Information on the wavelength is essential for most laser applications and a wide range of devices are available for measuring it. Commercially available wavemeters can provide femtometer resolution in a wide wavelength range but their refresh rate rarely goes into the kHz range. Streak cameras, on the other hand, provide extremely fast measurements with a wide spectrum. However, the spectral resolution is severely limited due to the use of a grating as the wavelength separating element. Here we present a wavemeter that combines a megahertz measurement rate and sub-picometer wavelength resolution. The technique uses the steep wavelength acceptance curve of a thick non-linear crystal to calculate the wavelength from just two power measurements. The bandwidth is limited only by the speed of a photodiode while the resolution and wavelength range can be engineered by choosing a suitable crystal type and geometry. We use the wavemeter to examine how the longitudinal mode evolves during a single pulse from a tapered diode laser. High resolution, high speed measurements of the wavelength can give new information about laser diodes, which is valuable for applications requiring short but wavelength stable pulses, such as pulsing of the second harmonic light.
A wide range of laser medical treatments are based on coagulation of blood by absorption of the laser radiation. It has, therefore, always been a goal of these treatments to maximize the ratio of absorption in the blood to that in the surrounding tissue. For this purpose lasers at 577 nm are ideal since this wavelength is at the peak of the absorption in oxygenated hemoglobin. Furthermore, 577 nm has a lower absorption in melanin when compared to green wavelengths (515 − 532 nm), giving it an advantage when treating at greater penetration depth. Here we present a laser system based on frequency doubling of an 1154 nm Distributed Bragg Reflector (DBR) tapered diode laser, emitting 1.1 W of single frequency and diffraction limited yellow light at 577 nm, corresponding to a conversion efficiency of 30.5%. The frequency doubling is performed in a single pass configuration using a cascade of two bulk non-linear crystals. The system is power stabilized over 10 hours with a standard deviation of 0.13% and the relative intensity noise is measured to be 0.064 % rms.
The use of visible lasers for medical treatments is on the rise, and together with this comes higher expectations for the laser systems. For many medical treatments, such as ophthalmology, doctors require pulse on demand operation together with a complete extinction of the light between pulses. We have demonstrated power modulation from 0.1 Hz to 10 kHz at 532 nm with a modulation depth above 97% by wavelength detuning of the laser diode. The laser diode is a 1064 nm monolithic device with a distributed feedback (DFB) laser as the master oscillator (MO), and a tapered power amplifier (PA). The MO and PA have separate electrical contacts and the modulation is achieved with wavelength tuning by adjusting the current through the MO 40 mA.
Semiconductor lasers are ideal sources for efficient electrical-to-optical power conversion and for many applications where their small size and potential for low cost are required to meet market demands. Yellow lasers find use in a variety of bio-related applications, such as photocoagulation, imaging, flow cytometry, and cancer treatment. However, direct generation of yellow light from semiconductors with sufficient beam quality and power has so far eluded researchers. Meanwhile, tapered semiconductor lasers at near-infrared wavelengths have recently become able to provide neardiffraction- limited, single frequency operation with output powers up to 8 W near 1120 nm.
We present a 1.9 W single frequency laser system at 562 nm, based on single pass cascaded frequency doubling of such a tapered laser diode. The laser diode is a monolithic device consisting of two sections: a ridge waveguide with a distributed Bragg reflector, and a tapered amplifier. Using single-pass cascaded frequency doubling in two periodically poled lithium niobate crystals, 1.93 W of diffraction-limited light at 562 nm is generated from 5.8 W continuous-wave infrared light. When turned on from cold, the laser system reaches full power in just 60 seconds. An advantage of using a single pass configuration, rather than an external cavity configuration, is increased stability towards external perturbations. For example, stability to fluctuating case temperature over a 30 K temperature span has been demonstrated. The combination of high stability, compactness and watt-level power range means this technology is of great interest for a wide range of biological and biomedical applications.
We investigate the temporal dynamics of Modal instabilities (MI) in ROD fiber amplifiers using a 100 μm core rod fiber in a single-pass amplifier configuration, and we achieve ~200W of extracted output power before the onset of MI. Above the MI threshold, we investigate the temporal dynamics of beam fluctuations in both the transient and chaotic regime. We identify a set of discrete frequencies in the transient regime and a white distribution of frequencies in the chaotic regime. We test three identical rods using a multiple ramp-up procedure, where each rod is tested in three test series and thermally annealed between each test series. We find that the MI threshold degrades as it is reached multiple times, but is recovered by thermal annealing. We also find that the test history of the rods affects the temporal dynamics.
Supercontinuum generation in photonics crystal fibers (PCFs) pumped by CW lasers yields high spectral power density
and average power. However, such systems require very high pump power and long nonlinear fibers. By on/off
modulating the pump diodes of the fiber laser, the relaxation oscillations of the laser can be exploited to enhance the
broadening process. The physics behind the supercontinuum generation is investigated by sweeping the fiber length, the
zero dispersion wavelength, and the fiber nonlinearity. We show that by applying gain-switching a high average output
power of up to 30 W can be maintained and the spectral width can be improved by 90%. The zero dispersion wavelength
should be close to but below the pump wavelength to achieve the most visible light. By increasing the nonlinearity the
fiber length can be reduced from 100 m to 25 m and the efficiency of visible light generation is improved by more than
High-power fiber lasers and amplifiers have gained tremendous momentum in the last 5 years. Many of the traditional manufacturers of gas and solid-state lasers are now pursuing the fiber-based systems, which are displacing the conventional technology in many areas. High-power fiber laser systems require reliable fibers with large cores, stable mode quality, and good power handling capabilities-requirements that are all met by the airclad fiber technology. In the present paper we go through many of the building blocks needed to build high-power systems and we show an example of a complete airclad laser system. We present the latest advancements within airclad fiber technology including a new 100 μm single-mode polarization-maintaining rod-type fiber capable of amplifying to megawatt power levels. Furthermore, we describe the novel airclad-based pump combiners and their use in a completely monolithic 350 W cw fiber laser system with an M2 of less than 1.1.
We demonstrate an all-fiber 7x1 signal combiner for incoherent laser beam combining. This is a potential key
component for reaching several kW of stabile laser output power. The combiner couples the output from 7 single-mode
(SM) fiber lasers into a single multi-mode (MM) fiber. The input signal fibers have a core diameter of 17 μm and the
output MM fiber has a core diameter of 100 μm. In a tapered section light gradually leaks out of the SM fibers and is
captured by a surrounding fluorine-doped cladding. The combiner is tested up to 2.5 kW of combined output power and
only a minor increase in device temperature is observed. At an intermediate power level of 600 W a beam parameter
product (BPP) of 2.22 mm x mrad is measured, corresponding to an M2 value of 6.5. These values are approaching the
theoretical limit dictated by brightness conservation.
We demonstrate for the first time an imaging fibre bundle ("hexabundle") that is suitable for low-light applications in
astronomy. The most successful survey instruments at optical-infrared wavelengths today have obtained data on up to a
million celestial sources using hundreds of multimode fibres at a time fed to multiple spectrographs. But a large fraction
of these sources are spatially extended on the celestial sphere such that a hexabundle would be able to provide
spectroscopic information at many distinct locations across the source. Our goal is to upgrade single-fibre survey
instruments with multimode hexabundles in place of the multimode fibres. We discuss two varieties of hexabundles: (i)
closely packed circular cores allowing the covering fraction to approach the theoretical maximum of 91%; (ii) fused noncircular
cores where the interstitial holes have been removed and the covering fraction approaches 100%. In both cases,
we find that the cladding can be reduced to ~2μm over the short fuse length, well below the conventional ~10λ thickness
employed more generally. We discuss the relative merits of fused/unfused hexabundles in terms of manufacture and
deployment, and present our first on-sky observations.
A 7+1 to 1 pump/signal combiner with single-mode (SM) polarization maintaining (PM) 15 μm mode-field-diameter
(MFD) signal feed-through is demonstrated. The combiner is designed for pulse amplification in an active Yb-doped airclad
fiber operated in backward pumped configuration. Signal coupling through the device is realized by a
microstructured taper element allowing single-mode guidance and constant MFD at a taper ratio of 3.4.
We demonstrate electrical tunability of a fiber laser using a liquid crystal photonic bandgap fiber. Tuning of the laser is
achieved by combining the wavelength filtering effect of a liquid crystal photonic bandgap fiber device with an
ytterbium-doped photonic crystal fiber. We fabricate an all-spliced laser cavity based on a liquid crystal photonic
bandgap fiber mounted on a silicon assembly, a pump/signal combiner with single-mode signal feed-through and an
ytterbium-doped photonic crystal fiber. The laser cavity produces a single-mode output and is tuned in the range 1040-
1065 nm by applying an electric field to the silicon assembly.
We demonstrate the fabrication of a multi-mode (MM) to 61 port single-mode (SM) splitter or "Photonic Lantern". Low
port count Photonic Lanterns were first described by Leon-Saval et al. (2005). These are based on a photonic crystal
fiber type design, with air-holes defining the multi-mode fiber (MMF) cladding. Our fabricated Photonic Lanterns are
solid all-glass versions, with the MMF defined by a low-index tube surrounding the single-mode fibers (SMFs). We
show experimentally that these devices can be used to achieve efficient and reversible coupling between a MMF and 61
SMFs, when perfectly matched launch conditions into the MMF are ensured. The total coupling loss from a 100 μm core
diameter MM section to the ensemble of 61 SMFs and back to another 100 μm core MM section is measured to be as
low as 0.76 dB. This demonstrates the feasibility of using the Photonic Lanterns within the field of astrophotonics for
coupling MM star-light to an ensemble of SM fibers in order to perform fiber Bragg grating based spectral filtering.
Liquid crystal photonic bandgap fibers represent a promising platform for the design of all-in-fiber optical devices,
which show a high degree of tunability and exhibit novel optical properties for the manipulation of guided light. In this
review paper we present tunable fiber devices for spectral filtering, such as Gaussian filters and notch filters, and devices
for polarization control and analysis, such as birefringence control devices and switchable and rotatable polarizers.