Distributed Fiber Optic Sensing (DFOS) systems rely on measuring and analyzing different properties of the backscattered light of an optical pulse propagating along a fiber cable. DFOS systems can measure temperature, strain, vibrations, or acoustic excitations on the fiber cable and to their unique specifications, they have many applications and advantages over competing technologies. In this talk we will focus on the challenges and applications of DFOS systems using outdoor grade telecom fiber networks instead of standard indoor or some specialty fiber cables.
A regeneratively mode-locked laser with simultaneous low noise radio frequency (RF) tone and optical comb generation is presented. The laser does not need any external RF signal and emits a pulse train at ∼10 GHz repetition rate with a 1.5-ps optical pulse width after compression. The generated RF tone has a signal-to-noise ratio of 121 dB/Hz and an RF fluctuation of 10−9 over 0.1 s. The optical frequency comb spacing is also at ∼10 GHz and the optical comb tooth has a linewidth of <1 kHz.
In the design of mode-locked lasers, single-walled carbon nanotube saturable absorbers (SWCNT-SAs) have emerged as important alternatives to semiconductor saturable absorber mirrors (SESAMs) due to their favorable optical
characteristics, low cost, and relatively simple fabrication scheme. Therefore, it is of great interest to explore the limits of energy scaling in solid-state lasers mode-locked with SWCNT-SAs. Due to their unique wavelength range for
biomedical applications, a room-temperature Cr4+:forsterite laser operating near 1.3 μm was used in the mode-locking experiments. The laser was end-pumped with a continuous-wave Yb-fiber laser at 1064 nm. Furthermore, a q-preserving multipass-cavity (MPC) was added to the short resonator to lower the pulse repetition rate to 4.51 MHz and to scale up the output pulse energy at low average power. The SWCNT-SA was fabricated with SWCNTs grown by the highpressure CO conversion (HiPCO) technique. With dispersion compensation optics, the net group delay dispersion of the resonator was estimated to be around -4440 fs2. When mode-locked with the SWCNT-SA, the resonator produced 10-nJ, 121-fs pulses at 1247 nm with a spectral bandwidth of 16 nm, corresponding to a time-bandwidth product of 0.37. To our knowledge, this represents the highest peak power (84 kW) generated to date from a bulk femtosecond solid-state laser, mode-locked by using a SWCNT-SA. The results also suggest that the peak power achieved in our experiments was limited only by the self-focusing in the Cr4+:forsterite gain medium and further increase in output energy should in principle be possible in other gain media mode-locked with SWCNT-SAs.
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.
It is critical to know the free spectral range (FSR) of an etalon for telecommunication applications. In this
paper, we have improved the Pound-Drever-Hall (PDH) based technique for measuring the FSR of an
etalon by 2 orders of magnitude. This improved technique results 1 part in 106 precision. To our knowledge
this is the most precise measurement of FSR.
We are investigating optical frequency comb generation by direct modulation of CW light. Our scheme is based on three cascaded modulators; one amplitude modulator and two phase modulators. The modulation scheme is optimized for flatness and power efficiency. A stable optical spectrum has been generated with ~100 comb lines with 0.625 GHz spacing and 3 dB flatness. We also investigate comb generation via phase only modulation.
Mode-locked laser designs for both time and frequency domain based applications are presented. It is shown that for
strictly time domain applications, simple laser cavity designs can produce pulse trains at 10 GHz with sub-5 fs relative
timing jitter (1 Hz-100 MHz) using only commercially available components. Frequency stabilized sources maintain
low timing jitter while achieving 1MHz maximum frequency deviations for optical spectra consisting of ~200 10 GHz
spaced frequencies. Methods of characterizing pulse-to-pulse timing jitter by examining the photodetected spectrum are
reviewed. The effects of the phase noise of an rf source used to drive an actively mode-locked laser on the laser's
amplitude and timing fluctuations is also discussed.
Frequency skewed optical pulses are generated via both a composite cavity structure in a fiberized semiconductor optical amplifier ring laser and a frequency skew loop outside the laser cavity. The composite cavity technique is similar to rational harmonic mode-locking, however it is based on cavity detuning rather than frequency detuning. These frequency skewed pulses are ideal for range detection applications since their interference results in a range dependent RF signal. The intracavity frequency skewed pulse train showed superior performance in both stability and signal quality.
We report on supermode noise suppression of a harmonically modelocked laser by optical injection. The modelocked
laser was injection locked to a CW narrow linewidth source. Injection locking selects a single supermode group
reducing the supermode noise spurs in the photodetected signal by 20 dB to a level of -130 dBc/Hz.
A novel method incorporating time division multiplexing technique with optical parabolic phase modulation has been introduced to overcome the limitations on optical generation of chirped RF signals. Extension of the frequency span and frequency sweep time of a RF chirp signal has been experimentally realized. A chirped RF signal with a center frequency of 100 MHz, frequency span of 20 MHz and sweep time of 200 ns has been generated via this novel method. This chirp signal agrees well with the chirp signal generated by conventional methods.
Frequency stabilized modelocked lasers have recently garnered much attention owing to their potential in metrology, communications, and signal processing applications. The possibility of optical source technology that is economical, compact, and electrically efficient suggests that semiconductor gain media could allow frequency stabilized ultrafast sources to rapidly gain a foothold in communication and signal processing applications. This work will summarize recent work in the area of stabilized modelocked semiconductor diode lasers, and highlight unique features that will impact photonic signal processing applications.
Using an intracavity Pound-Drever-Hall technique, simultaneous optical frequency comb stabilization within ±3 MHz range and supermode phase noise suppression were demonstrated for a 10 GHz harmonically modelocked semiconductor ring laser resulting in timing jitter of 63.5 fs integrated from 10 Hz to 10 MHz.
This paper discusses use of optical frequency combs generated by modelocked semiconductor lasers for coherent photonic signal processing applications. Key in our approach is a high Q cavity, supermode suppression and low spontaneous emission. Targeted applications of the stabilized optical frequency combs lie in areas of metrology, optical sampling, arbitrary waveform generation and communications using coherent detection.
A novel time division multiplexing technique has been incorporated with optical serrodyne phase modulation to generate a single microwave tone with up to 1 MHz tunability. Sideband spurs due to finite phase reset time in ordinary serrodyne phase modulation have been suppressed to more than 40 dB below the carrier. These results show that endless phase modulation can easily be achieved using this novel technique.
We report on low noise performance of a 10 GHz actively mode-locked laser. The laser is a fiberized ring laser using a commercially available semiconductor optical amplifier (SOA) as the gain medium. The noise properties as a function of cavity length and optical spectrum are investigated. It is found that supermode noise is reduced when the cavity length is increased past a certain threshold. Best performance is achieved with a 20 meter cavity whose pulses are down chirped. No active feedback control is utilized to reduce the noise, yet the integrated jitter is only 29 fs (10Hz-100MHz).
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