Especially in retinal imaging, OCT has become one of the most valuable imaging tools for diagnosing eye diseases. Considering the scattering and absorption properties of the eye, the nm OCT system is preferred for retinal imaging. In this study, we describe the use of an akinetic swept-source OCT system based on a pulse-modulated active mode locking AML fiber laser at a nm wavelength for in-vivo human retinal imaging.
The akinetic AML wavelength-swept fiber laser was constructed with polarization-maintaining fiber that has an average linewidth of 0. We successfully obtained in-vivo human retinal images using the proposed OCT system without the additional k-clock and the frequency shifter that provides a wide field of view of The main retina layers, such as the retinal pigment epithelium, can be distinguished from the OCT image with an axial resolution of 6.
Optical coherence tomography OCT is a noninvasive imaging modality that can provide high-resolution, cross-sectional images of tissues 1 , 2 , 3.
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OCT has evolved over the past decades as one of the most important clinically applicable tests in ophthalmology, cardiology, and dermatology 1 , 4 , 5 , 6. Especially in ophthalmology, OCT has become one of the most valuable imaging tools for diagnosing eye diseases 7 , 8 , 9. Although nm OCT is used for dermatological, cardiological, and corneal imaging, the high water absorption feature of the vitreous body of the human eye limits the depth of retinal OCT imaging.
The nm OCT system provides more detailed morphological information, microvascular mapping of the retina with a relatively high axial resolution, and it has been successfully popularized in the clinic setting. However, it is difficult to image deeper posterior areas of the eye, such as the choroid region, because of the strong absorption and reflection of the retinal pigment epithelium RPE and highly pigmented choroidal melanocytes.
Considering the scattering and absorption properties of the eye, the nm OCT system is preferred for retinal imaging In order to successfully apply a biomedical imaging modality to the clinical setting, it must be non-invasive, non-labeling, real-time, in-vivo imaging with high-resolution, a sufficient imaging range, and short acquisition time. One of the high-speed OCT systems is the spectral domain OCT SD-OCT system, which uses a broadband light source, such as a superluminescent diode, and a spectrometer to measure the optical interference signals.
However, with the SD-OCT system, the imaging speed and imaging depth range are limited by the characteristics of the spectrometer Silicon-based spectrometers are widely used for the SD-OCT system; however, this device has a low sensitivity at a nm wavelength than at an nm wavelength.
The characteristics of SS-OCT systems mainly depend on the performances of the wavelength-swept source. Many kinds of wavelength-swept sources have been reported, and they tune a lasing wavelength based on mechanical movements of a wavelength-tunable component in the laser cavity. The sweeping behaviors are determined by the mechanical movements of the wavelength-tunable components, such as a the Fabry-Perot tunable filter 18 , 19 , a polygon mirror scanner 20 , tunable microelectromechanical system filter with a semiconductor optical amplifier SOA 21 , or vertical cavity surface-emitting laser 7 , This mechanical movement requires depletion and accumulation of momentum for its completion, which can be affected to hysteresis or experience undesirable drifts of operational scheme, such as thermal drift because of friction.
Unstable or drifting mechanical movements can lead to overall degradation of source performance 11 , 14 , 15 , Recently, Insight Photonic Solutions, Inc.
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The company successfully showed an SS-OCT image of the human retina, skin, and tooth and an OCT angiogram of human skin owing to the phase-stable swept source. However, the swept source contains valid and invalid points that can limit available sampling points and cause data acquisition to be complex. Therefore, a k-clock has to be used to synchronize acquisition of the DAQ card with a delay on the start of the sweep in order to remove invalid data points 15 , and only sample numbers are used for OCT images, which can affect the image quality.
Another type of akinetic wavelength-swept laser is based on the active mode locking AML method with dispersive fiber cavity 11 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , Its feasibility for use with fiber Bragg grating sensor systems 25 and SS-OCT systems 11 , 23 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 with AML wavelength-swept sources has been reported.
Although the mechanical tunable filters are successfully eliminated with AML methods, the main limitation of in-vivo OCT imaging remains: a short image depth range coherence length of the laser. Therefore, in a previous study, a frequency shifter was inserted into the interferometer to extend the imaging depth range, but the whole corneal structure still could not be shown simultaneously In the present study, we describe the use of a pulse-modulated AML wavelength-swept fiber laser with a nm wavelength for in-vivo human retinal OCT imaging. The average linewidth of 0. The sweeping behavior was determined by a fully electrical signal without a mechanical tuning mechanism.
Therefore, we extracted a fully synchronized sweep spectrum without observing a difference between forward and backward sweeping. The -6 dB roll-off of 2. We successfully obtained in-vivo retina images of a human using the OCT system without the additional k-clock and the frequency shifter providing wide field of view FOV of The main retina layers such as internal limiting membrane, retinal nerve fiber layer, photoreceptor layers, RPE, choroidal stroma, were clearly distinguished through the OCT image with axial resolution of 6.
All components were made of a polarization-maintaining fiber to protect polarization-dependent properties and ensure environmental resistance. A radiofrequency RF signal from an RF signal generator was connected to the pulse generator. An arbitrary waveform from an arbitrary function generator AFG was used to electrically vary the center frequency of the RF signal generator by using the frequency modulation FM function. The optical components were spliced as short as possible to provide a better sweeping property. The total cavity length was 1. The FSR of the laser cavity is expressed as follows:.
However, in a chromatic dispersive medium, the FSR becomes a function of the wavelength or wavenumber. The relationship between the lasing wavelength and modulation frequency can be expressed as follows 11 :. The sensitivity is defined as follows:. As modulation frequency f m varied from The red line in Fig.
OSA | Endoscopic applications of optical coherence tomography
The measured R-square values of the first-order fitting and residual sum of square were 0. The experimentally measured sensitivity parameter S was 1. The theoretical value of the sensitivity was 1. A total tuning range of The theoretical maximum tuning range can be expressed as follows:. It was determined as The tuning bandwidth of the laser was primarily limited by the 3-dB reflection bandwidth of the CFBG and optical circulator. With the pulse-modulated AML wavelength-swept fiber laser, the relatively wide tuning range without linewidth broadening could be obtained in the whole wavelength range, unlike with direct sinusoidal modulation.
Since the wavelength components are almost continuously existed in different time in the laser cavity according to the dispersion, the RF signal should select wavelength component in the short time for narrow linewidth. In addition, with the direct sinusoidal modulated AML laser, a high DC current above the gain threshold should be applied to the SOA before RF modulation to achieve a wide tuning range. This high DC current caused a relatively high-amplified spontaneous emission ASE signal, which can also affect the linewidth broadening at the wavelength of the high-gain region.
Therefore, the DC current should be controlled to preserve a narrow linewidth, which limits wider tuning bandwidth As shown in Fig. The measured 3-dB linewidth of the lasing peak was 0.
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The measured output power of pulse-modulated AML fiber laser was 2. With the wavelength-swept laser, a lasing property could be preserved until the sweep rate reached a single round-trip frequency.
The single round-trip frequency was defined as follows 19 :. In this experiment, the sweep rate f single-roundtrip for a single round trip was about The spectra were measured by a peak-hold mode of an optical spectrum analyzer. In addition, the output power and other laser properties simultaneously degraded, such as the optical linewidth. For the same laser cavity configurations such as the cavity length, linewidth, gain, the operating spectral bandwidth should be narrower in the case of a kHz sweep rate.
In order to use the wide spectral bandwidth at a higher sweep rate, the cavity length should be as short as possible. A wider spectral bandwidth at a higher sweep rate with sufficient lasing properties are expected to be available when the length of the laser cavity is decreased further by using improved fiber splicing techniques. The higher sample number can be achieved at the higher high duty ratio, which makes it possible to obtain better imaging qualities. An advantage of our system is that the entire spectrum is single-directionally swept without the use of a buffering method.
The measured spectral bandwidth of the laser was Therefore, according to Eq. For better coherence properties during the sweeping, the laser should be swept from a short to a long wavelength region forward sweeping. When the optical pulse passes through the gain medium e. SOA of laser cavity, the optical redshift is induced by the self-phase modulation.
This red shifting from modulation of the gain medium allows stable pulsation for short to long wavelength tuning In contrast, the coherence properties were degraded for backward sweeping long to short wavelength region. To examine this experimentally, two types of waveforms from an AFG were applied to the laser. One was a forward sweeping, and the other was a backward sweeping with single-directional linear drive function.
The characteristics of these waveforms were analyzed and compared. The measured sensitivity of the system was The proposed laser is basically a pulse laser, therefore, the pulse duration can affect a fringe visibility as well as roll-off of the OCT system. In term of the coherence behavior, the pulse width is one of the factors which could affect the fringe visibilities because of the time mismatch of the time-delayed pulses pair from reference and sample arms.
In our previous study, we inserted a frequency shifter to enhance the imaging depth range because of the low coherence length of the sweeping source However, in this study, we obtained enough imaging range for in-vivo retina OCT imaging thanks to the linewidth enhancement of the pulse modulation.
The average incident power on the finger surface was 1. Because of the longer imaging range and high axial resolution of the system, the high contrast OCT image could be obtained with the pulse-modulated AML wavelength-swept laser compared with the sinusoidal-modulated AML wavelength-swept laser. For retinal OCT imaging, the optical lens at the sample arm was removed, and the light was focused by the eye of a patient. The average incident power on the eye surface was 1.
The main layers of the retina, such as the internal limiting membrane, retinal nerve fiber layer, photoreceptor layers, RPE, choroidal stroma, could be distinguished because of the high axial resolution of the proposed sweep source and the OCT system. The acquisition times were 5 milliseconds and 2.