3.1 Introduction

Fiber lasers can produce, in a compact device, outputs with remarkable beam quality and high output power and can operate with long life, high power efficiency, and low cost of ownership. In many cases, fiber lasers are replacing solid‐state lasers in research, defense, and industry. Extending the emission wavelength into the mid‐infrared is one of the avenues of fiber laser development, which will clearly benefit numerous existing and future applications, e.g. in the field of medicine, remote sensing, welding of polymers, and for pumping longer‐wavelength mid‐IR or terahertz optical parametric oscillators.

Fiber lasers are typically pumped by laser diodes with free‐space or fiber‐coupled output (Figure 3.1) and consist of a special optical fiber whose core has been infused with active ions that create the optical gain. For mid‐IR fiber lasers, the dopants in the gain medium are rare‐earth ions such as thulium (Tm³⁺), erbium (Er³⁺), holmium (Ho³⁺), or dysprosium (Dy³⁺) [1].

Although the gain media for fiber lasers are similar to those of solid‐state bulk lasers, the waveguiding effect and the small effective mode area usually lead to substantially different properties. For example, fiber lasers often operate with much higher laser gain and higher resonator losses. Also, the spatial beam quality in fiber lasers is typically close to diffraction‐limited. Similar to solid‐state crystalline lasers, the lasing wavelength is determined by the energy levels of the rare‐earth cation doped into the core of the fiber.

Figure 3.2 shows the laser transitions responsible for infrared emission of rare‐earth‐cation‐doped fiber lasers, while the emission cross sections of rare‐earth laser transitions used in mid‐IR fiber lasers as a function of emission wavelength are plotted in Figure 3.3.

3.2 Continuous‐wave Mid‐IR Fiber Lasers

3.2.1 Tm‐based Fiber Lasers

Fiber lasers employing the Tm³⁺ cation [2–5] with emission at around 2 μm are the most powerful, efficient, and developed of the mid‐IR fiber lasers. These devices are pumped with robust diode lasers emitting at ~790 nm, and their output can be tuned from 1.86 to 2.09 μm [6]. With high concentrations of Tm³⁺ dopant ions, exceeding 2.5 weight%, cross‐relaxation between neighboring Tm³⁺ cations can nearly double the slope efficiency [7]. Similar to the solid‐state Tm‐based lasers reviewed in Chapter 2, the cross‐relaxation between Tm³⁺ cations creates two excited Tm³⁺ cations in the upper laser level for every pump photon excitation (Figure 3.4). This “two‐for‐one” excitation is resonant in silicate glass and provides an efficient way to generate 2 μm light using a commercial laser diode pump.

Kilowatt‐class thulium‐doped silica fiber laser with the central wavelength of 2.045 μm was reported in [5, 8]. The layout of such a laser is depicted in Figure 3.5 and is based on a 50‐W seed laser at 2.045 μm and a two‐stage power amplifier configuration. High‐power operation of the Tm‐fiber laser has been made possible due to (i) development of high‐brightness, fiber‐coupled 790‐nm pump lasers with nearly 40% electrical‐to‐optical conversion, (ii) fiber‐based pump‐coupling optics that provide a highly efficient combining of light from several multimode sources into one fiber, and (iii) “two‐for‐one” photon recycling due to the cross‐relaxation process. The power levels achieved in this work are: >500 W at 62% slope efficiency with respect to diode pump optical power with 6 pump lasers and 1.05 kW at 53% slope efficiency with 12 pump lasers (Figure 3.6). With the 20‐μm core diameter of the fiber amplifier, the laser showed single‐spatial‐mode operation. These results represent the highest continuous wave (CW) power level ever generated in this wavelength range [8].

The Tm‐fiber laser technology is now commercialized. For example, single‐spatial‐mode Tm‐fiber lasers with the wavelength selectable anywhere between 1900 and 2000 nm and with output power in the range of 1–200 W are now standard commercial products offered by IPG Photonics.¹

3.2.2 Ho‐based Fiber Lasers

The ⁵I₇ → ⁵I₈ transition of Ho³⁺ has a peak emission wavelength of 2.1 μm, which overlaps with the important atmospheric transmission window. This Ho transition can be pumped by 1150‐nm laser diodes, but it is also well suited for resonant pumping by the output of Tm³⁺‐doped silicate glass fiber lasers, where the quantum defect can be as small as 7% [9].

Alternatively, a very practical solution is a codoped Tm³⁺, Ho³⁺‐fiber laser, where thulium provides efficient excitation using standard 790‐nm laser diodes, and where the resonant energy transfer process efficiently populates the Ho³⁺ ion. With such a fiber, and using a bidirectional 793‐nm diode pumping, 83 W of CW output power has recently been achieved at 2.105 μm, with up to 42% slope efficiency [10].

The long‐wavelength ⁵I₆ → ⁵I₇ (λ ≈ 2.9 μm) laser transition of Ho³⁺ is self‐terminating, because the lifetime of the upper laser level is shorter than that of the lower laser level. A simplified energy‐level diagram for the cascade laser is shown in Figure 3.7. As a solution to this problem, a cascaded lasing of both the 3 μm (⁵I₆ → ⁵I₇) and 2.1 μm (⁵I₇ → ⁵I₈) transitions of Ho³⁺ has recently been demonstrated [11]. The laser used Ho³⁺‐doped ZBLAN fiber (ZBLAN is the most common fluoride glass used in mid‐IR fiber lasers with the following composition: ZrF₄–BaF₂–LaF₃–AlF₃–NaF) and two commercial 1150‐nm diode lasers (total launched pump power of 7.6 W) to pump directly the upper laser level ⁵I₆. Cascading in this case helps to quickly unload the long‐living ⁵I₇ energy level, which serves as a lower laser level for the 3‐μm transition. The laser produced 0.77 W at 3.002 μm and 0.24 W at 2.1 μm, making this system the first watt‐level fiber laser operating at ~3 μm in the mid‐IR.

De‐excitation of the lower laser level can also be achieved by codoping of Ho³⁺‐fibers with praseodymium (Pr³⁺) ions. An efficient high‐power operation of a 2.94‐μm Ho³⁺, Pr³⁺‐doped fluoride glass fiber laser was demonstrated in [12]. The core of the laser was made of a Ho³⁺, Pr³⁺‐doped ZBLAN and the diode pumping was performed through fiber cladding. The laser produced a maximum output power of 2.5 W at 32% slope efficiency (that is 82% of the Stokes limit) with respect to the diode laser pump at 1150 nm. It is noteworthy that the emission wavelength of 2.94 μm measured at maximum pump power is particularly well suited for medical applications because it overlaps with the fundamental OH absorption peak of water in human tissue.

Finally, the laser action at the long‐wavelength ⁵I₅ → ⁵I₆ transition (λ = 3.9 μm) with a cascade laser scheme in a Ho³⁺‐doped fluoride fiber system was demonstrated under cryogenic (77 K) conditions [13]. With the fiber length of 340 cm, the pump wavelength of 885 nm, and a launched pump power of 900 mW, the researchers achieved 11 mW of CW output power at 3.9 μm. Simultaneously, λ = 1.2 μm (⁵I₆ → ⁵I₈ transition) radiation was emitted with 70‐mW output power. This result represents the longest wavelength emitted from a fiber laser.

3.2.3 Er‐based Fiber Lasers

The laser transitions between manifolds ⁴I_11/2 and ⁴I_13/2 of rare‐earth cation Er³⁺ (see Figure 3.2) are suitable for tuning across the mid‐IR range of 2.7–3.05 μm. There is a good overlap of the Er³⁺ absorption peak, which corresponds to the excitation of the upper laser level ⁴I_11/2 with the wavelength range of highly developed diode lasers emitting at ~980 nm. This makes Er‐doped fiber lasers suitable candidates for efficient mid‐IR sources. Operation in the mid‐IR has necessitated the use of fluoride glass fiber (e.g. ZBLAN), owing to its low phonon energy. As an additional bonus, the energy transfer upconversion process in Er³⁺ ions (see Figure 3.4b) effectively depopulates the lower laser level and recycles the excitation to produce slope efficiencies as high as 35.6%, a value that exceeds the Stokes efficiency limit [14].

The first 1‐W class 3‐μm laser was demonstrated nearly two decades ago by Jackson et al. [15]. It was based on diode‐pumped Er³⁺, Pr³⁺:ZBLAN fiber and emitted at 2.71 μm with the output power of 1.7 W and slope efficiency of 17.3%. Since then, Er‐based fiber lasers have shown markedly improved performance. Zhu and Jain demonstrated the first 10‐W‐level fiber laser of the 3‐μm range [16]. In their experiment, the authors used a 4‐m‐long heavily erbium‐doped Er:ZBLAN double‐clad fiber, pumped by a 975‐nm laser‐diode array. The output power of 9 W was obtained at 2.785 μm at the launched pump power of 42.8 W. The output, however, fluctuated showing a pulsing behavior, and at higher pump rates the authors observed optical damage of the fiber end facets [16]. This unstable behavior was attributed to the poor thermal conductivity of Er:ZBLAN, its low melting temperature, and the large amount of heat that was generated by Er³⁺ dopants in the process of optical excitation.

Tokita et al. used liquid cooling to demonstrate 24 W of power from an Er:ZBLAN fiber laser operating at λ = 2.8 μm with 975 nm diode pumping [17]. The whole fiber was soaked in a fluorocarbon liquid coolant, which circulated to maintain a constant temperature of 20 °C. The 4.2‐m‐long fiber had an active core diameter of 25 μm, and the maximum power of 24 W was obtained at 166 W of the diode pump [17]. Later, the same team developed a 10‐W wavelength‐tunable Er:ZBLAN fiber laser operating without direct liquid cooling [18]. Instead, the fiber was cooled conductively when placed between aluminum plates that were maintained at 20 °C by water cooling. Also, to remove the heat from the output fiber end, the latter was polished into a spherical shape and put in optical contact with a 2‐mm‐thick sapphire plate (see Figure 3.8). The pump power was 93 W at 975 nm and an external diffraction grating in the Littrow configuration was used to tune the wavelength from 2.71 to 2.88 μm.

Faucher et al. [19] obtained the output power of 20.6 W from an Er³⁺‐doped fluoride glass single‐mode all‐fiber laser source at λ = 2.825 μm. The fiber was passively cooled by using an aluminum spool; it was 4.6‐m long and had a single‐mode core diameter of 16 μm. An undoped fiber with highly reflective (99.4%) Bragg grating written in it was fusion‐spliced to the doped fiber and served as an end mirror. The pump source consisted of three 976‐nm, 30‐W fiber‐coupled modules, and the slope efficiency of the laser was up to 35.4% with respect to the absorbed pump power, which is higher than the Stokes efficiency of 34.3%.

Recently, Aydin et al. reported a passively cooled cascade Er³⁺‐doped fluoride fiber laser (λ = 2.825 μm) with an output power of ~13 W and a record slope efficiency of 50% (absolute optical efficiency of 37%) with respect to absorbed pump power at 976 nm [20]. The presence of an excited‐state absorption band centered at 1.675 μm, between the ⁴I_13/2 and ⁴I_9/2 levels (see Figure 3.2), which partially overlaps the cascaded (⁴I_13/2 to ⁴I_15/2) secondary emission at 1.614 μm, is believed to be responsible for the recycling of the excitation back to the upper laser level of the mid‐IR transition. One of the key features of this approach was to resonate the emission at 1.614 μm, in addition to resonating the 2.8‐μm emission.

Fortin et al. demonstrated a λ = 2.94 μm erbium‐doped fluoride glass fiber laser delivering a record output power of 30.5 W in CW mode [21]. The laser operated on the long‐wavelength transition – from the lowest Stark level of the ⁴I_11/2 to the highest Stark level of the ⁴I_13/2 manifold of the Er³⁺ ion. The passively cooled all‐fiber laser cavity was formed by fiber Bragg gratings, and the laser was pumped using seven laser diodes operating around 980 nm with combined power of 188 W. The overall laser efficiency with respect to the launched pump power at 980 nm was 16%. The laser produced a narrow (0.15 nm) linewidth and had a single‐mode beam quality with a M² < 1.2 [21]. Since the 2.94 μm wavelength is resonant with the liquid water OH absorption peak, this laser has potential to find many applications in the biomedical field.

On the longer wavelength side, a 3.44 μm all‐fiber laser emitting up to 1.5 W at room temperature was demonstrated in [22]. The laser operated on the ⁴F_9/2 → ⁴I_9/2 transition of erbium‐doped fluoride glasses and relied on a dual pumping scheme: at 974 and 1976 nm. In this case, a 974‐nm pump was used to excite Er³⁺ ions to the long‐lived level ⁴I_11/2, thus creating a virtual ground state, while a 1976‐nm pump was used to populate the 3.5‐μm transition upper level ⁴F_9/2 (Figure 3.9). By combining a dichroic mirror deposited on the input fiber tip and a fiber Bragg grating as an output coupler, a stable laser emission was produced with a bandwidth of less than 0.6 nm. The laser had an efficiency of 19% with respect to the launched pump power at 1976 nm and no saturation was observed, provided there is sufficient co‐pumping at 974 nm.

With an improved (so‐called “monolithic”) design of the dual‐wavelength‐pumped erbium‐doped fluorozirconate (Er³⁺:ZrF₄) laser cavity, a multi‐watt laser operation at 3.55 μm was demonstrated [23]. The fiber laser cavity was bounded by two fiber Bragg gratings, which allowed high‐power performance without damage of the ZrF₄ fiber tip. The maximum output power and total optical efficiency were, respectively, 5.6 W and 26.4%. The laser power was the highest ever achieved from a fiber laser at this wavelength.

Based on the idea of dual‐wavelength pumping, a broadly tunable long‐wavelength erbium‐doped Er:ZBLAN fiber laser was reported in [24]. The laser cavity consisted of a 2.8‐m‐long Er³⁺‐doped ZBLAN fiber gain medium with a 16‐μm‐diameter core and a diffraction grating. With the dual pumping at 977 and 1973 nm, the emission was centered at 3.5 μm, and the laser was tunable across 450 nm. A maximum power of 1.45 W at 3.47 μm was achieved with a 4‐W pump at 977 nm and a 5.5‐W pump at 1973 nm (overall optical efficiency 15%). The longest wavelength achieved was 3.78 μm, which is the longest wavelength emission from any fiber laser operating at room temperature [24].

3.2.5 Raman Fiber Lasers

Fundamentally, the Raman process relies on photon–phonon scattering, where an optical phonon (lattice vibration) mediates the process of frequency downconversion. The amount of red shift of the Stokes wave, with respect to the pump, depends on the frequency of the optical phonon for a given glass medium. In optical fibers, a variety of redshifted laser wavelengths extending beyond 2 μm were produced via stimulated Raman scattering. Typically, a Raman fiber laser consists of a piece of fiber enclosed between two highly reflective Bragg gratings written directly into the same fiber.

Using a germanate‐based Raman gain cavity, Dianov et al. produced the output at 2.193 μm, as the fourth Stokes component, with an average power of 210 mW. A 1608‐nm Er/Yb‐doped fiber laser with an average power of 5 W was utilized as a pump [26]. Cumberland et al. reported Raman 2.105‐μm emission wavelength from a high‐concentration GeO₂ fiber Raman laser, with a maximum output of 4.6 W. The pump was a 22‐W thulium‐doped fiber laser at 1.938 μm [27].

In order to access longer mid‐IR wavelengths, materials with extended spectral transmission windows, e.g. glasses based on fluoride, tellurite, or chalcogenide, must be considered. Fortin et al. reported a Raman laser based on a fluoride glass optical fiber [28]. The laser was pumped by a Tm³⁺:silica CW fiber laser operating at 1940 nm with an average power of 9.6 W. A maximum output power of 580 mW was measured at 2185 nm (frequency shift of 579 cm⁻¹), corresponding to 6% conversion efficiency with respect to the launched pump power. Later, the same team demonstrated a multi‐watt fluoride glass Raman fiber laser emitting at 2.23 μm. The laser was based on a nested (Raman laser inside pump laser) cavity with the pump being a Tm‐doped silica fiber laser at 1.98 μm. The maximum Stokes output power recorded at 2.23 μm was as high as 3.7 W [29].

Chalcogenide glasses, arsenic sulfide (As₂S₃) and selenide (As2Se3) – have long been identified as ideal candidates for mid‐IR Raman media because of (i) a high Raman gain coefficient (more than 50 times larger than that of a fluoride glass) and (ii) a transparency window that extends to beyond 6 and 9 μm respectively. Jackson et al. reported an As₂Se₃ chalcogenide Raman laser operating simultaneously on multiple Stokes bands. A 2.051‐μm Tm³⁺‐doped silica fiber laser with a 2‐W average power was employed as the pump source. The first Stokes emission was observed at 2.062 μm with an output power of 0.64 W. Two other Raman active vibrational modes of the fiber produced first Stokes outputs at 2.102 μm (0.2 W) and 2.166 μm (16 mW), respectively [30].

Bernier et al. reported the first Raman fiber laser emitting above 3 μm [31, 32]. The pump was an Er³⁺ fluoride glass fiber laser emitting at 3.005 μm (that is at the long‐wavelength edge of the Er³⁺ emission band) operating in the quasi‐CW mode with a pulse duration of 5 ms and a repetition rate of 20 Hz. Using a Fabry–Pérot cavity formed by a single‐mode As₂S₃ chalcogenide glass fiber and fiber Bragg gratings (Figure 3.10), the authors demonstrated a Raman fiber laser action at λ = 3.34 μm. The laser output average power was 50 mW (0.6 W peak) at the launched pump average power 250 mW [31].

A long‐wavelength quasi‐CW fiber Raman source, operating at room temperature, was reported in [33]. The 3.77‐μm output wavelength was generated by two cascades of Raman shifting; the Raman laser was based on a single‐mode As₂S₃ fiber and nested Fabry–Pérot cavities formed by two pairs of fiber Bragg gratings. Similar to [32], the pump was an Er³⁺:fluoride glass fiber laser at 3.005 μm operating in the quasi‐CW mode (5‐ms pulses, 20 Hz, duty cycle 10%). The first and second Stokes orders were at 3.345 and 3.766 μm, respectively. The maximum average output power of 9 mW was measured at 3.766 μm for an averaged launched pump power of 371 mW, corresponding to a peak power of 112 mW [33].

3.3 Q‐switched Mid‐IR Fiber Lasers

Mid‐IR fiber lasers operating in the Q‐switched mode with microsecond to nanosecond pulse duration are desirable for such applications as light detection and ranging (LIDAR), pumping longer‐wavelength mid‐IR optical parametric oscillations (OPOs), supercontinuum generation, polymer processing, and for laser surgery where efficient and painless tissue removal can be achieved due to fast energy deposition.

In the 2‐μm region, both Tm‐ and Ho‐fiber lasers are suitable for pulsed‐mode operation. Eichhorn and Jackson demonstrated a Q‐switched Tm³⁺‐doped fiber laser providing multi‐watt average power at λ = 1.98 μm [34]. The silica double‐clad fiber was 2.3 m long with a core diameter of 20 μm and was diode‐pumped at 792 nm. The laser was Q‐switched with an acousto‐optic modulator (AOM) and generated up to 30 W of average power at 110 kHz repetition rate, with 270 μJ pulse energy, pulse duration 41 ns, and peak power 6.6 kW. The maximum power, according to the authors, was limited by the buildup of amplified spontaneous emission (ASE). With a diffraction grating in the resonator, the laser was tuned from 1.93 to 2.05 μm with a pulse width of 41–50 ns at 125 kHz repetition rate [34]. Kadwani et al. reported an acousto‐optically Q‐switched oscillator based on Tm‐doped photonic crystal fiber (PCF). The pump was a 100‐W diode laser at 793 nm, and the active fiber had a large mode‐field area >1000 μm². The laser maintained single‐mode beam quality in a polarized beam and provided 435‐μJ, 49‐ns pulses at 10 kHz repetition rate at 2 μm wavelength, corresponding to an average power of 4.4 W and peak power as high as 8.9 kW [35].

A pulsed‐mode operation with an acousto‐optic Q‐switching of a Tm³⁺, Ho³⁺‐codoped double‐clad silica fiber laser was reported by Eichhorn and Jackson [36]. Here, thulium provides efficient excitation using standard 790‐nm laser diodes, while a resonant energy transfer process efficiently populates the Ho³⁺ ions. The fiber was symmetrically diode‐pumped from two sides at 792 nm, with up to 30 W per each side. With the use of an intracavity diffraction grating and by taking advantage of both Tm³⁺ and Ho³⁺ emission bands, the laser was tuned in the range 1.95–2.13 μm. In the Ho³⁺ band, the shortest pulses were reached at λ = 2.07 μm at 20 kHz repetition frequency with pulse duration of 58 ns and pulse energy of 250 μJ, corresponding to an average power of 5 W. In the Tm³⁺ emission band, at λ = 2.02 μm, the maximum average power was 15 W at 100 kHz repetition frequency (pulse energy 150 μJ) [36]. A high average power nanosecond system based on thulium‐doped fiber amplifier with 793‐nm diode pump was achieved in a two‐stage oscillator–amplifier setup [37]. The pulses with variable duration between 4 and 72 ns were generated by mode‐locking a long‐cavity low repetition rate oscillator with the aid of a nonlinear optical loop mirror. At 1.07 MHz repetition frequency and 72 ns pulse duration, 100.4 W of average power was produced at λ ≈ 2 μm.

A number of groups achieved Q‐switched fiber laser operation in the 3‐μm band. Hu et al. demonstrated a Q‐switched laser based on codoped Ho³⁺, Pr³⁺ fiber operating at 2.87 μm. (Codoping Ho with Pr ions allows de‐excitation of the ⁵I₇ state in holmium, providing high slope efficiencies.) With an acousto‐optical Q‐switching (Figure 3.11), the authors achieved 78‐ns‐long pulses at 120 kHz, with an average power of 0.72 W and slope efficiency of 20% with respect to the launched pump power at 1150 nm [38].

Tokita et al. demonstrated an impressive result in terms of 3‐μm fiber laser peak and average power [39]. A diode‐pumped 2.8 μm laser oscillator, based on 35‐μm‐core Er‐doped ZBLAN fiber, was Q‐switched with an AOM. At a repetition rate of 120 kHz and a maximum 975‐nm diode pump power of 75 W, the pulse energy reached 100 μJ at a pulse duration of 90 ns, corresponding to the average output power of 12.4 W (0.9 kW peak).

Compared to actively Q‐switched fiber lasers, passively Q‐switched lasers have the advantages of low cost and compactness. Several passively Q‐switched fiber lasers around 3 μm with ~μs pulse duration have been reported recently, using different saturable absorbers: graphene [40, 41], Fe²⁺:ZnSe [41], semiconductor saturable absorber mirror (SESAM) based on indium arsenide (InAs) [42], and using a topological insulator Bi₂Te₃ [43]. (Topological insulators represent a novel class of so‐called Dirac materials. In the bulk state, they have a narrow band gap while at the surface they are in the gapless metallic state and saturable absorption is observed in a broadband spectral range via the Pauli blocking effect.) The average output power in [42, 43] exceeded 300 mW at repetition frequencies around 50–100 kHz (see Table 3.1).

3.4 Mode‐locked Mid‐IR Fiber Lasers

Fiber‐based mode‐locked lasers are desirable sources of ultrashort pulses because of their compactness and environmental reliability, as has already been demonstrated by their near‐IR counterparts. Comparatively large gain bandwidths of fiber lasers, due to the amorphous nature of the glass host, typically allow achieving sub‐100‐fs pulse durations. Mode‐locked lasers in the mid‐IR can further fuel the ultrafast laser science and offer a wide variety of promising applications, such as generation of supercontinuum and optical frequency combs, and precision laser surgery, to name a few.

Ultrafast fiber lasers are usually based on passively mode‐locked oscillators. While erbium at λ = 1.55 μm and ytterbium at λ = 1.05 μm silica fiber lasers still dominate in the commercial realm, enormous progress took place recently in the development of thulium (Tm) and holmium (Ho) doped fiber lasers emitting near 2.0 μm and Er‐fluoride fiber lasers emitting near 3.0 μm [52].

Tm‐doped silica‐fiber‐based 2‐μm lasers are the most mature among the mid‐IR mode‐locked fiber lasers. In the early stage of their development, passively mode‐locked femtosecond Tm‐doped fiber oscillators were demonstrated in the mid‐1990s. For example, Nelson et al. [53] used additive‐pulse mode‐locking based on nonlinear polarization evolution to obtain tunable 1.8–1.9 μm pulses in the soliton regime, whereas Sharp et al. [54] reported a mode‐locked thulium‐doped silica fiber laser using an InGaAs saturable absorber. Later, watt‐level femtosecond Tm‐fiber lasers were developed. For example, by amplification of a Raman‐shifted output of a 1.56‐μm Er‐doped fiber laser in a Tm‐doped fiber having a 25‐μm‐diameter core, Imeshev and Fermann obtained 108‐fs pulses at 2 μm wavelength with an average power as high as 3.1 W, pulse energy of 31 nJ, and peak power of 230 kW [44]. Subsequently, stable 2‐μm Tm‐fiber‐based oscillators were developed using SESAM combined with nonlinear polarization evolution for mode‐locking mechanism. After amplification in a dispersion‐compensated Tm‐fiber amplifier, high‐power (>2.5 W) sub‐70‐fs pulses at ~2 μm were generated [45, 55]. These developments have also led to creating fully stabilized optically referenced frequency combs discussed in Chapter 6.

A mode‐locked soliton fiber laser based on Ho³⁺‐doped glass (λ = 2.08 μm) was demonstrated with graphene as a saturable absorber [56]. The laser was pumped by a CW Tm‐fiber laser (0.5 W at 1950 nm) and produced 811‐fs duration pulses at 34 MHz repetition rate and 44 mW average power.

Based on a variety of saturable absorbers, passively mode‐locked lasers in the 3‐μm range were also reported, with pulse durations ~20 ps and average power ~100 mW. These include an Er³⁺‐doped ZBLAN fiber laser at 2.8 μm, passively mode‐locked by a Fe²⁺:ZnSe crystal inside the laser cavity [57] and Ho³⁺, Pr³⁺ codoped ZBLAN fiber laser at 2.87 μm, which used an InAs‐based SESAM as passive mode‐locker [58]. Yet, the above lasers exhibited “pulsing” – both Q‐switching and mode‐locking behavior simultaneously.

Haboucha et al. reported a truly continuous mode‐locking from an Er³⁺‐ZBLAN glass fiber laser operating at 2.8 μm by using SESAM in conjunction with a fiber Bragg grating inside a linear cavity [46]. The Bragg grating allowed for the generation of a stable, self‐starting pulse train with a repetition rate of 52 MHz, pulse duration of 60 ps, and an average power of 440 mW. In parallel, Hu et al. demonstrated a stable mode‐locked fiber ring laser near 2.9 μm based on a Ho³⁺, Pr³⁺‐codoped ZBLAN fiber using an InAs saturable absorber. This configuration enabled the generation of 6‐ps pulses at 25 MHz repetition rate with an average power of 70 mW [47].

Most recently, two groups have independently demonstrated the possibility of generating femtosecond pulses from an Er³⁺‐doped fluoride (ZBLAN) fiber at 2.8 μm based on nonlinear polarization rotation [48, 49]. This technique relies upon the high Kerr nonlinearity of the fluoride fiber (n₂ = 2.1 × 10⁻²⁰ m²/W) to create an intensity‐dependent rotation of the polarization state inside the fiber, which, when combined with polarizing optics (quarter wave plate, half wave plate, and an optical isolator), makes an effective saturable absorber for mode‐locking. Hu and colleagues were able to produce 497‐fs pulses at 2.8 μm with 206 mW average power at 57 MHz repetition rate (Figure 3.12) [48]. Similarly, Duval et al. [49] reported a passively mode‐locked Er‐ZBLAN fiber ring laser based on nonlinear polarization evolution. The laser generated 207 fs pulses at λ = 2.8 μm with a repetition rate of 55 MHz at an average power of 40 mW. Since ZBLAN fiber has anomalous dispersion at 2.8 μm, both of these lasers were reported to operate in the soliton regime. By varying the Er³⁺:fluoride fiber length and the output coupling, Duval et al. were able to scale up the 2.8‐μm mode‐locked laser power. They demonstrated stable 270‐fs pulses with the repetition rate of 96.6 MHz and the average power of 675 mW (pulse energy 7 nJ, peak power 23 kW) [50].

The same team reported on a tunable (2.8–3.6 μm) and high‐power (>1 W) ultrafast fiber laser system based on erbium‐doped zirconium fluoride glass [51]. Tuning of the central wavelength of the pulses was achieved through a process known as soliton Raman self‐frequency shift, which is facilitated by the anomalous dispersion of fluoride glass fibers in this spectral range. An ultrafast fiber oscillator (the pump) operating at λ = 2.8 μm and a repetition rate of 58 MHz was similar to the one described in [50]. It was followed by a second erbium‐doped fluoride glass fiber, a portion of which (L = 1.25 m) served as an amplifier, while the remaining unpumped segment (L = 8 or 22 m) acted as a passive fiber to shift the amplified soliton to longer wavelengths. As the amplifier pump power was increased, the spectrum of a clean and isolated soliton continuously shifted toward longer wavelengths in the passive portion of the fiber, reaching a maximum central wavelength of 3.4 μm in the 8‐m fiber and 3.59 μm in the 22‐m fiber. Overall, high‐energy Raman soliton pulses tunable from 2.8 to 3.6 μm were generated. For example, at λ = 3.4 μm, 160‐fs pulses were produced with the pulse energy of 37 nJ (200 kW peak), corresponding to an average output power >2 W [51].

The main results for mid‐IR fiber lasers are presented in Table 3.1.

3.5 Summary

So far, the most successful fiber lasers in the mid‐IR are based on the Tm³⁺ (³H₄ → ³H₆) laser transition, the Er³⁺ (⁴I_11/2 → ⁴I1_3/2) transition, and the Ho³⁺ (⁵I₆ → ⁵I₇) transition. They cover, respectively, 1.9–2.1, 2.7–3.0, and 2.8–3.05 μm spectral ranges.

Other wavelengths can be reached via dual‐wavelength pumping (Er³⁺, 3.44–3.55 μm), Raman effect (chalcogenide glass, 3.34 μm), or via cryogenic cooling (Ho³⁺, 3.9 μm).

In terms of the output power, Tm‐doped fibers enable getting 1‐kW CW power in a single‐spatial mode at 2.045 μm with optical‐to‐optical conversion efficiency higher than that dictated by the quantum defect, thanks to the “two‐for‐one” cross‐relaxation process. At longer wavelengths, achieving high‐power operation is more challenging. The main cause of power decline is the increase of quantum defect since the most efficient high‐power laser diodes typically emit in the near‐IR. Thus, there is more energy dissipation, and created heat becomes an ever‐increasing fraction of the absorbed pump energy as the wavelength increases. Nevertheless, based on the 3‐μm transition in erbium, benefiting from “two‐for‐one” cross‐relaxation process, more than 30 W of average power has been obtained at 2.94 μm with passively cooled Er³⁺‐doped fluoride glass fiber.

Mid‐IR fiber lasers operating in the CW, Q‐switched, and mode‐locked modes open up a plethora of new applications in biomedicine, frequency combs generation, and in nonlinear optics and spectroscopy. For further reading, excellent reviews on mid‐IR fiber lasers can be found in [1, 32].

References

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