Shigetaka Itakura

High-Current Backside-Illuminated Photodiode Array Module for Optical Analog Links

We have developed a high-current photodiode (PD) array module that consists of a beam splitter, which splits a light beam from a single-mode fiber into four beams with identical powers, a backside-illuminated InGaAs p-i-n PD array, and a two-stage Wilkinson RF power combiner. To obtain a linear PD response, the PD is equipped with a partially depleted absorber, which reduces the bias voltage. To prevent thermal failure, the backside-illuminated PD structure is used, which reduces the thermal resistivity. The beam splitter is fabricated using optical contacts at the SiO2 interfaces between a coating film and a neighboring prism to prevent degradation of the adhesive glue at the prism interfaces along the optical path; therefore, light with a power of a watt level can be used. The two-stage Wilkinson combiner combines four RF outputs of the PD array into a single-RF output through a K connector; the combiner also works as an impedance-matching circuit. Experimental results reveal that an RF power output of 29.0 dBm can be obtained at a frequency of 5 GHz, and good intermodulation distortion characteristics with a third-order intercept point of 32.5 dBm can be achieved.

High-Current Back-Illuminated Partially Depleted-Absorber p-i-n Photodiode With Depleted Nonabsorbing Region

We demonstrate a high-current back-illuminated InGaAs/InP p-i-n photodiode (PD), whose depleted region comprises partially depleted-absorbing, depleted-absorbing, and depleted-nonabsorbing layers to increase RF power output. The back-illuminated PD has an advantage of small thermal resistance between a photoabsorber and a heat sink for avoiding catastrophic thermal failure. The thermal resistance decreases with an increase in the detecting area; however, it simultaneously increases the capacitance, imposing a limitation on the RF response. To reduce the capacitance, we have incorporated a depleted-nonabsorbing layer into a partially depleted absorber PD structure that photogenerated electrons can drift trough. We have fabricated two samples, one with a detecting area of 50 μm, and the other with a detecting area of 70 μm in diameter. Both PDs show high RF power outputs of 28.7 and 29.0 dBm at a frequency of 5 GHz, and 25.7 and 26.7 dBm at each -3-dB frequency of 10.5 and 7 GHz, respectively.

High-Power, Highly Efficient Second-Harmonic Generation in a Periodically Poled

We designed a single-pass quasi-phase-matched second-harmonic generation (SHG) device with a planar waveguide; the device comprised a Y-cut 5 mol% MgO-doped LiNbO3 (MgO:LiNbO3) crystal core that was 3 mum thick and SiO2 cladding. The waveguide provided a high coupling efficiency of 95% between an incident Gaussian beam and the fundamental guided mode of a fundamental wave; it also provided high electric-field confinement in the case of both the fundamental and SHG waves in the core. Thus, a high overlap between nonlinear polarization and an SHG-guided mode was attained. The bonding of the device with the waveguide side positioned downward to a heat sink provided a large heat radiation area when pumping with a near-collimated Gaussian beam, which reduced the temperature rise and its gradient along the waveguide to minimize the phase mismatch. We demonstrated the green light generation of 1.6 W with 40% conversion efficiency using a 7-mm-long sample and 1.2-W SHG with 60% efficiency using an 18-mm-long sample.

{\hbox {MgO}}{:}{\hbox {LiNbO}}_{3}

We designed a high-power microwave photodiode array consisting of an optical divider, a four-element InGaAs pin photodiode array and an RF power combiner. The developed array achieved RF output power of 1 W at 5 GHz.

Planar Waveguide

We designed a tapered unstable-resonator laser diode consisting of a 3-mm-long tapered amplifier and a fiber-Bragg-grating reflector, both of which were coupled with a biconical microlens at the fiber end. The amplifier was composed of an InGaAs single quantum well inside a waveguide, which had an optical confinement factor of 1.2%, a tapered current constriction of 6deg in full angle, and an input width of 10 mum. The grating reflector was fabricated in a polarization-maintaining fiber with a reflectivity of 99% and a reflection bandwidth of 0.19 nm. A biconical microlens with vertical and horizontal radii of 3.5 and 11 mum, respectively, was fabricated at the fiber end to increase the coupling efficiency between the tapered amplifier and the grating reflector. An output power of 3.2 W was obtained at a wavelength of 1064 nm in multimode operation for the grating reflection bandwidth.

High-power microwave photodiode array for radio over fiber applications

We designed a 1.06-mum single-quantum-well (SQW) InGaAs/AlGaAs planar tapered amplifier that was injected with seed light of a fiber Bragg grating stabilized laser diode through a fiber biconical microlens. To increase the amplifier output, the microlens with approximately 3- and 11-mum radii on vertical and horizontal axes, respectively, provides high coupling efficiency between the laser diode and the amplifier. The microlens also controls propagation in the tapered gain area to suppress the filament formation. In addition, the small radii of the microlens reduce near-end reflection at the amplifier input to prevent parasitic laser oscillation of the amplifier. We demonstrated near-diffraction-limited output of 5.5 W with the beam quality factor M2 of 1.5 by using a 3-mm-long amplifier having an optical confinement factor of 1.2%.

High-Power Tapered Unstable-Resonator Laser Diode With a Fiber-Bragg-Grating Reflector

We fabricated a backside-illuminated high-current InGaAs/InP photodiode, and demonstrated an RF power of 25.8 dBm at 5 GHz. The third-order intercept point was 34.2 dBm at the photocurrent of 100 mA, 5.2 GHz.

High-Power 1.06-

We designed an aspheric anamorphic microlens for coupling between a tapered amplifier with a length of 3 mm and a planar-waveguide second-harmonic-generation device consisting of a periodically poled MgO-doped lithium niobate crystal core with a thickness of 3 μm. A coupling efficiency of 35% and corresponding green light generation with a power of 65 mW and conversion efficiency of 7.4% was experimentally demonstrated.