Fiber coupled semiconductor laser

Definition: A diode laser in which the light generated is coupled into an optical fiber.

In many cases, it is necessary to couple the output light from a diode laser into an optical fiber so that the light can be transmitted to where it is needed. Fiber-coupled semiconductor lasers have the following advantages:

1. The intensity curve of the light emitted from the optical fiber is generally smooth and circular, and the beam quality is symmetrical, which is very convenient in application. For example, less complex optics are used to generate circular pump spots for end-pumped solid-state lasers.

2. If the laser diode and its cooling device are removed from the solid-state laser head, the laser becomes very small and there is enough space to place other optical parts.

3. Replacing unqualified optically coupled semiconductor lasers does not require changing the arrangement of the device.

4. The optical coupling device is easy to use in combination with other fiber optic devices.

Fiber Coupled Semiconductor Laser Types

Many finished diode lasers are fiber-coupled, containing very robust fiber-coupled optics in the laser package. Different diode lasers use different fibers and technologies.

The simplest case is that a VCSEL (Vertical Cavity Surface Radiation Laser) typically radiates a beam with very high beam quality, medium beam divergence, no astigmatism, and a circular intensity distribution. Imaging the radiation spot into the core of a single-mode fiber requires a simple spherical lens. The coupling efficiency can reach 70-80%. Optical fibers can also be coupled directly into the radiating surface of the VCSEL.

Small edge-emitting laser diodes also radiate a single spatial mode and thus can, in principle, couple efficiently into single-mode fibers. However, if only a simple spherical lens is used, the ellipticity of the beam will greatly reduce the coupling efficiency. And the beam divergence angle is relatively large in at least one direction, so the lens needs to have a relatively large numerical aperture. Another problem is the astigmatism present in the output light of the diode, especially the gain-guided diode, which can be compensated by using an additional cylindrical lens. If the output power reaches several hundred milliwatts, fiber-coupled gain-guided laser diodes can be used to pump erbium-doped fiber amplifiers.

Figure 2: Schematic of a simple low-power fiber-coupled edge-emitting laser diode. The spherical lens is used to image the light emitted from the surface of the laser diode onto the fiber core. Beam ellipticity and astigmatism reduce coupling efficiency.

Large-area laser diodes are spatially multi-mode in the direction of radiation. If you just shape the circular beam through a cylindrical lens (for example, a fiber lens, as shown in Figure 3) and then enter the multimode fiber, most of the brightness will be lost because the high-quality beam in the fast axis direction Quality cannot be used. For example, light with a power of 1W can enter a multimode fiber with a core diameter of 50 microns and a numerical aperture of 0.12. This light is sufficient to pump a low-power bulk laser, such as a microchip laser. Even emitting 10W of light is possible.

Figure 3: Schematic of a simple optically coupled large-area laser diode. Fiber optic lenses are used to collimate light in the fast axis direction.

An improved broadband laser technology would be to shape the beam to have a symmetrical beam quality (not just the beam radius) before firing it. This also results in higher brightness.

In diode arrays, the problem of asymmetric beam quality is even more serious. The output of each transmitter may be coupled into a different fiber in the fiber bundle. The optical fibers are arranged linearly on one side of the diode array, but the output ends are arranged in a circular array. A beam shaper can be used to achieve symmetrical beam quality before launching the beam into a multimode fiber. This allows 30W of light to be coupled into a 200 micron diameter fiber with a numerical aperture of 0.22. This device can be used to pump Nd:YAG or Nd:YVO4 lasers to obtain an output power of approximately 15W.

In diode stacks, fibers with larger core diameters are also commonly used. Several hundred watts (or even several kilowatts) of light can be coupled into an optical fiber with a core diameter of 600 microns and a numerical aperture of 0.22.

Disadvantages of Fiber Coupling.

Some disadvantages of fiber-coupled semiconductor lasers compared to free-space radiation lasers include:

higher cost. Costs can be reduced if the beam handling and transmission processes are simplified.

The output power is slightly smaller and more importantly the brightness. The loss of brightness is sometimes very large (greater than an order of magnitude) and sometimes small, depending on the fiber coupling technology used. In some cases this does not matter, but in other cases it becomes a problem, such as in the design of diode-pumped bulk lasers or high-power fiber lasers.

In most cases (especially multimode fiber), the fiber is polarization maintaining. Then the output light of the fiber is partially polarized, and if the fiber is moved or the temperature changes, the polarization state will also change. If the pump absorption is polarization dependent, this can create significant stability problems in diode-pumped solid-state lasers.