## Abstract

Based on fiber Bragg gratings (FBGs) and high nonlinear photonic crystal fiber (HN-PCF), a novel dual-wavelength erbium-doped fiber (EDF) laser is proposed and demonstrated. Experimental results show that, owing to the contributions of two degenerate four-wave mixings in the HN-PCF, the proposed fiber laser is quite stable and two output signals are uniform at room temperature. With adjustment of the attenuator, our fiber laser can selectively realize one wavelength lasing.

©2005 Optical Society of America

## 1. Introduction

Because of potential applications of multiwavelength fiber lasers, erbium-doped fiber (EDF) lasers emitting in multiple wavelengths simultaneously have attracted much interest recently [1–3]. Because of the homogeneous gain broadening of EDFs, various techniques for mitigating the mode competition have been employed to achieve stable multiwavelength (or dual-wavelength) oscillations [4–9]. For example, a filter was inserted into the EDF laser cavity [2, 4], and a single EDF was cooled in liquid nitrogen (77 K) to reduce the homogeneous broadening [8]. Moreover, with the assistance of four-wave mixing (FWM) of PCF, a new fiber laser configuration has been proposed [10] and laser pulses have been enhanced [11]. In this paper, on the basis of fiber Bragg gratings (FBGs) and FWM of HN-PCF, a stable dual-wavelength EDF laser is proposed and demonstrated. The experimental results show that the proposed EDF laser can stably and uniformly lase two wavelengths simultaneously.

## 2. Experimental setup

The schematic layout of the proposed EDF laser is shown in Fig. 1(a). This experiment is based on a standard EDF ring laser, with HN-PCF and two FBGs. The transmission spectra of FBG1 and FBG2 are exhibited in Fig. 1(b). A circulator and two FBGs form the ring, with EDFA for lasing wavelengths *λ*
_{1} and/or *λ*
_{2} and HN-PCF for creating FWM. A variable attenuator (VA) is used to adjust the reflection spectra of FBG1, helping to lase wavelength *λ*
_{1} or *λ*
_{2} or both. A 90:10 coupler (10% output) is used for the laser output. For simultaneously lasing two wavelengths *λ*
_{1} and *λ*
_{2}, the ring losses at *λ*
_{1} and *λ*
_{2} are balanced by adjusting VA and changing polarization controller (PC).

In Fig. 1(a), the commercial EDFA (Opto-Link Corporation Limited, Model EDFA-MP), which can offer 13 dBm output saturation power and a maximum 35 dB small signal gain, includes the 980 nm pump source, 980/1550 WDM coupler and optical isolator. A 51-m-long HN-PCF is with the nonlinear coefficient of ~11/W/km and the flat dispersion (0.5–1.5 ps/nm/km) over the wavelength range of 1480–1620nm. FBG1 and FBG2 have the central wavelength of 1563.51 nm and 1567.21 nm, the reflectivity of 12.1 dB and 13.9 dB, and the full-width at half maximum (FWHM) of 0.12 nm and 0.13 nm, respectively (see Fig. 1(b)). Moreover, except that Fig. 2 and Fig. 7 are measured by an optical spectrum analyzer (OSA) with the resolution of 0.1 nm, all other results are examined by the resolution of 0.01 nm.

## 3. Principle and results

When the attenuation of VA is equal to zero, the amplified spontaneous emission (ASE) reflection spectra from two FBGs are demonstrated in Fig. 2. Figure 2 shows that the normalized ASE power reflected from FBG1 is more than that reflected from FBG2. Therefore, *λ*
_{1} is lased in the ring cavity, and the output spectrum is illustrated in Fig. 3(a). On the other hand, when the attenuation of VA is more than zero and the reflection power from FBG1 is less than that from FBG2, only *λ*
_{2} is lased rather than *λ*
_{1}. In this case, Fig. 3(b) shows the output spectrum. Both Figures 3(a) and (b) are measured at the pump current *I*=55 mA. The wave without lasing in Fig. 3(b) is from the reflection of FBG, while the wave at the wavelength of 1567.21 nm in Fig. 3(a) is somewhat lased besides the FBG-reflected ASE. The experimental results also show that the output spectrum performance at other pump currents is similar to Fig. 3. Therefore, by adjusting VA in the EDF ring laser, the lased wavelength *λ*
_{1} or *λ*
_{2} is selective.

By adjusting VA and PC, both *λ*
_{1} and *λ*
_{2} can be lased simultaneously when the reflection powers from FBG2 and FBG1 are approximate equal. Unfortunately, the experiments show that the lased wavelengths *λ*
_{1} and *λ*
_{2} are unstable and nonuniform if without PCF. To improve the stability and increase the uniformity of dual-wavelength, the HN-PCF is introduced. The operation principle is as follows.

The continuous waves (CW) with wavelengths *λ*
_{1} and *λ*
_{2} (corresponding to frequencies *ω*
_{1} and *ω*
_{2}) are reflected from FBG1 and FBG2, respectively, and are launched into a 51-m-long HN-PCF. Due to higher nonlinear coefficient and lower dispersions of PCF, two new waves at frequencies *ω*
_{0}=2·*ω*
_{1}-*ω*
_{2} and *ω*
_{3}=2·*ω*
_{2}-*ω*
_{1} are created at the output by two degenerate-FWM processes of HN-PCF (*ω*
_{0}, *ω*
_{1}, *ω*
_{2} and *ω*
_{3} are shown in Fig. 4). Successively, four waves are inputted to EDFA and are amplified. Because of the mode competition caused by the homogeneous gain broadening of EDF [5], the power of one signal (e.g., *ω*
_{1}) is greater than that of other signal (e.g., *ω*
_{2}). After the reflection of FBGs with signals *ω*
_{1} and *ω*
_{2} and launching into HN-PCF again, two degenerate-FWMs (i.e., *ω*
_{0}+*ω*
_{2}=2·*ω*
_{1} and *ω*
_{3}+*ω*
_{1}=2·*ω*
_{2}) in the HN-PCF lead to the energy transfer from the higher-power signal *ω*
_{1} to the lower-power signal *ω*
_{2}. Therefore, FWM can effectively alleviate the gain competition in the EDF and significantly increase the stability and uniformity of two signals *ω*
_{1} and *ω*
_{2}.

Figure 4 demonstrates the experimental result of the proposed dual-wavelength EDF laser at *I*=240 mA, where two inserted figures are zoomed in two created waves *ω*
_{0} and *ω*
_{3}. From Fig. 4, we can see that two signals *ω*
_{1} and *ω*
_{2} produced by our EDF laser are great uniform and OSNR is more than 60 dB. Moreover, Figs. 3 and 4 illustrate that the FWHM bandwidth in each wavelength is approximately 0.09–0.11 nm, and then they are the multi-longitudinal mode lasers (the detailed explanations are shown in Appendix). To give a clearer understanding of the uniformity of signals *ω*
_{1} and *ω*
_{2}, we present a movie that illustrates the evolution of dual-wavelength EDF laser in terms of the EDFA pump current *I* (see Fig. 5).

The movie exhibits that (1) when the pump current *I*<45 mA, the oscillator does not produce any signal except noises because of the loss of the cavity; (2) if 45 mA <*I*<50 mA, two waves are created by the reflection of two FBGs; (3) when *I*>50 mA, only one signal is lased in the oscillator firstly, and successively two signals are generated simultaneously; (4) during the process of 50 mA <*I*<70 mA, two lased signals *ω*
_{1} and *ω*
_{2} are unstable and nonuniform; (5) when *I*>70 mA, *ω*
_{1} and *ω*
_{2} are great stable and uniform. Above results can be found from Fig. 6 in detail.

Figure 6 shows the power difference Δ*P* of two signals *ω*
_{1} and *ω*
_{2} in terms of the pump current *I*. The inset is zoomed in the red dashed-dot frame. From Fig. 6 and movie, one can see that (1) although the power difference Δ*P* is small when *I*<50 mA, no any signal is lased; (2) only one signal is lased when *I*=51 mA and 52 mA, as leads to Δ*P*>27 dB; (3) if *I*>70 mA, two lased signals are great uniform and their power difference Δ*P* is less than 1.8 dB (see the inset of Fig. 6); (4) Δ*P* is less than 0.6 dB when *I* exceeds 165 mA. Therefore, with the increase of the EDF gain (correspond to increase *I*), the lased spectra of the dual-wavelength tend to be more uniform. Our experiments also show that lengthening the length of nonlinear fiber can effectively equalize the lased spectra of two waves. The experimental results exhibit that, when *I*>70 mA, our dual-wavelength EDF laser is much stable. To show this advantage, Fig. 7 offers an experimental example at *I*=170 mA.

The variation of power in terms of time at *I*=170 mA is demonstrated in Fig. 7, where the unit of abscissa is second. One can see that, from Fig. 7, the signal powers fluctuate slightly and their relative fluctuation is less than 0.34%. Therefore, the proposed EDF laser with the assistance of two degenerate-FWMs are not only much uniform for two signals but also great stable. The experiments also show that the output power of each wavelength in the single-wavelength (e.g., Fig. 3) and dual-wavelength lasing is of the excellent stability.

From Figs. 4–7, we can see that the proposed fiber laser has the capacity of lasing the dual-wavelength with the excellent uniformity and stability. The physical reasoning can be explained as follow. When two degenerate-FWM processes are produced, we name as *ω*
_{1}+*ω*
_{1}=*ω*
_{0}+*ω*
_{2} and *ω*
_{2}+*ω*
_{2}=*ω*
_{3}+*ω*
_{1}. Then, two photons of frequency *ω*
_{1} are annihilated in order to create one photon of frequency *ω*
_{0} and another photon of frequency *ω*
_{2}. In the same way, two photons of frequency *ω*
_{2} are annihilated in order to produce one photon of frequency *ω*
_{3} and another photon of frequency *ω*
_{1}. Here, we designate *P*
_{1} and *P*
_{2} as the powers at *ω*
_{1} and *ω*
_{2}, respectively. To balance the number of photons annihilated and created in FWM processes, variation of power at frequency *ω*
_{1}, Δ*P*
_{1}, and at *ω*
_{2}, Δ*P*
_{2}, can be determined as

where the parameter *α* denotes the efficiency of FWM processes. From Eqs. (1) and (2), variation of the quotient *P*
_{2}/*P*
_{1} can be achieved as

From Eq. (3), it can be seen that ① if *ω*
_{1}
*P*
_{2}>*ω*
_{2}
*P*
_{1} (or *P*
_{2}>*P*
_{1}, approximately), Δ(*P*
_{2}/*P*
_{1})<0 and then *P*
_{2}→*P*
_{1};② if *ω*
_{2}
*P*
_{1}>*ω*
_{1}
*P*
_{2} (or *P*
_{1}>*P*
_{2}, approximately), Δ(*P*
_{2}/*P*
_{1})>0 and so *P*
_{1}→*P*
_{2};③ and if *ω*
_{1}
*P*
_{2}=*ω*
_{2}
*P*
_{1} (or *P*
_{2}=*P*
_{1}, approximately), Δ(*P*
_{2}/*P*
_{1})=0. Therefore, both powers can be equalized and stabilized by FWM processes.

## 4. Conclusions

On the basis of two FBGs and a flat dispersion HN-PCF, a new dual-wavelength EDF laser is proposed and demonstrated in this paper. With the assistance of two degenerate-FWMs in the HN-PCF, the lased dual-wavelength has great stability, e.g., the relative fluctuation of signal power is less than 0.34% in our experiments. At the same time, the two lased signals *ω*
_{1} and *ω*
_{2} are quite uniform; e.g., their power difference Δ*P* is less than 1.8 dB for the pump current *I*>70 mA and even it is less than 0.6 dB for *I*>165 mA at room temperature. By adjustment of the attenuator, the proposed fiber laser can easily realize one selection wavelength lasing.

## Appendix

The longitudinal-mode spacing Δ*v _{L}* in the laser can be achieved by use of the phase-matching condition [12], i.e.,

where *L _{R}* is the fiber length and

*β*is the propagation constant. Usually, Δ

*v*can be approximately given by Δ

_{L}*v*=1/

_{L}*T*[12], where

_{R}*T*is the round-trip time within the resonator. In our EDF laser, the total length is ~70 m, which includes the HN-PCF length of 51 m, the EDFA length of ~10 m and some jumpers and connecting fibers. As a result, the longitudinal-mode spacing Δ

_{R}*v*≅3 MHz. On the other hand, the effective linewidth of the EDF laser is less than 100 MHz [9, 13]. However, the FWHM of the spectral envelope of the lasers in Figs. 3–5 is ~12 GHz (i.e., ~0.1 nm). Therefore, such a spectral envelope should include any modes and our EDF laser is a multi-longitudinal-mode laser.

_{L}Actually, to realize a single-longitudinal-mode fiber laser, some special methods and techniques have to be implemented, e.g., inserting a linewidth-narrowing saturable absorption filter [13]. In our proposed EDF laser, such techniques are excluded. Therefore, the excellent stability and uniformity of our dual-wavelength laser are realized by the assistance of the FWM effect of HN-PCF.

## References and links

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