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A wide-band polymeric electro-optic modulator array based on unidirectional coupling between multi-mode waveguide array and a vertical configured dumping planar waveguide
ARTICLE IN PRESS
Optics & Laser Technology 38 (2006) 573–576 www.elsevier.com/locate/optlastec
A wide-band polymeric electro-optic modulator array based on unidirectional coupling between multi-mode waveguide array and a vertical configured dumping planar waveguide Xuejun Lua,, Linghui Wub, Xuping Zhangc, Ray T. Chend a
Electrical Engineering Department, University of Massachusetts, Lowell, MA 01854, USA b Omega Optics, Research Square, suite 108, 10435 Burnet Rd, Austin, TX 78758, USA c Institute of Optical Communication Engineering, Nanjing University, P.R. China d Electrical and Computer Engineering Department, The University of Texas at Austin, Austin, TX 78758, USA Received 27 August 2004; received in revised form 14 February 2005; accepted 24 February 2005 Available online 15 April 2005
Abstract A polymeric wide-band electro-optic (EO) modulator array based on unidirectional mode-coupling between the guiding multimode waveguide array and a vertical configured planar dumping waveguide was developed. A low insertion loss of o1.7 dB was obtained due to asymmetrically designed guiding waveguide and the dumping waveguide. A modulation depth of 91% was achieved with the device length of 3.5 cm at a low driving voltage of 3.2 V. The employment of the dumping planar waveguide not only provides an efficient way to achieve the unidirectional coupling mechanism, but also effectively confines the light coupled from the guiding waveguides within the planar dumping waveguide. The combination of unidirectional coupling and light confinement effects ensures low cross talk of o22 dB between adjacent guiding waveguides. Since the proposed modulator is based on the unidirectional coupling mechanism, it is intrinsically wide-band. A wide-band operation from 1308 to 1324 nm was experimentally obtained. The unidirectional mode-coupling-based wide-band modulation principle can be readily applied to other wavelengths, such as 1330 and 1550 nm ranges. r 2005 Elsevier Ltd. All rights reserved. Keywords: Electro-optical modulator; Photonic devices; Unidirectional coupling polymer waveguide; Multi-mode waveguide; Wide-band modulator; Optical network
1. Introduction The proliferation of data, voice, and video communications has strained the capacity of conventional networks and motivated research on all-optical networks [1–3]. In all-optical networks, wavelength division multiplexing (WDM), optical switching and wavelength routing technologies are employed to multiplex/demultiplex, reroute and split or tap optical signals to different networks and nodes [4–6]. To make the WDM optical networks reconfigurable and reduce the high Corresponding author.
E-mail address: [email protected] (X. Lu). 0030-3992/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2005.02.007
costs associated with individual designs and fabrications of different wavelengths, tunable laser sources and wideband electro-optic (EO) modulator are highly desired for low-cost metro-optical networks. Polymeric single-mode optical waveguide-based EO modulators have been investigated and high modulation bandwidth (410 Gbit/s) has been demonstrated [7–9]. However, most of the demonstrated polymer waveguide modulators are based on interference mechanism, which is intrinsically wavelength sensitive. The wavelength sensitivity not only introduces design and fabrication difficulties, but also seriously limits the modulator’s application in dynamic reconfigurable optical network. To address the wavelength sensitivity issue, we have
ARTICLE IN PRESS 574
X. Lu et al. / Optics & Laser Technology 38 (2006) 573–576
developed an EO modulator based on a unidirectional coupling mechanism [10,11]. In this paper, we present a wide-band EO modulator array based on the unidirectional coupling mechanism. A low insertion loss of o1.7 dB was obtained due to asymmetrically designed guiding waveguide and the dumping waveguide. A modulation depth of 91% was achieved with the device length of 3.5 cm at a low driving voltage of 3.2 V. A low cross talk of o22 dB between the guiding waveguides was achieved due to the unidirectional coupling mechanism and the effective light confinement provided by the dumping planar waveguide. Since the proposed modulator array is based on the unidirectional coupling mechanism, it is intrinsically wide band. A wide-band operation from 1308 to 1324 nm was experimentally demonstrated. Based on the same unidirectional modecoupling-based wide-band modulation principle, wideband EO modulators working other wavelengths, such as 1330 and 1550 nm ranges can be achieved.
The unidirectional coupling equations can be written as [11] 2ðibn an Þ
dAn þ kmn Am ðzÞ eðam an Þz eiDz ¼ 0, dz
(1)
dAm þ knm An ðzÞ eðan am Þz eiDz ¼ 0, (2) dz where kmn is the coupling constant between the guided modes of the guiding waveguide and the planar waveguide, D ¼ bm bn is the phase-mismatch, and b; a are the propagation constant and the loss coefficient, respectively. Ignoring the loss of the guiding waveguide, an , i.e. an 0, and solving the differential Eqs. (1) and (2), one can get 2ðibm am Þ
An ðzÞ ¼ eðiDþa=2Þz ½A eðcþjdÞz þ B eðcþjdÞz ,
(3)
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2v " #3 u 2 2 1 4u jkj jkj a =b m m 5, c ¼ Re ta2 D2 þ i 2Dam þ 2 1 þ ðam =bm Þ2 1 þ ðam =bm Þ2
(4)
2. Device design based on the unidirectional coupling mechanism The schematic structure of the device is shown in Fig. 1. The device consists of a multi-mode waveguide array designed for guiding optical signals, top and bottom modulating electrodes, and a planar dumping waveguide under the guiding multi-mode waveguide. The guiding multi-mode waveguides and the dumping planar waveguide, separated by a low-index polymer buffer layer, are intentionally designed to have different effective indices, which introduces coupling modes phase-mismatches between the guiding waveguides and the planar waveguide. The phase-mismatches ensure a low insertion loss for each guiding waveguide. The dumping planar waveguide was designed to be lossy (46.5 dB/cm) such that optical energy coupled from the guiding multi-mode waveguide can be efficiently dumped out; thus, a unidirectional coupling mechanism [11] can be achieved.
Fig. 1. Schematic structure of the multi-mode waveguide-based EO modulator array.
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2v " #3 u 2 2 u 1 jkj jkj a =b m m 5. d ¼ Im4ta2 D2 þ i 2Dam þ 2 1 þ ðam =bm Þ2 1 þ ðam =bm Þ2
(5) Considering the boundary condition ðd=dzÞAn ðzÞjz¼0 ¼ 0; one get B¼A
ðam =2 þ cÞ þ jðd D=2Þ . ðam =2 þ cÞ þ jðd þ D=2Þ
(6)
Substituting (4), (5) and (6) into (3), one get the intensity of the light: I n ðzÞ ¼ jAn ðzÞj2 az
¼e
jAj e
2 ðcþjdÞz
ðam =2 þ cÞ þ jðd D=2Þ ðcþjdÞz 2 e þ . ðam =2 þ cÞ þ jðd þ D=2Þ
ð7Þ The intensity of light in the guiding waveguides is shown in Fig. 2. Notice that when phase-matched and the planar waveguide is lossy, high-efficiency unidirectional coupling can be achieved. When the coupling is phase-mismatched, less than 1 dB power coupling loss to the dumping waveguide can be obtained with the device length of 3.5 cm. According to the simulation, shown in Fig. 2, the guiding waveguide and the planar waveguide of device are intentionally designed to have a large phasemismatch between the modes of the guiding waveguide and the planar waveguide. The phase-mismatch ensures a low mode-coupling loss of o1 dB. The EO-induced index change modulates the phase-mismatch and thus the coupling efficiency. When phase-matching condition is satisfied, efficient unidirectional coupling to the
ARTICLE IN PRESS X. Lu et al. / Optics & Laser Technology 38 (2006) 573–576
dumping planar waveguide can be obtained. Due to the employment of the unidirectional coupling mechanism [11], all the different modes in the guiding waveguide can be effectively coupled to the planar waveguide; thus, multi-mode EO modulator can be obtained [11]. The planar waveguide not only provides a dumping waveguide, but also effectively confines the light coupled from the guiding waveguides. The combination of unidirectional coupling and effective light confinement results in reduced multi-channel cross talk associated with the light scattering.
3. Device fabrication and characterization The sample was prepared using disperse red 1(DR1) side chained poly(methyl methacrylate) (PMMA/DR1) from IBM Almaden Research Center. The bottom electrode was first deposited and patterned using conventional thermal deposition and lift-off technology. A 2.2 mm U9020 was then spin coated as the bottom
Fig. 2. Simulation results of the unidirectional coupling, green: phasematched lossless; upper line: phase-mismatched lossless; lower line: phase-matched with loss of 6.5 dB/cm; middle line: phase-mismatched with loss of 6.5 dB/cm; oscillating line: phase-matched, no loss.
575
cladding. A 5.1 mm PMMA/DR1 was then spin coated as the dumping planar waveguide. The refractive index of PMMA/DR1 was measured to be 1.57 at 1330 nm under TM polarization. A single-mode PMMA/DR1 polymer waveguide was then formed by standard spincoating and subsequent photolithography and reactive ion etching (RIE) processes, followed by thermal depositing and patterning of the total electrode. The sample was poled using the contact poling technology [11,12]. An EO coefficient of 22 pm/V was obtained. The device was finished by removing the poling electrode and spin-coating 2.6 mm Master Bond UV15s top cladding layer and depositing and patterning of the top modulating electrode. The cross section and top view of the device are shown in Figs. 3(a) and (b), respectively. The width of the guiding rib waveguide is 50 mm, the rib height is 0.7 mm and the thickness of the dumping planar waveguide is 5.1 mm. The device length is 3.5 cm. To characterize the performance of the device, a TM polarized laser beam is endfire coupled into the device through a 40 objective lens. The output light was focused through another 40 objective lens and then detected by a photodectctor. A modulation depth of 91% was achieved at the driving voltage of 3.2 V. A lower driving voltage can be expected when polymer materials with higher nonlinear coefficient are employed. A wide-band operation from 1308 to 1324 nm was experimentally observed due to the intrinsic wavelength insensitivity of the proposed electro-absorption mechanism. The insertion loss was measured to be 1.7 dB. The high-speed response of the device is shown in Fig. 4. The modulation frequency is 2.5 GHz and modulation voltage is 3.2 V (peak to peak). The cross talk of the adjacent channels was measured by coupling light into adjacent channels and measuring the light intensity modulation, a low cross talk ofo22 dB was obtained. The low cross-talk results from the employment of unidirectional coupling mechanism and light confinement effect of the dumping planar waveguide.
Fig. 3. Cross section and top view of the multi-mode waveguide EO modulator array. (a) (1) Top cladding, (3) buffer layer, (4) dumping planar waveguide. The solid rectangle channel is the one of the guiding multi-mode waveguides. Note that the guiding multi-modes waveguide and dumping planar waveguide are vertically configured. (b) A: multi-mode waveguide, B and C: modulating electrodes.
ARTICLE IN PRESS 576
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Fig. 4. High-speed response and eye diagram of the device. (a) 2.5G modulation response of the device, upper trace device response, lower trace modulation signal; (b) eye-diagram of the modulated optical signal.
4. Conclusion We have reported a wide-band polymeric multi-mode waveguide EO modulator based on unidirectional coupling mechanism with a vertical configured planar waveguide. Unlike traditional interference-based EO modulator, the proposed modulator is based on the unidirectional coupling mechanism, which is intrinsically wavelength insensitive. A modulation depth of 91% was experimentally achieved from 1308 to 1324 nm. A low insertion loss of 1.7 dB was obtained. Due to the light confinement and the unidirectional coupling mechanism introduced by the vertically configured planar waveguide, low multi-channel cross talk of o22 dB was achieved. The unidirectional modecoupling-based wide-band modulation principle can be readily applied to other wavelengths, such as 1330 and 1550 nm ranges. This device shows that the multi-mode optical waveguide with the vertically configured dumping planar waveguide is promising in wide-band EA modulator designs.
[2] [3]
[4] [5]
[6]
[7]
[8]
[9]
Acknowledgment This program was partially supported by the National Natural Science Foundation of China under the contract No. 6 0277020.
[10]
[11]
References [1] Leisching P, Bock H, Richter A, Glingener C, Pace P, Keyworth B, Philipson J, Farries M, Stoll D, Fischer G. All-opticalnetworking at 0.8 Tb/s using reconfigurable optical
[12]
add/drop multiplexers. IEEE Photonic Technol Lett 2000;12(7): 918–20. Rayn JP. WDM North American development trends. IEEE Commun Mag 1998:40–4. Iannone E, Sabella R, de Stefano L, Valeri F. All optical network wavelength conversion in optical multicarrier networks. IEEE Trans Commun 1996;44:716–24. Kartalopoulos SV. Introduction to DWDM technology. SPIE Optical Engineering Press; 2000. p. 91. Brackett CA, Acampora AS, Sweitzer J, Tangonan G, Smith MT, Lennon W, Wang KC, Hobbs RH. Scalable multiwavelength multihop optical network: a proposal for research on all-optical networks. J Lightwave Technol 1993;11(5): 736–53. Hashimoto T, Kurosaki T, Yanagisawa M, Suzuki Y, Akahori Y, Inoue Y, Tohmori Y, Kato K, Yamada Y, Ishihara N. A 1.3/1.55mm wavelength-division multiplexing optical module using a planar lightwave circuit for full duplex operation. J Lightwave Technol 2000;18(11):1541–7. Y. Shi. Characterization and device applications of disperse red 19 containing second-order nonlinear optical polymers. PhD dissertation, University of Southern California, 1992. Chen A. Low Vp high thermal stability electro-optic polymer waveguide modulator using full potential of high mb chromophores with a DC bias voltage. Proc SPIE 1998;3281: 94–105. Teng CC. Traveling-wave polymeric optical intensity modulator with more than 40Ghz of 3-dB electrical bandwidth. Appl Phys Lett 1992;60(3):1538–40. Lu X, Jang CH, An D, Zhou Q, Sun L, Zhang X, Chen RT. Polymeric multimode waveguide based electro-optic modulator with a vertically configured dumping planar waveguide. Appl Phys Lett 2002;81(5):795–7. Sun D, Lu X, An D, Chen RT. High performance unidirectional electro-optic modulator based on polymeric highly multimode waveguide. Opt Laser Technol 1998; 30(8):481. Zhang X, Lu X, Wu L, Chen RT. Contact poling of the nonlinear optical film for polymer-based electro-optic modulator. Proc. SPIE 2002;4653:87–95.
Optics & Laser Technology 38 (2006) 573–576 www.elsevier.com/locate/optlastec
A wide-band polymeric electro-optic modulator array based on unidirectional coupling between multi-mode waveguide array and a vertical configured dumping planar waveguide Xuejun Lua,, Linghui Wub, Xuping Zhangc, Ray T. Chend a
Electrical Engineering Department, University of Massachusetts, Lowell, MA 01854, USA b Omega Optics, Research Square, suite 108, 10435 Burnet Rd, Austin, TX 78758, USA c Institute of Optical Communication Engineering, Nanjing University, P.R. China d Electrical and Computer Engineering Department, The University of Texas at Austin, Austin, TX 78758, USA Received 27 August 2004; received in revised form 14 February 2005; accepted 24 February 2005 Available online 15 April 2005
Abstract A polymeric wide-band electro-optic (EO) modulator array based on unidirectional mode-coupling between the guiding multimode waveguide array and a vertical configured planar dumping waveguide was developed. A low insertion loss of o1.7 dB was obtained due to asymmetrically designed guiding waveguide and the dumping waveguide. A modulation depth of 91% was achieved with the device length of 3.5 cm at a low driving voltage of 3.2 V. The employment of the dumping planar waveguide not only provides an efficient way to achieve the unidirectional coupling mechanism, but also effectively confines the light coupled from the guiding waveguides within the planar dumping waveguide. The combination of unidirectional coupling and light confinement effects ensures low cross talk of o22 dB between adjacent guiding waveguides. Since the proposed modulator is based on the unidirectional coupling mechanism, it is intrinsically wide-band. A wide-band operation from 1308 to 1324 nm was experimentally obtained. The unidirectional mode-coupling-based wide-band modulation principle can be readily applied to other wavelengths, such as 1330 and 1550 nm ranges. r 2005 Elsevier Ltd. All rights reserved. Keywords: Electro-optical modulator; Photonic devices; Unidirectional coupling polymer waveguide; Multi-mode waveguide; Wide-band modulator; Optical network
1. Introduction The proliferation of data, voice, and video communications has strained the capacity of conventional networks and motivated research on all-optical networks [1–3]. In all-optical networks, wavelength division multiplexing (WDM), optical switching and wavelength routing technologies are employed to multiplex/demultiplex, reroute and split or tap optical signals to different networks and nodes [4–6]. To make the WDM optical networks reconfigurable and reduce the high Corresponding author.
E-mail address: [email protected] (X. Lu). 0030-3992/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2005.02.007
costs associated with individual designs and fabrications of different wavelengths, tunable laser sources and wideband electro-optic (EO) modulator are highly desired for low-cost metro-optical networks. Polymeric single-mode optical waveguide-based EO modulators have been investigated and high modulation bandwidth (410 Gbit/s) has been demonstrated [7–9]. However, most of the demonstrated polymer waveguide modulators are based on interference mechanism, which is intrinsically wavelength sensitive. The wavelength sensitivity not only introduces design and fabrication difficulties, but also seriously limits the modulator’s application in dynamic reconfigurable optical network. To address the wavelength sensitivity issue, we have
ARTICLE IN PRESS 574
X. Lu et al. / Optics & Laser Technology 38 (2006) 573–576
developed an EO modulator based on a unidirectional coupling mechanism [10,11]. In this paper, we present a wide-band EO modulator array based on the unidirectional coupling mechanism. A low insertion loss of o1.7 dB was obtained due to asymmetrically designed guiding waveguide and the dumping waveguide. A modulation depth of 91% was achieved with the device length of 3.5 cm at a low driving voltage of 3.2 V. A low cross talk of o22 dB between the guiding waveguides was achieved due to the unidirectional coupling mechanism and the effective light confinement provided by the dumping planar waveguide. Since the proposed modulator array is based on the unidirectional coupling mechanism, it is intrinsically wide band. A wide-band operation from 1308 to 1324 nm was experimentally demonstrated. Based on the same unidirectional modecoupling-based wide-band modulation principle, wideband EO modulators working other wavelengths, such as 1330 and 1550 nm ranges can be achieved.
The unidirectional coupling equations can be written as [11] 2ðibn an Þ
dAn þ kmn Am ðzÞ eðam an Þz eiDz ¼ 0, dz
(1)
dAm þ knm An ðzÞ eðan am Þz eiDz ¼ 0, (2) dz where kmn is the coupling constant between the guided modes of the guiding waveguide and the planar waveguide, D ¼ bm bn is the phase-mismatch, and b; a are the propagation constant and the loss coefficient, respectively. Ignoring the loss of the guiding waveguide, an , i.e. an 0, and solving the differential Eqs. (1) and (2), one can get 2ðibm am Þ
An ðzÞ ¼ eðiDþa=2Þz ½A eðcþjdÞz þ B eðcþjdÞz ,
(3)
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2v " #3 u 2 2 1 4u jkj jkj a =b m m 5, c ¼ Re ta2 D2 þ i 2Dam þ 2 1 þ ðam =bm Þ2 1 þ ðam =bm Þ2
(4)
2. Device design based on the unidirectional coupling mechanism The schematic structure of the device is shown in Fig. 1. The device consists of a multi-mode waveguide array designed for guiding optical signals, top and bottom modulating electrodes, and a planar dumping waveguide under the guiding multi-mode waveguide. The guiding multi-mode waveguides and the dumping planar waveguide, separated by a low-index polymer buffer layer, are intentionally designed to have different effective indices, which introduces coupling modes phase-mismatches between the guiding waveguides and the planar waveguide. The phase-mismatches ensure a low insertion loss for each guiding waveguide. The dumping planar waveguide was designed to be lossy (46.5 dB/cm) such that optical energy coupled from the guiding multi-mode waveguide can be efficiently dumped out; thus, a unidirectional coupling mechanism [11] can be achieved.
Fig. 1. Schematic structure of the multi-mode waveguide-based EO modulator array.
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2v " #3 u 2 2 u 1 jkj jkj a =b m m 5. d ¼ Im4ta2 D2 þ i 2Dam þ 2 1 þ ðam =bm Þ2 1 þ ðam =bm Þ2
(5) Considering the boundary condition ðd=dzÞAn ðzÞjz¼0 ¼ 0; one get B¼A
ðam =2 þ cÞ þ jðd D=2Þ . ðam =2 þ cÞ þ jðd þ D=2Þ
(6)
Substituting (4), (5) and (6) into (3), one get the intensity of the light: I n ðzÞ ¼ jAn ðzÞj2 az
¼e
jAj e
2 ðcþjdÞz
ðam =2 þ cÞ þ jðd D=2Þ ðcþjdÞz 2 e þ . ðam =2 þ cÞ þ jðd þ D=2Þ
ð7Þ The intensity of light in the guiding waveguides is shown in Fig. 2. Notice that when phase-matched and the planar waveguide is lossy, high-efficiency unidirectional coupling can be achieved. When the coupling is phase-mismatched, less than 1 dB power coupling loss to the dumping waveguide can be obtained with the device length of 3.5 cm. According to the simulation, shown in Fig. 2, the guiding waveguide and the planar waveguide of device are intentionally designed to have a large phasemismatch between the modes of the guiding waveguide and the planar waveguide. The phase-mismatch ensures a low mode-coupling loss of o1 dB. The EO-induced index change modulates the phase-mismatch and thus the coupling efficiency. When phase-matching condition is satisfied, efficient unidirectional coupling to the
ARTICLE IN PRESS X. Lu et al. / Optics & Laser Technology 38 (2006) 573–576
dumping planar waveguide can be obtained. Due to the employment of the unidirectional coupling mechanism [11], all the different modes in the guiding waveguide can be effectively coupled to the planar waveguide; thus, multi-mode EO modulator can be obtained [11]. The planar waveguide not only provides a dumping waveguide, but also effectively confines the light coupled from the guiding waveguides. The combination of unidirectional coupling and effective light confinement results in reduced multi-channel cross talk associated with the light scattering.
3. Device fabrication and characterization The sample was prepared using disperse red 1(DR1) side chained poly(methyl methacrylate) (PMMA/DR1) from IBM Almaden Research Center. The bottom electrode was first deposited and patterned using conventional thermal deposition and lift-off technology. A 2.2 mm U9020 was then spin coated as the bottom
Fig. 2. Simulation results of the unidirectional coupling, green: phasematched lossless; upper line: phase-mismatched lossless; lower line: phase-matched with loss of 6.5 dB/cm; middle line: phase-mismatched with loss of 6.5 dB/cm; oscillating line: phase-matched, no loss.
575
cladding. A 5.1 mm PMMA/DR1 was then spin coated as the dumping planar waveguide. The refractive index of PMMA/DR1 was measured to be 1.57 at 1330 nm under TM polarization. A single-mode PMMA/DR1 polymer waveguide was then formed by standard spincoating and subsequent photolithography and reactive ion etching (RIE) processes, followed by thermal depositing and patterning of the total electrode. The sample was poled using the contact poling technology [11,12]. An EO coefficient of 22 pm/V was obtained. The device was finished by removing the poling electrode and spin-coating 2.6 mm Master Bond UV15s top cladding layer and depositing and patterning of the top modulating electrode. The cross section and top view of the device are shown in Figs. 3(a) and (b), respectively. The width of the guiding rib waveguide is 50 mm, the rib height is 0.7 mm and the thickness of the dumping planar waveguide is 5.1 mm. The device length is 3.5 cm. To characterize the performance of the device, a TM polarized laser beam is endfire coupled into the device through a 40 objective lens. The output light was focused through another 40 objective lens and then detected by a photodectctor. A modulation depth of 91% was achieved at the driving voltage of 3.2 V. A lower driving voltage can be expected when polymer materials with higher nonlinear coefficient are employed. A wide-band operation from 1308 to 1324 nm was experimentally observed due to the intrinsic wavelength insensitivity of the proposed electro-absorption mechanism. The insertion loss was measured to be 1.7 dB. The high-speed response of the device is shown in Fig. 4. The modulation frequency is 2.5 GHz and modulation voltage is 3.2 V (peak to peak). The cross talk of the adjacent channels was measured by coupling light into adjacent channels and measuring the light intensity modulation, a low cross talk ofo22 dB was obtained. The low cross-talk results from the employment of unidirectional coupling mechanism and light confinement effect of the dumping planar waveguide.
Fig. 3. Cross section and top view of the multi-mode waveguide EO modulator array. (a) (1) Top cladding, (3) buffer layer, (4) dumping planar waveguide. The solid rectangle channel is the one of the guiding multi-mode waveguides. Note that the guiding multi-modes waveguide and dumping planar waveguide are vertically configured. (b) A: multi-mode waveguide, B and C: modulating electrodes.
ARTICLE IN PRESS 576
X. Lu et al. / Optics & Laser Technology 38 (2006) 573–576
Fig. 4. High-speed response and eye diagram of the device. (a) 2.5G modulation response of the device, upper trace device response, lower trace modulation signal; (b) eye-diagram of the modulated optical signal.
4. Conclusion We have reported a wide-band polymeric multi-mode waveguide EO modulator based on unidirectional coupling mechanism with a vertical configured planar waveguide. Unlike traditional interference-based EO modulator, the proposed modulator is based on the unidirectional coupling mechanism, which is intrinsically wavelength insensitive. A modulation depth of 91% was experimentally achieved from 1308 to 1324 nm. A low insertion loss of 1.7 dB was obtained. Due to the light confinement and the unidirectional coupling mechanism introduced by the vertically configured planar waveguide, low multi-channel cross talk of o22 dB was achieved. The unidirectional modecoupling-based wide-band modulation principle can be readily applied to other wavelengths, such as 1330 and 1550 nm ranges. This device shows that the multi-mode optical waveguide with the vertically configured dumping planar waveguide is promising in wide-band EA modulator designs.
[2] [3]
[4] [5]
[6]
[7]
[8]
[9]
Acknowledgment This program was partially supported by the National Natural Science Foundation of China under the contract No. 6 0277020.
[10]
[11]
References [1] Leisching P, Bock H, Richter A, Glingener C, Pace P, Keyworth B, Philipson J, Farries M, Stoll D, Fischer G. All-opticalnetworking at 0.8 Tb/s using reconfigurable optical
[12]
add/drop multiplexers. IEEE Photonic Technol Lett 2000;12(7): 918–20. Rayn JP. WDM North American development trends. IEEE Commun Mag 1998:40–4. Iannone E, Sabella R, de Stefano L, Valeri F. All optical network wavelength conversion in optical multicarrier networks. IEEE Trans Commun 1996;44:716–24. Kartalopoulos SV. Introduction to DWDM technology. SPIE Optical Engineering Press; 2000. p. 91. Brackett CA, Acampora AS, Sweitzer J, Tangonan G, Smith MT, Lennon W, Wang KC, Hobbs RH. Scalable multiwavelength multihop optical network: a proposal for research on all-optical networks. J Lightwave Technol 1993;11(5): 736–53. Hashimoto T, Kurosaki T, Yanagisawa M, Suzuki Y, Akahori Y, Inoue Y, Tohmori Y, Kato K, Yamada Y, Ishihara N. A 1.3/1.55mm wavelength-division multiplexing optical module using a planar lightwave circuit for full duplex operation. J Lightwave Technol 2000;18(11):1541–7. Y. Shi. Characterization and device applications of disperse red 19 containing second-order nonlinear optical polymers. PhD dissertation, University of Southern California, 1992. Chen A. Low Vp high thermal stability electro-optic polymer waveguide modulator using full potential of high mb chromophores with a DC bias voltage. Proc SPIE 1998;3281: 94–105. Teng CC. Traveling-wave polymeric optical intensity modulator with more than 40Ghz of 3-dB electrical bandwidth. Appl Phys Lett 1992;60(3):1538–40. Lu X, Jang CH, An D, Zhou Q, Sun L, Zhang X, Chen RT. Polymeric multimode waveguide based electro-optic modulator with a vertically configured dumping planar waveguide. Appl Phys Lett 2002;81(5):795–7. Sun D, Lu X, An D, Chen RT. High performance unidirectional electro-optic modulator based on polymeric highly multimode waveguide. Opt Laser Technol 1998; 30(8):481. Zhang X, Lu X, Wu L, Chen RT. Contact poling of the nonlinear optical film for polymer-based electro-optic modulator. Proc. SPIE 2002;4653:87–95.