Masashi Ohkawa

Displacement measuring technique for satellite-to-satellite laser interferometer to determine Earth's gravity field

We present a new displacement measuring technique with simplicity, robustness, high sensitivity and wide measurement range. A set of a frequency shifter and a voltage–frequency converter is used to lock a homodyne interferometer on the half-bright fringe by eliminating the Doppler fringe resulting from mirror motion. The mirror displacement is directly retrieved from the feedback signal of a fringe control loop. By developing a table-top interferometer, we successfully demonstrated signal recovery without significant degradation. The achieved displacement sensitivity and measurement range of the interferometer were 24 nm Hz−1/2 and 1.3 mm at a Fourier frequency of 0.1 Hz, respectively. This technique was found to have a potential for application to precise displacement measurements. It is, in particular, suitable for a satellite-to-satellite laser interferometer to determine Earths gravity field.

Integrated Optic Accelerometer Employing a Cantilever on a Silicon Substrate

A novel integrated optic accelerometer employing a cantilever on a silicon substrate is proposed. An analysis is carried out to demonstrate the feasibility of the proposed sensor and obtain the basic design concept. The sensitivity and the detectable frequency range of the acceleration can be set by proper selection of the cantilever dimensions and the added weight.

Relationship between sensitivity and waveguide position on the diaphragm in integrated optic pressure sensors based on the elasto-optic effect

The sensitivities of integrated optic pressure sensors with diaphragms theoretically are known to be strongly dependent on the position of the sensing waveguide on the diaphragm. According to the theoretical results, the diaphragm edge is the best position for the waveguide of a sensor based on the elasto-optic effect. The relationship between sensitivity and the waveguide position, however, has not been investigated experimentally, although it is important in the designing of such a sensor and in determining the misalignment tolerance of the sensing waveguide. In this study, this relationship in a glass-based integrated optic sensor by use of an intermodal interference was examined experimentally.

Silicon-Based Integrated Optic Pressure Sensor Using Intermodal Interference between TM-Like and TE-Like Modes

In this paper, a silicon-based integrated optic pressure sensor using an intermodal interference between the fundamental TM-like and TE-like modes is described. The sensor consists of a micromachined rectangular diaphragm and a straight polystyrene optical waveguide passing over the diaphragm. Its sensitivity is theoretically known to be strongly dependent on the position of the waveguide over the diaphragm. To experimentally investigate such dependence, we fabricated a sensor with a 1.2 mm 2 10 mm 2 20 w m diaphragm, over which waveguides were placed at 50 w m intervals. The measured phase sensitivity was 98 mrad/kPa for the waveguide nearest to the diaphragm edge. The measurement was also carried out for the other waveguides. As theoretically expected, the largest sensitivity was obtained for the waveguide nearest to the edge.

Sensitivity dependences on side length and aspect ratio of a diaphragm in a glass-based guided-wave optical pressure sensor

According to our previous theoretical study, sensor sensitivity is proportional to the cube of the side length of the diaphragm in a guided-wave optical pressure sensor consisting of a glass diaphragm and a single-mode waveguide on the diaphragm. Also, to obtain higher sensitivity, an aspect ratio of the diaphragm should be approximately 1 for two waveguide positions: the center and the edge of the diaphragm. In this study, sensitivity dependences on side length and aspect ratio of the diaphragm were experimentally examined. The obtained experimental results strongly supported the theoretical predictions.

Bandwidth enhancement of second-harmonic generation with quadratic spatial-soliton generation versus conventional methods

A detailed numerical comparison of the bandwidth–efficiency trade-off for second-harmonic generation (SHG) achieved by the formation of two-dimensional quadratic spatial solitons versus the more conventional method of gentle focusing in the middle of a nonlinear crystal is presented. Numerical simulations for type II SHG in potassium titanyl phosphate with constant wave-vector mismatch show that the 3-dB drop-off in SHG conversion efficiency can be many multiples of π in detuning from the phase-match condition over a large range of walk-off angles and focal positions.

Scale-reduction rule without drop in the sensitivity of a silicon-based guided-wave optical pressure sensor using a micromachined diaphragm

In this study, an original scale-reduction rule without a drop in the sensitivity of a guided-wave optical pressure sensor was obtained using a micromachined diaphragm. The sensor has a rectangular diaphragm as a pressure-sensitive structure and a sensing waveguide across the diaphragm. Its sensitivity is theoretically known to be strongly dependent on the dimensions of the diaphragm. According to the theoretical results, the sensitivity can be kept constant even if the diaphragm dimensions are reduced as long as both the aspect ratio and the characteristic length of the diaphragm remain constant. Here, the characteristic length is introduced as the cube of either width or length of the rectangular diaphragm divided by the square of its thickness. Such a scale-reduction rule would be very useful for miniaturizing a sensor without reducing sensitivity, although it has not been experimentally confirmed. In this study, the scale-reduction rule was experimentally examined using three fabricated sensors with the same aspect ratio and the same characteristic length. The measured sensitivities of the three sensors were quite similar to each other, as theoretically predicted.

Sensitivity dependence with respect to diaphragm thickness in guided-wave optical pressure sensor based on elasto-optic effect

We experimentally investigated the relationship between sensitivity and diaphragm thickness in a glass-based guided-wave optical pressure sensor using intermodal interference between the fundamental TM-like and TE-like modes. The sensor consists of a rectangular diaphragm and a straight single-mode waveguide on the diaphragm. The sensitivity is theoretically known to be inversely proportional to the square of the diaphragm thickness. In this study, to examine this relationship, four sensors with diaphragm thicknesses of 0.30 mm, 0.22 mm, 0.20 mm, and 0.15 mm were fabricated. The area of the diaphragm was 10 mm×10 mm. For the waveguide position nearest to the center of the diaphragm, the measured sensitivities almost agreed with the theoretical ones.

Experimental study of sensitivity dependences on waveguide position and diaphragm thickness in silicon-based guided-wave optical accelerometer

Abstract. Our group has developed a silicon-based guided-wave optical accelerometer, consisting of a proof mass centered on a diaphragm and a waveguide across the diaphragm. The sensor operates based on phase retardation induced by the elasto-optic effect. Designing the sensor is considerably complicated since it is strongly suggested that sensitivity is related to many parameters, such as waveguide position, diaphragm dimensions, and size and weight of proof mass. Hence, these relations should be clearly identified to establish a design guideline. In this study, sensitivity dependences on waveguide position and diaphragm thickness were experimentally examined. Regarding sensitivity dependence on waveguide position, phase sensitivity was highest for the waveguide at the diaphragm edge and was also relatively high for the waveguide at the edge of proof mass, whereas the sensitivity was quite low around the midposition between the edges of the diaphragm and proof mass. Regarding diaphragm thickness, phase sensitivity was found to be inversely proportional to the square of the diaphragm thickness.

Optical microphone using a silicon-based guided-wave optical pressure sensor

In this paper, an optical microphone using a silicon-based guided-wave optical pressure sensor as an opto-mechanical transducer is reported. The pressure sensor consists of a rectangular diaphragm and a straight waveguide on the diaphragm. The sensitivity of the sensor and the resonance frequency of the diaphragm are important factors to determine the characteristics of the microphone, and depend on the diaphragm dimensions. In this study, to examine a feasibility of the proposed optical microphone, the target values of phase sensitivity and resonance frequency were set at 1.6 mrad/Pa and 7 kHz, respectively. By design considerations, the diaphragm dimensions were determined to be 7 mmX7 mmX23 μm. After fabrication of the optical microphone, sound pressure from 5 to 25 Pa, with a frequency of 1 kHz, was applied to the fabricated microphone with a 7 mmX7 mmX27 μm diaphragm. During measurement, a lock-in detection was taken because the fabricated pressure sensor had an unexpected low sensitivity, which resulted in an extremely low S/N ratio. The measured output voltage from the lock-in amplifier was proportional to the sound pressure as expected although the lock-in detection is not practical for the microphone.