JP4457000B2 - Optical amplifier - Google Patents

Description

  The present invention relates to an optical amplifying apparatus suitable for use in a semiconductor optical amplifying apparatus capable of controlling the gain with respect to signal light by, for example, injection of control light.

With the dramatic increase in communication demand in recent years, the application range of high-capacity and high-speed photonic networks has expanded to the metro access systems familiar to subscribers. A photonic network applied to a metro access system has a flexible network configuration using an optical add drop multiplexer (OADM) or the like.
The optical amplifier used in such a network is capable of high-speed gain control with respect to fluctuations in the number of multiplexed wavelengths and fluctuations in the intensity of input light, and a function that always obtains a constant optical output. (Level control function) is required.

  As an optical amplifier having a level control function, for example, as shown in FIG. 8, a semiconductor optical amplifier (SOA) is provided in both arms 101 and 102 of a Mach-Zehnder interferometer 100 including 3 dB couplers 107 and 108, respectively. There has been proposed an optical amplifier having a configuration in which 103 and 104 are provided and a control SOA 105 is integrated outside the interferometer 100 (see, for example, Patent Document 1).

In this optical amplifier, the signal light is amplified by the SOAs 103 and 104 disposed in both arms 101 and 102 of the Mach-Zehnder interferometer 100. Furthermore, an optical path of oscillation light (control light) is provided in the element so as to intersect the optical path of signal light, and an antireflection film (reflection) having a reflectance of about 0.1% as a feedback mechanism on the optical path of the oscillation light. Mirror) 106 and a gain medium such as SOA are arranged so that the oscillation light oscillates in the element. In addition, as a feedback mechanism, a diffraction grating, a loop waveguide, and the like are also disclosed. In this case, the SOAs 103 and 104 in the arms 101 and 102 also contribute to the laser oscillation of the oscillation light, and the gain with respect to the signal light is kept constant (clamped) at the gain (threshold gain) at the oscillation threshold of the oscillation light. . In this optical amplifier, the gain of the optical amplifier is controlled by changing the threshold gain of the SOAs 103 and 104 in both arms 101 and 102 by adjusting the gain of the control SOA 105 disposed outside the interferometer. I am doing so.
JP 2003-179289 A

By the way, an optical amplifier used in a photonic network is required to have a polarization-independent amplification characteristic whose gain does not change depending on the polarization state of signal light.
In the above-described conventionally proposed optical amplifier (see FIG. 8), a polarization-independent SOA is used for the SOA in the arm, so that a polarization-independent amplification characteristic for the signal light can be realized. it can.

In the above-described conventionally proposed optical amplifier (see FIG. 8), the SOA disposed in the arm and the control SOA disposed outside the interferor have a common active layer. It is normal. For this reason, the control SOA outside the interferometer also has polarization-independent amplification characteristics.
However, the above-described conventionally proposed optical amplifier (see FIG. 8) uses a control SOA having a polarization-independent amplification characteristic, and uses the oscillation so that the gain becomes a constant value. When gain control is performed by changing the injection current (control current) to the control SOA, mode hopping occurs in the control light (oscillation light) in the oscillation state, and further, kinks occur due to this mode hopping. It will be. Thus, when a kink occurs during gain control, the gain (optical gain) of the optical amplifier does not change smoothly with respect to the control current, and continuous gain control becomes difficult. Also, the control circuit becomes complicated.

  By the way, as an optical amplifier having a level control function, for example, as shown in FIG. 9, SOAs 113 and 114 are provided in both arms 111 and 112 of a Mach-Zehnder interferometer 110 including 3 dB couplers 116 and 117, respectively. A configuration in which a DFB laser 115 is integrated as a control laser light source outside the device 110 has also been proposed (see, for example, Japanese Patent Application No. 2004-508538). The end face of the optical amplifier is provided with an antireflection coating (reflectance <0.001%), and antireflection coating films 118 and 119 are formed.

  In this optical amplifier, signal light is amplified by SOAs 113 and 114 disposed in both arms 111 and 112 of the Mach-Zehnder interferometer 110. On the other hand, the control light emitted from the DFB laser 115 arranged outside the interferometer 110 is incident on both SOAs 113 and 114, amplified, and guided in the diagonal direction with the signal light in the element. ing. Then, by adjusting the power of the control light emitted from the DFB laser 115, the gain of the optical amplifier is controlled by the mutual gain modulation effect between the control light and the signal light.

  However, the optical amplifier having such a configuration has the same problem as the above-described optical amplifier (see FIG. 8). That is, even in an optical amplifier having such a configuration, it is usual that the DFB laser 115 has an active layer common to the SOAs 113 and 114. Therefore, the DFB laser 115 also has a polarization-independent amplification characteristic like the SOA. Further, the gain is set to a constant value using oscillation. In this case, when gain control is performed by changing the injection current (control current) to the DFB laser 115 to adjust the power of the control light, mode hopping occurs in the control light in the oscillation state. As a result, kinks are generated [see FIG. 10]. Thus, when a kink occurs during gain control, the gain (optical gain) of the optical amplifier does not change smoothly with respect to the control current, and continuous gain control becomes difficult. Also, the control circuit becomes complicated.

  The present invention was devised in view of such a problem, and an object thereof is to provide an optical amplifying device capable of realizing continuous gain control while realizing polarization-independent amplification characteristics. To do.

For this reason, the optical amplifying device of the present invention includes a symmetric Mach-Zehnder interferometer including a pair of arms connecting the input side coupler, the output side coupler, and the input side coupler and the output side coupler, and a pair of arms. Provided respectively are first and second optical amplifiers for amplifying signal light and control light sources for outputting control light, and the first and second optical amplifiers have polarization-independent characteristics. The control light source is configured such that the standardized gain difference ΔαL between polarizations is 0.15 or more .

The optical amplifying device of the present invention includes an input side coupler, an output side coupler, a symmetric Mach-Zehnder interferometer configured by a pair of arms connecting the input side coupler and the output side coupler, and a pair of arms. Provided with first and second optical amplifiers for amplifying signal light and a control light source for outputting control light, the first and second optical amplifiers having polarization-independent characteristics The control light source is a distributed reflection type laser having an end mirror having a predetermined loss difference between polarizations .

  Therefore, according to the present invention, there is an advantage that continuous gain control can be realized while realizing polarization-independent amplification characteristics.

Hereinafter, an optical amplifying device according to an embodiment of the present invention will be described with reference to the drawings.
In the optical amplifying device (optical amplifying element) according to the present embodiment, an optical amplifier having a polarization-independent characteristic is provided in the arm of a symmetric Mach-Zehnder interferometer, while a gain difference between polarizations or a loss difference between polarizations is provided outside the interferometer. By providing the light source for control light having the above, the mode polarization hopping of the control light is prevented from occurring, and as a result, the occurrence of kinks due to the mode hopping of the polarization mode is prevented.
(First embodiment)
An optical amplifying device according to a first embodiment of the present invention will be described with reference to FIGS.

  The present optical amplifying device is a semiconductor optical amplifying device having a gain control function by injection of control light, and as shown in FIG. 1A, a symmetric Mach-Zehnder interferometer 1 and two semiconductor lights for amplifying signal light. Amplifiers (SOA; first and second optical amplifiers) 2, 3, DFB laser (control light source; control laser light source) 4 that outputs control light, and DFB (Distributed Feed Back) laser 4 The control light waveguide 5 for guiding the control light output from is monolithically integrated on the same semiconductor substrate 6.

In the present embodiment, both end faces (end faces on the signal light input side and signal light output side) 50A, 50B of the element 50 are both provided with a non-reflective coating. That is, as shown in FIG. 1A, non-reflective coating films 7 and 8 (reflectance R; R <0.001%) are formed on both end faces 50A and 50B of the element 50, respectively.
Here, as shown in FIG. 1A, the symmetric Mach-Zehnder interferometer 1 includes two 3 dB couplers 9 and 10 and a pair of arms 11 and 10 connected to these 3 dB couplers 9 and 10 and having the same optical path length. 12 and has a symmetrical structure. Of the two 3 dB couplers, the one provided on the signal light input side is called the input side 3 dB coupler (input side coupler) 9, and the one provided on the signal light output side is the output side 3 dB coupler (output side coupler). ) 10

In the pair of arms 11 and 12, as shown in FIG. 1A, polarization-independent SOAs 2 and 3 are provided, respectively. In other words, two SOAs 2 and 3 having polarization-independent characteristics are arranged inside the symmetric Mach-Zehnder interferometer 1. These SOAs 2 and 3 are configured to have the same length (same SOA length).
Further, as shown in FIG. 1A, an input waveguide 13 for inputting signal light and an output waveguide 14 for outputting signal light are connected to the symmetric Mach-Zehnder interferometer 1, respectively. In other words, the input waveguide 13 is connected to the input side 3 dB coupler 9 of the symmetric Mach-Zehnder interferometer 1, and the output waveguide 14 is connected to the output side 3 dB coupler 10 of the symmetric Mach-Zehnder interferometer 1. In the symmetric Mach-Zehnder interferometer 1, since the phase conditions are matched between the arms 11 and 12 (functionally symmetric), the signal light input from the input waveguide 13 is output at a symmetrical position. The signal is output from the waveguide 14.

  Furthermore, a control light waveguide 5 is connected to the symmetric Mach-Zehnder interferometer 1 as shown in FIG. As shown in FIG. 1A, the control light waveguide 5 includes a waveguide portion (input side portion) 5A connected to the input side 3 dB coupler 9 and a waveguide connected to the output side 3 dB coupler 10. Part (output side part) 5B. The control light is input from the output side portion 5B and output from the input side portion 5A provided at a symmetrical position. Here, the control light propagates in the opposite direction to the input light, and the path (optical path) through which the signal light propagates intersects the path (optical path) through which the control light propagates.

  Further, the input side portion 5A of the control light waveguide 5 reaches the end face 50A on the signal light input side of the element 50, and the output side portion 5B of the control light waveguide 5 is on the signal output side of the element 50. It reaches the end face 50B. As described above, since the antireflection coating films 7 and 8 are formed on the end faces 50A and 50B of the element 50, the antireflection coating is applied to both end faces of the control light waveguide 5 (5A and 5B). Films 7 and 8 are formed.

A DFB laser 4 using laser oscillation is provided as a light source for control light in the waveguide portion 5B connected to the output side 3 dB coupler 10 of the control light waveguide 5. That is, the DFB laser 4 is arranged outside the symmetric Mach-Zehnder interferometer 1.
When the control light emitted by the DFB laser 4 is incident on the SOAs 2 and 3 in the arms 11 and 12, a mutual gain modulation effect is generated between the control light and the signal light. By using this, the gain of signal light by the SOAs 2 and 3 can be suppressed.

Therefore, the gain of the SOAs 2 and 3 can be controlled by changing the amount of control light incident on the SOAs 2 and 3 by controlling the injection current to the DFB laser 4 or the like.
By the way, in this embodiment, as shown in FIGS. 1B and 1C, the SOAs 2 and 3 are made of a strain bulk active layer 25 (InGaAs layer having a strained bulk structure; for example, a strain amount of −0.24%; 50 nm). That is, the SOAs 2 and 3 are polarization independent optical amplifiers having polarization independent characteristics.

Here, the polarization-independent SOAs 2 and 3 refer to those in which the polarization gain difference ΔG (dB) is smaller than a predetermined value (for example, 2 dB) (for example, ΔG <2 dB; within several dB).
On the other hand, the DFB laser 4 is configured to have a strained MQW active layer 28 (for example, a strain amount + 0.8%; for example, a thickness of 50 nm) having a strained multiple quantum well (MQW) structure. This is because it is possible to intentionally provide a large gain difference between polarizations by designing the distorted MQW structure. The strained MQW active layer 28 may be formed of, for example, an InGaAsP material, but other materials may be used.

Thereby, the DFB laser 4 as the light source for control light is configured to have a predetermined gain difference between polarizations. Here, when the length (resonator length) of the DFB laser 4 is L and the material gain difference between polarizations is Δα (α _TE −α _TM ; unit cm ⁻¹ ), The gain difference can be expressed by ΔαL (= α _TE L−α _TM L). Here, in order to avoid inter-polarization mode hopping in the DFB laser 4, the normalized inter-polarization gain difference ΔαL of the DFB laser 4 becomes a predetermined value (approximately 0.15) or more (ΔαL ≧ 0.15). I am doing so. Note that the gain difference between polarizations can be arbitrarily set by selecting the material and the layer structure of the strained MQW active layer 28.

For example, FIG. 2 shows a gain difference ΔαL (= α) between the TE mode gain α _TE L and the TM mode gain α _TM L when the length (resonator length) L of the DFB laser 4 is 300 μm. _TE L-α _TM L) (standardized by resonator length).
In FIG. 2, point A indicates the gain difference ΔαL when the active layer of the DFB laser 4 has a strained MQW structure (strain amount + 0.8%), and point B strains the active layer of the DFB laser 4. A gain difference ΔαL in the case of a bulk active layer (strain amount−0.24%) is shown.

  As shown by a point B in FIG. 2, when the active layer of the DFB laser 4 is a strained bulk active layer (strain amount -0.24%), the gain difference ΔαL between both polarization modes is almost zero. On the other hand, when the active layer of the DFB laser 4 is a strained MQW active layer (strain amount + 0.8%) as shown by a point A in FIG. 2, the gain difference ΔαL between both polarization modes is It is about 1.7, and it can be seen that a large gain difference between polarizations can be obtained.

In the present embodiment, the active layers (gain medium) of the SOAs 2 and 3 and the active layer (gain medium) of the DFB laser 4 have different structures. For this reason, the thickness and the amount of strain are also different from each other. The materials may be different from each other or the same.
Note that the active layers (gain medium) of the SOAs 2 and 3 and the active layer (gain medium) of the DFB laser 4 have the same structure (for example, both have a strained bulk structure), and different materials are used (for example, the SOA 2 and SOA 2). Even if InGaAs is used for the active layer 3 and InGaAsP is used for the active layer of the DFB laser 4, a predetermined gain difference between the polarizations can be obtained.

Here, a relationship between the above-described gain difference ΔG between the SOAs 2 and 3 and the normalized gain difference ΔαL between the DFB laser 4 will be described.
When the SOAs 2 and 3 and the DFB laser 4 have a common active layer having a material gain difference Δα (α _TE −α _TM ; unit cm ⁻¹ ) between polarizations, the length of the _SOAs 2 and 3 is L _SOA (cm). Assuming that the length of the DFB laser 4 is L _DFB (cm), the gain difference ΔG between the SOAs 2 and 3 and the normalized gain difference ΔαL between the DFB lasers 4 can be expressed by the following equations, respectively. .

ΔG = 4.34 * (L _SOA / L _DFB ) * ΔαL _DFB
ΔαL = ΔαL _DFB
As can be seen from these equations, the gain difference ΔG between the _{SOAs 2} and 3 is equal to the length L _{SOA of the SOAs 2} and 3 and the length L _{DFB of the} DFB laser 4 in addition to the normalized polarization difference ΔαL _DFB of the active layer. (L _SOA / L _DFB ).

Here, the gain difference ΔG between the polarizations of the SOAs 2 and 3 influences the final device characteristics, and therefore includes not only the active layer structure but also the lengths L _{SOA of the SOAs 2} and 3 and the length L _DFB of the DFB laser 4. As a total design, it is necessary to make the gain difference ΔG between the SOAs 2 and 3 smaller than, for example, 2 dB (ΔG <2 dB).
For example, when the SOAs 2 and 3 and the DFB laser 4 have a common active layer, the general lengths are L _DFB = 0.03 cm and L _SOA = 0.15 cm (in this case, the length of the SOAs 2 and 3 If the standardized gain difference ΔαL between the DFB lasers 4 is 0.15 or more (ΔαL ≧ 0.15), the SOAs 2 and 3 The gain difference ΔG between polarizations is larger than 3.3 dB (ΔG> 3.3 dB), and polarization-independent characteristics cannot be obtained. Of course, if the lengths of the SOAs 2 and 3 are made extremely short, the gain difference ΔG between the polarizations of the SOAs 2 and 3 can be made smaller than 2 dB (ΔG <2 dB). Since it becomes quite small, it is not realistic.

Therefore, as described above, the SOAs 2 and 3 having the polarization-independent characteristics have the gain difference ΔG (dB) between the polarizations smaller than a predetermined value (for example, 2 dB) (for example, ΔG <2 dB; within several dB). It prescribes.
On the other hand, the DFB laser 4 having a gain difference between polarizations is defined as having a standardized gain difference ΔαL between polarizations greater than or equal to a predetermined value (for example, 0.15) (ΔαL ≧ 0.15).

Thus, if the DFB laser 4 as the light source for control light is configured to have a gain difference between polarizations of a predetermined value (for example, 0.15) or more, the DFB laser 4 as the light source for control light during the gain control operation. Thus, stable single mode oscillation can be obtained, while continuous gain control without kink can be realized.
Next, a method for manufacturing the optical amplifying device according to the present embodiment will be described with reference to FIGS. 1B to 1D as appropriate.

Here, FIG. 1B shows the cross-sectional structure of the SOAs 2 and 3, FIG. 1C shows the cross-sectional structure of the DFB laser 4, and FIG. 1D shows the cross-sectional structure of the optical waveguide. ing.
First, as shown in FIGS. 1A to 1C, on an n-type InP substrate (n-InP substrate) 20, an n-InP clad layer 21 (for example, 1 μm or less in thickness), a diffraction grating layer (InGaAsP layer) For example, an emission wavelength of 1.2 μm; for example, a thickness of 60 nm) 22 and an n-InP cap layer (for example, a thickness of 10 nm) 23 are grown by, for example, a metal organic chemical vapor deposition method (MOVPE method).

Next, a SiO ₂ film is formed on the entire surface, a resist is applied thereon, and then a diffraction grating pattern (for example, a period of 236 nm) is formed only in the region of the DFB laser 4 [see FIG. 1A] by electron beam (EB) exposure. ) Is drawn and patterned to form a SiO ₂ mask. Next, for example, a diffraction grating having a depth of 60 nm is formed by dry etching such as RIE. Then, after removing the SiO ₂ mask, the formed diffraction grating is embedded with n-InP. This portion becomes a part of the n-InP cap layer 23. As a result, a diffraction grating is formed only in the region of the DFB laser 4 of the InGaAsP diffraction grating layer 22 [see FIG. 1 (a)].

  Next, as shown in FIG. 1B, the lower SCH layer 24 (InGaAsP layer; light guide layer; for example, emission wavelength 1.2 μm; thickness 100 nm, for example), strained bulk active layer 25 (InGaAs layer; Amount -0.24%; for example, thickness 50 nm), upper SCH layer 26 (InGaAsP layer; light guide layer; for example, emission wavelength 1.2 μm; for example, thickness 100 nm), p-InP cladding layer 27 (for example, thickness 300 nm) Are grown by, for example, metal organic vapor phase epitaxy (MOVPE) to form a stacked structure including active layers constituting the SOAs 2 and 3.

Next, a SiO ₂ mask (dielectric mask) is formed only in the SOA 2 and 3 regions [see FIG. 1A], and these regions are left until reaching the n-InP cap layer 23 (ie, on the surface side). The p-InP cladding layer 27 to the lower SCH layer 24) are removed by, for example, wet etching.
In the removed region, as shown in FIG. 1C, the lower SCH layer 24 (InGaAsP layer; light guide layer; for example, emission wavelength 1.2 μm; for example, thickness 100 nm), strained MQW active layer 28 ( For example, emission wavelength 1.58 μm; for example, strain amount + 0.8%; for example, thickness 50 nm), upper SCH layer 26 (InGaAsP layer; for example, emission wavelength 1.2 μm; for example, thickness 100 nm), p-InP cladding layer 27 (for example, A thickness of 300 nm) is butt-joint grown by, for example, the MOVPE method to form a laminated structure including an active layer constituting the DFB laser 4.

Thus, in this embodiment, a stacked structure including the active layer of the DFB laser 4 is formed separately from the stacked structure including the active layers of the SOAs 2 and 3. That is, the gain media of the SOAs 2 and 3 and the gain medium of the DFB laser 4 are formed independently (separately).
Next, an SiO ₂ mask (dielectric mask) is formed only in the areas of the SOAs 2 and 3 and the DFB laser 4 [see FIG. 1A], and these areas are left until the n-InP cap layer 23 is reached. (Ie, from the p-InP cladding layer 27 on the surface side to the lower SCH layer 24), for example, by wet etching.

  Next, as shown in FIG. 1D, the waveguide layer 29 (InGaAsP layer; for example, emission wavelength 1.3 μm; for example, thickness 200 nm) and the p-InP cladding layer 27 (for example, thickness 350 nm) are formed by, for example, MOVPE method. The two arms 11 and 12 of the symmetrical Mach-Zehnder interferometer 1, the control light waveguide 5 (5A and 5B), the input waveguide are formed in a region connected to the regions of the SOAs 2 and 3 and the DFB laser 4 by butt joint growth. 13 and the output waveguide 14 are stacked to form an optical waveguide.

Next, an SiO ₂ mask is formed in the areas of the SOAs 2 and 3 and the DFB laser 4 and the optical waveguide area, for example, ICP-RIE (Inductively Coupled Plasma Reactive Ion Etching) method (inductively coupled plasma reactive ion etching method). For example, a waveguide mesa structure having a height of 1.5 μm and a width of 1.5 μm is formed by dry etching such as [see FIGS. 1A to 1D].

Next, as shown in FIGS. 1B to 1D, on both sides of the mesa structure, a p-InP current blocking layer (first current blocking layer) 30 and an n-InP current blocking layer (second current blocking layer) are formed. ) 31 is grown by, for example, the MOVPE method to form a current confinement structure.
Then, after removing the SiO ₂ mask, as shown in FIGS. 1B to 1D, a p-InP cladding layer 32 (for example, a thickness of 3 μm) and an InGaAsP contact layer 33 (for example, an emission wavelength of 1.3 μm) are formed thereon. For example, a thickness of 100 nm) to complete the epitaxial growth.

Although not shown, the InGaAsP contact layer 33 is removed from the wafer having completed the epitaxial growth in this manner, leaving only the areas of the SOAs 2, 3 and the DFB laser 4, and the InGaAsP contact layer 33 in the areas of the SOAs 2, 3 and the DFB laser 4 is removed. Electrodes are formed on the front surface and the back surface of the n-InP substrate 1.
Further, after the cleavage, as shown in FIG. 1A, the non-reflective coating is applied to both end faces 50A and 50B of the element 50 to form the non-reflective coating films 7 and 8.

In the element 50 thus manufactured, as shown in FIG. 1A, signal light is input from the signal light input port to the input waveguide 13 and output from the signal light output port via the output waveguide 14. Will be.
Therefore, the optical amplifying apparatus according to the present embodiment has an advantage that continuous and stable gain control can be realized while realizing polarization-independent signal light amplification characteristics.

That is, according to the present optical amplifying device, since the polarization-independent SOAs 2 and 3 are provided on the arms 11 and 12 of the symmetric Mach-Zehnder interferometer 1, it is possible to realize polarization-independent signal light amplification characteristics. In addition, since the DFB laser 4 having a gain difference between polarizations greater than or equal to a predetermined value is used as the light source for control light, it is possible to prevent mode hopping from occurring, and in turn, generation of kinks due to mode hopping can be prevented. Can be prevented. As a result, it is possible to perform continuous gain control in which the optical gain (optical gain by SOA) smoothly changes with respect to the control current without degrading other characteristics of the optical amplifying device [ As shown in FIG. 3, a high-performance optical amplifying device can be realized. It is also possible to prevent the control circuit from becoming complicated.
(Second Embodiment)
Next, an optical amplifying device and a method for manufacturing the same according to a second embodiment of the present invention will be described with reference to FIGS.

  The optical amplifying apparatus according to the present embodiment is different in structure of the DFB laser from the above-described first embodiment. That is, in the first embodiment described above, the active layer of the DFB laser has a strained MQW structure, which is different from the SOA structure in the arm, whereas in this embodiment, as shown in FIG. The active layer 40 of the laser 4 has a strained bulk structure (strained bulk active layer), which is the same as the structure of the SOAs 2 and 3 in the arms 11 and 12, and is different in that these thicknesses are changed.

Other configurations are the same as those of the first embodiment described above. In FIG. 4, the same components as those in the first embodiment (see FIG. 1) are denoted by the same reference numerals.
In the present embodiment, as in the first embodiment described above (see FIG. 1B), the SOAs 2 and 3 are formed of a strained bulk active layer 25 (InGaAs layer having a strained bulk structure; for example, a strain amount of −0.24). %; For example, a thickness of 50 nm). That is, the SOAs 2 and 3 are polarization independent optical amplifiers having polarization independent characteristics.

Here, the polarization-independent SOAs 2 and 3 refer to those in which the polarization gain difference ΔG (dB) is smaller than a predetermined value (for example, 2 dB) (for example, ΔG <2 dB; within several dB).
On the other hand, the DFB laser 4 is configured to have a strained bulk active layer 40 (InGaAs layer; for example, a thickness of 150 nm) having a strained bulk structure.
Here, the thickness of the active layer 40 of the DFB laser 4 is made larger than the thickness of the active layer 25 of the SOAs 2 and 3.

Here, in order to change the thickness of the active layer 40 of the DFB laser 4 with respect to the thickness of the active layer 25 of the polarization-independent SOAs 2 and 3, for example, a selective growth technique may be used. That is, when the active layer 40 of the DFB laser 4 is formed, a selective growth mask (eg, SiO ₂ mask) 41 is formed around the region where the active layer 40 is formed, as shown in FIG. 5, for example. If the active layer 25 of the SOAs 2 and 3 and the active layer 40 of the DFB laser 4 are grown at the same time, the thickness of the active layer 40 of the DFB laser 4 is larger than the thickness of the active layer 25 of the SOAs 2 and 3. Can be thickened. Here, the SiO ₂ mask 41 has an opening 41A having a width of about 20 μm in the region of the active layer 40, and has a width of about 80 μm on both sides of the opening 41A. In FIG. 5, a dotted line indicates a waveguide including the active layer 40 of the DFB laser 4.

Here, in consideration of practicality, the thickness of the active layer 40 of the DFB laser 4 is increased. However, the thickness is not limited to this, and the thickness of the active layer 40 of the DFB laser 4 is decreased. May be. In short, the thickness of the active layer 40 of the DFB laser 4 may be changed with respect to the thickness of the active layer 25 of the polarization-independent SOAs 2 and 3 that can obtain a polarization-independent optical gain.
As described above, when the thickness of the active layer 40 of the DFB laser 4 is changed with respect to the thickness of the active layer 25 of the polarization-independent SOAs 2 and 3, the optical confinement coefficient differs between the polarization modes. (For example, as the film thickness increases, the optical confinement factor of the TM mode increases and the optical confinement becomes stronger.) This causes a gain difference between the two polarization modes.

  By utilizing this, the thickness of the active layer 40 of the DFB laser 4 is changed, so that the DFB laser 4 as the light source for control light has a predetermined gain difference between polarizations. Here, in order to avoid inter-polarization mode hopping in the DFB laser 4, the normalized inter-polarization gain difference ΔαL of the DFB laser 4 is set to a predetermined value (for example, approximately 0.15) or more (ΔαL ≧ 0.15). It is trying to become. Note that the gain difference between the polarizations can be arbitrarily set by changing the thickness of the strain bulk active layer 40.

For example, FIG. 6 shows a gain difference ΔαL (= α) between the TE mode gain α _TE L and the TM mode gain α _TM L when the length (resonator length) L of the DFB laser 4 is 300 μm. _TE L-α _TM L) (standardized by resonator length).
In FIG. 6, point A indicates the gain difference ΔαL when the thickness of the active layer 40 of the DFB laser 4 is 150 nm, and point B indicates that the thickness of the active layer 40 of the DFB laser 4 is 50 nm. In this case, the gain difference ΔαL is shown.

In FIG. 6, when the thickness of the active layer 40 of the DFB laser 4 is set to 50 nm, the gain difference ΔαL between the two polarization modes is almost zero, as shown by the point B in FIG. As indicated by point A, when the thickness of the active layer 40 of the DFB laser 4 is 150 nm, the gain difference ΔαL between the two polarization modes is about 0.45, and a large gain difference between the polarizations is obtained. I understand that
Thus, in the present embodiment, the active layers (gain medium) of the SOAs 2 and 3 and the active layer (gain medium) of the DFB laser 4 have the same structure and materials, but have different thicknesses. It is said. When the selective growth technique is used, there is a predetermined correlation between the thickness of the active layer and the amount of strain (for example, when the thickness increases in the selective growth of In _1-X Ga _X As, the amount of strain increases in the compressive strain direction. Since the thicknesses are different from each other, it can be defined that the distortion amounts are different from each other.

Thus, if the DFB laser 4 as the light source for control light is configured to have a gain difference between polarizations of a predetermined value (for example, 0.15) or more, the DFB laser 4 as the light source for control light during the gain control operation. Thus, stable single mode oscillation can be obtained, while continuous gain control without kink can be realized.
Next, a method for manufacturing the optical amplifying device according to the present embodiment will be described with reference to FIG. 4 as appropriate. FIG. 4 shows a cross-sectional structure of the DFB laser 4.

First, similarly to the first embodiment described above, a diffraction grating is formed, and the formed diffraction grating is embedded with n-InP.
Next, around the area of the DFB laser 4 [see FIG. 1A], for example, as shown in FIG. 5, the active layer 40 has an opening 41A of about 20 μm, and both sides of the opening 41A. Then, a selective growth mask (for example, SiO ₂ mask) 41 having a width of about 80 μm is formed.

  In this state, the lower SCH layer 24 (InGaAsP layer; light guide layer; for example, emission wavelength 1.2 μm; for example, thickness 100 nm), strained bulk active layer 25 (InGaAs layer; for example, strain amount −0.24%; For example, the thickness is 50 nm), the upper SCH layer 26 (InGaAsP layer; light guide layer; for example, emission wavelength 1.2 μm; for example, thickness 100 nm), the p-InP clad layer 27 (for example, thickness 300 nm), Growth is performed by a growth method (MOVPE method) to form a laminated structure including active layers constituting the SOAs 2 and 3 [see FIG. 1 (b)].

In this embodiment, the stacked structure of the SOAs 2 and 3 and the stacked structure of the DFB laser 4 are the same except for the thickness of the active layer, so that the stacked structure of the SOAs 2 and 3 is formed at the same time. Using the selective growth technique, a stacked structure of the DFB laser 4 is also formed as shown in FIG.
In this case, since the selective growth mask 41 is formed around the region of the DFB laser 4 as described above, the thickness of each layer constituting the DFB laser 4 is equal to the thickness of each layer constituting the SOAs 2 and 3. It becomes thicker than this. In particular, the thickness of the strained bulk active layer 40 (InGaAs layer) of the DFB laser 4 is about 150 nm, for example, and is thicker than the strained bulk active layer 25 of the SOAs 2 and 3 having a thickness of about 50 nm.

As described above, in the present embodiment, the stacked structure including the active layers of the SOAs 2 and 3 and the stacked structure including the active layer of the DFB laser 4 are formed by simultaneous growth using the selective growth technique. That is, the gain media of the SOAs 2 and 3 and the gain medium of the DFB laser 4 are formed simultaneously.
Next, after removing the selective growth mask 41, an SiO ₂ mask (dielectric mask) is formed only in the areas of the SOAs 2, 3 and the DFB laser 4 [see FIG. 1 (a)], leaving these areas. Until the n-InP cap layer 23 is reached (that is, from the p-InP cladding layer 27 on the surface side to the lower SCH layer 24), for example, by wet etching.

Thereafter, the optical amplifying device according to the present embodiment is manufactured through the same manufacturing process as that of the first embodiment.
Therefore, the optical amplifying device according to the present embodiment has the same operations and effects as those of the first embodiment described above.
(Third embodiment)
Next, an optical amplifying device and a method for manufacturing the same according to a third embodiment of the present invention will be described with reference to FIG.

The optical amplifying apparatus according to the present embodiment is provided with a DFB laser as a control light source in the first embodiment described above, whereas a DBR (Distributed Bragg Reflector; distribution) using laser oscillation as a control light source. The difference is that a (reflective) laser is provided.
In the present embodiment, the DBR laser 60 as the control light source is configured to have the same stacked structure as the SOAs 2 and 3 in the arm [see FIG. 1B]. For this reason, the DBR laser 60 is configured to have a strained bulk active layer 61 (InGaAs layer; for example, strain amount -0.24%; for example, thickness 50 nm) having a strained bulk structure.

As shown in FIG. 7, the DBR laser 60 is configured as a part of the control light waveguide 5B, and has an end face mirror 61 and a diffraction grating 62 provided on the end face of the control light waveguide 5B as a reflection mechanism. Configured as a thing.
Here, the end mirror 61 is a region including the control light waveguide 5B, for example, by etching, and is an etched mirror formed by forming a hole 61A at a position spaced apart from the element end surface 50B. What is necessary is just to comprise.

Here, similarly to the above-described first embodiment, the non-reflective coating is applied to both end faces 50A and 50B of the element 50, but the end face of the end mirror 61 is not coated. .
In this case, the reflectance of the end mirror 61 is larger in the TE mode than in the TM mode (TE mode> TM mode). That is, the light loss received when the control light is reflected by the end mirror 61 is larger in the TM mode than in the TE mode (TE mode <TM mode). The DBR laser 60 is configured to have a predetermined inter-polarization loss difference by utilizing the inter-polarization loss difference of the end mirror 61.

Thus, if the DBR laser 60 as the light source for control light is configured to have a predetermined difference in polarization loss, a single TE mode stable by the DBR laser 60 as the light source for control light can be obtained during the gain control operation. While mode oscillation can be obtained, continuous gain control without kink can be realized.
Other configurations are the same as those of the first embodiment described above. In FIG. 7, the same components as those in the first embodiment (see FIG. 1) are denoted by the same reference numerals.

Next, a method for manufacturing the optical amplifying device according to the present embodiment will be described with reference to FIG. 7 as appropriate.
First, similarly to the first embodiment described above, an n-InP clad layer 21, a diffraction grating layer 22, and an n-InP cap layer 23 are formed on an n-InP substrate 20.
Next, a SiO ₂ film is formed on the entire surface, a resist is applied thereon, and then an inner region of the DBR laser 60 region (region on the symmetric Mach-Zehnder interferometer 1 side; diffraction grating region) by electron beam (EB) exposure. A diffraction grating pattern (for example, a period of 236 nm) is drawn and patterned only in [see FIG. 7] to form a SiO ₂ mask. Next, for example, a diffraction grating 62 having a depth of 60 nm is formed by dry etching such as RIE. Then, after removing the SiO ₂ mask, the formed diffraction grating 62 is buried with n-InP, and this portion becomes a part of the n-InP cap layer 23. As a result, the diffraction grating 62 is formed only in the inner region [see FIG. 7] of the region of the DBR laser 60 of the InGaAsP diffraction grating layer 22.

  Next, as in the first embodiment described above (see FIG. 1B), the lower SCH layer 24 (InGaAsP layer; light guide layer; for example, emission wavelength 1.2 μm; for example, thickness 100 nm), strain bulk activity Layer 25 (InGaAs layer; for example, strain amount -0.24%; for example, thickness 50 nm), upper SCH layer 26 (InGaAsP layer; light guide layer; for example, emission wavelength 1.2 μm; for example, thickness 100 nm), p-InP cladding A layer 27 (for example, a thickness of 300 nm) is grown by, for example, metal organic vapor phase epitaxy (MOVPE method), and a stacked structure including active layers constituting the SOAs 2 and 3 and an active layer constituting the DBR laser 60 are formed. A laminated structure is formed.

As described above, in this embodiment, the stacked structure including the active layers of the SOAs 2 and 3 and the stacked structure including the active layer of the DBR laser 60 are formed by simultaneous growth. That is, the gain medium of the SOAs 2 and 3 and the gain medium of the DBR laser 60 are formed simultaneously.
Next, as in the first embodiment described above, an SiO ₂ mask (dielectric mask) is formed only in the regions of the SOAs 2 and 3 and the DBR laser 60 [see FIG. 7], and these regions are left to form n-InP. Until the cap layer 23 is reached (that is, from the p-InP clad layer 27 on the surface side to the lower SCH layer 24), it is removed by wet etching, for example.

  Next, similarly to the first embodiment described above (see FIG. 1D), the waveguide layer 29 (InGaAsP layer; for example, emission wavelength 1.3 μm; for example, thickness 200 nm), the p-InP cladding layer 27 (for example, thickness) 350 nm) is butt-joint grown by, for example, the MOVPE method, and the two arms 11 and 12 of the symmetrical Mach-Zehnder interferometer 1 and the control light waveguide 5 ( 5A, 5B), a laminated structure constituting the optical waveguide of the input waveguide 13 and the output waveguide 14 is formed.

Thereafter, electrodes are formed on the surface of the InGaAsP contact layer 33 and the back surface of the n-InP substrate 1 in the regions of the SOAs 2 and 3 and the DBR laser 60 through the same manufacturing process as in the first embodiment.
Thereafter, in the present embodiment, the control light waveguide 5B is located at a predetermined distance from the outer region of the region of the DBR laser 60 (region on the element end face 50B side; end face mirror region), that is, the element end face 50B. An SiO ₂ mask (insulator mask) having an opening in the upper region is formed. Next, as shown in FIG. 7, for example, a hole 61A having a depth of 5 μm is formed by dry etching such as ICP-RIE. The wall surface of the hole 61A thus formed becomes the end face of the control light waveguide 5B, and the end face mirror 61 as an etched mirror is formed.

Then, after the cleavage, as shown in FIG. 1A, the non-reflective coating is formed on the both end faces 50A and 50B of the element 50 to form the non-reflective coating films 7 and 8, and the light according to the present embodiment. An amplification device is manufactured.
Therefore, the optical amplifying device according to the present embodiment has the same operations and effects as those of the first embodiment described above.

  In addition, the manufacturing method of the optical amplifying device concerning each above-mentioned embodiment is only what showed an example, You may manufacture using another growth method and process. For example, a selective growth technique may be used. The layer structure of the optical amplifying device is merely an example, and other layer structures may be used. For example, in each of the above-described embodiments, the optical amplifying device is configured by an InGaAsP / InP-based material, but other materials such as an InAlGaAs / InP-based material may be used. Further, for example, a laminated structure having another configuration may be formed on a p-type substrate or a semi-insulating substrate.

In each of the above embodiments, the current confinement structure in the waveguide structures of the SOAs 2 and 3, the DFB laser 4, and the optical waveguide is a buried hetero structure (BH structure). However, the present invention is not limited to this. For example, a current confinement structure such as a high mesa structure, a ridge structure, or a semi-insulating buried hetero structure (SI-BH structure, SI-PBH structure) may be used.
In each of the above-described embodiments, the signal light and the control light are propagated in the opposite directions to the in-arm SOAs 2 and 3 in order to suppress an increase in the noise of the amplifier. For example, the signal light and the control light may be propagated in the same direction. In this case, a control light source may be provided in the input side portion 5A of the control light waveguide 5.

  In each of the above-described embodiments, a Mach-Zehnder interferometer having a symmetric structure having the same arm length and the same SOA length is used as the Mach-Zehnder interferometer. However, the present invention is not limited to this. For example, an asymmetric Mach-Zehnder interferometer having a pair of arms having different arm lengths may be used. Further, for example, Mach-Zehnder interferometers having an asymmetric structure in which the lengths of the SOAs 2 and 3 are different (having different SOA lengths) may be used. In this case, it is preferable to arrange a phase adjuster on one side or both sides of the arm. The phase adjuster can be integrated by forming electrodes so that a current can be injected into a part of the waveguide.

The present invention is not limited to the configurations described in the above-described embodiments and other columns, and various modifications can be made without departing from the spirit of the present invention.
(Appendix 1)
A Mach-Zehnder interferometer comprising an input side coupler, an output side coupler, and a pair of arms connecting the input side coupler and the output side coupler;
First and second optical amplifiers provided on the pair of arms, respectively, for amplifying signal light;
A control light source that outputs control light;
The first and second optical amplifiers are configured to have polarization-independent characteristics;
The optical amplification device, wherein the control light source is configured to have a predetermined gain difference between polarizations or loss difference between polarizations.

(Appendix 2)
The optical amplifying device according to appendix 1, wherein the gain medium of the first and second optical amplifiers and the gain medium of the light source for control light are configured with different structures or materials.
(Appendix 3)
The first and second optical amplifiers are configured to have an active layer having a strained bulk structure,
The optical amplifying apparatus according to appendix 2, wherein the control light source is configured to have an active layer having a strained multiple quantum well structure.

(Appendix 4)
2. The optical amplifying apparatus according to claim 1, wherein the gain medium of the first and second optical amplifiers and the gain medium of the control light source are configured to have different thicknesses or distortion amounts.
(Appendix 5)
The optical amplifying apparatus according to appendix 4, wherein the first and second optical amplifiers and the control light source are both configured to have an active layer having a strained bulk structure.

(Appendix 6)
The optical amplifying apparatus according to appendix 1, wherein the light source for control light is a laser using laser oscillation.
(Appendix 7)
The optical amplifying apparatus according to appendix 6, wherein the light source for control light is a distributed feedback laser having a predetermined gain difference between polarizations.

(Appendix 8)
7. The optical amplifying apparatus according to appendix 6, wherein the light source for control light is a distributed reflection type laser having an end face mirror having a predetermined polarization loss difference.
(Appendix 9)
The control light waveguide has a waveguide portion connected to the output-side coupler;
The optical amplification device according to any one of appendices 1 to 8, wherein the light source for control light is provided in the waveguide portion.

(Appendix 10)
Additional notes 1 to 9, wherein the Mach-Zehnder interferometer, the control light waveguide, the first and second optical amplifiers, and the control light source are all provided on the same semiconductor substrate. The optical amplification device according to any one of the above.
(Appendix 11)
A Mach-Zehnder interferometer comprising an input side coupler, an output side coupler, and a pair of arms connecting the input side coupler and the output side coupler;
First and second optical amplifiers provided on the pair of arms, respectively, for amplifying signal light;
A control light source that outputs control light;
The first and second optical amplifiers are configured such that the gain difference ΔG between polarizations is smaller than 2 dB,
The optical amplifying apparatus, wherein the control light source is configured such that a standardized gain difference ΔαL between polarizations is 0.15 or more.

BRIEF DESCRIPTION OF THE DRAWINGS (a)-(d) is a schematic diagram which shows the structure of the optical amplifier concerning 1st Embodiment of this invention, (a) is the top view, (b) is a cross section which shows the structure of SOA. It is a figure, (c) is sectional drawing which shows the structure of a DFB laser, (d) is sectional drawing which shows the structure of an optical waveguide. It is a figure for demonstrating the gain difference between polarization | polarized-lights in the DFB laser in the optical amplification apparatus concerning 1st Embodiment of this invention. It is a figure for demonstrating the effect of the optical amplifier concerning 1st Embodiment of this invention, Comprising: It is a figure which shows the change of the optical gain with respect to a control current. It is typical sectional drawing which shows the structure of the DFB laser contained in the optical amplification apparatus concerning 2nd Embodiment of this invention. It is a typical top view for demonstrating the formation method of the active layer of the DFB laser contained in the optical amplifier concerning 2nd Embodiment of this invention. It is a figure for demonstrating the gain difference between polarization | polarized-lights in the DFB laser in the optical amplifier concerning 2nd Embodiment of this invention. It is a typical top view which shows the structure of the optical amplifier concerning 3rd Embodiment of this invention. It is a figure for demonstrating the structure of the conventional optical amplifier. It is a figure for demonstrating the structure of the conventional optical amplifier. It is a figure for demonstrating the subject of the conventional optical amplifier, Comprising: It is a figure which shows the change of the optical gain with respect to control current.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Symmetric Mach-Zehnder interferometer 2,3 Semiconductor optical amplifier (SOA; 1st and 2nd optical amplifier)
4 DFB laser (light source for control light)
5 Waveguide for control light 6 Semiconductor substrate 7, 8 Non-reflective coating 9 Input side 3 dB coupler (input side coupler)
10 Output side 3dB coupler (Output side coupler)
11, 12 Arm 13 Input waveguide 14 Output waveguide 20 n-type InP substrate (n-InP substrate)
21 n-InP cladding layer 22 InGaAsP diffraction grating layer 23 n-InP cap layer 24 Lower SCH layer (InGaAsP layer; light guide layer)
25, 40 Strained bulk active layer (InGaAs layer)
26 Upper SCH layer 24 (InGaAsP layer; light guide layer)
27 p-InP cladding layer 28 MQW active layer 29 Waveguide layer (InGaAsP layer)
30 p-InP current blocking layer (first current blocking layer)
31 n-InP current blocking layer (second current blocking layer)
32 p-InP cladding layer 33 InGaAsP contact layer 41 SiO ₂ mask 41A opening 50 element 50A, 50B end face (end faces on the signal light input side and signal light output side)
60 DBR laser (light source for control light)
61 End mirror 61A Hole 62 Diffraction grating

Claims (10)

A Mach-Zehnder interferometer comprising an input side coupler, an output side coupler, and a pair of arms connecting the input side coupler and the output side coupler;
First and second optical amplifiers provided on the pair of arms, respectively, for amplifying signal light;
A control light source that outputs control light;
The first and second optical amplifiers are configured to have polarization-independent characteristics;
The optical amplifying apparatus, wherein the control light source is configured such that a standardized gain difference ΔαL between polarizations is 0.15 or more . A Mach-Zehnder interferometer comprising an input side coupler, an output side coupler, and a pair of arms connecting the input side coupler and the output side coupler;
First and second optical amplifiers provided on the pair of arms, respectively, for amplifying signal light;
A control light source that outputs control light;
The first and second optical amplifiers are configured to have polarization-independent characteristics;
The optical amplification device , wherein the light source for control light is a distributed reflection type laser having an end mirror having a predetermined polarization loss difference . The optical amplifying apparatus according to claim 1 or 2, wherein the gain medium of the first and second optical amplifiers and the gain medium of the control light source are configured with different structures or materials. . The first and second optical amplifiers are configured to have an active layer having a strained bulk structure,
4. The optical amplifying apparatus according to claim 3 , wherein the control light source is configured to have an active layer having a strained multiple quantum well structure. A gain medium of said first and said control light source and gain medium of the second optical amplifier, characterized in that the thickness or strain amount is configured as a different light according to claim 1 or 2 Amplification equipment. 6. The optical amplifying device according to claim 5 , wherein each of the first and second optical amplifiers and the light source for control light has an active layer having a strained bulk structure. 2. The optical amplifying apparatus according to claim 1, wherein the control light source is a distributed feedback laser having a predetermined gain difference between polarizations . The control light waveguide has a waveguide portion connected to the output-side coupler;
The optical amplification device according to claim 1, wherein the light source for control light is provided in the waveguide portion.   9. The Mach-Zehnder interferometer, the control light waveguide, the first and second optical amplifiers, and the control light source are all provided on the same semiconductor substrate. The optical amplification device according to any one of the above. Before Symbol first and second optical amplifiers, polarization dependent gain difference ΔG is characterized in that it is configured to be smaller than 2 dB, the optical amplifying device according to any one of claims 1-9.

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