Optical frequency domain imaging (OFDI) is an interferometric technique using a wavelength-swept source to obtain 2- or 3-dimensional images of a biomedical sample.1-3
Backscattered light from the biological sample is combined with reference light to generate an interference signal in a photodetector. Each frequency component of the detector signal is related, in amplitude and phase, to the backscattered light from a corresponding depth in the image. Therefore, an axial reflectivity profile of the sample (A-line) is obtained from a Fourier transform of the detector signal. Although this basic principle of frequency domain ranging has been well known in the context of optical frequency domain reflectometry4-7
, recently developed high-speed OFDI3
makes use of a wide-band rapidly-swept laser8
with a sweep repetition rate of a few tens of kHz to provide a vastly improved measurement speed.
In high-speed OFDI, the maximum ranging depth is typically limited by the finite width of the coherence function of the laser output because the coherence length associated with the instantaneous linewidth is often compromised to obtain higher tuning speed, higher output power, or wider tuning range. The finite coherence length causes the visibility of the interference fringe to decrease as the path length difference of the interferometer increases. This results in a degradation of signal-to-noise ratio (SNR) and therefore limits the useful ranging depth.2
Furthermore, the inability to distinguish between a positive and negative electrical frequency in a conventional interferometry leads to the ambiguity between positive and negative depths. To avoid the superposition or folding of the positive-delay image upon the negative-delay image, the reference delay of the interferometer can be adjusted to be outside of the sample. This, however, further limits the ranging depth for a given coherence length of the source. To avoid this limitation, researchers have measured quadrature interference signals based on active or passive phase biasing using a piezoelectric actuator9
or 3×3 coupler11
. These techniques could unfold otherwise overlapping images associated with positive and negative depths, but tended to leave residual artifacts due to the difficulty of producing stable quadrature signals.
In this paper, we propose and demonstrate a simple technique that effectively and instantaneously eliminates the ambiguity between positive and negative depths. The technique uses an optical frequency shifter in the interferometer to provide a constant frequency shift of the detector signal. This method allows both sides of the coherence range to be used without crosstalk and doubles the ranging depth. The same concept has been described previously in the context of 1-dimensional optical frequency domain reflectometry using rotating birefringence plates at 58 Hz5
or a recirculating frequency shifting loop.6
In this work we use an acousto-optic frequency shifter and apply the technique to high-speed OFDI with several orders of magnitude faster ranging speed. Furthermore, we demonstrate a signal processing algorithm to accommodate nonlinear tuning in the wavelength-swept OFDI source.