Ryan Stafford

Low threshold, singly-resonant CW OPO pumped by an all-fiber pump source

An oscillation threshold of 780mW has been demonstrated in a singly-resonant, continuous-wave optical parametric oscillator (CW SRO) using a fiber-amplified, distributed feedback (DFB) fiber laser as pump source. A linewidth of 1MHz was measured, and the idler frequency was fine-tuned by up to 130GHz by tuning the pump laser. To our knowledge, this is the first example of a single frequency CW SRO pumped by an all-fiber pump source, a reduction in threshold by a factor of three over previous 1- microm-pumped CW SROs, and a reduction by two orders of magnitude in the linewidth of CW SROs pumped by fiber pump sources.

Spectral broadening and stimulated Raman conversion in a continuous-wave optical parametric oscillator

We have confirmed that single-frequency oscillation of a continuous-wave singly resonant optical parametric oscillator is limited to operation below a critical value of the pumping ratio, as predicted by early theoretical treatments. We also report different regimes of spectral broadening as well as stimulated Raman conversion of the signal wave above this critical pump level. We show that spectral broadening may be eliminated by implementing output coupling of the signal wave and demonstrate 8.6 W of total signal and idler output with single-frequency spectra at both wavelengths for 14.5 W of pump power.

Spectral beam combining of high-power fiber lasers

We have demonstrated spectral beam combining of two high power fiber lasers and obtained more than 40 W output power from the system. The system consists of two 30 W fiber lasers, a diffraction grating and a resonator. Both fiber lasers have broadband fiber Bragg gratings on the rear sides and share the diffraction grating and output coupler on the output sides. The wavelengths of the fiber lasers are determined by the optical dispersion provided by the grating and the collimating lens, as well as the fiber spacing. A model that analyzes dependence of laser line-width on beam quality of an SBC system is given in this paper. We also model a novel configuration that can significantly improve beam quality. The experimental results have shown that control of fiber laser line-width is the key to achieving high power SBC lasers. In addition, a new approach using three gratings is proposed and has been proven by the experiment. The new approach consists of three gratings, in which one grating is used by the SBC cavity to lock wavelengths while the other two combine the collimated beams without beam quality reduction. This approach has successfully improved beam quality M2 from 11 to 2.0.

8.6-watt single-frequency CW OPO

A continuous wave singly resonant optical parametric oscillator (CW SRO) has been developed which produces a total of 8.6 Watts of single frequency output at two wavelengths. 5.1 Watts of signal output at 1.65 microns and 3.5 Watts of idler output at 3.0 microns was measured, using a 15 Watt, single frequency fiber laser pump source. Power stability of 3% peak to peak was measured over a period of 24 hours and six hours of operation without longitudinal mode hops was recorded. The beam quality of both outputs was near-diffraction-limit, with an M2 parameter < 1.1.We have also observed for the first time, the transition from single frequency to broadband oscillation of a CW SRO at pumping levels greater than three times threshold. At the highest pumping levels, Raman conversion of the signal frequency was observed. Based on these measurements we have been able to define an optimum operating point for CW SROs ensuring maximum conversion efficiency and single frequency oscillation.

Long-Term Frequency-Stable Operation of a Single Frequency CW OPO without Active Locking

Frequency stability of <400MHz without discontinuous jumps over a period of 30 hours has been demonstrated in a high-power, fiber laser-pumped, single frequency CW OPO operating at 3 microns wavelength without active frequency locking.

High-peak-power pulse amplification in SM fibers

In this paper we report on the performance of a modular single mode pulsed fiber laser system operating in the C-band. With off-the-shelf telecom components and specialty-designed electronics, 3 kW peak power can be generated in a short (1 ns) pulse at 10 kHz at 1545 nm; however, the onset of nonlinear optical effects (SRS, FWM, and SPM) is observed at a 1kW peak power level. Using highly doped erbium fibers, peak powers up to 13kW and pulse energies of up to 20μJ have been generated with a pulse duration range of 0.6-5 nsec, repetition rates between 3kHz to 1 MHz, and at a wavelength of 1545.3nm and 1567.5 nm before the onset of nonlinear effects became noticeable. Therefore, with the use of highly doped erbium fiber, the onset of nonlinear effects can be increased by an order of magnitude. For narrowband amplification, stimulated Brillouin scattering (SBS) is the limiting nonlinear process. In this regime we recorded the onset of SBS at 8μJ/pulse with a duration of 2.5 nsec. Depending on the pulse shape and pulse duration, self phase modulation (SPM) was also observed, which spectrally broadens the output centered at the signal wavelength; however, the spectral broadening due to SPM is only minor compared to SRS and FWM. It was also demonstrated that pulse steepening is minimized with an appropriate seed waveform. A 3 ns, shaped, input pulse nearly maintained its pulse duration after amplification. Without pulse shaping, the pulse shortened to 1.1 ns.

Radiant energy during infrared neural stimulation at the target structure.

Infrared neural stimulation (INS) describes a method, by which an infrared laser is used to stimulate neurons. The major benefit of INS over stimulating neurons with electrical current is its spatial selectivity. To translate the technique into a clinical application it is important to know the energy required to stimulate the neural structure. With this study we provide measurements of the radiant exposure, at the target structure that is required to stimulate the auditory neurons. Flat polished fibers were inserted into scala tympani so that the spiral ganglion was in front of the optical fiber. Angle polished fibers were inserted along scala tympani, and rotating the beveled surface of the fiber allowed the radiation beam to be directed perpendicular to the spiral ganglion. The radiant exposure for stimulation at the modiolus for flat and angle polished fibers averaged 6.78±2.15 mJ/cm2. With the angle polished fibers, a 90º change in the orientation of the optical beam from an orientation that resulted in an INS-evoked maximum response, resulted in a 50% drop in the response amplitude. When the orientation of the beam was changed by 180º, such that it was directed opposite to the orientation with the maxima, minimum response amplitude was observed.

Radiant energy during infrared neural stimulation at the target structure

Infrared neural stimulation (INS) describes a method, by which an infrared laser is used to stimulate neurons. The major benefit of INS over stimulating neurons with electrical current is its spatial selectivity. To translate the technique into a clinical application it is important to know the energy required to stimulate the neural structure. With this study we provide measurements of the radiant exposure, at the target structure that is required to stimulate the auditory neurons. Flat polished fibers were inserted into scala tympani so that the spiral ganglion was in front of the optical fiber. Angle polished fibers were inserted along scala tympani, and rotating the beveled surface of the fiber allowed the radiation beam to be directed perpendicular to the spiral ganglion. The radiant exposure for stimulation at the modiolus for flat and angle polished fibers averaged 6.78±2.15 mJ/cm2. With the angle polished fibers, a 90º change in the orientation of the optical beam from an orientation that resulted in an INS-evoked maximum response, resulted in a 50% drop in the response amplitude. When the orientation of the beam was changed by 180º, such that it was directed opposite to the orientation with the maxima, minimum response amplitude was observed.

Spectral Control of CW OPOs and Their Application to Mid-Infrared Spectroscopy

A fiber-laser-pumped CW OPO operating at 3170nm with a linewidth of 1MHz will be reported. We will describe spectral control of the output and its application to different spectroscopic measurement techniques.