4.3 Passive devices
4.3.7 Experimental results with single ring device in InGaAsP material
different material InGaAsP, the bandgap wavelength of the quaternary material with three quantum wells is λgap = 1.41 μm and thickness of 0.0654 μm. The straight waveguide losses in InGaAsP were 25 dB/cm at the wavelength of 1.55 μm.
A single ring resonator incorporating with MMI turning mirror coupler of 128 μm in length and 7.6 μm in width, the spectral response is shown in Fig. 4.12. The solid curves
show the output spectra at the throughput port and drop port, respectively. The free spectral range (FSR) is of 82 GHz for this ring resonator. The FWHM of the passband is ~0.24 nm, which corresponding to a quality factor of Δλ/FWHM~6461 for the drop port. The finesse (F) is 2.7, and the contrast of the drop port is 4 dB in this ring resonator. The cross coupling factors have been determined to be K=0.15 from the simulation. If the compensated loss (α=0), Tmax(Throughput port)/Tmin(Drop port)=10*log[4*(1-K)/K2] (dB). The coupling factor (K) is calculated to be 0.15 from the measured value Tmax(Throughput port)/Tmin(Drop port) of 21 dB. The experimental coupling factor coincides with the simulation. The coupling loss between the tapered lens fiber and the device is 17 dB (2*8.5 dB input and output) with a confinement factor of 0.09. The loss of the single ring resonator is about 6.2 dB, and the loss of MMI turning mirror couplers is about 5.52 dB including the reflector loss 1.8 dB/per reflector on the ring device. Then, the loss of the facets of the input and output waveguides tilted by 7 degrees is measured to be 7.74 dB (2*3.87 dB input and output facet), and the total length of the straight waveguide from the input to the output port is around 0.16 cm. The straight waveguide loss in InGaAs (λgap = 1.41 μm) is around 25 dB/cm at the wavelength of 1.55 μm.
The total insertion loss is 40.5 dB. Comparing the measurement with the simulation, we obtain the value α is 17 cm-1 for the ring device. In order to improve the contrast for throughput port and drop port and to compensate the loss of the devices, optical amplifiers have to be implemented into the ring devices. The amplifiers (gains) are operated around 1.55 μm. In the future, the ring devices combine active (λgap = 1.55 μm) and passive components (λgap = 1.41 μm) of SOAs, the confinement factor between active and passive components will become very high and the total insertion loss is lower after compensation.
Fig. 4.12 MMI width of 7.6 μm, FSR=82GHz, Δλ/FWHM ~6461 and simulation
Reference
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[4.3] P. Bhattacharya, Semiconductor Optoelectronic Devices (2nd, Prentice-Hall, New Jersey, 1994), pp.175.
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“Fold-Cavity Resonators as Key Elements for Optical Filtering and Low-Voltage Electroabsorption Modulation,” IEEE Journal of Lightwave Technol., vol. 24, pp.3464-3470, September 2006.
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Chapter Ⅴ Summary
For PIC purpose, the active device of ridge waveguide, the passive device of optical ring resonator with MMI turning mirror and the enhancement and blue shift of QWI technique, respectively on the basis of the multiple quantum wells InGaAlAs/InGaAs laser/SOAs on InP substrate have been developed, fabricated and characterized in this thesis. Moreover, the same mask and fabrication is used in different InGaAsP material. The characterization of the device also is the same.
z Passive devices:
1. We have demonstrated an innovative design for optical switches by using MMI waveguide crossing and turning mirrors. The perturbation is the minimum when the crossing occurs at the self image location for the MMI waveguides. Such MMI waveguides can cross 90 degrees or 60 degrees with minimal cross talk. From these simulation results, one can reflect the incident mode into an intersecting waveguide by introducing an idea reflecting plane. In practice, the reflector is replaced by a plane for total internal reflection with correction for Goos-Hanchen shift. The devices, which possess multimode waveguide crossings and turning mirrors, have the merits of smaller foot print, and lower crossing loss.
2. A novel design of a single-ring resonator with the designed λ= 1.41 μm MQWs laser/SOAs structure by MBE system using low-loss multimode waveguide turning mirror couplers results in a coupling factor 0.15 in this ring resonant. The coupling factor of the MMI turning mirror coupler can be changed from 0.85 to 0.15 and the length of the MMI turning mirror coupler (K=0.15, Lmmi= (3/4)*Lπ) is decreased
to 33% than the length of the MMI coupler (K=0.15, Lmmi=3*(3/4)*Lπ). The circumference of the curve waveguide (diamond-shape) in this ring resonator is decreased to 50%. Thus, we have demonstrated an optical ring resonator with MMI turning mirror couplers to obtain a coupling factor K=0.15 and a shorter ring length for the resonators in the passive region of photonic integrated devices. The experimental coupling factor coincides with the simulation. The 2X2 MMI turning mirror coupler gets a reverse coupling factor. Output spectral response shows a FSR of 83 GHz for the device, respectively. From the experimental value Tmax(Throughput port)/Tmin(Drop port) is 21 dB, The coupling factor (K) can be calculated to 0.15. A contrast of almost 2 dB for the throughput port and 3 dB for the drop port and full-width at half-maximum (FWHM) of 0.24 nm for the drop port have been achieved. The experiment results are in good agreements with the simulated results.
These results are a first step toward the realization of monolithic integration of semiconductor optoelectronic devices.
3. For a single ring resonator with MMI turning mirror couplers is performed in InGaAsP-InP material, we also design the bandgap of 1.41 μm of the epitaxial layers consisted of top and bottom InGaAsP cladding layers and a quaternary core In0.67Ga0.33As0.6P0.4 / In0.71Ga0.29As0.74P0.26 of three quantum wells grown on a 2-inch n-type InP substrate laser/SOAs structure by metal organic chemical vapor deposition (MOCVD)system. Using the same mask, the same fabrication process and the same measurement, we also have demonstrated an optical ring resonator with MMI turning mirror couplers to obtain a coupling factor 0.15 and a shorter ring length for this resonator in the passive region of the photonic integrated devices. The experimental result shows a FSR of 82 GHz for the device. A contrast of 4 dB and full-width at half-maximum (FWHM) of 0.24 nm for the drop port also have been achieved. The experiment results are in good agreements with the
simulated results. These results are a first important step for active and passive region integrated in the Ⅲ-Ⅴ semiconductor devices.
z Quantum well intermixing (QWI) technique: (depicted in Appendix - A)
1. Argon plasma induced photoluminescence enhancement and QWI of InGaAs/InGaAlAs MQWs are demonstrated that the enhancement of the PL intensity of MBE-grown multi-quantum-well structures by RTA has been improved by morn than an order of magnitude by adding an Ar+ plasma bombardment step before RTA. The PL signal enhancement does not necessarily cause a significant change of the PL wavelength. The maximum amount of blue shift that has been observed is only 15 nm. The etching rate during Ar+ plasma bombardment is found to be 6.5 nm/min. This information is useful for growing a sacrificial cap layer of the appropriate thickness.
2. The characterization of Ar+ plasma bombardment jointed the IFVD technique with the sputtered SiO2 capping layer induced QWI are investigated for In0.77Ga0.23As0.79P0.21/In0.57Ga0.43As0.64P0.36 MQWs structure by MOCVD system and In0.53Ga0.47As/In0.53Ga0.26Al0.21As MQWs structure by MBE system. The main influence of QWI process is contributed to three factors. First, the optimal thickness between MQWs and the upper cladding layer is 200 nm for InGaAlAs material and 300 nm for InGaAsP material by ICP-RIE. The majority of point defects are generated in this factor. Second, the enhanced defects are assisted by atomic bombardment under a porous SiO2 film deposited. Third, atoms and defects obtained enough energy to diffuse under higher annealing temperature. The samples with the Ar+ bombardment jointed the IFVD technique have a higher degree of intermixing than only Ar+ bombardment or the sputtered SiO2 capping layer under RTA treatment. This is attributed to the fact that the upper cladding
layer is too thickness to generate the point defects and thermal expansion effect.
Therefore, the maximum blue-shift of wavelength of 60 nm for InGaAlAs material and of 90 nm for InGaAsP material using Ar+ plasma bombardment with the sputtered SiO2 capping layer have been achieved under the RTA treatment. This behavior is very useful for re-grown and fabrication of PICs.
Appendix – A Quantum Well Intermixing Technique
A.1 Introduction
Semiconductor multiple-quantum-well (MQW) structures are often designed into various types of lasers and optical waveguides for use in the 1.55-μm optical-fiber communications band [5.1-5.4]. Epitaxial wafers intended for the fabrication of photonic integrated circuits (PICs) usually also rely on MQW structures to provide optical gain. For all these applications, the strength of photoluminescence (PL) is of crucial importance. In general, epitaxial growth at higher substrate temperature tends to reduce the density of non-radiative-recombination centers and leads to higher PL efficiency. For epitaxial structures grown on InP substrates by molecular-beam epitaxy (MBE), the maximum substrate temperature that can be used during growth without causing surface damage is lower than the typical growth temperature used in the metal-organic chemical vapor deposition (MOCVD) technique. Consequently, the PL efficiency of MBE-grown epitaxial materials on InP substrates is usually not as high as that of MOCVD-grown materials.
Therefore, it is important to have an effective post-growth process to enhance the PL efficiency of MBE-grown materials on InP. Although post-growth rapid thermal annealing (RTA) has been shown to improve the PL efficiency of this type of wafers, the amount of improvement is typically only a factor of two [A.1].
First, we demonstrate that much greater improvement of the PL efficiency (up to two orders of magnitude) can be obtained if the wafer is subjected to bombardment in an Ar plasma before RTA. The amount of blue shift of the PL peak wavelength resulting from this process has been found to be relatively small, with a maximum of 15 nm.
For PICs and optoelectronic integrated circuits (OEICs) are realized, many techniques
are currently under investigation. The quantum well intermixing (QWI) techniques are attracted a lot of attention because of their importance is to PICs and OEICs [A.2, A.3, A.8].
To fabricate a photonic integrated device, the band-gap of energy in passive section needs to be higher than that in active section. Generally, selective etching and re-growth are used to have different the band-gap of energy on a semiconductor chip, but these methods are a complicated process. In contrast, QWI technique is cost effective, which obtains the butt joint with perfect alignment on a chip. QWI is a powerful technique for monolithic integrated optoelectronic devices of varying functionalities on a single wafer and permits a post-growth modification of the absorption edge. So it can be used to create regions with different band-gap by post-growth techniques such as impurity-induced disordering (IID) [A.12], ion-implantation enhanced inter-diffusion (IIEI) [A.13], laser-induced disordering [A.14] and impurity-free vacancy diffusion (IFVD) [A.5, A.15]. Using Argon plasma bombardment and dry etching to the optimal depth produce point defects to promote QWI, which continued the deposition of a 300-nm-thick sputtered SiO2 capping layer is very useful to enhanced QWI effect by thermal treatment under a higher temperature annealing during QWI process. The point defects induced by Argon plasma and atomic bombardment [A.2, A.3, A.8] play a key role in QWI, which jointed the IFVD technique with the sputtered-SiO2 capping layer accelerate the out-diffusion of Ga into the capping layer and lead to the injection of Ga vacancies into the quantum wells. Therefore, the group Ⅲ vacancies generated in QW region promote the inter-diffusion between quantum wells and barriers.
Second, we investigate for the InGaAsP and InGaAlAs multi-quantum wells (MQWs).
Optimal distance is of 300-nm-thick for InGaAsP and of 200-nm-thick for InGaAlAs between MQWs and the upper cladding by ICP-RIE and bombardment, covering the 300-nm-thick sputtered SiO2 by RTA process resulted in a band-gap blue-shift of 90 nm for InGaAsP and of 60 nm for InGaAlAs.
Fig. A.1 Schematic cross section of the epitaxial layer structures for (a) sample 50d and (b) sample C071