## 1 Introduction  
Optical coherence tomography (OCT) has become a mainstay in biomedical imaging technology owing to its noninvasive imaging and higher spatial resolution, and has been one of the most rapidly developed optical imaging modalities of the last few decades. At present, the most developed domain is ophthalmology, and its ability to differentiate between layers of the retina has made it a standard diagnostic tool [1; 2; 3]. Various probes have been developed for OCT to extend the range of its applications [4; 5]. Compact, robust, and reliable endoscopic probes and imaging catheters have long been in demand in various fields of biomedical optics. The size of the imaging probe is important in achieving minimally invasive accessibility to deep regions. Most probe implementations can be divided into two groups based on their scan modes: Side-imaging or forward-imaging. In side-imaging probes [6; 7; 8; 9; 10], a deep skeletal muscle structure was imaged using an ultrathin optical needle probe (outer diameter (OD) = 310 \(\upmu\)m) consisting of a no-core fiber and a fiber gradient-index (GRIN) lens, with a lateral resolution of 20 \(\upmu\)m [7]. The structure of an optical probe using fiber GRIN lenses was first proposed in 2002 [10]. Forward-imaging probes are generally more complicated in design and require that their actuators be located at or very near the probe tip. So far, several types of probes have been demonstrated [11; 12; 13; 14; 15; 16]. Probes using a long GRIN lens (OD = 0.74 mm, length = 120 mm) have been reported to obtain OCT images of ex vivo human brains with lateral scanners [17]. Full-field OCT (FF-OCT), which utilizes spectrally modulated and spatially incoherent illumination, has also been demonstrated. The interference image obtained was transferred to a camera through a bundle fiber (OD = 1.93 mm, length = 1.76 m) [18]. These probes are useful in multiple fields of biology and in clinical applications. OCT with a high-numerical-aperture objective lens is often referred to as optical coherence microscopy (OCM) [19]. OCM, a variant of OCT, acquires _en face_ tomographic images, with 3D imaging achieved by acquiring a stack of _en face_ images. One of the OCM approaches is full-field OCM (FF-OCM). FF-OCM uses a temporally and spatially low-coherence light source associated with an area camera to acquire _en face_ images [20]. Many specific studies on FF-OCT and FF-OCM have been reported [21; 22; 23]. We have been studying the application of short multimode fibers (SMMFs) as an imaging fiber GRIN lens in a forward-imaging probe, because of its miniaturization, robustness, reliability, and low cost [24]. It is possible to miniaturize a forward-imaging probe without lateral scanning mechanisms using an area camera. The theoretical and experimental evaluations of imaging characteristics have been reported using an SMMF with a diameter of 125 \(\upmu\)m and a length of 8.8 mm. The dependence of contrast on wavelength was measured using surface images of chicken tendons [25]. By means of an SMMF with a diameter of 140 \(\upmu\)m and a length of 5 mm, we also obtained transmission images using ex vivo rat brains [26]. For designing an SMMF, the modulation transfer function was evaluated using a thin random phase screen model [27]. The mode patterns resulting from the propagation in SMMF and the distribution of depolarizations have been investigated [28]. Using an SMMF with a diameter of 125 \(\upmu\)m and a length of 5.12 mm, FF-OCM has been demonstrated for the first time. 3D images of a dried fish fin were obtained by placing the SMMF in contact with the sample [29]. We have also reported the images obtained by FF-OCM with SMMF of a diameter of 125 \(\upmu\)m and a length of 7.4 mm using ex vivo rat brains [30]. In the field of neurosciences, OCT has also been demonstrated as a promising imaging tool [31]. OCT enables non-invasive visualization of the structures and functionality of the brain, which is valuable for fundamental research and medical diagnosis.