Experimental demonstration of a multi-channel

Experimental demonstration of a multi-channel

micro-optical bridge for multi-Gb/s, free-space

intra-MCM interconnects

G. Verschaffelt, R. Buczynski, P.Tuteleers, P.Vynck, V.Baukens, H.Ottevaere, C.Debaes, S.Kufner, M.Kufner,

A.Hermanne*, J.Genoe**, D. Coppée**, R. Vounckx**, S. Borghs***, I. Veretennicoff and H.Thienpont

Laboratory for Photonics, Applied Physics Department, Vrije Universiteit Brussel, B-1050, Brussel, Belgium *Cyclotron Department, Vrije Universiteit Brussel, Laarbeeklaan 103, B-1090 Brussel, Belgium

**Laboratory for Microelectronics, Faculty of Applied Sciences, Vrije Universiteit Brussel, B-1050, Brussel, Belgium

***IMEC, Kapeldreef 75, B-3001 Leuven, Belgium

Abstract - We demonstrate a 2.5Gb/s optical intra-MCM data link with a four-channel micro-optical bridge. This bridge was fabricated by deep proton lithography and monolithically integrates cylindrical lenses and micro-mirrors.

1. Introduction

Future advances in the application of photonic interconnects will involve the insertion of parallel-channel links into Multi-Chip Modules (MCMs) [1]. These will make use of new device-level components such as arrays of Vertical Cavity Surface Emitting Lasers (VCSEL's) [2] or arrays of Micro Cavity Light-Emitting Diodes (MCLED’s) [3] and low power photoreceiver circuits [4]. One of the challenges associated with the development of free-space intra- and inter-MCM

optoelectronic interconnects is the fabrication of manufacturable, chip-compatible, and high precision monolithic micro-optical pathway blocks. These three-dimensional modules should integrate micro-optical components to optically interconnect surface-normal transmitters and receivers.

We use deep proton irradiation of Poly MethylMethAcrylate (PMMA) as a deep etch lithographic technique to fabricate monolithic structures integrating refractive micro-lenses, micro-mirrors, fiber positioning holes, stand-off and alignment

features [5] [6]. The technique works as follows: a proton beam in the energy range between 5 and 10 MeV, passing through a metal mask, impinges on a high molecular weight PMMA substrate. The protons break the polymer chains and

decrease the material's molecular weight. Using a specific developer the irradiated zone can then be selectively removed, making it possible to structure the sample to depths of several hundreds of microns with an optical surface quality of better

than 20 nm.

In this paper we will report on the data transcription at 2.5Gb/s with a multi-channel optical bridge made with this deep

proton lithographic technique. We have also fabricated a single-channel optical bridge and used this component to demonstrate the proof-of-principle of optical intra-chip interconnects by establishing a digital data link between

optoelectronic transceivers positioned on the same chip. Finally we project future performances of this approach for improved lens and emitter characteristics.

2. Multi-channel optical bridge

With the deep proton lithography we fabricated a monolithic micro-optical bridge consisting of two micro-mirrors, four

input and four output cylindrical micro-lenses (see Fig. 1). Each optical channel is formed by two of the lenses, so in total we have four parallel channels. The pitch between the different channels is 250 P m, the focal length of the lenses is 375 P m and the thickness of the sample is 500 P m. The input micro-lenses act as beam collimators, while the output micro-lenses refocus the light on the detector. Each micro-mirror surface makes an angle of 45° with respect to the optical axes

of the lenses in order to deflect the collimated beam by 90°. To demonstrate the potentialities for high bandwidth intra-MCM data communication, we have used this multi-channel micro-optical bridge to establish a free space optical link between a pigtailed laser diode and a pigtailed detector. This way we obtained data transcription at 2.5Gb/s over a distance of 8.3mm with a BER less than 10-13 (see Fig. 2). In this experiment, the speed of the interconnection was limited by the driving electronic circuitry and not by optical bridge itself. The efficiency of this bridge was measured to be 10%, while the cross-talk between neighboring channels was less than -30dB. This measurement was carried out for each of the 4 channels. The efficiency of the different channels was measured to be in the range 10%±1%, so the parallel channels have a good uniformity. During the measurement of the efficiency, we still made use of a single mode input fiber and a multi mode output fiber. The relatively low efficiency of this optical bridge is due to the cylindrical lenses, which perform a lens operation in only one direction.

3. Single-channel intra-chip interconnection

With the same technology we also fabricated a single-channel micro-optical bridge and used this to demonstrate an optical intra-chip interconnection between an emitter and a receiver positioned on the same chip. As optoelectronic transceiver elements we have used optical thyristors [7]. These are bistable devices with a S-shape quasi-static IV-curve. For a voltage below the break-over voltage V BR the thyristors have a high impedance and they do not emit light. When a voltage greater than V BR is placed across them, the impedance falls and they emit light as a LED. External illumination reduces V BR and they are thus optically sensitive. This optical sensitivity can be greatly enhanced by connecting two devices together with a common resistance in a differential pair configuration. When an optical input is applied to one of the thyristors of the differential pair, photo-induced electron-hole pairs are generated. Placing a voltage across the pair causes a differential competition to take place between the elements. The one which has the lowest break-over voltage (caused by precharging due to the optical input) will begin to switch on first, drawing charge carriers from the other and preventing it from switching on. The energy required to switch such a differential pair could be as low as 15 aJ per P m 2 optical window area.After emission, applying a negative voltage pulse resets the differential pair. The devices used at present have an active window of 16 P m x 19 P m, and we can thus estimate the switching energy E S of the differential pair to be (16x19) P m 2 x 15 aJ P m -2 = 6 fJ. The center separation of each thyristor in the pair is 26 P m. In our experiments we used two kinds of thyristor configurations: a thyristor pair where both thyristors are individually and electronically addressable, which we use as emitters, and a differential pair configuration where the two thyristors are optically addressable, which we use as receivers.

To establish an intra-chip interconnection between these two thyristor pairs, we have fabricated a single-channel micro-optical bridge consisting of two micro-mirrors and two cylindrical micro-lenses. The distance between the center of the lenses was chosen such to match the pitch of the two thyristor pairs (844 P m). The lenses have apertures of 844 P m and focal lengths of 1044 P m. The micro-mirrors cover only half of the lens aperture and thus form an aperture beamsplitter,such that a part of the can escape from the top of the bridge and can be monitored with a detector. After the fabrication of this optical bridge, we have positioned it above the thyristor chip. Micro-lens M 1, positioned above the individually addressable thyristors T EL and T ER acts as a beam collimator, while micro-lens M 2 refocuses this collimated beam onto the receiving thyristors T RL and T RR respectively. Each micro-mirror makes an angle of 45q with respect to the optical axis of the lenses in order to deflect the collimated beam by 90q .

The actual data transcription works as follows: when a high voltage is applied to the right-hand side emitter thyristor T ER ,this thyristor will switch on and emit light. This light is imaged by the optical bridge onto the right-hand side thyristor T RR of the differential pair. In the next phase, T ER is switched off and we apply a high voltage across the differential pair,causing the receiving element T RR to switch on and emit light because of the illumination in the previous phase. The light emitted by T RR is than detected by a detector positioned above the bridge.

To demonstrate an optical intra-chip interconnection between the two thyristor pairs, an electrical waveform was applied to the thyristor T ER . In case of correct data transcription, the corresponding receiving thyristor T RR should only switch on when a high voltage was applied to the emitter T ER

. This way we obtained reliable data transcription up to 1Mb/s.

four cylindrical input lenses four cylindrical output lenses

input single

mode fiber (NA=0.11)output multi mode fiber (NA=0.2, core diameter = 50P m)

micro-mirror

micro-mirror Input driving

waveform

Inverted

output signal

Figure 1 : Four-channel optical bridge with input and output fiber.

Figure 2 : Driving waveform of the pigtailed

laser diode (O =1.3P m) and the corresponding

optical output at the receiver.

Using a Monte-Carlo based radiometric simulation we found an optical efficiency K 0 of 0.06% for the single channel data transmission system described above. Together with the switching energy E s of the differential pair and the power P e emitted by the individual thyristors, we can determine the minimum duration W e min of the receiving face of the differential pair in order to obtain correct data transcription.

W e min E s P e K 0Using 6 fJ for E s and a measured value of 35 μW for P e , we find a value of 0.3 μs for W e min , which is in quit good

agreement with the experimental value of around 0.8 μs. The difference between the experimental and the expected value can probably be attributed to misalignment errors.

From the expression for W e min we can see that there are different ways to improve the performance of the interconnection.First it is possible to improve the efficiency of the optical system by replacing the cylindrical lenses with spherical lenses,which can also be fabricated by deep proton lithography. Next, we can replace the thyristors, which are Lambertian sources, by transmitters with better output characteristics and a higher emission efficiency, such as MCLED's and VCSEL’s. Table 1 gives an overview of the calculated optical efficiencies for different configurations. Note that all of these efficiencies can be multiplied with a factor of two by making the micro-mirrors longer, such that the mirrors cover the complete lens aperture. In the case of the MC-LED and the VCSEL based system, the overall system performance will not only increase due to a higher optical efficiency of the optical pathway block, but also because these sources have a higher emission efficiency.844 um Figure 3 : Setup used to demonstrate an intra-chip interconnection. The dashed arrows indicate the optical pathway. The inset shows a picture of the micro-optical bridge.

Figure 4 : Thyristor chip used to realize

the optical pathway represented in Fig 3M 1M 2

Figure 5 : Driving waveforms for emitter (T E ) and

receiver (T R ) and the optical output

Configuration Optical efficiency

(mirrors cover half of the lens aperture)

Cylindrical lenses, LED (div. Angle 180°)0.06 %

Spherical lenses, LED (div. Angle 180°)0.9 %

Spherical lenses, MC-LED (div. Angle 100°) 1.5 %

Spherical lenses, VCSEL (div. Angle 12°)46 %

Table 1

4.Conclusion

As a conclusion we can state that we have demonstrated the proof-of-principle of an optical intra-chip interconnection by demonstrating data transcription at Mb/s between two optoelectronic transceivers positioned on the same chip. We have shown how the performance can be enhanced by improving the lens and emitter characteristics. We also realized a multi-channel optical bridge, which we have used to demonstrate the potentialities for high bandwidth datacommunication with low cross-talk between the channels.

In the future we will try to further improve the performance of our optical modules by replacing the cylindrical lenses with spherical ones, which can also be fabricated with deep proton irradiation of PMMA. This way we should achieve a much higher efficiency and we will be able to make two dimensional arrays of parallel channels.

Acknowledgments

This work was funded by the EC-MEL-ARI project OIIC, the FWO, IUAP 13, the GOA research concerted action “Photonics in computing II” and the OZR of the Vrije Universiteit Brussel .

References

[1]R.F.Carson et al., “Low-power approaches for parallel free-space photonic interconnects”, SPIE CR 62, Eds.

R.T.Chen and P.S.Guilfoyle, pp. 35-63, 1996.

[2]K.H. Gulden, D. Ruffieux, K. Thelen, M. Moser, D. Leipold, J. Epler, H.P. Schweizer, E. Greger and P. Riel, “16*16

Individually Addressable Top Emitting VCSEL Array with High Uniformity and Low Threshold Voltages”, Proc.

E.O.S., Conf. On Optics and Information, pp. 6.1, Mulhouse, France, 1995.

[3]H. De Neve, J. Blondelle, P. van Daele, P.M.A. Demeester, R.G. Baets, G. Borghs, “Planar substrate-emitting

microcavity light-emitting diodes with 20% external QE”, Proc. of SPIE, vol. 3002, pp. 74-84, SPIE Photonics West, San Jose, California, 1997.

[4]T.K. Woodward, A.V. Krishnamoorthy, A.L. Lentine, L.M.F. Chirovsky, “Optical receivers for optoelectronic

VLSI”, IEEE Journal of Selected Topics in Quantum Electronics, vol. 2, pp. 106-116, 1996.

[5]M. Kufner and S. Kufner, “Micro-optics and Lithography”, VUB Press, Brussels, 1997.

[6]M. Kufner, S. Kufner, “Fabrication of monolithic integrated fiber-lens connector arrays by deep proton irradiation”,

Microsystem Technologies, pp. 114-118, 1996.

[7] B Knupfer, M Kuijk, R Vounckx, P Heremans, G Borghs, “Cascadable differential PnpN optoelectronic switch

operating at 50 Mbit/s with ultrahigh optical input sensitivity”, Electr. Letts. 31 pp 485-486, 1995.