Microwave modulation of laser diodes with optical feedback

It is shown theoretically and experimentally that significant variations in the microwave modulation responses of laser diodes occur due to optical feedback from the transmission fiber. These variations decrease if the laser spectrum is less coherent; this can be achieved, for example, by increasing the intensity modulation index. In a recent letter the authors (1989) discussed the small-signal transfer functions; large deviations in the amplitude and the phase response resulted from optical feedback. For large-signal modulation, weaker feedback-induced deviations of the transfer function were predicted by W.I. Way and M.M. Choy (1988). The microwave modulation characteristics are considered in more detail, including the effects of higher modulation indexes. For a typical intensity modulation index of 26%, variations of up to 3 dB in amplitude and 60° in phase were observed     

Properties of a tunneling injection quantum-well laser: Recipe for`cold' device with a large modulation bandwidth

A quantum-well laser in which electrons are directly injected into the lasing quantum well by resonant tunneling is proposed and demonstrated. The preliminary GaAs-based devices, grown by molecular beam epitaxy have an 80-Å In_0.1Ga_0.9As active single quantum well and AlAs tunneling barriers. I_th is 15 mA in a single-mode ridge device and the differential gain is ~2×10^-16 cm^-2. The principle of operation promises a `cold' laser at high injection levels, and therefore Auger recombination and chirp are expected to be suppressed. In addition, tunneling of carriers into the active well as the potential to achieve large modulation bandwidths     

Microwave modulation of a ruby laser output by absorption

Ruby exhibits the Zeeman effect in the energy levels responsible for the absorption of the R1line. When a ruby laser is directed to pass through a section of unexcited ruby rod, the intensity of the transmitted beam is strongly dependent upon the magnetic field applied to the absorber. This effect was used to modulate the amplitude of a pulsed ruby laser in the microwave frequency region. The experimental modulator consisted of a 0.2- inch long ruby absorber located at one end of anS-band TE111mode cylindrical cavity pumped by a 3.2-Gc/s microwave source. A dc bias magnetic field was also applied to the absorber. When both the laser and the absorber rods were cooled to 80°K, the modulated beam attained maximum modulation index with a bias field of 2.5 kOe. This and other experimental results agree well with calculations made from theoretical analysis of this modulation technique, and indicate that the various known relaxation mechanisms in the spin system of ruby do not impose a frequency limit to this method of modulation.     

Microwave frequency operation of quantum-well injection transittime (QWITT) diode

Present DC and microwave characteristics of three different QWITT structures. A peak output power of ~1 mW in the frequency range 2-8 GHz has been obtained. This is the highest output power obtained from any quantum-well oscillator at any frequency. For the same quantum-well structure, by systematically increasing the length of the depletion region (i.e. the drift region) on the anode side of the device, a corresponding increase in the specific negative resistance and output power is obtained. The significant increase in output power clearly suggests that the intrinsic device characteristics have been improved through the use of a drift region. This is in keeping with small-signal analysis for the QWITT diode, which predicts an improvement in the RF performance of the device with the use of an appropriate drift region length for a particular frequency of operation     

Unambiguous determination of quantum capture, carrier diffusion,and intrinsic effects in quantum-well laser dynamics usingwavelength-selective optical modulation

A novel wavelength-dependent optical modulation technique capable of explicitly delineating the effects of quantum capture, carrier diffusion, and other intrinsic effects in quantum-well laser dynamics is described. Results for a compressively strained multiple-quantum-well laser are presented     

Ultralow internal optical loss in separate-confinement quantum-well laser heterostructures

Internal optical loss in separate-confinement laser heterostructures with an ultrawide (>1 smm) waveguide has been studied theoretically and experimentally. It is found that an asymmetric position of the active region in an ultrawide waveguide reduces the optical confinement factor for higher-order modes and raises the threshold electron density for these modes by 10–20%. It is shown that broadening the waveguide to above 1 μm results in a reduction of the internal optical loss only in asymmetric separate-confinement laser heterostructures. The calculated internal optical loss reaches ∼0.2 cm⁻¹ (for λ≈1.08 μm) in an asymmetric waveguide 4 μm thick. The minimum internal optical loss has a fundamental limitation, which is determined by the loss from scattering on free carriers at the transparency carrier density in the active region. An internal optical loss of 0.34 cm⁻¹ was attained in asymmetric separate-confinement laser heterostructures with an ultrawide (1.7 μm) waveguide, produced by MOCVD. Lasing in the fundamental transverse mode has been obtained owing to the significant difference in the threshold densities for the fundamental mode and higher-order modes. The record-breaking CW output optical power of 16 W and wallplug efficiency of 72% is obtained in 100-μm aperture lasers with a Fabry-Perot cavity length of ∼3 mm on the basis of the heterostructures produced. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 38, No. 12, 2004, pp. 1477–1486. Original Russian Text Copyright ? 2004 by Slipchenko, Vinokurov, Pikhtin, Sokolova, Stankevich, Tarasov, Alferov.     

Small-signal analysis of modulation characteristics in asemiconductor laser subject to strong optical injection

Injection locking of a semiconductor laser can induce major changes in the modulation characteristics of the laser. A small-signal analysis using the lumped element model shows that both the frequency and damping of the characteristic resonances of the coupled complex field and free carriers (gain medium) are modified. The detuning between the injected field and the free-running oscillating field, the amplitude of the injection field relative to the free-running field, the linewidth enhancement factor, the cavity photon and spontaneous carrier decay rates, and the field enhancement of the decay rate are all key parameters in determining the changes to the modulation characteristics. For a broad range of parameters, there is simultaneous enhancement of the modulation bandwidth and stable, locked operation. The enhancement is a cavity phenomena and does not occur in a traveling wave amplifier. It requires that the frequency of the locking field be detuned from the injection-modified frequency of the cavity resonance. This causes a resonant enhancement of the modulation sideband associated with the preferred frequency of the optical cavity. Bandwidth enhancements beyond the free-running laser limit are possible over a range of injection levels and injection frequency detunings     

Four-wave mixing and optical modulation in a semiconductor laser

There is a direct connection between nearly degenerate four-wave mixing in a semiconductor laser and optical modulation in the laser field. It can be understood using a model of an unlocked, optically injected laser, which emphasizes the effect of the laser resonator on the optical interactions. This model correctly describes the observed spectral characteristics and their dependence on the intrinsic parameters of the semiconductor laser. This is used to develop a simple and accurate technique using a single experimental setup for the parasitic-free characterization of the intrinsic laser parameters, including the relaxation resonance frequency, the total relaxation rate, the nonlinear relaxation rate, and the linewidth enhancement factor. Other parameters, such as the spontaneous carrier lifetime, the photon lifetime, the differential and nonlinear gain parameters, and the K factor, are determined from the power dependencies of these parameters. This technique requires only two CW lasers closely matched in wavelength and is applicable to semiconductor lasers of any wavelength and any dynamic bandwidth     

Double-band generation in quantum-well semiconductor laser at high injection levels

Spectral and emission-power characteristics of semiconductor lasers based on quantum-dimensional asymmetric heterostructures with separate confinement in a system of InGaAs/GaAs/AlGaAs alloys are studied in the case of high pump levels in the pulsed mode of lasing (200 A, 100 ns, and 10 kHz). It is shown that, in lasers with a quantum-dimensional active region containing one or two levels of dimensional quantization, the spectrum consists of one or two lasing bands. It is established that the condition for inverse population of the second electron level and two-band lasing are attained due to leveling-off of the rate of stimulated recombination from the first electron level and a high density of states for the second level. It is shown that, in lasers with double-band lasing spectrum, the total emission spectrum exceeds appreciably the emission power of a laser with a single electron level and a single spectral band.     

Nonlinear dynamics of a laser diode with optical feedback systemssubject to modulation

The nonlinear dynamic behavior of a direct frequency-modulated diode laser with strong optical feedback is examined and compared to a laser diode subject to electro-optically modulated, strong optical feedback. Direct modulation is achieved by sinusoidal modulation of the diode laser injection current. Electro-optic modulation is achieved by applying a sinusoidal voltage to an intracavity phase modulating element. The output state (characterized by the output power versus time, the intensity noise spectrum and the optical frequency spectrum) for both types of modulation is dependent on the ratio of the modulation frequency to the external cavity resonant frequency, and the modulation power. A number of distinct states are observed: conventional amplitude modulation (with FM spectra); multimode, low-noise amplitude modulation; multimode, high-noise amplitude modulation; periodic limit-cycle operation; quasi-periodicity; chaos; low-frequency fluctuations; and mode-locking. There are significant differences between the direct and electro-optic frequency-modulation cases. The onset of the dynamic instability is characterized as a noisy period-one oscillation for direct modulation and a low-frequency fluctuation for intracavity electro-optic modulation. Phase portraits produced experimentally with the use of a digital phosphor oscilloscope are shown to agree well with those constructed from output power versus time data. This represents an experimental method for examining the dynamics phase portraits in real-time     

High-bit-rate pulse regeneration and injection laser modulation using a diode circuit

The push-pull diode circuit investigated regenerates pulse trains near 1 Gbit/s with a power amplification of 8.4 dB. As a particular application, the direct modulation of a laser diode was carried out with this circuit at a bit rate of 1 Gbit/s.     

Dispersion and modulation of the linear optical properties of GaAs-AlAs superlattice waveguides using quantum-well intermixing

We report on the large modulation of the optical properties of a 14:14 monolayer GaAs-AlAs superlattice waveguide following quantum-well intermixing. Low-temperature photoluminescence measurements illustrate a large 169-meV differential blue-shift obtained between the disordering-suppressed and disordering-enhanced materials. Effective index measurements are presented as a function of polarization, for both the as-grown and disordered material for near-bandedge and half-bandedge wavelengths, which is the wavelength range 775-1550 nm. The largest effective refractive index shift observed was 9/spl times/10/sup -2/ which exceeds that previously reported for disordered AlGaAs ternary multilayer structures, and illustrates the potential of the superlattice for the fabrication of etch-free, planar optical components with large index contrast.     

Vertical integration of a GaAs/AlGaAs quantum-well laser and along-wavelength quantum-well infra-red photodetector

A short-wavelength (~0.8 μm) GaAs/AlGaAs graded-index separate-confinement heterostructure quantum-well laser has been monolithically integrated with a long-wavelength (~8 μm) GaAs/AlGaAs multiple-quantum-well infra-red photodetector on a semi-insulating GaAs substrate by molecular beam epitaxy. The vertical integration method is used and the combined structure is a pinin structure. Both the laser and detector exhibit excellent characteristics. At room temperature, the ridge waveguide laser has an extremely low threshold current of 25 mA and a differential quantum efficiency above 65% with a stripe width of 20 μm. The quantum-well detector has a peak response at 8 μm and a responsivity of 0.7 A/W     

Electro-optic modulation in a chopped quantum-well electrontransfer structure

The authors demonstrate the first electron-transfer Mach-Zehnder modulators to operate in the 1.55 μm wavelength range. The use of chopped quantum wells provides a wide operating bandwidth compatible with Er-doped fibre amplified systems (>40 nm about 1.54 μm) for electrorefractive waveguide devices. In this wavelength range, obtained voltage-length products are for the 180° optical phase shift V_π×L between 2.7 and 4.0 Vmm     

High-frequency broad-band signal generation using a semiconductor laser with a chaotic optical injection

Chaotic signals with a flat power spectrum over 20 GHz have been generated using two commercially available semiconductor lasers coupled in a unidirectional master-slave scheme. The master laser has an external optical feedback that induces optical chaos in the laser output. A part of the chaotic light output from the master laser is injected into the slave laser. We experimentally demonstrated the generation of broad-band signals up to 22 GHz using lasers whose relaxation oscillation frequency in the free-running state is only around 6.4 GHz. We also show that the experimental results can be well reproduced by numerical simulations using two coupled rate equations. The numerical investigation shows that the high-frequency broad-band signal generation is owing to two key effects: high-frequency oscillations as a result of beating between the master and slave laser lights, and spectrum flattening due to the injection of the chaotic signal. The flatness, stability, and tunability of the power spectra demonstrated in our experiments suggests that the proposed system can be potentially useful for generation of high-frequency broad-band random signals.     

Optical gain in a strained-layer quantum-well laser

The optical gain and the refractive index change of a uniaxially stressed GaAs-Al₂Ga_1-xAs quantum-well laser is studied theoretically using the multiband effective mass theory (k -p method) and density matrix formalism with intraband relaxations. It is found that uniaxial strain of the quantum well substantially alters the subband structures and the optical gain of the quantum-well laser. In particular, the gain of the TM mode increases while the gain of the TE mode decreases with increasing stress. Thus, the threshold current either decreases or increases with the stress, depending on whether the laser is operating in a TM or TE mode     

Semiconductor laser FSK modulation and optical direct discrimination detection

FSK signals are generated in an AlGaAs double heterostructure laser by employing an electrical equalising circuit to compensate for its nonuniform FM modulation characteristics. A 10?9 bit error rate is achieved at a 100 Mbit/s data rate in the optical direct discrimination detection with a Michelson interferometer. The optical FSK discrimination detection performance is studied in terms of receiving signal power level and laser spectral linewidth.     

Band-structure engineering of a cubic GaN quantum-well laser

A theoretical model of a cubic GaN quantum-well laser is studied taking into account the effects of strong spin-orbit (SO) split-off band coupling on the valence-band structure and the optical gain within the 6/spl times/6 Luttinger-Kohn model. It is expected that a very narrow separation (10 meV) between the SO band and the heavy- and light-hole bands causes two undesirable effects on the lasing of GaN quantum well: (1) the TE and the TM polarizations have comparable magnitudes over the wide range of carrier densities and (2) the SO band will be easily occupied by the injected holes which in turn reduces the injection efficiency or increases the lasing threshold. A combination of strain and the use of alloy is proposed to reduce the hole and the electron masses and to increase the SO band separation in order to reduce the lasing threshold.