90 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 1, JANUARY 2004
S Band Gain-Clamped Erbium-Doped Fiber
Amplifier by Using Optical Feedback Method
Chien-Hung Yeh, Chien-Chung Lee, Chih-Yang Chen, and Sien Chi
Abstract—We have proposed and experimentally demonstrated
an band gain-clamped erbium-doped fiber amplifier, which pro-vides an operation range from 1480 to 1520 nm by using an optical feedback method. The behavior and performance of gain clamping in band have also been investigated experimentally under dif-ferent operation conditions.
Index Terms—EDFA, gain-clamped, band.
I. INTRODUCTION
W
IDE-BAND erbium-doped fiber amplifiers (EDFAs)have been intensively studied for wavelength-divi-sion-multiplexing (WDM) systems. The stabilized gain versus the variation of input signal power is one of the key issues for WDM networks. Several gain-clamping techniques have been reported, such as the all-optical gain-clamped method [1], or different optical filters including fiber Bragg grating filters, finer acoustooptic filters, and tunable bandpass filters (TBFs)
[2]–[4], covering both and bands (1530–1610 nm). In
addition, the gain-clamping effect by using an optical feedback has been shown [1], [3], [4]. Recently, an band (1450–1530 nm) amplification technique, which employs the erbium-doped silica fiber with depressed cladding design and 980-nm pump laser to generate EDF gain extension effect, has been reported [5]. Therefore, the gain clamping technique is expected to extend to band by using this band amplifier module. In this letter, we present a gain-clamped band amplifier with a forward optical feedback configuration over the operation range from 1480 to 1520 nm. The gain clamping behaviors have also been investigated experimentally.
II. EXPERIMENTS ANDDISSCUSSIONS
In an homogeneously broadened medium, lasing action at a wavelength fixes the total population inversion, therefore, the gain for all the wavelengths are only dependent on their ab-sorption and emission cross sections and the overlapping factor. Any variation in input signal powers will be compensated by the adjustment of the lasing signal power. As a result, each signal wavelength experiences a constant gain through this amplified system, independent of signal power variation caused by opera-tion such as channel adding or dropping. Based on this principle, Manuscript received June 24, 2003; revised August 27, 2003. This work was supported in part by the Academic Excellence Program of the R.O.C. Ministry of Education under Contract 89-E-FA06-1-4, and in part by the National Science Council of the R.O.C. under Contract NSC-91-2215-E-009-027.
The authors are with the Institute of Electro-Optical Engineering, Na-tional Chiao Tung University, Hsinchu, Taiwan 30050, R.O.C. (e-mail: [email protected]).
Digital Object Identifier 10.1109/LPT.2003.820468
Input 980 nm Pump Laser W EDF EDF W Isolator C
S-Band EDFA Module
C2 C1
FFP Filter 10
90 Output
Fig. 1. Experimental setup ofS band EDFA module with forward optical feedback for gain clamping.
Fig. 1 shows the experimental setup of band amplifier module with forward optical feedback for gain clamping. The system consists of an band amplifier module composed of two-stage amplifiers and a power-sharing 980-nm pump laser, two 1 2 optical couplers: and , and a FFP filter. However, with an input coupling ratio of 90% and with an output cou-pling ratio of 95%, 90%, 80%, and 70%, respectively. A tun-able laser source (TLS) is used to probe the gain spectrum of this proposed amplifier module. The band erbium-doped fiber (EDF) inside the amplifier module has a depressed cladding de-sign in order to provide a long wavelength cutoff filter for the fundamental mode (near 1530 nm) of the fiber. Then, the com-position of the core is approximately 2.5% GeO , 5.5% Al O , and 92% SiO , with 0.15 wt.% Erbium. The depressed cladding is approximately 3% Fluorine, 0.5% P2O5, and 96.5% SiO2. The numerical aperture of the core, relative to the depressed cladding, is 0.22. However, the EDFs in the first and second stages have different characteristics. The EDFs in the first and second stages have different characteristics. The fiber in the first stage has the fiber length of 20 m, and can provide low noise figure and medium gain by forward pumping. The fiber in the second stage has the fiber length of 30 m, and can produce large output power by backward pumping. The total pump power of this amplifier module can be up to 280 mW while the bias cur-rent is operated at 356 mA. In addition, the optical isolator be-tween these two stages can reduce backward amplified sponta-neous emission (ASE) and improve noise figure performance. The evolution from a standard EDFA to this band design by the introduction of a continuous long wavelength cutoff filter in the EDF. Although the spectrum indicates strong gain at band wavelengths, the gain cannot be realized because of strong ASE 1041-1135/04$20.00 © 2004 IEEE
YEH et al.: BAND GAIN-CLAMPED ERDFA BY USING OPTICAL FEEDBACK METHOD 91 Wavelength (nm) 1470 1480 1490 1500 1510 1520 1530 Gain (dB) 0 9 18 27 36 45 Noise Figure (dB) 0 5 10 15 20 G : Pin = 0 dBm G : Pin = -15 dBm G : Pin = -30 dBm NF : Pin = 0 dBm NF : Pin = -15 dBm NF : Pin = -30 dBm Resolution = 0.07 nm Wavelength (nm) 1470 1480 1490 1500 1510 1520 1530 Power Level (dBm) -50 -40 -30 -20 -10 0 10 20 λs1 (1493nm) λs2 (1514nm) 1470 Wavelength (nm)1490 1510 1530 Power Level (dBm)-50 -40 -30 -20 -10 b a
Fig. 2. (a) Optical gain spectra and noise figure of theS band EDFA module over the operation range from 1480 to 1520 nm when the input signal power Pin= 0, 015, and 030 dBm, respectively. (b) Two lasing wavelengths, 1493 nm ( ) and 1514 nm ( ), of the proposed setup with forward optical feedback while the output ratio ofC is 80%, and the insert is S band ASE spectrum without optical feedback.
at the 1530 nm peak, which limits the length of the population inversion. Introduction of a progressively sharper long wave-length cutoff filter suppresses the gain in the and bands, so that the band region can exhibit increasing gain, as ASE from the 1530-nm peak does not grow and limit the population inversion. Final result is a complete suppression of the longer wavelength gain, resulting in a usable high net gain in the band. Performance equivalent to typical band EDFAs is ob-tained with a nominally longer fiber. The band EDF integrates depressed cladding design and distributive filtering enhanced function for this amplifier, therefore, the quantitative analysis of Rayleigh scattering contribution is quite complicated and still needs further study. Fig. 2(a) shows the optical spectra of gain and noise figure for this band amplifier module over an oper-ation range from 1480 to 1520 nm when the input signal power sets at Pin , 15, and 30 dBm, and the saturated output power at 1498 nm can be up to 16.1 dBm for inoput power of 0 dBm, but the noise figure is 7.2 dB as seen in Fig. 2(a). The
λs1= 1493 nm Input Power (dBm) -40 -30 -20 -10 0 10 Gain (dB) 0 10 20 30 40 50 Noise Figure (dB) 0 5 10 15 20 G: without feedback G: 5/95 G: 10/90 G: 20/80 G: 30/70 NF: without feedback NF: 5/95 NF: 10/90 NF: 20/80 NF: 30/70 λs2 2 = 1514 nm Input Power (dBm) -40 -30 -20 -10 0 10 Gain (dB) 0 10 20 30 40 50 Noise Figure (dB) 0 5 10 15 20 G: without feedback G: 5/95 G: 10/90 G: 20/80 G: 30/70 NF: without feedback NF: 5/95 NF: 10/90 NF: 20/80 NF: 30/70 b a
Fig. 3. Measured gain and noise figure characteristics versus the different power level of the input signal at 1510 nm while the saturated tone at (a) 1493 nm or (b) 1514 nm, and the output ratios ofC are 95%, 90%, 80%, and 70%, respectively.
FFP filter is an all-fiber device having a widely tunable range, low insertion loss of 0.5 dB, low polarization-dependent loss (PDL) of dB, the free-spectral range (FSR) of 44.5 nm, the inesse of 200, and the 3-dB bandwidth of 0.4 nm. This FFP filter is placed into the intercavity to select a lasing wavelength ranging from 1493 to 1514 nm when the external voltage ( 12 V) is applied on the piezoelectric transducer (PZT) of the FFP filter. Fig. 2(b) shows the lasing powers of two different wave-lengths [resolution has also been shown in Fig. 2(b)], 1493 nm ( ) and 1514 nm ( , for this proposed setup with forward optical feedback while the output ratio of is 80%, and the insert is band ASE spectrum without optical feedback.
Fig. 3(a) and (b) shows the measured gain and noise figure characteristics versus the different power level of input signal at 1510 nm while the lasing wavelength at 1493 and 1514 nm, and the output ratios of are 95%, 90%, 80%, and 70%. Because some components placed at the signal input end have higher losses in band and the splice point of band EDF and WDM coupler possesses higher loss, the noise figure of this band EDFA module will be slightly degraded. Therefore, compared with the and bands gain-clamped EDFAs [2]–[4], the noise
92 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 1, JANUARY 2004 λs1 = 1493 nm C2 : 20/80 Wavelength (nm) 1470 1480 1490 1500 1510 1520 1530 Gain (dB) -10 0 10 20 30 40 50 Pin = 0 dBm Pin = -15 dBm Pin = -30 dBm λs2 = 1514 nm C2 : 20/80 Wavelength (nm) 1470 1480 1490 1500 1510 1520 1530 Gain (dB) 0 10 20 30 40 50 Pin = 0 dBm Pin = -15 dBm Pin = -30 dBm a b
Fig. 4. Gain spectra of aS band gain-clamped EDFA module with an optical feedback injection andC of 80% output ratio at the input signal-power Pin = 0, 015, and 030 dBm over the operation range from 1480 to 1520 nm whenthe lasing wavelength is (a) , or (b)
figure of an band gain-clamped amplifier was also slightly higher than that of them. The gain clamping effect is observed when the output ratio of is not larger than 90% and 95% for lasing wavelength at 1493 and 1514 nm, respectively. By using of 70% output ratio, the gain can be kept constant at the input power of 15 dBm at the expense of around 5.3-dB gain and 2.8-dB noise figure degradations for the lasing wavelength at 1493 and 1514 nm. The noise figure of 2.8 dB impairment
observed in Fig. 3(a) is mainly induced by the gain saturation of the lasing power and the insertion loss of optical coupler at the signal input end for optical feedback configuration. As a result, a dynamic range of input signal from 30 dBm to 15 dBm and the gain of 23 dB are retrieved for the optical feed-back scheme no matter the lasing wavelength is 1493 or 1514 nm.
Fig. 4(a) and (b) indicates the gain spectra of the
gain-clamped amplifier module with of 80% output
ratio at the input signal-power Pin , 15, and 30 dBm over the operation range from 1480 to 1520 nm form the lasing wavelengths and , respectively. Furthermore, the maximum gains at 1504 nm are 23.9 and 26.7 dB at the input power of 30 dBm, and the maximum gain variations are less then 1.2 and 2.4 dB over the operating wavelength range and 15 dB ( 30 to 15 dBm) input dynamic range for the lasing wavelengths at and , respectively.
III. CONCLUSION
We have proposed and experimentally demonstrated an band gain-clamped amplifier by using an optical feedback method. A dynamic range of input signal from 30 to 15 dBm and the gain of 23 dB over an operation range from 1480 to 1520 nm are retrieved. In addition, the gain clamping performance has also been investigated experimentally under different operation conditions such as the lasing wavelength, the cavity loss and the input signal wavelength. This band gain-clamped amplifier is very useful to the future band applications.
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