We demonstrated all-monolithically-integrated self-scanning vertical-cavity surface-emitting laser (VCSEL) arrays whose emitters emit light individually. Each array needs only three to five bonding pads even when the number of VCSELs is over hundreds. Recently, an array with hundreds of VCSELs that lase individually is needed for some applications that do not require mechanical devices, such as MEMS and polygon mirrors. However, conventional VCSEL arrays need the same number of bonding pads, gold wires, substrate wirings, and IC driver terminals as emitters, thus taking up space and increasing application costs. We applied the self-scanning light emitting device (SLED) technology to the VCSEL array to reduce the number of bonding pads and other components mentioned above. In this paper, we discuss the temperature characteristics of the all-monolithically-integrated self-scanning VCSEL array. All layers in the epilayer structure of the array were formed at a time by MOCVD. The array consists of AlGaAs based thyristors and conventional 850 nm oxide-confined VCSELs on a p-type GaAs substrate. The array includes a transmission region, an emission region with 16 emitters, five bonding pads on the top side, and one anode electrode on the bottom of the substrate. Switching of the thyristor and lasing of the VCSEL were achieved up to 90°C. Self-scanning of the array was also possible at a temperature as high as 90°C. This is a key technology to dramatically reduce the size and cost of the chip itself and its applicable applications that require many emitters.
We introduce the characteristics of vertical-cavity surface-emitting lasers (VCSELs) for use in optical communications. In the field of optical interconnections and networks, 850 nm VCSELs are key optical transmitters due to their high-speed modulation and low power consumption. One promising candidate for achieving high-speed modulations exceeding 50 Gbps is the transverse-coupled-cavity (TCC) VCSEL. In this talk, we demonstrate the characteristics of 850 nm transverse-coupled-cavity VCSELs, which helped us achieve a high 3dB modulation bandwidth (30 GHz) at 0 °C and realize eye-opening at the large-signal modulation rate of 48 Gbps. The VCSEL's epilayer structure was grown by MOCVD. The active region consists of three strained InGaAs QWs surrounded by AlGaAs barriers. The n-type and p-type DBRs are composed of AlGaAs/AlGaAs, respectively. A line-shaped H+ ion was implanted at the center of the bowtie-shaped post, dividing it into two cavities. The threshold current of the TCC VCSEL with an oxide aperture of 3.6 μm is 0.33 mA. Only the left-side cavity is pumped, while the right cavity is unpumped. The effect of modulation bandwidth enhancement was observed over a wide temperature range of 120K thanks to an optical feedback in the coupled cavities. These results show the possibility of achieving high-speed VCSELs without any temperature or bias control. We also demonstrate an ultra-compact photodetector-integrated VCSEL with two laterally-coupled cavities. An output power and a photocurrent exhibit similar tendencies under a wide range of temperature changes. This device could be also used for monitoring output power without a conventional photodetector mounted separately.
We review the characteristics of vertical-cavity surface-emitting lasers (VCSELs) for use in printers and optical
communications. In 2003, we launched the world's first laser printer with a 780-nm single-mode 8×4 VCSEL array
introduced to the light exposure system in order to meet the market demands for improving the image quality and speed
for laser printers. The design of the VCSEL array enabled us to increase the pixel density and the printing speed by
projecting 32 beams at a time to the photoconductor in the exposure process. High uniformity with less than 5% of
variation has been achieved for both the optical output and the divergence angle. Currently, our high-end color printer is
capable of producing the resolution of 2400 dpi (dots per inch) at the speed of 137 ppm (pages per minute). In the field
of optical interconnections and networks, 850-nm VCSELs are needed as high-speed optical transmitters (≥10Gbps). In
order to address communication traffic that will increase further as well as to reduce their power consumption to an even
lower level, we assessed the lasing characteristics of 850-nm VCSELs with InGaAs strained quantum-well (QW) active
layers by changing the ratio of Indium composition. As a result, we succeeded in reducing the power consumption per bit
to 43 fJ/bit at 10-Gbps, which is much lower than that of commercial GaAs QW VCSELs. Also, we studied 850-nm
transverse-coupled-cavity VCSELs, which enabled us to achieve a high 3dB modulation bandwidth (>23 GHz) and
realize eye-openings at the large-signal modulation rate of 36 Gbps.
We demonstrated highly strained GaInAs/GaAs QW VCSELs emitting at 1.16 micrometers . The fabricated device shows the record low threshold current density and high efficiency in 1.1-1.2 micrometers wavelength range. The VCSEL structure was monolithically grown on a (100) n-type GaAs substrate by a low-pressure metalorganic vapor phase epitaxy (MOVPE). The active region consists of triple 8 nm thick Ga0.64In0.36As TQWs separated by 25 nm GaAs barrier layers. The compressive strain of QWs is 2.3%. The threshold current is 3 mA for a 10micrometers ~10micrometers oxide device, corresponding to a threshold current density of 3 kA/cm2. We achieved the maximum output power of over 2 mW and a slope efficiency of 0.3 W/A at 25 degree(s)C, which are the record data for 1.2 micrometers band GaInAs VCSELs. The maximum CW operating temperature is 85 degree(s)C. The threshold current is almost constant in the temperature range of 20-70 degree(s)C which results from appropriate wavelength matching between gain peak and lasing mode. The temperature dependence of the lasing wavelength is 0.07 nm/K. We present the details of temperature characteristics of the fabricated VCSEL and discuss a possibility of uncooled GaInAs/GaAs VCSELs for high speed LANs.
This paper presents a new image retrieval method suitable for a large-scale image filing system used in, for example, medicine. Keywords for image retieval may be more ambiguous than those for document retrieval, and the meaning of the keywords depends on each user. In order to overcome this problem, we introduce two concept spaces into the retrieval method: one of the user, and the other of the system. The mapping relation between the two concept spaces is then updated through the learning process, so that the retrieval system can be optimized for each user. An optical implementation of this system is also proposed to realize a fast retrieval.