Optical communication system

Details Optical communication system Optical communication system

1999-06-10

2003-01-07

Chan, Jason (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S199200, C455S522000

Reexamination Certificate

active

06504636

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an optical communication system for transmitting a high-frequency analog signal such as a radio signal via an optical fiber.
This application is based on Japanese Patent Application No. 10-163561, filed Jun. 11, 1998 and Japanese Patent Application No. 10-309981, filed Oct. 30, 1998, the contents of which are incorporated herein by reference.
Along with recent development of mobile communication, expansion of radio communication service areas is required. To effectively utilize radio wave frequency resources and reduce cost of base station equipment, a scheme in which individual radio zones (cells) are made small, and instead, a number of radio zones are arranged at a high density has received a great deal of attention. This is called a picocell radio zone. To realize the picocell radio zone, a radio communication base station arrangement in which transmitting/receiving devices and transmitting/receiving stations are connected through optical fibers has been examined.
More specifically, a radio base station has transmitting/receiving stations and transmitting/receiving devices. A plurality of transmitting/receiving devices are prepared for one transmitting/receiving station. The output power from each transmitting/receiving device is made small for the picocell radio zones. The transmitting/receiving devices and the transmitting/receiving station are connected through optical fibers. The transmitting/receiving devices transmit signals received from a common transmitting/receiving station to subscribers and transmit signals received from subscribers to a common transmitting/receiving station. The output from each transmitting/receiving device is made small to reduce cost.
A transmitting/receiving device is mainly formed from an antenna section and placed in each cell. A transmitting/receiving station has a modem and a controller corresponding to the plurality of transmitting/receiving devices in the cells. Therefore, the transmitting/receiving station is also called a central control terminal station. An analog radio signal is optically transmitted through an optical fiber between the transmitting/receiving device and transmitting/receiving station. With this arrangement, each transmitting/receiving device can be made simple, compact, and low-cost, and one radio communication base station can provide a number of cells.
In this arrangement, the basic arrangement of a transmitting/receiving device includes only an antenna, and opto-electric and electro-optic conversion devices and does not depend on the data rate or modulation scheme of a radio signal. Therefore, even when the radio transmission scheme is changed, replacement of the transmitting/receiving device or change in constituent elements of the transmitting/receiving devices is unnecessary.
For the above optical analog transmission, an electro-optical converter (E/O converter) is required to convert an electrical signal into an optical signal. At the E/O converter, light intensity of a semiconductor laser element is modulated with a radio frequency signal. As the modulation scheme, a scheme of directly modulating a semiconductor laser element or a scheme using an external optical modulator is employed.
Advantages and disadvantages of these two schemes will be compared. In terms of modulation distortion characteristics, device scale, and device cost, the scheme of directly modulating a laser element is more advantageous.
However, the trend of technology obviously indicates carrier frequency shift to a higher frequency band, e.g., shift to the 2- to 5-[GHz] band as the capacity of a radio frequency signal increases. However, in a distributed feedback laser element (DFB-LD) as a representative laser element, the modulation frequency range with a relatively small modulation distortion is as low as 2 to 3 [GHz]. Therefore, direct modulation of a laser element using a radio frequency signal is becoming difficult.
As disclosed in, e.g., Japanese Patent Publication (KOKAI) No. 6-164427, a scheme (subcarrier transmission) of superposing an intermediate frequency subcarrier signal f
IF
modulated by a data signal on a pilot carrier signal f
LO
as a sinusoidal wave and optically transmitting the superposed analog signal from a transmitting/receiving station to a transmitting/receiving device has been proposed.
In the transmission scheme proposed in this prior art, the intermediate frequency subcarrier signal f
IF
is frequency-converted (up-converted) by a multiplied signal obtained by multiplying the received pilot carrier signal f
LO
on the transmitting/receiving device side, thereby obtaining a radio frequency signal. The laser element is used in a low frequency band with excellent modulation distortion characteristics, and the pilot carrier signal f
LO
is superposed on a frequency close to the intermediate frequency subcarrier signal f
IF
.
According to an embodiment described in the above prior art, a pilot carrier signal f
LO
having a frequency of 300 [MHz] is superposed near an intermediate frequency subcarrier signal f
IF
in the 200-[MHZ] band, as shown in FIG.
1
. In this scheme, on the transmitting device side, to ensure the noise characteristics of the radio frequency signal and increase the frequency stability, the CNRs (Carrier-to-Noise Ratios) of the received intermediate frequency subcarrier signal f
IF
and pilot carrier signal f
LO
must be high. That is, the noise level must be low.
However, in the frequency band near the pilot carrier signal f
LO
, the RIN (Relative Intensity Noise) increases. Therefore, when the pilot carrier signal f
LO
is arranged near the frequency band of the intermediate frequency subcarrier signal f
IF
, as in the prior art, the CNR decreases.
FIG. 2
shows the result of an experiment conducted by the present inventors. When the intermediate frequency subcarrier signal f
IF
is set at 500 [MHz] and the pilot carrier signal f
LO
is set at 550 [MHz], the RIN characteristics largely degrade in accordance with the optical modulation index of the pilot carrier signal f
LO
and, more particularly, at an optical modulation index of 15 [%] or more, as shown in FIG.
2
. Therefore, the communication quality of a radio frequency signal greatly degrades.
Especially, when the optical modulation index of the pilot carrier signal f
LO
increases, degradation in RIN becomes conspicuous. Hence, a radio frequency signal generated by frequency-converting the intermediate frequency subcarrier signal f
IF
using the pilot carrier signal f
LO
contains a number of noise components and therefore has poor transmission characteristics. When a radio frequency signal containing a number of noise components is transmitted, the noise components adversely affect other radio frequency signals to impede radio communication. Solutions to this problem are required.
To cope with a shortage in channels due to the recent increase in number of subscribers or an increase in transmission rate, extensive studies have been made for radio communication using a frequency band higher than the conventional frequency band, e.g., millimeter waves or submillimeter waves. For this system as well, an arrangement for connecting transmitting/receiving devices and transmitting/receiving stations through optical fibers has been examined.
As a connection form using optical fibers, a PON (Passive Optical Network) is used. In the PON, as shown in
FIGS. 3 and 4
, a transmitting/receiving station
1
and a plurality of transmitting/receiving devices
2
are connected through optical fibers
4
in which a passive optical divider
3
is inserted. An optical signal transmitted from the transmitting/receiving station
1
to the optical fiber
4
is divided by the optical divider
3
inserted into the optical fiber
4
, and distributed to the transmitting/receiving devices
2
.
In the PON, a passive optical divider
3
is inserted midway along optical fibers
4
to accommodate the plurality of transmitting/receivin

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