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Optical Coherence Tomography
Published in Margarida M. Barroso, Xavier Intes, In Vivo, 2020
Roshan Dsouza, Stephen A. Boppart
SS-OCT has additional advantages over SD-OCT systems. SS-OCT can be implemented using a balance-mode detection scheme to cancel the strong background reflection in the interferometer. SD-OCT typically uses silicon detectors for wavelengths in the 800–1060 nm range. More expensive and slower line-array cameras using indium gallium arsenide (InGaAs) detectors are required for wavelengths around 1300 nm. SS-OCT, however, can more readily operate at longer wavelengths and can often operate at much faster acquisition rates than SD-OCT systems. Additionally, signal roll-off over depth in SS-OCT has been significantly reduced, mainly because of improvements in the coherence properties of the laser source and the highly efficient broad bandwidth detectors. The conventional tunable laser uses bulk or fiber components that result in a relatively longer resonator cavity length and limit high sweep rates because of the long round trip time (Yun et al., 2003a; Choma et al., 2005; Yasuno et al., 2005; Okabe et al., 2012). To overcome this issue, two novel source technologies have been developed: a Fourier-domain mode locking (FDML) laser and a vertical cavity surface-emitting laser (VCSEL). Both sources have significantly improved sweep rates (in kHz–MHz) with an increased depth range from tens of millimeters to a meter (Jayaraman et al., 2012; Wieser et al., 2014; Wang et al., 2016b).
Anterior segment OCT
Published in Pablo Artal, Handbook of Visual Optics, 2017
SS-OCT system operating at the wavelength 1050 nm (Figure 4.5b) was equipped with either a short external cavity tunable laser (Axsun Technologies, United States) or a prototype of MEMS-tunable vertical cavity surface emitting laser (VCSEL) (Praevium/Thorlabs, United States). The light sources emitting light centered around 1310 nm used in laboratory SS-OCT instruments were a short external cavity tunable laser (Axsun Technologies, United States) and a custom-made Fourier-domain mode locking (FDML) laser. The systems are characterized by different performance parameters, as shown in Table 4.2. The laboratory systems are usually more flexible in adjusting the performance. The results presented in subsequent sections were obtained with those instruments.
Intraoperative Optical Guidance for Neurosurgery
Published in Yu Chen, Babak Kateb, Neurophotonics and Brain Mapping, 2017
Chia-Pin Liang, Cha-Min Tang, Yu Chen
Besides detecting the vessels, the probe can also monitor the pulsation (Figure 21.6) and differentiate the vessel type. These capabilities could be valuable for screening the vessels posing high risk in neurosurgery. The aliasing problem may hinder using DOCT signal to quantify blood flow; however, this problem can be solved by using velocity variance (Yang et al., 2003) or axial Kasai algorithm (Morofke et al., 2007). Also, the high-speed Fourier domain mode locking laser should be able to increase the velocity detection limit by one to two orders (Adler et al., 2007; Klein et al.).
Photonic broadband signal frequency conversion and bandwidth multiplication based on a Fourier domain mode-locked optoelectronic oscillator
Published in Journal of Modern Optics, 2021
Yalan Wang, Jin Zhang, Xiang Li, Jianghai Wo, Anle Wang, Xiaoniu Peng
Then the frequency conversion is implemented. The central frequency of the LFM signal generated by AWG is set to be 20 GHz, and the bandwidth of the signal is 1 GHz. Figure 3(a) shows the frequency down-conversion results by tuning the wavelength of the laser. The central frequency of the LFM signal is tuned from 4.6 to 11.2 GHz when the laser wavelength is adjusted from 1550.99 to 1550.94 nm. The frequency conversion range and the resolution are limited by the bandwidth of the FBG-FP filter and the free spectral range (FSR) of the OEO, respectively. Similar results are also confirmed when the central frequency of the AWG frequency is tuned at 30 GHz (see Figure 3(b)). Figure 3(c and d) show the frequency conversion results at different input frequencies. As shown in Figure 3(c and d), by tuning the central frequency of the LFM signal, the frequency of the converted signal changes correspondingly. Since we use an optical filter that is not narrow enough to select the desired optical sideband, only frequency down-conversion is demonstrated. If the CS-SSB modulation is employed in terms of the DPMZM, frequency up-conversion can also be achieved. Furthermore, by applying different modulation signal onto the MZM and utilizing the modulated optical sideband as the light source, the system can generate not only frequency-converted LFM signal, but also other arbitrary waveforms such as phase coded signal as long as Fourier domain mode locking is achieved. Figure 3(e and f) are the corresponding time-domain waveform and the pulse compression performance, the amplitude jitters are very small, and the full width at half maximum is 1.08 ns, respectively. Thus, a pulse compression ratio of ∼18865 is realized.