JP2010113084A - Optical signal processing device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve such a problem that in a modulator including an optical modulation element using a compound semiconductor, a mesa stripe of a laser and of the modulation element is formed in a direction giving a large change amount of the refractive index, that is, in a direction giving high refractive index modulation efficiency so as to secure appropriate crystal growth, which results in failure in achieving high efficiency refractive index modulation, and thereby, to achieve a structure in which a modulation element can be control-driven at a low bias voltage and a large refractive index control amount can be obtained with a small change in the optical loss. <P>SOLUTION: In a semiconductor optical controlling device performing refractive index modulation by voltage control, a refractive index control layer comprising a single layer or multiple layers interposed between a p-clad layer and an n-clad layer, includes, at least partially, a doping layer. The layer structure may be a bulk layer, a single layer or a multiple quantum well layer. The crystal orientation of the element stripe is controlled to a direction along [110] or [1-10]. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical signal processing apparatus that controls an optical signal. More specifically, the present invention relates to a semiconductor light control technique for controlling light intensity, phase and frequency by voltage drive.

  Due to the explosive increase in transmission capacity accompanying the spread of the Internet in recent years, demands for large capacity, high speed and long distance transmission in optical fiber communication are accelerating. One important basic technology in optical fiber communication is the light modulation technology. There are two typical types of light modulation techniques. One is a direct modulation system that generates a modulated optical signal output by superimposing a modulation signal on the laser drive current, and the other is an external that modulates CW (Continuous Wave) light oscillated from the laser by an external modulator. Modulation method. First, an overview of the configuration and operation of the modulation element in each modulation scheme will be given.

  As for the direct modulation method, a transmitter for 10 Gbps using a distributed reflection (DFB) laser or a surface emitting laser has already appeared, and 40 Gbps operation has been realized at the research level. The direct modulation method is excellent in that the configuration is very simple, but there is a problem that the drive current becomes large when performing a high-speed modulation operation. In addition, the fluctuation of the oscillation wavelength accompanying the modulation operation is unavoidable, and the waveform of the transmission signal is deteriorated during optical fiber transmission, so that the transmission distance is limited.

  The wavelength variation of the oscillation wavelength occurs for the following reason. When the drive current of the laser is modulated, the light density in the resonator changes. Since the gain changes with the change of the optical density due to the nonlinear effect, the carrier concentration further changes accordingly. The change in carrier concentration generates a refractive index change due to a carrier effect including a plasma effect and a band filling effect, and as a result, causes a longitudinal mode change of the resonator. More specifically, the wavelength fluctuation includes an excessive chirp caused by a sudden carrier fluctuation at the time of rising or falling of the waveform, and an adiabatic chirp that changes following the optical output level. When the above-described wavelength variation occurs, the spectrum of the transmitted optical signal widens, and the dispersion tolerance during optical fiber transmission deteriorates. Therefore, in the direct modulation type laser, the transmission distance at the time of 10 Gbps operation is normally limited to about 10 km, and the transmission distance at the time of 40 Gbps operation is limited to about 2 to 3 km.

On the other hand, with respect to the external modulation method, various methods for obtaining a modulated light output by utilizing the characteristic properties of the material used for the modulation element have been realized. Examples of the material system used for the modulation external modulation system include a compound semiconductor, a LiNbO ₃ element, and silicon.

  A typical example of a modulation element configuration using a compound semiconductor is an integrated element of a DFB laser and an electroabsorption modulator. In these element configurations, the absorption edge is shifted by applying an electric field to the semiconductor material, and intensity modulation is performed. At this time, the physical properties of the Franz Keldish effect are used in the bulk structure and the quantum confined Stark effect (QCSE) is used in the quantum well structure. These element configurations are excellent in integration of the light source and the modulation element, and can realize 40 Gbps operation by low power driving. However, as with the direct modulation method, wavelength variation during modulation still becomes a problem.

  Wavelength fluctuations in the electroabsorption modulator are caused by the following causes. When an electric field is applied to the modulation element, the band edge shifts and an abrupt change in absorption coefficient occurs. Using this absorption coefficient change, the intensity modulation signal can be superimposed on the CW signal. However, the change in absorption coefficient causes a change in refractive index due to the Kramers-Kronig relationship. This refractive index change causes a wavelength variation of the modulation signal. Since the wavelength variation is applied in the variation region of the refractive index variation, excessive chirp occurs at the rise and fall of the signal. Compared to the direct modulation method, the external modulation method has a smaller chirp amount, and further reduction in chirp is possible by adopting a quantum well structure design utilizing band engineering. However, even with an external modulation system, the transmission distance is generally 40 to 80 km at the time of 10 Gbps operation.

A Mach-Zehnder type modulator has been proposed in order to solve the above-described wavelength variation problem and to cope with high-speed and long-distance transmission. The Mach-Zehnder type modulator obtains a modulated light output by using the interference of the branched light. First, after the input light is branched by a 3 dB coupler, a phase difference is given between the branched lights by giving individual refractive index changes to the propagation paths of the branched lights, and then multiplexed. The phase amount and chirp amount of the output light can be freely controlled, and can be used as a phase modulator or a frequency modulator. When a refractive index change is applied to only one propagation path of the branched light, the phase difference Δφ between the branched lights is expressed by the following equation where the wavelength is λ, the element length is L, and the effective refractive index change is Δn. The
Δφ = 2πΔnL / λ Equation (1)

When used as an intensity modulator, the phase difference between the branched lights may be modulated between 0 and π. In the Mach-Zehnder type modulator, it is necessary to dynamically control the refractive index. Therefore, it is important to select a material for obtaining a large amount of change in refractive index that can be controlled at a low voltage and can respond at high speed. Currently, in the communication wavelength band, reports using LiNb0 ₃ , InGaAsP, InAlGaAs, and silicon have been made, and the mainstream is the LiNb0 ₃ (LN) element.

  The LN element can operate at a high speed exceeding 40 GHz by utilizing the electro-optic (EO) effect. However, there is a problem that the size of the element is large and the driving voltage is at least about 4 V or higher. As the future increase in communication transmission capacity is expected, the number of elements is expected to increase rapidly. Therefore, in order to save space and reduce costs, it is urgent to reduce the size and drive power of modulation elements. Yes. In order to solve this problem of low power drive, a Mach-Zehnder type modulation element using a compound semiconductor that has a greater electro-optic effect than an LN element and can be easily integrated with a light source has been developed.

  The refractive index change in the compound semiconductor utilizes physical properties of the quantum confined Stark (QCSE) effect, the Franz Keldish effect, and the electro-optic effect, similarly to the change in the absorption coefficient described above. As described above, the electro-optic effect is capable of very high frequency response, and Non-Patent Document 1 is reported as an element using an InAlGaAs-based material, and Non-Patent Document 2 is reported as an element using an InGaAsP-based material. Has been. A Mach-Zehnder modulator using these compound semiconductors and capable of supporting 40 Gbps transmission is very promising as an alternative to the LN element. To improve the performance of the modulation element, as shown in Equation (1), a large refractive index change Δn is obtained at a low voltage, and an absorption coefficient change accompanying a refractive index change is reduced in order to suppress wavelength fluctuation. Is important.

  In recent years, in addition to the development of a Mach-Zehnder type modulation element having a small wavelength variation as described above, another proposal has been made as a new method for controlling the wavelength variation in a modulation signal. For example, this is a technique for realizing long-distance transmission and high-speed modulation by converting an FSK signal into an ASK signal using an optical filter using a frequency modulation light source and performing chirp control. Non-Patent Document 3 proposes a technique in which a DFB laser is used as a frequency modulation light source, an intensity modulation signal is generated from a frequency modulation signal using a filter, and wavelength fluctuation is suppressed. 38.5 km transmission has been reported for 10 Gbps NRZ signals. In Non-Patent Document 3, the adiabatic chirp described above is used as a frequency modulation signal, and conversion from a frequency modulation signal to an intensity modulation signal is performed using a filter. This method has an advantage that a significant increase in transmission distance can be realized by chirp control. However, when trying to cope with a higher speed operation, there is a problem that the response speed is limited by the relaxation oscillation frequency. Therefore, it is desired to realize a frequency modulation element that can perform frequency modulation and maintain high-speed response characteristics.

  In addition to the method using the adiabatic chirp of the direct modulation laser, a method of controlling the longitudinal mode by providing a refractive index control region inside the laser resonator as a method to realize the frequency modulation function and phase modulation function capable of high-speed response Is mentioned. In Non-Patent Document 4, a voltage is applied to a refractive index control region of a distributed Bragg reflector (DBR) laser including a distributed Bragg reflector, an active layer region, and a refractive index control region for phase control, and the electro-optic effect is applied. A method for controlling the oscillation frequency by utilizing a change in refractive index has been reported. This method can realize extremely high-speed frequency modulation by voltage control, and can control the oscillation frequency and the optical output level separately. In addition, by suppressing optical loss during modulation by voltage control, ideal frequency modulation and phase modulation without optical output fluctuation can be realized. For example, when there is no change in absorption loss due to modulation, the frequency response is not limited to the relaxation oscillation frequency of the element, but is determined only by the resonator response. Therefore, extremely high-speed modulation is possible as compared with the direct modulation method.

The frequency modulation amplitude is determined by the resonator structure. Hereinafter, it is assumed the active layer region of length L _p, the refractive index control region for frequency modulation of the length L _p, the laser of the total resonator length L _a11 consisting waveguide region of length L _w. Frequency modulation amount Δf in this device, control of the effective refractive index in the refractive index control region (variation) of the [Delta] n _eff, the effective refractive index n _eff, is expressed as follows when the carrier frequency is f.

Here, Δn _eff / n _eff is defined as a ratio between the effective refractive index change amount Δn _eff and the effective refractive index n _eff when the bias voltage is changed from Vb to Vb + ΔV, and is expressed by the following equation. The

Therefore, as in the case of the Mach-Zehnder modulator described above, obtaining a large change amount Δn _eff / n _eff of the effective refractive index with a low control voltage is an important point for improving the element characteristics.

In the foregoing, the modulation light source element in the amplitude transition modulation (ASK) system has been overviewed. In recent years, studies on transmission format technology have further progressed, and multi-level modulation schemes have attracted attention. In particular, studies on a multi-level phase modulation (PSK) method in which a data sequence composed of a plurality of bits is associated with one phase state are rapidly progressing. Specifically, transmission over a longer distance is realized by using a phase modulation method such as DPSK or DQPSK and combining it with a wavelength multiplexing method. For example, Non-Patent Document 5 discloses an example of a long-distance transmission system that uses a multi-level phase modulation method, has a transmission capacity of 1 Tb / s, and has a transmission distance exceeding 100 km. As shown in Non-Patent Document 5, a method using a distributed feedback laser as a CW light source and combining with a LiNbO ₃ Mach-Zehnder modulator as a phase modulation element is the mainstream. In the future, the importance of frequency modulation elements and phase modulation elements using semiconductor materials capable of low power drive and high speed drive is expected to increase in the future.

  As described above, in a light source incorporating a current communication modulation element, it is an issue to realize high-speed response and high-efficiency phase control or frequency control using voltage control. That is, it is an important common problem to realize a modulation element structure that can drive the modulation element with a low voltage, has a small optical loss variation, and can obtain a large amount of refractive index control.

K. Tsuzuki, “A 40-Gb / s InGaAlAs MQW n-i-n Mach-Zehnder Modulator With a Drive Voltage of 2.3 V”, IEEE Photonics Technology letters, vol.17, no.1, pp.46-48. S. Akiyama, ISLC 2002, TuCl, “Novel InP-based Mach-Zehnder Modulator for 40 Gb / s Integrated Lightwave Source” P. A. Morton, IEE Electronics Letters, vol.33, no.13310-311, “38.5km error free transmission at 10 Gbit / s standard fiber using a low chirp, spectrally filtered, directly modulated 1.55μm DFB laser” J. Langanay, IEE Electronics Letters, vol.30, no.4, pp.311-321 A. Sano, “14-Tb / s (140 × 111-Gb / s PMD / WDM) CSRZ-DQPSK Transmission over 160km 27-THz Bandwidth Extended L-band EDFAs,” 32nd European Conference on Optical Communication (ECOC)

However, in a modulation element using a compound semiconductor, when the laser and the modulation element are integrated in a direction in which the refractive index modulation efficiency is high and manufactured to achieve high-efficiency refractive index modulation, the problems described below was there. Here, the refractive index control using the electro-optic effect in the semiconductor device will be described in more detail. The electro-optic effect that the electric field E applied to the substance affects the refractive index is generally expressed by the following equation, where Δn is the change in the refractive index.
Δn = n (E) −n (0) = AE ² + BE (3)

  For semiconductor materials, the first-order term in Equation (3) mainly indicates the Pockels effect, and the second-order term is the Franzkeldish effect for the bulk structure and the QCSE effect for the quantum well structure, respectively. Show. Usually, in the refractive index control element, the operating wavelength is set to the long wave side at least 50 nm or more with respect to the band gap wavelength in order to avoid light absorption. In this operating region, the effect A of the second-order term is generally a plus sign, and the anisotropy due to the crystal orientation is small. That is, the refractive index increases with the electric field application E. On the other hand, the Pockels effect, which is the first-order term, is characterized by having a large anisotropy depending on the crystal orientation.

  For example, when a waveguide element is formed on a zinc blende type InP substrate (001) surface, the coefficient of the Pockels effect is positive when a stripe is formed in the [1-10] ([-110]) direction. When the stripe is formed in the [110] ([-1-10]) direction, it is a minus sign. Therefore, when the mesa stripe of the waveguide element is formed in the [1-10] direction, the effect of the second-order term and the Pockels effect are intensifying effects. On the other hand, when the mesa stripe of the waveguide element is formed in the [110] direction, the second-order term effect and the Pockels effect cancel each other, so that a sufficient refractive index change cannot be obtained. Therefore, when producing a refractive index control element, it is effective to form a waveguide mesa stripe in the [1-10] direction.

  However, in a general embedded semiconductor laser, the mesa stripe of the waveguide is produced in the [110] direction, and it is difficult to integrate the modulation elements in the direction in which the refractive index control efficiency is high. This is due to the following reason. In the fabrication of the buried laser, it is necessary to re-grow the buried layer by forming a mask on the mesa stripe of the waveguide. This is because when the mesa stripe is formed in the [1-10] direction, the growth surface forms the (111) A surface, and the buried layer grows on the mask, thereby obtaining a flat growth surface. This is because it becomes difficult. Therefore, when the refractive index control region is formed in the resonator of the semiconductor laser, or when the semiconductor laser and the refractive index control element are integrated, the efficiency of the refractive index control (refractive index modulation) with respect to the bias voltage decreases. Occurs.

  (A) and (b) of FIG. 1 is a figure which shows the structural example of the refractive index control area | region in a prior art. FIG. 1A is a conceptual diagram showing a light source including a modulation element for communication. The light source 10 includes a modulation element 20 to which a modulation signal is input, and the light source 10 outputs modulated light. Here, the modulation element 20 can be, for example, a refractive index control region made of a semiconductor material.

FIG. 1B is a diagram showing the configuration of the refractive index control region in the prior art. Here, it should be noted that the refractive index control region 20 constitutes a part of a light source such as a laser in which the refractive index control region is integrated or an optical signal processing device. The refractive index control region 20 is configured by sequentially laminating the following layers on the InP cladding layer 101 having an n-type doping concentration of 1 × 10 ¹⁸ cm ⁻³ . That is, the refractive index control layer 102, the p-type cladding layer 103, and the contact layer 104 are sequentially stacked on the n-type cladding layer 101. The refractive index control layer 102 is made of non-doped bulk InGaAsP having a band gap wavelength of 1.3 μm and a thickness of 300 nm, and the p-type cladding layer 103 is made of p-type InP having a doping concentration of 1 × 10 ¹⁸ cm ⁻³ and a height of 1.5 μm. The layer 104 is made of InGaAsP having a doping concentration of 1 × 10 ¹⁹ cm ⁻³ and a band gap of 1.5 μm. An n-side electrode 105 is formed on the lowermost surface on the n-type cladding layer 101 side, and a p-side electrode 106 is formed on the uppermost surface on the p-type cladding layer 103 side. A voltage (hereinafter referred to as a bias voltage) for controlling the refractive index is applied between the n-type electrode 105 and the p-type electrode 106. Note that, as already mentioned, the mesa stripe is formed in the [110] direction in order to obtain a flat growth surface of the buried layer.

  (A) of FIG. 2 is a figure which shows the energy band figure of a refractive index control area | region. The horizontal axis indicates the position of the refractive index control region in the thickness direction, and the vertical axis indicates energy when no bias voltage is applied. When used in the communication wavelength band, a range of positions from approximately 1100 nm to 1450 nm corresponds to the refractive index control layer 102. In (a), the left side of the position corresponding to the refractive index control layer 102 corresponds to the n-type cladding layer, and the right side corresponds to the p-type cladding layer.

  FIG. 2B shows the potential distribution near the refractive index control layer when the bias voltage is changed. In the refractive index control layer 102, the potential distribution is an inclined straight line, which is a substantially uniform electric field distribution. When the bias voltage is changed, only the slope of the straight line changes with the deformation of the band structure, and the potential changes uniformly. That is, the refractive index of the refractive index control region changes according to the effect of the bias voltage.

FIG. 3 is a diagram showing the relationship between the bias voltage in the refractive index control region shown in FIG. 1B and the refractive index change amount due to the electro-optic effect. The horizontal axis indicates the bias voltage, where a positive voltage is a forward bias and a negative voltage is a reverse bias. The vertical axis represents the refractive index change amount Δn _eff / n _eff , and is calculated by the equation (3) with a bias voltage of 0 V as a reference (Vb = 0). Since the refractive index control layer in FIG. 1B uses a bulk structure and the mesa stripe is formed in the [110] direction, the second order term is the Franz Kelish effect, and the first order term is This effect is based on the Pockels effect.

  Referring to FIG. 3, in the low bias voltage region where the absolute value of the bias voltage is small, the Franz Keldisch effect and the Pockels effect cancel each other, so that a sufficiently effective refractive index change amount cannot be obtained. In order to obtain a desired amount of change in refractive index, it is necessary to use in a high bias voltage region where the effect of the second-order term is strong. This results in an increase in drive voltage (bias voltage) and an increase in drive power consumption. As described above, when a compound semiconductor is used as a constituent material of the element, the mesa stripe of the laser and the modulation element is arranged in a direction in which the refractive index change amount is large, that is, in a direction in which the refractive index modulation efficiency is high in order to ensure proper crystal growth. However, there is a problem that high-efficiency refractive index modulation cannot be realized.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a refractive index modulation element and an optical signal processing apparatus that can be driven with a low bias voltage and can perform high-efficiency and high-speed modulation. Is to realize.

  In order to achieve such an object, the invention described in claim 1 is directed to a light control device that performs refractive index modulation using a control voltage, and includes a p-clad layer, an n-clad layer, the n-clad layer, and the A light control device, comprising: a refractive index control layer including a doping layer sandwiched between p-cladding layers, wherein the refractive index of the refractive index control layer is modulated by the control voltage. .

  The invention according to claim 2 is the light control device according to claim 1, wherein a doping concentration of the doping layer in the clad layer subjected to the same kind of doping in the n clad layer or the p clad layer. It is characterized by being in the range of 1/10 to 1 times the doping concentration.

  A third aspect of the present invention is the light control device according to the first or second aspect, wherein the doping layer is a bulk layer or a multiple quantum well layer.

  Invention of Claim 4 is the light control apparatus of Claim 1 or 2, Comprising: The said doping layer is arrange | positioned adjacent to one of the said n clad layer or the said p clad layer, and the same kind as said one It has a laminated structure including a doped bulk layer and a multiple quantum well layer disposed adjacent to the bulk layer.

  The invention according to claim 5 is the light control device according to any one of claims 1 to 4, wherein the refractive index control layer has a stripe structure formed in a crystal orientation of [110]. .

  A sixth aspect of the present invention is the light control apparatus according to any one of the first to fifth aspects, wherein the doping layer is n-type doped or p-type doped.

  The invention according to claim 7 is the light control device according to any one of claims 1 to 6, wherein the p-cladding layer and the n-cladding layer are driven in the forward bias direction and the reverse bias direction as the control voltage. A modulation signal having such a voltage amplitude is applied.

  The invention according to claim 8 is a semiconductor laser comprising a DBR region, a refractive index control region including the refractive index control layer according to any one of claims 1 to 7, and an active layer region. is there.

  The invention according to claim 9 is the semiconductor laser according to claim 8, wherein the control voltage is applied to the refractive index control region, and an oscillation wavelength control current is supplied to the DBR region. To do. The control voltage may be applied to the DBR region, and an oscillation wavelength control current may be supplied to the refractive index control region.

  A tenth aspect of the invention includes the semiconductor laser according to the eighth or ninth aspect, and a frequency filter that converts a frequency modulation optical signal output from the semiconductor laser into an intensity modulation optical signal. It is a light modulation device.

  As described above, according to the present invention, it is possible to realize a refractive index modulation element that can be driven with a low bias voltage and can perform high-efficiency and high-speed modulation. Furthermore, a refractive index control structure that can be easily integrated with a laser can be realized. By applying the present invention to a DBR laser, it is possible to realize a frequency modulation operation driven by a low bias voltage. Furthermore, by using this DBR laser as a frequency modulation light source and performing FM / AM conversion of the frequency modulation light using an optical filter, an optical transmitter capable of long-distance transmission with low power consumption can be realized. .

  The light control apparatus of the present invention employs the following configuration. In a semiconductor optical control device that performs refractive index modulation using voltage control, at least a part of a refractive index control layer including a single layer or a plurality of layers sandwiched between a p-clad layer and an n-clad layer has a doping layer. As the doping of the refractive index control layer, n doping or p doping can be applied, and n doping is particularly effective. The layer structure can be a bulk layer, a single layer, or a multiple quantum well layer. The refractive index control layer is formed of a plurality of layers, and the n-cladding layer side is an n-doped bulk layer and the p-cladding layer side is an n-doped multiple quantum well layer, or the p-cladding layer side is a p-doped bulk layer and n-cladding The layer side may be a p-doped multiple quantum well layer.

  Furthermore, the material used was zincblende crystal, and the element had a configuration in which the crystal orientation of the stripe of the element was in a direction according to [110] or a direction according to [1-10]. In addition to the description of the configuration and operation of the refractive index control layer, an example in which the present invention is applied to a frequency modulation DBR laser that performs frequency modulation by controlling the electric field of the refractive index control layer is also shown. Furthermore, the structure which utilizes FM / AM conversion in combination with an optical filter is also shown. Hereinafter, the structure and operation of the refractive index control layer (region) included in the optical signal processing apparatus according to the present invention will be described in detail with reference to the drawings.

  [Layer Structure Example 1] FIG. 4 is a diagram showing the configuration of the refractive index control region of Structure Example 1 according to the present invention. Here, it should be noted that the refractive index control region 100a constitutes a part of a light source such as a laser in which the refractive index control region is integrated, or an optical signal processing device configured to be integrated.

The refractive index control region 100a is configured by sequentially laminating the following layers on the InP cladding layer 101 having an n-type doping concentration of 1 × 10 ¹⁸ cm ⁻³ . That is, the refractive index control layer 107, the p-type cladding layer 103, and the contact layer 104 are sequentially stacked on the n-type cladding layer 101. The refractive index control layer 107 is characterized in that it is composed of n-type InGaAsP having a Si doping concentration of 1 × 10 ¹⁸ cm ⁻³ , a band gap wavelength of 1.3 μm, and a thickness of 300 nm, whereas the prior art is non-doped. There is. The p-type cladding layer 103 is p-type InP with a doping concentration of 1 × 10 ¹⁸ cm ⁻³ and a height of 1.5 μm, and the contact layer 104 is InGaAsP with a doping concentration of 1 × 10 ¹⁹ cm ⁻³ and a band gap of 1.5 μm. Each is composed. An n-side electrode 105 is formed on the lowermost surface on the n-type cladding layer 101 side, and a p-side electrode 106 is formed on the uppermost surface on the p-type cladding layer 103 side.

  Hereinafter, the electro-optic effect when a bias voltage is applied between the p-type electrode 106 and the n-type electrode 105 of this structure will be described separately in the case of the mesa stripe production direction. First, consider the case where the mesa stripe production direction, that is, the light propagation direction is the [110] direction.

  (A) of FIG. 5 is a figure which shows the energy band figure of the refractive index control area | region of this structure which gave n dope. When n-doping is performed, electrons having the same concentration as the doping concentration exist in the refractive index control layer 107, and the band on the n clad layer 101 side in the refractive index control layer 107 is substantially flat. On the other hand, a steep built-in electric field is applied to the p-cladding layer 103 side.

  FIG. 5B shows the potential distribution in the vicinity of the refractive index control layer when the bias voltage is changed in Structural Example 1. In the case of the present invention, the region on the p-type cladding layer 103 side of the refractive index control region 107 is driven in a region with a high electric field. For this reason, the amount of change in the refractive index associated with the application of the bias voltage is dominated by the effect of the second-order term in Equation (3), that is, the Franz Keldish effect, and the change in the refractive index can be obtained even at a low bias voltage. Become.

FIG. 6 is a diagram showing the relationship between the bias voltage and the amount of change in refractive index due to the electro-optic effect in Structural Example 1 of the present invention. The horizontal axis indicates the bias voltage, where a positive voltage is a forward bias and a negative voltage is a reverse bias. The vertical axis represents the refractive index change amount Δn _eff / n _eff , and is calculated by the equation (3) with a bias voltage of 0 V as a reference (Vb = 0). The amount of change in the refractive index is greatly increased as compared with the case of the prior art in which the doping indicated by the broken line is not performed. Further, by doping the refractive index control layer, it is possible to obtain a refractive index change due to the carrier effect in addition to the electro-optic effect.

  FIG. 7 is a diagram showing the relationship between the bias voltage and the carrier concentration in the refractive index control layer in Structural Example 1 of the present invention. It can be seen that the carrier in the refractive index control layer 107 is decreased by increasing the negative bias voltage, and the effect of the carrier sweep by applying the bias voltage is remarkably caused by doping. As shown by the comparison with the broken line, the same effect is produced in the case of the prior art in which doping is not performed because residual carriers exist, but the effect of carrier sweeping is increased by doping the refractive index control layer. It is clear that they are A change in carrier concentration accompanying a change in bias voltage causes a change in refractive index due to a plasma effect and a band gap reduction effect.

  FIG. 8 is a diagram showing the relationship between the bias voltage and the refractive index change amount due to the plasma effect in Structural Example 1 of the present invention. It is clear that the amount of change in the refractive index is significantly increased as compared with the case of the prior art in which the doping indicated by the broken line is not performed. Therefore, a large change in refractive index can be obtained even in an element having a mesa stripe structure in the [110] direction, and can also be applied to an optical signal processing element having a buried structure.

  FIG. 9 is a diagram showing the bias voltage dependence of the refractive index change amount combining the electro-optic effect and the carrier effect. That is, FIG. 9 shows the addition of the refractive index variation due to the electro-optic effect shown in FIG. 6 and the refractive index variation due to the plasma effect shown in FIG. In Structural Example 1 according to the present invention, it can be seen that the amount of change in the refractive index is significantly increased as compared with the prior art. In addition to the increase in refractive index change in the reverse bias region (<0) where the bias voltage is negative, a significant change in refractive index that varies with the bias voltage is also obtained in the forward bias region (> 0). It has been.

Referring to FIG. 9, it can be seen that the linearity of the refractive index variation with respect to the bias voltage is improved by doping the refractive index control layer. Therefore, according to this structural example, the modulation element can be driven in a wide bias voltage range from the forward bias to the reverse bias. In particular, by setting the reference bias voltage near 0V and driving the bias voltage corresponding to the modulation signal in the vicinity of the boundary between the forward bias region and the reverse bias region, the absolute value of the bias voltage is suppressed and low power driving is realized. it can. For example, in FIG. 9, when the drive range of the bias voltage is −0.5 to +0.5 V, Δn _eff / n _eff hardly changes according to the conventional technique. On the other hand, according to the first structural example, Δn _eff / n _eff changes by 2 × 10 ⁻⁵ when the driving range is the same bias voltage. In the prior art, in order to obtain the same amount of change in the refractive index, for example, the drive range of the bias voltage must be −2 to −0.5V. The amount of change in the refractive index can be controlled by changing the doping concentration in the refractive index control layer, and can be further increased.

FIG. 10 is a diagram showing the doping concentration dependence of the effective refractive index variation when the bias voltage is −3V. The horizontal axis represents the doping concentration to the refractive index control layer 107, and the vertical axis represents the refractive index change amount. When the doping concentration of the n-cladding layer 101 containing the same kind of dopant as that of the refractive index control layer 107 is 1 × 10 ¹⁸ cm ^−3, an effect of increasing the effective refractive index change amount is obtained even if the refractive index control layer 107 is slightly doped. It is done. As can be seen from FIG. 10, the effective refractive index change amount increases rapidly when the doping concentration is increased to 1 × 10 ¹⁷ cm ⁻³ (0.1 on the horizontal axis scale), and the doping concentration is 2 × 10 ¹⁷ cm ^−. Increasing to ³ further increases. When the doping concentration increases to 3-4 × 10 ¹⁷ cm ⁻³ or more, the effective refractive index change amount will eventually begin to saturate. Furthermore, at 5 × 10 ¹⁷ cm ⁻³ , the maximum value is reached and it is saturated. The electric field generated in the refractive index control layer 107 described with reference to FIGS. 5A and 5B depends on the ratio between the doping concentration of the n-type cladding layer 101 and the doping concentration of the refractive index control layer 107. Considering this, when the doping concentration in the refractive index control layer 107 is 1/10 or more of the doping concentration in the n-type cladding layer 101, an effect of sufficiently increasing the effective refractive index variation can be obtained.

  When the doping concentration of the refractive index control layer 107 is higher than the doping concentration of the n-clad layer 101, an electric field is generated only in a region near the hetero interface between the refractive index control layer and the p-side cladding layer. Since the light confinement amount in this region is small, the effect of increasing the effective refractive index change amount by the layer structure of the present invention is small.

  From the above, the effect of increasing the effective refractive index variation in the layer structure of the present invention is effective when the doping concentration of the refractive index control layer is in the range of 1/10 to 1 of the doping concentration of the n-clad layer.

In this structure, the carrier absorption due to n-doping is as extremely small as 1 cm ⁻¹ per 1 × 10 ¹⁸ cm ⁻³ doping concentration, and a modulation operation with small loss variation can be realized. Since the modulation element according to this structure uses only the carrier drift, not the current injection operation, a high-speed modulation operation of 40 GHz class is also possible. In the above description, only the case of a very uniform doping profile in the refractive index control layer is shown, but the same effect can be obtained even with a non-uniform doping profile such as an inclined type.

  Up to this point, the structure example 1 of the present invention has been described in the case where the mesa stripe production direction, that is, the light propagation direction is the [110] direction. Next, consider the case where the light propagation direction is the [1-10] direction. It should be noted that each of the above cases includes an index indicating an equivalent orientation. For example, the [110] direction includes the [−1-10] direction and the [1-10] direction includes the [−110] direction and the like. Therefore, the concept including the [110] direction and all equivalent directions is also referred to as a direction according to the [110] direction.

  When the mesa stripe production direction, that is, the light propagation direction is the [1-10] direction, a larger amount of refractive index change can be obtained compared to the [110] direction. Also in the [1-10] direction, the change in the band structure and the potential shape when a bias voltage is applied are the same as those shown in FIGS. 5A and 5B, respectively. However, since the Pockels coefficient A has a positive sign in the electro-optic effect represented by the expression (3), the Pockels effect and the second-order term effect are intensified, and the refractive index variation is larger than that in the [110] direction. Is obtained. Since the amount of change in the refractive index due to the carrier has a small anisotropy, the effect of the carrier is equivalent to the [110] direction.

  FIG. 11 is a diagram showing the bias voltage dependence of the effective refractive index change amount in the [1-10] direction. When compared with the case of [110] direction shown in FIG. 9, it can be seen that a larger refractive index change is obtained.

FIG. 12 is a diagram showing the doping concentration dependence of the effective refractive index change amount in the [1-10] direction and the bias voltage of −3V. As in the [110] direction shown in FIG. 10, the effect of increasing the effective refractive index change amount by doping can be obtained. The effective refractive index change amount starts to increase rapidly when the doping concentration increases to 1 × 10 ¹⁷ cm ^−3, and further increases when the doping concentration increases to 2 × 10 ¹⁷ cm ⁻³ . When the doping concentration increases to 3-4 × 10 ¹⁷ cm ⁻³ or more, the effective refractive index change amount starts to be saturated. The electric field generated in the refractive index control layer depends on the ratio between the doping concentration of the cladding layer and the doping concentration of the refractive index control layer, and the doping concentration in the n cladding layer 101 is 1 × 10 ¹⁸ cm ^−3. When the doping concentration of the refractive index control layer is 1/10 or more of the doping concentration in the n-clad layer, an effect of increasing the effective refractive index change amount can be obtained.

  When the doping concentration in the refractive index control layer 107 is higher than the doping concentration in the cladding layer, the electric field is generated only in the region near the hetero interface between the refractive index control layer and the p-side cladding layer. Since the light confinement amount in this region is small, the effect of increasing the effective refractive index change amount by the layer structure of the present invention is small.

  As described above, the effect of increasing the effective refractive index variation in the layer structure of the present invention is effective when the doping concentration in the refractive index control layer is in the range of 1/10 to 1 of the doping concentration in the cladding layer.

[Layer Structure Example 2]: FIG. 13 is a diagram showing a configuration of the refractive index control region of Structure Example 2 according to the present invention. Instead of the refractive index control layer 107 in the layer structure example 1, a 20-layer unstrained InGaAsP / InP quantum well having an n-type doping concentration of 1 × 10 ¹⁷ cm ⁻³ with Si, a transition wavelength of 1.4 μm, and a thickness of 300 nm The layer 108 was included. The configuration of Structural Example 2 is the same as that of Structural Example 1 except for the quantum well layer 108. In the quantum well layer 108, the thickness of one well layer was 6 nm, and the thickness of one barrier layer was 10 nm.

  In this structural example 2, the Franz Keldisch effect in the structural example 1 is replaced with the QCSE effect, and by combining this QCSE effect and the carrier effect, the effective refractive index change amount is large and highly efficient refractive index modulation is possible. It becomes. In particular, since the QCSE effect is based on exciton absorption, the absorption edge of the absorption spectrum is steep, so that it can be used in a region near the band edge. Therefore, the structural example 2 has a larger refractive index change amount than the structural example 1 using the Franz-Keldish effect, and can realize a highly efficient refractive index modulation operation. The n-type doping is effective when applied to either the well layer or the barrier layer in the quantum well layer 108, but it is more preferable to dope the barrier layer from the viewpoint of waveguide loss. Further, in this structural example, the production of the mesa stripe is effective in either the [110] direction or the [1-10] direction. However, similar to the layer structure example 1, the mesa stripe can be more effectively produced in the [1-10] direction.

[Layer Structure Example 3] FIG. 14 is a diagram showing the configuration of the refractive index control region of Structure Example 3 according to the present invention. This structural example 3 is different from the structural example 1 in that it is composed of two types of layers instead of the single refractive index control layer 107 in the structural example 1. That is, a 1.3 μm refractive index control layer 109 having a thickness of 200 nm and an n-type doping concentration of 1 × 10 ¹⁷ cm ⁻³ is formed on the n-cladding layer 101 side as a first layer. On the first layer, there are further 13 unstrained InGaAsP / InP quantum well refractive index control layers 110 having a thickness of 100 nm and an n-type doping concentration of 1 × 10 ¹⁷ cm ⁻³ and a transition wavelength of 1.4 μm. Is formed. In the quantum well refractive index control layer 110, the thickness of the well layer was 6 nm, and the thickness of the barrier layer was 10 nm.

  As described above, the quantum well refractive index control layer is arranged on the p-type cladding layer side of the n-type bulk refractive index control layer, that is, on the cladding layer side doped with the opposite polarity to the doping in the refractive index control layer. And

  FIG. 15 is a diagram showing a band structure in the refractive index control region of Structural Example 3. The band structure in this structure is a structure in which the n-type bulk refractive index control layer on the n electrode side is flat and the n-type quantum well refractive index control layer on the p electrode side is steeply inclined. Therefore, since a high electric field is applied on the p electrode side, the QCSE effect with high refractive index control efficiency can be used effectively. On the n-electrode side, the effective applied electric field is small, but the use of the bulk structure increases the carrier extraction speed, making it possible to efficiently use the carrier effect.

  [Layer Structure Example 4] In the structure examples 1 to 3 described above, the case of n-type doping was shown as the type of dopant, but the effect of changing the refractive index on the carrier is also caused by p-type doping. Therefore, the same structure can be adopted by changing only the kind of dopant. Specifically, a p-doped bulk layer can be used for the refractive index control layer 107 as a structure according to Structural Example 1. Further, as a structure according to Structural Example 2, a p-doped quantum well layer can be used for the refractive index control layer 108.

  Similarly, as a structure according to Structural Example 3, a p-doped quantum well refractive index control layer is provided as the refractive index control layer 109 which is the first layer on the n electrode side, and the refraction which is the second layer on the p electrode side. A p-doped bulk refractive index control layer is provided as the rate control layer 110. As described above, the quantum well refractive index control layer is arranged on the n-type cladding layer side of the p-type bulk control layer, that is, on the cladding layer side doped with the opposite polarity to the doping in the refractive index control layer. .

  FIG. 16 is a diagram showing an energy band diagram of the refractive index control region when p-doping is used in the structural example 3 as an example of the structural example 4. In the refractive index control layer on the p-electrode side, the effective applied electric field is small, but because of the bulk structure, the carrier extraction speed is increased, and the carrier effect due to holes can be used efficiently. As the p-type dopant, for example, Zn can be used.

  As is apparent from the description of the operation principle of each structural example described above, the configuration of the refractive index control region of the present invention is not limited to these examples. For example, the effect of increasing the amount of change in the effective refractive index can also be obtained by introducing doping into a part of the multilayer refractive index control layer. Hereinafter, a plurality of device application examples will be described as a specific example of an optical signal processing device incorporating the above-described layer structure.

  [Element Application Example 1] FIG. 17 is a diagram showing the structure of a frequency modulation DBR laser having a semi-insulating embedded structure to which the refractive index control region of the present invention is applied. FIG. 17 is a cross-sectional view of a plane including the light propagation direction of the waveguide of the DBR laser. That is, the y-axis direction in the figure is the light propagation direction, and the mesa stripe is formed in the [110] direction in the y-axis direction. The DBR laser is sequentially manufactured while forming a waveguide and a mesa structure according to a process described later.

  The DBR laser 400a is composed of four regions in the y-axis direction. That is, distributed Bragg reflector mirror regions (hereinafter referred to as DBR regions) 111a and 111b having a length of 600 μm arranged at both ends in the y-axis direction, and a refractive index control region having a length of 200 μm that performs frequency modulation adjacent to one DBR region 111a. 112, and an active layer region 113 having a length of 200 μm adjacent to the refractive index control region 112.

  In the z direction, a waveguide layer 102 having n-type doping in each region is formed on the n-type cladding layer 101. That is, the DBR layer 102 is stacked in the DBR regions 111a and 111b, and the refractive index control layer 102 is stacked in the refractive index control region 112. In the active layer region 113, a 10-layer multiple quantum well active layer (MQW active layer) 114 having an emission wavelength of 1.55 μm is stacked. A diffraction grating 115 is provided in each of the waveguide layers 102 of the DBR regions 111a and 111b. Further, in each region, a p-type cladding layer 103 and a contact layer 104 are sequentially stacked, and a p-type electrode 106 and an n-type electrode 105 are provided above and below the DBR laser element 400a, respectively. In order to electrically isolate the four regions 111a, 112, 113, and 111b, the contact layer 104 near the region boundary is removed.

The p-type electrode of each region, the voltage controller (V) 116 with respect to the refractive index control region 112 for performing frequency modulation, DBR region 111a, the oscillation wavelength control current device for 111b _(I R, I _F ) 118 and 119 are connected to the active layer region 113 by an active layer current control device (I _D ) 117, respectively.

18A and 18B are views showing a cross section of the active layer region and a cross section of the DBR region or the refractive index control region of the DBR laser 400a, respectively. That is, (a) is a view of the active layer region 113 in FIG. 17 as viewed in the xz plane. FIG. 18B is a diagram when the DBR regions 111a and 111b or the refractive index control region 112 in FIG. 17 are viewed in the xz plane. In each region, a mesa stripe having a waveguide width of 1.5 μm is formed. A p-InP cladding layer 103 having a doping concentration of 1 × 10 ¹⁸ cm ⁻³ is formed on the waveguide 102. Both sides of the active layer stripe and the DBR stripe are each embedded with a semi-insulating InP clad 120. A SiO ₂ insulating film 121 is formed on the cladding layer 120 except for the top of the mesa stripe, and a p-side electrode 106 is formed on the uppermost surface of the element.

This DBR laser is manufactured according to the following procedure. First, the n-type cladding layer 101 and the MQW active layer 114 were formed on the entire surface of the n-type InP substrate by metal organic vapor phase epitaxy. Next, a SiO ₂ film was formed on the entire surface using plasma CVD, and a SiO ₂ mask was formed on the MQW active layer 114 region using photolithography and dry etching. Then, unnecessary MQW active layer was removed using wet etching. Subsequently, the n-doped InGaAsP refractive index control layer 102 and the DBR layer 102 were butt-joint grown using metal organic vapor phase epitaxy. Next, a resist pattern of the diffraction grating was drawn in the DBR region using the electron beam exposure method, and the diffraction grating 115 was manufactured using wet etching. After producing the diffraction grating 115, the resist and the SiO ₂ film were removed, and the p-cladding layer 103 and the contact layer 104 were grown using metal organic vapor phase epitaxy. Thereafter, a SiO ₂ mask was formed, and a SiO ₂ waveguide pattern was formed using photolithography and dry etching.

Next, using this pattern as a mask, a waveguide stripe was formed using dry etching. After forming the waveguide stripe, the semi-insulating Fe-doped InP layer clad 120 was buried and grown to the height of the stripe using metal organic vapor phase epitaxy. Next, after forming SiO ₂ for separation between waveguides, an SiO ₂ mask was formed using photolithography and dry etching, and the contact region 104 was removed by wet etching. Next, after removing SiO ₂ on the stripe, the SiO ₂ insulating film 121 was formed using plasma CVD. Furthermore, the window of the electrode part was formed using photolithography and dry etching. Subsequently, an electrode pattern was formed by lift-off, and an Au / Zn / Ni p-electrode 106 was formed using electron beam evaporation. Thereafter, a Ti / Pt / Au electrode 105 was formed as the n electrode 105. Finally, a non-reflective coating of SiO ₂ / TiO ₂ was applied to the end faces at both ends of the DBR laser element using vacuum deposition.

Next, the modulation operation in the present DBR laser will be described. The active layer current control device 117 injects a current into the active layer region 114 to perform laser oscillation. Current is injected into the DBR regions 111a and 111b by the oscillation wavelength control current devices 118 and 119 to adjust the oscillation wavelength. A modulation operation is performed by applying a modulation signal voltage from the voltage control device 116 to the refractive index control region 112. More specifically, the peak with respect to the bias voltage V _b - modulated signal voltage modulated signal of the peak amplitude V _PP is superimposed, it is applied to the refractive index control region 112. As a result of the effective refractive index of the refractive index control region 112 changing according to the modulation signal voltage, the frequency modulation operation is realized by changing the longitudinal mode of the oscillation wavelength.

FIG. 19 is a diagram showing the relationship between the frequency modulation amount and the applied voltage in the DBR laser element of element application example 1. The relationship when a refractive index control region according to the prior art is provided is also indicated by a broken line. Assuming that the refractive index change in the DBR region caused by the voltage amplitude change V _PP of the modulation signal is Δn _eff , the frequency modulation amount Δf is given by equation (2). L _p / L _a11 in this example is about 0.3. Therefore, for example, when a frequency modulation amount of 10 GHz (0.08 nm in wavelength change amount) is to be obtained and the bias voltage Vb is set to 0 V, the voltage amplitude change V _PP required for the modulation signal is 0. .75V. On the other hand, in the case of using the conventional refractive index control region, even when the bias voltage Vb is set to 0V, it can be seen from the fact that the slope in the vicinity of 0V of the characteristic of the prior art shown by the broken line in FIG. In addition, almost no effective frequency modulation amount can be obtained. Therefore, in order to obtain a frequency modulation amount of 10 GHz, the bias voltage Vb must be increased to −1.85V.

By applying the refractive index control region of the present invention to the DBR laser, the bias voltage can be greatly suppressed as compared with the structure of the prior art. In addition, even with the same bias voltage, the voltage amplitude change _VPP of the modulation signal can be suppressed. According to the present invention, a significant change in refractive index can be obtained even in a forward bias region of 0 V or higher. Therefore, modulation can be performed from the forward bias region to the reverse bias region as an operating point with the bias voltage of 0 V interposed therebetween. At this time, with respect to the forward bias, since the forward current does not flow if the operating point is set so that the instantaneous voltage due to the modulation signal is equal to or lower than the forward breakdown voltage, a high-speed modulation operation can be maintained.

  As described above, by performing the modulation operation from the forward bias region to the reverse bias region with the vicinity of 0V as the center, it is possible to perform the modulation operation with low driving power, which has been impossible in the past. In addition, since the linearity of the frequency change response to the bias voltage change is improved, it is possible to perform modulation with a stable frequency modulation amount in a wide voltage range from the forward bias region to the reverse bias region. Specifically, by improving the linearity of the response characteristic of the bias voltage-frequency change, the electric signal waveform of the modulation signal can be reflected on the optical modulation signal waveform without deterioration. In a communication system, it is possible to suppress waveform deterioration distortion due to nonlinearity of modulation characteristics.

  As described above in detail, according to the present invention, even with a conventional buried structure using a mesa stripe fabricated in the [110] direction, high-efficiency modulation can be obtained by driving with a low bias voltage. Significant improvement in characteristics of the optical signal processing apparatus including the modulator can be realized. The above description has been made from the viewpoint of frequency modulation. However, the frequency change is equivalent to the oscillation wavelength change, and the DBR laser can be applied as a wavelength tunable light source that realizes an ultrafast wavelength switching operation.

  [Element Application Example 2] FIG. 20 is a diagram showing the structure of a frequency modulation DBR laser having a ridge structure to which the refractive index control region of the present invention is applied. FIG. 20 is a cross-sectional view in a plane including the light propagation direction of the waveguide of the DBR laser. That is, the y-axis direction in the figure is the light propagation direction, and the mesa stripe is formed in the [110] direction in the y-axis direction. The DBR laser is sequentially manufactured according to a process described later while forming a waveguide and mesa structure pattern.

  The DBR laser 400b is composed of four regions in the y-axis direction. That is, distributed Bragg reflector mirror regions (hereinafter referred to as DBR regions) 111a and 111b having a length of 600 μm arranged at both ends in the y-axis direction, and a refractive index control region having a length of 200 μm that performs frequency modulation adjacent to one DBR region 111a. 112, and an active layer region 113 having a length of 200 μm adjacent to the refractive index control region 112.

  In the z direction, a waveguide layer 102 having n-type doping is formed on the n-type cladding layer 101 in each of the four regions. That is, the DBR layer 102 is stacked in the DBR regions 111a and 111b, and the refractive index control layer 102 is stacked in the refractive index control region 112. Also in the active layer region 113, a waveguide layer 102 having n-type doping is laminated, and further, a 10-layer multiple quantum well active layer (MQW active layer) 114 having an emission wavelength of 1.55 μm is laminated thereon. A diffraction grating 115 is provided in each of the waveguide layers 102 of the DBR regions 111a and 111b. Further, in each region, a p-type cladding layer 103 and a contact layer 104 are sequentially stacked, and a p-type electrode 106 and an n-type electrode 105 are provided above and below the DBR laser element 400b, respectively. In order to electrically isolate the four regions 111a, 112, 113, and 111b, the contact layer 104 near the region boundary is removed.

The p-type electrode of each region, the voltage controller (V) 116 with respect to the refractive index control region 112 for performing frequency modulation, DBR region 111a, the oscillation wavelength control current device for 111b _(I R, I _F ) 118 and 119 are connected to the active layer region 113 by an active layer current control device (I _D ) 117, respectively.

21A and 21B are views showing a cross section of the DBR region or the refractive index control region and a cross section of the active layer region of the DBR laser 400b, respectively. That is, (a) is a view of the DBR regions 111a and 111b or the refractive index control region 112 in FIG. FIG. 21B is a diagram when the active layer region 113 in FIG. 20 is viewed in the xz plane. In each region, a mesa stripe having a waveguide width of 2 μm and a height of 1.5 μm is formed. A p-InP cladding layer 103 having a doping concentration of 1 × 10 ¹⁸ cm ⁻³ is formed on the waveguide 102. Both sides of the active layer stripe and the DBR stripe are filled with benzocyclobutene (BCB) 122, respectively, and an SiO ₂ insulating film 121 is formed thereon. A p-side electrode 106 is formed on the uppermost surface of the element.

This DBR laser is manufactured according to the following procedure. First, an n-type doped refractive index control layer 102, an InP etch stop layer having a thickness of 10 nm, and an MQW active layer 114 were formed on the n-type InP substrate 101 by metal organic vapor phase epitaxy. Next, a SiO ₂ film was formed on the entire surface using plasma CVD, and a SiO ₂ mask was formed on the MQW active layer 114 region using photolithography and dry etching. Thereafter, unnecessary MQW layers were removed using wet etching. Next, a resist pattern of the diffraction grating was drawn in the DBR region using an electron beam exposure method, and the diffraction grating 115 was manufactured using wet etching. After producing the diffraction grating 115, the resist and the SiO ₂ film were removed, and a p-InP clad layer 103 and a contact layer 104 were grown in this order on the entire surface using a metal organic vapor phase epitaxy method.

Then SiO ₂ is formed on the entire surface to form a SiO ₂ waveguide pattern using photolithography and dry etching. Next, using this pattern as a mask, the p-InP cladding region 103 and the contact region 104 were removed except for the waveguide portion using dry etching and wet etching to form a waveguide stripe. Next, after forming SiO ₂ for separation between waveguides, a SiO ₂ mask was formed using photolithography and dry etching, and the contact region 104 between the electrodes was removed by wet etching.

Next, the waveguide stripe was embedded with BCB 122 using spin coating and heat treatment steps. Further, after removing the SiO ₂ on the stripe, the SiO ₂ insulating film 121 was formed using plasma CVD. Then, the window of the electrode part was formed using photolithography and dry etching. Subsequently, an electrode pattern was formed by lift-off, and a p-electrode 106 made of Au / Zn / Ni was formed using electron beam evaporation. Thereafter, a Ti / Pt / Au electrode 105 was formed as the n electrode 105. Finally, a non-reflective coating of SiO ₂ / TiO ₂ was applied to the end faces of both ends of the DBR laser element using vacuum deposition.

Next, the modulation operation in the present DBR laser will be described. The active layer current control device 117 injects a current into the active layer region 114 to perform laser oscillation. Current is injected into the DBR regions 111a and 111b by the oscillation wavelength control current devices 118 and 119 to adjust the oscillation wavelength. The modulation operation is performed by applying a modulation signal voltage from the voltage control device 116 to the refractive index control region 112. More specifically, the peak with respect to the bias voltage V _b - peak amplitude modulation signal voltage modulated signal is superimposed in the V _PP is applied to the refractive index control region. As a result of the effective refractive index of the refractive index control region changing according to the modulation signal voltage, the frequency modulation operation can be realized by changing the longitudinal mode of the oscillation wavelength.

  Also in this DBR laser, as in the element application example 1, high-efficiency modulation can be obtained by driving with a low bias voltage, and a significant improvement in characteristics of an optical signal processing device including an optical modulator can be realized. In this element application example, the regrowth of the waveguide region becomes unnecessary by using the substrate on which the waveguide and the active layer are grown together. Since a device can be manufactured by one re-growth process, it is advantageous in terms of manufacturing cost. In addition, n doping is effective in reducing the electric resistance in the active layer region 113.

  Further, the following advantages can be obtained by adopting the ridge structure. As described above, in the buried structure, the production in the stripe direction is limited to the [110] direction in order to realize a flat growth surface during the buried growth. However, in the ridge structure, since it is not necessary to consider the problems related to the burying growth, the stripe can also be produced in the [1-10] direction. When the stripe is formed in the [1-10] direction, as described above, the refractive index change due to the electro-optic effect when the bias voltage is applied and the refractive index change due to doping are in the same direction, and the effect of increasing the refractive index change amount is achieved. Can be further increased. Therefore, the increase in the refractive index change is higher in the [1-10] direction than in the [110] direction. Therefore, by adopting a ridge structure for forming the waveguide and further producing a stripe in the [1-10] direction, a laser capable of large frequency modulation and frequency modulation with low power consumption can be realized.

  [Element Application Example 3]: In the above-described two element application examples, a configuration in which a refractive index control region separate from the DBR region is provided and this refractive index control region functions as a phase modulation region has been described. However, the DBR region that controls the oscillation wavelength can also function as a phase modulation region. That is, the present invention can also be applied to a frequency modulation laser that causes the DBR region to function as a refractive index modulation region. In this case, the refractive index control region in the element application examples 1 and 2 can be used for controlling the oscillation wavelength. In the following element application examples, from the viewpoint of the element structure of the DBR laser, the same components as those in the element application examples 1 and 2 are described with the same names, but are used for different purposes in terms of function. I want to be. Hereinafter, description will be made while comparing with device application examples 1 and 2.

  FIG. 22 is a diagram showing a configuration of a frequency modulation type DBR laser having a semi-insulating embedded structure to which the refractive index control layer structure of the present invention is applied. FIG. 22 is a cross-sectional view of a DBR laser waveguide on a plane including the light propagation direction. That is, the y-axis direction in the figure is the light propagation direction, and the mesa stripe is formed in the [110] direction in the y-axis direction. The DBR laser is sequentially manufactured according to a process described later while forming a waveguide and mesa structure pattern.

  The DBR laser 400c is composed of three regions when viewed in the y-axis direction. That is, in the y-axis direction, the distributed Bragg reflecting mirror region (DBR region) 111 having a length of 400 μm, the refractive index control region 112 having a length of 200 μm adjacent to the DBR region 111, and the length adjacent to the refractive index control region 112 are provided. The active layer region 113 is 200 μm. This DBR laser is different from the DBR laser of the element application example 1 including two DBR regions 111a and 111b in that the DBR laser is configured by only one DBR region.

  In the z direction, a waveguide layer having n-type doping is formed in each region on the n-type cladding layer 101. That is, the DBR layer 102 is laminated in the DBR region 111, and the refractive index control layer 102 is laminated in the refractive index control region 112, respectively. In the active layer region 113, a 10-layer multiple quantum well active layer (MQW active layer) 114 having an emission wavelength of 1.55 μm is stacked. A diffraction grating 115 is provided in the waveguide layer 102 of the DBR region 111. Further, in each region, a p-type cladding layer 103 and a contact layer 104 are sequentially stacked, and a p-type electrode 106 and an n-type electrode 105 are provided above and below the DBR laser element 400c, respectively. In order to electrically isolate each of the three regions 111, 112, and 113, the contact layer 104 near the region boundary is removed.

For the p-type electrode in each region, a voltage control device (V) 116 for performing frequency modulation on the DBR region 111 and an oscillation wavelength control current device for performing wavelength adjustment on the refractive index control region 112 Note that (I _P ) 118 is connected to the active layer region 113 by an active layer current control device (I _D ) 117. This is different from the configuration of the element application example 1 in terms of the type of power source that drives each region.

The cross-sectional structure of each region is the same as the element application example 1. 18A and 18B are views showing a cross section of the active layer region and a cross section of the DBR region or the refractive index control region of the DBR laser 400c, respectively. That is, (a) is a view of the active layer region 113 in FIG. 22 as viewed in the xz plane. FIG. 22B is a diagram when the DBR region 111 or the refractive index control region 112 in FIG. 22 is viewed in the xz plane. In each region, a mesa stripe having a waveguide width of 1.5 μm is formed. A p-InP cladding layer 103 having a doping concentration of 1 × 10 ¹⁸ cm ⁻³ is formed on the waveguide 102. Both sides of the active layer stripe and the DBR stripe are each embedded with a semi-insulating InP clad 120. A SiO ₂ insulating film 121 is formed on the cladding layer 120 except for the top of the mesa stripe, and a p-side electrode 106 is formed on the uppermost surface of the element.

This DBR laser is manufactured according to the following procedure. First, the n-type cladding layer 101 and the MQW active layer 114 are formed on the entire surface of the n-type InP substrate using metal organic vapor phase epitaxy. Next, a SiO ₂ film was formed on the entire surface using plasma CVD, and a SiO ₂ mask was formed on the MQW active layer 114 region using photolithography and dry etching. Then, unnecessary MQW active layer was removed using wet etching. Subsequently, the n-doped InGaAsP refractive index control layer 102 and the DBR layer 102 were butt-joint grown using metalorganic vapor phase epitaxy. Next, a resist pattern of the diffraction grating was drawn in the DBR region using the electron beam exposure method, and the diffraction grating 115 was manufactured using wet etching. After producing the diffraction grating 115, the resist and the SiO ₂ film were removed, and the p-cladding layer 103 and the contact layer 104 were grown using metal organic vapor phase epitaxy. Subsequently, a SiO _2, was formed a waveguide pattern of SiO ₂ using photolithography and dry etching.

Next, using this pattern as a mask, waveguide etching was formed using dry etching. Subsequently, a semi-insulating Fe-doped InP layer clad 120 was buried and grown to a stripe height using metal organic vapor phase epitaxy. Next, after forming SiO ₂ for waveguide separation, a SiO ₂ mask was formed using photolithography and dry etching, and the contact region 104 was removed by wet etching. Next, after removing SiO ₂ on the stripe, the SiO ₂ insulating film 121 was formed using plasma CVD. Furthermore, the window of the electrode part was formed using photolithography and dry etching. Subsequently, an electrode pattern was formed by lift-off, and an Au / Zn / Ni p-electrode 106 was formed using electron beam evaporation. Thereafter, a Ti / Pt / Au electrode 105 was formed as an n electrode. Finally, a non-reflective coating of SiO ₂ / TiO ₂ was applied to the end surface of the DBR laser element on the DBR region side using vacuum deposition.

Also in this configuration, a highly efficient frequency modulation operation is possible with respect to a low bias voltage. A current is injected into the active layer region 113 by the active layer current control device 117 to perform laser oscillation. The oscillation wavelength control current device 118 injects current into the refractive index control region 112 to adjust the oscillation wavelength. The modulation operation is performed by applying a modulation signal voltage to the DBR region 111 by the voltage controller 116. More specifically, a voltage modulation signal in which a modulation signal having a peak-to-peak amplitude of voltage amplitude _VPP is superimposed on the bias voltage Vb is applied to the DBR region 111.

The oscillation frequency (oscillation wavelength) of the DBR laser having this configuration varies depending on the mechanism described below. The oscillation wavelength of the DBR laser is determined by the phase matching condition with the reflection spectrum centered on the Bragg wavelength of the DBR. When the refractive index changes in the DBR, the center wavelength determined by the Bragg wavelength of the DBR and the longitudinal mode wavelength of the resonator change. At this time, the change in refractive index caused by the voltage amplitude V _PP applied to the DBR region is _denoted by Δn _eff . The Bragg reflection wavelength (frequency) shift Δf _B is expressed by the following equation, where n is the refractive index of the DBR region and f _B is the frequency corresponding to the Bragg wavelength.

Further, deviation Delta] f _m of the longitudinal mode wavelengths, the effective length L _DBR of the total resonator length L _a11, DBR region, the longitudinal mode frequency When f _m is expressed by the following equation.

When the modulation operation is performed without causing mode jump, the shift (change) in the longitudinal mode wavelength contributes to the actual frequency modulation amount. In other words, the actual modulation frequency amount is determined by Delta] f _m as shown in equation (6). Equation (6) is one in _{which L P} of formula (2) is replaced by _{L DBR.} Therefore, in order to obtain a large amount of frequency modulation, it is important to increase the amount of change Δn _eff / n _eff of the effective refractive index as in the equation (2).

  As described above in detail, according to the present invention, even when a buried structure using a mesa stripe manufactured in the [110] direction of the prior art is used, high-efficiency modulation is obtained by low-voltage driving, and optical Significant improvement in characteristics of the optical signal processing apparatus including the modulator can be realized. The above description has been made from the viewpoint of frequency modulation. However, the frequency change is equivalent to the oscillation wavelength change, and the DBR laser can be applied as a wavelength tunable light source that realizes an ultrafast wavelength switching operation. Further, in this configuration, the refractive index control region 112 is provided for the function of adjusting the oscillation wavelength. However, the modulation operation can be realized even in the configuration without the refractive index control region 112 by the above-described modulation principle. Should be noted.

  [Element Application Example 4] FIG. 23 is a diagram showing another configuration of a frequency modulation type DBR laser having a semi-insulating embedded structure to which the refractive index control layer structure of the present invention is applied. FIG. 23 is a cross-sectional view in a plane including the light propagation direction of the waveguide of the DBR laser. That is, the y-axis direction in the figure is the light propagation direction, and the mesa stripe is formed in the [110] direction in the y-axis direction. The DBR laser is sequentially manufactured according to a process described later while forming a waveguide and mesa structure pattern.

  The DBR laser 400d is composed of three regions in the y-axis direction. That is, in the y-axis direction, a distributed Bragg reflector mirror region (DBR region) 111 having a length of 400 μm, a refractive index control region 112 having a length of 200 μm adjacent to the DBR region 111, and a length of 200 μm adjacent to the refractive index control region 112 Active layer region 113. This DBR laser is different from the DBR laser of the device application example 2 including two DBR regions 111a and 111b in that the DBR laser includes only one DBR region.

  In the z direction, a waveguide layer 102 having n-type doping is formed on the n-type cladding layer 101 in each of the three regions. That is, the DBR layer 102 is laminated in the DBR region 111, and the refractive index control layer 102 is laminated in the refractive index control region 112, respectively. Also in the active layer region 113, a waveguide layer 102 having n-type doping is laminated, and further, a 10-layer multiple quantum well active layer (MQW active layer) 114 having an emission wavelength of 1.55 μm is laminated thereon. A diffraction grating 115 is provided in each DBR layer 102 of the DBR region 111. Further, in each region, a p-type cladding layer 103 and a contact layer 104 are sequentially stacked. A p-type electrode 106 and an n-type electrode 105 are provided above and below the DBR laser element 400d, respectively. In order to electrically isolate each of the three regions 111, 112, and 113, the contact layer 104 near the region boundary is removed.

For the p-type electrode in each region, a voltage control device (V) 116 for performing frequency modulation on the DBR region 111 and an oscillation wavelength control current device for performing wavelength adjustment on the refractive index control region 112 Note that (I _P ) 118 is connected to the active layer region 113 by an active layer current control device (I _D ) 117. This is different from the configuration of the element application example 2 in terms of the type of power source that drives each region.

The cross-sectional structure of each region is the same as in the device application example 2. FIGS. 21A and 21B are views showing a cross section of the DBR region or the refractive index control region and a cross section of the active layer region of the DBR laser 400d, respectively. That is, FIG. 23A is a diagram when the DBR region 111 or the refractive index control region 112 in FIG. 23 is viewed in the xz plane. (B) is the figure which looked at the active layer area | region 113 in FIG. 23 in xz plane. In each region, a mesa stripe having a waveguide width of 2 μm and a height of 1.5 μm is formed. A p-InP cladding layer 103 having a doping concentration of 1 × 10 ¹⁸ cm ⁻³ is formed on the waveguide 102. Both sides of the active layer stripe and the DBR stripe are filled with benzocyclobutene (BCB) 122, respectively, and an SiO ₂ insulating film 121 is formed thereon. A p-side electrode 106 is formed on the uppermost surface of the element.

This DBR laser is manufactured according to the following procedure. First, an n-type doped refractive index control layer 102, an InP etch stop layer having a thickness of 10 nm, and an MQW active layer 114 were formed on the n-type InP substrate 101 by metal organic vapor phase epitaxy. Next, a SiO ₂ film was formed on the entire surface using plasma CVD, and a SiO ₂ mask was formed on the MQW active layer 114 region using photolithography and dry etching. Then, unnecessary MQW active layer was removed using wet etching. Next, a resist pattern of the diffraction grating was drawn in the DBR region using an electron beam exposure method, and the diffraction grating 115 was manufactured using wet etching. After producing the diffraction grating 115, the resist and the SiO ₂ film were removed, and the p-InP cladding layer 103 and the contact layer 104 were grown in this order on the entire surface by using a metal organic vapor phase epitaxy method.

Then SiO ₂ is formed on the entire surface to form a SiO ₂ waveguide pattern using photolithography and dry etching. Next, using this pattern as a mask, dry etching and wet etching were used to remove the p-InP cladding region 103 excluding the waveguide portion, thereby forming a waveguide stripe. Next, after forming SiO ₂ for separation between waveguides, a SiO ₂ mask was formed using photolithography and dry etching, and the contact region 104 between the electrodes was removed by wet etching.

Next, the waveguide stripe was embedded with BCB 122 using spin coating and heat treatment steps. Next, after removing SiO ₂ on the stripe, the SiO ₂ insulating film 121 was formed using plasma CVD. Then, the window of the electrode part was formed using photolithography and dry etching. Subsequently, an electrode pattern was formed by lift-off, and a p-electrode 106 made of Au / Zn / Ni was formed using electron beam evaporation. Thereafter, a Ti / Pt / Au electrode 105 was formed as the n electrode 105. Finally, a non-reflective coating of SiO ₂ / TiO ₂ was applied to the end face on the DBR region side of the DBR laser element using vacuum deposition.

Also in this configuration, a highly efficient frequency modulation operation is possible for a low bias voltage. A current is injected into the active layer region 113 by the active layer current control device 117 to perform laser oscillation. The oscillation wavelength control current device 118 injects current into the refractive index control region 112 to adjust the oscillation wavelength. The modulation operation is performed by applying a modulation signal voltage to the DBR region 111 by the voltage controller 116. More specifically, a voltage modulation signal in which a modulation signal having a peak-to-peak amplitude of voltage amplitude _VPP is superimposed on the bias voltage Vb is applied to the DBR region 111.

  Similar to the device configuration example 3, even when a buried structure using a mesa stripe manufactured in the [110] direction according to the prior art is used, high-efficiency modulation can be obtained by driving a low bias voltage, and light including an optical modulator can be obtained. Significant improvement in characteristics of the signal processing device can be realized. Although description has been made so far from the viewpoint of frequency modulation, the change in frequency is equivalent to the change in oscillation wavelength, and the DBR laser can be applied to a wavelength tunable light source that realizes an ultrafast wavelength switching operation. Further, in this configuration, the refractive index control region 112 is provided for the function of adjusting the oscillation wavelength. However, the modulation operation can be realized even in the configuration without the refractive index control region 112 by the above modulation principle. Should be noted.

  [Other device application examples]: In each of the above-described device application examples 1-4, a refractive index control region for performing frequency modulation is provided inside a resonator of a DBR laser including DBR reflectors on both sides or one side. The structure was shown. The application example of the modulation element having the refractive index control layer of the present invention is not limited to the above-described structure. In the device application examples 1 and 2, the four-electrode DBR laser structure including the front and rear DBR reflecting mirrors is shown, but the number of functional sections is not limited to four, and a structure with a different number of sections may be used. For example, a plurality of refractive index control regions may be provided in order to provide mode suppression and wavelength fine adjustment functions. Further, as in element application examples 3 and 4, a structure in which a DBR structure is provided only on one side of the element may be used.

  A semiconductor optical amplifier (SOA) region may be integrated before and after the modulation element including the refractive index control layer of the present invention. In addition, although the semi-insulating buried structure and the ridge type structure are illustrated as the element structures of the above-described embodiments, it is needless to say that the element structure can be applied to other waveguide structures such as a pn buried structure and a high mesa structure. Although an example using InGaAsP as the material used has been shown, the present invention can also be applied to materials such as InAlGaAs and GaInNAs.

  The present invention is not limited to an oscillator including a DBR structure, and can be applied to all structures having a refractive index control region for controlling a frequency and a wavelength in a resonator. For example, in place of the DBR reflector, wavelength selectivity such as a super-period diffraction grating type DBR (SSG-DBR), a sample type diffraction grating type DBR (SG-DBR), a ring resonator, an arrayed waveguide diffraction grating (AWG), etc. Any type can be used as long as it has a resonator. Further, in each of the above-described embodiments, an example of an element configuration in which the active layer, the reflecting mirror, and the refractive index control region are monolithically integrated has been described. However, the present invention is also applicable to an external resonator type element configuration in which the reflecting mirror is provided outside. it can.

  Furthermore, in the above-described application example of the element of the present invention, an application example to a frequency modulation light source capable of high-speed modulation has been described, but the application example is not limited to a frequency modulation laser. Needless to say, the present invention can be applied to many optical control elements and optical signal processing devices that require a large amount of change in refractive index with application of a bias voltage, such as a Mach-Zehnder type modulator.

  [Application Example to Modulated Light Source]: A specific example of an optical signal transmission method using a frequency modulated light source to which a modulation element including a refractive index control layer of the present invention is applied is shown below.

  FIG. 24 is a diagram showing a configuration of an optical modulation signal transmission system using a frequency modulation light source including a refractive index control layer of the present invention. An optical modulation signal generator 201 includes a light source 202 that frequency modulates an optical signal using an NRZ signal from an information signal source, and a frequency filter 203 that is connected to the output side of the light source 202 and has a function of converting a frequency modulation signal into an intensity modulation signal. Including. The optical signal output from the optical modulation signal generator 201 is received by the optical receiver 205 after propagating through the optical fiber 204. The modulation element including the refractive index control layer of the present invention is applied to the frequency modulation light source 202 of the light modulation signal generation device 201. Specifically, the DBR lasers from the element application example 1 to the element application example 4 can be applied to the frequency modulation light source.

  FIG. 25 is a diagram illustrating a configuration of a frequency filter used in the present optical modulation signal generation device. The frequency filter 203 has a configuration in which two etalon filters 301a and 301b are connected in series. The etalon filters 301a and 301b are provided with mirrors 303a and 303b at the input and output ends of the resonator 302a, and mirrors 303c and 303d at the input and output ends of the resonator 302b, respectively. The FSR (Free Spectral Range) of the frequency filter 203 is 100 GHz.

  FIG. 26 is a diagram illustrating transmission spectrum characteristics and phase characteristics of the frequency filter. FIG. 26A shows transmission spectrum characteristics, and FIG. 26B shows phase characteristics. Referring to (a) and (b), it can be seen that a phase change occurs with a change in transmittance, that is, a change in intensity, near the maximum peak of the transmittance. The frequency filter used in the present embodiment may be a frequency filter having an FM / AM conversion function. Specifically, there are a composite resonance etalon, an arrayed waveguide grating, a lattice filter, a Mach-Zehnder interferometer, a ring resonator filter, a fiber Bragg grating, and the like. When the frequency filter has a dispersion characteristic in addition to the FM / AM conversion function, it can also have a dispersion compensation function by setting the dispersion characteristic of the frequency filter to a value opposite to the dispersion value of the optical fiber. Here, the FM / AM conversion function of the frequency filter will be described.

  FIG. 26A illustrates the setting conditions for the operating frequency and the transmission frequency of the frequency filter when the optical modulation signal generating apparatus 202 is operated at a transmission rate of 20 Gbps. The carrier frequency is 193.4 THz (1.55 μm). The transmittance peak frequency of the frequency filter is set so as to be on the high frequency side of 5 GHz with respect to “1” of the NRZ signal. Further, the frequency of the optical signal corresponding to “0” of the NRZ signal is set to be 10 GHz lower than the NRZ signal “1”. Note the relationship between the frequencies corresponding to “0” and “1” displayed by arrows on the horizontal axis in FIG.

  As can be seen from the transmittance characteristic of FIG. 26A, the transmittance (approximately −5 dB) of the optical signal corresponding to “1” of the NRZ signal is the transmittance of the optical signal corresponding to “0” of the NRZ signal. (Approximately -20 dB). That is, there is a difference of 15 dB between the transmittance of the optical signal corresponding to “1” of the NRZ signal and the transmittance of the optical signal corresponding to “0” of the NRZ signal. For this reason, when the frequency modulation signal passes through the frequency filter 203, an intensity modulation signal corresponding to the input frequency modulation signal can be output.

  The condition for setting the half value of the transmission bit rate to the value of the modulation width of the frequency modulation like the setting condition described with reference to FIG. 26 is called MSK (Minimum Shift Keying). In the case of MSK, the phase difference between adjacent pulses is π. For this reason, the MSK condition has an effect of suppressing overlapping of pulse signals generated due to dispersion when the optical signal propagates through the optical fiber. Further, if the value of the modulation width of the frequency modulation is set between ¼ and ¾ of the bit rate transmitted by the optical signal, “1-0-1” is adjacent when the signal changes. The phase difference between pulses has an opposite sign. This setting condition has an effect of suppressing signal overlap due to dispersion of pulses after propagation through the optical fiber.

  As already described, when the element application example 1 of the present invention is used, the modulation voltage required to obtain the frequency modulation amount of 10 GHz is 0.75 V under the driving condition that satisfies the MSK condition described in FIG. On the other hand, in the structure using the prior art, the modulation voltage requires 1.85 V, and the drive bias voltage can be greatly reduced by employing the refractive index control layer of the present invention.

  27 and 28 are diagrams showing the waveforms of the modulated optical signal before and after passing through the frequency filter, respectively. FIG. 27 is a diagram showing a temporal change in the intensity component of the optical signal before and after transmission when a 20 Gbps NRZ frequency modulation signal is generated from the signal source 202 and transmitted through the frequency filter 203. The intensity component is shown together with the signal sequences “1” and “0”. FIG. 28 is a diagram illustrating a temporal change in the frequency component of the optical signal before and after transmission. The frequency components are shown together with signal sequences “1” and “0”. The vertical axis indicates the frequency deviation from the carrier frequency.

  The input optical signal from the light source 202 to the frequency filter 203 does not have an intensity modulation component but has only a frequency modulation component. This input optical signal passes through the frequency filter 203 and is converted into an NRZ intensity modulation signal by the FM / AM conversion function. Further, the frequency component of the input optical signal has a frequency component synchronized with the signal sequence before passing through the filter. On the other hand, after passing through the filter, the time (period) in which the frequency modulation component is in the vicinity of 0 GHz has increased, and it can be seen that large frequency fluctuation occurs only when the signal is “0”. That is, during the time when the optical signal exists with a large amplitude, the frequency fluctuation is suppressed, so that there is substantially no frequency fluctuation, and it is possible to suppress the deterioration of the transmission characteristics due to the dispersion of the optical fiber. Next, specific transmission characteristics will be described.

  FIG. 29 is a diagram illustrating an eye opening of an optical signal output from the optical modulation signal generation device. The eye opening when the optical modulation signal generated by the optical modulation signal generation device 201 is transmitted through the optical fiber 204 having a dispersion value of 16.3 ps / nm / km for a section of 50 km is shown. FIG. 29A shows the eye opening of the optical signal output from the optical modulation signal generating apparatus 201, and FIG. 29B shows the eye opening of the optical signal received by the optical receiving apparatus 205. Even after the optical fiber is propagated through the 50 km section, a sufficiently clear eye opening is obtained. As described above, by applying the refractive index control layer of the present invention to a frequency modulation light source, it is possible to realize a modulation light source that operates at a low bias voltage and low power consumption and can perform high-speed and long-distance transmission.

  As described above in detail, according to the present invention, it is possible to realize a refractive index modulation element that can be driven with a low bias voltage and can perform high-efficiency and high-speed modulation. It is also possible to realize a refractive index control structure that can be easily integrated with a laser. Further, by applying the present invention to a DBR laser, a frequency modulation operation with low voltage drive can be realized. Furthermore, by using this DBR laser as a frequency modulation light source and performing FM / AM conversion of the frequency modulation light using an optical filter, an optical transmitter capable of long-distance transmission with low power consumption can be realized. .

  The present invention can be used for an optical signal processing device used in an optical communication system, for example, a communication light source, an optical signal modulation device, and the like.

(A) is a conceptual diagram which shows the light source containing the modulation element for communication. (B) is a figure which shows the structure of the refractive index control area | region in a prior art. (A) shows the band structure of the refractive index control area | region of a prior art, (b) is a figure which shows the bias voltage dependence of the potential of a refractive index control area | region. It is a figure which shows the relationship between the bias voltage and the effective refractive index change amount in a prior art. It is a figure which shows the structure of the refractive index control area | region of the structural example 1 which concerns on this invention. (A) shows the band structure of the refractive index control region of Structural Example 1 of the present invention, and (b) shows the bias voltage dependence of the potential of the refractive index control region. It is a figure which shows the relationship between the bias voltage in the structural example 1, and the effective refractive index change amount resulting from an electro-optic effect. It is a figure which shows the relationship between the bias voltage in Example 1 of a structure, and carrier concentration. It is a figure which shows the relationship between the bias voltage in the structural example 1, and the effective refractive index change amount resulting from a plasma effect. It is a figure which shows the bias voltage dependence of the effective refractive index change amount which combined the electro-optic effect and carrier effect in a [110] direction. It is the figure which showed the doping concentration dependence of the effective refractive index variation | change_quantity in a [110] direction. It is a figure which shows the bias voltage dependence of the effective refractive index variation | change_quantity in a [1-10] direction. It is the figure which showed the doping concentration dependence of the effective refractive index variation | change_quantity in a [1-10] direction. It is a figure which shows the structure of the refractive index control area | region of the structural example 2 which concerns on this invention. It is a figure which shows the structure of the refractive index control area | region of the structural example 3 which concerns on this invention. It is a figure which shows the energy band figure of the refractive index control area | region by the structural example 3. FIG. It is a figure which shows the energy band figure of the refractive index control area | region by the structural example 4. FIG. It is a structural diagram of a frequency modulation type DBR laser having a semi-insulating embedded structure to which the refractive index control region of the present invention is applied. (A) is sectional drawing of the active layer area | region of this DBR laser, (b) is sectional drawing of a DBR area | region or a refractive index control area | region. It is a figure which shows the relationship between the frequency modulation amount and applied voltage in the DBR laser element of element application example 1. It is a structural diagram of a frequency modulation type DBR laser having a semi-insulating embedded structure to which the refractive index control region of the present invention is applied. (A) is sectional drawing of a DBR area | region or a refractive index control area | region, (b) is sectional drawing of the active layer area | region of this DBR laser. It is a figure which shows the structure of the frequency modulation type DBR laser which has a semi-insulation embedding structure to which the refractive index control layer structure of this invention is applied. It is a figure which shows the other structure of the frequency modulation type DBR laser which has the semi-insulation embedding structure to which the refractive index control layer structure of this invention is applied. It is a figure which shows the structure of the optical modulation signal transmission system using the frequency modulation light source containing the refractive index control layer of this invention. It is a block diagram of the frequency filter used with an optical modulation signal generation apparatus. (A) is a transmission spectrum characteristic of a frequency filter, (b) is a figure which shows a phase characteristic. It is a figure which shows the time waveform of the amplitude of the modulation | alteration optical signal before and behind permeate | transmitting a frequency filter. It is a figure which shows the time waveform of the frequency component of the modulation | alteration optical signal before and behind permeate | transmitting a frequency filter. It is a figure which shows the eye opening of the optical signal output from the optical modulation signal generation apparatus.

Explanation of symbols

101 n-type cladding layer (n-type cladding substrate)
102 bulk refractive index control layer 103 p-type cladding layer 104 contact layer 105 n-side electrode 106 p-side electrode 107 n-doped bulk refractive index control layer 108 n-doped multiple quantum well refractive index control layer 109 n-doped bulk refractive index control layer 110 n-doped multiple Quantum well refractive index control layer 111, 111a, 111b DBR region 112 Refractive index control region 113 Active layer region 114 Multiple quantum well active layer 115 Diffraction grating 116 Voltage control device 117 Active layer current control device 118 Oscillation wavelength control current device 119 Oscillation wavelength Control current device 120 Semi-insulating InP clad 121 SiO ₂ insulating film 122 BCB buried layer 201 Optical modulation signal generator 202 Frequency modulation light source 203 Frequency filter 204 Optical fiber 205 Optical receiver 301a, 301b Etalon filter 302a, 302b Resonator 303a, 303b, 303c, 303d Reflector 400a, 400b, 400c, 400d DBR laser

Claims (10)

In a light control device that performs refractive index modulation using a control voltage,
a p-cladding layer;
an n-clad layer;
A refractive index control layer including a doping layer sandwiched between the n clad layer and the p clad layer, wherein the refractive index of the refractive index control layer is modulated by the control voltage. Light control device.   The doping concentration of the doping layer is in the range of 1/10 to 1 times the doping concentration of the n-type cladding layer or the p-type cladding layer to which the same kind of doping is applied. Item 4. The light control device according to Item 1.   The light control device according to claim 1, wherein the doping layer is a bulk layer or a multiple quantum well layer.   The doping layer includes a bulk layer that is disposed adjacent to one of the n-cladding layer and the p-cladding layer and is doped with the same kind as the one, and a multiple quantum well layer disposed adjacent to the bulk layer. The light control device according to claim 1, wherein the light control device has a laminated structure including the light control device.   5. The light control device according to claim 1, wherein the refractive index control layer has a stripe structure formed in a crystal orientation of [110].   The light control device according to claim 1, wherein the doping layer is n-type doped or p-type doped.   7. The modulation signal having a voltage amplitude that is driven between the p-clad layer and the n-clad layer in a forward bias direction and a reverse bias direction is applied as the control voltage. The light control device according to claim 1. A DBR region;
A refractive index control region comprising the refractive index control layer according to claim 1;
A semiconductor laser comprising: an active layer region.   9. The semiconductor laser according to claim 8, wherein the control voltage is applied to the refractive index control region, and an oscillation wavelength control current is supplied to the DBR region.   10. A light modulation device comprising: the semiconductor laser according to claim 8; and a frequency filter that converts a frequency modulated optical signal output from the semiconductor laser into an intensity modulated optical signal.

Images