We proposed and experimentally demonstrate a simple and flexible scheme of all-fiber Fabry-Perot filter with continuous free spectral range tunability exploiting a superimposed chirped fiber Bragg grating. Then, we apply the proposed continuously tunable multichannel filter to a multiwavelength SOA ring laser at room temperature. The specially designed apparatus can induce the symmetrical strain gradient and modify the chirp ratio along the superimposed chirped fiber Bragg grating attached onto a flexible cantilever beam. Then the free spectral range of the all-fiber Fabry-Perot filter can be continuously controlled by the variation of reflection chirp bandwidth of the superimposed chirped fiber Bragg grating without center wavelength shift, which depends on the moving direction of translation stage. The proposed scheme can provide the effective multichannel filter with continuous free spectral range tunability. We successfully achieve the stable operation of a multiwavelength SOA laser with eleven lasing channels based on the proposed flexibly tunable multichannel filter at room temperature.
©2007 Optical Society of America
Multichannel filters have attracted significant interest in dense wavelength division multiplexed systems, testing instrument, and optical fiber sensors because of their plenty of advantages like multiwavelength operation, small size, low cost and insertion loss [1–6]. There are a plenty of multiwavelength fiber laser sources based on special multichannel filters [1–5]. In order to realize various multichannel filters, versatile technique based on Mach-Zehnder interferometer, and polarization maintaining fiber (PMF) loop mirrors, and fiber gratings have been proposed and demonstrated [1–10]. In the case of Mach-Zehnder interferometer, it has a lot of drawback like performance instability and difficulty in free spectral range (FSR) control . The PMF-based multichannel filters can provide various functionalities like both the peak wavelength tunability and FSR tunability, which are controlled by the effective length and birefringence of multiple PMF segments depending on the relative phase difference between two orthogonal polarization modes within the loop [2, 3]. The FSR, however, can not be controlled continuously but discretely. The fiber grating-based multichannel filters like cascaded long-period fiber gratings, sampled chirped fiber Bragg gratings (CFBGs), and a superimposed CFBGs have a lot of advantages like wavelength selective nature, mass production, and flexible implementation [4 – 8]. However, previously proposed methods do not also provide the FSR controllability. Recently FSR tunable multichannel filters with a sampled CFBG or s superimposed CFBG have been proposed, which were based on the spectral Talbot effect  or the bending technique of grating . However, the former has the degradation of the flatness of multichannel peaks and the discrete tunability. The latter has the narrow tuning range of the FSR tunability, difficulty in precisely controlling FSR, and phase errors due to the grating deflation during tuning. In addition, the continuous FSR tunability is the most important function to improve a real-time spectrum analysis based on a superimposed CFBG for wavelength division multiplexing (WDM) networks .
In this paper, we propose and experimentally demonstrate a novel and practical scheme for all-fiber Fabry-Perot (FP) filter with continuous FSR tunability and its application to a multiwavelength semiconductor optical amplifier (SOA) ring laser, which incorporates a superimposed CFBG. The specially designed devise can induce the symmetrical modification of the chirp ratio along the superimposed CFBG attached on a flexible cantilever beam. Two translation stages with gears and a sawtooth wheel can simultaneously induce the tension and compression strain along the superimposed CFBG attached onto a flexible cantilever beam, which corresponds to the moving direction of translation stage. The FSR of an all-fiber FP filter based on the superimposed CFBG can be continuously controlled by the variation of reflection chirp bandwidth because the tension and compression strain can effectively controls the chirp ratio along the grating. Based on the proposed technique, we achieve continuous FSR tunability of the all-fiber FP filter in a range from 0.29 to 0.81 nm and its tunability was measured to be ~ ±0.033 nm/mm, which depends on the moving direction of translation stage like ±y-direction, respectively. We obtained the stable operation of a multiwavelength SOA laser with eleven lasing channels and high extinction ratio of more than ~35 dB based on the proposed flexibly tunable multichannel filter at room temperature.
2. Flexible FP Filters Based on Superimposed Chirped Fiber Bragg Gratings with Continuous FSR Tunability and its application to a multiwavelength fiber laser
Figure 1(a) shows the proposed experimental scheme for an all-fiber FP filter with continuous FSR tunability . The proposed all-fiber FP filter has flexible multiwavelength selectivity with a large number of channels compared with previous method based on a sampled CFBG . The all-fiber FP filter is based on a superimposed CFBG embedded on a flexible cantilever beam. In order to fabricate a superimposed CFBG, two identical CFBGs were inscribed onto the same portion of the photosensitive fiber by exploiting a beam scanning technique with 244nm Ar+ laser. As seen in Fig. 1(a), since two CFBGs have a small longitudinal displacement (d) between two gratings, distributed FP interference can be created within the fiber core region by multiple reflections between of two CFBGs. In fact, the FSR of the all-fiber FP filber is inversely proportional to a longitudinal displacement (FSR = λ 2/2neff∙d, neff=effective index). The length of the chirp phase mask was 5 cm and its chirp ratio was 4.8 nm/cm. In order to further enhance photosensitivity for fabrication of strong CFBGs, we took the hydrogen loading process for 7 days with a 100 bar pressure. After fabricating the superimposed CFBG with 244nm Ar+ laser, we annealed the FBG at 100°C for 15 hours to remove unreacted hydrogen and to stabilize the quality of the superimposed CFBG.
We successfully achieved a high quality of the all-fiber FP filter with a channel number of 45 and a FSR of ~0.51 nm as seen Fig. 2(a). However, since the FSR is not controllable after the superimposed CFBG is fabricated, it is necessary to prove the flexible controllability of FSR to the all-fiber FP filter based on the superimposed CFBG. As seen in Fig. 1(a), the superimposed CFBG was attached onto the flexible cantilever beam to induce symmetrical modification of the chirp ratio within CFBGs, which can provide continuous tunability of the FSF to the all-fiber FP filter based the superimposed CFBG. The flexible cantilever beam is made up of a spring steel with high resistance against fatigue and corrosion. The length and thickness of cantilever beam are 15 cm and 0.2 mm, respectively. The sampled CFBG should be carefully attached to the cantilever beam using the UV curable epoxy to reduce the phase error along the fiber grating due to the microbending, which can induce additional phase error. The proposed technique to precisely control the FSR of the all-fiber FP filter is based on the sophisticated fiber bending technique to induce symmetrically linear strains gradient in the center of the superimposed CFBG. The proposed device consists of two translation stages with pivots, a sawtooth wheel, a micrometer, and two cantilever beam holder. The sawtooth wheel is positioned at the middle of fixed stage. Each of two translation stages has a gear, which can transform the linear motion of translation stage into the rotary motion of sawtooth wheel. As seen in Fig. 1(b), when the left translation stage is forward moved by the micrometer (+y-direction), its gear rotates the sawtooth wheel and the right translation stage is moved oppositely by the rotary motion of sawtooth wheel. As two translation stages are moved oppositely by the interaction between two gears and a sawtooth wheel, the position of pivots on two translation stages is changed oppositely and the symmetrical bending along the flexible cantilever beam can be induced. Since the compression and tension strain in the left and right side of the superimposed CFBG, respectively, can be induced by the symmetrically curved cantilever beam, the increase of the chirp ratio makes the reflection chirp bandwidth broaden and consequently the FSR will increase. Contrarily, as seen in Fig. 1(c), the FSR will decrease since the reflection chirp bandwidth is reduced by the decrease of the chirp ratio corresponding to opposite bending direction (-y-direction).
Considering the bending direction of the cantilever beam by the moving direction of the stage, the FSR (Δλ) of the superimposed CFBG after tuning can be given by
where FSR 0, and B 0, are the initial FSR and bandwidth of the superimposed CFBG, respectively. λ p is a center wavelength of the CFBG. ρe is the photo-elastic coefficient, t is the thickness of the cantilever beam. L and Lg are the length of the cantilever beam and the CFBG, respectively. Therefore, the FSR of the superimposed CFBG can be continuously controlled by the direction of the moving stage.
Figure 2 shows the transmission spectra of the superimposed CFBG as the left translation stage moves forward (+y-direction). The FSR of the superimposed CFBG increased because the reflection chirp bandwidth was enhanced by the bending of the cantilever beam. Oppositely, when the left translation stage moved backward (-y-direction), the FSR decreased because of the reduction of the reflection chirp bandwidth as seen in Fig. 3. Therefore, we could continuously control the FSR of the superimposed CFBG by the symmetrical bending technique in the range from 0.21 to 0.81 nm. The bending along the superimposed CFBG can induce the additional insertion loss and the total insertion loss increases to be less than ~0.2 dB over whole tuning range, which is negligible.
Figure 4 shows the FSR change as a function of the moving stage corresponding to the direction. We obtained the continuous tunability of the FSR of ±0.033 nm/mm depending of the ±y-direction of the moving stage, respectively. There is a small discrepancy of the continuous tunability of the FSR as the translation stage is moved toward the +y or -y direction. We believed that it is mainly caused by the imperfection by the imperfection in an UV epoxy, in curing process, and in a spring steel cantilever beam. We are trying to reduce the displacement of the continuous tunability regardless of the bending direction. Center wavelength shift of the superimposed CFBG was too small to be considered over the whole tuning range, which was measured to be less than 0.02 nm. We applied the proposed tunable FP filter to a multiwavelength SOA fiber ring laser at room temperature. Since the SOA is an inhomogeneous gain medium, the stable operation of a multiwavelength SOA laser can be easily realized [1, 2]. Figure 5 shows the experimental results of the output spectrum of the SOA ring laser incorporating the proposed FSR tunable multichnnel filter. The amplified spontaneous emission of the SOA was shown in the inset when the input current was 200 mA.
As seen in Fig. 5, it is clearly evident that a high quality of the multiwavelength SOA fiber ring laser with eleven lasing channels and high extinction ratio of more than ~ 35 dB could be obtained.
3. Discussion and conclusion
In summary, we investigated a novel and practical scheme of all-fiber FP filter with continuous FSR tunability based on a superimposed CFBG incorporating the specially designed apparatus for the induction of the sophisticated bending along the grating. The proposed technique is based on the symmetrical modification of the chirp ratio along the fiber grating attached on a flexible cantilever beam. First we achieved a high quality of the all-fiber FP filter with 45 channels and the FSR of ~0.51 nm by overwriting of two identical CFBGs with a small longitudinal displacement between two gratings. Then the proposed apparatus could induce the symmetrical strain gradient and modify the chirp ratio along the all-fiber FP filter based on the superimposed CFBG attached onto a flexible cantilever beam. Then the reflection chirp bandwidth of the grating was continuously changed by the symmetrical bending with the proposed device without center wavelength shift, which is depending on the moving direction of translation stage. Accordingly, the FSR of the all-fiber FP filter could be continuously controlled by the moving stage. We successfully achieved the wide FSR tuning range from 0.21 to 0.81 nm without center wavelength shift (< 0.02 nm). The continuous tunability of the FSR was measure to be ~±0.033 nm/mm, which was depending on the ±ydirection of the moving stage, respectively. Then, we apply the proposed continuously tunable multichannel filter to a multiwavelength SOA ring laser at room temperature. A high quality of the multiwavelength SOA laser with eleven lasing channels and high extinction ratio of more than ~35 dB could be achieved clearly. We believe that the proposed technique would be very useful for applications to optical communication systems, real time spectrum analyzers in WDM systems, tunable multiwavelength fiber lasers, tunable filters, optical interleavers, optical switches, and so on.
This work was supported by the Korea Science and Engineering Foundation through Quantum Photonic Science Research Center at Hanyang University.
References and links
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