We demonstrated a fully fiber-integrated widely tunable picosecond optical parametric oscillator based on highly nonlinear fiber. The ring cavity with a 50 m highly nonlinear fiber was synchronously pumped with a picosecond mode-locked fiber laser. The tuning range was from 1413 to 1543 nm and from 1573 to 1695 nm, which was as wide as 250 nm. A high-quality pulse was generated with a pulse width narrower than that of the pump. © 2009 Optical Society of America.published_or_final_versio

Cheung, KKY

Chui, PC

Wong, KKY

Yang, S

Zhou, Y

Optics Letters

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Title Widely tunable picosecond optical parametric oscillator usinghighly nonlinear fiberAuthor(s) Zhou, Y; Cheung, KKY; Yang, S; Chui, PC; Wong, KKYCitation Optics Letters, 2009, v. 34 n. 7, p. 989-991Issued Date 2009URL http://hdl.handle.net/10722/92281Rights Creative Commons: Attribution 3.0 Hong Kong LicenseApril 1, 2009 / Vol. 34, No. 7 / OPTICS LETTERS 989Widely tunable picosecond optical parametricoscillator using highly nonlinear fiberYue Zhou,* Kim K. Y. Cheung, Sigang Yang, P. C. Chui, and Kenneth K. Y. WongDepartment of Electrical and Electronic Engineering, The University of Hong Kong,Pokfulam Road, Hong Kong*Corresponding author: yzhou@eee.hku.hkReceived December 1, 2008; revised February 5, 2009; accepted February 18, 2009;posted February 23, 2009 (Doc. ID 104759); published March 20, 2009We demonstrated a fully fiber-integrated widely tunable picosecond optical parametric oscillator based onhighly nonlinear fiber. The ring cavity with a 50 m highly nonlinear fiber was synchronously pumped witha picosecond mode-locked fiber laser. The tuning range was from 1413 to 1543 nm and from1573 to 1695 nm, which was as wide as 250 nm. A high-quality pulse was generated with a pulse widthnarrower than that of the pump. © 2009 Optical Society of AmericaOCIS codes: 060.2320, 060.4370, 190.4380.Picosecond optical pulse sources have been playingan important role in ultrafast processes and opticalcommunications. Today there is a growing demandfor short pulse generation in nonconventional wave-length bands. However, a high-quality picosecond la-ser is generally not available at some nonconven-tional wavelengths due to the lack of efficient gainmedium. Thanks to the ultrafast response, wide-gainbandwidth [1], high gain [2], and large detune fromthe pump [1] of the fiber optical parametric amplifier(FOPA), efficient short pulse generation is possible atnonconventional wavelengths by using an opticalparametric oscillator (OPO) configuration.Conventionally, an OPO based on the 2 nonlineareffect of crystals is utilized for parametric oscillation[3]. This technique has a wide range of applicationsin optical research and development. However, thisapproach requires a dedicated alignment for optimaloperation. Fiber optical parametric oscillators(FOPOs), which are based on the 3 nonlinear effectof optical fiber, eliminate the need for alignment andallow further integration with fiber components [4].Using the FOPO configuration, CW [5–7] and nano-second [8] pulse generators have been demonstratedwith a wide wavelength tuning range. For picosecondand femtosecond pulses, since the walk-off betweenthe pump and the signal is quite large, it is requiredto use short fiber, especially microstructure fiber(MF) based FOPOs, which have a tuning rangemostly in the visible regime [9–11]. However, most ofthe telecommunications applications require sourcesin the spectral region near 1550 nm. Some other MFbased FOPOs [12], dispersion-shifted fiber (DSF)based FOPOs [13], and highly nonlinear fiber(HNLF) based FOPOs [14,15] were designed to oper-ated in this regime, but usually with limited tuningrange. Thus, to build a widely tunable FOPO oper-ated around 1550 nm is highly desirable.In this Letter, we demonstrated a fully fiber-integrated widely tunable picosecond optical para-metric oscillator based on HNLF. The tuning rangewas as wide as 250 nm around 1550 nm. The totallyfiber-integrated nature of this source allows completeself-alignment and further integration to other sys-0146-9592/09/070989-3/$15.00 ©tems. This scheme may be useful in generating ashort pulse for potential ultrafast communication innonconventional wavelength bands.When a highly nonlinear DSF (HNL-DSF) is de-ployed as the gain medium inside a cavity, the ampli-fied spontaneous emission (ASE) noise in either ofthe sidebands will be amplified and then fed back tothe HNL-DSF as a seed. After multiple paths therewill be significant signal and idler components at theoutput.A phase-matching technique is required to gener-ate signal and idler with substantial power. Thephase-matching condition is + 2P = 0, 1where  is the fiber nonlinear coefficient and P is theincident pump power.  is the total phase mis-match, which can be approximated by the followingequation [1]: = 22 + 44/12, 2where  is the frequency detune of the signal (andtherefore idler) from the pump; m is the mth orderderivative of the propagation constant.The shape of the gain spectrum is determined bythe fiber dispersion profile. When 20 and 40, it will operate as a conventional FOPA, whichhas a continuous gain spectrum around the pump.One the other hand, when 20 and 40, idlerand signal will be generated with frequency detunemuch larger than the bandwidth of the conventionalFOPA. For a given fiber, we can vary 2 over a largerange by tuning the pump wavelength, thus makingthe widely tunable FOPO feasible.The experimental setup of a widely tunable pico-second pulse generator is shown in Fig. 1. The pumpwas generated by a picosecond mode-locked fiber la-ser (MLFL), which generated a short pulse with rep-etition rate of 10 GHz and a pulse width of 10 ps. Theoutput from the pulsed laser was intensity-modulated by a 156.2 MHz electrical pulse with dutyratio of 1/64 to increase the peak power of the pump,and broadened to 15 ps by a variable-bandwidth tun-2009 Optical Society of America990 OPTICS LETTERS / Vol. 34, No. 7 / April 1, 2009able bandpass filter (VBTBPF), which has a 3 dBbandwidth of 0.3 nm to produce a pump pulse with aduty ratio of 0.23% and a repetition rate of156.2 MHz. The motivation of increasing the pulsewidth from 10 to 15 ps was to increase the interac-tion length of the pump and the signal to obtain ahigher gain. The pump was then amplified byerbium-doped fiber amplifiers (EDFAs), EDFA1 andEDFA2, and filtered by a tunable bandpass filter(TBPF) with a 1 nm bandwidth to produce a high-power, low-noise pump. The average power of thepump was measured to be 20 mW after the amplifier.If we take duty ratio into consideration, the peakpump power was estimated to be 8.7 W. The pumpthen passed through a circulator, and the reflectedpower was observed using a power monitor. Since thelinewidth of the pump pulse was larger than thebandwidth of the stimulated Brillouin scattering(SBS) gain, the SBS was low and therefore the phasemodulator used in [7] was not required to suppressthe SBS. It was then coupled into the cavity for para-metric amplification through the wavelength-divisionmultiplexing coupler (WDMC1), which had a cutoffwavelength of 1565 nm. Note that the cavity lengthwas about 60 m, including a 50 m long HNL-DSFas the gain medium, which had nonlinear coefficientof 14 W−1 km−1, zero-dispersion wavelength (ZDW)of 1554.7 nm, and dispersion slope of0.035 ps/nm2 /km. A 50/50 coupler in the cavityprovided 50% feedback and 50% output. The feed-back branch was filtered by the WDMC2 with a cutoffwavelength of 1565 nm, so that only the signal(Stokes wave) returned to the HNL-DSF throughWDMC1. As a result, the FOPO was only singly reso-nant with the signal. The polarization controller (PC)in the cavity was used to align the state of polariza-tion (SOP) with that of the pump, while the opticalline delay (ODL) in the cavity was used to adjust thecavity length, thus the round-trip time of the signalmatched the pump repetition rate, so as to synchro-nize the signal with the pump.After one round trip, the walk-off of the pump andthe signal along the fiber was approximately 8 ps fora 100 nm separation, which was comparable to theFig. 1. (Color online) Experimental setup of the FOPO.EDFA, erbium-doped fiber amplifier; TBPF, tunable band-pass filter; OSA, optical spectrum analyzer; MZM, Mach–Zehnder modulator; DCA, digital communication analyzer;PC, polarization controller; ODL, optical delay line; CIR,circulator.pump pulse width. However, by changing the cavitylength, the signal could be synchronized with a newpump pulse after one round trip, thus the walk-offbetween the pump and the signal could be compen-sated and would not aggregate. The round-trip loss ofthe cavity was measured to be 7.1 dB. The single-pass signal peak gain of the FOPO in the case of per-fect phase matching can be calculated using [8]GdB 10 log1014 exp2PL , 3where L is the fiber length, P is the peak power of thepump, and  is the nonlinear coefficient. The gainwas calculated to be 46 dB. The actual gain wasperceived to be lower than this value, since not onlythe walk-off between the signal and the pump wouldlower the gain: ZDW fluctuations and random bire-fringence along the HNL-DSF would also lower thegain [1]. However, the gain can still easily exceed thecavity loss. Stable mode-locked oscillations thus oc-curred.The FOPO output spectrum was monitored by anOSA through a 1/99 coupler. WDMC3 was used to fil-ter out the desired signal or idler. The waveform ofthe signal (idler) was recorded using a digital com-munication analyzer (DCA).Figure 2 shows the optical spectra measured at theFOPO output port. The pump wavelength was tunedfrom 1559 to 1544 nm and the pump power wasslightly adjusted to maintain almost the same peakand narrow linewidth for all spectra. When the ODLwas tuned slightly at the same time, the achievableoutput tuning range was from 1413.5 to 1543.7 nmand from 1573.4 to 1695.8 nm, which was over250 nm. The tuning range is larger than those CW[5–7] and picosecond FOPOs [12–15] reported. Thewidest separation from the pump was 151.8 nm. Thesmaller peaks that occurred when the pump wave-length was tuned to above 1554 nm were high-orderfour-wave mixing (FWM) components. The narrowlinewidth 2 nm of the output signal or idler thatrequires no additional bandpass filters inside thecavity to select the wavelengths is a highly desirableFig. 2. Optical spectra measured at FOPO output usingOSA.April 1, 2009 / Vol. 34, No. 7 / OPTICS LETTERS 991feature for this configuration, as the availability ofthe filter at certain wavelengths may limit the tuningrange of the FOPO [7].The long wavelengths’s amplitudes that werelarger than those of the short wavelengths whentheir separations to the pump wavelength were largemay be due to the Raman gain. Further tuning waslimited by the walk-off between the pump and thesignal, which increased with their wavelength sepa-ration and thereby decreased their interactionlength.Figure 3 shows the waveforms of the input pumppulse at 1544 nm and output signal pulse at 1695 nmmeasured from the DCA. It can be observed that thepulse width of the signal was narrower than that ofthe pump because of the pulse compression effect[16]. The compression ratio was less than that pre-dicted because of the walk-off between the pump andthe signal. The signal-to-noise ratio (SNR) of the sig-nal measured from the DCA was 17 dB. The noise atthe mark level was inherited from the ASE noise ofthe EDFAs. The actual signal pulse width was be-lieved to be shorter than the measured value as thehigh amplitude ringing at the trailing edge suggestedthat the measurement was limited by the photodetec-tor electrical bandwidth of about 32 GHz.Figure 4 shows the average output power of thesignal near 1695 nm from the FOPO output port ver-Fig. 3. (Color online) (a) Output pulse from the pumpsource and (b) signal pulse observed at the FOPO output.The horizontal scale is 20 ps/div.Fig. 4. Average output power of the signal from FOPO assus the average power of the pump of wavelength at1544 nm. Gain saturation began as the averagepump power exceeded 80 mW, indicating that furtherincreasing the pump power would not generate amuch higher signal output power as discussed in [6].The conversion efficiency was from 3.35% to 6.21%,which was higher than those picosecond FOPOs re-ported in [12,13]. The conversion efficiency was per-ceived to be limited by the walk-off between the sig-nal and the pump.In conclusion, an all-fiber widely tunable picosec-ond optical parametric oscillator based on highlynonlinear fiber was demonstrated. The output wascontinuously tunable from 1413 to 1543 nm and from1573 to 1695 nm, when the pump wavelength wastuned between 1559 and 1544 nm. A high-qualitypulse was generated with a pulse width narrowerthan that of the pump. This scheme could be founduseful in the generation short pulses for ultrafastcommunication in nonconventional wavelengthbands.The work described in this Letter was partiallysupported by grants from the Research Grants Coun-cil of the Hong Kong Special Administrative Region,China (projects HKU 7172/07E and HKU 7179/08E).The authors also acknowledge Sumitomo Electric In-dustries for providing the HNL-DSF and AlnairLaboratories for providing the VBTBPF.References1. M. E. Marhic, K. K.-Y. Wong, and L. G. Kazovsky,IEEE J. Sel. Top. Quantum Electron. 10, 1133 (2004).2. T. Torounidis, P. A. Andrekson, and B.-E. Olsson, IEEEPhoton. Technol. Lett. 18, 1194 (2006).3. M. H. Dunn and M. Ebrahimzadeh, Science 286, 1513(1999).4. J. E. Sharping, J. Lightwave Technol. 26, 2184 (2008).5. M. E. Marhic, K. K. Y. Wong, L. G. Kazovsky, and T. E.Tsai, Opt. Lett. 27, 1439 (2002).6. C. de Matos, J. Taylor, and K. Hansen, Opt. Lett. 29,983 (2004).7. T. Torounidis and P. Andrekson, IEEE Photon. Technol.Lett. 19, 650 (2007).8. G. K. L. Wong, S. G. Murdoch, R. Leonhardt, J. D.Harvey, and V. Marie, Opt. Express 15, 2947 (2007).9. Y. Deng, Q. Lin, F. Lu, G. Agrawal, and W. Knox, Opt.Lett. 30, 1234 (2005).10. Y. Q. Xu, S. G. Murdoch, R. Leonhardt, and J. D.Harvey, Opt. Lett. 33, 1351 (2008).11. J. E. Sharping, M. Fiorentino, P. Kumar, and R. S.Windeler, Opt. Lett. 27, 1675 (2002).12. J. Lasri, P. Devgan, R. Tang, J. E. Sharping, and P.Kumar, IEEE Photon. Technol. Lett. 15, 1058 (2003).13. D. K. Serkland and P. Kumar, Opt. Lett. 24, 92 (1999).14. P. Devgan, J. Lasri, R. Tang, V. Grigoryan, W. Kath,and P. Kumar, Opt. Lett. 30, 528 (2005).15. B. Cuenot, A. D. Ellis, and C. J. McKinstrie, inProcedings of the European Conference on OpticalCommunication (2006), paper We3.P.19.16. T. Torounidis, M. Karlsson, and P. A. Andrekson, J.Lightwave Technol. 23, 4067 (2005).the function of pump power.

Widely tunable picosecond optical parametric oscillator using highly nonlinear fiber

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Widely tunable picosecond optical parametric oscillator using highly nonlinear fiber

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