1. 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

2. 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)

3. 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

4. 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)

5. 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).

6. 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).

7. 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).

8. 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.

9. 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).

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

11. 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)

12. 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).

13. 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

14. 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).

15. 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

16. 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.