High power downconversion deep-red emission from Ho³⁺-doped fiber lasers

2.1 Spectral properties and energy levels of Ho³⁺-doped ZBLAN fiber

Ground state absorption (GSA) spectra of active fiber was measured at room temperature by a HL-2000 Halogen light source (Ocean Optics) and by an optical spectrum analyzers (OSA, AQ6315B, Ando) with resolution of 0.05 nm. In our experiment, the active fiber was provided by a Ho³⁺-doped ZBLAN fiber from Le Verre Fluoré, France (core/cladding: 7.5/125 μm, NA: 0.23, dopant concentration: 5000 ppm). The calculation of the absorption cross section is based on Beer Lambert’s law [29], which is expressed as:

Where I ₀ and I are the optical intensity of incident light and transmitted light, respectively. N is the doping concentration of rare earth ions in the fiber. For 0.5% mol Ho³⁺:ZBLAN, it has a value of 2.935 × 10¹⁹ cm⁻³. L is the fiber length.

The GSA cross section of Ho³⁺-doped ZBLAN fiber in the visible to near-infrared band is plotted by the red line in Figure 2. At 532 nm, the absorption cross section is about 0.5 × 10⁻²⁰ cm². It is worth noting that 532 nm is not at the GSA peak in the green band. Interestingly, the excited state ⁵I₇ absorption cross section represented by the black curve in Figure 2 is up to 0.5 × 10⁻²⁰ cm² [27]. As we all know, this will greatly shorten the lifetime of the ⁵I₇, which is as long as 13.6 ms. Moreover, the lifetime of the lower level of the laser is an important factor restricting the laser output characteristics of the four level structure.

Figure 2:

Spectra of GSA (red), ESA (black) and fluorescence emission (blue) cross sections along with assignments of Ho³⁺-doped ZBLAN fiber.

Emission spectra was measured by excitation with a commercial 532 nm continuous wave (CW) solid-state laser. According to the Fuchtbauer–Ladenburg theory, the emission cross section can be deduced from the emission spectrum of the relevant transition [30], and it can be written as follows:

Where, λ is the center wavelength, and I(λ) is the corresponding emission spectrum intensity. n is the refractive index of the core, and c is the speed of light. Here τ represents the fluorescent lifetime for related energy level.

The blue curve in Figure 2 shows the emission cross section of Ho³⁺-doped ZBLAN fiber in the visible band. As typical spectral resources, the emission cross sections of green (543 nm) and deep-red (750 nm) are 1.0 × 10⁻²⁰ cm² and 1.7 × 10⁻²⁰ cm², respectively. In view of the reabsorption effect of Ho³⁺-doped ZBLAN fiber in green band, and the strong ESA of ⁵I₇ at 532 nm, this paper mainly presents a deep-red downconversion fiber laser with 532 nm as the pump source.

The downconversion pumping process with a 532 nm pump is shown in Figure 3. First, ground state (⁵I₈) ions are excited to the higher state level and decay to the metastable ⁵F₄, ⁵S₂ level by multiphonon relaxation. These ions are re-excited to the ⁵G₅ level by ESA and decay to the ⁵F₄, ⁵S₂ level. This process reduces the population of ⁵I₇ and increases the population of ⁵F₄, ⁵S₂.

Figure 3:

Partial energy-level diagram of Ho³⁺-doped ZBLAN fiber pumped by a 532 nm solid-state laser.

Compared with our previous work [22, 24], the ground state particles are pumped to a higher excited state ⁵G₆, ⁵F₁ under the blue LD 450 nm pump downconversion, while the two-photon sequential absorption upconversion process under the red light 640 nm pump also causes energy loss due to multi-phonon relaxation transition. In this paper, a green light 532 nm solid-state laser was used as the pump source to significantly reduce the energy loss caused by non-radiative transitions, providing higher energy conversion efficiency. More importantly, due to the strong ESA of ⁵I₇ for 532 nm pump, the effective energy level equivalent lifetime is greatly reduced, which creates a new method to produce high-performance deep-red laser.

2.2 Experimental setup

Figure 4(a) shows the experimental setup of the Ho³⁺-doped ZBLAN visible fiber laser. Such a visible oscillation consists of a fiber end-facet mirror (i.e., M1 or M2) and a series of active fibers of different lengths. The pump source is a commercial diode-pumped solid-state laser based on frequency doubling technology operating at 532 nm. As presented in Figure 4(b), the green pump source has a maximum fundamental output power of 6 W and coupled into a passive fiber (core/cladding: 7.5/125 μm, NA: 0.26), in correspondence to a slope efficiency of 72%. Figure 4(c) gives the optical transmission properties of the fiber end-facet mirrors, which were designed with a high reflectivity at signal wavelengths (i.e., M1: R = ∼99.95%@∼543 nm; M2: R = ∼99.95%@∼750 nm). It is worth noting that the M1 mirror is specially designed to have a transmission of more than 90% in the deep-red band, while the M2 mirror also increases the transmission in the green band as much as possible. In combination with our description of the emission cross section of the Ho³⁺-doped ZBLAN fiber in the visible band in Figure 2, high-quality coating of the fiber end facets were required to ensure the single wavelength output of the target laser.

Figure 4:

The configuration of deep-red fiber lasers.

(a) Schematic of the experimental setup for the Ho³⁺-doped ZBLAN visible fiber laser. (b) Incident pump power as a function of the 532 nm solid-state laser output. (c) Optical transmission spectra of the fiber end-facet mirror (M1/M2).

3.1 High output performance of deep-red fiber laser

Figure 5 exemplifies the lasing characteristics of high performance deep-red fiber laser oscillation for the active fiber length of 30 cm. As mirrors in the laser resonator, M2 has a high reflectivity of >99.9% in the deep-red oscillation wavelength range, and the end facet of the Ho³⁺-doped fiber with a ∼4% Fresnel reflection was used as the laser output. The output power characteristic as a function of the incident pump power is presented in Figure 5(a). The slope efficiency with respect to the incident pump power was 50.2% with an oscillation threshold of 0.30 W. The maximum output power achieved is 1.64 W, which is the highest output power from Ho³⁺-doped fiber lasers in the visible range. It is worth pointing out that with the increase of pump power, the laser output power still increases linearly and does not tend to saturation. Thus, the power scaling is only limited by the currently available pump power. The output spectrum with a peak wavelength of 752.10 nm is shown in Figure 5(b). The FWHM of the spectrum at 752.10 nm is approximately 1.3 nm at an OSA resolution of 0.05 nm. As shown in Figure 5(c), the beam quality factors M ² at ∼1 W of output power are M x 2  = 3.2 and M y 2  = 3.4 in the x and y directions, respectively. As presented in Figure 5(d), to investigate the power stability of the deep-red fiber laser, we measured every 5 min for 40 min at the laser output power of ∼1 W. As can be seen from the measured data, the root mean square error of the output power is around 10 mW, which corresponds to a relative power fluctuation of only 0.1%, demonstrating the uniform stability of the deep-red fiber laser.

Figure 5:

High performances of deep-red fiber laser with a 30 cm Ho³⁺-doped ZBLAN fiber. (a) The output power as a function of the incident pump power. (b) The optical spectra under the maximum output power. Measured (c) beam quality M ² and (d) power stability at ∼1 W of deep-red output power.

3.2 Output characteristics of deep-red fiber lasers with different designs

Interestingly, due to the extremely strong ESA of ⁵I₇ energy level, the laser resonator mirror M ² is not necessary, and the deep-red laser output can be obtained by Fresnel reflection on the two end faces of the active fibers. Figure 6 shows the Ho³⁺-doped ZBLAN deep-red lasers output characteristics under different lengths and different transmissions. For an active fiber length of 27 cm, as is displayed in Figure 6(a), the maximum power of 96% output is 1.28 W with a threshold power of 0.20 W and the slope efficiency is 43.2%, while the maximum power of the double-terminal free cavity output is 0.97 W with a threshold power of 0.40 W and the slope efficiency is 34.5%. For another active fiber with a length of 35 cm shown in Figure 6(b), a maximum output power of 1.34 W was measured with a threshold power of 0.30 W, whose slope efficiency was 47.4% at a launched pump power of ∼3 W with the 96% laser output, while a maximum output power of 0.91 W is recorded with a threshold power of 0.42 W, and 32.4% slope efficiency with the double-terminal free cavity output. The output spectra are given in Figure 6(c), the wavelengths of the two laser cavities at the highest output power are around 750.50 nm and 752.20 nm, respectively. The results illustrate that despite the different gain fiber lengths, the output laser wavelengths are similar for the same cavity structure. The difference in wavelength between the two cavity structures can be attributed to the role of sublevels splitting in the lower energy level of the laser in the different pumping processes.

Figure 6:

Deep-red laser output characteristics at different active fiber lengths (27 cm, 35 cm) and different output transmission.