IEC PAS 62593:2008

Measurement method of a half-wavelength voltage for Mach-Zehnder optical modulators in wireless communication and broadcasting systems

IEC/PAS 62593:2008(E) gives a measurement method of half-wavelength voltage applicable to Mach-Zehnder optical modulators in wireless communication and broadcasting systems. In addition, this method is also effective for the estimation of the intermodulation distortion of Mach-Zehnder optical modulators.
- Frequency range: 10 MHz to 30 GHz.
- Wavelength band: 0,8 µm, 1,0 µm, 1,3 µm and 1,5 µm.
- Electro-optic material based Mach-Zehnder optical modulators and their modules.

General Information

• Status: Published
• Publication Date: 12-Nov-2008
• ICS: 33.060.20 - Receiving and transmitting equipment
• Technical Committee: TC 103 - Transmitting and receiving equipment for radiocommunication
• Current Stage: PPUB - Publication issued
• Start Date: 13-Nov-2008
• Completion Date: 31-Dec-2008

Standards Content (Sample)

IEC/PAS 62593
Edition 1.0 2008-11
PUBLICLY AVAILABLE
SPECIFICATION
PRE-STANDARD
Measurement method of a half-wavelength voltage for Mach-Zehnder optical
modulators in wireless communication and broadcasting systems

IEC/PAS 62593:2008(E)
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IEC/PAS 62593
Edition 1.0 2008-11
PUBLICLY AVAILABLE
SPECIFICATION
PRE-STANDARD
Measurement method of a half-wavelength voltage for Mach-Zehnder optical
modulators in wireless communication and broadcasting systems

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
V
ICS 33.060.20 ISBN 978-2-88910-812-1
– 2 – PAS 62593 © IEC:2008(E)
CONTENTS
FOREWORD.4
INTRODUCTION.5
1 Scope.6
2 Normative references .6
3 Terms, definitions and acronyms .6
3.1 Terms and definitions .6
4 Electro-optic material based Mach-Zehnder optical modulator .8
4.1 Type.8
4.2 Structure .8
4.3 Requirements for Mach-Zehnder optical modulators .8
5 Sampling .9
5.1 Sampling .9
5.2 Sampling frequency.9
6 Measurement method of half wavelength voltage.9
6.1 Circuit diagram.9
6.2 Circuit description and requirement .9
6.3 Measurement condition .10
6.4 Principle of measurement method .10
6.5 Measurement procedure.13
Annex A (normative) Conventional Measurement method of Optical Modulation Index .17
Annex B (informative) Calculation method of intermodulation distortions from driving
voltages and half-wavelength voltage for Mach-Zehnder optical modulator .19
Annex C (informative) Characteristics of Mach-Zehnder optical modulator .27
Annex D (informative) Points to consider for measurement .29
Bibliography.35

Figure 1 – A transfer curve of a Mach-Zehnder optical modulator .7
Figure 2 .8
Figure 3 – Circuit diagram.9
Figure 4 .11
Figure 5 – The schematic block diagram of the measurement setup.12
Figure 6 – Driving voltage measurement setup .14
Figure 7 .15
Figure 8 a) – The amplitude of the optical signal is almost zero .16
Figure 8 b) – The optical signal is modulated in phase with S2 element .16
Figure 8 c) – The optical signal is modulated in opposite phase with S2 element .16
Figure A.1.17
Figure A.2.18
Figure B.1 Mach-Zehnder interferometer type optical modulator .19
Figure B.2 – Quadrature points of a transfer cureve for a Mach-Zehnder optical
modulator .23

PAS 62593 © IEC:2008(E) – 3 –
Figure B.2 – Dependency of IM2 on NOMI and Bias voltage of a Mach-Zehnder optical
modulator .24
Figure B.4 – Relation between IM3 and OMI of a Mach-Zehnder optical modulator .24
Figure B.5.25
Figure B.6 – IMD2 and IMD3.26
Figure D.1 – Errors of half-wavelength voltage measurements caused by limitations
from the resolution of RF power supply.31
Figure D.2 – Relative errors of half-wavelength voltage measurement caused by
limitations from the resolution of RF power .32
Figure D.3 – Relation between NOMI and IM3 for the Mach-Zehnder modulator
(sample #1) .33
Figure D.4 – Relation between NOMI and IM3 for the Mach-Zehnder modulator
(sample #2) .33
Figure D.5 – Relation between NOMI and IM2 for the Mach-Zehnder modulator
(sample #1) .34

Table 1 – Acronyms .7
Table C.1.27
Table C.2.28
Table D.1.29
Table D.2 – Measurement results of half-wave voltages for Mach-Zehnder modulators.32

– 4 – PAS 62593 © IEC:2008(E)
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
MEASUREMENT METHOD OF A HALF-WAVELENGTH VOLTAGE
FOR MACH-ZEHNDER OPTICAL MODULATORS IN WIRELESS
COMMUNICATION AND BROADCASTING SYSTEMS

FOREWORD
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patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
A PAS is a technical specification not fulfilling the requirements for a standard, but made
available to the public.
IEC-PAS 62593 has been processed by IEC technical committee 103: Transmitting equipment
for radio communication.
The text of this PAS is based on the This PAS was approved for
following document: publication by the P-members of the
committee concerned as indicated in
the following document
Draft PAS Report on voting
103/74/PAS 103/81/RVD
Following publication of this PAS, which is a pre-standard publication, the technical committee
or subcommittee concerned may transform it into an International Standard.
This PAS shall remain valid for an initial maximum period of 3 years starting from the
publication date. The validity may be extended for a single 3-year period, following which it
shall be revised to become another type of normative document, or shall be withdrawn.

PAS 62593 © IEC:2008(E) – 5 –
INTRODUCTION
A variety of microwave-photonic devices are used in wireless communication and
broadcasting systems. An optical modulator is an interface which converts an electronic signal
into an optical signal. In the field of optical fibre communication systems, the IEC 62007
series "Semiconductor optoelectronic devices for fibre optic system applications" has been
published. In the field of wireless systems, specifications of inter-modulation and composite
distortion of modulators have been an important issue and have typically been negotiated
between users and suppliers. During an International Meeting on Microwave Photonics, a
proposal was announced to address standardizations for key-devices for Radio over Fibre
(RoF) systems.
The RoF system is comprised mainly of two parts; one is the RF to photonic converter (E/O),
and the other is photonic to RF converter (O/E). Radio waves are converted into an optical
signal at E/O, and the signal is transferred into the optical fibre, and then the radio waves are
regenerated at O/E. The nonlinear distortion characteristics of both E/O and O/E are
important for the performance of the system. Semiconductor photodiodes are commonly used
for O/E. Several types of optical modulator are used for E/O, such as Mach-Zehnder
modulators, electro-absorption modulators and directly modulated LDs.
This PAS has been prepared in order to provide industry standard measurement methods for
evaluating electro-optic material based Mach-Zehnder optical modulators to be used in
wireless communication and broadcasting systems. When the optical modulation index (OMI)
is calculated from the half-wavelength voltage measurement results, the intermodulation
distortion of the Mach-Zehnder optical modulator can be obtained. In this PAS, the
measurement method of the half-wavelength voltage for Mach-Zehnder optical modulators is
described. The details of calculations of the second order intermodulation distortion (IM2) and
the third order intermodulation distortion (IM3) are described in Annex B.

– 6 – PAS 62593 © IEC:2008(E)
MEASUREMENT METHOD OF A HALF-WAVELENGTH VOLTAGE
FOR MACH-ZEHNDER OPTICAL MODULATORS IN WIRELESS
COMMUNICATION AND BROADCASTING SYSTEMS

1 Scope
This PAS gives a measurement method of half-wavelength voltage applicable to Mach-
Zehnder optical modulators in wireless communication and broadcasting systems. In addition,
this method is also effective for the estimation of the intermodulation distortion of Mach-
Zehnder optical modulators.
– Frequency range: 10 MHz to 30 GHz.
– Wavelength band: 0,8 µm, 1,0 µm, 1,3 µm and 1,5 µm.
– Electro-optic material based Mach-Zehnder optical modulators and their modules.

2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 62007-1, Semiconductor optoelectronic devices for fibre optic system applications -
Part 1: Specification template for essential ratings and characteristics
IEC 62007-2 Semiconductor optoelectronic devices for fibre optic system applications -
Part 2: Measuring methods
3 Terms, definitions and acronyms
3.1 Terms and definitions
For the purpose of this document, the terminology concerning the physical concept, the type
of devices, the general terms, those related to rating and characteristics in IEC 62007-1 and
IEC 62007-2, as well as the following terms and definitions, apply.
3.1.1 Half-wavelength voltage: Vπ
The voltage required for a Pockels effect material based optical modulator to shift phase of
the light by one-half a wavelength relative to the other. It corresponds to an ON/OFF voltage
of the Mach-Zehnder optical modulator.

PAS 62593 © IEC:2008(E) – 7 –
V
π
Quad.
Input Voltage
Figure 1 – A transfer curve of a Mach-Zehnder optical modulator
3.1.2 Normalized optical modulation index: NOMI
For the Mach-Zehnder optical modulator, the ratio of driving voltage and half-wavelength
voltage of the modulator,
NOMI = (V / V ) × 100 [%] (3.1)
pp π
where
V is the driving voltage (peak to peak voltage);
pp
V is the half-wavelength voltage.
π
NOTE NOMI does not denote actual optical modulation index defined as the ratio of the optical modulated signal
power and the average optical power. The detailed explanations of OMI including measurement method are
described in Annex A.
3.1.3 Extinction Ratio
The ratio of two optical power levels of the optical signal generated by the optical modulator:
R = 10log(P /P) (3.2)
ext 1 2
where
P is the optical power level generated when the output power is "on,";
P is the power level generated when the output power is "off."
NOTE The extinction ratio is sometimes expressed as a fraction, not in dB.
3.1.4 Acronyms and symbols
The acronyms and symbols are shown in Table 3.1.
Table 1 – Acronyms and Symbols
V A Half wavelength voltage
π
OMI Optical Modulation Index
NOMI Normalized OMI
IM2 Second-order Inter-Modulation distortion
IM3 Third-order Inter-Modulation distortion
CSO Composite Second-Order distortion
CTB Composite Triple-Beats distortion
Output Optical Power
– 8 – PAS 62593 © IEC:2008(E)
4 Electro-optic material based Mach-Zehnder optical modulator
4.1 Type
The optical modulators and their modules consist of basic parts as follows
– Mach-Zehnder interferometer type optical modulator
– input and output fibre pigtails (where appropriate)
– bias control port (where appropriate)
– photodiode for bias monitoring (where appropriate)
– laser diode for light source (where appropriate)
– thermal sensor (where appropriate)
– Peltier element (where appropriate)
4.2 Structure
– Electrode: lumped type, traveling-wave type, etc.
– Options: optical isolator, photodiode, half-mirror, laser-diode, etc.

V
E
E out(t)
in(t)
V
Figure 2
4.3 Requirements for Mach-Zehnder optical modulators
This method is based on the theoretical transfer curve of an electro-optic material based
Mach-Zehnder interferometer, where the phase shift of traveling light on each arm of the
interferometer should be proportional to the applied voltage, and power of traveling lights on
each arm are almost same. Requirements for the modulator of this measurement method are
as follows:
4.3.1 Substrate material
The main Substrate materials of the modulator should be the materials such as LiNbO ,
LiTaO , KH PO , PZT, PLZT, InP, GaAs, InGaAs, InAlAs, InGaAsP, CLD type chromophore
3 2 4
containing polymer, FTC type chromophore containing polymer, etc., which realize an electro-
optic effect (Pockels effect). If strictly considered, semiconductor materials do not have pure
electro optic effect, however, the semiconductor Mach-Zehnder modulators can be adjudged
as electro-optic material based Mach-Zehnder modulators.
4.3.2 Optical waveguide design
The optical waveguide should be designed as a single Mach-Zehnder interferometer type
comprised of two y-junctions or symmetric directional couplers and parallel waveguides.
Reflection type Mach-Zehnder optical modulators are included.

PAS 62593 © IEC:2008(E) – 9 –
5 Sampling
5.1 Sampling
A statistically significant sampling plan shall be agreed upon by user and supplier. Sampled
devices shall be randomly selected and representatives of production population, and shall
satisfy the quality assurance criteria using the proposed test methods.
5.2 Sampling frequency
Appropriate statistical methods shall be applied to determine adequate sample size and
acceptance criteria for the considered lot size. In the absence of more detailed statistical
analysis, the following sampling plan can be employed.
Half wavelength voltage:  two units at least/manufacturing lot.
6 Measurement method of half wavelength voltage
6.1 Circuit diagram
3 5
1 2 4
Figure 3 – Circuit diagram
6.2 Circuit description and requirement
1 = Laser diode
2 = Polarization controller
3 = Device Under Test
4 = Photo Diode
5 = Oscilloscope
6 = Monitor Signal Source (SG2)
7 = Bias Tee
8 = (Step) Attenuator (Electrical)
9 = Microwave Amplifier
10 = Microwave Signal Source (SG1)
11 = Power Meter or Spectrum Analyzer (electrical)

– 10 – PAS 62593 © IEC:2008(E)

6.3 Measurement condition
6.3.1 Temperature and environment
The measurement should be carried out in the room from 5 °C to 35 °C. If the operation
temperature ranges of the measurement apparatus are narrower than the above range, the
specifications of the measurement apparatus should be followed. It is desirable to control the
measurement temperature within ±5 °C in order to suppress the influence of the temperature
drift of measurement apparatus to minimum.
6.3.2 Warming up of measurement equipment
The warming-up time shall be respected, typically 60 minutes, or the time written in the
specifications of the measurement equipments or systems. Moreover, the warming up time
should be that of to be the longest of all the measurement equipment.
6.4 Principle of measurement method
The Method for measuring half-wavelength voltage (AC half-wavelength voltage) of a Mach-
Zehnder type optical modulator is described here. In this method, the half-wavelength
voltages of Mach-Zehnder type optical modulators can be measured accurately without
depending on the bias voltage of an optical modulator. When the input RF signal to the
modulator is set to such a specific level that the zero-order Bessel function can be zero, the
average optical output power of the modulator becomes constant regardless of the bias
voltage. By measuring the input RF power or voltage at this condition, half-wavelength
voltage, V is determined. This measurement can be achieved through a wide frequency
π
range, though it needs a high-voltage signal source (of about 1,5 times of Vπ).
6.4.1 Measurement principle
The optical output power of MZ modulators is given by,
I
I =[]1+ cos()Φ + Φ (6.1)
1 2
πV
pp
Φ = sin()2πft (6.2)
2V
π
Φ = const. (6.3)
where φ1 and φ2 are the phase change caused by the high-frequency RF signal and that due
to the Bias voltage, respectively. V is the half-wavelength voltage at the RF signal frequency
π
f, V is the peak-to-peak voltage amplitude of the high-frequency wave, and I is the
pp 0
maximum optical output power. The time average power of I, I′ is calculated by,
1/ f
I
I' = f []1+ cos()Φ + Φ dt
1 2
∫
0 2
1/ f
I
= f[]1+ cos Φ cos Φ − sin Φ sin Φ dt (6.4)
1 2 1 2
∫
0 2
PAS 62593 © IEC:2008(E) – 11 –
After calculation from Eq. (6.4), we get,
⎡ ⎤
1/ f ⎧ πV ⎫ ⎧ πV ⎫
I
pp pp
I' = f 1+ cos sin()2πft cos Φ − sin sin()2πft sin Φ dt
⎢ ⎥
⎨ ⎬ ⎨ ⎬
2 2
∫
2 2V 2V
⎢ π π ⎥
⎩ ⎭ ⎩ ⎭
⎣ ⎦
∞ ∞
⎡ ⎤
πV πV
1/ f ⎧ ⎫ ⎧ ⎫
I
pp pp
= f ⎢1+ ε cos()2n ⋅ 2πft J cos Φ − 2sin{}()2n +1 2πft J sin Φ ⎥dt
⎨ ⎬ ⎨ ⎬
∑ n 2n 2 ∑ 2n+1 2
∫
0 2 2V 2V (6.5)
⎢ ⎥
π π
⎩ ⎭ ⎩ ⎭
⎣ n=0 n=0 ⎦
⎡ ⎤
⎛ πV ⎞
I
pp
⎜ ⎟
= 1+ J cos Φ
⎢ ⎥
0 2
⎜ ⎟
2 2V
⎢ π ⎥
⎝ ⎠
⎣ ⎦
where
1Ln = 0
⎧
ε
=
⎨
n
2Ln ≠ 0
⎩
/(2V ) = 2,405 can be satisfied,
When the input RF signal is tuned so that the relation πV
pp min π
the zero-order Bessel term in the Eq. (6.5) becomes zero, and the time average of the optical
output power becomes constant. As shown in Figure 4, there are many voltage amplitudes at
which the AC component of I’ goes down to zero. V denotes the lowest one of them.
p-p min
/ (2V )
V
π π
p-p min
(=2.405)
02 46 8 10
V / (2V )
π π
p-p
Figure 4
The schematic block diagram of the measurement setup is shown in Figure 3. In order to
easily find the state where the optical output is constant, a low frequency signal for monitor
(SG2) is superimposed on the RF signal. By adjusting the RF voltage amplitude of the high-
frequency signal (SG1), the status can be observed where the monitor signal (SG2) amplitude
shows the minimum value. At this status the wave form of monitor signal is observed as a flat
at the frequency of SG1 can be calculated from the
line on the screen of the oscilloscope. V
π
measured result of V using the following relation.
p-pmin
(P_S1/10-3) 1/2
πV
π･20(10 )
p−pmin
Vπ = = (6.6)
2× 2.405 2× 2.405
| J (πV /2Vπ) |
0 p-p
– 12 – PAS 62593 © IEC:2008(E)

6.4.2 Circuit diagram
3 5
1 2 4
Intensity
Intensity
I/2
I/2
Time
Time
Figure 5 – The schematic block diagram of the measurement setup
6.4.3 Circuit description and requirement
1 = Laser diode
2 = Polarization controller
3 = Device Under Test
...