An optical comb source based on a slab-coupled optical waveguide amplifier (SCOWA) is presented. The laser is
harmonically mode-locked at 10.287 GHz repetition rate and stabilized to an intra-cavity Fabry-Pérot etalon via Pound-
Drever-Hall locking. The Fabry-Pérot etalon serves as a reference for the optical frequency of the comb-lines and
suppresses the fiber cavity modes to allow only a single longitudinal mode-set to oscillate, generating a frequency comb
spaced by the repetition rate. The pulse-to-pulse timing jitter and energy fluctuations are < 2 fs and < 0.03%,
respectively (integrated from 1Hz to 100 MHz). Fundamental to this result is the incorporation of the SCOW amplifier
as the gain medium and the use of an ultra-low noise sapphire-loaded cavity oscillator to mode-lock the laser. The
SCOWA has higher saturation power than commercially available gain media, permitting higher intra-cavity power as
well as available power at the output, increasing the power of the photodetected RF tones which increases their signal-to-noise
ratio. A high visibility optical frequency comb is observed spanning ~3 nm (at -10 dB), with optical SNR > 60 dB
for a cavity with no dispersion compensation. Initial results of a dispersion compensated cavity are presented. A spectral
width of ~7.6 nm (-10 dB) was obtained for this case and the pulses can be compressed to near the transform limit at
~930 fs.
KEYWORDS: Mode locking, Oscillators, Frequency combs, Optoelectronics, Optical resonators, Phase shifts, Phase modulation, Laser systems engineering, Modulation, Signal to noise ratio
A CW injection locked Coupled Opto-Electronic Oscillator (COEO) is presented with a 10.24 GHz spaced optical frequency comb output as well as a low noise RF output. A modified Pound-Drever-Hall scheme is employed to ensure long-term stability of the injection lock, feeding back into the cavity length to compensate for cavity resonance drifts relative to the injection seed frequency. Error signal comparison to an actively mode-locked injection locked laser is presented. High optical signal-to-noise ratio of ~35 dB is demonstrated with >20 comblines of useable bandwidth. The optical linewidth, in agreement with injection locking theory, reduces to that of the injection seed frequency, <5 kHz. Low amplitude and absolute phase noise are presented from the optical output of the laser system. The integrated pulse-to-pulse energy fluctuation was found to be reduced by up to a factor of two due to optical injection. Additional decreases were shown for varying injection powers.
We report the fabrication of a saturable absorber made of a novel polymer SU8 doped with Single Wall Carbon
Nanotubes (SWCNTs). A passive mode-locked ring cavity fiber laser was built with a 100 μm thick SU8/SWCNT film
inserted between two FC/APC connectors. Self-starting passively mode-locked lasing operation was observed at 1572.04
nm, with a FWHM of 3.26 nm. The autocorrelation trace was 1.536 ps corresponding to a pulse-width of 871 fs. The
time-bandwidth product was 0.344, which is close enough to transform-limited sech squared pulses. The repetition rate
was 21.27 MHz, and a maximum average output power of 1 mW was also measured.
This work discusses the development of a frequency chirped, low repetition rate, semiconductor based mode-locked
laser having reduced noise over previous demonstrations. Specifically, we present a major upgrade on the 100 MHz
harmonically mode-locked Theta (Θ) laser cavity design in the form of the introduction of an intra-cavity fiberized
Fabry-Perot etalon. The initial demonstration of the Theta cavity design offered improved energy per pulse and a linearly
chirped pulse output compared to conventional cavity designs. Nonetheless, it suffered from pulse-to-pulse timing and
energy noise. The noisy operation arises from the harmonic nature of the laser. To mitigate this effect we have inserted a
fiberized etalon within the laser cavity.
The intra-cavity etalon stores and inter-mixes the pulses of the harmonically mode-locked laser, as well as enforces
lasing on a single optical mode-set from the multiple interleaved sets supported by the mode-locked laser due to its
harmonic nature. This leads to the generation of a stable optical frequency comb with 100 MHz spacing and the
suppression of the RF super-mode noise spurs, which results in a reduction of the laser noise. Due to fiber length drift in
both the fiberized laser cavity and the fiberized etalon, a long-term stabilization scheme is necessary. An intra-cavity
Hansch - Couillaud scheme is employed. The laser outputs chirped pulses with 10 nm of bandwidth.
This work provides an in depth analysis of both the development of the Theta cavity with the intra-cavity etalon and
the performance of the developed laser system.
The generation of optical pulses with parabolic time intensity profiles is experimentally demonstrated. A Mode Locked Laser
(MLL) that generates near transform limited pulses with a gaussian optical spectrum are temporally stretched using a linearly
chirped fiber Bragg grating. The temporal intensity profile of the stretched pulses matches the optical spectrum of the laser
due to frequency-to-time mapping. An amplitude modulator is driven by a carefully designed voltage signal to result in
parabolic pulses. Experimental results of pulse shaping with a MLL input source are presented, and show good agreement
with modeling results. Parabolic pulse generation using a CW laser source is also demonstrated and a deviation of less than
3% from an ideal parabola is observed.
Mode-locked lasers have applications in signal processing and communications such as analog to digital conversion,
arbitrary waveform generation and wavelength division multiplexing. For such applications low noise and phase
coherent frequency stabilized optical combs are needed. In this work we report a low noise, Pound-Drever Hall
frequency stabilized, semiconductor mode-locked laser at 10.287GHz centered at 1550nm with 1000-Finesse sealed,
ultralow insertion loss intracavity etalon. The output optical power of the mode locked laser is ~5mW.
KEYWORDS: Modulators, Modulation, Laser sources, Pulsed laser operation, Analog electronics, Transmittance, Electro optics, Electrooptic modulators, Signal generators, Digital signal processing
In this work we present a method for improving the uniformity of the optical spectrum or the temporal intensity
profile of a quasi-CW, linearly chirped laser source covering the time interval between subsequent pulses. A novel laser
cavity design, referred to as the Theta (Θ) cavity, provides linearly chirped pulses directly from the laser oscillator that
having non-uniform optical spectrum, that is mapped into the temporal intensity profile of the pulse, due to the
frequency-to-time mapping nature of this cavity design. The system developed in this work has been designed to
improve the spectral and temporal intensity profile of lasers for photonic signal processing.
A fiberized feed-forward system is implemented to reduce variations in the temporal intensity profile, or the optical
spectrum due to the time-to-frequency mapping, input to the system. In the feedforward scheme presented, the quasi-CW
pulse train generated from the laser is split and part of it is photodetected, while the electrical signal generated alters the
transmittance of the second part of the input as it goes through an amplitude modulator, resulting in increase in the
uniformity of the signal. The contrast of the optical spectrum of a chirped pulse at input to the system is improved from
51% to 16%, or 3.1 times.
A chirped fiber Bragg grating with a dispersion of 1651ps/nm is used to generate temporally stretched,
frequency chirped pulses from a passively mode locked fiber laser that generates pulses of ~1ps (FWHM)
duration at a repetition rate of 20MHz with 3.5mW average power (peak power of 175W). The use of a
chirped fiber Bragg grating enables the generation of temporally stretched pulses with low peak power so
that non-linear effects in the fiber can be avoided. A fiber based interferometeric arrangement is used for
interfering a reference signal with the reflected signal from the target to realize a coherent heterodyne
detection scheme. In the RF domain, the detected heterodyne beat frequency shifts as the target distance is
changed. A round trip target distance of 14km in air is simulated using 9.3km of optical fiber and a
resolution of less than a millimeter is observed.
An eXtreme Chirped Pulse Oscillator (XCPO) implemented with a Theta cavity and based on a semiconductor optical
amplifier (SOA) is presented for generating 10ns frequency-swept pulses and 3.6ps compressed pulses directly from the
oscillator. In this experiment, we show the two distinct characteristics of the XCPO which are the scalability of the
output energy and the mode-locked spectrum. The laser cavity design allows for low repetition rate operation <100MHz,
as well. The cavity, significantly, reduces nonlinear carrier dynamics, integrated self phase modulation (SPM), and fast
gain recovery in an SOA. Due to the laser's ability to generate directly frequency-swept pulses from the oscillator, this
oscillator can be used for high speed frequency-swept optical coherence tomography (OCT) and time-stretched photonic
analog to digital converters (P-ADC).
The development of a TEA HF (SF6:C3H8:He) oscillator-double amplifier laser system with one common switching element and one common power supply unit is presented. All three cavities employ the conventional charge transfer excitation circuit and the surface corona discharge preionization scheme. For each amplifier the small signal gain coefficient go and the saturation energy density Es are determined and their dependence on the charging voltage and the active gas concentrations is investigated. The performance characteristics of the laser system i.e. maximum output energy, pulse duration, spectral and beam profile evolution through all three stages of the system are given. Additionally, beam propagation measurements through a 150cm long, 250μm core diameter GeO2 mid-IR fiber are presented.
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