JPH09121071A - Semiconductor laser and operating method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor laser performing a mode synchronizing operation in which the position and length of saturable absorbing region can be altered arbitrarily after manufacturing. SOLUTION: An electrode 161 is used for injecting current and forming a gain region. An electrode for forming a saturable absorbing region by applying a reverse bias is split into a plurality of electrodes 162 -164 . The position for forming the saturable absorbing region and the number thereof can be altered arbitrarily after manufacturing a device by selecting the electrodes to be actually applied with reverse bias and the number thereof among the electrodes 161 -164 . Consequently, an optimal mode synchronizing operation can be realized.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an ultra-high-speed optical measurement,
The present invention relates to a mode-locked semiconductor laser for generating an ultrashort optical pulse train used for optical communication, optical information processing, and the like, and a method of operating the same, and more particularly to a semiconductor laser capable of optimizing mode-locked operation and a method of operating the same.

[0002]

2. Description of the Related Art In recent years, a technology for generating a pulse train of ultrashort light having a repetition frequency exceeding several tens of GHz (gigahertz) to over 100 GHz has been required as a basic technology of ultrahigh-speed optical measurement, optical communication, and optical information processing. ing. One type of optical oscillator that generates high-speed optical pulses is a semiconductor laser that performs a mode-locked operation. Such a semiconductor laser,
It is called a mode-locked semiconductor laser.

One type of mode-locked semiconductor laser has a structure in which electrodes are divided into two or three regions. In this semiconductor laser, a reverse bias is applied to an electrode in one region, a corresponding region acts as a saturable absorption region, and current is injected from an electrode in another region to form a gain region. By arranging the frequency intervals and the phase intervals between the modes oscillating in multiple modes by the saturable absorption region, self-excited pulse oscillation at a repetition frequency of 100 GHz or more is enabled.

In some mode-locked semiconductor lasers, a saturable absorption region is provided for a predetermined length from one end of an optical waveguide, and a gain region is formed in the remaining region. In this semiconductor laser, the saturable absorption region provided at one end provides
The mode frequency interval and phase interval are adjusted.

Japanese Patent Application Laid-Open No. 4-133487 discloses a mode-locked semiconductor laser by forced locking.
In this semiconductor laser, electrodes are provided at both ends of the optical waveguide for forcibly changing the gain by high frequency.
In addition, an electrode for applying a reverse bias to form a saturable absorption region is provided at a central portion of the optical waveguide. Then, between the electrodes at both ends and the center electrode,
An electrode for forming a gain region is provided.

In this semiconductor laser, an oscillation mode is forcibly set by a current supplied from electrodes at both ends. Further, this semiconductor laser has a symmetrical shape, and the two optical pulses generated in the gain region have the same waveform, amplitude, and speed. Thus, the two light pulses collide in the central saturable absorption region. The saturable absorption region has a property that the light absorption loss decreases as the intensity of the incident light increases. For this reason, the pulse becomes steeper in the central saturable absorption region, and an ultrashort optical pulse with a short pulse width can be obtained. The mode-locked semiconductor laser described so far is described in Y. K. Chen et al. By the I-E-E-Journal of
Quantum Electronics, Vol. 28, No. 10, 217
Pages 6 to 2185 (YK Chen and MCWu, IEEE Journal
of Quantum Electronics vol.28, no10, pp.2176-2185,1
992).

[0007]

The operation characteristics of a mode-locked semiconductor laser depend on the length and the position of the saturable absorption region. The optimum length of the saturable absorption region differs depending on the layer structure and the material system. Also, both ends of the optical waveguide are usually cleaved to form a Fabry-Perot resonator. Even if the length and position of the saturable absorption region are optimally designed, it is difficult to accurately determine the cleavage position, and thus the length and position of the saturable absorption region may deviate from the design values.

In a conventional mode-locked semiconductor laser having a saturable absorption region, one electrode for forming a saturable absorption region or a gain region is provided for each region. For this reason, there is a problem that the position and the length of the saturable absorption region of the semiconductor laser once manufactured cannot be adjusted after the manufacture, even if the design is deviated by cleavage. In addition, when the material system is changed, there is a problem that the position and length of the electrode for forming the saturable absorption region must be changed each time.

Therefore, a first object of the present invention is to provide a semiconductor laser capable of arbitrarily changing the position and length of a saturable absorption region after fabrication.

A second object of the present invention is to appropriately operate a semiconductor laser capable of arbitrarily changing the position and length of a saturable absorption region after fabrication.

[0011]

According to a first aspect of the present invention, there is provided an optical waveguide in which an active layer is sandwiched by clad layers having different refractive indices from both sides of the active layer, and a current is injected into a partial region of the optical waveguide. The electrodes for forming the gain region and the regions other than the region where the current is injected by this electrode are divided into a plurality of small regions, respectively. A semiconductor laser is provided with a plurality of electrodes for forming a saturable absorption region at an arbitrary position.

That is, according to the first aspect of the invention, an electrode for forming a gain region and a plurality of electrodes for applying a reverse bias to form a saturable absorption region are provided on the optical waveguide. By applying a reverse bias using any one of the plurality of electrodes forming the saturable absorption region, the length and position of the saturable absorption region can be adjusted appropriately.

According to a second aspect of the present invention, an optical waveguide in which an active layer is sandwiched by clad layers having different refractive indices from both sides of the active layer and a current is injected into a partial region of the optical waveguide to form a gain region. One or more of a plurality of electrodes of the semiconductor laser having an electrode for controlling the current and a plurality of electrodes respectively provided in each of a plurality of small regions obtained by dividing the region other than the region where the current is injected by the electrode. A saturable absorption region is formed by applying a reverse bias to the other electrodes, and current is injected from the electrodes other than the reverse biased electrode among the plurality of electrodes and the electrode for forming the gain region, and the saturable absorption region is formed. The length of the area is adjusted.

That is, according to the second aspect of the invention, one or two of the plurality of electrodes for forming the saturable absorption region are used.
Optimizing the length of the saturable absorption region by applying a reverse bias from the above electrodes and performing current injection from the other electrode prepared for forming the saturable absorption region and the electrode for forming the gain region. Adjust it to a good one. By adjusting the number of electrodes to which a reverse bias is applied, the length of the saturable absorption region can be easily changed even after the device is manufactured.

According to the third aspect of the present invention, an optical waveguide is formed by sandwiching the active layer with cladding layers having different refractive indices from both sides of the active layer, and a current is injected into a partial region of the optical waveguide to form a gain region. One or two or more of the plurality of electrodes of the semiconductor laser having an electrode for controlling the current and a plurality of electrodes respectively provided in each of the small regions obtained by dividing the region other than the region into which the current is injected by the electrode. A saturable absorption region is formed by applying a reverse bias to the electrodes, and current is injected from the electrodes other than the reverse biased electrode among the plurality of electrodes and the electrode for forming the gain region, in the optical waveguide. The position of the saturable absorption region of is adjusted.

That is, in the third aspect of the invention, one or two of the plurality of electrodes for forming the saturable absorption region are used.
The position of the saturable absorption region is optimized by applying a reverse bias from the above electrodes and performing current injection from the other electrode prepared for forming the saturable absorption region and the electrode for forming the gain region. adjust. The position of the saturable absorption region in the optical waveguide can be changed even after the device is manufactured, depending on which of the plurality of prepared electrodes is used to apply the reverse bias. For example, the position of the saturable absorption region can be accurately provided at the center of the optical waveguide.

According to the fourth aspect of the present invention, of the plurality of electrodes prepared for forming the saturable absorption region, the electrode to which current is injected is adjacent to the electrode prepared for forming the gain region. An arbitrary number of continuously arranged electrodes including electrodes are used.

In other words, according to the present invention, an arbitrary number of electrodes for the saturable absorption region, including electrodes adjacent to the electrode for the gain region, and an electrode for the saturable absorption region and an electrode for the gain region are provided. Injecting. Thus, a continuous gain region can be obtained by using the electrodes for the original gain region and the saturable absorption region. In the optical waveguide, the boundary position between the saturable absorption region and the gain region will be changed,
The length and position of the saturable absorption region can be adjusted without changing the number of regions.

According to the fifth aspect of the present invention, the plurality of electrodes for forming the saturable absorption region are arranged continuously at the center of the optical waveguide and in the vicinity thereof.

That is, in the fifth aspect of the present invention, the electrode group for forming the saturable absorption region is disposed near the center of the optical waveguide. If the saturable absorption region is formed using the electrode actually located at the center of the resonator after fabrication, the saturable absorption region can be accurately provided at the center of the optical waveguide.

According to the sixth aspect of the present invention, the plurality of electrodes for forming the saturable absorption region are arranged continuously from one end of the optical waveguide.

That is, in the invention according to claim 6, a plurality of electrodes for forming the saturable absorption region are provided continuously from one end of the optical waveguide. Of these electrodes,
By applying a reverse bias from a number of electrodes corresponding to the required length from the end, a saturable absorption region having an optimum length from the end can be formed after the device is manufactured.

[0023]

FIG. 1 shows a cross section of a semiconductor laser according to an embodiment of the present invention. In this semiconductor laser, an n-side cladding layer 12, an active layer 13, and a p-side cladding layer 14 are sequentially stacked on a substrate 11.
Further, on the p-side cladding layer 14, the cap layer 15 having a plurality of divided _{1-15 4} is formed. Also, a plurality of p divided into a plurality at the same location as the cap layer 15
The side electrodes 16 _{1 to} 16 ₄ are formed on the cap layer 15. On the opposite side of the substrate 11 from the n-side cladding layer 12, n
A side electrode 17 is provided.

The p-side electrode 16 ₁ is an electrode for forming a gain region 21 performs current injection. p-side electrode 16 ₂ ~
16 ₄ lower regions 22, 23 and 24, when a reverse bias is applied to the corresponding electrode to form a saturable absorption region. On the other hand, when current is injected from the corresponding electrode, a gain region is formed. p-side electrode 16 ₄ reverse bias is applied saturable absorption region in which the edge of at least the optical waveguide of the p-side electrode 16 _{2-16 4} is formed. The remaining p-side electrodes 16 ₂ and 16 ₃ are used for either current injection or reverse bias application. These p-side electrodes 1
Depending 6 _2, 16 ₃ is used for the one for the reverse bias current injection, the length of the saturable absorption region is adjusted.

The substrate 11, the n-side cladding layer 12, and the p-side cladding layer 14 are formed of InP (indium phosphide). The active layer 13 is composed of a quantum well of InGaAs (indium gallium arsenide) / InGaAsP (indium gallium arsenide). The capping layer 15 _{1-15 4} is composed of InGaAs.
Capping layer 15 _{1-15 4,} p-side electrodes _161-164
It has a function of facilitating the flow of the current injected from the lower layer. Further, the p-side electrodes 16 _{1 to} 16 ₄ are removed at the same locations as the p-side electrodes 16 _{1 to} 16 ₄ to achieve insulation between the electrodes.

FIG. 2 shows an example of wiring when current is injected and a reverse bias is applied to the semiconductor laser shown in FIG. The same portions as those in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted as appropriate. The layer indicated by reference numeral 31 in the drawing collectively represents the n-side electrode 17, the substrate 11, and the n-side cladding layer 12 in FIG.
The constant current source 32 is connected to the p-side electrode 16 ₁ .
On the other hand, a constant voltage source 33 for applying a reverse bias is connected to the p-side electrodes 16 _{2 to} 16 ₄ . This allows
Region 21 acts as a gain region. Also, the area 22,
23 and 24 both act as saturable absorption regions. Assuming that the lengths of the regions 21 to 24 are L1, L2, L3, and L4, the length of the gain region is L1, and the length of the saturable absorption region is (L2 + L3 + L4).

FIG. 3 shows another example of wiring when current is injected and a reverse bias is applied to the semiconductor laser shown in FIG. The same parts as those in FIG. 2 are denoted by the same reference numerals. Here, the constant current source 32 is a p-side electrode 16 ₁
And 16 _2, and current is injected from these electrodes. On the other hand, the constant voltage source 33 has a p-side electrode 16 ₃ ,
16 ₄ are connected to a reverse bias is applied from the electrodes. Therefore, the regions 21 and 22 function as gain regions, and their length becomes (L1 + L2). The regions 23 and 24 function as saturable absorption regions, and their length is (L3 + L4).

FIG. 4 shows still another example of wiring when current is injected and a reverse bias is applied to the semiconductor laser shown in FIG. The same parts as those in FIG. 2 are denoted by the same reference numerals. Here, the constant current source 32 is connected to the p-side electrodes 16 ₁ , 16 ₂ , and 16 _3, and a current is injected from these electrodes. On the other hand, the constant voltage source 33 is connected only to the p-side electrode 16 ₄ , and the reverse bias is applied only from this electrode. Therefore, the regions 21, 22, 2
3 acts as a gain region, and its length is (L1 + L2L
3) The region 24 functions as a saturable absorption region, and its length is L4.

For example, the resonator length (L1 + L2 + L3 +
L4) is 440 μm (micrometer), and the area 2
It is assumed that the lengths of 2 and 23 are each 25 μm, the length of the region 24 is 50 μm, and a reverse bias is applied only to the region 24 to initially design the saturable absorption region.
However, when the end face is cleaved in the manufacturing process of the device, the length of the region 24 to be the saturable absorption region is 25 μm.
It became m. Therefore, as shown in FIG.
₃ and 16 ₄ were connected, and about 0.5 V (volt) was applied as a reverse bias voltage from these to make the regions 23 and 24 saturable absorption regions.

Further, by connecting the p-side electrode 16 ₁ and the p-side electrode 16 _2, From these injected current of 50 mA (milliamps) was the region 21 and the region 22 in the gain region. Since the length of the region 24 is 25 μm due to cleavage, the total length of the region 23 and the region 24 is just the design value of 50 μm. With such a wiring, a mode-locked operation in which the repetition frequency of the optical pulse train was about 100 GHz and the pulse width was about 2 ps (pico second) was observed.

When the material system is changed, the optimum length of the saturable absorption region differs. In such a case, the number of the p-side electrodes to which the reverse bias is applied is appropriately changed. The length of the saturable absorption region can be adjusted.
Thus, even if the material system is changed, the number of parts to be newly redesigned is reduced, and the design can be labor-saving and speeded up.

Modification

Although the semiconductor laser described above has the saturable absorption region at the end of the optical waveguide, the semiconductor laser according to the modified example has the saturable absorption region substantially at the center of the optical waveguide.

FIG. 5 shows a cross section of a semiconductor laser according to a modification. In this semiconductor laser, a saturable absorption region is formed by an electrode arranged at the center. P-side electrode 41 _1, 41 ₅ for each forming a gain region from both ends of the optical waveguide is provided. The p-side electrodes 41 ₂ , 41 _2
3, 41 ₄ are provided. The cap layer is
Corresponding to the side electrode is divided into five parts 42 ₁ to 42 _5. The laminated structure of the other layers is the same as that shown in FIG. These are denoted by the same reference numerals as in FIG. 1 and their description is omitted. The length of the region 43 located below each electrode is represented by L1, the length of the region 44 is represented by L2, the length of the region 45 is represented by L3, the length of the region 46 is represented by L4, and the length of the region 47 is represented by L5.

When mode-locking operation is performed using a semiconductor laser in which a saturable absorption region is arranged at the center, it is important that the saturable absorption region is accurately located at the center of the resonator. For example, regions 45 corresponding to the electrode 41 ₃ is just
Was designed so as to be positioned at the center of the resonator, the cleavage of the end face, is actually manufactured the center position of the resonator, and was part of the electrode 41 _2. In such a case,
A reverse bias is applied only to the electrode 41 ₂ which is located in the device center, connecting the electrodes 41 ₁ and the electrode 41 ₃ to 41 ₅ from each other, a current is injected from these. Thus, the saturable absorption region can be provided just in the center of the element, and the regions on both sides thereof can be used as the gain regions, and the regions can be configured symmetrically to the left and right.

When the cavity length of the semiconductor laser is 880 μm,
An electrode 41 provided at the center of the device for the saturable absorption region
_2, 41 _3, 41 ₄ is assumed to be a length of 20μm, respectively. Upon cleavage of the device end face, the length of the region 47 is shorter than the designed value, the portion of the electrode 41 ₂ is located at the device center. Therefore, the 0.5V is applied as the reverse bias only to the electrode 41 ₂ and the other electrodes 41 ₁ and 41 ₃ to 41
₅ were connected to each other, and a current injection of about 70 mA was performed from these. As a result, mode-locked operation with a repetition frequency of about 100 GHz was confirmed.

As described above, even if the electrode portion different from the design is located at the center of the device due to cleavage, if the electrode near the center of the device is divided into small regions, the electrode actually located at the center of the device can be obtained. By configuring the saturable absorption region by using the saturable absorption region, an appropriate mode locking operation can be obtained.

Further, by applying a reverse bias to connect the electrodes 41 ₂ and the electrode 41 ₃ In the semiconductor laser shown in FIG. 5, the length of the saturable absorption region (L2 + L
3) By applying a reverse bias to connect the electrodes 41 _{2-41 4,} it is possible to set the length of the saturable absorption region (L2 + L3 + L4). As described above, the length of the saturable absorption region can be adjusted by changing the number of electrodes to which the reverse bias is applied.

Not only when the saturable absorption region is arranged at the center of the device but also when it is arranged at an arbitrary position of the semiconductor laser, if the electrodes in the vicinity thereof are finely divided, a deviation from the design due to cleavage will occur in the device. It can be corrected after production. For example, the saturable absorption region may be formed at a position that is one-third of the resonator length or a position that is one-fourth of the resonator length, such as a position that is an integral fraction of the resonator length. When a saturable absorption region is formed at one third of the length of the resonator, the pulse interval changes, and the repetition frequency can be set to three times the resonator circulating frequency. In the case of performing such a mode periodic operation, if the electrode at the one-third position and the region in the vicinity thereof is divided into a plurality of parts, the deviation from the design due to the cleavage is corrected after the device is manufactured, and just three-thirds is corrected. The portion located at 1 can be a saturable absorption region.

The material system of each layer of the semiconductor laser shown in the embodiment and the modification described above is not limited to the illustrated one. Also, the length of each electrode may be other than the values shown in the embodiments and the like, as long as the electrode in the vicinity of the saturable absorption region is divided into a size that can absorb an error due to cleavage.

[0041]

As described above, according to the invention described in claim 1, since the saturable absorption region is divided into small regions by a plurality of electrodes, it is possible to apply a reverse bias using any one of them. The length and position of the saturated absorption region can be adjusted appropriately. Thereby, the saturable absorption region can be formed at the designed position after the device is manufactured, and the optimum mode-locked operation can be obtained.

According to the second aspect of the invention, the length of the saturable absorption region can be easily changed even after the device is manufactured by adjusting the number of electrodes to which the reverse bias is applied. it can.

Furthermore, according to the third aspect of the present invention, the position of the saturable absorption region can be easily adjusted even after the device is manufactured, depending on which of the prepared electrodes is used to apply the reverse bias. Can be changed to For example, the position of the saturable absorption region can be accurately provided at the center of the optical waveguide.

According to the fourth aspect of the present invention, since the gain region can be formed continuously from the original gain region, the boundary position between the saturable absorption region and the gain region can be changed. The length and position of the saturable absorption region can be adjusted without changing the number of regions.

Further, according to the fifth aspect of the present invention, the electrode group for forming the saturable absorption region is arranged near the center of the optical waveguide. If the saturable absorption region is formed using the electrode actually located at the center of the resonator after fabrication, the saturable absorption region can be accurately provided at the center of the optical waveguide.

According to the invention, a plurality of electrodes for forming the saturable absorption region are provided continuously from one end of the optical waveguide. Of these electrodes,
By applying a reverse bias from a number of electrodes corresponding to the required length from the end, a saturable absorption region having an optimum length from the end can be formed after the device is manufactured.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a cross-sectional structure of a semiconductor laser according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing an example of wiring when current is injected and a reverse bias is applied to the semiconductor laser shown in FIG.

FIG. 3 is an explanatory diagram showing another example of wiring when current is injected and a reverse bias is applied to the semiconductor laser shown in FIG.

FIG. 4 is an explanatory diagram showing another example of wiring when current is injected and a reverse bias is applied to the semiconductor laser shown in FIG. 1;

FIG. 5 is a cross-sectional view illustrating a cross-sectional structure of a semiconductor laser in which a saturable absorption region according to a modification is arranged at a central portion.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Substrate 12 n side clad layer 13 active layer 14 p side clad layer 15, 42 cap layer 16, 17, 41 electrode 32 constant current source 33 constant voltage source

Claims (6)

[Claims] 1. An optical waveguide in which an active layer is sandwiched from both sides by a cladding layer having a refractive index different from that of the active layer, and an electrode for injecting a current into a partial region of the optical waveguide to form a gain region, An area other than the area where the current is injected by the electrodes is divided into a plurality of small areas, and a reverse bias is applied to any small area to form a saturable absorption area of any length at any position. And a plurality of electrodes of the semiconductor laser. 2. An optical waveguide in which an active layer is sandwiched from both sides by a clad layer having a refractive index different from that of the active layer, and an electrode for injecting a current into a partial region of the optical waveguide to form a gain region. A reverse bias is applied to one or more electrodes of the plurality of electrodes of the semiconductor laser having a plurality of electrodes provided for each of a plurality of small regions obtained by dividing the region other than the region where the current is injected by the electrode. To form a saturable absorption region, current is injected from the electrodes other than the reverse biased electrode among the plurality of electrodes and the electrode for forming the gain region, and the length of the saturable absorption region is increased. A method of operating a semiconductor laser, characterized in that the height is adjusted. 3. An optical waveguide in which an active layer is sandwiched from both sides by a cladding layer having a refractive index different from that of the active layer, and an electrode for forming a gain region by injecting a current into a partial region of the optical waveguide. A reverse bias is applied to one or more electrodes of the plurality of electrodes of the semiconductor laser having a plurality of electrodes provided in each of a plurality of small regions obtained by dividing the region other than the region into which the current is injected by this electrode. A saturable absorption region is formed by applying a current, and current is injected from an electrode other than the reverse biased electrode of the plurality of electrodes and an electrode for forming the gain region to saturate the optical waveguide. A method of operating a semiconductor laser, comprising adjusting the position of an absorption region. 4. An electrode on which current injection is performed among a plurality of electrodes prepared for forming the saturable absorption region,
4. The laser operation method according to claim 2, wherein an arbitrary number of continuously arranged electrodes including an electrode adjacent to an electrode prepared to form the gain region are provided. 5. The semiconductor laser according to claim 1, wherein a plurality of electrodes for forming said saturable absorption region are arranged continuously at and near a central portion of said optical waveguide. 6. The semiconductor laser according to claim 1, wherein the plurality of electrodes for forming the saturable absorption region are arranged continuously from one end of the optical waveguide.