Stabilization of a Fourier domain mode-locked (FDML) laser was achieved in a wide temperature range of over 30 degrees by adaptively tuning the sweep rate without using any complicated or massive temperature control equipment. The proposed FDML laser in a fiber ring cavity configuration consists of an optical tunable filter based on a KTN (KTa<sub>1-x</sub>Nb<sub>x</sub>O<sub>3</sub>) scanner. The FDML laser operates at the sweep rate of around 200 kHz. The output properties show an output power of 2 mW and coherence lengths of 8.5 mm for the sweep range of 100 nm and 11 mm for that of 80 nm at the center wavelength of 1300 nm.
Using light-beam scanning technology based on a potassium tantalate niobate (KTa<sub>1-x</sub>Nb<sub>x</sub>O<sub>3</sub>, KTN) single crystal, we constructed a wavelength-swept light source for industrial applications. The KTN crystal is placed in an external cavity as an electro-optic deflector for wavelength scanning without any mechanical operation. Cavity arrangement and mechanism elements are specially designed for long-term stability and environmental robustness. In addition, we updated the handling of the KTN crystal. We used a pair of thermistors for accurate temperature monitoring, and weakly irradiated the crystal with a 405-nm light during operation to achieve drift suppression. We selected a moderate repetition rate of 20 kHz to suit the practical application. The output of the light source was 6.2 mW in average power, 1314.5 nm in central wavelength, and 83.3 nm in bandwidth. The interference fringes of the light enable us to specify the thickness of a wafer sample by the peak positions of the point spread functions. We measured the thickness of a silicon wafer as 3651 μm in the optical path length using a reference quartz plate. The distribution of the obtained values is about 0.1 μm (standard deviation). We experimentally confirmed that this property persists continuously at least over 153 days. Our light source has a remarkable feature: extremely low timing jitter of the sweep. Thus, we can easily reduce the noise level by averaging several fringes, if necessary.
We developed high frame-rate en face optical coherence tomography (OCT) system using KTa<sub>1-x</sub>Nb<sub>x</sub>O<sub>3</sub> (KTN) optical beam deflector. In the imaging system, the fast scanning was performed at 200 kHz by the KTN optical beam deflector, while the slow scanning was performed at 800 Hz by the galvanometer mirror. As a preliminary experiment, we succeeded in obtaining en face OCT images of human fingerprint with a frame rate of 800 fps. This is the highest frame-rate obtained using time-domain (TD) en face OCT imaging. The 3D-OCT image of sweat gland was also obtained by our imaging system.
We have developed a highly stable electro-optic KTa1-xNbxO3 (KTN) deflector by enhancing electron transportation through KTN crystal. The amount of current is increased with 405-nm light irradiation to rapidly generate a stable refractive-index change, which induces deflection. The deflection angle is set at 160 mrad within tens of seconds and is kept at that angle for 3,000 hours. The developed deflector has been applied to a wavelength-swept light source to measure the thickness of Si wafers with a 3.6-mm optical length. The precision of 0.1-μm has been continuously achieved corresponding to the stability of the KTN deflector.
For high-speed optical beam scanning, we developed a novel planar optical deflector using KTa1-xNbxO3 (KTN) crystals. When a KTN deflector is operated at high frequencies, heat generated by KTN causes a decrease in its relative dielectric constant, which limits the deflection angle at frequencies above 200 kHz. To overcome this problem, we decreased the thickness of KTN to reduce its capacitance because the heat it generates is proportional to its capacitance. We arranged the two electrodes on the same surface, whereas previously reported structures have each electrode on opposite surfaces. We successfully reduced KTN’s capacitance to 1/30 of that previously reported. The deflection angle of this novel structure at 700 kHz is 16.89 mrad, which is more than half of that at 100 kHz, while the deflection angle of previously reported thick KTN rapidly decreases at more than 200 kHz. The experimental results indicate that our proposed planar optical deflector is effective for suppressing heat generation in KTN and improving the scanning speed of deflectors.
We must expand the operating wavelength range of the optical fiber amplifier if we are to achieve a large scale DWDM and CWDM optical communication system with high performance levels. In this report, we introduce the S-band amplification technique with a Tm<sup>3+</sup>-doped fluoride fiber amplifier and an Er<sup>3+</sup>-doped fiber amplifier, and a fiber Raman amplification technique with a wider application range realized by using tellurite fiber. Furthermore, we describe the use of our proposed wide optical fiber amplifiers in an 8-channel CWDM communication system.