JP2013251424A - Optical integrated device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress increase in oscillation stroke width of a semiconductor laser.SOLUTION: An optical integrated device 100 comprises an optical amplification region 2 configured to amplify output light 9, which is light output from a single mode LD1. The optical amplification region 2 includes a loss control region 6 capable of controlling loss added with the output light 9 passing through the optical amplification region 2.

Description

  The present invention relates to an optical integrated device using a single mode semiconductor laser.

  Along with a dramatic increase in communication demand in recent years, a wavelength division multiplexing communication system that multiplexes a plurality of signal lights having different wavelengths and enables large-capacity transmission with one optical fiber has been put into practical use. As the light source for the system, a single mode LD (Laser Diode) capable of obtaining a high side mode suppression ratio (SMSR) of at least 30 to 40 dB or more is preferable. The single mode LD is, for example, a distributed feedback laser diode (DFB-LD) or a distributed Bragg reflector laser diode (DBR-LD).

  In recent years, the development of wavelength-tunable LDs that can cover the entire wavelength band has been promoted by arraying single-mode LDs and using both element temperature adjustment and refractive index control by current injection. In addition, we have also developed an optical integrated device in which an external modulator such as an electro absorption (EA) type optical modulator or a Mach Zehnder (MZ) type optical modulator is integrated on the same substrate in a wavelength tunable LD. Has been done.

  In the array LD section and the MZ modulator section of the optical integrated element, there are widely used multimode interference (MMI) type optical multiplexing / demultiplexing elements (hereinafter referred to as “MMI elements”) having relatively little wavelength dependency. It is used. Further, a semiconductor optical amplifier (SOA) is integrated in a chip, and the chip is often used to compensate for output increase, waveguide branching loss, connection loss, and scattering loss. The SOA is composed of an epitaxial layer similar to the LD. The SOA functions as an amplifier for incident light when current injection is performed, but is designed so that laser oscillation does not occur by itself.

  Consider a case where a minute reflection point for guided light exists on the output side of an element in which the above-described wavelength tunable LD and SOA are integrated. The reflection point is a position where light is reflected. The return light from the reflection point becomes a level that cannot be ignored when amplified by the SOA. As a result, the operation of the LD becomes unstable and the oscillation line width increases.

  Therefore, for example, Non-Patent Document 1 discloses a technique for providing a curved waveguide on the exit side of the SOA in order to reduce reflected return light, and using an anti-reflection (AR) coating in combination (hereinafter, a conventional technique). (Also referred to as A). In Non-Patent Document 1, the conventional technique A succeeded in reducing the reflection of light from the vicinity of the end face to about −50 dB or less, and has been able to remove the influence on the oscillation line width of the DFB-LD. It has been reported.

“IEEE Journal of Selected Topics in Quantum Electronics”, Vol.11, No.5, 2005, pp919-923

  However, the conventional techniques have the following problems. Specifically, in the optical integrated device of the prior art A including SOA, a bent waveguide is provided to reduce reflection of light from the emission end face. This prevents an increase in the oscillation line width of the DFB-LD.

  However, in the optical integrated device, in addition to the emission end face, a chip (an optical integrated device) may be a reflection point such as a bonding interface between waveguides having slightly different refractive indexes or a bonding portion with a modulator. ) Are in large numbers.

  In particular, in an MMI element, generation of minute reflected light due to mode mismatch is inevitable near the boundary between the multimode waveguide portion and the port waveguide portion. If the reflected light at these locations becomes the return light to the LD via the SOA in the middle, there arises a problem that causes an increase in the oscillation line width as in the case of the emission end face.

  However, the curved waveguide of the prior art A is not effective for reflection points other than the emission end face. Therefore, the curved waveguide of the conventional technique A cannot be applied to reflection points other than the emission end face. For example, in an optical communication system using high-speed modulation of 40 Gbps or more, a narrow oscillation line width of at least about 1 MHz is required, and it is necessary to suppress an increase in the oscillation line width.

  The present invention has been made to solve such a problem, and an object thereof is to provide an optical integrated device capable of suppressing an increase in the oscillation line width of a semiconductor laser.

  In order to achieve the above object, an optical integrated device according to one embodiment of the present invention includes a single-mode semiconductor laser that outputs light, and an optical amplification region that amplifies output light that is light output from the semiconductor laser And the optical amplification region is provided in a waveguide that connects the semiconductor laser and a reflection point that reflects the output light and that exists in the optical integrated element, and through which the output light passes, Includes a loss control region capable of controlling a loss added to the output light passing through the optical amplification region.

  According to the present invention, the optical integrated device includes an optical amplification region that amplifies output light that is light output from the semiconductor laser. The optical amplification region includes a loss control region capable of controlling a loss added to the output light passing through the optical amplification region.

  Thereby, the intensity of the amplified output light can be controlled. Therefore, even if the reflected light reflected at the reflection point enters the light amplification region, the intensity of the reflected light can be suppressed from increasing. That is, it is possible to suppress the intensity of reflected light returning to the semiconductor laser from increasing. Therefore, an increase in the oscillation line width of the semiconductor laser due to the reflected light returning to the semiconductor laser can be suppressed.

It is a top view which shows the structure of the optical integrated element which concerns on Embodiment 1 of this invention. It is sectional drawing of the optical integrated element formed on the board | substrate. It is sectional drawing of the optical integrated element formed on the board | substrate. It is a schematic diagram which shows the relationship between output light and LD return light. It is a schematic diagram which shows the relationship between the input light and output light in SOA with a gain of AdB. It is a schematic diagram which shows the frequency spectrum of SOA input signal light and SOA spontaneous emission light.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following description, the same components are denoted by the same reference numerals. Their names and functions are also the same. Therefore, detailed description thereof may be omitted. Further, in this specification, the optical loss is simply expressed as loss.

  It should be noted that the dimensions, materials, shapes, relative arrangements, and the like of the constituent elements exemplified in the embodiments are appropriately changed depending on the configuration of the apparatus to which the present invention is applied and various conditions. It is not limited to those examples. Moreover, the dimension of each component in each figure may differ from an actual dimension.

<Embodiment 1>
FIG. 1 is a plan view showing a configuration of an optical integrated device 100 according to Embodiment 1 of the present invention. The optical integrated device 100 is an optical integrated circuit. FIG. 1A is a diagram showing a configuration of the optical integrated device 100 in which the reflection portion (reflection point) is inside the optical integrated device 100 (chip). The reflection point is a position where light is reflected. FIG. 1B is a diagram showing a configuration of an optical integrated device 100 having a configuration different from that in FIG. Specifically, FIG. 1B is a diagram illustrating a configuration of the optical integrated device 100 in which the reflection portion (reflection point) is on the light emission end face.

  The optical integrated device 100 according to the first embodiment may be either the optical integrated device 100 in FIG. 1A or the optical integrated device 100 in FIG.

  The optical integrated device 100 in FIG. 1A includes a single mode LD 1, an optical amplification region 2, an MZ modulator 3, and an emission end face 11. Each of the single mode LD 1, the optical amplification region 2, and the MZ modulator 3 is connected by a connection waveguide 4.

  The single mode LD1 is a single mode semiconductor laser that outputs light. The single mode LD1 is used for an optical fiber, for example. The single mode LD1 is a single LD such as a DFB-LD or DBR-LD, or a wavelength tunable LD in which the single LD has a wavelength variable function.

  Hereinafter, the light output from the single mode LD1 is also referred to as output light 9. The output light 9 is LD output light output from the single mode LD1. The output light 9 passes through the optical amplification region 2.

  The optical amplification region 2 amplifies the output light 9. The optical amplification region 2 includes an SOA 5 and a loss control region 6. The SOA 5 is a semiconductor optical amplifier that amplifies light passing through the SOA 5 (output light 9). The SOA 5 is connected to the loss control region 6 by the connection waveguide 4.

  The loss control region 6 is provided at a position adjacent to the SOA 5 (semiconductor optical amplifier) and a position through which output light amplified by the SOA 5 passes. The loss control region 6 adds a loss to light passing through the loss control region 6 according to a voltage applied from the outside, as will be described in detail later. More specifically, the loss control region 6 has a configuration capable of controlling the loss added to the output light passing through the optical amplification region 2. That is, the loss control region 6 controls the loss added to the light according to the voltage (electric field) applied from the outside. In other words, the loss control region 6 controls the insertion loss of light passing through the loss control region 6.

  The MZ modulator 3 modulates light. The MZ modulator 3 includes MMIs 7a and 7b and refractive index controllers 8a and 8b.

  The MMI 7a is an element that demultiplexes light. The MMI 7b is an element that synthesizes the demultiplexed light. That is, the MMIs 7a and 7b are elements for optical multiplexing / demultiplexing. The MMI 7 a is connected to the loss control region 6 and the refractive index control units 8 a and 8 b by the connection waveguide 4. Each of the refractive index controllers 8 a and 8 b is connected to the MMI 7 b by the connection waveguide 4. The MMI 7 b is connected to the emission end face 11 by the connection waveguide 4.

  The emission end surface 11 is a surface from which the optical integrated device 100 emits light to the outside.

  Each of the connection waveguide 4, the SOA 5, the loss control region 6, the MMIs 7a and 7b, and the refractive index control units 8a and 8b is an optical waveguide.

  Next, the operation in the optical integrated device 100 will be described.

  First, the output light 9 is amplified while passing through the optical amplification region 2 and reflected before entering the MZ modulator 3. Specifically, a part of the output light 9 is reflected by the MMI 7 a to become reflected light 10. In other words, the MMI 7 a is a reflection unit having a reflection point that reflects the output light 9. The reflected light 10 is reflected return light that returns to the SOA 5 (single mode LD1) by reflection. Note that most of the output light 9 is modulated by the MZ modulator 3 and then emitted to the outside from the emission end face 11 to become emitted light 12.

  The optical integrated device 100 in FIG. 1B differs from the optical integrated device 100 in FIG. 1A in the arrangement positions of the optical amplification region 2 and the MZ modulator 3. Since the other configuration is the same as that of the optical integrated device 100 of FIG. 1A, detailed description will not be repeated.

  In FIG. 1B, the output light 9 is modulated by the MZ modulator 3 and then enters the optical amplification region 2 and is amplified. A part of the amplified output light 9 is reflected by the emission end face 11 to become reflected light 10. That is, in FIG. 1B, the emission end face 11 is a reflection part having a reflection point that reflects the output light 9. Note that most of the output light 9 is emitted light 12.

  2 and 3 are cross-sectional views of the optical integrated device 100 formed on the InP substrate 14. FIG. 2 is a sectional view of the SOA 5 (buried waveguide) taken along the line A1-A2 of FIG. FIG. 3 is a cross-sectional view of the MMI 7a (high mesa waveguide) taken along line B1-B2 of FIG.

  As shown in FIG. 2, the optical integrated device 100 has a stacked structure in which an InP lower cladding layer 15, an InGaAsP active layer 16, and an InP upper cladding layer 17 are stacked in this order. That is, the SOA 5 of the optical integrated device 100 forms a double hetero waveguide.

  An InP current blocking layer 19 is provided in a region adjacent to the InGaAsP active layer 16.

  As shown in FIG. 3, the MMI 7a of the optical integrated device 100 has a stacked structure in which an InP lower cladding layer 15, an InGaAsP core layer 18, and an InP upper cladding layer 17 are stacked in this order. That is, the optical integrated device 100 constitutes a double hetero waveguide. The MMI 7b has the same configuration as the MMI 7a.

  Each of the InGaAsP active layer 16 and the InGaAsP core layer 18 may be a bulk crystal or a multiple quantum well (MQW).

  2 and FIG. 3 show the cross sections of the SOA 5 and the MMI 7a, respectively, the single mode LD1, the connection waveguide 4 and the loss control region 6 have the same laminated structure.

  Next, reflected return light (reflected light) generated in the optical integrated device 100 will be described.

  FIG. 4 is a schematic diagram showing the relationship between the output light 9 and the LD return light 20. In FIG. 4, only the vicinity of the optical amplification region 2 and the reflection portion 21 in the optical integrated device 100 according to the present embodiment is shown. The LD return light 20 is light that is reflected at the reflection point of the reflection unit 21 and returns to the single mode LD1. The reflection part 21 is the MMI 7a of FIG. 1A or the emission end face 11 of FIG.

  As shown in FIG. 4, the output light 9 passes through the SOA 5 and the loss control region 6 included in the optical amplification region 2, is reflected at the reflection point of the reflection unit 21, and becomes reflected light 10. In other words, the optical amplification region 2 is provided in a waveguide (path) through which the output light 9 passes, which connects the single mode LD 1 and the reflection point of the reflection unit 21.

  The reflected light 10 passes through the loss control region 6 again, so that a loss is added by the loss control region 6. That is, the intensity of the reflected light 10 decreases. The reflected light 10 that has passed through the loss control region 6 becomes LD return light 20 by passing through the SOA 5.

  Here, the intensity of the output light 9 is set as a reference (0 dB), and the gain of the SOA 5 is set to + AdB. In addition, loss (hereinafter also referred to as reflection loss) added to the light reflected by the reflection point of the reflection unit 21 is defined as -CdB. If the loss added by the loss control region 6 is 0 dB, the intensity of the reflected light 10 (reflected return light) is expressed by (AC) dB.

  On the other hand, when the loss added by the loss control region 6 is -BdB, the amount of reflected light is so small that it can be ignored. Therefore, the intensity of the outgoing light 22 from the optical amplification region 2 is approximately (A−B) dB.

  FIG. 5 is a schematic diagram showing a relationship between input light and output light in the SOA 5 having a gain of AdB. In FIG. 5, the SOA output light is output light emitted from the SOA 5. The SOA input light is input light input to the SOA 5.

  The SOA input light is, for example, reflected light 10 that enters the SOA 5 from the loss control region 6 side in FIG. The SOA output light is, for example, the LD return light 20 in FIG.

  As shown in FIG. 5, for example, input light (output light 9) having an intensity of 0 dB is amplified by the SOA 5 to become output light 9 having an intensity + AdB. Here, the loss B added by the loss control region 6 is assumed to be 0 dB. In this case, the intensity of the SOA input light (reflected light 10) is (AC) dB. Further, the intensity of the SOA output light (LD return light 20) amplified by the SOA 5 is (2A-C) dB. Therefore, if the reflection loss -CdB is not sufficiently reduced, the line width of the single mode LD1 into which the SOA output light (LD return light 20) is incident is affected.

  FIG. 6 is a schematic diagram showing frequency spectra of the SOA input signal light 23 and the SOA spontaneous emission light 24. The SOA input signal light 23 is light that enters the SOA 5. The SOA spontaneous emission light 24 is light that the SOA 5 spontaneously emits (outputs) when current is injected into the SOA 5 to generate a gain. In other words, the SOA spontaneous emission light 24 is light emitted when the SOA 5 (semiconductor optical amplifier) is driven.

  As shown in FIG. 6, the spectrum of the SOA spontaneous emission light 24 has a wide wavelength range (about 50 nm or more). On the other hand, when the SOA input signal light 23 is the output light 9 output from the single mode LD1, the spectrum of the SOA input signal light 23 has a unimodal shape.

  In order for the SOA input signal light 23 to be amplified by the SOA 5, the SOA input signal light 23 needs to have at least stronger intensity than the SOA spontaneous emission light 24. On the contrary, when the SOA input signal light 23 has a weak intensity that is comparable to the intensity of the SOA spontaneous emission light 24, an effective gain cannot be obtained, or attenuation due to absorption inside the SOA 5 occurs.

  Here, in FIG. 4, when light travels back and forth through the loss control region 6 where the loss is −B dB, a loss of −2 B dB is added to the light. Hereinafter, the loss that occurs when light reciprocates in the loss control region 6 is also referred to as a reciprocal loss. Therefore, as shown in FIG. 5, the intensity of the SOA input light is (AC-2B) dB. If the intensity of the SOA input signal light 23 is smaller than the intensity of the SOA spontaneous emission light 24, amplification in the SOA 5 hardly occurs.

  Therefore, the loss control region 6 according to the present embodiment adds a loss to the reflected light 10 to reduce the intensity of the reflected light 10 passing through the loss control region 6 in order to hardly cause amplification in the SOA 5. , And the intensity of the SOA spontaneous emission light 24 is reduced to the same level. Here, the reflected light 10 that passes through the loss control region 6 is light that is reflected from the reflection point of the output light 9 at the reflection portion 21 and returns to the SOA 5 (semiconductor optical amplifier). Specifically, the loss control region 6 has an intensity of the reflected light 10 passing through the loss control region 6 that is less than the intensity of the SOA spontaneous emission light 24 and an intensity in the vicinity of the SOA spontaneous emission light 24. Reduce to.

  That is, the loss control region 6 sets the intensity (light quantity) of the reflected light 10 (LD return light 20), which is reflected return light, toward the single mode LD1 to an intensity that does not affect the line width of the single mode LD1. To reduce. This can prevent an increase in the oscillation line width of the single mode LD1.

  In this case, the intensity of the LD return light 20 remains (AC-2B) dB. Therefore, the intensity of the LD return light 20 is improved by (A + 2B) dB as compared with the case where no loss is added by the loss control region 6.

  Next, it is assumed that the gain of the SOA 5 is increased to (A + x) dB in the above state. The xdB is an increase in the gain of the SOA 5. Further, the loss added in the loss control region 6 according to xdB is increased by −x / 2, and control is performed to (−B−x / 2) dB. That is, when the gain of the SOA 5 (semiconductor optical amplifier) is increased, the loss control region 6 adds a loss for canceling out about 1/2 of the increased gain to the output light 9 passing through the loss control region 6. . As a result, it is possible to suppress the LD return light from the reflecting portion 21 (reflection point) while increasing the gain, and to prevent the LD oscillation line width from increasing.

  In this case, the intensity of the output light from the loss control region 6 is increased by x / 2 dB from xx / 2. However, the increase in round-trip loss (−x / 2 × 2 = −xdB) in the loss control region 6 cancels the gain increase (xdB) in the SOA 5. Therefore, the intensity of the LD return light 20 is maintained at (AC-2B) dB as described above.

  In the present embodiment, the loss control region 6 is configured so that absorption (loss) occurs in the loss control region 6 in accordance with the application of an external voltage. The absorption is light absorption. In other words, the loss control region 6 is configured to control the magnitude of the loss added to the output light passing through the optical amplification region 2 by applying an electric field generated by voltage application from the outside. Configure.

  Specifically, electrodes are provided above and below a waveguide (for example, MQW waveguide) in the loss control region 6. Then, the loss control region 6 is configured such that there is almost no absorption when no voltage is applied to the electrode, and absorption occurs when a reverse bias voltage is applied to the electrode. The absorption is caused by the shift of the wavelength position of the absorption peak due to the quantum confined Stark shift. With this configuration, gain control of the optical amplification region 2 including the loss control region 6 can be performed.

  The loss control region 6 is not limited to the above configuration. The loss control region 6 may be configured, for example, by dividing a part of the electrodes of the SOA 5 and decreasing the forward injection current to increase the amount of absorption.

  As described above, the optical integrated device 100 includes the optical amplification region 2 that amplifies the output light 9 output from the single mode LD1. The optical amplification region 2 includes a loss control region 6 that can control the loss added to the output light 9 that passes through the optical amplification region 2. Thereby, the intensity of the output light 9 to be amplified can be controlled. Therefore, even if the reflected light reflected at the reflection point of the reflecting portion 21 enters the light amplification region 2, it is possible to suppress the intensity of the reflected light from increasing. That is, it is possible to suppress the intensity of the reflected light returning to the single mode LD1 from increasing. Therefore, an increase in the oscillation line width of the single mode LD1 (semiconductor laser) due to the reflected light returning to the single mode LD1 (semiconductor laser) can be suppressed.

  Further, the optical integrated device 100 (loss control region 6) according to the present embodiment reduces the intensity (light quantity) of the LD return light 20 from the reflection unit 21 (reflection point) to the extent that amplification by the SOA 5 hardly occurs. Suppress. This can prevent an increase in the oscillation line width of the single mode LD1 (LD). As described above, the integrated optical device 100 can achieve both a narrow oscillation line width and a high light output.

  In the present invention, the embodiments can be appropriately modified and omitted within the scope of the invention.

  The present invention can be used as an optical integrated device capable of suppressing an increase in the oscillation line width of a semiconductor laser.

  1 single mode LD, 2 optical amplification region, 3 MZ modulator, 5 SOA, 6 loss control region, 7a, 7b MMI, 8a, 8b refractive index control unit, 21 reflection unit, 100 optical integrated device.

Claims (5)

An optical integrated device,
A single-mode semiconductor laser that outputs light;
An optical amplification region that amplifies output light that is light output from the semiconductor laser,
The optical amplification region is provided in a waveguide through which the output light passes, connecting the semiconductor laser and a reflection point that reflects the output light, present in the optical integrated device,
The light amplification region is
An optical integrated device including a loss control region capable of controlling a loss added to the output light passing through the optical amplification region. The light amplification region further includes:
Including a semiconductor optical amplifier for amplifying the output light;
The optical integrated device according to claim 1, wherein the loss control region is provided at a position adjacent to the semiconductor optical amplifier and a position through which the output light amplified by the semiconductor optical amplifier passes. The loss control region is light that is output when the semiconductor optical amplifier drives the intensity of the reflected light that is the light reflected from the reflection point of the output light and returns to the semiconductor optical amplifier. The optical integrated device according to claim 1, wherein the optical integrated device is reduced to the same level as the intensity of spontaneous emission light. The optical integrated device according to claim 3, wherein the loss control region adds the loss for canceling out about ½ of the increased gain to the output light when the gain of the semiconductor optical amplifier is increased. The light according to any one of claims 1 to 4, wherein the loss control region controls a magnitude of loss added to the output light passing through the optical amplification region by applying an electric field from the outside. Integrated element.

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