An Erbium-Doped Fiber Source with Near-Gaussian-Shaped Spectrum Based on Double-Stage Energy Matching and LPFGs

Abstract

An Erbium-doped fiber source with a near-Gaussian-shaped spectrum consisting of only a single peak, based on the double-stage energy matching and LPFGs, is proposed and demonstrated. A double-stage Erbium-doped fiber source system is built. The first-stage structure adopts the single-pass forward pumping method with a 15 m Erbium-doped fiber, while the second-stage structure adopts the backward-pumping method with a 26.5 m Erbium-doped fiber. The energy of the output spectrum is concentrated near the long wavelength (1560 nm) through the double-pump energy matching of the two stages. Long period fiber gratings (LPFGs) are used to filter the excess light near the short wavelength (1530 nm) in order to obtain a near-Gaussian-shaped spectrum consisting of only a single peak. The output power, pump conversion efficiency, line width, 3 dB bandwidth and mean wavelength of this near-Gaussian-shaped spectrum are tested and analyzed. When the pump powers of the first and second stages are 50 mW and 360 mW, respectively, the results show that a near-Gaussian-shaped spectrum with a power of 10.17 mW, spectral line width of 19.767 nm, and mean wavelength stability of −0.978 ppm/mW can be obtained. This research provides a method for the generation of a near-Gaussian-shaped spectrum with high output power and excellent mean wavelength stability, and it can produce multiple forms and energies of near-Gaussian-shaped spectra via this Erbium-doped fiber source.

1. Introduction

An Erbium-doped optical fiber source is a kind of superfluorescent fiber source (SFS), which is based on the amplified spontaneous emission (ASE) from an Erbium-doped fiber and is also known as an ASE source. The Erbium-doped fiber source is widely used in fiber optic gyroscope (FOG) [1,2], signal processing, optical coherence tomography [3], wavelength division multiplexing systems (WDM) [4,5] and spectrum-sliced sources [6] because of its advantages, which include low spectral ripples, a short coherence length, a high output power, a good temperature stability, a wide spectral bandwidth and a long service life. Therefore, many researchers have carried out in-depth research and proposed abundant design schemes for realizing Erbium-doped fiber sources with a high performance. The spectral bandwidth of Erbium-doped fiber sources has been broadened by inserting a LPFG [7,8], adopting a two-stage configuration [9], employing a nonlinear optical loop mirror [10], using a piece of Bi/Er co-doped fiber [11] or by means of a mode-locked Erbium-doped fiber laser [12]. Some works have also been reported on L-band [13,14] or C+L band [15] Erbium-doped fiber sources for the immediate expansion of the fiber-optic communication window. Additionally, many methods, such as employing a long-period fiber grating [16], a chirped fiber grating [17], a bandpass filter [18], an Erbium-doped photonic crystal fiber [19] or a real-time adjustment of two pump ratios [20], have been proposed to improve the mean-wavelength stability of Erbium-doped fiber sources for optical sensor applications. Some researchers have enhanced the radiation resistance of SFS by optimizing the concentration of various dopants [21] or employing multiple self-compensating methods [22] for space applications. Graphdiyne-Polymer Nanocomposites have been developed as saturable absorbers for ultrafast Erbium-doped fiber lasers [23]. Some researchers have controlled multistability in an Erbium-doped fiber laser by using a novel neural controller [24] or employing error-feedback control in a system with a coexisting attractor [25].

The research on Erbium-doped fiber sources have been restricted mostly to a two-peak spectrum [26,27], which, compared with a single-peak spectrum, shows a worse intrinsic mean-wavelength stability [28] and is not conducive to the application of high precision FOG. Additionally, only few works have reported a single-peak spectrum. Hall et al. [29] achieved single-peak backward output spectra by using a pump wavelength at 950 nm, which is different from the conventional 980 nm pump wavelength. Li et al. [30] only obtained a single-peak backward spectrum by incorporating an Er-doped fiber filter and a Faraday rotator mirror when the pump power was fixed at 55 mW. From the above research, it can be seen that the pump energy of the light source system has important effects on the spectral profile, so that it is possible to design an Erbium-doped fiber source with a specific spectral profile that meets the application requirements by regulating the pump energy, such as an Erbium-doped fiber source with a near-Gaussian-shaped spectrum, which could serve as a promising candidate for fiber optic gyroscopes.

In this paper, a double-stage configuration combined with cascaded LPFGs has been used to achieve an Erbium-doped fiber source with a high-performance near-Gaussian-shaped spectrum, which can still maintain a single-peak profile under a variable pump power. The output spectra under different pump powers with two stages are demonstrated and tested, and the LPFGs are adopted to filter the spectral peak at 1530 nm. Then, the output power, pump conversion efficiency, spectral line width, 3 dB bandwidth and mean wavelength with different pump powers are analyzed, respectively. The single-peak near-Gaussian-shaped spectra with multiple forms and energies can be received by this proposed Erbium-doped fiber source system.

2. Principle of System

The diagram of the double-stage Erbium-doped fiber source system is shown in Figure 1. Two laser diodes (LD, PYOE-SM-974) were manufactured by Henan Minghai Optoelectronic Technology Limited Company with a work wavelength of 974 nm. The Erbium-doped fiber (EDF, EDFC-980-HP) was manufactured by Nufern with a core radius of 1.6 µm and a numerical aperture of 0.23. The length of first-stage EDF is 15 m, while that of the second-stage one is 26.5 m. The first-stage EDF is shorter to obtain C-band ASE light, and the second-stage EDF is longer to convert ASE light from a short wavelength to a long wavelength. The first-stage structure adopts the single-pass forward pumping method, while the second-stage structure adopts the backward-pumping method to achieve a high output power. The two-stage structure is followed by two cascaded long-period fiber gratings (LPFGs) as the filter. Additionally, the LPFGs were manufactured by Henan Minghai Optoelectronic Technology Limited Company using SM-28e fiber. The optical isolator (ISO) is used to suppress the optical feedback. An Optical Spectrum Analyzer (OSA, AQ6370D) measured the output spectra of this system.

When the C-band ASE light is generated by the first stage, the light will be transmitted into the second-stage EDF. The second-stage structure adopts the backward-pumping method, and the EDF employed is enough long, so that the inversion level in the front part of the EDF is low. The ASE light around 1530 nm generated by the first stage will be absorbed by the Erbium ion located in the ⁴I_15/2 ground level manifold in the front end of EDF as pump light in order to generate long-wavelength light around 1560 nm [31]; if the length of EDF in the second stage is longer, the C-band ASE light may be converted further to the L-band; the corresponding energy-level diagram is shown in Figure 2. Then, the ASE light around 1560 nm will be injected into the EDF back end and amplified because of the high inversion level of the back-end EDF excited by LD. Therefore, the system can realize energy matching in the double stage and obtain multiple forms and energies of the near-Gaussian spectrum according to the energy matching. The energy of ASE light output by the two-stage structure will be mostly concentrated in the vicinity of 1560 nm, while a small amount of light output near 1530 nm will be generated by the backward ASE light of the second-stage structure. After energy matching, the spectrum generated by the two-stage structure passes through the LPFGs, and the excess light near 1530 nm will be filtered out. Therefore, the final output ASE spectrum will be a near-Gaussian-shaped spectrum containing only one peak near 1560 nm.

3. Experimental Results and Analysis

In order to verify the influence of two pump sources on the output spectrum of the double-stage structure, we firstly tested the output spectrum of the experimental system without the LPFGs shown as Figure 1. The test results are shown in Figure 3a–g. P_e is the pump power of the first-stage structure, and P_p is the pump power of the second-stage structure. The adjustive range of pump power P_e is from 0 mW to 60 mW, and the step is 10 mW. The adjustive range of pump power P_p is from 20 mW to 100 mW, and the step is 10 mW.

It can be seen from Figure 3a,b that when P_e ≤ 10 mW, the ASE light generated by the first-stage structure is too weak and is fully absorbed by the low-inversion front end of the EDF, so that the output light of the system without LPFGs is the backward ASE light generated by the second-stage structure. The spectra have two peaks around 1530 nm and 1560 nm, and the peak power around 1530 nm is higher than that around 1560 nm, indicating that the spectral energy is dominated by the light around 1530 nm. When the first-stage pump starts working at 20 mW, as shown in Figure 3c, the peak around 1530 nm becomes significantly weaker, and the peak around 1560 nm becomes stronger. The above phenomenon is caused by the fact that the ASE light around 1530 nm generated by the first-stage structure will be absorbed by the ground-state Erbium ion in the low-inversion front end of second-stage EDF as pump light, and it generates long wavelength light around 1560 nm. Then, the ASE light around 1560 nm will be injected into the high-inversion back end of second-stage EDF as a seed light and amplified by LD. When the pump power P_e increases to 30 mW, as shown in Figure 3d, the peak around 1530 nm further dramatically decreases, while the peak around 1560 nm further dramatically increases. When the pump power P_e increases to 40 mW, as shown in Figure 3e, the peak around 1530 nm only slightly decreases, while the peak around 1560 nm only slightly increases. When the pump power P_e further increases to 50 and 60 mW, as shown in Figure 3f,g, the two spectral peaks only show a tiny change. When the pump power P_e satisfies P_e ≥ 30 mW, as shown in Figure 3e–g, the obtained spectra consist of a main peak around 1560 nm and a secondary peak around 1530 nm, the increase of the pump power P_p of the second stage will cause the two peaks to enhance simultaneously, and the intensification of the peak around 1560 nm is more obvious. The above results illustrate that spectral variation can be modulated by the matching of two pump energies.

However, in Figure 3e–g, there is still a weak peak around 1530 nm, which is caused by the backward ASE light output from the second-stage structure. The secondary peak around 1530 nm will degrade the stability of the mean wavelength with changes in the pump power or wavelength. In order to obtain only a single peak in the near-Gaussian-shaped spectrum, two cascaded LPFGs are employed to filter the light around 1530 nm. The LPFGs can produce a broad attenuation band at a specific wavelength by coupling light from the fundamental guided mode to forward-propagating cladding modes [32]. The transmission spectra of the LPFGs used are shown in Figure 4.

The peak loss wavelength of LPFG1 is 1531.14 nm, the peak loss is 15.85 dB, and the 3 dB bandwidth is 9.25 nm. The peak loss wavelength of LPFG2 is 1532.98 nm, the peak loss is 7.83 dB, and the 3 dB bandwidth is 14.77 nm. The peak loss wavelength of LPFG1+ LPFG2 is 1531.62 nm, the peak loss is 26.00 dB, and the 3 dB bandwidth is 8.53 nm.

Then, we added the LPFGs in this double-stage Erbium-doped fiber source system shown in Figure 1. Figure 5 shows the evolution of the output spectrum with pump powers P_e and P_p measured by the OSA. The adjustive range of pump power P_e is from 10 mW to 60 mW, and the step is 10 mW. The adjustive range of pump power P_p is from 20 mW to 100 mW, and the step is 10 mW.

As shown in Figure 5a–f, the cascaded LPFGs were added to this fiber source system as a filter to eliminate the peak around 1530 nm. When the first-stage pump power P_e is not high enough, as shown in Figure 5a,b, the spectra still contain an obvious peak around 1530 nm because the double-stage structure outputs a high spectral peak around 1530 nm, which cannot be completely filtered by the LPFGs. When the pump power P_e increases to 30 mW, as shown in Figure 5c, the spectral peak around 1530 nm after filtering by LPFGs is very tiny.

When the pump power P_e further increases and satisfies P_e ≥ 40 mW, as shown in Figure 5d–f, the spectral peak around 1530 nm can be completely filtered by the LPFGs. We have fitted the spectral data and obtained the corresponding function expression; Formula (1) is the expression, while P_e and P_p are 40 mW and 100 mW, respectively:

$$\mathrm{P}=1.170\times {10}^{-4}+1.219\times {10}^{-2}\mathrm{e}\mathrm{x}\mathrm{p}[-1.602\times {10}^{-2}\times {\left(\mathsf{\lambda }-1556.451\right)}^2],$$

(1)

The result indicates that the spectral profile has a near-Gaussian shape, and other spectral data also have similar functional forms. Thus, a near-Gaussian-shaped spectra with only a single peak around 1560 nm can be obtained when compared with the spectra shown in Figure 3e–g. Additionally, although Figure 5f only shows the spectrum with P_e and P_p up to 60 mW and 100 mW, respectively, the output spectra can, in practice, still perfectly maintain a single-peak near-Gaussian profile, while P_e and P_p continue to increase. The above results illustrate that the proposed Erbium-doped fiber source could achieve near-Gaussian-shaped spectra with a single peak under a variable pump power.

Then, the influence of the pump power on the output power P_o, pump conversion efficiency (PCE), spectral line width (Δλ), 3 dB bandwidth and mean wavelength $\overline{\mathsf{\lambda }}$ is analyzed and discussed. The adjustive range of pump power P_e is from 40 mW to 100 mW, and the step is 10 mW. The adjustive range of pump power P_p is from 20 mW to 500 mW, and the step is 20 mW. The output power and 3 dB bandwidth are directly measured by the OSA, while the pump conversion efficiency, spectral line width [26] and mean wavelength [1] are respectively obtained by the following three formulas:

$$\mathrm{P}\mathrm{C}\mathrm{E}=\frac{{\mathrm{P}}_{\mathrm{o}}}{{\mathrm{P}}_{\mathrm{e}}+{\mathrm{P}}_{\mathrm{p}}},$$

(2)

$$\Delta \mathsf{\lambda }=\frac{{\left|\int \mathrm{P}\left(\mathsf{\lambda }\right)\mathrm{d}\mathsf{\lambda }\right|}^2}{\int {\mathrm{P}}^2\left(\mathsf{\lambda }\right)\mathrm{d}\mathsf{\lambda }},$$

(3)

$$\overline{\mathsf{\lambda }}=\frac{\int \mathrm{P}\left(\mathsf{\lambda }\right)\mathsf{\lambda }\mathrm{d}\mathsf{\lambda }}{\int \mathrm{P}\left(\mathsf{\lambda }\right)\mathrm{d}\mathsf{\lambda }},$$

(4)

Those results are shown in Figure 6a–e.

As shown in Figure 6a,b, the output power and pump conversion efficiency at different pump powers are obtained. The output power P_o linearly increases with the increase of the pump power P_p at the second stage and also increases with the increase of the pump power P_e at the first stage. It is easy to observe that the effect of P_p on the output power is more obvious than that of P_e. The pump conversion efficiency first increases and then tends to be flat with the increase of pump power P_p. The pump conversion efficiency decreases with the increase of P_e, while the value of P_p satisfies P_p 400 mW. When the value of P_p satisfies P_p ≥ 400 mW, the pump conversion efficiency first increases and then decreases with the increase of P_e, and the maximum pump conversion efficiency is obtained at P_p = 50 mW.

In order to obtain a high output power and a better conversion efficiency, it is best to set the pump power P_e of the first-stage structure to 50 mW, and the pump power P_p of the second-stage structure should satisfy P_p ≥ 360 mW. The output power is up to 10.17 mW, and the corresponding conversion efficiency is 2.48%, while P_e = 50 mW and P_p = 360 mW. When P_p is increased to 500 mW, the maximum output power is 13.56 mW, and the corresponding pump efficiency is 2.465%.

The line width and 3 dB bandwidth at different pump powers are shown in Figure 6c,d, respectively. The spectral line width and 3 dB bandwidth decrease first and then increase slowly with the increase of P_p. When P_p ≥ 100 mW, the curves at different P_e almost coincide, indicating that the spectral line width and 3 dB bandwidth are insensitive to the change of P_e. In order to ensure a high output power and better pump efficiency, we consider the case where P_e is fixed at 50 mW and P_p increases from 360 mW to 500 mW: the spectral line width increases slowly from 19.767 nm to 19.872 nm, and the corresponding 3 dB bandwidth increases slowly from 11.927 nm to 12.029 nm.

The mean wavelength at different pump powers is shown in Figure 6e. When P_e ≤ 50 mW, the mean wavelength of the output spectrum first increases and then decreases with the increase of P_p. Additionally, the mean wavelength of the output spectrum always decreases with the increase of P_p, while P_e ≥ 60 mW. When P_p ≥ 100 mW, the curves at different P_e almost coincide, indicating that the mean wavelengths are insensitive to the change of P_e. In order to ensure a high output power and better pump efficiency, we consider the case where P_e is fixed at 50 mW and P_p increases from 360 mW to 500 mW: the mean wavelength decreases slowly from 1555.168 nm to 1554.955 nm. The stability of the mean wavelength with the pump power can be calculated as −0.978 ppm/mW.

4. Conclusions

In summary, an Erbium-doped fiber source with a near-Gaussian-shaped spectrum adopting double-stage energy matching and LPFGs is proposed and demonstrated. The output spectra under a variable pump power can maintain a single-peak near Gaussian profile. Additionally, the output power, pump conversion efficiency, spectral line width, 3 dB bandwidth and mean wavelength of the near-Gaussian-shaped spectrum at different pump powers are discussed in detail. When the pump power of the first and second stages are 50 mW and 360 mW, respectively, a near-Gaussian-shaped spectrum with an output power of 10.17 mW, spectral line width of 19.767 nm, and mean wavelength of 1555.168 nm can be obtained. The stability of the mean wavelength with the pump power is −0.978 ppm/mW, while P_p increases from 360 mW to 500 mW. The designed Erbium-doped fiber source is a promising candidate for fiber optic gyroscopes that require a high stability source. The results show that the laser spectral profile can be easily adjusted, controlled and transformed by the pumped-light-source energy control, which is an effective way to realize the active regulation of the light-source spectrum, and has good application prospects in the design, characteristic control and improvement of lasers.