We have presented a new fiber optic SD PS-OCT scheme based on PM fibers and demonstrated its use for UHR imaging of the human retina in vivo. The method transfers the principles of our previously reported bulk optics PS-OCT systems to fiber optics, thereby combining the main advantages of the bulk optics system with the flexibility and simple alignment of fiber optics based systems, an important step towards the development of commercial PS-OCT systems (the presently available commercial OCT systems are based on fiber optics).
Most of the previously reported fiber based PS-OCT systems used standard (non-PM) single mode fibers. These fibers do not maintain the polarization state of transmitted light: they introduce a phase retardation that varies with fiber bending. To overcome this problem, PS-OCT systems based on these fibers typically probe the sample with at least two polarization states, and the additional information gained in that way is used to compensate for the unknown retardation introduced by the fiber. Some of these systems record two polarization states in parallel while others record them successively. The latter approach has the advantage of a simpler setup requiring just one sensor, however, on the downside, a very stable phase relationship between successive A-scans is required.
Our method is based on PM single mode fibers. It records both polarization channels in parallel and requires only a single input polarization state per measurement location to derive reflectivity, retardation, axis orientation, and Stokes vector. This has some advantages compared to methods that require two or more input states or that record the two polarization channels subsequently: (i) the overall imaging speed can be increased; (ii) lateral oversampling is not required (although it still improves overall image quality by an averaging effect); (iii) the method is insensitive to phase shifts between adjacent A-scans that can occur by vibrations, bulk sample motions, or blood flow. Compared to PS-OCT schemes that modulate the polarization state within an A-scan, our method has the advantage that the full depth resolution is maintained (the sampling density is not split between the two polarization channels), and that neither expensive polarization modulators nor complicated triggering schemes are needed. While our system presently uses two separate spectrometer cameras, it can easily be adapted to single camera systems [44
] thereby further reducing costs.
The use of PM fibers for PS-OCT and low coherence interferometry has been reported previously with TD setups [28
]. The main problem that arises with PM fibers is that they just maintain linear polarization states, not arbitrary elliptical states as required with our scheme (elliptical states are decomposed into two linear states with a phase difference dependent on the fiber length). Previous solutions required either splicing of two PM fibers with exactly equal length (which is difficult to achieve), where the slow axis of the first fiber was coupled to the fast axis of the second fiber, and vice versa [46
], or the implementation of an additional pair of compensating birefringent wedges that had to be carefully adjusted to compensate the phase shift introduced by the PM fibers [28
]. Our method avoids these procedures by exploiting a very useful feature of SD OCT: the direct access to phase information after Fourier transform of the spectral data allows a simple compensation by a single post processing step. The results show that this method allows PS-OCT imaging with the same image quality as previously achieved with our bulk optics setups.
One drawback of our present setup are ghost images that occur at a distance of ~1 mm from the main image. These are probably caused by imperfect optical elements that cause cross coupling of polarization states into the wrong mode of the PM fibers. This problem has also been addressed in ref [28
]. In the ghost images were eliminated by simply cutting them out. For comparison, shows the intensity data set before this operation. The ghost image is clearly visible. While this simple elimination method usually works well with a normal, healthy retina (they are usually thinner than the separation of the ghost image), problems can arise in various cases of diseases where retinal thickness is increased and the ghost and real images overlap. Solutions could either be to use better optical elements that avoid cross coupling, or to use longer PM fibers that further separate the position of the ghost [28
Circumpapillary B-scan from human retina in vivo. Same data set as in , ghost images not removed.
While we have shown that our PM fiber based approach works well with a stationary setup, applications that require movements of fibers (e.g. endoscopy) might suffer from fiber bending and twisting. To investigate this influence, we made an experiment where we introduced a 360° loop into the fiber of the sample arm in between two measurements. The influence on the measured sample retardation was negligible (< 1°), however, the measured axis orientation changed by ~10 – 15°. This indicates that endoscopy based applications that measure only retardation should work well with our scheme. Applications requiring quantitative axis orientation would need an additional reflector (e.g. a weakly reflecting glass plate) at the distal fiber end that can be used for calibration.
A further improvement of the system reported in this work, as compared to our previous systems, is the use of a broadband light source of 110 nm bandwidth. Only few reports demonstrated such a light source for UHR PS-OCT imaging so far [34
], and they were limited to time domain systems. While the improved axial resolution in general is an obvious advantage, the reduction of speckle size (further improved by the smaller focal spot size) is of special interest for depolarization imaging. The size of the evaluation window for Stokes vector element averaging, which determines the resolution of DOPU images, is considerably reduced. This will allow RPE segmentation [23
] with better resolution, enabling improved detection of small RPE lesions which can be precursors of more advanced stages of, e.g., age related macular degeneration (AMD) [47
]. This possibly offers new perspectives for early diagnosis and follow up of AMD and other RPE related diseases.