Stable and high-performance multiwavelength erbium-doped fiber laser based on fiber delay interferometer

Abstract

In this paper, we proposed a novel scheme to realize the multiwavelength erbium-doped fiber lasers. By adding a length of dispersion shifted fiber (DSF) in the ring cavity, we can suppress the cavity mode competition resulting from homogeneous line broadening (HLB) effect. In addition, a comb filter based on fiber delay interferometer (DI) is used for frequency selecting. To enhance the extinction ratio while maintaining the free space range (FSR), the proposed isolator-assisted double-pass DI is utilized into the laser cavity, and a stable 7-wavelength simultaneous lasing spaced at 21.5 GHz is accordingly achieved with an extinction ratio of higher than 40 dB. The lasers are stable with a maximum power fluctuation per channel of less than 0.6 dB during an hour test.

Key words： multiwavelength; erbium-doped; fiber laser; delay interferometer (DI); dispersion shifted fiber (DSF)

Introduction

Multiwavelength fiber lasers have aroused interest because of their potential applications in optical communications, fiber-optic sensors, optical instrumentation, and microwave photonic systems [1]. Both the semiconductor optical amplifiers (SOAs) and erbium-doped fiber amplifiers (EDFAs) have been used as the gain medium for generation of multiwavelength fiber laser. Compared to SOA-based multiwavelength fiber lasers, multiwavelength lasers with EDFA have advantages in their higher saturated power, lower polarization-dependent gain (PDG), and flatter gain spectrum. The main challenges for erbium-doped fiber (EDF) ring lasers to achieve stable multiwavelength lasing at room temperature are the strong homogeneous line broadening (HLB) and the cross-gain saturation. Previously, several approaches have been proposed. Cooling the EDF to 77 K by liquid nitrogen can suppress the homogenous line broadening and the cross-gain saturation [2], but this technique is impractical in many applications. To obtain room temperature multiwavelength lasing, a specially designed erbium-doped two core fiber has been used to provide inhomogeneous gain through macroscopic spatial hole-burning [3]. More recently, multiwavelength lasing was reported by incorporating a highly nonlinear fiber (HNLF) in the ring cavity [4]. Based on the self-phase modulation and the four-wave mixing (FWM) in the HNLF, 488 channels with a wavelength spacing of 10 GHz were obtained. However, the signal-to-spontaneous-noise ratio of the laser was below 20 dB, which was not suitable for some applications. Meanwhile, Liu et al. [5] and Zhang et al. [6] proposed multiwavelength fiber ring lasers by adding a length of highly nonlinear photonic crystal fiber (HN-PCF) into the ring cavity. The HN-PCF has a flat and low dispersion profile over a wide bandwidth and therefore generates an FWM-induced dynamic gain flattening mechanism in the ring cavity, which further enables the multiwavelength operation.

In this paper, we investigated the stable room temperature multiwavelength lasing in an erbium-doped fiber laser by incorporating a section of dispersion shifted fiber and a delay interferometer. In order to narrow the lasing linewidth and enhance the extinction ratio, an isolator-assisted double-pass delay interferometer (DI) is utilized into the laser system and a high-performance multiwavelength erbium-doped fiber laser is demonstrated experimentally. The laser is able to realize 7-wavelength simultaneous lasing spaced at 21.5 GHz with an extinction ratio of higher than 40 dB and excellent stability within the power nonuniformity of less than 3dB.

Theoretical analysis

Figure 1 shows the schematic of the delay interferometer. The input light is split into two paths along the two arms through a 3 dB coupler. One arm is introduced by a \mathrm{\Delta }t time delay determined by the fiber length difference of the two arms, while the other is introduced by a phase shift of \mathrm{\Delta }\varphi determined by the phase difference of the two arms. The two lights superimpose at another 3 dB coupler.

Fig.1 Schematic diagram of DI

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By using either a single-pass DI or a double-pass DI always suffer from their respective drawbacks, such as a broader lasing linewidth or a lower signal to noise ratio. In this part, an isolator-assisted double-pass DI configuration is proposed to overcome those problems. The transmission characteristics of such configuration are then analyzed and compared with those of the single-pass and direct double-pass configurations, respectively. Figure 2 shows the schematic diagrams of the single-pass, direct double-pass, and isolator-assisted double-pass DI, respectively. The conventional single-pass DI simply consists of two 3 dB couplers and a delay element in one of the two arms, as shown in Fig. 2(a). The direct double-pass configuration shown in Fig. 2(b) is formed by directly connecting the two output ports of DI. The newly proposed isolator-assisted double-pass DI is configured by inserting an optical isolator between the two connected output ports as shown in Fig. 2(c). By using the coupled wave equations for couplers and the propagation equation along the delay arm, taking the polarization effect and the splitting ratio of couplers into account and assuming a linearly polarized input light, we obtain the new transmission functions of the three different types of DI configurations as follows: in the single-pass configuration,

(1)T_{\mathrm{1}}(\varphi )=A_{\mathrm{1}}-\mathrm{2}{A}_{\mathrm{2}}\cos \theta \cos \varphi ;

in the direct double-pass configuration,

(2)T_{\mathrm{2}}(\varphi )=\mathrm{8}{A}_{\mathrm{2}}^{\mathrm{2}}+A_{\mathrm{3}}^{\mathrm{2}}+\mathrm{8}{A}_{\mathrm{1}}{A}_{\mathrm{2}}\cos \theta \cos \varphi +\mathrm{8}{A}_{\mathrm{2}}^{\mathrm{2}}\cos (\mathrm{2}\theta )\cos (\mathrm{2}\varphi );

and in the isolator-assisted double-pass configuration,

(3)T_{\mathrm{3}}(\varphi )=A_{\mathrm{4}}+\mathrm{4}{A}_{\mathrm{2}}^{\mathrm{2}}-\mathrm{4}{A}_{\mathrm{1}}{A}_{\mathrm{2}}\cos \theta \cos \varphi +\mathrm{2}{A}_{\mathrm{2}}^{\mathrm{2}}\cos (\mathrm{2}\theta )\cos (\mathrm{2}\varphi ),

where

A_{\mathrm{1}}=\mathrm{1}-k_{\mathrm{1}}-k_{\mathrm{2}}+\mathrm{2}{k}_{\mathrm{1}}{k}_{\mathrm{2}},

A_{\mathrm{2}}=\sqrt{(\mathrm{1}-k_{\mathrm{1}})(\mathrm{1}-k_{\mathrm{2}})k_{\mathrm{1}}{k}_{\mathrm{2}}},

A_{\mathrm{3}}=\mathrm{2}{k}_{\mathrm{1}}-\mathrm{4}{k}_{\mathrm{1}}{k}_{\mathrm{2}}+\mathrm{2}{k}_{\mathrm{2}}-\mathrm{1},

A_{\mathrm{4}}={(\mathrm{1}-k_{\mathrm{1}})}^{\mathrm{2}}{(\mathrm{1}-k_{\mathrm{2}})}^{\mathrm{2}}+k_{\mathrm{1}}^{\mathrm{2}}{k}_{\mathrm{2}}^{\mathrm{2}}.

k_{\mathrm{1}} and k_{\mathrm{2}} are the splitting ratio of the two couplers, respectively; \theta denotes the polarization angle difference of the two beams after a single pass through the two arms of DI; \varphi is the phase difference between the two arms of DI at wavelength \lambda, \varphi =\mathrm{2}\pi n\mathrm{\Delta }L/\lambda, where n is the material refractive index, and \mathrm{\Delta }L is the length difference between the two arms of DI. It should be noted that the insertion losses of the two couplers, the delay arm, and the isolator are ignored in Eqs. (1)-(3), since they just induce an added loss factor for the transmission functions and do not influence on the transmission spectra. In addition, although Eqs. (1)-(3) are obtained by assuming a linearly polarized input light, they are also applicable to an unpolarized input light. An unpolarized light can be divided into two orthogonally linearly polarized components whose phase difference is not fixed. By applying Eqs. (1)-(3) to each of the two orthogonal components, the transmission function of each component can be obtained. Then, the overall transmission function can be obtained by adding the transmission functions of two orthogonal components.

Fig.2 Schematic diagram of three types of DI. (a) Single-pass; (b) direct double-pass; (c) isolator-assisted double-pass

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Fig.3 Measured transmission spectra of three different types of DI. (a) Single-pass; (b) direct double-pass; (c) isolator-assisted double-pass

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In the experiments, using a broadband amplified spontaneous emission (ASE) source and an optical spectrum analyzer (OSA) with a resolution of 0.01 nm, we measured the transmission spectra of the three different types of DI used in our experimental system. As shown in Fig. 3, the direct double-pass configuration reduces the free space range (FSR) to a half of the single-pass configuration, and accordingly, it considerably reduces the 3-dB passband. In contrast, the proposed isolator-assisted double-pass DI configuration can keep the same FSR and also significantly narrow the 3-dB passband as compared with the single-pass configuration. Furthermore, the configuration of double-pass with isolator provides a much higher extinction ratio (>35 dB), whereas the direct double-pass configuration has the lowest extinction ratio (only about 6 dB).

Experimental results and discussion

Figure 4 shows the schematic diagram of the proposed multiwavelength erbium-doped fiber laser. The multiwavelength laser consists of a commercial erbium-doped fiber amplifier (EDFA), a length of dispersion shifted fibers (DSFs), a delay interferometer, a polarization controller (PC), an isolator, and a 10∶90 output coupler. The EDFA, which can deliver 350 mW output saturation powers, provides the required gain. The DSF is used to suppress the cavity mode competition resulting from homogeneous line broadening effect. The DI is the key component of the proposed multiwavelength laser. As shown in Fig. 2, each of the three waveguide-based DI configurations, including the single-pass, direct double-pass, and isolator-assisted double-pass, is used as a comb filter in the laser cavity. In Fig. 3, the single-pass DI with an FSR of 43 GHz has a widest 3 dB passband and a medium extinction ratio of about 20 dB. The double-pass DI reduces the FSR to a half (i.e., 21.5 GHz) and certainly has a narrowest 3-dB passband; however, it also has a lowest extinction ratio of only about 6 dB. In contrast, the isolator-assisted double-pass DI can considerably narrow the 3-dB passband and enhance the extinction ratio to an extremely high value (>35 dB) while maintaining the FSR. The insertion losses of the single-pass, direct double-pass, and isolator-assisted double-pass waveguide-based DI in the experiments are 2.1, 4.7, and 5.3 dB, respectively.

Fig.4 Schematic diagram of experimental system

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Fig.5 ASE spectrum of EDFA

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In the experiments, all the laser outputs from the coupler were measured by an OSA (resolution 0.01 nm) and a power meter. The EDFA provides a saturation output power of 330 mW for all the experiments. And Fig. 5 shows the ASE spectrum of the EDFA. With the direct double-pass DI configuration, the flatten output optical spectrum was obtained by carefully adjusting the PC, as shown in Fig. 6(a). The total output power was -1.5 dBm. As can be clearly seen in Fig. 6(b), 12-wavelength simultaneous oscillations from around 1559 nm to 1561 nm are observed with a power nonuniformity of less than 3 dB and a channel spacing of 21.5 GHz. However, one can obviously find in Fig. 6 that the laser has a lower extinction ratio of less than 30 dB and a halved channel spacing of 21.5 GHz. This is attributed to the fact that the direct double-pass DI degrades the filter characteristics such as the extinction ratio and FSR. On the contrary, the proposed isolator-assisted double-pass DI configuration can be expected to greatly improve these characteristics as also included in our theoretical analysis. When the isolator-assisted double-pass DI configuration is utilized in the laser cavity, the corresponding output optical spectrum with the full wavelength band is shown in Fig. 6(c), and its close look around the peak wavelength is shown in Fig. 6(d). As shown in Fig. 6(d), 9 lasing lines with a channel spacing of 43 GHz are achieved with the power nonuniformity less than 3 dB. Furthermore, the extinction ratio is enhanced to be higher than 30 dB. Such a high extinction ratio is attributed to the excellent characteristics of our proposed isolator-assisted double-pass DI configuration.

Fig.6 Output spectra of laser with different DI configurations. (a) Full laser spectrum of direct double-pass DI configuration; (b) close look of laser spectrum of (a) for clearly 11 lasing lines; (c) full laser spectrum of isolator-assisted double-pass DI configuration; (d) close look of laser spectrum of (c) for clearly 8 lasing lines.

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Achieving a narrow linewidth of each lasing channel is one of important purposes of this work. We measured the linewidths of the three multiwavelength fiber lasers (MFLs) with different DI configurations using an OSA with a spectral resolution of 0.01 nm. As shown in Fig. 7, the single-pass configuration has the widest linewidth of 0.051 nm. The linewidth of the isolator-assisted double-pass configuration is considerably narrowed to 0.037 nm, absolutely benefiting from the double passing of DI. The direct double-pass configuration, because of reduction in the FSR to a half value, has the narrowest linewidth of 0.031 nm.

Fig.7 Measured linewidth of single lasing mode in different DI configurations. (a) Single-pass; (b) direct double-pass; (c) isolator-assisted double-pass

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To evaluate the stability of the above MFLs, the output spectra of both the direct double-pass and isolator-assisted double-pass MFLs are scanned at a five-minute interval for one hour, respectively. Accordingly, 13 series of spectral data are also obtained and recorded. For both the lasers, no significant variations per lasing channel in the amplitudes are directly observed from the optical spectra. To precisely investigate the power stability, the maximum power fluctuation per channel is obtained by analyzing the 13 series of the recorded spectral data. The experiment results show that the maximum power fluctuation is less than 0.6 dB.

Conclusion

In conclusion, an isolator-assisted double-pass DI configuration is proposed and theoretically analyzed to improve the filter characteristics of the conventional single-pass and direct double-pass DI. Using the proposed isolator-assisted double-pass and the conventional direct double-pass DI configurations as comb filters in an EDFA-based fiber laser cavity, we have experimentally demonstrated high-performance multiwavelength EDFA-based fiber lasers with 8-wavelength simultaneous oscillations spaced at 43 GHz and high extinction ratio of more than 40 dB. Compared with the linewidth 0.0518 nm of single-pass DI configuration, narrower linewidths of 0.037 and 0.031 nm have been obtained for both isolator-assisted and direct double-pass DI configurations, respectively. The lasers have a good stability with a maximum power fluctuation per lasing channel of less than 0.6 dB.

References

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