text: 1. Technical Field
  The present invention relates to a signal transmission device and signal reception device which send or receive multivalued data via optical transmission, a test device which tests a device under test, a test module and a semiconductor chip. Particularly, the present invention relates to a signal transmission device and signal reception device which perform short-distance optical transmission, a test device, a test module, and a semiconductor chip.
  2. Related Art
  One conventionally known method for transmitting data is optical transmission. In order to increase the amount of information to transmit through one line of optical fiber, trunk-line optical communications for long-distance data transmission use multiplexing by multivalued PSK (Phase Shift Keying), QAM (Quadrature Amplitude Modulation), WDM (Wavelength Division Multiplexing), DWDM (Dense Wavelength Division Multiplexing), etc. Digital modem circuits or wavelength multiplexing circuits that implement this method are complex and have problems of large amount of power consumption, large area occupation, high cost demand, etc. However, such multivalued transmissions or wavelength multiplexed transmissions are commonly used because lower costs are required by such transmissions than by laying a plurality of long-distance fibers in parallel.
  As compared with this, short-distance transmissions of about 10 m use parallel transmissions because a cost increase that arises in a case where the number of fibers to be laid in parallel is increased is smaller than that that required by the method of increasing the amount of transmission by PSK, QAM, WDM, etc. mentioned above. However, there is a physical limitation on the number of fibers that can be laid in parallel. Hence, if a further increase in the amount of transmission is required, it is necessary that the amount of information that can be transmitted on one optical fiber be increased.
  Conceivable solutions to increasing the amount of transmission per fiber are (1) increasing the transmission rate, (2) using multivalued transmissions, and (3) using wavelength multiplexed transmissions, likewise in the above-described long-distance transmissions. However, any further increase in the transmission rate is hardly available because the response speed of existing electronic circuits or opto-electric or electro-optic converter circuits has almost reached its possible maximum. Hence, multivalued transmissions or wavelength multiplexed transmissions are usually employed.
  Presently, no relevant patent literature has been identified, so no indication of such literature is herein given.
  However, wavelength multiplexed transmission in short-distance parallel optical transmission requires a multi-wavelength light source, a synthesizer, and a branching filter for each bit of the data transmitted. This worsens the cost performance. PSK or QAM multivalued transmission requires a wider margin on the axis of timing than the currently allowed margin which is almost as wide as possible already.
  ASK (Amplitude Shift Keying) for modulating the amplitude of laser light is another possible option. However, conventional ASK uses an indirect modulation device to modulate the amplitude of laser light showing CW-mode oscillation, which is output from a laser light source. An indirect modulation device requires a large area for mounting, to make highly dense packaging of the transmission system unavailable and worsen the cost performance. As such, conventional devices are not useful in increasing the amount of transmission through optical communications.