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Inband-pumped, broadband bleaching of absorption and refractive index changes in erbium-doped fiber
Optics Communications 255 (2005) 65–71 www.elsevier.com/locate/optcom
Inband-pumped, broadband bleaching of absorption and refractive index changes in erbium-doped fiber R.N. Liu a, I.A. Kostko a b
b,*
, R. Kashyap
a,b
, K. Wu a, P. Kiiveri
c
Department of Electrical Engineering, Ecole Polytechnique de Montreal, CP 6079 Succ. Centre-ville, Montreal, Que., Canada H3C 3A7 Department of Engineering Physics, Ecole Polytechnique de Montreal, CP 6079 Succ. Centre-ville, Montreal, Que., Canada H3C 3A7 c Liekki Oy, Sorronrinne 9, 08500 Lohja, Finland Received 25 February 2005; received in revised form 7 May 2005; accepted 31 May 2005
Abstract Broad-band absorption bleaching in erbium-doped fiber by in-band narrow-linewidth pumping is reported. For the first time, spectra of absorption bleaching of erbium-doped fiber is measured and the refractive index changes inferred through the Kramers–Kro¨nig relations in this true two-level system in order to understand its operation in the external cavity of a laser. 2005 Elsevier B.V. All rights reserved. Keywords: Erbium-doped fiber; External cavity lasers; Optical hole-burning; Saturable absorption
1. Introduction Erbium- and ytterbium-doped fibers have been used extensively in optical fiber amplifiers and lasers. Saturable absorption and the effect of pump power on the gain in these devices have been observed by various researchers when pumping in a three level [1–3] as well as in quasi-two level configurations [4,5]. In saturable absorptive medium, a * Corresponding author. Tel.: +1 514 3404711; fax: +1 514 3403218. E-mail addresses: [email protected] (I.A. Kostko), [email protected] (R. Kashyap).
standing wave can induce a periodic variation in absorption population to form a reflective Bragg grating. These filters have been observed at 1040 nm when pumping a saturable absorber (SA) at 975 nm [1], as well as in the 1530– 1560 nm band where a fiber laser was pumped at 1064 nm [6] or 1480 nm [7]. A filter at 1530 nm was also reported when a laser at 980 nm was used to pump a saturable absorber in a loop-mirror configuration [2]. Spatial hole-burning is also used in lasers with external cavity. A laser with intracavity erbium-doped fiber (EDF) has been shown to generate stable single-frequency output at 1535 nm [8]. A recently proposed device shows
0030-4018/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.05.054
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R.N. Liu et al. / Optics Communications 255 (2005) 65–71
Laser diode
Saturable absorber
Bragg grating
Fig. 1. Schematic of laser with saturable absorber in the external cavity.
similar characteristics using an Yb-doped fiber in the external cavity of a high-power 980 nm semiconductor laser (Fig. 1) [9]. Operation of these lasers in the ultra-stable, narrow-line-width regime makes them potential sources in radio-over-fiber and dense wavelength division multiplexing (DWDM) applications. Saturable absorption measurements are well known as a technique for measuring clustering in rare earth doped fibers [10,11]. If a laser with a saturable absorber in the external cavity is pumped at a wavelength of an external fiber Bragg grating mirror (1530 for Er-doped and 976 for Yr-doped fiber) the absorption and emission are at the same wavelength. We refer to this as inband-pumping. Even though a few devices demonstrated before use inband-pumping of a saturable absorber [8,9], there has been no reported measurement of saturable absorption in a true two-level system. In this paper, we study this behaviour for the first time by inband-pumping of EDF with a narrowbandwidth laser source and demonstrate strong broadband bleaching of the absorption. In this work, we show that fiber inhomogeneities can be probed by making broadband absorption measurements under single-wavelength, strong-signal, partial bleaching conditions. The technique can also be used in the design of laser cavities. We infer the changes in the refractive index via Kramers– Kro¨nig relations to estimate the coupling constants of the induced grating. The measured characteristics of saturable absorption of EDF are key parameters for dynamic modeling of lasers with saturable absorber in the external cavity. 2. Experiments Fig. 2(a) shows the measurement setup for the bleaching of absorption in a 50-cm-long piece of
Fig. 2. The variation of absorption of EDF per unit length as a function of input power at indicated pump wavelengths. (a) Experimental setup and (b) result.
highly doped Liekki Er 40-4/125 EDF. The EDF is pumped by a single narrow-linewidth, mWpower DFB laser only with wavelength selected in the range of 1480–1540 nm, with a full-width half-maximum (FWHM) less than 10 pm. The output power from a doped fiber followed by a narrow band filter is measured (at the same wavelength as the input) as a function of pump power. The measured output provides the net absorption, which is the sum of absorption and emission in the EDF. Absorption per unit length in this fiber for different pump wavelengths is shown in Fig. 2(b) as a function of pump power. The maximum unpumped absorption per meter at a pump wavelength of 1530 nm is 42.4 dB/m. Note worthy is the fact that the bleaching shows a threshold pump–power of approximately 15 dBm for the length of fiber used. If the power is increased above this threshold, absorption per unit length decreases rapidly. The slope of this bleaching depends on the peak unpumped absorption and pump wavelengths. This characteristic is particularly useful for lasers with a doped fiber in the cavity as the strength of a dynamic grating is defined by the maximum change in the absorption with power at a wavelength of the external fiber Bragg grating of a laser.
R.N. Liu et al. / Optics Communications 255 (2005) 65–71
Pumping the SA inband with a mW level power source alters the absorption at other wavelengths independent of the pump wavelength. Fig. 3(a) shows another setup for the measurement of absorption spectra for a piece of 46-cm-long EDF. The maximum unpumped absorption of this EDF is approximately 47 dB/m at 1528.7 nm and the net absorption at 1532 nm is 36.5 dB/m. A stable, narrow-line-width source, lasing at 1532 nm (FWHM <3 pm, amplitude fluctuation <0.1 dBm and with a side-mode suppression ratio 30 dB), with the maximum power of >5 dBm was used as a pump laser (PuL). A second counter-propagating tunable probe laser (PrL) was swept in the 1520–1570 nm wavelength range to measure absorption at each wavelength. The maximum power of PrL was deliberately kept very low in comparison with the PuL (<30 dB) so that the PrL does not substantially alter the ion-popula-
Fig. 3. The spectra of absorption bleaching of the EDF with a maximum pump power of 2.8 dBm at 1532 nm for a 46 cm piece of fiber. (a) Experimental setup and (b) result. The pump wavelength is indicated by the arrow.
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tions in the ground and upper states. In order to keep the wavelength of the pump laser constant, an in-line attenuator was used to change the power output of the PuL rather than by changing the drive current of the pump laser. The measurement resolution was ±1.5 pm in wavelength and ±0.05 dBm in power. The PrL was scanned and the absorption spectrum measured across the whole band with and without the EDF. With the PuL coupled to the same EDF, the absorption is re-measured at each wavelength within the absorption bandwidth of the EDF. The difference between pumped and unpumped absorption is the bleached spectrum, shown in Fig. 3 for different values of the pump power. When the EDF is pumped by a 2.8 dBmpump laser PuL at 1532 nm, bleaching at the absorption peak (1528.7 nm) is approximately 39.5 dB/m. It can be seen that bleaching of absorption also occurs at wavelengths shorter than the pump wavelength, which indicates that the upper levels of the ion-population are substantially affected by the pump photons. In Fig. 4, the wavelength of the pump laser PuL is 1554 nm, for which the net unpumped absorption for this piece of the EDF is approximately 14 dB/m. When a 3.3 dBm-PuL at 1554 nm is used to pump the 46-cm-long EDF, bleaching of 16 dB (equivalent to 33.9 dB/m) is observed at 1528.7 nm. The shape of the spectra with
Fig. 4. The spectra of absorption bleaching of the EDF (46 cm) at a pump wavelength of 1554 nm. The setup is the same as in Fig. 3(a). The pump wavelength is indicated by the arrow.
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1554 nm pumping is similar to that shown in Fig. 3. When the EDF is pumped at a wavelength with low net absorption (1554 nm), the largest change in absorption occurs at 1528.7 nm, where the unpumped absorption is highest. Since the EDF is considered to be spatially (axially) homogeneous, all pump photons have an equal probability to be absorbed [4]. However, the absorption-rate is related to the difference between absorption and emission cross-sections at the pump wavelength. When an EDF is pumped with a narrow-linewidth source, the absorption of photons is determined by the frequency dependence of absorption and emission rates at the pump wavelength. This causes less number of ions to be available for absorbing at other wavelengths outside of the pump wavelength resulting in broadband SA. Pumping at other wavelengths within the absorption band was also tried; the results showed only minor differences. It is clear from the above experiments that by changing the inband-pump power one can alter absorption at the all other wavelengths. Spectral hole-burning in EDF can significantly impair the performance of an EDFA. In an amplifier pumped at 980 or 1480 nm, spectral hole burning observed at 1550 nm depends on a host material [12,13], pump wavelength [14], saturated signal wavelength [15], and pump power [16]. The spectral holes in the 1530–1550 nm band have been reported to be about 1–2 dB in power and have a bandwidth of 3–10 nm [12–15]. We tried to observe the occurrence of spectral hole-burning in our fibers. In our experiments with a narrow linewidth inband-pump the absorption of the EDF and the stimulated emission are at the same wavelength. The ions should be excited to a very narrow band of the upper level and almost no non-radiative (no scattering) transitions should occur within this band. We specifically did not see any evidence of spectral hole-burning close to the pump-wavelengths in our measurements. In a laser with a doped fiber in the external cavity (shown in Fig. 1) high power at the resonant wavelength causes bleaching of absorption in the doped fiber. We have shown that a narrow linewidth resonance can bleach absorption over the whole band. When a standing wave is formed
in a semiconductor external-cavity laser with a saturable absorber [8,9] and external fiber grating mirror, modulation in the ion-population gives rise to spatial hole-burning and hence a spatially periodic modulation in refractive index, causing a dynamic grating to form via the Kramers–Kro¨nig effect. The reflectivity of this Bragg grating increases with the intra-cavity power that forms the longitudinal modulation of absorption. Since no obvious spectral hole-burning was found in the vicinity of the pump wavelength, broadband absorption bleaching, and therefore spatial holeburning is the key reason for linewidth narrowing and stabilization of the wavelength of the external cavity EDF laser. The described measurements, which can be equally applied to EDF as well as a Yb-doped fiber in the 980 nm band, allow us to estimate the maximum variation of refractive index in a dynamic grating at a given pump wavelength. The change in refractive index, Dn, due to the pump-induced change in absorption Da is derived from the Kramers–Kro¨nig equations [3]: Z x2 c Daðx0 Þ DnðxÞ ¼ P:V: dx0 ; 0 2 2 p x1 ðx Þ x where c is the velocity of light in vacuum, and P.V. is the principal value of the integral derived over the frequency range x1 < x < x2, where the absorption changes are significant. The wider the wavelength range considered, the more accurate are the results. In this work, the measurements were done in the narrow wavelength range (1520–1570 nm), limited by the bandwidth of the JDS tunable laser. We extrapolated the spectra of absorption bleaching in order to calculate the spectrum of the change in refractive index due to pumping using the Kramers–Kro¨nig relations. When the EDF is pumped at 1554 nm, bleaching of absorption is still higher at the peak of the absorption spectrum (1528 nm), rather than at the wavelength of pumping. The spectra of the refractive index change at low-power (27 dBm) and high-power (3 dBm) pumping at 1532 and 1554 nm are shown in Fig. 5. When the pump power is low (27 dBm) and the EDF is pumped at 1532 nm, the refractive index change is Dn3 · 108, and Dn 1.4 · 108 at 1554 nm when the EDF is pumped at the same wavelength.
R.N. Liu et al. / Optics Communications 255 (2005) 65–71
Fig. 5. The spectra of refractive index change of the EDF at pump wavelengths of 1532 nm (solid line) and 1554 nm (dashed line) at pump powers of 2.7 and 3.3 dBm, respectively. Dotted line shows spectrum of the refractive index change at 27 dBm pump power. Dashed line is the level of refractive index change at 1532 nm when the EDF is pumped at 1532 nm with a power of 2.8 dBm.
When the fiber is pumped with 2.8 dBm of a narrow-linewidth source at 1532 nm, the change in the refractive index at the same wavelength is almost two orders of magnitude larger, Dn 1.2 · 106; and Dn 0.98 · 106 when the EDF is pumped by 3.3 dBm at 1554 nm. For higher pump powers, e.g., 100 mW, the maximum possible change in absorption at 1532 nm may be an additional 6 dB/m, when the system is completely bleached and the population difference is saturated at high pump power. Thus, the refractive index change may be estimated by multiplying its value at 2.8 dBm pump power by 1.16. Therefore, if the fiber is pumped with 100 mW at a wavelength of 1532 nm a maximum refractive index change of 1.4 · 106 may be expected at this wavelength.
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The peak-to-trough pumped–unpumped change in refractive index is 3.76 · 106 when pumped at 1532 nm and 3.3 · 106 when pumped at 1554 nm. This value is an order of magnitude higher than that previously reported for the three-level system in EDF amplifier [3]. Normalized to the peak absorption coefficient of 47 dB/m, this gives a general value relating the pumped–unpumped refractive index change to the pumped–unpumped change in transmission in the EDF of 8.0 · 108/ (dB/m), which is close to the value, obtained in [3]. These results are summarized in Table 1. A coupling constant of an induced grating in the external cavity laser (Fig. 1) can be estimated from the refractive index change Dn [17] 2Dn k¼ ; nk where n is a refractive index of the doped fiber (n 1.45) and k is the central wavelength of the dynamic grating. Hence, an EDF with an induced refractive index change of Dn 1.4 · 106 at k = 1532 nm has a dynamic grating with the coupling constant of j 1.26. A dynamic grating with a smaller coupling constant of j 0.887 is formed at 1554 nm when an induced refractive index change is Dn 1.0 · 106. Therefore, a coupling constant of a dynamic grating is higher at 1532 than at 1554 nm. A shape of a reflectivity spectrum of a dynamic grating is defined by the coupling factor and the length of the EDF, L. As the reflectivity of the grating depends on a product of jL according to the formula R ¼ tanh2 ðjLÞ; it can be maximized by increasing either the length or the coupling factor of the EDF. For example, a
Table 1 Summary of refractive index changes at different pump wavelengths and pump powers Pump wavelength (nm)
Pump power (dBm)
Refractive index change at pump wavelength (Dn)
Peak-to-trough pumped–unpumped change in refractive index
Pumped–unpumped refractive index change to the pumped–unpumped change in transmission (dB/m)1
1532 1532 1554 1554
27.2 2.8 26.7 3.3
3 · 108 1.2 · 106 1.4 · 108 0.98 · 106
1.1 · 107 3.76 · 106 1.5 · 107 3.3 · 106
2.3 · 109 8.0 · 108 3.2 · 109 7.0 · 108
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grating with j 1.26 (k 1532 nm) and L = 0.50 m has the same reflectivity R 30% as a grating with j 0.87 (k 1554 nm) and L 0.73 m. Therefore, by optimization of the pump wavelength, the doped fiber section of a laser may be designed shorter by 23 cm and have the same reflectivity as a grating with j 0.87. The peak of the refractive index change in Fig. 5 gives the maximum coupling factor of the dynamic grating and thus is the optimal pump wavelength for the EDF. In a laser with a saturable absorber in the external cavity this is an optimal central wavelength of an external fiber Bragg grating of the laser. A side-mode suppression ratio and a linewidth of a laser with a saturable absorber in the external cavity are defined by interplay between the reflectivity spectra of the fixed external and dynamic gratings. It has been shown earlier that the higher contrast between the reflectivity of the external and dynamic gratings leads to the higher sidemode suppression ratio [18]. An external Bragg reflector has a wide bandwidth (0.2 nm in [8,9]) and does not select any of more than 100 external cavity modes. However, a dynamic grating has a very narrow bandwidth, which depends on the length of the doped fiber. For example, a dynamic grating of L 50 cm and j 1.26 (R 30%) has a bandwidth between the first zeroes of 3.3 pm, which is only 2.8 times wider than the longitudinal mode spacing in a laser with a total cavity length of 70 cm. Thus, the laser may oscillate on a single longitudinal mode. As a result of a long external cavity, the linewidth of this longitudinal mode is very narrow and has been measured to be less than 1 kHz [8]. A laser with a saturable absorber in the external cavity has a highly linear LI characteristic and a narrow linewidth when it operates in a stable single-mode regime [9]. A transition from coherence collapse to a single-mode regime in a laser with Yb-doped fiber and 980 nm pump diode has been shown earlier by simulations of an external cavity laser with two gratings [18]; the same approach can be applied to a laser with EDF, pumped at 1530 nm. A single-mode operation of these lasers depends on a drive current and the reflectivity of the dynamic grating. The measured refractive
index change and the coupling factor can be used in the dynamics and stability simulations of the lasers with saturable absorber in the external cavity.
3. Conclusions Effect of bleaching in EDF by pumping with an inband mW-power, narrow-linewidth laser source is reported for the first time. It is demonstrated that narrow-linewidth pumping at any single wavelength within absorption band results in bleaching of the entire absorption spectrum of EDF and the depth of bleaching in EDF depends on the pump power and the pump wavelength. 39.5 dB/m bleaching at the peak of the absorption spectrum was measured when a piece of EDF was pumped with 2.8 dBm at 1532 nm. The wide-band nature of saturable absorption in doped fibers plays a significant role in creating new devices such as single frequency lasers tunable over entire absorption spectrum of the doped fiber. The measured data is particularly useful for time-domain simulations of lasers with saturable absorber in the external cavity, the results of which will be reported in a future publication. Acknowledgements The authors acknowledge the Canadian Institute for Photonics Innovation (CIPI), the Natural Science & Engineering Research Council of CanadaÕs (NSERC) Canada Research Chairs program, and the NSERCÕs Discovery Grants program for their support of the research.
References [1] R. Paschotta, J. Nilsson, L. Reekie, A.C. Trooper, D.C. Hanna, Opt. Lett. 22 (1997) 40. [2] S.A. Havstad, B. Fischer, A.E. Willner, M.G. Wickham, Opt. Lett. 24 (1999) 1466. [3] S.C. Fleming, T.J. Whitley, IEEE J. Quantum Electron. 32 (1996) 1113. [4] E. Desurvire, Erbium-doped Fiber Amplifiers: Principles and Applications, Wiley, 1994. [5] J.L. Zyskind, C.R. Giles, E. Desurvire, J.R. Simpson, IEEE Photon. Technol. Lett. 1 (1999) 428.
R.N. Liu et al. / Optics Communications 255 (2005) 65–71 [6] Y. Cheng, J.T. Kringlebotn, W.H. Loh, R.I. Laming, D.N. Payne, Opt. Lett. 20 (1995) 875. [7] N. Kishi, IEEE Photon. Technol. Lett. 11 (1999) 182. [8] W.H. Loh, R.I. Laming, M.N. Zervas, Appl. Phys. Lett. 66 (1995) 3422. [9] F.N. Timofeev, R. Kashyap, Opt. Exp. 11 (2003) 515. [10] P. Myslinski, D. Nguyen, J. Chrostowski, Techn. Dig. Opt. Fiber Commun. Conf. (OFC) 8 (1995) 172. [11] Chen-Chun Ye, B. Jaskorzynska, S. Helmfrid, P.-Y. Fonjallaz, Appl. Opt. 37 (1998) 6362. [12] I. Joindot, F. Dupre, Electron. Lett. 33 (1997) 1239.
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[13] J.W. Sulhoff, A.K. Srivastava, C. Wolf, Y. Sun, J.L. Zyskind, Photon. Technol. Lett. 9 (1997) 1578. [14] M.J. Yadlowsky, J. Lightwave Technol. 17 (1999) 1643. [15] E. Rudkevich, D.M. Baney, J. Stimple, D. Derickson, G. Wang, IEEE Photon. Technol. Lett. 11 (1999) 542. [16] M. Bolshtyansky, J. Lightwave Technol. 21 (2003) 1032. [17] J. Carroll, J. Whiteaway, D. Plumb, Distributed Feedback Semiconductor Lasers, IEE, Stevenage, 1998. [18] I.A. Kostko, R. Kashyap, Proc. SPIE 5579 (2004) 367, Photon. North 2004.
Inband-pumped, broadband bleaching of absorption and refractive index changes in erbium-doped fiber R.N. Liu a, I.A. Kostko a b
b,*
, R. Kashyap
a,b
, K. Wu a, P. Kiiveri
c
Department of Electrical Engineering, Ecole Polytechnique de Montreal, CP 6079 Succ. Centre-ville, Montreal, Que., Canada H3C 3A7 Department of Engineering Physics, Ecole Polytechnique de Montreal, CP 6079 Succ. Centre-ville, Montreal, Que., Canada H3C 3A7 c Liekki Oy, Sorronrinne 9, 08500 Lohja, Finland Received 25 February 2005; received in revised form 7 May 2005; accepted 31 May 2005
Abstract Broad-band absorption bleaching in erbium-doped fiber by in-band narrow-linewidth pumping is reported. For the first time, spectra of absorption bleaching of erbium-doped fiber is measured and the refractive index changes inferred through the Kramers–Kro¨nig relations in this true two-level system in order to understand its operation in the external cavity of a laser. 2005 Elsevier B.V. All rights reserved. Keywords: Erbium-doped fiber; External cavity lasers; Optical hole-burning; Saturable absorption
1. Introduction Erbium- and ytterbium-doped fibers have been used extensively in optical fiber amplifiers and lasers. Saturable absorption and the effect of pump power on the gain in these devices have been observed by various researchers when pumping in a three level [1–3] as well as in quasi-two level configurations [4,5]. In saturable absorptive medium, a * Corresponding author. Tel.: +1 514 3404711; fax: +1 514 3403218. E-mail addresses: [email protected] (I.A. Kostko), [email protected] (R. Kashyap).
standing wave can induce a periodic variation in absorption population to form a reflective Bragg grating. These filters have been observed at 1040 nm when pumping a saturable absorber (SA) at 975 nm [1], as well as in the 1530– 1560 nm band where a fiber laser was pumped at 1064 nm [6] or 1480 nm [7]. A filter at 1530 nm was also reported when a laser at 980 nm was used to pump a saturable absorber in a loop-mirror configuration [2]. Spatial hole-burning is also used in lasers with external cavity. A laser with intracavity erbium-doped fiber (EDF) has been shown to generate stable single-frequency output at 1535 nm [8]. A recently proposed device shows
0030-4018/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.05.054
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R.N. Liu et al. / Optics Communications 255 (2005) 65–71
Laser diode
Saturable absorber
Bragg grating
Fig. 1. Schematic of laser with saturable absorber in the external cavity.
similar characteristics using an Yb-doped fiber in the external cavity of a high-power 980 nm semiconductor laser (Fig. 1) [9]. Operation of these lasers in the ultra-stable, narrow-line-width regime makes them potential sources in radio-over-fiber and dense wavelength division multiplexing (DWDM) applications. Saturable absorption measurements are well known as a technique for measuring clustering in rare earth doped fibers [10,11]. If a laser with a saturable absorber in the external cavity is pumped at a wavelength of an external fiber Bragg grating mirror (1530 for Er-doped and 976 for Yr-doped fiber) the absorption and emission are at the same wavelength. We refer to this as inband-pumping. Even though a few devices demonstrated before use inband-pumping of a saturable absorber [8,9], there has been no reported measurement of saturable absorption in a true two-level system. In this paper, we study this behaviour for the first time by inband-pumping of EDF with a narrowbandwidth laser source and demonstrate strong broadband bleaching of the absorption. In this work, we show that fiber inhomogeneities can be probed by making broadband absorption measurements under single-wavelength, strong-signal, partial bleaching conditions. The technique can also be used in the design of laser cavities. We infer the changes in the refractive index via Kramers– Kro¨nig relations to estimate the coupling constants of the induced grating. The measured characteristics of saturable absorption of EDF are key parameters for dynamic modeling of lasers with saturable absorber in the external cavity. 2. Experiments Fig. 2(a) shows the measurement setup for the bleaching of absorption in a 50-cm-long piece of
Fig. 2. The variation of absorption of EDF per unit length as a function of input power at indicated pump wavelengths. (a) Experimental setup and (b) result.
highly doped Liekki Er 40-4/125 EDF. The EDF is pumped by a single narrow-linewidth, mWpower DFB laser only with wavelength selected in the range of 1480–1540 nm, with a full-width half-maximum (FWHM) less than 10 pm. The output power from a doped fiber followed by a narrow band filter is measured (at the same wavelength as the input) as a function of pump power. The measured output provides the net absorption, which is the sum of absorption and emission in the EDF. Absorption per unit length in this fiber for different pump wavelengths is shown in Fig. 2(b) as a function of pump power. The maximum unpumped absorption per meter at a pump wavelength of 1530 nm is 42.4 dB/m. Note worthy is the fact that the bleaching shows a threshold pump–power of approximately 15 dBm for the length of fiber used. If the power is increased above this threshold, absorption per unit length decreases rapidly. The slope of this bleaching depends on the peak unpumped absorption and pump wavelengths. This characteristic is particularly useful for lasers with a doped fiber in the cavity as the strength of a dynamic grating is defined by the maximum change in the absorption with power at a wavelength of the external fiber Bragg grating of a laser.
R.N. Liu et al. / Optics Communications 255 (2005) 65–71
Pumping the SA inband with a mW level power source alters the absorption at other wavelengths independent of the pump wavelength. Fig. 3(a) shows another setup for the measurement of absorption spectra for a piece of 46-cm-long EDF. The maximum unpumped absorption of this EDF is approximately 47 dB/m at 1528.7 nm and the net absorption at 1532 nm is 36.5 dB/m. A stable, narrow-line-width source, lasing at 1532 nm (FWHM <3 pm, amplitude fluctuation <0.1 dBm and with a side-mode suppression ratio 30 dB), with the maximum power of >5 dBm was used as a pump laser (PuL). A second counter-propagating tunable probe laser (PrL) was swept in the 1520–1570 nm wavelength range to measure absorption at each wavelength. The maximum power of PrL was deliberately kept very low in comparison with the PuL (<30 dB) so that the PrL does not substantially alter the ion-popula-
Fig. 3. The spectra of absorption bleaching of the EDF with a maximum pump power of 2.8 dBm at 1532 nm for a 46 cm piece of fiber. (a) Experimental setup and (b) result. The pump wavelength is indicated by the arrow.
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tions in the ground and upper states. In order to keep the wavelength of the pump laser constant, an in-line attenuator was used to change the power output of the PuL rather than by changing the drive current of the pump laser. The measurement resolution was ±1.5 pm in wavelength and ±0.05 dBm in power. The PrL was scanned and the absorption spectrum measured across the whole band with and without the EDF. With the PuL coupled to the same EDF, the absorption is re-measured at each wavelength within the absorption bandwidth of the EDF. The difference between pumped and unpumped absorption is the bleached spectrum, shown in Fig. 3 for different values of the pump power. When the EDF is pumped by a 2.8 dBmpump laser PuL at 1532 nm, bleaching at the absorption peak (1528.7 nm) is approximately 39.5 dB/m. It can be seen that bleaching of absorption also occurs at wavelengths shorter than the pump wavelength, which indicates that the upper levels of the ion-population are substantially affected by the pump photons. In Fig. 4, the wavelength of the pump laser PuL is 1554 nm, for which the net unpumped absorption for this piece of the EDF is approximately 14 dB/m. When a 3.3 dBm-PuL at 1554 nm is used to pump the 46-cm-long EDF, bleaching of 16 dB (equivalent to 33.9 dB/m) is observed at 1528.7 nm. The shape of the spectra with
Fig. 4. The spectra of absorption bleaching of the EDF (46 cm) at a pump wavelength of 1554 nm. The setup is the same as in Fig. 3(a). The pump wavelength is indicated by the arrow.
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1554 nm pumping is similar to that shown in Fig. 3. When the EDF is pumped at a wavelength with low net absorption (1554 nm), the largest change in absorption occurs at 1528.7 nm, where the unpumped absorption is highest. Since the EDF is considered to be spatially (axially) homogeneous, all pump photons have an equal probability to be absorbed [4]. However, the absorption-rate is related to the difference between absorption and emission cross-sections at the pump wavelength. When an EDF is pumped with a narrow-linewidth source, the absorption of photons is determined by the frequency dependence of absorption and emission rates at the pump wavelength. This causes less number of ions to be available for absorbing at other wavelengths outside of the pump wavelength resulting in broadband SA. Pumping at other wavelengths within the absorption band was also tried; the results showed only minor differences. It is clear from the above experiments that by changing the inband-pump power one can alter absorption at the all other wavelengths. Spectral hole-burning in EDF can significantly impair the performance of an EDFA. In an amplifier pumped at 980 or 1480 nm, spectral hole burning observed at 1550 nm depends on a host material [12,13], pump wavelength [14], saturated signal wavelength [15], and pump power [16]. The spectral holes in the 1530–1550 nm band have been reported to be about 1–2 dB in power and have a bandwidth of 3–10 nm [12–15]. We tried to observe the occurrence of spectral hole-burning in our fibers. In our experiments with a narrow linewidth inband-pump the absorption of the EDF and the stimulated emission are at the same wavelength. The ions should be excited to a very narrow band of the upper level and almost no non-radiative (no scattering) transitions should occur within this band. We specifically did not see any evidence of spectral hole-burning close to the pump-wavelengths in our measurements. In a laser with a doped fiber in the external cavity (shown in Fig. 1) high power at the resonant wavelength causes bleaching of absorption in the doped fiber. We have shown that a narrow linewidth resonance can bleach absorption over the whole band. When a standing wave is formed
in a semiconductor external-cavity laser with a saturable absorber [8,9] and external fiber grating mirror, modulation in the ion-population gives rise to spatial hole-burning and hence a spatially periodic modulation in refractive index, causing a dynamic grating to form via the Kramers–Kro¨nig effect. The reflectivity of this Bragg grating increases with the intra-cavity power that forms the longitudinal modulation of absorption. Since no obvious spectral hole-burning was found in the vicinity of the pump wavelength, broadband absorption bleaching, and therefore spatial holeburning is the key reason for linewidth narrowing and stabilization of the wavelength of the external cavity EDF laser. The described measurements, which can be equally applied to EDF as well as a Yb-doped fiber in the 980 nm band, allow us to estimate the maximum variation of refractive index in a dynamic grating at a given pump wavelength. The change in refractive index, Dn, due to the pump-induced change in absorption Da is derived from the Kramers–Kro¨nig equations [3]: Z x2 c Daðx0 Þ DnðxÞ ¼ P:V: dx0 ; 0 2 2 p x1 ðx Þ x where c is the velocity of light in vacuum, and P.V. is the principal value of the integral derived over the frequency range x1 < x < x2, where the absorption changes are significant. The wider the wavelength range considered, the more accurate are the results. In this work, the measurements were done in the narrow wavelength range (1520–1570 nm), limited by the bandwidth of the JDS tunable laser. We extrapolated the spectra of absorption bleaching in order to calculate the spectrum of the change in refractive index due to pumping using the Kramers–Kro¨nig relations. When the EDF is pumped at 1554 nm, bleaching of absorption is still higher at the peak of the absorption spectrum (1528 nm), rather than at the wavelength of pumping. The spectra of the refractive index change at low-power (27 dBm) and high-power (3 dBm) pumping at 1532 and 1554 nm are shown in Fig. 5. When the pump power is low (27 dBm) and the EDF is pumped at 1532 nm, the refractive index change is Dn3 · 108, and Dn 1.4 · 108 at 1554 nm when the EDF is pumped at the same wavelength.
R.N. Liu et al. / Optics Communications 255 (2005) 65–71
Fig. 5. The spectra of refractive index change of the EDF at pump wavelengths of 1532 nm (solid line) and 1554 nm (dashed line) at pump powers of 2.7 and 3.3 dBm, respectively. Dotted line shows spectrum of the refractive index change at 27 dBm pump power. Dashed line is the level of refractive index change at 1532 nm when the EDF is pumped at 1532 nm with a power of 2.8 dBm.
When the fiber is pumped with 2.8 dBm of a narrow-linewidth source at 1532 nm, the change in the refractive index at the same wavelength is almost two orders of magnitude larger, Dn 1.2 · 106; and Dn 0.98 · 106 when the EDF is pumped by 3.3 dBm at 1554 nm. For higher pump powers, e.g., 100 mW, the maximum possible change in absorption at 1532 nm may be an additional 6 dB/m, when the system is completely bleached and the population difference is saturated at high pump power. Thus, the refractive index change may be estimated by multiplying its value at 2.8 dBm pump power by 1.16. Therefore, if the fiber is pumped with 100 mW at a wavelength of 1532 nm a maximum refractive index change of 1.4 · 106 may be expected at this wavelength.
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The peak-to-trough pumped–unpumped change in refractive index is 3.76 · 106 when pumped at 1532 nm and 3.3 · 106 when pumped at 1554 nm. This value is an order of magnitude higher than that previously reported for the three-level system in EDF amplifier [3]. Normalized to the peak absorption coefficient of 47 dB/m, this gives a general value relating the pumped–unpumped refractive index change to the pumped–unpumped change in transmission in the EDF of 8.0 · 108/ (dB/m), which is close to the value, obtained in [3]. These results are summarized in Table 1. A coupling constant of an induced grating in the external cavity laser (Fig. 1) can be estimated from the refractive index change Dn [17] 2Dn k¼ ; nk where n is a refractive index of the doped fiber (n 1.45) and k is the central wavelength of the dynamic grating. Hence, an EDF with an induced refractive index change of Dn 1.4 · 106 at k = 1532 nm has a dynamic grating with the coupling constant of j 1.26. A dynamic grating with a smaller coupling constant of j 0.887 is formed at 1554 nm when an induced refractive index change is Dn 1.0 · 106. Therefore, a coupling constant of a dynamic grating is higher at 1532 than at 1554 nm. A shape of a reflectivity spectrum of a dynamic grating is defined by the coupling factor and the length of the EDF, L. As the reflectivity of the grating depends on a product of jL according to the formula R ¼ tanh2 ðjLÞ; it can be maximized by increasing either the length or the coupling factor of the EDF. For example, a
Table 1 Summary of refractive index changes at different pump wavelengths and pump powers Pump wavelength (nm)
Pump power (dBm)
Refractive index change at pump wavelength (Dn)
Peak-to-trough pumped–unpumped change in refractive index
Pumped–unpumped refractive index change to the pumped–unpumped change in transmission (dB/m)1
1532 1532 1554 1554
27.2 2.8 26.7 3.3
3 · 108 1.2 · 106 1.4 · 108 0.98 · 106
1.1 · 107 3.76 · 106 1.5 · 107 3.3 · 106
2.3 · 109 8.0 · 108 3.2 · 109 7.0 · 108
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grating with j 1.26 (k 1532 nm) and L = 0.50 m has the same reflectivity R 30% as a grating with j 0.87 (k 1554 nm) and L 0.73 m. Therefore, by optimization of the pump wavelength, the doped fiber section of a laser may be designed shorter by 23 cm and have the same reflectivity as a grating with j 0.87. The peak of the refractive index change in Fig. 5 gives the maximum coupling factor of the dynamic grating and thus is the optimal pump wavelength for the EDF. In a laser with a saturable absorber in the external cavity this is an optimal central wavelength of an external fiber Bragg grating of the laser. A side-mode suppression ratio and a linewidth of a laser with a saturable absorber in the external cavity are defined by interplay between the reflectivity spectra of the fixed external and dynamic gratings. It has been shown earlier that the higher contrast between the reflectivity of the external and dynamic gratings leads to the higher sidemode suppression ratio [18]. An external Bragg reflector has a wide bandwidth (0.2 nm in [8,9]) and does not select any of more than 100 external cavity modes. However, a dynamic grating has a very narrow bandwidth, which depends on the length of the doped fiber. For example, a dynamic grating of L 50 cm and j 1.26 (R 30%) has a bandwidth between the first zeroes of 3.3 pm, which is only 2.8 times wider than the longitudinal mode spacing in a laser with a total cavity length of 70 cm. Thus, the laser may oscillate on a single longitudinal mode. As a result of a long external cavity, the linewidth of this longitudinal mode is very narrow and has been measured to be less than 1 kHz [8]. A laser with a saturable absorber in the external cavity has a highly linear LI characteristic and a narrow linewidth when it operates in a stable single-mode regime [9]. A transition from coherence collapse to a single-mode regime in a laser with Yb-doped fiber and 980 nm pump diode has been shown earlier by simulations of an external cavity laser with two gratings [18]; the same approach can be applied to a laser with EDF, pumped at 1530 nm. A single-mode operation of these lasers depends on a drive current and the reflectivity of the dynamic grating. The measured refractive
index change and the coupling factor can be used in the dynamics and stability simulations of the lasers with saturable absorber in the external cavity.
3. Conclusions Effect of bleaching in EDF by pumping with an inband mW-power, narrow-linewidth laser source is reported for the first time. It is demonstrated that narrow-linewidth pumping at any single wavelength within absorption band results in bleaching of the entire absorption spectrum of EDF and the depth of bleaching in EDF depends on the pump power and the pump wavelength. 39.5 dB/m bleaching at the peak of the absorption spectrum was measured when a piece of EDF was pumped with 2.8 dBm at 1532 nm. The wide-band nature of saturable absorption in doped fibers plays a significant role in creating new devices such as single frequency lasers tunable over entire absorption spectrum of the doped fiber. The measured data is particularly useful for time-domain simulations of lasers with saturable absorber in the external cavity, the results of which will be reported in a future publication. Acknowledgements The authors acknowledge the Canadian Institute for Photonics Innovation (CIPI), the Natural Science & Engineering Research Council of CanadaÕs (NSERC) Canada Research Chairs program, and the NSERCÕs Discovery Grants program for their support of the research.
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