May 1, 2004 / Vol. 29, No. 9 / OPTICS LETTERS 983

Continuous-wave, totally fiber integrated optical parametricoscillator using holey fiber

C. J. S. de Matos and J. R. Taylor

Femtosecond Optics Group, Imperial College, Prince Consort Road, London SW7 2BW, UK

K. P. Hansen

Crystal Fibre A/S, Blokken 84, DK-3460 Birkerod, Denmark

Received August 19, 2003

We present, for the first time to our knowledge, a cw, all-fiber optical parametric oscillator that uses a holeyfiber. The oscillator operates at 1.55 mm and can yield an oscillating parametric signal that consists of asingle line with a 30-dB extinction ratio and a 10-pm linewidth or that consists of multiple lines. In additionto the signal and the idler, five other pairs of spectral lines can be observed that are due to multiple parametricinteractions. The source reaches threshold for a pump power of 1.28 W and saturates for pump powers inexcess of �1.6 W. © 2004 Optical Society of America

OCIS codes: 060.2320, 190.4380.

Optical parametric oscillators have received consider-able attention throughout the years and have foundapplications as coherent sources at wavelengths atwhich practical conventional lasers are unavailable.1

These sources generally utilize nonlinear crystals asgain media and bulk optics elements, which requirecareful alignment for optimal operation. In fiberoptical parametric oscillators2 – 8 (FOPOs) crystals arereplaced with optical f ibers and the configurations canbe made totally f iber integrated, virtually eliminatingthe need for alignment and allowing for increased com-pactness. However, to date FOPOs have presentedtwo major limitations: As the nonlinearity of conven-tional fibers is considerably lower than that of crystals,most FOPOs reported used pulsed pumps such thathigh peak pump powers could be achieved and theparametric threshold reached. In addition, as insingle-mode fibers, the parametric process is uniaxial,and phase matching between pump and signal wavesis determined solely by the fiber’s dispersion profileand by the pump power through the f iber’s nonlinearrefractive index. In particular, it can be shown3 thatthe gain that is available is generally higher whenthe pump is in the anomalous dispersion region andthat a wider gain bandwidth is achieved with thepump near the fiber’s zero-dispersion wavelength.These gain characteristics have normally limitedthe operation of FOPOs to wavelengths longer than�1.3 mm, where the zero dispersion of silica is located.

Recently, FOPO configurations were reported thatsomewhat overcome one or the other of the above-mentioned limitations.4 – 7 In one experiment4 a cwFOPO was demonstrated at �1.55 mm through the useof a f iber with a small core diameter that resulted in in-creased pump intensities and therefore in a decreasedparametric threshold. In other experiments,5 –7 holeyor tapered fibers that present anomalous dispersionat wavelengths shorter than �1.3 mm were usedto achieve FOPO operation at �0.75 (Ref. 5) and�0.50 (Refs. 6 and 7) mm. However, in the latter

0146-9592/04/090983-03$15.00/0

experiments pulsed pumps were used and bulk ele-ments were present in the cavities, increasing theexperimental complexity. Another holey-fiber-basedFOPO was demonstrated8 that used 10-GHz pumppulses at 1.55 mm.

In this Letter we demonstrate, for the first time tothe best of our knowledge, a cw FOPO that uses a holeyfiber (HF). The FOPO was totally f iber integrated ina ring cavity configuration and operated at 1.55 mm.As many as six pairs of parametrically generated spec-tral lines were observed; the center of each pair wasthe pump wavelength. The main parametric signalpresented an �10-pm linewidth and carried �38% ofthe output power. Threshold was reached for a pumppower of 1.28 W.

The experimental conf iguration of the cw FOPOis depicted in Fig. 1. The pump consisted of anerbium-doped fiber amplif ier (EDFA) that yieldsoutput powers of as much as 3.5 W and was seeded byan external-cavity semiconductor laser, tunable from1500 to 1580 nm, with a nominal linewidth of 50 MHz.

Fig. 1. Experimental configuration of the all-f iber para-metric oscillator using HF.

© 2004 Optical Society of America

984 OPTICS LETTERS / Vol. 29, No. 9 / May 1, 2004

To suppress stimulated Brillouin scattering in theFOPO gain fiber we phase modulated the tunablelaser at 100 and 982 MHz in a single lithium niobatemodulator.

The FOPO cavity comprised a unidirectional ringformed by the gain fiber, an output coupler extracting1% of the cavity power, a polarization controller (PC),a broadband (.400-nm) optical circulator, and a fiberBragg grating (FBG). The gain fiber was a 100-mHF with a 2.3-mm core diameter and an estimatedmodal area of 7 mm2. It was directly spliced to thering cavity; the lowest splice loss was 0.42 dB. TheHF had dispersion and attenuation values at 1.55 mmof 34 ps nm21 km21 and 12.4 dB km21, respectively,and presented some birefringence because of itscross-sectional profile, also shown in Fig. 1. TheFBG ref lected 97% of the light at 1550.6 nm and hada 3-dB bandwidth of 0.8 nm. The pump was insertedinto the cavity through a circulator following the FBG,which had a ref lectivity of less than 0.1% at the pumpwavelengths used. As the attenuation between thetwo circulator ports connected to the ring was �40 dB,the experimental arrangement ensured that only thesignal resonant with the FBG was fed back into thecavity.

The cavity round-trip loss was measured to be10.6 dB and was due mainly to the HF’s attenuationand splice losses and to the double-pass loss in theoptical circulator. Because of this high loss, theeffective f iber length over which the resonant signaltravels is expected to approach the single-pass HFlength. However, as shown below, feedback plays animportant role in the FOPO dynamics.

The HF parametric properties can be theoreticallypredicted by use of the f iber parameters quotedabove. We estimated the nonlinear coefficient to be�16 km21 W21 by taking the nonlinear refractiveindex to be �2.7 3 10220 m2�W, as in typical pure-silica polarization-preserving f ibers.9 By using theequation for power gain in an unsaturated parametricamplifier given in Ref. 3 it is possible to estimatethe HF’s single-pass gain profile. For this purposewe neglected higher-order dispersion components andused a HF effective length of 87 m that takes into ac-count its attenuation.9 We found that the single-passparametric gain should equal the ring loss (FOPOthreshold condition) for a postcirculator pump powerof �1.5 W. Furthermore, a pump power of 1.9 W isexpected to give an unsaturated peak gain of 14.4 dBat wavelengths shifted by 1.4 nm from the pump, with3-dB gain bandwidths measuring 1.1 nm. Clearly,as the FOPO reaches threshold the gain becomessaturated and the gain profile changes somewhat.

Experimental output spectra of the FOPO underdifferent conditions can be seen in Fig. 2. Figure 2(a)shows the case of maximum conversion eff iciency, ob-tained for a pump wavelength of 1552.1 nm and a post-circulator pump power of 1.88 W. Six pairs of linescan be observed as a consequence of multiple four-wavemixing. The recirculating signal at 1550.8 nm isshown at the left of the pump and has respectivelinewidth and extinction ratios of �10 pm and �30 dB.As the optical spectrum analyzer used in the mea-

surements had a 10-pm resolution, the true signallinewidth can be slightly narrower. We obtained sucha narrow linewidth by operating the FOPO toward theedge of the FBG spectrum. Also, it is believed thatsignal recirculation led to further linewidth narrowing.The signal output power was 1.3 mW, correspondingto �38% of the total output power and indicatingthat strong pump depletion was achieved. The idlerappears to the right of the pump, at 1553.3 nm, with alinewidth of �60 pm and an output power of �1.1 mW.Note that the relatively low output powers achievedare due to the extraction of only 1% of the cavitypower. The conversion efficiency in the gain fiber,defined as the ratio between the idler power at the HFoutput and the pump power at the HF input, is �15%.As a comparison, the single-pass spectrum obtainedwith the same pump power launched in the HF is alsodepicted in Fig. 2(a). It is clear that the feedback iscrucial for obtaining spectrally narrow lines with highconversion efficiencies.

It was found that, for certain polarization states,modulation was visible across the spectral profile ofthe FOPO. It is believed that this feature resultedfrom having the polarization of both the pump and therecirculating signal at an angle to the HF’s principalaxes. The HF then acts as a Mach–Zehnder in-terferometer, with the HF’s principal axes acting asinterferometer arms. As light leaves the HF, couplingbetween the orthogonal polarizations occurs becauseof perturbations in the non-polarization-preserving

Fig. 2. Output spectra (a) of the FOPO (solid curve) andof the HF without feedback (dashed curve) adjusted for thehighest conversion eff iciency and (b) of the FOPO adjustedto yield birefringence-induced multiline operation.

May 1, 2004 / Vol. 29, No. 9 / OPTICS LETTERS 985

Fig. 3. Signal and idler output powers as functions ofpostcirculator pump power.

fibers. It can be shown that the wavelength spacingbetween the Mach–Zehnder transmission maxima isDl � l2�LB, where l is the signal wavelength, L is theHF’s length, and B is the HF’s degree of birefringence.This transmission modulation allowed for multilineoscillation to be achieved within the pump’s paramet-ric gain band by adjustment of the PCs. Figure 2(b)shows an example of such a condition, in which threespectral lines of similar power were obtained withinthe FBG’s ref lection spectrum. Three idler lines arealso obtained, as expected. The spectral line spacingis �0.21 nm, indicating a HF degree of birefringenceof 1.15 3 1024, which is comparable to that obtainedin polarization-preserving f ibers.9

The signal and idler output powers as a function ofpostcirculator pump power are shown in Fig. 3 for apump wavelength of 1552.0 nm and for polarizationstates that yield a single recirculating signal line.The parametric threshold is reached for a 1.28-Wpump power, above which a signal slope efficiencyof 3.3 3 1023 was observed. The �17% mismatchbetween theoretical and experimental threshold pumppowers can be attributed to errors in the rough esti-mates that led to the calculation of the HF’s nonlinearcoeff icient. It can be seen that gain saturation startsat a pump power of �1.6 W, indicating that furtherincreasing the pump power would not result in muchhigher output powers. Higher output power would beachieved if the cavity loss were lowered and the outputcoupling ratio increased.

The temporal characteristics of the FOPO wereinvestigated with a 15-ps rise time detector in asso-ciation with a 50-GHz oscilloscope and an electricalspectrum analyzer. As expected, the source wasfound to be cw, except for minor amplitude modula-tions of �5% or less at the frequencies with which thepump was phase modulated.

It was found that the peak parametric conversionefficiency occurred for a pump–signal separation of1.3 nm and that the source was tunable over 1.5 nm.

These values are in good agreement with the theo-retical parametric gain band characteristics discussedabove. Because of the small HF modal area, thistunability is �3 times larger than that expected ifa conventional fiber were used. Larger tunabilitywould be achieved through the use of a HF with alower dispersion value.

The fiber integrated ring configuration describedhere can also be used with a highly nonlineardispersion-shifted f iber as the gain medium. Sucha scheme would benefit from the lower attenuationsobtained in these f ibers. However, it is importantto note that HF-based FOPOs can operate at a widerwavelength range and that the experiment describedhere points to the feasibility of cw, all-f iber-formatFOPOs below 1.3 mm because all components usedhave counterparts in that spectral region.

In conclusion, an all-f iber, cw optical parametric os-cillator at �1.55 mm based on holey fiber was pre-sented and yields a 30-dB extinction ratio output signalthat has a 10-pm linewidth and carries �38% of thetotal output power. In addition, as many as six pairsof spectral lines generated from multiple parametricprocesses are visible in the output spectrum. Multi-line signals directly generated within the pump para-metric gain band can also be obtained if the FOPOpolarization is properly adjusted. The present demon-stration indicates that all-f iber cw parametric oscil-lators can now operate in the spectral region below1.3 mm with the use of holey f ibers.

C. J. S. de Matos is supported by Coordenação deAperfeiçoamento de Pessoal de Nível Superior, Brazil,and by an Overseas Research Student (UK) award.C. J. S. de Matos’s e-mail address is c.de-matos@imperial.ac.uk.

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