The voltage-controllable multichannel filter based on multiply cascaded long-period fiber gratings with a divided coil heater will be proposed and experimentally demonstrated. It has several advantages of the large tuning range in both C- and L-band, multichannel operation, multiwavelength selectivity, and bandwidth controllability. The tunable bandpass filter based on long-period fiber gratings with the broad bandwidth over 6.5 nm, large tuning range over 30 nm, and excellent side mode suppression more than 40 dB will be also discussed.
©2004 Optical Society of America
Fiber gratings, which are classified with fiber Bragg gratings (FBGs) and long-period fiber gratings (LPFGs) corresponding to their grating period (Λ), have been intensively investigated since they have a lot of advantages like the wavelength-selective nature, simple implementation, high flexibility, low back reflection, and so on . They have been also investigated for the physical sensing applications due to their high sensitivity to the external perturbation change [2, 3]. Recently, it has been reported that several LPFGs, which are cascaded in series to form comb filters, are very applicable to multichannel isolators and multi-channel filters for multiwavelength Raman fiber lasers [4–6]. It is necessary to make the multichannel filter with high extinction ratio, low loss, and wide tunability for the generation of the multiwavelength fiber laser. For example, the multiple wavelength fiber filters based on a high birefringence fiber depending on the polarization state have been reported [7, 8].
LPFGs are also very useful for application to other communication devices like add/drop multiplexers  and bandpass filters  in coarse wavelength division multiplexing systems due to their large bandwidth. However, as indicated in previous reports, it is difficult to control the spectral characteristics like the resonant wavelength and passband position since they did not provide the flexible controllability.
In this paper, the tunable multichannel filters based on LPFGs with the large tuning range in both C- and L-band will be proposed and experimentally demonstrated. The voltage-controllable coil heater can change the temperature of LPFGs and finally modify their transmission characteristics. The voltage-controllable bandpass filter based on cascaded LPFGs with a core mode blocker will be also experimentally investigated. The core mode blocker can be utilized to prevent the interaction between the core and cladding mode in fiber grating. The resonant wavelength and passband of the bandpass filter can be easily controlled by the coil heater depending on the cladding mode order.
2. Voltage-controllable multichannel filter and bandpass filter based on LPFGs
The principle of the proposed device is based on the transmission characteristics of multiply cascaded LPFGs. The first LPFG can couple the fundamental core mode (HE1,1) to the several kinds of cladding modes (HE1,m). When several LPFGs with the grating-free region are cascaded in turn, two modes can be interacted together in the second or more LPFGs and finally generate the interference fringe pattern [4–6].
Based on the co-directional coupled mode theory, the modal amplitude of the core and cladding modes in multiply cascaded LPFGs can be easily analyzed. For multiply cascaded LPFGs, the core and cladding modes simultaneously propagate through gratings and grating-free regions alternately. Once the transfer matrix of a unit composed of an LPFG and a grating-free region is obtained, the modal amplitudes of the core and cladding modes after passing through N units can be written in matrix form as [5, 6]
where a Co and a Cl represent the modal amplitudes of the core and cladding modes, respectively. Ti is the transfer matrix of a unit that comprises an LPFG and a grating-free region [5–7]. The transmission characteristics of multiply cascaded LPFGs strongly depend on the physical parameters like the separation distance (D), grating length (L) and number of gratings (N) . The channel spacing (Δλ) decreases as the separation distance between the LPFGs increases and can be approximated as
where Δn is the effective group index difference between the core and cladding modes. When two LPFGs with the separation distance of D are cascaded, the multi resonant peaks, which result from the interaction between the core and cladding mode in the second grating, can be created as shown in Fig. 1.
Figure 2(a) shows the experimental schematic of the proposed tunable multichannel filter based on cascaded LPFGs without a core mode blocker. The controller applies the electric power to each heater section to increase the temperature along the grating and finally to modify the properties of each grating. The effect of each grating on cascaded LPFGs can be easily controlled since the voltage-controllable coil heaters can modify the propagation constants of the core and cladding modes and change the resonant wavelength of each grating. When the voltage is applied to the coil heater, the heating value (Q) can be written as
where V, R are the voltage and resistance of each coil, respectively. N is the total number of pulses, n is the number of applied pulses, and T is the period of the pulse. The applied voltage can change the temperature of the divided coil heater and finally control the temperature of two gratings. Consequently, the resonant wavelength of the filter can be controlled by the applied voltage. The fabrication parameters of gratings were as follows: grating period Λ = 500 μm, L = 2 cm, UV energy = 150 mJ/pulse, and D = 30 cm.
Figure 1 shows the experimental results of the transmission characteristics of the tunable multichannel filter when the applied voltage changes. The extinction ratio of the filter can be maximized when the coupling strength of two gratings is same as 3 dB. In our experiment, the extinction ratio of the filter was more than 20 dB as shown in Fig. 1. Figure 3 shows the experimental results of the wavelength shifts of three channels as a function of the applied voltage. The resonant wavelength of each channel shifted into the short wavelength as the applied voltage increased due to the effect of boron, which has the negative temperature sensitivity [3, 11]. Each of channels has the same sensitivity to the applied voltage change due to the same cladding mode order (HE1,4). The linear fit has the slope of -3.4 nm/V. The tunable multichannel filter has the large tuning range in both C- and L-band.
On the other hand, once a core mode blocker is placed in the middle of two gratings as shown in Fig. 2(b), it can prevent the core mode from propagating through the core region. The cladding modes that are coupled from the core mode via the 1st grating can be coupled back to the core mode through the 2nd grating without the interaction between the core and cladding mode and consequently the bandpass filter can be fabricated. The core mode blocker was fabricated by exposing the H2 loaded Ge-B codoped fibers to the electric arc discharge . If the voltage-controllable coil heater modifies the transmission characteristics of two gratings as well as the amount of the coupling between two modes, the passband of the bandpass filter can be controlled depending on the cladding mode order. The fabrication parameters of gratings were as follows: grating period Λ = 500 μm, L = 2 cm, UV energy = 150 mJ/pulse, and D = 1 cm.
Figure 4 shows the experimental results of the transmission characteristics of the tunable bandpass filter with the voltage change corresponding to the cladding mode order. Since the core mode power is removed by a core mode blocker after the cladding modes are coupled from the code mode by the first LPFG, the bandpass filter can be formed without the interaction among the code and cladding modes in the second LPFG as shown in Fig. 4. Multiple passbands occur in the spectrum because the core mode is generally coupled to several cladding modes in the LPFGs. The proposed bandpass filter has the wide bandwidth over 6.5 nm, large wavelength tunability over 30 nm, and excellent side mode suppression more than 40 dB compared with that based on phase-shifted fiber Bragg gratings. The passbands corresponding to the cladding mode order shifted into the short wavelength due to the high concentration of boron [3, 11]. Figure 5 shows the wavelength shift with the applied voltage change corresponding to the cladding mode order. The sensitivity of the high order cladding mode to the applied voltage change is higher than that of low order mode due to its higher thermal sensitivity. The proposed device has the disadvantage of the slow response time in order of msec although it is enough for their applications to tunable devices like dispersion compensator , gain flattening filters , etc. Currently, we are trying to enhance the response time of the proposed device by developing the heating materials, fiber nonlinearity, etc. No significant drift in the resonant wavelength or transmission amplitude of the proposed devices with time is detected.
In summary, the simple voltage-controllable multichannel filter and bandpass filter based on cascaded LPFGs with the large tuning range in both C- and L-band were proposed and experimentally demonstrated. The voltage-controllable coil heater can effectively control the transmission characteristics of multichannel filter like the resonant wavelength of each channel. The proposed tunable multichannel filter has several advantages of multichannel operation, large tuning range in both C- and L-band, and multiwavelength selectivity. The tunable bandpass filter with broad bandwidth over 6.5 nm, large tuning range over 30 nm, and excellent side mode suppression more than 40 dB was also investigated. The proposed devices can be useful for applications to the multiwavelength-operational signal gating devices, optical switching devices, and multiwavelength fiber lasers in optical communication systems.
References and links
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