Long wavelength waveguide multiple quantum well optical modulators

The first measurements of large anisotropic modulation of long-wavelength light with a large on/off ratio and low driving voltage propagating along the plane of InGaAs/InAlAs multiple quantum well (MQW) structures grown by molecular beam epitaxy (MBE) are reported. Photocurrent response and optical modulation of waveguide pin diodes is measured for incident light polarization parallel and perpendicular to the MQW layers emitting from a color center laser. The incident-light power and wavelength dependence of on/off ratio are also determined.     

Long-wavelength waveguide multiple-quantum-well (MQW) optical modulator with 30:1 on/off ratio

Large on/off ratio optical modulation of long-wavelength light propagating along the plane of InGaAs/InAlAs multiple-quantum-well (MQW) structures grown by molecular beam epitaxy (MBE) is obtained for the first time. These waveguide MQW optical modulators have a modulation on/off ratio of 30:1 (15 dB) at a driving voltage as low as 9 V, and a capacitance-limited pulse response of 280 ps (FWHM). This measurement is the first step in achieving faster and higher extinction ratio devices.     

Dynamic frequency response for InGaAs/InAlAs multiple quantum well optical modulators

The frequency response of InGaAs/InAlAs multiple-quantum-well (MQW) optical modulators and their dependence on incident light power are described. No decrease in small-signal modulation bandwidth was observed up to ten times the initial value of 1 mW. The mechanism of the frequency-response difference between InGaAs/InAlAs MQW modulators and InGaAs/InP modulators is discussed.<>     

Spectral linewidth of MQW DFB lasers with intensity-modulatedTM-polarized light injection

The spectral linewidth of a multiple-quantum-well (MQW) distributed feedback (DFB) laser is measured when intensity-modulated orthogonally polarized (transverse magnetic (TM) mode) is injected into the laser. The spectral linewidth does not change when the modulation frequency is higher than several hundred megahertz and is almost the same as without light injection. However, it broadens when the injected orthogonally polarized light modulation frequency is close to zero. The line shape of the MQW DFB laser's lasing light becomes non-Lorentzian in shape below a modulation frequency of 500 MHz     

Modulation of visible light by AlAs-AlGaAs multiple-quantum-wellp-i-n structures

We report measurements on the modulation of visible light by a multiple-quantum-well (MQW) modulator designed for normal incidence. The MQW structure is composed of AlAs-Al_xGa_1-xAs with an Al concentration x=0.41. The structure exhibits an exciton absorption peak at 608 nm in the visible part of the spectrum. The phase and amplitude modulation properties of this structure mere studied using a Michelson interferometer that employed a novel double lock-in detection technique. Phase and amplitude modulation were studied as a function of the wavelength to determine their maxima. In our structure, phase modulation up to 200 and amplitude modulation up to 8% were observed. These values are comparable to what is typically achieved by using waveguide geometry     

Saturation intensity and time response of InGaAs-InGaP MQW opticalmodulators

We report modulation saturation and time response measurements on InGaAs-InGaP MQW modulators. The measurements yield a saturation intensity of (3.7±0.1) kW/cm² for a 0-10 V swing and switching times between 10 and 90 ns, depending on the bias voltage and incident light intensity. The observed dependence indicates that field screening due to carrier build-up is the dominant physical mechanism determining both the speed and the saturation intensity. This conclusion is supported by results of theoretical calculations     

High-speed InGaAlAs/InAlAs multiple quantum well optical modulators

High-speed modulation over 22 GHz for waveguided InGaAlAs/InAlAs multiple quantum well (MQW) optical modulators is described. A large on/off ratio of over 25 dB is demonstrated with a low-drive voltage (6 V) operating in the 1.55-μm wavelength region. The design and characteristics of MQW p-i-n modulators are discussed. The causes of large-insertion loss and the required drive voltage bandwidth figure of merit for the MQW modulator are discussed. The frequency response measurements show that the response speed is limited by the RC time constant of the device. This suggests that the speed can be further enhanced by decreasing the size and capacitance of the device     

GaInAs/InP waveguide multiple-quantum-well optical modulator with 9 dB on/off ratio

Clear excitonic peak wavelength shifts are obtained with an applied electric field and large on/off ratio optical modulation of long-wavelength light propagating along the plane of GaInAs/InP multiple-quantum-well (MQW) structures grown by metalorganic molecular beam epitaxy (MOMBE). These waveguide MQW optical modulators have a modulation on/off ratio of 8:1 (9 dB) at a driving voltage as low as 5 V operating at a wavelength of 1.55 ?m. This measurement is the first step towards faster and higher extinction ratio devices.     

Frequency response of an MQW DFB laser injected with intensity-modulated TM-polarized light

The frequency response of a multiquantum-well (MQW) distributed-feedback (DFB) laser was measured by injecting intensity-modulated transverse-magnetic (TM)-polarized light. With low bias currents, the 3-dB bandwidth of the MQW DFB laser's frequency response was anomalously enhanced by injection of intensity-modulated TM-polarized light. The maximum 3-dB bandwidth, 16 GHz, was observed when the normalized bias current (I/I/sub th/-1) was 1. The 3-dB bandwidth behavior of the MQW DFB laser with TM-polarized light injection looks promising for high-speed lasing mode control of semiconductor lasers.<>     

Tradeoff between phase- and intensity modulation in GaAs/AlGaAsdouble heterostructure and multiple quantum well phase-modulatorwaveguides

The wavelength dependence of phase sensitivity (°/mm V) and intensity modulation of a double-heterostructure (DH) device and a multiquantum-well (MQW) device in the GaAs/AlGaAs material system is studied. The results show the tradeoff between phase modulation and intensity modulation and also show that, for the same intensity modulation, the MQW device produces twice as much phase modulation as the DH device. For example, at an intensity modulation of -1 dB the MQW device has a phase sensitivity of 120°/mm V while the DH device gives a value of ≈60°/mm V     

1.3-μm InAsP modulation-doped MQW lasers

The effect of both n-type and p-type modulation doping on multiple-quantum-well (MQW) laser performances was studied using gas-source molecular beam epitaxy (MBE) with the object of the further improvement of long-wavelength strained MQW lasers. The obtained threshold current density was as low as 250 A/cm² for 1200-μm-long devices in n-type modulation-doped MQW (MD-MQW) lasers. A very low CW threshold current of 0.9 mA was obtained in 1.3-μm InAsP n-type MD-MQW lasers at room temperature, which is the lowest ever reported for long-wavelength lasers using n-type modulation doping, and the lowest value for lasers grown by all kinds of MBE in the long-wavelength region. Both a reduction of the threshold current and the carrier lifetime in n-type MD MQW lasers caused the reduction of the turn-on delay time by about 30%. The 1.3-μm InAsP strained MQW lasers using n-type modulation doping with very low power consumption and small turn-on delay time are very attractive for laser array applications in high-density parallel optical interconnection systems. On the other hand, the differential gain was confirmed to increase by a factor of 1.34 for p-type MD MQW lasers (N_A=5×10¹⁸ cm ^-3) as compared with undoped MQW lasers, and the turn-on delay time was reduced by about 20% as compared with undoped MQW lasers. These results indicate that p-type modulation doping is suitable for high-speed lasers     

Intermodulation distortion in a multiple-quantum-well semiconductor optical amplifier

The measurement of intermodulation distortion (IMD) induced by carrier-density modulation in a multiple-quantum-well (MQW) semiconductor amplifier is reported. The results show that MQW amplifiers have 15 dB less IMD than conventional buried-heterostructure semiconductor amplifiers. The IMD is dependent on the output power of the amplifiers, which confirms that the carrier-density modulation is the dominant nonlinear mechanism in MQW amplifiers. In addition, the results show that, unlike conventional buried-heterostructure amplifiers, MQW amplifiers have at least two time constants (200-250 ps and <10 ps) for the gain recovery process.<>     

Analysis of differential gain in InGaAs-InGaAsP compressive andtensile strained quantum-well lasers and its application for estimationof high-speed modulation limit

A simplified model that furnishes an intuitive insight for the change in quantum-well (QW) laser gain due to QW strain and quantum confinement is presented. Differential gain for InGaAs-InGaAsP compressive and tensile strained multi-quantum-well (MQW) lasers is studied using the model. The comparison between the calculated and experimental results for lattice-matched and compressive strained MQW lasers shows that this model also gives quantitatively reasonable results. It is found that the variance-band barrier height strongly affects the differential gain, especially for compressively strained MQW lasers. The tensile strained MQW lasers are found to have quite high differential gain, due to the large dipole matrix element for the electron-light-hole transition, in spite of the large valence-band state density. Furthermore, a great improvement in the differential gain is expected by modulation p doping in the tensile strained MQW lasers. The ultimate modulation bandwidth for such lasers is studied using the above results     

Monolithic integration of InGaAs/InP DFB lasers and InGaAs/InAlAs MQW optical modulators

The first successful monolithic integration of InGaAsP/InP distributed feedback (DFB) lasers and InGaAs/InAlAs multiple quantum well (MQW) optical modulators using LPE (liquid phase epitaxy)/MBE (modlecular beam epitaxy) hybrid growth reported. A 14% light output modulation is observed in this integrated device.     

High-speed InGaAlAs/InAlAs multiple quantum well electrooptic phasemodulators with bandwidth in excess of 20 GHz

High-speed phase modulation (in the frequency bandwidth of 20 GHz, the highest yet reported for multiple quantum well (MQW) phase modulators) for waveguided InGaAlAs/InAlAs MQW optical modulators is reported. The modulator successfully operates at a long wavelength of 1-55 μm with a low required voltage for phase shift (Vπ=3.8 V), small intensity modulation depth below 1.5 dB, and without any modulation bandwidth degradation up to 20 GHz under high input optical power of 0 dBm in single-mode fiber     

Effect of linear gain saturation on small-signal dynamics of MQWDFB lasers

The linear gain saturation effect is shown to be important in determining the dynamics of multiple-quantum-well (MQW) distributed-feedback (DFB) lasers. A more realistic logarithmic dependence of material gain on carrier density is assumed in a comprehensive MQW DFB laser model. It is found through simulation that because of the linear gain saturation, the interplay between modal gain and differential gain leads to an optimal κL for maximum small-signal modulation bandwidth in λ/4-shifted MQW DFB lasers     

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     

Effect of in-phase and antiphase gain coupling on high-speedproperties of MQW DFB lasers

Effect of in-phase and antiphase gain-coupling on high-speed properties is studied for MQW DFB lasers with periodically truncated quantum-wells. The enhancement of modulation bandwidth due to antiphase gain-coupling is found significantly suppressed, and gain-coupled DFB lasers with high κL are preferred for large modulation bandwidth due to the presence of linear gain saturation in MQW lasers     

Substantial reduction of dynamic wavelength chirp of 2 Gbit/s pseudorandom optical signal using three-contact InGaAsP/InP multi-quantum-well distributed-feedback laser

Linewidths as low as 0.08 nm (-20 dB) have been achieved for a three-contact multi-quantum-well (MQW) distributed-feedback (DFB) laser subject to NRZ pseudorandom modulation at 2 Gbit/s. These linewidths are substantially narrower than those observed using equivalent single-contact devices, the improved spectral performance being achieved by using the central device region to control the wavelength of the light generated.<>     

Bifurcation in 20-GHz gain-switched 1.55-μm MQW lasers and itscontrol by CW injection seeding

A gain-switched laser operating at a 20-GHz repetition rate exhibited chaos and period-doubling depending on the bias condition, which was the first observation for 1.55-μm strain-compensated InGaAlAs-InGaAsP multiple-quantum-well (MQW) lasers. Unlike the case previously reported for bulk lasers, the irregular behavior was set over wide bias current conditions for the MQW laser. We show that the instability is attributable to a small gain compression factor of the MQW laser and a high-repetition frequency of the gain-switching operation. Further, we report that the irregular behavior can be successfully suppressed by seeding the laser with external coherent-light injection. The numerical simulations of a set of rate equations using parameters extracted from small-signal modulation characteristics were carried out in order to clarify the conditions for the stabilization of the gain-switching operation. The scenario behind the inhibition of irregular behavior was due to the enhanced damping phenomena of a nonlinear oscillator induced by the injection seeding light. The influence of the α parameter on the optical pulse distortion under the external seeding and the parametric dependence for a single-period operation were investigated based on numerical simulation