F.V.C. Mendis

Enhancement of modulation bandwidth of laser diodes by injection locking

In this letter, we suggest a new method for significantly enhancing the intrinsic bandwidth of a laser diode through the use of an injection locking technique. Our analysis shows that for moderate and high-injection levels, the bandwidth of a laser diode can be increased to several times its free-running bandwidth.

Overmodulation in subcarrier multiplexed video FM broad-band optical networks

The probability of an SCM (subcarrier multiplexed) signal falling outside the linear region of a laser diode is calculated in order to determine the overmodulation that may be practically employed for multichannel video distribution in single-octave SCM networks. The overmodulation is predicted as a function of the number of subcarriers in single-octave SCM systems supporting up to about 70 FM video channels, for SNRs (signal-to-noise ratios) of 47 dB (I/PAL), and 56 dB (M/NTSC). The RMS optical modulation index (OMI) is demonstrated to be a more useful performance parameter than the linear OMI in SCM systems carrying a large number of channels. A 47 dB weighted SNR (with 4 dB margin) is deemed sufficient for acceptable picture quality in cable distribution systems, whereas a 56 dB value would be required for long-distance point-to-point optical links. RMS OMI values of around 70 and 50, respectively, can be employed. >

Generalized perturbation analysis of distortion in semiconductor lasers

We have carried out a generalized perturbation analysis of the second‐order nonlinear equation for photon density for semiconductor lasers. The second‐order equation is obtained with minimal approximations, from the single‐mode semiconductor laser rate equations, which unlike previous work, include both the effect of gain compression and the coupling of the spontaneous emission to the lasing mode. After deriving the general perturbation formula, expressions for second harmonic distortion as well as third‐order intermodulation distortion are obtained. Unlike previous work which is restricted to two subcarriers, the present work gives results for an arbitrary number of subcarriers.

Dynamic properties of optically switched bistable semiconductor lasers

The dynamic properties of optically switched semiconductor lasers biased from below-to-above threshold are presented theoretically. An analytic expression for the carrier density in the active region of a laser with respect to time is given to discuss the switching-off time. The numerical results show that the switching-on time and the switching-off time are governed by different mechanisms. They are related to the laser parameters for the free-running laser. They also depend on the optical power and the time duration of the input optical pulse and the frequency detuning between the frequency of the free-running laser and that of the input optical pulse. A small frequency detuning is desired to reduce both the switching-on and switching-off times. However, there is an optimal detuning to maximize the energy of the output optical pulse. On the other hand, for fixed detuning and injection power, a larger bias current results in a shorter switching-off time, but a lower bias current results in a shorter switching-on time.

Reduction of nonlinear distortion in semiconductor lasers with external light injection

It is demonstrated analytically that the static light-current characteristic of an injection-locked laser is highly linear away from the threshold and is suitable for direct intensity modulation. By using a small signal perturbative approach, expressions for the second order harmonic distortion and two-carrier third order intermodulation distortion are evaluated. At microwave frequencies, it is demonstrated that these distortions can be significantly reduced using injection locking. This is attributed to additional coupling effects between optical phase and photons under injection locking, in addition to the photon-electron interactions seen in free-running lasers.

Frequency characteristics of an avalanche photodiode in super-high-speed optical demodulation with electronic mixing

An analysis is presented for super-high-speed optical demodulation by an avalanche photodiode (APD) with electronic mixing. A normalized gain is defined to evaluate the performance of the demodulation. Unlike previous work, we include the effect of the non-linear variation of the APD capacitance with bias voltage as well as the effect of parasitic and amplifer input capacitance. As a result, the normalized gain is dependent on the signal frequency and the frequency difference between the signal frequency and the local oscillator frequency. However, the current through the equivalent resistance of the APD is almost independent of signal frequency. The mixing output is mainly attributed to the nonlinearity of the multiplication factor. We show also that there is an optimal local oscillator voltage at which the normalized gain is maximized for a given avalanche photodiode.

New method for reducing distortion in directly modulated lasers in subcarrier-multiplexed systems

In this paper we use small signal analysis to predict the intrinsic non-linear distortion of an injection-locked semiconductor laser diode directly driven by two subcarriers. We show that the second-order harmonic distortion may be reduced by between 12 and 27 dB over the free- running laser bandwidth of 3.4 GHz and that the third-order intermodulation distortion may be reduced by between 12 and 22 dB over a bandwidth of 5 GHz, for an injection ratio of 0.5.

Nonlinear distortion in semiconductor lasers in the presence of optical feedback

The rate equations of a semiconductor laser, with feedback due to optical reflections, are solved numerically under the condition of direct intensity modulation. The nonlinear distortion induced by the laser diode are investigated under the conditions of small and large signal modulations. The results show that for large signal modulation, the effect of optical feedback may be very small.<<ETX>>

Intermodulation distortion in semiconductor lasers in application to subcarrier multiplexed fiber optic video systems

Analytical techniques for determining intermodulation distortion in directly modulated semiconductor lasers are examined. Direct expressions for the two- and three-frequency third-order intermodulation distortion-to-carrier ratios are used to check the validity of a well-known 6-dB difference formula. It is found that this formula is not always valid, the discrepancy being a function of laser diode parameters and subcarrier spacing. The carrier-to-intermodulation-noise ratios are computed for a 25-channel subcarrier multiplexed system, with subcarrier spacings of 6 MHz for amplitude modulation and 40 MHz for frequency modulation, and it is found that the central subcarrier is not always the worst affected, although it has the maximum number of interfering intermodulation products. This is in contrast to memoryless nonlinearities where the central subcarrier may be expected to have the worst carrier-to-intermodulation-noise ratio.

Simulation of distortion in multicarrier subcarrier-multiplexed optical communication systems employing direct intensity modulation of semiconductor lasers

Direct intensity modulation of semiconductor lasers in subcarrier multiplexed (SCM) optical fiber communication systems is of interest because of its simplicity and the possibility of integration of the drive electronics with the laser diode. Direct modulation also produces distortion which degrades system performance. In this paper, we extend a static numerical simulation method for multi-carrier SCM systems to a dynamic simulation employing rate equations. We first show that the results of our simulation agree with analytical results for multi-carrier SCM systems obtained from perturbation theory in the absence of optical reflection into the semiconductor laser. We proceed with the calculation of distortion in the presence of weak reflections. We show that the distortion is lower than what is expected from previous analytical considerations for two subcarriers. Our simulation can also take into account the effect of overmodulation in a dynamic manner. However, our simulation does not take into account the distortion due to device imperfections.