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Widely-tunable cw laser action has been demonstrated in a diode-pumped Yb:KYF4 crystal. A comprehensive characterization of the active material and laser performance are reported, with particular attention to single-frequency laser operation. Single longitudinal-mode operation is achieved in the tuning range from 1020 to 1045 nm with a maximum output power of 150 mW. The relative intensity noise of the single-frequency laser is limited by quantum noise contribution for Fourier frequencies larger than 1 MHz.
©2007 Optical Society of America
1. Introduction
Diode-pumped solid-state lasers are currently employed in different application fields such as remote sensing, communications, high-resolution spectroscopy, precision optical measurements, and frequency metrology. Novel solid-state laser sources covering different wavelength ranges, from visible to near-infrared, are currently under investigation. In the wavelength range around 1 μm Yb-based solid-state lasers show several advantages compared with Nd oscillators for efficient generation of widely-tunable high-power cw radiation. The main advantage is the presence of only two electronic multiplets in the near infrared spectral region leading to a relatively small quantum defect, low thermal loading, and reduced up-conversion losses [1, 2]. Moreover, the stronger electron-phonon interaction provides wide wavelength tunability [3], which is needed in spectroscopy and frequency metrology applications.
Among the different potential crystal hosts, Yb-doped fluoride crystals, as compared to commonly used oxides and tungstates, have noticeable advantages, such as low phonon energy, which makes them particularly suitable for IR transitions, long radiative lifetime, and reduced thermal lens effect [4, 5]. We have recently reported on the first demonstration of laser action in Yb-doped KYF4 [6]. This crystal presents a slight internal disorder that favors broad emission bands, which can be exploited for cw wide-tunability oscillators at around 1.03 μm. A specific use of the wide tunability of Yb:KYF4 laser could be the realization of a 1-μm optical standard of high quality, frequency stabilized against hyperfine transitions of 127I2 in the second-harmonic wavelength region from 520 to 500 nm. In fact, the natural 127I2 linewidth decreases towards the dissociation limit (corresponding to a wavelength of ∼ 498 nm), leading to an improvement of the frequency standard performance [7].
In this paper, we report on a widely-tunable single-frequency diode-pumped Yb3+-doped KYF4 laser. Spectroscopic properties of the active material and cw laser performance are thoroughly investigated. Single axial-mode operation is obtained in the emission wavelength range from 1020 to 1045 nm with a maximum output power of 150 mW at the peak emission wavelength of 1032 nm. The single-frequency radiation is characterized by a low relative intensity noise that reaches the quantum noise floor for Fourier frequencies larger than 1 MHz.
2. Growth and spectroscopic characterization of the Yb3+:KYF4 crystal
KYF4 crystal is non-congruent melt and has the distorted fluorite structure [8]. It has trigonal symmetry with a space group P3112 and the hexagonal lattice parameters a = 14.060 Å and c = 10.103 Å. The rare-heart dopant can be substituted into one of the three possible sites, although KYF4 crystal sometimes is considered a disordered material [9, 10].
The Yb:KYF4 crystal is grown in a home-made computer-controlled resistive Czochralski furnace. Crystal growth is carried out in 5N-purity argon atmosphere at temperatures around 805 °C. The high-purity powders (by AC Materials, Tampa, FL, USA) are suitably mixed, in accordance to non-congruent melting characteristics of the crystal. The growth starts from an oriented seed. The 10% Yb nominal doping is obtained by adding proper amounts of YbF3 to KF-YF3 powders mixing, leading to an Yb ion density of 1.04 × 1021 ion/cm3. The growth and rotation rates were 0.5 mm/h and 7 RPM (rounds per minute), respectively. The crystal was oriented by X-ray backscattering Laue technique and cut in a 5 mm × 3 mm × 1.8 mm (w × h × l) sample with the c-axis parallel to the h direction [6].
Fig. 1. Cross sections of room-temperature Yb3+:KYF4 crystal for E ⊥ c polarization. (a) Absorption and emission cross sections; (b) gain cross section for different values of the exited-state fractional population β = N 2/Nt.
Figure 1(a) shows the room-temperature absorption and emission cross sections around 1 μm corresponding to radiative transitions between 2 F 7/2 and 2 F 5/2 energy levels for E ⊥ c polarization. The absorption cross section, blue curve in Fig. 1(a), is readily obtained from the absorption measurement performed by a Cary 500 spectrophotometer. The absorption cross section shows a sharp peak located at a wavelength of 973 nm, whose value is σa,peak = 11.9 × 10-21 cm2 with a Full Width at Half Maximum (FWHM) of 1.4 nm, which can be exploited for efficient diode laser pumping. A second broader peak at 961 nm (FWHM ∼ 12 nm) with lower intensity (σa = 3.95 × 10-21 cm2) can also be used for diode-pumping. We note that the absorption spectrum from the UV to the IR wavelength region does not show any spurious band due to hydroxyl radicals or other unwanted impurities within the sensitivity of the spectrophotometer.
Yb3+:KYF4 emission cross section is calculated by the integral β-τ method [11], from the corrected polarized fluorescence spectrum and the radiative lifetime measurement. The emission cross section σe,j for emitted fluorescence in the j polarization is given by:
where λ is the wavelength, η is the quantum efficiency, Ij(λ) is the emission spectrum for j-polarization, τrad is the radiative lifetime of the emitting level, n is the refractive index, c 0 is the speed of light in vacuum, and the summation is extended to all the polarizations. The room-temperature Yb2 F 5/2 lifetime was measured to be 2.8 ± 0.1 ms. Assuming unitary value for the quantum efficiency and a refractive index of 1.42 [12], the emission spectrum can be calculated through eq. (1) (see Fig. 1(a), red curve). It consists of several broadened peaks located in the wavelength region from 970 to 1070 nm; the maximum peak value σe,peak = 3.3 × 10-21 cm2 is located at 1026.5 nm and the FWHM is ≃ 20 nm. For the sake of comparison, Table 1 summarizes some important spectroscopic parameters of different Yb-doped crystals. It may be noted that Yb:KYF4 presents the longest radiation lifetime (2.8 ms) and the broadest emission cross-section (up to 1100 nm). It is therefore an excellent candidate for widely-tunable cw lasers and for efficient storage of pump radiation in pulsed (Q-switched) operation.
Table 1. Main spectroscopic properties of different Yb-doped crystals.
The potential tunability range of the Yb:KYF4 crystal can be obtained by evaluating the available logarithmic gain and gain cross section, which, for a quasi-three level laser such as Yb:KYF4 crystal, is given by the following relation:
where N 1,2 is the population density of the lower (upper) laser level, Nt ≃ N 1 + N 2 is the Yb ion density, β = N 2/Nt is the exited-state population fraction, l is the crystal length, and σg is the gain cross section. The gain cross section becomes higher than zero when the condition β ≥ βtr is fulfilled, being βtr the transparency value of the exited-state fractional population. Using the measured values of σa and σe, the transparency condition for the Yb:KYF4 is obtained at a wavelength of 1068 nm for βtr ≃ 0.07. Laser action starts when the logarithmic gain exceeds the total logarithmic losses within the laser resonator, i.e. g(λ) > γ. For example, assuming a crystal length l = 0.2 cm, a Yb ion density Nt = 1021 cm-3, and a typical total cavity losses γ = 0.02, the gain cross section has to be σg > 0.1 × 10-21 cm2. Figure 1(b) shows the gain cross section as a function of the emission wavelength for fractional populations equal to 2βtr, 4βtr, and 6βtr. It is apparent in Fig. 1(b) that laser action can be achieved in a broad wavelength emission range. For β = 2βtr the peak emission wavelength is 1033 nm and the tunability range is in the order of 20 nm; for β = 4βtr and β = 6βtr the peak emission shifts from 1033 to 1027 nm and the potential tunability ranges are 65 nm and 85 nm, respectively.
3. CW diode-pumped Yb:KYF laser
The laser cavity used in the cw laser experiments is based on a high-reflectivity (HR) plane mirror, a HR curved mirror, and a plane output coupler (see [6] for more details). The HR mirrors have a special dichroic coating with high-reflectance, R > 99.9%, in the wavelength range from 1020 to 1100 nm and high-transmittance (T > 80%) for wavelengths shorter than 980 nm. A 1.8-mm thick Yb:KYF4 crystal, mounted on a copper support without any temperature control, is placed at Brewster angle close to the input mirror and is longitudinally pumped by a cw fiber-coupled diode providing a maximum output power of 2.8 W in a 50-μm core diameter multimode fiber at 970 ± 2 nm. The pump beam is collimated and then focused into the active crystal through the input mirror using a pair of anti-reflection coated plano-convex lenses. A doubled pass configuration for the pump beam is implemented to increase the pump absorption to ∼ 65%. To tune the laser emission wavelength, a birefringent filter, based on 2-mm thick quartz plate with the optical axis lying on the crystal plane, is inserted at Brewster angle in the longest arm of the resonator. The birefringent filter has a free-spectral range of ∼ 65 nm at the Yb:KYF4 laser wavelength. Single-frequency operation is achieved by means of two additional intracavity uncoated etalons (1 mm and 0.08 mm thickness).
Table 2. Performance of the room-temperature diode-pumped Yb:KYF laser.
In the cw regime the laser is linearly polarized along the TM direction with the electric field normal to the c crystal axis and in single transverse mode TEM00. Table 2 summarizes the cw laser performance in multi-longitudinal mode operation (without the intracavity etalons). A maximum output power of 505 mW and a slope efficiency of 43% have been obtained for an incident pump power of 2 W and an output coupling of 2.5% [6]; with respect to the absorbed pump power, the slope efficiency turns out to be 66%. Similar results were obtained with the 4.5% output coupling, the only difference being an increased value of the threshold pump power (480 mW). The widest tunability range of 65 nm, from 1013 to 1078 nm, is obtained with 1% output coupling. Increasing the output coupling, the tuning intervals are slightly reduced to 62 nm and 60 nm for the 2.5% and 4.5% couplings, respectively. No laser action is observed at emission wavelengths shorter than 1013 nm. This discrepancy with the theoretical predictions is due to the limited bandwidth of both the birefringent filter and the dielectric mirrors. In particular, the reflectivity of the dichroic coatings of the input and folding mirrors rapidly decreases from R > 99.9% to R < 20% in a wavelength range of ∼ 40 nm, from 1020 to 980 nm, preventing in this way laser action below 1013 nm.
From the experimental data it is possible to retrieve few fundamental laser parameters such as the round trip losses, the reabsorption losses, and the pump rate performing a Findlay and Clay analysis [18]. In a quasi-three level laser system the threshold pump power can be related to the output coupler reflectivity by the relation:
Fig. 2. Single-frequency characteristics of the Yb:KYF4 laser for an output coupling of 2.5%. (a) Output power versus incident pump power; (b) output power versus emission wavelength (1.2 W incident pump power).
where α 0 is the absorption coefficient of the laser medium when all atoms are in the ground state, l is the active material length, δ is the round trip losses, R is the reflectivity of the output coupler, and K is the ratio between the pump rate and the pump power. The best interpolation of the experimental data by means of Eq. 3 gives for the Yb:KYF4 the following results: α 0 = 0.16±0.01 cm-1, δ = 0.015±0.007, and K = 4.6±0.1 W-1.
4. Single-frequency diode-pumped Yb:KYF laser
Figure 2(a) shows the laser output power as a function of the incident pump power in single-mode regime for 2.5% output coupling. A maximum single-frequency output power of 150 mW at 1.03 μm is achieved for 2.5 W incident pump power with an optical to optical slope efficiency of ∼18%. When compared to multimode operation, the single-frequency performance is clearly degraded (increasing of the threshold pump power and reduction of the slope efficiency) due to additional losses of the un-coated etalons. Without the intracavity etalons the laser operates on several longitudinal modes with a typical emission bandwidth of ∼0.5 nm. The single-frequency operation is monitored by means of a scanning Fabry-Perot interferometer with 50 GHz of free spectral range and a frequency resolution of 0.5 GHz, which can easily resolve the laser longitudinal mode separation of ∼0.8 GHz. Figure 2(b) shows the output power as a function of the emission wavelength for an incident pump power of 1.2 W. The single-mode 1-μm radiation can be continuously tuned from 1020 to 1045 nm combining the rotation of the birefringent filter with the tilting of the etalons. This tunability makes the Yb:KYF4 an excellent alternative to the Yb:YAG laser [19] for broad investigation of I2 absorption lines in the second-harmonic wavelength region from 510 to 522 nm, a spectral region close to the I2 dissociation limit where the hyperfine linewidths are extremely narrow (tens of kilohertz) [7].
Fundamental properties of optical sources for high-sensitivity measurements and frequency metrology are low intensity noise and high spectral purity. For the single-frequency Yb:KYF4 laser a typical emission linewidth of few hundreds of kilohertz is expected as demonstrated in similar diode-pumped KYF4-based near infrared laser [20]. Intensity noise in solid-state lasers is induced by external sources of acoustical, mechanical and thermal noise as well as by pump power and pump wavelength fluctuations. To describe the RIN in solid-state diode-pumped lasers a perturbation analysis of the steady-state solution of the rate equations needs to be performed. In particular, the relative intensity noise (RIN) spectrum is strongly enhanced at around the relaxation oscillation frequency, which is usually located in the range between a few tens of kilohertz and a few megahertz (depending on the laser resonator configuration, pump power, and laser medium population dynamics). To characterize the RIN of the Yb:KYF4 laser the single-mode 1 μm radiation is focused onto a 5-MHz bandwidth, low-noise photodetector. The power spectral density of the output photocurrent is then measured with an electrical spectrum analyzer and normalized to the DC output photocurrent.
Fig. 3. Relative intensity noise spectrum of the single frequency Yb:KYF laser for a laser output power of 100 mW. The red curve represents the best fit of the experimental data (K = 87 dB, f 0 = 29.6 kHz, and ξ = 0.018). The inset represents a magnification of the RIN spectrum at around the relaxation oscillation frequency.
The RIN spectrum is shown in Fig. 3, together with the quantum noise floor corresponding to the value of -145 dB/Hz. The relaxation oscillation of the laser system is located at the frequency of 29.6 kHz and the corresponding RIN peak value is –63.8 dB/Hz. For Fourier frequencies lower than 10 kHz, the RIN level is –90 dB/Hz; for frequencies higher than 30 kHz rapidly decreases as (–40 dB/dec), reaching the quantum-noise limit for Fourier frequencies higher than 1 MHz. From the data reported in Fig. 3 the RIN of the Yb:KYF laser, in contrast to that of monolithic Yb:YAG laser [19], is dominated by pump power noise contribution and can be well described by the following transfer function [21]
where K is the sensitivity of the Yb-laser system, f 0 is the relaxation oscillation frequency, and ξ is the damping factor of the complex conjugate poles, which is related to the cavity round-trip losses, stimulated and spontaneous emission rates, and pump rate [21]. The best interpolation of the RIN data using Eq. 4 (red curve in Fig. 3) leads to the following values of the parameters: K = 5×108 (corresponding to 87 dB), f 0 = 29.6 kHz, and ξ = 0.018.
5. Conclusion
A novel fiber-coupled diode-pumped CW Yb:KYF4 laser operating at room-temperature over a wide wavelength tuning range at around 1.03 μm has been investigated. A tunability range up to 65 nm, from 1011 to 1076 nm, is obtained with a maximum output power of ∼ 500 mW in a single transverse mode. Single-frequency operation is achieved by the combination of two intracavity uncoated etalons and a birefringent filter. The single-frequency emission can be continuously tuned in the wavelength range from 1020 to 1045 nm, with a maximum output power of 150 mW and a relative intensity noise limited by quantum noise contribution for Fourier frequencies larger than 1 MHz. The Yb:KYF4 laser has potential applications to high-resolution spectroscopy of I2 in the frequency-doubled region from 510 to 522 nm, optical frequency standard and metrology, as well as to Lidar and Dial systems for atmospheric remote sensing of CO2 and H2O.
Acknowledgment
The authors thank Elisa Sani and Ilaria Grassini for the sample preparation.
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