reply to comment on "passively q-switched fiber laser that uses saturable raman gain"

September 15, 2006 / Vol. 31, No. 18 / OPTICS LETTERS 2739

Reply to Comment on “Passively Q-switched fiberlaser that uses saturable Raman gain”

Yucheng Zhao and Stuart D. JacksonOptical Fibre Technology Centre, Australian Photonics Cooperative Research Centre, University of Sydney,

206 National Innovation Centre, Australian Technology Park, Eveleigh, Sydney, NSW 1430, Australia

Received July 13, 2006; accepted July 14, 2006;posted July 14, 2006 (Doc. ID 72999); published August 25, 2006

We show experimentally that clean fundamental and Stokes RF spectra are observed in the output from apassively switched fiber laser configuration that was first demonstrated by Zhao and Jackson [Opt. Lett. 31,751 (2006)]. Stable pulse trains are shown to be produced, thus verifying that the passively switched fiberlaser operates in a stable fashion. Describing the physical mechanisms underlying the behavior as nonsta-tionary within the cavity may only partly describe the observed behavior. © 2006 Optical Society of America

OCIS codes: 140.3510, 140.3540, 140.3550.

In a previous Letter1 we reported on the characteris-tics of a pulsed multiple-wavelength fiber laser thathas the Raman and fundamental fiber lasers operat-ing within the same resonator. The passivelyswitched Raman fiber laser was pumped by acontinuous-wave laser diode and produced stablepulses at the fundamental and Stokes wavelengths.It was suggested in the preceeding Comment2 thatthe pulsing mechanism was based on a nonstationaryprocess in which the stimulated Raman process lead-ing to the pulses relies on the collision between thefundamental and Stokes pulses close to the outputend of the fiber laser. Under these conditions it wassuggested that the laser would emit transient pulsetrains with a concomitant increase in the noise andthat it would be desirable to test this prediction ex-perimentally. In this Reply, we confirm that the out-put from the fiber laser does comprise a regular pulsetrain and that the RF spectrum does exhibit a cleanline at the pulse repetition rate.

To add further experimental evidence to the prob-lem, the same experimental setup as used in Ref. 1was constructed. A low-noise 1 GHz bandwidthphotoreceiver (New Focus 1611) in combination witha 2 GHz bandwidth oscilloscope (Tektronix TDS794D) was used to record the pulse train. The funda-mental and Stokes pulses trains from the fiber laserwere recorded with durations longer than the inversepulse repetition rate. Figure 1 shows the typical andstable pulse train characteristics for the fundamentalpulses, first Stokes pulses, and second Stokes pulsesunder the same operating conditions for the resultspresented in Fig. 4 of Ref. 1. The RF spectrum of theoutput was also recorded with an RF spectrum ana-lyzer (Hewlett Packard 8591). Figure 2 shows thetypical RF spectrum of the fundamental and secondStokes emissions from the fiber laser. It can beclearly observed that indeed a clean RF spectrumline is produced that is consistent with the train ofpulses that are emitted from the laser. As expected,the RF spectrum relating to the 2nd Stokes emissionhas more harmonic components than the RF spec-trum of the fundamental emission because the pulse

0146-9592/06/182739-2/$15.00 ©

With reference to the Comment, the use of the term“Q-switching” with the traditional definition3 is de-batable in the present case, since the overall cavitylosses are not varied directly. However, it is clearthat, in the light of the new experimental evidencepresented in this Reply, the physical picture has notbeen totally clarified and that more work is required

Fig. 1. Fundamental, first, and second Stokes pulse trainsof the passively switched three-wavelength fiber laser.

Fig. 2. Measured RF spectrum of the fundamental and the

2006 Optical Society of America

2740 OPTICS LETTERS / Vol. 31, No. 18 / September 15, 2006

to fully elucidate the physical mechanisms underpin-ning the behavior of the laser.

Y. Zhao’s e-mail address isy.zhao@oftc.usyd.edu.au.

References

1. Y. Zhao and S. D. Jackson, Opt. Lett. 31, 751 (2006).2. R. Paschotta, Opt. Lett. 31, 2737 (2006).3. A. E. Siegman, Lasers (University Science, 1986).