Record laser cooling of Yb-doped silica by 18.4 K and 6.3 K was observed in vacuum and atmospheric pressure conditions, respectively. We present a detailed investigation into the optical refrigeration of ytterbium doped silica glass for both in-air and in-vacuum conditions using various pump powers. Temperature measurements were made relative to the room temperature using thermal camera imaging and differential luminescence thermometry. Through analysis of the temporal behavior of the temperature differential at the start of the in-vacuum experiments, we find the cooling efficiency of the studied silicates to be 0.66 ± 0.07%.
Record laser cooling of Yb-doped silica by 18.4 K was recently observed in vacuum. There is a strong indication pointing to even stronger laser-cooling of Yb-doped silica. In light of these results, a radiation-balanced fiber laser in which cooling from spontaneous emission offsets waste heat generation appears more feasible than before. We will discuss the possibility of a radiation-balanced fiber laser, the similarities and differences with the conventional fiber lasers, and performance issues.
KEYWORDS: Silica, Glasses, Luminescence, High power lasers, Optical fibers, Optical amplifiers, High power fiber amplifiers, Absorption, High power fiber lasers, Fiber lasers
Anti-Stokes fluorescence cooling of a Yb-doped silica glass optical fiber preform is achieved using a high-power laser in a double-pass configuration. The coherent laser beam illuminates the silica glass preform in the red tail of its absorption spectrum, and the heat is carried out by anti-Stokes fluorescence of the blue-shifted photons. The high-purity Yb-doped silica glass preform has low parasitic absorption and is codoped with modifiers to mitigate the quenching-induced non-radiative decay for sufficiently high concentrations of Yb ions in silica glass. Therefore, sufficiently large laser absorption could be achieved to observe cooling while maintaining a near-unity external quantum efficiency.
Radiation balancing has been suggested as a possible path for heat mitigation, especially because Yb-doped silica has been recently shown to be amenable to solid-state refrigeration. In this work, we characterize a Yb-doped silica fiber preform and extract its laser cooling related parameters. We show that a fiber drawn from this preform is suitable for designing radiation-balanced fiber lasers (RBFLs). Numerical simulations based on measured values support such an observation.
Rare-earth-doped glass Anderson localizing optical fibers (g-ALOF) have great potential for novel power, coherence, and spectral properties as the gain medium in fiber lasers. We report on our investigation of the optical properties of Yb-doped g-ALOF near the peak absorption wavelength, especially the localization, and lasing- and gain-related parameters. These include the fluorescence lifetime, emission cross-section, absorption cross-section, saturation power, gain, scattering loss, and background absorption. We also elaborate on several new measurement techniques that were required to obtain these parameters, mainly due to the substantial structural difference between g-ALOFs and conventional fibers.
The modal instability due to heating has been a limiting factor in the ongoing progress towards power scaling in fiber lasers and amplifiers. Radiation balancing has been suggested as a feasible approach for heat mitigation, for which Yb-doped ZBLAN is a recommended gain medium. In this study, we introduce a non-destructive and non-contact method to characterize Yb-doped ZBLAN fibers and show that it is suitable for designing a radiation-balanced fiber laser. Numerical simulations based on measured values indicate that background absorption is the main hindrance to be overcome in such designs.
We report the observation of anti-Stokes fluorescence cooling of Yb-doped silica glass by 0.7 degrees Celsius. We conduct a detailed investigation of the cooling parameters of this glass, including the wavelength dependence of the cooling efficiency as a function of the wavelength and also the parasitic absorption of the pump laser. The measurements are performed on three different glass samples with different compositions and cooling is observed in all samples to varying degrees. The results highlight the possibility of using Yb-doped silica glass for radiation-balancing in fibers. Radiation-Balancing is a viable technique for heat mitigation in lasers and amplifiers.
Transverse Anderson Localizing Optical Fiber (TALOF) provides a novel waveguiding mechanism. The entire disordered transverse structure of the fiber supports compactly located Anderson localized modes. Each transversely localized mode of the fiber forms a guiding channel. The strong disorder-induced transverse confinement due to the Anderson localization suffices for single-mode light transmission. In this work, we analyze the beam quality of highly localized modes in a glass-TALOF. We use the M^2 factor as a broadly accepted metric in optical fiber community for the beam quality evaluation. A numerical analysis of the M^2 values on a large ensemble of calculated modes in a glass-TALOF hints to the presence of high-quality modes at various locations across the transverse profile of the fiber. Our experimental results on the statistics of M^2 values in glass-TALOF supports the numerical analysis where it is shown that high-quality modes can be easily excited across the entire transverse profile of the fiber. Specifically, we present M^2 values of 30 localized modes and show how the M^2 ~ 1 modes are dominant. When the gain is added to TALOF, it can support localized lasing modes. Recently, we have demonstrated a directional and spectrally stable random laser mediated by a glass-TALOF; localization of the lasing modes reduces mode competition and helps with the spectral stability. The disorder-induced high-quality wavefronts in TALOF in combination with the aforementioned advances in lasing mediated by TALOF can foster a new class of single-mode optical fiber laser.
Recent advances in power scaling of fiber lasers and amplifiers are hampered by the transverse mode instability, which deteriorates the output laser beam quality: the main cause is the overheating of the optical fibers. Radiative cooling has been suggested as a potentially viable heat removal scheme: the rare-earth-doped optical fiber is pumped at the pump wavelength, which is higher than the mean fluorescence wavelength of the active ions; therefore, the anti-Stokes fluorescence removes some and ideally most of the excess heat. In practice, the pump absorption cross section is considerably lower than its peak value, because the pump wavelength must be sufficiently longer than the mean fluorescence wavelength for efficient radiative cooling. Therefore, the design of such lasers and amplifiers must naturally depart from the conventional designs.
I will focus on general scaling laws that govern the design of radiation-balanced fiber amplifiers and lasers. In particular, I will show that the undesirable parasitic absorption can make conventional designs unrealistic or at best irrelevant. In other words, a sufficiently large (and realistic) value of parasitic absorption can totally dominate the thermal balance, because its contribution scales linearly with the power, while the radiative cooling saturates at high power values. I will show that in conventional designs, radiation balancing can only be achieved at power values sufficiently low that may not even warrant any sophisticated cooling effort. However, there exist unconventional design strategies that make radiation balancing relevant even at high powers and I will explore such designs and their consequences.
KEYWORDS: Cladding, Optical fibers, High power fiber amplifiers, Optical amplifiers, Fiber amplifiers, Thermal effects, Absorption, High power fiber lasers, Laser development, High power lasers
Currently heat management is a big hurdle for power-scaling of high-power fiber lasers and amplifiers. Different methods have been developed over the years to mitigate the heat generation in high-power lasers and amplifiers; radiation balancing is a new method that leverages the radiative cooling for heat mitigation. In this study, the effects of different design parameters on the operation of a doped-double-cladding (DC) fiber laser and amplifier for radiation balancing are investigated. The results show that the value of the internal quantum efficiency for an effective heat mitigation by radiation balancing is not required to be very close to unity. Conversely, it is shown that in high-power operation, even small values of background absorption can be detrimental to radiation balancing. It is argued that both the background absorption and the small dopant area of the doped-DC fibers are big hurdles in achieving an effective heat mitigation by radiation balancing in high power operation. In order to address this issue, we suggest to dope the inner cladding of the DC fiber such that at the pump wavelength, the inner cladding cools down by optical refrigeration. Due to the geometry of the fiber, the temperature is uniformly homogenized across the fiber and the effective fiber temperature decreases, allowing Yb-doped DC fiber lasers and amplifiers to operate at Kilo-Watt levels in radiation balanced mode.
To study the cooling efficiency and radiation-balanced condition in optical fiber lasers, it is essential to measure
the absorption coefficient over the appropriate spectral range accurately. The most common technique to measure the absorption coefficient in rare-earth doped optical fibers is the cut-back method. Unfortunately, the cut-back method is destructive and requires mechanical movement and optical realignment, which are troublesome for fragile fibers like ZBLAN fibers or tapered optical fibers. Moreover, the presence of the cladding modes is a source of inaccuracy in the final result, and it is challenging to properly remove the cladding modes in highly doped fibers to achieve high accuracy in the measurement of the absorption coefficient. We introduce a non-destructive method based on analyzing the side-light along the doped fiber. The method is presented and explained in detail, and it is successfully used for measuring the resonance absorption of the Yb-doped silica and ZBLAN fiber in single-mode and multi-mode rare-earth doped fibers.
Laser cooling in Tm:YLF and Tm:BYF crystals has recently been reported. We investigate high power laser cooling of Tm doped crystals under high vacuum using multiple-pass Herriott cell configuration. We also model potential mid-IR Radiation Balanced Lasers (RBLs) in available Tm:YLF and Tm:BYF crystals. Our experiments and modelling shows that our 1% wt. Tm:BYF sample is a promising 2 µm RBL candidate, since it has high gain and high external quantum efficiency as well as good room temperature cooling efficiency. We will attempt to demonstrate the first mid-IR RBL experimentally in Tm:BYF crystal as well.
Measurement of cooling efficiency and temperature of the doped optical fiber is critical for the development of optical refrigerators and radiation balanced lasers. Measuring the optical fiber temperature, especially for single mode fibers, is challenging. Non-contact thermometry is required because a temperature sensor which is in thermal contact with the fiber can potentially be a heat load when exposed to the scattered pump power and fiber luminescence and can lead to inaccuracies in thermometry. One of the best non-contact methods is using differential luminescence thermometry (DLT). DLT works based on the fact that the 4f electrons in rare-earths are shielded from the surroundings and host field transitions; therefore, the temperature-induced intensity changes in rare-earth material luminescence are mainly caused by changes in Boltzmann population of emitting states. We propose a variant of DLT for finding a relation between the spontaneous emission of the fiber and the temperature. Our method is based on the normalized correlation between the spontaneous emission spectrum at each temperature and the reference spontaneous emission. In this method, we chose a section of the spontaneous emission spectrum as the reference and calculate the normalized correlation factor of the spontaneous spectrum at each temperature with the reference spontaneous spectrum. We make a calibration curve, and based on the calibration curve we estimate the temperature difference from the reference. Comparisons with the conventional DLT will be presented.
A numerical study of laser cooling in a large mode area Single Mode Photonic Crystal Yb3+:ZBLAN glass fiber is presented. In a recent analyses on conventional single mode fibers (SMFs), strategies to maximize the cooling efficiency were highlighted and it was shown that the cooling scales quadratically with the core and inversely with the cladding radius. For conventional SMFs, heat source density can hardly be increased due to limitations in the total Yb doping concentration and the fact that the system easily operates in the pump saturation regime due to very low saturation intensities in small core single mode fibers. Therefore, it is essential to use the largest possible core radius and smallest cladding radius to obtain detectable cooling. A trivial design approach to obtain large mode areas is to decrease the numerical aperture (NA). However, there are several difficulties in applying this concept to rare-earth-doped fibers. Here, we propose large mode area single mode Yb3+:ZBLAN photonic crystal fibers as a robust alternative design. The radial distribution of the pump and laser mode intensities are numerically calculated using Finite Element Method and the heat source density is directly calculated using the pump and laser intensity distributions. The heat source density is fed back to the heat transfer module of COMSOL and radial temperature distribution across the large core of the fiber and its surrounding photonic crystal structure is calculated. Our results show that a much higher laser cooling efficiency is achievable in large mode area single mode photonic crystal fibers.
Radiation-balanced lasers are lasers where the heat generation in the gain medium is compensated by optical refrigeration due to anti-Stokes fluorescence emission. To investigate the feasibility of RBL operation in an optical fiber, a diagnostic test and a comprehensive model are essential. The model presented here is based on a two-level system and includes the intensity saturation effect, which has been usually neglected in bulk materials. We will show that for a material with a very low dopant area such as a single mode fiber (SMF) in which the saturation power is easily attainable, there is an optimum power at which the best cooling efficiency is obtained. The effect of the dopant density on the cooling power is investigated to find the maximum cooling efficiency which can be extracted for the material. We also present data for the extraction efficiency and other parameters of commercial Yb:ZBLAN glass and Yb:Silicate SMFs to discuss their cooling feasibility. Due to the structural defects, a double exponential behavior is usually observed in the fluorescence decay of the fibers that includes an slow and a fast decay channels. Some of the ions usually reside in the fast decay side and cause a large decrease in the heat extraction efficiency. Using our model, we will first analytically show that there is a maximum limit for the fast decay lifetime below which the cooling can still be functional and secondly discuss the effect of the measured decay lifetimes on the cooling efficiency.
Mode splitting (MS) in whispering gallery microresonators provides excellent noise suppression in sensing signal compared to mode shifting. Here, we theoretically studied the ability of hollow bottle microresonators for detection of a single nanoparticle in air and water medium by MS phenomenon. To find out the optimum condition of sensor for nanoparticle (NP) detection, the effects of bottle geometry parameters, mode orders, and mode polarization state was investigated. The first radial transverse electric mode demonstrated the best sensitivity when the resonator radius and wall thickness were 10 and 0.3 μm, respectively. However, transverse magnetic modes manifested slightly better detection limit. In the air core hollow microbottle resonator (HMBR), the best detection limit of 3.1 nm radius for polystyrene NPs was achieved at an optimum condition of 30-μm resonator radius and 0.8-μm wall thickness. While MS could not be resolved in deionized water filled HMBRs for all of the investigated conditions at 1550 nm, changing the wavelength to 780 nm provided a detection limit of 15.1 nm in water. Furthermore, it is found that the sensitivity of HMBR is increased by at least two times in comparison with a microtoroid sensor. HMBRs are optofluidic platforms, so employing them could drastically enhance the applicability of microresonator-based systems for label-free NP detection.
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