3D in vivo optical imaging on a mouse has been obtained using a 2D MEMS mirror for lateral scanning in a time-domain optical coherence tomography (OCT) system. The MEMS mirror aperture size is 1 × 1 mm2, and the device footprint is 2 × 2 mm2. The MEMS mirror scans ± 30° optical angles about both x and y-axis at only 5.5V DC voltage. An endoscopic probe with an outer diameter of 5.8 mm has been designed, manufactured and packaged. The probe scans an average transverse area of 2 mm × 2 mm. The imaging speed of the probe is about 2.5 frames per second, limited by the speed of the employed optical delay line.
© 2010 OSA
Optical coherence tomography (OCT) is an emerging optical imaging technique that is capable of obtaining real-time, in vivo, high-resolution cross-sectional information of biological tissues . By employing a broadband light source and a Michelson interferometer, OCT can achieve resolutions in the range of 1 to 15 µm and penetration depths of 1 to 3 mm in highly-scattered tissues. Researchers have applied OCT extensively in external imaging, such as ophthalmology [1–4] and dermatology [5–8]. The capability of OCT for internal organ imaging has also been demonstrated [9–17]. However, in vivo endoscopic OCT (E-OCT) imaging is very challenging since fast optical scanning must be realized inside a very small imaging probe. In an E-OCT imaging probe, the most critical part is the transverse light beam scanning. In 1990’s, some simple but slow transverse scanning mechanisms have been used in catheter-based OCT systems, such as rotating a fiber micro-prism module at the proximal end  or swinging the distal fiber tip by a galvanometric plate . In order to increase scanning speed, a cantilever fiber may be excited to its resonance by a piezoelectric tube . However, swinging the distal fiber tip may cause significant coupling non-uniformity especially at large scan range. A pair of scanning GRIN lenses with proper angle cut has been proposed to increase the coupling efficiency , but further miniaturization of the probe is difficult because two motors are required for this probe configuration.
On the other hand, microelectromechanical systems (MEMS) have emerged as a viable and mature technology field creating various microactuators and microsensors which are widely used in automotive and consumer electronics such as portable projectors, cell phones and video game controllers. The main advantages of MEMS technology include low cost, small size and fast speed, which are exactly what E-OCT probes need. In 2001, the first MEMS-based E-OCT probe was reported . Since then, various MEMS E-OCT probes employing either MEMS micromirrors or micromotors have been demonstrated [18–29]. Micromotors are used to achieve 360° circumferential scanning [20–23], but micromotor-based probes can only perform side-view imaging.
Micromirrors are more versatile and cost-effective. There are several types of micromirrors: electrostatic, electrothermal and electromagnetic. Electrostatic micromirrors are most popular mainly because of their fast speed and low power consumption, but they need high driving voltages, which are typically on the order of 100V; posing a potential risk to the patient. For example, Jung et al. reported a 4mm-diameter E-OCT probe based on an electrostatic micromirror which has a resonant frequency of about 2 kHz and the scanning optical angles are 20° for each axis at 100 Vdc . Electromagnetic micromirrors can achieve large scan angle at low drive voltage. For example, Kim et al. reported a 2.8mm-diameter E-OCT probe  based on an electromagnetic micromirror which has a maximum scan angle of ± 20° at only 3 Vdc, but the power consumption is as high as 150mW and the assembling process is relatively complex due to the permanent magnet required. Electromagnetic interference is also a concern especially when there are high-power systems nearby.
Another common problem for electrostatic and electromagnetic micromirrors is the small fill factor which is typically less than 5%. The fill factors of our micromirrors are typically 25%, compared to 10% or less for other MEMS mirrors. Fill-factor is the ratio between the mirror aperture size and the device footprint, so for the same effective mirror aperture size, our device footprint is much smaller, which makes further miniaturization of the probe possible. One solution to the small fill factor problem is to use hidden actuators [30,31], but the processes for the formation of the mirror tops are complicated and not reliable. Another solution is to use electrothermal bimorph actuators [32–37]. Thermal bimorph-based 2-axis micromirrors have relatively high (15-35%) fill factors and they can achieve large scan range at low driving voltage (usually less than 10V). These features make the thermal bimorph-based MEMS mirrors very suitable for miniaturizing OCT probes for endoscopic applications. A comparison of different actuation mechanisms is given in Table 1 . The large range, high fill factor and low drive voltage make the electrothermal actuation very attractive for being used for miniaturizing OCT probes for endoscopic imaging applications.
Our reported E-OCT probes are all based on our 1D electrothermal micromirrors . Those 1D electrothermal micromirrors have large initial tilt angle as well as large shift of the rotation center during scanning, which makes the optical alignment difficult and complicates optical design and mechanical assembly. In this paper, we propose to use a novel 2D electrothermal MEMS mirror. Compared to the 2D electrothermal MEMS mirror reported in , the basic bimorph designs are similar; the improvement of the MEMS mirror used in this work lies in several aspects: first, there is no lateral shift for the mirror center when scanning, so the effective mirror aperture size does not change during operation; second, with a more compact actuator design, the fill factor of the mirror increased; third, the MEMS mirror in  can only generate tip-tilt motion, while this newer version can realize both tip-tilt motion and piston motion, which can be applied to more applications, such as confocal imaging.
This 2D mirror has zero initial tilt and the rotational center can be fixed. A prototype 5.8 mm-diameter E-OCT probe has been designed and built with the 2D MEMS mirror installed. 3D in vivo images of a mouse tongue and ear have been successfully obtained with this MEMS probe. In the following, first the 2D MEMS mirror design and fabrication are introduced. Then the E-OCT probe design and manufacturing are described. Finally the OCT imaging results are presented.
2. Two-dimensional scanning MEMS mirror
A bimorph beam is a cantilever which consists of two layers of different materials. Thermal bimorph actuation is based on the thermal expansion coefficient difference of the two materials composing the bimorph beam. Both 1D and 2D micromirrors based on thermal bimorphs have been reported [19,32]. Large initial tilt angles of the thermal micromirrors with straight bimorph beams cause complication of optical alignment . Actuators based on folded bimorph beams have been developed to achieve initially-flat mirror plates . For example, the large-vertical-displacement (LVD) design utilizes two folded bimorphs that compensate each other’s tilting, which not only achieves zero initial-tilt angles but also large piston displacement . However the LVD design has large lateral shift and rotational center change during scanning. Its area fill factor is also relatively small due to the need of a gimbal for 2D scan.
In this work, a new 2D thermal bimorph micromirror is developed. First of all, it is a gimbal-less design. As shown in the schematic in Fig. 1(a) , the mirror plate is suspended by four actuators on its four sides. There is no gimbal. This arrangement significantly increases the area fill factor, and it is enabled by a unique actuator design, namely lateral-shift-free (LSF) LVD design . As shown in Fig. 1(b), each LSF-LVD actuator consists of three Al/SiO2 bimorph beams connected in series with a Pt heater embedded for electrothermal actuation and two rigid silicon-supported frames. In order to compensate the lateral shift and tilting of the vertical displacement, the lengths of the five components must be properly designed and it has been found that the following relations need to be satisfied:
A scanning electron micrograph (SEM) of such a 2D MEMS mirror is shown in Fig. 2 . The two bimorph materials are SiO2 and aluminum. Aluminum is also coated on the surface of the mirror plate for high reflectance. The size of the mirror chip is 2mm × 2mm, and the size of the mirror plate is 1mm × 1mm, resulting in a 25% fill factor. The mirror plate is suspended by four identical electrothermal actuators on four sides of the mirror plate. Each actuator is connected to a bonding pad, so voltage can be added to control the movement of the mirror plate. A fifth bonding pad is used for the common ground. A Pt heater is embedded in each of the actuators to provide heating power.
The process flow of fabricating this MEMS mirror is shown in Fig. 3 . A simple surface and bulk-combined micromachining process based on SOI wafer is used . Figures 3(a)–(e) show the multilayer thin-film deposition and etching steps to form the patterns for bimorphs and frames and the reflective coating of the mirror region. A 40-μm thick silicon layer is used for the rigid frames and mirror plates. A SiO2 reactive-ion-etch (RIE) and a silicon deep-reactive-ion-etch (DRIE) step are then performed from the backside to etch the bulk silicon of the SOI handle layer. The uniformity of the silicon underneath the frames and mirror plates is guaranteed by the buried SiO2 layer as an etch stop in the DRIE. The buried SiO2 layer is then etched and the SOI device layer is exposed (Fig. 3(f)). After that, the microstructures are finally released from the front side by another DRIE silicon etch followed by an isotropic silicon etch (Figs. 3(g–h)).
After the device is released, the residual stress will cause the bimorph beams to bend, giving a large initial elevation of more than 600µm to the mirror plate (Fig. 2). This large elevation provides enough room for the mirror plate to scan large angles and move in large vertical displacement. Upon actuation, voltages will be applied to the Pt heaters, and the Joule effect will heat up the bimorph beams. Since Al has much higher thermal expansion coefficient than SiO2, the bimorph beams will bend downward. When same voltages are applied to all four actuators, piston motion can be realized. When differential voltages are applied on opposite actuators, two-axis scanning can be realized. The rotation axes are along the center, so there is no lateral shift during scanning even at large scanning angles. Thus during scanning, the effective mirror aperture size will not change, greatly simplifying the optical alignment and improving mechanical design flexibility as well.
Figures 4(a) and 4(b) show the measured characteristics of the MEMS mirror. As we can see from Fig. 4(a), the piston displacement response is linear between 1.5V and 4.5V, and more than 600µm displacement can be achieved with only 5.5V voltage. Figure 4(b) shows that ± 31° optical scan angles can be obtained at a maximum 5.5V, of which ± (5~31°) optical scanning ranges are linear. As shown in Fig. 2(c), the average deviation of the measured results from the linear fit is only about 0.83% for a large optical scan range up to 26°. Figure 2(d) depicts the relationship between current consumption and optical scan angle; the largest current is about 15.5mA, it corresponds to the maximum optical angle of 31°. The frequency response of mirror is shown in Fig. 4(e). The mirror has a 3dB cutoff frequency of 13Hz. Figures 4(f) and 4(g) show some scanning patterns of the MEMS mirror. The raster scan pattern is obtained by differentially driving two opposite actuators with ramp waveforms of 0.5~3.8V at 320 Hz (~30°) and 0.5~3.8 V at 8 Hz (~36°) for the two orthogonal directions, respectively. The Lissajous pattern is obtained by shifting the fast axis frequency from 320 Hz to 300 Hz.
3. Design and manufacturing of the MEMS-based OCT probe
A miniature OCT imaging probe has been designed based on the 2mm × 2mm MEMS mirror. As shown in Fig. 5 , it is a side-viewing probe, and the probe head is composed of a mount base, an optical fiber, a graded index (GRIN) lens, and a MEMS mirror. The material for the mount base is stainless steel. The optical fiber is a single mode fiber (corning SMF-28) which delivers the light. The fiber tip is cut with an 8° angle to minimize back-reflection. The light coming out from the fiber is focused by a GRIN lens. Optical UV glue is used at the connection between the fiber tip and the GRIN lens to reduce the effect of index mismatch. A 0.5mm-deep cavity is made for the MEMS chip to fit in, and the mirror plate is 45° to the central axis of the probe. Figure 5 shows the diagram of the probe design.
The optical design is shown in Fig. 5(b). The focal length of the GRIN lens is 5mm and the spot size is about 20 µm at the focal plane. The distance between the end of the GRIN lens and the MEMS mirror plate is designed to give the probe a working distance of about 1.5 mm in air. If we assume the refractive index of the sample to be 1.5, because of the refraction at the tissue surface, the infrared light coming out of the probe will be focused about 2 mm beneath the sample surface. The beam is incident on the MEMS mirror plate with a 45° angle when no voltage is applied. When the mirror scans to its maximum optical angle, the incident angle of the beam to the mirror plate will increase to 75°, which results in a larger beam size on the mirror plate. The mirror plate size is 1mm × 1mm to accommodate for the variation of the beam size on the mirror plate during scanning. The insertion loss of the probe is 2.44dB, and the back coupling efficiency is 11.3%.
Electrical connection to the MEMS mirror is provided by wire bonding, soldering and gluing. First, five copper wires are soldered from the back side of a custom-designed printed-circuit board (PCB) to form the connection to the five pads on the front side of the PCB. Then the PCB is glued to the top slot of the probe, followed by wire bonding to the MEMS chip. Five gold wires are bonded to the pads on the MEMS mirror chip. After that, the MEMS chip is placed and glued in the cavity designed for it. The last step is to glue the gold bonding wires to the five pads on the front side of the PCB using silver epoxy. When all the above steps are completed, the probe is inserted into flexible, biocompatible, transparent fluorinated ethylene propylene (FEP) tube. A wooden stick is used to make the probe rigid, and the probe is sealed by super glue from the top and bottom ends. The diameter of the stainless steel base is 5 mm, the length of the base is 12mm; and the outer diameter of the final packaged tube is 5.8 mm.
Although this prototype probe based on the electrothermal MEMS mirror does not have smaller diameter than some existing MEMS probes mentioned in Section 1; its potential for further miniaturization is great because of its high fill factor. MEMS probes with diameters less than 2.5 mm are under development (See Fig. 6 ).
4. OCT system
We use a Time-Domain OCT system in our lab, and the schematic of the system is shown in Fig. 7 . The center wavelength of the employed broadband light source (DenseLight, DL-BX9-CS3159A) is 1310nm, and its FWHM (full width half maximum) is 75nm, resulting in a 10µm axial resolution in air. The input light is divided into the reference arm and the sample arm by a beam splitter. Depth scanning at the reference arm is realized by a rapid scanning optical delay line (RSOD), the scanning range is 0 to 1.6mm depth. The RSOD employs a galvanometer scanning at 1 kHz. We use a 500 kHz carrier frequency for the OCT signal, and it is generated by the small misalignment between the galvanometer center axis and the optical axis. The MEMS based probe is used in the sample arm to perform 2D transverse scanning. The OCT signal is then detected by a balanced photodetector, acquired by a DAQ card, and processed by a computer. The sensitivity of the system is measured to be 53dB. The frame rate of our system is 2.5 frames/s.
5. Experiment and results
The MEMS based probe has been applied for in vivo imaging experiments. The 2D transverse scan is done by simultaneously driving all four actuators with one opposing pair for fast scan and the other pair for slow scan. The actuator pair on the circumferential direction is used for fast-axis scan, driven by 0~4V differential ramp voltages at 1.25Hz; and the actuator pair on the longitudinal direction is used for slow-axis scan, driven by 0.5~3.5V ramp voltages at 0.0125Hz. This generates a transverse area of ~2.3 × 2.3mm2.
The sample used for in vivo imaging was a female athymic (nu/nu) nude mouse with a body weight of 20g to 26g (Harlan Laboratories, Indianapolis, IN). The mouse was anesthetized by injecting ketamine (100mg/kg) and xylazine (10mg/kg) intraperitoneally. Both 2D and 3D images of the mouse tongue have been obtained. The 2D OCT image shown in Fig. 8(a) was acquired using the MEMS OCT probe at the base of the tongue. A stratified squamous keratinized epithelium (SSKE) and lamina propria (LP) corresponding to the relatively strong signal bands from the upper layers of the tissue structure were observed. Figure 8(b) shows a reconstructed 3D image of the same mouse tongue.
2D and 3D OCT images of the mouse ear have also been obtained, and the results are shown in Fig. 9 . The ear thickness is about 500 µm. The dermal structures and the subcutaneously adjacent layers were observed using the MEMS OCT probe in this experiment. The cartilage (C) of the ear was represented in the middle by the dark band and the dense conjunctive capsule (cc) with two bright layers put around the cartilage. The superficial epidermis (E) of the mouse ear was detected at the first and last of the mouse ear longitudinally. The lower and upper areas from the dense conjunctive capsule to epidermis, the dermis (D), were observed. Figure 9(b) shows the 3D reconstruction image of the ear.
A 2-axis electrothermally-actuated MEMS mirror has been fabricated and it has also been successfully installed into a 5.8 mm-diameter prototype endoscopic OCT probe. The unique feature of this MEMS design is that large scan range, high speed and high fill factor and body-safe driving voltage are achieved simultaneously. Both 2D and 3D in vivo endoscopic OCT images using this probe have been demonstrated. The large fill factor of 25% makes it possible to further reduce the probe down to 2.5 mm or even smaller. The imaging speed is only limited by the RSOD of the time-domain OCT system. If the RSOD is fast enough, the MEMS scanner is capable of acquiring real-time images with over 30 frames per second. A frequency-domain OCT system will be also used with the MEMS probe for real time imaging on internal organs, such as GI track, stomach, small intestine, and Esophagus.
This project is supported by the National Science Foundation (NSF) under award#0725598.
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