Lingze Duan

A stable smoothly wavelength-tunable picosecond pulse generator

Smooth wavelength tuning over 10 nm has been realized in a dispersion-tuned harmonically mode-locked fiber ring laser. Supermode noise is suppressed by 13 dB by adding a semiconductor optical amplifier (SOA) into the cavity and the suppression improves as the SOA gain increases. 5.3-ps pulses are obtained at a repetition rate of 10 GHz. A simple numerical model demonstrates the effects of the intracavity dispersion and the SOA on the pulse characteristics.

Radio-frequency clock delivery via free-space frequency comb transmission

We characterize the instability of an rf clock signal caused by free-space transmission of a frequency comb (FC) under typical laboratory conditions. The phase-noise spectra show the involvement of multiple random processes. For a 10 m transmission, the rms timing jitter integrated over 1-10(5) Hz is 95 fs, and the root Allan variance over 1 s is 4x10(-13). The measured Allan variance has a tau(-1) behavior and an excellent agreement with the phase noise measurement. These results indicate the feasibility of FC-based free-space rf clock distribution over short distances.

Operation of a relativistic backward-wave oscillator filled with a preionized high-density radially inhomogeneous plasma

A modified relativistic (500 kV, 500 A) backward-wave oscillator (BWO) filled with a radially nonuniform preionized plasma of high-peak density (peak plasma frequency>operating frequency) is studied experimentally. The effects of high plasma density in the interaction region as well as the relativistic diode and the output horn regions were studied. By protecting both the diode and output regions against plasma penetration, the plasma effects on the BWO was studied up to a peak plasma density of 8/spl middot/10/sup 12/ cm/sup -3/. It was demonstrated that filling the BWO structure with high density radially nonuniform plasma leads to the following. 1) Suppression of high-order modes. This is a promising approach for implementing single mode operation of high-power large-diameter overmoded devices. 2) Frequency upshift of up to 30% (from 8.5 to 11 GHz). This demonstrates the possibility of electronic frequency tunability by controlling the plasma density. 3) Substantial increase in the measured microwave output power for n/sub p/>10/sup 12/ cm/sup -3/ over the corresponding vacuum value.

Smoothly wavelength-tunable picosecond pulse generation using a harmonically mode-locked fiber ring laser

A simple design of a stable, smoothly wavelength-tunable picosecond pulse generator has been demonstrated using a dispersion-tuned, harmonically mode-locked fiber ring laser with a directly modulated semiconductor optical amplifier (SOA). The SOA functions as both a polarization-insensitive mode locker and a supermode noise suppressor. Near-linearly chirped pulses are generated and compressed to less than 4 ps when the intracavity dispersion is anomalous while 11-ps, near-transform-limited pulses are generated without compression when the dispersion is normal. Smooth wavelength tuning is achieved over more than 11 nm by only tuning the modulation frequency and pulse characteristics are stable over the entire tuning span. A simple numerical model successively simulates the operation principle of the system. The tuning range is determined by both the gain profile and the total intracavity dispersion. The dispersion and the SOA ensure the long-term stability of the system.

Dynamic optical sampling by cavity tuning and its application in lidar

Optical sampling by cavity tuning (OSCAT) enables cost-effective realization of fast tunable optical delay using a single femtosecond laser. We report here a dynamic model of OSCAT, taking into account the continuous modulation of laser repetition rates. This allows us to evaluate the delay scan depth under high interferometer imbalance and high scan rates, which cannot be described by the previous static model. We also report the demonstration of remote motion tracking based on fast OSCAT. Target vibration as small as 15 µm peak to peak and as fast as 50 Hz along line-of-sight has been successfully detected at an equivalent free-space distance of more than 2 km.

Atmospheric Timing Transfer Using a Femtosecond Frequency Comb

We have experimentally demonstrated atmospheric transfer of microwave timing references using a femtosecond frequency comb. The excess timing jitter induced by the atmospheric propagation has been characterized, and evidence is provided to show that such characterization is not compromised by the parasitic effect of power-to-phase coupling in the photodetector. The fractional frequency stability for a 60-m total transmission distance is on the order of 10-12 with a 1-s averaging time. The Allan deviation shows a τ-1 dependence up to 500 s. Scale estimate confirms that the measured excess timing noise is caused by clear-air turbulence. Comparisons with previous works show that our results offer a more precise characterization of atmospheric timing transfer. The work may potentially help the development of high-fidelity synchronization for future free-space optical communications.

Carrier-envelope phase-sensitive inversion in two-level systems

We theoretically study the carrier-envelope phase-dependent inversion generated in a two-level system by excitation with a few-cycle pulse. On the basis of the invariance of the inversion under time reversal of the exciting field, parameters are introduced to characterize the phase sensitivity of the induced inversion. Linear and nonlinear phase effects are numerically studied for rectangular and sinc-shaped pulses. Furthermore, analytical results are obtained in the limits of weak fields, as well as strong dephasing, and by nearly degenerate perturbation theory for sinusoidal excitation. The results show that the phase-sensitive inversion in the ideal two-level system is a promising route for constructing carrier-envelope phase detectors.

Multiheterodyne Characterization of Excess Phase Noise in Atmospheric Transfer of a Femtosecond-Laser Frequency Comb

We report an experimental investigation on remote transfer of a femtosecond-laser frequency comb through an open atmospheric link. Optical multiheterodyne is used to measure the excess phase noise and the frequency stability of the transferred comb. The dispersion of air is found to have a minimal impact on the multiheterodyne signal, and the effectiveness of the technique to characterize the behaviors of comb lines under the influence of turbulence is theoretically analyzed. Large phase modulation due to the index fluctuation of the air over a 60-m transmission link is found to cause a significant linewidth broadening. Under low-wind conditions, a fractional frequency stability in the order of 10-14 has been achieved over several minutes with a 1-s averaging time. A comparison of this work with previous tests based on continuous wave (CW) lasers indicates that pulsed lasers can work as well as CW lasers for remote transfer of optical frequency references through the atmosphere.

Electromagnetic properties of periodic cavities coupled to a radiating antenna

The electromagnetic properties of spatially periodic cavities determines both the linear and nonlinear interaction between the waves and the electron beam in high-power backward-wave oscillators. A corrugated cavity is usually left open at one end for extraction of the useful microwave energy; however, reflections at the open end are large so that a cavity is still formed. In contrast to a previously studied N period closed cavity where the number of axial modes with frequencies falling in the lowest pass band of the structure is equal to N+1, an open cavity was found to support only N axial modes. In this paper the resonance frequencies, quality factors and the field patterns of the axial modes in an open cavity were all investigated experimentally and the results are in very good agreement with those obtained using a new, time-dependent, quasi-three dimensional code. It was also demonstrated that a short interface section between the periodic cavity and the radiating antenna can drastically reduce the quality factors to the diffraction limit over a very wide frequency band. This is expected to substantially increase the starting current and allow operation at high-beam current without degradation of spectral purity.

Atmospheric transfer of a radio-frequency clock signal with a diode laser

Remote transfer of a radio-frequency clock signal over a 60 m open atmospheric link has been experimentally investigated using a diode laser as the clock carrier. Phase-noise spectra and Allan deviations are both measured to characterize the excess clock instability incurred during the transfer process. Different detection schemes are used to assess the contributions from different noise sources. With an 80 MHz clock frequency, the total root-mean-square noise amplitude is measured to be about 5×10(-3)  rad, with fractional frequency instability on the order of 1×10(-10) at 1 s. The majority of this excess noise is attributed to the transmitter noise, with the amplitude fluctuations of the diode laser identified as the main source. The excess phase noise caused by air turbulence is at the level of 10(-4)  rad under the current experimental conditions. Our finding suggests that suppressing the transmitter noise is critical for improving the clock-transfer fidelity.