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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 26 — Dec. 21, 2009
  • pp: 23380–23388
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Dynamic switching characteristics of InGaAsP/InP multimode interference optical waveguide switch

Shinji Tomofuji, Shinji Matsuo, Takaaki Kakitsuka, and Ken-ichi Kitayama  »View Author Affiliations


Optics Express, Vol. 17, Issue 26, pp. 23380-23388 (2009)
http://dx.doi.org/10.1364/OE.17.023380


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Abstract

Multimode interference (MMI) waveguide switches show promise for switch in optical packet switching (OPS). In this work, we fabricated 1 × 4 InGaAsP/InP MMI waveguide switch device which consists of a 1 × 4 MMI splitter, 4 equally spaced single-mode waveguides with phase shifters, and a 4 × 4 MMI combiner. Good crosstalk and extinction ratio of −14.47 dB and 23.39 dB, respectively, are obtained. In addition, we experimentally demonstrate dynamic switching, and the rise and fall time of 1.4 ns and 1.2 ns, respectively, are obtained.

© 2009 OSA

1. Introduction

Low-power consumption is demanded for electric IP router as well as high bit rate interface beyond 40 Gbit/s. Optical packet switching (OPS) might be a promising solution to these issues. Optical delay line is only an option for buffer memory in the optical domain [1

1. D. B. Sarrazin, H. F. Jordan, and V. P. Heuring, “Fiber optic delay line memory,” Appl. Opt. 29(5), 627–637 (1990). [CrossRef] [PubMed]

]. There has been an optoelectronic approach to optical buffer, which consists of static random access memory (RAM), optical serial-to-parallel converter, and optical parallel-to-serial converter [2

2. R. Takahashi, T. Nakahara, K. Takahata, H. Takenouchi, T. Yasui, N. Kondo, and H. Suzuki, “Ultrafast optoelectronic packet processing for asynchronous, optical-packet-switched networks,” J. Opt. Netw. 3(12), 914–930 (2004). [CrossRef]

]. Lack of optical RAM for the buffer is one of obstacles to realize OPS. A challenge to all-optical RAM remains. In the R&D program where we are involved, nano-structured optical bit memory is a promising device because the retention time of the memory has been drastically improved up to a few hundred nanoseconds recently [3

3. A. Shinya, S. Matsuo, T. Yosia, T. Tanabe, E. Kuramochi, T. Sato, T. Kakitsuka, and M. Notomi, “All-optical on-chip bit memory based on ultra high Q InGaAsP photonic crystal,” Opt. Exp. 16(23), 19382–19387 (2008). [CrossRef]

]. Hence, the two-dimensional array of the optical bit memory can be constructed for the all-optical RAM.

An optical addressor is required in order to store a series of bits of optical packet, an optical pulse sequence, in the all-optical bit memories. As shown in Fig. 1
Fig. 1 Schematic diagram of Optical Addressor and Optical Bit Memory Array.
, the optical addressor can spatially steer and position a light beam of optical pulse to a designated array of optical bit memory. We will present optical switch devices, which can be used for the addressing to the optical bit memory. Requirements of the optical addressor are large port count and high speed response. To meet these requirements, many types of the optical space switch have been proposed because they enable us to obtain a bit rate free and signal format free operations [4

4. R. Ulrich and G. Ankele, “Self-imaging in homogeneous planar optical waveguides,” Appl. Phys. Lett. 27(6), 337–339 (1975). [CrossRef]

]. Examples are the broadcast and select type switch using the SOA gates [5

5. S. Tanaka, S.-H. Jeong, S. Yamazaki, S. Tomabechi, A. Uetake, M. Ekawa, and K. Morito, “Polarization-insensitive monolithically-integrated 8:1 SOA gate switch with large gain and high extinction ratio,” OFC 2008, OWE2 (2008).

] and the waveguide switch using the phased-array [6

6. T. Tanemura, M. Takenaka, A. Abdullah, K. Takeda, T. Shioda, M. Sugiyama, and Y. Nakano, “Design and fabrication of integrated 1×5 optical phased array switch on InP,” LEOS 2007, 780–781 (2007).

,7

7. K. Shimomura and Y. Kawakita, “Wavelength selective switch using arrayed waveguides with linearly varying refractive index distribution,” IPAP Books 2, 341–354 (2005).

]. About the latter switch, it is reported that the average crosstalk is −15.5 dB, the dynamic extinction ratio is larger than 13 dB, and the switching time is less than 10 ns [8

8. T. Tanemura, K. Takeda, and Y. Nakano, “320-Gbps wavelength-multiplexed 1x5 optical packet switching using broadband InP phased-array switch,” OFC 2009, OMU3 (2009).

]. We have also proposed and demonstrated the optical space switches using the multimode interference (MMI) structure, because of their potentials of large port count, small footprint, and good fabrication tolerance [9

9. S. Niwa, S. Matsuo, T. Kakitsuka, and K.-i. Kitayama, “Experimental demonstration of 1×4 InP/InGaAsP optical integrated multimode interference waveguide switch,” IPRM 2008, ThA1.7 (2008).

]. There have been proposed a wide variety of MMI waveguide switch devices such as type of Si waveguides [10

10. Z. Jin and G. Peng, “Designing optical switches based on silica multimode interference devices,” PIER 2005, 58–61 (2005).

] and GaAs/AlGaAs waveguides [11

11. R. M. Jenkins, J. M. Heaton, D. R. Wight, J. T. Parker, J. C. H. Birbeck, G. W. Smith, and K. P. Hilton, “Novel 1×N and N×N integrated optical switches using self-imaging multimode GaAs/AlGaAs waveguides,” Appl. Phys. Lett. 64(6), 684–686 (1994). [CrossRef]

]. About the latter type, it is reported that the average crosstalk is −13.6 dB for 1 × 10 device. However, there are few studies on the MMI waveguide switch which consists of InGaAsP/InP waveguides, which could be monolithically integrated with a semiconductor optical amplifier (SOA) to compensate the loss in the switch. Furthermore, there are no experimental results on dynamic switching characteristics.

In this paper, we prepare and experimentally demonstrate the dynamic switching of 1 × 4 InGaAsP/InP MMI waveguide switch. First, the operation principle of optical switch is introduced, followed by the device design and fabrication. Finally, the experimental results of the test optical switch will be presented. Good crosstalk and extinction ratio of −14.47 dB and 23.39 dB, respectively, are obtained. The rise and fall times are 1.4 ns and 1.2 ns, respectively for dynamic switching.

2. Operation principle

A schematic diagram of 1 × N MMI waveguide switch is shown in Fig. 2
Fig. 2 Schematic diagram of proposed MMI waveguide switch.
. In general, 1 × N switch consists of a single-mode input waveguide, a 1 × N MMI splitter, N equally spaced single-mode waveguides with phase shifters, an N × N MMI combiner, and N equally spaced single-mode output waveguides.

Here, we discuss the operation principle of the proposed MMI waveguide switch. In general, N multiple images are exited at the output of the MMI section (z=Lmmi) in N × N MMI structure based on the self-imaging property when the light signal is launched from the input arms j [12

12. M. Bachmann, P. A. Besse, and H. Melchior, “Overlapping-image multimode interference couplers with a reduced number of self-images for uniform and nonuniform power splitting,” Appl. Opt. 34(30), 6898–6910 (1995). [CrossRef] [PubMed]

]. The phase relations at the output arms k in relation to the input arms j are given by following expression:
Δφjk={φ0+π+π4N(N+j+k1)(3Njk+1)                                                                                                       forN+j+k:oddφ0+π4N(Nj+k)(N+jk)                                                                                                     forN+j+k:even
(1)
where ϕ0 is a constant phase shift depending on the MMI length. Under this condition, the field distribution at the input of the MMI section (z=0) is denoted Ψj(x,0) and the phase relations of output multiple images at the output of the MMI section (z=Lmmi) is denoted (θj1,θj2,...,θjN).

Now, we consider that N light signals with phase relationship of (θj1,θj2,...,θjN) are inserted from N input arms. The field distribution of these light signals at the input of the MMI section (z=0) is denoted Ψj(x,0). By assuming Ψj(x,0) as the sum of the field distribution of the single light signal Ψj(x,0), Ψj(x,0) can be expressed as follows:
Ψj(x,0)=1Nq=0N1Ψj(xxq,0)exp(iθjq)=Ψj(x,Lmmi)
(2)
where xq is a lateral position of qth light signal. This expression shows that the field distribution Ψj(x,0) merges at the output arms j by traveling a distance of Lmmi. The proposed MMI waveguide switch uses this merge function of the N × N MMI structure as switch operation. To switch the light to desired output port, the phases of the light signals split by 1 × N multimode waveguides are changed by phase shifters.

For example, when switching the input light to output port 1 in 1 × 4 switch, the relative phases of the four input lights of 4 × 4 MMI combiner set to be (0, −5π/4, -π/4, 0) by using the phase shifters, because the relative phases at the front of 4 × 4 MMI combiner are (0, 5π/4, π/4, 0) when the light is launched reversely from output port 1 [12

12. M. Bachmann, P. A. Besse, and H. Melchior, “Overlapping-image multimode interference couplers with a reduced number of self-images for uniform and nonuniform power splitting,” Appl. Opt. 34(30), 6898–6910 (1995). [CrossRef] [PubMed]

]. To switch the input light to port 1, required phase changes by the phase shifters are (-π/4, -π, 0, -π/4). In the same way, to switch the input light to port 2, required phase changes are (0, -π/4, -π/4, -π). To switch the input light to port 3, required phase changes are (-π, -π/4, -π/4, 0). To switch the input light to port 4, required phase changes are (-π/4, 0, -π, -π/4).

3. Design and device fabrication

In order to design the proposed switch device, we perform numerical modeling using the finite-difference beam propagation method (FD-BPM). To simplify designing the MMI waveguide switch device, we performed 2-D modeling of the waveguide by the effective index method. Then, we used FD-BPM to calculate the field evolution on the basis of the 2-D switch structure. By using FD-BPM, we got designed 1 × 4 MMI switch device parameter as follows. MMI length (Lmmi), MMI width (We), and waveguide width is 411.8 μm, 14.40 μm, and 1.6 μm, respectively. Each arm is separated by a lateral distance of 2.0 μm to decrease the current outflow among the arms in the actual device. The designed high-mesa waveguide structure is shown in Fig. 3
Fig. 3 Designed high-mesa waveguide structure.
. Refractive index of InP in clad layer is 3.168, and InGaAsP (1.4Q) in core layer is 3.440. The light intensity pattern in the 1 × 4 MMI waveguide switch device for two different output ports is illustrated in Fig. 4
Fig. 4 Theoretical light intensity pattern in the 1 × 4 MMI waveguide switch device for two different output ports.
. Figure 4 shows the output lights, which are switched from input port 1 to output port 1 in (a) and port 2 in (b), respectively, by changing amount of phase shift in phase shifters. Figure 5
Fig. 5 Theoretical light intensity distribution at the output for two different output ports.
shows the light intensity distribution at the output for two different output ports. We obtained good extinction ratio of more than 23 dB for each port in theory.

4. Experimental results

We conducted static switching operation. To switch the light to desired output port of 4 × 4 MMI combiner, the phases of light split by 1 × 4 MMI splitter are changed by phase shifters. Figure 7(a)
Fig. 7 Near field pattern of the output light.
shows the near field pattern of the output light without applying any current to the phase shifters. Figures 7(b), (c), (d), and (e) show the near field patterns with applying current to the phase shifters for switching to port 1, 2, 3, and 4, respectively. Table 1

Table 1. Measured Performance of the Fabricated MMI Waveguide Switch

table-icon
View This Table
summarizes the crosstalks and extinction rations, measured in experiment. The wavelength of the tunable laser diode is set at 1550 nm. The input light is launched into the input port by using a lensed single mode fiber, and the light emerging from output ports are focused into a lensed single mode fiber array. The fiber array outputs are terminated in photodetectors to measure output power. The phase shifters are operated by using four independent current sources. The output power variation for each output port was ± 14%. The typical values of crosstalk and extinction ratio were −14.47 dB and 23.39 dB, respectively.

Figure 8
Fig. 8 Relation between wavelength and output power.
shows the output power from port 3 against the wavelength of the input light. The output power is normalized so that the maximum power is 0 dB. Figure 8(a) shows the experimental result when we changed only the wavelength of the tunable laser diode. The output power has a peak at 1550 nm, and decreases as the wavelength gets shorter or longer from 1550 nm. The experimental wavelength dependence is in good agreement with the simulation results denoted by (b). This means the fabricated device has wavelength dependence. Figure 8(c) shows the experimental result when we adjusted the injection current values optimally, in order to obtain constant output power against the wavelength. We can see that it is possible to mitigate the decrease of the output power depending on the wavelength by adjusting the injection current values. The decrease of the output power reduced from −2.0 dB to −1.3 dB at 1530 nm, and from −1.7 dB to −1.3 dB at 1570 nm. Figure 8(d) shows the experimental result when we used the device whose MMI length is 8 μm shorter than optimal length. The wavelength where the output power has a peak is longer than that of (a). This means that center wavelength of MMI waveguide switch depends on the MMI length.

Dynamic switching operation by applying the electrical signal pattern to the phase shifter will be practically important. The laser diode outputted at 1550 nm is launched into the input port, and the light from output ports are converted into electrical signals by photodetectors to be observed by an oscilloscope. The phase shifters are operated by using a 4-channel arbitrary waveform generator (AWG) (Tektronix, AWG5004B). The electrical signal pattern from AWG changes at intervals of 20 ns to switch cyclically the light to ports 1, 2, 3, and 4, in order. Figure 9(a)
Fig. 9 Timing chart of output power from with dynamic switching.
shows the timing chart of the output power with changing the electrical signal pattern to the phase shifters dynamically. We can find that the signal from each output port is high level when the light is switched to the port, and it becomes low when the light is switched to the other port. The typical value of extinction ratio is 10 dB. Figure 10(a)
Fig. 10 Timing chart of electrical signal pattern to the phase shifters.
shows the timing chart of the electrical signal pattern to the phase shifters. The voltage of electrical signal changes from 0 mV to 930 mV. Figure 9(b) and (c) show the time variation of the output power at the moment of rising and falling with dynamic switching, respectively. The typical values of rise time and fall time are 1.4 ns and 1.2 ns, respectively. These values of switching time are limited by the changing speed of electrical signal pattern from AWG. Figure 10(b) and (c) show the time variation of the electrical signal to the phase shifter 1 at the moment of rising (port 1 to port 2) and falling (port4 to port 1) with dynamic switching, respectively. Both the rise and fall time of electrical signal are 0.8 ns. The switching time of 1.4 ns and 1.2 ns would be shorter if the changing speed of electrical signal pattern from AWG were higher. We could reduce the switching times by using carrier plasma effect to change the refractive index at phase shifters. The refractive index changes in only a few microseconds by carrier plasma effect.

5. Conclusion

We have experimentally demonstrated dynamic switching of 1 × 4 InGaAsP/InP MMI waveguide switch. First, the operation principle of optical switch was introduced, followed by the device design and fabrication. Finally, the experimental results of the test optical switch were presented. Good crosstalk and extinction ratio of −14.47 dB, and 23.39 dB, respectively, are obtained. The rise and fall times were 1.4 ns and 1.2 ns, respectively for dynamic switching. These characteristics are adequate to spatially steer and position the light beam of optical packet to the designated array of optical bit memory.

Acknowledgments

This research has been conducted under the program contract financially supported by the National Institute of Information and Communications Technology (NICT).

References and links

1.

D. B. Sarrazin, H. F. Jordan, and V. P. Heuring, “Fiber optic delay line memory,” Appl. Opt. 29(5), 627–637 (1990). [CrossRef] [PubMed]

2.

R. Takahashi, T. Nakahara, K. Takahata, H. Takenouchi, T. Yasui, N. Kondo, and H. Suzuki, “Ultrafast optoelectronic packet processing for asynchronous, optical-packet-switched networks,” J. Opt. Netw. 3(12), 914–930 (2004). [CrossRef]

3.

A. Shinya, S. Matsuo, T. Yosia, T. Tanabe, E. Kuramochi, T. Sato, T. Kakitsuka, and M. Notomi, “All-optical on-chip bit memory based on ultra high Q InGaAsP photonic crystal,” Opt. Exp. 16(23), 19382–19387 (2008). [CrossRef]

4.

R. Ulrich and G. Ankele, “Self-imaging in homogeneous planar optical waveguides,” Appl. Phys. Lett. 27(6), 337–339 (1975). [CrossRef]

5.

S. Tanaka, S.-H. Jeong, S. Yamazaki, S. Tomabechi, A. Uetake, M. Ekawa, and K. Morito, “Polarization-insensitive monolithically-integrated 8:1 SOA gate switch with large gain and high extinction ratio,” OFC 2008, OWE2 (2008).

6.

T. Tanemura, M. Takenaka, A. Abdullah, K. Takeda, T. Shioda, M. Sugiyama, and Y. Nakano, “Design and fabrication of integrated 1×5 optical phased array switch on InP,” LEOS 2007, 780–781 (2007).

7.

K. Shimomura and Y. Kawakita, “Wavelength selective switch using arrayed waveguides with linearly varying refractive index distribution,” IPAP Books 2, 341–354 (2005).

8.

T. Tanemura, K. Takeda, and Y. Nakano, “320-Gbps wavelength-multiplexed 1x5 optical packet switching using broadband InP phased-array switch,” OFC 2009, OMU3 (2009).

9.

S. Niwa, S. Matsuo, T. Kakitsuka, and K.-i. Kitayama, “Experimental demonstration of 1×4 InP/InGaAsP optical integrated multimode interference waveguide switch,” IPRM 2008, ThA1.7 (2008).

10.

Z. Jin and G. Peng, “Designing optical switches based on silica multimode interference devices,” PIER 2005, 58–61 (2005).

11.

R. M. Jenkins, J. M. Heaton, D. R. Wight, J. T. Parker, J. C. H. Birbeck, G. W. Smith, and K. P. Hilton, “Novel 1×N and N×N integrated optical switches using self-imaging multimode GaAs/AlGaAs waveguides,” Appl. Phys. Lett. 64(6), 684–686 (1994). [CrossRef]

12.

M. Bachmann, P. A. Besse, and H. Melchior, “Overlapping-image multimode interference couplers with a reduced number of self-images for uniform and nonuniform power splitting,” Appl. Opt. 34(30), 6898–6910 (1995). [CrossRef] [PubMed]

OCIS Codes
(080.1238) Geometric optics : Array waveguide devices
(130.4815) Integrated optics : Optical switching devices
(060.6719) Fiber optics and optical communications : Switching, packet

ToC Category:
Integrated Optics

History
Original Manuscript: September 10, 2009
Revised Manuscript: November 21, 2009
Manuscript Accepted: November 23, 2009
Published: December 7, 2009

Citation
Shinji Tomofuji, Shinji Matsuo, Takaaki Kakitsuka, and Ken-ichi Kitayama, "Dynamic switching characteristics of InGaAsP/InP multimode interference
optical waveguide switch," Opt. Express 17, 23380-23388 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-26-23380


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References

  1. D. B. Sarrazin, H. F. Jordan, and V. P. Heuring, “Fiber optic delay line memory,” Appl. Opt. 29(5), 627–637 (1990). [CrossRef] [PubMed]
  2. R. Takahashi, T. Nakahara, K. Takahata, H. Takenouchi, T. Yasui, N. Kondo, and H. Suzuki, “Ultrafast optoelectronic packet processing for asynchronous, optical-packet-switched networks,” J. Opt. Netw. 3(12), 914–930 (2004). [CrossRef]
  3. A. Shinya, S. Matsuo, T. Yosia, T. Tanabe, E. Kuramochi, T. Sato, T. Kakitsuka, and M. Notomi, “All-optical on-chip bit memory based on ultra high Q InGaAsP photonic crystal,” Opt. Express 16(23), 19382–19387 (2008). [CrossRef]
  4. R. Ulrich and G. Ankele, “Self-imaging in homogeneous planar optical waveguides,” Appl. Phys. Lett. 27(6), 337–339 (1975). [CrossRef]
  5. S. Tanaka, S.-H. Jeong, S. Yamazaki, S. Tomabechi, A. Uetake, M. Ekawa, and K. Morito, “Polarization-insensitive monolithically-integrated 8:1 SOA gate switch with large gain and high extinction ratio,” OFC 2008, OWE2 (2008).
  6. T. Tanemura, M. Takenaka, A. Abdullah, K. Takeda, T. Shioda, M. Sugiyama, and Y. Nakano, “Design and fabrication of integrated 1×5 optical phased array switch on InP,” LEOS 2007, 780–781 (2007).
  7. K. Shimomura and Y. Kawakita, “Wavelength selective switch using arrayed waveguides with linearly varying refractive index distribution,” IPAP Books 2, 341–354 (2005).
  8. T. Tanemura, K. Takeda, and Y. Nakano, “320-Gbps wavelength-multiplexed 1x5 optical packet switching using broadband InP phased-array switch,” OFC 2009, OMU3 (2009).
  9. S. Niwa, S. Matsuo, T. Kakitsuka, and K.-i. Kitayama, “Experimental demonstration of 1×4 InP/InGaAsP optical integrated multimode interference waveguide switch,” IPRM 2008, ThA1.7 (2008).
  10. Z. Jin and G. Peng, “Designing optical switches based on silica multimode interference devices,” PIER 2005, 58–61 (2005).
  11. R. M. Jenkins, J. M. Heaton, D. R. Wight, J. T. Parker, J. C. H. Birbeck, G. W. Smith, and K. P. Hilton, “Novel 1×N and N×N integrated optical switches using self-imaging multimode GaAs/AlGaAs waveguides,” Appl. Phys. Lett. 64(6), 684–686 (1994). [CrossRef]
  12. M. Bachmann, P. A. Besse, and H. Melchior, “Overlapping-image multimode interference couplers with a reduced number of self-images for uniform and nonuniform power splitting,” Appl. Opt. 34(30), 6898–6910 (1995). [CrossRef] [PubMed]

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