Reference Lightwave Research Articles

Single-shot measurement of the Jones matrix for anisotropic media using four-channel digital polarization holography.

Dynamic measurement of the Jones matrix is crucial in investigating polarization light fields, which have wide applications in biophysics, chemistry, and mineralogy. However, acquiring the four elements of the Jones matrix instantly is difficult, hindering the characterization of random media and transient processes. In this study, we propose a single-shot measurement method of the Jones matrix for anisotropic media called "four-channel digital polarization holography" (FC-DPH). The FC-DPH system is created by a slightly off-axis superposition of reference light waves, which are modulated by a spatial light modulator (SLM), and signal light waves that pass through a Ronchi grating. The SLM enables flexible adjustment of the spatial carrier frequency, which can be adapted to different anisotropic media. The four elements of the Jones matrix can be obtained from the interferogram through the inverse Fourier transform. Optical experiments on anisotropic objects validate the feasibility and accuracy of the proposed method.

Ultra-long-distance distribution of low-phase-noise reference lightwave for optical communications.

The remote delivery of optical reference with highly stable oscillation frequency and carrier phase can eliminate the need of digital signal processing for the estimation of these parameters in optical communication. The distribution distance of the optical reference has been limited, however. In this paper, an optical reference distribution over 12,600 km is achieved while maintaining low-noise characteristics, using an ultra-narrow-linewidth laser as a reference source and a fiber Bragg grating filter for noise removal. The distributed optical reference enables 10 GBaud, 5 wavelength-division-multiplexed dual-polarization 64QAM data transmission without using carrier phase estimation, which significantly reduces off-line signal processing time. In the future, this method can enable all coherent optical signals in the network to be synchronized to a common reference ideally, thereby improving overall energy efficiency and cost.

Computer-Generated Hologram Based on Reference Light Multiplexing for Holographic Display

In this article, we propose a reference light wave multiplexing scheme to increase the information capacity of computer-generated holograms. The holograms were generated by different reference light waves and superimposed together as a multiplexed hologram. A modified Gerchberg–Saxton algorithm was used to improve image quality, and different images could be reconstructed when the multiplexed hologram was illuminated by corresponding reference light waves. We performed both numerical simulations and optical experiments to demonstrate the feasibility of the proposed scheme. Numerical simulations showed that the proposed method could reconstruct multiple images successfully by a single multiplexed hologram and optical experiments are consistently good with numerical simulations. It is expected that the proposed method has great potential to be widely applied in holographic displays in the future.

Photonics-Assisted Multi-Band Dual Linear/Nonlinear Chirp Waveform Generation Based on Optical Phase Modulation

A novel photonic scheme for generating multi-band dual-chirp waveforms with linear or nonlinear chirp rates is proposed via two cascaded polarization modulators (PolMs) paralleled with a phase modulator (PM). The cascaded PolMs produce a flat optical frequency comb (OFC) as a multi-frequency reference light, while the PM is driven by the power-function-type baseband signals to obtain the lightwave with different order phase chirp. The phase-chirped lightwave and the multi-frequency reference lightwave are orthogonally coupled by a polarization beam combiner (PBC) and then recombined via a polarization beam splitter (PBS) to make them in-phase in one output port of the PBS and anti-phase in the other. The two recombined lightwaves are detected by a balanced photodetector (BPD), and the different order dual-chirp signals, covering six bands, can be generated in their differential photocurrent without interference components. Compared with the single-chirp signal of the knife-edge-type ambiguity function, the dual-chirp signal with the peak-saliency-type ambiguity function significantly suppresses sidelobe and shows better range-Doppler resolution performance. With the 1GHz-bandwidth and 102.4ns-duration quadratic baseband driving signal, six-band linear dual-chirp signal, centered at 10, 30, 50, 70, 90 and 110GHz, are generated by simulation, and each linear dual-chirp signal has a bandwidth of 4.1GHz, the pulse compression ratio of 465.45 and the peak-to-sidelobe ratio (PSLR) of 12.8dB. As baseband driving signal has higher (3rd) order power, the six-band nonlinear dual-chirp signals with the same central frequencies can also be generated but with increased bandwidth (6.3GHz) and the improved PSLR (15.51dB). Their autocorrelation functions show that compared with the linear chirp signal, the nonlinear chirp signal has better sidelobe suppression capability, which is improve as the degree of nonlinearity increase.

Rapid scanning optical delay line based on a diffraction grating pair for a low-coherence reflectometer.

We present a simple low-coherence time-domain interferometric reflectometer with a rapidly scanning optical delay line (RSODL) based on a non-parallel diffraction grating (DG) pair. The novelty of the solution is that the lightwave in the reference channel is focused on a galvo-mirror in a sub-mm static spot, which allows implementation of fast microelectromechanical systems scan optics. It is shown that the DG pair can be operated as a non-dispersive element that provides dynamic group delay of a reference lightwave. The DG pair system is also capable of tuning the RSODL dispersion from negative to positive values. The experimental depth range in air was obtained as large as 2.5mm for axial resolution of 20μm.

An effective method in calibrating nonlinear errors for phase-shifting interferometry

In phase measurement or digital holography for phase-shifting interferometry, the key role is the variation of reference light wave and recover algorithm based on interferograms and reference phase, so the calculation result is directly affected by phase-shift accuracy. However, because of the errors of nonlinear and other random factors, it is difficult to control the actual phase-shifting amount accurately. In this paper, we aim to propose an efficient method for phase-shifting interferometry which does not require accurate initial estimation of phase-shift amounts, only a few pixels with several randomly shifted interferograms are sufficient for accurate extraction of phase information. This method has reduced the dependence of reference phase, and can obtain phase-shifting amount directly without using complex revised algorithm for correcting phase-shifting nonlinear errors.

SNR enhancement in Brillouin optical time domain reflectometer using multi-wavelength coherent detection

A novel scheme using multi-wavelength coherent detection for SNR enhancement in a Brillouin optical time domain reflectometer (BOTDR) is presented. The probe pulse of the BOTDR contains multiple wavelengths with small equal-wavelength spacing, and the Brillouin scattering light waves of the multiple wavelengths beat with the corresponding local reference light waves with the same equal spacing to generate overlapped multiple Brillouin scattering spectra. The overlapped scattering spectra can be detected by a conventional photodetector. A three-wavelength heterodyne detection BOTDR is demonstrated and it provides 4.2 dB SNR enhancement compared to a conventional single wavelength heterodyne detection system.

Experimental study on the existence and properties of speckle phase vortices in the diffraction region near random surfaces

We design an optical setup to extract phase vortices in which the interference intensity of reference light wave and speckle fields produced by random screens with different roughness values in the diffraction region near random screens is obtained. Random screens with different roughness are used as samples. Fourier transform is used to extract speckle phase vortices from the interference intensity, and the experimental results show that the phase vortices can be produced when the roughness of the screen is large enough, and they even may appear on the surface. The density of phase vortices would become larger with an increase of the distances in the diffraction region near the random screen. When the distance is certain, the density of phase vortices becomes larger with the increase of roughness. These results would be helpful for understanding the formation of phase vortices.

Computer generated holography based on Fourier transform using conjugate symmetric extension

A new computer generated holography (CGH) algorithm based on Fourier transform is proposed. The complex amplitude of the light wave to be recorded is extended to produce a conjugate symmetric function. The function is then Fourier transformed to generate a real valued distribution containing both the amplitude information and the phase information of the light wave. The obtained real distribution is coded to give a gray-scale hologram，which can be used to reconstruct the original light wave. Unlike the existing CGH algorithms based on interference of object and reference light waves，the proposed method does not require simulatingthe interference to produce the hologram. The algorithm is highly efficient，and can produce good results both in digital and electroholographic reconstruction. Theoretical and experimental results are presented to verify the validity of the method.

F-1106 高感度トワイマン・グリーン干渉計による固体表面の微小変形計測(J25-2 光学的手法・弾性的性質)(J25 機械力学,材料力学,エンジン関連の計測技術)

High-sensitivity Twyman-Green interferometer is developed to measure the small change of the thickness of transparent plate specimens. A half-mirror is placed in an arm of a common Twyman-Green interferometer. Multiple reflection of light occurs between the half-mirror and a mirror of the Twyman-Green interferometer. A specimen exists between them. The light wave that passes through the specimen 2n times interferes with the reference light wave, and makes interference fringes that show the contours of the specimen thickness. The contrast of interference fringes decreases with increasing the number of the multiple reflection, that is, with increasing sensitivity. A common image processing software is applied to increase the contrast of the fringes. It is possible to make the interferometer that is twice as sensitive as the common Twyman-Green interferometers.

Coherent optical frequency domain reflectometry using phase-decorrelated reflected and reference lightwaves

We report a new scheme for coherent optical frequency domain reflectometry (C-OFDR) where the coherence length of the lightwaves does not limit the measureable fiber length. In this scheme, we use the beat spectrum which results when we mix reflected and reference lightwaves whose phases are not correlated. We demonstrated this scheme using a narrow-linewidth-lightwave source and an external electro-optical phase modulator. We measured Rayleigh backscattering and Fresnel reflections from a 30-Ion optical fiber, and achieved a spatial resolution of 5 m for two neighboring Fresnel reflectors located at the far end of the fiber. We estimated the expected spatial resolution and single-way dynamic range for our new scheme and show that it is capable of measuring long optical fibers with high-spatial resolution.

Performance improvement of a combined OTDR for distributed strain and loss measurement by randomizing the reference light polarization state

We successfully improve the ability of an OTDR, which combines both BOTDR and COTDR functions, to measure strain and optical loss distribution along optical fibers, by randomizing the polarization state of the reference lightwave for optical coherent detection. The OTDR is upgraded to provide a high-spatial resolution of up to 1 m in the BOTDR mode and a fading noise as low as 0.05 dB in the COTDR mode.

Time shared lightwave reference frequency distribution for photonics networks

Time shared lightwave reference frequency distribution is demonstrated using a pulsed lightwave sweep frequency generator, tracking generators, and a linear repeater. The reference lightwave, which contains over 1000 frequencies in 250 MHz steps, is generated using a dynamically gain controlled erbium-doped fiber amplifier and acoust-optical deflector. Continuous lightwave frequencies locked to the distributed time shared reference frequency are generated over a 200 GHz range with an instability of only /spl plusmn/5 MHz. Absolute lightwave frequency from the tracking generator is confirmed by a frequency chain between two hydrogen cyanide gas cell stabilized lightwave sources. The effect of fiber nonlinearity encountered in frequency distribution, a dynamically gain stabilized linear repeater, and system design are also discussed.

Phase-modulating optical coherence domain reflectometry by synthesis of coherence function

Phase-modulating optical coherence domain reflectometry by synthesis of the coherence function is proposed as a promising method for diagnosing optical waveguide devices and systems, and basic experiments are carried out. In this method, a delta-function-like optical coherence function is synthesised by the direct frequency modulation of a laser diode, and scanned by the phase modulation of a reference lightwave in an interferometer. A spatial resolution of ~1.8 mm is achieved with ~18 cm measuring range in basic experiments.

Coherent self-heterodyne detection of spontaneously Brillouin-scattered light waves in a single-mode fiber

Time-domain reflectometry of spontaneous Brillouin scattering in a single-mode optical fiber is performed with a coherent self-heterodyne detection system containing a recently proposed external frequency translator and a single light-wave source. The light wave is divided into probe and reference light waves. The frequency of the probe light wave is upconverted by the translator by an amount approximately equal to the Brillouin frequency shift. The frequency-converted probe is launched into the fiber and spontaneously Brillouin scattered. As the frequency of the scattered probe is downconverted to near that of the reference light wave, coherent self-heterodyne detection of spontaneous Brillouin scattering becomes possible without having to use a fast-speed detector.

Sound velocity measurement based on optical heterodyne detection techniques using two successive ultrasonic waves

A new optical method for measuring the velocity of sound has been investigated; it is based essentially on optical heterodyne detection techniques using a pair of ultrasonic light modulators of slightly different frequency. A light beating signal can be easily generated from the frequency difference between these modulators if signal and reference light waves obtained from the first order dual diffraction spectrum are photomixed with each other. The only optical components are the two modulators, and their alignment is not critical to the generation of a stable beating signal. Since the beating signal includes the sound velocity in each modulator in its phase factor, the sound velocity is immediately measured by making use of a conventional electronic phasemeter.