ModBox Pulse Generation 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 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. - 1 -
General Description : the Pulse Generation ModBoxes P H O T L I N E T e c h n o l o g i e s Puse Generation ModBoxes are Modulation Units designed to generate controlled optical pulses from a continuous optical laser source and a pulsed RF signal. The Pulse Generation ModBox is a 19 rackable Modulation Unit integrating a LiNb0 3 intensity modulator (MX-LN type), its RF driving electronics and bias control circuit and optional laser source and RF generator. The example below shows a 1053 nm Pulse Generation ModBox integrating a Near Infrared intensity modulator, a specific driver for pulse application. Other Pulse Generation ModBoxes can be built, at virtually any wavelength between 780 nm and 2.0 µm. Options - Choice of operating wavelength in the 780 nm 2000 nm range - Choice of pulse width and repetition rate - Choice of optical connectors Principle LiNb0 3 intensity modulators are appealing modulation solution with strong benefits : - high bandwidth up to 40 GHz - rise/fall time as low as 10 ps - chirp free : they do not induce wavelength distortion of the optical modulated signal - relatively low optical loss (4 db typ.) - low driving voltage (< 5 V @ 10 GHz) - proven technology LiNb0 3 intensity modulators are Mach-Zehnder waveguide interferometers, whose transfer function is a sine curve and they are operated at quadrature point, minimum point or any other point of the curve. In the example below, the modulator is operated at the positive quadrature point, with a peak-to-peak modulation voltage equal to the half-wave voltage so as to get an output signal between zero (0) and the maximum level (1). Modulation Curve Signal Output Bias Voltage MIN Point Electrical Signal Figure 1 : Transfer function of a Mach-Zehnder modulator and output signal vs input signal at QUAD+ operating point - 2 -
Your Pulse Generation ModBox: Modbox-Pulse-Gene-1550nm-10nm-30dB The Modbox-Pulse-Gene-1550nm-10nm-30dB is a Pulse Generation ModBox optimized to generate short optical pulses from 5 ns up to 200 ns at low repetition rate from 100 khz up to 500 khz with 30 db dynamic extinction ratio from customer supplied 1550 nm ±10 nm continuous laser signal and pulse RF electrical modulation signal. The Modbox-Pulse-Gene-1550nm-10nm-30dB assume easy configuration and operation long term stability Block Diagram RS 232 Pulse Generation Unit MBC-Board DC DC in External Photodiode PM Modulator High Extinction ratio PM FC/APC Coupler PM 1/99 %, 5/95 % PM Driver DR-PL-10/20 RF in Main Input FC/APC - PM Gain control Pulse RF Input SMA Main Output FC/APC - PM Figure 2 : Schematic and components integration The Modbox-Pulse-Gene-1550nm-10nm-30dBintegrates: one electro-optical LiNbO 3 modulator with high extinction ratio modulators (Photline MXPE-LN-10), one bias controller board (Photline MBC-board-1001) to lock the working point of each Mach-Zehnder modulator in MIN mod one driver optimised to fit the electrical pulse specification in terms of pulse width, repetition rate and rise and fall time (Photline DR-PL-10-LR/HR). The ModBox features a RS-232 interface for adjustment of the bias control parameters. - 3 -
Specifications - 4 - P H O T L I N E T e c h n o l o g i e s Unit Min Typ Max input signal (user supplied, not a ModBox specification) Laser type TBD Mode CW wavelength nm 1540 1550 1560 Polarization Linear and controlled Spectral width nm TBD SMSR db TBD Power (average) TBD 20 Pulse modulation signal (user supplied, not a ModBox specification) Pulse width ns 5-200 Repetition rate khz 100-500 Rise time ps - 60 - Signal amplitude mv p-p 100 350 500 Impedance matching Ω - 50 - External Modulation stage Modulator Photline NIR-MX-LN-10 Waveguides Polarizing Insertion loss (at maximum transmission, 1053 nm) db - 4 5 Modulators electro-optic bandwidth S21 @-3 db GHz 10 12 - Dithering signal for bias control of the intensity modulator 1 khz Modulated optical signal (obtained with above typical pulse modulation signal) Pulse width ns 5-200 Repetition rate khz 100-500 Duty cycle 1 % - 1 - Rise time ps - 100 - Dynamic Extinction ratio 2, 3 db 30 35 - Interfaces (front panel) Input and Output fiber and connector Polarization maintaining, Panda type, FC/APC, polarization in slow axis // key Modulation interface (front panel) Intensity modulation connector 50 Ω SMA-type female Bias Control Circuit interface RS-232 Environmental Operating temperature 15 C to 35 C Storage temperature -20 C to +50 C Power Supply Rear pannel AC Voltage V 90 220 240 Hz - 60 - Maximum ratings Maximum RF input power +28dBm Maximum optical input power +25dBm Dimensions 471 mm x 271 mm x 102 mm 1: The extinction ratio is optical source dependant. Additional information of the optical source is needed to assume minimum and typical values. 2: Duty cycle is defined as the ratio between pulse width and repetition period (inverse of repetition rate). Duty cycle at 1 % will define the maximum pulse width whatever the 100 khz 500 khz repletion rate. 3: Refer to appendix 1 for measurement method
Front and Rear Panels P H O T L I N E T e c h n o l o g i e s CW Input 1 Driver On/Off 5 Driver Gain Control 6 2 3 4 7 Modulated Output RF Input MBC Reset Unit On/Off Figure 2 : Front Panel 8 AC Power Interface Figure 3 : Rear Panel 9 MBC Interface Features Notice 1. input FC-APC optical connector (fiber slow axis // to connector key) 2. output FC-APC optical connector (fiber slow axis // to connector key) 3. RF input SMA type (female) RF connector 4. MBC reset Reset of the bias control circuit 5. RF driver ON/OFF RF driver supply switch 6. RF driver gain adjust 7. ON/OFF Unit ON/OFF allows to adjust the driver gain, and thus the optical pulse amplitude, over 3 db 8. AC Power Plug General On/Off switch Main switch 9. MBC interface Sub-D15 female connector for MBC parameter adjustment - 5 -
Appendix 1: Dynamic Extinction Ratio measurement method This appendix describes the method to measure the high Dynamic Extinction Ratio ( DER ) when it is > 50 db using LiNBO 3 modulators in pulse mode. The equipment used are the following: - a scope to observe the pulse shape, - an optical power meter to measure the average optical power from the modulator atput, - a stable laser source, - a RF pulse programmable generator for short pulse and repetition rate. The figure 1 below gives the pulse definition, we define Pulse width and Repetition rate as δt, 1/T respectively. ρ Max power δτ ρ moy ρ min T t Figure 1: Pulse definition We define the Static Extinction Ratio ( SER, also DC Extinction Ratio) and Dynamic Extinction Ratio as below: P SER = 10.log min Pmax ρ DER = min eff 10.log ρmax Equation 1 The SER is measured with no electrical signal applied on the RF input port of the modulator. The bias voltage on the DC port (connected to the DC electrode) is adjusted to get an ON state (P max ) then an OFF state (P min ). SER is calculated based on formula given at equation 1. - 6 -
The DER is measured by applying a RF pulse electrical signal at the RF input port of the modulator, the DC bias voltage being tuned to set the modulator at the minimum operating point of the modulator s transfer function. The DER cannot be directly measured due to pulses train, and DER evaluation becomes a difficult task. We define here a method for DER to be measured. We cannot have access to the ρ min value, but only to the ρ average using an optical power meter. The ρ average optical power can be expressed as: δτ ρaverage = ( ρmax ρmin ) + ρmin, Equation 2 T The equation 2 is true if the condition cited below are respected. It means that pulses are not distorted by optical and electrical components. It assumes: - pulse driver needs low rise and fall time, - modulator has enough bandwidth, - modulator is stabilized at the MIN point by MBC whatever the duty cycle, - pulse width significantly wider than rise time: δτ > pulse rise time In equation 2, we have: δτ δτ ρmin = ρaverage ρmax, if << 1 T T Equation 3 Using Equations 1, 2, 3, this gives the final expression of the DER eff for a given duty cycle we can measure: DEReff ρaverage δτ = 10.log where ρ max = P max. Equation 4 ρmax T Before DER evaluation, we have first to ensure that conditions given in Equation 2 are respected. This is the case when the measured and calculated average optical power curves superimposed, (Figure 2). - 7 -
Duty Cycle 1.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-03 1.00E-04 1.00E-05 Paverage/Pmax 1.00E-06 Calculation Measurement Figure 2: Average power at the output of the modulator versus duty cycle during pulse modulation. The repetition arte is 2.5 khz, and the pulses width in the 100 ps to 50 ns range. The figure 2 above shows good working conditions (coming from equation 2) where the modulator is correctly bias whatever the duty cycle, and modulator and driver do not create pulse distortion (they have enough bandwidth). - 8 -