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Biasing lasers below threshold in interconnect links yields a lower effort for the monitoring circuitry, but it leads to a significant turn-on jitter due to the bit-pattern effects and spontaneous emission. An analytical expression describing the probability density function (pdf) of the total turn-on delay for a single-mode vertical-cavity surface-emitting laser (VCSEL) biased below threshold is derived, which accounts for both bit-pattern effects and spontaneous emission. In a high speed digital transmission system both timing jitter as well as the signal-to-noise ratio (SNR) limit the system-performance, which can be measured by the resulting bit-error rate (BER). The measured BER is compared with the calculated BER yielding good agreement. Therefore, following the quite general guideline as presented here, the operation range for "below-threshold"-biased VCSEL-interconnect links can be determined.