Er:YAG three-micron laser: performances and limits

Mathematical modeling is used to estimate the performances of the three-micron Er:YAG laser in various generation regimes. The model, based on simple rate equations, uses exclusively spectroscopic data and includes upconversion from both initial (/sup 4/I/sub 11/2/) and terminal (/sup 4/I/sub 13/2/) levels as well as the cross-relaxation from the pump level (/sup 4/S/sub 3/2/). Despite the unfavorable ratio between the lifetimes of the laser levels, the recirculation of the excitation on the metastable levels produced by the effective energy transfer processes at high erbium concentrations leads to rather high emission efficiency in the continuous wave (CW) regime. In contrast, in the Q-switch regime, the energy transfer processes are practically frozen during the giant pulse generation and the access to the stored energy is limited. In this paper, simple analytical expressions for emission efficiency in CW and Q-switch regimes are presented. Due to the growing interest in short laser pulses for medical applications, we discuss in more detail the Q-switch regime (pump conditions, co-doping, etc.).     

Low threshold laser-diode side-pumped Tm:YAG and Tm:Ho:YAG lasers

We report results from transverse laser-diode pumping of Tm:YAG and Tm:Ho:YAG rods. Using two 60-W quasicontinuous-wave laser-diode bars and a special dielectric coating structure on the barrel surface of the laser rod, laser operation was obtained at room temperature with threshold pump energies below 100 mJ and with output pulse energy above 10 mJ in free-running operation and 2 mJ in Q-switched operation. The Tm:Ho:YAG laser was more susceptible to a temperature increase in the material and performed significantly poorer in the Q-switched mode of operation than the Tm:YAG laser. This was predicted by a model accounting for up conversion and the dynamic equilibrium between the upper levels in thulium and holmium in Tm:Ho:YAG     

Dielectric Solid-State Planar Waveguide Lasers: A Review

This review takes a look at the historical development of the dielectric planar waveguide laser leading to key state-of-the-art technologies that fall within this broad subject area. Discussed herein are many of the advantages offered by the waveguide geometry such as high optical gain, and thus, low threshold-power requirements, suitability for quasi-three-level laser transitions, integration with functional devices on single substrates, guided spatial-mode control, and its considerable immunity to thermal effects and external environmental conditions. A detailed snapshot is made of many active host media for which there has been reported laser action in the planar waveguide geometry, covering many of the major rare-earth-ion transitions. Several fabrication techniques are highlighted and appraised for their applicability to different host media, touching on their benefits and drawbacks. Challenges and future prospects for these lasers are considered.     

Tm-Doped Fiber Lasers: Fundamentals and Power Scaling

We describe fundamental measurements of the properties of thulium (Tm)-doped silica and power scaling studies of fiber lasers based on the material. Data on the high-lying Tm:silica energy levels, the first taken to our knowledge, indicate that pumping at 790 nm is unlikely to lead to fiber darkening via multiphoton excitation. Measurement of the cross-relaxation dynamics produces an estimate that, at the doping levels used, as much as 80% of the decay of the Tm level pumped is due to cross relaxation. Using a fiber having a 25- $mu$m-diameter, 0.08 numerical aperture (NA) core, we observed fiber laser efficiencies as high as 64.5% and output powers of 300 W (around 2040 nm) for 500 W of launched pump power, with a nearly diffraction-limited beam. At these efficiencies, the cross-relaxation process was producing 1.8 laser photons per pump photon. We generated 885 W from a multimode laser using a 35-$mu$ m, 0.2-NA core fiber and set a new record for Tm-doped fiber laser continuous-wave power.      

Single-Wavelength Silicon Evanescent Lasers

The silicon evanescent device platform provides electrically pumped active device functionality on a low-loss silicon-on-insulator waveguide platform. We present here recent research in the area of single-wavelength silicon evanescent lasers that utilize distributed feedback, distributed Bragg reflector (DBR), and sampled grating (SG) DBR laser topographies.      

Microring-Resonator-Based Widely Tunable Lasers

We describe widely tunable lasers incorporating microring resonators. By using a double-ring-resonator-coupled filter, a wide wavelength tuning range is achieved with a low tuning current. A double-ring-resonator-coupled tunable laser has series-connected microring resonators in the laser cavity. A tuning range of 50 nm is achieved with injection current of less than 5.2 mA. This low injection current reduces the frequency drift caused by thermal transients to less than 5 GHz. We also describe an integrated filtered feedback tunable laser consisting of a Fabry–Perot (FP) laser and integrated filtered feedback sections. In this device, the filter section is removed from the laser cavity. The device exhibits a 24-nm tuning range with a side-mode suppression ratio of 50 dB. The frequency drift is less than 1 GHz because the longitudinal mode of the laser is mainly determined by the FP laser section. We have also developed a filter-free widely tunable wavelength converter by monolithically integrating a tunable laser and a wavelength converter based on a semiconductor optical amplifier. Wavelength conversion for a 10-Gb/s nonreturn-to-zero signal is also demonstrated, tunable over a 40-nm range.      

Comparative Performance of Passively Q-Switched Diode-Pumped Yb 3+-Doped Tungstate and Garnet Lasers Using Cr 4+:YAG Saturable Absorber

We investigated the continuous wave (CW) free-running and repetitive modulation in the kilohertz frequency domain of a passively Q-switched diode-pumped Yb:YAG, Yb:GGG, and Yb:KYW lasers by using Cr⁴⁺:YAG as a saturable absorber. The results presented in this paper are focused on the design of a passively Q-switched Yb-doped garnets or Yb-doped tungstates microlasers. The free-running performance of Yb:YAG, Yb:GGG, Yb:KGW, and Yb:KYW were characterized, and experimental parameters such as gain and loss were evaluated. We carried out a fit between our experimental results and an existing numerical model, which relates the experimental and the physical parameters of the ytterbium diode-pumped system to the minimal threshold pumping power. The best performance among the laser crystals was obtained for Yb:YAG laser. A maximum peak power of ap4.5 kW at an average output power of 1.32 W was extracted with an extraction efficiency of ap25%.     

Enhanced Modulation Characteristics of Optical Injection-Locked Lasers: A Tutorial

In this paper a tutorial of optical injection locking of semiconductor lasers is given, with particular emphasis on the enhancement of system parameters. Furthermore, physical intuition of each parameter enhancement is explained and practical design rules and trends are also shown.      

Optofluidic Distributed Feedback Dye Lasers

We review our recent work on poly(dimethylsiloxane) (PDMS)-based optofluidic dye lasers using a guided wave distributed feedback (DFB) cavity. We show experimental results of single-mode operation, an integrated laser array, multiple color dye lasing, mechanical and fluidic tuning, and monolithic integration with microfluidic circuits. Potential applications and future directions are discussed     

All solid-state continuous-wave frequency-quadrupled Nd:YAG laser

We report all solid-state, continuous-wave, high-power, deep ultraviolet generation. An intracavity frequency doubled Nd:YAG laser pumped by fiber-coupled diodes is used to generate the high-power green output. The significance of nonlinear coupling and spatial hole burning of the two-mode oscillations in the laser is discussed. When the incident 532 mn power on an external resonant doubler was 2.9 W, we generated 1.5 W of continuous-wave 266-nm radiation using a Czochralski-grown β-BaB₂O₄. To achieve stable external resonance, a novel voice coil motor actuator is employed with servo bandwidth as large as 50 kHz     

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.     

Discharge Mechanisms in Neutral Gas Lasers

The equations which govern the discharge phenomena in neutral gas lasers are cast into simplified form which brings out the dependence of laser properties on the external parameters of importance?gas pressure, tube diameter, and tube current. The well known empirical scaling laws which govern neutral gas laser operation follow quite naturally from this procedure. Specific examples are given to illustrate agreement with experiment.     

Solid-State Lasers From an Efficiency Perspective

Solid-state lasers have remained a vibrant area of research because several major innovations expanded their capability. Major innovations are presented with emphasis on the laser efficiency. A product of efficiencies approach is developed and applied to describe laser performance. Efficiency factors are presented in closed form where practical and energy transfer effects are included where needed. In turn, efficiency factors are used to estimate threshold and slope efficiency, allowing a facile estimate of performance. Spectroscopic, thermal, and mechanical data are provided for common solid state laser materials.     

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     

Advanced Ti:Er:LiNbO3 waveguide lasers

This paper reviews the latest developments of diode-pumped Ti,Er:LiNbO₃ waveguide lasers emitting at wavelengths around 1.5 μm. In particular, harmonically mode-locked lasers, Q-switched lasers, distributed Bragg reflector (DBR)-lasers, and self-frequency doubling lasers are discussed in detail. Supermode stabilized mode-locked lasers have been realized using a coupled cavity concept; a side mode suppression ratio of 55 dB has been achieved at 10-GHz pulse repetition rate with almost transform limited pulses. Q-switched lasers with a high extinction ratio (>25 dB) intracavity electrooptic switch emitted pulses with a peak power level up to 2.5 kW and a pulsewidth down to 2.1 ns at 1-kHz repetition frequency. Numerical simulations for both lasers are in a good, almost quantitative agreement with experimental results. A DBR-laser of narrow linewidth (≈3 GHz) with a permanent (fixed) photorefractive grating and 5 mW output power has been realized. Self frequency doubling lasers have been fabricated with a periodic microdomain structure inside an Er-doped laser cavity; simultaneous emission at the fundamental wavelength, 1531 nm, and at the second harmonic wavelength, 765 nm, has been obtained     

Yb:YAG absorption at ambient and cryogenic temperatures

We have performed absorption measurements and generated absorption cross sections as a function of wavelength for the laser material YAG doped with ytterbium at 300, 175, and 75 K. This data was generated to enable a direct comparison of the absorption intensity and linewidths at room and cryogenic temperatures, and in particular near the temperature of liquid nitrogen at 77 K. The data have been used to compute universal absorption contour plots that display absorption as a function of the incident light center wavelength and optical thickness (doping density times penetration depth) for a number of bandwidths, and assuming that the spectrum of the incident light can be described as a Gaussian. Curves are presented for both 300 and 75 K, and may be used to optimize the absorption and laser efficiency.     

Fifteen Years of Work on Thin-Disk Lasers: Results and Scaling Laws

The principal ideas of the thin-disk laser design will be illustrated and the advantages for operating different laser materials will be explained. The results for continuous-wave (CW) and Q-switched operation as well as for amplification of short (nanosecond) and ultrashort (picosecond, femtosecond) pulses demonstrate the potential of the thin-disk laser design. The scaling laws for this laser design show that the power limit for CW operation is far beyond 40 kW for one single disk and the energy limit is higher than 3 J from one disk in pulsed operation. Also, the applicability of the thin-disk laser concept to optically pumped semiconductor structures will be discussed. When pumping directly into the quantum wells, the energy defect between the pump photon and the laser photon can be smaller than 5%, thus reducing the waste heat generated inside the semiconductor structure. First results demonstrate the potential of this new concept. Finally, a short overview of the industrial realization of the thin-disk laser technology will be given.     

Introduction to the Issue on Semiconductor Lasers

The 63 papers in this special issue describe some of the most exciting work in the field of semiconductor lasers.