IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 1, JANUARY 1, 2006
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Broad-Band Optical Coupler Based on Evanescent-Field Coupling Between Three Parallel Long-Period Fiber Gratings Yunqi Liu and Kin Seng Chiang, Member, IEEE
Abstract—We propose a broad-band 3 3 optical coupler based on the principle of evanescent-field coupling between three parallel identical long-period fiber gratings. In particular, we demonstrate experimentally the situation where light is launched into one fiber and coupled equally to the other two fibers. The effects of changing the surrounding refractive index and introducing an offset distance between the gratings on the transmission characteristics of the coupler are investigated. Using three identical 35-mm-long gratings, we find that the peak coupling efficiency increases by 9 and 14 dB, as the surrounding refractive index and the offset distance increase from 1.0 to 1.440 and from 0 to 50 mm, respectively. The side-lobe suppression ratio varies between 16.0 and 23.0 dB. We achieve a total power conversion efficiency of 85% at the resonance wavelength with a surrounding refractive index of 1.420 and an offset distance of 50 mm. Index Terms—Evanescent-field coupling, long-period fiber gratings, optical couplers, optical filters.
A
long-period fiber grating (LPFG) produced in a single-mode fiber with a pitch of the order of 100 m enables light coupling from the guided mode to selected cladding modes at specific wavelengths (known as the resonance wavelengths); therefore, it is inherently a band-rejection filter [1]. To expand the functionality of an LPFG, techniques have been proposed to turn an LPFG into a bandpass filter [2], [3] or even an add–drop multiplexer [4]–[7]. To form an add–drop multiplexer, two LPFGs are placed in parallel so that light is transferred between the two fibers through evanescent-field coupling between the cladding modes of the two fibers. Under appropriate conditions, complete power transfer at the resonance wavelength should be possible [7]. The structure of parallel LPFGs provides additional output ports with complementary transmission characteristics and thus offers more flexibility in terms of applications. In this letter, we extend the study to three parallel LPFGs. The structure of three parallel LPFGs is a six-port 3 3 broad-band coupler, where light launched into one grating can be coupled to the other two gratings with maximum coupling occurring at the resonance wavelength. The output from the fiber where light is launched shows band-rejection characteristics; whereas, the outputs from the other two fibers show bandpass characteris-
Manuscript received August 5, 2005; revised October 13, 2005. This work was supported by the Research Grants Council of Hong Kong Special Administrative Region, China, under Project CityU 112 005. The authors are with the Optoelectronics Research Centre and the Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong, China (e-mail:
[email protected];
[email protected]). Digital Object Identifier 10.1109/LPT.2005.861534
Fig. 1. (a) Schematic diagram showing experimental setup, where three bare fibers that contain LPFGs are secured in two separate V grooves on both sides of gratings. (b) Three identical LPFGs with length L are placed side by side and in close contact with each other with offset distance s introduced between gratings in tapping fibers and grating in transmission fiber.
tics. We demonstrate experimentally the effects of changing the surrounding refractive index and introducing an offset distance between the gratings on the transmission characteristics of the coupler and achieve a maximum total power conversion efficiency of 85% using three 35-mm-long gratings. The LPFG used in our experiments was written in a B-Ge codoped photosensitive fiber by irradiating the fiber with a 248-nm UV excimer laser through an amplitude mask. The grating period was 300 m. Two groups of three identical LPFGs were fabricated, all of which had the same length of 35 mm. For Group A, the resonance wavelength, the contrast at the resonance wavelength and the 3-dB bandwidth were 1561.0 nm, 16 dB, and 17 nm, respectively. For Group B, the resonance wavelength and the corresponding contrast were 1562.5 nm and 18 dB, respectively, while the 3-dB bandwidth was the same as that for Group A. These characteristics were measured for bare LPFGs in air. The corresponding cladding mode was identified to be the LP mode. In our setup, the three fibers that contained the LPFGs, with the soft jackets removed, were secured in two separate V grooves on the two sides of the LPFGs, as shown in Fig. 1(a). Suitable tension was applied along the fibers to keep them straight and in close contact to each other. If necessary, index-matching liquid was applied to the LPFGs to change the surrounding refractive index experienced by the LPFGs. A schematic diagram of the three parallel LPFGs is shown in Fig. 1(b). We consider the situation where light is launched into only one fiber, which is referred to as the transmission fiber. The other two fibers are referred to as tapping fibers (tapping fiber 1 and tapping fiber 2). The
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 1, JANUARY 1, 2006
Fig. 2. Experimental results using Group A gratings, where gratings are perfectly aligned, i.e., s = 0. (a) Normalized output spectra of tapping fiber 1 and transmission fiber measured at different surrounding refractive indexes, n = 1:0; 1:377; 1:404; 1:420, and 1.440. (b) Dependence of resonance-wavelength shift on surrounding refractive index. (c) Dependence of peak coupling efficiency on surrounding refractive index. (O) shows tapping fiber 1. ( ) shows tapping fiber 2.
Fig. 3. Experimental results using Group B gratings, where surrounding medium is air, i.e., n = 1:0. (a) Normalized output spectra from tapping fiber 1 and transmission fiber measured at different offset distances, s = 0; 10; 20; 30; 40, and 50 mm. (b) Dependence of peak coupling efficiency on offset distance. (O) is tapping fiber 1. ( ) is tapping fiber 2. (c) Dependence of 3-dB bandwidth of output spectra of tapping fibers on offset distance.
gratings in the tapping fibers were allowed to displace from that in the transmission fiber in the longitudinal direction by an equal offset distance . Light coupling between three parallel LPFGs can be understood in the following way. The LPFG in the transmission fiber couples light from the guided mode to the cladding mode of the fiber. At the same time, the cladding modes of the two tapping fibers are excited through evanescent-field coupling between the three parallel fibers. They are coupled to the guided modes of the tapping fibers through the LPFGs in the tapping fibers. Because the spectrum rejected from the transmission fiber is emerged from the tapping fibers, the outputs from the two tapping fibers show identical bandpass characteristics that are complementary to the band-rejection characteristics of the transmission fiber. The efficiency of the whole process is expected to depend critically on the efficiency of the evanescent-field coupling and, hence, the surrounding refractive index and the offset distance , as in the case of two parallel LPFGs [4], [5], [7]. In our experiments, light from a commercial C L band amplified spontaneous emission (ASE) source was launched into one fiber (the transmission fiber), and the output spectra of the three fibers were measured with an optical spectrum analyzer. The power conversion efficiency of the coupler is given as , where is the sum of the output powers from is the input power launched into the both tapping fibers and transmission fiber. In the first experiment, the three LPFGs of Group A were placed side by side (i.e., ) and in close contact with each other. To vary the surrounding refractive index, different index-matching liquids were applied to the LPFGs. Fig. 2(a) shows the normalized output spectra of tapping fiber 1 and the transmission fiber measured at different surrounding indexes, (air), 1.377, 1.404, 1.420, and 1.440. The output spectra
of tapping fiber 2 are similar to those of tapping fiber 1 and therefore not shown. The results in Fig. 2(a) confirm significant light coupling from the transmission fiber to the tapping fibers and the peak coupling efficiency occurs at the resonance wavelength. The 3-dB bandwidth of the output spectra of the tapping fibers is 20 nm, which is somewhat broader than the bandwidth of the individual LPFG. The slight broadening in the bandwidth is believed to be due to the fact that the transmission spectra of the three gratings are not perfectly identical, so the location of the peak coupling in the transmission spectrum becomes less sharp. The side-lobe suppression ratio, defined as the ratio of the peak coupling efficiency to the coupling efficiency of the most significant side lobe, varies between 16.0 and 23.0 dB over the range of the surrounding refractive indexes used. It should be possible to further reduce the side lobes by using apodized LPFGs [8]. The dependences of the resonance wavelength shift and the peak coupling efficiency on the surrounding refractive index are shown in Fig. 2(b) and (c), respectively. The results in Fig. 2(b) are similar to those reported for a single LPFG [9], [10]. When the surrounding index is increased from 1.0 to 1.440, the resonance wavelength shifts to the shorter wavelength by more than 30 nm, and the peak coupling efficiency increases by 9 dB. The maximum peak coupling efficiencies are 12.7 dB (5.4%) and 12.6 dB (5.5%) for the two tapping fibers, respectively. The increase in the peak coupling efficiency with the surrounding index is due to the enhancement of the evanescent fields of the cladding modes [7]. In the second experiment, the three LPFGs of Group B were placed in close contact with each other and an offset distance was introduced between the gratings in the tapping fibers and the grating in the transmission fiber in the longitudinal direction. Fig. 3(a) shows the normalized output spectra of tapping fiber 1 and the transmission fiber measured with air
LIU AND CHIANG: BROAD-BAND OPTICAL COUPLER BASED ON EVANESCENT-FIELD COUPLING
Fig. 4. Normalized output spectra of two tapping fibers and transmission fiber measured with surrounding refractive index of 1.420 and offset distance of 50 mm for Group B gratings.
as the surrounding medium at different offset distances, , and mm. The side-lobe suppression ratio varies between 16.0 and 23.0 dB. Fig. 3(b) shows how the peak coupling efficiency increases with the offset distance as a result of increasing the interaction length for evanescent-field cou) and Fig. 2(c) pling. A comparison between Fig. 3(b) (at (at ) shows that the peak coupling efficiency achieved with the Group B gratings is approximately 2 dB higher than that achieved with the Group A gratings. This is because the Group B gratings have a larger contrast at the resonance wavelength. As shown in Fig. 3(b), the peak coupling efficiency increases by 14 dB when the offset distance is increased from 0 to 50 mm. The peak coupling efficiencies, measured at mm, reach 5.06 dB (31.2%) and 5.04 dB (31.3%) for the two tapping fibers, respectively, which gives a total power conversion efficiency of 62.5%. These results confirm the effectiveness of increasing the offset distance for the improvement of the power conversion efficiency. Fig. 3(c) shows the dependence of the 3-dB bandwidth of the output spectra of the tapping fibers on the offset distance. The 3-dB bandwidth decreases with an to 11.8 nm increase in the offset distance from 20 nm at mm. The reduction in the 3-dB bandwidth is due to the at limited bandwidth of the evanescent-field coupling process (i.e., the bandwidth of evanescent-field coupling decreases with an increase in the interaction length). By using proper grating parameters, the bandwidth of an LPFG can be varied in the range of 1–50 nm [1], [6]. So, the bandwidth of the coupler can be controlled by using properly designed gratings. Nevertheless, the offset distance between the transmission fiber and the tapping fibers can be used to tune the 3-dB bandwidth, though at the expense of changing the power conversion efficiency. It should be clear from Figs. 2 and 3 that a combined use of a high enough surrounding index and a long enough offset distance should lead to an optimal power conversion efficiency. Fig. 4 shows the normalized output spectra of the two tapping fibers and the transmission fiber measured with a surrounding index of 1.420 and an offset distance of 50 mm (using Group B gratings). The peak coupling efficiencies achieved are 3.73 dB (42.4%) and 3.69 dB (42.8%) for the two tapping fibers, respectively, which give a total power conversion of 85.2% at the resonance wavelength. The side-lobe suppression ratios are
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18 and 23 dB for the two tapping fibers, respectively. With a grating contrast of 18 dB (98.4%), the actual coupling efficiency from the cladding mode of the transmission fiber to the cladding mm. modes of the tapping fibers was 88.0% at As for the packaging of the coupler, the length of the gratings should be made as short as possible, yet a high evanescent-field coupling efficiency should be maintained. This can be achieved by using a higher order cladding mode (i.e., a shorter grating pitch) and an external index closer to the cladding index [7]. In practice, the gratings should be embedded in a stable low-index material (polymer or glass). The temperature compensation techniques for fiber-grating packaging could also be adapted for the packaging of LPFG couplers. We are now working on a solution to the packaging problem. In conclusion, we propose a broad-band 3 3 coupler based on the principle of evanescent-field coupling between three parallel identical LPFGs. Although we demonstrate only the symmetrical situation where light is launched into one fiber and distributed equally into the other two fibers, it is understood that flexibility exists for the realization of a wide range of splitting ratios among the fibers at the resonance wavelength by introducing another offset distance between the two tapping fibers and/or controlling the contrasts of the gratings. In addition, the structure offers the possibility of tuning the splitting ratios by changing the surrounding refractive index or the offset distance. It is envisaged that the configuration of three parallel coupled LPFGs can be used for the construction of many novel six-port devices with applications in add–drop multiplexing, signal tapping, and broad-band filtering.
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