A small-sized wavelength-tunable semiconductor laser

Abstract

The present invention discloses a small-sized wavelength tunable semiconductor laser. It comprises two resonant cavities with different optical lengths, both of which comprise a first reflection surface, a polymer imaging coupler and a second reflection surface, and the two resonant cavities are arranged in a straight line along the same axis, and the second reflection surfaces of the two cavities are connected by a deep-etched waveguide to couple the two resonant cavities; the two polymer imaging couplers, the two second reflection surfaces and the deep-etched waveguide constitute a half-wave coupler, which has wavelength selective loss to achieve the best single-mode characteristics, and at the same time achieves wide-range wavelength tuning through the vernier effect. The present invention achieves small-sized tunability of a channel spacing of 400 GHz to 800 GHz through the small volume feature of the polymer imaging coupler, and does not require the production of gratings, simplifies the process, has a simple and compact structure, and is simple to control by tuning the channel through a single electrode, and has the potential to integrate modulators and photodiodes on a chip, and has the advantages of small size, low cost, and wide-range wavelength tunability.

Description

Technical Field

[0001] The invention relates to the field of semiconductor lasers, and in particular to a small-size wavelength-tunable semiconductor laser based on a polymer imaging half-wave coupler. Background Art

[0002] Wavelength tunable semiconductor lasers are an indispensable key technology in current optical communications and sensing fields due to their relatively low cost, small size and low power consumption. In medium and long-distance optical communication scenarios such as metropolitan area networks, tunable semiconductor lasers have replaced fixed wavelength lasers in many application scenarios due to their excellent interchangeability. In recent years, with the development of 5G technology, new generation access network technology, high-speed and high-capacity data center interconnection needs, and sensing fields, the demand for tunable semiconductor lasers has expanded dramatically and new technical requirements have been proposed, among which the most core demand is low cost.

[0003] At present, the most widely used tunable semiconductor lasers in the field of optical communication are: Y-type tunable semiconductor lasers (MG-Y) based on modulated gratings, distributed feedback lasers (SG-DBR) based on sampled gratings, distributed feedback lasers (DS-DBR) based on digital supermode gratings, and external cavity tunable lasers based on silicon photonics technology. Most of these solutions contain gratings, and they are gratings with certain modulation structures (SG-DBR and MG-Y) or multi-segment gratings (DS-DBR). These gratings are used to achieve good single-mode characteristics and tunable performance, but the complex grating structure requires multiple exposure etching and secondary epitaxy steps to make, which greatly increases the cost; and these tunable semiconductor laser solutions all contain structures such as multi-segment grating areas, gain areas, and phase areas. The laser needs to adjust the electrodes on all areas to achieve single-mode stable operation and wavelength tuning. The multi-electrode coordinated tuning control algorithm is very complex, and it also complicates the laser drive circuit, which is also an important factor in increasing costs. The external cavity tunable laser based on silicon photonics technology is a rapidly developing solution in recent years. However, this solution basically has no monolithic integration. The external cavity laser is large in size and the coupling between the gain chip and the mode selection structure device based on silicon photonics technology is technically complex, making it impossible to achieve low cost.

[0004] Chinese patent CN100463312B proposes a low-cost tunable semiconductor laser based on a V-shaped cavity structure. This solution does not require a grating, but uses two resonant cavities with a certain difference in optical length. The vernier effect between the two resonant cavities is used to amplify the change in refractive index to tune the wavelength. And the phase difference between the two output ports and the specific energy coupling ratio between the two optical resonant cavities are realized through a uniquely designed half-wave coupler to achieve good single-mode characteristics. This solution does not require secondary epitaxy and other processes, and has only three electrodes. By changing one of the two resonant cavity electrodes, single-electrode wavelength tuning can be achieved, which greatly simplifies the wavelength control algorithm. These factors work together to make the V-shaped cavity tunable semiconductor laser an excellent low-cost tunable laser solution. However, in this solution, each resonant cavity needs to include a tuning waveguide in addition to a half-wave coupler of a certain length, so that the resonant cavity cannot be too short, and it is difficult to achieve a wavelength switchable laser with a channel spacing of more than 400GHz. In addition, a V-shaped curved structure is used instead of a conventional linear structure, which causes certain losses and a large device width, and a compact laser array cannot be achieved. Summary of the invention

[0005] In view of the shortcomings of the prior art, the object of the present invention is to provide a small-sized wavelength tunable semiconductor laser.

[0006] The present invention does not require the production of gratings, simplifies the process, has a simple and compact structure, can tune the channel through a single electrode, has a simple control algorithm, has the potential to integrate modulators and photodiodes and other devices on a chip, and is a low-cost, wide-range wavelength tunable semiconductor light source.

[0007] The technical solution adopted by the present invention is:

[0008] The present invention comprises two optical resonant cavities, each of which is mainly composed of a first partial reflection surface, a polymer imaging coupler and a second partial reflection surface connected in sequence. The two optical resonant cavities are distributed along the same straight line and are coupled to each other through a deeply etched waveguide connecting the second partial reflection surfaces of each.

[0009] The polymer imaging coupler is a multimode interference coupler that is subjected to size compression processing, so that the length and width are compressed to a range of 0.6 of the original size, and the final length and width are 50-100 μm and 8-12 μm respectively.

[0010] The polymer imaging coupler constituting the optical resonant cavity in the present invention utilizes the principle of multimode interference coupling. On the basis of designing a multimode interference coupler with suitable width and length and specific imaging conditions, the formed image is aggregated to a certain extent by compressing the overall size, thereby adjusting the energy distribution of the output end face to meet specific energy distribution requirements.

[0011] The present invention designs the polymer imaging coupler to have a small size feature, and the polymer imaging coupler is the main body of the optical resonant cavity, so that the overall size of the tunable semiconductor laser is small, and the length of the optical resonant cavity is suitable for designing a laser with a channel spacing of 400GHz to 800GHz.

[0012] Each optical resonant cavity has two second partial reflection surfaces, and the two second partial reflection surfaces are respectively located on two symmetrical sides of the connection between the end of the polymer imaging coupler and the deep etched waveguide;

[0013] The two side parts of one end of the polymer imaging coupler are connected to the second part reflection surface, and the remaining part in the middle is coupled to the deep etched waveguide in the middle; the two second part reflection surfaces on both sides constitute a group of total reflection structures, which reflect part of the energy distribution of the end face of the polymer imaging coupler end of the optical resonant cavity back to the optical resonant cavity where it is located to form a straight-through coupling, and the other part of the energy is coupled to another optical resonant cavity through the deep etched waveguide to form a cross coupling.

[0014] The first partial reflection surface is a first pit etched on the wafer to penetrate the quantum well layer, and the plane of the first pit wall is used as the first partial reflection surface; the second partial reflection surface is two second pits etched on the wafer at the position of the end of the polymer imaging coupler and on both sides of the symmetry of the straight line where the polymer imaging coupler is located, each second pit is set to a 45-degree cut angle at a corner close to the end of the polymer imaging coupler, and the plane of the pit wall with a 45-degree cut angle is used as the second partial reflection surface; and there is a gap distance between the two second pits with a width less than the width of the polymer imaging coupler, and a deep etched waveguide is set in the gap distance.

[0015] The two second pits of the second partial reflection surface of the two optical resonant cavities are shared, that is, two second pits engraved through the quantum well layer are also etched between the opposite ends of the polymer imaging type couplers of the two optical resonant cavities and on both sides of the symmetry of the straight line where the polymer imaging type couplers are located. Each second pit is set to a 45-degree cut angle at a corner close to the end of the polymer imaging type coupler, and the plane of the pit wall with a 45-degree cut angle is used as the second partial reflection surface.

[0016] The deep etching of the deep etched waveguide refers to deep etching through the quantum well layer on both sides to form a waveguide, and the deep etching depth is greater than 4 μm. Both sides may be second pits, that is, the deep etching of the second pit forms a deep etched waveguide.

[0017] The first partial reflection surface and the second partial reflection surface of the two optical resonant cavities are also deeply etched reflection surfaces, where the deep etching means that the depth of the corresponding pit / etching depth is greater than 4 μm.

[0018] The optical lengths of the two optical resonant cavities are different, that is, the lengths of the aggregate imaging couplers of the two optical resonant cavities are different; and within the gain spectrum of the laser material, only one of the resonant frequencies of the two optical resonant cavities overlaps, ensuring single-mode operation, and the overlapping resonant frequency is used as the operating wavelength.

[0019] Typically, the ratio of the optical lengths of the two optical resonant cavities to the optical lengths of the polymer imaging coupler is 1.02-1.1, and the cavity length difference between the two optical resonant cavities is typically 2%-10%.

[0020] The resonant frequency of the first optical resonant cavity corresponds to a series of equally spaced target working channels, the optical length of the second optical resonant cavity is greater than the optical length of the first optical resonant cavity, and the resonant frequency interval of the second optical resonant cavity is smaller than the resonant frequency interval of the first optical resonant cavity. The two optical resonant cavities have different optical lengths, the longer second optical resonant cavity is called a long cavity, and the first optical resonant cavity is called a short cavity.

[0021] The two optical resonant cavities each have a set of resonant frequencies determined by their own optical lengths. The resonant frequency of one optical resonant cavity corresponds to a series of equally spaced working channels (called a fixed gain cavity or a non-tuned cavity, and the corresponding other optical resonant cavity is called a channel selection cavity or a tuned cavity). The interval between the resonant frequencies of the other optical resonant cavity is different from the interval between the working channels. The two sets of resonant frequencies periodically overlap at specific frequency positions, and the frequency at the overlap is used as the working frequency of the laser. The distance between the two overlapping frequencies is called the free spectral range (FSR).

[0022] Injecting current into any optical resonant cavity changes its overall equivalent refractive index, thereby changing the optical length and the operating wavelength, thereby achieving tuning. The resonance frequency interval corresponding to the long cavity is smaller, and there is a larger tuning range under the same current, making the long cavity a tunable cavity, and the overall operating wavelength of the tunable semiconductor laser is determined by the short cavity.

[0023] The above design makes only one resonant frequency of the two optical resonant cavities overlap within the gain spectrum of the laser quantum well material, avoiding multi-wavelength oscillation. During tuning, the optical length of the tuning cavity is changed by injecting current, so that its resonant frequency moves, and then the frequency of the two cavities overlapping is changed to achieve channel tuning. The overlapping frequency is determined by the group of resonant frequencies of the non-tuned cavity. This method of achieving a larger operating wavelength change through a smaller current change forms a vernier effect. Since the interval between the group of resonant frequencies of the long cavity is smaller than that of the short cavity, under the premise of the same refractive index change, the number of resonant frequency shifts of the long cavity is greater, and more channels can be tuned, so the long cavity is usually used as the tuning cavity. The size of the tuning range is determined by the optical length difference between the long cavity and the short cavity, and this length difference also affects single-mode performance such as the side mode suppression ratio (SMSR).

[0024] The small-size wavelength tunable semiconductor laser of the present invention has two designs for electrode distribution.

[0025] The first scheme is to have three electrodes on the front, namely the coupler electrode, the long cavity electrode and the short cavity electrode. The two optical resonant cavities are divided into two parts along the axial direction, and the parts close to the center of the laser are covered by the coupler electrode. The remaining parts of the two optical resonant cavities are covered by their respective electrodes (correspondingly called the first resonant cavity electrode and the second resonant cavity electrode, or simply the long cavity electrode and the short cavity electrode). The optical length ratio of the two optical resonant cavities covered by the coupler electrode is consistent with the overall optical length ratio of the two optical resonant cavities, and is usually designed to be selected between -. The coupler electrode is the main electrode that provides gain and is also an electrode for modulation. The above design can ensure that the laser does not jump mode after adding the modulation signal. In addition, the coupler electrode can also be used to slightly change the working state of the half-wave coupler and fine-tune the single-mode performance and wavelength position of the laser. The long cavity electrode and the short cavity electrode are tuning electrodes while providing gain. Both electrodes can be used for tuning, but as long as the current of one electrode changes, tuning can be achieved. As mentioned above, the long cavity electrode is basically used for tuning. Single-electrode tuning makes the laser tuning algorithm simple and simplifies the drive circuit. The back of the laser is a ground electrode corresponding to the three electrodes.

[0026] The second electrode design has only two electrodes on the front side, and the two electrodes cover two optical resonant cavities respectively (also called the first resonant cavity electrode and the second resonant cavity electrode, or simply the long cavity electrode and the short cavity electrode). This solution is suitable for situations where the laser does not need to be used for direct modulation. The long cavity electrode and the short cavity electrode are also used as gain electrodes and tuning electrodes at the same time. As mentioned above, the long cavity electrode is basically used for tuning, and the single electrode can be tuned. The elimination of the coupler electrode simplifies the structure, making the laser more compact and smaller in size, while further simplifying the peripheral circuit design, making it suitable for array applications. The back of the laser is also the ground electrode corresponding to the two electrodes on the front side.

[0027] The laser is provided with two electrodes, a short cavity electrode and a long cavity electrode; the long cavity electrode and the short cavity electrode completely cover the longer optical resonant cavity and the shorter optical resonant cavity respectively. The length of the long cavity electrode covering the corresponding optical resonant cavity is greater than the length of the short cavity electrode covering the corresponding optical resonant cavity.

[0028] The short cavity electrode and the long cavity electrode are both covered on the polymer imaging type coupler of the optical resonant cavity, and the optical length covered on the polymer imaging type coupler is used as the optical length covered on the optical resonant cavity.

[0029] The laser is provided with three electrodes, namely, a coupler electrode located in the middle and a short cavity electrode and a long cavity electrode located on both sides; the coupler electrode covers a portion of each of the two optical resonant cavities at the same time, and the ratio of the optical lengths of the two optical resonant cavities covered by the coupler electrode is kept the same as the ratio of the overall optical lengths of the two optical resonant cavities;

[0030] The long cavity electrode and the short cavity electrode cover the longer optical resonant cavity and the shorter optical resonant cavity respectively and cover the remaining part not covered by the coupler electrode. The length of the long cavity electrode covering its corresponding optical resonant cavity is greater than the length of the short cavity electrode covering its corresponding optical resonant cavity.

[0031] The coupler electrode, the short cavity electrode and the long cavity electrode are all covered on the polymer imaging coupler of the optical resonant cavity, and the optical length covered on the polymer imaging coupler is used as the optical length covered on the optical resonant cavity.

[0032] Among the first partial reflection surfaces of the two optical resonant cavities, one first partial reflection surface is coated with a high reflectivity film as a total reflection surface, and the other first partial reflection surface is a semi-transparent and semi-reflective surface, serving as the output end of the laser.

[0033] In the laser, a tapered waveguide is arranged between the first partial reflection surface as the output end of the laser and the polymer imaging type coupler connected thereto, and the width of the tapered waveguide gradually decreases from the polymer imaging type coupler to the first partial reflection surface;

[0034] Or / and, waveguides and corresponding electrodes for lateral heat tuning are added on both sides of the aggregate imaging coupler of the two optical resonant cavities.

[0035] Therefore, waveguides and electrodes for lateral heating tuning can be added on both sides of the two optical resonant cavities, and the cavity length of the optical resonant cavity can be fine-tuned by injecting current to heat the cavity sideways, thereby fine-tuning the operating wavelength of the laser. Adding a degree of freedom in tuning the wavelength can more accurately lock the wavelength position without adjusting the current of the long cavity and short cavity electrodes and thus without affecting the working conditions of the half-wave coupler. In addition, the lateral heating electrodes can synchronously increase or decrease the current to allow the operating wavelength to move continuously without mode hopping, achieving quasi-continuous tuning.

[0036] The innovation of the present invention is that a polymer imaging coupler is introduced in the structural design, so that the overall laser cavity length (that is, the length of the polymer imaging coupler) can be designed to be as small as about 50μm, thereby realizing a small-volume tunable laser. At the same time, a tunable laser with a channel spacing of 400GHz to 800GHz is realized through the short cavity length. In addition, the present invention realizes excellent single-mode performance by combining a polymer imaging coupler with a deep-etched waveguide to form a half-wave coupler with wavelength selective loss. There is no need to introduce a grating structure, the structure is compact, easy to integrate, the tuning capability is strong and the tuning control is simple, while achieving low cost.

[0037] The present invention includes two resonant cavities with different optical lengths to form a vernier effect. Both resonant cavities are composed of a first reflection surface, a polymer imaging coupler and a second reflection surface. The two resonant cavities are arranged in a straight line along the same axis, and the second reflection surfaces of the two cavities are connected by a section of deeply etched waveguide to couple the two resonant cavities. The two polymer imaging couplers, the two second reflection surfaces and the deeply etched waveguide in the middle form a half-wave coupler with wavelength selective loss to achieve optimal single-mode characteristics, and at the same time, combined with the vernier effect to achieve wide-range wavelength tuning. The present invention uses the small size of the polymer imaging coupler to achieve a small-size tunable laser with a channel spacing of 400GHz and above.

[0038] Compared with the background technology, the present invention has the following beneficial effects:

[0039] 1. The present invention does not require the production of gratings, simplifies the process flow and reduces costs.

[0040] 2. The half-wave coupler based on the principle of aggregate imaging and a deep-etched waveguide in the middle improves the single-mode performance by introducing wavelength selective loss; at the same time, the design direction is clearer and the design freedom is higher, so the working parameters of the coupler can be designed more accurately to improve the performance.

[0041] 3. By utilizing the vernier effect between two optical resonant cavities with a certain optical length difference, the tuning range of the laser is increased; and only single-electrode tuning is required, which simplifies the tuning control algorithm and external circuit design and reduces costs.

[0042] 4. It can realize small-size tunable lasers, which are suitable for designing tunable lasers with channel spacing of 400GHz to 800GHz. At the same time, it can flexibly design lasers on a large cavity length scale to realize tunable lasers with different wavelength intervals.

[0043] 5. The structure is simple, and the minimum cavity length can reach 50μm. It is easy to integrate with other devices, reduce the overall device size, and realize chips with more complex functions. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is a schematic diagram of the basic structure of a small-size wavelength tunable semiconductor laser.

[0045] Figure 2 This is a schematic diagram of the half-wave coupler structure in a small-size wavelength tunable semiconductor laser.

[0046] Figure 3 It is an electrode arrangement design for small-size tunable semiconductor lasers.

[0047] Figure 4 It is a schematic diagram of a small-sized wavelength tunable semiconductor laser with an added lateral fine-tuning hot electrode.

[0048] Figure 5 It is a schematic diagram of the vernier effect principle and a schematic diagram of the long cavity and short cavity resonant frequency and free spectral range of a small-size wavelength tunable semiconductor laser.

[0049] Figure 6 Schematic diagram of the phase and amplitude of the amplitude reflection coefficient correction factor η when the coupling phase is π / 2, where (a) represents the phase and (b) represents the amplitude.

[0050] Figure 7 Schematic diagram of the phase and amplitude of the amplitude reflection coefficient correction factor η when the coupling phase is π, where (a) represents the phase and (b) represents the amplitude.

[0051] Figure 8 is the threshold gain difference of each longitudinal mode when the through coupling coefficient is 0.8 and 0.2.

[0052] Fig. 9 This is the relationship between the difference in primary and secondary mode thresholds and the normalized cross-coupling coefficient for devices with 5% and 7% cavity length differences.

[0053] Fig.10 This is a schematic diagram of the structure of a traditional 1×N multimode interference coupler.

[0054] Fig.11 This is the calculated result of the energy distribution on the output end face of a 1×4 multimode interference coupler in the initial state of the design example of the polymer imaging coupler.

[0055] Fig.12 This is the final calculation result of the output end face energy distribution of the polymer imaging coupler design example.

[0056] Fig.13 This is a periodic calculation verification of the imaging principle of the multi-mode interference coupler.

[0057] Fig.14 It is a schematic diagram of the basic structure and electrode arrangement design of Example 2 of the present invention.

[0058] Fig.15 It is a schematic diagram of the basic structure and electrode arrangement design of Example 3 of the present invention.

[0059] Fig.16 It is a schematic diagram of the basic structural design of Example 4 of the present invention. DETAILED DESCRIPTION

[0060] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0061] The main structure of the optical resonant cavity of the small-sized wavelength tunable semiconductor laser described in the present invention is a polymer imaging coupler. Due to the aforementioned design method and other factors, the polymer imaging coupler naturally has a small size feature, so the volume of the optical resonant cavity is also very small, and its length can be as small as 50μm. The advantage of the small size feature of the present invention also makes the present invention suitable for designing a tunable laser with a channel spacing of 400GHz and above, to 800GHz or even higher. In addition, the design of the polymer imaging coupler (that is, the optical resonant cavity) in the length direction is more flexible. If the optical length of the optical resonant cavity to be designed is long, the polymer imaging coupler may be too wide and lose the polymer imaging condition. At this time, the periodicity of the multi-mode interference imaging principle can be used to design a polymer imaging coupler at a shorter length, and the multi-mode interference imaging period length of the corresponding width is calculated, and the designed polymer imaging coupler is added with several times the multi-mode interference imaging period length to achieve the target cavity length. Therefore, while the present invention has the advantage of small size, it can design a tunable laser in a large cavity length selection range to meet different wavelength spacing requirements.

[0062] like Figure 1 As shown, the laser implemented specifically includes two optical resonant cavities, each of which is mainly composed of respective first partial reflection surfaces 101, 201, aggregate imaging type couplers 102, 202 and second partial reflection surfaces 103, 203 connected in sequence, the aggregate imaging type couplers 102, 202 are mainly in a strip-shaped structure, the two optical resonant cavities are distributed along the same straight line, and are coupled to each other through a deeply etched waveguide 301 connecting respective second partial reflection surfaces 103, 203.

[0063] The polymer imaging couplers 102, 202 and the second partial reflection surfaces 103, 203 of the two optical resonant cavities, together with the middle deeply etched waveguide 301, constitute a complete half-wave coupler. Both the first partial reflection surface and the second partial reflection surface are deeply etched reflection surfaces, which accurately define the size of the polymer imaging coupler and the cavity length of the optical resonant cavity, and at the same time provide feedback as the reflection surface of the laser. One of the two first partial reflection surfaces is coated with a high reflectivity film to improve feedback and lower the threshold of the laser, and the other first partial reflection surface has a certain transmittance and is used as the output of the laser. In order to simplify the process, the two sides of the middle deeply etched waveguide are also designed as deeply etched surfaces. The main structure of the laser 301 is composed of two optical resonant cavities and the middle deeply etched waveguide 301.

[0064] The polymer imaging couplers 102 and 202 are dimensionally compressed based on the multimode interference couplers, so that the length and width are compressed to 0.6 of the original size, and the final length and width are 50-100 μm and 8-12 μm, respectively.

[0065] The multimode interference coupler refers to a 1×N multimode interference coupler, which is prepared and formed on a waveguide layer.

[0066] The polymer imaging coupler of the present invention is an unconventional multimode interference coupler. Based on the principle of multimode interference imaging, the overall size is appropriately proportionally compressed so that the distance between the required positions on the output end face is close enough, so that the energy distribution after the multimode interference coupler is imaged is further superimposed at the corresponding output position, and different energy distributions are achieved at different positions of the output end face instead of N image points with uniform energy. This process is called polymer imaging. This process avoids the problem that in the energy distribution of the output end face of a conventional multimode interference coupler, there is always energy distribution at positions where output is not required, thus causing loss.

[0067] The half-wave coupler is composed of two polymer imaging couplers, their respective second partial reflection surfaces, and a middle deeply etched waveguide. The cross-coupling energy and the through-coupling energy of the polymer imaging coupler are designed to meet certain ratio requirements. At the same time, the phase difference between through-coupling and cross-coupling is controlled by the length of the middle deeply etched waveguide to meet the working requirements of the half-wave coupler. The two core working parameters are independently controlled by the polymer imaging coupler and the middle deeply etched waveguide, which increases the design freedom and flexibility. The half-wave coupler introduces wavelength-related selective loss to ensure the excellent single-mode performance of the laser represented by high side mode suppression ratio.

[0068] Each optical resonant cavity has two second partial reflection surfaces 103, 203, which are respectively located on the symmetrical sides of the connection between the end of the polymer imaging coupler 102, 202 and the deep etched waveguide 301; the two side parts of one end of the polymer imaging coupler 102, 202 are connected to the second partial reflection surfaces 103, 203, and the remaining part in the middle is coupled to the deep etched waveguide 301 in the middle; the two second partial reflection surfaces located on both sides of the end of the same polymer imaging coupler 102, 202 constitute a group of total reflection structures, which fully reflect a part of the energy distribution of the end face of the polymer imaging coupler 102, 202 of the current optical resonant cavity back to the optical resonant cavity where it is located to form a straight-through coupling, and the remaining other part of the energy is coupled to another optical resonant cavity through the deep etched waveguide 301 to form a cross coupling.

[0069] In the present invention, the energy at the output end face of the aggregated imaging coupler is designed to be distributed in three positions, one of which is the middle of the output end face coupled into the middle deep etched waveguide 301 and then coupled into another optical resonant cavity. This part of energy is called cross-coupling energy; the other two places are respectively located in the two 45-degree angle reflection surfaces of the second part of the reflection surface, and are fully reflected back into the optical resonant cavity. This part of energy is called through-coupling energy.

[0070] The above features are combined with the design of controlling the energy distribution at the output end face of the polymer imaging coupler 102, 202 to achieve the specific cross-coupling and through-coupling energy ratio requirement required by the half-wave coupler.

[0071] like Figure 1 As shown, the first partial reflection surface 101, 201 is formed by firstly etching a first pit through the quantum well layer, and the plane of the wall of the first pit is used as the first partial reflection surface, and the first partial reflection surface is a plane.

[0072] like Figure 2 As shown, the second partial reflection surface 103, 203 is formed by etching two second pits on the waveguide layer at the end of the polymer imaging type coupler 102, 202 and on both sides of the symmetric line where the polymer imaging type coupler 102, 202 is located, and each second pit is set to have a 45-degree cut angle at a corner close to the end of the polymer imaging type coupler 102, 202, and the plane of the pit wall with a 45-degree cut angle is used as the second partial reflection surface, and the second partial reflection surface is a plane; this makes the second partial reflection surface 103, 203 form an angle of 45 degrees with the central axis of the laser. The second partial reflection surface 103, 203 is also a deeply etched reflection surface.

[0073] There is a gap distance between the two second pits, which is smaller than the width of the polymer imaging coupler 102, 202, and a deep etched waveguide 301 is arranged in the gap distance. The deep etched waveguide 301 and the polymer imaging coupler 102, 202 are arranged in a straight line and on the same center line, that is, along the central axis of the laser axis.

[0074] The two second pits of the second partial reflection surfaces 103, 203 of the two optical resonant cavities are shared, that is, between the ends of the polymer imaging type couplers 102, 202 of the two optical resonant cavities that are opposite to each other and along the symmetrical sides of the straight line where the polymer imaging type couplers 102, 202 are located, the quantum well layer is engraved to form two second pits, and a corner of each second pit close to the end of the polymer imaging type coupler 102, 202 is set to a 45-degree cut angle, and the plane of the pit wall with a 45-degree cut angle is used as the second partial reflection surface, and the second partial reflection surface is a plane.

[0075] Specifically, the second pit on the upper side is set to a 45-degree cut angle at the lower left corner near the end of the first polymer imaging coupler 102, and the vertical plane of the pit wall where the 45-degree cut angle is located is used as the second partial reflection surface of the first optical resonance cavity; the second pit on the lower side is set to a 45-degree cut angle at the upper left corner near the end of the first polymer imaging coupler 102, and the vertical plane of the pit wall where the 45-degree cut angle is located is used as another second partial reflection surface of the first optical resonance cavity; the second pit on the upper side is set to a 45-degree cut angle at the lower left corner near the end of the second polymer imaging coupler 202, and the vertical plane of the pit wall where the 45-degree cut angle is located is used as the second partial reflection surface of the second optical resonance cavity; the second pit on the lower side is set to a 45-degree cut angle at the upper left corner near the end of the second polymer imaging coupler 202, and the vertical plane of the pit wall where the 45-degree cut angle is located is used as another second partial reflection surface of the second optical resonance cavity. This also makes the two corners of the polymer imaging coupler 102 , 202 connecting the end of the deeply etched waveguide 301 also form a 45-degree cut angle that fits tightly with the second partial reflection surface 103 , 203 .

[0076] In a specific implementation, the phase difference between the cross coupling and the through coupling can be adjusted by adjusting the length of the deep etched waveguide 301 in the middle. In a specific implementation, the phase difference can be easily controlled to meet the functional requirements of the half-wave coupler by adjusting the length of the deep etched waveguide 301. When a specific cross coupling and through coupling energy ratio is met and the cross coupling phase and the through coupling phase difference are, the half-wave coupler achieves selective loss for the wavelength, ensuring the mode selection performance of the laser.

[0077] In a specific implementation, it is required that the optical lengths of the two optical resonant cavities are different, that is, the lengths of the polymer imaging couplers 102 and 202 of the two optical resonant cavities are different, and the two optical resonant cavities have an optical length difference.

[0078] In specific implementation, the two-electrode scheme is:

[0079] The laser is provided with two electrodes, a short cavity electrode 104 and a long cavity electrode 204, which are respectively located on both sides; the long cavity electrode 204 and the short cavity electrode 104 completely cover the longer optical resonant cavity and the shorter optical resonant cavity respectively. In a specific implementation, the length of the long cavity electrode 204 covering its corresponding optical resonant cavity can be set to be greater than the length of the short cavity electrode 104 covering its corresponding optical resonant cavity, and the ratio between the length of the long cavity electrode 204 covering its corresponding optical resonant cavity and the length of the short cavity electrode 104 covering its corresponding optical resonant cavity is kept the same as the ratio between the overall optical lengths of the two optical resonant cavities.

[0080] The two-electrode design is suitable for applications that do not require modulation, and is convenient for forming arrays or inheriting modulator functions on a chip, making the laser more compact while simplifying the peripheral packaging and control circuit design. The back of the same laser is the ground electrode corresponding to the two electrodes.

[0081] In specific implementation, the three-electrode scheme is:

[0082] The laser is provided with three electrodes, namely, a coupler electrode 302 located in the middle and a short cavity electrode 104 and a long cavity electrode 204 located on both sides; wherein the coupler electrode 302 in the middle covers a part of each of the two optical resonant cavities at the same time, and the ratio between the optical lengths of the two optical resonant cavities covered by the coupler electrode 302 is kept the same as the ratio between the overall optical lengths of the two optical resonant cavities;

[0083] The coupler electrode 302 is an electrode that mainly provides light emission and gain, and is also an electrode for modulation. The above design can ensure that mode hopping does not occur after the modulated signal is added. At the same time, the coupler electrode 302 can also be used to slightly change the working state of the half-wave coupler and fine-tune the single-mode performance of the laser.

[0084] The long cavity electrode 204 and the short cavity electrode 104 are respectively covered on the longer optical resonant cavity and the shorter optical resonant cavity and cover the remaining part not covered by the coupler electrode 302, and the ratio between the length of the optical resonant cavity covered by the long cavity electrode 204 and the length of the optical resonant cavity covered by the short cavity electrode 104 is kept the same as the ratio between the overall optical lengths of the two optical resonant cavities.

[0085] The remaining parts of the two optical resonant cavities are covered by two electrodes, which are called the first resonant cavity electrode and the second resonant cavity electrode, or the short cavity electrode 104 and the long cavity electrode 204 according to the length of the optical resonant cavity. The long cavity electrode 204 and the short cavity electrode 104 are tuning electrodes while providing gain. Both electrodes can be used for tuning, but only one electrode current needs to be changed for tuning, which makes the laser tuning algorithm simple. The back of the laser is the ground electrode corresponding to the three electrodes.

[0086] Furthermore, under the scheme of the present invention, tuning only requires changing the current of one electrode, usually the long cavity electrode 204, and the single electrode tunability greatly simplifies the control algorithm.

[0087] In the specific implementation, the polymer imaging coupler serves as the main body of the optical resonant cavity, which is manufactured on the waveguide layer. Below the waveguide layer is a quantum well layer that emits light after receiving current injection. After current is injected into all electrodes, the quantum well layer can emit light and then propagate in the two optical resonant cavities.

[0088] The first partial reflection surfaces 101, 201 and the second partial reflection surfaces 103, 203 of the two optical resonant cavities are all deeply etched reflection surfaces, where deep etching means that the etching depth penetrates the quantum well layer, and generally the etching depth is greater than 4 μm.

[0089] This makes the deep etched reflective surface have a larger reflectivity, which is usually 0.4, and can be close to 0.99 if it is plated with a high reflectivity film. The size of the polymer imaging coupler 102, 202 is precisely controlled by forming the deep etched reflective surface during the manufacturing process instead of the usual cleavage method to ensure the realization of the function of the polymer imaging coupler 102, 202. The two sides of the middle deep etched waveguide 301 are also defined by the deep etched surface for the purpose of simplifying the process.

[0090] Among the first partial reflection surfaces 101 and 201 of the two optical resonant cavities in the laser, one first partial reflection surface 101 is a total reflection surface and can be coated with a high reflectivity film to further increase the reflectivity reduction threshold, and the other first partial reflection surface 201 is a semi-transparent and semi-reflective surface, serving as the output end of the laser.

[0091] In the laser, a conical waveguide 401 is arranged between the first partial reflection surface 201 serving as the output end of the laser and the polymer imaging type coupler 202 connected to it at the output end, so that the polymer imaging type coupler 202 is connected to the first partial reflection surface 201 through the conical waveguide 401, and the width of the conical waveguide 401 gradually decreases from the polymer imaging type coupler 202 at the output end to the first partial reflection surfaces 101, 201.

[0092] The size of the output spot of the laser of the present invention is determined by the multi-mode interference imaging result, rather than the usual optical waveguide constraint, so the size of the polymer imaging coupler must be precisely defined by deep etching. The spot size is slightly larger than that of the ordinary optical waveguide constraint, which is beneficial for subsequent coupling packaging. In the above, a tapered waveguide 401 can be added between the output end polymer imaging coupler 202 and its first partial reflection surface 201, and the spot can be appropriately compressed to the required size before output, which is more conducive to the on-chip integration of other optical devices.

[0093] Waveguides 105, 205 for lateral heating tuning and their corresponding electrodes 106, 206 are flexibly added to both sides of the aggregate imaging coupler 102, 202 of the two optical resonant cavities. The laser operating wavelength is fine-tuned by injecting current into the waveguides 105, 205 for lateral heating tuning, thereby adding a degree of freedom for tuning the wavelength and facilitating accurate wavelength locking. At the same time, it can be used in applications such as quasi-continuous tuning.

[0094] Figure 1 The basic cavity structure of the present invention except for the electrodes is shown in the figure. The material used in the present invention is a multi-layer quantum well wafer based on III-V semiconductor compounds, and the main structures from top to bottom are the waveguide layer, the quantum well layer and the substrate layer. The main structure of the present invention includes a long optical resonant cavity (called a long cavity) and a short optical resonant cavity (called a short cavity). The two optical resonant cavities are arranged in a straight line, with a compact structure and convenient for array application. The left and right optical resonant cavities each include their own first partial reflection surface 101, 201, a polymer imaging coupler 102, 202 and a second partial reflection surface 103, 203. The two optical resonant cavities are coupled by connecting the two second partial reflection surfaces 103, 203 through a middle deeply etched waveguide 301. All reflection surfaces are composed of deeply etched surfaces that are engraved through the quantum well layer during the wafer fabrication process.

[0095] The main body of the optical resonant cavity is a polymer imaging coupler 102, 202 based on the principle of multi-mode interference imaging and superimposed with a polymer imaging effect. The polymer imaging coupler 102, 202 has a certain light field energy distribution on its output end surface. The total reflection structure of the second partial reflection surface 103, 203 returns part of the energy to its own resonant cavity, which is called straight-through coupling, and the remaining energy is coupled into another resonant cavity through the middle deep etched waveguide 301, which is called cross coupling. The two polymer imaging couplers 103, 203 and the middle deep etched waveguide 301 form a half-wave coupler, such as Figure 2 as shown in .

[0096] The half-wave coupler has a selective loss of wavelength to improve the single-mode performance of the laser. The operation of the half-wave coupler requires that the cross-coupling energy and the through-coupling energy meet a certain proportional relationship and their phase difference is. The polymer imaging effect can be adjusted by size control, and the proportion of cross-coupling energy and through-coupling energy can be designed as needed. The length of the deep etched waveguide 301 in the middle determines the additional phase of the cross-coupling energy. Therefore, adjusting the length of the deep etched waveguide in the middle can flexibly control the phase difference between the cross-coupling energy and the through-coupling energy. The two key parameters of the cross-coupling and through-coupling energy proportions and phase difference are independently designed in two dimensions, which increases the design freedom and reduces the design complexity.

[0097] The polymer imaging couplers 102 and 202 are designed by compressing the size on the basis of the multimode interference coupler, so their volume is naturally small, which is the key reason why the present invention achieves small size. The polymer imaging couplers 102 and 202 constitute the main body of the optical resonant cavity, so the small size also makes the cavity length shorter, making the present invention suitable for designing tunable lasers with channel spacing of 400 GHz to 800 GHz. The adjustment of the cavity length, that is, the channel spacing, can be achieved within a certain range by scaling the size of the polymer imaging couplers 102 and 202. If the required cavity length is longer, the width is larger after scaling, so that the polymer imaging effect is lost, then a polymer imaging coupler of a shorter length is designed first, and then the periodicity of the multimode interference imaging principle is used to add a certain period to the design to achieve the target cavity length. Therefore, the present invention is not only suitable for small-sized lasers, but also can be used to design tunable lasers in a large cavity length requirement range, that is, a large frequency interval requirement range.

[0098] The two first partial reflection surfaces 101 and 201 serve as the end face reflections of their respective optical resonant cavities, one of which is coated with a high reflectivity film 101, usually metal gold, to increase the reflectivity and reduce the laser threshold. The high reflectivity film is completed synchronously with the metal electrode process during the tape-out process, and no additional evaporation process is required. In addition to reflection, the other first partial reflection surface 201 has a certain transmittance for laser output. The output spot size is determined by the multi-mode interference imaging effect. Figure 3A basic electrode arrangement design of the present invention is shown in the figure, which includes three electrodes, namely the middle coupler electrode 302 and the electrodes of the two optical resonant cavities, which are respectively called the long cavity electrode 204 and the short cavity electrode 104. The two optical resonant cavities are divided into two inner and outer sections, and the length ratio of the inner and outer sections of the two optical resonant cavities is consistent. The coupler electrode 302 covers the inner sections of the two optical resonant cavities, so that the length ratio of the inner sections of the two cavities covered by the coupler electrode is consistent with the overall length ratio of the two cavities, and no jump in the working wavelength occurs when the modulation signal is applied. The coupler electrode 302 is the main electrode for providing gain and the electrode for modulation. The long cavity electrode 204 covers the outer section of the long cavity, and the short cavity electrode 104 covers the outer section of the short cavity. Changing the current of the long cavity electrode 204 or the short cavity electrode 104 can change the resonant frequency of the corresponding optical resonant cavity, and the change amount is amplified by the vernier effect to tune the working wavelength of the laser. The long cavity electrode 204 and the short cavity electrode 104 can also provide gain at the same time.

[0099] Thermal tuning waveguides 105, 205 and electrodes 106, 206 may be arranged on the sides of the two optical resonant cavities as required, such as Figure 4 As shown in . The equivalent optical length of the optical resonant cavity is fine-tuned by lateral heating to fine-tune the operating wavelength. An additional degree of freedom for tuning the wavelength is added to accurately lock the wavelength position. The thermal tuning electrodes 106 and 206 on both sides of the two optical resonant cavities can also be tuned synchronously, so that the resonant frequencies of the two optical resonant cavities move synchronously in one direction, and the position where the two sets of resonant frequencies are aligned does not change but moves, thereby realizing quasi-continuous tuning of the laser.

[0100] The manufacturing process of the present invention is simple, and only requires conventional process steps and deep etching processes similar to the DFB laser after removing the grating manufacturing step, and does not require a complex grating manufacturing process and a complex docking coupling process required by silicon photonics technology.

[0101] The implementation principle of the present invention is as follows:

[0102] First, let's look at the effect of the vernier effect formed by the different optical lengths of the two optical resonant cavities. Assuming the length of the short cavity is ls and the corresponding length of the long cavity is ll, then the comb resonance frequency interval Δf corresponding to the short cavity is s As shown in formula 1.1, it is:

[0103] △f s =c / (2×n g ×ls) (1.1)

[0104] Where c is the speed of light in a vacuum, n gis the group refractive index of the laser waveguide. The resonant frequency interval of the short cavity is selected according to the requirements. If the laser volume is very small, the resonant frequency interval is very large, so the frequency interval of 400GHz to 800GHz can be designed. The length of the long cavity is ll=ls×(1+α), where α is the percentage difference in cavity length between the long cavity and the short cavity.

[0105] Similarly, the resonant frequency and frequency interval of the long cavity can be calculated. The frequency interval is smaller than that of the short cavity, such as Figure 5 as shown in .

[0106] The control makes sure that there is only one frequency overlap point of the two comb spectra within the gain spectrum width, ensuring the single-mode oscillation of the laser. At the same time, the distance between the frequency overlap points of the two comb spectra is the free spectrum range FSR, which is also determined by Figure 5 As shown in , it characterizes the tuning range FSR of the present invention, and the calculation is shown in formula 1.2.

[0107] FSR=(△f l ×△f s ) / |△f s -△f l |=(1+α) / α×△f l (1.2)

[0108] Among them, △f l represents the comb spectrum frequency interval of the long cavity, △f s Represents the comb spectrum frequency interval of the short cavity.

[0109] It can be seen that by injecting current to change the refractive index of one of the optical resonant cavities, its corresponding comb spectrum can be moved, causing the coincidence frequency to change and wavelength tuning to be achieved. This phenomenon is called the vernier effect. The vernier effect requires only a small change in the refractive index to achieve a large change in wavelength, thereby improving the tuning efficiency. That is, because the comb resonance frequency interval of the long cavity is smaller than that of the short cavity, the refractive index change required to tune a channel is smaller. More channels can be tuned under the same current range with higher tuning efficiency, making the long cavity called a tuning cavity. The long cavity electrode is the main tuning electrode, while the short cavity is usually not tuned. Its comb resonance frequency interval determines the frequency interval of the entire laser operating wavelength. The wavelength that the laser can tune occurs on a series of discrete comb frequencies determined by the short cavity. This phenomenon is called digital tuning.

[0110] Then, we analyze the required working conditions of the half-wave coupler based on the threshold equation. The threshold equation is as follows:

[0111] C 11 ×r1×r2×e 2×(g+i×k)×ls + C 22 ×r1×r2×e 2×(g’+i×k’)×ll - (C 11×C 22 -C 12 ×C 21 )×r1 2 ×r2 2 ×e 2×(g+i×k)×ls ×e 2×(g’+i×k’)×ll =1 (2.1)

[0112] Where g and k are the gain coefficient and propagation constant in the short cavity, g' and k' are the gain coefficient and propagation constant in the long cavity, subscript 1 indicates a short cavity, and subscript 2 indicates a long cavity. 11 With C 22 The energy amplitude direct coupling coefficient of the half-wave coupler returning from the short cavity to the short cavity and from the long cavity to the long cavity, C 12 With C 21 are the energy amplitude cross-coupling coefficients of the half-wave coupler from the short cavity to the long cavity and from the long cavity to the short cavity, respectively. i is the imaginary unit, e is the power function, r1 and r2 represent the reflection coefficients of the first partial reflection surfaces of the two optical resonators, respectively. The threshold equation is equal to 1 at the end.

[0113] The reference position for calculating the four coupling coefficients is the end of the second partial reflection surface of the two optical resonant cavities. Due to the symmetry of the optical paths of the two optical resonant cavities, the short cavity is taken as the main analysis target in the following analysis, and the influence of the long cavity and the deep etched waveguide in the middle on the short cavity is reflected as a complex reflection coefficient η. η can be expressed as:

[0114] η = C 11 +C 21 ×C 12 ×r1×r2×e 2×(g’+i×k’)×ll / (1-C 22 ×r1×r2×e 2×(g’+i×k’)×ll ) (2.2)

[0115] From the symmetry of the structure of the present invention, it can be seen that C 11 =C 22 =C b , C 12 =C 21 =C x , using C b represents the through coupling coefficient, C x represents the cross coupling coefficient. Both are amplitude complex coupling coefficients, C b With C x The imaginary difference of reflects the phase difference between the through coupling coefficient and the cross coupling coefficient. Therefore, the amplitude reflection coefficient correction factor η is simplified to:

[0116] η = C b +C x 2 ×r1×r2×e2×(g’+i×k’)×ll / (1-C b ×r1×r2×e 2×(g’+i×k’)×ll ) (2.3)

[0117] Figure 6 and Figure 7 The phase and amplitude of the amplitude reflection coefficient correction factor η in the two special cases of phase difference π / 2 and π are given in the figure. The phase and amplitude of the amplitude reflection coefficient correction factor η in the case of π / 2 are as follows: Figure 6 As shown in (a) and (b), the phase and amplitude of the amplitude reflection coefficient correction factor η at π are respectively as follows: Figure 7 As shown in (a) and (b). It can be seen that when the phase difference is π / 2, the amplitude of η is constant at 1 in a large wavelength range, that is, the half-wave coupler at this time has no loss for any wavelength. This shows that the long cavity and the deep etched waveguide in the middle have no effect on the resonance of the short cavity, and the two optical resonant cavities are completely independent. Such a half-wave coupler does not provide any mode selectivity and cannot be used in the present invention.

[0118] Figure 7 As shown in (a) and (b), when the phase difference is π, η plays a wavelength selective effect, and the equivalent amplitude reflection coefficient at one end of the short cavity is a wavelength-dependent comb spectrum with a very narrow bandwidth. The reflection coefficient comb spectrum, the resonance spectrum of the short cavity itself, and the gain spectrum of the material work together to ensure the mode selection function of the present invention, and achieve excellent side mode suppression ratio SMSR performance. Although no grating is used, single-mode performance similar to that of a grating is achieved. It can be seen that the phase difference of π between the through coupling coefficient and the cross coupling coefficient is the first performance indicator of a half-wave coupler that can provide a mode selection mechanism, and it is also one of the core indicators of the present invention. Because the phase difference is π, it is called a half-wave coupler.

[0119] On the other hand, under the premise that the phase difference between the through coupling coefficient and the cross coupling coefficient is π, optimizing the amplitude relationship between the cross coupling coefficient and the through coupling coefficient can further improve the single-mode performance. The influence of the proportional relationship between the two coupling coefficients on the amplitude reflection coefficient correction factor η is mainly reflected in Figure 8 The width of the comb spectrum shown and the contrast that characterizes the height of its peak. The SMSR of a laser is an important indicator for measuring static single-mode characteristics. It represents the power difference between the main mode and the side mode. Theoretically, it can be characterized by the threshold gain difference between the two modes. The greater the difference in threshold gain between the main mode and the side mode, the better the single-mode performance. Starting from the threshold equation 2.1, consider the equal loop gain condition (that is, consider g×ls=g×ll). At this time, the threshold gain g0 of the main mode is only determined by the reflection coefficients r1 and r2 of the cavity end face. The half-wave coupler does not introduce loss to the main mode. The thresholds of other modes can also be solved from the threshold equation. The results are shown in Figure 8 as shown in . Figure 8The threshold conditions of the main mode and the side mode are calculated in the figure, considering the two cases of through coupling coefficient of 0.8 and 0.2, and assuming no loss at this time. It can be seen that the larger the through coupling coefficient, the smaller the ratio of the cross coupling coefficient to the through coupling coefficient, and the narrower amplitude reflection coefficient correction factor η brings a larger threshold gain difference, which is about 6cm -1 , while the threshold gain difference is only 0.32cm when the through coupling coefficient is 0.2 -1 .

[0120] The normalized intensity cross-coupling coefficient χ of the half-wave coupler is defined as follows:

[0121] χ=|C 21 | 2 / (|C 11 | 2 +|C 21 | 2 )=|C 12 | 2 / (|C 12 | 2 +|C 22 | 2 ) (2.4)

[0122] Fig. 9 The relationship between the threshold difference between the main mode and the side mode and χ is calculated in the case of two cavity length differences of 5% and 7%. It can be seen that for a specific cavity length difference, there is an optimal χ, at which the threshold difference between the main mode and the side mode is the largest. The normalized intensity cross-coupling coefficients required for different cavity length differences are different. It is calculated that a 7% cavity length difference corresponds to 0.043 and a 5% cavity length difference corresponds to 0.023. The larger the cavity length difference is, the greater the main and side mode threshold difference is, but the larger the normalized intensity cross-coupling coefficient χ is. This also means a larger cross-coupling coefficient and a smaller through-coupling coefficient, which also means a higher SMSR. The normalized intensity cross-coupling coefficient χ is also an important performance indicator of the half-wave coupler. However, it can be seen from formula 1.2 that a larger cavity length difference leads to a smaller FSR, which is not conducive to the tuning range of the present invention. Therefore, the normalized intensity cross-coupling coefficient is required to make special optimizations for the specific design (specific cavity length difference). Between 5% and 7% is a more suitable design to balance SMSR and tuning range.

[0123] The main function of the polymer imaging couplers 102 and 202 constituting the half-wave coupler is to achieve excellent single-mode performance by making the cross-coupling coefficient and the through-coupling coefficient satisfy a certain proportional relationship. In specific implementation, this is achieved by selecting a suitable traditional multi-mode interference coupler MMI based on the principle of multi-mode interference imaging, and making certain dimensional compression fine-tuning on this basis. The specific design method is described as follows. Common 1×N type complete MMI structures are as follows: Fig.10As shown in , N is the number of images at the imaging surface at a specific distance. The 1×N type MMI is selected as the basis because the input position is selected as the center of the coupler in consideration of the requirements of the second part of the reflection surface structure and the position of the deep etched waveguide in the middle and the convenience of light output coupling. That is, the output spot position of the present invention is located at the center of the coupler. Assuming that the width of the 1×N type MMI is W and the effective refractive index is n r The positions and phases of its N self-image points can be obtained by Fourier analysis and summing the generalized Gaussian function. By approximating each order of guided mode as a sine function, we can obtain that L=p / n×3×L π The imaging distribution at , where p represents the periodicity of the axial imaging result, and n represents the position of n images.

[0124] Where L π See the calculation in equation 3.1, which is defined as the beat length of the zero-order guided mode and the first-order guided mode.

[0125] L π =π / (β0-β1)=(4×n r ×W 2 ) / (3×λ0) (3.1)

[0126] Among them, L π is the beat length of the zero-order guided mode and the first-order guided mode, β0 and β1 represent the transmission constants of the zero-order guided mode and the first-order guided mode, n r is the equivalent refractive index, W is the width of the multimode interference coupler, and λ0 is the operating wavelength.

[0127] The imaging distribution at a distance L from the input end is defined by equation 3.2:

[0128] Ψ(y,L)=1 / C×∑Ψ in (yy q )exp(i×ψ q ), q=0,1,..,N-1 (3.2)

[0129] in:

[0130] y q =p×(2×qN)×W / N (3.3)

[0131] ψ q =p×(Nq)×(q×π) / N (3.4)

[0132] Among them, Ψ(y,L) refers to the distribution of energy along the longitudinal coordinate y at length L, Ψ in () refers to the energy longitudinal distribution function at the input, q is the number of each image, y q For each image vertical position, exp() represents the natural exponential function, ψ qis the phase of each image. C is a normalized constant that satisfies |C|=N 1 / 2 .

[0133] Formula 3.3 represents the position of each image at the imaging surface with a distance of L, and the energy of each image is evenly distributed. Formula 3.4 represents the phase of each image at the imaging surface with a distance of L, and p represents the periodicity of the MMI imaging effect along the MMI axis. Therefore, under the premise that the position and size of the MMI input waveguide are determined, the distribution, intensity and phase of each imaging point on the MMI imaging surface can be obtained. Since the energy of each image at the imaging surface is evenly distributed, for the imaging output surface with N images, although the energy ratio of cross-coupling and straight-through coupling can be adjusted by designing and controlling the ratio of the number of images coupled into the middle deep-etched waveguide 301 and the number of images fully reflected back into the cavity by the second partial reflection surface 103, 203, the designable energy ratio is greatly restricted, not flexible enough and may not meet the use requirements. Therefore, on the basis of the above 1×N type MMI, the scale of the MMI is proportionally reduced, and the distance between adjacent imaging points is compressed to aggregate its energy distribution, thereby forming a new non-evenly distributed light field energy distribution, which provides the possibility of achieving different cross-coupling and straight-through coupling ratios. This MMI after polymerization operation is called a polymerization imaging coupler.

[0134] The following is a design example of the polymer imaging coupler 102, 202. Starting from the 1×4 MMI, its imaging is calculated using the finite difference time domain method FDTD. The results are shown in Fig.11 As shown in , it can be seen that at a specific distance, four images are clearly formed. Then the MMI size is compressed. There are two factors to consider for the compressed MMI size. One is the ratio of the cross-coupling and straight-through coupling energy of the core, and the other is the length of the polymer imaging coupler 102, 202, that is, the length of the optical resonant cavity. If the ratio of the cross-coupling and straight-through coupling energy does not meet the requirements after the MMI length direction is compressed to the required cavity length, one way is to go back to the beginning and select a different 1×N type MMI to re-optimize, and another way is to adjust the width of the deep etched waveguide 301 in the middle. The width of the middle deeply etched waveguide 301 determines how much aggregated energy can be coupled into the middle deeply etched waveguide 301 to become real cross-coupling energy on the imaging surface of the compressed aggregate imaging coupler. On the other hand, the width of the middle deeply etched waveguide 301 and the scale of the second partial reflection surface 103, 203 are complementary. When adjusting the width of the middle deeply etched waveguide 301, the scale of the second partial reflection surface 103, 203 is appropriately adjusted, that is, the straight-through coupling part of the energy in the total reflection back cavity is optimized, thereby adjusting the ratio of cross-coupling to straight-through coupling energy. Therefore, there is enough design freedom to meet the design index requirements of the aggregate imaging coupler 102, 202. The energy distribution at the output imaging surface of an example of the aggregate imaging coupler 102, 202 after the final design is shown in Fig.12 As shown in , the non-uniformly distributed light field energy is basically divided into three parts, and the energy of the two side parts is totally reflected back into the cavity by the second part reflection surfaces 103 and 203. At this time, the width of the middle deeply etched waveguide 301 is designed to be 2.5μm, so that the normalized intensity cross-coupling coefficient χ is 0.0245, which meets the requirements of the half-wave coupler. The width of the middle deeply etched waveguide 301 is usually designed to be in the range of 2-4μm. The length of the aggregate imaging type coupler that meets the above design requirements can be very short, and can be designed to be about 50μm-100μm. The corresponding tuning wavelength frequency interval is designed to be 800GHz or 400GHz. Therefore, the present invention is naturally suitable as a small-size tunable laser solution. In addition, if the required target cavity length is long and cannot be properly compressed to aggregate the imaging points, the periodicity of the multi-mode interference imaging principle described in Equations 3.2 and 3.3 can be used. The periodicity of the imaging is calculated as shown in Fig.13 As shown in , after an additional cycle, the output imaging surface still forms N images, and the degree of aggregation is also consistent. In the design, the same size of the compressed 1×N type MMI is considered first, and the width of the compressed aggregate imaging coupler is calculated to calculate the period length corresponding to its width, to ensure that one or several period lengths plus the original compressed aggregate imaging coupler length meet the cavity length requirements, and then fine-tune the width of the middle deep etched waveguide 301 to adjust the cross-coupling and through-coupling energy ratio, and complete the final design of the aggregate imaging coupler 102, 202. Therefore, the present invention can not only be used as a small-sized laser, but also can flexibly design tunable lasers with different cavity lengths, that is, different tuning wavelength frequency intervals, in a wide range.

[0135] The main function of the middle deeply etched waveguide 301 constituting the half-wave coupler is to match the appropriate cross-coupling and through-coupling energy ratio by changing the width as mentioned above, and to match the phase difference between the cross-coupling and through-coupling energy to π. As can be seen from formula 3.4, there is a phase difference between each imaging point on the output imaging plane, and the phase situation after the aggregate imaging compression is more complicated, and the superimposed mode field distribution needs to be superimposed and calculated. When the aggregate imaging couplers 102 and 202 mentioned above were designed, only the requirements for the cross-coupling and through-coupling energy ratio were considered. At this time, the phase relationship between the cross-coupling and through-coupling energy is complicated. After calculating the phase difference between the two through FDTD, an additional phase difference is introduced by designing the length of the middle deeply etched waveguide 301 to match, so that the phase difference between the cross-coupling and through-coupling energy is finally π. In the above design example, the length of the middle deeply etched waveguide 301 is 3μm, and the final cross-coupling and through-coupling phase difference is 0.994π. In the present invention, the two indicators of the core half-wave coupler (the proportional relationship between the cross-coupling and through-coupling energies and the phase difference of π) are independently designed through different structures. Unlike the V-cavity tunable laser, the two indicators are interrelated in the design, which increases the flexibility and feasibility of the design and simplifies the design process.

[0136] The specific embodiments of the present invention are as follows: Embodiment 1:

[0137] like Figure 1As shown in , the left optical resonant cavity is composed of a first partial reflection surface 101, a polymer imaging coupler 102 and a second partial reflection surface 103 from left to right, and the right optical resonant cavity is composed of a second partial reflection surface 203, a polymer imaging coupler 202 and a first partial reflection surface 201 from left to right. The second partial reflection surfaces 103 and 203 are composed of two reflection surfaces that are 45 degrees to the axis, forming a group of total reflection structures. The two optical resonant cavities are arranged along the same straight line with the middle deep-etched waveguide 301, and the middle deep-etched waveguide 301 connects the second partial reflection surfaces 103 and 203 of the two optical resonant cavities to couple the two optical resonant cavities. All the first partial reflection surfaces 101, 201 and the second partial reflection surfaces 103 and 203 are deep-etched reflection surfaces, and the positions of the two sides of the middle deep-etched waveguide 301 are also defined by deep etching. Usually, the left side is a short cavity and the right side is a long cavity. The first partial reflection surface 101 of the left optical resonant cavity is coated with a high reflectivity film, and the first partial reflection surface 201 of the right optical resonant cavity is not coated, and is used as a semi-transmissive and semi-reflective structure for laser output. The light forms a certain energy distribution at the output end face of the polymer imaging coupler 102, 202, and the two side parts are reflected back into the cavity through the total reflection structure of the second partial reflection surface 103, 203 to form the straight-through coupling part of energy, and the middle part is coupled into the middle deep etched waveguide 301 and finally coupled into another optical resonant cavity to form the cross-coupling part of energy.

[0138] The electrode structure in this embodiment is shown in Figure 3 As shown in . The two optical resonant cavities are divided into two parts along the axis, and the parts of the two close to the middle deeply etched waveguide 301 are covered by the coupler electrode 302, which is also the modulation electrode of the laser. The remaining outer parts of the two optical resonant cavities are covered by their respective electrodes, called short cavity electrode 104 and long cavity electrode 204. There is a certain length difference between the two optical resonant cavities, and the length ratio of the inner parts of the two optical resonant cavities covered by the coupler electrode 302 is consistent with the overall length ratio of the two optical resonant cavities, so that the working wavelength does not jump when a high-speed modulation signal is applied. All three electrodes need to provide the gain of the laser, and the wavelength can be tuned by changing the current of the long cavity electrode 204 or the short cavity electrode 104. The three electrodes share a ground electrode located on the back of the laser chip. Embodiment 2:

[0139] The basic structure of the laser is the same as Figure 1 As shown in, it is consistent with Example 1, except for the electrode arrangement. The electrode structure in this embodiment is shown in Fig.14 As shown in . It only contains two electrodes, which cover two optical resonant cavities respectively, called long cavity electrode 204 and short cavity electrode 104. The ground electrodes corresponding to the two electrodes are also located on the back of the laser chip. Similarly, changing the current of the long cavity electrode 204 or the short cavity electrode 104 can tune the wavelength.

[0140] This embodiment is not used in applications that require high-speed modulation. It omits one electrode to simplify the external circuit design and can be used in quasi-continuous tuning applications. Quasi-continuous tuning refers to synchronously tuning the long cavity electrode 204 and the short cavity electrode 104, so that the comb frequency spectra of the two optical resonant cavities move synchronously and in the same direction due to thermal effects, so that the positions where the two sets of comb frequency spectra overlap do not change, but the tuning method in which the working wavelength changes continuously is suitable for fields such as frequency sweeping and sensing. At the same time, the structure of this embodiment is more compact and more suitable for applications where laser arrays are required. Embodiment 3:

[0141] In this embodiment, the main structure of the laser and the electrode arrangement are designed as follows: Figure 4 and Fig.15 As shown in , the main two optical resonant cavities and the corresponding first partial reflection surfaces 101, 201, the polymer imaging coupler 102, 202 and the second partial reflection surfaces 103, 203 and the middle deep etched waveguide 301 and other structural arrangements are consistent with those in Example 1, and the main electrode arrangements can adopt the schemes in Example 1 and Example 2. In this embodiment, two additional waveguides 105, 205 for heat fine-tuning and corresponding electrodes 106, 206 are added to the sides of the two optical resonant cavities, as shown in FIG. Fig.15 As shown in . The short cavity side is a short cavity thermal tuning waveguide 105, which is covered with a short cavity thermal tuning electrode 106; the long cavity side is a long cavity thermal tuning waveguide 205, which is covered with a long cavity thermal tuning electrode 206. The thermal tuning structures on the two sides affect the optical lengths of the two optical resonant cavities by injecting current to generate heat from the sides, thereby tuning the laser and adding a degree of freedom for tuning the wavelength. This effect is relatively weak compared to direct injection of the long cavity electrode 204 and the short cavity electrode 104, so it is mainly a means of fine-tuning. Fine-tuning the wavelength allows the laser to maintain good single-mode performance while accurately operating at the wavelength specified by various standards. The thermal tuning structures on the two sides can also be used for quasi-continuous tuning of the laser. Embodiment 4:

[0142] In this embodiment, the main structure of the laser is shown in Fig.16As shown in . Its main two optical resonant cavities and the corresponding first partial reflection surfaces 101, 201, the polymer imaging type couplers 102, 202 and the second partial reflection surfaces 103, 203 and the middle deep etched waveguide 301 and other structural arrangements are basically the same as those in Example 1, except that there is a tapered waveguide 401 with a width gradually decreasing from left to right between the polymer imaging type coupler 202 on the output side and its first partial reflection surface 201. Since the output light spot size of the previous embodiments is determined by the multi-mode interference imaging effect and is slightly larger than the ordinary light spot constrained by the optical waveguide, the introduction of the tapered waveguide 401 can control the size of the output light spot to the required scale, which is more conducive to on-chip integration with other functional optical devices (such as modulators, etc.). At the same time, the introduction of the tapered waveguide 401 causes a certain change in the length design of the optical resonant cavity in which it is located. The polymer imaging type couplers 102, 202 corresponding to the two optical resonant cavities can adopt the same design, and the length difference corresponding to the two optical resonant cavities can be compensated by adjusting the length of the tapered waveguide, which simplifies the design process. The polymer imaging couplers 102 and 202 corresponding to the two optical resonant cavities in the above-mentioned embodiments need to be designed independently considering the different cavity lengths. The electrode design in this embodiment can adopt the design scheme in embodiment 1 or embodiment 2.

[0143] The above design scheme is of a descriptive nature, and any scheme similar to its spirit falls within the scope of protection of this patent.

Claims

1. A small-sized wavelength tunable semiconductor laser, characterized in that: It comprises two optical resonant cavities, each of which is mainly composed of respective first partial reflection surfaces (101, 201), polymer imaging couplers (102, 202) and second partial reflection surfaces (103, 203) connected in sequence, the two optical resonant cavities are distributed along the same straight line, and are coupled to each other via a deeply etched waveguide (301) connecting respective second partial reflection surfaces (103, 203); The polymer imaging coupler (102, 202) is a multi-mode interference coupler that is subjected to size compression processing, so that the length and width are compressed to a range of 0.6 of the original size, and the final length and width are 50-100 μm and 8-12 μm respectively.

2. A small-size wavelength tunable semiconductor laser according to claim 1, characterized in that: Each optical resonant cavity has two second partial reflection surfaces (103, 203), which are respectively located on two symmetrical sides of the connection between the end of the polymer imaging coupler (102, 202) and the deep etched waveguide (301); The two side portions of one end of the polymer imaging coupler (102, 202) are connected to the second partial reflection surface (103, 203), and the remaining portion in the middle is coupled to the deep etched waveguide (301) in the middle; the two second partial reflection surfaces (103, 203) on both sides form a group of total reflection structures, which reflect a portion of the energy in the end face energy distribution of the polymer imaging coupler (102, 202) at the end of the optical resonant cavity back to the optical resonant cavity where the optical resonant cavity is located to form a straight-through coupling, and the other portion of the energy is coupled to another optical resonant cavity through the deep etched waveguide (301) to form a cross coupling.

3. The small-size wavelength tunable semiconductor laser according to claim 1, characterized in that: The first partial reflection surface (101, 201) is a first pit etched on the wafer to penetrate the quantum well layer, with the plane of the pit wall of the first pit serving as the first partial reflection surface; The second partial reflection surface (103, 203) is formed by etching two second pits at the end of the polymer imaging type coupler (102, 202) and on both sides of the symmetry of the straight line where the polymer imaging type coupler (102, 202) is located, and each second pit is set at a 45-degree cut angle at a corner close to the end of the polymer imaging type coupler (102, 202), and the plane of the pit wall with a 45-degree cut angle is used as the second partial reflection surface; There is a gap distance between the two second pits, the width of which is smaller than the width of the polymer imaging type coupler (102, 202), and a deep etched waveguide (301) is arranged in the gap distance.

4. The small-sized wavelength tunable semiconductor laser according to claim 3, characterized in that: The two second pits of the second partial reflection surfaces (103, 203) of the two optical resonant cavities are shared, that is, two second pits that penetrate the quantum well layer are etched between the ends of the polymer imaging type couplers (102, 202) of the two optical resonant cavities that are opposite to each other and along the symmetrical sides of the straight line where the polymer imaging type couplers (102, 202) are located, and each second pit is set to have a 45-degree cut angle at a corner close to the end of the polymer imaging type coupler (102, 202), and the plane of the pit wall surface with a 45-degree cut angle is used as the second partial reflection surface.

5. The small-size wavelength tunable semiconductor laser according to claim 1, characterized in that: The optical lengths of the two optical resonant cavities are different, that is, the lengths of the aggregate imaging couplers (102, 202) of the two optical resonant cavities are different; and within the gain spectrum of the laser material, only one of the resonant frequencies of the two optical resonant cavities overlaps, and the overlapping resonant frequency is used as the working wavelength.

6. The small-size wavelength tunable semiconductor laser according to claim 5, characterized in that: The laser is provided with two electrodes, a short cavity electrode (104) and a long cavity electrode (204), which are respectively located on two sides; the long cavity electrode (204) and the short cavity electrode (104) are respectively covered on the longer optical resonant cavity and the shorter optical resonant cavity.

7. The small-sized wavelength tunable semiconductor laser according to claim 5, characterized in that: The laser is provided with three electrodes, namely, a coupler electrode (302) located in the middle and a short cavity electrode (104) and a long cavity electrode (204) located on both sides; the coupler electrode (302) simultaneously covers a portion of each of the two optical resonant cavities, and the ratio of the optical lengths of the two optical resonant cavities covered by the coupler electrode (302) is consistent with the ratio of the overall optical lengths of the two optical resonant cavities; The long cavity electrode (204) and the short cavity electrode (104) respectively cover the longer optical resonant cavity and the shorter optical resonant cavity and cover the remaining portion not covered by the coupler electrode (302), and the length of the optical resonant cavity covered by the long cavity electrode (204) is greater than the length of the optical resonant cavity covered by the short cavity electrode (104).

8. The small-size wavelength tunable semiconductor laser according to claim 1, characterized in that: Of the first partial reflection surfaces (101, 201) of the two optical resonant cavities, one first partial reflection surface (101, 201) is a total reflection surface, and the other first partial reflection surface (101, 201) is a semi-transmissive and semi-reflective surface, serving as the output end of the laser.

9. The small-size wavelength tunable semiconductor laser according to claim 1, characterized in that: In the laser, a tapered waveguide (401) is arranged between a first partial reflection surface (101, 201) as an output end of the laser and a polymer imaging type coupler (202) connected thereto, and a width of the tapered waveguide (401) gradually decreases from the polymer imaging type coupler (202) to the first partial reflection surface (101, 201); Or / and, waveguides (105, 205) and corresponding electrodes (106, 206) for lateral heating tuning are added to both sides of the aggregate imaging coupler (202) of the two optical resonant cavities.

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