Tapered amplifiers are a specialized type of semiconductor optical amplifier designed with a tapered geometry in which the width of the amplifier region gradually increases along the propagation direction of light. They are primarily used to amplify laser diode outputs efficiently. A tapered amplifier amplifies the optical power of the beam but does not inherently change the beam's height or width. They offer a unique advantage because they can simultaneously provide both high optical power and narrow beam quality, making them ideal for applications in laser systems, such as optical tweezers, atomic physics experiments, and frequency doubling for laser cooling, where a combination of high power and good beam quality is essential. 

They can deliver output powers in the range of several watts while keeping a high-quality beam, often with a narrow linewidth and the ability to tune the wavelength, sometimes over 20 nm or even 50 nm. These amplifiers typically offer a gain of around 20 dB. 

The wavelength of a tapered amplifier can vary depending on its design and application. Tapered amplifiers can be designed to operate at different wavelengths within the electromagnetic spectrum, and their specific wavelength range is determined by factors such as the semiconductor material used and the intended use case. Common wavelength ranges for tapered amplifiers include near-infrared (NIR) wavelengths around 800 nm to 1600 nm and mid-infrared (MIR) wavelengths beyond 1600 nm. 

The output beam from a tapered amplifier has an uneven shape – it is much broader horizontally and diverges more vertically due to some factors in the optical system, such as the properties of the input beam, the optics used to collimate or focus the beam, or the characteristics of the laser cavity. As a result, more advanced optics are needed, including cylindrical lenses, to create a collimated beam with a circular profile. These devices have found applications in various fields, including telecommunications, spectroscopy, medical imaging, and material processing.

Working of Tapered Amplifiers

Tapered amplifiers work based on the principle of optical amplification within a semiconductor gain medium, typically made of materials like gallium arsenide (GaAs) or indium phosphide (InP). These devices are designed with a unique geometry where the cross-sectional area of the optical path gradually increases along the length of the amplifier. This tapering design allows for efficient amplification of the input laser beam.

The key components of a tapered amplifier include the seed laser diode, ridge waveguide, taper region, and electrode. The process begins with the seed laser diode, which generates the initial laser beam, referred to as the seed. This seed laser beam is then precisely directed into the tapered amplifier through a narrow ridge waveguide. The ridge waveguide guides and controls the laser light, ensuring it enters the amplifier efficiently.

Inside the tapered amplifier, a region called the taper region plays a central role. In this region, the width of the optical path gradually increases, facilitating the efficient amplification of the laser light. Simultaneously, an electrode is actively engaged in the process. It supplies an electric current, typically a few amperes, to the semiconductor chip housing the amplifier. This injection of current populates the valence band within the semiconductor material, leading to the phenomenon of optical gain.

MOPA Systems with Tapered Amplifiers

Using a tapered amplifier in a master oscillator power amplifier (MOPA) setup, as depicted in the figure above, is a common practice. The initial laser source, known as the seed laser, is typically a laser diode, such as a distributed-feedback laser with a narrow linewidth, or a different type of laser diode that can adjust its wavelength. An external-cavity diode laser can also serve as the seed laser.

Typically, these systems operate continuously without interruption. Amplifying pulses is somewhat challenging because semiconductor amplifiers have a relatively low saturation energy.

Due to the challenging task of fully eliminating the Fresnel reflection from the input, it's often necessary to incorporate a Faraday isolator between the seed laser and the amplifier. Also, collimation and focusing optics are needed both before and after the isolator. To mitigate potential feedback issues with a high-gain amplifier, an extra Faraday isolator may be necessary at the output. This type of amplified laser system can be designed to be compact and robust, and it can also be equipped with fiber coupling for added convenience.

External-cavity Diode Lasers

A tapered amplifier can also be integrated into an external-cavity diode laser (ECDL), extra optical components are introduced for tuning the wavelength or reducing the linewidth. Typically, these components are added on the amplifier's input side, and the Fresnel reflection from the output side, possibly altered with a coating, can serve as the output coupler mirror. For instance, such an amplifier might include a collimation lens and a rotatable diffraction grating in the Littrow configuration to create a tunable laser, as seen in the figure above.

Frequency Doubling

To access shorter wavelength ranges, a method called frequency doubling can be used. This can be challenging with most other semiconductor laser systems, but tapered amplifiers make it relatively straightforward due to their high power and excellent beam quality. One option is to use resonant frequency doubling, especially if achieved single-frequency operation is achieved. Alternatively, frequency doubling can be efficiently achieved in a single pass through a nonlinear waveguide.

Advantages of Tapered Amplifier

• High Output Power
• High Beam Quality
• Wavelength Tunability
• Narrow Linewidth
• Compact Size
• Frequency Doubling
• Cost-Effectiveness
• Compatibility

Applications of Tapered Amplifier

Tapered amplifiers find a wide range of applications in various fields due to their unique properties. They are used in optical communication systems to amplify signals in optical fibers. These amplifiers can boost the power of optical signals without introducing excessive noise, making them essential components in long-distance optical communication networks.

In medical imaging, OCT is used to create detailed cross-sectional images of biological tissues. Tapered amplifiers are employed to amplify the low-power light source used in OCT systems, allowing for deeper tissue penetration and improved image resolution.

These amplifiers are vital in experiments related to quantum optics and quantum information processing. They can amplify single photons or weak quantum states, enabling researchers to manipulate and study quantum phenomena such as entanglement and superposition. In atomic and molecular physics, tapered amplifiers are used to amplify laser beams used for laser cooling and trapping experiments. These experiments require precise control over the laser intensity and frequency, which tapered amplifiers provide.

Tapered amplifiers are employed in biophotonics applications, such as flow cytometry and fluorescence spectroscopy. They enhance the intensity of laser light for efficient cell analysis and fluorescence excitation.

These amplifiers are used in LiDAR (light detection and ranging) systems, particularly in remote sensing and atmospheric studies. They amplify the laser pulses used to measure distances and gather information about the Earth's atmosphere.

Tapered amplifiers are crucial in high-resolution spectroscopy experiments that require precise control over the laser beam properties. They can amplify narrow linewidth lasers used for studying atomic and molecular spectra.

In the field of high-speed electronics, tapered amplifiers are used to amplify electrical signals in semiconductor devices. They find applications in high-speed data transmission, enabling faster and more efficient communication.

Tapered amplifiers can be used to boost the power of free-space optical communication systems, which are used for high-speed data transfer over short distances, often in situations where traditional wired connections are impractical. Theses amplifiers are employed in various sensing applications, including gas sensing, strain sensing, and temperature sensing. They enhance the sensitivity and precision of the measurement systems.