Beam shaping to suppress phase mismatch in high power second-harmonic generation

A deliberate pattern of distributing the radiation energy of a high power laser can minimize the steady-state thermal gradients in a nonlinear crystal, thus permitting a large increase of the average powers of such systems. The theory of the two main techniques of beam shaping-fanning and scanning-is developed. Results of several experiments based on this theory are given. Experimental techniques, including some not yet attempted, are discussed.     

Fabrication of optically nonlinear semiconductor mirrors for modelocking of neodymium-doped fiber lasers

A recipe is given for an all-semiconductor, optically nonlinear mirror for passive modelocking of neodymium-doped fiber lasers. InGaAs-GaAs quantum wells (QWs) are used for a saturable absorber. It was empirically found that the best modelocked operation occurred when QWs had a heavy-hole excitonic absorption peak at a wavelength about 10-20 nm longer than the lasing wavelength. An AlAs-GaAs quarter-wavelength stack is used for a rear high reflector. Nonlinear mirrors are etched after epitaxial growth so that the lasing wavelength coincides with a reflectance maximum, giving maximum optical modulation. Nonlinear mirrors are implanted with large doses (2/spl times/10/sup 15//cm/sup 2/) of hydrogen ions in order to increase electron-hole recombination rates, speed material recovery, increase saturation intensity, and inhibit spontaneous Q-switching. Studies of material growth by molecular beam epitaxy and metal-organic chemical vapor deposition (MOCVD) are reported. Modelocked fiber lasers exhibited a pseudorandom output that was produced by several optical pulses that circulated simultaneously in the fiber. It was found that a very high reflectance mirror is not required for lasing or modelocking. To our knowledge, this is the first report of the use of a nonlinear mirror grown by MOCVD to modelock a neodymium fiber laser.     

A range-resolved Doppler imaging sensor based on fiber lasers

We have developed a unique eyesafe (1.5 μm) laser with a pseudorandom pulse output. The laser is based on a mode-locked pulse format; the pulse pattern repeats over a time consistent with the cavity round-trip period but appears random within that period. The pseudorandom pulse format occurs when we overdrive a passive mode-locking mirror in a long fiber laser. This laser is used for generating range-resolved Doppler images     

Low-heat high-power scaling using InGaAs-diode-pumped Yb:YAG lasers

We report to our knowledge the highest to date quasi-CW output power, 600 W and pulse energy, >1 J, for an InGaAs diode-pumped Yb:YAG laser. In separate preliminary results, we have also obtained 225 W of average output power under true CW diode pumping. This performance was obtained using a laser head designed to be part of a master oscillator power amplifier (MOPA) operating at 3 kW. We summarize why the diode-pumped Yb:YAG crystal laser is ideal for scaling to high average powers and the different approaches being pursued. We also report our latest results for side-pumped rod devices     

Scalable visible Nd:YAG pumped Raman laser source

Experimental and analytical studies of a 60 Hz, 0.45 W, 630 nm Raman laser source with a 12 ns pulse duration have demonstrated an overall electrical to optical efficiency of 0.12 percent. This is the first demonstration of a short pulse, high repetition rate red laser at such a high average output power without the need for a visible pump laser. Additional significance arises from the fact that simple extensions of the present work will produce many wavelengths in the visible and near infrared (IR) spectral region. A 1064 nm Nd:YAG pump laser operating at 60 Hz was used to pump a methane gas Raman laser operating at 1544 nm. This wavelength was mixed with the remaining 1064 nm laser output in noncritically phase matched lithium niobate to produce 630 nm radiation. The optical energy conversion efficiencies for the three steps were 1.4, 30, and 20 percent, respectively, for output energies of 86 mJ at 1064 nm, 15 mJ at 1544 nm, and 7.5 mJ at 630 nm. rms pulse amplitude variation measured 6 percent or less. A 10.7 million pulse life test was conducted, and the average output energy did not vary more than ±10 percent from its initial value.     

A 2.65-kW Yb:YAG single-rod laser

We report a continuous-wave average output power of 2.65 kW from a single Yb:YAG laser rod pumped with 9000 W from 940 nm InGaAs laser diodes. To the best of our knowledge, this is the highest average output power ever reported from a single Yb:YAG gain element. The optical-to-optical efficiency (i.e., output power to raw laser diode optical power) was 28%. We also obtained 860 W with an M/sup 2/ of 2.1 when pumping with 6000 W, obtaining 14% optical-to-optical efficiency.