Phased-array wavelength demultiplexer with flattened wavelength response

Phased-array wavelength demultiplexer with flattened

wavelength response

Citation for published version (APA):

Amersfoort, M. R., Boer, de, C. R., Ham, van, F. P. G. M., Smit, M. K., Demeester, T., Tol, van der, J. J. G. M., &

Kuntze, A. (1994). Phased-array wavelength demultiplexer with flattened wavelength response. Electronics

Letters, 30(4), 300-302.

Document status and date:

Published: 01/01/1994

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10

1508131

1 frequency, GHz

Fig. 3 Coupler refection loss

Output magnitude balance when

fed

to

H-port

the band 4.5-9.2GHz are shown in Table 2.

As

expected, very good output balance performances are obtained. As predicted by theory, a second f q u e n c y where the coupler is perfectly matched

exists near 8.8GHz.

Table 2: Coupler measured performances in frequency range 4.5

-

9.2GHz

mads, /

SA

0.3

*

O.OSdB

Pmumeter

I

Electrical defmiiion

I

Measured vulue

Return loss

I

ma&>

I

< -1odB

ance when fed to Isolation

E-port phase& /

SA

180" 4"

m d S A man(&)

< -23dB

Output phase bal-

I

I

I

0 " * 4 "

I

an& when fed to

I

phase(S,, I

,

)

S

H-UOrt

I

Output magnitude

balance when fed to

1

mag@, /

SA

I

0.4 t 0.25dB

E-port

Output phase bal-

1

I

Conclusion: We have presented a new small size, wideband hybrid

ring

coupler. Its circumference is only 0.6% which is to OUT

knowledge the smallest

size

ever attained by a 180" ring hybrid. All the coupler ports are fed by coplanar waveguides. The coupler demonstrated a band slightly larger than one octave. Excellent output magnitude and phase balance is achieved. The design sim- plicity and the absence of any transition make the coupler suitable for monolithic integration.

Acknowledgment: The authors wish to thank A. Boulouard from CNET Lannion for his helpful suggestions.

IEE 1994 21 Dcccmber 1993

Electronics Letters Online No: 19940234

M.-H.

Murgulescu, E. Moisan, P. Legaud and E. Penard (France

Telecom, CNET LAB/OCM/MLS, Route de Tdgmtel. BP 40, 22301.

Lmvlwn Cedex, h c e )

I. Zaquine (Irtihdt National des T6I.4cunununicatwns. DEC. 9 rue

Charles Fourirer. 91011. Evry Cedex. France) RdeFeoces

I MARCH.S.: 'A wideband stripline ring hybrid', IEEE Trans., 1968, MTT-16, pp. 361

300

I

2 KIM, D.I., and YANG, G.S.: 'Broad-band design of improved hybrid- ring 3dB directional coupler', IEEE Truns, 1982, MTT-30, pp. 2040-2046

3 KIM. D.I., and Y A W . G.S.: 'Design of a new hybrid-ring directional coupler using

M8

or U6 sections', IEEE Trans.. 1991, MTT-39, pp. 1179-1783

COUF'EZ. J.P., PEDEN, A., and PERSON, C.: 'Analysis and design of ultra miniature hybrid ring directional coupler'. Roc. European Miaowan Cod., 1991, pp. 443447

5 MURGULESCU, M.-H., LEGAUD. P., MOISAN, E., PENARD, E., and

U Q U I N e I.: 'Miniaturized wideband hybrid ring couplers: theory and experiment'. Submitted to IEEE MlT-S Symp., 1994 6 COHN, S.B.: 'Slot-line on a dielectric substrate', IEEE Trans.. 1969,

MTT-19, pp. 773-774 4

Phased-array wavelength demultiplexer

with flattened wavelength response

M.R. Amersfoort, C.R. de Boer, F.P.G.M.

van Ham,

M.K. Smit, P. Demeester, J.J.G.M. van der To1 and

A.

Kuntze

Indexing terms: Optical waveguides. Integrated optics, Wavelength

diviswn multipkxing

A four-channel phased-array wavelength demultiplexer with a Ilattcncd wavelength rcsponsc has heen realised for the fmt time in InP/InGaAsP at 1 . 5 4 ~ by employing multimode output waveguides. The device. has 2 m channel spacing and a flat respons (within IdB) of 17nm.

Inirohction: Optical phased-array wavelength demultiplexers

combine low loss with excellent spectral resolution [I-31. However the parabolic shape of the spectral output characteristics of those demultiplexers requires accurate matching of the laser wavelength to the transmission maximum of the demultiplexer. In this Letter we report the realisation of a phased-array demultiplexer with a flattened wavetength response. This has been achieved by using wide multimode output waveguides.

Iaqement see fig. 2 ,e'

rn

Fig. 1 Schemotic representation ofpkrrcamroy demultiplexer

~ monomode waveguide

-multimode waveguide

Design: A phasar demultiplexer consists of two star couplers, con- nected by a dispersive waveguide array (set Fig. 1). The electrical field distribution of the input waveguide is reproduced in the image plane of the demultiplexer

with

a lateral displacement dependent

on

the input wavelength,

thus

allowing

for the

spatial separation of d f l e m t wavelengths, as

described

by Smit 141. The shape of the spectral

response cum

is determined by the overlap

of

the field

distribution in the image plane with the eigenmode(s) of

the

output waveguide. In the phasar demultipkxers reported to

date, monomode input and output waveguides were used with identical dimensions, giving a parabolic-like spectral response. To flatten

this

response, we applied relatively wide multimode output waveguides, which

is

a common technique in bulk-optic demulti-

ELECTRONICS LETTERS

17th February 1994

Vol.

30

No.

4

~~ ~ ~

plexers 151. Within a certain wavelength range, the focused spot in the image plane (Fig. 2) will couple efficiently to the output waveguide. Although the power distribution between these modes

strongly depends on the relative position

d

the image, the total coupling efficiency will be close to 100%. as long as the image is not too close to the edge of the output waveguide.

mm

Fig. 2 Representation of image plane of demultilpexer

Use of wide multimode output waveguides will flatten the spectral response of the demultiplexer

Owing to the multimode excitation of the output waveguide, the demultiplexer outputs Cannot be coupled efficiently to monomcde output fibres. If they are however coupled to detectors, the advan- tage of the flattened response can be fully exploited.

A four-channel demultiplexer with 2 nm wavelength spacing at 1 . 5 4 ~ has been designed. The array has 46 waveguides with a path length difference of 5 9between adjacent arms. The radius ~ of curvature in the array vanes from 500 to 8 0 5 ~ . The width of the input waveguides is 2 ~At the output, . 6pm wide multimode output waveguides separated by a 3pm gap are used. Calculations predict a IdB transmission bandwidth of 1.12nm, taking into account that the highest order mode of the multimode output waveguide will be radiated out of the bend. A diffraction loss of 2dB was allowed for the outermost receiver waveguides. Calcula- tions predict a TE-TM shift of 4.lnm due to waveguide birefrin- gence. The total device size is 3.3x3.9mm2 including the input and output branches.

Fabrication: The demultiplexer is realised on InP substrate for integration with photodetectors. A double-beterostructure waveguide structure was grown by LP-MOWE: a 1 . 5 ~ InP buffer layer, a 0 . 6 6 ~ InGaAsP(1.3) waveguide core and a 0.32

pm InP top cladding. The demultiplexer was fabricated by etching 0.4 pm deep waveguide ridges using CH/He RIE with an S O 2 etching mask. In a second step the output waveguides were etched another 60 nm to reduce the radiation loss of the bends. Finally the sample was cleaved and AR-coated by evaporation of an SiO, layer onto the waveguide facets.

Results: Light from an HP 8168A tunable laser source was end- fire coupled into the input waveguide using an NA=O.65 micro- scope objective. The light emanating from the multimode output waveguide was imaged onto a Ge photodiode. The attenuation of 2 p wide straight reference waveguides was found to be 2dB/cm for both polarisations by Fabry-Perot contrast ratio measurements on uncoated waveguides. Fig. 3 shows the output power of the four receiver channels for TE polarised light. Measurements are calibrated against straight reference waveguides. The excess loss of the device is 3.5-4.5dB with a crosstalk level below -18dB. On- chip losses are estimated to be 4-5dB by adding the loss of a straight reference waveguide. The average 1dB bandwidth is 1.05nm. For the TM polarisation the spectrum is blue shifted 4.0nm due to waveguide birefringence.

ELECTRONICS LETTERS

17th February 7994

Vol.

0,

,

,

,

,

,

,

,

,

,

,

wavelength.nrn

lmm

Fig. 3 Spectral response of phasar demultiplexer

Measurements are calibrated against straight reference waveguides

D i s m s i o n t The insertion loss of this device is comparable to demultiplexers in I d ’ that we realised before 131. The measured 1 dB bandwidth agrees quite well with theoretical calculations. The crosstalk level is -5dB higher than for previous demultiplexers. This is mainly attributed to the wider output waveguides, that will pick up more of the incoherent background radiation that is due to local variations in the propagation constant in the array.

The realised 1dB bandwidth in excess of lnm will relax the matching requirements for the laser source wavelength with respect to the transmission maximum of the demultiplexer. In addition this flattened wavelength mponse can be used for polari- sation independent demultiplexers [6, 7] based on a TE-TM s h i

equal to the demultiplexer periodicity. In that case the flattened wavelength response will relax the ttquired tight control of the TE-TM shift of such a device.

Conclusions: A four-channel phased-array demultiplexer has been

realised on InP substrate with a IdB bandwidth of lnm at

2nm

wavelength spacing. The on-chip loss of the device is 4-5dB with a crosstalk level below -18dB.

The

flattened wavelength response will alleviate several tuning and trimming problems in WDM applications.

Acknowledgments: This work has been supported by the Nether- lands Technology Foundation (STW), the Dutch Ministry of Eco- nomic Affairs (IOP) and part of it has been camed out within the RACE 2070 MUNDI project. We thank A.M.J. Koonen and G.D. Khoe for useful discussions.

0 IEE 1994

Electronics Letters Online No: 19940249

M.R. Amersfoort, C.R. de Boer, F.P.G.M. van Ham and M.K. Smit

(Deyt University of Technology, Department of Elecrrical Engineering, PO Box 5031. 2600 GA Delft. The Netherlandr)

P. Demeester (University of Gent/lMEC, Departmenr of Informarion

Technology, Belgium)

J.J.G.M. van der To1 ( P l T Research. Leidrchendam. The Netherlands)

4 January I994

A. Kuntze (De&? Universify of Technology, Deparrment of Applied

Physics. The N e t h e r h i s )

References

1 TAKAHASHI. H., NISHI, I., and HIBINO, Y.: ‘1OGHz spacing optical frequency division multiplexer based on arrayed-waveguide grating’, Electron. Lett., 1992, 28, (4), pp. 38CL382

2 Z~RNGIBLM., DRAG ONE,^., and JOYNER,~.: ‘Demonstration of a 15 x 15 arrayed waveguide multiplexer on I&”, IEEE Photonics Techno]. Lett., 1992, 4, (ll), pp. 125CL1253

AMERSFOORT, M., DE BOER, C., VERBEEK. E., OEI, Y., DEMEESTER, P.,

GROEN, F., and PEDERSEN, I.: ‘High performance 4-channel PHASAR wavelength demultiplexer integrated

with

photodetectors’. Roc. 19th Eur. Conf. on Optical Communication (ECOC ’93), September 1993, Postdeadline paper ThC12.13, pp. 49-52

4 SMIT, M.: ‘Integrated optics in silicon-based aluminium oxide‘. PhD Thesis, Delft University of Technology, 1991

3

30 No.4

30

1

5 FUIII, Y., AOYAMA, K., and MMOWA, 1.: ‘Optical demultiplexer using a silicon echelette grating’, IEEE J. Quantum Electron., 1980, Q E 16, (Z), pp. 165-169

6 VELLEKOOP. A., and SMIT. M.: ‘Four-channel integrated-optic wavelength demultiplexer with weak polarization dependence’, J.

Lightwave Technol., 1991, LT-9, (3). pp. 31&314

7 ZIRNGIBL, M, JOYNER. c., STUETZ,L., GAIFFE,T., and DRAG ONE,^.:

‘Polarisation independent 8x 8 waveguide grating multiplexer on InP’, Electron. Lutt., 1993, 29, (2), pp. 201-202

SOGbit/s, 1.55km strained-InGaAsP MQW

modulator integrated DFB laser module

K. Wakita, K. Sato,

I.

Kotaka,

M.

Yamamoto and

T.

Kataoka

Indexing terms: Integrated oploelectronics, Elecrroabsorption modulators, Disiribuied feedback lasers

The high-sped (MGbiUs) and highly efficient (2V peak to peak

for a UdB onioff ratio) operation of an MQW integrated electroabsorption modulatorDFB laser module is demonstrated. Output power from the module is over +3dBm in the pigtailed singlemode fibre. To the authors’ knowledge, this is the frst

report of 2OGbiVs operation with a monolithically integrated light

source.

Introduction: For long-haul optical fibre transmission over l00km at a wavelength of 1.55pn, chirpless or low-chirp light sources are required. An electroabsorption intensity modulator monolithically integrated with a DFB or DBR laser is the most promising light source. Since an external modulator and a DFB laser diode (LD) were first integrated [I], many structures have been reported [2-91.

In lightwave systems with greater than IOGbit/s capacity, the modulation voltage must be reduced to produce a drive voltage in high-speed IC drivers. Multiquantum well (MQW) structures based on the quantum-confined Stark effect are promising because of their highly efficient electroabsorption [4,5,9]. We reported the first successful 15 - 2OGbiffs lOOkm transmission experiments [IO] using an ultrahigh-speed driverless InGaAsiInAlAs MQW inten- sity modulator with a small penalty of less than IdB. However, the stability of the optical coupling between the fibre and the soli- tary modulator was a serious problem. Recently, we succeeded in fabricating a strained InGaAsP-MQW modulator integrated DFB laser operating at low driving voltage and high emitting power [9]. In this Letter, we report, for the first time, 20Gbiffs modulation using an integrated light source module with a low driving voltage of 2v.

Device structure and design: The chip structure is the same as reported in [9]. It has two kmds of MQW layer: one a laser active layer and the other an electroabsorption layer with photolumines- cence wavelengths of 1.55 and 1.49p1, respectively. After fabricat-

ing first-order gratings on the top of the separate confinement heterostructure layer of the MQW LD and etching the LD section selectively, the modulator section was formed. The lengths of the modulator, the separation region and LD are 200, 50, and 3OOp, respectively. An antireflection coating with lower than 0.5% reflec- tivity was deposited on the front of the modulator facet to e l i i - nate residual reflection. To reduce device capacitance, polyimide was spin-coated under the bonding pad of the modulator.

The extinction ratio of the chip output power was 22dB at -2V and 30dB at

-

3V, at the coupled spherical end fibre. The 3dB bandwidth for a 5OQ load was more than ISGHz with a parasitic caoacitance of onlv 0.5aF.

cal isolator for low noise operation. Moreover, temperature con-

trollability and a hermetically d e d package improve stability and reliability. To reduce the impedance mismatch between the monolithic chip and the transmission line, a thin f h resistor of 50 Q was mounted close to the modulator chip. This matching circuit has a return loss of more than lOdB from 0 to IOGHz. The bond- ing wire between the chip and the line is designed as short as pos- sible.

For efficient optical coupling of the chip to a singlemode fibre, we used an optical coupling scheme where a spherical lens with a 6 O O p diameter and one with a 2.0mm diameter were placed in a confocal arrangement to accommodate the optical isolator. The minimum coupling loss of this lens system was 4.0dB, including isolator loss.

To stabilise operation, we introduced a thermoelectric cooler. The module can be operated from 0 to 50°C with a stable item temperature of 25

*

0.01 “C. All elements except the isolator were permanently fixed by YAG laser welding.

-5

-

E . m -10

-

W . -15

-

I . g-20

-

-25

-

-30

.

I

.

I

.

I

.

I

.

1 0 -1 - 2 -3 -4

applied voltage , V Ra0,ll Fig. 1 Output power against applied voltage from module at injection

current of IOOmA

Emitting wavelength is 1.546pm

Module performance: The output power from the module as a function of applied voltages is shown in Fig. 1. The average out- put power of the chip was +3dBm in the singlemode fibre. The threshold current of the DFB laser was 1SmA.

The small signal frequency response is shown in Fig. 2. The injection current was 80mA and the

DC

applied voltage for the modulator was - 0.4V. The

-

3dB electrical bandwidth of the transmitter module was more than 1SGHz which was consistent with the measured capacitance. Large signal measurements were performed at 2OGbit/s for a pseudorandom sequence 2’

-

1 pulses

long with 2V non-return-to-zero (NRZ) modulation. The centre oscillation wavelengths and the full widths at 20dB down with and without modulation were 1.5466 and 1.5468pn, and 0.2nm and 0.42nm, respectively. A sidemode suppression ratio of more than 40dB was obtained. The output power of the module was

-

0.7dBm under 20Gbit/s modulation and injection current of

88mA. I I I m c m 9 - 9 -

-

I I

I

0 5 10 15 20 frequency, GHz --, r -

Fa.

2 Small signalfrequency respome of module

Injection current for laser diode is 8 O d , and modulator is 4.4V

Module design: Optical transmitter modules for 10Gbit/s systems have heen developed using a Franz-Keldysh electroabsorption modulator integrated light source [l I]. In the module in this Let- ter, we reduced parasitic reactance caused by the assembly for high-speed modulation, introduced an efficient optical coupling scheme for high output power, and used an efficient compact opti-

for

Fig. 3 shows the eye pattern for 20GbiUs modulation driven by a 2.0V peak to peak electrical signal. A clear eye opening was