High-order collision mode-locked laser with tunable repetition frequency and optical frequency comb

Abstract

The invention discloses a repetition frequency tunable high-order collision mode-locked laser and an optical frequency comb, and the laser comprises a laser resonant cavity, the two ends of the laser resonant cavity are respectively provided with a high reflection coating end face and a low reflection coating end face, a saturable absorber SA region and a gain Gain region, the total length ratio of SA to Gain is 1: A, the resonant cavity is divided into (1 + A) large sections, each large section is divided into B small sections, and (1 + A) * B unit sections are arranged, the adjacent unit sections are electrically isolated and can be combined into an SA or Gain region through electrical connection; sA is composed of continuous B unit sections, Gain is composed of the remaining unit sections, and collision mode locking of different orders and repetition frequency tuning are achieved by changing the position of the SA. By changing the connection mode of the resonant cavity and combining with the conical ridge width structure, the high-power pulse light source with tunable repetition frequency is realized under the fixed resonant cavity, the system complexity and cost are reduced, the pulse stability and peak power are improved, and a scheme is provided for low-cost and miniaturized application.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor optoelectronic technology, and in particular relates to a high-order collisional mode-locked laser with tunable repetition rate and an optical frequency comb. Background Technology

[0002] Mode-locked lasers, also known as ultrafast lasers, produce pulses with narrow pulse widths, high peak power, high repetition rates, and broad spectra. With the rapid development of laser technology, mode-locked lasers have been widely applied in numerous cutting-edge technologies, such as optical clocks, nanomaterial processing, lidar detection, and millimeter-wave and terahertz generation. Traditional semiconductor mode-locked lasers utilize the nonlinear optical properties of saturable absorbers: achieving a bleaching effect on high-intensity input light pulses while completely absorbing weak light, thus realizing phase locking between longitudinal modes within the cavity and easily generating ultrashort pulses. Semiconductor mode-locked lasers, with their small size, low threshold voltage, simple structure, and ease of operation, have remained popular and are considered one of the most promising comb-spectrum emitters.

[0003] Currently, the most common semiconductor mode-locked lasers on the market are passive mode-locked lasers or multi-segment collisional mode-locked lasers. In passive mode-locked lasers, the SA (saturable absorber) region is typically located at the end face of the device, meaning its repetition frequency is directly determined by the length of the laser resonant cavity. It cannot be tuned by simple electrical or optical means. Changing the frequency often requires replacing the laser module with one of different cavity lengths, which not only increases the complexity and cost of the system design but also reduces the equipment's versatility and adaptability.

[0004] In contrast, multi-segment collisional mode-locked lasers offer a more promising solution. These lasers typically contain multiple gain regions and / or saturable absorption regions. By carefully designing the relative positions and optical properties of these regions, the collisions and interactions of pulses in different areas within the cavity can be controlled, thereby achieving more complex mode-locking states, such as higher-order mode-locking (generating multiple pulses operating and interacting simultaneously within the cavity). Theoretically, by changing the specific position of the saturable absorption region within the laser cavity, the pulse dynamics can be effectively adjusted, thus achieving tuning of the repetition frequency or switching of the mode-locking state. This offers hope for solving the problem of fixed frequency in passively mode-locked lasers.

[0005] However, multi-segment collisional mode-locked lasers face significant challenges in practical applications. The core issue lies in their high structural complexity and the extremely stringent requirements for the relative positional accuracy between their internal segments. In current mainstream semiconductor manufacturing processes, the device geometry, especially the positions of critical functional regions like the saturation absorption region, is typically precisely determined during the chip design layout stage and solidified in subsequent micro / nano fabrication steps such as photolithography and etching. This means that once chip manufacturing is complete, the position of the saturation absorption region becomes a physical "dead point," impossible to change.

[0006] Therefore, if researchers or engineers find after testing that the current SA position cannot achieve the expected mode-locking state (e.g., the required high-order mode-locking mode cannot be achieved, or the repetition frequency tuning range is not ideal), or if the SA position needs to be adjusted to adapt to new working requirements in practical applications, the only way is to redesign the layout, adjust the SA position, and then re-enter the expensive fabrication run process. This process is not only time-consuming and labor-intensive, usually taking weeks or even months, but each fabrication run is also accompanied by high manufacturing costs, which is a huge burden on R&D cycles and cost control. This "one-shot" structural fixation severely restricts the rapid iterative optimization and flexible application deployment of multi-segment collision mode-locked lasers, making it difficult to meet the increasingly diverse market demands and rapidly changing scientific research needs.

[0007] In summary, existing passive mode-locked lasers lack flexibility due to their non-adjustable frequency, while multi-segment collision mode-locked lasers, although possessing frequency tuning potential, suffer from extremely difficult, costly, and time-consuming adjustments to the SA position due to their fixed structure. Both of these technologies have significant limitations, necessitating a novel mode-locked laser structure that can overcome these shortcomings and achieve convenient and low-cost control of the SA position. Summary of the Invention

[0008] To address the technical problems of fixed repetition frequency in passive mode-locked lasers and high cost and long cycle time in adjusting SA position in multi-segment collision mode-locked lasers, this invention provides a high-order collision mode-locked laser with tunable repetition frequency and an optical frequency comb.

[0009] The technical solution adopted by this invention to solve its technical problem is: A high-order collisional mode-locked laser with tunable repetition rate includes: The laser resonant cavity includes a saturable absorber (SA) region and a gain (Gain) region, with the ratio of the total length of the SA region to the total length of the Gain region being 1:A. The resonant cavity is divided into (1+A) large segments, and each large segment is further divided into B small segments, for a total of (1+A)×B unit segments. Adjacent unit segments are electrically isolated by etching, and each unit segment can be combined to form an SA region or a Gain region through electrical connection; where A is an integer greater than 1 and B is a positive integer. The SA region consists of B consecutive unit segments, and the Gain region consists of the remaining unit segments. By changing the position of the SA region in the resonant cavity, different orders of collision mode-locking states and repetition frequency tuning can be achieved.

[0010] Preferably, the length of each unit segment is: ; Where A is the proportionality coefficient, ranging from 4 to 9, and B is the number of segments. The total length of region SA. The length of each unit segment, is the length of the laser resonant cavity.

[0011] Preferably, the total length of the SA region is: ; The total length of the Gain region is: .

[0012] Preferably, one end of the resonant cavity has a high-reflection coated end face, and the other end has a low-reflection coated end face, with a distance of [missing information] from the low-reflection coated end face. The region at point Gain1 is a gain region with a tapered ridge waveguide structure, and the widths of its left and right ridges are w1 and w2, respectively. , Furthermore, the width configuration of w2 ensures that the device outputs single-mode light.

[0013] Preferably, the distance between the center point of the SA region and the high-reflectivity coating end face is x, and the relationship between x and the collision mode-locking order n is as follows: ; Where n is a positive integer, and the value of n is determined by the adjustment of the position of the SA region within the resonant cavity.

[0014] Preferably, all unit segments in the SA region are electrically connected and a reverse voltage is applied by means of gold wire connection, and all unit segments in the Gain region are electrically connected and a forward current is applied. A stable mode-locked operating point is obtained by adjusting the voltage and current levels.

[0015] Preferably, when in the nth-order collision mode-locked state, n-1 suppressed wavelengths appear within the interval between adjacent emission wavelengths in the output spectrum. This spectral characteristic is determined by the position of the SA region and the corresponding order n.

[0016] Preferably, the laser is based on a GaAs substrate or a silicon substrate and employs a III-V quantum dot active layer.

[0017] An optical frequency comb includes a high-order collision mode-locked laser with tunable repetition rate. The spacing between the comb teeth is determined by the repetition frequency of the laser, and the tooth distribution satisfies the following: when the laser is in an n-order collision mode-locked state, there are n-1 suppressed comb teeth in the spacing between adjacent comb teeth. The frequency spacing between the comb teeth is related to the length of the laser resonant cavity and the mode-locking order n.

[0018] Preferably, the spectral width of the optical frequency comb is determined by the gain bandwidth of the III-V quantum dot active layer in the laser, and the mode-locking order n is changed by adjusting the position of the SA region to achieve the tuning of the comb tooth spacing.

[0019] The aforementioned high-order collision mode-locked laser with tunable repetition rate and optical frequency comb can achieve high-order collision mode-locking state switching and repetition frequency tuning under a fixed resonant cavity by flexibly adjusting the position of the SA region, while reducing equipment cost and operation complexity. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of a high-order collision mode-locked laser with tunable repetition rate according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the spectrum of a high-order collision mode-locked laser with tunable repetition rate under different connection conditions in one embodiment of the present invention. Figure 3 This is one example structure of a high-order collision mode-locked laser with tunable repetition rate in one embodiment of the present invention; Figure 4 This is another example structure of a high-order collision mode-locked laser with tunable repetition rate in one embodiment of the present invention. Detailed Implementation

[0021] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings.

[0022] In one embodiment, such as Figure 1 As shown, a high-order collisional mode-locked laser with tunable repetition rate includes: The laser resonant cavity includes a saturable absorber (SA) region and a gain (Gain) region, with the ratio of the total length of the SA region to the total length of the Gain region being 1:A. The resonant cavity is divided into (1+A) large segments, and each large segment is further divided into B small segments, for a total of (1+A)×B unit segments. Adjacent unit segments are electrically isolated by etching, and each unit segment can be combined to form an SA region or a Gain region through electrical connection; where A is an integer greater than 1 and B is a positive integer. The SA region consists of B consecutive unit segments, and the Gain region consists of the remaining unit segments. By changing the position of the SA region in the resonant cavity, different orders of collision mode-locking states and repetition frequency tuning can be achieved.

[0023] Specifically, in this embodiment, the laser operates in the O-band.

[0024] In one embodiment, the length of each unit segment is: ; Where A is the proportionality coefficient, ranging from 4 to 9, and B is the number of segments. The total length of region SA. The length of each unit segment, is the length of the laser resonant cavity.

[0025] Specifically, experimental verification shows that if the value of A is too small (A < 4), it means that the proportion of the saturable absorber in the resonant cavity is relatively too large, which will introduce excessively high intracavity losses, causing laser oscillation to fail to be generated or difficult to maintain. Conversely, if the value of A is too large (A > 9), it means that the proportion of the saturable absorber is too small, and its modulation depth is insufficient to initiate and stabilize the mode-locking process. Therefore, configuring the value of A between 4 and 9 can effectively balance intracavity losses and modulation capabilities, which is the key to ensuring that the device achieves stable self-mode-locking operation.

[0026] In one embodiment, the total length of the SA region is: ; The total length of the Gain region is: .

[0027] In one embodiment, a high-reflection coated end face is provided at one end of the resonant cavity, and a low-reflection coated end face is provided at the other end. The distance from the low-reflection coated end face is... The region at point Gain1 is a gain region with a tapered ridge waveguide structure, and the widths of its left and right ridges are w1 and w2, respectively. , Furthermore, the width configuration of w2 ensures that the device outputs single-mode light.

[0028] Specifically, in laser resonant cavities, high-reflectivity coatings typically have high reflectivity at their end faces to minimize optical loss and maintain laser oscillation. For typical semiconductor lasers, the reflectivity of high-reflectivity coatings is generally greater than 90%. Common coating materials include multilayer dielectric films, which are deposited alternately to form a distributed Bragg mirror structure to achieve high reflectivity.

[0029] Low-reflectivity coatings, used as laser output couplers, have low reflectivity to allow partial light transmission. For single-mode output lasers, the reflectivity of low-reflectivity coatings is typically set between 10% and 30%, depending on the gain medium and resonator design. The coating material may include single or multiple anti-reflective layers to reduce reflectivity.

[0030] Furthermore, by flexibly combining adjacent laser resonators, collision mode-locking forms of different orders can be obtained. That is, using the same cavity length, different connection methods can be used to change the position of the SA region in the entire resonator, thereby obtaining different repetition frequencies.

[0031] In one embodiment, the distance between the center point of the SA region and the high-reflectivity coating end face is x, and the relationship between x and the collision mode-locking order n is as follows: ; Where n is a positive integer, and the value of n is determined by the adjustment of the position of the SA region within the resonant cavity.

[0032] Specifically, the position of the SA (Section A) determines the frequency and phase conditions of the pulse collisions. When the SA divides the cavity into several segments, the pulses will meet at the SA at specific time intervals during their round-trip propagation. This compression of the pulses through nonlinear absorption effects enhances mode-locking stability, thus forming a higher-order mode-locking. The larger the order n, the closer the SA is to one end of the cavity. This design allows for flexible control of the mode-locking order by adjusting the position of the SA region.

[0033] In one embodiment, all cell segments in the SA region are electrically connected and a reverse voltage is applied by means of gold wire connection, and all cell segments in the Gain region are electrically connected and a forward current is applied. A stable mode-locked operating point is obtained by adjusting the voltage and current levels.

[0034] In one embodiment, when in an nth-order collision mode-locked state, n-1 suppressed wavelengths appear within the interval between adjacent emission wavelengths in the output spectrum. This spectral characteristic is determined by the position of the SA region and the corresponding order n.

[0035] Specifically, by coupling the light generated by the device to a spectrometer via single-mode fiber, it is possible to determine whether the device is mode-locked and the specific order of collisional mode-locking. (See attached diagram.) Figure 2As shown, if the higher-order collisional mode-locking is in an unlocked state, its spectrum is consistent with that of a typical FP laser, i.e., wavelengths 1 to 7, with each wavelength emitting light and the distance between adjacent wavelengths being a fixed value determined by the device cavity length. If the device is under second-order collisional mode-locking, one of the two adjacent emission wavelengths will be suppressed, i.e., wavelengths 2, 4, and 6 will not emit light, while wavelengths 1, 3, 5, and 7 will emit light. If the device is under fourth-order collisional mode-locking, three of the two adjacent emission wavelengths will be suppressed, i.e., wavelengths 1 and 5 will emit light, while wavelengths 2, 3, and 4 will be suppressed. Similarly, the spectrum exhibited by n-order collisional mode-locking is such that n-1 wavelengths of the two adjacent emission wavelengths will be suppressed, thus allowing determination of the device's mode-locking state.

[0036] Attached Figure Figure 3 and Figure 4 This presents a specific implementation case, such as Figure 3 As shown, the overall SA:Gain region length is 1:6. The entire device is divided into 7 major segments, and each major segment is further divided into 4 minor segments, i.e., A=6, B=4, for a total of 28 Units. The 12 Units closest to LR (Low Reflection) form a fixed light-emitting taper structure called Gain1.

[0037] If SA is adjacent to Gain1, the entire resonant cavity is divided into symmetrical Gain1 and Gain2 parts by SA. The center of the SA region is 14 units away from HR (High Reflection) and can be deduced to be n=2, that is, the structure is a second-order collision mode-locked structure.

[0038] If SA is as shown in the appendix Figure 4 At the position shown, where x is 4 Units in length, Gain2 is divided into Gain21 and Gain22 by SA. According to the formula, n=7, which means that under this structure, it is a seventh-order collision mode-locked state.

[0039] In one embodiment, the laser is based on a GaAs substrate or a silicon substrate and employs a III-V quantum dot active layer.

[0040] An optical frequency comb includes a high-order collision mode-locked laser with tunable repetition rate. The spacing between the comb teeth is determined by the repetition frequency of the laser, and the tooth distribution satisfies the following: when the laser is in an n-order collision mode-locked state, there are n-1 suppressed comb teeth in the spacing between adjacent comb teeth. The frequency spacing between the comb teeth is related to the length of the laser resonant cavity and the mode-locking order n.

[0041] In one embodiment, the spectral width of the optical frequency comb is determined by the gain bandwidth of the III-V quantum dot active layer in the laser, and the mode-locking order n is changed by adjusting the position of the SA region to achieve the tuning of the comb tooth spacing.

[0042] The above-mentioned high-order collisional mode-locked laser with tunable repetition rate and optical frequency comb has the following beneficial effects: (1) Tunable repetition frequency: Without replacing the laser module or re-fabrication, the mode-locking order n can be adjusted by changing the electrical connection position of the SA area, so as to achieve flexible tuning of the repetition frequency and reduce system complexity and cost.

[0043] (2) High stability: The tapered ridge waveguide structure is used to ensure single-mode output. Combined with the gain characteristics of the quantum dot active layer, the stability and peak power of the mode-locked pulse are improved.

[0044] (3) Easy to operate: Stable mode locking can be achieved by connecting gold wires and adjusting voltage / current, without the need for complex optical debugging, and is suitable for rapid iteration and diverse application scenarios.

[0045] (4) Optical frequency comb extension: The comb tooth spacing based on the laser can be tuned by the mode-locking order, and the spectral width is determined by the quantum dot gain bandwidth, providing a solution for low-cost, miniaturized optical frequency comb applications.

[0046] The above provides a detailed description of a high-order collisional mode-locked laser with tunable repetition rate and an optical frequency comb provided by the present invention. Specific examples have been used to illustrate the principles and implementation methods of the invention, and the descriptions of the embodiments are merely for the purpose of helping to understand the core ideas of the invention. It should be noted that those skilled in the art can make various improvements and modifications to the invention without departing from its principles, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A repetition rate tunable high order collision mode-locked laser, characterized in that, The application relates to a high-order collision mode-locked laser with adjustable repetition frequency. The laser resonant cavity comprises a saturable absorber (SA) region and a gain (Gain) region, and the ratio of the total length of the SA region to the total length of the Gain region is 1:A. The resonant cavity is divided into (1+A) large sections, each large section is divided into B small sections, and a total of (1+A)*B unit sections are obtained, adjacent unit sections are electrically isolated by etching, and each unit section can be combined to form the SA region or the Gain region through electrical connection; wherein A is an integer greater than 1, and B is a positive integer. The SA region is composed of continuous B unit sections, and the Gain region is composed of the remaining unit sections, and different orders of collision mode-locked states and repetition frequency tuning are realized by changing the position of the SA region in the resonant cavity.

2. The laser of claim 1, wherein, The length of each unit section is: ； Wherein, A is a proportional coefficient, the value range is 4-9, B is a number of segments, is the total length of the SA region, is the length of each unit segment, is the length of the laser resonant cavity.

3. The laser of claim 2, wherein, The total length of the SA region is: ； The total length of the Gain region is: 。 4. The laser of claim 3, wherein, The resonant cavity is provided with a high-reflective coating end face at one end and a low-reflective coating end face at the other end, and the distance between the low-reflective coating end face and the gain region Gain1 is The left and right ridge widths of the gain region Gain1 with the tapered ridge waveguide structure are w1 and w2 respectively, wherein, , The width of w2 is configured to ensure that the output light of the device is single mode.

5. The laser of claim 4, wherein, The distance between the center point of the SA region and the high-reflection-coated end face is x, and the relationship between x and the order n of the collision mode-locked state is: ； Wherein n is a positive integer, and the value of n is determined by the position adjustment of the SA region in the resonant cavity.

6. The laser of claim 5, wherein, All unit sections of the SA region are electrically connected and a reverse voltage is applied through gold wire connection, all unit sections of the Gain region are electrically connected and a forward current is applied, and a stable mode-locked operating point is obtained by adjusting the voltage and current level.

7. The laser of claim 6, wherein, When in the n-order collision mode-locked state, n-1 suppressed wavelengths appear in the interval between adjacent light-emitting wavelengths in the output spectrum, and the spectral characteristics are determined by the position of the SA region and the corresponding order n.

8. The laser of any one of claims 1 to 7, wherein, The laser is based on a GaAs substrate or a silicon substrate and adopts a III-V quantum dot active layer.

9. An optical frequency comb, characterized in that, The application relates to a high-order collision mode-locked laser with adjustable repetition frequency, and the comb tooth interval of an optical frequency comb is determined by the repetition frequency of the laser, and the comb tooth distribution satisfies that when the laser is in an n-order collision mode-locked state, n-1 suppressed comb teeth exist in the interval between adjacent comb teeth, and the frequency interval of the comb teeth is related to the length of the laser resonant cavity and the mode-locked order n.

10. The optical frequency comb of claim 9, wherein, The spectral width of the optical frequency comb is determined by the gain bandwidth of the III-V quantum dot active layer in the laser, and the mode-locked order n is adjusted by adjusting the position of the SA region to realize the tuning of the comb tooth interval.

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