JP2947702B2 - Tunable laser device and manufacturing method thereof - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength tunable laser device, and more particularly to a wavelength tunable laser device capable of continuously controlling and changing the laser oscillation wavelength by voltage control from the outside and having a sufficiently large tunable width. The present invention relates to a tunable laser device and a manufacturing method thereof.

[0002]

2. Description of the Related Art As a light source for coherent optical communication and various light sources for measurement, a semiconductor laser device capable of controlling and changing a laser oscillation wavelength is required. In these coherent optical communication and various measurement applications, in most cases, laser light satisfying the following conditions is required.

A single mode (including pulse driving) and a laser beam of an arbitrary wavelength can be oscillated within a wavelength tunable range. In order to satisfy these conditions, a voltage or a current applied from the outside must be It is necessary to continuously change the oscillation wavelength of the semiconductor laser device.

Conventionally, as a wavelength variable semiconductor laser device which has been developed for the purpose of continuously changing the oscillation wavelength, there is a type which changes the refractive index of a laser waveguide with a diffraction grating. This type of laser device is a distributed feedback (DFB) type semiconductor laser or a plug reflection type (DB).
R) A semiconductor laser has a basic configuration. This type of laser device has a diffraction grating combined with an optical waveguide inside the laser device. In such a configuration, the wavelength of the oscillation laser is controlled by controlling the refractive index of a part of the waveguide having the diffraction grating by increasing or decreasing the number of injected carriers or the temperature. Light wavelength)
This is done by changing

As a first example, a wavelength tunable semiconductor laser device based on the prior art, of the type in which the wavelength of an oscillation laser is controlled using a temperature effect, will be described. FIG. 22 (a) to FIG.
FIG. 2 (d) shows a manufacturing process of this conventional laser device. The manufacturing process of the present laser device will be described below with reference to the drawings.

[0006] First, as shown in FIG.
1.2 μm thick n-A1GaAs on aAs substrate 700
s clad layer 701, 0.15 μm thick n-A1GaAs
s gradient refractive index layer 702, 7 nm thick GaAs single quantum well active layer 703, 0.15 μm thick p-A1 GaAs gradient refractive index layer 704, 0.1 μm thick p-A1 GaAs carrier barrier layer 705, and 0.10 μm A thick p-guide layer 706 is formed in this order by metal organic chemical vapor deposition (MOC).
The crystal is continuously grown by the VD) method.

Next, as shown in FIG. 22B, a part of the wavelength control region 710 of the p-guide layer 706 is subjected to a period of 129n by using an electron beam exposure method and a normal etching method.
A diffraction grating 707 having a depth of 80 nm and a depth of 80 nm is manufactured. In this state, Si ions are implanted into the wavelength control region 710, and the interface between the quantum well active layer 703 and the gradient refractive index layers 702 and 704 in the wavelength control region 710 is disordered to increase the forbidden band width. Thereby, the active region 72
The waveguide of the wavelength control region 710 is made transparent to the light generated at 0. Subsequently, a mesa stripe 730 having a width of 3.5 μm and a depth of 0.12 μm is formed by a normal photolithography technique and an etching technique.
Is formed substantially perpendicularly to both the wavelength control region 710 and the active region 720 to obtain a wafer with a diffraction grating 707.

Further, as shown in FIG. 22C, a 1.2 μm-thick p-A1G
aAs cladding layer 708 and 0.3 μm thick p-GaAs
An s-contact layer 709 is grown in this order by MOCVD.

Finally, as shown in FIG. 22D, a width of 3.5 μm and a depth of 1.3 are placed immediately above the mesa stripe 730.
A current constriction mesa stripe 750 of μm is formed, and two grooves 713 and 714 having a depth of 0.5 μm are formed perpendicularly to the stripe direction. Thereby, the active region 7
20 and the wavelength control region 710, and the phase control region 711 and the wavelength variable region 71 in the wavelength control region 710.
2 is electrically separated. Also, the light emitting electrode 751 on the active region 720, the phase control electrode 752 on the phase control region 711, and the wavelength variable electrode 75 on the wavelength variable region 712.
3, after forming a common electrode 754 on the entire surface of the substrate 700, a chip is cut out by cleavage to form a laser device. In order to inject current only into the current constriction mesa stripe 750, the current confinement mesa stripe 75 is used.
The electrodes 751, 752, and 753 are formed after forming an SiO ₂ film 740 on the element surface excluding the top surface.

The wavelength tuning principle in the conventional laser device manufactured as described above will be described below. Generally, in a semiconductor laser device having a diffraction grating 707 inside a waveguide, its oscillation wavelength is determined by the optical pitch of the diffraction grating 707 (actual pitch × refractive index of the waveguide). In this conventional example, a current is passed through the wavelength variable region 712 having the diffraction grating 707 on the surface of the guide layer 706, so that layers (n- and p-cladding layers 701, 709, n-) through which laser light is guided.
Since the temperatures of the p-gradient refractive index layers 702 and 704 and the active layer 703) increase, the optical pitch of the diffraction grating 707 increases. This utilizes the property that the refractive index increases as the temperature of the semiconductor increases. As a result, when the amount of current injected into the wavelength tunable region 712 is increased,
The oscillation wavelength continuously shifts to the longer wavelength side.

FIG. 23 shows a wavelength tunable characteristic of the laser device according to the conventional example. As can be seen from the figure, the current (wavelength tunable current) flowing through the wavelength tunable region 712 is from
When changed to mA, the oscillation wavelength of the laser device changes from 849.7 nm to 852.5 nm,
The wavelength variable width is about 3 nm. However, a wider wavelength tunable range (10 nm or more) is desired as a wavelength tunable laser device in applications such as a wavelength division multiplex communication system and a coherent detection type measurement system.
For such an application, the wavelength tunable range of the conventional laser device is insufficient.

As described above, in order to change the wavelength, it is necessary to inject a current, and the power required for the wavelength change is as large as 200 mW (2 V × 100 mA), which is a problem in application.

As a second example, a wavelength tunable laser device utilizing the change in refractive index due to temperature as in the first example, but utilizing other principles will be described. The present laser device utilizes a change in the refractive index due to the carrier density. FIG. 24 shows a structural diagram of the present laser device. The structure of the present laser device will be described below with reference to the drawings.

The present laser device has a normal DFB laser device structure having a diffraction grating 900 inside the device. This laser device includes current injection electrodes 901, 902, and 903 divided into three in order to add a wavelength variable function. Of these electrodes 901, 902, 903, two electrodes 901 and 903 located near both end faces are short-circuited to each other and connected to the light output control circuit 904, respectively. The center electrode 902 is a wavelength control circuit 905.
Connected to.

In the conventional laser device, since the electrodes of the DFB laser are divided into three, the carrier distribution state in the resonator direction of the laser device can be changed. Generally, a semiconductor material has a property that a refractive index is reduced when a current is injected and a carrier density is increased (carrier plasma effect). By utilizing this property, it is possible to electrically control the optical pitch averaged in the resonator direction of the diffraction grating 900 that the laser beam senses. Actually, the current I _S from the light output control circuit 904 to the active region and the current I _C from the wavelength control circuit 905 to the wavelength control region are controlled independently, and only the oscillation wavelength is maintained while the laser light output is constant. To change. In this conventional device, the current I _{C is set} to 70
By flowing the current up to mA, the oscillation wavelength could be changed from 1.3525 μm to 1.3501 μm, and a variable range of 2.4 nm was obtained.

The wavelength tunable laser device utilizing the plasma effect of the carrier shown in FIG. 24 has features such as obtaining a wavelength tunable characteristic at a higher speed as compared with the wavelength tunable laser device of the first example due to the temperature effect. . However, since the change in the refractive index of the semiconductor due to the plasma effect of carriers is small, the wavelength variable range is as small as 2 to 4 nm. Further, since the principle of changing the wavelength by injecting a current as in the case of the temperature effect is used, the power required to change the wavelength is as large as 140 mW (2 V × 70 mA).

[0017]

As described above, in the wavelength tunable semiconductor laser device according to the prior art, it is fundamental to control the refractive index of the semiconductor material by changing the carrier density or the temperature of the portion corresponding to the waveguide. Principle, and has the following problems.

In the laser device of the first example, in which the oscillation wavelength is controlled by heat, the wavelength can be changed only within a range of about several nm, and the heat for changing the wavelength generates an active region in which laser light is generated. Therefore, the life of the laser device is shorter than that of a normal laser device, and the power consumed for heat generation is large.

On the other hand, in the laser device of the second example in which the wavelength is controlled by the carrier density, high-speed wavelength tunable characteristics can be obtained. The oscillation wavelength range that can be achieved is not only as small as about 2 to 4 nm, but also a large amount of electric power is required to change the wavelength because a current needs to flow for carrier injection.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art, and has a wavelength tunable laser device with low power consumption and a sufficiently wide wavelength tunable range that can be controlled continuously. An object is to provide a manufacturing method thereof.

[0021]

SUMMARY OF THE INVENTION A wavelength tunable laser device according to the present invention is provided with an active portion having a first waveguide for generating and guiding light, and is guided in the first waveguide. A second light guide including a first light reflecting means for reflecting light into the first wave guide, a periodic uneven structure, and a liquid crystal layer in contact with the periodic uneven structure, and the liquid crystal A wavelength control unit comprising control means for applying a voltage to the layer, located at a position opposite to the first light reflection means with the first waveguide means interposed therebetween, and optically coupled to the active unit And second light reflection means for reflecting light guided in the second waveguide means into the second waveguide means, thereby achieving the above object.

The active section and the wavelength control section may be formed on the same substrate.

The first waveguide means is formed with a first layer for generating a laser beam and the first layer interposed therebetween, and has a different conductivity type and a refractive index smaller than the refractive index of the first layer. A second layer and a third layer having a refractive index, wherein the periodic concavo-convex structure of the wavelength control unit guides at least a part of light generated from the first layer, and guides the guided light. May be a diffraction grating for distributed feedback.

[0024] The second waveguide means may include two of the periodic uneven structures, and the liquid crystal layer may be sandwiched between the two periodic uneven structures.

According to the method of manufacturing a wavelength tunable laser device of the present invention, a plurality of first semiconductor layers serving as first waveguide means for generating and guiding light on a substrate, and a plurality of first semiconductor layers adjacent to the first semiconductor layer are provided. A plurality of second waveguides serving as second waveguide means of the wavelength controller.
Crystal growing a semiconductor layer, forming a periodic concavo-convex structure on the second semiconductor layer, and using a selective etching method, at least one layer excluding the uppermost layer of the plurality of second semiconductor layers. Removing the semiconductor layer to expose at least a part of the periodic uneven structure, and contacting a liquid crystal material to the exposed surface of the periodic uneven structure. Thereby, the above object is achieved.

[0026]

In the wavelength tunable laser device according to the present invention, light is generated and guided by the first waveguide means of the active portion. The light guided in the first waveguide is reflected into the first waveguide by the first light reflector. On the other hand, the second waveguide unit of the wavelength control unit is optically coupled to the first waveguide unit, and a part of the light generated in the active unit is guided in the second waveguide unit. . The light guided in the second waveguide is reflected by the second light reflector into the second waveguide. The first light reflecting means and the second light reflecting means are located at positions opposing each other with the first waveguide means interposed therebetween, and the active part and the wavelength control part have a single resonator structure. The second waveguide means includes a periodic uneven structure and a liquid crystal layer in contact with the periodic uneven structure, and a voltage is applied to the liquid crystal layer by the control means. When the voltage is applied, the alignment state of the liquid crystal molecules in the liquid crystal layer changes,
As a result, the refractive index felt by the light guided in the second waveguide changes. As a result, the optical pitch of the periodic uneven structure changes, and the oscillation wavelength of the laser device changes. Therefore, by continuously changing the voltage applied to the liquid crystal layer, the oscillation wavelength of the laser device can be changed continuously.

[0027]

Embodiments of the present invention will be described below.

(Embodiment 1) FIG. 1 shows a configuration diagram of a wavelength tunable laser device according to Embodiment 1 of the present invention. This laser device
It comprises an active part 110 having a normal semiconductor laser configuration, a wavelength control part 120 having a liquid crystal waveguide, and a lens 130 for optically coupling the active part 110 and the wavelength control part 120.

The active part 110 includes a GaAs substrate 111,
An A1GaAs lower cladding layer 112, a GaAs active layer 113, an A1GaAs upper cladding layer 114, and a contact layer 115 are formed on the substrate 111. The electrodes (not shown) have a stripe structure in the direction indicated by arrow A. The front surface 116 of the active part 110 is a cleaved surface of the crystal, and its reflectance is 30%. Active part 1
By forming a dielectric film (not shown) corresponding to a thickness of １ ／ of the wavelength of the generated light on the rear surface 117 of 10, the reflectance is set to 2% or less.

The wavelength control unit 120 includes a Si substrate 121,
A lower electrode (not shown), a lower SiO2 cladding layer 122, a liquid crystal core layer 12 formed on the substrate 121.
3, an upper cladding layer 124 and an upper electrode (not shown). The surface of the SiO2 lower cladding layer 122 which is in contact with the liquid crystal core layer 123 is provided with a diffraction grating 125 (period 27) by a normal holographic exposure method.
0 nm and a depth of 15 nm). Therefore, the liquid crystal molecules in the liquid crystal core layer 123 in a state where no electric field is applied are oriented in a direction parallel to the diffraction grating 125, reflecting the shape of the diffraction grating 125. The end surface 126 of the wavelength control unit 120 on the active portion 110 side has a dielectric film (shown in the drawing) having a thickness corresponding to １ ／ of the wavelength of light generated by the active portion 110 in order to reduce the reflectance to 1% or less. ) Are formed.

The waveguide of the active part 110 (the lower cladding layer 1
12, active layer 113, and upper cladding layer 114)
And the waveguide (the lower cladding layer 12
2. Liquid crystal core layer 123 and upper cladding layer 124)
Are arranged on substantially the same axis, and between the waveguides,
A lens 130 is provided. Thereby, the active unit 110
The efficiency of the optical coupling between the wavelength controller 120 and the wavelength controller 120 is improved.

The principle of operation of the laser device according to the present embodiment having the above configuration will be described below.

First, the active portion 110 has a higher refractive index than that of the cladding layers 112 and 114 of GaAs constituting the active layer 113.
Is a core layer. Light generated in the active layer 113 by current injection into the active part 110 is guided in the direction indicated by the arrow A. The light guided to the front surface 116 reaches the reflection portion of the front surface 116, and a part of the light is
The light is reflected toward the inside 10 and guided toward the rear surface 117. Some of the light reaching the front surface 116 is
0 is released outside. On the other hand, most of the light guided to the rear surface 117 is emitted to the outside of the active part 110 because the reflectance of the rear surface 117 is low.

The light emitted from the rear surface 117 of the active part 110 is made incident on the liquid crystal core layer 123, which is the waveguide of the wavelength control part 120, by the lens 130. In this way,
The light generated by the active unit 110 propagates through the liquid crystal core layer 123 in the direction indicated by the arrow B, but is converted by the diffraction grating 125 in the wavelength control unit 120 into light sequentially traveling in the direction indicated by the arrow C. The light reflected by the diffraction grating 125 in the direction indicated by the arrow C is emitted from the wavelength control unit 120 and is again incident on the waveguide of the active unit 110 via the lens 130.

By repeating such an operation, laser oscillation occurs. That is, the active unit 110, the lens 130,
In addition, a laser resonator is formed by the wavelength control unit 120.

In the wavelength tunable laser device having such a configuration, the oscillation wavelength is determined by the peak position of the superposition of the gain distribution inside the resonator and the reflectance distribution of the two reflection structures constituting the resonator. Is determined by In the case of this laser device, the gain distribution and the front surface 1
16 has a relatively wide distribution with respect to the wavelength, the diffraction grating 12
5, the wavelength having the highest reflectance is oscillated selectively. As is generally well known, the wavelength selectively reflected by the diffraction grating (Bragg wavelength) is substantially equal to an even multiple of the optical pitch (refractive index × actual pitch) of the diffraction grating. Since the pitch of the diffraction grating 125 of the wavelength control unit 120 in the present laser device is 270 nm and the equivalent refractive index of the waveguide in the absence of an electric field is 1.63, the Bragg wavelength is 880 nm. Therefore, the present laser device is 880
nm light is selectively oscillated.

FIG. 2 shows the refractive index distribution in the layer thickness direction in the wavelength control section 120. FIG. 2A shows a refractive index distribution in a state where no electric field is applied, and FIG. 2B shows a refractive index distribution in a state where an electric field is applied. As can be seen from the figures, the refractive index n of the lower cladding layer 122 and the upper cladding layer 124 in any state.
_Since the refractive indexes n _LC0 and n _{LC1 of} the liquid crystal core layer 123 are higher than _c , the light introduced into the wavelength controller 120 becomes guided light having a peak in the liquid crystal core layer 123.

As shown in FIG. 2A, in the absence of an electric field, all the liquid crystal molecules distributed in the liquid crystal core layer 123 have an uneven structure (the diffraction grating 12) on the surface of the lower cladding layer 122.
(Equivalent structure to 5). The laser light generated in the structure of the laser device of this embodiment is TE
A wave (a direction in which the optical electric field component is parallel to the semiconductor crystal layer).
Therefore, the direction of the long axis of the liquid crystal molecules in the state of no electric field matches the direction of the laser light electric field. In such an alignment state, the refractive index felt by the laser beam is n _LC0 .

On the other hand, when an electric field is applied, liquid crystal molecules generally rotate so as to be parallel to the direction of the applied electric field.
The state is as shown on the left side of FIG. In other words, in the vicinity of the lower cladding layer 122, it is not responded to an electric field while being oriented in the horizontal direction, being affected by the uneven structure. As can be seen from the figure, when the liquid crystal molecules are oriented in a direction perpendicular to the electric field direction of the laser light, the refractive index n _LC1 that the laser light senses is the refractive index n _LC1 in the above-mentioned electric field-free state.
_Becomes smaller than _LC0 . Accordingly, the refractive index of the liquid crystal core layer 123 with respect to the guided light decreases, and the equivalent refractive index of the waveguide also decreases. Therefore, the diffraction grating 125 provided in the waveguide
Is smaller than that in the non-electric field state. Accordingly, the wavelength of the light selectively reflected by the diffraction grating 125 (Bragg wavelength) shifts to the shorter wavelength side, and the wavelength of the oscillation laser light shifts to the shorter wavelength side.

The refractive index of the liquid crystal core layer 123 can be variously set by selecting the liquid crystal molecular layer and the solvent material. In this embodiment, n _LC0 = 1.72 and n _LC1 = 1.64. Further, the refractive index n _C of the cladding layers 122 and 124 =
1.45.

FIG. 3 shows the liquid crystal core unit 1 of the wavelength control unit 120.
The relationship between the voltage applied to 23 and the oscillation wavelength of the present laser device is shown. In this laser device, 10 V is applied to the wavelength control unit 120.
By applying a voltage to the waveguide, the equivalent refractive index of the waveguide changes from 1.63 to 1.61 in the absence of an electric field,
Thereby, as can be seen from FIG. 3, the oscillation laser wavelength can be continuously changed from 880 nm to 869 nm. That is, the wavelength variable range in this case is 11n
m, and a sufficient wavelength tunable range required for applications such as a wavelength division multiplexing communication system and a coherent detection type measurement system can be obtained.

Further, in the laser device of this embodiment, unlike the above-mentioned conventional example, the wavelength is controlled by voltage and almost no current is required, so that the power required for changing the wavelength is several tens μW. Can be very small.

Embodiment 2 As described above, by applying the present invention, it is possible to realize a wavelength tunable laser device having low power consumption and a wide continuous variable range. However, in the first embodiment, since the active section 110 and the wavelength control section 120, which are elements having different waveguide structures, are optically coupled via the lens 130, the relative section of about 0.5 μm is required. Active part 1 due to misalignment
The optical coupling efficiency between 10 and the wavelength control unit 120 is reduced to 5% or less. As a result, the current injected into the active portion 110 necessary for generating the laser beam may be abnormally large, and there may be a problem in improving the yield in manufacturing the laser device and in the long-term stability of the laser device. .

A laser device according to a second embodiment which solves such a problem will be described below.

FIGS. 4 to 11 show manufacturing process diagrams of the laser device of this embodiment. The structure and manufacturing method of the present laser device will also be described based on the drawings.

First, as shown in FIG. 4, an n-InP cladding layer 20 having a thickness of 0.5 μm is formed on an n-InP substrate 201.
2 and a 0.2 μm thick n-InGaAsP light guide layer 2
03 are grown in this order. Next, a diffraction grating 204 is engraved on the surface of the light guide layer 203 by a photolithography technique using ordinary holographic exposure and a wet etching technique. The pitch の of the diffraction grating 204 is 208 nm and the depth is 100 nm.

Next, as shown in FIG.
04 on the substrate 201 with a thickness of 0.15 μm.
P lower insulating layer 205, 2 μm thick high-resistance InGaAs etching removal layer 206, 0.3 μm thick high-resistance InP upper insulating layer 207, 1.0 μm-thick p-InP cladding layer 208, and 0.5 μm-thick p-type The InGaAsP contact layer 209 is grown continuously in this order. As a crystal growth method at this time, for example, a metal organic chemical vapor deposition (MOCVD) method is used. Also, as can be seen from FIG. 5, the growth conditions are selected such that the surface shape of the InP lower insulating layer 205 grown on the diffraction grating 204 substantially reflects the shape of the diffraction grating 204.

Next, as shown in FIG.
A mask 2 made of a silicon nitride film only in a portion to be 10
11 is formed, and a portion of the growth layer corresponding to a region (active region 220) other than the wavelength control region 210 is removed by etching until the growth layer reaches the n-InP cladding layer 202.
At this time, a method of selectively etching each layer using a hydrochloric acid-based etchant for InP etching and a sulfuric acid-based etchant for InGaAsP etching is employed. With this method, the surface of the n-InP cladding layer 202 was exposed.

Next, as shown in FIG. 7A, using the silicon nitride film mask 211 used for the etching as it is as a mask for selective growth, the exposed n-InP cladding of the etched region 220 is used. On the layer 202, an n-InGaAsP light guide layer 2 having a thickness of 0.12 μm is formed.
21 and an active layer 222 composed of an InGaAsP / InP multiple quantum well having a thickness of 0.075 μm and a p-type layer having a thickness of 3.0 μm.
Inp clad layer 223 and 0.5 μm thick p-I
The nGaAsP contact layer 224 is continuously grown in this order. At this time, the crystal growth method may be, for example, MO
The CVD method is used. The composition of the p-InGaAsP contact layer 224 is a composition (corresponding to a wavelength of 1.15 μm) that is not etched by the sulfuric acid-based etchant.

FIG. 7B is an enlarged view of the active layer 222. As shown in the figure, the multiple quantum well forming the active layer 222 includes five 7-nm thick InGaAs well layers 222A,
The InP barrier layer 222B having a thickness of 10 nm includes four layers, and the well layers 222A and the barrier layers 222B are alternately stacked. With such a configuration, the oscillation laser wavelength becomes about 1.5 μm. Further, in order to improve the precision of fine processing in a later step, the active region 220 selectively grown is used.
The height of the crystal surface is substantially matched with the height of the crystal surface of the wavelength control region 210, and the thickness of the selectively grown film is controlled so that the unevenness remaining on the wafer surface is reduced.

Next, as shown in FIG. 8, after the silicon nitride film mask 211 is removed by etching with hydrofluoric acid,
The mesa stripe 230 having a width of 2 μm and a height of 5 μm is formed by etching with a bromine-based etchant using a silicon nitride film having a width of 5 μm as a mask. This etching is performed until reaching the lower insulating layer 205 in the wavelength control region 210, and the cladding layer 2 is formed in the active region 220.
This is performed so as to leave a part of 23.

Subsequently, as shown in FIG. 9, using the silicon nitride film mask used for forming the mesa stripe 230 as a selective growth mask, a high resistance In is formed by MOCVD.
The P layer 231 is selectively grown and the mesa stripe 230 is embedded. Next, an inter-region insulating groove 240 is formed at the boundary between the active region 220 and the wavelength control region 210 by a reactive ion etching method. The width of the insulating groove 240 is 10 μm, and the depth is 3.5 μm. A p-type electrode 241 for applying a wavelength control electric field is formed on the surface of the p-contact layer 209 of the wavelength control region 210 of the wafer thus manufactured.
After forming a p-type electrode 242 for current injection on the surface of the p-contact layer 224 of the active region 220 and an n-type ohmic electrode 243 below the substrate 201, the chip is divided into chips by cleavage and dicing.

Next, as shown in FIG.
An Al ₂ O ₃ film 125 is formed on the 0-side laser end face by electron beam evaporation. The thickness of the Al ₂ O ₃ film is set to be さ と of the optical thickness with respect to the wavelength of the laser light to be generated.

Next, the chip is immersed in a sulfuric acid-based etching solution. The sulfuric acid-based etchant has a property of selectively etching InGaAs. In the case of the laser device of the present embodiment, the InGaAs layer that comes into contact with the sulfuric acid-based etching solution is the InGaAs etching removal layer 206 in the wavelength control region 210 and the end surface 212 of the wavelength control region 210.
The etching gradually proceeds from the insulating groove 240. The etching removal layer 206 having a thickness of 2 μm and a width of 2 μm can be completely removed over a length of 200 μm by performing etching for about 1.5 hours while applying ultrasonic vibration to the etching solution. Since the layers other than the etching removal layer 206 are made of InP or InGaAsP close to InP, the cavity 20 thus formed is formed.
6a is an InP lower insulating layer 205, as shown in FIG.
As shown in FIG. 7, an uneven structure reflecting the shape of the diffraction grating 204 located thereunder appears on the surface.

Next, 2 is added to the portion of the etching removal layer 206.
A hollow portion 206a having a cross section of × 2 μm ^{2 and} a depth of 200 μm
The temperature is raised to 150 ° C. in a vacuum for about 30 minutes in a vacuum to completely remove water.
Next, the chip temperature is lowered to room temperature in a vacuum atmosphere, and the end surface 212 of the wavelength control region 210 is brought into contact with the liquid crystal in that state, and as shown in FIG. Inject. Hollow part 2
Since the uneven structure having the diffraction grating shape on the surface of the InP lower insulating layer 205 is exposed inside 06a, the injected liquid crystal molecules are aligned in a direction parallel to the uneven structure having the diffraction grating shape.

Finally, the end surface 212 on the wavelength control region 210 side
An ultraviolet curable resin (not shown) is applied to the upper and insulating grooves 240 for electrode separation, cured by irradiating ultraviolet rays, and the liquid crystal material is confined to form a laser device.

The present laser device basically constitutes a distributed plug reflection (DBR) semiconductor laser device, and one of a plurality of layers constituting the waveguide for the plug reflector is formed of liquid crystal. Made up of The operation principle of the wavelength tunable semiconductor laser device will be described below.

When a current flows through the electrode 242 of the laser active region 220, carriers are injected into the multiple quantum well type active layer 222, and light is emitted according to the same operation principle as that of a normal semiconductor laser device. Light emitted from the active layer 222 is guided to the wavelength control region 210 which is in contact with the active region 220. In the wavelength control region 210, the diffraction grating 20
4 is formed on the surface of the light guide layer 203 which is the center of the structure of the waveguide. The diffraction grating 204 in the laser device of this embodiment is designed so that the pitch ２０８ is 208 nm and the waveguide equivalent refractive index of the wavelength control region 210 in the state where no electric field is applied to the liquid crystal is 3.12. Therefore, the plug reflection wavelength is 1.3 μm, and functions as a distributed reflection mirror (primary grating) for light having a wavelength of 1.3 μm generated in the active region 220.

On the other hand, since the end face on the active region 220 side is constituted by a cleaved face coated with an Al ₂ O ₃ film 125, it is about 1
It functions as a mirror having a reflectance of 0%. Therefore,
The laser device of the present embodiment has a resonator configuration sandwiched between a cleavage plane mirror on the front surface of the active region 220 and a distributed reflection mirror of the wavelength control region 210, and its oscillation wavelength is determined by the diffraction grating in the wavelength control region 210. The wavelength coincides with the wavelength selectively reflected by 204.

As described above, although the oscillation laser wavelength is determined, as described in the conventional example, the optical pitch of the diffraction grating 204 is electrically controlled by the refractive index of a part of the waveguide layer. It can be changed through changing, thereby changing the oscillation laser wavelength. In this embodiment, the refractive index of the guide layer 203 is designed to be higher than the refractive index of the liquid crystal layer 260. Therefore, the laser light in the waveguide of the wavelength control region 210 of the laser device of the present embodiment has a peak in the guide layer 203 with the diffraction grating 204, and the lower n-InP cladding layer 202 and the upper lower portion The light propagates to the InP insulating layer 205 and the liquid crystal layer 260 in a seeping mode. Therefore, the oscillation laser wavelength can be changed by electrically changing the refractive index of the liquid crystal layer 260.

FIGS. 12A and 12B show the refractive index distribution in the layer thickness direction of the waveguide structure in the wavelength control region 210. FIG. FIG. 12A shows the wavelength control electrode 241.
FIG. 12 shows a state in which no voltage is applied to FIG.
(B) shows a state in which a voltage is applied to the wavelength control electrode 241.

First, the case shown in FIG. 12A where no voltage is applied to the wavelength control electrode 241 will be described. When no voltage is applied, the liquid crystal layer 260 is in an electric field-free state.
All the liquid crystal molecules distributed in the liquid crystal layer 260 are aligned in parallel with the uneven structure (equivalent to the diffraction grating 204) on the surface of the lower insulating layer 205. The laser light generated in the structure of the laser device of the present embodiment is a TE wave (a state in which the optical electric field component is in a direction parallel to the semiconductor crystal layer, that is, in a direction indicated by an arrow D). Therefore, the direction of the long axis of the liquid crystal molecules in the state of no electric field matches the direction of the laser light electric field. In such an orientation state, the refractive index felt by the laser beam is n
_LC0 . Incidentally, n _c in the drawing is the refractive index of the cladding layer 202, n _g is the refractive index of the optical guide layer 203.

On the other hand, when a voltage is applied to the wavelength control electrode 241, an electric field is applied to the liquid crystal layer 260 via the upper and lower InP insulating layers 205 and 207 in the layer thickness direction. Generally, the liquid crystal molecules rotate so as to be parallel to the direction of the applied electric field, so that the state is as shown on the left side of FIG. That is, the vicinity of the lower insulating layer 205 is affected by the concavo-convex structure and does not respond to the electric field while being oriented in the direction indicated by the arrow D. As can be seen from the figure, when the liquid crystal molecules are oriented in the direction of the electric field of the laser light, that is, in the direction perpendicular to the direction shown by the arrow D, the refractive index n that the laser light feels
_LC1 is smaller than the refractive index _nLC0 in the above-mentioned field-free state.

The optical pitch of the diffraction grating 204 is
When the refractive index of the liquid crystal layer 60 is large,
When the refractive index of 0 is small, it increases. Therefore, when a voltage (positive or negative DC voltage) is applied to the wavelength control electrode 241 with respect to the current injection electrode 241, the oscillation laser shifts to the shorter wavelength side.

FIG. 13 shows the actual wavelength tunable characteristics of the laser device of this embodiment. As can be seen from the figure, the oscillation wavelength which was 1305 nm when no voltage was applied to the wavelength control electrode 241 became 1296 nm when a voltage of 13.5 V was applied, and the laser oscillation wavelength was continuously extended over a wide range of about 9 nm. Can be shifted.

Further, in the laser device of this embodiment, the wavelength can be changed by the voltage driving method, so that it is not necessary to inject a current as in the conventional example, and the power consumed for changing the wavelength is also 10 μW. It can be as small as possible.

Third Embodiment Next, a third embodiment in which the present invention is applied to a semiconductor laser device of another material will be described. FIG.
19 to 19 show the manufacturing process diagrams. The structure and manufacturing method of the present laser device will also be described based on the drawings.

First, as shown in FIG. 14, p-GaAs
1.6 μm thick p-Al _0.4 Ga _0.6 A on the substrate 300
s clad layer 301, 0.06 μm thick Al _0.07 Ga
_0.93 As active layer 302, 0.07 μm thick n-Al _0.5
Ga _0.5 As carrier barrier layer 303, 0.15 μm thick n-Al _0.25 Ga _0.75 As guide layer 304, and 3n
A m-thick GaAs antioxidant layer (not shown) is grown by gas source molecular beam epitaxy in this order.
A diffraction grating 306 is formed on the surface of the n-Al _0.25 Ga _0.75 As guide layer 304 by an electron beam lithography method and a normal etching method. The diffraction grating 306 has a pitch of 120 nm and a depth of 60 nm.

Next, an n-Al _0.45 Ga _0.55 As clad layer 307 having a thickness of 1.5 μm and an n-GaAs contact layer 3 having a thickness of 1.0 μm are formed on the substrate 300 having the diffraction grating 306.
08 are grown in this order by metal organic chemical vapor deposition (MOCVD). Subsequently, a stripe-shaped Si ₃ N ₄ film 309 having a width of 4 μm is formed by a normal photolithography technique, and selective etching is performed using this as a mask.
As shown in FIG. 7, a mesa stripe 310 is formed.

In this selective etching step, first, an ammonia-based etching solution (Al _x Ga _{1 -x} As: 0 ≦ x ≦ 0.
4 can be etched only) and only the GaAs contact layer 308 on the surface can be selectively removed, and then a hydrofluoric acid-based etching solution (Al _x Ga _{1 -x} As: only 0.42 ≦ x ≦ 1 can be used for etching) _{The 0.45} Ga _0.55 As clad layer 307 is selectively removed. The shape of the mesa stripe 310 has a width of 3 μm and a depth of 2.5 μm. Further, since the hydrofluoric acid-based etchant does not etch Al _0.25 Ga _0.75 As, the etching stops on the surface of the guide layer 304. As a result, the diffraction grating 306 can be exposed on the surface outside the mesa stripe 310 without deforming the fine unevenness as shown in FIG.

Next, using the striped Si ₃ N ₄ film 309 used for the etching as it is as a mask, a 50 nm thick n-Al _0.5 Ga _0.5
An As lower etching protection layer 311, a 1.0 μm thick high-resistance GaAs etching removal layer 312, a 1.35 μm thick p-Al _0.5 Ga _0.5 As upper etching protection layer 313,
A 0.1 μm thick p-GaAs contact layer 314 is grown in this order by MOCVD. Also, 50 nm
Thick n-Al _0.5 Ga _0.5 As lower etching protection layer 31
One surface has Al _0.25 because the thickness of the protective layer 311 is thin.
Diffraction grating 30 formed on Ga _0.75 As guide layer 304
6 has a shape substantially reflecting the shape. Due to this crystal growth, the wafer surface becomes almost flat as shown in FIG.

Next, an active region electrode 316 is formed on the mesa stripe 310, a wavelength control electrode 317 is formed on both sides of the mesa stripe 310, and an electrode 318 is formed on the entire back surface of the substrate 300.

Subsequently, as shown in FIG. 17, a plurality of holes 315 each having a width of 5 μm and a depth of 1.6 μm are formed at positions 10 μm away from the mesa stripe 310 using photoresist 500 (see FIG. 19) as a mask. Are formed at intervals of 10 μm in a direction parallel to the mesa stripe 310. In the etching step in this step, first, a hole 315 is formed in the wavelength control electrode 317 using an ion etching method using Ar, and then a sulfuric acid-based etchant whose etching rate is relatively independent of the Al composition ratio is used. Then, etching is performed through the p-GaAs contact layer 314 and the p-Al _0.5 Ga _0.5 As upper etching removal layer 313 to reach the GaAs etching removal layer 313. Next, by baking the wafer at 150 ° C. for 20 minutes,
As shown in FIG. 9, the photoresist 5 of the etching mask
00 is deformed to cover the side surface exposed in the groove of the p-GaAs contact layer 314. In this state, only GaAs is selectively etched by performing etching using an ammonia-based etchant. As a result, the GaAs etching removal layer 312 is etched around the hole 315 and the periphery thereof. That is, the hatched portion in the figure is removed, and the cavity region 501 is formed. In the cavity region 501, regions removed by etching proceeding from the adjacent hole 315 in the direction of the mesa stripe 310 are in contact with each other.
It becomes a hollow area of 0 μm. Also, as shown in FIG.
The ammonia-based etching in this step is performed by n-Al
It is possible to expose the _0.5 Ga _{0 .5} As lower etching protection layer 311 surface.

Next, the wafer surface is brought into contact with the liquid crystal under a reduced pressure atmosphere of 100 Pa or less, and the cavity region 50 is inserted through the hole 315.
A liquid crystal layer 419 is formed as shown in FIG. The reason why the reduced pressure atmosphere is required in this step is that the gas in the hollow area 501, which is a closed space, is exhausted, and the liquid crystal enters the hollow area 501 due to surface tension, so that the liquid crystal enters the hollow area 501.
The purpose is to satisfy 1 completely. Subsequently, an ultraviolet curing resin (not shown) is applied to the surface of the wafer, and the liquid crystal is confined in the cavity region 501 by irradiating ultraviolet light only around the hole 315, and finally, the chip is divided by cleavage.

The operating principle of the laser device of the present embodiment manufactured as described above will be described below.

The mesa stripe 310 has a pn diode structure, and carriers can be injected into the active layer 303 located immediately below the mesa stripe 310 by passing a current between the electrode 316 and the substrate electrode 318.
That is, the mesa stripe 310 becomes an active region for generating light. On the other hand, no current flows because the outside of the mesa stripe 310 has a pinp structure. Therefore, the current flowing through the electrode 316 can be efficiently injected into the active layer 303 immediately below the mesa stripe 310 without spreading in the horizontal direction. The electrode 317 and the electrode 3
16 is applied to the liquid crystal layer 319 or the pn reverse bias interface. The liquid crystal molecules in the liquid crystal layer 319 to which no electric field is applied are oriented in parallel to the uneven structure having the same shape as the diffraction grating 306 existing on the surface of the lower etching protection layer 311 as in the first and second embodiments. ing. On the other hand, when a voltage is applied between the electrodes 317 and 316, the liquid crystal molecules rotate and become parallel to the direction of the electric field. Therefore, it is possible to change the optical pitch of the diffraction grating 306 using these two states,
The area outside the mesa stripe 310 corresponds to the wavelength control area. The principle of the actual change in the refractive index is the same as that in FIG. 12 in the second embodiment, and thus the detailed description is omitted here.

The principle described above is the same as that of the second embodiment, but the present embodiment is different from the first embodiment in that the wavelength control region is located not on the light traveling direction but on the side surface of the mesa stripe 310 as the active region. This is different from the second embodiment. From the liquid crystal layer 319, n-
Since the refractive index of the AlGaAs cladding layer 307 is high,
Light generated in the active region is guided immediately below the mesa stripe 310 in the stripe direction. At this time, since the light seeps and propagates in both side directions of the mesa stripe 310, by changing the value of the voltage applied to the wavelength control region where the liquid crystal layer 319 exists, the diffraction grating 306 equivalent to the guided light can be sensed. Can be changed. Therefore, the laser device of the present embodiment has a configuration that enables the diffraction grating pitch of the DFB semiconductor laser device to be continuously changed by an external voltage. Since the oscillation wavelength of the DFB semiconductor laser device is determined by the optical pitch of the diffraction grating formed therein, the oscillation laser wavelength can be continuously changed by applying a voltage to the wavelength control region. . In addition, since the change in the refractive index of the liquid crystal layer 319 due to the application of a voltage is very large as compared with that of the semiconductor material, the wavelength can be changed in a wider range than the conventional laser device.

FIG. 20 shows the results of actually measuring the wavelength tunable characteristics of the laser device of this embodiment. As can be seen from the figure, by changing the voltage applied to the wavelength control region from 0 V to 15 V, the voltage is changed from 852.2 nm to 845.7.
It is possible to continuously change the oscillation laser wavelength over a wide range of about 6.5 nm down to nm.

In the case where the wavelength is changed by the laser device of this embodiment, the liquid crystal layer 319 is also applied by applying a voltage.
Since it is applied that the refractive index greatly changes, there is no need to supply a current, and the power required to change the wavelength can be controlled as low as about 15 μW.

In the laser device of this embodiment shown in FIG.
Although a single electrode 316 is used to inject a current (carrier) into the mesa stripe 310 corresponding to the active region,
This can be divided into a plurality in the traveling direction of light as in the conventional example, and a high-speed wavelength tunable function as shown in the conventional example can be provided by modulating the current flowing through the electrodes. Generally, the change in the refractive index due to the liquid crystal is large, but the response speed is as slow as 1 ms or more. Conversely, the change in the refractive index due to the plasma effect of the carriers in the active region has a very small change width, but the response speed is very high at 10 GH or more. Therefore, in the wavelength tunable laser device in which the active region electrode is divided into a plurality of pieces, adjustment of a rough wavelength, for example, adjustment for canceling a wavelength shift due to device temperature,
Adjustment for tuning with the oscillation wavelength of other laser devices
Adjustment required by applying a voltage to the wavelength tunable region and requiring high-speed wavelength tunable characteristics, such as optical frequency modulation for signal transmission in optical coherent communication, is performed by injecting current (carrier) into a plurality of divided electrodes in the active region. This is convenient for system application.

Further, in the second embodiment, the liquid crystal enters from the end face of the device in order to inject the liquid crystal material into the inside of the semiconductor laser device. However, in the present embodiment, the liquid crystal can enter from the upper surface of the laser device. Liquid crystal injection processing can be performed in a wafer state. Similarly, a cavity region for injecting liquid crystal can be formed by processing from the wafer surface. For this reason, in this embodiment, the manufacturing process is greatly simplified, and the manufacturing yield of the laser device can be improved and the time required for manufacturing can be significantly reduced.

(Embodiment 4) FIG. 21 shows a configuration diagram of a liquid crystal layer portion in a laser device according to another embodiment 4 of the present invention.
As shown in the figure, the laser device of the present embodiment includes a liquid crystal layer 40.
3 has diffraction gratings 401 and 402 having an uneven structure above and below. The method of fabricating this structure is the same as that of the laser device of the above-described embodiment, except that the growth conditions for crystal growth of the etching-removed layer on the diffraction grating 401 are changed. By lowering it, the shape of the diffraction grating is also reflected on the surface of the etching removal layer. Thus, when a cavity (corresponding to the liquid crystal layer 403) formed by selective etching is formed, it is possible to expose a diffraction grating-like uneven structure above and below the cavity.

In the laser apparatus according to the present embodiment, the wavelength can be changed with a small power similarly to the above-described embodiment, and the wavelength variable range can be obtained in a wide range of 8 nm. Further, as described above, the diffraction grating 40 is provided above and below the liquid crystal layer 403.
Providing 1 and 402 makes it possible to more strictly control the alignment state of the liquid crystal molecules when no electric field is applied to the liquid crystal layer 403 so as to be parallel to the diffraction grating 401, thereby improving the inter-device yield of wavelength tunable characteristics. Becomes possible.

Further, the present invention is not limited to the above embodiment, and similar effects can be expected in the following cases.

When the specific configuration of the semiconductor laser device is different: specifically, when the structure in the layer thickness direction such as a carrier / light separation / confinement structure and the structure in another layer direction such as a buried laser structure are different. Is equivalent.

When the materials constituting the semiconductor laser device are different: specifically, InGaAlAs / GaAs
System, InGaAs1P / GaAs system, II-VI group semiconductor material and the like.

When the position of the diffraction grating is different: More specifically, there may be a case where the diffraction grating exists on the substrate side from the active layer, a case where the diffraction grating exists only in a part of the stripe direction in the second embodiment, and the like.

When the shape of the diffraction grating is different: More specifically, a case where secondary or tertiary grading is applied, a case where rectangular or triangular waveform grading is applied, and the like.

[0089]

As is clear from the above description, according to the wavelength tunable laser device and the method of manufacturing the same of the present invention, the wavelength tunable operation can be performed with low power consumption and can be changed continuously. A tunable laser device having a sufficiently wide tunable range can be realized.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a laser device according to a first embodiment of the present invention.

2 is a graph showing a refractive index in a layer thickness direction in the laser device shown in FIG. 1, wherein (a) shows a state where no voltage is applied, and (b) shows a state where a voltage is applied.

FIG. 3 is a graph showing wavelength tunable characteristics in the laser device shown in FIG.

FIG. 4 is a diagram illustrating a manufacturing process of the laser device according to the second embodiment of the present invention.

FIG. 5 is a diagram illustrating a manufacturing process of the laser device according to the second embodiment of the present invention.

FIG. 6 is a diagram illustrating a manufacturing process of the laser device according to the second embodiment of the present invention.

FIG. 7A is a diagram illustrating a manufacturing process of a laser device according to a second embodiment of the present invention, and FIG. 7B is an enlarged view of an active layer portion illustrated in FIG.

FIG. 8 is a diagram illustrating a manufacturing process of the laser device according to the second embodiment of the present invention.

FIG. 9 is a diagram illustrating a manufacturing process of the laser device according to the second embodiment of the present invention.

FIG. 10 is a diagram illustrating a manufacturing process of the laser device according to the second embodiment of the present invention.

FIG. 11 is a perspective view illustrating a laser device according to a second embodiment of the present invention.

12 is a graph showing a refractive index in a layer thickness direction of the laser device shown in FIG. 11, wherein (a) shows a state where no voltage is applied, and (b) shows a state where a voltage is applied.

FIG. 13 is a graph showing wavelength tunable characteristics in the laser device shown in FIG.

FIG. 14 is a diagram illustrating a manufacturing process of the laser device according to the third embodiment of the present invention.

FIG. 15 is a diagram illustrating a manufacturing process of the laser device according to the third embodiment of the present invention.

FIG. 16 is a diagram illustrating a manufacturing process of the laser device according to the third embodiment of the present invention.

FIG. 17 is a diagram illustrating a manufacturing process of the laser device according to the third embodiment of the present invention.

FIG. 18 is a perspective view illustrating a laser device according to a third embodiment of the present invention.

FIG. 19 is a conceptual diagram of side protection by a resist in a manufacturing process of a laser device according to a third embodiment of the present invention.

20 is a graph showing a wavelength tunable characteristic in the laser device shown in FIG.

FIG. 21 is a diagram illustrating a structure around a liquid crystal layer of a laser device according to a fourth embodiment of the present invention.

FIGS. 22A to 22C are views showing a manufacturing process of the laser device of the first conventional example, and FIG. 22D is a perspective view of the laser device.

FIG. 23 is a graph showing wavelength tunable characteristics in the laser device shown in FIG. 22;

FIG. 24 is a view showing a second conventional laser device.

[Explanation of symbols]

 Reference Signs List 110 active portion 112 lower cladding layer 113 active layer 114 upper cladding layer 120 wavelength controller 122 lower cladding layer 123 liquid crystal core layer 124 upper cladding layer 125 diffraction grating 130 lens

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Jun Shimonaka 22-22, Nagaikecho, Abeno-ku, Osaka-shi, Osaka Inside Sharp Corporation (56) References JP-A-64-35978 (JP, A) JP-A-4 -142090 (JP, A) JP-A-1-140124 (JP, A) JP-A-63-235904 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H01S 3/18 JICST File (JOIS)

Claims (5)

(57) [Claims] An active part having first waveguide means for generating and guiding light; and reflecting light guided in the first waveguide means into the first waveguide means. A first light reflecting means, a second waveguide means including a periodic uneven structure, a liquid crystal layer in contact with the periodic uneven structure, and a control means for applying a voltage to the liquid crystal layer, A wavelength control unit which is located at a position opposite to the first light reflecting unit with the first waveguide unit interposed therebetween and is optically coupled to the active unit; A second light reflecting means for reflecting the reflected light into the second waveguide means. 2. The tunable laser device according to claim 1, wherein said active section and said wavelength control section are formed on the same substrate. 3. The first waveguide means is formed with a first layer for generating a laser beam and a first layer sandwiching the first layer. The first waveguide means has different conductivity types from each other and has a refractive index higher than that of the first layer. A second layer having a small refractive index, and a third layer having a small refractive index, wherein the periodic uneven structure of the wavelength control unit guides at least a part of light generated from the first layer; The wavelength tunable laser device according to claim 2, wherein the wavelength tunable laser device is a diffraction grating for performing distributed feedback of generated light. 4. The wavelength tunable according to claim 1, wherein the second waveguide has two of the periodic uneven structures, and the liquid crystal layer is sandwiched between the two periodic uneven structures. Laser device. 5. A plurality of first semiconductor layers serving as first waveguide means for generating and guiding light on a substrate;
Crystal growing a plurality of second semiconductor layers adjacent to the semiconductor layer and serving as second waveguide means of the wavelength control unit; forming a periodic uneven structure in the second semiconductor layer; Exposing at least a part of the periodic concavo-convex structure by removing at least one or more semiconductor layers excluding the uppermost layer of the plurality of second semiconductor layers using Bringing a liquid crystal material into contact with the surface of the periodic uneven structure.