- Email: [email protected]
s based on a monolithic-integrated photonic circuit
Journal of Crystal Growth 209 (2000) 471}475
Interferometric wavelength converter operating at 10 Gb/s based on a monolithic-integrated photonic circuit C. Rigo*, C. Coriasso, D. Campi, A. Stano, C. Cacciatore, D. Re, G. Fornuto, D. Soldani, R. De Franceschi, F. Ghiglieno, M. Vallone, P. Valenti, L. Zucchelli, S. Lupo, P. Gambini CSELT - Centro Studi e Laboratori Telecomunicazioni, Via G. Reiss Romoli 274, 10148 Torino, Italy
Abstract In this work we present a wavelength converter based on a Michelson interferometer. It is obtained by monolithic integration of two-semiconductor optical ampli"ers with a passive waveguided X-coupler, incorporating turning mirrors. It operates in the 1.55 lm spectral window and allows the wavelength conversion of data streams up to 10 Gb/s, showing open-eye diagrams and extinction}ratio regeneration capabilities. Comparison of two structures with di!erent active layers and their in#uence on the polarization sensitivity is also presented. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 42.79.Sz; 72.80.Ey; 78.30.Fs; 85.30.St Keywords: Chemical beam epitaxy; Interferometric wavelength converter; Semiconductor optical ampli"er
1. Introduction and working principle The wavelength conversion of a #ux of optical data at high bit-rate is one of the key functions required by future wavelength division multiplexing (WDM) systems for optical cross-connect and fast packet switching purposes [1]. An attracting solution to realise this functionality in a completely optical way, exploits cross-phase modulation (XPM) in an interferometric structure deriving from the monolithic integration of semiconductor optical ampli"ers and passive wave-
* Corresponding author. Tel.: #39-11-228-7025; fax: #3911-228-5085. E-mail address: [email protected] (C. Rigo)
guides [2]. In particular, the realised device is based on a Michelson interferometer con"guration, in which two-strained multi-quantum well (MQW) semiconductor optical ampli"ers (SOA) are integrated with the passive region consisting of an X-coupler and eight 903 micromirrors (Fig. 1). Taking advantage of the total internal re#ection at the semiconductor}air interface, these integrated mirrors reduce the overall dimensions of the device. Beside this, the advantages of this device are the wide operation bandwidth, the high signal-to-noise ratio and the regeneration capability of the signal. The passive ports (A, B) of the chip in Fig. 1 are coated with an antire#ection medium consisting of a double dielectric layer (TiZrO/SiO ), while the 2 two active ports (C, D) are as cleaved and behave as semire#ecting mirrors.
0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 6 0 0 - 4
472
C. Rigo et al. / Journal of Crystal Growth 209 (2000) 471}475
Fig. 1. Wavelength converter with close-up view of micromirrors regions.
A local CW laser (j ) is coupled to the port 1 A and routed to the X-coupler which is evenly split and directed to the active SOA region. The propagating beams are consequently ampli"ed and re#ected at the facets C and D and then re-routed to the X-coupler region where the phase interference occurs. Adjusting the injection current of the SOA's makes it possible to vary the phase of the travelling "elds, "nding the balancing condition in which the re#ected beam is completely addressed to port A and achieving extinction at port B. The working principle is based on the perturbation of the balancing condition induced by the signal to convert, that in operating conditions is an incoming #ux of data at wavelength j . 2 In fact, in the presence of the incoming data #ux (j ) every bit `1a relaxes the balancing conditions 2 and, through the XPM e!ect, addresses the re#ected signal (j ) at port B. Using spectral "ltering the 1 component j is then removed and therefore the 2 data sequence (originally encoded at j ) is transfer2 red onto the optical channel j . 1 2. Structure and geometry The structure for the device was grown in a VG80H CBE system whose description was
given in earlier references [3]. After a Si-doped InP bu!er layer, a 0.15 lm thick unstrained 1.24 lm wavelength In Ga As P layer was 0.76 0.24 0.52 0.48 deposited underneath the active layer to evascently couple the light into and out of the active sections: it served as a separate-con"nement layer within the active sections, and as a waveguide core in the passive section. A 50 nm thick undoped InP layer was then grown as stop-etch for the following technological steps. The active QW region was deposited using growth interruptions and evacuating the hydrides during the switches. A "nal Be-doped InP layer was used as a cladding layer. The growth temperature was 5203C for the quantum well region and the quaternary layer and 5003C for the InP layers. In the active layer of the SOAs we used a multiple quantum well (MQW) sequence, similar to the one documented in Ref. [4]. In order to reduce the polarization-dependence of the gain, we stacked up QWs of alternative compressive (#1%) and tensile (!0.8%) strain. The three compressive strained In Ga As QWs provide TE gain 0.675 0.325 through electron}heavy hole recombination, while three tensile-strained In Ga As QWs 0.417 0.583 provide predominantly TM gain through electron}light hole recombination. The barrier material (In Ga As P ) was also compressive 0.88 0.12 0.42 0.58 strained (#0.5%). Thickness was 5.5, 20 and
C. Rigo et al. / Journal of Crystal Growth 209 (2000) 471}475
15 nm for the compressive wells, the tensile wells and the barriers, respectively. Each gain sections had a length of 700 lm, and width of 4 lm, and it was buried in a semi-insulating InP : Fe layer grown by metal organic chemical vapor deposition (MOCVD). In the passive section, the waveguide region is of ridge type, with a ridge width of 4 lm. The central part of the passive section is a 3 dB X-coupler, with two arms (of length 800 lm) forming a 13 angle; the two arms are connected to parallel input/output waveguides through turning mirrors (Fig. 1), optimized according to Ref. [5]. Losses at the mirrors were measured to be less than 1 dB/mirror. The passive layout (X-coupler and micromirrors) was designed with a beam propagation method (BPM) model taking into account the process tolerances of the variables in the technological steps like tilts, surface roughness, layer thickness and waveguide geometry. In following runs we also tested a second-active QW structure designed to improve the polarization independence: in this case we used a 6 period stack of 14.5 nm thick tensile-strained In. Ga. As wells 48 52 (corresponding to a!0.3% tensile strain), separated by 7 nm thick lattice matched 1.24 lm InGaAsP barriers. The photoluminescence spectrum of this MQW structure presents a single emission peak at 1.552 lm wavelength with a good intensity comparable to the tensile/compressive structure. The TE/TM power output ratio of laser devices can be used as a guideline for the comparative evaluation of the residual polarization independence in SOA devices with di!erent active layers. In Fig. 2 the reduction of the TE/TM output power ratio is clearly shown moving from compressively to tensile-strained active layers. Preliminary results on this structure con"rm these "ndings and they will be described later.
3. System performance characterization The static characteristic of the device with alternate compressive/tensile wells is shown in Fig. 3. The bias current of the "rst SOA is kept at 180 mA while in the second was varied in the range from 150 to 270 mA. The beam of the local laser
473
Fig. 2. TE/TM power output ratio for lasers with the same active QW materials used for the SOA, as a function of bias current normalized to threshold current.
Fig. 3. Normalized output power from port B without optical signal (dashed line) and with signal (continuous line)at port C. I and I are bias currents at the SOA's of the chip. 1 2
(j "1533 nm) extracted from the port B was mea1 sured without (dashed curve) and with (continuous curve) the optical pumping beam (j "1559 nm) at 2 the port C. These curves represent the output signal of the device referred to the states `0a and `1a of the input signal. A pump o!/pump on extinction ratio higher than 10 dB is measured.
474
C. Rigo et al. / Journal of Crystal Growth 209 (2000) 471}475
Fig. 4. BER measurement at 2.5 Gb/s in back to back con"guration (solid line) and after conversion (dashed line).
Fig. 6. Eye diagrams at 5 (a) and 10 Gb/s (b).
Fig. 5. Eye diagram of the input (j ) and output signal (j ) 1 2 showing optical `0a level recovery.
Another important feature required in wavelength converters is the capability of regeneration of the input signal. The nonlinear transfer function of this interferometric device determines a compression of the noise band of the input signal. This e!ect generates an improvement of the converted output signal and consequently a negative penalty in the bit-error rate (BER) measured at 2.5 Gb/s as in Fig. 4. In this experiment we also observed (Fig. 5) a recovery of the optical `0a level of a purposely distorted input signal after the conversion. The conversion was also evaluated at higher bit rate, namely at 5 and 10 Gb/s with well-resolved eye diagrams as demonstrated in Fig. 6. The spectral bandwidth of these SOA is 85 nm centered at
Fig. 7. Static conversion measurement at j "1530 and 1 j "1570 nm of tensile well SOA, induced by pumping at 1 j "1550 nm. 2
1550 nm. This device shows a rather marked polarization dependence ranging from 4 to 8 dB with a predominant TE mode in the spectral band of erbium ampli"ers. For this reason these dynamical conversion measures were performed under optimized conditions with polarization controllers on the local signal. No polarization control was required on the input signal to convert. To eliminate this polarization depencence we studied the behavior of the solely tensile well structure. Up to now, static conversion tests (Fig. 7)
C. Rigo et al. / Journal of Crystal Growth 209 (2000) 471}475
show extinction ratio better than 15 dB over a wide wavelength range from 1530 to 1570 nm. As expected from data reported in Fig. 2 the polarization sensitivity was drastically reduced and even reversed: in the tested wavelength range from 1540 to 1560 the gain di!erence was between 0.4 and 1.8 dB with a predominant TM component. It seems therefore possible, by slightly reducing the amount of strain in the wells, to "nd the proper conditions to further improve the polarization dependence of the converter. 4. Conclusions An interferometric wavelength structure with compact con"guration is presented. This device operates at bit rate as high as 10 Gb/s showing a recovery of the input signal. The polarization dependence of a device with tensile wells active layer was reduced in the range 0.4}1.8 dB and further improvements are expected with a "ne tuning of the strain in the active region. Extinction ratios
475
as high as 18 dB are achieved over wide spectral operation bandwidth.
References [1] K. Stubkyaer, B. Mikkelsen, C. Joergensen, S.L. Danielsen, M. Vaa, R.J. Pedersen, H. Povlsen, P.B. Hansen, M. Shilling, K. Daub, K. Dutting, W. Idler, M. Klenk, E. Lach, G. Laube, K. Wunstel, P. Doussiere, A. Jourdan, F. Pommerau, G. Soulange, L. Goldstein, J.Y. Emery, N. Vodjdani, F. Ratovelomanana, A. Enard, G. Glastre, D. Rondi, R. Blondeau, in: G. Prati (Ed.), Photonics Networks, Springer, London, 1997, p. 103. [2] D. Campi, A. Stano, C. Coriasso, C. Cacciatore, C. Rigo, D. Re, M. Vallone, G. Fornuto, F. Ghiglieno, R. Defranceschi, D. Soldani, P. Valenti, L. Zucchelli, S. Lupo, P. Gambini, 11th LEOS Annual Meeting, Orlando, USA, Dec. 1}4 1998. [3] C. Rigo, R. Vincenzoni, A. Stano, R. Defranceschi, Cryst. Growth 164 (1996) 327. [4] M.A. Newkirk, B.I. Miller, U. Koren, M.G. Young, M. Chien, R.M. Jopson, C.A. Burrus, IEEE Photon. Technol. Lett. 5 (1993) 406. [5] L. Faustini, C. Coriasso, A. Stano, C. Cacciatore, D. Campi, IEEE Photon. Technol. Lett. 8 (1966) 1355.
Interferometric wavelength converter operating at 10 Gb/s based on a monolithic-integrated photonic circuit C. Rigo*, C. Coriasso, D. Campi, A. Stano, C. Cacciatore, D. Re, G. Fornuto, D. Soldani, R. De Franceschi, F. Ghiglieno, M. Vallone, P. Valenti, L. Zucchelli, S. Lupo, P. Gambini CSELT - Centro Studi e Laboratori Telecomunicazioni, Via G. Reiss Romoli 274, 10148 Torino, Italy
Abstract In this work we present a wavelength converter based on a Michelson interferometer. It is obtained by monolithic integration of two-semiconductor optical ampli"ers with a passive waveguided X-coupler, incorporating turning mirrors. It operates in the 1.55 lm spectral window and allows the wavelength conversion of data streams up to 10 Gb/s, showing open-eye diagrams and extinction}ratio regeneration capabilities. Comparison of two structures with di!erent active layers and their in#uence on the polarization sensitivity is also presented. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 42.79.Sz; 72.80.Ey; 78.30.Fs; 85.30.St Keywords: Chemical beam epitaxy; Interferometric wavelength converter; Semiconductor optical ampli"er
1. Introduction and working principle The wavelength conversion of a #ux of optical data at high bit-rate is one of the key functions required by future wavelength division multiplexing (WDM) systems for optical cross-connect and fast packet switching purposes [1]. An attracting solution to realise this functionality in a completely optical way, exploits cross-phase modulation (XPM) in an interferometric structure deriving from the monolithic integration of semiconductor optical ampli"ers and passive wave-
* Corresponding author. Tel.: #39-11-228-7025; fax: #3911-228-5085. E-mail address: [email protected] (C. Rigo)
guides [2]. In particular, the realised device is based on a Michelson interferometer con"guration, in which two-strained multi-quantum well (MQW) semiconductor optical ampli"ers (SOA) are integrated with the passive region consisting of an X-coupler and eight 903 micromirrors (Fig. 1). Taking advantage of the total internal re#ection at the semiconductor}air interface, these integrated mirrors reduce the overall dimensions of the device. Beside this, the advantages of this device are the wide operation bandwidth, the high signal-to-noise ratio and the regeneration capability of the signal. The passive ports (A, B) of the chip in Fig. 1 are coated with an antire#ection medium consisting of a double dielectric layer (TiZrO/SiO ), while the 2 two active ports (C, D) are as cleaved and behave as semire#ecting mirrors.
0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 6 0 0 - 4
472
C. Rigo et al. / Journal of Crystal Growth 209 (2000) 471}475
Fig. 1. Wavelength converter with close-up view of micromirrors regions.
A local CW laser (j ) is coupled to the port 1 A and routed to the X-coupler which is evenly split and directed to the active SOA region. The propagating beams are consequently ampli"ed and re#ected at the facets C and D and then re-routed to the X-coupler region where the phase interference occurs. Adjusting the injection current of the SOA's makes it possible to vary the phase of the travelling "elds, "nding the balancing condition in which the re#ected beam is completely addressed to port A and achieving extinction at port B. The working principle is based on the perturbation of the balancing condition induced by the signal to convert, that in operating conditions is an incoming #ux of data at wavelength j . 2 In fact, in the presence of the incoming data #ux (j ) every bit `1a relaxes the balancing conditions 2 and, through the XPM e!ect, addresses the re#ected signal (j ) at port B. Using spectral "ltering the 1 component j is then removed and therefore the 2 data sequence (originally encoded at j ) is transfer2 red onto the optical channel j . 1 2. Structure and geometry The structure for the device was grown in a VG80H CBE system whose description was
given in earlier references [3]. After a Si-doped InP bu!er layer, a 0.15 lm thick unstrained 1.24 lm wavelength In Ga As P layer was 0.76 0.24 0.52 0.48 deposited underneath the active layer to evascently couple the light into and out of the active sections: it served as a separate-con"nement layer within the active sections, and as a waveguide core in the passive section. A 50 nm thick undoped InP layer was then grown as stop-etch for the following technological steps. The active QW region was deposited using growth interruptions and evacuating the hydrides during the switches. A "nal Be-doped InP layer was used as a cladding layer. The growth temperature was 5203C for the quantum well region and the quaternary layer and 5003C for the InP layers. In the active layer of the SOAs we used a multiple quantum well (MQW) sequence, similar to the one documented in Ref. [4]. In order to reduce the polarization-dependence of the gain, we stacked up QWs of alternative compressive (#1%) and tensile (!0.8%) strain. The three compressive strained In Ga As QWs provide TE gain 0.675 0.325 through electron}heavy hole recombination, while three tensile-strained In Ga As QWs 0.417 0.583 provide predominantly TM gain through electron}light hole recombination. The barrier material (In Ga As P ) was also compressive 0.88 0.12 0.42 0.58 strained (#0.5%). Thickness was 5.5, 20 and
C. Rigo et al. / Journal of Crystal Growth 209 (2000) 471}475
15 nm for the compressive wells, the tensile wells and the barriers, respectively. Each gain sections had a length of 700 lm, and width of 4 lm, and it was buried in a semi-insulating InP : Fe layer grown by metal organic chemical vapor deposition (MOCVD). In the passive section, the waveguide region is of ridge type, with a ridge width of 4 lm. The central part of the passive section is a 3 dB X-coupler, with two arms (of length 800 lm) forming a 13 angle; the two arms are connected to parallel input/output waveguides through turning mirrors (Fig. 1), optimized according to Ref. [5]. Losses at the mirrors were measured to be less than 1 dB/mirror. The passive layout (X-coupler and micromirrors) was designed with a beam propagation method (BPM) model taking into account the process tolerances of the variables in the technological steps like tilts, surface roughness, layer thickness and waveguide geometry. In following runs we also tested a second-active QW structure designed to improve the polarization independence: in this case we used a 6 period stack of 14.5 nm thick tensile-strained In. Ga. As wells 48 52 (corresponding to a!0.3% tensile strain), separated by 7 nm thick lattice matched 1.24 lm InGaAsP barriers. The photoluminescence spectrum of this MQW structure presents a single emission peak at 1.552 lm wavelength with a good intensity comparable to the tensile/compressive structure. The TE/TM power output ratio of laser devices can be used as a guideline for the comparative evaluation of the residual polarization independence in SOA devices with di!erent active layers. In Fig. 2 the reduction of the TE/TM output power ratio is clearly shown moving from compressively to tensile-strained active layers. Preliminary results on this structure con"rm these "ndings and they will be described later.
3. System performance characterization The static characteristic of the device with alternate compressive/tensile wells is shown in Fig. 3. The bias current of the "rst SOA is kept at 180 mA while in the second was varied in the range from 150 to 270 mA. The beam of the local laser
473
Fig. 2. TE/TM power output ratio for lasers with the same active QW materials used for the SOA, as a function of bias current normalized to threshold current.
Fig. 3. Normalized output power from port B without optical signal (dashed line) and with signal (continuous line)at port C. I and I are bias currents at the SOA's of the chip. 1 2
(j "1533 nm) extracted from the port B was mea1 sured without (dashed curve) and with (continuous curve) the optical pumping beam (j "1559 nm) at 2 the port C. These curves represent the output signal of the device referred to the states `0a and `1a of the input signal. A pump o!/pump on extinction ratio higher than 10 dB is measured.
474
C. Rigo et al. / Journal of Crystal Growth 209 (2000) 471}475
Fig. 4. BER measurement at 2.5 Gb/s in back to back con"guration (solid line) and after conversion (dashed line).
Fig. 6. Eye diagrams at 5 (a) and 10 Gb/s (b).
Fig. 5. Eye diagram of the input (j ) and output signal (j ) 1 2 showing optical `0a level recovery.
Another important feature required in wavelength converters is the capability of regeneration of the input signal. The nonlinear transfer function of this interferometric device determines a compression of the noise band of the input signal. This e!ect generates an improvement of the converted output signal and consequently a negative penalty in the bit-error rate (BER) measured at 2.5 Gb/s as in Fig. 4. In this experiment we also observed (Fig. 5) a recovery of the optical `0a level of a purposely distorted input signal after the conversion. The conversion was also evaluated at higher bit rate, namely at 5 and 10 Gb/s with well-resolved eye diagrams as demonstrated in Fig. 6. The spectral bandwidth of these SOA is 85 nm centered at
Fig. 7. Static conversion measurement at j "1530 and 1 j "1570 nm of tensile well SOA, induced by pumping at 1 j "1550 nm. 2
1550 nm. This device shows a rather marked polarization dependence ranging from 4 to 8 dB with a predominant TE mode in the spectral band of erbium ampli"ers. For this reason these dynamical conversion measures were performed under optimized conditions with polarization controllers on the local signal. No polarization control was required on the input signal to convert. To eliminate this polarization depencence we studied the behavior of the solely tensile well structure. Up to now, static conversion tests (Fig. 7)
C. Rigo et al. / Journal of Crystal Growth 209 (2000) 471}475
show extinction ratio better than 15 dB over a wide wavelength range from 1530 to 1570 nm. As expected from data reported in Fig. 2 the polarization sensitivity was drastically reduced and even reversed: in the tested wavelength range from 1540 to 1560 the gain di!erence was between 0.4 and 1.8 dB with a predominant TM component. It seems therefore possible, by slightly reducing the amount of strain in the wells, to "nd the proper conditions to further improve the polarization dependence of the converter. 4. Conclusions An interferometric wavelength structure with compact con"guration is presented. This device operates at bit rate as high as 10 Gb/s showing a recovery of the input signal. The polarization dependence of a device with tensile wells active layer was reduced in the range 0.4}1.8 dB and further improvements are expected with a "ne tuning of the strain in the active region. Extinction ratios
475
as high as 18 dB are achieved over wide spectral operation bandwidth.
References [1] K. Stubkyaer, B. Mikkelsen, C. Joergensen, S.L. Danielsen, M. Vaa, R.J. Pedersen, H. Povlsen, P.B. Hansen, M. Shilling, K. Daub, K. Dutting, W. Idler, M. Klenk, E. Lach, G. Laube, K. Wunstel, P. Doussiere, A. Jourdan, F. Pommerau, G. Soulange, L. Goldstein, J.Y. Emery, N. Vodjdani, F. Ratovelomanana, A. Enard, G. Glastre, D. Rondi, R. Blondeau, in: G. Prati (Ed.), Photonics Networks, Springer, London, 1997, p. 103. [2] D. Campi, A. Stano, C. Coriasso, C. Cacciatore, C. Rigo, D. Re, M. Vallone, G. Fornuto, F. Ghiglieno, R. Defranceschi, D. Soldani, P. Valenti, L. Zucchelli, S. Lupo, P. Gambini, 11th LEOS Annual Meeting, Orlando, USA, Dec. 1}4 1998. [3] C. Rigo, R. Vincenzoni, A. Stano, R. Defranceschi, Cryst. Growth 164 (1996) 327. [4] M.A. Newkirk, B.I. Miller, U. Koren, M.G. Young, M. Chien, R.M. Jopson, C.A. Burrus, IEEE Photon. Technol. Lett. 5 (1993) 406. [5] L. Faustini, C. Coriasso, A. Stano, C. Cacciatore, D. Campi, IEEE Photon. Technol. Lett. 8 (1966) 1355.