Biomedical subjects

N R Newbury

Publications and source records attributed to N R Newbury.

8 recordsLinked to original sources

PubMed2006

Fiber-laser frequency combs with subhertz relative linewidths.

We investigate the comb linewidths of self-referenced, fiber-laser-based frequency combs by measuring the heterodyne beat signal between two independent frequency combs that are phase locked to a common cw optical reference. We demonstrate that the optical comb lines can exhibit instrument-limited, subhertz relative linewidths across the comb spectra from 1200 to 1720 nm with a residual integrated optical phase jitter of approximately 1 rad in a 60 mHz to 500 kHz bandwidth. The projected relative pulse timing jitter is approximately 1 fs. This performance approaches that of Ti:sapphire frequency combs.

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PubMed2006

Elimination of pump-induced frequency jitter on fiber-laser frequency combs.

Optical frequency combs generated by femtosecond fiber lasers typically exhibit significant frequency noise that causes broad optical linewidths, particularly in the comb wings and in the carrier-envelope offset frequency (f(ceo)) signal. We show these broad linewidths are mainly a result of white amplitude noise on the pump diode laser that leads to a breathing-like motion of the comb about a central fixed frequency. By a combination of passive noise reduction and active feedback using phase-lead compensation, this noise source is eliminated, thereby reducing the f(ceo) linewidth from 250 kHz to <1 Hz. The in-loop carrier-envelope offset phase jitter, integrated to 100 kHz, is 1.3 rad.

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PubMed2006

Frequency-resolved coherent lidar using a femtosecond fiber laser.

We demonstrate a coherent lidar that uses a broadband femtosecond fiber laser as a source and resolves the returning heterodyne signal into N spectral channels by using an arrayed-waveguide grating. The data are processed incoherently to yield an N-times improvement in the Doppler measurement of a surface vibration. For N=6, we achieve a sensitivity of 153 Hz, corresponding to a 0.12 mm/s motion, in 10 ms despite a signal that is speckle broadened to 14 kHz. Alternatively, the data are processed coherently to form a range image. For a flat target, we achieve a 60 microm range resolution, limited mainly by the source bandwidth, despite the dispersion of 1 km of optical fiber in the signal path.

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PubMed2005

Stabilized frequency comb with a self-referenced femtosecond Cr:forsterite laser.

A frequency comb is generated with a Cr:forsterite femtosecond laser, spectrally broadened through a highly nonlinear optical fiber to span from 1.0 to 2.2 ,m, and stabilized using the f-to-2f self-referencing technique. The repetition rate and the carrier-envelope offset frequency are stabilized to a hydrogen maser, calibrated by a cesium atomic fountain clock. Simultaneous frequency measurement of a 657-nm cw laser by use of the stabilized frequency combs from this Cr:forsterite system and a Ti:sapphire laser agree at the 10(-13) level. The frequency noise of the comb components is observed at 1064, 1314, and 1550 nm by comparing the measured beat frequencies between cw lasers and the supercontinuum frequency combs.

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PubMed2004

Absolute-frequency measurements with a stabilized near-infrared optical frequency comb from a Cr:forsterite laser.

A frequency comb is generated with a chromium-doped forsterite femtosecond laser, spectrally broadened in a dispersion-shifted highly nonlinear fiber, and stabilized. The resultant evenly spaced comb of frequencies ranges from 1.1 to beyond 1.8 microm. The frequency comb was referenced simultaneously to the National Institute of Standards and Technology's optical frequency standard based on neutral calcium and to a hydrogen maser that is calibrated by a cesium atomic fountain clock. With this comb we measured two frequency references in the telecommunications band: one half of the frequency of the d/f crossover transition in 87Rb at 780 nm, and the methane v2 + 2v3 R(8) line at 1315 nm.

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PubMed2003

420-MHz Cr:forsterite femtosecond ring laser and continuum generation in the 1-2-micrometre range.

We demonstrate a chromium-doped forsterite femtosecond ring laser that generates 30-fs pulses at a 420-MHz repetition rate with nearly 500 mW of average power. The compact solid-state design and broad spectral output make this laser attractive for telecommunications applications in the 1.3-1.5-micrometre region. Additional spectral broadening of the laser output in highly nonlinear optical fiber leads to octave-spanning spectra ranging from 1.06 to 2.17 micrometre. The octave is reached at a level of 18 dB below the peak. The underlying optical frequency comb can be linked to existing optical frequency standards.

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PubMed2003

Noise amplification during supercontinuum generation in microstructure fiber.

Supercontinua generated by femtosecond pulses launched in microstructure fiber can exhibit significant low-frequency (<1-MHz) amplitude noise on the output pulse train. We show that this low-frequency noise is an amplified version of the amplitude noise that is already present on the input laser pulse train. Through both experimental measurements and numerical simulations, we quantify the noise amplification factor and its dependence on the supercontinuum wavelength and on the energy and duration of the input pulse. Interestingly, the dependence differs significantly from that of the broadband white-noise component, which arises from amplification of the input laser shot noise.

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PubMed2003

Fundamental noise limitations to supercontinuum generation in microstructure fiber.

Broadband noise on supercontinuum spectra generated in microstructure fiber is shown to lead to amplitude fluctuations as large as 50% for certain input laser pulse parameters. We study this noise using both experimental measurements and numerical simulations with a generalized stochastic nonlinear Schrödinger equation, finding good quantitative agreement over a range of input-pulse energies and chirp values. This noise is shown to arise from nonlinear amplification of two quantum noise inputs: the input-pulse shot noise and the spontaneous Raman scattering down the fiber.

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