JPH0575212A - Optical nonlinear amplifying element - Google Patents

Abstract

PURPOSE:To make the wavelengths of incoming and outgoing waves the same and amplify light nonlinealy by amplifying the light with a light amplifier after regenerating the waves with a saturable absorber. CONSTITUTION:Waveguide paths consisting of amplifiers 1G1-1G(n+1) and saturable absorbers 1A1-1An are arranged on a semiconductor substrate 102. The gain is saturated by lengthening the amplifier 1G(n+1) at the final state, whereby the output of an element has strong limiter property. Moreover, nonreflective structures 101 are arranged on the end face the light amplifier 1G1 at the first stage an the end face of the light amplifier 1G(n+1) at the last stage. Currents blow in forward direction in the amplifiers 1G1-1G(n+1), and those perform nearly linear light amplification. Reverse voltage is applied to the saturable absorbers 1A1-1An, and those show nonlinear input properties. And the lights are amplified with the light amplifiers 1G1-1G(n+1), after the waves of high-speed optical signals are regenerated in the saturable absorbers 1A1-1An. Hereby, the lengths of the incoming and outgoing waves can be made nearly the same, and it becomes possible to amplify the light nonlinearly.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical non-linear amplification element having a non-linear optical input / output characteristic.

[0002]

2. Description of the Related Art FIG. 11 shows an example of a conventional optical nonlinear amplification element of this type. This device is a bistable laser in which the electrode of a semiconductor laser is divided into two, and the forward injection current of one electrode is increased to serve as a gain section, and the forward injection current of the other electrode is reduced to act as a saturable absorption section. Is. here,
201 is a gain part electrode, 202 is an InGaAsP contact layer, 203 is an InP (p) cladding layer, and 204 is In.
GaAsP active layer, 205 InP (n) cladding layer, 206 InP (n ⁺ ) substrate, 207 lower surface electrode,
Reference numeral 208 is a saturable absorption electrode, and 209 is a separation groove formed in the electrode 201 and the layers 202 and 203.

FIG. 12 is a graph showing the characteristics of the injection current I ₂ into the gain region of this element versus the optical output. By reducing the injection current I ₁ to the saturable absorber, the threshold characteristic (I
_It can be seen that when ₁ = 6 mA) and bistable characteristics (when I ₁ = 5 mA) are obtained.

In this figure, the injection current I ₂ into the gain region is I _B
And I ₁ = 6 mA. Optical input P _{in at} this time
FIG. 13 shows the characteristics of the light output P _out and the light output P _out . As is clear from FIG. 13, this element functions as an optical nonlinear amplification element.

The conventional device has the following problems. First, the operation response time is limited by the carrier recombination life in the active layer of the saturable absorber, and high-speed operation cannot be performed. Second, the emission wavelength is defined by the resonator structure of the bistable laser and the gain spectrum, and generally the incident and emission wavelengths are different. Thirdly, it is difficult to avoid the critical throwdown phenomenon in which the switching time is delayed when the input light intensity is near the oscillation threshold.

Another example of a conventional optical nonlinear element is shown in FIG. This element is an integrated semiconductor laser with an optical modulator. Here, LD is a semiconductor laser and MD is an optical modulator. 211 is an InGaAsP contact layer, 2
12 is an InP (p) clad layer, 213 is InGaAs
P guide layer, 214 MQW active layer, 215 InP
(N) clad layer, 216 is an InP (n ⁺ ) substrate, 21
7 is a lower surface electrode, 218 is a separation groove formed on the boundary between the electrodes 221 and 222 and the layers 211 and 222, 21
9 is a non-reflective coating, 220 is a grating, 2
Reference numeral 21 is a modulator electrode, and 222 is a laser electrode.

In this conventional device, when the semiconductor laser LD and the optical modulator MD are integrated, in order to obtain a sufficient contrast, the portion of the modulator MD is lengthened to about 100 μm and a signal with a large voltage amplitude is applied. It was necessary to apply. Therefore, there were the following three problems. First, there is a large loss in the modulator and a strong output cannot be obtained. Secondly,
The electrode area is large, that is, the RC time constant is large, and the modulation band is likely to be limited. Thirdly, it is difficult to generate a signal having a frequency with a large voltage amplitude.

[0008]

The present invention has been made under such a background, and it is possible to operate at high speed and to make incident and outgoing wavelengths the same.
An object of the present invention is to provide an optical non-linear amplification element that performs non-linear optical amplification.

Still another object of the present invention is to provide an optical non-linear amplification element integrated with a semiconductor laser for obtaining high speed intensity modulated light.

[0010]

In order to solve the above-mentioned problems, the invention according to claim 1 is characterized in that an optical amplification section and a saturable absorption section are alternately arranged in a semiconductor optical waveguide, and the first stage optical amplification is carried out. It is characterized in that a non-reflection structure is provided on the end face of the section and the end face of the final stage optical amplification section.

According to a second aspect of the present invention, in the optical nonlinear amplification element according to the first aspect, the active layers of the optical amplification section and the saturable absorption section are each formed of a multiple quantum well structure. ..

The invention according to claim 3 is the invention according to claim 1 or 2.
In the described optical nonlinear amplifier element, the length of the final stage optical amplifier section is longer than the length of the intermediate stage optical amplifier section.

The invention according to claim 4 is the invention according to claims 1 to 3.
The distributed feedback semiconductor laser is arranged in front of the optical non-linear amplification element described in any one of 1.

The invention as defined in claim 5 is defined by claim 1 through claim 3.
A distributed Bragg reflector semiconductor laser is arranged in the preceding stage of the optical nonlinear amplifier element described in any one of the above paragraphs.

[0015]

According to the present invention, since the high-speed optical signal is regenerated by the saturable absorption section and then amplified by the optical amplification section, high-speed operation is possible and the incident and emission wavelengths can be controlled. It is possible to provide the optical non-linear amplification element which can make the same wavelength and amplifies light non-linearly.

[0016]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of the present invention, in which 1G _{1 to} 1G _{(n + 1)} are amplifying units and 1A ₁ to 1A.
A _n is a saturable absorber, 1E _{1 to} 1E _{(2n + 1)} is an electrode, 10
Reference numeral 1 is a non-reflective structure, 102 is a semiconductor substrate, and a waveguide composed of the amplification units 1G _{1 to} 1G _{(n + 1)} and the saturable absorption units 1A _{1 to} 1A _n is arranged on the semiconductor substrate 102. By making the amplifying section 1G _{(n + 1)} at the final stage long, gain saturation is achieved and the output of the element has a strong limiter characteristic.

FIG. 2 shows the k-th amplification unit 1G _k in FIG.
FIG. 3 is a cross-sectional view of the saturable absorber 1A _{k in} the waveguide direction. Here, 103 is a contact layer, 104 is a cladding layer, 105 is an MQW active layer, 106 is a cladding layer, 10
7 is a bottom electrode, 1G _k is a k-th amplification section, 1A _k is a k-th saturable absorption section, 1S _(2k-1) is a (2k-1) -th electrode separation structure, 1E _(2k-1) and 1E _2k is k
Th saturable absorber 1A _k and kth amplifier 1G _k
Of the upper electrode layer.

A current of I _k is supplied to the _kth amplifying section 1G _k, and a reverse voltage of −V _k is applied to the _kth saturable absorbing section 1A _k .

In this structure, since the MQW structure is used as the active layer 105, it is possible to apply QC by applying a reverse voltage.
SE (Quantum Confined Stark)
Effect) to control the absorption in the saturable region.

Next, the operation principle of the optical non-linear amplification element of this embodiment will be described with reference to FIGS.

FIG. 3 shows the optical input / output characteristics of the _kth amplifying section 1G _k and the saturable absorbing section 1A _k .
Here, a current is flowing in the forward direction in the amplification unit 1G _k ,
Performs almost linear optical amplification. Amplification factor is 3 as a typical value
It is about 00 (1 / cm), and the light intensity is amplified to about 100 kW / cm ² before entering the saturable absorber 1A _k . The absorption coefficient of the saturable absorber 1A _k is 1500 (1 /
cm) it is about, in order to compensate for the amplification in the amplifier section 1G _k, a length of about 1/5 with respect to the amplification unit 1G _k. In the saturable absorber 1A _k , the higher the light intensity is, the smaller the absorption coefficient is, and therefore the nonlinear input / output characteristic is exhibited as shown in FIG.

Generally, the saturable absorber has a light intensity of 20 kW.
If it is equal to or more than / cm ^2, it does not work effectively and the light intensity sharply decreases at the saturable absorber, so in this embodiment, the saturable absorber 1A _{k has} a length of about 10 μm, and therefore the amplifier 1G
The length of _k is about 50 μm (the amplification unit 1G _{(n + 1) at the} final stage has a length of about 500 μm to cause gain saturation)
Is.

The lengths of the saturable absorber 1A _k and the amplifier 1G _k are such that the light input / output in the amplifier 1G _k is linear and the light input / output in the saturable absorber 1A _k is non-linear. And the effective amplification factor and absorption coefficient are
It depends on the material and the confinement coefficient of the optical waveguide, but in the intermediate stage it is obtained from the condition of the ratio of length so that amplification and absorption are compensated.

As shown in FIG. 3, since sufficient nonlinearity cannot be obtained in the amplification section 1G _k and the saturable absorption section 1A _k in one stage, they are arranged in multiple stages.

FIG. 4 shows the light input / output characteristics of the entire device. Here, the broken line shows the case where the length of the amplification unit 1G _{(n + 1) at} the final stage is not sufficient, the amplification is not saturated, and the limiter characteristic is not sufficient. A good threshold characteristic can be obtained by lengthening the amplification unit 1G _{(n + 1)} at the final stage to cause amplification saturation.

FIG. 5 shows an example of the optical output P _out when the optical signal P _in having a broken waveform is input to the device of this embodiment. The signal is reproduced by the non-linearity of the element to improve the S / N of the signal and then optically amplified. In this case, the incident signal and the emitted signal have exactly the same wavelength.

In the present invention, since the reverse voltage is always applied to the saturable absorption region, the device response time is rate-controlled not by the carrier recombination lifetime in the saturable absorption region but by the tunnel time of the barrier layer in MQW. Therefore, the device can be made to respond at high speed. Moreover, since the absorption coefficient of the saturable absorber can be easily controlled, the threshold value of the non-linear amplification element can be easily controlled.

As shown in FIG. 2, in the present embodiment, the active layer 1 of the amplification section 1G _k and the saturable absorption section 1A _k.
Although the same MQW layer is used for 05, it goes without saying that MQW layers having different band gaps may be used. Alternatively, an MQW active layer having a thin barrier layer can be used as the MQW layer that produces Wannier excitons in the MQW structure. In this case, when a reverse voltage is applied to the saturable absorber, the band edge shifts to the short wave side.
Therefore, it becomes easy to set the operating wavelength to a wavelength near the band edge of the absorption spectrum of the saturable absorber, increase the nonlinearity of the saturable absorber, and then set the gain of the amplifier at that wavelength to be large. ..

FIG. 6 shows an example of this. In this case, the maximum value of the gain spectrum of the gain section is at the wavelength of 1.53 μm.
In the case of a normal MQW thick barrier layer, the absorption spectrum of the saturable absorber moves to the long-wave side as shown by the broken line with the application of a reverse voltage. Therefore, at a wavelength of 1.53 μm, where the gain is large, the band edge is moved away, and the non-linearity of the saturable absorber is reduced. However, in the case of MQW that produces Wannier excitons, a large gain and a large nonlinearity can be obtained at a wavelength of 1.53 μm. That is, it is possible to configure the optical non-linear amplification element with a small number of stages of saturable absorbers.

Needless to say, a bulk active layer or a strained quantum well active layer can be used. Alternatively, it goes without saying that a similar structure can be made with a GaAs-based semiconductor material.

FIG. 7 is a constitutional view of a specific embodiment of the present invention and is a perspective view showing the appearance of the element. FIG. 8 is a sectional view on the axis of the waveguide in this specific example.
FIG. 9 is a sectional view in the direction perpendicular to the waveguide in this specific example. In this example, four saturable absorbers and 5
An optical non-linear amplification element is composed of the stepped gain section.

Here, 5E _{1 to} 5E ₉ are electrodes, and 502 is an InP (n ⁺ ) substrate. 5G _{1 to} 5G ₅ are amplification units,
5A _{1 to} 5A ₄ are saturable absorbers, 501 is a non-reflective coating layer, 502 is an InP (n ⁺ ) substrate, and 503 is In.
GaAsP contact layer, 504 is InP (p) clad layer, and 505 is InGaAs (70 angstrom)
/ InP (30 angstrom) 30 periods MQW active layer, 506 is InP (n) cladding layer, 50
Reference numeral 7 is a lower surface electrode, and 5S _{1 to} 5S ₈ are electrode separation grooves. 5
09 is an InP (p) layer, 510 is an InP (n) layer, 51
1 is an InP (p) layer.

Each of the saturable absorbers 5A _{1 to} 5A _{4 has} a length of 1
0 μm, the length of the preceding _four gain sections 5G _{1 to} 5G ₄ is 5
The length of the final gain section 5G ₅ is 0 μm and 400 μm.

Although the pn type buried structure is used in this embodiment, it is needless to say that other electric / optical confinement structure such as buried by iron-doped high resistance InP, ridge waveguide structure and the like can be used.

Further, although the electrodes are separated by etching to form the grooves, it goes without saying that the electrodes can be separated by ion implantation.

FIG. 10 is a block diagram of another specific embodiment of the present invention, in which a semiconductor laser light source and an optical non-linear amplification element are integrated together. Here, 6L is a distributed feedback semiconductor laser, and 6P is an optical non-linear amplification element. 6G ₂
~ 6G ₅ is an amplifier, 6A ₁ ~ 6A ₄ is a saturable absorber, 6
E _{1 to} 6 E ₉ are electrodes, 601 is a non-reflective coating layer,
602 is an InP (n ⁺ ) substrate, 603 is InGaAsP
Contact layer, 604 is InP (p) clad layer, 60
5 is an InGaAsP clad layer, 606 is InGaAs
(70 angstrom) / InP (30 angstrom) MQW active layer consisting of 30 periods, 607 is In
P (n) clad layer, 608 lower surface electrode, 609 grating, 610 grating pitch shift part,
6S _{1 to} 6S ₈ are electrical isolation grooves. For length,
The semiconductor laser 6L is 300 μm, the saturable absorber 6A ₁ is 20 μm, the saturable absorbers 6A _{2 to} 6A ₄ are 10 μm, the amplifiers 6G _{2 to} 6G ₄ are 50 μm, the final stage amplifier 6G ₅ is 400 μm, the separation groove 6S ₁ ˜6S ₈ is 5 μm.

In this element, a high-speed electric modulation signal is applied to the electrode 6
It is applied to E ₂ , that is, saturable absorber 6A ₁ . Here, the light emitted from the distributed feedback semiconductor laser 6L is
The waveform is slightly modulated and shaped as it passes through the optical non-linear amplification element, and is output as modulated signal light with high contrast. Compared with the conventional technique, the length of the waveguide for applying electrical modulation is short (the electrode area is small), and a large contrast can be obtained even if the applied voltage amplitude is small. Therefore, the frequency limitation by the RC time constant is less likely to occur, and the electric pulse for modulation can be easily generated. Moreover, since the loss is guaranteed, a large light output can be obtained.

Although the distributed feedback type semiconductor laser is used as the semiconductor laser in this embodiment, it goes without saying that a distributed reflection type semiconductor laser can also be used. Furthermore,
Although the active layer and the clad layer of the semiconductor laser and the optical non-linear amplification element are common, it goes without saying that the active layer and the clad layer having different band gaps may be used respectively.

[0040]

As described above, according to the present invention,
Since the high-speed optical signal is regenerated by the saturable absorption section and then amplified by the optical amplification section, high-speed operation is possible and the incident and emission wavelengths can be the same. It is possible to provide an optical non-linear amplification element that performs non-linear optical amplification.

Moreover, since the optical nonlinear amplification element of the present invention can be modularized in a small size, it is easy to handle and can be incorporated into other devices. It can be used for optical signal transmission devices and the like.

In addition, a light source for high-speed intensity modulation can be constructed by integrating the optical nonlinear amplification element of the present invention with a semiconductor laser.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a configuration of an exemplary embodiment of the present invention.

FIG. 2 is a sectional view of a part of a waveguide according to an embodiment of the present invention.

FIG. 3 is a light input / output characteristic diagram of a k-th amplification section and a saturable absorption section in the present invention.

FIG. 4 is a light input / output characteristic diagram of an example of the present invention.

FIG. 5 is a signal waveform diagram showing the optical signal input and output of the embodiment of the present invention in comparison.

FIG. 6 is an explanatory diagram of an absorption spectrum of a saturable absorber according to the present invention.

FIG. 7 is a configuration diagram of a specific embodiment of the present invention.

8 is a sectional view taken along the waveguide axis of the specific example shown in FIG.

FIG. 9 is a cross-sectional view taken in a direction perpendicular to the waveguide of the specific example shown in FIG.

FIG. 10 is a configuration diagram of another specific embodiment of the present invention.

FIG. 11 is a schematic diagram showing a configuration of a conventional example.

FIG. 12 is a characteristic diagram of injection current vs. optical output of a conventional example.

FIG. 13 is a light input / output characteristic diagram of a conventional example.

FIG. 14 is a schematic diagram showing the configuration of another conventional example.

[Explanation of symbols]

1G _{1 to} 1G _{(n + 1)} amplification section 1A _{1 to} 1A _n saturable absorption section 101 non-reflective structure 102 semiconductor substrate 103 top electrode layer 104 contact layer 105 clad layer 106 MQW active layer 107 clad layer 108 bottom electrode 109 optical waveguide 1G _k k-th amplification section 1A _k k-th saturable absorption section 1S _(2k-1) (2k-1) -th electrode separation groove 201 Saturable absorption section electrode 202 InGaAsP contact layer 203 InP (p) cladding layer 204 InGaAsP active layer 205 InP (n) clad layer 206 InP (n ⁺ ) substrate 207 bottom electrode 208 amplification part electrode 209 isolation groove LD semiconductor laser MD optical modulator 211 InGaAsP contact layer 212 InP (p) clad layer 213 InGaAsP guide layer 214 MQW active layer 215 InP (n) Cladding layer 216 InP (n ⁺ ) substrate 217 Lower surface electrode 218 Separation groove 219 Antireflection coating 220 Grating 221 Modulation part electrode 222 Laser part electrode 5E _{1 to} 5E ₉ electrode 502 InP (n ⁺ ) substrate 5G _{1 to} 5G ₅ amplification part 5A _{1 to} 5 A ₄ saturable absorber 501 non-reflective coating layer 502 InP (n ⁺ ) substrate 503 InGaAsP contact layer 504 InP (p) clad layer 505 MQW active layer 506 InP (n) clad layer 507 lower surface electrode 5S _{1 to} 5S ₈ Electrode separation groove 509 InP (p) layer 510 InP (n) layer 511 InP (p) layer 6L Distributed feedback semiconductor laser 6P Optical nonlinear amplification element 6G _{2 to} 6G ₅ Amplification section 6A _{1 to} 6A ₄ Saturable absorption section 6E ₁ ～6E ₉ electrodes 601 anti-reflective coating layer 602 InP (n ⁺⁾ Plate 603 InGaAsP contact layer 604 InP (p) cladding layer 605 InGaAsP clad layer 606 MQW active layer 607 InP (n) cladding layer 608 lower electrode 609 grating 610 grating pitch shift unit 6S ₁ ~6S ₈ electrode separation grooves

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location H01S 3/108 8934-4M

Claims (5)

[Claims] 1. An optical amplification section and a saturable absorption section are alternately arranged in a semiconductor optical waveguide, and a non-reflection structure is arranged on an end surface of the first stage optical amplification section and an end surface of the final stage optical amplification section. An optical nonlinear amplification element characterized by. 2. The optical nonlinear amplification element according to claim 1, wherein the active layers of the optical amplification section and the saturable absorption section each have a multiple quantum well structure. 3. The optical nonlinear amplification element according to claim 1, wherein the length of the final stage optical amplification section is longer than the length of the intermediate stage optical amplification section. .. 4. An optical non-linear amplification element characterized in that a distributed feedback semiconductor laser is arranged in the preceding stage of the optical non-linear amplification element according to any one of claims 1 to 3. 5. An optical non-linear amplification element characterized in that a distributed Bragg reflector semiconductor laser is arranged in the preceding stage of the optical non-linear amplification element according to any one of claims 1 to 3.