We overview recent advances in visible single- and double-clad fluoride fiber lasers pumped by blue GaN laser diodes. The spectroscopic properties of ZBLAN glasses doped with Pr3+, Ho3+ and Dy3+ ions are revised. Power scalable efficient continuous-wave visible fluoride fiber lasers emitting in the green, yellow, red and deep-red spectral ranges are presented. Pumped by a single-emitter 6-W 443-nm GaN laser diode, a continuous-wave red double-clad Pr:ZBLAN fiber laser delivered 1.51 W at 634.5 nm with a slope efficiency of 31.0%, a laser threshold of 0.63 W and a spatially single-mode output (M2 ~1.02). Employing a high-power fiber-coupled laser module, power scalability up to 4.61 W was achieved at the expense of a lower slope efficiency of 22.8% and an increased laser threshold of 1.74 W. Green Ho:ZBLAN (543 nm) and yellow Dy:ZBLAN (575 nm) fiber lasers with high-brightness core pumping at 450 nm are also reported delivering 100 mW-level output with slope efficiencies of 31.2% and 19.6%, respectively, operating on the fundamental mode. A numerical model to predict the visible laser performance is presented and guidelines for further engineering of visible fiber laser sources are given.
Holmium ions (Ho3+) are attractive for generation of green emission according to the 5F4+ 5S2 → 5 I8 transition. We report on the assessment of the potential of Ho3+ -doped fluoride glasses for green fiber lasers exploiting the double-clad fiber geometry. ZBLAN glasses doped with HoF3 (0.1 – 0.9 mol%) were studied. The absorption cross-section for the 5 I8 → 5F1+ 5G6 transition is 1.75×10-20 cm2 at 448.5 nm. Under excitation in the blue, the glasses exhibit intense green luminescence. The stimulated-emission cross-section for the 5F4+ 5S2 → 5 I8 transition is 0.67×10-20 cm2 at 549 nm. With increasing the HoF3 doping level, the luminescence lifetime of the 5F4+ 5S2 states slowly decreases in the range of 291 – 180 μs. The output performance of a diode-pumped green Ho fiber laser was simulated for a fiber geometry with a double D-shaped inner cladding. The variable parameters were the HoF3 doping level, the fiber length, and the output coupling. It is shown that the generation of watt-level green output from such a laser is possible when using relatively short (<1 m) fibers with low doping levels (about 0.5 mol% HoF3), as well as high transmissions of the output coupler (<60%, depending on the passive losses in the fiber).
Many applications such as high power laser locking and seeding, atom cooling and trapping or optical heterodyning and coherent communication, require a single-frequency emission with high frequency stability and low noise. Narrow linewidth single-frequency emission at 1.064µm is well known and usually monolithic ring cavities (such as Non Planar Ring Oscillators) is a good solution. In this paper, we demonstrate a stable single-frequency emission at 1.064μm with noise reduction at a power of 500mW based on a linear monolithic cavity with Nd:YAG amplifier. In the past, we have already demonstrated the efficiency and the reliability of monolithic cavities used in our standard product line (LaserBoxx LCX-532S, LCX-553S and LCX-561S). In this presentation, single-frequency operation is achieved by a double Lyot filter, the first filter selecting the emission band of Nd:YAG and the second filter selecting the longitudinal mode and achieves the single-frequency operation. We report narrow linewidth <100kHz. By introducing a nonlinear crystal (KTP for second harmonic generation (SHG) at 532nm), we modify the laser dynamics and reduce the oscillation relaxation and consequently the laser noise (<0.2% RMS). We also report a good laser frequency stability due to our monolithic cavity (<100 MHz for short-term behavior and <1pm for long-term behavior), a frequency tuning capabilities (up to 10pm without mode hopping and up to 1nm with mode hopping), a power stability less than 2% for a laser base temperature from 15°C to 45°C at 500mW, a good optical efficiency (<25%) and a high beam quality (M2<1.2).
Narrow linewidth laser diodes (LDs) emitting in the near-UV (NUV) are gaining attention for applications ranging from spectroscopy to atom cooling and interferometry or other applications requiring high spectral purity. InGaN edge-emitting LDs can exhibit a power of hundreds of mW in an unstable multimode regime detrimental to aforementioned uses. In this paper we report on a compact and robust design based on a low-cost blue LD, a beam shaping optical system and a fiber Bragg grating (FBG) acting as a wavelength selective reflector. One longitudinal mode of the non-antireflection coated laser diode is selected by a close to 30 pm bandwidth FBG allowing a few mW output power around 400 nm and a sidemode- suppression-ratio approaching 50 dB exceeding our last published results. Our previous studies showed that a single-frequency regime with a sub-MHz integrated linewidth and an estimated intrinsic linewidth of 16 kHz was possible by a carefully engineered external cavity. We will study the influence of the cavity length with different fiber types (SM or PM). Assessment will focus on the linewidth and a detailed intensity and frequency noise analysis of the emission. We will also investigate for the first-time the stability of several types of UV-FBG submitted to tens of mW of 400 nm light guided into the fiber core. This work demonstrates state-of-the-art performances by connecting low-cost components and opens the way to the fabrication of highly coherent laser sources that could meet the markets for the NUV applications.
Visible 561 nm and 532 nm laser emissions from 14-mm long DPSS monolithic cavities are frequency converted to deep
UV 280 nm and 266 nm in 16-mm long monolithic external cavities. Wavelength conversion is fully insensitive to
mechanical vibrations and the whole UV laser sources fit in a miniaturized housing. More than 20 mW deep UV laser
emission is demonstrated with high power stability, low noise and good beam quality. Aging tests are in progress but
long lifetimes are expected thanks to the cavity design. Protein detection and deep UV resonant Raman spectroscopy are
applications that could benefit from these laser sources.
Diode lasers have been demonstrated to operate over a great part of the visible spectrum: InGaN diodes cover the violet-blue-
green part (<530 nm) and InGaAlP diodes cover the red part (>635 nm). Some fluorophorus in biotechnology
applications are excited by intermediate wavelengths, from 540 to 630 nm. Optically pumped InGaAs lasers were
demonstrated from 460 nm up to 580 nm. Standard frequency doubled diode pumped solid state (DPSS) lasers lack of
suitable transition to cover the 565-650nm region. It is possible to modify the semiconductor composition to extend the
frequency range or to frequency mix DPSS laser wavelengths, but it comes either with a significant R&D effort or with a
complexity in the design.
Raman scattering can red-shift the strong transitions of Nd or Yb lasers so that many wavelengths lying in the 1080-1300
nm range can be achieved. Recently several CW diode pumped Raman lasers were demonstrated, some of them
including intra-cavity frequency doubling or mixing. The problems with these Raman lasers are the high pump threshold
and the high noise. Based on monolithic cavities, we have built several visible Raman lasers with a reduced loss
presenting a low pump threshold (<1W) and a high slope efficiency. Output powers in excess of 100 mW were achieved
at 588 nm with a 2.5W 808 nm pump. Laser emissions from 556 nm up to more than 610 nm were demonstrated. Noise
of these lasers was analyzed and means to reach low noise operation will be discussed at the conference.
Single longitudinal mode visible DPSS are more and more used in devices where performance can be affected by short
term (minutes) frequency drifts and hops. Long exposure holography and Raman spectrometry are applications requiring
high frequency stability. Resonant external cavity frequency doubling (to generate CW deep UV) and pumping doubly
resonant OPOs may be even more demanding applications in terms of frequency stability. Mechanical vibrations and
thermal fluctuations are usual sources of short term frequency variations or instabilities. Monolithic ring cavities (such as
NPRO) are known to solve this problem but they are quite expensive to manufacture. We will show that much simpler
linear monolithic cavities used in our standard product line (SLIM-532, SLIM-550, SLIM-561 and SLIM-660) present
best of class frequency stabilities compatible with the most demanding applications. Frequency tuning capabilities will
be discussed and could be used in an active stabilization of the laser.
Some applications can benefit from long term wavelength stability as well. Raman spectra can be monitored without
control of the pump wavelength if the long term stability is good. In addition, narrow filters can be used to measure small
Stokes shifts. We are monitoring several monolithic laser sources. After more than 6000 hours of operations, the
wavelength shift is within 1 pm. The laser source has been restarted more than 1000 times without any change of the
operating wavelength. Finally, thermal cycles do not impact the wavelength.
In conclusion, we demonstrate that monolithic linear cavities are best suited for all applications requiring wavelength
stability.
Since the first introduction of DPSS lasers at 561 nm in 2004, the power level required by some biotechnical applications
has always increased. Oxxius has contributed to fulfill the demand thanks to the introduction of the SLIM-561 100mW
in 2008 and of the SLIM-561 200mW and 300mW in 2009. More recently, new dyes or nano-dots have required shorter
wavelengths (such as 553 nm) and new applications such as Laser Doppler Velocimetry requiring both high power and
single frequency operation have appeared.
In this presentation, we demonstrate how to further increase the power. 553nm and 561 nm emission are obtained by
frequency doubling the 1106 nm and 1123 nm lines of Nd:YAG. The latter transitions are significantly weaker than the
1064 nm line. As a consequence, any loss in the cavity significantly increases the laser threshold. Because of the perfect
alignment of the crystal interfaces and the low divergence of the intracavity beam, monolithic cavities demonstrate
significantly reduced round-trip losses compared to standard cavities. Consequently, laser threshold can be dramatically
reduced and the nonlinear loss, responsible for the visible emission, can easily dominate the linear losses. We have taken
standard monolithic cavities of our commercial SLIM-561 products and have increased the pumping power up to 2.8 W.
Yellow and Yellow-green powers have not shown any sign of saturation and 0.5 W could be achieved at both
wavelengths. This results in a record 18% pump to signal optical efficiency. We have checked that the emission
remained single frequency whatever the pumping power.
We demonstrated for the first time three-level operation at 981 nm with a Yb:KYW crystal inserted into the cavity of a
diode pumped Nd:YVO4 laser operating at 914 nm. We achieved an output power up to 1.4 W at 981 nm. Moreover we
demonstrated that crystal heating favored laser emission at 981 nm rather than emission at higher wavelength.
We present what is, to the best of our knowledge, the first experiment of intracavity pumping at 914 nm of an
Yb:SFAP crystal emitting at 985 nm on the three-level laser transition. This configuration enabled us to indirectly
diode-pump this ytterbium doped crystal, and to obtain 1.4 W output power at 985 nm for 20 W of incident pump
power at 808 nm. Intracavity second harmonic generation has also been demonstrated in a KNbO3 crystal with a
total of 120 mW linearly polarized output power at 492.5 nm on two output beams.
Low noise CW UV lasers are needed for applications in bioinstrumentation (cell sorting, cytometry...) and
semiconductors (wafer inspection, micromachining ...). We have recently demonstrated that such laser sources
can be obtained with diode pumped solid state (DPSS) architectures. One key to success was the quasimonolithic
structure of the laser. The advantages of quasi-monolithic DPSS lasers for UV generation are
simplicity of design, compactness, efficiency and thus low power requirements and limited heating.
In this paper, we present for the first time a long term characterization of the diode pumped CW 355 nm laser.
The interplay between pump absorption, cavity length, 1064 mode size, walk-off angle, acceptance angle has
been optimized. In our experiments, the temperature of each element of the laser was controlled and UV power,
noise and spectra were monitored versus these temperatures. At 2.5 W pump power, low noise UV power
beyond 30 mW was measured on most samples built. At a reduced pump power of 1.65 W, all lasers were
operating at 10 to 15 mW and could be maintained at 10 mW over days. The noise level remained below the 1%
rms level. More long-term measurements will be presented at the conference.
Low noise lasers in the UV spectrum (355 nm) are important for many applications like spectroscopy, confocal microscopy, flow cytometry, cell sorting, CD mastering, semiconductor inspection and reprographics. The UV CW laser sources currently commercially available, are restricted to excimers lasers, gas ions lasers which suffer from low power efficiency (0.01% or less), high electrical consumption and bulky dimensions. These products, which are expensive, are still waiting for practical replacements. To our best knowledge, we report in this paper the first efficient low noise diode-pumped neodymium doped solid-state laser operating at 355 nm by intra-cavity third harmonic generation (THG). The fundamental infrared laser light is generated by a diode-pumped Nd:YVO4 crystal optically contacted with others components of the laser cavity. Intra-cavity SHG and THG are achieved with a KTP and LBO respectively. Several configurations have led to low noise 355 nm single-frequency operation at a power exceeding 10 mW. We believe that this power can still be improved.
Low noise CW milliWatt scale UV lasers are needed for many analysis applications in the semiconductor and the
biological fields. Intracavity tripling has been widely used to improve the UV output power of Q-switched or
modelocked lasers, but no efficient diode-pumped CW UV laser was ever reported. One of the key to success is the use
of a monolithic laser structure which both eliminates the birefringence interference issue and facilitates the single
frequency operation. The monolithic structure is obtained by optically contacting crystals. It does not require any
alignment, reduces the manufacturing cost and improves reliability. The optimization of the amplifying medium and
doubling and tripling crystals involves as many parameters as pump absorption, thermal lens, cavity length, 1064 nm
mode size, walk-off, acceptance angles, polarizations, phases... The interplay between these parameters will be
discussed. Finally, several amplifying media (Nd:YAG and Nd:YVO4), doubling crystals (KTP, KNbO3, BBO, BiBO
and LBO) and tripling crystals (BBO, BiBO, LBO) were tested. With a 2.4W 808 nm diode pump, several
configurations have led to low noise 355 nm single frequency operation exceeding 5 mW. We believe that this power
can still be improved.
Diode pumped frequency doubled Nd :YAG microchip lasers should become an alternative to air-cooled argon ion lasers. The main issues to be solved are the long-term stabilization of the single frequency operation and the power control of the multi-frequency operation. These questions are mostly related to the laser dynamics. In this paper, we present an accurate modelling of the laser dynamics, including quenching processes, non uniform pumping, partial overlap of optical signals and excited-state populations, hole burning and type-I frequency doubling. We theoretically predict that even in multimode operation, only one mode is oscillating at a time, with a mode hoping at a rate of about 50 kHz. This behavior, quite different from the well known dynamics of intra-cavity type-II frequency doubled lasers (green noise) is experimentally confirmed. Diode pumped frequency doubled Nd:YAG 473 nm lasers based on a simple linear cavity are built and exhibit high output power (90+ mW) and record slope efficiency (45% with respect to absorbed power). The understanding of its 2-mode operation allowed us to stabilize the average output power. A similar laser was operated on a single frequency. No wavelength drift could be measured and no mode hopping was observed over 24 hours.
Although powerful (2.8 W) blue emission at 473 nm was recently reported, the design of low-cost compact and efficient 473 nm source failed so far. We have theoretically and experimentally analyzed the operation of a simple 2.1 mm long microchip assembly pumped by a 100x1 μm laser diode and found several ways to improve the efficiency of such a laser. As a first result, we achieved high beam quality (M2<1.2) 40 mW CW operation @ 473 nm with less than 900 mW launched power (5% efficiency) and small low-frequency (10Hz-20 kHz) noise. Multi-frequency operation however led to high-frequency noise (green problem). To our best knowledge, this is the highest efficiency obtained with a linear cavity. In this paper, we model and measure thermal lensing, pump, 946 nm and 473 nm beam size and power evolution within the MCA. Nd concentration was left unoptimized in previously reported experiments to its standard value (1.1%). Optimum Nd concentration is shown to generally be greater than 1% and is compatible with efficient low-noise single-frequency operation.
Dispersion-managed solitons exhibit superior transmission quality as soon as Kerr nonlinearity in non negligible. Long transmission distances, large span loss and high spectral efficiency are expected for the next generation of transmission systems based on 20 Gbit/s and 40 Gbit/s data rate.
A new kind of instrumentation based on Optical Fiber Bragg grating sensors is proposed for the main relevant monitoring needs in the electric power industry. The challenging metrological properties of these components are presented and their good resistance to (gamma) -ray irradiations experimentally proved.
Photoinscriptions of Bragg gratings have been carried out within several germanosilicate fibers via UV side writing near 243 nm. Real-time measurements of the spectral transmission, of the FWHM bandwidth and of the resonance wavelengths of the Bragg gratings lead to noteworthy observations when long irradiation times are used. Thus, second order diffraction efficiency has been observed from Bragg gratings written within some fibers. Second order diffraction can be detected once the first order grating reflectivity has begun to saturate. Whereas the dynamics of the UV induced excess losses may vary from one sample to another, no obvious direct correlation can be established between the dynamical evolutions of the loss spectra and the features of the grating growth.
The spectra of Nd3+ and Pr3+ doped fiber lasers utilizing two intracore Bragg grating reflectors written at 1084 nm Bragg wavelength have been investigated under 30 MHz resolution. A spectral vernier effect was induced by the subcavities created by reflections from the fiber end faces and from Bragg gratings. We made use of this effect to enhance the sensitivity of the measurement of the fiber temperature rise which arises from absorption of the pump power. Fresnel reflection was also used to reduce the linewidth of the laser emission.
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