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Laser writing correction of polymer waveguide fanouts
Optics Communications 244 (2005) 171–179 www.elsevier.com/locate/optcom
Laser writing correction of polymer waveguide fanouts Namkhun Srisanit, Zhiqiang Liu, Xianjun Ke, Michael R. Wang
*
Department of Electrical and Computer Engineering, University of Miami, 1251 Memorial Drive, McArthur Bldg. Room 406, Coral Gables, FL 33146, USA Received 13 July 2004; received in revised form 10 September 2004; accepted 13 September 2004
Abstract Conventional optical waveguide fabrication by lithographic techniques can result in waveguide devices that cannot be repaired or corrected after the fabrication process. Especially when encountering error due to imperfect fabrication, a time consuming costly error correction re-fabrication would be required. We report on the non-lithographic laser writing fabrication of polymer waveguide on a newly developed 4 0 -hydroxy-4-nitroazobenzene dye functionalized polymer film that can avoid the error correction re-fabrication. The laser writing correction of power splitting ratio on waveguide directional coupler and y-branch fanouts has been demonstrated. 2004 Elsevier B.V. All rights reserved. Keywords: Polymer channel waveguides; Laser-direct writing; Waveguide fanout
1. Introduction Waveguide based optical fanout devices in directional coupler and y-junction configurations are basic building block elements for realizing various guided wave photonic devices for optical interconnection, modulation, switching, filtering, and multiplexing and demultiplexing applications. Conventional fabrication of waveguide optical * Corresponding author. Tel.: +1 305 284 4041; fax: +1 305 284 4044. E-mail addresses: [email protected] (Z. Liu), [email protected] (M.R. Wang).
fanouts is accomplished by photolithographic techniques resulting in static fanouts with fixed optical power splitting ratio among fanout channels. Such static fanout is stable but in case of fabrication error the fanout device cannot be corrected or repaired. The time consuming costly fabrication process would have to be repeated to correct any error encountered in the prior fabrication. It would be desirable that the fabricated waveguide optical fanouts could be corrected or repaired to demonstrate the required device functionality and to serve as experimental verification of any waveguide device with simulation results. Especially for the verification purpose, fast low
0030-4018/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2004.09.042
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cost waveguide device prototyping is critically important. We report herein a non-lithographic laser-direct write fabrication of waveguide optical fanouts with correctable fanout performance using a newly developed 4 0 -hydroxy-4-nitroazobenzene (HNA) dye functionalized polymer film [1,2]. The polymer film can be spin coated on a glass substrate. The programmed laser-direct writing yields the waveguide optical fanouts. The fanout optical power splitting ratios can be corrected by the additional laser writing at the corresponding fanout junction location. The concept of selective writing correction has been simulated and experimentally demonstrated.
2. Technique for fanout device correction The well-known waveguide directional coupler fanout is composed of an incident branch and a coupling branch. Using the commercial BeamPROP software from RSoft, Inc., we simulated the directional coupler fanout of various coupling conditions with the three-dimensional modal. For the simulation we considered a channel type step index rectangular waveguide, 8 lm channel waveguide width, 5 lm waveguide depth, 1 lm waveguide gap, waveguide core refractive index 1.529, substrate refractive index 1.523, laser wavelength 1550 nm, and fanout output branching angle 2. BeamPROP Launch Type is Computed Mode, Launch Power is Unit Power, and Monitor Types are Fiber Mode Power (for pathways) and Total Power (for the calculated optical field). The incident and coupling branch power exchange as a function of coupling length is shown in Fig. 1(b). Clearly, the increase of coupling length from zero will increase the coupling branch power while decrease the incident branch remaining power as expected. The even power splitting is achieved at about 300 lm coupling length of the present simulation example. When the incident branch remaining power drops to zero, further increasing of the coupling length will couple the laser power from the coupling branch back to the incident branch. This is a well-known phenomenon and will not be discussed further. The non-cosine square cou-
pling response is due to the three-dimensional simulation model. Our focus is on the initial increase of the coupling branch power and the decrease of the incident branch remaining power. At a suitable coupling length, we can achieve even output beam powers from both the incident and coupling branches. This serves as a guideline that when the coupling branch power is lower than the incident branch power and desires an increase in coupling branch power we can control the laser writing to increase the coupling length while monitoring the power splitting ratio at real time. The increasing coupling length writing should stop when we achieve the desired power splitting ratio. On the other hand, when the laser writing already results in a lower incident branch remaining power and a higher coupling branch power and the coupling branch power is to be lowered, we can widened the waveguide width (deviating from the resonant coupling condition) near the coupling region as schematically shown in Fig. 1(c). This is considered as a reverse correction. Reverse correction can be done on both the incident branch and the coupling branch. However, reverse correction on coupling branch is not as profound as on incident branch that results in a lower excess loss. Fig. 1(d) shows the simulation results of the present example. Clearly, the reverse correction is possible but not as significant as the forward correction. Since the reverse correction is more difficult than forward correction, the initial fabrication of the directional coupler fanout should be done carefully to avoid over coupling to the coupling branch. The forward correction should also be carefully performed to avoid over correction. The y-branch waveguide fanout is another wellknown fanout structure. Because of the small branching angle and the sharp feature at the y-branch junction, the fabrication imperfection often yields the imbalance fanout power splitting ratio. The adjustment of the y-branch waveguide power splitting ratio can be done by slightly increasing the channel width at the opposite side of the branching channel that desires power increase. For example, to increase the power for the upper branching channel as shown in Fig. 2(a) we should slightly increase the bottom channel width. Similarly, to increase the bottom branching channel
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Fig. 1. (a) The schematic of the 1 · 2 waveguide directional coupler fanout that can increase the coupling branch power (forward correction) through increasing the coupling length by laser writing; (b) simulation results showing such power exchange as a function of the coupling length; (c) the schematic of the reverse correction by increasing the channel waveguide width near the coupling region; and (d) simulation results of the reverse correction of the present example when both the coupling length and the reverse correction writing length are 400 lm.
power we should increase the upper-side channel width. Fig. 2(b) shows the simulation results of splitting ratio adjustment for an ideal y-branch. It appears that the slight change in the channel width is very effective to the power splitting ratio adjustment. Assuming the bottom branch waveguide width is 0.9 times of the upper branch as shown in Fig. 2(c) and an even splitting ratio is desired, we
should increase the upper side branch width. Fig. 2(d) shows a simulation result of the current examples. Because of the high correction sensitivity, the simulation results can only be used as a general guideline. Quantitative correction guideline appears not practical since it depends largely on various device parameters especially channel waveguide index profile and waveguide width that are usually
174
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Fig. 2. (a) Schematic of increasing the channel width for power splitting ratio adjustment in a symmetric y-branch waveguide; (b) the simulation results showing the sensitivity of the width adjustment effect; (c) schematic of increasing the channel width for power splitting ratio correction in an asymmetric y-branch waveguide; and (d) simulation results of the correction of the present example when the bottom branch waveguide width is 0.9 times of the upper branch.
different from the simulation. It is important to perform the correction writing while observing the correction results in real time to avoid over correction. The technique of correcting the fanout power splitting ratio by laser writing can be extended to
various cascaded structure such as 1 · 4, 1 · 8, and 1 · N directional coupler and/or y-branch waveguide fanouts. The laser writing correction is nothing more than to perform one by one correction on all 1 · 2 fanout elements. There are some other ways to change the power splitting ratio in
N. Srisanit et al. / Optics Communications 244 (2005) 171–179
the waveguide fanout [3,4]. The laser writing is clearly an effective technique for fanout fabrication and fanout power splitting ratio correction.
3. Experiments In our previous work [1,2], we introduced low loss, low cost HNA dye functionalized polymeric waveguide fabricated by the one-step nonlithographic laser writing technique. The fabricated polymer film has high absorption in the wavelength range of 400–550 nm and very low absorption near 1550 nm. Details on the polymer film properties will be reported elsewhere. The laser writing using a highly absorptive 532 nm wavelength laser beam can increase the local refractive index of the film through local heating for the realization of channel optical waveguides. Fig. 3 shows our experimental setup of the laser writing correction and real time monitoring of the waveguide fanout performance. The laser beam from a 532-nm diode-pumped solid-state laser is intensity controlled with an acousto-optics modulator (AOM) and is reflected to the polymeric waveguide sample positioned on the high accuracy X–Y servo motion stages. The waveguide
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fanout patterns are loaded to a personal computer (PC) to control the initial device writing process. A CCD camera located on top of the beam splitter monitors the real-time waveguide patterning process. A 1550-nm infrared (IR) semiconductor laser is fiber butt-coupled to the writing samples and the output intensities are monitored by an IR Vidicon camera and processed by the laser beam profiler software from New Span Opto-Technology Inc. The whole laser writing process including auto focus is controlled by a personal computer with an in-house written LabView based software code. The waveguide fanout patterns were designed using BeamPROP software that was converted to laser writer execution codes [5]. On-screen monitor captures real-time laser spot patterns from the stages. The continuous wave (CW) laser writing intensity was 1.8 mW, writing beam-focused spot size was 2 lm, writing speed was 0.28 mm/s, parallel writing line-to-line separation was 1.6 lm, and designed waveguide width in BeamPROP simulation was 8 lm. Under such situation, single mode polymer waveguide was realized with a broadened waveguide width of about 12 lm after laser writing. We believe this broadened width is caused by local spread heating of the film by the writing laser spot. Considering this broadening effect, we
Fig. 3. The experiment setup for the waveguide fanout correction and monitoring.
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set the initial channel gap of the designed directional coupler pattern to be 6 lm. The resulting channel gap will be smaller. Both directional coupler and y-branch waveguide splitter were written and corrected to achieve uniform fanouts successfully as discussed below. The waveguide directional coupler fanout intensity corrections are shown in Fig. 4. The results are in good agreement to the simulations such that the
incident branch remaining power decreases while the coupling branch output power increases as the coupling length increases until all the power are coupled from the incident branch to the coupling branch. Experimental results show that the output powers from the two branches reach uniformity at about 500-lm coupling length, different from the 300-lm simulation coupling length. The differences are due to the fact that the physically
Fig. 4. The output image patterns and center intensities of the directional coupler waveguide fanouts in the process of correction writing at different coupling length of (a) 0 lm, (b) 200 lm, (c) 400 lm, (d) 600 lm, (e) 800 lm, and (f) after the reverse correction by broadening writing width of local incident branch to 10 lm.
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Fig. 4 (continued ).
realized device parameters are different from that used in simulation such as waveguide refractive index profile, channel waveguide width, and channel gap. Thus, the simulation results can only be used as general guideline. The experimentally realized power splitting ratios in Fig. 4 are (a) 0:1, (b) 0.093:1, (c) 0.341:1, (d) 1:0.645, (e) 1:0.459, and (f) 1:0.864 achieved by the correction writing. The result of Fig. 4(f) is achieved after reverse correction writing by increasing the local incident branch channel width after laser writing to the
result of Fig. 4(e). CCD camera saturation should be avoided in order to ensure correct determination of the power splitting ratios. A 1 · N directional coupler fanout may consist several 1 · 2 branches. To reach uniform fanout for a 1 · 4 directional coupler, we can first correct the two second-order 1 · 2 branches and then to correct the first-order 1 · 2 branch to power splitting uniformity. Fig. 5(a) shows the original uneven 1 · 4 directional coupler fanouts with an power splitting ratio of 0.097:1:0.908:0.052 before
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Fig. 5. A 1 · 4 directional coupler fanout (a) before and (b) after the laser writing correction.
laser writing correction. After the laser writing correction using the technique presented above we achieved the final output power splitting ratio of 0.984:0.978:1:0.875 as shown in Fig. 5(b). Similarly, we have examined the feasibility to correct the y-branch waveguide fanout by the laser
writing technique. A non-uniform fanout caused by fabrication imperfections could be corrected by adjusting the branching width as shown in Fig. 2. The sequence to correct 1 · N y-branch fanout is also higher order branches first then lower order branches similar to that demonstrated for
Fig. 6. Results of a corrected 1 · 2 y-branch waveguide fanout.
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1 · 4 directional coupler fanout. Fig. 6 shows our correction results of a 1 · 2 y-branch fanout. A splitting ratio of 1:0.976 has been experimentally achieved.
4. Conclusion In conclusion, we have successfully demonstrated 1 · N correctable waveguide directional coupler and y-branch fanouts by laser writing on a newly developed 4 0 -hydroxy-4-nitroazobenzene (HNA) dye functionalized polymer film. The green writing laser beam induced local film heating and thus the local refractive index increase is responsible for the instant waveguide pattern generation for the 1550 nm wavelength laser beam without subsequent processing and fixing. The non-lithographic waveguide writing fabrication supports the waveguide pattern correction for the 1 · N fanout output power splitting ratio adjustment. The laser writing correction is effective for both directional coupler and y-branch waveguide fanouts. The experimental correction results agree with simulation in general and the simulation results can be used as guideline on how to achieve the writing correction of the fanout power splitting ratio. Quantified guideline from simulation is however impractical because of the difficulty to experimentally realize the exact waveguide simulation parameters such as refractive index profile
179
and waveguide width, depth, and gap. The waveguide fanout correction technique can be used to demonstrate required waveguide device functionality when in error without going through costly correction re-fabrication process used by conventional lithographic fabrication approaches. The correctable waveguide writing fabrication can also benefit low cost fast waveguide device prototyping and for fast experimental verification of any waveguide device with simulation results.
Acknowledgment This development was supported in part by the National Science Foundation.
References [1] N. Srisanit, X. Yan, J.J. Yang, M.R. Wang, in: 2002 Conference on Lasers and Electro-Optics, Long Beach, CA, May, 2002, paper TuF62, p. 140. [2] N. Srisanit, Z. Liu, X. Ke, M.R. Wang, J.J. Yang, Opt. Commun. (submitted). [3] A. Chen, V. Chuyanov, F.I. Marti-Carrera, S. Garner, W.H. Steier, S.S.H. Mao, Y. Ra, L.R. Dalton, Y. Shi, IEEE Photon. Technol. Lett. 9 (11) (1997) 1499. [4] W.H. Steier, A. Chen, S.-S. Lee, S. Garner, H. Zhang, V. Chuyanov, L.R. Dalton, F. Wang, A.S. Ren, C. Zhang, G. Todorova, A. Harper, H.R. Fetterman, D. Chen, A. Udupa, D. Bhattacharya, B. Tsap, Chem. Phys. 245 (1999) 487. [5] X. Ke, M.R. Wang, J.J. Yang, in: OSA Annual Meeting, Orlando, FL, 2002, MK13, p. 52.
Laser writing correction of polymer waveguide fanouts Namkhun Srisanit, Zhiqiang Liu, Xianjun Ke, Michael R. Wang
*
Department of Electrical and Computer Engineering, University of Miami, 1251 Memorial Drive, McArthur Bldg. Room 406, Coral Gables, FL 33146, USA Received 13 July 2004; received in revised form 10 September 2004; accepted 13 September 2004
Abstract Conventional optical waveguide fabrication by lithographic techniques can result in waveguide devices that cannot be repaired or corrected after the fabrication process. Especially when encountering error due to imperfect fabrication, a time consuming costly error correction re-fabrication would be required. We report on the non-lithographic laser writing fabrication of polymer waveguide on a newly developed 4 0 -hydroxy-4-nitroazobenzene dye functionalized polymer film that can avoid the error correction re-fabrication. The laser writing correction of power splitting ratio on waveguide directional coupler and y-branch fanouts has been demonstrated. 2004 Elsevier B.V. All rights reserved. Keywords: Polymer channel waveguides; Laser-direct writing; Waveguide fanout
1. Introduction Waveguide based optical fanout devices in directional coupler and y-junction configurations are basic building block elements for realizing various guided wave photonic devices for optical interconnection, modulation, switching, filtering, and multiplexing and demultiplexing applications. Conventional fabrication of waveguide optical * Corresponding author. Tel.: +1 305 284 4041; fax: +1 305 284 4044. E-mail addresses: [email protected] (Z. Liu), [email protected] (M.R. Wang).
fanouts is accomplished by photolithographic techniques resulting in static fanouts with fixed optical power splitting ratio among fanout channels. Such static fanout is stable but in case of fabrication error the fanout device cannot be corrected or repaired. The time consuming costly fabrication process would have to be repeated to correct any error encountered in the prior fabrication. It would be desirable that the fabricated waveguide optical fanouts could be corrected or repaired to demonstrate the required device functionality and to serve as experimental verification of any waveguide device with simulation results. Especially for the verification purpose, fast low
0030-4018/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2004.09.042
172
N. Srisanit et al. / Optics Communications 244 (2005) 171–179
cost waveguide device prototyping is critically important. We report herein a non-lithographic laser-direct write fabrication of waveguide optical fanouts with correctable fanout performance using a newly developed 4 0 -hydroxy-4-nitroazobenzene (HNA) dye functionalized polymer film [1,2]. The polymer film can be spin coated on a glass substrate. The programmed laser-direct writing yields the waveguide optical fanouts. The fanout optical power splitting ratios can be corrected by the additional laser writing at the corresponding fanout junction location. The concept of selective writing correction has been simulated and experimentally demonstrated.
2. Technique for fanout device correction The well-known waveguide directional coupler fanout is composed of an incident branch and a coupling branch. Using the commercial BeamPROP software from RSoft, Inc., we simulated the directional coupler fanout of various coupling conditions with the three-dimensional modal. For the simulation we considered a channel type step index rectangular waveguide, 8 lm channel waveguide width, 5 lm waveguide depth, 1 lm waveguide gap, waveguide core refractive index 1.529, substrate refractive index 1.523, laser wavelength 1550 nm, and fanout output branching angle 2. BeamPROP Launch Type is Computed Mode, Launch Power is Unit Power, and Monitor Types are Fiber Mode Power (for pathways) and Total Power (for the calculated optical field). The incident and coupling branch power exchange as a function of coupling length is shown in Fig. 1(b). Clearly, the increase of coupling length from zero will increase the coupling branch power while decrease the incident branch remaining power as expected. The even power splitting is achieved at about 300 lm coupling length of the present simulation example. When the incident branch remaining power drops to zero, further increasing of the coupling length will couple the laser power from the coupling branch back to the incident branch. This is a well-known phenomenon and will not be discussed further. The non-cosine square cou-
pling response is due to the three-dimensional simulation model. Our focus is on the initial increase of the coupling branch power and the decrease of the incident branch remaining power. At a suitable coupling length, we can achieve even output beam powers from both the incident and coupling branches. This serves as a guideline that when the coupling branch power is lower than the incident branch power and desires an increase in coupling branch power we can control the laser writing to increase the coupling length while monitoring the power splitting ratio at real time. The increasing coupling length writing should stop when we achieve the desired power splitting ratio. On the other hand, when the laser writing already results in a lower incident branch remaining power and a higher coupling branch power and the coupling branch power is to be lowered, we can widened the waveguide width (deviating from the resonant coupling condition) near the coupling region as schematically shown in Fig. 1(c). This is considered as a reverse correction. Reverse correction can be done on both the incident branch and the coupling branch. However, reverse correction on coupling branch is not as profound as on incident branch that results in a lower excess loss. Fig. 1(d) shows the simulation results of the present example. Clearly, the reverse correction is possible but not as significant as the forward correction. Since the reverse correction is more difficult than forward correction, the initial fabrication of the directional coupler fanout should be done carefully to avoid over coupling to the coupling branch. The forward correction should also be carefully performed to avoid over correction. The y-branch waveguide fanout is another wellknown fanout structure. Because of the small branching angle and the sharp feature at the y-branch junction, the fabrication imperfection often yields the imbalance fanout power splitting ratio. The adjustment of the y-branch waveguide power splitting ratio can be done by slightly increasing the channel width at the opposite side of the branching channel that desires power increase. For example, to increase the power for the upper branching channel as shown in Fig. 2(a) we should slightly increase the bottom channel width. Similarly, to increase the bottom branching channel
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Fig. 1. (a) The schematic of the 1 · 2 waveguide directional coupler fanout that can increase the coupling branch power (forward correction) through increasing the coupling length by laser writing; (b) simulation results showing such power exchange as a function of the coupling length; (c) the schematic of the reverse correction by increasing the channel waveguide width near the coupling region; and (d) simulation results of the reverse correction of the present example when both the coupling length and the reverse correction writing length are 400 lm.
power we should increase the upper-side channel width. Fig. 2(b) shows the simulation results of splitting ratio adjustment for an ideal y-branch. It appears that the slight change in the channel width is very effective to the power splitting ratio adjustment. Assuming the bottom branch waveguide width is 0.9 times of the upper branch as shown in Fig. 2(c) and an even splitting ratio is desired, we
should increase the upper side branch width. Fig. 2(d) shows a simulation result of the current examples. Because of the high correction sensitivity, the simulation results can only be used as a general guideline. Quantitative correction guideline appears not practical since it depends largely on various device parameters especially channel waveguide index profile and waveguide width that are usually
174
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Fig. 2. (a) Schematic of increasing the channel width for power splitting ratio adjustment in a symmetric y-branch waveguide; (b) the simulation results showing the sensitivity of the width adjustment effect; (c) schematic of increasing the channel width for power splitting ratio correction in an asymmetric y-branch waveguide; and (d) simulation results of the correction of the present example when the bottom branch waveguide width is 0.9 times of the upper branch.
different from the simulation. It is important to perform the correction writing while observing the correction results in real time to avoid over correction. The technique of correcting the fanout power splitting ratio by laser writing can be extended to
various cascaded structure such as 1 · 4, 1 · 8, and 1 · N directional coupler and/or y-branch waveguide fanouts. The laser writing correction is nothing more than to perform one by one correction on all 1 · 2 fanout elements. There are some other ways to change the power splitting ratio in
N. Srisanit et al. / Optics Communications 244 (2005) 171–179
the waveguide fanout [3,4]. The laser writing is clearly an effective technique for fanout fabrication and fanout power splitting ratio correction.
3. Experiments In our previous work [1,2], we introduced low loss, low cost HNA dye functionalized polymeric waveguide fabricated by the one-step nonlithographic laser writing technique. The fabricated polymer film has high absorption in the wavelength range of 400–550 nm and very low absorption near 1550 nm. Details on the polymer film properties will be reported elsewhere. The laser writing using a highly absorptive 532 nm wavelength laser beam can increase the local refractive index of the film through local heating for the realization of channel optical waveguides. Fig. 3 shows our experimental setup of the laser writing correction and real time monitoring of the waveguide fanout performance. The laser beam from a 532-nm diode-pumped solid-state laser is intensity controlled with an acousto-optics modulator (AOM) and is reflected to the polymeric waveguide sample positioned on the high accuracy X–Y servo motion stages. The waveguide
175
fanout patterns are loaded to a personal computer (PC) to control the initial device writing process. A CCD camera located on top of the beam splitter monitors the real-time waveguide patterning process. A 1550-nm infrared (IR) semiconductor laser is fiber butt-coupled to the writing samples and the output intensities are monitored by an IR Vidicon camera and processed by the laser beam profiler software from New Span Opto-Technology Inc. The whole laser writing process including auto focus is controlled by a personal computer with an in-house written LabView based software code. The waveguide fanout patterns were designed using BeamPROP software that was converted to laser writer execution codes [5]. On-screen monitor captures real-time laser spot patterns from the stages. The continuous wave (CW) laser writing intensity was 1.8 mW, writing beam-focused spot size was 2 lm, writing speed was 0.28 mm/s, parallel writing line-to-line separation was 1.6 lm, and designed waveguide width in BeamPROP simulation was 8 lm. Under such situation, single mode polymer waveguide was realized with a broadened waveguide width of about 12 lm after laser writing. We believe this broadened width is caused by local spread heating of the film by the writing laser spot. Considering this broadening effect, we
Fig. 3. The experiment setup for the waveguide fanout correction and monitoring.
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set the initial channel gap of the designed directional coupler pattern to be 6 lm. The resulting channel gap will be smaller. Both directional coupler and y-branch waveguide splitter were written and corrected to achieve uniform fanouts successfully as discussed below. The waveguide directional coupler fanout intensity corrections are shown in Fig. 4. The results are in good agreement to the simulations such that the
incident branch remaining power decreases while the coupling branch output power increases as the coupling length increases until all the power are coupled from the incident branch to the coupling branch. Experimental results show that the output powers from the two branches reach uniformity at about 500-lm coupling length, different from the 300-lm simulation coupling length. The differences are due to the fact that the physically
Fig. 4. The output image patterns and center intensities of the directional coupler waveguide fanouts in the process of correction writing at different coupling length of (a) 0 lm, (b) 200 lm, (c) 400 lm, (d) 600 lm, (e) 800 lm, and (f) after the reverse correction by broadening writing width of local incident branch to 10 lm.
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Fig. 4 (continued ).
realized device parameters are different from that used in simulation such as waveguide refractive index profile, channel waveguide width, and channel gap. Thus, the simulation results can only be used as general guideline. The experimentally realized power splitting ratios in Fig. 4 are (a) 0:1, (b) 0.093:1, (c) 0.341:1, (d) 1:0.645, (e) 1:0.459, and (f) 1:0.864 achieved by the correction writing. The result of Fig. 4(f) is achieved after reverse correction writing by increasing the local incident branch channel width after laser writing to the
result of Fig. 4(e). CCD camera saturation should be avoided in order to ensure correct determination of the power splitting ratios. A 1 · N directional coupler fanout may consist several 1 · 2 branches. To reach uniform fanout for a 1 · 4 directional coupler, we can first correct the two second-order 1 · 2 branches and then to correct the first-order 1 · 2 branch to power splitting uniformity. Fig. 5(a) shows the original uneven 1 · 4 directional coupler fanouts with an power splitting ratio of 0.097:1:0.908:0.052 before
178
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Fig. 5. A 1 · 4 directional coupler fanout (a) before and (b) after the laser writing correction.
laser writing correction. After the laser writing correction using the technique presented above we achieved the final output power splitting ratio of 0.984:0.978:1:0.875 as shown in Fig. 5(b). Similarly, we have examined the feasibility to correct the y-branch waveguide fanout by the laser
writing technique. A non-uniform fanout caused by fabrication imperfections could be corrected by adjusting the branching width as shown in Fig. 2. The sequence to correct 1 · N y-branch fanout is also higher order branches first then lower order branches similar to that demonstrated for
Fig. 6. Results of a corrected 1 · 2 y-branch waveguide fanout.
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1 · 4 directional coupler fanout. Fig. 6 shows our correction results of a 1 · 2 y-branch fanout. A splitting ratio of 1:0.976 has been experimentally achieved.
4. Conclusion In conclusion, we have successfully demonstrated 1 · N correctable waveguide directional coupler and y-branch fanouts by laser writing on a newly developed 4 0 -hydroxy-4-nitroazobenzene (HNA) dye functionalized polymer film. The green writing laser beam induced local film heating and thus the local refractive index increase is responsible for the instant waveguide pattern generation for the 1550 nm wavelength laser beam without subsequent processing and fixing. The non-lithographic waveguide writing fabrication supports the waveguide pattern correction for the 1 · N fanout output power splitting ratio adjustment. The laser writing correction is effective for both directional coupler and y-branch waveguide fanouts. The experimental correction results agree with simulation in general and the simulation results can be used as guideline on how to achieve the writing correction of the fanout power splitting ratio. Quantified guideline from simulation is however impractical because of the difficulty to experimentally realize the exact waveguide simulation parameters such as refractive index profile
179
and waveguide width, depth, and gap. The waveguide fanout correction technique can be used to demonstrate required waveguide device functionality when in error without going through costly correction re-fabrication process used by conventional lithographic fabrication approaches. The correctable waveguide writing fabrication can also benefit low cost fast waveguide device prototyping and for fast experimental verification of any waveguide device with simulation results.
Acknowledgment This development was supported in part by the National Science Foundation.
References [1] N. Srisanit, X. Yan, J.J. Yang, M.R. Wang, in: 2002 Conference on Lasers and Electro-Optics, Long Beach, CA, May, 2002, paper TuF62, p. 140. [2] N. Srisanit, Z. Liu, X. Ke, M.R. Wang, J.J. Yang, Opt. Commun. (submitted). [3] A. Chen, V. Chuyanov, F.I. Marti-Carrera, S. Garner, W.H. Steier, S.S.H. Mao, Y. Ra, L.R. Dalton, Y. Shi, IEEE Photon. Technol. Lett. 9 (11) (1997) 1499. [4] W.H. Steier, A. Chen, S.-S. Lee, S. Garner, H. Zhang, V. Chuyanov, L.R. Dalton, F. Wang, A.S. Ren, C. Zhang, G. Todorova, A. Harper, H.R. Fetterman, D. Chen, A. Udupa, D. Bhattacharya, B. Tsap, Chem. Phys. 245 (1999) 487. [5] X. Ke, M.R. Wang, J.J. Yang, in: OSA Annual Meeting, Orlando, FL, 2002, MK13, p. 52.