Hirohisa Sano

InGaAs/InGaAsP MQW electroabsorption modulator integrated with a DFB laser fabricated by band-gap energy control selective area MOCVD

The fabrication and basic characteristics of a InGaAs/InGaAsP multi-quantum-well (MQW) electroabsorption modulator with a novel structure integrated with a distributed-feedback (DFB) laser are presented. A fundamental study was performed on the applicability of the InGaAs/InGaAsP MQW structure to an electroabsorption-type modulator. Efficient attenuation small hole pileup and small chirp characteristics of a discrete modulator based on this MQW structure were demonstrated experimentally. A study of the controllability of in-plane band-gap energy by the use of selective-area metal-organic chemical vapor deposition (MOCVD) was also demonstrated. The modulator was monolithically integrated with a MQW DFB laser of the same material. Using a low-capacitance semi-insulating buried heterostructure, over 14 GHz modulation under high-light-output operations up to +10 dBm was achieved. Modulation at 10 Gb/s with a modulation voltage swing of only 1 V/sub pp/ demonstrates the potential value of this system for 1.55- mu m lightwave communications. >

Ultra-high-speed multiple-quantum-well electro-absorption optical modulators with integrated waveguides

Integrating the input and output waveguides with a multiple-quantum-well (MQW) electro-absorption (EA) optical modulator is shown to achieve ultra-high-speed modulation while keeping the total device length long enough for easy fabrication and packaging. Testing with fabricated modulators showed that a shorter modulation region results in a larger modulation bandwidth. The additional loss due to the waveguide integration was less than 1 dB. An optimized modulator showed a large modulation bandwidth of 50 GHz, a low driving voltage of less than 3 V, and a low insertion loss of 8 dB. A prototype module of this modulator had a bandwidth of greater than 40 GHz. Optimizing the MQW structure makes the modulator insensitive to polarization. These results demonstrate that MQW-EA modulators with integrated waveguides are advantageous in terms of fabrication, packaging, and ultra-high-speed modulation.

Strained InGaAs/InAlAs MQW electroabsorption modulators with large bandwidth and low driving voltage

We demonstrate the improved modulation properties of a new strained InGaAs/InAlAs MQW electro-absorption modulator. This tensile strained MQW modulator shows low driving voltage (V/sub 15 dB/=1.2 V), large modulation bandwidth (f/sub 3 dB/>20 GHz), and a 10 Gbit/s eye pattern with a clear eye opening and high extinction ratio. The effective /spl alpha/ parameter determined from waveform deterioration is 0.6, which is low enough for multigigabit long-haul fiber transmission systems.<<ETX>>

High-extinction-ratio MQW electroabsorption-modulator integrated DFB laser fabricated by in-plane bandgap energy control technique

The local bandgap energy of an InGaAs/InGaAsP multiple quantum well (MQW) structure was precisely adjusted by in-plane Eg control in one-step selective area metalorganic chemical vapor deposition (MOCVD) growth. The technique was then applied to an MQW electroabsorption-modulator integrated distributed feedback (DFB) laser. Experimental results showed superior device characteristics, such as a high extinction ratio of 25 dB and low threshold current of 15 mA.<<ETX>>

Electrooptic effects in an InGaAs/InAlAs multiquantum well structure

The field-induced refractive index change of an InGaAs/InAlAs MQW waveguide is examined for various wavelengths and TE/TM modes using a MZ modulator. The quadratic EO coefficients in the MQW waveguide for both TE and TM modes due to quantum confined Stark effect (QCSE) is on the order of 10/sup -18/ (m/sup 2//V/sup 2/), which is dominant compared with the linear EO effect. An effective linear EO effect, however, is significant because of the bias field arising from the built-in potential. This effect is more than one order greater than the conventional laser EO effect, which may suggest new device applications for QCSE.<<ETX>>

Guided-wave multi/demultiplexers with high stopband rejection

A new guided-wave multi/demultiplexer with high stopband rejection is made using high-silica embedded channel waveguides. This consists of three directional couplers of the same structure, that is, the second and third directional couplers are connected with two output waveguides from the first directional coupler, which suppresses undesired optical signals. This optical device is fabricated using low temperature CVD, lithography, and a reactive ion etching process. It is possible to apply a high-reliability fail-safe module, two-way repeater transmission module using wavelength division multiplexing; it also has other uses.

Frequency-domain measurement of carrier escape times in MQW electro-absorption optical modulators

Frequency-domain measurement of carrier escape times in reverse-biased multiple-quantum wells (MQWs) is proposed and demonstrated. Measurement and analysis of opto-to-electrical (OE) frequency response give the escape times of both electrons and holes with excellent time resolution. Using this technique, we measured escape times in an InGaAs-InAlAs MQW electro-absorption modulator and estimated the carrier density in the wells during optical input. This measurement can clarify the optical saturation effect in optical devices such as MQW electro-absorption modulators.

Calculation of photogenerated carrier escape rates from GaAs/AlGa/sub 1-x/As quantum wells

We present a new theory for photogenerated carrier escape rates from single quantum wells, as a function of an applied electric field, that includes thermionic emission, direct tunneling, and tunneling via thermal occupation of upper subbands, and compare the results for GaAs/Al/sub x/Ga/sub 1-x/As quantum wells with recent experiments. We account for the two dimensional (2D) density of states below the barrier, assume thermal equilibrium of carriers within the well, allow for the possibility of strain in the well and/or barrier, and include the contribution to electron thermionic emission from indirect conduction band minima. Our expressions for thermionic emission reduce, in the limit of large well width, to those derived by assuming a three-dimensional (3D) density of states. The results for electron emission from GaAs/Al/sub x/Ga/sub 1-x/As quantum wells with x=0.2 and x=0.4 barriers at room temperature agree well with experiment. For wells with x=0.2 barriers, thermally assisted tunneling overtakes thermionic emission around 40 kV/cm, while for wells with x=0.4 barriers thermionic emission from the L valley conduction band minima dominates for fields less than 70 kV/cm. For holes we show that the escape rates are very sensitive to the in-plane effective masses, and results using simple expressions for the in-plane masses that do not include light/heavy-hole mixing agree poorly with experiment. The agreement with experiment is improved using in-plane masses that include light/heavy-hole mixing, particularly for wells with high barriers. We suggest that agreement with experiment would be improved by using more accurate in-plane hole masses for all of the subbands. >

High-speed MQW electroabsorption optical modulators integrated with low-loss waveguides

An MQW electro-absorption optical modulator integrated with low-loss input and output waveguides is proposed to achieve larger modulation bandwidth with a shorter modulation region, keeping the total device length large enough for easy fabrication and packaging. A fabricated 1-mm-long modulator with a modulation-region length of 50 /spl mu/m shows a low insertion loss (7 dB), low driving voltage (V/sub 10/ /sub dB/=2.6 V), and large modulation bandwidth f/sub 3 dB/=40 (GHz) extrapolated from measurements up to 20 GHz. This modulator is suitable for application to ultra-high-speed fiber transmission systems.<<ETX>>

Performance of strained InGaAs/InAlAs multiple-quantum-well electroabsorption modulators

The performance of a strained InGaAs/InAlAs multiple-quantum-well (MQW) electroabsorption (EA) modulator was evaluated theoretically and experimentally, The theoretical analysis showed that adding tensile strain to the InGaAs wells widens the wells and reduces the driving voltage; it also showed that adding compressive strain to the InAlAs barriers reduces the band discontinuities and increases the optical saturation power. A fabricated InGaAs/InAlAs MQW modulator with tensile strain in the wells and compressive strain in the barriers had a large modulation bandwidth (f/sub 3 dB/>20 GHz) and a lower driving voltage (V/sub 15 dB/=1.2 V) compared to an unstrained InGaAs/InAlAs MQW modulator, so it had a higher modulation efficiency (17 GHz/V). This strained MQW modulator produced a 10 Gb/s optical signal with a clear eye opening and small chirping (/spl alpha//sub eff/=0.6). Moreover, it had shorter carrier escape times, so it has better optical saturation behavior. Introducing strain thus significantly improves the performance of the MQW-EA modulators.