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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 10, MAY 15, 2006
Fiber-Optical Parametric Amplifier With 70-dB Gain Thomas Torounidis, Peter A. Andrekson, and Bengt-Erik Olsson
Abstract—A record optical fiber gain of 70 dB was obtained in a continuous-wave pumped fiber-optical parametric amplifier. Limitations due to saturation effects from amplified spontaneous emission (ASE) and due to stimulated Brillouin scattering in this unidirectional amplifier are discussed. The spectral density of ASE was up to 180 mW/nm in agreement with theoretical expectations, illustrating the possible use as a high brightness optical noise source. Index Terms—Four-wave mixing, highly nonlinear fiber (HNLF), optical parametric amplifier.
Fig. 1.
Experimental setup for the FOPA.
II. EXPERIMENT I. INTRODUCTION
F
IBER-OPTICAL parametric amplifiers (FOPAs) are multifunctional devices that have been used in a wide range of applications, especially in telecommunications where they have been used as pulse sources, demultiplexers, preamplifiers, wavelength converters, and in all-optical sampling [1]. The FOPA can also provide very large gain as well as very broadband gain, even though, not necessarily at the same time. One of the unique features with FOPAs is the unidirectional gain, which results in no backward propagating amplified spontaneous emission (ASE) noise. This feature is expected to allow the design of very high gain amplifiers in contrast to most other types of amplifiers, e.g., erbium-doped fiber amplifiers (EDFAs). For EDFAs, there has been proposed a fundamental limit of the gain at 57–70 dB due to internal Rayleigh scattering being amplified in the backward direction of the EDFA [2]. Previously, a continuous-wave pumped FOPA was reported with a gain of 49 dB by removing the optical bandpass filter (OBPF) after the high output power EDFA used to amplify the pump signal in order to reach higher pump power into the amplifying fiber [3]. This was later increased by another group to 60 dB by using an in-line isolator between two highly nonlinear fibers (HNLFs) in order to suppress stimulated Brillouin scattering (SBS) [4]. In this letter, a single-stage FOPA with 70-dB fiber gain is presented without removing the OBPF, and thus maintaining better noise performance and also without using any isolator. The high gain is achieved using efficient SBS suppression on both pump and amplified signal. High gain amplifiers could be useful as ASE sources, preamplifiers in low bit rate communication systems, or in free space communication systems. Another motivation is to understand the limitations of the FOPA.
Manuscript received November 9, 2005; revised February 27, 2006. This work was supported by the Swedish Foundation for Strategic Research (SSF). The authors are with the Photonics Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-412 96 Göteborg, Sweden (e-mail:
[email protected];
[email protected];
[email protected];
[email protected]). Digital Object Identifier 10.1109/LPT.2006.874714
The experimental setup for the FOPA is shown in Fig. 1. A tunable laser (TL) served as the pump signal. The pump is phase modulated using two phase modulators (PMs) in series in order to increase the SBS threshold [5]. The first PM is modulated using a 10-GHz frequency. The second PM is modulated using four frequencies at 105, 325, 1000, and 3110 MHz. Polarization controllers (PCs) were used to align the state of polarization (SOP) to the principal axis of the PMs. The phase modulated pump was subsequently amplified using an ordinary EDFA and the ASE noise was filtered using an OBPF with a full-width at half-maximum (FWHM) bandwidth of 0.8 nm. The output from the EDFA was then connected to a high-power EDFA, capable of emitting output powers up to several watts. The output from the high-power EDFA was filtered using a flat top and high-power handling filter with an FWHM bandwidth of 2 nm. The signal to be amplified originates from a TL which was connected to a dual-drive Mach–Zehnder modulator. This was used as a PM with the purpose to reduce SBS from the amplified signal. In at this case, a pseudorandom binary sequence of length 2.5 Gb/s was used. The phase modulated signal power could be adjusted using the variable optical attenuator (VOA). The PC at the output of the VOA was used to align the SOP of the signal to coincide with the pump so that the most efficient parametric amplification could occur. The signal and pump were then combined using a 10-dB fused fiber coupler and the output was connected to an HNLF. The fiber consisted of three spools with different zero dispersion wavelengths at 1557.2, 1561.9, and 1561.2 nm with lengths of 200, 100, and 200 m, respectively, of 1559.7 nm. The HNLF had a which results in an average W km , dispersion slope nonlinear coefficient ps/nm km, a fiber loss of 0.7 dB/km, and an effective core area of 12 m . The power levels of the pump spectrum frequency components were adjusted to obtain a flat profile, which was resolved using a high resolution ( 100-MHz FWHM) scanning Fabry–Pérot etalon. The number of generated components (shown in Fig. 1). This when using five frequencies are gives a theoretical increase of the SBS threshold of around 24 dB. The threshold of the HNLF without PM was 16 dBm and the experimentally achieved increase was approximately 20 dB.
1041-1135/$20.00 © 2006 IEEE
TOROUNIDIS et al.: FOPA WITH 70-dB GAIN
Fig. 2. Gain versus pump power for = 1566:5 nm and = 1562:0 nm. Resolution bandwidth (RBW) = 0:5 nm. Theoretically obtained values using analytic expressions (dotted line), and numerical model (dashed lines). The gain was measured at the signal wavelength of maximum gain.
III. FIBER GAIN Fig. 2 shows the obtained gain versus input pump power nm and nm, (into the HNLF) at respectively. The experimental values were measured at the signal wavelength of peak gain of the FOPA, which means was adjusted for each power that the signal wavelength level due to the power dependent gain and bandwidth of the from 1572.6 nm at FOPA. This resulted in a tuning of low input pump power to 1582.9 nm at high pump power. is given by [1], The theoretical gain slope which results in a gain slope of 50 dB/W for the fiber used in the experiment. The theoretical maximum gain is plotted (dotted line) in Fig. 2. The maximum gain is calculated using , where is the pump power and is the length of the HNLF. The experimentally obtained nm , which agrees quite well value is 45 dB/W with theory. For high pump powers, the signal and ASE will saturate the FOPA due to the very high gain which the theory does not account for. It was confirmed here that the signal power was low enough such that it did not saturate the gain nm, dB/W which is significantly. At not in agreement with the simple theory. This was explained by using a numerical model based on [6], which takes into account that there are three sections of fiber with different . The obtained from this model are 47 dB/W at nm and 36 dB/W at nm and these values agree well with the experimental ones. The obtained gains for the two pump wavelengths using the numerical model are also plotted nm, the onset of (dashed lines) in Fig. 2. At saturation appears at lower gain than when nm, and the reason is due to more significant saturation from ASE nm. Details of in this case compared to when this will be discussed later. The differential group delay of the FOPA was 0.04 ps and this could also contribute somewhat to the differences among the measured curves in Fig. 2. An ON–OFF gain of 71 dB was obtained, however, this gain does not account for the losses in the fiber, e.g., splice losses, isolators,
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Fig. 3. (a) Gain versus input signal with = 1566:5 nm, P = 1:6 W, and = 1582:1 nm. (b) Gain versus wavelength with = 1566:5 nm, = 48 dBm. P = 1:9 W, and P
0
etc. A more appropriate measure of gain is to use the fiber gain which is found by taking the ON–OFF gain and reduce it with the loss between the input and output of the fiber. The highest fiber gain measured was 70 dB when the input signal was 48 dBm, which resulted in an output signal power of 22 dBm. It is worth noting that it is not the available pump power that limits the gain or signal output power in our experiment. The output power was limited due to both saturation effects from ASE and by SBS on the amplified signal. Fig. 3(a) shows the gain versus nm and nm and a input signal for pump power of 1.6 W. The measurement range was limited at low input powers due to poor optical signal-to-noise ratio and for higher input powers by SBS on the amplified signal. The gain as a function of wavelength was also measured and the results are shown in Fig. 3(b). The input signal in this case was 48 dBm, the pump power was 1.9 W, and was 1566.5 nm. From the figure it is seen that it is possible to obtain gains higher than 60 dB for a bandwidth of 11 nm. IV. AMPLIFIED SPONTANEOUS EMISSION The generated spontaneous emission noise from within the FOPA will be amplified in the FOPA, and will thus eventually saturate the gain. However, this will only occur in the forward direction in contrast to most other amplifiers. By increasing the pump wavelength (i.e., moving further away from ), the gain bandwidth decreases, which in turn can decrease the influence of gain saturation caused by ASE. In order to investigate this, nm, the gain at three pump wavelengths ( nm, and nm) was measured. The longest wavelength was limited by the tunability of the high-power filter and of the HNLF. For each pump the lowest wavelength by the wavelength, the signal was placed at the wavelength where maximum gain could be achieved. As seen in Fig. 4(a), the highest W increases achieved gain for a fixed pump power with increasing pump wavelength [consistently with Fig. 2(a)] and simultaneously the gain bandwidth of the FOPA decreases. The generated ASE was also measured for a fixed pump power
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 10, MAY 15, 2006
in the ASE as in the pump. There is also a tilt in the spectra for the highest pump powers which is attributed to Raman gain. This means that part of the high gain achieved could also be attributed to Raman assisted FOPA gain [8]. A spectral density higher than 10 mW/nm was measured over a bandwidth of 90 nm. The highest achieved spectral density (i.e., when the gain was 70 dB) at the peak was measured to 180 mW/nm. This value agrees well with the theoretical value of 160 mW/nm (found by . Since the single-pumped using FOPA is a single-polarization amplifier, the generated ASE is also polarized, which was also confirmed experimentally. The noise figure (NF) of the FOPA was measured to be 7 dB dB with an input power of 43.5 dBm using and at electrical method [9]. Further analysis of the FOPA in terms of NF and bit-error rate are subject for further research. V. CONCLUSION We have reported the highest fiber gain achieved in FOPAs so far with a peak gain of 70 dB. It was achieved using highly efficient SBS suppression on the pump. The gain was limited due to mainly saturation effects caused by ASE. The FOPA can also be used as a tunable ASE source with high spectral density. A spectral density of 180 mW/nm was measured in agreement with theory. ACKNOWLEDGMENT The authors thank Sumitomo Electric Industries for providing the HNLF. S. Radic and P. Kylemark are acknowledged for fruitful discussions. Fig. 4. (a) Maximum obtained gain and generated spectra for three pump wavelengths ( = 1562 nm, = 1564 nm, and = 1567 nm). Insets show typical spectra at each pump wavelength. (b) Measured spectra at the output of the HNLF for a fixed pump wavelength and at four different pump powers (P = 1:4 W, P = 1:7 W, P = 2:2 W, and P = 2:8 W).
of 1.3 W and an input signal of 49 dBm. As the pump wavelength increases, the width of the spectra decreases. For nm, the 10-dB bandwidth of the right-hand side gain lobe was 8.4 nm whereas for nm, the bandwidth increased to 14.6 nm. By tuning the pump wavelength, it is possible to obtain ASE with high spectral density over a specific bandwidth around a center wavelength determined by , which is similar to a tunable filter [7]. When moving from nm to nm, the total amount of generated ASE increased from 10 to 20 dBm. At the same time, the maximum gain increased from 38 to 50 dB. Since the ASE does not increase as much as the gain, there is less internally generated ASE that can saturate the FOPA, which leads to a higher gain. The generated spectra from the FOPA was measured for W, W, four different pump powers ( W, and W), and the result is shown in Fig. 4(b). As the pump power into the HNLF was increased, the width of the spectrum also increased. At a pump power of 1.4 W, almost all of the applied pump power was still at the pump wavelength at the output of the FOPA, whereas for a pump power around 2.2 W, there was an equal amount of the power
REFERENCES [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. Topics Quantum Electron., vol. 8, no. 3, pp. 506–520, May/Jun. 2002. [2] S. L. Hansen, K. Dybdal, and C. C. Larsen, “Gain limit in erbiumdoped fiber amplifiers due to internal Rayleigh scattering,” IEEE Photon. Technol. Lett., vol. 4, no. 6, pp. 559–561, Jun. 1992. [3] J. Hansryd and P. A. Andrekson, “Broad-band continuous-wave-pumped fiber optical parametric amplifier with 49-dB gain and wavelength-conversion efficiency,” IEEE Photon. Technol. Lett., vol. 13, no. 3, pp. 194–196, Mar. 2001. [4] K. K. Y. Wong, K. Shimizu, K. Uesaka, G. Kalogerakis, M. E. Marhic, and L. G. Kazovsky, “Continuous-wave fiber optical parametric amplifier with 60-db gain using a novel two-segment design,” IEEE Photon. Technol. Lett., vol. 15, no. 12, pp. 1707–1709, Dec. 2003. [5] S. K. Korotky, P. B. Hansen, L. Eskildsen, and J. J. Veselka, “Efficient phase modulation scheme for suppressing stimulated Brillouin scattering,” in Tech. Dig. Int. Conf. Integrated Optics and Optical Fiber Communications, vol. 2, Hong Kong, 1995, Paper WD2-1, pp. 110–111. [6] R. H. Stolen and J. E. Bjorkholm, “Parametric amplification and frequency conversion in optical fibers,” IEEE J. Quantum Electron., vol. QE-18, no. 7, pp. 1062–1072, Jul. 1982. [7] M. E. Marhic, K. K. Y. Wong, and L. G. Kazovsky, “Wide-band tuning of the gain spectra of one-pump fiber optical parametric amplifiers,” IEEE J. Sel. Topics Quantum Electron., vol. 10, no. 5, pp. 1133–1141, Sep./Oct. 2004. [8] D. A. Chestnut, C. J. S. de Matos, and J. R. Taylor, “Raman-assisted fiber optical parametric amplifier and wavelength converter in highly nonlinear fiber,” J. Opt. Soc. Amer. B, vol. 19, no. 8, pp. 1901–1904, Aug. 2002. [9] P. O. Hedekvist and P. A. Andrekson, “Noise characteristics of fiberbased optical phase conjugators,” J. Lightw. Technol., vol. 17, no. 1, pp. 74–79, Jan. 1999.
