Monolithically integrated DWDM receiver - IEEE Xplore

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Oct 7, 1992 - Abstract: A grating spectrometer integrated monolithically in the InGaAsP system with a photodiode array for a dense WDM system is pre-.
Mono1ith ically integrated DWDM receiver C. Cremer N. Emeis M. Schier G. Heise G. Ebbinghaus

Indexing t e r m : D W D M , Optical integration, Gratings, Photodiode arrays

Abstract: A grating spectrometer integrated monolithically in the InGaAsP system with a photodiode array for a dense WDM system is presented for the 1.5 pm wavelength region. The chip provides more than 30 wavelength channels with a spacing of 4 nm, a channel crosstalk of approximately - 15 dB, an internal photodiode efliciency of 90% and a photodiode capacitance of 0.33 pF. The chip needs no optical adjustments. It is therefore well suited to mass production.

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Introduction

Wavelength division multiplexing (WDM) in fibre-optic links allows superior utilisation of optical fibres and therefore attracts interest for optical communication [ 11 and for local area networks [2]. WDM may also be used for board-to-board and chip-to-chip interconnects in complex high-speed multiprocessor environments. In all these fields one takes advantage of the very high bandwidth and the freedom from electromagnetic interference in DWDM systems. Originally WDM was thought of as an intermediate system on the way to coherent systems, but the impressive development of optical amplifiers, and thus the improved sensitivity of direct receivers, has made WDM systems even more attractive. All WDM networks require demultiplexers to separate the wavelength channels in the optical domain. Gratingtype demultiplexers allow the simultaneous detection of numerous wavelength channels. Up to the present most systems have employed bulk grating spectrometers. Some examples of planar grating spectrometers in InGaAsP/ InP [3, 41 are known from the literature. Compact, reliable and cheap optical demultiplexers are needed for the widespread utilisation of WDM. These demands can be best met by the optical integration of filters and photodiodes. First results on such a device were presented recently [SI. We present here results on grating demultiplexers in InGaAsP/InP and recent progress in integrating with photodiode arrays. 2

fibre into a slab waveguide, allowing to strike the specified wavelength channels by selecting the appropriate waveguide and thereby tuning in between the channel

Design and technology

Fig. 1 shows a schematic view of the receiver. Several waveguides enable well-defined injection of light from a Paper 93573 (E3, E13). first received 6th July and in revised form 7th October 1992 The authors are with Siemens AG, Research Laboratories, 8OOO Munich 83, Germany

Fig. 1 nels

Sketch o J D W D M receiver chip showing Jour wavelength chan-

spacing. The light from this waveguide diverges freely in the slab with an aperture according to its width and is then diffracted and focused by the blazed and curved grating into one of the output waveguides, thus obviating the need for both lenses and focusing mirrors. The size of the slab waveguide is approximately 2.5 x 3.2 mm2 and gathers light with an aperture up to a full angle of 53”. The area used for the output waveguides is approximately 2.2 x 3.5 mm’. The area of the slab and the output waveguides could be reduced still further. We report here on chips with integrated photodiodes as well as chips without photodiodes but with output waveguides which reach the chip end face. The data showing low polarisation dispersion were taken from chips with output waveguides only.

2.1 Grating The mounting of the spectrometer is similar to a Rowland type mounting (Rowland radius of 1.5 mm or 1.8 mm) with a retrodiffraction wavelength of 1.4 pm. However, we can freely choose the positions of the foci, of the locus of the grating and of the pitch of the grating as our spectrometer is made by planar technologies (conventional lithography and etching). So we use a mounting with high dispersion but improve the resolution by making a design with two stigmatic foci at wavelengths 1, = 1.47 pm and I , = 1.58 pm located at the corresponding points of the Rowland circle [SI. Fig. 2 shows the spherical aberrations of a conventional This work has been supported by the Deutsche Bundespost. 71

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Rowland spectrometer compared to those of our modified mounting with stigmatic foci. One can clearly see the effect of the stigmatisation at the stigmatic wavelengths

more than a 1 pm thick InP layer one gets a remaining difference of approximately 0.5 nm in wavelength for TE and TM polarised light. For the chips with integrated photodiodes we used a InP cover layer of only 0.2 pm thickness (Fig. 4) to simplify the technology, but with a birefringence of approximately 2.5 nm. photodiode

output waveguide

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Fig. 4 Cross-sections of the photodiode and the passiue waueguide regions of the demultiplerer chip

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a Longitudinal cross-section h Transverse cross-section

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Fig. 2 Spectrum of main aberrations of a classical Rowland mounting and a mounting with two stigmatic foci in multiples of the %,acelength a Classical Rowland mounting b Two stigmatic focii

and in the spectral range close to these. The geometrical arrangement of the mounting changes only very slightly from the Rowland mounting. Astigmatism does not occur in slab waveguides and so theoretically the resolution is only diffraction limited. The lithography was made by a g-line stepper with a NA of 0.42, so we could resolve a blazed grating with a minimum period of 3.2 pm diffracting in the eighth order. Each grating tooth is blazed by an optimum angle. The grating is etched by RIE with CH,/H, to a depth of 4 pm and is finally metallised with gold to achieve a high diffraction efficiency (Fig. 3).

The layers were grown by MOVPE on an n-type InP substrate with a lattice mismatch Aula < 3 x lo-,. Even small inhomogeneities in terms of the composition and thickness of the slab waveguide yield deformations of the phase fronts and prevent the diffraction limited operation of the device. High quality layers are therefore needed. Deviations of the absolute values of the thickness and composition of the layers can be adjusted on the chip by choosing different input waveguides and temperatures. The temperature dependence of the refraction index of InP causes a wavelength shift of 0.1 nm/”C.

SEM micrograph of the etched and metolised grating

2.3 Input and output waveguides The lateral confinement of the input waveguides is achieved by removing the InP cover layer and thus making strip-loaded waveguides. At the focal line eight input monomodal waveguides (width 2 pm) end to couple the light into the slab waveguide at well defined positions. Coupling in the light at a neighbouring input waveguide gives a wavelength shift of 7 nm at one single output waveguide. Fine tuning of the wavelength detected in an output waveguide can be achieved by temperature control. The output waveguides on the chip with integrated photodiodes are multimode to get a wide passband and are deeply etched to avoid coupling between adjacent waveguides. These waveguides fan out to gain space for the contact pads of the photodiodes. Each output waveguide is terminated at its own photodiode. The fan out is realised by using circular bows of different bowlength and different radii. Waveguides with radii smaller than 2 mm showed additional radiation loss. The output waveguides on the chip without integrated photodiodes are striploaded and monomode.

2.2 Slab waveguide The polarisation dispersion of the device is given by the difference of the effective indices in the slab waveguide. To reduce this difference we choose a gap wavelength for the quaternary material of the slab waveguide of i.,= 1.05 pm which is fairly close to the binary InP. Furthermore, by covering the 0.6 pm thick slab waveguide with

2.4 Photodiodes The photodiodes are integrated vertically with the waveguides [7, 81. To simplify the technology, the p-contact metallisation of each photodiode is located on top of the 100 pm long and 50 pm wide photodiodes. Fig. 4 shows a longitudinal cross-section of a photodiode (Fig. 4a) and a

Fig. 3

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transverse cross-section of the input and output waveguides (Fig. 4b). A 0.5 pm thick InP buffer layer, a 0.6 pm thick InGaAsP waveguide layer (ig = 1.05 pm), a 0.2 pm thick InP intermediate layer, a 1 pm thick InGaAs absorption layer and a 0.5 pm thick InP cladding layer are grown on an n-type InP substrate by low-pressure MOVPE. In a first step, the pn junctions of the photodiodes are formed by Zn indiffusion through an SiN, mask. After metallisation of the contact pads by lift off, mesas for the photodiodes are etched down to the InP intermediate layer by RIE (CHJH,).

2.5 Test structures In addition to the spectrometer, the chip also contains some test structures: straight strip-loaded waveguides and straight waveguides which are strip loaded in the first part and deeply etched in the second part. Some of these straight waveguides are terminated by photodiodes whereas others reach the end face and allow coupling of light out of the chip. The chip also contains elliptical mirrors in the end face of a slab waveguide to evaluate the reflectivity of the etched end faces. Input and output waveguides terminate in the focal points of the elliptical mirrors. The output waveguides end either in a photodiode or reach front face of the chip. 3

Results

The response of the spectrometer was tested using a tunable colour centre laser (2 = 1.461.8 pm) with a bandwidth of 5GHz. The light was focused onto the cleaved end face of the sample by a microscope lens (50 x , NA = 0.6). The current of the photodiodes was measured using probe needles. The light from output waveguides without photodiodes was imaged with a small mirror and a second microscope lens through a pinhole onto a external photodiode as shown in Fig. 5. t o I R camera

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Measuremenr set up for rhe spectrometer wthout inteyrated Fig. 5 phorodiodes

3.I Spectrometer with integtrated photodiodes O n the chips with spectrometer and integrated photodiodes we measured 35 of the 42 channels in the wavelength range from 1464 nm to 1604 nm with the designed channel spacing of 4 nm. One channel at a wavelength of 1488 nm missed because of a defect in the photodiode. Fig. 6 shows the spectral response of the 35 adjacent photodiodes at 4.2 V bias on a logarithmic scale. Most of the channels show a crosstalk of less than - 15 dB at the adjacent wavelength channels ( A i = 4 nm). The passband of the filter is approximately 2.5 nm (FW) at a -1 dB level and 3.3 nm FWHM. IEE P R O C E E D I N G S - J , Vol. 140, N o . I , F E B R U A R Y 1993

O n the chips with photodiodes we found some inhomogeneities in the slab waveguide which gave some scattered light as background optical noise at approximately - 17 dB. By improving the quality of this slab

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wauelength, pm

Superposition of the phorocurrrnt per external laser power of Fig. 6 35 integrated Photodiodes as a function of waoelength

waveguide the same crosstalk should be achieved as in the chips without photodiodes (-25 dB) which have slab waveguides of better quality. The polarisation dispersion between TE and TM polarised light in the chips with photodiodes is 2.5 nm due to the design of the slab waveguide with only a thin InP cover layer. The photodiodes exhibit a dark current of 1.5 nA and a capacitance of 0.33 pF at 5 V reverse bias. They show an external sensitivity of 0.03 AIW for a single wavelength channel. It is known from other experiments that the photodiodes absorb more than 90% of the light in the waveguide, corresponding to a loss of less than 0.5 dB. The sensitivity of the wavelength channels fluctuates by approximately 3 dB due to varying loss in the output waveguides. Output waveguides with radii R < 2 mm have additional radiation loss from the bending of the waveguides. In Fig. 6 the channels with wavelength greater than 1.54pm have output waveguides with R < 2 mm. The outer channel at a wavelength of 1.604 pm has a waveguide with a bending radius of 1.06 mm. Obviously the reduction of the intensity for channels with wavelengths greater than 1.54 pm results from additional bending loss in the output waveguides. Neglecting the loss due to the waveguide-photodiode coupling, we estimate from measurements on the test structures the following contributions to the total insertion loss of - 14 dB ( - 17 dB): -4 dB coupling loss to the chip; -2 dB loss for the input waveguides; -3 dB for the slab waveguide; -2 dB for the output waveguides with R > 2 m m ( - 5 d B f o r R = l m m ) a n d -3dBdiffraction efficiency for the grating. The waveguides give the main contribution (7 dB-10 dB) to the loss. By using shorter waveguides and improved technology this value can be reduced. 3.2 Spectrometer with output waveguides only Fig. 7 shows the polarisation dispersion of the spectrometer with a 1 pm thick InP cover layer. O n this chip the output waveguide had a smaller width than the output waveguides on the chip with integrated photodiodes. Therefore the passband shown in Fig. 7 is only 0.4nm (FW) at - 1 dB and 0.6 nm FWHM. The two maxima are therefore well separated. The maximum of the peak 73

with TE polarised light is separated by approximately 0.4nm from the maximum with TM polarised light, which is in agreement with theory. The crosstalk from the

The on-chip attenuation is 10 dB but it can be reduced to less than 10dB by improving the waveguides. A powerful way to overcome the attenuation is to integrate an optical amplifier onto the chip. In this case the noise of the optical amplifier can be reduced by filtering the spontaneous emission in the grating spectrometer. This would give a very sensitive DWDM receiver chip. The entire chip is produced in the InGaAsP system. It is quite compact and needs no adjustments. It is therefore suitable for mass production. 5

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Fig. 7 Superponed signals f o r T E and T M polarised light from the external laser as a firnction of the wawlength for the chip without integrated photodiodes ~

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_ _ _ _ TM

adjacent wavelength channel obviously is lower than -25 dB. By improving the technology in processing the chips with integrated photodiodes the same crosstalk should be achieved on these chips. 4

Conclusion

A DWDM receiver chip is presented that demultiplexes 35 optical channels into 35 parallel electric signals. The crosstalk of the chip is better than - 15 dB. This already meets the requirements for DWDM broadcast systems. The crosstalk may even be reduced to -25 dB as has been shown.

References

1 BRACKETT, C.A.: ‘Dense wavelength division multiplexing networks: Principles and applications’, IEEE J . Sel. Areas Commun., 1990,8, pp. 948-964 2 DONO, N.R., GREEN, P.E. Jr., LIU, K., RAMASWAMI, R., and TONG, F.F-K.: ‘A wavelength division multiple access network for computer communications’, IEEE J. Sel. Areas Comun., 1990,ft, pp. 983-994 3 CREMER, C., EBBINGHAUS, G., HEISE, G., MOLLERNAVRATH, R., SCHIENLE, M., and STOLL, L.: ‘Grating spectrometer in InGaAsP/lnP for dense wavelength division multiplexing’, Appl. Phys. Lett., 1991,59,(6).pp. 627429 4 SOOLE, J.B.D., SCHERER, A., LEBLANC, H.P., ANDREADAKIS, N.C., BATH, R., and KOSZA, M.A.: ‘Monolithic InP - based grating spectrometer for wavelength division multiplexing systems at 1.5 jm’, Electron. Lett., 1991,27,pp. 132-134 5 CREMER, C., EMEIS, N., SCHIER, M., HEISE, G., and EBBINGHAUS, G.: ‘Grating spectrograph integrated with photodiode array in InGaAsPflnGaAsilnP’. IEEE Photonics Technol. Lett., 1992.k pp. 108-110 6 MXRZ, R., and CREMER, C.: ‘On the theory of planar spectronraphs’. IEEE J . Liahtwaw Technol.. 1992.10. (12) 7 EAVAILLES, J.A.,”RENAUD, M., JARRY, P., ERMAN, M., VINCHANT, J.F., and GOUTELLE, A.: ‘Integration of detectors with InGaAsPpnP carrier depletion optical switches’, Electron. Lett., 1990, 26, p. 1783 8 WINZER, G., DOLDISSEN, W., CREMER, C., FIEDLER, F., HEISE, G., KAISER, R., MARZ, R., MAHLEIN, H.F., MORL, L., NOLTING, HJ., REHBEIN, W., SCHIENLE, M., SCHULTEROTH, G., UNTERBORSCH, G., UNZEITIG, H., and WOLFF, U.: ‘Monolithidly integrated detector chip for a twochannel unidirectional WDM link at 1.5 pm’, IEEE J. Sel. Areas Commun., 1990, 8, pp. 1183-1189

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