Quadrature Amplitude Modulation (QAM) format Features of QAM format: Two carriers with the same frequency are amplitude-modulated independently. The phase of the two carriers is 90 deg. shifted each other. 2N QAM processes N bits in a single channel, so it has N times spectral efficiency compared with OOK. Quadrature-phase (Q) Quadrature-phase (Q) 0000 0100 1100 1000 0001 0101 1101 1001 1 In-phase (I) In-phase (I) 0011 0111 1111 1011 0010 0110 1110 1010 Constellation map for 16 (=24) QAM With OOK

Various modulation formats for microwaves and their spectral efficiencies [1] Large Small Amplitude change Fixed amplitude (-1.6 dB) ASK type PSK type MSK type FSK type Quadrature modulation type Shannon limit 1024 256 64 16 ・16 QAM Multi-level FSK Quadrature modulation Associated quadrature M-QAM C/W (bit/s/Hz) 4 ・64 QAM Coded Correlation PSK Duobinary FSK Satellite communication ・256 QAM Adoption of coding technique Mobile communication Coded modulation Fixed wireless communication Eb/N0 (dB) Increase in spectral efficiency Increase in power efficiency C: Channel capacity　(bit/s), W: Bandwidth (Hz) Eb/N0: Energy to noise power density ratio per bit Eb/N0 at BER = 10-4 is shown assuming synchronous detection Modulation schemes and their application fields Spectral efficiency of various modulation schemes [1] Y. Saito, “Modulation and demodulation in digital wireless communication,” IEICE (in Japanese)

Advantages of QAM optical transmission Microwave transmission Drawbacks of QAM wireless or metallic cable transmission: Obstacle Transmitted point Fading noise caused by obstacles Narrow bandwidth transmission Free space Received point Metallic cable Optical fiber transmission Advantages of QAM optical transmission: Regional IP backbone network Integrated global 10 Mb/s~1 Gb/s 10 Gb/s~40 Gb/s per wavelength User access 100 Gb/s~1 Tb/s No fading noise in optical fibers Broad bandwidth transmission

Configuration for QAM coherent transmission IF signal Optical fiber fIF=fs- fL fs Coherent light source Demodulator PD QAM modulator fL Local oscillator (LO) Optical phase- locked loop (OPLL) circuit Key components of QAM coherent transmission: - Coherent light source: C2H2 frequency-stabilized laser - QAM modulator: Single sideband (SSB) modulator - OPLL circuit: Tunable tracking laser as an LO - Demodulator: Digital demodulator using a software (DSP)

A C2H2 frequency-stabilized fiber laser[1] LN Frequency Modulator 1.5 GHz Reflection [dB] Wavelength [nm] 1.48 mm LD Feedback Circuit Low Pass Filter VPZT PZT WDM EDF Electrical Amp PM- FBG[2] Cavity Length ~ 4 m (FSR= 49.0 MHz) DBM Circulator MLP Electrical Amp 80/20 Coupler Phase Sensitive Detection Circuit PD Coupler 13C2H2 Cell LN Frequency Modulator 1.54 mm Optical Output (No Frequency Modulation) Single-frequency Fiber Ring Laser Laser Frequency Stabilization Unit Frequency stability: 2x10-11 Line width: 4 kHz [1] K. Kasai et al., IEICE ELEX., vol. 3, 487 (2006). [2] A. Suzuki et al., IEICE ELEX., vol. 3, 469 (2006).

QAM modulator[1] Optical input Optical output p/2 Electrical magnitude of optical signal I data RFA: F1(t)+DCA time MZA MZC Optical input Optical output MZB Electrical magnitude of optical signal p/2 Q data RFB: F2(t)+DCB DCC time MZ: Mach-Zehnder interferometer Configuration of QAM modulator I data Q data DCC [1] S. Shimotsu et al., IEEE Photon. Technol. Lett., vol. 13, 364 (2001).

OPLL circuit with a tunable fiber laser as an LO[1] Resolution: 10 Hz RF spectrum analyzer 500 Hz Less than 10 Hz IF signal: fIF=fs-fL Loop filter1 (Fast operation: 1 MHz) DBM fs PD fsyn Loop filter2 (Slow operation: 10 kHz) Synthesizer fL to PZT LO IF signal spectrum to LN phase modulator Tunable fiber laser - Linewidth: 4 kHz - Bandwidth of frequency response：1.5 GHz SSB phase noise [dBc/Hz] SSB phase noise spectrum Frequency offset 10 Hz 1 MHz -40 -60 -80 -100 -120 -140 Phase error: 0.3 deg. [1] K. Kasai et al., IEICE ELEX., vol. 4, 77 (2007).

Configuration of digital demodulator DSP (Software Processing) QAM Signal I(t) LPF S(t) = I(t)cos(wIFt+f0) -Q(t)sin(wIFt+f0) 2cos(wIFt+f) Clock signal Save to file A/D Decode 0, 1, 0, 0, • • • p/2 -2sin(wIFt+f) LPF Q(t) Bit Error Rate Measurement Accumulation of QAM Data Signal Digital Demodulation Circuit 　Our system operates in an off-line condition by using softwares.

Polarization-multiplexed 1 Gsymbol/s, 64 QAM (12 Gbit/s) coherent optical transmission system[1] Optical Frequency Pilot QAM data signal Intensity 2.5 GHz Arbitrary Waveform Generator I QAM( ) Delay Line ⊥ QAM Modulator Q Arbitrary Waveform Generator Q QAM(//) QAM Modulator Optical Filter (~ 5nm) C2H2 Frequency-Stabilized Fiber Laser PBS Att OFS PC Pilot DSF 75 km DSF 75 km EDFA (MUX) (fOFS =2.5 GHz) 2 GHz FBG IF Signal fIF=fsyn+fOFS=4 GHz Digital Signal Processor PD A/D Att PBS ( or ) (DEMUX) DBM EDFA: Erbium-doped Fiber Amplifier PC: Polarization Controller OFS: Optical Frequency Shifter PBS: Polarization Beam Splitter DSF: Dispersion-shifted Fiber FBG: Fiber Bragg Grating PD: Photo-detector DBM: Double Balanced Mixer PD (fsyn= 1.5 GHz) Synthesizer Feedback Circuit LO [1] M. Nakazawa, et al., OFC2007, PDP26 (2007).

Electrical spectrum of IF signal (//) (//) ( ) ( ) Demodulation bandwidth Demodulation bandwidth 2 GHz 2 GHz (a) Orthogonal polarization (b) Parallel polarization

Experimental result for polarization-multiplexed 1 Gsymbol/s, 64 QAM (12 Gbit/s) transmission over 150 km Constellation diagram Eye pattern (I) Eye pattern (Q) (a) Back-to-back (Received power: -29 dBm) (b) 150 km transmission for orthogonal data (Received power: -26 dBm) (c) 150 km transmission for parallel data (Received power: -26 dBm)

Improvement of spectral efficiency by using a Nyquist filter[1] Bandwidth reduction of data signal without intersymbol interference Transfer function Impulse response Data signal spectrum Bandwidth narrowing f [1] H. Nyquist, AIEEE Trans, 47 (1928).

Electrical spectrum of IF data signal ( ) (//) (//) ( ) ( ) ( ) Demodulation bandwidth Demodulation bandwidth 2 GHz 1.5 GHz (a) Without Nyquist filter (b) With Nyquist filter Roll off factor: 0.35

Experimental result for polarization-multiplexed 1 Gsymbol/s, 64 QAM (12 Gbit/s) transmission over 150 km[1] Q Q Q Constellation diagram Eye pattern (I) Eye pattern (Q) (a) Back-to-back (Received power: -29 dBm) (b) 150 km transmission for orthogonal data (Received power: -26 dBm) (c) 150 km transmission for parallel data (Received power: -26 dBm) [1] K. Kasai et al., OECC2007, PDP, PD1-1 (2007).

Bit error rate (BER) characteristics Orthogonal polarization (Back-to-back) Orthogonal polarization (150 km transmission) Parallel polarization (Back-to-back) Parallel polarization (150 km transmission) 3 dB

Conclusion Two emerging optical transmission technologies were described. (1) Ultrahigh-speed OTDM transmission 160 Gbit/s-1,000 km transmission was successfully achieved by combing DPSK and time-domain OFT. OFT has crucial potential especially for high bit rate, thus it is expected to play an important role for OTDM transmission at 320 Gbit/s and even faster. (2) Coherent QAM transmission We have successfully transmitted a polarization-multiplexed 1 Gsymbol/s, 64 QAM (12 Gbit/s) coherent optical signal over 150 km within an optical bandwidth of 1.5 GHz using a Nyquist filter. Thus, a spectral efficiency of 8 bit/s/Hz has been achieved in a single-channel.