What is the Single-mode laser diode?
A single-mode laser diode is a type of laser diode that produces a single output wavelength. It operates on a single kind of transverse resonator mode, which is called the Gaussian mode. The laser emits quasi-monochromatic radiation with a very small linewidth and low phase noise. It is also called a single-frequency diode. The single-mode laser diodes are available in both free space and fiber-coupled package configuration. In a free space single-mode laser diode, the light beam travels in the free space. In a fiber-coupled type, the output of the single-mode laser is coupled with fiber cable to transfer the beam to a very long distance with low power loss and low disturbance. These diodes are suitable for the applications, such as confocal fluorescence microscopy, laser seeding, gas sensing, and telecommunications.
Single-mode laser diode (BTF package)
Understanding Single-mode (TEM00) or Gaussian mode
The type of material and their compositions used in the fabrication of laser diode decides the wavelength of light. The ridge width of a single-mode laser diode typically measures from 3 to 10 microns. Because of the smaller width, this type of diode emits a light beam with only one mode of propagation. The ridge width also decides the capacitance value of the diode. The capacitance value of the single-mode laser diode is low, which allows for quick switching operation, and hence it is suitable for high-speed applications. Due to the lower width, the output power will be low, usually less than 1 W.
Specifications of a single-mode laser diode:
Central wavelength: Represents the wavelength of laser emitted from the diode. The wavelength is represented in nm (nanometre). The single-mode laser diode can have a wavelength range of 375nm to 2128nm.
Longitudinal modes: Single
Output power: Represents the output power of the laser diode. Usually, it is in the range 8 mW to 800 mW (milliwatts).
Transversal mode: TEM00
Beam diameter at aperture: It is in mm range.
Threshold current (Ith): It is the current at which the output optical power increases sharply. When the laser diode is biased below this threshold current, the output optical power will be very low. Usually, the threshold current is in the range of 17 mA - 250 mA.
Working current: It is the driving current or input currents when the laser diode reaches the rated power. Usually, the operating current is in the range of 20 mA to 200 mA.
Operating voltage: Represents the supply voltage. Normally, it has a 4.5 V to 5 V range.
Reverse voltage: Represents the maximum allowable reverse voltage, when it is applied across the laser diode. Beyond the reverse voltage, the diode will damage. It is usually in the range of 2 V.
Vertical Divergence Angle (θ⊥): The angle at which the light-emitting band spreads in a direction vertical to the PN junction. It is usually in the range of 15˚-40˚.
Horizontal Divergence Angle (θ∥): The angle at which the light-emitting band spreads in a direction parallel to the PN junction. Typically, it is in the range of 6˚-10˚.
Operating temperature: Represents the safe operating temperature of the laser diode. Typically, it has a range of -10 °C to +80°C.
Storage temperature: Represents the storage temperature range of laser diode. Typically, it has a range of -45 °C to +80°C.
Package type: Represents the type package used to pack the laser diode. Usually, the single-mode laser diodes are available in the SOT, Flatpack, Butterfly (BTF), bar, TO-can, TO-Can pigtailed, HHL, and DIL packages.