Takefumi Hara

Optical frequency domain ranging by a frequency-shifted feedback laser

This paper describes the theoretical and experimental study of a new technique for optical frequency domain ranging (OFDR) by a frequency-shifted feedback (FSF) laser. In conventional OFDR, a frequency chirped single-mode laser is used as a light source to convert a distance into a beat frequency, and a tradeoff exists between measurement range and resolution. The FSF laser output consists of periodically generated chirped frequency components whose chirp rate is faster than 100 PHz/s (P=10/sup 15/), By use of the FSF laser, the tradeoff is removed and long-distance high-resolution OFDR is realized In the experiment, a distance of 18.5 km was measured with a resolution of 20 mm.

A new technique of optical ranging by a frequency-shifted feedback laser

We report an application of the frequency-shifted feedback (FSF) laser to optical ranging. The FSF laser output consists of periodically generated chirped frequency components whose chirp rate is faster than 100 PHz/s (P=10/sup 15/). By taking advantage of the spectral features of the FSF laser, the conventional limits on operating range and spatial resolution are removed. In the experiment, a distance of 3.7 km was measured with a resolution of 9.4 mm.

Spectral characteristics of an all solid-state frequency-shifted feedback laser

Summary form only given. A laser cavity closed via the first-order diffracted light of an acousto-optic modulator is referred as a frequency-shifted feedback laser (FSFL). The laser exhibits outstanding features in its oscillation spectrum. We previously demonstrated the first oscillation of all-solid-state Nd:YVO/sub 4/ FSFL, and it was made clear that internal structure of the spectrum consists of a continuously-chirped comb of frequency components. Furthermore, oscillation dynamics have been under our investigation both experimentally and theoretically for a better understanding of FSFL. Here we discussed its oscillation bandwidth as a function of the output coupler reflectance.

Diffraction Properties and Beam-Propagation Analysis of Waveguide-Type Acoustooptic Modulator Driven by Surface Acoustic Wave

A waveguide-type acoustooptic modulator (AOM) driven by a surface acoustic wave (SAW) in a tapered crossed-channel waveguide on a 128°-rotated Y-cut LiNbO3 substrate has been proposed for an optical wavelength of 1.55 µm. In this study, to clarify the conditions for a higher diffraction efficiency and a lower driving power, the diffraction properties of the waveguide-type AOM were measured and simulated. First, an AOM with an AO interaction region length of 3 mm was fabricated and the diffraction efficiency of 65% was obtained. Next, the measured values of the SAW power required for 100% diffraction (P100) for the driving frequencies of 125 MHz and 200 MHz were found to be in agreement with the calculated P100, which shows that there is an optimum driving frequency. Furthermore, optical frequency domain ranging using a frequency-shifted-feedback fiber laser with the waveguide-type AOM was demonstrated. Finally, the diffraction properties of the waveguide-type AOM are simulated using a beam-propagation method (BPM) and compared with the experimental results.

Improvement of Diffraction Properties in Waveguide-Type Acoustooptic Modulator Driven by Surface Acoustic Wave

A waveguide-type acoustooptic modulator (AOM) driven by a surface acoustic wave (SAW) in a tapered crossed-channel waveguide on a 128°-rotated Y-cut LiNbO3 substrate has been proposed for an optical wavelength of 1.55 µm. In this study, to improve the diffraction properties of the waveguide-type AOM, the dependences of the diffraction properties on the relative position of the SAW beam for the diffraction region were simulated using a beam-propagation method (BPM) and measured. By decreasing SAW beam width, although the input voltage needed to obtain the maximum diffraction efficiency increased, diffraction efficiency improved to 84% from the previous value of 65%. Furthermore, to obtain a lower driving voltage, the utilization of a unidirectional transducer was investigated for the AO interaction on a planar optical waveguide at an optical wavelength of 0.633 µm.

Monolithically Integrated Tandem Waveguide-Type Acoustooptic Modulator Driven by Surface Acoustic Waves

A waveguide-type acoustooptic modulator (AOM) using coplanar AO coupling due to a surface acoustic wave (SAW), that is, Bragg diffraction, in a tapered crossed-channel proton-exchanged (PE) optical waveguide on a 128°-rotated Y-cut LiNbO3 substrate for an optical wavelength of 1.55 µm has been proposed. In this study, a monolithically integrated tandem waveguide-type AOM driven by SAW was designed and fabricated. The structure considered was a 2×4 optical switch, in which the input ports of two second 1×2 switches were connected to the two output ports of the first 2×2 switch on the same substrate. An interdigital transducer (IDT) with a period length of 32 µm and an overlap length of 2 mm was fabricated in the first and second stages of the AO interaction region. Diffraction efficiency was measured at a driving frequency of approximately 120 MHz. A diffraction efficiency of approximately 90% was obtained for each stage. When both stages were driven at the same frequency, a peak diffraction efficiency of 63% was obtained. Furthermore, the optical frequency shifts for the sum of two driving frequencies and the difference frequency ranging from DC to 5 MHz were observed.

Frequency-Shifted Feedback Fiber Laser Oscillation with Monolithically Integrated Tandem Waveguide-Type Acoustooptic Modulator Driven by Surface Acoustic Waves

Frequency-shifted feedback fiber laser oscillation and optical frequency domain ranging using a monolithically integrated tandem acoustooptic modulator driven by surface acoustic waves (SAWs) were demonstrated. The linear relationships between the beat frequency change and the optical path difference in the Mach–Zehnder interferometer with three slopes were obtained by combining the upward and downward frequency shifts in the first and second SAWs.

Average atmosphere temperature measurement by a frequency-shifted feedback laser

A Frequency-Shifted Feedback (FSF) laser has an intracavity acousto-optic modulator (AOM) and the spectral output consists of a chirped frequency comb evenly spaced at the cavity free spectral range (FSR). An FSF laser is a useful source for optical frequency domain reflectometry (OFDR). We present a new average atmospheric temperature sensor by OFDR using an FSF laser for the first time. The beat signal, which is detected through the self-delayed heterodyne detection of an FSF laser, is proportional to the path difference, and measurements can be done within the frequency bandwidth of a cavity FSR. Furthermore, the beat frequency characteristics are unrelated to the beat order. Therefore, the path measurement resolution is consist and unrelated to the path difference. Changes in atmospheric refractive index primarily depend on variation of temperature and pressure. Observing variation in path difference with an FSF laser should allow calculation of the average atmospheric temperature along the path if the change in pressure is known. As the path difference increases, the temperature resolution improves. This paper outlines the principle of the average atmospheric temperature measurement using an FSF laser and presents preliminary experimental result.

Profilometry using a frequency-shifted feedback laser

A frequency-shifted feedback laser is used as chirped light source for optical profilometry based on optical frequency-domain reflectometry. Measurement ranges of 1 m and accuracies better than 50 /spl mu/m have been demonstrated.

Spectral Characteristics of a Frequency-Shifted Feedback Ring Laser Using a Semiconductor Optical Amplifier

We report the spectral characteristics of a frequency-shifted feedback laser using a semiconductor optical amplifier as a gain medium in a free-space ring cavity configuration. Oscillation bandwidths as broad as 130 GHz have been measured. Using self-delayed heterodyne interferometric experiments, we confirm the generation of a comb of chirped frequency components from this laser, with chirp rates faster than 85 PHz/s.