Device and method for signal transmission and optical communications

An optical pulse having time-dependent spectral variation is modulated by a time-dependent signal so that, upon determination of the spectral content of the modulated pulse, a representation of the signal can be obtained. Frequency in the optical pulse preferably varies essentially unidirectionally, i.e., aside from noise, each frequency is encountered just once as, e.g., in a linear relationship. Such an optical pulse can be produced by passing a short laser pulse through a dispersive medium such as, e.g., an optical fiber whose zero-dispersion wavelength is away from wavelengths to be dispersed. Modulation of the optical pulse may be by electrooptic interaction. The technique can be used for the characterization of device and material responses; envisaged also are communications applications.

TECHNICAL FIELD 
The invention is concerned with methods and devices for transmitting 
high-speed signals such as, e.g., picosecond and subpicosecond signals 
originating in devices and materials, as well as with methods and devices 
for optical communications. 
BACKGROUND OF THE INVENTION 
As ever higher levels of performance are being sought in computing and 
communications devices, there is a need for increasingly sensitive methods 
and instruments for evaluating material and device characteristics such 
as, e.g., response times and pulse shapes. In this respect, however, the 
development of electronic sampling oscilloscopes of the currently 
available type is estimated as having reached a point at which significant 
further increases in response speed are unlikely. As a result, alternate 
methods are being sought especially for investigating signals having 
time-dependent characteristics which may range from a few hundred 
picoseconds down to the subpicosecond range. 
One approach which represents an alternative to the use of an electronic 
sampling oscilloscope is based on the generation of electrical signals 
upon illumination of a device or photoconductive material by optical 
pulses as disclosed in U.S. Pat. No. 4,482,863, issued Nov. 13, 1984 to D. 
H. Auston et al. In this approach, optical pulses are used both to 
generate as well as to sample electrical events and, if the device or 
material being tested is photosensitive itself, optical pulses may be made 
incident directly on the device or material being tested. For further 
details in this respect see the papers by P. R. Smith et al., "Measurement 
of GaAs Field-effect Transistor Electronic Impulse Response by Picosecond 
Optical Electronics", Applied Physics Letters, Vol. 39 (1981), pp. 739-741 
and by D. H. Auston et al., "Picosecond Optical Electronics for High-speed 
Instrumentation", Laser Focus Magazine, April 1982, pp. 89-93. 
Another approach to the characterization of ultrafast signals is based on 
electrooptic sampling; this represents an electric field-sensitive 
technique in which ultrashort optical pulses serve as "sampling gates" for 
electrical transients as they simultaneously propagate through an 
electrooptic medium. In this respect, see U.S. Pat. No. 4,446,425, issued 
May 2, 1984 to J. A. Valdmanis et al., the paper by J. A. Valdmanis et 
al., "Picosecond Electro-optic Sampling System", Applied Physics Letters, 
Vo. 41 (1982), pp. 211-212, and the paper by J. A. Valdmanis et al., 
"Subpicosecond Electrooptic Sampling: Principles and Applications", IEEE 
Journal of Quantum Electronics, Vol. QE-22 (1986), pp. 69-78. 
While methods disclosed in the cited references are based on sampling a 
repetitive train of pulses, methods are desired also for characterizing 
signals which are non-repetitive. Methods of such latter type, and devices 
for their implementation, not only are potentially useful for testing and 
measurement purposes but may be used also in the field of 
telecommunications. 
SUMMARY OF THE INVENTION 
An optical pulse having time-dependent spectral variation is 
amplitude-modulated in a time-dependent fashion so as to alter the 
spectral content of the pulse. Upon determination of the spectral content 
of the pulse, a representation of the modulating signal can be obtained. A 
signal of interest typically has characteristics ranging from a few 
hundred picoseconds down to the subpicosecond range, and its sensing may 
involve any physical phenomenon which interacts with light; e.g., 
electrical, magnetic, or electromagnetic phenomena are suitable in this 
respect. Spectral variation in the optical pulse is preferably essentially 
unidirectional, i.e., aside from noise, frequency in the pulse is either 
steadily increasing or else steadily decreasing. 
A signal representation obtained in this fashion may be of interest, e.g., 
for the determination of a characteristic of a device or material being 
tested. Also, such representation may contain digital or analog 
information to be transmitted for the purpose of communications.

DETAILED DESCRIPTION 
Shown in FIG. 1 are laser 1, beam splitter 2, frequency-dispersive optical 
fiber 3 with input and output focusing lenses 4, optical polarizer 5, 
electrooptic modulator 6, compensators 7, polarization analyzer 8, 
spectrograph 9, spectrum analyzer 10, mirrors 11 forming a fixed-length 
delay line, and chopper 12 which is electrically connected to spectrum 
analyzer 8 for synchronization. Positioned in the light path between 
chopper 12 and modulator 6 is a device 13 to be tested. 
Functioning of the arrangement depicted in FIG. 1 can be described as 
follows: A laser pulse is split into two beams by splitter 2, one beam 
propagating into optical fiber 3 via a lens 4, the other passing along a 
delay path formed by mirrors 11. The latter, here designated as trigger 
beam, initiates the generation of a signal of interest; the former, here 
designated as probe beam, undergoes frequency dispersion resulting, e.g., 
in lower frequencies moving ahead of higher frequencies as it emerges from 
optical fiber 3. 
Such a pulse may be designated as a frequency-swept, wavelength-swept, or 
"chirped" pulse, such pulse having preferred frequency dispersion which, 
aside from noise, is unidirectional as a function of time of passage. 
Accordingly, light of any one frequency preferably is encountered not more 
than once as the pulse passes, an essentially linear relationship between 
frequency and time being convenient and preferred. Frequency dispersion 
can be positive or negative, i.e., lower frequencies may travel ahead of 
higher frequencies or vice-versa, and both types of dispersion can be 
produced, e.g., by suitably chosen optical fibers. Dispersion can also be 
produced by other means, gratings being considered as particularly 
suitable for this purpose. 
The frequency-swept probe pulse is polarized by polarizer 5, and the 
polarizied light passes through the electro-optical modulator 6 where it 
is phase-modulated by the electrical signal originating from device 13. 
The length of the delay line formed by mirrors 11 is chosen to ensure that 
the electrical signal arrives within the optical "measurement window" 
defined by passage of the probe pulse. Thus, an entire signal of interest 
can be measured with the passing of a single optical pulse. 
The modulated light passes through analyzer 8 so that, at this point, its 
intensity becomes modulated as a function of phase modulation. 
(Compensators 7 are included between polarizer 5 and polarization analyzer 
8 to ensure that the modulator 6 operates in the zeroth order of net phase 
retardance). The intensity-modulated output pulse is directed into a 
spectrograph whose detector array is coupled to a spectrum analyzer. 
Output from the detector array may be viewed as a spatial representation 
of the modulated pulse as a function of frequency and thus, due to 
time-dependent frequency dispersion, as a function of time. 
At this point of operation it is possible, upon subtraction of the shape of 
the unmodulated phase from that of the modulated pulse, to recover the 
shape of the waveform of interest. If the shape of the unmodulated pulse 
is sufficiently well known in advance, this subtraction can be carried out 
by reference to a permanently stored waveform. Alternatively, it may be 
appropriate to transmit, from time to time, an unmodulated pulse along the 
path of the probe beam to furnish the spectrum analyzer with the 
unmodulated pulse shape; this is conveniently effected by having chopper 
12 prevent the triggering of device 13. For example, under conditions of 
repetitive operation, sensing a signal of interest and furnishing the 
spectrum analyzer with the unmodulated pulse form may be effected by 
simple alternation between triggered and untriggered operation. Repetitive 
operation can further be used to obtain a smoothed representation of a 
signal by averaging over a series of individual representations. 
Optical polarizer 5, modulator 6, and polariziation analyzer 8 are shown 
also in FIG. 2 which further shows a frequency-swept, modulated optical 
pulse 21 traveling away from polarization analyzer 8. The corresponding 
modulating signal 31 is shown moving along modulator 6. Shown also is a 
second, unmodulated probe pulse 22 traveling towards modulator 6, and a 
second electrical signal 32 traveling along modulator 6. 
As described above, short-duration (high-bandwidth) optical pulses are 
conveniently obtained from a femtosecond-pulsed laser. Suitable pulses 
having frequencies in an interval in the optical range from ultraviolet to 
infrared can also be obtained by other means such as, e.g., the nonlinear 
frequency expansion of a picosecond pulse. 
While the description above is in terms of the modulation of a probe pulse 
whose duration is comparable to that of the modulating signal, it is 
possible also, in the case of a longer signal of interest, to obtain 
representations of portions of such signal by simple adjustment of the 
delay line to different lengths. Conversely, adjustment of delay line 
length can be used with signals much shorter than the measurement window 
to investigate the frequency distribution in the probe pulse. For example, 
linearity of frequency distribution in the probe pulse can be induced if 
the shape of a received waveform is independent of the portion of the 
probe pulse used for modulation. 
Finally, as mentioned above, the invention has communications aspects in 
that a modulating signal may represent information such as, e.g, voice, 
data, or graphical information. In such applications it may be desirable, 
in the interest of reduced pulse length, to pass the modulated pulse 
through a medium having dispersive characteristics complementary to those 
of the medium used for dispersion prior to modulation. In this way, 
frequency compression and pulse shortening can be realized. 
In communications use of the invention, spectral analysis of a modulated 
optical pulse typically occurs after transmission over a considerable 
distance and, in the course of such transmission, a considerable amount of 
unintentional frequency dispersion may result in a dispersive transmission 
medium such as, e.g., an optical fiber. Spectrographic analysis at the 
receiver results in a special benefit of the invention as compared with 
other methods of communication in that demodulation is not affected by 
dispersion during transmission. 
EXAMPLE 
An arrangement as schematically depicted in FIG. 1 was used to determine 
the electrical pulse response of an ion-bombarded gallium arsenide 
photoconductive detector. Optical pulses having a width of 50 femtoseconds 
and a transform-limited bandwidth of approximately 9.5 nanometers were 
generated at a rate of 100 megahertz by a balanced, colliding-pulse, 
mode-locked dye laser. The probe pulses were stretched out to a width of 
approximately 300 picoseconds by a polarization-preserving, single-mode 
optical fiber having a length of approximately 50 meters. 
The gallium arsenide device was attached to the end of a 
250-micrometer-thick, balanced-line, traveling-wave lithium tantalate 
electrooptic modulator across which the electrical signal propagated. The 
modulator arrangement was optically biased close to zero transmission in 
order to maximize the modulation depth of the transmitted light even at 
the low overall intensity level at this point. (Such biasing in the 
interest of enhanced modulation of small-amplitude signals is desirable 
because, even near zero transmission, the modulation depth resulting from 
picosecond signals can be less than 10.sup.-3, and the dynamic range of 
the multichannel analyzer is approximately 10.sup.4.) 
A 0.3-meter spectrograph was used in combination with a 1024-element 
Reticon array and OMA-II spectrum analyzer. This system has a resolution 
of approximately 0.04 nanometer and thus can resolve 237 points within the 
spectrum of the probe pulse, yielding a resolution of 42 femtoseconds. 
Convolution with the original pulse duration yields a true temporal limit 
of approximately 65 femtoseconds, far exceeding the modulator's 
capabilities. (The temporal resolution of this system is determined by 
convolving the original pulse duration with a time given by the stretched 
pulse duration divided by the ratio of pulse bandwidth to spectrograph 
resolution. 
A pulsed signal representation was obtained having a width of approximately 
10 picoseconds as measured at a level corresponding to one-half of the 
peak level. The signal had a rise time of approximately 5 picoseconds.