Kazuhiro Imai

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.

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.

Towards an accurate frequency standard at 1.5 /spl mu/m based on the acetylene overtone band transition

We have made a frequency standard operating at the 1.5 /spl mu/m region using the saturated absorption of the v/sub 1/+v/sub 3/ band of /sup 13/C/sub 2/H/sub 2/. To enhance the moderate power from a laser diode and to saturate a very weak molecular overtone transition, we used a Fabry-Perot (FP) cavity and inside of it we put an acetylene cell. In order to obtain a longer life time for the cell, we designed a new scheme consisting of a cavity with a ULE (ultralow expansion) glass spacer and a baked and sealed-off cell with Brewster windows. We observed the saturated spectrum having a 1 MHz spectral linewidth. The signal obtained using the same cell ten months later had the same signal-to-noise ratio (SNR) and spectral linewidth. We made two versions of the frequency standard and observed a beat note between these two lasers from which we derived the frequency stability, in terms of the square root of the Allan variance /spl sigma/(/spl tau/)=2.4/spl times/10/sup -13/ at an integration time /spl tau/=1000 s.

Continuous tuning of an electrically tunable external-cavity semiconductor laser

We propose and demonstrate a novel approach to achieving rapid and phase-coherent continuous broadband tuning of a single-mode external-cavity semiconductor laser. Two internal acousto-optic devices control the angle of incident light on a diffraction grating and the effective round-trip phase electronically. Although the present implementation derives the phase from a long delay line and is not yet mode-hop free, the new approach lends itself to fully electronic control.

Absolute frequency measurement of the saturated absorption lines of methane in the 1.66 μm region

Abstract We have determined the absolute frequencies of the R(0) and Q(1) transitions of the 2 ν 3 band of 12 CH 4 in the 1.66-μm region to be 180.345u2008065u200808(37) and 180.021u2008253u200810(61) THz. The numbers in parentheses are the standard deviations in units of the last digit of the quoted frequencies. We have used the 1.54-μm saturated absorption line of 13 C 2 H 2 as a frequency reference and an optical frequency comb generator for bridging the frequency difference of 14 THz between the 1.66- and 1.54-μm radiations.

Accurate relative frequency cancellation between two independent lasers.

For high-precision frequency-based applications of lasers, the frequency difference between two independent lasers is accurately stabilized and maintained. We describe a simple and novel feed-forward method with an acousto-optic modulator. This method can be used in optical phase-locked loops.

Second-harmonic generation of an optical frequency comb at 1.55 ?m with periodically poled lithium niobate

To expand the span of the optical frequency comb (OFC), we generated the second harmonics of an OFC at 1.55microm , using a multiperiod periodically poled lithium niobate (PPLN) crystal. A coupled-cavity OFC generator with an average output power of 0.2 mW was amplified and expanded with a fiber amplifier and a dispersion-flattened fiber. The fundamental OFC average power and span were 100 mW and 45 THz, respectively. The second-harmonic combs span was 3.2 THz; however, we tuned the center frequency over 30 THz by changing the poling period. We also demonstrated that the second-harmonic comb can be used for frequency-difference measurement.

ACCURACY OF OPTICAL FREQUENCY COMB GENERATION IN OPTICAL FIBER

We evaluated the accuracy of an optical frequency comb in optical fibers by measuring the frequency shift after a sideband from an electro-optic modulator had passed through the fiber. We found that a frequency drift of a few hertz was due largely to a variation in the ambient temperature that corresponded to an increase in the square root of the Allan variance to 0.66 Hz.

Optical frequency measurement of the acetylene transitions at 1.5 /spl mu/m

We report on an optical frequency measurement of the acetylene transitions at 1.5 /spl mu/m. Using an optical frequency comb generator and second harmonic generation, we can directly connect an acetylene-stabilized diode laser to a 778 nm diode laser stabilized on a reference Rb two-photon transition.

Generation of 10-GHz 2-ps optical pulse train over the C band based on an optical comb generator and its application to 160-Gbit/s OTDM systems

We demonstrate stable generation of a 10-GHz 2-ps optical pulse train from 1530 to 1555 nm based on an optical comb generator. Such pulse train is multiplexable to 160 Gbit/s.