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Characterization of the electro-optical transceivers in the KM3NeT optical network

 S.Pulvirenti, F. Ameli, A. D’Amico, G. Kieft, J-W Schmelling for KM3NeT 

Collaboration

15-09-2015

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Summary• Introduction• The electro-optical transceiver in KM3NeT• OESolution SFP Transceiver• Wavelength tuning• (First) Tests at Nikhef• Device aging and wavelength drift• Test bench• Test Procedure• Preliminary results• Conclusion and future Test

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IntroductionThe presentation shows preliminary results about the characterization of the tuneable features of the electro-optical transceivers used in KM3NeT network.Over the telescope life time, precise temperature control of the laser is required to maintain stability of the central frequency, complying with the ITU-T G.694.1 recommendations of the International Telecommunication Union.

This characterization is required to compensate for the expected wavelength drift due to aging factors.

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SFP Transceivers in KM3NeTThey are a key element in the KM3NeT network, present in DOM, JB, Tower Floor, Shore Station

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OESolution SFP Transceivers The data transport over the KM3NeT optical network is based on the Dense Wavelength Division Multiplexing (DWDM) technique with optical channels spaced 50 GHz apart, a bit rate of 1.25Gbps and up to 100 km span length.

The digital diagnostic monitoring interface allows real time access to the device operating parameters:

• Transmitted Optical Power• Received Optical Power• Transmitter Bias Current• Transceiver Temperature• Laser Temperature• Supply Voltage• TEC Current

KM3NeT SFP Requirements

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Wavelength tuningThe wavelength tuning feature of a SFP is a special feature which is not required by the DWDM SFP MSA standards. The principle of the wavelength tuning is based on changing the temperature of the TOSA (Transmission Optical SubAssembly). To be more precise change the temperature of the laser cavity. Inside a SFP there is a temperature control circuitry which on its turn is connected to a microcontroller. The actual change of temperature is done via a TEC (Thermoelectric cooling) controller. This TEC controller on its turn is connected to the microcontroller where it gets its set-point from. The actual wavelength tuning by the user is done by changing data in the microcontroller memory which is translated to the temperature setting of the TEC and thus the TOSA temperature. All the communication is done via the I2C bus of the SFP transceiver.

The expected wavelength change is typically 100 pm/°C

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(First) Tests at Nikhef

Wavelength measurements at Nikhef performed using an Ando wavelength meter, model

model AQ6140, instead of an OSA.

Main specifications:

• Wavelength range 1270 to 1650 nm

• Wavelength accuracy ±2 ppm (1550 nm, 1310 nm: ±0.003 nm)

• Minimum resolvable seperation 10 GHz or less (1550 nm: 80 pm, equal power line input)

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DAC offset vs Wavelength change

DAC variation Max variation-100pm 6%+100pm 10%

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Wavelength tuning response

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Device aging and wavelength driftLong-term device reliability is characterized by the increase in threshold current and the decrease in optical output power. The aging of the laser is associated with loss of optical efficiency resulting in a drop of the emitted optical power.In the application it is preferred to operate the device in constant output power mode. As the device undergoes aging the dissipated power increases resulting in a rise in the junction temperature. The lasing wavelength is a sensitive function of the junction temperature.

The lasing wavelength of a DFB laser diode is given by the simple equation:

where is the n effective refractive index and is the grating pitch. In order to obtain a desired wavelength accurately, both parameters have to be well controlled.

Moreover, the effective refractive index can change with the driving conditions such as laser temperature or laser current. Due to the above reasons, temperature control is often used to fine-tune the DFB wavelength precisely to an ITU-T grid.

uBlaze I2C

RS232

SFP Laser

Xilinx Spartan6 - FPGA

FCM Board

Test Bench

Communicate with Host PC (RS232)

Optical Spectrum Analyzer

• A MicroBlaze microcontroller is embedded in the FCM FPGA:

Soft IP core (synthesizable); Xilinx development environment; Peripheral availability for standard communication

protocol with drivers and APIs;• A dedicated SW has been written to:

read/write Laser registers (I2C); implement tuning procedure for OESolutions Laser; communicate with Host PC (RS232);

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Test bench tools

OSA – Optical Spectrum Analyzer:

• Wavelength range [nm] 1250 to 1650• Resolution bandwidth FWHM 0.065 nm• Wavelength uncertainty [nm] ± 0.05• Wavelength repeatability [nm] ± 0.003• Wavelength linearity [nm] typical ± 0.01

FCM – Floor Control Module is KM3NeT-IT Floor Control Module:• Communication with On-Shore through SFP Laser;

- Use of Custom Protocol• Collection of digital streams from 6 PMTs located in the Optical

Modules;• Collection of data from 2 hydrophones;• Communication with floor oceanographic instrument• Management of local sensors (Temperature, Relative Humidity,

Currents and Voltages, etc.) See Carlo Nicolaus’s Talk

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Test Procedure

• Warm up time: > 15 minutes• Initial settings of the transceiver: Factory settings• Disable the transceiver alarms• Store the factory tuning value• Temperature scan 2°C

• Wavelength scan:- Acquisition of factory settings- Four samples before scan start (at nominal wavelength)- Eight samples during the wavelength scan. - Four samples after scan stop (at nominal wavelength)

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Preliminary results (over 8 scan samples)

Wavelength vs Laser temperature

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Preliminary Results (Linear Fit)

Example CH 30C (λ = 1553,33 nm)

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Preliminary results – CH 59C

λ = 1530,33 nm

Factory settings (4 samples scan)

Factory settings (4 samples scan)

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Preliminary results – CH 59C

Linear fit between Laser temperature and Tuning Value

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Conclusion and Future Test

• Aging stress test – Monitor the SFP behavior in a long period and check the center wavelength stability at high temperature

• Accumulate more statistics (more devices of the same wavelength)

• Characterize more devices covering the whole C band within ± 100 pm from the nominal wavelength

Aim to predict the correction to be applied in case of a drift of more than ± 50 pm 

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Thank you for your attention!