Monolithically integrated 80-Gb/s AWG-based all-optical
wavelength converter
Citation for published version (APA):
Tangdiongga, E., Liu, Y., Besten, den, J. H., Geemert, van, M., Dongen, van, T., Binsma, J. J. M., Waardt, de, H., Khoe, G. D., Smit, M. K., & Dorren, H. J. S. (2006). Monolithically integrated 80-Gb/s AWG-based all-optical wavelength converter. IEEE Photonics Technology Letters, 10(20), 1627-1629.
https://doi.org/10.1109/LPT.2006.878152
DOI:
10.1109/LPT.2006.878152 Document status and date: Published: 01/01/2006
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 15, AUGUST 1, 2006 1627
Monolithically Integrated 80-Gb/s AWG-Based
All-Optical Wavelength Converter
E. Tangdiongga, Y. Liu, Member, IEEE, J. H. den Besten, M. van Geemert, T. van Dongen,
J. J. M. Binsma, H. de Waardt, Member, IEEE, G. D. Khoe, Fellow, IEEE, M. K. Smit, Fellow, IEEE, and
H. J. S. Dorren, Member, IEEE
Abstract—We present a monolithically integrated all-optical
wavelength converter. The converter consists of four semi-conductor optical amplifiers for four separate inputs and an arrayed-waveguide grating. Error-free wavelength conversion with reasonable penalties for a 27 1 pseudorandom binary sequence was shown for a single input 80-Gb/s signal. The device exploits cross-gain/phase modulation in a single amplifier and se-lects with a filter the blue-chirped spectrum of the new wavelength signal in order to speed up the device response. This device has a dimension of 1.7 3.5 mm2 and it can be operated to convert simultaneously four 80-Gb/s wavelength channels.
Index Terms—Optoelectronic devices, semiconductor nonlinear
optics, semiconductor optical amplifier (SOA).
I. INTRODUCTION
A
LL-OPTICAL wavelength converters (AOWCs) are expected to become key components in future broad-band networks. Semiconductor optical amplifiers (SOAs) are favoured for use as AOWCs because they can be made compact, cascadable, and operationable at low optical powers. Wavelength conversion can be realized either by cross-gain modulation (XGM) or by cross-phase modulation (XPM) [1]. For both techniques, the conversion is severely limited by the finite recovery time of the SOA, causing distortion and pattern independence of the converted signal. An attractive approach to artificially improve the recovery speed was proposed in [2] in which a fiber Bragg grating is applied at the SOA output.We have recently demonstrated that a single SOA assisted by an optical filter due its ultrafast effects can be employed to wavelength-convert 160-Gb/s optical signals with extremely low powers and small penalties [3]. The 160-Gb/s AOWC was constructed by using commercially available fiber pigtailed components. In this letter, we present a compact monolithically integrated wavelength converter that can switch 80-Gb/s data streams with reasonable penalties. The wavelength converter Manuscript received January 12, 2006; revised May 1, 2006. This work was supported by The Netherlands Technology Foundation STW project EET6491, by the European Commission funded Programme IST-LASAGNE (FP6-507509), and by the European Network of Excellence IST-ePIXnet (FP-004525).
E. Tangdiongga, Y. Liu, H. de Waardt, G. D. Khoe, and H. J. S. Dorren are with COBRA Research Institute, Eindhoven University of Technology, Electro-Optical Communication Systems, Eindhoven 5600 MB, The Nether-lands (e-mail: E.Tangdiongga@tue.nl).
J. H. den Besten and M. K. Smit are with COBRA Research Institute, Eindhoven University of Technology, Opto-Electronic Devices, Eindhoven 5600 MB, The Netherlands.
M. van Geemert, T. van Dongen, and J. J. M. Binsma were with JDS Uniphase, Eindhoven, The Netherlands.
Digital Object Identifier 10.1109/LPT.2006.878152
consists of an SOA array and an arrayed-waveguide grating (AWG) to form a 1.7 3.5 mm AOWC chip.
II. DEVICEFABRICATION
Our AOWC consists of four 2- m-wide, 750- m-long ridge waveguide type SOAs integrated with a four-channel AWG. The AWG was designed to have 5-nm channel spacing , 20-nm free-spectral range (FSR), and 1.2-nm bandwidth (BW). Fabrication of the device is summarized as follows. All epi-taxial layers for the AOWC chip were grown by low-pressure metal–organic vapour phase epitaxy at 625 C. The SOA ac-tive layer consists of a 0.12- m-thick InGaAsP layer (
m; bg: bandgap) embedded between two quaternary con-finement layers. The structure was covered by a 0.2- m-thick p-InP layer. Next, the active sections were defined by lithog-raphy and (selective) wet chemical etching using a SiO layer as etching mask. In the second epitaxy step, a InGaAsP layer ( m) was selectively grown for the passive sec-tions with the SiO mask protecting the active secsec-tions [4]. In the third epitaxy step, a 1.3- m-thick p-InP cladding layer and the p-InGaAs contacting layer were grown. All waveguides were etched to the same depth by reactive ion etching using Cl –CH –Ar–H . The etch depth was limited to 0.1 m into the InGaAsP film ( m), to minimize propagation loss. Intolerances in etch depth of 50 nm were found not to affect the AWG BW significantly. The fabrication process was completed by standard isolation, metallization, and facet coating proce-dures. The chip was equipped with a thermal controller for fine tuning the AWG transmission bands.
III. SETUP ANDOPERATIONPRINCIPLE
The experimental setup as shown in Fig. 1 consists of an 80-Gb/s optical time-division-multiplexed (OTDM) transmitter, the proposed wavelength converter, and an 80-Gb/s OTDM re-ceiver. The transmitter and receiver were constructed by using commercially available pigtailed components. For the trans-mitter, a 2-ps, 10-GHz optical clock signal ( nm) that outputs from an actively mode-locked fiber ring laser is optically quadrupled to 40 GHz and is subsequently modulated to form a 40-Gb/s base rate of return-to-zero (RZ) pseudorandom binary sequence (PRBS). This 40-Gb/s data signal is further time-multiplexed to 80 Gb/s by using an optical doubler . The short PRBS length is required because of the multiplexer and the receiver limitations. The 80-Gb/s data signal is combined with a continuous-wave (CW) probe signal using a 3-dB optical coupler. This probe signal is set at a wavelength of nm which coincides with one of the passbands of the AWG filter. The data pulse and the CW signal are injected into the AOWC chip that is made out 1041-1135/$20.00 © 2006 IEEE
1628 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 15, AUGUST 1, 2006
Fig. 1. AOWC: (a) setup, (b) monolithically integrated wavelength converter. MLFRL: mode-locked fiber ring laser,22= 2 4: optical doubler/quadrupler, 1t: delay line for channel selector, AR: anti reflection.
of an SOA array and one AWG filter. The four SOAs make, in principle, possible simultaneous wavelength conversion of four input signals of 80 Gb/s each. However, for proof of concept only one input port, thus one SOA, is used. The AWG spectral response can be thermally controlled, i.e., 0.1 nm/ C, in order to find an optimum conversion point. The receiver comprises a channel selector , an electroabsorption modulator (EAM), and a 40-Gb/s photodetector. We used the nonlinear behaviour of the EAM to create 5-ps switch window for gating 40 Gb/s from 80-Gb/s wavelength-converted signals. The receiver performance is measured at the input of the 40-Gb/s receiver.
The working principle of this concept utilizing the SOA ultra-fast chirp dynamics has been expeditiously explained in [3]. The principle can be summarized as follows. In the SOA, the injected data signal at acts as pump and modulates the SOA gain. As a result, the CW probe signal at is affected via XGM, causing an inverted operation of wavelength conversion. More-over, the injected data signal also modulates the refractive index of the SOA, resulting in a chirped probe signal. The leading edge of the probe signal is red-shifted, whereas the trailing edge is blue-shifted. As a consequence, the spectrum of the probe signal is broadened. The probing wavelength is tuned to the red side of the AWG transmission band where the filter has a negative slope. Due to the SOA gain compression and the filter nega-tive slope, the contrast ratio and the temporal response of the wavelength switch are optimized. The output signal is an in-verted copy of the input signal, i.e., inin-verted mode. To operate the AOWC in noninverted mode, we flip the signal polarity by using a delay interferometer (DI) that is made out of polariza-tion controllers, a polarizapolariza-tion-maintaining fiber (PMF), and a polarizer [3]. This DI-PMF produces a maximum differential group delay of 4 ps.
IV. RESULTS
The spectral response of the wavelength converter chip is shown in Fig. 2. The measured FSR and BW are 20 and 1 nm, respectively. The AWG crosstalk is less than 15 dB. The AWG reponse is highly polarization-dependent. The polarization-dependent loss for transverse-electric (TE) and
Fig. 2. AWG transfer profile of the realized AOWC chip. The AWG acting as a multiplexer, hence only one passband, is observed to be polarization dependent (>3 nm), TE/TM loss differs by >6 dB. PDL: polarization-dependent loss.
Fig. 3. Eye diagrams from various points of the setup: (a)–(b) inverted and (c)–(d) noninverted wavelength conversion. An EAM demultiplexes 80 Gb/s to 22 40 Gb/s. (Color version available online at http://ieeexplore.ieee.org.)
transverse-magnetic (TM) mode differs by 6 dB and the TE/TM spectral shift is about 3 nm. Polarization controllers are used for each input signal to minimize the effect of polarization dependence. The slope of the AWG transfer function at the probe wavelength is approximately 0.005/GHz. During the experiment, the SOA is biased to 200 mA. We injected into the AOWC chip an RZ-PRBS 80-Gb/s data signal and evaluated with an optical sampling scope first the eye performance at various points in the setup and second the bit-error-rate (BER) performance. The eye diagrams are depicted in Fig. 3. Two conversion schemes are investigated: inverted and noninverted schemes. For an inverted operation of the AOWC, no addi-tional optical function is employed, giving the eye diagram in Fig. 3(b). For noninverted operation, the DI-PMF interferom-eter inverts the polarity of the AOWC output, producing the eye diagram in Fig. 3(c). The eye is somewhat noisy at the mark level because the trailing edge of the pulse does not coincide perfectly with the leading edge of its delayed copy. The AOWC output signal is sent to the EAM-based optical gate which
TANGDIONGGA et al.: MONOLITHICALLY INTEGRATED 80-Gb/s AWG-BASED AOWC 1629
Fig. 4. Detection performance of the wavelength-converted 80 Gb/s for PRBS 2 0 1: (a) BER curves, 40-Gb/s demultiplexed eyes for (b) inverted and (c) noninverted wavelength conversion. Penalties are related to the input power which causes BER = 10 . (Color version available online at http://ieeex-plore.ieee.org.)
retrieves the 40-Gb/s base-rate signal. The output of the gate is depicted in Fig. 3(d) for the inverted and Fig. 3(e) for the noninverted operation. All eyes show a clear opening around the center.
The 40-Gb/s base-rate signal is then coupled into the 40-Gb/s detector and a BER tester. The tester used the clock signal of the PRBS generator. Fig. 4(a) shows BER values of the 80-Gb/s receiver. In absence of the AOWC chip, the two 40-Gb/s tributaries show almost an identical BER performance. For comparison purposes, we also present in Fig. 4(a) BER values of the 40-Gb/s basic channel before the multiplexing stage. The 80-Gb/s receiver only causes penalties of less than 1.5 dB for BER when compared to the BER curve of the basic channel. The demultiplexing penalty can be considered small because it also includes the contribution of the multi-plexer. For inverted and noninverted wavelength conversion, the two 40-Gb/s signals show a similar result. Eye diagrams in Fig. 4(b) and (c) of the demultiplexed 40-Gb/s signal show very clear opening. The penalty for inverted and noninverted AOWC increases by 8 and 10 dB, respectively. These penal-ties are largely attributed to the fiber-to-chip coupling losses ( 4.5 dB/facet), the detuned AWG ( 10 dB), and the output signal distortion. All curves can reach BER values smaller than without any form of error floors. This performance indicates an excellent function of the monolithically integrated AOWC for 80-Gb/s operation. Simultaneous wavelength con-version of four input signals of 80 Gb/s each is feasible and is currently under investigation.
V. DISCUSSION ANDCONCLUSION
The 750- m SOA in the AOWC initially has 500-ps re-covery time. In AOWCs based on classical XGM/XPM, this SOA only allows operating bit rates of 10 Gb/s or less. Our ap-proach utilizing ultrafast chirp dynamics achieved much shorter recovery times, i.e., 10 ps. The crucial point here is that the center of AWG is slightly blue-shifted from the probe wave-length. By carefully matching the slope of AWG to the blue chirp, we can obtain a narrow switch window with steep slopes. The level of detuning is a compromise between the maximum contrast ratio and the minimum power loss in the AOWC. An op-tical filter with steep transmission edges in combination with a fast SOA will improve the pulse contrast ratio, reduce the signal distortion, and allow higher speed operation. Polarization-in-dependent AOWCs require polarization-inPolarization-in-dependent SOAs and AWGs. The chip uses a shallow etched AWG, which unfortu-nately causes polarization dependence. A deeply etched AWG can significantly reduce polarization dependence. Polarization-independent SOAs are feasible using current integration tech-nology. At the input and output port of the AOWC, we used er-bium-doped fiber amplifiers (EDFAs) to compensate for losses affected by the detuned filter and fiber-to-chip coupling. The use of EDFAs as optical amplifiers is not essential here since optical gain can also be delivered by SOAs. For noninverted operation, we used a DI-PMF after the AOWC. The DI-PMF can be re-placed by an integrated delayed interferometer, as was shown in [5]. The chip design allows multichannel operations. For simul-taneous multichannel wavelength conversion, the chip should have low optical crosstalk levels ( 30 dB). A chip with such low crosstalk levels can be realized using existing processing technology.
In conclusion, we have demonstrated a compact device for wavelength conversion that utilizes chirp in a single SOA. The device has been tested error free in wavelength-converting a 2 40-Gb/s OTDM signal for inverted and noninverted opera-tion. The chip can be operated to wavelength-convert up to four channels simultaneously. Upgrading to 160-Gb/s wavelength conversion is feasible within this technology for example by adapting the filter and minimizing the coupling loss and the in-ternal reflection. Finally, we wish to underline that this mono-lithically integrated device requires effectively very low input powers and offers high conversion stability.
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