An integrated reconfigurable four-channel wavelength-division-multiplexed drop module for use in the long-wavelength was demonstrated using a tunable wavelength-selective photodetector array. The array consists of an InP-based p-i-n absorption structure and a GaAs-based multistep Fabry-Pérot filtering cavity. The high quality GaAs/InP heteroepitaxy was realized by employing a thin low temperature buffer layer. The GaAs-based multistep cavity was fabricated by wet etching and regrowth. The dropped central wavelengths were 1538, 1550, 1559, and 1570nm. The tunable range reached 10nm with a tuning power efficiency of 14.2nm/W. A spectral linewidth less than 0.5nm (FWHM), a 3dB bandwidth of 9.2GHz, and the peak quantum efficiencies from 13% to 20% were simultaneously obtained, in agreement with the theoretical simulation.
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Tunable wavelength-selective photodetector (WSPD) arrays around 1550nm will be important for the development of the reconfigurable optical add-drop multiplexer (ROADM). Thus far, several ROADMs based on integrated wavelength demultiplexers, such as the tapered Fabry-Pérot (F-P) filters [1,2], microring resonator filters [3–5], and array waveguide gratings (AWGs) [6,7], have been proposed and demonstrated. However, discrete photodetectors are needed for the scheme of the tapered F-P filters, the precise control of the coupling strength presents practical difficulties for the microring resonator filters, and the manufacture process is complicated and costly for the AWGs scheme.
In order to achieve large-scale integration and reduce manufacturing cost, it is desirable to develop an integrated long-wavelength tunable WSPD array for multi-wavelength receiving applications. By integrating an InP-based p-i-n absorption structure (PIN) with a GaAs-based multistep F-P filtering cavity (FPC), the tunable WSPD array is realizable and a small optical bandwidth, a large tuning range and a high speed can be attained. To our knowledge, there is still no report on the photodetector array based on the multistep FPC. In addition, by utilizing a tapered glass optical waveguide, the array can be used as an integrated reconfigurable multi-channel wavelength-division-multiplexed (WDM) drop module. The signal input from the fiber is coupled to the input port of the tapered glass. The light propagating in the waveguide is demultiplexed by the multistep FPC and routed to the corresponding PIN photodetectors.
The paper reports our work on design, fabrication, and testing of an integrated reconfigurable four-channel WDM drop module based on the tunable WSPD array, which operates at 1538, 1550, 1559, and 1570 nm. Each photodetector of the array can be tuned respectively, and the tunable range is about 10nm. A spectral linewidth less than 0.5nm (FWHM), a 3dB bandwidth of 9.2GHz, and the peak quantum efficiencies from 13% to 20% were obtained, in agreement with the theoretical simulation.
2. Design and fabrication
2.1 Modeling of the integrated reconfigurable four-channel WDM drop module
The schematic structures of the reconfigurable four-channel WDM drop module and a tunable WSPD of the array are shown in Fig. 1 . Figure 1(b) is the proposed structure of the WSPD, which combines an InP-based PIN photodetector array with a GaAs-based multistep FPC. The GaAs-based multistep FPC comprises two identical distributed Bragg reflectors (DBRs) and a GaAs multistep resonator cavity layer. The top and bottom DBRs are made of GaAs/AlGaAs quarter-wave stacks designed for high reflectivity at 1550nm center wavelength. Each step height of the GaAs multistep resonator cavity layer is about 20nm, for the wavelength interval of the spectral responses is about 10nm. The device can be fabricated by GaAs/InP heteroepitaxy growth of an InP-based PIN photodetector array on a GaAs/AlGaAs multistep FPC. The high quality GaAs/InP heteroepitaxy is realizable by employing a thin low temperature buffer layer [8–10]. The high reflecting film is formed by depositing a mirror on the bottom surface of the glass. The glass has two vertical tapers, whose angles are designed to ensure the input beam and output beam perpendicular to the tapers. The array can be bonded onto the tapered glass using the polymer benzocyclobutene (BCB) . As shown in Fig. 1(a), The light input from the taper are guided by the FPC and the bottom mirror with low loss, only that meet the resonant condition can pass through the filters and be routed to the corresponding PIN photodetectors. Each photodetector of the array operates at a different wavelength due to the multistep resonator cavity layer, which can be tuned respectively based on the thermal-optic effect [12–14].
When the photocarrier generation occurs throughout the device including the contacts, assuming that only those carriers generated in the high electric field i-region can be collected, the quantum efficiency η can be written as :Eq. (2) and the assumption that there is no downward-traveling light from the top of the device, Eout− = 0, Ein+ = S11× Eout+, and Ein− = S21× Eout+. Thus, the reflectivity of the device is given by:Eqs. (1) and (3).
The wavelength dependencies of the quantum efficiency are shown in Fig. 2 for different pairs. The three curves correspond to the cases of 5, 10 and 20 pairs DBR. The photodetector exhibits the peak quantum efficiency of 23% regardless of optical loss in the FPC, which is independent of pairs. The FWHM decreases with additional pairs, down to less than 0.3nm over twenty GaAs/AlGaAs pairs. If the reflectivity of the bottom mirror is 99.5%, the calculated spectral responses at the drop port of the integrated reconfigurable four-channel WDM drop module are shown in Fig. 3 , which correspond to different resonator cavity thicknesses.
2.2 Growth of the WSPD array
The details of the epitaxial structure used in this work are given in Table 1 . The device was fabricated by growth of an InP-based PIN photodetector array on a GaAs/AlGaAs multistep FPC. The GaAs/AlGaAs multistep FPC comprises two identical DBRs and a GaAs multistep resonator cavity layer. The DBRs are quarter-wave stacks consisting of 22 periods of alternating GaAs and Al0.9Ga0.1As layers, and the designed operating wavelength is 1550 nm. The InP-based PIN comprises a 50 nm InGaAs etching-stop layer, a 256 nm n-doped InP layer, a 40 nm InGaAs etching-stop layer, a 468 nm InP spacer layer, a 400 nm InGaAs absorption layer, a 240 nm InP spacer layer, a 200 nm p-doped InGaAs layer, and a 120 nm InP cap layer. The etching-stop layer enables precise deposition of the electrode on the n-doped InP layer during the device fabrication.
The epitaxy was performed by metal-organic chemical vapor deposition (MOCVD) with a Thomas Swan CCS-MOCVD system at a pressure of 100 Torr. Trimethylgallium, trimethylindium, trimethylaluminum, arsine, and phosphine were used as the precursors. Diethylzinc and silane were used as the p-type and the n-type dopant sources, respectively. In this work, the InP and the GaAs substrates were (100)-oriented and epiready grade.
As shown in Fig. 4 , the growth of the WSPD array was carried out as the following steps.
- (1) 22 pairs of quarter-wave stacks of GaAs/AlGaAs layers (λ0 = 1550nm) and an 800nm GaAs cavity layer were grown on a semi-insulating GaAs substrate.
- (2) The four steps on the epitaxial layer surface of the GaAs cavity are formed by two-step wet etching. The widths of the strip-geometry pattern are 1600μm and 800μm for the first and second photolithography mask, respectively. The patterned sample was etched by H2SO4/H2O2/H2O (1:1:30) solution with the etch rate of 1.5nm/s. The first etch depth h1 is 40nm and the second depth h2 is 20nm. So each step has a width of 800μm and a step height of 20nm.
- (3) After being degreased in organic solvents, another 800nm GaAs cavity layer and 22 pairs of quarter-wave stacks of GaAs/AlGaAs layers were regrown, and then the 48nm InP low temperature buffer layer was grown at 450°C, which realized the high quality GaAs/InP heteroepitaxy growth. The structure of InP-based PIN was grown later. The cross sectional SEM view of the device epitaxial structure is given in the Fig. 5.
The double-crystal X-ray diffraction scans of the epitaxial layer are shown in Fig. 6 , in which the left peak B is introduced by the InP-based PIN; the full-width at half-maximum value is 480 arcsec, which indicates the high quality of InP/GaAs heteroepitaxy. The point A shows the influence of the p-type or n-type doping. The right two peaks C and D correspond to the GaAs-based FPC, in which the peak C corresponds to AlGaAs, as its crystal lattice constant is larger than GaAs. The satellite peaks S are contributed to the GaAs/AlGaAs DBRs.
2.3 Fabrication of the WSPD array
The device was fabricated as the following procedures. After lithography, Ti-Pt-Au were evaporated and patterned by a lift-off process to form an annular p+ Ohmic contact with 30μm inside diameter. A 42μm diameter top round mesa was formed by etching down to the n-type InP contact layer. InP layers and InGaAs layers were selectively removed in HCl/H3PO4 (1:1) and H2SO4/H2O2/H2O (1:1:2) solution, respectively. The etch rate had been very well controlled by inspecting the test samples. The n+ Ohmic contact was achieved by a Pt/Ti/Pt/Au lift-off. Then a 61μm × 64μm bottom square mesa was etched down to the InP buffer layer. The device was covered with Polyimide and annealed at 210°C for passivation. Two windows on the InP buffer layer were opened at both sides of the bottom mesa by standard photolithography and etching, and two holes were then formed by extending the windows vertically down to the GaAs cavity layer of the GaAs-based FPC. Ti/Au was evaporated and patterned by a lift-off process to form electrodes. The device was accomplished after polishing. The optical microscope image of the device is shown in Fig. 7 .
2.4 Design of the tapered glass optical waveguide
The input laser beams with different wavelengths from the fiber pigtail are coupled into the tapered glass. The light propagating in the waveguide are demultiplexed by the multistep FPC and routed to the corresponding PIN photodetectors.
The geometry structure of the tapered glass was designed by using the ray optics method. The spacing between the two photodetectors is 800μm, and the thickness of the glass is 5mm. If the taper angle is 4.75°, all the input beams can be assumed to be vertically transferred into the glass waveguide. The 99.5% reflecting film was formed by depositing SiO2/TiO2 DBRs on the bottom surface of the glass.
3. Results and discussion
The spectral response measurements were carried out in the 1520-1590nm wavelength range by using an Anritsu Tunics SCL tunable laser with a single-mode fiber pigtail as the light source. An input beam was obtained by collimating the light from the fiber with a fiber collimator.
The spectral response of the device was plotted on a linear scale and a log scale, as shown in Fig. 8(a) and 8(b), respectively. The dropped wavelengths are 1538nm, 1550nm, 1559nm and 1570nm. The right curve corresponds to the as-grown wafer, while others correspond to cumulative recess etches of 20, 40 and 60nm in the FPC, respectively. The spectral linewidth is less than 0.5nm (FWHM) and the peak quantum efficiencies are from 13% to 20% with an extinction ratio of 50dB. The linewidth of the device is mainly dependent on DBR layers. For the narrower linewidth, more pairs of DBR layers are required. The peak quantum efficiency depends mainly on the 400nm In0.53Ga0.47As absorber layer and the propagation loss in the tapered glass. The higher quantum efficiency can be achieved by increasing the absorber layer thickness, by using a resonant cavity enhanced (RCE) structure, or by using a one-mirror inclined three-mirror cavity (OMITMC) structure [15–19].
The WSPD can be tuned based on quantum-confined Stark effect, carrier-injection effect, thermal-optic effect, or Kerr effect . Generally, these effects cannot be considered in isolation, as the thermal-optic effect and the carrier-injection effect. For low injection currents, the carrier-injection effect plays the major role, and a very small blue shift of the wavelength is observed. A large red shift of the wavelength can be obtained with increasing injection currents because the thermal-optic effect predominates . Thus, the wavelength tuning of our photodetector is mainly decided by the thermal-optic effect. When tuning power was increased, the dropping central wavelength shifted to longer wavelength.
The tunable spectral responses of one photodetector in the WSPD array were plotted in Fig. 9 . The wavelength tuning ranges of 2.9, 7.0, and 10.0nm (1538–1548nm) were achieved with the applied tuning powers of 0.2, 0.5, and 0.7W, respectively. At the tuning power of 0.7W, a 10nm wavelength shift was obtained. The tuning power efficiency was about 14.2nm/W. The dropped wavelength shift dependence on tuning power was found to be linear, as shown in Fig. 10 . The spectral linewidth less than 0.5nm were obtained simultaneously.
The room temperature current-voltage characteristic of the device was measured without illumination. As shown in Fig. 11 , the dark current slowly increases from 2nA at 0V reverse bias to 20nA at 6V. At larger reverse biases, the dark current increases more rapidly due to tunneling at a higher electric field. The dark current is 10nA at a reverse bias of 3V. There are several sources contributing to the dark current, including the imperfect surface passivation , diffusion current, generation recombination current, and tunneling current at high bias voltages . The generation recombination current is mainly caused by the background dopant and the mismatch dislocations . The high quality heteroepitaxy growth, by using a thin low temperature buffer layer, reduces the generation recombination current due to lower mismatch dislocations.
Measurement of the bandwidth was made with a tunable laser and a HP8703A lightwave component analyzer in the frequency range from 130MHz to 20GHz at 1550nm wavelength. The device was contacted by a microwave probe of 50Ω characteristic impedance and biased through an internal bias tee. The measured frequency response is shown in Fig. 12 , where the 3dB bandwidth of the device is 9.2GHz with a reverse bias of 3.0V. In our device, the bandwidth is mainly limited by the resistance-capacity (RC) time constant. In the future, smaller area photodetectors or uni-traveling carrier (UTC) structures should be fabricated to increase the response speed [25–27].
The feasibility of a reconfigurable multi-channel WDM drop module using a WSPD array with a multistep FPC has been shown and the performance of the device has been characterized. The four-channel drop function was realized, and the dropped central wavelengths were 1538, 1550, 1559, and 1570nm. At the tuning power of 0.7W, a 10nm wavelength shift was obtained with a tuning power efficiency of 14.2nm/W. A spectral linewidth less than 0.5nm (FWHM), a 3dB bandwidth of 9.2GHz, and the peak quantum efficiencies from 13% to 20% were simultaneously obtained, in agreement with the theoretical simulation. As the key part of ROADMs, the WSPD array with the multistep FPC can increase integrated level and reduce cost, and has great potential for future low cost WDM networks.
This work was supported by National Basic Research Program of China (2010CB327601), National High Technology R&D Program of China (2007AA03Z418), 111 Project of China (B07005), Changjiang Scholars and Innovative Research Team in University of China (IRT0609), and Program of Key International Science and Technology Cooperation Projects (2006DFB11110).
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