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We demonstrate an individually-switchable multi-wavelength fiber ring laser using semiconductor optical amplifier with novel switchable band-pass filters. The switchable band-pass filter is based on a fiber Sagnac loop interferometer incorporating both fiber Bragg gratings and polarization controller. The proposed multi-wavelength fiber ring laser shows a high output performance of independent switching at arbitrary lasing wavelength and high extinction ratio for wavelength-division multiplexing source.
©2007 Optical Society of America
1. Introduction
Multi-wavelength fiber ring lasers have been attractive sources in various applications, such as wavelength division multiplexing (WDM) communications, fiber sensors, optical testing, and RF photonics [1-12]. In order to achieve multi-wavelength lasing oscillation with an erbium doped-fiber amplifier, there have been various approaches including cryogenic cooling [9], frequency-shifted feedback [10], phase modulation [11], and high nonlinearity [12]. A semiconductor optical amplifiers (SOA) is also considered as a good active gain medium due to its inhomogeneous linewidth broadening characteristic for the stable dense-spacing multi-wavelength lasing output at room temperature [1] [13].
The most important key component of the multi-wavelength fiber ring laser is a wavelength selection filter. Among versatile techniques to realize the wavelength selection filter, a fiber Bragg grating (FBG) is an ideal intra-cavity device to select the lasing wavelength. The FBG has significant advantages such as fiber compatibility, superior spectral reflectivity and cost effectiveness. For the implementation of switchable multi-wavelength fiber lasers, various techniques have been used based on the FBG written in multimode fiber [1], FBG written in a high-birefringence fiber [2], variable optical attenuator (VOA) in a ring cavity [3], VOA between cascaded FBG segments [4], high-birefringence fiber loop mirror [5], Lyot filter with a sampled FBG [6], spectral polarization-dependent loss element [7] and control of Raman pump [8]. However, most of these methods are limited to only 2 or 3 switchable wavelengths and the individual switching of each FBG cannot be accomplished yet.
In this research, we investigate an independently switchable multi-wavelength fiber laser incorporating a novel switchable FBG filter in the ring cavity. In fact, the circulator is an essential element to convert the reflective spectrum of a FBG into a transmissive response in the fiber ring cavity. In the proposed technique for a switchable FBG filter, the reflective spectra of multi-FBG’s can be converted into transmissive responses without the help of a circulator. The proposed scheme has a lot of advantages, such as independent turning on/off at each FBG filter, flexible selections of multiple lasing wavelengths, the increased number of multi-wavelength outputs, the removal of the high-cost circulator, and simple all-optical fiber configuration. We show the theoretical analysis and experimental results of the switchable FBG filter incorporating a fiber Sagnac loop interferometer (FSI) and also present the switching operation of multi-wavelength lasing outputs corresponding to the polarization state within the FSI with FBG’s.
2. Multi-FBG band-pass filter
The experimental scheme for the individually controllable multi-wavelength ring laser is shown in Fig. 1. The proposed fiber ring laser is composed of a SOA gain medium, an optical isolator, a wavelength selection filter, an in-line polarization controller (PC), and an output coupler. By convention, the wavelength selection filter consists of both circulators and multi-FBG segments to implement the reflection property of the FBG as a band-pass spectrum, as shown in Fig. 1(a). In order to remove the help of a high-cost circulator, a FSI was recently suggested to convert the reflective response into a transmissive response [14-16]. The FSI devices consist of a 50:50 fiber coupler and a single broadband FBG to demonstrate the routing switch [14], the notch filter [15], and the comb filter [16].
Fig. 1. Schematic of the multi-wavelength semiconductor ring laser including the wavelength selection filter, which can be filled with (a) conventional FBG filter including circulator, (b) proposed multi-FBG band-pass filter without circulator.
In the proposed configuration in Fig. 1(b), it comprises of multiple narrowband FBG-segments within the Sagnac loop based on the polarization insensitive coupler. The transmission spectrum of multiple band-pass peaks can be tuned in terms of changing the polarization state by the all-fiber PC within the Sagnac loop. The principle is as follows. The incident beam, Iin, sent to the 50:50 coupler is split into two counter propagating beams in the loop. When those two beams meet the FBG in the loop, the beams at the FBG resonance bandwidth will be reflected from the FBG; next, those inversely-directed beams will recombine at the 50:50 coupler, which result in Michelson-like interference in the spectrum of the output beam, Iout. If the path length from the FBG to the 50:50 coupler in the clockwise and counterclockwise directions are different, as in L 1 and L 2, there are multiple periodic peaks within the FBG resonance bandwidth of the transmission spectrum. The period of those sinusoidal Michelson interference peaks in the FBG resonance bandwidth is shown as [14, 16],
where λ is the operation wavelength, n is the effective refractive index, and ΔL12 is the path length difference between L 1 and L 2. It is easily derived that there is no Michelson interference pattern in the bandwidth of the FBG resonance when the FBG is located exactly at the center of the loop, ΔL12 = 0.
Simultaneously, the other beams in the out of the FBG resonance bandwidth will rotate though the FBG and result in a Sagnac interference according to the phase difference, which is proportional to the birefringence difference between the fast and slow axes in the loop. Depending on the birefringence component in the loop, the transmission spectrum will show the sinusoidal characteristic in the out of the FBG resonance bandwidth, where the spectral Sagnac period is shown as [17],
where Δneo and Leff are the birefringence and the effective length of the loop path, respectively. By selecting the type of optical fiber in the loop, such as high birefringence polarization-maintaining fiber and conventional single mode fiber, we can easily control the transmission period of the Sagnac loop filter [17]. The overall transmission spectra of the proposed filter in Fig. 1(b) can be easily derived by using Jones matrixes [16-18] and can be written as
where TFBG(λ) and RFBG(λ) are the transitivity and the reflectivity of the FBG, respectively, along the wavelength. β is the propagation constant. ϕ T and ϕ R are the phase retardations in the transitive path through FBG and reflective paths from FBG in the loop, respectively.
Experimental demonstration is performed with 4 FBG samples (named as FBG 1, 2, 3 and 4 in Fig. 2). Their characteristics are easily controlled by various fabrication parameters, such as a grating length, a grating pitch, a resonance wavelength, a resonance bandwidth, a transitivity, and a reflectivity. The transitivity (TFBG(λ)), and reflectivity (RFBG(λ)) of the 4 FBG samples correspond to Fig. 2(a) and Fig. 2(b), respectively. As shown in Fig. 1(a), when the circulator is conventionally inserted in the SOA ring laser cavity to convert the multi-band reflective spectrum (RFBG(λ)) in Fig. 2(b) into the multi-band transmissive spectrum, the multiple band-pass peaks at difference resonance wavelengths (λ1,λ2,λ3, and λ4) can determine the lasing wavelengths of multi-peak lasing outputs. However, it is still hard to switch the individual lasing outputs with more than 4 outputs flexibly, even though various techniques have been proposed to switch a few outputs based on the FBG and circulator in fiber ring cavity [1-8].
In the proposed scheme, the switchable multiple filter with multiple band-pass peaks can be demonstrated by using the novel FBG band-pass filter configuration of Fig. 1(b) without a circulator. Figure 3 shows the experimental results of the proposed filter including the same 4 FBG’s. Since the path length difference from the multiple position of each FBG to the 50:50 coupler, ΔL 12, is larger than 20 cm, the period of Michelson-interference peaks in each FBG resonance bandwidth is less than 0.005 nm in Eq. (1). This value is smaller than the measurement resolution (0.05nm) of a conventional optical spectrum analyzer (OSA). Thus, it is almost impossible to count or monitor the fine interference spectrum within each FBG resonance bandwidth region. Instead, the transmission intensity in the FBG resonance bandwidth shows the constant averaged level value between the peak and depth levels, even though the polarization state, ϕ, is varied by the PC in the loop.
Contrarily, the spectrum of Sagnac interference in the out of the FBG resonance bandwidth shows a broad sinusoidal period of more than 100 nm from Eq. (2) with a short polarization-maintaining fiber segment (Δneo = 0.00038 and Leff= 5 cm) in the loop. By tuning the phase value (ϕ) using the PC in the loop, the periodic spectrum shifts linearly and the almost flattened spectrum in the out of the FBG resonance bandwidth can be adjusted to be optimal transmission values [17]. When the wave plate of PC shows the retardance (Γ(λ)) of π and the axes orientation (θ) of π/4 with respect to the laboratory coordinates (the function of a half-wave plate), the Sagnac loop interferometer shows the maximum transmission over the wavelength of interest [19]. The minimum transmittance and maximum reflection is monitored when either Γ or θ is 0, which corresponds to the fiber loop mirror condition of the Sagnac loop interferometer [19]. As shown in Fig. 3 and Eq. (3), the flat transmission spectrum in the out of the FBG resonance bandwidth is tuned to be maximum or minimum values by tuning the value of ϕ; however, the transmission intensity in the FBG resonance bandwidth is almost independent of the variation of ϕ.
The multi-band transmissive spectrum, with the PC condition of Γ= 0 and θ = 0, of Fig. 3 is used as a FBG band-pass filter for the multi-peak lasing outputs as shown in Fig. 4. The pump current to the SOA is measured to 200 mA. In order to increase the stability of lasing peaks, an isolator is used to prevent the back-reflection from the FBG band-pass filter without a circulator. The time variation of peak amplitude is measured to be less than 1 dB and the signal to amplified spontaneous emission (ASE) ratio is more than 35 dB in each FBG resonance wavelength. The tuning of lasing wavelength positions can be independently obtained with various techniques of the temperature and strain tuning on each FBG segment [12].
3. Cascaded individual-switchable multi-FBG band-pass filters.
The individual switching performance of multiple band-pass filters is also demonstrated by cascading multiple Sagnac loop filters. As described in the previous section, the dual modes of the band-stop filter and band-pass filter can be easily switched by adjusting the PC in the loop. With the cascaded two Sagnac loop filters, (a) and (b), as shown in Fig. 5, it is possible to obtain a flexible transmission spectra of different band-pass channels by controlling the PC(a) and PC(b) of each filter.
Fig. 5. Schematic of the multi-wavelength laser including the cascaded individual-switchable multi-FBG band-pass filters.
Fig. 6. Transmission of the cascaded individual-switchable FBG filters including FBG 1, 2, 3 and 4, respectively
For the experimental demonstration, we placed FBG1 and FBG3 in the Sagnac loop filter (a), and also FBG2 and FBG4 into the other Sagnac loop filter (b), respectively. Figure 6 shows the transmission of the cascaded individual-switchable filters with various switching combinations, such as (a) FBG1, FBG3 On / FBG2, FBG4 Off, (b) FBG2, FBG4 On / FBG1, FBG3 Off, and (c) all FBG’s On. From these spectra, we can clearly demonstrate that it is possible to obtain independent wavelength-switching characteristics of band-pass channels with a high extinction ratio of more than 10 dB.
Figures 7(a), 7(b), and 7(c) show the various lasing spectra by using the cascaded individual-switchable multi-FBG band-pass filters, which correspond to the switching combinations of FBG’s of Fig. 6(a), 6(b), and 6(c), respectively. It is notable that the relative intensity distribution of the filter spectrum is the most important parameter to consider when selecting the lasing output positions among 4 FBG resonance wavelengths, λ1, λ2, λ3, and λ4. We have repeatedly switched the various lasing combinations, and the reliable switching operation is monitored by the state of the PC in the loop of the individual filter. When the pump current to the SOA is 200 mA, the signal to ASE ratio becomes more than 35 dB in Fig. 7 (a) and 6(b), but the value is decreased to be less than 30 dB for Fig. 6(c). Since the filter transmission in Fig. 6(c) is less than those of Figs. 6(a) and 6(b), the amplitudes of lasing peaks in Fig. 7(c) are smaller than the other spectral peak values in Figs. 7(a) and 7(b). When the transmission in the background level may decrease in various switching combinations, the pump current into the SOA can be increased to compensate for the extra insertion loss of the filter. The PC(c) in the fiber ring cavity is used to optimize the lasing output spectrum by tuning the beam condition before the stage of SOA.
4. Conclusion
We have proposed a simple configuration of the multi-FBG band-pass filter based on the cascaded individually-switchable FBG filters, which is a fiber Sagnac loop interferometer incorporating both FBG and PC. By using novel FBG filters as a wavelength selection component in the fiber ring cavity, the individual-switchable multi-wavelength SOA ring laser is demonstrated without a circulator. The proposed individually switchable laser has excellent lasing output characteristics, such as independent turning on/off at flexible lasing wavelength positions, an increased number of stable multi-wavelength outputs, removal of the high-cost circulator, and simple all-optical fiber configuration.
Acknowledgments
This work was supported by grant No. R01-2005-000-10239-0 from the Basic Research Program of the Korea Science & Engineering Foundation.
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