Apparatus for measurement of an optical pulse shape

An apparatus for measurement of an optical pulse shape intended to measure the temporal waveform of an ultrashort single optical pulse is disclosed. The present invention comprises a linearly chirped supercontinuum light source that is synchronized with an optical pulse to be measured; a nonlinear optical interferometer to transform the temporal waveform of an incident optical pulse into a spectrum using an incident light from said supercontinuum light source; and an optical spectrum analyzer to measure the wavelength of the light passing through the nonlinear interferometer so that it can measure the temporal waveform of a single optical pulse. The present invention employs the method that transforms the temporal waveform of an incident optical pulse into a spectrum and measures the wavelength using a linearly chirped supercontinuum light source, a nonlinear optical interferometer, and an optical spectrum analyzer. The present invention provides an apparatus for measurement of an optical pulse shape that is able to measure the waveform not with the repeated measurements but with a single shot measurement.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to an apparatus for measurement of an optical
 pulse shape intended to measure the temporal waveform of an ultrashort
 single optical pulse.
 2. Description of the Prior Art
 In the prior art, the conventional method of measuring the waveform of an
 ultrashort optical pulse converts an incident optical pulse into an
 electrical signal by use of a fast-photo diode, and measures the signal by
 a wide-bandwidth oscilloscope. However, the pulsewidth of the optical
 pulse that is able to be measured by a fast-photo diode and wide-
 bandwidth oscilloscope is limited in about 50 ps. Therefore, a streak
 camera is used for measuring the waveform of an optical pulse having a
 pulsewidth of a few ps, however, it can only measure in the range from
 near ultraviolet to near infrared and has a high-price. Thus, it is
 required to develop a measurement apparatus that is able to measure the
 waveform of a short pulse having narrower than a few ps pulsewidth or for
 a spectrum in which a streak camera can not measure. On the other hand,
 dislike the direct measurement method of the waveform, a pulsewidth, which
 is one of the important informations of an optical pulse, can be
 determined by measuring the second order auto-correlation using a
 nonlinear optical effect. In this method, the second harmonic lightwave
 and two photons absorption is mainly used to produce a nonlinear effect.
 FIG. 1 is a view illustrating the structure of a prior-art apparatus for
 measurement of the pulsewidth of an optical pulse using the second
 harmonic lightwave.
 A nonlinear optical effect causes interactions between numerous incident
 lights and produces a new light with a frequency of the sum or difference
 of the frequencies of the incident lights. In order words, it produces a
 light with different wavelength (Here, the frequency and the wavelength
 are inversely proportional to each other.). The process that two lights,
 each has the same frequency of .omega., interact each other in a nonlinear
 optical medium and produce a light with a frequency of 2 .omega. is called
 the second harmonic lightwave generation. The procedures to measure the
 pulsewidth using the second harmonic lightwave is a follows:
 At first, one divides the optical pulse 1 to be measured into two optical
 pulses using a 50:50 optical beam splitter 2 and makes the two divided
 lights proceed in different routes respectively. At this stage, a time
 delayer 3, which is able to control the time delay between the two lights,
 is located in one route and a reflection mirror 4 is located in the other
 route. The two lights is combined by a 50:50 optical coupler 2, then the
 combined light passes through a lens 5 and incidents to a nonlinear
 optical crystal 6. The second harmonic lightwave is generated thereon
 according to the intensity of the light. Since the second harmonic
 lightwave comes out with the light having a frequency of .omega., one
 should eliminate the light having a frequency of .omega. by a filter 7 and
 thereafter, measures the second harmonic lightwave by a photo diode 18 or
 a photo multiplying tube (PMT).
 In the procedures, the intensity of the second harmonic lightwave is
 determined by the overlapped amount of two optical pulses when they are
 coupled. In other words, in case that the time delay between the two
 lights is so large that they can not be overlapped when they are coupled,
 the intensity of the second harmonic lightwave becomes very small, and in
 case that the time delay is zero, the intensity of the second harmonic
 lightwave gets its maximum value. Therefore, if one operates the time
 delayer 3 with a fixed velocity and measures the intensity of the second
 harmonic lightwave generated, one can find the amount of overlapped pulses
 and measure the pulsewidth thereby. This method, however, has a drawback
 that one should assume the waveform of a pulse to find an accurate
 pulsewidth.
 FIG. 2 is a view illustrating the structure of a prior-art apparatus for
 measurement of the pulsewidth of an optical pulse using two photons
 absorption effect.
 A light-absorbing material has an energy band gap corresponding to the
 energy of the incident light. In the energy band gap of the material is
 bigger than the energy of the incident light, the light is not absorbed
 but transmits. However, the intensity of the incident light gets high, an
 absorption still happens in this case. This phenomenon is one of nonlinear
 phenomena of the medium, and it happens in a medium of which the energy
 band gap is twice as much as the energy of the incident light. This is
 called two photons absorption effect. The two photons absorption effect
 increases proportional to the intensity of the incident light. The
 apparatus for measurement of the pulsewidth of an optical pulse using the
 two photons absorption effect is similar to that of the method using the
 second harmonic lightwave as described in FIG. 1. The procedures to
 measure the pulsewidth using the two photons absorption effect is as
 follows:
 One divides the optical pulse 11 to be measured into two optical pulses
 using a 50:50 optical beam splitter and makes the two divided lights
 proceed in different routes respectively. At this stage, a time delayer
 13, which is able to control the time delay between the two lights, is
 located in one route and a reflection mirror 14 is located in the other
 route. The two lights is combined by a 50:50 optical coupler 12. The
 combined light passes through a lens 15 and is measured by a photo diode
 18.
 The difference between the two apparatuses described in FIGS. 1 and 2 is
 that the latter does not use a nonlinear medium but make use of the two
 photons absorption effect of a photo diode used for measurement. This
 method has advantages that it has simpler structure than the method of
 using the second harmonic lightwave and the fabricating price is low.
 However, it has a drawback that the sensitivity is lower than that of the
 method of using the second harmonic lightwave. And with this method, one
 should also assume the waveform of a pulse to find an accurate pulsewidth.
 SUMMARY OF THE INVENTION
 It is therefore an object of the present invention to provide an apparatus
 for measurement of an optical pulse shape that is able to measure the real
 waveform of an ultrashort optical pulse by transforming the temporal
 waveform of an incident optical pulse into a spectrum using a linearly
 chirped supercontinuum light source and a nonlinear optical
 interferometer.
 To achieve the object, the apparatus for measurement of an optical pulse
 shape in accordance with the present invention comprises a linearly
 chirped supercontinuum light source that is synchronized with an optical
 pulse to be measured; a nonlinear optical interferometer to transform the
 temporal waveform of an incident optical pulse into a spectrum using an
 incident light from the supercontinuum light source; and an optical
 spectrum analyzer to measure the wavelength of the light passing through
 the nonlinear interferometer so that it can measure the temporal waveform
 of a single optical pulse.

DETAILED DESCRIPTION OF THE INVENTION
 Referring to appended drawing, detailed description of the present
 invention is now described.
 FIG. 3 is a view illustrating the structure of an apparatus for measurement
 of an optical pulse shape in accordance with the present invention. It
 employs the method that transforms the temporal waveform of an incident
 optical pulse into a spectrum and measures the wavelength using a linearly
 chirped supercontinuum light source 30, a nonlinear optical interferometer
 40, and an optical spectrum analyzer 50.
 A linearly chirped supercontinuum light source 30 should be synchronized
 with an optical pulse 21 to be measured. To produce a linearly chirped
 supercontinuum light, it is composed of the generator 31 to generate a
 supercontinuum optical pulse and the dispersion medium 32 to linearly
 chirp the optical pulse. A nonlinear interferometer 40 can comprise
 various types of interferometers such as Michelson interferometer, Sagnac
 interferometer, Mach-Zehnder interferometer and so on. However, we will
 mainly describe Mach-Zehnder interferometer here. A nonlinear
 interferometer 40 comprises the third nonlinear medium 42, the first 50:50
 optical coupler 41 to divide an incident light from the supercontinuum
 light source 30 into two routes, the first dichroic optical coupler (WDM1)
 43 to couple the light that passes through the first optical coupler 41
 and the optical pulse to be measured and thereafter to incident the
 coupled light to an arm of the interferometer, the second dichroic optical
 coupler (WDM2) 44 to extract the optical pulse to be measured, which
 incidents from the first dichroic optical coupler 43 and passes through
 the third nonlinear medium 42, from the interferometer, the first mirror
 45 to reflect the light that passed through the second dichroic optical
 coupler 44, the second mirror 46 to reflect the light that is reflected at
 the first optical coupler 41, and the second 50:50 optical coupler 47 to
 couple the lights that are reflected from the first and second mirrors 45
 and 46. A supercontinuum light that passed through the interferometer is
 measured by an optical spectrum analyzer 50 which employs a monochrometer
 51 and an optical multi-channel analyzer 52 as a detector.
 A linearly chirped supercontinuum light is the light of which the spectrum
 distribution is also linearly distributed in time. Therefore, if it is
 distributed in the time interval [t.sub.0, t.sub.0 +T], it can be
 described as follows in Equation 1:
EQU .lambda.=.lambda..sub.0 +A/T(t-t.sub.0), t.sub.0.ltoreq.t [Equation 1]
 When the signal light to be measured incidents to Mach-Zehnder
 interferometer through WDM143, the interferometer transmittance,
 T(.lambda.), of the linearly chirped supercontinuum light is given as
 follows Equation 2:
 ##EQU1##
 Here, I(.lambda.) is the intensity of the linearly chirped supercontinuum
 light at the wavelength of .lambda., .PHI.(.lambda.) is the phase
 difference between the two arms of the interferometer, .PHI..sub.0 is the
 initial phase difference of the interferometer, .delta..PHI. is the amount
 of nonlinear phase change caused by the optical pulse that incidents to
 the interferometer by WDM143, n.sub.2 is the nonlinear refraction index of
 the medium, and I.sub.p is the intensity of the incident light to be
 measured. By controlling the initial phase to have .PHI..sub.0 =.pi./2,
 T(.lambda.) can be approximated as follows Equation 3:
 ##EQU2##
 .delta.T is the difference between the transmittance when there exist an
 optical pulse to be measured, I.sub.p, and does not exist.
 .delta.T(.lambda.) and I.sub.p (.lambda.) are easily measured by an
 optical spectrum analyzer 50, and if using an optical multi-channel
 analyzer 52, the wave characteristics of an optical pulse can be measured
 with only a single shot. The waveform of the optical pulse to be measured,
 I.sub.p (t), can be obtained as follows Equation 4 by coupling Equation 1
 and Equation 3:
 ##EQU3##
 For example, if measuring a Gaussian optical pulse of 1 ps using a linearly
 chirped pulse having a width of 100 nm and a time interval of 100 ps, a
 waveform having about 1 nm full-width at half-maximum appears in the
 spectrum. Considering that the minimum resolution of the conventional
 optical spectrum analyzer is 0.05 nm, the resultant waveform is enough to
 be measured the real waveform.
 As described above, if an optical pulse to be measured incidents to an arm
 of Mach-Zehnder interferometer through WDM141, a nonlinear phase change
 happens in the corresponding route. The nonlinear phase change causes the
 state changes of Mach-Zehnder interferometer, thereafter results the
 change of the output to H.sub.2 port. The change of the output is
 determined by the intensity of the incident light. Therefore, the output
 change occurs corresponding to the waveform of the incident optical pulse.
 A linearly chirped supercontinuum light source 30 takes an important role
 to convert a temporal behaviour into a spectrum. Conclusively, it is a
 measuring method that employs a nonlinear optical effect to convert a
 temporal waveform of an incident optical pulse into a spectrum.
 Since those having ordinary knowledge and skill in the art of the present
 invention will recognize additional modifications and applications within
 the scope thereof, the present invention is not limited to the embodiments
 and drawings described above.
 As mentioned above, according to the present invention, the real time
 analysis of a waveform of the light source, that is used in a ultra-high
 speed optical communication system or in a measurement of the
 characteristics of elements, can be achieved.