In this chapter, categories of laser diodes are explained such as FP laser diode, DFB laser diode, quantum dot laser diode and so on. Characteristics and applications of the each packages are explained.
Table of Contents
Laser oscillators are made of laser gain mediums and resonators. Resonator of a laser diode is a Fabry-Perot resonator, and in this the cleavage plane plays the role of a plane mirror.
In a crystal, atoms are periodically arranged, where the atomic force is strong and weak can be clearly distinguished. If you put a scratch in the specific place where the atomic force is weak in the crystal, the crystal cracks along the direction of the scratch. This is called a cleavage, and a surface made of a cleavage is called a cleavage plane. Laser light is reflected or transmitted through the cleavage plane, the reflectance depends on the material of the active layer. Because of this the laser diode, immediately after the cleavage is made, the cleavage plane’s reflectance becomes the same with the side where the laser light is emitted from and the opposite side. So in reality, the surface that emits light is subjected to low-reflection coating, and the opposite side is subjected to a high reflection coating. The coating is very important also to protect the surface.
Distributed Feedback : DFB laser diode is a mode-hop-free single longitudinal mode (single-mode) laser that has a diffraction grating structure (a wavy structure) inside the resonator. Diffraction grating structure is formed on top of the active layer (The boundary of the cladding layer and the active layer has a diffraction grating pattern). In a DFB laser, out of the light generated in the photoactive layer, only the light that has been affected by the diffraction grating comes back in to the resonator (feedback). Because only this specific wavelength light that has been selected by the diffraction grating structure strengthening, it single-mode oscillates. Because diffraction gratings are distributed along the resonance direction of the active layer, it is called a Distributed Feedback laser.
The clip bellow explains DFB laser in three minutes.
Characteristics of DFB laser diodes
Laser diodes constructed of normal Fabry-Perot resonator structure, easily multi-mode oscillates (Multi-mode oscillation becomes remarkable when in pulsed operation), oscillation wavelength changes with the environmental temperature or injection current. On the other hand with DFB lasers, wavelength stability is very high, it single-mode oscillates and line width is very narrow. Unlike Fabry-Perot semi-conductor lasers it stays single mode even in high-speed modulation (stays single mode even in pulsed operation), it is used for high-capacity, long-distance optical communication. Applications that are not optical communication are, fiber Inspection, raman spectroscopy, wavelength conversion, and sensors.
Mainly DFB lasers are supplied as Coaxial Pigtailed package, 14 Pin Butterfly package and TO-CAN package (there are also Bare Die).
The normal oscillation wavelength of a DFB laser is 760 ~ 2300 nm. In recent years nanoplus manufactures and sales DFB laser with a wavelength of 3〜6 um. Output power depends on the wavelength, however usually manufacturers sell ones that is between a few mW and 150 mW.
In addition, DFB lasers can be tuned in a range of < 5 nm without mode hop, by change the pitch of the diffraction grating or by controlling the drive current and temperature. This is the biggest difference to DBR lasers and External cavity lasers which are also single mode lasers.
A Distributed Bragg Reflector: DBR is a laser diode that has a diffraction grating structure at the Fabry-Perot laser’s extension of the waveguide of the active layer. In a diffraction grating only a certain wavelength (one mode) is reflected in the active layer’s direction, other light doesn’t return to it. Because of this DBR laser diodes become a single mode laser that only oscillates a single wavelength.
Below is a illustration explaining the structure of External cavity laser diode, DFB laser diode, and DBR laser diodes.
Fig. 3.3.1. (a) external cavity laser (Littrow), (b) DFB lasers, (c) DBR laser
Between the DBR laser diodes active layer and feedback layer is the phase section. By controlling the phase in this area, you can aim and control only the phase. Because this is passible, a DBR laser diode can be single wavelength operated over a wide range.
DBR laser and DFB laser
DFB laser diode is mode-hop-free, DBR laser diode is not mode-hop-free. However, the tuning range of the DBR laser diode is wider than the DFB laser diode.
Reference and Links
-  Linglin Jiang,”High-Power DBR Laser Diodes Grown in a Single Epitaxial Step,” Proc. of SPIE Vol. 7320 (2009)
-  Photodigm, Wavelength Tuning of Distributed Bragg Reflector Lasers
-  R. Victor Jones,”Injection Lasers”(2000)
External Cavity Laser Diode: ECL is a laser diode with a diffraction grating (or a mirror and a diffraction grating) installed on the outside of a normal free-space output-type gain chip (AR coating laser diode). In this AR coating is applied to the cleaved surface of the gain chip’s exit side (=entrance side of the diffraction grating).
Laser emitted from the gain chip is incidents to the diffraction grating after being collimated. In the diffraction grating, all the modes incident into the gain chip is diffracted in the diffraction angle corresponding to the each mode. At this moment, one mode that has been selected by the diffraction grating goes back (is fed back) to the gain chip. With this mode resonating in the gain chip the output becomes single mode.
There are two types in ECL, Littman type and Littrow type laser diode.
The Littman type laser diode feed backs the first-order diffracted light in to the gain chip by reflecting it with a mirror. Because the laser is diffracted twice by the diffraction grating the output level is limited.
The Littrow type laser diode feed backs the first-order diffracted light in to the gain chip directly, and takes out the laser as 0-order diffracted light. Because the laser is diffracted only once the output level is higher than the Littman type laser diode.
Also for the Littrow type laser diode, it is possible to apply a normal Fabry-Perot lasers diode that have no AR coating on the diffraction grating’s side. Because of this it is possible to make it wideband, high output, and low price.
The fig bellow is a conceptual diagram of the Littman type laser diode and the Littrow type laser diode that Thorlabs manufactures and sales. Thorlabs manufactures and sales external cavity tunable laser of 770, 1050, 1220, 1310, 1450, 1550, and 1900 nm.
Fig. 3.4.1. Littman type laser laser diode
Fig. 3.4.2. Littrow laser laser diode
Characteristics of external cavity laser diode
With external cavity laser diode the design of the optical system enables a broadband tuning of more than 100 nm. The Littrow type laser diode tunes the wavelength by rotating the diffraction grating, and the Littman type laser diode tunes the wavelength by rotating the mirror.
An external cavity laser oscillates at the same wavelength band as a normal laser diode. In other words it can oscillate at a wavelength of under 630 nm, which DFB laser diode and DBR laser diode cannot.
The main factors that determine the line width of an external cavity laser laser diode are, electrical noise, acoustic noise and vibration. Considering from the structure the compact and robust Littrow type laser diode has its advantageous, however, with the Littman type laser diode it easily becomes a narrow line width due to the light passing through the diffraction grating twice.
References and Links
In amongst the 14 Pin Butterfly package laser diodes, for wavelength stabilization, have Fiber Bragg Grating written in the pigtail fiber. The light emitted from the gain chip is coupled to a pigtail fiber, and by the FBG the wavelength corresponding to the reflection band of the FBG returns (is fed back) to the gain chip. As a result, the light of reflection wavelength of the FBG gain chip resonates selectively obtaining a narrow spectral width laser.
Characteristics of the FBG with fiber pigtail type laser diode
FBG with fiber pigtail type laser diode’s spectral width is about 0.5 ~ 1 nm, and the spectral shift with temperature is about 0.01 nm / deg. From 3SP Group a 1050 mW（Kink-free） model is available. Compared to DFB laser diode and DBR laser diode , the spectral width is wide, but it can cope with high output. Because of this products of 976 nm wavelength is often used as excitation light source for fiber lasers.
The quantum dot is fine particles composed of a oxide semiconductor or compound semiconductor, consisting of 10 to 50 atoms. Quantum dot laser diode is a type of laser diode with quantum dots spread over the active layer. There are three types of structures of a semiconductor with the name “quantum” attached to it, quantum well, quantum wire and quantum dot. Quantum well is those that have an active layer thickness of about 10 nm. Quantum wire’s active layer is a thin thread. Make that thread short, makes it a quantum dot. The video below explains quantum wells, quantum wires, quantum dots.
By controlling the size of the quantum dot, it is possible to control the emission wavelength. The strong point of quantum dot laser diodes are: low power consumption, high efficiency, and temperature dependence being very low. From these features, adjustment of the modulation current and bias current is not required for the quantum dot laser diode. As for the usage for quantum dot laser diode, optical communication is most promising. Note here that, there are LED, SLD, and SOA using quantum dots are also commercially available.
Videos explaining quantum dot laser diodes
Carriers in a laser diodes are usually electrons and holes. It emits by recombination of holes in the valence band and electrons in the conduction band. On the other hand, quantum cascade laser diode, semiconductor quantum wells are connected in multiple stages. It is a type of laser diode, using the transition between sub-bands. Carriers in the semiconductor is one type. Emission wavelength due to band-to-band transition depends on the energy band gap of the semiconductor materials. The size of the inter-sub-band-energy can be changed by changing the width of the quantum well, so emission wavelength can be flexibly designed. Oscillation region of quantum cascade laser is mid-infrared to terahertz band. At room temperature, output of 20 mW has been done.
Below is a video that Alpes Lasers, Inc. has published, showing how the quantum cascade laser oscillates.
VCSEL emits the laser light perpendicular to the substrate surface, while normal laser light from laser diode is emitted from the semiconductor’s cleavage plane. Below is a video explaining VCSEL.
VCSEL does not need to have the semiconductor facet cleaved, and manufacturing of the VCSEL chip can be carried out in one go on the wafer, making it suitable for mass production, thus lowering the price. Also inspection can be done while it is still a chip.
Performance wise, its oscillation threshold current is under mA, has low power consumption, and is not suitable for high power applications. This is due to the active region being short. Output of one VCSEL basic element is a few mW, by arraying several hundred basic elements it is possible to output more than 1 W. This sort of VCSEL packages are commercially available. Emission light is not elliptical cone shape as an edge-emitting laser, but conical. Allowing high-density array is also a great feature. Important parameters of VCSEL are: Linewidth, Bias Current, Data Rate, Contact Configuration, and Channel Configuration.
VCSEL from being mass-produced in 1994, is used in a wide range of applications such as: Sensing, Optical interconnects, Laser printer, Bar code reader, Computer mouse, and Light Peak. A famous maker is II-VI Laser Enterprise. They have MOCVD and CVD suited for mass production.