University of Wisconsin-Madison

First Year Performance Paper - Section 2.1

2.1 The Digital Optical Module (DOM)

The Digital Optical Module is the fundamental detector element of IceCube. It consists of a 25 cm diameter R7081-02 Hamamatsu Photo-multiplier Tube (PMT) and a suite of electronics board assemblies contained within a 35.6 cmdiameter glass pressure housing (Fig. 1). The DOM achieves high accuracy and a wide dynamic range by internally digitizing and time-stamping the photonic signals and transmitting packetized digital data to the surface. This scheme, which was first demonstrated in AMANDA String-18 [23], allows each DOM to operate as a complete and autonomous data acquisition system. Each DOM can independently perform functions such as PMT gain calibration, time calibration with the master clock system (section 3), data packaging and response to remote commands for change of configuration.

A schematic view of an IceCube Digital Optical Module.
A schematic view of an IceCube Digital Optical Module.

Since the DOMs are inaccessible once deployed in the deep ice, they have been designed to operate reliably for 15 years in a cold, high-pressure environment. High-reliability commercial parts were used where possible, and all subassemblies underwent a rigorous screening process involving functional and environmental stress testing, and/or inspection before being integrated into a DOM. In the remote environment, energy is expensive, so power minimization is another key concern. The DOM derives its internal power, including the PMT high voltage, from the nominal ±48V DC supplied by the cable. Under normal operating conditions, power consumption is about 3.5 Watts/DOM. All communication with a pair of In-Ice DOMs occurs over a single twisted copper pair; this includes power distribution, bidirectional data transmission, and timing calibration signals.

Most of the electronics reside on the Main Board (MB), which holds the analog front-end and two digitizer systems. The fast digitizer system uses a 128-sample switched-capacitor array, implemented in a custom Analog Transient Waveform Digitizer (ATWD) chip, which can run between 200 and 700 megasamples per second (MSPS). The ATWD sampling frequency is controlled with a digital-to-analog converter (DAC). A calibration procedure measures the ATWD sampling rate by capturing the MB oscillator 20MHz signal and counting the ATWD bins between oscillation cycles at several sampling frequencies, resulting in a linear relation between sampling frequency and DAC setting. Two ATWDs are used in a ping-pong fashion to minimize dead-time. The slow digitizer system uses a commercial flash ADC, operating at 40 MSPS digitizer, and allows a capture window of 6.4μs.

The digital functions on the MB are performed by a 400K-gate Field Programmable Gate Array (FPGA) containing a 32-bit ARMTM CPU, 8 MBytes of flash storage, and 32 MBytes of random access memory. Aside from a small non-volatile boot-up program, the operating parameters of the DOM, FPGA code, and ARM software are all remotely reconfigurable. Timing on the Main Board is controlled by a 20 MHz quartz oscillator, which is doubled to 40 MHz.

Within a DOM, data acquisition is initiated when the PMT signal exceeds a programmable threshold, typically 0.3 photo-electrons (PE) for DOMs in the ice. This trigger is given a coarse time stamp by the 40 MHz local clock. The trigger initiates acquisition of data by the ATWD and capture of data from the 40 MSPS ADC, 10 bit parallel output pipeline ADC. The ATWD collects 128 samples of 10-bit data, and the commercial ADC collects 256 samples of 10-bit data. To capture the entire waveform, before and after the trigger was issued, the signal to the ATWD is delayed 75 ns, using a 11.2 m long stripline on a separate, dedicated circuit board. The delayed signal is split among three (of 4) input channels of each one of the ATWDs with gains differing by successive factors of 8. In this manner the digitizers cover the entire dynamic range of the PMT, which is linear up to currents of 400 PE/15 ns. The fourth ATWD channel is used for calibration and monitoring.

The ATWD sampling speed is variable and is currently set at 3.3 ns/sample, allowing acquisiton of 422 ns long waveforms. The precise timing of a signal is determined from the waveform referenced to the coarse time stamp. This is supplemented by timing information from the 40 MSPS ADC, which continually samples the amplified and shaped output of the PMT. The DOMs can operate in one of several local coincidence modes, to reduce the noise-trigger-related data traffic to the surface. String 21 currently operates in a nearest-neighbor local coincidence mode, in which a local coincidence pulse is transmitted from a DOM to its immediate neighbors, above and below, whenever its discriminator fires. The DOM transmits its data to the surface only when it receives a local coincidence pulse within 800 ns of the trigger, signaling that at least one of its neighbors also had a trigger.

The flasher-board is an optical beacon integrated into each DOM. It contains 12 gallium nitride LEDs pointing radially outward from the DOM, 6 of which horizontally and the other 6 pointing upwards at an angle of 48°. Each LED is capable of producing 1×107 to 1×1010 photons per pulse with peak wavelength in the range of 400 nm - 420 nm at a repetition rate up to 610 Hz. Special operating modes allow an arbitrary ensemble of DOMs to be scheduled to emit optical beacon signals into the ice for (1) calibration of local coincidence, timing and geometry; (2) inter-string timing and geometry calibration; (3) verification of optical properties of the ice; (4) linearity calibration of surrounding DOMs; (5) high energy cascade calibration; etc. A more detailed description of the operation of the DOM and its components is given elsewhere [24,25].