JPH0321934A - Nonlinear optical device - Google Patents

Abstract

PURPOSE:To enable action at low intensity of light and ultrahigh-speed action by using a solid material based on Ge, As, S and Se as an optical medium having a nonlinear refractive index. CONSTITUTION:A solid material based on Ge, As, S and Se used as an optical medium having a nonlinear refractive index in a nonlinear optical device. The nonlinear optical medium belongs generally to a material called chalcogenide glass, has a significant tertiary nonlinear effect and can be accurately evaluated like an org. material by measuring the intensity of third harmonics (THG light). A high response speed, a relatively high nonlinear refractive index, a small loss and superior workability are ensured, a prolonged size can be ataained and high-speed action at low intensity of light is enabled.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to nonlinear optical devices such as optical switches, optical memories, or optical signal processing devices used in optical data/information processing and optical communication systems. .

[Conventional technology and its sections! i] Nonlinear optical effect means that the electric polarization P in a substance is as follows (1
), in addition to the term proportional to the electric field E of light, E1, E
This is an effect caused by having a higher-order term proportional to 3.

p = x (1 1 E + x (!) E R
+x (3) E 3 + − ... (1) χ
(0 is linear susceptibility, χ(0 and χ(3) are each 2
They are called second- and third-order nonlinear susceptibilities. In particular, the third term indicates third harmonic generation (a phenomenon in which light with a frequency of 3ω is emitted for incident light with a frequency of ω), which is well known as a third-order nonlinear effect, and is also expressed by the following equation (2). This results in a change in the refractive index that depends on the intensity of the light being received.

n-no→-ntl ●●●●(2
) In equation (2), nt (unit: cm''/W) is given by the following equation (3) Cn0 with respect to the third-order nonlinear susceptibility χ0》 (unit: Hesu). However, n. is the normal refractive index, C is the speed of light.

When an optical medium that exhibits this effect (hereinafter referred to as a nonlinear optical medium) is combined with other optical elements such as a polarizer, an optical collector, or a reflector, it is possible to create an optically controlled optical switch or an optical bistable element. It is also possible to construct important devices that will be used in the future in optical information processing and optical communication systems, such as phase conjugate wave generators.

Hereinafter, a conventional example of a nonlinear optical device that applies a nonlinear refractive index will be explained with reference to the drawings.

FIG. 7 shows an example of the configuration of an optical Kahn Yatta using an optical medium having a nonlinear refractive index. The optical Kerr shutter switches input light Pi with gate pulse light Pg to obtain output light Pt corresponding to the time waveform of the gate pulse.

In the figure, numeral 1 is a nonlinear optical medium made of CS* (carbon disulfide) liquid sealed in a glass cell with a length lIII1, and numerals 2a and 2b are nonlinear optical media 2a and 2b arranged so that their polarization axes are perpendicular to each other. This is an orthogonal polarizer system consisting of two polarizers. In this configuration, only while the gate pulse light pg is incident, the linearly polarized wave of the input light Pi that has passed through the polarizer 2a changes to an elliptically polarized wave due to the change in the refractive index of the nonlinear optical medium l. Part of the light is crossed by the orthogonal polarizer 2b
can pass through. That is, the input light Pi is optically switched by the pulse of the gate light Pg.

The transmittance T of the human power light Pi when it is switched on is the gate light P
The maximum value is reached when the polarization direction of g and the polarization direction of the human-powered light Pi are tilted by 45 degrees. In this case, the transmittance T is T=s i n ” (△φ/2)
−・− (4′)2πntLIin Δφ1 ・・・・
(5) Represented by λ. However, L is the medium length, λ is the input optical wavelength, and f
in is the gate light intensity, and nt is the nonlinear refractive index.

Equations (4) and (5) show that when △φ is sufficiently small, T QC
Since n*"L"lin" (6), it can be seen that the transmittance T is proportional to the square of n.

The results of our re-examination of the CSf optical power output shown in Fig. 7 show that λ = 0.83μ polarization (input light from two semiconductor lasers), L = Ims, and I in = 300 MW/c.
vi'' (gated dichromate pulsed laser), a transmittance T=0.8% was obtained.

From this result, using equations (4) and (5), the nonlinear refractive index n
, is calculated, n ,=7. 7X 1 0-”C-
″/W. From this value of n, using equation (3) to calculate the nonlinear susceptibility χ(0), we get χ(0=3.6Xl
It is calculated as O-”esu.

The mechanism of the nonlinear refractive index effect of the above C St material is based on the fact that the refractive index shows light intensity dependence due to the rotational orientation of molecules in response to the optical electric field (molecular rotational nonlinear effect). Although it is advantageous in that the possible input light wavelengths cover a wide range from the visible to the near-infrared region, it has the problem that nx is not very large and therefore requires a large operating input light intensity (as is clear from equation (6)). there were. Another problem is that the response time is limited by the rotational relaxation time of the molecule, and it cannot be used for signal processing faster than ro-II to IO-''s.

FIG. 8(a) shows the configuration of an etalon type optical bistable element which is another conventional example of a nonlinear optical device. In FIG. 8, reference numeral 1 is a nonlinear optical medium having a nonlinear refractive index, and reference numerals 3a and 3b are dielectric multilayer mirrors with a reflectance of about 90%, which are arranged in parallel with the nonlinear optical medium 1 in between. constitute an optical resonator. In this configuration, if the wavelength of the input light Pi or the mirror spacing between the two dielectric multilayer mirrors 3a and 3b is slightly changed to improve the resonance conditions of the resonant body, the output light will change with respect to the human power light intensity Pi. Strength P
L or Figure 8(b). It exhibits the characteristics shown in (c). (For the operating principle, see the literature A ppl. Phys. L.
ett. ) vol. 35, p451 (1976) for details. ) These are compatible with remisota operation and bistable operation, respectively, and are useful for waveform shaping of input optical pulses, optical switches, optical signal memory, optical logic operation, etc. in optical communications and optical information processing systems. It is applicable.

Minimum human power light intensity required for operation of etalon type optical bistable device 14. ■in is analytically expressed as the following formula (7) 1i, min=K
λ/n,L...It is expressed as (7). however,
λ is the human light wavelength, n is the nonlinear refractive index, and medium length is K.
(~0.01) is a coefficient determined by mirror reflectance and resonator length adjustment.

A typical etalon-type optical bistable device that has been reported so far uses a superlattice crystal produced by alternately and repeatedly growing semiconductor thin films of GaAs and GaAIAg as a nonlinear refractive index medium.

The operating principle of this optical medium is that light is absorbed within the crystal and excitons are excited, and as a result, the refractive index shows light intensity dependence (absorption nonlinear effect).
(ie, χ(3)) is large, and the prior input light intensity necessary for operation is as small as about IOOW/c-2, which is an advantage.

However, the usable input light wavelength is limited to an extremely narrow range near the exciton absorption spectrum L, and the response time is determined by the exciton lifetime, and the
There was a problem that it could not be used for optical signal processing faster than 0-''s.

As is clear from the above, the performance of a nonlinear optical device is mostly determined by the characteristics of the nonlinear optical medium. Therefore, the development of a nonlinear optical medium with a wide usable wavelength range, good third-order nonlinear effect efficiency, and fast response speed is eagerly awaited, and active research is currently being carried out toward this end.

As the third-order optical nonlinear optical medium, the above C S t and Ga
In addition to As/GaAIAs superlattice materials, benzene rings and 2
Organic nonlinear optical materials with π-electron conjugated systems such as double or triple bonds have recently attracted particular attention. Ichiro of this material was developed by the inventors of the present application, for example, in Japanese Patent Application No. 62-2487.
60 and Japanese Patent Application No. 63-120269.

Polynoacetylene bis(paratoluenesulfonate) (abbreviated as PTS) is the most representative organic nonlinear optical material.
is the value of the cubic nonlinear susceptibility χ(3), Z0)=I
XIO-"esu (converted to nonlinear refractive index n, n
The large value of t=2xloichimoku cm'/W
S (two orders of magnitude larger than t). Furthermore, the mechanism of this nonlinear optical effect is not due to absorption or interaction with molecules or crystals, but rather originates from the separation of pure intramolecular π electrons (third-order polarization effect), which reduces the intensity of the optical signal. The response time that can follow changes is extremely fast, on the order of 1 O-''s.

However, before at! Polynoacetylene materials, typified by PTS, are generally insoluble and infusible, and have major problems in processability. Therefore, it is very difficult to obtain an optical medium having a desired shape, surface smoothness, etc. In fact, a nonlinear optical device using PTS as an optical medium has not yet been realized.

In order to eliminate these drawbacks, the present invention has a fast response speed, a relatively large nonlinear refractive index, low loss, excellent workability, and, as a result, a nonlinear refractive index that allows for long lengths. The present invention provides an optical medium and provides a nonlinear optical device that operates at high speed and low light intensity.

[Several Steps to Solve the Problems] In order to solve the above problems, the present invention has the following configuration.

In other words, the present invention provides a nonlinear optical device that includes an optical medium having a nonlinear refractive index and an optical element such as a polarizer, an optical resonator, or a reflecting mirror or an optical coupler as basic elements. (T) The optical medium is germanium (Ge), arsenic (As), sulfur (S), selenium (
It is characterized by being a solid material mainly composed of Se).

The nonlinear optical medium used in the present invention can be defined as belonging to a material generally called chalcogenide glass, and its chemical formula is represented by the following general formula (A).

GexAsyS zsew”(A>
However, x. y and Z i are numbers representing the composition, x+y+z+v=l O O, O≦X . y.
z, w < 1 0 0.

Such chalcogenide glasses are already known as a low-loss glass material for transmitting infrared light.

However, the fact that the chalcogenide glass material used in the present invention has a large third-order nonlinear effect has been clarified for the first time by the present invention.

In addition, this material was used as a nonlinear optical medium to construct nonlinear optical devices such as optical switches, optical bistable devices, and vertical conjugate wave generators.
This is the most important point of the present invention, and in this point, the present invention is completely different in gist from the prior art in which the above-mentioned lucogenide glass is used for transmitting infrared light. It can be said that it belongs to a separate invention.

FIG. 1 is a graph showing measurement data of an experiment conducted to evaluate the efficiency of the third-order nonlinear effect of the chalcogenide glass of the present invention. The third-order effect of glass materials can be evaluated with considerable accuracy by measuring the intensity of the third harmonic (THC; light), similarly to organic materials. Figure l shows thickness 1.2
On a plate of Asao Sao glass optically polished to 2+as,
A laser pulse beam with a wavelength of 1 and a wavelength of 90μ is input, and while the sample is rotated around an axis perpendicular to the direction of incidence of the laser beam, the output T H (; is the result of measuring the light intensity. This pattern is This is called a maker fringe, and is measured using the same observation system as a reference sample using a material whose efficiency is known (quartz glass was used here).
By comparing the G peak intensities, the cubic effect efficiency χ0) can be determined. Regarding the measurement principle, please refer to the literature Electronics Letters (Electron.
tt. ) Vol. 2 3, No. I 1, p5
I am familiar with 9 5 (1 9 8 7).

Table 1 summarizes the results obtained by this measurement, and shows the results of other chalcogenide glasses such as Ge■S? 6
The results for glass and quartz glass as a reference sample are also shown together. The value of nt in Table 1 is a calculated value using equation (3).

As can be seen from Table 1, the n! The value of is about 25 times larger than that of quartz glass. (χ (approximately 100 times larger in terms of value) Also, the absorption coefficient of the measured sample is α<
It showed an extremely small value of 1×10−”cm−’.

Considering this low loss property and the fact that the third-order nonlinear effect mechanism of this glass is a third-order polarization effect that does not require absorption, it is concluded that the usable wavelength range of this material is extremely wide.

Similarly, based on the mechanism, the response time of this material is estimated to be on the order of IO-'s, similar to that of organic materials, and is sufficiently fast.

Hereinafter, the features of the nonlinear optical device using chalcogenide glass according to the present invention will be explained in detail using Examples.

[Example] (Example 1) 684 as chalcogenide glass. S. An example of an optical car shutter device using glass is shown below.

A84 of nonlinear optical media. Sao glass was produced by the following method. Commercially available arsenic and sulfur powder raw materials were weighed so that the molar ratio was 2:3, sealed in a vacuum-evacuated quartz glass ampoule, sufficiently heated in an electric furnace at 850-900°C, and then taken out into the atmosphere and rapidly cooled. and vitrified it.

The surface of the samble was optically polished in parallel, and the thickness was set to L=lcm. A nonlinear optical device was constructed using the same components as shown in FIG. 7 except for the nonlinear optical medium.

As shown in Table I. Value of nonlinear refractive index of glass n 2-1 . 8 X l O −”cm
Calculating the operating characteristics of the above optical Kern Yatta from equations (4) and (5) using '/W, λ10, 83μ (input light, semiconductor laser), L = lOcm, lin = 300
When MW/cs' (gated dichromate pulsed laser), the value of switch-on transmittance is calculated as T=4.1%. The actual characteristics were also found to be in good agreement with this. The response speed was also verified to be as fast as expected.

AS4 for nonlinear optical media. S. Chalcogenide glasses with compositions other than glasses, such as Ge shown in Table 1
tsS7s glass, AsaoSeso glass or
Gex Asy made by appropriately mixing these materials
S z Sew glass (X + y + z + w - 1
00, 0≦X. y , Z i <100), a front-car shutter device having almost the same characteristics as the case using A 840 680 glass was realized.

(Example 2) Figure 2 is a diagram explaining another example of the nonlinear optical device of the present invention, in which the AstS3 glass described in the example was processed into a fiber shape and used as the nonlinear optical medium l. be.

In FIG. 2, reference numerals 4a, 4b. 4c is an optical lens for combining each light beam, 5 is an optical fiber coupler, and 51a and 5lb are fiber couplers 5.
51c and 51d are the exit side arms,
Reference numerals 6 and 6 indicate fiber-type polarizers arranged such that their polarization axes are perpendicular to each other, and are constructed by winding a polarization-maintaining fiber into a loop shape. Also, reference numeral 9 indicates gate light Pg.
This is an optical filter for blocking the output light Pt and extracting only the output light Pt. The optical fiber polarizer 6 is connected to the input side arm 5la, and the nonlinear optical medium fiber 1 is connected to the output side arm 51d. Input light Pi and gate light Pg enter from optical fiber polarizer 6 and entrance arm 5lb via optical lenses 4a and 4b, respectively. The optical fiber coupler 5 branches the light from the input side arm 51a to the output side arms 51c and 51d at a branching ratio of 0.1:99.9 for the wavelength of the input light Pi, and For the wavelength of light Pg, the light from the input side arm 5lb is split into the output side arm 51 with a splitting ratio of approximately 50:50.
It has the function of branching into c and 51d.

In the apparatus having the configuration shown in FIG. 2, the optical lens 4a
, 4b, 4c, and the optical filter 9 are all fiber-type products, so they are integrally constructed using terminal connections, and the device is characterized by being compact, lightweight, and highly stable. .

When the size of the optical fiber is 10 μm in diameter and the length is 1001, and the same configuration as in Example 1 is used for the manual light source and the gate light source, the input light intensity is It decreased to about 171,000 in Example 1.

It was also possible to use a semiconductor laser instead of the dye pulse laser as the gate light source.

(Example 3) FIG. 3 is a diagram illustrating an example of the nonlinear optical device of the present invention, in which the chalcogenide glass fiber 1 used as the nonlinear optical medium in Example 2 is used in one arm,
This is a Mann-Zehnder interferometer type optical gate switch.

In FIG. 3, reference numeral 1 is AsaoSao's optical fiber-shaped nonlinear optical medium, reference numerals 4a and 4b are lenses, reference numeral 5l is the first optical fiber coupler, and reference numbers 5la and 51h are the first optical fiber coupler 51. Incident side arm, code 51c
, 51d is an output 0111 arm.

52 is a second optical fiber coupler, 52a, 52
b is the input side arm of the second optical fiber coupler 52, and 52c and 52d are the output side arms.

Reference numeral 7 is a semiconductor laser for input generation, reference numeral 8 is a semiconductor laser for gate light generation, reference numeral 10 is a photodetector, and reference numeral 11 is a quartz optical fiber.

The fiber-shaped nonlinear optical medium 1 and the Ishifu optical fiber 1l are
The lengths are both IO-, and the output side arm 51c of the first fiber coupler 51 and the second optical fiber coupler 5
2, an output arm 5ld of the first optical fiber coupler 5l, and an input arm 52b of the second fiber coupler 52, respectively. Jinriki Hikari] 冫l and Game 1 Hikari Pg are the first
The input side arms 51a, 5lb of the optical fiber coupler 5l
It is incident from In this embodiment, as the game 1 light source,
Frequency 1 by directly modulating a semiconductor laser with a wavelength of 1.3 μm
Light of 0GI-1z and a peak value of 100 mW was used.

In addition, human-powered light is generated from a semiconductor laser with a wavelength of 1.52 μm.
OmW continuous light was used.

The l-th optical fiber coupling 851 receives input light Pi with a wavelength of 1.52 μm incident from the incident side arm 5la at a ratio of 50:5.
Branches into output side arms 51c and 51d with a branching ratio of 0,
In addition, the wavelength l. 3μm
It has a function of branching the gate light Pg to the output side arms 51c and 51d at a branching ratio of 99.9+0.1. On the other hand, the second optical fiber coupler 52 has an incident measurement arm 52a.
and 52b, the human-powered light Pi with a wavelength of 1.52 μm is input to the output side arm 52 at a branching ratio of 50:50.
The gate light Pg with a wavelength of 1.3 μm, which is branched to c152d and incident from the incident side arm 52a, is sent to OI99,9.
At a branching ratio of , the output side gates h 5 2 c and 52 d are branched. Further, the gate light having a wavelength of 1.3 μm inputted from the input side arm 52b has a function of branching into the output side arms 52c and 52d at a branching ratio of 99 90 l.

With this configuration, the input light Pi with a wavelength of 1.52 μ- is transmitted to the l-th optical fiber coupler 5! by 50
:50, pass through the fiber-shaped nonlinear optical medium 1 and the quartz fiber It, and are combined by the second optical fiber coupler 52.

This configuration is a Mach-Zehnder interferometer, and the optical intensities 1c and Id of the incident light Pi emitted from the output side arms 52c and 52d are expressed by the following equation (8) Iccx:cos'
(△φ/2) (8) Id sin ″ (△φ/2) where △φ is the phase difference between the light that has passed through the fiber-like nonlinear medium l and the quartz fiber 1l. Yes, given by equation (5) above.

However, the nonlinear refractive index of the quartz optical fiber 11 was ignored.

In this example, in a state where no excitation light is incident (Ic
=l, [d=O, but the peak peak value of the excitation light is 1
At 0 0m W, Ic=0.02, Id=0.98
Met.

FIG. 4 shows the time waveforms of the input gate light and the output signal light from the output side arm 52c in this embodiment. Input light Pi incident as continuous light is converted into gate light P of lOGHz.
It can be seen that high-speed switching is performed by g.

(Embodiment 4) FIG. 5 is a diagram showing another embodiment of the nonlinear optical device of the present invention, and is AS4. An etalon-type optical bistable element is constructed using Sea's chalcogenide glass as the nonlinear optical medium l. In FIG. 5, reference numerals 3a and 3b indicate dielectric multilayer mirrors deposited directly on the surface of the nonlinear optical medium 1. The operating method of this device is as already explained in FIG. 8, and the resonator conditions of the resonator can be adjusted by slightly changing the wavelength of the human-powered light Pi. In the case of this embodiment, a wavelength-tunable YAG laser-excited dye laser (wavelength Q
．． 77z+a) light was used.

There is a difference between the manual light intensity Pi and the output light intensity pt as shown in Fig. 8 (
Limiter operation and bistable operation as shown in b) and (c) were obtained. Minimum input light intensity required for operation 1i. m
in was about 50 MW/cm', but this value is calculated analytically from equation (7) as a logical value [ i , m
in −4 0 MW/c yA”.

(However, L, λ-1.064 lza, and L-1 cm were set.

) The response time t of this chalcogenide glass material is t~l O
As already mentioned, the response time of the device in this example is the larger of the response time t of this medium and the intra-cavity photon lifetime tp of equation (9) below. Determined by value.

Lp = −Lop/c−+n R”=
(9) However, Lop is the optical length of the resonator, C is the high speed, and R
is the reflectance of the dielectric multilayer mirrors 3a and 3b.

In this embodiment, since (p~8XIO-''s, tp>t, this tp value becomes the response time of the device.

In this example, it was confirmed that the response time was shorter than IO-Ils.

(Embodiment 5) FIG. 6 is a diagram showing another embodiment of the nonlinear optical device of the present invention, A84. A phase conjugate wave generator is constructed using Sea's chalcogenide glass as the nonlinear optical medium l.

In FIG. 6, numerals 8a and 8b are both semi-transparent mirrors, and numeral 9
is a total reflection mirror. This structure is an optical arrangement called reduced iA four-wave mixing, in which AI, A
When three light waves, 2 (in the opposite direction to At) and Ap (inclined incidence), are incident, a fourth light wave (Ac) whose only spatial phase term is conjugate to Ap is generated. This α-phase conjugate wave is attracting attention as an effective means for image correction in image information processing technology and real-time holography. For applications, see the literature O Plus E.
Details in March issue, ρ, 73 (1982).

In this example as well, the high-speed response of the device and the low operating input light intensity were confirmed.

[Effects of the Invention] As explained above, chalcogenide glass, which is the basic element of the present invention, has a third-order nonlinear susceptibility of approximately χ"-10-"esu, which is two orders of magnitude higher than that of silica glass. It also has low loss and excellent processability, and as a result, it can be made into long lengths, so it can be used for optically controlled optical switches and optical bistable devices that can operate with low light intensity, as well as for phase conjugate waves. It is possible to realize a generator, etc.

In addition, since the mechanism of the nonlinear refractive index effect is a third-order polarization effect that does not use an absorption process, the usable wavelength range is extremely wide, and the response time is 10-"s, which is about the same as that of R1" mechanical materials. This realizes ultra-high-speed operation.

Therefore, the nonlinear optical device of the present invention includes an optical switch,
It has the advantage of being applicable to future optical communication and information processing technologies such as optical memory and optical signal processing.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the THG intensity pattern of a chalcogenide glass material, which is a basic element of the nonlinear optical device of the present invention, and FIG. 2 is a diagram showing a second embodiment of the present invention. Figure 3 shows a third embodiment of the present invention, in which a Manoha-Zehnder interferometer type using a chalcogenide glass fiber is used. A schematic diagram of the configuration of the light control optical system; FIG. 4 is a graph showing the time waveforms of the input gate light and the output signal light in the third embodiment of the present invention; FIG. 6 is a schematic diagram of an etalon-type optical bistable device using chalcogenide glass. FIG. A schematic structural diagram of the device, FIG. 7 shows a conventional nonlinear optical device and a first embodiment of the present invention, and a schematic structural diagram of an optical Kerr shutter, which is one of the optically controlled optical switches, This figure shows another conventional nonlinear optical device, and is a schematic configuration diagram of an etalon type optical bistable element. DESCRIPTION OF SYMBOLS 1... Optical medium, 2... Polarizer, 3... Dielectric multilayer mirror 4... Optical lens, 5... Optical fiber coupler, 6... Optical fiber polarizer, 10... photodetector,
12 semi-transmissive mirror, l3...total reflective mirror. Figure 1: Input of force storage camera 4 corners (/L) Koichi\ Figure 5: Irihan optical night (Fig. 6: 72゜half-noodle sharp; Irihan optical gyoi: Figure 8: 1 [Linear optical medium contribution (O) 4 Kenpachi diagram? Power light strength ■ Pi Input value 5 Sagasa P (b) Human output characteristics (Rimai 7 Ku movement I) (C) Input and output characteristics [ (Double The constant movement is)

Claims (1)

[Claims] An optical medium having a nonlinear refractive index, a polarizer, an optical resonator,
Alternatively, in a nonlinear optical device that consists of optical elements such as reflectors and optical couplers as basic elements, the optical medium with a nonlinear refractive index is germanium (Ge).
A nonlinear optical device characterized in that it is a solid material mainly composed of arsenic (As), sulfur (S), and selenium (Se).