This work proposes an electrically tunable infrared light source based on a new compact structure, i.e., an AlGaInAs semiconductor multiple quantum well (MQW) integrated with a liquid crystal Fabry-Pérot filter. The AlGaInAs MQW is used as a luminance layer that emits broadband light. By sandwiching the AlGaInAs and LC material with two conducting mirrors, the active light source with an optical filter can be tuned with a wide wavelength range. The filter filled with nematic liquid crystal enables continuous tuning of emission along the extraordinary mode and provides a 58 nm tuning range with a bias of 14 V. The simulation results of wavelength and tunability are consistent with the experimental results. Cholesteric liquid crystal with a planar texture is also used to examine the properties of the tunable light source. Under an electric field, all the helical liquid crystal molecules tend to be aligned parallel to the field. The variation of the refractive index is normal to the substrate surface, and the polarization-independent tuning range is 41 nm. The wide tuning range and the polarization properties observed when NLC and CLC are respectively incorporated into the AlGaInAs based Fabry-Pérot cavity suggest that this integration scheme has potential for applying to optical communication system.
© 2012 OSA
Tunable light sources have attracted lots of attention due to their inherent wavelength-selective properties, which enable exploitation of wavelength-division-multiplexing (WDM) techniques and thus increased flexibility of usage and optical communication capacity. Generally, a tunable light source combines a broadband light source with a tunable filter. Of the various general light source elements, semiconductors based on quantum well (QW) structures have been widely used due to their small size, their capability to induce strong electro-optical interactions by electron confinement, and their potential for mass production. Through both QW and layer-structure bandgap engineering of heterogeneous materials and integration with other optoelectronic elements, optical processing can be realized in a compact size. Bandwidths of 1300nm or 1550nm are generally used in optical fiber communications because of their low loss and low dispersion in fiber. The InAlGaAs-based multiple QW (MQW) is the typical material system for fitting such wavelength region, so it can be used as an efficient near-infrared light source.
Optical filter characteristics such as tuning range, speed, and finesse play important roles in tunable light sources. The commonly used filter is the tunable Fabry-Perót (FP) filter, a simple optical-resonator composed of two highly reflective mirrors. The two common methods of achieving wavelength selection of FP filter are changing the cavity length by applying mechanical force on the mirrors [1–3] or varying the refractive index by incorporating a nematic liquid crystal (NLC) material into the cavity [4–7]. The former method is mechanically complex, and the actuation force may causes nonparallel reflection due to weakening of membranes . By contrast, the merits of the latter method include low power consumption, relatively wide tuning range, and a compact size. However, the NLC FP filter is polarization-dependent because of the anisotropic properties of the liquid crystal molecules, which limit its applications. Thus, the many proposed approaches for improving polarization-dependency in the NLC FP filter include twisting the structure of the NLC film and using an axially symmetrical configuration [9–12]. However, the alignment conditions required for these methods complicate the fabrication process. Cholesteric liquid crystals (CLCs) are widely used in various optical devices, including displays, filters, and distributed feedback lasers [13–15]. Under a homogeneously aligned condition, a CLC presents a planar state, which has a periodic helical structure with its axis normal to the cell surface. If the light wavelength substantially differs from those in the CLC reflection band, such a helical structure offers an average refractive index for incident light with an arbitrary polarization. By applying an electric field normal to the substrate, all the liquid crystal molecules tend to oriented from a planar texture to a homeotropic state and the change of refractive index is parallel to the electrical field . Therefore, CLC material is suitable for polarization insensitive tunable filters.
The tunable infrared light source in this study integrated a AlGaInAs quantum well with an NLC FP filter. The AlGaInAs quantum well provides a wide luminescence spectrum in the near-infrared (NIR) region while the NLC FP filter controls the wavelength of output light. The tunable properties in the NIR region were investigated and the simulations were performed to clarify the experimental results. Finally, the tunable properties of a polarization-independent tunable infrared light source with CLC were analyzed.
2. Sample fabrication and measurement
Figure 1(a) shows that the tunable infrared light source structure includes an active region and a phase modulator layer. The active region is a p-i-n heterogeneous semiconductor layer structure grown on an InP wafer by Metal Organic Chemical Vapor Deposition (MOCVD). The luminescence layers contain eight AlGaInAs quantum wells (λg = 1700nm) and nine AlGaInAs barriers (λg = 1300nm) sandwiched between an n-InP layer and a p-InP layer. The luminescence wavelength of the MQW is centered at 1560 nm.
A highly reflective (R>0.99) and wide stop-band (1370~1670 nm) filter with 18 periods of SiO2/Ta2O5 multi-film is deposited on an ITO glass substrate for use as the bottom optical reflection mirror. The luminescent layer is incorporated in the cavity by spinning an adhesive benzocyclobutene (BCB) layer on the top of the bottom mirror to bond the AlGaInAs/InP in the active region . After bonding the bulk semiconductor, a selective wet etching solution is used for selective removal of the InP substrate from the As-related material. The top cover of the cavity is fabricated by metalizing the Al layer on an ITO glass substrate used for electrical conducting and top of the optical mirror, where the Al mirror has 78% reflectively in 1300-1700 nm wavelengths. Finally, an empty cell is formed by placing the top mirror with a polyimide-coated and pre-rubbed layer for planar alignment of LC on top of the semiconductor layer with a 12.6 μm gap. To investigate optical polarization in the optical cavity, the cells were filled with positive NLC (BL006 from Merck) and green-reflecting CLC for comparison.
Figure 1(b) schematically depicts the measurement setup. The sample was placed on a stage and pumped by a CW laser (1064 nm). A focusing lens with a 10 cm focal length was used to focus the pump laser onto the sample at an incidence angle of 30°. The emission light created by the active region was collected by an optical fiber positioned in the normal direction. The polarization controller (PC) and the polarization beam splitter (PBS) were used to separate TE and TM orthogonal modes to compare their polarizing properties. An optical spectrum analyzer (OSA) was used for further characterization of the device. During measuring process, we used intermittent pumping instead of continuous pumping in order to avoid the thermal accumulation inside the MOW and variation of refractive index of LC inside the cavity.
3. Result and discussion
Because of the anisotropic NLC material used in the tunable light source, there are two orthogonal propagation modes, extraordinary or ordinary modes which depend on the polarization direction of light. When the polarization direction of propagation light is perpendicular to the optic axis of the LC molecules, the light passes through a refractive index of the no, which is defined as ordinary mode. The resonance condition is given by
When an electric field is applied to the device, orientation of the LC molecules changes from homogeneous alignment to the direction of the electric field. The refractive index for extraordinary mode varies from ne to no, whereas the refractive index for ordinary mode remains constant, no. Figure 2 shows the spectra of resonant emission at the ordinary and extraordinary modes under various applied voltages. As the voltage increases, the emission peaks of ordinary mode do not shift because the refractive index is still no [Fig. 2(a)], while the emission peaks of extraordinary mode shift to a shorter wavelength as the refractive index decreases [Fig. 2(b)]. The maximum shift range can be observed with applied voltage of 14 V, which means the LC molecules have been tilted to homeotropic state. When the voltage rises from 0V to 14 V, the total shift in extraordinary mode is about 58 nm. Figure 2(c) plots the wavelengths of the emission peaks as a function of applied voltage.
Generally, for liquid crystal material in the near IR region, some overtone molecular vibration bands begin to appear and cause the optical loss by absorption. Fortunately, the absorption coefficient of 5CB, which is the major component of BL-006, is lower than 0.5 cm−1 around luminescence wavelength of the MQW (~1560 nm) . Even though, some absorption loss of LC still inevitable because of the multiple passes inside the cavity.
To confirm that the change in the refractive index of liquid crystal layer causes the shift in resonant peaks in extraordinary mode, the emission peaks of the NLC-based device were simulated. The transmittance of the Fabry-Pérot filter under different reflectance mirrors was presented in 
Simulations were performed by first using Eq. (4) to find a dispersion relation of the hybrid liquid crystal AlGaInAs structure based on the emission peaks measured under a bias of 14 V. The polynomial fit of the refractive index was then entered into Eq. (3) to derive the emission peaks of the NLC-based tunable light source as shown in Fig. 3(a) . The tunability of the light source was examined by separately measuring the change of refractive index under various operating voltages. The reflection-voltage (R-V) curves of the LC film were obtained as follows. A light beam from a He–Ne laser (632.8 nm) was passed through the LC sample and reflected by dielectric mirror along the same path. One beam-splitter and two polarizers, which were oriented at ± 45° relative to the rubbing direction of the LC film, were used to exam the reflectance. The calculated phase difference between extraordinary and ordinary modes was about 14π. Finally, the variation in reflectance was substituted into Eq. (3), and emission peaks were derived under various voltages. Figure 3(b) shows that the simulation results correspond with experimental results, which confirm that the change in the effective refractive index of the NLC causes the shift in the emission peaks of the NLC-based tunable light source.
We further demonstrated a polarization-independent tunable light source by replacing the NLCs with CLCs. With a homogeneously aligned condition, a CLC presents planar state, where the helical axis is perpendicular to the substrate surface, offering an average refractive index for the incident light with polarization-independency. Under the influence of the small electric field, the planar state can be switched to focal conic or fingerprint texture, depending on the pitch of CLC and cell gap. However, the change of the refractive index at low voltages is not obvious. Hence, we focus on the behavior of CLC based tunable light source under the large electric field. When a high enough voltage is applied on the device, the CLC molecules orientate from planar state to homeotropic state, and thus the refractive index varies from (ne + no)/2 to no. Notably, all the operating processes are polarization insensitive. Figure 4(a) shows the spectra of resonant emission under various applied voltages. With the increase in voltage from 0 V to 110 V, the maximum shift range at 110 V is about 41 nm. Figure 4(b) plots the wavelength of the transmission peaks as a function of the applied voltage.
The electro-tunable infrared light source presented in this study integrates a liquid crystal Fabry-Pérot filter with an AlGaInAs active layer. The tunable light source based on NLCs exhibits wavelength-tuning emissions along the extraordinary direction with a tuning range of 58 nm in the 1.55 μm regime. The phase retardation of the extraordinary wave is about 14 π, and simulations of peak wavelength and their tunability agree well with the experiment results. For the CLC-based tunable AlGaInAs light source, application of an external field orientates the director, and the CLCs change from planar state to homeotropic state. The refractive index in CLC can be tuned from (ne + no)/2 to no, which provides an overall tuning range of 41 nm for this light source. Owing to the helical distribution of LC director along light propagation, tunability is polarization independent. Therefore, the tunable infrared light source has potential applications in optical communications and in related optical processing applications.
The authors would like to thank the National Science Council (NSC) of Taiwan, for financially supporting this research under the contract no. 100-2628-E-10-007-MY3 and 99-2119-M-10-006-MY3.
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