Ultrafast Optical Switches and Wavelength Division Multiplexing

Ultrafast Optical Switches and Wavelength Division Multiplexing(WDM)Amplifiers Based on Bismuth Oxide GlassesNaoki Sugimoto *Research Center,Asahi Glass Co.,Ltd.,Yokohama 221-8755,JapanGlasses with a high refractive index exhibit interesting prop-erties.All optical switching and broadband amplification performances have been demonstrated using glasses based on bismuth oxide (Bi 2O 3).Optical Kerr shutter (OKS)switching and degenerated four-wave mixing experiments for nonresonant-type Bi 2O 3–B 2O 3–SiO 2glasses have been per-formed using femtosecond lasers.This glass exhibits an ultra-fast response (<150fs)in OKS operation.Moreover,terahertz-range (THz-range)optical switching has been suc-cessfully performed with this glass,using a 1.5-THz pulse train.Erbium-doped bismuth-based oxide glasses also have been prepared for wavelength division multiplexing (WDM)amplifiers.These glasses exhibit broadband emission and negligible concentration quenching,which indicates that the bismuth-based glass is suitable for broadband amplifiers and highly doped short-length fiber applications for metro use.I.IntroductionHEAVY -METALoxide glasses that contain Bi 2O 3,TeO 2,PbO,andGa 2O 3are well-known to exhibit a high refractive index.This feature is very important for advanced optical telecommunication and processing devices.Rapidly increasing internet and data traffic are stimulating the demand for greater information transmission capacity of backbone and “metro”optical networks.To meet the capacity demand,optical time division multiplexing (OTDM)and wavelength division multiplexing (WDM)have been proposed and used practically.In the OTDM system,the bit rate is planed to be 100Gbit/s to 1Tbit/s,which indicates that the time interval between pulses is 10–1ps.To process the high-speed optical signals,ultrafast (Ͻ1ps)all-optical switching is required.Third-order optical nonlinearities and their response time are the most-important properties for the realization of all-optical switching.Much research on glass composition has been performed to yield a large third-order optical susceptibility (␹(3))value,because glass that has a high refractive index exhibits a large ␹(3)value.1–7We have prepared new,nonresonant-type glasses that contain a high concentration of Bi 2O 3,and we have shown that these glasses exhibit large ␹(3)values and an ultrafast response.8,9On the other hand,optical Kerr shutter (OKS)switching with a response time of several picoseconds has been observed when a chalcogenide glass fiber was used.10However,more information on the response properties of optical switching in nonresonant-type glasses is needed for the realization of all-optical switching with response times of less than a picosecond.In addition,more information on the response properties and applied properties at telecommunica-tion wavelengths of 1.5␮m is needed.In this paper,we report on the femtosecond OKS operation and the first demonstration of terahertz-range (THz-range)switching using glass that contains a high concentration of Bi 2O 3.We also compare the ultrafast response property and the threshold to optical damage at a wavelength of 1.5␮m with a chalcogenide (As 2S 3)glass,using a degenerated four-wave mixing (DFWM)method.In the WDM system,an increasing channel count is required to increase the transmission capacity.To meet this capacity demand,the practical use of erbium-doped fiber amplifi-ers (EDFAs)has been the key technology for the massive development of WDM.Much effort has been exerted to broaden and flatten the EDFA bandwidth,to increase the capacity in the WDM system.Moreover,compact amplifiers such as erbium-doped planar waveguide amplifiers have been proposed to meet the low-spatial and low-cost demands for metro use.We have prepared novel erbium-doped Bi 2O 3-based glasses that exhibit broadband emission and negligible concentration quenching up to an erbium concentration of 26000ppm.11,12In this paper,we report on the broadband and short-length amplification properties of Bi 2O 3-based erbium-doped fiber (Bi-EDF).II.Ultrafast Switching(1)Experimental ProcedureThe glass,which had a composition of 63.3Bi 2O 3–32.6B 2O 3–4.1SiO 2–0.24CeO 2(based on molar ratio),was prepared using a melting method.13The concentration of Bi 2O 3in this glass corre-sponds to 92wt%.Reagent-grade Bi 2O 3,SiO 2,B 2O 3,and CeO 2were used as the raw materials.The raw materials were mixed together and melted in a platinum crucible in a SiC furnace in air at 1150°C for 1h.Then,the melt was poured onto a stainless-steel plate and annealed.The obtained glass plate was polished to a thickness of 0.7mm for OKS and DFWM measurements.OKS measurements were performed at a wavelength of 810nm in the noncolinear configuration,using 150-fs laser pulses that were derived from a regenerated amplified Ti:sapphire laser that was operating at a frequency of 200kHz.The experimental setup is shown in Fig.1.The output beam of the laser was split into two partial beams:one was used as a gate beam,and the other was used as a probe beam.The two beams were focused onto the same spot,using a lens with a focal length of 160mm,which led to a focal diameter of ϳ100␮m and a interaction length of ϳ1mm.The power intensity of the gate beam was maintained at a value that was a factor of Ͼ20greater than that of the probe beam.The sample was positioned between a polarizer and an analyzer in a cross-Nicol configuration.The gate power density was varied over a range of 3–80GW/cm 2.The transmitted beam (i.e.,the OKS signal)was detected by a photomultiplier.A time delay was inserted into the probe beam,with respect to the gate beam,to measure the response time of the OKS switching.CS 2in a quartz cuvette with a thickness of 1mm was used as a reference.DFWM measurements were performed at a wavelength of 1.5␮m in the boxcar configuration,using 150-fs laser pulses derived from an optical parametric oscillator pumped by regenerated amplified Ti:sapphire laser that was operating at a frequency of 1kHz.The experimental setup is shown in Fig.2.The pump power R.K.Brow—contributing editorManuscript No.187865.Received March 12,2001;approved March 25,2002.Presented at the Fall Meeting of the Glass and Optical Materials Division,Oct.1–4,2000,Corning,NY.*Member,American Ceramic Society.Glass and Optical MaterialsJ.Am.Ceram.Soc.,85[5]1083–88(2002)1083density was varied over a range of 30–200GW/cm 2.The dif-fracted beam (i.e.,the DFWM signal)was detected by a photo-diode.A time delay was inserted into the probe beam,with respect to the two pump beams,to measure the ␹(3)response time of the third-order optical nonlinearity.As 2S 3glass with a thickness of 1mm was used as a reference.(2)Results and DiscussionIn the OKS measurement,the probe transmittance T for the OKS can be written as a function of the angle ␪B between the gate and the probe beam;that is,T ϭsin 2͑2␪B ͒(1)The experimental value of T changed according to the polarization dependence between the gate and the probe beams,as given in Eq.(1).If the thermal effect were the cause of the switching operation,T would not exhibit such a dependence on ␪B .When the value of T is sufficiently small,the probe transmit-tance can be written asT ϰn 2͑OKS ͒2I g 2L2(2)where n 2(OKS)is the OKS nonlinear refractive index,I g the gate power intensity,and L the sample length.The experimental probe transmittance T increased in proportion to I g 2.These observed results of polarization dependence and the gate power dependence indicate that the switching operation using Bi 2O 3glass is driven by the optical Kerr effect that originates from third-order optical nonlinearity.Figure 3shows the decay property of the OKS signal in Bi 2O 3glass and CS 2with a gate power density of 40GW/cm 2.The CS 2reference material had a decay component of ϳ1ps,whose origin is molecular orientation relaxation.14On the other hand,the OKS signals in Bi 2O 3glass were detected only when the probe beam overlapped the gate beam.Hence,the response time of the OKS operation in Bi 2O 3glass was considered to be Ͻ150fs.We observed no damage in the sample,even at a gate power density of 85GW/cm 2,nor was any slow-decay component detected.This finding leads to the conclusions that the thermal effect or the slow process that originated from the two-photon absorption did not occur,even under such high gate power densities;in addition,one can conclude that this glass has a high damage threshold.Moreover,we have demonstrated OKS switching at a THz-range repetition rate for the first time,using a 1.5-THz pulse train as the gate beam.15The THz-range pulse train was synthesized with 150-fs laser pulses derived from an optical parametric oscillator that was pumped by a regenerated ampli-fied Ti:sapphire laser operating at a frequency of 1kHz and a wavelength of 633nm,via a pulse shaping technique that used gratings and a liquid-crystal phase modulator.16The interval of the gate pulses was ϳ670fs.The experimental setup for the OKS measurement was the same scheme as that shown in Fig.1,except for the laser system and the pulse synthesizer.The gate power density of the maximum pulse was estimated to be 20GW/cm 2,and the probe transmittance of the Bi 2O 3glasswasFig.1.Experimental setup for optical Kerr shutter (OKS)measurement using a femtosecond laser system.Inset describes the angle ␪B between the polarization vector of the probe and the gatebeams.Fig.2.Experimental setup for degenerated four-wave mixing measure-ment at a wavelength of 1.5␮m.Fig.3.Decay properties of optical Kerr shutter (OKS)signals in Bi 2O 3glass and the CS 2reference material.1084Journal of the American Ceramic Society—Sugimoto Vol.85,No.5ϳ1%.The OKS signals in the CS 2reference material over-lapped each other,because of the slow-decay component.Therefore,we could not separate the individual pulses,as shown in Fig.4.However,Fig.4also shows that the OKS signals in Bi 2O 3glass could be completely separated from each other,which indicates that high-speed switching in the THz range can be realized using Bi 2O 3glass.Figure 5shows the pump power dependence of the DFWM signal intensity in As 2S 3and Bi 2O 3glasses on the DFWM measurements at a wavelength of 1.5␮m.The DFWM signal intensity of As 2S 3and Bi 2O 3glasses varied in proportion to the cube of pump power density.These results indicate that the DFWM signals are generated by third-order optical nonlinearity in both glasses.The DFWM signal intensity of Bi 2O 3glass varied in proportion to the cube of the pump power density,from 30GW/cm 2to 200GW/cm 2;on the other hand,the DFWM signal intensity of As 2S 3glass was saturated when the pump power density was Ͼ80GW/cm 2.These results indicate that As 2S 3glass was optically damaged at pump power densities of Ͼ80GW/cm 2.Although the absorption edges of these glasses are almost the same,the optical damage threshold of Bi 2O 3glass is higher than that of As 2S 3glass at a wavelength of 1.5␮m.The glass-transition temperature of Bi 2O 3glass is 345°C,and that of As 2S 3glass is 200°C.17The thermal stability of Bi 2O 3glass is greater than that of As 2S 3glass;this is the one of the reasons why the damage threshold of Bi 2O 3glass is higher than that of As 2S 3glass.The decay properties of As 2S 3and Bi 2O 3glasses at various pump power densities are shown in Figs.6and 7.DFWM signals in As 2S 3glass were detected only when the probe beam over-lapped the pump beams at 30and 80GW/cm 2.Hence,the response time of the third-order optical nonlinearity in As 2S 3glass was considered to be Ͻ150fs.At pump power densities of 140and 190GW/cm 2,however,a slow-decay component was detected at the tail of the decay curves.As described previously,As 2S 3glass was optically damaged at pump power densities of Ͼ80GW/cm 2.Therefore,this slow-decay component is related to the origin of optical damage.We believe that multiphoton absorption caused a slow-decay process and thermal damage in glass under high pump power density.On the other hand,DFWM signals in Bi 2O 3glass were detected only when the probe beam overlapped pump beams at pump power densities of 30–190GW/cm 2.Even with a pump power density of 190GW/cm 2,we observed no damage in the samples,and no slow-decay component was detected either.This finding leads to the theory that no thermal effect or slow process that originates from the multiphoton absorption occurs,even under such a high pump power density,and that this glass has a high damage threshold at a wavelength of 1.5␮m.Fig.4.Transmitted probe signals in Bi 2O 3glass and CS 2when the probe beam has been delayed against the gate beam to measure the response time of the OKSswitching.Fig.5.Pump-power dependence of DFWM signal intensity at a wave-length of 1.5␮m in (a)Bi 2O 3glass (t ϭ0.7mm)and (b)As 2S 3glass (t ϭ1mm).May 2002Ultrafast Optical Switches and WDM Amplifiers Based on Bi 2O 3Glasses1085III.Erbium-Doped Bi 2O 3Glass(1)Experimental ProcedureErbium-doped Bi 2O 3-based glasses were prepared using the same melting method that was mentioned in the former section.We fabricated single-mode Bi-EDF with a plastic coating.The erbium concentration was 6500ppm in this fiber.The refractive index of the core glass was 2.03,and the numerical aperture (NA)was 0.2at 1550nm.The propagation loss of 2dB/m was estimated at 1300nm,using the cut-back method.The Bi-EDF was pumped using a 975-nm laser diode.A tunable laser diode (1525–1625nm)was used as the signal source.Pump and signal beams were coupled with a 980/1550fiber coupler.We used pump lasers with powers of 150and 100mW and applied bidirectional pumping at high power pumping.Type FC connectors with physical contact (FC/PCs)were used to connect the Bi-EDF to 6/125␮m SiO 2fibers.The insertion loss of the Bi-EDF using this connection was estimated to be 5dB at 1625nm.(2)Results and DiscussionsFigure 8shows the gain profile of a 22-cm-long Bi-EDFwithFig.6.Decay properties of the DFWM signal in As 2S 3glass at various pump power densities ((a)30,(b)80,(c)140,and (d)190GW/cm 2).Fig.7.Decay properties of the DFWM signal in Bi 2O 3glass at various pump power densities ((a)30,(b)80,(c)140,and (d)190GW/cm 2).Fig.8.Gain bandwidth of 22-cm-long Bi-EDF with different signal powers ((a)Ϫ32.7,(b)Ϫ2.7,and (c)3.3dBm).1086Journal of the American Ceramic Society —Sugimoto Vol.85,No.5different signal powers.The applied pump power was 239mW.We can obtain high gain (Ͼ20dB)in the C-band region with a low input signal power of Ϫ32.7dBm;we also can obtain a gain of Ͼ9dB,even in the L-band region,using only 22-cm-long fiber.The gain profile became flat at an input signal power of 3.3dBm,and we can obtain gain flatness within 2dB at wavelengths of 1530–1610nm without a gain equalizer.These results indicate that the Bi-EDF exhibits broadband amplifying performance with very short length fiber.This performance reflects the bulk property of Bi 2O 3glass,which shows broadband emission and negligible concentration quenching.Figure 9shows the net-gain property of the 22-cm-long Bi-EDF at a wavelength of 1560nm.The input signal power was Ϫ32.7dBm.A net gain of 18dB was achieved,using an input pump power of 130mW.The internal gain of this fiber was estimated to be 23dB,because the insertion loss of Bi-EDF in this experiment was 5dB.Figure 10shows the measured pump-to-signal power conversion at 1560nm,using an input signal power of Ϫ2.6dBm.The power conversion efficiency for pumping at a wavelength of 975nm was estimated to be 15%.Figure 11shows the gain saturation characteristics at wave-lengths of 1525,1540,1560,1580,1600,and 1620nm.The saturation output power (3dB)at 1560nm is 10dBm,and the maximum saturation output power of each wavelength is estimated to be ϳ13dBm.This result is consistent with the results shown in Fig.8.These results indicate a broadband and flat amplifier can be achieved under saturating conditions using only 22-cm-long Bi-EDF.Er ions can be doped up to a level of 26000ppm without quenching,which means that much-shorter Bi-EDF can be avail-able with sufficient gain performances.Moreover,we have found that the Bi-EDF can be spliced with SiO 2fiber,using a conven-tional fusion splicer.The water durability of Bi 2O 3-based glass was evaluated by measuring the weight loss in water at 90°C for 20h.The mechanical properties of Bi 2O 3-based glass were evaluated using the Vickers indentation method.We compared these properties with tellurite glass (Na 2O –ZnO –TeO 2)and fluoride glass (ZBLAN).The water durability of Bi 2O 3-based glass was a factor of 1000better than that of fluoride glass and a factor of 2better than that of tellurite glass.The Vickers hardness (H v )of Bi 2O 3-based glass was 430,which was higher than that of tellurite glass (H v ϭ260)and fluoride glass (H v ϭ240).The crack-initiation load that was evaluated using the indentation method for Bi 2O 3-based glass was a factor of 3greater than those for tellurite and fluoride glasses.These results indicate the Bi 2O 3-based glass has practical potential as a host material of optical amplifiers.IV.ConclusionsWe have used a melting method to prepare nonresonant-type Bi 2O 3–B 2O 3–SiO 2glass that contained a high concentration of Bi 2O 3,and we have performed ultrafast optical switching opera-tions.Optical Kerr shutter (OKS)switching has been operated with a response time of Ͻ150fs,and terahertz-range (THz-range)optical switching has been successfully performed using a 1.5-THz pulse train as the gate beam.This glass also has exhibited an ultrafast response and a high threshold for optical damage at the communication wavelength of 1.5␮m.These results indicate that high-speed switching beyond the THz range can be realized using Bi 2O 3glass.-gain property of 22-cm-long Bi-EDF at a wavelength of 1560nm;the input signal power was Ϫ32.7dBm.Fig.10.Relationship between pump power and output power at 1560nm,using 22-cm-long Bi-EDF with an input signal power of Ϫ2.6dBm.Fig.11.Gain-saturation characteristics of 22-cm-long Bi-EDF at wavelengths of 1525–1620nm,with a pumping power of 239mW.May 2002Ultrafast Optical Switches and WDM Amplifiers Based on Bi 2O 3Glasses1087We have also fabricated Bi2O3-based erbium-doped fiber(Bi-EDF)with an erbium concentration of6500ppm and evaluated the amplifying performance.A net gain of18dB was achieved at1560 nm using only22-cm-long Bi-EDF,and we also can obtain a gain ofϾ9dB in the CϩL band using this fiber.The maximum saturation output power at wavelengths of1530–1610nm was estimated to beϳ13dBm.We believe that Bi-EDF is suitable for the advanced broadband and compact amplifiers that are strongly required in metro wavelength division multiplexing(WDM)net-work systems.AcknowledgmentsThe study of ultrafast optical switching was the result of a collaboration with Hirao Active Glass Project,Science and Technology Corporation,Japan,and the study of the optical properties of Er ions in glasses was the result 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