The mainframe of the PDL 828 holds the power supply and the computer interface via USB bus and provides space for the necessary modules. The number of slots for the modules depends on the version of the PDL 808:

• The short version provides space for one oscillator module and two laser driver modules.
• The large version provides space for one oscillator module and up to eight laser driver modules.

The oscillator module has eight (individual) trigger outputs (channels) that can each be addressed individually. For example, channels can be combined to be activated at the same time or each channel can be individually activated in a sequence. To enable burst operation, multiple pulses can be output from one channel (or from combined channels) before the next channel becomes active. Additionally, the oscillator has its own synchronisation output, an external trigger input as well as an auxiliary input and output connector.

Working principle

Generally speaking, the oscillator module works in a rotary fashion, which means that the programmed sequence of channel (C) 1 through channel 8 must be completed before channel 1 becomes active again as illustrated in the figure below.

All channels can therefore be activated individually in a sequence. Adjacent channels can also be combined for simultaneous operation. The channels can, however, not be combined in any arbitrary fashion as the rotary working principle must be preserved.
The auxiliary input can be used to inhibit the oscillator to start the output period, i.e. to "start" the output on channel 1. If the signal is provided during the rotation through the channels, one full period will be finished nonetheless. The auxiliary output is used to signal the start of a period. It is active when a period is finished (i.e. if channel 8 has been active) and a new one is about to start.This special time is defined as the moment, when the the first pulse is output from channel 1.

Repetition rate

The oscillator module of the Sepia II provides a wide range of user selectable repetition rates to generate pulses at the eight output channels of the module. In principle all repetition rates are derived from an internal crystal oscillator along with a frequency divider. The oscillator module has three internal user-selectable crystal oscillators with base frequencies (F) of 80, 64 and 50 MHz. Each base frequency can be further reduced by division (D) though any integer value between 1 and 255. The highest repetition rate is therefore 80 MHz and the lowest repetition rate is 50 MHz/255~196 kHz. Instead of using one of the three internal oscillators, it is also possible to provide an external trigger signal. The frequency divider is also active on external trigger signals thereby allowing virtually all repetition rates between single shot and 80 MHz.

Defining bursts and combining channels

The oscillator module allows to output any number of pulses between 1 and 16772215 (16.7 million) to one channel (C) before the next channel becomes active (bursts - B). Adjacent output channels can also be combined to emit pulses at the same time, if the burst length is set to 0.

The output of each channel can be enabled or disabled. This does, however, not mean that the channel is "eliminated" from the rotary working principle. Instead, the programmed number of pulses is still processed, but simply no signal is present at the output. This is useful to insert time gaps between two bursts.

The synchronization signal

Each pulse that is output at any of the eight channels can be accompanied by a synchronisation signal (S) at the sync out connector of the oscillator module. This synchronisation signal can be used to control external hardware like e.g. provide the start pulse for a TCSPC system. It can be enabled and disabled individually for each channel. One special feature of the Sepia II is that the synchronisation signal can be time shifted at lower repetition frequencies. If the divider ratio is >1, the sync pulse can be moved within the main clock period (the divided signal) in steps of the crystal oscillator's period (pre-sync - P). If the pre-sync is set to a value equal or higher than the divider, the synchronisation signal will be disabled.

An additional option to influence the synchronisation signal is to mask (M) a defined number of pulses. Masking in this context means that no synchronisation signals are output. The number of omitted pulses can be set to any integer value between 0 and 255.

The sync-mask can also be inverted (iM), i.e. not the number of omitted pulses is specified, but the number of actual output pulses. Again, the number of inverted masked synchronisation pulses can be set to any integer value between 0 and 255. Of course, a value of 0 leads to no synchronisation output at all.

A laser driver module is necessary for each laser head that needs to be connected and controlled at the same time by the Sepia II. Besides the connector for the laser head, the module provides its own synchronisation output, a trigger input and two gating inputs (fast and slow gate).

Triggering modes

Each Laser Driver Module can operate either independently from the other modules driven by its internal oscillator at six fixed, user-selectable frequencies (80, 40, 20, 10, 5 or 2.5 MHz) or synchronized to each other when using the external trigger signal from the oscillator module or any other source of a NIM compatible signal.

Emission intensity

The intensity of the laser emission is controlled via an internal voltage in steps of 1% of its full scale value. However, due to the non-linear behaviour of laser didoes, the voltage setting does not linearly correspond to the output power, i.e. a voltage of 50% does not correspond to 50% of the maximum output power.

A special feature of the Sepia II is to switch between cw and pulsed mode with the new LDH-D-C Series of laser heads. Of course, older laser heads can also be driven by the Sepia II.

Gating functions

For special applications like scanning devices, the laser driver module has two gating functions which allow to suppress the laser emission by an external signal: A slow gate, that reduces setting times of the laser heads to a minimum at slow on / off periods (seconds) and a fast gate, that can perform transitions within nanoseconds, i.e. in between two pulses. The fast gate also provides high pulse stability when the on / off signal is periodic and fast (milliseconds).