Polarizing EDFA tunable laser

Abstract

The invention discloses a polarized EDFA (erbium-doped fiber amplifier) tunable laser, which is characterized in that non-polarized light is emitted through a gain medium, wavelength selection is carried out on the non-polarized light through a filter in a forward process, and then the light enters a polarization beam splitter to be split into a first polarization direction and a second polarization direction in a transmission direction and a reflection direction of the beam splitter; the light in the first polarization direction is subjected to power beam splitting through the power beam splitting coupler, the first beam of light serves as laser output and leaves the laser resonant cavity, the second beam of light serves as laser feedback, the polarization direction of the second beam of light is converted into the second polarization direction through the axis converter, the second beam of light is reflected back to the filter through the polarization beam splitter, and in the reverse process, the second beam of light is output. And the light in the second polarization direction sequentially and reversely passes through the filter and the gain medium and reaches the Faraday mirror, the light in the second polarization direction which is reversely incident is changed into the light in the first polarization direction, the light travels on a forward light path, and the light beam forms laser at an output port through continuously and circularly repeating a forward process and a reverse process.

Description

Technical Field

[0001] The invention belongs to the technical field of tunable lasers, and in particular to a polarization EDFA tunable laser. The 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 are widely used in various fields. The key components of tunable laser sources are gain media and wavelength selectors, which are also called tunable filters (TF). Most gain media, especially those based on semiconductors, have gain characteristics for only one polarization, which makes them naturally suitable for producing single-polarization output. Other types of gain media, such as single-mode erbium-doped fiber amplifiers (EDFAs), have gains that are roughly uniform in each polarization direction, resulting in unpredictable output polarization directions of the laser source. In applications that require 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 through 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 through an optical fiber;

[0007] The non-polarized light is emitted through the gain medium. In the forward process, the wavelength of the non-polarized light is selected by the filter, and then it enters the polarization beam splitter. The polarization beam splitter divides 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 of the first polarization direction is propagated 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 resonance 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 through the axis converter and is 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 to reach the Faraday reflector. The Faraday reflector converts the reverse incident light of the second polarization direction into the forward traveling light of the first polarization direction, and continues to travel on the forward optical path and passes through the filter in the forward direction again. The light beam circulates in the resonance cavity for many times according to the previous order rule. By continuously repeating the forward process and the reverse process, a laser 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 through 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 light entering the output port in the reverse direction is isolated by the second isolator.

[0012] Optionally, the filter is a tunable filter, and the wavelength of 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 optical 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] In the present invention, a polarization light output tunable laser using a non-polarized gain medium is proposed without sacrificing output power and stability. The single polarization laser manufactured by this setup allows the gain of the gain medium to be fully utilized in both polarization directions, rather than losing half of the light to obtain a single polarization laser output. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The drawings constituting a part of the present application are used to provide a further understanding of the present application. The illustrative embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the 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 optical fiber configuration according to an embodiment of the present invention;

[0021] Figure 3 This is a schematic diagram of the structure of a tunable fiber laser that uses a non-polarized gain medium to construct a 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 in the embodiments of the present 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 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 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, which is reflected back to the right side of the filter by the polarization beam splitter, and then reversed in sequence ( Figure 3 -Z direction) through the filter, gain medium, and reaches the Faraday reflector. The Faraday reflector converts the reverse incident second polarization direction light into the forward traveling first polarization direction light, which continues to travel on the forward optical path and passes through the filter again in the forward direction. The light beam circulates in the resonant cavity many times according to the above rules and forms a laser at the output port.

[0025] In this article, the EDFA gain medium can be replaced by 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) The tunable filter selects the wavelength of light emitted from the third port, and the light then travels to the 30 / 70 power beam splitter, with 70% being used as the laser output. 30% of the light is used as the laser feedback, returns to the first port of the circulator, and travels backward from the second port of the circulator through the gain medium and is reflected by the reflector, so that the light beam travels forward again.

[0031] 3) Place an isolator before the output to prevent the retrograde 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, there are light waves traveling 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 manner;

[0036] 2) A 70 / 30 power beam splitter between the tunable filter and the gain medium, 70% of the light is used as laser output 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 optical fibers in the cavity must be PM fibers to maintain the polarization direction 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 lasers are used to convert non-polarized gain media into polarized lasers without reducing losses; 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 connected in sequence through optical fibers, wherein the feedback end of the power beam splitter is also connected to an axis converter, wherein the axis converter is connected to the polarization beam splitter to guide the feedback light back to 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 through an optical fiber. The gain medium uses a single-mode erbium-doped fiber amplifier.

[0042] It should be noted that in this article, the EDFA gain medium can be replaced by 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) optical fibers, that is, the optical fibers between points M and A, between B and C, and between D and E are all single-mode (SM) optical fibers, and light propagates in two directions. This area belongs to the standing wave area of ​​the laser resonant cavity.

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

[0046] 3) For the light running clockwise in the annular region of the laser resonant cavity, the power beam splitter feeds 30% of the light back to the laser resonant cavity at point F, and 70% of the light leaves the resonant cavity as laser output. The ratio of feedback and 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, and 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 penetrating 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, and at the same time converts the polarization direction from the original direction to its orthogonal direction.

[0049] Specific transmission 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, traveling in two directions (+Z and -Z), with one side of the emitted light transmitting to point C and the other side transmitting to point M. -Z is toward the Faraday reflector, and +Z is toward the tunable filter.

[0051] 2) The light emitted from the gain medium is non-polarized 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 area of ​​the laser cavity.

[0052] 3) The polarization beam splitter splits the light into two paths according to the polarization direction of the incident light. The polarized light in the X direction is reflected downward to point H, which enters the second interface of the axis converter, and propagates counterclockwise to point G, which is output at the first interface. After entering the power beam splitter counterclockwise, the light is blocked by the first isolator and disappears. On the other hand, the Y polarized light continues to be transmitted through the first isolator to the power beam splitter, that is, point F, where 70% of the light is output as laser and 30% is directed to the first interface of the axis converter.

[0053] 4) At the first interface, namely 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 so that the polarization direction of the light is turned to the X direction at the second interface, point H and reflected by the polarization beam splitter (PBS), and begins to propagate 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 moves towards the +Z direction.

[0056] 7) In this structure, after a stable laser is formed, the light propagating to the right (+Z) is Y polarized. The light propagating to the left (-Z) is X polarized. Therefore, the light traveling in the +Z direction in the standing wave region can only travel in the clockwise direction after entering the annular region. This setting allows the gain of the gain medium in both polarization directions to be used.

[0057] In order to obtain polarized output light, this setup allows the gain of the gain medium to be fully utilized in both polarization directions. It is different from the traditional non-polarized gain medium fiber laser. In order to obtain single polarization output light, a polarizer must be added at the exit to obtain single polarization laser output by losing half of the light.

[0058] The above are only preferred specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope 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 through 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 through an optical fiber; The non-polarized light is emitted through the gain medium. In the forward process, the wavelength of the non-polarized light is selected by the filter, and then it enters the polarization beam splitter. The polarization beam splitter divides 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 of the first polarization direction is propagated 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 resonance 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 through the axis converter and is 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 to reach the Faraday reflector. The Faraday reflector converts the reverse incident light of the second polarization direction into the forward traveling light of the first polarization direction, and continues to travel on the forward optical path and passes through the filter in the forward direction again. The light beam circulates in the resonance cavity for many times according to the previous order rule. By continuously repeating the forward process and the reverse process, a laser is formed at the output port.

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

3. The tunable laser according to claim 1, characterized in that: A first isolator is also 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 through 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 also connected to a second isolator via an optical fiber, and the light entering the output port in the reverse direction is isolated by the second isolator.

5. The tunable laser according to claim 1, characterized in that: The filter adopts 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, characterized in that: The power beam splitter adopts 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.

Images