Single-mode Injection Research Articles

Influence of semiconductor-laser phase noise on coherent optical communication systems

Several authors have recently investigated phase noise in semiconductor lasers and the related problems that arise when such lasers are employed in coherent optical communication systems. We report accurate measurements of high-frequency phase noise in single-mode injection lasers that show the presence of a peak in the phase-noise spectrum at the same frequency as that of the amplitude-noise peak. This peculiar phenomenon must be taken into account when one studies the characteristics of coherent optical communication systems.

Theory of FM noise of single-mode injection lasers

The FM noise of single-mode injection lasers is studied in terms of the root-mean-square frequency deviation Δfrms against the angular modulation frequency ωm off-carrier. Spontaneous emission causes phase fluctuations of the laser wave. Enhanced FM noise may occur due to radiation-induced fluctuations of the carrier density. The influence of the coupling between laser field and active region on this modulation noise is investigated.

Characterization of phase noise in semiconductor lasers

Phase noise in semiconductor lasers has been investigated by many authors in the range of low frequencies (<1 MHz). In this letter we present for the first time phase noise measurements extended up to frequencies greater than 1 GHz. Experimental results showing the power spectral density Sφ̇(ω) of the instantaneous frequency φ̇(t) and the variance σ2[Δτφ(t)] of the phase shift Δτφ(t) are presented. The peculiar behavior of Sφ̇(ω), which presents a sharp peak at the same frequency of the amplitude noise peak, can account for the excessive broadening of the linewidth of single-mode injection lasers.

Spectral characteristics of single-mode injection lasers: The power-gain curve from weak stimulation to full output

Experimental study of the mode powering in an assortment of single-mode lasers covering samples of most of the contemporary structures reveals a common property: the functional dependence on gain of the power in the strong mode. The high-power asymptote is a linear function of the gain, while the low-power asymptote of the light-emitting diode (LED) state is linear in the reciprocal of the departure of the gain from its threshold value. The entire dependence is expressed by a universal dimensionless function of the gain, which is independent of laser structure, and which describes both asymptotes, provides a reasonable connecting link, and gives an experimental fiduciary of threshold. The fiduciary is the current at the sharp maximum in the logarithmic derivative of mode power with respect to junction voltage. Exerimental study reveals that this fiduciary corresponds closely to the conventional extrapolated threshold, with the inflection point in the junction resistance, and with the peak in noise. The universal function yields the dependence of power in the dominant mode on junction voltage given by the steady-state solution of North’s P* theory. It is not consistent with the gain clamping at threshold that is described by familiar hole-burning theories of injection lasers, which limit the lasing region to far too small a range of gain. The same function is found to describe a published measurement of the transition of threshold of an He-Ne laser.

(Invited) Theory of Single Mode Injection Lasers Taking Account of Electronic Intra-band Relaxation

A new theory of injection lasers is given by taking account of the electronic intraband relaxation with the density matrix method. “Band tail” of the gain profile in nondoped crystal is explained with this relaxation. Gain saturations of lasing modes at band to band and band to impurity level transitions are theoretically given. In the case of band to band transition with k-selection rule, a small excess gain suppression affects the lasing behaviour. At the band to impurity level transition with no k-selection rule, the excess gain suppression disappears. The former effect corresponds to the “inhomogeneous gain profile”, and the latter to the “homogeneous gain profile”. Theoretical calculations are in good agreement with experiments. As an extention of this theory, a condition of single longitudinal mode oscillation is shown.

Single-mode and single-frequency injection lasers (review)

The factors responsible for the multimode emission of injection lasers are considered and a review is given of experimental investigations of this subject. Various methods of spatial and spectral selection of the oscillation modes are described, including particularly the use of external and composite resonators. The applications of single-mode injection lasers are discussed. It is reported that such lasers are used very widely in high-resolution infrared spectroscopy.