First Infrared Optical Output Device

Recently, there has been remarkable progress in development of optical communications networks and parts, such as semiconductor lasers, optical fibers, fiber amplifiers, etc. There are various optical output devices emitting light in the infrared wavelengths, such as semiconductor lasers, super-luminescent diodes, and semiconductor optical amplifiers, forming key parts used by communications networks. Additionally, in the optical-sensing field, infrared optical output devices, such as optical-fiber gyroscopes and optical coherence tomography (OCT) equipment, which is of key significance in ophthalmology diagnostics, are already deeply embedded and in widespread use in society. This article explains the basic structure of these infrared optical output devices, as well as the applicable wavelengths and features.

Basic Structure

Infrared optical output devices are fabricated from layers of various compound semiconductors over a substrate base of either indium phosphide (InP) or gallium arsenide (GaAs). The wavelengths of the optical output are determined by the combination of substrate material and the ratio of semiconductor materials forming the optical output layer called the active layer. Many different structures have been investigated so far, but two types are used currently: “Buried heterostructure” and “Ridge waveguide structure”. In the former, the active layer is first grown on the substrate and is then mostly etched away before being buried again under semiconductor materials. In the latter, the etching is shallow and does not reach the active layer, creating channels that restrict the region where current can flow. The choice of device structure depends on the required wavelength and usage. The communications field uses 0.8-µm band ridge devices with a GaAs substrate as well as both 1.3 and 1.6-µm band buried and ridge devices using a InP substrate.

Examples of Cross-section Structure

Applicable Wavelengths

Signal light sources for optical communications mostly use wavelengths close to 1.3 and 1.55 µm, which are least affected by degraded signal quality and optical-fiber transmission losses. Additionally, light sources for pumping optical-fiber amplifiers use the 1.4-µm band. On the other hand, light sources for sensing applications use wavelengths matching the required measurement resolution, light absorption properties of the target material, scattering properties, light water-absorption properties, etc. Some concrete examples include the 0.8-µm band used by optical-fiber gyroscopes, the 0.8 and 1.06-µm bands used for ophthalmology OCT, the 1.3-µm band used for dermatology treatments, and the 1.6-µm band used for measuring blood-glucose levels. In the gas-sensing field, the best wavelength required for the gas type is used, such as 1.65 µm for methane (CH₄) and 1.53 µm for ammonia (NH₃).

There are several candidates for use as the active layer, but the following three are commonly used. AlGaAs and InGaAs active layers on a GaAs substrate emit light at wavelengths up to 0.9 µm. On the other hand, an InGaAsP active layer on an InP substrate emits light at wavelengths at 1.2 µm or more commonly used by communications. The following table shows the output light wavelengths of the active layer and substrate combinations. Today, the optical output band can be widened using a repeating quantum-well structure fabricated from extremely thin multiple active layers.

Crystal Materials and Output Optical Wavelengths

Infrared Optical Output Device Spectrum Features

(1) Fabry-Perot Laser Diode (LD)

This laser uses a resonator with periodic transmission properties vs wavelength by reflecting light back and forth between two parallel internal mirrors of reflectance R. The resonator contains a material to increase gain which amplifies light leaving the resonator and soon leads to oscillation. This Fabry-Perot LD is the simplest form of laser and is fabricated by etching a semiconductor wafer with a narrow striped active layer. It has a high output but with multimode spectrum characteristics. It is used as a pumping laser for optical amplifiers and can achieve a maximum output exceeding 600 mW from a single mode fiber (SMF). It can be used as a FBG laser with a fixed oscillation wavelength using an FBG fiber with a diffraction grating for pumping a Raman amplifier.

Fabry-Perot Laser

FBG Laser

(2) Distributed Feedback (DFB) Laser

The lack of a single wavelength mode makes the Fabry-Perot LD unsuitable as a light source for optical communications compared to the single-mode DFB laser using an internal diffraction grating to select a specific wavelength. More recently, the DFB laser is being integrated into modulators. It is also used in remote gas-sensing applications by irradiating the gas absorption lines with laser light.

DFB Laser Structure

(3) Super-Luminescent Diode (SLD)

The Fabry-Perot LD has a periodic spectrum due to reflections at the end faces. The super-luminescent diode (SLD) lowers the reflectance of both end faces as much as possible using coatings to control the resonance mode and widen the spectrum. It features wide bandwidth, low coherence, and high output plus high coupling efficiency with SMF and is already in widespread use as a light source for optical-fiber gyroscopes and medical sensing.

SLD Structure

(4) Semiconductor Optical Amplifier (SOA), Gain-Chip

The Semiconductor Optical Amplifier (SOA) is a semiconductor device for amplifying optical input signals. Like the SLD, it implements a wide bandwidth and high gain by reducing the reflectance at the element end faces. Unlike the SLD, noise reduction is required to suppress noise from coupling optical fibers to both end faces as well as self-generated noise in the SLD design. Although the SOA gain makes it unsuitable for optical-amplifier applications, it has advantages of compact size, low power consumption, and a wide center wavelength. It is used increasingly in optical transceivers as transmission distances become longer. It is also in widespread use as a so-called ‘Gain-Chip’ for variable-wavelength light sources. The Gain-Chip is configured with a low-reflectance (about 0%) (LR) face at one side plus an external oscillator using a wavelength filter and diffraction grating, etc.