B. Widiyatmoko

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.

12-THz frequency difference measurements and noise analysis of an optical frequency comb in optical fibers

A span up to 50 THz of optical frequency comb (OFC) has been obtained by self-phase modulation in an optical fiber. The coherent nature of the process was verified by heterodyne-detecting the sidebands offset by up to 12 THz from the carrier. The signal-to noise ratio (SNR) of the beat signal between a sideband at 12 THz offset and another single-mode laser was 32 dB in a 1-MHz bandwidth. Although the linewidth of each beat signal was maintained within a few megahertz, phase noise pedestal power increased with the offset frequency.

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 180.345 065 08(37) and 180.021 253 10(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.

3.17-THz frequency-difference measurement between lasers using two optical frequency combs

In order to increase the signal-to-noise ratio in the measurement of frequency-difference between two lasers by using an optical frequency comb generator (OFC-G), two laser beams were combined into the OFC-G. Then many sidebands that contribute to the beat signal generation were generated. We observed the spectrum of the beat signal with a signal-to-noise ratio of 30 dB when the frequency-difference between two lasers was 3.17 THz.

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.

Linking two optical frequency combs by heterodyne optical phase locking between diode lasers at 2.6-THz frequency-difference

Two optical frequency combs (OFC) with total span of 8 THz have been realized and linked by injecting two laser beams into one OFC generator. Two lasers whose frequencies were separated by 2.6 THz were heterodyne phase-locked to link the two combs. The phase-locked loop bandwidth was 1 MHz and the phase error variance was estimated to be 0.08 rad/sup 2/.

Generation of Expanded Optical Frequency Combs

We present a method to expand optical frequency combs (OFC) using self-phase modulation in an optical fiber. The initial OFC is generated using a resonant electro-optic modulator and exhibits a span of less than 10 THz. The span of a broadened OFC can reach up to 50 THz in the 1.5 µm wavelength domain. Second-Harmonic Generation (SHG) of this OFC has also been demonstrated. The span of a second-harmonic comb can also reach up to 50 THz but in the 0.8 µm region. We also demonstrate an innovative method to make frequency-difference measurements between two laser signals when the difference frequency between the lasers is larger than the span of the OFC.

9. The cavity-enhanced optical-frequency comb generator and its applications

Publisher Summary An optical-frequency comb (OFC) is useful for applications requiring precise measurement of optical frequencies distributed over a wide frequency range. This type of measurement is required for applications as varied as dense frequency-division-multiplexed communication networking, high-resolution spectroscopy, and the measurement of fundamental physical constant. An OFC simplifies the accurate optical-frequency measurements needed in these applications. For example, by using an OFC with a tunable laser, a stable and accurately tunable coherent light source can be realized, which is nothing other than an optical-frequency synthesizer. OFCs have not only frequency-domain applications, but also novel applications in time domain. This chapter discusses an OFC generator based on an electro-optic modulator within a Fabry-Perot cavity. The chapter describes the basics of OFC generation, including expansion of the comb by self-phase modulation and discusses the methods used for optical difference frequency measurements using an OFC generator. The chapter also describes how OFCs are used, focusing on applications in precision spectroscopy and a novel implementation of optical coherent tomography.