Baets, Roel

DAGENS, B

DECOBERT, J

LAGAE, L

LE GOUEZIGOU, O

MAKE, D

Van Parys, Wouter

Van Thourhout, Dries

English

Ghent University Academic Bibliography

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 24, DECEMBER 15, 2007 1949Amplifying Waveguide Optical Isolator With anIntegrated ElectromagnetWouter Van Parys, Student Member, IEEE, Dries Van Thourhout, Member, IEEE, Roel Baets, Fellow, IEEE,Beatrice Dagens, Jean Decobert, Odile Le Gouezigou, Dalila Make, and Liesbet LagaeAbstract—We have demonstrated an amplifying waveguideoptical isolator with an integrated electromagnet. This providesa solution to the generally poor magnetic remanence of this typeof isolator. The proof of principle is presented and optimizationroutes are discussed.Index Terms—Magnetooptic Kerr effect, optical isolators, semi-conductor optical amplifiers (SOAs).I. INTRODUCTIONAN optical isolator is indispensable in an optical telecomlink to protect semiconductor lasers from optical feedback.A planar, waveguide version of the optical isolator is highly de-sirable as it would greatly reduce the packaging cost—hencethe overall cost—of a laser diode module, because it avoids theexpensive alignment required with current commercial bulk iso-lators.An integratable isolator configuration that is getting a lot ofattention in recent years is the amplifying waveguide opticalisolator [1]–[4]. The device basically is a semiconductor op-tical amplifier (SOA) with a transversally magnetized ferromag-netic film close to the guiding core. The magnetooptic (MO)Kerr effect causes a nonreciprocal shift of the complex effec-tive index of the guided modes, implying that the modal loss isdependent on the propagation direction. Electrical biasing de-creases the overall loss level in the device. The result is a devicewhich, being transparent in one direction while providing loss inthe opposite direction, is isolating. The main advantage of thisisolator concept is that monolithic integration with a semicon-ductor laser is straightforward, as both components are essen-tially the same. Configurations operating in transverse-magnetic(TM) polarization [1], [3], [4] and transverse-electric (TE) po-larization [2] have been demonstrated.One of the main issues for this kind of device is the mag-netization of the ferromagnetic metal film. The very high as-pect ratio of the length of the film to the width results in verylow remanent magnetization of the film due to large demagne-tizing fields. One of the solutions to this problem is to have aManuscript received July 23, 2007; revised September 6, 2007. This researchhas been carried out in the framework of the IST-ISOLASER Project.W. Van Parys, D. Van Thourhout, and R. Baets are with the Photonics Re-search Group, Department of Information Technology (INTEC), Ghent Uni-versity-IMEC, B-9000 Gent, Belgium (e-mail: Wouter.VanParys@intec.UGent.be).B. Dagens, J. Decobert, O. Le Gouezigou, and D. Make are with AlcatelThales III-V Lab, 91767 Palaiseau, France.L. Lagae is with IMEC vzw, B-3001 Leuven, Belgium.Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LPT.2007.909634magnet integrated with the isolator. In this letter, we demon-strate a TM-mode amplifying waveguide isolator with an in-tegrated electromagnet. This magnet is a gold strip depositedalong the longitudinal direction of the isolator in close vicinityto the ferromagnetic film, which in this configuration both actsas the source of the nonreciprocity and as the electrical contactfor the SOA. Current flowing through the gold strip from oneside of the cavity to the other generates a TM field causing theMO Kerr effect.Apart from the application as an optical isolator, this result isinteresting in itself as it is now possible to modify the internalloss of an SOA dynamically and nonreciprocally.II. DESIGNThe design of the amplifying waveguide optical isolatorand the integrated electromagnet can be considered as twoseparate parts. The design of the isolator itself was extensivelydescribed in our previous work [4], hence we will limit our-selves here to a description of the component. The amplifyingcore is an AlGaInAs nm tensile-strained ( 1.6%strain) multiple quantum-well structure, where the built-intensile strain provides TM-selective material gain while sup-pressing TE-gain. Among the studied ferromagnetic metalsCo Fe was found to be the best compromise between a highMO effect and low absorption [5]. Low electrical resistivityof the metal–semiconductor contact combined with minoradditional absorption is provided by a hybrid Be p -dopedIn Ga As P –In Ga As bilayer [6]. Properdesign of the thickness of the top cladding layer and the twoseparate confinement heterostructure layers gives the opti-mized theoretical performance; 62 mA suffices to achievetransparency on a 2-mm-long isolator providing 10-dB opticalisolation [7], where the ridge width was set at 2 m.A schematic cross section of the amplifying waveguide op-tical isolator with integrated electromagnet is given in the toppart of Fig. 1. The design parameters for the electromagnet arethe width of the gold stripe and the distance between the elec-tromagnet and the ferromagnetic metal film. Other parametersare the thickness of the gold stripe, which influences the currentdensity in the electromagnet but not the strength of the magneticfield, and the variation of the generated magnetic field in the lat-eral direction ( -direction), which is of only minor importanceif the stripe width is much larger than the width of the ferro-magnetic film. The latter has to be the case for the isolator toensure that the magnetic field is aligned along the lateral direc-tion ( -direction).The partial differential equation describing the magneticfield problem was solved numerically. The bottom part ofFig. 1 shows the result of these calculations. The simulatedvalues of the magnetic field (in Oe) are plotted for variation of1041-1135/$25.00 © 2007 IEEE1950 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 24, DECEMBER 15, 2007Fig. 1. (top) Schematic cross section of the TM-mode amplifying waveguideisolator with integrated electromagnet. (bottom) Simulation of the lateral mag-netic field (x-direction) as a function of the width of the electromagnet goldstripe and the distance (y-direction) from the electromagnet, expressed in Oeper milliampere of current injection.the width of the electromagnet and the distance from the elec-tromagnet per unit (milliampere) current injection in the goldstripe. For sufficiently wide stripes the generated magnetic fieldis quasi-independent of the distance from the electromagnet.This is an advantage for the fabrication because the use of athick separation layer between bias contact and electromagnetreduces the risk of short-circuiting both metal layers.With the available lithography mask it was only possible tohave an electromagnet gold stripe as wide as 30 m, resultingin a magnetic field that is far from optimized. However, it issuitable for a proof-of-principle experiment [8].III. FABRICATIONThe isolator was grown with metal–organic vapor phase epi-taxy on an n S-doped InP substrate N cm .The layer structure is a Si n-doped N cmInP cladding layer, the nonintentionally doped (n.i.d.) op-timized active core, a Be p-doped InP cladding Ncm cm and the optimizedcontact structure. The sputter-deposited 50-nm-thick Co Fefilm, capped with a 40-nm/150-nm Ti–Au protective bilayer,was patterned into 3- m-wide stripes with standard lift-off.Ridge waveguides were defined with CH H plasma reactiveion etching using these metallic stripes as an etch mask. Withthis technique, full covering of the ridge with metal has beenachieved. An additional 30- m-wide gold stripe is evapo-rated to enable contacting of the isolator, separated from thesemiconductor surface by a benzocyclobutene (BCB) currentisolation layer. On top of the isolator bias contact a 30- m-wideand 215-nm-thick gold stripe is deposited, serving as the elec-tromagnet. Both gold stripes are separated by a 1- m-thickBCB layer. Isolators with a cavity length of 1.3 mm have beencleaved and mounted for characterization.In Fig. 2, two scanning electron microscope (SEM) imagesof a cross section of the amplifying waveguide optical isolatorwith integrated electromagnet are given. The left picture showsFig. 2. SEM pictures of the cross section of the amplifying waveguide opticalisolator with integrated electromagnet.Fig. 3. Optical power emitted by the isolator as a function of the externallyapplied magnetic field at a bias current of 140 mA (= above threshold).that the fabricated device is in agreement with the design. Whenzooming in on the ridge waveguide (right part of Fig. 2), it canbe seen that the definition of the ridge is not perfect, with a ridgewidth that is slightly wider than designed and roughness presentat the side of the ridge. While this has definitely its influence onthe isolator performance, these inaccuracies are not expected tochange the operation of the integrated electromagnet.IV. CHARACTERIZATIONAn easy and fast characterization method for the amplifyingwaveguide optical isolator is to pump as-cleaved isolators abovethreshold and extract the emitted power as a function of the ap-plied lateral magnetic field—switching the magnetic field di-rection is equivalent to switching the light propagation direc-tion. Because the material gain is clamped above threshold, thenonreciprocal loss shift is independent of the isolator bias cur-rent and the optical isolation can be extracted from the ratio offorward to backward intensity—or equivalently the ratio of theintensity at magnetization in either lateral direction(1)This method is used throughout this letter. The amplifyingwaveguide isolator was initially characterized with an externalelectromagnet as a reference for the integrated electromagnet.The laser threshold current equals 103 mA and the lasingwavelength is 1288 nm. In Fig. 3, the emitted power is plottedas a function of the externally applied magnetic field for abias current of 140 mA on a 1.3-mm-long isolator. The cor-responding maximal isolation, at saturation magnetization, is6.2 dB. This graph shows that the remanent isolation—at zeromagnetic field—is only 8.3% of the maximum value.Next, the integrated electromagnet was characterized by ap-plying current to the gold strip while pumping the isolator abovethreshold. To avoid heating, the electromagnet is driven withVAN PARYS et al.: AMPLIFYING WAVEGUIDE OPTICAL ISOLATOR WITH AN INTEGRATED ELECTROMAGNET 1951Fig. 4. (top) Time evolution of the injected (pulsed) current in the electro-magnet. (bottom) Corresponding (normalized) intensity emitted by the opticalisolator at a bias current of 140 mA (= above threshold).pulsed current (pulsewidth 0.3 s, duty cycle 3%). The chiptemperature is controlled with a thermoelectric cooler at a valueof 20 C. The isolator bias current is continuous wave. Theoutput signal is evaluated on a digital sampling oscilloscope.The measurement is done for both directions of the current flowin the electromagnet, corresponding to a magnetic field in eitherlateral direction. The bottom part of Fig. 4 shows the measuredtime evolution of the intensity. The corresponding pulsed cur-rent is plotted in the top part of Fig. 4, where negative currentcorresponds to reversed current flow direction. The intensityis normalized to compensate for variations in the fiber-to-chipcoupling. At the on-set of the current pulse the emitted powerrapidly increases or decreases, depending on the current flowdirection. This is a clear proof of the on-set of the nonrecip-rocal loss shift. After the off-set of the pulse, the emitted powerquickly becomes identical again in both cases and then slowlyrecovers from the heating of the device. The ratio of the inten-sity at “forward” to that at “backward” current injection equals1.64, which for an isolator of 1.3-mm cavity length correspondsto an isolation of 4.3 dB, or 70% of what is achieved at satura-tion magnetization.We repeated the measurements for different values of theelectromagnet current injection. In Fig. 5, the correspondingevolution of the optical isolation with injected (pulsed) currentis plotted (bottom axis). For comparison, the optical isolationas a function of the strength of an externally applied magneticfield is also given (top axis). Fitting both graphs shows that witha current injection of 500 mA, the achieved magnetic field ishalf the value needed to saturate the ferromagnetic film.The results in Fig. 5 shows that a very high current of 1.1 A isneeded to achieve magnetic saturation with the fabricated elec-tromagnet. However, as previously mentioned, due to limita-tions in the fabrication, the demonstrated electromagnet designis far from optimized. Decreasing the width of the gold stripto 5 m will decrease the required current by a factor of 5. Inorder not to increase the current density—hence the dissipatedFig. 5. Evolution of the optical isolation with current in the electromagnet(bottom axis) and with externally applied magnetic field (top axis).heat—the thickness of the gold film must accordingly be in-creased by a factor of 6. This has only a minor influence on thegenerated magnetic field. Furthermore, modification of the fer-romagnetic film might decrease the magnetic field—hence thecurrent level—required to saturate the film, a point which needsfurther investigation.V. CONCLUSIONWe have developed an optical isolator with an integrated elec-tromagnet. While the demonstrated drive current is beyond theacceptable level, possibilities for major improvement have beenidentified. Further research is however required, as the electro-magnet drive current should be lower than 50 mA to limit itscontribution to the overall power consumption of the amplifyingwaveguide optical isolator.REFERENCES[1] M. Vanwolleghem, W. Van Parys, D. Van Thourhout, R. Baets, F.Lelarge, O. Gauthier-Lafaye, B. Thedrez, R. Wirix-Speetjens, andL. Lagae, “Experimental demonstration of nonreciprocal amplifiedspontaneous emission in a CoFe clad semiconductor optical amplifierfor use as an integrated optical isolator,” Appl. Phys. Lett., vol. 85, pp.3980–3982, 2004.[2] H. Shimizu and Y. Nakano, “Fabrication and characterization of anInGaAsP/InP active waveguide optical isolator with 14.7 dB/mm TEmode nonreciprocal attenuation,” J. Lightw. Technol., vol. 24, no. 1,pp. 38–43, Jan. 2006.[3] V. Zayets and K. Ando, “Isolation effect in ferromagnetic-metal/semi-conductor hybrid optical waveguide,” Appl. Phys. Lett., vol. 86, pp.261105–261107, 2005.[4] W. Van Parys, B. Moeyersoon, D. Van Thourhout, R. Baets, M. Van-wolleghem, B. Dagens, J. Decobert, O. Le Gouezigou, D. Make, R.Vanheertum, and L. Lagae, “Transverse magnetic mode nonreciprocalpropagation in an amplifying AlGaInAs/InP optical waveguide iso-lator,” Appl. Phys. Lett., vol. 88, p. 071115, 2006.[5] M. Vanwolleghem, P. Gogol, P. Beauvillain, W. Van Parys, and R.Baets, “Design and optimization of a monolithically integratable InP-based optical waveguide isolator,” J. Opt. Soc. Amer. B, vol. 24, pp.94–105, 2007.[6] W. Van Parys, D. Van Thourhout, R. Baets, M. Vanwolleghem, F.Lelarge, B. Thedrez, and L. Lagae, “Study of a magnetooptic contactfor an amplifying waveguide optical isolator,” IEEE Photon. Technol.Lett., vol. 19, no. 9, pp. 659–661, Sep. 2007.[7] W. Van Parys, D. Van Thourhout, R. Baets, B. Dagens, J. Decobert,O. Le Gouezigou, D. Make, R. Vanheertum, and L. Lagae, “11.4 dBisolation on an amplifying AlGaInAs/InP optical waveguide isolator,”presented at the OFC 2006, U.S., 2006, Paper OFA2.[8] W. Van Parys, D. Van Thourhout, R. Baets, B. Dagens, J. Decobert, O.Le Gouezigou, D. Make, and L. Lagae, “Amplifying waveguide opticalisolator with integrated electromagnet,” presented at the ECIO 2007,Denmark, 2007, Paper ThA5.

Amplifying waveguide optical isolator with an integrated electromagnet

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Amplifying waveguide optical isolator with an integrated electromagnet

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