Linear and nonlinear characteristics of devices using millimeter-scale spools of highly nonlinear fiber are experimentally investigated within 2000-2400nm spectral range. Coils with radius larger than 3.5 mm indicate that macro-bending induced radiation loss is negligible up to 2400nm. Devices with smaller diameter coiling resulted in macro-bending losses that dominate over micro-bending losses beyond 2200nm. A tunable short-wave infrared source was constructed using a coin-sized fiber module to demonstrate an efficient nonlinear conversion from 1.26 to 2.2 μm.
©2010 Optical Society of America
Recent advances in dispersive and nonlinear engineering of silica highly nonlinear fibers (HNLFs) has lead to the development of multiple classes of nonlinear devices that include supercontinuum (SC) sources, Raman amplifiers, ultra-short pulse generators, all-optical signal processors, broadband parametric amplifiers and oscillators [1–4]. While number of non-silica glasses and crystals possess large nonlinear index, they are impaired by high losses, two-photon absorption, free carrier generation and complex coupling . In contrast, HNLF offers the combination of low loss, precise dispersion tailoring and nearly lossless coupling. As a result, reported continuous-wave HNLF figure of merit, defined as product of the effective interaction length, nonlinearity and pump power is at least an order of magnitude higher than that of any comparable platform. Unfortunately, HNLF devices are also intuitively associated with large packaging size and commonly invoke an image of conventional (12-inch diameter) fiber bobbin. This notion stands in sharp contrast with physical size of monolithic platforms, where nonlinear waveguide possess only centimeter scale .
To address this issue, recent efforts investigated the feasibility of compact HNLF packaging suitable for nonlinear device construction [6–9]. The work was motivated by a simple fact that HNLF possesses an order of magnitude higher core-cladding index contrast  than that of the conventional fiber types. Consequently, high HNLF confinement should allow sharp bending before radiative losses become significant. The macro-bending loss was studied as a function of the spool radius  in near-infrared (1.5 μm) band and led to 100 nm wide SC source construction using HNLF coils with only 25 mm radius .
This paper investigates the practical limits of HNLF device size and its operational performance within contiguous near infrared (NIR) and short wave infrared (SWIR) bands spanning 1000 to 2400 nm. We report linear and nonlinear characteristics of HNLF coiled in millimeter-scale bobbins and their characteristics in contiguous 1000-2400 nm spectral range. Measurements in fibers coiled with radius larger than 3.5 mm show that macro-bending induced radiation is negligible up to 2400 nm; smaller radius coiling resulted in macro-bending dominating over micro-bending beyond 2200 nm wavelength. Finally, to demonstrate that ultra-compact HNLF modules do not compromise the performance of broadband nonlinear devices, a tunable short-wave infrared transmitter was constructed using a coin-sized HNLF package for efficient nonlinear conversion between 1260 nm and 2190 nm.
2. Experimental setup for attenuation characterization
The experimental setup for characterization of spooled and unspooled fibers is shown in Fig. 1 . An SC served as primary light reference and is propagated though the fiber under test (FUT) to retrieve the spectral response in contiguous NIR/SWIR band. The SC source was seeded by an external cavity laser at 1560 nm, which was amplitude modulated by low duty-cycle pulses and subsequently amplified by cascaded Erbium doped fiber amplifiers (EDFAs). Amplified pump pulse had a peak power of 600 W and was launched into a 20 m segment of HNLF to generate the broadband radiation spanning NIR/SWIR bands. A variable attenuator was placed at SC output to prevent instrumentation damage and, more importantly, to prevent any nonlinear interaction in FUT that would distort calibration of loss measurements. The SC was stable over extended period, as required to rigorously characterize the fiber loss and did not require any additional tracking procedure. All spectra were measured using an optical spectrum analyzer (OSA) operating in contiguous 1.2-2.4 μm band.
The attenuation of HNLF was measured in 8m-long segments that were spooled on steel rods with radius varying from 1.5 to 15 mm. The procedure allowed for a constant spooling diameter and precise characterization of the diameter at which macro-bending induced radiation loss becomes significant when compared to losses in unspooled fibers. All tested HNLF segments were cut from a single 1 km long HNLF section designed with effective area A eff = 11 μm2, dispersion slope S 0 = 0.027 ps/nm2-km, and zero dispersion wavelength λ0 = 1583 nm.
3. Results: Attenuation characterization of HNLFs and SMFs
In first set of experiments, a variable-radius loss characterization was performed in spooled and unspooled standard single mode fiber (SMF) and HNLF in order to compare two types; Fig. 2 shows two examples of the SC transmission spectra obtained for these fibers. Figure 2a shows the measurement of 8-m long, unspooled SMF, indicating SC spectrum before and after SMF segment. Attenuation induced by SMF segment becomes non-negligible beyond 1900 nm and is attributed to micro-bending induced radiation. The subtraction between these spectral measurements results in calibrated loss characteristics of unspooled 8 m long SMF in contiguous NIR/SWIR band. A similar procedure was applied to HNLF type: Fig. 2b shows the response of an 8m long HNLF section. A much smaller attenuation is observed and might be related to the tighter confinement of the optical mode in HNLFs.
Figure 3 shows the SC transmission spectra obtained for the 8-m long HNLF with spooling radius of 4, 2, and 1.5 mm. Attenuation induced by HNLF segment becomes increasingly non-negligible as the spooling radius is decreased to 2 and 1.5 mm.
Figure 4 summarizes HNLF and SMF loss measurements for the bending radii that were analyzed. We note that both types, when unspooled, have non-negligible loss beyond 2200 nm. The loss is attributed to micro-bending induced radiative loss and, to smaller extent, inherent silica absorption properties in SWIR band. This observation is supported by the fact that HNLF has significantly lower micro-bending induced loss than SMF: 0.6 dB versus 2 dB at 2.4 μm. SMF spooling to 12.75 mm radius coil resulted in strong macro-bending starting at 1400 nm and reaches 5 dB/m at 1700nm. In contrast, spooling HNLF to smaller (9 mm and 3.95 mm) radius results in loss spectrum that is nearly identical to that of an unspooled fiber. Indeed, it was necessary to reduce the spooling radius to 2 mm in order to induce a non-negligible loss in SWIR band (2 dB/m at 2.37 μm).
Decreasing the HNLF radius to 1.5 mm results in an increased macro-bending loss that starts to grow beyond 2.1 μm and becomes 3.2 dB/m at 2.32 μm. Consequently, the summary shown in Fig. 4 indicates that there is a negligible macro-bending induced radiation loss up to 2.2 μm in HNLFs for all tested spool radii. All measurements were performed using multiple different segments of HNLF, and were repeated several times to verify the repeatability and accuracy of the results; no reportable variance between physically distinct section was observed. Finally, a last set of measurements was performed with HNLFs sections possessing different zero dispersion wavelength (ZDW) to obtain the identical results. Consequently, all loss bending properties were independent from dispersive HNLF tailoring, allowing us to construct nonlinear devices with varying ZDWs while retaining the same (linear) loss characteristics.
Finally, we note that the 8 m HNLF segments had 15cm-long SMF pigtails spliced at both ends. Their attenuation was taken into account within the entire NIR/SWIR band and used in plotting the HNLF-only loss in Fig. 4.
4. Results: supercontinuum generation and parametric conversion in ultra-compact HNLF devices
High-confinement nonlinear fibers exhibited negligible macro-bending induced loss up to 2200 nm for spooling radius as small as 1.5 mm. However, as no special control regarding the twist or tension was exercised, it was expected that such bending would induce finite birefringence in otherwise nearly isotropic fiber [11–16]. As no prior analysis has been reported regarding this regime, to the best of our knowledge, it was a not a priori clear that newly induced birefringence would impair the performance of a nonlinear device. To assess this practical, but important limitation, a broadband SC source was constructed using a spooled and unspooled 8-m long HNLF. The HNLF segment was cut from the same (1km-long) master coil used for loss measurements described in the previous section. The 5-ns pump was positioned at 1590 nm (in the anomalous dispersion region) to generate SC from the well known modulation instability induced break-up mechanism: in the first stage the modulation instability sidebands induce a fast modulation of the pump envelope, subsequently producing a break into a train of ultra short solitonic pulses. Finally, Raman soliton self-frequency shift and dispersive wave generation give rise to the broadband SC .
The supercontinuum performance is shown in Fig. 5 for two different spooling radii. A negligible difference between supercontinuum generated in spooled and unspooled configurations were observed in both cases. This indicates that the induced birefringence did not play any significant role in supercontinuum performance for coils with radius as small as 3.5 mm.
A second set of nonlinear characterization experiments focused the attention to distant wavelength conversion using a tightly spooled HNLF. The experimental setup used for this purpose was previously described in SWIR generation report . A pulsed pump was centered at 1600 nm and had a peak power of 214 W. The pump was combined with a weak signal in 1200 nm range and launched into 8m-long HNLF section used previously for SC generation. The signal was tunable between 1260 and 1360 nm and the corresponding converted wave was generated between 1945 and 2190 nm as shown in Fig. 6 (a) . The conversion transparency was achieved over the entire tuning range, with maximum conversion efficiency of 15 dB at 1980 nm. For comparison, Fig. 6(b) shows the parametric conversion using the same HNLF, but now unspooled. Even though the pump power was the same in both cases, the parametric fluorescence spectral shapes are rather different: spooled HNLF leads to a reduced bandwidth and intensity of parametric noise if compared to spooled case.
A comparison of the details of the conversion efficiency for the spooled and unspooled HNLF is shown in Fig. 7 . Note that the conversion efficiency and optical signal to noise ratio are better in the spooled case. For example, at the longest converted wavelength (2191 nm) we measured a conversion efficiency of 3.5 dB for the spooled HNLF and −2 dB in the unspooled case. The explanation of this different parametric behavior might be related to the strong induced birefringence and is still under investigation.
The lower limits of parametric devices using HNLF were investigated. The operational performance of millimeter-scale HNLF devices was measured within contiguous near infrared and short wave infrared bands spanning 1000 to 2400 nm. Attenuation measurements in HNLF spooled with radius larger than 3.5 mm show that macro-bending induced radiation is negligible up to 2400 nm; smaller radius coiling resulted in macro-bending dominating over micro-bending beyond 2200 nm wavelength. A tunable short-wave infrared transmitter was constructed using a coin-sized HNLF package for efficient nonlinear conversion between 1260 nm and 2190 nm. The results demonstrate that ultra-compact HNLF modules do not compromise the performance of broadband nonlinear devices, and millimeter-scale packaging can be routinely reached. We also believe that induced birefringence, expected from tight spooling reported in this paper, can be used to benefit in controlling the efficiency of devices relying on distant frequency conversion.
We gratefully acknowledge Sumitomo Electric Industries for supplying HNLF fiber. National Science Foundation is acknowledged for partial financial support.
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