In this paper, we report a highly effective relative humidity (RH) sensor implemented on graphene oxide (GO) coated long period grating (LPG). The GO nanocolloides bonded onto a cylindrical fibre cladding enables the LPG with strong evanescent waves to absorb more water molecules increasing its RH sensitivity. In an LPG, the phase matching condition occurs when a forward propagating core mode is coupled with the co-propagating lower order cladding modes generating evanescent waves to interact with the surrounding medium. This unique effect of LPGs can be more enhanced with multilayer GO deposition. There is an expansion of GO film with the absorption of more water molecules as RH increases. The absorption of water molecules on GO coating increases the conducting carrier (holes) density on it, thus decreasing the refractive index of GO film. The combined effect of increasing evanescent waves and modulated refractive index makes the GO coated LPGs as effective RH sensors. Our recently achieved results have shown the RH sensitivity of the GO coated LPG is about 0.01 dB/%RH. We have also investigated the effect on GO layer thickness, showing thicker layer increases the RH response of the LPG cladding mode resonances in lower wavelength region.
Dual-frequency solid-state microchip lasers have advantages, such as large frequency differences, high pumping efficiency, high beam quality, and narrow laser linewidth. In addition, they are characterized by simplicity, compactness, stability, long lifetime, etc. However, the frequency difference of the dual-frequency solid-state microchip laser cannot be modulated and is only determined by its internal stress. The stress distribution inside the microchip laser is uneven and the stability of the frequency difference needs to be improved furthermore, which limits its applications. The structure of a Nd:YAG laser resonator, the mechanism of dual-frequency generation, and the output characteristics are studied. The relationship between the polarization direction of the pumping laser and the intensities of the dual-frequency components is theoretically and experimentally researched, realizing the maximum amplitude of the beat signal. Based on the photoelasticity theory, the relationship between the frequency difference and the external force is analyzed and the PID closed-loop frequency difference controlling system based on the piezoelectric transducer is proposed and realized. By applying the closed-loop control, the frequency difference of the Nd:YAG laser can be continuously modulated in the range from 24 to 30 MHz and the fluctuation is less than 450 kHz. The high-performance Nd:YAG dual-frequency solid-state microchip laser with stable and adjustable frequency difference is achieved.
A method is presented in this paper for resolution calibration of the laser feedback displacement sensor based on Fabry-Perot (F-P) high-order feedback cavity using a conventional feedback system to measure the same displacement with it simultaneously. By calibrating the ratio of the intensity modulated curves gotten by this two different systems, the accurate optical resolution of the integrated system can be obtained. Without using any other subdivision method, the optical resolution can be about 11nm, which is traceable to light wavelength. By adding 20 times electric subdivision, the final resolution of this system is about 0.55nm. The integrated system can fit the requirement of the industrial application, and can also be used for nanometrology.
The noncooperative and high sensitivity optical displacement measurement technology is very relevant to the study and the determination of high-precision thermal expansion coefficients (TECs) of materials. This paper describes a measurement technology based on Nd:YVO4 laser feedback systems, which can realize fully non-
contact measurement of many kinds of materials with surface reflectivity greater than 10-5. A muffle furnace is
designed with two coaxial holes opened on the opposite furnace walls. This length determination technique is
based on the frequency-shifted optical feedback effects and the heterodyne phase measurement technique. For
validation, the samples are determined in the temperature range 298 to 748K, confirming high sensitivity non-
contact measurement of the materials and demonstrating TEC-measurement capabilities with uncertainties in the range of 10-7 or less.
We present the experimental observation of a phenomenon in which the reflection loss, induced by an uncoated glass sample placed in a laser cavity, significantly reduces at a series of incident angles. The light amplification condition for a laser to work can be satisfied by means of this phenomenon, though the gain is less than the loss when the sample is placed in the normal incidence. The angle ranges for the laser can keep working are intermittent, and both of the lasing range and no-lasing range become narrow with the incident angle increasing. Six kinds of optical glass samples and one birefringent sample have been tested, and three types of lasers are used to confirm this phenomenon. This phenomenon may make the anti-reflection film be not necessary for a transparent sample in some techniques or instruments based on the characteristics of laser resonant cavity. Principle and properties of this phenomenon are analyzed, and the theoretical analyses are coincident to the experimental observations. Three conditions for this phenomenon to occur, as well as the potential applications, are given finally.
A simple and effective displacement sensor based on external anisotropic feedback in Nd:YAG lasers has been presented
and demonstrated. When the system operates in anisotropic feedback induced by placing a birefringence element with
phase difference about 45 degree(such as a wave plate) in the external cavity, both the laser intensities in two orthogonal
directions are sinusoidal-modulated by the external reflector with a period of half wavelength displacement, but with a
phase difference about 90 degree between them. When threshold intensity is introduced, a period of intensity fringe can
be divided into four equal zones. Each zone corresponds to &lgr;/8 displacement of the external feedback reflector.
According to the appearing sequence of the four states, the movement direction of external reflector can be discriminated.
Thus, a novel displacement sensor with a resolution of up to 133 nm, as well as a function of direction discrimination, is
believed to be achieved. The chief advantages of this sensor are that it is compact, small size, flexible, low cost, and
robust. Most importantly, this sensor has a great potential to be improved in resolution by electric subdivision methods
applied in the grating encoders. Experimental results have shown that the uncertainty (3&sgr;) of displacement measurement
is 0.2&mgr;m in a 7mm range, and the linearity is better than 2.5710-5.
The characteristic of laser intensity modulation in microchip Nd:YAG lasers with anisotropic feedback is presented, on
which a force measurement scheme based is demonstrated. The measurement system is composed of a microchip
Nd:YAG laser, a birefringence element (BFE), and an external feedback mirror. Due to the birefringence effect of BFE,
the external cavity modulates the laser intensities in two orthogonal directions with a phase difference (PF), which is
twice as large as that of the BFE. If a photoelastic element with force loaded on is served as BFE, the PF between two
in-quadrature laser intensities is proportional to the force loaded on the photoelastic element. Thus, the force can be
easily and conveniently obtained from the PF between two in-quadrature laser intensities. A theoretical model is put
forward and is in good agreement with the experimental results. Moreover, the results here can also be applied to
displacement measurement. Our researches broaden the optical feedback in application for precision measurement.