Coherent polarization locking: an approach to mitigating optical damage in a pulsed Ho:YAG laser.

Intracavity optical damage is mitigated in a pulsed Ho:YAG laser cavity using the coherent polarization locking (CPL) technique. By splitting the available pump power into two individual Ho:YAG laser rods, we passively coherently locked two orthogonal polarization lasers with 9.13 mJ output pulse energies and 14 ns pulsewidths, and operating at 800 Hz repetition rate. A conventional Ho:YAG laser cavity with the same pump and cavity configuration results in severe optical damage when operating at <2 kHz repetition rate, thus limiting the output pulse energies to <5 mJ. We also demonstrated, to the best of our knowledge, the first pulsed operation within the entire CPL Ho:YAG laser cavity by Q-switching in one of the polarization arms, producing nanosecond pulses with no sign of pulse instability.

High energy and average power femtosecond laser for driving mid-infrared optical parametric amplifiers.

We have developed the first (to our knowledge) femtosecond Tm-fiber-laser-pumped Ho:YAG room-temperature chirped pulse amplifier system delivering scalable multimillijoule, multikilohertz pulses with a bandwidth exceeding 12 nm and average power of 15 W. The recompressed 530 fs pulses are suitable for broadband white light generation in transparent solids, which makes the developed source ideal for both pumping and seeding optical parametric amplifiers operating in the mid-IR spectral range.

Preserving a diffraction-limited beam in Ho:YAG laser using coherent polarization locking.

We overcome several thermal issues present in Ho:YAG lasers by distributing the gain over a larger volume and achieve a diffraction-limited beam using coherent polarization locking. Increased single-pass absorption, suppression of output power saturation, and improvement in beam quality were shown using the coherent polarization locking technique as compared to a conventional Ho:YAG laser cavity with the same pump and cavity configuration. Ten watts of CW Ho:YAG laser power was generated with >96% coherent combining efficiency.

High power coherent polarization locked laser diode.

We have coherently combined a broad area laser diode array to obtain high power single-lobed output by using coherent polarization locking. The single-lobed coherent beam is achieved by spatially combining four diode emitters using walk-off crystals and waveplates while their phases are passively locked via polarization discrimination. While our previous work focused on coherent polarization locking of diode in Gaussian beams, we demonstrate in this paper, the feasibility of the same polarization discrimination for locking multimode beams from broad area diode lasers. The resonator is designed to mitigate the loss from smile effect by using retro-reflection feedback in the cavity. In a 980 nm diode array, we produced 7.2 W coherent output with M2 of 1.5x11.5. The brightness of the diode is improved by more than an order of magnitude.

Femtosecond laser pumped by high-brightness coherent polarization locked diodes.

We demonstrate, for the first time to our knowledge, the use of a coherent polarization locked diode as the high-brightness pump source for a femtosecond laser. Four diode emitters are coherently locked to produce more than 5 W linearly polarized, narrow linewidth, and single-lobed pump beam. This gives >10× brightness improvement over the conventional diode array. The diode beam is then used to pump a Yb:KYW laser to obtain 2 W output with 57% slope efficiency in cw laser operation. By using a saturable absorber mirror, we achieved cw mode-locking operation with a 177 fs pulse width at an average power of 0.55 W.

Multipoint laser Doppler vibrometry with single detector: principles, implementations, and signal analyses.

A 20-point laser Doppler vibrometer with single photodetector is presented for noncontact dynamic measurement. A 5×4 beam array with various frequency shifts is generated by a 1.55 µm distributed feedback laser and four acousto-optic devices, and illuminating different points on vibrating objects. The reflected beams are coupled into a single-mode fiber by a pigtailed collimator and interfere with a reference beam. The signal output from a high-speed photodetector is amplified and then digitized by a high-speed analog-to-digital converter with a sampling rate of 1 gigasample per second (1 GS/s). Several methods are introduced to avoid the cross talk among different frequencies and extract the vibration information of 20 points from a one-dimensional signal. Two signal processing algorithms based on Fourier transform and windowed Fourier transform are illustrated to extract the vibration signals at different points. The experimental results are compared with that from a commercial single-point laser vibrometer. The results show simultaneous vibration measurement can be realized on multiple points using a single laser source and a single photodetector.

Spatially encoded multibeam laser Doppler vibrometry using a single photodetector.

A laser Doppler vibrometer with single photodetector is introduced to measure the vibration on multiple points of target simultaneously. A 2 x 5 beam array with various frequency shifts is generated by three acousto-optic devices, illuminating different points on a vibrating object. The reflected beams interfere with a reference beam on a high-speed photodetector, and the signal is amplified and digitized with a rate of 500 megasamples/s. To extract vibration information of different points, the carrier frequencies of each beam are elaborately designed so that they can be separated from cross-talk regions in the spectrum. The experimental results are compared with that from a commercial single-point vibrometer, and the comparison shows that it is possible to do a precise measurement on multiple points simultaneously using a single photodetector.

Generation of radially polarized beam with a segmented spiral varying retarder.

We convert a linearly polarized Gaussian beam into a radially polarized doughnut beam with an eight-segment spirally varying retarder (SVR) at wavelength of 808 nm. The SVR is designed based on the linear birefringence of alpha-barium borate (alpha-BBO) crystal and fabricated using a dry etching process. Radially polarized light of high purity (> 96% at far-field distribution) was generated experimentally using the segmented SVR positioned between two quarter waveplates with orthogonal slow axes. The emergent polarization can be switched between radially and azimuthally polarized cylindrical vector beams with a pair of half-wave plates.

Simple coherent polarization manipulation scheme for generating high power radially polarized beam.

We present a simple novel scheme that converts a Gaussian beam into an approximated radially polarized beam using coherent polarization manipulation together with Poynting walk-off in birefringent crystals. Our scheme alleviates the interferometric stability required by previous schemes that implemented this coherent mode summation using Mach-Zehnder-like interferometers. A symmetrical arrangement of two walk-off crystals with a half-wave plate, allows coherence control even when the laser has short temporal coherence length. We generated 14 watts of radially polarized beam from an Ytterbium fiber laser, only limited by the available fiber laser power.

Fast recursive algorithm for broadband APFs using complex cepstrums.

Integrated-optical All-Pass Filters are of interest for their potential compactness and economy of production. For broadband applications, the number of APFs involved can be as large as 50. To find optima for all the large number of parameters involved, we need a fast and efficient algorithm based on recursive equations. APF design algorithms based on complex cepstrum are proposed in digital signal processing. In this paper, we enhance these algorithms to efficiently fit the differential phase profile required for in-line broadband Polarization Mode Dispersion and Polarization Dependent Loss compensation.

Coherent polarization locking of a diode emitter array.

We present our work on the coherent combining of an array of diode emitters in a conventional diode bar configuration using the coherent polarization locking technique. An external laser cavity is designed so that the diode emissions from four diode emitters are spatially overlapped and passively phase locked via a series of birefringent walk-off crystals and a polarizing beam splitter. This concept was experimentally demonstrated up to 1030 mW of coherently combined power and was shown to increase the laser brightness of the diode bar by approximately 50 times.

Multi-mJ, 200-fs, cw-pumped, cryogenically cooled, Yb,Na:CaF2 amplifier.

Using a novel (to our knowledge) broadband Yb-doped Yb3+,Na+:CaF2 crystal cooled in a closed loop to 130 K we demonstrate a chirped pulse regenerative laser amplifier delivering the energy of up to 3 mJ at a repetition rate of 1 kHz and an average output power of 6 W at 20 kHz. The gain narrowing in the laser crystal is compensated by shaping the amplitude of the seed pulse spectrum. As the result, at the highest amplified pulse energy we obtain a 12 nm FWHM bandwidth supporting a 130 fs pulse duration, assuming ideal compression. Amplified pulses were recompressed from 250 ps to 195 fs with a 1700 lines/mm transmission grating compressor.

Fast switchable electro-optic radial polarization retarder.

A fast and switchable electro-optic radial polarization retarder (EO-RPR) fabricated using the electro-optic ceramic Pb(Mg(1/3)Nb(2/3))O(3)-PbTiO(3) is presented. This EO-RPR is useful for fast and switchable generation of a pure cylindrical vector beam. When used together with a pair of half-wave plates, the EO-RPR can change circularly polarized light into any cylindrical vector beam of interest, such as radially or azimuthally polarized light. Radially and azimuthally polarized light with purities greater than 95% are generated experimentally. The advantages of using EO-RPR include fast response time, low driving voltage, and transparency in a wide spectral range (500 to 7,000 nm).

Mimicking optical activity for generating radially polarized light.

We propose a new scheme for generating radially polarized light by mimicking optical activity using linear birefringence. It involves a birefringent spirally varying retarder sandwiched between two orthogonally oriented quarter-wave plates. Using Poincaré sphere representation, we show that the polarization transformation of such a scheme is equivalent to that of a spirally varying optical activity and is capable of generating radially polarized light. We demonstrate the proof-of-concept using y-cut crystalline quartz.

Coherent polarization locking with near-perfect combining efficiency.

We demonstrate the coherent locking of two orthogonal polarized lasers by using polarization selective loss. The two orthogonal polarizations are locked coherently to produce a resultant polarization state that sees minimal cavity loss. In contrast to the Michelson locking schemes, our scheme has the advantage of easy tunability, which helps to routinely achieve near-perfect (>99%) combining efficiency even when the power of the two arms is highly imbalanced and is varied from a power ratio of unity to >5. We also demonstrate an interesting phenomenon in which a miniscule injection of an antiphase field component from one arm into another can significantly inhibit the locking mechanism.

High-average-power mid-infrared ZnGeP2 optical parametric oscillator with a wavelength-dependent polarization rotator.

A wavelength-dependent polarization rotator is used to transform the orthogonal polarizations of the signal and idler of a near-degenerate type II KTP optical parametric oscillator (OPO) into a common polarization state. This common polarization allows a single ZnGeP2 OPO to fully utilize the 2 microm signal and idler KTP OPO outputs in a mid-IR conversion. The simple design of the wavelength-dependent polarization rotator yields a compact source that simultaneously generates four mid-JR wavelengths collinearly with a total mid-IR average power of 5.5 W at a >15 kHz pulse repetition rate.

Linearly polarized 100-W output from a diode-pumped Nd:YAlO laser.

We demonstrate a compact cw diode-pumped Nd:YAlO laser that can produce 100 W of power at 1079 nm and 18.3 W at 1341 nm. Lasing and nonlasing thermal lensing data are presented.

Multiwatt High-Repetition-Rate 2-microm Output from an Intracavity KTiOPO(4) Optical Parametric Oscillator.

We have obtained 6.5 W of 2-mum output from a near-degenerate, type II KTiOPO(4) (KTP) optical parametric oscillator (OPO) pumped within the cavity of a Q-switched diode-pumped Nd:YAG laser that operates at 3 kHz. We adopted the simplest configuration with a compact diode-pumped Nd:YAG module pumping the doubly resonant KTP OPO in its randomly polarized resonator with an acousto-optic Q switch. Attempts to increase the 2-mum output power by pumping this intracavity KTP OPO with a polarized laser beam by use of thermal birefringence compensation configurations are discussed.

Coupled tandem optical parametric oscillator (OPO): an OPO within an OPO.

We have demonstrated, for the first time to our knowledge, a coupled tandem optical parametric oscillator (OPO) configuration in which a second ZnGeP(2) OPO is placed within the resonator of the first, KTiOPO(4), OPO. A significant enhancement in the overall cascaded efficiency of this OPO compared with standard two-stage OPO's was observed. With a multimode Nd:YAG laser, an overall optical-to-optical efficiency (from 1.06 microm to the mid IR) of 5.2% was obtained from operating only ~1.4 times above oscillation threshold. The measured overall slope efficiency was attractively high at 35%. With a single set of mirrors we obtained a broad wavelength-tuning range from 2.7 to 8 microm.

High-efficiency mid-infrared ZnGeP2 optical parametric oscillator in a multimode-pumped tandem optical parametric oscillator.

We demonstrate a high-efficiency ZnGeP(2) optical parametric oscillator (OPO) pumped by another KTP OPO in a multimode-pumped tandem OPO configuration. The maximum optical-to-optical and slope efficiencies were 32% and 42.5%, respectively. Our setup also provides tunable multiband radiation in the 2.03-2.32-microm range and the 2.9-6.2-microm range simultaneously.