External cavity laser

Abstract

This paper provides an improved external cavity laser (1) comprising a gain section (2) and an external cavity passive chip (3), the external cavity passive chip having a first end (31) optically coupled to the gain section (2) and a second end (35) providing laser output, the external cavity passive chip (3) having a cavity (33) between the first end (31) and a phase-shifted Bragg grating (FBG) (34).

Description

Technical Field

[0001] This invention relates to external cavity lasers. An external cavity laser includes a gain section that serves as an incoherent light source and an external cavity that receives incoherent light and outputs a coherent laser beam. Background Technology

[0002] In his conference paper "Hybrid external cavity laser with a 160-nm tuning range" presented at the 2020 Laser & Electrooptics Conference, Guo described a known external cavity laser. For example... Figure 6 As schematically shown, the disclosed device includes an InP reflective semiconductor optical amplifier (RSOA) 2 dockably coupled to a SiN chip 3. The SiN chip specifically includes a thermo-optical (TO) phase shifter A, a dual-loop Vernier filter B, and a tunable Sagnac loop reflector C. The Sagnac loop reflector C provides feedback to the laser cavity, and the dual-loop Vernier (frequency) filter B is used to select the laser output wavelength. Known external cavity lasers require a relatively large area.

[0003] There is a need for a more compact external cavity laser. Summary of the Invention

[0004] This invention provides an improved external cavity laser.

[0005] This improved external cavity laser comprises a gain section and an external cavity passive chip having a first end optically coupled to the gain section and a second end providing laser output. The external cavity passive chip has a cavity between the first end and a phase-shifted Bragg grating (FBG). This phase-shifted Bragg grating (FBG) has a discontinuity that results in a phase shift between the forward and reverse traveling waves. The unmodified Bragg grating is a notch-filter, while the FBG provides a narrow transmission peak within the blocked transmission band. In the improved external cavity laser, the phase-shifted Bragg grating (FBG) functions both as a reflector, providing optical feedback to the cavity, and as a filter, transmitting laser radiation with a narrow linewidth. This enables a compact structure.

[0006] In one embodiment, the discontinuity is a π-phase discontinuity. Thus, the FBG has a single transmission peak within the reflection band. However, other embodiments are also conceivable, where the FBG has a longer discontinuity. In this case, the FBG has multiple transmission peaks within the reflection band.

[0007] In one embodiment, the wavelength of the transmission peak is controllable. Wavelength control is simplified compared to the conventional structure shown in Figure 6. The wavelength at the center of the transmission peak can be adjusted by controlling the discontinuity formed in the FBG. Therefore, the improved device's operational control is simpler compared to conventional devices that require simultaneous adjustment of the Sagnac loop and the Vernier loop. To achieve optimal efficiency, a cavity length control element can be placed at the first end of the optical coupling between the cavity and the gain section to control the optical path length of the cavity. For this purpose, a heating element, such as a resistive heating element, can be provided. The heat generated near the discontinuity causes a change in its length. Thus, by appropriately heating the discontinuity, the wavelength that achieves a predetermined phase shift (e.g., a π phase shift) can be adjusted, thereby achieving a phase shift between the forward and reverse traveling waves. Similarly, the cavity length control element located near the first end can also be a (resistive) heating element, through which the cavity length can be controlled to an integer multiple of the desired wavelength. This method is applicable to almost all materials suitable for realizing passive optical chips, including those commonly used in the semiconductor industry, such as silicon and its compounds, such as silicon oxide, silicon nitride, silicon oxynitride, silicon carbide, and silicon phosphide, as well as other materials such as calcium fluoride, magnesium oxide, and magnesium fluoride. Furthermore, other methods can be employed, such as using electro-optically active materials. Examples of such materials include lithium niobate (LiNbO3), lithium tantalate (LiTaO3), potassium titanium phosphate (KTP), and barium β-borate (BBO). In this case, the discontinuity or the optical length of the cavity can be controlled by an electric field. In other examples, the optical length is controlled by mechanical stress. Additionally, a combined approach can be used, where the optical length of the discontinuity is first controlled by a first method, and then the optical length of the cavity is controlled by a second method different from the first method.

[0008] Surprisingly, the study found that more efficient operation can be achieved when the π-phase discontinuity is closer to the first end of the FBG facing the cavity, rather than closer to the second end facing the output laser. In this case, the sum of reflected and transmitted radiation power is higher than when the π-phase discontinuity is located at the center or closer to the second end.

[0009] In an exemplary embodiment, the π phase discontinuity is located at a distance D from the first end of the FBG, which is selected within the range of 15% to 45% of the length L of the FBG. For example, the distance D is selected within the range of 30% to 40% of the length L of the FBG.

[0010] A very compact external cavity laser structure can be realized using meandering waveguides. In one example, the external cavity passive chip has a photonic waveguide that includes, in sequence from a first end to a second end, a first linear segment, a first U-shaped bend segment, a second linear segment, a second U-shaped bend segment, and a third linear segment.

[0011] In one embodiment, the FBG is a sidewall-modulated FBG. Alternatively, the FBG can be designed as a series of longitudinal segments with alternating refractive indices. For example, one method is to change the refractive index by implanting ions into the structure. Alternatively, the refractive index can be changed by laser radiation. However, it is preferable to design the FBG as a sidewall-modulated FBG with longitudinal segments having alternating cross-sections. This allows for a smaller size. Attached Figure Description

[0012] These and other details will be described in more detail in the accompanying drawings. Among them: Figure 1 A first embodiment of the improved external cavity laser disclosed herein is schematically illustrated; Figure 2 Another embodiment of the improved external cavity laser is schematically illustrated; Figure 3 The spectral characteristics of various embodiments of the improved external cavity laser are schematically illustrated; Figure 4 Components of another embodiment of an improved external cavity laser are shown; Figure 4A It shows Figure 4 Details of the components shown; Figure 4B It shows Figure 4 Details of the variants of the shown components; Figure 5 Another embodiment of the improved external cavity laser is schematically illustrated; Figure 6 An external cavity laser according to the prior art is schematically shown. Detailed Implementation

[0013] Unless otherwise stated, the same reference numerals in the figures denote the same elements.

[0014] Figure 1 A first embodiment of the improved external cavity laser 1 as disclosed herein is schematically shown. Figure 1 As shown, the improved external cavity laser 1 includes a gain section 2 and an external cavity passive chip 3. A first end 31 of the external cavity passive chip is optically coupled to the gain section 2, while a second end 35 of the external cavity passive chip 3 provides laser output. The external cavity passive chip 3 includes a phase-shifted Bragg grating (FBG) 34 near its second end 35. A cavity 33 is defined between the first end 31 and the FBG 34. In the illustrated embodiment, the FBG 34 includes a series of longitudinal segments 344, 345 with alternating refractive indices. In the illustrated example, the refractive index n1 of the darker shaded segments 345 is higher than the refractive index n0 of the lighter shaded segments 344. These segments have thicknesses d1, d2, such that d1 n1 = d2 n2 = λ / 2, where d1 and d2 are the thicknesses of the low-refractive-index and high-refractive-index segments, respectively, and λ is the center wavelength of the reflection band of FBG 34. FBG 34 has a discontinuity 342, such as a π-phase discontinuity. In the example shown, the discontinuity 342 is located closer to the first end 341 of FBG 34 facing the cavity 33 than the second end 343 of FBG 34 facing the laser output. For example, the cavity 33 is made of silicon nitride, and the materials of the low-refractive-index segment 344 and the high-refractive-index segment 345 are silicon dioxide and silicon nitride, respectively. The discontinuity 342 in FBG 34 provides a transmission peak within the reflection band of the FBG, as discussed in more detail below. Gain segment 2 is, for example, a reflective semiconductor optical amplifier. Suitable semiconductor materials for this purpose, depending on the desired wavelength range, include gallium arsenide (GaAs), indium phosphide (InP), and gallium nitride (GaN).

[0015] Figure 2 Another embodiment of the improved external cavity laser 1 is schematically shown. (As shown) Figure 1 As shown in the embodiment, the improved external cavity laser 1 includes a gain section 2 and an external cavity passive chip 3. A first end 31 of the external cavity passive chip 3 is optically coupled to the gain section 2, while a second end 35 of the external cavity passive chip 3 provides laser output. In addition, the external cavity passive chip 3 includes a phase-shifted Bragg grating (FBG) 34 near its second end. Figure 2 The right side shows FBG 34 in this embodiment in more detail. FBG 34 has a discontinuity 342, which in this case is a π-phase discontinuity. Figure 4 As shown more clearly, FBG 34 in this embodiment is a sidewall modulated FBG. That is, the FBG has longitudinal segments with alternating cross-sections. Figure 4A In the example shown, the cross-sectional diameter alternates between w1 = 600 nm and w2 = 1000 nm, and the period P of the FBG is 425 nm. Discontinuity 342, in this case a π-phase discontinuity, is provided as an elongated segment with a length equal to the period P and a diameter of 1000 nm. Alternatively, the discontinuity can have a smaller diameter of 600 nm. Figure 2 The sidewall modulation FBG 34 shown in the embodiment enables the achievement of higher performance than... Figure 1 The refractive index modulated FBG shown has a smaller footprint. Figure 4BThe discontinuity can be optionally configured as a long, thin segment with a small diameter w1. Discontinuities exceeding the Bragg period P are also possible. Therefore, the transmission peak is located far from the center of the reflection valley. Thus, a laser wavelength different from the midwavelength of the reflector can be selected. However, by further increasing the segment length, the free spectral range decreases, resulting in more and more transmission peaks within the reflection valley, rather than a single transmission peak. Preferably, the discontinuity is designed such that the FBG has a single transmission peak within the reflection valley, thus eliminating the need for additional filtering to remove unwanted transmission peaks.

[0016] exist Figure 2 In the illustrated embodiment, the external cavity laser is provided in a compact arrangement with a meandering photonic waveguide 33. More specifically, in this example, the photonic waveguide 33 sequentially includes a first linear segment 301, a first U-shaped bend segment 302, a second linear segment 303, a second U-shaped bend segment 304, and a third linear segment 305 in the direction from the first end 31 to the second end 35. In other examples, depending on the application requirements, the photonic waveguide 33 is used as a single linear element or consists of more linear segments coupled by the bend segment. In one embodiment, the external cavity passive chip 3 has a size of 400 μm x 2 μm.

[0017] exist Figure 2 In the example shown, the π phase discontinuity 342 is set at the center between the two ends 341 and 343 of FBG 34.

[0018] In a more general case, the optical properties of FBG can be modeled by the following equation: (1) in It is the incident light on end 341 of FBG 34. It is the reflected light at end 341. It is the transmitted light at end 343 of FBG 34.

[0019] On the right side of the equation, the first ABCD matrix describes the first part of the Bragg reflector between the first end 341 and the discontinuity 342, the middle matrix describes the discontinuity 342, and the last ABCD matrix describes the second part of the Bragg reflector between the discontinuity 342 and the second end 343. The exponent of ) is the phase shift imposed by discontinuity 342. A, B, C, and D are transmission matrix elements that transform the incident electromagnetic field from the input side to the output side, and are described by the following equation, assuming TE polarization:

[0020]

[0021]

[0022]

[0023] Where k, k1, and k2 are wave vectors determined by the characteristics of the FBG segment. In one example, k1 is the wave vector within a segment with a diameter of w2 (see...). Figure 4A k2 is the wave vector within the segment with diameter w1, and k is the wave vector at the discontinuity. k can be equal to k1 or k2, depending on whether the discontinuity is wide or narrow.

[0024] Furthermore, the value of k in the exponent (±ik.P / 2) can be adjusted to tune the resonant wavelength, thereby determining the output wavelength of the laser.

[0025] Figure 3 Exemplary spectral characteristics of an embodiment of an FBG are schematically illustrated. For an example where the first and second portions of the FBG are of equal length, the transmission characteristic is represented by T50. The efficiency characteristic is represented by TR50, which is the sum of transmission and reflection. Figure 3 As can be seen, FBG 34 exhibits relatively high reflectivity in the 1540-1550 nm and 1560-1570 nm ranges. Between these ranges, FBG 34 has a relatively narrow transmission peak centered at 1555 nm. In this embodiment, the transmission T50 in the transmission peak is approximately 80%. The sum of transmission and reflection in the transmission peak, TR50, is also approximately 80%. Therefore, approximately 20% of the incident radiation at the transmission peak wavelength is lost.

[0026] In other embodiments, such as Figure 1 and Figure 4 As shown, discontinuity 342 is closer to the first end 341 of FBG 34 than the second end 343 of FBG 34 facing the laser output, that is, the end of FBG 34 facing cavity 33. Figure 4 The location of the discontinuity 342 at a distance D from the first end 341 in an FBG of length L is shown.

[0027] like Figure 3 As shown, reducing the ratio D / L to below 0.5 lowers the height of the transmission peak but increases the efficiency of FBG 34, i.e., reduces the amount of absorbed radiation. For example, T40 represents the transmission of the FBG, where the π-phase discontinuity 342 is located at a distance D from the first end 341 of FBG 34, equal to 40% of the FBG length L. It can be seen that the transmittance is still relatively high, approximately 55%, but the loss is reduced. That is, approximately 92% of the radiation at wavelength 1555 is reflected or transmitted. Therefore, only 8% is lost.

[0028] In another embodiment, the π phase discontinuity 342 is located at a distance D from the first end 341 of the FBG 34, equal to 35% of the FBG length L. It can be seen that the transmission T30 is now reduced to approximately 35%, but the loss is further reduced. That is, approximately 95% of the radiation at wavelength 1555 is either reflected or transmitted. Therefore, only a few percent is lost.

[0029] It can also be shown that when the discontinuity is located closer to the cavity, the decrease in output efficiency is less than the decrease in transmittance, because most of the radiation that is not directly output is output after one or more reflections within the cavity.

[0030] Furthermore, by increasing the reflectivity r of the FBG, the coherence length of the output radiation was also improved.

[0031] The table below shows some other exemplary embodiments (EMB1-6). N1 and N2 are the number of Bragg cycles of the FBG 34 on the cavity side and away from the cavity, respectively. N is the sum of N1 and N2. L is the length of the FBG 34, where the tooth pitch P of the FBG is assumed to be 425 nm. The last column indicates the D / L ratio. For example, these designs are fabricated in 800 nm thick silicon nitride with a design width of 1 micrometer and an etched tooth depth of 200 nm on each side (in the direction perpendicular to light propagation).

[0032]

[0033] like Figure 5 As shown, the wavelength of the transmission peak can be tuned more easily compared to the conventional arrangement. This conventional arrangement requires coordinated control of the Sagnac loop, a pair of Vernier loops, and the cavity length tuner to control the wavelength of the transmission peak.

[0034] In such Figure 5In the embodiment of the improved external cavity laser shown, the peak transmission wavelength can be more easily controlled by a pair of control signals c342, c32, one signal for controlling the FBG discontinuity 342 and the other signal for controlling the length tuner 32. For this purpose, the external cavity laser 1 includes a controller 4 that provides control signal c342 for controlling the resistance heating element 342c near the discontinuity 342. Controller 4 also provides control signal c32 for controlling the resistance heating element 32c of the cavity length control element 32. In one embodiment, a lookup table with predetermined settings is prepared for the resistance heating element 342c to obtain the desired peak transmission wavelength. These settings are, for example, settings for an appropriate temperature of the resistance heating element to be achieved, or settings for the electrical power to achieve an appropriate temperature. The lookup table can then be used to control the temperature of the resistance heating element to achieve the desired transmission peak wavelength. Alternatively, the relationship between electrical power and transmission peak wavelength, or the relationship between resistance heating element temperature and transmission peak wavelength, can be modeled as a polynomial function. Therefore, the transmission peak wavelength can be controlled in a feedforward manner using a lookup table or a polynomial function. In another embodiment, a feedback control loop is used to control the transmission peak wavelength by measuring the actual transmission peak wavelength, determining the difference from the desired transmission peak wavelength, and tuning the power supplied to the resistive heating element to minimize that difference. In another embodiment, these methods are combined because the feedforward controller provides a control signal indicating the power expected to be suitable for achieving the desired transmission peak wavelength, and the feedback controller provides a tuning signal to minimize the remaining difference between the measured wavelength and the desired wavelength. Similarly, the resistive heating element 32c can be controlled according to one of these methods to tune the length of the laser cavity to match the desired wavelength.

[0035] As mentioned above, other mechanisms, such as mechanical strain and electric field, can also control the transmission peak wavelength and cavity length. Similarly, feedforward, feedback, or combinations thereof can be used to achieve the desired transmission peak wavelength and cavity length using these mechanisms.

[0036] Therefore, the wavelength of the transmission peak of the FBG, i.e. the wavelength of the laser output, can be maintained at a predetermined value to prevent it from drifting due to the external environment, or it can be changed to meet the requirements of the application.

[0037] In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite articles "a" or "an" do not exclude multiple. A single component or other unit can perform the function of several items described in the claims. The fact that certain measures are referenced in mutually different claims does not mean that a combination of these measures cannot be advantageous. No reference numerals in the claims should be construed as limiting the scope.

Claims

1. An external cavity laser (1) comprising a gain section (2) and an external cavity passive chip (3), the external cavity passive chip having a first end (31) optically coupled to the gain section (2) and a second end (35) providing laser output, the external cavity passive chip (3) having a cavity (33) between the first end (31) and a phase-shifted sidewall modulation Bragg grating (FBG) (34), the cross-sectional dimension of the FBG (34) alternating between a smaller and a larger value with a period P, and wherein, The discontinuity is manifested as an elongated segment with a length equal to or greater than the period P, a cross-sectional dimension equal to or greater than the smaller value, and wherein the discontinuity (342) in the FBG is closer to the first end (341) of the FBG (34) facing the cavity (33) than to the second end (343) of the FBG (34) facing the laser output.

2. The external cavity laser (1) according to claim 1, wherein, The FBG (34) is provided with a π phase discontinuity (342).

3. The external cavity laser (1) according to claim 1 or 2, wherein, The larger dimension is equal to the cross-sectional dimension of the cavity.

4. The external cavity laser (1) according to claim 3, wherein, The discontinuity (342) is located at a distance (D) from the first end (341) of the FBG (34), the distance being selected in the range of 15% to 45% of the length (L) of the FBG.

5. The external cavity laser (1) according to any one of the preceding claims, comprising a meandering photonic waveguide (33).

6. The external cavity laser (1) according to claim 5, wherein, The photonic waveguide (33) includes, in sequence, a first linear section (301), a first U-shaped bend section (302), a second linear section (303), a second U-shaped bend section (304), and a third linear section (305) in the direction from the first end (31) to the second end (35).

7. The external cavity laser (1) according to any one of the preceding claims includes a wavelength tuning element (342c) for tuning the transmission peak wavelength of the FBG (34) and a controller (4) for controlling the tuning element.

8. The external cavity laser (1) according to claim 7, wherein, The wavelength tuning element (342c) is configured to thermally control the wavelength of the transmission peak.

9. The external cavity laser (1) according to claim 7 or 8, wherein, The external cavity passive chip (3) further includes a length tuning element (32c) at its first end (31), and wherein the controller (4) is configured to jointly control the wavelength tuning element (342c) and the length tuning element (32c) to maintain the ratio of the cavity length to the wavelength of the transmission peak as an integer value.

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