CN102638003A - Distributed feedback laser array - Google Patents

Abstract

The invention discloses a distributed feedback laser array. It consists of multiple distributed feedback lasers arranged in parallel on the same semiconductor epitaxial wafer. The semiconductor epitaxial wafer at least includes an upper cladding layer, an active layer, a spacer layer, a refractive index control layer, a lower cladding layer and a substrate. The active layer or the refractive index control layer contains a Bragg grating forming a distributed feedback. Layers above the index control layer form the upper mesa of the distributed feedback laser. The refractive index control layer forms the wave guiding structure of the distributed feedback laser, and the width of the wave guiding structure of each distributed feedback laser in the array is different. Any distributed feedback laser in the distributed feedback laser array is composed of an upper electrode, an upper table, a waveguide structure, a substrate and a lower electrode. The waveguide structure with different width realizes that each distributed feedback laser has a Bragg grating with the same period and at the same time makes the lasing wavelength shift to a certain extent. The array can be used as a multi-wavelength laser and a tunable laser.

Description

Distributed Feedback Laser Array

technical field

The invention relates to a distributed feedback laser array, in particular to a distributed feedback laser array with tunable lasing wavelength and multi-wavelength lasing.

Background technique

The popularization and development of information technology make the requirements for network technology higher and higher, especially for bandwidth. The development of next-generation technologies such as dense wavelength division multiplexing and passive optical networks has raised the need for multi-wavelength light sources. Although multiple discrete lasers can meet the wavelength requirements, the discrete devices are independent of each other, and the working conditions of each device are quite different, which leads to the inability to guarantee the overall performance of the light source equipment in terms of stability and reliability. In this context, tunable lasers have significant advantages. One type of tunable laser affects the parameters of the laser by injecting current or heating, thereby changing the lasing wavelength of the laser; the other type uses the integration of multiple lasers with different lasing wavelengths to realize the tuning of the lasing wavelength. The integration of multiple lasers can obtain a larger tuning range, a more compact structure and higher stability.

Distributed feedback lasers have the characteristics of high emission power and good single-mode performance during high-speed modulation, and have been widely used as light sources in optical communication, optical sensing and other fields. The distributed feedback laser array inherits the advantages of a single distributed feedback laser, while achieving superior tunable performance, simple structure, high reliability, and good stability.

The distributed feedback laser array is composed of several distributed feedback lasers, and the lasing wavelength of each laser has a certain offset relative to the previous one. When a small wavelength change is required, the lasing wavelength of one laser can be tuned by injecting current, controlling temperature, etc.; when a large wavelength change is required, another laser can be selected to lase.

The key to realizing the distributed feedback laser array is to make the lasing wavelengths of different lasers have a certain offset. The most commonly used method at present is to change the grating period. Fabricating Bragg gratings by electron beam direct writing exposure can precisely control the grating period, but due to the slow speed of electron beam exposure technology, its production capacity is limited, and it is difficult to meet the requirements of large-scale and low-cost production. Canada's Quantum Device Company has described a method of using a phase mask to make a distributed feedback laser array. However, the cost of the phase mask is relatively high, and large-scale applications are still problematic. In China, Wuhan Institute of Posts and Technology has successfully produced a 13-channel distributed feedback laser array by using the emerging nanoimprinting technology. However, the hard mask used in nanoimprinting technology is easy to damage the active layer when making distributed feedback lasers. There are still many problems to be solved in the fabrication of distributed feedback laser arrays using this technology. ``

Another way to achieve lasing wavelength shift in distributed feedback lasers is to use the sampling Bragg grating technique. The position of the first-order interference wavelength of the Bragg grating is related to the sampling period. As long as the resonance of the zero-order interference wavelength is suppressed, the laser is lased at the first-order interference wavelength, and the lasing wavelength of the laser can be controlled by controlling the sampling period. Since the sampling period of the grating is on the order of microns, the fabrication of the sampling grating can be realized by holographic exposure plus one UV lithography. But so far, according to reports from Zhu Hongliang's research group of Beijing Institute of Semiconductors, Nanjing University's Professor Chen Xiangfei's research group, and South Korea's Electronics and Communications Research Group, which mainly study this technology, the wavelength of the distributed feedback laser array produced by this technology can be The variable range is still small (no more than 11nm), which is difficult to meet the actual application requirements. In addition, since the lasing threshold of the 1st order interference wavelength generated by grating sampling is higher than that of 0th order, the threshold value of distributed feedback lasers with sampled gratings is usually higher than that of general distributed feedback lasers.

Another method is that the grating adopts holographic exposure to make a grating with a fixed period, and then changes the equivalent refractive index of the waveguide through the change of the waveguide structure to realize the wavelength shift. Infinera Corporation of the United States uses selective epitaxy technology to epitaxially produce active layers with different thicknesses in different regions at one time, so as to realize the control of the refractive index of the waveguide. However, according to the results reported so far, the wavelength shift of this method is limited, generally no more than 10nm, and a single chip is difficult to meet the needs of practical applications. Multiple chips need to be packaged to cover the required band, which increases the complexity of chip packaging. sex and cost.

To sum up, in addition to the use of high-precision exposure methods such as electron beams, the wavelength variable range of distributed feedback laser arrays produced by methods such as sampling gratings is very limited. There is a lack of low-cost and high-reliability distributed feedback laser array fabrication methods.

Contents of the invention

In order to overcome the shortcomings in the background technical solutions, the object of the present invention is to provide a distributed feedback laser array with simple structure, convenient manufacture, low cost, high reliability, and large tuning range, and at the same time, it is easy to integrate passive waveguides such as multiplexers.

The technical scheme adopted in the present invention is:

Technical solution one:

It is composed of multiple distributed feedback lasers arranged in parallel on the same semiconductor epitaxial wafer; the semiconductor epitaxial wafer includes upper cladding layer, active layer, spacer layer, refractive index control layer, lower cladding layer and The substrate, the Bragg grating used to form the distributed feedback is set in the active layer or the refractive index control layer; the period of the Bragg grating is kept consistent on the entire semiconductor epitaxial wafer; all the distributed feedback lasers are perpendicular to the Bragg grating; the upper cladding, the The source layer and the spacer layer constitute the upper mesa of the distributed feedback laser; the refractive index control layer forms the waveguide structure; any distributed feedback laser in the distributed feedback laser array consists of an upper mesa and its corresponding waveguide structure and the lower cladding, The substrate, the upper electrode and the lower electrode are jointly constituted.

Technical solution two:

It is composed of multiple distributed feedback lasers arranged in parallel on the same semiconductor epitaxial wafer; the semiconductor epitaxial wafer includes an upper cladding layer, a refractive index control layer, a spacer layer, an active layer, a lower cladding layer and The substrate, the Bragg grating used to form the distributed feedback is set in the active layer or the refractive index control layer; the period of the Bragg grating is consistent on the entire semiconductor epitaxial wafer; all the distributed feedback lasers are perpendicular to the Bragg grating; the upper cladding layer constitutes the distribution The upper mesa of the feedback laser; the refractive index control layer forms a waveguide structure; any distributed feedback laser in the distributed feedback laser array consists of an upper mesa and its corresponding waveguiding structure, as well as a lower cladding layer, a substrate, an upper electrode, and a lower cladding layer. electrodes together.

In the above two technical solutions: the width of the upper mesa and the waveguide structure increases with the increase of the lasing wavelength among different distributed feedback lasers. The refractive index of the refractive index control layer is higher than that of the upper cladding layer (11) and the lower cladding layer.

Compared with background technology, the beneficial effect that the present invention has is:

By introducing a refractive index control layer, it is laterally etched by lateral wet etching or selective oxidation to form waveguide structures with different widths, thereby affecting the equivalent refractive index and shifting the lasing wavelength. Sufficient offset can be obtained. Varying the width of the waveguide structure from 0.8 microns to 1.6 microns can shift the lasing wavelength by 30 nm. The shift of the lasing wavelength is obtained by adopting the method that each distributed feedback laser in the distributed feedback laser array has a different width guide wave structure, and it is not required to make corresponding changes to the Bragg grating period of each distributed feedback laser in the distributed feedback laser array. Each laser in the feedback laser array uses a Bragg grating with the same period, so it can be produced by holographic exposure, which is convenient for the integration and production of the distributed feedback laser array, and is also conducive to packaging, which simplifies the process, reduces the cost, and improves the finished product. Rate. In addition, the refractive index control layer is also conducive to making passive structures such as multiplexers, and is conducive to coupling the outputs of different distribution feedback lasers in the distribution feedback laser array, which is convenient for use.

Description of drawings

Fig. 1 is a schematic diagram of a semiconductor epitaxial wafer structure of a distributed feedback laser array.

Fig. 2 is a side view of a distributed feedback laser array.

Fig. 3 is a schematic diagram of another semiconductor epitaxial wafer structure of a distributed feedback laser array.

Fig. 4 is a side view of another distributed feedback laser array.

Fig. 5 is a schematic diagram showing the variation of the equivalent refractive index of the base film with the width of the waveguide structure.

Fig. 6 is a schematic diagram showing the variation of lasing wavelength offset with the width of the waveguide structure.

Fig. 7 is a schematic diagram of the relationship between the laser wavelength shift and the thickness of the spacer layer.

In the figure: 1. Semiconductor epitaxial wafer, 2. Distributed feedback laser; 11. Upper cladding layer, 12. Active layer, 13. Refractive index control layer, 14. Spacer layer, 15. Lower cladding layer, 16. Substrate, 21. Upper table, 22. Waveguide structure, 23. Upper electrode, 24. Lower electrode, 121. Bragg grating.

Detailed ways

The present invention will be further described below in conjunction with accompanying drawings and examples.

As shown in Figure 1, a distributed feedback laser array is made up of a plurality of distributed feedback lasers 2 arranged in parallel on the same semiconductor epitaxial wafer 1; the semiconductor epitaxial wafer 1 includes an upper cladding layer 11, Active layer 12, spacer layer 14, refractive index control layer 13, lower cladding layer 15 and substrate 16, the Bragg grating 121 that is used to form distributed feedback is arranged in active layer 12 or refractive index control layer 13; Bragg grating 121 The period of is kept consistent on the whole semiconductor epitaxial wafer 1 . the

As shown in Figure 2, all the distributed feedback lasers 2 in a distributed feedback laser array are perpendicular to the Bragg grating 121; the upper cladding layer 11, the active layer 12 and the spacer layer 14 constitute the upper table 21 of the distributed feedback laser 2; the refractive index The control layer 13 forms a waveguide structure 22; any distributed feedback laser 2 in the distributed feedback laser array consists of an upper mesa 21 and its corresponding waveguide structure 22, a lower cladding layer 15, a substrate 16, an upper electrode 23, a lower The electrodes 24 are formed together.

As shown in Figure 3, another distributed feedback laser is made up of a plurality of distributed feedback lasers 2 arranged in parallel on the same semiconductor epitaxial wafer 1; the semiconductor epitaxial wafer 1 includes an upper cladding layer 11, Refractive index control layer 13, spacer layer 14, active layer 12, lower cladding layer 15 and substrate 16, the Bragg grating 121 that is used to form distributed feedback is arranged in active layer 12 or refractive index control layer 13; Bragg grating 121 The period of is kept consistent on the whole semiconductor epitaxial wafer 1 .

As shown in Figure 4, all the distributed feedback lasers 2 in another distributed feedback laser array are perpendicular to the Bragg grating 121; the upper cladding layer 11 constitutes the upper table 21 of the distributed feedback laser 2; the refractive index control layer 13 forms the waveguide structure 22 Any distributed feedback laser 2 in the distributed feedback laser array is composed of an upper mesa 21 and its corresponding waveguide structure 22, lower cladding layer 15, substrate 16, upper electrode 23, and lower electrode 24.

In the distributed feedback laser arrays in the two schemes, the direction of the distributed feedback laser 2 is perpendicular to the direction of the Bragg grating 2, so that the distributed feedback effect of the Bragg grating can be utilized.

The Bragg grating 121 is once fabricated in the semiconductor epitaxial wafer 1 during the growth process, so that the period of the Bragg grating 121 of the distributed feedback laser array is consistent. Two cases where the Bragg grating 121 is in the active layer 12 or in the refractive index control layer 13 are included. The solution of the Bragg grating 12 in the active layer 12 includes gain coupling in addition to the refractive index coupling conventionally provided by the distributed feedback laser 2 . Gain coupling overcomes the problem that the reflection peaks on both sides of the forbidden band of the Bragg grating 121 have the same threshold, thereby reducing the single-mode yield of the distributed feedback laser, so that the distributed feedback laser 2 can work more in the mode near the edge of the forbidden band and in the long-wave direction , which ensures the single-wavelength lasing of the distributed feedback laser 2. The solution of the Bragg grating 121 in the refractive index control layer 13 avoids the etching of the active layer 12 and lowers the threshold value, and the problem of yield can be compensated by methods such as end face coating. Both cases have their own advantages.

The distributed feedback lasers 2 in the same distributed feedback laser array have upper mesas 21 with different widths, which is due to the need to fabricate waveguide structures with different widths at one time through processes such as lateral wet etching. The entire distributed feedback laser array is fabricated once.

The distributed feedback laser array can be used as a tunable laser or a multi-wavelength laser because different distributed feedback lasers 2 have different lasing wavelengths. The lasing wavelength depends on the equivalent refractive index of the base film and the period of the Bragg grating 121 . Because the method for changing the period of the Bragg grating 121 requires a high cost, the present invention adopts an easy-to-implement and mature process, all distributed feedback lasers 2 in the same distributed feedback laser array have the same period of the Bragg grating 121, by making different The distributed feedback lasers 2 have different equivalent refractive indices of the base film to achieve different lasing wavelengths. The difference in the equivalent refractive index of the base film is realized by the difference in the width of the waveguide structure 22 . The number of distributed feedback lasers 2 in the distributed feedback laser array depends on the need for the lasing wavelength shift range in use or the need for the number of simultaneous lasing different wavelengths.

The equivalent refractive index of the base film depends on the optical field distribution. The active layer 12 is a high-refractive-index layer in which the optical field is mainly distributed, but applying operations to the active layer 12 will affect the threshold and output power of the laser. Therefore, near the active layer 12, a layer of refractive index control layer 13 with a higher refractive index than the upper cladding layer 11 and the lower cladding layer 15 is introduced. The refractive index of the refractive index control layer 13 is close to the active layer 12. The high refractive index The layer can affect the original fundamental mode light field distribution and gather part of the light field distribution around it. Applying operations to the refractive index control layer 13 can have a slightly different influence on the optical field distribution, so as to achieve the purpose of changing the equivalent refractive index of the base film without affecting the performance of the distribution feedback laser 2 . At the same time, the refractive index control layer 13 is used as a passive layer on which passive structures such as multiplexers can be fabricated, which is conducive to coupling the outputs of all distributed feedback lasers 2 in the entire distributed feedback laser array.

As shown in Figure 5, it reflects the result of the change of the equivalent refractive index of the base film with the width of the waveguide structure, where the abscissa is the width of the waveguide structure, and the ordinate is the equivalent refractive index of the fundamental mode. In the present invention, the waveguide structure 22 is made on the introduced refractive index control layer 13, and different distributed feedback lasers 2 in the distributed feedback laser array have waveguide structures 22 of different widths, thereby having different equivalent refractive indices of the base film, Furthermore, the lasing wavelength is different.

纵向的传播常数

来描述。通过亥姆霍兹方程： In a certain waveguide structure 22, the optical field is distributed by the transverse electric field

longitudinal propagation constant

to describe. Via the Helmholtz equation:

                   （1）

(1)

就是等效折射率。对一个特定的导波结构，分析出等效折射率以后再结合具体的光栅参数通过传输矩阵和耦合波等方法就可以得到它的激射波长。布拉格方程： and the boundary conditions in the specific structure.

is the equivalent refractive index. For a specific guided wave structure, its lasing wavelength can be obtained by analyzing the equivalent refractive index and combining the specific grating parameters through transmission matrix and coupled wave methods. Bragg equation:

(2)

为等效折射率，为布拉格光栅周期，

为激射的波长阶数，一阶光栅取1。（2）两边取微分可得：

is the equivalent refractive index, is the Bragg grating period,

is the wavelength order of the lasing, and the first-order grating takes 1. (2) Differentiate both sides to get:

                     （3）

(3)

In this way, the relationship between the change of the equivalent refractive index of the base film and the laser wavelength shift is obtained. Equation (3) analyzes the offset of the Bragg wavelength, not the real lasing wavelength of the distributed feedback laser, but the distance between the lasing wavelength and the center of the forbidden band (that is, the Bragg wavelength) changes in a small equivalent refractive index No noticeable changes will occur. It can be considered that the offset of the Bragg wavelength is the offset of the lasing wavelength of the distributed feedback laser 2 .

As shown in FIG. 6 , it is a schematic diagram reflecting the variation of the lasing wavelength offset with the width of the waveguide structure 22 . The abscissa is the width of the waveguide structure 22, and the corresponding ordinate is the offset of the lasing wavelength relative to the lasing wavelength when the width of the waveguide structure is 0.8 microns, and the thickness of the spacer layer 14 is 0.01 microns. It can be seen that under the designed structure of the semiconductor epitaxial wafer, when the width of the waveguide structure 22 changes from 0.8 microns to 1.6 microns, the lasing wavelength shift can reach 32 nanometers, which is enough to meet the interval band of 32 channels of 50 GHz. If the refractive index of the refractive index control layer 13 is lower than that of the upper cladding layer 11 and the lower cladding layer 15, sufficient light field distribution cannot be collected thereon, thereby failing to have a sufficient influence on the equivalent refractive index, thereby failing to achieve significant excitation. wavelength shift.

The core difference between the two technical solutions in the present invention is that the refractive index control layer 13 is above or below the active layer 12 . This reflects the effect of the thickness of the spacer layer 14 between these two layers. The refractive index control layer 13 is below the active layer 12 , and the refractive index control layer 13 can be made as close as possible to the active layer 12 in the process, so as to increase the function of the refractive index control layer 13 .

As shown in Figure 7, it reflects the influence of the thickness of the spacer layer 14 on the lasing wavelength offset, wherein the abscissa is the thickness of the spacer layer, and the ordinate is that when the width of the waveguide structure is 1.6 microns, it is 0.8 microns relative to the width of the waveguide structure. The laser wavelength offset at . The lasing wavelength shift decreases obviously when the thickness of the spacer layer increases.

The scheme that the refractive index control layer 13 is above the active layer 12 avoids the problem of increased loss that may be caused by cutting through the active layer 12 during the process of manufacturing the upper mesa 21 , and at the same time simplifies the manufacturing process. Both technical solutions have their own advantages.

The above-mentioned embodiments are used to illustrate the present invention, rather than to limit the present invention. Within the spirit of the present invention and the protection scope of the claims, any modification and change made to the present invention will fall into the protection scope of the present invention.

Claims (6)

1. distributed feedback laser array is characterized in that: it is to be made up of a plurality of distributed feedback lasers that are arranged in parallel (2) that are produced on the same block semiconductor epitaxial wafer (1); Semiconductor epitaxial wafer (1) comprises top covering (11), active layer (12), wall (14), refractive index key-course (13), under-clad layer (15) and substrate (16) from top to bottom successively, and the Bragg grating (121) that is used to form distributed feed-back is arranged in active layer (12) or the refractive index key-course (13); The cycle of Bragg grating (121) is consistent on monoblock semiconductor epitaxial wafer (1); All distributed feedback lasers (2) are vertical with Bragg grating (121); Top covering (11), active layer (12) and wall (14) constitute the upper table surface (21) of distributed feedback laser (2); Refractive index key-course (13) forms guided wave structure formed (22); Any distributed feedback laser (2) constitutes by a upper table surface (21) and its pairing guided wave structure formed (22) and under-clad layer (15), substrate (16) and top electrode (23), bottom electrode (24) jointly in the distributed feedback laser array.

2. distributed feedback laser array according to claim 1 is characterized in that: the width of said upper table surface (21) and guided wave structure formed (22) increases progressively with the increase of excitation wavelength between different distributed feedback laser (2).

3. distributed feedback laser array according to claim 1 is characterized in that: said refractive index key-course (13) refractive index is greater than the refractive index of top covering (11), under-clad layer (15).

4. distributed feedback laser array is characterized in that: it is to be made up of a plurality of distributed feedback lasers that are arranged in parallel (2) that are produced on the same block semiconductor epitaxial wafer (1); Semiconductor epitaxial wafer (1) comprises top covering (11), refractive index key-course (13), wall (14), active layer (12), under-clad layer (15) and substrate (16) from top to bottom successively, and the Bragg grating (121) that is used to form distributed feed-back is arranged in active layer (12) or the refractive index key-course (13); The cycle of Bragg grating (121) is consistent on monoblock semiconductor epitaxial wafer (1); All distributed feedback lasers (2) are vertical with Bragg grating (121); Top covering (11) constitutes the upper table surface (21) of distributed feedback laser (2); Refractive index key-course (13) forms guided wave structure formed (22); Any distributed feedback laser (2) constitutes by a upper table surface (21) and its pairing guided wave structure formed (22) and under-clad layer (15), substrate (16) and top electrode (23), bottom electrode (24) jointly in the distributed feedback laser array.

5. distributed feedback laser array according to claim 4 is characterized in that: the width of said upper table surface (21) and guided wave structure formed (22) increases progressively with the increase of excitation wavelength between different distributed feedback laser (2).

6. distributed feedback laser array according to claim 4 is characterized in that: said refractive index key-course (13) refractive index is greater than the refractive index of top covering (11), under-clad layer (15).

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