JP2009003310A - Electroabsorption type optical modulator, semiconductor optical integrated device and control method of them - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electroabsorption type optical modulator which operates in a high-temperature environment (wide temperature range) and is capable of long-distance transmission, to provide a semiconductor optical integrated device equipped with the electroabsorption optical modulator, and to provide a control method of the electroabsorption optical modulator and the semiconductor optical integrated element. <P>SOLUTION: The electroabsorption optical modulator has a quantum well structure 3 constituted of a well layer including In, Ga and As and a barrier layer containing Al, In, Ga and As, wherein a conduction band offset value of the quantum well structure 3 is set to be 120 meV or higher, in order to operate the electroabsorption optical modulator at a high temperature and is set to be 250 meV or lower, in order to transmit an optical signal modulated by the electroabsorption optical modulator over a long distance. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an electroabsorption optical modulator, a semiconductor optical integrated device, and control methods thereof, and more particularly to a semiconductor optical integrated device suitable for use in an optical communication module and an optical communication system.

  With the spread of optical fiber communications, metro access communications (up to 80km) operate semiconductor lasers and optical modulators that are currently operated with temperature control coolers without temperature control coolers, thereby reducing costs. , Demand is growing. In order to satisfy such demands, it is required that semiconductor lasers and optical modulators have high performance so that no significant characteristic deterioration is observed even at high ambient temperatures.

  When considering high-temperature operation of an electroabsorption optical modulator (hereinafter referred to as an EA modulator) having a multiple quantum well structure, an Al-based semiconductor material (InGaAlAs-based material) as compared with a conventional P-based semiconductor material (InGaAsP-based material). There have been many reports that optical modulators have been fabricated using) and obtained good characteristics. (Non-Patent Document 1)

  An EA modulator having a multiple quantum well structure uses the physical phenomenon of exciton absorption to turn on / off light intensity. An exciton is a state in which electrons and holes are bound to each other, but the exciton absorption strength increases with increasing exciton binding energy, exciton intensity, and confinement of the wave function. Increase. In order to increase exciton absorption, the confinement of the electron wave function within the quantum well layer may be increased. FIG. 1 shows a band schematic diagram of an Al-based semiconductor material and a P-based semiconductor material. It is known that an Al-based semiconductor material has a characteristic that a conduction band offset value (hereinafter referred to as ΔEc) is larger when a quantum well structure is produced than a P-based semiconductor material. A quantum well structure having a large ΔEc value has a large confinement of the wave function, so that exciton absorption appears strongly. Therefore, in recent years, there have been many reports on producing an electroabsorption optical modulator using an Al-based semiconductor material. (Non-Patent Document 2)

  In a situation where the ambient temperature is high, the energy distribution of electrons shifts to the high energy side. In a quantum well structure with a small ΔEc value, the wave function of electrons in the well layer is less likely to be confined within the energy level in the well layer. Therefore, a quantum well structure having a large ΔEc value is desirable from the viewpoint of operating at a high temperature. FIG. 2 shows a schematic diagram of light absorption spectra at room temperature and high temperature. In a quantum well structure with a small ΔEc value, the absorption coefficient decreases as the temperature increases, whereas in a quantum well structure with a large ΔEc value, the change in absorption coefficient is small even at high temperatures.

  However, when considering long-distance transmission, a quantum well structure having a large ΔEc value is not always good. In the EA modulator, wavelength chirping occurs at the time of modulation, and this causes a problem of waveform deterioration after fiber transmission. That is, due to wavelength chirping, the optical signal spectrum after modulation becomes wider than before modulation. When this optical signal is transmitted through an optical fiber, waveform deterioration occurs due to the dispersion effect of the fiber medium, which adversely affects transmission characteristics. This phenomenon becomes more prominent as the transmission speed is faster and the transmission distance is longer.

  Wavelength chirping increases as the change in light absorption during transmission increases. The greater the change in the amount of light absorption when turning light on / off, the greater the wavelength chirping. The Al-based semiconductor material has a large ΔEc value, and therefore exciton absorption is large, and has an absorption spectrum similar to the large ΔEc value in FIG. On the other hand, since the P-based semiconductor material has a small ΔEc value, it has an absorption spectrum as small as the ΔEc value in FIG. The change in the amount of light absorption when turning on / off light is larger when the ΔEc value is larger. That is, when the ΔEc value is increased, the wavelength chirping is increased, and long-distance transmission becomes impossible.

M, R, Gokhale, etc., Optical Fiber Communications Conference, 2003, PD42 H.Arimoto et al., Electronics letters vo141, No.1, pp35-37

  From the above, it has been considered difficult to achieve both high-temperature operation and long-distance transmission.

  Therefore, in view of the above circumstances, the object of the present invention is to operate in a high temperature environment (that is, operate in a wide temperature range from 0 ° C. to 85 ° C.) and to enable long-distance transmission in this entire temperature range. It is an object of the present invention to provide an EA modulator having a low chirp multiple quantum well structure using a semiconductor material, and to provide a semiconductor optical integrated device including the EA modulator and a control method thereof.

  In order to achieve the above object, the present inventor provides an EA modulator and a composition of a well layer and a barrier layer of a quantum well structure and a ΔEc value of the quantum well structure in a semiconductor optical integrated device including the EA modulator. Furthermore, a method for controlling these EA modulators and semiconductor optical integrated devices is provided. Specifically, the EA modulator, the semiconductor optical integrated device and the control method thereof according to the present invention have the following characteristics.

That is, the electroabsorption optical modulator of the first invention that achieves the above object has a quantum well structure including a well layer having In, Ga, and As and a barrier layer having Al, In, Ga, and As. And
The conduction band offset value of the quantum well structure is 120 meV or more and 250 meV or less.

The electroabsorption optical modulator of the second invention is the electroabsorption optical modulator of the first invention.
The conduction band offset value is 120 meV or more for operating the electroabsorption optical modulator at a high temperature, and 250 meV or less for transmitting the optical signal modulated by the electroabsorption optical modulator over a long distance. It is characterized by.

The electroabsorption optical modulator of the third invention is the electroabsorption optical modulator of the first or second invention.
The barrier layer is an In (1-yz) Ga (y) Al (z) As layer (0.11 ≦ y ≦ 0.24, 0.18 ≦ z ≦ 0.31). To do.

The electroabsorption optical modulator of the fourth invention is the electroabsorption optical modulator of any of the first to third inventions,
The well layer is an In (x) Ga (1-x) As layer (0.46 ≦ x ≦ 0.49).

The electroabsorption optical modulator of the fifth invention is the electroabsorption optical modulator of the fourth invention.
The well layer has a thickness of 7 nm to 10 nm.

The electroabsorption optical modulator of the sixth invention is the electroabsorption optical modulator of any of the first to fifth inventions,
The semiconductor crystal filling both sides of the multiple quantum well structure is a Ru-doped semi-insulating semiconductor crystal.

The electroabsorption optical modulator of the seventh invention is the electroabsorption optical modulator of any of the first to sixth inventions,
The voltage applied to the electroabsorption optical modulator is changed in accordance with a change in environmental temperature in an applied voltage range in which the extinction ratio is lowered and the chirp parameter value is a negative value.

  A semiconductor optical integrated device of the eighth invention is characterized in that the electroabsorption optical modulator of any of the first to seventh inventions and a DFB laser are monolithically integrated on the same substrate.

The semiconductor optical integrated device of the ninth invention is the semiconductor optical integrated device of the eighth invention,
The DFB laser has a buried structure and has a Ru-doped semi-insulating buried layer.

A control method for an electroabsorption optical modulator according to a tenth aspect of the invention is the control method for an electroabsorption optical modulator according to any of the first to sixth aspects, wherein
The voltage applied to the electroabsorption optical modulator is changed in accordance with a change in environmental temperature in an applied voltage range in which the extinction ratio is lowered and the chirp parameter value is a negative value.

The semiconductor optical integrated device control method according to the eleventh aspect of the invention is a semiconductor optical integrated device obtained by monolithically integrating the electroabsorption optical modulator of any of the first to sixth inventions and a DFB laser on the same substrate. Control method,
The voltage applied to the electroabsorption optical modulator is changed in accordance with a change in environmental temperature in an applied voltage range in which the extinction ratio is lowered and the chirp parameter value is a negative value.

  According to the present invention, the conduction band offset value (ΔEc) of the quantum well structure in the electroabsorption optical modulator is set to 120 meV or more and 250 meV or less, so that the electroabsorption optical modulator can be operated in a high temperature environment (that is, a wide temperature range). In addition, the optical signal modulated by the electroabsorption optical modulator can be transmitted over a long distance.

  The features of the ΔEc value of a quantum well structure manufactured using a P-based semiconductor material and the ΔEc value of a quantum well structure manufactured using an Al-based semiconductor material are listed below. In addition, the calculation results for the ΔEc value that enables long-distance transmission will be described. In addition, experimental results of ΔEc value capable of high temperature operation will be described. In the present invention, attention is paid to the ΔEc value of the quantum well structure in the electroabsorption optical modulator, and the ΔEc value of the quantum well structure that provides both operation in a wide temperature range and enabling long-distance transmission is provided. Therefore, it is possible to produce the quantum well structure that can operate in a wide temperature range and enable long-distance transmission, and an EA modulator that is an object of the present invention can be realized.

FIG. 3 shows the calculation result of the relationship between the Al composition and the ΔEc value. This is a well layer of quantum well structure: In _0.485 Ga _0.515 As (strain −0.4% fixed), barrier layer: In (1-xy) Ga (y) Al (x) As (strain + 0.4%) (Fixed) results. It can be seen that the ΔEc value increases as the Al composition increases.
(1) When a quantum well structure is made of a P-based semiconductor material, the ΔEc value is 80 meV to 150 meV, and the valence band offset value (hereinafter referred to as ΔEv) is about 70 meV to 100 meV, although it varies depending on the composition.
(2) When a quantum well structure is made of an Al-based semiconductor material, ΔEc> 100 meV can be easily achieved as shown in FIG. 3, and ΔEc can be increased to about 400 meV. In addition, ΔEv can be greatly changed to about 40 meV to 150 meV.

  FIG. 4 shows the calculation results for the ΔEc value that enables long-distance transmission. FIG. 4 shows calculation results of respective chirp parameter values when the composition of the barrier layer is changed using the well layer and the ΔEc values of the quantum well structure are 120, 160, and 250 meV. In order to enable long-distance transmission of 80 km or more, the chirp parameter value is required to be a negative negative chirp.

  As shown in FIG. 4, the chirp parameter value shows a peak when the applied voltage is about 1.5V, and tends to decrease when the applied voltage is 1.5V or more, regardless of the value of ΔEc. When ΔEc is 120 meV, the chirp parameter value shows a negative value in the entire applied voltage range. When ΔEc is 160 meV, the applied voltage is −2.7 V or less and the chirp parameter value is negative. When ΔEc is 250 meV, the applied voltage is -4 V or less and the chirp parameter value is negative. As described above, when a quantum well structure having ΔEc of 250 meV or less is used, if the EA modulator is operated with an applied voltage of −4 V or less, the chirp parameter value becomes a negative value, thereby realizing long-distance transmission. Can do. Here, the withstand voltage when the EA modulator is operated is at most about −20 V, and when a voltage higher than this is applied, the operating characteristics of the EA modulator deteriorate with the deterioration of the pn junction characteristics.

各温度に対してそのＤＣ電圧を、例えばデータシートとして記憶させておき、そのデータに基づいて制御することにより、全温度に対応することができることになる。
なお、ここではＤＣ電圧を温度に対して線形的に変化させて制御したが、非線形的に変化させてもよいし、事前に取得したデータに基づいて変化させてもよい。但し、線形的に変化させた方がデータ処理が簡便で計算が速くできるので制御速度の点で有効である。 Next, a case where the EA modulator is used in a wide temperature range (from 0 ° C. to 85 ° C.) will be described. FIG. 7 shows a change in temperature from 0 ° C. to 85 ° C. of the extinction characteristic when a quantum well structure having ΔEc of 170 meV is used. The difference between the DFB laser oscillation wavelength and the absorption edge wavelength determined by the composition of the quantum well structure of the EA modulator is referred to as a detuning amount (hereinafter referred to as Δλ), where Δλ = 120 nm.
In this characteristic curve, the portion where the extinction ratio suddenly decreases due to the application of a DC voltage in the reverse direction is related to the performance of switching on / off the guided light during the operation of the modulator. In actual use, light is turned on / off at a high speed by applying a modulation amplitude voltage in a state where a DC voltage of this rapidly changing portion is applied at each temperature. For example, when the applied voltage is changed from −2 V to −4 V, the modulation amplitude voltage width may be set to 2 V while the DC voltage is applied to −3 V. Therefore, changing the DC voltage in accordance with the temperature change is effective for operating the EA modulator satisfactorily over a wide temperature range. From FIG. 7, it can be seen that, when a quantum well structure having ΔEc of 170 meV is used, it is effective to change the applied voltage in the range of −2V to −6V with respect to the temperature change from 0 ° C. to 85 ° C. For example, the DC voltage may be changed in the range of −3V to −5V and the modulation amplitude voltage width may be applied as 2V. Here, the range of the DC voltage varies depending on the amount of Δλ. When Δλ is small, it shifts to a lower DC voltage, and when Δλ is large, it shifts to a higher DC voltage. Thus, by changing the DC voltage, the EA modulator can be favorably operated over a wide temperature range.
Actually, the temperature is measured by a temperature sensor such as a thermistor, and a DC voltage corresponding to the temperature is applied. In this case, the DC voltage is changed according to the temperature change, but the modulation amplitude voltage may be kept constant. For example, when an element with ΔEc = 160 meV is used, the chirp parameter value becomes a negative value with a DC voltage of −2.7 V or less. In that case, for example, as illustrated in Table 1 below, even if the transmission experiment is performed such that the DC voltage changes linearly with respect to the temperature, the transmission is successful.

By storing the DC voltage for each temperature, for example, as a data sheet and controlling based on the data, it is possible to cope with all temperatures.
Here, the DC voltage is controlled by linearly changing with respect to the temperature, but may be changed nonlinearly or may be changed based on data acquired in advance. However, the linear change is more effective in terms of control speed because the data processing is simpler and the calculation is faster.

Next, long-distance transmission over a wide temperature range will be described.
The slope of the portion where the extinction ratio decreases in the characteristic curve of FIG. 7 correlates with the chirp parameter value. In FIG. 7, since this slope is almost the same from 0 ° C. to 85 ° C., the chirp parameter value is considered to be substantially constant in the temperature range from 0 ° C. to 85 ° C. This means that the applied voltage dependency of the chirp parameter value in FIG. 4 hardly changes in the temperature range from 0 ° C. to 85 ° C.
From FIG. 4, when using a quantum well structure in which ΔEc is 160 meV or less in a temperature range from 0 ° C. to 85 ° C. with Δλ = 120 nm, the chirp parameter value is negative if the DC voltage is operated at −2.7 V or less. Therefore, the optical signal modulated by the EA modulator can be transmitted over a long distance. As described above, when the DC voltage is changed in the range of −2 V to −6 V according to the temperature change, the EA modulator can be operated well in a wide temperature range. It is effective to realize long distance transmission. Thus, long distance transmission is possible in a wide temperature range by changing the applied voltage according to the value of ΔEc.

  That is, according to the change of the environmental temperature, the applied voltage to the EA modulator is changed in the applied voltage range (−4V to −6V) in which the extinction ratio is decreased and the chirp parameter value is a negative value. This is effective in achieving both the operation (operation in a wide ambient temperature range) and the extension of the transmission distance.

  FIG. 5 shows the experimental results of the extinction characteristics of the EA modulator when the well layer is used to change the barrier layer composition and the ΔEc value of the quantum well structure is 120 meV, 160 meV, and 250 meV. The extinction characteristic is directly linked to the ability to switch on / off the guided light when operating the modulator, and the extinction ratio difference must be at least 5 dB in absolute value to achieve good modulator characteristics. It is. In particular, when a quantum well structure with ΔEc of 120 meV is used, the extinction ratio is −2 dB when the applied voltage is −4 V, and the extinction ratio is −8 dB when the applied voltage is −6 V. Therefore, the applied voltage is from −4 V to −6 V In other words, if a DC bias is applied with -5V and modulation is performed with a modulation amplitude voltage width of 2V, an extinction ratio difference (6 dB) of 5 dB or more can be obtained. Further, when a quantum well structure having ΔEc values of 160 meV and 250 meV is used, an extinction ratio difference of 10 dB or more can be obtained. Therefore, if a quantum well structure having a ΔEc value of 120 meV or more is used, an extinction ratio difference of 5 dB or more can be obtained, and good modulator characteristics can be obtained. Therefore, it can be seen that the ΔEc value is desirably larger than 120 meV.

  From the above, if the ΔEc value of the quantum well structure is set to be in the range of 120 meV to 250 meV, the operation of the EA modulator in a wide temperature range and the long distance of the optical signal modulated by the EA modulator It becomes possible to make transmission compatible.

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

[Example 1]
FIG. 6 is a cross-sectional view of an EA modulator having a quantum well structure according to the present invention. Below, the structure of this EA modulator is demonstrated with the preparation method.

As shown in FIG. 6, a strain-free InGaAlAs guide layer 2 having a PL wavelength of 1.1 μm and a quantum well structure 3 are formed on an n-InP substrate 1 by MOCVD, and a strain-free InGaAlAs guide layer having a PL wavelength of 1.1 μm is formed thereon. 4. A p-InP overclad layer 5 was grown sequentially. Here, the quantum well structure 3 is an 8-period structure of a quantum well layer In _0.473 Ga _0.526 As ₁ having a thickness of 8 nm and a quantum barrier layer In _0.5855 Ga _0.172 Al _0.242 As ₁ having a thickness of 5 nm. Thereafter, a mesa stripe having an inverted mesa-shaped ridge structure is formed so that the mesa stripe width becomes 2 μm, and the sidewall of the stripe is embedded with the organic film 6. Specifically, a BCB film was used as the organic film 6. Further, a p-electrode 7 and an n-electrode 8 were formed and cleaved so that the resonator length was about 200 μm. By this production method, an EA modulator that controls light having a wavelength of 1.55 μm was produced.

  The prototype device (EA modulator) has ΔEc = 170 meV that combines high-temperature operation (operation in a wide environmental temperature range) and longer transmission distance, and the actual ambient temperature is 0 to 85 ° C and 80 km. After transmission, it showed no waveform degradation. FIG. 7 shows the extinction characteristics at each temperature, and an extinction ratio of 15 dB or more was obtained at all temperatures ranging from 0 ° C. to 85 ° C. FIG. 8 shows a waveform after 80 km transmission, and shows that there is no waveform deterioration after 80 km transmission.

[Example 2]
FIG. 9 is a sectional view of an EA-DFB laser (semiconductor optical integrated device) in which an EA modulator having a quantum well structure and a DFB laser according to the present invention are monolithically integrated on the same substrate, and FIG. 10 is an aa line arrow in FIG. FIG. 11 is a sectional view taken along the line bb in FIG. 9. Below, the structure of this EA-DFB laser is demonstrated with the preparation method.

  First, the configuration of the DFB laser unit 11 will be described. As shown in FIGS. 9, 10 and 11, an unstrained InGaAlAs guide layer 13 having a PL wavelength of 1.15 μm is grown on an n-InP substrate 12, and an active layer is grown thereon. This active layer is composed of a quantum well structure 14, which has a PL wavelength of 1.55 μm, + 1.0% compressive strain InGaAlAs 7 nm thick well layer, and a PL wavelength of 1.20 μm, −0. It consists of 8 periods of a 10 nm thick barrier layer of InGaAlAs with 4% tensile strain. An InGaAlAs guide layer 15 is grown on the quantum well active layer 14, a p-InAlAs layer 16 is grown, and a p-InGaAsP layer (diffraction grating) 17 is formed thereon.

In order to couple the above-described DFB laser unit 11 and the electroabsorption optical modulator unit 19 of the present invention, a butt joint technique is used. Specifically, a SiO ₂ mask is attached on the laser structure growth layer, and the quantum well active layer 14 is formed into an island shape having a width of 15 μm and a length of 450 μm by wet etching. That is, the quantum well active layer 14 is left only in a region having a width of 15 μm and a length of 450 μm. By growing the modulator structure in that state, the modulator structure is grown and integrated around the DFB laser portion.

The configuration of the electroabsorption optical modulator portion 19 is the same as that of the first embodiment. That is, the quantum well structure 21 has an 8-period quantum well structure 21 of an 8 nm-thick quantum well layer In _0.473 Ga _0.526 As ₁ and a 5 nm-thick quantum barrier layer In _0.5855 Ga _0.172 Al _0.242 As ₁ . More specifically, the InGaAlAs guide layer 20, the quantum well structure 21, the InGaAlAs guide layer 24, and the InGaAsP guide layer 25 are grown on the n-InP substrate 12 in the same manner as in the first embodiment. Here, the InGaAsP guide layer 25 is a layer for preventing the surface of the InGaAlAs guide layer 24 from being exposed and oxidized.

After the butt joint, the mask was removed, and a p-InP overclad layer 18 having a layer thickness of 1.6 μm was grown on the entire surface. A p-InGaAsP contact layer 26 was grown on the p-InP overclad layer 18. SiO ₂ is deposited again, and a striped mask is newly formed by photolithography. Using this as a mask, a mesa stripe having a width of 2 μm was formed by RIE (reactive ion etching). Subsequently, a Ru-doped InP layer 27 was grown as a current blocking layer on both sides of the mesa stripe by a MOVPE method to a thickness of 3 μm. Bisethylcyclopentadienyl ruthenium (II) was used as a Ru raw material.

Further, the mask made of SiO _{2 is} removed by HF, the over cladding layer 18 of the EA modulator section 19 is processed into a mesa stripe shape, and electrical insulation is performed, so that the DFB laser section 11 and the EA modulator section 19 are electrically insulated. The contact layer between was removed. Thereafter, an organic film 28 is applied to the entire surface, and the organic film is removed leaving only the mesa side of the EA modulator. Thereafter, each p-electrode 22 is formed, and after polishing the n-InP substrate 12, an n-electrode 23 is formed and cleaved.

  According to the EA-DFB laser of Example 2, a dynamic extinction ratio of 10 dB or more during modulation was obtained at an optical output of 10 mW at 85 ° C., a characteristic temperature of 80 k, and 10 Gbit / s.

[Example 3]
FIG. 12 is a cross-sectional view of an EA-DFB laser (semiconductor optical integrated device) in which an EA modulator having a quantum well structure and a DFB laser according to the present invention are monolithically integrated on the same substrate, and FIG. FIG. For integration, a structure called a loading type is used instead of a butt joint.

  As shown in FIGS. 12 and 13, an EA modulator layer (quantum well structure 35) of the present invention is grown on the same n-InP substrate 31, and further an active layer (LD quantum well structure) of a DFB laser is further formed thereon. 40) grow. Thereafter, only the active layer of the DFB laser part on the EA modulator part is wet etched with RIE or a mixed solution of sulfuric acid and hydrogen peroxide. This structure is called a loading type and is an EA-DFB laser structure that can be produced by a simple process.

Specifically, an unstrained InGaAlAs guide layer 34 having a PL wavelength of 1.15 μm is grown on the n-InP substrate 31, and then an EA modulator layer (quantum well structure 35) of the present invention is grown. The EA modulator layer quantum well structure 35 has an eight-period structure of a quantum well layer In _0.473 Ga _0.526 As ₁ having a thickness of 8 nm and a quantum barrier layer In _0.5855 Ga _0.172 Al _0.242 As ₁ having a thickness of 5 nm. Thereafter, an unstrained InGaAlAs guide layer 36, an InP etch stop layer 37, and an InGaAsP etch stop layer 38 are grown. Further, an InGaAlAs layer 39 as a laser guide layer is grown thereon, and an active layer 40 is grown thereon. The active layer 40 has a quantum well structure, a PL layer of 1.55 μm, a compressive strained InGaAlAs 7 nm thick well layer, a PL wavelength of 1.20 μm, a tensile strained InGaAlAs of −0.4%. Consisting of 6 periods of a 10 nm thick barrier layer. An InGaAlAs guide layer 41 is grown on the active layer 40, a p-InAlAs layer 42 is grown, a p-InGaAsP layer (diffraction grating) 43 is formed thereon, and a p-InP diffraction grating stacking layer 48 is used for p -An InGaAsP layer (diffraction grating) 43 is embedded.

A SiO ₂ mask is attached on the DFB laser part 32, and etching is performed up to the InP etch stop layer 37 by wet etching to form the DFB laser part 32. The quantum well active layer 40 is formed in an island shape having a width of 40 μm and a length of 450 μm. In this state, the SiO ₂ mask is peeled off with HF, a p-InP overclad layer 44 is grown on the entire surface, and a p-InGaAsP contact layer 47 is grown thereon.

Thereafter, SiO ₂ is deposited and a stripe mask is newly formed by photolithography. Using this as a mask, a mesa stripe having a width of 2 μm and a height of about 1.6 μm was formed by RIE (reactive ion etching). Thereafter, an organic film 49 is grown on the entire surface, and the organic film 49 is removed leaving only the sides of the EA modulator section 33. Thereafter, each p-electrode 45 is formed, and after polishing the n-InP substrate 31, an n-electrode 46 is formed and cleaved.

  According to the EA-DFB laser of Example 3, an optical output of 6 mW at 85 ° C., a characteristic temperature of 70 k, and a dynamic extinction ratio of 10 dB or more during modulation were obtained at 10 Gbit / s. It was also confirmed that the waveform after 80 km transmission did not deteriorate.

[Example 4]
The fourth embodiment is substantially the same as the first embodiment, except for the EA modulator layer quantum well structure. That is, in the EA modulator layer quantum well structure 3 of FIG. 6, an 8-period structure of a quantum well layer In _0.5888 Ga _0.398 Al _0.0714 As ₁ with a thickness of 8 nm and a quantum barrier layer In _0.4707 Ga _0.3441 Al _0.1852 As ₁ with a thickness of 5 nm. Is used.
In this embodiment, the example in which the InGaAlAs layer is used for the well layer has been described. In the above, the InGaAs layer is used for the well layer, because the tensile strain is introduced into the well layer. The quantum well structure may be formed so as to have the above-described ΔEc value by introducing compressive strain into the well layer and using the InGaAlAs layer as the well layer.

[Example 5]
The fifth embodiment has substantially the same structure as that of the first embodiment except for the EA modulator layer quantum well structure. That is, in the EA modulator layer quantum well structure 3 of FIG. 6, a 10-period structure of a quantum well layer In _0.473 Ga _0.526 As ₁ with a thickness of 10 nm and a quantum barrier layer In _0.5855 Ga _0.172 Al _0.242 As ₁ with a thickness of 10 nm is used. It was.

  As described above, a structure other than the 8-period quantum well structure having the well layer thickness of 8 nm and the barrier layer thickness of 5 nm shown in the first to fourth embodiments may be used. In particular, it is effective that the well layer thickness is 7 nm or more and 10 nm or less.

  The present invention achieves both high-temperature operation and long distance in a semiconductor optical modulator (electroabsorption optical modulator), and integrates the optical modulator and a DFB laser to provide a low-cost modulated light source without a temperature control cooler. (Semiconductor optical integrated device) can be realized, and this modulated light source can be applied to a low-cost optical transmission system without a temperature control cooler.

It is a band schematic diagram of a P-type semiconductor material and an Al-type semiconductor material. It is a schematic diagram of the relationship between the magnitude of ΔEc and the absorption coefficient. It is a figure which shows the calculation result of the relationship between Al composition and (DELTA) Ec. It is a figure which shows the calculation result of (DELTA) Ec dependence of EA modulator DC bias and a chirp parameter value. It is a figure which shows the experimental result of (DELTA) Ec value and EA modulator extinction characteristic. It is sectional drawing of the semiconductor modulator (EA modulator) in connection with Example 1 of this invention. It is a figure which shows the temperature dependence of the extinction characteristic of the EA modulator of Example 1 of this invention. It is a figure which shows the mode of the waveform of the optical transmission characteristic of the EA modulator of Example 1 of this invention. It is sectional drawing of the EA-DFB laser (semiconductor optical integrated element) in connection with Example 2 of this invention. FIG. 10 is a cross-sectional view taken along line aa in FIG. 9. Bb line arrow sectional view of FIG. It is sectional drawing of the EA-DFB laser (semiconductor optical integrated element) in connection with Example 3 of this invention. FIG. 13 is a cross-sectional view taken along the line cc in FIG. 12.

Explanation of symbols

1 n-InP substrate 2 In _0.527 Ga _0.235 Al _0.2377 As guide layer 3 In _0.473 Ga _0.526 As ₁ / In _0.5855 Ga _0.172 Al _0.242 As ₁ quantum well structure 4 In _0.527 Ga _0.235 Al _0.2377 As guide layer 5 p-InP overclad Layer 6 BCB film 7 p electrode 8 n electrode 11 DFB laser part 12 n-InP substrate 13 In _0.527 Ga _0.235 Al _0.2377 As guide layer 14 In _0.67 Ga _0.22 Al _0.11 As ₁ / In _0.48 Ga _0.34 Al _0.18 As ₁ quantum well structure 15 In _0.527 Ga _0.235 Al _0.2377 As guide layer 16 p-InAlAs layer 17 p-InGaAsP layer (diffraction grating)
18 p-InP overclad layer 19 EA modulator portion 20 In _0.527 Ga _0.235 Al _0.2377 As guide layer 21 In _0.473 Ga _0.526 As ₁ / In _0.5855 Ga _0.172 Al _0.242 As ₁ quantum well structure 22 p electrode 23 n electrode 24 In _0.527 Ga _0.235 Al _0.2377 As guide layer 25 InGaAsP guide layer 26 p-InGaAsP contact layer 27 Ru-doped InP layer 28 Organic film 31 n-InP substrate 32 DFB laser part 33 EA modulator part 34 In _0.527 Ga _0.235 Al _0.2377 As guide layer 35 In _0.473 Ga _0.526 As ₁ / In _0.5855 Ga _0.172 Al _0.242 As ₁ EA modulator layer quantum well structure 36 In _0.527 Ga _0.235 Al _0.2377 As guide layer 37 InP etch stop layer 38 InGaAsP etch stop layer 39 In _0.527 Ga _0.235 Al _0.2377 As Id layer _{40 In 0.67 Ga 0.22 Al 0.11 As} 1 / In 0.48 Ga 0.34 Al 0.18 As 1 LD quantum well structure 41 In _0.527 Ga _0.235 Al _0.2377 As guide layer 42 p-InAlAs layer 43 p-InGaAsP layer (diffraction grating)
44 p-InP over clad layer 45 p electrode 46 n electrode 47 p-InGaAsP contact layer 48 p-InP diffraction grating additional layer 49 organic film

Claims (11)

A quantum well structure including a well layer including In, Ga, and As and a barrier layer including Al, In, Ga, and As;
An electroabsorption optical modulator, wherein the conduction band offset value of the quantum well structure is 120 meV or more and 250 meV or less.   The conduction band offset value is 120 meV or more for operating the electroabsorption optical modulator at a high temperature, and 250 meV or less for transmitting the optical signal modulated by the electroabsorption optical modulator over a long distance. The electroabsorption optical modulator according to claim 1.   The barrier layer is an In (1-yz) Ga (y) Al (z) As layer (0.11 ≦ y ≦ 0.24, 0.18 ≦ z ≦ 0.31). The electroabsorption optical modulator according to claim 1 or 2.   The electroabsorption according to claim 1, wherein the well layer is an In (x) Ga (1-x) As layer (0.46 ≦ x ≦ 0.49). Type optical modulator.   The electroabsorption optical modulator according to claim 4, wherein the well layer has a thickness of 7 nm to 10 nm.   6. The electroabsorption optical modulator according to claim 1, wherein the semiconductor crystal filling both sides of the multiple quantum well structure is a Ru-doped semi-insulating semiconductor crystal.   The applied voltage to the electroabsorption optical modulator is changed in the applied voltage range in which the extinction ratio is lowered and the chirp parameter value is a negative value according to a change in environmental temperature. 7. The electroabsorption optical modulator according to any one of 6 above.   8. A semiconductor optical integrated device, wherein the electroabsorption optical modulator according to claim 1 and a DFB laser are monolithically integrated on the same substrate.   9. The semiconductor integrated device according to claim 8, wherein the DFB laser has a buried structure and has a Ru-doped semi-insulating buried layer. A method for controlling an electroabsorption optical modulator according to any one of claims 1 to 6,
An electroabsorption light characterized by changing an applied voltage to the electroabsorption optical modulator in an applied voltage range in which the extinction ratio is reduced and the chirp parameter value is a negative value in accordance with a change in environmental temperature. Modulator control method. A method for controlling a semiconductor optical integrated device, wherein the electroabsorption optical modulator according to any one of claims 1 to 6 and a DFB laser are monolithically integrated on the same substrate,
A semiconductor optical integrated device characterized in that the applied voltage to the electroabsorption optical modulator is changed in the applied voltage range in which the extinction ratio is lowered and the chirp parameter value is a negative value in accordance with a change in environmental temperature. Control method.

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