# Ten Years of Gabor-Domain Optical Coherence Microscopy
## 3 Gabor-Domain Optical Coherence Microscopy  
Gabor-domain optical coherence microscopy (GDOCM) is a high transverse resolution variant of spectral domain optical coherence tomography (SD-OCT) [20]. A schematic of the GDOCM system is shown in Figure 3.  
In GDOCM, the choice of wavelength range in the near infrared was made prioritizing lateral resolution over imaging depth and penetration into tissue. The light source is a superluminescent diode with a center wavelength of 840 nm and 100 nm bandwidth, yielding axial resolution of 3.1 \(\upmu\)m in air and 2 \(\upmu\)m in tissue (at a refractive index of 1.3). A 50:50 fiber coupler is used for the interferometer. The reference arm performs optical path length matching and dispersion compensation by incorporating a dispersive element [21]. Polarization controllers are used to maximize fringe interference by matching the polarization of the light in the two arms of the interferometer. Optionally, the dispersion and polarization adjustments can be performed to compensate changes in dispersion and polarization introduced by the sample itself. The spectral interference signal is acquired at a line rate of 80 kHz,  
Figure 3: Schematic of a Gabor-domain optical coherence microscopy (GDOCM) microscope consisting of a fiber-based Michelson interferometer. CMOS: Complementary metal oxide semiconductor; MEMS: Microelectromechanical systems. with imaging depth in the sample greater than 2 mm with a custom Czerny-Turner spectrometer, which includes a reflective dispersive element and a line camera with CMOS (complementary metal oxide semiconductor) sensor. The spectrometer design eliminates coma by incorporating two off-axis spherical mirrors, and a cylindrical lens is used to compensate astigmatism in the beam introduced by the off-axis mirrors [22]. A 2 \(\upmu\)m transverse resolution is achieved over a field of view of \(2\times 2\) mm in the GDOCM microscope design with a numerical aperture of 0.2, with experimental depth of focus of \(\sim\)100 \(\upmu\)m [23]. A compact dual-axis MEMS (microelectromechanical system) mirror integrated in the microscope is used to scan the beam over the 2D field of view [24]. The microscope can be operated in two modalities, to be selected for the desired application: In contact with the sample (with gel medium to create optical contact between the distal surface of the microscope and the sample), or with a working distance of 15 mm. The contact imaging modality may be more advantageous to maximize signal collection in highly scattering tissues such as skin, while the 15 mm working distance is preferred for ophthalmic applications, or for imaging areas that are more difficult to reach with a contact probe, such as certain locations on the face. In order to obtain micron-resolution imaging over the entire volume, multiple volumetric images of the sample (termed zones) are acquired, each corresponding to a different focusing depth. A good rule of thumb for the number of zones to be acquired is to consider the ratio between total sample depth and the depth of focus; for example, given the depth of focus of \(\sim\)100 \(\upmu\)m, for a total sample depth of 600 \(\upmu\)m, six zones are required. A liquid lens integrated in the optical design of the microscope achieves dynamic refocusing with no moving parts over a 2 mm range [25]. The in-focus portions of each volumetric zone are extracted and merged together to produce a high-definition volume with invariant 2 \(\upmu\)m resolution, both axially and transversally. The process of combining together the in-focus portions of the zones is referred to as fusing. Fusing can be performed either in the spatial or spectral domains [26; 27]. As example of a GDOCM image of a human fingertip acquired with three zones and the corresponding fusing procedure is shown in Figure 4.  
Figure 4: Example of GDOCM image acquisition and fusing of a human fingertip. The field of view is \(1\times 1\) mm.