- Email: [email protected]
The design and error analysis of 90° hybrid based on InP 4×4 MMI
Optics Communications 351 (2015) 63–69
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
The design and error analysis of 90° hybrid based on InP 4 4 MMI Pan Pan, Junming An n, Hongjie Wang, Yue Wang, Jiashun Zhang, Liangliang Wang, Qin Han, Xiongwei Hu State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
art ic l e i nf o
a b s t r a c t
Article history: Received 10 March 2015 Received in revised form 16 April 2015 Accepted 17 April 2015 Available online 21 April 2015
A 90° hybrid based on InGaAsP/InP deep-ridge 4 4 Multi-Mode Interference (MMI) structure is designed. Three-dimensional beam propagation method (3-D BPM) is used in the simulation and error tolerance investigation process. The calculated common-mode rejection ratio (CMRR) and phase deviation are larger than 30 dB and smaller than 1.4° within 1530–1565 nm, respectively. Error tolerance of refractive index, thickness of core layer, waveguide width and length of MMI are analyzed. The designed hybrid is fabricated in the laboratory and measured in the coupling test platform. The test results show that CMRR of the hybrid is almost greater than 10 dB in the wavelength from 1530 to 1565 nm. The estimated phase deviation of the hybrid is less than 12° within 1533–1548 nm and even larger in other wavelengths. The performance degradation is explained well by the error-tolerance analysis. & 2015 Published by Elsevier B.V.
Keywords: 4 4 MMI 90° hybrid MZI-delay line Error analysis
1. Introduction During the past few years, the rapidly growing demand for high-capacity communication indicated that the age of big data is coming [1]. Thanks to the advancement of both high-performance photonic integrated circuits (PIC) and high-speed electronics [2,3], dense wavelength division multiplexing (DWDM) system [4] has made the aggregate transmission speed up to 100 Gb/s, and realtime coherent optical super channels make it 500 Gb/s [5,6]. DWDM line system operated with a fixed relationship between client data rates and line side data rates. However, super-channel offers new levels of flexibility which is software selectable. This system consists of a dual polarization, quadrature phase shift keyed (DP-QPSK) transmitter combined with coherent detection and real time electronic processing of received data [7,8]. A coherent receiver using the balanced detection of modulated optical signals with polarization and phase diversity through an unmodulated external local oscillator (LO) has been used and standardized. 90° optical hybrids with two paired balanced PIN photodiodes are key components of coherent receiver [9,10]. Silicabased planar lightwave circuit and silicon-photonics have been widely used for hybrid integration [11,12], while InP-based photonics are used in monolithic integration. The InP-based photodiodes and 90° optical hybrids have the advantage that they can realize the monolithic integration by regrowth process which can n
Corresponding author. E-mail address: [email protected] (J. An).
http://dx.doi.org/10.1016/j.optcom.2015.04.047 0030-4018/& 2015 Published by Elsevier B.V.
offer smaller footprint in packaging and eliminate complicated alignments in assembly process [13]. The 90° hybrid is widely applied to coherent receiver. In this paper, we designed and fabricated a 90° hybrid, and firstly analyzed the error tolerance of 90° optical hybrid based on 4 4 MMI using beam propagation method (BPM) [14].
2. Design of a 90° hybrid based on 4 4 MMI 2.1. Design of the 90° hybrid A 90° hybrid based on 4 4 MMI structure was designed operating at C-band. The basic structure of this device is shown in Fig. 1(a), input port 1 and 3 carry the signal light (S) and local oscillator (LO) light, respectively. Output channel 1 and channel 4 (2 and 3) are subtracted to obtain I(Q) component of the QPSK signal. The key parameters of the 4 4 MMI are width (W) and length (L). According to the self-imaging principle [15], by defining Lπ as the beat length of the two lowest-order modes,
Lπ ≅
4nr we2 π ≅ β0 − β1 3λ 0
(1)
where β0 and β1 are the propagation constants of fundamental mode and first order mode, We is the effective width of MMI (take into account the lateral penetration depth of each mode field), λ0 is a free-space wavelength which is set as 1550 nm here, and nr is ridge (effective) refractive index. For N N MMI (N ¼4), the first
64
P. Pan et al. / Optics Communications 351 (2015) 63–69
Fig. 1. The structure of 90° hybrid and the cross-section of access waveguides. (a) Layout of 90° hybrid based on 4 4 MMI (b) cross-section of deep ridge waveguide.
4-fold image appears at
L=
3 Lπ 4
(2)
In this work, the deep ridge InP waveguide is used as seen in Fig. 1(b), traditional sandwich structure is adopted and the core layer is 1.05Q InGaAsP with the thickness of 0.5 μm. For deep ridge waveguide, nr ¼3.18341, the confinement of optical field in the sidewall of the waveguide is strong so that the penetration depth is negligible. Thus we can consider that
W ≈ We
(3)
Eqs. (1)–(3) are used for rough estimating of W and L of the 4 4 MMI, the waveguide width and length are chosen to be 20 μm and 845 μm, respectively. The access waveguides width is set as 2.6 μm to satisfy the single mode condition. Between the single mode access waveguides and multimode waveguide, taper waveguides are introduced to reduce coupling loss. 2.2. Introduction of a delay line For a fabricated 90° hybrid device, we can receive the intensity information of output field easily by light detection. However, the phase information is difficult to detect directly. In this work, a delay line based on Mach Zehnder Interferometer (MZI) is introduced in front of the 90° hybrid as shown in Fig. 2. The delay line consists of a 3 dB coupler connected with two arms which have a fixed length difference, and the length difference determines the Free Spectrum Range (FSR) of the MZI. When broadband light go through the delay line, light with different wavelengths will obtain different phase deviations. Thus, the phase information can be converted into wavelength information. The FSR of the MZI is corresponding to the resonant period of the output power which is defined as follows:
FSR ≅
λ 02 neff ΔL
(4)
where λ0 is the central wavelength, neff is the effective index of the waveguide and ΔL is the length difference between two arms. Here, we set the bend arm with bend radius of 500 μm and ΔL is 172.6 μm. According to Eq. (4), FSR is calculated to be about 3.9 μm.
Fig. 2. Structure of hybrid with MZI-based delay line.
3. Simulation and error analysis of the 90° hybrid Three-dimensional beam propagation method (3-D BPM) of the commercial software called Rsoft is used here to simulate the field transmission of the 90° hybrid (in the case of TE mode), as well as to analyze the error tolerance. BPM module of Rsoft is based on finite-difference beam propagation method (FD-BPM) which is suitable for unidirectional transmission waveguides with large cross-section. Thus BPM is appropriate for simulating the 90° hybrid based on 4 4 MMI. 3.1. Simulation of the 90° hybrid In the simulation process, S and LO are launched into input ports 1 and 3, respectively. Fig. 3(a) indicates that light is recovered to output channel 1 to 4 uniformly as the self-imaging method for single input from port 1 or 3, and the common-mode rejection ratio (CMRR) is represented the nonuniformity of I/Q channel which is defined as, 20log[|P1 P4|/|P1 þP4|]/ 20log[|P2 P3|/|P2 þP3|], where P1/2/3/4 are the output power of output channel 1/2/3/4 [16]. If signal and local oscillator are inputted simultaneously, interference will occur in the multimode waveguide, and the power of output signal will oscillate with the phase difference between the two input light beams which can be seen in Fig. 3(b). When two pairs of balanced PDs integrated with the output ports, the oscillation of power is detected and the phase information can be estimated [17]. For this application, power and phase accuracy are the most important performance. According to discussions above, we calculated the CMRR that represents the power nonuniformity and phase deviations across C-band. From Fig. 4(a) and (b) we can see that the CMRR is greater than 30 dB and the phase deviation
P. Pan et al. / Optics Communications 351 (2015) 63–69
65
Fig. 3. The beam-propagation map of the 90° hybrid. (a) For single input. (b) For two input signal with different phase difference.
6
100
Phase deviation/deg.
90
I-channel Q-channel
CMRR/dB
80 70 60 50 40 30 1530
1535
1540
1545
1550
1555
1560
1565
CH-1 to CH-2 CH-1 to CH-3 CH-1 to CH-4
4 2 0 -2 -4 -6 1530
1535
1540
wavelength/nm
1545
1550
1555
1560
1565
wavelength/nm
Fig. 4. Simulated CMRR and phase deviation of the hybrid. (a) Simulated CMRR. (b) Simulated phase deviations.
3.2. Error-tolerance analysis of the 90° hybrid The performance simulated above is under ideal conditions, but deviations will be introduced in the fabrication process. In this section, the error tolerance of the 90° hybrid is discussed using 3-D BPM.
0
Loss/dB
between every two channels is less than 1.4° in the full C-band. The spectral response of the hybrid with a delay line is shown in Fig. 5, the relative positions of peaks and troughs also indicate that the phase deviations are small.
-10
-20
-30 1545
CH-1 CH-2 CH-3 CH-4
1550 1555 wavelength/nm
1560
Fig. 5. Simulated spectral response with a delay line.
3.2.1. Error analysis of the materials growth process During the materials growth process, the index difference Δn of the sandwich structure as well as the thickness of the core layer are set as 0.083 and 0.5 μm, respectively. Δn and thickness of the core layer are two important parameters of the vertical structure of hybrid. For a single input from either port 1 or 3, the excess loss and power nonuniformity in the four output channels change with the deviations of the parameters. Fig. 6(a) and (b) shows the impacts on the hybrid caused by Δn and thickness of core layer. To ensure decrease in excess loss smaller than 0.1 dB, deviations in Δn and thickness of core layer must be controlled within 0.01 and 0.1 μm, respectively. The changes of excess loss caused by deviations in Δn and thinkness of core layer can also introduce errors of CMRR and phase deviations. In Fig. 7(a) and (b), we can indicated that CMRR affected by the deviations of Δn and thickness of core layer in a cotrollable range are still kept greater than 20 dB. Moreover, CMRR is almost kept the same with changes in thickness of core layer. Fig. 8 shows the simulated spectral response of the hybrid with a delay line and biased parameters. In Fig. 8(a), Δn decrease 0.01 and in Fig. 8(b) thickness of core layer decrease 0.1 μm. The phase deviations are all estimated less than 3°. To increase accuracy, 3-D BPM is used so that the camputational
process is time consuming and limited data points are selected, the valleys of the spectrum are not low enough. 3.2.2. Error analysis of the pattern transfer process W and L are basic geometrical parameters of the 4 4 MMI based hybrid. In the lithography and etching process, waveguide size including width and length is always changed. For TE type hybrid, the MMI based on InP deep ridge waveguide was optimized about at a waveguide width of 20 μm, which is a relatively small width achievable to enable the 4-fold imaging to appear. And the length of MMI was optimized around at 845 μm to make light couple to the 4 isolated access waveguides. As can be seen in Fig. 9(a) and (b), powers in the four output channels change with the deviations of W and L. The hybrid is highly sensitive to W. When the width changes 0.1 μm, power losses of the four output channels increase 1 dB. When L changes about 3 μm, the loss of output signals only increase 0.1 dB, and therefore the hybrid is not sensitive to L. Changes in width and length of MMI can introuce deviations of CMRR and phase information. As the hybrid is not so sensitive to L,
P. Pan et al. / Optics Communications 351 (2015) 63–69
0
-0.1
-0.5
-0.11 -0.12
-1
output channel 1 output channel 2 output channel 3 output channel 4
-1.5 -2 -2.5 -3 0.04
Loss/dB
Loss/dB
66
0.06
0.08
0.1
-0.13
output channel 1 output channel 2 output channel 3 output channel 4
-0.14 -0.15 -0.16
0.12
-0.17 0.4
0.14
0.45
Δn
0.5
0.55
0.6
Thickness/μm
Fig. 6. The dependence of excess loss on Δn and thickness of core layer. (a) The dependence of excess loss on Δn. (b) The dependence of excess loss on thickness.
100
100 I-channel Q-channel
90
80
CMRR/dB
CMRR/dB
80 70 60
70 60
50
50
40
40
30 0.04
0.06
0.08 Δn
0.1
I-channel Q-channel
90
30 0.4
0.12
0.45
0.5
0.55
0.6
Thickness/μm
Fig. 7. The dependence of CMRR on Δn and thinkness of core layer. (a) Dependence of CMRR on Δn. (a) Dependence of CMRR on thickness of core layer.
-4
-4
-6
-6 -8 Loss/dB
Loss/dB
-8 -10 -12 -14
-18 1545
-12 -14
CH-1 CH-2 CH-3 CH-4
-16
-10
-16
1550 1555 wavelength/nm
1560
-18 1545
CH-1 CH-2 CH-3 CH-4 1550 1555 wavelength/nm
1560
Fig. 8. Simulated spectral response with deviated Δn and thickness of core layer. (a) Spectral response with Δn decrease 0.01. (b) Spectral response with thickness of core layer decrease 0.1 μm.
0
0
-1
-1
Loss/dB
Loss/dB
-2 -3
output channel 1 output channel 2 output channel 3 output channel 4
-4 -5
output channel 1 output channel 2 output channel 3 output channel 4
-2
-3
-6 -7 19.7
19.8
19.9
20
W/μm
20.1
20.2
20.3
-4 830
835
840
845
850
L/μm
Fig. 9. The dependence of addtional loss on different widths and lengths of MMI. (a) The dependence of excess loss on width. (b) The dependence of excess loss on length.
P. Pan et al. / Optics Communications 351 (2015) 63–69
80
60
I-channel Q-channel
50
CMRR/dB
70 CMRR/dB
67
60 50 40
40 30
I-channel Q-channel
20 10
30 840
842
844
846
848
850
0 19.6
19.8
20
20.2
20.4
W/μm
L/μm
Fig. 10. The dependence of CMRR on Δn, thinkness of corelayer, L and W. (a) Dependence of CMRR on L. (b) Dependence of CMRR on W.
Phase deviations/deg.
20
CH-1 to CH-2 CH-1 to CH-3 CH-1 to CH-4
10 0 -10 -20 19.7
19.8
19.9
20 20.1 width/μm
20.2
20.3
Fig. 11. Dependence of phase deviations on W.
-6 CH-1 CH-2 CH-3 CH-4
Loss/dB
-8 -10 -12 -14 1545
1550 1555 wavelength/nm
1560
Fig. 12. Simulated spectral response with W decrease 0.2 μm.
we can see in Fig. 10(a) that CMRR is kept almost larger than 40 dB with the changes of L within 71 μm. However, performances of hybrid is highly affected by the width of MMI as can be seen in Figs. 10 (b) and 11, when W changes more than 0.3 μm, CMRR dropped to 10 dB and phase deviations rise to 10° in the wavelength of 1550 nm. The spectral response of the hybrid with a delay line is also affected by changes in width of MMI obviously which is displayed in Fig. 12. When MMI width with a decrease of 0.2 μm, the spectra deformed as the phase deviations increased. In addition, when wavelength greater than 1553 nm, the intensity oscillation of spectral response becomes less obvious which even cannot indicate the phase information. Change in W affects the operating wavelength range of hybrid seriously.
(MOCVD) technology. Then the plasma enhanced chemical vapor deposition (PECVD) technology is used to deposit a SiO2 layer as hard mask. The patterns of designed hybrid are transmitted on the chip by ultra-violet lithography, and two etching processes including SiO2 etching and InP etching are followed to form deep ridge waveguides using inductively coupled plasma (ICP) technology. Finally, a 1 μm-thick SiO2 layer is deposited on the chip to protect the device from external influence. In Fig. 13, the overall structure of the fabricated 90° optical hybrid is shown by the microscope image, and the scanning electronic modules (SEM) images display the details of the structure. The fabricated 90° hybrid is measured in the test platform. Firstly the power nonuniformity is measured. The TE-mode beam is launched via lensed fiber into single input port 1 of hybrid without delay line. Then the output signals from the output channels are collected by lensed fibers and sent to spectrograph. The CMRRs of I and Q channels are measured as can be seen in Fig. 14. In the wavelength range from 1530 nm to 1565 nm, CMRR of Q-channel is almost greater than 20 dB. However, CMRR of I-channel is greater than 10 dB in 1530–1545 nm and greater than 20 dB in 1545–1565 nm. Fig. 15 shows the spectral response of hybrid with a delay line. From the figure we can estimate that, in the wavelength range from 1533 nm to 1548 nm, the phase deviation of the hybrid is less than 12° as demonstrate in Fig. 16. But for output channel 2 and 3, the intensity oscillations of spectral response become less obvious between 1548-1560 nm and the phase deviation is increased. From the measured results, we can see that the performance is deviated from designed value. As analyzed in Section 3, the measurement result in Fig. 15 is similar to the simulated spectral response with waveguide width decrease 0.2 μm in Fig. 12. In our laboratory, the material growth technology is based on metalorganic chemical vapor deposition (MOCVD) which can control Δn and thickness of core layer accurately. The accuracy of waveguide size is limited by lithography technology, the contact exposure is used here and a 0.2 μm or more error is always introduced. For hybrid is sensitive to its waveguide width, the error is mainly due to deviation of waveguide width which is introduced by lithography process. Analysis in Section 3 can explain the measurement results well.
5. Conclusions 4. Fabrication and results The designed optical 90° hybrid was fabricated in the laboratory. Firstly, the waveguide materials including InP buffer layer, InGaAsP core layer (1.05Q) and InP cap layer are deposited on the InP substrate by metal-organic chemical vapor deposition
To realize InP-based PM DQPSK receiver, the 90° optical hybrid based on 4 4 MMI is designed and simulated. The theoretical design shows that CMRR 4 30 dB and phase deviation is less than 1.4° over the full C-band. We also analyze the error tolerance of the fabrication process using 3-D BPM. The index difference of core layer and cladding layer, the thickness of core layer, width of MMI
68
P. Pan et al. / Optics Communications 351 (2015) 63–69
Fig. 13. Microscope and SEM Images of fabricated hybrid with a MZI-delay line.
experimental conditions, error-tolerance analysis can explain the unsatisfactory measurement results of the 90° hybrid well.
40
CMRR/dB
30
Acknowledgments
20
This work has been supported by a Project supported by the National High Technology Research and Development Program of China (Nos. 2015AA012302 and 2013AA031402).
I-channel Q-channel
10
0 1530
1535
1540
1545
1550
1555
1560
1565
wavelength/nm
Reference
Fig. 14. Measured CMRR of the hybrid.
0
CH-1 CH-2 CH-3 CH-4
Loss/dB
-5
-10
-15
-20 1530
1535
1540
1545 1550 1555 wavelength/nm
1560
1565
Fig. 15. The spectral response of the fabricated hybrid with delay line.
CH-1 to CH-2 CH-1 to CH-3 CH-1 to CH-4
Phase deviation/deg.
20
10
0
-10
-20 1534
1536
1538
1540
1542
1544
1546
1548
wavelength/nm
Fig. 16. The estimated phase deviations.
(W) and length of MMI (L) are analyzed, respectively. Among them, width of MMI is the most sensitive parameter to hybrid which needs more accurate fabrication technology. For limited
[1] H. Ishibuchi, Big data era computational intelligence magazine, IEEE 9 (2014) 2–11. [2] L.A. Coldren, High-performance photonic integrated circuits (PICs), in: Proceedings of the Conference on Optical Fiber Communication – OFC 2010 Collocated National Fiber Optic Engineers Conference (OFC/NFOEC), 2010, pp. 1–52. [3] M. Moller, High-speed electronic circuits for 100 Gb/s transport networks, in: Proceedings of the Conference on Optical Fiber Communication – OFC 2010 Collocated National Fiber Optic Engineers Conference (OFC/NFOEC), 2010, pp. 1–3. [4] A. Aloisio, F. Ameli, A. D’Amico, R. Giordano, G. Giovanetti, V. Izzo, Performance analysis of a DWDM optical transmission system, IEEE Trans. Nucl. Sci. 59 (2012) 251–255. [5] R. Nagarajan, M. Kato, D. Lambert, P. Evans, S. Corzine, V. Lal, J. Rahn, A. Nilsson, M. Fisher, M. Kuntz, J. Pleumeekers, A. Dentai, H.-S. Tsai, D. Krause, H. Sun, K.-T. Wu, M. Ziari, T. Butrie, M. Reffle, M. Mitchell, F. Kish, D. Welch, Terabit/s class InP photonic integrated circuits, Semicond. Sci. Technol. 27 (2012) 094003. [6] J. Rahn, S. Kumar, M. Mitchell, R. Malendevich, H. Sun, K. Wu, P. Mertz, K. Croussore, H. Wang, M. Kato, V. Lal, P. Evans, D. Lambert, H. Tsai, P. Samra, B. Taylor, A. Nilsson, S. Grubb, R. Nagarajan, F. Kish, D. Welch, 250 Gb/s real-time PIC-based superchannel transmission over a gridless 6000 km terrestrial link, in: Proceedings of the Conference on Optical Fiber Communication Conference and Exposition and the National Fiber Optic Engineers Conference (OFC/NFOEC), 2012, pp. 1–3. [7] R. Kunkel, H.G. Bach, D. Hoffmann, C.M. Weinert, I. Molina-Fernandez, R. Halir, First monolithic InP-based 900 hybrid OEIC comprising balanced detectors for 100GE coherent frontends, in: Proceedings of the IEEE International Conference on Indium Phosphide & Related Materials (IPRM'09), 2009, pp. 167–170. [8] R. Ludwig, A. Matiss, H.G. Bach, L. Molle, C.C. Leonhardt, R. Kunkel, D. Schmidt, Coherent detection of a 50 Gb/s QPSK signal using an InP 900 hybrid monolithically integrated with balanced photodetectors, in: Proceedings of the 35th European Conference on Optical Communication (ECOC'09), 2009, pp. 1–2. [9] H.G. Bach, A. Matiss, C.C. Leonhardt, R. Kunkel, D. Schmidt, M. Schell, A. Umbach, Monolithic 900 hybrid -with balanced PIN photodiodes for 100 Gbit/s PM-QPSK receiver applications, in: Proceedings of the Conference on Optical Fiber Communication-includes Post Deadline Papers (OFC), 2009, pp. 1–3. [10] M. Seimetz, C.M. Weinert, Options, feasibility, and availability of 2 4 90° hybrids for coherent optical systems,, J. Lightw. Technol. 24 (2006) 1317–1322. [11] Y. Sakamaki, H. Yamazaki, T. Mizuno, T. Goh, Y. Nasu, T. Hashimoto, S. Kamei, K. Hattori, H. Takahashi, One-chip integrated dual polarization optical hybrid using silica-based planar lightwave circuit technology, in: Proceedings of the 35th European Conference on Optical Communication (ECOC'09), 2009, pp. 1–2. [12] L. Zimmermann, K. Voigt, G. Winzer, K. Petermann, C.M. Weinert, C-band optical 90°-hybrids based on silicon-on-insulator 4 4 waveguide couplers, IEEE Photonics Technol. Lett. 21 (2009) 143–145. [13] H. Yagi, N. Inoue, R. Masuyama, T. Kikuchi, T. Katsuyama, Y. Tateiwa, K. Uesaka, Y. Yoneda, M. Takechi, H. Shoji, InP-based p-i-n-photodiode array integrated with 90° hybrid using butt-joint regrowth for compact 100 Gb/s coherent receiver, IEEE J. Sel. Top. Quantum Electron. 20 (2014) 374–380. [14] F. Fogli, G. Bellanca, P. Bassi, I. Madden, W. Johnstone, Highly efficient full-vectorial
P. Pan et al. / Optics Communications 351 (2015) 63–69
3-D BPM modeling of fiber to planar waveguide couplers, J. Lightw. Technol. 17 (1999) 136–143. [15] L.B. Soldano, E.C.M. Pennings, Optical multi-mode interference devices based on self-imaging: principles and applications, J. Lightw. Technol. 13 (1995) 615–627. [16] M. Boudreau, M. Poirier, G. Yoffe, B. Pezeshki, An integrated inp coherent receiver for 40 and 100 Gb/Sec telecommunications systems, in: Proceedings of the
69
Conference on Optical Fiber Communication-incudes Post Deadline Papers (OFC), 2009, pp. 1–3. [17] C.F. Clarke, R.A. Griffin, T.C. Goodall, Highly integrated DQPSK modules for 40 Gb/s transmission, in: Proceedings of the Conference on Optical Fiber Communicationincudes Post Deadline Papers (OFC), 2009, pp. 1–3.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
The design and error analysis of 90° hybrid based on InP 4 4 MMI Pan Pan, Junming An n, Hongjie Wang, Yue Wang, Jiashun Zhang, Liangliang Wang, Qin Han, Xiongwei Hu State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
art ic l e i nf o
a b s t r a c t
Article history: Received 10 March 2015 Received in revised form 16 April 2015 Accepted 17 April 2015 Available online 21 April 2015
A 90° hybrid based on InGaAsP/InP deep-ridge 4 4 Multi-Mode Interference (MMI) structure is designed. Three-dimensional beam propagation method (3-D BPM) is used in the simulation and error tolerance investigation process. The calculated common-mode rejection ratio (CMRR) and phase deviation are larger than 30 dB and smaller than 1.4° within 1530–1565 nm, respectively. Error tolerance of refractive index, thickness of core layer, waveguide width and length of MMI are analyzed. The designed hybrid is fabricated in the laboratory and measured in the coupling test platform. The test results show that CMRR of the hybrid is almost greater than 10 dB in the wavelength from 1530 to 1565 nm. The estimated phase deviation of the hybrid is less than 12° within 1533–1548 nm and even larger in other wavelengths. The performance degradation is explained well by the error-tolerance analysis. & 2015 Published by Elsevier B.V.
Keywords: 4 4 MMI 90° hybrid MZI-delay line Error analysis
1. Introduction During the past few years, the rapidly growing demand for high-capacity communication indicated that the age of big data is coming [1]. Thanks to the advancement of both high-performance photonic integrated circuits (PIC) and high-speed electronics [2,3], dense wavelength division multiplexing (DWDM) system [4] has made the aggregate transmission speed up to 100 Gb/s, and realtime coherent optical super channels make it 500 Gb/s [5,6]. DWDM line system operated with a fixed relationship between client data rates and line side data rates. However, super-channel offers new levels of flexibility which is software selectable. This system consists of a dual polarization, quadrature phase shift keyed (DP-QPSK) transmitter combined with coherent detection and real time electronic processing of received data [7,8]. A coherent receiver using the balanced detection of modulated optical signals with polarization and phase diversity through an unmodulated external local oscillator (LO) has been used and standardized. 90° optical hybrids with two paired balanced PIN photodiodes are key components of coherent receiver [9,10]. Silicabased planar lightwave circuit and silicon-photonics have been widely used for hybrid integration [11,12], while InP-based photonics are used in monolithic integration. The InP-based photodiodes and 90° optical hybrids have the advantage that they can realize the monolithic integration by regrowth process which can n
Corresponding author. E-mail address: [email protected] (J. An).
http://dx.doi.org/10.1016/j.optcom.2015.04.047 0030-4018/& 2015 Published by Elsevier B.V.
offer smaller footprint in packaging and eliminate complicated alignments in assembly process [13]. The 90° hybrid is widely applied to coherent receiver. In this paper, we designed and fabricated a 90° hybrid, and firstly analyzed the error tolerance of 90° optical hybrid based on 4 4 MMI using beam propagation method (BPM) [14].
2. Design of a 90° hybrid based on 4 4 MMI 2.1. Design of the 90° hybrid A 90° hybrid based on 4 4 MMI structure was designed operating at C-band. The basic structure of this device is shown in Fig. 1(a), input port 1 and 3 carry the signal light (S) and local oscillator (LO) light, respectively. Output channel 1 and channel 4 (2 and 3) are subtracted to obtain I(Q) component of the QPSK signal. The key parameters of the 4 4 MMI are width (W) and length (L). According to the self-imaging principle [15], by defining Lπ as the beat length of the two lowest-order modes,
Lπ ≅
4nr we2 π ≅ β0 − β1 3λ 0
(1)
where β0 and β1 are the propagation constants of fundamental mode and first order mode, We is the effective width of MMI (take into account the lateral penetration depth of each mode field), λ0 is a free-space wavelength which is set as 1550 nm here, and nr is ridge (effective) refractive index. For N N MMI (N ¼4), the first
64
P. Pan et al. / Optics Communications 351 (2015) 63–69
Fig. 1. The structure of 90° hybrid and the cross-section of access waveguides. (a) Layout of 90° hybrid based on 4 4 MMI (b) cross-section of deep ridge waveguide.
4-fold image appears at
L=
3 Lπ 4
(2)
In this work, the deep ridge InP waveguide is used as seen in Fig. 1(b), traditional sandwich structure is adopted and the core layer is 1.05Q InGaAsP with the thickness of 0.5 μm. For deep ridge waveguide, nr ¼3.18341, the confinement of optical field in the sidewall of the waveguide is strong so that the penetration depth is negligible. Thus we can consider that
W ≈ We
(3)
Eqs. (1)–(3) are used for rough estimating of W and L of the 4 4 MMI, the waveguide width and length are chosen to be 20 μm and 845 μm, respectively. The access waveguides width is set as 2.6 μm to satisfy the single mode condition. Between the single mode access waveguides and multimode waveguide, taper waveguides are introduced to reduce coupling loss. 2.2. Introduction of a delay line For a fabricated 90° hybrid device, we can receive the intensity information of output field easily by light detection. However, the phase information is difficult to detect directly. In this work, a delay line based on Mach Zehnder Interferometer (MZI) is introduced in front of the 90° hybrid as shown in Fig. 2. The delay line consists of a 3 dB coupler connected with two arms which have a fixed length difference, and the length difference determines the Free Spectrum Range (FSR) of the MZI. When broadband light go through the delay line, light with different wavelengths will obtain different phase deviations. Thus, the phase information can be converted into wavelength information. The FSR of the MZI is corresponding to the resonant period of the output power which is defined as follows:
FSR ≅
λ 02 neff ΔL
(4)
where λ0 is the central wavelength, neff is the effective index of the waveguide and ΔL is the length difference between two arms. Here, we set the bend arm with bend radius of 500 μm and ΔL is 172.6 μm. According to Eq. (4), FSR is calculated to be about 3.9 μm.
Fig. 2. Structure of hybrid with MZI-based delay line.
3. Simulation and error analysis of the 90° hybrid Three-dimensional beam propagation method (3-D BPM) of the commercial software called Rsoft is used here to simulate the field transmission of the 90° hybrid (in the case of TE mode), as well as to analyze the error tolerance. BPM module of Rsoft is based on finite-difference beam propagation method (FD-BPM) which is suitable for unidirectional transmission waveguides with large cross-section. Thus BPM is appropriate for simulating the 90° hybrid based on 4 4 MMI. 3.1. Simulation of the 90° hybrid In the simulation process, S and LO are launched into input ports 1 and 3, respectively. Fig. 3(a) indicates that light is recovered to output channel 1 to 4 uniformly as the self-imaging method for single input from port 1 or 3, and the common-mode rejection ratio (CMRR) is represented the nonuniformity of I/Q channel which is defined as, 20log[|P1 P4|/|P1 þP4|]/ 20log[|P2 P3|/|P2 þP3|], where P1/2/3/4 are the output power of output channel 1/2/3/4 [16]. If signal and local oscillator are inputted simultaneously, interference will occur in the multimode waveguide, and the power of output signal will oscillate with the phase difference between the two input light beams which can be seen in Fig. 3(b). When two pairs of balanced PDs integrated with the output ports, the oscillation of power is detected and the phase information can be estimated [17]. For this application, power and phase accuracy are the most important performance. According to discussions above, we calculated the CMRR that represents the power nonuniformity and phase deviations across C-band. From Fig. 4(a) and (b) we can see that the CMRR is greater than 30 dB and the phase deviation
P. Pan et al. / Optics Communications 351 (2015) 63–69
65
Fig. 3. The beam-propagation map of the 90° hybrid. (a) For single input. (b) For two input signal with different phase difference.
6
100
Phase deviation/deg.
90
I-channel Q-channel
CMRR/dB
80 70 60 50 40 30 1530
1535
1540
1545
1550
1555
1560
1565
CH-1 to CH-2 CH-1 to CH-3 CH-1 to CH-4
4 2 0 -2 -4 -6 1530
1535
1540
wavelength/nm
1545
1550
1555
1560
1565
wavelength/nm
Fig. 4. Simulated CMRR and phase deviation of the hybrid. (a) Simulated CMRR. (b) Simulated phase deviations.
3.2. Error-tolerance analysis of the 90° hybrid The performance simulated above is under ideal conditions, but deviations will be introduced in the fabrication process. In this section, the error tolerance of the 90° hybrid is discussed using 3-D BPM.
0
Loss/dB
between every two channels is less than 1.4° in the full C-band. The spectral response of the hybrid with a delay line is shown in Fig. 5, the relative positions of peaks and troughs also indicate that the phase deviations are small.
-10
-20
-30 1545
CH-1 CH-2 CH-3 CH-4
1550 1555 wavelength/nm
1560
Fig. 5. Simulated spectral response with a delay line.
3.2.1. Error analysis of the materials growth process During the materials growth process, the index difference Δn of the sandwich structure as well as the thickness of the core layer are set as 0.083 and 0.5 μm, respectively. Δn and thickness of the core layer are two important parameters of the vertical structure of hybrid. For a single input from either port 1 or 3, the excess loss and power nonuniformity in the four output channels change with the deviations of the parameters. Fig. 6(a) and (b) shows the impacts on the hybrid caused by Δn and thickness of core layer. To ensure decrease in excess loss smaller than 0.1 dB, deviations in Δn and thickness of core layer must be controlled within 0.01 and 0.1 μm, respectively. The changes of excess loss caused by deviations in Δn and thinkness of core layer can also introduce errors of CMRR and phase deviations. In Fig. 7(a) and (b), we can indicated that CMRR affected by the deviations of Δn and thickness of core layer in a cotrollable range are still kept greater than 20 dB. Moreover, CMRR is almost kept the same with changes in thickness of core layer. Fig. 8 shows the simulated spectral response of the hybrid with a delay line and biased parameters. In Fig. 8(a), Δn decrease 0.01 and in Fig. 8(b) thickness of core layer decrease 0.1 μm. The phase deviations are all estimated less than 3°. To increase accuracy, 3-D BPM is used so that the camputational
process is time consuming and limited data points are selected, the valleys of the spectrum are not low enough. 3.2.2. Error analysis of the pattern transfer process W and L are basic geometrical parameters of the 4 4 MMI based hybrid. In the lithography and etching process, waveguide size including width and length is always changed. For TE type hybrid, the MMI based on InP deep ridge waveguide was optimized about at a waveguide width of 20 μm, which is a relatively small width achievable to enable the 4-fold imaging to appear. And the length of MMI was optimized around at 845 μm to make light couple to the 4 isolated access waveguides. As can be seen in Fig. 9(a) and (b), powers in the four output channels change with the deviations of W and L. The hybrid is highly sensitive to W. When the width changes 0.1 μm, power losses of the four output channels increase 1 dB. When L changes about 3 μm, the loss of output signals only increase 0.1 dB, and therefore the hybrid is not sensitive to L. Changes in width and length of MMI can introuce deviations of CMRR and phase information. As the hybrid is not so sensitive to L,
P. Pan et al. / Optics Communications 351 (2015) 63–69
0
-0.1
-0.5
-0.11 -0.12
-1
output channel 1 output channel 2 output channel 3 output channel 4
-1.5 -2 -2.5 -3 0.04
Loss/dB
Loss/dB
66
0.06
0.08
0.1
-0.13
output channel 1 output channel 2 output channel 3 output channel 4
-0.14 -0.15 -0.16
0.12
-0.17 0.4
0.14
0.45
Δn
0.5
0.55
0.6
Thickness/μm
Fig. 6. The dependence of excess loss on Δn and thickness of core layer. (a) The dependence of excess loss on Δn. (b) The dependence of excess loss on thickness.
100
100 I-channel Q-channel
90
80
CMRR/dB
CMRR/dB
80 70 60
70 60
50
50
40
40
30 0.04
0.06
0.08 Δn
0.1
I-channel Q-channel
90
30 0.4
0.12
0.45
0.5
0.55
0.6
Thickness/μm
Fig. 7. The dependence of CMRR on Δn and thinkness of core layer. (a) Dependence of CMRR on Δn. (a) Dependence of CMRR on thickness of core layer.
-4
-4
-6
-6 -8 Loss/dB
Loss/dB
-8 -10 -12 -14
-18 1545
-12 -14
CH-1 CH-2 CH-3 CH-4
-16
-10
-16
1550 1555 wavelength/nm
1560
-18 1545
CH-1 CH-2 CH-3 CH-4 1550 1555 wavelength/nm
1560
Fig. 8. Simulated spectral response with deviated Δn and thickness of core layer. (a) Spectral response with Δn decrease 0.01. (b) Spectral response with thickness of core layer decrease 0.1 μm.
0
0
-1
-1
Loss/dB
Loss/dB
-2 -3
output channel 1 output channel 2 output channel 3 output channel 4
-4 -5
output channel 1 output channel 2 output channel 3 output channel 4
-2
-3
-6 -7 19.7
19.8
19.9
20
W/μm
20.1
20.2
20.3
-4 830
835
840
845
850
L/μm
Fig. 9. The dependence of addtional loss on different widths and lengths of MMI. (a) The dependence of excess loss on width. (b) The dependence of excess loss on length.
P. Pan et al. / Optics Communications 351 (2015) 63–69
80
60
I-channel Q-channel
50
CMRR/dB
70 CMRR/dB
67
60 50 40
40 30
I-channel Q-channel
20 10
30 840
842
844
846
848
850
0 19.6
19.8
20
20.2
20.4
W/μm
L/μm
Fig. 10. The dependence of CMRR on Δn, thinkness of corelayer, L and W. (a) Dependence of CMRR on L. (b) Dependence of CMRR on W.
Phase deviations/deg.
20
CH-1 to CH-2 CH-1 to CH-3 CH-1 to CH-4
10 0 -10 -20 19.7
19.8
19.9
20 20.1 width/μm
20.2
20.3
Fig. 11. Dependence of phase deviations on W.
-6 CH-1 CH-2 CH-3 CH-4
Loss/dB
-8 -10 -12 -14 1545
1550 1555 wavelength/nm
1560
Fig. 12. Simulated spectral response with W decrease 0.2 μm.
we can see in Fig. 10(a) that CMRR is kept almost larger than 40 dB with the changes of L within 71 μm. However, performances of hybrid is highly affected by the width of MMI as can be seen in Figs. 10 (b) and 11, when W changes more than 0.3 μm, CMRR dropped to 10 dB and phase deviations rise to 10° in the wavelength of 1550 nm. The spectral response of the hybrid with a delay line is also affected by changes in width of MMI obviously which is displayed in Fig. 12. When MMI width with a decrease of 0.2 μm, the spectra deformed as the phase deviations increased. In addition, when wavelength greater than 1553 nm, the intensity oscillation of spectral response becomes less obvious which even cannot indicate the phase information. Change in W affects the operating wavelength range of hybrid seriously.
(MOCVD) technology. Then the plasma enhanced chemical vapor deposition (PECVD) technology is used to deposit a SiO2 layer as hard mask. The patterns of designed hybrid are transmitted on the chip by ultra-violet lithography, and two etching processes including SiO2 etching and InP etching are followed to form deep ridge waveguides using inductively coupled plasma (ICP) technology. Finally, a 1 μm-thick SiO2 layer is deposited on the chip to protect the device from external influence. In Fig. 13, the overall structure of the fabricated 90° optical hybrid is shown by the microscope image, and the scanning electronic modules (SEM) images display the details of the structure. The fabricated 90° hybrid is measured in the test platform. Firstly the power nonuniformity is measured. The TE-mode beam is launched via lensed fiber into single input port 1 of hybrid without delay line. Then the output signals from the output channels are collected by lensed fibers and sent to spectrograph. The CMRRs of I and Q channels are measured as can be seen in Fig. 14. In the wavelength range from 1530 nm to 1565 nm, CMRR of Q-channel is almost greater than 20 dB. However, CMRR of I-channel is greater than 10 dB in 1530–1545 nm and greater than 20 dB in 1545–1565 nm. Fig. 15 shows the spectral response of hybrid with a delay line. From the figure we can estimate that, in the wavelength range from 1533 nm to 1548 nm, the phase deviation of the hybrid is less than 12° as demonstrate in Fig. 16. But for output channel 2 and 3, the intensity oscillations of spectral response become less obvious between 1548-1560 nm and the phase deviation is increased. From the measured results, we can see that the performance is deviated from designed value. As analyzed in Section 3, the measurement result in Fig. 15 is similar to the simulated spectral response with waveguide width decrease 0.2 μm in Fig. 12. In our laboratory, the material growth technology is based on metalorganic chemical vapor deposition (MOCVD) which can control Δn and thickness of core layer accurately. The accuracy of waveguide size is limited by lithography technology, the contact exposure is used here and a 0.2 μm or more error is always introduced. For hybrid is sensitive to its waveguide width, the error is mainly due to deviation of waveguide width which is introduced by lithography process. Analysis in Section 3 can explain the measurement results well.
5. Conclusions 4. Fabrication and results The designed optical 90° hybrid was fabricated in the laboratory. Firstly, the waveguide materials including InP buffer layer, InGaAsP core layer (1.05Q) and InP cap layer are deposited on the InP substrate by metal-organic chemical vapor deposition
To realize InP-based PM DQPSK receiver, the 90° optical hybrid based on 4 4 MMI is designed and simulated. The theoretical design shows that CMRR 4 30 dB and phase deviation is less than 1.4° over the full C-band. We also analyze the error tolerance of the fabrication process using 3-D BPM. The index difference of core layer and cladding layer, the thickness of core layer, width of MMI
68
P. Pan et al. / Optics Communications 351 (2015) 63–69
Fig. 13. Microscope and SEM Images of fabricated hybrid with a MZI-delay line.
experimental conditions, error-tolerance analysis can explain the unsatisfactory measurement results of the 90° hybrid well.
40
CMRR/dB
30
Acknowledgments
20
This work has been supported by a Project supported by the National High Technology Research and Development Program of China (Nos. 2015AA012302 and 2013AA031402).
I-channel Q-channel
10
0 1530
1535
1540
1545
1550
1555
1560
1565
wavelength/nm
Reference
Fig. 14. Measured CMRR of the hybrid.
0
CH-1 CH-2 CH-3 CH-4
Loss/dB
-5
-10
-15
-20 1530
1535
1540
1545 1550 1555 wavelength/nm
1560
1565
Fig. 15. The spectral response of the fabricated hybrid with delay line.
CH-1 to CH-2 CH-1 to CH-3 CH-1 to CH-4
Phase deviation/deg.
20
10
0
-10
-20 1534
1536
1538
1540
1542
1544
1546
1548
wavelength/nm
Fig. 16. The estimated phase deviations.
(W) and length of MMI (L) are analyzed, respectively. Among them, width of MMI is the most sensitive parameter to hybrid which needs more accurate fabrication technology. For limited
[1] H. Ishibuchi, Big data era computational intelligence magazine, IEEE 9 (2014) 2–11. [2] L.A. Coldren, High-performance photonic integrated circuits (PICs), in: Proceedings of the Conference on Optical Fiber Communication – OFC 2010 Collocated National Fiber Optic Engineers Conference (OFC/NFOEC), 2010, pp. 1–52. [3] M. Moller, High-speed electronic circuits for 100 Gb/s transport networks, in: Proceedings of the Conference on Optical Fiber Communication – OFC 2010 Collocated National Fiber Optic Engineers Conference (OFC/NFOEC), 2010, pp. 1–3. [4] A. Aloisio, F. Ameli, A. D’Amico, R. Giordano, G. Giovanetti, V. Izzo, Performance analysis of a DWDM optical transmission system, IEEE Trans. Nucl. Sci. 59 (2012) 251–255. [5] R. Nagarajan, M. Kato, D. Lambert, P. Evans, S. Corzine, V. Lal, J. Rahn, A. Nilsson, M. Fisher, M. Kuntz, J. Pleumeekers, A. Dentai, H.-S. Tsai, D. Krause, H. Sun, K.-T. Wu, M. Ziari, T. Butrie, M. Reffle, M. Mitchell, F. Kish, D. Welch, Terabit/s class InP photonic integrated circuits, Semicond. Sci. Technol. 27 (2012) 094003. [6] J. Rahn, S. Kumar, M. Mitchell, R. Malendevich, H. Sun, K. Wu, P. Mertz, K. Croussore, H. Wang, M. Kato, V. Lal, P. Evans, D. Lambert, H. Tsai, P. Samra, B. Taylor, A. Nilsson, S. Grubb, R. Nagarajan, F. Kish, D. Welch, 250 Gb/s real-time PIC-based superchannel transmission over a gridless 6000 km terrestrial link, in: Proceedings of the Conference on Optical Fiber Communication Conference and Exposition and the National Fiber Optic Engineers Conference (OFC/NFOEC), 2012, pp. 1–3. [7] R. Kunkel, H.G. Bach, D. Hoffmann, C.M. Weinert, I. Molina-Fernandez, R. Halir, First monolithic InP-based 900 hybrid OEIC comprising balanced detectors for 100GE coherent frontends, in: Proceedings of the IEEE International Conference on Indium Phosphide & Related Materials (IPRM'09), 2009, pp. 167–170. [8] R. Ludwig, A. Matiss, H.G. Bach, L. Molle, C.C. Leonhardt, R. Kunkel, D. Schmidt, Coherent detection of a 50 Gb/s QPSK signal using an InP 900 hybrid monolithically integrated with balanced photodetectors, in: Proceedings of the 35th European Conference on Optical Communication (ECOC'09), 2009, pp. 1–2. [9] H.G. Bach, A. Matiss, C.C. Leonhardt, R. Kunkel, D. Schmidt, M. Schell, A. Umbach, Monolithic 900 hybrid -with balanced PIN photodiodes for 100 Gbit/s PM-QPSK receiver applications, in: Proceedings of the Conference on Optical Fiber Communication-includes Post Deadline Papers (OFC), 2009, pp. 1–3. [10] M. Seimetz, C.M. Weinert, Options, feasibility, and availability of 2 4 90° hybrids for coherent optical systems,, J. Lightw. Technol. 24 (2006) 1317–1322. [11] Y. Sakamaki, H. Yamazaki, T. Mizuno, T. Goh, Y. Nasu, T. Hashimoto, S. Kamei, K. Hattori, H. Takahashi, One-chip integrated dual polarization optical hybrid using silica-based planar lightwave circuit technology, in: Proceedings of the 35th European Conference on Optical Communication (ECOC'09), 2009, pp. 1–2. [12] L. Zimmermann, K. Voigt, G. Winzer, K. Petermann, C.M. Weinert, C-band optical 90°-hybrids based on silicon-on-insulator 4 4 waveguide couplers, IEEE Photonics Technol. Lett. 21 (2009) 143–145. [13] H. Yagi, N. Inoue, R. Masuyama, T. Kikuchi, T. Katsuyama, Y. Tateiwa, K. Uesaka, Y. Yoneda, M. Takechi, H. Shoji, InP-based p-i-n-photodiode array integrated with 90° hybrid using butt-joint regrowth for compact 100 Gb/s coherent receiver, IEEE J. Sel. Top. Quantum Electron. 20 (2014) 374–380. [14] F. Fogli, G. Bellanca, P. Bassi, I. Madden, W. Johnstone, Highly efficient full-vectorial
P. Pan et al. / Optics Communications 351 (2015) 63–69
3-D BPM modeling of fiber to planar waveguide couplers, J. Lightw. Technol. 17 (1999) 136–143. [15] L.B. Soldano, E.C.M. Pennings, Optical multi-mode interference devices based on self-imaging: principles and applications, J. Lightw. Technol. 13 (1995) 615–627. [16] M. Boudreau, M. Poirier, G. Yoffe, B. Pezeshki, An integrated inp coherent receiver for 40 and 100 Gb/Sec telecommunications systems, in: Proceedings of the
69
Conference on Optical Fiber Communication-incudes Post Deadline Papers (OFC), 2009, pp. 1–3. [17] C.F. Clarke, R.A. Griffin, T.C. Goodall, Highly integrated DQPSK modules for 40 Gb/s transmission, in: Proceedings of the Conference on Optical Fiber Communicationincudes Post Deadline Papers (OFC), 2009, pp. 1–3.