Linewidth Of Semiconductor Lasers Research Articles

Linewidth determination from electrical noise measurements on semiconductor lasers

A novel method to determine the linewidth of a semiconductor laser is described. By measuring the noise voltage at the electrical terminal, the electron density variations in the gain medium can be determined. Since these variations are the major cause of the linewidth of semiconductor lasers, both the linewidth and the lineshape can be determined from electrical measurements if the α-parameter is known and the parasitic impedances of the laser are small.

Dependence of semiconductor laser linewidth on measurement time: evidence of predominance of 1/ f noise

Linewidths of 1.3 µm InGaAsP lasers were measured by using delayed self-heterodyne set-ups with two different delay-line lengths, i.e. with two equivalent measurement times. It has been found that in high-power operation the linewidth increases as the measurement time becomes longer, and that this dependence is explained well by a calculation assuming that the 1/f noise in the FM noise spectrum is the predominant cause of the spectral broadening. The significance of this fact in coherent optical communications is discussed.

Measurement of linewidth broadening factor in gain-switched InGaAsP injection lasers by CHP method

First measurements of the semiconductor laser linewidth broadening factor α using a recently proposed CHP (chirp-halfwidth product) method are reported. Gain-switched optical pulses have been generated by sinusoidal RF and comb generator drive. The simultaneous recording of streak-camera and monochromator traces provides the required data for determination of α. The values of α between 6 and 7.5 are obtained for a 1.3 μm mesa laser. Computer simulations predict that the CHP formula should provide an estimate of α within ±15% for either comb generator or hard sinusoidal drive.

Intrinsic lineshape and FM response of modulated semiconductor lasers

A new method for measuring the FM response and the intrinsic linewidth of modulated semiconductor lasers is reported. No additional stochastic line broadening has been observed for direct modulation of buried-heterostructure 1.3 μm lasers with modulation depths of less than 20%. Thus FSK systems employing heterodyne detection are expected not to exhibit any additional phase noise penalty due to direct current modulation.

Spectral linewidth of external cavity semiconductor lasers with strong, frequency-selective feedback

The spectral linewidth of 1.5 μm external cavity semiconductor lasers is investigated. For nonzero intermediate facet reflectivity, detuning from exact resonance is found to reduce linewidths substantially for strong, frequency-selective feedback. A 10 cm cavity gives linewidths below 1 kHz, while coherent systems requirements will be met by cavities under 2.5 cm.

Multimode effects in the spectral linewidth of semiconductor lasers

We present experimental results on the spectral linewidth of one longitudinal laser mode of multimode semiconductor lasers with different emission wavelengths and different wave guiding. The dependence of linewidth on mode power cannot be described by a single-mode theory (modified Schawlow-Townes formula), for all lasers. The observed power independent contribution to the linewidth as well as the slope of the linewidth versus optical power data show significant multimode effects. A theoretical model for the linewidth of a two-mode laser provides a qualitative description of the experimental results.

Error-rate floor in optical ASK heterodyne systems caused by nonzero (semiconductor) laser linewidth

Using a simple model to account for the effects of non-negligible linewidths for the transmitter and local oscillator lasers in a heterodyne ASK system, we show that there is an error probability floor which is a function of the IF bandwidth and the relative decision threshold level. We describe the physical reason for this behaviour and present some calculated results for total error probability in terms of signal power, IF linewidth and bandwidth, and threshold setting. The implications of this ‘floor’ for practical receiver design are discussed.

Influence of (semiconductor) laser linewidth on the error-rate floor in dual-filter optical FSK receivers

We discuss an error-rate floor in dual-filter optical FSK receivers in terms of the IF linewidth and the frequency shift between 0s and 1s, using a comprehensive model for the receiver which takes into account nonzero laser linewidth, noise correlation and signal overlap between the filters. We present calculated results for total error probability in terms of signal power, IF linewidth and frequency shift, illustrating the effect of the floor on receiver sensitivity, and we contrast it with a related effect in heterodyne ASK systems.

Frequency modulation noise and linewidth reduction in a semiconductor laser by means of a negative frequency feedback technique

Electrical negative frequency feedback control has been shown to reduce frequency modulation (FM) noise linewidth in semiconductor lasers. The method is based on the direct frequency modulation capability of a semiconductor laser. An error signal is extracted through optical heterodyne frequency discrimination detection using a stable master laser. FM noise is reduced by more than 20 dB and linewidth is reduced by one order of magnitude.

Reduction of dynamic linewidth in single-frequency semiconductor lasers

Carrier density oscillations and frequency chirping of pulse code modulated single frequency semiconductor lasers are shown to be greatly reduced by use of a small current step in the leading edge of the drive pulse. For a 100 km single mode fibre link the dispersion power penalty at 1.55 μm resulting from dynamic line broadening is predicted to be reduced from 3.7 to 1.0 dB by use of the modified current pulse.

Semiconductor laser linewidth in optical feedback configurations

We present a theoretical analysis for semiconductor laser linewidth, employing optical feedback. This analysis is valid not only for weak feedback, but also for strong feedback including multiple reflections. The experimental result reported so far under a strong feedback condition is well explained.

Semiconductor laser linewidth broadening due to 1/ f carrier noise

The effect of low frequency (1/f) carrier noise on the linewidth of a semiconductor laser is investigated. Theoretical results suggest that broadening is strongly dependent on the 3 dB corner frequency of the 1/f noise. As an example, a linewidth of 80 MHz broadens to 110 MHz with a 5 MHz corner frequency.

Measurements of the semiconductor laser linewidth broadening factor

A new method is presented for determining the semiconductor-laser linewidth-broadening factor α. Measurements on 1.5 μm devices suggest that α decreases with decreasing laser length. 300 μm-long devices emitting at 1.3 and 1.5 μm were found to have similar values of α (6.6 cf. 6 4), but at 0.85 μm significantly lower results were measured (2.8)

Direct observation of visibility curve of semiconductor lasers

A method for the determination of the linewidth of semiconductor lasers is demonstrated. It employs tuning of the laser frequency, observation of the backscattered light from a single-mode fibre and Fourier-transform evaluation.

Experimental 1.5 μm coherent optical fibre transmission systems

The authors consider the design requirements for 1.52 μm coherent optical fibre transmission systems. Some of the essential requirements are: narrow linewidth sources, polarisation stable transmission media, low insertion loss optical mixers and frequency tracking or phase locked local oscillators. Having been successful in reducing the linewidth of semiconductor lasers to less than 30 kHz, by using injection locking techniques, and in fabricating optical mixers with insertion losses of the order of dB, they have been able to experiment with coherent optical fibre transmission systems to assess their practicability. In this paper it will be shown that coherent systems can be operated over long lengths of conventional low loss monomode fibre without any polarisation penalty and give far better performance than direct detection systems.

Oscillation center frequency tuning, quantum FM noise, and direct frequency characteristics in external grating loaded semiconductor lasers

Simple analytical expressions for the oscillation frequency, quantum FM noise spectrum, oscillation power spectrum, spectral linewidth, and direct optical frequency modulation efficiency in semiconductor lasers with external grating feedback are presented. Experimental results for the grating loaded AlGaAs lasers are in good agreement with the theoretical predictions. Remarkable reduction in spectral linewidth to less than 50 kHz is achieved by feeding back only 10-3of the output power. Oscillation frequency stabilization, frequency jump behavior, and reduction in direct optical frequency modulation efficiency are also discussed.