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Q-Switch-Pumped Supercontinuum for Ultra-High Resolution Optical Coherence Tomography

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Q-Switch-Pumped Supercontinuum for Ultra-High Resolution Optical Coherence Tomography 4744 Vol. 42, No. 22 / November 15 2017 / Optics Letters Letter Q-switch-pumped supercontinuum for ultra-high resolution optical coherence tomography 1,2,3, 3 1 1 MICHAEL MARIA, *IVAN BRAVO GONZALO, THOMAS FEUCHTER, MARK DENNINGER, 1 1 1,3 2 PETER M. MOSELUND, LASSE LEICK, OLE BANG, AND ADRIAN PODOLEANU 1NKT Photonics A/S, Birkerød 3460, Denmark 2University of Kent, Applied Optics Group, School of Physical Sciences, Canterbury CT27NH, UK 3Technical University of Denmark, DTU Fotonik, Fiber Sensors and Supercontinuum, Kgs. Lyngby 2800, Denmark *Corresponding author: [email protected] Received 20 September 2017; accepted 11 October 2017; posted 18 October 2017 (Doc. ID 307518); published 14 November 2017 In this Letter, we investigate the possibility of using a samples, other aspects, such as optical power, spectral range, commercially available Q-switch-pumped supercontinuum and penetration depth are more important for NDI. The (QS-SC) source, operating in the kilohertz regime, for ultra- SC is the only optical source that can offer a large freedom high resolution optical coherence tomography (UHR-OCT) of wavelength choice compatible with a large bandwidth, while in the 1300 nm region. The QS-SC source proves to be delivering a high spatial coherence and a high optical power. more intrinsically stable from pulse to pulse than a mode- Supercontinuum generation is the spectral broadening of a locked-based SC (ML-SC) source while, at the same time, high-power laser pulse in nonlinear media, such as silica pho- is less expensive. However, its pumping rate is lower than tonic crystal fibers (PCFs). Commercially available SC sources that used in ML-SC sources. Therefore, we investigate here are based on pumping a PCF (in the anomalous dispersion specific conditions to make such a source usable for OCT. regime close to the zero-dispersion wavelength) with a high- We compare images acquired with the QS-SC source and power picosecond or nanosecond laser. In these conditions, SC with a current state-of-the-art SC source used for imaging. generation is initiated by modulation instability, which breaks We show that comparable visual contrast obtained with the two technologies is achievable by increasing the readout up the pulse into solitons. As a result of soliton interaction and time of the camera to include a sufficient number of QS-SC stimulated Raman scattering, the SC is extended towards longer pulses. © 2017 Optical Society of America wavelengths. Such SC generation is known to be noisy [5] and to display significant pulse-to-pulse intensity fluctuations [6,7]. OCIS codes: (320.6629) Supercontinuum generation; (110.4500) SC sources reported so far in UHR-OCT use mode-locked Optical coherence tomography; (110.4280) Noise in imaging systems. lasers as pump sources. This is due to the many advantages of- fered by these lasers, namely high peak power and megahertz https://doi.org/10.1364/OL.42.004744 repetition rate operation. In fact, a high repetition rate reduces the noise in the UHR-OCT images [8] due to pulse-to-pulse intensity averaging. An alternative as a pump laser for SC gen- Ultra-high resolution optical coherence tomography (UHR-OCT) eration is a Q-switched laser. Q-switched lasers offer longer is a non-invasive imaging modality relying on the principle of pulse duration than mode-locked lasers with enough peak low coherence interferometry. Since its first demonstration, power for SC generation due to their high pulse energy. UHR-OCT [1] has constantly been improved through progress Furthermore, an important advantage of Q-switch technology in optical sources. Today, ultra-high axial resolution within the over mode locking is the much lower cost. micrometer scale is achievable using mainly supercontinuum In this Letter, we demonstrate that a low-cost commercially (SC) light sources for visible OCT or near-infrared OCT [2,3]. Q Other optical sources are also used, such as several combined available SC source using a -switched pump laser (QS-SC) super-luminescent diodes or broadband swept sources, but (SuperK Compact, NKT Photonics) can be used for UHR- their available bandwidth is smaller than that of SC sources. OCT. The pump laser operates at 22.222 kHz with a pulse A common drawback of all broadband sources is their relative length of 1.6 ns. This Letter shows that sufficiently good OCT complexity and consequent cost, which contributes signifi- images can be achieved with the only disadvantage of increase cantly to the high price of UHR-OCT systems. in the exposure time of the camera. We compare this QS-SC to The primary UHR-OCT application field is biomedical im- an SC source recommended for OCT, which is based on a aging; however, more and more reports can be found regarding mode-locked pump laser (ML-SC) (SuperK Extreme, NKT non-destructive imaging (NDI) [4]. NDI requirements are dif- Photonics) with a repetition rate of 320 MHz and a pulse ferent compared to eye or skin imaging. Indeed, while speed is length of 10 ps. The QS-SC source currently costs less than one of the main parameters to consider when imaging in vivo 15% of the ML-SC price. 0146-9592/17/224744-04 Journal © 2017 Optical Society of America Letter Vol. 42, No. 22 / November 15 2017 / Optics Letters 4745 eter includes several components, such as fiber and optics, which act as overall spectral filters. The spectra measured by the spectrometer after the interferometer for each SC source are shown in Fig. 2(b). Due to their similar spectral shapes, the achievable point spread functions (PSFs), characterizing the axial resolution of the UHR-OCT system, are also alike. Fig. 1. Sketch of the UHR-OCT system with DC, directional cou- The measured axial resolution is slightly below 5 μm (in air), pler; PC, polarization controller; C1, C2, parabolic collimator; Disp. as shown in Fig. 2(c), corresponding to a resolution in tissue of C, dispersion compensation block; NDF, neutral density filter; M1, 3.5 μm(n 1.4). flat mirror; OBJ, scanning lens. In terms of noise, the main contributions are the detector noise, the shot noise, and the relative intensity noise (RIN). Detector noise contains the noise due to the thermal photo- The UHR-OCT setup, shown in Fig. 1, is a Michelson electron generation and signal digitization. Shot noise is the interferometer with an ultra-broadband 50/50 directional cou- noise caused by the random arrival of photons at the detector. pler (DC) splitting the light into a reference arm and a sample Finally, the RIN is the noise due to the amplitude fluctuations arm. The reference arm consists of a reflective collimator (C2— of the source. Optimally, an OCT system should operate in a Thorlabs RC04APC-P01), a dispersion compensation block regime where shot noise is dominant as this would give the (Thorlabs LSM02DC), a variable neutral density filter (NDF), highest signal-to-noise ratio (SNR) [8,9]. In case of a noisy and a flat mirror (M1). The sample arm consists of a reflective optical source, RIN can dominate leading to a reduced SNR. collimator (C1—Thorlabs RC04APC-P01), a set of galvanom- The RIN of a series of measured pulse peak powers, at a eter-based XY-scanners (Thorlabs GVSM002/M), and a scan- given wavelength, is defined as RIN σM ∕hMi, where hMi ning lens (OBJ—Thorlabs LSM02) offering a spot size of is the mean peak power and σM is the standard deviation of the 11 μm at a wavelength of 1315 nm. The spectrometer is a time series of peak powers. To measure RIN, a filter, a photo- Cobra 1300 (Wasatch Photonics) with an optical bandwidth diode, and an oscilloscope are used. According to the conver- from 1070 to 1470 nm operating at a maximum line rate of sion performed by the photodetector, if fast enough, the train of 76 kHz and with 2048 pixels. Then the imaging range of our measured voltage peaks is proportional to the pulse energy, system is around 2 mm. Finally, the processing unit is made of a which is also proportional to the peak power. frame grabber card (NI PCIe-1433) connected to a workstation The RIN of both SC sources was measured over a wave- (Dell—CPU Intel i7 = 3.33 GHz—12 GB RAM). All data are length range of 1100–1450 nm, covering almost the entire acquired and processed using a home-designed LabVIEW in- spectrometer range from 1070 to 1470 nm. To do so, the light terface and Matlab algorithm. No particular synchronization is from the SC source was filtered using several 10 nm bandpass applied between the SC sources and the spectrometer readout filters (Thorlabs) stepped in their central wavelengths by 50 nm, in order to keep the system as simple as possible in terms of then detected by an InGaAs photodiode (Thorlabs— software and hardware. DET08CFC—800–1700 nm, 5 GHz). The pulse train was Figure 2(a) shows the spectrum of the two SC sources recorded with a fast oscilloscope (Teledyne LeCroy— measured using an integrating sphere connected with a fiber to HDO9404—10 bits resolution, 40 GS/s, and 4 GHz). an optical spectrum analyzer (OSA) limited to 1750 nm. The Figure 3(a) summarizes the measured RIN as a function of spectra extend in reality to 2.4 μm for the QS-SC source and to wavelength. The QS-SC source has a low RIN, between only 2.0 μm for the ML-SC source. However, the OCT interferom- 2.5%–5%. In contrast, the ML-SC shows strong fluctuations with a RIN ranging from 10% to almost 45% at longer Fig. 2. Spectra of both SC sources measured using (a) a commercial OSA and (b) a spectrometer after the interferometer.
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