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Abstract
The photodarkening suppression effect of an 813-nm ${{\rm{Tm}}^{3 +}}$-doped ZBLAN fiber amplifier has been investigated. We have experimentally observed that a photodarkened fiber can be bleached by an 813-nm light and that the photoinduced loss during the amplifier operation is effectively suppressed with the help of the high-power signal. To the best of our knowledge, this is the first investigation of the photodarkening suppression by using a higher power signal in ${{\rm{Tm}}^{3 +}}$-doped fiber amplifiers. Based on these signal photodarkening suppression effects, we have designed a multistage fiber amplifier, and demonstrated the stable operation of the 1.15-W fiber master oscillator power amplifier at 813 nm without additional photobleaching light.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. INTRODUCTION
The Sr optical lattice clock (Sr-OLC) is a promising candidate for the next-generation frequency standards, whose frequency accuracy has been progressively improved [1]. Long-term stable operation and minimizing its size are indispensable for further applications such as portable OLCs and spaceborne precision clocks. The requirements of the lattice laser are as follows: output power of more than 1 W, linewidth of less than 1 MHz, single-frequency with high signal-to-noise ratio (SNR), and single-transverse-mode at 813 nm. As the lattice laser of Sr-OLC, a titanium–sapphire laser (Ti:Sap) or a master oscillator power amplifier (MOPA) system with a semiconductor amplifier have been currently used. The Ti:Sap and the semiconductor MOPA have disadvantages because of the difficulty of long-term stable operation and the deterioration of beam quality, respectively. For the next-generation Sr-OLC, these problems should be overcome. Therefore, we have developed a fiber-based laser system as a new lattice laser for Sr-OLC. Though ${{\rm{Tm}}^{3 +}}$ has emission of only around 810 nm in the rare-earth ions, ${{\rm{Tm}}^{3 +}}$-doped silica fiber cannot amplify the 810-nm signal light due to its shorter upper-state lifetime. By using ${\rm{Zr}}{{\rm{F}}_4}{ - }{\rm{Ba}}{{\rm{F}}_2} {-} {\rm{La}}{{\rm{F}}_3} {-} {\rm{Al}}{{\rm{F}}_3} {-} {\rm{NaF}}$ (ZBLAN) glass as the host material, a ${{\rm{Tm}}^{3 +}}$-doped fiber shows much longer upper-state lifetime (1350 µs) than that of a ${{\rm{Tm}}^{3 +}}$-doped silica fiber (14.2 µs) [2]. We reported 1.95 W of narrow-linewidth output at 813 nm by a ${{\rm{Tm}}^{3 +}}$-doped ZBLAN fiber MOPA system, which is the highest 810-nm band output power from the fiber amplifier ever reported [3]. Though more than 1 W of the 813-nm light was obtained from the ytterbium-doped fiber laser (YbFL)-pumped ${{\rm{Tm}}^{3 +}}$-doped fiber amplifier, the output power was quite susceptible to photodarkening. The photodarkening effect is a time-dependent loss-creation induced by light irradiation, which absorbs the wavelength range from visible to near infrared [4], and photodarkening in rare-earth-doped fibers with the several dopant elements such as ${{\rm{Tm}}^{3 +}}$, ${{\rm{Tb}}^{3 +}}$, and ${{\rm{Yb}}^{3 +}}$ have been reported [5–7]. Though the mechanism of the photodarkening is still unclear, it is known that the photodarkening can be suppressed by several methods, for example, thermal bleaching, Ce codoping, ${{\rm{H}}_2}$ gas loading, and photobleaching [8–11]. In ${{\rm{Tm}}^{3 +}}$-doped fibers, the photodarkening-induced loss is attributed to the formation of color centers created by highly excited ${{\rm{Tm}}^{3 +}}$ ions, and photodarkening-mitigation methods such as codoping (La, Ce) and photobleaching (520 nm) have been demonstrated [12,13]. By using 532-nm breaching light during high-power operation, we successfully obtained the high output power of nearly 2 W at 813 nm [3]. However, additional bleaching light makes our MOPA system too complicated. As the next step, we try to make use of the photobleaching effect of an 800-nm signal light for suppressing the photodarkening effect in high-power operation. Though the photobleaching effect at 800 nm have been reported in silica fibers [14–16], there has been no report in ZBLAN fibers. In our present paper, the photobleaching effect obtained from 813-nm light is investigated in Section 2. Photodarkening-induced loss in the main amplifier is investigated by using the pump-probe experiment (Section 3). A 200-mW preamplifier is inserted before a main amplifier, whose strong signal light would suppress the photodarkening effect in the main amplifier. In the main amplifier, the strong signal light results in not only the photobleaching effect, but also the suppression of the excited state absorption (ESA), which is the main cause of the photodarkening. The photodarkening suppression effect by the strong signal light allows the multistage amplifier to realize high-power stable operation. The photobleaching effect of the signal power during amplifying operation is called “signal breaching.”
2. PHOTOBLEACHING EFFECT AT 813 NM
In order to investigate the photobleaching effect at 813 nm, an incoherent white-light (SC-5, YSL Photonics) was transmitted through a ${{\rm{Tm}}^{3 +}}$-doped ZBLAN fiber whose transmitted light spectra were measured before and after photobleaching. The ${{\rm{Tm}}^{3 +}}$ -doped ZBLAN fiber under testing (FiberLabs Inc.) has the concentration of 5000 ppm, a core diameter of 4.5 µm, and a length of 48 cm. The transmitted white light from the Tm-doped ZBLAN fiber is measured by an optical spectrum analyzer (AQ6374, Yokogawa) with the resolution bandwidth of 0.5 nm. The blue trace in Fig. 1 indicates a transmitted spectrum through a pristine fiber. The pristine fiber was deteriorated by introducing 2.5-W light at 1050 nm from an YbFL whose transmitted spectrum is depicted as a sky-blue trace in Fig. 1. After launching 150-mW of the 813-nm light into the fiber core, the photodarkening-induced loss was recovered, which is indicated by a red trace. From this result, it is found that the 813-nm light acts as the photobleaching light for suppressing the photodarkening-induced loss in ${{\rm{Tm}}^{3 +}}$ -doped ZBLAN fibers. However, the photodarkening-induced loss still remained after introducing photobleaching-light at 813 nm. We formerly tested the photobleaching effect by applying 532-nm light, and it showed a stronger photobleaching effect than that by 813-nm light. Though the photobleaching effect depends on the photon energy of the applied light, the 813-nm light has a certain amount of photobleaching effect on the ${{\rm{Tm}}^{3 +}}$-doped ZBLAN fiber.
Fig. 1. Transmitted spectra of supercontinuum-light source from pristine (blue), photodarkened (sky blue) and photobleached (red) fiber.
3. PHOTODARKENING INHIBITION IN MOPA SYSTEM AT 813 NM
A. 813-nm Preamplifier System with a ${{\rm{Tm}}^{3 +}}$-Doped ZBLAN Fiber
We have developed a preamplifier with a ${{\rm{Tm}}^{3 +}}$-doped ZBLAN fiber (Fig. 2). A lab-made external-cavity laser diode (ECLD) at 813 nm is used as a master laser, which generates 40-mW single-frequency continuous-wave light at the injection current of 100 mA with a linewidth of less than 200 kHz, and a wavelength that is set at 813.42 nm. The frequency selectivity is provided by using a narrowband dielectric interference filter in the cavity with the full width at half-maximum of 0.4 nm. The filter-type ECLD has higher mechanical stability than the diffraction grating designs such as those of Littrow and Littman–Metcalf [17]. The 10-mW launched signal power is amplified by an YbFL-pumped ${{\rm{Tm}}^{3 +}}$-doped ZBLAN fiber. Since ${{\rm{Tm}}^{3 +}}$ ions have reabsorption around 810 nm, a ${{\rm{Tm}}^{3 +}}$-doped fiber should be intensely core-pumped by using a high-brightness YbFL. The ${{\rm{Tm}}^{3 +}}$-doped ZBLAN fiber (FiberLabs Inc.) has a length of 50 cm and a concentration of 15,000 ppm and is mechanically spliced to a silica fiber (Corning: HI 780) with a splicing loss of less than 1 dB. Both facets of the fiber are angled-polished with 8 deg for suppressing parasitic lasing. The ${{\rm{Tm}}^{3 +}}$-doped ZBLAN fiber is core-pumped backward by our lab-made YbFL, which generates 1064-nm light from single-transverse-mode fiber. ${{\rm{Tm}}^{3 +}}$ ions are pumped by the upconversion process with a 1064-nm laser based on the photon avalanche mechanism [18]. In upconversion pumping, successive ground-state absorption (GSA) and excited-state absorption (ESA) excite ${{\rm{Tm}}^{3 +}}$ ions to the upper lasing state $^3{{\rm{H}}_4}$, and 1064-nm upconversion pumping shows higher efficiency because the strong ESA at 1064-nm pumping assists weak GSA through a cross-relaxation process. The signal power is amplified up to 200 mW with the launched pumping power of 500 mW, whose output power and the spectrum are shown in Fig. 3. The temperature of the fiber holder was measured during operation. After 300 min passed after the amplifier operation started, the temperature of the fiber holder became stable, and its trends resembled that of the amplified power. Therefore, the fluctuation of the amplified power shown in Fig. 3(a) is partly attributed to the temperature-dependent photo-induced loss of the fiber [19]. The SNR of the amplified signal is more than 50 dB [Fig. 3(b)], which is the same as that obtained from a forward-pumped preamplifier.
Fig. 2. Experimental setup of an 813-nm 200-mW preamplifier. ECLD, external cavity laser diode; ISO, isolator; WDM, wavelength division multiplexer; TDZF, ${{\rm{Tm}}^{3 +}}$-doped ZBLAN fiber; YbFL, ytterbium fiber laser.
Fig. 3. Temporal variation of the (a) output power and (b) output spectrum generated from the preamplifier at 813 nm.
B. Photodarkening Inhibition Effect with Signal Light during the Operation of Fiber Amplifier
As the next step, we developed an 813-nm multistage amplifier system that consists of a master laser, preamplifier, and main amplifier. The dependence of the photodarkening effects in the main amplifier on the input signal power are investigated by using the pump-probe experiment, whose schematic is shown in Fig. 4. The experiment was conducted with several input signal powers to the main amplifier by changing the output power from the preamplifier, and both amplified power and transmitted probe power were simultaneously recorded. A 520-nm light from a laser diode (LD) was used as a probe laser because the ${{\rm{Tm}}^{3 +}}$ ion has little absorption in the 520-nm range. 20 μW of the probe laser light is introduced into the main amplifier through a wavelength division multiplexer (WDM), whose power is low enough to neglect the photobleaching effect. Figure 5 shows the temporal variation of the transmitted power at 520 nm during the amplifying operation, which was obtained by a lock-in detection technique. The vertical axis in Fig. 5 is the induced loss at 520 nm, which is obtained from the probe light and is normalized by the initial transmitted power of the experiment. The input probe power is modulated at 2.2 kHz by a mechanical chopper. A ${{\rm{Tm}}^{3 +}}$-doped ZBLAN fiber in the main amplifier has a concentration of 15,000 ppm, a core diameter of 4.6 µm, a length of 1 m, and an NA of 0.12. The ${{\rm{Tm}}^{3 +}}$-doped ZBLAN fiber is pumped by a 1050-nm YbFL from the back with the launched pumping power of 0.5 W. The ${{\rm{Tm}}^{3 +}}$-doped ZBLAN fiber is mechanically spliced with a silica fiber (HI780) whose splicing loss is less than 1 dB.
Fig. 4. Schematic of a pump-probe experiment for investigating photodarkening-induced loss in the main amplifier during amplifying operation of an 813-nm MOPA. ECLD, external cavity laser diode; WDM, wavelength division multiplexer; DM1, dichroic mirror with high transmittance at 813 nm and high reflectance at ${\gt}{{1}}\;{\rm{\unicode{x00B5}{\rm m}}}$; DM2, dichroic mirror with high transmittance at 813 nm and high reflectance at 520 nm; PM, power meter; PD, photodetector; LIA, lock-in amplifier.
Fig. 5. Temporal variations of photodarkening-induced excess loss at 520 nm during the operation of 813-nm MOPA and fitting results with the launched signal power of 14 mW (red), 27 mW (blue), 49 mW (green), 69 mW (orange), 92 mW (sky blue), and 105 mW (violet).
Stretched-exponential function fitting was applied to the measured data in Fig. 5, where the R-squared is greater than 98.7%. The stretched-exponential function is generally used for describing the temporal variation of photodarkening-induced loss, whose expression is as follows:
Fig. 6. Equilibrium loss at (a) 520 nm and (b) 813 nm as a function of launched signal power during the operation of the fiber amplifier at 813 nm.
Fig. 7. Output power evolution generated from multistage ${{\rm{Tm}}^{3 +}}$-doped ZBLAN fiber MOPA at 813 nm.
Fig. 8. (a) Output spectrum and (b) temporal variation of maximum output power generated from multistage ${{\rm{Tm}}^{3 +}}$-doped ZBLAN fiber MOPA.
C. High-Power Operation of Multistage ${{\rm{Tm}}^{3 +}}$-Doped ZBLAN Fiber MOPA System at 813 nm
High-power operation of the 813-nm multistage MOPA is conducted where the 100-mW of a preamplified signal is launched into the main power amplifier. Figure 7 shows the output power evolution as a function of pumping power in the main power amplifier. The maximum output power of 1.15 W with the pumping power of 3 W has been achieved, which is limited by the available pump power. The slope efficiency of 40% has been obtained. However, with the increase of pumping power, the photodarkening effect became stronger, which resulted in the decreasing of the efficiency of the amplifier at higher pumping power. The power spectrum of the power amplifier at the maximum output power is depicted in Fig. 8(a), which was obtained with the resolution bandwidth of 0.5 nm. Compared with the optical spectrum of the preamplifier output, no degradation of the SNR appeared and amplified spontaneous emission (ASE) noise at the shorter wavelength range has been decreased by the absorption of ${{\rm{Tm}}^{3 +}}$ ions. The temporal variation of the maximum output power is shown in Fig. 8(b). Since the photodarkening effect was suppressed, the output power of more than 1 W was maintained over 1 day. The output power was gradually decreased due to the mechanical drift of the pumping optics. As the next step, we tried to fix all the optical components directly to the optical bench by using adhesives, which would realize longer stable high-power operation.
4. CONCLUSIONS
We investigated the photobleaching and photodarkening effect of an 813-nm ${{\rm{Tm}}^{3 +}}$-doped ZBLAN fiber amplifier pumped by YbFL for realizing high-power stable operation. It was revealed that the 813-nm signal light shows the photobleaching effect on a ${{\rm{Tm}}^{3 +}}$-doped ZBLAN fiber. It was also observed that the photodarkening in the operation of the ${{\rm{Tm}}^{3 +}}$-doped ZBLAN fiber amplifier has a strong dependence on the input signal power, and the photodarkening-induced loss in the fiber was dramatically suppressed by increasing the input signal power. The photodarkening suppression effect during the amplifier operation was investigated for the first time (to our knowledge). It is considered that the photodarkening suppression effect of 813-nm light was caused not only from the photobleaching effect at 813 nm but also from ESA suppression because highly excited ${{\rm{Tm}}^{3 +}}$ ions result in the photodarkening. We designed a multistage fiber MOPA system in which the higher-power signal from the preamplifier effectively suppresses the photodarkening in the power amplifier, and the stable output power of more than 1 W was maintained for over a day. This is the longest continuous stable operation time in fiber amplifiers in the 810 nm region. Furthermore, a high SNR of more than 50 dB has been obtained. By increasing the multistage fiber amplifier stage or using higher pumping power, the output power of more than 2 W could be achieved. And also, by fixing optical components with adhesives or a welding process, the operational stability would be secure, and our fiber MOPA could pave the way for realizing a high power, low noise, robust, and small-sized system at 813 nm for a future space-borne Sr-OLC.
Acknowledgment
We thank FiberLabs Inc. for fabricating Tm3+-doped ZBLAN fibers and for discussion.
Disclosures
The authors declare no conflicts of interest.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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