Fiber lasers produce a high-power high-quality optical beam and have led to many major advances in optics and photonics, including fiber laser-based frequency combs,1 laser eye surgery,2 and industrial machining.3The vast majority of these laser systems have been based on doping rare-earth ions into silica glass fiber, the fabrication and performance of which has reached a level of maturity rarely found in optics. This has enabled the construction of robust, high-performance fiber laser systems operating in the near-IR. However, silica fibers are only transparent at wavelengths shorter than 2μm, and many important molecules have strong fundamental rotational-vibrational transitions in the mid-IR range (3–5μm).4As a result, lasers based on silica fibers have not seen extensive use in applications such as trace gas detection, ablation of water-based tissues (e.g., human skin), and sensitive spectroscopy. Extending the wavelength range of fiber lasers to this region is desirable as the advantages of fiber lasers could open up new applications.
To overcome this wavelength limitation, one must use a type of fiber based on a fluoride glass known as ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF): this allows for transparency and lasing up to nearly 4μm. This glass fiber is relatively new and little known, but many advances are being made and high-performance systems are starting to emerge.5, 6 Until now, many systems have used erbium ions (Er3+) as the dopant. The relevant transition in Er3+ has a central wavelength of 2.75μm, and to date the highest power output in the mid-IR has been achieved using erbium-doped ZBLAN.6
Figure 1. (a) Output power of the laser at five different pump levels. At the highest pump level, we achieve >1W average power over a tuning range of 130nm, with a 0.5nm linewidth. (b) Characterization of various mid-IR fibers using this source. By tuning over a large range, we reveal OH-peaks in undoped chalcogenide (arsenic trisulfide, As2S3) and fluoride (ZBLAN) glass fibers. MM: Multi-mode fiber. SM: Single-mode fiber.
Figure 2. (a) Pulse profile of the Q-switched laser as measured on a fast indium arsenide (InAs) photodetector. (b) Peak power and pulse width as a function of repetition rate of the laser. As the repetition rate decreases, and thus the pulse energy increases, the pulse width reduces. Arb. u.: Arbitrary units.
In our work, however, we have used holmium ions (Ho3+) as the dopant, the relevant transition of which has a slightly longer central wavelength and has significantly less pump-excited-state absorption relative to Er3+. This absorption is a problem because pump light is absorbed by the upper laser level of the 3μm laser transition, and this saturates the output power when the pump power is increased. Our models show that performance should be comparatively better with Ho3+ at high pump power, although it has not been shown experimentally (because pump power from the diodes pumping Ho3+ is currently much lower than the power from diodes pumping Er3+). The recent development of strained indium gallium arsenide (InGaAs) quantum well diodes has enabled direct diode pumping of the Ho3+ ion at 1150nm. A peculiarity of the energy level structure in Ho3+ is that it requires a praseodymium (Pr) sensitizer ion to ensure efficient operation of the laser. Using Ho/Pr-doped ZBLAN fiber, we have recently demonstrated several laser systems including: a continuous-wave (cw) laser that can be tuned over 130nm with a 0.5nm linewidth,7,8 a Q-switched system that produces nearly 600W peak power,9 and a mode-locked system that produces 24ps pulses.10
The cw laser utilizes a grating feedback (Littrow configuration, where the diffraction angle is the same as the incidence angle) for line narrowing and wavelength tuning. At the highest pump powers, we achieved more than 1W average power over a wavelength range of 130nm, with a linewidth 1(a). With this broad tuning range, we were able to characterize the loss in several commercial mid-IR fibers (both single-mode and mutli-mode, indicating the number of transmission paths, or modes) around 3μm, an important region due to the resonance of the OH− ion (a common contaminant in mid-IR fibers). These measurements show that the commercially available ZBLAN had significantly less absorption than the commercially available chalcogenide (0.3dB/m vs. >1dB/m): see Figure 1(b).
To access applications that rely on high peak power such as material machining, laser tissue surgery, and nonlinear optics, the laser cavity must be modified to produce short pulses. We have investigated two approaches to this: Q-switching and mode-locking. In Q-switched operation, the Q of the cavity (i.e., the ‘quality factor’ or ratio of the resonance frequency and the full-width at half-maximum linewidth) is held artificially low while the gain medium is pumped to a high inversion. Once the gain is saturated, the cavity Q is then suddenly increased so that the laser is instantaneously well above threshold. This results in the build-up and output of a giant pulse. In our system, we use an intra-cavity mid-IR transparent acousto-optic modulator to modulate the laser between high-Q and low-Q states. The performance of our system indicates that we can achieve sub-40ns pulses at repetition rates as low as 1kHz, yielding peak powers up to nearly 600W: see Figure 2.8 We believe these systems will soon be used for characterization and material processing in the mid-IR wavelength range.
One of the more exciting applications of fiber lasers is their use as part of an optical frequency comb. These systems, which have become a groundbreaking tool in metrology and sensing, rely on a mode-locked laser producing a regular train of ultrashort pulses. In the near-IR, mode-locked Er-doped silica fiber lasers provide optical frequency comb coverage at 1–2μm. However, a fiber laser frequency comb in the mid-IR has yet to be developed. The first step in creating this mid-IR fiber laser frequency comb is to demonstrate a short-pulse mode-locked mid-IR fiber laser.
To this end, we recently investigated using a semiconductor saturable absorber mirror (SESAM) in one of our Ho/Pr fiber lasers. The SESAM, which was based on saturable absorption in indium arsenide (InAs), enabled us to generate a train of pulses with a repetition frequency of 27MHz. To measure the actual pulse width of the pulses, we constructed a two-photon autocorrelator based on InGaAs and recorded 24ps pulses.10
In summary, we have demonstrated several Ho/Pr-co-doped ZBLAN fiber laser systems, including a cw laser that can be tuned over 130nm with a 0.5nm linewidth,7,8 a Q-switched system that produces nearly 600W peak power,9 and a mode-locked system that produces 24ps pulses.10While this pulse width is too long (and the corresponding peak power too low) to create a frequency comb, we are pursuing several improvements, including dispersion management in the laser cavity and graphene to provide saturable absorption for mode-locking. While much work remains to be done, this experiment represents a crucial step toward realizing mid-IR fiber laser frequency combs. We are now working on advancing the performance of each of these laser systems. For the narrow linewidth laser, we have recently written a fiber Bragg grating into our fiber and used it to produce a single longitudinal mode laser operating at 2914nm. With advances in fluoride fiber technology and increasingly high-performance laser systems being demonstrated, the rise of mid-IR fiber lasers is well underway.
Darren D. Hudson, Stuart D. Jackson
School of Physics
University of Sydney
1. B. Washburn, S. Diddams, N. Newbury, J. W. Nicholson, M. F. Yan, C. G. J⊘rgensen, Phase-locked, erbium-fiber-laser-based frequency comb in the near infrared, Opt. Lett. 29, p. 250, 2004.
2. F. Morin, F. Druon, M. Hanna, P. Georges, Microjoule femtosecond fiber laser at 1.6μm for corneal surgery applications, Opt. Lett. 34, p. 1991-1993, 2009.
4. F. Tittel, D. Richter, A. Fried, Mid-infrared laser applications in spectroscopy, Top. Appl. Phys. 89, p. 445-529, 2003.
5. D. Faucher, M. Bernier, N. Caron, R. Vallee, Erbium-doped all-fiber laser at 2.94μm, Opt. Lett. 34, p. 3313, 2009.
6. S. Tokita, M. Murakami, S. Shimizu, M. Hashida, S. Sakabe, Liquid-cooled 24W mid-infrared Er:ZBLAN fiber laser, Opt. Lett. 34, p. 3062, 2009.
7. D. Hudson, E. Magi, L. Gomes, S. Jackson, 1W diode-pumped tunable Ho3+, Pr3+-doped fluoride glass fibre laser, Elec. Lett. 47, p. 985, 2011.
8. S. Crawford, T. Hu, D. Hudson, S. Jackson, Widely tunable 2.9mm Ho3+, Pr3+-doped fluoride glass fibre laser used to identify 317cm-1 Raman shift, Proc. Austral. Conf. Opt. Fibre Technol. (ACOFT) , 2012.
9. T. Hu, D. Hudson, S. Jackson, Actively Q-switched 2.9mm Ho3+ Pr3+-doped fluoride fiber laser, Opt. Lett. 37, p. 2145-2147, 2012.
10. J. Li, D. Hudson, Y. Liu, S. Jackson, Efficient 2.87μm fiber laser passively switched using a semiconductor saturable absorber mirror, Opt. Lett. 37, p. 3747-3749, 2012.