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 present different methods of generating light in the blue-green spectral range by nonlinear frequency conversion of
tapered diode lasers achieving state-of-the-art power levels. In the blue spectral range, we show results using single-pass
second harmonic generation (SHG) as well as cavity enhanced sum frequency generation (SFG) with watt-level output
powers. SHG and SFG are also demonstrated in the green spectral range as a viable method to generate up to 4 W output
power with high efficiency using different configurations.
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 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.
High-power fiber lasers and amplifiers have gained tremendous momentum in the last five years, and many of the
traditional manufactures of gas and solid-state lasers are pursuing the attractive fiber-based systems, which are now
displacing the old technology in many areas. High-power fiber laser systems require specially designed fibers with large
cores 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 70
μm single-mode polarization-maintaining rod-type fiber capable of amplifying to MW 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. Finally, we briefly touch upon the subject of photo darkening and its origin.
We present record-breaking experimental data on high power transmission through novel 7X1 and 19X1 multimode
combiners based on Photonic Crystal Fiber technology. Both combiners are monolithic, have losses of ~0.2 dB, show
very high thermal robustness and can handle record high optical powers. We have transmitted 100 W through the 7X1
and 310 W through the 19X1 combiner without evidence of any degradation or critical heating. The powers were limited
only by available pump power.
The combiners are based on Air-clad technology, where a ring of air-holes running along the length of the device provide
guiding for the light. This Air-clad offers three major advantages for the device: 1: It is well suited for high optical
powers as no polymer coating gets into contact with the light; 2: it is much easier to package as mechanical contact can
be made anywhere on the device without risk of optical performance penalty; 3: the Numerical Aperture of the light can
be increased beyond the limits imposed by polymer coatings.
The presented pump combiners are especially well suited for high power fiber lasers, since such combiners can be
spliced directly onto the active fiber, thereby enabling a robust, stabile laser solution with excellent efficiency and beam
Fiber lasers deliver excellent beam-quality and high efficiency in a robust and largely maintenance-free format, and are now able to do so with output powers in the kilowatt regime. Consequently, fiber lasers have become an attractive alternative to solid-state and gas lasers for e.g. material processing like welding, cutting and marking.
The all-glass air-clad photonic crystal fibers (PCFs) combine large mode-field diameters (currently up to 40 μm), high numerical aperture (typically in the 0.6-0.65 range), high pump absorption (30 dB/m demonstrated in ytterbium) and excellent high-power handling (kW CW and mJ pulses demonstrated). These properties have made this fiber type one of the most promising candidates for the future high-power fiber laser and amplifier systems that are expected to replace many of the traditional systems in use today.
To utilize the high numerical aperture and large mode-field diameters of the air-clad PCFs, special care must be taken in the system integration. In this paper, we will show examples of how these fibers can be integrated in laser and amplifier sub-assemblies with standard fiber pump-interfaces for use with single-emitter diodes or diode-bar pump sources. Moreover, we report on the most recent advances in fiber design including rod-type fibers and broadband polarizing ytterbium-doped large-mode-area air-clad fibers. Finally, we will review the latest results on PCF-based amplifier and laser configurations with special focus on high-power CW systems and high-energy pulsed configurations.
We report on the latest development within active photonic crystal fibers for high power lasers and amplifiers with special focus on how the fibers can be improved with both polarization-maintaining and polarizing properties. We describe rod-type fibers for which a record-high power extraction of 250W/m is achieved. Moreover, we describe how active characterization is used to optimize fibers for laser and amplifier sub-assemblies with respect to beam quality, efficiency and robustness. Finally, we illustrate how the fibers can be integrated with high NA tapers and passive air-clad fibers containing Bragg grating to form an all-fiber, alignment-free, high-power fiber laser subassembly.
Air-clad photonic crystal fibers hold promise to bring the single mode power levels past the 1kW limit through the utilization of extremely high numerical apertures, large mode field diameters and short fiber lengths. Here we discuss design, fabrication and handling issues of such fibers.
We outline physical models and simulations for suppression of self-focusing and filamentation in large aperture semiconductor lasers. The principal technical objective is to generate multi-watt CW or quasi-CW outputs with nearly diffraction limited beams, suitable for long distance free space transmission, focusing to small spots or coupling to single-mode optical fibers. The principal strategies are (1) optimization of facet damage thresholds, (2) reduction of the linewidth enhancement factor which acts as the principal nonlinear optical coefficient, and (3) design of laterally profiled propagation structures in lasers and amplifiers which suppress lateral reflections.
Microcavity semiconductor lasers are interesting for fundamentals of controlled spontaneous emission, thresholdless lasing, quantum physical interactions between photons, excitons and cavity polaritons. They also promise novel engineering applications such as low noise oscillators, filters and optoelectronic switches. Microdisk lasers which support whispering gallery modes are especially useful in that they provide 2D wavelength scale confinement. Novel InGaAsP/InP microdisks supported on glass substrates have recently been fabricated in our laboratories and have been pumped optically, resulting in the first CW room temperature lasing in these devices.
Monolithically integrated flared amplifier master oscillator power amplifier (MFA-MOPA) semiconductor lasers are studied theoretically using a high resolution computational model which resolved times and longitudinal and transverse space dependencies and includes Lorentzian gain and dispersion spectra. The simulations show that, by altering the linear flare of the power amplifier into a nonlinear, trumpet- shaped flare, the dynamic stability range of the MOPA is increased by a factor of 3. This enables the MOPA to maintain a stable, nearly diffraction limited output beam for higher currents before the onset of transverse instabilities, large beam divergence and facet damage due to filamentation. Thus the MOPA will be able to emit an output beam of significantly higher power and brightness.