# Functional photonic-crystal mini-stopband demux integrated in emitting and detecting devices

Henri Benisty, Cyril Cambournac, Melanie Ayre, Frederick van Laere, Dries van Thourhout, R. Baets, Tiziana Stomeo, T.F. Krauss, M. Kamp, H. Hoffman, et al.

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## To cite this version:

Henri Benisty, Cyril Cambournac, Melanie Ayre, Frederick van Laere, Dries van Thourhout, et al. Functional photonic-crystal mini-stopband demux integrated in emitting and detecting devices. 14th European Conference on Integrated Optics, Jun 2008, Eindhoven, Netherlands. pp.293-296. hal-00573288

HAL Id: hal-00573288

https://hal.science/hal-00573288v1

Submitted on 24 Aug 2022

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Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
Functional photonic-crystal mini-stopband demux integrated in emitting and detecting devices.

H. Benisty¹, C. Cambournac¹, M. Ayre¹, F. Van Laere², D. Van Thourhout², R. Baets²,

T. Stomeo³, T.F. Krauss³, M. Kamp⁴, H. Hofmann⁴, A. Forchel⁴, D.F.G. Gallagher⁵

¹Institut d'Optique, CNRS, Campus Polytechnique, RD128, 91127 Palaiseau cedex, France
henri.benisty@institutoptique.fr

² Ghent University - IMEC, Department of Information Technology, Belgium

³ St Andrews University, Physics and Astronomy, Fife, KY16 9SS, Scotland, U.K.

⁴Technische Physik, Am Hubland, Universität Würzburg, Würzburg D-97074, Germany

⁵Photon Design, 34 Leopold Street, Oxford, OX1 4TW, UK

**Abstract.** We successfully implement mini-stopband DeMUX – the wavelength-selective routing in multimode photonic-crystal “Wn” n-missing-rows waveguides – in two devices: (i) a tunable laser for wavelength monitoring based on W3 guides with < 20 GHz accuracy; (ii) a membrane-based InP DeMUX receiver, based on W(5+x) guides, with polarization diversity and Coarse WDM capability.

Introduction

Demultiplexing is a crucial function in integrated telecom devices. Photonic Crystal (PhC) multimode waveguides support anticrossing phenomena, so-called MSB (mini-stopband) that have been exploited to demonstrate a demux action. We show here two devices where integration of such a demux has been made possible, a laser to monitor its wavelength, and a polarization-independent surface coupled receiver, with integrated photodiodes. This work was part of the EU project FUNFOX (IST 04582).

Fig. 1 explains the basics of MSB demux. Fig.1a depicts the typical band structure of a multimode “Wn” photonic crystal waveguide consisting of holes on a triangular array of pitch *a* with *n* missing rows (here *n*=7, matrix index *n*<sub>eff</sub>=2.7 to 3.3 depends on the layer structure). Higher order modes and the fundamental mode couple together in tiny frequency and wavevector windows [1], ignoring each other elsewhere. The higher-order mode easily tunnels through many rows of PhC (Fig.1d-e), while the fundamental mode cannot. Hence, in Fig.1b, we exploit this difference to produce highly wavelength-dependent photocurrents in two ‘side’ and ‘through’ photodiodes. The ratio can be used to monitor in an integrated fashion a tunable laser wavelength, and provide data for feedback, especially along the edges of the spectral response.

Fig.1c shows the demux WDM action, that we shall use rather in a Coarse WDM (CWDM) spirit, according to Refs [2-4]. Thanks to its small footprint and satisfactory response, our device competes well with various approaches (phasar/AWG [5], cut-off waveguides [6], multiple cavities [7]) and has some intrinsic design robustness.

The existing attempts of an MSB-based demux [2-4] were all made in a substrate approach. This will still be the case of the laser monitor of Fig.1b, but we will demonstrate here an MSB-based demux implemented on an InP membrane[8,9], with surface couplers and polarization diversity [10].

Wavelength monitor

The wavelength monitor we implemented was based on the principle of Fig.1b. The basic PhC-tunable laser makes use of two cavities of similar length, and uses two
injection currents to tune the laser monomode emission across several tens of nm around 1550 nm, with good SMSR in general [11-12]. The rear mirror of such a laser was thinned to three rows to allow sufficient signal to leak through. The short taper was simple, its efficiency was not a critical issue, as it had a smooth spectral behaviour.

Several other hurdles had to be tackled. Firstly, electrical cross-talk between diodes and with lasers had to be controlled by means of insulation trenches and grounded guard rings to avoid leakage currents. Next, reflections in the laser had to be tamed, thanks to extensive FDTD simulations [13]. For this, the few rows at the start of the PhC waveguide were made different so as to break coherence of the back reflected beam, as will be detailed. The thinned side was also made with a simple trench instead of one of the rows to diminish the feedback and broaden the response range.

After all these steps, the system looked as shown in Fig.2. The ratio of the photocurrent was monitored as a function of the two laser currents with our home made setup and gave the result of Fig.3a. Compare with the lasing wavelength map of Fig.3b. The impact of the MSB can clearly be seen. Using the ratio to monitor, we obtained the correlation data of Fig.4a,b, which show an unambiguous correlation between lasing wavelength and ratio over several nm.

This particular monitor, with associated electronics, could therefore be of interest to finely lock laser arrays in PICs. The issue of temperature dependence will be discussed.

## Integrated receiver with photonic-crystal integrated demux

Thanks to the progresses in surface couplers on membrane [14], we could design a device that gathers three key elements: (i) a polarization diversity surface coupler, (ii) a demux device working as indicated in Fig.1c, and (iii) integrated photodiodes.

The basic system is an InP membrane bonded thanks to BCB onto a holder. A single e-beam-defined mask is defined. It is first used to perform deep etching of PhC holes, while the coupler areas are completely protected by another coarse masking. Then, these area are de-protected, and a shallow-etch step is practised, of no influence for the “through” PhC holes. The photodiode array is included on the back side, as will be described in more detail on slides in correspondence with the BCB bonding procedure.

Several subparts were tested at intermediate steps. They revealed the high performance limits of our approach, notably the polarization diversity performance (Fig.4) and the wavelength demux capabilities (Fig.5, on devices with cleaved optical readout rather than photodiodes).

The overall chip is described in Fig.6 as well as its key demux performances on all but one functional channel. The overall footprint is small and can be shrunk to 100x150 µm with curved grating coupler. Device-wise, 50 MHz operation of the demux will be shown, the photodiodes having a designed 10 GHz cut-off (not measured).

## Acknowledgements

Authors acknowledge the FP6 European Project IST-04582 “FUNFOX” (2004-2007).

## References

[1] S. Olivier et al. Phys. Rev. B, vol. 63, pp. 113311 (1-6), 2001.

[2] E. Viasnoff-Schwoob et al. Appl. Phys. Lett., vol. 86, pp. 101107, 2005.
[3] L. Martinelli IEEE J. Lightwave Technol., vol. 25, pp. 2385-2394, 2007.

[4] L. Martinelli et al. IEEE Phot. Technol. Lett., vol. 19, pp. 282-284, 2007.

[5] W. Bogaerts et al. "Compact Wavelength-Selective Functions in Silicon-on-Insulator Photonic Wires," IEEE J. Sel. Top. in Quantum Electron., vol. 12, pp. 1394-1401, 2006.

[6] T. Niemi et al. IEEE Phot. Technol. Lett., vol. 18, pp. 226-228, 2006.

[7] Y. Akahane et al. Opt. Express, vol. 13, pp. 2512-2530, 2005.

[8] T. Stomeo et al. "Integration of Grating Couplers with Compact PhC Demux on InP Membrane", submitted to Optics Letters (2008).

[9] G. Roelkens et al., J. Electrochem. Soc., vol. 153, pp.G1015-G1019 (2006).

[10] F. Van Laere, D. Van Thourhout, R. Baets, T. Stomeo, T. F. Krauss, M. Ayre, C. Cambournac, and H. Benisty, "Multifunctional Photonic Crystal Compact Demux-Detector on InP," paper OThM5, presented at OFC/NFOEC 2008, 25-28 Feb., San Diego, 2008.

[11] S. Mahnkopf, M. Kamp, A. Forchel, et al. Optics Comm., vol. 239, pp. 187-191, 2004.

[12] M. Kamp et al. "Nanophotonic integrated lasers" SPIE Proc. Photonics West (San Jose, 2007), Integrated Optics: Devices, Materials, and Technologies X 64750Z (Feb. 14, 2007).

[13] http://www.photondesign.co.uk

[14] See F. Van Laere et al. IEEE Phot. Technol. Lett., vol.19, pp.1919-1921 (2007) and refs therein.

Figure 1: (a) dispersion relation of a typical Wn PC waveguide (here W7), with highlighted crossing of higher order mode with fundamental mode; (b) use of MSB to get complementary spectral selectivity between the two photo-currents, 'side' and 'through'; (c) use of variable width to produce a demux; (d) schematic profile and tunneling of higher-order mode through the thinned cladding. (e) fundamental mode profile.

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Figure 2: Micrograph of the wavelength monitor situated at the rear side of a two section PhC laser. The fate of different wavelengths is illustrated by arrows with solid line or dotted line. Note the strong taper just at the entrance of the monitor, partly hidden by the guard ring.

Figure 3 (page 4 top): (a) Correlation of wavelength and ratio along a particular plateau (high SMSR), ensuring 20 GHz potential stability over 110 GHz (dots at the top are the SMSR data, cf. left scale); (b) Tuning map of the laser: wavelength (color) as a function of front and rear section currents. Note well-defined parabola-shaped regions; (c) Map of photocurrent ratio PD2/PD1 as a function of the same parameters. Note the overall similarity with the (b) map. features.
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Figure 4 : (a) Grating coupler and integrated detectors for polarization diversity, test layout; (b) Polarisation dependent loss as a function of wavelength (see Fig.5c for the typical coupler spectral characteristics)

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Figure 5 : (a) modeled response of an seven-channel demux based on a wedged W<sub>5+x</sub> PhC (x<0.26), largest wavelength channels are extracted first, hence the trough on the right wing of shorter wavelength channels; (b) experimental data from test device with cleaved edge, dB scale ; (c) spectral response of surface coupler used in this test device for incoupling.

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Figure 6 : (a) Overall layout of the compact integrated demux showing the two polarization diversity paths circulating in the pair of demux 1&2, and recombining in the array of photodiodes, including the ‘through’ one at the right end; (b) Raw spectral response (photo-current, linear scale) of the three rightmost channels and the through detector. Channel4 was damaged.

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