April 25, 2008
The Furukawa Electric Co., Ltd. (hereinafter called Furukawa Electric) has developed “optical phase-locked loop (Note 1) technology” to realize synchronization of ultrafast optical signals, and, using this technology succeeded in generating a synchronized optical clock signal (Note 2) with repetition rate of 1-THz (Note 3) in the telecommunications band for the first time in the world.
The synchronized optical clock signal is used, in repeaters such as routers that link communication networks, as a “timing signal (i.e., time reference)” to realize all-optical signal processing (Note 4). The all-optical signal processing in the repeaters where communication traffic particularly concentrates can significantly improve the operation speed limit of the electrical signal processing now in use, so that it is expected to make the terabit-class signal processing possible in the future. For your information, a communication speed of 1 terabit per second means that 25 volumes of 2 hour-long movies can be transmitted in a second.
In the future, Furukawa Electric intends to pursue technology development further utilizing this technology aiming at realization of innovative equipment such as optical regenerative repeaters (Note 5) and optical routers.
This technology is the product of a joint research effort with the Japan Science and Technology Agency for three and half years since November, 2003.
Developmental Background
To effect long-distance optical communications, it is necessary to use repeaters to compensate for the degradation of optical signals due to transmission, rendering the signals ready for retransmission. As for the attenuated optical power, the signals are amplified by optical amplifiers installed every 50 to 100 km. In addition, reshaping and retiming of optical signals are needed.
Whereas the optical power is amplified directly as light using an optical amplifier, reshaping and retiming of optical signals are carried out by electrical means in which optical signals are first converted into electrical signals for processing and subsequently converted back again into optical signals for retransmission. Thus, the limit of processing speed is thought to be around 100 gigabit per second.
Recent years have seen a remarkable increase in telecommunications traffic, so that to support higher transmission speed, hopes are raised for “all-optical network” in which optical signals are processed as light in high speed without electrical conversion. In addition, all-optical processing in repeaters gives a cost advantage due to the lower power consumption and the reduced space.
Accordingly, Furukawa Electric has developed “optical phase-locked loop technology” which is an elemental technology indispensable for carrying out “timing compensation for optical signals” (i.e., one of the functions of an optical repeater) directly as light, and has succeeded in generating 1-THz repetition rate synchronized optical clock signals for the first time in the world.
Technological Features
To regenerate transmitted optical signals at a repeater, a clock signal which serves as a time reference to make out signals is needed, whereby in order to achieve exact regeneration processing, the clock signal is made to match (i.e., synchronize) with the time reference (i.e., timing) of the optical signal.
In other words, for regeneration processing of optical signals with higher speeds, it is essential to enhance the speed of the clock signal and to establish a synchronization technology of the clock signal with the optical signal.
The optical phase-locked loop that Furukawa Electric has developed here has a simple structure consisting of: 1) a clock light source to generate ultra high-speed optical clock signals, 2) a timing error detector that detects the timing error between the optical clock signal and the reference optical signal, and 3) a controller that minimizes (i.e., synchronizes) the timing error.
| (1)Clock light source | A distributed feedback laser diode (DFB-LD) (Note 6) is used to generate ultra high-speed optical clock signals with controllable timing. |
| (2)Timing error detector | Two photon absorption effect at a silicon photodiode (Note 7) is utilized to detect a timing error signal between ultra high-speed optical signals. |
| (3)Controller | The timing error signal detected is fed back to the clock light source via a controller achieving synchronization of the optical clock signal with the reference optical signal. |
Results of the Development
Configuration of optical phase-locked loop
Using the optical phase-locked loop developed here, generation of synchronized optical clock signals up to 1 THz repetition rate has been confirmed for the first time in the world. Timing jitter (Note 8) which stands for an index of synchronization performance was 0.016 rad2 (corresponding to 126 femtosecond (Note 9)) at a synchronized optical clock signal repetition rate of 0.16 THz, and 0.03 rad2 (corresponding to 28 fs) at a clock signal repetition rate of 1 THz, respectively, thus achieving a practical value for synchronization performance. Also regeneration of optical signals using the synchronized optical clock signals developed here has been carried out successfully.
Glossary
(Note 1) Optical phase-locked loop:
A feedback circuit for generating optical signals, timing of which coincides with that of the reference signal.Back to Main Content
(Note 2) Synchronized optical clock signal:
An optical signal modulated with a timing corresponding to the transmitted optical signal, so as to retain the timing information. Back to Main Content
(Note 3) 1 Terahertz:
Oscillation frequency of 1012 cycles per second. It is 100 times the frequency of 10-Gigabit optical communications which became widespread at present. Back to Main Content
(Note 4) All-optical signal processing:
A technology in which signals are processed by optical means without using electrical means. Because ultra high-speed optical phenomena are used, ultra high-speed processing of signals becomes possible.Back to Main Content
(Note 5) Optical regenerative repeater:
A repeater with functions of restoring optical signals degraded due to transmission back to original clean signals, comprising the functions of optical signal amplification, optical signal reshaping and optical signal retiming.Back to Main Content
(Note 6) Distributed feedback laser diode (DFB-LD):
A laser diode with a structure of wavelength-selective cavity allowing for single wavelength operation. Because its lasing mode is stabilized even at high-speed modulation, the laser is used as a signal light source for optical communications.Back to Main Content
(Note 7) Silicon photodiode:
An optical-electrical conversion device using silicon.Back to Main Content
(Note 8) Timing jitter:
A kind of statistics indicating the timing fluctuation of an optical signal. It is expressed in terms of variance or deviation of time intervals between signals. The smaller the value, the higher the quality of the signal with lower fluctuation. Back to Main Content
(Note 9) Femtosecond:
An extremely short period of time of a 10-15 second. Back to Main Content