Extended nonlinear parametric process in anomalously pumped linear cavity oscillator L. T. Lim,1 K. S. Yeo,2 M. H. Abu Bakar,1 N. Tamchek,1 and M. A. Mahdi1,* 1

Wireless and Photonics Networks Research Centre, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia 2 Department of Electrical Engineering, Faculty of Engineering, University of Malaya, Malaysia * [email protected]

Abstract: We demonstrate a linear cavity fiber optical parametric oscillator with extended pump-signal separation of 14.3 THz (116 nm). The signal laser is provided by a pair of 1675nm fiber Bragg gratings and a tunable idler from 1456.12 nm to 1462.48 nm is generated by detuning the pump wavelength in the anomalous dispersion regime of a highly nonlinear fiber. At such large pump-signal separation, we are still able to record a parametric conversion efficiency of more than −35 dB and idler optical signal-to-noise-ratio of 50 dB on average. The stability of the lasing signal and idler is examined and result shows both signal and idler peak power fluctuation is less than 1 dB over a period of 30 minutes. ©2014 Optical Society of America OCIS codes: (190.4970) Parametric oscillators and amplifiers; (060.4370) Nonlinear optics, fibers; (140.3550) Lasers, Raman.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

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#216967 - $15.00 USD Received 14 Jul 2014; revised 13 Aug 2014; accepted 26 Aug 2014; published 5 Sep 2014 (C) 2014 OSA 8 September 2014 | Vol. 22, No. 18 | DOI:10.1364/OE.22.022190 | OPTICS EXPRESS 22190

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1. Introduction Early optical parametric oscillators (OPOs) utilize the effect of second-order χ2 in nonlinear crystals for tunable laser source generation. Following the invention of low loss optical fiber, fiber-based optical parametric oscillators (FOPOs), which operate based on the third-order χ3 nonlinearity of optical fiber, then come into the OPO research stream. The early optical fiber has low nonlinearity values, thus pulsed pump are often preferred over continuous wave (CW) pump to achieve sufficient gain for lasing until the advent of silica-based highly nonlinear fiber (HNLF) [1, 2]. Unlike exotic fibers such as microstructured fibers and nonsilica-host fibers, HNLF are conventional germanium-doped fiber with reduced core effective area [3], hence allowing seamless integration in standard fiber system. A lot of works have been performed on fiber optical parametric oscillators (FOPO) in the past decades due to unique features reported in fiber optical parametric amplifiers experiments such as very high gain [4, 5] and large gain bandwidth over 100 nm [6–8]. The demand of fast growing optical communication system has intensified the research efforts in FOPO to produce tunable radiation in optical region that are unavailable in conventional laser system. In addition to this, FOPO has been investigated to support other applications. The noteworthy research works include multiwavelength FOPOs based on Mach-Zehnder interferometer [9, 10], single-longitudinal mode FOPO with linewidth of less than 400 kHz at 1549 nm [11] and tunable single-longitudinal mode lasers over 14 nm for each signal (1567.9 to 1544 nm) and idler (1567.9 to 1582 nm) bands [12]. Additionally, a mode-locked FOPO can be realized as well with stable 10 GHz pulse train tuned over 21 nm bandwidth [13]. In [14], the first anomalously pumped CW linear cavity FOPO is demonstrated by placing fiber Bragg gratings (FBGs) between a HNLF. Results show that the generated idler can be tuned from 1500 nm to 1580 nm, corresponds to total tuning bandwidth of 80 nm at a threshold power of 240 mW. By pumping in fiber anomalous dispersion region, wide parametric gain is generated in wavelength region symmetrical around the pump. If a suitable tunable spectral filtering device is placed within the FOPO, a tunable laser with consistent linewidth can be obtained. On the other hand, by detuning the pump into normal dispersion region in a ring cavity FOPO, the tuning bandwidth limit can be extended to 240 nm at threshold power of 3.5 W [15]. Nevertheless, positioning the pump in normal dispersion of gain fiber will invite other limitations to FOPO as the lasing signal full-width half maximum is a function of the distance of the pump, thus leading to inconsistent linewidth laser across spectrum, in addition to its susceptibility to wavelength variation caused by pump laser instability [2]. Despite the inconsistent linewidth and laser wavelength instability, this type of FOPO has attracted many research works pertaining to the effect of stimulated Raman scattering (SRS) on the parametric amplification [15–17]. Recently, Malik et al. presented an astonishing 254 nm of continuous tuning bandwidth laser from S- to U-band (1476 to 1730 nm) in an anomalously pumped ring cavity FOPO utilizing relatively high pump power (3 W) and long HNLF of 520 m [18]. The combination of high CW pump power and long fiber favored the SRS process in the gain fiber, which in turn assisted in extending the nonlinear parametric process to longer tuning bandwidth [19]. Hence, it would be interesting to investigate the impact of SRS to other FOPO structure, namely linear cavity, which to the best of our knowledge, has not been reported yet. In this paper, we experimentally demonstrate a linear cavity FOPO where the parametric pump is positioned in the HNLF anomalous dispersion region. The linear cavity consists of a

#216967 - $15.00 USD Received 14 Jul 2014; revised 13 Aug 2014; accepted 26 Aug 2014; published 5 Sep 2014 (C) 2014 OSA 8 September 2014 | Vol. 22, No. 18 | DOI:10.1364/OE.22.022190 | OPTICS EXPRESS 22191

pair of specially fabricated FBGs at 1675 nm, and a 500 m long HNLF pumped at 1559 nm, realizing the furthest pump-signal distance (116 nm or 14.3 THz) in a linear cavity FOPO experiment to our best knowledge while maintaining the linewidth stability of this pumping method. The long wavelength conversion process is made possible due to strong SRS assisting the weak parametric process in FBG center wavelength region. A consistent linewidth and stable idler is generated in the S-band region with conversion efficiency exceeding −35 dB, and is tunable from 1456.12 nm to 1462.48 nm by detuning the pump wavelength. 2. Experimental setup The experimental setup of FOPO is shown in Fig. 1. A tunable laser source, TLS (Santec WSL-100) is deployed as a pump source to emit a 5.5 dBm pump light, which is positioned in the anomalous wavelength region of the gain fiber. The TLS has 3 dB linewidth of 5 MHz with wavelength tuning range from 1529 to 1567 nm. Two polarization controllers, PC1 and PC2 are used to align the polarization of the pump light into the phase modulator (PM) and into the FOPO cavity to obtain optimum laser output. The phase modulator is driven with four sinusoidal tones at 100, 300, 900 and 2700 MHz to suppress stimulated Brillouin scattering (SBS) effect on the pump light. The parametric pump power is boosted by an erbium-doped fiber amplifier (EDFA) to a maximum power of 1.5 W. A tunable bandpass filter (TBF) is placed in the path after the EDFA to remove excessive amplified spontaneous emission (ASE) generated by the EDFA. A 99/1 coupler (C1) of 2 × 2 ports is used before FBG1. The 1% port at the input is utilized to monitor the SBS back reflection while the other output is to detect the amount of pump power into the linear cavity. A spool of 500 m HNLF with nonlinearity coefficient, γ = 11.5 W−1km−1, zero dispersion wavelength, ZDW = 1557.6 nm, dispersion slope, S = 0.016 ps/nm2/km, β2 = −2.888 × 10−29 s2/m, β3 = 2.665 × 10−41 s3/m, β4 = 1.321 × 10−55 s4/m, attenuation coefficient, α = 0.82 dB/km, and optical fiber effective area, Aeff = 11μm2 is used as the gain fiber, placed in between two FBGs. The reflectivity of FBG1 and FBG2 is 99.99% and 91.34% respectively, and both FBGs have a central wavelength at 1675 nm with 1.2 nm full-width half-maximum reflectance bandwidth. The FOPO output spectrum is then observed using an optical spectrum analyzer (OSA) through 5% port of a 95/5 coupler (C2). This is to minimize the damage of OSA while measuring the performance of the proposed FOPO. All the measurements can be back calculated to point “x” as the actual output. The 95% arm of C2 is terminated with an isolator (ISO3) to prevent any backreflection signals that can interfere the FOPO stability during the measurement.

Fig. 1. Experimental setup of the proposed FOPO in a linear cavity.

2. Results and discussions Figure 2 shows the output spectrum at point “x” when a 1559 nm pump of 500 mW is injected into the proposed linear cavity FOPO. This output was initially measured at the 5% port of C2 and then back calculated to the point “x”. An oscillating signal at 1675 nm and an idler at 1457.8 nm with optical signal-to-noise ratio of 50 dB are observed, making the total

#216967 - $15.00 USD Received 14 Jul 2014; revised 13 Aug 2014; accepted 26 Aug 2014; published 5 Sep 2014 (C) 2014 OSA 8 September 2014 | Vol. 22, No. 18 | DOI:10.1364/OE.22.022190 | OPTICS EXPRESS 22192

signal-idler distance 218 nm, which is comparable to [19]. The converted idler has a peak power of −12.8 dBm and an average power of 0.212 mW (−6.74 dBm). From this result, its conversion efficiency is calculated to be −33.1 dB, indicating that the parametric conversion bandwidth can be extended by oscillating a seed laser in the SRS-dominant region. Pump

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Figure 3 depicts that the signal/idler power increases linearly with pump power. The measurement is performed at pump wavelength 1559 nm, signal wavelength 1675 nm and idler 1457 nm. The threshold power (Pth) is found to be 215 mW, which is lower compared to the threshold power measured in [14]. Since pump power is limited and the maximum signal/idler output power is limited by the availability of pump power, both lights do not exhibit any saturation characteristic. 700

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In extended nonlinear of linear-cavity parametric oscillator, the S-band idler wavelength is tunable from 1456.12 nm to 1462.48 nm, which gives a total tuning range of 6.4 nm, by tuning the pump wavelength from 1558.0 to 1561.6 nm, which is in the fiber anomalous dispersion region, as depicted in Fig. 4(a). The overlap spectra in Fig. 4(a) also show that the idler achieves an average optical signal-to-noise-ratio (OSNR) of 50 dB with an average linewidth of 0.8 nm. The OSNR by definition describes the degree of signal quality to its signal noise level, in terms of power ratio, which is usually expressed in decibel. The linewidth fluctuation is around 0.2 nm for all generated idlers. This indicates that by pumping the HNLF in the anomalous dispersion region, the linewidth variation of the idler can be minimized as opposed to its counterpart pumping technique in the normal dispersion region

#216967 - $15.00 USD Received 14 Jul 2014; revised 13 Aug 2014; accepted 26 Aug 2014; published 5 Sep 2014 (C) 2014 OSA 8 September 2014 | Vol. 22, No. 18 | DOI:10.1364/OE.22.022190 | OPTICS EXPRESS 22193

[7]. Figure 4(b) indicates that the maximum idler average power variation across its tuning range is within 6 dB with conversion efficiency above −35 dB.

Fig. 4. (a) Idlers tunability from 1456.12 nm (black line in left) to 1462.48 nm (dashed line in right) when detuning the pump wavelength from 1558 nm to 1561.6 nm at pump power of 400 mW, and (b) average power of the idlers and their calculated conversion efficiency.

The output power stability of the FOPO is also evaluated. Figure 5(a) illustrates the scanned output spectra of the FOPO, and Fig. 5(b) shows the recorded peak power of pump, idler and signal over 30 minutes in room temperature. There is no significant variation of the output spectrum as portrayed in Fig. 5(a). In addition, the peak power fluctuation for signal and idler is less than 1 dB.

Fig. 5. (a) Output spectra of fiber OPO at pump wavelength, λp = 1559 nm scanned for a time period of 30 minutes and (b) the peak power fluctuation during the scanning.

In order to verify the importance of SRS in extending the parametric conversion bandwidth at long wavelength region, the FBGs pair in the cavity is taken out from the setup and a TLS is used to provide seed signal of + 11 dBm into the HNLF through a 90/10 coupler. The TLS wavelength (Yokogawa AQ 2200-136) can only be tuned from 1440 nm to 1640 nm, thus here we only perform the parametric amplification in that region to mimic the long wavelength and short wavelength parametric conversion. Figures 6(a) and 6(b) show the output spectrum of the parametric amplifier (FBG1 and FBG2 are removed) when the signal wavelength is set at 1485 nm and 1640 m, respectively. The signal wavelength is chosen such that comparison can be made between two cases: S-band idler (1485 nm) is generated when signal is at TLS’s longest wavelength limit (1640 nm), and L-band idler (1640 nm) is generated when the signal is set at 1485 nm. Referring to Fig. 6(a), the signal is located at wavelength shorter than the pump wavelength (hence not SRS amplified), while the signal in Fig. 6(b) is located at wavelength longer than pump wavelength (in SRS dominant region). This condition has allowed the signal in Fig. 6(b) to achieve higher peak power due to SRS amplification, and consequently boost up its associated idler’s peak power. Under the same pump power, it is visually clear that the idler peak power (−40.5 dBm) in Fig. 6(b) is higher than that (−48.5 dBm) in Fig. 6(a), showing an improvement of idler power benefited from

#216967 - $15.00 USD Received 14 Jul 2014; revised 13 Aug 2014; accepted 26 Aug 2014; published 5 Sep 2014 (C) 2014 OSA 8 September 2014 | Vol. 22, No. 18 | DOI:10.1364/OE.22.022190 | OPTICS EXPRESS 22194

SRS. Based on this finding, we can verify that SRS is a crucial element to extend FWM wavelength conversion and helps to further extend the total conversion bandwidth. In addition, Fig. 6(c) (which resembles to signal/idler wavelength locations in Fig. 2) shows weak FWM conversion efficiency when the signal is located at 1457 nm and its generated idler at 1675 nm. However, when FOPO with oscillation wavelength of 1675 nm is operated in depleted-pump mode, the signal-idler peak power gap becomes closer and attains good conversion efficiency as shown in Fig. 2. 10

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4. Conclusion In conclusion, an anomalously pumped linear cavity FOPO that generates a laser at 1675 nm and an S-band idler in linear cavity is experimentally demonstrated. We have successfully shown that far pump-signal separation (116 nm) laser oscillation is possible at pump power less than 1 W. At long oscillation wavelength region where SRS is dominant, weak parametric process is boosted up by SRS and a short-wavelength idler can be created symmetrically around the pump. The idler is found to have a tuning range of 6.4 nm with 50 dB OSNR, with consistent linewidth and flat peak power, which is potentially useful in optical fiber communication systems. Acknowledgments This work is partly supported by the Ministry of Education, High Impact Research #A000007-50001 and the Ministry of Science, Technology and Innovation (National Science Fellowship).

#216967 - $15.00 USD Received 14 Jul 2014; revised 13 Aug 2014; accepted 26 Aug 2014; published 5 Sep 2014 (C) 2014 OSA 8 September 2014 | Vol. 22, No. 18 | DOI:10.1364/OE.22.022190 | OPTICS EXPRESS 22195

Extended nonlinear parametric process in anomalously pumped linear cavity oscillator.

We demonstrate a linear cavity fiber optical parametric oscillator with extended pump-signal separation of 14.3 THz (116 nm). The signal laser is prov...
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