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Controllable graphene Q-switching in an Yb:YAG fs-laser-written beam-splitter-type waveguide laser is demonstrated by tunable beam-splitting ratio and varying pump power. Desired channels are regulated to generate Q-switched pulses separately or simultaneously with a single pumping.

We report on evanescent-field Q-switched operation of fs-laser-written Yb:YAG surface channel waveguide lasers employing single-walled carbon nanotubes in single- and double-pass pumping geometries.

We demonstrated GHz modelocking of femtosecond-laser inscribed Yb:YAG channel waveguide lasers using SESAM and single-walled carbon nanotube saturable absorber. Sub-2-ps pulses at multi-GHz repetition rates are generated from a dispersion-managed compact laser cavity with watt-level output powers.

We report on detailed spectroscopic investigations and efficient visible upconversion laser operation of Er3+:LiLuF4. This material allows for efficient resonant excited-state-absorption (ESA) pumping at 974 nm. Under spectroscopic conditions without external feedback, ESA at the laser wavelength of 552 nm prevails stimulated emission. Under lasing conditions in a resonant cavity, the high intracavity photon density bleaches the ESA at 552 nm, allowing for efficient cw laser operation. We obtained the highest output power of any room-temperature crystalline upconversion laser. The laser achieves a cw output power of 774 mW at a slope efficiency of 19% with respect to the incident pump power delivered by an optically-pumped semiconductor laser. The absorption efficiency of the pump radiation is estimated to be below 50%. To exploit the high confinement in waveguides for this laser, we employed femtosecond-laser pulses to inscribe a cladding of parallel tracks of modified material into Er3+:LiLuF4 crystals. The core material allows for low-loss waveguiding at pump and laser wavelengths. Under Ti:sapphire pumping at 974 nm, the first crystalline upconversion waveguide laser has been realized. We obtained waveguide-laser operation with up to 10 mW of output power at 553 nm.

This paper summarizes our recent results on crystalline waveguides and waveguide lasers fabricated by the femtosecond-laser writing technique. Highly efficient waveguide lasers emitting in the near infrared spectral range based on ytterbium and neodymium doped Y 3 Al 5 O 12 with slope efficiencies above 70% and continuous wave output powers of more than 5 W were demonstrated. Furthermore, pulsed laser operation of passively Q-switched Yb:YAG and Nd:YAG wavequide lasers was achieved by incorporating the saturable absorber Cr 4+ :YAG in a monolithic setup. Based on fs-laser written curved structures in Yb:YAG, efficient curved waveguide lasers could be demonstrated. Also, waveguide lasers in the visible spectral range were investigated. We achieved laser emission in the green, orange, red and deep-red spectral region and could demonstrate output powers of more than 1 W with waveguides based on praseodymium doped materials. Additionally, switchable and dual wavelength operation between the orange and red laser emission was achieved. By utilizing the nonlinear crystal KTiOPO 4 as substrate material for fs-laser inscription, nonlinear waveguides were realized. With these devices, efficient second harmonic generation into the blue and green spectral range was achieved.

We present the amplification of a mode-locked vertical external-cavity surface-emitting laser (VECSEL) using an Yb:YAG crystalline waveguide as gain medium. The VECSEL seed laser operates at a center wavelength of 1030 nm and generates 300-fs pulses at a repetition rate of 1.77 GHz. An average seed power of 60 mW was launched onto a 8.3 mm long fs-laser written Yb:YAG waveguide pumped by 7.7 W from a 969-nm continuous-wave VECSEL. The amplifier achieves an average output power of up to 2.9 W, corresponding to an amplification factor of 17 dB. Due to gain narrowing, the pulse duration increases to 629 fs. Our results show that crystalline waveguides are a promising technique for the realization of compact multi-watt ultrafast amplifier systems.

In this paper the recent advances in waveguide lasers fabricated by ultrafast laser inscription (ULI) are reported with the focus on Yb:YAG waveguide based lasers in two different operation regimes. Watt level output powers in single longitudinal mode (SLM) as well as modelocked (ML) operation are demonstrated.

Compact pulsed laser sources are of interest for diverse application areas. Due to their broad gain bandwidth, high Stokes efficiency and low thermal load near the 1 μm spectral region, ytterbium doped laser materials are promising for such devices. Channel waveguides in such gain media provide compact laser systems and deliver efficient single-mode laser operation with low laser thresholds. One of the widely-used techniques for manufacturing channel waveguides in transparent materials including laser gain media is fs-laser inscription. Inscribing two parallel tracks inside a laser crystal with a distance of few 10 μm allows for highly efficient laser operation in few mm-long monolithic resonators [1]. Pulsed operation of fs-laser inscribed channel waveguide lasers has been demonstrated with saturable absorbers (SAs) including SESAMs and low-dimensional carbon nanostructures. Intrinsic fast response time, linear and nonlinear absorption in a broad spectral range and relatively simple fabrication processes make graphene and single-walled carbon nanotubes (SWCNTs) a cost effective approach for saturable absorbers for various laser types including channel waveguide lasers. Recently, Q-switched [2-4] and even mode-locked operation at 1.94 GHz repetition rate [5] have been demonstrated with fs-laser inscribed Yb3+-doped waveguides. However, mode locking of fs-laser inscribed waveguide lasers by employing carbon nanotubes is not yet reported.

We present a detailed study on selective chemical etching of fs-laser modified single crystalline Y 3 Al 5 O 12 (YAG) toward the fabrication of microchannels for micro- and optofluidic as well as photonic applications. The precise knowledge of parameters such as the etching rate, selectivity and roughness of the etched surfaces is essential for this purpose. In particular their dependence on structuring parameters, such as pulse energy and translation velocity as well as etching parameters, such as agent and temperature needs to be determined.

We report on modelocking of fs-laser-inscribed Yb:YAG channel waveguide lasers using single-walled carbon nanotube saturable absorbers and SESAMs. Sub-2-ps-pulses at few-GHz-repetition rates are obtained at watt-level output powers in both cases.

We demonstrate an Yb:YAG femtosecond-laser-written channel-waveguide-laser with record-high average-output-power of 5.7 W and 78% slope-efficiency in CW and 5.3 W for Q-switched operation with 11-ns pulses and ∼1-μJ energy at 5.4-MHz repetition-rate.

We report on single longitudinal mode Yb:YAG waveguide lasers with 2 W of output power and high efficiency. The distributed feedback (DFB) lasers are based on ultrafast laser inscribed waveguide Bragg gratings.

We demonstrate the first amplification of an ultrafast semiconductor disk laser inside a crystalline waveguide. The 8.3-mm long fs-laser written Yb:YAG channel waveguide generates 2.89 W in 630 fs pulses at 1030 nm.

We studied operation characteristics of an Yb:YAG channel waveguide laser, Q-switched by employing monolayer graphene. Uniform monolayer graphene grown by thermal chemical vapor deposition is transferred directly onto one end facet of the channel waveguide which simultaneously serves as an output coupling mirror (OC), making a monolithic Q-switched waveguide laser possible. In this cavity configuration, the Q-switched laser delivers a maximum average output power of 85 mW, corresponding to a pulse energy of 64 nJ at 1.33-MHz repetition rate. The laser performance of this device is compared with another cavity configuration, in which the monolayer graphene is coated onto separate OCs. In this case a shorter pulse duration of 79 ns is achieved, but the laser operation performance is worse with respect to efficiency and output power. The proposed monolithic approach demonstrates the potential for developing more compact Q-switched laser devices. (C) 2016 Optical Society of America

Using the direct femtosecond-laser writing technique tracks of modified material were written in an Yb(15%):YAG ceramic. Due to a stress-induced change of the refractive index between two parallel tracks waveguiding was achieved in channels between the tracks. Laser oscillation of these channel waveguides at a wavelength of 1030 nm was demonstrated with a maximum output power of 0.7 W. The investigation of two different outcoupling transmissions of 99% and 91% revealed that for the higher output coupling the slope efficiency is 35%, which is much lower in comparison to the case of lower output coupling with a slope efficiency of 65%. This effect may be explained by inversion dependent loss mechanisms.

Tracks of modified material were written with femtosecond-laser pulses in neodymium-doped YAG crystals. Due to a stress-induced change of the refractive index, waveguiding beside the tracks and between two adjacent tracks with a distance of approximately 25 μm was observed. Loss measurements resulted in guiding losses of about 1.6 dB/cm for the double track waveguide. Spectroscopic investigations of the 4F3/2→4F11/2 transmission lines of the neodymium ions, which are close to the modified region, revealed a small stress-induced red shift of the lines. Laser oscillation of single-track waveguides and double-track waveguides was demonstrated with Ti:Sapphire laser pumping at a wavelength of 808 nm. Best laser performance with about 1.3 W output power at 2.25 W launched pump power was achieved using a double-track waveguide with a separation of 27 μm at an outcoupling transmission of 95%.

We demonstrate GHz-repetition rate mode-locked operation of a femtosecond-laser-inscribed Yb:YAG channel waveguide laser using single-walled carbon nanotube saturable absorber mirror (SWCNT-SAM). A 6.3-mm-long, type II Yb:YAG waveguide laser with an extended cavity configuration delivers mode-locked picosecond (ps) pulses at GHz repetition rates. The dispersion of the laser cavity is compensated by the combination of a multi-functional output coupler and the Gires-Tournois interferometer (GTI) effect arising from an air-gap between the facet of the waveguide and the output coupler. The incident beam fluence on the SWNCNT-SAM is controlled by adjusting two intracavity lenses to avoid optical damage on the polymer nanocomposite matrix containing the SWCNTs. The average output power of our mode-locked waveguide laser is 322 mW at a pump power of 3.2 W. Nearly Fourier-limited, stable 2-ps-short pulses are generated at a repetition rate of 2.08 GHz.

Laser written waveguides in crystalline materials can be used to make highly efficient, high gain lasers. The bi-directional emission from such lasers however is typically broadband with poor spectral control. Hybridizing a tapered, mode matched laser written Bragg grating with a broadband Yb:YAG crystalline waveguide laser, we demonstrate single longitudinal mode output from one end of the device. Careful control of the grating characteristics led to laser thresholds below 90 mW, slope efficiencies greater than 42% and output powers greater than 20 mW.

We report on, to the best of our knowledge, the first fs-laser-written waveguide laser in Yb3+:CaGdAlO4 (Yb:CALGO). With Yb:CALGO crystals grown in our labs, we obtained a slope efficiency of 69% and up to 2.4 W of continuous wave (cw) output power in a waveguide-laser configuration. Moreover, bulk laser experiments with Yb:CALGO were performed, and slope efficiencies up to 73%, optical-to-optical efficiencies of 65%, and maximum cw output powers of 3.3 W were reached. These are the highest efficiencies in the laser configuration with Yb:CALGO.

We propose a s-switched channel waveguide laser operating near 1 μm in mirrorless resonator configuration. PMMA-assisted monolayer graphene acting as saturable absorber is grown by chemical vapor deposition and transferred onto an end facet of the waveguide.

In this Letter, we report on laser emission from Yb:YAG beam-splitters fabricated by direct femtosecond-laser inscription. These devices consist of two adjacent mirrored s-curved waveguides merged into a section allowing for multi-mode interference (MMI) and a straight waveguide. Nine-millimeter-long one-to-two beam-splitters with radii of curvature between 10 and 80 mm have been fabricated resulting in separations between the two arms of up to 0.65 mm. Due to the MMI-section, the splitting ratio was variable by slightly changing the incoupling conditions. Maximum laser output powers of 2.29 W and slope efficiencies of 52% were achieved with the best beam-splitter waveguide laser. For all radii of curvature exceeding 10 mm, similar efficiencies were demonstrated.

By proper selection of pump light wavelength, polarization, and crystal orientation, linearly polarized laser output, independent dual wavelength operation, and intracavity frequency doubling can be obtained in the cubic crystal Pr3+:KY3F10.

Diode pumped fs-laser written Pr:KY3F10 waveguide lasers with 42 mW output power in the visible spectral region are presented. Utilizing polarization dependent spectroscopic properties of cubic Pr:KY3F10 dual wavelength and switchable laser operation was achieved.

We present highly efficient high power single longitudinal mode waveguide lasers. Single mode operation was achieved by combining a tapered waveguide Bragg grating with a Yb:YAG crystalline waveguide laser in a hybrid approach. Both structures were fabricated by fs-laser writing. We achieved 1.59 W of output power in a single longitudinal mode and 4.71 W of output power with a spectral bandwidth of 38 pm. The slope efficiency was 66% and the laser threshold 92 mW.

Single longitudinal mode (SLM) lasers are important tools for many scientific and commercial applications. SLM operation can be achieved in distributed-feedback (DFB) lasers based on Bragg structures. Semiconductor waveguide DFB lasers are well-established devices, but their output power is limited to a few hundred milliwatts. DFB lasers have also been demonstrated in dielectric waveguides. However, in this case the output power was even lower. Here we present the first monolithic Yb:YAG DFB laser. The waveguide and the DFB structure were fabricated in the volume of an Yb:YAG crystal by ultrafast laser inscription. The DFB laser delivered 2 W of output power at a slope efficiency of 61% in SLM operation under pumping with an optically pumped semiconductor laser. This power level outperforms previously demonstrated dielectric DFB waveguide lasers by nearly an order of magnitude. Our approach paves the way for compact, robust, and highly efficient high-power SLM laser sources.

Waveguides were inscribed into a 9.5 mm long periodically poled KTiOPO4 crystal with a Ti:sapphire femtosecond-laser. Waveguiding was achieved between two parallel written tracks with spacings of 17-19 μm. The fundamental power of 1.6 W for frequency doubling in the waveguide was delivered by a continuous wave Ti:sapphire laser at a wavelength of 943.18 nm. A maximum output power of 76 mW in the blue spectral region was achieved. This corresponds to a single pass normalized conversion efficiency of 4.6% W(-1) cm(-2).

The common scheme of femtosecond-laser inscription of waveguides in fluorides is a cladding geometry. The first crystalline Er:LiLuF4-upconversion waveguide laser is reported somewhere else. The poor stability and efficiency of the green emission were caused by nonoptimized cavity and fabrication parameters. In this paper, the mentioned drawbacks were tried to be overcome and an improvement in stability and output power is shown by the application of a circular cladding geometry. An output power of 10 mW at 552.6 nm could be achieved, with an incident pump power on the crystal of 1.5 W at 974 nm.

We report to the best of our knowledge on the first green laser operation of a Pr3+-doped crystalline waveguide laser. Furthermore red and deep red laser operation with output powers of > 1 W and >500 mW was achieved. The optical-to-optical efficiency was as high as 31% and 19%, respectively.

Ti:Sapphire-laser pumped s-curve and straight fs-laser-written Yb(7%):YAG waveguide lasers are demonstrated. With slope efficiencies of 73% for straight and more than 60% for curved waveguide lasers the possibility of fs-laser-written complex optical devices is demonstrated.

In this Letter, we present high-power continuous wave (CW) and Q-switched femtosecond laser-written Yb:YAG channel waveguide lasers. In Q-switched operation, obtained by a semiconductor saturable absorber mirror (SESAM), as well as in CW operation, the laser generates average output powers of more than 5.6 W and reaches slope efficiencies above 74%. The Q-switched laser operated at a maximum repetition rate of 5.4 MHz with a minimum pulse duration of 11 ns, and with a maximum pulse energy of 1 μJ. This laser has almost an order of magnitude higher average output power than previously reported Q-switched channel waveguide lasers.

Waveguides were fabricated in a 5 mm long Pr(0.5 at%):LiYF4 (YLF) crystal with a femtosecond Ti:sapphire laser system. Waveguiding was achieved inside a core surrounded by eight single modified tracks building a rhombic structure. The waveguide was pumped at a wavelength of 444 nm with an InGaN laser diode. Orange and deep red laser oscillation were realized. Maximum output powers of 25 mW at 604 nm and 12 mW at 720 nm with respect to the incident pump power were achieved.

Femto-second laser writing was used to fabricate waveguides in a z-cut KTP sample with losses below 0.8 dB/cm. They were used for efficient, broad bandwidth, Type II birefringent second harmonic generation to the green. The temperature and wavelength bandwidth were, 28 C ∙ cm and 0.85 nm ∙ cm, respectively.

A Ti:Sapphire-pumped Q-switched waveguide laser was fabricated by fs-laser-writing of two parallel tracks in a bonded Nd:YAG/Cr4+:YAG-crystal. We achieved 1-ns-pulses with an average output power of 300 mW and 300 kHz repetition rate.

We fabricated waveguides in Pr:SrAl(12)O(19) crystals by direct femtosecond laser writing. The propagation losses were calculated to be as low as 0.16 dB/cm at a wavelength of 633 nm. Laser oscillation in a diode-pumped waveguide at a wavelength of 643.5 nm was realized. The output power of the waveguide laser was 28.1 mW at a slope efficiency of 8%.

In recent years the direct laser-writing technique with ultrashort pulses became a great tool for the fabrication of three dimensional integrated optical devices in dielectric media. Highly efficient miniaturized lasers, emitting in the near infrared have been realized by the inscription of waveguides in various rare earth doped crystals [1, 2].

Using a femtosecond laser tracks were written in Yb:YAG. Due to stress induced birefringence waveguiding was possible in channels surrounding the tracks. Laser oscillation was achieved with an output-power of 719mW at 1223mW of pump-power.

Using a fs-laser tracks were written in Nd:YAG. Due to stress induced birefringence waveguiding is possible in channels surrounding the tracks. Laser oscillation was achieved with an output-power of 132mW at 284mW of pump-power.

We report on, to the best of our knowledge, the first fs-laser-written waveguide laser in Yb3+:CaGdAlO4 (Yb:CALGO). With Yb:CALGO crystals grown in our labs, we obtained a slope efficiency of 69% and up to 2.4 W of continuous wave (cw) output power in a waveguide-laser configuration. Moreover, bulk laser experiments with Yb:CALGO were performed, and slope efficiencies up to 73%, optical-to-optical efficiencies of 65%, and maximum cw output powers of 3.3 W were reached. These are the highest efficiencies in the laser configuration with Yb:CALGO.

We demonstrate a diode-pumped femtosecond-laser-inscribed Yb:YAG channel waveguide laser, Q-switched by using single-walled carbon nanotubes (SWCNTs) near 1029 nm. We used saturable absorber mirrors (SAMs) fabricated by depositing SWCNTs on three different output couplers. Best performance of the 9.3-mm-long ultra-compact Q-switched waveguide laser is obtained with an output coupling transmission of 20%. In this case, a maximum average output power of 60 mW with a corresponding pulse energy of 37.7 nJ and a pulse duration of 88 ns at 1.59-MHz repetition rate were achieved. The highest pulse energy of 39.2 nJ and the shortest pulse duration of 78 ns were obtained with 30% and 10% output couplers, respectively. (C) 2015 Optical Society of America

Waveguides were inscribed into a 9.5 mm long periodically poled KTiOPO4 crystal with a Ti:sapphire femtosecond-laser. Waveguiding was achieved between two parallel written tracks with spacings of 17–19 μm. The fundamental power of 1.6 W for frequency doubling in the waveguide was delivered by a continuous wave Ti:sapphire laser at a wavelength of 943.18 nm. A maximum output power of 76 mW in the blue spectral region was achieved. This corresponds to a single pass normalized conversion efficiency of 4.6% W−1 cm−2.

Straight and s-curve Yb(7%):YAG waveguides have been fabricated with the femtosecond laser writing technique. By employing a novel writing scheme an increase of the refractive index change could be achieved in comparison to waveguides written with the standard procedure. Straight waveguides, fabricated with this scheme, enabled highly efficient Ti:sapphire laser pumped waveguide lasers with slope efficiencies of 79% and output powers of more than 1 W. With slope efficiencies from 50% to 60% for the curved waveguide lasers with radii of curvature of R ≥ 20 mm the possibility of fs-laser written complex optical devices is demonstrated.

We report the fabrication and visible laser operation of Pr,Mg:SrAl12O19 waveguides. Waveguiding structures were created by focusing the radiation of a femtosecond Ti:sapphire laser into bulk material. Guiding losses were determined to be as low as 0.12 dB cm(-1) at 632.8 nm. By employing a frequency-doubled optically pumped semiconductor laser, waveguide laser operation was realized at wavelengths of 525.3, 644.0, and 724.9 nm with output powers as high as 36, 1065, and 504 mW, respectively. To the best of our knowledge this is the first demonstration of green laser operation in a Pr(3+)-doped crystalline waveguide laser.

Waveguides were written into single crystalline Yb(7%):YAG with a femtosecond laser. Laser oscillation of the waveguides without external mirrors at an output coupling transmission of 99% was demonstrated. The laser performance of the waveguide lasers, pumped with various light sources, was examined. With an optically pumped semiconductor laser (OPS) as pump source a slope efficiency of 51% regarding incident pump power and a maximum output power of 1.76 W could be achieved. By using a high brightness DBR tapered diode laser as pump source the possible miniaturization of the waveguide laser device was demonstrated. With this pump source even 2.35 W of output power from the waveguide laser was achieved. The beam quality at highest output power turned out to be excellent with an M2-factor of less than 1.3.

Using a femtosecond Ti:Sapphire laser, micro-tracks of material damage were written into Yb:YAG crystals. Waveguiding was achieved in a channel between pairs of tracks with guiding losses of 1.3 dB/cm at a wavelength of 1063 nm, due to a stress induced change of the refractive index. Pumped at a wavelength of 941 nm, highly efficient laser oscillation in a Yb:YAG channel waveguide at a wavelength of 1030 nm was demonstrated. An output power of 0.8 W at 1.2 W of launched pump power was achieved, resulting in a record slope efficiency of 75%.

The Pulsed Laser Deposition (PLD) technique has several advantages such as the possibility to produce thin films of any compound with a stoichiometric material transfer from the target to the substrate. Also the high energy of the ablated material is a benefit, leading to crystalline film growth due to the high mobility of the atoms on the substrate surface. In combination with a recently demonstrated efficient Nd:YAG waveguide laser, the PLD technique is suitable for producing passively Q-switched waveguide lasers by depositing thin films of highly doped Cr4+:YAG on one endface of the waveguide [1–2].

Stress induced channel waveguides were written in crystalline Yb (7%)∶YAG with fs-laser pulses. Pumped by an OPS-laser at 969 nm laser oscillation with up to 1.76 W output power was demonstrated.

We review the progress in performance of Yb:YAG, Yb:LuAG, and Yb:Lu2O3 for ultrahigh power generation. Future challenges are the prevention of losses at high Yb inversion densities, further power scaling of thin disk and crystalline waveguide lasers.

In recent years highly efficient waveguide lasers based on femtosecond-laser (fs-laser) written waveguides in crystalline materials could be realized. One approach for the fabrication of waveguiding structures is the inscription of two parallel tracks of modified material with distances in the order of 20 μm and the waveguiding region in between. During the fabrication process the sample is moved perpendicular to the incident beam of the fs-laser.