Overview:
The mid-infrared (2um-5um) fiber laser technology has unique applications, covering various atmospheric windows. It can be used in laser radar, atmospheric communication, laser ranging, high-resolution astronomical spectrograph calibration, and optoelectronic detection [1]. The mid-infrared band contains characteristic spectral lines known as "molecular fingerprints," which are useful for high-speed, high-resolution, high-spectral sensitivity, and high signal-to-noise ratio mid-infrared spectroscopy [2]. Strong water absorption peaks near 3um make it suitable for many medical procedures. Absorption spectra in the covalent bond region allow for the detection of molecular content and identification of molecular types, enabling molecular imaging, among other applications.
Currently, commercial mid-infrared laser sources include OPO parametric oscillation lasers, supercontinuum sources, quantum cascade lasers, and fiber lasers. Mid-infrared fiber lasers can be categorized into active and passive categories based on their implementation, including rare-earth-doped mid-infrared lasers such as Er3+ and Dy3+ doped ZBLAN fiber lasers, nonlinear effect-based mid-infrared lasers like Raman lasers and supercontinuum lasers, and special waveguide structure hollow-core fibers, combined with various gases to achieve different wavelengths of mid-infrared lasers. In recent years, with the continuous development and maturity of fiber laser technology, research on mid-infrared laser technology has become increasingly active, with numerous experimental and product reports. Here, we will discuss single-wavelength mid-infrared fiber lasers based on gain-active fibers.
Er:ZBLAN Fiber:
Erbium (Er) has rich energy level structures, and particles are excited from the ground state at pump wavelengths of 655nm, 790nm, and 980nm to higher energy levels. Emission at 1.55um is generated through radiative transitions from the 4I13/2 energy level to the 4I15/2 level, emission at 2.8um results from transitions from the 4I11/2 level to the 4I13/2 level, and 3.5um emission comes from transitions from the 4F9/2 level to the 4I9/2 level. Currently, achieving 2.8um lasers with high Er doping concentrations in ZBLAN fibers is a relatively mainstream method [4].
Fluoride fibers are used for 2-3um light output, sulfide fibers are used for 3-6.5um light output, and longer wavelengths beyond 6.5um can be achieved with halide fibers. Fluoride fibers are mainly composed of fluoride glass materials such as aluminum fluoride (AlF3), ZBLAN (53%ZrF4-20%BaF2-4%LaF3-3%AlF3-20%NaF), or indium fluoride (InF3). Among these, ZBLAN is commonly used, as it allows rare-earth doping, and the fusion splicing process with silica-based fibers is relatively mature. InF and AlF fibers can be used to make fiber devices (such as couplers) and fiber end caps. However, the main drawback of fluoride fibers is their susceptibility to moisture.
2.8um Mid-Infrared Continuous Fiber Laser:
In 1988, Brierley reported the first 2.7um Er3+-doped fiber laser [5]. In 1999, the output power of Er:ZBLAN fiber lasers reached the watt level, with Jackson et al. achieving 1.7W laser output using Er3+/ Pr3+ co-doped ZBLAN fibers [6]. In the 21st century, the power of 3um band lasers has further increased, with impressive progress reported in laboratories worldwide, including Kyoto University in Japan, the University of Adelaide in Australia, Laval University in Canada, and Shenzhen University in China.
In 2015, Fortin et al. from Laval University reported a 30.5W output power laser at 2938nm using Er3+-doped fluoride fiber [7]. This system used fiber Bragg gratings (FBG) inscribed in both ZBLAN and Er:ZBLAN fibers to form a 10m long resonant cavity, and the fiber end was equipped with an AlF3 end cap to reduce degradation due to moisture. The overall laser efficiency was 16% under 980nm pumping.
In 2018, Aydin et al. from Laval University achieved 41.6W of laser output at 2.8um using Er:ZBLAN fiber, inscribing gratings directly within the Er:ZBLAN fiber along its entire length, and employing a dual-pump scheme [8]. This is the highest reported output power for Er:ZBLAN mid-infrared fiber lasers to date.
In 2021, Guo et al. from Shenzhen University reported a 20W all-fiber 2.8um mid-infrared laser, using an Er3+: ZrF4 fiber with a diameter of 15um, a numerical aperture (NA) of about 0.12, a total length of 6.5m, and absorption coefficient of 2-3dB/m at 976nm. High reflectivity fiber Bragg gratings (FBG) and low reflectivity FBG were directly inscribed on the gain fiber to form a resonant cavity with a central wavelength of 2825nm. The output power reached 20.3W at a pump power of 140W, with a conversion efficiency of 14.5% [10].
In 2023, using coated reflectors and self-made high-performance mid-infrared fiber end caps to provide cavity feedback, along with efficient coupling of high-power pump light, the output power of single-ended pump mid-infrared fiber lasers was increased to 33.8W, achieving the highest laser efficiency at power levels exceeding 30W [21].
After years of effort, researchers in fiber laser technology have greatly optimized the processing of mid-infrared fibers. With the help of commercial specialized fiber processing equipment, they have achieved lower fusion loss. These fibers are used in various devices such as mid-infrared mode field adaptors, combiners/splitters, and output end caps, leading to the development of product-grade all-fiber mid-infrared light sources.
Mid-Infrared Q-Switched Pulse Fiber Laser:
In 2020, Sojka et al. used a 30W 975nm laser-pumped, 15um core diameter, 7% Er:ZBLAN double-clad fiber to achieve Q-switched output at 2.8um with a repetition rate of 10kHz. A 1.1m long Er:ZBLAN fiber was used to generate laser pulses with an energy of 46uJ, peak power of 0.821 kW, and pulse width of 56ns [11]. In 2021, they used a 35um core diameter Er:ZBLAN multimode fiber to achieve pulses with a width of 26ns, peak power of 12.7kW, and pulse energy of 330uJ [12].
In 2021, Shen et al. achieved pulsed laser output at 2.8um using electrical-optical Q-switching for the first time. They used a 33um core diameter ZBLAN fiber doped with
6% Er as the gain medium, NA 0.12, and an RTP crystal-based electro-optic modulator. This setup generated pulses with a width of 13.1ns, pulse energy of 205.7uJ, and peak power of 15.7kW, making it the highest reported peak power for Er:ZBLAN Q-switched fiber lasers to date.
Mid-Infrared Mode-Locked Ultrafast Fiber Laser:
In the realm of mode-locked ultrafast fiber lasers, there has been significant progress, especially in the mid-infrared region beyond 2um.
In 2018, Jena University reported a 2um ultrafast laser with 1000W average power and 256fs pulse duration using a large-mode-area Tm-doped photonic crystal fiber, 50/250-Tm-PM-PCF, marking one of the highest achievements in similar experiments [14].
For wavelengths beyond 2um, most research on fiber lasers employs passive mode-locking techniques, primarily utilizing saturable absorbers (SESAM) and nonlinear effects. In 2020, Guo et al. used WSe2 thin films as a saturable absorber (SA) to achieve 2.8um Er:ZBLAN fiber laser mode-locking with a pulse width of 21ps, repetition rate of 42.43MHz, and an average power of 360mW [14]. In 2022, Qin et al. from Shanghai Jiao Tong University used molecular beam epitaxy to create InAs/GaSb superlattice SESAM, enabling stable mode-locking at 3.5um for Er:ZBLAN fiber lasers. The pulses had a width of 14.8ps, average power of 149mW, and repetition rate of 36.56MHz [15]. Further advancements led to the shortening of mode-locked pulse widths to 215fs with pulse energies of 9.3nJ and peak powers of 43.3kW [16].
In 2020, Guo et al. reported a 131fs mode-locked output at 2.8¦Ìm for Er:ZBLAN fiber lasers using the nonlinear polarization rotation (NPR) technique, achieving a peak power of 22.68kW for soliton pulses with an energy of 3nJ [17]. Huang et al. also used the NPR technique to obtain 126fs pulses with a pulse energy of 10nJ in 3.3m long Er:ZBLAN fibers under 980nm pumping, and further compression to 15.9fs was achieved using Er:ZBLAN amplifiers and ZBLAN nonlinear fibers, resulting in a peak power of 500kW [18].
In 2022, Yu et al. utilized Er:ZBLAN fibers doped with 7% Er at a length of 2.4m to produce 283fs pulse seed sources. They further compressed the pulse width to 59fs using nonlinear amplification, achieving an impressive average output power of 4.13W. This represents the highest average output power for sub-picosecond mode-locked fiber lasers to date [19].
In Summary:
Mid-infrared fiber lasers offer numerous advantages, including compactness, low maintenance, high stability, and high beam quality. Various mid-infrared fibers, including fluoride, sulfide, halide, and hollow-core fibers, have greatly propelled the development of mid-infrared lasers in terms of power, spectrum, and fiber device applications. As mid-infrared materials and fiber technology continue to mature, there will be more high-quality mid-infrared fiber laser products emerging, playing an increasingly significant role in areas such as defense, research, industrial manufacturing, and healthcare.
1.Hao Q, Zhu G S, Yang S, et al. Mid-infrared transmitter and receiver modules for free-space. optical communication [J]. Applied Optics, 2017, 56(8), 2260-2264.
2.Ren X Y, Dai H Li, D T, et al. Mid-infrared electro-ptic dual-comb spectroscopy with feedforward frequency stepping [J]. Optics Letters, 2020, 45(3), 776-779.
3.GOLDING P, JACKSON S, KING T, et al. Energy-transfer processes in Er3 + -doped and Er3 + , Pr3 + -codoped ZBLAN glasses[ J]. Physical Review B, 2000, 62: 856-864.
4.ZHAO Wenkai, et al. Research Progress on Er ¡Ã ZBLAN Mid-Infrared Fiber Lasers Emitting at 2.8¦Ìm. BULLETIN OF THE CHINESE CERAMIC SOCIETY, vol 11, 2022.11.
5.BRIERLEY M C, FRANCE P W. Continuous wave lasing at 2. 7 ¦Ìm in an erbium-doped fluorozirconate fibre[J]. Electronics Letters, 1988, 24(15): 935.
6.JACKSON S D, KING T A, POLLNAU M. Diode-pumped 1. 7 W erbium 3 ¦Ìm fiber laser[J]. Optics Letters, 1999, 24(16): 1133-1135.
7.FORTIN V, BERNIER M, BAH S T, et al. 30 W fluoride glass all-fiber laser at 2. 94 ¦Ìm[J]. Optics Letters, 2015, 40(12): 2882-2885.
8.AYDIN Y O, FORTIN V, VALLÉE R, et al. Towards power scaling of 2. 8 ¦Ìm fiber lasers[J]. Optics Letters, 2018, 43(18): 4542-4545
9.´ÞÓîÁúµÈ£¬ÖкìÍâ¹âÏ˼¤¹â¼¼ÊõÑо¿½øÕ¹ÓëÕ¹Íû¡£¹âѧѧ±¨£¬42¾í9ÆÚ£¬2022.5
10.¹ù´ºÓê, ¶·±Áú, ÉòÅôÉúµÈ, 20WÖкìÍâ2.8umÈ«¹âÏ˼¤¹âÆ÷Ñо¿ [J]. Öйú¼¤¹â 2021, 48 (14), 1416001
11.SOJKA L, PAJEWSKI L, LAMRINI S, et al. Experimental investigation of actively Q-switched Er3 + :ZBLAN fiber laser operating at around 2. 8 ¦Ìm[J]. Sensors (Basel, Switzerland), 2020, 20(16): 4642.
12.SÓJKA L, PAJEWSKI L, LAMRINI S, et al. High peak power Q-switched Er¡Ã ZBLAN fiber laser[J]. Journal of Lightwave Technology, 2021,39(20): 6572-6578.
13.SHEN Y L, WANG Y S, ZHU F, et al. 200 ¦ÌJ, 13 ns Er¡Ã ZBLAN mid-infrared fiber laser actively Q-switched by an electro-optic modulator[J].Optics Letters, 2021, 46(5): 1141-1144.
14.GUO C Y, WEI J C, YAN P G, et al. Mode-locked fiber laser at 2. 8 ¦Ìm using a chemical-vapor-deposited WSe2 saturable absorber mirror[J]. Applied Physics Express, 2020, 13(1): 012013.
15.QIN Z P, CHAI X L, XIE G Q, et al. Semiconductor saturable absorber mirror in the 3 ~ 5 ¦Ìm mid-infrared region[J]. Optics Letters, 2022,47(4): 890-893
16.Qin Z P Xie G Q Gu H G et al Mode-locked 2.8¦Ìm fluoride fiber laser from soliton to breathing pulse J Advanced Photonics 2019 16 065001
17.GU HONGAN, QIN Z P, XIE G Q, et al. Generation of 131 fs mode-locked pulses from 2. 8 ¦Ìm Er ¡Ã ZBLAN fiber laser[ J]. Chinese Optics Letters, 2020, 18(3): 031402.
18.HUANG J, PANG M, JIANG X, et al. Sub-two-cycle octave-spanning mid-infrared fiber laser[J]. Optica, 2020, 7(6): 574.
19.YU L P, LIANG J H, HUANG S T, et al. Average-power (4. 13 W) 59 fs mid-infrared pulses from a fluoride fiber laser system[ J]. Optics Letters, 2022, 47(10): 2562-2565.
20.Huang J Pang M Jiang X et al Sub-two-cycle octave-spanning mid-infraredfiberlaser J Optica 2020 7 6 574-579
21.ÕžûÏèµÈ£¬33.8W¸ßЧÂÊÖкìÍâ2.8¦Ìm¹âÏ˼¤¹âÆ÷¡£Öйú¼¤¹â£¬50¾í7ÆÚ£¬2023.4