Polarization EDFA tunable laser

Abstract

The present invention discloses a polarization EDFA tunable laser, which includes: emitting non-polarized light through a gain medium; in a forward process, wavelength selection of the non-polarized light is performed through a filter; the non-polarized light then enters a polarization beam splitter to split the light into a first polarization direction and a second polarization direction, respectively; power splitting of the light in the first polarization direction is performed through a power beam splitting coupler; the first beam of light is output as laser light and leaves a laser resonant cavity; the second beam of light is fed back as laser light and is converted into a second polarization direction through an axis converter; the second beam of light is reflected back to the filter by the polarization beam splitter; in a reverse process, the light passes through the filter and the gain medium in reverse order and reaches a Faraday reflector to convert the reversely incident light in the second polarization direction into light in the first polarization direction, which then travels along a forward optical path; the light beam forms a laser at an output port by continuously repeating the forward and reverse processes.

Description

Technical Field

[0001] The present invention belongs to the technical field of tunable lasers, and in particular relates to a polarization EDFA tunable laser. This technical method can be used for any laser that uses a non-polarized gain medium to generate a single polarization output. Background Art

[0002] Tunable laser sources have a wide range of applications in various fields. The key components of a tunable laser source are the gain medium and the wavelength selector, which is also called a tunable filter (TF). Most gain media, especially those based on semiconductors, have gain characteristics for only one polarization, which makes them naturally suitable for generating single-polarization output. Other types of gain media, such as single-mode erbium-doped fiber amplifiers (EDFAs), have gains that are roughly uniform in all polarization directions, resulting in unpredictable output polarization directions of the laser source. In applications requiring polarized light, a polarizer must be added between the laser source and the device under test (DUT). This operation reduces the laser power and may also cause unstable output power due to uncontrolled polarization. Summary of the Invention

[0003] In order to solve the above technical problems, the present invention proposes a polarization EDFA tunable laser to solve the above problems in the prior art.

[0004] To achieve the above object, the present invention provides a polarization EDFA tunable laser, comprising:

[0005] A gain medium, a filter, a polarization beam splitter, and a power beam splitter connected in sequence through optical fibers;

[0006] The feedback port of the power beam splitter is connected to one end of the axis converter via an optical fiber, the other end of the axis converter is connected to the polarization beam splitter, and the side of the gain medium away from the polarization beam splitter is connected to the Faraday reflector via an optical fiber;

[0007] Unpolarized light is emitted through the gain medium. In the forward process, the wavelength of the unpolarized light is selected by the filter, and then it enters the polarization beam splitter. The polarization beam splitter splits the two orthogonally polarized lights into the transmission and reflection directions of the beam splitter. The polarization direction of the light in the transmission direction is the first polarization direction, and the polarization direction of the light in the reflection direction is the second polarization direction. The transmitted light with the first polarization direction propagates to the power beam splitter, which splits the light with the first polarization direction into a first beam and a second beam. The first beam is used as laser output and leaves the laser resonator. The second beam is used as laser feedback. The polarization direction of the second beam is converted to the second polarization direction by the axis converter and reflected back to the right side of the filter by the polarization beam splitter. In the reverse process, it then passes through the filter and the gain medium in reverse order and reaches the Faraday reflector. The Faraday reflector converts the reverse incident light with the second polarization direction into forward traveling light with the first polarization direction. The light continues to travel on the forward optical path and passes through the filter in the forward direction again. The light beam circulates multiple times in the resonator according to the previous rule. By repeating the forward and reverse processes, laser light is formed at the output port.

[0008] Optionally, the gain medium is a single-mode erbium-doped fiber amplifier.

[0009] Optionally, a first isolator is further connected between the polarization beam splitter and the power beam splitter via an optical fiber;

[0010] The reflected light transmitted forward to the polarization beam splitter is isolated by the first isolator after passing through the axis converter and the power beam splitter to avoid returning to the laser resonance cavity.

[0011] Optionally, the output port of the power beam splitter is further connected to a second isolator via an optical fiber, and the second isolator isolates light entering the output port in the reverse direction.

[0012] Optionally, the filter is a tunable filter, and the wavelength of the non-polarized light is selected by the tunable filter.

[0013] Optionally, the optical fibers connecting the Faraday reflector, the gain medium, the tunable filter and the polarization beam splitter in sequence are all single-mode optical fibers.

[0014] Optionally, the optical fibers connecting the polarization beam splitter, the first isolator, the power beam splitter, the axis converter and the polarization beam splitter in sequence are polarization-maintaining fibers, and the light travels in a clockwise direction between the polarization beam splitter, the first isolator, the power beam splitter and the axis converter in sequence.

[0015] Optionally, the power beam splitter uses a 70 / 30 coupler, in which 70% of the light is used for output and 30% is used for laser feedback, transmitted to the axis converter, and returned to the laser resonance cavity. The ratio of feedback and output can be adjusted as needed.

[0016] Compared with the prior art, the present invention has the following advantages and technical effects:

[0017] This paper proposes a polarization-tunable laser using a non-polarized gain medium without sacrificing output power and stability. This setup allows the gain medium's gain to be fully utilized in both polarization directions, rather than sacrificing half the light to achieve single-polarization laser output. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The accompanying drawings, which constitute part of this application, are intended to provide a further understanding of this application. The exemplary embodiments and descriptions of this application are intended to explain this application and do not constitute an improper limitation on this application. In the accompanying drawings:

[0019] Figure 1 A schematic diagram of a standing wave tunable fiber laser configuration according to an embodiment of the present invention;

[0020] Figure 2 A schematic diagram of a ring cavity tunable fiber configuration according to an embodiment of the present invention;

[0021] Figure 3 This is a schematic structural diagram of a tunable fiber laser that uses a non-polarized gain medium to construct polarized light output according to an embodiment of the present invention. DETAILED DESCRIPTION

[0022] It should be noted that, in the absence of conflict, the embodiments and features of the embodiments in this application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0023] It should be noted that the steps shown in the flowcharts of the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and that, although a logical order is shown in the flowcharts, in some cases, the steps shown or described can be executed in an order different from that shown here.

[0024] The present invention discloses a polarization EDFA tunable laser, comprising: emitting non-polarized light through a gain medium, forward ( Figure 3+Z direction)) The wavelength of the non-polarized light is selected by the filter, and then enters the polarization beam splitter. The beam splitter divides the two orthogonally polarized lights into the transmission and reflection directions of the beam splitter. The polarization of the light of the aforementioned transmission part is the first polarization direction, and the polarization of the light of the reflection part is the second polarization direction. The selected light is polarized and the transmitted light of the first polarization direction is propagated to the power beam splitter coupler. The power beam splitter coupler performs power splitting on the light of the first polarization direction to form two separated beams of light. The first beam of light is used as laser output and leaves the laser resonance cavity. The second beam and the guide light are used as laser feedback. After passing through the axis converter, their polarization is converted to the second polarization direction and reflected by the polarization beam splitter back to the right side of the filter, and then reversed in sequence ( Figure 3 The light then passes through the filter and gain medium (in the -Z direction) before reaching the Faraday reflector. The Faraday reflector converts the reverse-incident second polarization light into forward-traveling first polarization light. The light then continues along the forward optical path and passes through the filter again in the forward direction. The beam circulates multiple times within the resonant cavity according to the aforementioned rules, forming a laser beam at the output port.

[0025] In this article, the EDFA gain medium can be replaced with other non-polarization gain media. The filter can be placed Figure 3 Any location within the laser resonator.

[0026] 1) Standing wave cavity laser and ring cavity laser

[0027] There are two types of fiber lasers: standing wave cavity lasers and ring cavity lasers, such as Figure 1-2 shown.

[0028] 1. Standing wave cavity tunable laser, such as Figure 1 As shown,

[0029] 1) The light emitted from the gain medium is transmitted forward to the second port of the circulator and emitted from the third port.

[0030] 2) A tunable filter selects the wavelength of light emitted from the third port. The light then passes through a 30 / 70 power splitter, with 70% of the light being used as laser output. The remaining 30% of the light, as laser feedback, returns to the first port of the circulator and travels backward from the second port through the gain medium. It is then reflected by a mirror, allowing the beam to travel forward again.

[0031] 3) Place an isolator before the output to prevent the reverse light from entering the laser cavity.

[0032] 4) In this configuration, if the gain medium of the standing-wave cavity tunable laser has gain in both polarization directions, single-mode (SM) fiber can be used; otherwise, polarization-maintaining (PM) fiber is used.

[0033] 5) It is important to note that in a standing wave cavity tunable laser setup, light waves travel in both directions between the reflector and the node at the second port, forming a standing wave.

[0034] 2. Ring cavity tunable laser, such as Figure 2 As shown,

[0035] 1) In a ring cavity tunable laser, light propagates clockwise in a closed loop: gain medium → isolator → tunable filter → power beam splitter → gain medium, and propagates in a closed loop in the above clockwise propagation manner;

[0036] 2) A 70 / 30 power beam splitter between the tunable filter and the gain medium, 70% of the light is output as laser and 30% of the light is fed back into the laser resonant cavity. Figure 1-2 In both configurations shown, the gain medium can be uniaxial or biaxial.

[0037] 3) For uniaxial gain media, all fibers in the cavity must be PM fibers to maintain polarization and ensure that the light entering the gain medium has the correct polarization direction. In this case, the polarization direction of the output laser is fixed.

[0038] 4) If the gain medium of the ring cavity tunable laser has gain for both polarization directions, the fiber can be PM or SM, but the polarization direction of the output beam will not be controlled.

[0039] 2) Polarization EDFA tunable laser

[0040] Polarization EDFA tunable laser is used to convert non-polarized gain medium into polarized laser without reducing loss; Figure 3 A polarization EDFA tunable laser is shown as a structure for converting an unpolarized gain medium into polarized laser light.

[0041] The polarization EDFA tunable laser includes a gain medium, a tunable filter, a polarization beam splitter, a first isolator, a power beam splitter, and a second isolator, all connected in sequence via optical fibers. The feedback end of the power beam splitter is also connected to an axis converter, which, in turn, connects to the polarization beam splitter to guide the feedback light back into the standing wave region of the laser cavity. The end of the gain medium away from the tunable filter is also connected to a Faraday reflector via optical fiber. The gain medium utilizes a single-mode erbium-doped fiber amplifier.

[0042] It should be noted that in this article, the EDFA gain medium can be replaced with other non-polarization gain media. The filter can be placed Figure 3 Any position in the laser resonator.

[0043] Related configuration:

[0044] 1) The optical fibers used for light propagation between the Faraday reflector, gain medium, tunable filter, and polarization beam splitter are all single-mode (SM) fibers. That is, the optical fibers between points M and A, between points B and C, and between points D and E are all single-mode (SM) fibers, allowing light to propagate in both directions. This region is the standing wave region of the laser resonator.

[0045] 2) Polarization-maintaining (PM) fiber is used for light propagation between the polarization beam splitter, the first isolator, the power beam splitter, the axis converter, and the polarization beam splitter. Specifically, PM fiber is used between points E and F, between F and G, and between H and E. Light propagates in a clockwise direction. This region forms the annular region of the laser resonant cavity.

[0046] 3) For light traveling clockwise within the annular region of the laser resonator, the power beam splitter at point F feeds 30% of the light back into the laser resonator, leaving the resonator as laser output. The ratio of feedback to output can be adjusted as needed.

[0047] 4) The axis converter includes a first interface and a second interface, wherein the first interface is connected to the power beam splitter, and the second interface is connected to the polarization beam splitter. In the axis converter, if light propagates from the second interface to the first interface, that is, from point H to point G, the polarization direction of the light is converted from the X axis to the Y axis. If light propagates from the first interface to the second interface, that is, from point G to point H, the polarization direction of the light is converted from the Y axis to the X axis. X is the polarization direction of the light reflected by the polarization beam splitter; Y is the polarization direction of the light transmitted by the polarization beam splitter.

[0048] 5) In a polarization EDFA tunable laser, a Faraday reflector changes the direction of the input light from -Z to +Z, while simultaneously converting the polarization direction from the original direction to its orthogonal direction.

[0049] Specific propagation process:

[0050] 1) When the power is turned on, the amplified spontaneous emission (ASE) source emitted by the gain medium is emitted from both surfaces B and A simultaneously, traveling in two directions (+Z and -Z). The emitted light is transmitted to point C on one side and to point M on the other side. -Z is toward the Faraday reflector, and +Z is toward the tunable filter.

[0051] 2) The light emitted from the gain medium is unpolarized light. The light emitted to the tunable filter and the light reflected by the Faraday reflector both travel to the right, enter the polarization beam splitter at point E, and enter the annular region of the laser cavity.

[0052] 3) The polarization beam splitter splits the incident light into two paths based on its polarization direction. X-polarized light is reflected downward to point H, where it enters the second interface of the axis converter. It then propagates counterclockwise to point G, where it is output at the first interface. After entering the power beam splitter counterclockwise, this light is blocked by the first isolator and disappears. Meanwhile, Y-polarized light continues through the first isolator and travels to the power beam splitter, reaching point F. 70% of this light is output as laser light, while 30% is directed to the first interface of the axis converter.

[0053] 4) At the first interface, ie, point G, the polarization direction of the light directed to the first interface remains in the Y direction.

[0054] 5) At the first interface, point G, the PM fiber is remelted, causing the polarization direction of the light to be redirected to the X direction at the second interface, point H. The light is reflected by the polarization beam splitter (PBS) and begins propagating in the -Z direction. The node of the remelted PM fiber is called an "axis converter," which converts the polarization direction from one direction to its orthogonal direction.

[0055] 6) The light beam reflected by the polarization beam splitter hits point M of the Faraday reflector. After being reflected, the polarization direction of the light returns to the Y direction and travels towards the +Z direction.

[0056] 7) In this configuration, after stable lasing is achieved, light propagating to the right (+Z) is Y-polarized, while light propagating to the left (-Z) is X-polarized. Therefore, light traveling in the +Z direction in the standing wave region can only travel in a clockwise direction after entering the annular region. This setup allows the gain medium to utilize its gain in both polarization directions.

[0057] To obtain polarized output light, this setup allows the gain of the gain medium to be fully utilized in both polarization directions. This differs from traditional non-polarized gain medium fiber lasers, which require a polarization selector at the output to obtain single-polarization output light, thereby losing half of the light to obtain single-polarization laser output.

[0058] The above are merely preferred embodiments of the present application, but the scope of protection of the present application is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in this application should be included in the scope of protection of the present application. Therefore, the scope of protection of the present application should be based on the scope of protection of the claims.

Claims

1. Polarization EDFA tunable laser, characterized in that, include: A gain medium, a filter, a polarization beam splitter, and a power beam splitter connected in sequence through optical fibers; The feedback port of the power beam splitter is connected to one end of the axis converter via an optical fiber, the other end of the axis converter is connected to the polarization beam splitter, and the side of the gain medium away from the polarization beam splitter is connected to the Faraday reflector via an optical fiber; Unpolarized light is emitted through the gain medium. In the forward process, the wavelength of the unpolarized light is selected by the filter, and then the light enters the polarization beam splitter. The polarization beam splitter splits the two orthogonally polarized lights into the transmission and reflection directions of the beam splitter. The polarization direction of the light in the transmission direction is a first polarization direction, and the polarization direction of the light in the reflection direction is a second polarization direction. The transmitted light of the first polarization direction propagates to the power beam splitter. The power beam splitter performs power beam splitting on the light of the first polarization direction to form a first beam of light and a second beam of light. The first beam of light is used as laser output and leaves the laser resonant cavity. The second beam of light is used as laser feedback. The polarization direction of the second beam of light is converted to the second polarization direction by the axis converter and reflected back to the right side of the filter by the polarization beam splitter. In the reverse process, the light then passes through the filter and the gain medium in reverse order and reaches the Faraday reflector. The Faraday reflector converts the reverse incident light of the second polarization direction into forward traveling light of the first polarization direction. The light continues to travel on the forward optical path and passes through the filter in the forward direction again. The light beam circulates multiple times in the resonant cavity according to the previous rule. By repeating the forward and reverse processes, a laser is formed at the output port. This setup allows the single-polarization laser to be fabricated, allowing the gain of the gain medium to be fully utilized in both polarization directions, rather than losing half of the light to obtain single-polarization laser output.

2. The tunable laser according to claim 1, characterized in that The gain medium adopts a single-mode erbium-doped fiber amplifier.

3. The tunable laser according to claim 1, characterized in that A first isolator is further connected between the polarization beam splitter and the power beam splitter via an optical fiber; The reflected light transmitted forward to the polarization beam splitter is isolated by the first isolator after passing through the axis converter and the power beam splitter to avoid returning to the laser resonance cavity.

4. The tunable laser according to claim 1, characterized in that The output port of the power beam splitter is further connected to a second isolator via an optical fiber, and the second isolator isolates light entering the output port in the reverse direction.

5. The tunable laser according to claim 1, wherein: The filter is a tunable filter, and the wavelength of non-polarized light is selected by the tunable filter.

6. The tunable laser according to claim 1, characterized in that: The optical fibers connecting the Faraday reflector, the gain medium, the tunable filter and the polarization beam splitter in sequence are all single-mode optical fibers.

7. The tunable laser according to claim 1, characterized in that: The optical fibers connecting the polarization beam splitter, the first isolator, the power beam splitter, the axis converter and the polarization beam splitter in sequence use polarization-maintaining optical fibers, and the light travels in a clockwise direction among the polarization beam splitter, the first isolator, the power beam splitter and the axis converter in sequence.

8. The tunable laser according to claim 1, wherein: The power beam splitter uses a 70 / 30 coupler, in which 70% of the light is used for output and 30% is used for laser feedback, transmitted to the axis converter, and returned to the laser resonance cavity. The ratio of feedback and output can be adjusted as needed.

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