ISSN 1054-660X, Laser Physics, 2008, Vol. 18, No. 10, pp. 1192–1195.
FIBER OPTICS
© MAIK “Nauka /Interperiodica” (Russia), 2008. Original Text © Astro, Ltd., 2008.
Gain-Clamped Discrete Raman Amplifier with Suppressed Low-Frequency Relative Intensity Noise Pump-to-Signal Transfer G. Sun, A. Lin, D. Hwang, W.-T. Han, and Y. Chung* Department of Information and Communications, Gwangju Institute of Science and Technology, Oryong-Dong, Buk-gu, Gwangju, 500-712 Korea *e-mail:
[email protected] Received April 30, 2008
Abstract—We demonstrate an optically gain-clamped discrete Raman amplifier with a suppressed low-frequency relative intensity noise transfer from pump sources to an input amplified signal, which is accomplished by employing a cascaded second-order Raman fiber resonator as an optical amplification. The input signal dynamic range for a 3-dB gain compression is controllable without affecting the clamping level. In addition, we find that the Raman net gain is unsusceptible to fiber parameters such as the gain and attenuation coefficients at the pump, first-, and second-order Stokes lines. These characteristics fit our proposed gain-clamped discrete Raman amplifier for practical deployment although it works at the expense of the efficiency. PACS numbers: 42.60.Da, 42.60.Lh DOI: 10.1134/S1054660X08100149
1. INTRODUCTION Discrete Raman amplifiers (DRAs) have attracted more and more attention on account of their wideband and flexible applications. DRAs based on dispersion compensation fibers (DCFs) can compensate for both the fiber dispersion and loss simultaneously, and have been successfully employed in many long-haul transmission systems [1]. However, the generated noises originating from the amplified spontaneous emission (ASE), the double-Rayleigh scattering (DRS) [2], the relative intensity noise (RIN) [3], and etc., degrade performances and induce a system penalty. In order to overcome the fundamental noise limits arising from the use of a large length of the gain medium, a Raman fiber oscillator [4] is experimentally applied to an intracavity amplification to shorten the required DCF length. But, no efforts are made to clamp the Raman net gain and suppress the RIN transfer. The Raman net gain is also dependent on the relative polarization states of the pump laser and signal radiation [1]. The high-power Raman fiber laser often exhibits relatively high RIN levels. Worst of all, the RIN transfer function is larger than 0 in dB, especially by higher-order pumping schemes [5] and in the pump undepletion regime [3]. By fluctuating the first-order pump power, the RIN transfer has been suppressed in a dual-order Raman pumping configuration [6]. But, it complicates the system setup and is infeasible to the DRA [4], since the first-order pump is generated in the Raman resonator not inputted from outside. In this paper, a gain-clamped DRA is achieved by utilizing a pair of fiber Bragg gratings (FBGs) at both
ends of the DRA to generate the second-order Stokes field near the input signal line. In our calculation, we find that the Raman net gain is independent of the pump power, which shows that the low-frequency RIN transfer is suppressed from a high-power pump laser to the signal radiation and the gain clamping degree is strengthened without changing the Raman gain. In addition, the Raman net gain is also unrelated to the DCF parameters such as the gain and attenuation coefficients at the pump, first-, and second-order Stokes lines, which reduces the demands on the gain medium and avoids the use of the polarization diversity pumping. Therefore, our proposed DRA is fit for practical application. 2. SCHEMATIC SETUP AND THEORETICAL MODEL Figure 1 shows the schematic representation of our proposed DRA. The cascaded second-order Raman resonator as the signal amplification comprises a piece of DCF with length L = 1 km and FBGs at the input and output end. One FBG at pump wavelength λp is used for the feedback pump power and the others to generate resonated first- and second-order Stokes lines, respectively, at wavelength λs1 and λs2. The former is employed to amplify the input signal at wavelength λs and the latter to accomplish the gain clamping. At the output end, only the signal radiation can pass the bandpass filter (BPF). The evolutions of the pump, Stokes, signal, and noise powers along the fiber length could be
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GAIN-CLAMPED DISCRETE RAMAN AMPLIFIER
Psin
λ s2
WDM
λ s1
DCF fiber
1193
λ s1 λ s2 BPF
λp
FBGs
Psout
FBGs
Ppin Fig. 1. Schematic configuration of the proposed gain-clamped discrete Raman amplifier.
described by a set of ordinary nonlinear differential equations as follows [7]:
The boundary conditions at the input end are given by +
in
Pp ( 0 ) = Pp ,
±
dP p + – s1 ± − gλ − α p P p± + ------ P ( P + P s1 ), --------- = + λ p p s1 dz
+
0
–
–
in
+
Ps ( 0 ) = Ps ,
N s ( 0 ) = 0.
At the output end, the boundary conditions are given
±
by –
λ s2 ± + – + – + – − + g 0 ------- P s1 ( P s2 + P s2 + P s + P s + N s + N s + 4hν∆ν ), λ s1
+
± ± + – ----------- = − + α s2 P s2 ± g 0 P s2 ( P s1 + P s1 ) , dz
(1)
±
dP s ± ± + – --------- = − + α s P s ± g 0 P s ( P s1 + P s1 ) , dz ± d Ns
± + – ± ---------- = − + α s N s ± g 0 ( P s1 + P s1 ) ( N s + 2hν∆ν ) dz − +
− +
± γ ( P s + N s ), ±
±
±
where P p , p s1 , p s2 , p s , and N s , respectively, represent the pump, first-, second-order Stokes, signal and noise power (+ stands for the forward and – for the backward propagating), and αp, αs1, αs2, and αs are the corresponding linear attenuation coefficients. The parameters g and g0 are the Raman gain coefficients. h is the Planck’s constant, ν is the signal frequency, ∆ν is the resolution bandwidth of the ASE light, and γ is the Rayleigh scattering coefficient.
–
P p ( L ) = R p P p ( L ), –
L
L
+
P s1 ( L ) = R s1 P s1 ( L ), +
P s2 ( L ) = R s2 P s2 ( L ), 0, L
± dP s2
±
0
+
P s2 ( 0 ) = R s2 P s2 ( 0 ),
dP s1 ± ± + – ----------- = − + gP s1 ( P p + P p ) + α s1 P s1 − dz
±
+
P s1 ( 0 ) = R s1 P s1 ( 0 ),
–
N s ( L ) = 0,
0, L
where Rp, R s1 , and R s2 , respectively, represent the reflectivities of the FBGs at the pump, first-, and second-order Stokes wavelengths; 0 and L label the input in in and output ends; and P p and P s is the launched pump and signal power, respectively. For convenience, we set L + – + G = 0 [–α s + g( P s1 + P s1 )]dz, then P s (L) =
∫
+ P s (0)exp(G).
The numerical integration of (1) has been performed using the monoimplicit Runge–Kutta methods within a defect-control framework [8]. The model parameters are listed in the table. 3. RIN SUPPRESSION TECHNIQUE Figure 2 demonstrates the characteristics of the Raman net gain and noise figure (NF) versus the input signal power. For a fixed pump power (0.4 W) and a low signal input power, the DRA without gain clamping exhibits a gain saturation and a higher NF that is consistent with previous results [9]. But, the gain-clamped DRA possesses relatively uniform gain values. For a high input signal, the lasing second-order Stokes wave no longer dominates the DRA performance. Conse-
Model parameters αp = 0.75 dB/km λp = 1355 nm
αs1 = 0.55 dB/km λs1 = 1450 nm
Rp = 1.0
R s1 = 1.0
R s1 = 1.0
g = 2.7 (W km)–1
g0 = 2.7 (W km)–1
γ = 9.14 × 10–5 (km)–1
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αs2 = 0.35 dB/km λs2 = 1550 nm L
αs = 0.35 dB/km λs = 1554 nm ∆ν = 0.2 nm
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SUN et al.
Net gain, dB 35 30 25 20
NF, dB 2.8
NF, dB
Clamped No clamped
2.75 2.70 2.65 2.60 2.55 2.50
2.0
25 20 15 10 5
1.6 –10
0
10
15
1.2 0.8
10 5 –20 –15 –10
Raman on–off gain, dB 35 30
2.4
0.4 –5
0
5 10 15 20 Signal power, dBm
0
0
Clamped No clamped
100 200 300 400 500 600 700
100 200 300 400 500 600 700 Pump power, mW
0
Fig. 2. Raman net gain and noise figure (NF) versus the L
R s2 = 5%.
quently, the gain-clamped DRA behaves similarly to its undamped counterpart. Unlike the NF dynamic properties in [10], the NF of our DRA decreases with the input signal, because the use of shorter DCF results in a dominant ASE over the DRS noise, alleviating the limit on the input signal power. Figure 3 depicts the calculated Raman on–off gain and the NF versus the launched pump power. Obviously, the undamped Raman on–off gain and NF monotonically increases with the pump power, shown as the curve labeled by the downtriangle. But, the gain-clamping curve labeled by the uptriangle is markedly divided into three different parts. Firstly (Pin < 0.03 W), the pump power is below the threshold to resonate the firstorder Stokes without mentioning the second-order Stokes. Secondly (0.03 W < Pin < 0.16 W), the pump power is above the threshold to resonate the first-order Stokes, but below the threshold to resonate the secondorder Stokes. In both cases, the Raman on–off gain and the NF behave similarly to their unclamped counterparts. Lastly (Pin > 0.16 W), the second-order Stokes is resonated. As a result, the Raman on–off gain is unvaried with the pump power given that the higher pump power is indirectly consumed by the second-order Stokes and only has an effect on the first-order Stokes distribution, but no effect on integral G. This strengthens the degree of gain clamping and broadens the input signal dynamic range for a 3-dB gain compression without impacting the clamping level. Furthermore, the RIN transfer from a pump laser to a signal radiation is dramatically suppressed, relaxing the RIN requirements for the pump sources. In short, our proposed DRA is quite suitable for use as optical amplifiers although it operates at the expense of efficiency.
Fig. 3. Raman on–off gain and noise figure (NF) versus the pump power with and without gain clamping. The input sig0
L
nal power is 0.1 mW and R s2 = R s2 = 5%.
4. TUNABILITY OF THE RAMAN NET GAIN The characteristics of our DRA are further investigated by varying model parameters. We find that the Raman net gain is insusceptible to the values of param0 L eters αp, αs1, αs2, αs, g, g0, Rp, R s1 , and R s1 only if the second-order Stokes is resonated. The reasons are as follows: When the gain coefficient g0 is decreased (increased) or αs2 and αs are increased (decreased), the intracavity first-order Stokes power is increased (decreased) to resonate the second-order Stokes. Consequently, the tendency of the G, namely, the Raman net gain variation is automatically and completely off0 L set. When αs1, αp, g, Rp, R s1 , and R s1 are varied, the lasing second-order Stokes will adjust its power to control the value of G for clamping the Raman net gain. 0
9
–5
3
5
11 9
13
–10 –15
17
1
7
15
19
7 9
13
11 15
17
–20
3
5
11
L , dB Rs2
0
input signal power. The pump power is 0.4 W and R s2 =
–15
7 9
13
–10 0 , dB Rs2
11
–5
0
Fig. 4. Contour plot of the net gain in dB as a function of the 0
L
reflectivities R s2 and R s2 in decibells. The pump power is 0.8 W. LASER PHYSICS
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GAIN-CLAMPED DISCRETE RAMAN AMPLIFIER
These special properties reduce demands on the DCF and makes the polarization diversity pumping unnecessary. Although the Raman net gain is independent of the above-mentioned parameters, it relates to the reflectiv0 L ities R s2 and R s2 . The calculation presented only depends on their product, as illustrated in Fig. 4. When 0 L the product R s2 R s2 decreases, the threshold first-order Stokes power is increased to keep the second-order Stokes lasing, but no feedback to offset its effect on the signal radiation, and then increases the Raman net gain. 5. CONCLUSIONS In summary, we have demonstrated, for the first time to our knowledge, the simultaneous realization of gain clamping and the low-frequency RIN transfer suppression by resonating the intracavity second-order Stokes field in our DRA. The input signal dynamic range for a 3-dB gain compression can be broadened without influencing the clamping level. Furthermore, the Raman net gain is not related to the DOF parameters such as the gain and loss coefficients at the pump, first-, and second-order Stokes waves, decreasing demands on the gain medium and the system cost. The proposed technique should also be feasible to the multichannel Raman amplification. ACKNOWLEDGMENTS This work was performed under partial support from the Brain Korea-21 (BK-21) project, Ministry of Education, Korea.
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