Toshihiro Fujita

Intensity fluctuation in semiconductor lasers coupled to external cavity

Broad peaks are observed in the intensity noise spectrum for external cavity AlGaAs lasers. Oscillating longitudinal mode spectrum measured with a high resolution Fabry-Perot interferometer indicates that the intensity noise peaks result from the lasing of several closely spaced external cavity modes. A new mechanism is proposed to explain the observed noise spectrum; the intensity fluctuation is induced by modulation of the dielectric constant in the semiconductor cavity due to the beating of the electric fields for lasing modes. Qualitative agreements between the theory and experiments are obtained.

Theoretical analysis of longitudinal mode coupling in external cavity semiconductor lasers

The single longitudinal mode behavior in long external cavity semiconductor lasers is discussed. Experimentally, the laser exhibits a single frequency oscillation even for an external cavity length of 100 cm. The mode selectivity of a composite cavity is shown to be insufficient to explain the experiments. Longitudinal mode coupling in semiconductor lasers is found to arise from an interference effect between the modes on the interband transition probability of electrons. Mode coupling equations are derived, which indicates that the single mode oscillation in long external cavity semiconductor lasers is brought about when the coupling strength goes beyond a critical value. It is shown that the effect of the hole burning in the external cavity semiconductor lasers is similar to that in the solitary laser.

Single longitudinal mode operation of long, integrated passive cavity InGaAsP lasers

We propose a new 1.3‐μm wavelength InGaAsP laser—the integrated passive cavity (IPC) laser—and demonstrate its device performances compared with conventional lasers fabricated under similar procedures. The long IPC laser (3.55‐mm‐long passive cavity), as well as the short IPC laser, exhibited single frequency oscillation even just above the threshold, and the maximum ratio of longitudinal main to submode exceeded 30 dB. They also showed favorable effects in the oscillation frequency stabilization.

Second-order distortion generated by amplification of intensity-modulated signals with chirping in erbium-doped fiber

A rate equation analysis for second-order distortion generated in an erbium-doped fiber amplifier is reported. It is shown that the relevant mechanism of distortion is the interaction between the frequency chirping of input light and the local gain tilt in the erbium-doped fiber. It is found that the output light distortion is highly affected by the phase difference between the input light distortion and the chirping-induced distortion. This indicates that second-order distortion is not always degraded by erbium-doped fiber amplifiers.<<ETX>>

Correlation between intensity noise and longitudinal modes of a semiconductor laser coupled to an external cavity

Correlation between intensity noise and fine structures of longitudinal modes in a semiconductor laser coupled to an external cavity has been investigated. It is experimentally shown that the intensity noise is perfectly suppressed when the laser oscillates in ‘‘pure’’ single frequency oscillation even with strong optical feedback. Intensity noise, which arises only when several external cavity modes exist, is interpreted in terms of the time‐dependent refractive index in the semiconductor cavity caused by the light beatings among the lasing modes.

Energy transfer in Tm:Eu codoped fluorozirconate fiber

Excited state absorption‐induced up conversion at 452 and 480 nm in Tm:Eu:ZBLAN fiber was observed for a single pump laser source at 650 nm. Compared to Tm:ZBLAN fiber, codoping with Eu3+ increases the 452 nm emission while the 480 nm emission is reduced. The effective lifetime of the terminal 3H4 level of the 452 nm transition in Tm3+ is shortened by energy transfer to the 7F6 level in Eu3+. This energy transfer scheme is useful in discriminating radiative transitions.

Cancellation of second-order distortion of directly modulated laser in erbium-doped fiber amplifier

Experimental verification of cancellation of second-order distortion from a directly modulated laser diode by amplification in an erbium-doped fiber is demonstrated. This cancellation occurs when the distortion of laser diode and the distortion generated in the erbium-doped fiber amplifier are out of phase. >

Second-order distortion of amplified intensity-modulated signals with chirping in erbium-doped fiber

Second-order distortion produced in an erbium-doped fiber amplifier (EDFA) is investigated theoretically and experimentally. Rate equations for intermodulation products are led by the perturbation method. From numerical calculation, it is shown that optimum combinations of input power to EDFA and signal wavelength exist that give a distortion-free condition, Improvement of the composite second-order (CSO) distortion, which results from the cancellation of the laser distortion and the gain tilt induced distortion, was confirmed for VHF multichannel transmission. Furthermore, CSO distortion-free transmission in cascaded EDFAs has been demonstrated for a 42-channel AM-SCM signal by experimentally determining the optimum operating condition of the EDFA such that each amplifier generates no additive distortion.

Low-loss MOVPE-grown ZnSeZnS superlattice optical waveguide

Abstract Low-loss ( α = 0.7 cm −1 ) short-wavelength optical waveguides using ZnSeZnS strained-layer superlattices (SLSs) were successfully fabricated on GaAs substrates by low-pressure metalorganic vapor phase epitaxy. These waveguides exhibited a large difference in the propagation loss between TE and TM polarizations under some conditions. It is supposed that the difference in the losses may be related to the birefringence of the SLSs. Therefore, we investigated the phase retardation between the TE and TM modes in the waveguide output. From these results, the numerical values for the birefringence were estimated to be 5.8 × 10 −3 for 80 periods of ZnSe(50 A)ZnS(50 A) and 4.25×10 −3 for 100 periods of ZnSe(25 A)ZnS(50 A).

Spectral linewidth broadening due to self-modulation of the refractive index of semiconductor lasers

We propose a new mechanism for the spectral linewidth broadening of semiconductor lasers, which is self‐modulation of the refractive index of the semiconductor cavity caused by interference between the lasing modes. It is shown that the broadening of the main‐mode linewidth due to submodes is explained reasonably well by the self‐modulation term. A new interpretation of the power‐independent linewidth problem is also suggested.