optical device

Abstract

This suppresses fluctuations in light output intensity, enabling the frequency of the semiconductor laser to be modulated. [Solution] This optical device comprises a semiconductor laser 100 composed of a laser region 101 and a modulation region 102 formed continuously with respect to the laser region 101. Furthermore, this optical device includes an operating signal generation circuit 103 that supplies modulation signals with inverted phases to the laser region 101 and the modulation region 102. The modulation region 102 is an electric field absorption type modulator with an electric field absorption type optical waveguide structure having a second active layer 121 formed on a substrate 141 as its core.

Description

Technical Field

[0001] The present invention relates to an optical device.

Background Art

[0002] Direct modulation type semiconductor lasers have attracted attention for application as frequency modulation semiconductor lasers for optical ranging systems that enable autonomous driving of automobiles (Non-Patent Document 1). Since the oscillation frequency of this semiconductor laser can be modulated by directly modulating the injection current, it has attracted attention as a low-cost light source for optical ranging systems.

[0003] In a semiconductor laser, the oscillation frequency changes due to the effect of heat generated by the resistance of the element for modulation signals of several kHz or less, but at modulation frequencies higher than that, the oscillation frequency changes due to the change in carrier density in the resonator. Therefore, at modulation frequencies of several kHz or higher, the carrier density in the resonator of the semiconductor laser can be modulated by modulating the injection current, and the oscillation frequency can be modulated through the carrier plasma effect. When the injection current into the semiconductor laser is increased to increase the electron density by ΔN, the refractive index changes due to the carrier plasma effect as shown in the following equation.

[0004]

Equation

[0005] In the above equation, e is the elementary charge, λ is the wavelength, c is the speed of light, η0 is the permittivity in vacuum, n is the refractive index, m

[0004] ,

[0006] , , , , , , , ,

[0005] , , e , represents the effective mass of an electron. By inducing this refractive index variation in the semiconductor laser resonator, the oscillation frequency can be modulated.

[0006] The ranging system employs FMCW (Frequency Modulated Continuous Wave)-LiDAR (Light detection and ranging) technology, which enables high sensitivity, high resolution, and long-range ranging. However, it requires linear sweeping of the laser beam frequency. Furthermore, the laser beam intensity must remain constant during frequency sweeping. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] N. Yokota et al., "Role of modulation bandwidth in narrow spectral linewidth semiconductor lasers under optical negative feedback", Japanese Journal of Applied Physics, vol. 59, no. 10, 102001, 2020. [Non-Patent Document 2] M. Kanno et al., "Measurement of Intrinsic Modulation Bandwidth of Hybrid Modulation Laser", IEEE Photonics Technology Letters, vol. 32, no. 13, pp. 839-842, 2020. [Overview of the project] [Problems that the invention aims to solve]

[0008] However, conventional semiconductor lasers have a problem in that changing the injection current to achieve frequency modulation also changes the output light intensity. In conventional semiconductor lasers, changing the injection current to achieve frequency modulation also changes the output light intensity, as shown in Figure 5.

[0009] This invention was made to solve the above-mentioned problems, and aims to suppress fluctuations in light output intensity and enable modulation of the frequency of a semiconductor laser. [Means for solving the problem]

[0010] The optical device according to the present invention comprises a laser region consisting of a distributed feedback laser formed on a substrate, a modulation region optically connected to the laser region and formed on the substrate, which receives the laser light output from the laser region and modulates the frequency of the laser light by modulating the optical loss through modulation of the input current, and an operating signal generation circuit that supplies modulated signals with inverted phases to the laser region and the modulation region. [Effects of the Invention]

[0011] As described above, the present invention includes an operating signal generation circuit that supplies a modulation signal with inverted phases to the laser region and the modulation region of the semiconductor laser, thereby suppressing fluctuations in optical output intensity and enabling modulation of the frequency of the semiconductor laser. [Brief explanation of the drawing]

[0012] [Figure 1] Figure 1 is a configuration diagram showing the configuration of an optical device according to an embodiment of the present invention. [Figure 2] Figure 2 is a characteristic diagram showing the optical output and frequency response characteristics during modulation of an optical device according to an embodiment of the present invention. [Figure 3] Figure 3 is a configuration diagram showing the configuration of another optical device according to an embodiment of the present invention. [Figure 4] Figure 4 is a configuration diagram showing the configuration of another optical device according to an embodiment of the present invention. [Figure 5] Figure 5 is a diagram showing the configuration of a conventional optical device. [Modes for carrying out the invention]

[0013] Hereinafter, an optical device according to an embodiment of the present invention will be described with reference to Figure 1. This optical device comprises a semiconductor laser 100 composed of a laser region 101 and a modulation region 102 formed continuously with respect to the laser region 101. The modulation region 102 is optically connected to the laser region 101. This optical device also comprises an operating signal generation circuit 103 that supplies modulation signals with inverted phases to the laser region 101 and the modulation region 102. In Figure 1, the semiconductor laser 100 is shown in a schematic cross-section of a plane parallel to the waveguide direction and perpendicular to the substrate 141.

[0014] The laser region 101 is an optical waveguide structure with a first active layer 111 formed on the substrate 141 as its core, and is a distributed feedback laser equipped with a diffraction grating 112 formed in the region where the first active layer 111 is formed as a resonator. The first active layer 111 can be a multi-multilayer quantum well structure made of InGaAlAs with a band edge wavelength of 1.55 μm. For example, the first active layer 111 can be a multilayer quantum well layer using five well layers with a thickness of 6 nm. An anti-reflective film 107 is formed at the edge of the laser region 101.

[0015] The modulation region 102 is an electric field absorption type modulator with an electric field absorption type optical waveguide structure centered on a second active layer 121 formed on a substrate 141. The modulation region 102 receives laser light output from the laser region 101 as input, and modulates the frequency of the laser light by modulating the optical loss through modulation of the input current. A reflector 108 is provided at the end of the modulation region 102. The second active layer 121 can be a multiple quantum well structure made of a quantum well layer of InGaAlAs with a band edge wavelength of 1.43 μm. For example, the second active layer 121 can be a multilayer quantum well layer using five well layers with a thickness of 6 nm. The modulation region 102 modulates the guided light by the quantum confinement Stark effect in the second active layer 121 with a multiple quantum well structure.

[0016] Also, over the entire regions of the laser region 101 and the modulation region 102, a semiconductor layer 142 is formed over the first active layer 111 and the second active layer 121. The semiconductor layer 142 can be formed of, for example, p-type InP. The substrate 141 can function as a lower cladding layer, and the semiconductor layer 142 can function as an upper cladding layer. Although not shown, electrodes can be formed on the back surface of the substrate 141 common to each region.

[0017] Also, the laser region 101 and the modulation region 102 can be formed in a ridge (strip) shape extending in the waveguide direction, with the side surfaces embedded by an embedded insulating layer (not shown) (embedded waveguide structure). The embedded insulating layer can be formed of, for example, BCB (Benzocyclobutene), a resin with a low dielectric constant.

[0018] Also, a first electrode 113 is formed on the semiconductor layer 142 of the laser region 101, and a second electrode 122 is formed on the semiconductor layer 142 of the modulation region 102.

[0019] The operation signal generation circuit 103 outputs a modulation signal obtained by directly amplifying the modulation signal input to the input line 130 to the first output line 132, and outputs an inverted modulation signal obtained by inverting and amplifying the waveform of the modulation signal input to the input line 130 to the second output line 133. In this example, the configuration is such that the modulation signal and DC power are supplied through the bias tees 143 and 144. The modulation signal output to the first output line 132 is combined (superimposed) with DC power by the bias tee 143 and input to the second electrode 122 (modulation region 102). Also, the modulation signal output to the second output line 133 is combined (superimposed) with DC power by the bias tee 144 and input to the first electrode 113 (laser region 101).

[0020] <000010​​​​First, in the first process, as the voltage applied to the laser region 101 increases, the injection current increases, the carrier density in the laser region 101 increases, and the optical output increases.

[0022] Furthermore, in the second process, the reverse bias applied to the modulation region 102 increases, which increases the amount of light absorbed in the modulation region 102. As a result, the threshold value of the semiconductor laser 100 increases, and the carrier density in the resonator increases. In addition, the light output of the semiconductor laser 100 decreases as the threshold value increases.

[0023] In the first and second processes, the carrier density fluctuations in the laser region 101 and the modulation region 102 are in the same direction, which increases the amount of carrier density fluctuation and thus the amount of change in the optical frequency of the oscillating light. Furthermore, since the sign of the optical output fluctuation is reversed between the first and second processes, the fluctuation in output light intensity can be suppressed, and frequency modulation can be achieved while suppressing intensity modulation, as shown in Figure 2.

[0024] The waveguide length of the laser region 101 is 300 mm, and the coupling coefficient of the diffraction grating 112 is 40 cm. -1 By implementing an operating signal generation circuit 103 on a semiconductor laser 100 with an anti-reflective coating 107 reflectivity of 10%, a waveguide length of the modulation region 102 of 150 mm, and a reflectivity of the reflective section 108 of 80%, it was possible to suppress the output light intensity fluctuation to less than 10% and achieve a frequency sweep width of 10 GHz.

[0025] Incidentally, in the optical device according to this embodiment, the operating signal generation circuit 103 can be integrated onto the substrate 141.

[0026] For example, as shown in Figure 3, a laser region 101, a modulation region 102, and an operating signal generation circuit 103a can be integrated on a substrate 141a made of a compound semiconductor such as InP. The laser region 101, the modulation region 102, and the operating signal generation circuit 103a can be made of compound semiconductors. Although not shown in Figure 3, a diffraction grating acting as a resonator is formed in the region of the first active layer 111 of the laser region 101.

[0027] The modulated signal applied to the modulation signal input electrode 131 is amplified by the operation signal generation circuit 103a. The in-phase modulated signal is output to the DC bias and the first wiring 132a, and then output to the modulation region 102 of the semiconductor laser 100. The out-of-phase modulated signal is output to the DC current and the second wiring 133a, and then output to the laser region 101 of the semiconductor laser 100. The operation signal generation circuit 103a also integrates a bias tee, and the DC bias supplied to the DC bias supply electrode 134 is supplied to the modulation region 102 via the first wiring 132a. The DC current supplied to the DC current supply electrode 135 is supplied to the laser region 101 via the second wiring 133a.

[0028] The waveguide length of the laser region 101 is 300 mm, and the coupling coefficient of the diffraction grating is 40 cm. -1 By monolithically integrating a semiconductor laser 100 with an anti-reflective coating 107 having a reflectivity of 10%, a waveguide length of the modulation region 102 being 150 mm, and a reflectivity of the reflective section 108 being 80%, along with an operating signal generation circuit 103a with a mounting area of ​​350 mm x 200 mm, onto a substrate 141a, a compact one-chip optical device measuring 450 mm x 300 mm was realized, achieving a frequency sweep width of 10 GHz while suppressing output light intensity fluctuations to less than 10%.

[0029] Furthermore, as shown in Figure 4, for example, the laser region 101, the modulation region 102, and the operating signal generation circuit 103b can be integrated on the silicon substrate 141b. The operating signal generation circuit 103b is made of silicon, and the laser region 101 and the modulation region 102 are made of compound semiconductors and are hybrid integrated on the silicon substrate 141b. Although not shown in Figure 4, a diffraction grating as a resonator is formed in the region of the first active layer 111 of the laser region 101.

[0030] The modulated signal applied to the modulation signal input electrode 131 is amplified by the operation signal generation circuit 103b. The in-phase modulated signal is output to the DC bias and the first wiring 132a, and then output to the modulation region 102 of the semiconductor laser 100. The out-of-phase modulated signal is output to the DC current and the second wiring 133a, and then output to the laser region 101 of the semiconductor laser 100. The operation signal generation circuit 103b also integrates a bias tee, and the DC bias supplied to the DC bias supply electrode 134 is supplied to the modulation region 102 via the first wiring 132a. The DC current supplied to the DC current supply electrode 135 is supplied to the laser region 101 via the second wiring 133a.

[0031] The waveguide length of the laser region 101 is 300 mm, and the coupling coefficient of the diffraction grating is 40 cm. -1 By hybrid integrating a semiconductor laser 100, with an anti-reflective coating 107 having a reflectivity of 10%, a waveguide length of the modulation region 102 of 150 mm, and a reflectivity of the reflective section 108 of 80%, onto a silicon substrate 141b on which an operating signal generation circuit 103b with a mounting area of ​​350 mm x 200 mm was fabricated, a compact one-chip optical device measuring 450 mm x 300 mm was realized, achieving a frequency sweep width of 10 GHz while suppressing output light intensity fluctuations to less than 10%.

[0032] As described above, according to the embodiment of the present invention, an operating signal generation circuit is provided that supplies modulation signals with inverted phases to the laser region and the modulation region of the semiconductor laser, thereby suppressing fluctuations in optical output intensity and enabling modulation of the frequency of the semiconductor laser. By using this semiconductor laser, frequency modulation sensitivity can be improved, the frequency sweep width can be increased, and the distance measurement resolution in the distance measurement system can be increased.

[0033] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be implemented within the technical concept of the present invention by those with ordinary skill in the art. [Explanation of symbols]

[0034] 100...Semiconductor laser, 101...Laser region, 102...Modulation region, 103...Operation signal generation circuit, 107...Anti-reflective film, 108...Reflective part, 111...First active layer, 112...Diffraction grating, 113...First electrode, 121...Second active layer, 122...Second electrode, 130...Input line, 131...Modulation signal input electrode, 132...First output line, 133...Second output line, 141...Substrate, 142...Semiconductor layer, 143,144...Bias tees.

Claims

1. A laser region consisting of a distributed feedback laser formed on a substrate, A modulation region is formed on the substrate and optically connected to the laser region, and laser light output from the laser region is input. The frequency of the laser light is modulated by the modulation of the input current, thereby modulating the optical loss. An operating signal generation circuit that supplies modulated signals with inverted phases to the laser region and the modulation region, An optical device equipped with [the necessary components].

2. In the optical apparatus according to claim 1, The aforementioned operating signal generation circuit is an optical device integrated on the substrate.

3. In the optical apparatus according to claim 1 or 2, The substrate is composed of a compound semiconductor, and the laser region, the modulation region, and the operating signal generation circuit are composed of compound semiconductors in this optical device.

4. In the optical apparatus according to claim 1 or 2, The optical device comprises a silicon substrate, a silicon operating signal generation circuit, and a compound semiconductor for the laser region and the modulation region.

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