JP2002033549A - Semiconductor ring laser and its manufacturing method as well as method for driving - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve problems of a difficulty in manufacturing a conventional semiconductor ring laser, a large power consumption and a low yield. SOLUTION: The semiconductor ring laser for constituting a ring resonator comprises an optical waveguide on a semiconductor substrate, and an optical crystal structure formed on a periphery of the waveguide in a direction parallel to the substrate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor ring laser used for a measuring instrument, a sensor, and the like, and a method of manufacturing and driving the same.

[0002]

2. Description of the Related Art A gyro is a general term for a measuring device for detecting a rotational angular velocity with respect to an inertial space. Among them, the characteristics of light,
For example, an optical gyro applied to a gyro utilizing high frequency characteristics (several hundred THz or more) and coherence is called an optical gyro. This optical gyro can be roughly classified into an active optical gyro and a passive optical gyro. The active optical gyro is also called a ring laser gyro, and the entire ring resonator is an active medium. For example, He-N as an active medium
As the ring resonator, a tube formed by cutting a fused quartz block into a triangular shape as an e-gas is widely used.

As can be seen from this configuration, an active optical gyro requires an additional device to stabilize the output drift due to a laser gas flow, requires large power for laser oscillation, and requires measures against thermal expansion and distortion. Therefore, miniaturization and cost reduction are essentially difficult, and applications are aircraft and the like (for example, Boeing 757/767, Airbus A310, etc.).
Was limited to very limited uses.

A typical passive optical gyro is an optical fiber gyro in which an active medium and a resonator are separated, and a semiconductor amplifier is used as the active medium and a single mode fiber is used as the resonator. General. Also in this case, there are problems that high mechanical accuracy is required and that the size cannot be reduced.

On the other hand, with the development of semiconductor lasers in recent years,
Attempts have been made to realize a ring laser with a semiconductor active medium and a semiconductor process. The semiconductor ring laser is, for example, a semiconductor laser gyro described in Japanese Patent Application Laid-Open No. 4-174317.

[0006]

However, this conventional example has advantages but has problems. That is,
Since it is formed by a semiconductor process, there is an advantage that positioning of optical components and the like are not required unlike a fiber gyro, but there are the following problems. First, the corner mirror portion of the quadrangular ring-shaped waveguide needs to be vertical and smooth in order to increase the reflectance by total reflection, but it has been difficult to satisfy both at the same time.

Second, since the reflection of the corner mirror portion uses total reflection of the semiconductor and air, there is a disadvantage that it is difficult to form a ring resonator having an arbitrary shape. Third, the ridge waveguide portion needs to be manufactured accurately so that only the fundamental transverse mode is selected, and needs to be formed smoothly so that scattering does not occur. Actually, it is difficult to completely satisfy this requirement, which causes disadvantages such as an increase in power consumption and a decrease in yield.

The present invention has been made in view of the above-mentioned conventional problems, and has as its object to provide a semiconductor ring laser which is excellent in power consumption and can be manufactured at low cost, and a manufacturing method and a driving method thereof. To provide.

[0009]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a semiconductor ring laser having an optical waveguide on a semiconductor substrate and constituting a ring resonator, having an optical crystal structure around the optical waveguide in a direction parallel to the substrate. This is achieved by a semiconductor ring laser characterized by the following.

Another object of the present invention is to form a slab waveguide structure comprising a core layer and a pair of clad layers sandwiching the core layer on a semiconductor substrate, a step of forming a nanohole array on the slab waveguide structure, Forming a resist mask in a region to be an optical waveguide on the nanohole array, transferring the nanohole array pattern to a region other than the region to be the optical waveguide using the resist mask and the nanohole array as a selection mask, and forming a pair of cladding layers. Forming an optical crystal structure in a direction parallel to the substrate on the upper cladding layer, removing the nanohole array,
The method is attained by a method of manufacturing a semiconductor ring laser, comprising a step of forming an electrode for injecting carriers only in a region to be the optical waveguide.

Further, an object of the present invention is to form a first cladding layer on a semiconductor substrate, a step of forming a nanohole array on the first cladding layer, and a region of the nanohole array to be an optical waveguide. And a method of forming a second cladding layer on the nanohole array, and a step of forming an electrode.

[0012]

Embodiments of the present invention will be described below in detail with reference to the drawings. First, the principle of the semiconductor ring laser of the present invention will be described. In the present invention, by forming a two-dimensional optical crystal structure on a plane parallel to the substrate, the optical confinement in the lateral direction of the optical crystal and the ring resonator structure are simultaneously provided. An optical crystal is a structure having a periodic structure with a refractive index difference, and it is theoretically revealed that there is a region where no light can exist, that is, a photonic band gap (PHB), depending on design parameters. (For example, JDJoannopoulos et a
l; Nature, Vol 386, p143 (1997)).

The non-optical crystal region surrounded by this structure is 2
An optical waveguide having almost no loss in a dimensional plane can be obtained. Light confinement in the direction perpendicular to the substrate is confined by a slab waveguide structure as in the past. If the non-optical crystal region is set as the gain region and the ring resonator region, there is no scattering loss or radiation loss, so that a ring laser operating at an extremely low threshold can be realized. Conventionally, the shape of a ring laser is limited by a critical angle determined by an oscillation wavelength and a refractive index difference between a substance inside and outside a waveguide, but this restriction is eliminated by providing an optical crystal structure in a lateral direction. Since the ring laser does not necessarily need to extract light to the outside, the compatibility with the optical crystal is high.

Conventionally, in a conventional two-dimensional optical crystal,
Good results have not been obtained due to insufficient confinement of light in three dimensions. The present invention solves this problem by forming light confinement in the thickness direction with a normal slab waveguide. Regarding photonic band gap formation conditions,
Reference M. Plihal et al; "Photonic band structure oftwo
-dimensional systems: The triangular lattice ", Physi
cal Review B, 44, p8565 (1991), etc.

Consider a simple arrangement, for example, a case in which a two-dimensional aperture array on a triangular cylinder is formed on a flat thin film as shown in FIG. When the length of the unit cell is r and the radius of the cylindrical hole is a, 0 <a / r <0.5
It is. Here, when a / r is called an aperture ratio, the aperture ratio a / r = 0.3 and the dielectric constant ε = 1 of the thin film.
In the case of 1 (in the case of GaAs), it can be calculated that the photonic band gap appears in the range of 0.23 <a / λ <0.3. λ is the wavelength. here,
For example, when the emission wavelength is 0.8 μm, the hole diameter where PHB appears is about 180 nm <a <240 nm.

In this range, a reflectance of almost 100% with respect to the incident light can be obtained. Nanohole arrays of this size anodize previously proposed aluminum,
This can be realized by a method using an alumina nanohole array. Increasing the aperture ratio leads to an increase in the range of PHB, but it is extremely difficult to fabricate. Also,
The method of making the optical crystal region a non-optical crystal region or a gain region can be realized by using an alumina nanohole array as a selective growth mask or a selective etching mask.

Therefore, a specific embodiment of the present invention will be described. FIG. 1 is a schematic diagram showing the configuration of a first embodiment of the semiconductor ring laser of the present invention. In FIG. 1, 100 is a semiconductor substrate, 101 is a two-dimensional optical crystal region, 102 is a ring resonator region, and 103 is an electrode pad. The two-dimensional optical crystal region 101 is formed on the semiconductor substrate 100 around the ring resonator region 102 in a direction parallel to the substrate, as described in detail later. Reference numeral 104 denotes an electrode.

Next, a method of manufacturing the semiconductor ring laser will be described. FIG. 2 and FIG. 3 are schematic diagrams for explaining a manufacturing process of the semiconductor ring laser. First, FIG.
As shown in (a), an n-type A
lGaAs cladding layer 202, AlGaAs / GaAs
The MQW active layer 203 and the p-type AlGaAs first cladding layer 204 are crystal-grown. The crystal growth method may be arbitrary. Here, MBE method (molecular beam epitaxial growth method)
Is used. Next, as shown in FIG. 2B, an Al thin film (thickness: 0.3 μm) 205 is laminated on the wafer by sputtering or the like. Alternatively, an Al thin film may be continuously deposited by MBE.

Next, as shown in FIG. 2C, the Al thin film 205 is anodized to form a nanohole array 206 made of alumina. The spacing and diameter of the nanoholes are smaller than the emission wavelength of the gain region and are regularly arranged in a two-dimensional direction. This forming method is described in detail in the literature, Solid Physics Vol. 31, pp. 493-499, 1996 and the like. As specific anodic oxidation conditions, for example, phosphoric acid is used as the electrolytic solution, and the voltage is 80 V.
After the anodization, the hole diameter can be increased by etching with phosphoric acid.

Under these conditions, a uniform nanohole array having a unit cell of an equilateral triangle having a cell interval of 200 nm and a hole diameter of 150 nm is formed. In addition, regularity can be improved by forming fine concave portions on the surface of the Al thin film 205 before anodic oxidation. The specific method is a stamp method (Applied Physics Letters 71, p27
70 (1997)) and a focused ion beam method (FIB). In particular, FIB is an effective method because a desired concave pattern can be directly drawn at an arbitrary position on the substrate. In addition, 2
As the two-dimensional optical crystal structure, it is also possible to adopt a structure in which casts are regularly arranged in a two-dimensional direction.

Next, as shown in FIGS. 2C and 2D, a resist pattern 207 is formed by a usual photolithography technique in a portion to be a waveguide region of the ring laser. Here, a circular pattern having a diameter of 300 μm is used. Also, as shown in FIG. 2E, Reactive lon Beam Etching (RIB)
E) or Inductively Coupled Plasma Etching (ICP) etc.
Then, using the alumina nanohole array layer as a mask, the pattern is transferred to the underlying wafer. For example, when RIBE is used, the etching conditions are chlorine gas pressure 0.5 Pa, E
CR power 100W, drawer 350V, substrate temperature 30
C is appropriate. The etching is performed until the etching depth reaches at least the active layer 203 as shown in FIG.

When the etching is performed as described above, FIG.
As shown in (e), the nanoholes are transferred to the cladding layer 204 in a region other than the region serving as the optical waveguide, and a two-dimensional optical crystal is formed in the direction parallel to the substrate. This is 204 '
Indicated by. Thereafter, only the alumina nanohole array 206 is selectively removed by hydrofluoric acid etching. As a result, as shown in FIG. 3A, an optical crystal structure is formed by air and an AlGaAs layer in the two-dimensional optical crystal region, and only the gain is formed without forming the optical crystal in the ring resonator region. . The refractive index difference and the size forming this structure are sufficiently small to form a photonic band gap for light in the wavelength band of 0.8 μm.

Next, as shown in FIG. 3B, a p-type AlGaAs second cladding layer 20 is further formed on the cladding layer 204 '.
8 is formed. At this time, by using the lateral growth mode of MOCVD (metal organic chemical vapor deposition), the nanohole can be grown only on the surface without growing in the nanohole. Thereafter, as shown in FIG.
The s-contact layer 209 is stacked. Further, as shown in FIG. 3B, a positive electrode 210 is formed on the resonant waveguide portion, and a negative electrode 211 is formed on the entire back surface. As the positive electrode 210 and the negative electrode 211, both Ti / Pt / Au are used.

In this case, the positive electrode 210 has a ring shape, and by applying a potential from the ring electrode, carriers are injected only into the non-optical crystal region, that is, the region serving as a waveguide in the active layer 203. Is done. FIG. 3 (c)
Shows a plan view at the time of completion. Here, a circular shape is selected as the ring resonator shape, but it may be a triangular shape, a hexagonal shape, or an arbitrary shape as shown in FIG. In particular, in the case of an equilateral triangle or an equilateral hexagon, the ring laser characteristics can be improved most because the shape can be matched to the unit cell of the nanohole array.

Next, the operation principle will be described. The carriers injected from the positive electrode 210 and the negative electrode 211 are recombined in the active region, become photons, select a waveguide mode, and propagate. First, in the thickness direction, only the waveguide mode of a normal slab waveguide is selected. In the lateral direction, a two-dimensional optical crystal is formed, and photons generated in the active layer are set so as to be in a forbidden band. Since the non-optical crystal region forms a ring resonator and also serves as a gain region, the ring laser can be oscillated as a ring laser by giving a gain exceeding the loss, that is, by increasing the carrier injection amount. In the case of the present embodiment, since the emitted light is all returned to the resonator and reused, a major feature is that the operating power can be extremely reduced.

Next, a method of using the semiconductor ring laser of this embodiment will be described. FIG. 4 is a schematic diagram of a measuring device when a semiconductor ring laser is used as a rotating gyro. In FIG. 4, reference numeral 301 denotes a semiconductor ring laser,
302 is a current source, 303 is a resistor, and 304 is a voltmeter. As the semiconductor ring laser 301, the one shown in FIG. 1 is used (or one manufactured by the manufacturing method shown in FIGS. 2 and 3). The semiconductor laser 301 is attached to a substrate (not shown) or the like, and is rotatable around an axis perpendicular to the substrate.

Here, when the current is increased to reach the oscillation threshold, the two modes oscillate almost simultaneously in the ring laser mode. In the case of the present embodiment, the oscillation cannot be confirmed by a normal photodetector because no light is emitted to the outside, but the oscillation threshold can be monitored by monitoring the differential resistance. That is, the diode characteristics are normal until oscillation, but after oscillation, since the carrier density in the active layer becomes constant, a kink is generated as the differential resistance, so that it can be easily determined. If rotation is applied after oscillation, an optical path difference occurs in the clockwise and counterclockwise modes, and this appears in the voltage change as a beat signal. Normally, this beat frequency has a linear relationship with the rotational angular velocity, so that the rotational angular velocity can be quantified from this.

In this embodiment, since a two-dimensional optical crystal region is formed on the semiconductor substrate to confine light in the direction parallel to the substrate, the resonator loss can be extremely reduced, and the power consumption can be significantly reduced. Further, since the resonator shape can be designed almost arbitrarily, a device structure suitable for the purpose can be obtained. The shape of the ring resonator is not limited to the shape shown in the figure, and may be, for example, a square ring shape.

In particular, if an asymmetric tapered region (region for giving a difference between loss of clockwise light and counterclockwise light) is provided in the waveguide, light in both directions can be obtained even when the ring laser is stationary. , An oscillation frequency difference, that is, a beat is generated. By comparing the beat signal when stationary and the beat signal when rotating, not only the angular velocity but also the direction of rotation can be detected. Regarding the taper shape, refer to Japanese Patent Application No. 12-3895.
Familiar with the issue.

Next, a second embodiment of the method for manufacturing a semiconductor ring laser according to the present invention will be described. The semiconductor ring laser is the same as in FIG. 5 and 6 are schematic views for explaining the manufacturing process. The difference from the first embodiment is that the gain medium is selectively embedded later to selectively form a gain region and reduce the difference in refractive index to form a non-optical crystal. Hereinafter, a specific description will be given. First, as shown in FIG. 5A, a p-type AlGaAs cladding layer 202 is crystal-grown on a p-type GaAs substrate 201. The crystal growth method may be arbitrary. Here, the MBE method (molecular beam epitaxial growth method) is used. Next, FIG.
As shown in (b), an Al thin film (thickness: 0.5 μm) 205
Is laminated on this wafer by sputtering or the like. Or MB
Al may be continuously deposited by the E method.

Next, a nanohole array 206 made of alumina is formed in the same manner as in the first embodiment. As specific anodic oxidation conditions, for example, phosphoric acid is used as the electrolytic solution, and the voltage is 120V. Under these conditions, finally, the cell interval is 300 nm and the hole diameter is 250
A nanohole array 206 having a uniform thickness of nm is formed. This state is shown in FIG. The reason why the cell spacing and the hole diameter are different from those in the first embodiment is to obtain an optical crystal structure using alumina and air. After this, FIG.
The wafer is returned to the crystal growth apparatus again as shown in FIG.
FIG. 6 shows a state in which the metal mask 407 is arranged close to the wafer.
As shown in (a), the MQW active layer 203 (wavelength 0.8 μm)
m) is selectively grown so as to fill only the optical waveguide (non-optical crystal region) forming the ring resonator.

Here, an MOCVD method advantageous for selective growth is used. The pattern of the optical waveguide in the metal mask 407 is a regular hexagon as shown in FIG. 6C, and is set so as to match the period of the nanohole array 206. After the active layer is grown, the metal mask 407 is removed, and the entire wafer is n-type AlG as shown in FIG.
The aAs cladding layer 208 is grown. At this time, by using the lateral growth mode, in the two-dimensional optical crystal region, the holes are epitaxially grown in the lateral direction without being filled in the holes. As a result, although the alumina remains in the region where the active layer 203 has grown, the difference in the refractive index is small.
It becomes a doping region that optically disturbs periodicity, and becomes a non-optical crystal structure. In addition, by injecting carriers, it also becomes a gain region. The region where the active layer 203 is not filled is
It is maintained as an optical crystal region. This is a major difference from the first embodiment.

Next, as shown in FIG. 6B, after successively growing an n-type GaAs contact layer 209, a positive electrode 210 is formed only on the optical waveguide portion, and a negative electrode 211 is formed on the entire back surface. To complete manufacturing. A plan view after the final step is shown in FIG. Here, the case where the ring resonator is a regular hexagon is shown. If the ring shape is a regular hexagon, it is advantageous because the close-packed structure of the nanohole array is not disturbed, and the periodicity of the optical crystal is easily maintained. However, other shapes may be used. The second
In the embodiment, the positive electrode is formed only on the optical waveguide portion, but may be formed on the entire surface. This is because when carriers are injected from the positive electrode, both sides of the active layer 203 are made of alumina oxide, and current flows only in the active layer 203, so that it is not necessary to limit the shape of the electrode.

The principle of operation, the method of use, and the like are the same as those described in the first embodiment, and a description thereof will not be repeated. In the above embodiment, it is more effective to use GaInNAs for the active layer. An embodiment in this case will be described below.
That is, in the first embodiment and the second embodiment, Al
Instead of a GaAs / GaAs MQW active layer, GaAs
The use of the / GaInNAs MQW active layer has the following effects. (1) Since the effective refractive index of the active layer can be increased, the difference in refractive index from air can be further increased. (2) Since the oscillation wavelength can be made longer, the process accuracy can be reduced when the optical crystal region is manufactured. (3) Since the band offset of the conductor can be made large,
A ring laser having excellent temperature characteristics can be manufactured.

[0035]

As described above, according to the present invention, since the optical crystal structure is formed in the direction parallel to the substrate around the optical waveguide, light confinement in the lateral direction of the optical crystal can be increased.
Since the operating current can be reduced, power consumption can be significantly reduced. In addition, the production can be performed at low cost without lowering the yield. Further, since the light is confined in the lateral direction using the optical crystal structure, the ring shape can be set freely. Also, by using GaInNAs for the active layer, the process accuracy can be relaxed and the temperature characteristics can be improved.

[Brief description of the drawings]

FIG. 1 is a perspective view showing one embodiment of a semiconductor ring laser of the present invention.

FIG. 2 shows a first example of a method for manufacturing a semiconductor ring laser according to the present invention.
It is a figure showing an embodiment.

FIG. 3 shows a first example of a method for manufacturing a semiconductor ring laser according to the present invention.
It is a figure showing an embodiment.

FIG. 4 is a diagram illustrating an example of a method of using the semiconductor ring laser of FIG. 1;

FIG. 5 shows a second example of the method for manufacturing a semiconductor ring laser according to the present invention.
It is a figure showing an embodiment.

FIG. 6 shows a second method of manufacturing the semiconductor ring laser according to the present invention.
It is a figure showing an embodiment.

FIG. 7 is a diagram for explaining a photonic band gap.

[Explanation of symbols]

 Reference Signs List 100 semiconductor substrate 101 optical crystal region 102 ring resonator region 103 electrode pad 104 electrode 201 substrate 202, 204 clad layer 203 active layer 204 'clad layer 205 Al thin film 206 alumina nanohole array 207 resist mask 208 clad layer 209 contact layer 210 positive electrode 211 Negative electrode 301 Semiconductor ring laser 302 Current source 303 Resistance 304 Voltmeter

Claims (11)

[Claims] 1. A semiconductor ring laser comprising an optical waveguide on a semiconductor substrate to form a ring resonator, wherein the semiconductor ring laser has an optical crystal structure in a direction parallel to the substrate around the optical waveguide. 2. The semiconductor ring according to claim 1, wherein the optical crystal structure comprises a two-dimensional optical crystal structure in which holes or columns having a constant diameter are regularly arranged in a two-dimensional direction. laser. 3. A step of forming a slab waveguide structure including a core layer and a pair of clad layers sandwiching the core layer on a semiconductor substrate, a step of forming a nanohole array on the slab waveguide structure, and a light guide on the nanohole array. Forming a resist mask in a region to be a waveguide, transferring the nanohole array pattern to a region other than the region to be the optical waveguide using the resist mask and the nanohole array as a selection mask, and forming an upper clad layer of a pair of clad layers. Forming an optical crystal structure in a direction parallel to the substrate, removing the nanohole array, and forming an electrode for injecting carriers only in a region to be the optical waveguide. Method. 4. A step of forming a first cladding layer on a semiconductor substrate, a step of forming a nanohole array on the first cladding layer, and filling an active layer in a region of the nanohole array to be an optical waveguide. Forming a second clad layer on the nanohole array, and forming an electrode. 5. The method according to claim 3, wherein a cladding layer is further formed on the upper cladding layer. 6. The method according to claim 4, wherein the electrode is formed only in a region to be an optical waveguide or on the entire surface. 7. In the step of forming a nanohole array, an aluminum thin film is formed and anodized to form a nanohole array in which nanohole diameters and hole intervals are regularly arranged at a size smaller than the emission wavelength of the gain region. 5. The method of manufacturing a semiconductor ring laser according to claim 3, wherein the semiconductor ring laser is formed. 8. In the step of forming the nanohole array, an aluminum thin film is formed, a focused ion beam is irradiated to a desired position on the thin film surface, and then anodized, so that the nanohole diameter and the hole interval are in the gain region. 5. The method for manufacturing a semiconductor ring laser according to claim 3, wherein a nanohole array regularly arranged with a size smaller than the emission wavelength of (a) is formed. 9. The method of manufacturing a semiconductor ring laser according to claim 3, wherein a part of said optical waveguide contains a group III-V nitride material typified by GaInNAs. 10. The method of driving a semiconductor ring laser according to claim 1, wherein the semiconductor ring laser is rotated around an axis perpendicular to a substrate, and the terminal voltage of the ring laser changes. A method for driving a semiconductor ring laser, wherein an angular velocity is measured by monitoring the angular velocity. 11. The method according to claim 10, wherein the angular velocity is measured by monitoring a differential resistance of a terminal voltage of the semiconductor ring laser.