SUBHASH SINGH, NICK REILLY, and CHUNLEI GUO
Fiber lasers use rare-earth-doped optical fibers as active media and laser diodes for pumping, giving them a number of key advantages that make them attractive for the generation of ultrashort pulses through mode locking. The large gain bandwidth and efficiency of doped fibers allows fabrication of comparatively cheap, compact, and rugged fiber laser systems with fiber-coupled output beams that suit a wealth of applications.
Femtosecond architectures
Fiber provides a high surface-to-volume ratio that allows for efficient cooling and customizable recipes for tailored performance parameters. Initially limited to continuous-wave (CW), low-power, single-mode operation, fiber lasers have been transformed over three decades into single- and multimode varieties with ultraviolet (UV) to far-infrared (far-IR) wavelengths that have very high power levels, variable repetition rates, and—perhaps most significantly—millisecond to femtosecond pulse durations.
Unlike conventional free-space lasers, fiber lasers use optical fiber and fiber Bragg gratings (FBGs) that replace conventional dielectric mirrors for optical feedback. Most high-power fiber lasers use a double-clad fiber architecture where the gain medium is at the core of the fiber and is surrounded by two layers of cladding. A multimode pump beam, produced either from the laser diode or from another fiber laser propagating in the inner and confined by the outer cladding layers, excites the active medium and generates the lasing mode that propagates in the core of the fiber.
To create ultrafast laser pulses, either active or passive mode-locking techniques are required. Some techniques used today for passive mode locking include nonlinear polarization rotation and saturation absorption techniques, while electro- or acousto-optic modulators are used for active mode locking.1
Semiconductor saturable absorber mirrors (SESAMs), where semiconductor quantum wells are grown on semiconductor distributed Bragg reflectors, have been used to successfully fabricate femtosecond fiber lasers operating at approximately 1.0 and 1.5 μm wavelengths. And self-starting mode locking and stable soliton pulse generation have been demonstrated with a graphene saturable absorber in an erbium (Er)-doped fiber laser. These are just a few of the femtosecond fiber laser architectures being implemented by commercial laser manufacturers to address both well-known and yet-to-be-imagined scientific and industrial applications.
Nonlinear mode locking
For reproducible and long-term stable operation, figure 9 technology from Menlo Systems (Martinsried, Germany) uses the well-established nonlinear optical loop mirror (NOLM) mode-locking mechanism. Both the oscillator and amplifier use only polarization-maintaining (PM) fiber components for high stability and low-noise, maintenance-free operation.
Menlo's Er-doped fiber lasers exhibit a broad gain bandwidth at a center wavelength of 1560 nm and 780 nm with standard and high-power models that have <90 fs pulse duration and repetition rates in the range of 50–250 MHz. Its ytterbium (Yb)-doped Orange femtosecond fiber lasers operate at 1040 and 520 nm, available with average power levels of up to >10W and providing <150 fs pulse duration. As a developer for optical frequency combs, all laser systems can be synchronized with high accuracy. These systems are widely used in spectroscopy, microscopy, metrology, gravitational observatories, and materials processing.2,3BlueCut, which includes an oscillator and an amplifier with pulse-picking unit and compressor for short and high-energy pulses, is Menlo's industrial-grade microjoule fiber laser system. Based on all-fiber integrated technology, the system is robust and stable for microprocessing applications (see Fig. 1).
Fiber CPA
Based on its fiber chirped-pulse amplification (FCPA) technology, the FCPA μJewel series from IMRA America (Ann Arbor, MI) consists of Yb-doped fiber lasers with sufficient pulse energy, even at wavelengths of 1045 nm (see Fig. 2).4 The FCPA architecture allows users to choose between high-energy operation up to 50 μJ at a repetition rate of 100 or 200 kHz, and high average power of 10 and 20 W at 1 MHz. This option allows for material processing at faster rates, depending on the application requirements.
IMRA's Raman-shifting technology creates a clean pulse shape and spectrum in the form of Er-doped, Raman-shifted femtosecond fiber lasers at an 810 nm wavelength, making the Femtolite model a replacement for the titanium sapphire (Ti:sapphire) laser—a femtosecond workhorse in clinical and industrial settings. Femtolite power levels range from 150 to 200 mW at both 810 and 1620 nm wavelengths, which are useful in terahertz wave generation and detection, multiphoton fluorescence microscopy, and second-harmonic imaging.5
Moreover, femtosecond pulses can be delivered by optical fiber to the end-user apparatus. Femtolite FD series provide a fiber-coupled output delivering an average power up to 1 W at a repetition rate of 50 MHz. This level of pulse energy is sufficient for many nonlinear imaging or metrology applications requiring flexible integration of the femtosecond pulse source to the apparatus.
Similarly, the fiber laser chirped-pulse amplifier (FLCPA) is the basis for Calmar Laser (Palo Alto, CA) Cazadero high-energy (up to 30 μJ) ultrafast laser pulses (<0.5 ps) at 1 or 1.5 μm wavelengths with repetition rates of hundreds of kilohertz. These ultrafast laser models are also available in green (515 nm) and UV (343 nm) wavelengths, and can be coupled to the company's Bodega OPA to provide broad wavelength coverage throughout the near-infrared (near-IR).
Cazadero FLCPA ultrafast lasers start with a 27 MHz passively SESAM mode-locked seed fiber laser, sampled down to a pulse rate of 120 kHz or higher. The ultrafast pulse is time-stretched by frequency (chirped) for amplification through a high-power fiber amplifier stage at lower peak intensity. Up to 30 μJ of short-pulse energy is delivered into free space. This FLCPA is a cost-effective alternative to solid-state femtosecond laser amplifiers for applications in precision biomedical materials processing and nanostructuring.7
Able to produce high melting temperatures of more than 4000°C for materials melting, microstructure manipulation, or multi-materials synthesis, high-power femtosecond fiber amplifiers have been increasingly used for material processing and manufacturing. In 2014, Laser-Femto (San Jose, CA) achieved the first 0.5 mJ femtosecond fiber laser, pushing high-power limit of fiber-based femtosecond technology.
Passive mode locking
Passive SESAM mode-locking technology also forms the basis of Calmar's Carmel-CFL ultrafast fiber laser seed platform. The Carmel X-series is a range of high-power, air-cooled, 780 nm (and optional 1550 nm), fiber-based femtosecond lasers with output powers from 0.2 to greater than 1.0 W (up to 2.5 W at 1550 nm; see Fig. 3).
The X-series includes remote data logging, power monitoring, system diagnostics, and automated adjustment of the second-harmonic crystal for prolonged lifetime and OEM service support. For multiphoton microscopy applications, the Carmel ultrafast laser is useful for cellular tissue imaging with minimal scatter and reduced risk of photodamage. The compact laser head and associated armored fiber cable allow integration into existing microscopes with minimal delivery optics.
Another passively mode-locked, industrial-grade femtosecond laser with low phase noise and timing jitter is the Onefive (Regensdorf, Switzerland) Origami. Its transform-limited soliton pulse emission provides diffraction-limited beam quality and pointing stability at various wavelengths and repetition rates.
Packaged in an air-cooled and sealed enclosure for operation in harsh environments, Origami is designed for high stability and low drift. The Origami (<100 fs, up to 5 nJ), Origami HP (<100 fs, up to 100 nJ), and Origami XP (<400 fs) femtosecond models are good for ultralow-noise timing and frequency comb applications.6 The Origami XP (<400 fs, >40 μJ) is established in femtosecond ophthalmology (LASIK and cataract surgery), as well as material processing (micromachining).Multi-wavelength output
Using Er and Yb optical fibers, the FemtoFiber series of femtosecond fiber lasers from Toptica Photonics (Munich, Germany) include numerous output options: 1560/780 nm, visible/near-IR tunable output, IR/near-IR supercontinuum, and short-pulse varieties, which have utility in nonlinear microscopy, two-photon polymerization, time-domain terahertz, and attoscience applications, and can also function as seed lasers.
Saturable absorber mirror (SAM) mode locking and PM fiber technology allow the turnkey FemtoFiber ultra series to be used in life science, attoscience, optical coherence tomography (OCT), and industrial/OEM integration applications that are usually lacking engineers with laser expertise. The fiber oscillators of the FemtoFiber lasers may be used for seeding either one or more optical amplifiers. Synchronized multiple laser systems are beneficial in applications like frequency metrology and pump-probe experiments.
The Toptica FemtoFiber dichro series simultaneously provides two synchronized laser beams at different wavelengths out of one box and from the same aperture. The platform has been designed with a special focus on applications that frequently need more than one color such as biophotonics applications, including two-photon fluorescence and second-harmonic generation (SHG) microscopy.
The FemtoFiber dichro bioMP model emits sub-150 fs pulses from one aperture at 780 nm (>500 mW) and 1050 nm (>1000 mW) that can be modulated independently in intensity, pulse duration (chirp), and respective inter-pulse delay-adjustable parameters that have uses in live-cell multicolor two-photon imaging, broadband coherent anti-Stokes Raman spectroscopy (CARS), and stimulated Raman spectroscopy (SRS) studies, as well as pump-probe experiments and pulsed stimulated emission-depletion (STED) microscopy (see Fig. 4).9
MOPA power
The Fianium (Southampton, England; acquired by NKT Photonics in early 2016) fiber laser product portfolio is based on master oscillator power amplifier (MOPA) building blocks to produce mode-locked laser sources with picosecond or femtosecond optical pulses. High average power (>20 W) and high-energy systems are available at megahertz to single-shot repetition rates and spectral operation from 240 to 2500 nm.
Fianium's supercontinuum (SC450-4) or "white-light" fiber lasers emit an ultra-broad optical spectrum, typically from UV to beyond 2 μm, with laser-like beam quality. All Fianium supercontinuum lasers provide picosecond pulses at megahertz repetition rates, making them equally effective as a CW source in steady-state applications or a pulsed source for lifetime measurements. A single supercontinuum laser, in combination with a tunable filter, can replace an unlimited number of single-frequency lasers in a range of applications.
Fianium's FemtoPower lasers are high-average-power, fixed-repetition-rate, passively mode-locked MOPA products at 1064 nm. The second harmonic at 532 nm is also available with pulse duration of 200 fs.
The rack-mountable Fianium High-Energy series is capable of delivering up to 10 μJ of pulse energy. Inside, there is an integrated output modulator that allows the user to customize the output from single-shot to continuous mode at up to 1 MHz. This product line has <500 fs pulse-duration options that produce a maximum average power of 2 W and <5 ps options available up to 5 W for even-higher-energy applications, including micro- and nanostructuring, tissue ablation, and ophthalmic surgery.10LightWire FF1000 is optimized for nonlinear microscopy (two-photon, SHG) applications. High average power (1.5 W), short pulse duration (80 fs), and good beam quality combine to achieve sharp, bright images of samples. Laser emission wavelength of 1030 nm is optimal both for deep excitation and collecting light from the tissue. High peak power (625 kW) of the femtosecond pulses is also useful in many other nonlinear optical applications, like terahertz generation or two-photon polymerization.
Based on a well-established MOPA scheme, the EKSPLA (Vilnius, Lithuania) 1030 nm, 1.5 W average power LightWire FF1000 laser is optimized for nonlinear microscopy (two-photon, SHG) applications with pulse durations down to 80 fs. And its LightWire FF50 femtosecond fiber laser provides pulse durations of <130 fs at a 1064 nm wavelength—a low-cost, compact, and rugged fiber-based replacement for traditional neodymium yttrium-aluminum-garnet (Nd:YAG) lasers.
Mid-IR options and more
Infrared pulsed fiber lasers from IPG Photonics (Oxford, MA) are available from 1.03 to 1.06, 1.55 to 1.65, 2.09, and 2.1 to 2.6 μm wavelengths. Nonlinear external conversion produces green output, and picosecond and femtosecond pulsed options are available up to 10 W at 1.5 μm, 100 W at 1.06 μm, or as a second-harmonic source up to 5 W at 0.52 μm.
While these femtosecond options range from 400 to 600 fs with up to 3 MHz repetition rates, IPG's CLPF ultrafast oscillators provide 40 fs pulses at a customer-selected fixed wavelength in the range of 2.1–2.6 μm with 80–800 MHz pulse repetition rate and 2 W output power. IPG's ultrafast amplifiers provide access to multiwatt output powers in the 2–3 μm spectral range. The Kerr-lens mode-locked oscillator and ultrafast amplifier heads are pumped by IPG's CW fiber lasers, and address a range of scientific and biomedical applications.
All femtosecond fiber laser manufacturers continue to expand their arsenal of ultrafast architectures with broader wavelength ranges, shorter pulses, and a variety of power output options that will address the next generation of materials probing and processing challenges.
REFERENCES
1. D. Y. Tang et al., Phys. Rev. Lett., 101, 15, 153904 (2008).
2. T. Steinmetz et al., Science, 321, 5894, 1335–1337 (2008).
3. J. Kim et al., Nat. Photon., 2, 12, 733–736 (2008).
4. K. Hartinger and R. Holzwarth, Laser Tech. J., 11, 2, 34–35 (2014).
5. U.S. Patent No. 8,503,069 B2 (Aug. 6, 2013).
6. A. C. Millard et al., Appl. Opt., 38, 36, 7393–7397 (1999).
7. M. Eisele et al., Nat. Photon., 8, 841–845 (2014).
8. N. G. Horton et al., Nat. Photon., 7, 205–209 (2013).
9. W. R. Zipfel et al., Proc. Nat. Acad. Sci., 100, 12, 7075–7080 (2003).
10. P. Blandin et al., Appl. Opt., 48, 3, 553–559 (2009).
Subhash Singh is a scientist in the Department of Physics at the University of Allahabad, Allahabad, India; www.allduniv.ac.in, while Nick Reilly is a graduate student and Chunlei Guo is a professor of Optics & Physics and director of the Center for Laser Material Processing, both at the University of Rochester, Rochester, NY; e-mail: [email protected]; www.optics.rochester.edu/workgroups/guo.
For More Information
Companies mentioned in this article include:
Calmar Laser
Palo Alto, CA
http://calmarlaser.com
EKSPLA
Vilnius, Lithuania
www.ekspla.com
Fianium (NKT Photonics)
Southampton, England
www.fianium.com
IMRA America
Ann Arbor, MI
www.imra.com
IPG Photonics
Oxford, MA
www.ipgphotonics.com
Laser-Femto
San Jose, CA
www.laser-femto.com
Menlo Systems
Martinsried, Germany
www.menlosystems.com
Onefive
Regensdorf, Switzerland
www.onefive.com
Toptica Photonics
Munich, Germany
www.toptica.com
For a complete listing of companies making femtosecond and other ultrafast lasers, visit the Laser Focus World Buyers Guide (http://buyersguide.laserfocusworld.com/index.html).