JP4301925B2 - Semiconductor laser, driving method of semiconductor laser, and wavelength conversion element - Google Patents

Description

  The present invention relates to a semiconductor laser for generating an ultrafast optical pulse used in the field of optical fiber communication, particularly an all-optical signal processing technique, a semiconductor laser driving method, and a wavelength conversion element.

  Optical fiber communication technology is an important infrastructure that supports the modern information society. Conventionally, the development of optical fiber communication technology has progressed rapidly, such as submarine optical cables and land trunk line communication networks connecting cities. Currently, the communication speed per channel of the trunk line system ranges from 10 to 40 Gbps, and in the future, it is expected to realize ultra high speed and large capacity communication of 80 to 160 Gbps or more.

  In the current system configuration, an optical signal is once converted into an electrical signal (EO conversion) at the node portion of the network, re-timed and waveform-shaped, and then converted again into an optical signal (OE conversion). It has been sent out. However, in an ultrahigh-speed optical communication system exceeding several tens of Gbps, it is no longer difficult to process an optical signal by such control via an electric signal. That is, the signal processing speed at the node is gradually becoming a bottleneck that limits the signal processing speed of the entire network. All-optical signal processing is a key technology for solving such problems and realizing ultrahigh-speed and large-capacity communication.

  In all-optical signal processing, from the technical and economic viewpoints, there is a need for processing to send an optical signal sent to a network node after performing waveform shaping and amplification as it is without converting it to an electrical signal. . Advantages of using the light-light control method include that the operation speed is not limited by the CR time constant of the electric circuit and that light pulses capable of generating ultrashort pulses can be used directly.

  Various optical elements are required to realize such all-optical signal processing, but an optical clock pulse in which a short optical pulse is sustained at a constant frequency is indispensable, and it is stable and jitter, that is, signal fluctuation on the time axis. Realization of an optical clock pulse generating element with a small number of elements is demanded. Generation of an optical clock pulse by a semiconductor element is important from the viewpoint of miniaturization of a network system and robustness against vibration.

  As a conventional semiconductor device for generating an optical clock pulse capable of operating at high speed, a self-pulsating DFB laser (self-disclosed) disclosed in US Pat. No. 6,215,805 (Patent Document 1) or US Pat. No. 6,122,306 (Patent Document 2) is used. pulsating distributed feedback laser). As shown in FIG. 1 of US Pat. No. 6,215,805, a conventional self-pulsating DFB (distributed feedback) laser has a ridge waveguide structure and has at least two electrically separated electrodes. In the case of a three-electrode configuration, two active regions, a front DFB region provided with a diffraction grating and a rear DFB region, are integrated with a phase control region provided between them. Each region is electrically isolated by an etching groove, and current can be injected independently. Further, anti-reflection (AR) coating is applied to both end faces.

  The repetition frequency of the self-pulsating DFB laser can be adjusted by direct current injection current into the front DFB region and the rear DFB region. The phase control region is provided to adjust on / off of the self-pulsation by adjusting the phase of the light wave in the resonator composed of the front DFB region and the rear DFB region, and to stabilize the self-pulsation operation. .

  Next, a driving method of the conventional self-pulsating DFB laser disclosed in the above-mentioned US Pat. No. 6,215,805 will be described. Although the operating principle of the device has not yet been fully elucidated, dispersive self Q-switching described below has been proposed. In general, in a Q-switched laser, a high inversion distribution is generated in the active layer due to strong excitation, but since there is a high resonator loss in the initial state, laser operation is hindered. Once the resonator loss is canceled, a short pulse with high impulse intensity is emitted. In order to achieve the above Q switching operation, the loss of the resonator reflector provided outside is rapidly changed from a high state to a low state, or an internal loss is initially formed in the resonator and then the internal There was a way to get rid of the loss.

  The front DFB region is strongly excited so as to sufficiently exceed the laser threshold current, and the rear DFB region is excited weakly so as to be in a substantially transparent state in the vicinity of the laser threshold current. At this time, the front DFB region functions as a laser, and the rear DFB region functions as a so-called reflection mirror having strong dispersibility, that is, having a large wavelength dependency of reflectance. The Bragg wavelengths of the front DFB region and the rear DFB region change depending on the carrier density injected into the region. When strong asymmetric excitation is performed between the front DFB region and the rear DFB region, a detuning state in which the Bragg wavelengths of the two regions slightly shift occurs. On the long wavelength side of the stop band, laser oscillation is likely to occur because the light density is increased by light reflection from the rear DFB region, that is, feedback.

  When the reflectivity of the rear DFB region is high, the threshold current of the laser decreases. Conversely, when the reflectivity of the rear DFB region is low, the threshold current of the laser increases. On the long wavelength side of the stop band, the laser threshold current is modulated very efficiently due to a slight change in wavelength near the bottom of the steep reflection peak of the dispersive reflecting mirror.

  For example, when the reflectivity of the rear DFB region is low and the laser threshold current increases as a result, the carrier density inside the resonator also increases and the refractive index decreases, so that the laser oscillation wavelength shifts to the short wavelength side. At that time, the reflectivity of the rear DFB region is increased, and as a result of the rapid decrease in the laser threshold current, a short pulse is output as in the Q-switched laser. There is a time delay until carriers in the active region consumed by laser oscillation are replenished by current injection, and laser oscillation stops during this time. Thus, in the vicinity of the minimum point of feedback from the rear DFB region, the feedback from the rear DFB region, that is, the Q value of the resonator greatly changes due to the fluctuation of the carrier density. By repeating the above process, the self-pulsation operation can be maintained despite the use of a direct current excitation current.

In the conventional self-pulsating DFB laser, an optical clock pulse is generated, and the optical clock pulse is regenerated by synchronizing with an input signal close to the oscillation frequency. When the input signal is absorbed in the active region, a carrier density fluctuation occurs in the laser region, and the self-pulsation operation is affected, so that frequency pull-in occurs. In this case, if the active region is formed of a tensile strain bulk crystal having no polarization dependence, it is possible to perform optical clock regeneration even if it is polarization independent and deviates from the input wavelength.
US Pat. No. 6,215,805 US Pat. No. 6,122,306

In order to stably generate optical pulses having a high frequency on the order of gigahertz (GHz) with a conventional self-pulsating DFB laser, for example, an extremely high diffraction grating coupling coefficient (κ) of 300 cm ⁻¹ or more is required. In the case of dispersive Q-switching, self-pulsation is more likely to occur when receiving strong feedback from the mirror part. In the case of beat-type vibration, if the diffraction grating coupling coefficient κ is high, the lasers in the two regions are almost independent. It will be in a state that can be considered. Further, it is because a normal self-pulsation frequency optical pulse cannot be generated with a normal diffraction grating coupling coefficient κ. However, in order to stably realize a high coupling coefficient, it is necessary to fabricate the diffraction grating very close to the optical waveguide layer, and in order to realize this, an ultra-fine processing technique is indispensable. It was. Therefore, in the conventional self-pulsating DFB laser, the range of conditions under which stable self-pulsation operation can be realized is extremely narrow, and self-pulsation operation exceeding 100 GHz has not been reported experimentally.

  The present invention has been made to solve the above problems, and is a semiconductor laser capable of realizing a self-pulsation operation with a stable and high oscillation frequency under a normal manufacturing method, a semiconductor laser driving method, and An object is to provide a wavelength conversion element.

A semiconductor laser according to the present invention includes a first conductivity type semiconductor substrate, a first conductivity type cladding layer, an optical waveguide layer, a front DFB region, a rear DFB or DBR region, and a second conductivity type cladding layer. I have. The first conductivity type cladding layer is formed on the semiconductor substrate. The optical waveguide layer is formed on the first conductivity type cladding layer. The front DFB region includes a diffraction grating that includes the optical waveguide layer as a part and is positioned in front of the laser beam emission direction and close to the optical waveguide layer surface. The rear DFB or DBR region includes a diffraction grating that partially includes the optical waveguide layer and is positioned rearward with respect to the laser beam emission direction and close to the surface of the optical waveguide layer, and is electrically separated from the front DFB region. . The second conductivity type cladding layer is formed so as to embed the diffraction grating. The diffraction grating in the front DFB region has a phase shift portion in part. Both the laser oscillation wavelength in the stop band of the front DFB region and the laser oscillation wavelength at the end of the stop band of the front DFB region generated by injecting current into each of the front DFB region and the rear DFB or DBR region are The diffraction grating in the rear DFB or DBR region is adjusted so as to be positioned in a high reflectivity portion in the stop band generated by the DFB or DBR region. The period of the diffraction grating in the front DFB region is different from the period of the diffraction grating in the rear DFB or DBR region.

  In the semiconductor laser according to the present invention, the laser oscillation wavelength in the stop band of the front DFB region generated by injecting current into each of the front DFB region and the rear DFB or DBR region, and the laser at the end of the stop band of the front DFB region Since the diffraction grating in the rear DFB or DBR region is adjusted so that both the oscillation wavelength and the oscillation wavelength are located in the high reflectivity part in the stop band generated by the rear DFB or DBR region, stable self-pulsation operation is achieved. A semiconductor laser that can be realized is obtained.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(Embodiment 1)
FIG. 1 is a schematic view showing a configuration of a semiconductor laser according to Embodiment 1 of the present invention.

  Referring to FIG. 1, in the semiconductor laser of the present embodiment, an n-type InP clad layer (first conductivity type InP clad layer) is formed on an n-type indium phosphorus (InP) substrate (first conductivity type semiconductor substrate) 1. 2 is formed. An indium gallium arsenide phosphorus (InGaAsP) optical waveguide layer 3 is formed on the n-type InP cladding layer 2. A p-type InP cladding layer (second conductivity type InP cladding layer) 4 is formed on the optical waveguide layer 3. A diffraction grating 5 is embedded in the p-type InP cladding layer 4. A phase shift portion 6 is formed in a part of the diffraction grating 5. A p-type InGaAsP contact layer 7 is formed on the p-type InP cladding layer 4. A high resistance InP current confinement layer 8 for confining current is formed on the side surface of the optical waveguide layer 3. Each region is electrically separated by separation grooves 9a and 9b formed by etching. An n-type electrode 20 is provided on the back side of the n-type InP substrate 1. On the p-type InGaAsP contact layer 7, p-type electrodes 21a, 21b, and 21c are provided so as to be separated from each other and electrically separated by separation grooves 9a and 9b. Thus, the semiconductor laser of the present embodiment is a multi-electrode laser.

  In the semiconductor laser according to the present embodiment, a tensile strain bulk crystal layer of about 0.2% is used as the optical waveguide layer 3, and it is independent of polarization with respect to the external input signal light and is operated synchronously over a wide wavelength range. To be able to.

  2 is a cross-sectional view of the semiconductor laser shown in FIG. 1 in the direction of the resonator (surface represented by II in FIG. 1).

  Referring to FIG. 2, low reflectance (AR) films 30 are formed on both end faces of the semiconductor laser. The semiconductor laser has a front DFB region 101, a phase control region (front-rear phase control region) 102, and a rear DFB region 103. The phase control region 102 is located between the front DFB region 101 and the rear DFB region 103 and is electrically separated from each of the front DFB region 101 and the rear DFB region 103.

  Each of the front DFB region 101 and the rear DFB region 103 is provided with a diffraction grating 5 that is close to the optical waveguide layer 3 and embedded in the p-type InP cladding layer 4. A 2λ / 9 shift portion 6 is formed in a part of the diffraction grating 5 in the front DFB region 101.

  Between the front DFB region 101 and the rear DFB region 103, a phase control region 102 for controlling on / off of the self-pulsation operation is provided as in the conventional example. However, if the phase shift amount between the front DFB region 101 and the rear DFB region 103 can be adjusted to the range of −λ / 10 to λ / 4 at the element fabrication stage, the phase control region 102 is omitted. A two-electrode configuration having only the front DFB region 101 and the rear DFB region 103 may be employed.

  The period of the diffraction grating 5 in the front DFB region 101 and the period of the diffraction grating 5 in the rear DFB region 103 may be different.

  Next, the operation of the semiconductor laser according to the present embodiment will be described.

  As described above, the semiconductor laser of the present embodiment is divided into three regions in which the p-type electrodes 21a, 21b, and 21c are electrically separated by the separation grooves 9a and 9b, and the front DFB region 101 and the phase control region 102 are divided. In addition, a current can be injected independently into each of the optical waveguide layers 3 in the rear DFB region 103. That is, current is independently injected from the p-type electrode 21a to the front DFB region 101, from the p-type electrode 21b to the phase control region 102, and from the p-type electrode 21c to the rear DFB region 103.

  When a current is injected into the front DFB region 101 through the p-type electrode 21a, a strong excited state is generated in the optical waveguide layer 3 of the front DFB region 101. As a result, in the front DFB region 101, the phase shift 6 is caused. An oscillation mode close to the oscillation mode of the single DFB laser is generated at the wavelength in the stop band. On the other hand, at the end of the stop band of the front DFB region 101, when the phase magnitude given by the phase control region 102 is adjusted, the front DFB region 101 and the rear DFB region 103 form a coupling cavity, and the oscillation mode Produces different oscillation modes. Since these two types of oscillation modes oscillate simultaneously, self-pulsation occurs at a repetition frequency corresponding to the wavelength difference between the modes.

  Next, the results of simulation analysis of the element structure of the semiconductor laser according to the present embodiment will be described.

In the semiconductor laser in which the coupling coefficient of the diffraction grating 5 is about 150 cm ⁻¹ , the lengths of the front DFB region 101 and the rear DFB region 103 are both 300 μm, and the phase shift amount in the phase control region 102 is set to λ / 10. A characteristic simulation based on time domain analysis was performed on an element structure in which a phase shift was introduced into the front DFB region 101.

  The relationship between the complex electric field F (t, z) of the forward wave and the complex electric field R (t, z) of the backward wave in the single mode optical waveguide including the diffraction grating 5 and having gain is generally as follows. It can be described by two coupled wave equations that depend on time.

Here, v _g is the group velocity of light in the waveguide, kappa is the coupling coefficient of the diffraction grating, g is the electric field gain, [delta] is the amount representing the deviation from the Bragg wavelength of the grating, i _sp is from spontaneous emission Represents the contribution. The element to be analyzed is divided into a number of sections, and the carrier density and electric field density within each section are uniformly approximated, and the spatial step size corresponding to the time step size is appropriately selected. The time evolution of the complex electric field of the forward wave and the backward wave was numerically calculated, and finally the time change of the output light intensity was obtained. Further, the output light intensity was subjected to Fourier transform to obtain an oscillation spectrum.

  An element having a structure in which a λ / 4 shift is introduced at the center of the front DFB region 101 and the diffraction grating 5 of the rear DFB region 103 is detuned by 2 nm to the longer wavelength side than the diffraction grating 5 of the front DFB region 101. An example of the calculation results of the output optical time waveform obtained by simulating the pulsation operation, the laser oscillation spectrum, and the reflection spectrum calculated from each of the front DFB region 101 and the rear DFB region 103 is shown in FIG. 3, FIG. 4, and FIG. The reflection spectrum showed a value calculated considering only the effect of the real part, with the imaginary part of the waveguide refractive index of the element being zero.

  Referring to FIG. 3, an ultra high frequency self-pulsation waveform is shown. Further, as described above, the oscillation spectrum consists of two main oscillation modes as shown in FIG. The oscillation mode wavelength on the short wavelength side in FIG. 4 is located in the stop band of the front DFB region 101 as shown in FIG. 5 and has a property close to the oscillation mode of the single DFB laser caused by the introduced phase shift 6. On the other hand, the oscillation mode wavelength on the long wavelength side shown in FIG. 4 is located at the end of the stop band of the front DFB region 101 as shown in FIG. 5, and the front DFB region 101 and the rear DFB region 103 are coupled to each other. This is an oscillation mode generated by forming. Since these two types of oscillation modes oscillate simultaneously, self-pulsation occurs at a repetition frequency corresponding to the wavelength difference between the modes.

  The oscillation wavelength of the longer wavelength mode can be adjusted within the range of the end of the stop band by varying the phase given by the phase control region 102, so that the wavelength difference between the two modes, that is, the repetition frequency can be adjusted. Thus, if this element is operated near a predetermined repetition frequency, it can be finally operated at a predetermined repetition frequency by phase adjustment. For example, the clock regeneration function at the node can be realized by operating this element at the carrier frequency of optical fiber communication and synchronizing the light injection.

  In order to stably generate the self-pulsation by the operation mechanism described above, the wavelengths of the two oscillation modes shown in FIG. 4 (the laser oscillation wavelength in the stop band of the front DFB region 101 and the laser oscillation wavelength at the end of the stop band) are shown. As shown in FIG. 5, the Bragg wavelength of the diffraction grating 5 in the rear DFB region 103 is set in advance so that the element driving conditions are adjusted so that both are located in the high reflectivity portion in the stop band of the rear DFB region 103. ing. The reason is that when the oscillation mode wavelength is located at the end of the stop band of the rear DFB region 103, unlike the operation mechanism described above, the self-pulsation at a relatively low frequency of about 20 GHz is repeatedly performed by the distributed Q switch mechanism. This is because it tends to occur, and when it is located in the low reflectivity portion of the rear DFB region 103, feedback to the front DFB region 101 is weakened, and oscillation in a single mode is likely to occur.

  Further, the laser oscillation wavelength in the stop band of the front DFB region 101 generated by the introduction of the phase shift unit 6 is set to a desired wavelength interval of 0.3 nm to 5.5 nm with respect to the end wavelength of the stop band. The amount of phase shift is adjusted. Although it is difficult to make the wavelength interval larger than the width of the stop band in the front DFB region, the stop band width should be increased to about 5.5 nm by optimizing the specifications of the element length and diffraction grating coupling constant. Is possible. Further, in order for self-pulsation due to beat-type vibration to occur stably, the wavelength interval is desirably 0.3 nm or more.

  The frequency of self-pulsation by the above-described operation mechanism can be changed greatly by changing the phase shift amount of the front DFB region 101. FIG. 6 shows a calculation result of the pulsation frequency when the magnitude of the phase shift is changed in the above-described element.

Referring to FIG. 6, it can be seen that the pulsation frequency increases as the value of the phase shift amount increases. This is because the oscillation mode on the short wavelength side is shortened. In addition, since the coupling coefficient of the diffraction grating 5 affects the stop band width, it affects the pulsation frequency. As described above, the coupling coefficient, phase shift, DFB region length, and detuning of the diffraction grating 5 are design parameters of the element, and various self-pulsation waveforms can be selected by changing the design values of these parameters. For example, in the element of phase shift λ / 4, when the detuning value of the diffraction grating 5 is in the range of 0 to 3 nm, for example, and the coupling constant of the diffraction grating 5 is about 100 to 150 cm ⁻¹ , Self-pulsation occurs.

  Further, as described above, the wavelength of the oscillation mode on the long wavelength side can be adjusted by the phase of the phase control region 102, but a possible adjustment range is shown by a rectangular region in FIG. For example, when the phase shift amount is λ / 9, the pulsation frequency is 80 ± 20 GHz, when the phase shift amount is 2λ / 9, the pulsation frequency is 160 ± 20 GHz, and when the phase shift amount is 4λ / 9, the pulsation frequency is about 320 ± 20 GHz. Thus, it can be used as a signal pulse light source or an element suitable for clock recovery in ultra-high-speed time division multiplexing optical communication.

  Note that the self-pulsation operation by the above-described mechanism similarly occurs even in an element having a structure in which the rear DFB region 103 is replaced with a DBR (Distributed Bragg Reflector) region configured by a transparent optical waveguide layer at a laser oscillation wavelength. It is possible to make it. Further, even in an element having a structure in which the phase control region 102 is formed of an active layer having a gain at the laser oscillation wavelength, the self-pulsation operation by the above-described mechanism can be similarly caused. The number and position of introducing the phase shift 6 are not limited to the single and the center of the diffraction grating, and the laser oscillation wavelength and pulsation frequency can be adjusted by adjusting the number and position of introduction. .

  In the semiconductor laser according to the present embodiment, the stripe shape in the resonator direction of the optical waveguide layer 3 is formed by etching, and then a current confinement layer 8 is formed by embedding crystal growth on both side portions of the optical waveguide layer 3 to obtain a current. It has a configuration for narrowing. For this reason, in addition to being able to inject current efficiently into the optical waveguide layer 3, an active region contributing to laser oscillation in the optical waveguide layer 3 in the direction of the resonator and an inactive state such as a reflection mirror and phase adjustment. As a result of significantly reducing the coupling loss with the region, it is possible to form a low-loss optical waveguide and to perform self-pulsation operation with high efficiency.

  In the semiconductor laser of this embodiment, both sides of the active region are buried and grown with the current confinement layer 8 to confine the current. However, like the conventional semiconductor laser, a ridge waveguide is used. However, almost the same effect can be obtained.

  Further, in the semiconductor laser of the present embodiment, a tensile strain bulk of about 0.2% is used as the optical waveguide layer 3, but the wavelength range capable of responding to the external input signal light is narrowed, and the polarization dependence If it is allowed to occur, the optical waveguide layer 3 may be composed of a multiple quantum well (MQW).

  As described above, according to the first embodiment, both of the wavelengths of the two oscillation modes (the laser oscillation wavelength in the stop band of the front DFB region 101 and the laser oscillation wavelength at the end of the stop band) are the stop band of the rear DFB region 103. Since the diffraction grating 5 in the rear DFB region 103 is adjusted so as to be located in the high reflectance portion, a semiconductor laser that can be manufactured by a normal manufacturing method and can realize a stable self-pulsation operation is obtained. It is done.

(Embodiment 2)
FIG. 7 is a schematic cross-sectional view showing the configuration of the semiconductor laser according to the second embodiment of the present invention.

  Referring to FIG. 7, in the semiconductor laser according to the present embodiment, phase shifts 6a and 6b corresponding to 2λ / 9 are provided in the central portion of front DFB region 101 and the central portion of rear DFB region 103, respectively. Since the other structure of the present embodiment is the same as that of the first embodiment, the same constituent elements are denoted by the same reference numerals, and the description thereof is omitted.

  According to the same simulation as in the first embodiment, the oscillation spectrum is composed of two main oscillation modes: an oscillation mode in the stop band of the front DFB region 101 and an oscillation mode at the end of the stop band of the front DFB region 101. It can be seen that the nature of the oscillation mode is the same as in the first embodiment. Since these two types of oscillation modes oscillate simultaneously, self-pulsation occurs at a repetition frequency corresponding to the wavelength difference between the modes. Also in the present embodiment, in order to stably generate self-pulsation, the rear DFB region is such that the wavelengths of the two oscillation modes are both located in the high reflectivity portion in the stop band of the rear DFB region 103. It is desirable to set the Bragg wavelength of the diffraction grating 5 103 in advance and adjust the element driving conditions.

  In the present embodiment, the shape of the stop band of the rear DFB region 103 is different from that of the first embodiment, but the two types of oscillation mode wavelengths are both positioned in the high reflectivity portion in the stop band of the rear DFB region 103. It is possible to adjust to the same as in the first embodiment. The variable range of the pulsation frequency by changing the phase shift amount mainly corresponds to the stop bandwidth of the front DFB region 101, and is the same as in the first embodiment. Since the phase shift 6b is also provided in the diffraction grating 5 of the rear DFB region 103, the reflectance of the rear DFB region 103 can be controlled, and the self-pulsation operation by the above-described mechanism can be easily obtained. .

(Embodiment 3)
FIG. 8 is a schematic cross-sectional view showing the configuration of the semiconductor laser according to the third embodiment of the present invention.

  Referring to FIG. 8, in the semiconductor laser of the present embodiment, front DFB region 101 includes two DFB regions 101a and 101b, and a phase control region (forward DFB phase control region) 102a provided therebetween. And 3 regions. This phase control region 102a is electrically separated from each region. The semiconductor laser according to the present embodiment further includes a rear DFB region 103 and a phase control region 102b, and is composed of five regions as a whole. The phase control region 102 b is provided between the front DFB region 101 and the rear DFB region 103.

The coupling coefficients of the diffraction gratings 5 are each about 150 cm ⁻¹ , and the diffraction gratings 5 in the rear DFB region 103 are detuned by 2 nm from the diffraction gratings 5 in the front DFB region 101 toward the long wave side. The lengths of the two DFB regions 101a and 101b in the front DFB region 101 are each 150 μm, and the length of the rear DFB region 103 is 300 μm.

  By driving the two DFB regions 101a and 101b of the front DFB region 101 with the same current density and varying the phase applied by varying the drive current of the phase control region 102a, the front DFB region of the present embodiment 101 can be operated in the same manner as a single DFB region having a variable phase shift amount. That is, the oscillation mode wavelength having a property close to that of the single DFB laser in the front DFB region 101 described in the first embodiment can be varied within the stop band. On the other hand, as in the first embodiment, at the end of the stop band of the front DFB region 101, when the magnitude of the phase applied in the phase control region is adjusted, the front DFB region 101 and the rear DFB region 103 are coupled to each other. Thus, an oscillation mode different from the oscillation mode is generated. Since these two types of oscillation modes oscillate simultaneously, self-pulsation occurs at a repetition frequency corresponding to the wavelength difference between the modes.

  As described in the first embodiment, the self-pulsation frequency can be significantly changed by changing the phase shift amount of the front DFB region 101. In the present embodiment, the pulsation frequency can be changed over a wide frequency range of 40 to 400 GHz with the same element by electrically varying the phase shift amount of the phase control region 102a included in the front DFB region 101. it can. Such an element can be used as a pulse light source for signals or an element suitable for clock recovery in ultrahigh-speed time division multiplexing optical communication.

(Embodiment 4)
FIG. 9 is a schematic cross-sectional view showing the configuration of the semiconductor laser in the resonator direction according to the fourth embodiment of the present invention.

  Referring to FIG. 9, the semiconductor laser according to the present embodiment has a front DFB region 101, a phase control region 102, and a rear DBR region 103. The band gap energy of the crystal constituting the optical waveguide layer 3 a in the rear DBR region 103 and the phase control region 102 is larger than the band gap energy of the crystal constituting the optical waveguide layer 3 in the front DFB region 101. Since the other structure of the present embodiment is the same as that of the first embodiment, the same constituent elements are denoted by the same reference numerals, and the description thereof is omitted.

  In the semiconductor laser of the first embodiment, the rear DFB region 103 has the same optical waveguide layer 3 as the front DFB region 101, and light absorption and gain in the rear DFB region 103 of the generated laser light are completely ignored. I can't. Therefore, the semiconductor laser of the present embodiment is configured to replace the rear DFB region 103 in the semiconductor laser of the first embodiment with a passive DBR mirror that has a large band gap and can ignore light absorption and gain.

  The operation of the semiconductor laser according to the fourth embodiment will be described below. Since the basic operation of the semiconductor laser is almost the same as that of the semiconductor laser according to the first embodiment, a characteristic operation portion in this embodiment will be described.

  An operating current Ilas1 that is equal to or greater than the laser oscillation threshold is supplied to the front DFB region 101, and a refractive index control current Ilas2 is supplied to the rear DBR region 103. In this case, the front DFB region 101 functions as a laser, while the rear DBR region 103 operates as a passive DBR mirror having strong dispersibility, that is, having a large wavelength dependency of reflectance.

  Further, in the semiconductor laser of the present embodiment, the optical waveguide layer 3 of the front DFB region 101, the phase control region 102, and the optical waveguide layer 3 formed of a single crystal layer in the semiconductor laser of the first embodiment. The optical waveguide layer 3a in the rear DBR region 103 is crystal-grown by butt joint growth so that the band gap energy of the optical waveguide layer 3a in the phase control region 102 and the rear DBR region 103 is larger than the band gap energy of the front DFB region 101. A so-called butt joint structure formed of a simple crystal may be applied. By applying the butt joint structure, it is possible to eliminate the coupling loss between the active region and the optical waveguide region along the cavity direction, and to form the optical waveguide layers 3 and 3a with low loss. Furthermore, since reflection due to a difference in refractive index at the interface between both regions of the optical waveguide layers 3 and 3a is reduced, the semiconductor laser can be driven with high efficiency.

  In the semiconductor laser of the fourth embodiment, since the rear DFB region of the semiconductor laser of the first embodiment is replaced with a passive DBR mirror, fluctuations in carrier density due to unexpected light absorption that occurs when the DFB region is used can be suppressed. Stable self-pulsation operation can be continued. Furthermore, the self-pulsation frequency can be adjusted more easily by applying a passive DBR mirror. Since the rear DBR region 103 does not change the gain and the refractive index at the same time, phase adjustment is easy, and since the rear DBR region 103 has no gain, the laser oscillation wavelength of the front DFB region 101 and the stop of the passive DBR mirror This is because laser oscillation outside the band hardly occurs.

(Embodiment 5)
FIG. 10 is a schematic cross-sectional view showing the configuration of the semiconductor laser according to the fifth embodiment of the present invention.

  Referring to FIG. 10, in the semiconductor laser of the present embodiment, a semiconductor optical amplifier (SOA) 104 is integrated adjacent to the front portion of front DFB region 101. The configuration of the front DFB region 101, the phase control region 102, and the rear DFB region 103 is the same as that of the first embodiment. The length of the SOA region 104 is, for example, 900 μm. The SOA active layer has the same specifications as the front DFB region 101 except that the diffraction grating is not provided. In the SOA region 104, a p-type InGaAsP contact layer 7 and a p-type electrode 21d are formed on the p-type InP cladding layer 4. The p-type InGaAsP contact layer 7 and the p-type electrode 21d in the SOA region 104 are separated from the p-type InGaAsP contact layer 7 and the p-type electrode 21a in the front DFB region 101 by the separation groove 9d.

  In the case where the SOA is not integrated, the dependence of the clock recovery characteristic due to the light injection locking on the injected light wavelength is, for example, as shown in FIG. In FIG. 11, the vertical axis represents the injected light intensity required for synchronization, and the horizontal axis represents the injected light wavelength. The polarization direction of the injected light was the same as that of the laser beam. When the injection light wavelength is the same as the laser oscillation wavelength, synchronization can be achieved with the smallest intensity. When the injection light wavelength is changed from the laser oscillation wavelength, the required injection light intensity increases when the difference is about the stop bandwidth or less, and decreases when the difference is about the stop bandwidth or more. This behavior is caused by the fact that the reflectivity by the DFB region is larger at the injection light wavelength within the stop band than the wavelength outside the stop band, and the injection light cannot sufficiently propagate through the DFB region. This is because it becomes difficult to occur. On the other hand, in the case of the injection light wavelength close to the laser oscillation wavelength, reflection by the stop band is small, and the injection light propagates in the DFB region while being multiplied, so that it can be synchronized with a small light intensity.

  In this embodiment, external CW (continuous wave) light (continuous light) having the same wavelength and polarization direction with respect to laser oscillation light and signal pulse light for clock reproduction are incident as incident signals to the SOA section. Is done. Due to the effect of mutual gain modulation that occurs when propagating through the SOA, the incident external CW light undergoes intensity modulation by the signal pulse light. Since the modulated external CW light has the same wavelength as that of the laser oscillation light, after reaching the DFB region, injection locking can be generated with a small light intensity. The signal pulse light intensity required to cause injection locking can be reduced when the SOA mutual gain modulation is used than when the SOA is not provided. Therefore, the injection locking can be generated with high sensitivity by providing the SOA.

  Further, regarding the wavelength dependency of the signal pulse light, when the wavelength is within the SOA gain wavelength band, the above-described mutual gain modulation in the SOA similarly occurs, and therefore, 30 nm or more corresponding to the SOA gain wavelength band. Injection locking can be generated in a wide wavelength range. Thus, by providing the SOA, the wavelength dependency is small with respect to a wide signal light wavelength range, and injection locking can be generated with high sensitivity.

  In this embodiment, the SOA part 104 is provided in the front part of the front DFB area 101. However, even when the SOA part 104 is provided in the rear part of the rear DFB area 103 as shown in FIG. By making the signal pulse light incident from the rear side, an injection locking operation similar to the above can be performed. In this case, the SOA active layer has the same specification as that of the rear DFB region 103 except that the diffraction grating is not provided. In the SOA region 104, a p-type InGaAsP contact layer 7 and a p-type electrode 21d are formed on the p-type InP cladding layer 4. The p-type InGaAsP contact layer 7 and the p-type electrode 21d in the SOA region 104 are separated from the p-type InGaAsP contact layer 7 and the p-type electrode 21c in the rear DFB region 103 by the separation groove 9d.

  The same injection locking operation can be performed also in other combinations of elements using the semiconductor laser of the present invention and SOA described in the first, second, third, and fourth embodiments.

  Further, as shown in FIGS. 13A and 13B, a wavelength conversion element using the element of the present invention can be configured. The wavelength conversion element includes an incident side optical waveguide 202 into which signal light is incident from one end, an incident side coupler 203 and a branch optical waveguide optically coupled to the incident side optical waveguide, and an SOA integrated self-pulsating semiconductor laser. It mainly has an injection light generation (injection locking) semiconductor laser 204 that oscillates at a wavelength that matches the oscillation wavelength 211. The incident-side optical waveguide and the injection so that the signal light propagated through the incident-side optical waveguide 202 and the output light of the semiconductor laser 204 for generating the injection light can enter the SOA via the incident-side coupler 203 and the branch optical waveguide. A synchronization semiconductor laser is arranged.

  FIG. 13A is a configuration example in which output light is obtained in a direction opposite to the propagation direction of input signal light, and FIG. 13B obtains output light in the same direction as the propagation direction of input signal light. This is an example of the configuration. The injection light generating semiconductor laser 204 includes a known λ / 4 phase-shifted DFB laser and a wavelength tunable semiconductor laser designed to oscillate in the vicinity of a wavelength that matches the oscillation wavelength of the SOA integrated self-pulsating semiconductor laser 211. Is used. In the case of a wavelength tunable semiconductor laser, the wavelength can be easily selected because the wavelength tunable range is large. In the case of the DFB laser with λ / 4 phase shift, the DFB laser with λ / 4 phase shift is set so that the oscillation wavelength of the SOA integrated self-pulsating semiconductor laser 211 falls within a range in which the oscillation wavelength can be varied by the element drive current. Produced.

  Incident light from the injection light generating semiconductor laser 204 is cross-phase-modulated in the SOA section, and causes an injection locking operation of the element. In the case of the configuration shown in FIG. 13A, the wavelength-converted light that is the clock reproduction output can be obtained in the same direction as the propagation direction of the incident light via the output-side waveguide 202a. In the case of FIG. 13B, wavelength-converted light that is a clock reproduction output can be extracted from the incident-side optical waveguide 202 through the incident-side coupler 203 in the direction opposite to the propagation direction of incident light. The output light can be separated using an optical circulator. By constructing an element in which such an injection light generating semiconductor laser is integrated, an external CW light, a polarization adjustment mechanism accompanying it, and a coupling coupler for signal pulse light become unnecessary, and the entire system can be made compact. It is possible to easily generate the injection locking operation.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  The present invention can be advantageously applied to a semiconductor laser for generating an ultrafast optical pulse and a method for driving the semiconductor laser used in the field of optical fiber communication, particularly all-optical signal processing technology.

1 is an overview diagram showing a configuration of a semiconductor laser according to a first embodiment of the present invention. FIG. 2 is a cross-sectional view (a surface represented by II in FIG. 1) in the resonator direction of the semiconductor laser of FIG. Self-pulsation with an element having a structure in which the λ / 4 phase shift in the semiconductor laser of the first embodiment is introduced and the diffraction grating in the rear DFB region is detuned by 2 nm longer than the diffraction grating in the front DFB region. It is a figure which shows the simulation result of the output optical time waveform obtained by simulating operation | movement. Self-pulsation with an element having a structure in which the λ / 4 phase shift in the semiconductor laser of the first embodiment is introduced and the diffraction grating in the rear DFB region is detuned by 2 nm longer than the diffraction grating in the front DFB region. It is a figure which shows the simulation result of the laser oscillation spectrum obtained by simulating operation | movement. Self-pulsation with an element having a structure in which the λ / 4 phase shift in the semiconductor laser of the first embodiment is introduced and the diffraction grating in the rear DFB region is detuned by 2 nm longer than the diffraction grating in the front DFB region. It is a figure which shows the simulation result of the reflection spectrum obtained by simulating operation | movement. FIG. 3 is a diagram showing the relationship between the phase shift magnitude and the pulsation repetition frequency in the semiconductor laser of the first embodiment. It is a schematic sectional drawing which shows the structure of the semiconductor laser in Embodiment 2 of this invention. It is a schematic sectional drawing which shows the structure of the semiconductor laser in Embodiment 3 of this invention. It is a schematic sectional drawing which shows the structure of the semiconductor laser in Embodiment 4 of this invention. It is a schematic sectional drawing which shows the structure of the semiconductor laser in Embodiment 5 of this invention. It is a figure which shows the injection light wavelength dependence of the clock reproduction | regeneration characteristic by light injection synchronization in case SOA is not integrated. It is a schematic sectional drawing which shows the structure of the semiconductor laser which provided the SOA part in the rear part of back DFB area | region. It is a figure which shows the structural example of the wavelength conversion element which integrated the semiconductor laser (self-pulsating DFB laser) of this invention, SOA, and the semiconductor laser for injection light generation, (a) is reverse to the propagation direction of incident signal light. In the configuration example, the output light is obtained in the direction, and (b) is the configuration example in which the output light is obtained in the same direction as the propagation direction of the incident signal light.

Explanation of symbols

  1 n-type InP substrate, 2 n-type InP clad layer, 3 InGaAsP optical waveguide layer, 4 p-type InP clad layer, 5 diffraction grating, 6, 6a, 6b phase shift portion, 7 p-type InGaAsP contact layer, 8 high resistance InP Current confinement layer, 9a, 9b separation groove, 20 n-type electrode, 21, 21a, 21b, 21c p-type electrode, 30 low reflectivity (AR) film, 101, 101a, 101b front DFB region, 102, 102a, 102b phase Control region, 103 Rear DFB region, 104 Semiconductor optical amplifier, 202 Incident side optical waveguide, 202a Outgoing side waveguide, 203 Incident side coupler, 204 Injected light generating semiconductor laser, 211 SOA integrated self-pulsating semiconductor laser.

Claims (11)

A first conductivity type semiconductor substrate;
A first conductivity type cladding layer formed on the semiconductor substrate;
An optical waveguide layer formed on the first conductivity type cladding layer;
A front DFB region including a diffraction grating that includes the optical waveguide layer in part and is positioned forward with respect to the laser light emitting direction and close to the optical waveguide layer surface;
A rear DFB or DBR that includes a part of the optical waveguide layer and includes a diffraction grating that is located rearward with respect to the laser light emitting direction and is close to the surface of the optical waveguide layer, and is electrically separated from the front DFB region. Area,
A second conductivity type cladding layer formed so as to embed the diffraction grating,
The diffraction grating of the front DFB region has a phase shift portion in part;
The laser oscillation wavelength in the stop band of the front DFB region and the laser oscillation wavelength at the end of the stop band of the front DFB region, which are generated by injecting current into each of the front DFB region and the rear DFB region or the DBR region, The diffraction grating of the rear DFB or DBR region is adjusted so that both are located in the high reflectivity part in the stop band generated by the rear DFB or DBR region ,
2. A semiconductor laser according to claim 1, wherein a period of the diffraction grating in the front DFB region is different from a period of the diffraction grating in the rear DFB or DBR region .   The phase of the laser oscillation wavelength in the stop band of the front DFB region is a desired wavelength interval of 0.3 nm to 5.5 nm with respect to an end wavelength of the stop band of the front DFB region. 2. The semiconductor laser according to claim 1, wherein the size of the shift portion is adjusted.   3. The semiconductor laser according to claim 1, wherein the diffraction grating in the rear DFB or DBR region has a phase shift portion.   The front DFB region is composed of two DFB regions and a front DFB phase control region, and the front DFB phase control region is provided between the two DFB regions, and the two DFB regions. The semiconductor laser according to claim 1, wherein the semiconductor laser is electrically isolated from each of the regions.   A front-rear phase control region that is located between the front DFB region and the rear DFB or DBR region and is electrically separated from each of the front DFB region and the rear DFB or DBR region; The semiconductor laser according to any one of claims 1 to 4, wherein   The rear DFB or DBR region is composed of a rear DBR region, and the band gap energy of the crystal constituting the optical waveguide layer in the front-rear phase control region and the rear DBR region is the optical wavelength in the front DFB region. 6. The semiconductor laser according to claim 5, wherein the semiconductor laser is larger than a band gap energy of a crystal constituting the wave layer. And further comprising a current confinement layer formed on both sides of the optical waveguide layer, a semiconductor laser according to any one of claims 1-6. 8. The semiconductor laser according to claim 7 , wherein the current confinement layer is formed by buried crystal growth. A first conductivity type semiconductor substrate;
A first conductivity type cladding layer formed on the semiconductor substrate;
An optical waveguide layer formed on the first conductivity type cladding layer;
A front DFB region including a diffraction grating that includes the optical waveguide layer in part and is positioned forward with respect to the laser light emitting direction and close to the optical waveguide layer surface;
A rear DFB or DBR that includes a part of the optical waveguide layer and includes a diffraction grating that is located rearward with respect to the laser light emitting direction and is close to the surface of the optical waveguide layer, and is electrically separated from the front DFB region. Area,
A second conductivity type cladding layer formed so as to embed the diffraction grating,
The diffraction grating of the front DFB region has a phase shift portion in part;
The laser oscillation wavelength in the stop band of the front DFB region and the laser oscillation wavelength at the end of the stop band of the front DFB region, which are generated by injecting current into each of the front DFB region and the rear DFB region or the DBR region, The diffraction grating of the rear DFB or DBR region is adjusted so that both are located in the high reflectivity part in the stop band generated by the rear DFB or DBR region,
And further comprising a semiconductor optical amplifier which is integrated adjacent to the rear of the front or the rear DFB or DBR region of the front DFB region, semiconductors lasers. 10. A driving method using the semiconductor laser according to claim 9 as a laser having a multi-electrode structure for performing a self-pulsating operation, wherein an input signal light and a continuous light having a laser oscillation wavelength are incident on the semiconductor optical amplifier to generate a clock. A method for driving a semiconductor laser, wherein a reproducing operation is performed. A wavelength conversion element comprising the semiconductor laser according to claim 9 ,
An incident-side optical waveguide into which signal light is incident from one end;
An injection locking semiconductor laser that oscillates at a wavelength that matches the oscillation wavelength of the semiconductor laser,
The incident-side optical waveguide and the injection-locking semiconductor laser are arranged so that the signal light propagated through the incident-side optical waveguide and the output light of the injection-locking semiconductor laser can enter the semiconductor optical amplifier of the semiconductor laser. A wavelength conversion element which is arranged.

Images