Abstract
Coupled-cavity mode-locked (CCM) NaCl:OH− lasers are attractive sources of high power, broadly tunable, femtosecond pulses around 1.55 μm. Although selfstarting operation has been reported1,2 synchronous pumping using a cw mode-locked pump laser is desirable as it relaxes the power requirements for starting. However, when synchronously pumped, the output from the CCM NaCl:OH− laser is amplitude modulated.2 The origin of this modulation is from the passive nature of coupled-cavity lasers. The synchronous pumping serves only to initiate mode-locking after which the laser pulse frequency is determined by the cavity length. This frequency can be different from the mode-locked pump frequency, and therefore the CCM laser pulses walk through the pulse train from the pump laser. The upper state lifetime of NaCl:OH− is 150 ns, so in 12.5 ns (the period of the pump laser pulses) the small-signal gain decreases by −8%. This gain modulation is sufficient to cause a sawtooth modulation of the laser output. In this paper we describe a method of eliminating the modulation by referencing the pulse repetition frequency (hence the cavity length) of the color center laser to that of the mode-locked pump laser.
© 1992 Optical Society of America
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