Playing with tapered amplifier (TA)

Introduction

Tapered amplifier, is a device with name resembling its design. It is an laser amplifier(or a gain chip). Its gain area is fabricated to be a tapered shape. The tapered shape help guide the light to a larger output facet, which prevent gain saturation as well as optical damage to the facet.

A drawing of a typical tapered amplifier chip is presented on the left. A drawing of the assembled C-mount tapered amplifier is on the right.

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Tapered Amplifier System

When there is no input seed light, a powered tapered amplifier will still emit florescence in both direction. The output florescence (also known as ASM) from the two facet is very different, thus can be used to determine the direction of the chip. Using ASM as a guide light, we can easily align the seed light into the input facet. It is important that you may not use high current as well as high seed power when adjusting seed light coupling. Tapered amplifier will be killed when pumped with high current without proper seeding. Misaligned seed light may also destroy the input facet of tapered amplifier.

When the seed light is partially aligned, you can see laser coming out at the center section of the output ASM. With the help of a laser power meter, I can optimize the amplified laser to maximum. Now, it is safe to turn up the current! I can easily get over 450mW single frequency laser after tapered amplifier.

The tapered amplifier system is shown below.

ta_system_photo

The seed light comes from a standard ECDL amplified by a injection locking laser.

ta_seeding_photo

Since tapered amplifier has a very bad output spatial mode, I have to use several cylindrical lens to shape the beam. In order to prevent retro-reflected laser damaging tapered amplifier(because the reflected laser will be also amplified and collimated to the small input facet which may kill the coating), a good optical isolator is put at the output of the system. Lenses between the tapered amplifier and the isolator are also slightly misaligned to reduce reflection.

AOM Sideband Generation

For experiment needed, I need to generate a sideband(frequency shift a portion of laser) at several hundred MHz. I use an AOM to do that.

ta_setup

Finally, I get around 70mW pure linear polarized output at the output of PM fiber with equal power in the two frequencies. I also set up a scanning FP-cavity beside by system to monitor the system ensuring single frequency output. The yellow channel is the seed light, the green channel is the AOM output. You can clearly see the sideband generated by the AOM 100MHz apart.

ta_output_fpcavity

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