A device and method for local isolation in a composite cavity laser.

Abstract

This invention belongs to the field of optoelectronic device technology and discloses a device and method for local isolation in a composite cavity laser. The isolation device provided by this invention includes a first optical isolator disposed in the optical path between the gain chip and the interference reflection element, and a second optical isolator disposed between the interference reflection element and the resonant cavity mirror. Both the first and second optical isolators include an analyzer and a Faraday crystal, and the polarization directions of the analyzers of the two optical isolators are orthogonal. Along the laser emission direction, the laser sequentially passes through the analyzer and Faraday crystal of the first optical isolator, and sequentially passes through the Faraday crystal and analyzer of the second optical isolator. This invention utilizes the first and second optical isolators to allow reflected light after passing through the resonant cavity mirror to pass through, while isolating reflected light from the interference reflection element. This invention can achieve local isolation when the incident angle of the interference reflection element is close to 0°, while ensuring the linewidth characteristics of the laser.

Description

Technical Field

[0001] This invention belongs to the field of optoelectronic device technology, and more specifically, relates to a device and method for local isolation in a composite cavity laser. Background Technology

[0002] In composite cavity lasers (e.g., external cavity single-frequency lasers) that include interfering reflection elements (e.g., FP etalons and filters), to achieve lower peak loss, the collimated light source must be incident directly onto the FP etalon. However, since the FP etalon is coated with high-reflectivity films on both sides, the reflected light during direct incidence can affect the active cavity, i.e., the gain chip. To prevent the reflected light from the interfering reflection elements (e.g., FP etalons) from returning to the gain chip and affecting it, existing research has proposed some isolation schemes, but these have some problems.

[0003] For example, such as Figure 1 As shown, if a conventional isolator (a conventional single-stage isolator consists of two analyzers, a Faraday crystal, and a magnetic ring; since the difference between a single-stage isolator and a two-stage or even three-stage isolator is only in the degree of isolation, a single-stage isolator will be used as an example) is placed in the optical path between the gain chip and the interference reflection element, although the light emitted by the gain chip can pass through the isolator with low loss and the reflected light from the interference reflection element is isolated, the reflected light from the resonant cavity mirror will also be isolated. Since external cavity lasers need to use the resonant cavity mirror to return the light to the cavity of the gain chip to form a stable oscillation, this type of scheme will result in the inability to form a resonant cavity. To solve the problem of not being able to form a resonant, in this type of external cavity laser, the interference reflection element (such as the FP etalon) can usually only be placed in the optical path at a non-0° incident angle. However, the nature of the etalon determines that the larger the incident angle, the greater the peak loss. In an external cavity single-frequency laser, the greater the cavity loss, the wider the linewidth, resulting in poor linewidth performance of the laser.

[0004] For example, see Figure 2If a quarter-wave plate and a polarization beam splitter cube are used to form an isolation device and placed between the gain chip and the interference reflection element (e.g., FP etalon), the laser beam is incident perpendicularly into the polarization beam splitter cube, and the outgoing light is vertically polarized linear light (P-light). The P-light is incident into the quarter-wave plate, and the outgoing light is converted into left-hand circularly polarized light (or right-hand circularly polarized light). The right-hand circularly polarized light (left-hand circularly polarized light) reflected back enters the quarter-wave plate again and is converted into horizontally polarized linear light (S-light). The S-light is emitted in another direction after passing through the polarization beam splitter cube. Although the above isolation device can also prevent the reflected light from the interference reflection element from returning to the gain chip, it has the following main disadvantages: (1) Limited isolation: The theoretical maximum isolation of the isolator using a polarizer plus a quarter-wave plate is about 30dB, but in practice, due to factors such as the quality of the device, the polarization purity of the light, and the accuracy of the waveplate, the isolation can usually only reach about 20dB. In addition, the isolation scheme using a quarter-wave plate also has the following problems: (2) The polarization state is changed: the light after passing through the quarter-wave plate will become circularly polarized light, which is not suitable for laser systems that require polarized light output; (3) High requirements for polarization state: the input light must be linearly polarized light, and the polarization direction needs to be precisely at a 45° angle with the fast axis of the quarter-wave plate. If there is any change in the polarization state of the input light (e.g., imperfect polarization degree or polarization direction drift), it will significantly affect the isolation effect; (4) Wavelength sensitivity: the phase delay of the quarter-wave plate is strictly dependent on the wavelength of the light. A slight wavelength shift (e.g., wavelength drift due to temperature changes or unstable light source) will cause the phase delay to deviate from 90°, resulting in incomplete circular polarization conversion, thereby reducing the isolation performance; (5) Poor environmental stability: the quarter-wave plate is sensitive to environmental factors such as temperature and mechanical stress. These factors will change the optical thickness and phase delay characteristics of the plate, thereby affecting the isolation performance; (6) Narrow bandwidth: due to the material dispersion of the quarter-wave plate, the isolator can only work effectively in a narrow wavelength range, which is a limitation for optical systems that require broadband operation.

[0005] In summary, achieving local isolation (isolating the reflected light from the interfering reflector without isolating the reflected light from the resonant cavity mirrors while maintaining the laser's linewidth characteristics) when the incident angle of the interfering reflector is close to 0°, is a key technical problem that needs to be addressed in this field. Furthermore, further improving optical isolation and widening the operating bandwidth are also research topics in this area. Summary of the Invention

[0006] This invention provides a device and method for local isolation in a composite cavity laser, solving the problem in the prior art that it is difficult to achieve local isolation when the incident angle of the interfering reflection element is close to 0°, while ensuring the linewidth characteristics of the laser.

[0007] This invention provides a device for local isolation in a composite cavity laser, comprising: a first optical isolator disposed in the optical path between a gain chip and an interference reflection element, and a second optical isolator disposed between the interference reflection element and a resonant cavity mirror; both the first and second optical isolators include a polarizer and a Faraday crystal, and the polarization directions of the polarizer of the first optical isolator and the polarizer of the second optical isolator are orthogonal; along the laser emission direction, the laser first passes through the polarizer of the first optical isolator and then through the Faraday crystal of the first optical isolator, and the laser first passes through the Faraday crystal of the second optical isolator and then through the polarizer of the second optical isolator.

[0008] Preferably, the composite cavity laser includes the gain chip, collimating lens, interference reflection element and resonant cavity reflector arranged coaxially along the laser emission direction.

[0009] Preferably, both the first optical isolator and the second optical isolator are two-piece optical isolators.

[0010] Preferably, the two-piece optical isolator consists of an analyzer, a Faraday crystal, and a magnetic ring, with the analyzer and the Faraday crystal both disposed within the magnetic ring.

[0011] Preferably, the Faraday crystal is used to rotate the polarization direction of linearly polarized light by 45°.

[0012] Preferably, the interference reflection element includes an FP etalon and a filter arranged coaxially along the laser emission direction, and both ends of the FP etalon are coated with a high-reflectivity film; the resonant cavity reflector is a beam splitter.

[0013] Preferably, the composite cavity laser is an external cavity single-frequency laser.

[0014] On the other hand, the present invention provides a method for local isolation in a composite cavity laser, which is implemented by the above-mentioned device for local isolation in a composite cavity laser. The method for local isolation in a composite cavity laser includes: using a first optical isolator and a second optical isolator to allow reflected light after passing through the resonant cavity mirror to pass through, and isolating reflected light that interferes with the reflecting element.

[0015] Preferably, the linearly polarized light emitted by the gain chip is collimated by a collimating lens with a polarization direction of 0° and then passes through the analyzer of the first optical isolator. The polarization direction of the analyzer of the first optical isolator is 0°. After passing through the Faraday crystal of the first optical isolator, the polarization direction of the linearly polarized light is rotated by 45°. The reflected light from the interference reflection element passes through the Faraday crystal of the first optical isolator again, and the polarization direction of the linearly polarized light is rotated by 45° in the same direction. At this time, the polarization direction of the linearly polarized light is orthogonal to the polarization direction of the analyzer of the first optical isolator, thereby isolating the reflected light from the interference reflection element.

[0016] Preferably, the linearly polarized light emitted from the gain chip is collimated at 0° by a collimating lens and then passes through the analyzer of the first optical isolator. The polarization direction of the analyzer of the first optical isolator is 0°. After passing through the Faraday crystal of the first optical isolator, the polarization direction of the linearly polarized light rotates by 45°. After passing through the interference reflection element, the linearly polarized light is incident on the second optical isolator. After passing through the Faraday crystal of the second optical isolator, the polarization direction of the linearly polarized light rotates again by 45° in the same direction. The polarization direction of the analyzer of the second optical isolator is 90°. At this time, the polarization direction of the linearly polarized light coincides with the polarization direction of the analyzer of the second optical isolator. After passing through the analyzer of the second optical isolator, the linearly polarized light is incident on the resonant cavity mirror. The reflected light after passing through the resonant cavity mirror returns along the original optical path. After passing through the Faraday crystals of the second and first optical isolators, the polarization direction of the linearly polarized light rotates again by 90° in the same direction. At this time, the polarization direction of the linearly polarized light coincides with the polarization direction of the analyzer of the first optical isolator. The linearly polarized light returns to the gain chip.

[0017] One or more technical solutions provided in this invention have at least the following technical effects or advantages:

[0018] (1) The device for local isolation in a composite cavity laser provided by the present invention includes a first optical isolator disposed in the optical path between the gain chip and the interference reflection element, and a second optical isolator disposed between the interference reflection element and the resonant cavity mirror; wherein, both the first optical isolator and the second optical isolator include a polarizer and a Faraday crystal, the polarizer of the first optical isolator and the polarizer of the second optical isolator are orthogonal in polarization direction; the first optical isolator and the second optical isolator are opposite in direction, that is, along the laser emission direction, the laser first passes through the polarizer of the first optical isolator and then through the Faraday crystal of the first optical isolator; along the laser emission direction, the laser first passes through the Faraday crystal of the second optical isolator and then through the polarizer of the second optical isolator. This invention utilizes a first optical isolator and a second optical isolator to allow reflected light after passing through the resonant cavity mirror to pass through, while isolating reflected light from interfering reflective elements. This invention can achieve local isolation (preventing reflected light from interfering reflective elements from returning to the gain chip cavity, while simultaneously not affecting the return of reflected light from the resonant cavity mirror to the gain chip cavity) when the incident angle of the interfering reflective element is close to 0°. This solves the problem of interference of reflected light from interfering reflective elements to the active cavity, and can reduce the reflected light by about 30dB, that is, the isolation can reach 30dB, further reducing the noise in the resonant cavity and ensuring the linewidth characteristics of the laser. For example, it can enable an external cavity single-frequency laser with a linewidth performance of 3kHz to reach the 1kHz level.

[0019] (2) The optical isolator in this invention is based on a Faraday crystal. Since the isolation of an optical isolator using a Faraday rotator can theoretically reach 40-60dB, this invention can improve the optical isolation compared with an isolation device using a polarizer and a quarter-wave plate.

[0020] (3) The optical isolator in this invention is based on a Faraday crystal. Therefore, this invention can overcome the problem of narrow bandwidth of isolators containing quarter-wave plates. This invention is more suitable for some optical systems that require broadband operation.

[0021] (4) The optical isolator in this invention is based on a Faraday crystal. Since the working principle of the Faraday rotator is to rotate the polarization plane of light without changing the polarization state, this invention is a better choice than isolation devices containing quarter-wave plates for laser systems that need to maintain a specific polarization state.

[0022] (5) The optical isolator in this invention is based on a Faraday crystal. Therefore, this invention can overcome the problems of high polarization state requirements, wavelength sensitivity and poor environmental stability of isolation devices containing quarter-wave plates. This invention can ensure good isolation effect in practical applications. Attached Figure Description

[0023] Figure 1This is a schematic diagram of the structure of an existing conventional isolator;

[0024] Figure 2 A schematic diagram of an existing isolation device containing a quarter-wave plate;

[0025] Figure 3 This is a top view of a device for local isolation in a composite cavity laser provided in Embodiment 1 of the present invention;

[0026] Figure 4 This is a device for local isolation in a composite cavity laser provided in Embodiment 1 of the present invention, and a side view of the composite cavity laser;

[0027] Figure 5 This is a schematic diagram of the optical isolator in a device for local isolation in a composite cavity laser provided in Embodiment 1 of the present invention;

[0028] Figure 6 The diagram shows the device for local isolation in the composite cavity laser provided in Embodiment 1 of the present invention, which is used for the forward light, the reflected light from the FP etalon, and the reflected light after the beam splitter.

[0029] Among them, 1-gain chip, 2-collimating lens, 3-first optical isolator, 4-FP etalon, 5-filter, 6-second optical isolator, 7-beam splitter, 8-Faraday crystal, 9-magnetic ring, 10-analyzer. Detailed Implementation

[0030] To better understand the above technical solutions, the following will provide a detailed explanation of the technical solutions in conjunction with the accompanying drawings and specific implementation methods.

[0031] The device for local isolation in a composite cavity laser provided by the present invention includes: a first optical isolator disposed in the optical path between the gain chip and the interference reflection element, and a second optical isolator disposed between the interference reflection element and the resonant cavity mirror; both the first optical isolator and the second optical isolator include a polarizer and a Faraday crystal, and the polarization directions of the polarizer of the first optical isolator and the polarizer of the second optical isolator are orthogonal; along the laser emission direction, the laser first passes through the polarizer of the first optical isolator and then through the Faraday crystal of the first optical isolator, and the laser first passes through the Faraday crystal of the second optical isolator and then through the polarizer of the second optical isolator.

[0032] The interference reflection element in this invention can be a combination of an FP etalon and a filter, or a combination of other devices or multiple devices. Since this invention focuses on the problem of how to suppress reflection in this area, it is named an interference reflection element. We do not specifically limit the other functions of this element.

[0033] The following example illustrates the interference reflection element, which includes an FP etalon and a filter arranged coaxially along the laser emission direction, and a beam splitter as the resonant cavity reflector.

[0034] Example 1:

[0035] Example 1 provides a device for local isolation in a composite cavity laser, see [link to example]. Figures 3 to 6 It includes: a first optical isolator 3 disposed in the optical path between the gain chip 1 and the FP standard etalon 4, and a second optical isolator 6 disposed between the filter 5 and the beam splitter 7; both the first optical isolator 3 and the second optical isolator 6 include an analyzer 10 and a Faraday crystal 8, and the polarization directions of the analyzer 10 of the first optical isolator 3 and the analyzer 10 of the second optical isolator 6 are orthogonal; along the laser emission direction, the laser first passes through the analyzer 10 of the first optical isolator 3 and then through the Faraday crystal 8 of the first optical isolator 3; along the laser emission direction, the laser first passes through the Faraday crystal 8 of the second optical isolator 6 and then through the analyzer 10 of the second optical isolator 6.

[0036] The composite cavity laser includes a gain chip 1, a collimating lens 2, an FP etalon 4, a filter 5, and a beam splitter 7 arranged coaxially along the laser emission direction.

[0037] See Figure 3 and Figure 4 The composite cavity laser and isolation device generally includes a gain chip 1, a collimating lens 2, a first optical isolator 3 (which consists of an analyzer and a Faraday crystal, and can be referred to as a forward optical isolator), an FP etalon 4, a filter 5, a second optical isolator 6 (which consists of a Faraday crystal and an analyzer, and can be referred to as a reverse optical isolator) and a beam splitter 7, arranged coaxially along the laser emission direction.

[0038] The FP standard etalon 4 has high-reflectivity films coated on both ends.

[0039] The composite cavity laser can be an external cavity single-frequency laser.

[0040] See Figure 5 Specifically, the first optical isolator 3 and the second optical isolator 6 are both two-piece optical isolators. The two-piece optical isolator is composed of an analyzer 10, a Faraday crystal 8 and a magnetic ring 9. The analyzer 10 and the Faraday crystal 8 are both disposed in the magnetic ring 9.

[0041] Specifically, the Faraday crystal 8 is used to rotate the polarization direction of linearly polarized light by 45°.

[0042] See Figure 6The light emitted by the gain chip 1, with a polarization direction initially set to 0°, is collimated by the collimating lens 2 and then passes through the analyzer 10 in the first optical isolator 3 with a loss of less than 0.2dB (the polarization direction of this analyzer is 0°). After passing through the Faraday crystal 8 of the first optical isolator 3, the polarization direction is rotated by 45°. At this time, the reflected light from the FP etalon 4 passes through the Faraday crystal 8 of the first optical isolator 3 again, and the polarization direction of the linearly polarized light is rotated by 45° in the same direction. At this time, the polarization direction of the linearly polarized light is orthogonal to the polarization direction of the analyzer 10 in the first optical isolator 3, which plays an isolation role, and the isolation degree can reach 30dB. The transmitted light, after passing through the Faraday crystal 8 of the first optical isolator 3, has a polarization direction of 45°. It then passes sequentially through the FP etalon 4 and the filter 5 before entering the second optical isolator 6. Here, the light first passes through the Faraday crystal 8 of the second optical isolator 6, rotating its polarization direction by 45°. The polarization direction of the analyzer 10 of the second optical isolator 6 is set to 90°. At this point, the polarization direction of the linearly polarized light coincides with the polarization direction of the analyzer 10 of the second optical isolator 6, allowing it to pass through the analyzer with a loss of less than 0.2 dB. The light is then reflected by the reflector 7 and returns to the second optical isolator 6 along the same path. Here, the polarization direction remains 90°, coinciding with the polarization direction of the analyzer 10 of the second optical isolator 6, allowing it to pass through with a loss of less than 0.2 dB. During the return journey, after the linearly polarized light passes through the Faraday crystal 8 of the second optical isolator 6 and the Faraday crystal 8 of the first optical isolator 3, its polarization direction is rotated by 90° in the same direction again. This causes the polarization direction of the linearly polarized light to coincide with the polarization direction of the analyzer 10 of the first optical isolator 3, allowing it to pass through with a loss of less than 0.2 dB. Therefore, the linearly polarized light can return to the gain chip 1. Overall, this isolates the reflection from the FP etalon 4 without isolating the reflection from the beam splitter 7.

[0043] In summary, the local isolation device in the composite cavity laser provided in Example 1 can prevent the reflected light from the FP etalon from returning to the gain chip cavity when the incident angle of the FP etalon is close to 0°, while not affecting the reflected light from the beam splitter returning to the gain chip cavity. It can achieve local isolation and ensure the linewidth characteristics of the laser.

[0044] Example 2:

[0045] Example 2 provides a method for local isolation in a composite cavity laser, implemented using the device for local isolation in a composite cavity laser as described in Example 1. See [link to example]. Figures 3 to 6The method of local isolation in the composite cavity laser includes: using a first optical isolator 3 and a second optical isolator 6 to allow the reflected light after passing through the beam splitter 7 to pass through, and to isolate the reflected light of the FP etalon 4.

[0046] Specifically, the linearly polarized light emitted by the gain chip 1 is collimated by the collimating lens 2 with a polarization direction of 0° and then passes through the analyzer 10 of the first optical isolator 3. The polarization direction of the analyzer 10 of the first optical isolator 3 is 0°. After passing through the Faraday crystal 8 of the first optical isolator 3, the polarization direction of the linearly polarized light is rotated by 45°. The reflected light from the FP etalon 4 passes through the Faraday crystal 8 of the first optical isolator 3 again, and the polarization direction of the linearly polarized light is rotated by 45° in the same direction. At this time, the polarization direction of the linearly polarized light is orthogonal to the polarization direction of the analyzer 10 of the first optical isolator 3, thereby isolating the reflected light from the FP etalon 4.

[0047] Linearly polarized light emitted from gain chip 1 is collimated at 0° by collimating lens 2 and then passes through analyzer 10 of the first optical isolator 3. The polarization direction of analyzer 10 of the first optical isolator 3 is 0°. After passing through Faraday crystal 8 of the first optical isolator 3, the polarization direction of the linearly polarized light rotates by 45°. After passing through FP etalon 4 and filter 5, the linearly polarized light is incident on the second optical isolator 6. After passing through Faraday crystal 8 of the second optical isolator 6, the polarization direction of the linearly polarized light rotates again by 45° in the same direction. The polarization direction of analyzer 10 of the second optical isolator 6 is 90°. At this time, the polarization direction of the linearly polarized light is the same as that of the first optical isolator 3. The polarization direction of the analyzer 10 of the second optical isolator 6 coincides with that of the linearly polarized light. After passing through the analyzer 10 of the second optical isolator 6, the linearly polarized light is incident on the beam splitter 7. The light reflected by the beam splitter 7 returns along the original optical path and passes through the analyzer 10 of the second optical isolator 6. After passing through the Faraday crystal 8 of the second optical isolator 6, the polarization direction of the linearly polarized light is rotated by 45° in the same direction. After passing through the Faraday crystal 8 of the first optical isolator 3, the polarization direction of the linearly polarized light continues to rotate by 45° in the same direction. At this time, the polarization direction of the linearly polarized light coincides with the polarization direction of the analyzer 10 of the first optical isolator 3, and the linearly polarized light returns to the gain chip 1.

[0048] For example, by using the aforementioned method of local isolation in a composite cavity laser, the interference of reflected light from the FP etalon to the active cavity in an external cavity single-frequency laser can be resolved, reducing the reflected light by 30dB and further reducing the noise in the resonant cavity, enabling the external cavity single-frequency laser, which originally had a linewidth performance of 3kHz, to reach the 1kHz level.

[0049] In summary, this invention utilizes two optical isolators with opposite directions (i.e., one optical isolator passes through the analyzer first and then the Faraday crystal, while the other optical isolator passes through the Faraday crystal first and then the analyzer, and the polarization directions of the two analyzers are orthogonal) to achieve local isolation. This invention provides a new method and approach for the design of optical resonant cavities by locally isolating a portion of the composite cavity without affecting the optical path outside that portion.

[0050] Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to examples, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A device for local isolation in a composite cavity laser, characterized in that, include: A first optical isolator is disposed in the optical path between the gain chip and the interference reflection element, and a second optical isolator is disposed between the interference reflection element and the resonant cavity mirror; Both the first optical isolator and the second optical isolator include an analyzer and a Faraday crystal. The polarization directions of the analyzer in the first optical isolator and the analyzer in the second optical isolator are orthogonal. The Faraday crystals in both the first and second optical isolators are used to rotate the polarization direction of linearly polarized light by 45°, and the rotation directions of the Faraday crystals in the first and second optical isolators are the same. Along the laser emission direction, the laser first passes through the analyzer of the first optical isolator and then through the Faraday crystal of the first optical isolator, and the laser first passes through the Faraday crystal of the second optical isolator and then through the analyzer of the second optical isolator.

2. The device for local isolation in a composite cavity laser according to claim 1, characterized in that, The composite cavity laser includes the gain chip, collimating lens, interference reflection element and resonant cavity reflector arranged coaxially along the laser emission direction.

3. The device for local isolation in a composite cavity laser according to claim 1, characterized in that, Both the first optical isolator and the second optical isolator are two-piece optical isolators.

4. The device for partial isolation in a composite cavity laser according to claim 3, characterized in that, The two-piece optical isolator consists of an analyzer, a Faraday crystal, and a magnetic ring, with the analyzer and the Faraday crystal both located within the magnetic ring.

5. The device for local isolation in a composite cavity laser according to claim 1, characterized in that, The interference reflection element includes an FP etalon and a filter arranged coaxially along the laser emission direction, and both ends of the FP etalon are coated with a high-reflectivity film; the resonant cavity reflector is a beam splitter.

6. The device for partial isolation in a composite cavity laser according to claim 1, characterized in that, The composite cavity laser is an external cavity single-frequency laser.

7. A method for local isolation in a composite cavity laser, characterized in that, The method of local isolation in a composite cavity laser as described in any one of claims 1 to 6 includes: using a first optical isolator and a second optical isolator to allow reflected light after passing through the resonant cavity mirror to pass through, and to isolate reflected light that interferes with the reflecting element.

8. The method for local isolation in a composite cavity laser according to claim 7, characterized in that, The linearly polarized light emitted by the gain chip is collimated by a collimating lens with a polarization direction of 0° and then passes through the analyzer of the first optical isolator. The polarization direction of the analyzer of the first optical isolator is 0°. After passing through the Faraday crystal of the first optical isolator, the polarization direction of the linearly polarized light is rotated by 45°. The reflected light from the interference reflection element passes through the Faraday crystal of the first optical isolator again, and the polarization direction of the linearly polarized light is rotated by 45° in the same direction. At this time, the polarization direction of the linearly polarized light is orthogonal to the polarization direction of the analyzer of the first optical isolator, thus isolating the reflected light from the interference reflection element.

9. The method for local isolation in a composite cavity laser according to claim 7, characterized in that, Linearly polarized light emitted from the gain chip is collimated at 0° by a collimating lens and then passes through the analyzer of the first optical isolator. The polarization direction of the analyzer of the first optical isolator is 0°. After passing through the Faraday crystal of the first optical isolator, the polarization direction of the linearly polarized light rotates by 45°. After passing through the interference reflection element, the linearly polarized light is incident on the second optical isolator. After passing through the Faraday crystal of the second optical isolator, the polarization direction of the linearly polarized light rotates again by 45° in the same direction. The polarization direction of the analyzer of the second optical isolator is 90°. At this time, the polarization direction of the linearly polarized light coincides with the polarization direction of the analyzer of the second optical isolator. After passing through the analyzer of the second optical isolator, the linearly polarized light is incident on the resonant cavity mirror. The reflected light after passing through the resonant cavity mirror returns along the original optical path. After passing through the Faraday crystals of the second and first optical isolators, the polarization direction of the linearly polarized light rotates again by 90° in the same direction. At this time, the polarization direction of the linearly polarized light coincides with the polarization direction of the analyzer of the first optical isolator. The linearly polarized light returns to the gain chip.

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