Design Study of Integrated Optical Coherence Tomography Using MEMS Technology

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Department of Applied Physics & Optoelectronics, Shri G. S. Institute of Technology & Science, Indore 452 003 MP India \affiliationDepartment of Applied Physics & Optoelectronics, Shri G. S. Institute of Technology & Science, Indore 452 003 MP India \emailjtandrews@sgsits.ac.in \affiliationSchool of Physics, Laser Bhawan, Devi Ahilya University, Indore 452 017 MP India

AbstractIntegrate chip based optical coherence tomography is an combination of passive optical waveguides optoelectronics devices such as simple optical waveguide, 3dB bidirectional coupler, active optical delay line, source and detector etc. OCT has a multi-function modality and the integrated multi-modality imaging system can readily switch between different imaging modalities, which will make it a powerful diagnostic tool in a clinical environment. In this article we explored the possibility of implementation to miniaturized integrated OCT on a chip.
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jtandrews@sgsits.ac.in

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Integrated OCT, MEMS, Optical MEMS

Introduction

Optical Coherence Tomography (OCT) is becoming a multipurpose modality in area of biomedical imaging as well as a diagnostics tool. OCT offers high resolution, non-contact, non-invasive or minimal invasive and non-destructive medical diagnosis applications at low cost with an option for portability in measurement. The measurement technique behind OCT is based on the principles of low coherence interferometry (Huang 1991). OCT has sufficient special resolution about 1 to 15 \(\mu\)m depending on the light source employed. The reported penetration depth in skin up to 3 mm is achieved depending on the numerical aperture of the focusing lenses. At this resolution, skin appendages and blood vessels can be visualized. Repeatability of measurement at point of study sample or imaging point greatly increased due to the non-destructive capabilities of OCT (Solanki 2013).

On the other hand, to take the OCT to next level of applications the device needed to be downsized without compromizing on resolution and speed of measurements. The optical components could be downsized with micro meter sized optical elements. Micro-electromechanical systems (MEMS) devices have been adopted for OCT applications and OCT utilizing a scanning MEMS device in the the sampling arm of a Michleson interferrometer have been presented (Pan 2001, McCormick 2007). Towards miniaturization of OCT as endoscopic probe based on MEMS micro-mirror technology has been significant role in the development of integrated miniature OCT. A variety of MEMS device based on electro-thermal, electrostatic, electromagnetic and pneumatic actuation mechanisms have been demonstrated in various endoscopic OCT application (Sun 2011). But, In spite of MEMS endoscopic probe, the other ideal active and passive optical components still have to be fabricate for complete OCT on a single chip.

Recent technological advancements in the optoelectronic and MEMS industry have led to a revival of interest in more applications area. Micro-opto-electro-mechanical-systems (MOEMS) devices have inclusion of integrated optoelectronics and MEMS technologies, which are includes several active and passive monolithically or heterogeneously (Hybrid) integrated or 2D/3D micro-machined optical and MEMS components. This new family of devices and systems is generally called Optical MEMS or MOEMS. Further, MOEMS devices take advantage of high integration density, high reliability, high bandwidth, and low cost fabrication for mass production. While in some cases MOEMS technology focuses on the replacement of conventional big devices. Miniaturization of MOEMS base device has been achieved with the use of integrated optics and micro fabrication techniques. Various optical micro machined devices of this type have been demonstrated over the past two decades (Madou 1997). MOEMS is a most rapidly growing area of research and market development with great potential to impact daily life. The basic concept is the miniaturization of combined optical, mechanical, and electronic functions into an integrated assembly. Micro-opto-electro-mechanical-systems (MOEMS) are very new, but, have the potential to be broadly utilized in much area such as telecommunication, military, particularly in display arenas, remote sensing, guided surgery, optical data storage and imaging. The ability to integrate micro-optical elements with movable structures and micro-actuators has opened up many new opportunities for optical and optoelectronic systems. It allows us to manipulate optical beams more effectively than conventional methods, and is scalable to large optical systems. The most active researches have been conducted in the fiber-optic communications industry while such a device is often used in wavelength-division multiplexing (WDM) technology. Another industry looking into spatial light modulator/diffraction devices is the digital display industry. These display industry aims to develop MEMS based micro-array devices capable of modulating the wavelength of projected light, such as the color pixels of a typical projector or DLP display (Kim 1999, Kessel 1998). MEMS are fabricated using the techniques and materials of microelectronics. These technology has opened up many new possibilities and optical functions on chip has been demonstrated for optical and optoelectronic systems (Wu 1997, Tuantranont 2000, Noell 2002). Recently, MEMS based system have received great attention and found numerous applications in biomedical instrumentation and allow the realization on miniature optical system exhibiting low power consumption, small size, high operation speed. A variety of scanners for imaging in OCT have been developed based on MEMS technology and these MEMS mirrors/scanning lenses have previously been employed in bio-optic applications such as confocal microscopy (Kwon 2003) cell sorting (Pei 2004) florescence (Piyawattanametha 2008) and endoscopic OCT (E-OCT) (Pan 2001, McCormick 2007). MEMS based endoscope is the part of the OCT, which is employe with sample are to generate tomography image. This part only part that have been miniaturized and demonstrated with other bulk optical components such as beam splitter, reference arm, focusing elements etc.

OCT systems have been orignally implemented using free-space or fiber optical components. Although systems based on fiber optics allow for the compact, inexpensive and portable instrumentation, the final size and price of the devices are still not adequate for many applications. OCT systems are composed of many bulk optical components, such as: optical fibers, (focusing) lenses, beam splitters or directional coupler (DC), delay line and beam delivery system as scanners etc., that all are necessary component to relay the optical signal through the OCT system. The combination of all these components makes OCT systems bulky, expensive, and complex. Integrated optics can integrate these bulk components that make an OCT system onto a single optical chip. It is expected that much more economic, batch fabrication and compact devices can be obtained by means optical MEMS technology which is employed earlier. Therefore, integrated optics can make OCT systems robust, portable and repeatability in experiment. Integrated OCT is inclusion of many active and passive waveguide components such as waveguide, waveguide 3dB couplers, optical modulators, reflectors, focusing elements, sources and detectors etc. Thus, we have reviewed in these section only active and passive components for OCT which was earlier integrated.

Optical Elements for OCT

Towards the integration of OCT various Low-loss and compact optical elements were designed, studied and fabricated specially for optical coherence tomography applications. An elliptical couplers were created for focusing/collimating beams in the off-chip region (Nguyen 2010). As a focusing element SiON based integrated elliptic couplers was fabricate and applied them to Fizeau-based spectral-domain low-coherence depth ranging. Because of it has good focusing property in plane direction for biological tissue. Optical delay line most important functional components of OCT that provides optical delay at the reference arm to get interference signal with the backscattered optical signals at sample arm and it also attends fast scanning speed and scan depth. Fiber optics delay lines have already been demonstrated (Choi 2005). Compact Silicon-Based Integrated Optic Time Delays line has been fabricated (Yegnanarayanan 1997) and demonstrated optical time delay, in which they used silicon-on-insulator (SOI) technology with an incremental time delay of 12.3 ps. Guided- wave optical delay lines provide the required precision and are more compact than optical fibres. A novel broadband motion free micro delay generator was performed (Alameh 2005), in which they used two Opto-VLSI processors operating in the steering mode to generate variable true time delays suitable for high resolution OCT imaging. They demonstrated a maximum delay of 0.5 mm and measured 3dB bandwidth of more than 55 nm a center wavelength 1550 nm. E. Margallo-Balbas at al. (Margallo-Balbas 2010) realized fast scanning delay line based on thermo-optics of silicon on silicon-on-insulator platform at wavelength 1.3 \(\mu\)m. Fabricated optical delay line performed line scans rates of 10 kHz and demonstrated a scan range of 0.95 mm. TD-OCT system with thermo optic resolution was measured 15 \(\mu\)m throughout the full scan range. Sources and detectors are integral part of any optical waveguide devices and direct coupling, decoupling of light in integrated waveguide is very difficult task. It may have optical losses with many of complex coupling mechanism processes. In this manner perfect optical alignment of the laser and the waveguide subsystem is ensured. Researchers have also been made significant progress towards integration of laser source and detectors on chip either on silicon-on-insulator (SOI) platform or hybrid integration approaches. However, MEMS based optoelectronic sources—due to their feasible ease of use especially in regard to production processes—make them highly attractive for integrated, low cost and niche lab-on-chip applications.

Of late, the demand of integrated laser source is growing up and cause of the intense increase in the use of light sources in lab-on-chip measurement systems. The first time integration of laser emission and photodetection was demonstrated by Roelkens at al (al 2006) with heterogeneous epitaxial growth of InP/InGaAsP on developed silicon-on-insulator (SOI) platform. Integrated light sources formed on SOI substrates have been reported (Liu 2010, Sysak 2007, Fujioka 2010) as well. However, because high power operation of the light source is required and the laser diode (LD) mounting technology is well-developed in silica waveguide-based platforms (Takeuchi 2009), a hybrid integrated light source with an LD Array (Takanori 2011) was mounted on silicon optical waveguide platform. Grivas at al (grivas 2004) demonstrated the integrated as well as compact fluorescence devices based on Ti:sapphire are particularly interesting for applications in biological imaging for optical coherence tomography (OCT) due to their emission in the near infrared, which ensures an adequate penetration depth of the light in the tissue. Other attributes are the improved longitudinal resolution as a result of their large spectral bandwidth and hence short coherence length, as well as the potential for detection sensitivity and imaging of weakly backscattering structures in tissues due to their high irradiance and wide dynamic range. The fluorescence output power was in the order of 300 \(\mu\)W when the structures were pumped by a 3 W multiline argon laser. These characteristics make the rib structures suitable as light sources for optical coherence tomography applications. They also represented (Bourquin 2005) light source for parallel OCT based on multiple waveguides written in Ti:sapphire. They have parallelized such a Ti:sapphire channel-waveguide emitter by structuring parallel rib into a planar waveguide and pumping theses ribs simultaneously by creating an Ar+ laser light focus and coupling this pump light through a cylindrical lens and a cylindrical microlens array, thus creating broadband luminescence output in parallel channels. It could generate fluorescence centred at 772 nm with a bandwidth of 174 nm. System resolution of 1.9um is achieved per channel, with an optical power of around 30 \(\mu\)W on sample per waveguide. In additionally, integrated optics can enhance the performance of OCT that would be too difficult or complex to achieve with bulk optics, for example, parallelization (Bourquin 2001) of OCT devices on a chip. They demonstrated simple way to improve the frame rate of such an imaging system is to use a parallel detection scheme. This approach allows one to remove the transverse scanner used in preferred OCT setups and to acquire a complete image. They used two dimensional smart detector arrays that performed simultaneous heterodyne detection at all pixels in parallel. This approach allowed us to acquired 3D images with more than 100,000 voxels at sensitivity of -58 dB and at a rate of 6 Hz. Furthermore, Tilma at al. (Tilma 2012) presented the design and characterization of a monolithically integrated tunable laser containing quantum-dot amplifiers, phase modulators, and passive components for optical coherence tomography in medicine at 1700 \(\mu\)m Wavelength Region. They realized electro-optical tuning capabilities over 60 nm between 1685 and 1745 nm, which is the largest tuning range found for an arrayed waveguide grating controlled tunable laser. It performed that the tunable laser has a 0.11 nm effective linewidth and an approximately 0.1 mW output power. But output power is not sufficient at detection side at detector arms. Andrews at al. (Andrews 2012) also proposed MEOMS based OCT for the Non-invasively blood glucose monitoring device and simulated all the passive waveguide components as well as active component such as electro-optics modulator for reference arm with single chip. It has been also designed, simulated and fabricated a MOEMS based optical delay line towards optical coherence tomography on a chip (Choudhary 2014).

Waveguide technology has been proven that the low loss waveguide structure can be easily fabricated with either silicon on insulator waveguide technology or polymer waveguide technology. Interferometer is heart of OCT system, which give interference signal that contain depth information of the biological tissue. Towards the integration of interferometer for OCT, Culemann at al. (Culemann 2000), first time fabricated an integrated eight Michelson interferometer including all reference arms in one small chip in glass with ion-exchanged low-index contrast glass for time-domain OCT system, with all other components external to the optical chip. Each interferometer has its own two SLD sources. The couplers for the combination are integrated on the glass chip, too. The obtained FWHM of the combined light source is approximately 100 nm and thus the longitudinal resolution is 7.4 \(\mu\)m in air and 5.2 \(\mu\)m in skin tissue, respectively. With an appropriate lens system the lateral resolution in the tissue probe can be measured up to 4 \(\mu\)m. Yurtsever et al. (Yurtsever 2010) demonstrated silicon-on-insulator based Michelson interferometer for FD-OCT as well as swept-source OCT system. Only the waveguide splitter and the reference arms have been implemented using integrated optics, the source and detectors were coupled with optical fibers externally. The size of the interferometer is 1500 \(\mu\)m x 50 \(\mu\)m with 40 \(\mu\)m of axial resolution and a sensitivity of 25 dB. Recently, they also demonstrated passive optical components as y-splitter and 190 mm long reference arm for Mach-Zahnder interferometer with a foot-print of only \(10\times 33\) mm\({}^{2}\) (Yurtsever). It is employed for SD-OCT and found 8dB sensitivity of system. Axial resolution, sensitivity and bandwidth of system insufficient for imaging with variety of spectral band.

Integrated waveguide structure with bidirectional coupler was demonstrated for swept-source optical coherence tomography (SS-OCT) and fabricate through TriPleX technology (Nguyen 2012). Chip has a footprint of 0.4 cm \(\times\) 1.8 cm with external fiber based Michelson interferometer to couple light on it; the lateral resolution is 21\(\pm 1\mu\)m, maximum depth of 5.09 mm and sensitivity of −80 dB was taken in account. Fiber optics Michelson interferometer externally used and feasibility of integrated-optics-based SS-OCT is also demonstrated with the tissue phantom. Arrayed-waveguide grating (AWG) is the component that avoid the moving reference arm in FD-OCT and the reference arm length is fixed at a position approximately corresponding to the position of the sample. AWG as a spectrometer have been implemented for spectral domain optical coherence tomography (SD-OCT) (Nguyen 2011) in silicon oxynitride. The AWG has a footprint of only 3.0 cm \(\times\) 2.5 cm, that operates at a center wavelength of 1300 nm, and has 78 nm free spectral range. OCT measurements are performed with multi-layered tissue phantom that demonstrate imaging up to a maximum depth of 1 mm with an axial resolution of 19 \(\mu\)m. Moreover, in another experiment pair of AWG spectrometer fabricated at 0.4 nm and 0.16 nm channel spacing with the silicon oxynitride technology. Axial resolution was found 25 \(\mu\)m and 20 \(\mu\)m in air for at cantered wavelength 800 nm and 1300 nm ranges, respectively (B. I. Akca 2012). However, this system has the disadvantage that it uses optical amplifiers, is extremely costly, and has a high complexity. In another recent study, I. B. Akca et al. demonstrated adiabatic 3dB beam splitter and AWG spectrometer on chip for SS-OCT in SiON (Akca 2013). Whereas, the bandwidth of the adiabatic 3dB beam splitter is showing weakly wavelength and polarization dependence over a range larger than 150 nm. Both of these properties may degrade performance of the device. Therefore, the design of integrated OCT always will have to consider various parameter such as resolution, sensitivity, depth of focus etc, these are all parameters can be enhanced using large-bandwidth of source, low coherence length, Power (high irradiance) of the light source and device bandwidth. Device bandwidth is a important parameter that influences performance of system and other OCT parameters.

Over the years many attempts have been made for fabricating various optical elements required for an optical coherence tomography. Looking into these facts and the need for an miniaturised optical coherence tomography integrating all the optical components of the OCT into a single chip similar to a Lab-on-chip technology. We have simulated and analysed the possibility of fabricating all optical integrated optical coherence tomography setup having footsize of 3mm \(\times\) 3mm. The design parameter, simulation results, process etc are discussed in the following sections.