Showing 9 results for Laser
Sh. Mohammad Nejad, M. H. Haji Mirsaeidi,
Volume 1, Issue 1 (1-2005)
Abstract
In this paper altitude measurement from water surface using laser beam is
presented. Research data indicate that the reflection of infrared waves from water surface is
about zero and it is less than 2% for visible radiations. Phase-shift and heterodyne
technique was used for the measurement, and the laser beam ( mW p nm 10 , 700 = = l ) was
modulated by a sine wave having a fixed frequency. The optimum design and low-noise
elements made it possible to detect a light power about 20 nW at operating frequency.
M. Esfand Abadi, M. H. Miran Baygi, A. Mahloojifar, S. Moghimi,
Volume 1, Issue 4 (10-2005)
Abstract
In this paper, thermal effects of laser irradiance on biological tissue is
investigated using computer simulations. Earlier attempts in this field made use of finite
difference and finite element techniques. Here a novel approach is adopted to improve the
results. The effect of our implicit approach on the convergence procedure and accuracy of
results, with different timing steps, is explored. Monte Carlo method is used in combination
with the finite volume algorithm in order to obtain a profile of light distribution and heat
transport in tissue. It is shown that implicit finite volume technique has not only acceptable
accuracy, but also high stability for different timing steps.
Sh. Mohammad-Nejad, M. Pourmahyabadi, A. Lajevardizadeh,
Volume 2, Issue 3 (7-2006)
Abstract
In this paper, the performance of a Ring Laser Gyro based inertial navigation is
investigated. Dynamic and stochastic modeling are applied to gyro simulation and
performance evaluation. In the dynamic model, some parameters such as scale factor and
environmental sensitivity have been determined, whereas in the stochastic model, the other
parameters such as random drift and measurement noise have been computed. The
performance of the system is evaluated for several inputs. Also, the parameter variation of
output noise as a result of changing the dither characteristics is analyzed.
S. Olyaee, Sh. Mohammad-Nejad,
Volume 3, Issue 3 (7-2007)
Abstract
A new heterodyne nano-displacement with error reduction is presented. The
main errors affecting the displacement accuracy of the nano-displacement measurement
system including intermodulation distortion error, cross-talk error, cross-polarization error
and phase detection error are calculated. In the designed system, a He-Ne laser having
three-longitudinal-mode is considered as the stabilized source. The free spectral range of
the 35cm laser cavity is about 435-MHz at 632.8-nm wavelength, which a secondary beat
frequency equal to 300-kHz is produced by combining the reference and measurement
beams. The resolution of the displacement measurement resulting from intermodulation
distortion, cross-talk and cross-polarization errors is limited to 18-pm. Also, the phase
detection uncertainty causes an error of only 5.9-pm in the displacement measurement.
Furthermore, frequency-path models of two- and three-longitudinal-mode laser
interferometers are modeled as the ac interference, ac reference, dc interference and optical
power terms. A comparison study between two- and three-longitudinal-mode laser
interferometers confirms that the performance of the designed system is considerably
improved.
Shahram Mohammad Nejad, Saeed Olyaee,
Volume 5, Issue 2 (6-2009)
Abstract
In this paper, we present a high accuracy laser range finder and velocimeter using ultra-fast time-to-digital converter (TDC). The system operation is based on the measuring the round-trip time of a narrow laser pulse. A low-dark current high-speed PIN photodiode is used to detect the triggered laser beam and to produce start signal. The pulsed laser diode generates 45W optical power at 30ns duration time and 905nm wavelength. A high-responsivity avalanche photodiode (APD) detects the reflected beam from the target. An optical head including beam splitter, lenses and optical filters is also designed and implemented. The signal conditioner of the system includes pre- and post-amplifiers, comparator, opto-isolators and monostable. By using a 3MV/W reach-through structure avalanche photodiode and a wideband pre-amplifier, the pre-amplifier output reaches 15.9mV, resulting from the minimum detectable optical power. The APD temperature and as a result its responsivity is controlled by a thermoelectric controller unit. The start and stop signals from PIN and APD are led to the time-to-digital converter to count the round-trip time of the laser beam. The system is tested by a retro-reflector as a target for 30-1200m distances. The resolutions of the distance and velocity measurement are limited to 18.75mm and 1.2m/s, respectively. In the worst condition, the minimum reflected optical power is limited to about 5.3nW in 1.2km distance.
M. Moravvej-Farshi, F. Esmailifard, K. Saghafi,
Volume 7, Issue 1 (3-2011)
Abstract
We present an optimized design for GaAs/AlGaAs quantum cascade lasers operating at 4.1THz. This was based on a three-well active module with diagonal radiative transition. This was performed by modifying the existing model structure, to reduce the parasitic anticrossings (leakage currents) as well as the optical gain linewidth. While the gain FWHM was reduced by more than 50% the gain peak was increased by about 23.3%.
F. Dabbagh Kashani, M. R. Hedayati Rad, E. Kazemian, A. Kahrizi, M. R. Mahzoun,
Volume 10, Issue 1 (3-2014)
Abstract
In this paper, we investigate the effects of auto-tracking subsystem together with different beam divergences on SNR, BER and stability of FSO communication links. For this purpose we compute the values of power, SNR and BER on receiver, based on analytic formula of Gaussian beam on receiver plane. In this computation the atmospheric effects including absorption, scattering and turbulence are considered. Using mentioned computed values, the laser link stability and its reliability in presence of auto-tracking subsystems are evaluated. The results of simulation and computation are shown with the help of figures and tables.
S. Mohammad Nejad, H. Arab, N. Ronagh Sheshkelani,
Volume 14, Issue 3 (9-2018)
Abstract
In this paper, after a brief overview on laser warning system (LWS), a new structure for an optical array that is used in its optical subsystem is introduced. According to the laser threats’ wavelengths (0.5 – 1.6 µm) and our desirable field of view (FOV), we used 6 lenses for gathering the incident radiation and then optimized the optical array. Lenses’ radius, their semi diameter, their distance from each other, their thickness and the kind of glass used in them was chosen in which we access a very high transmission coefficient. Also the optical reflection and absorption of the array decreases at the same time. After optimization, the obtained optical transmission in our desirable FOV is up to 82% and the obtained optical reflection and absorption is less than 15%. Total aberration of the incident ray decreased notably and the results showed that this parameter is less than 2µm. The laser spot diameter which is focused on the detector is smaller than 400 µm in the worst case which is the laser radiation with 1.54 µm wavelength and field of 10 degrees. Total track of the array is 66.819 mm and effective focal length and F/# parameter are as small as possible which leads to high quality of the light’s focus on the detector and smaller dimension and lighter weight for the receiver. Using optical devices with such appropriate arrangement and very good optical transmission coefficient, the offered structure has a remarkable signal to noise ratio (SNR) which is up to 160 dB. The receiver’s operation in far distances from laser sources (beyond 15 km) and in hazy conditions and low temperatures is quite suitable as well.
E. Y. Burkin, F. A. Gubarev, V. V. Sviridov, D. V. Shiyanov,
Volume 19, Issue 3 (9-2023)
Abstract
A two-channel pulsed power supply for an imaging system with brightness amplification and independent synchronous laser illumination is designed. The power supply generates synchronized high-voltage pulses with a frequency of 16–24 kHz, an average electrical power of up to 1.2 kW, and an adjustable amplitude of up to 6.2 kV to pump copper bromide gas discharge tubes with independent control of the temperature parameters of the active medium. To generate pumping pulses for laser media, we used a two-channel thyratron circuit with a common source of stabilized voltage provided by a step-down pulse stabilizer and a bridge inverter-based circuit for the pulsed charge of storage capacitors. The voltage equalization on the storage capacitors is carried out by means of magnetic coupling of the charging inductances wound on a common core. Adjustable delay lines based on variable inductances provide synchronous operation of two brightness amplifiers with a synchronization accuracy of lasing pulses of ±1 ns. The power supply demonstrated stable operation with two gas discharge tubes having different characteristics, including those with different types of electrodes. It has been integrated into a laboratory facility for the study of high-energy materials combustion.
