An optical RZ transmitter comprises an optical signal source and a pair of electro-optical modulators in tandem, one arranged to receive a NRZ electrical data signal and the other a clock signal at the data rate of the data signal. The electrode guide comprises an internal passage profile that transitions along the machining axis from a substantially circular cross section comprising a diameter d2 to an apexed cross section. Once the sleeve is in place, a hose, to introduce the explosive charge, is lowered through the sleeve to bottom of the borehole.