JPH10223971A - Semiconductor pulse laser device and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make passive mode synchronization to occur easily by providing a diffraction grating having a sampled grating structure in which recessing and projecting section forming areas and planar areas are repetitively formed at prescribed intervals in a Bragg reflecting mirror area. SOLUTION: When a current is supplied to a gain area 121 from a power source 131, the area 121 acts as a gain medium and makes the light amplification required for light emission and laser oscillation. A saturable absorbing area 122 causes passive mode synchronization when a reversely biased voltage is applied across the area 122 from another power source 132. In addition, a Bragg reflecting mirror area 123 works as a wavelength filter incorporated in a resonator, because the area 123 is provided with a diffraction grating 105a. Namely, the area 123 controls the oscillation wavelength and limits the oscillation wavelength area. Therefore, the passive mode synchronization can be made to occur easily by suppressing the influence of refractive index dispersion.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor pulse laser device for generating an ultrashort optical pulse train used for, for example, optical measurement and optical communication.

[0002]

2. Description of the Related Art Conventionally, as a semiconductor pulse laser device for generating an ultrashort optical pulse train, a device called a distributed reflection type or a distributed feedback type is known.

Documents disclosing such a semiconductor pulse laser device include, for example, documents by the present inventors such as Shin Arahira, Saeko Oshiba, Yasuhiro Matui, and Tat.
suoKunii, and Yoh Ogawa, "Terahertz-rate optical pul
se generation from a passively mode-locked semicon
ductor laser diode ", Optics Letters, vol.19, No.11,19
94. (hereinafter referred to as “Reference 1”), and semiconductor lasers and optical integrated circuits, written by Yasuharu Suematsu, Ohmsha, pages 325-339 (hereinafter referred to as “Reference 2”), and the like.

The semiconductor pulse laser device described in the above-mentioned document 1 has a gain region for generating and amplifying light by current injection, a saturable absorption region having the same structure as this gain region, a bandgap wavelength. A passive waveguide (phase adjustment region) having a composition shorter than that of the gain region, and a Bragg reflector region having the same bandgap composition as the passive waveguide and having a diffraction grating provided on the surface. It is configured by integrating parts.
Further, in Reference 1, the length of each region in the waveguide direction is 750 μm for the gain region, 75 μm for the saturable absorption region, 150 μm for the passive waveguide, and 120 μm for the Bragg reflector region. 1100 μm.

In such a distributed reflection type or distributed feedback type semiconductor pulse laser device, the diffraction grating in the Bragg reflector region is constituted by providing a periodic uneven structure in the guide layer. By providing such a periodic structure (grating), it is possible to generate Bragg reflection between reflected light and generate an optical pulse output.

[0006]

In such a semiconductor pulse laser device, the repetition frequency of the light pulse output is
It is determined by the total length of the element, the injection current into the gain region, and the like. For example, in the semiconductor pulse laser device (total length of about 1100 μm) disclosed in Document 1, when a current of 76 mA is injected into the gain region with the saturable absorption region grounded, the repetition frequency becomes 3
8.8 GHz (this repetition frequency is referred to as “basic repetition frequency”). Further, the injection current of the gain region is set to 1
When increasing to 37 mA, 154 mA and 179 mA, the repetition frequency becomes 10 times the basic repetition frequency,
400 and 800 GHz, which are 20 times and 40 times,
It increases to 1540 GHz, and higher-order mode locking can be obtained.

On the other hand, according to the study by the present inventors, it is possible to set the repetition frequency of the optical pulse output without depending on the total length of the element (see the above-mentioned reference 1).
reference). Hereinafter, this method will be described.

FIG. 4 is a graph conceptually showing the reflection characteristics of a Bragg reflector region in a conventional semiconductor pulse laser device. As shown in the figure, the Bragg reflector region has a periodic small reflectance peak α ₁₁ near a reflectance peak α ₀ coincident with the Bragg wavelength (wavelength satisfying the Bragg reflection condition) λ _b. , Α ₁₂ ,... And α ₂₁ , α ₂₂ ,.

According to the study of the present inventors, each of these reflectance peaks α ₀ , α ₁₁ , α ₁₂ ,... And α ₂₁ ,
By causing mode locking between the reflected lights reflected at α ₂₂ ,..., an optical pulse output with a very high repetition frequency can be obtained without depending on the total length of the element.

Here, the Bragg wavelength λ _b in the Bragg reflector region is given by the following equation (1). Here, Λ is the period of the unevenness (grating pitch) of the diffraction grating in the Bragg reflector region, and n _eq is the equivalent refractive index of the waveguide in the Bragg reflector region.

Λ _b = 2 · n _eq · Λ (1) Further, each reflectance peak α ₀ , α ₁₁ , α ₁₂ ,...
alpha _21, alpha _22, · · · interval (i.e., wavelength difference of the reflected light reflected by each of these reflectance peak) [Delta] [lambda] is given by equation (2) using the Bragg wavelength lambda _b. Note that L
_eff is that the intensity of light entering the Bragg reflector region is 1 /
e (e = 2.72). This L _eff is
The grating becomes shorter as the grating becomes deeper (that is, as the difference in height of the unevenness becomes larger).

Δλ = λ _b / (2 · n _eq · L _eff ) (2) Here, in the semiconductor pulse laser device disclosed in Reference 1, L _eff is about 100 μm. , And the wavelength difference Δλ is calculated to be about 3.1 nm. When this wavelength difference Δλ is converted into a frequency, 400 GHz
z.

As described above, the mode locking is performed between the reflected lights reflected at the respective reflectance peaks α ₀ , α ₁₁ , α ₁₂ ,... And α ₂₁ , α ₂₂ ,. By so doing, a very high fundamental repetition frequency can be obtained.

Actually, in the case of the method not using the mode locking of each reflectance peak shown in FIG. 4 (the method in which the basic repetition frequency is determined by the circulating frequency) as described above, the basic frequency is set to 400 GHz. Then
The total length of the device must be about 100 μm. Actually manufacturing such a short element is, in terms of manufacturing technology,
Very difficult. On the other hand, in the method using the mode locking of each reflectance peak, a very high repetition frequency can be obtained without extremely shortening the entire length of the element, and thus such a problem in the manufacturing technique occurs. Never.

In practice, in order to cause mode locking between the wavelengths of the wavelength interval Δλ = 3.1 nm, the recovery time of the gain, that is, the gain once reduced after the pulse in the gain region has passed, is recovered. Therefore, it is necessary to increase the pumping rate in the gain region. For example, by increasing the injection current into the active waveguide in the gain region to 137 mA, 10th-order mode locking becomes possible. Further, the injection current is increased by 15
When increasing to 4 mA, for every other wavelength of the wavelength interval Δλ = 3.1 nm, between every other mode (ie, 6.3)
(interval at nm), and the repetition frequency at this time is 800 GHz. Subsequently, when the injection current is increased to 179 mA, the wavelength interval Δλ = 3.
Oscillation can occur between every two modes (i.e., at intervals of 12.5 nm) of each wavelength of 1 nm, and the repetition frequency at this time is 1540 GHz.

However, this method (method using the mode locking of each reflectance peak shown in FIG. 4) involves the method of making each peak appearing near the reflectance peak α ₀ (that is, the reflectance peak corresponding to the Bragg wavelength λ _b ). The reflectance peaks α ₁₁ , α ₁₂ ,
, Α ₂₁ , α ₂₂ ,... Decrease as the distance from the reflectance peak α ₀ decreases, so that the threshold gain required for oscillation increases, and higher-order mode locking is performed. However, there is a drawback that oscillation becomes more difficult as the temperature becomes higher.

That is, as can be seen from FIG. 4, the reflectance at the reflectance peak α ₀ corresponding to the Bragg wavelength λ _b is 10
If it is 0%, the reflectance of the reflectance peaks α ₁₁ and α ₂₁ on both sides is usually several percent. This trend is
The effect becomes more pronounced as the grating becomes deeper (that is, as the difference between the heights of the irregularities increases), and becomes more noticeable as the grating becomes longer.

As described above, if it is difficult to oscillate a higher-order mode, the number of modes contributing to mode synchronization is limited, which is an obstacle to improving the repetition frequency of optical pulse output. I was

For this reason, a technique for improving the reflectance of each reflectance peak appearing on both sides of the reflectance peak corresponding to the Bragg wavelength has been desired.

[0020]

[Means for Solving the Problems]

(1) A semiconductor pulse laser device according to a first aspect of the present invention, wherein a gain region, a saturable absorption region, and a Bragg reflector region are respectively formed on a substrate. A diffraction grating having a sampled grating structure in which a region and a planar region are repeated at predetermined intervals.

According to such a configuration, the reflectance of each reflectance peak appearing on both sides of the reflectance peak corresponding to the Bragg wavelength can be improved.

(2) A method of manufacturing a semiconductor pulse laser device according to a second aspect of the present invention is a method of manufacturing a semiconductor pulse laser device, comprising: Forming a thin film to be a diffraction grating on a waveguide in a Bragg reflector region
And a second step of forming irregularities arranged along the waveguide direction on the entire surface of the thin film, and etching and flattening only a predetermined region of the thin film to form an irregularity forming region and a planar region. And a third step of forming a sampled grating structure by repeating at a predetermined interval.

According to such a configuration, a semiconductor pulse laser device capable of improving the reflectance of each reflectance peak appearing on both sides of the reflectance peak corresponding to the Bragg wavelength can be manufactured by a simple process. .

[0024]

Embodiments of the present invention will be described below with reference to the drawings. In the drawings, the size, shape, and arrangement of each component are only schematically shown to an extent that the present invention can be understood, and numerical conditions described below are merely examples. Please understand that.

FIG. 1 is a diagram conceptually showing the configuration of a semiconductor pulse laser device according to this embodiment.

As shown in FIG. 1, in a semiconductor pulse laser device 100, an active waveguide 102 is formed on the surface of a semiconductor substrate serving as an n-type InP cladding layer 101.

This active waveguide 102 is made of InGaAsP.
It is composed of guide layers 103 and 105 and a quantum well structure layer 104.

That is, InGa having a thickness of about 150 μm
On the surface of the AsP guide layer 103, a quantum well structure layer 10 having a value near 1.55 μm as a bandgap wavelength is provided.
4 are formed. In this embodiment, as the quantum well structure layer 104, a well made of an InGaAs layer having a thickness of about 7 nm and a band gap wavelength of about 1.67 μm, and a band gap wavelength of 1.3 μm and a thickness of 14 nm are used.
And a five-layer structure in which InGaAsP layers are alternately stacked.

An InGaAsP guide layer 105 is formed on the surface of the quantum well structure layer 104. Here, in the InGaAsP guide layer 105, a sampled grating structure (FIG. 2 described later) in which the irregularity forming region and the planar region are repeated at predetermined intervals is provided on the surface of the region to be the Bragg reflector region 123.
) Is formed, and as a result, In
The GaAsP guide layer 105 forms a diffraction grating 105a.

Further, the InGaAsP guide layer 10
5, a p-type InP cladding layer 106 is formed.

On the entire back surface of the n-type InP cladding layer 101, an ohmic electrode 111 made of Au or the like is formed. The ohmic electrode 111 is grounded.

On the other hand, of the surface of the p-type InP cladding layer 106, the diffraction grating 10 is formed on the InGaAsP guide layer 105.
Ohmic electrodes 112 and 113 made of Au or the like are formed on regions where 5a is not formed. Then, the ohmic electrode 112 and the above ohmic electrode 11
1, a power supply 131 is connected in the forward direction.
Furthermore, ohmic electrodes 113 and 111
, A power supply 132 is connected in the reverse bias direction. In the semiconductor pulse laser device 100, a region where the ohmic electrode 112 is formed on the surface is a gain region 121.
The region where the ohmic electrode 113 is formed on the surface becomes the saturable absorption region 122.

Further, the semiconductor pulse laser device 10
0, the diffraction grating 1 is formed on the InGaAsP guide layer 105.
05a is formed in the Bragg reflector region 1
23. On the surface of the p-type InP cladding layer 106 in this region 123, four ohmic electrodes 114 made of Au or the like are arranged along the waveguide direction.
A power supply 133 is connected between the ohmic electrodes 114 and 111 in the forward direction.

In this embodiment, the length of the gain region 121 is 800 μm, and the length of the saturable absorption region 122 is 50 μm.
m, and the length of the Bragg reflector region 123 was 1600 μm. Therefore, the total length of the semiconductor pulse laser device 100 is 2450 μm.

Each of these areas 121, 122, 1
It is assumed that there is sufficient electrical isolation between the 23 and no electrical interference effect.

Here, the InGaAsP guide layer 103 and the quantum well structure layer 1 are formed on the surface of the n-type InP clad layer 101.
04, InGaAsP guide layer 105 and p-type InP
As a method of forming the cladding layer 106, a normal thin film deposition technique can be used.

In order to form a sampled grating structure on the InGaAsP guide layer 105, for example, first, a concavo-convex structure is formed on the surface of the InGaAsP guide layer 105 using a conventional uniform grating structure manufacturing technique. Then, a plane region may be formed by subjecting a part of the uneven structure to a known chemical etching process.

Ohmic electrodes 112, 113, 114
Is formed by depositing a thin film made of an electrode material such as Au on the entire surface of the p-type InP cladding layer 106 and removing a part of the thin film by chemical etching or the like using ordinary photolithography technology. can do.

FIG. 2 is a sectional view schematically showing a sampled grating structure according to this embodiment.

As shown in the drawing, the surface of the InGaAsP guide layer 105 forming the diffraction grating 105a has a planar region 20 of the same length as the unevenness forming region 201 of the same length.
2 are alternately formed. That is, in this way, a sinusoidal sampled grating structure is formed on the surface of the InGaAsP guide layer 105 at a uniform period.

In this embodiment, in this sampled grating structure, the width L ₁ of each of the concavo-convex formation regions 201 is set.
Is set to, for example, 15 μm, and the width L ₂ of each planar region 202 is set to, for example, 65 μm. That is, the length Λ ₂ of one cycle of the sampled grating structure was about 80 μm. Also, the grating in the unevenness forming region 201, and one period lambda ₁ and about 240 nm, about the amplitude A 50n
m.

Next, the operation principle of the semiconductor pulse laser device according to this embodiment will be described.

The semiconductor pulse laser device 10 shown in FIG.
At 0, the gain region 121 acts as a gain medium when a current is injected from the power supply 131, and generates light and amplifies light necessary for laser oscillation. The saturable absorption region 122 is applied with a reverse bias voltage by the power supply 132 to cause passive mode locking. Further, the Bragg reflector region 123 operates as a wavelength filter incorporated in the resonator due to the provision of the diffraction grating 105a. That is, the Bragg reflector region 123 controls the oscillation wavelength and limits the oscillation wavelength range, thereby suppressing the influence of the dispersion of the refractive index and making it easy to cause passive mode locking.

FIG. 3 is a graph showing a simulation result of the reflection characteristics of the Bragg reflector region 123 according to this embodiment. The vertical axis represents the reflectance, and the horizontal axis represents the light wavelength (n).
m). In this graph, 上述₂
= 80 μm, L ₁ = 15 μm, and the coupling coefficient κ is 1
00 cm and a waveguide loss of 0 cm ^-1 .

In the semiconductor pulse laser device according to this embodiment, the interval Δλ _b between the reflection peaks in the Bragg reflector region 123 is given by the following equation (3).

Δλ _b = 2 · n _eq · Λ ₂ (3) Here, Λ ₂ = 80 μm as described above, and the equivalent refractive index n _eq of the active waveguide 102 is about 3.25. Therefore, the interval Δλ _b between the reflection peaks is about 5 nm.

As described above, in the conventional semiconductor pulse laser device, the interval Δλ _b between the reflection peaks depends on L _eff (see equation (2)). Therefore, this Δλ _b is the length of the Bragg reflector region. It depends on you. On the other hand, in the semiconductor pulse laser device according to this embodiment, the interval Δλ _b between the reflection peaks is different from the Bragg reflector region 1.
It depends on the length の 一₂ of one period of the sampled grating structure, not the entire length of 23.

As can be seen from FIG. 3, the normal line connecting the peaks of the reflection peaks is the reflection peak when the diffraction grating 105a has a uniform grating structure (ie, the structure of the diffraction grating of the conventional semiconductor pulse laser device). It has almost the same shape as the shape. Here, the interval Δ between the reflection peaks when the diffraction grating 105a has a uniform grating structure.
λ _b is obtained by substituting the width L ₁ of the concavo-convex formation region for the length Λ ₂ of one period of the sampled grating structure, and is given by the following equation (4).

Δλ _b = 2 · n _eq · L ₁ (4) Therefore, the pitch of the above-mentioned normal line also matches Δλ _b given by the equation (4). In the Bragg reflector region 123 according to this embodiment, since L ₁ = 15 μm, the pitch of the normal line is 40 nm from equation (4). That is, according to this embodiment, a plurality of reflection peaks having a high reflectance can be obtained in a very wide range of 40 nm (the wavelength range of 1535 nm to 1575 nm in FIG. 3).

In the semiconductor pulse laser device according to this embodiment, mode locking is caused between these high reflectance reflection peaks (reflection peaks in the vicinity of wavelengths 1535 nm to 1575 nm in FIG. 3), so that the optical pulse output is increased. To obtain the repetition frequency.

To cause mode locking between these reflection peaks, first, the power supply 131 is used to set the gain region 121 of the semiconductor pulse laser device 100 (see FIG. 1).
Current injection. When the laser is oscillated in this state, the Bragg reflector region 1 is located near the peak wavelength of the spectrum (gain spectrum) generated in the gain region 121.
It is possible to perform laser oscillation in a plurality of modes at wavelengths corresponding to the 23 reflection peaks.

For example, in the case of this embodiment, among the reflection peaks of the Bragg reflector region 123, the wavelength width in which the reflection peak to be used exists is 40 nm (that is, the wavelength range of 1535 to 1575 nm). 8
Up to about nine longitudinal modes can be oscillated simultaneously.

Then, in this state, the bias power supply 132
By applying a reverse bias voltage to the saturable absorption region 122 using, the absorption recovery time of the saturable absorption region 122 can be shortened, the phase relationship between the oscillation modes can be fixed, and mode synchronization can be realized. it can.

In this embodiment, since the wavelength interval of the mode is 5 nm, the repetition frequency is 650 GHz.
Can be obtained.

As described above, according to the semiconductor pulse laser device of this embodiment, the intensity of each reflection peak appearing on both sides of the Bragg wavelength reflection peak can be improved. As a result, the threshold gain required for oscillation can be reduced, and oscillation by higher-order mode locking can be easily performed.

That is, according to the invention according to this embodiment, laser oscillation using each reflection peak appearing on both sides of the Bragg wavelength reflection peak can be easily realized.

[0057]

As described in detail above, according to the semiconductor pulse laser device of the present invention, the intensity of each reflection peak appearing on both sides of the Bragg wavelength reflection peak can be improved. Laser oscillation can be easily realized by mode locking using the reflection peak. Thus, the interval between the oscillation wavelengths can be set not by the resonator length of the element but by the length of one period of the sampled grating structure. Therefore, according to the semiconductor pulse laser device of the present invention, it is not necessary to extremely shorten the element length even when the repetition frequency is set to be extremely high, so that the element can be easily manufactured. Further, the range of the spectrum (range of wavelength) that contributes to mode locking can be expanded, so that an element having a small pulse width can be easily obtained.

Further, according to the method of manufacturing a semiconductor pulse laser device according to the present invention, the above-described semiconductor pulse laser device according to the present invention can be manufactured in a simple manufacturing process at low cost.

[Brief description of the drawings]

FIG. 1 is a diagram conceptually showing a configuration of a semiconductor pulse laser device according to an embodiment of the present invention.

FIG. 2 is a sectional view schematically showing a sampled grating structure according to the embodiment of the present invention.

FIG. 3 is a graph showing a simulation result of reflection characteristics of a Bragg reflector region according to the embodiment of the present invention.

FIG. 4 is a graph conceptually showing reflection characteristics of a Bragg reflector region according to a conventional semiconductor pulse laser device.

[Explanation of symbols]

 REFERENCE SIGNS LIST 100 semiconductor pulse laser element 101 n-type InP cladding layer 102 active waveguide 103, 105 InGaAsP guide layer 104 quantum well structure layer 105a diffraction grating 106 p-type InP cladding layer 111-114 ohmic electrode 121 gain region 122 saturable absorption region 123 Bragg Reflector area 131-133 Bias power supply

Claims (4)

[Claims] 1. A semiconductor pulse laser device comprising a gain region, a saturable absorption region, and a Bragg reflector region formed on a substrate, wherein the Bragg reflector region is provided at a predetermined interval between the concavo-convex formation region and the plane region. A semiconductor pulse laser device comprising a diffraction grating having a sampled grating structure formed repeatedly. 2. The semiconductor pulse laser device according to claim 1, wherein said gain region, said saturable absorption region, and said Bragg reflector region have semiconductor waveguides of the same composition. 3. The semiconductor pulse laser device according to claim 1, wherein the unevenness forming region is formed in a sine wave shape having a uniform period. 4. A method of manufacturing a semiconductor pulse laser device having a Bragg reflector region provided with a diffraction grating having a sampled grating structure in which a concavo-convex forming region and a plane region are repeated at a predetermined interval, wherein: A first step of forming a thin film to be the diffraction grating on a waveguide; a second step of forming irregularities arranged along the waveguide direction on the entire surface of the thin film; A third step of forming a sampled grating structure in which the irregularities forming region and the planar region are repeated at a predetermined interval by performing etching to planarize the semiconductor pulse laser. Device manufacturing method.