J. Eur. Opt. Society-Rapid Publ. 22, 44( 2026) 431
Figure 1. Schematic of the in-band-pumped Tm: KYW CW laser inside the dry-air-purged enclosure. LD – 1720 nm fiber-coupled laser diode; L1, L2 – AR-coated lenses imaging the fiber output into the crystal; Tm: KYW – 3-mm gain crystal with a dichroic coating( DC) that is anti-reflective( AR) for the pump and highly-reflective( HR) for the laser deposited on the facet towards the pump; CM – concave mirror( ROC �100 mm); PM – plane mirror; OC – plane output coupler; BRF – 2-mm-thick quartz birefringent filter used only for wavelength-tuning experiments( dashed box).
have a custom HR coating in the 1.85 – 2.1 lmrange. Thecavity is terminated with an output coupler( OC). To investigate laser performance over a wide operating range, we vary the output coupling rate from 0.6 % to 7.5 %. To study the wavelength tunability, an optional 2-mm-thick crystal-quartz birefringent filter( BRF) with its optical axis lying in the surface plane at Brewster’ s angle is introduced before the OC. This cavity leads to an estimated laser beam radius of 55 lm( 1 / e 2) in the crystal, based on a formalism of ray transfer matrices for Gaussian beam. The laser beam is polarized along the N m axis, corresponding to p-polarization in our optical setup, in agreement with the Tm: KYW emission cross section, whose largest magnitude is along the N m axis [ 27, 32 ].
The output power was measured with a calibrated thermopile power detector( Gentec UP19K-15S-H5). The transverse intensity profile was recorded using a DataRay WinCamD-FIR2 camera( pixel size 1717 lm 2). The output spectrum was acquired with an APE WaveScan USB spectrometer( spectral resolution < 0.5 nm).
The targeted application requires mode-hop-free tuning, hence the entire laser setup was enclosed in a sealed box continuously purged with dry air to drastically reduce water-vapor absorption in the Tm: KYW emission region. The relative humidity was monitored with a Thorlabs TSP01 sensor( accuracy ± 4 % below 20 % humidity). We measure a residual relative humidity of 1 % below the accuracy of the detector. All diagnostic devices were placed inside the purged enclosure to minimize the influence of water absorption on measured data.
3 Experimental results
Figure
2a shows the laser output power as a function of absorbed and incident pump power for the three OCs( 0.6 %, 2 %, and 7.5 %), in unpurged( dashed lines) and in the purged configuration( solid lines). For all pump powers and output-coupler( OC) transmissions, we obtained stable CW operation of the Tm: KYW laser in the fundamental TEM 00 mode.
The slope efficiency was extracted from a linear fit to the linear scaling region of the output laser power as a function of absorbed pump power. The lasing threshold was taken as the x-intercept of this fit. For the 7.5 % output coupler, the power scaling remains essentially linear over the fully investigated pump range. For 0.6 % and 2.0 % output coupling, the scaling is also close to linear over most of the range, with only a slight roll-off appearing at the highest absorbed pump powers( J 5 W), therefore, the linear fit was restricted to the range where the dependence remains linear. Overall, increasing the output coupling rate increases the extracted slope efficiency and yields higher output powers for a given pump power. With the 7.5 % output coupler under purged operation, the laser delivers up to 4.55 W for 5.7 W of absorbed pump power, corresponding to an optical-to-optical efficiency of 80 %( 40 %) with respect to absorbed( incident) pump power. The lasing threshold is reached at 0.16 W of absorbed pump power, corresponding to 0.31 W of incident pump power.
Compared to previously reported in-band-pumped CW Tm: KYW lasers, these values represent around 3.1-fold increase in output power and almost 33 % higher slope efficiency than Ref. [ 29 ], where a 1750 nm Tm-fiber laser was used for pumping. Relative to Ref. [ 27 ], which employed a 1750 nm multimode diode pump, we see an increase in output power by a factor > 50 and an almost threefold improvement in slope efficiency. These results show the strong advantage of multimode high-power in-band pumping for achieving both high power and high efficiency operation with a simple and compact design.
For comparison, we also measured the power in unpurged operation( dashed lines), showing a few-percent