Experiments on modulation properties and optical feedback characteristics of laser diodes stabilized by an external cavity or injection locking

In the wavelength range near 1.3-μm, single-mode operation of laser diodes is achieved by means of an external cavity and injection locking. The influence of digital modulation and optical feedback from a directly taper-coupled fiber is investigated. Lasers stabilized by injection locking showed excellent side-mode suppression at CW-operation but strong side modes when the laser was modulated. The frequency of the main mode is neither affected by modulation nor by optical feedback from the fiber. On the contrary the laser stabilized by an external cavity exhibited good side-mode suppression even when it was modulated. However, the frequency stability of the main mode was influenced severely by optical feedback and modulation. A transmission experiment over 21 km at a bit rate of 2.24 Gbit/s showed that this simple stabilization method is sufficient for applications in optical wide-band IM-PCM transmission systems.     

Theory of direct frequency modulation of semiconductor lasers withintegrated external cavity

Numerical calculations of the frequency modulation responses of monolithically integrated external-cavity semiconductor lasers are performed using G.P. Agrawal's (J. Appl. Phys., vol.56, p.3110-15, 1984) two-section model. Theoretical results show good agreement with the reported experimental results. The comparison of the modulation responses of the solitary semiconductor laser diodes, the conventional external-cavity semiconductor lasers, and the integrated passive external-cavity semiconductor lasers demonstrates that the integrated external-cavity semiconductor laser is the best choice for use in direct frequency modulation     

0.81 ?m band AlGaAs/GaAs double-channel planar buried-heterostructure laser with large optical cavity

0.81 ?m-band AlGaAs/GaAs double-channel planar buried-heterostructure laser diodes with large optical cavity have been developed as light sources for use in analogue optical fibre communication systems. Linear light-output/current characteristics and a single-longitudinal-mode spectrum at a modulation frequency f = 100 MHz with modulation index m = 0.9 have been attained.     

Optimizing Optical Transmitters and Receivers for Transmitting Multichannel Video Signals Using Laser Diodes

Analog transmission using laser diodes is quite attractive for transmitting multiplexed video signals, owing to the wide modulation bandwidth of laser diodes. Optimizations for transmitting color-TV signals (7 channel frequency division multiplexed) in the VHF band using buried heterostructure laser diodes are investigated. The major transmission objectives are a carrier to noise ratio of larger than 48 dB and intermodulation products of less than -55 dB. Design curves for minimizing distortion at the transmitter are demonstrated. A modulation index of 70 percent is shown to be capable of attaining the objective. Both the avalanche photodiode (APD) and the pinphotodiode (PD) are considered for use in the receiving amplifier. Minimum receiving power of -11 dBm obtained with the PD receiver is in good agreement with theoretical values. Deterioration of signal to noise ratio due to fiber guided transmissions is also reviewed in comparison with aerial transmissions.     

Stability regimes and high-frequency modulation of laser diodeswith short external cavity

The results of a linear analysis of stationary generation stability for short external cavity laser diodes in a wide range of coupling coefficients, with the effect of external cavity mode beating being taken into account, are presented. Besides the coherence-collapse transition, high-frequency instability due to mode interaction is revealed and investigated. The results of linear theory are confirmed by computer simulations. They may be considered as guidelines for the design of high-speed laser diodes with integrated passive cavities     

Nonlinear dynamics in directly modulated multiple-quantum-welllaser diodes

A theoretical and experimental analysis of the nonlinear dynamics of Fabry-Perot (FP) and distributed feedback (DFB) multiple-quantum-well (MQW) laser diodes is presented. The analysis is performed under single-tone and two-tone direct modulation. In the FP laser, we observe period doubling and in the DFB laser both period doubling and period tripling are identified. Period doubling is found over a wide range of modulation frequencies in both lasers. The reason for this wide modulation frequency range is attributed to the large relaxation frequencies found in MQW laser diodes. The spontaneous emission factor is measured for both FP and DFB lasers. The dependencies of period doubling on output power and RF input power level are also analyzed. The nonlinear dynamics of the laser are found to be enhanced when modulated under two-tone modulation. Numerical simulations carried out show good agreement with the measured results     

Small-signal analysis of semiconductor lasers modulated atfrequencies on the order of the beat frequency

A small-signal analysis of direct modulation of laser diodes is presented. The analysis is valid over a wide frequency range, up to the frequency corresponding to the round trip time of the active region. This theory is best for laser diodes with small active regions but with large and complicated optical cavities. Optical resonances may occur in this range, resulting in a large response to direct modulation as a result of interaction of the lasing mode with the optical resonance. This theory can predict the linear response of the laser to direct modulation throughout this frequency range. It also predicts the lower frequency relaxation oscillation effect, thus unifying these different effects with a single treatment. The form of the modulation response is found to be the same as the standard relaxation oscillation formula, modified by a factor the form of which depends on the optics of the laser cavity alone     

Microwave/millimeter-wave frequency subcarrier lightwavemodulations based on self-sustained pulsation of laser diode

A method of subcarrier lightwave modulation based on the self-sustained pulsation (SSP) of laser diodes is presented. The tunable microwave/millimeter-wave frequency subcarrier is generated by the laser diode itself under proper DC biases. The subcarrier is frequency-modulated when the DC bias is superimposed with a small AC signal. No external modulation is needed. A theoretical analysis based on a double-section laser diode reveals that Hopf bifurcation is the origin of SSP. Theoretical calculations also show that the modulator has the potential of generating subcarriers up to 100 GHz by proper control of the device parameters V-channeled substrate inner-stripe (VSIS) GaAlAs laser diodes were used to generate 1-7-GHz SSP subcarriers. A preliminary subcarrier multiplexing (SCM) experiment demonstrated the transmission of 1-GHz modulation and video signals on a readily FM-modulated 6.3-GHz SSP subcarrier     

单量子阱激光器小信号调制时的啁啾噪声

量子阱激光器具有良好的小信号调制频率响应 ,能作为高速光通信光源采用直接调制方式进行信号传输。与普通半导体激光器一样 ,直接调制将引起啁啾 ,从而影响光纤通信系统的性能。文中对小信号调制下单量子阱激光器的啁啾特性进行了研究和分析。得出了啁啾幅度和啁啾相位与调制频率的关系。对考虑与不考虑啁啾两种情况下 ,光脉冲在常规光纤中传输时的色散特性进行了模拟分析 ,发现了啁啾对系统的高阶色散有较大影响     

高速直接调制激光器的研究进展

高速直接调制半导体激光器,是目前城域网和高速以太网的关键器件.文章综述了影响高速半导体激光器的调制带宽的各种因素,并探讨了目前所实现的提高半导体激光器调制带宽的各种方法.     

Experimental determination of gain compression factors of a DFBlaser in the presence of TM light injection

Compared to direct current modulation, an optical modulation scheme of laser diodes exhibits no rolling off characteristics due to the electrical parasitics existing in the laser diodes. In particular, in the TM light injection scheme, no injection locking phenomena take place. A number of applications of TM light injection have been reported. In relation to these, determining the gain compression factors of laser diodes in the presence of TM light injection is essential. A new technique for determining these factors is presented. The technique is shown to be viable for measuring the gain compression of distributed feedback (DFB) lasers     

Measurement accuracy of chromatic dispersion by the modulation phase technique

A simple measuring method for the chromatic dispersion measurement is described. This technique uses group delay measurement by modulation phase of laser diodes with different wavelengths. An experiment showed that this technique is capable of measuring the chromatic dispersion of a 42.3-km single-mode fiber. The accuracy of measurement is discussed in view of fiber length, modulation frequency, and the number of employed laser diodes. The accuracy is improved when a higher modulation frequency is used and a longer fiber is measured since the detected phase fluctuation depends little on the modulation frequency and very little on the fiber length. The estimated measurement error of chromatic dispersion was 0.1 ps/km/nm for a four-laser diode system.     

Dynamic behaviour of a GaAs-AlGaAs MQW laser diode

A comparative study is given on the dynamic properties existing between a GaAs-AlGaAs multi-quantum-well (MQW) laser diode fabricated from an MBE-grown wafer and a conventional double-heterostructure laser diode. Spectral broadening and wavelength shift associated with deep injection current modulation were found to be much smaller in MQW laser diodes than in conventional DH laser diodes.     

New Fiber Optic Analog Baseband Transmission Plan for Color TV Signals

A new fiber optic baseband analog transmission plan is proposed. It arranges the original signal into a pulse train for driving the laser diode to generate baseband components at the transmitter. The receiver can be just the same as those used in conventional direct linear intensity modulation systems using light emitting diodes (LED). Advantages of this new plan are (i) potentially cheap fibers can be used, since the plan requires neither an extra wide bandwidth nor special light acceptance conditions, (ii) long repeater spacings can be attained, since abundant light can be coupled into fibers using laser diodes and also the simple structures required for fibers are suitable for realizing low transmission losses, and (iii) the transmitter and receiver can be quite simple. Some candidate pulse arranging plans for generating baseband components are investigated. A pulse frequency arranging plan and pulse width arranging plan seem to be promising for efficient picture transmissions. An experiment using pulse frequency arrangement proves the theory developed here to attain a color TV transmission using a 10 km long step index low-loss fiber for a signal to noise ratio of 44 dB. Future improvements are expected by redesigning the transmitter and receiver and also by improving the mode stability of laser diodes.     

Functional specifications and evaluation of commercially available laser diodes for optical space links

Commercially available GaAlAs laser diodes have been tested to predict their behaviour in intersatellite communications. Three types of device with various structures have been investigated: Sharp lasers, Hitachi lasers (single longitudinal mode emission in c.w. operation) and Ortel lasers (multi-longitudinal modes). Two modulation schemes were applied to devices (NRZ and 4-PPM), corresponding to the two possible modulation formats selected for ISL experiments. Each laser had an average power output of 30 mW, whatever the modulation mode. The output signal, emission spectrum and beam divergence were measured. A reduction in peak power was found with some devices. Multimode spectra were detected under 4-PPM modulation.     

Systematic Study of the Effects of Modulation p-Doping on 1.3-μm Quantum-Dot Lasers

The effects of modulation p-doping on 1.3-mum InGaAs-InAs quantum-dot (QD) lasers are systematically investigated using a series of wafers with doping levels from 0 to 18 acceptors per QD. Various characterization techniques for both laser diodes and surface-emitting light-emitting diode structures are employed. We report: 1) how the level of modulation p-doping alters the length dependant laser characteristics (in turn providing insight on various key parameters); 2) the effect of modulation p-doping on the temperature dependence of a number of factors and its role in obtaining an infinite T₀; 3) how increasing concentrations of modulation p-doping affects the saturated gain, differential gain, and gain profile of the lasers; and finally, 4) the effect modulation p-doping has on the small signal modulation properties of 1.3-mum QD lasers. In each of these areas, the role of modulation p-doping is established and critically discussed.     

The effect of reflections on the system performance of intensitymodulated laser diodes

The performance degradation incurred by unwanted external reflections in high-speed digital optical communication systems using intensity-modulated single-longitudinal-mode laser diodes is investigated by numerical simulation of the rate equations. The effects of intensity modulation on the chaotic behavior in the laser output are examined and performance estimates for 0.5, 2.5, and 5 Gb/s systems are made. For example, it is found that to incur less than 0.25-dB power penalty in a 2.5-Gb/s system using a positive-intrinsic-negative photodetector, the reflected light in the laser pigtail should be at least 36 dB below that of the transmitted light     

Frequency-doubled monolithic master oscillator power amplifierlaser diode

Single-pass frequency doubling of laser diodes extends the wavelength range of infrared laser diodes to blue-green wavelengths. We describe the first experiments of frequency doubling of a coherent, high-power, monolithic master oscillator power amplifier (M-MOPA) laser diode. The output from a l-W M-MOPA is frequency doubled in a single pass through an 8.2-mm-long KNbO₃ crystal. We obtained 3.7-mW diffraction-limited output power at a wavelength of 491 nm and modulation at 20 MHz was demonstrated     

1200 nm GaAs-based vertical cavity lasers employing GaInNAs multiquantum well active regions

Elemental source molecular beam epitaxy has been used to grow vertical cavity laser diodes on GaAs substrates that employ GaInNAs multiquantum well active regions and AlAs-GaAs distributed Bragg reflectors. The laser diodes emitted light at 1200 nm and had threshold current densities of 2.5 kA/cm/sup 2/ and efficiencies of 0.066 W/A under room temperature pulsed operation.