Thermally induced transverse mode instability (TMI) has been recognized as one of the major limits to average power scaling of single-mode fiber laser. Mitigating the thermal load in single-mode high-power fiber lasers by operating lasing closer to the pump wavelength is one of the effort directions. Here, we demonstrate 220w single–mode output power at 1018nm from an ytterbium-doped all-solid photonic bandgap fiber (ASPBF) pumped at 976nm. The quantum defect is only 4.1%, helping to mitigate the thermal load. The ASPBF fiber has the multiple-cladding-resonant design, leading to better higher-order modes (HOM) suppression in its ~50µm core. The large core/cladding ratio also benefits the 1018nm lasing, providing the higher cladding pump absorption so shorter fiber length is needed with better ASE suppression at longer wavelength. In addition, the use of a phosphosilicate host in this fiber also enhances ytterbium gain at 1018nm, leading to a reduction in the required inversion, further increasing efficiency. In the laser test, one end of fiber is spliced to a high-reflective fiber-Bragg-grating at 1018nm and the other end is right-angle cleaved. ~62% and ~77% lasing efficiency has been achieved around maximum power with respective to the launched and absorbed pump power. The M2 was measured at 130W as 1.06 and 1.17 with respective to the x and y axis.
Transverse mode instability (TMI) has been recognized as a major limit to average power scaling of single-mode fiber laser besides the optical nonlinear effects. One key to mitigate TMI is to suppress the higher-order modes (HOMs) propagation in the optical fiber. By implementing additional cores in the optical fiber cladding, HOMs can be resonantly coupled from the main core to the surrounding cladding cores, leading to better HOMs suppression. Here, we demonstrate an Yb-doped multiple-cladding-resonant all-solid photonic bandgap fiber with a ~60μm diameter core for high power fiber lasers. The fiber has a multiple-cladding-resonant design in order to provide better HOMs suppression. Maximum laser power of 910w is achieved for a direct diode-pumped fiber laser without TMI with a 9m long fiber at 60cm coil diameter, breaking the TMI threshold of 800w that has been observed in large-mode-area PCFs with ~40μm core. This result is limited by fiber end burning due to the un-optimized thermal management. Later experiment demonstrates maximum laser power of 1050w with 90% lasing efficiency versus absorbed pump power in a 8m long fiber coiled at 80cm diameter, limited by the pump source. However, the fiber bending condition needs to be optimized in order to produce a better laser beam quality.
Thermal management is critical for kw-level power lasers, where mode instability driven by quantum defect heating is a major challenge. Tandem pumping using 1018nm fiber lasers are used to enable both high brightness and low quantum defect. It is, however, difficult to realize efficient 1018nm YDFL. The best demonstration to date is limited by the use of both conventional aluminosilicate host and smaller core diameters. In these cases, higher inversion is required due to the aluminosilicate host and higher pump brightness is required due to the smaller core, which results in high signal brightness for the same output power. These factors lead to large pump power to exit fiber, resulting in poor efficiency. Phosphosilicate host, on the other hand, requires much lower inversions to reach the gain threshold at 1018nm. The combination of phosphosilicate host and large-core leakage channel fibers (LCF) is a perfect candidate for efficient 1018nm fiber laser. We report a highly efficient Yb-doped phosphosilicate LCF laser with a quantum defect of 4.1% using a ~50μm-core diameter and ~420μm cladding diameter. The slope efficiency with respect to the launched pump power at 1018nm is 70%. The ASE suppression is <60dB. The large cladding of 420μm demonstrates a combination of high efficiency, ~4% quantum defect and high-power low-brightness diode pumping. We have also studied the limits of operating ytterbium fiber lasers at shorter wavelengths and found the efficiency to fall off at shorter wavelengths due to the much higher inversions required.
Fiber lasers are in the process of revolutionizing modern manufacturing. Further power scaling is still much desired to increase throughput and to break new frontiers in science and defense. It has become very clear now that highly single-mode fibers with large effective mode areas are required to overcome both nonlinear effects and mode instability [1-3]. We have been studying all-solid photonic bandgap fibers (AS-PBF), which have open and highly dispersive cladding, making them ideal for higher-order-mode controls in large-mode-area fibers. I will review our recent progress in this area and, especially in ytterbium-doped AS-PBF lasers and amplifiers.
Ytterbium-doped large mode area all-solid photonic bandgap fiber amplifiers were used to demonstrate <400 W of
output power at 1064 nm. In an initial set of experiments, a fiber with a core diameter of ~50 μm, and a calculated
effective area of 1450 μm<sup>2</sup> in a straight fiber, was used to generate approximately 600 W. In this case, the input
seed was modulated using a sinusoidal format at a frequency of 400 MHz. The output, however, was multimode as
the fiber design did not allow for single-mode operation at this wavelength. A second fiber was then fabricated to
operate predominantly in single mode at 1064 nm by having the seed closer to the short wavelength edge of the
bandgap. This fiber was used to demonstrate 400 W of single-frequency output with excellent beam quality. As the
signal power exceeded 450 W, there was significant degradation in the beam quality due to the modal instability.
Nevertheless, to the best of our knowledge, the power scaling results obtained in this work far exceed results from
prior state of the art all-solid photonic bandgap fiber lasers.
Polarizing optical fibers are important components for building compact fiber lasers with linearly polarized laser output. Conventional single-mode optical fibers with birefringence can only preserve the polarization when the incident beam is launched properly. Recent reports demonstrate that the birefringence in photonic bandgap fibers (PBFs) can provide single-polarization operation near the edge of transmission band by shifting the transmission band for the light with orthogonal polarizations. Here, we demonstrate a 50μm core Yb-doped polarizing photonic bandgap fiber (PBF) for single-polarization operation throughout the entire transmission band from 1010nm to 1170nm with a polarization extinction ratio (PER) of >5dB/m, which is >15dB/m near the short wavelength edge of the transmission band. The polarizing effect is due to the differential polarization transmission loss presented in this fiber, which is benefited from the fiber birefringence of 3.2x10<sup>-4</sup>, obtained by incorporating low-index boron-doped rods on either side of the core. The achievement is based on the fact that light at fast axis has lower effective mode index which is closer to the modes in the photonic cladding and thus to be easily coupled into cladding. A 2.6m long straight fiber was tested in a laser configuration without any polarizers to achieve single polarized laser output with a PER value of 21dB at 1026nm lasing wavelength.
Power scaling of fiber lasers is highly desirable in many applications but is mainly limited by nonlinear effects. Large-mode-area fibers have been used to mitigate this limit, such as the leakage channel fiber (LCF). The mode intensity profile in these fibers typically exhibits Gaussian-like structure with much reduced effective mode-area compared to the physical fiber core area. Thus, a flat-top mode with a uniform intensity distribution is more suitable for larger effective mode-area without having to increase core size. In this work, we demonstrate the first flat-top mode generated in a 50 μm-core Yb-doped LCF fiber. The mode flattening from Gaussian beam to a flat-top one is achieved by using a 30 μm uniform Yb-doped area in the core center with a refractive index very slightly below that of the background silica glass by 2×10<sup>-4</sup>. The resulting flat-top mode has a significantly increased effective mode area of ~1880 um<sup>2</sup>, which is ~50% larger than that of a conventional uniform core and ~6 times the effective mode area of the flat-top mode record demonstrated previously. A 6m-long fiber is also tested in a laser configuration with a slope efficiency of ~84% at 1026 nm with respect to the absorbed pump power at 976 nm.
We present here a novel full field pump-probe photothermal dynamics microscopy (PTDM) which uses a numerical lockin mechanism for capturing full field photothermal responses and is capable of imaging 2D thermal dissipation dynamics by varying the time delay between the probing and pump nano-second pulses. PTDM may find interesting applications in biology and medicine. As one example, we report PTDM imaging nuclei contained on intact hematoxylin and eosin (H&E) stained prostate cancer specimens may potentially be used to distinguish low grade and high grade prostate cancer.
There are still very strong interests for power scaling in high power fiber lasers for a wide range of applications in medical, industry, defense and science. In many of these lasers, fiber nonlinearities are the main limits to further scaling. Although numerous specific techniques have studied for the suppression of a wide range of nonlinearities, the fundamental solution is to scale mode areas in fibers while maintaining sufficient single mode operation. Here the key problem is that more modes are supported once physical dimensions of waveguides are increased. The key to solve this problem is to look for fiber designs with significant higher order mode suppression. In conventional waveguides, all modes are increasingly guided in the center of the waveguides when waveguide dimensions are increased. It is hard to couple a mode out in order to suppress its propagation, which severely limits their scalability. In an allsolid photonic bandgap fiber, modes are only guided due to anti-resonance of cladding photonic crystal lattice. This provides strongly mode-dependent guidance, leading to very high differential mode losses. In addition, the all-solid nature of the fiber makes it easily spliced to other fibers. In this paper, we will show for the first time that all-solid photonic bandgap fibers with effective mode area of ~920μm<sup>2</sup> can be made with excellent higher order mode suppression.
Further power scaling of single frequency fiber lasers is of significant interests for many scientific and defense applications. It is currently limited by stimulated Brillouin scattering (SBS). In recent years, a variety of techniques have been investigated for the suppression of SBS in optical fibers. A notable example is to design transverse acoustic properties of optical fibers in order to minimize optical and acoustic mode overlap. It was pointed out recently that SBS suppression from such transverse acoustic tailoring is limited when considering the existence of acoustic leaky modes. We demonstrate, for the first time, a post-processing technique where hydrogen is diffused in to a fiber core and then locally and permanently bonded to core glass by a subsequent UV exposure. Large local acoustic property can be altered this way for significant SBS suppression. It is also possible to use this technique to implement precisely tailored acoustic properties along a fiber for more optimized SBS suppression in a fiber amplifier. Change in Brillouin Stokes frequency of ~320MHz at 1.064μm has been demonstrated using hydrogen, corresponding to a SBS suppression of ~8dB. Much higher SBS suppression is possible at higher hydrogen concentrations.
There are still very strong interests for power scaling in high power fiber lasers for a wide range of applications in
medical, industry, defense and science. In many of these lasers, fiber nonlinearities are the main limits to further scaling. Although numerous specific techniques have studied for the suppression of a wide range of nonlinearities, the fundamental solution is to scale mode areas in fibers while maintaining sufficient single mode operation. Here the key problem is that more modes are supported once physical dimensions of waveguides are increased. The key to solve this problem is to look for fiber designs with significant higher order mode suppression. In conventional waveguides, all modes are increasingly guided in the center of the waveguides when waveguide dimensions are increased. It is hard to couple a mode out in order to suppress its propagation, which severely limits their scalability. In an all-solid photonic bandgap fiber, modes are guided due to anti-resonance of cladding photonic crystal lattice. This provides strongly modedependent guidance, leading to very high differential mode losses. In addition, the all-solid nature of the fiber makes it easily spliced to other fibers. In this paper, we will show for the first time that all-solid photonic bandgap fibers with effective mode area of ~800m2 can be made with excellent higher order mode suppression.
There are very strong interests for power scaling in high power fiber lasers for a wide range of applications in medical,
industry, defense and science. In many of these lasers, fiber nonlinearities are the main limits to further scaling.
Although numerous specific techniques have studied for the suppression of the wide range of nonlinearities, the
fundamental solution is scaling mode areas in fibers while maintaining sufficient single mode operation. Here the key
problem is that more modes are supported once physical dimensions of waveguides are increased. There are two basic
approaches, lower refractive index contrast to counter the increase of waveguide dimension or/and introduction of
additional losses to suppress higher order modes. Lower index contrast leads to weak waveguides, resulting in fibers no
longer being coil-able. Our research has been focused on designs for significant higher mode suppression. In
conventional waveguides, modes are increasingly guided in the center of the waveguides when waveguide dimensions
are increased. It is hard to couple the modes out to suppress them. This severely limits the scalability of all designs based
conventional fibers. In an all-solid photonic bandgap fiber, modes are guided due to anti-resonance of cladding photonic
crystal lattice. This leads strongly mode-dependent guidance. Our theoretical study has shown that it can have some of
the highest differential mode losses among all designs with equivalent mode areas. Our design and experimental works
have shown the potential of this approach for all-glass fibers with >50μm core which can be coiled for high power
Convergence of light towards a desired location in optically diffusive and aberrative media is highly relevant to
optical methods of biomedical imaging. In this study, we demonstrated the feasibility of employing photoacoustic
signals originating from an optically absorptive target as feedback for shaping the incident wavefront to increase
optical energy density at the absorptive target. The wavefront of a collimated laser beam was shaped by an array of
two-dimensional MEMS deformable mirrors and then transmitted through optically scattering paraffin. The phase of
light reflected by each mirror was varied (0-2π) iteratively to maximize the amplitude of the photoacoustic signal.
The photoacoustic signal potentially provides a non-invasive and reliable feedback for manipulating spatial phase
distribution of light to achieve focusing in diffusive media and may facilitate optical imaging at greater depths.
Purpose: Ultrasound and optical coherence tomography (OCT) are widely used techniques for diagnostic imaging
of the eye. OCT provides excellent resolution, but limited penetration. Ultrasound provides better penetration, but an
order-of-magnitude poorer resolution than OCT. Photoacoustic imaging is relatively insensitive to scattering, and so
offers a potential means to image deeper than OCT. Furthermore, photoacoustic imaging detects optical absorption,
a parameter that is independent of that detected by conventional ultrasound or OCT. Our aim was to develop a
photoacoustic system suitable for imaging the eye.
Methods: We developed a prototype system utilizing a focused 20 MHz ultrasound probe with a central aperture
through which optics were introduced. The prototype system produced 1-μJ, 5-nsec pulses at 532 or 1064 nm with a
20-μm spot size at a 500 Hz repetition rate. The photoacoustic probe was mounted onto computer-controlled linear
stages and pulse-echo ultrasound and photoacoustic images obtained on ex vivo pig eyes and in vivo mouse eyes.
Results: Lateral resolution was significantly improved by use of a laser spot size much smaller than the acoustic
beamwidth. Photoacoustic signals were obtained primarily from melanin in ex vivo tissues and from melanin and
hemoglobin in vivo. Image fusion allowed superposition of photoacoustic signals upon the anatomic features
detected by conventional ultrasound.
Conclusion: Photoacoustic imaging detects the presence of clinically relevant pigments, such as melanin and oxyand
deoxy-hemoglobin, and, potentially, from other pathologic pigments occurring in disease conditions (tumors,
nevii, macular degeneration). Fine-resolution photoacoustic data provides information not detected in current
ophthalmic imaging modalities.
In this paper, we discussed different mechanisms which are proved to ensure phase locking in a laser array. We also reported the phase locking in solid-state (crystal) laser array and fiber laser array by using the self-imaging confocal resonator which provides high feedback efficiency, quite insensitive to power variations among the pump beams, simply modal profile and also can achieve phase locking passively. A passive approach is to utilize the process of self-adjustment in lasing frequency to adapt to changes in the optical path lengths. The phase-locked mode is highly stable despite the phase variations in the individual elements caused by thermal and mechanical effects.