JP3505509B2 - Semiconductor light emitting device, semiconductor light emitting device, and method for modulating semiconductor light emitting device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor light emitting device in which output light having a wavelength different from that of input light is modulated at high speed by high speed modulated input light, a semiconductor light emitting device using this semiconductor light emitting device, and further this semiconductor. The present invention relates to a modulation method for modulating a light emitting element at high speed.

[0002]

2. Description of the Related Art Due to the rapid development of optical technology, large-capacity optical transmission technology is about to be applied not only to backbone networks and optical LANs but also to optical access systems, microwave millimeter-wave photonics, home networks and the like. In these systems, the cost is far more important than the backbone system, and a high-performance and low-cost light source is required. Furthermore, in applications such as home networks, it is considered that not only the near-infrared region used in conventional optical communication but also the visible light region will be used. Modular light sources are needed.

However, the operating speed of the conventional semiconductor light source is
It is limited by electrical capacity, resistance, etc., and the optical communication wavelength is limited to about 20 GHz, and the visible wavelength region is limited to several tens MHz to several GHz. The problems of the related art will be described below with respect to examples of microwave photonics and home networks.

At present, the mobile Internet is rapidly spreading, and in the future, 22 GHz will be used for high speed wireless access, ultra high speed wireless LAN, wireless home link, ITS, etc.
It is considered that band expansion using millimeter waves such as band, 26 GHz band, 38 GHz band, and 60 GHz band is required. The locality of the narrow range of millimeter waves is also an important factor in increasing the number of subscribers. For millimeter-wave transmission, ROF (Radio OnFiber) using optical fiber, which has small attenuation even after long-distance transmission and enables large-capacity transmission by multiplexing
The introduction of technology is considered essential, and the realization of that requires a low-cost analog light source for millimeter-wave transmission.

In the semiconductor laser, the relaxation oscillation frequency increases as the carrier injection level increases, so that the 3 dB modulation band also increases. However, since the gain saturation occurs as the optical power becomes stronger, the band decreases conversely when the injection level is too high. The limit band due to gain saturation of the semiconductor laser in the optical communication wavelength band is usually several tens GHz. However, in most cases, the 3 dB band of the device is rather limited by the CR time constant. When the active layer (light emitting layer of the semiconductor laser) is a quantum well, carriers are accumulated in an optical waveguide layer or an SCH layer provided to enhance optical confinement in the active layer, so-called "carrier transport effect", The modulation band may be limited.

Due to these factors, the 3 dB band of the semiconductor laser for communication is limited to a range of several GHz to 20 GHz. Although it can operate at higher speed than semiconductor lasers of other wavelengths, the band is still insufficient for optical modulation in the millimeter wave band (22 GHz or higher). Therefore, in the millimeter wave band, a semiconductor electro-absorption (EA) type optical modulator or LiNbO _{3 is used.}
Experiments using a Mach-Zehnder interferometer (MZ) type optical modulator such as the above are under way. However, the millimeter wave band (22 GHz
High speed optical modulator module operating at ~ 60 GHz)
It is difficult to mount the high frequency drive circuit and the connection part, and it is not practical because it is expensive and the mass productivity is low.

On the other hand, in home links and optical LANs, expectations for low cost optical links using plastic optical fibers (POF) are increasing. The low loss wavelength band of POF includes a 650 nm band, a 570 nm band, and a 500 nm band. A light emitting diode (LED) is used as a light source when the modulation speed is relatively low, and a semiconductor laser is used when the modulation speed is high.

Since the semiconductor laser in the visible region has a large electrode resistance, the donor and acceptor levels are deep, and the hetero barriers that carriers must pass through are large, the semiconductor laser has a series resistance higher than that of the semiconductor laser in the near infrared region. Tends to grow. In addition, because of the structure, the active layer (light emitting layer) has a wide stripe width and mesa width, and the number of carriers piled up at a heterojunction interface with large discontinuities is large, so the parallel capacitance tends to be larger than in the near infrared semiconductor laser. There is. Therefore, even if the relaxation frequency is increased by high injection, it is difficult to perform high-speed modulation of several GHz or more due to the limitation due to the CR time constant of the power feeding system.

In any case, driving circuits and mountings for high-frequency signals and high-speed signals cause high cost.

[0010]

As described above, the operating speed of the conventional semiconductor light source is limited by the phenomena such as the electric CR time constant and the carrier transport effect. The limit is about 20 GHz, and several tens of MHz to several GHz in the visible wavelength range. Further, although it was possible to manufacture an optical modulator capable of operating in the millimeter wave band of several tens GHz at the optical communication wavelength, it was not practical in terms of mass productivity and cost.

The present invention has been devised in view of the above problems, and an object thereof is to use it as a semiconductor light source capable of high-speed modulation of several GHz to several tens GHz (millimeter wave band). An object of the present invention is to provide a semiconductor light emitting element that can be used, a semiconductor light emitting device using the semiconductor element, and a modulation method for modulating the semiconductor light emitting element.

[0012]

(Structure) In order to solve the above problems, the present invention adopts the following structure. That is, according to the present invention, a light emitting region having a first semiconductor layer to be a light emitting layer and second and third semiconductor layers having a forbidden band width wider than the first semiconductor layer formed so as to sandwich the light emitting region. When,
A semiconductor light emitting device comprising means for electrically injecting electrons and holes into a first semiconductor layer to emit light, wherein at least a fourth semiconductor layer and a fourth semiconductor layer are provided in the vicinity of the first semiconductor layer. A quantum well layer including a fifth semiconductor layer having a narrower forbidden band than the semiconductor layer is formed, and the quantum well layer has an intersubband absorption wavelength different from the emission wavelength of the first semiconductor layer. It is a semiconductor light emitting device characterized in that

Preferred embodiments of the semiconductor light emitting device are as follows. Between the quantum well layer and the second and third semiconductor layers, carriers excited in the excitation subband due to intersubband absorption overflow into the second and third semiconductor layers (cladding layer, optical waveguide layer, etc.). Means are provided to prevent this. Specific examples include:
There are hetero barriers for corresponding carriers, multiple quantum barrier layers with superlattice structure, and the like.

Further, according to the present invention, a light emitting region in which a plurality of first quantum well layers having a first semiconductor layer to be a light emitting layer are stacked and arranged, and a first semiconductor formed by sandwiching the light emitting region. What is claimed is: 1. A semiconductor light emitting device, comprising: second and third semiconductor layers having a forbidden band width wider than that of a layer; and means for electrically injecting electrons and holes into the first semiconductor layer to emit light. Second quantum well layers each including at least a fourth semiconductor layer and a fifth semiconductor layer having a bandgap narrower than that of the fourth semiconductor layer are formed between the first quantum well layers. Is a semiconductor light emitting device characterized by having an intersubband absorption wavelength different from the emission wavelength of the first quantum well layer.

The preferred embodiments of the semiconductor light emitting device are as follows. Between the second quantum well layer and the second and third semiconductor layers, carriers excited in the excitation subband due to intersubband absorption are contained in the second and third semiconductor layers.
Means are provided for preventing the semiconductor layer from overflowing into the semiconductor layer (clad layer, optical waveguide layer, etc.). Specific examples thereof include a hetero barrier for a corresponding carrier, a multiple quantum barrier layer having a superlattice structure, and the like.

Further, according to the present invention, means for injecting the modulated signal light within the band of the intersubband absorption wavelength of the quantum well layer having the fourth and fifth semiconductor layers is provided to the semiconductor light emitting device having the above structure, The semiconductor light emitting device is characterized in that the emitted light is modulated by modulating the ratio of carriers that contribute to the light emission of the first semiconductor layer by inputting the modulated light.

The following are preferred embodiments of the semiconductor light emitting device.

(1) The semiconductor light emitting element and a light source that emits light having an absorption wavelength between subbands are integrated on the same substrate. This integration may be monolithically integrated on the same substrate, or elements formed on another substrate may be hybridly integrated.

(2) Means for coupling light having an absorption wavelength between sub-bands is provided from the outside of the semiconductor light emitting device to the second quantum well layer. Specific examples of the means for coupling light include an end face of an optical waveguide structure formed by cleavage or etching, a low reflectance coating for incident light, an optical fiber, a lens, an optical filter, an optical coupler, a multiplexer, and a polarization device. Beam splitter, optical circulator, optical isolator,
There are optical waveguides and diffraction gratings.

Further, according to the present invention, with respect to the semiconductor light emitting device having the above structure, the modulated signal light within the band of the absorption wavelength between subbands of the quantum well layer having the fourth and fifth semiconductor layers and the modulated signal light are provided. A means for injecting CW light having a different center frequency by a predetermined frequency is provided, and the emitted light is emitted by modulating the ratio of carriers contributing to light emission in the first semiconductor layer by the beat of the modulated signal light and the CW light. A semiconductor light emitting device characterized by being modulated.

The following are preferred embodiments of the semiconductor light emitting device. (1) The semiconductor light emitting device and a light source that emits light having an absorption wavelength between sub-bands are integrated on the same substrate.
This integration may be monolithically integrated on the same substrate, or elements formed on another substrate may be hybridly integrated.

(2) A means for coupling light having an absorption wavelength between subbands from the outside of the semiconductor light emitting device is provided to the second quantum well layer. Specific examples of the means for coupling light include an end face of an optical waveguide structure formed by cleavage or etching, a low reflectance coating for incident light, an optical fiber, a lens, an optical filter, an optical coupler, a multiplexer, and a polarization device. Beam splitter, optical circulator, optical isolator,
There are optical waveguides and diffraction gratings.

The present invention also provides a semiconductor light-emitting device modulation method for modulating emitted light when driving a semiconductor light-emitting device having the above structure, wherein a sub-band of a quantum well layer having fourth and fifth semiconductor layers is provided. A modulation method characterized by modulating the emission light in the semiconductor light-emitting element by injecting a modulation signal light in the inter-absorption wavelength band to modulate the ratio of carriers contributing to the emission of the first semiconductor layer. is there.

The following is a preferred embodiment of the above-mentioned modulation method. (1) The modulated signal light is modulated in an analog manner. (2) The power of the input modulated signal light is strong enough to partially saturate the intersubband absorption.

The present invention also provides a semiconductor light emitting device modulation method for modulating emitted light when driving a semiconductor light emitting device having the above-mentioned structure, wherein a subband of a quantum well layer having fourth and fifth semiconductor layers is used. The modulated signal light within the band of the inter-absorption wavelength and the CW light having a center frequency different from the modulated signal light by a predetermined frequency are injected, and the beat of the modulated signal light and the CW light causes The modulation method is characterized in that the emitted light in the semiconductor light emitting element is modulated by modulating the ratio of carriers contributing to light emission.

The following are preferred embodiments of the modulation method. (1) The modulated signal light is modulated in an analog manner. (2) The power of the injected CW light is strong enough to partially saturate the inter-subband absorption, and the power of the modulated signal light is sufficiently smaller (at least two digits) than this CW light.

(Operation) When the semiconductor light emitting device of the present invention is a semiconductor laser, electrons and holes are accumulated in the first semiconductor layer by a CW current to form a population inversion as in the conventional semiconductor light emitting device. , Laser oscillation due to stimulated emission occurs.
The total carrier density in the active layer in the laser oscillation state hardly changes, and the number of photons corresponding to the number of carriers (pair of electrons and holes) injected in excess of the oscillation threshold state is output. .

In this state, when the signal light within the intersubband absorption wavelength band is injected into the quantum well layer having the fourth and fifth semiconductor layers, the fifth semiconductor layer is absorbed by the intersubband absorption of electrons or holes. The carrier distribution between the sub-bands of the (well layer) is modulated, and as a result, the proportion of carriers contributing to light emission of the first semiconductor layer in equilibrium with the base sub-band of the well layer is also modulated. The modulation efficiency is such that when the power of the input signal light partially saturates the absorption between subbands,
The highest.

At this time, if the carriers excited from the lower base subband to the upper excitation subband do not overflow into the cladding layer or the optical waveguide layer (second and third semiconductor layers), the carriers in the active layer. The total number of
This modulation is almost unaffected by carrier lifetime, CR time constant, and carrier transport effect.

The intersubband relaxation time is about three orders of magnitude faster than the interband relaxation time (carrier life), and the time constant is around 1 ps. In particular, when the semiconductor quantum well layer is made of a nitride semiconductor, the time constant is on the order of several hundred fs, and extremely fast response can be realized. The cutoff frequency fc of the carrier distribution modulation between subbands is generally expressed by 1 / (relaxation time × 2π), and therefore 80 GHz when the relaxation time is 2 ps and 800 G when the relaxation time is 200 fs.
It is wide band at Hz.

If the carrier tunneling coupling and the like are properly designed, the carrier moving speed between the first semiconductor layer and the fifth semiconductor layer can be set to several ps or less, so that the carrier transfer is required. Even with time taken into consideration, modulation of several tens of GHz is possible.

The relaxation oscillation frequency can be set to several tens GHz by devising the laser structure and appropriately setting the bias current. However, since the relaxation oscillation frequency is greatly reduced when it is transferred near the threshold value, the present invention has a relatively high bias condition as compared with the case where digital modulation is performed up to or below the threshold value or near the threshold value. Great effect when analog modulation is performed.

When light is input at two wavelengths within the absorption band between subbands, if the beat frequencies of these two input lights are equal to or lower than the cutoff frequency (several tens of GHz) due to absorption between subbands, The number of carriers contributing to light emission of the first semiconductor layer is modulated at this beat frequency. Therefore, one input light is CW light and the other input light is several hundred MHz
When modulated with, the light modulated at the intermediate frequency (beat frequency) in the millimeter wave band is output.

At this time, only when the power of the CW light is strong enough to partially saturate the absorption between sub-bands, and the power of the modulated signal light is sufficiently smaller than this CW light, the efficiency is high and the linearity is good. Modulation is possible. If the power of the modulated signal light is too strong, the linearity is lost and the harmonic component increases. Moreover, the spectrum of the carrier component of the output light is also widened by the baseband modulation by the modulated signal light.

Thus, according to the present invention, it is possible to realize a semiconductor light emitting device capable of ultra-high-speed modulation exceeding the electrical modulation limit, and to realize a semiconductor light emitting device operating in the several GHz band to the millimeter wave band. . The power of the emitted light changes due to the modulation, but at the same time, the oscillation frequency and the phase thereof also change. Therefore, the present invention can be applied to not only the intensity modulation but also the frequency modulation and the phase modulation.

In the case where the present invention is applied to the speedup of a semiconductor light emitting device such as a visible semiconductor laser which can only electrically modulate at 1 GHz or less due to CR limitation or the like, in the first semiconductor layer and the first semiconductor layer, It is not necessary to increase the frequency limitation due to carrier balance between the semiconductor layers of No. 5 and the relaxation oscillation frequency so much, and a response of several GHz is sufficient.

The present invention is directed to amplified stimulated emission light (AS
E) semiconductor light emitting device, light emitting diode (LE)
It can also be applied to D) and the like. In this case, the mechanism for clamping the carrier density of the active layer does not work. In particular, the LED is affected by the carrier life. However, in the case where the carrier life is short and the electric CR time constant and the carrier transport effect are the dominant limiting factors of the response speed, the present invention can remarkably improve the response speed.

[0038]

DETAILED DESCRIPTION OF THE INVENTION The details of the present invention will be described below with reference to the illustrated embodiments.

(First Embodiment) FIG. 1 shows a first embodiment of the present invention.
Of the semiconductor light-emitting device (distributed feedback type (DF
(B) A semiconductor laser) is a diagram schematically showing a band structure near a light emitting region 1. Note that the thickness, the scale of energy, the bending degree of the band, etc. are not accurate because they are drawn for easy understanding.

The light emitting region (active layer) 1 is made of InGaA.
A first quantum well layer 4 including an sP well layer 2 (first semiconductor layer) and an InGaAsP barrier layer 3 having a band gap wider than the well layer 2, and two InGaAs well layers (fifth semiconductor layer) 5 And a coupled quantum well layer (second quantum well layer) 8 in which the thin strained AlAs intermediate barrier layer 6 formed between them is sandwiched between AlAsSb barrier layers (fourth semiconductor layer) 7.
It is laminated alternately. An n-type InGaAsP optical waveguide layer (second semiconductor layer) 9 is provided below the light emitting region 1, and a p-type InGaAsP optical waveguide layer (third semiconductor layer) 1 is provided above the light emitting region 1.
0 is stacked.

The outermost layers 7a and 7b of the AlAsSb barrier layer 7 are thicker than the other barrier layer portions,
The electrons in the excited subband are prevented from overflowing into the cladding layer. 1 shows the case where the potential (valence band) of holes in the InGaAs well layer 5 is higher than the valence band of the AlAsSb barrier layer 6 (type II quantum well). The heights of can be reversed (type I quantum well).

When there is no tunneling coupling with the InGaAsP well layer 2, a total of four subbands are formed in the conduction band of each coupled quantum well layer 8 so that two subbands are adjacent to each other. The energy difference from the lowest energy base subband to the highest energy subband is about 0.8 eV, and intersubband absorption occurs near the wavelength of 1.55 μm. The fact that inter-subband absorption in the 1.55 μm band can be realized with a coupled quantum well having such a structure is described in the literature (for example,
A. Neogi, et al., IEEE Photonics Technology Letter
s, Vol. 11, No. 6, pp. 632-634).
The band-to-band absorption of the coupled quantum well layer 8 does not affect the wavelengths of 1.3 μm and 1.55 μm considered here, if the well layer is thinned and the absorption edge wavelength is set to 1.2 μm or less.

The lowest subband of the conduction band of each InGaAsP well layer 2 and the two lower subbands of the coupled quantum well layer 8 adjacent to each other have almost the same energy, and are coupled by tunneling to form a plurality of levels. Is divided into
However, since the energies of these levels are close to each other, these levels will be collectively treated as one miniband in the following description.

FIG. 2 is a schematic sectional view taken along the light emitting direction of the DFB semiconductor laser. Below the n-type InGaAaP optical waveguide layer 9, an n-type InP clad layer 11 and an n-type In are formed.
There is a P substrate 12, and an n-type ohmic electrode 13 is formed below the n-type InP substrate 12. n-type InGaA
At the interface between the sP optical waveguide layer 9 and the n-type InP clad layer 11, a diffraction grating 14 for oscillating a single mode in the wavelength band of 1.3 μm is formed. In addition, p-type InGaAs
A p-type InP clad layer 15 and a p-type InP clad layer 15 are formed on the P optical waveguide layer 10.
Type InGaAsP contact layer 16 is laminated,
A p-type ohmic electrode 17 is formed on top of it.

A quarter wavelength phase shift portion 18 is formed near the center of the diffraction grating 14. Further, low-reflection coating films 19 are formed on both end surfaces. Electrode 13,
A power supply circuit is connected to 17, and the InGaAsP well layer 2 produces a gain in the vicinity of a wavelength of 1.3 μm by current injection and oscillates in a single mode due to the resonance effect of the diffraction grating 14.

Although not shown in the drawing, the n-type InP clad layer 11, the n-type InGaAsP optical waveguide layer 9, the light emitting region 1, the p-type InGaAsP optical waveguide layer 10, the p-type InP clad layer 15, and the p-type InGaAsP contact. The layer 16 is processed into a stripe shape, and is embedded with an n-type InP layer, a p-type InP layer, an n-type InP layer, and a p-type InP layer from the substrate side so that current can be concentrated in the stripe-shaped light emitting region 1. Has become.

FIG. 3 shows the DFB according to the first embodiment.
A diagram showing a schematic configuration of a millimeter-wave light source using a semiconductor laser,
FIG. 4 is a diagram schematically showing spectra at points (a), (b), and (c) among them.

The DFB semiconductor laser 20 having the structure shown in FIGS. 1 and 2 is mounted on a heat sink via a submount, and has an optical isolator 21, an optical filter 22, and an optical filter 22.
A polarization beam splitter (PBS) 23 for separating incident light and emitted light, a monitor light receiving element 24, a temperature control system 25 including a temperature sensor and a Peltier element, and light for coupling the incident light and the emitted light into an optical fiber. Coupling system or fiber
It is housed in the laser module 26 along with a pigtail (not shown) and the like. This laser module 2
A control circuit 27 for controlling the drive current and temperature of the DFB semiconductor laser 20 is connected to 6.

The laser module 26 also includes
Two 1.55 μm band DFB lasers 29 and 30 are connected via an optical fiber (not shown) and a multiplexer 28. A signal multiplexing / driving system 31 for driving the DFB laser with a multiplexed RF analog signal (several tens MHz to several hundreds MHz) is connected to the DFB laser 29. In addition, the two DFB lasers 29 and 30 have a wavelength control system that keeps the frequency difference of the output light from the DFB lasers 29 and 30 constant by using the difference between the beat frequency and the reference frequency obtained by the monitor light receiving element 24 as an error signal. 32 is also connected.

The DFB laser 29 is modulated by the RF signal by the signal multiplexing / driving system 31, and outputs the modulated signal light having the center frequency ν _{1 in} the wavelength band of 1.55 μm. On the other hand, the other DFB laser 30 has a C distance 60 GHz away from ν _1.
Outputs W light. These two DFB lasers 29, 30
The light output from the optical fiber is multiplexed by the multiplexer 28 and is transmitted through the polarization beam splitter 23 in the TM mode to the first mode of the present invention.
Is input to the DFB semiconductor laser 20 according to the embodiment.

The spectrum multiplexed by the multiplexer 28 is shown in FIG.
It shows in (a). It is assumed that the CW light power from the DFB laser 30 is set so as to partially saturate the intersubband absorption in the coupled quantum well layer 8 of the DFB semiconductor laser 20. The modulation signal light power from the DFB laser 29 is set to be at least two orders of magnitude smaller than the output light power of the DFB laser 30.

DFB semiconductor laser according to the first embodiment
The 20 has a relaxation vibration frequency of about 60 GHz.
Biased to the center frequency in the 1.3 μm band
Number ν ₂Single mode oscillation in TE basic mode
And This oscillation wavelength is a subband of the coupled quantum well layer 8.
It is outside the absorption band, and the absorption between sub-bands is TM
Excess loss for oscillation light
Is almost negligible.

The 1.55 μm band TM mode light incident on the DFB semiconductor laser 20 is guided by the light emitting region 1 and the optical waveguide layers 9 and 10 while being attenuated by the intersubband absorption of the coupled quantum well layer 8. Be waved. Due to this intersubband absorption, some of the electrons injected into the light emitting region 1 are excited to the upper subband, and as a result, the proportion of electrons that can contribute to the light emission of the InGaAsP well layer 2 is modulated.

Since the total carrier density in the light emitting region 1 hardly changes unless the transfer is performed below the threshold value, this modulation is hardly affected by the CR time constant and the carrier transport effect. Since the relaxation time between sub-bands and the tunnel time are ps order, it is possible to follow the beat frequency of 60 GHz of two incident lights. Therefore, the output light intensity in the 1.3 μm wavelength band is modulated at the beat frequency of 60 GHz. Since the power of the modulated signal light is set small and the power of the CW input light is set large, it is possible to perform modulation with high efficiency and good linearity.

The output light in the 1.3 μm band (center frequency ν ₂ ) is output through the optical isolator 21 and the optical filter 22. The optical filter 22 cuts one of the modulated sidebands and prevents the interference of the two sidebands due to the dispersion of the transmission optical fiber. The spectra before and after the optical filter 22 are shown in FIGS. 4 (b) and 4 (c), respectively.

As described above, according to this embodiment, the first quantum well layers 4 contributing to light emission and the second quantum well layers 8 contributing to modulation are alternately laminated to enable high-speed modulation of emitted light. By using the semiconductor light emitting device described above, a semiconductor light source that can be modulated in the millimeter wave band is realized. Since a millimeter-wave band high-power drive circuit, an optical modulator, and a shield are not required, the price can be lower than that of a conventional millimeter-wave band light source.

The present invention is not limited to the above embodiment, and various modifications can be made without departing from the gist of the invention.

For example, instead of disposing the polarization splitter between the DFB semiconductor laser 20 and the monitor light receiving element 24, dispose it between the DFB semiconductor laser 20 and the optical isolator 21, and from the emission side of the 1.3 μm oscillation light, 1 .55μ
If the m-band light is injected, the adjustment of the optical coupling of the DFB semiconductor laser 20 can be simplified.

As shown in FIG. 5, the DF in the 1.55 μm band
The same substrate 4 as the B lasers 41 and 42 and the monitor light receiving element 43
0 may be integrated. In this case, 1.55μ
The incident light of m band is DF of 1.3 μm band by the optical waveguide 44.
It is guided to the B semiconductor laser (semiconductor light emitting device of the invention) 45. Along the way, a half-wave plate 46 for converting the light oscillated in the TE mode into the TM mode and 1.55 μ
A wavelength filter 47 is inserted which transmits light in the m band but reflects light in the 1.3 μm band.
The 1.3 μm light from 5 is guided to the monitor light receiving element 43. Although not shown in the drawing, the drive circuit and the control circuit can be integrated on the same substrate 40.

Further, in this embodiment, two DFB lasers in the 1.55 μm band are used, but the output of one DFB laser is branched, one of them is subjected to RF modulation, and the other has a frequency of 60 GHz. You can also use it to shift and combine again. Further, the DFB laser may be used only for the light source that generates the modulation signal, and the fiber laser may be used for the light source that generates the CW light. The CW light source can also be used by amplifying and branching the output as a light source common to a plurality of semiconductor light emitting elements. The intermediate frequency is also not limited to 60 GHz. If the beats are set so as not to interfere with each other, it is possible to multiplex and use a plurality of RF frequency bands.

In the present embodiment, all InGaA are used.
Although the sP well layer (light emitting layer) 2 was tunneling-coupled with the coupled quantum well layer (intersubband absorption layer) 8, each well layer (light emitting layer) was made independent, and each well layer had a plurality of subband absorptions. The layers may be combined. Regarding the wavelength,
It is not limited to the combination of 1.55 μm and 1.3 μm, and the material and structure are not limited to those of the present embodiment.

(Second Embodiment) FIG. 6 shows a second embodiment of the present invention.
FIG. 3 is a diagram schematically showing a band structure near an active layer of the semiconductor light emitting device (blue semiconductor laser) according to the embodiment of FIG. Although a large built-in electric field due to the piezoelectric effect or the like exists in the nitride semiconductor heterostructure, in order to simplify the explanation, in this figure, the electric field including the diffusion potential of the pn junction is neglected and drawn. There is. Also, as in the case of FIG. 1, the scale is also deformed and drawn.

The semiconductor layers laminated here are n-type GaN well layer 51 and Al _0.2 Ga _0.8 N from the left side of the figure.
An n-type superlattice cladding layer (second semiconductor layer) 53 composed of the barrier layer 52, an Al _0.85 Ga _0.15 N barrier layer 54, an n-type Ga
An _intersubband absorption quantum well layer 57 including an N layer (fifth semiconductor layer) 55 and an Al _0.85 Ga _0.15 N layer (fourth semiconductor layer) 56, an n-type GaN layer 58, an In _0.18 Ga _0.82 N well layer ( First semiconductor layer) 59 and In _0.05 Ga _0.95 N barrier layer 60 active layer (light emitting layer) 61, p-type GaN layer 6
2, a p-type superlattice clad layer (third semiconductor layer) 66 including an Al _0.2 Ga _0.8 N barrier layer 63, a p-type GaN well layer 64 and an Al _0.2 Ga _0.8 N barrier layer 65.

In the conduction band of the intersubband absorption quantum well layer 57, two subbands, a base subband 67 and an excitation subband 68, are formed, and their absorption wavelength is 1.5.
It is 5 μm. In addition, here, the level group formed by tunneling coupling of the sub-bands of a plurality of wells is collectively treated as one sub-band. AlGaN
/ GaN quantum wells can realize intersubband absorption in the wavelength band of 1.55 μm in the literature (eg, C. Gmachl et al.
al., Applied Physics Lette, Vol. 77, No.23, pp. 3
722-3724).

FIG. 7 is a diagram schematically showing the sectional structure of this blue semiconductor laser. The above laminated structure 53 to 66
Are formed on the n-type GaN conductive layer 72 laminated on the sapphire substrate 71. A p-type GaN conductive layer 73 is stacked on the p-type superlattice clad layer 66, and an n-shaped opening 74 is formed on the p-type GaN conductive layer 73.
-Type GaN current confinement layer 75 and p-type GaN contact layer 7
6 are stacked. n-type GaN conductive layer 72 and p-type Ga
Ohmic electrodes 77 and 78 are formed on the N contact layer 76, respectively. n-type superlattice cladding layer 5
The layer structure above 3 is processed into a stripe-shaped mesa 79, and a laser resonator is formed by an end face formed by cleavage. 1.55 μm wavelength on one of the end faces
Is formed, but a dielectric multilayer film (not shown) is formed so as to transmit the light having a wavelength of 0.41 μm but reflect the light having a wavelength of about 0.41 μm.

When a current is injected into this semiconductor laser through the electrodes 77 and 79, a population inversion is formed between the quantum levels 69 and 70 formed in the conduction band and the valence band of the active layer 61,
Laser oscillation occurs around a wavelength of 0.41 μm. Active layer 6
In 1, the light emission mode is controlled by concentrating the current in the vicinity immediately below the opening 74. Wide-gap semiconductors such as nitride semiconductors have high resistivity,
Since the potential drop at the hetero interface is also large, the series resistance of the device becomes high. Further, since it is necessary to widen the width of the mesa 79 in order to reduce the electrode resistance, it is difficult to reduce the parallel capacitance. Therefore, the electric modulation band of this semiconductor laser is limited to the order of 100 MHz by the CR limitation.

With a CW current being oscillated in this semiconductor laser to oscillate, a modulation signal light having a wavelength of 1.55 μm is injected from the end face on which the dielectric multilayer film is formed. The optical power is sufficient to significantly change the energy distribution of carriers in the intersubband absorption quantum well layer 57, and the polarized wave has an electric field component perpendicular to the well layer. The incident signal light is assumed to be modulated at 5 GHz. At this time, the In _0.18 Ga _0.82 N well layer (first semiconductor layer) 59
Since the supply of electrons to the is modulated by the modulated signal light,
The oscillation light of 0.41 μm is also modulated at 5 GHz.

As described above, by applying the present invention, it becomes possible to modulate a semiconductor laser, which can be electrically modulated only at a frequency of about 100 MHz, at several GHz. In addition, instead of directly modulating the wide-gap semiconductor laser, a semiconductor laser of a near infrared wavelength having a low operating voltage is modulated, so that the voltage of the modulation circuit can be lowered. Since the technology developed for optical communication can be used, the cost can be reduced.

This blue semiconductor laser can be applied to an ultrahigh speed writing head for an optical disc. That is, the optical disc is rotated at a high speed and the band is several GH in the 1.55 μm band
By modulating the oscillation light of the high-power blue semiconductor laser at high speed through the DFB laser of z, it becomes possible to write data much faster than the conventional technique.

The present invention can be applied in various other ways. For example, there is a technique called high frequency superposition as a method of oscillating a reading semiconductor laser for optical discs in a stable multimode to reduce noise, but it is not possible to electrically superpose high frequency on a blue semiconductor laser having a large CR time constant. It was difficult. By injecting near-infrared light RF-modulated at a frequency corresponding to the relaxation oscillation frequency into the blue semiconductor laser of this embodiment, the same effect as high frequency superposition can be obtained.

In the case where the relaxation oscillation frequency cannot be made so high, the same operation can be performed even if the two input light beats are resonated in the longitudinal mode of the laser. That is, assuming that the cavity longitudinal mode interval of the blue semiconductor laser according to the second embodiment of the present invention is Δf, the 1.55 μm band semiconductor laser that stably oscillates at two wavelengths whose frequency difference is approximately equal to Δf. By combining the outputs of, stable high frequency superposition can be applied. Alternatively, if Δf is lowered to a practical frequency range by using an external resonator, RF modulation can be performed without using beat.

The element structure, material, wavelength, etc. are not limited to those in the second embodiment. For example, B
Intersubband absorption of eTe / ZnSe quantum well layer (1.
55 μm) and the blue-green light emission of the ZnCdMgSSe-based quantum well layer are also possible.

(Third Embodiment) FIG. 8 shows a third embodiment of the present invention.
It is a figure which shows typically the cross-section of the semiconductor light-emitting device (red photon recycling LED) concerning embodiment of this.

The LED of this embodiment is composed of n-type GaAs.
With a reflectance of 9 sequentially grown on the 10 ° off substrate 80.
8% n-type Al_0.95Ga_0.05As / Al_0.5Ga_0.5A
DBR layer 81 made of s, n-type Al_0.95Ga_0.05As layer
82, wavelength band 1.55 μm formed concentrically on it
Second-order diffraction grating 83 for this light,
N-type Al buried flat_0.5Ga_0.5As layer 84, n-type
Al_0.35Ga_0.15In _0.5Thin n-type strained AlA between P layers
sSb / GaInNAs quantum well layer (second quantum well
N-type clad layer 85 in which a plurality of layers are formed,
Ga Ga_0.5In _0.5P well layer and undoped Al_0.25G
a_0.25In_0.5Quantum well light emitting layer (first P barrier layer)
Quantum well layer) 86, p-type Al_0.95Ga_0.05As layer 8
7, p-type Al _0.35Ga_0.15In_0.5P clad layer 88,
It is composed of a p-type GaAs contact layer 89.

Two sub-bands are formed in the strained AlAsSb / GaInNAs well layer formed inside the n-type cladding layer 85, and the absorption wavelength between the sub-bands is 1.6 μm.

This layer structure has two concentric circles as shown in the figure.
Processed into a mesa shape, p-type Al _0.95Ga_0.05As layer
The outer edge of 87 was selectively oxidized by annealing in steam.
To form an insulation / aperture area 90. Also,
The p-type GaAs contact layer 89 is etched at the center.
And the dielectric reflection film mirror 91 is formed.
Has been. Ring-shaped p-type GaAs contact left
A p-type ohmic electrode 92 is formed on the layer 89, and
N-type ohmic on the backside of the LED except under the LED mesa
The electrode 93 is formed.

When a current is injected into this LED, the light emitting layer 8
At 6, electrons and holes are recombined to generate light emission in the 0.65 μm band. Since this light is confined by the DBR layer 81 having a large reflectance and the dielectric multilayer mirror 91 having a medium reflectance, strong light emission is generated by photon recycling. The red surface emitting laser (VCSEL) has a problem of poor temperature characteristics, but since the semiconductor light emitting element of the present embodiment is an LED, it is resistant to temperature fluctuations.

From the window portion on the back of the substrate 80 without the electrode 93,
A modulated signal light having a wavelength of 1.55 μm is incident. The DBR layer 81 reflects light having a wavelength of 0.65 μm.
It is set so that incident light in the 55 μm band is transmitted.
The modulated signal light in the wavelength band of 1.55 μm is used as the secondary diffraction grating 83.
Diffracts in a direction parallel to the growth layer. Since this diffraction component has an electric field component perpendicular to the well, the strained AlAsSb / GaInNA formed inside the n-type cladding layer 85 is formed.
It is absorbed by the intersubband transition of the s quantum well layer. Therefore, the carrier injection from the n-type cladding layer 85 to the light emitting layer 86 is modulated, and the output of the LED is modulated. Since the distance between the quantum well layer related to intersubband absorption and the light emitting layer is large, the equilibrium time of electrons between the quantum well layer related to intersubband absorption and the light emitting layer is the same as that in the first embodiment. Although it is longer than the above, it responds sufficiently to modulation of several GHz.

Since this modulation is not affected by the large series resistance of the n-type DBR layer 81 or the p-type cladding layer 88 or the capacitance of the thin oxide layer 90, it is much faster than the electrical modulation. Is. Therefore, a semiconductor light emitting device having a GHz-order modulation band comparable to that of a VCSEL and having excellent temperature characteristics is realized. The modulation voltage can also be suppressed low. The LED 94 can be used for POF transmission in the 0.65 μm band.

The LED 94 of this embodiment may be integrated on the front and back sides of the surface emitting laser (VCSEL) 95 that emits light in the 1.55 μm band and the GaAs substrate 80, as shown in FIG. As a method of manufacturing a 1.55 μm band VCSEL, an A layer having a large difference in refractive index is stacked on a GaAs substrate 80.
A technique is known in which a DBR layer made of 1GaAs / GaAs and an InGaAsP-based laminated structure formed on an InP substrate are integrated by a direct bonding technique, and such a technique is used in combination with the present invention. It is also possible.

The present invention can be variously modified and applied in addition to the above-described three embodiments without departing from the spirit of the invention.

[0082]

As described in detail above, according to the present invention, C
A high-speed semiconductor light emitting device that is not limited by electrical constraints such as R time constant and carrier transport effect can be realized. When the present invention is applied to a millimeter-wave transmission light source, an expensive millimeter-wave band drive circuit, optical modulator, shield, etc. are not required, and low-cost millimeter-wave band modulation can be realized. Further, when the present invention is applied to a visible light emitting semiconductor device, a driving circuit with a large voltage is not needed.

[Brief description of drawings]

FIG. 1 is a diagram showing a band structure near a light emitting region of a DFB laser according to a first embodiment.

FIG. 2 is a diagram showing a cross-sectional structure along a light emitting direction of the DFB laser according to the first embodiment.

FIG. 3 is a diagram showing a schematic configuration of a millimeter wave light source using the DFB laser according to the first embodiment.

4] Points (a) and (b) in the millimeter wave light source of FIG.
The figure which shows the spectrum of a point and a (c) point typically.

FIG. 5 is a diagram showing an example in which the DFB laser according to the first embodiment is integrated and integrated with another device.

FIG. 6 is a diagram schematically showing a band structure near an active layer of a blue semiconductor laser according to a second embodiment.

FIG. 7 is a view showing a sectional structure perpendicular to a mesa of a blue semiconductor laser according to a second embodiment.

FIG. 8 is a diagram showing a cross-sectional structure of a red photon recycling LED according to a third embodiment.

FIG. 9 is a diagram showing a cross-sectional structure when the red photon recycling LED according to the third embodiment is integrated with a 1.55 μm band VCSEL.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Light emission area 2 ... InGaAsP well layer (first semiconductor layer) 3 ... InGaAsP barrier layer 4 ... First quantum well layer 5 ... InGaAs well layer (fifth semiconductor layer) 6 ... AlAs intermediate barrier layer 7, 7a , 7b ... AlAsSb barrier layer (fourth semiconductor layer) 8 ... Coupled quantum well (second quantum well layer) 9 ... n-type InGaAsP optical waveguide layer (second semiconductor layer) 10 ... p-type InGaAsP optical waveguide layer ( Third semiconductor layer) 11 ... n-type InP clad layer 12 ... n-type InP substrate 13, 17, 77, 78, 92, 93 ... Electrode 14, 83 ... Diffraction grating 15 ... p-type InP clad layer 16 ... p-type InGaAsP Contact layer 18 ... Phase shift part 19 ... Low reflectance coating 20, 45 ... DFB semiconductor laser (semiconductor light emitting element of embodiment) 21 ... Optical isolator 22 ... Optical filter 23 ... Polarization Beam splitters 24, 43 ... Monitor light receiving element 25 ... Temperature control system 26 ... Semiconductor laser module 27 ... Control circuit 28 ... Multiplexer 29, 30, 41, 42 ... 1.55 .mu.m band DFB laser 31 ... Signal multiplexing Drive system 32 ... Wavelength control system 44 ... Optical waveguide 46 ... Half wave plate 47 ... Wavelength filters 51, 55, 58, 72, 75 ... N-type GaN layers 52, 63, 65 ... Al _0.2 Ga _0.8 N barrier layer 53 ... n-type superlattice cladding layers 54, 56 ... Al _0.85 Ga _0.15 N barrier layer 57 ... intersubband absorption quantum well layer 59 ... In _0.18 Ga _0.82 N layer (first semiconductor layer) 60 ... In _0.05 Ga _0.95 N layer 61, 86 ... Quantum well light emitting layer 62, 64, 73, 76 ... P-type GaN 66 ... P-type superlattice cladding layer 67, 68, 69, 70 ... Quantum level 71 ... Sapphire substrate 74 ... Opening 79 ... 80 ... GaAs substrate 81 ... n-type DBR layer 82 ... n-type Al _0.95 Ga _0.05 As layer 83 ... diffraction grating 84 ... n-type Al _0.5 Ga _0.5 As layer 85 ... n-type cladding layer 87 ... p-type Al _0.95 Ga _0.05 As layer 88 ... P-type cladding layer 89 ... P-type GaAs contact layer 90 ... Oxide film 91 ... Dielectric multilayer mirror 94 ... Red photon recycling LED 95 ... 1.55 .mu.m band VCSEL

─────────────────────────────────────────────────── --- Continuation of the front page (56) References JP-A-4-136823 (JP, A) JP-A-4-120784 (JP, A) JP-A-2-210332 (JP, A) JP-A-9- 222619 (JP, A) JP-A-10-90738 (JP, A) IEEE TRANSACTIONS ON MICROWAVE THEO RY AND TECHNIQUES, 1997, Vol. 45 No. 8, p. 1416-1423 IEICE TRANS. ELEC TRON. , 2002, Vol. E85-C No. 1, p. 174-180 (58) Fields investigated (Int.Cl. ⁷ , DB name) G02F 1/00-7/00 H01S 5/00-5/50

Claims (6)

(57) [Claims] 1. A light emitting region having a first semiconductor layer to be a light emitting layer, and second and third semiconductor layers having a forbidden band width wider than that of the first semiconductor layer sandwiching the light emitting region. When,
A semiconductor light emitting device comprising means for electrically injecting electrons and holes into a first semiconductor layer to emit light, wherein at least a fourth semiconductor layer and a fourth semiconductor layer are provided in the vicinity of the first semiconductor layer. A quantum well layer including a fifth semiconductor layer having a narrower forbidden band than the semiconductor layer is formed, and the quantum well layer has an intersubband absorption wavelength different from the emission wavelength of the first semiconductor layer. The absorption wavelength band between these sub-bands.
Input of modulated signal light in the region causes light emission of the first semiconductor layer
Emitting light by modulating the proportion of contributing carriers
A semiconductor light-emitting device characterized by modulating a . 2. A light emitting region in which a plurality of first quantum well layers having a first semiconductor layer to be a light emitting layer are stacked and arranged, and a first semiconductor layer formed by sandwiching the light emitting region. What is claimed is: 1. A semiconductor light-emitting device comprising: second and third semiconductor layers having wide forbidden bands; and means for electrically injecting electrons and holes into the first semiconductor layer to emit light. Second quantum well layers each including at least a fourth semiconductor layer and a fifth semiconductor layer having a narrower band gap than the fourth semiconductor layer are formed between the quantum well layers of The layer has an intersubband absorption wavelength different from the emission wavelength in the first quantum well layer ,
For input of modulated signal light within the absorption wavelength band between sub-bands of
The ratio of carriers contributing to light emission of the first semiconductor layer
A semiconductor light emitting element characterized by modulating emitted light by modulating . 3. The semiconductor light emitting device according to claim 1 or 2, and for this semiconductor light emitting device, a modulation signal within a band of an intersubband absorption wavelength of a quantum well layer having fourth and fifth semiconductor layers. And a means for inputting the modulated light, and the emitted light is modulated by modulating the proportion of carriers that contribute to the light emission of the first semiconductor layer by inputting the modulated light. 4. The semiconductor light emitting device according to claim 1, and the semiconductor light emitting device, together with the modulated signal light.
Means for injecting CW light in the band of the absorption wavelength between the sub-bands and having a center frequency different from the modulated signal light by a predetermined frequency, the beat signal of the modulated signal light and the CW light. 1. A semiconductor light emitting device, characterized in that emitted light is modulated by modulating a ratio of carriers contributing to light emission in one semiconductor layer. 5. When the semiconductor light emitting device according to claim 1 or 2 is driven, a modulation signal light within an intersubband absorption wavelength band of a quantum well layer having fourth and fifth semiconductor layers is injected. The method for modulating a semiconductor light emitting device is characterized in that the emitted light in the semiconductor light emitting device is modulated by modulating the ratio of carriers that contribute to light emission of the first semiconductor layer. 6. The semiconductor light emitting device according to claim 1, wherein the modulated signal light is within the band of the absorption wavelength between subbands of the quantum well layer having the fourth and fifth semiconductor layers, and the modulated signal light. By injecting CW light having different center frequencies by a predetermined frequency, and modulating the ratio of carriers contributing to light emission in the first semiconductor layer by the beat of the modulated signal light and the CW light. A method for modulating a semiconductor light emitting device, which comprises modulating light emitted from the device.