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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 1, JANUARY 2003
Dual-Wavelength (1050 nm + 1550 nm) Pumped Thulium-Doped Fiber Amplifier Characterization by Optical Frequency-Domain Reflectometry J. F. Martins-Filho, Member, IEEE, C. J. A. Bastos-Filho, M. T. Carvalho, M. L. Sundheimer, and A. S. L. Gomes
Abstract—We present experimental results of distributed 1550 nm) gain measurements from a dual-pumped (1050 nm thulium-doped fiber amplifier using optical frequency-domain reflectometry. We show that significant reductions in total pump power and/or fiber length are realized with the addition of a few milliwatts at 1550 nm. For our experimental conditions, the addition of 5 mW of 1550 nm allows for a reduction of 100 mW of pump power at 1050 nm or a reduction of 44% of doped fiber length to reach the same gain as with 1050-nm pumping alone.
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Index Terms—Fluoride fiber, optical fiber amplifiers, optical frequency-domain reflectometry (OFDR), thulium-doped fiber amplifier (TDFA).
I. INTRODUCTION
T
HE EVER-INCREASING demand for bandwidth in wavelength-division-multiplexing (WDM) optical-telecommunication systems is leading the research toward extending the transmission bands above and below the C-band (conventional band, from 1530 to 1560 nm). To realize this band expansion, new devices such as optical amplifiers have to be developed. Due to availability of low-loss single-mode fluoride fibers doped with rare earth ions, thulium-doped fiber amplifiers (TDFAs) appear to be strong candidates for amplifiers in the S-band (short band, from 1460 to 1520 nm) [1], [10]. In order to optimize gain and power conversion efficiency and to reduce total pump powers, several pumping schemes for TDFA have been proposed, including different single wavelength direct absorption and up-conversion pumping [1], [2], [10], and also dual-wavelength pumping schemes, where several combinations of pump wavelengths have been studied [3]–[7]. As shown in the literature and verified here, the addition of the 1550-nm pump has proven to increase gain and power conversion efficiency and to reduce the required total pump power [4]–[7]. However, there has been little attention paid to important issues such as gain dynamics and distributed gain along the fiber Manuscript received June 5, 2002; revised August 14, 2002. This work was supported by the Research and Development Center, Ericsson Telecomunicações S.A., Brazil. The work of C. J. A. Bastos-Filho and M. T. Carvalho was supported by the Brazilian Agency CAPES. J. F. Martins-Filho and C. J. A. Bastos-Filho are with the Departamento de Eletrônica e Sistemas, Universidade Federal de Pernambuco, 50740-530 Recife, Brazil (e-mail:
[email protected]). M. T. Carvalho, M. L. Sundheimer, and A. S. L. Gomes are with the Departamento de Física, Universidade Federal de Pernambuco, 50670-901 Recife, Brazil (e-mail:
[email protected]). Digital Object Identifier 10.1109/LPT.2002.805774
Fig. 1. Experimental setup for OFDR measurements of a dual-wavelength-pumped TDFA. Components are: TDF, WDM coupler, EDFA, reflector acting as a local oscillator (LO), fiber Bragg grating (BG), InGaAs photodiode (PD), and FFT spectrum analyzer.
for these dual-pump schemes. In this letter, we present for the first time for the dual-pumping scheme with 1050 and 1550 nm, measurements of distributed gain along the fiber length by optical frequency domain reflectometry (OFDR) and we show that the addition of a few milliwatts of 1550 nm in this dual-wavelength scheme allows for a significant reduction of total pump power or doped fiber length to reach the same gain as with 1050-nm pumping alone. II. EXPERIMENTAL SETUP Fig. 1 shows the experimental setup for OFDR measurements in our dual-wavelength pumped TDFA. Pump source 1 is a multilongitudinal mode diode laser operating at 1550 nm, whose output power is boosted from 2 mW up to 10 mW by a home-made erbium-doped fiber amplifier (EDFA). Pump source 2 is a diode-pumped ytterbium fiber laser emitting at 1050 nm. The laser signal to be amplified is provided by
1041-1135/03$17.00 © 2003 IEEE
MARTINS-FILHO et al.: DUAL WAVELENGTH (1050 nm
1550 nm) PUMPED THULIUM-DOPED FIBER AMPLIFIER
a commercially available single longitudinal-mode tuneable diode laser, which emits continually in the range from 1456 to 1584 nm. Pump and signal lasers are coupled together by an arrangement of two commercial WDM couplers. The output port of the WDM coupler is fusion spliced to a thulium-doped fiber module. This module was acquired from Le Verre Fluoré and it contains 18 m of ZBLAN (ZrF -BaF -LaF -AlF -NaF) fiber-doped with 2000-ppm molar Tm with a 2.8- m core and 125- m cladding diameter, 0.24-NA and 880-nm cutoff wavelength, spliced to standard silica fiber pigtails. Due to component and splice losses the maximum 1550-nm power at the input of the doped fiber module is 6 mW. We limited the maximum 1050-nm power to 450 mW to avoid possible damage to the doped fiber module and other components. The OFDR technique is based on the detection of the beat signal produced by the interference between a reference reflection (local oscillator) and the reflected or backscattered signal coming from the amplifier or device under test when the frequency of the light is swept linearly. Hence, a Michelson interferometer, a fine-tuneable narrow linewidth laser source, a low noise detector, and a fast Fourier transform (FFT) spectrum analyzer are used for this purpose. The optical circulator and the fiber Bragg grating are used to filter out most of the broadband amplified spontaneous emission (ASE) light, thus avoiding detector saturation and minimizing undesirable noise. The linewidth of the tuneable laser is of the order of 100 kHz, corresponding to a coherence length much greater than the range of our measurements. Reflectivity traces were obtained by fine sweeping ( 5 GHz full sweep) the optical frequency around the desired wavelength. Because the backscattered power is much smaller than the forward traveling signal, the gain distribution is determined by the pump and forward signal. The gain at a given fiber position is obtained by calculating the ratio of measured backscattered signal with and without pump and dividing it by two due to the double pass along the doped fiber. More details about the OFDR technique can be found in [8], [9] and references therein. III. EXPERIMENTAL RESULTS AND DISCUSSIONS Single wavelength up-conversion pumping is a two-step process in which the first photon populates the lower amplifying level ( F ) while the second photon is responsible for populating the upper amplifying level ( H ) by excited state absorption (ESA), simultaneously depopulating the lower level and creating population inversion. As is well known, 1050-nm (1047- or 1064-nm) pumping alone is useful to provide amplification but at the expense of high pump power due to the very low ground state absorption (GSA) to level F through H [1], [2], [10]. Therefore, it is not a very efficient process. In a dual-wavelength pump scheme with 1050 and 1550 nm, photons at 1550 nm help primarily to populate the F level due to strong GSA [4], [5]. Therefore, the addition of the 1550-nm pump increases the gain and power conversion efficiency of TDFAs, as described in previous works [4]–[7]. Fig. 2 shows the contribution of the 1550-nm pump to the amplifier gain as a function of pump power at 1050 nm, measured by traditional methods using an optical spectrum analyzer. An increase of
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Fig. 2. Amplifier gain measured by optical spectrum analyzer (OSA) versus pump power at 1050 nm for 0 mW of 1550 nm (triangles), and 5 mW of 1550-nm pump (squares). Signal power is 31 W ( 15 dBm) and signal wavelength is 1470 nm. Estimated error in gain measurements is 1 dB.
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Fig. 3. Photodetected backscattered signal as a function of beat frequency, as obtained from the FFT spectrum analyzer. Lower line is for signal only (no pump). Middle line is for 150 mW of 1050 nm only. Top line is for 150 mW of 1050 nm and 6 mW of 1550 nm. Signal power is 0.35 mW ( 4.5 dBm) and signal wavelength is 1470 nm.
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about 4 to 5 dB in gain is observed when the 1550-nm pump is added, as observed in previous works [4], [7]. It is more important to note from Fig. 2 that to reach 25-dB gain, we need 450 mW of 1050-nm pump in a single pump scheme. To reach the same 25-dB gain in this dual-wavelength pump scheme we need 350 mW of 1050-nm pump and 5 mW of the 1550-nm pump (355 mW total pump power). Therefore, in this dual-pump scheme we have a reduction of 95 mW from the total pump power (100 mW of 1050-nm pump power) to achieve the same gain level of 25 dB, which may reflect on costs and long-term reliability of system-implemented TDFAs, since components, doped fibers, and splices are sensitive to high power levels. It is also evident from Fig. 2 that this total pump power reduction effect depends on pump power. We performed OFDR measurements in order to have insight into the distributed gain along the fiber. This technique is also a nondestructive method of fiber length optimization for a given pump power and configuration. In Fig. 3 we show the FFT spectrum measured for three pumping conditions: no pump
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 1, JANUARY 2003
pump on fiber gain, since the curves for 4 and 6 mW are almost identical. This is due to the higher signal power ( 4.5 dBm) employed in order to improve signal-to-noise ratio for OFDR measurements. IV. CONCLUSION
Fig. 4. Fiber gain as a function of distance along the fiber for different pump powers at 1550 nm: squares 0 mW; filled circles 2 mW; triangles 4 mW; and open circles 6 mW. Signal power is 0.35 mW ( 4.5 dBm), signal wavelength is 1470 nm, 1050 nm pump power is 150 mW. Estimated error in gain measurements is 0.5 dB.
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(signal only), single pumping with 1050 nm only, and dual pumping with 1050 and 1550 nm. The sharp peaks represent reflections from TDFA components, such as WDM couplers and fiber splices. The doped fiber input and output splices are indicated. Since beat frequency in OFDR is directly proportional to distance, one can use the total fiber length to convert frequency into distance to obtain the distributed gain along the fiber. In Fig. 4 we present the fiber gain as a function of distance along the fiber for different pumping conditions. Note that the fiber gain measured by OFDR can be converted into amplifier gain if we subtract the system losses at the signal wavelength. In our TDFA we measured 5.1-dB loss for the signal, from amplifier input to doped fiber input. At the beginning of the doped fiber the low-intensity backscattered signal may lie below the noise floor level (backward ASE white beat noise within the detection bandwidth [8]), masking the distributed gain results. As the signal travels along the doped fiber and experiences gain the backscattered signal becomes greater than the noise floor level, giving reliable results of distributed gain. The noise floor can be obtained from Fig. 3, just before and after the doped fiber. Therefore, in Fig. 4 we removed the points where the noise is not negligible (beginning of the doped fiber), for every pumping condition. Approximate gain distributions in these regions can be inferred by interpolation (assuming zero gain at the doped fiber input). It is interesting to note that these OFDR measurements indicate that the dual-pump scheme can be used to reduce the doped fiber length required to achieve a certain gain level. For example, if we assume the desired gain is 14 dB, we need 150 mW of pump power using 1050 nm only and a fiber length of 18 m. In the dual-pump scheme the addition of 4 mW of 1550 nm allows for a reduction in fiber length from 18 to 10 m to achieve the same 14 dB gain. Due to the high cost of thulium-doped ZBLAN fibers, the use of this dual-pump scheme may represent a significant total amplifier cost reduction. From Fig. 4 we can also see some indication of saturation of the effect of the 1550-nm
We have characterized a dual-wavelength pumped TDFA using 1050- and 1550-nm sources by traditional measurements with an optical spectrum analyzer and also by optical frequency domain reflectometry. We obtained up to 28-dB amplifier small-signal gain from this dual-pump scheme, in accordance with previous works. We showed that the addition of 5 mW of 1550-nm pump resulted in an increase of about 5 dB in small-signal gain, or a reduction of 95 mW in total pump power, or a reduction of 44% on doped fiber length to achieve the same gain as with 1050-nm pumping alone. Actual numbers should depend on experimental conditions such as pump powers, signal power, fiber length, and doping. For example, for higher pumping powers at 1550 nm, further reduction on doped fiber length and total pumping powers is expected for small signal powers. However, for higher signal powers, the degree of fiber and pump power reduction is expected to be smaller. Other amplifier configurations, such as bidirectional or backward pumping, are likely to yield significantly different results. REFERENCES [1] T. Komukai, T. Yamamoto, T. Sugawa, and Y. Miyajima, “Upconversion pumped thulium-doped fluoride fiber amplifier and laser operating at 1.47 m,” IEEE J. Quantum Electron., vol. 31, pp. 1880–1889, Nov. 1995. [2] S. Aozasa, T. Sakamoto, T. Kanamori, K. Hoshino, K. Kobayashi, and M. Shimizu, “Tm-doped fiber amplifiers for 1470-nm-band WDM signals,” IEEE Photon. Technol. Lett., vol. 12, pp. 1331–1333, Oct. 2000. [3] A. S. L. Gomes, M. L. Sundheimer, M. T. Carvalho, J. F. Martins-Filho, C. J. A. Bastos-Filho, and W. Margulis, “Novel dual wavelength (1050 nm 800 nm) pumping scheme for thulium doped fiber amplifiers,” in Postdeadline Papers Tech. Dig. OFC 2002, Anaheim, CA, Mar. 17–22, 2002, pp. FB2-1–FB2-3. [4] T. Kasamatsu, Y. Yano, and H. Sekita, “1.50-m- band gain-shifted thulium-doped fiber amplifier with 1.05- and 1.56-m dual-wavelength pumping,” Opt. Lett., vol. 24, no. 23, pp. 1684–1686, Dec. 1999. [5] F. Roy, D. Bayart, A. Le Sauze, and P. Baniel, “Noise and gain band management of thulium-doped fiber amplifier with dual wavelength pumping schemes,” IEEE Photon. Technol. Lett., vol. 13, pp. 788–790, Aug. 2001. [6] T. Kasamatsu, Y. Yano, and T. Ono, “Laser-diode-pumped highly efficient gain-shifted thulium-doped fiber amplifier operating in the 1480–1510-nm band,” IEEE Photon. Technol. Lett., vol. 13, pp. 433–435, May 2001. [7] S. Tanabe and T. Tamaoka, “Gain characteristics of Tm-doped fiber amplifier by dual-wavelength pumping with tunable L-band source,” in Tech. Dig. OFC 2002, Anaheim, CA, Mar. 17–22, 2002, pp. 572–574. [8] J. P. von der Weid, R. Passy, B. Huttner, O. Guinard, and N. Gisin, “High-Resolution distributed-gain measurements in erbium-doped fibers,” IEEE Photon. Technol. Lett., vol. 10, pp. 949–951, July 1998. [9] M. Wegmuller, P. Oberson, O. Guinnard, B. Huttner, C. Vinegoni, and N. Gisin, “Distributed gain measurements in Er-doped fibers with high resolution and accuracy using an optical frequency domain reflectometer,” J. Lightwave Technol., vol. 18, pp. 2127–2132, Dec. 2000. [10] T. Komukai, T. Yamamoto, T. Sugawa, and Y. Miyajima, “Correction to ’Upconversion pumped thulium-doped fluoride fiber amplifier and laser operating at 1.47 m’,” IEEE J. Quantum Electron., vol. 32, p. 173, Jan. 1996.
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