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C-band three-port tunable band-pass thin film optical filter with low polarization-sensitivity
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Optics Communications 281 (2008) 3709–3714 www.elsevier.com/locate/optcom
C-band three-port tunable band-pass thin film optical filter with low polarization-sensitivity Kan Yu *, Wen Liu, Dexiu Huang, Jing Chang Wuhan National Laboratory for Optoelectronics and School of Optoelectronic Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China Received 22 November 2007; received in revised form 11 March 2008; accepted 11 March 2008
Abstract According to the oblique incidence characteristics of thin film narrowband filter, the stack of 100 GHz DWDM four cavities angletuned thin film filter has been designed and optimized. The two polarization modes’ central wavelengths of the thin film filter can be centered at the same one in oblique incidence, and it has a stable tuning range of 20 nm without the phenomenon of polarization central wavelength separation. Using this kind of angle-tuned thin film filter and the polarization beam-splitters, the tuning range of the angletuned thin film filter can be further expanded due to the reason that it can transmit only the s-polarization light. In this paper we also developed a three-port bandpass tunable filter device with new structure, which tuning range can cover the whole C-band and its adjacent channel isolation degree is high. The experiments show that the three-port tunable filter has an effective tuning range of 33 nm, and its adjacent channel isolation degree is more than 35 dB. It has a bright application opportunity for its flexibility and effective performance. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Tunable filter; Multiple cavities; Oblique incidence; Central wavelength; Angle tuning
1. Introduction A tunable optical filter is a useful component for optical wavelength selection in DWDM system. Conventional tunable optical filters employed the acousto-optic tunable filter(AOTF), the Fiber Bragg Grating (FBG), Fabry-Perot interferometer, etc. The AOTF which operates on the principle of acousto-optic interaction in an anisotropic medium has very rapid tune speed [1], but its adjacent channel isolation is low. The FBG filter which operates on the principle of the Bragg diffraction may have narrow bandwidth and high adjacent channel isolation [2], but the tuning range is very small and it is sensitive to temperature change. Due to the sensitivity to the temperature, it may be affected easily by the environment and its long-term sta*
Corresponding author. Tel.: +86 02787544509. E-mail addresses: onlyfi[email protected], onlyfi[email protected] (K. Yu). 0030-4018/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2008.03.019
bility is not so good. The Fabry-Perot interferometer which operates on the principle of multiple beams interference has been widely used [3]. However, due to its sharp transmission peak, it may be harmful to the payload signal. Its adjacent channel isolation is also very low so that it can only be used in the channel monitoring but can not add or drop a signal wavelength from multiple wavelengths for the DWDM system. With wide passband, low insertion loss and good temperature stability, the multiple cavities interference narrowband thin film filter is widely used in the DWDM system [4]. The thin film interference filter operates with the same principle as the Fabry-Perot interferometer [5]. When the incident angle is increasing, its passband and the transmission peak will shift to the shorter wavelength (blue shifted). This characteristic can be used to fabricate tunable filter by modulate the incident angle of the thin film filter [6–8]. At the same time, the stability of the transmission curve will change and the central wavelength of two polarization
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modes will separate obviously. It will cause the serious polarization dependent loss which adversely affects the system performance [9]. In oblique incidence, the wavelength shifts to shorter wavelength in low index spacer filter for s-polarization is larger than that for p-polarization, and the shift in high index spacer filter for s-polarization is less than that for p-polarization. So the central wavelengths of the two polarization modes can be centered at the same wavelength by using both high and low index materials as its spacer. By optimizing the stack structure, a 100 GHz angle-tuned filter without central wavelength separation of the two polarization modes has been designed and optimized. In order to get larger tuning range, we use the polarization beam-splitters and the half wave plates to transmit only the s-polarization light, which can expand the tuning range to more than 33 nm. In this paper we also proposed a three-port bandpass tunable filter based on the angle-tuned thin film filter and the polarization beam-splitters, and its tuning range can cover the whole C-band. The structure of the three-port tunable filter can adequately utilize the tune performance of the angle-tuned thin film and obviously decrease the intraband crosstalk. The experimental results show that it has a wide stable tuning range (from 1528 nm to 1561 nm) and high adjacent channel isolation degree (more than 35 dB). 2. Principle 2.1. Angle-tuned thin film filter stack design As the incident angle is increasing, the central transmission wavelength and the passband of the thin film interference filter will shift to shorter wavelength, which is caused by the change of spacer phase thickness [10]. The central wavelength k of the thin film filter used in oblique incidence can be expressed as [11] qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi k ¼ k0 1 e sin2 h0 ð1Þ where k0 is the central wavelength of the filter in normal incidence, e is a constant that depends on the stack configuration of the interference thin film filter and h0 is the oblique incident angle. In numerical value, e is the reciprocal of the spacer stack’s effective refractive index. The configuration of spacer in each cavity is the most important factor to the multiple cavities thin film filter. Conventional thin film filter is usually used in normal incidence, and it only uses the high or low index materials as its spacer. The Ta2O5 and SiO2 are the most common high and low refractive index materials. When the thin film filter is used in oblique incidence, the wavelength shifts to shorter wavelength in low index spacer filter for s-polarization is larger than that for p-polarization. On the contrary, the shift in high index spacer filter for s-polarization is less than that for p-polarization. So the central wavelength of two polarization modes will separate more and more obviously when the incident angle is increasing. Using both
high and low index materials as the spacer of the stack, the central wavelengths of the two polarization modes may be centered at the same one. In oblique incidence, the refractive index of the two polarization modes will change. The refractive index of spolarization is ns = n cos h0 and the refractive index of the p-polarization is np = n/cos h0, where n is the refractive index in normal incidence. According to the theory of the thin film matrix, we can calculate the transmission curves of the s-polarization and p-polarization modes from the stack of the multiple layers thin film. So we can get the central wavelength of the p-polarization kp and the central wavelength of the s-polarization ks. Hence, the central wavelength separation degree CWL of the two polarization modes can be expressed as CWL ¼ jkp ks j
ð2Þ
using both high and low refractive index materials as a spacer of the multiple cavities thin film filter, the stack of each cavity can be defined as follows: p
ðHLÞ ðs1 H Þðs2 LÞ ðsi H ÞðLH Þ
p
ð3Þ
In the stack (3), the high index material H is Ta2O5 (nH = 2.06) and the low index material L is SiO2 (nL = 1.465). They are both quarter wavelength coating, and the design central wavelength is at 1563 nm in normal incidence. Between each cavity, we use the low index material L as the coupling layer. Therefore, the spectrum characteristics of the stack can be determined by the cavity number q, reflect coatings number p, the high and low refractive index materials number i of the spacer and the interference grade order si of each index material. The characteristic parameter of the whole stack can be expressed as {p, q, si, i}. As a quarter stack, the independent variables q, p, i and si are all positive integers, and their parameters range are finite: q 2 (2–5), p 2 (5–8), si 2 (2–8). So we can get the appropriate results through optimizing the independent variables by the computer calculation. In 100 GHz DWDM system, the maximal bandwidth of the thin film interference filter at 25 dB is b = 1.2 nm, the minimum bandwidth of it at 0.5 dB is a = 0.4 nm, the maximal insertion loss is ‘ = 0.1 dB and the maximal central wavelength separation degree is r = 0.05 nm. So the design index of the angle-tuned thin film filter should content the constraint condition as follows: sðk a=2; k þ a=2Þ 6 ‘; BW 0:5 dB P a; BW 25 dB 6 b; CWL 6 r
ð4Þ
Where s(k) = 10 log(T(k)). Under the constraint condition, the effective range of the convergence {p, q, si, i} is finite. Therefore, the optimization can be described as the minimization of the convergence {p, q, si, i} which can content the constraint condition. We also proposed an evaluation function w({p, q, si, i}) to evaluate the stack result which contents the constraint condition.
K. Yu et al. / Optics Communications 281 (2008) 3709–3714 2
2
wðfp; q; si ; igÞ ¼ -fð10fÞ þ -eð1 eÞ þ -rð10rÞ
2
ð5Þ
Where the rectangular degree of the filter is e = BW0.5 dB/ BW25 dB and the ripple coefficient is n = (Tmax Tmin/T0. -f, -e and -r are the weighted factors of the rectangular degree, ripple coefficient and central wavelength separation degree. The value of the three weighted factors all can be defined as 1. The incident angle range is from 0° to 15° and the central wavelength is at 1563 nm in normal incidence. The value of the evaluation function can indicate the tune performance of all the stacks which content the constraint condition, and the best stack can have the minimum value of the evaluation function. We start the initial characteristic parameter {p, q, si, i} as {5, 2, 2, 1}. So the initial calculation stack is (HL)5(2H)(LH)5L(HL)5(2H)(LH)5, which a double cavities thin film stack is. Then the computer can calculate all the appropriate stacks within the parameter range which content the constraint condition. Then we can get the best one by comparing the evaluation function values of all the possible stacks. By tolerance optimizing analysis and comparing the evaluation function values of all the possible stacks, one of the best stacks of 100 GHz multiple cavities angle-tuned thin-film filter is designed as follows, which the final values of {p, q, si, i} is {8, 4, 4, 3}. ," G
7
7
8
8
8
8
7
7
ðHLÞ 2L3H 4L3H 2LðLH Þ LðHLÞ 2L3H 4L3H 2LðLH Þ L ðHLÞ 2L3H 4L3H 2LðLH Þ LðHLÞ 2L3H 4L3H 2LðLH Þ
Fig. 1 shows the simulation transmittance curves of the stack at different incident angle. This design is a four cavities structure stack. We can see from the simulation results that the central wavelength is at 1563 nm when the angle-tuned thin film filter is in normal incidence, and it shifts to 1544.5 nm at the incident angle of 14.5°. Meanwhile, the transmission curve has not distort and the central wavelengths of two polarization modes are approximately centered at the same wavelength. When the incident angle is more than 15°, the central wavelength separation degree of the two polarization modes will exceed
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the constraint condition. If the central wavelength separation degree r can add to 0.1 nm, the tunable range of the filter can be enlarged to 26 nm. In order to get larger tuning range under the same constraint condition, we can use the polarization beam-splitters to modulate only one polarization light. As the spacer stack of the angle-tuned thin film filter is 2L3H4L3H2L, the effective refractive index of the stack is 1.64, so the value of e is 0.608. 2.2. Structure of the three-port tunable filter In the DWDM system, a three-port tunable filter can select the specific wavelength out of the multiple wavelengths, and the residual wavelengths will transmit in the fiber without any disruption. In this paper we also developed a three-port tunable filter, as shown in Fig. 2, which using the angle-tuned filter we designed above as the tune core. Although the central wavelengths of the two polarizations are centered at the same wavelength, but the tuning range of the angle-tuned thin film is finite (less than 20 nm), and the bandwidths of two polarization modes are changing while the incident angle of the thin film filter is increasing. The bandwidth of p-polarization light is large than that of the average light and the bandwidth of s-polar#, A
ð6Þ
ization light is less than that of the average light. When the incident angle is larger than 15°, the bandwidth of the p-polarization at the 25 dB is 1.26 nm, which exceeds the constraint condition. However, the bandwidth of the s-polarization at both 25 dB and 0.5 dB contents the design indexes within the incident angle of 20°. The central wavelength of the s-polarization light can shift to 1528 nm at the incident angle of 20°. In order to get larger tuning range, we can use the polarization beam-splitters and the half wave plates to transmit only the s-polarization light. It will greatly enlarge the tuning range of the angle-tuned thin film
Fig. 1. The simulation transmittance curves of the stack (6). (a) At the incident angle of 0°, (b) at the incident angle of 14.5°.
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Fig. 2. The structure of the three-port tunable filter.
filter which can cover the whole C-band. Furthermore, due to the angle-tuned thin film filter only modulates the spolarization light, the polarization dependent loss of it will be also greatly reduced. This three-port tunable filter including: an angle-tuned interference thin film filter; a pair of polarization beamsplitters A and B which position on the two sides of the filter in the same light path; a pair of half wave plate C and D which position on the two sides of the filter in the different light path. The polarization beam-splitter A divides the input signal from the optical circulator into two parallel light paths which are the p-polarization and the s-polarization light. The half wave plate C at the p-polarization light path rotates the p-polarization light into the s-polarization light. Then the two paths of s-polarization light arrive the filter which normal is tilting to the light paths. Filter transmits the wavelength ki form multiple wavelengths and reflects the residual wavelengths to the mirror E which is keeping parallel to the filter all along. The half wave plate D at another s-polarization light path rotates the s-polarization light into the p-polarization light behind the filter. The polarization beam-splitter B then converges the ppolarization and the s-polarization light into a random polarization light to the drop port. The mirror F keeps uprightness to the light paths behind the mirror E. The residual signal with different wavelengths will back to the polarization beam-splitter A through the two mirrors. Then it goes out from the output port at the three-port optical circulator. That’s the whole process to choose one wavelength from the multiple wavelengths. It can choose another wavelength when the oblique angle of the filter and the mirror E which is keeping parallel to the filter are changed. This is the principle of the TFF (thin film filter) based three-port tunable optical filter. The polarization beam-splitters are coupled with the single fiber collimators via fiber. By using a pair of the polarization beam-splitters and the half wave plates, the
polarization mode of the input light at the thin film filter will be only the s-polarization mode, so the polarization dependent loss of the transmission light will be greatly reduced. As the tuning range of the s-polarization is larger than the p-polarization and the average light, the tuning range of the filter can be greatly enlarged which can easily cover the whole C-band (from 1528 nm to 1561 nm). The oblique incidence angle range of thin film filter is from 4.8° to 20°. In the output port and the drop port, there is no change in the signals, because the half wave plates rotate one s-polarization light back to the p-polarization light. So the light at any port is in random polarization. This three-port tunable filter has a simple structure and it is easy to be fabricated. The control of the filter is flexible for using a stepping motor to tune which wavelength should be dropped. The transmission of its light path is also very easy. By using the mirrors, the couple of polarization beam-splitters can be fixed because the residual reflected signal will back to the input port along the same light path. Without continuously collimating the optical signal, it can reduce the difficulty of the adjusting, improve the stability and much easier to be realized in practice. In the tunable filter, the reflected light path ensures the incidence and the reflection beam arrive the thin film filter in the same position twice, therefore there will be less residual filtered signal. So the intraband crosstalk will decrease obviously. However, the modulation time of it is a little slow (the maximum tuning time of the sample tunable filter is less than 6 seconds) due to the mechanism modulation of the stepping motor. 3. Experimental results and analysis The Filtech Corporation in China has prepared the angle-tuned thin film filter designed by us on the Veeco instruments. The sample device of the three-port tunable filter is fabricated in the Accelink Technologies Co.,Ltd in China. The incident angle range of the tunable filter is from 4.8° to 20° so that it can cover the whole C-band (1528–1561 nm). Figs. 3 and 4 show the measured transmission and reflection spectrum of the tunable filter at different incident angle. From Figs. 3 and 4 we can see that the shape of the transmission spectrum is good for high rectangular degree, the insertion loss is less than 2 dB. The central transmission wavelength is at 1561 nm when the incident angle is 4.8°, and it shifts to the 1528 nm at the incident angle of 20°. The shape of the reflection spectrum is also good and we can see from the spectrum that the isolation degree is more than 30 dB. In the 100 GHz DWDM system, the tuning range can cover the whole C-band and it can choose almost 40 channels. The spectrum shape at 20° incidence is coincide with the 4.8° incidence. In the whole modulation process, the bandwidth at 0.5 dB is more than 0.3 nm and the bandwidth at 25 dB is less than 1.3 nm, which absolutely satisfy the specification of the 100 GHz DWDM system.
K. Yu et al. / Optics Communications 281 (2008) 3709–3714
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Fig. 3. The measured transmission spectrum. (a) At the incident angle of 4.8°, (b) at the incident angle of 20°.
Fig. 4. The measured reflection spectrum. (a) At the incident angle of 4.8°, (b) at the incident angle of 20°.
Fig. 5. The measured spectrum of three DWDM adjacent channels.
In order to test the influence of the drop channel to its adjacent channels, we input three adjacent channels of the 100 GHz DWDM system. Fig. 5 shows the three channels at the middle of the tuning range (1543–1546 nm) without drop any one. Then we drop the middle channel of the
Fig. 6. The measured spectrum when middle channel is dropped.
three channels, and the Fig. 6 shows the measured spectrum. From Fig. 6 we can see that the drop channel has no adverse effect on the performance of the adjacent channels, and the adjacent channel isolation degree N is more than 35 dB.
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4. Conclusion
References
In this paper, we have developed a stack of 100 GHz DWDM four cavities interference narrowband thin film angle-tuned filter and a three-port tunable filter with new configuration. It has a simple structure, stable transmission characteristics, simple light path and it is easy to be fabricated. As the stack is optimized and the system only transmits the s-polarization light, the tunable filter has eliminated the phenomenon of polarization light central wavelength separation, so the polarization-sensitivity of the tunable filter is very low. The experiments also demonstrate that its insertion loss is low, its adjacent channel isolation degree is very high, and its effective tuning range can cover the whole C-band. This type of three-port tunable filter can be the core unit of the reconfigurable optical add/ drop multiplexer (ROADM), which is a key apparatus of the DWDM system. It has a bright application opportunity for its flexibility, low cost, and wide tuning range.
[1] P. A Gauss, J.R. Sambles, Opt. Lett. 16 (1997) 429. [2] Xinyong Dong, P. Shum, Xiufeng Yang, et al., Opt. Commun. 259 (2006) 645. [3] Shen Weidong, Liu Xiangdong, Huang Biqin, et al., Opt. Commun. 239 (2004) 153. [4] Junlchiro Minowa, Yohji Fujii, Appl. Opt. 23 (1984) 193. [5] S. Eliahou-Niv, R. Dahan, G. Golan, Microelectronics 37 (2006) 302. [6] Lawvence Domash, Ming Wu, Nikolay Nemchuk, et al., J. Lightwave Technol. 22 (2004) 126. [7] Yoshihiko Suemura, Ahio Tajima, Naoya Henmi, et al., J. Lightwave Technol. 14 (1996) 1048. [8] Anatoly Frenkel, Chinlon, J. Lightwave Technol. 4 (1989) 615. [9] N. Mekada, A. Al-Hamdan, T.H. Chong, et al., IEEE Photon. Technol. Lett. 9 (1997) 782. [10] H.A. Macleod, Thin-Film Optical Filters, Adam Hilger Ltd., Bristol, 1986, p. 260. [11] Yoshihiko Suemura, Akio Tajima, Naoya Henmi, et al., J. Lightwave Technol. 14 (1996) 1048.
Optics Communications 281 (2008) 3709–3714 www.elsevier.com/locate/optcom
C-band three-port tunable band-pass thin film optical filter with low polarization-sensitivity Kan Yu *, Wen Liu, Dexiu Huang, Jing Chang Wuhan National Laboratory for Optoelectronics and School of Optoelectronic Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China Received 22 November 2007; received in revised form 11 March 2008; accepted 11 March 2008
Abstract According to the oblique incidence characteristics of thin film narrowband filter, the stack of 100 GHz DWDM four cavities angletuned thin film filter has been designed and optimized. The two polarization modes’ central wavelengths of the thin film filter can be centered at the same one in oblique incidence, and it has a stable tuning range of 20 nm without the phenomenon of polarization central wavelength separation. Using this kind of angle-tuned thin film filter and the polarization beam-splitters, the tuning range of the angletuned thin film filter can be further expanded due to the reason that it can transmit only the s-polarization light. In this paper we also developed a three-port bandpass tunable filter device with new structure, which tuning range can cover the whole C-band and its adjacent channel isolation degree is high. The experiments show that the three-port tunable filter has an effective tuning range of 33 nm, and its adjacent channel isolation degree is more than 35 dB. It has a bright application opportunity for its flexibility and effective performance. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Tunable filter; Multiple cavities; Oblique incidence; Central wavelength; Angle tuning
1. Introduction A tunable optical filter is a useful component for optical wavelength selection in DWDM system. Conventional tunable optical filters employed the acousto-optic tunable filter(AOTF), the Fiber Bragg Grating (FBG), Fabry-Perot interferometer, etc. The AOTF which operates on the principle of acousto-optic interaction in an anisotropic medium has very rapid tune speed [1], but its adjacent channel isolation is low. The FBG filter which operates on the principle of the Bragg diffraction may have narrow bandwidth and high adjacent channel isolation [2], but the tuning range is very small and it is sensitive to temperature change. Due to the sensitivity to the temperature, it may be affected easily by the environment and its long-term sta*
Corresponding author. Tel.: +86 02787544509. E-mail addresses: onlyfi[email protected], onlyfi[email protected] (K. Yu). 0030-4018/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2008.03.019
bility is not so good. The Fabry-Perot interferometer which operates on the principle of multiple beams interference has been widely used [3]. However, due to its sharp transmission peak, it may be harmful to the payload signal. Its adjacent channel isolation is also very low so that it can only be used in the channel monitoring but can not add or drop a signal wavelength from multiple wavelengths for the DWDM system. With wide passband, low insertion loss and good temperature stability, the multiple cavities interference narrowband thin film filter is widely used in the DWDM system [4]. The thin film interference filter operates with the same principle as the Fabry-Perot interferometer [5]. When the incident angle is increasing, its passband and the transmission peak will shift to the shorter wavelength (blue shifted). This characteristic can be used to fabricate tunable filter by modulate the incident angle of the thin film filter [6–8]. At the same time, the stability of the transmission curve will change and the central wavelength of two polarization
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modes will separate obviously. It will cause the serious polarization dependent loss which adversely affects the system performance [9]. In oblique incidence, the wavelength shifts to shorter wavelength in low index spacer filter for s-polarization is larger than that for p-polarization, and the shift in high index spacer filter for s-polarization is less than that for p-polarization. So the central wavelengths of the two polarization modes can be centered at the same wavelength by using both high and low index materials as its spacer. By optimizing the stack structure, a 100 GHz angle-tuned filter without central wavelength separation of the two polarization modes has been designed and optimized. In order to get larger tuning range, we use the polarization beam-splitters and the half wave plates to transmit only the s-polarization light, which can expand the tuning range to more than 33 nm. In this paper we also proposed a three-port bandpass tunable filter based on the angle-tuned thin film filter and the polarization beam-splitters, and its tuning range can cover the whole C-band. The structure of the three-port tunable filter can adequately utilize the tune performance of the angle-tuned thin film and obviously decrease the intraband crosstalk. The experimental results show that it has a wide stable tuning range (from 1528 nm to 1561 nm) and high adjacent channel isolation degree (more than 35 dB). 2. Principle 2.1. Angle-tuned thin film filter stack design As the incident angle is increasing, the central transmission wavelength and the passband of the thin film interference filter will shift to shorter wavelength, which is caused by the change of spacer phase thickness [10]. The central wavelength k of the thin film filter used in oblique incidence can be expressed as [11] qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi k ¼ k0 1 e sin2 h0 ð1Þ where k0 is the central wavelength of the filter in normal incidence, e is a constant that depends on the stack configuration of the interference thin film filter and h0 is the oblique incident angle. In numerical value, e is the reciprocal of the spacer stack’s effective refractive index. The configuration of spacer in each cavity is the most important factor to the multiple cavities thin film filter. Conventional thin film filter is usually used in normal incidence, and it only uses the high or low index materials as its spacer. The Ta2O5 and SiO2 are the most common high and low refractive index materials. When the thin film filter is used in oblique incidence, the wavelength shifts to shorter wavelength in low index spacer filter for s-polarization is larger than that for p-polarization. On the contrary, the shift in high index spacer filter for s-polarization is less than that for p-polarization. So the central wavelength of two polarization modes will separate more and more obviously when the incident angle is increasing. Using both
high and low index materials as the spacer of the stack, the central wavelengths of the two polarization modes may be centered at the same one. In oblique incidence, the refractive index of the two polarization modes will change. The refractive index of spolarization is ns = n cos h0 and the refractive index of the p-polarization is np = n/cos h0, where n is the refractive index in normal incidence. According to the theory of the thin film matrix, we can calculate the transmission curves of the s-polarization and p-polarization modes from the stack of the multiple layers thin film. So we can get the central wavelength of the p-polarization kp and the central wavelength of the s-polarization ks. Hence, the central wavelength separation degree CWL of the two polarization modes can be expressed as CWL ¼ jkp ks j
ð2Þ
using both high and low refractive index materials as a spacer of the multiple cavities thin film filter, the stack of each cavity can be defined as follows: p
ðHLÞ ðs1 H Þðs2 LÞ ðsi H ÞðLH Þ
p
ð3Þ
In the stack (3), the high index material H is Ta2O5 (nH = 2.06) and the low index material L is SiO2 (nL = 1.465). They are both quarter wavelength coating, and the design central wavelength is at 1563 nm in normal incidence. Between each cavity, we use the low index material L as the coupling layer. Therefore, the spectrum characteristics of the stack can be determined by the cavity number q, reflect coatings number p, the high and low refractive index materials number i of the spacer and the interference grade order si of each index material. The characteristic parameter of the whole stack can be expressed as {p, q, si, i}. As a quarter stack, the independent variables q, p, i and si are all positive integers, and their parameters range are finite: q 2 (2–5), p 2 (5–8), si 2 (2–8). So we can get the appropriate results through optimizing the independent variables by the computer calculation. In 100 GHz DWDM system, the maximal bandwidth of the thin film interference filter at 25 dB is b = 1.2 nm, the minimum bandwidth of it at 0.5 dB is a = 0.4 nm, the maximal insertion loss is ‘ = 0.1 dB and the maximal central wavelength separation degree is r = 0.05 nm. So the design index of the angle-tuned thin film filter should content the constraint condition as follows: sðk a=2; k þ a=2Þ 6 ‘; BW 0:5 dB P a; BW 25 dB 6 b; CWL 6 r
ð4Þ
Where s(k) = 10 log(T(k)). Under the constraint condition, the effective range of the convergence {p, q, si, i} is finite. Therefore, the optimization can be described as the minimization of the convergence {p, q, si, i} which can content the constraint condition. We also proposed an evaluation function w({p, q, si, i}) to evaluate the stack result which contents the constraint condition.
K. Yu et al. / Optics Communications 281 (2008) 3709–3714 2
2
wðfp; q; si ; igÞ ¼ -fð10fÞ þ -eð1 eÞ þ -rð10rÞ
2
ð5Þ
Where the rectangular degree of the filter is e = BW0.5 dB/ BW25 dB and the ripple coefficient is n = (Tmax Tmin/T0. -f, -e and -r are the weighted factors of the rectangular degree, ripple coefficient and central wavelength separation degree. The value of the three weighted factors all can be defined as 1. The incident angle range is from 0° to 15° and the central wavelength is at 1563 nm in normal incidence. The value of the evaluation function can indicate the tune performance of all the stacks which content the constraint condition, and the best stack can have the minimum value of the evaluation function. We start the initial characteristic parameter {p, q, si, i} as {5, 2, 2, 1}. So the initial calculation stack is (HL)5(2H)(LH)5L(HL)5(2H)(LH)5, which a double cavities thin film stack is. Then the computer can calculate all the appropriate stacks within the parameter range which content the constraint condition. Then we can get the best one by comparing the evaluation function values of all the possible stacks. By tolerance optimizing analysis and comparing the evaluation function values of all the possible stacks, one of the best stacks of 100 GHz multiple cavities angle-tuned thin-film filter is designed as follows, which the final values of {p, q, si, i} is {8, 4, 4, 3}. ," G
7
7
8
8
8
8
7
7
ðHLÞ 2L3H 4L3H 2LðLH Þ LðHLÞ 2L3H 4L3H 2LðLH Þ L ðHLÞ 2L3H 4L3H 2LðLH Þ LðHLÞ 2L3H 4L3H 2LðLH Þ
Fig. 1 shows the simulation transmittance curves of the stack at different incident angle. This design is a four cavities structure stack. We can see from the simulation results that the central wavelength is at 1563 nm when the angle-tuned thin film filter is in normal incidence, and it shifts to 1544.5 nm at the incident angle of 14.5°. Meanwhile, the transmission curve has not distort and the central wavelengths of two polarization modes are approximately centered at the same wavelength. When the incident angle is more than 15°, the central wavelength separation degree of the two polarization modes will exceed
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the constraint condition. If the central wavelength separation degree r can add to 0.1 nm, the tunable range of the filter can be enlarged to 26 nm. In order to get larger tuning range under the same constraint condition, we can use the polarization beam-splitters to modulate only one polarization light. As the spacer stack of the angle-tuned thin film filter is 2L3H4L3H2L, the effective refractive index of the stack is 1.64, so the value of e is 0.608. 2.2. Structure of the three-port tunable filter In the DWDM system, a three-port tunable filter can select the specific wavelength out of the multiple wavelengths, and the residual wavelengths will transmit in the fiber without any disruption. In this paper we also developed a three-port tunable filter, as shown in Fig. 2, which using the angle-tuned filter we designed above as the tune core. Although the central wavelengths of the two polarizations are centered at the same wavelength, but the tuning range of the angle-tuned thin film is finite (less than 20 nm), and the bandwidths of two polarization modes are changing while the incident angle of the thin film filter is increasing. The bandwidth of p-polarization light is large than that of the average light and the bandwidth of s-polar#, A
ð6Þ
ization light is less than that of the average light. When the incident angle is larger than 15°, the bandwidth of the p-polarization at the 25 dB is 1.26 nm, which exceeds the constraint condition. However, the bandwidth of the s-polarization at both 25 dB and 0.5 dB contents the design indexes within the incident angle of 20°. The central wavelength of the s-polarization light can shift to 1528 nm at the incident angle of 20°. In order to get larger tuning range, we can use the polarization beam-splitters and the half wave plates to transmit only the s-polarization light. It will greatly enlarge the tuning range of the angle-tuned thin film
Fig. 1. The simulation transmittance curves of the stack (6). (a) At the incident angle of 0°, (b) at the incident angle of 14.5°.
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Fig. 2. The structure of the three-port tunable filter.
filter which can cover the whole C-band. Furthermore, due to the angle-tuned thin film filter only modulates the spolarization light, the polarization dependent loss of it will be also greatly reduced. This three-port tunable filter including: an angle-tuned interference thin film filter; a pair of polarization beamsplitters A and B which position on the two sides of the filter in the same light path; a pair of half wave plate C and D which position on the two sides of the filter in the different light path. The polarization beam-splitter A divides the input signal from the optical circulator into two parallel light paths which are the p-polarization and the s-polarization light. The half wave plate C at the p-polarization light path rotates the p-polarization light into the s-polarization light. Then the two paths of s-polarization light arrive the filter which normal is tilting to the light paths. Filter transmits the wavelength ki form multiple wavelengths and reflects the residual wavelengths to the mirror E which is keeping parallel to the filter all along. The half wave plate D at another s-polarization light path rotates the s-polarization light into the p-polarization light behind the filter. The polarization beam-splitter B then converges the ppolarization and the s-polarization light into a random polarization light to the drop port. The mirror F keeps uprightness to the light paths behind the mirror E. The residual signal with different wavelengths will back to the polarization beam-splitter A through the two mirrors. Then it goes out from the output port at the three-port optical circulator. That’s the whole process to choose one wavelength from the multiple wavelengths. It can choose another wavelength when the oblique angle of the filter and the mirror E which is keeping parallel to the filter are changed. This is the principle of the TFF (thin film filter) based three-port tunable optical filter. The polarization beam-splitters are coupled with the single fiber collimators via fiber. By using a pair of the polarization beam-splitters and the half wave plates, the
polarization mode of the input light at the thin film filter will be only the s-polarization mode, so the polarization dependent loss of the transmission light will be greatly reduced. As the tuning range of the s-polarization is larger than the p-polarization and the average light, the tuning range of the filter can be greatly enlarged which can easily cover the whole C-band (from 1528 nm to 1561 nm). The oblique incidence angle range of thin film filter is from 4.8° to 20°. In the output port and the drop port, there is no change in the signals, because the half wave plates rotate one s-polarization light back to the p-polarization light. So the light at any port is in random polarization. This three-port tunable filter has a simple structure and it is easy to be fabricated. The control of the filter is flexible for using a stepping motor to tune which wavelength should be dropped. The transmission of its light path is also very easy. By using the mirrors, the couple of polarization beam-splitters can be fixed because the residual reflected signal will back to the input port along the same light path. Without continuously collimating the optical signal, it can reduce the difficulty of the adjusting, improve the stability and much easier to be realized in practice. In the tunable filter, the reflected light path ensures the incidence and the reflection beam arrive the thin film filter in the same position twice, therefore there will be less residual filtered signal. So the intraband crosstalk will decrease obviously. However, the modulation time of it is a little slow (the maximum tuning time of the sample tunable filter is less than 6 seconds) due to the mechanism modulation of the stepping motor. 3. Experimental results and analysis The Filtech Corporation in China has prepared the angle-tuned thin film filter designed by us on the Veeco instruments. The sample device of the three-port tunable filter is fabricated in the Accelink Technologies Co.,Ltd in China. The incident angle range of the tunable filter is from 4.8° to 20° so that it can cover the whole C-band (1528–1561 nm). Figs. 3 and 4 show the measured transmission and reflection spectrum of the tunable filter at different incident angle. From Figs. 3 and 4 we can see that the shape of the transmission spectrum is good for high rectangular degree, the insertion loss is less than 2 dB. The central transmission wavelength is at 1561 nm when the incident angle is 4.8°, and it shifts to the 1528 nm at the incident angle of 20°. The shape of the reflection spectrum is also good and we can see from the spectrum that the isolation degree is more than 30 dB. In the 100 GHz DWDM system, the tuning range can cover the whole C-band and it can choose almost 40 channels. The spectrum shape at 20° incidence is coincide with the 4.8° incidence. In the whole modulation process, the bandwidth at 0.5 dB is more than 0.3 nm and the bandwidth at 25 dB is less than 1.3 nm, which absolutely satisfy the specification of the 100 GHz DWDM system.
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Fig. 3. The measured transmission spectrum. (a) At the incident angle of 4.8°, (b) at the incident angle of 20°.
Fig. 4. The measured reflection spectrum. (a) At the incident angle of 4.8°, (b) at the incident angle of 20°.
Fig. 5. The measured spectrum of three DWDM adjacent channels.
In order to test the influence of the drop channel to its adjacent channels, we input three adjacent channels of the 100 GHz DWDM system. Fig. 5 shows the three channels at the middle of the tuning range (1543–1546 nm) without drop any one. Then we drop the middle channel of the
Fig. 6. The measured spectrum when middle channel is dropped.
three channels, and the Fig. 6 shows the measured spectrum. From Fig. 6 we can see that the drop channel has no adverse effect on the performance of the adjacent channels, and the adjacent channel isolation degree N is more than 35 dB.
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4. Conclusion
References
In this paper, we have developed a stack of 100 GHz DWDM four cavities interference narrowband thin film angle-tuned filter and a three-port tunable filter with new configuration. It has a simple structure, stable transmission characteristics, simple light path and it is easy to be fabricated. As the stack is optimized and the system only transmits the s-polarization light, the tunable filter has eliminated the phenomenon of polarization light central wavelength separation, so the polarization-sensitivity of the tunable filter is very low. The experiments also demonstrate that its insertion loss is low, its adjacent channel isolation degree is very high, and its effective tuning range can cover the whole C-band. This type of three-port tunable filter can be the core unit of the reconfigurable optical add/ drop multiplexer (ROADM), which is a key apparatus of the DWDM system. It has a bright application opportunity for its flexibility, low cost, and wide tuning range.
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