The undercut long-wave infrared (LWIR) waveguide components with air-gap beneath are analyzed and fabricated on the Si-wafer with simple manufacturing process. A 1 × 2 multimode interference (MMI) splitter based on this structure is presented and measured under the 10.6μm wavelength experimental setup. The uniformity of the MMI fabricated is 0.76 dB. The relationship among the output power, slab thickness and air-gap width is also fully discussed. Furthermore, undercut straight waveguides based on SOI platform are fabricated for propagation loss evaluation. Ways to reduce the loss are discussed either.
© 2011 OSA
Silicon photonics has raised intensive interests in recent years, as well as silicon based optical interconnection. Taking advantages of the silicon-on-insulator (SOI) material and mature complementary metal-oxide-semiconductor (CMOS) technology, high volume manufacturability becomes possible. However, research on silicon based photonic integrated circuits (PICs) has primarily been devoted to operation within the near-infrared (NIR) (1.4-1.6μm) wavelengths, especially at 1.55μm telecommunication waveband. Recently, waveguide components for the mid-infrared (MIR) and long-wave-infrared (LWIR) wavelength (3-100μm) have been considered and proven to be meaningful for many applications, such as sensing, imaging, signal-processing, missile detection and free-space communication .
Low loss LWIR waveguides could be attainable due to the relatively low silicon absorption loss from 1.2 to 8μm and 24-100μm [1,2], which has triggered the research and exploration in longer wavelength region. Moreover, the free carrier plasma dispersion effect becomes much stronger at MIR and LWIR than at 1.55μm wavelength , which means silicon modulators utilizing carrier accumulation or depletion can be much more efficient and mode attenuation based on free carrier absorption can be decreased since lower doping level is required. And the two photon absorption is obviously reduced at the wavelengths beyond 1.55μm which reduces intensity-dependent loss [3–7]. In addition, optical fibers at mid–long-wave infrared wavelength are available during the past several years . Finally, the dimension tolerances for LWIR waveguide components would be more relaxed than for those in communication waveband, which deceases the technical requirements .All of these will bring benefits to the silicon waveguide devices and broaden the application area of silicon photonics into the LWIR region. Applications for biological and chemical detection in mid–long-wave infrared, make the waveguide components own a bright future, since the fundamental vibration of various important gases is in the wavelength of 3-14μm, such as CO2, NH3, and SF6 .
CMOS technology could apply to silicon LWIR waveguide components undoubtedly . To realize the low loss propagation, several novel waveguide structures are proposed, such as undercut SOI waveguides [1,8,11], freestanding waveguides , Hollow-core  and heterostructure waveguides, such as, Ge/silicon  and silicon/sapphire . The silicon on sapphire waveguide with loss of 4.0 ± 0.7 dB /cm is achieved at 5.5μm , and photonic crystal cavities in 4.4μm are also reported in the latest paper .
The popular SOI platform has not been considered as a qualified candidate because the absorption loss of silica is highly extensive in most of LWIR waveband. Take 10.6μm as an example which is the wavelength of applied CO2 gas laser in the following experiment, the absorption losses of SiO2 is ~3000 dB/cm. Furthermore, considering the dispersion effect, the refractive index of silicon and SiO2 is 3.42 and 2.52, respectively , which are different from large refractive index difference (Δn ~ 2) in the 1.55μm wavelength. As a result, the buried silicon oxide (BOX) which is usually 500nm-2μm thick cannot confine the light mode at 10.6μm, which induces strong mode leaking in the substrate silicon. Therefore, utilizing the undercut structure on SOI must etch away the oxide or suspend the rib over an air-filled cavity which comprises the lower cladding.
In this paper, first silicon wafer (100) is adopted rather than SOI so that the manufacturing process can be simplified and the additional loss of SiO2 would be avoided. We adopt the idea of undercut structure proposed by Soref  and fabricate an undercut based 1 × 2 MMI splitter device with a quite large area of air-gap under the silicon slab. The techniques required to fabricate the above undercut structure are mainly thermal oxidation, photolithography and wet etching which is much easier than those for freestanding, heterostructure and hollow-core. The whole procedure avoids utilization of state-of-art apparatus like MOCVD and bonding machine. The details of experimental technique are described in Section3, followed by the experimental set-up and results. At last, the undercut straight waveguides based on SOI platform is also fabricated in order to evaluate its propagation loss and to prove available commercial SOI is not a candidate for LWIR waveguide devices in the end of Section 4.
According to the single-mode condition proposed by R. A. Soref , the waveguides all ensure monomode propagation in the following simulation and experiments. An air gap is etched to achieve an undercut structure for light confinement within LWIR wavelength region on silicon wafer. Figure 1 (a) is the schematic cross section of an undercut waveguide where Wair is the width of the air gap. Figure 1 (b) shows the fundamental TE mode profile of the undercut silicon waveguide by applying finite difference (FD) solution. It illustrates that the undercut based silicon waveguide can confine the optical mode well at 10.6μm wavelength by choosing proper parameters. Rib is 4μm height and 5μm width. Slab is 5μm thick and air gap is 155μm wide, as shown in Fig. 1 (b). These parameters are chosen to ensure efficient coupling from the applied ZnSe lens since the width of its oval shape facula is 23μm which is also similar to the dimension of waveguide optical mode. In addition, polarization-insensitive light propagation can be obtained in large scale waveguides because polarization maintaining fibers for LWIR region are not yet available. The existing evanescent field can be further utilized for sensing. Therefore, undercut structure can be a good choice for waveguide devices at 10.6μm wavelength.
Wair and slab thickness are determined parameters for LWIR waveguide design. During the simulation, beam propagation method (BPM) is utilized. The rib is chosen as 4 × 5μm as illustrated in Fig. 1 (a) and propagation length is 1000μm. Relationship among Wair, Slab thickness and normalized output power which is confined in the rib and slab region is shown in Fig. 2 . If the slab thickness is fixed, take 5μm which is adopted in Fig. 1 (b) as an example, when the air gap is wider than 90μm, over 96% power is confined for 1000μm propagation. The normalized output power decreases with slab thickness increasing. The loss is mainly due to the optical mode spreading to the slab as the slab thickness grows or Wair narrows, and then the optical power partially leaks to the substrate at the two sides of the air gap. Besides, according to Fig. 2, the thinner the slab is, the narrower the air-gap is needed to obtain the same normalized output power due to better light confinement.
3 Device fabrication
An undercut based LWIR 1 × 2 (MMI) splitter is designed and fabricated to verify the analysis above. The input and output waveguides are 4 × 5μm rib waveguides, the width and length of MMI area are 200μm and 6552μm, respectively. The S bends with 133cm radius are necessary to separate output port 1 and port 2 to 300μm for measurement and to minimize the bend losses at 10.6μm wavelength. Tapers with 14μm width and 300μm length are positioned at both the input and output of the MMI region for the coupling efficiency enhancement from the 5μm wide input waveguide and optical loss reduction. Therefore the total length of the MMI splitter which includes the input and output waveguides, tapers and MMI area is 15600μm. The BPM simulation of the 1 × 2 MMI splitter is conducted by commercial software RSOFT . Results are given in Fig. 3 . The right hand side of Fig. 3 shows that outputs of the splitter both hold 0.5 normalized power and the S bend does not affect the propagation. Figures 4 (a) and (b) show the input and output tapers of the MMI region of LWIR splitter.
The experimental technique used for LWIR undercut waveguides is introduced as follows. First, the thickness of p(100) silicon wafer is reduced to about 50μm, and then a masking layer of 1μm oxide is formatted by adopting thermal oxidation at 1200 °C. UV lithography is performed to transfer the devices pattern with positive photoresist. Followed by lifting off silicon dioxide which is not covered by photoresist in a solution of HF:NH4F:H2O with a ratio of 3:6:9 in 45°C water bath heating. After that, the photoresist is removed. The sample is then wet etched in a solution of KOH:IPA:H2O with a ratio of 23.3:13.3:63.4 in 60°C water bath heating. When the MMI devices with 4 × 5μm rib waveguides are fabricated, the same procedure is conducted to achieve the air-gap. The air gap pattern is also developed by UV lithography at the back of the silicon wafer. The depth of the air gap is decided by wet etching duration.
The cross section of input waveguides and multimode interference area are shown in Fig. 5 (a) and Fig. 5 (b), respectively. As the wet etching process is utilized, the side wall angle about 35.3°inevitably exists as illustrated. Thus, the widest air gap is 481μm. Figure 5 (b) clearly exhibits the air-gap which is beneath the slab, though the multimode interference area is too wide to be fully displayed. The rib is 5μm wide and 4μm high. The slab is 8μm thick.
4 Experimental setup and results
The measurements are conducted by using the experimental setup shown in Fig. 6 . A 10.6μm CO2 gas laser with tunable output power is applied as light source. Light is end-fire-coupled to the waveguide sample via a ZnSe objective lens and then through another ZnSe objective lens to a LWIR detector (PVM-10.6 from VIGO company) for signal detection. A chopper (Stanford Research SR540) is used to generate square wave signal and supply reference signal to the following lock-in amplifier (Stanford Research SR830) which is applied to detect and amplify small signals and meanwhile to increase the signal-to-noise ratio (SNR). A computer is connected to collect the data.
The voltage signal obtained from the lock-in amplifier without sample under test is 0.069 mV. Then the undercut MMI waveguide sample is put to measure the insertion loss and device uniformity. Two MMI outputs are 0.0036 mV and 0.0033mV, respectively. The uniformity of the MMI splitter fabricated is 0.76 dB. The insertion loss of this system is about 23.0dB. It is a total loss associated with introducing 15600μm long MMI splitter into the system, and therefore includes 7.8dB inherent absorption loss based on absorption properties of silicon at 10.6μm wavelength which is 5dB/cm . From modeling, coupling at each facet contributes 2.6dB. Since there are three ports in 1 × 2 MMI, the total coupling loss is 7.8dB. The other loss may come from Fresnel reflection from the lens to the chip, the quality of the waveguide end face and S bend loss as well.
We also fabricate undercut straight waveguides based on SOI to evaluate the propagation loss. The SOI wafer has 6μm top silicon, 1μm BOX and 450μm silicon substrate. Since 1μm BOX cannot confine the light well at 10.6μm, undercut structure is still required to realize large refractive index difference. Besides, for SOI wafer, wet etching depth of the air-gap could be controlled precisely for taking advantage of etch-stop of the SiO2 layer to eliminate all the silicon substrate. Main difficulty is that the total thickness of top silicon and BOX are about 6μm. Eliminating all the substrate will weaken the waveguide strength. In order to better support the waveguide, 3μm silicon substrate is maintained in our experiment which brings additional loss undoubtedly. The rib is also chosen as 4 × 5μm.
As shown in Fig. 7 , the cut-back method , conceptually the simplest method, is utilized to measure propagation loss of the undercut SOI straight waveguide. The propagation loss is 11 ± 0.7d B/cm at the wavelength of 10.6μm. The loss owes to the SOI inherent properties at LWIR wavelength, including high SiO2 absorption loss and weak light constraint as discussed in the introduction.
To further reduce the propagation loss of SOI based waveguides, it is meaningful to etch away the SiO2 layer as well or to eliminate the oxide locally beneath the rib due to ultra absorption loss of SiO2 . And drawback is that top silicon layer of available commercial SOI is too thin to hold the whole waveguide. Thus, employing the SOI with thicker top silicon layer may be a good way to solve the manufacturing difficulty.
The novel undercut silicon waveguides for long-wave infrared at the wavelength of 10.6μm are proposed and fabricated in this paper. Based on the analysis, the optical mode is well confined in the rib waveguide area when the air-gap is applied as cladding. We have investigated the characteristics of the output power with different air-gap width and slab thickness, introduced the manufacturing process easier to adopt to realize undercut silicon based 1 × 2 MMI splitter device, and measured its insertion loss with 23.0dB under 10.6μm experimental set-up. Employing novel structures or materials is required for LWIR waveguide components. Fabricating undercut waveguide on silicon wafer, as discussed in this paper, is a good way to realize LWIR waveguide devices and reduce the fabrication technique requirements at the same time.
We also fabricate straight waveguides based on SOI platform to evaluate the propagation loss. Although commercial SOI wafers suffer light confinement and high SiO2 absorption loss issues within most LWIR wavelength region, adaptation SOI with thicker top silicon and etching away high loss silica or suspend the rib over an air-filled cavity which comprises the lower cladding can be a better choice in future LWIR devices development. Therefore, with the growing effects on the research of LWIR waveguide components, waveguides with lower loss would be achieved and used for a variety of applications in the near future.
This work is supported by the Natural Basic Research Program of China (No. 2007CB613405).
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