The paper presents a fiber optic tunable laser built with a band pass tunable optical filtering cascade made of two tiltedmirrors
(Optune) interferometers as feedback element of an optical amplifier. A dynamic wavelength reference monitors
the laser wavelength. The optical cascade has 200 nm tuning range, the band pass has 0.2 dB insertion loss flatness
across the entire tuning range, 0.1 nm bandwidth at 3 dB, 45 dB rejection ratio and 160 dB/nm roll-off. According to the
measurements, the line width of this tunable laser is below 41 MHz. The dynamic wavelength reference generates
wavelength markers with 0.1 pm relative accuracy and with 1 pm absolute accuracy across 40°C temperature interval.
These markers could be used to mitigate the non-linearity of the laser tuning. It was achieved 800 nm/ms tuning speed
with the tuning element working below its resonant frequency. The experiments performed in open-loop operation in
1550 nm spectral region revealed 1 pm tuning accuracy and 0.1% tuning non-linearity versus the tuning voltage across
40 nm tuning interval. Laser line roll-off is steeper than 160 dB/nm. With appropriate reflective coatings, the filtering
cascade can operate also in other spectral regions (visible, UV) with tuning accuracy limited by the wavelength reference
and by the laser controller. A digital signal processor monitors the operation of the tunable laser to achieve optimum
performance. This tunable laser source has applications in interrogators for fiber optic sensors and in optical coherence
Proc. SPIE. 7056, Photonic Fiber and Crystal Devices: Advances in Materials and Innovations in Device Applications II
KEYWORDS: Optical filters, Digital signal processing, Fiber Bragg gratings, Interferometers, Sensors, Signal attenuation, Linear filtering, Fiber optics sensors, Signal processing, Filtering (signal processing)
An optical system built with two Optune interferometers cascaded according to Vernier principle has attractive tunable
band pass filtering properties for numerous applications. Several characteristics of Optune interferometer such as 0.2 dB
insertion loss flatness across at least 90 nm interval, no tuning holes across 240 nm tuning range, quasi-periodic free
spectral range and 1 dB insertion loss are key parameters to obtain a cascade with 0.1 nm band pass tunable across
minimum 90 nm. Several properties of Optune interferometers are analyzed to build a cascade tunable across minimum
90 nm: the relationships between the free spectral ranges, bandwidths and tuning conditions. It is presented also a
cascade prototype with two interferometers having 9.72 nm free spectral range and respectively 11.12 nm free spectral
range. The cascade band pass is 0.1 nm tunable with 1 pm accuracy to any arbitrary wavelength across 150 nm free
spectral range, without any tuning hole. It has 0.125 ms / 100 nm tuning speed, the insertion loss is less than 3 dB, 50 dB
contrast, 0.5 dB flatness and 0.2 dB polarization dependent loss. A controller based on digital signal processor monitors
the operation of the cascade to achieve optimum tuning performance.
Although structural health monitoring and patient monitoring may benefit from the unique advantages of optical fiber
sensors (OFS) such as electromagnetic interferences (EMI) immunity, sensor small size and long term reliability, both
applications are facing different realities. This paper presents, with practical examples, several OFS technologies ranging
from single-point to distributed sensors used to address the health monitoring challenges in medical and in civil
OFS for medical applications are single-point, measuring mainly vital parameters such as pressure or temperature. In the
intra-aortic balloon pumping (IABP) therapy, a miniature OFS can monitor in situ aortic blood pressure to trigger
catheter balloon inflation/deflation in counter-pulsation with heartbeats. Similar sensors reliably monitor the intracranial
pressure (ICP) of critical care patients, even during surgical interventions or examinations under medical resonance
imaging (MRI). Temperature OFS are also the ideal monitoring solution for such harsh environments.
Most of OFS for structural health monitoring are distributed or have long gage length, although quasi-distributed short
gage sensors are also used. Those sensors measure mainly strain/load, temperature, pressure and elongation. SOFO type
deformation sensors were used to monitor and secure the Bolshoi Moskvoretskiy Bridge in Moscow. Safety of Plavinu
dam built on clay and sand in Latvia was increased by monitoring bitumen joints displacement and temperature changes
using SMARTape and Temperature Sensitive Cable read with DiTeSt unit. A similar solution was used for monitoring a
pipeline built in an unstable area near Rimini in Italy.
A novel optical tuning technology based on new non-resonant interferometer (Optune interferometer) is described.
This interferometer has a totally reflective layer either parallel with a partially reflective layer or tilted with a small
angle, with an adjustable air gap between them. An input fiber optic collimator delivers a free space collimated beam
that is incident first on the totally reflective layer at a small incidence angle. This beam bounces many times between the
two reflective layers. An output fiber optic collimator collects all the beams going through the partially reflective layer
making them to interfere at the entrance aperture of the output fiber. The optical configuration has no resonant
frequencies. A broadband signal at the input is available at the output as a comb with even spacing. Any arbitrary
wavelength can be selected by adjusting accurately the gap size. Tuning across 90 nm range could require less than 10 &mgr;m
change of the gap size. Some properties of Optune interferometer are: 240 nm tuning range, no tuning holes, 0.2 ms /
100 nm tuning speed, 1 pm tuning accuracy, 0.15 nm bandwidth, 1 dB insertion loss, 45 dB contrast, 0.2 dB flatness,
0.15 dB polarization dependent loss. Optune interferometer can be used either for filtering or for generating optical
wavelengths in a broad range of applications such as optical monitoring of structures (FBG and Brillouin technologies),
and in optical communications. U.S. Patent No. 7,002,696 covers Optune interferometer and also optical tuning
technology based on it.
The theoretical model for the heat diffusion in the case of a high power IR electrically calibrated laser powermeter, developed at the Institute for Atomic Physics in Bucharest, is presented. The IR laser beam falls onto a laser detector, a special design copper disc wafer which absorbs the laser beam, heats its center. A daisy-chain of thermocouple elements having one set of junctions thermally connected to the central region of the disc and the other ones to the disc's boundary is used to detect temperature rise induced by the exposure to the laser beam. For calibration, the copper disc is electrically heated and the electric power that produces the same temperature rise as one induced by an incident laser beam, should equal the laser beam power. The electric heater is designed to provide a uniform heating of the copper disc. The solution for heat diffusion equation was searched as a series of Bessel functions of zero order, the cold junction's temperature was imposed as boundary condition and the heat induced by the laser beam in the disc's center was regarded as input data. To find the correct solutions, there must be taken into account the designing elements of the copper disc: termic material's properties (caloric capacity, termic conductibility), laser detector's geometry, copper's density. The electric power for calibration was injected using a precision power injection circuit which allows a stability of the calibration power, better than 0.1%.
The paper is a description of a dedicated software for performing the mask layout of diffractive optical elements (DOE). This computer-aided design (CAD) tool is a high productivity tool developed to speed-up the layout process of DOE masks. This utility designs the mask layout, starting from the phase diffraction pattern of DOE expressed as re-usable objects, which are generated by different computing methods and optimization algorithms. The diffraction phase pattern of some frequently used diffraction structures (gratings, lenses), can be computed from the optical parameters entered as input data. The mask layout design can be made for any number of phase levels between 2 and 64, resulting in a set of binary encoding masks. Many DOE functions with the same number of levels can be combined on the same substrate. The mask layout, containing not only the diffractive pattern, but also all the process control patterns needed for manufacturing, is generated in GDS format. The capabilities of this design software are illustrated in an example containing many 4-phase level elements on the same substrate. This CAD mask design software was designed to run on PCs and on SUN Sparc workstations.
A simplified formula to accurately compute the influence of the temperature, pressure and humidity on the refractive index of the air is deduced. It is studied also the contribution of the measuring errors of the air parameters (temperature, pressure, humidity) and of the object investigated by laser interferometry (expansion coefficient), to the final error of the measurement.
Two high accuracy, high productivity measuring devices for checking mechanical products in fine mechanics factories are described. The first one enables the evaluation of the mechanical characteristics of dial indicators. The second is dedicated to metrological checking of graduated rules, used as length standards for the traveling parts of machine tools. The operating principle as well as the parameters of both devices are given.
A study on Nd:YAG media active laser pumped with millisecond variable pulses in free- running generation and passive Q-switched regime is presented. Different resonator configurations were used: stable plane-plane and spherical-plane resonators usually with reflectivity mirrors or with super-Gaussian variable reflectivity mirrors (VRM) as output couplers and super-Gaussian unstable resonators. In Q-switching operation, laser emission consists in trains of 20 - 50 nanosecond pulses with repetition frequency of 20 - 200 KHz. Peak power pulses of 0.5 - 1 MW were obtained and the laser energy contained in a train of pulses varies from 1 to 8 J depending on the length and energy of the pumping pulse. The beam quality of the Q-switched laser was studied comparatively with free-running laser operation.
The paper reports the results obtained at the Institute of Atomic Physics, Department of Laser, in the field of absolute measurements of laser radiation. For the IR range of wavelengths an electrically calibrated detector was developed. For the visible laser radiation a Romanian made, predictable quantum efficiency silicon detector was used, which was compared with similar detectors provided by the National Institute of Standards and Technology, during an international intercomparison.
Graduating rules used as length etalons in optical measuring instruments and tools need to be checked for graduation accuracy. The best way to perform this is by using a laser interferometer as a linear displacement transducer, some specially designed optoelectronic interfaces, and a computer for control and data processing. Implementation of measuring equipment consisting of these assemblies is described below.