ModBox - Spectral Broadening Unit

The ModBox Family
The ModBox systems are a family of turnkey optical transmitters and external modulation
benchtop units for digital and analog transmission, pulsed and other specific applications.
The Modbox design integrates within a bench-top or 2U 19” enclosure a laser source (optional), a
complete modulation stage featuring an external LiNb03 modulator with its RF driver and bias
control circuit, and a receiver stage (optional).
ModBoxes can be tailored to specific needs in order to provide systems engineers with reliable
performance and high speed modulation capabilities together with the peace of mind of a readyto-plug equipment.

General Description : the 1053 nm Spectral Broadening ModBoxes
A Spectral Broadening ModBox is designed to suppress the Stimulated Brillouin Scattering (SBS) caused
in optical fibers by high fluxes of highly coherent light. The SBS can lead to the destruction of the fiber andthe optical components along the fiber. When the temporal coherence of the signal is destroyed, the SBS
power threshold is significantly increased and thus its effects can be eliminated. The Spectral Broadening
ModBox achieves this by modulating the phase of the optical signal and creating a number of lines over aspectral width that can reach more than 100 GHz.
The Pulse Broadening ModBox is a 19’’ rackable Modulation Unit integrating a LiNb03 phase modulator(MPX-LN type) with a 38 dBm RF generator.
The exemple below shows a Spectral Broadening ModBox integrating a 14.25 GHz internal RF generator.
This frequency is selected to obtain the widest possible spectral broadening. Other Spectral Broadening
ModBoxes with lower frequency RF generator can be built.

Options-choice of operating wavelength in the 790 nm – 2200 nm range
choice of RF frequency : up to 14.25 GHz
choice of optical connectors

Principle
The spectral analysis of the optical signal transmitted by a phase modulator driven at a fixed frequency fand with a fixed RF input power yields a spectrum with multiple lines centered around the optical frequency
F, each line being spaced of f from its neighbors.
The spectral broadening is basically the product of the lines spacing by the number of lines. It depends on
the RF power and the RF frequency, that last one being equal to the line spacing.
Broaden Spectrum = 2 x m x fwith m = modulation depth and f = RF modulation frequency.

Figure 1 : Spectral broadening : approximate theoretical model

The 14.25 GHz 1053 nm Spectral Broadening ModBox
The 14.25 GHz Pulse Broadening ModBox is optimized to produce a ≥ 0.5 nm spectral width optical pulsed
signal from a customer supplied, 1053 nm pulsed , narrow spectrum (typ ; few MHz) optical signal.
The 14.25 GHz RF generator deliver a pulsed sine wave signal to an internal phase modulator. This signal
is gated by the pulse laser and is sent to the phase modulator only in presence of an optical pulse, so as toreduce the RF power absorbed by the modulator. The resulting phase shift during the pulses is thus a sinefunction and the optical spectrum of the laser source is modified : a number of lines appears.
The lines are separated from a frequency spacing equal to the modulation frequency 14.25 GHz. The
number of lines depends on the RF applied power. The proposed ModBox, when used at maximum RFpower allows to make appear at least 10 lines so as to create an optical spectrum broadened to minimum
135 GHz (0,5 nm).
The ModBox is supplied with a USB interface for instrument remote.

Typical set-up

ModBoxOptInput

OptOutput

NIR-MPX-LN-05

AmplifierOpticalSource
14.25 GHz
Sinewave generator

Figure 2 : ModBox Spectral broadening schematic

Hi-Resolution
OpticalSpectrum
AnalyserPanel features

Figure 3 : ModBox front panel

Figure 4 : ModBox back panel

Features

Notice1. RF Power LED Statuslights up green when the RF switch is turned ON.

2. Alarm LED Indicatorlights up red when an error occured.

3. RF Power switchTurn it on to proceed the pulse broadening

4. Optical I/O Ports

FC/APC fiber optical connector – Panda fiber withpolarization in slow axis // key

5. Pulse monitoring output

BNC connector.
1V when power is maximum

6. RS232 female socket

For RF Generator remote control and monitoring. Astandard RS232 direct cable is required

7. RF Power adjustement
8. AC Power Plug
9. Synchronization Input

Full range trimmer
System On/Off switch
BNC connector. TTL Signal

Specifications
INPUT SIGNALUnitMinTypMax

OPTICAL INPUT SIGNAL (user supplied, not a ModBox specification)
Wavelength of operation
Polarization extinction ratio
Optical power

nmdBdBm

25-1053-

20RF SOURCE

Unit

Min

TypMaxGHzMHzKHz/°CdBm
dBdBc.Hz-1
KHz.V-1
dBc-538-35

14.25
100
13100
40+5-95-

External trigger isignal type
External trigger repetition rate
Rise time
Fall time
JitterPulse width

Hznsns
nsµs

1150
2

TTL
10 M
-200 M
150500400-

OPTICAL MODULATION STAGE

Unit

Min

TypMax

INTERNAL RF SIGNAL
Frequency
Frequency accuracy
Stability
PowerPower tenability
Phase noise @10kHzPushingHigh order harmonics rejection

INTERNAL PULSE SIGNAL

MODULATOR
Modulator reference
CrystalModulator electro-optic bandwidth S21 @-3 dB
Insertion loss
Optical return loss

NIR-MPX-LN-05
Lithium Niobate z-cut, y-propagating
GHz
14dB
34dB-40-MAXIMUM RATINGS

Unit

Maximum optical input power

Min

Typ100 mWMax

GENERAL SPECIFICATIONS

Unit

Min

TypMax

INTERFACES

Input / Output fiber and connector

Input external trigger connectors
Output pulse monitoring
RF modulating signal power
RF port remoting
MBC Interface

Front Panel - 980 nm Polarization maintainingfibre- Panda type - FC/APC Polarization in slow axis // key
BNC
BNC
Manual trimmer
RS 232 – SubD9 - Female
RS 232 – SubD9 – Female or USB

ENVIRONMENTAL
Operating temperature
Storage temperature

15 °C – 35 °C-20 °C – +50 °C

POWER SUPPLYAC Voltage (Automatic Switch)

VHz

90
50

Electrical plug

110

240
60

Rear panel

DIMENSIONS
Modulation unit
Weight19” 2UKg

-3

COMPLIANCE
SafetyOther

EN 60825-1
CE marking-Test Results
Next screenshots show the spectra of the input optical signal (in green), and of the spectrally broadened
optical signal (in black). In regards of OSA resolution, bandwidth and sampling characteristics, therepetition rate was increased up to 100 kHz to obtain a better rendering.
NB : input laser spectrum is limited by the OSA resolution (0,06 nm = 16 GHz)
One can see that the optical signal width has been increased from a few MHz to nearly 500 GHz (1.8 nm),a sufficient spectral width to eliminate the SBS and its undesirable effects.

Figure 5 : Spectral broadening

Figure 6 : Modulation frequency lines

Appendix 1 : the internal 14.25 GHz RF Source
Inside the Spectral Broadening ModBox, a sine-wave synthesizer generates a 14.25 GHz pure modulating
frequency.
This carrier is then amplified through a two stage amplifier in order to obtain a maximum output power of 38
dBm. The modulator RF line has limited thermal dissipation capacity, thus the Modbox operates in pulsed
mode. The RF signal is externally triggered by a customer supplied signal and the RF power is applied only
during 2 µs with a repetition rate adjustable between 1 Hz and 200 kHz.

Figure 6 shows a basic sheme of the RF source.
TTLPower
SupplyAttenuator

RS232

ControlBoard

1st stage
amplifier2nd stage
amplifier

Isolator

Figure 7: basic scheme of the RF source
Next figures show measures of the 14.25 GHz source generator RF spectrum. On figure 8, the firstharmonic can not be seen at 28.5GHz, which is the sign of a good linearity.
On figure 9, with a 10 MHz span, no spurious frequencies are around the fundamental.

Figure 8 : Harmonic Rejection

Figure 9 : 14.25 GHz source spectral purity

Figure 10 : 14.25 GHz source spectrum (phase noise)Test reportThe 14.25 GHz Pulse Broadening ModBox is delivered with an individual test report.
The tests are performed at 25°C using the following equipments :•AWG = Agilent 33120•

Optical Spectrum Analyzer = Agilent 86142B•

Optical Source = 1053nm DFB fiber laser (linewidth <50 kHz)The test report includes :•Optical and microwave measurement

Content
The 14.25 GHz Pulse Broadening ModBox comes with•

110 V US main cord•

CD driver for MBC•

Test report•

User manual