Generation of a Soliton Pulse Train in an Optical Fibre Using Two by asafwewe

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									Generation of a Soliton Pulse Train in an Optical Fibre
Using Two CW Single-Frequency Diode Lasers
S.V. Chernikov^, J.R. Taylor2, P.V. Mamyshev^, andE.M. Dlanov1
^General Physics Institute, 38 Vavilov Str., 117942 Moscow, Russia
2Femtosecond Optics Group, Physics Department,
Imperial College, London SW7 2BZ, UK
Abstract. Tunable repetition rate ( 80 - 130 GHz) soliton pulse trains, with pulse durations
in the range 1.5 - 3.0 picoseconds at 1.53 /rm have been derived from cw DFB diode laser
sources, amplified in an erbium doped fibre and propagated in a special optical fibre which
exhibited a decreasing dispersion over its length.
1. Introduction
The generation of simple sources of high repetition soliton pulse trains has particular relevance
to future all-optical communication systems. Of the several techniques, see for example the
references in [1], it has been proposed that the cw beat signal derived from two distinct
frequencies can be used for the generation of a cw train of solitons, at repetition rates in the
Gigahertz regime [1,2]. The transformation of the beat signal into a soliton train results from
the nonlinear propagation in a fibre exhibiting a slow amplification or decreasing dispersion
over its length. The combined effect of self phase modulation and group velocity dispersion,
influenced by a weak amplification, or a decreasing fibre dispersion, leads to an adiabatic
reshaping and pulse compression of the sinusoidal beat frequency into a train of solitons,
where the repetition frequency is determined by the frequency difference of the two cw input
modes. It has been theoretically predicted that using this technique, a cw train of high quality
solitons can be generated with a negligible pedestal component. Here we describe the
experimental realization of this technique.
2. Experimental
A schematic of the experimental arrangement is shown in figure 1.
M2y E3 0
c
0.532 /rm Pump
h
(—®
Decreasing
Dispersion
Fibre
BS2
DFB1 M
O
<p
1
T,
C~2Er Doped
DFB 2
-dHT \ B 0-0/¬
2 BSj 1, F ®
Fibre
Diagnostics
Figure 1. Experimental schematic
Chopper
Two cw DFB diode lasers operating around 1.53 ftm were used to generate the beat frequency,
the period of which could be varied by changing the stabilized relative temperature of the
devices. Amplification of the beat signal to nonlinear power levels was achieved in a single
through pass through a conventional erbium doped fibre. The nonlinear transformation was
undertaken in a specially fabricated [3], fibre which had a dispersion decreasing from 6.5
ps/nm.km to 1.1 ps/nm.km over its 2.2 km length. In order to completely suppress the effects
318
Springer Series in Chemical Physics, Vol. 55 UllraTast Phenomena VIII
Editors: J.-L. Martin ■ A. Migus ■ O.A. Mourou ■ A.H. Zcwail
© Springer-Vcrlag Berlin Heidelberg 1993
of stimulated Brillouin scattering, the output from the DFB lasers was phase modulated around
100 MHz via the device drive current.
3. Results
Input and output spectra to the tapered fibre were recorded, see figure 2(a) and (d) respectively.
In the time domain, both linear autocorrelation functions at input and output ( see figure 2 (b)
and (e) respectively ) were made using the Michelson Interferometric technique. Conventional
background-free SHG intensity autocorrelations were correspondingly taken ( figure 2(c) and
(0 ). The representative results in figure 2 were recorded at a soliton pulse train repetition rate
of 95 GHz (ie wavelength difference of cw DFB laser signals AX = 0.74 nm ). Analysis of the
results of figure 2 indicated a good fit to a train of fundamental solitons with durations of 1.8
psec with negligible pedestal component.
Gi(t) = | J E(t+-c)E*(t)dtp
G2(-c) = j'|E(t+-c)p.|E(t)|2dl
(c) 3
(b)
(a)
2
G?
G2
l
o-V
0-
0 10
® 2 AX (nm)
0 10
-2
td (psec)
td (psec)
(e)
(0
(d)
G?
G2
o-
o-
^ 2 AX(nm)
0 10
-2
0	tj (psec)
td (psec)
Figure 2. Input signal to tapered fibre (a) spectrum, (b) linear autocorrelation and (c) background free SHG
intensity autocorrelation. The corresponding measurements at the output are shown (d) - (f) respectively
4. Conclusions
The experimental generation of a train of high repetition rate solitons from a sinusoidal beat
signal through propagation in a fibre with decreasing dispersion has shown good agreement
with theoretical prediction. Through operating at various input powers and frequency
separations from those described above, trains of solitons with durations in the range 1.5-3.0
psec at repetition rates of 80 - 130 GHz were readily generated [4].
5. References
[1]	P.V.Mamyshev, S.V.Chernikov & E.M.Dianov; IEEE J.Quant. Elect. 27, 2347 (1991)
[2]	E.M.Dianov, P.V.Mamyshev, A.M.Prokhorov, S.V.Chernikov; Opt.Lett 14, 1008 (1989)
[3]	V.A. Bogatyrev et al.; J. Lightwave Technology LT9, 561 (1991)
[4]	S.V. Chernikov, J.R.Taylor, P.V.Mamyshev, E.M.Dianov; Electron. Lett. 28, 931 (1992)
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