Mehmetcan Akbulut

Advanced Ultrafast Technologies Based on Optical Frequency Combs

This paper presents recent results in the development of novel ultrafast technologies based on the generation and application of stabilized optical frequency combs. By using novel active resonant cavity injection locking techniques, filtering, modulation and detection can be performed directly on individual components of the frequency comb enabling new approaches to optical waveform synthesis, waveform detection and matched filtering, with effective signal processing bandwidths in excess of 1 THz.

A Semiconductor-Based 10-GHz Optical Comb Source With Sub 3-fs Shot-Noise-Limited Timing Jitter and

In this work, a 10.287-GHz semiconductor-based harmonically mode-locked laser with 1000 finesse intracavity etalon is demonstrated. The timing jitter integrated from 1 Hz to 100 MHz (Nyquist) is 3 fs (14 fs). The optical linewidth is ~500 Hz and the optical frequency stability is <150 kHz over 30 s.

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We propose an intensity modulator based on injection locking of a resonant cavity with gain that has a linear transfer function, multigigahertz bandwidth, possible optical gain, and very low V(pi). The arcsine phase response of the injection-locked resonant cavity placed in one arm of a Mach-Zehnder interferometer is the key to the true linear performance of this modulator. The first (to our knowledge) demonstration of this modulator with 5 GHz bandwidth, V(pi) of approximately 2.6 mV, and 95 dB spur-free dynamic range is reported here.

500-Hz Comb Linewidth

A 10.287 GHz optoelectronic oscillator is experimentally demonstrated that uses a 1000 finesse Fabry-Perot etalon as the mode selector instead of an rf filter. The results are compared with a standard optoelectronic loop with an rf filter. The substitution of the rf filter with the optical filter results in a higher rf stability and lower phase noise.

Resonant cavity linear interferometric intensity modulator

In this paper, we present results on a master-oscillator Yb-doped fiber amplifier with 1 kW cw output power (at 1064nm), and near-diffraction limited beam quality (M2<1.4), with internal quantum efficiency >83%. The final amplifier stage uses a very high Yb-doped 35-um core LMA fiber, using a new process recipe that virtually eliminates photo-darkening. As a result, high efficiency, SBS-free operation to 1 kW cw power level is obtained, with a phase modulation bandwidth of only 450MHz, well below other reported results. To enable single-frequency cw power scaling to kW levels, we investigate LMA fiber waveguide designs exploiting gain-discrimination, using partially Yb-doped LMA fiber cores, with various diameters up to 80-um. SBS-free, singlefrequency (few kHz) operation is demonstrated up to 0.9kW cw power. At the lower cw powers (<200W) neardiffraction limited beam-quality is obtained, but is observed to deteriorate at higher cw powers. We discuss potential causes, and present a detailed simulation model of kW large-core fiber-amplifiers, that includes all guided modes, fiber bend, transverse spatial hole burning, gain-tailoring, mode-scattering, SBS nonlinearity, and various thermal effects. This model shows good agreement with the observed single-frequency power scaling and beam-quality characteristics.

Optoelectronic loop design with 1000 finesse Fabry-Perot etalon

A novel method of measuring the optical frequency stability of a continuous-wave (CW) laser by using an etalon-based optoelectronic oscillator is demonstrated. A 1000 finesse Fabry-Pérot etalon is used as a reference which eliminates the need of using an independent optical source as a frequency reference. Using this technique, the optical frequency stability of a CW laser is measured with ~3.5-kHz optical frequency resolution at update rates of 90 Hz.

1 kW cw Yb-fiber-amplifier with <0.5GHz linewidth and near-diffraction limited beam-quality for coherent combining application

A semiconductor-based mode-locked laser source with low repetition rate, ultralow amplitude, and phase noise is introduced. A harmonically mode-locked semiconductor-based ring laser is time demultiplexed at a frequency equal to the cavity fundamental frequency (80MHz), resulting in a low repetition rate pulse train having ultralow amplitude and phase noise, properties usually attributed to multigigahertz repetition rate lasers. The effect of time demultiplexing on the phase noise of harmonically mode-locked lasers is analyzed and experimentally verified.

Optical Frequency Stability Measurement Using an Etalon-Based Optoelectronic Oscillator

Heavy doping of common silica gain fibers is not practical; therefore long fibers are required for efficient amplification (usually 5-10m). This is undesirable due to nonlinearities that grow with fiber length. In contrast, NP Photonics phosphate-glass based fibers can be heavily doped without any side-effects, and hence can provide very high gain in short lengths (less than 0.5m). This enables an ideal pulsed fiber amplifier for a MOPA system that maximizes the energy extraction and minimizes the nonlinearities. We demonstrate 1W average power, 200μJ energy, and >10kW peak power from a SBS-limited all-fiber MOPA system at 1550nm, and 32W average power, 90μJ energy, and 45kW peak power from a SRS and SPM limited all-fiber MOPA system at 1064 nm. These results were limited by the seed and pump sources.

Low-noise, low repetition rate, semiconductor-based mode-locked laser source suitable for high bandwidth photonic analog–digital conversion

We have demonstrated a novel coherent optical detection architecture that serves as a matched filter and successfully discriminates between various encoded optical waveforms with high confidence. The waveforms are produced by encoding the spectral phase of an optical frequency comb with codes from a Walsh-Hadamard set. The encoded optical waveforms are decoded with a coherent detection technique implemented using an optical frequency comb as a set of local oscillators, an interferometer, and a pair of differential balanced photodetectors. The use of only linear optical devices in the receiver results in a higher sensitivity compared to the use of nonlinear optical thresholding and gating techniques. Decoding is demonstrated at power levels of ~ 25 μW, which are much lower than previously reported. Possible applications of the receiver architecture include laser radar and optical communications in an optical code-division multiple access network.

High energy, high average and peak power phosphate-glass fiber amplifiers for 1micron band

A low noise, frequency stabilized, semiconductor based, 10.287 GHz mode-locked laser with 1000 finesse intracavity etalon is demonstrated with a timing jitter (1Hz – 100MHz) of 10.9 fs and optical frequency fluctuations less than 150 kHz.