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Efficient, wideband and tunable optical wavelength conversion over 180 nm by four-wave mixing (Bragg scattering) in a fiber is demonstrated experimentally. This process has the potential to translate optical data (states of light) without the noise pollution associated with parametric amplification and spontaneous Raman scattering.
©2006 Optical Society of America
1. Introduction
All-optical wavelength conversion schemes have been intensively studied in recent years for applications in transparent optical telecommunication networks [1, 2]. Four-wave mixing (FWM) in optical fibers was found to be an attractive candidate for efficient and tunable wavelength conversion within a 30-nm range [3,4]. Moreover, wavelength conversion using the Bragg scattering (BS) process was predicted to be noise-free, in contrast to the modulation instability (MI), phase conjugation (PC), or stimulated Raman scattering (SRS) processes [5]. Scalar MI in the normal dispersion regime was shown to generate wavelengths that are widely separated from the pump wavelength [6, 7], and to generate idlers that are widely separated from input data signals [8], which enables data translation from the C-band to the visible band. Of potentially greater interest is the BS process, which can also generate large wavelength shifts. By injecting two pump waves and a signal into an optical fiber, with the average wavelength of one of the pumps (pump 1) and the signal slightly in the normal dispersion regime, an idler is generated at a wavelength that is widely separated from the signal, and which can be tuned by changing the wavelength of the second pump (pump 2). The size of the wavelength shift is determined by the availability of suitable pumps and the properties of the fiber, in particular its dispersion profile and spatial uniformity. In this paper we demonstrate not only the efficiency of the process but also the ability to translate data from a wavelength of 1545 nm (C-band) to 1365 nm (O-band).
2. Theory
FWM is usually classified as 3 different processes depending on the relative positions of the pump, signal and idler frequencies (Fig. 1). MI is a one pump process, whereas PC and BS are two-pump processes. The main advantage of BS is that power is transferred directly from the signal to the idler (and not from the pumps to the signal and idler, as in PC and MI). Since the total sideband power is constant, the vacuum fluctuations are not amplified, so no noise is produced if the frequencies and polarizations of the waves are chosen judiciously. On the other hand, as no amplification is involved, BS cannot be used for this purpose and an external amplifier should be used if needed.
As for other FWM processes, energy and momentum conservation are required for BS to occur, resulting in two conditions for the case in which both pumps have the same power [5]:
where ωb=(ω 1-ω s)/2 and ω c=(ωi-ω 2)/2, and β 2 and β 4 are respectively the second- and fourth-order dispersion coefficients at the average frequency ωa=(ω 1+ωs)/2. The phasematching condition (2) can be satisfied in the normal dispersion regime (β 2>0) if β 4<0. Equation (1) implies that ωi-ωa=ωa-ω 2: By choosing pump 2 with a frequency that is widely separated from the average frequency, an idler can be generated with a symmetric frequency difference. The signal-idler frequency shift ωi-ωs=ω1-ω 2.
3. Experiment
The average frequency of the signal and pump 1 (or of the idler and pump 2) should be slightly in the normal dispersion regime of the fiber, in order to convert efficiently a signal with the widest frequency shift. To demonstrate this BS conversion, a 1-km length of highly nonlinear fiber (HNLF) from Sumitomo Electric Industries, with a zero-dispersion wavelength (ZDW) of 1554.3 nm and a nonlinear coefficient of 13 (km-W)-1, was used in our experimental setup, which is illustrated in Fig. 2. Erbium-amplified tunable L-band (1605 nm) and C-Band (1540 nm) continuous-wave lasers were used as pumps, whereas a distributed-feedback (DFB) laser diode (1567 nm) was used as a signal. Before being amplified to 1 W, both pumps were phase-modulated with a 27-1 pseudo-random bit sequence (PRBS) signal at 2.5 Gb/s to suppress backward stimulated Brillouin scattering (SBS), which would decrease the efficiency of BS by reducing the powers of the pumps driving the interaction. A co-phased pump modulation scheme was used to prevent idler spectral broadening [9]. By analogy with the MI case [6], this phase-matching curve, which was simulated using Eq. (2) and β 2 and β 4 derived from the dispersion parameters given by the manufacturer, is plotted as a function of the average frequency of pump 2 and the idler [Fig. 3 (right)]. The output spectrum displayed in Fig. 3 (left) shows that an idler was generated at 1505 nm, which corresponds to a 62-nm shift from the signal wavelength, as expected from the simulated phase-matching curve. This agreement confirms that the influence of dispersion terms of order higher than fourth is negligible, so these terms can be omitted from Eq. (2) in the current case. Once the polarization controllers were optimized, most of the signal power was transferred to the idler. Notice that pump 2, whose frequency is in the anomalous dispersion regime of the fiber, experiences MI, whereas pump 1, which is located in the normal dispersion regime, does not. Fig. 3 (left) also illustrates the effect of these MI-generated sidebands of pump 2 on the BS process as sidebands on the idler are also generated. The good agreement between the theoretically-predicted and experimentally-observed idler wavelength demonstrates that the wavelength shift obtained using this fiber is only limited by the availability of a pump at wavelengths longer than 1620 nm. In principle, a wider shift could also be obtained by using a fiber with a shorter ZDW, and a pump 1 with a shorter wavelength.
Fig. 2. Experimental set-up. TLS=Tunable laser source, DFB=Distributed-feedback laser, ISO=Isolator, PC=Polarization controller, PM=Phase modulator, PRBS=Pseudorandom bit sequence signal, EDFA=Erbium-doped fiber amplifier, HNLF=Highly nonlinear fiber, VOA=Variable optical attenuator, OSA=Optical spectrum analyzer. Wavelength division multiplexing (WDM) couplers have been used to combine the different waves.
Fig. 3. Left: Output spectrum of the HNLF with and without the pumps. Right: Phase matching curves for Bragg scattering in the HNLF which was simulated using Eq. (2) and β 2 and β 4 derived from the dispersion parameters given by the manufacturer. Output wavelengths as a function of the average frequency. Markers represent experimental data.
In order to demonstrate the ability of BS to produce larger wavelength shifts, a second experiment was carried out with a different fiber. Due to the lack of a suitable HNLF, we performed the experiment using a 1.35-km length of Corning LEAF fiber with low water-peak attenuation, high SBS threshold, a ZDWof 1487.5 nmand a nonlinear coefficient of 1.5 (km-W)-1. Tunable and powerful pumps at shorter wavelengths are available using Raman lasers, but at the expense of broader pump bandwidths (which eliminate the need for phase modulation). By using such a laser in the previous set-up instead of the amplified C-band laser source, a signal in the C-band (1530–1565 nm) can be converted to the upper range of the O-band (1260–1360 nm). Results are shown in Fig. 4 (right) for pump powers of 2W and signals with frequencies in three different ranges (about 1515, 1530 and 1545 nm). The markers are experimental measurements and the lines represent the predictions of simulations based on Eq. (2). Once again, there is good agreement between the predicted and observed results. The output spectrum, which shows an idler generated 180 nm from the signal, is displayed in Fig. 4 (left).
Fig. 4. Left: Output spectrum of the LEAF. Right: Phase matching curves for Bragg scattering in the LEAF which was simulated using Eq. (2) and β 2 and β 4 derived from the dispersion parameters given by the manufacturer. Output wavelengths as a function of the average frequency. Markers represent experimental data.
The low conversion efficiency obtained in our experiment could be increased by using a fiber with higher nonlinearity, or by using more powerful and/or more coherent pumps. Moreover, although the relative polarizations of four waves with small frequency separations should remain constant as the waves propagate through the fiber, the polarization mismatch caused by polarization-mode dispersion (PMD) could also be a reason for the low efficiency of BS involving waves with large frequency separations [10].
For fixed signal and pump 1 frequencies, one can still tune the idler frequency by changing the frequency of pump 2. However, the tuning range of the idler will be lower compared to the case in which the frequency of pump 1 is also optimized. This idler-tuning 3-dB bandwidth was measured to be 15 nm in this last case and is displayed in Fig. 5 (left). This demonstrates that the frequency of the idler can be shifted by the same amount (15 nm), without changing any parameters in the set-up except the frequency of pump 2 (if both pumps are adjusted, the signal can be translated over the full 180 nm). This effect also explains why MI-generated sidebands of pump 2 also appear on the signal, as seen in Fig. 3 (left). Conversely, for fixed pump frequencies, one can tune the signal and determine the range of frequencies for which efficient idler generation occurs. This is a way of determining how many fixed-frequency signal channels can be converted simultaneously. In our case, this signal-conversion 3-dB bandwidth was 2 nm [Fig. 5 (center)]. Similarly, if the frequencies of the signal and pump 2 are both fixed, we found that the frequency of pump 1 should stay within a 2-nm bandwidth in order to not decrease the conversion efficiency by more than 3 dB [Fig. 5 (right)]. These bandwidths are limited by third-order dispersion [5].
Fig. 5. Power of the generated idler as a function of the wavelength of the pump 2 (left), signal (center) and pump 1 (right), while keeping other wavelengths fixed, for a wavelength shift of 165 nm (central wavelength of the waves: λS=1545nm, λI=1380nm, λP2=1600nm, λP1=1426nm).
4. Conclusion
Distant (180 nm shift) and efficient tunable wavelength conversions based on Bragg scattering were demonstrated experimentally. The lack of suitable pump sources at wavelengths longer than 1620 nm and shorter than 1400 nm is the main current limitation of the system, which could potentially provide efficient translation of an optical data signal from the C-band to the visible band using a highly nonlinear fiber (or a holey fiber) with suitable dispersion. The nonuniformity of the fiber diameter is an additional limiting factor. The predicted noise-free nature of this process makes it a promising candidate for developing wavelength switches in future all-optical, high-speed telecommunication networks.
References and links
1. J. Hansryd, P. A. Andrekson, M. Westlund, J. Li, and P.-O. Hedekvist, “Fiber-based optical parametric amplifiers and their applications,” IEEE J. Sel. Top. Quantum Electron. 8, 506–520 (2002). [CrossRef]
2. S. Radic and C. J. McKinstrie, “Optical amplification and signal processing in highly nonlinear optical fiber,” IEICE Trans. Electron. E88C, 859–869 (2005). [CrossRef]
3. K. Inoue, “Tunable and selective wavelength conversion using fiber four-wave mixing with two pump lights,” IEEE Photon. Technol. Lett. 6, 1451–1453 (1994). [CrossRef]
4. T. Tanemura, C. S. Goh, K. Kikuchi, and S. Y. Set, “Highly efficient arbitrary wavelength conversion within entire C-Band based on nondegenerate fiber four-wave mixing,” IEEE Photon. Technol. Lett. 16, 551–553 (2004). [CrossRef]
5. C. J. McKinstrie, J. D. Harvey, S. Radic, and M. G. Raymer, “Translation of quantum states by four-wave mixing in fibers,” Opt. Express 13, 9131–9142 (2005). [CrossRef] [PubMed]
6. J. D. Harvey, R. Leonhardt, S. Coen, G. K. L. Wong, J. C. Knight, W. J. Wadsworth, and P. S. J. Russell, “Scalar modulation instability in the normal dispersion regime by use of a photonic crystal fiber,” Opt. Lett. 28, 2225–2227 (2003). [CrossRef] [PubMed]
7. A. Y. H. Chen, G. K. L. Wong, S. G. Murdoch, R. Leonhardt, J. D. Harvey, J. C. Knight, W. J. Wadsworth, and P. S. J. Russell, “Widely tunable optical parametric generation in a photonic crystal fiber,” Opt. Lett 30, 762–764 (2005). [CrossRef] [PubMed]
8. R. Jiang, R. Saperstein, N. Alic, M. Nezhad, C. McKinstrie, J. Ford, Y. Fainman, and S. Radic, “375THz parametric translation of modulated signal from 1550nm to visible band,” Proc. OFC, paper PD16 (2006).
9. S. Radic, C. J. McKinstrie, R. M. Jopson, J. C. Centanni, A. R. Chraplyvy, C. G. Jorgensen, K. Brar, and C. Headley, “Selective suppression of idler spectral broadening in two-pump parametric architectures,” IEEE Photon. Technol. Lett. 15, 673–675 (2003). [CrossRef]
10. H. Kogelnik, R. M. Jopson, and L. E. Nelson, “Polarization-mode dispersion,” in Optical Fiber Telecommunications IVB, edited by I. Kaminow and T. Li (Academic Press, 2002), pp. 725–861.
