M. Kourogi

Wide-span optical frequency comb generator for accurate optical frequency difference measurement

An optical frequency comb (OFC) generator was realized for accurate optical frequency difference measurement of 1.5 mu m wavelength semiconductor lasers by using a high frequency LiNbO/sub 3/ electrooptic phase modulator which was installed in a Fabry-Perot cavity. It was confirmed that the span of the OFC was wider than 4 THz. By using semiconductor lasers whose spectrum linewidths were narrowed to 1 kHz and a sensitive optical balanced-mixer-receiver for measuring beat signal between the sideband of the comb and the laser, we demonstrated a frequency difference measurement up to 0.5 THz with a signal-to-noise ratio higher than 61 dB, and a heterodyne optical phase locking with a heterodyne frequency of 0.5 THz in which the residual phase error variance was less than 0.01 rad/sup 2/. The maximum measurable frequency difference, which was defined as the sideband frequency with the signal-to-noise ratio of 0 dB, was estimated to be 4 THz. >

Accurate optical frequency atlas of the 1.5-µm bands of acetylene

We have measured the absolute optical frequencies of 90 rovibrational lines of the 12C2H2 and 13C2H2 1.5-µm bands. Using an optical frequency comb generator, we could precisely measure the relative frequencies between two lasers stabilized on the saturation dip of the rovibrational lines up to 2 THz apart. The absolute frequencies were determined with respect to the Rb two-photon transition at 0.778 µm with a HCN rovibration line at 1.556 µm used as an intermediate reference. This measurement provides an accurate frequency atlas of acetylene 1.5-µm bands with an uncertainty of ∼10-9.

Increasing throughput of a near-field optical fiber probe over 1000 times by the use of a triple-tapered structure

We fabricated a new probe with extremely high throughput introducing a triple-tapered structure to reduce the loss in a tapered core, to focus the light, and to excite effectively the HE11 mode. A focused ion beam and selective chemical etching were used for fabrication. Over a 1000-fold increase in the throughput of the triple-tapered probe with the aperture diameter D<100 nm was realized in comparison with the conventional single-tapered probe. Furthermore, due to the third taper with a small cone angle, the localized optical near field on the triple-tapered apertured probe with D=60 nm has been confirmed.

A monolithic optical frequency comb generator

A monolithic optical frequency comb (OFC) generator was realized by coating high reflection films on the facets of a LiNbO/sub 3/ crystal used for an electro-optic (EO) phase modulator. The optical round-trip loss in the monolithic OFC generator was reduced to extend the span of an OFC. It is confirmed that the envelope of the OFC extended to a span as wide as 48 nm (or 6.1 THz) around 1.5 /spl mu/m. The maximum measurable frequency difference, which was defined as the sideband frequency with the signal-to-noise ratio of 0 dB, was estimated to be also 6.1 THz.<<ETX>>

Plasmon waveguide for optical far/near-field conversion

A plasmon waveguide was designed and fabricated using a metal-coated silicon wedge structure that converts propagating far-field light to the near field. Illumination (λ=830 nm) of the waveguide (plateau width 150 nm) caused transverse magnetic plasmon-mode excitation. Use of a near-field microscope allowed us to determine its beam width and propagation length as 150 nm and 2.5 μm, respectively.

30-THz span optical frequency comb generation by self-phase modulation in an optical fiber

The width of an optical frequency comb (OFC) was increased to 30 THz by using self-phase modulation (SPM) in an optical fiber. This value is 2.7 times larger than the maximum OFC span obtained by the OFC generator alone. We compare the resulting spectrum to numerical simulations to confirm that the SPM and the higher order dispersion of the fiber contribute to broaden the spectral profile.

Blue shift in room temperature photoluminescence from photo-chemical vapor deposited ZnO films

Abstract Highly transparent ZnO films were deposited on (001) α-Al 2 O 3 at substrate temperature in the range of room temperature to 300°C by photo-chemical vapor deposition. This enabled selective deposition of ZnO films on the selected substrate area irradiated by the light source. With the descending substrate temperature as low as 100°C, band gap widening of the films has been observed, which resulted in the shift of room temperature ultraviolet photoluminescence of ZnO from 380 nm to the shorter wavelength of 360 nm. It implies that in situ patterning of ZnO with different wavelength emission characteristics is possible on the same substrate by controlling the deposition temperature.

Ultrahigh Scanning Speed Optical Coherence Tomography Using Optical Frequency Comb Generators.

A novel optical coherence tomography system without any moving parts for depth scanning was proposed and demonstrated. It was accomplished using two optical frequency comb generators, as broad-band spectrum generators, instead of moving parts for depth scanning. A high scanning speed of more than 12.5 km/s and also a high repetition rate of 500 kHz were achieved in a long scan range of 25 mm. The resolution was about 100 µm.

Spectral Line-by-Line Pulse Shaping on an Optical Frequency Comb Generator

We demonstrate optical processing based on spectral line-by-line pulse shaping of a frequency comb generated by an optical frequency comb generator (OFCG). The OFCG is able to generate a smooth, broad, stable, known-phase frequency comb, which is ideal for recently developed spectral line-by-line pulse shaping technology applications. We demonstrate line-by-line pulse shaping on 64 lines at 10-GHz line spacing, generate transform-limited 1.6-ps short pulses at 10 GHz by combining two pulses in each period directly from the OFCG, and show various examples for optical arbitrary waveform generation. Further, we demonstrate that these pulse sources are of sufficient quality to support optical fiber communication applications as confirmed by bit error rate measurements.

FM noise reduction and subkilohertz linewidth of an AlGaAs laser by negative electrical feedback

Negative electrical feedback was applied to a CSP-type AlGaAs laser, reducing its FM noise at the Fourier frequency range of f >