Isamu Kotaka

Monolithic strained‐InGaAsP multiple‐quantum‐well lasers with integrated electroabsorption modulators for active mode locking

Active mode locking by monolithic lasers with integrated electroabsorption modulators using strained‐InGaAsP multiple quantum wells is described. The electroabsorption modulator acts as a short optical gate when a sinusoidal voltage is driven at a deep bias point. Pulse widths as short as 2 ps have been obtained at a repetition rate of 16.3 GHz for a 2.5‐mm‐long monolithic laser.

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- mu m 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. >

Chirping characteristic and frequency response of MQW optical intensity modulator

The spectral linewidth enhancement factor and frequency responses of electro-absorption-type optical-intensity modulators, especially InGaAs/InAlAs MQW modulators, are described. A method of exactly estimating the value of the alpha factor is presented under the nonlinearity of extinction-ratio characteristics. For measuring the frequency response of modulators, the sideband strength of the modulated output light with an optical spectrum analyzer, is analytically compared with the microwave power of photodiode direct detection with an electrical spectrum analyzer. >

Active mode locking at 50 GHz repetition frequency by half‐frequency modulation of monolithic semiconductor lasers integrated with electroabsorption modulators

Active mode locking achieved at a 50 GHz repetition frequency by modulation at half (25 GHz) the cavity resonance frequency using a monolithic mode‐locked InGaAsP laser integrated with an electroabsorption modulator is described. A pulse width of around 3 ps and a high suppression ratio of more than 33 dB of the intensity modulation at the driving frequency are obtained.

40-GHz bandwidth InGaAs/InAlAs multiple quantum well optical intensity modulator

High-speed waveguide InGaAs/InAIAs multiple quantum well (MQW) optical intensity modulators are demonstrated. To minimize the modulator capacitance, an undoped InAIAs cladding layer is added over the MQW core layer in the optical waveguide. In addition, polyimide is spin coated under the bonding pad. As a result, a very wide bandwidth in excess of 40 GHz is developed with a driving voltage of 6 V for a 10-dB extinction ratio and a linewidth broadening factor alpha of < 1.0 at an operating wavelength of 1.54 microm. The frequency response of the modulator is limited by the device capacitance and inductance.

Transform-limited 7-ps optical pulse generation using a sinusoidally driven InGaAsP/InGaAsP strained multiple-quantum-well DFB laser/modulator monolithically integrated light source

A 20-GHz optical pulse train is generated using a sinusoidally driven InGaAsP/InGaAsP strained multiple-quantum-well (MQW) DFB laser/intensity modulator monolithically integrated light source operating at low voltages (from -4- to -5-V DC bias with a 3.2- and 4.0-V peak-to-peak RF signal). An approximately-transform-limited 7-ps-wide optical pulse, with a spectral width of 47 GHz, is obtained.<<ETX>>

Observation of low-chirp modulation in InGaAs-InAlAs multiple-quantum-well optical modulators under 30 GHz

Modulated light spectra were measured in long-wavelength InGaAs-InAlAs multiple-quantum-well intensity modulators under 30-GHz large-signal modulations. The linewidth broadening factor alpha is determined from the relation between the intensity modulation index and the sideband strength relative to the carrier. The minimum alpha value is estimated to be 0.70 at 1.54 mu m, which is almost the same as the lowest value so far reported in a bulk Franz-Keldysh modulator. This is significantly lower than what is obtained from direct-intensity modulation of injection lasers, making this a useful device for application to high-bit-rate long-haul optical communication systems.<<ETX>>

High-speed InGaAlAs/InAlAs multiple quantum well optical modulators with bandwidths in excess of 20 GHz at 1.55 mu m

The modulator has a large on/off ratio, a low driving voltage (4 V), and operates in the 1.55- mu m wavelength region. Small device capacitance (0.2 pF) has been obtained by using spin-coated polyimides under the bonding pads, and small stray capacitance (0.07 pF) and bonding wide inductance (0.3 nH) have been realized. The modulator requires the lowest power yet reported for a high-frequency-operation external modulator.<<ETX>>

Design of InGaAs-InAlAs multiple-quantum-well (MQW) optical modulators

Design of InGaAs-InAlAs multiple-quantum-well (MQW) optical modulators is described by employing the step segment method (SSM), the three-dimensional beam propagation method (3-D BPM), and the lumped element circuit model. Optical waveguides are treated as materials with complex refractive index. Field distributions, power confinement factors ( Gamma factors), propagation losses, etc. are calculated. It is found that the overlap integral is needed to calculate accurately coupling losses between the optical waveguides and single-mode fibers in place of the simple Gaussian beam-coupling formula and that quasi-single-mode propagation can be obtained even for thick core structures. Design procedures for the driving voltage, and frequency characteristics are also discussed. >

Actively mode-locked strained-InGaAsP multiquantum-well lasers integrated with electroabsorption modulators and distributed Bragg reflectors

This paper describes picosecond pulse generation at 20 Gb/s by monolithic mode-locked lasers integrated with electroabsorption modulators and distributed Bragg reflectors. The electroabsorption modulator using strained-InGaAsP multiquantum wells acts as a pulse shortening gate when a sinusoidal voltage is driven at a large reverse bias voltage. To obtain transform-limited picosecond pulses, the required spectral bandwidth of the distributed Bragg reflector is estimated. Pulse generation around 4 ps with a time-bandwidth product of 0.5 has been performed at a repetition rate of 20 GHz. Driving conditions of the modulator, such as bias voltage and modulation frequency, are investigated. It is shown that an increase in the intensity noise is the main factor limiting performance.