1 Introduction

Stable multiwalength single-mode erbium-doped fiber-ring lasers (MEDFRLs) are very attractive sources for many applications in optical fiber sensing, sensor network multiplexing schemes and instrument testing due to their advantages: simple structures, narrow linewidth, and compatibility with other optical fiber components [1, 2].

To achieve multiwavelength laser operation, versatile methods based on various gain media, such as EDF amplifiers (EDFAs) [3–5], semiconductor optical amplifiers (SOAs) [6], and Raman amplifiers [7–10] have been investigated.

Unfortunately, to get stable fiber-ring lasers is not a straightforward task because they are made by a rather long cavity length and consequently narrow longitudinal mode spacing. This fact contributes to generate multiple densely spaced longitudinal modes around the central lasing wavelength, thus, laser output is usually unstable because of the multimode oscillation, mode competition and mode hopping at room temperature of multiwavelength EDF lasers [1]. This aspect can restrict their practical applications.

Furthermore, ring fiber lasers are also susceptible to output power instabilities. These instabilities can degrade the performance characteristics of a sensor multiplexing network based on a laser interrogation scheme. The optimization of the ring laser configuration can improve considerably the characteristics of these lasers.

To achieve single longitudinal mode (SLM) operation, several approaches have been proposed [11–14]. In [12, 13] a multi-ring cavity is proposed to guarantee a SLM operation. However, it is precise to adjust properly the laser parameters. In [13] a SLM operation of a fiber-ring laser is achieved with a saturable absorber. Nevertheless, the efficiency of the fiber laser is reduced with this technique. A fiber Bragg grating Fabry-Pérot etalon can be also used to obtain SLM fiber lasers, but the spacing between the different longitudinal modes must be as big as possible in order to achieve SLM easily [14].

Dual-wavelength illumination is an elegant process of interrogation for such remote Fabry–Pérot cavities. Therefore, a fiber Bragg grating (FBG) based dual-wavelength laser may be an appealing solution for an interferometer interrogation which improves the accuracy of the measurements [15].

In this study, a stable dual-wavelength erbium-doped fiber-ring laser that operates in SLM condition is experimentally proposed and demonstrated. An optical signal to noise (OSNR) as high as 65 dB is obtained owing to the low noise configuration. Besides, the topology allows an independent control of the losses of each emission line. The time stability of the proposed fiber laser is also analyzed and discussed.

2 Experimental setup and results

The experimental setup of the proposed dual-wavelength erbium-doped fiber-ring laser is shown in Fig. 1(a). The designed laser consists of an amplifying fiber-ring cavity wherein a double FBG reflection for each wavelength is utilized. For this purpose, we included a four port circulator and a couple of FBGs and tunable FBGs for each wavelength. The non-tunable FBGs were connected at port 2 of the circulator, because their bandwidths are narrower than the tunable FBGs, so most part of the Amplified Spontaneous Emission (ASE) was filtered at first reflection. The FBGs are centered at 1547 nm and 1550 nm with a corresponding full-width at half maximum (FWHM) of 0.19 nm and 0.165 nm; a reflectivity of 99% and 98.3% and an extinction ratio of 33 dB and 35.3 dB, in this order. The tunable FBGs tuned at the same wavelength, 1547 nm and 1550 nm, have a FWHM of 0.609 nm and 0.390 nm, a reflectivity of 98.9% and 99.7% and an extinction ratio of 30 dB and 26.9 dB, respectively. In order to achieve an independent control of the system’s equalization, the order of connection of the tunable FBGs was inverted at the port number 3 of the circulator, the \(\lambda'_{1}\) wavelength was tuned at 1547 nm and \(\lambda'_{2}\) to 1550 nm, this way the variable attenuator 1 (VA1) controlled the gain at λ 2 and at the same mode the (VA2) only affected to λ 1. In particular, in order to achieve power equalization of both channels the attenuation of VA1 was set to 0.86 dB, while the losses induced by VA2 were fixed to 0.15 dB.

Fig. 1
figure 1

(a) Schematic diagram of the proposed dual-wavelength erbium doped fiber ring laser configuration; (b) heterodyne detection system. EDF Erbium doped Fiber; VA variable attenuator; ESA Electrical spectra analyzer;  index matching gel

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The gain of the fiber laser was provided by a 5 m-long highly erbium-doped fiber (EDF) (Er-30 by Liekki, with an absorption of 30 dB/m at λ=1530 nm). A 980/1550 nm wavelength-division multiplexer (WDM) was used to introduce the 70 mW of optical pump power from a 980 nm laser diode. The laser output was extracted from the cavity using a 70:30 fiber coupler, with which 30% power was fed back into the ring cavity.

The circulator guided both emission lines inside the ring cavity and served also as an isolator in our lasing structure ensuring the unidirectional cavity and avoiding the undesired spatial hole-burning (SHB) effect. All the free terminations were immersed in refractive-index-matching gel to avoid undesirable reflections.

The laser output, as usual, was monitored by an optical spectrum analyzer (OSA) with highest spectral resolution of 0.01 nm. The single-longitudinal-mode operation was verified by a heterodyne detection system, which is depicted in Fig. 1(b). Each lasing wavelength from our fiber laser was combined with the power of a commercial tunable laser source (TLS) using a 3 dB coupler. The TLS has a linewidth of 100 kHz and its wavelength was placed close to the fiber laser emission wavelengths. Thus the beating signal was observed by means of an ESA, whose resolution bandwidth can be as good as 1 Hz.

Figure 2 displays the output spectra of the proposed dual-wavelength EDF fiber-ring laser scheme corresponding to the reflection bands of both FBGs when pump power is 70 mW. The measured power of both channels was around −10 dBm, which is more than 65 dB higher than the amplified spontaneous emission (ASE) noise floor. An interesting conclusion to be drawn from Fig. 2 is that the proposed scheme is a very low noise topology because the ASE is filtered twice, this is a crucial advantage over some previously reported lasers in the literature [11–16].

Fig. 2
figure 2

Output optical spectrum measured of the proposed dual-wavelength EDFRL configuration for a pump power of 70 mW

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A good stability and single-longitudinal-mode operation are requirements for EDF ring lasers for some applications. Before looking at the stability of the output power laser, let us first consider the SLM operation.

As explained previously, fiber-ring lasers usually support a vast number of spaced longitudinal modes. Specifically, accounting for the theory of fiber-ring lasers the number of longitudinal modes within an emission line is:

$$ \mbox{Number of modes} = \frac{\mathrm{FWHM}_{\mathrm{FBG}}}{\Delta\lambda}$$
(1)

and

$$ \Delta\lambda= \frac{\lambda^{2}}{nL}$$
(2)

where Δλ is the mode spacing between longitudinal modes of the ring; n is the refractive index; L is the ring length; λ is the central mode wavelength; FWHMFBG is the bandwidth of the corresponding fiber Bragg grating.

Taking into account: n=1.5; L=17 m and the corresponding values of FWHM and λ for each FBG, provided previously; 2024 and 1757 are the number of modes therein the emission line centered at 1547 nm and 1550 nm, respectively.

However, when both lasing wavelengths are oscillating simultaneously with similar output powers by using the VAs to adjust the cavity losses the laser presents a SLM operation behavior as it is shown in Fig. 3. To corroborate it, we have calculated the mode spacing for each wavelength region which is Δλ=9.385×10−5 nm and Δλ=9.421×10−5 nm; accordingly Fig. 3 shows a bandwidth of 3 GHz or in other words 2.4×10−2 nm, the observed bandwidth is around 25 times higher than the mode spacing between longitudinal modes. Thus, we can conclude that indeed the dual-wavelength EDF laser works in single-mode operation. The picture presents the results of the heterodyne detection system measured by the ESA with a resolution of 100 kHz: the beating between the tunable laser source and both emission lines when the tunable laser source was tuned close to the first emission line (Fig. 3(a)) and the second (Fig. 3(b)). As aforementioned, Figs. 3(a) and (b) only can show the ESA spectrum for one of the wavelengths laser emission due to the wavelength difference between both emission lines is around 3 nm while the bandwidth of Fig. 3 is 2 orders of magnitude lower and the mode spacing approximately 5 orders lower.

Fig. 3
figure 3

Output optical spectrum measured by the ESA for the dual-wavelength EDFRL configuration when the tunable laser was tuned close to the first (a) and second (b) wavelength laser emission

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In [17] a SLM fiber-ring laser is achieved annihilating the mode competition with an auxiliary lasing. From this, one can infer that owing to the interaction of the seed light produced from one channel to the other and vice versa, or, in other words, one channel works as auxiliary lasing to the other; the channels multiple-longitudinal-mode oscillation is suppressed, and thus the mode competition and mode hopping is not produced. Previous works of the authors also corroborate this fact [11, 18].

The output power imbalance between both lasing wavelengths has a direct influence in the mode of operation. Therefore, a throughout assessment of the maximum power difference between both channels in order to the system keeps the SLM operation has been carry out, this value must be lower than 9.23 dB. This case is shown in Fig. 4.

Fig. 4
figure 4

(a) OSA and (b) ESA spectrum when the output power imbalance between both emission lines is 9.23 dB

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The measurements have been repeated at different pump power from 28 to 150 mW. In all cases, a SLM operation in both channels is achieved when the two lasing wavelengths are oscillating simultaneously with similar output powers by using the VAs to adjust the cavity losses. The pump power chosen was 70 mW in order to work further than the pumping threshold needed to obtain laser emission, since it improves the power stability.

The output power of the proposed dual-wavelength EDF ring laser was further investigated because the power of this kind of laser usually suffers some changes with time which can restrict its applicability [19, 20]. Therefore, the short term stability of the proposed configuration was characterized. The instability is defined as the output power variation for a given interval of time and a specific confidence interval (CI), given as a percentage. The confidence interval (CI) is the estimated range of values where the parameter of interest is included [19].

The laser has been tested during a period of 10 minutes at room temperature. The measured data have been stored each 15 s and a CL if 90% is considered. Figure 5 shows the power fluctuation for both emission lines of the fiber-ring lasers. The instability was 1.08 and 0.67 dB for the emission line centered at 1550 nm and 1547 nm, respectively. Unsurprisingly, the reduction of the number of longitudinal modes inside the cavity till the SLM operation has an outstanding positive effect in the output power variations as it is shown in Fig. 5. Power fluctuations of FBG centered at 1547 nm vary from 1.52 dB when the laser operation is in multiple-longitudinal mode to 0.67 dB when it works in SLM operation.

Fig. 5
figure 5

(a) Output power fluctuation for the emission line centered at 1547 nm when the laser works in SLM and MLM operation; (b) output power fluctuation for the emission line centered at 1550 when the laser operation is SLM

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3 Conclusions

In summary, we propose and demonstrate experimentally a new stable dual-wavelength EDF ring laser scheme The laser is based on ring resonators and employs fiber Bragg gratings to select the operation wavelengths. The topology of the laser has a significant influence on its performance: allowing an independent control of the losses and achieving a low noise configuration. As a result, it is experimentally demonstrated that both emission lines work in single-longitudinal-mode operation, showing an optical signal-to-noise ratio as high as 65 dB and a power stability which varies from 1.1 to 1.25 dB for both emission lines.