Stress-birefringence measurement is critical in optical glass materials. A laser self-mixing interference (SMI) stress measurement system with a measuring range of 1 deg to 179 deg is presented. The light source of the system is a He-Ne laser with a variable frequency difference. Birefringent external cavity SMI experiments are conducted with the laser with different frequency differences. Experimental results show that a large frequency difference decreases the cross-saturation effect between the two laser eigenstates, and alternate oscillations are more likely to occur, which directly improves the sensitivity and expands the measuring range of the system. A piece of neodymium glass is tested with the system and the results are given, proving that the large frequency difference laser SMI system provides a simple, low-cost, and high-sensitivity stress measurement method.
A nanometer-resolution displacement measurement instrument based on tunable cavity frequency-splitting method is presented. One beam is split into two orthogonally polarized beams when anisotropic element inserted in the cavity. The two beams with fixed frequency difference are modulated by the movement of the reflection mirror. The changing law of the power tuning curves between the total output and the two orthogonally polarized beams is researched, and a method splitting one tuning cycle to four equal parts is proposed based on the changing law, each part corresponds to one-eighth wavelength of displacement. A laser feedback interferometer (LFI) and piezoelectric ceramic are series connected to the sensor head to calibrate the displacement that less than one-eighth wavelength. The displacement sensor achieves to afford measurement range of 20mm with resolution of 6.93nm.
The progress on laser feedback interferometry technology is reviewed. Laser feedback interferometry is a demonstration of interferometry technology applying a laser reflected from an external surface, which has features including simple structure, easy alignment, and high sensitivity. Theoretical analysis including the Lang–Kobayashi model and three-mirror model are conducted to explain the modulation of the laser output properties under the feedback effect. In particular, the effect of frequency and polarization shift feedback effects are analyzed and discussed. Various applications on various types of lasers are introduced. The application fields range from metrology, to physical quantities, to laser parameters and other applications. The typical applications of laser feedback technology in industrial and research fields are discussed. Laser feedback interferometry has great potential to be further exploited and applied.
A glass birefringence measurement system utilizing the reflective laser feedback (RLF) effect is presented. The measurement principle is analyzed based on the equivalent cavity of a Fabry–Perot interferometer, and the experiments are conducted with a piece of quartz glass with applied extrusion force. In the feedback system, aluminum film used as a feedback mirror is affixed to the back of the sample. When the light is reflected back into the cavity, as the reinjected light is imprinted with the birefringence information in the sample, the gain and polarization states of the laser are modulated. The variation of optical power and polarization states hopping is monitored to obtain the magnitude of the stress. The system has advantages such as simplicity and low-cost with a precision of 1.9 nm. Moreover, by adjusting the position of the aluminum, large-area samples can be measured anywhere at any place.
The combination of volumetric heating of the laser material by the absorbed pump radiation
and surface cooling required for heat extraction leads to a no uniform temperature distribution in the
rod. With the coactions of pump field and coolant, the temperature gradient is formed within laser
working medium, and then the thermal effects including thermal lens, thermal stress birefringence, etc.
They all seriously restrict the output characteristics of laser. The uniform temperature field distribution
in laser working medium weakens the influences of thermal effects in laser. The thermal effect of
Tm:YAG laser generated by laser-diode pumping the Tm:YAG crystal is analyzed. After considering
the quasi three-level structure of the crystal and the distribution of transmission power in the cavity, a
more actual temperature field in the crystal is obtained by revamping the heat conversion coefficient.
The thermal effects mechanics were analyzed at first, and then the physical and mathematical thermal
analysis models were established based on the theoretical knowledge of thermal effects in LD pumped
Tm:YAG laser. The method can be applied to the laser thermal effect research of quasi three-level. The
analysis and the result can be referred to the thermal effect research of the solid state laser end-pumped
by the LD and the optimal design of resonant cavity.
Diode-pumped solid-state lasers are high efficiency, long lifetime, compact and reliable, so they have been covering a wide
range of applications. Thermal effect is a major limiting factor in scaling the average power of high-power solid-state lasers,
so it is a critical issue in designing diode-pumped solid-state lasers. The uniform pump intensity distribution in laser rod
can weaken the influence of thermal effects in laser, and the research of improving the pump distribution uniformity has
attracted a great deal of attention. People usually establish a model of single diode-bar pumped laser rod to calculate the
distribution. However, for diode-array pumped high-power lasers, the model is limited and has deviation with the actual
pump distribution, which cannot reflect the real working conditions in the laser.
In this paper, the theoretical model of diode-array pumped laser rod is built. Based on the actual working environment of
diode-array side-pumped Tm:YAG laser rod, the expression of pump intensity distribution in the laser medium is deduced.
Additionally, the influence of total pump power, pump structure, Tm:YAG rod characteristic parameters and pump beam
radius on pump intensity distribution are simulated and analyzed. Moreover, the parameters are optimized in order to
obtain the optimistic results which are efficient to improve the uniformity of pump distribution. The results show that when
the pumping distance from diode-array to the rod’s surface is 3mm, the distance between two rows of diode-bars is 1mm,
the absorption coefficient is 330m<sup>-1</sup>,the pump beam width is 2.5mm,the pump intensity distribution of five-way pumped
laser rod is improved, and then the thermal effects could be weakened. The presented results can provide theoretical
guidance to design and optimization of high-power lasers.
High-power laser systems are getting more and more widely used in industry and military affairs. It is necessary to develop
a high-power laser system which can operate over long periods of time without appreciable degradation in performance.
When a high-energy laser beam transmits through a laser window, it is possible that the permanent damage is caused to the
window because of the energy absorption by window materials. So, when we design a high-power laser system, a suitable
laser window material must be selected and the laser damage threshold of the window must be known.
In this paper, a thermal analysis model of high-power laser window is established, and the relationship between the laser
intensity and the thermal-stress field distribution is studied by deducing the formulas through utilizing the
integral-transform method. The influence of window radius, thickness and laser intensity on the temperature and stress
field distributions is analyzed. Then, the performance of K9 glass and the fused silica glass is compared, and the
laser-induced damage mechanism is analyzed. Finally, the damage thresholds of laser windows are calculated. The results
show that compared with K9 glass, the fused silica glass has a higher damage threshold due to its good thermodynamic
properties. The presented theoretical analysis and simulation results are helpful for the design and selection of high-power
The output window of a high-power laser system is vulnerable to damage, and this is the main limiting factor on the power scaling and structure integrity of the laser system. In endeavoring to obtain higher output powers from the laser system, the impact of the thermal and mechanical effects and the damage mechanism of the output window must be considered. In order to study these issues, a thermal model of the laser window is established based on the heat transfer and thermoelastic theories, and the expressions for the transient thermal and mechanical stress distributions of the output window are deduced in terms of the integral-transform method. Taking the infrared quartz window material as an example, the temperature and mechanical field distributions of a high-power all-solid-state 2-μm laser system window are simulated, and the laser-induced damage mechanism is deeply analyzed. The calculation results show that the laser window-induced damage is mainly caused by melting damage when the temperature exceeds the melting point of the material. The presented theoretical analysis and numerical simulation results are significant for the design and optimization of high-power laser windows.