What are Polarization maintaining laser diodes?
Polarization-maintaining laser diode (Butterfly package)
A polarization-maintaining laser diode is a type of laser diode that is coupled with a polarization-maintaining fiber to maintain the polarization for applications that need this. Polarization-maintaining optical fiber (PM-fiber) is a specially designed optical fiber for polarization-maintaining purposes. Whenever a linearly polarized light is properly launched at the one end of the polarization-maintaining fiber, it will propagate through the fiber and exit at the other end with the same specific polarization (as in the sending end). i.e., the PM optical fiber maintains the polarization of the light that is passing through it. The output wavelength of the PM-laser diodes varies from 532 to 1617 nm. These diodes are used to give an input to a modulator in telecommunication systems, used in electro-optics, the aerospace industry, fiber optic sensors, and metrology applications.
Understanding polarization-maintaining fiber (PM-fiber):
Cross-sections of three types of PM fiber
This polarization-maintaining characteristic is achieved by intentionally introducing a systematic linear birefringence ( optically anisotropic materials) near the core of the fiber during the manufacturing process of polarization-maintaining fiber. Based on the shape of the birefringence material used near the core of the fiber, three types of polarization fibers are available. They are Panda PM-fiber, elliptical-clad PM-fiber, and the Bow-Tie type PM fiber. In a panda PM-fiber, the birefringence is introduced as the two stress rods near the core. The birefringence shape is elliptical in an elliptical-clad PM-fiber and bow-tie in a bow-tie PM fiber.
Due to the birefringence, the two polarization modes (vertical and horizontal polarization) have distinct phase velocities (because of different propagation constant) and hence the cross-talk between these modes is eliminated. This results in the polarization state of the transmitted light and it is preserved.
In a conventional fiber (Polarization-non maintaining fiber), the two polarization modes of light (vertical and horizontal polarization) have the same nominal phase-velocity (because of the same propagation constant) due to the circular symmetry of fiber. This same phase-velocity leads to cross-talk between the vertical and horizontal polarization. Cross-talk occurs under the fiber irregularities, external temperature, and mechanical stress. Hence, the regular fiber cannot maintain the polarization state of the laser beam.
Specifications details of polarization-maintaining laser diodes (PM-laser diodes):
Central wavelength: Represents the wavelength of laser emitted from the PM-laser diode. The wavelength is represented in nm (nanometre). Mostly, the PM-laser diodes can have a wavelength range of 532 nm to 1617 nm.
Technology: These diodes are available in the following technology. Distributed feedback laser (DFB) / Distributed-Bragg reflector (DBR) / Fabry-perot (FP) laser / Quantum well/ Quantum dot/ Multi-quantum well / Fiber-Bragg-Grating (FBG).
Configuration: Single emitter
Operation mode: CW laser / Pulsed laser
Type: Fiber coupled laser diode. A fiber coupled laser diode emits light through the fiber optic medium.
Fiber modes: Mostly available in Single-mode, polarization maintaining. Few multi-mode PM-laser diodes are also available.
Fiber connector type: Represents the type of fiber connector used with the PM- laser diode. Usually, these types of laser diodes are available with FC or SMA905 or FC/APC connectors.
Spectral width (FWHM): It is the width of the optical spectrum at the half-maximum power. It is represented in nanometre. Usually, the spectral width of the PM-laser diodes can vary in the range of 3.5 nm to 10 nm.
Output power: Represents the output power of the laser diode. The PM-laser diodes output power can vary from milli-watts to hundreds of milli-watts 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. Mostly, the threshold current of the PM-laser diodes can vary from 5 mA to 180 mA.
Operating current: It is the driving current or input currents when the laser diode reaches the rated power. Usually, the operating current of the PM-laser diodes can from milli-Ampere to hundreds of milli-Amperes
Operating voltage: Represents the supply voltage. Mostly, a PM-laser diode can have the operating voltage range of 1.4 V to 3 V.
Reverse voltage: Represents the maximum allowable reverse voltage, when it is applied across the laser diode. Beyond the reverse voltage, the diode will damage. Usually the reverse voltage of a PM-laser diode is 2 V.
Operating temperature: Represents the safe operating temperature of the PM-laser diode. Typically, it has a range of -20 °C to +75°C.
Storage temperature: Represents the storage temperature range of PM-laser diode. Typically, it has a range of -40 °C to +85°C.
Package type: Represents the type package used in the PM-laser diodes. Mostly, the PM-laser diodes are available in the Butterfly (BTF), TO-Can pigtailed, and DIL packages.
