## InP-based compact transversal filter for monolithically integrated light source array |

Optics Express, Vol. 22, Issue 7, pp. 7844-7851 (2014)

http://dx.doi.org/10.1364/OE.22.007844

Acrobat PDF (2171 KB)

### Abstract

We developed an InP-based 4x1 transversal filter (TF) with multi-mode interference couplers (MMIs) as a compact wavelength multiplexer (MUX) 1700 μm x 400 μm in size. Furthermore, we converted the MMI-based TF to a reflection type to obtain an ultra-compact MUX of only 900 μm x 50 μm. These MUXs are made with a simple fabrication process and show a satisfactory wavelength filtering operation as MUXs of monolithically integrated light source arrays, for example, for 100G bit Ethernet.

© 2014 Optical Society of America

## 1. Introduction

1. T. Fujisawa, S. Kanazawa, H. Ishii, N. Nunoya, Y. Kawaguchi, A. Ohki, N. Fujiwara, K. Takahata, R. Iga, F. Kano, and H. Oohashi, “1.3-μm 4 x 25-Gb/s Monolithically Integrated Light Source for Metro Area 100-Gb/s Ethernet,” IEEE Photon. Technol. Lett. **23**(6), 356–358 (2011). [CrossRef]

4. T. Fujisawa, S. Kanazawa, Y. Ueda, W. Kobayashi, K. Takahata, A. Ohki, T. Ito, M. Kohtoku, and H. Ishii, “Low-Loss Cascaded Mach–Zehnder Multiplexer Integrated 25-Gbit/s x 4-Lane EADFB Laser Array for Future CFP4 100 GbE Transmitter,” IEEE J. Quantum Electron. **49**, 1001–1007 (2013). [CrossRef]

5. H. Ishii, K. Kasaya, and H. Oohashi, “Spectral Linewidth Reduction in Widely Wavelength Tunable DFB Laser Array,” IEEE J. Sel. Top. Quantum Electron. **15**(3), 514–520 (2009). [CrossRef]

6. Y. Hibino, “Recent Advances in High-Density and Large-Scale AWG Multi/Demultiplexers with Higher Index-Contrast Silica-Based PLCs,” IEEE J. Sel. Top. Quantum Electron. **8**(6), 1090–1101 (2002). [CrossRef]

7. Y. Barbarin, X. J. M. Leijtens, E. A. J. M. Bente, C. M. Louzao, J. R. Kooiman, and M. K. Smit, “Extremely Small AWG Demultiplexer Fabricated on InP by using a Double-Etch Process,” IEEE Photon. Technol. Lett. **16**(11), 2478–2480 (2004). [CrossRef]

7. Y. Barbarin, X. J. M. Leijtens, E. A. J. M. Bente, C. M. Louzao, J. R. Kooiman, and M. K. Smit, “Extremely Small AWG Demultiplexer Fabricated on InP by using a Double-Etch Process,” IEEE Photon. Technol. Lett. **16**(11), 2478–2480 (2004). [CrossRef]

8. K. Takiguchi, T. Kitoh, M. Oguma, Y. Hashizume, and H. Takahashi, “Integrated-optic OFDM Demultiplexer using Multi-mode Interference Coupler-based Optical DFT Circuit,” in Proc. OFC 2012 OM3J.6 (2012). [CrossRef]

4. T. Fujisawa, S. Kanazawa, Y. Ueda, W. Kobayashi, K. Takahata, A. Ohki, T. Ito, M. Kohtoku, and H. Ishii, “Low-Loss Cascaded Mach–Zehnder Multiplexer Integrated 25-Gbit/s x 4-Lane EADFB Laser Array for Future CFP4 100 GbE Transmitter,” IEEE J. Quantum Electron. **49**, 1001–1007 (2013). [CrossRef]

## 2. Design of MMI-TF

*x(t)*, and the discrete Fourier transform (DFT) output,

*X(f)*, of a standard TF is as follows:

*x(t)*is sampled at every interval time,

*Δt*, from the start time,

*t*, and as a result, four

_{0}*X(f)*values are output as the DFT coefficients of

*x(t)*.

*Δf ( = 1/4Δt)*and

*f*are the frequency resolution and center frequency of the TF, respectively. In the MMI-TF we express Eq. (1) with the transfer matrices of a 1x4 MMI (

_{0}*T*), a 4-array delay line (

_{1x4MMI}*T*), and a 4x4 MMI (

_{4delay}*T*). Namely, we can obtain the four sampled

_{4x4MMI}*x(t)*values by splitting the input light with a 1x4 MMI and adding adequate time delays to each beam with a delay-line array. And we input the four beams into a 4x4 MMI designed as a 4x4 DFT circuit.

*T*, is expressed as

_{1x4splitter}*T*, as follows:

_{1x4MMI}*T*by multiplying

_{1x4splitter}*T*by a diagonal matrix whose elements compensate for the phase difference between the output ports of a 1x4 MMI. In an actual device, multiplying

_{1x4MMI}*T*by the diagonal matrix means that we add a phase-shifter array at the output of the 1x4 MMI, as shown on the right in Fig. 1(a).

_{1x4MMI}*ΔL = c/n*steps. Here,

_{g}ν_{FSR}*c*,

*n*and

_{g}*ν*are the speed of light in a vacuum, the group refractive index in the delay lines and the FSR of the filter, respectively. If we consider a single frequency, or propagation constant,

_{FSR}*β*,

*T*is expressed with

_{4delay}*ΔL*and

*β*, in the form

*T*, by a two-step conversion of

_{4x4DFT}*T*. The first step consists of interchanging the row and column elements of

_{4x4MMI}*T*as follows:

_{4x4MMI}*T*.

_{4x4DFT}8. K. Takiguchi, T. Kitoh, M. Oguma, Y. Hashizume, and H. Takahashi, “Integrated-optic OFDM Demultiplexer using Multi-mode Interference Coupler-based Optical DFT Circuit,” in Proc. OFC 2012 OM3J.6 (2012). [CrossRef]

*and*

_{4x4MMI}*T*as

_{4x4DFT}*and*

_{4x4MMI}≡ [b_{ij}]*T*, the conversion of

_{4x4DFT}≡ [d_{ij}]*T*to

_{4x4MMI}*must be implemented so that*

_{4x4MMI}*satisfies*

_{4x4MMI}*to obtain*

_{4x4MMI}*T*in Eq. (5) should be diagonal matrices. In the actual device, the conversion means that we interchange the order of the input/output (I/O) ports of the MMI and add phase shifters at each port as shown on the left in Fig. 1(a).

_{4x4DFT}*T*, from the product of Eqs. (2), (3), and (5), namely

_{MMI-TF}*T*. Figure 1(b) shows the schematic structure of an MMI-TF. Since we interchange the rows and columns of

_{MMI-TF}= T_{4x4DFT}T_{4delay}T_{1x4splitter}*T*, we need to cancel the interchange to avoid intersecting delay lines. And, when we use the MMI-TF as the MUX of an LD array, we can remove the phase-shifters at the DFT output ports (MUX input ports).

_{4x4MMI}_{MMI-TF}, which is obtained by modifying

*T*through canceling the interchange and removing the phase-shifters at the DFT output ports. Labels #1 to #4 correspond to the numbers in Fig. 1(b). Wavelength dependence is not considered in the calculation. So, in a practical design, we need to adopt adequate MMIs so that transmittance peaks of MMIs correspond to the wavelength band of the input light.

_{MMI-TF}_{MMI-TF}expresses the wavelength filtering operation, and transmittance peaks appear periodically in the order channel #1-#2-#4-#3-#1…, according to the increase in the input light wavelength. Since we interchange the 4x4-MMI ports in the design stage, as shown on the left in Fig. 1(a), DFT outputs,

*X(f)*,

*X(f + Δf)*,

*X(f + 2Δf)*,

*X(f + 3Δf)*, appear in ports #4, #2, #1, #3 respectively. The order is reversed for the wavelength domain expression in Fig. 2.

## 3. Experimental characteristics of MMI-TF

1. T. Fujisawa, S. Kanazawa, H. Ishii, N. Nunoya, Y. Kawaguchi, A. Ohki, N. Fujiwara, K. Takahata, R. Iga, F. Kano, and H. Oohashi, “1.3-μm 4 x 25-Gb/s Monolithically Integrated Light Source for Metro Area 100-Gb/s Ethernet,” IEEE Photon. Technol. Lett. **23**(6), 356–358 (2011). [CrossRef]

4. T. Fujisawa, S. Kanazawa, Y. Ueda, W. Kobayashi, K. Takahata, A. Ohki, T. Ito, M. Kohtoku, and H. Ishii, “Low-Loss Cascaded Mach–Zehnder Multiplexer Integrated 25-Gbit/s x 4-Lane EADFB Laser Array for Future CFP4 100 GbE Transmitter,” IEEE J. Quantum Electron. **49**, 1001–1007 (2013). [CrossRef]

*ΔL*) is about 25 μm and each width is 2 μm. Since we adjust the length of each line to obtain suitable phase shifters as shown in Fig. 1(b), the length differences among the lines are not strictly equal to the integral multiple of

*ΔL*. And it is found that the lengths of the lines are determined almost by the distance of the two MMIs because even the length of 3

*ΔL*is about 75 μm.

1. T. Fujisawa, S. Kanazawa, H. Ishii, N. Nunoya, Y. Kawaguchi, A. Ohki, N. Fujiwara, K. Takahata, R. Iga, F. Kano, and H. Oohashi, “1.3-μm 4 x 25-Gb/s Monolithically Integrated Light Source for Metro Area 100-Gb/s Ethernet,” IEEE Photon. Technol. Lett. **23**(6), 356–358 (2011). [CrossRef]

## 4. Conversion to reflection-type MMI-TF

9. T. Goh, S. Suzuki, and A. Sugita, “Estimation of Waveguide Phase Error in Silica-Based Waveguides,” J. Lightwave Technol. **15**(11), 2107–2113 (1997). [CrossRef]

*ΔL*and less than half the wavelength of the phase shifters as shown in Fig. 1(b). Although an essentially required line length is less than 100 μm even for the longest delay line of our 4x1 MMI-TF, in the actual device there is a “reference” delay line whose length is the shortest and is determined by the distance between the 4x4 and 4x1 MMIs. Therefore, the actual length of each delay line is the sum of the above “essential” delay length and the reference length. Although we can reduce the reference line length by placing the two MMIs closer together, such an MMI-TF requires bent waveguides with a smaller radius of curvature for the delay lines in which propagating beams tend to exhibit phase error and excite higher order modes.

## 5. Experimental characteristics of RTF

10. T. Segawa, S. Matsuo, T. Kakitsuka, Y. Shibata, T. Sato, Y. Kawaguchi, Y. Kondo, and R. Takahashi, “Monolithically Integrated Wavelength-Routing Switch using Tunable Wavelength Converters with Double-Ring-Resonator Tunable Lasers,” IEICE Trans. Electron. **E94-C**(9), 1439–1446 (2011). [CrossRef]

*ΔL*of

*~20/2 = ~10*μm for delay lines, and the factor of 1/2 is derived from the fact that the lines of the RTF are reflection type. The width of each line is 2.6 μm which is wider than that of our conventional MMI-TF, since we do not need any bent structure for the delay lines of the RTF. Thanks to the removal of the reference delay line as described in the previous section, we obtain very compact RTF whose length is about half that of the conventional MMI-TF. As regards the width, it is less than 50 μm and is determined solely by the 5x5-MMI width. The conventional MMI-TF needs a width of ~400 μm because of its bent delay lines.

10. T. Segawa, S. Matsuo, T. Kakitsuka, Y. Shibata, T. Sato, Y. Kawaguchi, Y. Kondo, and R. Takahashi, “Monolithically Integrated Wavelength-Routing Switch using Tunable Wavelength Converters with Double-Ring-Resonator Tunable Lasers,” IEICE Trans. Electron. **E94-C**(9), 1439–1446 (2011). [CrossRef]

## 6. Conclusion

## References and links

1. | T. Fujisawa, S. Kanazawa, H. Ishii, N. Nunoya, Y. Kawaguchi, A. Ohki, N. Fujiwara, K. Takahata, R. Iga, F. Kano, and H. Oohashi, “1.3-μm 4 x 25-Gb/s Monolithically Integrated Light Source for Metro Area 100-Gb/s Ethernet,” IEEE Photon. Technol. Lett. |

2. | S. Kanazawa, T. Fujisawa, A. Ohki, H. Ishii, N. Nunoya, Y. Kawaguchi, N. Fujiwara, K. Takahata, R. Iga, F. Kano, and H. Oohashi, “A Compact EADFB Laser Array Module for a Future 100-Gb/s Ethernet Transceiver,” IEEE J. Sel. Top. Quantum Electron. |

3. | T. Fujisawa, S. Kanazawa, K. Takahata, W. Kobayashi, T. Tadokoro, H. Ishii, and F. Kano, “1.3-μm, 4 × 25-Gbit/s, EADFB Laser Array Module with Large-output-power and Low-driving-voltage for Energy-efficient 100GbE Transmitter,” Opt. Express |

4. | T. Fujisawa, S. Kanazawa, Y. Ueda, W. Kobayashi, K. Takahata, A. Ohki, T. Ito, M. Kohtoku, and H. Ishii, “Low-Loss Cascaded Mach–Zehnder Multiplexer Integrated 25-Gbit/s x 4-Lane EADFB Laser Array for Future CFP4 100 GbE Transmitter,” IEEE J. Quantum Electron. |

5. | H. Ishii, K. Kasaya, and H. Oohashi, “Spectral Linewidth Reduction in Widely Wavelength Tunable DFB Laser Array,” IEEE J. Sel. Top. Quantum Electron. |

6. | Y. Hibino, “Recent Advances in High-Density and Large-Scale AWG Multi/Demultiplexers with Higher Index-Contrast Silica-Based PLCs,” IEEE J. Sel. Top. Quantum Electron. |

7. | Y. Barbarin, X. J. M. Leijtens, E. A. J. M. Bente, C. M. Louzao, J. R. Kooiman, and M. K. Smit, “Extremely Small AWG Demultiplexer Fabricated on InP by using a Double-Etch Process,” IEEE Photon. Technol. Lett. |

8. | K. Takiguchi, T. Kitoh, M. Oguma, Y. Hashizume, and H. Takahashi, “Integrated-optic OFDM Demultiplexer using Multi-mode Interference Coupler-based Optical DFT Circuit,” in Proc. OFC 2012 OM3J.6 (2012). [CrossRef] |

9. | T. Goh, S. Suzuki, and A. Sugita, “Estimation of Waveguide Phase Error in Silica-Based Waveguides,” J. Lightwave Technol. |

10. | T. Segawa, S. Matsuo, T. Kakitsuka, Y. Shibata, T. Sato, Y. Kawaguchi, Y. Kondo, and R. Takahashi, “Monolithically Integrated Wavelength-Routing Switch using Tunable Wavelength Converters with Double-Ring-Resonator Tunable Lasers,” IEICE Trans. Electron. |

**OCIS Codes**

(130.5990) Integrated optics : Semiconductors

(230.7408) Optical devices : Wavelength filtering devices

**ToC Category:**

Integrated Optics

**History**

Original Manuscript: January 23, 2014

Revised Manuscript: February 19, 2014

Manuscript Accepted: February 22, 2014

Published: March 27, 2014

**Citation**

Yuta Ueda, Takeshi Fujisawa, Kiyoto Takahata, Masaki Kohtoku, and Hiroyuki Ishii, "InP-based compact transversal filter for monolithically integrated light source array," Opt. Express **22**, 7844-7851 (2014)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-7-7844

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### References

- T. Fujisawa, S. Kanazawa, H. Ishii, N. Nunoya, Y. Kawaguchi, A. Ohki, N. Fujiwara, K. Takahata, R. Iga, F. Kano, H. Oohashi, “1.3-μm 4 x 25-Gb/s Monolithically Integrated Light Source for Metro Area 100-Gb/s Ethernet,” IEEE Photon. Technol. Lett. 23(6), 356–358 (2011). [CrossRef]
- S. Kanazawa, T. Fujisawa, A. Ohki, H. Ishii, N. Nunoya, Y. Kawaguchi, N. Fujiwara, K. Takahata, R. Iga, F. Kano, H. Oohashi, “A Compact EADFB Laser Array Module for a Future 100-Gb/s Ethernet Transceiver,” IEEE J. Sel. Top. Quantum Electron. 17(5), 1191–1197 (2011). [CrossRef]
- T. Fujisawa, S. Kanazawa, K. Takahata, W. Kobayashi, T. Tadokoro, H. Ishii, F. Kano, “1.3-μm, 4 × 25-Gbit/s, EADFB Laser Array Module with Large-output-power and Low-driving-voltage for Energy-efficient 100GbE Transmitter,” Opt. Express 20(1), 614–620 (2012). [CrossRef] [PubMed]
- T. Fujisawa, S. Kanazawa, Y. Ueda, W. Kobayashi, K. Takahata, A. Ohki, T. Ito, M. Kohtoku, H. Ishii, “Low-Loss Cascaded Mach–Zehnder Multiplexer Integrated 25-Gbit/s x 4-Lane EADFB Laser Array for Future CFP4 100 GbE Transmitter,” IEEE J. Quantum Electron. 49, 1001–1007 (2013). [CrossRef]
- H. Ishii, K. Kasaya, H. Oohashi, “Spectral Linewidth Reduction in Widely Wavelength Tunable DFB Laser Array,” IEEE J. Sel. Top. Quantum Electron. 15(3), 514–520 (2009). [CrossRef]
- Y. Hibino, “Recent Advances in High-Density and Large-Scale AWG Multi/Demultiplexers with Higher Index-Contrast Silica-Based PLCs,” IEEE J. Sel. Top. Quantum Electron. 8(6), 1090–1101 (2002). [CrossRef]
- Y. Barbarin, X. J. M. Leijtens, E. A. J. M. Bente, C. M. Louzao, J. R. Kooiman, M. K. Smit, “Extremely Small AWG Demultiplexer Fabricated on InP by using a Double-Etch Process,” IEEE Photon. Technol. Lett. 16(11), 2478–2480 (2004). [CrossRef]
- K. Takiguchi, T. Kitoh, M. Oguma, Y. Hashizume, and H. Takahashi, “Integrated-optic OFDM Demultiplexer using Multi-mode Interference Coupler-based Optical DFT Circuit,” in Proc. OFC 2012 OM3J.6 (2012). [CrossRef]
- T. Goh, S. Suzuki, A. Sugita, “Estimation of Waveguide Phase Error in Silica-Based Waveguides,” J. Lightwave Technol. 15(11), 2107–2113 (1997). [CrossRef]
- T. Segawa, S. Matsuo, T. Kakitsuka, Y. Shibata, T. Sato, Y. Kawaguchi, Y. Kondo, R. Takahashi, “Monolithically Integrated Wavelength-Routing Switch using Tunable Wavelength Converters with Double-Ring-Resonator Tunable Lasers,” IEICE Trans. Electron. E94-C(9), 1439–1446 (2011). [CrossRef]

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