The availability and reliability of the high data rate space-to-ground optical coherent communication links are critically challenged because the wave front of the signal beam is distorted and impaired when propagating through atmospheric turbulence. Based on the free-space interference of two successive data bits delayed by an unequal-arm-length Mach–Zehnder interferometer (MZI), a pupil-matching time-delay self-homodyne interference differential phase-shift keying (DPSK) optical receiver with 2 × 4 90-deg optical hybrid is designed for the high data rate space-to-ground optical communication links due to its immunity from wave front impairment. The delayed optical path difference (OPD) in the unequal-arm-length MZI corresponds to the duration of one bit and can be stabilized to below 1000th of the wavelength by the closed-loop feedback control. The maximum system insertion loss is <1 dB. The measurement accuracy of OPD in unequal-arm-length MZI is 0.01 mm by the deramping method from a chirped laser. The double-bit-rate 2.5- / 5-Gbps DPSK optical receiver has been presented. Parallel and separate atmospheric measurement along the optical communication link is also performed simultaneously. The 2.5-and 5-Gbps optical communication links have already been verified with a ϕ300-mm receiving telescope between two buildings in downtown over a distance of 2.4 km in the worst-case atmospheric conditions. The measured bit-error-rate is better than 10 − 6. Without wave front compensation of the adaptive optics, local oscillator, and optical phase-locked loops, the pupil-matching time-delay self-homodyne interference DPSK optical receiver has great significance for future developments of space-to-ground optical communication links.
A strip-mode side-looking large-aperture synthetic aperture imaging ladar (SAIL) based on free-space optics is designed to experimentally verify the key performances for further airborne application. The optical difficulties in the spatial domain, including optical antenna and imaging resolution for large-aperture SAIL, are considered systematically. The optical antenna consists of a circulated duplex including the tunable transmission and reception channels for the flexible wavefront control. An irradiating beam with a spatial quadratic phase-bias for the suitable and controlled phase history can be transmitted from the transmission channel, and the wavefront aberration in the reception channel resulted from the diffraction can be eliminated for the optimized heterodyne detection. The construction and 2-D imaging algorithm of the SAIL are also proposed. The performances, including wavefront, heterodyne efficiency, field of view, imaging resolution, and 2-D imaging algorithm, have been systemically verified in the short-distance of laboratory space. The well-focused 2-D SAIL imaging of a diffusely scattering target with submillimeter imaging resolution is given. The experimental results are in good agreement with the theoretical design. The results have the great significance for the future development of airborne large-aperture SAIL.
Success of optical measurement methods such as interferometry, holography and fringe projection methods
mainly depends on the availability of robust phase recovery (PR) algorithms. In presence of some problems such as
strong noise, abrupt phase changes, low fringe contrast and discontinuities, it’s necessary to introduce a highly
reliable algorithm to overcome them. As for the experimental phase map from holographic interferometry of China
coin’s micro-surface, some typical PR algorithms including the proposed (weighted) DCT (Discrete Cosine
Transform) are studied to try to recover the satisfactory continuous phase. The experimental phase map
(13921040 pixels) exhibits the local surface characteristic of one China coin. It features excessive noise and many
bad data points. It’s very difficult to achieve the continuous phase by the general algorithms. So some robust
algorithms are studied to solve this problem. These algorithms include the quality map guided method (QG),
(weighted) least square method (LS or WLS), mask cut method (MC) and (weight) DCT method (DCT or WDCT).
According to the tested results, the fastest algorithm is the LS method, and it only needs about 10s. The clearest
pattern profile results from the DCT and WDCT algorithms that need about 30s and 31s in the consuming time,
respectively. However, it’s very hard to clearly and completely reflect the smooth top of imprinted letters for all the
above algorithms. This could be due to the excessively much noise, the limited resolutions of CCD setup, and so
A novel car camera lenses have been designed. To reduce the cost, the system gives up the glued lenses. The whole
optical system is made of six separated lenses and an IR-Cut Filter, among which there is two aspheric lenses and four
spherical lenses. All lenses are made of glass. The resolutions of car camera system is three million pixels and the field
angle is 150°.The Tangential MTF is 0.2 at spatial frequency of 300 cycles/mm when field angle is equal to 150°. The
axis MTF is 0.7 at spatial frequency of 150 cycles/mm. The overall length of system is 18.23mm. The F-number is 2.8
and the effective focus length is 1.6mm. The new-designed car camera lenses can provide high-resolutions and very wide
field angle. At the same time, the glass structure of lenses enjoys better thermal and mechanical stability than polymer
lenses. The novel car-camera lenses will be then tested in the experiment.
A novel one-shot in-line digital holography based two-dimensional Hilbert demodulation is proposed. By weakening the
object wave compared with the reference wave and applying natural logarithmized operation on the in-line digital
hologram, the real part of object wave can be well extracted. Then utilizing two-dimensional Hilbert transform to
digitally realize π / 2 phase-shift makes it possible to reconstruct the object wave front from single-exposure in-line
digital hologram. Preliminary experimental results are presented to demonstrate the proposed method. This technique
can be used for real time imaging and monitoring moving objects.
A simple channel model of wireless optical links based spatial coding is described and two basic channel constraints are
outlined. For representing the spatial patterns as signal points, a signal space based 2-D orthonormal basis functions is
defined. After that, critical measures of performance are also defined as metrics for comparing candidate modulation
schemes. Then three candidate modulation schemes exploiting spatial dimensions are proposed, which are spatial on-off
keying, spatial pulse position modulation and spatial quadrature amplitude modulation schemes. At last, the three
modulation schemes are compared by power efficiency and bandwidth efficiency. The appropriateness of each scheme is
A temporal-spatial continuous Gaussian communication channel model is built by information theory. And the channel capacity is analyzed. Then a send-receive system scheme for imaging communication is proposed and the basic constrain conditions for system design are derived. The system performance is also analyzed and evaluated with spatial bandwidth product. After that an experiment is carried out for transmission 13X15 2-D pattern image through 30m distance. The expected performance is examined by the experiment results. At last a signal detection technique for imaging communication and corresponding pattern code and decode scheme are proposed.
Based on electro-optic effect in crystal, a novel laser ranging method is proposed. A special designed mono-block LiNbO<sub>3</sub> crystal is laid after the laser transmitter. The <i>CW-</i>laser emitted from the transmitter propagates through the crystal and travels forward to the measured target. After being reflected by the target, the laser goes back and crosses the same crystal. Electric pulses with the steep enough edges are loaded on the crystal. Based on electro-optic effect, double refraction and internal double reflection effect in crystal, the crystal cuts off the round-trip light beams, and reflects a light pulse cut out by the crystal to a detector aside from the original beam path. The pulse width <i>T</i> is the period that laser goes forward and back between the crystal and the target. From the <i>T</i> one can get the measured range <i>R</i>. The feasibility of this method is proved by our experiments and a brand-new way for the laser ranging is provided.