M.K. Haldar

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.

A simplified analysis of direct intensity modulation of quantum cascade lasers

Although it has been reported that quantum cascade lasers (QCLs) can theoretically achieve terahertz direct intensity modulation (IM) bandwidth, simple explanation in terms of a second-order system model of interband lasers is unsatisfactory. This paper uses simplified rate equations to analyze IM of QCLs. For fast carrier removal rates, the IM response does not have the resonant behavior of IM response of interband lasers. An approximate formula for IM bandwidth is presented. The bandwidth tends to a constant value of the order of 100 GHz with increase in light power. For slow carrier removal rates, associated with large bias currents, a peak can appear in IM response for reasons entirely different from those for the peak in IM response of interband lasers. Slow removal rates can also raise the 3-dB frequencies to terahertz values. However usable terahertz bandwidth cannot be attained even theoretically and practical bandwidths may be much less than 100 GHz.

Optical frequency modulation and intensity modulation suppression in a master-slave semiconductor laser system with direct modulation of the master laser

An injection-locked laser system where the master laser is directly modulated is analyzed using rate equations. Both quasi-static and dynamic analyses are carried out in order to ensure that the parameters lie inside the locking range. The analysis is valid for all injection levels. The quasi-static analysis provides a good basis for explaining the phenomena. It is shown that, for a given detuning frequency, maximum suppression of intensity modulation (IM) occurs at a specific value of the injection ratio. At low frequencies, the frequency modulation (FM) index of the slave laser bears a constant ratio to the FM index of the master laser of less than unity. It is illustrated that the direct FM scheme is only viable for modulation frequencies up to about 100 MHz. Large IM suppression can only be achieved for large values of the linewidth enhancement factor of the slave laser, small magnitude of the detuning frequency, and low injection ratio. The latter two conditions are associated with narrow limits on stable operation, and care should be taken to avoid instability.

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.

A circuit for sharing peripherals and instruments with the IEEE-488 interface between two or more microcomputers

Abstract The IEEE-488 bus forms the basis of most automated test systems. Several microcomputers provided with this bus are available cheaply, but the instruments and peripherals may be expensive. Unfortunately, it is a characteristic of the interface standard that there can be only one controller on the bus and hence only one microcomputer can independently access a shared peripheral or instrument on the bus. This paper describes a low-cost circuit that will allow two or more microcomputers to share and independently control peripherals and instruments equipped with the IEEE-488 interface. The circuit does not affect communication between a microcomputer and a peripheral or instrument dedicated to it.

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>>