Mark S. Bowers

Multiwavelength mid-infrared zinc germanium phosphide and PPLN optical parametric oscillator

A tandem PPLN and ZGP optical parametric oscillator (OPO) has demonstrated the generation of multiple wavelength output in the mid-infrared. A pulsed Nd:YVO4 laser, oeprating at 1.342 microns, pumped a PPLN OPO that generated 2.12 microns and 3.66 microns. 8.4W of 1.342 microns was available at the PPLN. The PPLN OPO produced 2.6W at 2.12 microns, of which 2.35W was available to pump the ZGP OPO. The ZGP OPO generated a total combined output of 0.85W at 3.9 microns and 4.6 microns. The threshold of the ZGP OPO was measured to be 60 micro-Joules (10.6 MW/cm2, ~18 ns FWHM pulse).

Pulsed blue laser source based on frequency quadrupling of a thulium fiber laser

We describe a pulsed blue (485 nm) laser source based on frequency quadrupling a pulsed Tm fiber laser. Up to 1.2 W at 485 nm was generated with an M2 of 1.3. At 10 kHz pulse repetition frequency, the output pulse at 485 nm was 65 ns FWHM resulting in an estimated peak power of 1.8 kW. We anticipate further improvements in power scaling with higher power Tm fiber lasers and improved conversion efficiency to the blue with optimized AR coatings and nonlinear optical crystals.

High peak power, short-pulse, eyesafe fiber laser for radar applications

We present a novel high power all-fiber-based master oscillator power amplifier (MOPA) laser system operating in the C-band (1.5 mm) with pulses <5ns and a repetition rate range of 200 kHz. This system generates >4 Watts of average power and a maximum pulse energy of 20 mJ and peak power of 5 kW at 200 kHz using custom designed Er:Yb co-doped double-clad fibers. This system was also operated at reduced repetition rates of 6 kHz and pulse energy of 165 mJ was generated with a peak power of 28 kW. By shortening the seed pulses a peak power of up to 33.9 kW with a pulse energy of 73 mJ was achieved at 20 kHz. A beam quality of M2=1.2 was achieved, which makes this system very suitable for scanning lidar applications.

Stimulated Brillouin scattering in optical fibers excited by broad-band pump waves in the presence of feedback

The transient theory of stimulated Brillouin Scattering (SBS) in optical fibers is used to investigate the effects of weak feedback of an incident broadband laser field. When the linewidth of the laser approaches the Brillouin frequency shift, a weak reflection of the laser field overlaps with the SBS gain and can be amplified in the fiber resulting in an apparent reduction in SBS threshold. We show that a reflection of 0.01% (-40 dB) of a laser field with a linewidth equal to the Brillouin shift can reduce the SBS threshold by a factor of 2. These results have implications on the design of high power fiber laser systems that utilize spectral broadening to suppress SBS.

Modeling a diode pumped Er:Yb:glass laser with CO2+:spinel as a passive Q-switch

In order to develop an efficient eye-safe laser, operating in the 1.53 μm region, we have written software that models the performance of a passively Q-switched Er:Yb:glass laser with a divalent cobalt Co2+:spinel saturable absorber. At present a 0D model, which uses a plane-wave approximation, has been completed. The model is based on a set coupled first order differential equations that describe the laser dynamics. These equations represent a two-level Yb3+ diode pump scheme, a five-level Er3+ gain medium, and a four-level Co2+ Q-switch. The model takes into account cooperative upconversion and excited state absorption (ESA) in both the gain and absorber media. Solutions to the rate equations and optimization examples are presented.

Modeling gain-medium diffraction in super-Gaussian coupled unstable laser cavities

The diffractive effects of a single laser rod in an unstable super-Gaussian coupled cavity are modeled for a range of cavity configurations, with an intracavity, zero-thickness aperture. After fundamental mode propagation through a maximally flat output coupler, beam quality (M2) and far-field power loss values are related. Beam quality is most sensitive to cavity magnification and aperture Fresnel number, both correlated to the aperture-equivalent Fresnel number. In contrast, variation of M2 with aperture position is sufficiently conservative to predict the intensity profile of a solid-state laser with a typical gain length, in good agreement with experimental data.

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.

Stimulated Brillouin scattering in multimode fibers

The propagation of high power narrowband laser pulses in multimode fibers and the limitations due to SBS are presented. An injection seeded pulsed Nd:YAG laser operating at 10 Hz was used to pump undoped step index fibers to determine the SBS threshold under various conditions. Measurements on 50μm core diameter fibers with various fiber lengths and pulse durations at 1064 nm were performed and simulated with a computer code. The code considers the time dependent coupling between the pump wave, the Stokes wave, and the acoustic wave. The experimental results are in good agreement with the numerical predictions. Our results quantify the limitations of high power narrowband pulse transmission in multimode fibers.

Stimulated Brillouin scattering in optical fibers with end reflections excited by broad-band, phase-modulated pump waves

The transient theory of stimulated Brillouin Scattering (SBS) in optical fibers is used to investigate the effects of feedback of an incident periodic phase-modulated laser field. Phase modulation with a single sine wave and a pseudo-random binary sequence (PRSB) are investigated. It is shown that a reflection as small as 0.001% (-50 dB) of a laser field from the fiber end can lead to enhancement in the reflected power and effectively reduce the SBS threshold if the laser optical spectrum overlaps with the SBS gain spectrum. These results have implications on the design of high power fiber laser systems that utilize periodic phase modulation techniques for broadening the laser field to suppress SBS.

High efficiency, high pulse energy fiber laser system

We report a master-oscillator/power-amplifier laser system featuring a polarizing and coilable 40-micron-core Yb-doped photonic crystal fiber as the final-stage amplifier. The laser source generates 3.4 ns pulses at a repetition rate 19 kHz, with maximum pulse energy 1.2 mJ, maximum average power 22.8 W, near diffraction-limited (M2 < 1.1) beam quality, and 20% electrical to optical efficiency in a compact package. This pulsed-fiber laser flight system provides high pulse energy, average power, peak power, diffraction limited beam quality, and high efficiency all in a thermally and mechanically stable compact package.