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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 26 — Dec. 12, 2011
  • pp: B531–B536
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InP monolithically integrated wavelength selector based on periodic optical filter and optical switch chain

Nicola Calabretta, Ripalta Stabile, Aaron Albores-Mejia, Kevin A. Williams, and Harm J. S. Dorren  »View Author Affiliations


Optics Express, Vol. 19, Issue 26, pp. B531-B536 (2011)
http://dx.doi.org/10.1364/OE.19.00B531


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Abstract

We present an InP monolithically integrated wavelength selector that implements a binary search for selecting N modulated wavelengths. The wavelength selector filter is realized using log2N an active Mach-Zehnder interferometer filter and broadband optical gating elements. Nanosecond reconfigurable operation with a spectral-alignment over 3.2nm free spectral range is achieved with an extinction ratio exceeding 25dB. Error-free operation of the wavelength selector for four modulated wavelengths with 2 dB of power penalty is demonstrated.

© 2011 OSA

1. Introduction

2. Principle of operation of the wavelength selector

Figure 1(b) shows the WS operation with 8 channels. Given ‘1 0 1’ as binary controls, λ6 is selected by the WS circuit. Indeed, at the first node the PF in combination with the optical switch selects λ5,.., λ8. At the second node, the PF with FSR/2 separates the λ5, λ6 at port 1 and λ7, λ8 at port 2 and the optical switch selects λ5, λ6. At the third node, the PF with FSR/4 separates the λ5 at port 1 and the λ6 at port 2 and the optical switch selects λ6. The example in Fig. 1 employed 8 channels and required 3 = log28 optical switches and PFs. The operation of the WS can be generalized to N channels by using log2N optical switches and PFs.

As a proof of concept we have fabricated an InP monolithically integrated WS, shown in Fig. 2
Fig. 2 Microscope image of the fabricated reconfigurable wavelength selector.
, capable to select one out of four wavelengths. The PFs were implemented by using MZI filters, and each of the two optical switches consists of two SOAs driven by two complementary electrical signals.

3. Device design and fabrication

The reconfigurable wavelength selector is implemented with a broadband gate stage, a wavelength selective switch stage and a final broadband gate stage monolithically integrated on InP-InGaAsP. Figure 2 shows the schematic layout for the waveguides and the control electrodes. The details at the Mach-Zehnder interferometer (MZI) are enlarged as an inset. The device is fabricated from a four quantum well active InGaAsP/InP epitaxy with a gain spectrum covering the range 1600-1620 nm. A three-step reactive ion etch is performed to define deep, shallow and electrically-isolated waveguides for the required operation. The MZI filter uses deep-etched waveguides to ensure tolerable loss in the 100 μm radius bends. The arrangement can be conveniently concatenated with arbitrary free spectral ranges at each filter stage by changing the differential length in the bends. In this study the filter arm lengths are 500 μm and 272 μm, respectively, and designed to provide a free spectral range of 3.2 nm. The three stages occupy a total area of 1 mm × 4 mm, although this can be readily reduced using shorter amplifier gates. Multimode interference couplers are employed as splitters and combiners. Input and output waveguides are angled at 7° with respect to the facets to suppress reflection. Using a feed-forward filter as the Mach-Zehnder interferometer filter also potentially increases the allowed gain before the occurrence of on-chip oscillation. Nonetheless, here operating currents are restricted to avoid oscillations from the uncoated facets, indicative of high levels of on-chip gain.

The gold shaded electrodes in Fig. 2 are wire bonded and electronically tuned to study spectral reconfigurability. The first broadband selector stage is operated by complementary biasing of SOA gate 1 and SOA gate 2. The wavelength selective Mach-Zehnder filter stage is operated by biasing the inner short arm with a fixed current near the transparency (20mA), and by varying the outer electrode current. The final broadband switch stage is operated by complementary biasing of SOA gates 3 and 4. The scheme is readily scaled through concatenation.

4. Experimental setup

The experimental set-up employed to demonstrate WS operation with optical packetized signals at different wavelengths is shown in Fig. 3
Fig. 3 (a) Experimental set-up. (b) Transfer function of the periodic filter.
.

First, the static characterization of the WS chip has been performed. Lensed fibers were employed for coupling the light in/out of the chip. DC currents of 110 mA and 114 mA were applied to SOA1 and SOA3, while 27.3 mA, 20.2 mA and 3.2 mA and 29.2 mA were applied to the two arms and the two couplers of the active filter, respectively. The SOA at the output of the chip was biased with 35 mA. Figure 3 (b) shows the normalized filter transfer function obtained by scanning an input tunable laser wavelength from 1585 nm to 1615 nm. The transfer function presents a periodicity of 3.2 nm. The −3dB bandwidth of the MZI filter was 1.1nm. The cross-talk between channels spaced by 1.6nm was around −16.5 dB. Cross-talk and flat-top passband can be further improved by using higher order filters [10

10. Z. Wang, S.-J. Chang, C.-Y. Ni, and Y. J. Chen, “A high-performance ultracompact optical interleaver based on double-ring assisted Mach–Zehnder interferometer,” IEEE Photon. Technol. Lett. 19, 1072–1704 (2007). [CrossRef]

].

The passbands are registered to the incoming WDM signals. Fine spectral alignment is demonstrated by scanning a tunable laser over the range from 1600 nm to 1607 nm for a signal launched at SOA1 [11

11. R. Stabile, N. Calabretta, H. J. S. Dorren, M. K. Smit, and K. A. Williams, “Reconfigurable Monolithic Wavelength Filter Using Gated Amplifying Mach-Zehnders,” IEEE Photonics 2011 Conference (IPC11) Arlington, Virginia, MR4 (2011).

]. An optical spectrum analyzer (OSA) measures the optical transmission through the filter device for a range of bias conditions at the longest MZI arm, SOA5 (see Fig. 2). Wavelength is tuned by scanning current from 40.0 to 100.0 mA in Fig. 4(a)
Fig. 4 (a) Output optical signal as a function of the current at the long MZI arm, SOA5, for four input wavelengths covering one free spectral range. (b) Contour map of optical transmission showing the fine wavelength tuning as a function of the current of SOA5.
. The wavelength is changed in the range from 1604.57 to 1609.97 nm covering the overall free spectral range. DC currents of 4.1 mA and 22.8 mA are applied to the two MMI splitters. The shortest MZI arm is biased with a 46.7 mA DC current. SOA1 and SOA3 DC currents are set to 99.4 and 103.1 mA, respectively. The output SOA current is fixed at 46.5 mA and the second last SOA current is 24.5 mA. Figure 4(a) shows the current tuning of high contrast nulls. The extinction ratio increases with the current, and this may be enhanced through the amplified filters themselves. The extinction ratio exceeds 25 dB for 1605.57 wavelength case when increasing the bias current of the long MZI arm from 66.0 to 97.0 mA.

Figure 4(b) shows the continuously programmable fine wavelength tuning: the peak wavelength shifts of 1 nm for almost every 10 mA tuning. In this case the DC current for the shorter arms is fixed at 33.6 mA. SOA1, SOA3 and SOA4 DC currents are changed to 118.3, 72.6 and 96.7 mA, respectively.

Once the WS is spectrally aligned, the static operation of the WS chip was tested by injecting four CW optical signals, λ1 = 1600.9 nm and λ2 = 1602.5 nm into port 1, and λ3 = 1604.1 nm and λ4 = 1605.7 nm into port 2, respectively (see Fig. 3). The optical switches in the WS chip have been electronically switched to select a distinct CW wavelength: when optical switch 1 selects the output port 1 of the MZI filter, only λ1 or λ2 is selected whenoptical switch 2 selects output port 1 or output port 2 of the MZI filter, respectively. Setting optical switch 1 to select the output port 2 of the MZI filter, only λ3 or λ4 is selected when optical switch 2 selects output port 1 or output port 2 of the MZI filter, respectively. The four measured spectra at the chip output for the four combinations are shown in Fig. 5 (a-d)
Fig. 5 Optical spectra recorded at the WS output for four different operation of the selector. Selecting (a) λ1. (b) λ2. (c) λ3. (d) λ4. (e) Time response of the SOA of the optical switch.
. These static results clearly show the WS operation. The measured cross-talk was lower than −16 dB, and the OSNR of the selected CW signals were larger than 30 dB. Scaling the WS operation to a larger number of channels will be limited by the OSNR degradation caused by the accumulated ASE noise of the SOAs in the chain. An OSNR larger than 26 dB after 8 SOA-based recirculation loops was measured in [12

12. E. F. Burmeister, J. P. Mack, H. N. Poulsen, J. Klamkin, L. A. Coldren, D. J. Blumenthal, and J. E. Bowers, “SOA gate array recirculating buffer for optical packet switching,” in Proceedings of OFC 2008, San Diego, USA, OWe4 (2008).

]. This indicates that potentially a WS with a chain of 8 passive filters and 8 SOA switch stages is possible enabling selection from 256 modulated wavelengths.

We have also investigated the time response of the WS chip. We fed a CW signal into the WS and we applied an electrical pulsed signal with 5 V of amplitude and a rising and falling time of 2 ns to the optical switch. Figure 5 (e) shows the photo-detected output of the WS showing a rise time and fall time of around 4.6 and 3.2 ns (10-90% transitions), respectively. Electrical reflections are seen to lead to a dip in the time resolved gain 46 ns after the turn-on transient. This is expected to be eliminated by implementing high speed drivers in close proximity to the chip.

To investigate the dynamic operation of the WS chip, we generated optical packets at λ1,…, λ4 by using an amplitude modulator driven by 10 Gb/s pattern generator with a 211-1 PRBS interleaved with 512 bits sequence of zeros (see Fig. 6a
Fig. 6 (a) Input packets. (b-c) Complementary controls applied to SOA1, SOA2 of the optical switch 1. (d-e) Complementary controls applied to SOA3, SOA4 of the optical switch 2. (f-i) WS output traces for the four wavelengths. BER curves of the back-to-back and static selected wavelength at λ1, and of the dynamic packet selected operation. Inset, eye diagrams of the signal before and after the WS at λ1.
). This results in a packet guard-time of 51.2 ns, which is sufficient to guarantee the response of the SOA to be flat with respect to the applied control, avoiding the dip 46 ns after turn on. The colored optical packets were amplified, wavelength demultiplexed and decorrelated, before being fed into the WS chip. Two pairs of modulated signals λ1, λ2 and λ3, λ4 were fed into the two inputs of the WS, respectively. The optical power of each signal was −2 dBm at the input fiber lens. The output power was −13dBm per channel. Assuming 6 dB/facet coupling losses, the chip losses are compensated by the SOAs. SOA1, SOA2, SOA3, and SOA4 of the two optical switches were driven by electronic control signals with 5.2 V, 5.1 V, 5.4 V, and 5.8 V, respectively. Note that most of the voltage is dropped across the 39 Ω matching resistor between the 50 Ω controller and the chip. By using a regular current source, the required voltage would be around 1.5 V. Figures 6(b-e) show the control signals appropriately delayed to dynamically select one distinct wavelength at a time. Figures 6 (f-i) report the time-domain traces for each of the four wavelengths at the output of the WS. Those traces clearly show that according to the control pattern only the one optical wavelength packet is selected by the WS. The average extinction ratio was higher than 15 dB.

The eye diagrams measured at the input and output of the WS chip are shown in Fig. 6 (j), respectively. The eye diagram at the WS output is clearly open but it is slightly degraded due to cross-gain modulation in the SOA and noise. The BER curves of the selected packets are reported in Fig. 6 (j). The BER curve in back-to-back configuration is provided as reference. We also report the BER curve in static operation of the WS recorded when only one wavelength (λ1) is transmitted through the WS. Error-free operation with a power penalty of 0.5 dB was measured. Error-free operation is also obtained for the dynamic selection of the packets at different wavelengths with a power penalty of 1.7- 2.1 dB, which is around 1.2 dB larger than the static case. The penalty is mainly due to cross-gain modulation between the signals and ASE from the SOAs.

5. Conclusions

We have fabricated and demonstrated a new fast InP monolithically integrated WS based on a cascade of periodic filters and optical switches that requires log2N optical switches for selecting N wavelength signals. The wavelength selector can be spectrally aligned to the incoming WDM signals electronically to select the required wavelength with an extinction ratio exceeding 25 dB at a tuning rate of 0.1nm/mA. Experimental results show error-free wavelength selection of four modulated signals at distinct wavelengths by using two optical switches with a power penalty of less than 2 dB.

References and links

1.

N. Amaya, I. Muhammad, G. S. Zervas, R. Nejabati, D. Simeodinou, Y. R. Zhou, and A. Lord, “Experimental demonstration of a gridless multi-granular optical network supporting flexible spectrum switching”, in Proc. OFC/NFOEC, San Diego, CA, OMW3 (2011).

2.

A. d’Alessandro, D. Donisi, L. De Sio, R. Beccherelli, R. Asquini, R. Caputo, and C. Umeton, “Tunable integrated optical filter made of a glass ion-exchanged waveguide and an electro-optic composite holographic grating,” Opt. Express 16(13), 9254–9260 (2008). [CrossRef] [PubMed]

3.

S. T. Chu, B. E. Little, V. Van, J. V. Hryniewicz, P. P. Absil, F. G. Johnson, D. Gill, O. King, F. Seiferth, M. Trakalo, and J. Shanton, ‘Compact full C-band tunable filters for 50 GHz channel spacing based on high order micro-ring resonators,’ OFC 2004, Anaheim, PDP9 (2004).

4.

X. Lu, M. Li, R. Samarth, and L. Zheng, “Electro-optic tunable bandpass filter based on long-period-grating-assisted asymmetric waveguide coupling,” Opt. Eng. 46, 405081 (2007).

5.

E. J. Norberg, R. S. Guzzon, J. S. Parker, L. A. Johansson, and L. A. Coldren, “Programmable Photonic Microwave Filters Monolithically Integrated in InP–InGaAsP,” J. Lightwave Technol. 29(11), 1611–1619 (2011). [CrossRef]

6.

E. L. Wooten, R. L. Stone, E. W. Miles, and E. M. Bradley, “Rapidly tunable narrowband wavelength filter using LiNbO3 unbalanced Mach-Zehnder Interferometers,” J. Lightwave Technol. 14(11), 2530–2536 (1996). [CrossRef]

7.

J. Tae Ahn, S. Park, J. Yun Do, J.-M. Lee, M.-H. Lee, and K. Hon Kim, “Polymer Wavelength Channel Selector Composed of Electrooptic Polymer Switch Array and Two Polymer Arrayed Waveguide Gratings, ” Photon. Technol. Lett. 16, 1567–1570 (2004).

8.

S. Khalfallah, B. Martin, J. Decobert, S. Fabre, C. Fortin, P. Peloso, I. Guillemot, J. Le Bris, M. Renaud, A. Goth, A. Dupas, L. Gilbert, and D. Pennincks, “First optical packet switching demonstration with sixteen-channel InP monolithically integrated wavelength selector module,” in Proceedings of the ECOC 2001, (2001), pp. 80–81.

9.

N. Kikuchi, Y. Shibata, H. Okamoto, Y. Kawaguchi, S. Oku, Y. Kondo, and Y. Tohmori, “Monolithically integrated 100-channel WDM channel selector employing low-crosstalk AWG,” IEEE Photon. Technol. Lett. 16, 2481–2483 (2004).

10.

Z. Wang, S.-J. Chang, C.-Y. Ni, and Y. J. Chen, “A high-performance ultracompact optical interleaver based on double-ring assisted Mach–Zehnder interferometer,” IEEE Photon. Technol. Lett. 19, 1072–1704 (2007). [CrossRef]

11.

R. Stabile, N. Calabretta, H. J. S. Dorren, M. K. Smit, and K. A. Williams, “Reconfigurable Monolithic Wavelength Filter Using Gated Amplifying Mach-Zehnders,” IEEE Photonics 2011 Conference (IPC11) Arlington, Virginia, MR4 (2011).

12.

E. F. Burmeister, J. P. Mack, H. N. Poulsen, J. Klamkin, L. A. Coldren, D. J. Blumenthal, and J. E. Bowers, “SOA gate array recirculating buffer for optical packet switching,” in Proceedings of OFC 2008, San Diego, USA, OWe4 (2008).

OCIS Codes
(200.4740) Optics in computing : Optical processing
(250.5980) Optoelectronics : Semiconductor optical amplifiers
(060.6719) Fiber optics and optical communications : Switching, packet
(130.7408) Integrated optics : Wavelength filtering devices

ToC Category:
Waveguide and Opto-Electronic Devices

History
Original Manuscript: October 3, 2011
Manuscript Accepted: October 17, 2011
Published: November 29, 2011

Virtual Issues
European Conference on Optical Communication 2011 (2011) Optics Express

Citation
Nicola Calabretta, Ripalta Stabile, Aaron Albores-Mejia, Kevin A. Williams, and Harm J. S. Dorren, "InP monolithically integrated wavelength selector based on periodic optical filter and optical switch chain," Opt. Express 19, B531-B536 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-26-B531


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References

  1. N. Amaya, I. Muhammad, G. S. Zervas, R. Nejabati, D. Simeodinou, Y. R. Zhou, and A. Lord, “Experimental demonstration of a gridless multi-granular optical network supporting flexible spectrum switching”, in Proc. OFC/NFOEC, San Diego, CA, OMW3 (2011).
  2. A. d’Alessandro, D. Donisi, L. De Sio, R. Beccherelli, R. Asquini, R. Caputo, and C. Umeton, “Tunable integrated optical filter made of a glass ion-exchanged waveguide and an electro-optic composite holographic grating,” Opt. Express16(13), 9254–9260 (2008). [CrossRef] [PubMed]
  3. S. T. Chu, B. E. Little, V. Van, J. V. Hryniewicz, P. P. Absil, F. G. Johnson, D. Gill, O. King, F. Seiferth, M. Trakalo, and J. Shanton, ‘Compact full C-band tunable filters for 50 GHz channel spacing based on high order micro-ring resonators,’ OFC 2004, Anaheim, PDP9 (2004).
  4. X. Lu, M. Li, R. Samarth, and L. Zheng, “Electro-optic tunable bandpass filter based on long-period-grating-assisted asymmetric waveguide coupling,” Opt. Eng.46, 405081 (2007).
  5. E. J. Norberg, R. S. Guzzon, J. S. Parker, L. A. Johansson, and L. A. Coldren, “Programmable Photonic Microwave Filters Monolithically Integrated in InP–InGaAsP,” J. Lightwave Technol.29(11), 1611–1619 (2011). [CrossRef]
  6. E. L. Wooten, R. L. Stone, E. W. Miles, and E. M. Bradley, “Rapidly tunable narrowband wavelength filter using LiNbO3 unbalanced Mach-Zehnder Interferometers,” J. Lightwave Technol.14(11), 2530–2536 (1996). [CrossRef]
  7. J. Tae Ahn, S. Park, J. Yun Do, J.-M. Lee, M.-H. Lee, and K. Hon Kim, “Polymer Wavelength Channel Selector Composed of Electrooptic Polymer Switch Array and Two Polymer Arrayed Waveguide Gratings, ” Photon. Technol. Lett.16, 1567–1570 (2004).
  8. S. Khalfallah, B. Martin, J. Decobert, S. Fabre, C. Fortin, P. Peloso, I. Guillemot, J. Le Bris, M. Renaud, A. Goth, A. Dupas, L. Gilbert, and D. Pennincks, “First optical packet switching demonstration with sixteen-channel InP monolithically integrated wavelength selector module,” in Proceedings of the ECOC 2001, (2001), pp. 80–81.
  9. N. Kikuchi, Y. Shibata, H. Okamoto, Y. Kawaguchi, S. Oku, Y. Kondo, and Y. Tohmori, “Monolithically integrated 100-channel WDM channel selector employing low-crosstalk AWG,” IEEE Photon. Technol. Lett.16, 2481–2483 (2004).
  10. Z. Wang, S.-J. Chang, C.-Y. Ni, and Y. J. Chen, “A high-performance ultracompact optical interleaver based on double-ring assisted Mach–Zehnder interferometer,” IEEE Photon. Technol. Lett.19, 1072–1704 (2007). [CrossRef]
  11. R. Stabile, N. Calabretta, H. J. S. Dorren, M. K. Smit, and K. A. Williams, “Reconfigurable Monolithic Wavelength Filter Using Gated Amplifying Mach-Zehnders,” IEEE Photonics 2011 Conference (IPC11) Arlington, Virginia, MR4 (2011).
  12. E. F. Burmeister, J. P. Mack, H. N. Poulsen, J. Klamkin, L. A. Coldren, D. J. Blumenthal, and J. E. Bowers, “SOA gate array recirculating buffer for optical packet switching,” in Proceedings of OFC 2008, San Diego, USA, OWe4 (2008).

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