Probe design can be varied by the selection and arrangement of components, such as DCPCF, gradient-index (GRIN) lens, MEMS mirror, etc. shows three different designs that we have investigated. Our objective is to compare the advantages and disadvantages based on the fundamentals of the different configurations. Sample images are obtained and demonstrated for each design. Information about the effective NA, field of view, and resolution are further extracted from the design configurations and the images to show the difference in performance.
Fig. 5 Three optical designs of the MPM probe and the corresponding images acquired with the designs. Design I: A GRIN lens is located before the MEMS mirror. Design II: A GRIN lens is located after the MEMS mirror. Design III: A GRIN lens and an aspheric lens (more ...)
Design I is the simplest case that could be realized for an endoscopic probe. This is a very similar concept with our previous 3-D endoscopic optical coherence tomography probes.19
It has the advantage of relatively easy alignment and packaging as well as size efficiency. However, it requires a lens with a long working distance in order for the beam to escape from the packaging and reach the sample. The drawback of design I is the long working distance between the GRIN lens and the sample, which usually produces poor resolution at the focal point.
A GRIN lens is characterized by its parabolic radial refractive index profile
, where n0
is the refractive index at the lens axis, and g
is the gradient parameter. Light is gradually bent toward the axis because of the gradient of the refractive index profile. Similar to a step-index fiber, the NA of GRIN lens is defined as
, where nR
is the refractive index at the margin of the profile. However, the acceptance angle θ
of a GRIN lens is determined by its effective NA, which varies sensitively with the gap between the lens and the object or image such as23
is the radius of the GRIN lens, and d
is the gap between the lens and the object or image. For the GRIN lens used in design I (0.29 pitch, NA
=0.9 mm), the variation of NAeff
with respect to d
is shown in . For d
=5 mm, the NAeff
is only 0.17 in design I. With this low NAeff
, the MPM imaging resolution is low as well as the excitation and collection efficiency. A typical image acquired with design I shows 20 μ
m beads in , where the image resolution is ~5 μ
The variation of NAeff with respect to the space between the GRIN lens and its focal spot.
In design II, the GRIN lens (0.23 pitch, NA =0.64, a =0.9 mm) is located after the MEMS mirror. Thus, the GRIN lens can be very close to the sample and the working distance can be very short. With a working distance of 0.21 mm, the NAeff is 0.54. Thus, the short working distance required over-comes the resolution problem of design I. With design II, we also imaged 20-μm beads, and the resolution is improved to ~2 μm. However, there are several problems associated with design II. With a single lens, the distances between fiber to GRIN lens and between GRIN lens to sample are restricted. Therefore, there is a lack of flexibility to adjust those distances in order to fit in the MEMS scanner. The light shining on the MEMS mirror and GRIN lens is a diverged beam, and the GRIN lens has a limited diameter of ~1.8 mm. Thus, the scannable field of view that can be achieved by the MEMS mirror and the GRIN lens is largely limited. The distance between fiber and GRIN lens also affects the beam diameter that can be achieved at the back aperture of the GRIN lens based on the propagation of a Gaussian beam. Therefore, design II is difficult to optimize and its field of view is small—in this case, ~100 μm.
In design III, light from the DCPCF is collimated with a GRIN lens (0.22 pitch, NA =0.6, a =0.9 mm). The collimated beam is reflected perpendicular to the propagation path of the lens by the two-axis MEMS mirror, which is positioned at 45 deg. The MEMS mirror further scans the laser beam in two axes, nominally in a raster scanning motion in a point-topoint manner. However, any desired scan pattern may be implemented, and either point-to-point scanning with adjustable dwell time or constant velocity scanning may be selected. An aspheric microlens (NA =0.62, a =2.5 mm, focal length =4.03 mm) then focuses the laser beam into a tight spot onto tissue samples. The span between GRIN lens and DCPCF is adjusted for beam collimation, which prohibits the divergence of incident light on the MEMS mirror. The focusing lens has the same length as the GRIN lens in design II but a larger diameter. The collimated beam and the large diameter of the focusing lens can significantly increase the imaging field of view. Because the beam is collimated between the GRIN lens and the aspherical lens, the span between them can be conveniently varied to fit in the MEMS mirror without changing the beam property. Thus, the probe design has the flexibility to be optimized independently at the MEMS mirror and the sample locations, respectively. The improvement is shown in , where the resolution is ~2 μm and the field of view is ~200 μm.
As design III has the optimum combination of resolution and field of view, a handheld probe is packaged based on this design.16
In assembling the probe, the GRIN lens is first assembled with the DCPCF to provide a collimated beam. The pigtailed GRIN lens is then mounted on one side of a custommade alignment bench, and the MEMS mirror is mounted on a 45 deg platform located on the other side of the alignment bench. The assembled MEMS mirror, GRIN lens, and DCPCF are then inserted into an aluminum housing, and last, the focusing lens is mounted onto the housing and aligned at the center of the MEMS mirror. Pictures of the assembled MEMS mirror and packaged probe are shown in . The probe is 1 cm in outer diameter and 14 cm in length. The size is mainly limited by the mechanical housing that is used to hold the MEMS mirror and the focusing lens. The total outer diameter of the probe can be reduced to ~5 mm diameter by using improved machining of the housing and a smaller diameter focusing lens.
Schematic of the endoscopic MPM system using a two-axis MEMS scanner. (a) Cross section of the DCPCF. (b) MEMS mirror assembled with the DCPCF and GRIN lens. (c) Packaged MPM probe.