High-frequency intensity modulation in orthogonal polarized dual frequency lasers with optical feedback

High-frequency modulation of laser output intensity is studied with asymmetric feedback induced by the misalignment of an external feedback reflector in an orthogonal polarized dual frequency laser. The fringe frequency of the optical feedback system is seven times higher than that of a conventional optical feedback system, due to multiple feedback effects. The output characteristics of two orthogonal polarized modes are also investigated. Mode competition is observed between the two modes. When initial intensities of the two modes are unequal, the mode competition will be strong. The difference in initial intensity between the two orthogonally polarized modes plays an important role in the mode competition with optical feedback. Experimental results are presented, as well as a theoretical explanation. The high-frequency modulation of laser intensity can greatly increase the resolution of an optical feedback sensing system.     

频差3～40MHz的HeNe双频激光器

报道了自行研制的3－40MHz频差的双频激光器。用两种方法成功地得到3－40MHz的频差：第一种方法是将寻常光（o光）２和非寻常光（e光）２在空间分离,使其各自使用各自行进进路径上的放大Ｎe原子介质;第二种方法是对双折射双频激光器外加横向磁场,将放大Ｎe原子介质分成两类,一类只放大o光,另一类只放大e光。     

Optical circulator for an in-line-type compact lidar

The application of an optical circulator is demonstrated for an in-line-type lidar. The lidar's transmitter and receiver are installed in a telescope. The optical circulator of interest here can separate the transmitting laser beam and the echo lights on the same optical axis. It can also divide the echo lights simultaneously into orthogonally polarized components. An insertion loss of 2.2 dB and isolation of >60 dB for the developed optical circulator are obtained in a laser-transmitting situation. This optical circulator makes it possible to measure the polarization ratio caused by cloud phases with a narrow field of view in an in-line-type lidar operation.     

Effects of unwanted feedback on synchronized chaotic optical communications

The effects of unwanted external optical feedback on synchronized chaotic optical communication systems are studied numerically. We consider an open-loop configuration consisting of a transmitter laser with double external optical feedbacks and a receiver laser with optical injection from the transmitter laser. First, including the effects of unwanted optical feedback, the synchronization performances of both the complete synchronization and the generalized synchronization are examined. Then the encoding and decoding performances of the generalized synchronization and the effects of the introduced feedback are investigated, respectively. Finally, we study the control of the unwanted feedback on the dynamics of the transmitter laser and briefly discuss the system security when the transmitter laser is driven to operate in a steady state or periodic oscillation state by the additional feedback.     

Simultaneous measurement of refractive index and thickness of birefringent wave plates

A nondestructive measurement system based on a position sensing detector (PSD) and a laser interferometer for determining the thickness and refractive indices of birefringent optical wave plates has been developed. Unlike previous methods presented in the literature, the proposed metrology system allows the refractive index and thickness properties of the optical plate to be measured simultaneously. The experimental results obtained for the e-light and o-light refractive indices of a commercially available birefringent optical wave plate with refractive indices of n(o)=1.542972 and n(e)=1.552033 are found to be accurate to within 0.004132 and 0.000229, respectively. Furthermore, the experimentally derived value of the wave plate thickness deviates by no more than 0.9 microm from the analytically derived value of 453.95 microm. Overall, the experimental results confirm that the proposed metrology system provides a simple yet highly accurate means of obtaining simultaneous measurements of the refractive indices and thickness of birefringent optical wave plates.     

Simple interferometric technique for generation of a radially polarized light beam

We present a theoretical and experimental investigation of an interferometric technique for converting a linearly polarized Gaussian beam into a radially polarized doughnut beam. The experimental setup accomplishes the coherent summation of two orthogonally polarized TEM01 and TEM10 beams that are obtained from the transformation of a TEM00 beam by use of a simple binary diffractive optical element. We have shown that the degree of radial polarization is maximum at a given distance from the interferometer output port that depends on the diameter of the incident beam at the interferometer input port.     

Laser optical feedback imaging insensitive to parasitic optical feedback

We present an optical architecture for the laser optical feedback imaging (LOFI) technique that makes it possible to avoid the effect of the optical parasitic reflections introduced by the optical components located between the laser source and the studied object. These reflections damage phase and amplitude information contained in the images. This phenomenon is a leading problem that strongly limits the LOFI performance for weak feedback detection. Consequently, it is essential to be able to limit or avoid the effect of these parasitic reflections to reach the optimal LOFI performance.     

Separation technique of a mixing of two uncorrelated and perfectly polarized lights with different coherence and polarization properties

The sum of two uncorrelated and totally polarized lights with different coherence and polarization properties usually results in a partially polarized light. It is shown in this paper that the initial totally polarized lights can be recovered from the mixed partially polarized light. The proposed technique is based on coherence analysis and does not require the knowledge of the polarization states or the coherence properties of the initial perfectly polarized beams as long as these properties are different for the two waves. Some practical optical implementations of this technique are discussed on different illustrative applications.     

Polarization changes at Lyot depolarizer output for different types of input beams

Lyot depolarizers are optical devices made of birefringent materials used for producing unpolarized beams from totally polarized incident light. The depolarization is produced for polychromatic input beams due to the different phase introduced by the Lyot depolarizer for each wavelength. The effect of this device on other types of incident fields is investigated. In particular two cases are analyzed: (i) monochromatic and nonuniformly polarized incident beams and (ii) incident light synthesized by superposition of two monochromatic orthogonally polarized beams with different wavelengths. In the last case, it is theoretically and experimentally shown that the Lyot depolarizer increases the degree of polarization instead of depolarizes.     

In-line interferometer for direction-sensitive displacement measurements by optical feedback detection

We demonstrate a compact in-line interferometer for direction-sensitive displacement measurement by optical feedback detection with a semiconductor laser (SL) light source. Two reflected beams from a semitransparent reference mirror and a reflecting test object interfere in the SL medium, causing a variation in its output power. The reference mirror is located between the SL output facet and the test object. The performance of the interferometer is investigated numerically and experimentally to determine its optimal operating conditions. We have verified the operating conditions where the behavior of the SL output power profile could indicate accurately the displacement magnitude and direction of the moving test object. The profile behavior is robust against variations in optical feedback and scale of the interferometer configuration.     

Opaque optics thickness measurement using a cyclic path optical configuration setup and polarization phase shifting interferometry

Thickness measurement of an opaque optics using a cyclic path optical configuration (CPOC) setup and polarization phase shifting interferometry (PPSI) is presented. The CPOC setup is used to simultaneously focus two orthogonally polarized counterpropagating converging beams at its hypotenuse arm. The opaque optics is placed at the hypotenuse arm of the CPOC setup such that one of its surfaces reflects back one of the counterpropagating focusing beams. Because of the thickness of the opaque optics, the other focusing beam suffers a longitudinal shift in the beam focus. Applying PPSI, the longitudinal shift in the beam focus which is twice the thickness of the opaque optics is determined. The results obtained for a silicon plate of thickness 0.660 mm with a measurement uncertainty of 0.013 mm are presented.     

Sensitivity of synthetic aperture laser optical feedback imaging

In this paper, we compare the sensitivity of two imaging configurations, both based on laser optical feedback imaging (LOFI). The first one is direct imaging, which uses conventional optical focalization on target, and the second one is made by a synthetic aperture (SA) laser, which uses numerical focalization. We show that SA configuration allows us to obtain good resolutions with high working distance and that the drawback of SA imagery is that it has a worse photometric balance in comparison to a conventional microscope. This drawback is partially compensated by the important sensitivity of LOFI. Another interest of SA relies on the capacity of getting three-dimensional information in a single x-y scan.     

Fault tolerant function of dynamic refreshing holographic memory with shutter-less optical feedback circuit

The optical system, consisting of two photorefractive memories and a shutter-less optical feedback circuit, will be demonstrated to function as data mirroring. This function is known to automatically detect the data dropout and restore data, using unimpaired data in another memory, in the event that part or all of the data in either of them were lost for some reason. This memory system also can cope with a damaged hologram, a result of reading beams, which is a disadvantage of rewritable photorefractive memory, to ensure non-destructive holographic reading. It can be achieved by using no electronic circuits or mechanical structures; our optical experimental method in particular obtains this basic action.     

Absolute distance measurement with heterodyne optical feedback on a Yb:Er glass laser

Absolute distance measurement based on optical feedback using a single-frequency Yb:Er glass laser is demonstrated via the combination of heterodyne detection and frequency sweep. The technique allows for the enhancement of the sensitivity of the laser response to self-mixing thanks to resonant excitation close to the relaxation-oscillation frequency peak. The experimental results on noncooperative targets are in good agreement with the theory, and the shape of the resulting signal is analyzed in both the temporal and the frequency domains considering the specific dynamic of the class B solid-state laser. Suggestions are provided for further improvements on the signal processing.     

Two-dimensional synthetic aperture laser optical feedback imaging using galvanometric scanning

We have improved the resolution of our laser optical feedback imaging (LOFI) setup by using a synthetic aperture (SA) process. We report a two-dimensional (2D) SA LOFI experiment where the unprocessed image (i.e., the classical LOFI image) is obtained point by point, line after line using full 2D galvanometric scanning. The 2D superresolved image is then obtained by successively computing two angular SA operations while a one-dimensional angular synthesis is preceded by a frequency synthesis to obtain a 2D superresolved image conventionally in the synthetic aperture radar (SAR) method and their corresponding laser method called synthetic aperture ladar. The numerical and experimental results are compared.     

Design considerations of stacked multilayers of diffractive optical elements for optical network units in optical subscriber-network applications

The detailed design process and experimental results of stacked multilayer diffractive optical elements are reported for an optical network unit used in optical subscriber-network applications. The optical network unit accepts two incoming light beams of 1.3- and 1.55-mum wavelengths through a single-mode optical fiber. A laser diode is also placed for bidirectional communications. The optical network unit consists of five diffractive optical elements that perform the following functions: collimation of incoming beams, focusing of the outgoing 1.55-mum beam, 3-dB splitting of the 1.3-mum beam, focusing of the 1.3-mum beam onto the photodiode, and collimation of the light emitted from a laser diode. Possible cost reductions as a result of mass production and the ease of alignment of the stacked diffractive optical elements could be ideal for constructing low-cost optical network units.     

Thickness measurement of transparent glass plates using a lateral shearing cyclic path optical configuration setup and polarization phase shifting interferometry

We present a measurement technique to determine the thickness of a transparent glass plate (GP) by using a lateral shearing cyclic path optical configuration (CPOC) setup and polarization phase shifting interferometry (PPSI). In the technique, the GP introduces a longitudinal shift in the focus of the beam and, as a result, a spherical wavefront emerges from the lens, which is otherwise set for producing a collimated beam. Using CPOC, two laterally sheared orthogonally polarized beams are generated from the incident spherical wavefront. By applying PPSI, the slope of the optical path difference variation between the laterally sheared interfering beams is evaluated, and the radius of the spherical wavefront and the longitudinal shift of the beam focus are calculated. The thickness of the GP is determined from the standard relation between the longitudinal shift of the focus introduced by the GP and the thickness of the GP. Results obtained for a GP of 9.810mm thickness are presented.     

Circularly polarized optical heterodyne interferometer for optical activity measurement of a quartz crystal

Phase retardation between two orthogonal circularly polarized light waves that propagate in an optical active medium is proportional to its optical activity. The measurement of optical activity of a quartz depolarizer in terms of the phase difference of two orthogonal circularly polarized waves is proposed. A circularly polarized optical heterodyne interferometer with a Zeeman laser to measure the optical activity of a quartz crystal is demonstrated experimentally. The accuracy of the measurement is discussed. In addition, the effect of elliptical polarization and nonorthogonality of linearly polarized light waves of a Zeeman laser on the optical activity measurement is analyzed.