Shigehisa Tanaka

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>>

Green-to-Yellow Continuous-Wave Operation of BeZnCdSe Quantum-Well Laser Diodes at Room Temperature

We have grown laser diode structures using highly strained BeZnCdSe quantum wells by molecular beam epitaxy and successfully obtained continuous-wave lasing in the green-to-yellow spectral region (543–570 nm) at room temperature. The green-to-yellow lasing color was tuned by simply varying the Cd content of the quaternary BeZnCdSe quantum well. The threshold current densities of 20-µm-wide lasers were found to be sufficiently low (less than 0.85 kA/cm2). This result demonstrates that BeZnCdSe is a promising material for use as the active layer in high-performance green-to-yellow laser diodes.

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.

Direct Modulation at 56 and 50 Gb/s of 1.3-

Direct modulation at 56 and 50 Gb/s of 1.3-μm InGaAlAs ridge-shaped-buried heterostructure (RS-BH) asymmetric corrugation-pitch-modulation (ACPM) distributed feedback lasers is experimentally demonstrated. The fabricated lasers have a low threshold current (5.6 mA at 85°C), high temperature characteristics (71 K), high slope relaxation frequency (3.2 GHz/mA1/2 at 85°C), and wide bandwidth (22.1 GHz at 85°C). These superior properties enable the lasers to run at 56 Gb/s and 55°C and 50 Gb/s at up to 80°C for backto-back operation with clear eye openings. This is achieved by the combination of a low-leakage RS-BH and an ACPM grating. Moreover, successful transmission of 56and 50-Gb/s modulated signals over a 10-km standard single-mode fiber is achieved. These results confirm the suitability of this type of laser for use as a cost-effective light source in 400 GbE and OTU5 applications.

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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>>

m InGaAlAs Ridge-Shaped-BH DFB Lasers

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.

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

We propose using MQW electroabsorption (EA) modulators as optical gates in wavelength-division-multiplexing (WDM) switching systems. A fabricated MQW-EA gate with integrated waveguides showed a high extinction ratio (>30 dB), a low polarization-dependent loss (0.3 dB), and a low wavelength-dependent loss (1.1 dB) within the gain band (1545-1560 nm) of erbium doped fiber amplifiers (EDFAs). Ultra-high-speed (<40 ps) switching of a WDM signal was demonstrated.

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

High-speed 1.3-μm directly modulated lasers (DMLs) have been developed to support sharply rising optical communications traffic. The most crucial requirement for these lasers is the ability to operate over a wide temperature range with no power-consuming electric cooler. Due to poor temperature characteristics resulting from its poor electron confinement in the multiquantum well, DMLs with InGaAsP material systems have been replaced by DMLs with InGaAlAs material systems, which have stronger electron confinement. In this paper, progress in uncooled 25-Gb/s 1.3-μm InGaAlAs DMLs is summarized. A 160-μm-long ridge-waveguide-type laser showed a lower threshold current Ith of 14.9 mA at high temperatures up to 95 °C, and a small signal-frequency-response bandwidth f3dB of 14 GHz was achieved at a bias current of 60 mA at 95 °C. Using this laser, clear 25-Gb/s operation was obtained at 95 °C. In addition, stable operation was achieved for up to 4000 h at 85 °C, indicating the basic applicability of the device to next-generation 25-Gb/s data communication systems. Furthermore, the developed technology was applied to a novel uncooled 25-Gb/s 1.3-μm surface-emitting laser for optical interconnects, in which a 45° mirror and a lens were integrated monolithically.

Polarization and wavelength insensitive MQW electroabsorption optical gates for WDM switching systems

We developed a 25-Gbps receiver employing a small, cost effective coaxial package and flexible printed circuits. Our fabrication method for reducing electrical resonance and loss was effectively applied. The fabricated receiver had a 24-GHz 3-dB bandwidth and was successfully operated at 25 Gbps.