A New Concept in Picosecond Lasers

A New Concept in Picosecond Lasers New solutions successfully demonstrated within BMBF joint project iplase Rico Hohmuth, Peer Burdack, Jens Limpert Over the last decade, mode-locked laser sources in the ten picosecond pulse regime expanded into medical applications and high-precision micro-material machining. However, the complexity and the alignment sensitivity of picosecond lasers translated to higher cost and impeded a broader market entry. The goal of the iplase project was to develop a novel laser concept (Fig. 1) which is capable of generating sub-10 picosecond pulses. To this end, we shortened the pulses of a novel amplified microchip laser using a pulse compressor. This article outlines the key achievements of the project. Motivation Picosecond lasers enable ultra-precise and non-contact cutting tools. They are proving indispensable in many industrial applications such as cutting, scribing and marking of different glasses, dicing and repair of semiconductor wafers, and drilling holes in metals, ceramics, or polymer materials. Depending on the specific application and material properties, one has to select the right set of processing parameters. A key attribute of ultra-short laser pulses is high peak power intensities in the infrared, green, or ultraviolet spectrum, combined with low pulse energies. These qualities enable high quality processing of temperature sensitive materials. Cold ablation is a term frequently associated with material processing that occurs without significant heat associated damage due to the very short pulses employed. Picosecond pulses are routinely and to this point exclusively obtained from mode-locked laser systems. Mode-locked lasers are, for the most part, rather complex systems and sensitive to external pertur- Microchip laser Fig. 1 Diagram of a novel picosecond laser incorporating a microchip laser, a pulse-stretching amplifier, and a bulk compressor. bations such as temperature and vibration transients. In contrast, classical Q-switched lasers are more robust and simple, but they emit longer pulses, usually in the nanosecond range, with significantly higher energies and lower repetition frequencies than modelocked oscillators. Nevertheless, the usage of nanosecond lasers represents the right choice for many applications consciously accepting a reduced process quality in lieu of higher material removal rates. Analysis of other variables like production throughput, product reliability, and cost of ownership usually contribute to a decision of one technology over another. It is clear, however, that the availability of robust, significantly less expensive and less complex picosecond laser sources would surely enable new market opportunities. Laser concept Recently, the further development of miniaturized passively Q-switched seed sources, so-called microchip lasers (MCLs), has opened up new parameter regimes. Characterized by short cavity length, these microchip lasers enable pulse durations in the sub-200 ps regime and an emission of only one Compressor Fiber amplifier longitudinal mode. Both are essential ingredients of the investigated laser concept. The pulse duration of a passively Q-switched laser is mainly determined by its cavity length; therefore, these microchip lasers likely deliver the shortest available durations emitted by a Q-switched laser. Furthermore, the pulse repetition rate of MCL varies proportionally with the pump power. Laboratory demonstrations have reached pulses as short as 20 ps [1]. Company BATOP GmbH Jena, Germany BATOP GmbH is an innovative company founded in 2003 as spin-off from the University of Jena, Germany. BATOPs areas of expertise are low temperature molecular beam epitaxy, dielectric sputter coating, wafer processing, and chip mounting technologies. During the last years BATOP became a worldwide leading supplier of saturable absorbers for passive laser mode locking. Second product families are photoconductive antennas (PCA) for terahertz radiation emission and detection as well as complete THz-time-domain spectrometers. www.batop.de Laser Technik Journal 1/2016 33

www.laser-journal.de Laser crystal Nd:YVO 4 Saturable output coupler (SOC) Laser Fig. 3 Compact SOC based microchip laser with integrated pump optic AR @ 808nm / HR @ 1064nm Company COHERENT As part of the BMBF joint project iplase, further reductions of pulse duration based on external compression techniques were investigated. Approaches such as these use nonlinear spectral broadening by self-phase modulation (SPM) in fibers, followed by a chirp removal using dispersive elements, or alternatively, simple spectral filtering to address the targeted pulse duration of sub-ten picosecond or even femtosecond pulses. Microchip laser AR @ 1064nm Fig. 2 Schematic setup of the microchip laser. The microchip laser is the seed source of the novel laser system, providing optical pulses with durations down to 100 ps. The setup of a microchip is rather simple and comprises a laser Founded in 1966, Coherent, Inc. is a Standard & Poor s Small Cap 600 and a Russell 2000 Index company. Headquartered in Santa Clara, CA, USA, Coherent is a world leader in providing laser-based solutions to commercial and scientific research markets. We have the broadest technology portfolio in the industry with solutions for any application. In 2016, Coherent celebrates its 50 years anniversary. www.coherent.com www.coherent.de crystal and a saturable absorber acting as passive Q-switch. In contrast to traditional microchip lasers using Cr 4+ :YAG crystals as a passive Q-switch, the novel design employs a semiconductor saturable absorber (SSA) for this purpose. The monolithic setup and the thin film optical design of the saturable absorber shorten cavity length and realize single frequency laser emission. As a result, the novel microchip laser provides a single frequency laser signal, since only one longitudinal mode matches the gain spectrum of the laser material. Well-known microchip lasers using SSAs are based on a reflective design with a saturable absorber mirror (SAM) [2], where the pump and laser light travel along the same path. To separate both beams a dichroic mirror is needed. During the project, a new concept with a novel transmittive design was developed. The key element in this configuration is a saturable output coupler (SOC) which acts as both a passive Q-switch and output coupler, simultaneously. The device is a semi-transparent mirror with a saturable absorber, fabricated via III-V semiconductor epitaxy. The SOC is bonded to the Nd:YVO 4 laser crystal. Fig. 2 shows the schematic of the microchip laser design. The pump light from a laser diode enters the chip from the laser crystal side, while the emitted laser pulses leave 1 st pulse (triggered) 5 µs/div 2 nd pulse the cavity by passing through the semiconductor device. The advantage of this novel concept is the spatial separation of pump and laser light path without the need for an additional optical element, such as a dichroic mirror. Additionally, delivering pump and laser radiation through separate optics opens the possibility for easier fiber integration. The new design is more compact and stable in comparison to the older reflective microchip designs [3]. The miniaturized pulsed lasers with integrated pump optic incorporate a laser crystal with an active area of 1.5 mm 1.5 mm and are, therefore, extremely compact (Fig. 3). In the project, microchip lasers with different cavity lengths and pulse durations of 90 ps and 220 ps have been evaluated. We achieved output powers of up to 65 mw, repetition rates of up to 400 khz, and pulse energies of up to 170 nj. High oscillator pulse energy is essential for simple and direct fiber or bulk amplification. Furthermore, we performed investigations for compact and rigid fiber connections for pump and laser light, and demonstrated a fiber-integrated microchip laser. Microchip laser stabilization Unlike mode-locked lasers, in which a laser pulse continuously circulates 2 nd pulse zoom 50 ns/div Fig. 4 Timing jitter of two following pulses (left) and expanded scale for second pulse (right). The jitter was measured over 60 s. 34 Laser Technik Journal 1/2016

within the cavity, Q-switched laser pulses initiate repeatedly out of the noise. Thus, the timing of the emission of a laser pulse is not exactly determined. The intrinsic time fluctuation between two pulses is called timing jitter and has a bigger uncertainty compared to mode-locked lasers. This timing jitter may limit the usability of the Q-switched laser for industrial applications, especially for processes that require a low pulse overlap. Steinmetz et al. previously demonstrated a method to reduce the timing jitter of Q-switched lasers via selfinjection seeding using a fiber delay line [4], achieving excellent jitter values of approximately 20 ps. However, the method requires a 1 to 1.5 km long fiber delay line, and works with a single pulse repetition frequency (PRF) only. Due to practical and cost limitations, this approach is not suitable for industrial lasers. A simplified method is the electronic modulation of the pump power of the microchip laser, which directly improves the laser repetition stability and reduces the residual timing jitter. For a free running, continuously pumped MCL, the timing jitter was 1500 ns at 40 khz PRF. We achieved a jitter reduction of approximately a factor of three with pump light modulation. If the electronics actively switches the pump light off after detecting a laser pulse with an internal photodiode, the jitter can be further reduced to 100 ns (measured over 60 s) [5]. This value is still significantly higher than mode-locked lasers; however, it is low enough for many applications. Fiber amplifiers Fiber-based laser systems are generally immune against any thermo-optical problems due to their special geometry. Fibers achieve excellent heat dissipation by virtue of the large ratio of surface-to-active volume. Additionally, the beam quality of the guided mode is determined by the fiber core design and is therefore power-independent. Due to the confinement of both the laser and pump radiation, the overlap is maintained over the entire fiber length and is not limited to the Rayleigh length as is the case in longitudinally pumped bulk lasers. The gain of the laser medium is determined by the product of pump light intensity and interaction length with the laser radiation in the gain medium. Therefore, the decisive product can be orders of magnitude higher in fibers than in other bulk solid-state lasers. This leads to very simple amplification setups and fiber laser systems, which exhibit very high gain and low pump threshold values. Complete integration of the laser process in a waveguide provides an inherent compactness and long-term stability. In particular, ytterbium-doped glass fibers, which possess a quantum defect of less than 10 %, can provide optical-to-optical efficiencies well above 80 % with very low induced thermal load. When high average power is desired, ytterbium is the first choice among of all rare earth ions. Ytter biumdoped fibers can amplify radiation in a wavelength span ranging from 970 to 1200 nm. Thus, the 1064 nm emission of a Nd:YVO 4 microchip laser fits well into the amplification bandwidth. Input Seed In an experimental demonstration, a microchip laser producing 70 ps pulses at 1 MHz was amplified in a two-stage ytterbium-doped fiber amplification system. The preamplifier stage has a double-pass configuration based on a 1.3 m photonic crystal fiber DC 170/40, and is end-pumped at 976 nm. The benefit of using a double-pass configuration is very high gain, which is essential for amplification from low power levels at flexible repetition rates. The use of a narrow bandpass filter between the first and the second pass leads to a clean and low ASE signal spectrum and a high signal-to-noise contrast ratio. The main amplifier consists of a 1.2 m long ytterbium-doped large-pitch fiber (LPF) with a pitch of 35 μm, mode field diameter of 55 μm, air clad of 200 μm, and is end-pumped at 976 nm. This system was capable of pulse energy as high as 103 μj, corresponding to 103 W average power with excellent beam quality [6]. Bulk amplifier Output Faraday isolator Laser crystal As mentioned previously, fiber amplifiers have many advantages compared to bulk systems. However, they are definitely more sensitive to optical feedback and limited to lower pulse energies. Dichroic Fig. 5 Double-pass configuration of the bulk amplifier stage Laser Technik Journal 1/2016 35

www.laser-journal.de (a) 75µJ / 6.2 ps measured (b) 280 µj / 8.8 ps measured Norm. intensity Norm. intensity -20-10 0 10 20-20 -10 0 10 20 Time delay / ps Time delay / ps Fig. 6 Autocorrelator traces after the first (a) and after the second bulk amplifer stage (b) Institute As an alternative, we evaluated bulk amplifier concepts to boost the microchip pulses to high pulse energies. The microchip laser was operated in the 10 to 100 khz range. Typically, the laser mode diameter in a bulk amplifier crystal is an order of magnitude higher than in fiber amplifiers. The thresholds of nonlinear effects like Raman or stimulated Brillouin Scattering (SBS) are dependent on peak power densities and the effective length of the active medium. These characteristics create the possibility for bulk amplifiers that are very tolerant against nonlinear effects and are physically robust. This robustness is a key benefit for bulk amplifier versus fiber amplifier for generating high pulse energies. For the experimental demonstration a longitudinally, mode-selective pumped Nd:YVO 4 amplifier (Fig. 5) was the preferred concept. It combines Institute of Applied Physics (IAP) Jena, Germany The fiber and waveguide laser group has a world-leading reputation in fiber laser development, a number of performance records have been achieved in the recent decade. The work focuses on new fiber designs and new experimental strategies to overcome e.g. limitations given by nonlinear effects. In addition, the IAP has successfully investigated passively Q-switched microchip lasers as compact seed source over the recent years. www.iap.uni-jena.de high beam quality and high energy extraction. This required a good pump and laser mode overlap to obtain beam qualities values close to the diffraction limit. A single-frequency 40 khz microchip laser seed was used and equipped with a longitudinal pumped pre-amplifier, a dispersive single-mode fiber for spectral broadening, and gratings for pulse compression. The seed source had an average power up to 0.3 W and the pulse duration was compressed from 115 ps to 5.9 ps. We achieved pulse energies of 110 µj with one double-pass amplifier unit. Output power scaled to 380 µj with a second amplifier stage. The pulse duration (auto correlator traces) for the first and second amplifier stage are shown in Fig. 6. With in creasing pulse energy, the pulse duration increases to 6.2 ps for pulse energies of 75 µj and 8.8 ps for 280 µj, respectively [5]. As such, the aspired project goal of < 10 ps and 400 µj was nearly fulfilled. Further investigations are necessary to optimize the setup and performance. Nevertheless, it demonstrates that this approach is an interesting alternative to mode-locked picosecond lasers. Pulse compression We employed two different compressors to demonstrate the pulse compression of passively Q-switched microchip lasers (MCL). One compressor was based on a chirped volume Bragg grating (CVBG) with fixed dispersion rate of 90 ps/nm and 1064 nm central wavelength. The CVBG enables a very small footprint, compressing the pulses of the MCL with 92 ps initial duration at repetition rates of 500 khz to less than 5 ps (Fig. 7). A pair of transmission gratings with 1740 lines/mm was the nucleus of the second compressor. This design element provides a flexible adjustment of dispersion for different pulses by changing the grating separation, and compresses to 3 ps from an initial duration of 70 ps at 1 MHz in another MCL. Analysis shows that a passively Q-Switched microchip laser combined with a fiber amplifier and a compact compressor based on chirped volume Bragg-gratings can reach > 100 W average power, > 100 μj pulse energy and < 10 ps pulse duration with diffraction-limited beam quality [6]. Nonlinear spectral broadening followed by spectral filtering forms the basis for an alternative technique of pulse compression (Fig. 8). This approach is particularly interesting for seed-sources, as it constitutes a simple, easy to adjust, inexpensive solution because it employs a fiber as waveguide structure and a reflective volume Bragg grating (VBG) as bandpass filter. Through SPM in a passive fiber, the nearly transform-limited pulse emitted from a MCL acquires a chirp, but its temporal pulse shape remains unchanged. Subsequently, this spectrally broadened pulse is directed to the bandpass filter where only a small spectral and, therefore, temporal (due to the chirp) part is reflected, i.e., the pulse is shortened in time domain. This method works as long as the propagation in the passive fiber is dominated by SPM and the bandwidth of the filter is narrower than the SPM-broadened spectrum. With this technique, 118 ps long pulses could be reduced to 32 ps while 36 Laser Technik Journal 1/2016

AC-trace: experiment (dots-line) simulation (filled curve) Microchip laser SMP in fiber Amplification Spectral filtering Norm. SHG intensity 1.35 x 4.7ps Active fiber -75-50 -25 0 25 50 75 Delay time / ps Fig. 8 Alternative pulse compression technique for an amplified microchip laser based on spectral filtering. Fig. 7 Simulated and measured compressed pulse duration of a MCL using a chirped volume Bragg grating preserving a high temporal pulse quality. Using a subsequent fiber amplifier stage, such filtered pulses can be easily boosted to energies higher than 20 μj. Hence, the presented pulse shortening method seems very suitable for the integration into all-fiber systems, resulting in very compact seed sources delivering pulses in the 10 ps range [7]. Summary The novel laser system presented here delivers pulse durations in the ten picosecond range, and as such is a promising light source for material processing applications. With a microchip laser as the seed source, the innovative concept uses nonlinear spectral broadening by selfphase modulation (SPM) in a fiber. Sub- 10 picosecond pulses can be obtained by additional subsequent chirp removal with the help of dispersive elements or simple spectral filtering. For further amplification of the compressed pulses a fiber and a bulk amplifier setup has been evaluated. We achieved pulse energies up to 280 µj with 8.8 ps duration using a double-stage bulk amplifier. The relative high pulse-to-pulse timing jitter of the Q-switched seed laser has been reduced by a factor of 15 using electronic modulation of the pump power. As a special benefit, this pump power modulation offers the possibility to alter the repetition rate of the laser system. In comparison to a typical mode-locked laser, this system allows a wide range of pulse repetition rates without any pulse picking. The achieved results clearly show the potential for robust and more cost efficient picosecond laser systems in the future. Acknowledgments The authors would like to thank the Federal Ministry of Education and Research (BMBF) for funding the project iplase within the framework Ultrashort pulse lasers for high precision processing (UKP). DOI: 10.1002/latj.201600001 [1] E. Mehner et al.: Opt. Lett. 39 (2014) 2940 2943 [2] G. Spühler et al.: J. Opt. Soc. Am. B 16 (1999) 3, 376-388 [3] A. Steinmetz et al.: Appl. Phys. B 97 (2009) 317 320 [4] A. Steinmetz et al.: Optic Lett. 35 (2010) 17, 2885-2887 [5] R. Hohmuth: iplase Innovative Picosecond Laser System for High-Precision Material Processing, Laser World of Photonics 2015, Session Ultrashort Pulse Lasers for High Precision Processing, Munich, 25th June 2015 [6] A. Steinmetz et al.: Opt. Lett. 37 (2012) 2550-2552 [7] R. Lehneis et al.: Opt. Lett. 37 (2012) 4401-4403 Authors Rico Hohmuth received his diploma in physics from the University of Jena in 2002. He is co-founder and chief technical officer of the company Batop GmbH in Jena. Peer Burdack earned his doctorate in physics from the University of Hanover in 2004. Currently, he is R&D manager and project leader for sub-nanosecond Q-switched lasers for Coherent LaserSystems GmbH & Co. KG in Luebeck. Jens Limpert received his MSc in 1999 and PhD in Physics from the Friedrich Schiller University of Jena in 2003. He is currently leading the Laser Development Group (including fiber- and waveguide lasers) at the Institute of Applied Physics at the University of Jena. Dipl.-Phys. Rico Hohmuth, BATOP GmbH, Wildenbruchstr. 15, 07745 Jena, Germany, info@batop.de; Dr. Peer Burdack, Coherent Laser Systems GmbH & Co. KG, Seelandstrasse 9, 23569 Luebeck, Germany, peer.burdack@coherent.com; Prof. Dr. rer. nat. Jens Limpert (Jun.-Prof.), Institute of Applied Physics, Friedrich Schiller University Jena, Albert-Einstein-Str. 15, 07745 Jena, Germany, jens.limpert@uni-jena.de Laser Technik Journal 1/2016 37