62 IEEE PH0”ICS TECHNOLOGY LE’ITERS, VOL. 6. NO. 1, JANUARY 1994
Low-Loss
Phased- Array Based
4-Channel Wavelength Demultiplexer
Integrated with Photodetectors
M.
R.
Amersfoort,C. R. de Boer, B.
H.
Verbeek, P.
Demeester, A.Looyen, J.
J.
G. M. van der To1
Abstract- A 4-channel phased-array wavelength division de- multiplexer with 1.8 nm channel spacing at 1.54pm has been monolithidy integrated with photodetedors in WhGaAsP. On chip losses are 3.5 to 4.5 dB. These are the lowest losses reported so far for demultiplexers monolithically integrated with photodetectors. Nearest neighbor crosstalk ranges from -12 to -21 dB.
I. ~ O D U C T I O N
KEY component of wavelength division multiplexed
A
(WDM) direct detection systems is a low-loss wave- length demultiplexer monolithically integrated with a pho- todetector array. Wavelength demultiplexers integrated with photodetectors on InP have been reported previously 111, [2]. Those devices, demultiplexing many channels, were based on curved reflection grating demultiplexers. On-chip losses typically range from 10 to 20dB.
An 8-channel transmission grating type demultiplexer on InP with 6.5 dB on-chip loss has been reported recently [3]. As an alternative, demultiplexers based on the dispersive and focusing properties of an optical- phased waveguide-array have demonstrated low insertion losses in glass waveguide technology [4], [5]. Zirngibl etal. [6] have realized a 15-channel phased-array demultiplexer
on InP substrate with 2 to 7 dB on-chip loss.
In this paper we report the realization of a k h a n n e l low-loss wavelength demultiplexer on InP monolithically in- tegrated with photodetectors. The demultiplexer, based on the phased-array concept, combines low-loss with good crosstalk properties and can be fabricated with a simple process tech- nology.
II. DESIGN
A k h a n n e l receiver with 2 nm wavelength spacing has been designed for operation at 1.54 pm wavelength. The
Manuscript received July 14, 1993; revised September 16, 1993. This work was supported in part by the Netherlands Technology Foundation (STW), the IOP (Dutch Ministry of Economic Affairs) and in part carried out in the RACE 2070 MUNDI project.
M. R. Amersfoort, C. R. de Boer and B. H. Verbeek are with Delft University of Technology. Faculty of Electrical Engineering, P. 0. Box 5031, 2600 GA Delft, The Netherlands. Verbeek is also with the Philips Optoelectronic Center, Eindhoven. The Netherlands.
P. Demeester is with the University of Gent/IMEC, Department of Infor- mation Technology, Belgium.
A. Looyen is with the Research Group for Optics, Dept. of Applied Physics, Delft University of Technology, The Netherlands.
J. J. G. M. van der To1 is with the FTT Research, Leidschendam, The Netherlands.
IEEE Log Number 9214969.
Waveguide array
Transmitters
Detectors Fig. 1. Schematic representation of the phased-array wavelength division demultiplexer principle.
demultiplexer consists of a dispersive waveguide array con- nected to slab waveguide regions for radiatively coupling light into and out of the array (Fig. 1). h o micrometer wide waveguides with a lateral refractive index contrast of 0.037 and Neff=3.29 (TE-polarization) were used. At the input and output aperture the waveguides are closely spaced (1 pm gap) in order to limit the amount of power coupled into higher diffraction orders. At the image plane the spacing of the receiver waveguides, which direct the light toward the photdiodes, was designed to be 5pm, theoretically giving more than -40 dB isolation between the different channels. The length of the slab waveguides is 280 pm, allowing for 1 dB diffraction loss for the outermost receiver channels. This re- quirement fixes the array order at 88 for the center wavelength.
To collect the light diffracted from the input waveguide the array contains 46
arms
with a path length difference of 41 pm between adjacent arms. Each arm consists of two straight waveguides of variable length smoothly connected to a non- concentric waveguide bend. The bending radius in the array varied from 500 to 750 pm. Corrections have been made to account for the radius-dependent propagation constant in the bend using the conformal transformation technique [7]. Four input waveguides were used to allow tuning of the wavelength response of the demultiplexer. The total device size is 3.0 x 2.3 mm2 including photodetectors and input branches, separating the input guides to a 100 pm pitch. A TE-Th4 shift of 4.1 nm is predicted due to waveguide birefringence.AMERSFOORT et 01.: LOW-LOSS PHASED-ARRAY BASED 4-CHANNEL 0 -5
:
-20 0 a -25 -30 63 1534 153E 1538 1540 1542 1544 Wavelength [ n m ]Fig. 3. Wavelength response of the reference demultiplexer. Measurements are calibrated against straight reference waveguides.
Fig. 2. Micrograph of the realized phased-array demultiplexer.
The light in the output waveguides is fed into the photode- tectors using evanescent field coupling. The layer structure has been optimized to enhance detector absorption [8]. The photodetector size was conservatively chosen to be 150 x 80 pmz for ease of fabrication and probing. The intemal quantum efficiency of these diodes is better than 90%.
111. FABRICATION
The layer structure was grown on an n+-InP substrate by low pressure MOVPE [9]. It comprised a 1.5 pm undoped InP buffer layer, a 0.6 pm undoped InGaAsP (A,= 1.3 pm) guiding layer, a 0.3pm undoped upper waveguide cladding, a 0.27 p m n-InGaAs (lx1017 ~ m - ~ ) absorption layer, a 0.5pm p- InP ( 1 ~ 1 0 ' ~ ~ m - ~ ) layer and finally a O.1pm p-InGaAs ( 2 ~ 1 0 ' ~ ~ m - ~ ) contact layer. Another layer structure, containing only the waveguide layers, has been grown for fabrication of a demultiplexer without photodetectors for reference purposes [lo].
In the first step, detector mesa's were defined by wet selec- tive etching down to the InP upper waveguide cladding layer. C&/He reactive ion etching [ 111 was used to define 2 pm wide and 0.35 pm deep waveguide ridges for the demultiplexer. The p-contact was performed by lift-off of Ti/Pt/Au and the n-contact by evaporating the same metallization on the back of the wafer. Finally the contacts were alloyed in an RTP. For the reference demultiplexer the waveguide facets were AR-coated by evaporation of an SiO, layer onto the cleaved waveguide facets. Fig. 2 shows a micrograph of the realized phased-array demultiplexer.
IV.
RESULTSThe demultiplexers were characterized with an
HP
816819 tunable laser source. Light was end-fire coupled into the input waveguides using a microscope objective. For the reference demultiplexer, the light emanating from the waveguides was imaged onto a Ge photodiode. The photocurrent of the in- tegrated demultiplexer was measured with a standard wafer prober. For both samples waveguide losses were found to be 1.3 and 2.0 dB/cm for TE andTM
polarization respectively, as determined from Fabry-Perot measurements on waveguides with non-coated facets.The wavelength response of the reference demultiplexer is displayed in Fig. 3. Excess losses were determined to be 2
to 3
dB
by comparison with straight reference waveguides. Excellent uniformity of the output signal level can be observed. The on-chip insertion losses are estimated to be 2.5 to 3.5 dB (by adding the loss of a straight reference waveguide). Nearest neighbor crosstalk is better than -23 dB. Measured channel spacing is 1.8 nm. The full-width at half maximum (FWHM)value is 0.7 nm. The measured free spectral range of the device is 15.1 nm. A TE-TM shift of 4.8 nm has been determined.
Figure 4 shows the response of the demultiplexer integrated with photodetectors. Excess losses were found to be 3 to 4 dB for TE polarization. TM excess losses were typically 0.5 dB higher. An external responsivity of 0.12 A/W (-10
dB
total insertion loss, including coupling losses to the waveguide) has been determined, consistently with power budget calculations assuming 100% detector efficiency. Nearest neighbor crosstalk ranges from -12 to -21 dB. A TE-TM shift of 4.6 nm has been measured.V. DISCUSSION
The on-chip losses of the demultiplexer and the external responsivity are, as far as we know, the best values reported for wavelength demultiplexers integrated with photodetectors. Integration with detectors resulted in a 1 dB loss penalty. This
is most probably due to deviation of the waveguide width from the design value, causing additional coupling loss at the input and output apertures of the array. The relatively high crosstalk level of the integrated demultiplexer is caused by light which directly couples into the detectors from the input
64 IEEE PHOTONICS TECHNOLOGY L E W R S , VOL. 6, NO. 1, JANUARY 1994 I I I I I
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m U aJ VI C 0 a VI a, Y L L M 0 U a, W Y n 1536 1538 1540 1542 1544 1546 W a v e l e n g t h [ n m ]Fig. 4. Wavelength response of the demultiplexer integrated with detec- tors. Measurements are calibrated against detectors integrated with straight reference waveguides with the same length as the demultiplexer.
waveguide, indicated by the fact that detector number 3, which shows the highest crosstalk level (-12 dB), is in line with the input waveguide. Choosing another input waveguide reduces the crosstalk level of this detector to -18 dB. An absorber between the input waveguide and the detectors would elimi- nate this problem in future designs. Deviation of the channel spacing from the design value of 2 nm is fully explained by material dispersion, which was not taken into account in the orginal design. Compared to curved diffraction grating type demultiplexers phased-array devices are less suitable for (de)multiplexing large numbers of channels, but exhibit lower losses and can be realized with relatively simple and tolerant process technology. Fabrication of the demultiplexer itself requires only one (optical) lithographic step and no deep mirror etching is required.
VI. CONCLUSIONS
A k h a n n e l wavelength demultiplexer has been integrated with photodetectors. With a relatively simple process technol- ogy we realized the lowest on-chip losses so far reported for
integrated demultiplexers. Nearest neighbor crosstalk ranges from -12 to -21
dB.
It is expected to be reduced to less than-23 dB for all channels by providing optical isolation between input and output.
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