Chapter 1 Introduction
1.3 Organization of this thesis
In this thesis, the technologies for the gain-clamped wide-band Erbium-doped fiber amplifiers and Erbium-doped waveguide amplifiers are studied. The researches on the development of the new gain-clamped constructions and the applications of EDFA and EDWA in WDM optical communication networks are the core of our study. In chapter 1, the overviews of technique including EDFA, EDWA, and gain-clamped technique in optical amplifiers are described. The evolution and development of EDFA and EDWA are also illustrated. A new gain-clamped wide-band Erbium-doped fiber amplifier is studied in the chapter 2. We have proposed and experimentally demonstrated the new architecture for gain-clamped wide-band Erbium-doped fiber amplifiers. The prosperities are also investigated in detail. For the WDM systems, optical amplifiers are frequently required to concurrently to amplify signals of several channels. In chapter 3, a simultaneously gain-clamped and
gain-flattened technique for EDFA has been investigated. The performance of the gain clamping and gain flattening will be analyzed through the simulation and experiments.
The study of the gain-clamped technique continues in chapter 4. Moreover, the novel module of gain-clamped EDWA has been proposed. Its amplifying and gain clamping behavior are discussed and experimentally shown. Chapter 5 provides an attractive module of hybrid S- to C- band optical fiber amplifiers. The design aspect is expounded. It also described and discussed the experimental results in this chapter. Erbium-doped fibers are not only used in the optical amplifiers but also used in the fiber ring lasers to act as gain medium.
An application of EDF amplification in S-band Erbium-doped fiber double ring laser has been proposed and experimentally demonstrated in Chapter 6. Finally, we make a conclusion of our research and some suggestions for further works in chapter 7.
Chapter 2
Gain-Clamped Hybrid C-Band to L-Band Optical Fiber Amplifier
EDFAs are generally used in the C-band (1530 - 1560 nm) for optical communication systems. The applications of the L-band (1560 - 1610 nm) to optical communication systems, such as monitoring for the strain of embed fibers and the troubleshooting for the optical communication systems operating at 1550nm, are more interested recently. There are some techniques have been investigated for the L-band fiber amplifiers by using a longer EDF than that of the C-band EDFAs [33], [34], the fiber Raman amplifiers [35], and the different hybrid amplifiers [36]. Besides, a wide-band EDFA from C- to L-band in parallel structure has also been studied [37]. The semiconductor optical amplifier (SOA) is another attractive optical amplifier in WDM systems due to their high on-off ratio. The feature of high on-off ratio is needed to improve the power penalty by crosstalk [38]. Furthermore, if using hybrid SOA and EDFAs, one can achieve a broadband gain bandwidth and reduce the crosstalk in WDM systems.The SOAs, on the other hand, generally have low gain and worse noise figure than the EDFAs. How to combine the advantage of the SOAs and EDFAs to construct a C- to L-band optical amplified is becoming a valuable research issue.
The reasons why Erbium-doped fiber amplifiers have strongly contributed to recent
advances in wavelength division multiplexing (WDM) networks are due to their high transmission capacities and optical gain. The aggregated transmit power would be time-variant in the WDM system when there are some channels are added and dropped. Thus, the stabilized gain versus the variation of input signal power is one of the important issues for WDM systems. However, because of the nature of Erbium-doped fibers, the gain profiles of the Erbium-doped fiber amplifiers present input-dependent behavior [39]. Therefore, the gain-clamped function is seriously needed for EDFAs dynamic working on wavelength-division-multiplexing networks. In other words, how to stabilize gain at different input signal powers is one of the critical issues for EDFAs when they are applied in DWDM systems. Several gain-clamped techniques have been studied, such as the all-optical gain-clamped methods [40], [41], [42] , or the concatenation of Erbium-doped fiber with different codopants [43], or variously optical filters, including long-period fiber gratings [44], fiber Bragg gratings [45], fiber acousto-optic filters [44], [46], [47], and Mach-Zehnder interferometer (MZI) [48], to compensate for the variations in the gain profile. In addition, the gain stabilizing methods by employing the optical feedback were also reported [49], [50]. A Gain-clainped EDFA using stimulated Brillouin scattering had been demonstrated in 10 Gbps based eight-channel WDM systems [51]. In this chapter, a new hybrid three-stage C- plus L-band fiber amplifier module, which is compose of two EDFAs and an SOA, over the operation range from 1540 to 1600 nm will be proposed and experimentally investigated. Moreover, we propose the gain-clamping technique on the configuration. The behavior and performance of gain clamping for this proposed amplifier have also been studied under different operation conditions.
2. 1 Principle of Operation and Theory
As described in previous section, SOAs have the advantage of high on-off ration and EDFAs have advantages of good performance of the amplification in the C-band and L-band.
We could use the output power of a broadband SOA to pump an EDF in order to extend the gain bandwidth from C-band to L-band. For the SOA, it will affect the extending gain bandwidth if using the smaller operating current. Thus, the bias current of the SOA is operated at maximum value to get a maximum of the extending gain bandwidth. However, the SOA suffer from the inter-channel crosstalk induced by carrier-density modulation occurring at the beat frequency of the neighbour optical carriers. Therefore, it will cause the worse noise figure. Since SOA having worse noise figure, the SOA will influence and degrade the noise figure spectra in this hybrid configuration. To improve this drawback, we could use another EDFA, putting in the front of the SOA, to provide the gain medium of low noise figure. By this way, this construction constitutes a hybrid three-stage C- plus L-band fiber amplifier module.
In homogeneously broadened gain medium, the gain for all the wavelengths is only dependent on the absorption and emission cross-sections and the overlapping factor while the total population inversion is fixed by a saturation tone, which lasing at a certain wavelength.
Any variation in powers of the input signals can be compensated by the properly adjusting lasing signal power. In other words, the lasing wavelength (or saturated tone) will cause the gain saturation due to the population inversion maintained. It is important to realize that even in a simple one-dimensional model of the fiber amplifier, the transverse shape of the optical mode and its overlap with the transverse Erbium ion distribution profile are important. [50]
Therefore, each signal wavelength undergoes a constant gain through this amplification system, independent of any signal power variation resulted from channels adding or dropping.
Based on this principle, we can use a fraction of out power of the hybrid three-stage C- plus
L-band fiber amplifier proposed above to be the saturated tone by the backward optical feedback method for gain-clamping. According this principle described above, we proposed a new gain-clamped hybrid three-stage C- plus L-band fiber amplifier module, which is compose of two EDFAs and an SOA. The proposed configuration and experimental setup will be described in the next section.
2.2 Proposed Configuration and Experimental Setup
Figure 2-1 shows the proposed gain-clamped hybrid C- plus L-band fiber amplifier module and experimental setup. The configuration consists of three amplifier blocks and a backward optical feedback loop in the third amplifier block. First stage is an EDFA module.
This module has a 10-m long EDF and a 980 nm pump laser, which has the pump power of 70 mW. The second stage is a semiconductor optical amplifier, which is biased at 260 mA bias current, and third stage is an EDFA module with a 35-m long EDF and 1480 nm pump laser of 110 mW. The isolators are used to prevent the ASE noise from spreading.
The backward optical feedback loop in the third amplifier stage is constructed by a 1×2 coupler, a tunable band-pass filter (TBF), and an optical circulator (OC). The TBF can be adjusted at 1559 and 1570 nm to act as the saturated tone for clamping gain. Four different couplers, with 95, 90, 70, and 50 % coupling ratio, respectively, are used to study the effect of the power level of the saturated tone. To study the performance of this proposed amplifier module, a tunable laser source (TLS) acts as the input signal and an optical spectrum analyzer (OSA) is used to measure the gain and noise figure spectra.
Traditional C- to L-band EDFA, which uses with a longer EDF to act as gain medium, can be achieved. To compare with the proposed amplifier, we construct a L-band EDFA using 100-m long EDF and 1480 nm pump laser of 160 mW. The experimental results will be
discussed in the following section.
2.3 Results and Discussion
Figure 2-2 show the gain (G) and noise figure (NF) spectra of the SOA over the operation range from 1530 to 1610 nm as the input signal power is 0 and –30 dBm, respectively. As we expected, the noise figure of this SOA is worse as shown in Figure 2-2.
Therefore, the SOA will influence and degrade the noise figure spectra in this proposed configuration when the first stage is neglected in this proposed hybrid amplifier.
Figure 2-3 shows the gain spectra of first and third amplifier stage when the input signal power Pin is 0 and –30 dBm, respectively. From Figure 2-3, the gain spectrum of first EDFA stage is distributed at C-band and the third EDFA stage presents the gain spectrum slightly shifting to longer wavelength. The maximum peak gain of 32.5 dB (6.4 dB noise figure) is at 1556 nm for the third EDFA stage. Figure 2-4 describe the gain and noise figure spectra of the proposed hybrid amplifier module without the backward optical feedback when the input power Pin is 0, –20 and –30 dBm, respectively. Furthermore, 41 dB gain and 4 dB noise figure are achieved at 1560 nm over the bandwidth of 1540 to 1600 nm while the input signal power is –30 dBm, as shown in Figure 2-4. Figure 2-5 shows the gain and noise figure spectra of the L-band EDFA from 1540 to 1600 nm when the Pin is 0, –20 and –30 dBm, respectively.
Compared with Figure 2-3 and Figure 2-5, the gain spectra of the two amplifiers are similar.
However, the total length of the EDF can be reduced in the proposed hybrid amplifier. The reason is that the signal is pre-amplified through the SOA stage and thus it can reduce the EDF gain medium to get the same gain.
Figure 2-6 shows the measured gain and noise figure characteristics versus the different power levels of input signal at 1550 nm while the lasing wavelength is at 1559 nm and the
input ratios of 1×2 coupler (C) is 95, 90, 70, and 50 %, respectively. The gain clamping effect cannot be observed when the input ratio of C is larger than 95 % for lasing wavelength at 1559 nm. There are only very little degradation in noise figure. This phenomenon seems better than the method using the conventional loop feedback with injected lasing signal. [43]
The reason is owing to the low noise figure of the first EDFA stage and the high on-off ratio of the SOA stage. As shown in Figure 2-6, although the gain-clamped behavior of using the C of 70 % or 50% input ratio are better than those of using the C of 90% input ratio, the gains of the former are smaller than the latter. Therefore, there is a trade-off between the gain clamping and signal gain. The optimum selection shall be dependent on the operating conditions of the systems, such as the range of the power level and the wavelength of the input signal. In the case of using the 90 % input ratio coupler, the gain can be almost kept constant up to the input power of –25 dBm. Meanwhile, only less than 2.4 dB variation is observed. Thus, a dynamic range of input signal from –45 dBm to –25 dBm and the gains of >
30 dB are retrieved for this optical feedback scheme when the input signal wavelength is 1550 nm. When we change the lasing wavelength to 1570 nm, Figure 2-7 and 2-8 present the measured gain and noise figure characteristics versus the different power levels of input signal at L-band (1566 nm) and C-band (1550 nm). The gain is also clamped up to the input power of –25 dBm while the C of 90 % input ratio used for Figure 2-8. As shown in Figure 2-7 and Figure 2-8, for a dynamic range of input signal from –45 dBm to –25 dBm, the gains of >
34.3 and >27.4 dB are retrieved in this proposed scheme when the input signal wavelength is 1566 and 1550 nm, respectively. Moreover, from Figure 2-6 and Figure 2-7, it reveals the gain variation is less then 2.4 dB over the operating wavelength range and 20 dB input dynamic range, from –45 to –25 dBm.
To compare the effect of the clapping among different schemes, we define clamping index (CI) as the ratio of gain variation (∆G) and average signal gain (G) :
Clamping Index (CI) = ∆G / G (4)
According the definition of the CI, the gain clamping is lower at the price of signal gain and thus the clamping index is smaller. Therefore, It can be used to evaluate the effective performance by comparing the CI values of different gain clamping scheme. Now, compared with the CI before and after using the proposed clamping scheme in this chapter, the clamping indexes are 0.133 and 0.045 for with and without clamping scheme, respectively. It reveals this scheme of the gain clamping shows very well performance.
2. 4 Summary
We have proposed and experimentally demonstrated a gain-clamped hybrid stages C- to L-band fiber amplifier module over the operation range from 1540 to 1600 nm. The proposed amplifier consists of two EDFA and one SOA in cascade. Compared with traditional L-band EDFA having longer EDF length, this amplifier can effectively reduce the total EDF length Meanwhile, 41 dB gain and 4 dB noise figure are achieved at 1560 nm over the bandwidth of 1540 to 1600 nm while the input signal power is –30 dBm. Moreover, the behavior and performance of the proposed gain-clamping amplifier module over C- to L-band have been investigated experimentally under different operation conditions. Compassion with other optical amplifiers reported before, the experimental results reveal this module features not only wide gain bandwidth over C- to L-band but also low gain variation in 20 input dynamic range of the input signal power. This gain clamping scheme also has low clamping index of 0.045. This proposed module is useful in applications of the WDM networks when the input dynamic range of the signal power is from –45 to –25 dBm. We also find that there is a trade-off between the gain clamping and signal gain. Hence, to get the optimum performance, we shall well control the coupling ratio of the coupler dependent on the operating range of the
signal power and wavelength.
Figure 2-1 Experimental setup of the proposed gain-clamped hybrid C- plus L-band fiber amplifier module
C1: 980/1550 nm Coupler C2: 1480/1550 nm WDM Coupler C3: Optical Coupler C1: 980/1550 nm Coupler
C2: 1480/1550 nm WDM Coupler C3: Optical Coupler
SOA
Wavelength (nm)
1520 1540 1560 1580 1600 1620
Ga in / N o ise Fig u re (d B)
0 3 6 9 12 15 18
G: Pin = 0 dBm G: Pin = -30 dBm NF: Pin = 0 dBm NF: Pin = -30 dBm
Figure 2-2 Gain and noise figure spectra of an SOA in the operating range from 1530 to 1630 nm. The probe input signal power Pin is 0 and –30 dBm, respectively.
Figure 2-3 Gain spectra of the first and third amplifier stages. The signal power of the probe Pin is 0 and –30 dBm, respectively.
Wavelength (nm)
1500 1520 1540 1560 1580 1600
Ga in ( d B )
0 10 20 30 40 50
Noise Fig u re (d B )
0 5 10 15 20
1st Stage (G): Pin = 0 dBm 1st Stage (G): Pin = -30 dBm 3rd Stage (G): Pin = 0 dBm 3rd Stage (G): Pin = -30 dBm 1st Stage (NF): Pin = 0 dBm 1st Stage (NF): Pin = -30 dBm 3rd Stage (NF): Pin = 0 dBm 3rd Stage (NF): Pin = -30 dBm
Figure 2-4 Gain and noise figure spectra of the proposed hybrid amplifier module without the backward optical feedback when the input power Pin is 0 and –30 dBm, respectively
Wavelength (nm)
1520 1540 1560 1580 1600 1620
Gain / Noise Figure (dB)
1520 1540 1560 1580 1600 1620
Gain / Noise Figure (dB)
Figure 2-5 Gain and noise figure spectra of conventional L-band EDFA with an EDF of 100 m long and a 1480 nm pump laser of 160 mW
Wavelength (nm)
1520 1540 1560 1580 1600 1620
Gain / N o ise Figure (dB)
1520 1540 1560 1580 1600 1620
Gain / N o ise Figure (dB)
Figure 2-6 Gains and noise figures versus the different power level of input signal at 1550 nm while the lasing wavelength at 1559 nm, and the input ratio of C is 95, 90, 70 and 50 %, respectively
Saturated Tone: 1559 nm Input Signal: 1550 nm
Input Power (dBm)
-50 -40 -30 -20 -10 0 10
Gain / Noise Figure (dB)
0 10 20 30 40 50 60
G: Orginal G: C = 95:5 G: C = 90:10 G: C = 70:30 G: C = 50:50 NF: Orginal NF: C = 95:5 NF: C = 90:10 NF: C = 70:30 NF: C = 50:50
Figure 2-7 Gains and noise figures versus the different power level of input signal at 1566 nm while the lasing wavelength at 1570 nm, and the input ratio of C is 95, 90, 70 and 50 %, respectively
Saturated Tone: 1570 nm Input Signal: 1566 nm
Input Power (dBm)
-50 -40 -30 -20 -10 0 10
Gain / Noise Figure (dB)
0 10 20 30 40 50 60
G: Orginal G: C = 95:5 G: C = 90:10 G: C = 70:30 G: C = 50:50 NF: Orginal NF: C = 95:5 NF: C = 90:10 NF: C = 70:30 NF: C = 50:50
Figure 2-8 Gains and noise figures versus the different power level of input signal at 1550 nm while the lasing wavelength at 1570 nm, and the input ratio of C is 95, 90, 70 and 50 %, respectively
Saturated Tone: 1570 nm Input Signal: 1550 nm
Input Power (dBm)
-50 -40 -30 -20 -10 0 10
Gain / Noise Figure (dB)
0 10 20 30 40 50
60 G: OrginalG: C = 95:5
G: C = 90:10 G: C = 70:30 G: C = 50:50 NF: Orginal NF: C = 95:5 NF: C = 90:10 NF: C = 70:30 NF: C = 50:50
Chapter 3
Simultaneously Gain-Clamped and Gain-Flattened Technique for EDFA
An advantage of optical amplifiers is that they can be used to amplify several communication channels simultaneously as long as the bandwidth of the multi-channel is smaller than the amplifier bandwidth. Even though the gain spectrum of an EDFA is relatively broad, the gain profile is far from uniform over a wide wavelength range and input dependent.
This problem would become quite severe in long-haul system employing a cascade chain of EDFAs. Therefore, the gain flatness and gain-clamped functions are simultaneously needed for EDFAs dynamic working on dense wavelength-division-multiplexing networks.
Many gain spectrum equalization and gain stabilizing techniques have been studied.
Using the concatenation of Erbium-doped fiber with different codopants [43], multiple side-tap gratings[36], and Mach-Zehnder interferometer (MZI) [48], have been reported to compensate for the variations in the gain profile. In this chapter, we present a simple gain profile control technique based on an EDFA with a backward injected light from Fabry-Perot laser diode (FP-LD). The simultaneously gain-flattened and gain-clamped profile has also been demonstrated experimentally.
3.1 Principle of Operation and Theory
The total population inversion in the homogeneously broadened gain medium is fixed if this gain medium is lasing at a wavelength. Consequently, the gain for all the wavelengths is only decided by the overlapping factor and the absorption and emission cross-sections. We can use this principle to cause the gain saturation in order to clamp the gain of the Erbium-doped fiber amplifier. In order words, the gain can be clamped at the desired level by clamping the average population inversion in the amplifier. Furthermore, because the bandwidth of saturated tone will affect the level of population inversion in the amplifier, we can inject multiple saturation tones into the gain medium to broadly clamp the average inversion over a wide spectrum and thus realize to flat the gain profile. Because of the Fabry-Perot laser diode featuring multiple longitudinal modes, we use Fabry-Perot laser diode to as a probe source. Then we inject this probe source into the Erbium-doped fiber amplifier to simultaneously suppress the gain variation and improve the gain flatness.
Due to the homogeneously broadened gain characteristics, the multi-wavelength input signal in WDM system can be simulated by a saturation tone with the power which equals to the aggregated power of multi-wavelength input signal [52]. Therefore, the output gain profile can be measured by a probe light source with tuneable wavelength. In the following section, we will describe the proposed configuration and the experimental setup.
3.2 Proposed Configuration and Experimental Setup
Figure 3-1 shows the proposed and experimental set-up for the simultaneously
Figure 3-1 shows the proposed and experimental set-up for the simultaneously