April 1, 2014 / Vol. 39, No. 7 / OPTICS LETTERS
2133
Stable, self-starting, passively mode-locked fiber ring laser of the 3 μm class Tomonori Hu,* Darren D. Hudson, and Stuart D. Jackson Centre for Ultrahigh bandwidth Devices for Optical Systems (CUDOS), Institute of Photonics and Optical Science, School of Physics, University of Sydney, New South Wales 2006, Australia * Corresponding author:
[email protected] Received January 21, 2014; revised March 3, 2014; accepted March 4, 2014; posted March 4, 2014 (Doc. ID 205195); published March 31, 2014 We report a passively mode-locked Ho3 Pr3 -doped fluoride fiber laser, producing 6 ps pulses at a repetition rate of 24.8 MHz, with a peak power of 465 W. For the first time, a ring cavity was demonstrated in a fluoride fiber laser arrangement which was essential to the generation of stable and self-starting mode-locked pulses. © 2014 Optical Society of America OCIS codes: (140.3510) Lasers, fiber; (140.4050) Mode-locked lasers; (140.3070) Infrared and far-infrared lasers; (140.3560) Lasers, ring. http://dx.doi.org/10.1364/OL.39.002133
Pulsed lasers in the mid-infrared wavelength range (2–20 μm) have gained attention due to their potential in the areas of defense, sensing, and mid-infrared photonics. As a specific example, picosecond pulses near 2.9 μm, targeting a water absorption resonance [1] can be used as a minimally invasive device for laser microsurgery [2]. As a result, achieving ultrashort pulses at mid-infrared wavelengths has become a point of interest for both fundamental research and commercial applications. There have been numerous reports of high power pulse production between 2.8 and 3.0 μm via the use of doped fluoride fibers which remain transmitting up to 4 μm [3–5]. In particular, early studies focused on Qswitching erbium and holmium transitions that produced pulses as short as 78 ns [4], with peak powers up to 0.9 kW [5]. More recent studies have implemented mode-locking methods that typically offer higher peak powers and shorter pulses. The erbium (Er3 ) transition at 2.7 μm was nominally mode-locked with the use of an Fe2 :ZnSe saturable absorber and gave predictions for a pulse width of 19 ps [6]. Similarly, a holmium-praseodymium (Ho3 Pr3 ) co-doped fiber laser at 2.9 μm was demonstrated to produce pulse widths of 24 ps using an InAs semiconductor saturable absorber mirror (SESAM) [7]. It was evident, however, that the laser was only partially mode-locked due to a nonoptimal cavity configuration. In these two studies, the transition from Q-switched modelocked to continuous wave (cw) mode-locked regimes appeared to be in disagreement. One study [7] showed that increasing the optical intensity on the saturable absorber caused a transition from Q-switched modelocked to cw mode-locked, whereas results of [6] indicated the reverse trend. This suggests the need for further experiments to optimize the cavity arrangement so that stable and well-characterized pulses can be obtained. The Ho3 Pr3 co-doped system works on a single transition (5 I7 → 5 I8 , details described in [8]) as opposed to the Er3 system, which involves energy upconversion processes [9]; therefore, in this work we pursued the simpler Ho3 Pr3 system in order to thoroughly study the mode-locking process. 0146-9592/14/072133-04$15.00/0
In this Letter, we report the demonstration of a passively mode-locked Ho3 Pr3 co-doped fluoride laser that produced pulses at 2.86 μm using an InAs-based saturable absorber. Unlike previous studies that implemented a linear cavity [6,7], we designed a ring cavity geometry that allowed for unidirectional operation, which was immune from in-cavity back reflections that are likely to be a source of unstable mode- locking. The layout of the laser is shown in Fig. 1. It contained 8 m of double clad Ho3 Pr3 co-doped ZBLAN fiber (FiberLabs) that consisted of an octagonal shaped inner cladding with diameter 125 μm and a circular core with diameter 10 μm. The length of the fiber was chosen to produce a repetition rate for an optimal pulse energy that became important in obtaining stable cw mode-locking, as discussed later. The NA of the core was 0.2, which set the single mode cut-off wavelength at 2.61 μm. The concentration of the Ho3 and Pr3 were 30,000 and 2500 ppm, respectively. Pumping was provided by commercially available diode lasers (Eagleyard Photonics), which output up to 4 W of power at 1150 nm. The ZBLAN fiber was placed on polarization controllers (PC) in a 2-4-2 loop configuration, each of diameter 65 mm, to compensate for the birefringence in the fiber. The coupling to
Fig. 1. Schematic of the passively mode-locked Ho3 Pr3 doped ring laser. The isolator set the anticlockwise operation, and the InAs saturable absorber placed in a confocal arrangement provided the saturable absorption. © 2014 Optical Society of America
2134
OPTICS LETTERS / Vol. 39, No. 7 / April 1, 2014
the fiber was provided by a biconvex CaF2 lens with a focal length of 12.5 mm that gave a pump launch efficiency of 80%. In order to avoid parasitic lasing from the fiber tips both ends were angle cleaved at 12°, which prevented any feedback from Fresnel reflections. The output was then separated from the pump using a dichroic mirror at 45° that has a reflection >98% at 2.9 μm. An optical isolator (Thorlabs) based on Faraday rotation set the unidirectional operation of the laser cavity (anticlockwise in Fig. 1) and had a transmission of 80% at 2.9 μm and an isolation of 29 dB, which prevented feedback from the reflections from intracavity freespace optics back into the fiber. The mode-locking element was an AR-coated InAs saturable absorber used in transmission (BATOP GmbH) inside a confocal setup between two ZnSe lenses (Innovation Photonics) with focal lengths of 6 mm each. The unsaturated loss of the saturable absorber was 8%, a modulation depth of 4%, and a relaxation time of ∼10 ps (as stated by the manufacturer). The output was taken from a CaF2 -based reflector (Rocky Mountain Instruments) which output 20% of the in-cavity light before it was coupled back into the fiber. This was then measured using a MCT detector with a rise time
