Optical Modulation Depth Research Articles

Optical modulation depth improvement in SCM lightwave systems using a dissymmetrization scheme

A scheme is proposed that permits the increase of the rms optical modulation depth (OMD) in SCM lightwave systems, without correlative increase of clipping-induced nonlinear distortion. This scheme is based on the addition to the SCM signal of low-pass filtered pulses, produced in synchronism with the peaks of the SCM signal, in order to prevent the clipping occurring during these peaks. An experiment demonstrating the feasibility is described. An rms OMD improvement of 4 and 5 dB has been obtained for a SCM band of 200-800 and 200-600 MHz, respectively.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

FM‐FDM optical CATV system employing demand access technology

AbstractA demand‐access optical CATV system using FM frequency multiplexing is developed. This system allows us to transmit 40 video signals including MUSE high vision signals over 40 km on a single‐mode fiber trunk‐line and can transmit arbitrary four video signals to subscribers at 10 km on subscribers' single‐mode optical fibers. The feature of the present system is the possibility to fabricate an inexpensive hub by using a newly developed channel selector which combines single heterodyne and RF switch and also to make use of a cheap conventional DBS tuner at the subscribers' receiving terminal. For a trunkline, an optical modulation depth has to be carefully determined by considering an inter‐modulation distortion. The present paper describes the degradation in effective CN ratio due to a white noise produced by the FM modulated third‐order distortion and shows an optimum optical modulation depth for a given transmitting distance. Also, a system design is discussed for certain distances of a trunkline and a subscriber line. One can estimate using the present design the CN ratio at a subscriber terminal. The present system can provide the video quality of 4.5 (CN ratio 17.5 dB) for high vision.

A new compound modulation technique for multichannel analog video transmission on fiber

An analog transmission technique for multichannel TV transmission on fiber using frequency-modulated/time-division multiplexing (FM/TDM) is described and compared with present digital and analog systems. Analysis and simulations indicate that the proposed system has a very low sensitivity to nonlinearities, similar to digital systems, and much better than current frequency-modulated/frequency-division multiplexed (FM/FDM) systems. This permits the use of higher optical modulation depth (as high as digital systems), which results in achieving high signal-to-noise ratio, large power budget, and/or the use of lower cost optical components. Furthermore, a better spectrum efficiency is possible.

An electrooptic intensity modulator with improved linearity

A highly linear electrooptic modulator has been designed, fabricated, and evaluated. The design of this modulator consists of only a simple modification to the directional coupler. Two-tone testing has demonstrated that, for an optical modulation depth of 30% per channel, the third-order intermodulation distortion is more than 30 dB lower than that of the conventional directional coupler or Mach-Zehnder modulators. This improvement was not observed to be accompanied by any increase in second harmonic distortion. Also included are results of two-tone computer simulations which predict the improvement in linearity of this device for a range of modulation depths.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Multi-gigahertz-bandwidth intensity modulators using tunable-electron-density multiple quantum well waveguides

We report the first measurement of modulation bandwidth in electron transfer quantum well modulators. A device with 1 pF capacitance provides ≳10 dB optical modulation depth at 1.537 μm wavelength with a 3 dB electrical bandwidth of 5.7 GHz. Optical pump-probe measurements indicate that the fundamental response time is determined by the voltage-dependent speed of carrier escape from the well.

In search of a linear electrooptic amplitude modulator

An electrooptic modulator which offers high linearity and low intermodulation distortion (IMD) is proposed. The modulation characteristics, harmonic and intermodulation distortions, are calculated. In the two-tone test with an optical modulation depth of 16% for each channel, the third-order IMD is expected to be as low as -95 dB. Such a linearized electrooptic modulator used in conjunction with diode-pumped solid-state lasers can be expected to outperform directly modulated injection lasers for CATV transmission systems.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Distortion characteristics of optical directional coupler modulators

A theoretical analysis is made of the nonlinearities of the intensity modulation response of the optical directional coupler, as a function of bias point for the case of phase-mismatch modulation, and the results are compared with those of interferometric modulators. The interferometric, standard 2*2 directional coupler and the 1*2 y-fed directional coupler modulators are shown to exhibit very similar intermodulation distortion effects. At 4% optical modulation depth, the third-order intermodulation products are -74 dB, -72 dB, and -73.6 dB respectively, below the carrier level. >

FM-FDM optical CATV transmission experiment and system design for MUSE HDTV signals

FM-FDM (frequency division multiplexing) optical transmission equipment has been developed for 34-channel MUSE HDTV (high-definition television) signals to realize optical CATV (cable television) systems. The equipment uses an LD (laser diode) with a 1.3 mu m wavelength, a single-mode optical fiber, and an avalanche photodiode (APD). A good picture is received after a 42 km transmission. A part of the multiplexed signals is distorted near or below the threshold of an LD. When the number of transmission channels is small and the total optical modulation depth is large, this nonlinearity governs the power ratio of an FM signal to one distortion component-the DU ratio. However, when the number of transmission channels is large, the DU ratio is determined by the effective optical modulation depth rather than the total optical modulation depth. Furthermore, the method of system design is clarified for an optical trunk line CATV system. If no restriction on the transmission bandwidth of optical devices exists, approximately 30 km transmission of 100-channel MUSE HDTV signals is available with a received CN ratio of 17.5 dB.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A small-area light intensity modulator implemented in silicon

Summary form only given. The authors demonstrate an optical modulator designed to provide a practical interface between silicon ICs and fiber-optic systems. The device uses the strongest optical effect in silicon, free-carrier optical dispersion, and the mode-filtering properties of single-mode optical fibers as the basis for a 200-mHz bandwidth reflection-type intensity modulator at 1.3- mu m wavelength. In contrast to the more familiar waveguide modulators, this silicon modulator's small size and vertical structure give it a much higher bandwidth (albeit with lower modulation depth) and allow it to be monolithically integrated anywhere on a silicon integrated circuit. The device was demonstrated in a simple fiber experiment using a CW 1.3- mu m laser diode. The modulator achieves 10% optical modulation depth over a 200-MHz bandwidth when under a forward bias of 10 mA and RF root-mean-square current also of 10 mA. >

Fully connectorized high-speed Ti:LiNbO3switch/modulator for time-division multiplexing and data encoding

We demonstrate a fully connectorized and packaged, 2×1 high-speed Ti:LiNbO 3 directional coupler switch suitable for both optical time-division multiplexing/demultiplexing and modulation at a 1.3- μm wavelength. The device has an intrinsic switching time of 60 ps and a small-signal 50-percent optical modulation depth bandwidth of 6 GHz. We have actively multiplexed two optical pulse streams with output non-return-to-zero (NRZ) center-to-center bit spacings of less than 100 ps, while incurring an average optical power penalty of only 0.5 dB. In addition, we have used this switch as a modulator to encode a pseudo-random data stream at 2-Gbit/s NRZ without degradation of the eye pattern or the semiconductor laser source spectrum.

Longitudinal mode spectrum of semiconductor lasers under high-speed modulation

Experimental observations of the lasing spectrum of a single-mode semiconductor laser under continuous microwave modulation reveal that the lasing spectrum is apparently locked to a single-longitudinal mode for optical modulation depths up to ∼ 80 percent, beyond which the lasing spectrum breaks into multimode oscillation. The width of the envelope of the multimode spectrum increases very rapidly with further increase in modulation depth. These results are satisfactorily explained by a theoretical treatment which gives simple analytic results for the time evolution of the individual longitudinal modes. It also yields considerable insight into spectral dynamics, and enables one to predict the dynamic lasing spectrum of a laser from its CW lasing spectra at various output powers. The results can also be used to predict the amount of spectral envelope broadening under single or pseudo-random pulse modulation.

Longitudinal mode spectrum of GaAs injection lasers under high-frequency microwave modulation

Experimental observations of the lasing spectrum of a single mode semiconductor laser under continuous microwave modulation reveal that the lasing spectrum is apparently locked to a single longitudinal mode for optical modulation depths up to ∼80%, beyond which the lasing spectrum becomes multimoded, whose envelope width increases very rapidly with further increase in modulation depth. These results are satisfactorily explained by a theoretical treatment which enables one to predict the dynamic lasing spectrum of a laser from its cw lasing spectra at various output powers.

Dynamic behavior of optically induced superconducting weak links

We report experimental measurement of the dynamic response of optically induced superconducting weak links modulated at up to 1 GHz. Increasing the optical modulation depth from 7 to 70% as the frequency varies from low frequency to 30 MHz extinguishes the ac Josephson effect which is consistent with a calculated thermal response time of 150×10−9 sec.

On the possibility of increasing the optical frequency modulation depth of an electron beam due to diffraction Pendellösung

It is shown that the electron current modulation depth may reach a value of ∼0.2 for an electron beam (incident or diffracted) transmitted through a crystal illuminated with a laser. This considerably exceeds the value of the photon absorption amplitude which is ∼0.01.