Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-11-30
2003-11-25
Mack, Ricky (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S264000, C257S347000, C257S014000
Reexamination Certificate
active
06654153
ABSTRACT:
BACKGROUND OF THE INVENTION
The sampling oscilloscope is among the measurement devices used to determine the modulation waveform of modulated optical signals such as those used in optical telecommunications. Conventional sampling oscilloscopes incorporate a conventional optical signal sampling device that first converts the optical signal into an electrical signal using a photodetector. The waveform of the electrical signal is then measured using an electrical sampling circuit.
FIG. 1
is a block diagram of an example of a conventional optical signal sampling device
10
for sampling an optical signal. The optical signal sampling device is composed of the photodetector
12
, the electrical sampling circuit
14
and the optical signal input
16
. The optical signal-under-test is received via the optical signal input. In this example, the optical signal input is composed of the input fiber
18
and the converging element
20
. The converging element converges the optical signal-under-test received via the input fiber onto the photodetector. The photodetector generates an electrical signal in response to the optical signal-under-test, and the electrical signal is sampled by the electrical sampling circuit.
The maximum frequency capability of the photodetector
12
needs to be several times the maximum frequency of the modulation waveform of the optical signal-under-test. In the example shown, the photodetector includes the photodiode
22
. The photodiode is DC biased by the DC bias source
24
. Currently-available PIN diodes suitable for use as the photodiode have a maximum frequency of about 110 GHz. The photodetector may alternatively include an avalanche photodiode, but an avalanche photodiode has a maximum frequency similar to that of a PIN diode.
The electrical sampling circuit
14
is composed of the sampling element
30
, the sampling pulse generator
32
and the clock pulse generator
34
. The sampling element includes the electrical signal input
42
, the sampling signal input
44
and the electrical sample output
46
. The sampling element is typically composed of a resistor, a capacitor, and a diode, none of which is shown. The output of the clock pulse generator is connected to the input of the sampling pulse generator. The output of the sampling pulse generator is connected to the sampling signal input
44
of the sampling element. The electrical signal input
42
of the sampling element is connected to the output of the photodetector
12
. The electrical sample output
46
of the sampling element provides electrical samples each of which represents a portion of the modulation waveform of the optical signal-under-test. Also shown are the integrating capacitor
50
and the buffer amplifier
52
that respectively integrate and buffer the electrical samples generated by the electrical sampling circuit
14
to generate an electrical output signal whose waveform represents the modulation waveform of the optical signal-under-test.
The clock pulse generator
34
generates electrical pulses having a duration of a few nanoseconds. The sampling pulse generator
32
compresses the electrical pulses received from the clock pulse generator to generate electrical sampling pulses having a duration of a few tens of picoseconds. The sampling pulse generator may include, for example, non-linear transmission line (NLTL), a step recovery diode and a strip line, none of which is shown. The sampling pulse generator supplies the sampling pulses to the sampling element
30
.
Currently, the maximum modulation frequency that can be measured using a sampling oscilloscope incorporating the conventional optical signal sampling device as just described is about 50 GHz. As the capacity of optical telecommunications has grown in recent years, the modulation rates of optical signals have reached 80 Gbps or more. It is predicted that sampling devices for measuring the waveforms of optical signals modulated at such high modulation rates will need a frequency response that extends to over 200 GHz. It is difficult to sample optical signals with such high modulation rates using conventional electrical sampling. A frequency response that extends to over 200 GHz needs samples having a duration of about 2 ps. It is extremely difficult to generate the electrical sampling pulses required to generate such short-duration electrical samples using a conventional NLTL. Moreover, propagation losses in the strip line become significant at such high frequencies. Therefore, a need exists for an optical signal sampling device capable of accurately sampling a modulated optical signal, and especially an optical signal modulated at a modulation frequency in the frequency range in which electrical sampling is difficult or impossible.
In addition, many optical telecommunication systems employ wavelength-division multiplexing (WDM). WDM increases the transmission capacity of an optical telecommunication system without increasing the transmission rate. In WDM, a transmission channel carries many optical signals, each of a different wavelength. What is also needed is the ability to measure the modulation waveforms of the individual optical signals, each of which has a different wavelength and a different modulation waveform.
SUMMARY OF THE INVENTION
The invention provides an optical domain optical signal sampling device for sampling an optical signal-under-test. The optical domain optical signal sampling device comprises a semiconductor saturable absorber, an optical sampling pulse source, an optical signal input and a light detector. The optical sampling pulse source is arranged to illuminate the semiconductor saturable absorber with optical sampling pulses. The optical signal input is arranged to illuminate the semiconductor saturable absorber with the optical signal-under-test. The light detector is arranged to receive optical samples of the optical signal-under-test output by the semiconductor saturable absorber in response to the optical sampling pulses.
The semiconductor saturable absorber enables the optical domain optical signal sampling device to sample the optical signal-under-test in the optical domain. Optical sampling pulse sources capable of repetitively generating pulses of light having a duration of a few picoseconds and with an intensity sufficiently high to induce saturable absorption in the semiconductor saturable absorber are commercially available. When illuminated with such short optical sampling pulses and the optical signal-under-test, the semiconductor saturable absorber outputs optical samples of the optical signal-under-test. The optical samples have a duration similar to that of the optical sampling pulses. The light detector converts the optical samples into an electrical signal that represents the modulation waveform of the optical signal-under-test. The frequency response of the light detector need not extend significantly higher in frequency than that of the modulation waveform of the optical signal-under-test. Such light detectors are also commercially available. Thus, the optical domain optical signal sampling device according to the invention can sample modulated optical signals and can provide a frequency response extending to beyond 200 GHz.
The optical sampling pulse source may include a laser that generates pulses of light having a pulse width of no more than 10 ps and the semiconductor saturable absorber may include a semiconductor material with a carrier lifetime of no more than 10 ps.
The semiconductor saturable absorber may include a semiconductor layer comprising semiconductor material having a short carrier lifetime. Semiconductor material having a short carrier lifetime generates the optical samples with a duration comparable with that of the optical sampling pulses and increases the wavelength range over which saturable absorption is induced. The semiconductor material having a short carrier lifetime may include low-temperature-deposited semiconductor material or ion implanted semiconductor material, for example.
The semiconductor layer may be a layer of bulk semiconductor materi
Hardcastle Ian
Mack Ricky
Thomas Brandi N
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