We have developed all-optical modulation of photonic crystal fiber (PCF) by infiltrating photoresponsive liquid crystal (LC) into the voids within the cladding structure. The photo-induced phase transformation of the photoresponsive LC modulates the effective refractive index of the photoresponsive LC-filled cladding, thereby creating an environment of modifiable total internal reflection that tunes the output intensity of guided light upon the stimulus of optical field. The modulation range for the 632 nm wavelength is 10 dB and the response time for switching is less than 1 second by manually obstructing the pumping light path. In addition to altering the power of the pumping laser to actively tune the output intensity, the polarization direction of the pumping laser can also tune the output intensity by 5 dB.
© 2008 Optical Society of America
Photonic crystal fibers (PCF) guide light by introducing periodically arrayed voids located in the cladding structure to create a forbidden bandgap [1–3]. Light cannot propagate through the fiber-void interface within the cladding body due to the guidance mechanism of modified total internal reflection (m-TIR) or photonic bandgap (PBG) effect from the designed contrast of refractive indices between solid or hollow core and air-hole cladding [4–7]. The intriguing design of hollow structure in core or cladding extends PCF’ use from optical fiber guidance [8, 9] and fiber laser [10, 11] into nonlinear optics [12–14], fiber sensing [15–17], and tunable fiber devices [18–20] by separately infiltrating gas, liquid, and solid materials into the voids within the core or the cladding structure. Infiltration of liquid crystal (LC) into the voids efficiently creates an actively tunable environment and has demonstrated a controllable intensity and spectrum of guided light confined within the PCF [21–23]. The LC-filled PCF has shown potential application in optical devices, such as tunable switches and filters [21, 24]. The tunability is due to LC’s unique properties of large and controllable birefringence that can be modulated by heat or electric field. The external stimulus changes the average refractive index (RI) of the LC-filled cladding and modulates the optical properties of the LC-filled PCF.
In this paper, we demonstrate a light controllable PCF achieved by infiltrating photoresponsive LC  into the air holes located within the cladding. The guiding mechanism of solid-core PCF used here is similar as conventional fiber where the RI of air-hole filled cladding is smaller than the solid core . After LC infiltration, we observed that the guiding ability of LC-filled PCF was still valid. The LC-filled cladding increases the RI of cladding and changes the guiding mechanism of PCF in which the LC-filled cladding regions act as a scattering center to modulate the output intensity. The photoresponsive LC contains a photochromic azobenzene dye in which the RI modulation of LC is achieved by the photochromatic deformation of the azobenzene under an applied external optical field. Recently, the thermal-optical modulation of PCF infiltrating dye-doped nematic LC has been demonstrated; however, the tuning mechanism was due to the local heating of the LC using a pulsed laser . Here, we present a light-controllable PCF where the optical switching is generated by directly reorientating the LC upon the exposure of optical field. We also investigated the all-optical tuning properties of the photoresponsive LC-filled PCF that yielded the on-off switching of 10 dB attenuation and the respective rise and decay times of 0.6 s and 0.3 s at 20 mW of pumping laser illumination. 5 dB modulations of output intensity could be obtained by tuning the polarization of the external optical field.
A commercial solid core, large mode area, and polarization maintaining PCF (Crystal fabre, LMA-PM-15) was used in the experiment. The fiber contains pure silica of 15µm diameter solid core and hollow air-filled cladding of 230µm in diameter. The size of each air-hole is 6µm in diameter and the distance between the centers of each cladding holes is 10µm in diameter. The fiber coating jacket (acrylate material) was removed before infiltrating photoresponsive LC. The photoresponsive LC was a homogeneous mixture of nematic LC, photochromic azobenzene and chiral dopant . The LC mixture was infiltrating into air holes by capillary force without the use of vacuum. No pre- or post-aligning treatment was applied in the LC or LC-filled PCF. Fig. 1. shows the two geometries of experimental setup for measuring the optical tuning properties of photoresponsive LC-filled PCF.
The probe light source was either a white light from halogen lamp or a 632 nm wavelength light from He-Ne laser. The detector was a CCD camera (Fig. 1(a)) or a photodetector (Fig. 1(b)) depending on the probe light source. The sample was positioned on the precision 3-axis stage between two 10× microscope objectives in both optical measuring experiments. The first objective focused the probe light into one end of PCF and the second objective collected the expanded beam from the opposite end of PCF and then focused the output light into the detecting devices. The lock-in amplifier and chopper were used to measure the output intensity from the He-Ne laser. The pumping light source was a violet laser diode (Nichia, NDHV220APA) which was adjusted to different driving currents to offer different optical powers. A half-wave plate was placed in front of the pumping laser to adjust the polarization of pumping light when He-Ne laser used as probe. The beam diameter of pumping laser was maintained at 2 mm-long, and the size of photoresponsive LC-infiltrated PCF was 10 mm-long.
3. Results and discussion
The photochemical isomerization generated by the guest (azobenzene)/host (nematic LC) mixture of the photoresponsive LCs is shown in Fig. 2(a). When a small amount of photochromic azobenzenes are added into nematic LC, an isotropic phase of LC mixture is generated. Under external light stimulus the nematic LC changes its orientation due to the cis to trans photoisomerization of the azobenzene molecules. The trans form of azobenzene stabilizes the phase structure of rod-like nematic LC under the external stimulus of light . The cis form of azobenzene further destabilizes the phase structure of photoresponsive LC mixture when the external light energy is released. Figure 2(b) shows the polarized micrograph of photoresponsive LC inside the PCF holes. The multicolor of photograph within the fibers indicates that the LC was not aligned in specific direction after the infiltration of photoresponsive LC into the air-hole cladding under capillary force.
Figure 3 shows the output beam profile captured from CCD camera using white light as probe when the pumping light switched off and on. Before applying the pumping light, the photoresponsive LC filled cladding still remains the guidance of probe light in solid-core PCF, which is clearly indicated in Fig. 2(a). When the pumping light switched on, the bright region in the center of PCF quickly disappeared, as shown in Fig. 2(b). For the common solid core PCF, the fiber core (pure silica, nsilica=1.45) has a higher RI than the cladding (nave=1.40, mixed silica and air holes, nair=1) which was calculated by the volume percentage of air-hole (12%) and silica (88%) areas according the geometry of commercial solid-core PCF. After photoresponsive LC infiltration, the RI of cladding changed from 1.40 to 1.47 which was calculated by using the average RI of nematic LC as 1.61 (nave=1.47 for TL213, no=1.527, ne=1.766, nave=2/3no+1/3ne) . Because the average RI of cladding is larger than the solid core after LC infiltration, the mechanism of m-TIR guidance is not suitable to explain the guidance of probe light in photoresponsive LC-filled PCF. Therefore, we speculate that the LC-filled cladding regions in PCF act as a scattering center and modulate the input intensity by changing the RI in the cladding structure. The pumping light changes the preferred orientation of the photoresponsive LC and further decreases the average RI of photoresponsive LC-filled air-hole cladding. At this moment, no guidance of probe light was observed because the RI of solid core and LC-filled cladding reached the equal value. The amount of RI modulation of photoresponsive LC-filled PCF after the exposure of pumping light was estimated as 0.02. The details of RI modulation of photoresponsive LC associated with the LC alignment inside PCF need to be studied further to clarify the guiding mechanism after LC infiltration.
Figure 4(a) shows a sequence of the CCD camera images of solid core profile observed from photoresponsive LC-filled PCF upon different optical power of pumping light. When no pumping light was applied, the solid core guided the white light as a Gaussian beam profile. Increasing the power of pumping light first decreased the central intensity of core-guided mode and eventually decreased the intensity of other mode of guided light. Figure 4(b) shows the temporal switching behavior of light guided in the photoresponsive LC-filled PCF. The images were monitored by CCD camera that recorded the profile of guided light in real time. The response times for the “on” and “off” photoswitching processes were estimated to be 0.6 s and 0.3 s, respectively. Since the modulation of the external light was achieved by manually obstructing the pumping laser path, the recorded temporal profiles at different time intervals do not reflect the intrinsic speed of the photoswitching capability of the photoresponsive LC .
To quantitatively characterize the tuning properties of photoresponsive LC-filled PCF, a 633 nm He-Ne laser was used as the probe light (Fig. 1(b)). The recording setup was attained using a photodiode, a chopper, and a lock-in amplifier to yield a stable output signal. The optical field-dependent intensity of the fiber output with a full hysteresis cycle of increasing and decreasing pumping light is shown in Fig. 5. The output intensity was normalized to the initial output intensity from LC-filled PCF without the stimulus of pumping light. At zero power of pumping light, the laser beam is mainly guided in the silica solid core. As the power of the external stimulating light increases, the modulation of RI makes the guided light gradually leak from the center mode of the PCF and eventually the output intensity decreases. The optical tuning range was measured to be over 10 dB at pumping light power of 20 mW. Large hysteresis was observed as forward and backward scans of pumping light were applied. Since the direction of the photoresponsive LC was in isotropic phase and not aligned in a particular direction to the fiber axis after LC infiltration, it is difficult for the reoriented LC after forward pumping light to come back its original director at backward pumping light.
We also characterized the optical tuning properties of the photoresponsive LC-filled PCF by changing the polarization of pumping light, as shown in Fig. 6. The corresponded angle between pumping light and PCF axis was modulated by a half-wave plate. We observed that the output intensity guided from photoresponsive LC-filled PCF could be tuned by changing the polarization of pumping light due to the dye-induced reorientation that make the host nematic LC molecules tend to become aligned in parallel to the polarization of pumping light . The polarization-dependent tuning range was measured to be 5 dB at an external pumping power of 20 mW.
We have achieved an optically tunable photonic device by infiltrating photoresponsive LC into a solid-core PCF. The output intensity could be optically controlled by changing the RI of cladding infiltrated with photoresponsive LC. The optical-field induced LC reorientation actively modulates the photonic bandgap effect in solid-core PCF and leads to a dynamic and rapid appearance and disappearance of guided light. The modulation of 633 nm HeNe laser is 10 dB and the response time is less than 1s using a pumping laser power of 20 mW and pumping laser spot size of 2 mm in diameter. The modulation of polarization direction of pumping laser can also tune the intensity of guided light by 5 dB. The realization of a light controllable PCF is applicable to integrated all-optical devices.
We thanks for John R. Waldeisen and Prof. Peng-Chun Peng for their technical discussions and paper preparation. This work is support by the National Science Council, Taiwan, under project No. 96-2112-M-260-002.
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