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Tutorial Note #5
Modulation Schemes

For high-speed communication (10 GBit/sand beyond) it becomes extremely difficult tomodulate the laser directly, therefore external
optical modulators are used. The electroabsorption modulator is such a modulator. It
can be understood as a reverse-biased PIN
detector.
Although it improves the chirp performance
considerably compared to direct modulationof the laser, there is still enough chirp tomake long haul high speed transmission
impossible. More importantly, the modulator
chirp is dynamic and changes with the actual
drive voltage.

Another possibility to modulate light is a
Mach Zehnder structure in a material showing
strong electro-optic effect (such as LiNbO3 orthe 3-5 semiconductors such as GaAs andInP). By applying a voltage the optical signal
in each path is phase modulated as the optical
path length is altered by the electric field.
Combining the two paths with different phasemodulation converts the phase modulation
into intensity modulation.
If the phase modulation is exactly equal ineach path but different in sign, the modulator
is chirp free, this means the output is only
intensity modulated without incidental phase(or frequency-) modulation.

Here you see the schematic of a Mach Zehnder modulator. MZ modulators areeither dual drive or single drive. The dualdrive configuration uses data and inverted
data to generate the intensity modulation. Ifthe electrical field in both MZ branches isidentical the dual drive configuration will bechirp free. The single drive configuration canbe either Z-cut or X-cut, since X-cut is
intrinsically balanced - this means
symmetrical - it is chirp free, whereas the zcut will exhibit chirp.

2One way of implementing a Mach Zehnder
modulator is a dual electrode structure or apush pull modulator: If we apply data andinverted data, the optical output will be chirp
free, if we change the amplitudes of thedriving signals the chirp can be adjusted.
To obtain a chirp free signal the two drivers
have to be very carefully matched (Option
matched pairs for SHF amplifiers).
The benefit of a push pull modulator
compared to a single electrode modulator isthat you need lower driving voltages in eacharm. The drawback is that you need two
amplifiers that are carefully matched and that
it is difficult to quantify the chirp of such aconfiguration.

As the electro-optic transfer characteristic ofa Mach Zehnder modulator is cos2-shaped,
the modulator will be biased in the linear
range (between the maximum and minimum)
and the modulation signal will besuperimposed onto the bias voltage. If
electrical NRZ data with an amplitude of Vπ
are fed into the modulator, the optical output
signal will be NRZ as well. In thisapplication the non-linear characteristic of
the modulator will improve the signal quality
as overshoot and patterning will be clipped.

For some applications (mainly analog linkapplications)
thenon-linear
transfercharacteristic of the modulator is a
disadvantage, if the modulating signal has anamplitude much less than Vπ the modulator
will operate in its linear range.
Keep in mind that by improving the linearity
we have sacrificed the extinction ratio.

3One application where the modulator isoperated within its linear range is afrequency response measurement.
To avoid measurement errors due toharmonic signal content, the driving signal ischosen to be small enough not to generateharmonics. The lower extinction ratio doesn’t
harm as the electrical receiver is narrow bandand therefore has a much higher sensitivity
than a broadband data receiver.

When a balanced modulator structure (e.g.
X-cut or dual-drive Z-cut modulator) isdriven by an electrical signal with 2 . Vπ, themodulator shows interesting behavior:- As the input signal passes through apositive and a negative slope of thetransfer characteristic, the output
signal will be doubled in frequency.
- When transiting the minimum of thetransfer characteristic, the phase of
the optical signal will be altered byπ.

To summarize:
EA modulators have more chirp than Mach
Zehnder modulators and are therefore mainly
used in short reach applications.
Z-cut modulators can have lower, and wellbehaved chirp than EA modulators but stillsome chirp, whereas X-cut modulators havezero chirp. Z-cut modulators offer lower Vπ
but show higher drift than X-cut modulators.

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A lot of research is currently done in the areaof novel modulation schemes. Just as radio
evolved from basic on/off keying throughamplitude modulation to phase- andfrequency
modulation,
opticalcommunication is progressing today – at amuch faster speed though.
The key driving factor is cost. To reduce cost
the following alternatives are investigated:- Higher spectral efficiency- Higher robustness and tolerances
against fiber non-linearities

To understand the new modulation schemes
let’s review the plain on/off keying called
NRZ transmission in optical communication:
When biased in the linear region and driven
with Vπ the LiNbO3 modulator acts as anintensity (amplitude) modulator.
The non-linear characteristic of themodulator clips the signal; it is therefore
sometimes possible to have a better Q factor
for the output (optical) signal than for theinput (electrical) signal.

For generating 40 GHz RZ pulses a LiNbO3
modulator is also very helpful:
When driven with a 20 GHz sine wave with
an amplitude of 2.Vπ and biased at themaximum, we will get optical pulses with awidth of 9 to 10 ps. Provided that themodulator is chirp free, these pulses are
perfectly suited for 40 Gb/s RZ transmission.

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Of course the 40 GHz pulses can also begenerated by a 40 GHz sine wave with anamplitude of Vπ and a modulator that isbiased in its linear region. In this case themodulator does not have to be chirp free.

A third possibility for generating 40 GHz RZ
pulses is to bias the modulator at its
minimum and to drive it with a 20 GHz sine
wave having an amplitude of 2.Vπ.
The RZ pulses will be wider, around 15 ps
and the modulator has to be chirp free.
With carrier suppressed RZ adjacent pulses
will have a difference of 180° or in their
optical phase.

When a stream of RZ pulses has been
generated, the data have to be encoded on thestream of equally spaced RZ pulses. A
second modulator, the “gating modulator“,commonly does this.
The gating modulator is driven with the NRZ
data and the output will be an optical RZ
signal.

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When the modulator is biased in its
maximum and driven by a 20 GHz sine wave
with 2.Vπ or if it is biased in its linear region
and driven by a 40 GHz sine wave with Vπ
we get conventional RZ.
Here you see the spectrum of a PRBS
sequence modulated in conventional RZ
format.

When the modulator is biased in its
minimum and driven by a 20 GHz sine wave
with two Vπ we have carrier suppressed RZ.
(CS-RZ)

Here we show a 40 Gb/s PRBS sequencemodulated in CS RZ format. Note that thespectrum shows that the optical carrier issuppressed and that the two modulation
sidebands are closer than they are forconventional RZ.

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As RZ has some benefits compared to NRZ
the higher technical effort in generating RZ
signals is justified especially in long-range
transmission systems.

A new modulation scheme that reduces thebandwidth and therefore the vulnerability
due to dispersion is duo-binary modulation.
The electrical data are precoded and encoded
and the modulator is driven with an electricalsignal having an amplitude of 2.Vπ to obtain
modulation of the phase of the optical signal.

If we modulate the phase of the opticalcarrier we can potentially improve thesensitivity, as the light will not be “switchedoff” when a logical 0 is transmitted. A Mach
Zehnder modulator is perfectly suited togenerate a binary phase shift keyed signal as
the phase of the optical carrier is altered by
180 degrees when the bias of the modulator
goes through the minimum of the transfer
characteristic.

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With a BPSK signal a single error would
cause all subsequent received bits to bedetected wrongly – therefore differential
phase shift keying is used. With DPSK thedata are precoded in the same way as they
are in duo binary, a transmitted “1” meansthat the data changed whereas a transmitted“0” corresponds to no change of the data.
Note: as PRBS signals are not altered by thepre-coding, a pre-coder is not needed in a test
transmitter.

Generating an NRZ-DPSK signal isstraightforward, the only difference to NRZ
is that the modulator is biased at its minimum
and that it is driven with 2.Vπ. Each time thedata change their logical value the phase of
the optical carrier is altered by a 180°.
The benefit of using a MZ modulator and nota phase modulator is the limiting
characteristic of a MZ modulator and the factthat the phase is altered by exactly a 180°.

If we use the above described phasemodulator together with a RZ generating MZmodulator we will have RZ-DPSK.

9When CS-RZ pulses are gated with our MZphase modulator we obtain CS-RZ DPSK.

Phase modulation requires that the modulator
is chirp free; therefore electro-absorption
modulators and single drive Z-cut LiNbO3modulators cannot be used.
Dual drive Z-cut LiNbO3 modulators – if thetwo input signals are carefully balanced –and of course X-cut modulators can beemployed for phase modulation.
Single drive X-cut modulators have theadvantage that you only need one amplifier
and you do not have to align the delay(phase) of two input signals.

Eachmodulation
schemesbehaves
differently regarding spectral efficiency,
tolerance against fiber imperfections andimplementation complexity.

10Here is a summary of the electro-optical andopto-electrical converters we offer.
With these converters you can upgrade ourbit error rate testers or our multiplexers anddemultiplexers. Depending on yourapplication you can choose between threetransmitters: NRZ or NRZ, RZ and CS-RZ ora transmitter that supports six modulation
formats: NRZ, RZ and CS-RZ and the threecorresponding DPSK versions.
As receivers we offer a photo receiver for
NRZ, RZ and CS-RZ as well as a receiver for
DPSK with built in decoder and a balanced
detector.

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