The Dissertation Organization of Part II - Broadband Erbium-Doped Fiber Sources

Part II Broadband Erbium-Doped Fiber Sources

4.5 The Dissertation Organization of Part II

= ⋅

Ω m

B LD

c _e

m it

lim 2 (4.12)

Equation (4.12) tells us that △ν determines Ωlimit, which is improved by using a light source with a broader linewidth.

4.5 The Dissertation Organization of Part II

There are six chapters organized in this part of the dissertation. In Chapter 5, we investigate the single-pumped L-band EDF ASE light source. In order to generate the ASE light source with broadband linewidth with low spectral ripple, high output power, high pumping efficiency, the double-pass bi-directional pumping L-band EDF ASE source and double- pass dual forward-pumping EDF ASE source are demonstrated and investigated in Chapter 6 and Chapter 7, respectively. In Chapter 8, the high power and broadband including C- and L-band EDF ASE source by using dual forward-pumping configuration is investigated. Chapter 9 gives a brief conclusion of all the work and research in the dissertation.

Chapter 5 Single-Pumped L-Band Erbium-Doped Fiber ASE Sources

Recently, the ASE in-coherent sources around 1.55 µm have been extensively studied due to their important applications for DWDM device characterization, spectrum sliced DWDM systems [28], [29], fiber-optic gyroscopes [35], and laser spectroscopy. The erbium-doped fiber based ASE source is a good candidate for simultaneous offering large spectral linewidth, high output power, and excellent mean wavelength stability to meet the application requirements.

In this chapter, we experimentally investigate the single-laser-pumped L-band EDF ASE sources without using external flattening filters. Because the 1550 nm ASE generated by the 1480 nm pumping is the pump source for 1580 nm band amplification and the quantum conversion efficiency to 1580 nm ASE with the 1480 nm band pumping is higher than that with the 980 nm pumping [63], the 1480 nm pumping is thus considered. The characteristics of SPF, SPB, DPF, and DPB configurations are examined and compared in terms of the output power, mean wavelength, linewidth, and pumping efficiency.

5.1 Configurations and Experimental Setup

Figure 5.1 shows the L-band ASE sources in (a) forward and (b) backward pumping configurations. Each ASE source consists of a piece of EDF, a 1480/1580 nm wavelength selective coupler (WSC), a 1480 nm pump laser diode (LD), an end fiber mirror, and an optical isolator (ISO) at the output port. Pump light from the pump LD is coupled to the EDF through the WSC. In forward pumping scheme, the ASE output light has the same direction as the pump light, and the opposite direction with the pump light in backward pumping scheme. In the experiments, a fiber terminator with reflectance, R, of 0%, a cleaved fiber end with R = 4%, and a fiber mirror with R = 80% or 8% were separately used. If we replace the mirror with a fiber terminator (R = 0%), the configuration is called SPF in Fig. 5.1(a) and SPB in Fig. 5.1(b) else the configuration is called DPF in Fig. 5.1(a) and DPB in Fig. 5.1(b). In the double-pass configurations, the fiber mirror was used to reflect the backward and forward ASE lights into the EDF gain medium to be amplified again in DPF and DPB configurations, respectively, and then causes the ASE light to emerge from the output port. The EDF used was HighWave Optical Technologies ER741 with a mode field diameter of 5 µm, a cutoff wavelength of 950 nm, and the absorption coefficient of 7 dB/m at 1532 nm and 3 dB/m at 1480 nm. The output

and 1580 nm bands. The fiber mirror with reflectance of 80% or 8% within a wide and flat 1450-1630 nm range was made with thin-film coating mirror. The insertion loss and isolation of optical isolator are about 1 dB and 42 dB at 1580 nm, respectively. Both the output power, Pout, and the output spectra of each ASE source were measured by an optical spectrum analyzer with a resolution of 0.1 nm. The mean wavelength (λm) and linewidth (∆λ) were calculated based on the measured spectra in the wavelength range of 1510-1630 nm by using eq. (4.7) and eq. (4.8).

5.2 Experimental Results and Characteristics Comparison

First, three EDF segments with lengths of 60 m, 100 m, and 130 m were used. Figure 5.2 shows the (a) measured output power and (b) linewidth of these ASE source configurations versus the EDF length with a pumping power of 100 mW. Several data points for DPF configuration corresponding to some EDF lengths were not indicated in Fig. 5.2(a) and 5.2(b) due to the instantaneous resonant lasing effect occurred at those operating points. The lasing effect was due to the instantaneous optical cavity, which was formed between the end mirror and the virtual mirror generated by the strong amplified Rayleigh scattering of the reflected ASE light in such long gain medium, especially while either the pump power or the mirror reflectance increased. From Fig. 5.2(a) and 5.2(b), the EDF length with 100 m seems suitable for most configurations to achieve a ∆λ > 24 nm with Pout ≥ 10 mW. For the EDF length of 60 m, the linewidth (∆λ) of all configurations was ≤ 24 nm, and each λm was < 1567 nm (not shown), which was not right at the L-band region. For the EDF length of 130 m, the ∆λ as high as 50 nm can be achieved for SPF configuration, but its output power was trivial with Pout < 0.1 mW. For other configurations, degradations of the output power or linewidth occurred as compared with the case with EDF length of 100 m. Therefore, the EDF length of 100 m was used in the following experiments.

Figure 5.3 shows the measured characteristics of (a) output power, (b) mean wavelength and (c) linewidth of these ASE configurations as a function of the pump power. Figure 5.4 shows the measured output spectra characteristics. From Fig. 5.3(a), three backward-pumped sources (SPB, DPB with R = 4%, and DPB with R = 80%) have almost the same and the highest Pout of 28.8 mW when the pump power was at 100 mW , but their mean wavelengths as shown in Fig. 5.3(b) are all shorter than 1558 nm, which is not properly located in L-band region. Such mean wavelengths limit their use for L-band ASE sources. In addition, there was

a main ASE hump around 1560 nm as shown in Fig. 5.4 for these three ASE sources. Their output spectra were quite similar, and the spectral profiles of both SPB and DPB with R = 4%

were completely overlaid. The spectral component of these three sources dominated at C-band region is resulted from the backward ASE power generated from the EDF segment at the left front end, and hence each moderate linewidth of about 24.5 nm as shown in Fig. 5.3(c) was obtained. So, both SPB and DPB configurations are intrinsically hard to be an L-band ASE source by optimizing the EDF length or the pump power. Instead, the external spectral flattening techniques are required for these backward pumping configurations.

In contrast, the SPF configuration has a poor Pout ≤ 0.1 mW as shown in Fig. 5.3(a). This is due to the used 100-m long EDF, which absorbed the forward ASE generated from the EDF segment at the left front end. The output spectrum of SPF configuration is sensitive to the EDF length and its spectrum was expanded into shorter wavelength region (not shown), however, C-band ASE spectrum occurred when the EDF length decreased to less than 35 m. For DPF configurations, the maximum Pout, as shown in Fig. 5.3(a), of DPF with R = 4% and 8% using 100-mW pump power was 11.3 mW and 13.8 mW, respectively, and was 11 mW for DPF with R = 80% using 80 mW pump power. While the pump power greater than 84 mW, the DPF with R = 80% was susceptible to the resonant lasing, and therefore higher pump power operation is inhibited for the DPF with R = 80% using either 100 m or 60 m long EDF length. Note that there is no region where the mean wavelength is independent of the pump power. This means that the condition of ∂λ_s /∂P_pump = 0 could not obtain. The mean wavelengths in the high-pump region (80 to 100 mW) were much more stable than in the low-pump regions as shown in Fig. 5.3(b). The slope of mean wavelength in the high-pump-power region was approximately –36.4 parts in 10⁶/mW. The linewidth of DPFs with R = 80%, 8%, and 4% was 33.7, 40.9, and 39.1 nm, respectively, as shown in Fig. 5.3(c).

Table 5.1 summarizes the characteristics of these ASE sources in terms of output power (Pout), mean wavelength (λm), linewidth (∆λ), and pumping efficiency (η). From the viewpoints of high pumping efficiency and wide linewidth, the DPF configuration with R = 8% is the best one to be an L-band ASE source offering Pout = 13.8 mW with a ripple of 0.6 dB, η = 13.8%, λm = 1585.7 nm, and ∆λ = 40.9 nm. Other configurations are intrinsically hard to be an L-band ASE source. The behavior of SPF, SPB, and DPB configurations was significantly different from the corresponding C-band counterparts. For C-band ASE sources, all configurations are satisfied and, among them, the DPB configuration has demonstrated the highest output power

with stable wide spectrum. However, for L-band ASE sources, the output spectra of DPB and SPB configurations always have a main hump at around 1560 nm regardless of the EDF length.

Although the SPF configuration can achieve an L-band spectrum using a proper EDF length, the output power is too small for most applications.

5.3 Summary

We have experimentally investigated the 1480 nm pumped L-band ASE source in SPF, SPB, DPF, and DPB configurations. The characteristics have been examined and compared in terms of the output power, mean wavelength, linewidth, and pump conversion efficiency. Among them, we found that the DPF configuration with the EDF length of 100 m and the effective mirror reflectance of 8% is the appropriate one to implement an L-band ASE source with a large linewidth of 40.9 nm and moderate output power 13.8 mW with a pump efficiency of 13.8%. Other configurations are intrinsically hard to be an L-band source for applications. This investigation result provides the configuration selection of L-band EDF ASE source.

Chapter 6 High Pumping-Efficiency L-Band Erbium-Doped Fiber ASE Source Using Double-Pass Bi-Directional Pumping

Configuration

In a recent study [64], we found that the DPF configuration is the better one to implement a single-laser pumped L-band ASE source with a pumping conversion efficiency of about 14%.

Other configurations (SPF, SPB, and DPB) are intrinsically hard to act as L-band sources.

However, high pumping-power operation for DPF configuration is inhibited by the instantaneous resonant lasing effect [64]. Therefore, high output-power operation with high pumping efficiency for such configuration is difficult to realize. In this chapter, we present an improved L-band ASE configuration to offer a high output power by using asymmetric bi-directional pumping configuration with an end fiber mirror. Such a configuration gives rise to relaxing the danger in resonant lasing and, thus allowing high pumping-power operation and therefore enhancing the pumping efficiency. The pumping-power arrangement and the effect of mirror reflectance on the characteristics of output power, mean wavelength, and spectral linewidth are examined. The high output power and wide spectral linewidth with small spectral ripple are obtained without using any external spectral filters. The pumping efficiency of such an L-band ASE source is significantly improved as compared with the conventional one in [44]

and [64].

6.1 Experimental setup

Figure 6.1 shows the proposed asymmetric bi-directional pumping configuration for L-band ASE source, which consists of a piece of EDF with an optimal length of 93 m, two 1.48/1.58-µm wavelength selection couplers (WSC1 and WSC2), two 1.48-µm pump laser diodes (LD1 and LD2), an end fiber mirror, a variable optical attenuator (VOA), and an optical isolator (ISO) at the output port. The fiber mirror with reflectance of 80% within a wide and flat spectral range of 1450 ~ 1630 nm was made with a thin-film coating mirror. The effective mirror with controllable reflectance, R, realized by incorporating a fiber mirror and a VOA, is used to reflect both the backward ASE light and the forward ASE light. There are two kinds of pump lights within the long EDF gain medium. One is the 1480 nm light (the forward pumping

power PF and the backward pumping power PB) from each pump LD, and the other is the C-band ASE lights, in which there are forward and backward ASEs (i.e. ASEFF and ASEFB, respectively) generated by the front EDF segment by the forward pumping LD, and forward and backward ASEs (i.e., ASEBF and ASEBB, respectively) generated by the rear EDF segment by the backward pumping LD. These C-band ASE lights (i.e., the mirror-reflected ASEFB and ASEBF and the traveling ASEFF lights) are also the pump lights for L-band amplification. After reflecting by the mirror, the total reflected ASE lights are launched into the EDF gain medium to be amplified and then causing the wide and flat L-band ASE light to emerge from the output port. Consequently, this ASE is a double-pass bi-directional pumping configuration to generate L-band ASE light.

The EDF used is HighWave Optical Technologies ER741 with a mode field diameter of 5 µm, a cutoff wavelength of 950 nm, and an absorption coefficient of 7 dB/m at 1532 nm, 0.9 dB/m at 1580 nm, and 3 dB/m at 1480 nm. The insertion loss of WSC1 at 1480 and 1580 nm is about 0.6 and 0.5 dB, respectively, and that of WSC2 is about 0.7 dB at both 1480 and 1580 nm. The insertion loss and isolation of the optical isolator are about 0.8 and 50 dB at 1580 nm, respectively. The output spectrum was measured by using ADVANTEST Q8384 optical spectrum analyzer with a resolution of 0.1 nm. The mean wavelength (λm) and linewidth (∆λ) were calculated based on the measured spectra in the wavelength range of 1500 - 1650 nm by using eq. (4.7) and eq. (4.8). The ∆P is the output power variation between the peak and valley power in 1565-1605 nm region. The spectral ripple is a half of the output power variation.

6.2 Experimental Results and Discussions

The criteria, besides the requirements of linewidth of ≧ 40 nm with a spectral ripple of

≦0.7 dB and the mean wavelength (λm) falling in the L-band region for the best design of ASE source, is to achieve the maximum pumping efficiency. Here the pumping efficiency (η) is defined as η = Pout / (PF + PB), in which Pout is the total output power of the ASE source. In the experiment, for each reflectance case of 4%, 10%, 20%, 30%, 40%, and 80%, we examined all optical spectral characteristics of the ASE source by first setting PF and then adjusting PB. Table 6.1 summarizes the characteristic comparison of the double-pass bi-directional pumping EDF ASE source in terms of output power (Pout), mean wavelength (λm), linewidth (∆λ), spectral ripple, and pumping efficiency (η) with EDF length of 93 m and 103 m for different effective mirror reflectance and pumping power. The better characteristics of output power of

71.8 mW, pumping efficiency of 42.2%, mean wavelength of 1584.3 nm, and spectral linewidth of 41.6 nm with a spectral ripple of 0.5 dB have been achieved by operating this 93 m EDF ASE source at PF of 40 mW and PB of 130 mW with a effective mirror reflectance of 30%. Fig. 2 illustrates the measured output spectra of ASE source, operating with an EDF length of 93 m pumped by PF of 40 mW and PB of 130 mW, against different effective reflectance R. There is a hump around 1570 nm for the ASE source with R of 4%, in contrast, with a hump around 1596 nm for R of 80%. The reason of spectral hump is described as followis. The insufficient C-band ASE lights resulted from insufficient reflection (due to the low R of 4%) is not enough to saturate the EDF at the front segment and creates a high inversion level in the system. Hence the ASE peak shifted toward the 1570 nm region and thus gave rise to a spectral ripple of 2.9 dB. Similarly, the excessive C-band ASE lights resulted from the high reflection (due to the high R of 80%) deeply saturates the EDF that makes the inversion level becomes too low. Hence the ASE peak shifted toward the 1600 nm region and, thus, gave rise to a spectral ripple of 0.9 dB. Only while using the effective mirror with R of 30%, the spectral density of both 1570 nm and 1600 nm ASE bands are nearly the same.

Therefore, we found that R of 30% is the appropriate one to achieve the maximum pumping conversion efficiency, to be described later, for the proposed L-band ASE source with rather broad and flat spectral characteristics as the heavy curve shown in Fig. 6.2.

Figure 6.3 illustrates the effect of pump powers on the measured L-band characteristics of output power Pout and a spectral ripple △P/2 for the ASE source operating at R of 30%. For each fixed PF, the output power of the ASE source increases with PB. Several data points corresponding to some pump powers were not indicated in Fig. 6.3 for higher power levels of PB since the instantaneous resonant lasing effect occurred at those operating points. The case with PF of 50 or 60 mW supported PB to operate down to 100 mW because of such lasing effect.

Such a lasing effect was due to the instantaneous optical cavity, which was formed between the effective mirror and the virtual mirror generated by the strong amplified Rayleigh scattering in such long gain medium and the optical reflection of WSC2, especially while either the pump power or the effective mirror reflectance increased. Note that the larger PF, the easier lasing effect, and hence the smaller allowed PB is used. The case with PF of 40 mW provides the highest ASE output power. However, the resonant lasing effect restricts PB to operate up to 130 mW. The spectral ripple is less than 0.6 dB for the pumping power ranges with PB of 100 ~ 140 mW and PF of 30 ~ 60 mW.

linewidth on the pumping power. The mean wavelengths shift to shorter wavelength as PB

increases, but they are still properly located within the L-band wavelength region. The pump power independent mean-wavelength operation with ∂λ_m ∂(P_B)= 0 is unable to exist for this ASE source. The ∂λ_m ∂(P_B) is about -22.6, -23.6, -24.6, and -23 ppm/mW for PF of 30, 40, 50, and 60 mW, respectively. In Fig. 6.4, the linewidthes of ASE source for most operation with PF of 30 ~ 60 mW and PB of 30 ~ 140 mW are always greater than 41 nm. Figure 6.5 illustrates the dependence of pump efficiency on the pump-power. We find that η increases as the PB increasing and the maximumη of 42.2% is achieved for the source with PF of 40 mW and PB of 130 mW. Though there is a maximum linewidth of about 42.6 nm with a power ripple of about 0.6 dB occurred at PF of 30 mW and PB of 80 mW, but it can not achieve the maximum pumping efficiency. Other EDF lengths (e.g., 80 m, and 103 m) with the corresponding optimal pump powers, have been examined in this proposed configuration but offering worse characteristics of pumping efficiency and /or spectral ripple than that with EDF length of 93 m. Therefore, based on the design criteria, the best characteristics of output power of 71.8 mW, pumping efficiency of 42.2%, mean wavelength of 1584.3 nm, and spectral linewidth of 41.6 nm with a spectral ripple of 0.5 dB have been achieved by operating this ASE source at PF of 40 mW and PB of 130 mW with a effective mirror reflectance of 30%. This pumping efficiency of 42.2% is significantly improved as compared with the conventional ones of 14.3% in [44] and 14% in [64]. The asymmetrical pumping ratio of forward and backward pump power is about 24% and 76%. Therefore, a single pump LD with pump power of about 200 mW and a pump coupler with a coupling ratio of 24/76% can be used to replace pump LD1 and LD2 in practical application to give compact and non-expensive design.

6.3 Summary

We have experimentally demonstrated a high pumping efficiency L-band EDF-ASE source by using asymmetric bi-directional pumping configuration, operating at the effective mirror reflectance of 30% and the EDF length of 93 m pumped by PF of 40 mW and PB of 130 mW, to achieve high output power, flat, broadband with low spectral ripple characteristics without using any external spectral filters. The characteristics of output power of 71.8 mW, high pumping efficiency of 42.2%, a broad linewidth of 41.6 nm with a spectral ripple of 0.5 dB, and the mean wavelength of 1584.3 nm have been obtained. Such flat, high-power ASE source is essential for L-band DWDM device characterization and spectrum sliced DWDM systems.

Chapter 7 High-Power Flat L-Band Erbium-Doped Fiber ASE Source Using Dual Forward-Pumping Configuration

We have found that only the DPF configuration is the better one for a single-laser pumped L-band erbium-doped fiber ASE source with a pumping efficiency of 14% [64]. Other configurations (SPF, SPB, and DPB) are hard to act as L-band ASE sources. However, such low pumping efficiency and output power of L-band ASE source may limit its applications. In this chapter, we demonstrate a novel dual forward-pumping configuration for enhancing the output power and pumping conversion efficiency. The pumping power arrangement and the effect of mirror reflectance on the characteristics of the ASE source are examined.

在文檔中光纖傳輸即時監控系統中受激拉曼散射效應與寬頻掺鉺光纖光源之研究 (頁 54-0)