Optical Modulation and

Demodulator

Broadray technology, 2011

Signal Modulation and Communication

� Similar to other telecommunication systems, signal modulation format is a key issue, which determines transmission quality and spectral efficiency.� The degradation effects can be categorized into random noise and waveform distortion.

� For multi-span optically amplified systems, the major contribution to the receiver noise

is the amplified spontaneous emission (ASE) generated by optical amplifiers.

� Signal waveform distortion can be generated by linear chromatic dispersion, fiber

nonlinearity (cross phase modulation, four-wave-mixing) and their combination.

� signal optical spectral bandwidth, tolerance to chromatic dispersion, resistance to

nonlinear crosstalk, susceptibility to accumulated noise, and other system

performance measures are directly related to the optical modulation format.

� Modulation Formats:

� OOK: NRZ-OOK, RZ-OOK,

� PSK, BPSK, DPSK and NRZ-DPSK.

� DQPSK

� PM-DQPSK

Modulation Formats

� OOK (On-Off Keying, one bit per symbol)

NRZ has been the modulation of choice for 10 Gbit/s and below.

Not self-clocking. Need scrambling the stream to increase number of transitions.

Improved clocking recovery. Need scrambling to avoid long stream zeros.

Simple detection can be used.

Modulation Format

� PSK, BPSK– Phase shift keying or Binary PSK

� Binary data are encoded by CHANGING preceding signal phase by π (“0”) or 0 (“1”)

� Signal intensity remains constant

� Better OSNR, use more bandwidth

Modulation Format

� DQPSK –Differential Quadrature Phase-Shift Keying

� Split the data stream in two, and use DPSK to modulate each stream, then

introduce π/2 delay to one of the streams before combining them, resulting in

four different phase (0, -π/2, π/2, and π).

� DQPSK transmits 2 bits for every symbol and narrower bandwidth. The

symbol info is encoded as a phase from one symbol to the next rather than

an absolute phase value.

� Tolerate more dispersion (PMD and CD). Allow stronger optical filtering and

closer channel spacing

� Suitable for 50GHz channel spacing 40Gb/s network

Modulation Format� PM-DQPSK –Polarization Multiplexed DQPSK (4 bit per symbol)

� Splits the bit stream in half, modulates each half rate using DQPSK,

polarizers each sub-signal using two orthogonal polarization planes, and

mixes them to form final modulated signal which is modulated in intensity

and phase and transmitted in two different polarization states.

� Lower tolerance to PMD.

� For 100Gbit/s network

DPSK Demodulator

� For Phase Shift Keying modulation format transmission,

demodulator is needed at receiving side.

� DPSK is widely used for long haul and untra LH transmission

and the required demodulator at receiving is relatively simple,

namely just need to split the signal into two and delay one bit

period in one optical path. The combined signal is intensity

modulated and can be detected directly.

DPSK Demodulator

� Design Principle

� Delay Interferometer produces two intensity outputs that are logically conjugated.

� The DI has a relative time delay difference Y between the two optical fields that is about equal to the symbol duration T . The output called “constructive port” and “destructive port”

� Transmission Function

Where ε is related DI extinction ratio,

φ the phase difference between two arm

FSR

DPSK Demodulator Parameters*

� FSR, Required by transmission modulation

� IL & difference: collimator selection, optical component tolerance, alignment

� Extinction Ratio: beam collimation, optical surface, polarization plate tolerance

� PDL: waveplate quality, optical surface cleanliness, alignment

� Return Loss: AR coating, component surface angle control

� Polarization Dependent Frequency Shift: most important parameter,

determined by optical component tolerance, alignment

� Optical delay between two ports: fiber length of two output

� PMD: component tolerance similar to PDFS

� Tuning Range/time constant: required by customers, selection of materials,

power used to control

* Consideration based on free-space, polarized optical design

DPSK Demodulator Specs

� FSR, Required by transmission modulation, 20-100GHz

� IL, ~2.0 dB

� IL Uniformity: 0.5 dB

� Extinction Ratio: >20 dB

� PDL: 0.2 dB

� Return Loss: >35dB

� Polarization Dependent Frequency Shift: <1% FSR

� Optical delay between two ports: 1ps

� PMD: 0.2 ps

� Tuning Range: >1.5 GHz

� time constant: 1s

* Consideration based on free-space, polarized optical design

DQPSK Demodulator

� Similar principle and structure as DPSK can be

used for DQPSK demodulator.

� Parameters and design considerations are also

similar.

� Input signal needs to be split into two and each of

them follows the optical path of DPSK demodulator.

� Main design considerations

� Athermal design

� Low PDFS

� IL and PDL