The background energy content of a mode-locked Nd-glass laser is determined with photodetectors and saturable absorbers. By comparing the signal height of a fast detector with the readout of an integrating energy meter the noise energy E U2 outside the rise-time of the fast detector is measured. The background energy E Ul within the rise-time is analysed by transmission measurements through two subsequent absorber cells. The obtained mean background to pulse energy (and intensity) ratios of (E u jE v ) m = 0.015 ± 0.012 ((/ u l // o L ) m = 1.8 X 10" 4 ± 1.5 X 10' 4 ) and (E U2 /E v ) m = 0.05 ± 0.01 ((/ U2 // 0 L) m = 3 X 10** ± 6 X 10~6) indicate a high degree of mode-locking.In mode-locked lasers the radiation is concentrated in a train of picosecond light pulses. The efficiency of mode-locking is characterized by the energy content in the train of picosecond pulses compared to the background energy (noise energy) between the pulses. Various techniques were applied to determine the signal to noise ratio of picosecond pulse trains. By overexposing fast photodetectors the background signal outside the pulse width of an oscilloscope-detector system could be displayed [1,2]. The contrast ratio of two-photon fluorescence traces was used as an indication of the mode-locking quality [2,3]. The efficiency of third harmonic generation [4,5], of threephoton fluorescence [2,6] and of four-photon parametric interaction [7] allowed the calculation of the background energy content. The transmission of picosecond pulse trains through saturable absorbers of various thickness [8] was applied to estimate the peak pulse intensity and the beam noise level. A direct measurement of the pulse duration and of the background energy was carried out with a picosecond streak camera [9]. The limited dynamic range reduced the background intensity resolution in the region following the intense picosecond pulse.In this letter we describe a new technique to monitor the background energy content between two successive picosecond pulses. The background energy content E u2 outside the risetime t x of a fast photodetector-oscilloscope system is measured by comparing the signal heights of a fast detector with that of an integrating detector. The detectors are calibrated by removing the background energy with a saturable absorber of low small signal transmission TQ. The background energy E u ± within the integration time t x of the fast photodetector is analyzed by transmission measurements through two bleachable dye cells in series. In the first cell the energy transmission is reduced by background absorption, while the input into the second cell is nearly free of background and a higher energy transmission results. A comparison of the energy transmission allows to determine the background content.The situation is illustrated in fig. 1. The separated part of the pulse train with duration t s = 10 ns (equal to the cavity round trip time) is shown in a semilogarithmic plot in fig. la. As an example it is assumed that the selected part consists of a...