Optical semiconductor element, light control device

Abstract

It comprises: a substrate; an optical modulator (2) disposed on the substrate; an optical waveguide (3) disposed on the substrate, one end of which is connected to the light emission side of the optical modulator (2), and the other end of which is located at the end of the substrate; a phase adjustment unit (4) disposed on the path of the optical waveguide (3) and adjusting the phase of the guided light in the optical waveguide (3); and an optical amplification unit (5) disposed on the path of the optical waveguide (3) and amplifying the power of the guided light in the optical waveguide (3). Through the phase adjustment of the phase adjustment unit (4), the minimum or maximum value of the transmittance spectrum of the ripples that periodically change with frequency due to multiple reflections of light generated between one end and the other end of the optical waveguide (3) is made to match the wavelength of the light input to the optical modulator (2), thereby minimizing the error vector amplitude.

Description

Technical Field

[0001] This disclosure relates to optical semiconductor elements and optical control devices. Background Technology

[0002] In optical communication, to generate optical modulation signals, direct modulation methods are used, where the magnitude of the emitted light power carries the signal by controlling the amount of current injected into the laser diode. Alternatively, external modulation methods are employed, such as using an EA (Electro-Absorption) modulator that absorbs light according to the applied voltage, or an MZ (Mach-Zehnder) modulator that changes the phase according to the applied voltage and extincts the light through interference, to absorb continuous light emitted from the laser diode, thereby enabling the emitted light power and phase to carry the signal. In recent years, with the maturity of semiconductor integration technology, the modulators used in these external modulation methods are mostly integrated with the laser diode, which serves as the light source. Furthermore, to increase the emitted light power, structures integrating optical amplification components such as SOA (Semiconductor Optical Amplifier) ​​into the modulator have been proposed.

[0003] However, at the boundary between these different functions, it is generally difficult to ensure that the light passing through the optical waveguides follows a completely uniform pattern, resulting in some degree of light reflection. Furthermore, at the emission end of the optical semiconductor element, to prevent light from reflecting back into the interior at the interface, an AR (Anti-Reflection) coating is typically formed on the light emission end, or a waveguide is constructed with its shape tilted relative to the end. However, similar to the boundary between different functions, it is difficult to completely suppress light reflection. In the presence of multiple reflection points, light undergoes multiple reflections between these points, thus functioning as a Fabry-Perot etalon, whose transmittance spectrum exhibits a periodic variation with respect to the light frequency. Since the modulated light becomes a spectrum whose width corresponds to the modulation speed, the spectrum of the modulated light after passing through multiple reflection points is distorted, resulting in a deterioration in the quality of the modulated signal.

[0004] Normally, their reflectivity is suppressed to a very low level, so it does not have an effect on the degree of degradation of the modulation characteristics of optical semiconductor elements. However, in the case of SOA formed between multiple reflection boundaries, the reflected light is also amplified, resulting in a degree of characteristic degradation that cannot be ignored.

[0005] As a technology for solving the above-mentioned problems, a technique is disclosed in which reflected light is suppressed into the optical amplification region by tilting the interface between the passive waveguide and the optical amplification region in an MZ-type semiconductor modulator with integrated SOA (Patent Document 1). In this method, by tilting the interface, it is possible to prevent the reflected light generated at the interface from coupling with the optical waveguide, thereby reducing the reflected light into the optical amplification region.

[0006] Patent Document 1: International Publication No. 2018 / 131227

[0007] Although Patent Document 1 discloses a structure for suppressing reflected light within the optical amplification region caused by reflections at the interface between the waveguide and the optical amplification region, it does not consider reflections from other reflection points, such as reflections from the optically combined wave section formed at the exit portion of an MZ-type semiconductor modulator. Furthermore, when manufacturing deviations are also considered, it is difficult to completely suppress reflected light at the tilted interface. Especially when the optical amplification gain of the SOA is high, even if only a trace amount of reflection remains, the transmittance spectrum changes significantly. Summary of the Invention

[0008] This disclosure was made to solve the above-mentioned problems, and aims to provide an optical semiconductor element and optical control device that suppresses negative effects on the spectrum and temporal waveform of signal light even if the reflected light from the reflection point within the element to the optical amplification region cannot be completely suppressed.

[0009] The optical semiconductor element disclosed in this application comprises: a substrate; an optical modulator disposed on the substrate; an optical waveguide disposed on the substrate, one end of which is connected to the light emission side of the optical modulator, and the other end of which is located at the end of the substrate; a phase adjustment unit disposed on the path of the optical waveguide and adjusting the phase of the guided light in the waveguide; and an optical amplification unit disposed on the path of the optical waveguide and amplifying the power of the guided light in the waveguide. By adjusting the phase of the phase adjustment unit, the minimum or maximum value of the transmittance spectrum having ripples that periodically change with frequency due to multiple reflections of light generated between the one end and the other end of the optical waveguide is made to match the wavelength of the light input to the optical modulator, thereby minimizing the error vector amplitude.

[0010] Other features of this disclosure will be set forth below.

[0011] According to the disclosure, by adjusting the phase of the phase adjustment unit, the minimum or maximum value of the transmittance spectrum of the waveguide with which the ripples periodically change with frequency due to multiple reflections of light generated between one end and the other end of the optical waveguide are made to match the wavelength of the light input to the optical modulator, thereby suppressing the negative impact on the spectrum and time waveform of the signal light. Attached Figure Description

[0012] Figure 1 This is a diagram illustrating an example of the structure of an optical semiconductor device.

[0013] Figure 2 This is a diagram illustrating an example of the structure of an optical semiconductor device.

[0014] Figure 3 This is a diagram showing an example of a modulated optical spectrum.

[0015] Figure 4 This is a diagram representing an example of a transmittance spectrum.

[0016] Figure 5 This is a diagram representing an example of a transmittance spectrum.

[0017] Figure 6 This is a diagram representing an example of a transmittance spectrum.

[0018] Figure 7 This is a diagram representing an example of a transmittance spectrum.

[0019] Figure 8 This represents the calculated result of the constellation of the modulated light.

[0020] Figure 9 This represents the calculated result of the constellation of the modulated light.

[0021] Figure 10 This represents the calculated result of the constellation of the modulated light.

[0022] Figure 11 This represents the calculated result of the constellation of the modulated light.

[0023] Figure 12 This is a graph representing the calculation results of EVM.

[0024] Figure 13 This is a graph representing the calculation results of EVM.

[0025] Figure 14 This is a diagram illustrating an example of the structure of a light control device.

[0026] Figure 15 This is a diagram illustrating an example of the time response waveform of a phase-adjusted signal.

[0027] Figure 16 This is a diagram representing an example of a transmittance spectrum.

[0028] Figure 17 This is a diagram illustrating an example of the time response waveform of a phase-adjusted signal.

[0029] Figure 18 This is a diagram representing an example of a transmittance spectrum.

[0030] Figure 19 This is a diagram illustrating an example of the time response waveform of a phase-adjusted signal.

[0031] Figure 20 This is a diagram representing an example of a transmittance spectrum.

[0032] Figure 21 This is a diagram illustrating an example of the time response waveform of a phase-adjusted signal.

[0033] Figure 22 This is a diagram illustrating an example of the structure of a light control device.

[0034] Figure 23 This is a diagram illustrating an example of the structure of an optical semiconductor device.

[0035] Figure 24 This is a diagram illustrating an example of the structure of an optical semiconductor device. Detailed Implementation

[0036] The optical semiconductor element and optical control device according to the embodiments are described with reference to the accompanying drawings. The same or corresponding components are labeled with the same reference numerals, and sometimes repeated descriptions are omitted.

[0037] Implementation Method 1

[0038] Figure 1 This is a top view showing a structural example of the optical semiconductor element 1 according to Embodiment 1. Each functional part of the optical semiconductor element 1 is formed on a substrate. The optical semiconductor element 1 includes an optical modulator 2 disposed on the substrate. The frequency response characteristics of the optical modulator 2 do not have a peak value exceeding 3 dB above the DC component at any frequency. An optical waveguide 3 disposed on the substrate is connected to the optical modulator 2. One end of the optical waveguide 3 is connected to the light emission side of the optical modulator 2, and the other end is located at the end of the substrate. The interface between the optical modulator 2 and the optical waveguide 3 is a first interface 31. The interface between the end of the substrate and the optical waveguide 3 is a second interface 32. The second interface 32 can also be described as the interface between the end of the optical semiconductor element 1 and the optical waveguide 3. This second interface 32 is the port where light guided in the optical waveguide 3 exits to the outside of the optical semiconductor element 1. Light emitted from the optical modulator 2 is guided by the optical waveguide 3 and emitted to the outside.

[0039] Along the path of the optical waveguide 3, there are: a phase adjustment unit 4, which adjusts the phase of the guided light in the optical waveguide 3; and an optical amplification unit 5, which amplifies the power of the guided light in the optical waveguide 3. In other words, the phase adjustment unit 4 and the optical amplification unit 5 are provided on the optical waveguide 3 between the first interface 31 and the second interface 32. The phase adjustment unit 4 can adjust the phase of the guided light by injecting current, applying voltage, or heat. The optical amplification unit 5 can amplify the power of the guided light by injecting current.

[0040] Due to multiple reflections of light generated between the first interface 31 and the second interface 32, a transmittance spectrum with ripples that periodically vary with the frequency of the light is generated. In other words, due to multiple reflections of light generated between one end and the other end of the optical waveguide 3, a transmittance spectrum with ripples that periodically varies with the frequency is generated. In Embodiment 1, the minimum value of this transmittance spectrum is made to match the wavelength of the light input to the optical modulator by phase adjustment of the phase adjustment unit 4.

[0041] When two light reflection points exist, the transmittance spectrum exhibits the same periodic ripples as the Fabry-Perot etalon due to multiple reflections. Since the modulated light has a constant spectral width corresponding to the modulation speed, the spectrum may be distorted when passing through this filter, affecting the signal characteristics. However, by constructing the optical semiconductor element 1 as described above and performing phase adjustment, the signal quality of the modulated light after passing through multiple reflection points can be improved.

[0042] The effects of the above-mentioned structure and adjustment method will be explained below. Figure 2 This diagram illustrates the construction of the optical semiconductor element used in the study. The optical semiconductor element 1 has a substrate based on indium phosphide. As the optical modulator 2, an MZ modulator with a nested structure suitable for polarization multiplexing multi-value intensity phase modulation is employed. The optical semiconductor element 1 includes an input waveguide 6 and an optical branch 7, which branches the light input from the input waveguide 6 into two powers. The optical branch 7 is constructed, for example, of a Y-branch waveguide or MMI (Multi-Mode Interference).

[0043] The light after the seven optical branches is input to nested MZ modulators, namely optical modulators 2a and 2b, which perform multi-value intensity phase modulation. Optical waveguides 3a and 3b are connected to the outputs of optical modulators 2a and 2b, respectively. A phase adjustment section 4a and an optical amplification section 5a are arranged along the path of optical waveguide 3a. A phase adjustment section 4b and an optical amplification section 5b are arranged along the path of optical waveguide 3b. These elements have corresponding... Figure 1 Each element has the same function. Phase adjustment units 4a and 4b are independent and related to... Figure 1 Similarly, phase adjustment is performed. Phase adjustment can be referred to as adjusting the amount of phase adjustment.

[0044] Figure 3 It means to use Figure 2An example of the spectrum of modulated light when optical modulators 2a and 2b are modulated with 16-QAM (Quadrature Amplitude Modulation) at 65.7 Gbaud is given. Here, the frequency response characteristics of optical modulators 2a and 2b are assumed to be those from the following literature.

[0045] F.Hirose et al, "Compact InP MZM Optical Sub-Assembly with Built-inElectrical Filters by Three-Dimensional Packaging Technique," WD2-3, Proc.OECC2019, 2019.

[0046] Depend on Figure 3 As can be seen, the modulated light spectrum has a constant width depending on the modulation speed or the frequency response band of the modulator. Especially in the case of phase modulation such as QAM, the bright line corresponding to the wavelength of the light input to optical modulator 2 at a relative frequency of 0 GHz in the figure disappears, resulting in a broad spectrum.

[0047] Figures 4 to 7 This is a graph showing an example of a transmittance spectrum caused by multiple reflections. These spectra were obtained under the following conditions: the distance between interface 31 and interface 32 is 1.82 mm, the effective refractive index of optical waveguide 3 is 3.3, making the reflectivity of light at interfaces 31 and 32 equivalent to 5.75%, the FSR (Free Spectral Range: resonant frequency interval) is approximately 25 GHz, and the ripple amplitude is approximately 1 dB. In reality, the reflectivity of light is much smaller than this; instead, the amplification effect of light based on SOA is considered. However, it should be noted that the reflectivity of light at interfaces 31 and 32 is an effective value that includes the effect of SOA in the reflectivity. Figures 4 to 7 This also indicates the change when the adjustment amount of the phase adjustment unit 4 is changed. By changing the phase adjustment amount of the phase adjustment unit 4, the transmittance spectrum can be shifted in the frequency direction. When the wavelength of the light input to the light modulator 2, i.e., the relative frequency 0 GHz, coincides with the maximum value of the ripple of the transmittance spectrum, it is defined as 0 degrees, and the coincidence with the minimum value is defined as 180 degrees. Figure 4 It shifted by 0 degrees. Figure 5 It has shifted by 60 degrees. Figure 6 It has shifted by 120 degrees. Figure 7 It is the transmittance spectrum when shifted by 180 degrees.

[0048] Figures 8 to 11This represents the calculated constellation of the modulated light emitted from the optical semiconductor element 1 when the phase adjustment amount of the phase adjustment unit 4 is changed. The circles in the figure represent the complex amplitude of the modulated light at the sign determination time, and the lighter-colored lines represent the migration trajectories of the complex amplitude of the modulated light. Regarding the phase adjustment amount, Figure 8 It is 0 degrees. Figure 9 It is 60 degrees. Figure 10 It is 120 degrees. Figure 11 It is 180 degrees.

[0049] Figure 12 This indicates the result of calculating the modulation precision (Error Vector Magnitude, EVM) used to represent the difference between the complex amplitude at the ideal symbol position and the complex amplitude of the modulated light at the actual symbol determination time, for each phase adjustment amount. Figure 12 In the diagram, circles represent calculation results, and dashed lines represent approximate curves.

[0050] according to Figure 12 When the phase adjustment is 180 degrees, meaning the wavelength of the light input to the optical modulator 2 is adjusted to match the minimum value of the ripple in the transmittance spectrum, the EVM is minimized, resulting in modulated light with good signal quality. That is, the error vector amplitude is minimized. Under these conditions, the modulated light spectrum after passing through multiple reflection points suppresses low-frequency components from DC and enhances high-frequency components, thus achieving the same effect as equalization.

[0051] like Figure 2 In this way, even in a structure consisting of multiple unit structures having optical modulators, optical waveguides, phase adjustment units, and optical amplification units arranged side by side, the EVM can be reduced by allowing each phase adjustment unit belonging to each of the multiple unit structures to perform phase adjustment independently.

[0052] Furthermore, when the maximum phase adjustment of the EVM is 0 degrees, i.e., when the wavelength of the light input to the optical modulator 2 is adjusted to match the maximum value of the ripple of the transmittance spectrum, the modulated light spectrum after passing through multiple reflection points becomes the same phenomenon as the case of modulation band degradation because the high-frequency components are suppressed.

[0053] The shape, material, and positional relationship of the optical semiconductor element 1, optical modulator 2, optical waveguide 3, phase adjustment unit 4, and optical amplification unit 5 are not limited to the above-described cases and can be modified in various ways. For example, the positions of the phase adjustment unit 4 and the optical amplification unit 5 can be interchanged, or the optical amplification unit 5 can be configured to be connected to the end face of the optical semiconductor element 1. In this case, it has the advantage of miniaturizing the size of the optical semiconductor element 1. In addition, the optical modulator 2 does not have to be a nested MZ modulator; it can be a single MZ modulator or an EA modulator. Furthermore, the input waveguide 6 for guiding the input light can be integrated into the optical semiconductor element 1 instead of the input waveguide 6 itself, which serves as the light source.

[0054] In the following embodiments, the differences from Embodiment 1 will be described.

[0055] Implementation Method 2

[0056] Although the optical semiconductor element involved in Embodiment 2 is similar to that in Embodiment 1, the frequency response characteristics of the optical modulator 2 in Embodiment 2 have a peak value exceeding 3 dB above the DC component in high-frequency components that are not 0 GHz. Furthermore, in Embodiment 2, the maximum value of the transmittance spectrum is made consistent with the wavelength of the light input to the optical modulator 2 through phase adjustment of the phase adjustment unit 4. That is, the phase adjustment amount of the phase adjustment unit 4 is adjusted so that the maximum value of the transmittance spectrum, which has ripples that periodically change with respect to the light frequency due to multiple reflections of light between interface 31 and interface 32, is consistent with the wavelength of the light input to the optical modulator 2.

[0057] In Embodiment 1, since the original optical modulators 2a and 2b have frequency response characteristics that do not have peak values ​​exceeding 3 dB above the DC component at any frequency, the phase adjustment amount of the phase adjustment unit 4 is adjusted so that the minimum value of the transmittance spectrum with ripples that periodically vary with the frequency of light matches the wavelength of the light input to the optical modulator 2. This makes it most effective at suppressing low-frequency components and enhancing high-frequency components from a DC perspective.

[0058] However, in cases where, for example, the peaking is large due to the relaxation frequency or the inductive component of the wire, and the optical modulator 2 has frequency response characteristics such as peak values ​​exceeding 3 dB above the DC component in high-frequency components that are not 0 GHz, a solution opposite to that in Embodiment 1 is required. That is, by adjusting the wavelength of the light input to the optical modulator 2 to match the maximum value of the ripple in the transmittance spectrum, high-frequency components can be suppressed in the modulated light spectrum after passing through multiple reflection points, thus enabling the shaping into modulated light with good signal quality.

[0059] In implementation method 2, it is also possible to... Figure 2In this way, multiple optical semiconductor elements are arranged side by side in the structure following the optical modulator.

[0060] Implementation Method 3

[0061] In Embodiment 3, the optical semiconductor element is similar to that in Embodiment 1, and the frequency response characteristics of the optical modulator 2 do not have a peak value exceeding 3 dB above the DC component at any frequency. Furthermore, through phase adjustment by the phase adjustment unit 4, the minimum value of the transmittance spectrum is made consistent with the wavelength of the light input to the optical modulator 2.

[0062] In embodiment 3, the distance from one end of the optical waveguide 3 to the other, i.e., the distance from interface 31 to interface 32, is set such that the resonant frequency interval (FSR) of the ripples of the transmittance spectrum is approximately consistent with 0.5 to 1.25 times the modulation baud rate of the communication signal input to the optical modulator. In other words, the length of the optical waveguide 3 is set in such a way that the resonant frequency interval (FSR) of the ripples of the transmittance spectrum, which varies periodically with respect to the frequency of light, is approximately consistent with 0.5 to 1.25 times the modulation baud rate of the optical modulator 2.

[0063] The effects involved in this implementation method will be explained. Figure 13 This is a diagram showing the results of calculating the EVM using the same method as in Embodiment 1, based on the constellation of the modulated light emitted from the optical semiconductor element 1 when the adjustment amount of the phase adjustment unit 4 is changed. Figure 13 In this embodiment, the resonant frequency interval (FSR) of the ripples in the transmittance spectrum caused by multiple reflections of light generated between interface 31 and interface 32 is set to 25 GHz, 32.9 GHz, 49.3 GHz, 65.7 GHz, 82.1 GHz, 98.6 GHz, and 131.4 GHz, respectively. As an example, it can be seen that when EVM < 13% is used as a reference, the improvement in signal quality is greater when the FSR is 32.9 GHz, 49.3 GHz, 65.7 GHz, and 82.1 GHz. This is because, in this embodiment, by obtaining the effect of emphasizing the frequency response characteristics near the fundamental frequency of half the baud rate, the deviation of the complex amplitude at the symbol determination time is strongly suppressed.

[0064] Implementation Method 4

[0065] The optical semiconductor element involved in Embodiment 4 is similar to that in Embodiment 3, but the frequency response characteristics of the optical modulator 2 have a peak value exceeding the DC component by more than 3 dB in the frequency components that are approximately consistent with the modulation baud rate of the input communication signal between 0.5 and 1.25 times. Similarly to Embodiment 3, the distance from one end to the other of the optical waveguide 3 in Embodiment 4 is set such that the resonant frequency interval of the ripples in the transmittance spectrum is approximately consistent with the modulation baud rate of the communication signal input to the optical modulator between 0.5 and 1.25 times.

[0066] Furthermore, through phase adjustment by phase adjustment unit 4, the maximum value of the transmittance spectrum is made to match the wavelength of the light input to light modulator 2.

[0067] In embodiment 3, since the original optical modulators 2a and 2b have frequency response characteristics that do not have peak values ​​exceeding 3dB above the DC component at all frequencies, it is most effective to adjust the phase adjustment amount of the phase adjustment unit 4 in such a way that the minimum value of the transmittance spectrum with ripples that periodically vary with the frequency of light is consistent with the wavelength of the light input to the optical modulator 2.

[0068] However, for example, sometimes the peaking is large due to the relaxation frequency or the inductive component of the wire, and the optical modulator 2 has a frequency response characteristic with a peak value exceeding 3 dB of the DC component in a frequency component that is approximately consistent with the modulation baud rate between 0.5 and 1.25 times. In this case, contrary to embodiment 3, by performing phase adjustment in a manner that makes the wavelength of the light input to the optical modulator 2 consistent with the maximum value of the ripple of the transmittance spectrum, high-frequency components in the modulated light spectrum after passing through multiple reflection points can be suppressed, thus enabling the shaping of modulated light with good signal quality.

[0069] Implementation Method 5

[0070] Figure 14 This diagram illustrates a structural example of the optical control device according to Embodiment 5. The optical control device according to Embodiment 5 is a variation of Embodiment 1. The frequency response characteristics of the optical modulator 2 according to Embodiment 5 do not have a peak value exceeding 3dB above the DC component at any frequency. A portion of the power of the modulated light 16 emitted from the other end of the optical waveguide 3, i.e., the second interface 32 of the semiconductor element, is split at the beam splitter 17. The light split at the beam splitter 17 is converted into a photocurrent corresponding to the optical power by the light receiving unit 8. The light receiving unit 8 is, for example, a semiconductor photodiode. This photocurrent is detected by the control unit 10. The control unit 10 detects the photocurrent and outputs a phase adjustment signal 11 to the phase adjustment unit 4.

[0071] In addition to applying a phase adjustment amount as a DC component, the control unit 10 applies a low-frequency dither signal with an amplitude of about 1 / 10 of the phase adjustment amount, which the light-receiving unit 8 can respond to, to determine the value of the DC component of the phase adjustment signal 11, thereby minimizing both the AC and DC components of the photocurrent 19. Here, the dither signal is, for example, a rectangular wave AC component of about 1 kHz. Thus, in addition to applying a phase adjustment amount as a DC component, the control unit 10 applies a sufficiently small dither signal with an amplitude of about 1 / 10 of the phase adjustment amount, which the light-receiving unit 8 can respond to, to determine the value of the DC component of the phase adjustment signal 11, thereby minimizing both the AC and DC components of the photocurrent 19 generated in the light-receiving unit 8.

[0072] Explain the effects involved in this implementation method. Figure 15 This is a schematic diagram illustrating the time response waveform of the phase adjustment signal 11. The AC component is superimposed on the DC component. At this point, as... Figure 16 As shown, the transmittance spectrum oscillates relative to the frequency and AC components, thus changing relative to the transmittance spectrum of light. As a result, the time response waveform of the photocurrent 19 changes according to the phase relationship between the light and the transmittance spectrum.

[0073] exist Figure 17 In, such as Figure 16 In this case, when the phase adjustment point is in the middle of the ripple of the transmittance spectrum, the photocurrent 19 becomes a time waveform similar to the phase adjustment signal 11. Figure 18 , 19 This describes the phase relationship between the light and the transmittance spectrum, and the time response waveform of the photocurrent 19, when the adjustment amount of the phase adjustment unit 4 is 0 degrees, i.e., the wavelength of the light input to the light modulator 2 coincides with the maximum value of the ripple of the transmittance spectrum. In this case, since the transmittance fluctuates around the maximum point of the ripple of the transmittance spectrum, the amplitude of the AC component becomes more... Figure 17 The state is small, and the DC component becomes the largest.

[0074] Figure 20 , 21 This describes the phase relationship between the light and the transmittance spectrum, and the time response waveform of the photocurrent 19, when the adjustment amount of the phase adjustment unit 4 is 180 degrees, i.e., the wavelength of the light input to the light modulator 2 coincides with the minimum value of the ripple of the transmittance spectrum. In this case, since the transmittance fluctuates around the minimum point of the ripple of the transmittance spectrum, the amplitude of the AC component becomes more... Figure 17 The state is small and with Figure 19 To the same extent, the DC component becomes the smallest.

[0075] Based on the above, as in this embodiment, by determining the value of the DC component of the phase adjustment signal 11 in a manner that minimizes both the AC and DC components of the photocurrent 19 generated by the photoreceiving unit 8, the phase adjustment can be automatically adjusted to the desired amount.

[0076] Implementation Method 6

[0077] The light control device according to embodiment 6 has the same characteristics as... Figure 14 The optical control device has the same structure as the optical control device. Therefore, the optical control device according to Embodiment 6 includes: a beam splitter 17 that branches a portion of the power of the modulated light 16 emitted from the other end of the optical waveguide 3 to the outside of the optical semiconductor element 1; a light receiving unit 8 that converts the light after being branched by the beam splitter 17 into a photocurrent corresponding to the optical power; and a control unit 10 that detects the photocurrent 19 and outputs a phase adjustment signal 11 to the phase adjustment unit 4.

[0078] The optical control device according to Embodiment 6 is a variation of Embodiment 2. The frequency response characteristics of the optical modulator 2 have a peak value exceeding the DC component by more than 3 dB in the high-frequency components that are not 0 GHz. In addition to applying a phase adjustment amount as a DC component, the control unit 10 applies a low-speed jitter signal with an amplitude of about 1 / 10 of the phase adjustment amount that the light-receiving unit 8 can respond to, in order to determine the value of the DC component of the phase adjustment signal, so that the AC component of the photocurrent is minimized and the DC component is maximized. The jitter signal is, for example, a rectangular wave AC component of about 1 kHz. In this way, by applying a sufficiently small jitter with an amplitude of about 1 / 10 of the phase adjustment amount that the light-receiving unit 8 can respond to, in addition to applying a phase adjustment amount as a DC component, the DC component of the phase adjustment signal 11 is determined in such a way that the AC component of the photocurrent 19 generated in the light-receiving unit 8 is minimized and the DC component is maximized.

[0079] In embodiment 5, the original optical modulators 2a and 2b have frequency response characteristics that do not have peak values ​​exceeding 3 dB above the DC component at any frequency. Therefore, it is most effective to adjust the phase adjustment amount of the phase adjustment unit 4 in such a way that the minimum value of the transmittance spectrum with ripples that periodically vary with the frequency of light is consistent with the wavelength of the light input to the optical modulator 2.

[0080] However, in cases where, for example, the peaking is large due to the relaxation frequency or the inductive component of the wire, and the optical modulator 2 has a frequency response characteristic with peak values ​​exceeding 3 dB above the DC component in high-frequency components that are not 0 GHz, contrary to embodiment 5, by adjusting the wavelength of the light input to the optical modulator 2 in a manner that matches the maximum value of the ripple in the transmittance spectrum, high-frequency components can be suppressed in the modulated light spectrum after passing through multiple reflection points. This allows for shaping into modulated light with good signal quality.

[0081] like Figure 18 , 19 As shown, when the adjustment amount of the phase adjustment unit 4 is 0 degrees, that is, when the wavelength of the light input to the light modulator 2 coincides with the maximum value of the ripple of the transmittance spectrum, the transmittance fluctuates around the maximum point of the ripple of the transmittance spectrum. Therefore, the amplitude of the AC component becomes more... Figure 17 When the state is small, the DC component becomes the largest. Based on the above, as in this embodiment, by determining the value of the DC component of the phase adjustment signal 11 in a way that minimizes the AC component of the photocurrent 19 generated by the photoreceiving unit 8 and maximizes the DC component, the phase adjustment can be automatically adjusted to the desired amount.

[0082] Implementation Method 7

[0083] The light control device in Embodiment 7 is a variation of Embodiments 5 and 6. Figure 22 This diagram illustrates a structural example of the optical control device according to Embodiment 7. The optical control device includes an optical branching section 7, which has the function of splitting the input light into two equal-power branches. The optical branching section 7 is, for example, a Y-branch waveguide or an MMI.

[0084] The system comprises a first unit structure having an optical modulator 2a, an optical waveguide 3a, a phase adjustment section 4a, and an optical amplification section 5a, and a second unit structure having an optical modulator 2b, an optical waveguide 3b, a phase adjustment section 4b, and an optical amplification section 5b. The first and second unit structures are arranged side-by-side. According to one example, the optical modulators 2a and 2b are nested MZ modulators performing multi-valued intensity phase modulation. The phase adjustment section 4a and the optical amplification section 5a are located between the first interface 31a and the second interface 32a. The phase adjustment section 4b and the optical amplification section 5b are located between the first interface 31b and the second interface 32b. The optical branch 7 equally divides the light power into two parts, one incident on the optical modulator 2a of the first unit structure and the other incident on the optical modulator 2b of the second unit structure.

[0085] A polarization rotation combiner is provided, which rotates the polarization of one of a first modulated light 38a emitted from a first unit structure and a second modulated light 38b emitted from a second unit structure, while keeping the polarization of the other unrotated, and combines them. As an example of a polarization rotation combiner, in... Figure 22 Optical prism groups 39 and 40 are shown. Through optical prism groups 39 and 40, the polarization of one of the modulated light 38a and the second modulated light 38b emitted from the second interfaces 32a and 32b is rotated, and they are combined.

[0086] A portion of the power of the modulated light after polarization multiplexing by the polarization rotating combiner is split by the beam splitter 17. Specifically, a portion of the power of the polarization multiplexed modulated light 38c after polarization multiplexing by optical prism groups 39 and 40 is split by the beam splitter 17.

[0087] The light-receiving unit 12 converts the light split by the beam splitter 17 into a photocurrent corresponding to the optical power. The light-receiving unit 12 is, for example, a semiconductor photodiode. The control unit 14 detects the photocurrent 13 generated in the light-receiving unit 12 and outputs phase adjustment signals 15a and 15b from the phase adjustment units 4a and 4b.

[0088] The two phase adjustment signals 15a and 15b will be explained. The control unit 14, acting as the two phase adjustment signals 15a and 15b, applies not only the phase adjustment amount as a DC component, but also a jitter signal with an amplitude of approximately 1 / 10 of the phase adjustment amount and a low frequency that the light-receiving unit can respond to, separated into two different frequencies to a degree of electrical separation. This determines the value of the DC component of the two phase adjustment signals, minimizing the AC components of the two different frequencies of the photocurrent and maximizing or minimizing the DC component. Here, the "two different frequencies of jitter signal separated to a degree of electrical separation" are, for example, rectangular wave AC components of approximately 1 kHz and 100 kHz, i.e., jitter.

[0089] Here, control unit 14,

[0090] • When the frequency response characteristics of the optical modulator do not have a peak value exceeding 3dB above the DC component at all frequencies, the DC component of the photocurrent should be minimized.

[0091] • When the frequency response characteristics of the optical modulator have a peak value exceeding 3dB above the DC component in high-frequency components that are not 0GHz, the DC component of the photocurrent is adjusted to the maximum.

[0092] As in this embodiment, by applying different frequencies of jitter to the phase adjustment units 4a and 4b, the AC component of the photocurrent 13 after polarization multiplexing of the two modulated lights 38a and 38b can be frequency-separated in the control unit. As a result, two different controls can be executed by detecting only one light, enabling a small and low-cost optical control device. Furthermore, the shape, material, and positional relationship of the optical control device are not limited to this embodiment. For example, although the optical prism groups 39 and 40, beam splitter 17, and light-receiving unit 12 are formed by a spatial optical system in this embodiment, they may also be planar optical systems with the same function formed on semiconductor materials such as silicon or indium phosphide, or glass materials such as PLC (Planer Lightwave Circuit), or combinations thereof.

[0093] Implementation Method 8

[0094] Figure 23 This diagram illustrates a structural example of the optical semiconductor element according to Embodiment 8. The optical semiconductor element according to Embodiment 8 is a variation of Embodiment 1 or 2, and includes a first diffraction grating 50 disposed in the path of the optical waveguide 3 at the portion of the phase ratio adjustment section 4 and the optical amplification section 5 near the optical modulator 2. The first diffraction grating 50 reflects a portion of the wavelength of light input to the optical modulator 2. According to one example, in addition to the first diffraction grating 50, a second diffraction grating 52 may also be provided. The second diffraction grating 52 is disposed in the portion of the path of the optical waveguide 3 at the end of the phase ratio adjustment section 4 and the optical amplification section 5 near the substrate. The second diffraction grating 52 reflects a portion of the wavelength of light input to the optical modulator 2.

[0095] Due to multiple reflections of light generated between the diffraction grating 60 and the end face of the optical semiconductor element 1, or between the diffraction gratings 50 and 52, the transmittance spectrum has ripples that periodically vary with the frequency of the light. By adjusting the phase of the phase adjustment unit 4, the minimum or maximum value of the transmittance spectrum is made to match the wavelength of the light input to the optical modulator.

[0096] Here, the minimum value of the transmittance spectrum is consistent with the wavelength of the light input to the optical modulator when the frequency response characteristic of the optical modulator 2 does not have a peak value exceeding 3 dB above the DC component at all frequencies. On the other hand, the maximum value of the transmittance spectrum is consistent with the wavelength of the light input to the optical modulator when the frequency response characteristic of the optical modulator 2 has a peak value exceeding 3 dB above the DC component in high-frequency components that are not 0 GHz.

[0097] According to the optical semiconductor element of Embodiment 8, by pre-forming light reflection points artificially, the FSR (Frequency Separation of Resonant Frequencies) of the ripples in the transmittance spectrum that periodically vary with the light frequency can be set to a desired value. This reduces the impact of manufacturing deviations in the optical semiconductor element 1 and improves the signal quality of the modulated light after passing through multiple reflection points. From the viewpoint of controllability over manufacturing deviations, it is superior to have diffraction gratings in two locations. On the other hand, while controllability is poor when the diffraction grating is in one location, the device can be manufactured to a smaller size, improving both size and tolerance to manufacturing deviations.

[0098] Implementation Method 9

[0099] Figure 24 This diagram illustrates a structural example of the optical semiconductor element according to Embodiment 9. The optical semiconductor element according to Embodiment 9 is a variation of Embodiment 1 or 2, and includes a refractive index modulation region 70 in a portion of the substrate. The refractive index modulation region 70 is provided from the position between the optical modulator 2 and the phase adjustment unit 4 to the end face of the optical semiconductor element 1, or from the position between the optical modulator 2 and the phase adjustment unit 4 to the end face between the optical amplification unit 5 and the optical semiconductor element 1. By providing the refractive index modulation region 70, the effective refractive index of the mode of light guided on the optical waveguide 3 differs from others in this region.

[0100] By providing the refractive index modulation region 70, a first refractive index variation section is generated in the portion of the phase ratio adjustment section 4 and the optical amplification section 5 in the path of the optical waveguide 3 near the optical modulator 2, thereby generating an effective refractive index variation of the guided wave's light mode. Furthermore, when the end of the refractive index modulation region 70 is located between the end of the optical amplification section 5 and the end of the optical semiconductor element 1, a second refractive index variation section is generated in the portion of the phase ratio adjustment section 4 and the optical amplification section 5 in the path of the optical waveguide near the substrate, thereby generating an effective refractive index variation of the guided wave's light mode.

[0101] Due to multiple reflections of light occurring at the interface between the refractive index modulation region 70 and other regions, ripples that periodically vary with the frequency of light are generated in the transmittance spectrum. By adjusting the phase adjustment amount of the phase adjustment unit 4, the minimum or maximum value of the transmittance spectrum is made to match the wavelength of the light input to the light modulator 2.

[0102] Here, the minimum value of the transmittance spectrum is consistent with the wavelength of the light input to the optical modulator 2 when the frequency response characteristic of the optical modulator does not have a peak value exceeding 3 dB above the DC component at all frequencies. On the other hand, the maximum value of the transmittance spectrum is consistent with the wavelength of the light input to the optical modulator 2 when the frequency response characteristic of the optical modulator has a peak value exceeding 3 dB above the DC component in high-frequency components that are not 0 GHz.

[0103] The effective refractive index of the light mode formed on the optical waveguide 3 and the specific construction of the refractive index modulation region 70 with different waveguide structures can be considered as follows: for example, a structure in which an air gap is formed directly below the optical waveguide; a structure in which a material with a different refractive index, such as BCB, is embedded as the cladding of the optical waveguide; a structure in which the width is slightly varied while keeping the center position of the core layer that mainly confines the light equal in the optical waveguide 3.

[0104] According to the optical semiconductor element of Embodiment 9, similarly to Embodiment 8, by artificially forming light reflection points in advance, the FSR of the ripples in the transmittance spectrum that periodically varies with the frequency of light can be set to a desired value. This reduces the impact of manufacturing deviations in the optical semiconductor element 1 and improves the signal quality of the modulated light after passing through multiple reflection points. From the viewpoint of controllability of manufacturing deviations, it is superior to position the refractive index modulation region 70 from between the optical modulator 2 and the phase adjustment unit 4 to between the optical amplification unit 5 and the end face of the optical semiconductor element 1. On the other hand, while the controllability is poor when the refractive index modulation region 70 is positioned from between the optical modulator 2 and the phase adjustment unit 4 to the end face of the optical semiconductor element 1, the device can be manufactured to a smaller size, improving both size and manufacturing deviation tolerance.

[0105] Description of Reference Numerals

[0106] 2... Optical modulator; 3... Optical waveguide; 4... Phase adjustment section; 5... Optical amplification section; 31... First interface; 32... Second interface.

Claims

1. An optical semiconductor element, characterized in that, The optical semiconductor element comprises: substrate; An optical modulator is disposed on the substrate; An optical waveguide is disposed on the substrate, with one end connected to the light emission side of the optical modulator and the other end located at the end of the substrate; A phase adjustment unit is disposed on the path of the optical waveguide and adjusts the phase of the light guiding the wave in the optical waveguide; as well as An optical amplification section is disposed along the path of the optical waveguide and amplifies the power of the light guiding the wave in the optical waveguide. The frequency response characteristics of the optical modulator do not have a peak value exceeding 3dB above the DC component at any frequency. By adjusting the phase of the phase adjustment unit, the minimum value of the transmittance spectrum of the waveguide with which the ripples periodically vary with frequency due to multiple reflections of light generated between one end and the other end of the optical waveguide is made to match the wavelength of the light input to the optical modulator, thereby minimizing the error vector amplitude.

2. The optical semiconductor element according to claim 1, characterized in that, Equipped with optical branching, Multiple unit structures, each comprising the optical modulator, the optical waveguide, the phase adjustment unit, and the optical amplification unit, are arranged side by side. The light after being branched at the optical branching point is respectively input into multiple of the unit structures. Each of the phase adjustment units belonging to each of the multiple unit structures independently performs phase adjustment.

3. The optical semiconductor element according to claim 1, characterized in that, The distance from one end to the other end of the optical waveguide is set in such a way that the resonant frequency interval of the ripples of the transmittance spectrum is approximately consistent with the modulation baud rate of the communication signal input to the optical modulator between 0.5 and 1.25 times.

4. The optical semiconductor element according to claim 1, characterized in that, A first diffraction grating is provided in the path of the optical waveguide, with a portion closer to the optical modulator than the phase adjustment section and the optical amplification section.

5. The optical semiconductor element according to claim 4, characterized in that, A second diffraction grating is provided in the path of the optical waveguide, at the end of the substrate closer to the phase adjustment section and the optical amplification section.

6. The optical semiconductor element according to claim 1, characterized in that, In the path of the optical waveguide, at a portion closer to the optical modulator than the phase adjustment section and the optical amplification section, a first refractive index changing section is provided to effectively change the refractive index of the light mode of the guided wave.

7. The optical semiconductor element according to claim 6, characterized in that, A second refractive index changing section is provided in the portion of the optical waveguide path that is closer to the end of the substrate than the phase adjustment section and the optical amplification section. This second portion is used to change the effective refractive index of the light mode of the guided wave.

8. A light control device, characterized in that, The optical control device includes: The optical semiconductor element according to claim 1; A beam splitter branches a portion of the power of the modulated light emitted from the other end of the optical waveguide toward the outside of the optical semiconductor element. The light-receiving section converts the light after it has been branched by the beam splitter into a photocurrent corresponding to the optical power. as well as The control unit detects the photocurrent and outputs a phase adjustment signal to the phase adjustment unit. In addition to applying a phase adjustment amount as a DC component, the control unit also applies a low-speed frequency jitter signal with an amplitude of about 1 / 10 of the phase adjustment amount that the light-receiving unit can respond to, in order to determine the value of the DC component of the phase adjustment signal, so as to minimize both the AC and DC components of the photocurrent.

9. A light control device, characterized in that, The optical control device includes: The first unit structure and the second unit structure are provided by arranging two unit structures having the optical modulator, the optical waveguide, the phase adjustment section and the optical amplification section as described in claim 1 in parallel; The light branch divides the light power into two equal parts, one of which is incident on the light modulator constructed in the first unit, and the other is incident on the light modulator constructed in the second unit. A polarization-rotating combiner rotates the polarization of one of a first modulated light emitted from the first unit structure and a second modulated light emitted from the second unit structure, while keeping the polarization of the other unrotated, and combines them. A beam splitter branches a portion of the power of the modulated light after polarization multiplexing by the polarization rotating combiner. The light-receiving section converts the light after it has been branched by the beam splitter into a photocurrent corresponding to the optical power. as well as The control unit detects the photocurrent and outputs two phase adjustment signals to the phase adjustment unit constructed in the first unit and the phase adjustment unit constructed in the second unit. In addition to applying a phase adjustment amount as a DC component, the control unit also applies a low-speed jitter signal with an amplitude of about 1 / 10 of the phase adjustment amount that the light-receiving unit can respond to, and which is separated into two different frequencies to a degree of electrical separation, in order to determine the value of the DC component of the two phase adjustment signals, so that the AC component of the two different frequencies of the photocurrent is minimized, and the DC component is minimized.

10. An optical semiconductor element, characterized in that, The optical semiconductor element comprises: substrate; An optical modulator is disposed on the substrate; An optical waveguide is disposed on the substrate, with one end connected to the light emission side of the optical modulator and the other end located at the end of the substrate; A phase adjustment unit is disposed on the path of the optical waveguide and adjusts the phase of the light guiding the wave in the optical waveguide; as well as An optical amplification section is disposed along the path of the optical waveguide and amplifies the power of the light guiding the wave in the optical waveguide. The frequency response characteristics of the optical modulator have a peak value exceeding that of the DC component by more than 3 dB in high-frequency components that are not 0 GHz. By adjusting the phase of the phase adjustment unit, the maximum value of the transmittance spectrum of the waveguide with which the ripples periodically change with frequency due to multiple reflections of light generated between one end and the other end of the optical waveguide is made consistent with the wavelength of the light input to the optical modulator, thereby minimizing the error vector amplitude.

11. The optical semiconductor element according to claim 10, characterized in that, Equipped with optical branching, Multiple unit structures, each comprising the optical modulator, the optical waveguide, the phase adjustment unit, and the optical amplification unit, are arranged side by side. The light after being branched at the optical branching point is respectively input into multiple of the unit structures. Each of the phase adjustment units belonging to each of the multiple unit structures independently performs phase adjustment.

12. The optical semiconductor element according to claim 10, characterized in that, The frequency response characteristics of the optical modulator have a peak value exceeding that of the DC component in frequency components that are approximately consistent with the modulation baud rate of the input communication signal between 0.5 and 1.25 times. The distance from one end to the other end of the optical waveguide is set in such a way that the resonant frequency interval of the ripples of the transmittance spectrum is approximately consistent with the modulation baud rate of the communication signal input to the optical modulator between 0.5 and 1.25 times.

13. The optical semiconductor element according to claim 10, characterized in that, A first diffraction grating is provided in the path of the optical waveguide, with a portion closer to the optical modulator than the phase adjustment section and the optical amplification section.

14. The optical semiconductor element according to claim 13, characterized in that, A second diffraction grating is provided in the path of the optical waveguide, at the end of the substrate closer to the phase adjustment section and the optical amplification section.

15. The optical semiconductor element according to claim 10, characterized in that, In the path of the optical waveguide, at a portion closer to the optical modulator than the phase adjustment section and the optical amplification section, a first refractive index changing section is provided to effectively change the refractive index of the light mode of the guided wave.

16. The optical semiconductor element according to claim 15, characterized in that, A second refractive index changing section is provided in the portion of the optical waveguide path that is closer to the end of the substrate than the phase adjustment section and the optical amplification section. This second portion is used to change the effective refractive index of the light mode of the guided wave.

17. A light control device, characterized in that, The optical control device includes: The optical semiconductor element according to claim 10; A beam splitter branches a portion of the power of the modulated light emitted from the other end of the optical waveguide toward the outside of the optical semiconductor element. The light-receiving section converts the light after it has been branched by the beam splitter into a photocurrent corresponding to the optical power. as well as The control unit detects the photocurrent and outputs a phase adjustment signal to the phase adjustment unit. In addition to applying a phase adjustment amount as a DC component, the control unit also applies a low-speed frequency jitter signal with an amplitude of about 1 / 10 of the phase adjustment amount that the light-receiving unit can respond to, in order to determine the value of the DC component of the phase adjustment signal, so that the AC component of the photocurrent is minimized and the DC component is maximized.

18. A light control device, characterized in that, The optical control device includes: The first unit structure and the second unit structure are provided by arranging two unit structures having the optical modulator, the optical waveguide, the phase adjustment section and the optical amplification section as described in claim 10 in parallel; The light branch divides the light power into two equal parts, one of which is incident on the light modulator constructed in the first unit, and the other is incident on the light modulator constructed in the second unit. A polarization-rotating combiner rotates the polarization of one of a first modulated light emitted from the first unit structure and a second modulated light emitted from the second unit structure, while keeping the polarization of the other unrotated, and combines them. A beam splitter branches a portion of the power of the modulated light after polarization multiplexing by the polarization rotating combiner. The light-receiving section converts the light after it has been branched by the beam splitter into a photocurrent corresponding to the optical power. as well as The control unit detects the photocurrent and outputs two phase adjustment signals to the phase adjustment unit constructed in the first unit and the phase adjustment unit constructed in the second unit. In addition to applying a phase adjustment amount as a DC component, the control unit also applies a low-speed jitter signal with an amplitude of about 1 / 10 of the phase adjustment amount that the light-receiving unit can respond to, and which is separated into two different frequencies to a degree of electrical separation, in order to determine the value of the DC component of the two phase adjustment signals, so that the AC component of the two different frequencies of the photocurrent is minimized and the DC component is maximized.

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