Optical parametric diffuser

An optical parametric diffuser is provided with an optical mixer which receives inputs of a probe light of a wavelength .lambda..sub.1 and a pumping light of a wavelength .lambda..sub.2, multiplexes them, and generates an output signal light including a wavelength .lambda..sub.5 which is apart from the pumping light wavelength on the opposite side of the probe light wavelength by n.DELTA..lambda. (.DELTA..lambda.=.vertline..lambda..sub.1 -.lambda..sub.2 .vertline., n.gtoreq.2). The signal of a wavelength .lambda..sub.5 is then extracted by a band-pass filter. The optical mixer is made to have a peak in output signal generation efficiency at the wavelength .lambda..sub.5 so that an output light beam of the wavelength .lambda..sub.5 can be obtained from the probe light and the pumping light.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a very high speed signal processing, more 
specifically to an optical parametric diffuser for performing wavelength 
conversion at very high speeds. 
2. Description of the Prior Art 
A conventional wavelength conversion device will be described first. 
FIG. 12 is a block diagram for describing a conventional example. In the 
following description, it is assumed that the symbols, .lambda..sub.1, 
.lambda..sub.2, .lambda..sub.3, . . . not only denote wavelengths but also 
signals of those wavelengths. 
The figure shows a multiplexing section 11 for multiplexing two input 
signals .lambda..sub.1, .lambda..sub.2 and outputting their multiplexed 
waves .lambda..sub.1, .lambda..sub.2. A mixer section 12 uses a device 
such as an optical semiconductor amplifier or a differential frequency 
generator using a KTP crystal. The mixer section 12 receives inputs of 
multiplexing signals .lambda..sub.1, .lambda..sub.2, mixes them, and 
outputs signals of wavelengths .lambda..sub.1, .lambda..sub.2, and 
.lambda..sub.3, .lambda..sub.4 as shown in FIGS. 3(a) and 3(b). The 
signals of the wavelengths .lambda..sub.3, .lambda..sub.4 are multiplexed 
waves generated by nonlinear optical effect at wavelengths distant from 
the input wavelengths .lambda..sub.1, .lambda..sub.2 by the difference 
.DELTA..lambda. (.DELTA..lambda.=.vertline..lambda..sub.1 -.lambda..sub.2 
.vertline.) between the two wavelengths. A band-pass filter (B. P.F.) 13 
as shown in FIGS. 3(a) and 3(b) has a passage band wavelength of 
.lambda..sub.4. 
Function of the conventional wavelength conversion device shown in FIG. 12 
will be described in reference to FIGS. 3(a), 3(b), 4(a)-4(d) and 
5(a)-5(c). 
A probe light of a wavelength .lambda..sub.1 and a pumping light of a 
wavelength .lambda..sub.2 are inputted to the multiplexing section 11. The 
pumping light is for increasing the nonlinear optical effect. The probe 
light is a reference light. The multiplexing section 11 multiplexes the 
two input signals and outputs a multiplexed wave signal to the mixer 
section 12. 
When the multiplexed wave signal is inputted to the mixer section 12, the 
mixer section 12, as shown in FIG. 5(a), with nonlinear optical effect, 
outputs multiplexed wave signals of wavelengths .lambda..sub.3, 
.lambda..sub.4 distant by .DELTA..lambda. from the input signals 
.lambda..sub.1, .lambda..sub.2. Here, as shown in FIG. 5(c), while 
multiplexed waves of wavelengths .lambda..sub.5, .lambda..sub.6 
respectively distant by 2.DELTA..lambda. from the wavelengths of the input 
signals .lambda..sub.1, .lambda..sub.2 are produced, since the gain of the 
mixer section 12 increases in the vicinity of the wavelengths 
.lambda..sub.1, .lambda..sub.2, and decreases in the vicinity of the 
wavelengths .lambda..sub.5, .lambda..sub.6, the multiplexed wave 
components of the wavelengths .lambda..sub.5, .lambda..sub.6 are buried 
below the noise level as shown in FIG. 5(c) and cannot be taken out as 
signals. 
The band-pass filter 13 has its passage band wavelength at .lambda..sub.4 
which is distant by a differential wavelength .DELTA..lambda. from the 
pumping light wavelength .lambda..sub.2 on the opposite side of the probe 
light wavelength .lambda..sub.1. 
In this way, the probe light signal of the wavelength .lambda..sub.1 is 
converted to the output light signal of the wavelength .lambda..sub.4. 
Conventional mixers for wavelength conversion include those described 
below. 
First, a mixer using an optical semiconductor amplifier will be described. 
Inputting two input signals of wavelengths .lambda..sub.1, .lambda..sub.2, 
outputs of wavelengths .lambda..sub.3, .lambda..sub.4 are produced which 
are distant by the differential wavelength 
.DELTA..lambda.=.vertline..lambda..sub.1 -.lambda..sub.2 .vertline. from 
the input signals on both sides of the input signals. 
Here, while either .lambda..sub.3 or .lambda..sub.4 may be used as the 
mixer output, since the wavelength is not so distant from that of the 
input signal, even if it is filtered with a band-pass filter of the 
passage band .lambda..sub.4 as shown in FIG. 3(b), the signals 
.lambda..sub.1, .lambda..sub.2 are not easy to cut off. 
Furthermore, the gain characteristic peak of the optical semiconductor 
amplifier is in the vicinity of the two input wavelengths .lambda..sub.1, 
.lambda..sub.2 and the gain is very low in the vicinity of the output 
signal wavelength .lambda..sub.4. Therefore, conversion efficiency (output 
signal to input signal) is very low (10.sup.-4 or less). 
Next, a mixer using a KTP crystal will be described. 
As shown in FIG. 4(a), when two input signals .lambda..sub.1, 
.lambda..sub.2 as spatial light beams are inputted to a KTP crystal 14, a 
wavelength-converted output signal comes out as a spatial light beam 
either in the form of a sum frequency wave, differential frequency wave, 
or second harmonic wave. 
FIGS. 4(a)-4(d) show relationship between the input wavelength and output 
wavelength (frequency). 
FIG. 4(b) shows the case in which the frequency of the output signal is the 
sum of the frequencies of the two input signals at wavelengths 
.lambda..sub.1, .lambda..sub.2. 
FIG. 4(c) shows the case in which the frequency of the output signal is the 
difference between the frequencies of the two input signals at wavelengths 
.lambda..sub.1, .lambda..sub.2. 
FIG. 4(d) shows the case in which the wavelengths of the output signals are 
respectively half the two input signal wavelengths by cutting off the 
input waves. 
When the KTP crystal is used, the process using the spatial light beam 
requires a certain size and makes installation less easy. Furthermore, 
with the differential frequency generation, like the optical semiconductor 
amplifier, since the wavelengths of input and output signals are not 
largely different from each other, it is less easy to cut off the input 
signal with a filter. 
Furthermore, nonlinear optical effect is not great and therefore the amount 
of the output signal is small. Therefore, conversion efficiency (output 
signal to input signal) is very low (10.sup.-5 or less). 
As described above, conventionally, when the probe light of the wavelength 
.lambda..sub.1 and the pumping light of the wavelength .lambda..sub.2 are 
inputted to obtain the output light of the wavelength .lambda..sub.4 which 
is apart from .lambda..sub.2 by .DELTA..lambda.=.vertline..lambda..sub.1 
-.lambda..sub.2 .vertline., if the .DELTA..lambda. is small, even if a 
band-pass filter is used, the input wave cannot be removed effectively and 
the output signal cannot be obtained efficiently. 
When the conventional wavelength conversion is applied to the measurement 
of the probe light of the wavelength .lambda..sub.1, a pumping light of 
the wavelength .lambda..sub.2 is used to produce an output signal which is 
apart by the differential wavelength .DELTA..lambda., and the output 
signal is measured. A disadvantage in that case is that, when the two 
input wavelengths .lambda..sub.1, .lambda..sub.2 are close to the 
wavelength .lambda..sub.4, the input wave component contained in the 
output cannot be completely cut off and the property of the probe light 
cannot be known accurately. 
Since the conventional mixer using the optical semiconductor amplifier or 
the KTP differential frequency generator is low in efficiency, levels of 
the pumping light and the probe light must be raised. Furthermore, since 
the wavelengths of the input and output waves are close to each other, it 
is difficult to cut off the input wave with a filter. Another problem is 
that, in order to obtain a large output signal, a large input signal is 
required. 
On the other hand, it is easy to filter and cut off the input signal with a 
mixer using KTP sum frequency generation or KTP second harmonic wave 
generation, in which the output wavelength is separated from the input 
wavelength band. However, such a mixer must handle spatial light, cannot 
be made compact in size, is very low in efficiency, and therefore is not 
suitable for practical use. 
Furthermore, with the conventional mixer using the optical semiconductor 
amplifier, the peak of gain is made to agree with the input wavelengths 
.lambda..sub.1, .lambda..sub.2 as shown in FIG. 5(b), the input signals 
are mixed and amplified to obtain an output by nonlinear optical effect at 
a wavelength which is apart by a differential wavelength 
.DELTA..lambda.=.vertline..lambda..sub.1 -.lambda..sub.2 .vertline.. With 
this wavelength characteristic, since the peak of gain is near the input 
wavelength, the more apart from the input wavelength, the lower the gain, 
and the nonlinear optical effect is less likely to occur. As a result, the 
output signal of .lambda..sub.5 occurring at a wavelength apart by 
n.DELTA..lambda. (n is any integer not less than 2) is buried below the 
noise level and cannot be utilized. 
SUMMARY OF THE INVENTION 
The present invention provides an optical parametric diffuser comprising: 
a multiplexing section for receiving a probe light of a wavelength 
.lambda..sub.1 and a pumping light of a wavelength .lambda..sub.2 and 
multiplexing said prove light and said pumping light; 
a mixer section for receiving an output from said multiplexing section to 
produce a multiplexed output including a wavelength .lambda..sub.5 
represented by: 
EQU .lambda..sub.5 =(1+n).lambda..sub.2 -n.lambda..sub.1 
wherein n is an integer of at least 2; and 
a band-pass filter for receiving an output from said mixer section and 
selectively outputting a multiplexed output of said wavelength 
.lambda..sub.5. 
It is an object of the invention to provide an optical parametric diffuser 
capable obtaining a mixer output of a wavelength .lambda..sub.5 which is 
apart from the input wavelength by an amount n times greater than that 
conventionally practicable, filtering and cutting off unwanted wavelengths 
contained in the probe light and the pumping light even when the probe 
light wavelength .lambda..sub.1 and the pumping light wavelength 
.lambda..sub.2 are close to each other, and to obtain an output light of a 
good quality.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a block diagram showing a constitution of an embodiment of the 
invention. 
In reference to FIG. 1, a multiplexing section 1 is for multiplexing two 
input signals .lambda..sub.1, .lambda..sub.2 and outputting their 
multiplexing two input signals .lambda..sub.1, .lambda..sub.2 and 
outputting their multiplexed wave signals .lambda..sub.1, .lambda..sub.2. 
A mixer section 2 receives inputs of multiplexing signals .lambda..sub.1, 
.lambda..sub.2 from the multiplexing section 1, mixes them, amplifies 
them, and outputs signals of wavelengths .lambda..sub.1, .lambda..sub.2, 
and .lambda..sub.3, .lambda..sub.4, .lambda..sub.5 as shown in FIG. 6. The 
signals of the wavelengths .lambda..sub.3, .lambda..sub.4, .lambda..sub.5 
are those of multiplexed wave signals generated by the nonlinear optical 
effect and generated at wavelengths apart from the input signal 
wavelengths .lambda..sub.1, .lambda..sub.2 by the differential wavelength 
.DELTA..lambda.=.vertline..lambda..sub.1 -.lambda..sub.2 .vertline. and 
n.DELTA..lambda. (n is 2 or greater integer, hereinafter the same). A 
band-pass filter 3 has its pass band at the wavelength .lambda..sub.5 as 
shown in FIG. 2(c). 
Next, the function of the optical parametric diffuser (hereinafter referred 
to OPD for brevity) of the embodiment shown in FIG. 1 will be described in 
reference to FIGS. 2(a)-2(c), 6(a)-6(b) and 11. 
The probe light of the wavelength .lambda..sub.1 and the pumping light of 
the wavelength .lambda..sub.2 are inputted to the multiplexing section 1. 
The pumping light is for starting up the nonlinear optical effect and the 
probe light (also called as reference light) is for converting its 
wavelength from now on. The multiplexing section 1 multiplexes the two 
input signals and outputs the multiplexed signals to the mixer section 2. 
Upon receiving the multiplexed signal, the mixer section 2 produces as 
shown in FIG. 6(a) by the nonlinear optical effect multiplexed signals of 
wavelengths .lambda..sub.3, .lambda..sub.4 which are apart from the two 
input signals .lambda..sub.1, .lambda..sub.2 by the differential 
wavelength .DELTA..lambda.=.vertline..lambda..sub.1 -.lambda..sub.2 
.vertline. and multiplexed signals .lambda..sub.5, .lambda..sub.6, which 
are apart from the two input signals by n.DELTA..lambda.. 
In the conventional arrangement, the gain of the mixer section has its peak 
in the vicinity of the wavelengths .lambda..sub.1, .lambda..sub.2 and the 
gain is small in the vicinity of the wavelengths .lambda..sub.5, 
.lambda..sub.6. Therefore, the multiplexed wave components of the 
wavelengths .lambda..sub.5, .lambda..sub.6 are buried below the noise 
level, and cannot be taken out as output signals. In the embodiment of the 
invention, however, since the peak is shifted toward the wavelength 
.lambda..sub.5 as shown in FIG. 6(b), the wavelength .lambda..sub.5 can be 
taken out as a multiplexed wave signal. 
Furthermore, since it is possible to bring the peak of the signal 
generating efficiency to the wavelength .lambda..sub.5, the efficiency of 
the mixer is extremely enhanced to produce very large output signals. 
The band-pass filter 3 as shown in FIG. 11 has its pass band at the 
wavelength .lambda..sub.5 which is apart from the probe light wavelength 
.lambda..sub.1 by n.DELTA..lambda. (which is n times the wavelength 
difference .DELTA..lambda. between the two input signals) on the opposite 
side of the pumping light wavelength .lambda..sub.2. 
In this way, the probe light signal of the wavelength .lambda..sub.1 is 
converted to the output signal of the wavelength .lambda..sub.5. 
In this embodiment, a laser diode having either of the following two 
characteristics may be used as the mixer section 2. 
(1) As shown in FIG. 6(b), a laser diode having a broad range, including 
the wavelengths of peak of output signal generating efficiency 
.lambda..sub.4, .lambda..sub.5. Although the output on the wavelength 
.lambda..sub.3 side is small, there is no problem because only the 
wavelengths .lambda..sub.4, .lambda..sub.5 side are used. 
(2) As shown in FIGS. 2(a), (b), a laser diode having two peaks in its 
signal generating efficiency, namely at the pumping light wavelength 
.lambda..sub.2 and at the wavelength .lambda..sub.5. 
Here, a conventional diode having the broad wavelength range mentioned in 
(1) above will be described using a Japanese Laid-open Patent Application 
Hei 6-283824 as an example. 
The active region of the laser diode of the Application is constituted with 
a light containment layer, a plural number of quantum well layers, and 
barrier layers placed among the quantum well layers and in contact with 
them. At least one of the plural number of quantum well layers is 
different in thickness from the others and with lattice non-alignment. And 
an embodiment is disclosed which amplifies wavelength from 1.45-1.54 .mu.m 
in the 1.5 .mu.m band by adjusting the layer thickness and amount of 
distortion of the plural number of quantum well layers. 
Next, the laser diode having two peaks in the output signal generation 
efficiency mentioned in (2) above will be described. 
In order that the diode has two peaks in the output signal generation 
efficiency, as shown in FIG. 7, energy is accumulated to a higher energy 
level than that of conventional laser diode by increasing the carrier 
injection, and mixing and amplification are carried out. 
When the reflectivity of end surface mirrors 21 is kept low, which will be 
described later in reference to FIG. 8, energy loss increases and OPD does 
not oscillate unless a larger energy is injected than that with the 
conventional laser diode. The increased energy increases the distance 
between the two wavelength. 
The higher the energy level, the higher the output signal generation 
efficiency at a wavelength distant from the original peak. As shown in 
FIG. 2(b), the laser diode having the second peak is made with the second 
output signal generation efficiency peak at .lambda..sub.5 by adjusting 
the layer thickness to agree with the wavelength at the time of 
manufacture. 
That is to say, the laser diode having a plural number of layers is made of 
a wide band type by adjusting each layer thickness so that one wavelength 
has its signal generation efficiency peak at a low level and the other at 
the second level. 
Generally with the pn junction type of laser diode, the energy gap Eg 
between the conduction band and the valence band, and the frequency .nu. 
are in the relationship h.nu.=Eg, where h is the Planck's constant. 
Therefore, the energy gap Eg is adjusted by adjusting the pn juction layer 
thickness so that the wavelength of the light outputted from the laser 
diode becomes .lambda..sub.5. 
With the ordinary laser diode, the reflection factor of the end surface 
mirrors is made less than 1/1000 and carrier is injected. When energy is 
accumulated and reaches the minimum level, OPD amplifies the signal and 
oscillates. With the laser diode of this embodiment, however, the 
reflectivity of the end surface mirrors 21 shown in FIG. 8 is kept as low 
as 1/1000-1/100,000 to raise the amplification-oscillation threshold so 
that OPD oscillates when the energy reaches the second level at an 
intended layer as shown in FIG. 9. 
Next, a quantum well laser diode appropriate for the mixer section 2 will 
be described in reference to FIGS. 7 and 8. 
The quantum well laser diode uses two types of alternately laminated 
composition materials for the active layer. 
One shown in FIG. 8 uses a quantum well laser diode 20 as the mixer section 
2 shown in FIG. 1. 
The quantum well laser diode 20 as shown in FIG. 8 is provided with end 
surface mirrors 21a, 21b on both ends between which is disposed an active 
layer consisting of two types of composition materials stacked alternately 
in heterojunction quantum well structure 22. The symbol 23 denotes a 
carrier injection power source. FIG. 10 shows the energy band of the 
quantum well laser diode 20. 
With the quantum well laser diode in which very thin films of two different 
materials are alternately laminated, electrons are stable at a low energy 
level. As a result, electrons may be locally distributed in the potential 
well layer where the energy of the conduction band bottom is low. The 
potential barrier layer becomes a barrier because of its high energy of 
the conduction band bottom. Since only the number of waves which become a 
standing wave are allowed to the electrons bound to the potential well 
layer, the wavelength is determined with the layer thickness. Therefore, 
the layer thickness should be adjusted to obtain the light of the intended 
wavelength. 
With the quantum well laser diode 20, electrons are stable at a low energy 
level. When energy is gradually injected, the energy collects in the 
potential well layer as shown in FIG. 10, and generally when the layer is 
saturated, oscillation begins. Since the reflection factor of the end 
surface mirrors 21a, 21b is kept as low as about 1/10,000, this quantum 
well laser diode does not oscillate here but oscillates when the energy 
reaches the next level. 
It is also possible to form the potential well layer in multiple layers 
with some layers in different thicknesses and different energy levels so 
as to cause oscillation in a plural number of wavelengths. 
The state density of the above arrangement as shown in FIG. 9 occurs in 
stepped values. Carrier is injected to accumulate energy to the second 
level and oscillate. Here, since energy value is four times that of the 
minimum level, the wavelength may be set more widely apart than that of 
wavelength adjustment with a normal layer thickness. 
The energy band structure of the quantum well laser is as shown in FIG. 10 
and oscillation tends to occur with energy gap between the lowest of the 
conduction band of the potential well layer and the highest of the valence 
electron band. However, due to the quantum effect, electrons cannot exist 
in the potential well layer at the lowest energy of the conduction band, 
and can only exist when a standing wave occurs in the potential well 
layer. The energy level when the number of waves in the standing wave is 
minimum is called the minimum level. When the number of waves in the 
standing wave increases, the energy level that electrons can take becomes 
n.sup.2 times the minimum level. Likewise, in the valence electron band, 
the energy level that the hole can take is only that when the standing 
wave is generated, and the value is n.sup.2 times the minimum level. 
For example, it is assumed that the thickness of the potential well layer 
and the potential barrier layer is 1 micrometers, and the energy gap of 
the potential barrier layer is 0.24 eV on the valence electron band side 
and 1.35 eV on the conduction band side, and the wavelength due to the 
energy gap Eg between the minimum energy in the potential well layer of 
the conduction band and the maximum energy in the valence band is 1600 nm. 
In that case, the energy at the minimum level corresponds to the 
wavelength 30 nm on the conduction band side and 5 nm on the valence band 
side. Therefore, oscillation wavelengths at the minimum and the second 
levels are respectively, 
(Minimum level oscillation): 1600-(30+5)=1565 nm 
(Second level oscillation) : 1600-(30.times.2.sup.2 +5.times.2.sup.2)=1460 
nm 
In this way, wavelength setting is made possible in a wider range than with 
the conventional layer thickness adjustment at the minimum level, even 
when the probe light wavelength .lambda..sub.1, the pumping light 
wavelength .lambda..sub.2, and the output signal of the wavelength 
.lambda..sub.5 are apart from each other. 
The output spectrum of the mixer section 2 using the quantum well type of 
laser diode is shown in FIG. 2(a) and FIG. 11. When the probe light 
wavelength .lambda..sub.1 =1553 nm, and pumping light wavelength 
.lambda..sub.2 =1548 nm are inputted and an output is taken out with n=2, 
the output wavelength .lambda..sub.5 is 1538 nm. Thus, it is known that 
unnecessary wavelength output such as the pumping light of the wavelength 
.lambda..sub.2 =1548 nm is removed with the filter band of about 3 nm or 
less. 
The signal generation efficiency when the multiple layer quantum well type 
of laser diode described above is used as the mixer 2 is shown in FIG. 
2(b). As shown, it is possible to set the peak resulting from the two 
input wavelengths .lambda..sub.1, .lambda..sub.2 with a slight shift 
toward .lambda..sub.5 to produce a peak resulting from the second level at 
the wavelength .lambda..sub.5. 
This optical parametric diffuser may be used for example in optical 
sampling observation of the output wavelength .lambda..sub.5 converted 
from the probe light wavelength .lambda..sub.1 with the pumping light in 
pulses. 
The above embodiment is described in the case in which the pumping light 
wavelength .lambda..sub.2 is shorter than the probe light wavelength 
.lambda..sub.1 and the output wavelength .lambda..sub.5 is shorter than 
the pumping light wavelength .lambda..sub.2. However, it is a matter of 
course that this invention may also be applied to the case in which the 
pumping light wavelength .lambda..sub.2 is longer than the probe light 
wavelength .lambda..sub.1 and the output wavelength .lambda..sub.5 is 
longer than the pumping light wavelength .lambda..sub.2 by interchanging 
the peak of the gain of the mixer section resulting from the minimum level 
and the peak of the gain of the mixer section resulting from the second 
level. 
This invention makes it possible to cut off an input signal easily by 
utilizing an output wavelength .lambda..sub.5 which is apart from the 
input signal by n.DELTA..lambda. (n is any integer not less than 2). 
Also, since two peaks in signal generation efficiency of the laser diode 
are provided at the pumping light wavelength .lambda..sub.2 and output 
wavelength .lambda..sub.5, influence from the output wavelength 
.lambda..sub.4 apart form the input signal by .DELTA..lambda. is 
minimized. 
Furthermore, the apparatus is made more compact than that using a KTP 
crystal and with an extremely high conversion efficiency (10.sup.-2 or 
more).