# Laser Lens Size Measurement Using Swept-Source Optical Coherence Tomography
## 1 Introduction  
The radius of curvature (ROC) is a key parameter for focusing a lens. The measuring accuracy of the ROC is essential for the focusing intensity of the laser beam. One of the non-contact measuring methods commonly used in ROC measurement is the interferometer [1]. [PERSON] et al. introduced a ROC-measuring system for spherical mirrors that can easily obtain high measurement-accuracy at reasonable cost [2]. [PERSON] et al. measured the ROC and other optical parameters by means of a digital holographic interferometer that compared the wavefront from the test lens with a digitally inserted plane wavefront while changing the incident angle of this digital wavefront [3]. [PERSON] et al. incorporated concave mirror into plano-concave Fabry-Perot cavity to measure ROC by using the principle of multi-beam interference [4]. [PERSON] et al. demonstrated a differential radius-measuring system to perform high-precision ROC measurement through the interference-length measuring technology [5]. [PERSON] et al. proposed a differential confocal self-collimation decentration measurement method, which can obtain ROC of optical element by using the differential confocal to precisely identify the center and vertex positions of the tested lens surface [6]. However, the interferometer has also some disadvantages, such as complicated structure and high cost. Moreover, the measuring accuracy is directly related to the focusing sensitivity. The measurement repeatability is unstable. If optical element is fixed, the ROC measurement will be restricted because of the limited propagating light. The laser confocal microscope has measuring advantages in ROC measurement with high measuring repeatability. Unfortunately, the inner structure cannot be detected with this method.  
Optical coherence tomography (OCT) is a non-contact, non-invasive optical imaging technique with high resolution. OCT can find the 1D depth image, 2D cross-sectional image and 3D topography by measuring the backscatter light of different depth in the sample using the low-coherence interferometry principle [7]. OCT has been widely used in ophthalmic imaging, medical endoscopy, cell and material detection [8; 9; 10; 11; 12; 13; 14]. It can provide the cross-sectional images of samples in vivo with micron scale resolution. OCT was first, performed in time-domain OCT, which called TD-OCT [15; 16].  
Fourier-domain OCT (FD-OCT) have attracted more attention than TD-OCT because of its high sensitivity, high signal-to-noise rate and high imaging speed. There are two implementations of the FD-OCT, spectral domain OCT (SD-OCT) and swept-source OCT (SS-OCT). A broadband light source and a spectrometer were used in SD-OCT system. A swept-source, a common photoelectric detector and high-speed data acquisition card were used in SS-OCT system. Compared with SD-OCT, SS-OCT can reach real-time measurement because of its high imaging speed [17; 18; 19; 20; 21; 22].  
SS-OCT is a nondestructive detection method which is suitable for measurement the ROC of optical element with high accuracy, which can get the 1D depth image and the 2D cross-sectional image simultaneously. In this study, the SS-OCT system was set up to measure the ROC of focusing lens, which can perfectly avoid the limitations of traditional measurement methods. Furthermore, it can also get the real-time images of the contour of the ROC.