choose a small optical thickness and large tilting angles, which may increase the output discrepancy standing in contrast with the demand for scanning window of small incident angles. Several design configurations of thin film optical filter and simulations were carried out by the demo version of the TFCalc thin film design software, by Software Spectra, Inc. using the ‘needle’ option. Fig. 6 represents the dependence of typical transmittance profile of thin film optical filter, which was designed for perpendicular incident radiation, in the incident angle. The filter consists of two materials: TiO2 and SiO2. From the simulation results, it can be seen that the optical response for short wavelengths are wider and lower in amplitude than the optical response for longer wavelengths on the spectral window. These results can be explained by realizing that the intensity of reflected and transmitted beams is a function of the angle of incidence due to refraction effects.
Incident Angle [deg.]
90
Transmission (%)
80 70 60 50
1.5
2
thickness (nm)
Fig. 6. Thin film filter transmittance dependence on the incident radiation angle.
100
1
10 11 12 13 14 15 16 17 18 19 20
2.5 x 104
Fig. 8. Typical design of TiO2 and SiO2 thin layers filter.
Thin film optical filter was designed while two important considerations were taken into account in order to achieve good line spacing in the transmittance profile. The first is 108 range of the incident angle. This range characterized by 108 in the lower incident angle and 208 in the higher incident angle. The incident angle range is determined by the filter mechanism as was described above. The second is the spectral width, which varies between 1530 and 1560 nm, in order to cover all the C-band range. Fig. 7 represents the thin film filter transmittance that was designed according to the consideration limits which are described above. The filter materials are TiO2 and SiO2. From Fig. 7 one can realized that the spectral width is characterized by an upper and lower wavelengths of 1530 and 1560 nm, respectively. The simulation was carried out for 11 different wavelengths, which are corresponding to 11 different incident angels, it can be seen that the 11 wavelengths are almost linearly distributed in the filter transmittance profile. These results are well suitable for the C-band range DWDM applications as was required from the design considerations. Fig. 8 represents the TiO2 and SiO2 layers design of the filter, which is described above, and its transmittance is illustrated in Fig. 8. Looking at the TiO2 and SiO2 layers distribution, one can clearly observe that the design consists of two different structures: one side consists of relatively thick layer of TiO2 which is characterized by high index of refraction containing thin layers of SiO2 which is characterized by low index of refraction. The other side consists of relatively thick layer of SiO2 containing thin layers of TiO2.
40
6. Conclusion
30 20 10 0 1520
1525
1530
1535
1540
1545
1550
1555
1560
Wavelength (nm)
Fig. 7. TiO2 and SiO2 filter transmittance.
1565
1570
It was shown that the developed optical scanning device is capable of a sufficiently large angular displacement with a large planar-coupled motion. It was demonstrated that a thin-film optical filter could be designed to match this mechanism output. Finally, it was shown that the proposed
S. Eliahou-Niv et al. / Microelectronics Journal 37 (2006) 302–307
device can serve as an optical tunable filter in the C-band range of fiber optics communications.
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