Transistor laser (TL) is a novel three-port dual electrical and optical transmitter based on heterojunction bipolar transistor (HBT). Reversed bias in the BC junction causing Franz-Keldysh (F-K) effect acts as the internal electro-absorption modulator which is also called the intracavity photon-assisted tunneling (ICPAT). Therefore, transistor lasers can be modulated by both base current injection and the voltage of BC junction. We study the frequency responses of the base current modulation and BC voltage modulation of TLs by introducing the ICPAT into laser rate equations. Furthermore, we modify the small-signal model of HBT with the addition current caused by the F-K absorption (Fig. 1(a)). Then, we simulate the optical frequency responses [J. Appl. Phys. 125, 023105 (2019)]. Beside the amplitude modulation, chirp is also important in the optical communication of lasers. We study the small-signal chirping of the voltage modulation of TLs which is different from the diode laser. The frequency shift of the Gaussian pulse is neither positive nor negative chirp. Thus, the pulse can be compressed regardless of dispersion type of the fiber [Opt. Lett. 44, 2109 (2019)].

Professor Hsiang-Chieh Lee

Optical Coherence Tomography (OCT) provides real-time and cross-sectional images of in vivo tissue architectural information without requiring exogenous contrast agents. Recently, the functional extension of OCT technology has been an emerging research topic, promising to further boost various medical applications with OCT. Particularly, studies have demonstrated the use of either optical micro-angiography (OMAG) or OCT angiography (OCTA) technique to identify the subsurface volumetric microvascular information of the biological tissues in vivo by measuring the variation of the intensity or the phase of the OCT signals between neighboring A-scans or B-scans of the structural OCT images. Due the flowing nature of the red blood cells with the microvascular network, it results in the variations of the OCT signal aforementioned when compared to the static scattering characteristic of the surrounding tissues. When compared to microvascular imaging techniques, for example, fluorescence angiography with the indocyanine green (ICG), OCTA, or OMAG exhibit several advantages, including high imaging contrast, free of exogenous contrast agents, high temporal and spatial imaging resolution. However, it has been challenging to provide blood flow information in the OCTA images.

Therefore, our group has developed and implemented the variable interscan time analysis (VISTA) algorithm to provide the relative blood flow information on the OCTA images generated. Also, in order to perform more comprehensive hemodynamic information of the imaged tissue, we have developed an OCT instrument allowing OCT and OCTA images of the mouse ear skin model with two different A-scan or axial (depth) scan speeds, 100 kHz and 200 kHz. Figure 1 shows the projected en face OCTA images of the mouse ear skin with an effective interscan time of 12.5 milliseconds (ms) and 25 ms by using the swept source laser either with an A-scan rate of 100 kHz or 200 kHz. The longer effective interscan time, the more blood vessels are identified, as marked by orange arrows in Fig. 1(a) and Fig. 1(b). The OCT system with a higher detection sensitivity with a lower speed swept source laser is sensitive to detect tiny signals marked by the red arrows shown in Fig. 1(a) compared with Fig. 1(c). Fig. 1(d) shows a color-coded OCTA image where red and blue color indicates blood flow with a relatively high and slow flow speed, respectively, by using the VISTA algorithm and scanning protocol.