Variable Wavelength Laser and Control Method Therefor

Abstract

A first current injection unit that injects a DBR current into a rear DBR region and a front DBR region and a second current injection unit that injects a phase adjustment current into a phase adjustment region are included. The second current injection unit injects the phase adjustment current that changes at a frequency that is twice as much as that of the DBR current into the phase adjustment region in synchronization with the DBR current. The first current injection unit inverts the DBR current to a positive value in a region in which the DBR current is a negative value.

Claims

1-4. (canceled)

5. A wavelength-variable laser, comprising: a rear DBR region; a phase adjustment region after the rear DBR region; a laser active region after the phase adjustment region; a front DBR region after the laser active region; an amplification region after the front DBR region; a first current injection circuit configured to inject a DBR current into the rear DBR region and the front DBR region; and a second current injection circuit configured to inject a phase adjustment current that changes at a frequency that is twice a frequency of the DBR current into the phase adjustment region in synchronization with the DBR current.

6. The wavelength-variable laser according to claim 5, wherein the first current injection circuit is configured to invert a modulation signal for modulating the DBR current in a region in which the modulation signal is a negative value.

7. A method for controlling a wavelength-variable laser, comprising: injecting a phase adjustment current that changes at a frequency that is twice a frequency of a DBR current injected into a rear DBR region and a front DBR region into a phase adjustment region in synchronization with the DBR current, wherein the wavelength-variable laser comprising: the rear DBR region; the phase adjustment region after the rear DBR region; a laser active region after the phase adjustment region; the front DBR region after the laser active region; and an amplification region after the front DBR region.

8. The method for controlling the wavelength-variable laser according to claim 7, further comprising inverting a modulation signal for modulating the DBR current in a region in which the modulation signal is a negative value.

9. A wavelength-variable laser, comprising: a rear DBR region; a phase adjustment region adjacent to the rear DBR region; a laser active region, wherein the phase adjustment region is between the laser active region and the rear DBR region; a front DBR region, wherein the laser active region is between the front DBR region and the phase adjustment region; a first current injection circuit configured to inject a DBR current into the rear DBR region and the front DBR region; and a second current injection circuit configured to inject a phase adjustment current that changes at a frequency that is twice a frequency of the DBR current into the phase adjustment region in synchronization with the DBR current.

10. The wavelength-variable laser according to claim 9, further comprising an amplification region, wherein the front DBR region is between the amplification region and the laser active region.

11. The wavelength-variable laser according to claim 9, wherein the first current injection circuit is configured to invert a modulation signal for modulating the DBR current in a region in which the modulation signal is a negative value.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] FIG. 1 is a configuration diagram illustrating the configuration of a wavelength-variable laser according to an embodiment of the present invention.

[0036] FIG. 2A is a characteristic diagram showing the change of a modulation signal of a DBR current and a modulation signal of a phase adjustment current with respect to time change of the wavelength-variable laser according to the present invention.

[0037] FIG. 2B is a characteristic diagram showing the relationship between the DBR current and the phase adjustment current of the wavelength-variable laser according to the present invention.

[0038] FIG. 3A is a characteristic diagram showing the change of a DBR current and a phase adjustment current with respect to time change of a conventional wavelength-variable laser.

[0039] FIG. 3B is a characteristic diagram showing the relationship between the DBR current and the phase adjustment current of the conventional wavelength-variable laser.

[0040] FIG. 4A is a characteristic diagram showing the change of the DBR current and the phase adjustment current with respect to time change of the wavelength-variable laser according to the embodiment.

[0041] FIG. 4B is a characteristic diagram showing the relationship between the DBR current and the phase adjustment current of the wavelength-variable laser according to the embodiment.

[0042] FIG. 5 is a cross-sectional view illustrating the configuration of a wavelength-variable laser according to a DBR laser.

[0043] FIG. 6 is computer graphics showing an oscillation wavelength map of the wavelength-variable laser according to the DBR laser.

[0044] FIG. 7 is computer graphics showing an SMSR map of the wavelength-variable laser according to the DBR laser.

[0045] FIG. 8A is a characteristic diagram showing the change of a modulation signal of a DBR current and a modulation signal of a phase adjustment current with respect to time change.

[0046] FIG. 8B is a characteristic diagram showing the relationship between the DBR current and the phase adjustment current.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0047] A wavelength-variable laser according to an embodiment of the present invention is described below with reference to FIG. 1. The wavelength-variable laser includes a rear DBR region 101, a phase adjustment region 102 disposed following the rear DBR region 101, a laser active region 103 disposed following the phase adjustment region 102, a front DBR region 104 disposed following the laser active region 103, and an amplification region 105 disposed following the front DBR region 104.

[0048] The regions are formed so as to share a semiconductor substrate. In the rear DBR region 101 and the phase adjustment region 102, a core formed by a bulk semiconductor is formed on the semiconductor substrate. In the rear DBR region 101, a grating is formed on the core. In the laser active region 103, an active layer having a multi-quantum well structure is formed on the semiconductor substrate. In the front DBR region 104, a core formed by a bulk semiconductor is formed on the semiconductor substrate, and a grating is formed on the core. In the amplification region 105, an active layer having a multi-quantum well structure is formed on the semiconductor substrate. An overclad is formed in the regions in a sharing manner. Those configurations are similar to those of the wavelength-variable laser according to the DBR laser described with reference to FIG. 5.

[0049] The wavelength-variable laser includes a first current injection unit 111 that injects a DBR current into the rear DBR region 101 and the front DBR region 104, and a second current injection unit 112 that injects a phase adjustment current to the phase adjustment region 102. The first current injection unit 111 applies a DBR current obtained by modulating the bias current by a modulation signal to the DBR regions. The second current injection unit 112 injects a phase adjustment current obtained by modulating the bias current by a modulation signal. The first current injection unit 111 inverts the modulation signal in regions in which the modulation signal is a negative value. The wavelength-variable laser includes a third current injection unit 113 that injects a current into the laser active region 103 and a fourth current injection unit 114 that injects a current into the amplification region 105.

[0050] Light generated in the laser active region 103 by injecting a predetermined current into the laser active region 103 by the third current injection unit 113 causes laser oscillation by a resonator formed by the rear DBR region 101, the phase adjustment region 102, and the front DBR region 104. The light is amplified by the amplification region 105 into which a predetermined current is injected by the fourth current injection unit 114, and exits from the right side of the paper of FIG. 1. The oscillation wavelength is determined by the DBR current injected by the first current injection unit 111 and the phase adjustment current injected by the second current injection unit 112.

[0051] In the wavelength-variable laser according to the embodiment, the second current injection unit 112 injects a phase adjustment current that changes at a frequency that is twice as much as that of the DBR current into the phase adjustment region 102 in synchronization with the DBR current. The first current injection unit 111 inverts the modulation signal for modulating the DBR current to a positive value in regions in which the modulation signal is a negative value.

[0052] The abovementioned control is described with reference to FIG. 2A and FIG. 2B. The horizontal axis in FIG. 2A represents time (or phase). The vertical axis in FIG. 2A represents the intensity of the modulation signals. As shown in FIG. 2A, for the regions in which the modulation signal of the DBR current is a negative value, the modulation signal is inverted, and the modulation signal of the phase adjustment current is changed at a frequency that is twice as much as the modulation signal of the DBR current. By controlling (the modulation signals of) the currents as above, a locus drawn when the horizontal axis represents the DBR current and the vertical axis represents the phase adjustment current becomes a locus as that shown in FIG. 2B. By controlling the modulation signal of the DBR current and the modulation signal of the phase adjustment current, sweeping in a form along the form of the wavelength map (see FIG. 6) becomes possible, and the degradation of the SMSR can be suppressed. As described above, when the degradation of the SMSR can be suppressed, the oscillation of the laser light becomes possible with a higher signal-to-noise ratio (S/N).

[0053] Next, the conventional control and the control of embodiments of the present invention are described in comparison with each other. First, the conventional control is described with reference to FIG. 3A and FIG. 3B. In the wavelength-variable semiconductor laser having a DBR structure, a current of 100 mA is applied to the laser active region and a current of 100 mA is applied to the amplification region. Periodically changing currents as those shown in FIG. 3A are applied to the DBR regions and the phase control region. As shown by the relationship in FIG. 3A, the DBR current and the phase adjustment current that change in the same phase with respect to time are applied. Specifically, the bias current is set to 4 mA and the amplitude is set to 3 mA for the DBR current, the bias current is set to 10 mA and the amplitude is set to 9 mA for the phase adjustment current, and oscillation is performed by a cosine wave with a period 0.1 ms. The locus described by the DBR current and the phase adjustment current set as described above forms a straight line as shown in FIG. 3B. When the SMSR of the laser oscillation light at this time is measured, the worst value is 20 dB.

[0054] Next, embodiments of the present invention are described with reference to FIG. 4A and FIG. 4B. In the wavelength-variable semiconductor laser having a DBR structure, a current of 100 mA is applied to the laser active region and a current of 100 mA is applied to the amplification region. Periodically changing currents as those shown in FIG. 4A are applied to the DBR regions and the phase control region. Specifically, with respect to time, for the DBR current, the bias current is set to 0.5 mA, the amplitude is set to 3 mA, oscillation is performed by a cosine wave with a period of 0.1 ms, and then the modulation signal is inverted for the part where the phase is from 90° to 270°. For the phase adjustment current, the bias current is set to 10 mA, the amplitude is set to 9 mA, and oscillation is performed by a cosine wave with a period of 0.051 m.

[0055] The locus described by the DBR current and the phase adjustment current set as described above forms a curved line as shown in FIG. 4B. When the SMSR of the laser oscillation light at this time is measured, the worst value is 40 dB. Therefore, according to embodiments of the present invention, usage as a light source in which the wavelength is continuously variable becomes possible in addition to sufficiently ensuring the S/N ratio of the signal. Therefore, the absorption lines of a plurality of gas can be accurately detected by using the wavelength-variable laser according to embodiments of the present invention. In the description of the abovementioned embodiment, cosine waves are applied to the DBR regions and the phase control region. However, the same effect can be obtained even if other waveforms such as a triangle wave or a sawtooth wave are applied thereto as long as the relationship between the phase and the amplitude is the same because the locus drawn by the DBR current and the phase adjustment current does not change.

[0056] As described above, according to embodiments of the present invention, the phase adjustment current that changes at a frequency that is twice as much as that of the DBR current injected into the rear DBR region and the front DBR region is injected into the phase adjustment region in synchronization with the DBR current, and hence the degradation of the SMSR in the wavelength-variable laser is suppressed.

[0057] The present invention is not limited to the embodiment described above, and it is obvious that various modifications and combinations can be carried out by a person skilled in the art within the technical idea of the present invention.

REFERENCE SIGNS LIST

[0058] 101 Rear DBR region [0059] 102 Phase adjustment region [0060] 103 Laser active region [0061] 104 Front DBR region [0062] 105 Amplification region [0063] 111 First current injection unit [0064] 112 Second current injection unit [0065] 113 Third current injection unit [0066] 114 Fourth current injection unit.