Diffraction-limited Output Beam Research Articles

Curved Waveguide Array Diode Lasers for High-Brightness Applications

We have developed a novel structure for achieving high-brightness output from a semiconductor laser array using coherent spatial beam combination. The structure consists of an array of curved ridge waveguides with in-phase, coherent output. Interference of the multiple outputs leads to a narrow, bright, on-axis, far-field beam shape. Experimental results demonstrate a diffraction limited output beam width, ideal for high-brightness applications.

Thermal lensing in an end-pumped Yb:KGW slab laser with high power single emitter diodes

We demonstrate that it is possible to pump an Ng-cut Yb:KGW crystal slab with up to 18W by a single emitter laser diode and still achieve a nearly diffraction limited output beam with a M(2) value below M(2) < 1.1 and optical to optical efficiencies of more than 44%. Furthermore, we have measured the focal length of thermally induced lenses in an Yb:KGW crystal slab which was end-pumped by 12 W and 18 W single-emitter diodes. We have observed that the focal lengths not only depend on the pump power and beam waists, but are also dependent on the specific diode beam profile, as well as the collimation optics. We have been able to correlate this behavior with specific patterns of the beam profile.

Dilute nitrides tailor the wavelength of semiconductor disk lasers

Interest is growing rapidly in efficient and compact optically pumped semiconductor disk lasers (OP-SDLs) since they provide a practical solution for generating high-power radiation at visible wavelengths. In particular, red-green-blue SDLs are thought to satisfy the demanding requirements of laser displays and projectors. Other applications, such as spectroscopy and biomedicine, are also expected to benefit greatly from the wavelength versatility and excellent beam quality offered by OPSDLs. Also known as vertical external cavity surface-emitting lasers,1, 2 OP-SDLs combine many of the advantages of traditional solid-state lasers with the versatility offered by semiconductor gain materials. Such laser sources can deliver Watt-level diffraction-limited output beams in a broad spectral range determined by the gain material. The key element of an OP-SDL is the semiconductor gain mirror, which is placed in an external laser cavity configuration, as shown in Figure 1. This arrangement allows a nonlinear crystal to be placed into the cavity for frequency conversion. The most notable results demonstrated with visible SDLs have been obtained in the blue-green region by frequency doubling 940nm and 1060nm laser radiation.3 However, the development of frequency-doubled OP-SDLs at red-orange wavelengths has been hindered by the lack of suitable semiconductor materials for fabricating high-quality gain mirrors operating at around 1200–1250nm. To tackle this problem, we have developed gallium indium nitride arsenide / gallium arsenide (GaInNAs/GaAs) gain regions that can be integrated with highquality gallium arsenide / aluminum arsenide (GaAs/AlAs) distributed Bragg reflectors (DBRs). DBRs act as mirrors for a specific wavelength range related to the optical thickness of Figure 1. Schematic of an OP-SDL with an intra-cavity heat-spreader.

Vertical integration of ultrafast semiconductor lasers

Lasers generating short pulses – referred to as ultrafast lasers – enable many applications in science and technology. Numerous laboratory experiments have confirmed that ultrafast lasers can significantly increase telecommunication data rates [1], improve computer interconnects, and optically clock microprocessors [2, 3]. New applications in metrology [4], supercontinuum generation [5], and life sciences with two-photon microscopy [6] only work with ultrashort pulses but have relied on bulky and complex ultrafast solid-state lasers. Semiconductor lasers are ideally suited for mass production and widespread applications, because they are based on a wafer-scale technology with a high level of integration. Not surprisingly, the first lasers entering virtually every household were semiconductor lasers in compact disk players. Here we introduce a new concept and make the first feasibility demonstration of a new class of ultrafast semiconductor lasers which are power scalable, support both optical and electrical pumping and allow for wafer-scale fabrication. The laser beam propagates vertically (perpendicularly) through the epitaxial layer structure which has both gain and absorber layers integrated. In contrast to edge-emitters, these lasers have semiconductor layers that can be optimized separately by using different growth parameters and with no regrowth. This is especially important to integrate the gain and absorber layers, which require different quantum confinement. A saturable absorber is required for pulse generation and we optimized its parameters with a single self-assembled InAs quantum dot layer at low growth temperatures. We refer to this class of devices as modelocked integrated external-cavity surface emitting lasers (MIXSEL). Vertical integration supports a diffraction-limited circular output beam, transform-limited pulses, lower timing jitter, and synchronization to an external electronic clock. The pulse repetition rate scales from 1-GHz to 100-GHz by simply changing the laser cavity length. This result holds promise for semiconductor-based high-volume wafer-scale fabrication of compact, ultrafast lasers.

Picosecond pulse amplification in a core-pumped large-mode-area erbium fiber

Amplification in a single-clad, large-mode-area erbium fiber as an alternative to double-clad Er-Yb amplifiers is presented. Both signal and pump are coupled through a mode-matched splice into the fundamental mode, which ensures preferential gain in the fundamental mode while minimizing the amplified spontaneous emission (ASE). The 875 microm(2) effective area of the Er fiber enables amplification of 6 ps pulses at 1.55 microm wavelength by approximately 33 dB in a single stage to >25 kW peak power with low nonlinear pulse distortion and a diffraction-limited output beam with M(2)<1.1.

Thermally Induced Birefringence Compensation in High Average Power Nd:YAG Laser

Efficient compensation of thermally induced birefringence with thermal lensing in a rod geometry Nd:YAG laser amplifier was achieved by an in-cavity 90° polarization rotator with three sets of image relaying optics between the laser rods. The measured birefringence loss per transit was 2.8% for pumping power of 580 W per rod. A nearly diffraction limited output beam of 38 W was observed.

2-kW Average Power CW Phase-Conjugate Solid-State Laser

We have demonstrated stable operation of a 2-kW Yb:YAG phase-conjugate master oscillator power amplifier (PC-MOPA) laser system with a loop phase-conjugate mirror (LPCM). This is the first demonstration of a continuous wave (CW)-input LPCM MOPA operating at a power greater than 1 kW with a nearly diffraction-limited output beam. The single-pass beam quality incident on the LPCM varied with the specific operating conditions, but it was typically <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">~</sub> 20 times diffraction-limited (XDL). The measured beam quality with an MOPA output power of 1.65 kW was 1.3 XDL.

300?W quasi-continuous-wave diffraction-limited output from a diode-pumped Nd:YAG master oscillator power amplifier with fiber phase-conjugate stimulated Brillouin scattering mirror

A master oscillator power amplifier system comprising a cw Nd:YAG master oscillator, a quasi-cw diode-pumped power amplifier, and a fiber-based stimulated Brillouin scattering phase-conjugate mirror is reported. A 12-pass amplifier configuration is employed to achieve high gain in a small-length slab amplifier. Residual and pump-induced optical inhomogeneities in the slab are corrected by the fiber phase conjugator to achieve diffraction-limited output beam quality. 300 W quasi-cw output and a maximal attainable gain of approximately 150 for the system are obtained.

Guiding of laser modes based on self-pumped four-wave mixing in a semiconductor amplifier

We propose that self-pumped degenerate four-wave mixing may be used to produce novel diode laser systems where lasing is based on nonlinear guiding of the laser beams inside the active semiconductor. The fundamental process responsible for the interaction is spatial hole burning in semiconductor amplifiers. The gain and index gratings created by the modulation of the carrier density in the active gain medium lead to selective amplification of one spatial mode and suppression of all other modes. This mechanism allows the laser system to be operated far above its threshold with an almost diffraction limited output beam. The third order nonlinear susceptibility of the non-linear material, which determines the strength of the induced gratings, depends on the angle between the interacting beams in the four-wave mixing configuration. It is shown theoretically that a narrow range of angles exist where the induced gratings are strong and where mode suppression of higher order spatial modes are obtained simultaneously. Experimental evidence sustaining these findings is given.

High-gain low-noise mid-infrared quantum cascade optical preamplifier for receiver

Optical preamplifier (OPA) in the mid-infrared can boost weak signals in systems such as lidars or optical wireless links to improve the receiver sensitivity. Quantum-cascade-based traveling-wave OPA with off-normal Bragg-grating (BG) coupled surface-emitting configuration have been developed with a net small-signal gain up to 13 dB over a spectral range /spl sim/70 nm around 4.7 /spl mu/m, limited by the wafer gain bandwidth, and the linear dynamic range was >20 dB. Amplified spontaneous emission (ASE) was measured and the spontaneous emission factor was determined N/sub sp//spl sim/1, indicating near quantum-limit noise behavior. Surface-emitting BG devices yielded diffraction-limited output beam, and provided simultaneous wavelength dispersion and ASE filtering. The OPA was coupled with a detector to form a receiver that is polarization-selective, has a net gain 8-13 dB with a corresponding optical filter bandwidth 25-50 GHz. These results suggest that BG-coupled OPA receivers can be useful for multispectral/wavelength-division-multiplexing systems in free space applications.

A new 3D multipass amplifier based on Nd:YAG or Nd:YVO 4 crystals

We report on new simple and efficient multipass amplifiers using prisms or corner cubes to perform several passes in different planes of incidence. This scheme provides an optimized overlap between the signal passes and the pumped volume. We investigated our amplification geometry with Nd:YAG and Nd: YVO4 crystals : the use of a low doped (0.3%) Nd:YVO4 crystal allowed better thermal behaviour and higher performances. We amplified a pulsed microlaser (110 mW of average power at 1064 nm) and obtained a diffraction-limited output beam with an average power of 5.7 W for 15 W of pump power and a small-signal gain of 56 dB in a 6-pass configuration.

High-power passively mode-locked semiconductor lasers

We have developed optically pumped passively mode-locked vertical-external-cavity surface-emitting lasers. We achieved as much as 950 mW of mode-locked average power in chirped 15-ps pulses, or 530 mW in 3.9-ps pulses with moderate chirp. Both lasers operate at a repetition rate of 6 GHz and have a diffraction-limited output beam near 950 nm. In continuous-wave operation, we demonstrate an average output power as high as 2.2 W. Device designs with a low thermal impedance and a smooth gain spectrum are the key to such performance. We discuss design and fabrication of the gain structures and, particularly, their thermal properties

High-average-power (&gt;20-W) Nd:YVO_4 lasers mode locked by strain-compensated saturable Bragg reflectors

Strain-compensated double InGaAs quantum-well saturable Bragg reflectors (SBR’s) with high damage thresholds have been developed for use as mode-locking elements in high-average-power neodymium lasers. Nd:YVO4 lasers have been developed with these new SBR’s, which produce transform-limited pulses of 21-ps duration at 90 MHz and an average power of 20 W in a diffraction-limited output beam. The peak pulse power at an output power of 20 W was 10.6 kW. A comparison of the operating parameters of strained single and strain-compensated double-well SBR’s indicates that the damage threshold increased by a factor of at least 2–3. Long cavity laser variants were investigated to assess the limitations of further power scaling. At a repetition frequency of 36-MHz stable mode-locked pulses with peak pulse powers of 24.4 kW and pulse energies of 0.6 µJ could be generated.

Diffraction limited output from a CW Nd:YAG master oscillator/power amplifier with fibre phase conjugate SBS mirror

A CW Nd:YAG master oscillator power amplifier with fibre based SBS phase conjugate mirror is reported. A four-pass amplifier configuration is employed to compensate pump induced thermal lens and birefringence in the amplifier rod in conjunction with a fibre phase conjugator to achieve diffraction limited output beam quality. Maximal attainable gain of ∼1.9 for the system is obtained. Application of the technique to diode pumped systems for high gain and high power delivery is proposed.

Efficient, low-noise, SESAM-based femtosecond Cr 3+:LiSrAlF 6 laser

We report what we believe to be the first demonstration of a SESAM (semiconductor saturable absorber mirror)-based femtosecond Cr 3+:LiSrAlF 6 laser pumped by narrow-stripe AlGaInP laser diodes that have diffraction-limited output beams. A highly asymmetric, four-mirror laser cavity design is described for which 57 fs duration pulses at an average output power of 6.5 mW are obtained for only 72 mW of incident pump power. Pump powers of this order were maintained for over 14 h using just six 1.5 V AA batteries as the electrical power source. We have shown that mode-locking can be sustained for incident pump powers as low as 21.5 mW.

Beam divergence studies on hard edge unstable resonators for a long pulse XeCl excimer laser

The focusability of a long pulse XeCl excimer laser has been improved using confocal positive branch unstable resonators where the outcoupling is done through the convex mirror. A nearly diffraction limited output beam is obtained from hard edge unstable resonators. An improvement of the far field energy distribution is achieved with partial reflecting hard edge outcoupling mirrors. The phase delay between the central part of the beam and the beam edge caused by these partial reflecting mirrors can be reduced by using phase unifying mirrors. Using unstable resonators a brightness of 1.4 × 10 15 W/cm 2 sr has been obtained from a long pulse XeCl excimer laser

Ultralow-pump-threshold, femtosecond Cr^3+:LiSrAlF_6 laser pumped by a single narrow-stripe AlGaInP laser diode

We report what we believe to be the first demonstration of a Kerr-lens mode-locked Cr(3+):LiSrAlF(6)laser that is pumped by a single narrow-stripe AlGaInP laser diode with a diffraction-limited output beam. A novel low-loss three-mirror laser cavity design is described in which strong, localized Kerr lensing was exploited such that 75-fs-duration pulses were obtained for only 36mW of incident pump power. This pump power was maintained for 18h by just three AA batteries as the electrical power source. We have shown that mode locking can be sustained for pump powers as low as 22mW.

Periodically poled lithium niobate optical parametric amplifiers pumped by high-power passively Q-switched microchip lasers

High-power passively Q-switched microchip lasers produce 157-muJ pulses of 1-ns duration in a single-frequency, diffraction-limited output beam. The unfocused 1.064-mum output of these devices has been used to drive periodically poled lithium niobate optical parametric amplifiers at wavelengths between 1.4 and 4.3 mum . With a peak conversion efficiency of nearly 100%, these devices generate 100-kW, subnanosecond pulses in the mid IR, with a beam quality that is better than two times diffraction limited.

Near-diffraction-limited, high-energy, high-power, diode-pumped laser using thermal aberration correction with aspheric diamond-turned optics

We achieved a 1.3× diffraction-limited output beam with a pulse energy of 0.76 J at 60 Hz (average power of 46 W) at 1.064 μm from a diode-pumped Nd:YAG master oscillator power amplifier rod laser using a diamond-turned aspheric optic to compensate thermally induced phase distortion of the gain medium. The output was frequency doubled in KTP to 30 W (0.5-J pulse energy) and 2.4× diffraction-limited at 532 nm.

&lt;italic&gt;Monolithic Diode‐laser Arrays&lt;/italic&gt;

Over the last two decades, the search for a compact, high-power semiconductor source has produced many designs and concepts for monolithic diode-laser arrays and optical amplifiers. However, only a few design approaches have emerged with the potential for producing high-power, high-brightness monolithic sources. Although semiconductor-diode lasers are the most compact, highest gain and most efficient laser sources, challenges remain in developing structures that will produce high-quality, diffraction-limited output beams. This book presents and analyzes the results of work performed over the last twenty years in the development of high-brightness diode-laser arrays.