Integrated all optical fibre source of multigigahertz soliton

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					Integrated all optical fibre source of                                             been transformed into a pulse train. The laser was pumped by
multigigahertz soliton pulse train                                                 -300mW from a Tisapphire laser operating at 980nm. The unab-
                                                                                   sorbed pump power transmitted by the laser was used to pump an
S.V. Chernikov, J.R. Taylor and R. Kashyap                                         optimised Er doped fibre preamplifier spliced to the laser output.
                                                                                   The residual pump was blocked by a filter (F). A polarisation sen-
                                                                                   sitive optical isolator suppressed back reflections into the laser
                                                                                   from the rest of the system and ensured the single polarisation
         Indexing terms: Fibre lasers, Soliton transmission, Optical               state output. The two laser frequencies bad well defined, stable,
         dispersion, Opticalfibres                                                 but generally different polarization states. A polarisation control-
         A 59.1GHz train of 2.2 ps solitons with low phase noise ( ~ X I O - ~ )   ler (PC) was introduced to equalise their relative intensities. Such
         is generated by a novel, passive, all optical fibre source. The           a compact integrated combination of laser and preamplifier
         technique is based on transformation of a dual frequency beat             (MOPA) provided an efficient use of the pump power [E]. The
         signal generated by a 16kHz linewidth erbium fibre DBR laser              amplified dual frequency signal at the output of the MOPA was as
         having a selectable and highly stable frequency separation, into a        high as 5mW.
         soliton train in a comblike dispersion profiled fibre, a novel fibre         The MOPA output was then amplified in an Ermb doped fibre
         design which uses conventional optical fibres.                            amplifier pumped by an NdYAG laser. The E r N b fibre had a
                                                                                   length of 10m with concentrations of Er = 300 p p m and Yb =
Simple and reliable sources of optical pulses capable of operating                 10000 ppm, and a cutoff wavelength of 1.25pm. For 600mW of
at high repetition rate in the 10-1000 gigahertz range beyond the                  pump power the amplifier provided -190mW of amplified power.
limits of conventional electronics are of considerable interest for
the next generation of optical telecommunication, optoelectronics
systems and optical computers. In this Letter, we report a 6OGHz                         :20,
soliton pulse source which uses all optical fibre technology. The                        5   15
principle of operation is based on an all optical technique using an                     a
optical beat signal as a source of a periodic optical signal, and the                    L 10
transformation of the beat half-periods into optical pulses using                        0
nonlinear propagation in optical fibre [1,2]. This approach has
been previously applied in several schemes where the beat signal
                                                                                         a 5
was transformed into a pulse train due to adiabatic conversion in                        g o
a special dispersion decreasing fibre [ 2 4 (or alternatively, in a                                        2000       4000        6000          8000
stepwise dispersion profiled fibre [2] or fibre with distributed
                                                                                                                    length, m
amplification [1,2]), the Raman self-scattering of a beat signal [5] ,
and switching in a nonlinear fibre loop mirror [6]. Generally in the               Fig. 2 Dispersion profile of comblike dispersion profiledfibre designed
experimental realisation, two independent DFB laser diodes were                    and used in experiment
used to generate beat signals. Our novel source uses a compact
and stable, self-contained dual-frequency erbium fibre laser incor-                   For the generation of the pulse train, a special fibre which we
porating fibre grating reflectors which provides narrow linewidths                 called a comblike dispersion profiled fibre, was designed. This
and high stability of frequency separation. A new technique for                    fibre consisted of segments of dispersion shifted (DSF) and stand-
transformation of the beat signal into soliton pulse train is demon-               ard telecommunication fibres (STF) spliced together alternating
strated. The technique is based on implementation of a new type                    (DSF-STF-DSF-STF. ..) Thus, the dispersion profile had a
of optical fibre, comblike dispersion profiled fibre (CDPF), which                 comb-like form as shown in Fig. 2. We used commercially availa-
was designed using conventional, standard optical fibres. This all                 ble DSFs. Their absolute dispersion at 1545nm was less than Ips/
optical fibre passive source generated highly stable, extremely low                nmikm. The measured splicing loss was -0.1 dB/splice. The princi-
phase noise train of soliton pulses.                                               ple of operation of this fibre consists in spatially separating the
                                                                                   effects of fibre nonlinearity and dispersion. When the pulses prop-
                    E r / Yb fibre                                                 agate through DSF the self-phase-modulation effect dominates
                                              comb 4 k e dispersion
                         amplifier              profiled fibre                     leading to their frequency modulation. Propagating through STF
        MOPA                                                                       (. 17ps/nm/km at 1545nm), the effect of anomalous group velocity

    r                                                                              dispersion provided the pulse compression and chirp compensa-
                                                                                   tion. Thus, comblike dispersion profiling allows control of the

                                                                                   pulse propagation in the fibre. Our numerical simulations demon-
                                                                                   strated that CDPF is appropriate for reshaping of a beat signal
                                                                                   into a high quality train of transform limited soliton like pulses.
        pump              dual - frequency                                         An optimised fibre design was found for a particular input power
                          doped fibre laser            F   150
                                                                                   and beat frequency. The dispersion profile of the CDPF fabricated
             WDM                         Er-dopedU                                 and used in the experiment described here is shown in Fig. 2.
           980/1550                         fibre                                     Additionally, the generation of a CW pulse train in optical fibre
Fig. 1 Integrated all opticalfibre configuration of source                         through the optical beating technique requires the suppression of
                                                                                   the stimulated Brillouin scattering (SBS) [3]. In previous experi-
   The source had a totally integrated configuration incorporating                 ments the SBS suppression was achieved by broadening the laser
three key components: a dual frequency erbium fibre laser with                     linewidths. Using CDPF, the SBS can be easily suppressed ena-
integrated master oscillator-power amplifier (MOPA), EriYb                         bling the use of a narrow linewidth dual-frequency laser having
doped fibre amplifier and CDPF, as illustrated in Fig. 1. A dual                   extremely low phase noise. For SBS suppression, the segments of
frequency beat signal was generated by a single coupled cavity                     CDPF were chosen to have different germanium concentration
erbium fibre laser [7]. The laser was formed by four intracore fibre               resulting in different SBS frequency shifts and suppression of SBS
grating reflectors holographically written in optical fibre. This                  growth through the system [9].
compact integrated laser of total length 16.5cm provided very sta-                    The SHG autocorrelation trace and optical spectrum character-
ble dual frequency operation (without mode hops) at 1545nm with                    ising the generated pulse train are shown in Fig. 3. The autocorre-
a selected frequency separation of V = 59.1GHz (0.47nm). The                       lation function consist of a periodical train (period T = 16.9~s) of
average linewidth was -16kHz, and the stability of frequency sep-                  well separated pulses without pedestal or background (Fig. 3a).
aration was better than A V = 3MHz (limited by the resolution of                   Individual pulses are well fitted by sech' shape pulses with dura-
the measuring instrument). The phase noise parameter AViV char-                    tion AT = 2.2ps as shown in Fig. 36 by the dashed line. The corre-
acterising the relative fluctuations of the beat signal period, is                 sponding mark space ratio ( M S R = A d 7 J was as high as 1:7.7. In
extremely low in this dual frequency laser (less than 5 x        This              optical spectrum (Fig. 3c), the shape and relative intensities of
is an important issue characterising this laser. because the beat fre-             spectral components are in good agreement with a train of soliton
quency noise corresponds to timing jitter when the beat signal has                 pulses with this MSR.
                                                                                      In conclusion, we have demonstrated a novel integrated all

1788                                                       ELECTRONICS LETTERS                        30th September 1993            Vol. 29         No. 20

                                                                                   6   CHERNIKOV, s.v.,and TAYLOR. J R : ‘MultigigabiVs pulse source
                                                                                     based on switching of an optical heat signal in a nonlinear fibre
                                                                                     loop mirror’, Electron. Lett., 1993, 29, (S), pp. 658460
                  60                                                               7 CHERNIKOV, s v , KASHYAP, R., and TAYLOR, J.R.: ‘Coupled cavity
         ;8                                                                          erbium fibre lasers incorporating fibre grating reflectors’, submitted
         5:                                                                            to Opt. Lett.
                                                                                   8 BALL,G.A, MOREY. ww.: ‘Narrow linewidth fibre laser with
         2 ? 30
         Gp                                                                          integrated master oscillator - power amplifier’. Conf. Optical Fibre
         05                                                                          Communication, 1992, Vol. WA 3, pp. 97
         5s                                                                        9 TKACH, R w., CHRAPLYVY, A R. and DEROSIER, R.M.: ‘Spontaneous
                                                                            0        Brillouin scattering for single-mode optical fibre characterisation’,
                   a                           time deloy.Ps                         Electron. Lett., 1986, 22, (19), pp. 1011-1013
     ~        I         z          o       y      I            ;~‘      :

     0 1

                                                                                   Phase stepping in projected-fringe fibre-
                  -20       -10        0   10     20                 1545          based moire interferometry
                                                        (0 5nmldivision)           J.D. Valera and J.D.C. Jones
Fig. 3 SHG background free autocorrelation traces on tw’o time scales
and spectrum of generated soliton pulse train
                                                                                            Indexing term: Light interferometry
The ulse train period is 16.9s corresponding to a repetition rate
69.1~ H ZThe wavelength separation between spectral components is
            .                                                                               An optical fibre based moire interferometer for surface shape
0.47nm. The dashed line in b represents a computer calculated fit of                        measurement is described. The technique uses an optical fibre
the experimental curve by an autocorrelation trace of a sech’ shape                         interferometer to project interference fringes and is readily phase
pulse with duration -2.2~s
                                                                                            stepped without mechanical movement of components.
a h SHG background free autocorrelation pulses
c’ Spectrum of generated soliton pulse tram
                                                                                   Moire interferometry is a well established technique for the meas-
                                                                                   urement of shape, static deformation and vibration analysis with
optical fibre source of a 59GHz train of 2.2ps soliton pulses. The                 sensitivities ranging from a fraction of a millimetre to centimetres
generated pulse train possess an extremely low phase noise (< 5 x                  [I]. The technique involves illuminating the test object with struc-
     owing to the use of the very stable dual frequency laser as the               tured light. For example, an interferometer may be used to pro-
source of periodic optical signals. A comblike dispersion profiled                 duce straight, parallel interference fringes which are used for
fibre was introduced for the first time and used for soliton pulse                 illumination. Viewed on the object, these straight fringes are mod-
generation. Such a type of fibre which can be easily fabricated                    ified because of object shape, deformation or vibration. An image
based on commercially available standard fibres, will find further                 of the illuminated object is formed on a reference grid (for exam-
applications for the control of pulse propagation in soliton                       ple, a Ronchi ruling) and gives rise to a moire pattern from which
systems. The technique demonstrated can be used for the                            the shape, deformation or vibration of the object can be inferred.
generation of pulse trains for a wide range of readily selectable                     Phase stepping is a fringe analysis technique used to recover
repetition rates from tens of gigahertz to the subterdhertz range.                 surface shape information from the moire pattern. Phase stepping
                                                                                   has been accomplished by in-plane translation of the reference rul-
Acknowledgments: This work was in part supported by the SERC                       ing [2]. However, accurate mechanical translation of the ruling
(UK). The E r N b fibre amplifier was supplied by the IRE-POLUS                    without rotation is critical. Phase stepping with projected interfer-
(Russia). We are pleased to acknowledge LYCOM A / S (Denmark)                      ence fringes may also be achieved by mechanical movement of one
for providing precisely specified dispersion shifted fibres used in                of the mirrors within the interferometer.
the experiment. We also thank B.J. Ainslie and D.L. Williams for                      We report an optical fibre based moire system that facilitates
provision of the photosensitive fibres and P.S. McKee, all of BTL,                 phase stepping. An optical fibre interferometer is used to produce
for generation of the phase masks used in grating production                       high quality projected fringes. A voltage controlled fibre optic
                                                                                   phase modulator is used to achieve phase stepping without intro-
0 IEE 1993                                          10 August 1993                 ducing misalignment or requiring mechanical translation of bulk
Electronics Letter Online No: 19931113                                             optic components.
S. V. Chernikov and J. R. Taylor (Femtosecond Optics Group, The                       In the basic form of moire interferometry the light illumination
Blackett Lahoratory, Imperial College, Prince Consort Road, London                 has a plane parallel structure in the plane normal to the optical
S W7 2BZ. United Kingdom)                                                          axis. The structure is transformed by the surface shape of the test
R. Kashyap (British Telecom Research Laboratories, Martlesham                      object. The illuminating straight line pattern can be obtained by
Heath, Ipswich IP5 7RE. United Kingdom)                                            projecting a Ronchi ruling into the surface or by illuminating with
                                                                                   two coherent plane waves (at an angle) that give rise to a straight
References                                                                         line interference pattern. The deformed straight line pattern is
                                                                                   imaged onto a Ronchi ruling of similar period to give rise to
1   DIANOV, E.M ,       MAMYSHEV, P v ,        PROKHOROV, A M.,          and       moire fringes that contour the surface shape.
    CHERNIKOV, s v : ‘Generation of a train of fundamental solitons at
                                                                                       The light transmitted through the reference Ronchi ruling is
    a high repetition rate in optical fibre’, Opt. Lett., 1989, 14, (18), pp.      given by the product of the ruling transmittance and the image of
    1008-1 0 10
2   MAMYSHEV, P v , CHERNIKOV. s v , and DIANOV. E M : ‘Generation of
                                                                                   the test surface when illuminated with a straight line interference
    fundamental soliton train for high-bit-rate optical fibre                      pattern. This product has several terms related to the surface
    communication lines’, IEEE J Quantum Electron., 1991, QE-27,                   shape and the periods of the projected fringes and the reference
    (IO), pp. 2347-2355                                                            Ronchi ruling [3]. The term of interest is the one that contours the
3                 sv,
    CHERNIKOV. TAYLOR, J.R., MAMYSHEY, P V , and DIANOV, E M :                     surface shape. This moire fringe term is represented by
    ‘Generation of soliton pulse train in optical fibre using two cw
    single-mode diode lasers’, Electron. Lett., 1992, 28, pp. 931-932                                  I ( z , y ) = 1+Vcos[(2n/p)ttanO]                 (1)
    PAYNE, D N : “iOGbit/s fibre based             of fundamental solitons         where Vis a constant, p is the grating period, 13 is the illumination
    at 1550 nm’, Electron. Lett., 1992, 28, (13), pp. 1210-1212                    angle and z is the height of a point on the object conjugate to (x,y)
5   CHERNIKOV, s v , RICHARDSON. D J , LAMING, R I , DIANOV, E.M , and             in the image plane, measured relative to a reference plane parallel
    PAYNE D N : ‘114Gbitis soliton train eeneration throueh Raman                                                               _.
                                                                                   to the obiect surface. Ean. 1 incoruorates auuroximations valid in
    self-scattering of a dual frequency beat-signal in dispersion                  the experiment the grating period and the fringe spacing imaged
    decreasing optical fibre’ App Phy L e t t , 1993, 63, pp 293-295

ELECTRONICS LETTERS 30th September 1993                                         Vol. 29     No. 20                                                      1789

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