Rare-earth-doped solid-state lasers utilize distributed-feedback (DFB) resonators to generate ultra-narrow-linewidth emission down to a few kHz. The longitudinal modes of a single Fabry-Pérot and a DFB resonator are influenced by the structure, the optical excitation configuration, and external feedback, as has been observed experimentally. Here we investigate the influence of bi-directional launching of light and external feedback on the longitudinal modes of single Fabry-Pérot, double Fabry-Pérot, and DFB resonators. These results can be exploited for the technological advancement of DFB lasers.
A simple recursive method based on the circulating field approach to obtain the exact electric-field and intensity distributions in an arbitrary multi-resonator structure is presented. Reflectivity curves obtained via this method and the coupled-mode theory are compared.
SrAl2O4 that is optically activated by Eu2+, often additionally co-doped with Dy3+, is a non-radioactive persistent phosphor which is known for its excellent afterglow properties. It has found various applications, e.g. in the watch industry, for security signs, in medical diagnostics, and in photovoltaics. The monoclinic SrAl2O4 was synthesized in polycrystalline form and structurally characterized. Its luminescence and afterglow properties were studied. Wavelength-dependent thermoluminescence experiments were performed on SrAl2O4:Eu and SrAl2O4:Eu,Dy polycrystalline samples. Substitution of Sr2+ by Eu2+ on two different Sr sites in the crystal is associated with blue and green Eu2+ emission. Excitation at 445 nm allows to selectively excite one of the two different Eu2+ ions, whereas excitation at 375 nm excites both Eu2+ ions. Incorporation of dysprosium increases significantly (by a factor of about 4 to 8) the total number of traps involved in the afterglow of this persistent phosphor. Increasing the temperature at which the samples are irradiated (loaded) from 173 K to 248 K reveals that many new traps can only be occupied or activated at higher temperatures, leading to a strong increase of the integrated thermoluminescence intensity, in particular for the Dy-codoped samples. The results of this study reveal that the diversity of traps leading to the long afterglow is much larger than previously reported in the literature. We propose that the presence of dysprosium induces an excitation-induced charge-transfer reaction Eu2+ + Dy3+ → Eu3+ + Dy2+. However, the principal traps responsible for the efficient afterglow are temperature-activated and appear to be associated with the green-emitting Eu2+ ion on the Sr2 site coupled to a nearby dysprosium ion.
Bragg-grating-based distributed-feedback waveguide resonators, with a discrete phase shift introduced inside the Bragg grating, exhibit within their grating reflection band a Lorentzian-shaped resonance line with an ultranarrow linewidth. If the phase shift is π/2, the resonance is located at the center of the reflection band, i.e., at the Bragg wavelength, where the grating reflectivity is maximum, hence the resonance linewidth is minimum. Alternatively, the required π/2 phase shift is often introduced by a distributed change in effective refractive index, e.g. by adiabatically widening the waveguide. Despite careful design and fabrication, the experimentally observed resonance wavelength deviates from the designed one. Besides deviations owing to fabrication errors, a fundamental, systematic shift towards shorter wavelengths occurs. We show theoretically and experimentally that the decay of light intensity during propagation from the phase-shift center into both sides of the Bragg grating due to (i) reflection by the periodic grating and (ii) the adiabatic refractive-index change causes an incomplete accumulation of designed phase shift by the oscillating light, thereby systematically shifting the resonance to a shorter wavelength. Calculations are performed based on the characteristic-matrix approach. Experimental studies are carried out in distributed-feedback channel-waveguide resonators in an amorphous aluminum oxide thin film on silicon with a distributed phase shift introduced by adiabatic widening of the waveguide according to a sin2 function. Calculations and experiments show good agreement. Considering in the design the overlap integral between distributed phase shift and light intensity provides a performance that is much closer to the desired value.
In the literature one finds several conflicting accounts of the phase difference of stimulated and spontaneous emission, as well as absorption, with respect to an existing (triggering) electromagnetic field. One of these approaches proposes that stimulated emission and absorption occur in phase and out of phase with their driving field, respectively, whereas spontaneous emission occurs under an arbitrary phase difference with respect to an existing field. It has served as a basis for explaining quantum-mechanically the laser linewidth, its narrowing by a factor of 2 around the laser threshold, as well as its broadening due to amplitude-phase coupling, resulting in Henry’s α-factor. Assuming the validity of Maxwell’s equations, all three processes would, thus, violate the law of energy conservation. In semi-classical approaches, we investigate stimulated emission in a Fabry-Pérot resonator, analyze the Lorentz oscillator model, apply the Kramers-Kronig relations to the complex susceptibility, understand the summation of quantized electric fields, and quantitatively interpret emission and absorption in the amplitude-phase diagram. In all cases, we derive that the phase of stimulated emission is 90o in lead of the driving field, and the phase of absorption lags 90o behind the transmitted field. Also spontaneous emission must obey energy conservation, hence it occurs with 90o phase in lead of an existing field. These semi-classical findings agree with recent experimental investigations regarding the interaction of attosecond pulses with an atom, thereby questioning the physical explanation of the laser linewidth and its narrowing or broadening.
Active media with high rare-earth concentrations are essential for small-footprint waveguide amplifiers. When operating at high population inversion, such devices are often affected by undesired energy-transfer processes and thermal effects. In this work, we study a 32-μm-thick epitaxial layer of potassium gadolinium ytterbium double tungstate with a high Yb content of 57at.%, representing an Yb3+ concentration of ~3.8 × 1021 per cubic centimeter, grown onto an un-doped KY(WO4)2 substrate. The pump absorption, luminescence decay, and small-signal gain are investigated under intense pumping conditions. Spectroscopic signatures of an energy-transfer process and of quenched ions, as well as thermal effects are observed. We present a gain model which takes into account excessive heat generated due to the abovementioned experimental observations. Based on finite-element calculations, we find that the net gain is significantly reduced due to, firstly, a fraction of Yb3+ ions not contributing to stimulated emission, secondly, a reduction of population inversion owing to a parasitic energy-transfer process and, thirdly, degradation of the effective transition cross-sections owing to device heating. Nevertheless, a signal enhancement of 8.1 dB was measured from the sample at 981 nm wavelength when pumping at 932 nm. The corresponding signal net gain of ~800 dB/cm, which was achieved without thermal management, is promising for waveguide amplifier operating without active cooling.
Distributed-feedback waveguide lasers based on Bragg-grating resonators generate ultranarrow-linewidth emission. Oscillation at the center of the reflection band ensures maximum reflectivity, hence minimum laser linewidth. The required μ/2 phase shift is often introduced by a distributed change in effective refractive index, e.g. by adiabatically widening the waveguide. Despite careful design and fabrication, the experimentally observed resonance wavelength deviates, thereby placing the resonance and laser emission at a position with lower reflectivity inside the reflection band. This effect is usually incorrectly attributed to fabrication errors. Here we show theoretically and experimentally that the decay of light intensity during propagation from the phase-shift center into both sides of the Bragg grating due to (i) reflection by the periodic grating and (ii) the adiabatic refractive-index change causes an incomplete accumulation of designed phase shift, thereby systematically shifting the resonance to a shorter wavelength. Calculations are performed based on the characteristic-matrix approach. Experimental studies are carried out in a distributed-feedback channel-waveguide resonator in amorphous Al2O3 on silicon with a distributed phase shift introduced by adiabatic widening of the waveguide according to a sin2 function. Calculations and experiments show good agreement. Considering in the design the overlap integral between distributed phase shift and light intensity provides the desired performance.
The spectral response of a distributed-feedback resonator with a thermal chirp is investigated. An Al2O3 channel waveguide with a surface Bragg grating inscribed into its SiO2 top cladding is studied. A linear temperature gradient along the resonator leads to a corresponding variation of the grating period. We characterize its spectral response with respect to wavelength and linewidth changes of the resonance peak. Simulated results show good agreement with the experimental data, indicating that the resonance wavelength is determined by the total accumulated phase shift. The calculated grating reflectivities at the resonance wavelength largely explain the observed changes of the resonance linewidth. This agreement demonstrates that the linewidth increase is caused by the increase of resonator outcoupling losses.
Energy-transfer processes strongly affect the performance of lanthanide-doped photonic devices. In this work, we introduce a simple stochastic model of energy-transfer processes and successfully apply it to the example of crossrelaxation (CR) and energy-transfer upconversion (ETU) in amorphous Al2O3:Tm3+ waveguides on silicon intended for lasers operating at ~2 μm. The stochastic model is based on the rate-equation formalism and considers two spectroscopically distinct ion classes, namely single ions and ions with neighbours (pairs and clusters), with the corresponding ion fractions being dependent on the doping concentration. We prove that a more accurate description of the luminescence properties of amorphous Al2O3:Tm3+ is obtained when accounting for the presence of these distinct ion classes. Based on the developed model, we derive microscopic CR and ETU parameters of CCR = 5.83×10-38 cm6 s -1 , CETU1 = 0.93×10-40 cm6 s -1 , and CETU2 = 7.81×10-40 cm6 s -1 , and determine the laser quantum efficiency ηq of excitation of Tm3+ ions in the upper laser level. For the maximum Tm3+ concentration of 5.0×1020 cm-3 studied experimentally in this investigation, ηq reaches 1.73. Furthermore, the transition cross-sections at the pump and laser wavelengths are determined. For the 3H6 → 3F4 transition, the maximum stimulated-emission cross-section is σe = 0.47 × 10-20 cm2 at 1808 nm.
We thoroughly investigate the Fabry-Pérot resonator, avoid approximations, and derive its generic Airy distribution, equaling the internal resonance enhancement, and all related Airy distributions, such as the commonly known transmission. We verify that the sum of the mode profiles of all longitudinal modes is the fundamental physical function characterizing the Fabry-Pérot resonator and generating the Airy distributions. We investigate the influence of frequency-dependent mirror reflectivities on the mode profiles and the resulting Airy distributions. The mode profiles then deviate from simple Lorentzian lines and exhibit peaks that are not located at resonant frequencies. Our simple, yet accurate analysis greatly facilitates the characterization of Fabry-Pérot resonators with strongly frequency-dependent mirror reflectivities.
Distributed-feedback (DFB) laser resonators are widely recognized for their advantage of generating laser emission with extremely narrow linewidth. Our investigation concerns ytterbium-doped amorphous Al2O3 channel waveguides with a corrugated homogeneous Bragg grating inscribed into its SiO2 top cladding, in which a λ/4 phase-shift provides a resonance and allows for laser emission with a linewidth as narrow as a few kHz. Pump absorption imposes a thermal chirp of the grating period, which has implications for the spectral characteristics of the resonator. Thermal effects on the spectral response of a DFB passive resonator were investigated via simulations using Coupled Mode Theory by considering (i) a constant deviation of the grating period or (ii) a chirp with a linear profile. We report an increase of the resonance linewidth up to 15%. This result is due to two factors, namely changes of the grating reflectivity at the resonance frequency up to 2.4% and of the shift of resonance frequency up to 61 pm due to an accumulated phase shift imposed on the grating by the chirp profile. The linewidth decrease due to gain is on the order of 106, which is a much larger value. Nevertheless, according to the Schawlow-Townes equation the linewidth increase of the passive resonator due to a thermal chirp quadratically increases the laser linewidth.
We report the spectroscopy of crystalline waveguide amplifiers operating in the telecom C-band. Thin films of erbiumdoped gadolinium lutetium potassium double tungstate, KGdxLuyEr1-x-y (WO4)2, are grown by liquid- phase epitaxy onto undoped potassium yttrium double tungstate (KYW) substrates and micro-structured by Ar+- beam etching. Channel waveguides with erbium concentrations between 0.45–6.35 × 1020 cm-3 are characterized. The transition cross-sections of interest are estimated. The effect of energy-transfer up-conversion (ETU) is experimentally investigated. Microscopic and macroscopic ETU parameters are extracted from a simultaneous analysis of 20 decay curves of luminescence on the transition 4I13/2 → 4I13/2. The correlation between ETU and the doping concentration is studied. Pump excited-state absorption (ESA) on the transition 4I11/2 → 4F7/2 is investigated via a direct ESA measurement using a double-modulation pump-probe technique. The effect of ESA is studied for different pump wavelengths. The pump wavelength of 984.5 nm is found to be favorable for the complete range of erbium concentrations.
Ytterbium-doped potassium rare-earth double tungstate thin films are excellent candidates for highly efficient waveguide lasers, as well as high-gain waveguide amplifiers, with a record-high optical gain per unit length of 935 dB/cm recently demonstrated. However, the spectroscopic properties of these highly ytterbium-doped thin films and, in particular, their temperature dependence are not well investigated. These characteristics are required for the understanding of the behavior of the fabricated optical devices and crucial for further device optimization. We experimentally determined the absorption cross-sections for a potassium ytterbium gadolinium double tungstate, KYb0.57Gd0.43(WO4)2, thin film grown lattice matched onto an undoped KY(WO4)2 substrate. At room temperature, the peak cross-section value at 981 nm and the overall absorption spectrum are very similar to those of Yb-doped bulk potassium double tungstate crystals, although Yb is now the dominating rare-earth content. The temperature-dependent study shows a significant decrease of the absorption cross-section values at 933 nm and 981 nm with increasing temperature. We verify theoretically that this is due to the temperature dependence of fractional populations in the individual Stark levels of the absorbing crystal-field multiplet, in combination with the linewidth broadening with increasing temperature. Further investigations suggest that the broadening of absorption linewidth at 981 nm originates in the intra-manifold relaxation between the two lowest Stark levels of the ground state. Finally, the implications of the spectroscopic findings on the operating characteristics of waveguide amplifiers are investigated. Amplifiers operating at 80 °C are expected to exhibit only 67% of the maximum theoretical gain at room temperature.
Rare-earth-ion-doped materials are of high interest as amplifiers and lasers in integrated optics. Their longer excited-state lifetimes and the weaker refractive-index change accompanied with rare-earth-ion excitation compared to electron-hole pairs in III-V semiconductors provide spatially and temporally stable optical gain, allowing for high-speed amplification and narrow-linewidth lasers. Amorphous Al2O3 deposited onto thermally oxidized silicon wafers offers the advantage of integration with silicon photonics and electronics. Layer deposition by RF reactive co-sputtering and micro-structuring by chlorine-based reactive-ion etching provide low-loss channel waveguides. With erbium doping, we improved the gain to 2 dB/cm at 1533 nm and a gain bandwidth of 80 nm. The gain is limited by migration-accelerated energy-transfer upconversion and a fast quenching process. Since stimulated emission is even faster than this quenching process, lasers are only affected in terms of their threshold, allowing us to demonstrate diode-pumped micro-ring, distributed-feedback (DFB), and distributed-Bragg-reflector (DBR) lasers in Al2O3:Er3+ and Al2O3:Yb3+ on a silicon chip. Surface-relief Bragg gratings were patterned by laser-interference lithography. Monolithic DFB and DBR cavities with Q-factors of 1.35×106 were realized. In an Er-doped DFB laser, single-longitudinal-mode operation at 1545 nm was achieved with a linewidth of 1.7 kHz, corresponding to a laser Q-factor of 1.14×1011. Yb-doped DFB and DBR lasers were demonstrated at 1020 nm with output powers of 55 mW and a slope efficiency of 67% versus launched pump power. A dual-phaseshift, dual-wavelength laser was achieved and a stable microwave signal at ~15 GHz was created via the heterodyne photo-detection of the two laser wavelengths.
DNA sequencing in a lab-on-a-chip aims at providing cheap, high-speed analysis of low reagent volumes to, e.g., identify genomic deletions or insertions associated with genetic illnesses. Detecting single base-pair insertions/deletions from DNA fragments in the diagnostically relevant range of 150−1000 base-pairs requires a sizing accuracy of S < 10-3. Here we demonstrate S = 4×10-4. A microfluidic chip was post-processed by femtosecond-laser writing of an optical waveguide. 12 blue-labeled and 23 red-labeled DNA fragments were separated in size by capillary electrophoresis, each set excited by either of two lasers power-modulated at different frequencies, their fluorescence detected by a photomultiplier, and blue/red signals distinguished by Fourier analysis. Different calibration strategies were tested: a) use either set of DNA molecules as reference to calibrate the set-up and identify the base-pair sizes of the other set in the same flow experiment, thereby eliminating variations in temperature, wall-coating and sieving-gel conditions, and actuation voltages; b) use the same molecular set as reference and sample with the same fluorescence label, flown in consecutive experiments; c) perform cross-experiments based on different molecular sets with different labels, flown in consecutive experiments. From the results we conclude: Applying quadratic instead of linear fit functions improves the calibration accuracy. Blue-labeled molecules are separated with higher accuracy. The influence of dye label is higher than fluctuations between two experiments. Choosing a single, suitable dye label combined with reference calibration and sample investigation in consecutive experiments results in S = 4×10-4, enabling detection of single base-pair insertion/deletion in a lab-on-a-chip.
Spiral-waveguide amplifiers in erbium-doped amorphous aluminum oxide are fabricated by RF reactive co-sputtering of
1-μm-thick layers onto a thermally-oxidized silicon wafer and chlorine-based reactive ion etching. The samples are
overgrown by a SiO2 cladding. Spirals with several lengths ranging from 13 cm to 42 cm and four different erbium
concentrations between 0.5−3.0×1020 cm-3 are experimentally characterized. A maximum internal net gain of 20 dB in
the small-signal-gain regime is measured at the peak emission wavelength of 1532 nm for two sample configurations
with waveguide lengths of 13 cm and 24 cm and erbium concentrations of 2×1020 cm-3 and 1×1020 cm-3, respectively.
The obtained gain improves previous results by van den Hoven et al. in this host material by a factor of 9. Gain
saturation as a result of increasing signal power is investigated. Positive net gain is measured in the saturated-gain
regime up to ~100 μW of signal power, but extension to the mW regime seems feasible. The experimental results are
compared to a rate-equation model that takes into account migration-accelerated energy-transfer upconversion (ETU)
and a fast quenching process affecting a fraction of the erbium ions. Without these two detrimental processes, several
tens of dB/cm of internal net gain per unit length would be achievable. Whereas ETU limits the gain per unit length to 8
dB/cm, the fast quenching process further reduces it to 2 dB/cm. The fast quenching process strongly deteriorates the
amplifier performance of the Al2O3:Er3+ waveguide amplifiers. This effect is accentuated for concentrations higher than
2×1020 cm-3.
A potassium double tungstate layer with the composition KY0.40Gd0.29Lu0.23Tm0.08(WO4)2 was grown onto a pure KY(WO4)2 substrate by liquid-phase epitaxy, microstructured by standard lithography and Ar-ion etching, and overgrown by a pure KY(WO4)2 layer. The end-facets were polished. Laser experiments were performed on these buried, ridge-type channel waveguides in a resonator with one butt-coupled mirror and Fresnel reflection from the other end-facet, resulting in a high output-coupling degree of 89%, compared to intrinsic round-trip losses of only 2%. By pumping with a Ti:Sapphire laser at 794 nm, 1.6 W of output power at 1.84 μm with a maximum slope efficiency of ~80% was obtained. To the best of our knowledge, this result represents the most efficient 2-μm channel waveguide laser to date. We determined the optimum Tm3+ concentration in double tungstate channel waveguides to be at least 8at.% for efficient lasing. The theoretical limit of the slope efficiency depends on the Stokes efficiency which here is 43.2%, the outcoupling efficiency which here is 99%, and the pump quantum efficiency. The pump quantum efficiency of a 2-μm Tm3+ laser pumped around 800 nm hinges on the efficiency of its cross-relaxation process. By fitting the macroscopic cross-relaxation parameter which linearly depends on the Tm3+ concentration to concentration-dependent luminescence- decay data, calculating the overall decay rate of the pump level, and deriving the concentration-dependent pump quantum efficiency, we obtain a theoretical limit for the slope efficiency of 83% for the chosen Tm3+ concentration. The experimental slope efficiency of ~80% closely approaches this limit.
We extend the existing theory of continuous-wave lasers by systematically considering spontaneous emission. In a
simple rate-equation approach, the laser eigenvalue, defined as the ratio of coherent photons coupled out of the resonator
divided by the number of photons coupled in via spontaneous emission, emerges as the fundamental parameter
describing a continuous-wave laser. We derive a general equation for the laser eigenvalue and confirm the point at which
coherence manifests itself in the resonator. The theory describes all types of lasers of three-level, four-level, or any
intermediate nature.
We report the fabrication and optical characterization of long, spiral-shaped erbium-doped aluminum oxide (Al2O3:Er3+) channel waveguides for achieving high overall signal amplification on a small footprint. Al2O3:Er3+ films with Er3+ concentrations in the range between 0.44−3.1×1020 cm-3 were deposited by reactive co-sputtering onto standard, thermally oxidized silicon substrates. Spiral-shaped waveguides were designed and structured into the films by chlorinebased reactive ion etching. In the current design, each spiral waveguide occupies an area of 1 cm2. Typical background propagation losses near 1500 nm are (0.2±0.1) dB/cm. A commercially available, pigtailed diode laser at 976 nm was employed as the pump source. The erbium-doped waveguide amplifiers were characterized in the small-signal-gain regime at the peak-gain wavelength (λ = 1532 nm) of Al2O3:Er3+. A maximum of 20 dB of internal net gain was measured for a 24.5-cm-long spiral waveguide with an Er3+ concentration of 0.95×1020 cm-3. Similar results were obtained for a shorter spiral with an Er3+ concentration about twice as high. Samples with lower concentration exhibited lower gain because of insufficient pump absorption, while samples with higher concentration showed less gain because of migration-accelerated energy transfer up-conversion and, more importantly, a fast quenching process.
Optical coherence tomography (OCT) has enabled clinical applications that revolutionized in vivo medical diagnostics.
Nevertheless, its current limitations owing to cost, size, complexity, and the need for accurate alignment must be
overcome by radically novel approaches. Exploiting integrated optics, the central components of a spectral-domain OCT
(SD-OCT) system can be integrated on a chip. Arrayed-waveguide grating (AWG) spectrometers with their high spectral
resolution and compactness are excellent candidates for on-chip SD-OCT systems. However, specific design-related
issues of AWG spectrometers limit the performance of on-chip SD-OCT systems. Here we present advanced AWG
designs which could overcome the limitations arising from free spectral range, polarization dependency, and curved
focal plane of the AWG spectrometers. Using these advanced AWG designs in an SD-OCT system can provide not only
better overall performance but also some unique aspects that a commercial system does not have. Additionally, a
partially integrated OCT system comprising an AWG spectrometer and an integrated beam splitter, as well as the in vivo
imaging using this system are demonstrated.
We report on diode-pumped distributed-feedback (DFB) and distributed-Bragg-reflector (DBR) channel waveguide lasers in Er-doped and Yb-doped Al2O3 on standard thermally oxidized silicon substrates. Uniform surface-relief Bragg gratings were patterned by laser-interference lithography and etched into the SiO2 top cladding. The maximum grating reflectivity exceeded 99%. Monolithic DFB and DBR cavities with Q-factors of up to 1.35×106 were realized. The Erdoped DFB laser delivered 3 mW of output power with a slope efficiency of 41% versus absorbed pump power. Singlelongitudinal- mode operation at a wavelength of 1545.2 nm was achieved with an emission line width of 1.70 0.58 kHz, corresponding to a laser Q-factor of 1.14×1011. Yb-doped DFB and DBR lasers were demonstrated at wavelengths near 1020 nm with output powers of 55 mW and a slope efficiency of 67% versus launched pump power. An Yb-doped dualwavelength laser was achieved based on the optical resonances induced by two local phase shifts in the DFB structure. A stable microwave signal at ~15 GHz with a –3-dB width of 9 kHz and a long-term frequency stability of ± 2.5 MHz was created via the heterodyne photo-detection of the two laser wavelengths. By measuring changes in the microwave beat signal as the intra-cavity evanescent laser field interacts with micro-particles on the waveguide surface, we achieved real-time detection and accurate size measurement of single micro-particles with diameters ranging between 1 μm and 20 μm, which represents the typical size of many fungal and bacterial pathogens. A limit of detection of ~500 nm was deduced.
We review our recent results on integrating biomedical optical systems onto a silicon chip. Light collection by integrated
waveguides has been investigated. Confocal light delivery and collection by a combination of two arrayed-waveguide
gratings has been achieved. Using an arrayed-waveguide grating as an integrated spectrometer, Raman spectroscopy and
spectral-domain optical coherence tomography have been demonstrated.
The optical amplifier performance of Nd3+-doped polymer and amorphous Al2O3 channel waveguides with single-mode and multi-mode behavior around 880 nm is compared. Internal net gain in the wavelength range 865-930 nm is
investigated under continuous-wave excitation near 800 nm, for Nd3+ dopant concentrations typically in the range of 0.6-
1.0 × 1020 cm-3. A peak gain of 2.8 dB at 873 nm is obtained in a 1.9-cm-long polymer waveguide at a launched pump
power of 25 mW. The small-signal gain measured in a 1-cm-long sample is 2.0 dB/cm. In Al2O3, a peak gain of 1.57
dB/cm in a short and 3.0 dB in a 4.1-cm-long waveguide is obtained at 880 nm. Tapered multi-mode Nd3+-doped
amplifiers are embedded into an optical backplane and a maximum 0.21 dB net gain is demonstrated in a structure
consisting of an Al2O3:Nd3+ amplifier placed between two passive polymer waveguides on an optical backplane. The
gain can be further enhanced by increasing the pump power and improving the waveguide geometry, and the wavelength
of amplification can be adjusted by doping other rare-earth ions.
Integrated optical probes for detecting backscattered light in, e.g., Raman spectroscopy show desirable characteristics
compared to conventional optical fiber probes, although the latter ones may have better collection efficiency in many
cases. Major advantages of integrated probes include reduced size; reduced background noise due to scattering in the
probe because of reduced propagation length; potential for monolithic integration with filters and spectrometers; very
small collection volume, providing high spatial resolution; and polarization maintenance. We demonstrate that when
scattered light needs to be collected from a thin layer close to the probe surface, integrated probes can have better
collection efficiency than fiber probes do. We modeled a multimode integrated waveguide probe by adapting an
analytical model that had been developed for fiber probes. The model was extended in order to account for arbitrary
waveguide geometries and a low number of discrete waveguide modes compared to the quasi-continuum of modes in a
typical multimode fiber. Using this model we compared the collection efficiencies of integrated and fiber probes for a
thin scattering sample. We found that the integrated probe has a higher collection efficiency for scattering layer thickness
and probe-to-layer distance both smaller than ~100 μm.
Erbium-doped aluminum oxide channel waveguides were fabricated on silicon substrates and their characteristics were
investigated for Er concentrations ranging from 0.27 to 4.2 × 1020 cm-3. Background losses below 0.3 dB/cm at 1320 nm
were measured. For optimum Er concentrations in the range of 1 to 2 × 1020 cm-3, internal net gain was obtained over a
wavelength range of 80 nm (1500-1580 nm) and a peak gain of 2.0 dB/cm was measured at 1533 nm. Integrated
Al2O3:Er3+ channel waveguide ring lasers were realized based on such waveguides. Output powers of up to 9.5 μW and
slope efficiencies of up to 0.11 % were measured. Lasing was observed for a threshold diode-pump power as low as 6.4
mW. Wavelength selection in the range 1530 to 1557 nm was demonstrated by varying the length of the output coupler
from the ring.
Erbium-doped aluminum oxide channel waveguides were fabricated on silicon substrates and their characteristics were
investigated for Er concentrations ranging from 0.27 to 4.2 × 1020 cm-3. Background losses below 0.3 dB/cm at 1320 nm
were measured. For optimum Er concentrations in the range of 1 to 2 × 1020 cm-3, internal net gain was obtained over a
wavelength range of 80 nm (1500-1580 nm) and a peak gain of 2.0 dB/cm was measured at 1533 nm. 170 Gbit/s high-speed
data amplification was demonstrated in an Al2O3:Er3+ channel waveguide with open eye diagrams and without
penalty. A lossless 1×2 power splitter has been realized in Al2O3:Er3+ with net gain over a wavelength range of 40 nm
(1525-1565 nm) across the complete telecom C-band.
We report on the use of femtosecond laser pulses to fabricate photonic devices (waveguides and interferometers) inside
commercial CE chips without affecting the manufacturing procedure of the microfluidic part of the device. The
fabrication of single waveguides intersecting the channels allows one to perform absorption or Laser Induced
Fluorescence (LIF) sensing of the molecules separated inside the microchannels. Microfluidic channels, with access
holes, are fabricated using femtosecond laser irradiation followed by chemical etching. Mach-Zehnder interferometers
are used for label-free sensing of the samples flowing in the microfluidic channels by means of refractive index changes
detection.
This paper reviews our and our colleagues' recent results concerning planar waveguide lasers in KY(WO4)2:Yb3+ near 1
μm and channel waveguide lasers in α-Al2O3:Ti3+ near 800 nm.
Amorphous Al2O3 is a promising host material for active integrated optical applications such as tunable rare-earth-ion-doped
laser and amplifier devices. The fabrication of slab and channel waveguides has been investigated and optimized
by exploiting reactive co-sputtering and ICP reactive ion etching, respectively. The Al2O3 layers are grown reliably and
reproducibly on thermally oxidized Si-wafers at deposition rates of 2-4 nm/min. Optical loss of as-deposited planar
waveguides as low as 0.11±0.05 dB/cm at 1.5-μm wavelength has been demonstrated. The channel waveguide
fabrication is based on BCl3/HBr chemistry in combination with standard photoresist and lithography processes. Upon
process optimization channel waveguides with up to 600-nm etch depth, smooth side walls and optical losses as low as
0.21±0.05 dB/cm have been realized. Rare-earth-ion doping has been investigated by co-sputtering from a metallic Er
target during Al2O3 layer growth. At the relevant dopant levels (~1020 cm-3) lifetimes of the 4I13/2 level as high as 7 ms
have been measured. Gain measurements have been carried out over 6.4-cm propagation length in a 700-nm-thick Er-doped
Al2O3 waveguide. Net optical gain has been obtained over a 35-nm-wide wavelength range (1525-1560 nm) with
a maximum of 4.9 dB.
Fluorinated Epoxy waveguides doped with Nd complexes have been studied for optical amplification applications. The
fluorescent complex was Nd(TTA)3phen (TTA = thenoyltrifluoroacetone, phen = 1, 10-phenanthroline), which was
mixed with the host material 6-FDA (6-fluorinated-dianhydride ). The solution was spin coated in order to obtain
Nd(TTA)3phen-doped 6-FDA/epoxy slab and channel waveguides. The emission spectra of the Nd-complex doped
waveguides were measured at different pump powers by pumping at 800nm, and emission was observed at 890nm,
1060nm and 1330nm. The luminescence lifetime of the Nd complex within the waveguides was experimentally
determined. The results demonstrate that the neodymium ions within the polymer host have good transition properties.
Based on experimentally obtained parameters the optical gain of the Nd-complex doped waveguides was estimated with
the aid of rate equations. The results show that Nd complex doped polymer waveguides are promising gain media for
optical amplification.
Our recent results on planar and channel waveguide fabrication and lasers in the dielectric oxide materials Ti:sapphire
and rare-earth-ion-doped potassium yttrium double tungstate (KYW) are reviewed. We have employed waveguide
fabrication methods such as liquid phase epitaxy and reactive ion etching as well as ion beam implantation and
femtosecond laser writing. We have obtained surface and buried channel waveguide lasers in Ti:sapphire near 800 nm
and planar waveguide lasers in KYW:Yb near 1 μm and KYW:Tm near 2 μm.
Reactively co-sputtered amorphous Al2O3 waveguide layers with low propagation losses have been deposited. In order to define channel waveguides in such Al2O3 films, the etching behaviour of Al2O3 has been investigated using an inductively coupled reactive ion etch system. The etch rate of Al2O3 and possible mask materials was studied by applying various common process gases and combinations of these gases, including CF4/O2, BCl3, BCl3/HBr and Cl2.
Based on a comparison of the etch rates and patterning feasibility of the different mask materials, a BCl3/HBr plasma and
and standard resist mask were used to fabricate channel waveguide structures. The etched structures exhibit straight
sidewalls with minimal roughness and etch depths of up to 530 nm, sufficient for defining waveguides with strong
optical confinement and low bending losses. Low additional propagation losses were measured in single-mode Al2O3 ridge waveguides defined using the developed etch process. In initial investigations, Al2O3:Er layers fabricated using the same deposition method applied for the undoped layers show typical emission cross-sections, low green upconversion luminescence and lifetimes up to 7 ms.
We present a new light source for parallel Optical Coherence Tomography (OCT) based on multiple waveguides written in Ti:sapphire. Each channel can generate a spectrum of 174 nm bandwidth centered at 772 nm, with an optical power on sample of 30 uW. A system depth resolution of 1.9 um is obtained, which correspond to 1.5 um in tissue.
We report on the fabrication of Ti:sapphire channel waveguides. Such channel waveguides are of interest, e.g., as low-threshold tunable lasers. We investigated several structuring methods including ion beam implantation followed by wet chemical etching strip loading by polyimide spin coating and subsequent laser micro-machining, direct laser ablation or reactive ion etching through laser-structured polyimide contact masks. The later two methods result in ribs having different widths and heights up to ~5 μm. By reactive ion etching we have obtained channel waveguides with strong confinement of the Ti:sapphire fluorescence emission.
The erbium 2.7-micrometer fluorozirconate fiber laser is investigated in computer simulations and experiments. An output power of 158 mW at 2.7 micrometer is achieved from an erbium- doped single-mode fiber cascade laser. The output power is limited only by the pump power available from the Ti:sapphire laser. The slope efficiency of 23.3% in the cascade-lasing regime is close to the calculated limit of 27%. It is shown that this cascade regime represents the most efficient system for the operation of an erbium 2.7-micrometer fluorozirconate fiber laser.
The limit of the slope efficiency with respect to absorbed pump power is investigated in erbium-doped 3-micrometer crystal lasers. It depends on the major population mechanisms of the system. Fluoride hosts are favorable due to the long lifetime of the upper laser level. The calculated slope efficiency in Er:YLF approaches 56% when pumping at 970 nm. This value clearly exceeds the Stokes limit of 35% because of energy recycling via interionic upconversion. A laser slope efficiency of 40% in Er(15%):YLF is experimentally obtained under Ti:sapphire pumping.
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