53 KAWANISHI Tetsuya and SAKAMOTO Takahide 1 Introduction Today we are witnessing the spread of high-speed Internet access services such as ADSL and FTTH, with a variety of services provided via continuous connection even in homes. Some of these services require large transmission capacity, as in the expanding range of audio and video network applica- tions. In terms of audio applications, manufac- turers have shifted emphasis in their main portable audio equipment products toward devices based on MP3 technology and have already begun a number of network music dis- tribution services. On the other hand, while audio services on the Internet offer quality equivalent to that of audio equipment, video distribution urgently requires further advances in network performance. Specifically, innova- tive optical devices must be developed to enable digital high-definition image distribu- tion. Optical communication technology is already used in many fields, from international communications to home FTTH. With these services, optical signals are mainly used to implement point-to-point transmission. When complicated processing is required, optical signals are generally converted into electrical signals for processing in electrical circuits. Processing in the electric circuit forms a bot- tleneck in this case, and various studies are underway to address this problem. As is wide- ly known, light has a dual wave/particle nature. Most existing optical communications systems do not take advantage of this nature and use only two states of light: on or off. Here we will discuss the fundamentals of an ultra-high-density optical technology that takes full advantage of the wave properties of light, increases transmission capacity, and pro- vides for a number of new functions required for optical signal processing. We also describe a unique device developed by NICT enabling free use of all three elements of a wave: inten- sity, phase, and frequency (or wavelength). We also describe an ultra-high-speed transmis- sion system and a packet system based on this device. 3-6 Advanced Optical Modulators for Next-generation Photonic Networks KAWANISHI Tetsuya and SAKAMOTO Takahide This article describes recent research activities using an NICT novel optical device, the opti- cal frequency-shift-keying (FSK) modulator, which can provide high-speed control of optical fre- quency, phase and amplitude. The FSK modulator can generate various types of high-speed optical signals, precisely. We show a couple of examples of recent results on applications of the FSK modulator for next-generation optical communications systems, such as, optical FSK label processing, tunable optical buffer techniques, high-speed differential quadrature-phase-shift- keying (DQPSK) signal generation for 100 GbE and continuous-phase FSK signal generation for dense wavelength-domain-multiplexing. Keywords Optical modulator, Optical buffer, Optical label, Phase, Frequency
Transcript
53KAWANISHI Tetsuya and SAKAMOTO Takahide
1 Introduction
Today we are witnessing the spread ofhigh-speed Internet access services such asADSL and FTTH, with a variety of servicesprovided via continuous connection even inhomes. Some of these services require largetransmission capacity, as in the expandingrange of audio and video network applica-tions. In terms of audio applications, manufac-turers have shifted emphasis in their mainportable audio equipment products towarddevices based on MP3 technology and havealready begun a number of network music dis-tribution services. On the other hand, whileaudio services on the Internet offer qualityequivalent to that of audio equipment, videodistribution urgently requires further advancesin network performance. Specifically, innova-tive optical devices must be developed toenable digital high-definition image distribu-tion. Optical communication technology isalready used in many fields, from internationalcommunications to home FTTH. With theseservices, optical signals are mainly used to
implement point-to-point transmission. Whencomplicated processing is required, opticalsignals are generally converted into electricalsignals for processing in electrical circuits.Processing in the electric circuit forms a bot-tleneck in this case, and various studies areunderway to address this problem. As is wide-ly known, light has a dual wave/particlenature. Most existing optical communicationssystems do not take advantage of this natureand use only two states of light: on or off.Here we will discuss the fundamentals of anultra-high-density optical technology thattakes full advantage of the wave properties oflight, increases transmission capacity, and pro-vides for a number of new functions requiredfor optical signal processing. We also describea unique device developed by NICT enablingfree use of all three elements of a wave: inten-sity, phase, and frequency (or wavelength).We also describe an ultra-high-speed transmis-sion system and a packet system based on thisdevice.
3-6 Advanced Optical Modulators for Next-generation Photonic Networks
KAWANISHI Tetsuya and SAKAMOTO Takahide
This article describes recent research activities using an NICT novel optical device, the opti-cal frequency-shift-keying (FSK) modulator, which can provide high-speed control of optical fre-quency, phase and amplitude. The FSK modulator can generate various types of high-speedoptical signals, precisely. We show a couple of examples of recent results on applications of theFSK modulator for next-generation optical communications systems, such as, optical FSK labelprocessing, tunable optical buffer techniques, high-speed differential quadrature-phase-shift-keying (DQPSK) signal generation for 100 GbE and continuous-phase FSK signal generation fordense wavelength-domain-multiplexing.
KeywordsOptical modulator, Optical buffer, Optical label, Phase, Frequency
54 Journal of the National Institute of Information and Communications Technology Vol.53 No.2 2006
2 Fundamental technology forultra-high-density optical com-munications
In the field of wireless technologies, sys-tems that take full advantage of the wavenature of radio communications are in widepractical use. On the other hand, in opticalcommunications, very few practical systemsmake use of the wave nature of light. Amongthe three elements of a wave—intensity,phase, and frequency (or wavelength)—mostpractical systems use a change in intensity forthe transmission of information. In addition toa change in intensity, some cutting-edgeresearch has incorporated the use of phasechange, with the aim of improving system per-formance in long-haul high-capacity commu-nications. However, a change in frequency(i.e., a change in wavelength) is only rarelyused, as conventional devices have beenunable to handle optical frequency control athigh speeds. In March 2004, NICT successful-ly developed an optical FSK modulator thatfunctions as an ultra-high-speed frequencycontrol device. NICT has already completedtransfer of this technology to a manufacturer,and the device is now commercially availableas a modulation device. To date we have per-formed a variety of experiments using thisdevice. One such experiment involved high-speed optical frequency shift keying (FSK)transmission, while another examined simulta-neous transmission of a light intensity modu-lation (IM) signal and an optical FSK signal,intended for use in optical packet systems.The development of this optical FSK modula-tor has enabled control of the intensity, phase,and frequency of a light wave at high speedand with high precision. As a result we mayexpect implementation of a range of compli-cated functions comparable to the signal pro-cessing seen in wireless technologies. (SeeFig. 1.)
3 Optical FSK modulation tech-nology
The optical FSK modulator integrates fouroptical phase modulators, as shown in Fig. 2.The device has three electrodes. When high-frequency electric signals are sent to two ofthe electrodes (RFa and RFb) with the phaseof one signal shifted 90 degrees from that ofthe other, the frequency of the output light isshifted to the same extent as the shift in thesignal frequency (this shift is referred to as the“modulation frequency”). The direction of theshift (designated as USB for the componentwith higher frequency, or designated as LSBfor the component with lower frequency) canbe controlled by the voltage applied to theremaining electrode (RFc). To switch thedirection of the shift in light frequency at highspeed, we have adopted a traveling-wavestructure for the electrode［1］. (This structureenables high-speed operation by propagating
Fig.1 Integrated control of three ele-ments of light wave (intensity,phase, and frequency)
the light and electric signals in the same direc-tion at the same rate.)
Figure 3 shows the operating principle.The horizontal axis is the frequency, the direc-tion of the arrow indicates the phase, and thedotted line indicates the frequency of the inputlight. At Points P and Q, both the USB andLSB components are generated. For the outputlight (Point R), the LSB components fromPoints P and Q have a phase difference of180 degrees (i.e., of opposite signs) and canceleach other out. Consequently, only the USBcomponents are output. The relationshipbetween the phases of the LSB and USB com-ponents can be controlled by the voltageapplied to the electrode RFc. Figure 4 shows acase in which the USB components cancelledout and the LSB components are output. Thespeed of this switching depends on theresponse speed of the electrode RFc. Theupper limit of the frequency change dependson the operable frequency of the electrodesRFa and RFb. All three electrodes (RFa, RFb,and RFc) of the optical FSK modulator devel-oped and evaluated by NICT feature the fol-
lowing characteristics: operable frequency (3-dB band) of approximately 18 GHz andswitching speed of approximately 55 picosec-onds.
The output light includes a slight amountof unnecessary components due to the genera-tion of harmonics from the phase modulation.Nevertheless, these effects may be further sup-pressed by simultaneously supplying the thirdharmonics. In this manner, we achieved a con-version efficiency of －12.9 dB and a suppres-sion ratio of 33.7 dB for unnecessary compo-nents with a frequency change of 7.5 GHz［2］.Figure 5 shows the configuration of the opticalFSK transmission experimental system. Withthis system, we were able to achieve error-freetransmission of 10-Gbps FSK signals over95 km of a single mode fiber (SMF) withoutdispersion compensation. The frequencychange in this case was 12.5 GHz［3］. Withbalanced reception using both the USB andLSB signals, we were able to improve thereceiver sensitivity even further, succeeding intransmission over 130 km［4］.
Fig.3 Operating principle of optical FSKmodulator (USB: generation ofcomponents with upward frequen-cy shift)
Fig.4 Operating principle of optical FSKmodulator (LSB: generation of com-ponents with downward frequencyshift)
Fig.5 10-Gbps FSK transmission experiment
56 Journal of the National Institute of Information and Communications Technology Vol.53 No.2 2006
The FSK/IM signal (an IM signal for thepayload and an FSK signal for the label sig-nal) can be generated by an optical FSK mod-ulator and an optical intensity modulator con-nected in series. Figure 6 shows the experi-mental system and the results of measurement(the waveforms of the demodulated IM andFSK signals and the signal waveform afterremoving the label). The IM signal is trans-mitted at 10 Gbps and the FSK signal is trans-mitted at 1 Gbps. We can demodulate the IMsignal by directly inputting the FSK/IM signalinto the optical detector. On the other hand,we can demodulate the FSK signal with theoptical detector by passing the FSK signalthrough an optical filter that can separate theUSB and LSB signals and processing this sig-nal by FM-IM conversion. We have confirmederror-free demodulation of both FSK and IMsignals［5］.
When we treat the FSK/IM signal with car-rier-suppressed double-sideband modulation(amplitude modulation with the input compo-nents suppressed by interference) and use anoptical filter to extract the same frequencycomponent as that comprising the input light,we can obtain a signal equivalent to a pure IMsignal, independent of the state of the FSK sig-nal. As shown in Fig. 6, we obtained a good
waveform for the demodulated IM signal with-out the FSK signal component and confirmederror-free demodulation. We can superpose anew FSK signal onto the demodulated IM sig-nal when we input the latter into another FSKmodulator. This method can change the labelinformation (FSK signal) without convertingthe optical signal into an electrical signal(referred to as a “label swap”)［5］.
5 High-density transmission tech-nology
When the frequency shift is larger than thebit rate of the FSK signal (i.e., with widebandFSK), the two components have little overlapon the frequency axis, their spectral forms areindependent of the phase relationship betweenthe components, and the effect of the phasechange on the demodulation characteristics isassumed to be small. To improve the efficien-cy of frequency use, FSK with small frequen-cy changes (i.e., narrowband FSK) is effec-tive. However, as the USB and LSB compo-nents overlap in such a case, the spectral formand the demodulation characteristics signifi-cantly depend on the phase relationshipbetween these components. An FSK signalwith phase continuity secured during frequen-cy switching (with CPFSK, or ContinuousPhase FSK) has a compact spectral form andsuperior demodulation characteristics. TheFSK modulator does not generally provide
Fig.6 Optical label transmission based on optical FSK modulation and label swapping
57KAWANISHI Tetsuya and SAKAMOTO Takahide
optical phase continuity during frequencyswitching, and this switching is usuallyaccompanied by rapid phase change. Howev-er, a CPFSK signal can be generated by syn-chronizing the signals used to generate theUSB and LSB components (RFa and RFb) andthe base band signal for frequency switching(RFc)［6］. The amount of the frequency shift isadjusted to half of the bit rate, B; in otherwords, the frequency interval between USBand LSB is adjusted to equal B, and switchingis performed when the phases of the USB andLSB are in agreement. In CPFSK, the phasechanges continuously by 180 degrees per bit.Figure 7 shows the results of the 10-GbpsCPFSK modulation experiment. We have con-firmed that our method provides approximate-ly the same sensitivity as a phase modulationmethod studied widely by other organizations(specifically, DPSK, or Differential PhaseShift Keying). CPFSK is also characterized bygreater suppression of higher frequency com-ponents relative to DPSK, so that we canexpect CPFSK to reduce interference betweenadjacent channels in high-density transmis-sion. It is important to change phase continu-ously during frequency switching in CPFSKmodulation. In contrast, it is possible to use arapid phase change during switching to gener-
ate optical UWB (ultra-wideband) signals［7］.Setting as our goals a further increase in
speed and more complicated functions, we havesucceeded in developing a multi-functionalmodulator that supports high-speed signals—at40 Gbps or higher—and have demonstrated 40-Gbps optical FSK modulation using this modu-lator［8］. This device can perform accurate fre-quency modulation, amplitude/intensity modu-lation, and multi-level phase modulation; more-over, the design incorporates 100-GbpsDQPSK (Differential Quadrature Phase ShiftKeying) to ensure compliance with the100 GbE next-generation Ethernet standard［9］.The modulator offers the highest transmissionspeed per channel available anywhere, and isalso the fastest modulator using a signal formatwith highly efficient frequency use. As ofMarch 2006, this multi-functional modulator isby far the world’s fastest device capable of con-trolling optical phase and frequency. It alsooffers superb accuracy, with higher quality out-put light than that provided by the conventionalcombination of two or mode modulators, asdemonstrated in the transmission experimentdescribed below based on 80-Gbps DQPSKmodulation (Fig. 8 (a)).
Fig.7 High-density transmission using opti-cal CPFSK modulation (a) opticalspectrum, (b) demodulated signalwaveform, and (c) bit error rate
Fig.8 High-density 80-Gbps transmissionbased on DQPSK modulation (a)optical spectrum, (b) bit error rate,and (c) demodulated signal wave-form
58 Journal of the National Institute of Information and Communications Technology Vol.53 No.2 2006
6 Variable delay technology
Next, we discuss variable optical delaybased on optical frequency control using mod-ulators［10］. To avoid collisions in opticalpacket exchange, variable optical delay tech-nology is used to construct an optical packetbuffer. A variety of methods have been pro-posed, including switching fibers and the useof many light sources. However, these meth-ods have a range of problems, including gen-erally complicated structures. On the otherhand, we can implement variable delay with asimple structure using an optical SSB modula-tor (optical frequency shifter). The amount ofthis delay can be electrically controlled. Asshown in Fig. 9, this structure involves anoptical input/output unit consisting of an opti-cal fiber loop equipped with an optical SSBmodulator, and an FBG (Fiber Bragg Grating)sandwiched between two circulators. Lightwithin the reflection band of the FBG circu-lates in the optical loop. When this light isinput from the optical input port, it is reflectedby the FBG and output without entering theloop. Light outside the reflection band propa-gates from the optical input port to the opticalloop and from the optical loop to the opticaloutput port. Thus, while the light within thereflection band is output without entering theoptical loop, the light outside the reflectionband is output through the optical loop with atime delay corresponding to a single lap of theloop. Operating the optical SSB modulator
shifts the optical frequency in the loop, so thatthe input light outside the reflection band maybe converted into light within the reflectionband. Figure 10 shows the spectrum of lightcirculating in the optical loop. The input light,at a frequency slightly outside the reflectionband, is processed with the SSB modulator toarrive at a frequency within the reflectionband and circles round the loop. The frequen-cy continues to change while the light is cir-cling through the loop, so that the frequencyexceeds the reflection band after a set numberof laps; the light then exits the loop and isextracted from the output port. Denoting thereflection bandwidth as fr and the frequencyshift by the optical SSB modulator as fm, thelight circles around the loop n times when therelationship n fm > fr > (n－1) fm holds. Thus,the number of laps can be controlled bychanging the value of fm. We processed theinput light using pulse intensity modulationand measured the change in delay in the loopfrom the time waveform of the output light.Figure 11 confirms that the amount of delaycan be controlled by the RF signal frequencyfm.
7 Conclusions
Here we have discussed the details andapplications of a high-speed optical modula-tion technology that can form an importantcomponent technology for next-generationphotonic network. We can now apply
Fig.9 Configuration of variable delay Fig.10 Principle of variable delay
59KAWANISHI Tetsuya and SAKAMOTO Takahide
advanced control over all elements of thewave/particle nature of light (amplitude,phase, and frequency). To date, the essentialrole of an optical modulator has been to con-vert the information expressed in electricalsignals into light. In the future, optical modu-lators are expected to find use in diversefields, from signal processing to control. Inorder to make these applications real, it will beimportant to promote research and develop-ment of modulators featuring new structures,optimized to suit the purposes of each applica-tion.
Fig.11 Delay controlled optical signal
References01 T. Kawanishi, K. Higuma, T. Fujita, J. Ichikawa, T. Sakamoto, S. Shinada, and M. Izutsu, “LiNbO3 high-
Senior Researcher, Advanced Commu-nications Technology Group, New Gen-eration Network Research Center (for-mer: Senior Researcher, PhotonicInformation Technology Group, Basicand Advanced Research Department)
High-speed Lightwave Modulation
SAKAMOTO Takahide, Ph.D.
Researcher, Advanced Communica-tions Technology Group, New Genera-tion Network Research Center (former:Expert Researcher, Photonic Informa-tion Technology Group, Basic andAdvanced Research Department)
