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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 20 — Oct. 7, 2013
  • pp: 23048–23057
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Colorless monolithically integrated 120° downconverter

P. J. Reyes-Iglesias, A. Ortega-Moñux, and I. Molina-Fernández  »View Author Affiliations


Optics Express, Vol. 21, Issue 20, pp. 23048-23057 (2013)
http://dx.doi.org/10.1364/OE.21.023048


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Abstract

We numerically demonstrate colorless reception of dense wavelength division multiplexed channels in the C-band for high-order QAM (16-64 QAM) signals on a 120° monolithically integrated downconverter, based on a 2x3 MMI with calibrated analog IQ recovery. It is shown that the proposed calibrated 120° downconverter can increase up to 80 the number of coincident channels in an efficient way, exhibiting good signal dynamic range and high fabrication yield. As this downconverter makes use of the minimum number of power outputs required for perfect recovery of IQ signals, it becomes an interesting alternative to conventional 90° based downconverters.

© 2013 Optical Society of America

1. Introduction

The deployment of reconfigurable optical add-drop multiplexers (ROADM) in transport optical networks has provided flexibility and configurability capabilities to network operators. The introduction in the near future of colorless ROADM will also allow any wavelength to be added/dropped to any port. In this situation colorless receivers can be used in the drop ports to increase the efficiency and reduce the cost of reconfigurable optical networks. In a colorless receiver, just by tuning the local oscillator (LO), an individual wavelength-division multiplexed (WDM) channel can be selected and detected without any optical filtering device (e.g. demultiplexer or filter).

The Optical Internetworking Forum (OIF) [1

1. Optical Internetworking Forum (OIF), “100G ultra long haul DWDM framework document,” document OIF-FD-100G-DWDM-01.0 (June 2009), http://www.oiforum.com/public/impagreements.html.

] has proposed dual polarization quadrature phase-shift keying (DP-QPSK) modulation format to reach 100 Gbps per channel in the amplified C-band over the existing optical network infrastructure. Higher quadrature amplitude modulation (e.g. 16-64 QAM) is a viable alternative to further increase system transmission capacity while reducing bandwidth requirements. Thus, in the framework of the MIRTHE project [2

2. Mirthe Project, “Monolithic InP-based dual polarization QPSK integrated receiver and transmitter for coherent 100–400Gb Ethernet,” http://www.ist-mirthe.eu/.

], 16-QAM monolithically integrated transmitter and receivers for 400 Gbps are being assessed. Required coherent receivers comprise a polarization diversity network (e.g. polarization beam splitters) and two phase diversity downconverters (one per polarization). In this paper we will focus on the last part, that is, the optical downconverter. A widespread solution is the monolithic integration of the 90° optical hybrid with four photodiodes in balanced configuration on the same chip [2

2. Mirthe Project, “Monolithic InP-based dual polarization QPSK integrated receiver and transmitter for coherent 100–400Gb Ethernet,” http://www.ist-mirthe.eu/.

, 3

3. R. Kunkel, H. G. Bach, D. Hoffmann, C. Weinert, I. Molina-Fernández, and R. Halir, “First monolithic InP-based 90 degrees-hybrid OEIC comprising balanced detectors for 100GE coherent frontends,” in International Conference on Indium Phosphide & Related Materials (IPRM, 2009), paper TuB2.2, pp. 167–170.

]. In colorless reception, a measure of the suppression of the interfering direct-detection terms from coincident WDM channels is the common-mode-rejection-ratio (CMRR) [4

4. B. Zhang, C. Malouin, and T. J. Schmidt, “Towards full band colorless reception with coherent balanced receivers,” Opt. Express 20(9), 10339–10352 (2012). [CrossRef] [PubMed]

, 5

5. L. E. Nelson, S. L. Woodward, S. Foo, M. Moyer, D. J. S. Beckett, M. O’Sullivan, and P. D. Magill, “Detection of a single 40 Gb/s polarization-multiplexed QPSK channel with a real-time intradyne receiver in the presence of multiple coincident WDM channels,” J. Lightwave Technol. 28(20), 2933–2943 (2010). [CrossRef]

]. Therefore, balanced 90° hybrid based coherent receiver with high CMRR and high LO-to-signal power ratio can be used as colorless receiver. However, a wideband high CMRR will require stringent fabrication tolerance requirements (resulting in high cost and low fabrication yield) to reduce hardware impairments (i.e. amplitude imbalances existing in phase diversity network or photodiode responsivity mismatch) [6

6. V. E. Houtsma, N. G. Weimann, T. Hu, R. Kopf, A. Tate, J. Frackoviak, R. Reyes, Y. K. Chen, L. Zhang, C. R. Doerr, and D. T. Neilson, “Manufacturable monolithically integrated InP dual-port coherent receiver for 100G PDM-QPSK applications,” Tech. Digest Optical Fiber Comm. (OFC) (2011), paper OML2.

, 7

7. P. J. Reyes-Iglesias, A. Ortega-Moñux, and I. Molina-Fernández, “Enhanced monolithically integrated coherent 120° downconverter with high fabrication yield,” Opt. Express 20(21), 23013–23018 (2012). [CrossRef] [PubMed]

].

A promising alternative to overcome the above problems is the 120° phase diversity receiver which, if properly calibrated, has shown to be highly tolerant to hardware impairments at microwave frequencies [8

8. P. Pérez-Lara, I. Molina-Fernández, J. G. Wangüemert-Pérez, and A. Rueda-Pérez, “Broadband five-port direct receiver based on low-pass and high-pass phase shifters,” IEEE Trans. Microw. Theory Tech. 58(4), 849–853 (2010). [CrossRef]

]. This is an interesting solution because, as it is known from multiport theory [9

9. F. M. Ghannouchi and R. G. Bosisio, “An alternative explicit six-port matrix calibration formalism using five standards,” IEEE Trans. Microw. Theory Tech. 36(3), 494–498 (1988). [CrossRef]

, 10

10. P. J. Reyes-Iglesias, I. Molina-Fernández, A. Moscoso-Mártir, and A. Ortega-Moñux, “High-performance monolithically integrated 120° downconverter with relaxed hardware constraints,” Opt. Express 20(5), 5725–5741 (2012). [CrossRef] [PubMed]

], three is the minimum number of power outputs to perfectly recover IQ signals from power readings by linear means, and thus the 120° based downconverter is the simplest multiport receiver. This type of 120° downconverter has been reported several times for optical communications by using 3x3 fiber couplers [11

11. T. Pfau, S. Hoffmann, O. Adamczyk, R. Peveling, V. Herath, M. Porrmann, and R. Noé, “Coherent optical communication: towards realtime systems at 40 Gbit/s and beyond,” Opt. Express 16(2), 866–872 (2008). [CrossRef] [PubMed]

, 12

12. C. Xie, P. J. Winzer, G. Raybon, A. H. Gnauck, B. Zhu, T. Geisler, and B. Edvold, “Colorless coherent receiver using 3x3 coupler hybrids and single-ended detection,” Opt. Express 20(2), 1164–1171 (2012). [CrossRef] [PubMed]

]. The authors have recently proposed a monolithic integrated downconverter, based on a 2x3 multimode interference coupler (MMI), with a simple linear calibration strategy to fully correct receiver impairments [7

7. P. J. Reyes-Iglesias, A. Ortega-Moñux, and I. Molina-Fernández, “Enhanced monolithically integrated coherent 120° downconverter with high fabrication yield,” Opt. Express 20(21), 23013–23018 (2012). [CrossRef] [PubMed]

, 10

10. P. J. Reyes-Iglesias, I. Molina-Fernández, A. Moscoso-Mártir, and A. Ortega-Moñux, “High-performance monolithically integrated 120° downconverter with relaxed hardware constraints,” Opt. Express 20(5), 5725–5741 (2012). [CrossRef] [PubMed]

]. Our proposal, compared to the balanced 90° downconverter (based on a 2x4 MMI), not only showed the same noise-induced penalty under ideal hardware, but exhibited a higher signal dynamic range, wider operating bandwidth and greater tolerance to fabrication errors for a single-channel reception. In this paper we compare the performance of colorless reception of 56 Gbps QAM WDM channels (enabling 112 Gbps under dual polarization), for two different types of monolithically integrated downconverters: the conventional 90°, based in a 2x4 MMI and differential transimpedance amplifiers, and the 120°, based on a 2x3 MMI [7

7. P. J. Reyes-Iglesias, A. Ortega-Moñux, and I. Molina-Fernández, “Enhanced monolithically integrated coherent 120° downconverter with high fabrication yield,” Opt. Express 20(21), 23013–23018 (2012). [CrossRef] [PubMed]

, 10

10. P. J. Reyes-Iglesias, I. Molina-Fernández, A. Moscoso-Mártir, and A. Ortega-Moñux, “High-performance monolithically integrated 120° downconverter with relaxed hardware constraints,” Opt. Express 20(5), 5725–5741 (2012). [CrossRef] [PubMed]

] with calibrated analog in-phase and quadrature (IQ) recovery. Simulation results show that hardware imbalances arising from typical fabrication errors reduce the CMRR and increase the interference from coincident channels in a much more limiting way for the monolithic integrated conventional 90° downconverter than for the calibrated 120° downconverter.

The paper is organized as follows: in Sections II and III monolithically integrated conventional 90° and proposed calibrated 120° downconverters are described, respectively, for colorless reception under hardware imbalances. In Section IV, the colorless performance of both schemes is numerically compared in a realistic fabrication scenario. In Section V different proposals of calibration for the 90° downconverter are briefly evaluated. Finally, Section VI provides the main conclusions.

2. Colorless operation of monolithic integrated conventional 90° hybrid downconverter

Conventional 90° hybrid integrated downconverter in Fig. 1
Fig. 1 Conventional 90° hybrid downconverter.
, is based on a monolithically integrated 2x4 MMI with four photodiodes followed by differential transimpedance amplifiers (TIA) with DC offset cancellation [3

3. R. Kunkel, H. G. Bach, D. Hoffmann, C. Weinert, I. Molina-Fernández, and R. Halir, “First monolithic InP-based 90 degrees-hybrid OEIC comprising balanced detectors for 100GE coherent frontends,” in International Conference on Indium Phosphide & Related Materials (IPRM, 2009), paper TuB2.2, pp. 167–170.

]. Output electrical IQ signal components are then digitized in two analog-to-digital converters (ADC) and combined to be further processed in the digital signal processor (DSP).

es(t)=Re{n=1Ne˜snejωnt}
(1)
eLO(t)=Re{PLOejωkt};k[1,N]
(2)

For homodyne detection, the LO (of power PLO) must be tuned to the angular frequency ωk of the channel to be detected. Considering an equal signal power for all multiplexed channelsPs=|e˜sn|2, individual slowly varying complex signal envelope of the nth channel can be written (neglecting fiber transmission impairments) in terms of the normalized IQ signal components, In and Qn, as

e˜sn=Ps(In+jQn)
(3)

The WDM signal and LO are combined in the 2x4 MMI, with scattering parameters Skij defined between their ports at frequency ωk, and detected from the photodiodes, with responsivities Ri. Therefore, when selecting kth channel, the four output photocurrents can be calculated (from i = 3, 4, 5 or 6) as

iik=Ri|n=1NSi1ne˜snejωnt+Si2kPLOejωkt|2
(4)

Neglecting the high frequency beating terms at ωnk, which will be completely filtered by the electronics, photocurrents iIk and iQk for IQ components can be obtained from idealized differential TIAs, and be described in matrix form as

[iIkiQk]=[i3ki4ki5ki6k]=[αIkαQk]+n=1N[γInγQn][In2+Qn2]+[Re(uk)Im(uk)Re(vk)Im(vk)][IkQk]
(5)

Three terms can be identified at the right-hand side of Eq. (5): DC offset term, an interfering direct-detection term from the self-beating of adjacent channels and linear axis transformation of IQ components. Their parameters (α, γ, u, v), which were first introduced in [13

13. A. Moscoso-Mártir, I. Molina-Fernández, and A. Ortega-Monux, “Signal constellation distortion and BER degradation due to hardware impairments in six-port receivers with analog I/Q generation,” Prog. Electromagnetics Res. 121, 225–247 (2011). [CrossRef]

], are shown again here in Table 1

Table 1. Parameters Derived in [13] to Characterize Conventional 90° Hybrid Integrated Coherent Receiver

table-icon
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for convenience. Linear terms cause a translation, rotation and imbalance of reference axes. Compensation of the linear distortion induced at each wavelength by hardware imperfections (hybrid and photodiode responsivity imbalance) will be removed in the DSP by the Gram-Schmidt orthogonalization procedure (GSOP) [14

14. I. Fatadin, S. J. Savory, and D. Ives, “Compensation of quadrature imbalance in an optical QPSK coherent receiver,” IEEE Photon. Technol. Lett. 20(20), 1733–1735 (2008). [CrossRef]

]. The second term of Eq. (5) causes a baseband interference current that cannot be removed and limits the colorless behavior of the receiver. This interfering current can therefore be expressed as

[iIkiQk]Interf=[i3ki4ki5ki6k]Interf=Psn=1N[R3|S31n|2R4|S41n|2R5|S51n|2R6|S61n|2][In2+Qn2]
(6)

The interference term depends on the signal power, the number of coincident channels and the performance of the coherent receiver in terms of power imbalance. It must be noticed that the baseband interference current shows a close relation with the CMRR for a single wavelength signal [1

1. Optical Internetworking Forum (OIF), “100G ultra long haul DWDM framework document,” document OIF-FD-100G-DWDM-01.0 (June 2009), http://www.oiforum.com/public/impagreements.html.

], since it is a direct measurement of the power imbalance behavior of a coherent receiver.

CMRRSI90°(ωn)=i3ni4ni3n+i4n|Interf=R3|S31n|2R4|S41n|2R3|S31n|2+R4|S41n|2CMRRSQ90°(ωn)=i5ni6ni5n+i6n|Interf=R5|S51n|2R6|S61n|2R5|S51n|2+R6|S61n|2
(7)

Therefore, we will use the CMRR as a figure of merit of the colorless behavior of the proposed integrated colorless receivers, as it is usually done in literature [4

4. B. Zhang, C. Malouin, and T. J. Schmidt, “Towards full band colorless reception with coherent balanced receivers,” Opt. Express 20(9), 10339–10352 (2012). [CrossRef] [PubMed]

, 5

5. L. E. Nelson, S. L. Woodward, S. Foo, M. Moyer, D. J. S. Beckett, M. O’Sullivan, and P. D. Magill, “Detection of a single 40 Gb/s polarization-multiplexed QPSK channel with a real-time intradyne receiver in the presence of multiple coincident WDM channels,” J. Lightwave Technol. 28(20), 2933–2943 (2010). [CrossRef]

].

For multichannel reception, from Eq. (5), the self-beating interference contribution from each adjacent channel will be weighted by the signal power and the CMRR at its respective wavelength. In this way, colorless reception will require a low Ps/PLO ratio and a high CMRR over the complete received multichannel frequency band.

Figure 2(a)
Fig. 2 (a) Transversal geometry of the InP/InGaAsP rib waveguides used in this work. H = 1µm, D = 0.5µm, nInP = 3.18, nInGaAsP = 3.27. (b) CMRR for input signal port versus wavelength in the C-band for the conventional 90° downconverter as a function of the fabrication tolerance (Case I/II).
shows the transversal geometry of the waveguides used in this work. We will consider only two relevant cases for simulations: I. Nominal design (i.e. no fabrication errors) and II. Moderate fabrication errors. Based on our experience working with a commercial InP fabrication platform [2

2. Mirthe Project, “Monolithic InP-based dual polarization QPSK integrated receiver and transmitter for coherent 100–400Gb Ethernet,” http://www.ist-mirthe.eu/.

, 3

3. R. Kunkel, H. G. Bach, D. Hoffmann, C. Weinert, I. Molina-Fernández, and R. Halir, “First monolithic InP-based 90 degrees-hybrid OEIC comprising balanced detectors for 100GE coherent frontends,” in International Conference on Indium Phosphide & Related Materials (IPRM, 2009), paper TuB2.2, pp. 167–170.

], we have chosen as moderate fabrication errors deviations in waveguide widths |δW|<150 nm and etch depth errors |δD|<45nm. A realistic photodiode imbalance responsivity of 5% has also been included in simulations. A more detailed description of the integrated downconverter including relevant physical dimensions can be found in [7

7. P. J. Reyes-Iglesias, A. Ortega-Moñux, and I. Molina-Fernández, “Enhanced monolithically integrated coherent 120° downconverter with high fabrication yield,” Opt. Express 20(21), 23013–23018 (2012). [CrossRef] [PubMed]

]. Figure 2(b) shows the resultant wavelength dependence of the maximum value of CMRR from Eq. (7). The OIF specifies a CMRR for the signal port greater than 20 dB in absolute value [1

1. Optical Internetworking Forum (OIF), “100G ultra long haul DWDM framework document,” document OIF-FD-100G-DWDM-01.0 (June 2009), http://www.oiforum.com/public/impagreements.html.

]. As expected, the nominal design (Case I) corresponds to an OIF-compliant 90° downconverter. However, the specification of the OIF for the CMRR will be fulfilled only in half of the C-band under reasonable fabrication errors (Case II).

3. Colorless operation of monolithic integrated calibrated 120° coupler downconverter

Figure 3
Fig. 3 Calibrated 120° coupler downconverter.
shows the proposed colorless calibrated 120° optical downconverter based on a 2x3 MMI coupler monolithically integrated with three photodiodes followed by their respective TIAs [7

7. P. J. Reyes-Iglesias, A. Ortega-Moñux, and I. Molina-Fernández, “Enhanced monolithically integrated coherent 120° downconverter with high fabrication yield,” Opt. Express 20(21), 23013–23018 (2012). [CrossRef] [PubMed]

, 10

10. P. J. Reyes-Iglesias, I. Molina-Fernández, A. Moscoso-Mártir, and A. Ortega-Moñux, “High-performance monolithically integrated 120° downconverter with relaxed hardware constraints,” Opt. Express 20(5), 5725–5741 (2012). [CrossRef] [PubMed]

]. Output electrical signals are then linearly combined with a calibrated analog circuit prior to be digitized in two ADCs and digitally processed.

Following a similar analysis to that of Section 2, the WDM signal and LO waves will be combined in the 120° coupler, being described the three photocurrents when selecting kth channel as i3k, i4k and i5k.

[i3ki4ki5k]=[α3kα4kα4k]+n=1N[γ3nγ4nγ5n][In2+Qn2]+[Re(u1k)Im(u1k)Re(u2k)Im(u2k)Re(u3k)Im(u3k)][IkQk]
(8)

Three parameters, showed in Table 2

Table 2. Parameters to Characterize 120° Coherent Receiver

table-icon
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, describe now the three terms that express the output photocurrents [10

10. P. J. Reyes-Iglesias, I. Molina-Fernández, A. Moscoso-Mártir, and A. Ortega-Moñux, “High-performance monolithically integrated 120° downconverter with relaxed hardware constraints,” Opt. Express 20(5), 5725–5741 (2012). [CrossRef] [PubMed]

]: a DC offset term, an interfering direct-detection term from the self-beating of adjacent channels (equivalent to the second term of Eq. (5) for the 90° downconverter) and LO-signal power-dependent linear combination from IQ signal components.

As it was shown previously for a single-channel reception in [10

10. P. J. Reyes-Iglesias, I. Molina-Fernández, A. Moscoso-Mártir, and A. Ortega-Moñux, “High-performance monolithically integrated 120° downconverter with relaxed hardware constraints,” Opt. Express 20(5), 5725–5741 (2012). [CrossRef] [PubMed]

], solving the linear equations formulated in Eq. (8), undistorted IQ components of the demodulated channel can be recovered by a linear combination of the three output photocurrents, while canceling self-induced intensity interfering term.

iIk=AI3i3k+AI4i4k+AI5i5kiQk=AQ3i3k+AQ4i4k+AQ5i5k
(9)

Under ideal hardware it is easily obtained that [10

10. P. J. Reyes-Iglesias, I. Molina-Fernández, A. Moscoso-Mártir, and A. Ortega-Moñux, “High-performance monolithically integrated 120° downconverter with relaxed hardware constraints,” Opt. Express 20(5), 5725–5741 (2012). [CrossRef] [PubMed]

, 12

12. C. Xie, P. J. Winzer, G. Raybon, A. H. Gnauck, B. Zhu, T. Geisler, and B. Edvold, “Colorless coherent receiver using 3x3 coupler hybrids and single-ended detection,” Opt. Express 20(2), 1164–1171 (2012). [CrossRef] [PubMed]

]

AI3=AI5=12,AI4=1;AQ3=32,AQ4=0,AQ5=32
(10)

For real operation, the required coefficients (AIi, AQi) are obtained following a simple calibration method [9

9. F. M. Ghannouchi and R. G. Bosisio, “An alternative explicit six-port matrix calibration formalism using five standards,” IEEE Trans. Microw. Theory Tech. 36(3), 494–498 (1988). [CrossRef]

] applied at the central wavelength of the C-band (i.e. 1550 nm). Due to the reduced wavelength-dependence of the 2x3 MMI scattering parameters [7

7. P. J. Reyes-Iglesias, A. Ortega-Moñux, and I. Molina-Fernández, “Enhanced monolithically integrated coherent 120° downconverter with high fabrication yield,” Opt. Express 20(21), 23013–23018 (2012). [CrossRef] [PubMed]

, 15

15. A. Besse, M. Bachmann, H. Melchior, L. B. Soldano, and M. K. Smit, “Optical bandwidth and fabrication tolerances of multimode interference couplers,” J. Lightwave Technol. 12(6), 1004–1009 (1994). [CrossRef]

], the calibrated coefficients calculated at 1550 nm can be used over the complete C-band to nearly cancel receiver imbalances. This allows us to electronically regenerate, as Fig. 3 shows, the IQ components of any channel in the C-band just by implementing the linear analog operation described by Eq. (9) (operation that was digitally assessed in [7

7. P. J. Reyes-Iglesias, A. Ortega-Moñux, and I. Molina-Fernández, “Enhanced monolithically integrated coherent 120° downconverter with high fabrication yield,” Opt. Express 20(21), 23013–23018 (2012). [CrossRef] [PubMed]

, 10

10. P. J. Reyes-Iglesias, I. Molina-Fernández, A. Moscoso-Mártir, and A. Ortega-Moñux, “High-performance monolithically integrated 120° downconverter with relaxed hardware constraints,” Opt. Express 20(5), 5725–5741 (2012). [CrossRef] [PubMed]

]). It must be highlighted that, contrary to the conventional balanced 90° hybrid approach, in this case it is not required the use of the digital GSOP algorithm, since the analog calibration almost compensates all the hardware impairments of the receiver over the complete C-band, as it will be shown later. From Eq. (8)-(9), it can be shown that the baseband interference current can be expressed as

[iIkiQk]Interf=Psn=1N[AI3R3|S31n|2+AI4R4|S41n|2+AI5R5|S51n|2AQ3R3|S31n|2+AQ4R4|S41n|2+AQ5R5|S51n|22][In2+Qn2]
(11)

In the ideal case (e.g. |Sij|2 = ⅓ within the working band), from Eq. (10)-(11), the interference term is zero and there is no limitation in the colorless operation of the device. When the 120° coupler is not ideal, deviations from the desired scattering parameters can be partially compensated using the calibrated coefficients (AIi, AQi), so the interference contribution from all WDM channels is highly reduced. It must be highlighted that the analog operation described by Eq. (9) has an important advantage with respect the digital approach proposed in [7

7. P. J. Reyes-Iglesias, A. Ortega-Moñux, and I. Molina-Fernández, “Enhanced monolithically integrated coherent 120° downconverter with high fabrication yield,” Opt. Express 20(21), 23013–23018 (2012). [CrossRef] [PubMed]

, 10

10. P. J. Reyes-Iglesias, I. Molina-Fernández, A. Moscoso-Mártir, and A. Ortega-Moñux, “High-performance monolithically integrated 120° downconverter with relaxed hardware constraints,” Opt. Express 20(5), 5725–5741 (2012). [CrossRef] [PubMed]

]. When the IQ recovery is performed in the analog domain, the interfering direct-detection term is cancelled prior to the ADC conversion, so the effective number of bits (ENoB) of the ADC in the presence of multiple adjacent channels is not seriously reduced [12

12. C. Xie, P. J. Winzer, G. Raybon, A. H. Gnauck, B. Zhu, T. Geisler, and B. Edvold, “Colorless coherent receiver using 3x3 coupler hybrids and single-ended detection,” Opt. Express 20(2), 1164–1171 (2012). [CrossRef] [PubMed]

]. As we stated in Section 2, the CMRR will be used as a figure of merit of the colorless behavior of the receiver. Since CMRR for a 120° downconverter is not defined in OIF, we propose to use the following expressions

CMRRSI120°(ωn)=AI3i3n+AI4i4n+AI5i5n|AI3|i3n+|AI4|i4n+|AI5|i5n|Interf=AI3R3|S31n|2+AI4R4|S41n|2+AI5R5|S51n|2|AI3|R3|S31n|2+|AI4|R4|S41n|2+|AI5|R5|S51n|2CMRRSQ120°(ωn)=AQ3i3n+AQ4i4n+AQ5i5n|AQ3|i3n+|AQ4|i4n+|AQ5|i5n|Interf=AQ3R3|S31n|2+AQ4R4|S41n|2+AQ5R5|S51n|2|AQ3|R3|S31n|2+|AQ4|R4|S41n|2+|AQ5|R5|S51n|2
(12)

It must be noticed that these expressions have a close relation with CMRR definition of the 90° downconverter from Eq. (7): the numerator coincides with the interference term defined in Eq. (11), whereas the denominator is just a normalization factor. Simulation results will show the appropriateness of Eq. (12).

4. Colorless performance comparison of conventional 90° and calibrated 120° downconverters

In this section the colorless performance of the downconverters presented in Sections 2 and 3 will be numerically simulated and compared. An external LO power of 10 dBm and WDM channels of equal power at 56 Gbps (enabling 112 Gbps under dual polarization) centered on the C-band (50 GHz grid) have been considered. The optical fiber has been modeled as an AWGN channel with uniform spontaneous amplified emission (ASE) noise contribution to each channel. Therefore, the effect of the residual dispersion or polarization orientation of adjacent channels on the receiver performance has not been assessed here (see [4

4. B. Zhang, C. Malouin, and T. J. Schmidt, “Towards full band colorless reception with coherent balanced receivers,” Opt. Express 20(9), 10339–10352 (2012). [CrossRef] [PubMed]

, 5

5. L. E. Nelson, S. L. Woodward, S. Foo, M. Moyer, D. J. S. Beckett, M. O’Sullivan, and P. D. Magill, “Detection of a single 40 Gb/s polarization-multiplexed QPSK channel with a real-time intradyne receiver in the presence of multiple coincident WDM channels,” J. Lightwave Technol. 28(20), 2933–2943 (2010). [CrossRef]

] for a deeper study of the scaling factor to be introduced in the intensity interfering term). Incoming OSNR has been adjusted for incidents channel at BER = 10−4 in an ideal coherent receiver in absence of internal noise sources. TIAs have been modeled with an input referred noise current density of 20 pA/√Hz. An ADC resolution of 5 bits and 6 bits has been considered for 16-QAM and 64-QAM respectively, thus obtaining a low quantization noise penalty (≈0.5 dB) [16

16. T. Pfau, S. Hoffmann, and R. Noé, “Hardware-efficient coherent digital receiver concept with feedforward carrier recovery for M-QAM constellations,” J. Lightwave Technol. 27(8), 989–999 (2009). [CrossRef]

]. More details of the simulation scenario can be found in [10

10. P. J. Reyes-Iglesias, I. Molina-Fernández, A. Moscoso-Mártir, and A. Ortega-Moñux, “High-performance monolithically integrated 120° downconverter with relaxed hardware constraints,” Opt. Express 20(5), 5725–5741 (2012). [CrossRef] [PubMed]

]. Figure 5
Fig. 5 OSNR penalty (for a BER = 10−4) versus input signal power in a conventional 90° hybrid (filled circles) and calibrated 120° coupler (empty circles) downconverters, following the nominal design (Case I), as a function of the number of WDM channels (a) 16-QAM transmission (b) 64-QAM transmission.
shows, for the nominal design (Case I), the OSNR penalty (for a BER = 10−4) versus the input signal power as a function of the WDM channel number under higher-order modulation (16-QAM and 64-QAM). Dashed line represents an additional 1dB OSNR penalty over the quantization noise floor (of 0.5 dB). Both receivers are limited by shot-noise in a similar way for low signal power levels. However the conventional 90° receiver performance will be particularly degraded for high signal power by the interference from the self-beating of adjacent channels. This interference contribution increases with the number of WDM channels and is due to CMRR degradation arising from non-perfect MMI coupler performance for all transmitted channels. In this way, for reasonable fabrication errors in the MMI (Case II), as Fig. 6
Fig. 6 OSNR penalty (for a BER = 10−4) versus input signal power in a conventional 90° hybrid (filled circles) and calibrated120° coupler (empty circles) downconverters, following moderate fabrication errors (Case II), as a function of the number of WDM channels (a) 16-QAM (b) 64-QAM transmission.
shows, the conventional 90° downconverter performance will be further degraded at high signal power, being limited its dynamic range. Please notice that colorless reception of 80 channels was not included in the results of Fig. 6(b) for the conventional 90° downconverter, as this type of receiver was not able to get the required BER for this scenario. The advantage of the calibrated120° downconverter is more obvious for high signal power when increasing the number of WDM channels, where the reduction of self-beating interference from adjacent channels becomes more appreciable. These results are also in close correspondence with those in Fig. 4, which showed a better value of CMRR along the complete C-band for the calibrated 120° receiver. Table 3

Table 3. Dynamic Range for the Conventional 90°/ Calibrated 120° Downconverter as a Function of the Number of WDM Channels

table-icon
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summarizes the dynamic range obtained for each type of downconverter as a function of the number of WDM channels, in a moderate fabrication error scenario (Case II) and for a maximum OSNR penalty of 1.5 dB.

5. Colorless operation of calibrated 90° downconverters

Figure 8
Fig. 8 OSNR penalty (for a BER = 10−4) versus input signal power in a calibrated 120° downconverter (empty circles) and the calibrated Option B of the 90° downconverter (filled squares), following moderate fabrication errors (Case II), as a function of the number of WDM channels (a) 16-QAM (b) 64-QAM transmission.
shows a comparison of performance for the calibrated downconverters: the 120° from Fig. 3 and the 90° Option B from Fig. 7(b). The OSNR penalty (for a BER = 10−4) versus the input signal power as a function of the WDM channel number under higher-order modulation (16-QAM and 64-QAM) are depicted in this figure for the moderate fabrication error scenario (Case II). It is observed that the calibrated 120° downconverter can still offer an improvement in the OSNR penalty of 0.8 dB for 16-QAM under 80 WDM channels and 2.8 dB for 64-QAM under 40 WDM channels (note that colorless reception of more than 40 channels was not included for 64-QAM transmission, as the Option B of the calibrated 90° downconverter was not able to get the required BER for this scenario).

6. Conclusions

We have compared the colorless performance of two monolithically integrated receivers: i) the conventional 90° hybrid downconverter based on a 2x4 MMI with balanced photodetection, ii) a 120° coupler downconverter based on a 2x3 MMI with calibrated analog IQ recovery. Passive components of both devices have been designed using standard InP/InGaAsP rib waveguides [7

7. P. J. Reyes-Iglesias, A. Ortega-Moñux, and I. Molina-Fernández, “Enhanced monolithically integrated coherent 120° downconverter with high fabrication yield,” Opt. Express 20(21), 23013–23018 (2012). [CrossRef] [PubMed]

], whereas typical fabrication errors (e.g. waveguide width and etch depth errors) have been included to define realistic simulation scenarios. Numerical results clearly show that, in a colorless multichannel high-order modulation (16-64 QAM) scenario, the calibrated 120° downconverter significantly outperforms the conventional 90° receiver. Specifically, it has been shown that, for realistic fabrication errors and 64-QAM transmission, the calibrated 120° downconverter can achieve colorless reception of 80 WDM channels within the whole C-band and over a wide dynamic range (~10 dB). In the same scenario, conventional 90° downconverter only supports up to 40 channels, with a much more reduced dynamic range (~4.5 dB). Additionally, other alternatives from the calibration of the 90° downconverter have been briefly evaluated.

Acknowledgments

The authors gratefully acknowledge the design support from Diego Pérez-Galacho. This work has been partially funded under Andalusian Regional Ministry of Science Innovation and Business project P09-TIC-5268, Spanish Ministry of Science and Innovation project TEC2009-10152 and EU 7th Framework Programme project MIRTHE ICT-2009-5 nº 257980.

References and links

1.

Optical Internetworking Forum (OIF), “100G ultra long haul DWDM framework document,” document OIF-FD-100G-DWDM-01.0 (June 2009), http://www.oiforum.com/public/impagreements.html.

2.

Mirthe Project, “Monolithic InP-based dual polarization QPSK integrated receiver and transmitter for coherent 100–400Gb Ethernet,” http://www.ist-mirthe.eu/.

3.

R. Kunkel, H. G. Bach, D. Hoffmann, C. Weinert, I. Molina-Fernández, and R. Halir, “First monolithic InP-based 90 degrees-hybrid OEIC comprising balanced detectors for 100GE coherent frontends,” in International Conference on Indium Phosphide & Related Materials (IPRM, 2009), paper TuB2.2, pp. 167–170.

4.

B. Zhang, C. Malouin, and T. J. Schmidt, “Towards full band colorless reception with coherent balanced receivers,” Opt. Express 20(9), 10339–10352 (2012). [CrossRef] [PubMed]

5.

L. E. Nelson, S. L. Woodward, S. Foo, M. Moyer, D. J. S. Beckett, M. O’Sullivan, and P. D. Magill, “Detection of a single 40 Gb/s polarization-multiplexed QPSK channel with a real-time intradyne receiver in the presence of multiple coincident WDM channels,” J. Lightwave Technol. 28(20), 2933–2943 (2010). [CrossRef]

6.

V. E. Houtsma, N. G. Weimann, T. Hu, R. Kopf, A. Tate, J. Frackoviak, R. Reyes, Y. K. Chen, L. Zhang, C. R. Doerr, and D. T. Neilson, “Manufacturable monolithically integrated InP dual-port coherent receiver for 100G PDM-QPSK applications,” Tech. Digest Optical Fiber Comm. (OFC) (2011), paper OML2.

7.

P. J. Reyes-Iglesias, A. Ortega-Moñux, and I. Molina-Fernández, “Enhanced monolithically integrated coherent 120° downconverter with high fabrication yield,” Opt. Express 20(21), 23013–23018 (2012). [CrossRef] [PubMed]

8.

P. Pérez-Lara, I. Molina-Fernández, J. G. Wangüemert-Pérez, and A. Rueda-Pérez, “Broadband five-port direct receiver based on low-pass and high-pass phase shifters,” IEEE Trans. Microw. Theory Tech. 58(4), 849–853 (2010). [CrossRef]

9.

F. M. Ghannouchi and R. G. Bosisio, “An alternative explicit six-port matrix calibration formalism using five standards,” IEEE Trans. Microw. Theory Tech. 36(3), 494–498 (1988). [CrossRef]

10.

P. J. Reyes-Iglesias, I. Molina-Fernández, A. Moscoso-Mártir, and A. Ortega-Moñux, “High-performance monolithically integrated 120° downconverter with relaxed hardware constraints,” Opt. Express 20(5), 5725–5741 (2012). [CrossRef] [PubMed]

11.

T. Pfau, S. Hoffmann, O. Adamczyk, R. Peveling, V. Herath, M. Porrmann, and R. Noé, “Coherent optical communication: towards realtime systems at 40 Gbit/s and beyond,” Opt. Express 16(2), 866–872 (2008). [CrossRef] [PubMed]

12.

C. Xie, P. J. Winzer, G. Raybon, A. H. Gnauck, B. Zhu, T. Geisler, and B. Edvold, “Colorless coherent receiver using 3x3 coupler hybrids and single-ended detection,” Opt. Express 20(2), 1164–1171 (2012). [CrossRef] [PubMed]

13.

A. Moscoso-Mártir, I. Molina-Fernández, and A. Ortega-Monux, “Signal constellation distortion and BER degradation due to hardware impairments in six-port receivers with analog I/Q generation,” Prog. Electromagnetics Res. 121, 225–247 (2011). [CrossRef]

14.

I. Fatadin, S. J. Savory, and D. Ives, “Compensation of quadrature imbalance in an optical QPSK coherent receiver,” IEEE Photon. Technol. Lett. 20(20), 1733–1735 (2008). [CrossRef]

15.

A. Besse, M. Bachmann, H. Melchior, L. B. Soldano, and M. K. Smit, “Optical bandwidth and fabrication tolerances of multimode interference couplers,” J. Lightwave Technol. 12(6), 1004–1009 (1994). [CrossRef]

16.

T. Pfau, S. Hoffmann, and R. Noé, “Hardware-efficient coherent digital receiver concept with feedforward carrier recovery for M-QAM constellations,” J. Lightwave Technol. 27(8), 989–999 (2009). [CrossRef]

OCIS Codes
(000.4430) General : Numerical approximation and analysis
(060.1660) Fiber optics and optical communications : Coherent communications
(060.2330) Fiber optics and optical communications : Fiber optics communications
(250.3140) Optoelectronics : Integrated optoelectronic circuits
(250.5300) Optoelectronics : Photonic integrated circuits

ToC Category:
Integrated Optics

History
Original Manuscript: June 10, 2013
Revised Manuscript: July 25, 2013
Manuscript Accepted: September 11, 2013
Published: September 23, 2013

Citation
P. J. Reyes-Iglesias, A. Ortega-Moñux, and I. Molina-Fernández, "Colorless monolithically integrated 120° downconverter," Opt. Express 21, 23048-23057 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-20-23048


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References

  1. Optical Internetworking Forum (OIF), “100G ultra long haul DWDM framework document,” document OIF-FD-100G-DWDM-01.0 (June 2009), http://www.oiforum.com/public/impagreements.html .
  2. Mirthe Project, “Monolithic InP-based dual polarization QPSK integrated receiver and transmitter for coherent 100–400Gb Ethernet,” http://www.ist-mirthe.eu/ .
  3. R. Kunkel, H. G. Bach, D. Hoffmann, C. Weinert, I. Molina-Fernández, and R. Halir, “First monolithic InP-based 90 degrees-hybrid OEIC comprising balanced detectors for 100GE coherent frontends,” in International Conference on Indium Phosphide & Related Materials (IPRM, 2009), paper TuB2.2, pp. 167–170.
  4. B. Zhang, C. Malouin, and T. J. Schmidt, “Towards full band colorless reception with coherent balanced receivers,” Opt. Express20(9), 10339–10352 (2012). [CrossRef] [PubMed]
  5. L. E. Nelson, S. L. Woodward, S. Foo, M. Moyer, D. J. S. Beckett, M. O’Sullivan, and P. D. Magill, “Detection of a single 40 Gb/s polarization-multiplexed QPSK channel with a real-time intradyne receiver in the presence of multiple coincident WDM channels,” J. Lightwave Technol.28(20), 2933–2943 (2010). [CrossRef]
  6. V. E. Houtsma, N. G. Weimann, T. Hu, R. Kopf, A. Tate, J. Frackoviak, R. Reyes, Y. K. Chen, L. Zhang, C. R. Doerr, and D. T. Neilson, “Manufacturable monolithically integrated InP dual-port coherent receiver for 100G PDM-QPSK applications,” Tech. Digest Optical Fiber Comm. (OFC) (2011), paper OML2.
  7. P. J. Reyes-Iglesias, A. Ortega-Moñux, and I. Molina-Fernández, “Enhanced monolithically integrated coherent 120° downconverter with high fabrication yield,” Opt. Express20(21), 23013–23018 (2012). [CrossRef] [PubMed]
  8. P. Pérez-Lara, I. Molina-Fernández, J. G. Wangüemert-Pérez, and A. Rueda-Pérez, “Broadband five-port direct receiver based on low-pass and high-pass phase shifters,” IEEE Trans. Microw. Theory Tech.58(4), 849–853 (2010). [CrossRef]
  9. F. M. Ghannouchi and R. G. Bosisio, “An alternative explicit six-port matrix calibration formalism using five standards,” IEEE Trans. Microw. Theory Tech.36(3), 494–498 (1988). [CrossRef]
  10. P. J. Reyes-Iglesias, I. Molina-Fernández, A. Moscoso-Mártir, and A. Ortega-Moñux, “High-performance monolithically integrated 120° downconverter with relaxed hardware constraints,” Opt. Express20(5), 5725–5741 (2012). [CrossRef] [PubMed]
  11. T. Pfau, S. Hoffmann, O. Adamczyk, R. Peveling, V. Herath, M. Porrmann, and R. Noé, “Coherent optical communication: towards realtime systems at 40 Gbit/s and beyond,” Opt. Express16(2), 866–872 (2008). [CrossRef] [PubMed]
  12. C. Xie, P. J. Winzer, G. Raybon, A. H. Gnauck, B. Zhu, T. Geisler, and B. Edvold, “Colorless coherent receiver using 3x3 coupler hybrids and single-ended detection,” Opt. Express20(2), 1164–1171 (2012). [CrossRef] [PubMed]
  13. A. Moscoso-Mártir, I. Molina-Fernández, and A. Ortega-Monux, “Signal constellation distortion and BER degradation due to hardware impairments in six-port receivers with analog I/Q generation,” Prog. Electromagnetics Res.121, 225–247 (2011). [CrossRef]
  14. I. Fatadin, S. J. Savory, and D. Ives, “Compensation of quadrature imbalance in an optical QPSK coherent receiver,” IEEE Photon. Technol. Lett.20(20), 1733–1735 (2008). [CrossRef]
  15. A. Besse, M. Bachmann, H. Melchior, L. B. Soldano, and M. K. Smit, “Optical bandwidth and fabrication tolerances of multimode interference couplers,” J. Lightwave Technol.12(6), 1004–1009 (1994). [CrossRef]
  16. T. Pfau, S. Hoffmann, and R. Noé, “Hardware-efficient coherent digital receiver concept with feedforward carrier recovery for M-QAM constellations,” J. Lightwave Technol.27(8), 989–999 (2009). [CrossRef]

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