PDD - AMANDA Data Transmission Techniques

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• 7.1 Overview
• 7.2 Digital Optical Module
  • 7.2.1 Pressure Housing
  • 7.2.2 Optical Sensor
  • 7.2.3 PMT HV Generator
  • 7.2.4 Optical Beacon
  • 7.2.5 Signal Processing Circuitry
  • 7.2.6 Local/Global Time Transformation
  • 7.2.7 Cable Electrical Length Measurement
  • 7.2.8 Data Flow and Feature Extraction
  • 7.2.9 Local Coincidence
  • 7.2.10 System Design Aspects
• 7.3 Network
  • 7.3.1 Copper Links
  • 7.3.2 Time-Base Distribution
• 7.4 Surface DAQ
  • 7.4.1 Overview
  • 7.4.2 DOM Hub
  • 7.4.3 String Processor
  • 7.4.4 IceCube and IceTop System Integration
  • 7.4.5 Experiment and Configuration Control
  • 7.4.6 Security Environment
  • 7.4.7 DAQ Components
  • 7.4.8 Calibration Operations
  • 7.4.9 DAQ and Online Monitoring
  • 7.4.10 DAQ Computing Environment
• 7.5 AMANDA Data Transmission Techniques

7.5 AMANDA Data Transmission Techniques

For historical reference, we present here the data transmission techniques used in the past by the AMANDA detector. The present AMANDA II detector has been deployed over several seasons, with a stepwise implementation of detectors with improved data transmission techniques. An array of four strings (AMANDA-B4) was installed in 1995/96, with the OMs connected to the surface by an electrical coaxial cable that transmits both the HV and the analog anode signal; see fig. 74, left. PMTs are operated at a gain of 10⁹ to drive the pulses over 2 km of coaxial cable without in-situ amplification. Hamamatsu R5912-2 PMTs with 14 dynodes are used to achieve the high gain; they require positive high voltage (HV) at the anode. The signal at the surface is picked up via a DC blocking high-pass filter. The time-offset t₀ is measured by sending a laser-generated light pulse through an optical fiber from the surface to a diffuser ball close to the OM. The anode signal is considerably attenuated and dispersed by the 2 km cable, with typical time over threshold of 550 ns and rise time of 180 ns. A correction for the amplitude-dependent offset of the time when the signal exceeds a fixed threshold (time slewing correction) is applied off-line, resulting in a time jitter of roughly 5 ns. Coaxial cables have been replaced by twisted-pair cables for the 216 OMs deployed on six strings (strings 5-10) in 1996/97. Both attenuation and dispersion were significantly reduced compared to coaxial cables. The rise time decreased from 180 ns to 100 ns. Some test-OMs were instrumented to transmit analog optical signals through the laser calibration fibers.

In 1997/98 the array was expanded by 3 test strings (strings 11-13) that used optical fibers on all OMs, both for calibration and for analog transmission of the PMT pulses. The main goal was to evaluate this technology under realistic conditions and to compare various optical fibers and optical connectors. Electrical transmission was installed as a backup. The PMTs were operated at a gain near 10⁹. Figure 74 shows the functionality of these OMs. The fiber transmitting the calibration signal from the surface penetrates the OM. The light is directed to the diffuser ball via an optical splitter. The PMT anode current is converted into an infrared signal by an LED operating at 1300 nm, which is transmitted to the surface where it arrives essentially without dispersion. Optical fibers, connectors and penetrators are more vulnerable to the high pressures when the hole refreezes than their electrical counterparts. For electrical connectors, the losses were close to 10% for string 1-4 but, after using penetrators with an appropriate shape, this value was reduced to 3% for strings 5-10. The failures of the optical connections have been, and still are, about 10%, although half of these operate satisfactorily with attenuated amplitudes.

The advantages of the optical analog transmission compared to the electrical analog technique are:

• High bandwidth (rise time of 5 ns);
• Good double pulse resolution;
• Higher dynamic range due to lower HV;
• No correction for amplitude-dependent time slewing needed and, therefore, easier calibration;
• No pickup of electromagnetic noise, no cross-talk;
• Operation during drilling operations is possible.

The analog fiber optic signal transmission became the default technology for the expansion to AMANDA II.

Experience with AMANDA II has shown that before establishing analog fiber optical transmission as an adequate technology for IceCube, the following improvements are required: 1) the light transmitter, whether LED or laser diode, has to be operated with an independent and adjustable bias current, and local electronics in the OM must provide that function, and 2) a higher current-to-light conversion factor must be achieved. This resulted in the dAOM (digitally controlled Analog Optical Module) concept. An alternate technical approach is the DOM (Digital Optical Module), which represents a departure from traditional AMANDA techniques because the digitization of the PMT signal is performed under ice. The right part of fig. 74 gives a functional description of both. Both technologies were deployed and tested with the 1999/00 completion of AMANDA II.

The dAOM concept is depicted as second from right. A DC/DC converter in the OM converts the 60V-supply voltage to the low voltage required to operate a microprocessor and other integrated circuits. The HV is generated by a Cockcroft-Walton generator, which is implemented in the PMT base. Voltages between the cathode and the first dynode, and between the first dynode and the anode, can be set separately. Slow control and reply signals are exchanged over the electrical cable. Also, an electrical analog signal is transmitted as a backup to prevent a complete loss of signal in case of damage to the optical fiber.

Figure 74: AMANDA OMs (from left to right): electrical signal transmission, passive optical signal transmission, active optical signal transmission (dAOM) and digital signal transmission (DOM).

Rightmost, the DOM is shown, which makes use of the same HV generator as the dAOM. The waveforms are digitized in the OM and transmitted to the surface via the electrical cable. Determination of the t0 offset and the synchronization between clocks at the surface and the OM are done using the electrical cable. There is no need for a fiber cable, which results in a savings of approximately $1.8k hardware costs per channel; other significant but less direct savings, such as ∼30 fewer plane flights, deployment simplifications, etc., are also realized.

In the 1999/2000 season six strings (strings 14-19) with 42 (41) OMs each were deployed. Of the 251 OMs, 189 use an improved passive, analog optical transmission system (with electrical back-up). The PMT anode current that drives the LEDs is now amplified in the OM by a transformer circuit. The HV is supplied from the surface. The rest of the OMs use DC/DC conversion inside the OM. Twenty-three OMs were of the dAOM type, with 10 using LED transmitters and 13 using Laser Diode (LD) transmitters. Forty-one modules are of the DOM type. The DOM modules have been equipped with LEDs and fiber optic read-out for implementation in the AMANDA II DAQ. Both the dAOM and the DOM are operated at considerably lower voltages than the standard analog OMs. They did not show any significant gain loss during the year 2000.

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