We propose and experimentally demonstrate a switchable multiwavelength erbium doped fiber laser based on a highly nonlinear dispersion shifted fiber and multiple fiber Bragg gratings. A nonlinear optical loop mirror based on a highly nonlinear dispersion shifted fiber is implemented in the ring laser cavity to stabilize the multiwavelength output at room temperature. Multiple fiber Bragg gratings with the wavelength spacing of 0.8 nm are connected with an arrayed waveguide grating to establish a multichannel filter. The high quality of the multiwavelength output with a high extinction ratio of ~60 dB and high output flatness of ~0.5 dB is realized. The nonlinear polarization rotation based on the nonlinear optical loop mirror can provide the switching performance of the proposed multiwavelength fiber laser. The lasing wavelength can be switched individually by controlling the polarization controller and the cavity loss.
©2008 Optical Society of America
Multiwavelength fiber lasers have been attracted considerable interest because of their potential applications in wavelength division multiplexing (WDM) systems and optical fiber sensor [1–11]. Multiwavelength fiber lasers have been realized by using versatile gain media including erbium doped fiber amplifiers (EDFA), Raman amplifiers, and semiconductor optical amplifier (SOA). Multiwavelength fiber lasers based on EDFAs have advanced significantly because of their advantages such as high power conversion efficiency, low threshold, and low cost. However, since the homogeneous line broadening of erbium ions leads to strong mode competition, it is not easy to obtain the stable multiwavelength operation at room temperature. Various techniques including the cooling of the EDF in the liquid nitrogen , four wave mixing (FWM) effect by utilizing high nonlinear fiber such as photonic crystal fibers (PCFs)  or dispersion shifted fibers (DSFs)  or bismuth-oxide fibers (Bi-NLFs) , hybrid gain medium with both Raman fiber and EDF , have been proposed to suppress the homogeneous line broadening of erbium ions. A nonlinear optical loop mirror (NOLM) based on a SMF was also proposed to obtain the stable multiwavelength EDF laser . Recently switchable multiwavelength EDF lasers have been investigated intensively [7–12]. Most of techniques for switchable multiwavelength EDF laser are based on fiber Bragg gratings (FBGs) such as cascade FBG cavities , cascade FBGs inscribed in birefringence fibers , sampled FBGs , and multimode FBGs . Both the four wave mixing effect and the nonlinear polarization rotation based on a nonlinear optical loop mirror were exploited to stabilize the multiwavelength output at room temperature [6, 12].
In this paper we propose and experimentally demonstrate a switchable multiwavelength EDF laser based on a NOLM incorporating multiple FBGs. The NOLM is utilized as an amplitude equalizer to induce intensity-wavelength dependent loss and nonlinear polarization variation. By inserting the NOLM in the laser ring cavity with multiple FBGs, the stable multiwavelength operation at room temperature can be achieved. Four FBGs with the wavelength spacing of 0.8 nm are connected with an array waveguide grating (AWG). The high quality of the multiwavelength output with a high extinction ratio of ~60 dB and high peak flatness of ~0.5 dB are achieved. The output power of the proposed multiwavelength EDF laser is stable and its power fluctuation is measured to be less than ~1 dB. The lasing wavelengths are effectively switched by two polarization controllers (PCs) because the nonlinear polarization phenomenon based on NOLM induces the polarization-dependence loss and the birefringence-induced wavelength-dependent loss in the laser ring cavity. Since the number of lasing wavelengths is effectively controlled by adjusting two PCs appropriately, the proposed multiwavelength can be operated in the single-, dual, triple- and quadruple-lasing wavelength states.
2. Switchable multiwavelength EDF laser based on a nonlinear optical loop mirror incorporating multiple fiber Bragg gratings
The experimental configuration for the proposed switchable multiwavelength EDF laser is shown in Fig. 1. The proposed laser consists of an EDFA, a NOLM with a highly nonlinear DSF with the length of 1 km, multiple FBGs, an array waveguide grating (AWG), a circulator, two PCs, an optical isolator, and a 10/90 optical coupler. The saturated output power of the EDFA at an input signal of 0 dBm was 26 dBm. Its small-signal gain at an input power of -30 dB was ~25 dB. The polarization-dependent gain of the EDFA was less than ~0.5 dB. An isolator with the insertion loss and the isolation of 0.5 dB and 55 dB, respectively, is implemented for the unidirectional operation of the laser. Two PCs are exploited to adjust the polarization state at the ring cavity, the input of the NOLM, and inside of the NOLM for the polarization state biasing the loop. An optical spectrum analyzer with 0.1 nm resolution was used for all measurement of the multiwavelength laser output through the 10 % output port of the optical fiber coupler. Four FBGs with the center wavelengths of 1552.5 nm (λ 1), 1553.3 nm (λ 2), 1554.1 nm (λ 3), and 1554.9 nm (λ 4) incorporating the NOLM were employed to generate the multiwavelength output. The NOLM was constructed by splicing two output ports of the optical coupler with a 30/70 power splitting ratio. A PC and a highly nonlinear DSF with the length of 1 km were inserted within the NOLM. The zero-dispersion wavelength and the nonlinear coefficient of the highly nonlinear DSF were 1552 nm and 15W-1km-1, respectively. Its insertion loss is 1.8 dB. The AWG was used for the parallel connection of multiple FBGs to reduce insertion loss and multipath interference. Such device was inserted in the laser cavity via the optical circulator with the insertion loss between port 1 to port 2 and port 2 to port 3 of 0.7 and 0.9 dB. The insertion loss and the 3 dB channel bandwidth of the AWG were ~4 dB and less than 0.4 nm, respectively. The polarization dependent loss was less than ~0.3 dB.
The reflection spectrum of four FBGs passing through the AWG is shown in Fig. 2. The channels spacing of FBGs were 0.8 nm. The 3-dB bandwidth and reflectivity of the FBGs were measured to be less than ~0.12 nm and more than ~80%, respectively. The highly nonlinear DSF has been exploited to stabilize the output of the multiwavelength EDF laser [2–4]. The FWM effect induced by the high nonlinear DSF introduces a dynamic gain flattening so that the mode competition is suppressed effectively. In the previous report, the NOLM based on a single mode fiber (SMF) with the length of 2.1 km could provides the phase difference between two lights within the loop and finally intensity-wavelength dependent loss . Based on these characteristic of the NOLM, the stable operation of the multiwavelength EDF laser at room temperature could be realized readily. In the proposed multiwavelength laser, we believe that the NOLM with the highly nonlinear DSF with the length of 1 km can effectively improve the amplitude equalization function and the lasing output because of the high nonlinearity and the short length of the highly nonlinear DSF. The transmission characteristic of the NOLM is given by as [9, 13]
where r is the power splitting ratio of the NOLM, γ is the nonlinear phase shift. P t and P i is the transmitted and input powers, respectively. L is loop length, λ is the operating wavelength, and A eff is effective area of the fiber. ϕ is the additional phase difference produced by two PCs. The transmission of the NOLM can be changed by the input power depending on polarization states . The NOLM can be operated as a saturable absorber or a gain equalizer by setting two PC appropriately [6, 14]. The high intensity light in the laser ring cavity can experience larger loss than that of the low intensity one because of intensity-wavelength dependent loss of the NOLM . Therefore, the NOLM can alleviate the mode competition in the EDF and generate the stable operation of the multiwavelength EDF laser at room temperature.
Figure 3(a) shows the output spectrum of the proposed switchable multiwavelength EDF laser based on the NOLM with a 1 km highly nonlinear DSF incorporating four FBGs. Since the intensity-wavelength dependent loss induced by the NOLM can effectively mitigate the mode competition of the EDF, the stable multiwavelength EDF laser at room temperature could be obtained by adjusting two PCs. The spectral bandwidth of the lasing wavelengths was less than ~0.06 nm. We could achieve four channel lasing wavelengths with the 0.8 nm spacing and the peak power was about -23 dBm with high extinction ratio of more than ~60 dB, which resulted from the combined effect of both multiple FBG cavities and the NOLM. The output peak flatness among four lasing channels was measured to be ~0.5 dB. In our work, the number of lasing wavelengths was limited to 4 channels because the lack of the nonlinearity of the 1km DSF. We believe that the specialty fiber with stronger nonlinearity in the NOLM can increase the number of laser wavelengths. After selecting a single lasing peak, we measured the relative intensity noise (RIN) spectrum of the proposed laser with a photodetector (Newfocus model 1611, 1-GHz bandwidth)) and an electrical spectrum analyzer (Advantest U3772). The RIN of the proposed multiwavelength laser was higher than that of normal DFB laser (~-140 dB/Hz) as seen in Fig. 3(b). We measured the stability of the laser by monitoring the output laser with 10 minute’s interval for period of 1 hour. The output power of the proposed multiwavelength EDF laser was stable and the power fluctuation was measured to be less than ~1 dB, as shown in Fig. 4.
The lasing wavelength of the proposed multiwavelength EDF laser could be switched individually by appropriately adjusting two PCs in the fiber ring cavity and the NOLM. The lasing wavelengths were switched individually by two PCs because the nonlinear polarization phenomenon based on the NOLM induces the polarization-dependence loss and the birefringence-induced wavelength-dependent loss in the laser ring cavity, which can determine the total loss of the laser ring cavity . Consequently, the number of lasing wavelength could be changed by two PCs. Figure 5 shows output spectra of the proposed switchable multiwavelength EDF laser by controlling two PCs. As seen in Fig. 5(a), a single lasing operation could be achieved because the cavity loss at the lasing wavelength was minimized by the nonlinear polarization phenomenon based on the NOLM by controlling the PC inside the NOLM. If the PC in the ring cavity is controlled appropriately, double- or triple-lasing wavelengths can be achieved because intensity-dependent loss is also changed, as shown in Fig. 5(b) or 5(c), respectively. The combination of the lasing wavelengths could be arbitrarily selected from four wavelengths by controlling two PCs. For all lasing combination, the output power was stable, which was measured to be less than ~1 dB.
3. Discussion and conclusion
In conclusion, we experimentally investigated a simple technique of a switchable multiwavelength EDF laser based on the NOLM with a highly nonlinear DSF incorporating multiple FBGs. The NOLM with the 1 km highly nonlinear DSF could induce the intensitydependence loss within the ring cavity, which could equipoise between the mode competition of erbium ions and the gain-clamping effect. Consequently the homogeneous line broadening of erbium ions could be suppressed effectively and the stable multiwavelength EDF laser at room temperature could be realized. We achieved four channel lasing wavelengths with the 0.8 nm spacing. The extinction ratio of the multiwavelength output was as high as ~60 dB. The multiwavelength EDF laser output was very stable and the peak fluctuation was less than ~1 dB. The flatness of the multiwavelength output was measured to be ~0.5. By adjusting two PCs precisely, a single, dual, or triple wavelength can be lasing simultaneously because of the polarization-dependence loss and the birefringence-induced wavelength-dependent loss based on the NOLM. The proposed multiwavelength EDF laser is useful for applications to WDM systems, multiwavelength optical switching devices, and optical sensors.
References and links
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