Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-09-30
2002-05-07
Mullen, Thomas (Department: 2632)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S341430
Reexamination Certificate
active
06384948
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to high speed optical digital transmission systems in general, and more particularly, the present invention relates to an optical photoreceiver in a fiberoptic digital transmission system.
2. Description of the Related Art
The transmission of speech, data, video and other information using the visible and infrared portion of the electromagnetic spectrum is commonly known as optical communication. A basic optical communication system is illustrated in FIG.
1
. Information is transmitted from an information source
100
to an information user
102
using an optical transmitter
104
, an optical channel
106
, and an optical receiver
108
. The most common light source used as an optical transmitter
104
generally includes a light emitting diode, a laser diode or a laser and modulator pair. The optical channel
106
, which refers to a transmission path between the optical transmitter
104
and the optical receiver
108
, is a glass fiber made of silicon dioxide. Some optical fibers are also made of transparent plastic. Finally, the optical receiver
108
is generally a semiconductor photodiode, with the two most commonly used semiconductor photodiodes being the p-i-n photodiode and the avalanche photodiode.
When analog signals, such as voice, are transmitted digitally, the transmission rate, or bit rate is dependent upon both a rate at which the analog signal is sampled and a coding scheme that is used. The analog signal can be accurately transmitted if the signal is sampled at a rate of at least twice the highest frequency contained in that signal. For example: since most of the energy in normal speech is contained in frequencies below 4 kHz, standard telephone channels need only transmit messages with frequencies up to 4 kHz. Therefore, the standard 4 kHz telephone channel is sampled 8000 times a second, and since a decoding procedure uses 8 bits to describe the amplitude of each sample, a total of 64,000 bits/second are transmitted for a single telephone message.
Random fluctuations in a received signal are commonly referred to as “noise”. One of the problems associated pith optical receivers in telecommunications systems involves the existence of noise at the photoreceiver side. For example, when a low level of light is detected directly in a photodiode, the electrical signal level generated by the photodetector is too low, or is small compared to thermal noise at an output of the photodetector. As a result, the signal is lost in the noise. For a typical p-i-n photodiode, this loss limits the sensitivity of the photodiode to −30 dBm of optical power at the input in order to maintain a reasonable bit error rate at 1 Giga-bit per second (Gbps). Therefore, in order to increase the data transmission rate above 1 Gbps, it is necessary to amplify the signal before the signal reaches the photodetection stage.
Although many attempts have been made, increasing the transmission rate in conventional telecommunication systems has proven to be problematic. For example, an avalanche photodiode, which has been used to increase the transmission rate, is limited to a net gain-to-noise ratio of approximately 10, which limits the avalanche photodiode to a transmission rate of approximately 10 Gbps. As a result, in order to extend the transmission rate to 100 Gbps using the avalanche or pin photodiodes, it is necessary to include optical preamplification. Conventional use of optical preamplification relies on post detection amplification, or amplification of the signal after the signal has been detected, which tends to be very difficult. To give a specific example, since significant low frequency information is contained in the data, it is necessary to have an electrical amplifier positioned after the photodetector that is both broadband (10 GHz bandwidth, for example) and that has a low frequency cut-off in the 10 kHz range. Because of the multiple decade difference that exists between the two ranges, constructing an electrical amplifier having those properties has been difficult. As a result, increasing the data transmission rate above 10 Gbps has proven to be a very demanding and difficult task.
A conventional optical receiver is illustrated in FIG.
2
. An incoming intensity modulated light signal is detected and converted to an electrical pulse stream, or electrical signal by a photoreceiver
20
, such as a p-i-n photodiode or an avalanche photodiode. The electrical signal is amplified by a low-noise preamplifier
30
and a linear amplifier
34
. The signal is further amplified and leveled by a limiting amplifier
32
. The electrical signal from the limiting amplifier
32
is input to a retiming stage
22
that includes a clock recovery circuit
24
and a retiming circuit. or flip flop
26
. After receiving the electrical signal, the retiming stage
22
outputs a retimed electrical signal.
As a result of standards which have been developed for a synchronous optical network, commonly referred to as the “SONET” standards, a received optical power level of −30 dBm (1 micro W) is required at a data rate of, for example, 10 Gbps. Therefore, the photoreceiver output electrical signal is usually in a range of tens of microvolts. Typically, the highest frequency for which the sensitivity of a photoreceiver utilizing a p-i-n photodiode is better than −30 dBm, is approximately 1 Gbps. As the frequency of transmission is increased to 10 Gbps, the power level necessarily increases to approximately −20 dBm, and to −10 dBm when the transmission rate is increased to 100 Gbps, due to the thermal noise at the output of the photoreceiver.
The electrical signal from the photoreceiver
20
is usually below the required output level needed by the clock recovery
22
and the flip flop
24
. Thus, once the digital signal is converted to an electrical signal, the electrical signal is amplified in an electrical amplification stage
28
prior to being input to the retiming stage
22
. The electrical amplification stage
28
includes a low noise preamplifier
30
, a limiting amplifier
32
, and in some cases a linear amplifier
34
. The amount of amplification is dependent upon the input optical signal level, the conversion factor of the photoreceiver
20
, and the signal level required for the limiting amplifier
32
.
The preamplifier
30
of the amplification stage
28
is a fixed gain low noise amplifier. The limiting amplifier
32
amplifies the signal from the preamplifier
30
and linear amplifier
34
, with variable gain, to adjust the gain to a fixed level. The signal is further processed in order to recover the timing in the retiming stage
22
using the clock recover circuit
24
. The recovered clock is input to the flip flop
26
and used to reshape and retime the amplified digital data stream to account for pulse distortion and broadening so that the integrity of the data is preserved after many, possibly thousands of retiming/regenerating operations. The electrical amplification stage
28
provides a fixed output electrical signal, typically in the range of 1 volt peak-to- peak, which is needed by the clock recovery circuit
24
and flip flop
26
to reliably extract information.
Since light is attenuated as it travels in a fiber, the electrical gain required in conventional intensity modulated digital fiber optic systems is usually high. As described above, typical photoreceiver optical input sensitivities are −30 dBm at 1 Gbps for a bit error ratio of 1.0×10
−9
with a p-i-n photodiode integrated with a low noise preamplifier, and −36 dBm for the same system operating with an avalanche photodiode. With an input of one microwatt (−30 dBm), the photocurrent produced can be as high as 1 &mgr;A through 50 ohms. Therefore, the electrical power out of the photodetector is −76 dBm. A typical flip flop requires at least 100 mV (−16 dBm) to operate reliably, which yields a minimum of 60 dB electrical gain. The gain increases to 80 dB when 1 volt is needed
Dennis Michael L.
Duling, III Irl N.
Esman Ronald D.
Villarruel Carl A.
Williams Keith J.
Karasek John J.
Mills John G.
Mullen Thomas
The United States of America as represented by the Secretary of
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