Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1997-09-12
2002-05-14
Chan, Jason (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200, C359S199200, C359S359000, C359S199200
Reexamination Certificate
active
06388784
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a digital optical communication device for transmitting and receiving optical signals involving subcarriers.
2. Description of the Background Art
Digital optical communication has been recently utilized over a wide range of application. For example, infrared digital optical communication is widely applied to remote control for household electric products such as televisions, videos and the like.
Various methods have heretofore been contrived as systems of digital optical communication. Typical examples of these systems are amplitude shift keying (ASK) modulation, frequency shift keying (FSK) modulation, phase shift keying (PSK) modulation, and the like. When applied to digital optical communication, these modulation systems can be roughly classified into two types of communications, coherent optical communications and incoherent optical communication. The coherent communication is adapted to perform modulation by employing an optical medium itself as carriers, and the incoherent communication is adapted to perform modulation by carriers simulatively created by on-off controlling light in a cycle considerably slower than its wavelength. The carriers simulatively created in case of the incoherent communication are generally called subcarriers.
FIGS. 1A
to
1
C show pulse signals of the respective modulation systems.
FIG. 1A
shows the pulse signal of the ASK modulation system. When sections {circle around (
1
)} to {circle around (
5
)} divided by dotted lines are referred to as symbols, each symbol involves a plurality of pulses (subcarriers). The symbols {circle around (
1
)}, {circle around (
3
)} and {circle around (
4
)} involve subcarriers of the same frequency respectively, and indicate logic “1”. On the other hand, the symbols {circle around (
2
)} and {circle around (
5
)} involve no subcarriers respectively, and indicate logic “0”.
FIG. 1B
shows the pulse signal of the FSK modulation system. The symbols {circle around (
1
)}, {circle around (
3
)} and {circle around (
4
)} involve subcarriers, and indicate logic “1”. The symbols {circle around (
2
)} and {circle around (
5
)} also involve subcarriers, which are different in frequency from those in the symbols {circle around (
1
)}, {circle around (
3
)} and {circle around (
4
)}. Due to the different frequency of the subcarriers, the symbols {circle around (
2
)} and {circle around (
5
)} indicate logic “0”.
FIG. 1C
shows the pulse signal of the PSK modulation system. The symbols {circle around (
1
)}, {circle around (
3
)} and {circle around (
4
)} involve subcarriers, and indicate logic “1”. The symbols {circle around (
2
)} and {circle around (
5
)} also involve subcarriers, which are identical in frequency to but out of phase with those in the symbols {circle around (
1
)}, {circle around (
3
)} and {circle around (
4
)}. Due to the phase difference between the subcarriers, the symbols {circle around (
2
)} and {circle around (
5
)} indicate logic “0”.
The subcarriers, which are generally formed by simply controlling light on-off as described above, are substituted not as sine waves but as rectangular waves in general. As shown in
FIG. 2
, light emission and no emission are repeated in each symbol in a constant cycle for generating subcarriers in the ASK modulation system. An interval corresponding to one cycle of the subcarriers is hereinafter referred to as a slot.
In the aforementioned infrared remote control, a technique of modulating transmit data by a PPM (pulse position modulation) system and modulating certain carriers again by a waveform modulated in the PPM system is generally employed as one of many data transmission systems.
FIGS. 3A and 3B
illustrate an exemplary waveform of this data transmission system. While
FIG. 3A
illustrates a PPM modulated waveform in a broad view, each pulse consists of subcarriers, as shown in FIG.
3
B. The PPM modulation system is adapted to transmit data by pulse positions, and pulse spaces Tp and Tp/2 indicate “1” and “0” respectively in FIG.
3
A.
FIGS. 4A and 4B
illustrate a conventional digital optical transmitter
721
and a conventional digital optical receiver
725
for the transmission system generally employed in the aforementioned infrared remote control, for example. The digital optical transmitter
721
includes a PPM modulation part
722
for receiving transmit data and PPM-modulating the same, an ASK modulation part
723
for ASK-modulating a PPM modulated signal, and an electrical/optical (E/O) conversion part
724
for converting an electrical modulated signal to an optical modulated signal. The PPM modulation part
722
receives the transmit data, for generating and outputting the aforementioned PPM modulated signal shown in
FIG. 3A
with no superposition of subcarriers. The ASK modulation part
723
modulates subcarriers by the PPM modulated signal inputted therein, and outputs the signal shown in FIG.
3
B. The E/O conversion part
724
converts the electrical modulated signal received from the ASK modulation part
723
to an optical on-off signal and outputs the same.
The digital optical receiver
725
includes an O/E (optical/electrical) conversion part
726
for converting an optical modulated signal to an electrical modulated signal, an ASK demodulation part
727
for demodulating the electrical modulated signal from the O/E conversion part
726
in the ASK system, and a PPM demodulation part
728
for receiving the ASK demodulated signal and converting the same to receive data. The O/E conversion part
726
receives the optical modulated signal, and converts the optical on-off signal to an electrical modulated signal. The ASK demodulation part
727
outputs an ASK demodulated signal (PPM modulated signal) obtained by removing the subcarriers involved in the electrical modulated signal. The PPM demodulation part
728
converts the ASK modulated signal to receive data and outputs the same.
FIG. 5A
is a circuit diagram of the O/E conversion part
726
and the ASK demodulation part
727
, and
FIGS. 5B
to
5
E illustrate output waveforms of the respective components. Referring to
FIG. 5A
, the O/E conversion part
726
includes a photoreceptor
731
, which is an element converting received light to an electric current. The ASK demodulation part
727
includes an amplifier
732
, a limiter
733
, a bandpass filter (BPF)
734
, a rectifier
735
, an integrator
736
, and a comparator
737
. The amplifier
732
converts the current received from the photoreceptor
731
to a voltage and amplifies the same.
The limiter
733
suppresses a voltage exceeding a certain value. The BPF
734
, which is adapted to remove noise components from subcarriers, matches its center frequency with the frequency of the subcarriers. When the photoreceptor
731
receives the optical signal shown in
FIG. 5B
, its output is converted to the signal shown in FIG.
5
C through the amplifier
732
, the limiter
733
and the BPF
734
.
The rectifier
735
extracts only a plus component of the voltage. The integrator
736
integrates the output from the rectifier
735
, and outputs the signal shown in FIG.
5
D.
The comparator
737
, which is formed by a Schmidt buffer, converts the output of the integrator
736
to a rectangular waveform as shown in
FIG. 5E
, and outputs the same.
While the digital optical transmitter and the digital optical receiver employing subcarriers have been described, an advantage of employment of the communication waveform using subcarriers is now described.
While the spectrum of a non-modulated signal (baseband signal) is generally distributed in a low-frequency region, this signal spectrum shifts to a band around the frequency of subcarriers when the subcarriers are modulated by a modulated signal. Particularly in the case of infrared communication, a number of external noises exist in the low-frequency region. Therefore, it is possible to improve the signal-to-noise ratio by modulating the modulated signal (baseband signal) by subcarriers and moving its s
Ikeda Yutaka
Kaminokado Tsukasa
Nakajima Takashi
Nakamura Makoto
Nakaoka Hiroyuki
Chan Jason
Sedighian M. R.
Sharp Kabushiki Kaisha
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